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

The variations of the heat capacity DeltaC(p) = [C-pl - C-pg](T=Tg) at the glass transition, and the value of the fragility index m were determined fo r two melamine formaldehyde resins by calorimetric investigations. These va lues characterise respectively the thermodynamic aspect and the kinetic asp ect of the Angell "Strong-Fragile" concept. For resins cured with a neutral pH, the 3D network formed is made of massive molecular units connected tog ether by small length chains, and the values are DeltaC(p) = 0.13 J K-1 g(- 1) and m = 143. For acid pH curing conditions, the 3D network is made of on ly massive molecular units connected together and we obtain DeltaC(p) = 0.1 2 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. (C) 2001 Elsevier Science Ltd. All rig hts reserved.