Insights into Structure and Mechanical Behavior of α and γ Crystal Forms of Nylon-6 at Low Strain by Infrared Studies

Abstract

Stress-induced frequency shifts of spun-cast nylon-6 films with α and γ crystals are studied at low strains with particular emphasis on vibrations related to hydrogen bonding and to the methylene chains. With increasing stress in the α films, there is a frequency increase of the NH stretching mode in the crystals where the NH bonds are preferentially aligned parallel to the tensile stretching direction. The CH2 stretching modes undergo a downward shift. This is correlated with the weakening of the hydrogen bonds and opening up of the tightly packed α crystal structure during deformation, thereby increasing both the N−H- - -O and CH bond lengths. In the α crystals where the NH bond and stretching directions are perpendicular to each other, compressive forces do not lead to significant amounts of frequency shifting of the NH stretching peak. Tensile deformation has less effect in weakening the hydrogen bonds in the more dynamic amorphous regions. For the γ films, however, there is negligible frequency shift of the NH and CH2 stretching modes with deformation. The high sensitivity of the NH stretching frequency to mechanical stress (frequency shift coefficient, α = 100 cm-1/GPa) demonstrates the utility of using FTIR to study deformation micromechanics in hydrogen-bonded systems. An analysis was performed utilizing bond potentials which revealed that the observed FTIR shifts correspond to changes in X−H bond distances that are as small as 0.001%.

Role of interface in dispersion and surface energetics of polymer nanocomposites containing hydrophilic POSS and layered silicates

“…The addition of the different nanofillers does not change the crystalline form of PA6, but the intensity of a0 0 2 reflection (at 2h = 24°) decreases relative to the a2 0 0 peak. This indicates that there is a preferential arrangement of hydrogen-bonded planes of the a crystals parallel to the surface of pure PA6 film [35,36]. Similar XRD spectra were also obtained for other nanofiller-modified samples.…”

Investigation of polymer and nanoclay orientation distribution in nylon 6/montmorillonite nanocomposite

“…In our previous work with spun cast nylon 6 film of similar molecular weight containing primarily a crystals, the A x to A z ratio for the NH stretching band was 2.34 [14], also indicating preferential arrangement of the NH bonds along the plane of the film. For the nanocomposite in this work, the A x to A z ; ratio has dropped to 1.65, showing that the arrangement of the NH bonds in the nanocomposite was more random compared to the homopolymer, even though the processing history is similar.…”

“…A sample with isotropic arrangement of a crystals should show reflections of about equal intensity from both the (002) and (200) planes which give a peak at 208 [13]. Since the reflection from the (200) planes were suppressed in the spun-cast film, this indicates that there is a preferential arrangement of hydrogen-bonded planes of the a crystals parallel to the surface of the film [14].…”

Probing deformation of double-walled carbon nanotube (DWNT)/epoxy composites using FTIR and Raman techniques

“…However, there are a few exceptions, where the shift is positive with tensile stress, and the difference has theoretically been attributed to stretching of the harmonic versus the anharmonic potential energy functions for C-C stretching component [15]. More recently, the FTIR technique was used to probe molecular behavior due to low strain imposed on spun cast Nylon-6 films [19]. It was found that the NH bonds underwent an upward frequency shift, while simultaneously the CH 2 stretching bonds exhibited a downward shift.…”