不同分子量与构型结构的聚乳酸均聚物与立体共聚物的凝聚态、热力学及结晶动力学研究
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摘要
近二十年来,生物降解高分子在基础研究和化学工业领域都是研究热点。多种脂肪族聚酯都具有良好的水解性质,降解产物对环境和生物体没有毒性,应用前景广阔。其中,聚乳酸(PLA)被认为是最有吸引力的可生物降解聚酯材料。因为PLA原料乳酸可以通过生物技术制备,PLA为热塑性聚合物,具有高强度和高模量,可以应用现有技术工艺进行成型加工。目前,大量文献报道了PLA及其共聚物在生物医用方面的研究成果。PLA可作为组织工程支架材料,制备可被人体吸收的医用植入体,药物释放系统中可作为药物载体,也可作为人体创口敷膜等。PLA的各种应用研究依赖于其特定的物理性质,如PLA作为骨科材料必须具有高模量,则需要较高的分子量和特定的凝聚态结构,而如果作为药物缓释材料,降解周期是一个重要参数,分子量相对较低。所以,PLA的分子量、分子结构、热行为、凝聚态结构和形貌等相关基础研究具有重要意义,这也是本论文的主要研究内容。
目前PLA的合成多采用辛酸亚锡为催化剂,PLA的结晶结构、热行为等物理性质研究也是基于辛酸亚锡体系。但是,辛酸亚锡对于生物体具有一定毒性,可能限制PLA的医学应用。本论文中,PLA通过锌催化开环聚合制备,而锌催化剂对生物体无毒性作用。在此基础上,合成制备了不同分子量、不同分子结构的PLA,研究PLA的热力学行为、结晶结构和形貌等。主要内容如下:
(1)制备了左旋聚乳酸(PLLA)样品,研究了PLLA基本热力学行为、微观形貌等,实验结果表明PLLA的结晶行为与催化剂没有明显的联系。等温结晶研究中,PLLA的熔融重结晶会引起双重熔融行为,等温可观察到有限的片晶增厚现象,结晶结构以稳定的α晶型结构存在。Avrami方程分析表明PLLA等温结晶为异相成核、三维生长,在105℃附近达到最大总结晶生长速率。PLLA球晶形貌观察到同心裂纹、环形或者六边形,形成原因是球晶的周期性生长特点和冷却时的热收缩效应。使用自成核、非等温方法研究了PLLA的球晶生长速率,并应用Lauritzen-Hoffman方程非线性拟合PLLA球晶生长速率,得到PLLA的结晶动力学参数。
(2)研究了分子量对PLLA的热力学以及动力学的影响。Flory相关公式可以推测,当分子量趋向于极大时,PLLA玻璃化转变温度为60.0℃,熔融温度为178.8℃。Avrami方程表明,分子量越高,PLLA总结晶速率越低,高过冷度时结晶诱导时间也越长。最大总结晶速率和分子量之间存在着标度律,幂律为-0.5。PLLA熔体等温结晶研究中,采用线性和非线性Hoffman-Weeks方程计算PLLA的平衡熔融温度,阐明了线性HW方程对于低分子量PLLA熔融温度计算失效的原因,得到极大分子量PLLA的平衡熔融温度为207.6℃。PLLA玻璃化转变研究中,发现了双吸热峰对应的热焓恢复现象,PLLA结晶时出现的受限无规态和自由无规态分子链是形成双峰的原因。通过化学刻蚀观察到板条状晶沿径向生长,片晶结构规整。Lauritzen-Hoffman理论分析得到不同分子量PLLA的结晶动力学上的区域Ⅲ与区域Ⅱ的转变温度在115-120℃。
(3)研究不同立体构型的PLA的结晶行为、形貌和酶降解过程。DSC研究显示不同右旋乳酸单元含量的PLA立体共聚物有着相同的结晶机制。ESEM研究确证了蛋白酶K对于PLA熔体等温结晶薄膜的自由非晶和受限结晶区域的降解作用。球晶中的片晶在降解过程中保持其空间排布,而并不会坍塌。球晶的成核位置在材料表面或是内部可能是观测到的两类PLA降解形貌的形成原因。FTIR研究确证了半结晶PLA中晶体特征峰和非晶特征峰的存在。921 cm~(-1)晶体特征峰的分裂说明了受限非晶区的降解影响了晶区链段的振动状态,而955 cm~(-1)非晶特征峰附近出现肩峰确证了蛋白酶K降解过程中PLA非晶组分的物理老化现象。
(4)研究了PLLA/PDLA的立体复合晶体。共混比例1:1,左旋和右旋PLA分子量接近。实验应用了三种PLLA/PDLA体系,目的是探讨不同分子量和不同旋光度对立体复合晶体的影响,研究立体复合晶体的晶体结构、热行为、酶降解过程等等。立体复合结晶受熔体状态的影响,当熔融温度升高或者熔融时间延长时,立体复合结晶将急剧减弱甚至消失,但通过溶液结晶,立体复合结晶可以再次恢复。异相和均相熔体的模型假设可以解释此实验现象。
During the past two decades,biodegradable polymers have attracted much interest in both basic research and chemical industry.Aliphatic polyesters present excellent degradation properties,with degradation by-products harmless to the environment and animal body.Especially,polylactide(PLA) is regarded as the most promising biodegradable polyester.The raw material of PLA,lactic acid,can be obtained from renewable sources.PLA is a thermoplastic with high strength and high modulus,and can be processed by using conventional industrial equipments and techniques.Up to now,a great deal of work has been reported on biomedical applications of PLA such as tissue engineering scaffolds,resorbable medical implants, sustained drug delivery systems and wound dressings.All these applications strongly depend on the specific physical properties of PLA.For example,high molecular weigh(MW) and highly condensed structure are required for higher-modulus PLA used as osteosynthetic materials,while drug delivery systems generally require amorphous copolymers of relatively low MW.This work consists in investigating the morphology and crystallization of PLA,including the thermodynamics,crystallization kinetics,condensed structure,morphology,and the influence of MW and configurational structure.
Many researchers have studied the crystallization of PLA obtained by using Sn(Oct)_2 as catalyst.However,Sn(Oct)_2 is more or less cyto-toxic,which could limit the potential medical applications of PLA.Zinc lactate was used as catalyst in this work for the sake of biocompatibility.PLA with various MWs and configurational structures were thus synthesized for studies on the thermodynamics,crystal structure, and morphology etc.The main contents are shown as follows.
1.Poly(L-lactide)(PLLA) was synthesized by ring opening polymerization of L-lactide using zinc lactate as catalyst.The basic thermal behaviors and morphologies were examined by using DSC,POM and SEM.The results show that the catalyst has no significant effect on the crystallization.Isothermally crystallized PLLA exhibits double melting behavior,which could be ascribed to melt recrystallization mechanism. Limited lamellar thickening was detected at certain temperatures,withαform crystal structure.Avrami analysis showed that PLLA crystallization starts with heterogeneous nucleation,followed by three-dimensional growth.The maximal overall crystal growth rate was obtained at approximately 105℃.Concentric cracks,either circular or hexagonal,were observed during melt crystallization at 135℃and quenching in liquid nitrogen,which was assigned to rhythmic growth and thermal shrinkage.In addition,the crystal growth rate of PLLA spherulites was evaluated by using self nucleation combined with non-isothermal method.Lauritzen-Hoffman equation was used for nonlinear fitting of the obtained data,which allowed deducing the kinetic parameters of PLLA crystallization.
2.The influences of MW on the thermodynamics and kinetics of PLLA crystallization were investigated.The glass transition temperature of PLLA with infinite MW was estimated to be 60.0℃,and the melting temperature 178.8℃,based on the relevant equations from Flory.In Avrami analysis,PLLA with higher MW presents lower overall crystal growth rate,higher super-cooling degree leading to longer induction period for crystallization.And there exists a scale law between the maximal crystal growth rate and MW with power of-0.5.Linear and nonlinear Hoffman-Weeks expressions were applied to calculate equilibrium melting temperature of isothermally melt crystallized PLLA.Linear extrapolation was found not applicable to PLLA with low MW.The equilibrium melting temperature of PLLA was determined to be 207.6℃.Double endothermic peaks were observed during PLLA glass transition.The two peaks result from enthalpy recovery of both confined and free amorphous fractions in crystallized PLLA.SEM shows that lathlike lamellae align along radial direction and are well organized in chemically etched PLLA spherulites.The transition temperature from RegimeⅡto RegimeⅢwas found to be around 115-120℃for PLLA with different MWs according to Lauritzen-Hoffman theory.
3.PLA samples with various configurational structures were synthesized from different L-lactide and DL-lactide feeds.DSC shows that the PLA samples present the same crystallization mechanism.ESEM observations confirmed that proteinase K could degrade both free and confined amorphous fractions of PLA through L-lactic acid units.During enzymatic degradation,lamellae inside the spherulites preserve their structural organization without collapsing.Two kinds of PLA spherulitic morphology were observed,which probably result from different nucleation locations, i.e.inside or at the surface.Furthermore,both crystal and amorphous characteristic peaks were detected on FTIR spectra.Splitting of the crystal peak at 921 cm~(-1) seems to indicate that degradation of confined amorphous fraction influences the vibration state of chains in lamellae,while a shoulder peak appearing near the amorphous peak at 955 cm~(-1) is considered as an evidence of physical aging of PLLA amorphous fraction.
4.The crystallization behaviors of PLLA/PDLA stereocomplex were investigated by using DSC and ESEM.PLLA and PDLA with similar MWs were mixed at 1:1 ratio.Three different types of PLLA/PDLA blends were utilized to illustrate the influence of MW and configurational structure on PLA stereocomplexation.In particular,sterecomplexation was found to weaken or even disappear when PLLA/PDLA was melted at higher temperature or for prolonged period.This interesting phenomenon was assigned to the initial melt state,i.e.homogeneous or heterogeneous.
目前PLA的合成多采用辛酸亚锡为催化剂,PLA的结晶结构、热行为等物理性质研究也是基于辛酸亚锡体系。但是,辛酸亚锡对于生物体具有一定毒性,可能限制PLA的医学应用。本论文中,PLA通过锌催化开环聚合制备,而锌催化剂对生物体无毒性作用。在此基础上,合成制备了不同分子量、不同分子结构的PLA,研究PLA的热力学行为、结晶结构和形貌等。主要内容如下:
(1)制备了左旋聚乳酸(PLLA)样品,研究了PLLA基本热力学行为、微观形貌等,实验结果表明PLLA的结晶行为与催化剂没有明显的联系。等温结晶研究中,PLLA的熔融重结晶会引起双重熔融行为,等温可观察到有限的片晶增厚现象,结晶结构以稳定的α晶型结构存在。Avrami方程分析表明PLLA等温结晶为异相成核、三维生长,在105℃附近达到最大总结晶生长速率。PLLA球晶形貌观察到同心裂纹、环形或者六边形,形成原因是球晶的周期性生长特点和冷却时的热收缩效应。使用自成核、非等温方法研究了PLLA的球晶生长速率,并应用Lauritzen-Hoffman方程非线性拟合PLLA球晶生长速率,得到PLLA的结晶动力学参数。
(2)研究了分子量对PLLA的热力学以及动力学的影响。Flory相关公式可以推测,当分子量趋向于极大时,PLLA玻璃化转变温度为60.0℃,熔融温度为178.8℃。Avrami方程表明,分子量越高,PLLA总结晶速率越低,高过冷度时结晶诱导时间也越长。最大总结晶速率和分子量之间存在着标度律,幂律为-0.5。PLLA熔体等温结晶研究中,采用线性和非线性Hoffman-Weeks方程计算PLLA的平衡熔融温度,阐明了线性HW方程对于低分子量PLLA熔融温度计算失效的原因,得到极大分子量PLLA的平衡熔融温度为207.6℃。PLLA玻璃化转变研究中,发现了双吸热峰对应的热焓恢复现象,PLLA结晶时出现的受限无规态和自由无规态分子链是形成双峰的原因。通过化学刻蚀观察到板条状晶沿径向生长,片晶结构规整。Lauritzen-Hoffman理论分析得到不同分子量PLLA的结晶动力学上的区域Ⅲ与区域Ⅱ的转变温度在115-120℃。
(3)研究不同立体构型的PLA的结晶行为、形貌和酶降解过程。DSC研究显示不同右旋乳酸单元含量的PLA立体共聚物有着相同的结晶机制。ESEM研究确证了蛋白酶K对于PLA熔体等温结晶薄膜的自由非晶和受限结晶区域的降解作用。球晶中的片晶在降解过程中保持其空间排布,而并不会坍塌。球晶的成核位置在材料表面或是内部可能是观测到的两类PLA降解形貌的形成原因。FTIR研究确证了半结晶PLA中晶体特征峰和非晶特征峰的存在。921 cm~(-1)晶体特征峰的分裂说明了受限非晶区的降解影响了晶区链段的振动状态,而955 cm~(-1)非晶特征峰附近出现肩峰确证了蛋白酶K降解过程中PLA非晶组分的物理老化现象。
(4)研究了PLLA/PDLA的立体复合晶体。共混比例1:1,左旋和右旋PLA分子量接近。实验应用了三种PLLA/PDLA体系,目的是探讨不同分子量和不同旋光度对立体复合晶体的影响,研究立体复合晶体的晶体结构、热行为、酶降解过程等等。立体复合结晶受熔体状态的影响,当熔融温度升高或者熔融时间延长时,立体复合结晶将急剧减弱甚至消失,但通过溶液结晶,立体复合结晶可以再次恢复。异相和均相熔体的模型假设可以解释此实验现象。
During the past two decades,biodegradable polymers have attracted much interest in both basic research and chemical industry.Aliphatic polyesters present excellent degradation properties,with degradation by-products harmless to the environment and animal body.Especially,polylactide(PLA) is regarded as the most promising biodegradable polyester.The raw material of PLA,lactic acid,can be obtained from renewable sources.PLA is a thermoplastic with high strength and high modulus,and can be processed by using conventional industrial equipments and techniques.Up to now,a great deal of work has been reported on biomedical applications of PLA such as tissue engineering scaffolds,resorbable medical implants, sustained drug delivery systems and wound dressings.All these applications strongly depend on the specific physical properties of PLA.For example,high molecular weigh(MW) and highly condensed structure are required for higher-modulus PLA used as osteosynthetic materials,while drug delivery systems generally require amorphous copolymers of relatively low MW.This work consists in investigating the morphology and crystallization of PLA,including the thermodynamics,crystallization kinetics,condensed structure,morphology,and the influence of MW and configurational structure.
Many researchers have studied the crystallization of PLA obtained by using Sn(Oct)_2 as catalyst.However,Sn(Oct)_2 is more or less cyto-toxic,which could limit the potential medical applications of PLA.Zinc lactate was used as catalyst in this work for the sake of biocompatibility.PLA with various MWs and configurational structures were thus synthesized for studies on the thermodynamics,crystal structure, and morphology etc.The main contents are shown as follows.
1.Poly(L-lactide)(PLLA) was synthesized by ring opening polymerization of L-lactide using zinc lactate as catalyst.The basic thermal behaviors and morphologies were examined by using DSC,POM and SEM.The results show that the catalyst has no significant effect on the crystallization.Isothermally crystallized PLLA exhibits double melting behavior,which could be ascribed to melt recrystallization mechanism. Limited lamellar thickening was detected at certain temperatures,withαform crystal structure.Avrami analysis showed that PLLA crystallization starts with heterogeneous nucleation,followed by three-dimensional growth.The maximal overall crystal growth rate was obtained at approximately 105℃.Concentric cracks,either circular or hexagonal,were observed during melt crystallization at 135℃and quenching in liquid nitrogen,which was assigned to rhythmic growth and thermal shrinkage.In addition,the crystal growth rate of PLLA spherulites was evaluated by using self nucleation combined with non-isothermal method.Lauritzen-Hoffman equation was used for nonlinear fitting of the obtained data,which allowed deducing the kinetic parameters of PLLA crystallization.
2.The influences of MW on the thermodynamics and kinetics of PLLA crystallization were investigated.The glass transition temperature of PLLA with infinite MW was estimated to be 60.0℃,and the melting temperature 178.8℃,based on the relevant equations from Flory.In Avrami analysis,PLLA with higher MW presents lower overall crystal growth rate,higher super-cooling degree leading to longer induction period for crystallization.And there exists a scale law between the maximal crystal growth rate and MW with power of-0.5.Linear and nonlinear Hoffman-Weeks expressions were applied to calculate equilibrium melting temperature of isothermally melt crystallized PLLA.Linear extrapolation was found not applicable to PLLA with low MW.The equilibrium melting temperature of PLLA was determined to be 207.6℃.Double endothermic peaks were observed during PLLA glass transition.The two peaks result from enthalpy recovery of both confined and free amorphous fractions in crystallized PLLA.SEM shows that lathlike lamellae align along radial direction and are well organized in chemically etched PLLA spherulites.The transition temperature from RegimeⅡto RegimeⅢwas found to be around 115-120℃for PLLA with different MWs according to Lauritzen-Hoffman theory.
3.PLA samples with various configurational structures were synthesized from different L-lactide and DL-lactide feeds.DSC shows that the PLA samples present the same crystallization mechanism.ESEM observations confirmed that proteinase K could degrade both free and confined amorphous fractions of PLA through L-lactic acid units.During enzymatic degradation,lamellae inside the spherulites preserve their structural organization without collapsing.Two kinds of PLA spherulitic morphology were observed,which probably result from different nucleation locations, i.e.inside or at the surface.Furthermore,both crystal and amorphous characteristic peaks were detected on FTIR spectra.Splitting of the crystal peak at 921 cm~(-1) seems to indicate that degradation of confined amorphous fraction influences the vibration state of chains in lamellae,while a shoulder peak appearing near the amorphous peak at 955 cm~(-1) is considered as an evidence of physical aging of PLLA amorphous fraction.
4.The crystallization behaviors of PLLA/PDLA stereocomplex were investigated by using DSC and ESEM.PLLA and PDLA with similar MWs were mixed at 1:1 ratio.Three different types of PLLA/PDLA blends were utilized to illustrate the influence of MW and configurational structure on PLA stereocomplexation.In particular,sterecomplexation was found to weaken or even disappear when PLLA/PDLA was melted at higher temperature or for prolonged period.This interesting phenomenon was assigned to the initial melt state,i.e.homogeneous or heterogeneous.
引文
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