David E. Henton

Acrylic acid polymerization kinetics

The kinetics of the isothermal polymerization of acrylic acid were determined utilizing 1H-NMR spectroscopy. The polymerization rate was observed to depend approximately on the power of monomer and the power of sodium persulfate concentration. This is consistent with a model in which the rate of initiation is itself dependent on the monomer concentration. The polymerization rate was also observed to have a strong dependence on percent neutralization, decreasing with increasing level of neutralization up to 75 to 100% neutralization, and then increasing again. The activation energy for the rate of polymerization was between 9 and 13 kcal/mol except for 100% neutralized acrylic acid, which had an activation energy of 18 kcal/mol. These data suggest that a transition in mechanism occurred at 100% neutralization. Increasing the ionic strength by the addition of sodium chloride also increased the rate. The dependence of the molecular weight on the above variables was also quantified for use in the model. It decreased with increasing conversion, decreasing ionic strength and increasing initiator.

Crosslinker reactivity and the structure of superabsorbent gels

A 13C-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 13C-NMR. This allowed easy quantification of the concentration of the minor component (crosslinker) in the polymer and monomer mixture to levels as low as 0.02%. Polymerizations were conducted in 5 mm NMR tubes under varied temperature, percent neutralization (pH), and percent solids. Reactivity ratios were determined from the rates of incorporation of the components into the gel by use of the integrated form of the copolymerization equation, and their sensitivity to the above variables was quantified. The relative rate of incorporation of the crosslinker into the gel was exceedingly fast. The reactivity ratio, r1 for acrylic acid, varied from 0.31 (65% neutralization) to 0.77 (unneutralized). The reactivity ratio was affected by the percent solids (solvent effect), but was insensitive to temperature over the range of 55–80°C. It was observed that all of the double bonds of TMPTA were incorporated into gel network as opposed to prior models predicting only two bonds reacting. The reported inefficiency of TMPTA is postulated to be caused by a solubility problem in the monomer mixture. Very low levels of extractables were found in the products even though the crosslinker was consumed by 70% conversion. Based on these data, we propose that a major component of the gel network is graft polymer that forms late in the polymerization onto the crosslinked gel formed earlier.

The effects of MEHQ on the polymerization of acrylic acid in the preparation of superabsorbent gels

The effect of the monomethyl ether of hydroquinone (MEHQ) on the polymerization of acrylic acid was studied. The rate of polymerization was quantified at various levels of MEHQ by use of an in situ NMR technique. While oxygen functions as an inhibitor in acrylic acid polymerizations, MEHQ was shown to function as a retarder. The decrease in the rate of polymerization allowed the calculation of an inhibition constant for this system. MEHQ was found to remain in the polymerizing mixture throughout the course of the reaction, significantly reducing the rate of polymerization, but not reducing the molecular weight of the polymer. The data are consistent with direct reaction of MEHQ with initiator fragments, but not termination of growing chains. Superabsorbent polyacrylic acid gels were prepared and the properties measured.

The decomposition of sodium persulfate in the presence of acrylic acid

The activation energy and rate constants for the decomposition of sodium persulfate in the presence of acrylic acid have been determined for monomer compositions representative of those used in a polymerization process to prepare sodium polyacrylate superabsorbent polymers. It was found that, relative to buffered aqueous conditions, the rate of decomposition of sodium persulfate is increased by a factor of two- to sevenfold in the presence of acrylic acid, low levels of neutralized acrylic acid, or high levels of acrylic acid at low % neutralization, due to an induced decomposition mechanism by the monomer. However, high concentrations of partially neutralized acrylic acid (high solids) decrease the decomposition rate of persulfate relative to that in the presence of acrylic acid (not neutralized). An ionic gel cage mechanism that influences the rate of decomposition of sodium persulfate is proposed.

Allyl endcapped polyethylene oxide crosslinkers and their use in superabsorbents

Several new crosslinkers have been synthesized for evaluation in superabsorbent polymers. These crosslinkers are allyl endcapped polyethylene glycols (PEG) of 200, 600, and 3400 molecular weight. A branched polyethylene oxide of 600 molecular weight, initiated with glycerin, was also synthesized as a trifunctional crosslinker. The allyl functionality was chosen because it is less reactive during radical polymerization than acrylate crosslinkers, an attribute that was necessary to achieve a more uniform gel network. A synthesis route was devised to make the crosslinkers in high purity and yield. The purity of the crosslinkers was determined by 13 C NMR, liquid chromatography, and size exclusion chromatography. Gels that were produced with the allyl crosslinkers gave excellent soluble polymer levels and swelling characteristics. The mechanism of incorporation of the allyl functionality was determined to be exclusively vinyl polymerization rather than through hydrogen abstraction. This was determined using NMR spectroscopy, monitoring the polymerization of a model system consisting of acrylic acid and allylacetate.