Temperature and strain rate dependence of yield strain and deformation behavior in polyethylene

The deformation behavior of a range of polyethylene materials which differ with respect to both their short-chain branch content and molecular weight has been studied. Mechanical measurements carried out over a wide range of temperatures have shown that there is a sudden transition in the measured tensile yield strain at a temperature which is dependent on both the grade of material and the applied strain rate. Above the transition temperature all of the materials behave in a nonlinear viscoelastic manner and the wide-angle X-ray scattering patterns obtained have shown that at low applied strains reorientation of the lamellae is observed before necking. Below the transition temperature the materials all behave in an elastic-plastic manner and there is no evidence of lamellar reorientation before necking. This transition in yield mechanism is not apparent when considering the yield stress data alone. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 545–552, 1997     

Adiabatic Calorimetry of a Very Low Density Polyethylene Copolymer

The heat capacity of a very low-density polyethylene copolymer (VLDPE) was measured between 10 and 410 K. Using two sets of literature data for the fully crystalline and the fully amorphous states, crystallinities were calculated as a function of temperature. During the stabilisation periods in the melting process no equilibrium is reached, because of an exothermic process, which is attributed to (re)crystallization. Values for the enthalpy of melting and for the heat capacity of the liquid are given. This revised version was published online in August 2006 with corrections to the Cover Date.     

Liquid–liquid phase separation in linear low-density polyethylene

The issue of multiple equilibrium phases in compositionally heterogeneous random copolymers is studied with an ethylene-butene copolymer representative of many linear low-density polyethylenes (LLDPE). This material has a dispersed minority phase (volume fraction ?^β ≈ 0.02) of highly branched, amorphous chains. A thermodynamic calculation of the equilibrium liquid state is done using the distribution of chain branching from temperature rising elution fractionation and the Flory-Huggins interaction parameter χ_AB for linear and ethyl branched C₄H₈ repeat units. The calculation indicates that this copolymer is metastable, between the binodal and spinodal at a melt temperature of 150°C. The predicted volume fraction of the second phase, ?^β = 0.016, is in good agreement with experiment. This work is the first to compare directly the observed and calculated two-phase behaviors in a random copolymer. © 1994 John Wiley & Sons, Inc.     

Effects of cooling rate on crystallinity of i-polypropylene and polyethylene terephthalate crystallized in nonisothermal conditions

Crystallization of polyethylene terephthalate and i-polypropylene in nonisothermal conditions is studied by means of differential scanning calorimetry. Measurements, carried out at several constant cooling rates, are interpreted in terms of a new theory^1,2 that takes into account effects related to a transient, nonsteady-state course of the process as well as athermal nucleation, which may occur under such circumstances. This article gives preliminary results based on analysis of final crystallinity reached at the end of cooling. Results indicate that the classical isokinetic approach is not adequate to describe crystallization kinetics at high cooling rates. A parameter quantizing the magnitude of deviations from isokinetic law is evaluated. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2821–2827, 1999     

Recoverable strain storage capacity of shape memory polyethylene

Shape memory polymers (SMPs) are an important class of smart materials. So far the focus of such polymers was to find suited triggers for various application fields. Thus, the potential of most of these macromolecular networks regarding their maximally storable strain capability was not explored. In this study, the polyethylenes HDPE, LDPE, and ethylene‐1‐octene (EOC) were systematically investigated with respect to their strain storage potential. To achieve maximum strains, the polymers were chemically cross‐linked in such a way that they are at the borderline between thermoplastics and elastomers. All investigated polymers showed higher strain storage than literature reported systems and exhibited excellent shape memory parameters. The highest stored strain was found for networks of EOC with fully recoverable 1400%. Interestingly, this value could not be enlarged by using EOCs with higher molecular weight, which is probably due to increasing content of entanglements as confirmed by Mooney‐Rivlin. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1033–1040     

Structure transfer from a polymeric melt to the solid state. Part I: The influence of chain entanglements in linear polyethylene

Melt-spinning experiments were carried out at high quenching rates. Mechanical properties (elongation at break, natural draw ratio, and elastic recovery) have been measured. Significant variations of these quantities were observed when extrusion conditions were changed. This has been attributed to different states of the entanglements within the melt, which are directly transferred into the solid state. This intercorrelation between melt and solid-state properties has been substantiated in the case of rapidly cooled samples, where a poor crystallization on one side and a simultaneous good conservation of melt history on the other side are provoked.     

Dynamic study of the noncrystalline phase of 13C-labeled polyethylene by variable-temperature 13C CP/MAS NMR spectroscopy

The ¹³C spin-lattice relaxation times T₁ of ¹³C-labeled polyethylene crystallized under different conditions were measured at temperatures from ?120 to 44°C by variable-temperature solid-state high-resolution ¹³C nuclear magnetic resonance (NMR) spectroscopy, in order to determine accurately the dynamics of the noncrystalline region of the polymer. From these results, it was found that the T₁ minimum for the CH₂ carbons in the noncrystalline region of solution-crystallized polyethylene with high crystallinity appears at higher temperature by about 20°C than that of melt-quenched polyethylene with low crystallinity. This means that the molecular motion of the CH₂ carbons in the noncrystalline regions is more constrained at a given temperature in the material of higher crystallinity. Furthermore, dynamics of the noncrystalline region is discussed in terms of the ¹³C dipolar dephasing times.     

The dependence of slow crack growth in a linear polyethylene on test temperature and morphology

The slow crack growth behavior of a linear polyethylene with different morphologies was studied by using three point bending with a single edge notched specimen at testing tem-peratures from 30 to 80°C. The morphology was varied by annealing the quenched material at temperatures from 86°C to 135°C. It was found that at test temperatures of 60°C or less, the changes in failure time with annealing temperature are very similar to the change in density with a maximum at 130°C. At testing temperatures above 60°C, the relationship of between failure time and annealing temperature is altered when the test is in the range of the α transition temperature. These results indicate that with respect to slow crack growth in the case of a homopolymer the strength of the crystals is relatively more important than the number of tie molecules. © 1993 John Wiley & Sons, Inc.     

Detection of long-chain branches in polyethylene via rheological measurements

The detection of long-chain branches(LCB) in polyethylene is of considerable importance as the processing properties of polyethylene are strongly affected by even a small amount of LCB. While the conventional characterization techniques such as GPC-MALS and13 C NMR fail or take very long time to detect low content of LCB, we turn to the rheological method, which is more sensitive to LCB. In our study, we performed oscillatory shear test, creep test and stress relaxation test on two series of metallocene linear low density polyethylene(LLDPE), revealing that the resins with LCB show higher zero-shear-rate viscosity, retarded relaxation and higher flow activation energy than those without or with less LCB. The resins with LCB showed shear thinning at very low shear rate and their zero-shear-rate viscosities were obtained via creep test. The content of LCB was quantitatively estimated from the flow activation energy. In addition, the modulus-time curves during stress relaxation of melt of the different resins obeyed the power law. The exponent of the resins with more LCB was 0.7, different from that of the resins with less LCB, around 1.7.     

Nanoscale calorimetry of isolated polyethylene single crystals

We used thin‐film differential scanning calorimetry to investigate the melting of isolated polyethylene single crystals with lamellar thicknesses of 12 ± 1 nm. We observed the melting of as few as 25 crystals. Over a wide number of crystals (25–2000 crystals), the heat of fusion was 40% larger than the bulk value. The melting temperature of the isolated single crystals was 123 ± 2 °C, 9 °C lower than that of the bulk material. We also measured the heat of fusion of quenched crystals (±15%) over a wide range of heating rates (20,000–100,000 K/s). Annealing the quenched crystals resulted in shifts in the endotherm peak by as much as 15 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1237–1245, 2001     

Thermoelastic behaviour of polyethylene terephthalate on approaching the glass transition range

The mechanical and thermal properties of the simple linear reversible extension of a solid can be well described by means of Young's modulus and linear thermal expansion coefficient. In this paper it is shown that on stretching near the glass transition temperature the temperature dependence of Young's modulus must also be taken into account.Dedicated to Prof. Dr. H.-G. Kilian on the occasion of this 60th birthday.     

Identification and quantitative analysis of the intermediate phase in a linear high-density polyethylene

The observation of chain conformation and mobility in polyethylene by solid-state ¹³C magic angle spinning (MAS) nuclear magnetic resonance (NMR) permits unambiguous identification and quantitative analysis of an intermediate phase. The carbon-carbon bonds in the intermediate phase adopt, on the average, an all-trans conformation and are more mobile than in the crystalline state (room temperature rate of reorientation ≈ 10⁷ Hz). Comparisons of crystallinities by differential scanning calorimetry, wide-angle x-ray diffraction, and NMR support the high orientation of the intermediate phase and suggest a lower heat of fusion than for the crystals. Results from ¹³C spin-lattice relaxation and ¹H spin diffusion show that the mass fractions are ≈ 20% and the domain sizes ≈ 36 Å. Both change with crystallization and annealing conditions. © 1994 John Wiley & Sons, Inc.     

The deformation behaviour of polyethylene shish-kebabs produced by stirring-induced crystallization

Polyethylene mats of shish-kebab fibrils were prepared from solution by stirring-induced crystallization, and subjected to deformation. A morphological study by scanning electron microscopy showed that the elementary shish-kebabs are elongated during drawing. For low draw ratios, the average distance between the lamellae on the fibrils increases proportionally to the draw ratio. The invariance of the fibril diameter upon drawing indicates a transformation of lamellar into fibrillar material. The molecular topology which underlies this deformation mode is discussed and related to the crystallization process.     

Grafting of maleic anhydride onto linear polyethylene: A Monte Carlo study

Monte Carlo simulation was used to study the graft of maleic anhydride (MAH) onto linear polyethylene (PE‐g‐MAH) initiated by dicumyl peroxide (DCP). Simulation results revealed that major MAH monomers attached onto PE chains as branched graft at higher MAH content. However, at extremely low MAH content, the fraction of bridged graft was very close to that of branched graft. This conclusion was somewhat different from the conventional viewpoint, namely, the fraction of bridged graft was always much lower than that of branched graft under any condition. Moreover, the results indicated that the grafting degree increased almost linearly to MAH and DCP concentrations. On the other hand, it was found that the amount of grafted MAH dropped sharply with increasing the length of grafted MAH, indicating that MAH monomers were mainly attached onto the PE chain as single MAH groups or very short oligomers. With respect to the crosslink of PE, the results showed that the fraction of PE‐(MAH)_n‐PE crosslink structure increased continuously, and hence the fraction of PE‐PE crosslink decreased with increasing MAH concentration. Finally, quantitative relationship among number average molecular weight of the PE, MAH, and DCP contents was given. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5714–5724, 2004     

Crystallization regime characteristics of exfoliated polyethylene/vermiculite nanocomposites

Polyethylene (PE)–vermiculite (VMT) nanocomposites containing 0.5, 2, 4, 6, and 8 wt % clay loadings were fabricated via direct melt compounding in a twin‐screw extruder. Crystallization kinetics was investigated by means of polarized optical microscopy and differential scanning calorimetry. Moreover, the kinetics of the spherulitic growth of PE–VMT nanocomposites was evaluated with the Lauritzen–Hoffman (LH) secondary nucleation theory. The results showed that the PE–VMT nanocomposite exhibits crystallization regime characteristics. The nucleation constants for regimes I and II were determined from the slope of the LH plots. The fold surface energies (σ_e) of the PE–VMT nanocomposites were estimated from these slopes. The σ_e values were found to decrease with an increasing VMT content, up to 2%, in regime I. Further increasing the VMT content resulted in a slight increase of the σ_e values. In regime II, the σ_e values of the nanocomposites were generally lower than that of pure PE. These results demonstrated that the exfoliated silicate layers acted as effective nucleation sites for the secondary nucleus of the nanocomposites. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 253–263, 2005     

Small strain behavior of polyethylene: In situ SAXS measurements

Stack lamella deformation depends on their orientation with respect to the loading axis, the intrinsic properties of the lamellae, and the mechanical coupling between crystalline and amorphous phases. The aim of this work is to investigate the influence of the stress transmitter (ST) density and the crystallinity X_c on the local deformation. A wide experimental campaign has been undertaken on several polyethylenes with controlled molecular parameters and subjected to different thermal treatments. The ST density has been evaluated by the natural draw ratio and calculated by the Brown's model. The local deformation was measured by SAXS along a tensile test by using the long period stretching of the equatorial lamella stacks. The ratio ε_local/ε_macro was found to be a constant close to 0.5. This surprising low value has highlighted that the equatorial regions could be either the stiffest zone of the spherulite or submitted to a lower stress. It is proposed that the stability of the ratio ε_local/ε_macro is the result of two opposite phenomena: On one hand, the increase of X_c leads to unload the equatorial regions due to partial percolation of the crystalline phase and so decreases the stresses. On the other hand, when increasing X_c, the ST density decreases which causes the decrease in the local equatorial modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1535–1542, 2010     

Relationship between fracture toughness and intrinsic deformation parameters in isotropic and flow‐oriented linear low‐density polyethylene

The fracture toughness of isotropic and flow‐oriented linear low‐density polyethylene (LLDPE) is evaluated by the Essential Work of Fracture (EWF) concept, with a special setup of CCD camera to monitor the process of deformation. Allowing for the molecular orientation, flow‐oriented sample, prepared via melt extrusion drawing, is stretched parallel (oriented‐0°) and perpendicular (oriented‐90°) to its original melt extrusion drawing direction, respectively. The obtained values of specific EFW w_e are 34.6, 10.2, and 4.2 N/mm for the oriented‐0°, isotropic and oriented‐90° sample, respectively. With knowledge of intrinsic deformation parameters deduced from uniaxial tensile tests, moreover, a relationship between specific EFW w_e the ratio of true yield stress to strain hardening modulus σ_ty/G is well established. It means that the fracture toughness of polyethylene is determined by both crystalline and amorphous parts, rather than by one of them. Moreover, the true yield stress seems to be nondecisive factors determining the fracture toughness of polyethylene. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2880–2887, 2006     

Calorimetry in the science of building materials

A new conceptual approach is proposed for evaluation of the most important properties of cement-containing composites on the basis of the rate and degree off completeness of hardening processes and study of the porous structure during material formation, within the limits of the system “composition-structure-process-property”.