Induced changes of polymer organization in a gel of poly(vinyl alcohol) crosslinked by Congo red

Different gel microstructures are induced at variable poly(vinyl alcohol) (PVA) and Congo red concentrations, as revealed by ultrarapid freezing and a replica technique for transmission electron microscopy. The polymer microstructures observed include random coils, rigid polymer rods, and long fibers. The development of the different polymer conformations is proposed to be dependent on the degree of intramolecular and intermolecular crosslinking and on the electrostatic interactions of the Congo red ions. The rigid-rod conformation appears to be the most energetically stable form; it is disrupted by electrostatic effects around the polymer overlap concentration (C*(PVA)). We propose that the gel microstructure influences the physical properties of the gel. Gels possessing the rigid-rod microstructure have increased Young's storage modulus values. Two possible mechanisms of gelation are suggested. The first describes a one-stage reaction when the polymer concentration approximates C*(PVA) where polymers in an extended random-coil conformation undergo intermolecular crosslinking without any microstructural changes. The second describes a two-stage reaction when the polymer concentration is less than or greater than C*, where a disorder-order transition results in the formation of rigid polymer rods and fibers followed by the formation of a macromolecular network. (C) 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1471-1483, 2001.