The effect of processing parameters on glass fiber birefringence development and relaxation

Birefringence of oxide glass fibers drawn from glass melts through an orifice has been detected previously [H. Stockhorst, R. BruÈckner, J. Non-Cryst. Solids 49 (1982) 471; H. Stockhorst, R. BruÈckner, J. Non-Cryst. Solids, 86 (1985) 105.] This birefringence is a measure of anisotropy in glass structure that can influence fiber performance properties. Birefringence is produced during fiber drawing as the fiber is rapidly stretched in the viscoelastic glass transition range. The birefringence is `frozen' into the glass during rapid cooling. With a simple drawing apparatus using Borosilicate glass (Corning code 7740) preforms, we produce glass fibers for a range of process conditions and measure their as-drawn birefringence. The development of birefringence in glass fibers is found to depend on the amount of deformation, the deformation rate, and temperature. Results for various process parameters show that increasing draw ratio, increasing elongation rate, and decreasing draw temperature increase birefringence. Post-process annealing is used to examine the time and temperature dependent glass fiber birefringence relaxation. Birefringence is found to completely relax in the temperature range close to the glass transition range as expected, but it is also noted that birefringence shows substantial (although incomplete) relaxation in a temperature range well below the glass transition. This low temperature relaxation indicates that the relaxation process may be due to a distribution of relaxation times. A modified stretched exponential and a pair of Jeffrey elements in parallel are used to describe this distribution and capture birefringence relaxation in a wide range of temperatures below the glass transition range. # 1999 Elsevier Science B.V. All rights reserved.