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%.