Modulated Phases Observed in Reacting Polymer Mixtures with Competing Interactions

A systematic study on immiscible binary systems undergoing thermal/photo reversible chemical reactions

In this work, we systematically study an immiscible binary system undergoing thermal/photo reversible reactions in theory. For the thermal reaction case, no dissipative structures can be formed and only uniform equilibrium states are observed but the dynamical evolution to these trivial states witnesses a new type of sophisticated phase amplification phenomenon-temporary phase separation (TPS). Linear analysis and light-scattering calculations confirm that TPS is predominated either by spinodal decomposition or nucleation and growth mechanism, or by both successively. For the photo reaction case, steady dissipative patterns exist and are maintained by the external energy input of lights. Linear analysis together with simulations reveals that the characteristic wavelength (ξ) of these structures shortens as the input energy density increases and they obey the relation of ln ξ∝ 1/T_b with T_b the effective temperature of lights. The TPS phenomenon and length-scale dependency of dissipative patterns observed in this simple binary system might have rich implications for the non-equilibrium thermodynamics of biological systems.

Polymer networks with bicontinuous gradient morphologies resulting from the competition between phase separation and photopolymerization

Poly(ethyl acrylate)/poly(methyl methacrylate) (PEA/PMMA) polymer networks (IPNs) with spatially graded bicontinuous morphology were designed and controlled by taking advantage of the spinodal decomposition process induced by photopolymerization of the MMA monomer. Spatial gradients of the quench depth, induced by the gradients of light intensity, were generated along the path of the excitation light travelling through the mixture. Bicontinuous structures with uniaxial gradient of characteristic length scales were obtained by two different methods: simply irradiating the mixture with strong light intensity along the Z-direction and using the so-called computer-assisted irradiation (CAI) method with moderate intensity to generate the light intensity gradient exclusively in the XY plane. These experimental results suggest that the combination of these two irradiation methods could provide polymer materials with biaxially co-continuous gradient morphology. An analysis method using the concept of spatial correlation function was developed to analyze the time-evolution of these graded structures. The experimental results obtained in this study suggest a promising method to design gradient polymers in the bulk state (3D) as well as on the surface (2D) by taking advantage of photopolymerization.

Autocatalytic phase separation and graded co-continuous morphology generated by photocuring

Interpenetrating polymer networks (IPNs) with spatially graded co-continuous structures were constructed by photo-cross-linking a homogeneous mixture of photo-reactive polystyrene and methyl methacrylate monomer. For a given thickness, irradiation with weak ultraviolet (UV) light results in a co-continuous morphology uniform throughout the sample. However, as the irradiation intensity increases to some certain extent, spatially graded co-continuous morphology in the micrometre scales emerges due to the significant effect of the gradient of the light intensity along the irradiation direction. These 3-dimensional graded structures were observed by using laser scanning confocal microscopy (LSCM) and were analyzed by digital image analysis. The depth dependence of these graded structures can be well expressed by a power law with an exponent depending strongly on the irradiation intensity. The time-evolution of the graded morphology was monitored at different depths of the same irradiated sample. It was found that the phase separation does not follow conventional laws of kinetics, but instead exhibits the autocatalytic behavior, reflecting the effects of the heat produced by the photopolymerization of MMA monomer on the phase separation process. The experimental data obtained in this study suggest a method of producing polymeric materials with spatially graded structures in the micrometer scales by solely changing the irradiation intensity.

Spatial modulation in cross-linked binary polymer blends

Spatial modulation means that the free energy of the composite mixture is minimized at a finite wavenumber, which defines the primary length scale, the maximum size possible, of the evolving phases. Although spatial modulation may appear in different systems in different ways and is not necessarily a thermodynamic precept, here it is expressed as a thermodynamic consequence of short-range interfacial forces pitted against long-range elastic forces. Cross-linked chains bring about new thermodynamic interactions based on their conformations and the fractal dimensions of the chain networks to which they tether and belong. Interplay among these different interactions during phase ordering brings about the spatial modulation and the spatial pattern. Here, the underlying processes are only succinctly outlined.

Free energy of mixing of cross-linked polymer blends

Free energy of mixing of cross-linked polymer blends is derived, as a modification to the Flory-Huggins-de Gennes free energy functional for linear polymer blends. The latter arrives from the assumption of mean-field, short-range thermal interactions among ideal Gaussian chains. However, upon cross-linking a linear chain, the chain no longer remains Gaussian; new chain architectures belying the threadlike image of linear chains emerge. Fractal dimensions of these nonlinear chain clusters convene and command new entropic interactions. Topological constraints by cross-links introduce long-range nonequilibrium elastic forces. Relatively shorter range steric repulsions between fractal network surfaces may arrive if cross-linking is carried out inside the blend's thermodynamically unstable region. Modified free energy has been used to highlight experiments on phase instability of cross-linked polymer blends.

Crystal-like pattern formation in polymerization-induced phase separation

A unique domain structure was found in liquid-liquid phase separation induced by radical polymerization of 4-chlorostyrene monomer in the presence of random copolymer poly(dimethylsiloxane-co-diphenylsiloxane). Droplets with a narrow size distribution were generated by phase separation, and these droplets hardly coalesced by collisions. With the growth of droplet size, droplets gradually filled the space and spontaneously arranged themselves in a regular array like a crystal. The mechanism of regular array formation was discussed based on the large difference in viscoelastic properties between two segregating components.

Fast growth in phase-separating A-B-copolymer ternary mixtures with a chemical reaction

We study the dynamics of phase separation of a binary A-B- polymer mixture with copolymer C, which is produced by the reaction of two counterpart reactive polymers A and B at the interface via the chemical reaction A+B right harpoon over left harpoon C. For low interfacial energy between the A and B phases, where the copolymer prefers to locate at interfaces, we show that the chemical reaction accelerates the phase separation of the system dramatically, because the backward reaction always drives the creation of immiscible A and B pairs at interfaces, which speed up the phase separation of the system, while the forward reaction process becomes more and more difficult as the interfaces are gradually saturated by copolymers. We also indicate that for a fixed chemical reaction rate constant, as the initial concentration of the copolymers increases, the domain growth at the late stage is speeded up as a result of the backward chemical reaction. However, when the interfacial energy is high, both forward and backward reactions coexist due to the occurrence of unsaturated interfaces, but the relative strength of reaction rates has no appreciable effect on domain growth during spinodal decomposition, because the interfacial energy dominates phase separation.

Kinetics of phase separation in polymer-solvent mixtures

We investigate the kinetics of phase separation in systems with a strong asymmetry in the mobility. This simple model system mimics the segregation kinetics of polymer-solvent mixtures, where the polymer-rich phase forms a low-mobility gel. We obtain detailed numerical results for this model, both without and with thermal noise. In the absence of thermal noise, we find that domain growth is slowed down drastically when the low-mobility phase percolates the system. However, thermal noise restores rapid domain growth through a Brownian coalescence mechanism.

Phase separation with multiple length scales in polymer mixtures induced by autocatalytic reactions

A ternary polymer blend with two components photo-cross-linked independently in its miscible region undergoes phase separation, exhibiting morphology with multiple length scales. Contrary to the case of thermally induced phase separation, the morphology exhibits a unimodal-->multimodal transition. It is shown that these multiple length scales are caused by the inhomogeneous freezing kinetics of the cross-linking process. This inhomogeneity arises from the autocatalytic feedback driven by the couplings between concentration fluctuations and the photo-cross-linking reactions.

Modes selection in polymer mixtures undergoing phase separation by photochemical reactions

Phase separation kinetics and morphology of binary polymer mixtures (A/B) in the presence of photochemical reactions were investigated by using phase-contrast optical microscopy combined with digital image analysis. The polymers were chemically designed in such a way that two types of chemical reactions, intermolecular photodimerization and intramolecular photoisomerization, of polymer segments can be induced and controled by irradiation with ultraviolet light. Unlike the conventional case, the phase separation in the presence of these reactions is spontaneously frozen due to the suppression of the long-wavelength instabilities, resulting in stationary spatial structures with intrinsic periodicities. These characteristic length scales are determined by the competition between the two antagonistic interactions: phase separation as a relatively short-range activation and the photochemical reaction as a long-range inhibition. Furthermore, it was found that the spatial symmetry breaking of concentration fluctuations can emerge from the elastic stress associated with the nonhomogeneous kinetics of the reactions. Experimental data obtained with three types of reactions: A-A only cross-link, A-A and B-B simultaneous cross-links and the reversible A<-->B photoisomerization are described. These results do not only indicate that combination of chemical reactions and phase separation could provide a novel method to control the morphology of multiphase polymer materials, but also suggest that photoreactive polymers can be used as a chemical system to study the mode-selection process in polymers far from thermodynamic equilibrium. (c) 1999 American Institute of Physics.