Jean Louis Halary

DSC studies on the transitions in poly(vinylidenefluoride) and some related copolymers

SummaryThe transition properties of poly(vinylidenefluoride) and of some related copolymers, either semicrystalline or amorphous, were studied by differential scanning calorimetry in the temperature range from 200 K to 500 K. The amorphous copolymers exhibit a single glass transition. Melting endotherms and a lower glass transition [Tg(L)] are systematically observed in the semi-crystalline materials as well as an upper glass transition [Tg(U)] for certain thermal and mechanical histories of the samples. Conditions for [Tg(U)] existence are related to solid state morphology of the macromolecules and loop length in the folded chains.

Structure–Property Relationships in Epoxy-Amine Networks of Well-Controlled Architecture:

Series of epoxy-amine networks of well-controlled architecture were prepared by varying (a) the chemical nature of both diepoxide and primary diamine, (b) the nature and relative amount of the difunctional amine co-hardener and (c) the stoichiometric ratio. The 22 systems under study here proved to be very suitable for establishing connections between network structure and various physical and mechanical properties including glass transition, primary and secondary mechanical relaxations, modulus in the glassy state, plasticization and antiplasticization effects, water uptake and development of residual stresses. Most of these relationships were based on the consideration of the cross-link density, which affects the properties to a larger extent than the chain flexibility.

Molecular analysis of the plastic deformation of amorphous semi-aromatic polyamides

Abstract The plastic behavior of a series of amorphous semi-aromatic polyamides (SAPA-A) was investigated in compression mode at temperatures ranging from −100°C to their glass transition temperatures. Data analysis was mainly based on the inspection of the temperature dependence of yield stress, σy, plastic flow stress, σpf, and strain softening, SSA=σy−σpf. The activation volumes, V0, and the index of non-elastic behavior, I, were also determined, to account for the sensitivity of plastic deformation to strain rate and for the relative easiness of elastic and non-elastic processes, respectively. Some connections have been established between the nature of the molecular motions and the values of σy, σpf, SSA, V0, and I, thanks to the knowledge of the relaxation behavior of these materials. The role played by the secondary relaxation motions (and especially by those presenting a cooperative character) was highlighted, in agreement with our conclusions in earlier reports on the subject. In addition, the molecular analysis proposed for describing the plastic deformation of the SAPA-A materials was shown to hold for other polymers also bearing phenylene rings in the main chain, namely some semi-aromatic polyamides of another series (SAPA-R) and aromatic polycarbonates. Interestingly, the line of reasoning proposed here proved to be suitable also to account for the plastic deformation characteristics of various vinyl polymers and copolymers.

On the viscoelastic and plastic behavior of semiaromatic polyamides

Different semiaromatic polyamides (SAPA) have been synthesized by step-growth polymerization of an aliphatic diamine, M (the 2-methyl 1,5-pentanediamine), and isophthalic acid, I, or terephthalic acid, T, or mixtures of these two diacids. The influence of the relative amount of randomly distributed MT units on the viscoelastic properties of the materials was investigated. It was shown that the glass transition T g , as deduced from DSC thermograms, and the relevant mechanical relaxation T α raise when the content of MT units increases. In contrast, the broad low-temperature secondary relaxation, called y, does not markedly depend on the MT content. Samples systematically studied in the absence of any moisture did not exhibit the intermediate-temperature secondary relaxation, called β, which is characteristic of the wet polyamides. The study of the plastic behavior was focused on the samples MI and I5, which are strictly amorphous, and contain 0% and 50 mol % of MT units, respectively. Mechanical experiments were carried out in both the compression and traction modes, at temperatures ranging from -80°C to T g . Analysis of the compression data was based on the inspection of the temperature dependence of elastic modulus, E(T), yield stress, σ y , plastic flow stress, σ pf , and strain softening σ y - σ pf . Whereas the plots of σ y as a function of temperature, T, reveal some differences between MI and I5 behavior, a unique master curve was obtained by plotting σ y /E(T) vs. T - T g , which means that the plastic behavior of these materials is controlled by their chain packing in the glassy state. The strain softening profile of MI and I5 is similar to that already reported in the case of brittle vinyl polymers. This observation is consistent with the traction data, which give evidence for the occurrence of the tensile yielding of MI and I5 at temperatures rather close to T g .

Relationships between thermally induced residual stresses and architecture of epoxy-amine model networks

The capability of epoxy-amine resins to develop residual stresses was studied as a function of temperature and network architecture. These residual stresses were induced while cooling epoxy-glass bilayers from temperatures higher than the network glass transition temperature, T g . This behavior was the result of the marked differences (α r - α g ), in linear thermal expansion coefficient of the two components, as evidenced by the measurement of α r for the epoxy networks under study. Various network architectures were selected, resulting from variation of (1) the chemical nature of both epoxide and curing agent, (2) the nature and relative amount of the chain-extensor agent, and (3) the stoichiometric ratio. Three ranges of cooling temperature were observed systematically: first, the range of temperatures above T g , where no stress has been detected, then an intermediate temperature range (from T g to T * ), where stresses develop quite slowly, and finally, the low temperature range (T < T * ), where a linear increase in stress accompanies the decrease of temperature. The two latter regimes were quantitatively characterized by the extent, T g -T * , of the first one and by the slope, SDR, of the second one. T g - T * values were shown to be governed by the T g of the network: the higher the T g , the larger the gap between T g and T * . This result was interpreted by accounting for the variation of relaxation rate at T g from one network to the other. It was also shown that a semiempirical relationship holds between SDR and T g : SDR decreases monotonically as T g increases. By inspecting the effects of network architecture in more details, it turned out that SDR is governed by the Youngs moduli, E r (T- T g ), of the epoxy resins in the glassy state: the lower E r (T- T g ), the lower SDR in a series of homologous networks. As E r (T - T g ) values are known to be related to the characteristics of the secondary relaxation β, which depends, in turn, on crosslink density, SDR values were finally connected to the amplitude of the β relaxation processes. This finding was corroborated by the measurements on an antiplasticized dense network. Finally, data relative to thermoplastic-filled networks showed that the addition of thermoplastic reduces the development of residual stresses, whatever the system, is homogeneous or biphasic.

Analysis of transport phenomena in cellulose diacetate membranes IV. Dependence of membrane desalination properties on annealing temperature: a Molecular Analysis

Abstract Using NaCl aqueous solutions, the volume flux and the salt rejection of various cellulose acetate membranes annealed at different temperatures, were determined from hyperfiltration experiments performed at 25°C and pressure up to 60 atm. Additional determinations including membrane hydration characteristics and dialysis-osmosis transport coefficients permitted analysis of the observed desalination properties in terms of polymer-polymer, polymer-water and water-water interactions. Whatever the membrane and the applied pressure both sharp decrease in volume flux and increase in salt rejection can be qualitatively explained by a decrease in the free water content of the membrane and an increase in the ratio of polymer-polymer to polymer-water hydrogen bonding. Practical efficiency of the heat-treatment depends on the formation conditions of the ascast membranes, owing to different sensitivity to pressure effects of each type of membrane structure.

Analysis of transport phenomena in cellulose diacetate membranes

SummaryOsmosis-dialysis transport properties and hydration characteristics were determined for various Manjikian type membranes. Some analogies were established between the behaviour of these membranes and the membranes of Loeb type. Particular attention was paid to show the generality of the correlations between the specific transport coefficients and the state of water (free or bound) in unilayer membranes.

Upgrading of the fracture properties of multi-layered PMMA-nanosilica materials by interface modification

Hybrid copolymers of methyl methacrylate and methacryloyl grafted nanosilica (MMA-MGS) exhibit a higher modulus, but also a marked embrittlement, as compared to pure poly(methyl methacrylate) (PMMA). With the aim of overcoming this latter feature detrimental for industrial applications, multilayered materials based on these hybrids were prepared by inserting pure polymer layers between the nanohybrid layers. Insertion of thin and soft layers of poly(ethyl acrylate) (PEA) between the nanohybrid layers leads to a marked toughening resulting from crack smoothing at each interface. However, this procedure is detrimental to the Youngs modulus, except for extremely thin (ca. 1 μm) PEA layers. Alternatively, insertion of 100 μm thick hard layers of PMMA also produces toughening, as the consequence of crack deviation at the layer interface. The latter procedure presents potential interest for applications because the Youngs modulus of the assembly remains very close to that of the monolithic hybrid. The observed behavior of these assemblages is analyzed within the framework of fracture mechanics.

Long period analysis in blends of high-density and lowdensity polyethylene

Abstract Long period calculations from small angle X-ray scattering experiments (SAXS) is a well-known way to describe biphasic polymeric systems. In the case of semi-crystalline homopolymers, more and more sophisticated models have been proposed to account for the SAXS intensity profiles. However, they use too many parameters to be applied to systems including two crystalline phases and an amorphous phase. Therefore, we propose a crude alternative, based on a generalization of the old paracrystalline Hoseman model, in order to consider the case of blends of high-density (HDPE) and low-density (LDPE) polyethylene. The crystalline lamella thickness distribution function is taken as the sum of two Gaussians (bimodal function 2G). As far as the amorphous domain thickness distribution function is concerned, either an exponential distribution (E) or a Gaussian distribution (G) is used. The model 2G/G proves to be much more realistic than the model 2G/E, with respect to the long period evolution as a function of HDPE/LDPE blend composition.