Extended Chain Crystalline Research Articles

Direct measure of crystalline domain size, distribution, and orientation in polyethylene fibers

High performance ultra high molecular weight polyethylene (UHMWPE) fibers are primarily composed of extended chain (shish) and lamellae (kebab) crystalline domains. Characterization of these architectures commonly utilizes x-ray scattering techniques, which rely upon models to transform from inverse scattering space to real space. Scattering models for lamellae and straight chain crystalline domains were developed and intended for use in ideal scattering scenarios; ie. purely extended chain or purely lamellae. Simultaneous implementation of both models with real (non-ideal) fibers takes for granted the validity of either model in the presence of the other crystalline feature. In this work, we utilize atomic force microscopy (AFM) to directly couple real space stiffness measurements to the analysis of scattering data. The validity of scattering models is tested against real space image analysis and conclusions are drawn regarding the analysis of scattering data to quantify crystalline domain sizes. We find that the lamellae long spacing estimated from scattering data is consistent with real space image analysis. Whereas significant differences are observed for the analysis of lamellae diameter. The characteristic size of extended chain crystalline domains is measured via the streak analysis method on both SAXS 2D scattering data and the Fast Fourier Transform (FFT) of AFM stiffness maps. The streak analysis results in comparable length scales when applied to SAXS and FFT AFM images. A comparison of the streak analysis with novel filtered AFM images suggest that the streak analysis is capturing continuity of straight chain crystalline domains.

Multiscale modeling of thermal conductivity of polymer/carbon nanocomposites

Molecular dynamics simulation was used to estimate the interfacial thermal (Kapitza) resistance between nanoparticles and amorphous and crystalline polymer matrices. Bulk thermal conductivities of the nanocomposites were then estimated using an established effective medium approach. To study functionalization, oligomeric ethylene–vinyl alcohol copolymers were chemically bonded to a single wall carbon nanotube. The results, in a poly(ethylene–vinyl acetate) matrix, are similar to those obtained previously for grafted linear hydrocarbon chains. To study the effect of non-covalent functionalization, two types of polyethylene matrices. -- aligned (extended-chain crystalline) vs. amorphous (random coils) were modeled. Both matrices produced the same interfacial thermal resistance values. Finally, functionalization of edges and faces of plate-like graphite nanoparticles was found to be only modestly effective in reducing the interfacial thermal resistance and improving the composite thermal conductivity.

Changing Molecular Orientation in Fluorocarbon Thin Films Deposited by Different Photo-Processing: Synchrotron Radiation Etching vs Laser Ablation

Flexible macromolecule poly(tetrafluoroethylene) (PTFE) has shown its unique capacity for crystallization and molecular orientation: it forms long extended-chain crystalline whiskers and well orients in its thin films. Formation of the highly oriented structure is essential for oriented growth of materials and for study of the crystal, having a great impact on materials science and engineering. Like laser ablation, synchrotron radiation (SR) can be used to directly etch materials without using any chemicals, offering a simple and versatile method for forming thin films. Using Fourier transfer infrared spectroscopy, the thin PTFE films deposited by the SR etching were found to have polymer chains highly oriented with molecular axes aligned to be perpendicular to substrate surface, whereas the molecular axes align to be parallel to the substrate surface in the films deposited by the laser ablation. Studying with mass spectrometry and X-ray photoelectron spectroscopy, we come to understand this change in mo...

On the interfacial energy of extended-chain crystals from polyethylene melt by revised Flory equations of fusion

To analyze extended-chain crystalline systems composed of linear polyethylene, Flory's conventional theory of fusion is reconsidered by introducing a new concept of crystallinity. When this new treatment is applied to a melting case of a low molecular weight polyethylene fraction (Mn = 5600) isothermally bulk crystallized, a certain result that very large lamellar thickness was caused by a very small increase in crystallization temperature can satisfactorily be explained by a significant change in interfacial free energy of the crystallite end. Further, it shows 14–17 kJ/mol as a nonequilibrium value range of interfacial free energy for highly crystalline polyethylene fractions of low molecular weight Mn ≦ 5600 by using the previous data presented by other workers. A similar result is also obtained on the Mn = 5600 fraction by analyzing from a standpoint of equilibrium crystallinity. In either case, the estimated range of interfacial free energy is consistent with the conventional range. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1293–1303, 1998

Low density foams produced from sheared ultra‐high‐molecular‐weight polyethylene gels

AbstractLow‐density foam (0.02–0.09 g/cc) was produced from gels of ultrahigh molecular weight (UHMW) polyethylenes in solvents with melting points above ambient. The hot PE solutions were stirred at high speed to induce extended chain crystalline gels. These gels were subsequently cooled to allow the solvent to solidify and then the solvent was extracted with alcohol or sublimed. This resulted in a tough, low density foam or a collapsed mass depending mainly on the concentration of the polymer. Shrinkage was significant until polymer concentrations reached 4–5%. Foam cell size varied between 5–30 μm, or 25–75 μm depending on the method of preparation. Polarizing microscopy revealed both spherulitic and row‐nucleated structures depending on the thermal history followed during the sample preparation. We were also interested in whether or not small particles could be used to influence the nucleation and growth of the solution crystallization process. These particles could be suspended in the gel and stabilized in something like a colloidal necklace as has been proposed recently. We successfully incorporated small (0.5 μm) tungsten particles in UHMWPE gels which remained in the foam on extraction of the solvent. Polyethylene containing carbon black particles produced less successful results. These effects may be related to absorption associated with gelation of UHMWPE.

Annealing of ultra‐oriented high density polyethylene extrudates

AbstractThe annealing characteristics of highly‐oriented, high density polyethylene (HDPE) fibers extruded at 90°C to constant extrusion draw ratios (EDR) of 5.8 to 30 have been studied using small‐(SAXS) and wide‐angle (WAXD) x‐ray diffraction, differential scanning calorimetry (DSC), thermal shrinkage and tensile tests. The thermal stability in macroscopic properties, such as transparency, sample dimension, and tensile modulus, remarkably increases with the sample EDR. However, the folded chain crystals within microfibrils sensitively reorganize on annealing at ≥ 110°C, even when the macroscopic properties exhibit no significant changes at high EDR's. Although the EDR has a minor effect on the reorganizability of the fold chain crystals, it has a major influence on the SAXS intensity and the extended chain crystalline component detected by WAXD for both unannealed and annealed extrudates. This specific crystalline component has an improved thermal stability and enhances the thermal stability of macroscopic properties at higher EDR. The fibers extruded at 90°C in this study and those prepared at 134°C in a prior study, exhibit significantly different melting behavior after annealing and for long period vs. EDR relationships. These facts combined with the independent observations of the effect of annealing and extrusion temperatures on the consequent long period strongly suggest that annealing during extrusion plays an important role in determining the microstructure of extrudates, especially for extrusion near the ambient melting point of the polymer.

Superheating effects on the melting of ultra-high modulus linear polyethylene

New results are reported on the superheating behaviour of ultra high modulus polyethylenes which have been restrained from shrinkage during the melting programme. Very high dagrees of superheating were observed, which were somewhat dependent on the polymer molecular weight. It is considered that the ultra high modulus polyethylenes do not contain extended chain crystalline material, and that the superheating effects are associated with the presence of a molecular network which can be prevented from relaxing by mechanical restraint.

Kinetics of two simultaneous second-order reactions occurring in different zones

Equations have been derived for the case of free radicals recombining according to the second-order kinetics with or without diffusion control under the conditions that there are two simultaneous spatially separated recombination reactions but that only the overall free-radical concentration can be observed. The properties of these equations are discussed and methods for determining the three independent parameters in the first case and five in the second developed. The resulting equations have been applied to the interpretation of data obtained in studying the decay of allyl chain free radicals in irradiated extended chain crystalline polyethylene. (auth)

Morphology of ultradrawn polyethylene strands. II. Nitric acid etching and melting behavior

AbstractA transparent, ultraoriented, high‐density polyethylene morphology has been produced by solid‐state (ultradraw) extrusion in a capillary rheometer. From the perspective of modulus and nitric acid etching behavior, the uniquely high draw ratios (<325) experienced by the polyethylene during extrusion result in a morphology with a high level of chain extension. The effect of nitric acid etching on strand thermal behavior has been determined by DSC. The observed melting points of unetched strands were sensitive to the thermal contact between sample and sample pan. Under conditions ensuring improved contact, strand superheating is reduced to one‐third of previously reported values. The negligible shrinkage evidenced by these strands up to 130° is consistent with the presence of a thermally stable component such as extended chain crystals or crystallized tie chains. The single, high‐melting peak is gradually replaced by a nonsuperheating, lower melting peak during the initial stages of acid etch. The resultant peak melting temperature is consistent with the value predicted for the peak crystal thickness of the etched polymer. No evidence is found for a higher melting peak attributable to the extended chain crystalline component. A highly constrained morphology produced by the large tie chain content is believed responsible for strand melting behavior. The melting point of the extended chain crystalline component is reduced by defects and a large ratio of lateral to basal surface area.

Nascent structures during the polymerization of ethylene

The thermal behaviour of nascent or “as grown” polyethylene was studied. The analysis of the melting curves of various high molecular weight samples show that the maximum melting points are usually sharper and higher than those corresponding to samples crystallized from the melt or solution in absence of external deformation. The melting points increase with polymerization time and reach at least 412 °K (139 °C) for samples polymerized for few minutes. This usually goes along with the appearance of a higher submaximum around 415°K (142°C) which corresponds to extended chain crystalline material having superheating properties. Such features are explained in terms of a scheme where continuous deformation is operative during polymerization and crystallization of the nascent samples.