Thomas L. Staples

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.

Wicking flow in irregular capillaries

Abstract Understanding capillary flow in porous media (wicking) is critical for predicting and improving the performance of absorbent structures. Capillary flow in straight circular tubes has long been known to be quite well described by the Lucas–Washburn equation. Experimental wicking results with beds of glass beads led us to investigate a minor modification of this equation to describe more complex systems. These results, as well as the mathematical treatment, are presented to describe capillary flow as a function of time in tubes irregularly shaped along their primary axis. The cross-section of the model tubes remains circular. Variations of the diameter that are periodic along the length of the tube are the focus of the study, and a sinusoidal description is argued to be sufficiently general to describe most systems. The variables used to describe the system can be divided into two groups. Fluid and surface chemistry are defined by the viscosity (η), surface tension (γ), contact angle (θ), and density (ρ). In the sinusoidal case, the capillary itself is described by Dcap, the diameter at the largest portion of the tube which determines the limiting capillary pressure, Dvis, the diameter of the throat which dominates the viscous drag, and λ, ‘wavelength’ of the fluctuations. The conclusions are: (1) as long as the wavelength of the fluctuation in diameter for the capillary is small relative to the length of travel, the actual value of this wavelength does not matter in the time derivative of the flow. This is typically the case for porous media in which the flow path dimensions fluctuate in the range of microns, and the wicking distances are measured in centimeters. (2) As a result of this simplification, a two-parameter equation can be written which yields the time t for the fluid front to reach a given distance L, analogous to the Lucas–Washburn equation: ln 1− L L eq + L L eq =− C 2 D cap 2 ρg 32ηL eq t The term Leq defines the equilibrium height and is a function of γ, θ, ρ, and Dcap. The rate is determined in addition by η and Dvis (=C×Dcap).

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.

Thermally stable, photoactive polymerizable diazenes. Applications in polymer technology

New and useful nonpolymerizable and polymerizable diazene free radical initiators have been designed, based on molecular orbital and empirical linear free energy calculations. The design of the polymerizable diazenes has been based on four requirements: independent reactivities of azo and vinyl moieties, high thermal stability of the trans-diazene isomers, efficient convertability of the trans isomer to the cis isomer, and high thermal reactivity (to produce free radicals) of the cis-diazene isomers. The cis-diazene isomers are obtained from the trans isomers by irradiation with visible light. A new free radical initiator has been shown to be useful in preparing graft copolymers and in crosslinking polymer chains by visible light activation. Poly(styrene-co-2-(meta-styrylazo)-2-methoxypropane) (2) was prepared by the copolymerization of styrene monomer and 2-(meta-styrylazo)-2-methoxypropane (1), either thermally or via a free radical initiator. Copolymer 2 then was reacted with methyl methacrylate monomer in the presence of visible light, to produce poly (styrene-g-methyl methacrylate) (6). Poly(styrene-co-2-(meta-styrylazo)-2-methylpropane) (17), prepared by the copolymerization of styrene monomer and 2-(meta-styrylazo)-2-methylpropane (15), was successfully crosslinked at elevated temperatures.

Chapter VIII - Synthetic Superabsorbents

Publisher Summary This chapter discusses synthetic superabsorbents. The chapter reviews the physical chemistry of aqueous fluid absorption and superabsorbents. The chapter discusses the manufacture of the major commercial synthetic materials, the process of performance evaluation, and also reviews the primary applications. The chapter focuses on crosslinked sodium polyacrylate gels because this is the dominant chemistry for commercial superabsorbents. Two quite different mechanisms are employed to retain fluids in absorbent products, capillarity, and osmosis. The essential characteristic of capillary absorption is illustrated by the spontaneous rise of fluid in a small tube. A familiar example of a good capillary absorbent for aqueous fluids is cellulose fluff. Osmosis, the other mechanism for fluid transport and storage, is driven by the same thermodynamics that cause dissolution. This chapter also discusses polymer networks or gels, crosslinked systems that do not form true solutions but stop at the swollen state noted by Billmeyer. The swelling process begins with the formation of a true polymer solution. Aside from swelling capacity, another property characteristic of a gel is its toughness or resiliency. In addition to sodium polyacrylate, certain other acrylate based polymer systems have been developed as superabsorbents.