Understanding the effect of constraint release on the dynamics of entangled polymers

In this thesis we mainly investigate multi-chain effects on the dynamics of entangled model polymers. We develop a systematic methodology for testing different relaxation mechanisms. For this purpose we use a stochastic single chain slip-spring model which quantitatively predicts chain dynamics at long time scales. In this model all relaxation mechanisms are naturally integrated. In order to test different assumptions related to respective contributions of different relaxation mechanisms we use a set of simplified slip-spring “toy” models, where different relaxation modes are systematically deactivated, analyzed and later reassembled in a controlled way. For testing existing constraint release (CR) theories we analyze different mixtures of linear and simple branched polymers. Implementation of the obtained results to tube models could be used for improving predictions of all stress relaxation components in the systems with complex constraint release environments, i.e. polydisperse in chain length and even architecture, which is typical for complex industrially relevant polymers.(FSA - Sciences de l) -- UCL, 201

The influence of molecular weight distribution of industrial polystyrene on its melt extensional and ultimate properties

We analyze the linear viscoelastic behavior and the strainrate dependence of nonlinear viscoelastic as well as the ultimate extensional properties of industrially relevant linear polystyrene mixtures (PS). The studied materials comprise different miscible binary mixtures of a well entangled matrix and unentangled diluent resulting in bimodal molar mass distribution (MWD). We also analyze the effect of the diluent weight average molar mass (Mw) by comparison with a mixture having broad but monomodal MWD. We show that the dilution effect on linear rheological properties is in agreement with the theoretical value of unity for the dilution exponent. We further show that the processing window, expressed as the ability of the material to withstand a given load without loss of homogeneity during elongation or ultimate loss of cohesion, is affected differently depending on the diluent Mw and concentration. Finally, we conclude that the existence of strain hardening is not sufficient for complete characterization of extension dominated operations. Our results demonstrate that significant enhancement of strain hardening achieved by adding small-Mw diluents is often accompanied by trade-off with respect to failure behavior of these mixtures

Contour length fluctuations and constraint release in entangled polymers: slip-spring simulations and their implications for binary blend rheology

We study the interaction between constraint release and contour length fluctuations in well entangled polymers, by means of a set of slip-spring simulations in which the rate of constraint release is precisely controlled. In the present simulations, a fraction f = 0.9 of the slip-springs undergo constraint release, while the remaining slip-springs do not, as an idealized model to mimic the constraint release environment of a bidisperse polymer melt. Both populations of slip-springs allow reptation of the chain. Our analysis of the data indicates the time and parameter regions in which contour length fluctuations occur effectively along a thin tube, or along a diluted fat tube. We predict the parameters for which case each is observed and the rate of relaxation due to contour length fluctuations in each region. Finally, we draw the implications of the simulations for bidisperse blend rheology, by revisiting the classic “Viovy diagram” for such melts. We relocate some of the lines on the original diagram, and identify new regimes, based on the physics and quantitative information supplied by the simulation data. In particular, we identify a new region in the diagram in which along-tube motion of the long chains is predominantly along the contour of the thin tube, yet contour length fluctuations occur in the fat tube, resulting in an acceleration of the terminal relaxation. We successfully and quantitatively locate a wide range of literature data on our redrawn diagram

Efficient Determination of Slip-Link Parameters from Broadly Polydisperse Linear Melts

We investigate the ability of a coarse-grained slip-link model and a simple double reptation model to describe the linear rheology of polydisperse linear polymer melts. Our slip-link model is a well-defined mathematical object that can describe the equilibrium dynamics and non-linear rheology of flexible polymer melts with arbitrary polydispersity and architecture with a minimum of inputs: the molecular weight of a Kuhn step, the entanglement activity, and Kuhn step friction. However, this detailed model is computationally expensive, so we also examine predictions of the cheaper double reptation model, which is restricted to only linear rheology near the terminal zone. We report the storage and loss moduli for polydisperse polymer melts and compare with experimental measurements from small amplitude oscillatory shear. We examine three chemistries: polybutadiene, polypropylene and polyethylene. We also use a simple double reptation model to estimate parameters for the slip-link model and examine under which circumstances this simplified model works. The computational implementation of the slip-link model is accelerated with the help of graphics processing units, which allow us to simulate in parallel large ensembles made of up to 50,000 chains. We show that our simulation can predict the dynamic moduli for highly entangled polymer melts over nine decades of frequency. Although the double reptation model performs well only near the terminal zone, it does provide a convenient and inexpensive way to estimate the entanglement parameter for the slip-link model from polydisperse data

Interplay of entanglement and association effects on the dynamics of semidilute solutions of multisticker polymer chains

International audienc

Improved Cell Morphology and Surface Roughness in High-Temperature Foam Injection Molding Using a Long-Chain Branched Polypropylene

Long-chain branched polypropylene (LCB PP) has been used extensively to improve cell morphologies in foaming applications. However, most research focuses on low melt flow rate (MFR) resins, whereas foam production methods such as mold-opening foam injection molding (MO-FIM) require high-MFR resins to improve processability. A systematic study was conducted comparing a conventional linear PP, a broad molecular weight distribution (BMWD) linear PP, and a newly developed BMWD LCB PP for use in MO-FIM. The effects of foaming temperature and molecular architecture on cell morphology, surface roughness, and mechanical properties were studied by utilizing two chemical blowing agents (CBAs) with different activation temperatures and varying packing times. At the highest foaming temperatures, BMWD LCB PP foams exhibited 887% higher cell density, 46% smaller cell sizes, and more uniform cell structures than BWMD linear PP. Linear PP was found to have a surface roughness 23% higher on average than other resins. The BMWD LCB PP was found to have increased flexural modulus (44%) at the cost of decreased toughness (−88%) compared to linear PP. The branched architecture and high molecular weight of the BMWD LCB PP contributed to improved foam morphologies and surface quality in high-temperature MO-FIM conditions

Tuning High and Low Temperature Foaming Behavior of Linear and Long-Chain Branched Polypropylene via Partial and Complete Melting

While existing foam studies have identified processing parameters, such as high-pressure drop rate, and engineering measures, such as high melt strength, as key factors for improving foamability, there is a conspicuous absence of studies that directly relate foamability to material properties obtained from fundamental characterization. To bridge this gap, this work presents batch foaming studies on one linear and two long-chain branched polypropylene (PP) resins to investigate how foamability is affected by partial melting (Method 1) and complete melting followed by undercooling (Method 2). At temperatures above the melting point, similar expansion was obtained using both foaming procedures within each resin, while the PP with the highest strain hardening ratio (13) exhibited the highest expansion ratio (45 ± 3). At low temperatures, the foamability of all resins was dramatically improved using Method 2 compared to Method 1, due to access to lower foaming temperatures (<150 °C) near the crystallization onset. Furthermore, Method 2 resulted in a more uniform cellular structure over a wider temperature range (120–170 °C compared to 155–175 °C). Overall, strong extensional hardening and low onset of crystallization were shown to give rise to foamability at high and low temperatures, respectively, suggesting that both characteristics can be appropriately used to tune the foamability of PP in industrial foaming applications

Dynamics of Star Polymers in Fast Extensional Flow and Stress Relaxation

We confirm the observation from Ianniruberto and Marrucci [ Macromolecules 2013, 46, 267−275] that entangled melts of branched polystyrenes behave like linear polystyrenes in the steady state of fast extensional flow, by measuring a linear, an asymmetric star, and a symmetric star polystyrene with the same span molecular weight (180 kg/mol). We show that all three melts reach the same extensional steady-state viscosity in fast extensional flow (faster than the inverse Rouse time). We further measure stress relaxation following steady extensional flow for the three melts. We show that initially they relax in a similar way, most likely via arm retraction, at short time, but behave differently at long time due to both the length of the arm and the branch point. The terminal relaxation is described by a Doi and Edwards based model, i.e., considering pure orientational relaxation