Structural heterogeneities and mechanical properties of vinyl/dimethacrylate networks synthesized by thermal free radical polymerisation

Dimethacrylate based networks usually have a poor impact resistance. In this work, we have tried to understand the origin of this brittleness. Two systems based on a same dimethacrylate monomer polymerised with styrene and divinylbenzene comonomers, respectively, have been chosen to correlate structural parameters with mechanical properties. The increase of structural heterogeneity, characterized by the width of relaxation time distribution, was measured as a function of double bonds conversion using dynamic mechanical tests. Atomic force microscopy observations of network structure after laser ablation show that the heterogeneity of networks is spacially organised due to the formation of microgels and their agglomeration. The presence of microgels strongly affects the polymerisation kinetics and controls the mechanical behaviour. Results show that the more densely crosslinked the network, the more heterogeneous is its structure. The impact resistance is shown to be related to the level of networks heterogeneities.     

PALS free volume and mechanical properties in dimethacrylate‐based thermosets

Dynamic mechanical thermal analysis, impact strength, bulk density and double‐bond conversion are reported for a series of dimethacrylate‐based networks in which the addition of styrene or/and divinylbenzene co‐monomer leads to variations in the morphology and crosslink density. The glass transition region was very broad for DVB‐rich networks and this was interpreted in terms of network heterogeneity. The mean size and the size distribution of free‐volume holes were estimated via analyses of positron annihilation lifetime spectroscopy (PALS) measurements at 22 °C and were found to be inversely correlated with the bulk density and dependent on the network heterogeneity. The PALS free volume was raised for DVB‐rich systems, due to the poor molecular packing in these highly crosslinked systems and because some regions in this heterogeneous network were in a rubbery state. The impact strength was found to be lower for the more highly crosslinked systems but was inversely correlated with the level of molecular mobility as measured by DMTA. Copyright © 2004 Society of Chemical Industry     

étude des composantes dépressives chez des adolescents à tendances suicidaires.

Studied symptoms of depression in adolescents with suicidal tendencies. Ss were 2,850 secondary school students (aged 12–18 yrs). Ss completed a questionnaire assessing sociodemographic variables, symptoms of depression, and suicidality. Results from the following subsamples were compared: (1) 105 Ss with suicidal ideation; (2) 206 Ss with suicidal ideation who had contemplated specific means of suicide; (3) 94 Ss who had attempted suicide; (4) 62 Ss with depression but without suicidal ideation; and (5) 336 age-, gender-, and grade-matched comparison Ss without suicidal ideation. The Dysphoria Scale by L. S. Radloff (1977) was used. (English abstract) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Endoglin regulates trophoblast differentiation along the invasive pathway in human placental villous explants

Successful invasion of the maternal vascular system by trophoblast cells is a prerequisite for the establishment of a normal hemochorial placenta. Transforming growth factor-beta (TGFbeta) has been implicated in the regulation of trophoblast invasiveness into the uterus. Endoglin is a component of the TGFbeta receptor complex that binds beta1 and beta3 isoforms and is expressed at high levels on syncytiotrophoblast throughout pregnancy and is also transiently up-regulated on extravillous trophoblasts differentiating along the invasive pathway. We investigated the role of endoglin in a serum-free human villous explant culture system that allows the study of trophoblast outgrowth, migration, and invasion and mimics events occurring in anchoring villi during the first trimester of gestation. Addition to explant cultures from 5-8 weeks gestation of a monoclonal antibody to endoglin or of antisense endoglin oligonucleotides significantly stimulated trophoblast outgrowth and migration. These responses were specific, as incubation of explants with nonimmune IgG or sense and scrambled oligonucleotides had no effect. Antisense endoglin-induced trophoblast outgrowth and migration were accompanied by cell division of villous-associated trophoblasts within the proximal region of the forming column and by the characteristic switch in integrins observed in anchoring villi in situ. Treatment of villous explants with antibody and antisense oligonucleotides to endoglin also resulted in an increased fibronectin release into the culture medium. The stimulatory effect of antisense endoglin on fibronectin production was overcome by the addition of exogenous TGFbeta2, but not TGFbeta1 and -beta3. These findings suggest that endoglin expression in the transition from polarized to nonpolarized trophoblasts in anchoring villi is necessary for mediation of the inhibitory effect of TGFbeta1 and/or TGFbeta3 on trophoblast differentiation along the invasive pathway.     

Characterization of epoxy thermosetting systems by differential scanning calorimetry

Differential Scanning Calorimetry (DSC) is used to study the curing behavior of epoxy systems. For non-catalyzed diamine-epoxy systems, the reaction enthalpy ΔH and the glass transition temperature T_g are evaluated and related to the structure of the hardener. A method is developed to determine the activation energy. The effect of variations in the amine-to-epoxy ratio r on T_g is also examined. A maximum value is observed for r = 1. Then, the influence of benzyldimethylamine as catalyst in an anhydride/DGEBA system and in three diamine/DGEBA systems is reported. In the cases of DDS and DDA curing agent the maximum value of T_g is shifted, to r = 0, 6. The results are explained by different reaction mechanisms.     

Comparison between reconstituted and fresh skim milk chemical and electrochemical acidifications

The aim of this study was to compare the electrochemical and chemical acidification of reconstituted and fresh skim milk in terms of electrodialytic parameters, precipitation kinetics, chemical composition and physicochemical and functional properties of isolates produced by bipolar membrane electro acidification (BMEA). The electrodialytic parameters were not influenced by the type of milk when both milks were compared at a similar protein and salt content. The difference in precipitation kinetics observed between the two milks, whatever the acidification procedure, can be explained mainly by a difference in salt content. Isolates produced by BMEA showed similar physicochemical and functional properties (except for foaming capacity) to isolates produced chemically. The main factor affecting the composition and the physicochemical and functional properties was the pre‐treatment of milk prior to acidification: it had a higher impact on the functional properties than the acidification treatment itself. Copyright © 2002 Crown in the right of Canada. Published for SCI by John Wiley & Sons, Ltd     

Effect of mild autoclaving and deamidisation on the nutritive value of protein fractions

The nutritive value of zein, rapeseed 12S and α-casein was assessed by apparent digestibility coefficient (ADC) and net protein ratio (NPR). Protein fractions were suspended (100 g litre^?1) in 0.1M HCl or in water and autoclaved for 1 h. The degree of deamidisation following acid autoclaving was 14.0% for zein, 7.3% for rapeseed 12S and 8.9% for α-casein. Results indicate that autoclaving in water or in HCl improved the ADC of zein and rapeseed 12S. However, acid autoclaving increased (P<0.05) the NPR of rapeseed 12S (from 3.07 to 3.44) whereas it decreased (P<0.05) that of α-casein (from 4.21 to 3.57).     

Synthesis and properties of polyamides from 2,5‐furandicarboxylic acid

The aim of this study is to investigate the synthesis of 2,5‐furandicarboxylic acid (FDCA)‐based copolyamides. Indeed, FDCA monomer may be a potential bio‐based alternative to phthalic acids. A series of polyamides and copolyamides, PA 6‐I(x)/6‐F(y), are synthesized in a pilot scale reactor by melt polycondensation of salts based on FDCA, isophthalic acid (IPA), and 1,6‐hexamethylenediamine. The chemical structure and composition of the resulting copolymers are extensively characterized by NMR (¹H, ¹³C, and 2D), MALDI‐TOF as well as size exclusion chromatography and solution viscosimetry. Their thermal properties are studied by differential scanning calorimetry and TGA and also by a molecular modeling technique. It is pointed out that, during the synthesis, FDCA‐rich polymers overcome a massive decarboxylation of FDCA, preventing the production of high molar mass polymers. It is also found that all polymers are amorphous and that the glass transition temperatures decrease as the amount of FDCA in the polymer increases. This is confirmed by the results from molecular dynamics. In addition, it is observed that the copolymers become more hygroscopic when the amount of FDCA in the copolyamides increases. However, the water uptake of PA 6‐I(x)/6‐F(y) with 90 > x > 50 is lower than those of PA6 and PA 66 polyamides. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45901.     

Synthesis and characterization of hybrid films from hyperbranched polyester using a sol–gel process

Hybrid organic/inorganic materials were prepared by an in situ sol–gel process using tetraethoxysilane (TEOS) in the presence of hyperbranched polyester. The influences of hyperbranched polyester molar mass as well as the amount of TEOS were examined. The condensation degree was characterized by solid state ²⁹Si NMR. The combination of solubility tests, calcination tests, SAXS and dynamic mechanical analysis allowed us to investigate the hybrid material nanostructure. The results show high compatibility between the inorganic silica phase and the organic polymer phase, due to the spherical shape of the hyperbranched polymer and its numerous hydroxyl groups. As a consequence, a continuous inorganic phase was formed even with a low silica precursor content without any macroscopic phase separation. These hybrid materials have a high T_g and high storage modulus even at an elevated temperature combined with improved thermal stability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39830.