Structure and Dynamics of Block Copolymer Micelles

Abstract

Block copolymers (BCPs) containing two or more distinct chains linked end-to-end will self-assemble into various nanostructures when dissolved in a selective solvent, including spherical and cylindrical micelles, and bilayer vesicles. The equilibrium structure and thermodynamic and dynamic properties are essential factors for processing BCP micelles in various applications. This work is motivated by a desire to investigate structure and chain exchange kinetics of BCP micelles. In this work, nanostructured micelles are formed by poly(styrene)-b-poly(ethylene-alt-propylene) (SEP) diblock, poly(ethylene-alt-propylene)-b-poly(styrene)-b-poly(ethylene-alt-propylene) (EPSEP’), and poly(styrene)-b-poly(ethylene-alt-propylene)-b-poly(styrene) (SEPS’) triblock copolymers in either squalane or binary mixtures of squalane and 1-phenyldodecane, where PEP and PEP’, and PS and PS’, refer to different chain lengths. The solvents are selective to the PEP block, leading to aggregation of PS blocks into the core, and swelling of PEP blocks as the corona. Micelle structures and thermodynamic properties of micelle solutions were characterized by static and dynamic light scattering (SLS and DLS), small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), while the kinetics of chain exchange was investigated by time-resolved small-angle neutron scattering (TR-SANS). These experiments and analyses quantitatively address the effects of corona block length, solvent selectivity, and corona block asymmetry on structure and chain exchange rates in BCP micelles. First, smaller core radii and aggregation numbers, but significantly thicker corona layers were observed in SEP diblock micelles with increasing corona block length. Two orders of magnitude faster kinetics was observed with increasing corona block length by four times. Second, the kinetics of chain exchange kinetics was accelerated by 10 orders of magnitude upon mixing squalane with 50 vol% 1-phenyldodecane for the same SEP block micelle. Third, the aggregation number, core radius, hydrodynamic radius, and critical micelle temperature decreased when varying the corona block asymmetry from asymmetric to symmetric EPSEP’ triblock micelles. The two asymmetric triblocks exhibited one order of magnitude faster exchange rates than the diblock, while the symmetric triblock was two orders of magnitude faster. Another symmetric triblock with two 1.6 times longer corona blocks accelerated the kinetics of chain exchange 10 times more. Finally, micelle ordering were suppressed up to 50 vol% in asymmetric EPSEP’ triblocks, while the equivalent SEP diblock micelles and symmetric EPSEP triblock micelles packed onto body-centered cubic structure at 10 – 30 vol% polymer concentration. These results offer a better understanding of the roles of corona block length, core block–solvent interaction parameter χ, and corona block length asymmetry in structure and chain exchange kinetics, which will ultimately aid in designing optimal block copolymer micelles (i.e., core and corona block length, chain architecture and solvent selectivity) for specific applications.