An Intriguing Array of Extrudate Patterns in Long‐Chain Branched Polymers During Extrusion

The present study highlights a range of surface and volume extrudate patterns that can be detected during the extrusion flow of long-chain branched polymers. Thus, four linear low-density polyethylenes (LDPEs) have been extruded using a single-screw extruder coupled to an inline optical imaging system. The selected LDPEs are selected to outline the influence of molecular weight and long-chain branching on the types of melt flow extrusion instabilities (MFEI). Through the inline imaging system, space–time diagrams are constructed and analyzed via Fourier-transformation using a custom moving window procedure. Based on the number of characteristic frequencies, peak broadness, and whether they are surface or volume distortions, three main MFEI types, distinct from those typically observed in linear and short-chain branched polymers, are identified. The higher molecular weight, low long-chain branching LDPEs exhibited all three instability types, including a special type volume instability. Independently of the molecular weight, higher long-chain branching appeared to have a stabilizing effect on the transition sequences by suppressing volume extrudate distortions or limiting surface patters to a form of weak intensity type

Polymer crystallinity and crystallization kinetics via benchtop 1 H NMR relaxometry: Revisited method, data analysis, and experiments on common polymers

Semi-crystalline polymers play an enormously important role in materials science, engineering, and nature. Two-thirds of all synthetic polymers have the ability to crystallize which allows for the extensive use of these materials in a variety of applications as molded parts, films, or fibers. Here, we present a study on the applicability of benchtop 1H NMR relaxometry to obtain information on the bulk crystallinity and crystallization kinetics of the most relevant synthetic semi-crystalline polymers. In the first part, we investigated the temperature-dependent relaxation behavior and identified T=Tg+100 K as the minimum relative temperature difference with respect to Tg for which the mobility contrast between crystalline and amorphous protons is sufficient for an unambiguous determination of polymer crystallinity. The obtained bulk crystallinities from 1 H NMR were compared to results from DSC and XRD, and all three methods showed relatively good agreement for all polymers. In the second part, we focused on the determination of the crystallization kinetics, i.e., monitoring of isothermal crystallization, which required a robust design of the pulse sequence, precise temperature calibration, and careful data analysis. We found the combination of a magic sandwich echo (MSE) with a short acquisition time followed by a CarrPurcell-Meiboom-Gill (CPMG) echo train with short pulse timings to be the most suitable for monitoring crystallization. This study demonstrates the application of benchtop 1H NMR relaxometry to investigate the bulk crystallinity and crystallization kinetics of polymers, which can lead to its optimal use as an in situ technique in research, quality control, and processing labs

Polymer crystallinity and crystallization kinetics via benchtop 1 H NMR relaxometry: Revisited method, data analysis, and experiments on common polymers

Semi-crystalline polymers play an enormously important role in materials science, engineering, and nature. Two-thirds of all synthetic polymers have the ability to crystallize which allows for the extensive use of these materials in a variety of applications as molded parts, films, or fibers. Here, we present a study on the applicability of benchtop 1H NMR relaxometry to obtain information on the bulk crystallinity and crystallization kinetics of the most relevant synthetic semi-crystalline polymers. In the first part, we investigated the temperature-dependent relaxation behavior and identified T=Tg+100 K as the minimum relative temperature difference with respect to Tg for which the mobility contrast between crystalline and amorphous protons is sufficient for an unambiguous determination of polymer crystallinity. The obtained bulk crystallinities from 1 H NMR were compared to results from DSC and XRD, and all three methods showed relatively good agreement for all polymers. In the second part, we focused on the determination of the crystallization kinetics, i.e., monitoring of isothermal crystallization, which required a robust design of the pulse sequence, precise temperature calibration, and careful data analysis. We found the combination of a magic sandwich echo (MSE) with a short acquisition time followed by a CarrPurcell-Meiboom-Gill (CPMG) echo train with short pulse timings to be the most suitable for monitoring crystallization. This study demonstrates the application of benchtop 1H NMR relaxometry to investigate the bulk crystallinity and crystallization kinetics of polymers, which can lead to its optimal use as an in situ technique in research, quality control, and processing labs

Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 2: crystallinity and supra molecular structure

Test methods including OM, SEM, TEM, DSC, SAXS, WAXS, and IR were used to characterise supra-molecular structure in three batches of polyethylene (PE), which had weight-average relative molar masses ¯¯¯¯ M w of approximately 0.6 × 106, 5 × 106, and 9 × 106. They were applied to compression mouldings made by the polymer manufacturer. Electron microscopy showed that powders formed in the polymerization reactor consisted of irregularly shaped grains between 50 and 250 μm in diameter. Higher magnification revealed that each grain was an aggregate, composed of particles between 0.4 and 0.8 μm in diameter, which were connected by long, thin fibrils. In compression mouldings, lamellar thicknesses ranged from 7 to 23 nm. Crystallinity varied between 70 and 75 % in reactor powder, but was lower in compression mouldings. Melting peak temperatures ranged from 138 to 145 °C, depending on processing history. DMTA showed that the glass transition temperature θg was −120 °C for all three grades of polyethylene. IR spectroscopy found negligibly small levels of oxidation and thermal degradation in mouldings. Optical microscopy revealed the presence of visible fusion defects at grain boundaries. It is concluded that relatively weak defects can be characterized using optical microscopy, but there is a need for improved methods that can detect less obvious fusion defects

Fourier-transform rheology applied on homopolymer melts of different architectures-Experiments and simulations

The detection and characterization of polymer architectures is an important subject for fundamental science and polymer industry. Large amplitude oscillatory shear (LAOS) combined with FT-Rheology is a newly established method to probe structural characteristics and quantify the non-linear rheological behaviour of different polymer topologies. This relates to short-chain branched (SCB) and long-chain branched (LCB), using the intensities and phases of the mechanical higher harmonics in the FT-spectrum of the stress signal. The resulting intensity of the odd harmonics and dominantly the 3rd harmonic at 31 as a function of strain amplitude, I3/1(ã0), can be described by empirical equations and the derived non-linear parameters are used to quantify the non-linear rheological behaviour. Measurements performed for linear polystyrene melts reveal a strong correlation between molecular weight and mechanical non-linearities. Finite element simulations of LAOS are performed to predict linear and non-linear rheological properties of model polystyrene branched architectures, using the Pom-pom model in the DCPP formulation (double-convected Pom-pom model) and the results are in qualitative agreement with experimental data. The method of combined experimental FT-Rheology with complementary NMR spectroscopy analysis and LAOS flow simulations is extended to industrial polyethylene of varying molecular weight and distribution, branching type and content. A dependence of the resulting non-linearities on Mw and PDI is derived. An incorporation of a small amount of LCB increases significantly the non-linearity of the stress response. Additionally results from blends of linear and LCB industrial polyethylenes reveal a monotonic dependence of I3/1 on LCB sample content. Additionaly, during polymer processing of polyolefines flow instabilities take place. When flow instabilities take place the resulting non-linearities depart significantly from the expected non-linear response of the material and even harmonics at 21 appear. The onset of these phenomena is quantified and correlated with molecular weight and polymer topology. The highly non-linear and in some cases non-periodic stress signals are qualitatively predicted via LAOS finite element simulations with a wall slip model which couples the fluid slip velocity with the wall shear stress. Finally, the FT-Rheology results from LAOS flow can be correlated to the behaviour during capillary extrusion, in order to predict extrudate distortions like sharkskin, stick-slip, gross fracture in polyethylene using only LAOS and FT-Rheology. Low molecular weight polyethylenes exhibit less flow instabilities and especially by increasing the amount of incorporated SCB, a shift of instabilities onset at higher critical deformations, or even a suppression of intense extrudate distortions, is observed. Samples containing LCB and accordingly high non-linearities during LAOS present instabilities at lower critical stresses and higher extrudate surface distortions

Predicting extrusion instabilities of commercial polyethylene from non-linear rheology measurements

Published online : 23 September 2014Processing at the highest possible throughput rates is essential from an economical point of view. However, various flow instabilities and extrudate distortions like sharkskin, stick slip, and gross melt fracture (GMF) may limit the production rate of high-quality products. Predicting the process conditions leading to the occurrence of rheological instabilities is the key for improving product quality, process control, and optimization. Large-amplitude oscillatory shear (LAOS) and FT-rheology were used to quantify the non-linear rheological behavior and instabilities of a series of wellcharacterized commercial polyethylene (PE). From the latter, we derive the critical non-linearity parameter, F0,c, which corresponds to the normalized intensity of the third harmonic at the critical strain amplitude, γ0,C (defined by the appearance of the second harmonic), normalized by γ0,C. The F0,c is correlated with the high molecular mass fraction of the polymers and with the Deborah numbers. Linear rheological parameters and molecular structures were related to F0,c. An experimental correlation between F0,c of commercial PE melts and pressure fluctuations associated with flow instabilities (sharkskin) was established both for capillary rheometry and extrusion.This work was supported by the Portuguese Foundation for Science and Technology through the Strategic Project PEst-C/ CTM/LA0025/2011 (Strategic Project-LA 25-2011-2012)

Extrusion-instabilities from non-linear rheology for industrial polyethylene

The instabilities that occur in the pressure-driven extrusion of molten polymers are fascinating from the scientific perspective but troublesome and sometimes catastrophic from the industrial one. Flow anomalies like sharkskin, Stick-Slip and Gross Melt fracture can occur in common extrusion operations such as the manufacture of polymeric rods, tubes, sheets, films, tanks or wire coating. Over the years, processors have learned to work around these processing defects by a variety of means: slowing the manufacturing rate, increasing the melt temperature or by- addition of processing additives. All these solutions can be proven extremely costly, thus a significant interest and effort is put in designing and manufacturing polymers with better processing-properties and lower potential to flow-instabilities. Sharkskin, here defined as periodic surface distortions of low amplitude and high frequency, is most commonly observed in polyethylene’s of sufficiently narrow molecular weight. One reason why sharkskin is of great importance is that as the extrusion flow rate increases, it is in many cases the first instability to occur, it is challenging to suppress and affects an important product quality parameter, namely the surface appearance. FT Rheology can be used to study different PE industrial samples with respect to their behavior under the LAOS flow. The resulting non-linearities showed a dependency on both the molecular weight and topology [1]. Filipe et al. [1] found out that for strain amplitudes above a critical value, the stress time signal exhibited an amplitude decay that indicates slip in qualitative agreement with Chen et al. [2] and Hatzikiriakos and Dealy [3]. The second harmonic was thus found to become significant (above noise level) at the onset of the stress amplitude decay and it is found to be a useful indicator of secondary flows or generally instabilities. [1]This work is a master thesis carried out in the frame of the Erasmus Mundus Master Course EURHEO (www.eurheo.eu). The Authors thank the European Union and EACEA for granting project 2008-0099-EURHEO: The Erasmus Mundus Master in Engineering Rheology, and LyondellBasell for sponsoring EURHEO. This work was supported by the Portuguese Foundation for Science and Technology through the Strategic Project PEst-C/CTM/LA0025/2011 (Strategic Project – LA 25-2011-2012)

Fourier-transform Dynamic mechanical thermal analysis - a new tool for the analysis of the thermo-rhelogical behavior of polymers and polymer sandwich composites

A Fourier transform approach to dynamic mechanical thermal analysis is here outlined. The method uses raw instrument data acquisition, data oversampling and subsequent post-processing to evidence nonlinearities in the input/output signals. First results on polymers during time and polymer composite sandwiches during temperature sweep measurements are presented

Linking Models of Polymerization and Dynamics to Predict Branched Polymer Structure and Flow

We present a predictive scheme connecting the topological structure of highly branched entangled polymers, with industrial-level complexity, to the emergent viscoelasticity of the polymer melt. The scheme is able to calculate the linear and nonlinear viscoelasticity of a stochastically branched “high-pressure free radical” polymer melt as a function of the chemical kinetics of its formation. The method combines numerical simulation of polymerization with the tube/entanglement physics of polymer dynamics extended to fully nonlinear response. We compare calculations for a series of low-density polyethylenes with experiments on structural and viscoelastic properties. The method provides a window onto the molecular processes responsible for the optimized rheology of these melts, connecting fundamental science to process in complex flow, and opens up the in silico design of new materials