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A theory of crystallization is formulated for random copolymers which crystallize with the non-crystallizable co-units incorporated into the crystalline lattice as defects. The appropriate melting point equation and other associated thermodynamic properties are derived for this model as a function of crystal thickness and comonomer concentration. The formation of lamellar type morphology is assumed to be a kinetically determined phenomena and nucleation theory is utilized accordingly. The isothermal lamella thickness is predicted to increase in a definitive manner as the noncrystallizable comonomer concentration X increases, while the associated isothermal growth rate is predicted to decrease. The variation of lamella thickness with X when the copolymer is quenched or cooled at a uniform rate is also qualitatively predicted. Under these conditions lamella thickness decreases with increasing X, which is in accord with previous experimental observations on random copolymers of tetrafluoroethylene and hexafluoropropylene as well as other random copolymers. Theory also suggests how the surface free energy parameters σ(e) and σ can be determined from isothermal crystallization experiments for a series of random copolymers of varying composition.

Between approximately 20 and 30 °C at atmospheric pressure, polytetrafluoroethylene exhibits a phase (denoted as IV), which has not been observed in P−V−T measurements at pressures above 10(8) Nm(−2) (1 Kilobar). Data are presented to resolve this phase in the temperature range 0 to 50 °C and the pressure range 0 to 0.686 × 10(8) Nm(−2). The II–IV (20 °C) transition pressure increases with temperature according to the equation P = (−820 + 26.9t + 0.68t(2)) × 10(5) and the IV–I (30 °C) transition according to the equation P = (− 91−57.8t + 1.99t(2)) × 10(5) suggesting a possible triple point near 2.6 × 10(8) Nm(−2) and 54 °C. However, the IV–I transition probably cannot be resolved by volume measurements at pressures above 2 × 10(8) Nm(−2) because its volume of transition is small and the two transitions overlap. The enthalpies of the transitions are calculated for different pressures and some thermodynamic properties of phase IV are determined indirectly. It is shown that, if the effect of volume change is subtracted, random reversals of the hand of the helical conformation of the molecule can account for appreciable fractions of the enthalpy and corresponding entropy change for the combined transitions at atmospheric pressure.

Polyimide (PI)–polyethylene oxide (PEO) block copolymers have a wide variety of applications in microelectronics, since PI–PEO films exhibit a high degree of thermal and chemical stability. The polymers consist of short, rigid rod T-shaped PI segments, alternating with flexible, PEO coil segments. The highly incompatible PI rods and PEO coils should phase-separate, especially in the presence of lithium ions used as electrolytes for lithium polymer batteries. The rigid rod phase provides a high degree of dimensional stability. In this paper, we provide evidence by DSC that the self-assembled ordered structure of the PI–PEO molecules is formed from concentrated solution rather than the bulk state. Tapping mode AFM and X-ray diffraction are applied to observe the nanodomains in the phase separation of the PI and PEO before and after doping with lithium ions. In addition, we report evidence of the ion transport primary mechanism, in the amorphous phase of the lithium salt-doped PI–PEO block copolymers' multinuclear NMR linewidth, spin-lattice relaxation time, and pulsed field gradient diffusion measurements.

In the prevailing thermodynamics associated with Gibbsian surface physics of a century ago, the surface of a crystal has an associated surface energy and surface stress that influences properties and morphology. This work uses molecular mechanics to investigate the surface energy and epitaxial packing of chain folds on the surfaces of adjacent crystals. It is shown that there are lower energy juxtapositions of the crystals resulting from energetically favorable epitaxial arrangements. These lower the excess surface energy with respect to the melt, but do not completely eliminate the discrepancy between the values of the surface energy from computational results and analysis of experimental data. The epitaxial arrangements are consistent with the experimentally observed rotational displacement of successive layers in multi-layer polyethylene crystals. Thus, they might influence the rotation of the successive layers resulting from screw dislocations.

In this initial work, we demonstrate a technique for preparing thin (10–20 μm) films of silk doped with green fluorescent protein (GFP) by casting/annealing at 20 °C and describe the resulting film characteristics. Notably, the GFP molecules maintain their nonlinear optical properties as evidenced by two-photon fluorescence microscopy and two-photon absorption measurements using near-infrared femtosecond pulses. The fractional transmission of focused near-infrared pulses of 775 nm wavelength, 140 fs pulsewidth was observed to decrease as the incident pulse energy is increased and/or the incident spot size is decreased, indicating that nonlinear absorption is taking place. Visible damage from the pulses is observed in a ∼10 μm film at the highest peak incident fluences, which were in the range of 0.1–0.2 J/cm 2 . Variations in thickness, morphology and GFP concentration of the films make precise specification of the two-photon absorption coefficient difficult. Since these films have potential applications in photonics, we suggest techniques for improving these properties in future generations of films. The suggestions present opportunities for future work.

Nanoscale fibers of natural silks of Bombyx mori and Nephila clavipes were produced from solutions in hexafluoro-2-propanol. Theelectrospun fibers were observed by optical, scanning electron, and transmission electron microscopy. These nanofibers showed opticalretardation, appeared to have a circular cross-section, and were thermally stable under nitrogen to 280 8C(N. clavipes) and to 245 8C(B. mori). The diameter of the fibers ranged from approximately 6.5–200 nm making them orders of magnitudes smaller than the naturalsilks spun by most silkworms and spiders. The smallest fiber diameters correspond to 200 molecules in the cross section of the N. clavipesfibers and 150 in B. mori. Electron diffraction patterns of annealed electrospun fibers of B. mori and N. clavipes exhibit diffraction peaksdemonstrating orientational and crystalline order comparable to that of naturally spun silks.q 2004 Elsevier Ltd. All rights reserved. Keywords: N. clavipes; B. mori; Electrospinning 1. IntroductionThe useful physical properties of natural silk fibers [1]and the ability to produce these proteins using biotechnol-ogy [2], have provided the impetus for the efforts in both thebiosynthesis and the spinning of these proteins. A greatmany organisms produce silk. Silkworms and spiders aretwo main ones. Silkworm silk, of the domesticated Bombyxmori, is well known and has been of interest for at least 5000years due to its importance as a textile fiber [3,4].Incomparison, the study of spider silks is relatively new. Orbweaving spiders are sophisticated users of silk. Somepossess up to seven different sets of glands which produce asmany different fibers, all designed for specialized appli-cations [5]. The most prominent set is the major ampulatewhich produces the protein for dragline silk. One of themost carefully studied spider silk fibers is the dragline ofthe orb weaver Nephila clavipes. This is partially due to thespider’s large size and the associated relatively largequantities of silk it can produce. Dragline fibers produced bythe orb weaver exhibit a highly desirable combination ofstrength, toughness and compressibility, not evident insynthetic high-performance fibers [6]. Some synthetic poly-mers such as Kevlar

Fifty-six N. clavipes spiders from the same region of Florida were kept in captivity under the same conditions and fed a similar diet of crickets. Their major ampulate glands were forcibly silked. Dates, silking times, and the colors of the dragline silk produced were recorded. The colors ranged from all white through various combinations of white and yellow upon different silkings to all yellow. If a spider had been producing white silk for at least 4 h, the color being produced could suddenly change to yellow 38% of the time. These observations indicate that factors beyond diet and environment influence the color of silk produced in captivity by forcible silking. They also indicate that the spiders store both pigmented and unpigmented silks and that some aspect of forcible silking precludes the spiders' choosing the color. The yellow and white silks exhibit similar exterior surface morphologies as well as similar tensile properties.

4 pags., 5 figs., 2 tabs., Ultra-microindentation was used to measure the microhardness and modulus of silk (Bombyx mori) membranes, cast from 20 to 80°C. The microhardness and modulus were determined from the loading/unloading curves. The membranes exhibit microhardness of about 400 MPa which is larger than the values for most common synthetic polymers (50-300 MPa) implying a greater scratch resistance. The moduli are of the order of those measured by the other means for B. mori silk membranes (5 GPa), and fibers (7-11 GPa). There is some correlation between microhardness and the dimensions of the grains/nanofibrils, but none with surface roughness. The results extend the range of an empirical correlation between microhardness and modulus. The present data together with previous data from other polymers fit the equation, H=0.55E0.74, with a correlation coefficient of 0.94. Finally, it is shown that elastic recovery of the silk membranes is an increasing function of the maximum load applied. © 2004 Elsevier Ltd. All rights reserved., This work was supported in part by Air Force Office of Scientific Research (AFOSR) Award number F49620-03-1-0169. Grateful acknowledgement is due to MCYT (grantBFM2000-1474) for the support of this investigation. Oneof us, I.P.O., also acknowledges the support of the FPIProgramme of MCYT (Spain).

The structures of membranes of PBZO and PBZT extruded with counter rotating dies (CRD) were studied by wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), atomic force, scanning electron and transmission electron microscopy (AFM, SEM, and TEM). The structure of CRD-extruded PBZO was compared with that of a solution-cast membrane. The extruded membranes have sheet structures typical of rigid-rod polymers. The heterocyclic rings of the extruded membranes are oriented approximately parallel to the membrane surface, while those of the cast membrane are oriented perpendicular to the surface. The parallel orientation of the rings of the extruded membranes may be due to the normal force exerted during extrusion. The polymer molecules near the surfaces of the extruded membranes are oriented along the shear directions of the extruder, while those in the middle are oriented randomly. There is little cholesteric nature. These materials have potential as microporous PEMs holding ion conducting polymers (ICPs).

Nanoindentation measurements are made on thermosetting materials using cantiever deflection vs. piezoelectric scanner position behavior determined by AFM. The spring model is used to determine mechanical properties of materials. The generalized Sneddon's equation is utilized to calculate Young's moduli for thermosetting materials at ambient conditions. Our investigations show that the force-penetration depth curves during unloading in these materials can be described accurately by a power law relationship. The results show that the accuracy of the measurements can be controlled within 7%. The above method is used to study oxidation profiles in Pl\1R-15 polyimide. The thermo-mechanical profiles ofPNIR-15 indicate that the elastic modulus at the surface portion of the specimen is different from that at the interior of the material. It is also shown that there are two zones within the oxidized portion of the samples. Results confirm that the surface layer and the core material have substantially different properties.

A concise literature survey concerning the processing and uses of silk membranes is presented in this note together with initial observations of new morphological data for the effect of processing temperature on morphology. Liquid silk from the middle section of the Middle Division of the silk gland of Bombyx mori was cast onto glass plates at 20, 40, 50, 60 and 80 °C. Silk from the anterior and posterior sections was cast at 20 °C. Samples cast at 20 °C exhibit particles, grains, nanofibrils and an irregular morphology. Each exhibits approximately the same dimensions for all the samples. Samples cast above 20 °C do not exhibit the irregular morphology. Samples cast above 50 °C exhibit larger grains and larger, more densely packed nanofibrils. All these changes might result from conversion of the amorphous structure to the β-pleated structure (Silk II). The nanofibrils appear to be self-assembled bio-nanofibrils. Membranes of regenerated fibroin treated with aqueous methanol solution exhibit grains and apparent nanofibrils. Opportunities for further work are pointed out.

X-ray diffraction with the assumption of uniform stress has been used for determining the crystal modulus of polymers. Values of the modulus of silk obtained previously using such an approach were found to be an order of magnitude lower than, those obtained by computational modeling. The differences are outside the limits of experimental and computational errors. This study re-examined the values in an improved manner. For the X-ray technique, moduli of different samples along the length of cocoon fibers were measured. The values obtained are 20–28 GPa depending on the crystallinity index and degree of orientation of the samples. For the computational calculation, the longitudinal acoustic mode (LAM) frequency of silk systems having three different c -axis dimensions were calculated. An acoustic dispersion curve was generated from the frequency values. The slope of the curve at the origin gave a crystal modulus of 13 GPa. A 3D-fluctuation analysis yielded an average modulus of approximately 16 GPa. These low values result from the out-of-plane torsioning of molecules in the β-pleated structure of the silk crystals. The experimental and computational results are in reasonable agreement with one another as well as with data reported in the literature for other pleated molecules.

Silk from 3-molted, 4-molted and sex-limited Bombyx mori, Antheraea pernyi and Antheraea yamamai have been investigated using Atomic Force Microscopy and Low Voltage High Resolution Scanning Electron Microscopy. Nanofibrils, bundles of nanofibrils, helical features and a layered structure with a cross angle between nanofibrils in different layers were observed for all silks. Similar log-normal distributions of fibril widths were obtained from all silks with the geometric mean fibril widths covering the range of 90–170 nm. There is no correlation of the fibril width with the fiber size from different types of silkworm. Similar normal distributions of cross angles were observed in 30% of the images with the arithmetic means covering the range of 30–50°. There is an apparent correlation of cross angle with fiber size. The larger fibers for which the crystals are primarily alanine exhibit a greater average cross angle, 46°, than do those of the smaller fibers for which the crystals are primarily alanine and glycine, 36°. Initial wide angle X-ray diffraction measurements on a single fiber of A. pernyi are consistent with the fraction of fibrils at different cross angles being 28%. A simplified computer calculation method and molecular modeling were used in a search for packing angles which minimize the interaction energy of packing between beta sheets. Initial results revealed no definitive evidence of epitaxy to account for the cross angles. It is proposed that a multifibrillar fiber offers a number of mechanical advantages over a solid fiber with the same cross sectional area.

Over a wide crystallization temperature (Tc) range, elongated rectangular lamellar single crystals of syndiotactic polypropylene (s-PP) grown from the melt of thin films exhibit sectorization along both diagonal directions to form the (010) [with the (010) free edge] and the (100) [with the (100) free edge] sectors. They can be observed by transmission electron and atomic force microscopy (TEM and AFM). Electron diffraction experiments show that the crystals in both sectors are form III orthorhombic structure, as recently proposed.1-4 The observation of this sectorization is due to the thickness difference between these two sectors with different fold planes. In situ observations of the single-crystal growth are monitored using an AFM coupled with a hot stage. The Tc-dependent crystal growth rates along both the (100) and (010) normal can be measured, and they are anisotropic. This leads to a large aspect ratio of the rectangular single crystal. AFM results also show that the thickness difference between ...

A small-angle peak has been found by X-ray scattering measurements on the dry dragline silk of Nephila clavipes. The weak peak, located on the equatorial axis, yields a long period of approximately 45 A. Probably it corresponds to the lateral spacing of the crystals and is determined kinetically during the formation of the nanofibers.

Small angle X-ray scattering (SAXS) and atomic force microscopy (AFM) measurements have been shown to be consistent with the presence of nanofibrils in the cocoon silk of Bombyx mori and the dragline silk of Nephila clavipes. The transverse dimensions and correlation lengths range from >> 59 to 220 nm and in the axial direction from >> 80 to 230 nm. Also, the two-dimensional Fourier transforms of the height profiles of AFM topographic images of interior surfaces of B. mori follow a power law approximately the same as that for the Porod region of the SAXS data. In this manner, the AFM can be used to help remove ambiguity about the scatterers responsible for SAXS patterns.

Solutions of Bombyx mori gland silk can be electrospun with the addition of some polyethylene oxide (PEO). Green fluorescent protein (GFP) can also be incorporated and electrospun without apparent phase separation from the silk. The dimensions of the fibers with and without the GFP are qualitatively similar. The results indicate the possibility of making fibers with uniform non-linear optical properties.

The methods of ‘histograms and cumulants’ have been applied to extract temperature dependent information from dynamic simulations of polyethylene. The use of these methods allows one to efficiently predict the coefficients of thermal expansion, and for the latter method, even to determine their temperature dependence from relatively short simulations. Using standard force fields and a run of only 470 ps, the latter method matches the coefficients of thermal expansion to 6% for the a unit cell dimension, 2% for b and 50% for the quite small, negative, coefficient in the c-dimension. © 1997 Elsevier Science Ltd.

Kinetic aspects of the transformation of poly(tetrafluoroethylene) (PTFE) from phase IV to phase II at 19°C and atmospheric pressure were studied using wide angle X-ray diffraction (WAXD) following a rapid decrease in temperature from 24.7 to 14.5°C. Information relating to changes in molecular packing was obtained from the position of an equatorial peak. Changes in the amount of disorder present were characterized by measuring the integrated intensity of peaks on the second layer line. Within the limits of error, the molecular packing and disorder changes occurred simultaneously as the transformation occurred. The kinetics of the transformation were evaluated following the approach of Avrami. The results indicate one-dimensional crystal growth in agreement with theory. The results from this study are discussed with respect to implications for the transformation mechanisms and previous work in this area.

Atomic force microscopy was used to examine the exterior surface of as-spun cocoon silk, raw silk, and degummed silk as well as the interior surface of peeled degummed silk of Bombyx mori. The images yielded a broad variety of features with a wide range of dimensions. They are qualitatively similar to those of the dragline silk of Nephila clavipes. Examination of the surface roughness, the surface profiles, and the Fourier transforms of the profiles showed the surface of the degummed silk to be quantitatively similar to the silk of N. clavipes. The average roughness is 4 nm. The average dimensions of the features are approximately 100 to 150 (> 2000 in peeled samples) nm axially and 50 to 300 nm laterally. They appear to emerge from and to enter the surface, to vary in width, and to twist about one another in an irregular manner. In the peeled degummed silk they extend more regularly for long distances in approximately the axial direction. In some cases they appear to be oriented at different sma...

The van der Waals parameters for Lennard-Jones (6–12) and 6–9 potentials and equilibrium geometries for use in molecular mechanics and dynamics calculations on perfluoroalkanes and polytetrafluoroethylene (PTFE) have been derived from MOPAC AM1 (Austin model 1) semiempirical calculations on the model molecule perfluorohexadecane (PFHD). Parameters derived from MOPAC AM1 energies scaled to yield higher barriers to torsional motion, as suggested by ab initio results, provide a large trans barrier and stronger intermolecular attractions, which will probably be important for dynamics investigations into the nature of the solid-state phase transitions and helical defects. Reasonable intramolecular geometries and intermolecular packing arrangements are obtained with all parameter sets reported.

Highly faceted, regular, lathlike lamellar single crystals of syndiotactic polypropylene (s-PP) fractions have been investigated through transmission electron microscopy (TEM), atomic force microscopy (AFM), and electron diffraction (ED). Single crystals of s-PP over 1 μm in size can be grown from the melt in thin films. ED results obtained from the s-PP single crystals indicate a unit cell III with a = 1.450 nm, b = 1.120 nm, and c = 0.740 nm as proposed by Lovinger and Lotz. At high crystallization temperatures, relatively low molecular weight s-PP fractions can grow lamellar single crystals with microsectors. The polyethylene decoration method has been used to identify the chain folding direction, and no preferred orientation has been observed on the nonsectorized lamellar crystals. Sectorized lamellar single crystals show two different regions. In the sectors along the long axis (the b-axis), the chain folding is found to be parallel to the 010 direction. In the sectors along the short axis (the a-axi...

The transformation of PTFE from phase II to phase IV at 19°C at atmospheric pressure has been investigated using wide angle X-ray diffraction. Diffraction patterns were taken as a function of temperature. The integrated intensity of an equatorial peak present only in phase II was used to monitor the extent of the transformation. Peaks on the second and fifth layer lines were used to determine the effect of transformation and/or disorder. Integrated intensities on these three layer lines were shown to decrease differently with increasing temperature. The decrease in intensity on the second and fifth layer lines could be described in terms of a model comprising the initial integrated intensity, fraction transformed and the amount of rotational disorder present. Molecular modeling also was used to investigate the transformation. A commercially available force field was modified to represent the equilibrium solid state of PTFE. The modified force field was used to examine aspects of the transformation including the influence of neighboring molecules on the molecular conformation.

A suite of 29 micas from miarolitic pegmatites associated with granitic units of the anorogenic Pikes Peak batholith (1.08–1.02 Ga), Colorado range in composition, and follow in paragenetic sequence, from 1M siderophyllite (N = 1), and 3T or 2M1 lithian biotite (N = 5) to 1M zinnwaldite (N = 20) and 1M ferroan lepidolite (N = 1). Locally, 1M (?) phlogopite (N = 1) and ferroan 2M1 muscovite (N = 1) are also present. Pervasive, late-stage hydrothermal alteration along with possible supergene weathering of many of these micas produced vermiculite. Additionally, some vugs and cavities were filled with chlorite and/or smectite. Early crystallized micas form tapered columnar crystals in graphic pegmatite, growing toward, and adjacent to the miarolitic cavity zone which contains the later crystallized micas. Principal associated minerals are quartz, microcline perthite (mostly amazonite), and albite, with local topaz or fluorite, and rarely tourmaline (schorl-elbaite). Progressively younger micas of the main crystallization sequence display increasing Si, Li, F, and Al/Ga, and decreasing total Fe, Mg, and octahedral occupancy. The zinc content of all micas is considerably elevated, whereas Mn, Rb, Cs, and Sc are moderate and T1 is very low. Early siderophyllite and lithian biotite show a narrow range of FeO/Fe2O3 (5.6–8.0), whereas later zinnwaldite is much more variable (2.4–40.3). Annite of the host granite and early graphic pegmatite is compositionally homogeneous, but most mica crystals from cavities show remarkable compositional and abrupt, sharp and distinct color zoning. Most cavity-grown zinnwaldite crystals show a decrease, from core to rim, in total Fe and Mg, whereas Si, Li and F increase and Mn, Rb, Cs and Na are essentially constant. A few to more than 100 color zones have been identified in some mica crystals. The zones are well correlated with the Ti content (

Previous calculations of surface stress in polyethylene and paraffin crystals are extended to include other experimental and computational data. There are significant changes in the magnitude of the stresses. For polyethylene, these might reflect variations in the nature of the fold surface. Computational modelling yields surface stresses and energies that agree with the experimentally based magnitudes for paraffins but disagree with those for polyethylene. The discrepancy is not reduced by taking account of the effect of neighbouring crystals in the modelling. The agreement of the paraffin modelling results with experiment suggests that the discrepancy for polyethylene has its origins in the model of the fold surface. This raises the possibility that modelling can be used to investigate the nature of the fold surface.

Molecular dynamics simulations were performed in an attempt to elucidate the molecular mechanism of a relaxation measured experimentally in poly(p-phenylene benzobisthiazole) (PBZT) fibres. Results for the coefficient of thermal expansion and tensile modulus parallel to the molecular axis agreed favourably with published experimental values. The X-ray peak intensity ratio of (200) (010) versus temperature obtained from the simulations showed a variation qualitatively similar to that from experiments. The torsion angle between the benzobisthiazole and phenyl moieties increased with temperature. A distortion of the phenyl and benzobisthiazole moieties also occurred and the molecules were observed to undergo increased oscillatory ribbon-like motions perpendicular to the (010) plane as the temperature increased. Finally, translational oscillations of the molecules parallel to the c-axis occurred. These molecular motions are coupled to the tensile stress and could play a role in the relaxation.

A microcompression apparatus to measure the compressive stress-strain curve of individual fibres has been developed. Illustrative results were obtained for a series of mesophase-pitch-based carbon fibres. A series of optical micrographs of a P100 fibre were taken as it failed in compression. The ultimate compressive strength (UCS) for any given type of fibre tended to cluster around two average values. The reason for this clustering is uncertain, but the two values may correspond to more and to less ‘perfect’ fibres. P55 exhibited a higher average UCS of 600 MPa and a lower average UCS of 300 MPa. P75 had a higher average UCS of 600 MPa and a lower average UCS of 300 MPa. P100 gave a higher average UCS of 400 MPa and a lower average UCS of 150 MPa. The compressive modulus for P75 was 470 GPa. The fibres exhibited non-linear elasticity, with the compressive modulus decreasing as the compressive strain increased.

X-ray diffraction measurements are reported for 27°C, pressures to 5.2 GPa and concentrations of CF3 units to 0.049 CF 3 CF 2 . These show both orthorhombic and monoclinic structures to exist under high hydrostatic pressure. It is proposed that shear stresses generated at elastic inhomogeneities in the sample lead to the monoclinic phase. Energy calculations are consistent with this concept. They also indicate that conformational and rotational disorders raise the entropy of the high pressure phase III. Perfluoromethyl branches increase the volume CF 2 of phase III more than that of the low pressure phase I. At high CF3 concentrations and pressures, both phases become metrically hexagonal. The volume of transition becomes zero at a concentration near 0.05 CF 3 CF 2 and no transition is observed to a pressure of 5.2 GPa. There appears to be a critical point near 27°C, a CF3 concentration of 0.05 and a pressure of 3.5 to 5 GPa.

The bond strength of paper-polypropylene-paper (PPP) laminates was measured using a peel test. Testing was performed on the as-received laminates of different manufacturers, after thermal aging in air and nitrogen, after static thermal aging in oil at room temperature and 90 degrees C, and after cyclic thermal aging in oil. Initial peel strengths ranged from 25 to 141 N per meter of width. The results of thermal aging ranged from a slight increase to a decrease of about 50% in peel strength. Aging in oil at room temperature had a smaller effect on peel strength. Thermal ageing in oil caused a decrease of up to 80% of the initial peel strength. Under cyclic thermal aging as in seen in actual cable use, some specimens delaminated while other were markedly weakened. It is believed that differential swelling of the paper and polypropylene was the primary cause of failure, as most specimens failed adhesively. The laminate with the highest initial peel strengggs not always the strongest under actual use. >

Under compression poly( p -phenylene terephthalamide) (PPTA) fibres fail in a ductile mode without apparent bond cleavage and fracture. However, the molecular origins of this are not well understood. We propose a possible molecular mechanism dominated by change in the amide-carbon-phenyl bond, as opposed to the amide bond.

Variations in lattice parameters for materials that form very thin lamellae are analyzed using a thermodynamic model that incorporates surface stress effects. It is predicted that lattice spacing variations should be proportional to the reciprocal of the lamella thickness, in agreement with experimental data for polyethylene, n-paraffins, and a copolymer of tetrafluoroethylene and hexafluoropropylene. The model is then used to calculate the surface stress associated with lamella interfaces in these crystalline materials. The calculated surface stress has the same order of magnitude as a surface tension, but is negative. The model is extended so that surface stresses associated with grain boundaries can be measured in very fine-grained metals and ceramics.

A normal coordinate approach was used to generate crystallographic packing candidates of a multitorsional polyimide synthesized from 3,3`,4,4`-benzophenonetetracarboxylic acid (BTDA) and 2,2-dimethyl-1,3-(4-aminophenoxy) propane (DMDA) (PI-2). Candidates were obtained under conditions of fixed axial advance of 24.6 {Angstrom} per monomer, and imposed 2/1 helical or 1/0 translational symmetry, consistent with the observed WAXD meridional layer line spacing. The ability of combinatorially generated torsional states to adopt the desired geometry was examined. Necessary corrections to the conformational parameter equations have been made. The procedure described allowed crystallographic conformations satisfying explicit geometric and MM3 intramolecular energy criteria to be generated for a linear multitorsional polyimide prior to the application of crystallographic screening or refinement procedures. 20 refs., 7 figs., 1 tab.

It is shown that experimentally determined values of the crystal modulus of Bombyx mori silk agree reasonably well with the computationally determined values, if account is taken of the stress relaxation, which occurs during the experimental measurements. The experimental are 16–22 GPa depending on the sample and the computational are 13 or 16 GPa depending on the method of analysis.

Energy analysis is used to determine the low energy crystal structure for poly(tetrafluoroethylene) in the 15 7 conformation. It is also used to determine the energies of various conformational defects. The perfect structure has a motif of two left- and two right-handed molecular stems in a triclinic unit cell with parameters, a , b , and γ of 1.102 nm, 1.142 nm, and 121 degrees, respectively. A variety of other perfect structures with higher energies exist. None of these corresponds to the experimentally observed metrically hexagonal cell containing one rotationally disordered stem. Excess energies were calculated for rotationally disordering defects such as molecules with the 15 7 conformation in a crystal of 54 25 molecules (and vice versa ), rotated setting angle, reversed hand, and the 2 1 conformation. Subject to the model and the minimization techniques used, these energies ranged in order from near zero to approximately 2 kcal/mol defect CF 2 with those for the 54 25 crystal being about 38 percent greater than those for the 15 7 crystal. The intramolecular ‘interfacial’ energies for these defects ranged from near zero to approximately 2 kcal/mol interfaces. These numbers are approximately 33 percent greater in the 54 25 crystal. The defects decrease the specificity of packing of the molecules. They also increase the volume, energy, rotational disorder and equality of the dimensions of the unit cell. They drive the structure toward the experimentally observed one.

Since the pioneering work of Carothers which led to the introduction of strong nylon fibers by DuPont in the 1930s, polymer scientists have pursued the development of high-performance fibers to replace natural or metallic products, both to improve mechanical properties and to reduce weight. That development was relatively slow and evenly paced until DuPont again revolutionized the field with the release of Kevlar™, an aromatic polyamide with unprecedented mechanical properties. Since then, the field has literally boomed with new developments, and now organic fibers are available with properties that compete with the best inorganics and are far superior to metal fibers.Strong motivation for the invention of new organic fibers comes from the aerospace industry, which seeks fibers to use in reinforced composite structural materials. Composites bring new advances in stiffness (airplane wings can't bend too much!) in weight savings (every kilogram saved in the structure of an airplane saves $120 over its lifetime, and in a spacecraft $10,000), and in radically new ideas, such as radar-invisibility (stealth)5 and mission-adaptive wings (in-flight variable-shape wings). Hence, for a variety of specialty applications, otherwise commercially indefensible materials become viable.It may be somewhat counter intuitive to materials scientists unfamiliar with polymers to expect polymer mechanical properties to be greater than in the best metals. The origin of high strength and stiffness in a polymer fiber is the covalent bond, especially when aligned in an ordered array of long chain molecules.

The phase diagram of polytetrafluoroethylene is enlarged by differential scanning calorimetry to include the concentration of hexafluoropropylene comonomer units. The two transitions near 292 and 303 K in the homopolymer move to lower temperatures and apparently become one at small concentrations. Analysis of the data yields 295 K for the temperature and 13.2 J/g for the heat of transition of an infinitely large homopolymer crystal. The heat of transition associated with the formation of a crystal defect is 0.021 eV. The qualitative features of the transition can be accounted for by a mean-field model which involves two order parameters corresponding to planar units and helix reversals. This model yields two transitions which move closer together and to lower temperatures with increasing comonomer concentration. Decreasing lamella thickness will have a qualitatively similar effect.

Semi-empirical energy calculations are reported for methyl branched hydrocarbon molecules in orthorhombic and monoclinic crystalline arrays of linear molecules. The 19 molecules in the array are flexible and twenty methylene units long. Two modes of packing the defect molecule in the crystals are found and for each the mechanical relaxation criterion of there being two or more sites available to the molecule is met. However, there is no significant relaxation strength because the differences in site energies are too large. This result holds for all cases because the unfavourable orientation of the molecular stem in one of the sites makes the energy much greater than in the other for which the stem has a favourable orientation. For the case with the lowest defect energy, the unit cell dimensions are comparable to those found experimentally for the same methyl group concentration. Drawings showing the molecular distortion are presented.

Calculations for diffusion in orthorhombic, monoclinic and triclinic subcells are reported. They show that self-diffusion can occur and support a vacancy mechanism. Vacancy energies are much larger than the barriers to molecular motion. Diffusion parallel to the basal plane can be by a rigid rod mechanism while perpendicular requires a conformational jog to traverse the crystal boundary. The diffusion coefficients are in order of magnitude agreement with the limited experimental data available. For a given subcell, diffusion parallel to the basal plane is about an order of magnitude greater than perpendicular. Diffusion coefficients for the triclinic subcell are larger than those in the other two. The computed coefficients are especially sensitive to some of the parameters used.

Energy analysis is used to determine the low energy crystal structure for polytetrafluoroethylene molecules in the 13 6 and 54 25 conformations. The structure for the 54 25 conformation contains an ordered repeating pattern of a left- and right-handed pair of molecules in a unit cell having projected parameters, a′, b′, and γ′, of 9.60 A, 5.62 A, and 91.4 degrees, respectively. These results, the relative orientations of the molecules, and the relative translations of the molecules are in good agreement with results obtained in a concurrent and independent analysis of diffraction data. Energies calculated for rotational and translational displacements in various modes suggest that a range of structures and a fairly large amount of disorder may be present, also in agreement with experiment.

X-ray diffraction patterns for oriented and unoriented copolymers of tetrafluoroethylene and hexafluoropropylene are presented. They show a transition between 229 and 243K for a copolymer with about 3.4 CF 3 per hundred main chain carbon atoms. The lower temperature phase (II) appears to be similar to that of the homopolymer. A primary difference is the presence in the copolymer crystals of small longitudinal displacements of the molecules caused by the randomly located CF 3 groups. The displacements increase with increasing CF 3 concentration. The higher temperature phase (I) is similar to that of the homopolymer with a difference again being the longitudinal disorder of the copolymer. The difference disappears at higher temperatures as longitudinal disorder is introduced thermally in the homopolymer. At 296K (phase I) the molecular stems in a copolymer with about 3.4 CF 3 per hundred main chain carbon atoms are at an angle of 37.1 degrees with respect to the normal to the basal plane of the lamellas. In phase II, this number is the same within the limits of error.

The unit‐cell dimensions of a given polyethylene have previously been shown to vary nearly linearly with the reciprocal of lamella thickness. Data obtained at 153.2 and 296.2°K are presented to show that the slope of this dependence is different for crystals of orthorhombic n ‐paraffins, melt‐crystallized polyethylene, and solution‐crystallized polyethylene. Within the limits of error, all extrapolate to the same basal area at infinite lamella thickness, and this agrees with the measured value for a sample crystallized from the melt under high pressure to yield a long period of about 3500 A. Since the effect is a surface one, it is proposed that these differences result from the differences between methyl interactions, fold interactions, different fold planes, domains, etc. It is shown that the variation of cell dimension with lamella thickness leads to a quadratic term in the variation of macroscopic density. This term permits the separation of the thickness and density of a lower‐density surface layer in a two‐phase model.

An electron diffraction pattern from polytetrafluoroethylene in Phase II was analysed to determine a chain conformation of 2.1598 CF2 units per turn of the helix. A structure was established from a combination of electron and X-ray diffraction data. The structure contains an ordered repeating pattern of a left- right-handed pair of chain stems. Although the structure is ordered in three dimensions, it cannot be described meaningfully in terms of classical space group nomenclature because very small changes in the structure result in very large changes in the cell parameters. The structure has a volume of 0.03542 nm3 per CF2 group, corresponding to a density of 2344 kg m−3.

The thickening of polymer crystals during isothermal annealing is usually observed to be an irreversible process. Phenomenological laws that govern such processes take the form of simple proportionalities—flux being proportional to force. For polymer crystals, a thermodynamic force capable of driving the thickening phenomenon arises from the unequal free energies of the fold and lateral surfaces. By analogy with other irreversible phenomena, the rate of crystal thickening is taken to be proportional to the derivative of the surface free energy with respect to crystal thickness. After certain assumptions, integration yields an equation in which three parameters characterize the system: an initial thickness l0, an equilibrium thickness l*, and a relaxation time τ which is a function of the ``undercooling''. The theory provides a basis for considering the effects of parameters such as time, temperature, thermal history, pressure, and liquids on the thickening rate. In particular, the theory adequately descri...

We have developed a method which uses laser-generated ultrasound to measure the Young's modulus of fibers as a function of temperature and static tensile stress. For fibers of PBZT, measurements have been made to 580°C and 1.7 GPa. The fibers are shown to exhibit nonlinear elasticity which changes systematically with temperature, tensile stress and fiber processing conditions. They exhibit a relaxation associated with a structure change at about 300–400°C.We have also used x-ray diffraction to measure both the crystal modulus and aspects of the ultrastructure such as crystal orientation as a function of static tensile stress, crystal size and unit cell. It is shown that improved crystal orientation with increased tensile stress is one of the most important mechanisms of the nonlinear elasticity. The measurements of orientation distribution are combined with other measurements to make calculations of the crystal modulus for the assumptions of uniform stress and uniform strain. These apparent crystal moduli are considerably greater than the measured ultrasonic ones and both are less than the theoretical values. The assumptions of uniform stress and uniform strain have also been used together with the orientation distribution and other parameters to calculate the macroscopic modulus. Both results exhibit less nonlinear elasticity than that observed experimentally, indicating that there is another mechanism in addition to the crystal reorientation which contributes to the nonlinear elasticity. The x-ray measurements of unit cell as a function of temperature to 450°C show a structure change at about 300-400°C. It is consistant with an oscillation of the phenyl and bisthiazole moities around the connecting single bonds.