Yuliang Yang

The phase diagrams of mixtures of EVAL and PEG in relation to membrane formation

Abstract The phase diagrams of various ethylene–vinyl alcohol (EVAL) copolymer with different hydroxyl (OH) group contents and poly(ethylene glycol) (PEG) with different molecular weights (ranging from 200 to 600) are determined. It is found experimentally that both the liquid–liquid (L–L) phase boundaries and the crystallization curves are shifted to higher temperature when the OH group contents in EVAL increase. On the other hand, only the L–L phase boundaries shift to higher temperature when the molecular weights of PEG increase. These phenomena are interpreted by using solubility parameters ( δ d , δ p ) to estimate the Flory–Huggins interaction parameters, χ ∗ , of the mixtures. By extrapolating the L–L bi-phasic curves from cloud points of temperature ( T cloud ), the experimental interaction parameters χ are obtained, which are linearly correlated to the theoretically estimated interaction parameters, χ ∗ .

MONTE CARLO SIMULATION OF SELF-AVOIDING LATTICE CHAINS SUBJECT TO SIMPLE SHEAR FLOW. I. MODEL AND SIMULATION ALGORITHM

The efficient on-lattice Monte Carlo (MC) method is extended to simulate, for the first time, the simple shear flow of multiple macromolecular chains with self-avoiding walk (SAW) on a molecular level by introducing a pseudopotential to describe the flow field. This pseudopotential makes sense only for the potential difference associated with each local microrelaxational movement of a bead in the chain strictly defined by the four-site lattice model and bond fluctuation approach. The free-draining bead-spring model is thus investigated at low and high shear rates, and the resultant shear stress and first normal stress difference are obtained by statistics according to the sampled configurational distributions under flow. As the first paper of a corresponding series, the pseudopotential is checked in detail and confirmed by the simulation outputs for both a single SAW chain and multiple SAW chains in two dimensions. The simulated velocity profile is found to greatly satisfy the requirement of the simple sh...

Synthesis of block copolymer by “living” radical polymerization of styrene with nitroxyl-functionalized poly(ethylene oxide)

Abstract A series of well-defined block copolymers of poly(ethylene oxide- b -styrene)s (PEO- b -PS)s with narrow polydispersity were synthesized by the following two-step approach: living anionic polymerization of ethylene oxide (EO) with sodium 4-oxy-2,2,6,6-tetramethy-1-piperidinyloxy (TEMPONa) as initiator to yield PEOn with a TEMPO moiety at chain end, was followed by a stable free radical polymerization (SFRP) of styrene (St) to give a block copolymer. In the process of anionic polymerization, TEMPONa could induce living anionic opening-polymerization of EO at low temperature under which the stable niroxyl radical at the end of the PEO chain could be not destroyed at all. The TEMPO moiety in the resulting PEOn acts as a radical trapper in quantitative efficiency and St can be designed for polymerization for good control of the SFRP. Such a polymerization proceeded with a living radical mechanism. However, PEOn should be more sterically hindered and possess lower diffusion ability and thus be less efficient at trapping propagating chain radicals, resulting in enhanced polymerization rate.

Phase behavior of semiflexible-coil diblock copolymers: a hybrid numerical SCFT approach

The phase behavior of semiflexible-coil diblock copolymer melts is studied by solving the self-consistent field theory (SCFT) equations of wormlike chains. Significant improvement of numerical accuracy and stability is achieved by a hybrid numerical implementation of SCFT, in which the space-dependent functions are treated using a spectral method and the orientation-dependent functions are discretized on a unit sphere (3D Euclidean space) with an icosahedron triangular mesh. The angular Laplacian is solved in real-space using a finite volume algorithm. Phase diagrams of the model system are constructed from SCFT. Phase transitions between various smectic phases such as monolayer and bilayer smectic-A, monolayer and bilayer smectic-C, as well as folded smectic phases, are predicted. In particular, the stability of the monolayer and bilayer smectic phases is associated with the competition between interfacial energy and coil-stretching entropy, which strongly depends on the interplay between orientational interaction and microphase separation and the topological disparity between the semiflexible and coil blocks.

Real-space self-consistent mean-field theory study of ABC star triblock copolymers

The phase behavior of ABC star triblock copolymers is examined using real-space self-consistent mean-field theory. The central part of the triangular phase diagram for ABC triblock copolymers with equal A/B, B/C, and C/A interactions is determined by comparing the free energy of a number of candidate ordered phases. In this region of the phase diagram, the dominant microstructures are cylinders with polygonal cross sections or two-dimensional polygon-tiling patterns. Most of the known polygon-tiling patterns observed in experiments and simulations, plus some neighboring morphologies, are considered in the construction of the phase diagram. The resulting phase behavior is consistent with experiments and computer simulations.

Birefringence Patterns of Nematic Droplets

Birefringence patterns of nematic liquid crystalline droplets are investigated both theoretically and experimentally. Gridding method is developed to calculate optical birefringence patterns of nematic droplets with different director configurations. Series of birefringence patterns of the four typical nematic droplets (radial, bipolar, axial and equatorial) with different orientations of droplets axes are obtained. By comparing experimental polarizing photographs with theoretical birefringence patterns, the authors find out that only bipolar configuration represents the self-organized structure of nematic droplets in our epoxy-based polymer dispersed liquid crystal (PDLC). It is also pointed out that the very gridding method can be easily extended to deal with other complicated anisotropic entities.

Self-Assembly of Star ABC Triblock Copolymer Thin Films: Self-Consistent Field Theory

Microphase separation and morphology of star ABC triblock copolymers confined between two identical parallel walls (symmetric wetting or dewetting) are investigated with self-consistent field theory (SCFT) combined with the masking technique to describe the geometric confinement of the films. In particular, we examine the morphology of confined near-symmetric star triblock copolymers under symmetric and asymmetric interactions as a function of the film thickness and the surface field. Under the interplay between the degree of spatial confinement, characterized by the ratio of the film thickness to bulk period, and surface field, the confined star ABC triblock copolymers are found to exhibit a rich phase behavior. In the parameter space we have explored, the thin film morphologies are described by four primary classes including cylinders, perforated lamellae, lamellae, and other complex hybrid structures. Some of them involve novel structures, such as spheres in a continuous matrix and cylinders with alternating helices structure, which are observed to be stable with suitable film thickness and surface field. In particular, complex hybrid network structures in thin films of bulk cylinder-forming star triblock copolymers are found when the natural domain period is not commensurate with the film thickness. Furthermore, a strong surface field is found to be more significant than the spatial confinement on changing the morphology of star triblock copolymers in bulk. These findings provide a guide to designing novel microstructures involving star triblock copolymers via geometric confinement and surface fields.

Self-Assembled Microstructures of Confined Rod-Coil Diblock Copolymers by Self-Consistent Field Theory

We employ the self-consistent field theory (SCFT) incorporating Maier-Saupe orientational interactions between rods to investigate the self-assembly of rod-coil diblock copolymers (RC DBC) in bulk and especially confined into two flat surfaces in 2D space. A unit vector defined on a spherical surface for describing the orientation of rigid blocks in 3D Euclidean space is discretized with an icosahedron triangular mesh to numerically integrate over rod orientation, which is confirmed to have numerical accuracy and stability higher than that of the normal Gaussian quadrature. For the hockey puck-shaped phases in bulk, geometrical confinement, i.e., the film thickness, plays an important role in the self-assembled structures transitions for the neutral walls. However, for the lamellar phase (monolayer smectic-C) in bulk, the perpendicular lamellae are always stable, less dependent on the film thicknesses because they can relax to the bulk spacing with less-paid coil-stretching in thin films. In particular, a very thin rod layer near the surfaces is formed even in a very thin film. When the walls prefer rods, parallel lamellae are obtained, strongly dependent on the competition between the degree of the surface fields and film geometrical confinement, and the effect of surface field on lamellar structure as a function of film thickness is investigated. Our simulation results provide a guide to understanding the self-assembly of the rod-coil films with desirable application prospects in the fabrication of organic light emitting devices.

Rate enhancement of nitroxide-mediated living free-radical polymerization by continuous addition of initiator

A new technique, which involves the continuous addition of a small amount of radical initiators, was developed with the assistance of computer simulation to increase the rate of nitroxide-mediated living free-radical polymerization. Using this method, the polymerization rate of styrene in the presence of 4-hydroxy-2,2,6,6-tetramethyl piperidinyl-1-oxy was increased more than 3-fold compared to that with one-batch addition of initiator, while the molecular weight and distribution remain the same, respectively, at higher monomer conversion.

Oscillatory shear induced anisotropic domain growth and related rheological properties of binary mixtures

Numerical simulation based on the modified time-dependent Ginzburg–Landau (TDGL) model has been performed on the domain growth and related rheological properties of binary mixtures under oscillatory shear. The simulation results reveal that the domain growth is anisotropic and depends on the quench depth. It is found that, in the deep quench case, the disclike domain with the normal parallel to the velocity gradient direction is observed, while in the shallow quench case, the rodlike domain with rod axis aligned along the flow direction is observed. The scattering functions for different light incident directions are calculated and suggest that the undulated rodlike morphology is formed in the shallow quench case. This undulated rodlike morphology shows the anomalous rheological response. A plausible interpretation for the anomalous rheological property is proposed based on the deformation of the undulated rodlike morphology under oscillatory shear.