Chain Exchange in Binary Copolymer Micelles at Equilibrium: Confirmation of the Independent Chain Hypothesis.

The mechanism of chain exchange in block polymer micelles at equilibrium is investigated using time-resolved small-angle neutron scattering (TR-SANS). The binary micelles are formed from blends of two poly(styrene-b-ethylene-alt-propylene) (PS-PEP) copolymers with different PS core block lengths, only one of which is contrast-matched with the solvent, squalane, so that the monitored scattering intensity only reflects the other species. Micelles prepared with an excess of deuterated PS chains (of the visible species) and those with the equivalent protonated PS chains are blended ("postmixed") at room temperature, where the exchange of chains is suppressed. At several elevated temperatures these samples were monitored by TR-SANS, in which mixing of isotope-labeled visible species gives systematic reduction of scattering signals with time and provides a quantitative way to characterize the micelle exchange kinetics. Within experimental error, the results for each labeled chain (i.e., longer or shorter) in the binary micelles are identical to those recently reported for the same labeled chains in the corresponding single block copolymer component micelles, thus proving that chain exchange in these micelles involves independent chain motion. This reinforces the important conclusions that the single-chain exchange mechanism dominates in the studied micelle solutions and that micelle fusion or fission events are rare.

Molecular Exchange in Diblock Copolymer Micelles: Bimodal Distribution in Core-Block Molecular Weights.

The role of core block size dispersity on the rate of molecular exchange in spherical micelles formed from 1% by volume poly(styrene-b-ethylenepropylene) (PS-PEP) diblock copolymers in squalane (C30H62) was investigated using time-resolved small-angle neutron scattering (TR-SANS). Separate copolymer solutions (total polymer 1% by volume) containing either deuterium labeled (dPS) or normal (hPS) poly(styrene) core blocks were prepared and mixed at room temperature, below the core glass transition temperature. Each preparation (dPS or hPS) contained equal volume fractions of Mn = 26 and 42 kg/mol (h-equivalent) poly(styrene) blocks. Heating to temperatures between 87 and 146 °C resulted in block copolymer exchange as evidenced by a systematic reduction in the SANS intensity; C30H62 and C30D62 were blended so as to contrast match the fully exchanged cores. Following a protocol established in a previous report, the time-dependent intensity data were shifted with respect to time and temperature, leading to a master curve covering nearly 7 orders of magnitude in reduced time. These results are quantitatively accounted for by summing the weighted relaxation functions obtained from the individual components, consistent with a previously published model that accounts for the dramatic sensitivity of the molecular exchange dynamics to core block dispersity.

Shear-induced nano-macro structural transition in a polymeric bicontinuous microemulsion.

Bicontinuous microemulsions arise in a narrow concentration range for ternary blends containing two immiscible homopolymers and the corresponding diblock copolymer. Steady shear reveals four distinct regimes of response as a function of shear rate, corresponding to flow-induced transitions in fluid structure. In situ neutron scattering shows flow-induced anisotropy in the nanometer-scale microemulsion structure at moderate shear rates, while higher rates induce bulk phase separation, with micron-size morphology, which is characterized with in situ light scattering and optical microscopy.