A. Tshmel

Effect of intercrystallite straight-chain segments on Young's modulus of gel-spun polyethylene fibers

The tensile stress and Youngs modulus of a series of gel-spun polyethylene fibers were measured in parallel to measurements of transverse and longitudinal sizes of crystallites, molecular orientation and straight-chain-segment (SCS) length distribution in order to find the correlation of the mechanical and microstructural characteristics during drawing. The low-frequency-Raman data evidence the predominance of the chain-straining process on the ends of crystallites. As a result, there appears a rigid intermediate substance (rigid amorphous phase (RAP)) between crystal cores and true amorphous phase. The RAP is composed of various-lengths SCS oriented in the fiber axis direction and arranged without longitudinal ordering. The RAP transforms gradually to uniform-length taut-tie molecules (TTM). The Youngs modulus correlates to the increase of the RAP in length, but there were no detectable response of the mechanical parameters to the formation of the TTM.

Comparative low-frequency Raman study of straight-chain-segment length distributions in gel-derived and melt-crystallized polyethylenes

The Raman longitudinal acoustic mode (LAM) has been examined for two ultra-high-molecular-weight reactor powders and melt-crystallized (MC) and gel-derived (GD) films and hot-drawn fibers prepared from them. The analysis of the straight-chain-segment (SCS) length distributions obtained from the LAM spectra demonstrates complete decomposition of the initial crystalline structure of the powders in the GD samples, while in MC films one could distinguish some features reminiscent of the powder structure. In addition, a certain amount of all-trans bridges linking adjacent crystallites in unoriented MC films was detected. In drawn MC samples, the average SCS length grows gradually with the draw-ratio increase, thus indicating the molecular ordering in the interface zones between the crystallite cores and the amorphous phase. A similar effect in GD fibers is accompanied by the formation of all-trans sequences whose length is much more than the crystallite core thickness, i.e., taut-tie molecules. The calculated SCS length distributions manifest clearly the presence of population of the taut-tie molecules entering a series of a few crystallites consecutively.

Remnant Features in Melt Crystallized Samples of Polyethylenes Originated from Reactor Powders

A series of reactor powders of ultrahigh molecular weight (UHMW) polyethylenes with different morphology was melted at 160°C for 5 min and cooled in ice water. Low‐frequency Raman spectroscopy was used to characterize the straight‐chain‐segment (SCS) length distribution of both initial and melt‐crystallized materials. A bimodal SCS length distribution was found in the melt‐crystallized sample originated from the reactor powder with a very tenuous amorphous phase. In other samples recrystallized from powders with more ordered structure, the SCS length distribution was unimodal and approximately identical. The result is explained in terms of perfection of lamellar crystals formed under limited time and thermal conditions from differently organized initial structure.

Effect of the morphology of reactor powders on the structure and mechanical behavior of drawn ultra-high molecular weight polyethylenes

Abstract Evolution of the straighten-chain-segment length distributions in three polyethylene reactor powders manufactured using different catalysts has been traced through the melting-drawing procedures carried out under the same temperature-deformation conditions. The powders exhibiting the presence of either folded or chain-extended crystalline entities yielded the oriented samples with quantitatively different interfibrillar structure, and, respectively, with different mechanical behavior concluded from their stress–strain curves and variation of the tangential modulus. The correlation between original and final properties of samples derived from reactor powders evidences a persistence of a kind of memory. The longitudinal disorder in nascent chain-extended crystals transforms virtually to the lateral disorder in the axially oriented interfibrillar material.

LAM Study of Chain Straightening During Stress Relaxation in Gel-Spun Polyethylene Fibers

The low-frequency Raman spectra of gel-spun polyethylene fibers were recorded in the frequency range of the longitudinal acoustic modes (LAM) localized on all-trans sequences, i.e., on straight-chain segments (SCS). During the experiment, the samples were stressed and held with their ends fixed. As soon as the stress was applied, there appeared a substantial amount of short (as compared to the size of the crystal) all-trans sequences randomly distributed in the amorphous phase. These primary SCS are not stable and decay (coil) within approximately half an hour. Further chain straightening continues in the interfacial regions between the crystalline and amorphous phases. In the course of this process, the individual all-trans sequences in the amorphous bulk are virtually replaced by the SCS adjacent to crystal cores. The final SCS length distribution in the relaxed sample is characterized by an increased fraction of the crystalline SCS penetrating deeply into the amorphous regions with formation of some quantity of taut-tie molecules.

Network junctions in polyethylene thermo-reversible gels

Abstract Infrared and low-frequency Raman data evidence the presence of the crystalline phase in the gels prepared from ultra-high molecular weight polyethylene. The straight chain segment length distribution covers the range from 1 to 10 nm with a sharp maximum at 3 nm. The latter peak is attributed to chain-folded crystals while the wings of the distribution are referred to the contribution of adjoined straightened portions of different molecules. The crystalline sites of both kinds serve as junctions in newly formed polymeric network.

Remote Structural Correlation in the Sequence Reactor Powder–Gel–Fiber in Polyethylenes

A series of gels originated from ultra-high-molecular-weight polyethylene reactor powders, differing in morphology due to their different synthesis prehistory, were utilized for obtaining oriented fibers through the gel technology. The source powders, gels, and drawn fibers were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and low-frequency Raman spectroscopy. A comparison of the SEM images of powders with their straight chain segment (SCS) length distributions derived from the Raman spectra showed that the samples with pronounced fibrous morphology exhibited bimodal distribution functions, whereas the granular morphological pattern was specified by the unimodal SCS length distribution. Some remnant features of the ordered structure inherent in the powders were revealed in the gels. The drawability of gel-derived fibers was found to be dependent on the SCS length distributions in the gels and, indirectly, on the morphology of the reactor powders.

Crystallisation process on the stage of the gel-to-solid transformation in thermo-reversible polyethylene gels

Abstract The crystalline phase in gels and solid (gel derived) samples of ultra-high molecular weight polyethylene was studied using the low-frequency Raman spectroscopy and the differential scanning calorimetry methods. The Raman data evidence the presence in gels of tiny crystallites whose thickness (3–5 nm) depends on the sort of solvent used. After removal of the solvent by drying or squeezing the original gels, there appear some extra ordered regions (6–12 nm in size) coexisting in solid material with the primary crystallites which remain unchanged. It is concluded that the gel-to-solid transformation is accompanied by a secondary crystallisation process which is weakly (if ever) dependent on the incipient crystalline entities in gels.

Structure of the tie chains in oriented polyethylene: dependence of all-trans stem lengths on draw ratio

Abstract By the Raman LAM experiments the length distributions of straight all-trans sequences were obtained for oriented polymers films of varied draw ratios λ. A shape of distribution function depended on λ and draw temperature as well: the higher λ the more long trans-stems could be observed in the oriented material. The experimental facts were matched to the results of the computer structure simulation based on ‘stencil crystallization’ model which was described previously [1] . Large deviation of experimental data from theoretical ones is discussed briefly. In essence, this deviation may be considered as a measure of superstructure imperfection of a real oriented polymer.

Stress-induced straightening of tie chains in oriented PE films as revealed by Raman (LAM) study

The Raman longitude acoustical mode (LAM) study of axially stressed polyethylene-made microfilms gave evidence of the formation of the straight tie molecules localized in two neighboring crystals and in an amorphous layer situated between them. The samples were bicomponent blends composed of polyethylenes with different MW (7 x 10 4 and 2 x 10 6 ). Each MW fraction exhibits an individual LAM band responding to crystals with the size specific to a given fraction. Under tensile stress, the bands shift insignificantly towards an exciting laser line. In addition, the LAM localized on all-trans sequencies with the length exceeding the mean crystallite size appear in the spectrum of a stressed sample. The reversibility of these spectral changes depends on the draw ratio of samples. We conclude the effect is due to stress-induced gauch-to-trans transformations taking place in the amorphous layers.