SYNOPSISUni-and biaxial stretching of various polymer films has been studied under well-defined conditions of temperature and elongational strain rate in order to determine the relationship between stress and recoverable strain for both modes of deformation. The extent of molecular orientation has been investigated with the aid of stress optical methods: In amorphous polymers birefringence was found to be directly proportional to the frozen-in internal entropic stress irrespective of the latter's relationship with recoverable strain. In a first approximation, molecular orientation can be understood in terms of deformation of a rubberlike network with temporary junction points. The total internal stress in an oriented glassy polymer may be significantly larger than the entropic stress. Experimental methods based on retractive force measurements, able to distinguish between internal stresses of different nature, are described.In uni-and biaxially drawn films of polyethylene terephtalate preferred planar orientation of ( 1 00) planes has been studied by means of X-ray diffraction and measurement of the three principal refractive indices. The intrinsic birefringence of completely oriented PET films has been estimated from these measurements with the aid of the Lorentz-Lorenz theory. The effect of draw ratio and temperature on the rate of crystallization and its consequences (increase of stretching stress and density, decrease of shrinkage) has also been studied.Finally, the effect of molecular orientation on various mechanical properties: modulus, tensile strength, impact resistance, creep compliance, is discussed for both amorphous and semicrystalline polymers, with special emphasis on the predominant influence of amorphous phase orientation.

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Uni-and biaxial stretching of poly(ethy1ene terephthalate) (PET) specimens of appropriate geometry at temperatures near the glass-rubber transition may lead to non-uniform deformation unless the draw ratio exceeds a critical value, the natural draw ratio, characteristic of the onset of strain hardening due to stress-induced crystallization. Experimental results obtained in the present.investigation show that natural draw ratios in uni-and biaxial stretching decrease with increasing resin molecular weight and with decreasing temperature. Undesirable uneven wall thickness distribution in biaxially stretched cylindrical parisons can only be prevented if draw ratios in both orthogonal principal stretching directions exceed the corresponding natural values. The minimum thickness reduction required for uniform biaxial stretching of a cylindrical parison at 95°C may vary between 12 and 5 depending on the resin's molecular weight or viscosity and this will affect the optimum design of parison geometry. The degree of unbalanced biaxial molecular orientation in the wall of cylindrical parisons stretched up to or beyond the natural draw ratios also depends on the resin molecular weight. Unbalanced biaxial orientation has been investigated by means ofwide angle X-ray diffraction and birefringence measurements as well as its effect on various properties: rigidity, yield stress, creep compliance, and dimensional stability.

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Dilute solution properties of twenty-five PVC fractions prepared from commercial and experimental bulk-polymerized PVC samples have been investigated by means of light-scattering, viscometry and exclusion-chromatography The following Mark-Houwink-Sakadura relationships: []2YC = 1,63 10-2 M°76° x 1.017 c.gs. I 125t -2 ir2 At 0.725 >< 1 02 Lflicyclohexanone -: ' c.g.s.have been found to be valid in a range of Mw-values between 6400 and 650000.The validity of various extrapolation procedures, proposed in order to calculate unperturbed dimensions and polymer-solvent interaction para meters from the empirical viscosity-molecular weight relationship, has been examined The Kurata-Stockmayer method applied to the measurements in tetrahydrofuran yields a Krvalue equal to 0.14 c.g.s. The polymer-solvent interaction parameter derived by the same method is not in agreement with the one deduced from second virial coefficient measurements.Molecular weight distributions determined in commercial PVC samples can be adequately represented, in a first approximation, by the usual exponential distribution function, first. proposed by Schulz. The chromatographic determination of MWD and its effect on the viscosity-molecular weight relationship are discussed.Finally, the possible occurrence of long chain branching in PVC has been investigated by means of an experimental study of the structure and properties of various polyethylene samples obtained by reduction with LiAIH4 of commercial PVC polymers. The melting temperatures and the intrinsic viscosity -molecular weight relationship for the polyethylene samples, so obtained, were found to be identical with those of High Density Polyethylene. It is concluded that most PVC polymers contain only very few long branches, if any