Sergei N. Chvalun

X-ray analysis and molecular modelling of the structure of aromatic copolyimides

Abstract X-ray diffraction and molecular mechanics modelling have been used to investigate the structures of two families of aromatic copolyimides. The first of these is synthesized by the reaction of 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) with a mixture of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA); the second is synthesized from BPDA and a mixture of o -tolidine (OTOL) and p -phenylene diamine (PPD). The X-ray fibre diagram of 70/30 copoly(BPDA-PFMB/PMDA-PFMB) is very similar to that of homopoly(BPDA-PFMB), except that it is more diffuse. It appears that the structure is highly blocky, and the main effect of introduction of comonomer is to reduce the crystallinity of the poly(BPDA-PFMB) blocks. In contrast, the X-ray data for the copoly(BPDA-OTOL/BPDA-PPD) are non-periodic along the fibre direction, and the layer-line positions are reproduced by a model consisting of parallel arrays of extended chains of completely random sequence. Interestingly, the correlation length for the extended chain is larger in the copolymer than in homopolymer BPDA-OTOL, in contrast to those seen for the copoly(BPDA-PFMB/PMDA-PFMB) series. It is suggested that this is due to the fact that ‘kink’ distortions are less disruptive in copoly(BPDA-OTOL/BPDA-PPD).

X-ray analysis of the internal rearrangement of the self-assembling columnar structure formed by a highly tapered molecule

Abstract Small angle and wide angle X-ray methods have been used to study the structure 2-{2-(2-hydroxyethoxy)-ethoxy}ethyl-3,4,5-tris( p -octadecyl-oxybenzyloxy)-benzoate (H18-ABG-3EO-OH), which is one of a series of precursors used to synthesize polymers with highly tapered side chains that form self-assembling, columnar hexagonal ( φ h ) phases. This material is crystalline at room temperature, and undergoes crystalline-to- φ h and φ h -to-isotropic transitions in the region of 65–70 and 75–80°C respectively, depending on the thermal history. Small angle X-ray data for oriented fibres drawn from the φ h phase and cooled to room temperature point to the presence of hexagonally packed columns with a diameter of 57xa0A. The corresponding wide angle data indicate that there is three dimensional order within the columns: we observe a six-point pattern probably due to packing of the C18 tails in a manner analogous to the hexagonal form of polyethylene. On annealing, there is a progressive increase in the internal order of the cylinders as the structure is converted to one analogous to the crystalline forms of the C12-tetraethoxy analog (H12-ABG-4EO-OH) and the methacrylate polymer thereof (H12-ABG-4EO-PMA). These results suggest that the internal ordering now extends to the aromatic and tri-ethoxy units. Heating and cooling cycles from room temperature to successively higher temperatures show that these improvements in the internal order are accompanied by a progressive reduction in the diameter of the cylinders.

A second columnar liquid crystalline phase formed by polymers with highly tapered side chains

Previous work has shown that polymethacrylates with highly tapered side groups, such as poly{2-{2-[2-(2-methacryloyloxyethoxy )-ethoxy]ethoxy] ethyl-3,4,5-tris-(p-dodecyloxy-benzyloxy)benzoate (H12-ABG-4EO-PMA ), form columnar hexagonal (Col h , previously designated o h ) liquid crystalline structures. The small angle X-ray data for drawn fibers of H12-ABG-4EO-PMA, and also those of its hydroxy-tcrminated monomer precursor and decyloxy-homologue, contain several wcak equatorial Bragg reflections in addition to those characteristic of the Col h phase. The intensities of these extra reflections decline progressively on heating and disappear before the temperature of the Col h -to-isotropic transition is reached. It is proposed that the as-drawn fibers contain a second phase, which is probably a columnar liquid crystalline structure in view of the decreases in the observed d-spacings with increasing temperature. The additional reflections are indexed by a two dimensional monoclinic lattice, which probably contains columns with elliptical cross-section. The lower symmetry is consistent with the formation of a tilted columnar phase analogous to those formed by certain low molecular weight discotic materials.

The structure of poly(cyano-p-xylylene)

Abstract Films of poly(cyano- p -xylylene) (CN-PPX) synthesized by vapor deposition polymerization are highly crystalline and can be prepared with uni-planar orientation. This orientation is maintained after drawing the film 700% at 350°C, when the chains become aligned parallel to the direction of draw, leading to a mosaic texture of doubly-oriented crystals. Three-dimensional wide angle X-ray data show that the polymer has a monoclinic unit cell with dimensions a=5.96 A , b=13.52 A , c=6.48 A and β =135.9°, containing two monomer units from symmetry related chains. The unit cell is almost identical to that of the α-form of the unsubstituted polymer, poly( p -xylylene), except for the increased b dimension ( b=10.64 A in PPX), which is due to the –CN substituent. Molecular modeling and X-ray simulations show that the two polymers have similar crystal structures. The best agreement between the observed and calculated structure amplitudes ( R =0.21) is for a model where there is random substitution of the –CN groups.

Molecular modelling of the structure of the copolyester prepared from p-hydroxybenzoic acid, biphenol and terephthalic acid

Abstract The X-ray data for melt-spun fibres of the 33/33/33 copolyester prepared from p-hydroxybenzoic acid, biphenyl and terephthalic acid are characteristic of a completely random microstructure. Nevertheless, these copolymers adopt three-dimensionally ordered structures in the solid state, in which the chains are packed on hexagonal or orthorhombic polymorphic lattices. We have used molecular mechanics modelling to optimize the packing of random sequences in the higher-density orthorhombic form. The models consisted of 48 non-identical chains of nine monomers each. The random sequences were restricted to three monomers of each type, so that they had approximately the same length, making it possible to apply a periodic boundary condition. The initial model had the extended chains in register, i.e. their central ester oxygens were in a plane perpendicular to the chain axis direction. This structure had high potential energy due to overlap between the non-identical sequences. Energy minimization eliminated these bad contacts, at a cost of only ∼1.8 kcal mol−1 per monomer, by relatively small changes in the torsion angles at the phenylenester linkage bonds. These small arrays also predict Bragg maxima on the equator and layer lines that match those observed. We conclude that non-identical chains can be packed in a stereochemically acceptable manner according to the geometry defined by the X-ray data.

Modeling the structure of the hard domains in HMDI-based polyurethane elastomers

The structure of the hard domains of polyurethanes based on 4,4′-trans,trans-dicyclohexylmethane diisocyanate (HMDI), with 1,4-butandiol (BDO) as the chain extender, has been derived by X-ray diffraction and molecular modeling. X-ray diffraction patterns of drawn annealed films of HMDI/BDO/poly(tetramethylene adipate) elastomers contain crystalline hard domains: the unit cell is triclinic and contains two chains, each of which has two monomers repeating in c=37.5 A. A number of chain conformations are compatible with this repeat, but these are reduced to two by the requirement that all the urethane groups should form hydrogen bonds. We have compared packing models for these two options in terms of the agreement between the simulated diffraction patterns for arrays of chain segments. The best agreement is obtained for a model in which the chain extender region has the gaucheplus-trans-gaucheminus conformation. Identical chains are linked in sheets by C=O···H−N hydrogen bonds along the α axis of the unit cell. The second chain is rotated by 180° about c relative to the first and positioned at 0,b/2. The crystallographic R-values for this model are 0.196 (observed data only) and 0.240 (observed plus unobserved data), which were much lower than those for the alternative models, and represent excellent agreement for a structure that is not fully refined. The hydrogen bonds have very similar geometry to those for diphenylmethane (MDI)/diol hard segments. Thus the higher melting point and other improved properties for HMDI-based polyurethanes are most likely due to stronger interactions between the stacked cyclohexane rings as compared to those between the phenylenes in MDI-based elastomers.

X-ray analysis of the structure of wholly aromatic copolyesters with thiophenyl and oxyphenyl side groups

Abstract X-ray methods have been used to investigate the solid-state structures of wholly aromatic thermotropic copolyesters prepared from equimolar amounts of p-hydroxybenzoic acid, biphenol and thiophenyl- or oxyphenyl terephthalic acid. We have also compared these structures with that of the unsubstituted copolymer. The scattering data for melt-spun fibres show that the presence of the side chains results in considerably less lateral order. Non-periodic layer lines are observed, and their positions are reproduced in simulated scattering patterns for extended chains of completely random sequence. For the unsubstituted copolymer, the chains are not fully extended but are contracted by approximately 1%, probably due to the requirements to pack random sequences with three-dimensional order. A further contraction of 1.5% occurs for the backbones of the substituted copolymers, probably in an effort to minimize the free volume. Nevertheless, the presence of the side chains results in a reduction of the density by ≈20%. The substituted copolymers adopt a distorted structure similar to that seen for poly(phenyl-p-phenylene teraphthalate), in which the side groups on adjacent chains are interdigitated. This is possible even for random sequences because there is only one side chain for every four aromatic units along the backbone. Nevertheless, this interdigitation appears to preclude any registration of adjacent random sequences, and thus there is no evidence for the three-dimensional order observed for the unsubstituted copolymer.

X‐ray analysis of the structure of a copoly(ester‐imide)

The structure of a thermotropic liquid crystalline copoly(ester-imide) prepared from p-hydroxybenzoic acid (48 mol %), 4,4-dihydroxybenzophenone (26 mol %), and N,N-bis(trimellitimide)hexane (26 mol %) has been investigated by X-ray diffraction. X-ray fiber diagrams of as-spun and annealed fibers contain a series of aperiodic layer lines reminiscent of those seen for fibers of other copolymers that have extended chain conformations and completely random monomer sequences. The positions of these layer lines were reproduced approximately in simulation of the X-ray scattering by a fully extended chain of completely random sequence, and the match was improved to within experimental error when we considered a stereochemically acceptable sinuous chain. This agreement was lost when the sequence statistics deviated were completely random.

The structure of wholly aromatic polyesters with bulky side chains: Poly(phenylene phenyl-terephthalate)

Abstract Poly(p-phenylene phenylthio-terephthalate) (PPTT) forms nematic melts and is highly crystalline in the solid state, despite the probable random 2- and 3-dispo-sition of the S-phenyl substituents. The X-ray pattern of melt-spun fibers of PPTT contains 24 Bragg reflections that are indexed by a monoclinic unit cell with dimensions a = 28.6 A, b = 4.81 A, c = 12.57 A (fiber axis), and γ= 101.6°. The cell contains monomer units of four chains that are arranged in pairs with the thiophenyl side chains interdigitated; successive pairs of chains are staggered by about c/2. We used molecular mechanics modeling to simulate arrays of chains with random 2- and 3-disposition of the side chains on the terephthalic acid units and compared the results with those for an idealized structure in which all the substituents were at the 2-position. The refined model for random substitution is more distorted, but the average separations of the monomer units are within the experimental errors of the observed unit cell d...