Macromolecules, Vol.52, No.22, 8651-8661, 2019
Relaxation Dynamics and Underlying Mechanism of a Thermally Reversible Gel from Symmetric Triblock Copolymer
We report a novel thermally responsive system from poly(butyl methacrylate)-b-poly(methacrylic acid)-b-poly(butyl methacrylate) (PBMA-b-PMAA-b-PBMA) triblock copolymer in dimethylformamide (DMF) solvent. This system shows a sol-gel transition by cooling below a critical temperature T-c. The network relaxation times of these gels rightly fall in the typical rheological experiment window, permitting us to investigate the relaxation dynamics and underlying mechanism, with a combination of linear rheology, time-resolved small-angle X-ray scattering (SAXS), and temperature-elevated nuclear magnetic resonance (NMR) measurements. Both time-temperature superposition (TTS) and time-concentration superposition (TCS) are well held in this system. The relaxation dynamics at temperatures below and above T-c are quite different, giving two different activation energies E-1a and E-2a, respectively. The E-1a, being polymer concentration independent, is related to micelle formation of end-blocks, while the concentration dependent E-2a is related to the chain friction of entangled polymer solution. By coupling linear rheology, SAXS, and NMR, we quantitatively estimated the fraction of bridge, loop, and dangling chains. The longest relaxation time of the gel, tau(L) strongly depends on polymer concentration, which is attributed to the increased connectivity of the micellar network. By combining the shift factors a(T) and a(C) determined from TTS and TCS, respectively, we can obtain the tau(L) at any given temperature and concentration from that at reference temperature and concentration.