No abstract

No abstract

SynopsisThe dynamic moduli G'(w) and G"(w) for two groups of linear polyethylene fractions (reported mu,/%,, < 1.2) were measured in the melt state using the eccentric rotating disk method. Values of zero shear viscosity 70 were obtained and compared with published results on similar fractions. Molecular weight data were converted to a common basis through intrinsic viscosities in trichlorobenzene (TCB) a t 135OC. With recent data on aW (light scattering) vs.[V]TCB. for linear polyethylene, the relationship a t 19O"C, 70 = 3.40 X 10-14(mu))3.m, was obtained. The flow activation energy E, was 6.4 kcal(7' = 140-195OC). The plateau modulus C$ a t 190°C was determined from the area under the loss modulus peak in one high-molecular-weight sample. The value obtained, G$ = 1.58 X lo7 dyn/cm2, corresponds to an apparent molecular weight between entanglements of 1850. The storage compliance J'(w) becomes anomalously large a t low frequencies. The recoverable compliance J $ could not be determined for any of the fractions.

In order to study the effect of the α, β crystalline structure of polypropylene (PP) on its mechanical properties, it is necessary to prepare samples with variable α/β-phase content but with constant crystallinity and constant spherulite size. With this objective, heat treatment was first defined to be applied to an isotactic PP containing a β nucleating agent in order to achieve these conditions. Then study of the effect of the β-phase content on the tensile properties and fracture behaviour has been done at room temperature. The mechanical properties at fracture were assessed by three-point bending tests and were analysed on the basis of the "Essential Work of Fracture" (EWF). The results show that the elongation at fracture under tensile stress and the "near" Plane-Strain Essential Work of Fracture, w Ie, increase substantially with the β-phase content. Besides, Young's modulus and the yield stress in tensile tests decrease slowly with the β-phase content. Finally, these results are analysed taking account the differences in structure of the α and β spherulites.

The mechanism of constraint release for an entangled linear chain of polystyrene diffusing by reptation is emphasized, dissolving large N chains in a matrix of shorter N, chains of the same species.Dynamic shear measurements allow us to define an average relaxation time f(NJVt) for the relaxation of the large chains, from which we derive another average relaxation time for the tube modification induced by constraint release. We find that, according to the Klein and Graessley theories, the tube could be considered as a Rouse chain (rmod(1V^VS) AT9*0-1) but constraint release is not directly connected to the reptation of the N, chains (rmod(A''ArB)2'3±0'1)-The process of constraint release included in the diffusion of chains in a binary blend at various component concentrations allows us to specify the idea of "cell" in the Graessley theory and to describe reasonably the variations of the limiting parameters and Je°w ith blend composition.

No abstract

No abstract

No abstract