Phase Separation of a Polymer Mixture Driven by a Gradient of Light Intensity

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.

Phase lines in mean-field models with nonuniform external forces

We look at the influence of external fields on systems described by generic free energy functional of the order parameter. The external force may have arbitrary spatial dependence, and the order parameter coupling may be nonlinear. The treatment generalizes seemingly disparate works, such as pure fluids, liquid and polymer mixtures, lipid monolayers, and colloidal suspensions in electric fields, fluids, and nematics in gravity, solutions in an ultracentrifuge, and liquid mixtures in laser radiation. The phase lines and thermodynamic behavior are calculated at the mean-field level. We find a "surface" critical point that can be shifted to higher or lower temperatures than the bulk critical point. Below this point, the transition from a "gas" phase to a "liquid" phase is first-order, while above it, the transition is second-order. The second-order line is affected by the spatial dependence of the force, while the first-order line is universal. Moreover, the susceptibility may diverge at a finite location r. Several analytical expressions are given in the limit where a Landau expansion of the free energy is valid.

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.

Using light to guide the motion of nanorods in photoresponsive binary blends: designing hierarchically structured nanocomposites

One of the challenges in creating high-performance polymer nanocomposites is establishing effective routes for tailoring the morphology of both the polymer mixture and the dispersed nanoparticles, which contribute desirable optical, electrical, and mechanical properties. Using computational modeling, we devise an effective method for simultaneously controlling the spatial regularity of the polymer phases and the distribution of the rods within this matrix. We focus on mixtures of photosensitive AB binary blends and A-coated nanorods; in the presence of light, the binary blends undergo a reversible chemical reaction and phase separation to yield a morphology resembling that of microphase-separated diblock copolymers. We simulate the effects of illuminating this sample with a uniform background light and a higher intensity, spatially localized light, which is rastered over the sample with a velocity v. The resulting material displays a periodically ordered, essentially defect-free morphology, with the A-like nanoparticles localized in lamellar A domains. The dynamic behavior of the rods within this system can be controlled by varying the velocity v and Γ2, the reaction rate coefficient produced by the higher intensity light. Specifically, the rastering light can drive the rods to be "pushed" along the lamellar domains or oriented perpendicular to these stripes. Given these attributes, we isolate scenarios where the system encompasses a complex hierarchical structure, with rods that are simultaneously ordered along two distinct directions within the periodic matrix. Namely, the rods form long nanowires that span the length of the sample and lie perpendicular to these wires in regularly spaced A lamellae. Hence, our approach points to new routes for producing self-organized rectangular grids, which can impart remarkable optoelectronic or mechanical properties to the materials.

Coassembly of nanorods and photosensitive binary blends: "combing" with light to create periodically ordered nanocomposites

Using computational modeling, we establish a means of controlling structure formation in nanocomposites that encompass nanorods and a photosensitive binary blend. The complex cooperative interactions in the system include a preferential wetting interaction between the rods and one of the phases in the blend, steric repulsion between the coated rods, and the response of the binary blend to light. Under uniform illumination, the binary mixture undergoes both phase separation and a reversible chemical reaction, leading to a morphology resembling that of a microphase-separated diblock copolymer. When a second, higher intensity light source is rastered over the sample, the binary blend and the nanorods coassemble into regular, periodically ordered structures. In particular, the system displays an essentially defect-free lamellar morphology, with the nanorods localized in the energetically favorable domains. By varying the speed at which the secondary light is rastered over the sample, we can control the directional alignment of the rods within the blend. Our approach yields an effective route for achieving morphological control of both the polymeric components and nanoparticles, providing a means of tailoring the properties and ultimate performance of the composites.

Using a single mask to create multiple patterns in three-component, photoreactive blends

Via simulations, we demonstrate a simple route for forming defect-free patterns in a photosensitive, immiscible ABC blend. The first pattern is established by irradiating the sample through a mask, which serves to pin the C regions and thereby promotes the self-assembly of A and B into ordered domains. When the mask is removed, the photoactivity of the AB blend leads to different periodic patterns. Thus, the use of one mask permits the creation of multiple ordered morphologies, which can be locked into the film by quenching the system at the appropriate time.

Modeling self-assembly and phase behavior in complex mixtures

Using a variety of computational techniques, I investigate how the self-assembly of complex mixtures can be guided by surfaces or external stimuli to form spatially regular or temporally periodic patterns. Focusing on mixtures in confined geometries, I examine how thermodynamic and hydrodynamic effects can be exploited to create regular arrays of nanowires or monodisperse, particle-filled droplets. I also show that an applied light source and chemical reaction can be harnessed to create hierarchically ordered patterns in ternary, phase-separating mixtures. Finally, I consider the combined effects of confining walls and a chemical reaction to demonstrate that a swollen polymer gel can be driven to form dynamically periodic structures. In addition to illustrating the effectiveness of external factors in directing the self-organization of multicomponent mixtures, the selected examples illustrate how coarse-grained models can be used to capture both the equilibrium phase behavior and the dynamics of these complex systems.

Dynamics of ternary mixtures with photosensitive chemical reactions: creating three-dimensionally ordered blends

Using computer simulations, we establish an approach for creating defect-free, periodically ordered polymeric materials. The system involves ABC ternary mixtures where the A and B components undergo a reversible photochemical reaction. In addition, all three components are mutually immiscible and undergo phase separation. Through the simulations, we model the effects of illuminating a three-dimensional (3D) sample with spatially and temporally dependent light irradiation. Experimentally, this situation can be achieved by utilizing both a uniform background light and a spatially localized, higher intensity light, and then rastering a higher-intensity light over the 3D sample. We first focus on the case where the higher-intensity light is held stationary and focused in a distinct region within the system. The C component is seen to displace the A and B within this region and replicate the pattern formed by the higher-intensity light. In effect, one can write a pattern of C onto the AB binary system by focusing the higher-intensity light in the desired arrangement. We isolate the conditions that are necessary for producing clearly written patterns of C (i.e., for obtaining sharp interfaces between the C and A/B domains). We next consider the effect of rastering a higher-intensity light over this sample and find that this light "combs out" defects in the AB blend as it moves through the system. The resulting material displays a defect-free structure that encompasses both a periodic ordering of the A and B domains and a well-defined motif of C. In this manner, one can create hierarchically patterned materials that exhibit periodicity over two distinct length scales. The approach is fully reversible, noninvasive, and points to a novel means of patterning with homopolymers, which normally do not self-assemble into periodic structures.

Exploiting photoinduced reactions in polymer blends to create hierarchically ordered, defect-free materials

Computer simulations reveal how photoinduced chemical reactions can be exploited to create long-range order in binary and ternary polymeric materials. The process is initiated by shining a spatially uniform light over a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. We then introduce a well-collimated, higher intensity light source. Rastering this secondary light over the sample locally increases the reaction rate and causes formation of defect-free, spatially periodic structures. These binary structures resemble either the lamellar or hexagonal phases of microphase-separated diblock copolymers. We measure the regularity of the ordered structures as a function of the relative reaction rates for different values of the rastering speed and determine the optimal conditions for creating defect-free structures in the binary systems. We then add a nonreactive homopolymer C, which is immiscible with both A and B. We show that this component migrates to regions that are illuminated by the secondary, higher intensity light, allowing us to effectively write a pattern of C onto the AB film. Rastering over the ternary blend with this collimated light now leads to hierarchically ordered patterns of A, B, and C. The findings point to a facile, nonintrusive process for manufacturing high quality polymeric devices in a low-cost, efficient manner.

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.

Harnessing light to create defect-free, hierarchically structured polymeric materials

Computer simulations reveal how photoinduced chemical reactions in polymeric mixtures can be exploited to create long-range order in materials with features that range from the submicron to the nanoscale. The process is initiated by shining a spatially uniform light on a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and a phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures. If a nonreactive homopolymer C is added to the system, this component localizes in regions that are irradiated with a higher intensity light, and one can effectively "write" a pattern of C onto the AB film. Rastering over the ternary blend with the collimated light now leads to hierarchically ordered patterns of A, B, and C. Because our approach involves homopolymers, it significantly expands the range of materials that can be fashioned into a periodic pattern. The findings point to a facile process for manufacturing high-quality polymeric components in an efficient manner.