E.D.T. Atkins

Chain-folded lamellar crystals of aliphatic polyamides. Investigation of nylons 4 8, 4 10, 4 12, 6 10, 6 12, 6 18 and 8 12

Abstract Chain-folded single crystals of the seven even-even nylons: 4 8, 4 10, 4 12, 6 10, 6 12, 6 18 and 8 12 have been grown from solution and their morphologies and structures studied using transmission electron microscopy: both imaging and diffraction. Sedimented mats were examined using X-ray diffraction. The solution grown single crystals are lath-shaped lamellae. Diffraction from these crystals, at room temperature, reveals that three crystalline forms are commonly present. The crystals are composed of chain-folded, hydrogen-bonded sheets; the linear hydrogen bonds within the sheets generate a progressive shear of the chains. The sheets are found to stack in two different ways: some of the sheets stack with progressive shear, to form ‘α-phase’ crystals; other sheets stack with alternate up and down stagger, to form ‘β-phase’ crystals. Both the α- and β-crystals give two strong diffraction signals at spacings of 0.44 and 0.37 nm; these signals represent a projected inter-chain distance within a hydrogen-bonded sheet (actual value 0.48 nm) and the inter-sheet spacing, respectively. Some crystals also show an additional diffraction signal at 0.42 nm; this signal is characteristic of the pseudo-hexagonal phase, a phase usually only found at high temperatures. The melting points of solution grown crystals of this even-even nylon series decrease with decreasing linear density of hydrogen bonds. On heating, the strong diffraction signals in both α- and β-phases move together and meet, as is the case for other even-even nylons. The lowest temperature at which the two signals first have the same spacing is termed the Brill temperature. For all the nylons of the present study the Brill temperature is coincident with the melting temperature, and the two strong signals meet at the spacing (0.42 nm) of the pseudo-hexagonal phase. The behaviour of these nylons is compared and contrasted with that of nylon 6 6, where only the α-phase is found at room temperature and, on heating, the Brill temperature is found to occur in the range 95-35°C below the melting point at 265°C.

Hyaluronic acid: a double-helical structure in the presence of potassium at low pH and found also with the cations ammonium, rubidium and caesium

An anti-parallel double-helical structure is proposed for hyaluronic acid in the presence of the cations K +, NH+4, Rb+ and Cs+. In particular the crystalline phase containing potassium ions, and in a defined pH range, has been determined and refined against the X-ray diffraction amplitudes. The reflections in the diffraction pattern index on a tetragonal unit cell (a = b = 1.714 nm, c (fibre axis) = 3.28 nm) with observed systematic absences of the form h + k + l = 2n for general hkl reflections and l = 4n (where n is an integer) for 001 reflections. Two double-helical molecules pass through the unit cell at positions (14,14, 0) and (34,34, 0) as defined by space group I4122. Each chain forms a contracted (axial advance per disaccharide repeat of 0.82 nm) 4-fold left-handed helix which intertwines with a neighbouring chain of opposite polarity about a common axis. Features of the structure are the positioning of the carboxyl group toward the core of the double helix and the presence of hydrophobic and hydrophilic pockets between adjacent double helices. The state of protonation of the hyaluronate molecule is discussed in the light of the infrared evidence. Common physicochemical features of these four cations which promote this particular doublehelical conformation for hyaluronate are considered.

The polyuronides: their molecular architecture☆

Abstract The polyuronides (uronic acid containing polymers) are considered for the first time as a coherent group. Until now the dearth of X-ray diffraction data resulted in many questions of polyuronide structure and conformation remaining unasked. The recent success in crystallizing the polyuronides is illustrated with X-ray diffraction patterns of hyaluronic acid, dermatan and chondroitin sulphates, heparan sulphate and heparin. The influence of the chemical structure, the glycosidic linkages and the charged substituents on the molecular conformations is considered together with the marked association with water which these molecules display. Computer-drawn projections of plausible molecular conformations are shown.

X-ray diffraction studies on the connective tissue polysaccharides. Two-dimensional packing schemes for threefold hyaluronate chains.

Abstract An extended conformation of sodium and calcium hyaluronate with helical parameters n (number of residues per turn) = 3 and h (axial rise per disaccharide) = 0.95 ± 0.01 nm has been found to pack in a number of two-dimensional arrays. Each of the structures exhibits a variant of a motif consisting of six chains surrounding a “vacant” site. The distribution of chains around this site, together with certain aspects of the periodicity, suggests that there might be a special relationship between this conformation of hyaluronic acid and water.

Investigation of solution-grown, chain-folded lamellar crystals of the even-even nylons: 6 6, 8 6, 8 8, 10 6, 10 8, 10 10, 12 6, 12 8, 12 10, and 12 12

Chain-folded lamellar crystals of the ten even-even nylons: 6 6, 8 6, 8 8, 10 6, 10 8, 10 10, 12 6, 12 8, 12 10, and 12 12 have been grown from solution and their morphologies and structures studied using transmission electron microscopy, both imaging and diffraction. Sedimented mats were examined using X-ray diffraction. The solution-grown crystals are lath-shaped lamellae and diffraction from these crystals, at room temperature, reveals that three crystalline forms are present in differing ratios. The crystals are composed of chain-folded, hydrogen-bonded sheets, the linear hydrogen bonds within which generate a progressive shear of the chains (p-sheets). The sheets are found to stack in two different ways. Some p-sheets stack with a progressive shear, to form the α α P structure; others sheets stack with an alternate stagger, to form the α β P structure. Both the α p and β p structures give two strong diffraction signals at spacings of 0.44 nm and 0.37 nm; these signals represent a projected intrasheet interchain distance (actual value 0.48 nm) and the intersheet spacing, respectively. Preparations of nylons 6 6, 8 6, 8 8, 12 6, and 12 8 consisted almost entirely of α p -structure material, with only a trace of β p -structure material being present. In contrast, nylons 10 6, 10 8, 10 10, 12 10, and 12 12 contained substantial quantities of both α p - and β p -structure material, with α p -structure material always being in the majority. Preparations of nylons 10 8, 12 10, and 12 12 also showed an additional diffraction signal at 0.42 nm; this signal is characteristic of the pseudohexagonal (high temperature) structure. The melting temperature of solution-grown lamellae of these even-even nylons decreases with decreasing linear amide density. On heating, the strong diffraction signals (0.44 nm and 0.37 nm) gradually moved together and merge at the Brill temperature to form a single diffraction signal (0.42 nm), characteristic of the pseudohexagonal structure. This single diffraction signal remained until melting. For nylons 6 6, 8 6, 8 8, 10 6, and 12 6, the Brill temperatures were substantially below the respective melting temperatures and the single 0.42 nm diffraction signal was stable over temperature ranges of 14 °C to 56 °C, depending on the nylon. Conversely, nylons 10 8, 10 10, 12 8, 12 10, and 12 12 had coincident melting and extrapolated Brill temperatures.

Structural studies of poly(p-phenylene benzbisthiazole) films

Poly(p-phenylene benzbisthiazole) (PBT) is one member of a new class of highly-rigid, linear, thermally-stable aromatic heterocyclic polymers. The role of heat-treatment in the improvement of the perfection of crystallinity and mechanical properties of oriented films is discussed. Part of the heat-treatment process seems to be to increase the conjugation length of the polymer chain by increasing the planarity of the molecule, as revealed by visual colour changes and by differential scanning calorimetry. This may in turn account for the improved quality of crystallinity. Considerable detail can be seen in the electron diffraction patterns of heat-treated films. With the exception of the equatorial diffraction peaks this scatter can be accounted for by the detailed molecular transform of the PBT polymer, suitably cylindrically averaged, indicating that the crystal structure is essentially two-dimensional, that is the chains while closely and regularly packed lack longitudinal register. A two-dimensional unit cell with the corresponding molecular packing is proposed which can satisfactorily account for the observed density and for the equatorial diffraction peaks.

Temperature‐induced changes in chain‐folded lamellar crystals of aliphatic polyamides. Investigation of nylons 2 6, 2 8, 2 10, and 2 12

Four members of the even-even nylon 2 Y series, for Y = 6, 8, 10, and 12, have been crystallized in the form of chain-folded lamellar single crystals from 1,4-butanediol and studied by transmission electron microscopy (imaging and diffraction), x-ray diffraction, and thermal analysis. The structures of these 2 Y nylons are different from those of nylon 6 6 and many other even-even nylons. At room temperature, two strong diffraction signals are observed at spacings 0.42 and 0.39 nm, respectively; these values differ from the 0.44 and 0.37 nm diffraction signals observed for nylon 6 6 and most even-even nylons at ambient temperature. Detailed analyses of the diffraction patterns show that all these 2 Y nylons have triclinic unit cells. The diamine alkane segments of 2 Y nylons are too short to sustain chain folds; thus, the chain folds must be in the diacid alkane segments in all cases. On heating the crystals from room temperature to the melt, the triclinic structures transform into pseudohexagonal structures and the two diffraction signals meet at the Brill transition temperature which occurs significantly below the melting point. The room temperature structures of these 2 Y nylons are similar to the unit cell of nylon 6 6 at elevated temperature, but below its Brill temperature. The room temperature structures and behavior on heating of the nylon 2 Y family is noticeably different from that of the even-even nylon X 4 family, although the only difference between these families of polyamides is the relative disposition of the amide groups within the chains. The results show that in order to understand the structure, behavior and properties of crystalline nylons, especially as a function of temperature, the detailed stereochemistry needs to be taken into account.

Dermatan sulfate: Molecular conformations and interactions in the condensed state

The molecular conformations and manner of aggregation has been determined for three allomorphs of the connective tissue polysaccharide dermatan sulfate by analysis of X-ray diffraction from oriented, polycrystalline fibers of sodium salts. One allomorph is unique among glycosaminoglycans in having right-handed (8(3)) helical chains. Two such chains pack antiparallel in a tetragonal unit cell (a = b = 1.267 nm, c = 7.353 nm) with P4(3)2(1)2 space group symmetry. The 3(2) chains of the second allomorph are organized in a trigonal unit cell (a = b = 1.460 nm, c = 2.823 nm, space group symmetry P3(2)21) containing two left-handed antiparallel polysaccharide molecules. (The chirality of this allomorph has been assumed to be the same as in other 3-fold glycosaminoglycan helices, since discrimination between 3(1) and 3(2) symmetries was found not to be possible.) The archiral 2(1) helices of the third allomorph, pack probably in an orthorhombic unit cell (a = 1.151 nm, b = 1.065 nm, c = 1.878 nm, space group symmetry P2(1)2(1)2(1)) that contains again two antiparallel polymer molecules. Each dermatan sulfate chain is stabilized intramolecularly by O3-O5 hydrogen bonds across the beta (1 leads to 4) linkage. There are two intermolecular hydrogen bonds per tetrasaccharide repeat in the tetragonal structure and two per disaccharide in the trigonal structure. Fourier difference syntheses indicated equivalents of four sodium ions per tetrasaccharide and two sodium ions per disaccharide in the tetragonal and trigonal structures, respectively. The cations are either partially or fully hydrated and link dermatan sulfate chains either intra- or intermolecularly by involving besides other polyanion oxygen atoms, carboxylate and sulfate oxygen atoms. The probable mode of packing in the orthorhombic structure indicates a pair of hydrogen bonds between adjacent antiparallel polysaccharide chains and suggests plausible cationic sites in the unit cell.

Cellulose trinitrate: molecular conformation and packing considerations

X-ray diffraction patterns from highly oriented fibres of cellulose trinitrate indicate that the molecule crystallizes in a five-fold helical conformation with an axial advance per monomer of 0.508 nm. Computerized molecular model building studies favour a 52 helix, i.e. a right-handed helix with two complete turns of the backbone in the layer line repeat of 2.54 nm. Two possible unit cells, one of which has been proposed previously (Happey, F. J. Text. Inst. 1950, 41, 381), are described and the packing of the chains is discussed.

Structural studies of polysiloxanes: crystal structure and fold conformation of poly(tetramethyl-p-silphenylene siloxane)

Abstract The crystal structure of poly(tetramethyl- p -silphenylene siloxane) has been determined using X-ray diffraction data from drawn fibres. The unit cell is tetragonal with dimensions a = b = 0.902 nm and c (fibre axis) = 1.543 nm. The space group is P4 3 2 1 2 and the unit cell contains four monomer units. Each chain adopts a 2 1 helical conformation so that two chains run through the unit cell. The crystallographic asymmetric portion of the molecule is one half of a monomer unit with the oxygen atom and the centre of mass of the phenyl group at special positions. The two halves of a monomer are symmetry related by a diad axis perpendicular to the fibre axis. In addition, this polymer [poly (TMPS)] can crystallize in the form of chain-folded lamellae, and detailed models for the fold structure have been examined using computer model building techniques. A unique conformational model was found for adjacent re-entry fold models and a stereochemically acceptable fold can be obtained using a single residue. The only acceptable models encompassing two or three residues in the fold turn out to be only small perturbations of the single residue adjacent re-entry fold.