Effect of Pressure on the Micellar Structure of PMMA-b-PNIPAM Diblock Copolymers in Aqueous Solution

The phase behavior as well as the pressure- and temperature-dependence of the micellar structure of the thermoresponsive amphiphilic diblock copolymer PMMA(21)-b-PNIPAM(283) in D2O are investigated. The coexistence line of PMMA(21)-b-PNIPAM(283) in D2O in the temperature-pressure frame is shifted to lower in temperature and pressure compared to PNIPAM homopolymers in D2O. High-pressure small-angle neutron scattering (SANS) provides insights into the structure and aggregation behavior of the micelles both in the one-phase and the two-phase regions. In the one-phase region, pressure does not noticeably alter the core-shell structure of the micelles. Upon heating the solution at atmospheric pressure into the two-phase region, the micellar shell strongly dehydrates and shrinks. Conversely, the micellar shell remains hydrated when the coexistence line is crossed either by heating at 35 and 75 MPa, or by increasing the pressure at 31.8 degrees C. Moreover, along both pathways, the aggregates in the two-phase region are significantly larger than at atmospheric pressure.