Helical Chain Conformation Research Articles

Chain conformation and unit cell in the crystalline phase of syndiotactic poly(4-methyl-1-pentene)

A preliminary characterization of the crystalline structure of syndiotactic poly(4-methyl-1 pentene) is presented. X-ray diffraction patterns of powder and fiber samples are reported. On the basisof X-ray diffraction data as well as conformational energy calculations, a helical chain conformation with a sequence of dihedral angles of the kind ≃(TTGG) n having symmetry s(12/7)2 and chain repetition c=46.91 A is suggested. A tetragonal unit cell with axis a=18.03 A is proposed

Structural Organization of syndiotactic polystyrene films crystallized in the β form

Abstract Syndiotactic polystyrene (sPS) films were obtained either by casting or by melt crystallization in the β form. Different thermal treatments of the melt led to mixed α and β forms. The structural organization was investigated by x-rays, transport properties, and Fourier transform infrared analysis. The β form is characterized by chains in zigzag planar conformation. For samples prevalently in β form, the fraction of impermeable phase, derived from sorption measurements, was found coincident with the fraction of crystalline phase derived from x-rays. In samples with mixed α and β forms, however, a fraction of mesophase is also present. The β form of sPS is stable to dichloromethane, whereas the α form is transformed into a crystalline form with the chains in helical conformation. Nevertheless, when the relative fraction of β form is lower than a critical value, it also dissolves in liquid dichloromethane, undergoing the conformational transition into the helical form.

Calorimetric and x-ray diffraction studies of rye glucocerebroside mesomorphism

Glucocerebrosides (GlcCer) isolated from the leaves of winter rye (Secale cereale L. cv Puma) differ from the more commonly investigated natural and synthetic cerebrosides, in that greater than 95% of the fatty acids are saturated and monounsaturated hydroxy fatty acids. Isomers of the trihydroxy long chain base hydroxysphingenine (t1(8:18 cis or trans)) and isomers of sphingadienine (d18:2(4trans, 8 cis or trans)) comprise 77% and 17%, respectively, of the total long chain bases. The phase behavior of fully hydrated and dry rye leaf GlcCer was investigated using differential scanning calorimetry (DSC) and x-ray diffraction. On initial heating, aqueous dispersions of GlcCer exhibit a single endothermic transition at 56 degrees C and have an enthalpy (delta H) of 46 J/g. Cooling to 0 degrees C is accompanied by a small exothermic transition (delta H = -8 J/g) at 8 degrees C. On immediate reheating, a broad exothermic transition (delta H = -39 J/g) is observed between 10 and 20 degrees C in addition to a transition at 56 degrees C. These transitions are not reversible, and the exothermic transition rapidly diminishes when the sample is held at low temperature. Using x-ray diffraction, it was determined that the endotherm at 56 degrees C represents a transition from a highly ordered lamellar crystalline phase (Lc) with a d-spacing of 57 A and a series of wide-angle reflections in the 3-10 A range, to a lamellar liquid crystalline (L alpha) phase having a d-spacing of 55 A and a diffuse wide-angle scattering peak centered at 4.7 A. Cooling leads to the formation of a metastable gel phase (L beta) with a d-spacing of 64.0 A and a single broad reflection at 4.28 A. Subsequent warming to above 15 degrees C restores the original Lc phase. Thus, rye GlcCer in excess water exhibit a series of irreversible transitions and gel phase metastability. Dry GlcCer undergo an initial heating endothermic transition at 130 degrees C, which is ascribed to a transformation into the HII phase from a two phase state characterized by the coexistence of phases with disordered (alpha) and helical (delta) type chain conformations but of unknown lattice identity: An exotherm at 67.5 degrees C observed upon subsequent cooling is of unknown origin. Since an undercooled HII phase persists down to 19 degrees C, the exotherm may derive in part from an alpha-to-delta type chain packing conformational change especially under slow cooling conditions. Upon reheating from low temperatures to 65 degrees C, a phase with a two-dimensional, primitive rectangular lattice and delta-like chain packing (R8 phase) in coexistence with the HI, phase emerges. With continued heating to 90 degrees C these coexisting phases give way to a phase with a two-dimensional, centered rectangular lattice and delta-like chain packing (P8phase) which again coexists with the HI, phase. Above 130 degrees C, the Pb phase disappears and the sample converts completely to the HI, phase as observed upon initial heating. These results indicate that the mesomorphic behavior of rye leaf GIcCer is distinct from that of other cerebrosides.

Solvent-induced crystallization of syndiotactic polystyrene in different liquids—sorption and diffusion phenomena

Transport properties of different liquids inducing crystallization in syndiotactic polystyrene (sPS) were investigated at 25°C. The weight uptake kinetics in the different liquids imply Fickian behaviour, with the exclusion of n-hexane and cyclohexane for which the weight uptake starts after an induction time. The equilibrium concentration of the different liquids in sPS is a function of the solubility parameter and shows a maximum with respect to chloroform, indicating a maximum in the polymer-solvent interaction. The swollen samples are birefringent but appear substantially amorphous on the basis of the X-ray diffractograms. After drying a well developed crystalline order appears, and the diffractograms indicate the presence of solvated structures, characterized by chains in helical conformation.

3,4'-Dihydroxybenzophenone terephthalate: an all-aromatic thermotropic polyester with a helical chain conformation

Preparation, structure et proprietes d'un cristal liquide qui a une conformation helicoidale a l'etat solide et une conformation etiree a l'etat nematique. Transition nematique a 285°C et isotrope a 360°C

Design for phase II testing of new agents within a phase III trial with crossover: D.J. Schaid, J.N. Ingle, and D.L. Ahmann Mayo Clinic, Rochester, Minnesota(53)

Four crystal types (α, β, γ, δ) and some mesophases/sub-modifications have been identified and discussed in syndiotactic polystyrene (sPS). The α- and β-forms are the main crystal packing forms in thermally-processed sPS, while the γ- and δ-crystals are identified only in solvent-treated sPS. In addition, the δ- and γ-forms are of a monoclinic crystal cell (with helical chain conformation) and the cell dimensions depend on the types and amount of residual solvent trapped in the crystal. The δ- and γ-crystal in solvent-treated sPS are more like mesophases that transform readily to the α-, β″- or β′-crystal upon heating the solvent-treated sPS to high temperatures near melting. This review thus focuses on studies of the α, β-crystals in sPS, and provides comprehensive discussions on the thermal behavior, crystal structures, thermodynamics, kinetics, and stability of these two major crystal packings (α vs. β) in sPS upon melt crystallization in comparison with cold crystallization. Analyses of melting behavior, diffractograms, or IR spectra, etc. of sPS can be complicated by the presence of co-existing polymorphic crystals. In general, a total of four melting peaks (labeled as P-I, P-II, P-III, P-IV from low to high temperatures) have been identified in a melt-crystallized sPS that typically contains mixed fractions of both crystals. By refining the techniques of obtaining sPS with individually isolated α- or β-crystal, recent studies have been able to correct suspected inaccuracy of some thermodynamic and kinetic measurements in earlier studies and to interpret the relative stability of the various crystals in sPS. sPS samples could be prepared such that they contained purely isolated α- or β-crystal, and the individual crystal types are used for more precise characterization of analysis. The P-I and P-III melting peaks are attributed to the β′-crystal while the P-II and P-IV peaks are attributed to the α″-type. In addition, kinetic and thermodynamic characterization has been thoroughly performed on individually isolated crystal types. The α-crystal of sPS has a lower melting temperature than the β-crystal, with Tm,α0=281.7°C and Tm,β0=288.7°C. The crystallization kinetics of the α-crystal is a heterogeneous nucleation with higher rates while the β-crystal is a homogeneous nucleation with lower rates. The β′-type is more thermodynamically stable than the α″-type; but the α″-type is kinetically more favorable. In addition, although there is literature report concerning a transformation of δ or γ mesophase crystals to α- or β-crystal; there is no evidence showing a solid–solid transition from the α- to β-crystal or β- to α-crystal during normal thermal processes. It suggests that both α- to β-crystal are stable solid and transformation between them can only be achieved by melting and re-packing. This could be fully explained using a stability/metastability chart of free energy vs. temperature. Nevertheless, the individual melting/reorganization of these two crystals might undergo crystal transformation via solid–liquid–solid transition. The crystallization kinetics of β′-crystal is a homogeneous nucleation with lower rates. By comparison, crystallization kinetics of the α-crystal is a heterogeneous nucleation with higher rates. Microscopy characterization also revealed a highly nucleated crystallization of the α-crystal. The effect of blend miscibility on the polymorphism behavior in sPS is also discussed. The effects of miscibility on polymorphism was investigated by studying miscible blends of sPS with atactic polystyrene (aPS) or sPS with poly(1,4-dimethyl phenylene oxide) (PPO). Miscible blends containing sPS have been found to favor growth of β-crystal than neat sPS when subjected to the same melt crystallization conditions.

Mercerization of cellulose. I. X‐ray diffraction evidence for intermediate structures

AbstractA study of the mercerization of ramie cellulose was begun using x‐ray diffraction, in order to determine the types of alkali–cellulose structures that occur as intermediates during the conversion of cellulose I to cellulose II. A total of five unique alkali–celluloses (Na–celluloses) could be generated reproducibly, depending only on the alkali concentration used. The reproducibility was enhanced by slowing down the mercerization process, by laterally compressing the fibers in capillary tubes. The five structures, named Na–celluloses I, IIA, IIB, III, and IV, were of two types based on their crystallographic fiber repeats. The first type was represented by Na–celluloses I, III, and IV, all exhibiting a ca. 10 Å repeat. The second type was represented by Na–celluloses IIA and IIB, with a ca. 15 Å repeat. The latter structures are possibly based on a threefold helical chain conformation, which has not been seen in crystalline celluloses. All Na–cellulose structures exhibited a reasonable degree of crystallinity and high degree of fibrous orientation. They formed a definite interconversion scheme, marked by crystalline‐to‐crystalline phase transformations.

Ring-Chain Transition in Liquid Selenium by a Disordered Chain Model

The structural transition between ring and chain molecules in liquid selenium is explained by a disordered chain model which includes a part of both the Se–8 ring and helical chain conformations in a single molecule. Also are discussed the enthalpy change for the dissociation of chain molecules and the mean length of a chain molecule in liquid selenium.

Packing analysis of carbohydrates and polysaccharides. V. Crystal structures of two polymorphs of pachyman triacetate.

AbstractThe crystal structures of two polymorphic forms of pachyman triacetate, the fully acetylated derivative of a naturally occuring β‐(1 → 3)‐D‐glucan, were determined by a combination of stereochemical and x‐ray diffraction analysis. The two polymorphs could be obtained depending on the temperature and the degree of stretching of film specimens of the substance: polymorph I resulted from stretching 25–50% at 125°C and polymorph II resulted from further stretching to 300% at 215°C. Both polymorphs had previously been shown to have sixfold helical chain conformations, but of unequal pitch. Subsequent detailed structure refinement performed with bond lengths, bond angles, conformational angles, and helix‐packing parameters as refinement variables, and the simultaneous minimization of packing and conformational energy and the crystallographic R‐factor as refinement criteria, resulted in a complete determination of the two crystal structures. Pachyman triacetate I was found to be a right‐handed helix packing with antiparallel polarity and space group P212121 symmetry (unit‐cell parameters a = 11.0, b = 19.0, c (fiber repeat) = 22.38 Å). The acetate groups were nearly planar and the O(2) and O(4) acetates were oriented in such a fashion that the carbonyl double‐bond nearly eclipsed the corresponding C—H bond of the ring. The O(6) was in the tg position and its acetate was oriented in such a fashion that the bond sequence C(6)—O(6)—C(6C)—C(6M) was nearly trans‐planar, with the carbonyl double‐bond bisecting the tetrahedral angle formed by C(6) and its two hydrogens. The final R = 0.221. Pachyman triacetate II was similarly found to be a right‐handed helix, but packing as a 50:50 mixture of parallel and antiparallel polarities (unit‐cell parameters a = 11.49, b = 20.13, c (fiber repeat) = 18.6 Å). The acetate positions in pachyman triacetate II were substantially the same as in pachyman triacetate I. The final R for the 50:50 mixture was 0.234. Probable reasons for the change in packing polarities are discussed, as are the difficulties encountered in the structure refinement of acetate derivatives.

A New Class of Polyethers-Poly(1,4-Dichloro-2,3-Epoxybutane)s - Synthesis, Mechanism and Property Aspects

Abstract The cationic ring opening polymerization and copolymerization of the cis and trans isomers of 1,4-dichloro-2,3-epoxybutane (CD and TD) with an iBu3Al - 0.7 H2O catalyst at -78°C has led to an interesting new class of polyethers containing 50% chlorine. The crystalline, racemic diisotactic polymer, obtained from CD in quantitative yield, has the most interesting combination of properties. It has a high melting point (235°C), a high glass transition (95°C), crystallizes readily from the melt, is unusually stable to heat and light, can be melt-processed up to 270°C, is flame resistant, has good barrier properties for air and water, and has good electrical properties. Its main deficiency is poor impact strength which can be overcome by orientation and other methods. This polymer is derived from low-cost raw materials, i.e. Cl2, butadiene, and O2. The interesting combination of properties of the CD polymer combined with its potential moderate cost should lead to such uses as flame resistant plastics, fibers, films and coatings. The CD polymer crystallizes with a helical chain conformation which appears to persist in the melt and in solutions and which appears to influence the crystallization and glass transition behavior of CD polymer and copolymers. The cationic polymerization of CD to highly stereoregular diisotactic polymer has unusual mechanistic aspects. A possible polymerization mechanism is proposed.

Cocrystallization in copolymers of α-olefins I—Copolymers of 4-methylpentene with linear α-olefins

The helical chain conformations in homopolymers of isotactic olefins, branched and unbranched, are shortly discussed with a view to predicting the degree to which cocrystallization may occur in random copolymers due to isomorphism of monomer units, and particularly in relation to copolymers of 4-methylpentene with linear α-olefins, pentene, hexene, octene, decene and octadecene. In accordance with prediction, it is shown that copolymers with pentene and hexene show a high degree of isomorphism and cocrystallization. Copolymers with the higher α-olefins show increasing disruption of the poly-4-methylpentene crystallinity with side-chain length and comonomer content, but octene and decene units can enter the poly-4-methylpentene crystal lattice to some extent, while octadecene units do not. Degrees of crystallinity, lattice parameters and melting points have been measured, and crystal phases in pentene copolymers are discussed. Although the polymerization conditions were designed to give random copolymers, evidence is adduced to show that copoly-merization has taken place but not entirely randomly; chains of varying composition and degrees of blocking are undoubtedly present.