Abstract
The objective of this paper is to explore the kinetics and mechanism ofemulsion polymerizations for systems of large particles.For the emulsifier-free emulsion polymerization of styrene with minoramount of acrylamide as water soluble comonomer and potassiumpersulfate asinitiator, experimental studies on the kinetics, molecular weightdistributions, and particle morphology have been carried out. It was foundthat the particle nucleation is likely via the mechanism of homogeneouscoagulative nucleation to yield primary particles and coagulation of thepromary particles to yield uniform particles. While the particle growth inthe post nucleation stage is via shell-growth mechanism, by which thepolymerization occurs mainly in the shell region (about 100 to 400 A inthickness) of the particles.The shell-growth mechanism was further supported with the morphologicalevidence of seeded emulsion polymerizations. The particles are consideredto be uniformly saturated with monomer before disappearance of the monomerdroplets. The existence of the reaction shell region is solely due tosurface anchoring of the sulfate ends of the growing radicals and to thelarge size of the particle (greater than about 1500-2000 A in diameter).The average number of growing radicals per particle in the growth periodis found to be from 2 to 6, much higher than that in the conventionalcase. The high n value and confined polymerization in the shell regioncause a termination of the growing radicals at lower molecular weight (MW)in the order of 104 to 105. This lower MW is consistent with thecalculated shell thickness, since the growing chain end must locate withinthe shell having a thickness equal to its root-mean-square end-to-enddistance.As the monomer droplets disappear (Interval III), in large emulsionparticles, monomer consumed in the shell region is supplied by monomerdiffusing from the core. As polymerization proceeds further, the viscosityin the shell increased, causing an increased resistance for monomerdiffusion, and the monomer concentration in the core further decrease.Eventually, the polymerization becomes monomer diffusion-control and thelimiting conversion occurs. Accordingly, limiting conversion is theconsequence of shell-region polymerization characteristic of the largeemulsion particles and diffusion-control mechanism in high conversionperiod. On the other hand, systems with small emulsion particles, 100%conversion can always be obtained.本研究之目的在探討大粒徑乳化聚合反應之動力學現象與其反應機構。本研究從分析苯乙烯╱丙烯醯胺系統之動力學數據為出發點，考慮粒子粒徑變化對乳化聚合系統的影響：（１）從低轉化率時的粒子成核期開始，由系統之分子量分佈(MWD)與反應條件對Np的影響發現其成核機構為均相成核(homogeneous nucleation);（２）在粒子成長期，除了根據系統之轉化率與分子量分佈數據提出”外殼層反應機構”(Shell-Growth Mechanisim) 外，並以種子乳化聚合法與粒子超薄切片技術提出粒子形態學上的直接證據；（３）探討高轉化率期之極限轉化率(limitingconversion) 的形成機構。綜合以上三個時期的反應機構完整地描述大粒徑（大於0.15-0.2 um)乳化聚合系統的整個反應過程。”外殼層反應機構”的理論基礎如下所述：主要之鏈終結反應機構為重合反應(combination)，而由於聚合體鏈端官能基的親水性與聚合體鏈本身的疏水性，使得此一鏈端官能基定錨(anchoring) 於粒子表面，所以雖然單體在粒子中為均勻分佈，但整個反應主要在靠近粒子表面的殼層進行，此殼層之厚度約為成長中聚合體鏈的平均鏈端距離。換言之，也只有在粒子半徑遠大其聚合體鏈之平均鏈端距離時才會有”外殼層反應機構”的發生，否則應在粒子內部進行均相聚合反應。如果粒子成長遵循外殼層反應機構，則具有（轉化率）2/3 對時間成正比之”特徵曲線”而且具有較傳統乳化聚合系統之分子量(105-106) 為低的分子量(104-105) 。對照不同粒子粒徑系統之轉化率曲線與其分子量時發現：當粒子之粒徑約大於0.15-0.2 時，粒子成長即遵循”外殼層反應機構”。由大粒徑粒子系統之分子量所估計出之聚合體鏈之鏈端距離為100-400 A ，而在小粒徑粒子系統中，其聚合體鏈之鈄端距離約為700A，其結果亦支持本文之論點。在大粒徑粒子中其活性自由基位於靠近粒子外殼層內，當單體滴消失後，單體必需由粒子內部擴散到粒子外殼層之反應區才可能進行反應。在高轉化率時(Ca., m>0.9)，粒子內部之黏度非常高，單體擴散困難，造成粒子內部單體呈現非均勻分佈的結果，亦即此時之粒子很可能具有由裡到外為一單體濃度遞減的粒子形態，甚至可能是由一個含單體的被困在粒子無法擴散到粒子外殼層的反應區域所致。因此控制極限轉化率的主要機構為外殼層反應機構與單體擴散控制反應機構共同作用的結果；只要粒子粒徑夠大(Ca., Dp>0.2 mu)，就都存在著極限轉化率的現象。而大粒徑粒子表面的高電荷密度、系統中起始劑的殘存量、與聚合體系統的玻璃轉變溫度(glass transitiontemperature, Tg)等因素都不是決定其極限轉化率的主要原因。