Dielectric behavior and morphological changes in nylon 6: Applicability of time‐temperature superposition

The humidity sensitivity of nylon polymers is well known, as is the fact that certain nylons (such as nylon 6) can crystallize in different lattice forms. As part of a study of the effect of swelling agents on nylon 6, particularly those which cause morphological changes (such as phenol and aqueous potassium triiodide), we have made dielectric measurements (of both ϵ′ and ϵ″) on nylon 6 films subjected to the action of swelling agents and absorbed water as a means of obtaining further information on the morphology of the polymer and the effects of crystal morphology changes on the properties of the amorphous regions (since it is essentially the behavior of the amorphous regions which is observed in dielectric measurements). These measurements were made in a special dielectric cell in which both temperature and atmospheric humidity could be regulated. The temperature and frequency ranges covered were 25–170dC and 20-2 × 106 Hz, and RH was varied from 0% to 100%. In studies of the effect of humidity alone, it was found that both time-temperature (frequency-temperature) superposition and humidity-temperature superposition could be applied to the experimental data within certain ranges, but not outside those limits. In the case of absorbed moisture, it was found that the first 2% absorbed produced effects which were different from those produced by additional amounts of absorbed water. The latter seemed to act as a simple plasticizer, and simple horizontal superposition shifting was possible in this range. In the lower humidity range, vertical as well as horizontal shifting of curves was required. This difference in nature of the first 2–3 % of water absorbed in nylon 6 has also been documented recently by Papir, Kapur, Rogers and Baer [J. Polym. Sci., A-2, 10, 1305 (1972)]. The region in which simple horizontal time-temperature superposition can be applied is evidently related to the type of state or system which Hopkins has denoted as “thermorheologically simple,” and the concept can be extended to plasticized systems. This implies a system where the structure is stable; i.e., in the present case, where no changes take place in the crystalline phase during the measurements, and the amorphous phase is therefore free to behave in a classically simple manner. The form of the dielectric loss curve, both as regards area under the curve (representing a total “dielectric dispersion strength”) and the width of the loss peak (representing the spread of the distribution of dielectric relaxation times) was analyzed as a function of temperature and RH. Both of these parameters showed a significant increase above the glass transition (80dC in dry nylon 6). The area actually goes through a maximum, and decreases at still higher temperatures. This behavior is essentially the same in the original polymer (containing smectric hexagonal crystallinity) and in samples with developed and stabilized α and γ crystal forms (produced by phenol and triiodide treatments), indicating that the nature of the amorphous regions is relatively unaffected by crystalline morphology changes in the polymer.