ESR studies of polymer transitions. III. Effect of molecular weight and molecular weight distribution on Tgvalues of polystyrene as determined by ESR spin-probe studies

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Abstract
The ESR spin-probe technique with the use of the nitroxide probe 2, 2, 6, 6-tetramethyl-4-hydroxypiperidin-1-oxyl benzoate (BzONO), has been used to study Tg variation in a series of polystyrenes of varying molecular weight and molecular weight distribution. Glass temperatures, as determined by ESR, for “monodisperse” polystyrenes vary smoothly as a function of molecular weight. From results on polystyrene binary blends it is shown that the technique is responsive to M n rather than M w. Data obtained by ESR are shown to agree with conventional DSC measurements. Some advantages, limitations, and implications of the technique are also presented and discussed. The actual shapes of extrema separation vs. temperature plots are functions of molecular weight (becoming broader as molecular weight increases) and are discussed in terms of free volume effects.