Self-seeding in one dimension: an approach to control the length of fiberlike polyisoprene-polyferrocenylsilane block copolymer micelles.
Self-seeding in one dimension: an approach to control the length of fiberlike polyisoprene-polyferrocenylsilane block copolymer micelles.
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DOI：
10.1002/anie.201006223
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发表时间：
2011-02
影响因子：
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通讯作者：
Jieshu Qian;G. Guerin;Yijie Lu;G. Cambridge;I. Manners;M. Winnik
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文献类型：
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作者：
Jieshu Qian;G. Guerin;Yijie Lu;G. Cambridge;I. Manners;M. Winnik
自播种是聚合物结晶过程中特有的现象。聚合物很难结晶，在大多数情况下，每个聚合物链只有一部分可以容纳在晶格中。结果，聚合物形成具有层状结构的晶体，其表面包含链折叠。如果长链必须在短时间内整合到晶体中，他们将以较低的结晶度为代价。因此，聚合物晶体不可避免地由具有不同链序和构象熵的区域组成。聚合物晶体具有广泛的熔化温度范围，其值取决于结晶过程的细节。在典型的自播种实验中，将块状或悬浮在溶剂中的结晶聚合物加热到略高于其正常熔点（如通过差示扫描量热法测定），这样光学或光谱就无法检测到残留的晶体。冷却这种熔体或溶液导致聚合物单晶的形成，通常以薄板的形式在尺寸和厚度上均匀，这可以理想地适合于进一步的应用。这些单晶被认为是由亚微观核在溶解过程中幸存下来而形成的。自20世纪60年代发现自播种以来，该工艺作为一种不需要外部成核剂就能控制聚合物结晶成核步骤的方法而引起了人们的注意，这种方法可以形成均匀的均聚物和嵌段共聚物单晶，也可以用于材料应用。聚二茂铁基二甲基硅烷（PFS）是一种晶体含金属聚合物，具有一系列有趣的性质。PFS嵌段共聚物和密切相关的材料自组装形成具有半晶核的细长胶束。它们是目前已知的唯一一种合成聚合物，通过类似于可溶性蛋白形成淀粉样纤维的机制形成纤维状胶束。因此，由嵌段共聚物单体组成的可溶聚合物“单体”凝聚到存在于溶液中或有意添加到溶液中的种子结构的两端。PFS嵌段共聚物纤维的形成涉及由PFS部分外延结晶到现有胶束或种子的PFS核心的开放端所驱动的构象变化，这些胶束或种子是通过将预制的纤维状胶束进行温和的超声处理而获得的。因此，生长过程结束时胶束的数量是由开始时存在的种子数量决定的。此外，种子生长实验允许对所获得的结构类型进行精确的控制。例如，一种类型的PFS嵌段共聚物如PI-PFS （PI =聚异戊二烯）可以用来形成种子结构，而另一种类型的PFS嵌段共聚物如PFSPDMS （PDMS =聚二甲基硅氧烷）可以从末端生长。通过这种方式，可以制备出引人注目的新型结构，称为“三嵌段共胶束”。[10]我们最近的工作目标是更深入地了解PFS嵌段共聚物的自组装过程，以便开发可扩展到其他线圈结晶嵌段共聚物的原理。这可能允许获得具有有用光学或电子特性的半柔性纳米线的可加工悬浮液。通过对其长度和尺寸的适当控制，这种结构可以集成到光电器件中或用于其他应用。2009年，Reiter和同事研究了熔体中PFS均聚物单晶和P2VP-PEO嵌段共聚物单晶的自播种机制。（P2VP =聚2-乙烯基吡啶），PEO =聚环氧乙烷)。结果表明，在这两种体系中，再生晶体的数量密度随溶解温度的升高呈指数下降，但不随溶解时间的增加而变化。他们还发现，在开始的单晶和通过自我播种过程形成的再生晶体克隆之间，分子取向存在相关性。他们的实验证实，单晶的自播种生长是在热力学控制下进行的，这与加热后，不太完美的晶体会融化，而更完美的晶体会存活的观点是一致的。它不是一种与聚合物构象记忆效应相关的动力学效应。[*]钱金生，Dr. G. Guerin， Y. J. Lu， G. Cambridge, Prof. M. A. Winnik，多伦多大学化学系，多伦多圣乔治街80号，安大略省，M5S 3H6（加拿大）传真：（+ 1）416-978-0541 E-mail: mwinnik@chem.utoronto.ca
Self-seeding is a phenomenon unique for polymer crystallization. Polymers have difficulty in crystallizing and, in most cases, only part of each polymer chain can be accommodated in the crystal lattice. As a result, polymers form crystals with lamellar structures terminated by surfaces containing chain folds. If long chains have to be integrated into the crystal in a short time, they will do so at the expense of lower crystallinity. As a consequence, polymer crystals inevitably consist of regions with different chain order and conformational entropy. Polymer crystals have a broad range of melting temperatures whose values depend upon the details of the crystallization process. In a typical self-seeding experiment, a crystalline polymer in the bulk state or suspended in a solvent is heated slightly above its normal melting point (as determined, for example, by differential scanning calorimetry; DSC) so that no residual crystals can be detected optically or spectroscopically. Cooling this melt or solution leads to the formation of polymer single crystals, normally in the form of thin plates uniform in size and thickness, which can be ideally suited for further applications. These single crystals are thought to be initiated by submicroscopic nuclei that survived the dissolution procedure. Since the discovery of self-seeding in the 1960s the process has attracted attention as a means of controlling the nucleation step of polymer crystallization without the need for external nucleating agents, to form uniform single crystals of homopolymers and block copolymers and also for materials applications. Polyferrocenyldimethylsilane (PFS) is a crystalline metalcontaining polymer with a range of interesting properties. PFS block copolymers and closely related materials selfassemble to form elongated micelles with a semicrystalline core. They are the only currently known synthetic polymers to form fiberlike micelles by a mechanism resembling that for the formation of amyloid fibers from soluble protein. Thus soluble polymeric “monomers” consisting of block copolymer unimers condense onto both ends of seed structures present in, or intentionally added to, the solution. PFS block copolymer fiber formation involves a conformation change driven by epitaxial crystallization of PFS moieties onto the open ends of the PFS core of existing micelles or seeds obtained by subjecting preformed fiberlike micelles to mild sonication. Thus, the number of micelles at the end of the growth process is determined by the number of seeds present at the beginning. Moreover the seeded growth experiments permit exquisite control over the types of structures obtained. For example, one type of PFS block copolymer such as PI-PFS (PI = polyisoprene) can be used to form the seed structure, and a different type of PFS block copolymer such as PFSPDMS (PDMS = polydimethylsiloxane) can be grown off the ends. In this way striking novel architectures referred to as “triblock co-micelles” can be prepared. 10] Our recent work targets a deeper understanding of the self-assembly process for PFS block copolymers in order to develop principles that can be extended to other coil-crystalline block copolymers. This may allow access to processable suspensions of semiflexible nanowires with useful optical or electronic properties. With proper control over their length and dimensions, such structures could be incorporated into optoelectronic devices or used in other applications. In 2009, Reiter and co-workers examined the mechanism of self-seeding in the melt for PFS homopolymer single crystals and for single crystals formed by P2VP-PEO block copolymers. (P2VP = poly(2-vinylpyridine), PEO = poly(ethylene oxide)). They showed for these two systems that the number density of the regenerated crystals decreased exponentially with the increase of the dissolution temperature but did not vary with the dissolution time. They also found a correlation in molecular orientation between a starting single crystal and the regenerated crystal clones formed through the self-seeding process. Their experiments established that single-crystal growth by self-seeding operates under thermodynamic control, consistent with the idea that upon heating, the less perfect crystals will melt and more perfect crystallites will survive. It is not a kinetic effect associated with polymer conformational memory effects. [*] J. S. Qian, Dr. G. Guerin, Y. J. Lu, G. Cambridge, Prof. M. A. Winnik Department of Chemistry, University of Toronto 80 St. George Street Toronto, Ontario, M5S 3H6 (Canada) Fax: (+ 1)416-978-0541 E-mail: mwinnik@chem.utoronto.ca