Rheological Characterization of the Gel Point: A New Interpretation

Winter and Chambon have recently shown that the gel point can be precisely identified in a crosslinking process from rheological measurements on the reaction medium. The cross-linking reaction of poly(ethylene oxide) (PEO), carrying at both chain ends hydroxyl functions, with various pluriisocyanates was chosen to check the validity of this new method. The experimental setup included Fourier transform infrared (FTIR) spectroscopy to follow the conversion of the isocyanate functions with time and a rheometer to investigate the mechanical response of the reaction medium to oscillatory solicitations at various stages of the reaction. In stoichiometric systems at a given conversion, the storage modulus G′ and the loss modulus G″ become congruent and proportional to _ω1/2 over the entire frequency range. Simultaneously, the extent of reaction reaches the critical value p_c predicted by branching theory. This excellent agreement substantiates Winter’s method and confirms its validity. This investigation has been extended to nonstoichiometric systems. When the end-standing hydroxyl functions of PEO are in excess, the rheological behavior observed at the gel point is quite different, and it was attempted to account for the phenomena observed.