Photoluminescence in small isotactic, atactic and syndiotactic PVA polymer molecules in water

Abstract

Polyvinyl alcohol (PVA) dispersed in small polymer molecules in water (∼4.0 g/dl) has photoluminescence (PL) in three distinct bands at 415, 437, and 465 nm. This occurs in a resonance excitation with vibronic level of O–H stretching vibration ν 1 (or overtone) at λ ex =400 or 350 nm wavelength of excitation. Relative band intensities, which lie in the 100:94:45 order in three bands in λ ex =350 nm excitation, vary depending on the excitation process with a function of the value of λ ex and PVA concentration in the solution. A single band occurs of the absorption spectrum at 196 nm in an altogether different π←π* electronic transition. The three PL bands thus attribute to n←π* electronic transition in non-bonding 2p 2 (O) electrons in free OH groups in three syndiotactic (s), atactic (a), and isotactic (i) conformers of PVA polymer molecules. ν 1 band appears in a resonance Raman transition at ∼3215, 3475, or 3560 cm −1 in the respective conformers. Solvent interaction with H 2 O molecules (destabilizes s- and i-PVA to convert into a-PVA) quenches the PL progressively in dilute solutions so that it no longer occurs in concentrations below 1.0 g/dl. The results are analyzed and modeled in terms of the localized n -electrons in small polymer molecules.