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Synthesis and characterization of silk-inspired thermoplastic polyurethane elastomers /
Abstract
Segmented polyurethane elastomers containing additional ordered structures within the hard or soft domains were developed to mimic the hierarchical structure and superior properties observed in spider silk fibers. The silk's toughness is related to a fiber morphology that includes P-pleated crystalline sheets within an amorphous matrix, as well as an additional interphase with an orientation and mobility between that of the two microphases. In the polyurethane mimics, bulky aromatic diisocyanates were incorporated between aliphatic hexamethylene diisocyanate (HDI) hard segments and poly(tetramethylene oxide) (PTMO) soft segments, to enhance the size and orientation of the interphase. The mixture of diisocyanates reduces the crystallinity of the HDI hard segments, allowing the polyurethane to form more well-organized domains observed by AFM imaging. The more interconnected hard domains allow the elastomers to deform to higher elongations and absorb more energy without a decrease of initial modulus. Shearing of the hydrogen-bonded hard domains orients the hard blocks at a preferred tilt angle of ±20⁰ from the strain direction during tensile deformation.
Shape memory polymers (SMPs) are functional materials, which find applications in a broad range of temperature sensing elements and biological micro-electro-mechanical systems (MEMS). These polymers are capable of fixing a transient shape and recovering to their original shape after a series of thermo-mechanical treatments. Generally, these materials are thermoplastic segmented polyurethanes composed of soft segments, usually formed by a polyether macroglycol, and hard segments formed from the reaction of a diisocyanate with a low molecular mass diol. The hard segment content is a key parameter to control the final properties of the polymer, such as rubbery plateau modulus, melting point, hardness, and tensile strength. The long flexible soft segment largely controls the low temperature properties, solvent resistance, and weather resistance properties. The morphology and properties of polyurethanes (PU) are greatly influenced by the ratio of hard and soft block components and the average block lengths. However, in some applications, SMPs may not generate enough recovery force to be useful. The reinforcement of SMPs using nanofillers represents a novel approach of enhancing the performance of these materials. The incorporation of these fillers into SMPs can produce performance enhancements (particularly elastic modulus) at small nanoparticle loadings (˜1-2 wt %). An optimal performance of nanofiller-polymer nanocomposites requires uniform dispersion of filler in polymers and good interfacial adhesion. The addition of nanofillers like cellulose nanofibers (CNF), conductive cellulose nanofibers (C-CNF), and carbon nanotubes (CNTs) allows for the production of stiffer materials with deformation capacity comparable to that of the unfilled polymer. Additionally, the use of conductive nanoreinforcements such as C-CNF and CNTs leads to new pathways for actuation of the shape memory effect. During this work, thermoplastic shape memory polyurethanes were synthesized with varying chemical composition and molecular weight. This was achieved by controlling the moles of reactants used, by using polyols with different molecular weights, and by using different diisocyanates. Using these controls, polymer matrices with different but controlled structures were synthesized and then reinforced with CNF, C-CNF, and CNTs in order to study the influence of chemical structure and polymer-nanoreinforcement interactions on polymer nanocomposite morphology, thermal and mechanical properties, and shape memory behavior.
Through billions of years of evolution nature has created and refined structural proteins for a wide variety of specific purposes. Amino acid sequences and their associated folding patterns combine to create elastic, rigid or tough materials. In many respects, natures intricately designed products provide challenging examples for materials scientists, but translation of natural structural concepts into bio-inspired materials requires a level of control of macromolecular architecture far higher than that afforded by conventional polymerization processes. An increasingly important approach to this problem has been to use biological systems for production of materials. Through protein engineering, artificial genes can be developed that encode protein-based materials with desired features. Structural elements found in nature, such as β-sheets and α-helices, can be combined with great flexibility, and can be outfitted with functional elements such as cell binding sites or enzymatic domains. The possibility of incorporating non-natural amino acids increases the versatility of protein engineering still further. It is expected that such methods will have large impact in the field of materials science, and especially in biomedical materials science, in the future.
Our observations on whole mounted major ampullate silk glands suggested that the thread is drawn from a hyperbolic die using a pre-orientated lyotropic liquid crystalline feedstock. Polarizing microscopy of the gland's duct revealed two liquid crystalline optical textures: a curved pattern in the feedstock within the ampulla of the gland and, later in the secretory pathway, the cellular texture previously identified in synthetic nematic liquid crystals. The behaviour of droplet inclusions within the silk feedstock indicated that elongational flow at a low shear rate occurs in the gland's duct and may be important in producing an axial molecular orientation before the final thread is drawn. Our observations suggested that the structure of the spider's silk production pathway and the liquid crystalline feedstock are both involved in defining the exceptional mechanical properties of spider dragline silk.
Spider major ampullate (dragline) silk is an extracellular fibrous protein with unique characteristics of strength and elasticity. The silk fiber has been proposed to consist of pseudocrystalline regions of antiparallel beta-sheet interspersed with elastic amorphous segments. The repetitive sequence of a fibroin protein from major ampullate silk of the spider Nephila clavipes was determined from a partial cDNA clone. The repeating unit is a maximum of 34 amino acids long and is not rigidly conserved. The repeat unit is composed of three different segments: (i) a 6 amino acid segment that is conserved in sequence but has deletions of 3 or 6 amino acids in many of the repeats; (ii) a 13 amino acid segment dominated by a polyalanine sequence of 5-7 residues; (iii) a 15 amino acid, highly conserved segment. The latter is predominantly a Gly-Gly-Xaa repeat with Xaa being alanine, tyrosine, leucine, or glutamine. The codon usage for this DNA is highly selective, avoiding the use of cytosine or guanine in the third position. A model for the physical properties of fiber formation, strength, and elasticity, based on this repetitive protein sequence, is presented.
Microphase separation in semicrystalline block copolymers can be driven by two forces: thermodynamic incompatibility between the blocks in the melt or crystallization of one or more blocks. The diblock copolymers investigated here are polyethylene-b-poly(3-methyl-1-butene), all containing ≈26 wt % ethylene. prepared by hydrogenating high-1,4-polybutadiene-b-high-3,4-polyisoprene. This block asymmetry leads to a hexagonally-packed cylindrical morphology when microphase separation is present in the melt. The molecular weights are varied to obtain differing degrees of incompatibility in the melt, ranging from a single-phase (disordered) melt to a strongly segregated melt. X-ray scattering and differential scanning calorimetry are used to investigate the morphology resulting from crystallization in these melts as a function of thermal history. Crystallization from strongly segregated melts is confined to the cylindrical microdomains and produces a morphology essentially independent of thermal history. In the most strongly segregated diblock crystallized at 10-20 °C/min. chain folding within these cylindrical microdomains is preferentially oriented with the chain axis tilted 18 ± 4° from normal to the cylinder axis. In contrast to strongly segregated melts, the morphology produced by crystallization from weakly segregated melts is highly dependent upon thermal history. Faster cooling kinetically confines crystallization to cylinders, while slower cooling results in complete disruption of the cylindrical melt mesophase upon crystallization, leading to a lamellar morphology with a larger domain spacing.
Liquid crystals in the last two decades have become part of a technological explosion, leading to advances in areas as diverse as oil recovery, the production of temperature sensors--from medical thermograms to "mood rings"--and biological research into nerve conduction and arteriosclerosis. Although they are as fundamental a phase of matter as solids, liquids, and gases, liquid crystals have over the past century puzzled scientists by their very existence. With this book Peter Collings is among the first to introduce the general reader to what is known of the chemistry and physics of liquid crystals, focusing on the basic principles behind their myriad of delicate properties. Written in a clear and lively style, this work is accessible to readers with a basic science background and the willingness to learn more about this ubiquitous technology. Collings discusses the discovery of liquid crystals and the theoretical research presently being performed. He also describes important applications, emphasizing the role of liquid crystal display technology in such devices as laptop computers, automobile dashboards, and pocket color televisions. Finally, the author covers new developments pertaining to polymers, emulsions, and biological systems as well as the importance of these advances for industry and medicine.
We present a method for analyzing the molecular scale orientation and structure of processed polymers using wide angle X-ray scattering data (WAXS). The technique is applied to the analysis of solution-spun fibers of a liquid crystalline polyester comprised of 1,4-hydroxybenzoic acid, isophthalic acid, and hydroquinone. The orientation distribution function (ODF) of the non-crystalline component of the polyester is constructed using a Legendre polynomial series expansion. To construct a consistent molecular scale description, a Monte Carlo sampling scheme which incorporates the standard Metropolis sampling was employed, coupled with a weighting factor that favors structures which are in closer accord with the experimental WAXS data. The members of the ensemble sampled by the simulation consist of rhombic lattices on which oligomers are placed in an aligned state. The conformation of each oligomer was obtained using a rotational isomeric state description. By direct comparison of the experimental and calculated structure factor coefficients, the ODF for the structural ensemble was deduced. For the polyester fiber considered here, a two-component system consisting of oriented and unoriented non-crystalline components is required for complete characterization of the scattering properties of the sample. The sample contains a negligible amount of crystalline material, but 85% of the sample is aligned locally (i.e. with respect to nearest neighbor chains). However, as the fibers are spun from an isotropic solution, the global orientation (i.e. orientation with respect to the sample axis) remains low.
Phase transfer catalysis has been found to be effective for the rapid conversion of carboxylic acids in high yield to the corresponding acid chlorides. The procedure is well suited for acids that exhibit low solubilities in normal organic solvents.
Side chain liquid crystalline polyurethanes are a new class of materials that show promise for mechanooptic applications. The rich morphology afforded by these materials also provides a chance to understand the interplay between polyurethane morphology and liquid crystalline ordering. In this paper, we study the response of a polyurethane with liquid crystals pendant to the soft segments to an applied strain using fourier transform infrared (FT-IR) linear dichroism. We find that this complex material follows the trend established in the literature for both side chain liquid crystalline homopolymers and segmented polyurethanes. At low strains, the soft segments align with strain inducing an orientation in “lone” hard segments. Up to strains of 40%, the LC mesogens align with the strain field and the hard segments in hydrogen bonded domains align perpendicular to the field. At strains above 40%, we find a rearrangement of the ordering that result in smectic layers and the hard segments aligning parallel to the field. A model is proposed to represent these findings, and reflections on the cooperative movement of the different macromolecular components of the polyurethane are offered.
Summary
This paper describes the synthesis and characterization of a new series of copolymers with a predominantly poly(ethylene oxide)
(PEO) backbone and phenyl and/or 4,4'-biphenyl structural units. Three copolymers, poly[oxyethylene]/poly[oxyphenylene] copolymer
(1), poly[oxyethylene] /poly[oxybiphenylene] copolymer (2), and poly[oxyethylene]/poly[oxyphenylene]/poly [oxybiphenylene] copolymer (3), were prepared based on modifications of hydroquinone and/or 4,4'-biphenol copolymerized with dimesylates of various length
poly(ethylene glycol)s (PEGs). Depending on their composition and chain length of PEGs used in the polymerization, the copolymers
show liquid crystallinity or non-liquid crystallinity.
Structural phase behavior of a thermotropic polyester, poly(hexamethylene 4,4'-biphenyl-dicarboxylate), abbreviated as BB-6, was studied under hydrostatic pressures up to 200 MPa by wide-angle X-ray diffraction apparatus equipped with a high-pressure vessel. The C alpha form of BB-6, cooled slowly from the melt under atmospheric pressure, is the most stable in the low-pressure region below 70 MPa, and this sample exhibits the reversible phase transition of C alpha <-> S-A <-> I. An application of high pressure above about 100 MPa induces another crystal polymorph (C delta) at high temperature: the C alpha --> C delta --> S-A --> I transition process is observed on heating under high pressure. Since the C delta <-> S-A <-> I transition occurs reversibly at high pressure, the C delta form can be regarded as the stable form in the high-pressure region. But this form becomes metastable in the low-pressure region because the C delta --> C alpha --> S-A --> I transition is observed at pressures below 50 MPa. The shrinkage of the fiber repeat of the C delta form by about 2 Angstrom would suggest the conformational change from all-trans to an introduction of gauche (g+ or g-) conformation in the aliphatic sequence. Since the X-ray fiber photograph of the C delta form shows intense fiber reflections along the direction tilted by 25 degrees from the equatorial line, the molecular chains in the unit cell would be arranged in a herringbone pattern. The phase stability between the crystal modifications of BB-6 was analyzed as s function of temperature and pressure.
Acetylated 4,4′-biphenydiol (ABD) and biphenyl-4,4′-dicarboxylic acid (BDCA) were polycondensed in inert aromatic reaction media at various temperatures and monomer concentrations. At temperatures ≤300°C oligoesters were obtained, which according to infra-red and 13C nuclear magnetic resonance cross-polarization/magic-angle spinning spectroscopic characterization preferentially possess two carboxyl end-groups. Furthermore, polycondensations were conducted at 400°C whereby high-molecular-weight polyesters were obtained. Electron microscopy revealed the formation of lengthy crystallites, but true needle-like whiskers were never formed. Analogous polycondensations were conducted with acetylated hydroquinone and BDCA or with ABD in combination with terephthalic acid. Furthermore, all three polyesters were prepared by polycondensation of the free diphenols with terephthaloyl chloride or biphenyl-4,4′-dicarboxylic acid dichloride in Marlotherm-S at 400°C. Highly crystalline polyesters were obtained whenever the polycondensations were conducted at 400°C in Marlotherm-S, but in all cases the morphology was that of irregular particles. Wide-angle X-ray diffraction powder patterns measured with synchrotron radiation up to 440°C revealed that all three polyesters undergo a gradual transition from an orthorhombic crystal lattice to pseudo-hexagonal and finally to hexagonal chain packing with increasing temperature. These transitions cover a temperature range of more than 400°C. Only in the case of the polyester derived from hydroquinone and BDCA was a reversible first-order phase transition observed around 506°C.
The elastomers investigated are cross-linked partly by primary and partly by secondary valencies, i.e., by allophanate, biuret, and cyanurate bonds, on the one hand, and by hydrogen bridges and other intermoiecular interactions, on the other. The so-called physical cross-linking effected by secondary valencies is associated with a segregation of the hard segments, whereby statistically defined systems of hydrogen bridges are formed within the segregation areas. These systems lead to quasiperiodical, paracrystalline arrangements of the hard segments within the segregation areas. Under certain conditions true crystallization of the hard segments can be observed.
The nanophase separated hard segment domains in solvent cast films of a segmented polyurethane and a segmented polyamide elastomer were imaged in real space using phase and topographical information from tapping-mode AFM techniques. A styrene triblock copolymer was used as a control to demonstrate the contrast mechanisms. For the segmented polyurethane and polyamide elastomers, contrast results from local stiffness variations of hard domains beneath a ca. 1 nm thick soft segment overlayer. Domain sizes and dispersity, shape, orientation, spacing, and uniformity in space are uniquely extracted from these real space AFM data. The ca. 7 nm diameter domains were relatively symmetric and uniformly space filling in the polyurethane. They were lamellar or sheetlike in the segmented polyamide elastomer, with a high aspect ratio and no curvature. There was no obvious correlation of lamellae orientation with macrocrystal aggregate (spherulite) position in the polyamide copolymer, while no such aggregates exist in the polyurethane.
The fiber morphology of the dragline silk of Nephila clavipes has been investigated by the detailed analysis of wide-angle X-ray diffraction (WAXD) patterns. WAXD gives the crystal lattice dimensions, the orientation distribution, the crystalline fraction, and an estimate of the crystal size. It is found that the crystals are very small and well oriented. The mean (minimum) crystal dimensions are 2 × 5 × 7 nm, and the angle, φ, between the molecular chains in the crystals and the fiber axis has a full width at half-maximum (fwhm) of 15.7° and an orientation function f = 0.981. The X-ray crystallinity is in the range 10−15%, and the amorphous diffraction is divided 60:40 between an isotropic ring and an oriented halo with fwhm 30°. This means one-third of the material is in the oriented amorphous state, with a chain orientation of fwhm 43° and f = 0.87. When the fiber is extended up to 10%, the orientation of the crystals increases as predicted for affine deformation at constant volume. There is no observable change in crystallinity and apparently a small reduction in the lateral crystal size on deformation.
The thermotropic polyesters based upon 4,4′-dihydroxybiphenyl and aliphatic dibasic acids exhibit a unique odd-even effect. The polymers formed from dibasic acids having an odd number of methylene units exhibit a nematic phase, while those having an even number show a smectic mesophase. DSC heating curves of the odd-membered series exhibit an additional transition which is shown to be due to a smectic phase. The temperature range of stability of the nematic phase for the odd-membered series is rather narrow and decreases with increasing molecular weight, so that the nematic phase becomes monotropic for the polyester having ηinh, = 1.90 dL/g. The smectic phase of the even-membered series is stable over a broader temperature range. Values are given for the enthalpy and entropy changes accompanying the transitions. Although the mutual miscibility method eliminated some possibilities, it failed to identify the type of smectic polymorph of the even-membered series. This is shown to be SH by X-ray diffraction. The degree of crystallinity is higher for the even-membered series. Structural information concerning the crystalline and smectic phases of the even-membered series is deduced from X-ray diffraction data taken using unoriented samples.
Drawn poly(ethylene terephthalate) (PET) fibers annealed under various conditions are investigated by proton spin diffusion as detected through nuclear magnetic resonance. The primary objective is to study morphology on the 1-50-nm scale, the smaller dimensions of which have proved difficult to characterize for PET by conventional techniques. The spin diffusion experiment is comprised of three periods: generation of a magnetization gradient among different domains, relaxation of the gradient by diffusion for a variable time, and separate detection of the magnetization corresponding to each domain. The use of a multiple-pulse sequence permits spin diffusion to be confined to the second period, resulting in enhanced resolution among the domains. This procedure allows the magnetization decay observed during the detection period to be decomposed into three components, which are assigned to mobile noncrystalline, constrained noncrystalline, and crystalline domains. Rates of polarization redistribution among these three components are studied as a function of the diffusion time. Computer modeling is carried out in order to relate these measurements to the spatial arrangement and size of the three components. The results quantify the increase in crystallinity and in crystallite size upon annealing. Information pertaining to the structure of the noncrystalline region, the importance of noncrystalline chain orientation, and the relative surface areas of the crystallites is also presented.
We introduce the first comprehensive molecular model of spider dragline elasticity which clearly integrates most of the information known to date about the structure of the fiber. In accordance with X-ray evidence, the dragline is represented by a large number of small crystallites separated by amorphous regions made of rubber-like chains. Our model results clearly indicate the important role of the crystallites which act as multifunctional cross-links and create inside the amorphous regions a thin layer with modulus higher than in the bulk. The role of the crystallites in spider silk is found to be amazingly similar to that speculated for carbon black in synthesized elastomers.
Segmented polyurethanes are multiblock copolymers which consist of the so-called hard and soft segments. Unlike A-B or A-B-A long-block copolymers, the blocks in segmented polyurethanes are usually very short. Consequently, there are many connecting points (chemical bonds) between the segments. By using small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC), they were able to show that the system viscosity, the hard-segment mobility, and the hard-segment interactions were the three controlling factors for determining the structure of segmented polyurethanes. The phase-separation process of segmented polyurethanes was usually very slow and behaved like a relaxation process. Earlier, Wilkes and co-workers also showed time-dependent mechanical properties. Once the multiphase-separated structure had formed, post treatments, such as annealing temperature, had only small effects. Thermodynamically, phase separation is more complete with aromatic hard segments because of increased thermodynamic incompatibility between hard segments and aliphatic soft segments. However, from a kinetic viewpoint phase separation becomes more complete with aliphatic hard segments because of increased mobility. Experimental results seem to support the kinetic viewpoint, which could also explain the annealing behavior and the spherulite growth of segmented polyurethanes.
A homologous series of polyesters was prepared from 4,4′-dihydroxybiphenyl and dibasic acids having 5-12 methylene units. The mesophases formed at elevated temperatures were studied by differential scanning calorimetry and polarized light microscopy. This family exhibits an unusual odd-even effect when the transition temperatures are plotted as a function of the number of methylene units in the dibasic acid. Not only do the points for odd and even members fall on different curves, but the odd members exhibit a nematic phase over a very short temperature interval, while the even members form a highly ordered smectic phase. For both the odd and even series, the transition temperatures are significantly depressed when the inherent viscosity falls below 0.2 dL/g. The largest depression occurs for the crystal melting transition, so that polymers of low ηinh show anisotropic and biphasic regions over wider temperature ranges. A copolymer formed from an equimolar mixture of sebacic and chiral (+)-3-methyl adipic acid forms a cholesteric phase. Evidently copolymerization destabilizes the smectic phase which would have been expected. The results are discussed in terms of existing theory.
Two theoretical models are presented to predict average molecular orientation properties of spider silk which has unknown molecular orientation and chemical structure. Birefringence and elongation (or shrinkage) data are required for two distinct lengths of a sample in order to fit the data. The first model is based on the assumptions that each molecule behaves as the average and that the deformations are affine. The second model is based on a random distribution of the molecules as the fiber is first formed and that the molecular deformations are affine when the fiber is elongated to any length. Data were taken from spider silks in the natural state and in a shrunken state (due to water). The second model gives a much better fit of the data. The models are applicable to other systems.
The synthesis, rheology, and spinning, as well as the mechanical and morphological properties of high-strength/high-modulus fibres made from lyotropic main chain liquid crystal polymers are reviewed. Emphasis is placed on those polymers that have attained (semi)-commercial status. Quantitative relations observed between the rheological and the spinning parameters, and between the structure and the mechanical properties, are extensively discussed. It is shown that these relations fit theoretical expectations as far as the modulus is concerned. A complete model for the strength of these fibers needs further development. A first attempt to provide a theoretical explanation of the long term properties, like creep and stress relaxation, is presented.
Approximately 30 years after their preparation, the nanoscale morphology of a series of ‘model’ segmented polyurethane elastomers has been further elucidated using the technique of tapping mode AFM. The materials investigated are based on 1,4-butanediol extended piperazine based hard segments and employ poly(tetramethylene oxide) soft segments. The chemistry of these polyurethanes was specifically controlled in a manner which yielded monodisperse hard segments precisely containing either one, two, three, or four repeating units. Phase images obtained via AFM, for the first time, enable visual representation of the microphase separated morphology of these materials. AFM images also confirmed the presence of a spherulitic morphology, as shown several years ago using SALS and SEM. In addition, applying AFM to films of freshly prepared solution cast samples, the observed lath-like hard domains are suggested to preferentially orient with their long axis along the radial direction of the spherulites, while the respective crystalline hard segments comprising the hard domains are, in turn, preferentially oriented perpendicular to the spherulitic radius. The hard domain connectivity was found to increase with increasing percentage hard segment content of the polymers.
Hydrophilic layered silicate/polyurethane nanocomposites were prepared via twin screw extrusion and solvent casting. Good dispersion and delamination was achieved regardless of processing route, illustrating that the need for optimised processing conditions diminishes when there is a strong driving force for intercalation between the polymer and organosilicate. Evidence for altered polyurethane microphase morphology in the nanocomposites was provided by DMTA and DSC. WAXD results suggested that the appearance of an additional high temperature melting endotherm in some melt-compounded nanocomposites was not due to the formation of a second crystal polymorph, but rather due to more well-ordered hard microdomains. Solvent casting was found to be the preferred processing route due to the avoidance of polyurethane and surfactant degradation associated with melt processing. While tensile strength and elongation were not improved on organosilicate addition, large increases in stiffness were observed. At a 7 wt% organosilicate loading, a 3.2-fold increase in Young's modulus was achieved by solvent casting. The nanocomposites also displayed higher hysteresis and permanent set.
A number of polyurethanes are synthesized, based on the novel diisocyanate 4,4′-dibenzyl diisocyanate (DBDI), the traditional 4,4′- methylene bis(phenyl isocyanate) (MDI), or combinations of these. Two equivalent series are prepared, one with hydroxyterminated poly(ethylene adipate) and one with poly(tetrahydrofuran) macrodiol. Mechanical measurements and X-ray crystallinity data are shown for the materials as prepared and after annealing. DBDI has a variable geometry, which allows crystallinity to develop, and leads to an increase in mechanical properties (tensile stress, ultimate strength stress, tear strength, hardness and strain energy): whereas the residual elongation is also dramatically increased. The general improvement in properties with DBDI is retained when both diisocyanates are included, especially when reacted together in a random fashion, rather than sequentially, in the prepolymer stage. Under these conditions, the residual elongation returns to that of a more conventional polyurethane, eliminating the major inconvenience derived from the high permanent deformability and high crystallinity which is undesirable for an elastomer. Scanning electron microscopy reveals coarse texture at the 10 μm level, and a finer texture at the 100 nm level which may be associated with the hard and soft polyurethane segments.
This paper reports a discrepancy found during the analysis of wide-angle X-ray diffraction patterns from a broad range of branched polyethylenes and describes its interpretation. The paper is the second in a sequence of three investigating the structure of polyethylene using new methods in X-ray diffraction and molecular modelling. The X-ray diffraction patterns were recorded from both unoriented and fibre sample forms, using reflection and transmission geometries, respectively. A common set of crystallisation conditions were used to prepare the unoriented samples and the fibre samples were drawn from these unoriented samples using a consistent set of drawing conditions. The X-ray diffraction patterns were fitted to two contributions, namely crystalline and amorphous components, according to standard practice for polymers. However, a discrepancy in crystalline peak positions between low and high angle regions of each diffraction pattern was found for all samples, in both unoriented and fibre forms. The discrepancy is interpreted in terms of an additional distinct contribution to the X-ray diffraction pattern of polyethylene, from a third structural component of intermediate order. The scattering is consistent with partially ordered material and is found to be correlated with not only the branch content but also the branch distribution of the polyethylene. This opens up the possibility of tailoring the influence of the partially ordered component on the polymer's microstructure, and hence properties, by varying the molecular architecture and thermal history.
Thesis (M.S.)--State University of New York at Stony Brook, 2000. Includes bibliographical references (leaves 26-28).
The molecular origin of the exceptional mechanical properties of spider silk is unclear. This paper presents solid-state 2H nuclear magnetic resonance data from unoriented, oriented, and supercontracted fibers, indicating that the crystalline fraction of dragline silk consists of two types of alanine-rich regions, one that is highly oriented and one that is poorly oriented and less densely packed. A new model for the molecular-level structure of individual silk molecules and their arrangement in the fibers is proposed. These data suggest that it will be necessary to control the secondary structure of individual polymer molecules in order to obtain optimum properties in bio-inspired polymers.