Unique Homo-Crystallization and Melting Behavior of Poly(L-lactic acid): Influence of Stereocomplex with Varied Structure

Homo-crystallization and melting behavior of poly(L-lactic acid) (PLLA) with poly(D-lactic acid) (PDLA) (≤10 wt.%) was studied. The different thermal history had been applied to exert structural variation on stereocomplex (SC). The PLLA/PDLA blend showed different crystallization and melting behavior when cooled from 250°C or 200°C. Double melting peaks were observed after the blend was cooled from 250°C. SC annealing at different temperatures exhibited significant effect for melt-crystallization of PLLA. Influence of initial melting condition before cooling was also investigated. The cold crystallization of amorphous blend initially was studied and some novel results had been observed.     

Effects of molten poly(3-hydroxybutyrate) on crystalline morphology in stereocomplex of poly(L-lactic acid) with poly(D-lactic acid)

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 T_c (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 T_c = 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.     

聚L-乳酸的热性能研究

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。     

Kinetics of crystal nucleation of poly(L-lactic acid)

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.     

Enhancing the PLA Crystallization Rate and Mechanical Properties by Melt Blending with Poly(styrene-butadiene-styrene) Copolymer

Poly(styrene-butadiene-styrene) (SBS) copolymer was used to enhance the crystallization ability and mechanical properties of poly(L-lactic acid) (PLA) through melt mixing. The thermogravimetric analysis and Fourier transform infrared results revealed the n–π interaction between PLA and SBS. Differential scanning calorimetry and X-ray diffraction analysis indicated that the crystallization ability of the PLA improved substantially. The PLA crystallized completely during the cooling process when the SBS content in the blend was above 20 wt.%. The elongation at break and the impact resistance were significantly improved compared with that of neat PLA when the SBS content was above 50 wt.%.     

聚乳酸的成型加工及其降解性能

研究了可生物降解材料－－聚乳酸（PLLA）的流变性能、纺丝成型性能及降解性能,发现PLLA熔体为切力变稀流体,非牛顿指数n=0.93,具有良好的挤出成型性能。同时,通过PLLA纤维和薄膜在不同环境中的降解实验,表明聚乳酸是一种良好的生物降解医用材料和环保塑料。     

一种新型成核剂对聚乳酸等温结晶行为的影响(英文)

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.     

增塑剂对生物降解塑料聚乳酸结晶形态的影响

采用熔融共混方法制备了聚乙二醇(PEG)和聚乳(酸PLA)的共混物,并借助差示扫描量热仪和广角X射线衍射仪研究了在80~140℃等温结晶时PEG对PLA结晶形态和熔融行为的影响。结果表明:100℃以上时PLA的结晶度随结晶温度的升高逐渐增加;100℃以下时PLA的结晶度随PEG含量的增加逐渐增大。110℃以下时,晶面间距随结晶温度的升高和PEG含量的增加而逐渐减小;110℃以上时,晶面间距保持不变;当加入15%的PEG时PLA的晶面间距不受结晶温度的影响结,晶生成的是稳定的α晶。     

聚乳酸/纳米二氧化钛复合材料的制备及其性能研究

以纳米二氧化钛对聚L-乳酸进行共混改性,并对所得PLLA/TiO2复合材料的结构与性能进行了分析表征。实验结果表明,所得复合材料处于无定型态,退火处理后结晶,TiO2不改变PLLA基体结晶的α晶型,但结晶衍射峰强度随TiO2添加量增加而减弱。TiO2对PLLA基体的结晶性能和熔融行为影响较小,表明TiO2对PLLA的异相成核作用有限。     

Study on the Crystallization Kinetic and Characterization of Poly(lactic acid) and Poly(vinyl alcohol) Blends

In this study, poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVA) blends, with PLA/PVA mass ratios of 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, and 40/60, were prepared by means of the melt blending method. The result of torque measurements and thermal gravimtric analysis tests showed that the addition of PLA can decrease the melt viscosity of PVA and that the second degradation step of PVA nearly disappeared for the PLA₈₀/PVA₂₀ blend. The absorbance peaks of the carbonyl group and the hydroxyl group in the Fourier transform infrared spectra of PLA/PVA blends had significant shifts to lower wave numbers, indicating that there were interactions between these two groups. Combined with the result of the differential scanning calorimetry curves, this interaction would be favorable for improving miscibility. The X-ray diffraction patterns and the polarized light microscope (PLM) micrographs showed that PVA can serve as a nucleating agent to promote the crystallization of PLA in PLA/PVA blends. Moreover, the PLA₈₀/PVA₂₀ blend gave the highest growth rate of PLA spherulite.     

熔融缩聚法合成高分子量聚L-乳酸

以L-乳酸为原料,采用分步除水、丙交酯回流、优选催化剂等工艺,通过熔融缩聚法制备了高分子量聚L-乳酸(PLLA),研究了脱水产物、反应装置、催化剂、聚合工艺等对产物分子量、产率及色泽的影响,并对产物分子结构进行了表征。在加装丙交酯回流装置后,以Sn(Ot)2/SnC l2.2H2O/TSA为催化剂,在170℃,反应12h所得PLLA黏均分子量(Mη)可达10.6万,产率为76%。     

Superstructure and mechanical properties of poly(L-lactic acid) microfibers prepared by CO2 laser-thinning

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.     

聚乙二醇增塑聚乳酸的等温结晶动力学研究

采用DSC方法对聚乙二醇(PEG)增塑聚乳酸的等温结晶动力学进行了研究。结果表明,PEG的加入提高了聚乳酸的结晶速度,在相同结晶温度下,随着PEG摩尔质量的增大,结晶速率增大,结晶活化能先减小后增大。不同摩尔质量PEG增塑PLA的结晶成核机理和生长方式一样。     

Studies on the Crystallization and Melting Behavior of Poly(ethylene 2,6-naphthalate)

Abstract

Crystallization behaviour of low molecular weight (oligomeric) and high molecular weight poly(ethylene 2,6-naphthalate)s (PEN) was studied using wide angle X-ray diffraction (WAXS) and differential scanning calorimetry (DSC). It was found that the crystallization conditions determine the nature of crystalline modification. Crystallization from the glassy state gives a-modification, whereas, crystallizing from the melt above 220°C results in β-modification. In contrast, the oligomers gives both the modifications up on crystallizing from the melt. The equilibrium melting temperatures of PEN were determined from DSC experiments, based on conditions of crystallization of a and β-modifications.     

聚乳酸成核剂的研究进展

回顾并展望了聚乳酸成核剂的研究与开发进展,比较和评价了不同类型成核剂对聚乳酸结晶行为的影响,在此基础上提出了开发聚乳酸专用成核剂的思考和建议。     

环氧扩链端羧基乳酸预聚物

L-乳酸先熔融缩聚成乳酸预聚物,再以丁二酸酐封端,最后以环氧树脂AG80进行扩链.采用傅里叶红外光谱仪、差示扫描量热仪与热重分析仪对扩链产物进行结构性能表征.探讨丁二酸酐用量、扩链剂用量、催化剂类型与用量、反应条件对扩链反应的影响.结果表明:丁二酸酐用量2.5％,环氧基团/羧基摩尔比1.5:1,催化剂为2-甲基咪唑,催化剂用量1.5％,在160℃下反应10 min时,扩链后聚乳酸粘均分子量达到145600.     

Hetero-stereocomplex formation between substituted poly(lactic acid)s with linear and branched side chains,poly(l-2-hydroxybutanoic acid) and poly(d-2-hydroxy-3-methylbutanoic acid)

Differential scanning calorimetry and wide-angle X-ray diffractometry first revealed the formation of hetero-stereocomplex (HTSC) between biodegradable, optically active, and isotactic poly(2-hydroxyalkanoic acid)s having different chemical structures and opposite configurations, i.e., l-configured substituted poly(lactic acid) (PLA) [poly(l-2-hydroxybutanoic acid), P(l-2HB)] with linear side chains (ethyl groups) and d-configured substituted PLA [poly(d-2-hydroxy-3-methylbutanoic acid), P(d-2H3MB)] with branched side chains (isopropyl groups) in solution and in bulk from the melt. The melting temperature of P(l-2HB)/P(d-2H3MB) HTSC crystallites was 197–204 °C, which is much higher those of P(l-2HB) and P(d-2H3MB) homo-crystallites (100–101 °C and 158–165 °C, respectively). The interplain distances and crystalline lattice sizes of P(l-2HB)/P(d-2H3MB) HTSC crystallites were respectively larger and smaller than those of P(l-2HB)/P(d-2HB) and P(l-2H3MB)/P(d-2H3MB) homo-stereocomplexes. The HTSC formation of substituted PLA with opposite configurations reported in the present study will provide a versatile way to prepare poly(2-hydroxyalkanoic acid)-based biodegradable materials having a wide variety of physical properties and biodegradability.     

聚乳酸纤维的静电纺丝及其形态结构研究

采用二氯甲烷为溶剂,以滚筒为收集装置,利用静电纺丝法制备了聚乳酸纳米纤维。分析了溶液体系和滚筒转速对纤维形态结构的影响。结果表明:在质量分数相同的条件下,采用相对分子质量较大的聚乳酸切片所纺纤维直径细而均匀;质量分数增加时,电纺丝产品由一些高分子微/纳米液滴渐变为成形较好、珠状较少的平滑纤维,其平均纤维直径先增加后减小;控制收集滚筒的转速在一定范围内,可以获得排列取向较好的纤维。     

Higher-order structures and mechanical properties of stereocomplex-type poly(lactic acid) melt spun fibers

Equimolar blend of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) was melt spun into fibers and the relations among the processing conditions, crystalline structures, thermal properties, and mechanical properties were investigated. Drawing and annealing were performed in order to obtain fiber mainly consisting of the stereocomplex crystal phase. Fibers drawn at various temperatures exhibited either amorphous, highly oriented homo crystal, or the mixture of homo and stereocomplex with a fairly low orientation depending on the drawing temperature. Annealing of the drawn fibers at an elevated temperature higher than the melting temperature of homo crystal increased the stereocomplex content significantly. The fractions of the homo and the stereocomplex crystals strongly depended on the higher-order structure of the drawn fibers and the annealing temperature.     

熔融缩聚法合成的聚乳酸熔体的流变性能研究

采用锥板流变仪研究聚L-乳酸(PLLA)的流变性能,并对直接法熔融缩聚制备的聚乳酸进行了测试。结果表明:PLLA熔体属于切力变稀流体;PLLA熔体的表观粘流活化能较高,其表观黏度对温度的敏感性高。