GEL TRANSITION STUDIES ON NONIDEAL POLYMER NETWORKS USING SMALL AMPLITUDE OSCILLATORY RHEOMETRY

Hydroxypropylguar (HPG) readily forms a physical gel in the presence o f berate ions. A discussion of gel properties is given, with emphasis on the rheological properties of berate crosslinked HPG at the structu ral transition point. A gel point characterization method developed by Winter and Chambon (1986) may be used to define the exact point of tr ansition from liquid behavior to that of a true chemical network with permanent crosslink sites. This technique has been applied to the HPG system described above whose crosslink sites (or junction zones) are a t a pH activated dynamic equilibrium. Analysis of the loss modulus at different stages of the solution transition presents a method for esta blishing the solution/gel transition of berate crosslinked HPG. A maxi mum in the loss modulus is observed to correspond very closely to the gel transition determined using the method of Winter and Chambon. The steady gel properties of the fluid are studied as a function of the co ncentration of both the polymer and the crosslinking species. The gel point relaxation exponent n varies between 0.14 and 0.5, depending on the concentration of the species which constitute the gel network. Res ults from both rheological techniques are presented as well as a discu ssion of their respective limitations. Small amplitude oscillatory mea surements are performed using a constant strain Weissenberg rheogoniom eter for all rheological measurements. (C) 1998 The Society of Rheolog y.