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

Abstract

The aqueous solution and aggregation behavior of polystyrene-b-poly­[styrene-r-(acrylic acid)] block-random copolymers have been examined. Previous work conducted by our group showed that these materials exhibit an unanticipated ease of dispersion in water compared to polystyrene-b-poly­(acrylic acid) block copolymers of similar molecular weight and composition. Herein, fluorescence labeling experiments and the presence of a critical aggregation concentration suggest the self-assembly of these materials into multichain aggregates as opposed to the self-folding behavior that was previously hypothesized. In our analysis, we demonstrate that caution must be used when interpreting particle size data from dynamic light scattering with polyelectrolyte solutions and that other characterization methods should be used to confirm findings. The fundamental understanding of block-random copolymer solution properties enables the widespread application of these easily dispersible materials in fields where amphiphilic copolymers are of interest, including biomedicine, catalysis, and stabilizers in emulsion polymerization.

Block–random copolymer stabilisers for semi-batch emulsion polymerisation

“…24 The block–random copolymers exhibit unusual behaviour when dispersed in alkaline media, where it was suspected they self-assemble into low aggregation number aggregates. 25 A preliminary study showed that a 200 nm diameter latex of polystyrene with a proportion of 5–10 w/v% based on monomer volume (BoM) could be efficiently polymerised. While classic surfactants and block copolymers exhibit a particle number dependence on the stabiliser concentration ( N p ∝ [S] α ) closer to square root or linear ( α = 0.6–1.0), polymerisations stabilised using BRCs exhibit a cubic relationship ( α = 3), specific and unique to this new class of copolymer when used in emulsion polymerisation.…”

Amphiphilic Block Random Copolymers: Influence of pH and Ionic Strength on Aqueous Solution Properties

“…In previous publications, we believed the BRCs formed either single-chain nanoparticles or low N agg aggregates, which contributed to the unique nucleation mechanism in emulsion polymerizations of styrene stabilized by BRCs. , It is now established that self-assembly of BRC chains into multichain aggregates occurs rather than a self-folding process at pH > 9.5 and Φ ∼ 100 mM . Moreover, Förster resonance energy transfer (FRET) experiments showed that the unimer exchange between aggregates over 6 h was not detectable at room temperature and became significant at elevated temperatures (90 °C).…”

“…26 Recently, our group has been studying PS-b-P(S-r-AA) block−random copolymers (BRC) as stabilizers in emulsion polymerization 27−29 and in solution. 30 Typical compositions of the BRC are approximately 60−85 mol % styrene and 15−40 mol % acrylic acid. Block copolymers of PS-b-PAA with >50 mol % acrylic acid are often applied in emulsion polymerizations to ensure ease of copolymer dispersion in aqueous conditions.…”

“…27,28 It is now established that selfassembly of BRC chains into multichain aggregates occurs rather than a self-folding process at pH > 9.5 and Φ ∼ 100 mM. 30 Moreover, Forster resonance energy transfer (FRET) experiments showed that the unimer exchange between aggregates over 6 h was not detectable at room temperature and became significant at elevated temperatures (90 °C). This section investigates the self-assembly of BR1−BR4 in comparison to other amphiphilic copolymer types, with an emphasis on the aggregate structure.…”

“…Previously, it was shown through FRET that the BRC unimer exchange between aggregates at room temperature within 6 h is not observed. 30 From the shape of the light scattering curves in Figure S9, it appears that dilution from a 50 g/L stock (initially at pH = 9.5, Φ ∼ 100 mM) causes disintegration and/or coalescence of aggregates, depending on their concentration relative to the CAC. If the final BRC concentration is less than the CAC at the given conditions, aggregates disintegrate into dissolved chains and the light scattering intensity is low.…”