Deuterium nuclear magnetic resonance of deuterium oxide in nylon 6 under active uniaxial deformation
Introduction
Polyamides are important engineering plastics whose mechanical properties are greatly influenced by its moisture content. Water plasticizes the nylon matrix and causes a reduction in the glass transition temperature (Tg) and Young's modulus [1], [2], [3] but enhances the toughening behavior [4]. It was postulated that such behavior is due to the “softening” of the polymer as water disrupts hydrogen bonds between amide groups [1], but to date there has been no work which is able to shed light upon the motional states of the water molecules or polymer chains during deformation.
Extensive evidence has shown that water only penetrates into the amorphous regions of nylon [5], [6], [7], but there has been some debate regarding its nature. Puffr and Sabenda have suggested the presence of both “loosely” and “tightly” bound water molecules in the matrix [8]. Using deuterium NMR techniques, we have demonstrated that water molecules exist only in one state in wet nylon [9], thus supporting the viewpoint held by le Huy and Rault [10].
We intend to continue to utilize this powerful tool to examine the motion of such absorbed water molecules while the polymer is undergoing active deformation, thereby giving insight into the deformation process and the role of the plasticizer. There have been various NMR studies on plasticizers in polymers [11], [12], [13]. The amount of experimental data available on actively deforming polymers is limited. In an earlier work we studied phenol-d5 in deforming nylon 6, the former being chosen because it does not undergo exchange with the hydrogen atom on the NH group and because of the resulting good signal to noise since it is absorbed in large quantities [14]. Other researchers have also used deuterium NMR to investigate small strain deformation in deuterated polycarbonate [15] and performed time resolved simultaneous X-ray of polyethylene undergoing active cold drawing [16].
Section snippets
Materials and methods
Two millimetre diameter nylon 6 rods were purchased from Goodfellow Corporation and deuterium oxide was obtained from Cambridge Isotope Laboratories. The molecular weight of the polyamide is 60,000 g/mol, determined from viscometry measurements using 2,2,2-trifluoroethanol as a solvent at 23.5±0.5°C. Differential scanning calorimetry (DSC) showed that the crystallinity of the material is about 30%. Without any further modification the rods were dried in vacuum at 100°C for 48 h to remove trace
Results and discussion
Fig. 1ashows plots of engineering stress versus engineering strain (strain, ε, is defined as ε=ΔL/L0 where ΔL is the increase in length and L0 is the initial length of the sample) of nylon 6 which is fully saturated with D2O (i.e. gained 12 wt% of D2O). Both curves show a yield point and strain hardening. In addition, the curve for the faster elongation rate exhibits a yield drop. The deformation was homogeneous and necking did not occur in either sample. Plasticization of the amorphous regions
Conclusion
During the active large strain tensile deformation of nylon 6 plasticized by D2O, the D2O molecules are found to associate with the amide groups. The quadrupolar splitting varies linearly with strain independent of the rate of elongation. Thus the order parameter of the D2O molecules is simply a function of strain and not of stress. This is similar to the linear dependence of quadrupolar splitting with draw ratio in the work done by Hutchison et al. on D2O in nylon 6 fibers [11]. Furthermore,
Acknowledgements
This work made use of MRSEC Shared Facilities supported by the National Science Foundation under Award Number DMR-9400334. This work was supported primarily by the MRSEC Program of the National Science Foundation under award number DMR 98-08941.
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