Session T18: Polymer and Polyelectrolyte Rheology II

Examination of non-universalities in entangled polymer melts and solutions during the startup of steady shear flow

The possibility of non-universality during inception of shear flow at large strain rates has recently been questioned so was recently examined using the discrete slip-link model (DSM). An expression for the Rouse relaxation time as a function of entanglement activity and number of Kuhn steps was found from a master curve of strain maxima, as predicted by the  theory. DSM predicts only a very weak dependence of Rouse time on chemistry [Macromolecules 54, 8033–8042 (2021)]. This expression is shown to collapse all entangled polymer solution and melt data to universal behavior for the maximum shear stress and the strain at maximum stress. The transition of these quantities from strain-rate-free values, to values that scale with dimensionless strain rate as 0.33 are shown to correspond to primitive path stretching. Furthermore, The scaling exponents for melt (0.1--0.15) and solution (0.2--0.3) in experimental data do not show the same scaling for steady-state shear stress, but the melts are in agreement with DSM (0.1). There is a small amount of data for the scaling of stress at undershoot, and strain at undershoot, which are predicted to scale as 0.1 and 0.33 for DSM, in agreement for melt data.   

Entanglement kinetics in polymer melts are chemically specific

Predictive modeling of the properties of polymeric material produced in next-generation manufacturing techniques requires knowledge of changes induced in the polymer microstructure during material processing. For instance, flow-induced disentanglement in fused-filament fabrication can decrease the strength of welds between printed layers. We have recently formulated a thermodynamically consistent constitutive equation that models entanglement kinetics to describe this behavior. The model predicts that the melt disentangles due to convective-constraint release and re-entangles on the Rouse time due to contour length fluctuations at the chain ends. A single fitting parameter β controls the rate of disentanglement; the remaining parameters in the model can be measured via model-independent experiments. We validate our model through comparison with molecular dynamics simulations. We determine β by fitting model predictions of disentanglement in steady-state shear to simulation measurements. Our model quantitively predicts re-entanglement following cessation of shear flow, confirming that the melt re-entangles on the Rouse time. We find that the rate of disentanglement is chemically dependent; stiffer melts disentangle faster more than more flexible melts.   

Bottom-up Multiscale Approach to Estimate Viscoelastic Properties of Entangled Polymer Melts with High Glass Transition Temperature

A multiscale computational method is presented for the prediction of the viscoelastic properties of entangled homopolymer melts with high glass transition temperatures. Starting from an atomistic model of a polymer, two coarser representations are introduced – a coarse-grained model and a slip-spring representation, that successively operate at longer time and length scales. The three models are unified by renormalizing the time and modulus scales, which is achieved through matching their normalized chain mean squared displacement and stress relaxation modulus, respectively. To facilitate the relaxation of entangled chains, the simulations are performed at temperatures higher than those accessible in experiments. Time-temperature superposition is then applied to extrapolate the viscoelastic properties calculated at high temperatures to experimentally accessible lower temperatures. This proposed approach can predict the linear rheology of the melt starting from an atomistic model and does not require experimental parameters as an input. Here it is demonstrated for syndiotactic polystyrene, where good agreement with experimental measurements is achieved.   

Yield Stress Dependence on the Nanostructure of 3D Printable Epoxy/Block Copolymer Inks using Rheological Creep Testing

Rheological characterization is critical in determining the printability of materials using direct ink writing (DIW). Our previous work showed that the results from transient creep testing correlated better to the printability of the DIW inks than results from traditional oscillatory amplitude sweeps. Creep testing was used to show that epoxy/block copolymer (BCP) inks had time-dependent flow properties resulting in the identification of critical apparent yield stress for the materials on time scales relevant to printing. In this presentation, we will build on this previous work to show the effects of the ink composition and the resultant BCP nanostructure on the yield stress of these materials. Specifically, the transition from spherical nanostructures to cylindrical structures results in three-fold increase in the yield stress. Additionally, we will demonstrate the importance of carefully selecting the preshear conditions to get repeatable results on these nanostructured epoxy/BCP inks.   

Structural and Rheological Investigation of Polymer-Grafted Cellulose Nanocrystals

Shear-flow induced organization of poly(acrylic acid)-grafted cellulose nanocrystals (CNCs) can influence water swelling and water permeability within CNC-based membranes. We will present the effect of polymer grafting parameters (graft density and length) on the network formation via hydrogen bondings between chains. The grafted nanocrystal networks formed by hydrogen bonds can be made to assemble in ordered and disordered states. These states will be studied for their chain swelling and effective ion bindings from water at different pH values. Concentration dependence of viscosity and phase transitions to the ordered and disordered networks will be evaluated in solution rheology. Structural and rheological investigations of polymer grafted CNCs hold potential applications for sustainable membrane technologies.   

Multiscale modeling of viscosity index improver (VII) polymers

Viscosity index improvers (VIIs) are compounds added to lubricants to help them maintain a uniform viscosity across a wide temperature range, allowing them to make important contributions to energy efficiency and wear protection. However, VIIs are typically high-molecular-weight polymers present at significant concentrations (~5 wt%), making them difficult to study using traditional particle-based methods (e.g., all-atom or coarse-grained molecular dynamics) due to length and time scale limitations. To overcome this obstacle, we employ a workflow where small-scale, atomistic simulations are used to parameterize statistical field theory models, which can then be used to probe the behavior of VIIs of realistic sizes while maintaining a connection to the underlying chemistry. This multiscale computational approach has the potential to accelerate discovery of novel VIIs that have improved performance and are more environmentally friendly. We demonstrate the capability of this approach by predicting various properties, including critical micelle concentrations and phase transitions, of a model system consisting of acrylic copolymers in a model base oil and discuss the possibility of using analytical theories to estimate rheological properties from field-based simulation outputs.   

Tuning the Relaxation Spectra of Vitrimers via Crosslinker Chemistry and Mixing

To design materials for vibrational damping applications, tuning the rheological relaxation spectra is key. Controlling the placement, breadth, and height of spectrum peaks determines which waves are dampened. Vitrimers, dynamic polymer networks with associative covalent bonds, provide an ideal platform for chemical and mechanical tunability. Here, four boronic acid crosslinkers of varying functionality and structure were reacted with silicone diols to form PDMS vitrimers. Three networks experienced relaxation times within one order of magnitude, with aromatic groups leading to faster times due to destabilization of the boronic ester. In contrast, a crosslinker with nitrogen neighboring groups led to a four order of magnitude drop in network relaxation time. The modulus at one temperature remains nearly constant regardless of various exchange kinetics. These samples, like all vitrimers, also experience modulus increase with temperature due to entropic elasticity. Effects of mixing multiple crosslinkers were also examined. When relaxation times are within an order of magnitude, the faster of two crosslinkers controls the relaxation time; when modes are further apart, a blending of dynamics occurs. These viscoelastic patterns and mixing rules are critical for damping applications.   

Defining the electrospinning window through solvent effects on extensional rheology

Ultrafine fibers produced by electrospinning are quickly becoming a frontrunner for functional fiber applications. Expanding the application field for ultrafine fibers necessitates knowledge of the behavior of polymer solutions during the electrospinning process. In this new application field, more complex fluids outside of the typical scope of electrospinnable solutions are required. These fluids have viscoelastic stresses and microstructural transitions that affect the flow, which may not be quantifiable in shear rheology. Hence, an analysis of the extensional rheology is required to fully characterize the behavior of the electrospinning jet. In this work, we study polymer solutions typically used in electrospinning (low and high MW PVP in methanol and water, and PEO and PVA in water). We see that the solutions differ in their extensional rheological behavior, especially with respect to surface tension which shows a strong effect on fiber formation. High surface tension solvents require higher extensional viscosities and relaxation times to form smooth fibers. Global dimensionless numbers such as the Deborah and Ohnesorge numbers appear to be a promising method of quantifying electrospinnability. Defining the electrospinning processing window through extensional rheology allows for more rational design of advanced ultrafine fibers for future applications.   

Spinnability and Rheology of PEO Solutions in Water/Acetonitrile Mixtures via Centrifugal Force Spinning

Centrifugal force spinning has recently emerged as a highly promising alternative technique for the production of nonwoven, ultrafine fiber mats.  Due to its high production rate and ability to utilize a wider range of solvents, it can provide a more technologically relevant fiber spinning technique than electrospinning.  In this contribution, we investigate the influence of polymer concentration and solvent properties on the centrifugal spinning process and fiber morphology.  We also correlate spinnability to the processing conditions and material properties using shear and extensional rheology carried out using torsional rheometry and dripping-onto-substrate (DoS) rheometry.  We eventually find that increasing polymer concentration will allow for continuous fibers to form after a beaded fiber regime.  We also find that fiber diameter is concentration-dependent, and that solvent choice and processing conditions affect the fiber morphology and mechanical properties of fiber mats.    

Dripping-onto-substrate extensional rheometry of the ultralong supramolecular polymer solutions

Supramolecular polymers which are utilized for viscosity modification, mist control, and drag reduction can give striking changes to the flow behavior of materials with their extremely high molecular weight and extraordinary chain extensibility. There have been several approaches to form supramolecular polymers such as assembly of associative polymers, however, the synthesis of ultralong polymers was only available to a limited extent. Accordingly, despite the importance of understanding not only shear rheological properties but also the dynamic response of ultralong supramolecular polymers under elongational flow, there have been few studies on them. In this research, ultralong supramolecular polymers were produced through a coordination complex between metal ions and ligands at both ends of linear building blocks. The size of the unit building block, the coordination number of metal ions, and pH conditions were manipulated to optimize the length of the supramolecular polymers. Finally, the extensional rheology and pinching dynamics of ultralong polymer solutions were analyzed using dripping-onto-substrate rheometry, and the relationship between the strength of the coordination bond and the chain extensibility has been investigated.   

Linear viscoelastic behavior of supramolecular polymer networks from non-telechelic associative block copolymers

Supramolecular polymer networks formed from physically associating polymers are a class of materials with special features such as self-healing, stimuli-responsiveness and reprocessibility. They owe this to the presence of associating groups, either along the chain or at the chain ends. Associating behavior can originate from various non-covalent interactions such as hydrogen bonds, electrostatic interactions, metal-ligand bonds, etc. A thorough understanding of the network topology and its effect on the rheological properties can pave the way for the development of transient networks with designed elasticity and relaxation spectra. In the present work, the focus is on block copolymers that associate into micelles via hydrophobic blocks distributed along the chain. The network formation upon increasing temperature is mapped out with rheology, differential scanning calorimetry and turbidimetry. The concentration dependent elasticity and relaxation dynamics of the networks is characterized via linear rheology. Using a combinatorics approach, the Annable’s mechano-statistical model for telechelic triblock copolymers was extended to more general multiblock copolymers. The model requires input about the spatial distribution of the micelles and their size, which was derived from X-ray scattering. A comparison between experimental data and model results shows the model’s predictive capability for the concentration-dependent plateau modulus. The evolution of the high-frequency plateau modulus and hence the elasticity with concentration hints towards a change in network topology upon increasing concentration. The structure evolves from loop-dominated with limited elasticity at low concentrations to bridge-dominated and highly elastic at higher concentrations. On the other hand, the concentration dependence of the relaxation time(s) reveals the importance of superstructures such as superbridges and superloops, on the sticky Rouse-like relaxation dynamics of the network.