Physical Gelation of Gelatin Studied with Rheo-Optics

Abstract

A rheo-optical device outfitted with a Peltier temperature control for rapid temperature changes has been constructed that allows simultaneous measurement of the optical rotation of light and the controlled-stress rheology. Optical rotation provides a direct in situ assessment of the extent of triple helix reversion in a gelatin solution undergoing physical gelation in the rheometer. Thermal gelation of gelatin was monitored over a wide range of concentrations and temperatures. Assuming dynamic scaling theory applies, viscosity data below the gel point were used to evaluate the gel point and determine the value of the viscosity exponent. Above the gel point, creep-recovery experiments are used to measure the shear modulus and determine the dynamic scaling elastic modulus exponent. During thermal gelation, the time-dependent optical rotation shows an initial rapid growth region where new helices are formed, followed by a slower growth region involving helix lengthening. For cases where the gel point occurs before the helix reversion slows appreciably, the viscosity and modulus exponents are found to depend on gelatin concentration, but not on temperature. However, anomalous exponents are measured using the same methods at higher temperatures, where the helix reversion slows appreciably before the gel point is reached. These results suggest that extreme caution must be used in evaluating dynamic exponents from any physical gelation process. The observed concentration dependences of the dynamic scaling exponents are discussed in terms of chain overlap and entanglement. For gelatin gelation, the plethora of different, reported percolation exponents in the literature are rationalized.

A study of gelatin gelation by Fourier transform mechanical spectroscopy

“…That observation notwithstanding, it is important to consider whether our rapid gelation tests could correspond to the fast gelling regime of Guo et al [4] who argue that large values of α (∼0.7) are only associated with the slow gelling regime and are physically meaningless. But we have clearly established herein the rheometrical conditions under which rapid gelation produces low values of α which are physically meaningless.…”

“…Recent studies have identified a need to reexamine reported rheological exponents measured on physical gels [4]. That is the purpose of the work reported herein in which we test the hypothesis that, in some instances, the recording of anomalous values of rheological exponents may be due solely to the application of rheometrically inappropriate procedures.…”

“…see [1][2][3]. In a recent rheo-optical study, Guo et al [4] point out that two temperature and concentration dependent processes (helix creation and helix lengthening) must be considered when interpreting the results of experiments involving gelatin gelation. They argue that only in the 'fast' gelling regime (involving helix creation) can scaling theories be easily tested and that their findings partially explain the wide range of rheological exponents reported from applying scaling ideas to the physical gelation of gelatin, most literature data having apparently been obtained in the slow gelling regime.…”

Preferential hydration fully controls the renaturation dynamics of collagen in water-glycerol solvents

“…The nucleation and growth of these cross-links leads to the formation of a percolating thermoreversible gelatin gel network. This process is revealed by the growth with time t of the shear elastic modulus G. After a lapse time t 0 associated with percolation of the polymer network, G first exhibits a rapid increase due to further nucleation and growth of cross-links [8,9]. It then crosses over to a slow regime in which G grows logarithmically with time, without any measurable trend towards saturation [10].…”

Experimental evidence of a helical, supercritical instability in pipe flow of shear thinning fluids

“…The concentration dependence of the zero-shear specific viscosity [19] [20] yields a concentration at the onset of the entanglement regime c E = 1 wt.%, so that the three systems tested in this work are well entangled polymer solutions. Their viscoelastic properties were characterized by the measurement of both the steady flow viscosity η(γ) and the first normal stress-difference N 1 (γ) for shear ratesγ in the range 10 −1 -10 2 s −1 .…”