Small Angle Neutron Scattering From Polymers In Supercritical Carbon Dioxide

George D. Wignall

Solid State Division

Oak Ridge National Laboratory*

Oak Ridge, TN 3783 1-6393

INTRODUCTION:

Carbon dioxide (CO2) is nontoxic, nonflammable, naturally occurring, recyclable and inexpensive. Above its critical pressure (Pc= 73.8 bar) and temperature (Tc = 31 OC),CO2 has properties intermediate between a gas and a liquid, i.e. high density and low viscosity. Because of these attributes, it has been proposed as an environmentally responsible replacement for the organic and aqueous media used in many solvent-intensive industrial applications. Despite these intrinsic advantages, CO2 has a significant drawback in that only two classes of polymers (amorphous fluoropolymers and silicones) have been shown to exhibit appreciable solubility at readily accessible temperatures and pressures (T ~ 100 OC, P ~ 450bar) [1,2].Thus, many polymers (e.g.longchain hydrocarbons, waxes, heavy greases, etc.) do not dissolve in CO2 and this necessitates the use of emulsifying agents to solubilize the "CO2-phobic" material. Such surfactants are generally amphiphilic (i.e. the different components of the molecule have different solubilities), and it is well known that hydrophobic oil may be solubilized in water by coating the oil droplets with the hydrophilic (water-soluble) component of a detergent. Thus, molecularly engineered diblock copolymer surfactants, consisting of "CO2 -phobic" and "CO2 -philic" blocks, have recently been developed [3] for a wide range of applications in liquid and supercritical CO2. These include the stabilization of polymer colloids during dispersion polymerizations [4], the formation of micelles [5,6], which can solubilize CO2-insoluble substances [3], and liquid-liquid extractions via the transfer of water-soluble substances from water into a surfactant-rich liquid CO2 phase. Small angle neutron and x-ray scattering (SANS and SAXS) methods allow the elucidation of the size and shape of both individual polymer chains and supramolecular structures [8,9] in the resolution range, 5-2000A. Over thepast two decades,

* Managed by Lockheed Martin Energy Research Corporation under contractDE-AC05-96OR-22464 for the U. S. Department of Energy.

SANS has emerged as the mostpowerful technique for studying polymer phase behavior [9] and the self assembly of amphiphiles in aqueous media [8]. In addition, neutron scattering is particularly suited to study the structure of matter under pressure, including supercritical fluids [10], due to the well known high transmission of many of the materials used in the construction ofpressure vessels. One otherfactthathas been used to advantage in the present studies is that fluorinated materials have greater SAXS contrast with the organic core and the solvent. Thus, SAXS and SANS are complementary techniques that highlight different components of the structure, and experiments to apply these techniques to study polymers in SC-CO2 have recently been undertaken [2,3,5,6,11]. In this publication we review structural information derived from SANS and SAXS studies at Oak Ridge.

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