Molecular Characterization of Polymer Networks.

Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.

Molecular design of self-coacervation phenomena in block polyampholytes.

Coacervation is a common phenomenon in natural polymers and has been applied to synthetic materials systems for coatings, adhesives, and encapsulants. Single-component coacervates are formed when block polyampholytes exhibit self-coacervation, phase separating into a dense liquid coacervate phase rich in the polyampholyte coexisting with a dilute supernatant phase, a process implicated in the liquid-liquid phase separation of intrinsically disordered proteins. Using fully fluctuating field-theoretic simulations using complex Langevin sampling and complementary molecular-dynamics simulations, we develop molecular design principles to connect the sequenced charge pattern of a polyampholyte with its self-coacervation behavior in solution. In particular, the lengthscale of charged blocks and number of connections between oppositely charged blocks are shown to have a dramatic effect on the tendency to phase separate and on the accessible chain conformations. The field and particle-based simulation results are compared with analytical predictions from the random phase approximation (RPA) and postulated scaling relationships. The qualitative trends are mostly captured by the RPA, but the approximation fails catastrophically at low concentration.

Nanolatticed Architecture Mitigates Damage in Shark Egg Cases.

Structural versatility and multifunctionality of biological materials have resulted in countless bioinspired strategies seeking to emulate the properties of nature. The nanostructured egg case of swell sharks is one of the toughest permeable membranes known and, thus, presents itself as a model system for materials where the conflicting properties, strength and porosity, are desirable. The egg case possesses an intricately ordered structure that is designed to protect delicate embryos from the external environment while enabling respiratory and metabolic exchange, achieving a tactical balance between conflicting properties. Herein, structural analyses revealed an enabling nanolattice architecture that constitutes a Bouligand-like nanoribbon hierarchical assembly. Three distinct hierarchical architectural adaptations enhance egg case survival: Bouligand-like organization for in-plane isotropic reinforcement, noncylindrical nanoribbons maximizing interfacial stress distribution, and highly ordered nanolattices enabling permeability and lattice-governed toughening mechanisms. These discoveries provide fundamental insights for the improvement of multifunctional membranes, fiber-reinforced soft composites, and mechanical metamaterials.

Small ion effects on self-coacervation phenomena in block polyampholytes.

Self-coacervation is a phenomenon in which a solution of polyampholytes spontaneously phase separates into a dense liquid coacervate phase, rich in the polyampholyte, coexisting with a dilute supernatant phase. Such coacervation results in the formation of membraneless organelles in vivo and has further been applied industrially as synthetic encapsulants and coatings. It has been suggested that coacervation is primarily driven by the entropy gain from releasing counter-ions upon complexation. Using fully fluctuating field-theoretic simulations employing complex Langevin sampling and complementary molecular dynamics simulations, we have determined that the small ions contribute only weakly to the self-coacervation behavior of charge-symmetric block polyampholytes in solution. Salt partitioning between the supernatant and coacervate is also found to be negligible in the weak-binding regime at low electrostatic strengths. Asymmetries in charge distribution along the polyampholytes can cause net-charges that lead to "tadpole" configurations in dilute solution and the suppression of phase separation at low salt content. The field and particle-based simulation results are compared with analytical predictions from the random phase approximation (RPA) and postulated scaling relationships. The qualitative trends are mostly captured by the RPA, but the approximation fails at low concentration.

Light-Induced Living Polymer Networks with Adaptive Functional Properties.

The advent of covalent adaptable networks (CANs) through the incorporation of dynamic covalent bonds has led to unprecedented properties of macromolecular systems, which can be engineered at the molecular level. Among the various types of stimuli that can be used to trigger chemical changes within polymer networks, light stands out for its remote and spatiotemporal control under ambient conditions. However, most examples of photoactive CANs need to be transparent and they exhibit slow response, side reactions, and limited light penetration. In this vein, it is interesting to understand how molecular engineering of optically active dynamic linkages that offer fast response to visible light can impart "living" characteristics to CANs, especially in opaque systems. Here, the use of carbazole-based thiuram disulfides (CTDs) that offer dual reactivity as photoactivated reshuffling linkages and iniferters under visible light irradiation is reported. The fast response to visible light activation of the CTDs leads to temporal control of shape manipulation, healing, and chain extension in the polymer networks, despite the lack of optical transparency. This strategy charts a promising avenue for manipulating multifunctional photoactivated CANs in a controlled manner.

Aqueous Formulation of Concentrated Semiconductive Fluid Using Polyelectrolyte Coacervation.

Conjugated polyelectrolytes (CPEs), which combine π-conjugated backbones with ionic side chains, are intrinsically soluble in polar solvents and have demonstrated tunability with respect to solution processability and optoelectronic performance. However, this class of polymers often suffers from limited solubility in water. Here, we demonstrate how polyelectrolyte coacervation can be utilized for aqueous processing of conjugated polymers at extremely high polymer loading. Sampling various mixing conditions, we identify compositions that enable the formation of complex coacervates of an alkoxysulfonate-substituted PEDOT (PEDOT-S) with poly(3-methyl-1-propylimidazolylacrylamide) (PA-MPI). The resulting coacervate is a viscous fluid containing 50% w/v polymer and can be readily blade-coated into films of 4 ± 0.5 µm thick. Subsequent acid doping of the film increased the electrical conductivity of the coacervate to twice that of a doped film of neat PEDOT-S. This higher conductivity of the doped coacervate film suggests an enhancement in charge carrier transport along PEDOT-S backbone, in agreement with spectroscopic data, which shows an enhancement in the conjugation length of PEDOT-S upon coacervation. This study illustrates the utilization of electrostatic interactions in aqueous processing of conjugated polymers, which will be useful in large-scale industrial processing of semiconductive materials using limited solvent and with added enhancements to optoelectronic properties.

Ion Pairing and the Structure of Gel Coacervates.

A scaling model for the structure of coacervates is presented for mixtures of oppositely-charged polyelectrolytes of both symmetric and asymmetric charge-densities for different degrees of electrostatic strength and levels of added salt. At low electrostatic strengths, weak coacervates, with the energy of electrostatic interactions between charges less than the thermal energy, k B T, are liquid. At higher electrostatic strengths, strong coacervates are gels with crosslinks formed by ion pairs of opposite charges bound to each other with energy higher than k B T. Charge-symmetric coacervates are formed for mixtures of oppositely-charged polyelectrolytes with equal and opposite charge-densities. While charge-symmetric weak coacervates form a semidilute polymer solution with a correlation length equal to the electrostatic blob size, charge-symmetric strong coacervates form reversible gels with a correlation length on the order of the distance between bound ion pairs. Charge-asymmetric coacervates are formed from mixtures of oppositely-charged polyelectrolytes with different charge-densities. While charge-asymmetric weak coacervates form double solutions with two correlation lengths and qualitatively different chain conformations of polycations and polyanions, charge-asymmetric strong coacervates form bottlebrush and star-like gels. Unlike liquid coacervates, for which an increase in the concentration of added salt screens electrostatic interactions, causing structural rearrangement and eventually leads to their dissolution, the salt does not affect the structure of strong coacervates until ion pairs dissociate and the gel disperses.

Insensitivity of Sterically Defined Helical Chain Conformations to Solvent Quality in Dilute Solution.

The interplay between polymer-polymer and polymer-solvent interactions as well as interactions that impose secondary structures determines the conformation of polymer chains in dilute solution. Polypeptoids-poly(N-substituted glycines) have been shown to form helical secondary structures primarily driven by steric interactions from chiral, bulky side chains, while polypeptoids with a racemic mixture of the same side chains lead to unstructured coil chains with a shorter Kuhn length. Small-angle neutron scattering (SANS) of the polypeptoids in dilute solution reveals that the helical polypeptoids are only locally stiffer than the coil chains formed from the racemic analogue, but exhibit overall flexibility. We show that chain conformations of both helical and coil polypeptoids (in terms of radius of gyration, Rg) are insensitive to solvent quality (parametrized by the second virial coefficient, A2). Potential effects from the bulky, chiral/racemic side chains dominating chain conformations are excluded by comparison with an achiral polypeptoid lacking side chain chirality. The specific interactions between polypeptoid segments are likely dominating the chain conformations in this type of polypeptoids as opposed to polymer-solvent interactions or energetic contributions from the helical secondary structure.

Complexation of a Conjugated Polyelectrolyte and Impact on Optoelectronic Properties.

Electrostatic assembly of conjugated polyelectrolytes, which combine a π-conjugated polymer backbone with pendant ionic groups, offer an opportunity for tuning materials properties and a new route for formulating concentrated inks for printable electronics. Complex coacervation, a liquid-liquid phase separation upon complexation of oppositely charged polyelectrolytes in solution, is used to form dense suspensions of π-conjugated material. A model system of a cationic conjugated polyelectrolyte poly(3-[6'-{N-butylimidazolium}hexyl]thiophene) bromide and sodium poly(styrenesulfonate) dissolved in tetrahydrofuran-water mixtures was used to investigate this complexation behavior of conjugated polyelectrolytes in terms of electrostatic strength, solvent quality, and polymer concentration. The balance of electrostatic interaction between the oppositely charged polyelectrolytes together with their charge compensating counterions and solvent quality for the hydrophobic π-conjugated backbone leads to a rich phase diagram of soluble complexes, precipitates, and complex coacervates. The conjugated polyelectrolyte in the polyelectrolyte complexes has an increased π-conjugation length and enhanced emissivity, with ideal chain configurations due to the reduction of kink sites and torsional disorder. The advantageous photophysical properties in the dense liquid phases makes the scheme attractive for the large-scale processing of optoelectronic devices, chemical sensors, and bioelectronics components.

Absence of Electrostatic Rigidity in Conjugated Polyelectrolytes with Pendant Charges.

The delocalization of electrons in conjugated polymers impacts their chain shape, affecting their local ordering, self-assembly, and ultimately charge transport. Conjugated polyelectrolytes introduce electrostatic interactions as a molecular design parameter to potentially tune chain rigidity by combining the π-conjugated polymer backbone with pendant ionic groups. In conventional polyelectrolytes, the self-repulsion of the bound charges induce extended rod-like chain configurations. Here, we leverage small-angle neutron scattering to measure the chain shapes of model conjugated polymers in dilute solution with controlled fractions of randomly distributed pendant charges. We find these model polythiophenes are semiflexible, with a persistence length of approximately 3 nm, regardless of charge fraction, suggesting the effective absence of electrostatic rigidity in conjugated polyelectrolytes. While the overall persistence length is negligibly impacted by pendant charges, optical spectroscopy indicates that the pendant charges increase the backbone torsion between thiophene rings without significantly impacting the π-conjugation length (the length of electron delocalization along a nearly planar backbone) in dilute solution. These results indicate the effective decoupling of the pendant ionic charges from the overall chain conformation with implications for solution processing of organic semiconductors.

Complete Phase Diagram for Liquid-Liquid Phase Separation of Intrinsically Disordered Proteins.

A number of intrinsically disordered proteins have been shown to self-assemble via liquid-liquid phase separation into protein-rich and dilute phases. The resulting coacervates can have important biological functions, and the ability to form these assemblies is dictated by the protein's primary amino acid sequence as well as by the solution conditions. We present a complete phase diagram for the simple coacervation of a polyampholyte intrinsically disordered protein using a field-theoretic simulation approach. We show that differences in the primary amino acid sequence and in the distribution of charged amino acids along the sequence lead to differences in the phase window for coacervation, with block-charged sequences having a larger coacervation window than sequences with a random patterning of charges. The model also captures how changing solution conditions modifies the phase diagram and can serve to guide experimental studies.