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.

Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high concentration dependence of the zero shear viscosity, with η0∼c8. Evidence for shear-induced structuring in steady shear, large amplitude oscillatory shear, and parallel superposed steady and dynamic shear is presented. Such structuring is more pronounced at lower polymer concentrations, consistent with the formation of intermolecular associations at the expense of intramolecular. In contrast to the simple linear telechelic associative polymers considered in a number of previous studies, the network dynamics of the polymer are no longer represented by a single characteristic time. This deviation from a classical Maxwellian response in oscillatory shear is interpreted as a broadening of the relaxation spectrum, arising from the coexistence of both hydrophobic associations and topological entanglements. Mechanistically, stress relaxation is better envisaged in terms of “hindered reptation” [Liebler et al. (1991)] of the chains, rather than Rouse-like behavior moderated purely by the hydrophobe disengagement rate [Annable et al. (1993)].

We report on the rheology and morphology of a hydrophobically modified alkali-swellable emulsion (HASE) polymer solubilized in alkaline media containing nonionic surfactants. The HASE polymer consists of complex alkylaryl hydrophobes composed of oligomeric nonylphenol condensates attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. The complex linear viscoelastic response of the polymer in alkaline solution suggests an unentangled network with an appreciable fraction of microgel. The concentration and hydrophile−lipophile balance (HLB) of nonionic surfactants profoundly affect the solution rheology. A surfactant of high HLB inhibits the dynamic network connectivity of the HASE polymer, as demonstrated by reductions of both the steady-shear viscosity and the dynamic storage modulus. The shear-induced structuring previously reported for this polymer is also progressively diminished as the surfactant concentration is increased. In contrast, the addition of a low-HLB surfactant promotes system structuring, as evidenced by (i) increases in the shear viscosity and the high-frequency plateau modulus and (ii) retention of the ability to undergo shear-induced structuring. We also employ cryofracture-replication transmission electron microscopy for the first time with regard to HASE associative polymers to examine the morphological characteristics of selected systems. The morphology of the HASE polymer in both latex and solubilized form appears more complex than previously anticipated, and a reasonable interpretation of these new data is provided.

The addition of low concentrations of very-high-molecular-weight polymers of the same chemistry as the bulk oil has the potential to increase the emulsification resistance of the tamponade agents while maintaining ease of injection and removal.