R.K. Eby

Investigation of the nanofibrillar morphology in silk fibers by small angle X-ray scattering and atomic force microscopy

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.

Effect of processing temperature on the morphology of silk membranes

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.

Investigation of oxidation profile in PMR-15 polyimide using atomic force microscope (AFM)

Nanoindentation measurements are made on thermosetting materials using cantilever deflection vs. piezoelectric scanner position behavior determined by atomic force microscope (AFM). The spring model is used to determine mechanical properties of materials. The generalized Sneddons equation is utilized to calculate Youngs 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 PMR-15 polyimide. The thermo-mechanical profiles of PMR-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.

Characterization of compressive properties of high-performance polymer fibres with a new microcompression apparatus

Abstract 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.

Crystal structure of poly(tetrafluoroethylene) homo- and copolymers in the high pressure phase

Abstract 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.

Fluoropolymer force fields derived from semiempirical molecular orbital calculations

Abstract 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.

High-temperature properties of PBZT fibres

Abstract 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.

Orientation changes in Kevlar® 49 under axial compression

The orientation of crystals in a single Kevlar® 49 fibre has been measured by X-ray diffraction using a microcompressive device and the Cornell High Energy Synchrotron Source (CHESS). The data show that the orientation with respect to the fibre axis decreases with increasing axial compressive strain. The birefringence has been measured on the same sample with a polarizing optical microscope and a tilting compensator. The results show that the birefringence decreases with increasing axial compressive strain. This also is consistent with a decreasing crystal orientation with increasing strain. Both effects are slightly nonlinear and a graph of the full width at half maximum of the crystal orientation versus birefringence is nearly linear. The decreasing crystal orientation in compression is opposite to the increasing orientation with increasing tensile strain. The tensile effect increases the fibre modulus. The compressive effect, on the other hand, should decrease the modulus. The former has a limiting effect, but the latter could have the opposite and might contribute to compressive failure.

Aspects of the Morphology of the Silk of Bombyx Mori

Abstract 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...

Surface stresses in paraffin and polyethylene

Abstract 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.