Enhanced Nucleation of Monomer Droplets in Miniemulsion Polymerization

摘要-El-Aasser-9628783将微乳液技术应用于乙烯基单体自由基聚合法制备胶乳的潜在优势尚未实现，原因之一是只有一小部分(约20%)的初始液滴(50-500 nm)成核成为聚合物粒子。这限制了粒子大小和共聚物组成的控制，因为需要单体从没有成核的液滴扩散到不断增长的聚合物粒子。对于不容易通过水相扩散的低水溶解度单体(例如大分子单体)来说，这是一个特别的缺点。细乳液聚合的最新进展表明，在细乳液滴中加入少量聚合物可促进其成核，在一定条件下可达100%，从而提高聚合速率和粒子数。这种现象归因于自由基进入这些液滴的速度增加，这提高了它们与现有聚合物颗粒竞争水相自由基的能力。对这一现象的机理解释以及将该方法推广到其他单体体系是本研究的重点。这项研究将采用多种技术相结合的方法，包括通过反应量热法连续测量反应速率，以及通过颗粒大小分布监测颗粒数量的演变。添加的聚合物的性质将根据其相对分子质量和端基的官能度而有所不同，以确定它是界面现象还是ObukO现象。同样，液滴/水界面的组成将是不同的，并具有不同的特征。除了常规的水溶性引发剂(即过硫酸盐)外，还将使用油溶引发剂来确定自由基的来源(水相与液滴相)对粒子成核的影响。其他单体/共聚体系也将被研究。最后，数学模型将与上述发现结合使用，以帮助从机理上理解细乳液聚合过程作为这些研究中选择的变量的函数。

Abstract - El-Aasser - 9628783 The potential advantages of applying miniemulsion technology to the preparation of latexes through free radical polymerization of vinyl monomers have not been realized, one reason being that only a fraction (circa 20%) of the initial droplets (50 - 500 nm) are nucleated to become polymer particles. This limits the control of the particle size and copolymer composition by requiring diffusion of monomers from the droplets which are not nucleated to the growing polymer particles. This is a particular disadvantage for monomers with low water solubility which do not readily diffuse through the aqueous phase (e.g., macromonomers). Recent developments in the miniemulsion polymerization of styrene have shown that the addition of a small amount of polymer to the miniemulsion droplets enhances their nucleation which can reach 100% under certain conditions, thereby increasing the polymerization rate and resulting particle number. This phenomenon has been attributed to an increased rate of radical entry into these droplets which improves their ability to compete with the existing polymer particles for aqueous phase free radicals. The mechanistic explanation for this phenomenon and the extension of the method to other monomer systems are the focus of this research. A combination of techniques will be applied in this investigation including continuous measurements of the reaction rate via reaction calorimetry and monitoring the evolution of the number of particles through the particle size distribution. The nature of the added polymer will be varied in terms of its molecular weight and the functionality of its end-groups in order to determine whether it is an interfacial phenomenon or a ObulkO phenomenon. Likewise, the composition of the droplet/water interface will be varied and characterized. In addition to the normal water-soluble initiator (i.e., persulfate), oil-soluble initiators will be used to determine the effect of the origin of the free radicals (aqueous vs. droplet phase) on particle nucleation. Other monomer/comonomer systems will also be investigated. And finally, mathematical modeling will be used in conjunction with the above findings to aid in mechanistic understanding of the miniemulsion polymerization process as a function of the variables chosen in these studies.