Coil Homopolymer Research Articles

Liquid-crystal Assembly of Semiflexible-coil/Homopolymer Blends: a Dissipative Particle Dynamics Study

The liquid-crystal assembly of semiflexible-coil diblock copolymers with coil or semiflexible homopolymers is studied by dissipative particle dynamics simulation. Phase diagrams of the blends and orientation ordering parameters among semiflexible blocks are constructed as a function of chain stiffness and homopolymer volume fraction. For semiflexible-coil/coil blends with varying stiffness of semiflexible blocks, we display the rich phase behaviors of the system transited from coil-coil/coil to rod-coil/coil blends. The disorder-lamellae or lamellae-liquid crystalline transition and “dry brush” phenomenon induced by coil homopolymers are observed. For semiflexible-coil/semiflexible blends, adding semiflexible homopolymers also leads to a disorder-order transition and even a transition between monolayer and bilayer smectic-A phase. The results demonstrate that blending homopolymers into semiflexible copolymers can induce liquid-crystal assembly and even improve the orientation ordering of semiflexible blocks effectively.

Crossover between activated reptation and arm retraction mechanisms in entangled rod-coil block copolymers.

Using a coarse-grained slip-spring model, the dynamics of rod-coil block copolymers is explored over a wide parameter space to fully capture the crossover between the short rod (activated reptation) and long rod (arm retraction) limits. An analytical, closed-form expression for curvilinear diffusion by activated reptation was derived by separating the drag into individual components for the rod and coil block. Curvilinear diffusion in the intermediate rod regime, where both mechanisms are important, was then found to be faster than predicted when both mechanisms are independently combined. The discrepancy in the crossover regime arises because the rod-coil copolymer's exploration of space is not accurately described by either a coil homopolymer (assumed by activated reptation) or a rod homopolymer (assumed by arm retraction). This effect is explored by tracking the rod orientation as the polymer reptates, confirming that the polymer reptates along a path that becomes more rodlike as the rod fraction is increased. Thus, activated reptation under-predicts diffusion because the rod can choose reptation paths that are more extended than the coil homopolymer by renewal of the entanglement tube from the ends. Arm retraction under-predicts diffusion because minor rotations of the rod allow some motion before full retractions of the coil block. Finally, more familiar 3-dimensional center-of-mass diffusion measurements are related to the curvilinear diffusion analysis because the ratio of these two quantities varies smoothly between the coil and rod homopolymer limits as the reptation path becomes more extended.

Self-Diffusion and Constraint Release in Isotropic Entangled Rod–Coil Block Copolymers

Understanding dynamic relaxation mechanisms in self-diffusion and constraint release processes of rod–coil block copolymers is important for many technological applications that employ neat melts or concentrated solutions. Using a model system composed of poly(alkoxyphenylenevinylene) rods and polyisoprene coils, reptation theories of entangled rod–coil block copolymers are investigated in the isotropic melt state. Self-diffusion was measured by forced Rayleigh scattering using a red laser line and a new blue photoswitchable dye that allow operation above the bandgap of most semiconducting polymers. In contrast to previous tracer studies where the diffusion of rod–coils through a coil homopolymer matrix is slowed relative to coil homopolymers because of a mismatch in the curvature of the rod and coil entanglement tubes, slowed diffusion is only present in self-diffusion measurements above a critical molecular weight. An activated reptation mechanism with constraint release is proposed as a modification to...

Diffusion Mechanisms of Entangled Rod–Coil Diblock Copolymers

Mechanisms of entangled rod–coil diblock copolymer diffusion are investigated using tracer diffusion simulations and experiments in a matrix of entangled coil homopolymers, demonstrating that the d...

Phase Behavior of the Blend of Rod–Coil Diblock Copolymer and the Corresponding Coil Homopolymer

Blending with a homopolymer is an effective approach to tailor the microphase-separated morphology of block copolymers. It has been established for the blends of coil–coil diblock copolymer (A-b-B) with the corresponding homopolymer (h-A) that h-A can be solubilized uniformly into A mcirodomain to induce structure transformation when its molecular weight is smaller than that of the A block, i.e., α = Mh-A/Mb-A < 1. Here we examine if the microdomain structure of a rod–coil diblock copolymer, poly(2,5-diethylhexyloxy-1,4-phenylenevinylene)-block-poly(methyl methacrylate) (DEH-PPV-b-PMMA), may be systematically tuned by blending with PMMA hompolymer (h-PMMA). The blends over the major composition window were found to undergo macrophase separation even when the value of α was as low as 0.3. The phase separation led to the formation of a copolymer-rich phase and a homopolymer-rich phase, in which microphase separation occurred, yielding a well-ordered lamellar structure and a sponge structure, respectively. T...

Experimental measurement of coil-rod-coil block copolymer tracer diffusion through entangled coil homopolymers.

The diffusion of coil-rod-coil triblock copolymers in entangled coil homopolymers is experimentally measured and demonstrated to be significantly slower than rod or coil homopolymers of the same molecular weight. A model coil-rod-coil triblock was prepared by expressing rodlike alanine-rich α-helical polypeptides in E. coli and conjugating coillike poly(ethylene oxide) (PEO) to both ends to form coil-rod-coil triblock copolymers. Tracer diffusion through entangled PEO homopolymer melts was measured using forced Rayleigh scattering at various rod lengths, coil molecular weights, and coil homopolymer concentrations. For rod lengths, L, that are close to the entanglementh length, a, the ratio between triblock diffusivity and coil homopolymer diffusivity decreases monotonically and is only a function of L/a, in quantitative agreement with previous simulation results. For large rod lengths, diffusion follows an arm retraction scaling, which is also consistent with previous theoretical predictions. These experimental results support the key predictions of theory and simulation, suggesting that the mismatch in curvature between rod and coil entanglement tubes leads to the observed diffusional slowing.

Diffusion of Entangled Rod–Coil Block Copolymers

The diffusion of entangled rod-coil block copolymers is investigated by molecular dynamics (MD) simulations, and theories are introduced that describe the observed features and underlying physics. The reptation of rod-coil block copolymers is dominated by the mismatch between the curvature of the rod and coil entanglement tubes, which results in dramatically slower diffusion of rod-coils compared to the rod and coil homopolymers. For small rods, a local curvature-dependent free energy penalty results in a rough energy surface inside the entanglement tube, causing diffusivity to decrease with rod length. For large rods, rotational hindrances on the rod dominate, causing the coil block to relax by an arm retraction mechanism and diffusivity to decrease exponentially with coil size.

Characterization of Copolymers and Blends by Quintuple-Detector Size-Exclusion Chromatography

The properties imparted, oftentimes synergistically, by the different components of copolymers and blends account for the widespread use of these in a variety of industrial products. Most often, however, processing and end-use of these materials (especially copolymers) is optimized empirically, due to a lack of understanding of the physicochemical phase-space occupied by the macromolecules. Here, this shortcoming is addressed via a quintuple-detector size-exclusion chromatography (SEC) method consisting of multiangle static light scattering (MALS), quasi-elastic light scattering (QELS), differential viscometry (VISC), ultraviolet absorption spectroscopy (UV), and differential refractometry (DRI) coupled online to the separation method. Applying the SEC/MALS/QELS/VISC/UV/DRI method to the study of a poly(acrylamide-co-N,N-dimethylacrylamide) copolymer in which both monomer functionalities absorb in the same region of the UV spectrum, we demonstrate how to determine the chemical heterogeneity, molar mass averages and distribution, and solution conformation of the copolymer all in a single analysis. Additionally, through the various mutually independent conformational and architectural metrics provided by combining the five detectors, including the fractal dimension (derived from two different detector combinations), two different dimensionless size parameters, the chemical heterogeneity, and the persistence length, it is shown that the monomeric arrangement is more alternating than random at lower molar masses, thus causing the copolymer to adopt a more extended conformation in solution in this molar mass (M) regime. At high M, however, the copolymer is shown to be and to behave more like a random coil homopolymer, after passing through a 250 kg mol(-1)-broad region of intermediate chain flexibility. Thus, the combination of five detectors provides a unique means by which to determine absolute properties of the copolymer, solution-specific physical behavior, and the underlying chemical basis of the latter. The quintuple-detector method was also extended to the study of blends of polyacrylamide and poly(N,N-dimethylacrylamide) homopolymers to quantitate their molar mass, solution conformation, and chemical heterogeneity and to shed light on the breadth of the distributions of the component species. The method presented should be applicable to the study of copolymers and blends in which either one or both component moieties or polymers absorb in the UV region and can be implemented using not only SEC but other size-based separation methods as well.

Self-Assembly of Rod−Coil−Rod Triblock Copolymer and Homopolymer Blends

The self-assembly structures and their transformation of blends of rod−coil−rod triblock copolymers and homopolymers were first detected. Adding coil homopolymers into block copolymers with a lamellar structure resulted in an increase of d-spacing. When coil homopolymers were added into triblock copolymer with a rod continuous hexagonally packed-cylinder structure, lamellar structures were obtained, and the lamellar d-spacing were smaller than that of the original structure. The addition of rod-like homopolymer into the block copolymers with a lamellar structure also resulted in the increase of the d-spacing. As a result of the interdigitation, the rod blocks must rearrange to a larger angle with respect to the interface.

Domain Size Control in Self-Assembling Rod−Coil Block Copolymer and Homopolymer Blends

The addition of homopolymers to a self-assembling rod−coil block copolymer is demonstrated to be a flexible route toward domain size control. Molecular weight matched rod-like homopolymers interdigitate with the rod-blocks within their respective lamellae. As a result of the interdigitation, the coil blocks must rearrange to occupy more interfacial area resulting in an unprecedented decrease in domain spacing with increasing rod homopolymers. Conversely coil homopolymers were locally solubilized within the coil microdomain resulting in an increase of domain spacing with increasing coil homopolymers. The mechanisms of homopolymers solubilization are in qualitative agreement with predictions made by self-consistent mean-field theory (SCFT) calculations.

Side-Chain Liquid Crystalline Block Copolymers with Well Defined Structures Prepared by Living Anionic Polymerization IV. Microphase Morphology in Blends with Coil Homopolystyrenes

The microdomain structure in the blends of liquid crystal (LC)-coil block copolymer with coil homopolymers was studied. The LC-coil block copolymer used is composed of poly(6-[4-(4’-methoxyphenyl) phenoxy] hexyl methacrylate) as a LC segment and polystyrene as a coil segment. It has the molecular weight Mn=47000 with the LC segment fraction of 45 wt%. As a coil homopolymer, three polystyrenes with different molecular weights of 37000, 26000, and 8200 were used. In all the blends with LC segment fractions of 35% to 10%, the microphase segregation is clearly recognized and LC segment in resulting microdomain undergoes the well defined crystal-SmA-isotropic phase transitions. The microdomain structure depends on both the molecular weight of homopolystyrene blended and the phase structure of LC segment. When the highest molecular weight polystyrene with Mn=37000 was blended (so called in a “dry-brush” regime), a lamellar type of morphology is invariably observed even if the composition and temperature are varied. In contrast, when the lowest molecular weight polystyrene with Mn=8200 was used (in a “wet-brush” regime), the type of morphology is significantly altered. At the isotropic temperatures of LC segment, the lamellar morphology is observed for the blends with LC contents from 45% to 35%, but the cylindrical or spherical domain becomes predominant with a decrease in the LC content. On the other hand, the lamellar morphology is commonly observed at the crystal temperatures. In some blends with the lower weight fractions of LC segment, hence, there can be seen the order–order transition from sphere or cylinder to lamella on decreasing temperature. This morphological transformation is caused by the formation of the layered structure that tends to orient perpendicularly to the interface of microdomain. The results show that the structural order of LC segment affects both the microdomain morphology and the solubility style of the homopolymer into the block copolymer.

Molecular dynamics simulation of a single-component molecular composite: poly(p-phenylene benzobisthiazole)/meta-poly(aryl ether ketone) block copolymer

Molecular dynamics simulations were performed with a model of a single-component molecular composite in the form of a block copolymer composed of poly( p -phenylene benzobisthiazole) rigid-rod and flexible meta -poly(aryl ether ketone) subunits. The molecular composite concept, applied to improve the compressive strength of rigid-rod polymers and to improve their solubility, relies on a uniform distribution of rods in a coil-like matrix. Pair distribution functions, orientation correlation functions and correlation volumes calculated from equilibrium dynamics trajectories of bulk copolymer, coil homopolymer and rigid-rod homopolymer systems imply that, while inter-rod spacing is only slightly increased in the copolymer, correlation of rod orientation is greatly reduced but to a somewhat lesser extent than previously found for a graft copolymer composed of identical subunits. Conformations of the flexible blocks extend to accommodate the partial rod alignment. The block copolymer topology appears to be a viable alternative to the hairy-rod graft copolymer as a single-component molecular composite.