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The use of static light scattering for the structure analysis of radiosensitive polymer gels: a literature survey

Static light scattering (SLS) could be a worthy technique to perform a structure analysis of the polymer structures inside radiation sensitive gels. The information obtained with SLS is a static characterization of the particle structures inside the gel. SLS will be combined with NMR relaxometry and NMR diffusion measurements, which deliver a hydrodynamic characterization of the microstructure of the gels.

The use of static light scattering for the structure analysis of radiosensitive polymer gels: a literature survey 1

. Introduction
Since Maryanski et al published their first paper on gel dosimetry [1], a lot of research has been done on the subject. After ten years of research a polymer gel (polyacrylamide gelatine gel) is obtained, that is clinically applicable [2]. The preparation of these gel dosimeters is unfortunately difficult and time consuming. The main reason is the removal of oxygen by bubbling nitrogen through the gel.
The removal of oxygen by use of an anti-oxidant as suggested by Fong et al [3] was first thought to be a solution for the oxygen problem. Until now, none of the applied 'normoxic' gels was found clinically applicable. An unpublished internal study showed dose rate dependence, temperature dependence and loss of spatial integrity.
In order to find a radiation-sensitive gel with an optimal composition, one should understand the chemical processes that occur in the gel during irradiation. In order to gain more information about these processes, a structure analysis of the radiation sensitive gels should be done. One of the techniques is static light scattering measurements.

Basic principle of static light scattering measurements
Static light scattering the angular dependency of the time-mean-intensity of laser light scattered by the particles is measured. The course of the scattered intensity as a function of the detector angle depends on size and structure of the particles. Rayleigh developed a theory that predicts the angular intensity distribution of light scattered by particles much smaller than the wavelength of light (R < λ/20). In this case the intensity of the scattered light depends only on the size of the particles and not on their structure or the concentration. If the intensity of the scattered light is measured in a plane orthogonal to the polarization plane of the incident light, there will be no angular distribution of the scattered light. (1) In this expression, R θ is the Rayleigh ratio, R is the particle size, n is the relative refractive index of the solute and λ is the wavelength of the incident light.  For larger particles, the influence of the particle structure and its concentration should be brought into consideration. In that case, the scattered light intensity for low particle concentrations can be described by: In this expression, R ex is the excess Rayleigh ratio, K is the optical constant, c is the particle concentration, M w is the mean molecular weight of the particles and P(θ) is the particle structure factor. If the particle concentration increase the influence of intraparticle optical interference and interparticle interactions increases. In that case the angular dependency of the scattered light can be described by: (3) in which, A 2 is the mean second virial constant.

Fitting of the data
Zimm [4] discovered that in the limit of low scatter vectors and low concentrations, the angular distribution of the scattered intensity becomes independent of the particle shape. By extrapolation of (Kc/R ex ) to zero concentration and zero angle the average molecular weight, radius of gyration and second virial constant can be estimated.
The structural parameters can be compared with theoretically derived values for several models such as from random coil, homogeneous sphere.

Applicability for gel dosimetry
About the structure of polyacrylamide polymers, there is already some information available [5,6]. N,N'-methylenebisacrylamide (Bis), much more hydrophobic as acrylamide (Ac), has the tendency to form clusters. As the polymerization starts, these clusters form cross-linked regions with a high concentration of Bis. As the concentration of Bis and the cluster formation decreases during polymerization less cross-linked side chains are formed that link the different microgel particles.
As a consequence, two phases will exist in the structure a dilute phase constituted by the side chains and a dense phase constituted by the Bis-clusters. The dense phase will contribute more to the light scattering as the dilute phase.
The separation of the polyacrylamide particles and the gelatin matrix is very difficult. This can be ascribed to the high crosslinked structures of polyacrylamide in which several gelatin molecules are caught. That is why we plan to perform the structure analysis of the polyacrylamide particles inside the gelatin medium. Tian et al [7] investigated the possibility of performing static light scattering measurements with paricles embedded in a hydrogel. They came to the conclusion that all parameters could be estimated correctly, except for the second virial constant.

Conclusion
Static light scattering (SLS) could be a worthy technique to perform a structure analysis of the polymer structures inside radiation sensitive gels. The information obtained with SLS is a static characterization of the particle structures inside the gel. SLS will be combined with NMR relaxometry and NMR diffusion measurements, which deliver a hydrodynamic characterization of the microstructure of the gels.