Tatiana Sadalla Collese

Augusto Cesar Ferreira De Moraes

Juan Miguel Fernandez-Alvira

Nathalie Michels

Stefaan De Henauw

Yannis Manios

Odysseas Androutsos

Anthony Kafatos

Kurt Widhalm

Myriam Galfo

Augusto César Ferreira

De Moraes

Juan Miguel Fernàndez-Alvira

Laurent Beghin

Michael Sjöström

Raquel Pedrero-Chamizo

Heràclito Barbosa Carvalho

Luis A Moreno

How do energy balance-related behaviors cluster in adolescents?

Keywords: Sedentary, Active, Unhealthy Eating, Healthy Eating, while boys differed (Brazilian: Sedentary, Active, Healthy Eating, European: Sedentary, Healthy, cluster analysis, energy balance-related behaviors, adolescents, socioeconomic status, parental education

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INTRODUCTION

WHO defines adolescents as people between 10 to 19 years of age [START_REF] Who | Core competencies in adolescent health and development for primary care providers: including a tool to assess the adolescent health and development component in pre-service education of health-care providers[END_REF]. Adolescence is a formative period in life that involves numerous physiological and psychological modifications, influencing nutritional needs and behaviors that directly affect lifelong health [START_REF] Viner | Adolescence and the social determinants of health[END_REF]). The early establishment of healthy energy balance-related behaviors (EBRB) is considered to be an important precursor of the prevention of disorders such as obesity and cardiovascular diseases [START_REF] Kavey | Determinants of fruit and vegetable consumption among children and adolescents: a review of the literature. Part II: qualitative studies[END_REF].

Nonetheless, unhealthy EBRB (e.g. sedentary behavior, unhealthy diet, and sleep deprivation) are highly prevalent in youth [START_REF] Johnson | Prevalence of risk factors for metabolic syndrome in adolescents: National Health and Nutrition Examination Survey (NHANES), 2001-2006[END_REF]. For instance, almost 50% of American adolescents showed at least one of these EBRB [START_REF] Johnson | Prevalence of risk factors for metabolic syndrome in adolescents: National Health and Nutrition Examination Survey (NHANES), 2001-2006[END_REF]. Additionally, more than 40% of adolescents in Finland, Portugal, Belgium, the United States, and Brazil exceed the screen time recommendation of a maximum of two hours per day [START_REF] Bueno | Chapter 20: Sedentary Behaviors and Obesity in Children and Adolescents[END_REF]. Furthermore, fruit and vegetable consumption among adolescents at this age is below the recommendation, regardless of country of origin or income status [START_REF] Kavey | Determinants of fruit and vegetable consumption among children and adolescents: a review of the literature. Part II: qualitative studies[END_REF].

EBRB are not isolated as they tend to cluster towards healthier or unhealthier combinations [START_REF] Leech | The clustering of diet, physical activity and sedentary behavior in children and adolescents: a review[END_REF]. For example, sedentary behavior is associated with the high consumption of energy-dense foods and sugar-sweetened beverages as well as a low fruit and vegetable intake [START_REF] Rodenburg | Clustering of diet-and activity-related parenting practices: cross-sectional findings of the INPACT study %U[END_REF].

It seems that there are possible synergistic effects of EBRB on health [START_REF] Santaliestra-Pasias | Clustering of lifestyle behaviours and relation to body composition in European children. The IDEFICS study[END_REF].

Consequently, there is increasing interest in analyzing the clustering of these behaviors to identify the populations that are at higher risk of developing chronic diseases [START_REF] Leech | The clustering of diet, physical activity and sedentary behavior in children and adolescents: a review[END_REF]). Furthermore, it has been observed that adolescents' EBRB are associated with economic and social factors within countries [START_REF] Viner | Adolescence and the social determinants of health[END_REF]). Nevertheless, there is no clear consensus regarding socioeconomic status as a determinant of these behaviors, given that such an association may be attributed to the demographic context and characteristics of the studied populations [START_REF] Seabra | Biological and socio-cultural determinants of physical activity in adolescents[END_REF]. The HELENA consortium previously published a description of EBRB clusters [START_REF] Ottevaere | Clustering patterns of physical activity, sedentary and dietary behavior among European adolescents: The HELENA study[END_REF]). However, this article disregarded gender differences and sleep duration. Sleep duration is crucial for health, and in adolescents, insufficient sleep is a risk factor of chronic diseases [START_REF] Nascimento-Ferreira | Validity and reliability of sleep time questionnaires in children and adolescents: A systematic review and meta-analysis[END_REF]. Moreover, little is known about gender differences in and the socioeconomic correlates of EBRB across different continents such as South America and Europe [START_REF] Elgar | Socioeconomic inequalities in adolescent health 2002-2010: a time-series analysis of 34 countries participating in the Health Behaviour in School-aged Children study[END_REF]. Hence, we aim to describe the clustering of several EBRB in adolescents in Brazil and Europe. Second, we aim to investigate whether these identified clusters are associated with socioeconomic status and parental education level.

METHODS

Study Design and Population

Data from the Brazilian Cardiovascular Adolescent Health (BRACAH) study were collected in 2007 in the city of Maringá, Brazil, which had approximately 330,000 inhabitants (15.6% of whom were adolescents). A total of 682 adolescents (14.0-17.5 years old; 54.25% girls) who were enrolled in public and private schools were selected using random sampling and evaluated on a broad range ofhealth-related behaviors and cardiovascular risk factors. The total sample size (n=991) and the methodology of this study were described previously [START_REF] Moraes | Factors associated with medicine use and self medication are different in adolescents Clinics[END_REF]. A formal request to conduct this survey was submitted and accepted by the boards of all participants' schools. This study was approved by the Ethics Committee on Research Involving Human Participants of the University Center of Maringá, and authorized by the Ethics Committee on Research Projects of the University of São Paulo, in accordance with Brazilian law. Written informed consent was obtained from participants' parents/guardians, and all research subjects agreed to participate in the study [START_REF] Moraes | Factors associated with medicine use and self medication are different in adolescents Clinics[END_REF].

The Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) study is a multicenter cross-sectional study based on a random sample of clusters of European adolescents (12.5-17.5 years old) who were tested on a wide range of nutrition and health-related parameters using standardized procedures [START_REF] Moreno | Design and implementation of the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study[END_REF]. The data were collected in 2006 and 2007 in ten European cities (Athens, Greece, Dortmund, Germany, Ghent, Belgium, Heraklion, Crete, Lille, France, Pecs, Hungary, Rome, Italy, Stockholm, Sweden, Vienna, Austria, and Zaragoza, Spain). A detailed description of the HELENA sampling and recruitment strategies, harmonization processes, data collection procedures, analytic strategies, and quality control activities was published elsewhere [START_REF] Moreno | Design and implementation of the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study[END_REF]). After receiving complete information about the aims and methods of the study, all parents/guardians signed an informed consent form, and the adolescents agreed to participate in the study [START_REF] Moreno | Design and implementation of the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study[END_REF]. The protocol was approved by the Human Research Review Committees of the centers involved. In HELENA, 3528 adolescents were recruited. However, only 1252 (52.72% girls) of them completed the food frequency questionnaire and provided all the information related to sedentary behavior, physical activity, sleeping duration, socioeconomic status and parental education level. Therefore, just these 1252 adolescents with complete information were included in the present analysis.

Clusters of Energy Balance-Related Behaviors (EBRB)

The EBRB were assessed using self-report questionnaires in both studies.

Individual ExposuresThe questionnaire that was employed to assess sedentary time was found to be a reliable tool for use with adolescents [START_REF] Ruiz | Reliability and validity of a screen time-based sedentary behaviour questionnaire for adolescents: The HELENA study[END_REF]. This questionnaire included items to determine the amount of time spent on habitual screen-based activities (e.g., television, the Internet, video games) in daily minutes. In both studies, the questionnaire included questions such as "During weekdays, how many hours do you usually spend watching television?" -"During weekdays, how many hours do you usually spend on computers?" -"During weekdays, how many hours do you usually spend playing video games?" The same information was collected for weekend days. The average time spent per day on these activities was calculated (separately for the week and the weekend).

Physical activity in the last week was assessed using an adapted version of the International Physical Activity Questionnaire (IPAQ-A). The IPAQ was originally developed to allow participants to self-report their physical activity and inactivity and is a valid and reliable tool for use with European adults (15-69 years) [START_REF] Craig | International physical activity questionnaire: 12-country reliability and validity[END_REF]. To adapt this questionnaire for use with adolescents, questions about physical activity "at work" were replaced by "at school", and activities in "the household domain" were shortened. To avoid over-reporting, the intensity of physical activity was adapted [START_REF] Craig | International physical activity questionnaire: 12-country reliability and validity[END_REF]. The IPAQ-A was validated with accelerometer data (an objective measure) in adolescents [START_REF] Hagstromer | Concurrent validity of a modified version of the International Physical Activity Questionnaire (IPAQ-A) in European adolescents: The HELENA Study[END_REF].

Sugar-sweetened beverage and fruit and vegetable consumption were assessed using a qualitative food frequency questionnaire (FFQ) that was adapted from the Health Behavior in School Children, which has been tested for reliability and validity [START_REF] Vereecken | A Belgian study on the reliability and relative validity of the Health Behaviour in School-Aged Children food-frequency questionnaire[END_REF]. This FFQ was administered in the classroom (under the supervision of a researcher). In the present study, we considered only the following food groups of interest: 1) Sugar-sweetened beverages; 2) Fruits; 3) Vegetables and greens. For example, one question on the FFQ asks, "How many times did you ingest fruit last week?" Therefore, research subjects were classified according to the number of servings consumed per week (Fewer than 1, 1-2, 3-4, 5-6, and 7 servings/week). No specific amount was recorded. That is, only the frequency (weekly consumption) associated with each food group was taken into account.

Habitual sleep duration was estimated using another questionnaire with items, including "During weekdays: How many hours (and minutes) do you usually sleep?" and "During weekend days: How many hours (and minutes) do you usually sleep?" The reliability of this questionnaire was assessed in the HELENA study (with a subsample of 183 adolescents), showing almost perfect agreement (0.81 and 0.96 for weekdays and weekends, respectively). In a recent meta-analysis of the validity and reliability of sleep time questionnaires in children and adolescents, the authors identified a positive effect on the correlation coefficients of sleep duration between the questionnaires and accelerometers (objective measure) [START_REF] Nascimento-Ferreira | Validity and reliability of sleep time questionnaires in children and adolescents: A systematic review and meta-analysis[END_REF].

The internal validity of all applied questionnaires were also tested in the BRACAH study (physical activity: Kappa coefficient = 0.91; sedentary behavior: Kappa coefficient = 0.91; FFQ: Kappa coefficient = 0.91; sleep time: Kappa coefficient = 0.81 and 0.96 on weekdays and weekend, respectively.

Socioeconomic Status

The Brazilian Criterion of Economic Classification was employed in the BRACAH study [START_REF] Abep | The Brazilian Criteria for Economic Classification (CCEB)[END_REF]. This questionnaire assessed parental education level and the presence and number of domestic appliances, vehicles, and rooms in the household. With this information, a specific score (range 0-46) was computed and, the family was classified into one of the following seven categories: (A1 [the wealthiest], A2, B1, B2, C, D and E [the poorest]) [START_REF] Abep | The Brazilian Criteria for Economic Classification (CCEB)[END_REF]. Finally, we grouped these categories into three socioeconomic levels: high (A1, A2, and B1), medium (B2 and C), and low (D and E).

In HELENA, the Family Affluence Scale, which measured the family structure, living conditions, and household goods, was used [START_REF] Iliescu | Socioeconomic questionnaire and clinical assessment in the HELENA Cross-Sectional Study: methodology[END_REF]. This scale has been found to be a valid indicator of socioeconomic status in adolescents [START_REF] Currie | Researching health inequalities in adolescents: the development of the Health Behaviour in School-Aged Children (HBSC) family affluence scale[END_REF]. The family was classified into one of three categories (range 0-8): A score from 0-3 indicates low socioeconomic status, 4-5 reflects medium socioeconomic status, and 6-8 indicates high socioeconomic status.

Parental Education Level

In both studies, data on mother and father education level were reported by the adolescents via a questionnaire [START_REF] Iliescu | Socioeconomic questionnaire and clinical assessment in the HELENA Cross-Sectional Study: methodology[END_REF]. Although there are some differences in the classification of education level between Brazil and Europe, it was categorized into four groups (in the two studies): elementary education, lower secondary education, higher secondary education, and tertiary education.

Potential Confounders

The potential confounders that were considered in this study were survey center (in HELENA study) and school (both studies) as a contextual factor, and these factors were used in the adjusted models to control the variability of the associations between them. The variables age (in years), the Clusters, socioeconomic status and parental education level are individual factors.

Statistical Analyses

To identify clusters of similar behaviors, a combination of hierarchical and non-hierarchical clustering analyses was used [START_REF] Fernandez-Alvira | Clustering of energy balance-related behaviors and parental education in European children: the ENERGY-project[END_REF]. Gender-specific cluster analyses were performed separately for each study due to significant differences in EBRB levels. Additionally, z-scores of all variables were calculated to standardize the dataset before clustering. This standardization prevents variables with larger ranges from contributing to cluster solutions to a greater extent than those with smaller ranges. Univariate and multivariate outliers (more than 3 standard deviations) were removed. First, we chose the Ward's method for applying hierarchical cluster analyzes based on the squared Euclidian distances [START_REF] Ward | HIERARCHICAL GROUPING TO OPTIMIZE AN OBJECTIVE FUNCTION[END_REF]. Consequently, a comparison of several possible cluster solutions was performed.

Using the resulting centers, nonhierarchical k-means cluster analyses were employed to fine-tune the preliminary hierarchical solution. ANOVA and Bonferroni post hoc tests were conducted to investigate the differences between clusters on all the variables.

The descriptive analyses are presented as the means (quantitative variables) or percentages (qualitative variables) and 95% confidence intervals (95% CI). We used Poisson multilevel regression models with fixed effects intercepts to estimate the prevalence ratios (PR), their respective confidence interval of 95% (CI 95%), and to analyze the relationship between each socioeconomic and parental education level with the clusters of EBRBs. We considered two levels of data organization: (i) clustering of EBRBs separately in each of the four groups (stratified by gender and region) and (ii) potential confounders (not shown) [START_REF] Victora | The role of conceptual frameworks in epidemiological analysis: a hierarchical approach[END_REF]). The context variable that was used was the school.

Homoscedasticity was graphically assessed in all regression models to meet the analyses' criteria.

Furthermore, p-values of <0.20 were adopted in the univariate analyses [START_REF] Victora | The role of conceptual frameworks in epidemiological analysis: a hierarchical approach[END_REF]) since they were necessary to include variables in the multivariate analyses and the hierarchical model according to the aforementioned levels. Moreover, p-values <0.05 or those representing >10% modification in the β of any variable that was already in the model were considered to be significant.

The multilevel analyses were performed with two objectives: to test the associations between socioeconomic status, parental education level, and indicators of behaviors (z-scores) and to test the extent in which country-specific characteristics and contextual variables influence the associations between socioeconomic status, parental education level and clusters of EBRB. The associations between socioeconomic status and parental education level with the clusters were estimated using Poisson multilevel regression models with mixed effects intercepts, considering the Sedentary cluster as the reference category. We also verified if there were differences between the included and excluded adolescents in both of our samples.

Stata 12.0 (Stata Corp., College Station, TX, USA) was employed for the statistical calculations.

All analyses were adjusted for the clustered nature of the sample using the "svy" set of commands and stratified by gender, as interactions between gender and the studied variables were observed (p<0.001). To avoid the effect of multiple tests, the Bonferroni correction was applied to assess the associations between the clusters of EBRB and socioeconomic status (p<0.017) and parental education level (p<0.013).

RESULTS

Characteristics of the Sample

Table 1 presents the characteristics of the participants (n=1,934) in the two cross-sectional studies.

We have found no significant difference between the included and excluded adolescents form both of our samples.

In BRACAH, adolescents had a longer period of sedentary time and shorter periods of physical activity and sleeping than adolescents in the HELENA study. However, the mean fruit and vegetable consumption was higher in BRACAH than in HELENA study.

Description of Clusters of EBRB

Figure 1 highlights the estimated clusters of EBRB. The main outcome measures are on the horizontal axis and their related standard deviation z-scores on the vertical axis. Each cluster is numbered and named at the bottom of the graphic (different line colors), with their respective prevalence in parentheses. We identified four comparable clusters of EBRB for girls (Figures 1 A,B: 1. Sedentary; 2.

Active; 3. Unhealthy Eating; 4. Healthy Eating), regardless of differences in prevalence. This wasn't the case in boys. Different clusters were found in BRACAH (Figure 1 Sedentary (Figures 1 A,B, C, D: n°1; represented by the purple line) was the only pattern identified in all four groups. It was characterized by z-scores above zero for screen time (e.g., watching TV, playing video games) and for sugar-sweetened beverage intake. Girls in both studies showed similar prevalence in the clusters. In Brazilian boys, the Sedentary cluster was dominant among the three clusters, whereas it was the least prevalent cluster in European boys.

The Active cluster (Figures 1 A,B,C: n°2; represented by the green line) was determined by zscores above 1.5 for physical activity and negative z-scores for screen time. This cluster was observed in girls in both studies and in boys in BRACAH, but it was the least prevalent cluster in those three groups.

The Healthy cluster (Figure 1 represented by the pink line) was defined by positive z-scores for fruit and vegetable consumption and negative z-scores for sugar-sweetened beverage intake. The Healthy Eating cluster was the most prevalent cluster among girls in the HELENA study.

Clusters of EBRB and Associations with Socioeconomic Status and Parental Education Level

The clusters of EBRB by socioeconomic status and parental education level in BRACAH and HELENA studies are presented in Table 2. Furthermore, the p-values for these associations are provided in the supplementary material (Table 3). The clusters that were significantly associated with either socioeconomic status or parental education are shown in Table 3 with an asterisk, and Figures 2 and3 illustrate their respective prevalence ratios (PR). In the BRACAH study, we did not identify any significant association between the clusters and socioeconomic status or parental education. However, in HELENA study, higher socioeconomic status (PR: 3.38; 95% CI: 1.58 -7.21) was associated with the Healthy Eating cluster among girls (Figure 2 A), while higher socioeconomic status (PR: 2.78; 95% CI: 1.28 -6.04) was positively associated with the Unhealthy Eating cluster in boys (Figure 2 B).

Regarding parental education, in the HELENA study, the mother's (PR: 6.98; 95% CI: 2.55 -19.09) and father's (PR: 6.48; 95% CI: 2.26 -18.62) university degree was significantly associated with the Healthy Eating cluster in girls (Figure 3A). Moreover, the mother's university degree (Figure 3B) was directly associated with the Healthy cluster (PR: 3.18;) in European boys.

DISCUSSION

We identified four equivalent EBRB clusters for girls and three different clusters for boys. One of the major findings is that the Sedentary cluster (mainly characterized by a high level of screen time and low level of physical activity) was common across studies and genders.

It is notable that, although they lived in geographically distant regions, surrounded by distinct types of physical, economic, socio-cultural, and political environments [START_REF] Delany-Moretlwe | Providing comprehensive health services for young key populations: needs, barriers and gaps[END_REF], girls in both studies had equivalent EBRB clusters (regardless of the distribution of their prevalence), and the most prevalent clusters were those that were primarily related to eating behavior (Unhealthy and Healthy).

This similarity in dietary behavior is in line with previous studies [START_REF] Ottevaere | Clustering patterns of physical activity, sedentary and dietary behavior among European adolescents: The HELENA study[END_REF][START_REF] Sabbe | A cluster-analytical approach towards physical activity and eating habits among 10-year-old children[END_REF]).

Hill [START_REF] Hill | Motivation for eating behaviour in adolescent girls: the body beautiful[END_REF] showed that girls tend to be more concerned with their appearance than boys because of the media's prominent role in setting "beauty ideals". Thus, they become more critical of their nutritional habits and pursue diets (that are not necessarily healthy) due to body image dissatisfaction (Djordjevic-Nikic and Dopsaj 2013). In boys, no such trend was analyzed. Most Brazilian boys were categorized in the Sedentary cluster, whereas most European boys were in the Healthy cluster. Regarding dietary intake, the observed clustering in the two studies diverged (Healthy Eating and Unhealthy Eating in boys from Brazil and Europe, respectively). These results among boys suggest that their behaviors may be more associated with individual-level (e.g., school and neighborhood) factors than those of girls [START_REF] Sirard | Physical activity and screen time in adolescents and their friends[END_REF].

Even though adolescents from the HELENA study had a higher prevalence of healthier clusters (girls: 35.7% Healthy Eating; boys: 42.7% Healthy), in both studies and genders, other unhealthy-related behaviors occurred, and Sedentary was the only cluster found in all of them. This finding requires attention, as studies indicate that time spent on screen-based activities is associated with high energy intake (via energy-dense snacks and sugar-sweetened beverages) and less fruit and vegetable consumption, rather than low energy expenditure [START_REF] Hobbs | Sedentary behaviour and diet across the lifespan: an updated systematic review[END_REF]. Furthermore, the Active cluster (which was mainly characterized by z-scores above 1.5 for physical activity and negative z-scores for screen time) was the least prevalent cluster in girls in both studies and in boys in the BRACAH study.

Boys in the HELENA study did not show this Active pattern. However, only they had the Healthy cluster. This infrequent cluster suggests that several lifestyle trends do not always discriminate in the same direction. Previous studies demonstrated that teenagers have difficulty meeting multiple recommended health guidelines concurrently because people often adhere to one healthy behavior to compensate for other unhealthy habits [START_REF] Ottevaere | Clustering patterns of physical activity, sedentary and dietary behavior among European adolescents: The HELENA study[END_REF].

In terms of usual sleep duration, negative z-scores primarily were related to the Unhealthy Eating cluster. A recent study noted that sleep restriction increases the time available for eating and promotes fatigue, reducing engagement in physical activity [START_REF] Thivel | Bedtime and Sleep Timing but not Sleep Duration Are Associated With Eating Habits in Primary School Children[END_REF]. In other research with adolescents, a later bedtime upregulated appetite, favoring higher energy intake (approximately 425 kcal/day) in teenagers with a low sleep duration than in those with an adequate sleep time [START_REF] Adamo | Later Bedtime is Associated with Greater Daily Energy Intake and Screen Time in Obese Adolescents Independent of Sleep Duration[END_REF].

The results pertaining to the second goal, the characterization of the clustering of EBRB with socioeconomic status and parental education level, revealed that associations were different according to the study and gender. Unexpectedly, no significant association was found in the BRACAH study, while high socioeconomic status was associated with only two clusters in the HELENA study (Healthy Eating and Unhealthy Eating in girls and boys, respectively). This association between high socioeconomic status and Unhealthy Eating cluster is controversial, however previous studies also found difficulties while associating socioeconomic status with behaviors, particularly in adolescents [START_REF] Beck | Indicadores antropométricos como preditores de pressão arterial elevada em adolescentes[END_REF][START_REF] Elgar | Socioeconomic inequalities in adolescent health 2002-2010: a time-series analysis of 34 countries participating in the Health Behaviour in School-aged Children study[END_REF][START_REF] Ferrar | Adolescent Diet and Time Use Clusters and Associations With Overweight and Obesity and Socioeconomic Position[END_REF]. This lack of an association in the BRACAH study could be due to several reasons, including that adolescents' behaviors are differently impacted by socioeconomic indicators and by peer-group influences [START_REF] Michels | Mediation of psychosocial determinants in the relation between socio-economic status and adolescents' diet quality[END_REF], the BRACAH sample had an economic distribution that was more concentrated in the medium class (approximately 80%), the methods used to estimate socioeconomic level in the two studies were distinct [START_REF] Beck | Indicadores antropométricos como preditores de pressão arterial elevada em adolescentes[END_REF], and the way that educational level is classified in Brazil is different from the European classification [START_REF] Abep | The Brazilian Criteria for Economic Classification (CCEB)[END_REF][START_REF] Iliescu | Socioeconomic questionnaire and clinical assessment in the HELENA Cross-Sectional Study: methodology[END_REF]).

In the HELENA study, both parents' university degrees were significantly associated with the Healthy Eating in girls, and the mother's university degree was associated with the Healthy cluster in boys.

These associations between higher parental education and adolescents' healthier behaviors seem to influence mediators such as parents' attitudes and beliefs, availability and accessibility, self-control, and knowledge, which are modifiable mediators that may hold promise for the development of obesity interventions [START_REF] Ferrar | Adolescent Diet and Time Use Clusters and Associations With Overweight and Obesity and Socioeconomic Position[END_REF]. We suppose that these associations were uncommon in Brazil because of the BRACAH's sample characteristics, with an important concentration of parents with upper secondary education (more than 40%). Still, it emphasizes that improved education is essential to promote health [START_REF] Gakidou | Increased educational attainment and its effect on child mortality in 175 countries between 1970 and 2009: a systematic analysis[END_REF].

Limitations

Our studies with Brazilian and European adolescents should be cautiously extrapolated to the corresponding overall populations; hence, we are taking into account geographic areas that have different peculiarities and environmental elements that may misrepresent the influence of socioeconomic status on clustering behaviors [START_REF] Beck | Indicadores antropométricos como preditores de pressão arterial elevada em adolescentes[END_REF]. In addition, it is important to notice that we have considered only the weekly consumption of two food groups (fruits and vegetables and sugar-sweetened beverages) in our analysis since they are reported by the American Heart Association as components of the Ideal Cardiovascular Health in Children and Adolescents [START_REF] Steinberger | Cardiovascular Health Promotion in Children: Challenges and Opportunities for 2020 and Beyond: A Scientific Statement From the[END_REF]. Furthermore, data were obtained by self-report questionnaires, which also have bias such as social desirability and under/overreporting of information [START_REF] Ottevaere | Clustering patterns of physical activity, sedentary and dietary behavior among European adolescents: The HELENA study[END_REF]). However, every assessment method comes with its own set of limitations, and no specific tool can be acknowledged as the best option [START_REF] Corella | Biomarkers: background, classification and guidelines for applications in nutritional epidemiology[END_REF]. Furthermore, questionnaires are frequently applied in centers with low financial resources, because they are more feasible and inexpensive than objective methods and are associated with relatively low participant burden [START_REF] Murakami | Accuracy of Wearable Devices for Estimating Total Energy Expenditure: Comparison With Metabolic Chamber and Doubly Labeled Water Method[END_REF].

Despite a few methodological differences (e.g., socioeconomic questionnaire) between studies, one strength of our research is the possibility of comparing adolescents' EBRB between Brazil and some European countries. This is an innovative study with cluster analysis and their associations with socioeconomic factors in adolescents.

Conclusions

Adolescents show Sedentary behavior, regardless of their sex, country of origin or socioeconomic condition. Girls showed equivalent energy balance-related behaviors, while boys differed, suggesting that boys' behaviors are more associated with individual-level factors than those of girls. In this sense, it is highly important that public health policies target different interventions for girls and boys.

Family socioeconomic conditions are more associated with Europeans' than Brazilians' behaviors.

However, both studies emphasize that improved education is essential to promote health and that identifying these energy-balanced behaviors may help to develop new interventions for the prevention of obesity.

LIST OF ABBREVIATIONS

• EBRB: Energy Balance-Related Behaviors

  C: 1. Sedentary; 2. Active; 3. Healthy Eating) and in HELENA (Figure 1 D: 1. Sedentary; 2. Healthy; 3. Unhealthy Eating).

  D: n°2; represented by the blue line) was exclusively identified in European boys with respect to low screen time and sugar-sweetened beverage intake and high physical activity, fruit and vegetable consumption, and sleep duration. The Unhealthy Eating cluster (Figures 1 A, B, D: n°3; represented by the red line) was characterized by positive z-scores for sugar-sweetened beverage intake and negative z-scores for fruit and vegetable consumption and sleep duration. This Unhealthy Eating behavior was the most prevalent among the girls in the BRACAH study. Inversely, the Healthy Eating cluster (Figures 1 A, B: n°4; C: n°3;

•

  BRACAH Study: Brazilian Cardiovascular Adolescent Health Study • HELENA Study: Healthy Lifestyle in Europe by Nutrition in Adolescence Study • IPAQ-A: International Physical Activity Questionnaire • FFQ: Food Frequency Questionnaire • PR: Prevalence Ratio COMPLIANCE WITH ETHICAL STANDARDS Data from the Brazilian Cardiovascular Adolescent Health (BRACAH) Study was approved by the Ethics Committee on Research Involving Human Participants of the University Center of Maringá, and authorized by the Ethics Committee on Research Projects of the University of São Paulo, in accordance with Brazilian law. The Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) Study was approved by the Human Research Review Committees of all the European centers involved. Both studies were conducted according to the guidelines laid down in the Declaration of Helsinki, and a formal request to conduct the survey was submitted and accepted by the boards of all participant schools. Written informed consent was obtained from all adolescents as well as from their parents/guardians.

Figure 1 :

 1 Figure 1: clusters solutions according to the standard deviation (SD) z-scores on energy balancerelated behaviors (EBRB) for the Brazilian Cardiovascular Adolescent Health (BRACAH) and the Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) studies. Brazil and Europe,2007.Clusters of EBRB are presented in different line colors. Clusters of EBRB are numbered, named, and with their respective prevalence (in parenthesis) at the bottom of each graphic.

Figure 2 A

 2 Figure 2 A and B: Clusters of energy balance-related behaviors (EBRB) that were significantly associated with socioeconomic status and their respective prevalence ratio. Brazilian Cardiovascular Adolescent Health (BRACAH) and the Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) studies. Brazil and Europe, 2007. Clusters of EBRB are indicated in different color boxes.

Figure 3 A

 3 Figure 3 A and B: Clusters of energy balance-related behaviors (EBRB) that were significantly associated with parental education level and their respective prevalence ratio. Brazilian Cardiovascular Adolescent Health (BRACAH) and the Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) studies. Brazil and Europe, 2007. Clusters of EBRB are indicated in different color boxes.

Table 1 .

 1 Characteristics of the samples of adolescents from the Brazilian Cardiovascular Adolescent Health (BRACAH) and the Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) studies,.Brazil and Europe, 2007.

	Variables	BRACAH GIRLS (n= 370)	BRACAH BOYS (n= 312)	HELENA GIRLS (n=660)	HELENA BOYS (n=592)
		Mean* or % (95% CI)	Mean* or % (95% CI)	Mean* or % (95% CI)	Mean* or % (95% CI)

Unhealthy Eating (33.3%) A) HELENA GIRLS B) HELENA BOYS Healthy Eating Cluster Unhealthy Eating Cluster Prevalence Ratio (95% CI) Socioeconomic Status


Sarah Christoph

Ahmed Hamraoui

Estelle Bonnin

Catherine Garnier

Thibaud Coradin

Francisco M Fernandes
email: francisco.fernandes@upmc.fr

Ice-templating beet-root pectin foams: Controlling texture, mechanics and capillary properties

Keywords:

Sugar beet pectin can be shaped into macroporous lightweight foams by freeze casting.

• Directional freezing enables enhanced liquid transport within pectin foams.

• Freeze cast pectin foams outperform previously reported low density pectin materials in compression testing.

Introduction

Macroporous materials are omnipresent in everyday life. From natural examples such as trabecular bone, balsa wood or cork [START_REF] Gibson | Cellular Materials in Nature and Medicine[END_REF] to man-made materials used in transport applications [START_REF] Badiche | Mechanical properties and non-homogeneous deformation of open-cell nickel foams: application of the mechanics of cellular solids and of porous materials[END_REF], antifrost surfaces [START_REF] Kim | Liquid-infused nanostructured surfaces with extreme anti-ice and anti-frost performance[END_REF], thermal insulation materials [START_REF] Wicklein | Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide[END_REF] or water harvesting devices [START_REF] Yang | Temperature-triggered collection and release of water from fogs by a sponge-like cotton fabric[END_REF], we are surrounded by outstanding, lightweight, functional materials. To balance functionality and apparent density, Nature has relied on highly specificoften cell-mediatedbottom-up processes to precisely shape the materials' porosity. Among these materials, a small portion, such as plant stems and tree trunks [START_REF] Gibson | Biomechanics of cellular solids[END_REF] or porcupine quills [START_REF] Karam | Biomimicking of animal quills and plant stems: natural cylindrical shells with foam cores[END_REF] displays highly anisotropic macropores. Two main reasons lie behind these aligned structures: mechanical performance [START_REF] Schulgasser | On the strength of herbaceous vascular plant stems[END_REF][START_REF] Orso | The elastic modulus of spruce wood cell wall material measured by an in situ bending technique[END_REF][START_REF] Gibson | The hierarchical structure and mechanics of plant materials[END_REF][START_REF] Karam | Elastic buckling of cylindrical shells with elastic cores-I. Analysis[END_REF] and/or fluid transport [START_REF] Gibson | Structure and mechanic of the iris leaf[END_REF][START_REF] Speck | Plant stems: functional design and mechanics[END_REF]. Several processing strategies are available to replicate such highly controlled porous structures [START_REF] Moritz | Oxide ceramics with unidirectional pore channels by electrophoretic deposition[END_REF][START_REF] Vesenjak | Mechanical and microstructural analysis of unidirectional porous (UniPore) copper[END_REF][START_REF] Nakajima | Fabrication, properties and application of porous metals with directional pores[END_REF][START_REF] Lin | Microarchitectural and mechanical characterization of oriented porous polymer scaffolds[END_REF], however few are as straightforward as freeze-casting, a technique based on oriented ice templating that has gained considerable during the last decade [START_REF] Lozinsky | Polymeric cryogels as promising materials of biotechnological interest[END_REF]. Freeze-casting relies on the use of ice crystal growth to create porosity. The method presents the advantage of being easy to implement, cost effective and applicable to a wide range of materials [START_REF] Gutiérrez | Ice-templated materials: sophisticated structures exhibiting enhanced functionalities obtained after unidirectional freezing and ice-segregation-induced self-assembly[END_REF]. In particular, since the technique relies on the use of low temperatures, it can be particularly relevant in the processing of materials prone to thermal denaturation such as biopolymers. The technique was first described in 1954 [START_REF] Maxwell | Preliminary Investigation of the Freeze-Casting Method for Forming Refractory Powders[END_REF] for the processing of refractory powders and has been used since to shape ceramics [START_REF] Deville | Freeze-casting of porous ceramics: a review of current achievements and issues[END_REF], metals [START_REF] Chino | Directionally freeze-cast titanium foam with aligned, elongated pores[END_REF][START_REF] Tong | Mechanism of lamellar spacing adjustment in directionally frozen agar gels[END_REF], clays [START_REF] Nakazawa | Texture control of clay-aerogel through the crystallization process of ice[END_REF], polymers [START_REF] Marras-Marquez | Robust and versatile pectin-based drug delivery systems[END_REF][START_REF] Tong | Formation of anisotropic ice-agar composites by directional freezing[END_REF] and composite materials [START_REF] Turco | Alginate/hydroxyapatite biocomposite for bone ingrowth: a trabecular structure with high and isotropic connectivity[END_REF][START_REF] Deville | Freezing as a path to build complex composites[END_REF].

Pectin is a polysaccharide present in plant cell wall that finds extensive application as gelling and thickening agent [START_REF] Willats | Pectin: new insights into an old polymer are starting to gel[END_REF]. The gelling properties of this heteropolysaccharide are strongly dependent on its source and extraction conditions [START_REF] Levigne | Characterisation of pectins extracted from fresh sugar beet under different conditions using an experimental design[END_REF]. Pectin from sugar beeta source yielding pectin with particularly low gelling powerdisplays fewer direct industrial application without extensive chemical modification than high gelling ability sources such as citrus peel or apple pomace [START_REF] Oosterveld | Pectic Substances from Sugar Beet Pulp: Structural Features, Enzymatic Modification, and Gel[END_REF]. Such difference in gelling power is mainly due to the presence of acetyl esters on the galacturonic acid chain [START_REF] Ralet | Polyelectrolyte behaviour and calcium binding properties of sugar beet pectins differing in their degrees of methylation and acetylation[END_REF]. As a result, sugar beet pectin remains a largely undervalued material, often considered as a byproduct of the sugar industry with few valorization alternatives beyond the production of arabinose and galacturonic acid [START_REF] May | Industrial pectins: sources, production and applications[END_REF]. Designing new processing approaches that enable the application of sugar beet pectin represents an opportunity to upcycle a largely overlooked industrial resource. Here we report how ice templating techniques can shape pectin to obtain biobased materials with adjustable porosity. We demonstrate that control over pore size and pore morphology can provide pectin-based materials with interesting mechanical and liquid transport properties. In summary, the finely controlled porous structure imposed during the ice templating unveils the possibility to apply a wide variety of water-soluble industrial sub-products for their potential application as lightweight structural elements, insulation or liquid transport devices.

Material and methods

Materials

Sugar beet pectin powder was obtained from CP Kelko and used without further purification. Degrees of methylation and acetylation were 49.00 and 20.90 respectively. The detailed composition is presented in Supplementary Information (Table S1). Pectin aqueous solutions ranging from 10 to 50 g.L -1 were prepared by dissolving the pectin powder in ultra-pure water followed by stirring under magnetic agitation for 24 h at room temperature.

Process

The macroporous pectin monoliths were shaped by growing ice crystals into aqueous pectin solutions, followed by freeze-drying in a Christ Alpha 2-4 LD freeze-dryer operating at 0.06 mbar for a 48 h period, in order to sublimate the ice crystals.

Four freezing conditions were investigated: two using laboratory freezers (-20 °C and -80 °C), one using a liquid nitrogen (LN) bath and one using a custom-made freeze-casting setup (FC) (Fig. S1). The samples frozen in the laboratory freezers (4 mL at 40 g.L -1 ) were placed in PE cylindrical molds (∅ = 19 mm) and left overnight in the freezer either at -20 °C or -80 °C before freeze-drying. Samples obtained through ice templating in LN were first poured in similar 19 mm diameter molds at room temperature and subsequently plunged for 5 min into LN before freeze-drying. Various samples were obtained by freeze-casting, using a previously reported setup [START_REF] Christoph | Cellularized cellular solids via freeze-casting[END_REF]. The device consists in a heat conductive copper rod partially immerged into LN. Temperature profiles at the top of the rod (interface between sample and copper rod) is set through a heating resistance controlled by a dedicated PID. The pectin solutions (3 mL) were placed at the top of the copper rod in 15 mm diameter cylindrical molds. The samples were then cooled down from 20 °C to -60 °C at a controlled cooling rate of 10 °C.min -1 . To ascertain the impact of the pectin solution concentration on the final material properties, samples were prepared from initial pectin solutions ranging from 20 g.L -1 to 50 g.L -1 .

Characterization

The cooling profiles for each freezing process were determined by inserting a K thermocouple at the center of 40 g.L -1 pectin solutions before freezing. The temperature profiles for the samples frozen inside the freezers (-20 °C and -80 °C) and by plunging into LN were recorded until thermalization. For the freeze-cast samples, the temperature profile was determined at the center of the pectin solution and at the interface between the copper rod and the pectin solution.

Samples obtained after freeze drying were manually cut with new scalpel blades. Scanning electron microscopy observations were performed on Hitachi S-3400N SEM. The samples were sputter coated with 20 nm of gold and observed under 3-4 kV acceleration and 30 µA probe current. SEM pictures were analyzed using the OrientationJ plugin [START_REF] Rezakhaniha | Experimental investigation of collagen waviness and orientation in the arterial adventitia using confocal laser scanning microscopy[END_REF] in ImageJ software for pore morphology characterization. The foams' pore dimensions and pore wall thickness were determined on 150 and 25 individual measurements, respectively.

Wetting behavior of the foams was assessed by capillary rise of a solution of Disperse Red at 0.2 g.L -1 in ethanol. Cylindrical pectin foams (triplicates with height between 6 and 8 mm) were placed above a Disperse Red solution and the impregnation recorded on camera at 30 fps. The resulting footage was analyzed using the "Reslice" operation to deduce ascension profiles in ImageJ software. The surface tension of Disperse Red in ethanol was measured using the Wilhelmy plate method on a Krüss K11model.

Mechanical behavior under compression was assessed using an Instron 5965 universal testing machine equipped with a 50 N load cell. Samples were cut into 1 cm 3 cubes and compressed between stainless steel plates up to 50% strain at constant displacement rate of 1 mm.min -1 . The corresponding stress-train curves were obtained for 5 replicas per sample.

Results and discussion

From freezing to freeze-casting pectin foams

Self-supporting, low density macroporous biopolymer materials were obtained by ice-templating pectin solutions. Ice templating is particularly adapted to the processing of biopolymers that are prone to thermal degradation such as pectin since the processing steps are carried out below ambient temperature. In addition, the technique does not require leaching with possible denaturating solvents to unveil the porosity as required for other porogens such as latex beads. Since most chemical species are largely insoluble in ice, an increase in concentration of the solutes occurs in the interstices formed between the growing ice crystals. When ice crystallization stops due to the lack of available water to freeze, the biopolymer located in between the ice crystals is structured into a solid macroporous network. The biopolymer foam obtained after ice sublimation therefore corresponds to the negative imprint of the formed ice crystals.

Different freezing conditions were explored to highlight the relationship between the conditions of ice formation and the final morphology of the porous monolith. Fig. 1 displays the temperature profile at the core of the samples during freezing. The first three conditions (freezers at -20 °C, -80 °C and LN bath) differ in the target temperature (-20 °C, -80 °C or -196 °C respectively) but the orientation of the temperature gradient within the samples is similar. In the case of freeze-casting however, multiple parameters differ. This setup allows controlling the freezing conditions both spatially (a specific temperature gradient is applied thanks to the marked difference in thermal conductivity between the copper rod and the surrounding sample environment) and in time (cooling rate can be tuned between 1 and 10 °C.min -1 ).

Samples frozen in the laboratory freezers display comparable cooling temperature profiles. After an initial cooling period, the temperature remains constant during the growth of the ice crystals in the whole sample. The temperature then decreases to reach the target temperature. At lower target temperature (-80 °C) the initial cooling rate is higher (3.6 °C.min -1 in the -80 °C freezer vs 1.2 °C.min -1 in the -20 °C freezer), and the ice growth time reduced (around 10 min at -80 °C vs 50 min at -20 °C). The sample frozen at -20 °C shows a small supercooling effect reaching -2.7 °C before rising back to 0 °C during the freezing step. By dipping in LN, no isothermal phase transition can be observed, but a significant delay in the initial cooling occurs. Using the freeze casting setup, the difference between the target temperature and lower boundary temperature is minimal throughout the whole freezing process. At the center of the sample (Fig. 1d) the temperature delay with respect to the cooling program reflects the unidirectional temperature gradient created with the freeze casting setup. The differences in the temperature profiles, and how these translate into different nucleation and growth kinetics of the ice crystals, can be linked to the final morphology of the porous materials.

Fig. 2 presents the micro-and macroscopic morphology of foams obtained in the different freezing conditions. For each sample a cross section and a longitudinal section at different length scales are presented. Samples obtained at -20 °C and -80 °C present similar morphologies in both orientations. In contrast, the sample prepared in LN shows a strong pore anisotropy. These observations are confirmed by orientation mapping (Fig. S2). The pores' morphologies can be related to the ice crystals nucleation and growth. As indicated in Fig. 1, the presence of moderate initial cooling rates and a temperature plateau for the samples prepared at -20 °C and -80 °C show that there is no or limited temperature gradient inside the sample. The cooling rate is slow enough for the sample temperature to remain homogeneous throughout the whole sample. As a result, the whole sample reaches transition temperature at the same time, resulting in simultaneous nucleation of ice in the whole volume. During the following ice growth phase there is limited temperature variation in all directions, resulting in non-specific ice crystal morphology, and therefore in non-aligned pores (Fig. 1e, top). When pectin solutions are immersed in LN, the temperature at the core of the sample remains stable before dropping sharply (243 °C.min -1 ). This indicates the presence of an important temperature gradient from the core of the sample to the sides. It is likely that ice crystals present in the outermost surface of the sample nucleate almost immediately when the sample is plunged into LN. Due to the radial temperature gradient, the ice crystals then grow in a radial fashion towards the center of the sample (Fig. 1e, middle), which dictates the radial orientation of the final porosity. However, this gradient which strongly depends on the mold geometry cannot be easily controlled.

In contrast, the freeze-casting configurationwhere the sample is placed in contact with a cooling element at the basis of the cylindrical moldproduces a longitudinal temperature gradient along the cylinder axis. The thermal gradient can be observed from the temperature difference at a given moment between the bottom and the center of the sample (Fig. 1d). Thus as soon as the temperature of the cooling element reaches the transition temperature, ice crystals nucleate in the bottom region of the sample and the crystals progressively grow along the temperature gradient (Fig. 1e, bottom). SEM observation of cross sections of the resulting foams reveals the presence of lamellar pores (Fig. 2g), well-aligned in longitudinal channels (Fig. 2h). The alignment of the pores was confirmed by the analysis of the pore orientation distribution, which also suggested the existence of domains of pores with similar orientation (Fig. S2).

Liquid transport properties

The pore orientation in these freeze-cast foams is reminiscent of the aligned channels observed in wood or plant stem structures. These highly anisotropic pores are responsible for the sap transport from roots to leaves. Here, we hypothesized that the oriented porosity generated by freeze casting pectin sols can be applicable for liquid transport devices. To highlight the influence of the porosity orientation on the transport properties within pectin foams, the capillary ascension of a solution of Disperse Red in ethanol was recorded in macroporous pectin monoliths obtained in a LN bath (radial structure) and by freeze-casting (axial porosity).

A circular capillary tube is the simplest model of a porous medium. In porous media, the driving forces that results from the Laplace's depression, i.e. capillary forces, are relatively large because of the small size of the pores, which are generally in competition with viscous and gravitational forces. In a more classical approach [START_REF] Washburn | The dynamics of capillary flow[END_REF], the height of a liquid in a vertical capillary tube of radius is obtained by solving the differential equation obtained by writing the equilibrium between these forces at any time t. If the porous medium is idealized as a single equivalent vertical capillary completely wetted by a given liquid, the height of advance of the meniscus h(t) is then given by the modified Washburn equation [START_REF] Hamraoui | Analytical approach for the Lucas-Washburn equation[END_REF] (Eq. ( 1)).

= -+ + + + h t η rξ r ξ η rξh ηh rσ t ( ) 1 4 [ 8 ( (0) 2 (0) ) ] 2 2 2 (1)
Where η (η = 1.17⋅10 -3 Pa.s) and σ (σ = 22.9 mN/m) are, respec- tively, the dynamic viscosity and surface tension of the liquid [START_REF] Hamraoui | Analytical approach for the Lucas-Washburn equation[END_REF][START_REF] Hamraoui | Can a dynamic contact angle be understood in terms of a friction coefficient?[END_REF]. The coefficient of friction ξ is defined as the three phase line friction coefficient that describes the interactions between the solid walls and the liquid molecules [START_REF] Hamraoui | Analytical approach for the Lucas-Washburn equation[END_REF]. By this means, we take into account the chemical nature of the substrate via the interactions at the three phase line, in terms of friction coefficient. The observed contact angle at the pectin-ethanol interface is equal to zero and the initial condition h (t = 0) = h(0) = 0.

The experimental data, for the rise of ethanol in the freeze cast sample as a function of time, together with the best fit to the modified Washburn equation (Eq. ( 1)) are presented in Fig. 3 (upper graph). A good agreement between theory and experimental data is found, with ξ = 0.045 Pa.s and the radius of the cylindrical equivalent capillary r = 1.935 µm. The capillary rise of the same solution for the sample obtained by immersion into LN deviates substantially from the best fit of the previous model. The r parameter can be considered also as characteristic size of the porous media. The ξ parameter is the expres- sion of the dynamical character of the contact angle between the liquid and the wall of the pore. The lower graph in Fig. 3 represents both the experimental and theoretical results for the foam obtained under LN freezing. The difference between the experimental and theoretical curves is most likely due to the change in dissipation regime. In the selected model, the main dissipation channel is due to the viscosity and the friction at the three-phase line (i.e. neglecting the gravitational dissipation). The orientation of the pores is the main source of the slowing down in this experiment. As a result, it seems that the axial porosity should be favored for the conception of devices requiring high liquid mass transport capacity.

Mechanical properties

Beside the interest in terms liquid transport of such channel-like porosity, the oriented morphology obtained by freeze casting also confers an advantage from a mechanical point of view. The mechanical properties of the different materials presented earlier were assessed under compression. Two orthogonal compression directions were used on 1 × 1 × 1 cm 3 samples. When applicable, these directions were specifically chosen along or orthogonally to the pores.

Table 1 shows the stress/strain behavior for materials with various pore organizations (Stress/strain curves can be found in Fig. S3). Compared to the other materials, the sample prepared by freeze-casting displayed a highly anisotropic behavior. When compressed in the direction of the channels, this material has a Young's Modulus of 2.8 MPa, which is typical for polymer foams of comparable apparent density [START_REF] Ashby | On the engineering properties of materials[END_REF]. However under compression orthogonal to the pores' orientation, the Young's Modulus drops to 0.13 MPa. This strong mechanical anisotropy is the direct consequence of the structural anisotropy described earlier. Macroporous materials with no specific orientation (samples obtained in freezers) have no or little mechanical anisotropy. The Young's Moduli are in the same order of magnitude in both directions (between 1.1 and 1.5 MPa for the -20 °C freezing vs 0.3 MPa for the -80 °C freezing). It is however important to note that the value of the Young's modulus differs greatly between these two samples. This might be attributed to the slightly elongated shape and more lamellar structure of the material prepared at -80 °C. The radial structure (samples obtained in liquid nitrogen), despite its specific pore orientation, displays little or no mechanical anisotropy. This may be due to the fact that the compression tests are not performed directly along the pores direction but rather along the diameter of the cylindrical mold. As a consequence, the measured Young's Modulus corresponds in fact to an average effect dictated by the distribution of the pore directions. These results can be interpreted according to simple composite models describing the elastic behavior of matrices (or continuous phase) reinforced with an anisotropic reinforcing phase. In porous materials the specificity is that the reinforcing phase can be modeled as an inclusion with an elastic modulus equal to zero. According to the relative orientation between the materials' pores and the compression axis the resulting material elastic properties can be describes according to two extreme models: the rule of mixtures (ROM) and the inverse rule of mixtures (IROM). If the inclusions (pores) are aligned along the compression axis, then the mechanical behavior can be roughly described by the ROM and its modulus will be maximal. On the contrary, if the pores are oriented perpendicular to the compression direction then the material compressive modulus will be roughly described by the IROM, where the elastic modulus depends strongly on the lowest modulus among the matrix/inclusion couple. These results prove that a precise control over the freezing conditions (i.e. the cooling profiles) confers control over the morphology of materials obtained by ice-templating, which has a direct influence on macroscopic properties such as the mechanical behavior. As in most of higher plants, where the xylem is oriented along the main mechanical solicitation direction, the morphology of freeze cast pectin samples displays a co-alignment between the most efficient liquid transport channels and the orientation exhibiting the highest mechanical resistance to compression.

Influence of pectin concentration

While freezing conditions allows for a precise control over the morphology, liquid transport behavior and mechanical properties of pectin foams, other parameters, such as the initial pectin concentrations may also impact the final porous structure. Foams prepared from aqueous pectin solutions ranging from 20 to 50 g.L -1 all display similar, well-aligned pores (Fig. 4 b, d, f andh). The appearance of longitudinal pores is mostly controlled by the thermal gradient developed throughout the sample during the freezing step, which is in all likelihood is barely affected by the pectin concentration. However, cross section SEM images (Fig. 4a,c, e and g) display notable differences depending on the pectin concentration.

The impact of pectin solution concentration on ice growth that can be evaluated from the size, morphology and organization of the pores (Figs. 5 andS4). Pectin foams obtained by freeze casting a solution prepared at 20 g.L -1 display a relatively large distribution of pores with little anisotropy in its cross-section. The average pore width (wall-towall distance) is centered at 15 µm and the cross-section length of the pores averages at ca. 100 µm. When pectin foams are prepared from solutions with increasing concentrations (up to 50 g.L -1 ), the pore width size decreases to 8.5 µm and their distribution becomes narrower (between 20 g.L -1 and 50 g.L -1 the standard deviation decreases from 7 µm to 2.6 µm, respectively). Inversely, the average pore length increases up to 245 µm. These morphological features correlate with the formation of thicker pore walls, from 39 nm to 208 nm at 20 g.L -1 and 50 g.L -1 , respectively. Overall, the increase in pectin concentration favors the formation of pores with smaller size distribution, higher anisotropy (Fig. 5d) and more effective packing of pores in ordered domains (Fig. S4). These consequences are coherent with the higher viscosity of the pectin solutions and how it may affect the growth of ice crystals.

Tuning the initial biopolymer concentration translates into variations of the final density of the materials and therefore provides a simple way to tailor their mechanical properties (Fig. 6 and Table S2). Young's Modulus measured for sugar beet pectin freeze cast foams are between 1 and 5 MPa, which are typical for polymer foams with density ranging from 20 kg.m -3 to 50 kg.m -3 [START_REF] Ashby | Material property charts[END_REF]. The stress-strain profiles show typical behavior for polymer foams, with an initial elastic behavior (up to about 7% strain), followed by a plastic deformation plateau (between 7 and 40% strain). The material then undergoes a densification, which translates in a strain increase beyond 40% deformation. Foams with different densities follow the same general profile, however the yield strength changes significantly (variation between 20 and 186 kPa), which can be linked to the changes in pore walls thickness previously mentioned.

Compared to previously reported pectin-based materials [START_REF] Chen | Biodegradable pectin/clay aerogels[END_REF][START_REF] Rudaz | Aeropectin: fully biomass-based mechanically strong and thermal superinsulating aerogel[END_REF], freeze cast sugar beet pectin is particularly efficient in terms of the specific moduli, E s , (i.e. the elastic modulus divided by the materials' apparent density) at bulk density below 50 kg.m -3 , ranging up to 107 kPa.m 3 .kg -1 . In Fig. 7, sugar beet pectin compressive modulus measured along the pores was plotted side by side with the literature Fig. 5. Influence of pectin initial concentration on the pore morphology in freeze cast foams. The boxplots a) to c) represent the different dimension of the macroporous features. The individual data points (black dots) used for each measurement along with their best fit to a gamma function (light grey line) are displayed next to the corresponding boxplots. Moustaches delimit the 5 and 95 percentile of the distribution and box limits represent the 1st and 3rd quartiles. The dependence between the average pore section anisotropy and the pectin concentration is plotted in d). values for other pectin-based foams (Fig. 7). From the plot, freeze cast sugar beet pectin foams' modulus are similar to citrus-based aeropectin but with significantly lower bulk densities. When compared with freezecast citrus pectin, sugar beet foams can display compressive moduli over one order of magnitude larger at similar bulk densities. Such an increased efficiency may arise from the higher aspect ratio and more controlled orientation of the porosity of the freeze cast sugar beet foams as compared with the radial orientation of the pores described by Rudaz [START_REF] Rudaz | Aeropectin: fully biomass-based mechanically strong and thermal superinsulating aerogel[END_REF] or the low anisotropy pores generated by the gelation/coagulation method [START_REF] Chen | Biodegradable pectin/clay aerogels[END_REF].

Conclusions

Freeze casting of sugar beet pectin solutions provides an easy and efficient way to prepare lightweight materials with high specific moduli and efficient liquid transport properties. The foams' properties are highly dependent on the processing parameters that allow for a precise control of the porous materials morphology. Other parameters, including mold geometry or presence of additive could be investigated to further widen the range of possible porous characteristics. Multi-step processing methods may also be devised to optimize the macroscopic properties of these materials. In a more general context, the design of all-polysaccharide lightweight materials has so far been mainly focused on the development of composite materials combining reinforcing moieties such as cellulose and its derivatives dispersed in other polysaccharide matrices. Here we show that structuring of a single polysaccharide material by freeze casting is possible and enables the upcycling of sugar beet pectin, a particularly undervalued polysaccharide. Extension to other water-soluble polysaccharides may prove instrumental in designing new materials out of undervalued resources. [START_REF] Chen | Biodegradable pectin/clay aerogels[END_REF]; green squares: supercriticaldried aerogels [START_REF] Rudaz | Aeropectin: fully biomass-based mechanically strong and thermal superinsulating aerogel[END_REF]). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 1 .

 1 Fig. 1. Temperature profiles during freezing of 40 g.L -1 pectin solutions using different ice templating conditions, a) -20 °C freezer, b) -80 °C freezer, c) liquid nitrogen (LN, -196 °C), d) freeze casting at 10 °C.min -1 (-60 °C). Full red line is the temperature at the core of the sample, full pale pink line is the temperature at the interface between the copper rod and the sample (for the freeze casting setup) and dashed line represents the target temperature. d) Schematic representation of ice growth for the different freezing processes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2 .

 2 Fig. 2. Morphology of pectin foams obtained under different freezing conditions applied on 40 g.L -1 pectin solution. SEM images of foams observed along two different planes (a-h). Insets display the macroscopic aspect of the foams obtained under the same conditions and observed along the same orientation (a′-h′). Sample preparation was conducted at -20 °C freezer (a, b), -80 °C freezer (c, d), LN, (e, f) and 10 °C.min -1 freeze-casting (g, h).

Fig. 3 .

 3 Fig. 3. Wetting behavior of freeze-cast samples (upper graph) and liquid nitrogen (LN) frozen foams (lower graph) by Disperse Red solution in ethanol. The grey circles are extracted from the analysis of capillary rise from video footage and the red circles correspond to the best fit to the modified Washburn model. (Fit values for the bottom graph, radial LN sample are: apparent radius r = 0.14 µm and ξ = 0.05 Pa.s). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .

 4 Fig. 4. SEM of sugar beet pectin prepared at different initial concentrations, scale bar is 500 µm. a) and b) 20 g.L -1 ; c) and d) 30 g.L -1 ; e) and f) 40 g.L -1 ; g) and h) 50 g.L -1 . Left hand-side images are taken along the ice growth direction, right-hand side images were taken perpendicular to the ice growth direction.

Fig. 6 .

 6 Fig. 6. Influence of pectin initial concentration on the mechanical properties of freeze cast foams. The Young's modulus (upper left panel) and yield stress (lower left panel) were inferred from the stress-train diagram obtained under compression of 1 × 1 × 1 cm 3 foams along the pores' longitudinal axis.

Fig. 7 .

 7 Fig. 7. Ashby plot of sugar beet pectin prepared by freeze casting in this work (black circles) and literature values for other lightweight materials prepared from citrus pectin (red triangles: freeze dried[START_REF] Chen | Biodegradable pectin/clay aerogels[END_REF]; green squares: supercriticaldried aerogels[START_REF] Rudaz | Aeropectin: fully biomass-based mechanically strong and thermal superinsulating aerogel[END_REF]). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1

 1 Mechanical properties and their standard deviation (sd) for the different foam morphologies and testing orientation.

		Freezing Setup	Freezing rate/°C.min -1	E/MPa	sd/MPa	σ Y /kPa	sd/kPa	Anistropy ratio
	Compression perpendicular to the pores	-20 °C (freezer)	1.2	1.1	0.5	65	17	40.7
		-80 °C (freezer)	3.6	0.3	0.1	NA	NA	19.4
		-196 °C (LN)	243.3	0.49	0.08	NA	NA	21.0
		FC at 10 °C.min -1	7.2	0.13	0.03	NA	NA	95.2
	Compression along the pores	-20 °C (freezer)	1.2	1.5	0.5	67	34	40.7
		-80 °C (freezer)	3.6	0.34	0.15	NA	NA	19,4
		-196 °C (LN)	243.3	0.62	0.78	NA	NA	21.0
		FC at 10 °C.min -1	7.2	2.61	0.65	92	2	95.2

Acknowledgements

This work was supported by French state funds managed by the ANR within the Investissements d'Avenir program under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE led by Sorbonne Universités. M. Turmine is warmly acknowledged for the measurement of the liquid surface tension.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2018.05.160.


Hugo Voisin

Carole Aimé

Anne Vallée

Thibaud Coradin

Cécile Roux
email: cecile.roux@upmc.fr

A flexible polymer-nanoparticle hybrid material containing triazole-based Fe(II) with spin crossover properties for magneto-optical applications †

In the perspective of elaborating functional devices based on inorganic molecular materials, it is important not only to continue research on their synthesis and to understand their properties but also to develop new strategies for their processing and shaping. In this context, we show here that centimeter-scale materials exhibiting solid state-like spin-crossover (SCO) properties and easy handling under ambient conditions can be obtained that preserve their thermochromic properties over months. Combining a triazole-based Fe(II) SCO compound with functionalized silica nanoparticles gives rise to nanocomposite soft gels. These gels were used as templates for the formation of a flexible macroporous hybrid silica network via a vapor phase sol-gel process. Further coating with PDMS provided both long-term protection of the material against drying and operationality in aqueous media. Our approach offers new hopes for the development of optical and magnetic devices based on SCO coordination polymers. † Electronic supplementary information (ESI) available: Detailed experimentals for Step 1, DSC and magnetic behaviors of the Si-SCO composite and the SCO system, DSC characterization of Si@Si-SCO aged for 28 days, XRPD of the Si-SCO composite system and of its compounds, XRPD of the Si-SCO and Si@Si-SCO systems and substraction of the amorphous background.

Introduction

The spin-crossover (SCO) phenomenon found in coordination compounds originates from the existence of metal-ligand interactions of intermediate strength, allowing for a stimuliinduced reversible switch from the high spin state (HS) to the low spin (LS), associated with profound modifications of their magnetic and optical properties making them very attractive for sensors, data storage or magneto-optical displays. [START_REF]Spin Crossover in Transition Metal Compounds I-III[END_REF][START_REF] Guionneau | [END_REF][3] Among SCO materials, the one-dimensional triazole-based Fe(II) family with the general formula [Fe(R-Trz) 3 ](A) 2/x (with R-Trz = 1,2,4-triazole ligand functionalized in the 4-position by an R group and A x-= counter-anion) is quite unique as it shows an abrupt transition, and is sometimes associated with wide thermal hysteresis around room temperature. [4][5][6] This is indicative of a high degree of cooperativity resulting from the bridging of triazole ligands. In the solid state, SCO characteristics (transition temperature and hysteresis width) depend on structural (intermolecular interactions and polymorphism) and chemical factors (ligands, counter-anions, and solvents) allowing one to obtain a broad diversity of compounds. In particular, the water molecule content can induce drastic changes in the SCO properties. [7][8][9][10][11][12] Key difficulties in converting these materials into functional devices are the low solubility of SCO compounds and the preservation of the SCO cooperativity, in particular in water, which is necessary for a broad range of applications. This explains why the diversity of SCO molecular structures is lost in SCO devices.

Over the past two decades, SCO materials were prepared as thin films, [13][14][15][16][17] liquid crystals, 18,19 and nanostructured objects. [20][21][22][23][24][25][26] Supramolecular gels have also been successfully obtained based on the alkyl functionalization of triazole ligands and/or counter-anions, [27][28][29][30][31][32][33] with few examples using hydrophilic ligands 34,35 or a polyol synthetic route. 36 The latter approach may be of particular interest as it should extend the possibility of obtaining gels from a broad diversity of SCO systems reported in the literature. In particular, we have recently used this polyol route combined with a hybrid approach. We have reported the synthesis of nanostructured hybrid gels (Si-SCO) that combine silica nanoparticles (NPs) with amorphous and microcrystalline SCO polymers [Fe(A-Trz) 3 ]SO 4 (with A-Trz = 4-amino-1,2,4-triazole). We have shown that the presence of NPs and the subtle balance between the three interacting components is a unique way to control the SCO properties of the gel. 36 In this case, transition temperatures (heating T ↗ and cooling T ↘ ) have been shown to increase with increasing SiO 2 content (Fig. S1 and S2 †).

However, the integration of switchable molecular systems as active elements into 3D centimeter-scale devices remains a challenge. 37 This is due to the fact that despite the purely molecular origin of the SCO phenomenon, the macroscopic behavior of these systems is strongly influenced by short-and long-range interactions between the transition-metal ions, giving rise to remarkable cooperative phenomena, which are in turn responsible for the robustness of the spin transition. [38][39][40] The engineering of composite materials is a promising way to address this ultimate goal. In this frame, SCO compounds have been encapsulated within polymer matrices of synthetic [41][42][43][44][45][46][47][48][49] and biological 38,[50][51][52][53] origin, providing nanoscale confinement suitable for maintaining cooperativity between SCO complexes. Sol-gel techniques were also used for encapsulation. These soft chemistry routes offer a unique versatility and processability for the engineering of hybrid materials. 54 Silica porous matrices have been preformed before the addition of SCO compounds that impose an important structural constraint and limit the loading and homogeneity of the composite material. 55 Nano-composite matrices have also been formed in situ by mixing alkoxysilanes with an SCO nanoparticle suspension leading to brittle materials with important changes of the spin transition properties. 56 We propose here an alternative strategy where the Si-SCO gel acts as a template for the growth of a silica coating by a post-diffusion route. This strategy is particularly well-suited for fragile objects. 57 In this work, we demonstrate that bulk materials exhibiting solid state-like SCO properties and amenable to easy handling and use in aqueous media can be prepared using a combination of liquid and vapor-phase sol-gel chemistry and a final PolyDiMethylSiloxane (PDMS) coating step.

Experimental

Synthetic procedures

Step 1. Synthesis of the Si-SCO composite Silica NPs functionalized with sulfonate groups. Silica NPs were synthesized following the Stöber process. Sulfonate functionalization of the surface proceeded through a previously described two-step procedure. 36 Zeta potential and size distribution are systematically measured using a Malvern Zetasizer Nano ZS90 instrument giving a negative zeta potential of ca. -53 mV in 0.5 M acetic acid ( pH 2.5) and a hydrodynamic diameter of ca. 200 nm.

Preparation of the Si-SCO composite. As previously described, 36 140 mg of silica NPs (200 nm in diameter) were suspended in 0.75 mL of an ethylene glycol : water mixture (v/v 9 : 1) under sonication for 45 minutes. 27.8 mg of iron sulfate heptahydrate (FeSO 4 •7H 2 O, 0.1 mmol) (Sigma-Aldrich, 99%) were added to the suspension and heated at 50 °C for 30 minutes with 20 mg of ascorbic acid. In parallel, 50.4 mg (0.6 mmol) of 4-amino-1,2,4 triazole (ATrz, C 2 H 4 N 4 , Alfa-Aesar, 99%) was separately dissolved in 0.25 mL of the same ethylene glycol : water mixture at RT. After completion of the dissolution of the iron salt, the ATrz solution was placed in an ice bath and the NP-FeSO 4 solution was added dropwise under stirring. The final content of Fe 2+ ions was 0.1 mol L -1 (0.42% w/w). This corresponds to a proportion of silica (SiO 2 ) to SCO polymer of 23.

Step 2. Si@Si-SCO: exposition to alkoxysilane vapors. Immediately after mixing the iron salt and the aminotriazole solutions, the resulting viscous suspension was casted in a PolyTetraFluoroEthylene (PTFE) mold of dimension typically 50 × 25 × 2 mm before being left to age in a closed chamber at RT for 9 days (Fig. 1b, Step 1). The mold with the Si-SCO composite was then introduced in a desiccator of which the bottom was filled with an organosilane. The closed desiccator was then placed in an oven at 25 °C (Fig. 1b, Step 2). The molds filled with Si-SCO composites were left open to silane vapors in the desiccator for a variable duration.

Step 3. Si@Si-SCO: aging. Once the exposure time was reached, the molds were taken out of the desiccator to be stored in a closed chamber at RT (Fig. 1b, Step 3). Their macroscopic aspect was monitored during their aging.

Step 4. PDMS@Si@Si-SCO: coating. PDMS coating of the Si@Si-SCO composites (Fig. 1b, Step 4) was carried out using commercial Bluesil RTV 141 (Bluestar Silicones). A PTFE mold was realized with slightly bigger dimensions than the piece of the material to be coated. A thin layer (1 mm) of the mixture of the constituents A and B of the silicone was laid down on the bottom of the mold before being degassed under vacuum. The layer was reticulated at 70 °C for 30 minutes. The piece of Si@Si-SCO material was then put on the hardened layer before being covered by a mixture of the constituents A and B of the silicone. The system was then degassed, before being reticulated at 70 °C for 30 minutes. The PDMS coated material could then be taken out for further characterization.

Crystallisation of the polymer in ethanol

The silica-free SCO polymer was prepared in an ethylene glycol : water mixture (v/v 9 : 1) as previously described. 36 To this suspension, 10 mL ethanol was added to induce the formation of a precipitate. The mixture was centrifuged, washed 3 times with 10 mL ethanol and the precipitate was recovered before drying under ambient air.

Differential scanning calorimetry

DSC measurements were performed on a TA Instruments model Q20 system. Aluminum capsules were loaded with 20-50 mg of Si-SCO and Si@Si-SCO samples and sealed. In the case of PDMS@Si@Si-SCO, the coating was directly performed within the capsule before sealing. The set-up and enthalpies were calibrated with respect to temperature using an indium reference (430 K, 3.3 J mol -1 ). Cooling and heating sweeps were performed at a scanning rate of 5 K min -1 under a N 2 (g) atmosphere. The transition temperatures were derived by considering the maximum of the thermal anomalies.

X-ray powder diffraction

XRPD measurements were carried out using the Cu Kα1 radiation on a Philips PW 1830 diffractometer. The samples were deposited on an Si monocrystal in order to have a flat background.

Dynamic mechanical analysis

The analysis was conducted on an MCR 302 Anton-Paar rheometer equipped with a solid rectangular fixture, SRF12. After diffusion of the silane vapour inside the composite gel and aging of the material, it was taken out of the PTFE cast and cut in pieces of dimensions typically 40 × 10 × 2 mm. The strips were then placed in between the instrument grips before being subjected to a strain sweep from 0.1% to 100% at a constant excitation frequency of 0.1 Hz. All experiments and their corresponding measurements were conducted in triplicate.

Electron microscopy

Scanning electron microscopy (SEM) imaging was performed using a Variable Pressure Hitachi S-3400N microscope after depositing a thin layer of the sample on aluminium pads without any further metallization.

Results and discussion

As previously described, 36 the Si-SCO system consists of a tripartite gel in ethylene glycol/water (v/v 90/10) with sulfonatemodified silica NPs and amorphous and microcrystalline SCO polymers [Fe(A-Trz) 3 ]SO 4 (with A-Trz = 4-amino-1,2,4-triazole) (Fig. 1a). Very importantly the subtle balance between these three interacting components allows reaching similar SCO properties as those of the [Fe(A-Trz) 3 ]SO 4 coordination polymer in the solid state (T ↗ = 328 K; T ↘ = 312 K, Fig. 4c and S1 †). While the spin transition finds its origin in the crystalline phase, it also induces a spin change in the amorphous phase favoured by the presence of silica NPs (Fig. S2 †).

The Si-SCO composite gels can be casted into PTFE molds before gelation. The gel then evolves significantly over 9 days at 25 °C with an increasing amount of the crystalline phase, hence of the SCO properties. The purple color is characteristic of the low spin state of the SCO polymer (Fig. 1a andb, Step 1). From this maturation state, the composite SCO gel exhibits a very good stability over months for both mechanical and SCO properties, although with relatively poor rheological properties (storage modulus G′ = 8-10 kPa). This high stability makes our mature SCO gel a very robust template for the postdiffusion of alkoxysilanes. Exposing samples to a silica precursor in the vapor phase to induce the sol-gel reaction is a technique commonly used to deposit silica thin films in situ at the surface of fragile samples without destroying their structure. 58 The Si-SCO gel was placed into a desiccator saturated with organosilane vapors (Fig. 1b, Step 2). Variations in the nature of the precursor have shown that for all organosilanes, solvent expulsion is systematically observed due to silica condensation. Organosilanes bearing a chemically reactive organic function, such as amino-or glycidopropyltriethoxysilane, oxidized the iron(II) ions thus destroying the coordination polymer (Fig. 2a). As a result, we observe the loss of thermochromism (low spin purple/high spin white). In contrast, the use of tetraalkoxysilanes, such as tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS), resulted in a silica matrix that was found to be too rigid to accommodate with the condensation-induced stress, leading to brittle materials. Alkyltrialkoxysilanes were thus used, with methyltriethoxysilane (MTEOS) leading to materials with optimal mechanical behavior. It is important to note that the presence of silica NPs is necessary to obtain a homogeneous material. In the absence of silica NPs, phase separation was observed during exposition to MTEOS vapors. Exposure time also had a strong influence on the material. In particular, short expositions (less than 3 days) led to inhomogeneous fragile gels that become oxidized with time after 25 days (Fig. 2b). Beyond 8 days of reaction, inhomogeneous gels were also obtained but without oxidation. In this case, important solvent expulsion from the material was observed together with its shrinkage, which can be attributed to an excessive condensation of the silica network.

In contrast, homogeneous gels can be obtained for intermediate exposure times (between 5 and 7 days) exhibiting a purple color and the thermochromism characteristic of the SCO phenomenon. After 5 days exposure, the increasing viscosity upon aging leads to pasty gels. This can be improved by increasing the exposure time to 7 days leading to centimeterscale flexible and easy to handle samples (Si@Si-SCO, Fig. 3a). Material flexibility at the macroscopic scale is reached after 15 days of aging. At this point, the gel state is confirmed by dynamic mechanical analysis with a storage modulus of ca. 1 MPa, which is higher than the loss modulus of 0.4 MPa (maximum deformation of 15%, Fig. 3b). This is three orders of magnitude higher than our previously reported hybrid system before exposure, 36 and more than four orders of magnitude higher to what is commonly reported for pure SCO gels. 28 The XRPD analysis of Si@Si-SCO shows two broad peaks at 2θ = 11 and 23°that are characteristic of the amorphous polymer and silica (Fig. S4 †). In addition, well-defined diffraction peaks corresponding to the solid SCO polymer alone are observed for Si@Si-SCO, which were also preserved in the Si-SCO nanocomposite (Fig. 4a and Fig. S5,S6 †). This reveals that the crystalline structure of the coordination polymer was not affected by the formation of the silica matrix. Characterization of the SCO properties of the Si@Si-SCO material was performed using the calorimetric method (DSC). Upon heating, a broad endothermic peak was observed at T ↗ = 333 K associated with the LS/HS transition. A broad exothermic peak at T ↘ = 316 K upon cooling is observed for the HS/LS transition (Fig. 4b). This is characteristic of a first order transition with a hysteresis width of 17 K. These values are similar to those of the Si-SCO system (Fig. 4c). The small increase in temperature observed for Si@Si-SCO can be attributed to the limited reorganization of the amorphous phase in the tri-partite system. Given the solvent evaporation, the iron concentration is estimated to be 0.5% w/w. The corresponding ΔH for the LS/HS transition can be evaluated to be 9.1 kJ mol -1 . This value is slightly lower than that of the integrated ΔH for the mature Si-SCO gel before alkoxysilane exposure.

SEM observations showed the presence of needle-shaped crystalline objects (Fig. 5a, white circles) and spherical nanoparticles (Fig. 5a, yellow arrows), reminiscent of the Si-SCO system (Fig. 5c), located within the cavities of a macroporous silica-based matrix. This model is schematically depicted in Fig. 5b. This supports the hypothesis that silane condensation occurs in the ethylene glycol solvent phase, around Si-SCO composite domains. The similarity of these results with those of the Si-SCO system shows that (1) the sample microstructure and thus the SCO amorphous and crystalline organizations are not significantly affected by the sol-gel reaction, and (2) the macroscopic flexibility of the silica matrix preserves the molecular elastic interactions between the iron sites.

However, we noticed that the material evolved during the days following its preparation, and its pink color faded after 28 days (Fig. 6a). In addition, the material became brittle after 50 days due to solvent expulsion (Fig. 6b). The color change could be attributed to the decrease in the amount of LS species. However, DSC measurements performed on Si-SCO and Si@Si-SCO after 28 days show similar profiles (Fig. S3 †). This shows that the crystalline phase is not affected and rather suggests a reorganization of the amorphous phase upon solvent expulsion. In addition, the presence of the silica matrix may contribute to the whitening of the final material and the loss of color contrast between the two spin states. To limit solvent evaporation, the Si@Si-SCO material was coated after 14 days of aging with a thin layer of PDMS reticulated at 70 °C for 30 min. The coated piece exhibited far better resilience against aging 80 days after preparation when compared to the uncoated sample (Fig. 6c). This coating also allowed plunging the sample into hot water without decomposition, highlighting the pronounced thermochromism of the material (Fig. 6d). The thermochromism phenomenon is associated with an LS/HS transition that occurs at T ↗ = 331 K associated with a ΔH of 7.5 kJ mol -1 and at T ↘ = 316 K as confirmed by DSC (Fig. 7a). Moreover the material displayed excellent preservation of its SCO behavior over 15 temperature cycles. From the second to the fifteenth cycle, T ↗ decreases from 327.95 to 327.35 K with a variation of 0.18%. In parallel, T ↘ increases from 311.33 to 312.05 K with a variation of 0.23% (Fig. 7b andc). The ΔH associated with heating is considered constant at a value of 10 kJ mol -1 . This value is much closer to that of the fresh Si@Si-SCO gel than to that of the aged one (9.1 vs. 7.5 kJ mol -1 , Fig. 4c) showing the good preservation of the Si@Si-SCO properties.

Conclusions

In conclusion, we show that hybrid materials consisting of organosilica-reinforced nanocomposite gels can exhibit SCO properties similar to their parent coordination polymer in the solid state. Moreover, when properly protected by a hydrophobic coating, these materials display a unique stability over time and cycling, under ambient conditions. Our approach is therefore very promising to achieve fully functional devices based on SCO properties. Further improvements in terms of the SCO compound content of the material, as well as transition temperature and hysteresis width are now required, taking advantage of both the versatility of our process and the wide variety of SCO systems described in the literature.

Fig. 1

 1 Fig. 1 (a) Structure of the SCO polymer [Fe(A-Trz) 3 ]SO 4 , and schematic representation and photograph of Si-SCO. (b) Scheme of the processing steps from Si-SCO (Step 1) to PDMS@Si@Si-SCO (Step 4).

Fig. 2

 2 Fig.2(a) Table describing the characteristics (thermochromism and macroscopic aspect) of Si@Si-SCO samples obtained after exposition at 25 °C of 9 days mature Si-SCO gel. Green and red check marks correspond to an observable thermochromism ( purple-white color change as a function of the temperature) or not, respectively. (b) Graphical representation of the macroscopic aspect of Si@Si-SCO samples as a function of exposure time to MTEOS and aging at 25 °C of the 9 days mature Si-SCO gel. IO-Gel = inhomogeneous oxidized gel; H-Gel = homogeneous gel; I-Gel = inhomogeneous gel. The star mark corresponds to the optimized parameters for the obtention of thermochromic flexible materials.

Fig. 3

 3 Fig. 3 (a) Photograph of Si@Si-SCO after 1 week of exposure and 14 days of aging. (b) An associated rheological profile under shear stress at a fixed excitation frequency of 0.1 Hz.

Fig. 4

 4 Fig.4(a) XRPD diffractograms of Si-SCO and Si@Si-SCO by deducing the amorphous phase (Fig.S5and S6 †). For comparison, the diffractogram of solid SCO without silica is plotted in green. (b) DSC at 5 K min -1 of Si-SCO (red lines) and of Si@Si-SCO (blue lines) aged for 14 days. DSC thermographs plotted as "exo up". (c) SCO characteristics from DSC results for the different samples.

Fig. 5

 5 Fig. 5 SEM images of (a) Si@Si-SCO (yellow arrows highlight the silica NPs, white circles the microneedles) aged for 14 days. (b) Scheme of the structure of Si@Si-SCO (the grey background represents the organosilica matrix, yellow disks the silica NPs, dark blue needles the crystalline SCO objects and light purple lines the amorphous SCO network). (c) SEM images of Si-SCO.

Fig. 6

 6 Fig.6Photographs of Si@Si-SCO after (a) 28 and (b) 50 days aging and (c) PDMS@Si@Si-SCO 80 days after their preparation. (d) Photograph illustrating the preserved thermochromic behavior of the flexible coated material PDMS@Si@Si-SCO.

Fig. 7

 7 Fig. 7 DSC characterization of PDMS@Si@Si-SCO (from 263 to 353 K at 5 K min -1 ): (a) with aging and (b) during thermal cycles. DSC thermographs are plotted as "exo up". (c) SCO characteristics over 15 temperature cycles.

Acknowledgements

The authors thank Isabelle Genois for her constant support for SEM imaging. H. V. thanks the ED 397 at the UPMC for a PhD grant.

Conflicts of interest

There are no conflicts to declare.