Studies on Two Types of Built-in Inhomogeneities for Polymer Gels:  Frozen Segmental Concentration Fluctuations and Spatial Distribution of Cross-Links

Abstract

The shrinking kinetics of poly(N-isopropylacrylamide) (PNIPA) gels has been studied for two types of PNIPA gels prepared by (i) copolymerization of constituent monomer and cross-linker (monomer cross-linked gels) and (ii) γ-ray irradiation in the PNIPA solutions(polymer cross-linked gels) in order to investigate the role of cross-linking on shrinking kinetics. The shrinking kinetics of the monomer cross-linked gels is quite similar to that of the polymer cross-linked gels. For example, the rapid shrinking is attained by simply lowering the cross-linking density for both types of gels with a skin formation with skin thickness of ca. 3 µm. On the other hand, a significant difference was found when the microscopic structure and the dynamics were investigated by small-angle neutron scattering (SANS) and static/ dynamic light scattering (SLS/DLS). The degree of built-in inhomogeneities and dynamic fluctuations were evaluated as a function of the cross-linking degree and the gel preparation temperature by intensity decomposition methods for both types of gels. It is concluded from the SANS and SLS/DLS results that the monomer cross-linked gels have extra built-in inhomogeneities due to the spatial distribution of crosslinks in addition to the frozen concentration fluctuations inherent in polymer gels.

Universality and Specificity of Polymer Gels Viewed by Scattering Methods

“…Figure 18 shows the time course of swelling ratio, d=d 0 , for (a) the PNIPA monomer cross-linked gels and (b) the -ray cross-linked gels with different cross-linking concentrations, after a T-jump from 20 to 45 C, where d is the diameter of the gel at observation and d 0 is the capillary diameter at which the gel was formed. 59 As can be seen from the figure, rapid shrinking was attained by simply lowering the cross-linker concentration or the irradiation dose for both types of gels indicated by filled symbols. A similar result has been reported for monomer cross-linked gels.…”

“…20 Figure 16 shows a comparison of hIi E and hI F i T of (a) monomer cross-lined gels and (b) polymer cross-linked gels as a function of cross-linker concentration. 59 Note that the scale of the ordinate is about ten times different. This figure also clearly indicates that the monomer cross-linked gels are more inhomogeneous.…”

“…Figure 17 schematically illustrates the difference of the network structure. 59 For the polymer cross-liking system, chains are loosely connected together but remain in sol state at low cross-linking degree (Fig. 17a).…”

Novel Self-Oscillating Polymer Actuators for Soft Robot

“…An N-isopropylacrylamide (NIPAAm) gel with a microscale phase separation that underwent a quick response has previously been reported [79]. By preparing a NIPAAm gel above the LCST, the NIPAAm gel forms a porous structure with polymer-rich domains and aggregations in the matrix of loosely bound network structures [80]. Consequently, polymer-rich domains inside the gel aggregate or disperse rapidly through the porous structure within the gel by an effluent pathway of the solvent.…”

Radiation Synthesis of Binary Hydrogels with Thermoresponsive Pores

“…However, the cross-linker induces the structural disorder in the gel system. On the contrary, γ-ray-induced gelation requires no cross-linkers and therefore the disorder brought by the external factors can be suppressed [13,14].…”