Poly[N-isopropylacrylamide-co-N-(3-methoxypropyl)acrylamide]-b-poly(D,L-lactide) (P(IPAAm-co-MPAAm)-b-PLA) as a thermoresponsive block copolymer and PMPAAm-b-PLA as a nonthermoresponsive block copolymer were co-assembled into thermoresponsive polymeric micelles in water. In addition, PMPAAm-b-P(IPAAm-co-MPAAm)-b-PLA triblock copolymer was assembled to form thermoresponsive micelles with a hydrophilic layer on the outermost surface of the thermoresponsive corona. Using both micelles, we investigated the effects of introducing hydrophilic polymer segments on micellar aggregation behavior at temperatures above the lower critical solution temperature (LCST) of the thermoresponsive micelles. Despite the external hydrophilic PMPAAm layer on PMPAAm-b-P(IPAAm-co-MPAAm)-b-PLA micelles, aggregation following dehydration of the thermoresponsive segments was not significantly suppressed at temperatures above the LCST due to the instability of the core-corona state. In contrast, intermicellar aggregation was successfully controlled by blending P(IPAAm-co-MPAAm) and PMPAAm in the thermoresponsive corona region, even above the LCST. In particular, PMPAAm chains longer than the P(IPAAm-co-MPAAm) chains could regulate the hydrodynamic diameter of micellar aggregates at temperatures above the LCST. The micelles showed enhanced drug release rates in response to temperature changes above the LCST without precipitating from solution. These results indicated that a side-by-side structure of hydrophilic/thermoresponsive chains in the corona region could effectively control the micellar aggregation state after a thermal phase transition. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1695-1704