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1.
The effect of a novel active nucleating agent(TBC8-eb) on the isothermal crystallization of poly(L-lactic acid) (PLLA) was studied by differential scanning calorimetry(DSC) and Fourier transform infrared spectroscopy(FTIR) . The analysis on kinetics demonstrates that TBC8-eb can not only accelerate the crystallization rate but also transform most of the original spherulite crystals of PLLA into sheaf-like crystals. Furthermore,the free energy of folding(σe) of PLLA and PLLA with TBC8-eb is 0.15 and 0.06 J·m-2,respectively,which suggests that the addition of TBC8-eb favors the regular folding of molecule chains in the crystallization of PLLA,improv-ing its crystallization rate. The FTIR results show that TBC8-eb can accelerate the conformational ordering of PLLA in the isothermal crystallization. The conformational ordering of PLLA nucleated with TBC8-eb begins with the interchain interaction of CH3,and then a short helix emerges where a couple of CH3 groups interact. 相似文献
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The kinetics of crystal nucleation of poly(L-lactic acid) (PLLA) has been analyzed by fast scanning chip calorimetry in a wide temperature range between 313 and 383 K, that is, between temperatures about 30 K below and 40 K above the glass transition temperature. The relaxed melt was rapidly cooled to the analysis temperature and subsequently aged between 10−1 and 104 s, to permit formation of nuclei. The number of formed crystal nuclei has been probed by analysis of the crystallization rate at 393 K. The nucleation rate is maximal at 370–375 K and decreases monotonously with decreasing temperature in the analyzed temperature range. The observation of a monomodal dependence of the nucleation rate on temperature points to predominance of a single nucleation mechanism in the analyzed temperature range, regardless nucleation occurs in the glassy or devitrified amorphous phase. The collected data suggest that nuclei formation at ambient temperature requires a waiting time longer than about 108 s. The performed study is considered as a quantitative completion of nucleation-rate data available for PLLA only at temperatures higher than 360 K, suggesting that the nucleation mechanism is independent on temperature in the analyzed temperature range between 313 and 383 K. 相似文献
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Ling Chang 《Polymer》2011,52(1):68-76
Effects of poly(3-hydroxybutyrate) (PHB) on crystalline morphology of stereocomplexing capacity of poly(L- and D-lactic acid) (PLLA and PDLA) were studied by differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), atomic-force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). When crystallized at high Tc (130 °C or above), morphology transition of stereocomplexed PLA (sc-PLA) occurs from original well-rounded Maltese-cross spherulites to dendritic form in blends of high PHB contents (50 wt.% or higher), where PHB acts as an amorphous species. Microscopy characterizations show that morphology of sc-PLA in PHB/sc-PLA blends crystallized at Tc = 170 °C no longer retain original complexed Maltese-cross well-rounded spherulites; instead, the spherulites are disintegrated and restructured into two types of dendrites: (1) edge-on feather-like dendrites (early growth) and (2) flat-on wedge-like crystal plates (later growth) by growing along different directions and exhibiting different optical brightness. The concentration and/or distribution of amorphous PHB at the crystal growth front, corresponding to variation of the slopes of spherulitic growth rates, is a factor resulting in alteration and restructuring of the sc-PLA spherulites in the blends. Despite of spherulite disintegration, WAXD result shows that these two PHB-induced dendrites still retain the original unit cells of complexes, and thus these two new dendrites are sc-PLA. 相似文献
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以聚乳酸(PLLA)/四氢呋喃(THF)为淬火溶液,无其他添加剂条件下,通过低温淬火、萃取、洗涤和干燥得到直径为30.92μm±1.55μm的PLLA多孔微球,多孔微球由直径为0.34μm±0.06μm向外辐射的纤维组成。偏光显微镜表明多孔微球为球晶结构。XRD结果表明,多孔微球属于α晶型,晶粒尺寸大小为17.25nm。DSC结果表明,PLLA多孔微球的结晶度为36.05%。与熔融挤出造粒得到PLLA原料(结晶度小于10%)相比,低温淬火得到的多孔微球的结晶度大大提高。N2吸附-脱附结果分析表明,多孔微球的平均孔径和孔体积分别为42.92nm和0.1135cm3/g,大部分为大孔和介孔结构,比表面积和孔隙率分别为14.18cm2/g和93.15%。采用等温DSC模拟低温淬火过程研究了PLLA在THF溶液中结晶动力学,利用Avrami方程得到Avrami指数n平均值为2.29,说明PLLA在THF溶液中为异相成核和三维生长。 相似文献
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Poly(L-lactic acid) (PLLA) microfibers were obtained by a carbon dioxide (CO2) laser-thinning method. A laser-thinning apparatus used to continuously prepare microfibers was developed in our laboratory; it consisted of spools supplying and winding the fibers, a continuous-wave CO2-laser emitter, a system supplying the fibers, and a traverse. The laser-thinning apparatus produced PLLA microfibers in the range of 100-800 m min−1. The diameter of the microfibers decreased as the winding speed increased, and the birefringence increased as the winding speed increased. When microfibers, obtained through the laser irradiation (at a laser power of 8.0 W cm−2) of the original fiber supplied at 0.4 m min−1, were wound at 800 m min−1, they had a diameter of 1.37 μm and a birefringence of 24.1×10−3. The draw ratio calculated from the supplying and winding speeds was 2000×. The degree of crystal orientation increased with increasing the winding speed. Scanning electron microscopy showed that the microfibers obtained with the laser-thinning apparatus had smooth surfaces not roughened by laser ablation that were uniform in diameter. The PLLA microfiber, which was obtained under an optimum condition, had a Young's modulus of 5.8 GPa and tensile strength of 0.75 GPa. 相似文献
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Yoshimitsu Uemura Yoh-suke Maetsuru Tomoharu Fujita Masahiro Yoshida Yasuo Hatate Kenji Yamada 《Korean Journal of Chemical Engineering》2006,23(1):144-147
One of the most promising applications of poly(L-lactic acid) (PLLA) is food packaging. However, PLLA has a poorer gas barrier
property than ordinary food packaging polymers. An improvement of the water-vapor barrier property is therefore required.
In this study, low temperature plasma treatment on a stretched PLLA film in the presence of tetramethoxysilane (TMOS) was
carried out in order to improve its water-vapor barrier property. The flux of water-vapor permeation through a stretched PLLA
film treated with low temperature plasma in the presence of TMOS was decreased to 60% of an untreated stretched PLLA. The
effects of plasma treatment parameters on the water-vapor permeability were investigated. 相似文献
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The crystallization behavior of polylactic acid (PLA) was studied in the presence of a crystal nucleating agent, ethylenebishydroxystearamide (EBH). The crystallization rate and crystallinity were significantly increased with addition of EBH. The isothermal crystallization half-time at 105°C was decreased from 18.8 minutes for neat PLA to 2.8 minutes for PLA with 1.0 wt % of EBH. The crystallinity of PLA with 1.0 wt % EBH was about 35% after 5-minute annealing at 105°C. Like neat PLA, the double melting peaks were also observed for nucleated PLA. The changes of the double melt peaks were investigated with various crystallization temperatures, heating rates, and annealing times. The heat deflection temperature (HDT) of nucleated PLA was up to 93°C after annealing. The correlation between crystallinity and HDT was demonstrated. A percolation threshold of crystallinity was found corresponding to HDT. The crystal size of nucleated PLA was significantly decreased with addition of EBH. The mechanical properties of annealed PLA blends simultaneously; showed improved modulus and impact strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012 相似文献
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《Polymer-Plastics Technology and Engineering》2013,52(17):1875-1888
ABSTRACTThe poly (lactic acid)-fulvic acid graft polymer (PLA-FA) was synthesized with lactic acid and fulvic acid (FA). The optimum parameters were determined by orthogonal experiment. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy exhibited that FA was successfully grafted onto PLA. Then, PLA/PLA-FA composites were prepared with PLA-FA as fillers by melt blending. The structure characterization and performance tests demonstrated that PLA-FA effectively enhanced the comprehensive performance of PLA composites. The rheological analysis demonstrated that PLA-FA had plasticization effect. The non-isothermal crystallization kinetics demonstrated that PLA-FA promoted the crystallization rate of PLA composites, improving toughness of PLA composites. 相似文献
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Effect of the addition of poly(D-lactic acid) (PDLA) as stereocomplex (SC) on crystallization behavior of poly(L-lactic acid) (PLLA) had been systemically investigated. The result indicated that the inclusion of PDLA with higher MW into PLLA exhibited lower t 1/2 and showed accelerated crystallization behavior. Meanwhile, SC formed in blends with higher MW of PDLA exhibited enhanced nucleation activity. In combination with both DSC and WAXD analysis, it was believed that nucleation process was more related to the crystalline size of SC. The result in this study would provide guidance for the application of SC as nucleating agent for the PLA-based products. 相似文献
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Soap-free emulsion polymerization of styrene using poly(methacrylic acid) macro-RAFT agent 总被引:2,自引:0,他引:2
Yeonhwa Wi 《Polymer》2008,49(26):5626-5635
A well-defined poly(methacrylic acid) (PMAA) macro-RAFT agent has been synthesized by the bulk polymerization using 4-toluic acid dithiobenzoate as a RAFT agent and successfully employed as a reactive emulsifier in the soap-free emulsion polymerization of styrene, leading to a formation of stable latex. The amphiphilic block copolymer, prepared from the in situ micelle formation, contains a hydrophilic PMAA block and a hydrophobic PS block, via styrene monomer transfer reaction to the dithioester function in PMAA macro-RAFT agent during the nucleation step. The chemical structure of the synthesized PS with the PMAA macro-RAFT agent was confirmed using FTIR and NMR. In addition, it was confirmed that the macro-RAFT agent is present on the particle surface via the ESCA measurement. The reaction mechanism was proposed that the stable spherical particles enlarged by the aggregation of small particles, which were also produced by the chemical or physical bonding between the tiny small particles. The results indicate that the PMAA macro-RAFT agent is used as emulsifier for the formation of PS particles and block copolymer [P(S-b-MAA)] in situ. 相似文献
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为了了解聚L-乳酸(PLLA)自身结构特点,更好地控制其成型加工过程,研究了PLLA的非等温结晶行为、熔融行为和热失重过程。结果表明:降温速率对PLLA的非等温结晶过程具有显著影响,在1℃/min的降温速率下,PLLA的结晶起始温度为121℃,结晶焓为3.363 J/g;PLLA的熔融双峰遵循熔融-再结晶的机理;PLLA热分解温度在300℃左右,且随升温速率的增加而增大。 相似文献
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Yuan Tian Muqing Liu Wentao Liu Jie Cheng Gensheng Wu Ting Han Yi Zhang Gutian Zhao Zhonghua Ni 《应用聚合物科学杂志》2021,138(22):50517
In this paper, the effects of annealing temperature on both radial supporting performance and axial flexibility of poly(L-lactic acid) (PLLA) braided stents are studied. Stents are annealed at a series of temperatures ranging from 80 to 160°C for 1 h, then indicators of shaping effect, radial supporting performance, and axial flexibility are compared. Stents not annealed and annealed at 80°C cannot be completely shaped. In contrast, stents annealed at 100 to 160°C are well shaped showing radial shrinkage rate of 1.0 ± 0.2% and almost no axial elongation. The radial compressive force and axial force are gradually increased by 51.2% and 89.2%, respectively with the annealing temperature gradually increasing from 80 to 160°C, indicating that the radial supporting performance is improved but axial flexibility is weakened by a higher annealing temperature. Taken together, PLLA braided stents can be annealed at 100 to 120°C to obtain sufficient radial force and lower axial force simultaneously for clinical applications. Moreover, tensile test, X-ray diffraction, and differential scanning calorimetry are performed for monofilaments annealed at different temperatures to further explore the effect mechanism of annealing temperature on the mechanical properties of stents. This study may provide helpful suggestions for the manufacture of biodegradable braided stents. 相似文献
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以异烟肼和均苯三酸为原料合成了新型均苯三酸三异烟肼,考察了其对聚L-乳酸(PLLA)结晶性能和热稳定性的影响。结果表明:均苯三酸三异烟肼起到了异相成核的作用,能有效提升PLLA的结晶速率,其中添加质量分数2.0%的均苯三酸三异烟肼可使PLLA有最大非等温结晶焓,但过量的均苯三酸三异烟肼却不利于PLLA的结晶。均苯三酸三异烟肼的加入不会改变PLLA的热分解行为,但随着均苯三酸三异烟肼含量的增加,其起始分解温度下降。 相似文献
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The isothermal and nonisothermal crystallization kinetics of nonnucleated and nucleated isotactic polypropylene (iPP) were investigated by DSC and a polarized light microscope with a hot stage. Dibenzylidene sorbitol (DBS) was used as a nucleating agent. It was found that the crystallization rate increased with the addition of DBS. The influence of DBS on fold surface energy, σe, was examined by the Hoffman and Lauritzen nucleation theory. It showed that σe decreased with the addition of DBS, suggesting that DBS is an effective nucleating agent for iPP. Ozawa's theory was used to study the nonisothermal crystallization. It was found that the crystallization temperature for the nucleated iPP was higher than that for nonnucleated iPP. The addition of DBS reduced the Ozawa exponent, suggesting a change in spherulite morphology. The cooling crystallization function has a negative exponent on the crystallization temperature. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2089–2095, 1998 相似文献
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The aerosol solvent extraction system process (ASES), which is one of the supercritical anti solvent processes (SAS), was
used to produce poly(L-lactic acid) (PLLA) into the submicron particles. Dichloromethane (DCM, CH2Cl2) and carbon dioxide were selected as a solvent and as an antisolvent for PLLA, respectively. The objective of this study
was to investigate the effect of the various process parameters such as temperature, pressure, and solution concentration
on PLLA particles. With increasing temperature and pressure, particle size was increased. Also, higher PLLA concentration
led to larger particle size and broader particle size distribution. A scanning electron microscope (SEM) was used to observe
the morphology and size of PLLA particles recrystallized by ASES process. The mean particle size and its distribution of processed
particles were measured by using a laser diffraction particle size analyzer (PSA). 相似文献