Hole structures of polymers and zeolites probed by two-dimensional angular correlation of annihilation radiation

The two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique has been developed to measure the hole structure of stretched and oriented polymers. The determination of polymer hole structure is demonstrated in a stretched semi-crystalline polyaryl-ether-ether-ketone (PEEK) sample. The atomic scale anisotropy is found to be consistent with the macroscopic scale stretch ratio. Applications of the 2D-ACAR method to image the three-dimensional hole, free-volume, and cavity structures of molecular systems are discussed.     

Thermal stability of shear-induced precursor structures in isotactic polypropylene by rheo-X-ray techniques with couette flow geometry

Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long-living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s⁻¹ and duration t_s = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s⁻¹ and duration, t_s = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two-phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006     

Structure determination of a membrane protein in proteoliposomes

An NMR method for determining the three-dimensional structures of membrane proteins in proteoliposomes is demonstrated by determining the structure of MerFt, the 60-residue helix-loop-helix integral membrane core of the 81-residue mercury transporter MerF. The method merges elements of oriented sample (OS) solid-state NMR and magic angle spinning (MAS) solid-state NMR techniques to measure orientation restraints relative to a single external axis (the bilayer normal) from individual residues in a uniformly (13)C/(15)N labeled protein in unoriented liquid crystalline phospholipid bilayers. The method relies on the fast (>10(5) Hz) rotational diffusion of membrane proteins in bilayers to average the static chemical shift anisotropy and heteronuclear dipole-dipole coupling powder patterns to axially symmetric powder patterns with reduced frequency spans. The frequency associated with the parallel edge of such motionally averaged powder patterns is exactly the same as that measured from the single line resonance in the spectrum of a stationary sample that is macroscopically aligned parallel to the direction of the applied magnetic field. All data are collected on unoriented samples undergoing MAS. Averaging of the homonuclear (13)C/(13)C dipolar couplings, by MAS of the sample, enables the use of uniformly (13)C/(15)N labeled proteins, which provides enhanced sensitivity through direct (13)C detection as well as the use of multidimensional MAS solid-state NMR methods for resolving and assigning resonances. The unique feature of this method is the measurement of orientation restraints that enable the protein structure and orientation to be determined in unoriented proteoliposomes.     

FTIR and X-ray study of polymorphs of nylon 11 and relation to ferroelectricity

Nylon-11 polymorphs have been studied by x-ray diffraction and by infrared spectroscopy. A γ or γ-like phase of Nylon-11 was produced by treating the metastable δ' phase with gaseous HCl or DCl and then exposing the resulting complex (amidonium salt) to 100% RH H₂O or D₂O vapor. An oriented form of the γ or γ-like phase results from treatment of the oriented δ' phase, but stretching of the unoriented γ or γ-like phase, obtained by treatment of the unoriented δ' phase, induces a transformation to the α phase stable at room temperature. The use of DCl and D₂O produced highly deuterated samples with Fermi resonances indicating the relative strengths of the hydrogen bonds are weakest in the α phase, intermediate in the δ' phase and strongest in the γ or γ-like phase. © 1994 John Wiley & Sons, Inc.     

Viscoelasticity of oriented poly(ethylene terephthalate)

Time–temperature superposition can be successfully applied to both the stress relaxation and dynamic mechanical properties of oriented PET fibers. Two curves result; one is the time dependence of the modulus at constant temperature, while the other is the shift, log aT, of this curve along the time scale as a function of temperature. This temperature dependence is less than that for both unoriented PET and typical amorphous polymers above T_g. It is about the same as that for oriented nylon 66 and unoriented glassy poly(methyl methacrylate). The isothermal modulus has the same time dependence as that of the unoriented PET; however, it is a factor of 3.3 larger. The modulus curve is almost identical in both shape and magnitude with that of oriented nylon 66. However, a temperature of 82°C. is required to place the viscoelastic dispersion region of PET at the same time scale as nylon 66 at 25°C. This temperature increase is the major difference in viscoelasticity between these two oriented polymers.     

Phase transitions and structural properties of a thermotropic polyester

The phase transition behavior and the structural properties of a thermotropic polyester, poly-(chloro-1,4-phenylene-trans-1,4-cyclohexanedicarboxylate), were studied by differential scanning calorimetry, x-ray diffraction, polarized-light microscopy, and electron microscopy. The polyester shows two first-order transitions in heating, i.e., a crystal-crystal transition and a crystal-liquid-crystal transition at 190–220 and 300–330°C, respectively. The texture in the liquid-crystalline phase is very similar to that of nematic low-molecular-weight materials. Oriented films of the polyester can be obtained from the liquid-crystalline state. Many fibrils occur in a fractured lateral surface of the film. Bandlike structures 50–100 nm wide were observed in surfaces of both oriented and unoriented films. The direction of the polymer chain axis is almost perpendicular to that of band elongation. In the oriented film, the band structures are perpendicular to the direction of orientation.     

Molecular and crystal structure of pyrylium squarylium dyes

The crystal structures of new dyes: 4-[[3-[[2,6-bis-(tert-butyl)-4H-pyran-4-ylidene]methyl]-2-oxido-4-oxo2-cyclobuten-]-ylidene]methyl]2,6-bis(tert-butyl)pyrylium (I) and its thio analog (II) were determined. Crystal data: space group P1 (I), P2₁/n (II); a = 5.960(9) Å (I), 10.400(4) Å (II); b = 9.366(3) Å (I), 12.242(4) Å (II); c = 13.948(3) Å (I), 14.482(6) Å (II); α = 70.43(2)° (I), 90.0° (II); ß = 84.82(9)° (I), 94.65(3)° (II); γ = 79.10(9)° (I), 90.0° (II); V = 720.0 ų (I), 1837.7 ų (II); Z = I (I), 2 (II); d_calc = 1.131 g/cm³ (I), 086 g/cm³ (II); R1(F) 0.049 (I), 0.045 (II). The substituents are trans-oriented relative to the planar 4-membered ring. Bond length distribution points to a considerable electron density delocalization over the whole molecule except the Bu^t groups. The pyrane rings are oriented differently relative to the central group; in II the rings lie in the same plane which is rotated through 7.7° with respect to the central fragment, whereas in I they lie in parallel planes which are 1.38 Å apart and deviate from the central fragment to opposite sides, forming with it dihedral angles of 12.8°. These conformational differences are possibly the result of the action of crystal field forces. In I molecules are arranged as overlapping stacks, whereas in II the pairwise parallel dye molecules are oriented in such a way that their long molecular axes are perpendicular to the long axes of the neighboring pair, resulting in a herringbone packing.     

Direct measurement of free-volume hole distributions in polymers by using a positronium probe

We have directly measured the free-volume hole distributions in semicrystalline polypropylene by positron lifetime annihilation spectroscopy. A Laplace inversion technique was engaged to analyze the positron lifetime spectra measured under quasi-isotropic external pressures of 0, 4.2, and 14.7 kbar into continuous lifetime distributions. The hole radii distributions as determined from the ortho-positronium lifetime distributions are found to be between 4.0 and 0.5 Å and to have maxima at 3.0, 1.9, and 1.1 Å under the external pressures of 0, 4.2, and 14.7 kbar, respectively. © 1992 John Wiley & Sons, Inc.     

Solid-state Structures for 2-Methoxy-1,3-xylyl-18-crown-5 and 2-Methoxy-1, 3-xylyl-21-crown-6: A Search for C–H···O Interactions

Solid-state structures have been determined for 2-methoxy-1,3-xylyl-18-crown-5 (4) (monoclinic, P2₁/n, a = 11.361(3) Å, b = 8.352(3) Å, c = 18.627(4) Å, β = 91.05(2)°, Z = 4) and 2-methoxy-1,3-xylyl-21-crown-6 (5) (monoclinic, Pc, a = 9.996(2) Å, b = 11.321(2) Å, c = 8.642(2) Å, β = 100.095(4)^o, Z = 2). In both molecules, the aromatic units are tilted with respect to the polyether ring and the methoxyl methyl hydrogens are oriented toward ether oxygen atoms of the rings. The interatomic distances, particularly the H···O distance and angles provide important information regarding the strength of the C–H···O interactions.     

Comparison of the supramolecular structures formed by a polymethacrylate with a highly tapered side chain and its monomeric precursor

Insight into the supramolecular structure formed by a polymethacrylate with a highly tapered side chain is obtained from parallel X-ray analysis of oriented fibers of the polymer and its monomeric precursor. The polymer is poly(2-{2-[2-(2-methacryloyloxyethoxy)ethoxy]ethoxy}ethyl 3,4,5-tris(p-dodecyloxybenzyloxy)benzoate) (abbreviated to 12-ABG-4EO-PMA); the monomeric precursor is the hydroxy-terminated side chain 2-{2-[2-(2-hydroxyethoxy)-ethoxy]ethoxy}ethyl 3,4,5-tris(p-dodecyloxybenzyloxy)benzoate (12-ABG-4EO-OH). The polymer and precursor both form ordered solid state structures that are converted to columnar hexagonal liquid crystalline (φ_h) phases at approximately 40°C and 50°C, respectively. The ordered solid state structures consist of ordered hexagonally packed cylindrical columns, in which the monomer units are probably packed with helical symmetry. For the polymer at 25°C, the column diameter is 60.4Å with an axial repeat of 5.03Å containing eight monomer units. For the precursor at 25°C, the column diameter is reduced to 53.5Å, probably due to the absence of the polymer backbone from the center of the column, and the axial repeat is doubled to 10.04Å. The X-ray data are compatible with a tighter winding of the monomers in a helical structure, but otherwise suggest that there are common features in the stacking of the aromatic groups in the two structures.     

Crystal structure of nylon 12

The crystal structure of nylon 12 prepared by polymerization of dodecalactam has been determined by x-ray diffraction. Nylon 12 fiber exhibits only the γ form as its stable crystal structure. The unit cell of nylon 12 was determined with the aid of the x-ray diffraction pattern of a doubly oriented specimen. The unit cell is monoclinic with a = 9.38 Å, b = 32.2 Å (fiber axis), c = 4.87 Å and β = 121.5° and contains four repeating monomer units. The chain is planar zigzag for the most part but is twisted at the position of amide groups, forming hydrogen bonds between neighboring parallel chains. The chain conformation is similar to that of the γ form of nylon 6 proposed by Arimoto. It was deduced from the calculations that there are two chain conformations statistically coexistent according to the direction of twisting. In each conformation, hydrogen bonds are formed between parallel chains to make pleated sheetlike structures. The sheets are nearly parallel to (200) and in the sheet the directions of the neighboring chains are antiparallel, as is the case with nylon 6.     

Determination of the persistence length of an aromatic polyamide-hydrazide by small-angle x-ray diffraction

Small-angle x-ray diffraction measurements were performed upon dimethyl acetamide (DMAc) solutions of four samples of X-500, an aromatic polyamide-hydrazide. The solubility improved as the polymer molecular weight increased, the most soluble sample being in the form of a highly oriented, crystalline fiber. Molecular weights determined from the scattering curves indicate that aggregation was present in all solutions. Its extent varied with the solubility of the sample, increased with polymer concentration and, for the lowest concentrations studied, could be reduced by the addition of acetic acid. Radius of gyration values were smaller than those obtained by light scattering, probably because of insufficient resolution of the camera. The mass per unit length along the chain was found to be 18–20 Å, while the radius of gyration of the cross section was 3.5–4.8 Å. Three of the samples gave nearly the same angular intersection for the coil-and rodlike regions of the scattering curve. Two of these were whole polymers and the other a narrow fraction. The fact that essentially the same intercept was found in all three cases indicates the persistence length as measured for linear chain polymers by small-angle x-ray diffraction is independent of the heterogeneity of the sample. The intercepts, after a small correction for excluded volume effects, correspond to persistence lengths in the range 71–88 Å for DMAc solutions, which approximate those determined for dimethyl sulfoxide (DMSO) solutions by light scattering the intrinsic viscosity without correction for excluded volume, but are substantially larger than the DMSO values, corrected for excluded volume of 30–49 Å. We can offer no explanation for this difference.     

Unit cell dimensions of isotactic polyvinylcyclopropane

The unit cell dimensions of isotactic polyvinylcyclopropane were determined, based on crystalline, oriented fiber and film samples. Two structures were found: (1) a hexagonal structure with a = 13.6 Å, c = 6.48 Å, 3₁ helix, space group P3₁ and P3₂, ρ(calc) = 0.9805 g/cm³, ρ(obs) = 0.975 g/cm³; (2) a tetragonal structure with a = 15.21 Å, c = 20.85 Å, 10₃ helix, space group I4 , ρ(calc) = 0.926 g/cm³.     

Structure of ultrathin polyethylene layers in multilayer films

The crystalline structures of “microlayer” and “nanolayer” polyethylene have been examined in coextruded films comprised of alternating layers of high-density polyethylene and polystyrene. Transmission electron microscopy (TEM), small-angle x-ray scattering (SAXS), and wide-angle x-ray scattering (WAXS) reveal that microlayer polyethylene, where the layer thickness is on the order of several microns, crystallizes with the normal unoriented lamellar morphology. In nanolayer films, where the film thickness of tens of nanometers is on the size scale of molecular dimensions, lamellae are oriented with the long axes perpendicular to the extrusion direction in a row-nucleated morphology similar to structures described in the literature. The lamellae are partially twisted about the long axes. The preferred twist angles of ±40° orient the lamellar surfaces normal to the layer surface. The row-nucleated morphology imparts highly anisotropic mechanical properties to the nanolayer polyethylene.     

The Role of the trans-Planar Mesophase in the Polymorphic Behavior of Syndiotactic Polypropylene

The transformations of the trans-planar mesophase of syndiotactic polypropylene (sPP) subjected to thermal, mechanical and solvent treatments, were investigated. The unoriented trans-planar mesophase, obtained by quenching the melt at 0°C, was annealed at 80°C and the thermal transformation was investigated by X-rays, infrared and dynamic-mechanical analysis. The presence of the helical form II was recognized in the annealed sample. The oriented trans-planar mesophase, obtained by drawing at room temperature and releasing the tension, was immersed in liquid dichloromethane for 24 hours. After drying the sample showed the presence of the oriented form II, although it was not possible to exclude a partial transition into form IV. On the basis of the present and literature results we suggested a scheme of the polymorphic transitions of sPP, in which the central role of the trans-planar mesophase is enlightened.     

An electron diffraction study of 3-methyldiaziridine and 1,2-dimethyldiaziridine

The structures of the title compounds, diaziridines, (the first to be studied in the gas phase) have been determined by electron diffraction. The following principal structural parameters were obtained with the estimated standard deviations parenthesized: 3-methyldiaziridine, N-C = 1.489(9) Å, N-N = 1.444(13) Å, C-C = 1.505(16) Å, C-H = 1.107(5) Å, α =∠ (C-C, NCN) = 61.3° (0.9); 1,2-dimethyldiaziridine, (parameters of the cycle CN₂ were assumed from the previous molecule), N-C (methyl) = 1.445(3) Å, C-H = 1.108(9) Å, ∠ C-N-Me = 112.0° (0.5), the two methyl groups are in the trans position. Vibrational amplitudes were also determined for all important distances.     

Molecular mechanism for elongation: poly(vinylidene fluoride) form II

Elongation of an unoriented sample of poly(vinylidene fluoride) form II at high temperature (above 140 °C) results in a uniaxially oriented sample of form II during so-called necking. The short-range and long-range order parameters of poly(vinylidene fluoride) form II and measures of the structural order on thec-projection are characterized by the intensity and half-width of the 120 reflection. The elongation under the same condition gives almost the same order parameters, independent of the order parameters before the elongation. Furthermore, the dependence of the order parameters on the elongation temperature behaves in the same way as the dependence of the quenched unoriented sample on the annealing temperature. This suggests that the elongation of the sample during necking corresponds to the crystallization or recrystallization after a much disordered state, approximately molten state; this supports the mechanism proposed by Yoon and Flory.     

Crystal Structures of the Newly Synthesized Kdy and Ktb Aluminosilicates

The crystal structures are determined for KDy[AlSi₄O₁₂]·0.41H₂O and KTb[AlSi₄O₁₂]·0.32 H₂O compounds synthesized under hydrothermal conditions, which crystallize in the space group C2/c with the unit cell parameters a = 26.6956(6) Å and 26.619(1) Å, b = 7.2477(2) Å and 7.2553(3) Å, c = 14.7705 Å and 14.8200(6) Å, β = 123.7721° and 123.5149° for KDy and KTb aluminosilicates respectively. Tetrahedral layers in the structures of the studied crystals differ in their packing order and can be matched in space with a twofold rotational symmetry axis. In the studied phases Dy and Tb present in a trivalent form.     

The crystal structures of AuTe2Cl and AuTe2I

The structures of AuTe₂Cl and AuTe₂I have been determined. Both compounds are orthorhombic: for AuTe₂Cl, a = 4.020 Å, b = 11.867 Å, c = 8.773Å, Z = 4, space group Cmcm; for AuTe₂I, a = 4.056 Å, b = 12.579 Å, c = 4.741 Å, Z = 2, Pmmb. Intensities were measured on an automatic diffractometer, and the structures were refined, with anisotropic temperature factors, to R = 2.1% and R = 3.5%, respectively. The structures consist essentially of corrugated two-dimensional nets of gold and tellurium atoms, with interleaving halogen atoms. The tellurium atoms form pairs coordinated to four gold atoms, and each gold atom is coordinated to four tellurium atoms.     

Morphology and Crystal Structure of Wholly Aromatic All-Para Polyamide-Hydrazide Polymers

The morphology of some amide-hydrazide polymers of the type useful for high-modulus X-500 class fibers has been characterized by transmission electron microscopy of thin films crystallized from dilute solution. Selected area electron diffraction was used to characterize the crystallinity and crystal structure of the thin films and precipitated polymer. The films were cast from concentrated solutions and crystallized by heating the films. The results of these studies revealed several unique features relative to the crystal structure of the all-para polymers. Thin films of the crystallized polymer showed a distinctive crystalline texture—the molecular chains were found to be preferentially oriented parallel to the film plane and randomly oriented about an axis normal to the film plane. Electron diffraction measurements showed equatorial reflection maxima at tilt angles of = 30, ±48, and =59 when the films were tilted on an axis parallel to the film plane. From these results a tentative crystal unit cell and theoretical crystal density were determined: a = 8.5 [Agrave], b = 4.9 Å, c (chain axis) = 29.6 Å, p (density) =1.51 g/cc. The value a/b = 1.735, which is very near 3¹/², implies essentially hexagonal packing of the chains. Crystallization from dilute solution revealed lamellar structures resembling “single crystals” in the electron microscope similar to those observed in other crystalline polymers. However, in contrast to these other polymers, these “crystals” are not likely to contain folded chains because of the very rigid nature of the all-para poiyamide-hydrazide.