Dynamic Light Scattering in Strong Glass-forming Liquids

Analysis is presented of photon correlation spectroscopy (PCS) measurements in several glass-forming liquids of varying fragility including alkali-modified boron trioxide. The measured dynamic structure factor of these liquids exhibits a two step relaxation that indicates a bimodal pattern in the distribution of relaxation times corresponding to regions of the fluid that are fast and slow, respectively. In a carefully calibrated experiment, the amplitude of the slow decay in the structure factor obtained by PCS is attributed to the volume fraction of slowly relaxing material in the liquid. A correlation between this amplitude and the fragility of the glass-forming liquid is demonstrated. This correlation then suggests that the distinction between strong and fragile behavior is directly related to volume fraction of slow material in the liquid; a quantity that should exhibit systematic trends with respect to network polymerization.