Effects of Casting Solvents on the Formation of Inverted Phase in Block Copolymer Thin Films

Abstract

Previously, an inverted phase (the minority blocks comprising the continuum phase) was found in solution-cast block copolymer thin films. In this study, the effect of casting solvents on the formation of inverted phase has been studied. Two block copolymers, poly(styrene-b-butadiene) (SB) (M w = 73 930 Da) and poly(styrene-b-butadiene-b-styrene) (SBS) (M w = 140 000 Da), with comparable block lengths and equal polystyrene (PS) weight fraction (∼30 wt %) were used. The copolymer thin films were cast from different solvents, toluene, benzene, cyclohexane, and binary mixtures of benzene and cyclohexane. Toluene and benzene are good solvents for both PS and PB, but have a preferential affinity for PS, while cyclohexane is a good solvent for PB but a ϑ solvent for PS (T ϑ = 34.5 °C). The differential solvent affinity for PS and PB was estimated in terms of a difference between the polymer−solvent interaction parameter, χ, for each block. Under an extremely slow solvent evaporation rate, the time-dependent phase behavior during such a solution-to-film process was examined by freeze-drying the samples at different stages, corresponding to different copolymer concentrations, φ. Our results indicate that the slight interaction difference between solvent and each block influences the effective volume fraction of each domain and drives the solution to form a transient inverted phase at the early stage of the microphase separation.

Protein Nanopatterning on Self-Organized Poly(styrene-b-isoprene) Thin Film Templates

“…More recently, solvent-annealing and the control of the solvent evaporation rate have been found to create ordered structures in a thin film of a similar diblock copolymer, polystyrene-b-polybutadiene (PS-b-PB). [33][34][35] Our approach is simpler: templates are created from the non-equilibrium in-frozen structure of the copolymer thin films after spin-casting. (46) (symmetric copolymer) ultrathin films with increasing thickness.…”

Morphological Transitions of Symmetric Polystyrene-block-Poly(1,4-butadiene) Copolymers in Thin Films upon Solvent-Annealing

“…[21][22][23]30 Thus, from the calculation of interaction parameters, it is seen that CHX used in this study is a θ-solvent for PS blocks and a good solvent for PBD blocks while HX is a selective solvent for only PBD blocks. Hence, CHX can provide a higher degree of swelling for PBD blocks than for PS blocks.…”

“…It is noted that some research groups reported that inversion of the contrast between PS and PBD, where the PS phase and PBD phase became dark and bright, respectively, could occur when the microscope was fulfilled at moderate tapping. 29,30 However, because our AFM experiments were operated at hard tapping, the glassy PS phase and rubbery PBD phase in phase mode of our AFM images appear as bright and dark regions, respectively.…”

“…To control the mobility of BCP chains as well as BCP ordering during the solvent-annealing process, it is a routine method to utilize mixtures of different solvents. 12,30 The morphological transition from the perforated lamellae to perpendicularly-oriented lamellae has been observed with increasing contents of HX in the binary mixture. Figure 3 shows AFM images in phase mode of PS-b-PBD thin films that were solvent-annealed in binary mixtures of CHX and HX in different mixing ratios.…”

“…They showed that slightly different affinities of solvents for blocks could dramatically affect their morphologies. 21,22,30 Moreover, the affinity of solvents for polymers is changeable in a given system. Therefore, it is essential to consider the relative affinity of solvents for BCPs systematically.…”