X‐ray and related studies of quenched, drawn, and annealed polypropylene

A variety of metastable conditions can be produced in polypropylene of normal commercial tacticity by quenching, cold drawing, and annealing. The resultant x‐ray patterns suggest a two‐component three‐phase model of the molecular organization, and such a model has suffcient scope to produce good agreement between calculated and observed data taken at tenth‐degree intervals along the equator of a number of specimens. The model consists of a noncrystallizable and nonorientable amorphous phase, a paracrystalline or quasicrystalline phase which is oriented and crystallizable and which contains short‐range three‐dimensional order of a limited nature, and a crystalline phase containing well ordered domains of varying lateral dimensions and varying thermal stability. The conversion from paracrystal to crystal is variable, depending on the preparative history of the sample but in a typical situation conversion started at 70°C., was half complete at 110°C. and nearly complete at 145°C. The crystallites formed at low temperature had lateral dimensions of 50 A. and at higher temperatures recrystallization involving a decrease in the number of crystallites led to corresponding dimensions in the range of 200 A. Extensive cold drawing was shown to produce severe disorder in addition to excellent orientation. Less severe drawing produces less disorder and less orientation, and in this case a subsequent isometric anneal actually produced further orientation along with increased order. In atonic annealing, essentially no disorientation of the crystalline phase was observed during the 50% gross shrinkage obtained at 145°C. Metric studies during heating at 1°C./min. showed shrinkage to begin at the same temperature as paracrystal to crystal conversion. During atonic annealing the shrinkage proceeded rapidly at first and then appeared to level off at a value characteristic of the temperature. After cooling, subsequent heating produced no shrinkage until the previous anneal temperature was exceeded, which is consistent with the x‐ray results demonstrating irreversible changes in order during annealing. Tensile tests following isometric annealing of well oriented samples revealed a slight increase in tenacity up to 100°C. with little change in modulus or elongation. At higher temperatures where recrystallization takes place and the crystals become larger, the tenacity decreased while the elongation increased and the 10% modulus decreased. The modulus after annealing at 160°C. was half what it was initially. These changes are interpreted on the basis of decoupling between crystals and between the crystalline and amorphous material with disentanglement and fractionation occuring during melting and recrystallization of the molten domains. Differential thermal analysis shows that early melting and recrystallization does occur in polymer crystallized isothermally from the melt. In the polypropylene sample studied, the melting peak depression was three tenths of the degree of super‐cooling at the time of crystallization, and the equilibrium melting point obtained by extrapolation was 178.6°C.