Integrated kinetic Monte Carlo-structure-rheology model for solution copolymerization of ethylene and α-olefins

\u3cp\u3eAn integrated Kinetic Monte Carlo-structure-rheology model is developed for polyolefins produced using solution copolymerization of ethylene and α-olefins. The proposed algorithm is completely first-principle and can be used to simulate any homogeneous single-site polymerization process resulting in full topological and chemical composition details of the product formed. The model maintains simple hierarchical data structures to track the structure and composition of polymer chains and efficiently generates a full ensemble of polymeric molecules under varying process conditions. The model has a control-volume description with no implicit assumptions regarding the dynamic state of the process, and hence the methodology can be used for any reactor configuration. We benchmark the model by simulating two semibatch pilot plant runs and use the benchmarked model to simulate production of three industrial grade polyolefins which differ significantly in their molecular weight distributions, long-chain branching fractions, and chemical composition. For modeling the continuous state production process with the control volume description, we propose a simple inflow/outflow step in the algorithm to model continuous stirred-tank reactors (CSTR) which eliminates use of idealized flow and steady-state approximations. The simulated molecular topology and bivariate long-chain branching (LCB) molecular weight distributions are used to select a representative ensemble of molecules that is used to calculate linear rheology using the branch-on-branch model of Das et al. The resulting product distributions for the semibatch trials reproduce the measured molecular weight distributions. For the polyolefins simulated with the continuous process the virtual products have closely matching molecular weight distributions and branching fractions compared to measured values of the industrial polyolefins. The simulated rheology also agrees quite closely with the experimental values without use of any fitting parameters in the full integrated approach. In addition, microstructural characteristics as measured by the crystallization elution fractionation technique also match closely with the simulated crystallization elution fractionation distributions calculated using the longest ethylene sequence distributions obtained for the three products.\u3c/p\u3

Novel approach to develop rheological structure-property relationships using genetic programming

\u3cp\u3eRheological structure-property models play a crucial role in the manufacturing and processing of polymers. Traditionally rheological models are developed by design of experiments that measure a rheological property as a function of the moments of molar mass distributions. These empirical models lack the capacity to apply to a wide range of distributions due the limited availability of experimental data. In recent years fundamental models were developed to satisfy a wider range of distributions, but they are in terms of variables not readily available during processing or manufacturing. Genetic programming can be used to bridge the gap between the practical, but limited, empirical models and the more general, but less practical, fundamental models. This is a novel approach of generating rheological models that are both practical and valid for a wide set of distributions.\u3c/p\u3

Correlations between high-density polyethylene viscoelasticity and annular extrudate swell

\u3cp\u3eExtradate swell is a key performance contributor in extrusion blow molding. The relation between extrudate swell and rheometric material characteristics is non-trivial. It is studied here for a large set of high-density polyethylenes. Extrudate swell data are derived from photographic analysis on a lab-scale annular extrusion set-up, while rheometric parameters are obtained from dynamic spectroscopy, uniaxial elongation, creep and creep recovery experimentation. The annular set-up produces thickness and diameter swell data, which are found to depend in different ways on the material viscoelasticity. The former correlates well with recoverable or storage compliance, while the latter correlates well with the ratio of storage to loss modulus.\u3c/p\u3

Pressure oscillations and periodic extrudate distortions of long-chain branched polyolefins

\u3cp\u3eCapillary rheometry is a much used technique for measuring pressure-flow rate behavior of polymer melts. The nature of such a flow curve depends on polymer architecture, die geometry, die material composition, and rheometer operating conditions. Typically, with increasing flow rates, monotonie flow curves have been associated with extradâtes that transcend from smooth to being volume distorted. Alternatively, nonmonotonic flow curves have been associated with a sequence of extradate appearances ranging from smooth via surface distortions and spurt to volume distortions. New experiments however indicate that monotonie flow curves can also be associated with spurtlike distorted extradates. For several long-chain branched polymers, it is reported that while the average pressure increases monotonically with increasing flow rate, the extradate distortions transition through an unanticipated regime where the extradate consists of alternating smooth and volume-distorted zones. Its origin is conjectured related to the specific viscoelastic flow properties of long-chain branched materials in the reservoir-die contraction region. Using a fast-response pressure transducer in the reservoir near the capillary die entry, the presence of small-amplitude pressure oscillations corresponding to the distortion period is confirmed. The critical conditions for the appearance of this phenomenon depend strongly on molecular mass and branching distribution.\u3c/p\u3

Expanding the industrial use of linear viscoelastic material functions

Polymer melts usually have relaxation times in a broad frequency range. To capture the full rheological behaviour, multiple rheometers are used. Due to the different modes of operation, combination of the data into a single rheological function often involves solving one or more inverse, ill-posed problems. Here, as an alternative approach, a new approximate direct analytical method is presented that allows the broadening of oscillatory shear data obtained from dynamic mechanical spectrometers, to low frequencies by using converted constant stress compliance data. The accuracy of the broadened curves depends on the experimental accuracy and the specific time scales of the polymer and the rheometers. The method is most effective when used in combination with inverse problem solvers. By doing so, translucence, insight, and directness from the analytic method is added to the potential accuracy of the full inverse problem solver

Compatibilization of polypropylene – polyethylene blends

Many plastic waste recycle streams are blends of various types of polypropylene (PP) and polyethylene (PE). When reprocessing these blends into products it is difficult to obtain good mechanical and optical properties due to the immiscibility of the components. Blend morphology is one of the governing factors for these properties. There-fore morphology control is a key challenge when turning plastic waste into valuable materials. In this work, the effect of a novel polymeric compatibilizer on the morphology of PP – PE blends was investigated via rheological and scanning electron microscopy experiments. Homopolymer javascript:void(0);PP was combined with PE of varying comonomer level. In addition to the effect of including compatibilizer, the effect of blend ratio and viscosity ratio is discussed. It was found that very fine dispersions could be obtained when including the compatibilizing polymer for all studied systems. The blend rhe-ology was compared with predictions from empirical and physics-inspired mixing rules. The difference between meas-ured and predicted rheology is expected to provide insight into the structure of the various blend systems

Compatibilization of polypropylene–polyethylene blends

\u3cp\u3eMany plastic waste recycle streams are blends of various types of polypropylene (PP) and polyethylene (PE). When reprocessing these blends into products it is difficult to obtain good mechanical and optical properties due to the immiscibility of the components. Blend morphology is one of the governing factors for these properties. Therefore morphology control is a key challenge when turning plastic waste into valuable materials. In this work, the effect of a novel polymeric compatibilizer on the morphology of PP–PE blends was investigated via rheological and scanning electron microscopy experiments. Homopolymer PP was combined with PE of varying comonomer level. In addition to the effect of including compatibilizer, the effect of blend ratio and viscosity ratio is discussed. It was found that very fine dispersions could be obtained when including the compatibilizing polymer for all studied systems. The blend rheology was compared with predictions from empirical and physics-inspired mixing rules. The difference between measured and predicted rheology is expected to provide insight into the structure of the various blend systems. POLYM. ENG. SCI., 58:460–465, 2018.\u3c/p\u3

From reactor to rheology in LDPE modeling

\u3cp\u3eIn recent years the association between molecular structure and linear rheology has been established and well-understood through the tube concept and its extensions for well-characterized materials (e.g. McLeish, Adv. Phys. 2002). However, for industrial branched polymeric material at processing conditions this piece of information is missing. A large number of phenomenological models have been developed to describe the nonlinear response of polymers. But none of these models takes into account the underlying molecular structure, leading to a fitting procedure with arbitrary fitting parameters. The goal of applied molecular rheology is a predictive scheme that runs in its entirety from the molecular structure from the reactor to the non-linear rheology of the resin. In our approach, we use a model for the industrial reactor to explicitly generate the molecular structure ensemble of LDPE's, (Tobita, J. Polym. Sci. B 2001), which are consistent with the analytical information. We calculate the linear rheology of the LDPE ensemble with the use of a tube model for branched polymers (Das et al., J. Rheol. 2006). We then, separate the contribution of the stress decay to a large number of pompom modes (McLeish et al., J. Rheol. 1998 & Inkson et al., J. Rheol. 1999) with the stretch time and the priority variables corresponding to the actual ensemble of molecules involved. This multimode pompom model allows us to predict the nonlinear properties without any fitting parameter. We present and analyze our results in comparison with experimental data on industrial materials.\u3c/p\u3