Many polymer gels display network defects and crosslinking inhomogeneity. This review reflects and interrelates investigations on the characterization of such polymer-network heterogeneity and on its impact on the swelling, elasticity, and permeability of polymer gels.

Microgels are soft deformable colloids that can be packed by external compression. Such packing transforms a suspension of loose microgel particles into an arrested state with properties similar to that of a macroscopic gel. This effect provides a way to purposely impart micrometer or submicrometer scale spatial inhomogeneities into these assemblies, allowing their effect to be studied. We follow this idea and prepare microgel packings that consist of major (50−99.7 No.%) fractions of soft, loosely cross-linked particles doped with defined minor (0.3−50 No.%) fractions of stiff, densely cross-linked particles. This approach creates soft microgel packings that contain defined submicrometer scale domains with very high degree of cross-linking, resembling the structure of inhomogeneous macroscopic gels. We study these inhomogeneous composites from macro-and microscopic perspectives by oscillatory shear rheology and fluorescence recovery after photobleaching to probe their macroscopic mechanics and the microscopic mobility of flexible linear tracer polymers that diffuse through them. These studies reveal an ambiguous behavior: whereas the presence of densely cross-linked domains does not exhibit any systematic effect on the bulk compressibility and microscopic tracer-chain diffusivity in the heterogeneous packings, it increases their macroscopic shear elastic modulus in a linear additive fashion. These results indicate that the impact of spatial inhomogeneities in polymer gels depends on whether the gels are probed in equilibrium or deformed states.

Polymer-network gels often exhibit local defects and spatial heterogeneity of their cross-linking density, which may differently affect their elasticity on microscopic and macroscopic scales. To appraise this effect, we prepare polymeric gels with defined extents of nanostructural heterogeneity and use atomic force microscopy to probe their local microscopic Young's moduli in comparison to their macroscopic elastic moduli measured by shear rheology. In this comparison, the moduli of the heterogeneous gels are found to be progressively smaller if the length scale of the probed gel region exceeds the size of the purposely imparted polymer-network heterogeneities. This finding can be explained with a conceptual picture of nonaffine deformation of the densely cross-linked polymer network domains in the heterogeneous gels. P olymer gels consist of three-dimensional assemblies of cross-linked polymer chains swollen in a solvent. 1,2 If they are formed by uncontrolled polymerization, these gels exhibit an inhomogeneous spacing of their network cross-linking junctions in the form of densely cross-linked local domains randomly distributed within a loosely cross-linked background. 3−6 As a result, such heterogeneous polymer gels display pronounced concentration fluctuations on length scales of several ten nanometers 7 that have been investigated by light, X-ray, neutron scattering, 8−11 NMR spectroscopy, 12,13 and microscopy techniques. 14,15 It has been shown that the elastic modulus of a gel, as probed by rheology, decreases with increasing degree of inhomogeneity in the gel, as probed by light scattering, 6,16,17 such that the elastic moduli of heterogeneous gels synthesized by free radical copolymerization of mono-and bifunctional monomers 6,16,18 are lower than the moduli of more homogeneous gels prepared by either controlled polymerization or by postpolymerization crosslinking of linear precursor chains. 19−21 Nevertheless, the elastic moduli of both of these types of gels are generally lower than what would be expected from their content of cross-linker on the basis of the statistical theory of rubber elasticity. 22,23 These observations have been explained by two different arguments. One argument is the formation of elastically ineffective network defects such as loops and dangling chains during the gelnetwork polymerization. 24 The other argument is the possible presence of very short network strands with a length close to the persistence length of the polymer, which are too rigid to deform and store elastic energy. In densely cross-linked domains that contain multiple of such short chains, several cross-links therefore just act as one single cross-linking supernode of high functionality but limited ability for elasticenergy storage. 25,26 In an extension of this conceptual picture, it was suggested that heterogeneous gels deform in a nonaffine fashion. 27−29 One hypothesis is that stiff, densely cross-linked gel domains deform less than the surrounding soft, loosely cross-linked background and, therefore, just pa...