Elisabeth Beyersmann
email: lisi.beyersmann@gmail.com

Jonathan Grainger

Séverine Casalis

Johannes C Ziegler

Effects of reading proficiency on embedded stem priming in primary school children

Keywords: morphological processing, embedded stem priming, reading development, reading proficiency, masked priming

Prior evidence from masked morphological priming has revealed conflicting findings regarding the acquisition of morpho-orthographic segmentation mechanisms in developing readers. Here, we examined changes in masked morphological priming across grade within a large sample of French primary school children (n = 191, grades 2-5) and how these effects are modulated by individual differences in reading proficiency, spelling proficiency, and morphological awareness. Target words were preceded by either (i) a suffixed word prime (e.g., tristesse-TRISTE), (ii) a suffixed nonword prime (e.g., tristerie-TRISTE), (iii) a nonsuffixed nonword prime (e.g., tristald-TRISTE), or (iv) an unrelated prime (e.g., direction-TRISTE), using very short prime durations (50 ms). Moreover, a frequency manipulation was included for suffixes and non-suffixes. The results revealed robust suffixed word priming across all children, independently of grade and proficiency. On the other hand, priming in the suffixed and non-suffixed nonword conditions was modulated by reading proficiency, with high proficiency children showing facilitation and low proficiency children showing inhibition. The effects of suffix and non-suffix frequency were modulated by grade, with the effects decreasing as grade increased. None of the observed priming effects were modulated by grade, spelling proficiency or morphological awareness. The results suggest that reading proficiency is an important predictor for embedded stem activation mechanisms in primary school children, which we discuss in the context of recent theories of morphological processing. (words = 220, max = 250)

Over the past decades, much research has been dedicated to understanding how morphemes are processed in the skilled reading system. Most of this research has used the masked primed lexical decision task in combination with behavioral and sometimes electrophysiological measures. This research has generated important insights into the mechanisms and the time-course of morphological processing in adults (e.g., [START_REF] Beyersmann | Semantic processingduringmorphologicalpriming: An ERPstudy[END_REF][START_REF] Diependaele | Semantic transparency and masked morphological priming: The case of prefixed words[END_REF][START_REF] Morris | Semantic transparency and masked morphological priming: an ERP investigation[END_REF]; for a review, see [START_REF] Rastle | Morphological decomposition based on the analysis of orthography[END_REF]. However, only recently scientists have begun to explore the development of such processes in children (e.g., [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Casalis | Morphological effects in children word reading: A priming study in fourth graders[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF]. We first outline the key milestones from morphological processing research in adults, followed by a summary of the related evidence from reading development.

Morphemes are the smallest meaningful entities that words are composed of. Based on a growing body of psycholinguistic evidence in adults, it is now known that the activation of morphemes is a rapid, highly automatized process which does not necessarily require the involvement of semantics. This now broadly accepted view primarily originates from the results reported in numerous masked morphological priming studies, across different languages, showing that not only words with a true morphological structure (e.g., farm-er), but also words with a pseudo-morphological structure (e.g., corn-er) produce priming of the stem target (e.g., farm, corn), even if presented with very short (40-50 ms) prime durations (for a review, see [START_REF] Rastle | Morphological decomposition based on the analysis of orthography[END_REF]. Since the meaning of pseudo-affixed words like corner cannot be derived from the meaning of its constituents corn and er, it has been argued that the activation of the embedded stem corn must be due to a semantically blind "morphoorthographic" decomposition process.

In addition to the evidence for morpho-orthographic processing, masked morphological priming studies in adults have further revealed that truly suffixed words tend to produce slightly stronger stem-target priming (e.g., farmer-FARM) than pseudo-suffixed words (e.g., corner-CORN), particularly when the prime is partially or fully visible (e.g., [START_REF] Diependaele | Semantic transparency and masked morphological priming: The case of prefixed words[END_REF][START_REF] Feldman | Early morphological processing is morpho-semantic and not simply morpho-orthographic: A violation of form-then-meaning accounts of word recognition[END_REF]. These findings suggest that the early morpho-orthographic processing stages are followed by a later "morpho-semantic" processing stage, by which truly affixed words (but not pseudo-affixed words) are segmented into semantically defined morphemic units.

Critically, the data discussed above have recently been complemented by masked priming studies using morphologically structured nonword primes. [START_REF] Longtin | Morphological decomposition in early visual word processing[END_REF] were the first to report priming with suffixed (e.g., rapidifier-RAPIDE) but not with non-suffixed nonwords (e.g., rapiduit-RAPIDE). However, recent morphological priming studies have found results that partially deviate from the Longtin and Meunier pattern [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF][START_REF] Morris | Effects of lexical status and morphological complexity in masked priming: An ERP study[END_REF]. Beyersmann et al. (2014) examined lexical decision times to target words preceded by four different types of primes: a suffixed word prime (e.g., tristesse-TRISTE), a suffixed nonword prime (e.g., tristerie-TRISTE), a non-suffixed nonword prime (e.g., tristald-TRISTE) or an unrelated prime (e.g., direction-TRISTE). The results revealed robust and equal priming in the two suffixed conditions across all participants, suggesting that not only suffixed real words, but also suffixed nonwords are rapidly decomposed into morpho-orthographic subunits at early stages during visual word recognition. This thus demonstrates that morphoorthographic decomposition mechanisms apply even in a nonword context in which the semantic stem-affix relationship is entirely non-interpretable.

Most importantly however, Beyersmann et al.'s findings revealed that priming in the non-suffixed nonword condition was modulated by individual language proficiency. While high proficiency adults showed as much priming in the non-suffixed condition as in the other two conditions, low proficiency adults did not show any evidence for non-suffixed priming.

The priming effect observed in the non-suffixed nonword condition is of particular relevance, as it suggests that high proficiency individuals rapidly activate embedded stems at early stages during visual word recognition, even in the absence of an affix. Hence, these nonword priming data suggest that affix-stripping in adult readers is complemented, and possibly even substituted by embedded stem activation mechanisms.

Prior evidence from lexical decision indicates that children already use morphological structure from as early as seven years old [START_REF] Lázaro | Morphological processing in reading disabled and skilled Spanish children[END_REF][START_REF] Perdijk | Effects of morphological Family Size for young readers[END_REF]. However, recent findings from masked morphological priming suggest that morpho-orthographic decomposition is quite a lateoccurring milestone in reading acquisition [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF]. One hypothesis that has been put forward is that children first learn to link the meanings of related whole words (e.g. a painter is someone who paints, a teacher is someone who teaches, etc.), which eventually results in a form-based morphological segmentation mechanism which is applied to every letter string with the mere appearance of morphological complexity (e.g., moth-er, numb-er, flow-er, etc.). Critically, previous studies using the masked priming paradigm to examine the role of morphological knowledge during reading development has revealed conflicting findings. [START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF] tested French third, fifth and seventh graders and found evidence for true morphological priming (e.g., farmer-FARM) and pseudomorphological priming (e.g., corner-CORN) across all age groups. The findings thus replicate the pattern standardly observed in adults, showing that French children rapidly decompose complex words into semantically blind morpho-orthographic subunits as early as grade 3. However, [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF], observed a different pattern in two groups of English speaking grade 3 and grade 5 children. Masked primes were presented for 50 ms and priming effects were found only for prime-target pairs sharing a true morphological relationship (farmer-FARM), but not in the pseudo-morphological condition (corner-CORN). Beyersmann et al.'s results thus demonstrate that English-speaking grade 3 and 5 children only produce priming when prime and target are semantically related, suggesting that they have not yet acquired abstract morphemic representation which are thought to be essential for the automatic segmentation into morpho-orthographic units.

One possible reason for these divergent findings is the fact, first pointed out by [START_REF] Baayen | An amorphous model for morphological processing in visual comprehension based on naive discriminative learning[END_REF], that prime words previously tested in the opaque/pseudo-morphological priming condition vary in the extent to which the suffix conveys its regular meaning. For example, as Baayen and colleagues (2011) pointed out, the etymological origin of archer ("someone who wields a bow") is Latin arcus (bow, arc), which is similar in structure to a denominal formation such as trucker ("someone who drives a truck"). On the other hand, a word like corner is clearly pseudo-suffixed, because the meaning of corner cannot be derived from the meaning of corn and er. In the present study, we avoided this problem by using complex nonword primes to test for morpho-orthographic segmentation in children, as in the prior research with adults summarized above [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF]. It is also possible that morphological parsing mechanisms in French and English are acquired at different speeds. French is morphologically more productive than English, and French children have been found to outperform English children in morphological knowledge (e.g., [START_REF] Duncan | Early metalinguistic awareness of derivational morphology: Observations from a comparison of English and French[END_REF]. It is therefore possible that French children begin to automatize morphological processing at an earlier age than English children, which would explain the absence of morpho-orthographic priming in Englishspeaking readers. A third fundamental confound within the assessment of morphological processing mechanisms in developing readers may be that children within the same age/grade group vary largely in their individual proficiencies in reading, spelling and morphology. For instance, some children in the last years of primary school have reading proficiency levels that are comparable to those of children in the lower grades and vice versa. One solution is to exclude children with reading problems who fall outside the expected proficiency range per grade level. However, this approach does not only result in the loss of valuable statistical power, but also narrows the analyses down to grade-level comparisons, rather than using more fine-tuned continuous proficiency measures irrespective of grade. Therefore, in order to more clearly capture the relationship between the acquisition of morpho-orthographic parsing mechanisms and the development of reading skills, it is essential to explore individual differences in reading proficiency continuously rather than analyze children per grade.

Similarly, individual differences in spelling proficiency and morphological awareness may present confounds in previous developmental priming studies. However, masked priming evidence in children is sparse and based on rather small participant samples from selected age groups (e.g., [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF]. Thus, the role of individual proficiency differences during the acquisition of morphological processing mechanisms is still not clear.

The goal of our present study was to examine the early processing of embedded stems and affixes within the developing reading system. Here, we tested a large sample of French primary school children (n = 191, from grades 2 to 5) with varying levels of reading proficiency, spelling proficiency, and morphological awareness, in order to explore whether or not these individual differences modulate morphological priming. We replicated the design previously used in adults [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF], comparing target words preceded by (i) a suffixed word prime (e.g., tristesse-TRISTE), (ii) a suffixed nonword prime (e.g., tristerie-TRISTE), (iii) a non-suffixed nonword prime (e.g., tristald-TRISTE) or (iv) an unrelated prime (e.g., direction-TRISTE), using very short prime durations (50 ms). Critically, in contrast to previous developmental work [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF], we used the exact same targets in all four prime conditions, to rule out that differences across conditions might be due to uncontrolled differences between target words.

If affixes form the primary recognition units in the developing reading system, we would expect significant priming in the suffixed word and suffixed nonword conditions, but not in the non-suffixed condition. If, however, children also draw upon an embedded stem activation mechanism during the early stages of visual word recognition, we would expect significant priming in all three priming conditions (including the non-suffixed condition). If morpho-orthographic segmentation mechanisms are acquired over the course of primary school, we would expect to see an increase of suffixed word and suffixed nonword priming across grades. If embedded stem activation mechanisms are acquired over the course of primary school, we would expect to see an increase in all three priming conditions across grades. Moreover, if morphological priming is modulated by children's performance in reading, spelling, and morphological awareness tasks, we would expect that the magnitudes of suffixed nonword and non-suffixed nonword priming should vary depending on individual differences in these scores. In addition, given that previous studies in developing readers have unambiguously reported evidence for true morphological priming effects (e.g., [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF], we expected that priming in the suffixed word condition should be robust across all children, independently of proficiency.

Finally, we adapted a further feature of the Beyersmann et al. materials, where suffixes and non-suffixes were divided into a high and a low frequency condition. If it is true that affixes form morpho-orthographic units in the developing reading system (e.g., [START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF], we would expect more priming for primes with high-frequency suffixes than low-frequency suffixes, but no difference between high-and low-frequency non-suffixes. If however, suffixes and non-suffixes have a comparable status during the first years of reading development, we would predict that high ending frequency should boost the activation of embedded target words, independently of the morphemic status of the ending-string.

Method

Participants

One hundred ninety-one children in grades 2-5 were recruited from a primary school in Lille and Arras, northern France (46 second graders, 48 third graders, 49 fourth graders, and 48 fifth graders). The mean participant age per grade was 7.7 years in Grade 2, 8.7 in Grade 3, 9.6 in Grade 4, and 10.6 in Grade 5. All were French native speakers with normal or corrected-to-normal vision. Parental consent was obtained prior to the testing. Children with a history of dyslexia and/or language impairment were excluded. No child was affected by hearing loss.

Materials

The materials were identical to the ones used in Beyersmann, et al. (2014). We first confirmed that the 50 word targets and suffixed word primes, originally selected from the adult corpus Lexique [START_REF] New | Lexique 2: A new French lexical database[END_REF], were also present in a lexical database for grade 1 children (MANULEX; [START_REF] Lété | MANULEX: A grade-level lexical database from French elementary school readers[END_REF]. Suffixed word primes were real words comprising stem (triste) and suffix (esse). Suffixed nonword primes were created using the same stem, but a different suffix of comparable frequency (rie), such that the whole letter string was not a word. Non-suffixed nonword primes were created by combining the stem with a non-morphemic ending (ald). Unrelated primes were suffixed words and all letter different from the target. All nonwords were orthographically legal and pronounceable. The four prime conditions were matched on length (for a full list of materials, see Beyersmann et al., 2014, Appendix 1).

Trials were divided into two subsets: one where primes comprised high-frequency suffixes (-ette, -ier, -eur, -ion, and -age) and one where primes comprised low-frequency suffixes (-esse, -oir, -ade, -ien, and -rie). Non-suffixed nonwords were created using five high-frequency (-nie, -ire, -ide, -ert, and -use) and five low-frequency non-morphemic endings (-uor, -ald, -abe, -uto, and -bli). String frequency was calculated based on the number of words in the Manulex database [START_REF] Lété | MANULEX: A grade-level lexical database from French elementary school readers[END_REF] which contained the given letter sequence in final position. Low frequency non-suffixes had a maximum frequency of 10 (mean: 4.2), and high frequency non-suffixes a minimum frequency of 40 (mean: 151.8). Low frequency suffixes had a maximum frequency of 170 (mean: 100.6) and high frequency suffixes a minimum frequency of 240 (mean: 402.0). Suffixed words and nonwords used the same suffixes, but with different stems. High and low frequency conditions were matched on length, frequency, suffix length and nonmorphemic ending length. For the purpose of the lexical decision task, 50 nonword targets (identical to the Set 2 nonwords in Beyersmann et al., 2014) were included and matched to the word targets on length.

Procedure

The procedure was identical to the one used in adults (Beyersmann et al., 2014).

Stimuli were presented in the centre of a computer screen, using DMDX [START_REF] Forster | DMDX: A Windows display program with millisecond accuracy[END_REF]. Each trial consisted of a 500-ms forward mask of hash keys, then a 50-ms prime in lowercase, then the uppercase target. The target remained present until the response or until 3 seconds had elapsed. Children were instructed to respond as quickly and accurately as possible. The experiment took between 3-6 minutes per participant.

Measures of individual differences

Upon completion of the masked primed lexical decision task, each child was assessed with a spelling proficiency test, a reading proficiency test, and a morphological awareness test. The morphological awareness task was used to measure children's awareness of morphological structures during spoken language processing. The reading and spelling tasks were used as two measures for children's proficiency in processing printed words.

Spelling proficiency

The 'Corbeau' subtest was used to assess spelling proficiency in Grade 3-5 children (Chevrie-Muller, Maillart, Simon, & Fournier, 2010). For each child, three different scores were obtained: (i) number of responses with correct phonetic form (e.g. oizo instead of oiseau), (ii) number of responses which are related existing lexical entries (e.g. oiseaux instead of oiseau; rendu instead of rendus), and (iii) number of correct responses. Spelling proficiency in Grade 2 children was examined based on a different dictation test ("dictée de phrases"), adapted for children with little spelling experience [START_REF] Chevrie-Muller | Langage oral, langage écrit, mémoire[END_REF].

Reading proficiency

The 'Alouette' was used to asses children's reading proficiency [START_REF] Lefavrais | Test de l'Alouette[END_REF][START_REF] Lefavrais | L' Alouette-R[END_REF]. Children were asked to read aloud a text of 265 words for which both reading accuracy (l'indice de precision) and speed (l'indice de vitesse; how many words are read within 3 minutes) were measured and evaluated based on the revised Alouette 2005 scoring system.

Morphological awareness

Morphological awareness was assessed with a test developed by [START_REF] Casalis | Morphological awareness and dyslexia[END_REF], consisting of three different spoken language tasks: two sentence completion tasks and an oddity task. In the sentence completion tasks, children were asked to produce a real affixed word (e.g. "Someone who works on a farm is a …") or a pseudo-affixed word (e.g. "Someone who works in a birk is a …"). In the oddity task, children were presented with 4 orthographically related words, amongst which only 3 shared the same morphemic stem (e.g. turn, turning, turner, turnip). The score was the number of correct responses in each task.

Results

Lexical decisions to word targets were analyzed as follows. Incorrect responses were removed from the RT analysis (11.4% of all data). Outliers below 200 ms were removed, which led to the removal of 0.03% of the data. 1/RTs were calculated for each participant to correct for RT distribution skews and used throughout the analyses. Untransformed RTs and error rates are presented in Table 1.

-Table 1-We used linear mixed-effect model modelling to perform the main analyses [START_REF] Baayen | Analyzing linguistic data: A practical introduction to statistics using R[END_REF][START_REF] Baayen | Mixed-effects modeling with crossed random effects for subjects and items[END_REF]. Fixed and random effects were only included if they significantly improved the model's fit in a backward stepwise model selection procedure. Models were selected using chi-squared log-likelihood ratio tests with regular maximum likelihood parameter estimation. Trial order was included to control for longitudinal task effects such as fatigue or habituation. To assess whether the obtained effects were modulated by individual differences of reading proficiency, spelling proficiency, or morphological awareness, the scores from the spelling dictation, reading aloud and morphological awareness tests were standardized for each grade. Composite measures were calculated by adding the standard scores of the subtests. Factor grade was included to explore the development of morphological segmentation across primary school grades. Ending-frequency was examined as a continuous variable. Finally, to rule out that any of the observed effects were due to differences in stem frequency (i.e. target frequency), we included this factor as a covariate in the analyses.

A linear mixed-effects model as implemented in the lme4 package [START_REF] Bates | lme4: Linear mixed-effects models using Eigen and S4. R package version 1[END_REF] in the statistical software R (Version 3.0.3; RDevelopmentCoreTeam, 2008) was fitted with eight fixed effect factors (primetype: suffixed-word, suffixed-nonword, nonsuffixed-nonword, unrelated word; ending-frequency; reading proficiency; spelling proficiency; morphological awareness; grade; stem-frequency; trial order), their interactions, and two random effects factors (random intercepts for subjects and items). P-values were determined using the package lmerTest [START_REF] Kuznetsova | lmerTest: Tests for random and fixed effects for linear mixed effect models[END_REF]. RT analyses revealed a significant priming effect in the suffixed word condition (t=4.17, p<.001), in the suffixed nonword condition (t=2.28, p=.022), and a marginally significant priming effect in the non-suffixed nonword condition (t=1.85, p=.065).

Priming in the suffixed word condition was significantly larger than priming in the nonsuffixed nonword condition (t=2.33, p=.020), and marginally significantly larger than in the suffixed nonword condition (t=1.88, p=.060). Although there was a significant main effect of grade (t=8.47, p<.001), grade did not modulate priming (see Figure 1, left panel).

-Figure 1-Ending-frequency, stem-frequency, spelling proficiency and morphological awareness did not significantly improve the model's fit and were therefore excluded from the analyses.

Interestingly however, the results revealed that while priming in the suffixed word condition (relative to the unrelated control condition) was not modulated by reading proficiency (t=1.63, p=.102), reading proficiency did modulate priming in the suffixed nonword condition and nonsuffixed nonword condition (t=3.56, p<.001; t=2.62, p=.009), showing that there was an inhibitory effect for children with low levels of reading proficiency, but a facilitatory effect for children with high levels of reading proficiency (see Figure 1, right panel).

Error analyses followed the same logic as the RT analyses. We applied a binomial variance assumption to the trial-level binary data using the function glmer as part of the Rpackage lme4. There was a significant effect of grade (z=.5.89, p<.001), showing that errors decreased with increasing grade level. There was a significant effect of reading proficiency (z=5.67, p<.001), showing that errors decreased with increasing proficiency. No other effects were significant.

Discussion

In the present study, we tested a large sample of French children in grades 2-5 of primary education in order to a) investigate the development of morpho-orthographic and morpho-semantic processing mechanisms across grades and b) examine changes in morphoorthographic and morpho-semantic processing as a function of individual differences in reading, spelling and morphological proficiency in beginning readers. Using the masked priming paradigm combined with a lexical decision task, priming in a truly suffixed condition (e.g., tristesse-TRISTE) was compared to a suffixed nonword (e.g., tristerie-TRISTE) and a non-suffixed nonword condition (e.g., tristald-TRISTE), using the same targets across conditions. Relative to an unrelated prime condition, robust suffixed word priming was observed across all children, independently of grade and proficiency. This suffixed priming effect was significantly larger than the priming effects observed in the suffixed nonword and non-suffixed nonword conditions, suggesting that the semantic interpretability of morphologically complex letter strings helps the segmentation into stem and affix. These results are consistent with previous developmental work from French and English-speaking primary school children (e.g., [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF].

Interestingly, priming in the suffixed and non-suffixed nonword conditions did not change as a function of grade. Moreover, there was no difference between the magnitude of suffixed and non-suffixed nonword priming (see comparable trajectories of both conditions in Figure 1, left panel). Our data thus do not provide evidence for morpho-orthographic segmentation, but rather speak in favor of children using embedded stem activation mechanisms [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF] during early morphological processing (we return to this point in more detail below). Most importantly, however, our present data suggest that priming did not change as a function of grade, as thus provide no evidence for developmental changes in morpho-orthographic and morpho-semantic processing across primary school, at least not when primes are presented subliminally (c.f., [START_REF] Ziegler | Orthographic and phonological contributions to reading development: Tracking developmental trajectories using masked priming[END_REF].

Our second key finding is that priming in the suffixed and non-suffixed nonword conditions was modulated by individual differences in reading proficiency (see Figure 1, right panel). While facilitatory priming effects were observed for high proficiency children, low proficiency children showed an inhibitory priming effect. These results suggest that high proficiency children benefit from the activation of embedded words, independently of whether they occur in combination with a real suffix (rie) or a non-suffix (ald). Children with higher levels of reading proficiency would be more expert in mapping sublexical orthography onto whole-word orthographic representations, and this would facilitate the activation of the embedded target word during prime processing (see also [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF] for related evidence from adults). That is, letters in the prime stimulus activate whole-word orthographic representations even when the orthographic overlap between prime and target is only partial [START_REF] Ziegler | Orthographic and phonological contributions to reading development: Tracking developmental trajectories using masked priming[END_REF]. Higher levels of efficiency in such whole-word processing lead to stronger priming effects from embedded stems, independently of whether the stem is accompanied by an affix or not.

The absence of embedded stem priming effects in previous developmental priming studies (e.g., [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF] is likely to be due to the fact that stems were embedded in strings which themselves are real words (e.g., cashew-CASH). The simultaneous activation of the lexical representations of cash and cashew generates competition between the two lexical entries, and prevents stem-target priming (for similar reasoning, see [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF].

What might be the cause of the inhibitory priming effect seen with low proficiency children in the suffixed nonword (tristerie-TRISTE) and non-suffixed nonword conditions (tristald-TRISTE)? This result suggests that the nonword primes (tristerie/tristald) generated some form of competitive process that affected activation of the embedded target word (triste). This competition could operate as children attempt to acquire novel lexical representations using orthographic learning (see [START_REF] Ziegler | Modelling reading development through phonological decoding and self-teaching: Implications for dyslexia[END_REF]. Successful creation of a new representation for a novel orthographic form (e.g., tristerie, tristald) would require suppressing the activation of orthographically similar known words (e.g., tristesse, triste, etc.), and this would result in slower responses to the embedded target word following a related nonword prime. These competitive processes would be countered in children with higher levels of reading proficiency by the nonword prime activating more strongly the embedded stem.

It is also important to note that in the present study we found no difference between the suffixed and non-suffixed nonword conditions, whatever the grade or the reading proficiency of the children. In the [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF] study, low proficiency adults showed priming with suffixed but not with non-suffixed nonwords. Beyersmann and colleagues argued that morpho-orthographic segmentation processes, such as affix-stripping, led to enhanced activation of embedded stems during the processing of suffixed nonword primes by low proficiency participants. That is, the drop in efficiency in processing wholeword orthographic information (including embedded words) is compensated by morphoorthographic segmentation processes. The present results suggest that, in primary school children, independent of their reading proficiency, morpho-orthographic processing is not yet sufficiently rapid and/or automatized in order to affect subliminal (i.e., automatic) priming. In contrast, our results suggest that embedded stems function as lexical representations, not as morpho-orthographic representations, which can be activated subliminally (i.e., automatically) during the process of learning to read. Morpho-orthographic segmentation mechanisms appear to be acquired quite late during reading development, and are likely to emerge at later stages throughout middle-school or high-school. We hypothesise that once morpho-orthographic segmentation has been incorporated in the reading system, it will gradually be used to increasingly compensate for lower proficiency in using lexical whole-word representations, as it has been shown to be the case in adults [START_REF] Beyersmann | Language proficiency and morpho-orthographic segmentation[END_REF].

  Each target was preceded by a suffixed word prime (tristesse-TRISTE [sadness-SAD]), a suffixed nonword prime (tristerie-TRISTE [sadation-SAD]), a non-suffixed nonword prime (tristald-TRISTE [sadald-SAD]), and an unrelated prime (direction-TRISTE [direction-SAD]).

Figure 1 .

 1 Figure 1. Inverse reaction times to targets preceded by unrelated, suffixed word, suffixed

Table 1 Table 1 .

 11 Reaction times (in ms) and error rates (in %), averaged across items and participants.Standard deviations are presented in parentheses. Reading age level is calculated based on the Alouette reading proficiency test[START_REF] Lefavrais | L' Alouette-R[END_REF].

	Reading	Suffixed	Suffixed	Non-suffixed	Unrelated	Number of
	age level	word	nonword	nonword		children
				Reaction times
	Grade 1	1451 (405) 1748 (477) 1721 (371)	1576 (505)	4
	Grade 2	1390 (377) 1412 (404) 1410 (359)	1400 (377)	45
	Grade 3	1086 (255) 1109 (283) 1112 (279)	1166 (306)	45
	Grade 4	961 (184)	971 (159)	967 (158)	982 (178)	31
	Grade 5	941 (178)	925 (168)	935 (161)	968 (190)	35
	Grade 6	933 (167)	977 (187)	952 (177)	957 (143)	21
	Grade 7	808 (158)	801 (141)	860 (211)	887 (117)	8
	Grade 8	674 (161)	681 (97)	685 (98)	716 (123)	2
				Error rates
	Grade 1	45.0 (6.0)	40.5 (9.5)	40.0 (10.7)	48.0 (5.1)	4
	Grade 2	32.0 (18.0) 37.2 (12.9) 33.6 (17.5)	32.5 (16.3)	45
	Grade 3	22.4 (18.9) 24.0 (17.8) 21.2 (18.8)	24.9 (19.0)	45
	Grade 4	20.7 (17.3) 20.1 (18.4) 16.8 (15.8)	18.4 (17.8)	31
	Grade 5	16.0 (16.8) 18.0 (19.0) 15.2 (18.0)	16.6 (18.6)	35
	Grade 6	13.6 (14.7) 14.3 (17.5) 17.3 (17.8)	14.9 (16.5)	21
	Grade 7	24.5 (16.0) 10.7 (14.7) 19.5 (16.5)	5.1 (14.3)	8
	Grade 8	15.1 (21.3) 18.8 (26.6) 23.4 (33.1)	20.2 (28.6)

Acknowledgments

This project was supported by a postdoctoral grant from the Fyssen Foundation to Elisabeth Beyersmann and a grant from the European Research Council (ERC grant 230313) to Jonathan Grainger. This research was carried out within the Labex BLRI (ANR-11-LABX-0036) and, as such, has benefited from support from the French government, managed by the French National Agency for Research (ANR), Investments of the Future A*MIDEX (ANR-11-IDEX-0001-02). We also thank Clelie Bigo, Maeva Veber and Marcia Becu for help with participant recruitment and testing.


J Bossu
email: juliebossu@hotmail.frn.lemoignenicolas.le-moigne@mines-ales.fr

• N Le Moigne

• S Corn

• P Trens

• F Di Renzo

F Di Renzo

Sorption of water-ethanol mixtures by poplar wood: swelling and viscoelastic behaviour

The swelling of wood upon impregnation by aqueous solutions of organic solvents is larger than swelling by pure water. This phenomenon, described as "hyperswelling", results from specific interactions with components of the complex wood microstructure. To understand the combined effect of mixed solvents on wood properties, the specific influence of each solvent on wo od biopolymers and structure must be first characterized. In this study, the effect of th e impregnation of aqueous solutions of ethanol with concentrations from 0 to 100% on the physico-mechanical properties of poplar wood was investigated. For each solution, the sorption behaviour of veneer sapwood samples and the effect of sorption on wood C2MA is member of the European Polysaccharide Network of Excellence (EPNOE), http://www. epnoe .eu.

Introduction

As an answer to the increasing demand in biomaterials for packaging, furniture industry and construction, wood and wood-based products could stand as competitive solutions. Indeed, the excellent physical and mechanical properties of wood fibres, the accessibility of the resources and the possible conversion into biocomposites are strong arguments making wood an attractive source of materials. Nonetheless, because of its hygroscopic nature, wood is subject to swelling in contact with liquids. This phenomenon needs to be better understood and controlled to promote the use of wood in technical applications.

Depending on the nature of the adsorbed liquid, the extent of swelling has been reported to be highly variable [START_REF] Gordy | Spectroscopic comparison of the proton-attracting properties of liquids[END_REF]. Similar effects were also observed in the swelling of lignocellulosic materials and cellulose [START_REF] Prusov | Interaction of cellulose and lignocellulosic polymers with water and aqueous systems[END_REF], as well as for wood-polymer composites [START_REF] Zhang | Water vapor adsorption and volumetric swelling of melt-impregnated wood-polymer composites[END_REF]. The correlation between the characteristics of the liquids and their effect on swelling has been mainly discussed in terms of molecular size and polarity. Several authors described a negative correlation between molar mass and swelling rate, due to the greater difficulty for larger molecules to diffuse into the fine structure of wood cell wall [START_REF] Ishimaru | Wood swelling and its transverse anisotropy in organic liquids having two or more functional groups in a molecule[END_REF][START_REF] Ishimaru | Swelling of wood in liquid mixtures. Part I: water-ethanol and water-acetone[END_REF][START_REF] Stamm | Variation in shrinking and swelling of wood[END_REF][START_REF] Stamm | Penetration of cellulose fibers[END_REF]. [START_REF] Nayer | Swelling of wood in various organic liquids[END_REF] and [START_REF] Nayer | Hydrogen bonding and the swelling of wood in various organic liquids[END_REF] also reported higher swelling in polar liquids characterized by high bonding capacity, which is explained by the greater and faster penetration of the liquids into wood cell walls. These results are confirmed by Mantanis et al. (1994aMantanis et al. ( , b, 1995)), who tested a large variety of pure solvents and wood species and reported that both hydrogen bonding capacity and size of the solvent molecules contribute to the extent of wood swelling.

However, there are still unexplained results: dimethyl sulfoxide (DMSO), for example, swells wood twice as water does despite lower polarity and higher molecular mass compared to water.

Wood swelling in pure solvents is well documented. Yet, in many industrial applications, organic solvents are often combined with water, already present in the wood. Studying the effect of aqueous solutions containing organic solvent on wood swelling has then a special technological relevance. The complexity of the phenomena involved, however, leads to surprising observations. [START_REF] Robertson | Cellulose-liquid interactions[END_REF] observed that binary mixtures of liquids can produce more swelling than either component alone and that this synergistic effect is particularly notable when water is one of the liquids. Later, O' Leary and Hodges (2001) mentioned that such systems behave "strangely" and reported that preferential adsorption of certain components within binary systems can cause disproportionate tangential swelling. In this study, the specific case of water-ethanol solutions, which are involved in a large range of industrial uses, is addressed. Studying poplar and pinewood in green and dry conditions, [START_REF] Chang | Deformation induced by ethanol substitution in normal and tension wood of chestnut (Castanea sativa Mill.) and simarouba (Simarouba amara Aubl.)[END_REF][START_REF] Chang | Solvent polarity and internal stresses control the swelling behaviour of green wood during dehydration in organic solution[END_REF] and [START_REF] Meier | Swelling of pinewood (pinus sylvestris) in binary aqueous solutions of organic substances[END_REF] reported that water-ethanol solutions generate "hyperswelling", namely a swelling more important than that observed for either pure water or pure ethanol. More precisely, the most important deformations were reported for a 50% volume fraction of ethanol. Similar results were observed on gels of acetylated lignin, showing no swelling in ethanol but eight times the initial volume for swelling in water/ ethanol 50% [START_REF] Nishida | Lignin gel with unique swelling property[END_REF].

High swelling rate strongly influences wood mechanical properties. But, albeit the process of adsorption on wood and its effect on wood viscoelastic properties have been widely studied for water [START_REF] Engelund | Acquisition of sorption isotherms for modified woods by the use of dynamic vapour sorption instrumentation. Principles and practice[END_REF][START_REF] Wang | Transient moisture effects in materials[END_REF]) and several pure solvents (Ishimaru et al. 2001a;[START_REF] Ma | The effect of donor-acceptor interactions on the mechanical properties of wood[END_REF], only a limited number of studies are available in the case of ethanol. In other lignocellulosic materials, it has been shown that aqueous solutions of ethanol can strongly affect the static mechanical properties. For example, alteration of both tensile and shear strength of bamboo fibres composites has been evidenced upon aqueous treatments with ethanol [START_REF] Hu | Influence of dyeing treatment on the performance of bamboo-based fibre composites[END_REF][START_REF] Qin | Research on surface color, properties of thermo-treated reconstituted bamboo lumber after artificial weathering test[END_REF]. Regarding wood, Meier et al. (2006b) found that both static bending and compression strength of pinewood swollen in aqueous solutions of ethanol were decreased by about the same amount as when using pure water. To date, there is no comprehensive explanation of these experimental observations.

To investigate these questions, it was considered in this work that studying the effect of water-ethanol sorption on both mechanical properties and wood fibres microstructure was a promising avenue of research. Wood microstructure is characterized by a non-homogeneous supramolecular organization. The specific composition of the layered structure of wood cell walls could explain why the sorption of binary solvents results in non-homogeneous hygroscopic and mechanical behaviours. The use of aqueous solutions of ethanol might lead to selective extraction of some of the cell wall and middle lamellae components, accompanied by alterations in stress transfer mechanisms within cell walls and between wood cells. Such phenomenon would result in the modification of the physico-mechanical and hygroscopic properties of lignocellulosic fibres. This hypothesis is supported by the observations of [START_REF] Meier | Swelling of pinewood (pinus sylvestris) in binary aqueous solutions of organic substances[END_REF]Meier et al. ( , 2006a, b), b), who reported that ethanol-water mixtures generate local decohesion of individual cells. Studies on other lignocellulosic materials also identified specific effects of the cooperative sorption of water and organics on the different layers of primary and secondary cell walls. Acera [START_REF] Fernández | Role of flax cell wall components on the microstructure and transverse mechanical behaviour of flax fabrics reinforced epoxy biocomposites[END_REF] showed that the removal of non-cellulosic components from the cell walls of flax fibres with various aqueous and organic solvents could strongly affect t he t ransverse mechanical properties of the fibres and their composites. In aqueous solutions of DMSO, Le [START_REF] Moigne | Gradient in dissolution capacity of successively deposited cell wall layers in cotton fibres[END_REF]Moigne et al. ( , 2010) ) and Le Moigne and Navard (2010) also observed gradients in swelling and dissolution between the different cell walls of lignocellulosic fibres.

To better understand the phenomenon of wood hyperswelling observed in aqueous solutions containing organic solvents, it is then proposed to use poplar wood and water-ethanol binary mixtures as a reference system to clarify the nature of the synergistic effect of these two solvents on wood samples. Investigating the specific contributions of water and ethanol to sorption, swelling and viscoelastic wood properties, it is aimed to improve the understanding of the swelling mechanisms of wood in mixed solvents. For this purpose, first the sorption behaviour of poplar wood veneers in water, ethanol and their binary mixtures was studied to observe the influence of ethanol concentration on sorption behaviour. Second, the effect of sorption on samples' dynamic mechanical properties was measured to see whether both solvents are sorbed in similar absorption sites, and assuming that different sorption sites could give rise to different changes in the dynamical mechanical behaviour. Finally, the impact of sorption on the deformation of cell walls and on the intercellular cohesion was evaluated by microscope monitoring, to characterize the effect of the different mixtures on wood microstructure.

Materials and methods

Wood sampling and mixed solvents

Poplar was chosen as a model wood material due to its regular microstructure and low extractive content, thus limiting the issues related to wood inherent variability. This species is also widely naturally spread and planted in Europe and North America and a future growth in poplar utilization is expected in the area of composite products, bioenergy and chemicals [START_REF] Balatinecz | Properties, processing and utilization[END_REF]. Unheated and untreated poplar veneers (Populus tremula) were cut with a hydraulic press in the same sapwood zone along the fibre direction (i.e. R = 5 mm × T = 0.5 mm × L = 45 mm) and stabilized for 2 months at room conditions (25 °C; 45% RH). The resulting samples were homogeneous and almost free from extractives, thus limiting the potential influence o f s econdary m etabolites o n wood s orption [START_REF] Bossu | New insights on wood dimensional stability influenced by secondary metabolites: the case of a fast-growing tropical species Bagassa guianensis Aubl[END_REF]. Solvents used for adsorption experiments were outgassed deionised water and ethanol (99.9%, Aldrich). Three binary mixtures made of water and ethanol were prepared with different m ass f ractions e thanol/(ethanol + w ater) (0.21, 0.44 and 0.70). The calculated molar fractions of ethanol in the resulting mixed solvents (named 21% EtOH, 44% EtOH and 70% EtOH) are 0.09, 0.24 and 0.48.

Gravimetric sorption measurements

Vapour sorption measurements were performed by dynamic vapour sorption (DVS), a technique largely used to determine sorptive properties of wood and biomaterials [START_REF] Engelund | Acquisition of sorption isotherms for modified woods by the use of dynamic vapour sorption instrumentation. Principles and practice[END_REF][START_REF] Hill | The water vapour sorption properties of Sitka spruce determined using a dynamic vapour sorption apparatus[END_REF][START_REF] Lequin | Adsorption equilibria of water vapor on cork[END_REF][START_REF] Ormondroyd | Water sorption, surface structure and surface energy characteristics of wood composite fibres refined at different pressures[END_REF][START_REF] Ormondroyd | Water sorption, surface structure and surface energy characteristics of wood composite fibres refined at different pressures[END_REF][START_REF] Hosseinpourpia | Dynamic vapour sorption of wood and holocellulose modified with thermosetting resins[END_REF][START_REF] Hosseinpourpia | Dynamic vapour sorption and waterrelated properties of thermally modified Scots pine (Pinus sylvestris L.) wood pre-treated with proton acid[END_REF]. In the isotherm gravimetric sorption instrument (DVS-Vacuum, Surface Measurement Systems, London, UK), the vapour pressure is obtained from temperature control of liquid sorbates placed in reservoirs. A mass flow controller ensures a constant admission of vapour generated from the liquid sorbates into the nitrogen flow to the balance cell. High vacuum can be obtained in the balance using a membrane pump coupled to a molecular pump. A butterfly valve on the pump allows reaching a targeted absolute pressure in the balance under continuous gas flow. The vapour pressure is controlled through the ratio of gas flows from the vapour reservoir and the dry carrier nitrogen bottle. Successive steps of relative pressures (p/p°) were set for determining the whole sorption isotherm. The equilibrium criterion for each step of the sorption isotherms was set with a mass variation ratio dm/dt of 0.005%/min. Each wood specimen was loaded and dried in situ at 40 °C under high vacuum (2 × 10 -6 Torr) for 10 h. Solvent sorption measurements were performed at 25 °C. The values of the saturation pressure of each component of the ethanol aqueous solutions are reported in Supplementary Materials 1. The typical duration of a sorption-desorption cycle was 30 h. These conditions have been shown to be adequate for reaching constant mass uptake in each step of water vapour sorption for sample size used in these experiments [START_REF] Bratasz | Analysis of water adsorption by wood using the Guggenheim-Anderson-de Boer equation[END_REF][START_REF] Wong | Characterizing the kinetics of free and bound water using a non-isothermal sorption technique[END_REF]. The sorption isotherms were best fitted by the GAB model with GraphPad Prism 6 software. Water sorption measurements were also conducted on a powdered wood sample prepared by 2-µm milling with a Retsch SM 100 cutting mill.

Dynamic mechanical characterization by DMA

Modified mechanisms of wood swelling can induce complex variations in the mechanical behaviour over time. In these cases, dynamical studies are more adapted to track transient mechanical changes occurring during sorption processes. For this reason, the effect of solvents sorption on the dynamic longitudinal mechanical behaviour of wood specimens were analysed by coupling a Dynamic Mechanical Analyser (DMA) (50N Metravib) with a tank connected to a thermostatic bath regulated at 40 °C (see Fig. 1). Samples were mounted in tensile mode and clamped between the jaws with a torque wrench at 2.5 N/m before being immersed in the different solvents. First trials were conducted for assessing the linear viscoelastic domain of the specimens. The optimal amplitude of the dynamic displacement was found to be 5 µm.

The variations of the complex modulus E*, which is composed of the storage modulus E′ (real part) and loss modulus E″ (imaginary part), and the damping (tanδ = E″/E′) were measured at a frequency of 0.05 Hz all along the absorption process during 2 h. Four repetitions were operated for water and ethanol and three repetitions for mixed solvents. To enable comparison between the different tests, relative E′ and relative tanδ were calculated, normalizing the current values by the initial values of E′ 0 and tanδ 0 measured after 500 s of stabilization and just before t 0 corresponding to the beginning of immersion. Dynamic mechanical behaviour in shear mode was also investigated using the same protocol.

The exact compositions of the solutions were checked before and after each measurement using gas chromatography-mass spectrometry (GC-MS). No modification of the composition could be observed during the experiments that would originate from selective evaporation, thus indicating a good sealing of the apparatus.

Optical microscopy observations

After mechanical testing, each sample was equilibrated with each solution before being cut into thin radial slices of 25 µm thickness with a WSL GSL1 sledge microtome in three different zones. The slices were then placed between two glass plates with the same solution to monitor the thickness and microstructure of the cell walls with a Leica DM LM/P optical microscope in transmission mode. In each slice, the average wall thickness of the cells was obtained by measuring the thickness of each visible cell on ten points and averaging the results.

Results and discussion

Sorption of vapours

Pure solvents

The sorption isotherms of water at 25 °C on poplar powder and solid specimens are shown in Fig. 2. Both sorption isotherms could be classified as type II, according to the IUPAC, which is characteristic for non-porous materials. The water uptake at saturation (17 or 19% w/w at p/p° = 0.95) as well as the steep slope at low relative pressure (p/p° < 0.1) are consistent with former studies on similar materials [START_REF] Peralta | Sorption of moisture by wood within a limited range of relative humidities[END_REF]. Sorption isotherms on biomass-based materials are frequently modelled by the GAB (Guggenheim, Anderson, de Boer) method, which has been developed to extend the isotherm modelling of the BET (Brunauer, Emmet, Teller) method [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF]) by considering the adsorption energy level not just of surface monolayers but also of interacting multilayers [START_REF] Anderson | Modifications of the Brunauer, Emmett and Teller equation[END_REF]. The GAB equation is formally equivalent to other extensions of the BET theory [START_REF] Dent | A multilayer theory for gas sorption. Part I: sorption of a single gas[END_REF] but also to models based on a different approach [START_REF] Hailwood | Adsorption of water by polymers: analysis in terms of a simple model[END_REF][START_REF] Okoh | Moisture sorption isotherms of the wood and inner bark of ten southern U.S. hardwoods[END_REF]. The Hailwood and Horrobin model was developed to account for water uptake by polymer gels and attributes the strongest sorption to absorbed "hydration" water molecules in direct interaction with the polymer and a weaker sorption to "dissolved" water contributing to the gel swelling. The formal equivalence of the models indicates that, based on uptake isotherms alone, it is impossible to differentiate between surface adsorption and bulk absorption phenomena [START_REF] Prothon | Application of the Guggenheim, Anderson and De Boer model to correlate water activity and moisture content during osmotic dehydration of apples[END_REF]). In the case of food samples, it was early realized that the sorption of vapours penetrating inside a low-surface area material could also be modelled by the GAB method, which allowed to evaluate the sorption of "bound" and "free" sorbate instead of, respectively, monolayer and multilayer adsorption [START_REF] Quirijns | Sorption isotherms, GAB parameters and isosteric heat of sorption[END_REF]). In the case of wood samples, this interpretation is strongly supported by the observation that typical GAB "monolayer" sorption values are two orders of magnitude larger than monolayer adsorption values measured by non-penetrating N 2 [START_REF] Bratasz | Analysis of water adsorption by wood using the Guggenheim-Anderson-de Boer equation[END_REF][START_REF] Clair | Characterization of a gel in the cell wall to elucidate the paradoxical shrinkage of tension wood[END_REF]).

In the GAB isotherm X is the mass uptake at p/p° = α, X m is the mass uptake at the conventionally called "monolayer" value and can be attributed to "bound" sorbate. C is an energetic constant related to the ratio of the Gibbs free energies of "bound" and "free" sorbate, and K is the ratio between the Gibbs free energy of the bulk liquid and the "free" sorbate. The values of the GAB constants for the different samples are reported in Table 1.

The values of the GAB parameters for water sorption on whole and powdered specimens of poplar veneer are alike, as expected by the similarity of the sorption isotherms. The X m and C values indicate that nearly 5% w/w of water is sorbed with high affinity for the material. The K parameter is essentially of entropic nature and its value, lower than 1, suggests that water molecules sorbed beyond the "monolayer" are significantly more structured than in bulk liquid water. The similarity of the water sorption isotherms on whole specimens and powders indicates that (1) the external surface area and the size of the material have a very limited influence on the sorption of water vapour and that (2) diffusion in the specimens-at least at the scale of the current samples-is not a limiting factor.

The desorption branches are shifted to a lower relative pressure as compared to the sorption branch. Full reversibility is observed, as the desorption branches go back to the origin as the relative pressure tends to zero. A significant feature of the hysteresis loop is the lack of the sudden step-down of the desorption branch with convergence to the adsorption branch at a relative pressure defined by the nature of the sorbent and by temperature [START_REF] Ravikovitch | Experimental confirmation of different mechanisms of evaporation from ink-bottle type pores: equilibrium, pore blocking, and cavitation[END_REF][START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. This phenomenon, typical of sorption in mesoporous materials, takes place at p/p° near 0.31 in the case of water at room temperature (Sing and Madeley 1954). This so-called catastrophic desorption occurs when capillary tension in small mesopores exceeds the tensile strength of the condensed liquid [START_REF] Burgess | Adsorption hysteresis in porous materials[END_REF][START_REF] Trens | A macrothermodynamic approach to the limit of reversible capillary condensation[END_REF]). This phenomenon is systematically absent in vapour sorption cycles on wood [START_REF] Bratasz | Analysis of water adsorption by wood using the Guggenheim-Anderson-de Boer equation[END_REF][START_REF] Hill | The water vapour sorption properties of Sitka spruce determined using a dynamic vapour sorption apparatus[END_REF][START_REF] Ormondroyd | Water sorption, surface structure and surface energy characteristics of wood composite fibres refined at different pressures[END_REF],

(1) indicating that the hysteresis loops cannot be attributed to a simple capillary condensation. The observed hysteresis is typical of an activated sorption process, compatible with a swelling mechanism in which the sorbate is absorbed within the bulk of the material.

X X m = CK (1 -K ) . (1 -K + CK )
The sorption isotherm obtained with pure ethanol has a type V sigmoidal shape (see Fig. 2). This type of isotherm is usually described as being the result of cooperative adsorption of species, in which a weak adsorption of isolated molecule is accelerated at the relative pressure corresponding to the formation of surface molecular clusters. However, at low relative pressure of ethanol, there is almost no adsorption (0.1% w/w up to p/p° = 0.3), whereas at saturation, the sorbed amount of ethanol is 11% (w/w). Such an extremely sigmoidal shape of the curve cannot be accounted for by simple models of surface adsorption and cannot be modelled by BET, BJH or GAB methods. To grasp the scale of the phenomenon, it is useful to observe that the formation of a monolayer of ethanol (cross-sectional area nearly 0.17 nm 2 ) on a poplar veneer with nitrogen surface area 0.5 m 2 g -1 [START_REF] Clair | Characterization of a gel in the cell wall to elucidate the paradoxical shrinkage of tension wood[END_REF]) would correspond to a limited mass uptake of just 0.02% w/w. The ethanol sigmoidal sorption is outside the field of monolayer/multilayer surface adsorption. It really seems that a high chemical potential is needed to overcome a sorption energy barrier. If the uptake up to p/p° 0.3 can reasonably be accounted for by surface adsorption, once the needed p/p° is reached ethanol penetrates inside the material. It is likely that the heterogeneity of the wood structure plays a role in the phenomenon, as experiment setting, pretreatment and wood species can do. Indeed, less strongly sigmoidal ethanol sorption isotherms have been obtained with very long equilibration times on vapour-treated birch and aspen [START_REF] Chirkova | Sorption properties of hydrothermally modified wood and data evaluation based on the concept of Hansen solubility parameter (HSP)[END_REF].

Another major difference between water and ethanol sorption is the shape of the desorption branch. In the case of ethanol, a larger hysteresis can be seen, with a pronounced irreversibility. Indeed, 4.6% w/w of ethanol remains adsorbed on the substrate after desorption. This irreversibility, strongly suggestive of chemisorption, confirms that the sorption of ethanol occurs in the bulk of the material.

Mixed solvents

The sorption isotherms of water-ethanol mixed vapours can be compared with that obtained with the pure components (see Fig. 3) with just one caveat: it has to be kept in mind that, unlike the case of the pure vapours, the composition of the adsorbed phase on the material does not necessarily correspond to the ethanol/water ratio in the vapour, due to a different affinity of each component for the substrate and a possible selectivity of adsorption.

The shapes of the sorption isotherms of mixed vapours are reminiscent of that found for the sorption of water. However, the initial mass uptake is steeper than in the case of pure water, as highlighted by the lower partial pressure needed to absorb up to 5% w/w (see Fig. 4, left side). Indeed, the GAB X m "monolayer" value for the 21% EtOH sorption (see Table 1) is higher than for pure water, suggesting that the presence of water triggers the co-sorption of ethanol, even at low vapour pressure. The GAB C parameter for the mixed vapours is always lower than for pure water vapour, indicating a lower average affinity of the absorbate for the absorbent. Quite interestingly, the K parameter increases with the ethanol fraction and reaches values higher than one, suggesting that the co-sorption of water and ethanol can lead to an absorbate phase more disordered than the corresponding bulk solvent. Globally, it can be deduced that the sorption of mixed vapours is a true co-sorption of water and ethanol and that the sorption of water wipes out the surface energy barrier for the sorption of ethanol.

On the other hand, the influence of ethanol in the sorption of the mixture can be seen in the hysteresis loop which is quite wide, especially for high ethanol contents. Irreversible retention upon desorption can also be observed, albeit to a smaller extent than for pure ethanol (see Fig. 4, right side). It seems likely that ethanol can strongly interact with some of the components present in wood specimens and the extent of residual mass uptake after desorption is more important as the ethanol molar fraction increases.

Influence of solvent sorption on viscoelastic behaviour

Pure solvents

Results from dynamic mechanical testing during immersion reveal that ethanol sorption has a much lower effect on the viscoelastic properties of poplar veneers samples compared to water (see Fig. 5). For ethanol sorption, the loss of relative storage modulus (namely E′/E 0 ) (-0.05 on average) and increase in relative damping (namely tanδ/tanδ 0 ) (+ 0.68 on average) occur quickly after immersion (extreme values reached at t = 219 s on average). At longer times, a slow recovery can be observed, i.e. slight variations from the extreme to the final values corresponding to + 0.01 and -0.26 of relative E′ and relative tanδ, respectively. It must be pointed out that the results for sorption of ethanol were highly reproducible.

The modification of the initial sample viscoelastic properties is clearly more pronounced for water sorption. Regarding storage modulus, the final loss of relative E′ is -0.35, which is 9 times larger than observed for ethanol. Nonetheless, comparing the average variations of E′ due to water or ethanol sorption, it can be noticed that the two curves have comparable shapes, even if the intensity of the phenomenon is more pronounced for water. Regarding damping, water sorption also entails much larger modification of relative tanδ (increase of + 2.21 on average, i.e. more than 5 times higher than ethanol), highlighting that the absorption of water within poplar wood cells induces a pronounced change in their viscoelastic properties. Indeed, an increase in damping is characteristic of an increase in the viscous contribution (E″) to the complex modulus (E*). It indicates a higher molecular mobility and a loss of intermolecular interactions between the biopolymers constituting cell walls and middle lamella. Besides, the effects of water and ethanol sorption on tanδ variations also differ in nature.

Wood-water interactions and its influence on dynamical mechanical properties are already well documented [START_REF] Ebrahimzadeh | Effect of impregnation on mechanosorption in wood and paper studied by dynamic mechanical analysis[END_REF]. The final modifications of E′ and tanδ after impregnation with water are comparable to the results obtained by Ishimaru et al. (2001b) and [START_REF] Obataya | The effects of adsorbed water on dynamic mechanical properties of wood[END_REF]. For ethanol, as no Fig. 5 Evolution of viscoelastic properties of poplar veneers specimens immersed in pure water and pure ethanol at 40 °C. Relative storage modulus E′ (E′/E 0 , left) and relative damping tanδ (tanδ/tanδ 0 , right) over time, after immersion at t 0 similar study has been conducted to the authors' knowledge, the influence of sorption on E′ and tanδ stands as a new result. Meier et al. (2006b) found that sorption of ethanol entails lower modifications of the static mechanical properties than water (two times lower compared to water), albeit this result was obtained for different loading modes.

The effect of variable humidity on E′ and tanδ after equilibrium has been widely studied in the case of water. Yet, regarding the immediate modification of the mechanical properties due to a sudden change of humidity, as performed in this study, there are fewer studies in the literature that can stand as a comparative reference. Here, it can be observed that water gives rise to a pronounced transitory state (d tanδ max = + 4.61), i.e. seven times more than ethanol, followed by a large recovery (average decrease of -2.40 from the extreme to the final values). Numerous studies have shown that transient moisture conditions have additional effects on the mechanical properties of hygroscopic materials [START_REF] Armstrong | Influence of moisture changes on deformation of wood under stress[END_REF][START_REF] Wang | Transient moisture effects in materials[END_REF]). These phenomena have been discussed in terms of instability originated by continual making and breaking of hydrogen bonds due to changes in moisture uptake in the path towards equilibrium [START_REF] Gibson | Creep of wood: role of water and effect of a changing moisture content[END_REF][START_REF] Nordon | Some torsional properties of wool fibers[END_REF]. The slip zone model proposed by [START_REF] Hoffmeyer | Mechano-sorptive creep mechanism of wood in compression and bending[END_REF] illustrates this hypothesis. Through the identification by microscopy of zones of minute failures as distinct planes extending through the S2 layer of the cell wall, they showed that the changes of the elastic, viscoelastic, and plastic properties of wood are proportional to the number of slip planes, which increases with increasing moisture change. Later, [START_REF] Navi | Transient moisture effects on wood creep[END_REF] showed that the transient hydrogen bonding between crystalline cellulose and the surrounding amorphous polymers, owed to the introduction or removal of water, may accelerate shear slip between the two phases in the presence of an external load. The recovery of E′ observed here is then likely the consequence of rebonding of the fibrils after sorption. However, though the breaking and remaking of hydrogen bonds during transient moisture conditions may provide a general explanation of these transient phenomena, it does not provide a detailed explanation for the quantitative differences observed between water and ethanol. Moreover, there is little information about the potential mechanism played by ethanol.

Based on the present results and literature data, it is assumed that the strong variations in viscoelastic behaviour observed for water absorption are the result of the diffusion of water molecules within the secondary S2 cell wall layer, i.e. the main body of cell walls, at the origin of stiffness and longitudinal strength of wood [START_REF] Bergander | Cell wall properties and their effects on the mechanical properties of fibers[END_REF]. Water molecules interact through H-bonding with hydrophilic biopolymers such as hemicelluloses located between more hydrophobic microfibrillar lattices, repulsing them one from another and giving rise to a pronounced macroscopic swelling of the specimens [START_REF] Barbetta | Impregnation and swelling of wood with salts: ion specific kinetics and thermodynamics effects[END_REF].

It was evidenced that ethanol molecules, like water molecules, are also absorbed by the material. Yet, the low resulting variations of E′ and tanδ indicate that the interactions of ethanol with wood biopolymers do not play a key role in the longitudinal mechanical properties of the fibres. This could be due to the nature of the interactions or to the location of the sorption sites. The large difference between mechanical properties after immersion in water or ethanol contrasts with the reasonably similar amounts of water and ethanol absorbed at saturation in the gravimetric adsorption experiments. If ethanol molecules were absorbed within cell wall layers at the same absorption sites as water molecules, their larger steric hindrance would generate more swelling and higher modifications in viscoelastic behaviour than water sorption, contrary to observed evidence. On these bases, it is assumed that the absorption sites of water and ethanol and the nature of their interactions with wood biopolymers are different and do not play the same role in the resulting viscoelastic behaviour of poplar wood.

Mixed solvents

In Fig. 6, the average results obtained upon immersion of poplar wood in pure and mixed solvents are compared. The variations of relative E' and tanδ after immersion in water-ethanol mixtures show a trend which is similar to the one observed for water at short immersion times: an initial transitory state, particularly evident in damping. At longer immersion times, the recovery of viscoelastic properties is slower and weaker than in the case of water. The average final modifications of relative E′ after immersion in 21% EtOH, 44% EtOH and 70% EtOH ethanol-water solutions are -0.44, -0.52 and -0.45, respectively. Regarding relative tanδ, the same solvents induced an increase of + 4.15, + 6.04 and + 4.05, respectively (see Supplementary Materials 2 for details of each test). The decrease in relative E′ and increase in relative tanδ in mixed solvents are always higher than those observed with pure water. This result indicates that ethanol-water mixtures have a specific synergistic contribution to wood swelling and variation in viscoelastic behaviour. The maximal variations are observed with 44% EtOH, where relative E′ and relative tanδ reach extreme values of -0.53 and + 7.15, respectively, i.e. 1.5 times more than for pure water. Variations in viscoelastic properties after sorption show a parabolic evolution as a function of the ethanol concentration (see Fig. 6), highlighting again the synergistic effect of mixed solvents.

The sluggish evolution of mechanical properties after their maximum variation is evidenced by several effects. In the curves of mechanical behaviour in Fig. 6, it can be remarked that tanδ reaches maximal values in a shorter time in pure water than in mixed solvents. In mixed solvents, the time to reach extreme values is maximal in 44% EtOH. Water sorption is also characterized by a larger recovery of relative tanδ (roughly -0.40), significantly higher than the value observed for mixed solvents (roughly -0.17) or for pure ethanol. This gradual decrease in relative tanδ, after reaching extreme variations corresponds to a local rearrangement of sorbate molecules that occurs within the cell walls structure after swelling. As shown above, the sorption of water alone is fully reversible, while ethanol sorption is partially irreversible, which means that the sorbed water molecules can be easily extracted from the material. In the presence of ethanol, either alone or in aqueous solutions, the recovery of mechanical properties after reaching extreme values is more sluggish and limited. This effect could be related to the irreversible retention of ethanol observed in gravimetric sorption experiments. Quite likely, the presence of retained ethanol molecules slows down the entire sorption-desorption process and prevents a complete recovery after the transient state.

A possible mechanistic interpretation of these results could be that irreversibly adsorbed ethanol molecules damaged the pristine microstructure of wood cells and/ or intercellular cohesion. The effects of these phenomena could be evidenced by mechanical analysis during drying. It might be impossible to return to the initial mechanical properties, and the difference between initial and final mechanical properties might depend on ethanol molar fraction. However, irreversible changes such as hornification could also occur, making it difficult to discriminate the effects of individual sorbed molecules on the viscoelastic behaviour of wood samples.

Optical microscopy observations

Pure solvents

Slices of 25 µm in thickness were sampled from specimens used for dynamic mechanical analysis in water, ethanol or mixed solvents, to be observed by optical microscopy. In Fig. 7, two microscopic slices tested in water and ethanol are shown, corresponding to the samples which displayed the most contrasting viscoelastic behaviour.

Regarding pure solvents, water and ethanol absorption result in two really contrasted effects on the cell wall layers. Wood cells from the samples immersed in water are tightly packed against each other and show thick secondary S2 layers. The different layers of the cell walls are compact and well bound to the middle lamella. On the contrary, in ethanol, secondary S2 layers swell to a lower extent and P wall seems less bound in middle lamella regions. Average wall thickness is nearly 3 µm after swelling in water and 2 µm after swelling in ethanol (see Fig. 9).

In addition, some peeling can be observed between the middle lamella and the cell walls after absorption of ethanol. The zoom on the sample tested with ethanol (see Fig. 7) enables a more precise definition of the damaged interface: the apparent separation seems to occur between the middle lamella and the primary wall (disbonding 1 in Fig. 7). These results are concordant with [START_REF] Meier | Swelling of pinewood (pinus sylvestris) in binary aqueous solutions of organic substances[END_REF] who observed a similar phenomenon on SEM images of pinewood swollen in ethanol. It also appears that ethanol absorption can entail the disbonding of the different layers of the secondary wall (disbonding 2 in Fig. 7), leading to the detachment of the internal part of the cell wall layers, which remain isolated in the lumen of the cells. This phenomenon can be the result of the weakening of the interface between S2 and S1 layers, as well as the detachment of the G-layer in the case of tension wood, as observed by [START_REF] Clair | On the detachment of the gelatinous layer in tension wood fiber[END_REF] on poplar specimens. It should also be pointed out that weakening of the intercellular cohesion due to ethanol absorption could have favoured the disbonding of wood cells during the cutting of the slices. In any case, ethanol absorption appears to be responsible for a weakening of the intercellular cohesion in poplar wood cell walls. Vapour sorption measurements have already shown that ethanol might preferentially react with lignin rather than cellulose [START_REF] Lequin | Sorption equilibria of ethanol on cork[END_REF]. [START_REF] Barrera-García | Role of wood macromolecules on selective sorption of phenolic compounds by wood[END_REF] showed a different behaviour of wood components towards sorption of phenolics from hydro-alcoholic solutions. Apparently, cellulose and hemicellulose absorbed several phenolic compounds without apparent distinction, whereas lignin preferentially extracted some of these compounds from the hydro-alcoholic solution. The middle lamella, which is mainly composed of lignin and other phenolic compounds, might then preferentially interact with ethanol, being altered or even partially dissolved. This could explain the disbonding between primary wall and middle lamella. This hypothesis is also supported by the work of [START_REF] Hosseinpourpia | Dynamic vapour sorption and waterrelated properties of thermally modified Scots pine (Pinus sylvestris L.) wood pre-treated with proton acid[END_REF], who observed that the removal of lignin on pinewood samples allows stronger swelling of the polysaccharide matrix. Within the secondary wall, S1 layer contains the greater amount of lignin, possibly explaining its disbonding from S2 wall. The weakening of the interface between S1 and S2 layers could allow the release of internal stresses originating from cell wall maturation. It is also likely that ethanol in aqueous solution leads to a better extraction of lignin. Indeed, in the field of papermaking, it has been shown that aqueous solutions of ethanol and acetone lead to higher lignin solubilization, with the highest efficiency obtained for volume concentrations around 50% [START_REF] Domínguez-Robles | Aqueous acetone fractionation of kraft, organosolv and soda lignins[END_REF][START_REF] Jääskeläinen | Water vapour sorption characteristics and surface chemical composition of thermally modified spruce (Picea abies karst)[END_REF].

The effect of ethanol sorption on the dynamic mechanical behaviour under shear was checked. It was assumed that the contribution of interfaces and intercellular cohesion to the mechanical properties of the samples would be more important in this loading mode (see Supplementary Materials 3). It was observed that the loss of E' modulus and increase of damping in ethanol was more pronounced in shear mode as compared to tensile mode, the relative variations of viscoelastic properties observed for ethanol and water absorption being similar. Even if ethanol can induce a pronounced intercellular decohesion and weaken the interfaces in wood microstructure, such results are not easy to interpret because of the superposition of different macromolecular and microstructural changes related to plasticization, hornification and decohesion. As pointed out above, this makes it difficult to discriminate the effects of individual sorbed molecules on wood samples.

Mixed solvents

In water-ethanol mixtures, a combination of high swelling due to water and intercellular decohesion due to ethanol can be observed (see Fig. 8). The micrographs of the wood cells from the samples immersed in 21% EtOH are similar to those observed for water, but cells' external shapes present more rounded profiles on average. Cells are mostly packed together and only little disbonding between the middle lamella and the primary wall can be observed. For the samples immersed in 44% EtOH, both high swelling of the secondary S2 layer and frequent local disbonding between the middle lamella and the primary wall can be noticed. The average thickness of cell walls (see Fig. 9) reaches a maximum of 3.6 μm for the 44% EtOH sample. The coupling of water sorption and disbonding by ethanol appears to be the best combination to maximize microscopic swelling of wood cells. Indeed, in this situation, cells are less mutually constrained and the secondary S2 layer swells more freely. Finally, after immersion in 70% EtOH, the micrographs show cells that are almost all dissociated from the middle lamella with a slight swelling of S2 wall. In this specific case, it can be inferred that even if cell disbonding induced by ethanol favours the free swelling of S2 wall, there is no sufficient water uptake to swell it significantly. If ethanol reacts preferentially in lignin-rich regions as the middle lamella and the outer parts of the cell wall, it is also likely that pre-swelling of the inner S3 layer by water enables ethanol to get access to these sites. This hypothesis is supported by previous studies reporting that the extraction of polyphenols from plant materials is more effective with an ethanol concentration of approximately 70% [START_REF] Yang | Ultrasound-assisted extraction of rutin and quercetin from Euonymus alatus (Thunb.) Sieb[END_REF]. Additionally, post-mortem optical microscopy observations evidence that ethanol and water molecules act synergistically to separate and swell the cell walls, resulting in turn in the observed large decrease of the elastic modulus and large increase of damping in poplar wood specimens.

Conclusion

This study aimed to investigate the effect of the impregnation of aqueous solutions of ethanol of variable composition on the physico-mechanical properties of veneer poplar wood, to better characterize the phenomenon of hyperswelling generated by mixed solvents. Vapour sorption gravimetry, dynamic mechanical analysis and microscopic observations were combined to evaluate the modifications of microstructure and properties of wood induced by the interactions between water-ethanol binary solutions and wood constitutive biopolymers.

These techniques allowed comparing the sorption behaviour of wood in pure ethanol with the known and confirmed sorption behaviour in water. In both cases, the solvent uptake was several orders of magnitude larger than the amount corresponding to a monolayer-multilayer adsorption at the surface of poplar wood and evidenced absorption in the core of the material. Albeit the amount of water or ethanol absorbed at saturation differed by less than a factor of two, the two different solvents had drastically different effects on the properties of wood. The sorption of water brought about an important swelling of the cell walls and a fast plasticization of the material, characterized by high loss of longitudinal stiffness and increased damping. Upon sorption of ethanol, the longitudinal stiffness of wood samples was almost unchanged, and the average thickness of the cell walls was slightly increased. Moreover, the sorption of ethanol caused disbonding and structural damage in the more lignin-rich layers, i.e. middle lamella. This effect must be correlated with a significant retention of ethanol upon desorption, strongly suggestive of chemisorption of ethanol in the polyphenolic fraction. Interestingly, the damage around the middle lamellae did not significantly affect longitudinal mechanical properties but the induced loss of cohesion between cells corresponded to a significant loss of mechanical properties in shear loading mode. The measurements of sorption from vapours usefully completed the experiments by immersion evidencing that ethanol is virtually not absorbed at low partial pressure, suggesting the presence of selective diffusional barriers between the components of cell walls.

The simultaneous sorption of water and ethanol highlighted synergistic effects. Hyperswelling was indeed observed, as immersion in ethanol aqueous solutions induced larger swelling and larger softening than immersion in pure water or pure ethanol. Sorption from mixed vapours showed that the presence of water molecules allows ethanol to be absorbed also at low partial pressure, strongly suggesting that lignin-rich regions are more easily accessible to ethanol through hydrated holocellulosic fractions. When co-absorbed with water, ethanol generates cell decohesion, and it seems likely that the release of constraints between the wood cell wall layers plays an important role in enabling hyperswelling of cell walls when mixed solvents are absorbed.

These results suggest several orientations for future research programmes. The selective interactions of solvent components with different wood biopolymers are worth to be investigated and validated by molecular dynamic simulations. The synergistic swelling mechanism by mixed solvents, implying a loss of stiffness by selective alteration of lignin-rich structures, suggests useful developments of theoretical physico-chemical models of wood swelling [START_REF] Barbetta | Impregnation and swelling of wood with salts: ion specific kinetics and thermodynamics effects[END_REF]. The mechanical study of the transitory states, a factor not to be underestimated in the evaluation of structural materials, represents a largely unexplored field when mixed solvents are concerned. It remains an open question to which extent the size of the specimens and diffusion mechanisms can affect the mechanical behaviour. As a last observation, the understanding of the mechanism of hyperswelling in mixed solvents other than water and ethanol remains terra incognita despite its technological relevance in industrial wood treatments and its service-life.

Fig. 1

 1 Fig. 1 Description of the protocol used for the dynamic mechanical characterization by DMA during the immersion of the poplar samples (here loaded in traction mode) into the tank of solvent connected to a thermostat bath maintained at 40 °C

Fig. 2

 2 Fig. 2 Isotherms of sorption of ethanol vapour on poplar veneers (red), water vapour on poplar veneers (dark blue) and water vapour on poplar powder (light blue) at 25 °C. Adsorption (solid lines) and desorption (dashed lines)

Fig. 3 Fig. 4

 34 Fig. 3 Isotherms of mixed water-ethanol solvents adsorption on poplar veneers specimens at 25 °C. Vapours at equilibrium of solutions with increasing ethanol mass fraction. Isotherms of pure water and ethanol are represented in dotted lines for comparison

Fig. 6

 6 Fig.6Comparison of the effect of sorption of pure ethanol and water (dashed lines) and mixed solvents of different concentrations (solid lines) on viscoelastic behaviour of poplar veneers specimens. Relative E′ (top left) and relative tanδ (top right) over time after immersion at t 0 . Influence of ethanol fraction on stabilized values of relative E′ and tanδ (bottom)

Fig. 7

 7 Fig. 7Micrographs of the cell wall microstructure of poplar veneer specimens after absorption and dynamic mechanical analysis in the pure solvents (×100). For each solvent, the two samples that displayed the most contrasting results during dynamical mechanical tests are presented

Fig. 8 Fig. 9

 89 Fig.8Micrographs of the cell wall microstructure of poplar veneer specimens after immersion and dynamic mechanical testing in the different mixed solvents (×100). For each solvent, the two tests of four that displayed the most contrasted viscoelastic behaviour are represented, to explore variability. Each test number is reported in the corresponding image

Table 1

 1

	GAB constants for the adsorption isotherms	C	K	X m (mg/g)
	Water on powder	11.6	0.75	56
	Water	10.2	0.72	54
	21% EtOH	7.1	0.85	67
	44% EtOH	7.4	1.07	55
	70% EtOH	8.9	1.43

Acknowledgements

We acknowledge funding from the Agence Nationale de la Recherche program Investissements d'Avenir in the framework of the contract ANR-10-LABX-05-01 (LabExCheMISyst). Poplar veneers were graciously provided by Maurice Orly, Danzer Company. The authors acknowledge the support of Surface Management Systems London and their team for DVS analyses, the technical support of Marc Longerey, Centre des Matériaux des Mines d'Alès (C2MA), for dynamic mechanical measurements, and the assistance of Olivier Arnoult, Laboratoire de Mécanique et Génie Civil (LMGC) of the Université de Montpellier, in microscopy techniques. The authors have especially appreciated the useful discussions with Thomas Zemb, Institut de Chimie Séparative de Marcoule (ICSM).