Self-assembly of H-shaped block copolymers

We have systematically studied the effect of the solvent nature on the self-assembly of H-shaped block copolymers with a poly(ethylene glycol) backbone and polystyrene branches, i.e., (PS)(2)-PEG(PS)(2). Two copolymers with different molecular weights (MW) and compositions, copolymer 1 and copolymer 2, were used, where copolymer 2 has a higher MW and higher PEG fraction than those of copolymer 1. In nominally neutral (nonselective) solvents, it is found that very subtle variation in the solvent affinity can remarkably influence the morphologies of the films spin-coated from the corresponding solutions. For example, as the solvent was changed from PS-affinitive to PEG-affinitive, the films of copolymer 1 changed from a disordered state into PEG cylinders. In contrast, the films of copolymer 2 changed from wormlike patterns into PEG spheres. Besides, the surface morphologies also showed some dependence on the solution concentration: with decreasing concentration, the PEG cylinders were changed into PEG spheres and a disordered state. In PEG-selective solvent (e.g., acetonitrile), copolymer 2 formed typical spherical micelles. In contrast, copolymer 1 formed a mixture of spheres and cylinders, which could be transformed into vesicles and lamellae upon dilution. The polymer-solvent interactions and their effects on the copolymer chain conformation are discussed.