Block Copolymer Strands with Internal Microphase Separation Structure via Self-Assembly at the Air−Water Interface

Abstract

Block copolymer microphase separation in the bulk is coupled to amphiphilic block copolymer self-assembly at the air-water interface to yield hierarchical Langmuir-Blodgett (LB) structures combining organization at the meso- and nanoscales. A blend of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (Mn=141K, 11.4 wt % PEO) and polystyrene-b-poly(butadiene) (PS-b-PB) (Mn=31.9K, 28.5 wt % PB) containing a PS-b-PB weight fraction of f=0.75 was deposited at the air-water interface, resulting in the spontaneous generation of aggregates with multiscale organization, including nanoscale cylinders in mesoscale strands, via evaporation of the spreading solvent. The resulting features were characterized in LB films via AFM and TEM and at the air-water interface via Langmuir compression isotherms. Blends containing lower PS-b-PB contents formed mesoscale aggregate morphologies of continents and strands (f=0.50) or mesoscale continents with holes (f=0.25), but without the internal nanoscale organization found in the f=0.75 blend. The interfacial self-assembly of pure PS-b-PB at the air-water interface (f=1) yielded taller and more irregularly shaped aggregates than blends containing PS-b-PEO, indicating the integral role of the amphiphilic copolymer in regulating the mesoscale organization of the hierarchically structured features.

Structural hierarchy in blends of amphiphilic block copolymers self-assembled at the air-water interface

“…We note that these tandem processes and the resulting aggregates are completely distinct from the cylinder-in-strand structures we have reported in ref. 29,29 since in that previous work phase separation between PS and PB blocks was constrained by chemical connectivity between the blocks, leading to nanoscale cylinders governed by thermodynamics. In contrast, in the current work, phase separation between PS and PB blocks is not constrained by chemical connectivity but instead is confined to the liquid surface within kinetically-determined surface aggregates, 18,19 leading to very different phase domains structures.…”

“…27,28 In other work, our group reported cylinder-in-strand features formed at the air-water interface via concurrent self-assembly of the amphiphilic block copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) and the hydrophobic block copolymer polystyrene-block-polybutadiene (PS-b-PB); spreading and solvent evaporation gave way to aggregation of various hydrophobic blocks to generate mesoscale strands, while microphase separation between covalently-connected PS and PB blocks within each strand resulted in an internal structure of nanoscale cylinders. 29 Investigating the self-assembly of multi-component polymer systems with different connectivities in interfacial environments introduces different constraints to repulsive and attractive interactions, providing new routes to hierarchical surface patterns for a variety of applications, including photolithography masks, display technology, surface-guided cell growth and tissue engineering. For example, a central pillar of soft nanomaterials research is the microphase separation between the covalently connected and chemically dissimilar blocks of block copolymers to generate ordered patterns of nanoscale dimensions.…”

Evaluating polymeric biomaterial–environment interfaces by Langmuir monolayer techniques

“…Almost all surface characterization methods are suitable for the description of the transferred layer, contributing to the knowledge of the monolayer assembly [24,69,70]. Several methods are used to quantify topography, thickness and morphologies, for example atomic force microscopy (AFM) [71 -73], scanning electron microscopy [74,75], transmission electron microscopy (TEM) [76,77] or ellipsometry [78]. Additionally, information on interfacial properties and orientation of molecules are provided by infrared [79], UV-vis and fluorescence spectroscopy [80], X-ray techniques [81] and Fourier transform infrared spectroscopy [82].…”

Preparation of cylindrical multi-compartment micelles by the hierarchical self-assembly of ABC triblock polymer in solution

“…1,[5][6][7][8][9][10] Large number of microsegments are formed in the cores and shells of MCMs, leading to a complex micellar morphology. 2 Generally, MCMs are prepared by the self-assembly of diblock copolymer blends, 11,12 self-assembly of triblock copolymers with 35 various topologies 2,13-32 and multiblock [33][34][35] copolymers in selective solvents, annealing of the self-assembled copolymer aggregates, changing the self-assembly behavior of the copolymers in the selected solvent by the fluorination of one block in the copolymers, self-assembly between copolymers and 40 assembly between copolymer and small molecules. Therefore, it is potentially useful in drug delivery and controllable release as well as the preparation of special nanostructured materials.…”