Phase Diagrams of Polynorbornene Amphiphilic Block Copolymers in Solution

Abstract

We report phase diagrams for amphiphilic block copolymers prepared via ring-opening metathesis polymerization (ROMP). A library of 30 block copolymers with variable hydrophilic functionality, block ratios, and degrees of polymerization was prepared, and the resulting assemblies were analyzed by small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). A phase diagram of the self-assemblies was constructed for each of the various copolymer systems screened, representing the first of its kind for polynorbornene block copolymers in dilute solutions. Furthermore, we take advantage of kinetic control in the preparation of an array of particle morphologies accessed from the same polymer structure. ■ INTRODUCTIONIn the design of nanomaterials, morphology is a key consideration in determining fundamental properties. Examples relevant to soft material design include arranging DNA on micelle coronas to increase resistance to nuclease degradation, 1,2 varying circulation patterns and clearance mechanisms of different particle shapes in vivo, 3−8 and shape effects for cellular uptake of nanoparticles in vitro. 9−11 Given the strong correlation between morphology and function, much effort has been expended in the attempt to define particle structure through the design of block copolymer amphiphiles. 12−18 However, it is known that the conditions under which a given micelle is formulated will greatly influence the outcome despite best efforts to dictate results through chemical structure. 15,16,19−25 Over the past few decades, experimental and theoretical studies have explored properties that contribute to selfassembly events of block copolymers (BCPs) in solution. 16,26−30 It has been established that the nanoparticle morphology is a result of a reduction in free energy achieved via three main parameters: (i) stretching of the hydrophobic core block, (ii) interfacial tension between the core and solvent, and (iii) repulsion between corona strands. 12,15,16,20,26,30,31 Each of these components involves contributions from myriad variables in the polymer structure and environment. To complicate the matter further, a delicate balance and interplay between thermodynamically and kinetically driven solution phase particle formation processes has a large influence over the final morphology of the nanostructure. 13,16,18−20,22−25,27−35 For many kinetically trapped systems, unimer exchange may be so unfavorable that the assemblies generated are stable on a time scale of months or years without outside perturbation, and no change in morphology is observed. 36,37 In contrast to BCPs in the bulk, where the influence of kinetics on morphology is less important, constructing phase diagrams for self-assemblies in solution is particularly challenging given that it is impossible to change one component of the polymer or solution without also perturbing the kinetic landscape of assembly. For example, small changes in core block size will not only alter polymer molecular weight or the hydrophilic−hydrophobi...

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