Flow Memory and Stability of Shear-Induced Nucleation Precursors in Isotactic Polypropylene

Abstract

The dissolution of flow-induced nucleation precursors in isotactic polypropylene is investigated indirectly by means of in situ rheo-SAXS measurements. The progress of crystallization and the evolution of crystal orientation are recorded in isothermal conditions after a controlled shear step followed by an annealing step of different duration at various melt temperatures. The results confirm that the survival time of shear-induced nucleation precursors is extremely large compared to typical rheological relaxation times and it is longer for the precursors originated at higher shear rate. Most important, we show that the effect of flow on the development of oriented morphologies is lost much earlier than that on the overall crystallization kinetics. A schematic model for precursors' dissolution involving gradual transformation from row into point-like nuclei is proposed.

Probing into the epitaxial crystallization of β form isotactic polypropylene: From experimental observations to molecular mechanics computation

“…Under quiescent condition, the long helices can form at the crystallization front, so the interchain ordering process and intrachain ordering process occur at the same time. This conclusion was also demonstrated by Alfonso et al via the observations of iPP and PB‐1 under POM . This investigation further provides additional evidence that the ordering structure of DCHT crystal can induce the formation of domains of chiral helices in β‐iPP crystal.…”

A few rediscovered and challenging topics in polymer crystals and crystallization

“…Even in the very early period of crystallization prior to detecting any ordered structure, flow‐induced crystallization may undergo the stage of precursor, as often found in the studies about the memory effect . A recent example is presented in Figure , in which it‐PP samples were sheared by pulling glass fibers at different high temperatures and then cooled down to 138°C for isothermal crystallization.…”

Influence of melt structure on the crystallization behavior and polymorphic composition of polypropylene random copolymer

“…They claimed that partly melting of orientated crystal structure plays very important role in the b-phase formation. Alfonso and coworkers [46,47] studied the formation and relaxation of shear-induced nucleation and found that shear flow and partially molten fiber have similar effect on the formation of b-phase.…”