Tom Brenner

Molecular mobility and microscopic structure changes in κ-carrageenan solutions studied by gradient NMR.

Changes in the molecular mobility of κ-carrageenan were observed by the pulsed field gradient stimulated echo (PGSTE) and Carr-Purcell-Meiboom-Gill (CPMG) methods for elucidating the molecular aspect of the sol-to-gel transition. The echo signal intensity of κ-carrageenan without a gradient, Ikap(0), decreased steeply near the sol-to-gel temperature (Tsg), suggesting that κ-carrageenan chains formed aggregates and a network structure. Below Tsg, the spin-spin relaxation time T2 and the diffusion coefficient of κ-carrageenan (Dkap) increased with decreasing temperature, indicating that the solute κ-carrageenan chains have a lower molecular weight Mw than chains involved in the aggregation. The diffusion coefficient of pullulan (Dpul) added as a probe molecule in κ-carrageenan solutions was measured, and the characteristic hydrodynamic screening length, ξ, was then estimated from the degree of diffusion restriction. Below a certain temperature, Dkap reached a higher value than that of Dpul, suggesting that the Mw of solute κ-carrageenan became lower than that of pullulan. GPC measurements confirmed the presence of κ-carrageenan chains with a lower Mw than that of pullulan. A simple physical model of the structural change in κ-carrageenan solution was proposed with a bimodal distribution of κ-carrageenan with higher and lower Mw than the pullulan probe. The higher Mw chains form the gel network restricting the probes diffusion, and the lower Mw chains increase the effective viscosity. The concentration of the high Mw solute κ-carrageenan chains in 1%, 2% and 4% κ-carrageenan solutions was estimated from Ikap(0) and the total κ-carrageenan concentration, and the relation with pullulan diffusion was studied.

Gelation of Iota/Kappa Carrageenan Mixtures

The rheological and thermal properties of kappa carrageenan (KC) and iota carrageenan (IC) solutions and their mixtures were studied using dynamic rheology and differential scanning calorimetry (DSC). Two-step gelation was observed for all KCl concentrations studied (0-70 mM), as seen from two steps of storage modulus (G’) increase and two DSC peaks both on cooling and heating. Ion and water migration between KC and IC solutions was studied by forcing a macroscopic separation across a dialysis membrane. No redistribution of water or ions was found after curing gels for 24hrs, unless the temperature was far below the gelation temperature of KC. This result suggests that gelation of both polysaccharides is not sufficient to cause ion and water migration between KC and IC rich phases (in case phase-separation takes place).

Diffusion Measurements of Water and Polymers in Hydrogels by Pulsed Field Gradient NMR

The basic theory and techniques of diffusion measurements by pulsed field gradient NMR are described, and experimental results for solutions and gels of poly(N,N-dimethylacrylamide), carrageenans, agar , and agarose are introduced and analyzed to give physical pictures for the gels. Discussion of experimental results for water and probe diffusion in synthetic polymer and polysaccharide gels and solution was offered. Relaxation times for the macromolecules and water give information on tumbling motion. The diffusion coefficient of probe molecules in hydrocolloid systems provided the information on the translational mobility of molecules, which can be used to infer the structure of the gel network . By comparing the results with other experimental techniques, a clear picture emerges, with clear correspondence of the microscopic events, namely, aggregation, polymer immobilization, and subsequent effects on molecular flexibility and probe diffusion, with the macroscopic (bulk) events, namely, gelation. The hydrodynamic shielding length, ξ, which represents within the mean field hydrodynamic approach the mesh size of the network, as a parameter that determines D/D 0 of probe molecules, has been discussed in detail. This parameter ξ was used to describe quantitatively the evolving structure of the gel network.