Session Y32: Polymer Structure and Dynamics Across Multiple Length and Timescales

Two-step, stratified crystallization in conjugated polymer thin films: the role of interfacial effects.

It is well known that crystallization of classical polymers is extremely sensitive to molecular characteristics and processing conditions, so even small changes can result in significantly different morphologies and properties. Likewise, charge transport in conjugated polymers—which are typically processed into thin films—critically depends on their semicrystalline structure, but the complex interplay between film interfaces, processing conditions, chain structure, and crystallization remains elusive and hinders the development of a predictive model to attain optimal properties. Here, we investigate the role of interfaces on solvent-free crystallization of poly-3-hexylthiophene (P3HT). In particular, we reveal a two-step crystallization process in P3HT films where the free surface induces distinct, highly edge-on oriented crystallization which occurs at temperatures much higher than for the underlayer and which remains confined within 20 nm of the air-polymer interface. We will discuss the potential of using interfacial effects to direct crystallization and morphology in films, as well as the role of molecular weight, regioregularity, and self-seeding.   

Elucidating the Rotator Phase in a Chemically Recyclable Polyolefin

Between the fully ordered crystal phase and isotropic liquid phase, there is a remarkable intermediate phase called rotator phase for chain molecules like alkanes. In this phase, chains have a long-range positional order but also have rotational freedom. The rotator phase has significance in both science and engineering. It plays an important role in crystal nucleation, lubrication behavior, and bioprocess of phospholipids. Recently, a novel family of recyclable hydrocarbon material, (1,n'-divinyl)oligocyclobutane (DVOCB) has been synthesized and found to have evidences of rotator phase. Different from the alkane chains, the rotator phase in DVOCB has a large temperature range and is stable in polymers, which allows wide applications. To investigate the rotator phase in DVOCB, we performed a comparison study of DVOCB and alkane with molecular dynamic simulations. It is found that the relaxation time of rotation has a distinct change from crystal phase to rotator phase. The timescale change in alkane chains is more dramatic than in DVOCB, because ring structure in DVOCB disrupts its chain axial correlation. This finding could also explain the rotator phase in other polymers, such as hydrogenated polynorbornene (hPN). In the end, this work provides fundamental understanding of rotator phase in DVOCB and illustrates design rules of rotator phase polymers.   

Shear Effects on Crystallization in Polyolefin Blends

Polyolefins, e.g., polyethylene (PE) and polypropylene (PP), constitute over half of the worldwide plastics production, but recycling rates for these materials remains low. Post-consumer PE and PP typically enter the recycling stream in mixture form and are processed together. PE/PP blends are often brittle due to thermodynamic incompatibilities that result micro-separated domains which crystallize separately upon cooling. The production of useful products from these materials requires an understanding of the fundamental parameters that affect crystallization during processing of polyolefin blends. In this work, we employ a multi-modal approach using a combination of rheo-Raman spectroscopy, differential scanning calorimetry, and optical and electron microscopy to explore the effects of shear on the morphology, crystallization kinetics, and rheological properties of blends of high-density PE and isotactic PP. Results show that flow-induced crystallization in the PP phase has a surprising composition dependence, which we attribute to the domain structure and micro-flow fields in the blends. These results underscore the importance crystallization and processing on the structure and properties of mixed recyclates.   

Quasi-Elastic Neutron Scattering Study on Dynamically Asymmetric Polymer Blends

Our current work explores how the segmental dynamics of poly(methyl acrylate) (PMA) chains are affected in the presence of other polymers of different miscibility and rigidity. Precisely, we studied using the differential scanning calorimetry (DSC) and quasi-elastic neutron scattering (QENS) dynamic heterogeneity in bulk blends of PMA within poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(ethylene oxide) (PEO), below and above the glass transition temperature of PMMA and PS. The double glass transition (T_g) for PMA/PMMA and PMA/PS blends, and the single transition for PMA/PEO indicate immiscible and miscible of the blends respectively. Also, the reduced cooperative motion was observed from the dynamic fragility measurement which suggests a corresponding reduced interchain cooperativity of PMA in the immiscible blends. Detailed QENS measurement revealed the findings: (i) an increased segmental jump distanced of PMA at lower temperatures, (ii) the suppression of effective diffusivity of PMA when in contact with PMMA or PS, and (ii) about ten times magnitude increase in the diffusivity of PMA in contact with PEO. These are explained in the context of free volume created by blending, the flexibility of chain environments, and the plasticizing effect of PEO respectively. These measurements were conducted in bulk, and hence confinement of the blends might seem worth exploring. We suffice that the miscibility will be improved by nanoconfinement and the expected dynamic response of the blends to deviate from that of the bulk. These findings will help us unravel the microscopic origin of property enhancement in these dynamically asymmetric blends with the addition of nanoparticles and pose a possible pathway for a wide range of applications of this system.     

Knot Theory Perspective to the Globular States of Linear Polymers

    Investigations into the globular states of linear polymers have traditionally utilized the density fluctuation model, distinguishing core-interface regions. However, under poor solvent conditions and near θ-temperature, computational results revealed a plateau region in the scaling of chain dimensions, with a Flory scaling exponent far below 1/3. Under these conditions, the density-based approach does not delineate the boundaries between distinct globular states due to the self-knotting phenomenon of chain, transcending the capabilities of mean-field methods as indicated by the Ginzburg criterion. By incorporating Molecular Dynamics simulations with knot analysis and comparing the results of linear chains to ring polymers (which lack chain ends and cannot form knots), we discovered that knot theory analysis can quantitatively distinguish between globular states. This offers a complementary perspective to traditional mean-field theories.

    This work was supported by NSF DMR 2114640   

Decoupled Main-Chain and Sticker Dynamics in Associating Comb Polymers

Associating polymer containing polar, acidic or ionic groups enable the design of responsive materials. Elucidating the lifetime (τ) and activation energy (E_a) of sticker association/dissociation is important in designing associating polymers. In this study of novel associating comb polymers, we use electrochemical impedance spectroscopy (EIS) to reveal an unexpected bifurcation of the subsegmental dynamics. This discovery enables us to directly measure the intrinsic τ and E_a of sticker association/dissociation process. This glassy phenomenon is attributed to the decoupled motions between the sulfur linkages adjacent to the backbone and the acid-terminated pendants and defines a new dynamic parameter, the bifurcation temperature T_b. T_b correlates strongly with the glass transition temperature (T_g) that is predominantly determined by pendant length rather than sticker concentration. Our results elucidate the critical role of polymer architecture and pendant length in the structural relaxations of glassy associating polymers.   

Theoretical study of the structure and thermodynamics of polymerized ionic liquids

We present a detailed study of ion-ion, ion-polymer and polymer-polymer real space pair correlations, their collective partial structure factor analogs, and the thermodynamic cohesive energy and bulk modulus of dense melts of polymerized ionic liquids (PILs) using PRISM integral equation theory with the modified-Verlet closure. Polymers are modeled as charged semiflexible chains, and mobile ions as spheres of sizes chosen to mimic Li, Na, Cs, Br, PF6 and TFSI. All species interact via a hard core repulsion plus variable strength screened Coulomb interactions. Rich interchain and collective correlations are predicted over a wide range of length scales with distinctive features such as ion-mediated bridging, intensity of monomer scale caging, Coulomb cage coordination number, and amplitude of longer length scale density fluctuations, which all depend on ion size, Coulomb interaction strength and packing fraction. Quantitative comparisons with the analogous neutral systems have also been determined. For some structural properties we find PILs fall in two qualitatively different classes of behavior corresponding to small and large ions. The results provide a foundation for creating microscopic theories of mobile ion diffusion, structural relaxation, and vitrification in PILs.   

Polyelectrolyte Solutions and Brushes

Polyelectrolytes exhibit unique solution properties compared to neutral polymers due to charge repulsion along the backbone that increases chain size and results in viscoelastic behavior even at low polymer concentration. Consequently, polyelectrolytes are extensively used in industrial applications, including as thickeners and rheology modifiers for aqueous coatings and flocculation agents for colloids and wastewater treatment. They also play a fundamental role in biological processes, including intracellular phase separation and joint lubrication. Polyelectrolytes may also be anchored onto surfaces to create brush architectures that offer flexible design parameters for imparting tailored interfacial functionality at the nanoscale. The influence of charge sequence and fraction on polyelectrolyte solution and brush behavior, however, is lacking. Here, we use solid phase peptide synthesis (SPPS) and surface-initiated copper(0) controlled radical polymerization (SI-CuCRP) to produce polymers with controlled sequence and charge fractions. Systematic studies using small-angle X-ray scattering (SAXS) and 3D single molecule tracking reveal that charge fraction and sequence influence polyelectrolyte solution conformation and phase behavior, as well as brush height and transport properties.   

Mechanisms for the creation of hierarchically structured block copolymer hydrogels via nonsolvent induced phase separation

Inspired by the extracellular matrix in biological tissues, researchers have envisioned a new generation of hydrogels that enable technologies like self-healing artificial tissues, soft bioelectronics, and bioresponsive actuators. However, replicating the complexity of biomaterials in synthetic systems requires control over both molecular structure and a hierarchical microstructure. Recently, our experimental collaborators have shown that nonequilibrium processing of hydrophobic-hydrophilic-hydrophobic ABA triblock copolymers creates hydrogels with diverse multiscale architectures. In this work, we strive to elucidate the theoretical mechanisms underpinning the formation of these microstructures. The task is not trivial, due to the presence of multiple dynamic modes across a wide range of length and time scales. We use multiple theoretical tools, including phase diagrams generated by a random phase approximation, 1D transport models, and phase-field models that incorporate Ohta-Kawasaki free energy functionals, to investigate the dynamics that leads to the nonequilibrium formation of hierarchical structures. Specifically, we find that the thermodynamic driving forces for microphase and macrophase separation in combination with mass transport are key elements to consider.   

Assembly and Dynamics of Random Heteropolymers in Aqueous Environments

Biological systems require immense complexity to sustain their inherent processes, but understanding and applying each of their intricacies to synthetic systems is extremely challenging. Synthetic random heteropolymers (RHPs) have recently been shown to capture the behaviors of proteins in biological environments by acting as ensembles with varying segmental properties. We hypothesize that RHPs can also leverage their side chain heterogeneity to capture the arrangements of proteins in time and space. We thus apply scattering techniques coupled with computational sequence analysis to probe the effects of segmental heterogeneity on RHP assembly and dynamics in solution. Our results indicate that in the absence of specific and monodisperse protein-like primary and secondary structures, RHPs can still achieve higher-order spatial correlations in aqueous environments. Owing to their methacrylate-based backbones, they retain sufficient backbone rigidity to allow exposed patches of chemically similar side chains to drive the assembly without unraveling at the single-chain level. These results provide fundamental insights into the use of randomness at the sequence level to achieve order at the chain level, while also defining the key parameters driving assembly tendencies seen in RHPs’ biological analogs.   

Investigating T1 Relaxation Times in Common Photopolymers: A Comparative Study

Utilizing ¹H Field Cycling Nuclear Magnetic Resonance (FC NMR) relaxometry measurements, the effects of several parameters such as the curing time and the presence and concentration of filler particles in common photopolymer resins is studied in the glassy dynamics of the material [1]. The filler surface can interact with the elastomers of the photopolymer, exhibiting different dynamics which depend on the characteristics of the filler surface, the polymer, and the temperature [2]. A Cryogen free magnet with incorporated VTI provides a range of magnetic fields between 0 and 9.4 T and a temperature range between 2 and 320 K for this study.

 

[1] F. Nardelli, F. Martini, E. Carignani, E. Rossi, S. Borsacchi, M. Cettolin, A. Susanna, M. Arimondi, L. Giannini, M. Geppi, and L. Calucci. “Glassy and Polymer Dynamics of Elastomers by 1H‑Field-Cycling

NMR Relaxometry: Effects of Fillers” J. Phys. Chem. B 2021, 125, 4546−4554

 

[2] G. Tsagaropoulos, A. Eisenberg. “Dynamic Mechanical Study of the Factors Affecting the Two Glass Transition Behavior of Filled Polymers. Similarities and Differences with Random Ionomers.” Macromolecules 1995, 28, 6067−6077.   

Oral: Exploring molecular mechanisms underlying mechanical and rheological response of dispersed polymer melts from molecular dynamics simulations

Motivated by the importance of interfacial properties of polymers in applications such as adhesion, there is a need to understand the molecular mechanisms which drives structure-property responses in the bulk. Due to their synthesis routes, commodity polymers have a large distribution of molecular weights, which is commonly characterized by dispersity (Ð). Recent studies have shown that dispersity can be used to tune polymer properties without the use of additives, or by changing the chemistry of the system. Here, using molecular dynamics simulation, we unravel molecular mechanisms that govern mechanical and rheological properties of entangled melts with dispersities ranging from 1.0 – 2.0. Specifically, we study the Schulz-Zimm distribution type, which models polymer samples synthesized by anionic and atom - transfer radical polymerization, across different average molecular weights (or chain lengths). We use the standard Kremer Grest bead – spring model which has been shown to capture the underlying physics of polymer melts.

 

First, we perform entanglement analysis on the basis of primitive paths to elucidate the topology of entangled melts. Next, we compute the stress relaxation of the melts from equilibrium simulations using the Green – Kubo relation. We also generate the frequency – dependent modulus of the melt and extract the storage and loss moduli. Finally, we perform uniaxial deformation to compute the stress-strain curve. All properties are compared with monodisperse systems. Future work includes simulating free standing films of these systems, to understand how dispersity affects interfacial melt properties.

 

 

Keywords: Polymer Melts, Dispersity, Viscoelastic Responses, Entanglements, Primitive Path Analysis, Stress Relaxation   

Anisotropic coarse-grained models for the structural characterization of unentangled linear polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is widely employed in corrosive or high-temperature applications due to its excellent chemical inertness and thermal stability. However, melt processing is commonly problematic owing to its high viscosity in the melt state. The symmetric carbon-fluorine chemical structure is the primary factor that makes PTFE melts have strong van der Waals interactions in high molecular weight. Molecular dynamics (MD) methods have been successfully applied to gain physical insights into all-atom (AA) and united-atom (UA) scales. However, the resolution of details requires expensive computational costs that prohibit their applicability to more extensive time and length scales. Given the inherent rigidity nature of PTFE, we propose the anisotropic coarse-grained (CG) models utilizing RE-squared potential in two CG levels, equivalent to 6 and 8 CF₂ groups. The bottom-up CG models are optimized through the mapping of critical structural properties based on atomistic simulations of C₉₆F₁₉₄ at 600K. These properties include segmental geometry, angular flexibility, pair correlations, and overall chain dimensions. Additionally, the impact of CG levels on dynamic characteristics, such as self-diffusivity and zero shear viscosity, is studied. Both CG models demonstrate adequate predictive capabilities regarding the structural characteristics of PTFE melts, encompassing molecular weights from 48 to 192 carbons per chain. Furthermore, the zero-shear viscosity presents a similar trend compared with AA systems across different molecular weights in both models. Benchmark evaluations for the C₉₆F₁₉₄ system reveal that the computational efficiency can be improved approximately 60 times than their AA counterparts. Therefore, our CG models offer accurate and efficient predictions of PTFE properties across different molecular weights, and are potentially applicable to study other PTFE-based polymers.