Ronald P. D’Amelia^1,, Evan H. Kreth¹

Glass transition temperature (Tg), termed the “melting point of amorphous materials” is the temperature at which an amorphous material changes from a hard, glassy state to a soft, rubbery one. As the number average molecular weight (Mn) of the amorphous material increases, its glass transition temperature also increases, but ultimately levels off at a maximum value labeled Tg_∞. Differential scanning calorimetry (DSC) was utilized to evaluate Tg for seventeen samples of polymethyl methacrylate (PMMA) whose Mn values ranged from three thousand to one and a half million. These values were then plotted against reciprocal Mn, producing a Flory-Fox equation of Tg = 135°C – 1.4 x 10⁵ °Cᐧgᐧmol^-1/Mn, with a correlation coefficient of 0.98. The Tg of binary mixtures of PMMA of different Mn values were also examined in this experiment. Tg values were calculated using the Fox equation: 1/Tg = w₁/Tg₁ + w₂/Tg₂ where w represents the weight percent of each PMMA sample. Correlation coefficients of 0.96 and 0.97 were achieved for the graphs plotting Tg against weight percent of the lower Mn value PMMA. Lastly, quantitative proton nuclear magnetic resonance spectroscopy (qHNMR) was utilized to determine the relative tacticity of binary mixtures of isotactic and syndiotactic PMMA. It was determined that the peak integrations for the methylene or methyl hydrogens, at their respective chemical shifts for each PMMA, could be used to determine relative tacticity. These experiments demonstrate the quantitative applications of DSC and NMR, as well as their suitability within the undergraduate chemistry laboratory.

Polymethyl Methacrylate (PMMA), Flory-Fox equation, glass transition temperature (Tg), differential scanning calorimetry (DSC), tacticity, quantitative proton nuclear magnetic resonance spectroscopy (qHNMR)