Plasticization by Water Clusters

Numerous studies have shown that water acts as a plasticizer when it is absorbed into amorphous sohds, resulting in reductions in [42a], decreases in NMR relaxation times [99b, 100], and increases in translational diffusion of water [99b] and other solutes [100b]. Plasticization of amorphous solids by water has also been a subject of several MD simulations where increasing water content has been associated with increases in polymer mobility and increases in diffusivities of water or other low-molecular-weight solutes [56, 61, 96,101], 

In PVP containing 10% w/w water, as shown previously in MD simulations (Fig. 13.5), water molecules tend to self-associate to form clusters or chains [24a]. This tendency for water to self-associate as its concentration inaeases has been reported in several other polymer glasses, including HPMCAS, polyimide [102], polyethylene [103], as well as in sugars above their T [96] and in monoglyceride/ triglyceride lipid vehicles [2b]. 

Busselez et al. [56] used neutron scattering with H/D substitution labeling to examine the structure and dynamics of glassy PVP and water in hydrated PVP systems, from which they identified two types of water motion. Consistent with the earlier simulations, they obtained structural evidence for the existence of water clusters, nanosegregation of PVP side-groups, and swelling and disorder within ring nanodomains in the presence of water. The Q-dependence observed for the relaxation time of water molecules indicated that water relaxation was a diffusive-like process in water-rich domains while between water clusters subdiffusive motions prevailed. 

FIGURE 13.14 Three snapshots of water clusters at different times in a simulation of a water saturated 60% tricaprylin/40% monocaprylin lipid mixture. A few water molecules are color coded to allow tracking of their facile movement within water clusters. A single molecule of benzamide remains nearly stationary throughout this time frame, (see insert for color representation of the figure.) 

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