D. T. Grubb

Analysis of small‐angle X‐ray scattering from fibers: Structural changes in nylon 6 upon drawing and annealing

The changes in the fibrillar and the lamellar structure in nylon 6 fibers resulting from drawing and annealing were studied by a detailed analysis of their two-dimensional small-angle scattering patterns. The scattering object that gives to rise the diffuse equatorial scattering in the angular range of Q = 0.02 to 0.3 A−1 is assumed to be a fibril. There are two distinct regimes in the equatorial diffuse scattering. The scattering at Q 0.1 A−1 to the lateral dimensions/organization of the fibril. The interfibrillar regions, unlike the interlamellar regions that are essentially made of amorphous chain segments, may have microvoids in addition to amorphous chain segments. The intensity distribution within the lamellar reflections was used to obtain the lamellar spacings and the dimension of the lamellar stacks. The length of the fibrils is between 1000 and 3000 A, the higher values being more prevalent at lower draw ratios. The fibril length is larger than the length of the lamellar stack, and approaches the latter at higher draw ratios. Annealing does not change the lengths of the fibrils, but the length of the lamellar stack increases. The fibrils form crystalline aggregates with a coherence length of ∼200 A at higher draw ratios. The diameter of the fibrils (50–100 A) determined from the lamellar reflection using both the Scherrer equation and the Guinier law are consistent with the lateral size of the crystallites derived from wide-angle x-ray diffraction. The longitudinal correlation of the lamellae between the neighboring fibrils improves upon drawing and decreases upon annealing. The degree of fibrillar and lamellar orientation is about the same as the crystalline orientation. Lamellar spacing increases upon drawing (from ∼60 to 95 A) and annealing (from ∼85 to 100 A). This is accompanied by an increase in the width of the amorphous domains from 30 to 50 A in drawn fibers, and from 45 to 55 A in annealed fibers. The diameter of the fibrils decreases slightly upon drawing and increases considerably upon annealing.

On the small-angle X-ray scattering of rigid-rod polymer fibres

Small-angle X-ray scattering was carried out on as-spun and heat-treated poly( p -phenylene benzobisoxazole) (PBO) fibre. While as-spun PBO fibre exhibits only an equatorial streak, generally attributed to elongated voids or fibrils, the heat-treated fibre shows in addition a radial four-point pattern. The long period and radial spacing of this pattern are 28 and 15 nm, respectively. The spacings do not vary with heat-treatment temperature. As the angle defined by the plane normal to the fibre axis and the X-ray beam is increased from 0° to 30° to 60°, the four-point pattern gradually changes to a two-point pattern. The measured long period in the tilted fibre is a projection of that in the untilted fibre. Various possible morphological models are discussed.

The effect of molecular weight on the lamellar structure, thermal and mechanical properties of poly(hydroxybutyrate-co-hydroxyvalerates)

Abstract A poly(hydroxybutyrate-co-hydroxyvalerate) with 9% hydroxyvalerate content has been thermally degraded to give a set of materials of different molecular weights. The effect of molecular weight on the lamellar structure, thermal and mechanical properties was investigated. The long period, lamellar and amorphous thickness all increase as molecular weight increases; their values vary linearly with 1/(molecular weight). Observed melting temperatures increase with molecular weight, following the same functional form, while melting enthalpy and non-isothermal crystallization temperature decrease. Youngs modulus varies by 13% with molecular weight; changes in crystallinity cannot explain this effect in detail. Ultimate tensile strength increases rapidly with molecular weight and then levels off at 28.5xa0MPa above 105xa0g/mol. This can also be seen as a linear variation with 1/(molecular weight). The strain at the point of ultimate tensile strength also increases rapidly up to 105xa0g/mol but then continues to increase at a slower rate.

Lamellar structure and properties in poly(ethylene terephthalate) fibers

The fibrillar and the lamellar structures in a range of poly(ethylene terephthalate) fibers were studied by small-angle X-ray scattering. The intensity maxima in the lamellar peaks lie on a curve that can be described as an ellipse. Therefore, the two-dimensional images were analyzed in elliptical coordinates. The dimensions of the coherently diffracting lamellar stack, the dimensions of the fibrils, the interfibrillar spacing, and the orientation of the lamellar surfaces were measured in addition to the lamellar spacing. The orientation of the lamellar planes and the size of the lamellar stacks had a better correlation with mechanical properties of the fibers than did the lamellar spacing. In particular, longer and wider lamellar stacks reduced fiber shrinkage, as did the closer alignment of the lamellar normal to the fiber axis. These structural features were also associated with lower tenacity.

Full-pattern analysis of two-dimensional small-angle scattering data from oriented polymers using elliptical coordinates

Abstract Two-dimensional small-angle scattering (SAS) data from oriented polymers are parameterized by profile fitting the intensity distribution to a product of two orthogonal functions. Elliptical cylindrical coordinates were found to best describe the observed small-angle scattering data. Each of the essential features of the small-angle X-ray/neutron scattering from uniaxially oriented polymers—the equatorial streak, lamellar reflections and interfibrillar interference peaks—is described completely by a single function in the elliptical coordinates. The parameters of the fit are used to describe the fibrillar and the lamellar structures. The analysis is illustrated with data from nylon 6 fibres, and the results are compared with those from a previous analysis of the same data as a series of one-dimensional scans. The method has enabled us to follow the changes in the weak equatorial scattering attributed to fluid-like organizations of the fibrils. The elliptical coordinates can be used to describe the wide variety of small-angle patterns that have been reported in the literature. The applicability of the elliptical coordinate system to SAS data is shown to be a natural consequence of the scattering object being elongated along the flow or the draw direction. Ellipticity could be used as a quantitative measure of the shape and the orientation distribution of the scattering object.

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.