Tiphaine Huyghebaert
email: tiphaine.huyghebaert@univ-tours.fr

Nicolas Gillet

Ophéline Audusseau

Evelyne Fouquereau

NURSES' WELL-BEING AND TURNOVER INTENTIONS Running head: NURSES' WELL-BEING AND TURNOVER INTENTIONS Perceived career opportunities, commitment to the supervisor, social isolation: Their effects on nurses' well-being and turnover

Keywords: Perceived career opportunities, social isolation from colleagues, affective commitment, wellbeing, turnover intentions. NURSES' WELL-

Perceived career opportunities, commitment to the supervisor, social isolation: Their effects on nurses' well-being and turnover

Tiphaine Huyghebaert, Nicolas Gillet, Ophéline Audusseau, Evelyne

employees and their supervisors, to employees' sense of emotional attachment and identification to their supervisors, and their desire to maintain a work relationship with them [START_REF] Landry | Role of commitment to the supervisor, leader-member exchange, and supervisor-based self-esteem in employee-supervisor conflicts[END_REF].

As most organizations have gotten more and more decentralized, supervisors have become "a more critical focus of exchange with employees than ever before" (Stinglhamber & Vandenberghe, 2003, p. 253). Supervisors are more visible and proximal to employees; they are the face of the organization to their subordinates, therefore they hold a pivotal place in keeping them willing to stay within the company. According to [START_REF] Lawler | An affect theory of social exchange[END_REF], such proximal entities (i.e., supervisors) have the power to support employees' autonomy and control over their work environment, and by doing so increase their attachment to said entities. In other words, when they are attached to their supervisor, employees are more likely to stay within the organization as they want to preserve the fulfilling identification with their supervisor's goals and values. Studies supported this by showing ACS' negative effect on intent to quit in various occupations (e.g., [START_REF] Stinglhamber | Organizations and supervisors as sources of support and targets of commitment: A longitudinal study[END_REF][START_REF] Vandenberghe | Affective commitment to the organization, supervisor, and work group: Antecedents and outcomes[END_REF] including in a sample of nurses attending courses organized by a nursing school [START_REF] Vandenberghe | A closer look at the relationship between affective commitment to supervisors and organizations and turnover[END_REF]). Yet, more research is needed to confirm whether these findings apply to general samples of nurses in other cultural settings.

Research also showed the importance of subordinate-supervisor relationship for nurses' well-being (e.g., [START_REF] Brunetto | The importance of supervisor-nurse relationships, teamwork, wellbeing, affective commitment and retention of North American nurses[END_REF]. Indeed, as suggested by social exchange theory [START_REF] Blau | Exchange and power in social life[END_REF], if nurses show commitment to their supervisor, the latter may reciprocate by rewarding them with more resources (e.g., support, information) to help them complete their tasks [START_REF] Lapointe | Psychological contract breach, affective commitment to organization and supervisor, and newcomer adjustment: A three-wave moderated mediation model[END_REF]. These resources not only allow workers to meet their professional goals but also to stimulate their personal development, hence they promote their well-being [START_REF] Bakker | The Job Demands-Resources model: State of the art[END_REF]. Indeed, because they are provided with more resources, nurses do not have to draw on their own resources to deal with their work challenges, leaving them with more energy and a higher sense of well-being [START_REF] Hobfoll | The Influence of culture, community, and the nested-self in the stress process: Advancing conservation of resources theory[END_REF]). Yet, prior research mostly studied work-related outcomes of ACS [START_REF] Lapointe | Psychological contract breach, affective commitment to organization and supervisor, and newcomer adjustment: A three-wave moderated mediation model[END_REF], while less attention has been paid to its effects on health-related consequences, even less so in nurses.

Hypothesis 2: Nurses' ACS negatively associates with their turnover intentions and positively relates to their well-being.

Social Isolation

Social isolation from colleagues refers to employees' "perceptions of lack of availability of support and recognition, missed opportunities for informal interactions with co-workers, and not being part of the group" (Marshall et al., 2007, p. 196). The understaffed context in which nurses often work puts them in a pressured work pace, which does not leave much time for social interactions with their peers. By missing these informal and spontaneous discussions, nurses may feel out of touch and sense a lack of affective bond and emotional support that may contribute to their feeling socially isolated from their co-workers.

Studies showed that social isolation from colleagues could alter employee well-being (e.g., [START_REF] Mulki | Workplace isolation, salesperson commitment, and job performance[END_REF]. Studies also showed that when employees feel isolated from their coworkers, they identify less with their organization, they show less organizational commitment and more turnover intentions (e.g., [START_REF] Marshall | Workplace isolation: Exploring the construct and its measurement[END_REF]. However, one should note that most of these deleterious effects of social isolation from colleagues were found in very specific samples such as salespeople or virtual workers (e.g., [START_REF] Mulki | Workplace isolation, salesperson commitment, and job performance[END_REF], and scholars called for future research to explore employees' perceptions of isolation in a variety of organizational settings.

Hypothesis 3: Social isolation from colleagues negatively relates to nurses' well-being and positively associates with their turnover intentions.

Indirect Effects

Not only did few studies examine the aforementioned relationships in nurses but little research was conducted on the possible mechanisms underlying such effects. Assuming that PCO, ACS, and social isolation have an effect on nurse's well-being and turnover intentions, then which common mechanism could explain such associations? ACO was demonstrated to be a valid mechanism to explain the relationship between employees' work experiences on one hand, and their turnover intentions as well as their well-being (e.g., [START_REF] Lapointe | Psychological contract breach, affective commitment to organization and supervisor, and newcomer adjustment: A three-wave moderated mediation model[END_REF][START_REF] Vandenberghe | Affective commitment to the organization, supervisor, and work group: Antecedents and outcomes[END_REF] on the other hand. Despite its sharing a common component (i.e., affective) with ACS, ACO reflects a different foci and these two forms of attachment do constitute two different constructs with different effects [START_REF] Vandenberghe | A closer look at the relationship between affective commitment to supervisors and organizations and turnover[END_REF]. Indeed, recent research showed workplace affective commitment to have a complex multifocal structure and supported the importance of considering specific affective commitment foci separately for they have unique relations with healthcare workers' attitudes and behaviors, above and beyond their global levels of general worklife affective commitment [START_REF] Perreira | The short form of the Workplace Affective Commitment Multidimensional Questionnaire (WACMQ-S): A bifactor-ESEM approach among healthcare professionals[END_REF].

Additionally, among the different facets of organizational commitment (i.e., affective, normative, continuance), ACO has drawn most of the attention because of its associations with important individual and organizational outcomes including turnover (e.g., [START_REF] Meyer | Affective, continuance and normative commitment to the organization: A meta-analysis of antecedents, correlates, and consequences[END_REF]. Scholars have offered a possible explanation for this by suggesting that, by definition and in essence, affective commitment reflects a mindset that implies attachment and involvement towards a target, which echoes a desire to continue employment and may therefore be "a stronger motive than the perceived cost of failing to do so (continuance commitment) or the perceived obligation to stay (normative commitment)" (Vandenberghe & Bentein, 2009, p. 331). Therefore, in this study, we focused on ACO as it has proven to be a valuable and reliable dimension to consider when exploring individual and organizational outcomes.

ACO is known to negatively associate with intended and actual turnover (e.g., [START_REF] Meyer | Affective, continuance and normative commitment to the organization: A meta-analysis of antecedents, correlates, and consequences[END_REF]. Indeed, employees orient their will to reciprocate towards the target they consider as being the source of their experiences at work [START_REF] Lavelle | Taking a multifoci approach to the study of justice, social exchange, and citizenship behavior: The target similarity model[END_REF]. When employees perceive that their organization allows them to have positive experiences at work, they may form affective commitment towards this specific target and reciprocate by less intent to quit said target. Prior research (e.g., [START_REF] Flinkman | Nurses' intention to leave the profession: integrative review[END_REF] showed ACO to negatively relate to nurses' turnover intentions.

Research has less documented the implications of ACO for employees themselves, in terms of their well-being. Some research did address this issue and showed ACO to be positively related to workers' well-being (e.g., [START_REF] Epitropaki | From ideal to real: a longitudinal study of the role of implicit leadership theories on leader-member exchanges and employee outcomes[END_REF]. When feeling strongly identified with their organization's goals and values, employees might sense that their work environment aligns with their true self, and experience higher well-being [START_REF] Meyer | Employee commitment and well-being: A critical review, theoretical framework and research agenda[END_REF]. Prior research found nurses' affective commitment to their occupation to positively relate to their well-being [START_REF] Sawada | The relationships among occupational and organizational commitment, human relations in the workplace, and well-being in nurses[END_REF]. Similarly, organizational commitment was found to have a positive effect on nurses' well-being (e.g., [START_REF] Vanaki | Organizational commitment, work environment conditions, and life satisfaction among Iranian nurses[END_REF].

Hypothesis 4: ACO negatively relates to nurses' turnover intentions and positively to their wellbeing.

Research indicated that ACO could be determined by a variety of work variables that are conceptually close to PCO, to ACS, and to social isolation from co-workers. For instance, career expectations (i.e., positive assessments of career opportunities within the organization; [START_REF] Chang | Organizational work-family culture and working mothers' affective commitment: How career expectations matter[END_REF] were shown to positively influence ACO. When allowed to customize their careers, employees may reciprocate by being committed towards the organization, as the latter's goals and values serve their interests in the first place [START_REF] Blau | Exchange and power in social life[END_REF]. Additionally, among the antecedents of ACO, ACS holds an important place (e.g., [START_REF] Panaccio | The relationships of role clarity and organization-based selfesteem to commitment to supervisors and organizations and turnover intentions[END_REF]. As organizational rules and objectives are passed on by supervisors, employees' identification and attachment to these rules and objectives may very well depend on their socio-emotional relationship with their supervisor. Similarly, when experiencing social isolation from colleagues, nurses may perceive the organizational setting as a hostile place, or at least a socially unfertile one, thus reducing their ACO. Indeed, research showed that resourceful relational job characteristics explained affective commitment to the hospital in nurses [START_REF] Santos | Relational job characteristics and nurses' affective organizational commitment: The mediating role of work engagement[END_REF]. These results suggest that the lack of social interaction would negatively associate with ACO.

Hypothesis 5: PCO and ACS positively relate to ACO, while social isolation negatively relates to ACO.

In line with this theoretical rationale, one could hypothesize ACO to explain the association between nurses' work environment (i.e., PCO, ACS, and social isolation from coworkers) and their turnover intentions and well-being. Indeed, work variables that are conceptually close to PCO (e.g., [START_REF] Chang | Organizational work-family culture and working mothers' affective commitment: How career expectations matter[END_REF], to ACS (e.g., [START_REF] Panaccio | The relationships of role clarity and organization-based selfesteem to commitment to supervisors and organizations and turnover intentions[END_REF], and to social isolation from coworkers (e.g., [START_REF] Santos | Relational job characteristics and nurses' affective organizational commitment: The mediating role of work engagement[END_REF] were shown to be related to ACO which is itself associated with turnover intentions (e.g., [START_REF] Flinkman | Nurses' intention to leave the profession: integrative review[END_REF] and well-being (e.g., [START_REF] Epitropaki | From ideal to real: a longitudinal study of the role of implicit leadership theories on leader-member exchanges and employee outcomes[END_REF]. In addition, previous studies also demonstrated ACO to partially mediate the relationships between these work variables and employee outcomes.

For instance, research showed ACO to mediate the relationship between human resource practices (e.g., offering career opportunities) and employees' behavior such as job performance or voice behavior (e.g., [START_REF] Harrison | How important are job attitudes? Meta-analytic comparisons of integrative behavioral outcomes and time sequences[END_REF]. Other studies found ACO to mediate the association between perceived career support and well-being at work [START_REF] Poon | Relationships among perceived career support, affective commitment, and work engagement[END_REF]. Similarly, previous investigations showed organizational commitment to mediate the effect of commitment to the supervisor on intent to quit, in diversified samples of employees [START_REF] Panaccio | The relationships of role clarity and organization-based selfesteem to commitment to supervisors and organizations and turnover intentions[END_REF][START_REF] Vandenberghe | Affective commitment to the organization, supervisor, and work group: Antecedents and outcomes[END_REF]. ACO thus appears to form a crucial mechanism between the work environment and employee functioning. It would therefore be interesting to verify whether ACO similarly mediates the effects of PCO, ACS, and social isolation on turnover intentions and well-being in a sample of nurses.

Hypothesis 6: ACO partially mediates the relationship between PCO, ACS, and social isolation on one hand, and turnover intentions and well-being on the other hand.

Methods Procedure and participants

A survey questionnaire was completed by 244 nurses from various French healthcare centers (e.g., public hospitals, nursing homes). A cover letter explaining the study's purposes and stressing the value of participation came with the questionnaire. According to local regulations, no formal ethical scrutiny was required as no ethics committee existed in the institution at the time of the study. However, nurses were surveyed only upon approval of their organization, and each participant signed an informed consent form stressing the confidential, anonymous, and voluntary nature of the study. No incentive was offered. Completed questionnaires were returned to a graduate student in charge of data collection.

Respondents' ages ranged from 20 to 59 years (M = 28.20, SD = 6.90). Average tenure in the current health organization was 3.91 years (SD = 5.27) and average tenure in the current job was 2.56 years (SD = 3.67). Two hundred and twenty nurses were full-time workers (90.2%). One hundred and eighty-two participants were permanent nurses (74.6%) and 62 were temporary workers (25.4%). Sixteen nurses (6.6%) worked in organizations with 11 to 49 employees, 60 in organizations with 50 to 249 employees (24.6%), 29 in organizations with 250 to 499 employees (11.9%), and 139 in organizations with over 500 employees (57.0%).

Measures

ACS was measured with [START_REF] Vandenberghe | A closer look at the relationship between affective commitment to supervisors and organizations and turnover[END_REF] 6-item scale (α = .91). Responses were provided on a 5-point Likert-type scale ranging from 1 (strongly disagree) to 5 (strongly agree).

Social isolation was assessed with the 6-item colleagues subscale (α = .92) from the workplace isolation scale developed by [START_REF] Marshall | Workplace isolation: Exploring the construct and its measurement[END_REF]. Responses were given on a 5-point scale ranging from 1 (strongly disagree) to 5 (strongly agree).

PCO were measured with 3 items (α = .85) developed by [START_REF] Kraimer | Antecedents and outcomes of organizational support for development: The critical role of career opportunities[END_REF]. Responses were anchored on a 5-point Likert-type scale ranging from 1 (never) to 5 (very often).

ACO was assessed with the 6 items (α = .86) used by [START_REF] Bentein | The role of change in the relationship between commitment and turnover: A latent growth modeling approach[END_REF]. Responses were given on a 5-point scale ranging from 1 (strongly disagree) to 5 (strongly agree).

Well-being was measured through the 5-item high pleasurable-high arousal subscale from the Jobrelated Affective Well-being Scale (Van Katwyk, Fox, Spector, & Kelloway, 2000) (α = .82). Participants answered on a 5-point Likert-scale ranging from 1 (never) to 5 (always).

Turnover intentions were assessed with 3 items (α = .89) used by [START_REF] Bentein | The role of change in the relationship between commitment and turnover: A latent growth modeling approach[END_REF]. Responses were given on a 5-point scale ranging from 1 (strongly disagree) to 5 (strongly agree).

Data analysis

We first conducted a confirmatory factor analysis. The maximum likelihood estimation method was employed and the analysis was conducted using a covariance matrix. We also addressed the issue of common method variance following Podsakoff, MacKenzie, Lee, & [START_REF] Podsakoff | Common method biases in behavioral research: A critical review of the literature and recommended remedies[END_REF] recommendations. Our hypotheses were then tested using path analysis procedures. Finally, a bootstrapping approach was used to confirm the mediating role of ACO in the relationships of ACS, social isolation, and PCO to well-being and turnover intentions.

Results

Preliminary analyses

First, a measurement model was estimated including all study dimensions as latent variables and their respective indicators. This model obtained good fit to the data, χ 2 (351) = 566.37, p < .001, IFI = .95, TLI = .94, CFI = .95, and RMSEA = .05. We then examined whether the hypothesized model was influenced by response bias (see [START_REF] Podsakoff | Common method biases in behavioral research: A critical review of the literature and recommended remedies[END_REF]. First, the Harman's single factor test revealed a poor fit to the data, χ 2 (366) = 2233.02, p < .001, IFI = .57, TLI = .52, CFI = .57, and RMSEA = .15. Second, we tested a model including an orthogonal latent method factor in addition to the theoretical factors. Items loaded on their theoretical constructs, as well as on a latent common method variance factor. The model fit was adequate: χ 2 (322) = 492.42, p < .001, IFI = .96, TLI = .95, CFI = .96, and RMSEA = .05. However, the method factor accounted for only fifteen percent of the total variance among the items. This suggests that our theoretical model was viable and that method bias was limited. Table 1 displays the correlations and descriptive statistics for the study. All correlations were in the expected direction and provided preliminary support for our hypotheses. Please insert Table 1 about here.

Main analyses

As hypothesized, ACO was defined as a partial mediator in the relationships of ACS, social isolation, and PCO to well-being and turnover intentions. The hypothesized model yielded a good fit to the data: χ 2 (1) = 1.86, p = .17, IFI = 1.00, TLI = .95, CFI = 1.00, and RMSEA = .06. We next tested an alternative model. Specifically, we removed the paths from ACS, social isolation, and PCO to wellbeing and turnover intentions (i.e., a fully mediated model). This model had a worse fit than the theoretical model, χ 2 (7) = 67.80, p < .001, IFI = .79, TLI = .54, CFI = .78, and RMSEA = .19. Overall, findings suggest that the best fitting model was the theoretical model (see Figure 1). Please insert Figure 1 about here.

ACS was positively linked to ACO (β = .26, p < .001) and well-being (β = .12, p < .05), and was negatively associated with turnover intentions (β = -.12, p < .05). PCO was positively linked to ACO (β = .30, p < .001), well-being (β = .19, p < .01), and negatively related to turnover intentions (β = -.40, p < .001). Social isolation was negatively linked to ACO (β = -.27, p < .001) and well-being (β = -.12, p < .05), and positively related to turnover intentions (β = .13, p < .05). Finally, ACO was positively linked to well-being (β = .28, p < .001) and negatively to turnover intentions (β = -.15, p < .05).

Finally, bootstrapping analyses revealed that the indirect effects of ACS on well-being (β = .07, CI = [.02, .16], p < .001) and turnover intentions (β = -.04, CI = [-.10, -.00], p < .01) through ACO were significant. The indirect effects of PCO on well-being (β = .08, CI = [.02, .19], p < .001) and turnover intentions (β = -.05, CI = [-.11, -.00], p < .01) through ACO were also significant. Moreover, the indirect effects of social isolation on well-being (β = -.08, CI = [-.16, -.02], p < .001) and turnover intentions (β = .04, CI = [.00, .11], p < .01) through ACO were significant.

Conclusions In a context where nurses' turnover and psychological health have become a major issue for healthcare organizations [START_REF] Aiken | Nurses' reports of working conditions and hospital quality of care in 12 countries in Europe[END_REF], the current study aimed to investigate whether PCO, ACS, and social isolation could offer leverage to increase nurses' well-being and reduce their turnover intentions. Moreover, we aimed to explore a potential common mechanism underlying such effects (i.e., ACO). Our results indicated that when nurses perceive that they have career opportunities and when they feel affectively committed to their supervisor, they experience a higher sense of well-being and less intent to leave their organization, thus confirming both Hypotheses 1 and 2. We therefore corroborate prior findings that showed PCO (e.g., [START_REF] De Beer | Job insecurity, career opportunities, discrimination and turnover intention in post-apartheid South Africa: Examples of informative hypothesis testing[END_REF] and ACS (e.g., [START_REF] Vandenberghe | Affective commitment to the organization, supervisor, and work group: Antecedents and outcomes[END_REF] to negatively relate to turnover intentions, and allow to generalize them to the nursing profession. We additionally contribute to answer scholars' call (e.g., [START_REF] Kraimer | Antecedents and outcomes of organizational support for development: The critical role of career opportunities[END_REF] to develop the nomological networks associated with PCO and ACS, by showing them to positively relate to a non-traditionally studied consequence (i.e., well-being).

Conversely, our findings showed that the more nurses feel socially isolated from their colleagues, the less they experience well-being and the more they intend to turnover, which provided support for Hypothesis 3. These results confirm previous research that similarly found social isolation from colleagues to relate negatively to well-being and positively to turnover intentions [START_REF] Marshall | Workplace isolation: Exploring the construct and its measurement[END_REF]. Indeed, when feeling socially isolated from their peers, nurses' fundamental need for relatedness may be thwarted and thus lead to decreased well-being [START_REF] Ryan | Living well: A self-determination theory perspective on 9udaimonia[END_REF], they may also want to quit their employer in search of an organization where they can fulfill their need to interact with others. Additionally, results revealed that when they feel strongly identified with their organization, nurses declare less intent to quit their employer and experience more well-being, thus providing support for Hypothesis 4 and corroborating previous findings demonstrated in the healthcare setting (e.g., [START_REF] Flinkman | Nurses' intention to leave the profession: integrative review[END_REF][START_REF] Li | Group cohesion and organizational commitment: Protective factors for nurse residents' job satisfaction, compassion fatigue, compassion satisfaction, and burnout[END_REF]. Indeed, when they feel strongly identified with their organization's goals and values, nurses may sense that their work environment aligns with their true self, and thus experience higher well-being, they may also want preserve this fulfilling environment and therefore have less intentions to quit their employer. Finally, our results demonstrated that when nurses sense that they have career opportunities and identify with their supervisor, they experience a stronger emotional bond with their organization. However, this emotional bond is impaired when nurses feel socially isolated from their coworkers, which confirms Hypothesis 5. These results are in line with prior research (e.g., [START_REF] Chang | Organizational work-family culture and working mothers' affective commitment: How career expectations matter[END_REF][START_REF] Panaccio | The relationships of role clarity and organization-based selfesteem to commitment to supervisors and organizations and turnover intentions[END_REF][START_REF] Santos | Relational job characteristics and nurses' affective organizational commitment: The mediating role of work engagement[END_REF].

More generally, this study confirmed our hypothesized model wherein ACO partially mediates the relationship between PCO, ACS, and social isolation on one hand, and turnover intentions and wellbeing on the other hand (Hypothesis 6). These findings thus build upon previous research which investigated different variables in various occupational settings (e.g. [START_REF] Poon | Relationships among perceived career support, affective commitment, and work engagement[END_REF][START_REF] Vandenberghe | Affective commitment to the organization, supervisor, and work group: Antecedents and outcomes[END_REF], by confirming ACO to be a valuable mechanism to explain the effect of nurses' work experience on their functioning. In other words, when they understand that they have career opportunities as well as a high-quality relationship with their supervisor, then nurses feel more committed to their organization, which in turn translates into higher well-being and lower turnover intentions. On the contrary, when feeling that they lack social interaction with their coworkers, nurses experience a lower sense of identification with their organization, which in turn converts into less well-being and more intent to quit.

In sum, the current study emphasized how nurses' well-being and turnover intentions vary according to various organizational (i.e., PCO), managerial (i.e., ACS), and interpersonal (i.e., social isolation from colleagues) factors. This research also showed ACO to be a valuable common mechanism to explain these effects. Altogether, these results show that the way nurses assess various aspects of their work experience pervades their emotional bond to their organization, which in turn explains their well-being and turnover intentions.

Theoretical implications

These results have several theoretical implications. First, this study offered an investigation of PCO and social isolation from colleagues in the nursing context, which, to the best of our knowledge, was never done in prior research. More specifically, this paper showed both of these variables to be important factors to consider in order to promote beneficial outcomes (i.e., ACO, well-being) and to prevent undesirable ones (i.e., turnover intentions). Second, this study adds up to the literature on ACS in the nursing profession. Indeed, even though few studies investigated this variable's effect on workrelated outcomes such as nurses' turnover intentions (e.g., [START_REF] Vandenberghe | A closer look at the relationship between affective commitment to supervisors and organizations and turnover[END_REF], prior research did not pay as much attention to health-related outcomes. The present research therefore broadens our understanding of the positive effects of ACS on nurses' well-being and confirms its negative relationship with their turnover intentions. Third, nurses' commitment to the organization was more documented in the literature but especially as a way to prevent their turnover intentions (e.g., [START_REF] Flinkman | Nurses' intention to leave the profession: integrative review[END_REF]. Our research also showed that ACO could as well be a relevant lever to promote nurse well-being. Fourth, and more specifically, in the present study we showed ACO to be a valuable mechanism to explain the effect of nurses' work experience on their well-being and turnover intentions. Therefore, this paper confirms ACO to be a key construct to explain the effects of organizational, managerial or social characteristics of the work environment on French nurses' functioning as well, thus allowing to generalize and broaden prior results found in different cultural backgrounds (e.g., El Akremi et al., 2014).

Limitations and future research

Despite its contributions, this paper presents some limitations. First, we only used self-report measures, which may induce common-method bias [START_REF] Podsakoff | Common method biases in behavioral research: A critical review of the literature and recommended remedies[END_REF]. Although we carried out confirmatory factor analyses to confirm that such bias was not a serious issue underlying our data, future research could, for instance, include objective physiological measures to assess nurses' wellbeing. Second, because we used a cross-sectional design, no changes in nurses' well-being or turnover intentions can be concluded. Longitudinal designs could be used to confirm the causality underlying our model and to offer a more parsimonious examination of the mediation. Third, our sample was relatively small. It would be fruitful to investigate a larger sample to allow for a more rigorous and generalizable analysis of the studied relationships. Moreover, our sample only consisted of French nurses. Yet, previous work emphasized that nurses' experiences of their jobs varied greatly across European countries [START_REF] Aiken | Nurses' reports of working conditions and hospital quality of care in 12 countries in Europe[END_REF]. Our findings could be replicated in different countries to be generalized to other cultures. Finally, we solely looked into one facet (i.e., affective) of both forms of commitment (i.e., to the supervisor and to the organization). Although affective commitment is known for being the most widely studied facet of commitment, commitment is a multi-dimensional construct. It would thus be interesting to explore how different facets of commitment (i.e., normative, continuance) (e.g., [START_REF] Panaccio | The relationships of role clarity and organization-based selfesteem to commitment to supervisors and organizations and turnover intentions[END_REF] might relate to our study variables.

Implications for nursing management

As healthcare organizations struggle to prevent nurses' turnover and protect their psychological health in a very constrained context, this study sheds light on some factors that could contribute to reduce nurses' intent to quit and promote their well-being, by the boosting of their emotional bond with their organization (i.e., ACO). Specifically, our research underscores the necessity for organizations to foster their career development policies so that nurses develop positive assessments about their career opportunities. Moreover, this study emphasizes the importance for nurses to have a high-quality relationship with their supervisor, and encourages health organizations to reinforce ACS in their employees. Some levers include promoting supervisor support or paying attention to justice issues (e.g., [START_REF] Vandenberghe | Affective commitment to the organization, supervisor, and work group: Antecedents and outcomes[END_REF]. Healthcare organizations could therefore offer training programs to their supervisors to encourage them to create such conditions for their subordinates. Finally, healthcare organizations would gain in creating the conditions to allow high-quality relationships among nurses themselves. Indeed, because of the many demands that are inherent to their profession, nurses may feel socially isolated from their coworkers, and sense that they do not have time for social and informal interactions with their peers. Healthcare organizations would therefore benefit from creating space and time for such interactions.

Table 1

 1 Correlation Matrix and Descriptive Statistics for the Study Variables Turnover intentions 2.52 1.04 -.31** .25** -.52** -.41** -.39** (.89) Note. Alpha values are on the diagonal in parentheses. (N = 244). All variables were measured on a 5-point scale (1-5). *p < .05; **p < .001 Figure 1. Results of the path analysis Note. Standardized coefficients are reported. All paths are significant (p < .05)

	Variables	M	SD	1	2	3	4	5	6	7
	1. ACS	2.75 .76	(.91)
	2. Workplace isolation 1.63 .62	-.09	(.92)
	3. PCO	2.56 1.02 .31** -.13*	(.85)
	4. ACO	2.90 .77 .37** -.33** .41** (.86)
	5. Well-being	3.15 .72 .29** -.25** .36** .44** (.82)
	6.


Lise Maciejewski
email: lise.maciejewski@agroparistech.fr

Emilien Kuhn
email: kuhn.emilien@brasserietumulte.fr

Anne Gégout-Petit
email: anne.gegout-petit@univ-lorraine.fr

Jean- Claude Gégout

Jean-Claude Gégout
email: jean-claude.gegout@agroparistech.fr

Natura 2000 forest habitats: climatic debt in lowlands and thermophilization in highlands

Keywords: climate change, forest ecosystem, global warming, habitat of Community

Natura 2000 is a European network of sites dedicated to the conservation of vulnerable habitats. The definitions of Natura 2000 habitats are mainly based on plant communities. We investigated if the increase of the dominance of warm-adapted species observed in plant communities, described as thermophilization, had already led to measurable changes in Natura 2000 forest habitats.

We created 5,701 pairs of neighboring forest plots by gathering plots surveyed before 1987 and after 1997 to reflect historical and recent climatic conditions. A Natura 2000 habitat type was assigned to each vegetation plot using an automatic classification program. We calculated a temperature index that synthesized the temperature range of each habitat, and compared the habitat temperature indexes of the recent and historical plots of each pair.

We highlighted a significant overall shift of 4.8% ± 1.78 (CI 95%) of the pairs toward warmer habitats over the studied period. While the shift was not significant in lowlands, 11.1% ± 3.0 (CI 95%) of the pairs evolved toward warmer habitats in highlands.

The excess of pairs with a warmer habitat in the recent period was interpreted as thermophilization of Natura 2000 forest habitats. Therefore, global warming has been strong enough to induce actual changes at the coarse-grained habitat resolution specifically targeted by public policies. The absence of significant results in lowlands suggests the existence of unrealized potential habitat changes, which can be considered as a climatic debt. These results call for differential prioritization levels and implementations of public policies for nature conservation in lowlands and highlands.

INTRODUCTION

Nature conservation was initially developed for species preservation [START_REF] Kiss | Du régional à l'universel : la généralisation des préoccupations environnementales[END_REF]. Faced with the homogenization of ecosystems and landscapes due to human omnipresence, conservationists aimed for a more integrated vision of nature. Plant ecologists have developed and used the concept of habitat for a century, close to the notion of ecosystem but initially based on plant community [START_REF] Yapp | The Concept of Habitat[END_REF]. In Europe, legal frameworks and most of the efforts dedicated to biodiversity conservation are committed to habitat management and restoration.

The European Directive 92/43/EEC (EC Council 2006) on the conservation of natural habitats and of wild fauna and flora (the Habitats Directive) sets the framework for the European Union's policies on nature conservation. The given definition of the term habitat is a "terrestrial or aquatic area distinguished by geographic, abiotic and biotic features, whether entirely natural or semi-natural" (art.1). The targeted habitats designated to be of Community interest (also named 'Natura 2000 habitat', 'habitat type' or 'habitat' hereafter) are rare, or in danger of disappearance, or display outstanding examples of typical characteristics of one of the biogeographical regions in Europe. Natura 2000 habitat types are classified and described in the EUR 28 Interpretation Manual (European Commission DG Environment 2013).

Phytosociology principles have been used to describe plant communities at the European level for one century, this is why phytosociology was chosen as a basis to define the terrestrial Natura 2000 habitats [START_REF] Guarino | Spatio-temporal variations in the application of the Braun-Blanquet approach in Europe[END_REF]. The basic unit of the phytosociological classification is the association, defined by its characteristic combination of plant species [START_REF] Braun | Remarques sur l'étude des groupements de plantes[END_REF].

Researchers, policymakers, and conservationists now widely agree that the Earth's climate is changing, with major impacts on biodiversity [START_REF] Guinochet | Conservation of Biodiversity in a Changing Climate[END_REF][START_REF] Araujo | Climate change threatens European conservation areas[END_REF]. Previous studies have shown that climate warming has led to an increase of the dominance of warmadapted species in plant communities described as thermophilization [START_REF] De Frenne | Thermophilization estimation is robust to the scale of species distribution data[END_REF], observed mainly in highlands [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF][START_REF] Gottfried | Continent-wide response of mountain vegetation to climate change[END_REF]. This thermophilization trend lags behind climate warming, particularly in lowlands [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF][START_REF] Bertrand | Ecological constraints increase the climatic debt in forests[END_REF][START_REF] Savage | Elevational shifts, biotic homogenization and time lags in vegetation change during 40 years of climate warming[END_REF][START_REF] Alexander | Lags in the response of mountain plant communities to climate change[END_REF]. Despite their potential usefulness for policymakers and Natura 2000 managers, published results dealing with the impact of climate warming on plant species and communities are often not expressed in the categories used by stakeholders, and translating them remains difficult [START_REF] Jeanmougin | Mismatch between Habitat Science and Habitat Directive: Lessons from the French (Counter) Example[END_REF].

A recent survey of the LIFE program Natur'adapt conducted among 497 nature conservation professionals in France and Europe showed that even if climate change is a priority issue for nine out of ten stakeholders, two thirds of nature professionals do not take climate change into account in their management practices [START_REF] De Sadeleer | Intégration du changement climatique dans la gestion des espaces naturels protégés. Initiatives existantes et attentes des gestionnaires européens[END_REF]. This situation can be explained by the lack of biologically relevant metrics of climate risk for local-scale climate vulnerability assessments and adaptation planning, but also the lack of results or a lack of translations of the results on ecosystem types specifically targeted by public policies and at a resolution used in local management by practitioners [START_REF] Ordonez | Projected climate reshuffling based on multivariate climateavailability, climate-analog, and climate-velocity analyses: implications for community disaggregation[END_REF][START_REF] De Sadeleer | Intégration du changement climatique dans la gestion des espaces naturels protégés. Initiatives existantes et attentes des gestionnaires européens[END_REF].

In a protected area, the habitat typology accounted for by the Natura 2000 habitat classification system (European Commission DG Environment 2013) provides the means to identify, map, manage ecosystems [START_REF] De Cáceres | A comparative framework for broad-scale plotbased vegetation classification[END_REF], and then restore them. Only few studies have investigated the future impacts of climate change on the Natura 2000 network, and they predict habitat range shifts [START_REF] Araujo | Climate change threatens European conservation areas[END_REF][START_REF] Bittner | Comparing modelling approaches at two levels of biological organisation -Climate change impacts on selected Natura 2000 habitats[END_REF][START_REF] Barredo | Mediterranean habitat loss under future climate conditions: Assessing impacts on the Natura 2000 protected area network[END_REF]. But the current impacts of recent climate warming on Natura 2000 habitats have not been studied yet.

Therefore, we investigated if the changes recently observed on plant communities linked with climate warming were sufficient to be already visible at a coarse-grained scale such as Natura 2000 habitats. Considering the previous studies [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF][START_REF] Bertrand | Ecological constraints increase the climatic debt in forests[END_REF][START_REF] Gottfried | Continent-wide response of mountain vegetation to climate change[END_REF][START_REF] Savage | Elevational shifts, biotic homogenization and time lags in vegetation change during 40 years of climate warming[END_REF][START_REF] Alexander | Lags in the response of mountain plant communities to climate change[END_REF], we also studied whether differences could be found between lowlands and highlands. Detecting early changes in Natura 2000 habitat with climate warming and differences between biomes is of crucial importance for political choices and the subsequent management practices.

METHODS

We studied Natura 2000 habitats in temperate and mountainous French forests excluding riparian and bog forests, and we investigated if the habitats under recent climatic conditions were characteristic of warmer temperature than the habitats under historical climatic conditions. We first characterized the temperature range of each habitat by creating a temperature index per habitat type that synthesized the range of temperature extracted for each plot where the habitat was found and considered as historically typical by experts. This temperature index enabled us to study the thermal responses of habitats. To this end, we used large databases of floristic plots to generate pairs of historical and recent neighboring plots. In order to disentangle temporal effects from potential spatial effects, we also paired two historical plots to highlight a potential effect of differences in plot location. Although pairing plots requires controlling the distance between the paired plots and their altitude, the huge advantage compared with permanent plot devices is that massive databases can be used and show slight trends over large areas. This was not feasible with the limited number of permanent plots available.

Floristic data

We extracted vegetation plots from three databases: Sophy, EcoPlant, and the national forest inventory (NFI) of France. A plot included a complete floristic inventory with the species cover and plot location variables. Sophy is a phytosociological database that gathers a compilation of complete floristic inventories carried out at the plot level from 1914 to 2000 [START_REF] Brisse | European vegetation survey. La banque de données phytosociologiques " SOPHY[END_REF].

EcoPlant is a phyto-ecological database with phytosociological inventories from 1922 to 2011 mainly focused on forest ecosystems [START_REF] Gégout | EcoPlant: A forest site database linking floristic data with soil and climate variables[END_REF]. In these two data sources, the plot size is the 400 m² standard size of a phytosociological inventory in a forested area [START_REF] Braun-Blanquet | Plant sociology: the study of plant communities[END_REF]. The database provided by the NFI of France [START_REF] Hervé | France[END_REF]) included floristic inventories of 700-m² plots inventoried between 1991 and 2014 and uniformly distributed across the forests of mainland France. We only kept plots with vegetation surveys conducted between April and October and whose coordinates were available with less than 1 km accuracy. A total of 116,760 floristic plots, including vascular plants and terricolous bryophytes, were extracted from these three databases [START_REF] Kuhn | Highlighting declines of cold-demanding plant species in lowlands under climate warming[END_REF].

Assignment of a Natura 2000 habitat type to each vegetation plot An automatic classification program based on a program developed in 2012 by Gégout and Coudun to classify floristic inventories of the French temperate and mountainous forests was used to assign a phytosociological association to the vegetation. This automatic classification program used the complete floristic composition of the plot for assignment, and its results were similar to those of expert judgments [START_REF] Gégout | The right relevé in the right vegetation unit: a new typicality index to reproduce expert judgement with an automatic classification programme[END_REF]. The list of phytosociological associations that defined each habitat available in [START_REF] Gégout | Les habitats forestiers de la France tempérée ; typologie et caractérisation phytoécologique[END_REF] was used to assign a habitat type to each plot whenever relevant.

Production of a temperature index per Natura 2000 habitat type

We defined a temperature index from plots whose habitat was assigned and considered as typical in terms of vegetation composition by experts, i.e. where most of the expected species were encountered [START_REF] Guinochet | Conservation of Biodiversity in a Changing Climate[END_REF]. We created a temperature index (TI) per habitat type that synthesized the range of temperature extracted for each plot considered as historically typical.

We used 3,379 plots to compute a TI for 15 frequent forest habitats in France. We calculated a TI for each habitat as the average value of the mean annual temperatures of the period for each plot. Annual temperatures were extracted at a 30-second resolution for each plot with the Worldclim climate database version 1.4 [START_REF] Hijmans | Very high resolution interpolated climate surfaces for global land areas[END_REF]. We avoided a possible disequilibrium between communities and climate linked to recent climate change by using plots and temperatures surveyed over the 1960-1990 period, before the warming period in France. We used at least 7 typical plots per habitat type to calculate a significant TI. The 15 studied Natura 2000 habitats were based on 80 phytosociological associations and covered the diversity of broadleaf and coniferous non hydromorphic Natura 2000 forest habitats of lowlands and mountainous areas of temperate France.

Pairing plots

Definition of the historical and recent periods

In France, recent climate warming is characterized by a relative stability of mean annual temperatures until the 1980s, a strong increase in temperature from the end of the 1980s until the end of the 1990s, followed by a period of relative stability [START_REF] Kuhn | Highlighting declines of cold-demanding plant species in lowlands under climate warming[END_REF]. A close look at annual average temperatures allowed us to define the period before 1987 as historical, and the period after 1997 as a warmer recent period. There was no significant temperature trend in the last three decades of the historical period or during the recent period (linear regressions p = 0.60 and 0.75, respectively). The mean temperature warming between the historical period and the recent period as measured by the French meteorological network is 1.13°C [START_REF] Kuhn | Highlighting declines of cold-demanding plant species in lowlands under climate warming[END_REF], and this increase was not significantly different in lowlands as compared to highlands before 2008 [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF].

The historical-recent dataset (HIS-REC dataset)

We created pairs of neighboring plots comprising one historical plot and one recent plot each (Fig. 1). We assumed that pairing historical and recent non-permanent plots spatially was an extreme case of resurveying historical plots with a large uncertainty on plot location. To limit uncertainty, we created pairs by iteratively selecting the geographically closest historical and recent plots among all pairing possibilities. The maximum allowed difference in elevation within each pair of plots was 200 m, and the maximum distance allowed between two plots within a pair was 5 km [START_REF] Kuhn | Highlighting declines of cold-demanding plant species in lowlands under climate warming[END_REF]. We selected pairs of plots including Natura 2000 habitat types present in the temperate and mountainous areas of France (pairs of the Mediterranean area excluded). Thus, we created a historical-recent dataset (HIS-REC dataset) of 5,701 pairs of historical and recent plots (Tab. 1 and supporting information Table S1 for the list of the studied habitats). The average distance between plots within pairs was 1,477 m (SE 16). The altitudinal difference between the recent and historical plots was negative or positive; the average difference was 2.0 m (SE 0.9). By calculating the average value among pairs without taking the + or -symbol into account, this difference was 44.9 m (SE 0.7) in absolute value. Once plots were paired, the average years of the historical and recent plots were 1974 and 2006, respectively. Data were also stratified according to the mean altitude of the pairs (from 2 to 2,389 m a.s.l.). Lowlands were defined as the areas below 500 m a.s.l., while highlands were defined as the areas at an altitude of 500 m a.s.l. or above.

The historical-historical dataset (HIS-HIS dataset)

To highlight a potential effect of differences in plot location, we used the previously described procedure to create 4,585 pairs of two historical plots (Tab. 1). The average distance between two plots was 586 m (SE 14). The average altitudinal difference between two plots was 0 m (SE 0.1) and 22.5 m (SE 0.6) in absolute value. The average difference between the two plot years was 2 years.

Light-controlled subset of the historical-recent dataset

Forest practices evolved over the last century [START_REF] Pignard | Evolution récente des forêts françaises : surface, volume sur pied, productivité[END_REF], with a potential impact on the forest cover. The forest cover influences incident light, which affects floristic composition and could impact habitat assignment. Therefore, we created subsets of the historical-recent dataset to control the potential forest canopy cover differences between the two periods. Light indexes for each species listed in the French forest flora [START_REF] Rameau | Flore forestière française -guide écologique illustrétome 1 : Plaines et collines[END_REF][START_REF] Rameau | Flore forestière française -guide écologique illustrétome 2 : Montagnes, Institut pour le Développement Forestier[END_REF] were extracted and recoded from qualitative assessment to a quantitative index ranging from 1 (shade tolerant) to 7 (heliophilous), following [START_REF] Ellenberg | Zeigerwerte der Pflanzen Mitteleuropas[END_REF]. Species absent from the French forest flora were considered as heliophilous species of open environments and were assigned an index of 8. Then, we calculated a bio-indicated light value for each vegetation plot as the average value of the light index of each species recorded in the floristic inventory. Next, we calculated the difference in bio-indicated light within the pairs of historical and recent plots.

The gradient of differences in bio-indicated light was divided into ten classes (five classes of negative differences and five of positive differences). An equal number of pairs was randomly selected in each positive class and its negative equivalent. It resulted in a zero-centered distribution of differences in bio-indicated light. This subsampling procedure resulted in the selection of 3,905 pairs of plots named "light-controlled subset of the historical-recent dataset" (Tab. 1). The average distance between two plots was 1,476 m (SE 20). The average altitudinal difference between two plots was -3.0 m (SE 1.1) and 45.3 m (SE 0.8) in absolute value. The average years of the historical and recent plots were 1974 and 2006, respectively.

Comparison of the habitat temperature indexes between periods

We calculated the differences between the temperature indexes of the habitats assigned to the recent and historical plots (∆TIpair=TIrecent -TIhist,) for each pair of plots. The objective was to determine whether the habitat in the recent climatic conditions was characteristic of warmer, colder or equal temperature conditions than the habitat in the historical climatic conditions. If the habitat assigned to the two plots was identical, the difference between the temperature indexes was null (∆TIpair=0). If the difference between the temperature indexes of the pair was negative (∆TIpair<0), we hypothesized that the evolution of plant communities led to a habitat characteristic of colder conditions, the pair had "cooled down". If the difference between the temperature indexes of the pair was positive (∆TIpair>0), we hypothesized that the evolution of plant communities led to a habitat characteristic of warmer temperatures, the pair had "warmed up". Then we calculated the percentage of pairs with null, positive and negative ∆TIpair for each dataset, and compared them. The proportion of pairs displaying the same habitat in the two plots was not excluded from the analyses.

From a theoretical viewpoint, in permanent plots undergoing no change in environmental conditions or in forestry practices, the habitats assigned to the two plots for each pair should be identical, with null values for ∆TIpair. But pairing plots can lead to habitat differences partly due to differences in plot location. Thus, part of the ∆TIpair could theoretically be null but also partly positive or negative. But with no environmental changes or in forestry practices, equal numbers of positive and negative ∆TIpair were expected. We checked this assumption by investigating if there was an identical percentage of pairs that had warmed up and cooled down using the HIS-HIS dataset. We tested the hypothesis of an impact of recent warming on habitat by searching for a significant excess of "warmed up" pairs versus "cooled down" pairs using the HIS-REC dataset. We tested for a potential effect of canopy changes between periods using the light-controlled subset of the HIS-REC dataset.

We applied a McNemar's test (χ²-test for paired data, df=1) to test the significant excess of the percentages of "warmed up" versus "cooled down" pairs (hereafter named "changing pairs").

The principle was to compare the observed percentage of "warmed up" and "cooled down" pairs to the theoretical percentage: an equal number of pairs warmed up and cooled down.

Therefore theoretical frequencies assigned the changing pairs an equal probability of 0.5 to each modality. Then, we subtracted the percentage of "cooled down" pairs from the percentage of "warmed up" pairs: [(% of pairs ∆TIpair>0) -(% of pairs ∆TIpair<0)]. This result was considered as the excess of pairs of plots with a warmer habitat temperature index in the recent period. Using a χ²-test, we calculated the associated 95% confidence interval considering that the plots had been paired. We used the same methodology for the light-controlled subset of the HIS-REC dataset. For the HIS-HIS dataset, the same analyses were run by calculating the temperature index differences between the two historical plots of each pair: ∆TIpair = TIhist_i -TIhist_ j. The positions of the two plots i and j of the pair in the equation were selected randomly.

Then, using a χ²-test, we checked for significant differences between the excess of pairs of plots with a warmer habitat temperature index in the recent period according to the dataset.

Finally, we also calculated the average value of all temperature indexes of the habitats present in the recent period, and the average value of all TIs of the habitats present in the historical period, and calculated the difference: ∆Mean(TI)dataset=Mean(TIrecent)-Mean(TIhist). Then, we performed a paired-sample Student's t-test to check for significant differences between the temperature indexes of the recent and historical periods. We used the same methodology for the light-controlled subset of the HIS-REC dataset and for the HIS-HIS dataset.

RESULTS

For a slight majority of the pairs of the HIS-REC dataset, the historical and recent plots displayed the same habitats (53.0% of the pairs). Pairs displaying the same habitats within the two plots were more frequent in lowlands (55.4%) than in highlands (48.7%) (Tab. 1).

In the HIS-REC dataset, the percentage of "warmed up" pairs (25.9%) was higher than the percentage of "cooled down" pairs (21.1%) (Tab. 1): the pairs of "warmed up" plots (∆TIpair>0) were in a significant excess of 4.8% ±1.78 (CI 95%) (Fig. 2). When we controlled the canopy cover conditions, the light-controlled subset also evidenced a significant excess of 3.6% ±2.15 (CI 95%) (Fig. 2). These two excesses were not significantly different between them (Fig. 2). In contrast, the HIS-HIS dataset showed no significant excess of pairs of plots with a positive ∆TIpair (Fig. 2).

The percentage of "warmed up" pairs in lowlands was not significantly different than the percentage of "cooled down" pairs in the HIS-REC dataset, in the light-controlled HIS-REC dataset, and in the HIS-HIS dataset (Tab. 1, Fig. 2). In contrast, in highlands we observed significant excesses of the pairs of plots with a warmer habitat in the recent period: 11.1% ±3.0 (CI 95%) in the HIS-REC dataset (Fig. 2), and 13.4% ±3.7 (CI 95%) in the light-controlled HIS-REC dataset (Fig. 2). The HIS-HIS dataset showed no significant excess of pairs of plots with a positive ∆TIpair (Fig. 2). Lastly, we observed the same pattern for the mean TI values of the habitats according to the periods: for the HIS-REC dataset and the light-controlled HIS-REC dataset, we observed a slight but significant increase in the average TI between the historical and recent periods in the total dataset. The difference was higher in highlands but not significant in lowlands (Tab. 2). We observed no significant difference between the mean TIs of the HIS-HIS dataset (Tab. 2).

DISCUSSION

This study highlights a significant shift toward warmer Natura 2000 forest habitats of temperate and mountainous forests in France between historical and recent periods over a large diversity of frequent forest habitats.

As permanent plots were not available in sufficient numbers, we paired historical and recent neighboring plots, following previous studies [START_REF] Hijmans | Comment on "Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species' Optimum Elevations[END_REF][START_REF] Kuhn | Highlighting declines of cold-demanding plant species in lowlands under climate warming[END_REF]. We used huge databases to investigate slight trends over a large area. Using the HIS-HIS datasets built with pairs of two historical plots, we checked for the absence of biases due to differences in plot location. Thus, the changes highlighted by the comparison between historical and recent plots were interpreted as a temporal trend. Changes in forestry practices and climatic changes over the last century could both have affected this temporal trend. The evolution of forestry practices [START_REF] Pignard | Evolution récente des forêts françaises : surface, volume sur pied, productivité[END_REF] brought about changes in the canopy cover that influenced plant communities [START_REF] Danneyrolles | Stronger influence of anthropogenic disturbance than climate change on century-scale compositional changes in northern forests[END_REF]. We controlled the canopy cover conditions with the light-controlled subset of the HIS-REC dataset, ran the tests again and obtained similar results. Thus, we ensured that the evolution of the canopy cover had not influenced the results.

Besides, a change in the rainfall regime could have influenced the changes in plant communities [START_REF] Harrison | Climate-driven diversity loss in a grassland community[END_REF]. But the decade-scale variation in rainfall remained the same before and after 1987 [START_REF] Lenoir | A Significant Upward Shift in Plant Species Optimum Elevation During the 20th Century[END_REF]), so that it cannot have directly influenced the changes we observed. Therefore, recent climate warming and perhaps other indirect climatic factors linked with temperature such as seasonality were the likely drivers of the observed increase in the dominance of habitats characteristic of warmer conditions. In agreement with the existing definition of plant communities of De [START_REF] De Frenne | Thermophilization estimation is robust to the scale of species distribution data[END_REF], the excess of pairs with a warmer habitat temperature index in the recent period was interpreted as thermophilization of Natura 2000 forest habitats induced by recent climate warming.

These results are congruent with previous studies regarding the changes induced by recent climate warming on plant communities in various temperate and mountainous ecosystems [START_REF] De Frenne | Microclimate moderates plant responses to macroclimate warming[END_REF][START_REF] Savage | Elevational shifts, biotic homogenization and time lags in vegetation change during 40 years of climate warming[END_REF]. This study complements previous community level results at the habitat level. It is the first evidence that global warming is strong enough to induce changes at the coarse-grained resolution of Natura 2000 habitat types specifically targeted by public policies and used in local management. This coarse-grained resolution is the likely reason why the average and highland increases of the habitat TI were of a lesser extent than the thermophilization highlighted at the community level which can exceed 0.5°C in mountains [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF].

We studied lowlands and highlands separately, and found a substantial shift toward warmer Natura 2000 forest habitats in highlands. In contrast, we observed no significant change in lowlands. However, the measured temperature increase exceeded one degree in lowlands and highlands across the French territory in the recent decades [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF][START_REF] Kuhn | Highlighting declines of cold-demanding plant species in lowlands under climate warming[END_REF]. Consequently, we can guess that unrealized potential habitat changes are under way in lowlands, and they can be considered as a climatic debt [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF].

Public policies for habitat conservation are currently implemented in the same way across the whole French territory. But this study shows that climate warming has differential impacts on habitats in lowlands and highlands. These differences call for differential prioritization levels and implementations of conservation strategies in lowlands and highlands. In lowlands where a climatic debt is being developed, several studies suggest that the current inertia of plants and plant communities is due to a high level of anthropogenic habitat fragmentation [START_REF] Bertrand | Changes in plant community composition lag behind climate warming in lowland forests[END_REF][START_REF] Wessely | Habitat-based conservation strategies cannot compensate for climate-change-induced range loss[END_REF]. Therefore, we advise to allow species and habitats to track their climatic envelope over time by facilitating dispersal to newly suitable areas. To do so, many studies call for improving the connectivity of Natura 2000 networks, even if the advice is mostly generic and devoid of practical instruments and criteria [START_REF] Van Teeffelen | How climate proof is the European Union's biodiversity policy?[END_REF]. The priority for mountain areas is to check that the existing network of sites represents the current diversity of natural habitats, but also to keep on updating this network to ensure that the future natural habitat diversity will also be accurately represented. Moreover, implementing management measures aimed at maintaining dynamics rather than a given structure and composition in each site can bring flexibility to managers [START_REF] Van Teeffelen | How climate proof is the European Union's biodiversity policy?[END_REF]. In that sense, setting conservation targets at regional scales rather than local scales can allow them to welcome the emergence of new habitats or the loss of previously existing ones.

Although conservation organizations and government agencies are developing "adaptation strategies" to facilitate the adjustment of human society and ecological systems to altered climate regimes [START_REF] Mawdsley | A Review of Climate-Change Adaptation Strategies for Wildlife Management and Biodiversity Conservation[END_REF], the impact of climate change is still poorly and hardly For example, forest managers still have reservations about the current usefulness of available scientific knowledge for their own areas and circumstances (de [START_REF] De Koning | Natura 2000 and climate change-Polarisation, uncertainty, and pragmatism in discourses on forest conservation and management in Europe[END_REF]. This study could help to take a step toward better including the climate change effects into local management because it is based on Natura 2000 habitats directly used by decision-makers and local managers and highlights regionalized impacts. For the HIS-HIS dataset, all the tests were performed considering one of the two plots randomly chosen as the recent one (∆TIpair=TIrecent -TIhist for the HIS-REC dataset and its light-controlled subsets; ∆TIpair = TIhist_i -TIhist_ j for the HIS-HIS dataset). For the HIS-HIS dataset, all the tests were performed considering one of the two plots randomly chosen as the recent one (∆TIpair=TIrecent -TIhist for the HIS-REC dataset and its light-controlled subsets; ∆TIpair = TIhist_i -TIhist_ j for the HIS-HIS dataset). Highland subset (>= 500 m) 1,620 a ∆TIpair<0: pairs of plots with a negative difference between the recent and historical temperature indexes associated to the habitats of each plot, i.e. pairs that have "cooled down." b ∆TIpair=0: pairs of plots with a null difference between the temperature indexes associated to the habitats of each plot.

FIGURE CAPTIONS

c ∆TIpair>0: pairs of plots with a positive difference between the temperature indexes associated to the habitats of each plot, i.e. pairs that have "warmed up". d For the HIS-HIS dataset, all the calculations were performed considering one of the two plots randomly chosen as the recent one (∆TIpair=TIrecent -TIhist for the HIS-REC dataset and its light-controlled subsets; ∆TIpair = TIhist_i -TIhist_ j for the HIS-HIS dataset). b Estimates = values of the differences between the mean value of all temperature indexes of the recent and historical periods.

c CI 95% = the associated 95% confidence interval of the mean.

d For the HIS-HIS dataset, all the tests were performed considering one of the two plots randomly chosen as the recent one (∆TIpair=TIrecent -TIhist for the HIS-REC datasets and its lightcontrolled subsets. ∆TIpair = TIhist_i -TIhist_ j for the HIS-HIS datasets).

Fig. 1 :

 1 Fig. 1: Location of the pairs composed of one historical and one recent forest floristic plot (HIS-REC dataset). Dots correspond to the centroid of each pair (n=5,701).

Fig. 2 :

 2 Fig. 2: Differences between the percentages of pairs of plots with a positive ∆TIpair (∆TIpair>0) and a negative ∆TIpair (∆TIpair<0) for each total dataset on the left (a.), and then stratified according to the mean altitude of the pairs, with lowlands in the center (b.) and highlands on the right (c.). The 95% confidence intervals were calculated considering that the plots had been paired. The lowercase letters (a, b, c) at the bottom of the figure indicate significantly different values, i.e., two values displaying the same letter are not significantly different.

Fig. 1 :

 1 Fig. 1: Location of the pairs composed of one historical and one recent forest floristic plot (HIS-REC dataset). Dots correspond to the centroid of each pair (N=5,701).

Fig. 2 :

 2 Fig. 2: Differences between the percentages of pairs of plots with a positive ∆TIpair (∆TIpair>0) and a negative ∆TIpair (∆TIpair<0) for each total dataset on the left (a.), and then stratified according to the mean altitude of the pairs, with lowlands in the center (b.) and highlands on the right (c.). The 95% confidence intervals were calculated considering that the plots had been paired. The lowercase letters (a, b, c) at the bottom of the figure indicate significantly different values, i.e., two values displaying the same letter are not significantly different.

Table 1 :

 1 Numbers of pairs of plots with a negative, null or positive temperature index difference (∆TIpair) within the pair in each dataset and subset and according to the mean altitude of the pairs (total dataset, lowland and highland subsets).

	Datasets and subsets	Pairs of plots N	∆TIpair< 0 a n (% of N)	∆TIpair= 0 b n (% of N)	∆TIpair> 0 c n (% of N)
	Historical-Recent dataset (HIS-REC dataset)
	Total (2-2,389 m)	5,701	1,204 (21.1)	3,021 (53.0)	1,476 (25.9)
	Lowland subset (< 500 m)	3,624	787 (21.7)	2,009 (55.4)	828 (22.8)
	Highland subset (>= 500 m)	2,077	417 (20.1)	1,012 (48.7)	648 (31.2)
	Light-controlled subsets of the HIS-REC dataset
	Total (2-2,389 m)	3,905	847 (21,7)	2,072 (53.1)	986 (25.2)
	Lowland subset (< 500 m)	2,512	580 (23.1)	1,400 (55.7)	532 (21.2)
	Highland subset (>= 500 m)	1,393	267 (19.2)	672 (48.2)	454 (32.6)
	Historical-Historical dataset (HIS-HIS dataset) d
	Total (2-2,389 m)	4,585	759 (16.6)	3,096 (67.5)	730 (15.9)
	Lowland subset (< 500 m)	2,965	459 (15.5)	2,053 (69.2)	453 (15.3)
			300 (18.5)	1,043 (64.4)	277 (17.1)

Table 2 :

 2 Paired-samples Student's t-test comparing the mean values of all temperature indexes of the recent period with those of the historical period according to the mean altitude of the pairs (total datasets, lowland and highland subsets).

	Datasets and subsets

a p-val = p-value (***: significant at 0.01%; ns: non-significant).

ACKNOWLEDGMENTS

The acquisition and the organization of historical floristic data were funded by the French Agency for Environment and Energy Management (ADEME), the National Forest Office (ONF) and the French National Research Agency (ANR) as part of the 'Investissement d'Avenir' program (ANR-11-LABX-0002-01, Lab of Excellence ARBRE). A part of this study was also funded through a grant to EK from the ADEME and the Regional Council of Lorraine.

DECLARATIONS Funding

The acquisition and the organization of historical floristic data were funded by the French Agency for Environment and Energy Management (ADEME), the National Forest Office (ONF) and the French National Research Agency (ANR) as part of the 'Investissement d'Avenir' program (ANR-11-LABX-0002-01, Lab of Excellence ARBRE). A part of this study was also funded through a grant to EK from the ADEME and the Regional Council of Lorraine.

Conflicts of interest/Competing interests

data, automatic classification program and code used in this study are available from the corresponding author on reasonable request.

SUPPLEMENTARY MATERIAL


domain: phys spi

text:
Thomas Lacour

Romain Poupart

Olivier Mondain-Monval

Christophe Aristégui

Olivier Poncelet

Thomas Brunet

Pressure effects on the resonant attenuation of soft porous beads-based materials for underwater acoustics

Dedicated coating materials for anechoism and furtivity in underwater acoustics must exhibit a strong reliability regarding their mechanical resistance to hydrostatic pressure. Soft porous materials, and especially, a distribution of soft porous beads within a polyurethane matrix have been previously proposed as an acoustic insulator device. The purpose of the present letter is to investigate the attenuation efficiency of soft porous silicone beads-based materials while being exposed to uniaxial loads mimicking hydrostatic pressures encountered in underwater acoustics. The acoustic performance of this locally resonant material is then compared to the classical coating technology using micro-balloons. The use of an adapted surfactant (a silicone alkyl polyether compound) in the fabrication process of the soft porous silicone-based beads, obtained through an emulsion templating process, leads to particles exhibiting an open porosity. The (resonant) attenuation of the soft porous beads-based material remains greater than the micro-balloons-based material until several bars. Above this critical resilience value, the mechanical stress irreversibly damages the soft porous beads.

I. INTRODUCTION

The design of innovative materials is of growing interest especially as a promising solution for underwater acoustic coatings [1,2]. These thin materials are used in two situations: the first case deals with the stealth of submersible structures while the second case aims at anechoism [3]. To achieve both, the sound emitted (or reflected) by an immersed structure towards its environment has to be reduced significantly. For that purpose, the most common approach for underwater acoustic coatings is to fill a polyurethane (PU) matrix, that shows an ideal impedance matching with water, with microballoons that strongly enhance the scattering and the attenuation of the acoustic waves [4]. Those micro-balloons are micro-bubbles of gas with a typical size of about 40 µm encapsulated by polymeric shells. However, in the context of underwater acoustics, a serious concern is the collapse of these spherical inclusions and the buckling instability of their polymeric shells when the composite material is subjected to high hydrostatic pressures [5]. This buckling can affect seriously the acoustic properties of the coatings making them ineffective in terms of attenuation [START_REF] Beretti | 10ème Congrès Français d'Acoustique[END_REF]. Other soft composite coatings involving simple air cavities have also shown a decrease in acoustic performances under hydrostatic pressures [START_REF] Thieury | 13th International Congress on Artificial Materials for Novel Wave Phenomena[END_REF][START_REF] Thieury | [END_REF][9].

To circumvent this issue, an innovative solution is to cluster these air cavities within submillimetric solid beads made of soft porous silicone rubber with a very low speed of sound [10][11][12], as such composite materials exhibit a strong resonant attenuation. Those soft porous beads have a large number of applications [13] and can be produced using an emulsion templating approach * thomas.brunet@u-bordeaux.fr [14]. In fact, it has been recently demonstrated that such emulsion-based materials could possess the ability to serve as acoustic insulators while dispersed in a PU matrix [15,16]. Note that the porosity of these beads can be closed or open. As a matter of fact, the emulsion leading to the soft porous silicone rubber is composed of isolated droplets. During the drying step (i.e. the revelation of the porosity), the pores tend to collapse due to the drying pressure [17,18]. Different ways have been proposed to circumvent that: either using supercritical carbon dioxide as a drying step [19], adding hydrogen peroxide in the emulsion formulation [19] or using another surfactant that leads to an open porosity [17]. Another benefit of soft porous beads is their ability to tune the frequencydependent resonant attenuation since the increase in the size of the beads decreases the resonance frequency at which the attenuation reaches its maximum value [16]. Therefore, it is possible to lower the frequency at which the attenuation peak occurs down to the targeted frequency domain for underwater acoustics (∼ kHz).

In this paper, we investigate the effect of hydrostatic pressure on the acoustic (resonant) attenuation of the two technologies: on the one hand, the micro-balloons commonly used in the naval industry and, on the other hand, our innovative soft porous beads. The pressure levels chosen, up to 10 and 20 bars, are aimed at mimicking an object diving to a depth of 100 and 200 meters.

II. ELABORATION OF THE COMPOSITE MATERIALS

A. Compounds

Silcolease UV Cata221, which is a 20 wt% alcoholic solution of (4-(1methylethyl)phenyl)-(4-methylphenyl)iodonium This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0144249

B. Soft porous beads generation

Our samples are composite materials made of the same polyurethane matrix embedding inclusions, either soft porous beads or commercially available micro-balloons. The protocol to generate soft porous beads [17] has been described previously in the literature but here, slightly adapted. Briefly, a water-in-oil emulsion is obtained by the emulsification of 5.4 g of salted water (1.5 wt% of NaCl) as the dispersed phase in a continuous oil phase containing 12 g of Silcolease UV Poly200, 0.5 g of Silcolease UV Cata221, 10 mg of ITX and 50 mg of Silube J208-812. In this way, large double emulsion droplets are formed into a water phase. During their transit through a microfluidic device, the droplets of double emulsion are crosslinked via an UV lamp (Thorlabs) and then collected and washed with water and ethanol. They are then dried at 75 o C. The beads are characterized by scanning electron microscopy (SEM, Hitachi TM-1000) and optical microscopy (Nikon SMZ1270i) to extract the mean size through an optical analysis as shown in Fig. 1. The final porosity of the produced soft porous beads is about 30%.

The measured beads size distribution indicated a mean size of 598 ± 82 µm. The white color of the beads is due to the presence of pores at the micrometer scale thus scattering light. SEM pictures also evidence the interconnected porosity, as seen in Fig. 1-C.

C. Sample fabrication

Following the beads preparation, our two sets of composite samples are made using either the DET 40 microballoons or the soft porous beads by adding 2 and 6 vol%, respectively, of the inclusions in the polyurethane (Sik-aBiresin) to obtain the same final air quantity in the whole composite, and thus the same mass density for all the samples. The PU is a bi-component system composed of an isocyanate (part A) and a polyol (part B). Both parts are degassed during 30 min at 7 mbar prior to use. Inclusions are dispersed in 11 g of part B and this mixture is added into 22 g of part A. The resulting mixture is kept under stirring for 8 minutes prior to being poured in tailor-made PTFE molds with a diameter of 30 mm and a height of 4 and 6 millimeters. After being fully filled with the mixture and sealed, the molds are kept at 75 o C for 2 h. The top and the bottom of the mold are subsequently removed, leaving the sample inside the PTFE cylinder mold for acoustic analysis. Samples made of pure PU without any inclusions are also fabricated and will serve as reference materials.

III. ATTENUATION PERFORMANCE UNDER UNIAXIAL LOAD

Before mechanical tests, the intrinsic attenuation is measured for each unloaded sample as shown in Fig. 2. As previously reported [16], the attenuation of the pure PU matrix is very low at the ultrasonic frequency range considered here [0.1 -1 MHz]. On the other hand, the acoustic attenuation is much higher in the sample containing soft porous beads compared to the one with micro-balloons, especially in a broad frequency range around 200 kHz for which the soft porous beads resonate [16]. As the micro-balloons-based material is the classical solution for underwater acoustics [20], the use of soft porous beads could be of crucial interest because of the large resonant attenuation they induce. In the following, acoustical tests are performed on all the samples to study the impact of an increasing static load on the acoustic attenuation of our materials at ultrasonic frequencies. Note that the (static) mechanical properties of This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0144249

these soft composite materials have already been exposed [21].

A. High frequency measurements

First, acoustic measurements are reported at high ultrasonic frequencies. To perform the acoustic characterization under uniaxial compressional loads, cylindrical shaped samples were synthesized following the manufacturing process described in section II. The sample is confined within a hollow PTFE cylinder placed between two broadband ultrasonic transducers (emitter and receiver, Olympus V301) with a diameter of 30 mm and a central frequency of 500 kHz. This stacking was then placed into a traction/compression testing machine to ensure a purely uniaxial load. The mechanical compression is sustained thanks to a Zwick/Roell AllroundLine Z010 machine and its dedicated software TestExpert. During the mechanical tests, the samples may undergo several cycles of compression/relaxation, where the force applied on the samples was set up to 700 N corresponding to a static pressure of 10 bars. A nonzero initial force of few tens of newtons was first applied on the sample to ensure a reproducible contact area between each element of the stack over the different measurements done. For each applied force varying from 70 to 700 N, the transmitted acoustic signals were recorded with a constant step in force of 70 N (corresponding to an applied pressure step of 1 bar) for the non-absorbing pure PU sample and then for the material to be characterized (with microballoons or with soft porous beads). By comparing the transmitted spectra obtained for samples with and without particles, the (longitudinal-wave) attenuation can be easily retrieved for the composite materials, knowing the sample thicknesses, as shown in Fig. 3. From these experimental results, one can draw meaningful conclusions. First, the attenuation of the sample containing the soft porous beads clearly decreases when the applied pressure increases as shown in Fig. 3-A. Moreover, the attenuation peak tends to shift slightly to higher frequencies. At around 8 bars, the attenuation is very low and almost no longer measurable as illustrated by the oscillations in the spectra that are due to multiple echoes reflecting on the two transducers, and thus interfering in the Fourier domain. This progressive decrease in the attenuation can be expected as the more compression goes, the higher it tends to close the porosity, thus leading to a dramatic reduction in the acoustic index contrast between beads and the surrounding matrix. The pressure release after this first ramp of applied loading on the sample does not allow the complete recovery of the initial attenuation level, as the attenuation is only about of 0.3 Np/mm at its maximum (Fig. 3-B) instead of the initial value of about 1 Np/mm. When the sample is submitted to additional pressure cycles, the attenuation continues to deteriorate because of the sample damage (Fig. 3-C) and cannot retrieve its initial level (before compression) even if the sample is left to rest for several days (Fig. 3-D).

It is worth noting that while the compressive strength of a polymeric foam decreased with increased interconnected pore ratio, the mechanical fatigue of the porosity is improved when opening the porosity [22]. It could be hypothesized that highly interconnected pores are equivalent to a unique large pore structure. The evidence of a critical pore size of porous PDMS below which the porosity tends to vanish has already been observed in the literature [18]. Indeed, optical microscopy observations performed before and after a cycle of compression/relaxation shows a disappearance of the whitish domains in the beads (see the insets of Fig. 3-A &B). Those domains are representative of the porosity, as the micrometer-sized pores promote the scattering of light at this scale. If the pores collapse, the beads lose their porosity and their sound speed is comparable to those in pure PDMS rubber (∼1000 m/s instead of the 100 m/s for porous silicone rubber [10]). The sound speed (or acoustic index) contrast between the inclusions and the surrounding matrix becomes thus much lower since the sound speed in pure PU is about 1500 m/s. This diminution of the acoustic index leads to the vanishing of the resonant scattering attenuation of the material containing soft porous beads.

On the opposite, acoustic measurements performed on the sample containing micro-balloons show that this material seems to be more resistant to the applied force as shown in Fig 4-A &-B. In fact, the higher the pressure, the higher the acoustic attenuation. This behavior can be explained through the alteration of micro-balloon shells. Indeed, these objects are composed of a very thin polymeric shell that tends to buckle at high pressure making them "softer". If the micro-balloon shells become less rigid, the scattering strength of these air-filled particles This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0144249 increases leading to higher attenuation levels as previously discussed in similar materials. As predicted theoretically, the buckling of these shells may lead to a decrease of the effective elastic moduli of the composite [23].

The micro-balloons seem thus to be more appropriate to maintain a significant attenuation at high pressure but the material made of soft porous beads exhibit higher attenuation at low pressure that is due to particle resonances.

For naval underwater applications, working frequencies are about few tens of kHz and not hundreds of kHz as considered in the previous section. It is therefore worth considering whether the acoustic performance of the material made of soft porous beads also deteriorate under high applied pressures at lower frequencies.

B. Low frequency measurements

For these low-frequency measurements, other samples were elaborated with the same diameter but greater in height (30 mm and not few millimeters). The samples were confined in hollow PVC cylinders and placed within the Zwick/Roell machine between the two same broadband transducers used for the high-frequency measurements presented in the previous section. Here, the emitter was excited with a 5-cycle tone-burst centered at 50 kHz, which is the lowest frequency at which our ultrasonic transducers can generate and detect a measurable acoustic signal. A first series cycles of compression/relaxation, with a maximum applied pressure of 10 bars, was applied like in previous experiments.

In order to compare both technologies based on the soft porous beads and on the micro-balloons, the attenuation This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0144249 between the two materials is shown in Fig. 5-A for few successive cycles of compression/relaxation. Contrary to what has been shown for higher frequencies in the previous subsection, the sample containing the soft porous beads remains more absorbing at low frequency than the material made of micro-balloons, over the whole mechanical test since the attenuation difference ∆α remains positive, even at 10 bars. Although the acoustic resonances tend to disappear under high applied pressures, some air cavities must remain in the soft porous beads (as seen in the inset of Fig. 3-B) maintaining a significant attenuation level.

Finally, a last series of cycles of compression/relaxation was conducted with higher pressure levels (Fig. 5-B). In this second series of ultrasonic experiments, the maximum force applied on the samples was 1400 N, corresponding to a hydrostatic pressure of 20 bars (and to an equivalent underwater depth of 200 m). Acoustic measurements shown in Fig. 5-B reveal that the soft porous beads do not resist to these high pressures anymore since the attenuation difference becomes negative beyond 12 bars. Moreover, the damage inflicted to the material increases, leading to an irreversible fatigue as shown in the next cycles.

IV. CONCLUSIONS

To conclude, the composite materials made of soft porous beads randomly but uniformly dispersed in a polyurethane matrix exhibit interesting acoustic properties. At ambient pressure, its (resonant) attenuation is much higher than the one reached with the common technology used in naval industry based on micro-balloons. However, the resonance of soft porous beads is strongly impacted and lessened under static pressures because of the collapse of the beads porosity. Moreover, the initial resonant properties are not recovered after one or This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0144249

several cycles of compression/relaxation indicating these mechanical tests irreversibly damage the structure of the soft porous beads. On the other hand, the composite materials made of micro-balloons demonstrate a much better pressure resistance that is probably due to the deformation of their shells. An alternative solution could be the reinforcement of the soft porous beads with a shell, mimicking the micro-balloons technology while keeping the advantages of the air-cavities clusterization in resonant particles. The mechanical reinforcement of the (soft) matrix could be also an efficient way to limit the collapse of such porous and compressible material.

Figure 1 .

 1 Figure 1. A. Optical microscopy of the soft porous beads. B. and C. SEM pictures of the soft porous beads showing their inner porosity (the white ruler correspond to 500 µm). D. Beads size distribution.

Figure 2 .

 2 Figure 2. Compared evolutions of the (longitudinal-wave) attenuation as a function of frequency for the pure PU matrix (green), for sample containing micro-balloons (orange) and soft porous beads (blue).

Figure 3 .

 3 Figure 3. Evolution of the (longitudinal-wave) attenuation for the sample containing the soft porous beads, as a function of frequency for different static loads. The applied pressure increases from 1 to 10 bars by steps of 1 bar (A), and decreases down to 1 bar with the same steps (B). The sample is then subjected to several cycles of compression/relaxation (C) and is finally left to rest for several days (D).

Figure 4 .

 4 Figure 4. Evolution of the (longitudinal-wave) attenuation for the sample containing the micro-balloons, as a function of frequency for different static loads. The applied pressure increases from 1 to 10 bars by steps of 1 bar (A), and decreases down to 1 bar with the same steps (B).

Figure 5 .

 5 Figure 5. Evolution of the attenuation difference ∆α = α beads -α µ-balloons measured at 50 kHz over 4 cycles of compression/relaxation for applied pressure ranging from 2 bars up to 10 bars (A) or to 20 bars (B). The areas represent the repeatability of the experiments and the solid line corresponds to the mean value.

ACKNOWLEDGMENTS

TL, RP and all the coworkers are indebted to the ANR and DGA for granting them a postdoctoral position through the PANAMA Project (ANR-17-ASTR-0002).

Fruitful conversations with J. Leng, from LOF; F. Le Lay, P. Rublon, J. Delcroix and Y. Renou from Naval Group are gratefully acknowledged. The authors thank Siltech Company for providing them with the Silube surfactant and also F. Marchal and T. Ireland from Elkem Silicones for providing them with silicone rubber and cationic initiator. They also thank J. Guitard for his assistance with the Zwick/Roell machine, A. Turani-i-Belloto for providing the MATLAB program used for the size distribution and V. Mouille & J.-E. Maigne for some additional acoustical experiments.

DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon reasonable request.

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0144249