Dynamic phase transition and self-assembly mechanism of thermosensitive poly(ethylene oxide)-b-poly(N-vinylcaprolactam) (PEO-b-PVCL) copolymer are explored deeply. A gradual dehydration process with predominated hydrophobic interactions among copolymer chains in the phase transition process distinguishes the copolymer from homopolymer. PVCL in the inner zone is restricted and counter-balanced by the PEO segments based on the sequence order of representative groups during the heating-cooling cycles. Remarkably, PEO shell experiences unusual hydration process, which is first discovered. This hydrophilic shell plays as water absorption sponge layer and captures expelled water from PVCL core, accompanied by gradient distribution of water existed in the assembly structures. Peculiarly, pseudo-linear changes of the integral area of free CO are presented compared with inflection point in the hydrated CO integral area, which propose that a part of hydrated CO forms incomplete dehydrated states. During the cooling process, perfect reversibility is observed without obvious hysteresis. (c) 2015 Wiley Periodicals, Inc.