Isothermally crystallization behavior of poly (L‐lactide) from melt under high pressure

Abstract

The poly (L-lactide) (PLLA) samples from melt were isothermally crystallized at 175°C and 190°C under pressures (P C ) ranging from 0.1 to 400 MPa. The crystalline structures and thermal properties were investigated by using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). On the basis of the DSC and WAXD results, it is confirmed that an α crystal was formed in the low-pressure region (P C ≤ 50 MPa), and the disordered α crystal (α′) was formed in the medium-pressure region (100 MPa ≤ P C ≤ 200 MPa) for the PLLA samples isothermally crystallized at 175°C. However, the only α crystal was formed in the pressure range from 0.1 to 200 MPa, and no α′ crystal could be found in the PLLA samples isothermally crystallized at 190°C. Interestingly, the full amorphous PLLA was obtained at 175°C and 190°C, when the P C was above 250 MPa. There seems to exist a cut-off pressure for the crystallization of PLLA, which divides the pressure into high and low regions.The peculiarities of crystalline structure could be explained by the enhanced melt supercooling and the confined crystallization behaviors of PLLA under high pressure.The effect of the compression rate on the formation of amorphous PLLA was also researched with different compression rates. The results indicate that the molten PLLA could be easy to solidify as full amorphous phase under the higher pressure at a lower critical compression rate. KEYWORDS high pressure, isothermally crystallization, melt, PLLA | INTRODUCTIONWith the development of polymer science, plastics bring us civilized conveniences. However, a large amount of waste plastics have been a serious environmental problem. We live in the plastic age (the "plasticene"), producing over 300 million tons (mt) of plastic every year globally, 5 to 15 mt of which flow into already polluted oceans. 1 Poly (L-lactide) (PLLA) has received much attention over the past few decades, because it is renewable, nontoxic, biodegradable, and biocompatible. [2][3][4] Furthermore, PLLA has unique properties, such as high modulus, high strength, and good clarity. Therefore, it has been widely applied in the fields of packing textile, and tissue engineering, making it one of the most potential alternatives for petroleum-based commodity polymers. 5,6 Due to its limited application because of its brittleness at room temperature, PLLA is usually blended with plasticizers to modify the brittle behavior for improving the application performance. 7,8 In contrast with the aforementioned approaches, regulating the crystalline structure and morphology of PLLA through controlling crystallization conditions can also effectively improve its impact resistance but can avoid aforementioned problems.Poly (L-lactide) is known to form three kinds of crystal modification, namely, the α, 9 β, 10 and γ 11 forms, along with a new α crystal modification, named the α′ form. 12 The resulting formation of crystal structure depends on various processing conditions, including temperature, shearing, and stretching. The most co...

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“…Isothermal crystallization behavior of PLLA has been broadly investigated and reported over the past 20 years [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Typically, PLLA can form different crystal structures through different processing routes.…”