Czajka Adam, Lovell Peter A, Armes Steven P
Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield, South Yorkshire S3 7HF, United Kingdom.
Department of Materials, School of Natural Sciences, The University of Manchester, Manchester M13 9PL, United Kingdom.
Macromolecules. 2022 Nov 22;55(22):10188-10196. doi: 10.1021/acs.macromol.2c01801. Epub 2022 Nov 9.
Recently, we reported time-resolved synchrotron small-angle X-ray scattering (TR-SAXS) studies during aqueous emulsion polymerization using a bespoke stirrable reaction cell ( , , 1474-1484). This proof-of-concept study utilized a semifluorinated specialty monomer (2,2,2-trifluoroethyl methacrylate) to ensure high X-ray contrast relative to water. Herein, we extend this approach to emulsion polymerization of methyl methacrylate (MMA) in the presence or absence of sodium dodecyl sulfate (SDS) at 70 °C. Solution conductivity measurements for this anionic surfactant indicated a critical micelle concentration (CMC) of 10.9 mM at this temperature. Thus, SDS was employed at either 1.0 or 20.0 mM, which corresponds to well below or well above its CMC. Postmortem analysis by H NMR spectroscopy indicated MMA conversions of 93-95% for these three formulations. We demonstrate that the X-ray contrast between water and PMMA is sufficiently large to produce high-quality scattering patterns during TR-SAXS experiments. Such patterns were fitted using a hard-sphere scattering model to monitor the evolution in particle diameter. This enabled (i) determination of the time point for the onset of nucleation and (ii) the evolution in particle size to be monitored during the MMA polymerization. The final particle diameters obtained from such TR-SAXS studies were consistent with postmortem DLS analyses, while TEM studies confirmed that near-monodisperse latex particles were formed. Micellar nucleation occurs within just 2 min when the SDS concentration is well above its CMC, resulting in a high particle number concentration and relatively small latex particles. In contrast, when SDS is either absent or present below its CMC, particle nuclei are formed by homogeneous nucleation over significantly longer time scales (14-15 min). In the latter case, adsorption of SDS onto nascent particles reduces their coagulation, giving rise to a larger number of smaller particles compared to the surfactant-free polymerization. However, the characteristic time required for the onset of nucleation is barely affected because this is mainly controlled by the kinetics of homogeneous polymerization of the relatively water-soluble MMA monomer within the aqueous phase. These results suggest that the aqueous emulsion polymerization of several other (meth)acrylic monomers, and perhaps also vinyl acetate, may be amenable to TR-SAXS studies.
最近，我们报道了在水乳液聚合过程中使用定制的可搅拌反应池进行时间分辨同步加速器小角X射线散射（TR-SAXS）研究（，，1474 - 1484）。这项概念验证研究使用了一种半氟化特种单体（甲基丙烯酸2,2,2 - 三氟乙酯），以确保相对于水具有高X射线对比度。在此，我们将这种方法扩展到在70°C下存在或不存在十二烷基硫酸钠（SDS）的情况下甲基丙烯酸甲酯（MMA）的乳液聚合。对这种阴离子表面活性剂的溶液电导率测量表明，在此温度下其临界胶束浓度（CMC）为10.9 mM。因此，SDS的使用浓度为1.0或20.0 mM，分别对应于远低于或远高于其CMC。通过¹H NMR光谱进行的事后分析表明，这三种配方的MMA转化率为93 - 95%。我们证明，在TR-SAXS实验期间，水和聚甲基丙烯酸甲酯（PMMA）之间的X射线对比度足够大，能够产生高质量的散射图案。使用硬球散射模型对这些图案进行拟合，以监测粒径的演变。这使得能够（i）确定成核开始的时间点，以及（ii）在MMA聚合过程中监测粒径的演变。从这种TR-SAXS研究中获得的最终粒径与事后动态光散射（DLS）分析结果一致，而透射电子显微镜（TEM）研究证实形成了近单分散的胶乳颗粒。当SDS浓度远高于其CMC时，胶束成核在短短2分钟内就会发生，导致高颗粒数浓度和相对较小的胶乳颗粒。相比之下，当不存在SDS或SDS浓度低于其CMC时，颗粒核通过均相成核在长得多的时间尺度（14 - 15分钟）内形成。在后一种情况下，SDS吸附到新生颗粒上会降低它们的凝聚，与无表面活性剂聚合相比，产生大量较小的颗粒。然而，成核开始所需的特征时间几乎不受影响，因为这主要由水相中相对水溶性的MMA单体的均聚动力学控制。这些结果表明，其他几种（甲基）丙烯酸单体以及可能还有醋酸乙烯酯的水乳液聚合可能适用于TR-SAXS研究。
