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Using Nuclear Magnetic Resonance Techniques to Determine Structural Characteristics of Gels, 15-9161

Principal Investigators
Qingwen Ni
J. Derwin King

Inclusive Dates: 09/23/99 - 01/23/00

Background - The structural characteristics of hydrogels is of great value in determining the gel's pore size distribution, a factor that varies in polymer gels as they are formed under different conditions. In hydrogel, for example, gel electrophoresis is a powerful method in separating DNA fragments with different molecular weights. Thus, the pore size and pore heterogeneity are important structural parameters that can be used for predicting hydrogel performance. No single technique is entirely adequate in determining these parameters. One purpose of this project was to demonstrate the applicability of a new method in determining pore size distribution, i.e., NMR relaxation (spin-spin or spin-lattice) spectra of saturated and partial saturated hydrogel. This NMR technique overcomes several of the disadvantages of the other methods and has recently been successfully applied to some nonmembrane types of porous media. This method avoids many of the problems inherent in the traditional techniques, such as the flow permeability/bubble point method, mercury porosimetry method, and the gas adsorption/condensation technique. The idea of using NMR for pore structure determination originated in the petroleum industry where NMR logging has been used to determine the pore rock properties. The basic principal is that the portion of probe fluid near a pore wall undergoes relaxation in a magnetic field faster than pore fluid removed from the pore wall. This factor, coupled with the fast diffusion exchange of fluid between regions within a pore, yields a unique average relaxation time constant for each different geometrical environment (i.e., pore size) within the material. This relaxation time constant is dependent on the ratio of surface fluid to nonsurface (or bulk) fluid in the pore, which is a measure of the pore hydraulic radius.

Approach - Efforts during this project included: 1) samples prepared for designed experiments including different kinds of agarose gels with different concentrations and types, buffer or nonbuffer, and formed procedures; 2) fiber cell model studies and modification for gel characterization; 3) experimental studies on different concentrations of polymer (1 - 3 percent), with or without derivatized carboxymethyle, with different synthesis temperature; and 4) experimental studies on the pylacrylamide gels at saturated and at different partial saturated conditions.

Accomplishments - Eleven agarose gels were received from the University of Texas Health Science Center at San Antonio. The samples included different concentrations of the polymer used to form the gel (1 to 3.5 percent) by agarose chemically underivatized and derivatized with carboxymethyl. Also some buffers (0.25 ppm) were used during gelation of agarose. Several samples were formed at different temperature conditions. The NMR measurements and its inversion relaxation spectral results were used to determine the radius of the effective pores of the gel. Results show that when synthesis is at same polymer concentrations but 1) with or without derivalization, the pore size changes are small; 2) with or without the buffer, the pore size change is increased but still not significant; and 3) for the same concentration of polymer hydrogel synthesized at different temperature, the pore size changes are significant. When the hydrogel is synthesized at 32 °C, both the pore size and heterogeneity are dramatically increased. In addition, the comparison of the inversion relaxation spectra among samples of 3.5, 2.0, and 1.0 percent polymer, formed at the same conditions except with different polymer concentrations, the pore size distributions are also significantly changed. The obtained information is very useful for design synthesizing narrow, small, large, and heterogeneity of the pore size for differing requirements. The pylacrylamistry gel was received from the Material Laboratory, University of North Texas, and the saturated and partial saturated pylacrylamide gel was characterized. In the fully saturated gel, the range of the T₂ distribution is narrower than in the partially saturated gel because, when the gel is fully saturated, the distributions of the pore size become uniform or most of the pores are connected together. Thus, the appearance of the difference of the pore size is small. However, in a partially saturated state, the nonuniformity of the pore size distribution is increased, i.e., some pore sizes are reduced or the fluids in the pores are less connected. These characterization methods are valuable for evaluating different polymer gel synthesis method and in predicting their performance.

      Measurement & Nondestructive Evaluation of Materials & Structures Program
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