A. Mahendrasingam

Characterization of strain-induced crystallization of poly(ethylene terephthalate) at fast draw rates using synchrotron radiation

Abstract Structural changes during fast drawing of poly(ethylene terephthalate) were studied by wide-angle X-ray scattering using synchrotron radiation. Drawing was studied at 80, 90, 100 and 110°C to a final draw ratio of ∼4:1 at a draw rate of ∼ 10s−1. Simultaneous video recording of the sample enabled variation in the X-ray pattern to be correlated with local extension. Essentially all oriented crystallization occurred after final extension. Primary crystallization fits a first-order transformation with little change in the rate of crystallization observed over the 30°C range of temperature. These results show that it can be misleading to rely on crystallinity information obtained when samples from interrupted draw experiments are quenched.

Real time SAXS/stress–strain studies of thermoplastic polyurethanes at large strains

Abstract Simultaneous small angle X-ray scattering (SAXS) and force measurements have been recorded during tensile deformation of two contrasting polyurethane elastomers. The elastomers comprise the same hard and soft chemical segments; in Sample A, the length of the hard blocks is randomised while in Sample B the hard blocks are monodisperse. During deformation of Sample A, the SAXS halo from the mesophase structure deforms to an ellipse with intensification on the meridian. In Sample B, the halo transforms into a four point pattern. The ellipse patterns of A are interpreted in terms of a model based on particles located on a statistical lattice which is subjected to an affine deformation scheme. According to this model, the SAXS patterns of A are consistent with the hard phase regions behaving as embedded particles which separate from each other in an affine manner and which are not impeded by interconnections during the mechanical yield process. In B, the interconnection of the hard phase prevents affine deformation of the structure and involves the formation of a four point ‘lattice’ structure which then subsequently deforms in an affine manner. The differences in behaviour are linked with the segment sequencing which result in the phase regions of Sample A having a lower volume fraction and are consistent with variation in applied stress.

A simultaneous SAXS/WAXS and stress-strain study of polyethylene deformation at high strain rates

Abstract An experimental system has been commissioned which allows the collection of time-resolved simultaneous two-dimensional small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) and stress-strain data with a temporal resolution of 40 ms during the drawing of synthetic polymers. X-ray data collection is achieved via two CCD-based area detectors which are positioned in order to maximize the amount of scattering that can be observed from a particular sample. True stress-strain measurements are obtained from video extensometer and load cell measurements. The system was used at the Daresbury SRS to study the structural changes which occur in a sample of isotropic high-density polyethylene when subjected to an overall strain rate of ≈ 3 s −1 . It is shown that the onset of both a partial stress-induced crystal phase change and micro-voiding in the sample can be directly correlated to the point of yield in the true stress-strain curve.

Observations of structure development during crystallisation of oriented poly(ethylene terephthalate)

Abstract Two types of SAXS and WAXS experiments have been made using synchrotron radiation to observe the transformation from smectic to crystalline phases in oriented poly(ethylene terephthalate) (PET). In step-anneal experiments, PET was drawn slowly at 30 °C and then observed after annealing at 5 °C steps up to 100 °C. In the other experiments, time-resolved observations were made while drawing at 90 °C at rates up to 10 s−1. Up to 70 °C the WAXS data in the step-anneal experiments showed the smectic meridional reflection reducing in lateral width, indicating an increase in lateral long range order with annealing. Between 70 and 100 °C, there was a reduction in the intensity of the smectic reflection which correlated with an increase in the intensity of crystalline reflections. The SAXS from the step-anneal experiments showed an intense equatorial streak which has a correlation peak around 20 nm and which diminishes with annealing above 70 °C. It is concluded that this feature is a characteristic of the presence of the mesophase in oriented PET and is due to elongated domains of smectic mesophase with a length >75 nm and with an interdomain spacing of around 20 nm. Between 70 and 100 °C the SAXS data showed additional diffuse diffraction which correlated quantitatively with the crystalline phase and evolved from a cross-like appearance to a well resolved four-point pattern. The time-resolved drawing experiments were limited by the time resolution of the SAXS detector. They showed the same development of four-point diffuse SAXS patterns as was observed in the step-anneal experiments and a very weak equatorial streak. Differences in phase transformation kinetics between the two types of experiment are attributed to the different chain relaxation processes available under different conditions.

Real-time FTIR and WAXS studies of drawing behavior of poly(ethylene terephthalate) films

The development of molecular orientation and crystallization was studied during uniaxial drawing of poly(ethylene terephthalate) (PET) films, which was immediately followed by subsequent taut annealing at the drawing temperature. The behavior was monitored in real time throughout the drawing and annealing using dynamic FTIR spectroscopy and in situ WAXS measurements using the Daresbury Synchrotron Radiation Source. Films were drawn at 80 and 85°C at varying strain rates (0.001–0.7 s−1). The true stress–strain behavior was determined at each of the drawing conditions and the density and optical anisotropy of unloaded samples was measured. The IR spectra were analyzed using curve reconstruction procedures developed previously, and they showed that orientation of the phenylene groups and the trans glycol conformers occurred before significant gauche–trans conformational changes could be seen. The onset of crystallization, defined as the point that the crystalline 105 reflection could be first observed using WAXS, was not found to correlate with any specific change in the proportions of trans and gauche isomers nor with any feature on the stress–strain curve. However, it was clear that, for these comparatively low strain rates, crystallization occurred during the drawing process while the crosshead was moving and the draw ratio was increasing. The orientation of the crystallites was calculated from the 105 reflection observed in a tilted film, transmission geometry. The crystallites were found to form at a draw ratio of about 2.5 with high orientation values (P2> 0.8) that increased during drawing and annealing to P2 values of 0.95, irrespective of the drawing conditions. Semiquantitative measurements of crystallinity showed that the fraction of crystalline material that developed during drawing decreased with increasing strain rate.

Neutron fibre diffraction study of DNA hydration

Interactions with water are crucial to the conformation assumed by the DNA double helix. The location of water around the D conformation has been investigated in a neutron fibre diffraction study which shows that water is ordered in the minor groove of the DNA. The D conformation is important since its occurrence is limited to specific DNA base pair sequences which have been identified as functionally significant. This study is of particular interest because the D conformation has not been reported in single crystal studies of oligonucleotides.

A high angle neutron fibre diffraction study of the hydration of the A conformation of the DNA double helix.

A high angle neutron fibre diffraction study of the distribution of water around the A-form of DNA has been performed using the diffractometer D19 at the Institut Laue-Langevin, Grenoble. These experiments have exploited the ability to replace H2O surrounding the DNA by D2O so that isotopic difference Fourier maps can be computed in which peaks are identified with the distribution of water in the unit cell. All peaks of significant height have been accounted for by four families of water molecules whose positions and occupancies have been determined using least squares refinement. The coordinates of the water peaks making up each family do not deviate significantly from a regular helical arrangement with the same parameters as the DNA. Two of these families are of particular interest. The first consists of water molecules in the major groove linking successive charged phosphate oxygens along the polynucleotide chains. The second is associated with bases in the major groove and forms a central core of density along the helix axis. These two families provide a layer of hydration lining the interior wall of the major groove leaving a central channel to accommodate cations. The relationship between these observations and conformational stability is discussed.

X‐ray camera for high‐ and small‐angle x‐ray diffraction studies of the drawing and annealing of polymers at Daresbury Synchrotron Radiation Source

A purpose‐designed x‐ray fiber diffraction camera has been constructed in the Keele University Physics Department to be used at the SERC Daresbury Laboratory Synchrotron Radiation Source. The camera allows time‐resolved studies of the change in both the high‐ and low‐angle diffraction patterns during drawing and annealing of polymer films to be recorded. Drawing of the films is achieved by two opposed stepper motors which allow films to be drawn uniaxially in both directions. The temperature of the sample environment can be controlled to within 1 °C by a radio spares proportional, integral, and derivative (PID) controller. Diffraction patterns can be recorded on the Enraf‐Nonius TV FAST detector or on photographic film. Exposure times using the FAST detector are typically 5 s, representing a gain of approximately a factor of 5 over photographic film. The FAST detector has a further advantage over photographic film in that essentially an unlimited number of diffraction patterns can be recorded end‐to‐end w...

An instrument for the collection of simultaneous small and wide angle x-ray scattering and stress–strain data during deformation of polymers at high strain rates using synchrotron radiation sources

A fully integrated system has been developed which allows the study of the deformation of synthetic polymers by simultaneous small- and wide-angle x-ray scattering (SAXS/WAXS) and stress–strain techniques at synchrotron radiation sources. Two-dimensional x-ray data collection is achieved via two charge coupled device based area detectors which provide video signal outputs. A video extensometer provides sample strain and cross-section data during deformation. All three video signals are processed by a powerful Synoptics i860 processor based video framegrabber, with no loss of data. With this data collection strategy a temporal resolution of 40 ms is possible. In order to study the mechanical yield of the sample, a bridge-type strain gauge is used which reveals the sample loading. An electronic trigger mechanism provides accurate synchronization of the x-ray data, sample video data, sample loading information and controls the onset of deformation. Two experiments are highlighted showing the drawing of polye...

Time‐Resolved SAXS/Stress–Strain Studies of Thermoplastic Polyurethanes During Mechanical Cycling at Large Strains

Abstract Simultaneous small‐angle x‐ray scattering (SAXS) and force measurements have been recorded during tensile load/unload deformation cycles of two contrasting polyurethane elastomers. The elastomers comprise the same hard and soft chemical segments; in sample A, the length of the hard blocks is randomized, while, in sample B, the hard blocks are monodisperse. During the initial extension of sample A, the SAXS halo from the mesophase structure deforms to an ellipse with intensification on the meridian. On subsequent loading and unloading, the elliptical halo reverses but with an off‐axis intensification giving the pattern a four‐point character. In sample B, the halo transforms into a four‐point pattern during the first extension. The azimuthal positions of the four‐point maxima move systematically with changing load. However, the azimuthal movement of sample A is in the opposite sense to sample B. The positions of the maxima have been analyzed in terms of an affine deformation scheme of the spatial correlations of the hard‐block domains in real space. The movement of the four points in sample B is consistent with the affine deformation of a two‐dimensional, four‐point reciprocal lattice correlation. The contrasting movement of the maxima in sample A is explained in terms of the affine separation of particulate hard domains in which local spatial correlations between domains is represented by a linear, one‐dimensional lattice.