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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.

Morphology is the decisive factor controlling practical properties such as impact strength or transparency in multiphase polymeric materials. The co-continuous structure formed by polymers has been of great interest to material scientists because of their superiority over those with random morphology. Although a number of efforts--including forcibly freezing the spinodal structure of polymer blends--have been made to produce materials with co-continuous structures, an efficient method for controlling their regularity is still lacking. Here, we demonstrate a novel method using periodic photo-crosslinking to control the length-scale distribution of the spinodal structure in binary polymer blends. It was found that the period distribution of the resulting co-continuous structure became significantly narrow under this periodic forcing. Also, there exists a particular irradiation frequency at which the periodic structure exhibits a minimum, indicating the existence of an ordering process driven by the external modulation frequency. Our findings reveal an easy way to produce polymer materials that is not only useful for optical applications, but also promising for biological separation, such as hemodialysis.

Interpenetrating polymer networks (IPNs) belong to a class of molecular composites that consisted of two different networks mutually entangled by cross-link during the polymerization process. In the past decades, a large number of IPNs have been synthesized by using various techniques with the expectation that superior mechanical properties may emerge from their unique networking structures. 1 However, except for some limited cases, 2 phase separation occurs when the yields of polymerization or cross-link density reach a threshold. Nevertheless, these two-phase materials have found their wide applications in specialty materials such as those with high impact strength, sound, or vibration damping or with controllability of gas transport. 3 Since the physical properties of IPNs strongly depend on morphology, it would be very useful to develop a convenient method to generate and control their phaseseparated structures.From the viewpoint of pattern formation process, IPN is a chemical system driven by two competing antagonistic interactions: cross-linking reactions vs phase separation. This competition is controlled by the socalled activator-inhibitor principle 4 where phase separation corresponds to an activator and cross-linking reaction plays the role of an inhibitor. It has been shown that such the competition process can result in a wide variety of morphologies. 5 Morphology control of IPNs using thermally activated reactions is generally limited because the heat used to induce chemical reactions also affects the miscibility of the mixture. As a consequence, some alternative methods are expected to efficiently manipulate the competition between phase separation and the chemical reaction. In this study, we demonstrate that photochemical reactions can provide a tool to generate and control hierarchical morphologies of IPNs in the micrometer ranges.We have utilized photo-cross-linking reactions to freeze the spinodal structure developing in polymer blends undergoing phase separation. 6 Though the cocontinuous morphology could be generated and controlled by this particular method, the development of the characteristic length scales in these experiments was quite limited because of the drastic increase in viscosity associated with the cross-linking reaction in the bulk state of polymer. Here, to remove this constraint on the morphological length scales, we performed experiments using an IPN system containing a photocross-linkable polymer dissolved in a photopolymerizable monomer of the second polymer. The rates of the cross-linking reactions as well as the photopolymerization were controlled by varying the light intensity. Furthermore, the mobility of polymer in the reacting mixture was regulated by changing the polymer molecular weight using different concentrations of photoinitiator. We show below that by simply changing the light intensity and/or the concentration of photoinitiator a variety of morphologies with different length scales as well as structural hierarchies can be generated and manipulated by irradiati...

Phase separation of polymer mixtures induced by photo-crosslinking and photopolymerization is summarized from the viewpoint of competing interactions in polymeric systems. First, the kinetics of photo-crosslinking and photopolymerization are reviewed with particular emphasis on the Trommsdorff − Norrish effect in polymerization, which leads to the formation of a bimodal distribution of molecular weight at long irradiation time. The emergence of a very large molecular weight and the characteristic time for this effect to set in are utilized as a tool to control the temporal development of the resulting morphology. The shrinkage associated with the reaction and particularly its effect on the phase separation and the morphology are discussed together with the influence of the Trommsdorff − Norrish effect. Experiments on polymerization-induced phase separation extended from binary to ternary mixtures are described where the interfacial interactions give rise to a wide variety of morphologies. Taking advantages of light, experiments using spatial and temporal modulation have been performed to control and manipulate the phase separation. These experimental results are summarized and briefly discussed in relation to the elastic deformation induced by polymerization. Finally, an overview of theoretical studies and numerical simulation of reaction-induced phase separation is given with some comments on the complexity of the phenomena.