Burroughs Michael C, Schloemer Tracy H, Congreve Daniel N, Mai Danielle J
Department of Chemical Engineering, Stanford University, Stanford, California94305, United States.
Department of Electrical Engineering, Stanford University, Stanford, California94305, United States.
ACS Polym Au. 2022 Dec 5;3(2):217-227. doi: 10.1021/acspolymersau.2c00051. eCollection 2023 Apr 12.
Embedding nanomaterials into polymer hydrogels enables the design of functional materials with tailored chemical, mechanical, and optical properties. Nanocapsules that protect interior cargo and disperse readily through a polymeric matrix have drawn particular interest for their ability to integrate chemically incompatible systems and to further expand the parameter space for polymer nanocomposite hydrogels. The properties of polymer nanocomposite hydrogels depend on the material composition and processing route, which were explored systematically in this work. The gelation kinetics of network-forming polymer solutions with and without silica-coated nanocapsules bearing polyethylene glycol (PEG) surface ligands were investigated using dynamic rheology measurements. Network-forming polymers comprised either 4-arm or 8-arm star PEG with terminal anthracene groups, which dimerize upon irradiation with ultraviolet (UV) light. The PEG-anthracene solutions exhibited rapid gel formation upon UV exposure (365 nm); gel formation was observed as a crossover from liquid-like to solid-like behavior during small-amplitude oscillatory shear rheology. This crossover time was non-monotonic with polymer concentration. Far below the overlap concentration (/* ≪ 1), spatially separated PEG-anthracene molecules were subject to forming intramolecular loops over intermolecular cross-links, thereby slowing the gelation process. Near the polymer overlap concentration (/* ∼ 1), rapid gelation was attributed to the ideal proximity of anthracene end groups from neighboring polymer molecules. Above the overlap concentration (/* > 1), increased solution viscosities hindered molecular diffusion, thereby reducing the frequency of dimerization reactions. Adding nanocapsules to PEG-anthracene solutions resulted in faster gelation than nanocapsule-free PEG-anthracene solutions with equivalent effective polymer concentrations. The final elastic modulus of nanocomposite hydrogels increased with nanocapsule volume fraction, signifying synergistic mechanical reinforcement by nanocapsules despite not being cross-linked into the polymer network. Overall, these findings quantify the impact of nanocapsule addition on the gelation kinetics and mechanical properties of polymer nanocomposite hydrogels, which are promising materials for applications in optoelectronics, biotechnology, and additive manufacturing.
将纳米材料嵌入聚合物水凝胶能够设计出具有定制化学、机械和光学特性的功能材料。能够保护内部物质并易于分散在聚合物基质中的纳米胶囊，因其能够整合化学性质不相容的体系以及进一步扩展聚合物纳米复合水凝胶的参数空间而备受关注。聚合物纳米复合水凝胶的性质取决于材料组成和加工路线，本工作对此进行了系统研究。使用动态流变学测量方法，研究了含有和不含有带有聚乙二醇（PEG）表面配体的二氧化硅包覆纳米胶囊的成网聚合物溶液的凝胶化动力学。成网聚合物由带有末端蒽基的四臂或八臂星形PEG组成，这些蒽基在紫外线（UV）照射下会二聚化。PEG-蒽溶液在紫外线照射（365 nm）下会迅速形成凝胶；在小振幅振荡剪切流变学过程中，凝胶形成表现为从类液体行为到类固体行为的转变。这个转变时间与聚合物浓度并非单调相关。远低于重叠浓度（/* ≪ 1）时，空间分离的PEG-蒽分子倾向于形成分子内环而非分子间交联，从而减缓了凝胶化过程。接近聚合物重叠浓度（/* ∼ 1）时，快速凝胶化归因于相邻聚合物分子的蒽端基理想的接近程度。高于重叠浓度（/* > 1）时，溶液粘度增加阻碍了分子扩散，从而降低了二聚化反应的频率。向PEG-蒽溶液中添加纳米胶囊比具有等效有效聚合物浓度的无纳米胶囊PEG-蒽溶液导致更快的凝胶化。纳米复合水凝胶的最终弹性模量随纳米胶囊体积分数增加而增加，这表明尽管纳米胶囊未交联到聚合物网络中，但它们具有协同机械增强作用。总体而言，这些发现量化了添加纳米胶囊对聚合物纳米复合水凝胶凝胶化动力学和机械性能的影响，聚合物纳米复合水凝胶是用于光电子学、生物技术和增材制造的有前景的材料。
