Dominant β-Form of Poly(l-lactic acid) Obtained Directly from Melt under Shear and Pressure Fields

Abstract

Shear and pressure fields unavoidably coexist in practical polymer processing operations, but their combined influence on the crystalline structure of poly(L-lactic acid) (PLLA) has never been studied due to the limit of experiment device. In the current work, we utilized a homemade pressuring and shearing device to study the crystalline morphology and structure of PLLA under the coexistence of shear and pressure. Interestingly, we obtained almost exclusive β-form directly from PLLA melt crystallization at our experimental condition (shear 13.6 s −1 , pressure 100 MPa, and crystallization temperature 160 °C). Undoubtedly, abundant β-form is helpful to tackle the major shortcoming of PLLA performance, i.e., poor toughness. This meaningful result is different from the common viewpoints that PLLA β-form can usually be obtained by hot-drawing or solid coextrusion under a high tensile ratio, suggesting that PLLA βform can be obtained through shear-induced crystallization. In addition, the fraction of β-PLLA strongly depends on supercooling and shear intensity. A higher supercooling (pressure 150 MPa and crystallization temperature 160 °C) could also induce predominant β-form even under a very low shear rate of 1.0 s −1 . While, under a lower supercooling (pressure 50 MPa and crystallization temperature 160 °C), we did not observe any trace of β-form. In the heating experiment to investigate crystal form transformation, we also found that partial β-form transformed into α-form through melting−crystallization, and meanwhile some β-form crystals melted directly without transformation. These results could beyond doubt help to comprehend the relationship between crystallization condition and inner crystal structure and thus afford guidance in practical processing to toughen final PLLA products via controlling crystalline structure.

A revisit to the flow and pressure jointly induced thick lamellae in isotactic polypropylene: A synchrotron radiation small‐ and wide‐angle X‐ray scattering study

“…The experiments were performed by using PSD (a detailed description of the PSD was released in our previous articles) . PSD can simultaneously impose flow and pressure on the polymer melt, where the shear rate increases linearly with radius ( r ) (see the Figure a [, where ω stands for the angular speed, and d is the gap, 0.5 mm].…”

“…Meanwhile, pressure has been revealed to slow down the crystallization kinetics, generate new crystal forms, and trigger the formation of extended chain crystals . Even though flow‐ and pressure‐induced crystallization have been well elucidated, the available theories of structure evolution under the coexistence of flow and pressure are rarely established ascribed to diverse challenges as documented in the previous articles …”

Flow‐induced crystallization of polylactide stereocomplex under pressure

“…Undoubtedly, the experimental results obtained in the conditions without considering influences of pressure cannot reflect the real situation, so cannot effectively guide practical processing. Moreover, our previous studies demonstrated that for iPP and PLLA there were notably different crystallization behavior when simultaneously applying flow and pressure . Ru et al .…”

“…Moreover, our previous studies demonstrated that for iPP and PLLA there were notably different crystallization behavior when simultaneously applying flow and pressure . Ru et al . obtained β‐form directly from PLLA melt at a shear rate of 13.6 s −1 and under a pressure of 100 MPa for the first time.…”

Quantification of pressure‐induced γ‐crystals in isotactic polypropylene: The influence of shear and carbon nanotubes

“…However, the combined effects of pressure, flow, and CNTs on γ‐phase formation of iPP still have not been well understood, which arouses our great interest in exploring the crystallization behaviors of iPP. Therefore, we used a homemade pressurizing and shearing device (PSD), which was introduced in detail in our group's previous work . With such a unique device, we were able to carry out a series of study of γ‐form formation for iPP/CNTs composite under coexistence of varies shear flow and pressures.…”