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Glass Transition Temperature, Tg 4 6 2 At T , ~ 10 -10 Ns/m g Below T , atomic rearrange- g ments are frozen in. Rigid fluid “Moon rocks” were produced millions of years ago T -melting point M 2 3.091 © H.L. Tuller-2003 Crystalline Versus Amorphous Solids Solids with simple structures and non-directional bonds, e.g. metals and alkali halides, have very low viscosity fluids above the melting point and easily crystallize upon cooling. Solids with complex structures and strong, highly directional bonds, e.g. silicates, polymers, have high viscosity fluids and tend to form amorphous or glassy solids 3 3.091 © H.L. Tuller-2003 A Crystalline Silicate Si O 4 3.091 © H.L. Tuller-2003 Crystalline Versus Amorphous Silicates Silicate melts tend to be highly viscous Variable bond angle & length Ordered SiO4 tetrahedra Disordered SiO4 tetrahedra 5 3.091 © H.L. Tuller-2003 Viscosity Measure of resistance to flow: elongation or strain, = ΔL/L = d/dt Liquid flow requires breaking and reformation of bonds 6 3.091 © H.L. Tuller-2003
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