Effect of pressure on the micellar structure and aggregation behavior of PMMA-<i>b</i>-PNIPAM diblock copolymers in a water/methanol mixture.

Abstract

The pressure-induced changes of the micellar structures and aggregation behavior of a thermoresponsive diblock copolymer, consisting of a short poly(methyl methacrylate) (PMMA) and a long poly(<i>N</i>-isopropylacrylamide) (PNIPAM) block, in a 90 : 10 v/v water/methanol mixture, are characterized in the temperature-pressure frame. The phase diagram of the polymer solution is established by turbidimetry. The maximum of the coexistence line is found at 33.7 °C and 83.3 MPa. Synchrotron small-angle X-ray scattering is used to determine the micellar structure and correlation over a temperature and pressure range of 28 to 36 °C and 10 to 250 MPa, respectively. In the one-phase region, the core size steadily decreases with increasing pressure, while the micellar shell slightly shrinks after featuring an initial swelling up to <i>ca.</i> 75 MPa. The micellar swelling is attributed to the higher degree of hydration of the PNIPAM blocks due to the weakening of the preferential binding of methanol with PNIPAM. In the two-phase region, two pressure regimes are found: At pressures up to <i>ca.</i> 75 MPa (low-pressure regime), the core size and shell thickness increase while the correlation between micelles diminishes with increasing pressure. Conversely, at pressures between 75 and 250 MPa (high pressure regime), these parameters exhibit the opposite behavior. This behavior in the high-pressure regime of the two-phase region occurs regardless of whether the pressure is increased across the coexistence line or occurs entirely within the two-phase region.