L-聚乳酸固相缩聚的工艺研究

以L-乳酸单体为原料,采用复合催化剂通过直接熔融法合成低聚左旋聚乳酸(PLLA),然后分段控温进行固相缩聚(SSP).对固相缩聚的工艺条件进行了详细的研究.采用粘度法和凝胶渗透色谱法(GPC)对固相缩聚产物PLLA的特性粘数([η])和分子量进行了表征.用差示扫描量热(DSC)研究了固相缩聚产物的熔融行为和结晶度.结果...     

PLLA/PDLA立构复合晶体的结晶、熔融行为

以水为引发剂合成了两种构型不同的左旋聚乳酸和右旋聚乳酸,运用DSC研究了分子量较低的PLLA/PDLA立构复合聚乳酸晶体的多重熔融行为,观察到随着180℃等温时间的增加,PLLA/PDLA共混物的立构复合晶体的熔点和熔融峰面积呈现震荡性变化行为,进一步的DSC和在线X射线研究结果显示,聚乳酸立构复合晶体多重熔融峰的震荡行为来自于晶体中就近生长的过量异构体链段的调整过程。     

三臂支化PLLA-PDLA嵌段共聚物的合成及立构复合结晶行为

以三羟甲基乙烷为起始剂, 开环聚合L-丙交酯(LLA), 合成三臂支化左旋聚乳酸(PLLA)预聚物. 采用端基活化技术对预聚物进行端羟基活化, 再与D-丙交酯(DLA)进行开环聚合, 合成了不同分子量的三臂支化左旋聚乳酸-右旋聚乳酸(PLLA-PDLA)嵌段共聚物. 采用核磁共振谱和凝胶渗透色谱等对样品的结构和分子量进行测试, 结果表明,合成的嵌段共聚物链结构具有链段立构规整度和高分子量的特点; 通过调节DLA单体与PLLA预聚物的投料比, 可实现对PLLA-PDLA嵌段共聚物的序列结构调控. 差示扫描量热仪和广角X 射线衍射结果表明, 三臂支化PLLA-PDLA嵌段共聚物的异构体分子间生成立构复合晶体, 其熔点高于200℃; 共聚物的嵌段序列结构对材料的凝聚态转变行为有很大的影响.     

熔融/固相缩聚法合成聚乙醇酸及其性能表征

采用熔融/固相缩聚法合成了聚乙醇酸(PGA)可降解高分子材料,其基本反应步骤为:以乙醇酸为原料,先在190℃熔融状态下将乙醇酸脱水制成分子量为2万左右的低聚物,然后将制得的低聚物在190℃下进行固相缩聚以进一步提高分子量,所制备的PGA产物通过IR、DSC、XRD等手段进行表征。重点考察了不同催化剂,催化剂用量、是否熔融、反应温度、反应时间等因素对固相缩聚的影响,并得出熔融/固相缩聚法合成高分子量的聚乙醇酸的较佳工艺条件:反应温度190℃,二水合醋酸锌与等摩尔量的对甲苯磺酸作为催化剂(质量分数为0.4%),熔融缩聚2h后制得低聚物,然后在190℃下进行固相缩聚,40h后熔融一次,产品粉碎后继续固相缩聚60小时,PGA的重均分子量可达74000左右。     

高耐热PLLA/PDLA共混物的热性能和结晶结构研究

通过熔融共混法制备了一系列等比例聚左旋乳酸(PLLA)/聚右旋乳酸(PDLA)共混试样,采用差示扫描量热法(DSC)、核磁共振(13C-NMR)及广角X射线衍射(WAXD)等方法对共混产物进行了表征和研究分析.结果表明,等比例PLLA和PDLA熔融共混生成了立构复合物(stereocomplex,sc),同时部分均聚物发生酯交换反应,生成了立体嵌段物(stereoblock,sb),从而在DSC升温曲线上sc晶体处出现特殊的熔融双峰现象;随着熔融共混温度升高,sc晶体的生成率和结晶度逐渐下降;聚乳酸立构复合物的加工稳定性较好,二次加工后,sc晶体熔点基本不变,生成率和结晶度提高;由于sc晶体的存在,PLLA/PDLA共混物的耐热性能提高,退火热处理后耐热性能得到进一步提高.这对于开发高耐热聚乳酸及其加工应用具有重要的应用价值.     

溶液法星型聚乳酸的合成与表征

探讨了采用辛酸亚锡为催化剂,多元醇及多元酸为引发剂,以溶液法制备星型聚乳酸的可行性,研究了不同引发剂对产物分子量的影响.采用核磁共振及DSC对产物进行了表征,结果表明:以溶液法合成星型聚乳酸是可行的,但与丙交酯开环聚合制备星型聚乳酸的方法相比,溶液法在产物结构和分子量控制上并不十分有效,由于反应受到多官能团核引发剂空间位阻和反应概率的影响,聚乳酸产物的结构除星型结构外也同时存在大量的线型结构.     

复合催化剂对聚乳酸熔融/固相聚合的影响

使用氯化亚锡/对甲苯磺酸(SnCl2/TSA)复合催化剂对聚乳酸熔融/固相聚合进行研究,通过使用乌氏粘度法,傅里叶变换红外光谱分析、差示扫描量热法对聚乳酸产品进行表征及分析。结果表明:聚乳酸熔融聚合过程中,使用的复合催化剂总含量为0.6%(wt)(SnCl_2和TSA分别为0.3%(wt)),能获得较高分子量的聚乳酸低聚物。使用单一组分0.5wt%的SnCl_2作催化剂,以熔融聚合获得的低聚物为原料进行固相聚合后,获得的聚乳酸粘均分子量是熔融聚合的3.7倍,且其相对分子量能达到最大值。     

聚苯乙烯嵌段对左旋聚乳酸/聚苯乙烯-b-右旋聚乳酸立构复合晶结晶行为的调控

聚乳酸较差的耐热性和较慢的结晶速率限制了其应用范围的扩展,左旋聚乳酸(PLLA)和右旋聚乳酸(PDLA)共混后形成的立构复合晶(SC)能够促进PLA均质晶(HC)的成核并提高其热稳定性,在改性PLA方面有着巨大应用前景.本研究通过可逆加成-断裂链转移聚合(RAFT)和开环聚合(ROP)法,制备了一系列不同聚苯乙烯(PS)分子量的PS-b-PDLA嵌段共聚物,并将其与PLLA共混,探究了嵌段共聚物及共混物的组成与结晶性能之间的关系.研究结果表明,分子运动性较差的PS嵌段的引入使共混物的结晶更加困难,而低分子量的PS嵌段由于抑制了均质晶(HC)的形成,反而有利于大量立构复合晶(SC)的形成,进而提高了共混物的结晶速率.     

一种基于立构复合及自组装方法构筑的耐受性疏水表面

利用丙交酯开环聚合法制备了聚(D-乳酸)-聚二甲基硅氧烷-聚(D-乳酸)(PDLA-b-PDMS-b-PDLA)三嵌段聚合物,将其溶液涂覆至充斥着非溶剂蒸汽的聚(L-乳酸)(PLLA)表面,PDLA-b-PDMS-b-PDLA在缓慢沉积的过程中与PLLA发生立构复合及自组装,得到由立构复合的亚微米颗粒组装体形成的聚乳酸表面疏水层。 研究了聚合物溶液的质量浓度、组装温度以及溶剂对聚乳酸表面的微观形貌和疏水性能产生的影响。 结果表明,随着PDLA-b-PDMS-b-PDLA聚合物溶液质量浓度的增加,可以实现聚乳酸表面Wenzel-Cassie-Wenzel的疏水行为转变;在0 ℃下,可得到最大疏水角151°的疏水层;选择对聚合物溶解性、挥发速度不同的溶剂,得到的表面微观形貌和疏水性也不同。 由于聚乳酸制品表面的PLLA链段与亚微米颗粒中的PDLA链段也能够立构复合,因此该表面疏水层对刀刮、胶带剥离和手指擦拭测试均表现出良好的耐受性。     

L-聚乳酸/低分子量壳聚糖共混膜的制备和特性

二甲基亚砜(DMSO)作为共溶刺，将两种极性不同的生物医用高分子L-聚乳酸(PLLA)与低分子量壳聚糖混合在一起．扫描电镜(SEM)研究结果表明两种组分共混时存在着明显的相分离．当壳聚糖含量在2％以内时，壳聚糖相在共混膜中呈现空心圆状的图案，空心圆的直径为几微米．PLLA用DMSO溶解后，在重新干燥的过程中，PLLA的堆积结构会发生变化，它的玻璃化转变温度由55.5℃变为34℃．共混膜中混入的壳聚糖含量在2％以内时，在保持的聚乳酸的力学性能的同时。可以降低聚乳酸的接触角。改善聚乳酸的亲水性．     

An efficient solid‐state polycondensation method for synthesizing stereocomplexed poly(lactic acid)s with high molecular weight

Simultaneous solid‐state polycondensation (SSP) of the powdery prepolymers of poly(L ‐lactic acid) (PLLA) and poly(D ‐lactic acid) (PDLA) can produce entire stereocomplexed poly(lactic acid)s (sc‐PLA) with high molecular weight and can be an alternative synthetic route to sc‐PLA. Ordinary melt polycondensations of L ‐ and D ‐lactic acids gave the PLLA and PDLA prepolymers having medium molecular weight which were pulverized for blending in 1:1 ratio. The resultant powder blends were then subjected to SSP at 130–160 °C for 30 h under a reduced pressure of 0.5 Torr. Some of the products thus obtained attained a molecular weight (M_w) as high as 200 kDa, consisting of stereoblock copolymer of PLLA and PDLA. A small amount of the stereocomplex should be formed in the boundaries of the partially melted PLLA and PDLA where the hetero‐chain connection is induced to generate the blocky components. The resultant SSP products showed predominant stereocomplexation after their melt‐processing in the presence of the stereoblock components in spite of containing a small amount of racemic sequences in the homo‐chiral PLLA and PDLA chains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3714–3722, 2008     

Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA) is incorporated into a 1:1 polymer blend system of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) that has a high molecular weight to study its addition effect on the stereocomplex (sc) formation of PLLA and PDLA. The ternary polymer blend films are first prepared by casting polymer solutions of sb-PLA, PLLA, and PDLA with different compositions. Upon increasing the content of sb-PLA in the blend films the sc crystallization is driven to a higher degree, while the formation of homo-chiral (hc) crystals is decreased. Lowering the molecular weight of the incorporated sb-PLA effectively increases the sc formation. Consequently, it is revealed that sb-PLA can work as a compatibilizer to improve the poor sc formation in the polymer blend of PLLA and PDLA.     

A Novel Synthetic Approach to Stereo-Block Poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA), consisting of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) in a blocky sequence, can successfully be synthesized by solid-state polycondensation of a stereocomplexed mixture of PLLA and PDLA. First, the melt polyconden-sation of L- and D-lactic acids is conducted to obtain PLLA and PDLA with medium molecular weights. Then, both polymers are melt-blended to easily form the stereocomplex. The resulting stereocomplexed mixture (melt-blend) is subjected to solid-state polycondensation for chain extension. The molecular weight (M_w) of the resultant sb-PLA is strongly affected by the lactide/oligomer content in the melt-blend, which is determined by the melt-blending conditions, because it is directly correlated with the polymer crystallinity of the polycondensation products.     

Different enantioselectivity of two types of poly(lactic acid) depolymerases toward poly(l-lactic acid) and poly(d-lactic acid)

Poly(lactic acid) (PLA) depolymerases are categorized into protease-type and lipase-type. Protease-types can hydrolyze poly(l-lactic acid) (PLLA) but not poly(d-lactic acid) (PDLA). Lipase-types, including cutinase-like enzyme (CLE) from Cryptococcus sp. strain S-2 preferentially hydrolyze PDLA. Both enzymes degraded not only PLA emulsion but also PLA film, in which amorphous region is preferentially attacked, but crystalline region can be also attacked. Stereocomplex PLA (sc-PLA) formed by 50:50 blending of PLLA and PDLA included no homo crystals, but a tiny homo crystallization peak appeared and crystallinity increased by 5% when attacked by CLE, although no significant change of molecular weight and crystalline size was found. Enantioselective degradation must occur in amorphous region of PLLA/PDLA film and preferentially hydrolyzed PDLA, resulting in a slightly excess amount of PLLA remained, which must be crystallized.     

三臂PPO-PDLA-PLLA 嵌段共聚物的制备与结构及其立构复合体的结晶行为

以环氧丙烷聚醚三元醇(PPO)为起始剂, 开环聚合D 型丙交酯(DLA), 合成三臂环氧丙烷聚醚三元醇-聚右旋乳酸(PPO-PDLA)嵌段预聚体. 采用端基活化技术对预聚体进行端羟基活化, 再与L 型丙交酯(LLA)进行逐步开环聚合,合成了不同分子量的三臂环氧丙烷聚醚三元醇-聚右旋乳酸-聚左旋乳酸(PPO-PDLA-PLLA)嵌段共聚物. 采用红外(FTIR)、核磁(NMR)和凝胶渗透色谱(GPC)等对三臂PPO-PDLA-PLLA 嵌段共聚物的测试表明, 合成的嵌段共聚物分子链具有很高的立构规整度; 通过调节LLA 单体与PPO-PDLA 预聚体的投料比, 不仅可控制产物的分子序列结构, 而且样品的数均分子量可大于100 kDa. 差示扫描量热仪(DSC)和广角X 射线衍射(WAXD)结果显示, 三臂PPO-PDLAPLLA嵌段共聚物的异构体链段分子间生成立构复合晶体, 其熔点约为200 ℃, 且没有PLLA 均聚物链段结晶现象. 实验结果表明, 这是一类具有实际应用价值的新型耐热聚乳酸(PLA)材料.     

Influence of graphene nanosheets on stereocomplex crystallization behaviors of star‐shaped poly (D(L)‐lactide) stereoblock copolymer

The nanocompsites of star‐shaped poly(D‐lactide)‐co‐poly(L‐lactide) stereoblock copolymers (s‐PDLA‐PLLA) with two‐dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X‐ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s‐PDLA‐PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0°C formed in isothermal crystallization for all samples. The crystallization temperatures of s‐PDLA‐PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3 wt% GNS was about 128.2°C, ie, 15°C higher than pure s‐PDLA‐PLLA. At isothermal crystallization processes, the halftime of crystallization (t_0.5) of the sample with 3 wt% GNS decreased to 6.4 minutes from 12.9 minutes of pure s‐PDLA‐PLLA at 160°C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three‐dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s‐PDLA‐PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s‐PDLA‐PLLA with good nucleation ability in the s‐PDLA‐PLLA material.     

Hydrogen bonding assists stereocomplexation in poly(l‐lactic acid)/poly(d‐lactic acid) racemic blends

In this communication, we reported the sequence variation of stereocomplex crystals (SC) and homocrystals (HC) in poly(l ‐lactic acid)/poly(d ‐lactic acid) (PLLA/PDLA) racemic blends melts. It was evidenced that the emerging sequence of the SC and HC depends on the hydrogen bond formation in the melt, and the hydrogen bond is required for the stereocomplexation in PLLA/PDLA racemic blend. First, by combining a commercial fast‐scan chip‐calorimeter (Flash DSC 1) and micro‐FTIR, we found that hydrogen bonds were formed in the melt during cooling at 2.5 K/s, but not at 3000 K/s. Second, annealing the melt without hydrogen bonds at 100 °C led to HC emerging first, while annealing the melt with hydrogen bonds resulted in SC emerging at first. Third, the crystallization kinetics of the racemic blends after cooling to predefined T_c at 2.5 or 3000 K/s further verified that the hydrogen bonding can be inhibited effectively by cooling the racemic blends isotropic melt at fast enough rate. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 83–88     

聚左旋乳酸/聚氧化乙烯共混物的拉伸行为及结构转变

采用熔融共混方法制备了聚左旋乳酸(PLLA)和超高分子量聚氧化乙烯(PEO)共混物, 通过差示扫描量热(DSC)、 扫描电子显微镜(SEM)和二维广角X射线散射(2D-WAXS)等方法系统研究了PEO的加入对不同温度下PLLA拉伸行为及拉伸过程中微观结构变化的影响. 结果表明, PLLA/PEO共混物为非均相体系, PEO粒子均匀分布在PLLA中形成两相结构. PEO的加入能够显著降低PLLA的玻璃化转变温度(T_g), 在25～60 ℃范围内显著提高PLLA的拉伸性能. 在60 ℃拉伸时, PEO的加入提高了PLLA在拉伸过程中的结晶和形变能力. 在80 ℃拉伸时, 共混物的拉伸断裂伸长率下降, 但共混物的结晶速度仍高于纯PLLA样品.     

Melt polycondensation of L‐lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight Poly(L‐lactic acid)

Poly(L ‐lactic acid) (PLLA) was produced by the melt polycondensation of L ‐lactic acid. For the optimization of the reaction conditions, various catalyst systems were examined at different temperature and reaction times. It was discovered that Sn(II) catalysts activated by various proton acids can produce high molecular weight PLLA [weight‐average molecular weight (M_w ) ≥ 100,000] in a relatively short reaction time (≤15 h) compared with simple Sn(II)‐based catalysts (SnO, SnCl₂ · 2H₂O), which produce PLLA with an M_w of less than 30,000 after 20 h. The new catalyst system is also superior to the conventional systems in regard to racemization and discoloration of the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1673–1679, 2000     

制备聚(L-乳酸-co-乙醇酸）/聚D-乳酸共混物立构复合物的新方法

首次以高分子量的聚(L-乳酸-co-乙醇酸）(PLLGA）和D-聚乳酸（PDLA)［m(PLLGA) : m(PDLA)=3:1, c 50 mL·g^-1］为原料,氯仿为溶剂,等体积的甲醇为沉淀剂,于50 ℃蒸发4 h形成了PLLGA和PDLA的立构复合物（sc-PLA）,其结构和性能经XRD, DSC和TGA表征。结果表明：sc-PLA的结晶度达96.2%,热失重5%温度为342 ℃（PLLGA为304 ℃）。