Dominique Juhé-Beaulaton

Historienne

Les bois sacrés face aux enjeux mondiaux de conservation de la biodiversité (Bénin et Togo)

Les bois sacrés représentent souvent les derniers îlots forestiers de l'aire d'étude (Sud du Bénin et du Togo). Ces formations forestières se trouvent menacées devant la pression foncière croissante combinée à l'évolution des pratiques culturelles. Une analyse rapide des facteurs de ces changements depuis l'arrivée des Européens dans cette région prendra en compte le contexte historique et culturel et l'identification des différentes catégories d'acteurs intervenant directement ou non dans la gestion des bois sacrés. Les recherches scientifiques orientées par les grandes conventions internationales ont précédé et accompagné la mise en place des politiques environnementales et culturelles et les actions des ONG. Ces différentes actions se traduisent par des modifications des modes de gestion de ces sites et une adaptation des pratiques culturelles dont les responsables politiques et religieux locaux sont les gardiens.. C'est ainsi que devant la diminution du pouvoir de ces chefs traditionnels (problèmes de succession, non respect des interdits religieux), l'administration forestière préconise la « restauration » de ces formations boisées par des plantations et que l'écotourisme apparaît comme une nouvelle forme de conservation de la biodiversité. Le renforcement du rôle de l'Etat passe par une définition du statut juridique de ces bois sacrés.

Introduction

Des « sanctuaires de la nature » d 'Auguste Chevalier (1933), aux « conservatoires de la faune et de la flore » des botanistes des années 1980 (V. J. [START_REF] Mama | Forêts fétiches; modèle de la conservation de la nature en République Populaire du Bénin[END_REF], la perception des bois sacrés 1 n'a guère évolué pour les scientifiques. Le thème du pavillon du Bénin à l'exposition universelle d'Aïchi au Japon en 2005 a porté notamment sur les « forêts sacrées 1 L'expression « forêt sacrée » couramment employée en français pour ces formations végétales, souvent de faible superficie -de 0,5 ha à 20 ha pour les plus grandes -s'avère peu appropriée. Une réflexion est menée par les ethnologues sur les définitions à partir de la classification des sites. Pour rendre compte de tous les sites qui composent un tel éventail, Stéphan Dugast (ethnologue IRD-UR 169) préfère celle de « lieux exceptés » au demeurant déjà employée dans la littérature consacrée à cette thématique par D. [START_REF] Liberski-Bagnoud | Les Dieux du territoire. Penser autrement la généalogie[END_REF] qui a elle-même emprunté l'expression à S. [START_REF] Czarnowski | Le morcellement de l'étendue et sa limitation dans la religion et la magie[END_REF]. L'expression « bois sacré » sera retenue ici par « commodité ». pour la conservation de l'environnement ». Pourtant, les études historiques montrent que leur conservation dépend plus de la pérennité des rites et des pratiques qui les régissent, transmis de génération en génération, que d'une gestion écologique de ces formations attribuée un peu rapidement aux responsables de ces sites. Jean-Pierre [START_REF] Chrétien | Les arbres et les rois, sites historiques du Burundi[END_REF] a d'ailleurs ouvert une nouvelle perspective de recherche en considérant ces bois comme des lieux de mémoire, témoins de l'histoire des relations des sociétés entre elles. En 1978, il préconisait déjà la préservation de ces bois par une réglementation adaptée, s'alarmant de la disparition de bosquets sacrés qui « font partie à plus d'un titre du patrimoine culturel ». Mon propos dans cette contribution sera de soulever les questions qui se posent aujourd'hui autour de leur conservation et d'apporter quelques éléments de compréhension des enjeux dont ces sites sont l'objet aujourd'hui. 2 Les bois sacrés dont il sera ici question sont situés dans l'aire culturelle Aja-Fon, dans le Sud du Bénin et du Togo,3 au niveau de l'interruption du domaine de la forêt dense humide et de l'arrivée jusqu'à la mer des formations de savanes guinéennes (« Dahomey Gap »). Les îlots forestiers encore présents sont généralement des lieux de cultes vodou qui sont sous la responsabilité de prêtres chargés de faire respecter des principes moraux et des interdits visant à conserver leur caractère « sacré » et leur intégrité. Ce sont généralement de simples lieux de culte, mais il existe différents types de bois sacrés, selon les vodou qui y résident ou les circonstances de leur fondation. Certains sont des lieux de réunions de sociétés secrètes qui avaient pour principale fonction d'assurer le maintien de l'ordre et une certaine cohésion sociale. D'autres abritent des couvents d'initiation, celle-ci pouvant durer plusieurs mois ou années selon les vodou. D'autres encore sont liées à l'instauration des royautés et certaines cérémonies particulières, comme l'intronisation des dignitaires politiques. Ces bois renferment très souvent une source, lieu de purification, qui se trouve ainsi protégée. Enfin, des rites funéraires particuliers sont parfois pratiqués dans certaines forêts. Ces sites sacrés représentent également des lieux où s'exercent plusieurs formes de pouvoirs, politique ou religieux, et de ce fait, ces lieux de culte ont connu de profondes perturbations depuis l'arrivée 2 Mon argumentation s'appuie sur des sources écrites et orales comprenant des récits de voyages européens, des documents d'archives, des documents administratifs, des rapports scientifiques et données recueillies sur le terrain jusqu'en juillet 2006. 3 Le peuplement est composé principalement des Ewe, des Aja, des Ayizo, des Xweda, des Fon, des Gun et des Yoruba. Ces populations originaires d'Oyo, une cité Yoruba du Nigeria se sont progressivement installées à partir du XV e siècle jusqu'à la fin du XIX e siècle. Elles présentent des caractéristiques culturelles communes, notamment sur le plan de la linguistique et de la religion : elles pratiquent le culte des vodou et la divination par le Fa. Les récits de leurs migrations relatent l'émergence de différents royaumes : Dahomey Ketou, Ouidah, Allada, Porto Novo.

des Européens sur cette côte. Actuellement, devant la pression foncière croissante combinée aux mutations socio-culturelles, ces bois sacrés se trouvent souvent menacés tout en étant considérés comme des conservatoires de la biodiversité.

les bois sacrés

Historicité des bois sacrés : reliques ou créations? 4

Depuis l'arrivée des Européens sur cette côte, au XVI e siècle, les sources écrites, récits Le Hollandais Bosman, un autre voyageur de la fin du XVII e siècle, remarque qu'« il n'y a point de village qui n'ait un petit bois sacré où les principaux vont faire leurs sacrifices soit pour le commun soit pour eux en particulier. Ils tiennent ces bois pour sacré et il y a des défenses très expressives de les polluer ou de les endommager en coupant quelques branches d'arbres. Outre la peine attachée à cette défense, ceux qui la violent attirent sur eux une malédiction universelle » (Bosman, 1705 : 155).

La relation village-bois sacré se retrouve encore dans ce texte qui précise en outre les interdits principaux et les risques encourus par les contrevenants. Aujourd'hui, on peut voir sur l'ancien emplacement du palais un îlot forestier où ont encore lieu des cérémonies religieuses. Lors de son séjour au Dahomey, l'explorateur anglais Burton observa les vestiges du palais du dernier roi xweda :

"Nothing now remains of the ancient glories of Savi; (...) a long trench, with a tall growth of trees, was the sole remnant of the palace occupied by the Whydah kings, whose descendants, even in their exile, held their ancient capital sacred". (Burton, 1864 : 94).

Les mutations sociales en cours depuis le XIX e siècle

La diffusion du christianisme à partir de la deuxième moitié du XIX e siècle entraîne des mutations sociales par les conversions de plus en plus nombreuses, l'abandon ou l'adaptation des pratiques et des rites vodou, le manque de respect des règles qui régissent les sites sacrés. En effet, dès leur arrivée, les missionnaires chrétiens rivalisent systématiquement avec les responsables des cultes vodou, à la fois dans leur prêches et dans l'implantation des églises qui bien souvent jouxtent des bois sacrés, dans le but non dissimulé de s'y substituer.

Ce processus s'accélère encore de nos jours avec la diffusion notamment des « nouvelles religions » issues du christianisme et dans une moindre mesure de l'islam5 . A la méfiance qu'inspirent les responsables religieux s'ajoutent les effets de l'exploitation agricole coloniale marquée par un accroissement considérable des défrichements aux dépens des zones forestières pour développer une économie de plantations (palmiers à huile, tecks) et de cultures de rentes (maïs, arachide, coton, café…). Le classement de zones de forêts, la construction de routes et de voies ferrées, les déplacements de villages pour des « raisons sanitaires » par l'administration coloniale provoquent le déguerpissement des habitants et donc l'abandon de nombreux lieux de culte. Le pouvoir colonial a donc appuyé, renforcé même, la position des missionnaires chrétiens. Pourtant, Le botaniste Auguste Chevalier fut l'un des premiers à s'alarmer de la déforestation accélérée de ces îlots boisés ; en 1933, il explique dans une communication à la Société de Biogéographie sur les bois sacrés, que « malheureusement, à notre contact, le primitif renonce à ses croyances, les bois sacrés disparaissent. Presque tous ceux du Bas-Dahomey qui entouraient des sources maintenant éteintes ou appauvries ont disparu depuis 20 ou 30 ans... » (Chevalier, 1933 : 37).

Ces changements dans l'exercice des pouvoirs locaux ont également contribué à altérer, modifier les perceptions que les habitants avaient de l'essence divine de ce pouvoir très souvent lié au culte rendu aux ancêtres et entraîné à plus ou moins long terme la désacralisation de nombreux lieux sacrés.

Enjeux actuels autour des bois sacrés

A côté de ce processus de revalorisation des pratiques religieuses locales, se pose cependant la question de la succession des chefs religieux et politiques locaux qui devient cruciale pour assurer la transmission des savoirs et des pratiques. Les responsables politiques 12 Il ne faut cependant pas généraliser la fixation des dates de cérémonies. Il peut y avoir un « dédoublement » du calendrier comme l'a observé S. Dugast à Bassar (Togo) : la date officielle, pour les autorités, et les dates traditionnelles, toujours en vigueur, où les vrais rites se déroulent, à l'abri des regards « officiels » (communication orale). et religieux sont de plus en plus jeunes, lettrés, souvent chrétiens, informés des actions politiques nationales et internationales dans les domaines culturels et naturels dont ils essaient de tirer parti pour le développement du territoire sous leur contrôle. En cas de vacance du pouvoir religieux et/ou politique, le site sacré peut être désinvesti ce qui peut conduire à son défrichement dans un but économique (mise en culture, création de marché, construction de dispensaire…). Les autorités politiques et religieuses villageoises se trouvent donc dans la nécessité de faire appel de plus en plus fréquemment aux agents forestiers pour faire respecter les règles de protection qui régissaient ces sites et dont ils perdent peu à peu le contrôle13 . De même, ils doivent en principe soumettre tout projet de gestion auprès des autorités administratives, que ce soit pour aménager, restaurer ou exploiter sur le plan touristique. Leur pouvoir politique s'exerce essentiellement dans le rôle d'intermédiaires qu'ils assurent entre les habitants et les autorités politiques administratives dont les communes représentent les unités de base. Il arrive même que les agents forestiers viennent appuyer les nouveaux chefs dont les pouvoirs ne sont pas toujours reconnus par tous les membres de la communauté.

Actions de l'administration forestière et politiques nationales

Dans ce contexte, les agents forestiers favorisent la mise en place de comités de gestion des forêts sacrées qui apparaissent au Bénin à partir de 199514 . Ces comités prennent parfois le relais des responsables religieux décédés restés sans successeur, ce qui n'est pas sans provoquer de conflits avec les habitants15 . Ces nouvelles instances, comités de gestion, conseils municipaux, essaient d'assurer la conservation des sites en mettant en place une gestion concertée (ou non), intégrant une restauration si besoin est, dont les objectifs ne sont plus seulement religieux et culturels mais aussi économiques et politiques.

Les opérations des agents forestiers sur les bois sacrés consistent essentiellement en actions de sensibilisation auprès des habitants, en plantations avec parfois la mise en place de plan de réhabilitation. Ces plans dépendent essentiellement des directeurs départementaux et donc de leur motivation. Ils ne s'observent pas dans tous les départements (Direction Départementale des Forêts et des Ressources Naturelles, départements du Mono et de l'Atlantique, Bénin). Ils sont inexistants au Togo pour le moment, même si cette année, la journée de l'arbre du 1 er juin a été célébrée en présence des autorités administratives dans le bois sacré d'Assouame dans la préfecture de Tsevie en vue de sa restauration par plantations. Les agents forestiers interviennent aussi pour constater les effractions et dresser des procès-verbaux 16 . Malheureusement, ils arrivent bien souvent trop tard pour arrêter le processus. C'est ainsi qu'un îlot forestier de 2 ha. situé sur la commune d'Akassato (Bénin) a pu être entièrement défriché par un collectif de villageois avant que les agents n'arrivent sur place à la demande de la famille détentrice des droits sur le bois. 17 Au Bénin, les agents appuient leurs actions sur l'inventaire national des forêts sacrées qu'ils complètent dans certains cas. Ces actions de l'administration forestière sont à mettre en relation avec les politiques nationales de protection de l'environnement. Au Bénin comme au 16 Ces rapports ne sont pas remontés à la direction centrale. L'archivage est souvent problématique et les mutations relativement fréquentes des agents ne permettent pas une conservation orale des actions entreprises sur le long terme. Ceci a été observé tant au Bénin qu'au Togo, où la situation semble encore plus préoccupante en relation avec la prise de conscience récente de l'intérêt de la conservation de ces formations. Jusqu'à présent les interventions des forestiers ne portaient que sur les forêts classées de l'Etat. 17 Dans le cadre de la loi sur la décentralisation, les communes élaborent des plans de développement local et dans ce cas, la création d'un marché avait pour objectif le développement de l'économie locale et l'emplacement choisi par les villageois a retenu le seul lieu non exploité et dont les détenteurs n'étaient pas sur place pour revendiquer leur droit. Cette opération de développement illustre la méconnaissance des acteurs locaux des textes législatifs en vigueur concernant l'exploitation des ressources naturelles. 18 Notons que l'évolution des mesures prises dans les deux pays est analogue quel que soit le régime politique en place, démocratique ou autoritaire, et suit en fait les grands courants internationaux de protection de l'environnement. Ceux-ci ont donné lieu aux grandes conférences et conventions internationales : les conventions sur la conservation des zones humides (Ramsar, 1971), la protection du patrimoine mondial naturel et culturel (Rio, 1972) ont précédé la conférence des Nations Unies sur l'environnement et le développement (Rio, 1992) qui a constitué une prise de conscience officielle des liens entre environnement et développement. Ce sommet a été suivi par plusieurs conventions. 19 Monographie du Bénin est non datée. 24 rapports de spécialistes sont cités en bibliographie datant au plus tard de 2000. Celle du Togo date de 2002. 20 l'article 94 de la loi 97-029 du 15 Janvier 1999, portant organisation des communes en République du Bénin stipule : « la commune veille à la protection des ressources naturelles, notamment des forêts, des sols, de la faune, de la flore, des ressources hydrauliques, des nappes phréatiques et contribue à leur bonne utilisation ». devrait permettre de procéder à l'inventaire de ce patrimoine et au classement d'un certain nombre de sites. Cependant, l'articulation entre ces différents textes de loi n'est pas aisée et leur application n'en sera pas facilitée. Dans le cadre de la décentralisation, un projet a été initié pour transférer la gestion de certaines forêts aux communes, le PAGEFCOM, Projet d'Appui à la gestion des Forêts Communales, dont le statut n'existe pas pour le moment.

L'aménagement et la gestion de près de 50 forêts sacrées du Sud Bénin est prévu, car selon le coordonateur 21 , « c'est surtout dans ces formations que se trouvent les espèces en voie de disparition. Il faut les reboiser et les faire revivre, car avec la révolution 22 , les forêts et les arbres ont été endommagés. » La phase 2 du Projet Bois de Feu, qui devrait bientôt commencer, prévoit de faire l'inventaire des ressources forestières en intégrant les bois sacrés. Entre 1985 et 2006, les bois sacrés sont passés des lieux de conservation des pratiques obscurantistes de sorcellerie à des hauts-lieux de conservation de la biodiversité. 22 . Sous le PRPB, parti de la république populaire du Bénin 23 Cette notion de paysage culturel a été définie en 1992 par le Comité intergouvernemental pour la protection du patrimoine mondial culturel et naturel et étendue aux critères retenus pour l'inscription de biens culturels sur la liste du patrimoine mondial [START_REF] Adjanohoun | La notion de paysage culturel et les liens nature-culture en Afrique[END_REF]. M. Rössler (2000 : 7) rappelle les trois catégories de paysages culturels reconnues par le Comité du patrimoine mondial (16 e session, 1992) : les paysages créés (jardins, parcs), les paysages évolutifs (reliques ou vivants, ils sont associés aux modes de vie traditionnels) et les paysages associatifs (association de phénomènes religieux, culturels à l'élément naturel comme les bois sacrés). Tous les lieux de conservation de la mémoire collective sont intégrés dans la construction de ce patrimoine national dépendant de la reconstruction-recomposition de sites en relation avec les actions de développement dont le tourisme et la conservation de la biodiversité sont deux composantes. Cependant, si les actions de protection de l'environnement intègrent la conservation de ces lieux en relation avec les politiques environnementales liées aux grandes conventions internationales, l'absence de concertation et la dilapidation des financements entre les différents projets de conservation du patrimoine culturel et de la biodiversité constituent un frein aux opérations de protection et de conservation.

Effets des décisions politiques sur les modes locaux de gestion

  : de l'histoire à la politique 1.1. La religion vodou, un panthéon dynamique L'histoire ou l'origine des sites sacrés renvoie à la constitution du panthéon vodou. Celui-ci est d'une grande complexité avec une très grande diversité des divinités et des interférences qui opèrent entre elles. Les principaux vodou émanent de la cosmogonie des populations de la région auxquels s'ajoutent un grand nombre de divinités en relation avec des événements historiques ou la manifestation de phénomènes interprétés comme surnaturels. L'interrogation du vodou Fa, intermédiaire entre les hommes et les dieux, par la divination confirme la présence d'une divinité. Ainsi la disparition d'un homme, la présence d'un serpent dans une termitière seront considérées comme des manifestations divines et des lieux de culte y seront alors aménagés. Chaque famille a ses propres divinités, dont l'ancêtre fondateur mythique ou déifié. Certains vodou, selon leur histoire ou leur rôle social, ont acquis une renommée en dehors du lignage. Ainsi, Dan, le dieu python des Xweda, a été adopté par la famille royale du Dahomey et des vodou de ce lignage se sont diffusés dans l'ensemble du royaume, au fur et à mesure des conquêtes, participant ainsi au contrôle politique du territoire. Les vodou contribuent aux stratégies de domination et d'alliances entre lignages et populations. Ils ont à la fois une connotation identitaire et politique qui dépasse la simple unité familiale. Chaque village a sa divinité tutélaire, liée aux circonstances de la fondation ou simplement installée par les premiers occupants, soit dans un bois sacré soit au pied d'un arbre, parfois planté, servant à matérialiser le lieu. En effet, les vodou, sous la forme de leur représentation matérielle, suivent les hommes dans leurs déplacements. Les relations entre les populations, les royaumes et les migrations humaines sont à l'origine de la création d'un grand nombre de lieux de cultes.

  Les exemples de ce type sont nombreux. Certains bois sacrés sont l'aboutissement de la sacralisation de « lieux historiques », devenus lieux de mémoire. Ces sites sont aussi respectés car un culte aux mânes des ancêtres continue à y être rendu. En effet, traditionnellement, les défunts étaient enterrés dans leur habitation qui était ensuite abandonnée. C'est ainsi que de nombreux lieux sacrés sont en réalité d'anciens sites d'habitats où les interdits religieux, notamment la culture, le défrichement, ont permis l'installation d'un couvert végétal forestier. Ces sites se remarquent dans les paysages par des groupements d'arbres ; les habitants continuent à s'y rendre pour célébrer les cérémonies en l'honneur des ancêtres qui y sont enterrés. Les études botaniques comme les sources historiques tant orales qu'écrites montrent que les bois sacrés sont soumis, et certainement depuis leur création, à une forte pression anthropique qui influence considérablement leur physionomie et leur composition floristique. Leur anthropisation est donc fort ancienne, ce qu'attestent les sources écrites, dès la fin du XVII e siècle. De plus, les récits de fondations de ces bois donnent à penser qu'ils ne peuvent pas tous être tenus pour des vestiges d'un couvert forestier originel. La sacralisation a permis le maintien d'une formation arborée lorsqu'il s'agit d'une relique de l'ancien couvert forestier ou son développement en cas de création sur le site d'anciens habitats ou d'installation d'un vodou.

  L'administration coloniale représente un autre facteur de déstabilisation. En effet, bien souvent, les administrateurs ont affermi leur position en nommant des chefs de villages qui n'étaient pas toujours les détenteurs du pouvoir politique « traditionnel », pouvoir qui était lié, voire imbriqué, au pouvoir religieux dont il pouvait dépendre. Les relations entre chefs religieux et administrateurs coloniaux ont souvent été conflictuelles, ce que montrent des documents d'archives de l'époque coloniale conservées à Porto Novo 6 . Dès les années 1900, nombre de responsables religieux sont arrêtés au Dahomey, emprisonnés et interdits de séjour dans leur village pour de longues années. En 1950, devant les « recrudescences de fétichisme criminel et délictueux », le gouverneur C. Valluy ordonne aux commandants de cercles d'établir un recensement et de recueillir des renseignements sur les couvents et les cultes traditionnels.

  Photo n°1 : Ouidah : Aménagements de la forêt de Kpassè après le Festival international des arts vodou de 1993 (© photo D. Juhé-Beaulaton)

  Photo n°1 : Le bois sacré a été sacrifié au développement de l'économie locale : les arbres abattus seront remplacés par les étalages du marché (© photo D. Juhé-Beaulaton)

  Togo 18 , les mesures de protection se sont renforcées au cours des années 1980-90. Depuis 1988, codes forestiers et codes de l'environnement ont remplacé la législation coloniale en vigueur depuis les années 1930. Des projets d'aménagement des forêts aux plans d'action environnementale mis au point par les ministères de l'Environnement créés entre 1993 et 1995 dans les deux pays, jusqu'à la Loi cadre sur l'environnement (Bénin) et les Lois sur la décentralisation (1999, transfert de gestion des ressources forestières aux communes), le cadre législatif évolue en application des Conventions internationales ratifiées par les deux pays : la diversité biologique (1993), les changements climatiques (1994) et la lutte contre la désertification (1996), même si les effets se font différemment sentir dans les deux pays étudiés. Suite à la ratification par le Bénin et le Togo de la Convention sur la diversité biologique, chaque pays a rédigé une monographie nationale de la diversité biologique faisant le point des connaissances des éléments de cette diversité afin de mettre en place la stratégie à adopter sur des bases rationnelles. 19 Parmi les domaines choisis pour être analysés figurent les forêts sacrées. Les travaux de N. Sokpon (2000) et K. Kokou ont contribué à la prise en compte de ces formations dans l'étude de la biodiversité. Les actions des scientifiques ont précédé la mise en place des politiques nationales. Au Bénin, en relation avec l'application de la loi de décentralisation 20 , une nouvelle loi en instance au parlement sur le foncier rural (2006) reconnaît les terrains portant des boisements naturels sur lesquels s'exercent des droits établis ou acquis selon les règles coutumières ou les pratiques et normes locales comme propriétés des particuliers, des organisations communautaires et relèvent du domaine forestier privé de ces particuliers. Les forêts sacrées ne sont pas clairement mentionnées mais elles sont directement concernées par ce texte législatif qui ne fait qu'entériner la situation existante. Il a le mérite de clarifier le statut de forêt privée dont les propriétaires doivent respecter les règles de la loi forestière. Parallèlement à toutes ces législations (forestière, environnementale, décentralisation et foncier rural), un projet de loi portant protection du patrimoine culturel et du patrimoine naturel à caractère culturel en république du Bénin est en cours d'élaboration et

  Le festival de Ouidah a joué un rôle de déclencheur et d'autres sites peuvent être cités comme lieux d'enjeux économiques en relation avec le développement de l'écotourisme, la reconnaissance des paysages culturels 23 et la volonté de conserver la biodiversité. Ces orientations politiques sont particulièrement observables à Bembé (Département de l'Ouémé -Bénin) où l'identification des différentes catégories d'acteurs autour de la conservation de la forêt sacrée de Bamèzoun montre une superposition d'actions sans réelle articulation. Un comité de gestion de la forêt sacrée est mis en place en 1995 dans l'objectif de pallier la vacance du pouvoir religieux. En 1998, la Direction départementale de la culture, de l'artisanat et du tourisme développe un projet d'exploitation touristique pour lutter contre la pauvreté et désenclaver la zone. Une signalétique des sites est mise en place. Des ONG, dont Nature tropicale, interviennent également dans la sensibilisation visant à la conservation et à la promotion de l'écotourisme. En 2002, l'Ecole du patrimoine africain y tient un atelier avec ses stagiaires qui prévoit un plan de gestion de la forêt. Enfin en 2004, démarre un Projet de réserve biologique intégrant les forêts sacrées (Ministère de l'Environnement/Banque 21 Entretien en juillet 2006 ; le texte du projet n'a pu être consulté.

  Photo n°2 : Bamèzoun (village de Bembé, Ouémé, Bénin) : forêt sacrée, site classé Ramsar, devient réserve biologique (© photo D. Juhé-Beaulaton)

  de voyages et documents d'archives, apportent des informations concernant l'état de la couverture forestière et relatent notamment l'existence de bois sacrés. Les descriptions de forêts sont rares. En effet, ces récits avaient comme principal objectif de faire connaître les ressources commerciales de cette région et non de présenter les paysages végétaux et leur

	richesse potentielle.
	Un auteur, resté anonyme, ayant séjourné à Ouidah à la fin du XVII e siècle et au début
	du XVIII e décrit ainsi la religion locale et ses sanctuaires : « Leurs temples sont des bouquets
	d'arbres fort hauts qui forment un bosquet. Tout le pays en est remply et dans chaque village il
	y en a pour le moins un. Dans ce bosquet est une petite maison en tourette... Tous les ans pour
	le moins une fois il se fait une assemblée de tout le pays à chaque principal bouquet d'arbre
	qu'ils nomment fétiche... Les vaudonous sont les prestres des dieux renfermés dans des
	bouquets d'arbres qui ont leurs chefs... ». (Anonyme, 1708 : 54) Ce voyageur a été marqué par la
	multitude de ces bosquets et leur omniprésence dans les paysages déjà ouverts à son époque. Les
	bois sacrés occupaient déjà des espaces restreints à proximité des lieux habités : ils se
	distinguaient parfaitement pour un observateur étranger qui a même noté la hiérarchie entre
	les bois.

Pour plus de détails sur l'historicité des forêts sacrées et leur évolution, voir : D.[START_REF] Juhé-Beaulaton | Arbres et bois sacrés : lieux de mémoire de l'ancienne Côte des Esclaves[END_REF] 2003) ; en collaboration avec B. Roussel, botaniste, professeur au MNHN de Paris : D.[START_REF] Juhé-Beaulaton | à propos de l'historicité des forêts sacrées de l'ancienne Côte des Esclaves[END_REF] 2003).

Les effets du Christianisme et des nouvelles religions semblent avoir autant d'impact au Togo et au Bénin.

Archives nationales du Bénin, Porto Novo, Série E : carton 3 E -2 : cultes traditionnels : correspondances.

Plusieurs décrets sur le régime forestier ont été publiés à l'époque coloniale, de 1935 à 1955, au Journal Officiel de l'AOF

Des projets de développement visent à résoudre la question énergétique des villes. Le projet Bois de Feu a débuté en 1985 au Bénin et s'est achevé en 1997. Des pépinières et des plantations forestières ont été créées pour accroître l'offre en bois et réduire la pression sur les formations naturelles.

De nombreux mémoires de fin d'études agronomiques de la Faculté des Sciences Agronomiques de l'université d'Abomy-Calavi portent sur différents départements du Bénin. Au Togo, l'inventaire n'a pu être réalisé faute de moyen, mais les botanistes travaillent aussi sur cette question[START_REF] Kokou | Les forêts sacrées littorales du Togo : rôle culturel et de conservation de la biodiversité[END_REF]. Quelques étudiants commencent à travailler sur ce sujet.

Ou bien les habitants se tournent vers les nouveaux mouvements religieux, les deux processus s'observant simultanément.

Voir « la réinvention de la tradition » développée par R.Banégas (2003 : 309 et suivantes).

Ce qui s'observe au Bénin comme au Togo.

Au Togo, l'administration forestière n'intervient quasiment pas dans la gestion des bois sacrés. Les agents rencontrés à la Direction centrale de Lomé pensent que toute intervention serait perçue comme une ingérence dans les affaires locales. Au niveau préfectoral, les agents reconnaissent des interventions ponctuelles, encore rares.

En effet, bien souvent, les responsables religieux, à leur mort, n'ont pas de successeur : cette fonction très contraignante impose au chef d'abandonner toute autre activité professionnelle et de rester au village ; la scolarisation, l'exode rural, la christianisation ou l'islamisation renforcent la difficulté de la sélection des « candidats ». La transmission des pratiques religieuses et des savoirs associés bien souvent n'est plus assurée. Ce problème se pose non seulement pour les fonctions religieuses mais également pour les pouvoirs politiques « locaux ».

En 1971, la convention sur la conservation des zones humides s'est tenue à Ramsar en Iran.

Au Togo, les forêts étudiées demeurent « fermées » au tourisme et les cérémonies secrètes, difficilement accessibles aux non-initiés.

Ce qui est par ailleurs un signe de l'affaiblissement du respect des interdits.

K. Kokou a effectué l'analyse biologique et biogéographique de la flore des forêts sacrées de la zone d'étude et ces travaux montrent que dans bien des cas, leur structure verticale est complètement perturbée ; les grands


Arindam Sannigrahi

Shogo Takamuku

Patric Jannasch
email: patric.jannasch@chem.lu.se

Block selective grafting of poly(vinylphosphonic acid) from aromatic multiblock copolymers for nanostructured electrolyte membranes

Keywords: Block copolymers, Polyelectrolytes, Ionomers, Microphase separation, Water uptake, Proton conductivity, Fuel cell membranes

Block selective grafting of poly(vinylphosphonic acid) from aromatic multiblock copolymers for nanostructured electrolyte membranes

Arindam Sannigrahi, Shogo Takamuku, Patric

Introduction

Phosphonated polymers are acidic, capable of hydrogen bonding and have a strong affinity for a broad variety of inorganic surfaces and metal ions. Their properties open up for a wide range of different application areas including flame-retarding additives, [1][2] anti-corrosion 3 and antimicrobial coatings 4 , dental materials, [5][6] thermoresponsive materials 7 , and materials for water purification and proton-exchange membranes. [8][START_REF] Lafitte | On the prospects for phosphonated polymers as proton-exchange fuel cell membranes[END_REF][START_REF] Rusanov | Proton-Conducting Polymers and Membranes Carrying Phosphonic Acid Groups[END_REF] To this end, both fluorinated [START_REF] Tayouo | [END_REF][12][13][14][15][16] and nonfluorinated [17][18][19][20][21][22][23][24][25] polymers have been functionalized with phosphonic acid and studied for different purposes. Poly(vinylphosphonic acid) (PVPA) in particular contains a very high concentration of phosphonic acid units directly attached to the polymer backbone. [26][27] The polymer is water soluble and shows a strong polyelectrolyte effect with only one proton per -PO(OH) 2 unit titratable. [START_REF] Lafitte | On the prospects for phosphonated polymers as proton-exchange fuel cell membranes[END_REF] Its very high ionic content enables the preparation of proton-exchange membranes with very high ion exchange capacities (IECs). PVPA thus shows high proton conductivity, although anhydride formation leading to P-O-P bridges seriously reduces the conductivity under low relative humidity and high temperature. [START_REF] Tayouo | [END_REF][12][13] Various block 28,29 and graft [30][31][32][33] copolymers, inorganic composites 34 a s w e l l a s interpenetrating networks 35 , based on PVPA have been prepared to circumvent the issues of water solubility and studied as membranes for fuel cells. We have previously grafted PVPA from polysulfone 31 (PSU) and poly(phenylene oxide) 32 (PPO) via anionic polymerisation. The former polymer was lithiated exclusively in positions ortho to the sulfone bridges using butyllithium, and used as polyfunctional macroinitiators for anionic polymerisation of diethyl vinylphosphonate. After hydrolysis and membrane casting, the PVPA grafted materials showed relatively high proton conductivity in the nominally dry and fully hydrated conditions in the range of 1-5 and 10-100 mS cm -1 , respectively. 31,32 Very high IECs are needed to reach high proton conductivity at low water contents. However, this leads to excessive water uptake at high relative humidity which, in turn, causes swelling, dilution effects and compromises the mechanical stability of the membranes. The PVPA grafts in the PSU and PPO materials were randomly distributed along the polymer backbone, giving relatively short PSU segments inbetween the grafts. This leads to the formation of quite small and non-uniform glassy PSU phase domains in the membranes to stabilize the membrane morphology and limit the water uptake.

One strategy to alleviate these issues is to only graft specific segments in the aromatic polymer, leaving larger aromatic segments to form large glassy phase domains to stabilize the membranes (Scheme 1). Retaining the "grafting from" approach, this would require an aromatic block copolymer structure where one segment type could be selectively lithiated and grafted. Aromatic multiblock copolymers have previously been prepared by coupling of chain-end functionalised precursor segments of, e.g., PSUs and polyketones formed by polycondensation via nucleophilic aromatic substitutions. [36][37][38][39][40] However, nucleophilic aromatic substitutions require the presence of electron-withdrawing groups, such as ketone, sulfone, and nitrile, to activate the halide atoms for substitution. With the exception of sulfone groups, these are in general incompatible with the lithiation reaction.

In the present work we have prepared multiblock copolymers by coupling PSU and polyfluoroether (PFE) precursor blocks. The latter block was prepared by polycondensation of hexafluorobenzene and bisphenol P to give a block structure where all the residual fluorine atoms were deactivated for substitution by their location in ortho positions to ether bridges.

Accordingly, we hypothesised that this specific PFE block would remain unaffected during the addition of butyllithium. The PSU blocks of the carefully designed multiblock copolymers were selectively lithiated and grafted with diethyl vinylphosphonate. The ester functions of the grafted multiblock copolymers were hydrolysed and membranes based on the PVPA containing multiblock copolymers were evaluated by thermal analysis, microscopy, and water uptake and proton conductivity measurements to evaluate their use as electrolyte materials.

Experimental

Materials

4,4 ′ -(1,4-Phenylenediisopropylidene)bisphenol (Bisphenol P, Sigma-Aldrich, 99%) and 4,4 ′dichlorodiphenylsulfone (DCDPS, Acros, 97%) were recrystallized from toluene and dried in vacuum oven at 80 °C before use. Resorcinol (RS, Acros, 98%), hexafluorobenzene (Aldrich, 99%), n-butyl lithium (BuLi, in hexanes, Acros, 2.5 M), 1-methyl-2-pyrrolidinone (NMP, Acros, 99%), N,N-dimethylacetamide (DMAc, Acros, 99%), dimethylsulfoxide (DMSO, Acros, 99.9%), 2-propanol (Honeywell, 98%), n-hexane (Scharlau, 96%), methanol (Honeywell, 98%), toluene (Fisher Scientific, HPLC grade), 1,1-diphenylethylene (DPE, Aldrich, 99%), diethyl vinylphosphonate (DEVP, Aldrich, 97%) and hydrochloric acid (Honeywell, 37%) were used as received. K 2 CO 3 (Acros, 99%) was dried at 120 °C and tetrahydrofuran (THF, Fisher Scientific, Analytical reagent grade) was dried with 5 wt% molecular sieves (Acros, 4 Å, 8-12 mesh) before use. The molecular sieves were activated at 250 °C for 48 h in an oven and subsequently cooled to room temperature under vacuum.

Synthesis of PSU precursor blocks

The PSU precursor block was prepared according to Scheme 2. Hydroxy terminated oligomers were first synthesised as follows. DCDPS (13.2797 g, 46.245 mmol), RS (5.5166 g, 50.101 mmol), K 2 CO 3 (7.67 g, 55.5 mmol) and DMAc (75 mL) were added to a two-neck flask (250 mL) equipped with a magnetic stirrer, a N 2 inlet and a condenser fitted with a CaCl 2 filter. The mixture was initially heated at 160 o C for 4 h and then at 175 o C for 24 h. A very low flow of N 2 was maintained throughout the polycondensation reaction. Finally, the product was precipitated in deionized water and washed several times with deionized water. The precipitate was filtered and dried under vacuum at 30 o C for 24 h (overall yield: 94%). The molecular weight was determined to be M n = 3.8 kDa using 1 H NMR data recorded with a CDCl 3 solution.

Reactive end groups were obtained by capping the PSU oligomer with hexafluorobenzene in NMP at 80 o C. In a 100 mL two neck flask, 7.00 g oligomer (1.81 mmol) was added together with K 2 CO 3 (2.45 g, 18.1 mmol), hexafluorobenzene (4.18 mL, 36.2 mmol) and NMP (50 mL). The mixture was kept at 80 o C for 24 h, before precipitating the product in 2propanol. The product was carefully washed with deionized water and dried in a vacuum oven at 30 o C for 24 h (overall yield: 87%). This sample was designated PSU4.

Synthesis of PFE precursor blocks

Two hydroxyl terminated PFE precursor blocks with different molecular weights were prepared by a polycondensation (Scheme 2) in a three-neck 100 mL flask equipped with a magnetic stirrer, a Dean-Stark trap filled with toluene, a N 2 inlet, and a condenser fitted with a CaCl 2 filter.

Bisphenol P (5.7689 g, 16.6505 mmol) and K 2 CO 3 (2.69 g, 19.5 mmol) were first heated in DMAc at 110 o C for 3 h for dehydration during azeotropic removal of the toluene. The mixture was then cooled to 30 o C before adding hexafluorobenzene (1.87 mL, 16.24 mmol). The reaction temperature was kept at 50 o C for 3 h and then at 80 o C overnight. The N 2 purging was stopped after adding the hexafluorobenzene. The oligomer was precipitated in methanol and washed with deionized water. It was filtered and dried in a vacuum oven at 30 o C for 24 h (overall yield: 94%). The molecular weights of the two hydroxy terminated PFE precursor blocks were determined to be M n = 6.1 and 8.0 kDa from 1 H NMR data recorded with a CDCl 3 solution.

These samples were designated PFE6 and PFE8, respectively.

Synthesis of PFE-PSU multiblock copolymers

Two alternating multiblock copolymers were synthesised by coupling of the two PFE precursor blocks (PFE6 and PFE8, respectively) with the PSU4 precursor block in NMP at 80 o C during 4 h (Scheme 2). The two copolymers were designated PFEx-PSU4 where x is either 6 or 8 for the two PFE precursor blocks. As an example, block copolymer PFE8-PSU4 was synthesised as follows. PFE8 (8.0 kDa, 2.6959 g, 0.337 mmol) and PSU4 precursor block (3.8 kDa, 1.30 g, 0.337 mmol), K 2 CO 3 (0.056 g, 0.404 mmol) and NMP (22 mL) were added to a 100 mL threeneck flask equipped with a magnetic stirrer, a N 2 inlet, and a condenser fitted with a CaCl 2 filter.

The mixture was heated to 80 o C under N 2 flow. After 4 h the mixture became viscous. The multiblock copolymer was precipitated in 2-propanol and washed with deionized water. It was then filtered out and dried in a vacuum oven at 30 o C for 24 h (overall yield: 93%).

Block selective grafting of PVPA

The PSU blocks of the multiblock copolymers were selectively lithiated and grafted with PVPA via anionic polymerisation of DEVP as shown in Scheme 3. The grafted multiblock copolymers were designated PFExPSU4-g-PVPAy where y is the content of PVPA in wt%. As an example, PFE8PSU4-g-PVPA57 was synthesised as follows. An amount of 0.500 g (0.456 mmol) multiblock copolymer PFE8PSU4 was dissolved in 100 mL of anhydrous THF in a round bottom glass reactor fitted with a gas inlet and outlet, a thermometer, and a rubber septum. The solution was cooled to -70 °C and degassed several times. Next, the required amount of n-BuLi (0.2 mL, 0.456 mmol) was added dropwise to form a yellow-orange solution. After 15 min, DPE (25 µL, 0.228 mmol) was added. After leaving the reaction for 10 min, the required amount of DEVP (0.35 mL, 2.28 mmol) was added all at once by syringe, and the polymerization was left to proceed for 5 h at -70 °C. At the addition of the DEVP monomer the solution turned opaque and violet and the viscosity increased. The grafted multiblock copolymer was precipitated in 200 mL of hexane and the yellow-white precipitate was filtered off and washed several times with distilled water. Finally, the copolymer was dried under vacuum during 24 h at 30 °C. The copolymer in the ester form was converted into the acid form in 37% aq. hydrochloric acid. In a typical reaction, 0.5 g of copolymer in the ester form was placed in 25 mL of the hydrochloric acid. After 24 h of reflux, the copolymer was filtered off and carefully washed with water. The copolymer was finally dried under vacuum for 24 h at 60 °C.

Polymer characterization

Size exclusion chromatography (SEC) was used to determine the molecular weight of the precursor blocks and the PFEx-PSU4 multiblock copolymers. All samples were analyzed on a Viscotek GPCmax VE-2001 equipped with ViscotekTriSEC Model 302 triple detector using two PLgel mixed-D columns from Polymer Laboratories with chloroform at an elution rate of 1 mL min -1 at room temperature. Calibration was done using polystyrene standards. 1 H, 19 F, and 31 P NMR data were recorded using a Bruker DRX 400 spectrometer. All the spectra were recorded in CDCl 3 and DMSO-d 6 solvents at room temperature. The ratio between the integrated peak of the CH 2 protons of the phosphonic ester at δ = 4.1 ppm and the integrated peaks of the aromatic protons of the main chain between δ = 6.8-8.1 ppm was used in the calculations of the PVPA content and the theoretical ion exchange capacity (IEC) of the hydrolyzed copolymers (Table 2). 31 In the phosphonic acid units of the PVPA side chains act as monoprotic acids, 26,31 and thus the theoretical IEC is defined as mmol P per g protonated membrane. FTIR spectra were recorded on a Bruker IFS 66 FT-IR spectrometer between 400 and 4000 cm -1 at a resolution of 4 cm -1 using 128 scans. The polymers were ground together with KBr before sample tablets were pressed.

Thermal analysis

Thermogravimetric analysis (TGA) was carried out on a Q500 TGA analyzer from TA instruments. The thermal stability of the polymers was measured under nitrogen and air, respectively, up to 600 o C at a scan rate of 10 and 1 o C min -1 , respectively. Initially the samples were kept at 150 °C isothermally for 10 min to remove potential water and solvent residues. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q1000 DSC to determine the glass transition temperatures (T g s) for all the precursors and multiblock copolymers. Samples were first heated from 50 to 300 °C and then cooled to 50 °C and finally heated to 300 °C at 10 °C min -1 . The reported T g s were determined from the second heating scan.

The PFExPSU4-g-PVPAy copolymers were analyzed using the following sequence of heating scans at a rate of 20 °C min -1 : 0 → 150 → 0 → 200 → 0 → 250 → 0 → 300 °C. In-between the heating scans the samples were cooled at a rate of 30 °C min -1 . The T g s were evaluated from the data recorded during heating scan by identifying the inflection points.

Microscopy

The surface morphologies of the grafted copolymers were investigated by Atom Force Microscopy (AFM) in tapping mode using an XE-100 Park system. Topology and phase images were recorded at a scanning frequency of 1 Hz. No image processing was done.

Membrane preparation

The required amount of PFExPSU4-g-PVPAy copolymer in the acid form was dissolved (5 wt%) in NMP at 100 °C. Membranes were then cast in Petri dishes at 100 °C under a continuous nitrogen flow during 24 h. The transparent and slightly yellow membranes were detached from the Petri dishes after immersion in deionized water. To remove impurities or possible solvent residues from the membranes, they were first boiled and washed several times in deionized water and then dried under vacuum at 30 °C for 24 h.

Water uptake measurements

The membranes were first thoroughly dried under vacuum for 24 h at 30 °C before measuring the weight of the dry membranes. Next, the membranes were immersed in deionized water for two days at room temperature. The surface water of the wet membranes was quickly wiped off with tissue paper before measuring the weight of the wet membranes. The water uptake (W water ) was then calculated as:

W water = % 100   dry dry wet W W W Eq.1 -8 -
where W wet and W dry were the weights of the wet and dry membranes, respectively. Similarly, the temperature dependent water uptake was investigated from 20 to 100 °C under immersed conditions.

Proton conductivity measurements

The proton conductivity was measured by electrochemical impedance spectroscopy (EIS) using a Novocontrol high-resolution dielectric analyzer V 1.01S equipped with a Novocontrol temperature system. The proton conductivity (S cm -1 ) of the membranes was measured between -20 and 120 °C under fully hydrated conditions over the frequency range of 10 -1 -10 7 Hz at 50 mV amplitude in a two-probe cell. The data was analyzed using the software WinDeta from Novocontrol to evaluate the proton conductivity.

Results and discussions

Multiblock copolymer synthesis

We have previously successfully grafted PSU homopolymers with PVPA side chains to prepare highly proton conducting polymers and membranes. 31 PSU is exclusively lithiated in the aromatic ortho positions of the sulfone bridges and thus forms an efficient macroinitiator for anionic polymerization after reaction with DPE. In the present study, our aim was to identify and synthesize multiblock copolymers with a combination of blocks that would enable one of the block types to be selectively lithiated and grafted without affecting the other block type. To that end, two different multiblock copolymers having PFE and PSU blocks were prepared by coupling of precursor blocks under mild conditions to avoid branching and other side-reactions (Scheme 2). We assumed that the PFE under suitable conditions would be left unmodified during the lithiation and the subsequent anionic polymerisation. The PFE precursor blocks were prepared by polycondensations of bisphenol P and hexafluorobenzene, and the PSU precursor block were prepared from DCDPS and RS. The slight excess of bisphenol P and RS, respectively, were employed in the reactions to ensure that the precursors were terminated by hydroxyl groups and that the predetermined molecular weights were reached. The PSU precursor was quantitatively end capped with hexafluorobenzene to enable efficient coupling reactions.

Hexafluorobenzene and bisphenol P were chosen for the hydrophobic PFE block in order to avoid any reaction with BuLi during the lithiation reaction and to favor solubility in THF, the preferred solvent during the lithiation. We hypothesized that the position of the fluorine atoms ortho to the ether bridges in the PFE would efficiently deactivate any nucleophilic substitution reactions potentially caused by the lithiated species. Concurrently, DCDPS and RS were selected for the PSU block in order to have a high density of diaryl sulfone sites for lithiation. Moreover, the meta configuration induced by RS facilitates solubility in THF. In all cases, the polycondensations gave hydroxyl terminated precursor blocks with well-controlled molecular weights and solubility in THF.

Two different PFExPSU4 multiblock copolymers were prepared by varying the PFE block length (6.1 and 8.0 kDa), keeping the PSU block length constant and smaller (3.8 kDa).

All the precursor blocks and the PFExPSU4 multiblock copolymers were characterized by 1 H NMR, SEC, DSC and TGA, and the data are collected in Table 1. Representative 1 H NMR spectra are shown in Fig. 1 and confirmed the structure of the respective samples. The integrals of the shifts originating from protons a and c ′ were used to calculate the molecular weight of the PSU precursor block (Fig. 1a), and the integrals of the signals from protons g and f' were used to calculate the molecular weights of the PFE precursor blocks (Fig. 1b).

By DSC analysis it was found that the T g of the PFE precursor blocks increased with molecular weight (Table 1), as expected. Each of the two multiblock copolymers showed two T g s to indicate microphase separation of the PFE and PSU blocks (Fig. 2). The T g1 of the PFE phase was approximately 10 o C higher than the respective neat precursor block, and the T g2 of the PSU phase was approximately the same as for the neat PSU precursor block. This may indicate a partial mixing of the PSU blocks in the PFE phase domain, and/or that the PFE chain segments were considerably constrained by the connectivity with the PSU blocks after the DSC cooling scan.

Selective grafting of multiblock copolymers

The PFExPSU4 multiblock copolymers were used as macroinitiators for anionic graft polymerization of DEVP (Scheme 3). The aim was to selectively and densely graft the PSU blocks with PVPA side chains to obtain multiblock copolymers having continuous phases with very high local concentrations of proton conducting phosphonic acid groups. After lithiation of the multiblock copolymers, DPE was added to form the active initiator sites for the polymerization of DEVP at -70 o C. We have previously used the same strategy to polymerize were hydrolyzed by reflux in aq. HCl. In this way, five different grafted multiblock copolymers were made with PVPA contents ranging from 57 to 82 wt% (Table 2). The phosphonated copolymers in both the ester and the acid forms were characterised by 1 H, 19 F, 31 P NMR and IR spectroscopy. Before hydrolysis, the signals of the CH 2 and CH 3 protons of the phosphonic ester were found by 1 H NMR at δ = 4.1 ppm and at δ = 1.2 ppm, assigned as b and a, respectively, in Fig. 3a. The CH and CH 2 protons from the poly(diethyl vinylphosphonate) chains were confirmed by the presence of two broad signals between δ = 1.3-1.7 ppm (see Fig. 3a).

One of the crucial points of the present work was to selectively graft the PSU blocks with PVPA without affecting the PFE blocks. BuLi is very reactive and there is a risk that the fluorine atoms of the PFE block may be substituted in connection with the grafting reactions. In order to ensure that the lithiation and the subsequent grafting were selective, we analyzed the copolymers before and after grafting by 19 F NMR spectroscopy. As expected, the solubility greatly changed after the grafting which complicated the analysis. The hydrophobic PFExPSU4 copolymers were readily soluble in CDCl 3 , while the hydrophilic PFExPSU4-g-PVPAy copolymers in the ester form easily dissolved in DMSO-d 6. The former copolymers just barely dissolved in DMSO-d 6 after ultrasonication and heating at 100 o C for 2 h. Fig. 4a shows the expected single sharp signal at δ = -154.3 ppm from the PFExPSU4 copolymers in CDCl 3 to indicate that all the fluorine atoms were chemically equivalent. The same sample shows a much broader asymmetrical signal at δ = -155.3 ppm in DMSO-d 6 , most probably because of the limited solubility (Fig. 4b). After grafting, the spectra recorded in DMSO-d 6 showed a single symmetrical rather broad signal at δ = -155.3 ppm, again indicating that all the fluorine atoms were equivalent (Fig. 4c). Thus, the fluorine signal was not shifted after the grafting which indicated that PFE was not affected by the grafting reactions. In addition, multiple signals in the 19 F NMR spectra would be expected if some of the fluorine atoms had been displaced. We concluded that the PSU blocks were selectively lithiated and grafted while the PFE blocks were left unmodified. In this context it should be mentioned that a corresponding multiblock copolymer having PFE blocks based on decafluorobiphenyl, instead of hexafluorobenzene, was not possible to lithiate selectively under the same conditions and multiple signals were found in the 19 F NMR spectrum. In contrast to the PFE blocks based on hexafluorobenzene, all the fluorine atoms in PFE blocks based on decafluorobiphenyl are not placed ortho to ether bridges, and are thus not deactivated for nucleophilic substitution during the lithiation.

The quantitative hydrolysis of the diethyl phosphonate ester groups was indicated by the disappearance of the characteristic signals of the phosphonic ester in the 1 H NMR spectra of the hydrolyzed products (Fig. 3c). Based on this result, it was possible to calculate the IEC from the spectra and these IEC values are collected in Table 2 for all five grafted multiblock copolymers.

In comparison with the 31 P NMR signal arising from the diethyl ester derivatives at δ = 32.9 ppm (Fig. 3b), the hydrolyzed copolymers showed a slightly lower chemical shift at δ = 29.2 ppm (Fig. 3d). The latter shift was attributed to the diacid form of the phosphonate units. 31 In some cases a small signal at δ = 21 ppm was observed in the 31 P NMR spectra of the hydrolyzed copolymers. This signal confirmed the presence of anhydrides produced through condensation of the phosphonic acid groups to form P-O-P bonds.

Representative FTIR spectra of the PFExPSU4 multiblock copolymers and the PFExPSU4-g-PVPAy multiblock copolymers in both the ester and acid forms are presented in Fig. 5. Before analysis, the PFExPSU4-g-PVPAy multiblock copolymers were dried in a vacuum oven at 30 °C overnight to remove traces of water. The P=O stretching is usually found to absorb at 1320-1200 cm -1 . 31 The FTIR spectrum of the grafted copolymer in the ester form (Fig. 5b) showed four new bands in comparison with those identified in the spectrum of the ungrafted copolymer in Fig. 5a. The bands at 1052 and 1023 cm -1 , as well as the band at 778 cm -1 , were assigned to (P)-O-C absorptions of the ester group, and the band at 952 cm -1 was ascribed to P-O-(C) vibrations. As shown in Fig. 5c, these bands disappeared after hydrolysis, and the (P)-O-H bending vibration at 990 cm -1 and a broad (P)-O-H stretching band at 2300 cm -1 appeared for the acidic copolymers (not shown). Thus, the FTIR spectra confirmed the successful hydrolysis of the phosphonate ester groups.

Thermal properties

The thermal stability of all the multiblock copolymers was analyzed up to 600 °C under nitrogen at a heating rate of 10 °C min -1 , as well as under air at 1 °C min -1 . Fig. 6a shows representative TGA curves of a PFE-PSU multiblock copolymer and grafted copolymers derived from this copolymer in the ester and acid form. The ungrafted multiblock copolymers showed decomposition temperatures (T d s) at approx. 450 o C. All the grafted copolymers in the ester form initially lost weight because of absorbed water. The first degradation step began at around 250 o C and was due to the decomposition of the ester groups by the formation and loss of ethylene. The magnitude of the weight loss was directly proportional to the PDEVP content of the samples, as previously noted for other copolymers. 31,41 The second main degradation step between 400 and 500 o C was assigned for the cleavage of the C-P bond and degradation of the PVPA chain. 18 For all the grafted copolymers in the acid form, an initial weight loss was observed above 150 o C attributed to the loss of water and formation of P-O-P anhydride bonds. This was in accordance with previous findings. 26,27,31,32 Fig. 6b shows the TGA curves of the grafted multiblock copolymers with different PVPA contents ranging from 57 to 82 wt% under air at 1 °C min -1 . The initial weight loss (less than 1%) above 150 o C was assigned to the reversible condensation of the phosphonic acid, as discussed already. The small weight increases observed above 300 o C are most probably due to oxygen uptake in connection with the formation of phosphates as the phosphonic acid is degraded, and has been reported previously. 31,32 The main degradation step was noted above 350 o C for all PFExPSU4-g-PVPAy samples and was attributed to the degradation of PVPA side chains. From Fig. 6b, it is clearly visible that the magnitude of the weight loss above 350 o C decreased with increasing PVPA content, which is mainly attributed to the flame-retardant property of the phosphonic acid. [START_REF]Plastics Flamability Handbook: Pronciples Regulations, Testing, and Approval[END_REF] The T d s for all the PFExPSU4-g-PVPAy copolymer samples in the acid form are collected in the Table 3.

All the multiblock polymer samples were analyzed by DSC to evaluate the glass transitions. Block copolymers having two immiscible amorphous blocks usually microphase separate and show two separate glass transitions. As mentioned previously, the PFE6PSU4 and PFE8PSU4 copolymers displayed two glass transitions to indicate phase separation between the blocks (Fig. 2). The grafted copolymers in the acid form showed two glass transition temperatures, one at T g = 131 o C from the PFE block and the other T g was found to increase with increasing annealing temperature during a temperature cycling with successively higher temperatures, as shown in Fig. 7. This glass transition originated from the PVPA phase for which the T g increased because of an increased level of anhydride formation with the temperature. The anhydride formation leads to intra-and intermolecular crosslinks which decreases the segmental mobility of the PVPA side chains leading to increased values of T g . [26][27][28]32 As expected, no glass transition from the PSU phase was detected for the acidic copolymers because of the dense grafting with PVPA.

Multiblock copolymer membrane morphology

PVPA grafted multiblock copolymer membranes were cast in the acid form from NMP solutions at 100 °C. The morphologies of the membrane surfaces that faced the air during the casting were analyzed by tapping mode AFM analysis. Fig. 8 shows phase and topography images of two membranes. It is evident from the images that the grafted multiblock copolymers were distinctly phase separated and nanostructured. The soft PVPA phase domains appear as dark areas while the hard hydrophobic phases appear as the brighter areas in the images. From Fig. 8 it is seen that the PVPA phase domain seemed to be continuous and that the size of the PVPA phase domain increased with the PVPA content, as expected. The size of the PVPA phase domains was estimated to be in the range between 10 to 20 nm for copolymer PFE8PSU4-g-PVPA82.

Water uptake and proton conductivity

The water uptake is a very important parameter that determines both the mechanical properties and the level and mechanism of the proton conductivity in both sulfonated and phosphonated membranes. Typically, sulfonated membranes absorb more water molecules per acid group compared to phosphonated membranes, allowing higher IECs of the latter membranes to be used.

Membranes based on the PFE6PSU4-g-PVPA75 and PFE8PSU4-g-PVPA82 copolymers were not stable in water at room temperature because of their high IEC, 6.9 and 7.7 mmol P g -1 , respectively. Fig. 9a shows the variation of the water uptake and  value between 20 and 100 °C for the other phosphonated membranes in the acid form. As expected, the water uptake increased with the temperature and the IEC value. At 20 °C, the water uptake of PFE6PSU4-g-PVPA68 was 75 wt% but increased excessively to 176 wt% at 100 °C. Membrane PFE8PSU4-g-PVPA57 showed a moderate increase of the water uptake with temperature and reached a value of 95% at 100 °C. Membranes PFE8PSU4-g-PVPA57 and PFE8PSU4-g-PVPA59 had a very similar structure and PVPA content and both absorbed 45% water at 20 °C. However, at 100 °C the latter membrane absorbed approximately 50% more water than the former membrane. The water uptake of the phosphonated membrane corresponded to λ = 10-16 water molecules per phosphonic acid unit at 100 °C (Fig. 9b). This can be compared to λ = 27 for Nafion ® NRE212.

In comparison, a previously studied PSU with randomly placed PVPA grafts along the backbone, containing 57 wt% PVPA (IEC=5.3 mmol g -1 ), took up 125 wt% water under immersed conditions at 20 °C. 31 Moreover, a poly(phenylene oxide) randomly grafted with PVPA (65 wt% PVPA, IEC=6.0 mmol g -1 ) had a water uptake of 111 wt% at 100% RH and 25 °C. 32 This is considerably more than for the present grafted multiblock copolymer membranes with similar PVPA content. Consequently, the grafted block copolymer architecture provides morphologies which restrict the water uptake much more efficiently than randomly grafted copolymers.

The proton conductivity of the phosphonated membranes was measured from -20 o C to 120 o C under fully hydrated conditions in water (Fig. 10). In order to remove possible impurities and/or residual solvent, the membranes were first boiled in deionized water, and then washed several times with deionized water before being dried under vacuum at 30 °C for 24 h. The membranes were finally immersed in deionized water before the conductivity measurements. As seen in Fig. 10, the conductivity of Nafion ® NRE212 increased sharply between -20 and 0 o C because of the melting of the absorbed water and reached high values at higher temperatures. At -20 o C, the conductivity of the phosphonated membranes exceeded that of the Nafion membrane.

For the phosphonated membranes, the conductivity increased less sharply than Nafion between -20 and 20 °C. The level of conductivity then followed the same trend as the water uptake.

Consequently, membrane PFE6PSU4-g-PVPA68 showed the highest conductivity and reached 85 mS cm -1 at 120 °C. The previously studied PSU with randomly placed PVPA grafts (57 wt% PVPA, IEC=5.3 mmol g -1 ) showed a conductivity of 79 mS cm -1 under fully hydrated conditions at 120 °C31 , while the poly(phenylene oxide) randomly grafted with PVPA (65 wt% PVPA, IEC= 6.0 mmol g -1 ) showed 78 mS cm -1 under the same conditions. 32 Thus, the grafted multiblock copolymers exceeded the proton conductivity of the randomly grafted polymers at a significantly lower water content.

Conclusions

In the present work, we have selectively lithiated the PSU blocks of PFE-PSU multiblock copolymers without affecting the PFE blocks. The multiblock copolymers were then used as macroinitiators for anionic graft polymerization of DEVP from the lithiated PSU blocks. The block selective grafting was made possible because of the careful choice of the PFE block. By using of hexafluorobenzene as the dihalide monomer in the polycondensation, all the fluorine atoms of the resulting tetrafluorobenzene rings in the PFE were deactiviated for substitution because of the position ortho to the ether bridges to the bisphenol residues. This allowed for an efficient lithiation of the PSU block without affecting the PFE block. After hydrolysis of the DEVP monomer residues to obtain PVPA grafts, the multiblock copolymers showed phase separation with separate T g s of the PFE blocks and the PVPA grafts, respectively. Moreover, AFM showed that the membranes derived from the grafted copolymers were nanostructured with seemingly continuous PVPA phase domains. The membranes had high thermal stability and reached high proton conductivity. At an IEC of 6.3 mmol P g -1 the membranes reached a conductivity of 85 mS cm -1 under fully hydrated conditions. In comparison to the previous studies of PSU and PPO grafted with PVPA, the current grafted multiblock copolymers reached similar proton conductivity, but at a significantly lower water content. This is an advantage for their use as electrolyte membranes. The tendency of PVPA to form anhydrides was found to increase with the temperature, which is still an issue under hot and dry conditions. In light of the numerous possibilities to introduce functional groups via lithiation chemistry, the approach of block selective lithiation and modification can be used as a general strategy to prepare durable and functional nanostructured polymer membranes and materials. -26 -

Scheme 1 .Scheme 2 .Scheme 3 .Fig. 1 1 HFig. 2 Fig. 3 1Fig. 4 19 F

 12312319 Scheme 1. Molecular structure of a grafted homopolymer (upper) and a selectively grafted multiblock copolymer (lower) containing PVPA (green), PSU (blue) and PFE (red) segments.

Fig. 5

 5 Fig. 5 FTIR spectra of multiblock copolymers PFE6PSU4 (a), and PFE6PSU4-g-PVPA68 in the ester (b) and acid (c) form.

Fig. 6 Fig. 7 Fig. 8

 678 Fig. 6 TGA curves of (a) multiblock copolymers PFE8PSU4, PFE8PSU4-g-PVPA57 in the ester and acid form, respectively (recorded at 10 o C min -1 under nitrogen), and (b) TGA curves of PFExPSU4-g-PVPAy samples in the acid form with PVPA contents ranging from 57 to 82 wt% (recorded at 1 o C min -1

Fig. 9

 9 Fig.9 The water uptake (a) and λ (b) for the phosphonated multiblock copolymer membranes as a function of temperature under immersed conditions. The data of Nafion ® NRE212 was added for reference.

a

  Determined via the weight loss at 350 o C recorded by TGA under nitrogen. b Calculated from 1

Table 1 .

 1 Multiblock copolymer data. Molecular weight calculated from NMR data and values in parenthesis calculated from SEC data using polystyrene standards. b Determined by DSC. c Temperature for 5% weight loss at 10 o C min -1 under N 2 .

a

Table 2 .

 2 Grafted multiblock copolymer data.

	Grafted multiblock copolymer	Precursor multiblock copolymer	Ratios of the reactants DEP:DEVP:PSU (mmol)	DEVP content (wt%) by by 1 H NMR TGA a	PVPA content (wt%)	IEC b (mmol P g -1 )

Acknowledgements

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2010-2013) under the call ENERGY-2010-10.2-1: Future Emerging Technologies for Energy Applications (FET) under contract 256821 QuasiDry.


Zhecheng Shao

Arindam Sannigrahi

Patric Jannasch

Poly(tetrafluorostyrenephosphonic acid) -Polysulfone Block Copolymer Electrolytes

Keywords: polystyrene, atom transfer radical polymerization (ATRP), phosphonation, ionomers, proton-exchange membrane fuel cells

A series of ionic ABA triblock copolymers having a central polysulfone (PSU) central block and poly(2,3,5,6,-tetrafluorostyrene-4-phosphonic acid) (PTFSPA) outer blocks with different lengths were prepared and studied as electrolyte membranes. PSU with terminal benzyl chloride groups was used as a bifunctional macroinitiator for the formation of poly(2,3,4,5,6-pentafluorostyrene) (PPFS) blocks by atom transfer radical polymerization (ATRP). Selective and complete phosphonation of the PPFS blocks was achieved via a MichaelisArbuzov reaction using tris(trimethylsilyl)phosphite at 170 °C. Copolymer films were cast from solution and subsequently fully hydrolyzed to produce transparent flexible proton conducting PTFSPA-b-PSU-b-PTFSPA membranes with a thermal stability reaching above 270 °C under air, and increasing with the PTFSPA content. Studies of thin copolymer electrolyte membranes by tapping mode atomic force microscopy showed phase separated morphologies with continuous proton conducting PTFSPA nano scale domains. Block copolymer membranes reached a proton conductivity of 0.08 S cm -1 at 120 o C under fully hydrated conditions, and 0.8 mS cm -1 under 45% relative humidity at 80 o C.

INTRODUCTION

Various polymers functionalized with phosphonic acid and its ester derivatives are interesting for use in a range of different applications including materials for dental cements, 1,2 implants and tissue engineering, 3,4 flame-retardants 5,6 and fuel cell membranes. [7][START_REF] Lafitte | Advances in Fuel Cells[END_REF][START_REF] Rusanov | Fuel Cells II[END_REF][START_REF] Peckham | [END_REF][11][12][13][14] Especially the protonexchange membrane fuel cell (PEMFC) currently attracts a great deal of interest because it has the potential to achieve higher efficiencies than current power sources at a significantly lower environmental effect. 15 To this end, both fluorinated [16][17][18][19][20][21] and non-fluorinated [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] polymers have been synthesized by polymerization or by functionalization of pre-polymers. Recently Atanasov and Kerres have reported on the phosphonation of poly(2,3,4,5,6-pentafluorostyrene) (PPFS) whereby the fluorine atom in the para position was substituted by phosphonic acid in a Michaelis-Arbuzov reaction with tris(trimethylsilyl)phosphite. 41 Because of the presence of the fluorine atoms on the aromatic ring, the pK a values of the resulting poly (2,3,5,6-tetrafluorostyrene-4-phosphonic acid) (PTFSPA) were calculated to be pK a 1 = 0.5 and pK a 2 = 6.2, which is lower than for typical arylphosphonic acids. 41 PTFSPA showed a high thermal stability and a high proton conductivity at reduced relative humidity above 100 o C. However, because of its high ionic content, PTFSPA has poor film forming properties and is highly water-soluble which makes it unsuitable for direct use as an electrolyte membrane.

Water solubility of hydrophilic polymer segments can be prevented by forming hydrophilichydrophobic block and graft copolymers. These copolymers characteristically self-assemble because of the thermodynamic incompatibility between the ionic and nonionic polymer segments. [START_REF] Peckham | [END_REF] The self-assembly leads to a variety of different morphologies that contain nanostructured ion-conducting phases. Several investigators have reported on the influence of ordered ionic nanostructures on the properties of proton conducting membranes. 11 However, there are still quite few reports on the morphologies and the proton conducing properties of phosphonated block 34,35,[42][43][44] and graft [36][37][38] copolymers.

By "clicking" PTFSPA side chains onto a PSU backbone, Dimitrov et al. very recently prepared graft copolymers which formed water non-soluble membranes with high contents of phosphonic acid. 45 In the present communication we report on the preparation and properties of the first block copolymers containing PTFSPA segments. An aromatic PSU was first prepared by polycondensation and functionalization with benzylchloride end groups. The PSU was then used as a macroinitiator for atom transfer radical polymerization (ATRP) of 2,3,4,5,6pentafluorostyrene to form PPFS-b-PSU-b-PPFS triblock copolymers with different block ratios.

Selective and complete phosphonation of the PPFS blocks was achieved with tris(trimethylsilyl)phosphite to obtain a series of PTFSPA-PSU-PTFSPA block copolymers.

Solvent cast membranes of the phosphonated block copolymers were investigated by thermal analysis, microscopy, and measurements of water uptake and proton conductivity.

EXPERIMENTAL SECTION

Materials

The following reagents and solvents were used as received: copper(I) bromide (Sigma-Aldrich, 98%), 2,2'-bipyridyl (Acros, 99+%), α,α′-dibromo-p-xylene (Acros, 97%

Macroinitiator Synthesis

The PSU macroinitiator was prepared in a polycondensation reaction followed by an end-group functionalization, as shown in Scheme 1. A pre-dried two-neck round bottom flask (50 mL) was firstly charged with DCDPS (2044 mg, 7.12 mmol, 1 eq.), bisphenol M (2540 mg, 7.32 mmol, 1.03 eq.), potassium carbonate (1180 mg, 8.56 mmol, 1.2 eq.), DMAc (28 mL) and toluene (8 mL) and was then equipped with a nitrogen inlet and a Dean-Stark trap. The Dean-Stark trap was filled with toluene and equipped with a condenser. The round bottom flask was kept at 160 °C for 4 h to dehydrate the reaction mixture. The toluene in the Dean-Stark trap was then removed and the temperature was raised to 175 °C. After 48 h, and the reaction mixture was diluted by adding 10 mL of DMAc, and the solution was then added drop-wise into methanol (400 mL) under vigorous stirring. The precipitate was collected by filtration and purified by stirring with a fresh portion of methanol (400 mL) overnight. After filtration, the precipitate was dried in a vacuum oven to afford a white compound 1. (3.7 g, yield = 91%).

The end-group functionalization was carried out as follows. A round bottom flask (500 mL)

was charged with 1 (3.5 g, 0.146 mmol, 1 eq.), α,α′-dibromo-p-xylene (1155 mg, 4.38 mmol, 30 eq.), potassium carbonate (605 mg, 4.38 mmol, 30 eq.) and THF (200 mL). The reaction mixture was kept at 80 °C for 5 days, and was then stopped by slowly pouring the mixture into 2propanol (1 L). The precipitate was collected by filtration, washed with water and dissolved in hot THF (200 mL). The solution was again slowly added to 2-propanol (1 L) and the precipitate was collected by filtration and washed with portions of fresh 2-propanol. The product was dried in vacuum to afford a white powder 2. (3.4 g, yield = 92%).

Triblock Copolymer Synthesis

The PPFS-PSU-PPFS triblock copolymers were synthesized by using the end-functionalized PSU as a macroinitiator for atom-transfer radical polymerization (ATRP). In a typical synthesis of a triblock copolymer, a Schlenk flask (25 mL) was first charged with 2 (500 mg, 0.02 mmol, 1 eq.). The solid was dissolved by adding o-xylene (2 mL) and heating to 110 °C. After cooling to room temperature, the Schlenk flask was charged with copper(I)bromide (6 mg, 0.04 mmol, 2 eq.), 2,2'-bipyridyl (13 mg, 0.08 mmol, 4 eq.) and a pre-determined amount of PFS (Table 1).

The molar ratio of Br-PSU-Br:CuBr:bipyridyl was kept at 1:2:4. After three freeze-thaw cycles, the flask was kept at 110 °C for 48 h under nitrogen atmosphere. The reaction mixture was then cooled and diluted with a portion of THF (10 mL). The solution was added drop-wise to diethyl ether (250 mL) under stirring. The precipitate was collected by filtration, washed with diethyl ether and dried in vacuum to afford a white-green compound. The triblock copolymers (3-7) are denoted as PPFS y -PSU 44 -PPFS y , where x and y are the number of repeating units in the respective blocks (y also represents the degree of polymerization of the PPFS blocks). The details of the synthesis of the five triblock copolymers are given in Error! Reference source not found..

Selective Phosphonation

In a typical phosphonation reaction, a Schlenk tube equipped with a condenser was charged with a PPFS y -PSU 44 -PPFS y copolymer (500 mg) and TSP (5 mL). The Schlenk tube was kept at 170 °C for 48 h. The reaction mixture was then cooled to room temperature and diluted with THF (10 mL). The solution was then slowly added into hexane (100 mL) under stirring. The precipitate was collected by filtration, washed with fresh hexane and dried in vacuum to afford a white compound. In order to obtain the acid form of the phosphonated triblock copolymers, the samples in the ester form was dissolved in DMAc (10 mL) and treated with aqueous hydrochloride acid (0.5 M, 10 mL). The mixture was refluxed at 110 °C overnight. Water was then removed under reduced pressure and the remaining DMAc mixture was poured into 2propanol (150 mL) under stirring. The precipitate was collected by filtration and dried under vacuum to afford a white-brown compound. Using this method, the five PPFS y -PSU 44 -PPFS y copolymers (3-7) were phosphonated and hydrolysed to obtain the PTFSPA y -PSU 44 -PTFSPA y copolymers (3p-7p, respectively) in the acid form.

Membrane Preparation. Phosphonated triblock copolymer membranes were cast both in the acid and ester form. The acid or ester form of 3p-7p was dissolved with DMAc (4 mL). In both cases, the solutions were stirred at 80 °C for 30 min and then filtered through a Teflon filter onto Petri dishes (60 mm diameter), before casting the membranes in an oven at 80 °C for 2 days. The membranes were detached after immersion in deionized water, and were then treated in deionized water at 50 °C for 24 h.

Polymer and Membrane Characterization

1 H, 19 F and 31 P NMR spectroscopy was performed with a Bruker DRX400 spectrometer at 400.13 MHz using either CDCl 3 (δ = 7.28 ppm) or DMSO-d 6 (δ = 2.50 ppm) solutions of the samples. Infrared spectra were recorded on a Perkin Elmer Spectrum One model 2000 FTIR system with a universal attenuated total reflection sampling accessory on a ZnSe/diamond composite. The size-exclusion chromatography (SEC) setup consisted of a Viscotek GPCmax VE-2001 equipped with ViscotekTriSEC Model 302 triple detector using two PLgel mixed-D columns from Polymer Laboratories. The samples were analyzed in chloroform at 1 mL min -1 , 30 °C, and molecular weight data were calculated using polystyrene standards.

The thermal stability of the samples was investigated by thermogravimetric analysis (TGA) between 50 and 600 °C at a heating rate of 10 °C min -1 under nitrogen or at a heating rate of 1 °C min -1 under air using a Q500 analyzer (TA Instruments). Prior to the TGA measurements, the sample was preheated at 150 °C for 10 min to remove any solvent residues. The temperature at 5% weight loss was taken as the degradation temperature (T d ). Samples were analyzed at 10 °C min -1 under N 2 by differential scanning calorimetry (DSC) using TA Instruments Q2000. The samples were first heated from 50 to 300 °C, then cooled to 50 °C and finally heated to 300 °C.

The data collected during the second heating scan were used to evaluate the glass transition temperature (T g ) which were taken as the midpoints of the transitions recorded.

Surfaces of the PTFSPA y -PSU 44 -PTFSPA y membranes were analyzed by Atom Force Microscopy (AFM) in tapping mode under ambient air using an XE-100 Park System.

Simultaneous topographic and phase imaging were carried out at a scanning frequency of 1 Hz.

The probe was Acta-50 (25-75 N m -1 , 200-400 kHz, Al coating). No image processing was done.

The water uptake (W water ) of the membranes was calculated as:

% 100    dry dry wet water W W W W ( E q . 1 )
, where W wet was the membrane weight after immersion in water for 2 days and W dry was the membrane weight after keeping the samples in vacuum for 2 days. The hydration value (λ), i.e., the number of water molecules per acid group, was calculated by combining the water uptake data and the IEC values, as:

  1000 IEC 18  W water (Eq. 2)
The proton conductivity (σ, S cm -1 ) of the membranes was measured by electrochemical impedance spectroscopy (EIS), both under fully hydrated conditions as a function of temperature and under reduced RH at 80 °C. The former measurement was performed in a sealed cell (1.4 cm × 1.4 cm × 1.0 cm) containing 0.3 mL of water with the membrane fully hydrated. A two-probe method was used in the range -20 to 120 °C with a Novocontrol high-resolution dielectric analyzer V 1.01S in the frequency range 10 1 -10 7 Hz at 50 mV. The humidity dependence of the proton conductivity from 25 to 90% RH was investigated at 80 °C by a four-probe method with a Gamry potentiostat/galvanostat/ZRA in the frequency range 10 -1 -10 5 Hz using a Fumatech MK3 conductivity cell. The humidity was equilibrated by deionized water in a closed system.

RESULTS AND DISCUSSION

Synthesis of Triblock Copolymers

Five different PTFSPA y -PSU 44 -PTFSPA y triblock copolymers having the same central PSU block but with terminal PTFSPA blocks of different lengths were prepared as shown in Scheme 1. First, the hydroxy-terminated oligomeric PSU (1) was prepared in a K 2 CO 3 -mediated polycondensation of DCDPS and a carefully controlled excess of bisphenol M. The structure of the PSU was tuned for the subsequent ATRP reactions. Thus, bisphenol M was employed to obtain a PSU oligomer which was soluble in o-xylene, the solvent of choice for the ATRP. 46,47 Notably, the corresponding PSU based on 1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene (bisphenol P) was found to be insoluble in the same solvent. The average number of repeating units of the PSU was determined to be x = 45 by comparing the integral of the most electron deficient diphenyl sulfone protons H c at 7.84 ppm with that of the phenolic protons H a of the chain ends at 6.70 ppm (Figure 1a). This corresponds to an overall average molecular weight of 25 kDa. The full assignment of the aromatic protons is found in Figure 1. Using polystyrene standards, analysis by SEC gave M n = 36 kDa and M w /M n = 1.9 (Figure 2).

The PSU oligomer was successfully functionalized by coupling α,α'-dibromo-p-xylene to the two phenolic chain ends to form a bidirectional macroinitiator for ATRP (2, Scheme 1). The dibromoxylene was added in a large excess to ensure complete functionalization and to prevent chain extension reactions. By comparing the integrals of signal H l (6.82 ppm, end group protons), H k (4.51 ppm) and H i (5.02 ppm) in the 1 H NMR spectrum, the full conversion of the phenolic end groups to bromobenzyl end groups was confirmed (Error! Reference source not found.b). SEC analysis showed that the peak of the PSU macroinitiator was similar in shape to that of the original PSU oligomer and had no shoulder to indicate any chain extension (Error! Reference source not found.).

The PPFS y -PSU 44 -PPFS y triblock copolymers (3-7) were prepared by using the PSU macroinitiator for ATRP of PFS, employing copper(I)bromide and 2,2'-bipyridyl as the catalytic system in o-xylene. The polymerizations were allowed to proceed for 48 h at 110 o C, which gave monomer yields between 60 and 80% (Table 1). The 1 H NMR spectra of the triblock copolymers indicated the expected structure (Figure 3a). The degree of polymerization of each of the two PPFS blocks (y) was determined by comparing the integrated values of the three broad signals H m , H n , and H o at 2.04, 2.41 and 2.78 ppm, respectively, corresponding to the hydrogen atoms on the two terminal PFS backbones, with the integral of the signal at 7.84 ppm arising from the diphenylsulfone hydrogens H c of the central PSU block. The degree of polymerization (y) was thus calculated as:

y  30I mno I c ( E q . 3 )
, where I mno is the summarized integrals of the H m , H n and H o signals and I c is the integral of the H c signal (Figure 3a). As seen in Figure 3b, the 19 F NMR spectrum shows three peaks at -143.4, -154.3 and -161.4 ppm corresponding to the three different kinds of fluorine atoms F p , F q and F r , respectively, in the PFS blocks. This further verified the success of the polymerization.

SEC analysis of the block copolymers indicated higher molecular weights than the macroinitiator and no new peaks at higher elution volumes to hint any homopolymer formation (Figure 2).

Copolymer Phosphonation

After some optimization, the selective phosphonation of the PPFS blocks was carried out as a classic S N Ar Michaelis-Arbuzov reaction with the copolymer dissolved in the phosphonating agent TSP at 175 °C over 48 h. 41 The molar ratio between PFS units and TSP was kept at 1:10 to ensure complete copolymer dissolution and phosphonation of the PPFS blocks. The triblock copolymers were insoluble in TSP below 140 °C and then gradually the powder dispersions turned to more homogenous fine white dispersions at 175 °C. After the phosphonation, the 19 F NMR spectra of the copolymers showed only two broad signals at approximately -132.9 and -142.7 ppm, corresponding to F p and F q (Error! Reference source not found.a). Notably, no signals from the original non-phosphonated PPFS units were found. Complete hydrolysis of the dimethylsilyl phosphonate ester groups was readily carried out by keeping the copolymers in refluxing dilute hydrochloric acid. The subsequent 31 P NMR analysis of the PTFSPA y -PSU 44 -PTFSPA y copolymers showed just one broad signal, consistent with the phosphonic acid derivative (Error! Reference source not found.b). 41 In addition, FTIR analysis supported the successful phosphonation by a broad band at 2000-3750 cm -1 from hydrogen bonding phosphonic acid and water, and peaks at 1475, 1275 and 555 cm -1 arising from stretching vibrations of PO 3 and P=O, and bending vibrations of PO 3 , respectively (Figure 5).

Thermal Properties

Amorphous block copolymers containing two types of blocks typically microphase separate to display two glass transitions. DSC traces of the non-ionic triblock copolymers showed the expected two transitions at approximately T g = 110 and 145 °C, originating from the PPFS and PSU phases, respectively (Figure 6). These values did not change with the composition of the copolymers and were nearly identical to those measured for the PPFS homopolymer and the PSU precursor block, 105 and 144 °C, respectively. As expected, the change in specific heat (C p ) increased with the content of the respective component (Table 2) for all the nonionic copolymers. DSC analysis of the phosphonated triblock copolymers in both the ester and acid form showed only the T g of the PSU phase (Figure 6). The PTFSPA homopolymer 8 did not show any T g in the acid form below the degradation temperature. Ionic polymers are known to generally have a higher T g than the corresponding non-ionic polymer because of increasing interactions as the ionic content increases. The T g of the PSU blocks decreased gradually with the PTFSPA content, from 149 °C for PTFSPA 15 -PSU 44 -PTFSPA 15 3p to 140 °C for PTFSPA 113 -PSU 44 -PTFSPA 113 7p (Table 2). The value of C p increased with the content of the PSU block as expected (Table 2). Notably, the phosphonated copolymers 3p-7p did not show any signs of anhydride formation like PVPA copolymers which typically show an increasing T g with annealing temperature. 34,37 The thermal stability of the precursors and the triblock copolymers were studied by TGA under air at 1 °C min -1 (Figure 7). The T d values of the precursor PSU 1 and PSU macroinitiator 2 were only 5 o C apart as expected due to their structural similarity (Table 2). The T d of the homopolymer PTFSPA in the acid form 8 was recorded at 335 °C, in good agreement with the literature value. 41 The non-ionic triblock copolymer 4 showed a lower T d at 289 °C which corresponded to the degradation of the PPFS blocks (T d at ~300 °C). The phosphonated copolymer 4p showed a gradual weight loss of 13 wt% from 200 to ~330 °C due to solvent residues, or possibly anhydride formation, before reaching the T d of the PTFSPA blocks at 335 °C.

Membrane Preparation

Both the acid and ester form of the PTFSPA y -PSU 44 -PTFSPA y (3p-7p) copolymers were readily soluble in polar aprotic solvents such as DMSO, DMF, DMAc and NMP. Initial attempts to directly cast copolymer membranes in the acid form using these solvents gave in most cases unstable membranes which disintegrated in contact with liquid water. This was most probably caused by the absorption of large amounts of water by the hydrophilic PTFSPA phase. Instead, membranes of the copolymers in the ester form were cast from DMAc solutions. Acidic membranes were obtained after hydrolysis in water at 50 °C for 24 h, whereafter the ester groups were completely removed. This was indicated by the loss of the ester signals at 0 ppm in the 1 H NMR spectra, corresponding to the -SiMe 3 groups. Membranes prepared from copolymers 3p-6p in this way were water-stable, but not the ones from copolymers 7p with the highest PTFSPA content. Thus, membranes of copolymer 3p-6p were further characterized.

Membrane Morphology

AFM tapping mode phase images of the triblock copolymer membranes in the acid form are shown in Figure 8. As seen, the images display distinct structures with bright and dark regions corresponding to the PSU (hard) and PTFSPA (soft) phases, respectively, formed by the respective block. The size of the PTFSPA phase domains was estimated to be in the range between 10 and 20 nm for copolymer 4p and 5p, respectively (Figure 8). Consequently, the AFM analysis clearly demonstrated the self-assembly of the block copolymers into phase separated morphologies on the nano level, although the DSC analysis only showed the glass transition of the PSU phase. The clear phase separation of the triblock copolymers can be expected because of the incompatibility of the ionic PTFSPA blocks and the hydrophobic PSU block. All AFM images except perhaps that of PTFSPA 15 -PSU 44 -PTFSPA 15 show morphologies with seemingly continuous proton conducting PTFSPA phase domains. In addition, all the morphologies are quite irregular and nonperiodic with poor long-range order. The formation of irregular non-periodic has been reported to be beneficial for transport properties over, e.g. periodic lamellar morphologies where the phase domains often orient parallel to the membrane surface and thus perpendicular to the direction of the proton transport. 48

Water Uptake and Proton Conductivity

The uptake of water is a fundamentally important property of PEMFC membranes that has a direct influence on the proton transport properties. Generally phosphonated polymers absorb less water molecules per acid group than the corresponding sulfonated polymers, allowing higher IEC values to be used for the former polymers. The theoretical IEC and the water uptake of the triblock copolymer membranes are summarized in Error! Reference source not found.. The membranes take up quite low amounts of water and, as expected, the amount increases with the IEC value. Also, the number of water molecules taken up per -PO 3 H 2 group () increases progressively with the IEC value. As already mentioned, the membrane with the highest PTFSPA content, 7p, disintegrated in contact with water and could not be characterized further.

The water uptake of the present triblock copolymers may be compared with that of PSU grafted with PTFSPA where a membrane with an IEC of 7.6 mmol g -1 took up 75 wt% water, corresponding to =11. 45 Kerres and Atanasov measured a water uptake of 50 wt% (=14) at 95% RH and 30 o C for the PTFPA homopolymer. 41 The temperature dependence of the proton conductivity of the triblock copolymer membranes in the fully hydrated state is shown in Error! Reference source not found.. As seen, the conductivity increased with the IEC and water uptake. The proton conductivity of all the membranes also increased sharply between -20 and 20 o C because of the melting of ice.

Membrane 6p showed the highest conductivity, 0.08 S cm -1 at 120 o C, and membrane 3p the lowest with 0.001 S cm -1 at the same temperature. This may be compared with the PTFSPA grafted PSU which also reached 0.08 S cm -1 at 120 o C, although at a significantly higher IEC (7.6 mmol g -1 ). 45 The proton conductivity under reduced humidity (RH) at 80 °C is given in Error! Reference source not found. for the three membranes with the highest PTFSPA content. As expected, the proton conductivity decreased with the RH, and the values for membrane 6p decreased from 0.08 S cm -1 under full humidification, to 5  10 -3 S cm -1 at 90% RH and further to 5  10 -5 S cm -1 at 25% RH. The much lower conductivity of membrane PTFSPA 46 -PSU 44 -PTFSPA 46 4p in relation to PTFSPA 62 -PSU 44 -PTFSPA 62 5p indicated that the former membrane had a morphology with a significantly lower percolation at low RH for the proton transport.

Membrane 6p reached a conductivity of approx. 10 -3 S cm -1 at 50% RH (80 o C) which may be compared with a conductivity of 0.04 S cm -1 at 50% RH (120 o C) measured for the PTFSPA homopolymer. 41

CONCLUSIONS

A series of block copolymers containing PTFSPA blocks have been prepared for the first time.

PTFSPA blocks with different molecular weights were successfully polymerized by ATRP from a difunctional macroinitiator to form PPFS y -PSU 44 -PPFS y triblock copolymers. The PPFS blocks were selectively and quantitatively phosphonated by displacing the aromatic fluorine atom in the para position by using TSP. Electrolyte membranes derived from the block copolymers were found to be phase separated and nanostructured, and showed a high thermal stability under air. If the PTFSPA content was too high, the membranes became unstable in liquid water. Still, a membrane containing 4.2 mmol g -1 was stable and reached a proton conductivity of 0.08 S cm -1 at 120 o C under fully hydrated conditions, and 0.8 mS cm -1 under 45% RH at 80 ˚C. The results also showed that PTFSPA has a low tendency to form anhydrides at high temperatures which significantly promotes the proton conductivity. This work opens up for the controlled synthesis of new of PTFSPA block copolymers with alternative architectures and block types, provided that the copolymers become reasonably soluble in TSP during the phosphonation. Copolymers containing PTFSPA should be interesting for a number of different applications because of the properties emanating from their high acidity and fluorinated character.

Supporting Information

DSC traces of the triblock copolymer 3-7 and their phosphonated acid form 3p-7p were shown in the supporting information.

SCHEME 1

 1 SCHEME 1 Synthetic pathway to phosphonated triblock copolymers 3p-7p via

FIGURE 1 1 HFIGURE 2

 12 FIGURE 1 1 H NMR spectra of the PSU oligomer 1 (a) and the functionalized PSU macroinitiator 2 (b)

FIGURE 3 1 HFIGURE 4 FIGURE 5

 145 FIGURE 31 H NMR (a) and19 F NMR (b) spectra of triblock copolymer 5 in CDCl 3 .

FIGURE 6

 6 FIGURE 6 DSC traces of PSU oligomer 1 (a), macroinitiator 2 (b), triblock copolymer 4 (c), and triblock copolymer 4p in the ester (d) and acid form (e), recorded during heating at 10 °C min -1 . The glass

FIGURE 7

 7 FIGURE 7 TGA traces of PSU oligomer 1 (a), macroinitiator 2 (b), homo PTFSPA 8 in the acid form (c), triblock copolymer 4 (d) and 4p in acid form (e), as recorded during heating at 1 °C min -1 under air.

FIGURE 8 FIGURE 9

 89 FIGURE 8 AFM tapping mode images of thin triblock copolymer membranes in the acid form. Phase (a)

FIGURE 10

 10 FIGURE 10 Proton conductivity of block copolymer membranes at 80 o C as a function of RH, as

  SCHEME 2

FIGURE 20 Figure

 20 FIGURE 20

Figure S2

 S2 Figure S2 DSC traces of phosphonated triblock copolymers (3p-7p) recorded during heating at

TABLE 2

 2 Thermal data of the precursors and the phosphonated copolymers in the acid form. Recorded under air at 1 °C min -1 . b Recorded under nitrogen at 10 °C min -1 . c Calculated from NMR data. n.d. = not detected

	Sample	Sample #	MW c (kDa)	T d5% a (°C)	T gPSU b (°C)	T gPPFS/ T gPTFSP b A (°C)	ΔC pPPFS J (g K) -1	ΔC pPSU J(g K) -1
	PSU 44	1	25	358	144	-	-	0.227
	Br-PSU 44 -Br	2	25	363	144	-	-	0.250
	PPFS 15 -PSU 44 -PPFS 15	3	31	274	145	n.d.	n.d.	0.328
	PPFS 46 -PSU 44 -PPFS 46	4	43	289	145	110	0.064	0.230
	PPFS 62 -PSU 44 -PPFS 62	5	49	270	145	109	0.085	0.105
	PPFS 72 -PSU 44 -PPFS 72	6	53	283	144	110	0.084	0.066
	PPFS 113 -PSU 44 -PPFS 113	7	64	294	145	107	0.154	0.052
	PTFSPA	8	-	335	-	n.d.	n.d.	-
	PTFSPA 15 -PSU 44 -PTFSPA 15	3p	33	243	149	n.d.	n.d.	0.402
	PTFSPA 46 -PSU 44 -PTFSPA 46	4p	49	256	145	n.d.	n.d.	0.105
	PTFSPA 62 -PSU 44 -PTFSPA 62	5p	56	269	145	n.d.	n.d.	0.121
	PTFSPA 72 -PSU 44 -PTFSPA 72	6p	62	274	141	n.d.	n.d.	0.083
	PTFSPA 113 -PSU 44 -PTFSPA 113	7p	78	297	140	n.d.	n.d.	0.018

a

TABLE 3

 3 IEC, water uptake and proton conductivity of phosphonic acid membranes under fully hydrated conditions.

	Phosphonated triblock copolymers	Sample #	Theoretical IEC b (mmol g -1 )	Water uptake c (wt%)	[H 2 O]/[-PO 3 H 2 ] () c	σ at 120 °C (S cm -1 )
	PTFSPA 15 -PSU 44 -PTFSPA 15		3p	1.4	1	0.8	0.001
	PTFSPA 46 -PSU 44 -PTFSPA 46		4p	3.2	5	1.7	0.028
	PTFSPA 62 -PSU 44 -PTFSPA 62		5p	3.8	7	2.0	0.028
	PTFSPA 72 -PSU 44 -PTFSPA 72		6p	4.2	25	6.6	0.008
	PTFSPA 113 -PSU 44 -PTFSPA 113	a	7p	5.0	-	-	-

a Unable to form water-stable membranes. b Determined from NMR data. c Immersed at 25 o C.

ACKNOWLEDGEMENTS

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2010-2013) under the call ENERGY-2010-10.2-1: Future