【作者】 方华高；

【导师】 王志刚；

【作者基本信息】 中国科学技术大学 ， 材料加工工程， 2014， 博士

【摘要】 聚乳酸(PLA)是一类热塑性脂肪族聚酯材料,具有生物降解性,生物相容性,较均衡的力学性能和可加工性等优点,被认为是最有潜力替代石油基聚合物的生物质材料。然而,聚乳酸也有些与生俱来的缺点,包括熔体强度低、结晶速度慢、韧性差等,严重限制了其应用范围。本论文工作中,针对聚乳酸的缺点,对其进行长链支化和共混增韧改性研究,提升了相关性能,为拓展其应用领域提供解决方案。同时,详细深入研究拓扑链结构改性,外加流动场,增韧改性剂与聚乳酸材料的各项性能之间关系,认识和理解其中的科学本质。主要内容包括以下四个部分：1.通过线性聚乳酸和三官能度单体三羟甲氧基丙烷三丙烯酸酯(TMPTA)共混,然后进行伽马射线辐射,引发自由基反应,在体系中引入长链支化结构,制备高熔体强度聚乳酸材料。黏度、储能模量、损耗角和Cole-Cole图等流变学特性显示长链支化聚乳酸(LCB PLA)的熔体强度获得了提高。同时,研究了长链支化结构的引入对拉伸流变性质的影响,长链支化聚乳酸样品展现出应变硬化的特性。发泡实验的研究表明,熔体强度的提升有利于提高聚乳酸的发泡能力。2.采用配备多角度光散射检测器的体积排除色谱(SEC-MALLS)和流变学参数分析法研究LCB PLA的拓扑链结构。SEC-MALLS测试表明,伽马射线辐射法制备的长链支化聚乳酸不仅有较高的重均分子量而且是由短的线性链组分和长支化链组分组成的二元链结构。通过热流变行为和活化能研究,长链支化聚乳酸的二元链结构特征被进一步确认。零剪切黏度与重均分子量的依赖关系(η0-MW图)表明长链支化聚乳酸是树枝状的拓扑结构。伽马射线辐射的剂量率显著低于电子束辐射的特点可以解释两种辐射条件导致不同链结构的原因。3.采用旋转流变仪和偏光显微镜研究长链支化结构在聚乳酸剪切诱导等温结晶过程中对成核密度提高和结晶形态演变的影响。剪切诱导等温结晶动力学结果表明,与静态条件相比,剪切作用显著地促进结晶过程且随着剪切时间的增加,结晶动力学加速明显。同一条件下,LCB PLA比线性PLA结晶速率更快。线性PLA和LCB PLA都存在着剪切时间对结晶动力学提高的饱和效应。原位偏光显微镜观察表明,相比于线性PLA,LCB PLA不仅具有在恒定剪切时间下更高的成核密度和较低的球晶生长速率,而且在足够长的剪切时间下形成了“shish-kebab"结构。LCB PLA在剪切作用下表现出的促进成核密度增加以及从球晶到"shish-kebab"结构的形态转变可归因于其拓宽且复杂的链段松弛行为。4.应用反应共混方法制备了具有生物相容和可降解性的高韧性聚乳酸和交联聚乙二醇二丙烯酸酯共混物。扭矩分析和FT-IR谱图证实聚乙二醇二丙烯酸酯(PEGDA)的原位交联反应是由丙烯酸酯基团按照自由基聚合机理进行的。差示扫描量热仪(DSC)和相差显微镜(PCOM)结果表明PEGDA原位聚合反应导致共混物呈现相分离的形貌结构,其中交联PEGDA为分散相,PLA为连续相。随着交联PEGDA含量提高,共混物熔体的黏度和弹性逐渐提高,当含量为15wt%时达到流变逾渗值。体系中引入交联PEGDA对PLA的结晶度影响很小,但力学性能获得很大的提升。分散相表面悬挂的PEG链段和两相间酯交换产物是获得有效界面相容性的主要原因。更多还原

【Abstract】 Polylactide (PLA) is a type of aliphatic thermoplastic polyester and possesses a number of interesting properties including biodegradability, biocompatibility, sufficient mechanical properties, and processability, which make it become one of the most competitive and promising candidates to substitute some petroleum-based polymers in future applications. However, PLA shows some inherent drawbacks including poor melt strength, low crystallization kinetics and brittleness. In this thesis, a series of studies is presented in an attempt to overcome these drawbacks of PLA and extend its application. Meanwhile, the thesis provides further scientific insights into the roles of chain topological modification, external flow field and toughening modifier on the properties of PLA. Main content includes four aspects:1. An easy procedure was applied to prepare high melt strength polylactide (PLA), which involved gamma radiation induced free radical reactions to introduce long chain branched structure on linear PLA precursor with addition of a trifunctional monomer, trimethylolpropane triacrylate (TMPTA). Various rheological plots including viscosity curve, storage modulus, loss angle and Cole-Cole plot are used to distinguish the improved melt strength for LCB PLA samples. The effect of LCB structure on elongational rheological properties is further investigated. The LCB PLA samples demonstrate the enhancement of strain-hardening under elongational flow. The enhanced melt strength substantially improves the foaming performance of LCB PLA samples.2. The topological structures of LCB PLA were investigated by SEC-MALLS and rheological analysis. SEC-MALLS measurements show that LCB PLA exhibits not only the increased weight-average molecular mass but also a bimodal architecture with a short linear chain fraction and a LCB fraction. By the analysis of the thermorheological behaviors and determination of activation energies, the bimodal architecture is confirmed. A conclusion with respect to the tree-like topography for LCB PLA samples is drawn from the molecular mass dependences of zero-shear viscosity (ηo-Mw, plot). An explanation to these findings is provided under the consideration of the radiation dose rate for the gamma radiation.3. The effects of long chain branching on the nucleation density enhancements and morphological evolution for polylactide (PLA) materials during shear-induced isothermal crystallization process were thoroughly investigated by using rotational rheometer and polarized optical microscopy (POM). The results of shear-induced isothermal crystallization kinetics show that the crystallization process under shear is greatly enhanced compared to the quiescent conditions and the crystallization kinetics is accelerated with the increases in shear rate and/or shear time. LCB PLA crystallizes much faster than linear PLA under the same shear condition. A saturation effect of shear time on crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations demonstrate that LCB PLA not only possesses higher nucleation density under the identical shear time and a constant lower value of spherulitic growth rate compared with that of linear PLA but also forms the shish-kebab structure after sheared for sufficient time. A saturation of nucleation density under shear can be reached for both linear PLA and LCB PLA. The enhancement of nucleation ability and the morphological evolution from the spherulitic to shish-kebab structures induced by shear flow can be ascribed to the broadened and complex relaxation behaviors of LCB PLA.4. Super-tough biocompatible and degradable binary blends of polylactide (PLA) and crosslinked poly(ethylene glycol) diacylate (CPEGDA) were fabricated by applying a novel and facile method involving reactive blending of PLA with PEGDA monomer with no addition of exogenous radical initiators. Torque analysis and FT-IR spectra suggest that crosslinking reaction of acylate groups occurs in melt blending process according to the free radical polymerization mechanism. Differential scanning calorimetry (DSC) and phase contrast optical microscopy (PCOM) results indicate the in-situ polymerization of PEGDA leads to a phase separated morphology with crosslinked PEGDA as the dispersion phase domains and PLA matrix as the continuous phase. The blends show increasing viscosity and elasticity with increasing crosslinked PEGDA content with a rheological percolation crosslinked PEGDA content of15wt%. Introduction of crosslinked PEGDA shows little effect on crystallinity of PLA in the blends. Mechanical properties of these blends are improved significantly. The effective interfacial compatibility is achieved by the dangling PEG chains and transesterification reactions at the interfaces between crosslinked PEGDA particles and PLA matrix.更多还原