Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK.
Soft Matter. 2018 Jun 6;14(22):4453-4463. doi: 10.1039/c7sm02488f.
We investigate the formation of poly(vinyl alcohol) microparticles by the selective extraction of aqueous polymer solution droplets, templated by microfluidics and subsequently immersed in a non-solvent bath. The role of polymer molecular mass (18-105 kg mol-1), degree of hydrolysis (88-99%) and thus solubility, and initial solution concentration (0.01-10% w/w) are quantified. Monodisperse droplets with radii ranging from 50 to 500 μm were produced at a flow-focusing junction with carrier phase hexadecane and extracted into ethyl acetate. Solvent exchange and extraction result in droplet shrinkage, demixing, coarsening and phase-inversion, yielding polymer microparticles with well-defined dimensions and internal microstructure. Polymer concentration, varied from below the overlap concentration c* to above the concentrated crossover c**, as estimated by viscosity measurements, was found to have the largest impact on the final particle size and extraction timescale, while polymer mass and hydrolysis played a secondary role. These results are consistent with the observation that the average polymer concentration upon solidification greatly exceeds c**, and that the internal microparticle porosity is largely unchanged. However, reducing the initial polymer concentration to well below c* (approximately 100×) and increasing droplet size yields thin-walled (100's of nm) capsules which controllably crumple upon extraction. The symmetry of the process can be readily broken by imposing extraction conditions at an impermeable surface, yielding large, buckled, cavity morphologies. Based on these results, we establish robust design criteria for polymer capsules and particles, demonstrated here for poly(vinyl alcohol), with well-defined shape, dimensions and internal microstructure.
我们研究了通过微流控模板形成的聚（聚乙烯醇）微球的形成，然后将其浸入非溶剂浴中。研究了聚合物分子量（18-105kgmol-1）、水解度（88-99%）和溶解度，以及初始溶液浓度（0.01-10%w/w）的作用。在带有载体相十六烷的流聚焦结处生成半径为 50-500μm 的单分散液滴，并将其萃取到乙酸乙酯中。溶剂交换和萃取导致液滴收缩、相分离、粗化和相反转，生成具有良好定义尺寸和内部微观结构的聚合物微球。通过粘度测量估计，聚合物浓度从低于重叠浓度 c到高于浓缩交叉浓度 c*变化，对最终颗粒尺寸和萃取时间尺度的影响最大，而聚合物质量和水解度则起次要作用。这些结果与以下观察结果一致，即凝固时的平均聚合物浓度大大超过 c，并且内部微球孔隙率基本不变。然而，将初始聚合物浓度降低到远低于 c*（约 100 倍）并增大液滴尺寸会得到薄壁（几百纳米）胶囊，这些胶囊在萃取时会可控地皱缩。通过在不渗透表面上施加萃取条件，可以轻易地破坏过程的对称性，从而产生大的、褶皱的、空腔形态。基于这些结果，我们建立了聚合物胶囊和颗粒的稳健设计标准，在此通过聚（聚乙烯醇）进行了演示，具有良好的形状、尺寸和内部微观结构。
