Temperature-Induced Aggregation Kinetics in Aqueous Solutions of a Temperature-Sensitive Amphiphilic Block Copolymer

Time effects for the temperature-induced association complexes in solutions of the thermoresponsive poly(N-isopropylacrylamide)-block-poly(ethylene glycol)-block-poly(N-isopropylacrylamide) (PNIPAAM(69)-b-PEG(23)-b-PNIPAAM(69)) copolymer that exhibit a lower critical solution temperature were studied by means of turbidimetry and dynamic light scattering (DLS). The DLS results clearly show that at temperatures below the cloud point (CP) unimers coexist with intermicellar structures, which contract as the CP is approached. At this stage, no time effect was detected. At temperatures above the CP, large association structures are formed, and these aggregates dominate the decay of the correlation functions. A novel time-dependent growth of the aggregates was observed over several hours. The growth of the clusters is strengthened as the temperature rises, and this feature is supported by the turbidity results and the reduced scattered intensity experiments. For a low polymer concentration, an initial growth of the clusters is observed, whereas at longer times the apparent hydrodynamic radius from DLS is virtually constant. The results from this work stress the importance to check possible time effects in solutions of thermosensitive copolymers as the cloud point is approached.