Hydrodynamic-flow-driven phase evolution in a polymer blend film modified by diblock copolymers

Abstract

We have studied surface-directed phase separation in thin films of deuterated polystyrene and poly(bromostyrene) (with 22.7% of monomers brominated) using 3 He nuclear reaction analysis, dynamic secondary ion mass spectroscopy and atomic force microscopy combined with preferential dissolution. The crossover from competing to neutral surfaces of the critical blend film (cast onto Au) was commenced: polyisoprene-polystyrene diblock copolymers were added and segregated to both surfaces reducing in a tuneable manner the effective interactions. Two main stages of phase evolution are characterised by i) the growth of two surface layers and by ii) the transition from the four-layer to the final bilayer morphology. For increasing copolymer content the kinetics of the first stage is hardly affected but the amplitude of composition oscillations is reduced indicating more fragmented inner layers. As a result, a faster mass flow to the surfaces and an earlier completion of the second stage were observed. The hydrodynamic flow mechanism, driving both stages, is evidenced by nearly linear growth of the surface layer and by mass flow channels extending from the surface layer into the bulk. The final bilayer structure, formed even for the surfaces covered by strongly overlapped copolymers, is indicative of long-range (antisymmetric) surface forces.PACS. 64.75.+g Solubility, segregation, and mixing; phase separation -68.55.-a Thin film structure and morphology

Surface-directed phase separation in nanometer polymer films: self-stratification and pattern replication

“…by changing the chemical nature of the substrate [9,11] Approaches using mixed SAMs [67] or end-grafted random copolymer brushes [68], both with variable composition, were used to tune the effective interactions between polymers and the substrate. A controlled variation of both effective surface fields was achieved by adding surface-active copolymers to the mixtures [10,12,69]. The co-polymers segregate predominantly to both surfaces of the blend film and reduce the fraction of the surface area exerting ordering interactions.…”

“…The co-polymers segregate predominantly to both surfaces of the blend film and reduce the fraction of the surface area exerting ordering interactions. Here we will discuss briefly this versatile approach to control the self-stratification process [10,12].…”

“…In our studies [10,12] polyisoprene-polystyrene PI-PS (PI-hPS and PI-dPS) diblock copolymers were used, added to two different critical mixtures: hPS 1 /dPS 1 and dPS 2 /PBrS 1 (hPS 2 /PBrS 1 ). Isotopic polymer counterparts were used to provide the contrast necessary for the depth profiling techniques.…”

“…1a). Self-stratification of thin films with initial compositional homogeneity, leading to a multilayer ordering, is the first [4,[10][11][12][13] of two main problems considered in the present paper. Phase separation in polymer mixtures is also altered by the presence of a pre-structured substrate with lateral pattern of surface energy [7,[14][15][16][17][18][19][20][21] (Fig.…”

“…the alignment of phase domains with respect to the patterned substrate (accompanied by free surface undulations), is the second main issue [19] discussed here. Selfassembly of polymer blends, cast as thin films on both homogeneous as well as patterned substrates, is a consequence of surface-directed phase separation [1,3,4,[9][10][11][12][13][23][24][25][26][27][28][29][30][31][32], discovered [23,27] only 10 years ago and described shortly below. Molecular mobility in the phase-separating films is promoted by the temperature elevated above glass transition (temperature quench applied for partly miscible or immiscible mixtures) or a common solvent added to the polymer blend (solvent quench encountered commonly for immiscible blends).…”