Control of Layer Thickness of Onionlike Multilayered Composite Polymer Particles Prepared by the Solvent Evaporation Method

Micrometer-sized, monodisperse, “onionlike” multilayered polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA, volume fraction of PS segment in the block copolymer; f PS ≈ 0.5) particles and PS/PS-b-PMMA/PMMA composite particles with different layer thicknesses (D) were successfully prepared by slow release of toluene by evaporation from polymer/toluene droplets dispersed in aqueous media formed using the membrane emulsification method. D is defined as one periodicity consisting of single PS and PMMA layers. The effects of number-average molecular weight (M n of PS-b-PMMA ≈ (7.8−29.0) × 104 g mol−1) and volume fraction of PS-b-PMMA (PS/PS-b-PMMA/PMMA = 0/10/0−4.5/1/4.5, v/v/v) on the D of multilayered particles were investigated. The D value increased with an increase in M n to the 2/3 power, in agreement with theory established in polymer blend film systems. PS (M n ≈ (1.1−48.9) × 104 g mol−1)/PS-b-PMMA (M n ≈ 29.0 × 104 g mol−1, f PS ≈ 0.5)/PMMA (M n ≈ (1.3−46.9) × 104 g mol−1) composite particles with various volume fractions were also prepared. When the molecular weights of the homopolymers were lower than those of the corresponding polymer segments in the block copolymer, multilayered structures were observed even at a low volume fraction of the block copolymer. On the other hand, when they were higher, microphase and macrophase-separated structures coexisted in all volume fractions. The D of multilayered particles containing low molecular weight homopolymers was proportional to the −1/3 power of the volume fraction of the block copolymer consistent with the theory of the polymer blend film systems, indicating that the D is controllable by proper selection of the experimental conditions.