Morphology and elasticity of oriented syndiotactic polypropylene from solvent cast films

A sample of syndiotactic polypropylene (sPP) was prepared by solvent casting in orthodichlorobenzene at 70 °C. Its structure and morphology were analyzed by X-rays, Atomic force microscopy (AFM), and transport properties of vapors at low activity. The structural parameters were compared to those of samples thermally crystallized at 25 and 100 °C. The presence of the helical form I was recognized in either sample with crystal dimensions strictly dependent on the crystallization temperature. The crystallite sizes, determined by X-rays, were found to be surprisingly high, pointing to an unusual ability of lamellar crystals of sPP helical modifications to host defects without loosing coherence. Different from the thermally crystallized samples, the phase composition of the sample obtained by casting was particularly simple with a wide fraction of amorphous component and an almost complete absence of trans-planar mesophase. The oriented sample obtained by drawing the solvent cast sample up to λ = 7.0 showed the presence of the trans-planar form III, as well as of the trans-planar mesophase. On release of the tension the crystalline form III almost completely transformed into the mesophase, and only weak reflections of the helical form appeared in the diffraction spectrum of the drawn sample. The elastic behavior of the oriented sample was investigated by obtaining the hysteresis curves at increasing draw ratios. The permanent set and the energy dissipated were obtained and compared to those of thermally crystallized samples. The elasticity was analyzed with the Mooney-Rivlin equation. A truly elastic range was recognized between λ = 4.0 and λ = 5.8. In this range, the X-ray analysis did not show any change of the structure. For higher strain values, an upturn appears in the curve ascribable, by X-rays studies, to a strain-induced crystallization.