Structure of pressure‐crystallized polypropylene

Abstract

The structure of polypropylene crystallized at pressures up to 5000 atm. has been studied. Upon slow cooling from the melt at 320 atm., the γ modification, previously found only in low molecular weight and stereoblock fractions, begins to appear in small amounts in addition to the normal α monoclinic form. As the pressure is increased further, a larger proportion of the sample crystallizes in the γ form until, at 5000 atm., only the γ modification is present. X‐ray and DTA studies show that the γ form of polypropylene transforms to the normal α modification at a temperature only slightly below the γ melting point. Evidence is presented which favors the occurrence of a solid‐state transition as a model of transformation to the α form. Results from isothermal crystallizations at low supercoolings and annealing experiments under high pressure show that the melting point of the γ modification of polypropylene is very sensitive to crystallite perfection.

Fast cooling of (non)-nucleated virgin and recycled poly(propylenes): Effect of processing conditions on structural and mechanical properties

“…The g-crystal phase is formed when crystallization takes place under high pressures and moderate cooling rates. This crystal phase is typically formed more easily in lower molecular weight polymers [14][15][16] or in the presence of an a-olefin comonomer [17]. The g-crystal phase has an orthorhombic unit cell (a = 8.54 Å, b = 9.93 Å, c = 42.4 Å) containing layers of non-parallel organized chain segments.…”

Crystallization of α and γ Phases in Isotactic Polypropylene with Low Ethylene Content: Isothermal Crystallization and Secondary Crystallization

“…Thus, the existence of the four states of the molecular structure results in various rules of the molecular packing in the crystalline phases and causes the complex polymorphous behavior. IPP shows three different crystalline phases, (2), (3) and (4,5) phases, the appearance of which depends on crystallization condition and on material grade.…”

The effect of matrix morphology on dynamic-mechanical properties of polypropylene/layered silicate nanocomposites

“…Hereafter, the peak at the higher temperature will be referred as peak-1 (P1), whilst those at lower temperatures will be called peak-2 to peak-4 (P2-P4). These peaks can be attributed to the complex melting behavior of the iPP [60][61][62][63][64]. This has been ascribed to several factors, among which are the presence of different crystal forms and differences in crystallite size and perfection.…”