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 β-form can be obtained through shear-induced crystallization. In addition, the fraction of β-PLLA str...