Drawing of fibers from irradiated polyethylene single crystals

The relationship between the structural defects present in crystalline polymers and the molecular structure of the drawn material is studied by drawing fibers from irradiated polyethylene single crystals. Irradiation introduces a structural defect in the crystal lattice since it generates crosslinks preferentially, the density of which is controlled by the radiation dose. Observations by means of transmission electron microscopy and electron diffraction indicate that fiber drawing occurs by breaking off of single blocks of folded chains and by subsequent thermally activated rearrangement of the molecules in the drawn material. With increasing crosslink density the size of the blocks decreases, and the fibers become increasingly interconnected so that a network of fibers is formed. If the dose is about 10 Mrad, the crosslinks present in the crystals result in the formation of bundles of fibers. Above 35 Mrad the formation of fibers (single fibers as well as bundles) ceases, and the drawn material forms a thin (15 Å), continuous film. The observed change in morphology with increasing dose (crosslink density) is interpreted in terms of the additional bonding forces between adjacent chains due to crosslinks.     

Growth of polyethylene single crystals from the melt: Morphology

Three-dimensional shape of polyethylene single crystals grown from the melt has been studied. Two distinct types of lateral habit have been obtained: lenticular shape (type A) and truncated lozenge (type B) in the range of regime I and II. Electron microscopy has revealed chair-like shape of type B crystal and reconfirmed the planar shape of type A crystal. In the type B crystal, spiral growth has occurred frequently in the {110} sectors and the sense of the handedness of spiral terraces has been maintained. It has been, suggested that the frequens occurrence of spiral growth is responsible for a morphological change (axialite-spherulite) accompanying the regime I–II transition. The origin of the chair-like crystals has been discussed and, a possible mechanism has been suggested for the formation of spiral terraces; the mechanism is based on a distortion caused by the three-dimensional shape of chair-like crystals. It has been found that the chair-like crystals are curved in the opposite way to S-shaped lamellae observed by Bassett and Hodge in banded spherulites. In fact, the present work has led to the recognition of further classes of crystal with curving cross-sections and of distinctions between them. In final analysis, a unifying thread has been identified between lateral habits, growth kinetics and three-dimensional shape of lamellae, in turn, leading to some rationalization of multilayer developments including twisting in banded spherulites, the latter based on existing suggestions in the literature.     

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     

Observations of slip traces in polyethylene single crystals

Single crystals of polyethylene are investigated by transmission electron microscopy. A pattern of lines is observed in the crystals. The properties of these lines suggest that the lines represent slip traces (slip steps) generated by the movement of dislocations in the polyethylene crystals.     

Melting behavior of drawn films from polyethylene single-crystal mats

Drawing of mats of linear polyethylene single crystals prepared from dilute solution is possible at temperatures above about 90°C. The structure and properties of the drawn specimens are much different from those ordinary drawn bulk polymer. Drawn mats have been investigated by differential scanning calorimetry. The characteristic experimental results are: (a) a broad melting curve, (b) considerable superheating depending on the rate of heating, (c) constancy of the melting point and the heat of fusion with annealing, (d) deviation from the relation between the heat of fusion and the density obtained for the drawn bulk specimens, (e) appearance of two melting peaks in samples annealed at temperatures above about 130°C. These results imply that the structure of the drawn mat is characterized by a larger number of the tie chains connecting the neighboring crystals (the structure postulated in earlier papers) than is the case in ordinary drawn bulk polymer. It can be concluded that the transformation of a fringed micellar type of structure to the folded lamellar structure may be difficult during annealing unless crystals melt and then recrystallize during cooling.     

Changes in defect concentration within polymer crystals: annealing of isotropic and drawn polyethylene

An estimation of the changes in the level of chain defects (branches) accommodated within crystals of undrawn and drawn branched polyethylene, subsequently annealed, is carried out in the light of unit cell expansion data. The results reveal that annealing affects the estimated crystalline defect concentration differently depending on whether the material was drawn or undrawn. Thus, annealing of drawn polyethylene with a defect concentration of 3 % results in a healing of crystal defects, due to a preferential rejection of branches from the crystals. In contrast, annealing of melt crystallized branched polyethylene does not influence the average concentration of chain defects within the crystal despite the increase in lamellar thickness. The branches are trapped into the much wider lamellar crystals with fewer grain boundaries available and defect sequences cannot diffuse out of the crystals.Cordially dedicated to Prof. Dr. H.-G. Kilian on the occasion of his 60th birthday.     

Fourier-transform infrared study of polyethylene single crystals in suspension

The infrared spectra of polyethylene single crystals suspended in liquid are reported and compared to dried mats of the same material. In order to observe the polyethylene bands, liquids with partial windows in complementary regions of the spectra were chosen. Any remaining bands of the suspending media were removed by substraction of the spectrum of the pure liquid. The crystalline bands of suspended polyethylene are much sharper than those of the dried material, indicating a decrease in lateral crystalline order upon drying. Drying also results in a change in the type and distribution of conformations associated with amorphous and fold structures, as determined by the observed frequencies of the methylene wagging mode.     

Properties and structures of films drawn from polyethylene single-crystal mats

The properties and structures of polyethylene films drawn from mats of single crystals were investigated in comparison with films drawn from bulk polymer. The most important result obtained is that the structural reorganization stimulated by mechanical stress and annealing occurs with much greater difficulty in the mat-drawn film. The chief mechanical characteristics of the film are a very low extensibility and a very high modulus. Structural characteristics, such as the double-orientation of the crystalline region and the morphology of the crystals, do suffer no substantial changes during annealing at high temperatures. This stability of the structure can be related to the characteristic fine structure of the mat-drawn film in which there are a much larger number of tie chains, connecting neighboring crystals, than in the bulk-drawn specimen. Relaxation of the amorphous region and a notable increase in long spacing take place in the mat-drawn sample as in ordinary bulk-drawn samples. However, the morphological changes of the crystals accompanied by the folding back of chains seem to take place with great difficulty during annealing even in the vicinity of the melting point.     

Thermoluminescence from extended-chain crystals of polyethylene

Thermoluminescence (TL) has been observed in γ-irradiated extended-chain crystals of polyethylene above room temperature. The TL curve, which exhibits four peaks at 50, 90, 120, and 140°C, is different from that given by folded-chain crystals, in both shape and intensity. In particular, a shape, strong glow peak is observed at 140°C, corresponding to the melting temperature of the extended chain crystals. These results are discussed in relation to independent measurements by differential scanning calorimetry and electron spin resonance.     

The effect of electron irradiation on the structure and mechanical properties of highly drawn polyethylene fibers

Two very different high-modulus polyethylene fiber samples, a low molecular weight melt-spun and drawn fiber, and a high molecular weight gel-spun and drawn fiber, have been subjected to electron beam irradiation to various doses in vacuum and in the presence of acetylene. The gel content after irradiation in acetylene was found to be much greater than for an equivalent dose in vacuum. The gel content–dose relationship could not be described by either Charlesby–Pinner analysis or the Inokuti equation. This is attributed to the polydispersity and the complications introduced by the unique morphologies of highly drawn fibers. Following previous studies, the tensile creep behavior was interpreted in terms of a model comprising two thermally activated processes in parallel, a low stress process relating to the amorphous network, and a high stress process relating to the continuous crystal fraction. Analysis of the creep behavior of the melt-spun, low molecular weight fiber irradiated in vacuum revealed crosslinking in the amorphous regions and chain scission in the crystal. Chain scission was found to be much reduced when irradiating in acetylene, for which a mechanism has been proposed. The creep rates and activation volumes of the high molecular weight, gel-spun fiber were found to be significantly lower, probably due to the unique morphology. In this case the dominant effect of irradiation on the mechanical properties can be attributed to chain scission rather than crosslinking.     

Investigations of morphological changes during annealing of polyethylene single crystals

The morphological evolution of isolated individual single crystals deposited on solid substrates was investigated during annealing experiments using in situ and ex situ atomic force microscopy techniques. The crystal morphology changed during annealing at temperatures slightly above the original crystallization temperature of the crystals, far below their melting temperature. Evenly distributed cavities penetrated the crystals, and the number of cavities increased with a rising annealing temperature until the adjacent cavities coalesced. The thickness of the crystals increased during annealing at temperatures slightly above the crystallization temperature. Annealing experiments at fixed temperatures showed that the reorganization process (cavity formation and single‐crystal thickening) was fast. Depending on the annealing temperature, the final morphology was formed in seconds. This behavior suggests high chain mobility as well as a homogeneous solid‐state reorganization of the entire single crystal at low annealing temperatures. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 763–770, 2001     

Halogenation of polyethylene. I. Bromination of single crystals

It has been demonstrated that the fold surfaces of polymers can be specific towards chemical attack, if the reaction is mild and nondestructive of the fold. Bromination of suspensions of single crystals of polyethylene in carbon tetrachloride has been shown to be such a system. This chemical modification of a fold surface is a powerful means of extending the applications of the physical methods available. Several methods were used, among them DTA, DSC, infrared spectroscopy and small-angle x-ray diffraction. Experimental results from these methods lead to the following conclusions. (a) Bromination takes place preferentially at the folds and is consistent with a regular adjacent reentry fold model. (b) Annealing of these brominated crystals demonstrates the major role played by the crystal surface in this process. (c) The preparation and properties of a novel copolymer system has been demonstrated. It is felt that this copolymer system may prove a useful addition to those systems presently available.     

Preparation and study of separated single crystals of extended-chain polyethylene

Etching of extended-chain crystals of polyethylene is shown to permit separation of the polycrystalline aggregate into single crystals. The single crystals are analyzed by density, molecular weight (lamellar crystal thickness), melting temperature, birefringence, superheating, and linear melting rate determination. The linear melting rate depends linearly on superheating and can be described by simple rate theory.