Dynamic mechanical analysis of supercooled water in nanoporous confinement

Dynamical mechanical analysis (DMA)(f = 0.2.100Hz) is used to study the dynamics of confined water in mesoporous Gelsil (2.6nm and 5nm pores) and Vycor (10 nm) in the temperature range from T = 80K to 300 K. Confining water into nanopores partly suppresses crystallization and allows us to perform measurements of supercooled water below 235K, i.e., in water's so-called no man's land', in parts of the pores. Two distinct relaxation peaks are observed in tan δ around T ₁ ≈ 145K (P ₁) and T ₂ ≈ 205K (P ₂) for Gelsil 2.6 nm and Gelsil 5 nm at 0.2Hz. Both peaks shift to higher T with increasing pore size d and change with f in a systematic way, typical of an Arrhenius behaviour of the corresponding relaxation times. For P1 we obtain an average activation energy of E _a = 0.47 eV, in good agreement with literature values, suggesting that P ₁ corresponds to the glass transition of supercooled water. The observation of a pronounced softening of the Young's modulus around 165K (for Gelsil 2.6 nm at 0.2 Hz) supports the conjecture of a glass-to-liquid transition in the vicinity of P ₁. In addition we find a clear-cut (1/d)-dependence of the calculated glass transition temperatures which extrapolates to T _g(1/d = 0) = 136 K, in agreement with the traditional value of water.