  Dynamical mechanical analysis (DMA)(f=0.2 - 100 Hz) is used to study the
dynamics of confined water in mesoporous Gelsil (2.6 nm and 5 nm pores) and
Vycor (10 nm) in the temperature range from T=80 K to 300 K. Confining water
into nanopores partly suppresses crystallization and allows us to perform
measurements of supercooled water below 235 K, i.e. in water's so called "no
man's land", in parts of the pores. Two distinct relaxation peaks are observed
around T1 = 145 K (P1) and T2 = 205 K (P2) for Gelsil 2.6 nm and Gelsil 5 nm at
0.2 Hz. 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 Ea=0.47 eV, in good agreement with literature values. It is suggested that
P1 corresponds to the glass transition of supercooled water far from pore
walls, whereas P2 reflects the dynamics of water molecules near the surface of
the pores. The observation of a pronounced softening of the Young's modulus
around 165 K (for Gelsil 2.6 nm at 0.2 Hz) is in agreement with a
glass-to-liquid transition in the vicinity of P1. In addition we find a
clear-cut 1=d-dependence of the calculated glass transition temperatures which
extrapolates to Tg(1/d=0)=136 K, i.e. the traditional value of water.
