Shear-Mediated Crystallization of Isotactic Polypropylene:  The Role of Long Chain−Long Chain Overlap

Abstract

The role of long chains in shear-mediated crystallization was studied by in situ rheo-optical measurements and ex situ microscopic observations. To elucidate the effects of long chains, we prepared model blends in which fractionated isotactic polypropylene (iPP) (denoted L-PP) with high molecular weight (MW) and narrow molecular weight distribution was blended with a metallocene iPP (Base-PP) with lower molecular weight. The concentration of L-PP (c) was varied ranging from 0 to twice the concentration (c*) at which L-PP coils overlap. The crystallization of all blends after cessation of transient shearing was accelerated, while the quiescent crystallization kinetics were not affected by the addition of L-PP. A distinctive change in the development of birefringence after shearing was observed when the wall shear stress (σ w) exceeded a critical value (σ*). Below σ*, irrespective of c, the birefringence after transient shearing increased gradually, reaching a small value at the end of crystallization. Above σ*, a brief interval of shear induced highly oriented growth, manifested in the birefringence after cessation of flow and growing stronger and reaching a large value as crystallization proceeded. Further, the rate of growth of the birefringence exhibited a strong, nonlinear c dependence. The morphology of the skin layer showed a shish kebab type structure observed by TEM for samples subjected to stresses above σ*. The number density and thickness of shish were affected by c and changed drastically at c near the overlap concentration of the long chains. This indicates that the role of long chains in shear-induced oriented crystallization is cooperative (rather than a single chain effect), enhanced by long chain-long chain overlap.

Modeling Flow-Induced Crystallization

“…The volume fraction of the shish is neglected because it is very small compared with the total volume of shish-kebabs; the radius of a shish is about 10 nm [108], whereas the radius of a shish-kebab grows to about 100 nm (cf. Roozemond et al [94], Seki et al [114]). Following the model proposed by Roozemond et al [89], daughter lamellae nucleate on the surface of the parents,…”

Crystallization of Polypropylene: Application of Differential Scanning Calorimetry Part I. Isothermal and Non-Isothermal Crystallization

“…In particular flow-induced crystallization of polymer melts can results in the formation of a so-called shish-kebab structure in semicrystaline polymers under appropriate conditions [33][34][35][36]. This special kind of chain crystalline assembly consists of foldedchain lamellae or kebabs periodically held together by fibrillar crystals or shishes [33][34][35][36][37][38][39][40][41][42][43] which is a result of a coil-to-stretch transition of polymer chains in the melt crystallization when the shear rate exceeds a critical value [33,37]. It has been predicted and observed that longer coiled-chains (longer than a critical length) of a polydisperse polymer melt can be stretch to the shishes while at the same flow rate shorter coiled-chains epitaxially grow into the kebabs.…”

Flow-induced crystallization studied in the RheoDSC device: Quantifying the importance of edge effects

“…with L the density of shish (typically 10 12 m/m 3 for isotactic polypropylene, which, at least in the point-like regime, shows comparable numbers to P1B (Seki et al 2002)), indeed gives a half crystallization time of ∼ 3 s. Crystal growth rate, which does not depend on morphology, is again denoted by G.…”