Molecular dynamics simulation of the structural and mechanical property changes in the Brill transition of nylon 10/10 crystal

Conformational disorder in the Brill transition of uniaxially-oriented nylon 10/10 sample investigated through the temperature-dependent measurement of X-ray fiber diagram

“…This conformational disordering results in the apparently hexagonal packing of the molecular chains above the Brill transition temperature, as supported by the actuallyobserved X-ray diffraction data: the 100 and 010 (110) reflections merge into one in the transition region. The conformational disordering causes a slight contraction of the repeating period of the skeletal chain and induces a remarkable decrement of the Young's modulus along the chain axis [5]. From such a point of view the conformational disordering and the associated mechanical property change of nylons are similar to those observed in the orthorhombicto-hexagonal phase transition of polyethylene (PE) crystal which occurs also immediately below the melting point [6][7][8].…”

“…In the latter case, the molecular chains experience a large-amplitude thermal motion around the chain axis and some of the chains rotate almost freely at enough high temperature. In the Brill transition of nylons, on the other hand, the amide groups of the neighboring chains are connected to each other by the intermolecular hydrogen bonds even in the high-temperature phase above the Brill transition temperature [1][2][3][4][5]. Therefore, different from the case of PE, the molecular chains of nylons cannot rotate perfectly freely around the chain axes because of such a partial constraint at the amide group positions.…”

“…In a series of papers [1][2][3][4][5], we have investigated the structural changes occurring in the so-called Brill transition of aliphatic nylons on the basis of X-ray diffraction, infrared spectra and molecular dynamics calculation. Quantitative analysis of a series of infrared progression bands intrinsic of methylene zigzag sequences clarified in a concrete manner that the chain conformation is disordered remarkably through the trans-gauche exchange around the methyleneamide and metylene-methylene skeletal bonds.…”

“…Therefore, it might be more suitable to define the Brill transition phenomenon, not in terms of the behavior of the X-ray equatorial reflections, rather in terms of the conformational disordering with the intermolecular hydrogen bonds kept unbroken even above the transition temperature region. So far, many people including us have investigated the structural changes in the Brill transition mainly on the basis of the X-ray diffraction [2][3][4][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and infrared spectral data [1][2][3][4]29,30] and also by the molecular dynamics (MD) calculation [5,31]. In most of the papers, however, the X-ray reflections used in the discussion are almost limited to the main 100 and 010/110 equatorial reflections only.…”

“…In the present paper, we have measured the temperature dependence of X-ray fiber diagram of a uniaxially-oriented nylon sample. As reported in the previous paper [5], we have the trajectory files created in the MD calculation, which contain the information about the atomic coordinates and velocities at the various temperatures as a function of time. These trajectory files allow us to predict the change of the X-ray fiber diagram in the Brill transition phenomenon.…”