Richard Lovell

Determination of the local conformation of PMMA from wide-angle X-ray scattering

Abstract Detailed conclusions about the local conformational structure of atactic PMMA have been derived from WAXS. The underlying conformation is near to the all-trans energy minimum proposed by Sundararajan and Flory. Precise values of backbone rotation angles (10°, 10°, −10°, −10°) and bond angles (110° and 128°) confirm the prediction that the chain consists of curved segments. The regular conformation persists over 16–20 backbone bonds for syndiotactic PMMA, and its curvature may explain why s-PMMA cannot be thermally crystallized.

CONFORMATIONAL ANALYSIS OF AMINE-CURED EPOXY RESINS

Abstract Analysis using molecular mechanics has been performed on the six-bond aliphatic link segment formed when diglycidyl ether of bisphenol-A (DGEBA) is cured with aromatic amines. It is found to have restricted conformational freedom owing to coulombic repulsion between oxygen and nitrogen atoms. Hence the segment can be approximated by a virtual bond about 9.5 A long. The study of the local network structure is therefore simplified.

The local structure of molten polyethylene

Abstract Wide-angle X-ray scattering provides a direct link to the local structure of non-crystalline materials. Methods of obtaining structural parameters from the scattering functions are described. Comparison of both scattering and real space functions calculated from a wide range of models with those obtained by experiment shows that the chains in molten polyethylene are irregular. The measurements confirm that the chain conformation comprises 60% trans bonds and 40% gauche, the sequences being random apart from the exclusion of pairs of gauche of opposite sign, and that the energy difference between the trans and gauche states is between 500–700 cal mol−1. The packing of the irregular chains is modelled using a novel approach based on the geometry of randomly packed spheres. There is no evidence for significant correlation of segmental orientation.

X-ray measurement of chain orientation in non-crystalline polymers

Abstract A new method of orientation measurement using wide angle X-ray scattering (WAXS) is described and compared with methods based on birefringence and spectroscopy. It is applied to PMMA deformed in the glassy or rubbery states. For specimens plastically deformed in uniaxial tension, a comparison is made between orientation parameters measured from WAXS and similar parameters obtained from the same specimens using n.m.r. by Ward. The WAXS method is further used to measure orientation in material deformed in plane-strain compression. It also demonstrates the absence of chain alignment in elastically strained PMMA despite the high birefringence observed.

An improved transformation procedure for radial distribution function analysis

Various methods of reducing the effect of termination error on radial distribution functions are briefly reviewed. A new approach is introduced in which the RDF is calculated by generating the Fourier transform at predetermined points. The resulting sampled transform is relatively free of termination ripple and the spacing of the points reflects the true resolution of the method for the particular, experimentally limited, Smax. The optimum choice of sampling points is determined in relation to special data terminations such as at a zero value of the interference functions or at a peak or trough position. The effectiveness of the sampled transform routine in reducing termination error is demonstrated by applying it to prematurely truncated interference functions derived from scattering data from atactic polystyrene. The advantage of the sampled transform approach to RDF analysis is that it prevents a termination discontinuity in the interference function, such as is often unavoidable even when special care is taken to apply precise corrections to the data, from causing an obscuring ripple on the RDF. In fact, in the extreme, it enables generation of useful RDFs of glassy polymers from data which have been neither corrected nor normalized.

WAXS investigation of local structure in epoxy networks

Abstract The wide-angle X-ray scattering peak which appears near s = 0.4 A −1 on curing diglycidyl ether of bisphenol-A (DGEBA) is investigated as a function of different amine curing agents and stoichiometry. Its development during the curing process is also observed. The intensity and position of the peak correlate with the presence and spacing of crosslinks (nitrogen atoms) in flexible chains of DGEBA units which form by secondary-amine reaction during the later stages of curing. Although the chains have high flexibility, the spacing between the nitrogen atoms is fixed at about 15 A. Such chains are components of all amine-cured DGEBA.

Wide-angle X-ray scattering study of structural parameters in non-crystalline polymers

The paper presents an analysis of WAXS (wide-angle X-ray scattering) data which aids an understanding of the structure of non-crystalline polymers. Experimental results are compared with calculations of scattering from possible models. Evidence is presented which supports the view that the chains in molten PE do not lie parallel but have a conformation in accord with the predictions of energy calculations. However, the evidence indicates that in “molten” PTFE the chains lie parallel over distances well in excess of their diameters. WAXS-based proposals are made for the conformations of a-PMMA and a-PS.

Analysis of wide-angle X-ray diffraction patterns of aligned glassy polymers with particular references to polystyrene

Abstract Improvement of WAXD patterns from aligned glassy polymers by a numerical desmearing technique is reported. This gives a fibre type diffraction pattern that can be more easily interpreted than radial or cylindrical distribution functions. Application to atactic and quenched isotactic polystyrene shows that the molecular conformations that are found are in agreement with those deduced from i.r. and n.m.r. spectroscopy. For isotactic polystyrene there is significant agreement between the fibre pattern of the aligned glassy polymer and that of the drawn crystalline polymer.

The Effect of Specimen Size on the Electric Breakdown of Unfilled and Filled Epoxy Polymers

Measurements are reported on the effect of specimen size and filler content on the electrical-impulse strength of epoxy polymer. For unfilled polymer, the strength was found to decrease more rapidly with increasing thickness than with increasing area. But the incorporation of as little as 1 percent by volume of filler removed the thickness effect and lowered the impulse strength. The area effect was, however, the same as for unfilled specimens. Since statistical time lags occur, it is concluded that breakdown is a random process occurring in an inhomogeneous material.

Radial distribution functions of non-crystalline polymers and their application to the structural analysis of PMMA

The use of Radial Distribution Functions (RDFs) in the determination of the structure of non-crystalline polymers is briefly reviewed. Particular aspects of the procedure for preparing RDFs from X-ray scattering are discussed in detail; namely the employment of an energy dispersive detector to remove the Compton component of the scattered X-rays and the application of the method of sampled transforms. A RDF is presented for atactic polymethylmethacrylate (PMMA) and its precision and reliability are discussed. It is analysed by comparison with RDFs calculated from computer-generated atom co-ordinates for isolated lengths of PMMA chains in different conformations. Methods are introduced by which the calculated RDFs are smeared to account for random disorder in the real chain and normalized so that, despite the finite range of the model, they can be immediately compared with the difference RDF which is directly obtained by transforming the data. Comparison between experimental and calculated RDFs shows that reasonable agreement is only obtained for a very limited range of conformations corresponding to sequences of backbone bond rotation angles of (10‡, 10‡, −10‡, −10‡) and the bond angles alternately 110‡ and 128‡. The form of the RDF appears very sensitive to important aspects of the molecular structure. The results both confirm and refine an earlier proposal from this laboratory which was based on comparisons between experimental and calculated functions in reciprocal space.