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 the description of entangled rod–coil block copolymer dynamics, where the slowing occurs when the time scale of rod block reptation is faster than the reorganization of the surrounding entanglement tube. This mechanism is supported by additional tracer diffusion experiments on polyalanine-<i>b</i>-poly­(ethylene oxide) diblocks in aqueous entangled poly­(ethylene oxide) matrix solutions and Kremer–Grest simulations where the matrix molecular weight is varied. The slowing of tracer diffusion in rod–coil block copolymers relative to coil homopolymers is significantly weaker for smaller matrix polymers, confirming the proposed constraint release effects

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 diffusion mechanisms first identified in coil–rod–coil triblocks are universal for various molecular architectures. Diffusion measurements were performed using both Kremer–Grest molecular dynamics simulations and forced Rayleigh scattering experiments. In the large rod regime, diffusivity decreases exponentially with increasing coil size as predicted by an arm retraction mechanism. The ratio of diblock to rod homopolymer diffusivity was approximately equal to the ratio for triblocks squared, suggesting that the two coil blocks of the coil–rod–coil triblock relax independently. In the small rod regime, both experiments and simulation show that the slowing of diffusion with increasing rod length is the same for rod–coil diblock and coil–rod–coil triblock copolymers. This behavior occurs because both types of molecules reptate through tubes with Gaussian statistics, so diffusional slowing results from entropic barriers to rod reptation through curved sections of the entanglement tube

Anomalous Self-Diffusion and Sticky Rouse Dynamics in Associative Protein Hydrogels

Natural and synthetic materials based on associating polymers possess diverse mechanical behavior, transport properties and responsiveness to external stimuli. Although much is known about their dynamics on the molecular and macroscopic level, knowledge of self-diffusive dynamics of the network-forming constituents remains limited. Using forced Rayleigh scattering, anomalous self-diffusion is observed in model associating protein hydrogels originating from the interconversion between species that diffuse in both the molecular and associated state. The diffusion can be quantitatively modeled using a two-state model for polymers in the gel, where diffusivity in the associated state is critical to the super diffusive behavior. The dissociation time from bulk rheology measurements was 2–3 orders of magnitude smaller than the one measured by diffusion, because the former characterizes submolecular dissociation dynamics, whereas the latter depicts single protein molecules completely disengaging from the network. Rheological data also show a sticky Rouse-like relaxation at long times due to collective relaxation of large groups of proteins, suggesting mobility of associated molecules. This study experimentally demonstrates a hierarchy of relaxation processes in associating polymer networks, and it is anticipated that the results can be generalized to other associative systems to better understand the relationship of dynamics among sticky bonds, single molecules, and the entire network

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 <i>E. coli</i> and conjugating coil-like poly­(ethylene oxide) (PEO) to both ends to form coil–rod–coil triblock copolymers. Tracer diffusion through entangled PEO homopolymer solutions was measured using forced Rayleigh scattering at various rod lengths and coil molecular weights for the tracer, and various concentrations for the coil homopolymer solutions. For rod lengths, <i>L</i>, that are close to the entanglementh length, <i>a</i>, the ratio between the diffusivity of a triblock and the diffusivity of a coil homopolymer of the same molecular weight decreases monotonically and is only a function of <i>L</i>/<i>a</i>, 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

<i>In Situ</i> Investigations of Microstructure Formation in Interpenetrating Polymer Networks

Interpenetrating polymer networks (IPNs) represent an effective strategy for compatibilizing immiscible polymers to enhance the mechanical properties of the final material. While it has been established that the macroscopic properties are dependent on the microstructure, it is unknown why various microstructures are formed in IPNs because the microstructure is often trapped in a nonequilibrium state. To explore this, we conducted a study to establish a relationship between polymerization kinetics and microstructure formation in polydimethylsiloxane/poly(methyl methacrylate) (PDMS/PMMA) IPNs. By manipulating the UV curing intensity, we observed three distinct morphologies: isolated PMMA-rich spheres within a PDMS matrix with a monomodal domain size distribution, spheres with a bimodal size distribution, and a clustered domain microstructure. To investigate the different phase separation mechanisms, we correlated in situ small-angle X-ray scattering (SAXS) to track microstructure formation and Fourier transform infrared spectroscopy (FT-IR) to track polymerization kinetics. Based on our findings, we propose that the monomodal sphere microstructure formed via spinodal decomposition. The positions of the domains are kinetically trapped in the PDMS network, preventing macrophase separation. Similarly, the clustered domain microstructure also arises from spinodal decomposition, but increased mobility within the PDMS matrix enables domains to aggregate after network percolation. In contrast, the bimodal spherical morphology is attributed to a combination of nucleation and growth, and spinodal decomposition. We postulate that these different mechanisms are dictated by changes in the PMMA molecular weight during polymerization. Through the examination of polymerization kinetics and microstructure formation, we have proposed multiple mechanisms that explain the microstructure formation in IPNs

Crossover Experiments Applied to Network Formation Reactions: Improved Strategies for Counting Elastically Inactive Molecular Defects in PEG Gels and Hyperbranched Polymers

Molecular defects critically impact the properties of materials. Here we introduce a paradigm called “isotopic labeling disassembly spectrometry” (ILDaS) that facilitates unprecedented precise experimental correlations between elastically inactive network defects (dangling chains and primary loops) and network formation kinetics and precursor structure. ILDaS is inspired by classical crossover experiments, which are often used to interrogate whether a reaction mechanism proceeds via an inter- or intramolecular pathway. We show that if networks are designed from labeled bifunctional monomers that transfer their labels to multifunctional junctions upon network formation, then the extent of junction labeling correlates directly with the number of dangling chains and cyclic imperfections within the network. We demonstrate two complementary ILDaS approaches that enable defect measurements with short analysis times, low cost, and synthetic versatility applicable to a broad range of network materials including polydisperse polymer precursors. The results will spur new experimental and theoretical investigations into the interplay between polymer network structure and properties