Extended-chain Crystals Research Articles

Effect of cross‐link density on the high pressure crystallization of UHMWPE

Ultrahigh molecular weight polyethylene (UHMWPE) is a bearing surface material for total joint implants. It is radiation cross-linked for high wear resistance and is melted or treated with vitamin E for oxidative stability. We investigated high pressure crystallization (HPC) of irradiated UHMWPE as an alternative method to improve the mechanical strength while stabilizing the residual free radicals from radiation cross-linking. HPC of uncross-linked UHMWPE has resulted in the formation of extended chain crystals and increased crystallinity, leading to improved strength. We hypothesized that increased cross-link density would hinder crystallization during HPC due to decreased chain mobility. Therefore, we investigated the crystalline structure and tensile mechanical properties of high pressure crystallized 25-, 65- and 100-kGy irradiated UHMWPE. We also determined free radical content and wear. The strength of 25- and 65-kGy irradiated UHMWPEs was improved by HPC with increased crystallinity and crystal size. 100-kGy irradiated UHMWPE did not show improved strength, supporting our hypothesis that decreased chain mobility would hinder crystal formation and strength improvement. None of the HPC irradiated UHMWPEs contained detectable free radicals and their wear properties were maintained, suggesting oxidative and mechanical stability in the long term. Therefore, HPC can be used effectively for imparting oxidative stability while strength improvement can be achieved for irradiated UHMWPE with low to moderate cross-link density.

Electrospinning as a New Technique To Control the Crystal Morphology and Molecular Orientation of Polyoxymethylene Nanofibers

Electrospinning is widely accepted as a simple and versatile technique for producing nanofibers. The present work, however, introduces a new concept of the electrospinning method for controlling the crystal morphology and molecular orientation of the nanofibers through an illustration of a case study of polyoxymethylene (POM) nanofibers. Isotropic and anisotropic electrospun POM nanofibers are successfully prepared by using a stationary collector and a rotating disk collector. By controlling the voltage and the take-up velocity of the disk rotator, the morphology changes between an extended chain crystal (ECC) and a folded chain crystal (FCC) as clarified by a detailed analysis of the X-ray diffraction and polarized infrared spectra of the POM nanofibers. Herman's orientation function and dichroic ratio lead us to a schematic conclusion--that (i) molecular orientation is parallel to the fiber axis in both isotropic and anisotropic POM nanofibers, (ii) a single nanofiber consists of a nanofibril assembly with a size of 60-70 A and tilting at a certain degree, and (iii) the higher the take-up velocity, the smaller the nanofibril under the (9/5) helical structure of the POM chains. It should be emphasized here that the electrospinning method is no longer a single nanofiber producer but that it can be applied as a new instrument to control the morphology and chain orientation characteristics of polymer materials, opening a new research field in polymer science where we can understand the relationship between structure at the molecular level and the properties and performance at the macroscopic level.

Orientation-induced crystallization of poly(ethylene terephthalate) fibers with controlled microstructure

Orientation-induced crystallization of PET fibers was studied by the in-situ wide-angle X-ray diffraction (WAXD) utilizing synchrotron radiation source combined with thermomechanical analysis. The noncrystalline as-spun fiber spun was heat-treated at 150, 165, 180 and 195°C for 0.1s under constrained condition. The heat-treatment allowed the fibers to have various amount of isotropic amorphous (IA), oriented noncrystalline (ON), and crystalline (Cr) phase. The structure evolution accompanying the crystallization of the fibers was then examined upon elevating temperature while the fiber length was held constant. The X-ray results clearly showed that the crystallization takes place first by ON phase (extended-chain crystallization) and then followed by the crystallization of IA phase (folded-chain crystallization). The on-set of extended-chain crystallization was dependent on the amount and degree of orientation of ON phase in the fiber that was derived from the various heat-treatment temperatures. It is also noted that the IA phase transforms into not only the CR phase but also the ON phase. The crystallization on the surface of preformed extended-chain crystals appeared to induce the spontaneous orientation of the chains. The thermomechanical data indicated that a stress emerges rapidly on fiber above glass transition temperature (Tg), which is associated with the entropic relaxation of the ON phase. The stress emerged on fiber then dropped drastically as the temperatures of fibers reached the temperatures of extended-chain crystallization, indicating that the stress drop is closely related with the extended-chain crystallization. The fibers heat-treated at the highest temperature showed the highest initial crystallinity but showed the slowest crystallization rate, resulting in the lowest final crystallinity among the fibers.

Free radical elimination in irradiated UHMWPE through crystal mobility in phase transition to the hexagonal phase

Ultra-high molecular weight polyethylene (UHMWPE) is radiation cross-linked to decrease wear in total joint applications. Irradiation decreases the strength of UHMWPE and introduces residual free radicals, which can cause oxidation in the long-term. We advanced a method eliminating the free radicals without a reduction in strength. UHMWPE exhibits a hexagonal phase at high pressure and temperature, where chain mobility in the crystalline phase is increased, leading to the formation of extended chain crystals. We hypothesized that the increased chain mobility during transformation from the orthorhombic to hexagonal phase could be used to eliminate the residual free radicals in irradiated UHMWPE. We eliminated the free radicals in 25-, 65- and 100-kGy irradiated UHMWPEs and these materials did not show oxidation after accelerated aging. The ultimate tensile strength and work-to-failure of 25 and 65-kGy irradiated UHMWPEs were improved significantly while that of 100-kGy irradiated UHMWPE was lower compared to irradiated UHMWPE melted at ambient pressure.

Formation and Stability of Shear-Induced Shish-Kebab Structure in Highly Entangled Melts of UHMWPE/HDPE Blends

The formation and stability of a shear-induced shish-kebab structure was investigated by in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) measurements of highly entangled polyethylene melts based on two polymer blends, containing small fractions (2 and 5 wt %) of ultra-high molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE). Immediately after shear, the combined SAXS and WAXD results at 142 °C confirmed the sole formation of shish without kebabs, indicating the interplay between the topological deformation of highly entangled UHMWPE chains and the extended-chain crystallization of stretched segments without the participation of coiled segments. The presence of HDPE chains influenced the entanglement of UHMWPE but they were not involved in the shish-kebab formation at the initial stage of crystallization. The final shish lengths in both blends were nearly identical at the same strain (ε = 500), even though the UHMWPE concentration was different. When the temperature was cooled to 134 °C, both sheared blends exhibited the kebab formation, following the diffusion-controlled growth process. Although the total kebab nucleation was higher in the 5/95 wt % UHMWPE/HDPE blend, the kebab density per shish was higher in the 2/98 wt % UHMWPE/HDPE blend. The thermal stability of the shish-kebab structure was also investigated by constrained melting. Both blends exhibited identical melting behavior of kebabs but different melting behavior of shish that is governed by the entanglement restraints of the stretched-chain network.

Electron microscopy of high pressure crystallised poly (p-phenylene sulfide)

High pressure crystallised poly (p-phenylene sulfide) (PPS) samples were investigated with wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The striation appearance, which is the most common feature of polymer extended chain crystals, was clearly observed. Poly (p-phenylene sulfide) extended chain crystals with a C-axis thickness around 10 μm were formed at high pressure. The DSC results showed that the melting temperature of high pressure crystallised PPS samples was up to 332·3°C, which was higher than the values of ideal PPS perfect crystals used by some researchers. Other morphologies formed at high pressure, including crystalbars and regular polygonal crystals of different characteristics, were also presented with the SEM measurements. The obtained PPS extended chain crystals may be used in the re-evaluation of certain parameters crucial to the processing of PPS products such as the equilibrium melting point and melting enthalpy of ideal crystal.

Crystallization and Dissolution of Flow-Induced Precursors

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (T(m)(0)=141.2 degrees C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120 s(-1) for 1 s at 142 degrees C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

In situ SAXS analysis of extended-chain crystallization during melt-drawing of ultra-high molecular weight polyethylene

Phase development during melt-drawing of ultra-high molecular weight polyethylene (UHMW-PE) was analyzed by in situ small-angle X-ray scattering measurements using synchrotron radiation at the SPring-8. Films with different entanglement characteristics were prepared by solution blending of higher and lower MW samples with a viscosity average MW of 1.07×107 (higher) and 1.73×106 (lower), followed by compression molding at 180°C. Independent of blend ratio, the strong streak attributed to extended-chain crystals (ECCs) appeared on the equator at the beginning point of the plateau stress region in the stress profile, which was one of the characteristic features of melt-drawing. A series of detailed analyses of these streaks suggested that two components with different dimensions were formed during melt-drawing: one was a precursory ECC, and the other was a mature ECC. These two ECC components grew synchronously at the beginning point of the plateau stress region for both films, independent of the entanglement characteristics of the film. However, the time of this synchronized growth was longer for the film with the lower MW component than for the film containing only the higher MW component. The resultant morphologies of the melt-drawn samples observed by transmission electron microscopy also reflected these characteristics. These results demonstrate that molecular entanglement characteristics dominate the unique crystallization mechanism that forms ECCs with different dimensions during melt-drawing of UHMW-PE.

Crystallization, glass transition and morphology of ( R)-3-hydroxybutyrate oligomers

The study focuses on the effect of the molecular length of isotactic hydroxybutyrate oligomers on the crystal morphology, crystallinity, and spherulitic superstructure. Furthermore, the process of solidification of the quiescent melt is evaluated by the analysis of the crystallization kinetics and of the glass transition. Melt-crystallization is strongly controlled by the chain length, and the regime of cooling. Crystallization can completely be avoided by rapid cooling. Slow cooling allows at best incomplete crystallization, with the crystallinity increasing with chain length. Typically, the maximum crystallinity is between 50% and 80% for OHB of molecular weights of 500 and 5000 g mol −1, respectively. The temperatures of the glass transition and of crystallization/melting increase with molecular length, and are discussed in terms of the Fox–Flory and Gibbs–Thomson equations, respectively. For all samples, regardless of the chain length, spherulitic crystallization is observed, with the perfection of spherulites increasing with decreasing crystallization temperature. The transition of formation of extended-chain crystals to formation of folded-chain crystals occurs at a molecular weight of about 2000 g mol −1, which corresponds to chain length of about 7 nm. Analysis of the heat-capacity increment at the glass transition temperature reveals the existence of a rigid amorphous fraction.

Quantum Beam Studies on Polymer Crystallization under Flow

One of the longstanding issues in polymer science is crystallization of polymers under flow, especially formation of the so-called shish-kebab. Recent progress in quantum beam technology shed light on the substantial nature in the shish-kebab formation. In this paper we review our recent experiments on polymer crystallization under flow using time-resolved depolarized light scattering, small-angle and wide-angle X-ray scattering and small-angle neutron scattering in a wide spatial scale from 0.1 nm to several tens μm. These studies revealed that the shish-kebab formation is governed by a competition between the crystallization rate and the chain relaxation rate. Small-angle neutron scattering study on an elongated blend of deuterated low molecular weight and protonated ultra-high molecular weight polyethylenes showed that a long cylindrical object 2 μm in diameter and 12 μm in length was formed from deformed network of ultra-high molecular weight chains, which included three shishs (or extended chain crystals) 9 nm in diameter.

Mechanical Force Induced Grafting Effect on Strengthening HDPE/GF Composites with Maleic Anhydride

Grafting of maleic anhydride (MAH) onto high density polyethylene (HDPE) was carried out in melt state through the mechanical force initiation caused by high-speed rotation of the twin-screw, investigating its functional reaction and the grafting degree influenced by the rotation of the twin-screw, observing the influence on its mechanical property. The results showed that the increase of grafting degree made the interface phase produce a firm chemical coupling, which improved the tensile-strength and flexural strength of the composites. The produced interfacial stress from the shrinkage of specimen melts in injection molding process can strain-induce the forming of the extended-chain crystals in the matrix and obviously improve the notched impact strength of the composites.

Self-Assembly and Chain-Folding in Hybrid Coil−Coil−Cube Triblock Oligomers of Polyethylene-b-Poly(ethylene oxide)-b-Polyhedral Oligomeric Silsesquioxane

Self-assembly and chain-folding in well-defined oligomeric polyethylene-block-poly(ethylene oxide)-block-polyhedral oligomeric silsesquioxane (PE-b-PEO-b-POSS) triblock molecules were studied by small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM). The triblock oligomers were synthesized by attaching two kinds of functional POSS molecules, namely, isocyanatopropyldimethylsilylisobutyl-POSS (Ib−POSS) and isocyanatopropyldimethylsilylcyclopentyl-POSS (Cp−POSS), to a hydroxyl-terminated PE-b-PEO-OH diblock oligomer (denoted as E39EO23) via urethane reactions. In these triblock oligomers, both PE and POSS were crystalline, whereas PEO became amorphous due to tethering of its both ends to other two blocks. In the crystalline state, PE chains tilted 32° from the lamellar normal, and both Ib−POSS and Cp−POSS molecules stacked into four-layer (ABCA) lamellar crystals, having the same trigonal (R3̄m) symmetry as in pure POSS crystals. Because the cross-sectional area for a PE chain in the PE crystals (0.216 nm2/chain) at the interface was much smaller than that for a POSS molecule in POSS crystals (1.136 nm2/molecule), the self-assembly and PE chain-folding were substantially affected by the sequence of PE and POSS crystallization when crystallizing from the melt. For example, PE crystallization induced the POSS crystallization in the bulk E39EO23−Ib−POSS, and thus extended-chain PE crystals were observed. The grains of crystalline lamellae again were small with often highly curved lamellar crystals. This could also be attributed to the unbalanced interfacial areas for POSS and PE blocks (the interfacial area ratio being 2.6 for interdigitated PE crystals, i.e., two PE chains per POSS molecule). For the E39EO23−Cp−POSS triblock oligomer, POSS molecules crystallized before PE crystallization, forming a well-defined lamellar structure. The preexisting POSS crystals confined the PE crystallization, and thus almost once-folded PE crystals were obtained. Because the interfacial area ratio between POSS and PE blocks decreased to 1.3, the crystalline lamellae were straight and their grains were relatively large. On the basis of this study, we conclude that confinement effect plays an important role on chain-folding of crystalline block oligomers, which in turn determines the unbalanced cross-sectional areas between chemically different blocks and finally dictates their final crystalline morphology.

Hierarchic Structure of Shish-Kebab by Neutron Scattering in a Wide Q Range

We performed small-angle neutron and X-ray scattering (SANS and SAXS) measurements on an elongated blend of low molecular weight deuterated polyethylene (PE) and high molecular weight hydrogenated one (97.2/2.8) in a very wide range of scattering vector of 1 × 10-4−3 A-1 to elucidate hierarchic structure of the so-called shish-kebab. Comparing the SANS and SAXS data, we found that the hydrogenated high molecular weight PE formed a large and long object aligned along the elongation direction. The SANS data were analyzed in terms of the multicore−shell cylinder model, and we found that the long oriented structure had radius of ∼1 μm and length of ∼12 μm and included about three extended chain crystals with radius of ∼45 A. We believe that this oriented long object with a relatively low aspect ratio of about 12:1, which is called the row structure in this paper, is a precursor formed from deformation of network of high molecular weight components due to entanglements.

Effect of pressure on the crystallization of polyethylene

Crystallization of polyethylene under pressure has been studied systematically with regard to the structure of the resultant crystal. It was found that in the isothermal crystallization under pressure, alteration of the form of the crystals from folded to extended occurred above certain pressures within a relatively narrow temperature range. The highest temperature that allows isothermal crystallization increases linearly with the pressure, while the lowest temperature to give the extended-chain crystals increases rather slowly with pressure, widening the temperature range to give the extended-chain crystals. The least pressure required was found between 2000 and 3000 kg/cm2 for the polymer studied in this work. It is considered that control by a particular nucleation or growth mechanism, different from the ordinary folded-chain type crystallization and possibly the formation of bundle-type nucleus is more likely than the effect of annealing or secondary growth of once folded molecules.

Melt Crystallization and Morphology of Poly(p-phenylene sulfide) under High Pressure

The high-pressure melt-crystallization behaviors of poly(p-phenylene sulfide) (PPS) were investigated using WAXD, DSC, TEM and SEM. PPS extended-chain crystals with c-axis thickness exceeding 4.5 μm were formed at high pressure. The DSC results showed that the melting temperature and melting enthalpy of high-pressure crystallized PPS samples were up to 327.53 °C and 94.96 J.g -1 , respectively, which were higher than the values of ideal PPS perfect crystals used by some researchers, and the melting enthalpy of the samples fluctuated regularly during the thickening growth of the PPS crystals. Other characteristic morphologies obtained at high pressure, i.e. spherulites and rod-like crystals, were also presented with the SEM measurements.

Extended‐chain crystals in high‐pressure crystallized poly(ethylene terephthalate)/bisphenol a polycarbonate blends

AbstractMorphologies of extended‐chain crystals with different characteristics were observed with scanning electron microscopy (SEM) in the high‐pressure crystallized polyethylene terephthalate/polycarbonate (PET/PC) blends. The crystals memorize their nucleation and growth process, which reveal an involvement of different mechanisms simultaneously. The presence of sliding diffusion during crystal thickening is indicated by a wedge shape of some crystals, while bent crystals suggest the occurrence of transesterification in the formation of the large extended‐chain crystals. The observation of two morphological features on one group of crystals shows that two mechanisms may work simultaneously. The connection between folded‐chain and extended‐chain crystals is demonstrated by the S‐shaped extended‐chain crystals as well as their direct morphological connection observed with SEM. Though transesterification plays the essential role in the formation of the large crystals, which acts in different aspects during the process, the thermodynamic driving force is the enthalpy gain associated with large crystals. This is a high‐pressure self‐assembly with a coupling between crystallization and transesterification, which may be instructive to grow such large crystals in similar polymer systems. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3148–3156, 2006

The role of the amorphous fraction for the equilibrium shape of polymer single crystals

The equilibrium state of polymer single crystals is considered by explicitly taking into account the amorphous fraction formed by loops and tails of the chains using a statistical model introduced by Muthukumar (Philos. Trans. R. Soc. London, Ser. A 361, 539 (2003)). We show that under realistic conditions below the equilibrium melting temperature, tight loops and close re-entries are favored, and that the amorphous fraction can be mapped into an excess surface free energy. The model is extended to many-chain crystals where it is shown that the lamellar thickness increases with the number of chains in the crystal and extended-chain conformations are thermodynamically favored if the number of chains in the crystal is sufficiently large. The number of chains necessary to form an extended-chain crystal in thermodynamic equilibrium scales with the square of the degree of polymerization of the chains. We discuss the temperature behavior of the equilibrium crystal thickness in the under-cooled state.

Molecular Segregation in Polymer Melt Crystallization: Simulation Evidence and Unified-Scheme Interpretation

The segregation of the low-molecular-weight fractions on the crystal growth of polydisperse polymers has been identified as a general phenomenon by means of dynamic Monte Carlo simulations of lattice polymer systems. Its mechanism was elucidated on the basis of the intramolecular nucleation model. A unified scheme has been proposed to interpret all three scenarios of molecular segregation according to the location of the crystallization temperature Tc demarcated by two melting points of short-chain fractions, namely for the bulk extended-chain crystals and Tm2D for two-dimensional single-folded-chain crystals on a smooth crystal-growth front. In the first scenario, Tc > , and short-chain fractions are thermodynamically forbidden in crystallization, like the small solvents in conventional monotectic polymer solutions. In the second scenario, > Tc > Tm2D, and short-chain fractions are fully excluded by their failures on the intramolecular secondary nucleation for folded-chain crystal growth. In the third sc...

In situ optical microscope study of the shear-induced crystallization of isotactic polypropylene

The shear-induced crystallization behavior of isotactic polypropylene (i-PP) has been investigated by in situ optical microscope under various thermal and shear histories. A shish-kebab structure during growth was observed under a well controlled, however, weak shear field. According to our results under these weak shear and thermal history, a modified model was proposed for the growing process of melt shear-induced crystallization of i-PP. Furthermore, many physical insights were provided on several still unsettled issues such as extended chain crystals, row nuclei, and smectic ordering questions.

Chain-Folding and Overall Molecular Conformation in a Novel Amphiphilic Starlike Macromolecule

The crystallization and self-assembly behavior of an amphiphilic starlike macromolecule (ASM) with a lipophilic core and four poly(ethylene oxide) (PEO) arms (Mn = 2000 g/mol) were characterized by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction at different crystallization temperatures (Tc). The overall d-spacing of the alternating amorphous core and crystalline PEO lamellae almost doubled from 11.4 nm for samples quenched into liquid nitrogen to 21.0 nm for Tc = 42 °C, indicating gradual transitions from nearly once-folded to fractionally folded and finally to extended chain crystals with increasing the Tc. On the basis of SAXS, transmission electron microscopy, and computer simulation results, it was suggested that the lamellae contained an interdigitated, single-layered PEO crystal, and an amorphous layer consisted of a double-layered lipophilic core and amorphous PEO. A folded (four PEO arms at one side of the core) rather than an extended (two PEO arms on each side o...