Shu-Gui Yang

Unexpected shear dependence of pressure-induced γ-crystals in isotactic polypropylene

Flow and pressure frequently coexist in practical polymer processing operations, but their combined influence on the microstructure of polymer parts has received very limited attention in the academic community. In the current work, we utilized a home-made pressuring and shearing device with a reliable dynamic sealing design to study the formation and microstructure of γ-form isotactic polypropylene (iPP) obtained under the coexistence of flow and pressure. We observed a strong shear dependence of pressure-induced γ-form iPP. There are three regions depending on shear flow intensity, i.e., facilitation ( 9.1 s−1) regions of the γ-form. As the shear rate is below 3.7 s−1, the pressure-induced γ-form dominates and the shear flow slightly facilitates formation of γ-form. Unexpectedly, above 3.7 s−1, the shear flow is unfavorable for γ-form growth. Even under a pressure of 100 MP, a flow field with a shear rate above 9.1 s−1 could entirely suppress the γ-form. Moreover, we did not observe any trace of the β-form in the obtained iPP that is generally generated under shear flow alone. These interesting results have never been reported, which undoubtedly help manipulate the inner structure and thus enhance the performance of final iPP products.

A Criterion for Flow‐Induced Oriented Crystals in Isotactic Polypropylene under Pressure

Flow-induced oriented crystals have attracted considerable attention because they significantly increase stiffness and strength of polymer products. Naturally, understanding the necessary condition of forming oriented crystals is of importance for both industry and polymer physics. Following the concept of specific work of flow proposed by Mykhaylyk and co-workers, the expression of the specific work of flow, w (T,P) , was carefully summarized and verified that when w (T,P) is above a critical specific work of flow, w c(T,P) = (1.7 ± 0.7) × 107 J m-3 , oriented crystals in isotactic polypropylene can be induced by flow at pressures (50, 100, and 150 MPa) and at a undercooling of 65 K. The influences of pressure on w c(T,P) stem from two facets: one is the influence on the melt viscosity (the Barus law), and the other one is the influence on the equilibrium melting temperature (the Clapeyron equation). The current study can guide real processing to fabricate high-performance polymer products with oriented crystals.

Thicker Lamellae and Higher Crystallinity of Poly(lactic acid) via Applying Shear Flow and Pressure and Adding Poly(ethylene Glycol)

In this work, we explored the crystallization of poly(lactic acid) (PLA) blended with poly(ethylene glycol) (PEG) under two inevitable processing fields (i.e., flow and pressure) that coexist in almost all processing for the first time. Here, the PEG was incorporated into PLA as a molecular chain activity promoter to induce PLA crystallization. A homemade pressuring and shearing device was utilized to prepare samples and necessary characterization methods, such as differential scanning calorimetry, scanning electron microscopy, and synchrotron radiation, and were used to investigated the joint effects of PEG, pressure, and shear flow on the crystallization behaviors and morphologies of PLA/PEG samples. The results reveal that adding 3-5 wt % PEG into PLA can significantly increase the PLA crystallinity due to the efficient plasticization effect of PEG, while the PEG content reaches 10 wt %, the PLA crystallinity decreases drastically as the phase separation between PEG and PLA occurs. We also find that applying a higher pressure (∼100 MPa) can facilitate the formation of thicker lamellae with fewer defects as well as higher crystallinity under an equal degree of supercooling compared to normal pressure or a low pressure condition because the slip of molecular chains during crystallization makes the lamellae thicker under higher pressures. The PLA crystalline structure in the PLA/PEG sample is not influenced by the shear flow, yet the crystallinity is largely enhanced by applying a shear flow with an appropriate intensity (0-3.5 s-1). It is worth noting that pressure and shear flow show a synergetic effect to fabricate PLA/PEG samples with high crystallinity. These meaningful results could beyond doubt help comprehend the relationship between crystallization conditions and crystallization behaviors of PLA/PEG samples and thus provide guidance to obtain high-performance PLA/PEG products via controlling crystallization conditions.