Dielectric relaxation studies of dipolar aromatics in polyethylene I. Spherulitic low-density polyethylene

Dielectric relaxation studies have been used as a means of investigating the behavior of aromatic molecules dissolved in a semicrystalline polymer. All molecules show a relaxation in the vicinity of the γ loss peak of polyethylene, while several show an additional peak at lower temperatures. The latter is interpreted as an indicator of molecular motions below the glass transition and the former is to be associated with the onset of molecular motions on a local scale as the glass transition is approached. The α loss process of polyethylene is seemingly unaffected by the presence of the molecules. Integration of loss curves shows that, at most, 50% of the molecules are able to relax but that the fraction relaxing increases with increasing temperature, suggesting the presence of a wide distribution of sites within the polymer.     

Triplet-state ESR spectra of aromatic molecules oriented in stretched polyethylene films

Triplet-state ESR spectra show that when the aromatic molecules anthracene, anthracene-d₁₀, terphenyl, and acridine are dissolved in stretched, low-density polyethylene films, a fraction of these solute molecules become oriented with their X axis along the stretch direction but with their XY planes oriented randomly to the stretch direction. The fraction of molecules involved in the orientation process is largest for terphenyl.     

The influence of morphology on the dielectric and dynamic mechanical behavior of a linear low-density polyethylene

The dielectric and viscoelastic relaxation behavior of linear low-density polyethylene has been investigated. All three usual relaxation regions occur; however, the α process is more dominant than in conventional low-density polyethylene made dielectrically active through the introduction of carbonyl groups. The observed behavior is very sensitive to orientation and morphology. There are significant differences between oriented samples (stretching or extrusion) and a single-crystal texture specimen. Effects are caused by dipolar orientation and dipolar immobilization, the relative importance of each effect varying with specimen preparation. Active dipolar groups are not carbonyls, or any common organic species, but appear to be due to chain ends having residual dipoles caused by the catalyst used in synthesis.     

Mechanical behavior of a highly oriented polyethylene rod

Dynamic mechanical and single step stress relaxation data are presented for a highly oriented linear polyethylene rod as produced by a high pressure extrusion technique. The results indicate that both major relaxation processes normally observed in unoriented polyethylene are virtually absent in torsion, whereas they are present in flexure. Stress relaxation in torsion data show a very highly nonlinear behavior when compared to similar data for an unoriented polyethylene rod.     

NMR study of diffusion of butane in linear polyethylene

The diffusion coefficient of butane in linear polyethylene at room temperature as a function of the vapor pressure of butane was measured by the spin-echo method with a pulsed magnetic field gradient. For the Special morphology of randomly oriented stacks of parallel lamellas the detour factor is 1/3. As long as the blocking factor and migration through the lamellas can be neglected, the local diffusion coefficient D_a of the small molecules through the amorphous layers in the direction parallel to the lamellas is three times the apparent diffusion coefficient D derived from the decay of the amplitude of the spin echo under the assumption of an infinitely extended homogeneous medium. The diffusion coefficient and the spin–spin relaxation time both increase exponentially with increasing pressure, i.e., butane concentration in the polymer, while the spin-lattice relaxation time is pressure independent and seems to be determined by interaction with the amorphous polyethylene matrix.     

Infrared studies of drawn polyethylene. II. Orientation behavior of highly drawn linear and ethyl-branched polyethylene

Infrared dichroism is employed to study the orientation of chain molecules in linear and ethyl-branched polyethylene in the crystalline and noncrystalline regions during drawing and subsequent annealing. A crystalline (1894 cm^?1) and a noncrystalline (1368 cm^?1) band, as well as the bands at 909 cm^?1 and 1375 cm^?1 resulting from vinyl endgroups and methyl endgroups and sidegroups, are studied. For these bands relative orientation functions are derived and compared as a function of draw ratio and annealing temperature. It is shown that the relative orientation functions as derived from the dichroism of the noncrystalline, vinyl and methyl bands follow the same curve while the orientation function for the crystalline bands does not. These results support a two-phase model for partially crystalline polyethylene and additionally favor segregation of the endgroups and sidegroups in the noncrystalline component during crystallization. It is further shown that shrinkage occurs at the temperature at which the noncrystalline chain molecules start to disorient. From the dichroism of the methyl groups in ethyl-branched polyethylene, a value for the mean orientation of the noncrystalline chain molecules is calculated. We obtain for the orientation function of the noncrystalline regions at highest draw ratios (λ = 15–20), f = 0.35–0.57, while the chain molecules in the crystallites are nearly perfectly oriented (f ≈ 1.0). On the assumption that the noncrystalline component consists of folds, tie molecules, and chain ends, the different contributions of these components to the overall orientation are estimated. From these the relative number of CH₂ groups incorporated into folds, tie molecules, and cilia can be derived. Further, on the basis of a simple structural model, the relative number of chains on the crystal surface contributing to the different noncrystalline components and their average length are estimated.     

Creep and yield behaviour of polyethylene

Extensive studies of the creep and recovery behaviour of oriented polyethylene have been undertaken because of the potential engineering applications of high modulus polyethylene fibres and large section oriented materials made by the Leeds die-drawing process. Two primary requirements are a valid protocol for the practical assessment of the creep behaviour and an adequate formal representation which can provide the basis for a physical interpretation of the creep processes in terms of the structure of the oriented materials. The practical requirements of improved creep performance lead to examination of the effects of molecular weight and copolymerisation on creep and recovery, together with studies of the effects of radiation cross-linking either before or after drawing. A particular new feature has been the development of a process for cross-linking in acetylene, where it has been shown that diene bridge crosslinks are produced with very little chain scission. The protocol adopted for assessing creep behaviour is to construct creep strain rate/strain (Sherby-Dorn) plots which, for highly oriented fibres, show constant strain rates after an initial period. These equilibrium strain rates can be modelled by two thermally activated processes acting in parallel. In recent research these studies have been extended to fibres of lower draw ratio and to isotropic polyethylene. It has been possible to establish links between creep and yield behaviour, and to show that slow crack propagation can be understood in terms of a combination of yield and creep to failure.     

Shearing of oriented polyethylene and polypropylene

To learn more about the out-of-plane deformation of polymer lamellae during drawing, we have measured the resistance to shear along various planes in uniaxially oriented polyethylene and polypropylene. Fissures parallel to the orientation axis in oriented materials always cause too small an experimental value for the resistance of crystal glide parallel to the chain axes, but a rough estimate for the resistance to crystal glide is obtained using the elastic anisotropy. Also, the results suggest that kinking can be easier than glide when glide is inhibited by tie molecules.     

NMR study of molecular motion in oriented long-chain alkanes. II. Oriented mats of polyethylene single crystals

Measurements of the NMR second moment of a uniaxially oriented sample of polyethylene single crystals in the range of temperatures from ?196°C to 130°C and its dependence on the alignment angle γ between the orientation axis (preferential direction of the molecular chains) and the NMR magnetic field are presented. The experimental results are discussed mainly with respect to the high temperature relaxation, called the α process, in polyethylene. They are compared to theoretical predictions made for a number of mechanisms of molecular motion in Part I of this work. Only one of the mechanisms considered is found to be in quantitative agreement with experiment, the mechanism here referred to as flip-flop motion. This consists of thermally activated rotational jumps of the crystalline chain segment between folds around its axis between two equilibrium sites in the lattice. Each rotational jump through 180° is accompanied by a shift of the molecule along its axis by one CH₂ group. The discussion of the low-temperature relaxation of polyethylene, the γ process, is based partly on the above measurements and partly on measurements of second moments for unoriented polyethylene samples varying widely in morphology and noncrystalline content. The decrease of the second moment observed with these samples between ?196°C and 20°C is taken as a measure of the intensity of the γ process. A linear correlation is found between the decrease in the second moment, designated ΔS, and the noncrystalline content, 1 ? α_m; this can be represented by ΔS = 1.4 + 22.1(1 ? α_m). It is shown that neither the crankshaft mechanism not the kink mechanism is able to account quantitatively for this result. The model of a chain end moving in a vacancy fails to adequately describe the angle dependence of ΔS in oriented polyethylene single crystals. The “sandwich model” of a polyethylene single crystal, in which the crystalline core is covered by noncrystalline surface layers, is in better agreement with observations.     

Single-molecule spectroscopy of uniaxially oriented terrylene in polyethylene.

Single terrylene molecules doped into linear low-density polyethylene can be oriented by tensile deformation of the matrix. In measurements on ensembles at ambient and on single terrylene molecules at cryogenic temperature, strong orientation along the stretching direction was observed by polarization-resolved confocal microscopy. At cryogenic temperatures narrow and spectrally stable zero-phonon lines were found. The low saturation intensity of 0.07 W cm(-2) is consistent with an uniaxial orientation of terrylene in the sample plane.     

Electronic States of Phenylene and Naphthylene Bicyclobutanes. Linear Dichroism in Stretched Polyethylene

Excited singlet states of bicyclobutylene-benzene (2) , 1,2:4,5-bis (bicyclo-butylene)benzene (3) , 2,3-bicyclobutylene-naphthalene (4) , and 1,8-bicyclobutylene-naphthalene (5) are investigated by means of linear dichroic absorption spectroscopy of molecules oriented in stretched polyethylene films and by semiempirical model calculations. The results indicate a strong hyperconjugative impact of the bicyclobutylene group on the aromatic chromophores in these compounds. Valence isomerization to aromatic products is predicted as the preferred photochemical pathway.     

Gel permeation chromatographic studies of the degradation of polyethylene with fuming nitric acid. II. Bulk polyethylene

Preliminary results are reported on the use of gel permeation chromatography in morphological studies of bulk polymers and fibres. Several samples of bulk isotropic and drawn polyethylene were analyzed by gel permeation chromatography following nitric acid oxidation. In all cases suitable samples showed two peaks in the molecular weight distribution, suggesting a qualitative similarity with results for single crystals. It is concluded that the present data are consistent with chain folding in bulk polymers, both in the isotropic and oriented states, with a less degree of regularity than exists in single crystals.     

Characterization and properties of polyethylene blends II. Linear low-density with conventional low-density polyethylene

Melting and crystallization phenomena in blends of a linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with a conventional low-density (branched) polyethylene (LDPE) are explored with emphasis on composition by differential scanning calorimetry (DSC) and light scattering (LS). Two endotherms are evident in the DSC studies of the blends, which suggests the formation of separate crystals. Light-scattering studies indicate that the blend system is predominantly volume filled by the LLDPE component whereby the LDPE component crystallizes as a secondary process within the domain of the LLDPE spherulites. In contrast to those of the LLDPE/HDPE blends, the mechanical and optical relaxation behavior of the LLDPE/LDPE blends are dominated by the LLDPE component in the vicinities of γ and β regions, whereas the trend reverses at high temperature α regions. This observation is accounted for on the basis of the relative restrictions imposed by the deformation of spherulites (which are primarily made up of the LLDPE component) at different time scales.     

Deuterium N.M.R. of liquid crystal solutions with proton-proton dipolar decoupling

Deuterium N.M.R. of solute molecules in liquid crystal solutions with the removal of proton-proton dipolar coupling has been explored. The deuterium N.M.R. spectrum of a partially deuterated compound dissolved in a liquid crystal solvent is usually complex or unresolved because of proton-deuterium coupling and extensive proton-proton dipolar coupling. When the latter is removed by a special dipolar decoupling sequence, the deuterium N.M.R. spectrum becomes first order, and deuterium-proton dipolar coupling constants can be readily obtained from the spectrum. Results of monodeuterated hexanes and heptanes dissolved in ZLI 2142 are reported.     

Crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene

The changes in crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene (UHMW PE) were investigated by means of wide-angle X-ray scattering. The orientation distribution of crystallites in drawn UHMW PE is composed of two components differing in width. The narrow and broad components revealed in this study indicate the existence of two classes of crystallites with different orientability. Some of the crystallites are oriented almost perfectly even at low-draw ratios, while the others do not orient so effectively. The analysis of melting behaviour of such a texture composed of orthorhombic crystals indicates that highly oriented crystallites are formed by taut molecules and transform first to the hexagonal phase, while the molecules constituting low-oriented crystallites melt directly to the typical amorphous phase. The increase in orientation of highly oriented crystallites during their partial melting, observed in the samples kept at constant length and even those allowed to shrink under constant load, can be explained by the kinetic factor proposed by Ziabicki. Received: 11 September 1998 Accepted in revised form: 18 February 1999     

Radiothermoluminescence. II. Radiothermoluminescence of polyethylene with different chain conformations

Molecular relaxation processes in the 77–260°K interval and the structure of polyethylene melt-crystallized under normal and high pressures have been studied. The positions of relaxation transitions and activation energy for molecular relaxation were determined by radiothermoluminescence. The most intense maximum in the glow curve of the sample crystallized under normal pressure is observed in the 200–240°K interval, i.e., in the range of the β transition. In this temperature interval the β relaxation activation energy changes from 15 to 25 kcal/mole. An increase of the pressure under which crystallization takes place results in a substantial decrease of the intensity of the β maximum. This indicates that the β transition of polyethylene is most probably due to the mobility of segments on the chain-folded lamellar surface. For samples melt-crystallized under pressure between 5000 and 7000 atm, relaxation transitions were found at 150 and 190°K. Various processes of molecular relaxation appear to be associated with the maxima observed on the polyethylene glow curve.     

Deuterium N.M.R. of liquid crystal solutions with proton–proton dipolar decoupling

Abstract

Deuterium N.M.R. of solute molecules in liquid crystal solutions with the removal of proton–proton dipolar coupling has been explored. The deuterium N.M.R. spectrum of a partially deuterated compound dissolved in a liquid crystal solvent is usually complex or unresolved because of proton–deuterium coupling and extensive proton–proton dipolar coupling. When the latter is removed by a special dipolar decoupling sequence, the deuterium N.M.R. spectrum becomes first order, and deuterium–proton dipolar coupling constants can be readily obtained from the spectrum. Results of monodeuterated hexanes and heptanes dissolved in ZLI 2142 are reported.     

1H NMR of probes oriented in stretched polyethylene

¹H NMR spectra of some small molecules oriented in stretched polyethylene rods have been recorded. The band splitting due to direct dipole-dipole couplings is obtained and the corresponding orientation parameters are shown as a function of the draw ratio of the polymer and the temperature. This method is compared to that of using liquid crystals as the orienting solvent.     

Characterization and properties of polyethylene blends I. Linear low-density polyethylene with high-density polyethylene

A blend system of linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with high-density (linear) polyethylene (HDPE) is investigated by differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), Raman longitudinal-acoustic-mode spectroscopy (LAM), and light scattering (LS). For slowly cooled or quenched samples, one single endotherm is evident in the DSC curve which depends on the composition. No separate peaks are observed in the WAXD, SAXS, Raman-LAM, and LS studies on the LLDPE/HDPE blends. This observation along with the fact that no peak broadening is observed suggests that these peaks are associated with the presence of a single component. In no case did we see double peaks or a broadened peak that might be associated with two closely spaced unresolved peaks. This suggests that segregation has not taken place at the structural levels of crystalline, lamellar, and spherulitic textures. A single-step drop in the scattered intensity (I_Hv) as a function of temperature is seen in the LS studies. It is therefore concluded that cocrystallization between the LLDPE and HDPE components occurs. The mechanical and optical α, β, and γ relaxations of these blends are explored by dynamic birefringence. The 50/50 blend displays the intermediate relaxation behavior between those of the components in all α, β, and γ regions. This observation is reminiscent of the characteristic of the typical miscible blends.     

Direct observation of polyethylene molecules with electron microscopy

Individual polyethylene molecules have been imaged in the electron microscope. Preparative difficulties are overcome by the following procedures. (1) The polymer is dissolved in n-hexadecane at 130°C; (2) the solution is deposited on a cooled substrate by spraying in an atmosphere of cold nitrogen; (3) the deposited polymer molecules were shadowed by platinum. Molecular weights obtained are in good agreement with those from light scattering.