Dynamics in Bridge-Rich Thermoresponsive Micellar Networks

Block copolymers containing thermoresponsive poly-N-isopropyl acrylamide (PNIPAM) and a persistently hydrophobic block, like polystyrene (PS), form micellar gels in water when triblock (PS-PNIPAM-PS) are added as they interconnect micelles. Neutron scattering methods utilizing specific deuteration strategies allow one to address the structure and dynamics with a focus on PNIPAM bridging blocks. Triblocks increase the mechanical stability depending on the number of bridges formed. Small-angle neutron scattering (SANS) allows us to observe the arrangement of triblocks around the micellar core, and confirms that a large number of blocks is in bridging configuration. The dynamics of the bridges is Zimm-like, with significant changes due to the fixed ends, but presents internal friction like free PNIPAM. A characteristic modulation in the dynamics related to the geometrical arrangement is found. Side chains are mobile but strongly constrained by a much slower backbone. The theoretical approach derived herein can serve as a framework for describing other systems in which polymeric junctions play a significant role.