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

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 (MEO₂MA; TT₁ = 25 °C) and poly(ethylene glycol) methyl ether methacrylate (OEGMA₃₀₀; TT₂ = 60 °C) with a molar ratio of 1:1. Aqueous solutions of PMEO₂MA-b-POEGMA₃₀₀ show a three-stage transition upon heating as seen with optical transmittance and small-angle X-ray scattering: dissolution (T < TT₁), self-assembled micelles with core-shell structure (TT₁ < T < TT₂), and aggregation of collapsed micelles (T > TT₂). Due to the restrictions in the polymer chain arrangement introduced by the solid Si substrate, spin-coated PMEO₂MA-b-POEGMA₃₀₀ 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 PMEO₂MA-b-POEGMA₃₀₀ films. After exposure to D₂O vapor for 3 h and then increasing the temperature above its TT₁ and TT₂, the ordered structure is still not observed. Only a D₂O enrichment layer is formed close to the hydrophilic Si substrate. Such PMEO₂MA-b-POEGMA₃₀₀ films show a linear shrinkage between TT₁ and TT₂ in a D₂O vapor atmosphere. This special behavior can be attributed to the synergistic effect between the restrained collapse of the PMEO₂MA blocks by the still swollen POEGMA₃₀₀ blocks and the impedance of chain arrangement by the Si substrate. Based on this unique behavior, spin-coated PMEO₂MA-b-POEGMA₃₀₀ films are further prepared into a temperature sensor by implementing Ag electrodes. Its resistance decreases linearly with temperature between TT₁ and TT₂.