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

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 beta-form directly from PLLA melt crystallization at our experimental condition (shear 13.6 s(-1), pressure 100 MPa, and crystallization temperature 160 degrees C). Undoubtedly, abundant beta-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 beta-form can usually be obtained by hot-drawing or solid coextrusion under a high tensile ratio, suggesting that PLLA beta-form can be obtained through shear-induced crystallization. In addition, the fraction of beta-PLLA strongly depends on supercooling and shear intensity. A higher supercooling (pressure 150 MPa and crystallization temperature 160 degrees C) could also induce predominant beta-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 degrees C), we did not observe any trace beta-form. In the heating experiment to investigate crystal form transformation, we also found that partial beta-form transformed into alpha-form through melting-crystallization, and meanwhile some beta-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.