Polymeric nanoparticles are used in many fields, e.g. for drug delivery. Poly(N-isopropylacrylamide) in aqueous solution forms nanoparticles (“mesoglobules”) above its cloud point. The coexistence line of this system in the temperature-pressure frame is an ellipse with a maximum at ~60 MPa and 35 °C. We investigate the formation and growth of mesoglobules as well as their dissolution after rapid pressure jumps across the coexistence line, both at low (below 20 MPa) and high pressures (above 101 MPa). Time-resolved small-angle neutron scattering at instrument D11 (ILL Grenoble) gives structural information on a large range of length scales and in a time range from 50 ms to ~1650 s after the jump. In the low-pressure regime, we find that, initially, growth proceeds via diffusion-limited coalescence, but this process is later slowed down by the appearance of a dense and rigid shell from dehydrated polymers. The further away the target pressure from the coexistence line, the earlier the slowing-down sets in and hinders further growth. In contrast, in the high-pressure regime, the chains stay hydrated and mobile, and the diffusion-limited coalescence proceeds without hindrance during the entire measuring time. The dissolution of mesoglobules is studied by pressure jumps from the two-phase into the one-phase region. Varying the target pressure allows altering the osmotic pressure of the solvent. At a target pressure close to the coexistence line, the release of single polymers from the surface of the mesoglobules is the dominating mechanism, whereas for target pressures deeper in the one-phase regime, the swelling of the mesoglobules by water prevails. The dissolution time decreases with increasing jump depth. The results point to the importance of the osmotic pressure of water.

Kinetics of mesoglobules formation and disintegration in solutions of thermo-responsive polymers after fast pressure jumps

ACS Spring Meeting 2021

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