Wall slip and melt fracture of poly(lactides)

The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 10⁴–10⁵ g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in the presence of PCL.     

Rheological and foaming behavior of linear and branched polylactides

In this work, a chain extender (CE) was added to polylactide (PLA) to improve its foamability. The steady and transient rheological properties of neat PLA and CE-treated PLA revealed that the introduction of the CE profoundly affected the melt viscosity and elasticity. The linear viscoelastic properties of CE-enriched PLA suggested that a long-chain branching (LCB) structure was formed from the reaction with the CE. LCB-PLA exhibited an increased viscosity, more shear sensitivity, and longer relaxation time in comparison with the linear PLA. The LCB structure was also found to affect the transient shear stress growth and elongational flow behavior. LCB-PLA exhibited a pronounced strain hardening, whereas no strain hardening was observed for the linear PLA. Batch foaming of the linear and LCB-PLAs was also examined at foaming temperatures of 130, 140, and 155 °C. The LCB structure significantly increased the integrity of the cells, cell density, and void fraction.     

Rheological and molecular characterization of long-chain branched poly(ethylene terephthalate)

Reactive extrusion with pyromellitic dianhydride (PMDA) and tetraglycidyl diamino diphenyl methane (TGDDM) was conducted to create long-chain branched poly(ethylene terephthalate) (LCB-PET). The mechanical and molecular properties were analyzed by linear and non-linear viscoelastic rheology in the melt state and by size-exclusion chromatography measurements with triple detection. The two tetra-functional chain extenders lead to strong viscosity increases, increasing strain hardening effects, and increasing LCB with increasing chain extender concentration. Molecular stress function model predictions show good agreement with the elongational data measured and allowed a quantification of the strain hardening. Analysis of SEC triple detection data shows a strong increase of the average molar mass, polydispersity, radius of gyration, and hydrodynamic radius with increasing chain extender concentration. Branching was confirmed by a decreasing Mark-Houwink exponent, and the analysis of the contraction of the molecule revealed either star-like, comb-like, random tree-like or hyperbranched structures depending on concentration and type of chain extender.     

The structure and properties of long-chain branching poly(trimethylene terephthalate)

A multifunctional epoxide chain extender (ADR4370S) was used to increase the molecular weight of poly(trimethylene terephthalate) (PTT). And the effects of ADR4370S content on the molecular structure, melt viscosity, and rheological properties of PTT were studied. It is found that a star-type topological structure is formed in PTT by introduction of ADR4370S, and the balance torque, intrinsic viscosity, and molecular weight are increased by increasing ADR4370S dosage. The rheological measurement results show that the elastic modulus, complex viscosity, and shear thinning behavior of long-chain branching PTT are increased with the concentration of ADR4370S. The presence of broadened relaxation time spectrum and a long relaxation time mode for the PTT with 1.50 wt% ADR4370S demonstrate that the cross-linking reaction occurs, and the gel forms in the PTT system.     

液晶高分子-各向异性流体挤出拉伸分岔研究

应用共转导数型本构方程研究了液晶高分子纺丝挤出过程的拉伸黏度,应用计算机符号运算软件 Maple得出解析表达式,拉伸黏度与拉伸率之间关系(随剪切速率变化)表明存在分岔现象,得出拉伸黏度显著高于相应的剪切黏度,解释了液晶高分子熔体挤出时不发生挤出胀大的物理机制．     

Linear viscoelastic models: Part I. Relaxation modulus and melt calibration

Constitutive models for the linear viscoelasticity of polymers are presented for the relation between the relaxation modulus and the molecular weight distribution (MWD). We also compute the MWD from a simulated relaxation modulus curve by first obtaining the rheologically effective distribution (RED) as a function of time, and converting this into the MWD by melt calibration; that is, the relation between timescale and the molecular weight. This procedure has similarities with the widely used universal calibration with solved polymers. The main principles of our technique are applied here to familiar relaxation modulus data, for which we present two models: (1) an analytical model derived from control theory, which is known capable of modelling partially observed system and (2) a practical characteristic model for obtaining usable results. No relaxation time or spectrum procedures are used to model the process of linear viscoelastic relaxation. The use of relative calculations and melt calibration dispenses with the need to know the real chain structures such as branching and entangled chain dynamics, and the model remains useful for future investigations of polymer chain structures and dynamics, such as using tube theory.     

Comparative investigations of surface instabilities (“sharkskin”) of a linear and a long-chain branched polyethylene

An experimental study of the physical origin and the mechanisms of the sharkskin instability is presented. Extrusion flows through a slit die are studied for two materials: a linear low density polyethylene (LLDPE) which exhibits sharkskin instability for flow rates larger than an onset value and a low density polyethylene (LDPE) which does not show any instability over a broad range of flow rates. By combining laser-Doppler velocimetry (LDV) with rheological measurements in both uniaxial extension and shear, the distributions of tensile and shear stresses in extrusion flows are measured for both materials. The experimentally measured flow fields appear to be qualitatively similar for both the unstable (LLDPE) and stable case (LDPE): around the die exit the flow accelerates near the boundaries and decelerates around the flow axis. The fields of the axial gradients of the axial velocity component are, however, quite different in the two cases. In the unstable case there exists a strongly non-uniform transversal distribution of velocity gradients near the die exit. This non-uniformity of the distribution of gradients is significantly smaller in the stable case. Significant differences in the extensional rheological properties of the two materials are found as well. Due to its branched structure, the LDPE is able to sustain higher tensile stresses prior to failure. Measurements of the distributions of tensile stresses around the die exit reveal a stress boundary layer and a stress imbalance between the boundaries and the bulk. The magnitude of the stress imbalance exceeds the melt strength in the experiments with the LLDPE which causes the failure of the material in the superficial layers and results in the emergence of the sharkskin instability. In the experiments with the LDPE, the magnitude of the stress imbalance remains smaller than the melt strength which explains the lack of an instability. The measured shear stresses around the die exit are significantly smaller than the tensile stresses, suggesting that the shear component of the flow plays no significant role in the emergence of the sharkskin instability.     

Dynamic and transient rheological properties of glass filled polymer melts

The dynamic and transient rheological properties of a low density polyethylene melt and a plasticized polyvinylchloride melt filled with glass beads were measured at 200 °C and 180 °C respectively in a modified Weissenberg Rheogoniometer R-17. Its main modification consisted of the use of a piezoelectric transducer instead of the conventional torsion bar, and of the interfacing of a microcomputer Apple II plus to the Rheogoniometer for data acquisition and analysis. The glass beads were pretreated with silane and titanate coupling agents to observe the effect of the chemical modification of the polymer filler interface on these properties. It was observed that both the dynamic viscosity and the storage modulus increased with the weight fraction, but this last parameter did appreciably affect the stress growth and stress relaxation curves of the polymeric matrices at low shear rates. The effect of coupling agents on these properties was varied.     

Comparison of squeeze flow and vane rheometry for yield stress and viscous fluids

Two principal squeeze flow modes are investigated for yield stress and Newtonian materials squeezed by a constant force, F, between plates of equal or unequal diameters. In mode A, the material fills the space between the plates and is extruded at their periphery as their separation decreases. Experiments are described to measure the contribution to F from the extrudate. In mode B, all the material remains in contact with the planes of the plates as their separation decreases; there is no extrudate. The results of mode B experiments agree closely with the predictions of theory and give rheological parameters in fair agreement with those measured by the rotational vane method. The material properties and extrusion behaviour which complicate mode A experiments are discussed.     

Stress relaxation after a step strain in uniaxial extension of polyisobutylene and polyethylene

The morphology of molten polymeric materials is known to be less sensitive to shear than to extensional deformations. However, it is not easy to characterise molten polymeric materials in simple extensional flows due to the large number of experimental difficulties involved. This has led to the effective absence of a structure-preserving, morphology probing technique similar to the ones commonly found in shear, i.e., the equivalent of stress relaxation and oscillatory experiments. It is the aim of the present work to demonstrate the usefulness of a recently developed experimental technique that enables stress relaxation experiments after a step strain in uniaxial extension to be performed. Results are presented for two model melts (polyisobutylene, PIB, of different molecular weights) and for a series of linear low-density polyethylenes, LLDPE, in which the molecular structure (molecular weight, MW, molecular weight distribution, MWD and degree of long chain branching, LCB) is changed systematically. It is shown that, for both types of materials, stress relaxation experiments in extension yield quantitatively correct results and that this technique is more sensitive to differences in molecular structure than oscillatory experiments in shear.     

Influence of thermal history on rheological properties of various bitumen

This paper focuses on the influence of thermal history on the rheological properties of unmodified and polymer modified bitumen (PMB), measured at elevated service temperatures, and contributes to the development of test methods for measuring binder properties, which can be used as indicators for asphalt rutting. It was found that the storing and preparation conditions prior to the rheological measurement can have a large influence, especially in the range of long loading times or low frequencies. For elastomer modification, the homogenization and sample pouring temperature and the corresponding change in microstructure, as revealed by fluorescence microscopy, have a large impact on the rheological measurements. For binders with semi-crystalline modifiers, the storage conditions between sample preparation and testing have the largest impact on the rheological behaviour. This can be related to variations in crystallinity, as shown by calorimetry. The main conclusion from this study is that sample preparation and handling is extremely important for the rheological properties of PMBs. Reproducibility can only be achieved when these conditions are controlled more accurately, especially in binder specification tests for rutting susceptibility.     

The use of the capillary rheometer for the rheological evaluation of extrudable cement-based materials

In this paper, the rheological properties of an extrudable cement-based paste are investigated by means of an original ram extrusion apparatus (capillary rheometer). The experimental results indicate that a careful measurement of the die pressure is necessary to obtain a realistic viscosity vs shear rate curve, as required in extrusion technology. In particular, it is shown that the optimal test configuration is when the pressure measurement is made directly inside the rheometer die. By applying this rheological methodology in steady-state conditions, it has been observed that the extrudable cement-based material here evaluated obeys to a simple power–law equation, in the range of shear rates investigated, which are suitable for an industrial extrusion process. This paper was presented at the third Annual European Rheology Conference (AERC) held in Hersonissos, Crete, Greece, April 24–27, 2006.     

A generalized relation between MWD and relaxation time spectrum

An inversion procedure for converting linear viscoelastic properties of polymer melts into molecular weight distribution (MWD) described by the generalized exponential function (GEX) has been implemented and applied in a previous work (Cocchini F, Nobile MR. Rheol Acta 42:232–242, 2003). It is based on the elegant relationship between the relaxation spectrum and the MWD function proposed by Thimm W, Friedrich C, Marth M. J Rheol 43:1663–1672 (1999). In the present paper, such a methodology has been generalized to properly account for sharp MWDs, in particular, nearly monodisperse or blends of nearly monodisperse polymer samples. The generalized relationship has been verified and calibrated using the BSW kernel (Baumgaertel M, Schausberger A, Winter HH. Rheol Acta 29:400–408, 1990) to describe the rheological behavior of some Polystyrene samples from the literature, in terms of the known MWD. Then, it has been successfully applied to the inversion problem for a wider set of samples, with both broad and sharp distributions. The Rouse contribution at high frequencies and the accelerating effect on the relaxation times due to polydispersion have been also addressed.     

Rheological evidence of modifications of polypropylene by β-irradiation

Electron beam irradiation can be used to induce chemical changes in polymers. The resulting reactions lead to both degradation and crosslinking of polymer chains, depending on reaction conditions. In neat polypropylene, degradation dominates and results in a decrease of molecular weight and worsening of mechanical properties. Enhanced crosslinking can be achieved by utilising a polyfunctional monomer. Triallylisocyanurate (TAIC) serves this purpose and can be used to crosslink polypropylene effectively. The corresponding changes of the rheological properties can be observed using oscillatory and creep experiments when changing the amount of TAIC added as well as the absorbed radiation dose. Depending on these parameters, we attribute the rheological properties to chain degradation or formation of a network and gelation. The phase angle vs the dynamic modulus plot is a useful analytical tool for characterisation of the resultant structures. Some samples showed behaviour that leads us to assume broadening of the molecular weight distribution and long chain branching. Resulting from the parameter dependencies, a topological state diagram is proposed to map parameter values to corresponding polymer structures.     

Rheological,morphological, and interfacial properties of compatibilized PLA/PBAT blends

The focus of this paper was to gain a true understanding of the impact of a multifunctional epoxide (Joncryl®;ADR-4368) on the interfacial properties of biopolymer blends based on poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT). The effect of Joncryl on the shear rheological, morphological, and interfacial properties of the blends was systematically investigated. For the deformed drop retraction experiments, different sandwich model systems (droplet/matrix), representing various scenarios of compatibilization, were prepared, aiming to probe the role of the epoxy-functionalized chains on the interface. The decrease of the interfacial tension in the modified/compatibilized PLA_PBAT and the formation of the PLA-Joncryl-PBAT copolymer were highlighted. A new relaxation peak relative to this copolymer was detected by the relaxation spectrum. Transient start-up shear and nonlinear stress relaxation experiments were carried out and confirmed the obtained results. In addition, the interface contribution was demonstrated using the Lee and Park model. The relaxation time increased with the amount of added Joncryl. Hence, the coexistence of chain extension/branching chains coupled to the PLA-Joncryl-PBAT copolymer formation had to be taken into account to explain the improved mechanical properties.     

Polymer motion as detected via dielectric spectra of 1,4-cis-polyisoprene under large amplitude oscillatory shear (LAOS)

In order to investigate the global polymer chain motion under large amplitude oscillatory shear (LAOS), the dielectric properties under LAOS are measured by a new rheo-dielectric combination. The design of the rheo-dielectric setup including a new fixture and modified oven is explained in detail. For 1,4-cis-polyisoprene, having type-A dipoles parallel to the backbone, the dielectric dipoles can detect the global polymer chain motion via the end-to-end vector. Thus rheological and dielectric (rheo-dielectric) properties reflect the dynamics of the polymer chain motion under LAOS. In this study, we investigate the rheo-dielectric properties under LAOS with 1,4-cis-polyisoprene as model component. As the strain amplitude was increased under LAOS, the relaxation strength from dielectric properties decreased for the whole spectra without changing the shape of the dielectric spectra. These results are analyzed on the basis of the molecular model of dynamic tube dilation (DTD) induced by the convective constraint release (CCR). It is found that the global chain motion under LAOS flow is affected by both rheological frequency and strain amplitude. It is also observed that segmental motion is affected via the oscillatory frequency under LAOS. This result differs from experiments under steady shear.     

Melt rheology of long-chain-branched polypropylenes

Rheological properties of long-chain-branched isotactic polypropylene (PP) via copolymerization with a very small amount of nonconjugated α,ω-diene monomer using metallocene catalyst system in both linear and nonlinear regions were investigated, comparing with conventional linear and long-chain-branched PP modified at postreactor. Although comonomer incorporation was equal to 0.05 mol% or less, it caused high molecular weight, broad molecular weight distribution, and long-chain branching. A detailed study on the effect of diene incorporation on the polymer properties was conducted, comparing with modified PP in postreactor. Polymer chain microstructures were characterized by gel permeation chromatography with multiangle laser light scattering (MALLS), differential scanning calorimetry, and rheological means: dynamic viscoelasticity, step-strain, uniaxial elongational flow measurements, and large amplitude oscillatory shear. The PP, which incorporated a small amount of diene monomer, showed significantly improved viscoelastic behaviors. The diene-propylene copolymer containing long-chain branches showed extremely long relaxation mode under shear and outstanding viscosity increase under elongational flow, so-called strain hardening. The difference in microstructure of diene-propylene copolymer with modified PP with long-chain branches is investigated by MALLS and rheological characterizations.     

Linear viscoelastic model for elongational viscosity by control theory

Flows involving different types of chain branches have been modelled as functions of the uniaxial elongation using the recently generated constitutive model and molecular dynamics for linear viscoelasticity of polymers. Previously control theory was applied to model the relationship between the relaxation modulus, dynamic and shear viscosity, transient flow effects, power law and Cox–Merz rule related to the molecular weight distribution (MWD) by melt calibration. Temperature dependences and dimensions of statistical chain tubes were also modelled. The present study investigated the elongational viscosity. We introduced earlier the rheologically effective distribution (RED), which relates very accurately and linearly to the viscoelastic properties. The newly introduced effective strain-hardening distribution (RED_H) is related to long-chain branching. This RED_H is converted to real long-chain branching distribution by melt calibration and a simple relation formula. The presented procedure is very effective at characterizing long-chain branches, and also provides information on their structure and distribution. Accurate simulations of the elongational viscosities of low-density polyethylene, linear low-density polyethylene and polypropylene, and new types of MWDs are presented. Models are presented for strain-hardening that includes the monotonic increase and overshoot effects. Since the correct behaviour at large Hencky strains is still unclear, these theoretical models may aid further research and measurements.