Crystal Structure and Properties of N6/AMCC Copolymer from Theory and Fiber XRD

Abstract

The MSXX force field developed previously from ab initio quantum calculations for studies of nylon are used to study the crystal structure and properties of the copolymer of nylon 6 with AMCC (4-aminomethylcyclohexanecarboxylic acid). For the isolated chain conformation of the copolymer, we consider both axial and equatorial connections of the chain with the cyclohexane ring and find that the best is chair-ee-St, which has equatorial connections on both ends of chair cyclohexane. We consider 12 possible crystal structures for the copolymer (the best four conformations of the isolated chain with the three forms of packing these chains:  α form, γ form, and δ form). With 12.5% of AMCC in the copolymer, we find that the γ form with the chair-ee-St chain structure is the most stable, even though the α form is most stable for nylon 6. The calculated X-ray diffraction patterns of the predicted crystal structure fit both equatorial and meridional scans of XRD very well. There are two reasons that make α form less stable for the copolymer. One is the bad contact between the axial hydrogen atoms of the cyclohexane ring and the CH2 hydrogens. The other is the difficulty of intramolecular H-bonds in the copolymer. The predicted chain-axis repeat distance of the copolymer (0 K) is 1.4 Å smaller than for the α form of Nylon 6, in good agreement with the X-ray results, which indicates that it is 1.5 Å smaller (at 300 K). Young's modulus in the chain direction is calculated to be 93 GPa for the copolymer (at 0 K), which compares to 135 and 295 GPa predicted for γ form and α form nylon 6, respectively. The introduced cyclohexane ring locates between the two amide pockets of the adjacent hydrogen bond sheets and has two major effects on the properties of the copolymer:  (i) It causes twisted conformations, which decreases Young's modulus of the copolymer in chain direction. (ii) It makes the chain rigid, which likely is responsible for the decrease in sensitivity of the copolymer to moisture.

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