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

在超临界流体中溶胀的交联聚合物结晶过程中的孔形成

• 批准号: 0107156
• 负责人: H. Henning Winter
• 金额: $ 21.78万
• 依托单位: University of Massachusetts Amherst
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-10-15 至 2004-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0107156&HistoricalAwards=false
• 关键词: Pore; Formation; during; Crystallization; Crosslinked

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.

CTS-0107156温特，H.HenningU of Massachusetts-Amherst这项研究探索了通过从凝胶状态结晶聚合物来创造开孔形态。所使用的凝胶是化学交联聚合物，在高温和压力下在超临界流体(SCF)中高度膨胀。当冷却到结晶温度以下时，聚合物分子开始组装成三维半结晶超结构，具有高比表面积和填充了SCF的狭窄、开放的孔，然后被释放。根据聚合物和加工条件的不同，孔的大小在10 nm到10 mm之间；孔体积可能在整个样品的0到90%之间。新的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)，但它整合了现有加工方法的各个方面，以提供所需形状的交联前驱体。这里使用SCF有几个原因。可以从聚合物中去除SCF，而不会塌陷毛孔。因此，超临界流体是可回收、自清洁和可重复使用的；对于重复使用，超临界流体的清洗是自然的，因为超临界烷烃(我们使用的)在每个处理周期结束时通过减压转化为劣质溶剂。得到的多孔聚合物特别干净(没有残留物)，并保持形状和结构的完整性。超临界流体还降低了结晶过程中的表面张力，增加了分子的流动性；这些性质可能会对结晶动力学和孔道形成产生潜在的影响。溶剂性质、聚合物的分子参数、交联法、交联剂密度和均匀性、结晶温度和溶胀条件有望成为控制孔体积和大小分布以及结晶度的参数。这项研究的目的是阐明这些关系，目的是实现可调的孔径和尺寸分布。本研究中的模型聚合物体系是在超临界丙烷中溶胀的各种交联型聚乙烯(xHDPE、xLDPE、xLLDPE)，关于聚乙烯的基本发现有望应用于广泛的半结晶聚合物的催化分离、高性能材料和生物医学应用。环境影响：日益严格的环境法规迫使许多行业重新评估其有害溶剂的使用。这一新工艺为制造多孔聚合物提供了一种全过程的设计替代方案。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取代有机溶剂。(B)完全消除最后的清洗步骤，否则这是多孔材料的典型步骤(相比之下，从传统工艺中清洗多孔聚合物目前是用特殊的清洗液进行的，这种清洗液往往是危险的)。(C)回收和重复使用超临界流体。