环氧丙烷聚醚三元醇/左旋聚乳酸三枝链嵌段共聚物的结晶行为

采用DSC对环氧丙烷聚醚三元醇/左旋聚乳酸三枝链嵌段共聚物(PPO-b-PLLA)的熔体结晶行为进行了研究. 在388～407 K范围内, 分别采用Avrami方程和Arrhenius方程进行了结晶动力学计算. Avrami指数n值约为2.2, 表明共聚物以二维生长方式进行晶体生长. 基于LH结晶理论, 对三枝链嵌段共聚物的结晶机理进行了探讨. 实验发现该体系共聚物的Regime II和Regime III转变温度随着n(PO)∶n(LA)的增大而变化, Kg (III)/Kg (II)＝2.0～2.2, 与LH理论预期值吻合. 实验结果表明三枝链的PPO链段对PLLA链段的结晶有很大影响, 使其成核较均聚物困难. 链折叠自由能σe和链折叠功q均高于PLLA的值.     

聚左旋乳酸-聚乙二醇二嵌段共聚物的非等温结晶行为研究

通过变温广角X射线衍射(WAXD)、 差示扫描量热法(DSC)和偏光显微镜(POM)研究了聚左旋乳酸-聚乙二醇(PLLA-PEG)二嵌段共聚物的非等温结晶行为, 并用Ozawa方程分析了PLLA-PEG的非等温结晶动力学. 实验结果表明, 高熔点的硬段PLLA结晶符合Ozawa理论, 而低熔点的软段PEG对PLLA的结晶起到了稀释剂的作用; 当软段PEG开始结晶时, 已经结晶完全的硬段PLLA限制了PEG的结晶, 使得软段PEG的结晶不符合Ozawa理论. 此外, 不同降温速率下的结晶形貌研究结果表明, 随着降温速率的增加, 晶体经历了从环带球晶、 环带和十字消光的混合球晶到典型的十字消光球晶的转变, 并且球晶的尺寸也明显变小.     

含氟两亲接枝共聚物的制备及其结晶行为

采用大分子单体技术合成了一系列以聚乙二醇为支链、甲基丙烯酸六氟丁酯为主链的含氟两亲接枝共聚物(PHFMA-g-PSPEG)。用1HNMR和凝胶色谱(GPC)对制备的大分子单体和两亲接枝共聚物的结构进行了表征。利用示差扫描量热法(DSC)、X射线衍射(XRD)和偏光显微镜(POM)测试技术对含氟两亲接枝共聚物的结晶行为进行了研究。DSC和XRD结果表明,随着共聚物中含氟链段质量分数的增加,其结晶温度(Tc)和结晶度(Xc)均降低,而结晶熔融温度(Tm)先减小后增加。POM发现,随着共聚物中含氟链段质量分数的增加,其结晶速度减慢,共聚物形成清晰球晶的能力减弱,当共聚物中含氟链段质量分数为57%时,含氟两亲接枝共聚物已不能形成清晰的球晶。     

含柔性链段聚右旋乳酸嵌段共聚物对聚左旋乳酸拉伸行为的影响

采用聚右旋乳酸(PDLA)与聚乙二醇(PEG)的三嵌段共聚物(PDLA-b-PEG-b-PDLA)对聚左旋乳酸(PLLA)进行改性,系统研究熔融共混法制备的PLLA/PDLA-b-PEG-b-PDLA共混物的热性能和不同温度下的拉伸行为,并通过原位X射线散射(WAXS)技术探索不同含量的PDLA-b-PEG-b-PDLA对PLLA在拉伸过程中结晶行为的影响.结果表明,加入PDLA-b-PEG-b-PDLA对PLLA的热稳定性影响较小;PLLA/PDLA-b-PEG-b-PDLA共混物中由于立构晶的存在,能有效提高PLLA的α晶的结晶速率;室温(30°C)拉伸时,样品均呈现脆性断裂;拉伸温度提高至50°C,纯PLLA和PLLA/PDLA-b-PEG-b-PDLA(95/5)的共混物仍然呈现脆性断裂,但是随着PDLA-b-PEG-b-PDLA含量的增加,PLLA发生屈服,断裂伸长率由纯PLLA的10%左右提高至200%以上;80°C拉伸时,PDLA-b-PEG-b-PDLA的加入显著提高了PLLA在拉伸过程中的结晶速率,出现α晶的应变从纯PLLA的400%降低至50%以下,立构晶含量在拉伸过程中基本保持不变.上述结果显示含柔性链段的PDLA的嵌段共聚物可有效提高PLLA的结晶速率和延展性,拓宽PLLA的应用范围.     

左旋聚乳酸的结晶行为研究

用DSC, WAXD和POM对Zn催化剂制备的左旋聚乳酸(PLLA)的熔体结晶行为进行了研究. 在95~125 ℃范围内, PLLA熔体结晶生成厚度约(14±1) nm的片晶, 该片晶不易发生熔体等温增厚. 对实验数据分别用Avrami方程和Arrhenius方程进行了计算, Avrami指数n=3±0.3, 表明PLLA以球晶形式生长, 其最大结晶速率温度为(105.0±0.5) ℃, t1/2约为5.2 min. 利用Lauritzen-Hoffmann（LH）理论对PLLA结晶机理进行了分析, 发现PLLA结晶的Regime Ⅱ和Regime Ⅲ的转变温度为107 ℃. Kg(Ⅱ)和Kg(Ⅲ)分别为4.57×105 K2和1.115×106 K2, 且Kg(Ⅲ)/Kg(Ⅱ)=2.4, 与LH理论值一致.     

PLLA-PEG共聚物的非等温结晶行为

采用熔融共聚法制备PLLA-PEG嵌段共聚物, 用WAXD和DSC方法研究其结晶行为, 并用Avrami方程的Jeziorny修正分析了非等温结晶动力学行为. 结果表明, PLLA结晶明显, 而PEG结晶难以观察到, PEG的柔性能促进PLLA结晶. PEG分子量的增加和投料量的增加都能使得结晶温度升高, 结晶度增大, 结晶速度加快.     

链拓扑结构对嵌段共聚物结晶行为的影响

通过等量的聚乙二醇(PEG)和聚苯乙烯(PS)合成了H型(PS)2-PEG-(PS)2(PEG链接在PS链的中间)和线型PS-PEG-PS(PEG链接在PS链的一端)三嵌段共聚物,并通过X射线小角散射(SAXS)、X射线广角衍射(WAXD)、示差扫描量热仪(DSC)和偏光显微镜(POM)等手段研究了其结晶行为,结果表明2种共聚物中PEG的晶型相同,但是H型比线形结晶温度更低,PEG片晶厚度更小,在相同温度下PEG结晶速度更慢,这是由于H型分岔结构引起空间位阻较大所致.     

溶剂诱导的聚乳酸/聚乳酸衍生物共结晶行为

通过溶液浇铸法制备不同组分的左旋聚乳酸(PLLA)和聚(L-2-羟基-3-甲基丁酸)(PL-2H3MB)共混物。运用差示扫描量热仪(DSC)、偏光显微镜(POM)、广角X射线衍射(WAXD)和热重分析仪(TGA)分析共混物的结晶、熔融行为和热稳定性。通过观察到DSC加热曲线中新的熔融峰判断PLLA和PL-2H3MB共晶的形成,共晶显著提高PLLA起始结晶温度和WAXD曲线中特征衍射峰的变化,证实了溶液浇铸共混物中的共结晶现象。同时,共混物中PL-2H3MB和PLLA还表现出优于单组分PLLA和PL-2H3MB的热稳定性。PLLA和PL-2H3MB的共结晶行为可能为调控PLLA的热稳定性、力学性能以及降解性能提供了一种新的潜在方法。     

接枝改性POSS对PLLA结晶行为的影响*

以二氯甲烷为溶剂,利用溶液共混法将三种聚笼形多面体倍半硅氧烷(POSS)分别与聚左旋乳酸(PLLA)进行共混,制备了不同POSS含量的单氨基POSS(POSS-NH₂)/PLLA、POSS接枝聚乙二醇(POSS-PEG)/PLLA和POSS接枝聚乳酸(POSS-g-PLLA)/PLLA复合材料。利用差示扫描量热仪(DSC)、热重分析仪(TGA)、偏光显微镜(POM) 分别对复合材料的本体结晶行为、热稳定性及结晶形貌和生长速率进行了观察。结果表明当加入不同质量分数(1 wt%, 5 wt%, 10 wt%)的POSS-PEG时,PLLA的结晶能力均得到改善,而POSS-NH2和POSS-g-PLLA仅在质量分数较低(1wt%)时对PLLA起成核剂的作用,具有较高质量分数时会阻碍PLLA分子链段的运动,从而限制其结晶。三种复合材料中仅POSS-PEG在一定程度上提高了PLLA的热稳定性,利用POM观察球晶生长过程发现POSS-PEG的加入提高了PLLA的球晶生长速率。     

聚(L-丙交酯)/聚(DL-丙交酯)的结晶性能及相溶性

用共溶液沉淀法制备了聚 (L 丙交酯 ) 聚 (DL 丙交酯 )共混物 (PLLA PDLLA) ,然后用成纤模压法压制成3 2mm的棒材 .用差示扫描量热法研究了共混物的结晶性能和相溶性 .结果表明 ,PLLA组分在共溶液沉淀过程中可生成结晶 ,共混物中PDLLA含量直到 30 %时 ,PLLA组分的结晶熔融温度和结晶度与纯PLLA相同 ,但PDLLA含量为 5 0 %时 ,PLLA组分的结晶熔融温度和结晶度明显下降 .由于加工成型条件的不一致性 ,共混物棒材中的PLLA组分的结晶熔融温度和结晶度呈较大的分散性 .共混物从熔体降温 ,在其后的升温DSC扫描中出现分别相应于PDLLA和PLLA的玻璃化转变 ,表明PDLLA与未结晶的PLLA形成的非晶相是不相溶的     

聚乙二醇-聚L-丙交酯嵌段共聚物结晶形貌研究

用偏光显微镜和原子力显微镜对比研究了PEG-PLLA嵌段共聚物在110℃或120℃等温结晶后的结晶形貌.发现在110℃时只有PEG5000-PLLA2300和PEG5000-PLLA6300在偏光显微镜下呈现环带球晶形貌,在原子力显微镜高度图中显示明显的环带,并具有交替凸凹起伏形貌.而PEG5000-PLLA12000球晶中没有出现环带形貌而是生成了规则的环线.在120℃时,PEG5000-PLLA12000的球晶中才生成了规则的环带图案,原子力显微镜也显示了其球晶具有明显的交替凸凹起伏形貌,说明过冷度直接影响环带球晶的生成.产生周期性凸凹起伏和明暗交替消光是由片晶沿着球晶的半径方向周期性扭转造成的,片晶在凸起部分是Edge-on取向,在凹下部分是Flat-on取向.     

Effects of functionalized graphenes on the isothermal crystallization of poly(L-lactide) nanocomposites

The effects of graphene oxide(GO) with polar groups and functionalized GO(f GO) with nonpolar groups on the isothermal crystallization of poly(L-lactide)(PLLA) were compared. Functionalized GO was obtained by grafting octadecylamine and characterized by FTIR, WAXD and TGA. Isothermal crystallization kinetics of PLLA/GO and PLLA/f GO nanocomposites were investigated by combining DSC data and Avrami equation. The results showed that f GO could improve PLLA crystallization rate more obviously than GO. By analyzing the morphology obtained from POM, SEM and TEM, it was found f GO with large layer space dispersed better in PLLA and supplied more nucleation sites than GO. Therefore, for the multilayer graphene, increasing the layer spaces is important to improve its dispersion in polymers, which will cause the crystal kinetics changing of polymers.     

Nonisothermal crystallization behavior of the poly(ethylene glycol) block in poly(L‐lactide)–poly(ethylene glycol) diblock copolymers: Effect of the poly(L‐lactide) block length

The confined crystallization behavior, melting behavior, and nonisothermal crystallization kinetics of the poly(ethylene glycol) block (PEG) in poly(L ‐lactide)–poly(ethylene glycol) (PLLA–PEG) diblock copolymers were investigated with wide‐angle X‐ray diffraction and differential scanning calorimetry. The analysis showed that the nonisothermal crystallization behavior changed from fitting the Ozawa equation and the Avrami equation modified by Jeziorny to deviating from them with the molecular weight of the poly(L ‐lactide) (PLLA) block increasing. This resulted from the gradual strengthening of the confined effect, which was imposed by the crystallization of the PLLA block. The nucleation mechanism of the PEG block of PLLA15000–PEG5000 at a larger degree of supercooling was different from that of PLLA2500–PEG5000, PLLA5000–PEG5000, and PEG5000 (the numbers after PEG and PLLA denote the molecular weights of the PEG and PLLA blocks, respectively). They were homogeneous nucleation and heterogeneous nucleation, respectively. The PLLA block bonded chemically with the PEG block and increased the crystallization activation energy, but it provided nucleating sites for the crystallization of the PEG block, and the crystallization rate rose when it was heterogeneous nucleation. The number of melting peaks was three and one for the PEG homopolymer and the PEG block of the diblock copolymers, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3215–3226, 2006     

Thermo-Responsive Hydrogels Based on Branched Poly(L-lactide)-poly(ethylene glycol) Copolymers

Summary: Branched poly(L -lactide)-poly(ethylene glycol) (PLLA-PEG) block copolymers were synthesized from trifunctional PLLA and amine functionalized methoxy poly(ethylene glycol)s. The copolymers in water formed hydrogels that showed thermo-responsive behavior. The hydrogels underwent a gel to sol transition with increasing temperature as determined with the vial tilting method and oscillatory rheology. For all copolymers, the transition temperature increased with increasing copolymer concentration. The transition temperature of corresponding branched copolymers also increased with increasing PEG molecular weight, and surprisingly decreased with increasing molecular weight of the PLLA branches. In general, the gel-sol transition is explained by disruption of micellar or aggregate interactions because of partial dehydration and shrinkage of the PEG chains. An increase in the molecular weight of the PLLA branches led to the formation of micelles and aggregates as observed with DLS at low concentrations. It is speculated that the non-uniform size distribution and possible crystallization of longer PLLA blocks may have a negative effect on the formation of micellar packing upon gelation, allowing the disruption of micellar or aggregate interactions to occur at lower temperatures. The transition temperature of the gels could be tuned closely to body temperature by varying the concentration of the solution or the molecular weight of the PEG block and the PLLA blocks, which implies that these polymers may be used as injectable systems for in-situ gel formation.     

Linear and four‐armed poly(l‐lactide)‐block‐poly(d‐lactide) copolymers and their stereocomplexation with poly(lactide)s

Linear and four‐armed poly(l ‐lactide)‐block‐poly(d ‐lactide) (PLLA‐b‐PDLA) block copolymers are synthesized by ring‐opening polymerization of d ‐lactide on the end hydroxyl of linear and four‐armed PLLA prepolymers. DSC results indicate that the melting temperature and melting enthalpies of poly (lactide) stereocomplex in the copolymers are obviously lower than corresponding linear and four‐armed PLLA/PDLA blends. Compared with the four‐armed PLLA‐b‐PDLA copolymer, the similar linear PLLA‐b‐PDLA shows higher melting temperature (212.3 °C) and larger melting enthalpy (70.6 J g^?1). After these copolymers blend with additional neat PLAs, DSC, and WAXD results show that the stereocomplex formation between free PLA molecular chain and enantiomeric PLA block is the major stereocomplex formation. In the linear copolymer/linear PLA blends, the stereocomplex crystallites (sc) as well as homochiral crystallites (hc) form in the copolymer/PLA cast films. However, in the four‐armed copolymer/linear PLA blends, both sc and hc develop in the four‐armed PLLA‐b‐PDLA/PDLA specimen, which means that the stereocomplexation mainly forms between free PDLA molecule and the inside PLLA block, and the outside PDLA block could form some microcrystallites. Although the melting enthalpies of stereocomplexes in the blends are smaller than that of neat copolymers, only two‐thirds of the molecular chains participate in the stereocomplex formation, and the crystallization efficiency strengthens. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1560–1567     

Crystallization and morphology of poly(ethylene oxide‐b‐lactide) crystalline–crystalline diblock copolymers

The crystallization behaviors and morphology of asymmetric crystalline–crystalline diblock copolymers poly(ethylene oxide‐lactide) (PEO‐b‐PLLA) were investigated using differential scanning calorimetry (DSC), wide angle X‐ray diffraction (WAXD), and microscopic techniques (polarized optical microscopy (POM) and atomic force microscopy (AFM)). Both blocks of PEO₅‐b‐PLLA₁₆ can be crystallized, which was confirmed by WAXD, while PEO block in PEO₅‐b‐PLLA₃₀ is difficult to crystallize because of the confinement induced by the high glass transition temperature and crystallization of PLLA block with the microphase separation of the block copolymer. Comparing with the crystallization and morphology of PLLA homopolymer and differences between the two copolymers, we studied the influence of PEO block and microphase separation on the crystallization and morphology of PLLA block. The boundary temperature (T_b) was observed, which distinguishes the crystallization into high‐ and low‐temperature ranges, the growth rate and morphology were quite different between the ranges. Crystalline morphologies including banded spherulite, dendritic crystal, and dense branching in PEO₅‐b‐PLLA₁₆ copolymer were formed. The typical morphology of dendritic crystals including two different sectors were observed in PEO₅‐b‐PLLA₃₀ copolymer, which can be explained by secondary nucleation, chain growth direction, and phase separation between the two blocks during the crystallization process. Lozenge‐shaped crystals of PLLA with screw dislocation were also observed employing AFM, but the crystalline morphology of PEO block was not observed using microscopy techniques because of its small size. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1400–1411, 2008