Pore Formation during the Crystallization of Crosslinked Polymers which are Swollen in Supercritical Fluids

这项研究通过从凝胶状态结晶聚合物来探索开孔形态的产生。使用的凝胶是化学交联聚合物，在超临界流体（SCF）中在高温高压下高度膨胀。当冷却到低于结晶温度时，聚合物分子开始组装成一个三维的、半结晶的上层结构，具有高比表面积和充满SCF的狭窄、开放的孔隙，然后释放出来。孔径根据聚合物和加工条件的不同在10nm ~ 10mm之间；孔隙体积可能在整个样品的0到90%之间。新策略不同于传统的发泡工艺（不需要气泡生长）和商业TIPS工艺（不涉及聚合物溶液），但它集成了现有加工方法的各个方面，以提供所需形状的交联前体。这里使用scf有几个原因。SCF可以在不破坏孔隙的情况下从聚合物中去除。因此，SCFs是可回收的、自清洁的和可重用的；对于重复使用，由于（我们使用的）超临界烷烃在每个处理周期结束时减压会转化为不良溶剂，因此对超临界流体的清洗是自然而然的。由此产生的多孔聚合物非常干净（无残留物），并保持形状和结构完整性。SCFs还降低了结晶过程中的表面张力，提高了分子的迁移率；这些性质可能对结晶动力学和孔隙形成有潜在的影响。这些都是有待解决的问题。溶剂性质、聚合物的分子参数、交联方法、交联密度和均匀性、结晶温度和溶胀条件将成为孔隙体积和大小分布以及结晶度的控制参数。该研究旨在阐明这些关系，以实现可调孔径和尺寸分布的目标。本研究的模型聚合物体系是在超临界丙烷中膨胀的各种交联聚乙烯（xHDPE, xLDPE, xLLDPE）。聚乙烯的基本发现有望广泛应用于催化分离、高性能材料和生物医学应用中的半结晶聚合物。环境影响：日益严格的环境立法迫使许多行业重新评估其有害溶剂的使用。这种新工艺为制造多孔聚合物提供了一种全工艺设计方案。它将通过以下方式减少废物的产生：(a)在生产过程中用超临界流体代替有机溶剂。(b)完全消除了多孔材料通常需要的最后清洗步骤（相比之下，传统工艺中多孔聚合物的清洗目前是用特殊清洗液进行的，这通常是危险的）。(c)回收和重复使用超临界流体。

CTS-0107156Winter, H. HenningU of Massachusetts - AmherstThis research explores the creation of open-pore morphology by crystallizing polymers from a gel state. The gels used are chemically crosslinked polymers which were highly swollen in a supercritical fluid (SCF) at elevated temperature and pressure. Upon cooling below their crystallization temperature, polymer molecules begin to assemble into a three-dimensional, semicrystalline superstructure of high specific surface area and with narrow, open pores filled with SCF which then gets released. Pore sizes are in the order of 10nm to 10mm, depending on polymer and processing conditions; pore volumes might be anywhere between 0 and 90% of the entire sample. The new strategy differs from the classical foaming process (no bubble growth is needed) and from the commercial TIPS process (no polymer solution involved), but it integrates aspects of established processing methods to provide crosslinked precursors of desired shape. SCFs are used here for several reasons. SCF can be removed from the polymer without collapsing the pores. Subsequently, SCFs are recoverable, self-cleaning, and reusable; for repeated use, cleaning of the supercritical fluid comes naturally since the supercritical alkane (which we use) transforms into a poor solvent by depressurizing at the end of each processing cycle. The resulting porous polymers are exceptionally clean (free of residues), and retain shape and structural integrity. SCFs also lower surface tension and increase molecular mobility during crystallization; these are properties which could potentially have an affect on crystallization kinetics and pore formation. These are open questions which need to be pursued.Solvent character, molecular parameters of the polymer, crosslinking method, crosslink density and homogeneity, crystallization temperature, and swelling conditions are expected to be the controlling parameters for pore volume and size distribution, and for crystallinity. The research is aimed at elucidating these relationships with the objective of achieving tunable pore size and size distributions. Model polymer systems in this study are various crosslinked polyethylenes (xHDPE, xLDPE, xLLDPE) swollen in supercritical propane.The fundamental findings on polyethylenes are expected to apply to a wide range of semicrystalline polymers in catalysis separations, high performance materials and biomedical applications. Environmental Impact: Increasingly strict environmental legislation has forced many industries to re-evaluate their use of hazardous solvents. The novel process offers a process-wide design alternative for making porous polymers. It will reduce waste generation by:(a) supercritical fluids substitution of organic solvents during production.(b) complete elimination of a final cleaning step which otherwise is typical for porous materials (in comparison, cleaning of porous polymers from traditional processing is currently performed with special cleaning fluids which often are hazardous).(c) recovery and repeated use of the supercritical fluid.