Amphiphilic Block-Random Copolymer Stabilizers: A “Seeded-Coagulative” Emulsion Polymerization Mechanism

Abstract

Polystyrene-b-[polystyrene-r-poly(acrylic acid)] block-random copolymers are effective stabilizers of emulsion polymerizations but exhibit unusual dispersion behavior, selfassembling with low aggregation numbers or possibly self-folding into single-chain nanoparticles. In emulsion polymerizations stabilized by these block-random copolymers, the total final particle surface area was directly proportional to the stabilizer concentration and was unaffected by kinetically driven processes such as initiation and particle growth rate; this behavior is not explained by any known emulsion polymerization mechanism. A "seeded-coagulative" emulsion polymerization mechanism is proposed, in which particles grow by polymerization and continuous coagulation, conserving total surface area. Mathematical equations for this proposed mechanism fit the experimental data well and explain the observed dependence of particle number on stabilizer concentration (N p ∼ [S] 3 ) and solids content (N p ∼ τ −2 ). The novel emulsion polymerization mechanism is attributed to the unique properties of the stabilizers used.

Block–random copolymer stabilisers for semi-batch emulsion polymerisation

“…As the feeding proceeds, the pH of the emulsion decreases to 6.5 because of the decomposition of APS initiator. 38 While the reference experiment ( pH i = 9.5) displays the previously reported trend for BRCs with increasing particle diameter (and a decrease of N p ) with increasing conversion because of the coagulation of the growing particles, 26 the pH = 7.9 experiment displays a different behaviour. The D n was found to decrease abruptly in the early stages and remain relatively constant until the end of polymerisation.…”

“…A unique, seeded-coagulative nucleation mechanism was proposed where a large population of small BRC aggregates act as seeds which coagulate as the polymer particles grow. 26 This seeded-coagulative mechanism showed no change in total stable particle surface area at a given concentration of BRC when kinetic factors ( e.g. initiation rate) were modified, suggesting a robust mechanism.…”

Methacrylic acid‐based amphiphilic block‐random copolymer stabilizers for emulsion polymerization

“…Detailed information on the evaluation of copolymer aggregates as well as the emulsion properties of the final latex is presented in a previous study. [16] 3 | RESULTS AND DISCUSSION…”

Amphiphilic Block-Random Copolymers: Shedding Light on Aqueous Self-Assembly Behavior

“…It was shown that PS- b -(PS- r -PAA) could successfully stabilize styrene emulsion polymerizations when at least a 3 wt % stabilizer relative to a monomer was used. In addition to the unique nucleation mechanism observed in emulsion polymerization, these copolymers presented an unanticipated ease of dispersion, especially given what has been observed for analogous block copolymers . It was proposed that the facile process leading to transparent dispersions at concentrations upwards of 300 g/L is in part due to self-folding into single-chain nanoparticles as opposed to the self-assembly behavior commonly observed with block copolymers and small-molecule amphiphiles.…”

“…Our group has previously reported the use of PS- b -(PS- r -PAA) “block-random” copolymers (BRCs) as stabilizers in emulsion polymerization. , These materials are highly hydrophobic, with less than 30 mol % acrylic acid overall, unlike previous reports of PS-PAA gradient copolymers, where it was assumed that more acrylic acid than styrene was required to enable dispersion of copolymers. It was shown that PS- b -(PS- r -PAA) could successfully stabilize styrene emulsion polymerizations when at least a 3 wt % stabilizer relative to a monomer was used.…”