Quantitative analysis of hydrogen bonding in electrospun fibers of poly(4-vinyl pyridine)/(4,4′-biphenol) complexes by ATR using liquid blends as models

To understand the characteristics of electrospun fibers of poly(4-vinyl pyridine) (P4VP) that are potentially crosslinked by hydrogen-bonding when blended with small molecules like 4,4′-biphenol (BiOH), it is necessary to determine the proportion of hydrogen-bonded pyridine rings (fb) and to contrast it with FOH, the mole percent of OH groups available for interacting with the pyridine rings. While this can be done by Fourier transform infrared spectroscopy (FTIR), two practical difficulties must be overcome. First, the correct intensities of the overlapped bands of free and hydrogen-bonded pyridine must be obtained, which is possible using the second derivative spectra. Second, the band absorption coefficient ratio (a) of the pair of bands must be known. In the P4VP/BiOH system, only the pair of free and hydrogen-bonded pyridine ring bands at 993 and 1007 cm−1 can be used for quantitative analysis. We determined, by analysis of liquid blends of BiOH as well as phenol with a model compound, 4-ethylpyridine (EtPy), using the attenuated total reflection mode (ATR), that a = 0.40. This led to values of fb = FOH, indicative of full complexation of the OH groups to pyridine, in the EtPy/BiOH liquid blends up to the BiOH solubility limit in EtPy (FOH = 60%) and in the EtPy/phenol blends up to FOH = 100%. In the electrospun P4VP/BiOH fibers prepared from solutions with FOH up to about 120%, fb averages 0.76FOH, and full complexation of the pyridine groups is achieved only at about FOH = 160%. In both P4VP/BiOH and EtPy/BiOH, the complexation occurs between one pyridine ring and one hydroxyl group. This work thus shows a new method to quantify the relative number of hydrogen-bonded pyridine rings in P4VP; it is expected to be applicable to other polymer-small molecule and polymer-polymer blends having suitable liquid models.