Temperature Dependence of Viscoelasticity of Polycaprolactone Critical Gels

Abstract

A systematic procedure was developed for determining the temperature dependence of critical gel properties. The polymer system was a poly(ccapro1actone) diol (PCL) end-linked with a three-functional isocyanate. Samples varied in prepolymer molecular weight and stoichiometric ratio. Time-temperature superposition was found to apply to the self-similar relaxation modulus G(t) = S P . A prerequisite of timetemperature superposition is that the relaxation exponent n is independent of temperature. As a consequence, criticalgels have theunique property that their loss tangent is independent of temperature. This was confirmed experimentally. The gel stiffness S(T) = S(To)a?/bTshifta with an Arrhenius type temperature dependence. The temperature shift factors for the PCL critical gels seem to be identical with those for the viscosity of the difunctional prepolymer ( t o ) and the modulus of the fully cross-linked polymer (G& This suggests that an experimental relationship of S = G,(T)[qo(T)/G,(T)ln is valid for our polymers at the experimental temperatures between 90 and 130 "C. IntroductionTemperature affects chemical gelation experiments in two major ways. High temperature accelerates the crosslinking reaction and thus speeds up the gelation process. It also accelerates relaxation processes so that the characteristic time scales of rheology become reduced. For analyzing the temperature dependence of gelation, we will use the rheology at the gel point as reference.Cross-linking polymers undergo a transition from liquid (sol) to solid (gel) when the extent of cross-linking @) reaches a critical value at the gel point (GP). The leading molecular cluster and the corresponding longest relaxation time diverge to infinity for the ideal material exactly at

Gelation and Cross-Linking in Multifunctional Thiol and Multifunctional Acrylate Systems Involving an in Situ Comonomer Catalyst

“…Since the relaxation exponent is also independent of the amine concentration, we conclude that the cross-linking mechanism remains unaffected by changes in the concentration of in situ catalyst. 43 This provides the ability to tune the gel time of the system by modulating the amine concentration without affecting the formation mechanism or spatial size of the critical gel.…”

Physico-chemical control of sol–gel transition of titanium alkoxide-based materials studied by rheology

“…It corresponds to the crossover to the glass transition or to the entanglement region, respectively for low and high molecular weight precursor materials [18]. The values of the relaxation exponent are restricted between 0 and 1 in order to ensure the zero value of the equilibrium shear modulus and the divergence of the steady shear viscosity at the gelation time.…”

“…Small amplitude oscillatory measurements have been successfully applied for either chemical [5,6,[16][17][18]25] or physical gels [26][27][28][29] and aggregating colloids [30].…”

Viscoelasticity of radiation-formed PVA/PVP hydrogel

“…Izuka et al [32,33] have shown that by using the relationship originally proposed by Scanlan and Winter [31], the gel strength S may be predicted simply from the value of the modulus of the fully cross-linked network G e and the zero shear viscosity of the precursor [´0 .pc/ ], i.e.…”