Influence of Cohesive Energy and Chain Stiffness on Polymer Glass Formation

Generalized entropy theory investigation of the relatively high segmental fragility of many glass-forming polymers

We utilize the generalized entropy theory (GET) of glass formation to address one of the most singular and least understood properties of polymer glass-forming liquids in comparison to atomic and small molecule liquids-the often relatively high fragility of the polymer dynamics on a segmental scale, ms. Based on this highly predictive framework of both the thermodynamics and segmental dynamics in terms of molecular structure, polymer backbone and side-group rigidities, and intermolecular interaction strength, we first analyze the relation between ms and the ratio, , where Sc is the configurational entropy density of the polymer fluid, equals Sc at the onset temperature TA for non-Arrhenius relaxation, and Tg is the glass transition temperature at which the structural relaxation time τα equals 100 s. While the reduced activation energy estimated from an Arrhenius plot (i.e., differential activation energy) normalized by kBTg is determined to be not equal to the actual activation energy, we do find that an apparently general nonlinear relation between ms and holds to a good approximation for a large class of polymer models, . The predicted ranges of ms and are consistent with experimental estimates for high molecular-mass polymer, oligomeric, small molecule, and atomic glass-forming liquids. In particular, relatively high values of ms are found for polymers having complex monomer structures and significant chain stiffness. The variation of ms with molecular mass, chain stiffness, and intermolecular interaction strength can be traced to the variation of , which is shown to provide a measure of packing frustration defined in terms of the dimensionless thermal expansion coefficient and isothermal compressibility. The often relatively high fragility and large extent of cooperative motion are found in the GET to derive from the often relatively large packing frustration in this class of polymer glass-forming liquids. Finally, we also develop a tentative model of the "dynamical segmental relaxation time" based on the GET, in which the polymers on a coarse-grained scale are modeled as strings of structureless "beads", as assumed in the Rouse and reptation models of polymer dynamics.

Alternatives to Fluoropolymers for Motion-Based Energy Harvesting: Perspectives on Piezoelectricity, Triboelectricity, Ferroelectrets, and Flexoelectricity

Fluoropolymers, including polytetrafluoroethylene (PTFE, Teflon), polyvinylidene difluoride (PVDF), and fluorine kautschuk materials (FKMs, Viton) are critical polymers for applications ranging from non-stick coatings, corrosion resistant seals, semiconductor manufacturing, membranes, and energy harvesting technologies. However, the synthesis of these fluoropolymers requires the use of per- and polyfluorinated alkyl substances (PFAS) known colloquially as "forever chemicals," and as such there is a pressing need to develop alternative technologies that can serve the end-use of fluoropolymers without the environmental cost of using PFAS. Further, fluoropolymers themselves fall under the PFAS umbrella. Here, alternative mechanical-to-electrical energy harvesting polymers are reviewed and benchmarked against the leading fluoropolymer energy harvesters. These alternative technologies include nonfluoropolymer piezoelectric polymers, triboelectric nanogenerators (TENGs), ferroelectric elastomers, and flexoelectric polymers. A vision towards sustainable, non-fluoropolymer-based energy harvesting is provided.

Confinement effect of inter-arm interactions on glass formation in star polymer melts

We utilized molecular dynamic simulation to investigate the glass formation of star polymer melts in which the topological complexity is varied by altering the number of star arms (f). Emphasis was placed on how the "confinement effect" of repulsive inter-arm interactions within star polymers influences the thermodynamics and dynamics of star polymer melts. All the characteristic temperatures of glass formation were found to progressively increase with increasing f, but unexpectedly the fragility parameter KVFT was found to decrease with increasing f. As previously observed, stars having more than 5 or 6 arms adopt an average particle-like structure that is more contracted relative to the linear polymer size having the same mass and exhibit a strong tendency for intermolecular and intramolecular segregation. We systematically analyzed how varying f alters collective particle motion, dynamic heterogeneity, the decoupling exponent ζ phenomenologically linking the slow β- and α-relaxation times, and the thermodynamic scaling index γt. Consistent with our hypothesis that the segmental dynamics of many-arm star melts and thin supported polymer films should exhibit similar trends arising from the common feature of high local segmental confinement, we found that ζ increases considerably with increasing f, as found in supported polymer films with decreasing thickness. Furthermore, increasing f led to greatly enhanced elastic heterogeneity, and this phenomenon correlates strongly with changes in ζ and γt. Our observations should be helpful in building a more rational theoretical framework for understanding how molecular topology and geometrical confinement influence the dynamics of glass-forming materials more broadly.

Understanding the thermomechanical behavior of graphene-reinforced conjugated polymer nanocomposites via coarse-grained modeling

Graphene-reinforced conjugated polymer (CP) nanocomposites are attractive for flexible and electronic devices, but their mechanical properties have been less explored at a fundamental level. Here, we present a predictive multiscale modeling framework for graphene-reinforced poly(3-alkylthiophene) (P3AT) nanocomposites via atomistically informed coarse-grained molecular dynamics simulations to investigate temperature-dependent thermomechanical properties at a molecular level. Our results reveal reduced graphene dispersion with increasing graphene loading. Nanocomposites with shorter P3AT side chains, lower temperatures, and higher graphene content exhibit stronger mechanical responses, which correlates with polymer dynamics. The elastic modulus increases linearly with the graphene content, which slightly deviates from the "Halpin-Tsai" micromechanical model prediction. Local stiffness analysis shows that graphene possesses the highest stiffness, followed by the P3AT backbone and side chains. Deformation-induced stronger chain alignment of the P3AT backbone compared to graphene may further promote conductive behavior. Our findings provide insights into the dynamical heterogeneity of nanocomposites, paving the way for understanding and predicting their thermomechanical properties.

Prolonged Glass Transition due to Topological Constraints in Polyrotaxanes

Topological constraints in polyrotaxanes significantly affected their glass transition dynamics. The effects of the constraints were systematically studied using a series of different coverage glass-forming polyrotaxanes consisting of a common polymer and threaded ring molecule of varying ratios. Although their ratios were similar and hence exhibited similar T_g values by differential thermal analysis, mechanical relaxation was considerably prolonged with increasing coverage. The relaxation became a two-step process: a faster step at a common temperature near the T_g and another which was prolonged by the coverage increase. Relaxation dynamics analysis revealed that segment motions, which are cooperative translations of different components, freeze at considerably higher temperatures than the T_g with increasing coverage. This suggests that although the rings are released from conventional interactions at the T_g, their cooperative translational motions are significantly constrained by the threading polymers with increasing coverage.

Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and Its Relation to Relaxation

To understand the role of collective motion in the often large changes in interfacial molecular mobility observed in polymer films, we investigate the extent of collective motion in the interfacial regions of a thin supported polymer film and within the film interior by molecular dynamics simulation. Contrary to commonly stated expectations, we find that the extent of collective motion, as quantified by string-like molecular exchange motion, is similar in magnitude in the polymer-air interfacial layer as the film interior and distinct from the bulk material. This finding is consistent with Adam-Gibbs description of the segmental dynamics within mesoscopic film regions, where the extent of collective motion is related to the configurational entropy of the film as a whole rather than a locally defined extent of collective motion or configurational entropy.

Influence of molecular weight on molecular dynamics and dynamic rheology of polypropylene glycol filled with silica

Molecular weight strongly influences the molecular dynamics and rheological responses of nanocomposites, which is far from being well understood. Herein molecular dynamics and rheological behaviors of hydrophilic fumed silica filled unentangled polypropylene glycol (PPG) were investigated as a function of weight averaged molecular weight (M w) of PPG and volume fraction (∅) of silica. It is shown that M w does not affect the glassy layers surrounding the nanoparticles and the segmental dynamics of the mobile PPG phase. On the other hand, the mobile PPG phase in the highly filled nanocomposites exhibits an abnormal "more fragile" to "stronger" transition with increasing M w. The reinforcement and thinning behaviors are stronger in lower-M w nanocomposites with the "more fragile" mobile PPG phase. The results suggest that reinforcement of nanocomposites affects the dynamic fragility of the mobile phase of the matrix.

Side-group size effects on interfaces and glass formation in supported polymer thin films

Recent studies on glass-forming polymers near interfaces have emphasized the importance of molecular features such as chain stiffness, side-groups, molecular packing, and associated changes in fragility as key factors that govern the magnitude of T_g changes with respect to the bulk in polymer thin films. However, how such molecular features are coupled with substrate and free surface effects on T_g in thin films remains to be fully understood. Here, we employ a chemically specific coarse-grained polymer model for methacrylates to investigate the role of side-group volume on glass formation in bulk polymers and supported thin films. Our results show that bulkier side-groups lead to higher bulk T_g and fragility and are associated with a pronounced free surface effect on overall T_g depression. By probing local T_g within the films, however, we find that the polymers with bulkier side-groups experience a reduced confinement-induced increase in local T_g near a strongly interacting substrate. Further analyses indicate that this is due to the packing frustration of chains near the substrate interface, which lowers the attractive interactions with the substrate and thus lessens the surface-induced reduction in segmental mobility. Our results reveal that the size of the polymer side-group may be a design element that controls the confinement effects induced by the free surface and substrates in supported polymer thin films. Our analyses provide new insights into the factors governing polymer dynamics in bulk and confined environments.

Energy Renormalization for Coarse-Graining the Dynamics of a Model Glass-Forming Liquid

Coarse-grained modeling achieves the enhanced computational efficiency required to model glass-forming materials by integrating out "unessential" molecular degrees of freedom, but no effective temperature transferable coarse-graining method currently exists to capture dynamics. We address this fundamental problem through an energy-renormalization scheme, in conjunction with the localization model of relaxation relating the Debye-Waller factor ⟨u2⟩ to the structural relaxation time τ. Taking ortho-terphenyl as a model small-molecule glass-forming liquid, we show that preserving ⟨u2⟩ (at picosecond time scale) under coarse-graining by renormalizing the cohesive interaction strength allows for quantitative prediction of both short- and long-time dynamics covering the entire temperature range of glass formation. Our findings provide physical insights into the dynamics of cooled liquids and make progress for building temperature-transferable coarse-grained models that predict key properties of glass-forming materials.

Energy-Renormalization for Achieving Temperature Transferable Coarse-Graining of Polymer Dynamics

The bottom-up prediction of the properties of polymeric materials based on molecular dynamics simulation is a major challenge in soft matter physics. Coarse-grained (CG) models are often employed to access greater spatiotemporal scales required for many applications, but these models normally experience significantly altered thermodynamics and highly accelerated dynamics due to the reduced number of degrees of freedom upon coarse-graining. While CG models can be calibrated to meet certain properties at particular state points, there is unfortunately no temperature transferable and chemically specific coarse-graining method that allows for modeling of polymer dynamics over a wide temperature range. Here, we pragmatically address this problem by "correcting" for deviations in activation free energies that occur upon coarse-graining the dynamics of a model polymeric material (polystyrene). In particular, we propose a new strategy based on concepts drawn from the Adam-Gibbs (AG) theory of glass formation. Namely we renormalize the cohesive interaction strength and effective interaction length-scale parameters to modify the activation free energy. We show that this energy-renormalization method for CG modeling allows accurate prediction of atomistic dynamics over the Arrhenius regime, the non-Arrhenius regime of glass formation, and even the non-equilibrium glassy regime, thus allowing for the predictive modeling of dynamic properties of polymer over the entire range of glass formation. Our work provides a practical scheme for establishing temperature transferable coarse-grained models for predicting and designing the properties of polymeric materials.

Stringlike Cooperative Motion Explains the Influence of Pressure on Relaxation in a Model Glass-Forming Polymer Melt

Numerous experiments reveal that the dynamics of glass-forming polymer melts are profoundly influenced by the application of pressure, but a fundamental microscopic understanding of these observations remains incomplete. We explore the structural relaxation of a model glass-forming polymer melt over a wide range of pressures (P) by molecular dynamics simulation. In accord with experiments for nonassociating polymer melts and the generalized entropy theory, we find that the P dependence of the structural relaxation time (τ_α) can be described by a pressure analog of the Vogel-Fulcher-Tammann equation and that the characteristic temperatures of glass formation increase with P, while the fragility decreases with P. Further, we demonstrate that τ_α for various P can quantitatively be described by the string model of glass formation, where the enthalpy and entropy of activation are found to be proportional, an effect that is expected to apply to polymeric materials under various applied fields.

Effect of chain stiffness and temperature on the dynamics and microstructure of crystallizable bead-spring polymer melts

We contrast the dynamics in model unentangled polymer melts of chains of three different stiffnesses: flexible, intermediate, and rodlike. Flexible and rodlike chains, which readily solidify into close-packed crystals (respectively, with randomly oriented and nematically aligned chains), display simple melt dynamics with Arrhenius temperature dependence and a discontinuous change upon solidification. Intermediate-stiffness chains, however, are fragile glass-formers displaying Vogel-Fulcher dynamical arrest, despite the fact that they also possess a nematic-close-packed crystalline ground state. To connect this difference in dynamics to the differing microstructure of the melts, we examine how various measures of structure, including cluster-level metrics recently introduced in studies of colloidal systems, vary with chain stiffness and temperature. No clear static-structural cause of the dynamical arrest is found. However, we find that the intermediate-stiffness chains display qualitatively different dynamical heterogeneity. Specifically, their stringlike motion (cooperative rearrangement) is correlated along chain backbones in a way not found for either flexible or rodlike chains. This activated "crawling" motion is clearly associated with the dynamical arrest observed in these systems, and illustrates one way in which factors controlling the crystallization versus glass formation competition in polymers can depend nonmonotonically on chain stiffness.

Self-assembly and glass-formation in a lattice model of telechelic polymer melts: Influence of stiffness of the sticky bonds

Telechelic polymers are chain macromolecules that may self-assemble through the association of their two mono-functional end groups (called "stickers"). A deep understanding of the relation between microscopic molecular details and the macroscopic physical properties of telechelic polymers is important in guiding the rational design of telechelic polymer materials with desired properties. The lattice cluster theory (LCT) for strongly interacting, self-assembling telechelic polymers provides a theoretical tool that enables establishing the connections between important microscopic molecular details of self-assembling polymers and their bulk thermodynamics. The original LCT for self-assembly of telechelic polymers considers a model of fully flexible linear chains [J. Dudowicz and K. F. Freed, J. Chem. Phys. 136, 064902 (2012)], while our recent work introduces a significant improvement to the LCT by including a description of chain semiflexibility for the bonds within each individual telechelic chain [W.-S. Xu and K. F. Freed, J. Chem. Phys. 143, 024901 (2015)], but the physically associative (or called "sticky") bonds between the ends of the telechelics are left as fully flexible. Motivated by the ubiquitous presence of steric constraints on the association of real telechelic polymers that impart an additional degree of bond stiffness (or rigidity), the present paper further extends the LCT to permit the sticky bonds to be semiflexible but to have a stiffness differing from that within each telechelic chain. An analytical expression for the Helmholtz free energy is provided for this model of linear telechelic polymer melts, and illustrative calculations demonstrate the significant influence of the stiffness of the sticky bonds on the self-assembly and thermodynamics of telechelic polymers. A brief discussion is also provided for the impact of self-assembly on glass-formation by combining the LCT description for this extended model of telechelic polymers with the Adam-Gibbs relation between the structural relaxation time and the configurational entropy.

Glass-Transition and Side-Chain Dynamics in Thin Films: Explaining Dissimilar Free Surface Effects for Polystyrene vs Poly(methyl methacrylate)

Despite having very similar bulk properties such as glass-transition temperature (T_g), density, and fragility, polystyrene (PS) and poly(methyl methacrylate) (PMMA) exhibit characteristically different T_g depression in free-standing ultrathin films due to free surface effects. Here we explain this difference using our recently established chemistry-specific coarse-grained (CG) models for these two polymers. Models capture the dissimilar scaling of T_g with free-standing film thickness as seen in experiments and enable us to quantify the size of the regions near free surfaces over which chain relaxation exhibits differences from bulk. Most interestingly, vibrational density of states (VDOS) analysis uncovers a relationship between the amplitude of side-chain fluctuations, associated with side-chain flexibility and T_g-nanoconfinement. We discover that increasing backbone to side-chain mass ratio in CG models increases the amplitude of side-chain fluctuations and suppresses the free-surface effect on T_g. We show that mass distribution and side-chain flexibility are central to explain dissimilar free surface effects on PS and PMMA. Our model predictions are further corroborated by experimental evidence showing the role of mass distribution in styrene thin films. Our study ascertains the significance of molecular characteristics on nanoconfinement and highlights the ability for chemistry-specific CG models to explore the thermomechanical properties of polymer thin films.

Molecular relaxation and dynamic rheology of “cluster phase”-free ionomers based on lanthanum(iii)-neutralized low-carboxylated poly(methyl methacrylate)

La(iii)-neutralized low-carboxylated poly(methyl methacrylate)-based ionomers free of cluster phase exhibit a fluid-to-solid transition assigned to an interconnected multiplets network.

Synthesis, properties, and antibacterial activity of polyphosphonium semi-interpenetrating networks

Polyphosphonium semi-interpenetrating networks were prepared and studied as antibacterial surfaces to elucidate the structural aspects leading to bacterial killing.

Polyrotaxane Glass: Peculiar Mechanics Attributable to the Isolated Dynamics of Different Components

The molecular dynamics of condensed polymers are directly linked to practically important mechanical properties, such as glass transition behavior and impact strength. We present a unique viscoelastic glass-forming polyrotaxane that is comprised of a main chain polymer and threaded cyclodextrins. The abnormally broad glass transition phase is attributed to the segment motion, which is unprecedentedly insusceptible to cooperative motion with neighboring chains until vitrification is completed. Even in the glass state, in which the cyclic components, occupying >80 wt % of the material, are vitrified, the main chain polymer retains high mobility within the glassy porous framework. The anomalous mobility of the polymer chains causes strong subrelaxation that changed the elastic modulus about 3-fold. These results demonstrate that the unique dynamics and mechanics are attributable to the loose correlation between different components, suggesting the possible creation of unprecedented mechanical properties based on the molecular design of mechanically interlocked polymers.

Free Volume in the Melt and How It Correlates with Experimental Glass Transition Temperatures: Results for a Large Set of Polymers

There is a continuing, strong interest in making connections between the polymeric glass transition (T_g) and bulk properties. In this Letter we apply the Locally Correlated Lattice (LCL) model to study a group of 51 polymers and demonstrate two broad correlations. In the first, we show that the theoretically determined polymeric free volume in the melt, all at a single common T, P (425 K, 1 atm), correlates noticeably with the experimentally determined T_g values, and that this trend sharpens considerably when families of polymers are examined. Further, we show a strikingly linear correlation between the experimental T_g and the LCL model calculation for the percent free volume expected at the polymeric T_g. We suggest that this trend has a predictive value, acting as a boundary of T-dependent minimum-required free volume separating the melt and glassy regimes. Our theoretical estimates of free volume values at a polymer's T_g range between 4 and 16%, and their evident temperature dependence indicates an important role for temperature in glassification.