Thermodynamically “strong” and kinetically “fragile” polymeric glass exemplified by melamine formaldehyde resins

The variations of the heat capacity ΔC p =[ C pl − C pg ] T= T g at the glass transition, and the value of the fragility index m were determined for two melamine formaldehyde resins by calorimetric investigations. These values characterise respectively the thermodynamic aspect and the kinetic aspect of the Angell “Strong–Fragile” concept. For resins cured with a neutral pH, the 3D network formed is made of massive molecular units connected together by small length chains, and the values are ΔC p =0.13 J K −1 g −1 and m=143. For acid pH curing conditions, the 3D network is made of only massive molecular units connected together and we obtain ΔC p =0.12 J K −1 g −1 and m=35. By comparing our results with the results of the literature concerning three-dimensional networks and inorganic polymers, we are able to conclude that the relaxation occurs in these systems mainly by movements involving triazine rings.