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%.