B. Rosi-Schwartz

A complete atomistic model of molten polyethylene from neutron scattering data: a new methodology for polymer structure

A new approach to the study of the local organization in amorphous polymer materials is presented. The method couples neutron diffraction experiments that explore the structure on the spatial scale 1–20 A with the reverse Monte Carlo fitting procedure to predict structures that accurately represent the experimental scattering results over the whole momentum transfer range explored. Molecular mechanics and molecular dynamics techniques are also used to produce atomistic models independently from any experimental input, thereby providing a test of the viability of the reverse Monte Carlo method in generating realistic models for amorphous polymeric systems. An analysis of the obtained models in terms of single chain properties and of orientational correlations between chain segments is presented. We show the viability of the method with data from molten polyethylene. The analysis derives a model with average C-C and C-H bond lengths of 1.55 A and 1.1 A respectively, average backbone valence angle of 112, a torsional angle distribution characterized by a fraction of trans conformers of 0.67 and, finally, a weak interchain orientational correlation at around 4 A.

A detailed single chain model for molten poly(tetrafluoroethylene) from a novel structure refinement technique on neutron scattering data

We present a new methodology that couples neutron diffraction experiments over a wide Q range with single chain modelling in order to explore, in a quantitative manner, the intrachain organization of non-crystalline polymers. The technique is based on the assignment of parameters describing the chemical, geometric and conformational characteristics of the polymeric chain, and on the variation of these parameters to minimize the difference between the predicted and experimental diffraction patterns. The method is successfully applied to the study of molten poly(tetrafluoroethylene) at two different temperatures, and provides unambiguous information on the configuration of the chain and its degree of flexibility. From analysis of the experimental data a model is derived with CC and CF bond lengths of 1.58 and 1.36 A, respectively, a backbone valence angle of 110° and a torsional angle distribution which is characterized by four isometric states, namely a split trans state at ± 18°, giving rise to a helical chain conformation, and two gauche states at ± 112°. The probability of trans conformers is 0.86 at T = 350°C, which decreases slightly to 0.84 at T = 400°C. Correspondingly, the chain segments are characterized by long all-trans sequences with random changes in sign, rather anisotropic in nature, which give rise to a rather stiff chain. We compare the results of this quantitative analysis of the experimental scattering data with the theoretical predictions of both force fields and molecular orbital conformation energy calculations.

Extracting force fields for disordered polymeric materials from neutron scattering data

We present a new approach that allows the determination of force-field parameters for the description of disordered macromolecular systems from experimental neutron diffraction data obtained over a large Q range. The procedure is based on a tight coupling between experimentally derived structure factors and computer modelling. We separate the molecular potential into non-interacting terms representing respectively bond stretching, angle bending and torsional rotation. The parameters for each of the potentials are extracted directly from experimental data through comparison of the experimental structure factor and those derived from atomistic level molecular models. The viability of these force fields is assessed by comparison of predicted large-scale features such as the characteristic ratio. The procedure is illustrated on molten poly(ethylene) and poly(tetrafluoroethylene).

Extracting quantitative structural parameters for disordered polymers from neutron scattering data

Abstract The organization of non-crystalline polymeric materials at a local level, namely on a spatial scale between a few and 100 A, is still unclear in many respects. The determination of the local structure in terms of the configuration and conformation of the polymer chain and of the packing characteristics of the chain in the bulk material represents a challenging problem. Data from wide-angle diffraction experiments are very difficult to interpret due to the very large amount of information that they carry, that is the large number of correlations present in the diffraction patterns. We describe new approaches that permit a detailed analysis of the complex neutron diffraction patterns characterizing polymer melts and glasses. The coupling of different computer modelling strategies with neutron scattering data over a wide Q range allows the extraction of detailed quantitative information on the structural arrangements of the materials of interest. Proceeding from modelling routes as diverse as force field calculations, single-chain modelling and reverse Monte Carlo, we show the successes and pitfalls of each approach in describing model systems, which illustrate the need to attack the data analysis problem simultaneously from several fronts.

The local structure of molten poly(tetrafluoroethylene)

Abstract We report wide angle neutron diffraction experiments performed on molten poly(tetrafluoroethylene) (PTFE). The experiments have been carried out both on the time of flight LAD diffractometer at ISIS (UK) and on the diffractometer D20 at the ILL (Grenoble). This study is part of a large programme concerned with the development of a detailed understanding of the local structure of non-crystalline polymers. Polytetrafluoroethylene is particularly suited for such an investigation, due both to the simplicity of its chemical structure and to the rigidity of its chain. We have carried out a quantitative analysis of the experimental results by comparing them with a computer built statistical model for the PTFE chain. In particular, a separation between correlations arising from chain segments belonging to the same molecule (intrachain correlations) and from segments of differing chains (interchain contributions) has allowed us to show the high degree of stiffness of the single polymer chain and the considerable level of correlation of segmental orientation between different chains.

The interplay between the chain stiffness and the local structure of fluorine containing polymers in the melt

A detailed study of the relationship between chain stiffness and the local structural arrangements in a series of molten Fluorine containing polymers is presented. Four polymers of the form -(CF2CFX)n- are considered with X = F, Cl, Br and D together with poly(ethylene), -(CH2CH2)n-. By comparing the experimental structure factor with those obtained from atomistic models quantitative structural parameters are obtained which describe the intrachain structure. The introduction of differing substituents into these Fluorine containing polymers has a marked impact on the nature of the local chain conformation. Using these models of the intrachain structure as a starting point, we have utilised reverse Monte Carlo techniques to derive information on the spatial and orientational correlations between these chains segments in the melt. Somewhat surprisingly the rather anisotropic chains segments of -(CF2CF2)n- show less orientational correlations between near neighbour segments than the rather flexible -(CH2CH2)n- system.

A new approach to neutron scattering for non-crystalline hydrogenous polymers

Abstract We report a wide angle neutron scattering experiment on atactic polystyrene, recently performed on the novel small angle neutron diffractometer for amorphous and liquid samples (SANDALS) commissioned at the pulsed neutron facility ISIS (UK). The use of SANDALS allows for the first time true quantitative neutron scattering data to be obtained over a large scattering vector range for hydrogenous polymers. Results for samples of both totally deuterated and totally protonated polystyrene are presented and compared with predictions from statistical models for the chain configuration. The possibility offered by SANDALS of investigating the local structure of hydrogenous polymers offers exciting prospects, particularly in the study of polymers of technological or commercial importance.

Hydrogen momentum distribution in anisotropic rigid-chain polymers

The authors have performed a trial deep inelastic neutron scattering (DINS) experiment on an aligned sample of a copolymer of poly(hydroxybenzoic acid) and poly(hydroxynaphthoic acid). The experiment has been carried out on the eVS inelastic spectrometer of the ISIS facility, at the Rutherford Appleton Laboratory (UK). The hydrogen atomic momentum distributions n/sub ///(p) and nperpendicular to (p), corresponding to momentum transfer Q parallel and perpendicular to the polymer main chain direction respectively, have been obtained as functions of the atomic momentum p. The experiment offers evidence of the insensitivity of the DINS technique to the degree of rigidity of the polymer main chain. Through the use of a quantum mechanical model they show that the hydrogen momentum distribution is defined by the C-H bond and that bonds connected to the carbon or other interactions have no significant influence.

Neutron scattering from hydrogenous polymers

Abstract We report a wide angle neutron scattering experiment on atactic polystyrene, performed on the novel SANDALS diffractometer at the pulsed neutron facility ISIS (UK). Results for samples of both totally deuterated and totally hydrogenated polystyrene are presented and compared with predictions from statistical models for the chain configuration. SANDALS enormous potential for the investigation of hydrogenated organic materials is presented.

Hydrogen momentum distribution in rigid chain polymers

Abstract We have performed a trial deep inelastic neutron scattering (DINS) experiment on an aligned sample of a copolymer of poly(hydroxybenzoic acid) and poly(hydroxynaphtoic acid), on the eVS inelastic spectrometer of the ISIS facility (UK). The hydrogen atomic momentum distribution n ∥ ( p ) corresponding to momentum transfer Q parallel to the polymer main chain direction has been obtained as a function of the atomic momentum p . The experimental results have been interpreted on the basis of a quantum mechanical model, which indicates that the hydrogen momentum distribution is essentially determined by the C-H bond and by its characteristic motions.