Structural transitions of the trans-planar mesomorphic form and crystalline form III of syndiotactic polypropylene in stretched and stress-relaxed fibers: a memory effect

A study of polymorphic transitions occurring during the stretching of samples of syndiotactic polypropylene (s-PP) in the trans-planar mesomorphic form is presented. Samples in the pure transplanar mesomorphic form or in mixture with crystals of the helical form I have been stretched at room temperature and at 4 °C. Both the stretching temperature and the crystalline modification of the starting material strongly influence the polymorphism of oriented s-PP fibers. Stretching procedures at room temperature induce the crystallization of the trans-planar form III, regardless of the crystalline modification of the starting unoriented sample. Both the trans-planar mesomorphic form and the helical form I transform at high deformation into the crystalline trans-planar form III. When the stretching is instead performed at 4 °C the structural transitions occurring upon stretching depend on the crystalline modification of the starting unoriented material. Indeed, if s-PP samples in the pure trans-planar mesomorphic form are stretched at 4 °C, only an orientation of the mesomorphic crystals occurs, and the transition into the crystalline form III is never observed, also for high deformation. When the s-PP samples are, instead, in mixtures of crystals of the mesomorphic and helical forms, stretching at 4 °C induces the transition of the helical form into the trans-planar form III at low deformations. At high deformations the mesomorphic form also transforms into the crystalline form III. In all the experiments, the removal of the tension in fibers stretched at room temperature and at 4 °C produces structural changes, which depend on the crystalline form of the starting unoriented samples and on the crystalline modifications formed during the stretching. The trans-planar form III transforms, indeed, into the crystalline forms present in the unoriented samples or formed during the stretching (helical form and/or mesomorphic form). These results indicate that the structural evolution of the trans-planar form III upon removing the tension in stretched fibers strongly depends on the memory of the crystalline forms present in the starting unoriented material and on the memory of the crystalline forms produced during the stretching.