Gelation Studies, 4

Abstract

Summary: In a recent paper [S. Richter et al., Macromol. Rapid Commun. 2004, 25, 1504] the sol‐gel transition of two thermoreversible gelling mixtures made of xanthan gum and locust bean gum has been studied by using time‐resolved dynamic light scattering (DLS) and low‐amplitude oscillatory shear rheology, and is indicated by the presence of the typical power‐law behaviors in the time‐intensity correlation function g2(t) − 1 ∝ t−µ and in the oscillatory shear experiment G′(ω) ∝ G″(ω) ∝ ωn. A significant and reproducible difference of 7 K for the estimated gelation temperatures was found again, with the higher one always determined by rheology. We now present a logical explanation together with new results obtained on this system that may also be of importance for other ones. The incipient infinite percolated cluster that is formed during the gelation process is more pronounced, and is detectable earlier by rheology than by DLS for this present system.Double‐logarithmic plot of the time‐intensity correlation functions g2(t) − 1 versus the delay time t and at a scattering angle of θ = 90° and at several temperatures of the mixture.imageDouble‐logarithmic plot of the time‐intensity correlation functions g2(t) − 1 versus the delay time t and at a scattering angle of θ = 90° and at several temperatures of the mixture.

Direct Observation of Early-Time Hydrogelation in β-Hairpin Peptide Self-Assembly

“…On the other hand, when peptide folding and self-assembly were triggered by increasing the solution ionic strength using the high salt buffer (κ -1 ≈ 0.5 nm), the count rate increased sigmoidally. An increase in scattered intensity fluctuations formed after t ≈ 26 min (Figure 10 inset), indicating the possible onset of intercluster interactions 29,39,40…”

Recent Gelation Studies on Irreversible and Reversible Systems with Dynamic Light Scattering and Rheology ‐ A Concise Summary

“…Sometimes it can be difficult to decide which of the TCFs during a temperature scan has the most linear behavior in the observed delay time window. As it was shown, [29,42] the effect in rheology is more pronounced, such that 1-2 K below or above the gelation threshold may influence the formed gel and pre-gel structures in the frequency dependence, and the values of G 0 (v) and G 00 (v) are stronger than in the shape of the TCFs. The DLS method seems not generally able to detect the gelation threshold, and DLS measures concentration fluctuations rather than the shear response.…”

“…Similar extended DLS and rheology studies to prove the validity of Equation (8) were performed on other thermoreversible systems (three mixtures made of xanthan gum and locust bean gum [29,42] (XG/LBG), and gelatin [30] ). The advantage of studying thermoreversible gelation over radical cross-linking reactions is that the gelation process is only influenced by temperature, which can be held constant to have enough measuring time for rheological and DLS investigations.…”

“…As an overall result, the prediction of Equation (8) is not valid! It must be mentioned that the difference in the estimated gelation temperature (power-law behavior in the oscillatory shear experiment and in the TCFs in the DLS) on all mixtures made of xanthan gum and locust bean gum was 5-7 K. [29,42] The higher sol-gel transition temperature indicated by the power-law scaling was always estimated by rheology. The reasons for this unusual behavior, which was observed for the first time, are given briefly in the conclusion section; for more details see ref.…”

“…The reasons for this unusual behavior, which was observed for the first time, are given briefly in the conclusion section; for more details see ref. [42] .…”