R. Séguéla

Influence of the β crystalline phase on the mechanical properties of unfilled and CaCO3-filled polypropylene. I. Structural and mechanical characterisation

Abstract The influence of β crystals on the mechanical properties of isotactic polypropylene is studied on compression-moulded sheets without filler or filled with stearate-coated calcium carbonate particles. A crystallisation procedure is setup for producing sheets with various amounts of β crystals, trying to keep constant crystallinity, spherulite size and crystal thickness. An optimum of β crystals among α crystals is first produced by isothermal crystallisation at 110°C. Then gradual transformation of β into α crystals is carried out through thermal annealing at 152°C after an intermediate cooling down to room temperature. Due to the thermal inertia of the large sheet thickness, the amount of β crystals does exceed 10% for unfilled sheets and 50% for filled sheets. Dynamic mechanical behaviour shows that molecular mobility is higher in the β crystals than in the α crystals, as judged from the temperature of the crystalline relaxation. Loss modulus in the temperature range of the crystalline relaxation also indicates greater damping capacity for the β crystals. This is discussed in terms of activation of conformational defects moving along the chain stems in the crystal. Plane strain compressive testing reveals better ductility for β rich samples. Interpretation is provided using an approach of semi-crystalline polymer plasticity based on dislocation-governed crystallographic slip. Correlation is made with the viscoelastic behaviour through the concept of conformational defects.

Physical and mechanical properties of polyethylene for pipes in relation to molecular architecture. I. Microstructure and crystallisation kinetics

Ethylene/α-olefin copolymers having bimodal molar weight distribution are investigated in comparison with unimodal copolymers in order to understand the incidence of the molecular architecture on the stress cracking resistance. The preferred introduction of the co-units in the longest chains of bimodal copolymers is suggested to favour the occurrence of intercrystalline tie molecules during crystallisation. The more complex is the molecular architecture, the greater is the difficulty for crystallisation by regular chain folding. Intermolecular chemical heterogeneity resulting from preferred incorporation of the co-units in the long chains enhances the co-unit disturbing effect on crystallisation without reducing crystallinity. Intra-molecular heterogeneity of the co-unit distribution is also suggested to be an efficient means to generate tie molecules and random chain folding at the expense of regular chain folding. Isothermal crystallisation is used to probe the effect of molecular architecture on the crystallisation kinetics. It appeared that the correlations between kinetics, molecular architecture and molecular topology of unimodal copolymers no longer hold when considering bimodal copolymers. In contrast, the crystal surface free energy proved to be sensitive to topological changes resulting from molecular architecture modifications.

Assessment of polyamide-6 crystallinity by DSC

AbstractnThis study addresses the question of the crystallinity determination of PA6 by means of DSC in the case when structural changes occur over a very large temperature domain during the heating scan. The temperature dependence of the melting enthalpy is then of crucial importance for determining the amount of crystalline phase involved in the various processes, and thus the initial crystallinity. Both DSC and WAXS measurements have been carried out of a PA6 sample submitted to various thermal treatments in order to identify the crystalline forms and the temperature-induced structural changes. The melting enthalpy dependence on temperature of PA6 was computed from heat capacity data of the solid and liquid borrowed from literature data tables. Similar computations were performed for PA66 which is likely to exhibit analogous structural changes during DSC analysis.