CROSS-LINKER REACTIVITY AND THE STRUCTURE OF SUPERABSORBENT GELS

A C-13-labeled crosslinker (trimethylolpropane triacrylate, TMPTA) was synthesized and copolymerized with acrylic acid while monitoring the relative rates of reaction of the crosslinker and acrylic acid by C-13 -NMR. This allowed easy quantification of the concentration of the min or component (crosslinker) in the polymer and monomer mixture to level s as low as 0.02%. Polymerizations were conducted in 5 mm NMR tubes un der varied temperature, percent neutralization (pH), and percent solid s. Reactivity ratios were determined from the rates of incorporation o f the components into the gel by use of the integrated form of the cop olymerization equation, and their sensitivity to the above variables w as quantified. The relative rate of incorporation of the crosslinker i nto the gel was exceedingly fast. The reactivity ratio, r(1) for acryl ic acid, varied from 0.31 (65% neutralization) to 0.77 (unneutralized) . The reactivity ratio was affected by the percent solids (solvent eff ect), but was insensitive to temperature over the range of 55-80 degre es C. It was observed that all of the double bonds of TMPTA were incor porated into gel network as opposed to prior models predicting only tw o bonds reacting. The reported inefficiency of TMPTA is postulated to be caused by a solubility problem in the monomer mixture. Very low lev els of extractables were found in the products even though the crossli nker was consumed by 70% conversion. Based on these data, we propose t hat a major component of the gel network is graft polymer that forms l ate in the polymerization onto the crosslinked gel, formed earlier. (C ) 1997 John Wiley & Sons, Inc.