Entropy reduction phenomenon in the non-equilibrium state of freeze-dried polymethyl methacrylate samples

Polymethyl methacrylate (PMMA) samples with different entanglements are prepared via a freeze-drying processing. Modulated differential scanning calorimetric (MDSC) and Fourier transform infrared spectroscopy (FT-IR) are combined to get insight into the evolution of glass transition behavior dependence of chain entanglements and the kinetics of the entanglements recovery process via annealing experiments. It is interesting to note that glass transition temperature (T-g) did not increase with concentration monotonically after annealing, and tended to be higher than the bulks. MDSC shows that two glass transition regions appeared on the reversing heat flow curves. There seems to be dual amorphous regions with quite different characters in the freeze-dried sample: the free amorphous region and the rigid amorphous region, which are analogous to that in semi-crystalline polymer. Meanwhile, the IR I (1240cm) (-1) /I (1270cm) (-1) ratio determined from the FT-IR curves showed the same progression as T-g, demonstrating that stronger interaction exists in the intermediate concentrated solution. It is proposed that, for intermediate concentration samples, the shielding effect due to the perfect intermolecular proximity and strong interaction between carbonyl group and backbone chain will make the system uneven, which results in entropy reduction and develops into a new quasi-metastable state, thus influencing the glassy-state relaxation behavior.