The crystal structures of two polyamides, poly(glycyl-β-alanyl-β-alanine) (nylon 2/3/3) and poly(methylene malonamide) (nylon 1,3), have been investigated by x-ray diffraction and electron microscopy. Crystallization of nylon 2/3/3 from a solution in a mixture of water and formic acid yields lamellar single crystals exhibiting a triangular habit. Doughnut-shaped morphologies diffracting as single crystals are obtained in the crystallization of nylon 1,3. A helical structure of the type known as polyglycine II is found for both polyamides. In such a structure, chains are intermolecularly linked by hydrogen bonds giving a hexagonal lattice of a = 4.79 Å. Insufficient data are available to determine precisely the conformation of the chains. We assume a threefold helix having c = 35.2 Å and c = 18.0 Å for nylon 2/3/3 and nylon 1,3 respectively. No sign of the layered structure familiar in polyamides has been detected for these polymers throughout the experiments made in the present study. 相似文献
Previously it was found that the lamellar thickness of solution-grown polyamide crystals as assessed by low-angle x-ray diffraction appeared to be unaffected by crystallization conditions. This point has now been re-examined in detail on nylon 6.6 with crystallization temperature Tc as the main variable. Over a very wide range of Tc the lamellar thickness was invariant; a notable increase could be produced only at the highest Tc. The “invariant” lamellar thickness occurs at larger supercoolings than can be realized in the much-studied polyethylene, which may explain why it has not yet been reported for polyethylene. The existence of this minimum invariant value raises important questions concerning the applicability of the current kinetic theories over such a wide range of supercoolings. Several other polyamides also revealed invariant long spacings after crystallization from solution. This provides the justification sought for comparing, in the paper which follows, the long periods observed for a large number of polyamides. 相似文献
A series of linear, aliphatic polyamides in which the number of carbon atoms in the repeat unit ranged from three to twenty-four was crystallized from solution. All gave lath-shaped crystallization products (usually aggregated in the form of sheaves) that were unmistakable lamellar. Sedimented mats of the crystals were examined by lowangle and wide-angle x-ray diffraction. Each polyamide had a characteristic layer thickness (fold length) which was determined by the length of the repeat unit and the number of hydrogen bonds in the lamella. The thickness was independent of other variables examined including crystallization conditions. The polyamides studied cover a wide range: they border on polypeptides at the one extreme and approach polyethylene at the other. For all these materials there emerged a unifying pattern which relates chemical structure directly to chain folding. 相似文献
The dependence of crystalline morphology of isotactic polypropylene crystallized from dilute solutions on its molecular weight and growing conditions and the mechanism of crystal growth were studied by electron microscopy and electron diffraction. Lathshaped lamellar crystals 150–300 A. in thickness are obtained from fractionated polypropylene powders of Mw (average molecular weight) = 600,000 and 240,000, but not from the samples of Mw = 82,000 and 44,000, by means of isothermal crystallization at 130°C. for 20 hr. in dilute α-chloronaphthalene solution (0.005 wt.-%). Precipitation of the fractionated polypropylene sample of Mw = 82,000 from a dilute solution of carbitol gives typical dendritic crystals under the same isothermal crystallizing conditions as mentioned above. The mode of chain folding in these crystals based on the orientation and the crystal structure of the lamellar crystals agrees with that proposed by Sauer, Morrow, and Richardson. From the morphological observations, the mechanism of growth pertinent to polypropylene lamellar crystals is presumed to be as follows: fibrils at first aggregate, then the molecular chains are folded to form small lamellae, and then these small lamellae accumulate compactly to grow to large, lath-shaped, lamellar crystals. 相似文献
The effect of isothermal crystallization temperature and time on the lamellar thickness and the melting behavior of polyamide 66 has been studied. Measurements were made of the melting temperature, crystallinity, and the long period. When calculated in the conventional direct manner, for samples crystallized isothermally, the calculated lamellar thickness was found to vary only from 2.4 to 3.2 nm over the entire range of conditions considered. When viewed in a non-critical fashion the polymer appears to conform to normal behavior including typical Tc vs. Tm behavior on a Hoffman-Weeks plot and apparent linearity in a Gibbs-Thompson plot. SAXS data indicates that there are only small changes in the lamellar thickness occurring over the entire crystallization range despite major changes in the melting point. Accordingly the Gibbs-Thompson plot shows major amounts of scatter, which are well beyond the experimental errors involved. The changes in melting temperature appear to be a result of changes in the structure of the fold surface on the basis of the conventional lamellar thickness analysis. In particular they appear to be due to changes in the character of the surface related to the hydrogen bonding and to the relative amounts of acid and amine segments present in the folds.When a more thorough analysis of the SAXS data are conducted, using a one dimensional correlation function approach, calculation of the crystal core thicknesses and “interfacial layer” thicknesses, a different picture emerges. In this case, the total lamellar thickness remains approximately constant at 2 repeat units in length with isothermal crystallization temperature, however, the core thickness increases with increasing crystallization temperature and time, from 1.5 to 2 repeat units in length, whereas the “interfacial layer” thickness is substantial at lower temperatures and times. When the core thickness is used in a Gibbs-Thompson plot the equilibrium melting temperature is found to be 303.7 °C (cf. 301 °C from solution grown crystals). However, the fold surface free energy is found to be 23.7 erg/cm2 much lower than the value of 74.6 erg/cm2 characteristic of solution grown crystals. Such a large discrepancy is believed to be a result of the highly polar solvents used in solution based studies generating the widely accepted “acid folds”. The difference may be because of a switch to folds containing six methylene groups from the diamine mer in the bulk case.Since the polymer is known to crystallize in the hexagonal state and reorganize during cooling to the regularly reported structure it is possible that the “interfacial thickness” is indeed a disordered surface layer within the crystalline lamella that originates from the precursor hexagonal phase during its formation, rather than the conventional disordered surface interpretation, applicable to polymers such as polyethylene. It is also possible that it is reflective of disorder induced in surface layers within the crystal due to the conformational changes occurring during this crystal-crystal transition. For these reasons, we prefer to refer to the “interfacial layer” obtained from SAXS calculations as an innerlayer. 相似文献
A comb-like polymer containing crystallized alkyl side chains and the intermolecular hydrogen bonds between the linking groups was fabricated by grafting long-chain fatty amine onto poly(styrene-co-acrylic acid)n (P(S-AA)n, wherein “n” denoted AA feed ratio). The chemical structures and crystallization behaviors of the comb-like polymer P(S-AA)n-g(p) (wherein “p” denoted the number of side-chain carbon atoms) were analyzed by Fourier transform infrared, gel permeation chromatography, X-ray photoelectron spectroscopy, and X-ray diffractometer, differential scanning calorimetry, atomic force microscopy, respectively. It was found that the lamellar morphology could be generated by controlling the grafting density and side chain length of P(S-AA)n-g(p). Moreover, it was identified that the hydrogen bonds between amide groups could enhance the crystallinity and then adjust the interlamellar spacing of lamellar phase. As a result, P(S-AA)70-g(18) with the highest degree of crystallinity and closely packed lamellar morphology showed a good gas-barrier performance, and the nitrogen permeability reached 1.78 × 10?14 cm3·cm/cm2·s·Pa. Furthermore, the permeation switch of the obtained comb-like polymer could reach 500 times traversing the melting point. 相似文献
Summary: Solution‐grown lamellar crystals of poly(p‐dioxanone) (PPDX) have been crystallized isothermally from butane‐1,4‐diol at 100 °C. The crystal structure of PPDX has been determined by interpretation of X‐ray fiber diagrams of PPDX fibers and electron diffraction diagrams of lozenge‐shaped chain‐folder lamellar crystals. The unit cell of PPDX is orthorhombic with space group P212121 and parameters: a = 0.970 nm, b = 0.742 nm, and c (chain axis) = 0.682 nm. There are two chains per unit cell, which exist in an antiparallel arrangement.
Transmission electron micrograph of PPDX chain‐folded lamellar crystals obtained by isothermal crystallization and its electron diffraction diagram. 相似文献
Being exposed to hydrochloric acid vapor, solutions of a surfactant and sodium tungstate form tungstic-acid-based materials
with a structure representing a system of interpenetrating hollow spheres 2–8 μm in diameter constructed from lamellar H2WO4 crystals with a thickness of 80–200 nm. The reduction of the tungstic-acid-based material with hydrogen gives rise to the
formation of a material based on tungsten(IV) oxide (WO2), which retains the initial structure. The adsorption capacity of the tungstic-acid-based materials is determined with respect
to benzene. The specific surface area of the obtained materials is 60–110 m2/g. 相似文献
Thin films of polyethylene prepared from blends of fractionated polymer and a linear hydrocarbon (n-C32H66) have been used to study the role of intercrystalline links in the deformation of semicrystalline polymer under uniaxial stress. These links have been found to be strong and virtually inextensible elements of the structure. It is shown that they are firmly attached to the chain-folded lamellar crystals they bridge (both within the same spherulite and across boundaries between adjacent spherulites) and that, by concentrating applied stress, they commonly induce these lamellae to begin yielding in regions close to their points of attachment. Where there are many closely spaced links the stress is distributed fairly evenly, and drawing is relatively smooth and uniform. With more sparsely distributed links, however, stresses tend to be concentrated at widely separated points; deformation then tends to be severe and highly localized, often resulting in failure of the material upon drawing. There are indications that stress is also transmitted between chain-folded lamellae in ways other than by intercrystalline links. One such way is by means of chain ends and molecular loops that emerge from the surfaces of these crystals and are embedded in interlamellar material. Experiments in which the deformed films were subsequently heated confirm earlier conclusions that extended chains in drawn polymer may undergo refolding during annealing. 相似文献
Polyethylene single crystals differing in lamellar thickness, both as-grown and annealed with different lamellar thickness, were irradiated by γ-rays to a dose of about 107 rad at liquid nitrogen temperature in vacuo, and then ESR measurements were made. It was found for the as-grown crystals that alkyl radicals were concentrated at the crystal surface. For the annealed crystals it was found that the radical concentration was greater than in the original crystals because of an increase in disorder with annealing. By assuming that the crystals form blocks upon annealing and that the surface and the interior of the blocks have the same trapping capacities for radicals as in the original crystals, the dependence of the size of the blocks upon variation in annealing temperature and the original lamellar thickness was estimated. This estimate is supported by the theory of the thickening process of single crystals. Two types of radical reactions with different reaction rates were found to occur simultaneously at room temperature. The rapid process was independent of lamellar thickness and was related to the reaction of radicals mainly in the surface region and the defects within the crystals. The slow process was strongly dependent on the lamellar thickness (i.e., the reaction rate was much depressed as the lamellar thickness was increased) and was inferred to be closely related to molecular motions manifested in viscoelastic measurements by the crystalline dispersion αc. 相似文献
The molecular and crystal structure of the title complex (I) obtained by addition of tin fluoride in a hydrofluoric acid solution to 18-crown-6 in methanol was investigated by X-ray structure analysis. The crystals are monoclinic, space group P21/n, a = 13.497(3), b = 7.806(2), c = 9.892(2) Å, β = 95.57(3)°, Z = 2 for C12H32F4O10Sn. In the polymer chain, the crown ether molecules alternate with the inorganic complexes [trans-SnF4(H2O)2] and are linked to them by O-H...O type hydrogen bonds involving the intermediate water molecules. The weak C-H...F interactions bind the chains into the layers which are parallel to the xz plane. 相似文献