Session G35: Padden Award Symposium

Photo-induced Melting of Semi-Crystalline Polymers via Azobenzene Isomerization

Incorporation of photo-isomerizable molecules into macromolecular systems is a powerful method to harness Angstrom-level geometry changes induced by light to drive macroscopic shape changes and force generation. The most commonly explored material platforms exploit azobenzene isomerization-driven disordering of nematic polymer networks. While these materials generally perform better than isotropic networks due to cooperative nanoscale organization, higher ordering of azo-molecules into crystalline lattices presents a potential route to further enhance photogenerated work output. To this end, we have explored semi-crystalline azo polymers as a means to enhance ordering while preserving the processability inherent to polymer systems. We show that upon UV light exposure these materials undergo an isothermal melting of crystalline lamellae due to trans-cis isomerization, and that crystalline order can be restored upon re-isomerization back to the trans state. Interestingly, this process is found to be strongly suppressed at temperatures sufficiently far below the melting point. Finally, using shear alignment, we demonstrate the fabrication of well-aligned crosslinked fibers wherein reversible and directional crystallization can be harnessed for photoactuation.   

Materials by design for hairy nanoparticle assemblies

A computational framework that combines machine learning and coarse-grained molecular dynamics (CGMD) simulations to tailor the mechanical properties of hairy nanoparticle assemblies (aHNP) is proposed. CGMD informed metamodel on PMMA-grafted nanocellulose assemblies revealed the necessity of having relatively low grafting density, high molecular weight, and high nanoparticle loading on achieving Pareto optimality. Utilizing theoretical scaling relationships derived from Daoud−Cotton theory, we identified the critical length scale (N_cr) governing Pareto optimality, originating based on conformational transition from concentrated to semi-dilute brush regime. We verified this finding by expanding our analysis to other common polymers, and quantified the role of polymer chemistry, backbone rigidity and side-group size of a polymer on the chain conformations and N_cr. Remarkably, normalization of the monomer radial distribution curves using N_cr and other key molecular parameters collapsed the curves for 110 distinct model aHNP systems to a universal curve governing the chain conformations in aHNPs. The novel modeling framework along with the new physical insights into the confinement and interface effects in aHNPs pave the way for superior designs of aHNPs.   

Multiscale Polymer and Nanoparticle Dynamics in Attractive Polymer Nanocomposite Melts

The addition of nanoparticles (NPs) to a polymer matrix, forming a polymer nanocomposite (PNC), has been used to extend and control macroscopic material properties. Many macroscopic properties are dictated by microscopic dynamic processes, including the dynamics of the polymer segments, chains, and NPs, which remain poorly understood in PNCs. This work experimentally explores the hierarchy of dynamics in model PNC melts with poly(2-vinylpyridine) and attractive NP-polymer interactions. In PNCs with 26-nm diameter NPs, we show that a bound polymer layer forms in solution and persists to the melt state. Using various techniques, we report that segmental relaxations near the NP interface are slower than in neat polymer and lead to polymer chain desorption that is ~10⁴ times slower than bulk diffusion. Interestingly, when the NP size is commensurate with the polymer segments (~2 nm), the dynamics are quite different. For example, polymer segments are slowed homogeneously, slow chain diffusion is quantitatively described by increased segmental friction, and the NPs diffuse anomalously fast. Overall, these results highlight the complex and synergistic effects of NPs and polymers in PNCs and provide insights to help design functional PNCs for a variety of applications.   

Polymers under Extreme Nanoconfinement

Polymers under nanoconfinement can show significantly deviated properties from bulk. Capillary Rise Infiltration (CaRI) can produce extreme polymer confinement by infiltrating the polymer into densely-packed nanoparticle (NP) films. Here, we use CaRI films to study physical and chemical properties of polymers under extreme nanoconfinement, including glass transition temperature (T_g), fragility, and thermal degradation. Polystyrene(PS)/SiO₂ and Poly(2-vinylpyridine)(P2VP)/SiO2 CaRI films were used as the model systems with weak and strong interfacial interactions. The degree of confinement was tuned by using polymers with different molecular weights and NPs with different sizes (forming pores with 3-30 nm average size). We show that T_g can increase by ~57 K for PS and ~100 K for P2VP confined in 3 nm pores. Furthermore, P2VP in CaRI films shows broader glass transition width but slightly lower fragility than bulk, indicating the gradient of dynamics in the pores. The thermal stability of both polymers are significantly enhanced in CaRI films with less char formation. The details of the process shows that the activation energy of PS degradation can increase by ~50 kJ/mol under nanoconfinement.   

Deformation response of a two-dimensional polymer

Kinetoplast DNA is the mitochondrial DNA of trypanosomes and related parasites. A kinetoplast is a complex two-dimensional network of topologically interlinked circular DNA. There has been much interest in studying the kinetoplast from a microbiology point of view, but little is known about the material properties of the network structure. The stretching response of a one-dimensional linear polymer has been studied extensively using single-molecule experiments. In this work, we probe experimentally the deformation response of the kinetoplast as a model two-dimensional polymer. We subject kinetoplasts to planar elongational fields in microfluidic channels and study the dynamics of kinetoplasts under a constant strain rate, as well as the relaxation dynamics of kinetoplasts.   

Decoupling the role of entanglements and mobility in the mechanics of ultrathin polymer glasses

As polymer glasses are processed into ultrathin films, neighboring polymer chains become less entangled, and surface-bound chains with altered states of mobility play an increasingly important role. Such changes in physical properties have long been studied, but changes in mechanical strength and deformation processes have remained difficult to quantify. We have developed a method to directly measure the uniaxial stress-strain response of ultrathin glassy polymer films of both liquid supported and freestanding films. In our work, we quantify the influence of thickness (10nm - 360nm), and molecular weight (61kDa - 2135kDa) on the deformation and failure response of ultrathin polystyrene films. We observe a molecular weight independent thickness-transition in strain localization and elastic modulus, and a molecular weight dependent decrease in maximum stress. We associate the changes in strain localization and elastic modulus to the surface mobile layer, and we form a model to capture the role of the average molecule size and the number of entanglements per chain on the decrease in maximum stress. These results provide new fundamental insights into how polymer behavior is altered due to changes in the entanglements and mobility in a polymer network upon confinement.   

Influence of Polymer Polarity on Ion Transport in Polymer Electrolytes

Polymer electrolytes are materials that could improve the safety and performance of lithium-ion and -metal batteries. Despite their promising attributes, such as high shear modulus and electrochemical stability, they tend to have sluggish ionic transport. Several properties have been shown to improve ionic transport, including host segmental dynamics. Recently, we showed that host polymer polarity, measured by the dielectric constant, is also an influencing factor in ionic transport. We found that higher polarity polymers more effectively break up ionic aggregates, which reduces correlated ionic motion. However, our study focused on polymers with a narrow dielectric constant range (~3–7), which prohibited a general understanding of the extent to which improving polarity could improve ionic transport.

In this work, we turn to a coarse-grained simulation model with which we explore a wide range of polymer polarities and simultaneously explore the influence of polymer molecular weight and salt concentration. We show that ionic transport maximizes at intermediate polarity due to a competition between reduced ionic aggregation and slowed polymer dynamics. We demonstrate that ionic transport only couples to molecular weight and salt concentration once a threshold polarity is exceeded.   

Solvation-Site and Dielectric Control of Ion Conduction in Polymer Electrolytes

Solid polymer electrolytes have the promise of improving safety and performance in energy storage devices. Metal cation-containing polymers provide a path to effective electrolytes, with dynamic metal-ligand interactions allowing mechanical properties and ionic conductivity to be widely and separately tuned. A modular synthetic platform based on thiol-ene click chemistry is presented that allows polymers to be post-functionalized with a variety of metal-binding ligands. This well-controlled model system has enabled the study of factors known or suspected to influence ionic conductivity, including segmental dynamics, dielectric constant, solvation site density, and ligand identity. Design rules based on this model platform have been developed by combining experimental results with field-theory–based simulations. The results underscore that low dielectric constant is not necessarily detrimental to ionic conductivity, especially in systems where the resulting ion aggregates form percolating domains.   

Self-assembly of Salt-Doped Ternary Polymer Blends

Ternary polymer blends comprising A and B homopolymers and an A-B block copolymer can self-assemble into various microstructures, depending on the polymer composition, chain architecture and temperature. When doped with salt, such blends are promising for applications such as lithium ion batteries due to the tunable morphology. The addition of lithium bis(trifluoromethane)sulfonamide (LiTFSI) to a polymer blend system containing low molar mass poly(ethylene oxide) (PEO) and polystyrene (PS) homopolymers, and a PS−PEO block copolymer (SEO) induces either macroscopic or microscopic phase separation. Blends with different total homopolymer compositions (fH) and volume ratios of PEO and PS homopolymer (fPEO/fPS) at specific temperatures are investigated. In some regions of the phase prism, addition of salt recovers the microphase separated structures of conventional ternary blends, and in particular creates a wide bicontinuous microemulsion (BμE) channel. With still higher fH, a surprising C15 Laves phase is found. This work will help understand the self-assembly of ion-containing A/B/A-B ternary polymer blends and guide the experimental design of polyelectrolyte systems with tunable nanostructures.   

SANS Partial Structure Factor Analysis for Determining Protein-Polymer Interactions in Semidilute Solution

Protein-polymer interactions play a crucial role in many processes, including protein crystallization, biofouling, and self-assembly of protein-polymer bioconjugates. However, it is often difficult to measure these interactions in multicomponent systems, especially in semidilute solutions. Here, contrast-variation small-angle neutron scattering (CV-SANS) was used to quantify the interactions between three water-soluble polymers (PNIPAM, POEGA, and PDMAPS) and a model protein mCherry. CV-SANS enables decomposition of SANS intensities into partial structure factors that describe polymer-polymer, protein-protein, and polymer-protein interactions. The three polymers span various chemistries and properties, with PNIPAM and POEGA being non-ionic polymers with different hydrogen-bonding capabilities and PDMAPS being a zwitterionic polymer. PNIPAM/mCherry interactions were repulsive and dominated by depletion forces. In contrast, POEGA/mCherry interactions are attractive, with polymer enrichment at the protein surface. PDMAPS/mCherry interactions are more complex, with both depletion and electrostatic contributions. CV-SANS thus represents a powerful method to separate, quantify, and reveal the nature of interactions in multicomponent systems.   

Self-regulating metal cross-linked hydrogels via competition

Polymer networks with dynamic cross-links have generated widespread interest as tunable and responsive viscoelastic materials. However, narrow stoichiometric limits in cross-link compositions are typically imposed in the assembly of these materials to prevent excess free cross-linker from dissolving the resulting polymer networks. Using both computational and experimental methods, we demonstrate how the presence of molecular competition allows for vast expansion of the previously limited range of cross-linker concentrations that result in robust network assembly. Specifically, we use metal-coordinate cross-linked gels to verify that stoichiometric excessive metal ion cross-linker concentrations can still result in robust gelation when in the presence of free ion competing ligands, and we offer a theoretical framework to describe the coupled dynamic equilibria that result in this effect. We believe the insights presented here can be generally applied to advance engineering of the broadening class of polymer materials with dynamic cross-links.   

Deciphering Low-Temperature Dielectric Relaxation of a Series of Amorphous Polymers

In contrast to the crystalline solid and gaseous phases, the physics behind the behavior of liquids and amorphous solids remains a significant challenge. Current descriptions of relaxation behavior in these materials are highly empirical, either utilizing fitting functions with tenuous physical significance, or fitting to a spectrum of Debye relaxation processes, where typical procedures implicitly make the unphysical assumption that the spectral density with respect to the characteristic times is constant and unvarying with temperature. Recently, we have found that the relaxation behavior of a moderately cross-linked epoxy resin is well described by a relaxation spectrum where the spectral strengths of individual components are constant while the spectral density is non-uniform and varies with temperature. The sub-Tg γ-relaxation is well described by a constant strength spectrum spanning over roughly 12 orders of magnitude in the frequency domain. In this work we use this newly developed approach to study the γ-relaxation of a series of epoxy resins with differing crosslink densities. From this, we develop maps showing the temperature dependence of the relaxation spectra, with which we aim to pinpoint the effect of crosslinking on the relaxation behavior of these materials.