Abstract
Waterborne latex is often called a product-of-process. Here, the effect of semi-batch monomer feed rate on the kinetics and gel formation in seeded emulsion polymerization was investigated for the copolymerization of n-butyl methacrylate (n-BMA) and ethylene glycol dimethacrylate (EGDMA). Strikingly, the gel fraction was observed to be significantly influenced by monomer feed rate, even while most of the experiments were performed under so-called starve-fed conditions. More flooded conditions from faster monomer feed rates, including seeded batch reactions, counterintuitively resulted in significantly higher gel fraction. Chain transfer to polymer was intentionally suppressed here via monomer selection so as to focus mechanistic insights to relate only to the influence of a divinyl monomer, as opposed to being clouded by contributions to topology from long chain branching. Simulations revealed that the dominant influence on this phenomenon was the sensitivity of primary intramolecular cyclization to the instantaneous unreacted monomer concentration, which is directly impacted by monomer feed rate. The rate constant for cyclization for these conditions was determined to be first order and 4000 s−1, approximately 4 times that typically observed for backbiting in acrylates. This concept has been explored previously for bulk and solution polymerizations, but not for emulsified reaction environments and especially for the very low mole fraction divinyl monomer. In addition, while gel fraction could be dramatically manipulated by variations in linear monomer feed rates, it could be markedly enhanced by leveraging non-linear feed profiles built from combination sequences of flooded and starved conditions. For a 2 h total feed time, a fully linear profile resulted in 30% gel while a corresponding non-linear profile with an early fast-feed segment resulted in 80% gel.
Highlights
IntroductionIntroduction published maps and institutional affilWaterborne resins with low VOC (volatile organic compound) content are commonly produced by seeded emulsion polymerization
Introduction published maps and institutional affilWaterborne resins with low VOC content are commonly produced by seeded emulsion polymerization
We have previously described key elements of crosslinking efficiency and mechanism for high monomer concentration environments, such as in bulk polymerization, that focus on Polymers 2021, 13, 596 chemical choices [17,18]
Summary
Introduction published maps and institutional affilWaterborne resins with low VOC (volatile organic compound) content are commonly produced by seeded emulsion polymerization. Care must be taken into account for other potential impacts of process strategy on the balance of available reaction mechanisms. Crosslinking during polymerization to achieve a target network topology is, quite sensitive to process conditions, with macromolecular architecture a key design property in applications such as dental materials, paints and coatings, and adhesives [3,4,5,6]. For multiphase polymers, such as latex, strategies to tune polymer properties must consider multiple length scales from the iations
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