Crystalline transition behaviors of nylons 12 20 and 10 20

The crystalline transition behaviors under different crystalline conditions of newly synthesized long alkane nylon 12 20 and nylon 10 20 are studied by wide-angle X-ray diffraction (WAXD) and real time Fourier transform infrared spectroscopy (FT-IR). The results show that their crystalline transition behaviors under WAXD were, to a large extend, related to the condition under which the crystals were prepared. The dilute solution-grown lamellar crystals of nylons 12 20 and 10 20 did not show distinct Brill transition behaviors before melting. Unlike the lamellar crystals of many other even-even nylons which display two crystal signals until melting temperature (T_M), they presented a broad amorphous-like signal when the temperature increased to around 10 °C below T_M. However, the post-annealing samples of nylons 12 20 and 10 20 displayed Brill transition at 155 and 157 °C, respectively, and the solution casting samples of nylons 12 20 and 10 20 at 110 and 135 °C, respectively. Furthermore, the IR spectra of nylons 12 20 and 10 20 displayed an interesting phenomenon: the intensity of the peak at 942 cm⁻¹ declined on heating and finally disappeared around Brill temperature (T_B), instead of T_M as is in usual nylons. This suggests that the long alkane segments, introduced by 18-octadecanedicarboxylic acid, may undergo a local melting at T_B.     

Structures of X 34‐nylons in chain‐folded lamellae and gel‐spun fibers

Structural studies and morphological features of a new family of linear, aliphatic even–even, X 34‐nylons, with X = 2, 4, 6, 8, 10, and 12, are investigated with X‐ray diffraction and electron microscopy. Solution‐grown crystals were obtained by isothermal crystallization from N,N‐dimethylformamide solutions. The thickness of lamellar‐like crystals was orders of magnitude less than the chain lengths of the polymer samples used, implying that the chains fold to form chain‐folded lamellae. The results bear a close resemblance, with the noticeable exception of 2 34‐nylon, to those reported for nylon 6 6 and other even–even nylon chain‐folded lamellar crystals. The basic structure of the straight‐stem lamellar core is similar to that of the classic nylon 6 6 triclinic α structure, and the chains tilt ≈42° relative to the lamellar normal. In the case of 2 34‐nylon, the structure resembles the 2 Y nylon series, and the chain tilt angle reduces to 36.6°. These combined results suggest that, even with a relatively low frequency of amide units along the backbone of these molecules, hydrogen bonding is still the dominant element in controlling the behavior, structure, and properties of these polymers. In addition, gels were prepared in concentrated sulfuric acid, and gel‐spun fibers were studied using X‐ray diffraction. The data are interpreted in terms of a modified nylon triclinic α structure that bears a resemblance to the structure of even–even nylons at elevated temperatures. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2685–2692, 2002     

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_p structure”; others sheets stack with an alternate stagger, to form the “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. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1209–1221, 2000     

Temperature‐induced structural changes in even‐odd nylons with long polymethylene segments

Structural transitions of nylons 8 9 and 12 9 heating and cooling processes were investigated using calorimetric, spectroscopic during and real time X‐ray diffraction data. These even‐odd nylons had three polymorphic forms related to structures where hydrogen bonds were established in two planar directions. Heating processes showed a first structural transition at low temperature where the two strong reflections related to the packing mode of the low temperature structure (form I) disappeared instead of moving together and merging into a single reflection, as observed for conventional even‐even nylons. The high temperature structure corresponded to a typical pseudohexagonal packing (form III) attained after the named Brill transition temperature. Structural transitions were not completely reversible since an intermediate structure (form II) became clearly predominant at room temperature in subsequent cooling processes. A single spherulitic morphology with negative birefringence and a flat‐on edge‐on lamellar disposition was obtained when the two studied polyamides crystallized from the melt state. Kinetic analyses indicated that both nylons crystallized according to a single regime and a thermal nucleation. Results also pointed out a secondary nucleation constant for nylon 12 9 higher than that for nylon 8 9, suggesting greater difficulty in crystallizing when the amide content decreased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2494–2506     

Structures for monodisperse oligoamides. A novel structure for unfolded three-amide nylon 6 and relationship with a three-amide nylon 6 6

Three-amide oligomers of nylon 6 and nylon 6 6 have been investigated using electron microscopy (imaging and diffraction), X-ray diffraction, and computational modeling. A new crystal structure has been discovered for the three-amide oligomer of nylon 6. This material crystallizes from chloroform/dodecane solutions into an unfolded crystal form that has progressively sheared hydrogen bonding in two directions between polar (unidirectional) chains. This structure is quite different from the usual room temperature α-phase structure of chain-folded nylon 6 crystals, in which alternatingly sheared hydrogen bonding occurs between chains of opposite polarity in only one direction. The occurrence of this new structure illustrates the extent to which progressively sheared hydrogen bonding is preferred over alternatingly sheared hydrogen bonding. Indeed, the progressive hydrogen bonding scheme occurs in the three-amide nylon 6 material even though it requires a disruption to the lowest potential energy all-trans conformation of the chain backbone, and requires all the chains in each hydrogen-bonded layer to be aligned in the same direction. We believe the presence of chain folding, which necessarily incorporates adjacent chains of opposite polarity into the crystal structure, prevents the formation of this new crystal structure in the nylon 6 polymer. In contrast, the three-amide nylon 6 6 crystal structure is analogous to the polymeric nylon 6 6 α-phase structure, found in both fibers and chain-folded crystals, and consists of progressive hydrogen-bonded sheets which stack with a progressive shear. In both structures, the molecules (≈ 3 nm in length) form smectic C-like layers with well-orchestrated stacking of 2.2 nm to form a three-dimensional crystal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2849–2863, 1998     

Structure and morphology of nylon 2 4 lamellar crystals: Comparison with polypeptides and relationship with other nylons

Chain-folded lamellar crystals of nylon 2 4 have been prepared from dilute solution by addition of poor solvent. Two crystal structures are observed at room temperature: a monoclinic form I, precipitated at elevated temperature, and a less-defined, orthorhombic form II, precipitated at room temperature. The unit cell parameters for both forms are similar to those reported for its isomer, nylon 3. Nylon 2 4 form II is a liquid–crystal-like or disordered phase, consisting of hydrogen-bonded sheets in poor register in the hydrogen bond direction. Form I crystals have two characteristic interchain spacings of 0.41 nm and 0.39 nm at room temperature and on heating, exhibit a structural transformation and a Brill temperature (250°C) characteristic of many other even–even nylons. Nylon 2 4 is a member of the nylon 2 Y and nylon 2N 2(N+1) families, and the form I crystals show behavior commensurate with both. We propose they contain a proportion of intersheet hydrogen bonds at room temperature, similar to that for the nylon 2 Y family, and the short dimethylene alkane segments mean that the structure consists of hydrogen-bonded a-sheets, with an amide unit in each fold, similar to that of nylon 4 6. The fold geometry and sheet structure is compared with chain-folded apβ-sheet polypeptides and nylon 3. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2401–2412, 1998     

Crystal structure of nylons 2/3/3 and 1,3

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.     

Structure and morphology of nylon 46 lamellar crystals: Electron microscopy and energy calculations

A detailed electron microscopy study of the structure and morphology of lamellar crystals of nylon 46 obtained by crystallization from solution has been carried out. Electron diffraction of crystals supported by X‐ray diffraction of their sediments revealed that they consist of a twinned crystal lattice made of hydrogen‐bonded sheets separated 0.376 nm and shifted along the a‐axis (H‐bond direction) with a shearing angle of 65°. The interchain distance within the sheets is 0.482 nm. These parameters are similar to those previously described for nylon 46 lamellar crystals grown at lower temperatures. A combined energy calculation and modeling simulation analysis of all possible arrangements for the crystal‐packing of nylon 46 chains, in fully extended conformation, was performed. Molecular mechanics calculations showed very small energy differences between α (alternating intersheet shearing) and β (progressive intersheet shearing) structures with energy minima for successive sheets sheared at approximately 1/6 c and 1/3 c. A mixed lattice composed of a statistical array of α and β structures with such sheet displacements was found to be fully compatible with experimental data and most appropriate to describe nylon 46 lamellar crystals. Annealing of the crystals at temperatures closely below the Brill transition induced enrichment in β structure and increased chain‐folding order. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 41–52, 2000     

Spherulites from polyamides with a structure characterized by three hydrogen‐bond directions

Polyamides constituted by glycine residues have a peculiar structure characterized by the establishment of intermolecular hydrogen bonds along three different directions. Spherulites of polyglycine and nylons 2/3, 2/3/3, 2/6, and 2/11 have been obtained from evaporation of concentrated solutions or from the molten state. In all cases, a negative birefringence was detected. This fact differs from the observations on spherulites of conventional nylons where intermolecular hydrogen bonds are formed along a single direction and sheet structures are postulated. Polarizing light, scanning electron, and transmission electron microscopy showed that banded spherulites are usually formed in the crystallization of nylons 2/n. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1719–1726, 2002     

Polymorphic forms of nylon 3

The crystal structure of nylon 3 was studied, and four crystalline modifications were observed. Modification I, as determined from the x-ray diffraction pattern of drawn fibers, is similar to the α crystal structure of nylon 6. The unit cell is monoclinic; a = 9.33 Å, b = 4.78 Å, (fiber identity period), c = 8.73 Å, and β = 60°. The theoretical density for nylon 3 with four monomeric units in the unit cell is 1.39 g/cm³, and the observed density is 1.33 g/cm³. The space group is P2₁. The nylon 3 chains are in the extended planar zigzag conformation. Although other odd-numbered nylon form triclinic or pseudohexagonal crystals when oriented, drawn nylon 3 crystals are monoclinic. In addition to modification I, modifications II, III, and IV were studied. Lattice spacings of modifications II and III are equal to those of modification I. However x-ray diffraction intensities are different. Infrared spectra of those forms indicate an extended planar zigzag conformation of the chains. Modification IV is thought to correspond to the so-called smectic hexagonal form. No γ crystals were found, and it appears that polyamide chains with short sequences of methylene groups cannot form crystals of this type.     

Thermal annealing of doubly oriented nylon 11 in contact with formic acid

The most striking feature of the mechanism of thermal annealing of doubly oriented samples of low-density polyethylene (LDPE) and probably of high-density polyethylene (HDPE) is a progressive tilt of lamellar crystals around their crystallographic b axis. Such a rotation does not occur on thermal annealing in doubly oriented nylons. However, this rotation mechanism occurs during the thermal annealing of doubly oriented samples of nylon 11 in contact with a solvent below its dissolution temperature. As for oriented samples of polyethylene (PE), a correlation between the changes of macroscopic dimensions and long spacing obtained from the small-angle x-ray pattern is difficult to establish. In doubly oriented samples of nylon 11, the basal faces of the lamellar crystals are parallel to the a axis of the unit cell. Nevertheless, simple Miller indices cannot be assigned to the basal planes of the lamellae. On thermal annealing in formic acid, the basal planes of the lamellar crystals are, in some cases, parallel to (00l) planes. Annealing in formic acid at room temperature induces a phase transition: the chain c axis remains oriented along the rolling direction and the (00l) planes become parallel to the limiting planes of the lamellar crystals. Bulk doubly oriented samples of nylon 11 annealed in formic acid just below the “dissolution temperature” have the same texture of orientation as filter mats of single crystals grown from dilute solution; moreover, as these bulk specimens remain doubly oriented, they can be used for further physicochemical investigations. The usual interpretation of the small-angle x-ray pattern is also discussed on the basis of the results reported in this paper.     

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. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 675–688, 1997     

Nylon–selected topics

Elements of the technology of nylon fibers and nylon plastics are reviewed. These include the development of equations to quantify the effects of end‐group imbalance and chain‐ending impurities, the kinetics of the polymerization of caprolactam, the solid‐state polymerization of nylon‐66, and the dependence of fiber tensile strength on molecular weight. This is followed by remarks that include comments on the significance of the amidation equilibrium for melt behavior and estimates of the activation energy of viscous flow, the pyrolysis of nylon, the use of the glass‐transition temperature (T_g) to predict the effect of moisture on properties, and the prospect for new nylons. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2565–2570, 2001     

Synthesis, Structure, and Superconductivity in Be1.09B3

The compound Be_1.09B₃ was prepared by arc-melting of the elemental constituents. The structure of single crystals taken from the arc-melted boule was determined from single-crystal X-ray data (T=120 K) and is hexagonal, having space group P6/mmm, and lattice parameters a=9.7738(7) Å and c=9.5467(6) Å, R=0.047. The structure consists of a hexagonal array of boronicosahedra, nonicosahedral B₁₂ cages, and B₁₈ cages. Stacked hexagonal layers of boron atoms, hexagons formed by B and Be, and equilateral triangles of boron atoms disordered by a 60° rotation exist along a 6-fold axis down the [001] direction. A superconducting transition at 0.72 K is clearly indicated by resistivity measurements.     

Design of Amphiphilic Peptide Geometry towards Biomimetic Self‐Assembly of Chiral Mesoporous Silica

In nature, diatoms and sponges are exquisite examples of well‐defined structures produced by silica biomineralisation, in which proteins play an important role. However, the artificial peptide templating route for the silica mesostructure remains a formidable and unsolved challenge. Herein, we report our effort on the design of amphiphilic peptides for synthesising a highly ordered two‐dimensional (2D)‐hexagonal and lamellar chiral silica mesostructure using trimethoxysilylpropyl‐N,N,N‐trimethylammonium chloride as the co‐structure directing agent (CSDA). The geometry of the peptide was designed by adding proline residues into the hydrophobic chain of the peptide to break the β‐sheet conformation by weakening the intermolecular hydrogen bonds; this led to the mesophase transformation from the most general lamellar structure to the 2D hexagonal P6mm mesostructure by increasing the amphiphilic molecules packing parameter g. Enantiomerically pure chiral mesostructures were formed thanks to the intrinsic chirality and relatively strong intermolecular hydrogen bonds of peptides.     

Incommensurate Phase in Λ-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive N−H⋅⋅⋅O and C−H⋅⋅⋅O Hydrogen Bond Networks**

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C₁₂H₃₀N₈)]³⁺ ⋅ 3 NO₃⁻ is reported to undergo a disorder-order phase transition at T_c1 ≈133 K upon cooling. Temperature-dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T_c1. The modulation wave vector varies as function of temperature and locks in at T_c3≈98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in T_c1<T<T_c3. Distinctive levels of competitions: trade-off between longer N−H⋅⋅⋅O and shorter C−H⋅⋅⋅O hydrogen bonds; steric constraints to dense C−H⋅⋅⋅O bonds give rise to pronounced modulation of the basic structure. Severely frustrated crystal packing in the incommensurate phase is precursor to optimal balance of intermolecular interactions in the lock-in phase.     

Studies on the oxidative degradation of nylons by nitrogen dioxide in supercritical carbon dioxide

Oxidative degradation of nylons was carried out using nitrogen dioxide (NO₂) as the oxidizing agent and supercritical carbon dioxide (scCO₂) as the reaction medium. Seven typical nylons were studied: three ring opening polymerization type nylons (nylon-6, -11 and -12) and four condensation co-polymerization type nylons (nylon-4/6; -6/6; -6/9 and -6/12). All the nylons decomposed in the NO₂/scCO₂ system under relatively mild conditions (140 °C, 1 h, and 10 MPa) and provided aliphatic α, ω-diacids such as succinic, glutaric and adipic acids in good yields. The product distribution of these α, ω-diacids strongly depended on the reaction conditions such as temperature, time and amount of NO₂, but not on the total pressure. Furthermore, the proportions of the products were affected by the type of nylon. A mechanism is proposed and a detailed discussion regarding the degradation of nylon in the NO₂/scCO₂ system is provided.     

Structure of crystals of nylon 6,3 and related polymers: Folding is stabilized by thermodynamic interactions

The n,3 polyamides have the structure: [-(CH₂)n NHCOCH₂ CONH-]×. Due to the stereochemistry of the malonamide unit, these polymers have a unique hydrogen bonding system with two different orientations at 120°: they do not form hydrogen bonded sheets as in conventional polyamides. We have obtained a very well oriented mat from crystals of this polymer which shows up to ten orders of the lamellar spacing. In this paper we analyze the structure of the fold in the crystal surface of nylon 6,3 and in related polyamides, including polyglycine. The thickness of these lamellar crystals is in agreement with the values determined for other polyamides. These results, taken together with some recent findings with other polymers, indicate that the thickness of polymer lamellar crystals may be thermodynamically controlled. An outline of this hypothesis is presented.     

Crystal morphology of the γ (triclinic) phase of isotactic polypropylene and its relation to the α phase

γ-phase crystals of isotactic polypropylene (iPP) obtained from low-molecular-weight extracts of pyrolyzed polymers are examined by electron microscopy and electron diffraction. γ-phase crystals differ from α-phase crystals in three important respects: (i) they are elongated along the b^* rather than the a^* axis, (ii) the chain axis is inclined at 50° to the lamellar surface (indexed as 101) rather than normal to it, and (iii) they show screw dislocations, while α crystals do not. γ crystals are nucleated on the lateral (010) faces of a α crystals; the b_α and b axes are parallel. Virtually no nucleation of the α phase takes place on the γ phase, which is therefore not involved in the repetitive lamellar branching leading to iPP quadrites. Crystallization of the γ phase appears to be favored by or linked to the absence of chain folds and may be involved in the macroscopic curvature of iPP branches.