B. L. Farmer

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.

Modeling of helix reversal defects in polytetrafluoroethylene: I. Force field development and molecular mechanics calculations

A force field suitable for modeling fluoropolymers and oligomers in the solid state has been derived from MOPAC semiempirical molecular orbital calculations on perfluorohexadecane. A conformational energy profile was generated using the PM3 Hamiltonian, and then valence parameters of a molecular mechanics (MM) energy expression, including a six-term cosine dihedral potential, were adjusted with a nonlinear least squares fitting algorithm to reproduce the profile. Minimum energy helical conformations of 48/22 and 13/6 were obtained when the geometries of C60F122 molecules in isolation and in a crystalline cluster, respectively, were optimized using the refined force field. The X-ray diffraction pattern calculated from the crystalline cluster indicated an equatorial d-spacing of 4.9685 A ˚. These intra and intermolecular structural characteristics for the cluster are in agreement with experimental X-ray diffraction data. Energy penalties of helix reversal defects in isolated chains and in a crystalline environment were also investigated. q 1999 Elsevier Science Ltd. All rights reserved.

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.

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.

Molecular dynamics simulations of siloxane-based side chain liquid crystalline polymers

Abstract Molecular dynamics simulations have been performed to study the intramolecular interactions of biphenyl ([B]) and cholesterol ([C]) substituents on linear oligomeric siloxanes. These interactions are of interest due to the liquid crystalline behaviour of these materials. Molecules with alternating diblock and triblock substituent arrangements were examined, as were unmixed substituents. The orientational behaviour of the mesogens and the stability of [B]-[B], [C]-[C], and [B]-[C] interactions were characterized by an order parameter for the mesogens as a function of simulation time. Results indicate that the [C]-[C] interaction is the strongest, with the [B]-[B] and [B]-[C] interactions being weaker. However, the arrangement of [B] and [C] mesogens in the different molecules strongly influenced the level of mesogen ordering. The flexibilities of the siloxane backbone and the spacer groups used to attach the mesogens give the mesogens a wide range of motion, allowing them to find energetically favourable arrangements and moderate to high order parameters.

Kinetic aspects of the IV-II phase transformation in PTFE

Abstract Kinetic aspects of the transformation of poly(tetrafluoroethylene) (PTFE) from phase IV to phase II at 19°C and atmospheric pressure were studied using wide angle X-ray diffraction (WAXD) following a rapid decrease in temperature from 24.7 to 14.5°C. Information relating to changes in molecular packing was obtained from the position of an equatorial peak. Changes in the amount of disorder present were characterized by measuring the integrated intensity of peaks on the second layer line. Within the limits of error, the molecular packing and disorder changes occurred simultaneously as the transformation occurred. The kinetics of the transformation were evaluated following the approach of Avrami. The results indicate one-dimensional crystal growth in agreement with theory. The results from this study are discussed with respect to implications for the transformation mechanisms and previous work in this area.

X-ray Diffraction Study of Intermolecular Interactions in Cholesteryl Ester-Substituted Cyclosiloxanes

Abstract The mesophase structure of a series of sixteen cholesterol-based liquid crystalline materials based on cyclosiloxane backbones is examined. The co-existence of two layer types (full and partial interdigitation of cholesterol mesogens) in the mesophase is examined as a function of ring size, composition, and spacer length. The relative X-ray intensities from each layer type are discussed as a function of these variables. An increase in the ring size and cholesterol composition, and a decrease in the spacer group all result in more partially interdigitated packing. Molecular modeling calculations performed on the mesogens and macromolecules examine relative differences in the orientational relationships and flexibility of mesogens.

Molecular Dynamics of Side Chain Liquid Crystalline Polysiloxanes

Molecular dynamics simulations have been used to characterize the development and longevity of associations between cholesterol and biphenyl mesogens when attached to linear siloxane oligomers by flexible spacer groups. Single substituents, alternating substituents, and diblock and triblock arrangements of the substituents were considered. The backbone and spacer groups allow sufficient flexibility that long-lived associations between cholesterol mesogens form quite rapidly, as do more fluid associations between biphenyl and cholesterol mesogens. The study of the individual mesogen interactions and how these lead to larger scale aligned structures has provided insight into the nature of the liquid crystalline state in these materials.

Molecular Modeling of Fluoropolymers: Polytetrafluoroethylene

A force field for solid state modeling of fluoropolymers predicted a suitable helical conformation but required further improvement in describing intermolecular effects. Though victory cannot yet be declared, the derived force fields improve substantially on those previously available. Preliminary molecular dynamics simulations with the interim force field indicate that modeling of PTFE chain behavior can now be done in an “all-inclusive” manner instead of the piecemeal focus on isolated motions and defects required previously. Further refinement of the force field with a backbone dihedral term capable of reproducing the complex torsional profile of perfluorocarbons has provided a parameterization that promises both qualitative and quantitative modeling of fluoropolymer behavior in the near future.

Modeling of Side-Chain Liquid Crystal Polysiloxanes

Molecular dynamics (MD) simulations of polysiloxanes havm& cholesterol and methoxynitrostilbene (MONS) substituents tethered 10 the siloxane backbone by an allyloxybenzoate spacer group demonstrated that ordering involving all of the substituents developed quite rapidly, even though only the cholesterol group by itself inherently mesogemc The observed ordering varied with the nature and arrangement of the substituents Generally, strong interactions between cholesterol groups iesulted in the longest lived pair-wise associations. MONS substituents also showed orientation, aosing primarily from their associations with cholesterol mesogens The overall order parameter was typically unaffected by the incorpoiation of MONS at tractions of the order of 25% The polymer having alternating MONS and cholesterol substituents did not develop any significant order Association lifetimes were not simply related to order parameter Polymers which achievea the highest ordei parameters did not exhibit the longest association lifetimes Cholesterol - cholesteiol associations tended to be the longest, MONS - MONS association the shortest.