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Researcher Liao Xia of Sichuan University and Professor Li Guangxian's group: the effect of carbon dioxide on the nucleation and growth of polylactic acid crystals
2020-01-09 Source: Polymer Technology

Recently, researcher Liao Xia and Professor Li Guangxian from the School of Polymer Science and Engineering of Sichuan University published a paper on Macromolecules about the crystallization behavior of polylactic acid under carbon dioxide conditions. The first author of the thesis is Li Shaojie, a PhD student.

As a bio-based polymer material, polylactic acid has attracted wide attention due to its biodegradability and good biocompatibility. In order to reduce the weight of polylactic acid material and expand the application range of polylactic acid, environmentally friendly carbon dioxide can be used as a foaming agent to prepare polylactic acid microcellular foam material. Previous studies have shown that carbon dioxide dissolves in polymers and causes the polymers to swell, which increases the free volume of the polymer and decreases the glass transition temperature and melting point. Due to the special Lewis acid-base interaction between carbon dioxide and polylactic acid, the solubility of carbon dioxide in polylactic acid is high, so that polylactic acid exhibits a special crystallization behavior under the condition of carbon dioxide. At present, the research on the crystallization behavior of polylactic acid under the condition of carbon dioxide is not deep enough, especially in terms of the formation and morphology control of polylactic acid crystal structure by carbon dioxide.

Based on the above problems, the author systematically studied the effect and mechanism of carbon dioxide on crystal nucleation and growth during polylactic acid crystallization. Studies have found that the introduction of carbon dioxide reduces the molecular chain's ability to move, reduces the possibility of forming embryos larger than the critical size during thermal fluctuations, and makes it difficult for the embryos to develop into stable nuclei; Previous research by the team found that carbon dioxide was dissolved in polylactic acid, which caused the polymer to swell and weakened the interaction between polylactic acid molecular chains. According to Flory's patch panel theory, the weakening of the interaction between molecular chains also makes it difficult to fix the molecular chains in the crystal lattice and form stable crystal nuclei. Therefore, under the experimental conditions of this paper, carbon dioxide inhibits the crystal nucleation of polylactic acid and reduces the number of crystals.

以上时引入二氧化碳会减缓晶体生长速率,在100 以下时二氧化碳的引入则促进了聚乳酸球晶的生长过程。 Through the research on the growth rate of polylactic acid crystals under the condition of carbon dioxide, it was found that the introduction of carbon dioxide at a temperature above 100 slowed down the crystal growth rate, and the introduction of carbon dioxide at a temperature below 100 promoted the growth of polylactic acid spherulites. The author believes that when the crystallization temperature is higher, the molecular chain's ability to move is stronger, and the introduction of carbon dioxide will further enhance the molecular chain's ability to move, so that the molecular chain is difficult to be fixed in the crystal lattice, thereby inhibiting the crystal growth; and the lower the crystallization temperature Below, the molecular chain has poor motion ability, which limits its migration into the crystal lattice. If carbon dioxide is introduced, it can accelerate the rate of molecular chain discharge into the crystal lattice and accelerate the spherulite growth process.

and 100 under air and different CO 2 pressures. Figure 1 POM photos of PLA after 4 hours' isothermal crystallization at 110 and 100 under air and different CO 2 pressures.

2 . Figure 2 Schematic representations for the mechanisms of nucleation of PLA crystals under (a) air and (b) CO 2 .

The authors found that the introduction of carbon dioxide increased the platelet thickness of the polylactic acid crystals, thereby increasing the melting point of the polylactic acid crystals. From the results of AFM, it was found that branching of polylactic acid platelets was suppressed under the condition of carbon dioxide. On the one hand, according to Keith and Padden's theory, the higher the crystallization temperature, the fewer the branching of the platelets. The effect of carbon dioxide is similar to temperature, so the introduction of carbon dioxide inhibits the formation of platelet branches; on the other hand, the platelet emits carbon dioxide outward during the growth process, which may form a local high pressure between platelets and hinder platelet branches. The generation. Therefore, during the growth of spherulite, the nucleation point of secondary nucleation may choose to grow in the z-axis direction or tile in the radial direction, thereby increasing the platelet thickness.

2 Figure 3 AFM height images and phase images of PLA treated under air and CO 2

This dissertation was strongly supported by the National Natural Science Foundation of China, the Special Independent Project of the State Key Laboratory of Polymer Materials Engineering of Sichuan University, and the Open Project of the Guangdong Key Laboratory of Advanced Polymer Manufacturing Technology and Equipment, South China University of Technology.

Article link: http://pubs.acs.org/doi/10.1021/acs.macromol.9b01601

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