Spatial gel inhomogeneity

Another non-ideal feature of hydrogels is the so-called spatial gel inhomogeneity (Shibayama 1998 Bastide and Candau 1996). In contrast to ideal gels with a homogeneous distribution of cross-links, hydrogels always exhibit an inhomogeneous... 

To have Fel for inhomogeneous cases, we distinguish the Cartesian coordinates of the deformed gel, X = (X, X2, X3), representing the real spatial position, and those in the isotropic, relaxed state, jc0 = (x , x , x ), representing the original position before deformation. We define the deformation tensor,... 

It is worth noting that the effect of inhomogeneities on the friction coefficient appears at much lower cross-linker concentrations than the appearance of the visual opacification in the gel. Indeed, the friction coefficient of the gel decreases with the increasing cross-linker concentration even in the lower concentration region of the cross-linker where the gel looks completely transparent. This result suggests that the seeds of the concentration fluctuations are already formed in the lower cross-linker concentration region, which have not been detected by optical measurements. Thus the friction coefficient of a gel reveals in an extremely sensitive way the spatial inhomogeneity of the gel. 

It is important to note that in the inhomogeneous gel, the average crosslinking density is not a relevant parameter for determining the frictional pore size of the gel. It is the spatial correlation length of the density fluctuations that determines the bulk frictional behavior of water in the gel. 

Karino T, Okumura Y, Ito K, Shibayama M, (2004) SANS studies on spatial inhomogeneities of slide-ring gels. Mactomolecules 37 6177-6182... 

When an infinite network is formed, the polymer chains in the network lose their freedom to move and are in a limited space. This results in non-cancellation of concentration fluctuations and in the emergence of position-dependent concentration fluctuations (i.e. fi ozen inhomogeneities). According to Shibayama et al. (Shibayama and Norisuye 2002 Shibayama 2006) there are four types of such frozen inhomogeneities in the gel state spatial, topological, mobility and coimectivity... 

Schmidt T, Quemer C, Amdt K-F (2003) Characterization methods for radiation cross-linked poly (vinyl methyl ether) hydrogels. Nucl Instr and Meth in Phys Res B 208 331-335 Shibayama M (1998) Spatial inhomogeneity and dynamic fluctuations of polymer gels. Macromol Chem Phys 199 1-30... 

Descriptions of networks in terms of cross-link index and density provide no information on the internal architecture, that is, the homogeneity, of the network. Depending on how they are produced, most networks are more or less inhomogeneous, that is, the local density has a distribution. With multifunctional polycondensation, the gel point is most often reached at relatively high yields, and the network formed is quite homogeneous. In addition polymerization, the gel point occurs already at relatively low yields. The polymerization continues around the spatially fixed network structured centers, and, so, densely cross-linked centers are produced within a less densely cross-linked matrix. 

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