STUDIES OF SELF-ASSOCIATION IN POLYAMIDES AND RELATED MIXTURES (NYLON, HYDROGEN BONDING, POLYMER BLENDS, INFRARED SPECTROSCOPY, FTIR)

Fourier transform infrared (FTIR) spectroscopic temperature studies of polyamides, low molecular weight amides and polymer blends containing polyamides are presented. The results strongly suggest that prior interpretations of the changes occurring in the N-H stretching region of the spectra of polyamides with temperatures were greatly oversimplified. In essence, these spectral changes were interpreted to be solely due to hydrogen bonded N-H groups transforming to "free" N-H groups. Subsequent use of these data to obtain thermodynamic parameters associated with hydrogen bond dissociation are erroneous. The primary factor not taken into account was the very strong dependence of the absorption coefficient on hydrogen bond strength. As the temperature is increased, the average strength of the hydrogen bonds decreases, which is observed in the infrared spectrum by a shift to higher frequency. Concurrently, the absorption coefficient decreases, leading to a reduction in the absolute intensity of the hydrogen bonded N-H band. The hydrogen bonded N-H band does not exhibit separable features attributed to ordered and disordered hydrogen bonded conformations, but rather reflects the overall distribution of hydrogen bonded strengths.; The Amide I mode is conformationally sensitive through dipole-dipole interactions, and separate bands are identified and assigned to ordered and disordered hydrogen bonded conformations in semicrystalline polyamides. Quantitative measurements of the area of the ordered band correlate well with the percent crystallinity determined from thermal analysis.; The Amide II mode is sensitive to conformation via mechanical coupling and hydrogen bonding and hence can be used to distinguish between different polymorphic crystal forms.; Studies of low molecular weight amides yield further information about the amide group hydrogen bond. In particular, the changes in the infrared spectrum with temperature are not as great as in the case of polyamides, suggesting that the polymer chain influences the hydrogen bond dynamics.; Miscible nylon blends were prepared from weakly self-associated polymers. Specifically, when poly(2-vinyl pyridine) (P2VP) and a copolymer of poly(ethylene oxide) : poly(propylene oxide) (PEO : PPO) were blended with an amorphous polyamide, infrared spectral changes occurred which were indicative of mixing. Thermal analysis and cloud point studies on nylon-P2VP blends established the existence of miscibility and revealed that the particular blends exhibited fairly low LCST's.