Electrospinning Physical Gels: The Case of Stereocomplex PMMA
Electrospinning Physical Gels: The Case of Stereocomplex PMMA
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DOI：
10.1021/ma9005395
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发表时间：
2009-07-14
期刊：
影响因子：
5.5
通讯作者：
Srinivasarao, Mohan
中科院分区：
文献类型：
--
作者：
Crne, Matija;Park, Jung Ok;Srinivasarao, Mohan
聚合物溶液的粘弹性和可纺性与溶剂类型、温度和聚合物浓度密切相关。对于良溶剂，其中聚合物-溶剂相互作用优于聚合物-聚合物相互作用，链在C * 以下表现得像单个分子，重叠浓度与分子量相关为C*<$Mra <$Mr 0。从Berry数和MarkrHouwinkrSakurada方程出发，[22]顾名思义，聚合物链在超过重叠浓度时开始重叠，因此体系的粘度随着浓度的增加而急剧增加。已经表明，立体复合物螺旋的形成增加了持久长度，并使聚合物链更像棒状。23在这种情况下，马克霍温克系数a变得更大，24并且在低得多的浓度C* 下发生重叠。因此，凝胶化可以在比针对高斯或柔性链计算的C* 更低的浓度下发生。虽然粘度的增加是线性的浓度为稀的制度，它增加了C4，5在半稀的制度。这是由于存在缠结，其充当临时交联，因此也赋予聚合物溶液凝胶状弹性。Gupta等人已经确定，a-PMMA在DMF中的溶液需要至少为4的浓度比C/C* 以产生光滑的电纺纤维。在我们的情况下，产生光滑纤维的5重量%立体络合混合物的C/C* 比为2，这远低于线性a-PMMA聚合物的报道。我们可以将其归因于静电纺丝过程中的螺旋形成和物理凝胶化。先前已经表明，物理凝胶需要较低的浓度来电纺光滑、连续的纤维。25在我们的情况下，与a-PMMA的比较更加相关，因为a-PMMA溶液和立体复合物PMMA溶液之间的唯一区别是物理凝胶化的发生。虽然在强络合溶剂（DMF是一种强络合溶剂）中的无规立构PMMA溶液中的立体络合物样相互作用已通过光谱方法（NMR，FTIR）26 r 28和DSC 28，29检测到，但未报告物理凝胶化的观察结果。最有可能的是，无规立构PMMA聚合物中的等规立构和间规立构序列的长度不足以促进长螺旋的形成以及随后的成束和凝胶化。为了说明，在无规立构PMMA中报道的全同立构序列的平均长度为1.33 r1。53个单体单元，28个，而一个螺距的螺旋需要9个全同立构单体单元。在凝胶化过程中，聚合物链彼此相互作用并聚集成3D交联结构，基本上增加了系统的有效分子量。聚合物链甚至在临界重叠浓度C* 以下也可以做到这一点。因此，C* 不是描述物理凝胶相互作用的最重要参数。弹性模量在凝胶化过程中也增加。最近，Yu等人已经确定，流体的弹性对于静电纺丝射流的稳定性比缠结的数量更重要。[31]对于会形成物理凝胶的溶液，这两种论点都可以解释它们的特殊行为。聚合物链在溶液中缔合，形成交联网络。这种结构增加了流体的弹性
DiscussionThe viscoelasticity and spinnability of a polymer solution are closely related to the solvent type, temperature, and polymer concentration. For a good solvent, where polymerrsolvent interaction is favored over polymerrpolymer interaction, the chains behave like a single molecule belowC*, the overlap concentration, related to molecular weight as C*∼ Mra∼ Mr0. 5 from the Berry number and MarkrHouwinkrSakurada equation. 22 As the name suggests, polymer chains start to overlap beyond the overlap concentration, and hence the viscosity of the system increases steeply with concentration above it. It has been shown that the formation of stereocomplex helices increases the persistence length and makes the polymer chain more rodlike. 23 In that case, the MarkrHouwink coefficient a becomes larger, 24 and the overlap occurs at a much lower concentration C*. Thus, gelation can occur at a lower concentration than the C* calculated for a Gaussian or a flexible chain. While the increase in viscosity is linear with concentration for dilute regime, it increases by C4, 5 in the semidilute regime. This is due to presence of entanglements that act as temporary cross-links and hence also impart gel-like elasticity to the polymer solution. Gupta et al. have determined that a-PMMA solutions in DMF need a concentration ratioC/C* of at least 4 to produce smooth electrospun fibers. 21 The C/C* ratio of the 5 wt% stereocomplexing mixture that produced smooth fibers was 2 in our case, which is much lower than reported for linear a-PMMA polymers. We can attribute this to the helix formation and physical gelation during the electrospinning process. It has been shown previously that physical gels require a lower concentration to electrospin smooth, continuous fibers. 25 In our case, the comparison with a-PMMA is all the more relevant, as the only difference between the a-PMMA solution and the stereocomplex PMMA solution is the occurrence of physical gelation. Although stereocomplex-like interactions in atactic PMMA solutions in strongly complexing solvents (DMF is a strongly complexing solvent) have been detected by spectroscopic methods (NMR, FTIR) 26r28 and DSC, 28, 29 the observation of physical gelation was not reported. Most likely the length of the isoatactic and syndiotactic sequences in atactic PMMA polymers was not sufficient to facilitate the formation of long helices and subsequent packing into bundles and gelation. For illustration, the average length of isotactic sequences reported in atactic PMMA was 1.33 r1. 53 monomer units, 28 while it takes 9 isotactic monomer units for one pitch of the helix. 30 During gelation, the polymer chains interact with each other and aggregate into a 3D percolated structure, essentially increasing the effective molecular weight of the system. The polymer chains can do this even below the critical overlap concentration C*. Therefore, C* is not the most important parameter that describes the interactions of a physical gel. The elastic modulus also increases during the gelation process. Recently, Yu et al. have determined that the elasticity of the fluid is more important for electrospun jet stability than the number of entanglements. 31 With solutions that will form physical gels, either argument can explain their peculiar behavior. The polymer chains associate in the solution, forming a crosslinked network. This structure increases the elasticity of the fluid