A. Saiter

Water barrier properties in biaxially drawn poly(lactic acid) films.

Crystallization is among the easiest ways to improve polymer barrier properties because of the tortuosity increase within the material and the strong coupling between amorphous and crystalline phases. In this work, poly(lactic acid) (PLA) films have undergone α thermal crystallization or different drawing processes. Although no effect of α thermal crystallization on water permeability is observed, the drawing processes lead to an enhancement of the PLA barrier properties. This work clearly shows that, in the case of PLA, the crystallinity degree is not the main parameter governing the barrier properties contrary to the crystalline and amorphous phase organizations which play a key role. X-ray analyses confirm that the macromolecular chain orientation in the amorphous phase is the main cause of the improvement of the drawn PLA water barrier property. This improvement is due to the orthotropic structure formation for sufficient draw ratios, particularly when using the Simultaneous Biaxial drawing mode. Moreover, independently of the draw conditions, the drawing process tends to reduce the plasticization coefficient. Consequently, the drawn material barrier properties are not much affected by the water passage.

Effect of molecular interactions on the performance of poly (isobutylene-co-isoprene)/graphene and clay nanocomposites

Poly(isobutylene-co-isoprene) (IIR)/graphene and cloisite10A nanocomposites were prepared successfully and the resulting mechanical, rheological and barrier properties were carefully evaluated and compared. Chemical treatments like maleic anhydride grafting were used to improve the dispersion of the clay in the IIR matrix. Blends with different loading (20, 40, 60, and 80xa0%) of maleic anhydride grafted poly(isobutylene-co-isoprene) (MA-g-IIR) and IIR were made to maintain a balance between the beneficial polarity induced by MA grafting and the inevitable decrease in molecular weight (due to chain scission) induced by the free radical grafting process. The highest moduli, tensile strength and elongation at break were achieved in the case of a 60:40 ratio of MA-g-IIR (grafting degree 0.75)/IIR mixture with 5xa0phr of cloisite 10A. IIR/graphene nanocomposites exhibited higher reinforcement (Young’s moduli) and lower gas permeability compared to the optimized clay nanocomposites with same weight percentage. The filler–elastomer and filler–filler interactions deduced from rheology, stress relaxation and Payne effect experiments emphasize the reinforcing ability in IIR/graphene and MA-g-IIR/clay. XRD, SEM and TEM results further substantiated the results from the obtained micro structure of the nanocomposites. The improved performances of IIR/MA-g-IIR/clay and IIR/graphene were successfully correlated with interactions between the filler platelets and elastomer chains occurring in the nanocomposites.

Entropy and fragility in vitreous polymers

Abstract The study of the sub T g relaxation in a semi-rigid polymer family for which the size of the lateral chain varies from one carbon to three carbon atoms have been performed. We used an entropic model proposed by Hutchinson et al. [Macromolecules 33 (2000) 5252] introducing a new parameter x s called the entropic non-linearity parameter. For our samples we found x s =0.45±0.05. On comparison with the data obtained on other linear polymers, we have shown that the rigidity of the main chain involves an increase in the entropic non-linearity parameter x s . Furthermore, we have shown, by using the fragility concept, that this new entropic model is conceptually equivalent to the Random Walk Model proposed by Arkhipov et al. [J. Non-Cryst. Solids 172 (1994) 396; J. Phys. Chem. 98 (1994) 662].

Application of random walk model to the glass transition of unsaturated polyester resins cured with different styrene contents

Abstract Dielectric measurements have been performed on unsaturated polyester resins, cured with different styrene contents. Based on the random walk model (RWM), we have determined, as a function of the styrene content, the width of the density of states kT 0 , the average energy barrier height 〈 E A 〉 that a structural unit must crossover in order to relax, the fragility index α and the value of T c at which qualitative changes in the dynamics occur. The RWM data are in good agreement with the strong-fragile concept.

Thermodynamically “strong” and kinetically “fragile” polymeric glass exemplified by melamine formaldehyde resins

Abstract The variations of the heat capacity ΔC p =[ C p l − C p g ] T = T g at the glass transition, and the value of the fragility index m were determined for two melamine formaldehyde resins by calorimetric investigations. These values characterise respectively the thermodynamic aspect and the kinetic aspect of the Angell “Strong–Fragile” concept. For resins cured with a neutral pH, the 3D network formed is made of massive molecular units connected together by small length chains, and the values are ΔC p =0.13 Jxa0K −1 xa0g −1 and m =143. For acid pH curing conditions, the 3D network is made of only massive molecular units connected together and we obtain ΔC p =0.12 Jxa0K −1 xa0g −1 and m =35. By comparing our results with the results of the literature concerning three-dimensional networks and inorganic polymers, we are able to conclude that the relaxation occurs in these systems mainly by movements involving triazine rings.

Combining Flash DSC, DSC and broadband dielectric spectroscopy to determine fragility

New experimental results focused on Flash DSC, DSC and broadband dielectric spectroscopy investigations are reported in this work. The fictive temperatures and fragility indexes are estimated from Flash DSC experiments and compared to values obtained from classical DSC. The consistency of the Tool–Narayanaswamy–Moynihan model and fragility concept is then investigated over a large range of cooling rates. Indeed, the Flash DSC allows exploring thermal properties of materials over a continuous and broad range of heating and cooling rates, complementary to rates usually available with DSC. The reliability of investigations is also demonstrated by comparing results obtained from two model amorphous polymeric systems: polystyrene and poly(ethylene terephthalate)-glycol. The temperature dependence of the cooling rate obtained by Flash DSC and DSC is also compared to the temperature dependence of the relaxation times obtained from broadband dielectric spectroscopy, experiment considered as the reference concerning the fragility measurements. The comparison of these two dependencies implies a better understanding about the origin of the temperature dependence of the cooling rate.

Cooperative rearranging region size and free volume in As–Se glasses

Glasses of the As-Se system have been used as model objects of the covalent disordered inorganic polymers to investigate the correlation between the cooperative rearranging region (CRR) size determined at the glass transition temperature and the free volume fraction in the glassy state. The CRR size has been determined using temperature modulated differential scanning calorimetry data according to Donths approach, while the free volume fraction in the investigated materials has been estimated using positron annihilation lifetime spectroscopy data. The obtained results testify that the appearance of open-volume defects greater than 80xa0Å(3) leads to a significant decrease in the CRR size.

Complex structural rearrangements in As-Se glasses.

Structural relaxation of As-Se glasses through the glass-to-supercooled liquid transition interval is studied with temperature-modulated differential scanning calorimetry. It is shown that connectivity of glass network and long-term physical ageing change not only the full width at half maximum of the out-of-phase component of complex heat capacity, which is conventionally used for analysis, but also its asymmetry value. The latter is shown to carry very important information on the dynamic heterogeneity in glasses. Raman spectroscopy is used as complementary technique to reveal possible structural rearrangements in the investigated glass network.

Semi‐rigid polyesters analysed using the “strong‐fragile” concept

Semi-rigid polyesters from diphenyl dicarbonic acid and some branched propyl and butyl spacers have been investigated using Differential Scanning Calorimetry. From Δc p at Tg and the determination of the fragility index m, we studied the strong-fragile behaviour of these materials. All the samples appear thermodynamically strong and kinetically fragile. A comparison of these results with those obtained from glass-forming liquids leading to linear polymers -such as PET, PCT, PEN or poly-methyl (a-n-alkyl) acrylates- shows that a modification of the polymer rigidity leads to change the fragility index m and the Acp at the glass transition.

Glass transition of anhydrous starch by fast scanning calorimetry

By means of fast scanning calorimetry, the glass transition of anhydrous amorphous starch has been measured. With a scanning rate of 2000Ks-1, thermal degradation of starch prior to the glass transition has been inhibited. To certify the glass transition measurement, structural relaxation of the glassy state has been investigated through physical aging as well as the concept of limiting fictive temperature. In both cases, characteristic enthalpy recovery peaks related to the structural relaxation of the glass have been observed. Thermal lag corrections based on the comparison of glass transition temperatures measured by means of differential and fast scanning calorimetry have been proposed. The complementary investigations give an anhydrous amorphous starch glass transition temperature of 312±7°C. This estimation correlates with previous extrapolation performed on hydrated starches.