A complete force field (MSXX) for simulation of all nylon polymers is derived from ab initio quantum calculations. Special emphasis is given to the accuracy of the hydrogen bond potential for the amide unit and the torsional potential between the peptide and alkane fragments. The MSXX force field was used to predict the structures, moduli, and detailed geometries of all nine nylons for which there are experimental crystal data plus one other. For nylon-(2n) with 2n ≤ 6, the α crystal structure (with all-trans CH2 chains nearly coplanar with the hydrogen bonding plane) is more stable, while for 2n > 6, γ (with the alkane plane twisted by 70°) is more stable. This change results from the increased importance of methylene packing interactions over H bonds for larger 2n. We find the highest Young's modulus for nylon-7.

http://www.wag.caltech.edu/publications/sup/pdf/353.pdf

Crystal Structures and Properties of Nylon Polymers from Theory

There are numerous possible γ-amino acids with different degrees of substitution and with various constitutions and configurations Of these the γ4- and the like- and unlike-γ2,4-amino acids have been previously used as building blocks in γ-peptides The synthesis of γ2-, γ3-, and γ2,3,4-peptides is now described The corresponding amino acids have been prepared by Michael addition of chiral N-acyl-oxazolidinone enolates to nitro-olefins, with subsequent reduction of the NO2 to NH2 groups Such additions to E-2-methyl-nitropropene provide (2R,3R,4R)-2-alkyl-3-methyl-4-amino-pentanoic acid derivatives (9, 10, 11) Stepwise coupling and fragment coupling lead to γ-di-, tri-, and hexapeptides (12–23), which were fully characterized The crystal structures of one of the γ-amino acids (2,3-dimethyl-4-amino-pentanoic acid⋅HCl, 9 a), of a γ2,3,4-di- and a γ2,3,4-tetrapeptide (20, 22) are described, and the NMR solution structure in MeOH of a γ2,3,4-hexapeptide (3) has been determined (using TOCSY, COSY, HSQC, HMBC and ROESY measurements and a molecular dynamics simulated-annealing protocol) A linear conformation (sheet-like), a novel (M) helix built of nine-membered hydrogen-bonded rings, and (M) 2614 helices have thus been identified NMR measurements at different temperatures (298–393 K) and H/D-exchange rates obtained for the γ2,3,4-hexapeptide are interpreted as evidence for the stability of the 2614 helix (no “melting”) and for its non-cooperative folding mechanism CD Spectra of the γ-peptides have been measured in MeOH and CH3CN, indicating that only the protected and unprotected γ2,3,4-hexapeptide is present as the 2614 helix in solution The structures of the γ2- and γ3-hexapeptides (1, 2) could not be determined

γ2‐, γ3‐, and γ2,3,4‐Amino Acids, Coupling to γ‐Hexapeptides: CD Spectra, NMR Solution and X‐ray Crystal Structures of γ‐Peptides

Solid-state deformation processes have been used to achieve polymers of high tensile moduli. The processes are based on achieving continuity for the oriented high-strength covalent bonds of the polymer chain. This is accomplished by the pulling out of chain folds and the subsequent extension of the long polymer chains which run both through and between crystal lamellae. Three major processes are used in solid-state deformation of semicrystalline polymers. In common cold drawing, the polymer is stretched at or below the crystalline melting point. In cold extrusion, a plug of solid polymer is forced by a ram through an orifice of smaller cross-sectional area to achieve draw. The third process, hydrostatic extrusion, is similar to cold extrusion except that the solid plug is surrounded by a pressure transmitting fluid which exerts a hydrostatic pressure on the plug that forces it through an orifice. Each method has distinct advantages and disadvantages that are described. Proposed molecular models for the three solid-state deformation processes generally consider the breakup of crystalline lamellae, their orientation in the deformation direction, and the pulling out of folded chains. These unfolded chains form tie-molecules between and among the disrupted lamellae. At highest deformation, fibril formation is observed which involves the partially extended tiemolecules. It is these chain-extended tie-molecules which are responsible for the unusually high tensile properties for drawn semicrystalline thermoplastics in the orientation direction.

Solid-state deformation of polyethylene and nylon and its effects on their structure and morphology

A re-examination of the crystal structure and molecular packing of α-nylon 6

Crystallographic Data and Melting Points for Various Polymers

The crystal structure of the α polymorph of nylon 4 has been determined from the x-ray diffraction patterns of uniaxially oriented monofilaments. In general the crystal structure of α nylon 4 is similar to that of α nylon 6. The unit cell is monoclinic with the following dimensions: a = 9.29 ± 0.05 A., b = 12.24 ± 0.05 A., c = 7.97 ± 0.05 A., and β = 114.5 ± 1.0°. There are eight monomeric units in the unit cell. The theoretical density is 1.37 g./cc. and the observed density 1.25 g./cc. The space group is P21. The nylon 4 chains are of the extended planar zigzag type, with the plane of the zigzag approximately parallel to the a axis of the unit cell. Along the a axis, every other chain is inverted—an antiparallel arrangement of chains—thus permitting complete hydrogen bonding and the formation of sheets of nylon 4 chains. Along the c axis of the unit cell, the second sheet is displaced by3/10 of the b axis, thus leading to a staggered arrangement of sheets. The sheets are held in place by van der Waals forces.

Crystallographic studies of nylon 4. I. Determination of the crystal structure of the α polymorph of nylon 4

In addition to the α polymorph, two crystalline forms of nylon 4, the β and δ polymorphs, have been observed and studied.

The β polymorph of nylon 4, which has not been found in a pure condition, appears to be similar to the β polymorph of nylon 6. The extended planar zigzag conformation of chains, found in the α polymorph, is present also in the β polymorph. Three models have been proposed to explain the observed data. In model 1, the staggered shear of van der Waals bonded sheets found in the α polymorph is abandoned; in model 2, the sheets are displaced by 1/2 the b axis, and in model 3 the chains are arranged in a parallel array. The β polymorph is converted to the α form in air upon heating for 11 min. at 227°C. and upon immersion in water for 2 hr. Models 1 and 2 would be converted to the α polymorph by a slippage of the van der Waals bonded sheets while the conversion from model 3 would involve a rupture of hydrogen bonds, a rotation of the chains through 90°, and the reformation of hydrogen bonds. The δ polymorph is formed by rapidly quenching extruded nylon 4 against chilled rolls. It is a metastable form which has only been observed in an unoriented condition. Upon orientation it is completely converted to the α polymorph. In addition to this, its conversion to the α form occurs under conditions similar to the β–α transition. The observed diffraction pattern can be indexed on the basis of a hexagonal packing of chains. High-temperature x-ray diffraction studies of the α polymorph suggest that the δ form, or a structure similar to it, is the high-temperature form of nylon 4.

Robert S. Sprague

Papers

Crystallographic studies of nylon 4. I. Determination of the crystal structure of the α polymorph of nylon 4

Crystallographic studies of nylon 4. II. On the β and δ polymorphs of nylon 4

Determination of strontium coprecipitated with barium by x-ray emission spectroscopy