Thermoresponsive Diblock Copolymer Films with a Linear Shrinkage Behavior and Its Potential Application in Temperature Sensors.

The linear shrinkage behavior in thermoresponsive diblock copolymer films and its potential application in temperature sensors are investigated. The copolymer is composed of two thermoresponsive blocks with different transition temperatures (TTs): di(ethylene glycol) methyl ether methacrylate (MEOMA; TT = 25 °C) and poly(ethylene glycol) methyl ether methacrylate (OEGMA; TT = 60 °C) with a molar ratio of 1:1. Aqueous solutions of PMEOMA--POEGMA show a three-stage transition upon heating as seen with optical transmittance and small-angle X-ray scattering: dissolution (T < TT), self-assembled micelles with core-shell structure (TT < T < TT), and aggregation of collapsed micelles (T > TT). Due to the restrictions in the polymer chain arrangement introduced by the solid Si substrate, spin-coated PMEOMA--POEGMA films exhibit an entirely different internal structure and transition behavior. Neutron reflectivity shows the absence of an ordered structure normal to the Si substrate in as-prepared PMEOMA--POEGMA films. After exposure to DO vapor for 3 h and then increasing the temperature above its TT and TT, the ordered structure is still not observed. Only a DO enrichment layer is formed close to the hydrophilic Si substrate. Such PMEOMA--POEGMA films show a linear shrinkage between TT and TT in a DO vapor atmosphere. This special behavior can be attributed to the synergistic effect between the restrained collapse of the PMEOMA blocks by the still swollen POEGMA blocks and the impedance of chain arrangement by the Si substrate. Based on this unique behavior, spin-coated PMEOMA--POEGMA films are further prepared into a temperature sensor by implementing Ag electrodes. Its resistance decreases linearly with temperature between TT and TT.