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The thermal-induced gelation process in a polyacrylonitrile (PAN) solution containing different amounts of water was investigated through both dynamic and steady-state rheologic measurements. During the cooling process, the storage modulus G 0 was found to intersect with the loss modulus G 00 at a temperature which can be considered as the incipient gel point Tig. This crossover point increased with the water content in the PAN solution. On the basis of the power law G 0 ~ G~ v n and the relation tan d ¼ GðvÞ=G 0 ðv Þ¼ tanðnp=2 Þ¼ const, the loss tangent tan d was traced with decreased temperature at various oscillation frequencies, and a crossover point was observed at the temperature a little higher than the Tig above for all PAN solutions. The water content might affect the value of the relaxation exponent n via two different mechanisms, that is, hydration decoupling of the pendant nitrile groups in PAN at low water content (2wt% water) and molecule aggregation at high water content (4 and 6wt%). Nevertheless, the slight difference in the n values reflects the unique structure of the critical gels irrespective of the water level. The apparent viscosity ha of either the PAN solution or the PAN gel was found to rise with increased water content and both showed pronounced shear-thinning behavior. Compared with temperature, the water effect on the apparent viscosity ha is more significant due to the water-induced aggregation of the molecular chains. Copyright 2010 John Wiley & Sons, Ltd.

Prediction of gel point conversion and network formation is of great importance in polycondensation during synthesis as well as processing. It enables one to estimate the safe conversions for reactor operation without gelation and the cycle time during processing, and plays an important role in controlling the molding parameters used for reinforced reaction injection molding (RRIM), reaction injection molding (RIM) and compression molding. Theories of gelation have been extensively published in the literature and supported by experimental data for various polycondensation systems. However, most such studies relate to unfilled systems. In this work, metal-filled polyurethanes have been synthesized in bulk by reacting toluene di-isocyanate with castor oil and its polyols possessing different hydroxyl values. Metallic aluminum powder (10–40% by weight) was dispersed thoroughly in castor oil and its polyols before reacting at different temperatures (30–60 °C) in a moisture-free, inert environment. The gel point conversions were measured experimentally and an empirical model from the experimental data has been developed to predict the gelation behavior. The proposed model could be of immense importance in the paints, adhesives and lacquers industries, which use castor oil in bulk. From these experiments it was concluded that: (i) fine metal powder gives a rise in viscosity; (ii) metal fillers not only restrict the molecular motion due to the increase in viscosity, but also lower the conversion; (iii) the vegetable oil and its polyols have a number of bulky groups, which also impart the delay tendency in gel time; (iv) there is a change in gelation dynamics at 50 °C – this is due to the change in reactivity of di-isocyanates; (v) the presence of metal filler does not initiate the intermolecular condensation; (vi) there is a gap between theoretical and experimental gel point owing to the unequal reactivity of the secondary alcohol position; (vii) there is an inverse relationship of gel time with the reaction temperature and hydroxyl value of polyols. An empirical model based on process parameters, i.e., hydroxyl value, temperature, shape factor and filler concentration, has been derived and found to be adequate for the metal-filled system. The correlation coefficient on the data is on the lower side in some cases because the following were not taken into account: (i) the first-order kinetics followed by the reaction in the second half while it is tending towards gelation; (ii) the error in observing the gel point viscosity; (iii) errors in assuming the spherical shape of aluminum metal powder; (iv) errors due to failure to maintain the constant speed in agitation. Copyright © 2003 John Wiley & Sons, Ltd.