The refractive index, n of a polymer was determined as a function of temperature.

Question

The results are given in the table. Determine the glass transition temperature of the polymer. T/°C 20 30 40 50 60 70 80 90 100 110 120 130 n 1.5913 1.5898 1.5883 1.5868 1.5853 1.5838 1.5822 1.5801 1.5766 1.5725 1.5684 1.5643

Answer 1

Build a plot in excel, or better yet, by hand on a graph paper. You'll see that the linear trend breaks at about 90°C. By plotting it carefully and extrapolating the both linear fragments towards each other, you can find a more precise glass transition temperature.