Session D70: Rheology and Dynamics of Polymer Liquids and Glasses

Molecular Simulations of Poly[n]catenane Dynamics and Rheology

With the recent synthesis of poly[n]catenanes – polymers composed of interlocking ring molecules – the possibility of incorporating catenated moieties into functional materials continues to grow. However, catenated polymers have been only sparsely studied, as previous investigations have primarily focused on statistical, topological, or structural features, rather than the dynamics. To characterize these new systems, we conducted molecular dynamics simulations of model poly[n]catenanes in the melt. We observe unusual monomer diffusion and present a Rouse-like model that can qualitatively explain the results. We show that this model fails to account for topological interactions and that various measures of friction in the system are not self-consistent. We examine the stress relaxation and find that the viscosity exhibits a surprising non-monotonic dependence on ring size. We briefly outline how these results may be applied to material design and synthesis.   

Graft Polymers and Entanglements: From Linear Chains to Filaments

Dynamics of melts and solutions of high molecular weight polymers is controlled by topological constraints (entanglements) imposing sliding chain motion along an effective confining tube. For linear chains, the tube size is determined by a universal packing number Pe, the number of polymer strands within a confining tube required for chains to entangle. Using coarse-grained molecular dynamics simulations, it is shown that in melts of graft polymers, the packing number is not universal and depends on the molecular architecture. The packing number of graft polymers is a nonmonotonic function of the degree of overlap between side chains belonging to the same molecule. Below side chain overlap, it decreases with increasing grafting density, then begins to increase as side chains start to interpenetrate, finally, in the limit of densely grafted side chains it approaches Pe for linear chains. This dependence reflects a crossover from chain-like entanglements in systems with loosely grafted side chains to entanglements between flexible filaments. This is in agreement with the experimental data for dependence of plateau modulus on the molecular architecture of graft poly(n-butyl acrylates) and poly(norbornene)-graft-poly(lactide) melts.   

Effect of Head-to-Head Association/Dissociation on Relaxation of Entangled Chains

For entangled high-cis polyisoprene (PI-COOH) undergoing mono-functional, head-to-head association/dissociation through hydrogen bonding of COOH groups attached at the chain head, viscoelastic and dielectric relaxation behavior was examined in bulk state. The relaxation was found to deviate significantly from that of a simple mixture (blend) of non-associative unimer PI and dimer (PI)₂ having the same composition as the associative PI-COOH (determined from FTIR spectroscopy for the COOH group). This deviation is attributed to an association/dissociation-induced motional coupling between the PI-COOH unimer and its dimer. Analysis based on the reptation eigenfunction expansion lends support to this conclusion.   

Unified analytic expressions for the entanglement length, tube diameter, and plateau modulus in polymer melts

By postulating that the contributions to the dimensionless plateau modulus from flexible-, semiflexible-, and stiff-chain entanglement mechanisms combine additively (in parallel), and testing this postulate with molecular dynamics simulations and topological analyses of Kremer-Grest bead-spring polymer melts, we obtain analytic expressions that quantitatively predict the plateau modulus G, the entanglement length N_e, and the tube diameter a in melts that span three orders of magnitude in l_K/p, where l_K and p are respectively the Kuhn and packing lengths. Our expressions resolve conflicts between previous scaling predictions for the loosely entangled [Lin-Noolandi: Gl_K³/k_BT ~ (l_K/p)³] and tightly-entangled [Morse: Gl_K³/k_BT ~ (l_K/p)^7/5] regimes.   

The Source of Strain Hardening in Glassy Polymers Investigated by Molecular Dynamics and Brownian Dynamics Simulations

Using both fine-grained Molecular Dynamics (MD) simulations and coarse-grained simulations we show how strain hardening in polymeric glasses under uniaxial extension arises from highly stretched strands that form as the polymer chains deform subaffinely on increasing length scales as strain increases. We find that although the HBD model ignores entanglements, it accurately predicts how the MD chain configurations evolve during deformation. Both models shows similar strain hardening modulus G_R that is much larger than the melt plateau modulus G_N because chain segments become highly stretched at modest Hencky strain (<1 ). Both models also capture the increase in strain hardening with increasing chain length that saturates in the long chain limit. We improve upon HBD’s ability to accurately capture stress-strain curves at small strains through yielding and strain softening by extending the theory to multiple segmental relaxation modes, whose strain-dependent relaxation times are obtained from small-molecule probe relaxation experiments by Ediger and coworkers [Bending, B. & Ediger, M. D. J. Polym. Sci. B 2016, 54, 1957-1967].   

Predicting time-temperature-superposition breakdown near the glass transition with the Heterogeneous Rouse Model

Since pioneering work by Plazek in the 1960’s, it has been known that time-temperature superposition breaks down in many polymers upon approach to the glass transition. Because TTS is fundamentally rooted in the Rouse model prediction of a shared temperature dependence for chain and segmental relaxation modes, this breakdown implies a significant gap in our understanding of polymer dynamics at low temperature. Here, we report on predictions of TTS breakdown by the Heterogeneous Rouse Model, which generalizes the Rouse model to incorporate dynamic heterogeneity – the emergence of a distribution of segmental relaxation times at low temperature. Predictions are in good agreement with simulations and with available experimentally observed relaxation spectra in the TTS-breakdown regime, suggesting that this theory provides a promising model for dynamics in the TTS breakdown regime.
  

Relationship Between Large Amplitude Oscillatory Strain (LAOS) Experiments and Commercial Pressure Sensitive Adhesives Applications Testing

Historically, analytical test methods on pressure sensitive adhesives (PSA’s) have shown poor correlation to applications test results. Since applications tests such as peel and shear involve large deformations of the adhesive, large amplitude oscillatory strain (LAOS) experiments may correlate more strongly to the applications test results than typical linear viscoelastic measurements. Here we report the applications and characterization results on nine different commercial PSA films. The primary applications test results are 90° stainless steel peel and stainless steel shear. Based on these results, the adhesives were classified as “tape”, “label”, or “removable”, with three samples in each category. The adhesives were characterized analytically by large amplitude oscillatory strain (LAOS) tests. The various LAOS parameters appear correlated to the applications peel and shear results indicating LAOS may provide a better indication of applications performance in a more efficient way than existing applications test protocols.   

Polymer rheology predictions from first-principles using the slip-link model

The discrete slip-link theory a hierarchy of strongly connected models that have great success predicting the linear and nonlinear rheology of high-molecular weight polymers. Three of the four parameters of the most detailed model can be extracted from primitive path analysis, which give quantitive experimental agreement for all examined chemistries (PS, PI, PBd and PE). Here we show that the remaining friction parameter can also be extracted from atomistic simulations. In particular, an available quantum chemistry-based force field for polyethylene oxide (PEO) was used to perform molecular dynamics simulations of a 12kDa melt. Once the four parameters are determined for any chemistry, all parameters for all members of the slip-link hierarchy are determined. Then, using a coarser member of the hierarchy the dynamic modulus and nonlinear rheology of a 256kDa PEO melt was predicted. The predictions are compared to experimental measurements performed at the same temperature. Unfortunately, the extracted friction differs by a factor of two from experiment, which presumably arises from insufficient accuracy in the force field. Nonetheless, the work demonstrates that theory predictions without adjustable parameters should be possible.   

A priori Determination of the Extensional Viscosity of Polydisperse Linear Polymer Melts

Compared to coarse grained MD, the COMOFLO algorithm is 100,000 times faster. The steady-state extensional viscosity function is calculated in a parameter free completely a priori manner as previously done for shear (Parameter Free Prediction of Rheological Properties of Homopolymer Melts by Dynamic Monte Carlo Simulation, Dorgan et.al., Macromolecules, (2012) DOI: 10.1021/ma301307d). The algorithm captures molecular details of extensional flow in three dimensions for entangled polymer melts of arbitrary molecular weight distribution across multiple flow regimes from the correct low deformation limit, through the strain-hardening regime, to the high deformation rate strain softening region. Comprehensive a priori shear rheology of polydisperse systems was established earlier (Finding the Missing Physics: Mapping Polydispersity into Lattice-Based Simulations. Rorrer & Dorgan, Macromolecules, 2014, DOI: 10.1021/ma5001207 ). The ability to predict steady shear and elongational rheology for linear polymer melts of arbitrary molecular weight distribution can be considered a solved problem.   

Thinning and break up of freestanding polymer solutions

The stability and break-up of thin liquid polymer films is a fascinating subject and reflects a complex interplay between intermolecular forces, hydrodyanic and viscoleastic phenomena, osmotic pressure and the effects of confinement. We have studied the thinning dynamics of concentrated polymer-solutions liquid films experimentally using a modified thin film balance technique. The variation of the capillary pressure that is driving the thinning of the film allows us to control the ratio of the the competing timescales of drainage and rupture. It is shown that rupture occurs through the random evolution of thickness fluctuations, whereas the drainaige dyanmcis are described well by continuum approaches, even down to lenght scales on the order of the radius of gysration of the films. Using time dependent positive, negative and oscillatory pressure jumps accros the interface, a wide range of dynamical phenomena can be evidenced. The implications of these results on macroscopic stability of multiphase systems and modelling events such as coalescence will be discussed.   

Uniaxial Extensional Rheology of Associating Polymers: from Processing to Performance

Polymers which include associating groups show increased performance characteristics over the base backbone polymer. The tunable properties of associating polymers are desirable for a wide range of applications, including as self-healing materials. However, there is typically a loss of processability for strongly associating polymers. In this work, we use filament stretching uniaxial extension to probe the processing window of strongly associating polymers. We show that, processing is limited by melt fracture in a wide range of Weissenberg numbers. The presence of entanglements increases the level of processability both in terms of the maximum Hencky strain before failure and the minimum achievable strain at low temperatures. Furthermore, the mechanical performance of associating polymers is confirmed to depend strongly on the associating group strength. Both the processability and performance of associating polymers can be captured by a spectrum of relaxation timescales related to the polymer backbone and various states of the associating groups. By reducing the critical behaviors of associating polymers to key timescales, clear design and processing parameters can be addressed in terms of fundamental physics.
  

Liquid to soft solid transition in block polymers via low strength magnetic fields

Achieving magnetic field-induced orientation in block polymers (BCPs) has typically relied on large field strengths (B≥5 T), the addition of liquid-crystalline (LC) or rod-like blocks, substantial chain anisotropy, or combinations therein. BCP ordering upon application of low-strength fields (≤0.5 T) has only been reported in systems of over 60% wt LC mesogen by mass. Here, we identify substantial field-induced rheological and structural changes in several coil-coil BCP variants using magneto-rheology and small-angle neutron scattering (SANS). Linear viscoelastic temperature ramps combined with B≥0.1 T magnetic field show a liquid to gel transition where the increase by three-to-six orders of magnitude beyond a critical time (t_t). Here, t_t is a function of field strength, polymer concentration and molecular weight, temperature, and ionic strength. The resultant gel state is stable for several hours after field removal, where the structural relaxation time scales with the maximum achieved modulus. SANS detects distinct gelation mechanisms based on BCP variant. In addition to structure enhancement, this approach has the potential to discover new structures not accessible through traditional self-assembly routes with minimal input from external fields.   

Multiscale simulation of a well-entangled polymer melt flow in between two coaxial cylinders under non-isothermal condition

We have successfully extended a multiscale simulation method to non-isothermal well-entangled polymer melt flows in between two coaxial cylinders. At the microscopic level, a dual slip-link model is employed as a model for well-entangled polymers. We found that the extended multiscale simulation method is quite effective to reveal the relation between non-isothermal polymeric flows and microscopic states of polymer chains expressed by primitive paths and slip-links. It is also found that the temperature-dependent reptation-time-based Weissenberg number is a suitable measure to understand how extent polymer chains are deformed in a range of the shear rate used in this study.