Rate‐, temperature‐, and structure‐dependent yield kinetics of isotactic polypropylene

Abstract

The influence of cooling rate on the structure and resulting mechanical performance is explored for a set of isotactic polypropylenes with varying molecular weight, insertion of counits, and addition of a nucleating agent. A continuous variation of crystal type (α–mesomorphic phase competition) and structural features is obtained with cooling rate. These variations are discussed in relation to the strain rate‐ and temperature‐dependent yield stress and time‐to‐failure kinetics. The deformation kinetics, characterized by constant activation volumes and energies in the Ree–Eyring theory, prove to be the same for various structures. Differences in thermal history are solely captured by two rate constants that are function of the structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

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

“…The non-nucleated virgin i-PP was supplied by Borealis, Linz, Austria (HD234CF, code PP-1) and was used in earlier studies [20,28]. The virgin nucleating agentcontaining grade homopolymer 578N (PP-2), was supplied by Sabic Europe, Geleen, the Netherlands.…”

“…The mechanical properties of semi-crystalline polymers are strongly dependent of their crystallinity, lamellar thickness, and type of crystal structure, which depend on the processing conditions [27]. A higher cooling rate during crystallization leads to a lower yield stress [28] and a higher strain-at-break. Lowering the yield stress of recycled i-PP could result in ductile instead of brittle behavior.…”

“…All contribute to a lower yield stress. On laboratory scale, the formation of these phases can be investigated by dilatometry and differential fast scanning calorimetry [20,28,38,39]. This type of DSC instrument allows the use of extremely high heating and cooling rates and is ideal for the investigation of metastable structures.…”

The chemical structure of the amorphous phase of propylene–ethylene random copolymers in relation to their stress–strain properties

“…A clear change in the rate dependence is displayed within the studied temperature and strain rate range. The change in slope indicates that there are two molecular deformation processes contributing to the yield stress [63]. The yield kinetics of polypropylene can be accurately captured using the Ree-Eyring modification of the original Eyring theory [57,64].…”

“…Essentially, this theory assumes that two processes act independently and that their contributions to the stress are additive. These processes can be linked to intralamellar ("crystalline") slip and interlamellar ("amorphous") slip [63]. Important is the observation that the relative contribution of the amorphous phase to the yield stress becomes increasingly important at lower temperatures when chain mobility in the amorphous phase decreases.…”