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The effect of colloidal silica fillers on the cross-linking behavior of a model UV-curable polymer system (thiol−ene) is studied using in situ rheology and real-time FTIR spectroscopy. The validity of the Winter−Chambon criterion (convergence of the loss tangents at the gel point) is examined for the cross-linking of these filled polymers, some of which are strongly flocculated dispersions (physical gels) prior to chemical cross-linking. Two different types of colloidal silica particles are studied:  one with octyl chains tethered to the surface and the other with methyl surface groups. The Winter−Chambon criterion is satisfied for all samples containing the methyl-terminated silica. However, the criterion breaks down for samples containing the octyl-modified silica, with the loss tangents not converging at any single point. This suggests the absence of a self-similar critical gel at the gel point in the latter case. Neither type of silica particles alters the mechanism of the cross-linking reaction, as revealed by FTIR spectroscopy, but they do retard the cross-linking kinetics. An alternate method is suggested for determining the chemical gel point of filled systems that fail to obey the Winter−Chambon criterion. This method involves monitoring the critical strain (limit of the linear viscoelastic region) at various UV exposure times. A dramatic increase is observed in the critical strain at the gel point, indicating a transition from weak, physical bonds to strong, covalent cross-links.

The rheological properties of different generations of hyperbranched polyesters in 1-methyl-2-pyrrolidinone solvent and their blends with a poly(2-hydroxyethyl methacrylate) are examined in this study. All the hyperbranched polyester solutions exhibit Newtonian behavior, with steady shear viscosities independent of shear rate. This indicates the absence of physical entanglements in these systems. In addition, solution viscosities are found to be only slightly affected by the different generations of the hyperbranched polymer. The polyesters have very small intrinsic viscosities, and their hydrodynamic radius scales as Rh ~ M^0.39, suggesting a less packed structure than dendrimers. All generations of the hyperbranched polyesters also show comparable apparent activation energies of flow over the temperature range studied. Replacing linear polymers with hyperbranched polymers causes large reductions in the blend viscosities. This behavior can be attributed to the decrease in both the number of physical entanglements between linear polymers and the blend's intrinsic viscosity upon addition of the hyperbranched polymer.

A dynamic rheological technique, Fourier transform mechanical spectroscopy (FTMS), was used to monitor in real time the evolving rheological properties during UV cross-linking of two thiol−ene systems. These systems comprised a trifunctional thiol (trimethylolpropane tris(2-mercaptoacetate)) together with a trifunctional allyl monomer (triallyl isocyanurate) and a tetrafunctional thiol (pentaerythritol tetrakis(2-mercaptoacetate)) with the same allyl monomer. FTMS, in conjunction with specially designed quartz plates, provided an in situ method to elucidate the effects of temperature and monomer functionality on the photoinitiated polymerization of these systems. It was found that the tetrafunctional thiol system cross-linked at a faster rate than the trifunctional thiol system over the temperature range (25−50 °C) studied. Moreover, increasing the temperature increased the cross-linking rates for both systems. The Winter−Chambon criterion was applied to determine the gel point and the two parameters which characterize the material at its gel point, the gel stiffness, S, and the relaxation exponent, n. The gel stiffness was found to be greater for the trifunctional thiol system, which was consistent with the higher value of conversion calculated from the Flory−Stockmayer theory of gelation. Relaxation exponents of 0.80 and 0.81−0.82 were determined for the tri- and tetrafunctional thiol systems, respectively, indicating similar fractal structures at the gel point. These relaxation exponents were also invariant over the temperature ranges studied, suggesting that the cross-linking mechanisms remained unchanged with temperature. From the temperature dependence of the gel times, apparent activation energies of 6.6 and 14 kcal/mol were calculated for the tri- and tetrafunctional thiol systems, respectively.

Real-time FTIR spectroscopy and in situ dynamic rheology were used to characterize the UV curing kinetics of a thiol-ene system containing trimethylolpropane tris(2-mercaptoacetate) and trimethylolpropane diallyl ether. The combination of these two techniques offered a powerful approach for monitoring changes in the chemical and rheological properties of the system during UV curing. Comparable gel times were independently obtained from both FTIR spectroscopy and rheology, thereby validating the comparison of data obtained from each method. The thiol conversion determined from FTIR spectroscopy was correlated with the elastic modulus obtained from rheology. The conversion increased very rapidly during the initial stages of UV curing. However, the elastic modulus did not have an appreciable value until after 65% of the thiol functional groups have reacted, following which the elastic modulus increased at a rapid rate. From the Flory−Stockmayer theory of gelation, the critical thiol conversion at the gel point was determined to be 0.71. This indicated that the elastic modulus had an appreciable value only when the sample is close to its gel point, consistent with the step growth kinetics of the polymerization reaction. From the conversion data, the photoinitiated thiol-ene reaction was determined to be a second-order reaction. This second-order behavior was inconsistent with the termination mechanisms currently found in literature. A possible reason for such a discrepancy is discussed.