Recording kinetics during a reaction is a challenging effort that provides significant insights into gelation. We recently published our work based on a novel custom-made rheometric setup for in situ crosslinking reaction [Besiri et al., Carbohydr. Polym., 2020, 246, 116615]. It facilitates the instant injection of CaCl2 solution into alginate via micro-holes of the lower plate configuration to initiate the process. Considering that the time evolution of the viscoelastic parameters is related to the developed structure, we can obtain the reaction kinetics. This study aims to improve the setup by increasing the number of micro-holes from two to four, investigating the mass ratio effects, and considering the proposed design as a batch reactor. As the volume and concentration of the reactants can be controlled during the initiation of the process, we investigate the molarity effect on the gelation. The long-term behavior of rheological oscillatory shear experiments indicates that the reaction is based on the mass of cations. The stoichiometry of reactants affects the diffusion of ions to alginate since, at high concentration and low volume of CaCl2, the mechanical properties are increased compared to lower concentration and higher volume of the cationic solution. Systematic time sweep experiments prove that at low angular frequencies, ω, the driving force of the reaction is the distribution of ions to the polymer. For higher values of ω, the force acting on the oscillating geometry of the rheometer is possibly the factor causing an enhanced mixing of the reactants, with a corresponding increase in moduli.

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A thorough assessment of gelation progress requires real-time recording of biopolymer kinetics. In this study, an in situ investigation with a customized parallel-plate rheometric setup examines how the initial distribution of CaCl 2 to alginate through micro-holes affects the process. At constant volume and concentration of reactants, stiffer gels are produced at the same probing time by increasing the number of suppliers (2-12). Due to the oscillation and the different local distribution of the cations into the biopolymer solution, the development of gel front is affected, which is for the first time in situ rheologically investigated by the temporal evaluation of complex modulus, G*. The alternative configurations of the same number of micro-holes have drastically contribution to this regard, too. Moreover, a two-kernel equation is introduced predicting the alginate-Ca 2+ gelation kinetics. It represents the initial fast increase in G*, determined by the concentration of cross-linker, and the slow diffusion of cations to the assembled gelling structures depicted on the successive evolution of G* in longer times. Finally, a continuous and an intermittent oscillatory time sweep influence the diffusion of the cross-linking solution resulting in considerably different gelation curves for the same concentration of reactants.

A triblock copolymer with hydrophobic end blocks and a polyampholytic middle block is investigated in a mixture of water and acetone with a focus on the dependence of the rheological properties and of the micellar structure and correlation on the content of acetone. The polymer under study is PMMA86-b-P(DEA190-co-MAA96)-b-PMMA86, where PMMA stands for poly(methyl methacrylate) and P(DEA-co-MAA) for poly(2-(diethylamino) ethyl methacrylate-co-methacrylic acid). The pH is chosen at 3. Rheological measurements reveal a transition from a viscoelastic solid over a viscoelastic liquid to a freely flowing liquid upon addition of 5 or 10 wt% of acetone to a 3 wt% aqueous polymer solution, respectively. Using small-angle neutron scattering on 0.5 wt% polymer solutions in water/acetone with the content of the latter ranging between 0 and 30 wt%, significant structural changes are observed as well, such as a decrease of the distance between the PMMA cross-links and of the size of the network clusters upon increasing acetone constant. These changes are attributed to the reduction of the dielectric constant by the addition of the cosolvent acetone, enhancing the flexibility of the middle blocks and their tendency to backfolding, as well as to the decrease of the solvent selectivity, inducing significant exchange rate enhancement of the core-forming PMMA blocks.

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