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