Polysiloxanes in Compressed Carbon Dioxide - American Chemical

Chapter 8

Polysiloxanes in Compressed Carbon Dioxide 1

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Sarah L. Folk and Joseph M. DeSimone * Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: March 10, 2003 | doi: 10.1021/bk-2003-0838.ch008

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Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695 2

Compressed carbon dioxide (CO ) is an environmentally friendly solvent alternative with potential for extensive use in industrial processes. Polysiloxanes are one of only a few classes of polymers that are soluble in this medium. This chapter reviews the properties and synthesis of polysiloxanes in liquid and supercritical CO , as well as their areas of utilization. 2

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As the negative impact of extensive water and traditional organic solvent use in industrial processes becomes more apparent, researchers look for environmentally friendly solvent alternatives. Carbon dioxide (C0 ) is an excellent choice because it is environmentally benign, nontoxic, nonflammable, and relatively inexpensive. Additionally, C 0 is readily availablefromnatural reservoirs and as a byproduct of current industrial processes as well as being recyclable in many applications. As a replacement solvent, C 0 is used in its compressed liquid or supercritical fluid (SCF) phases. SCFs have gas-like viscosities and liquid-like densities; the tuning of these properties, and hence the solvency of the medium, can be accomplished with minimal adjustments to temperature and pressure. A convenient advantage of the utilization of scC0 as compared to other SCFs, is its readily accessible critical temperature and pressure (T = 31 °C; P = 73.8 bar) (7). 2

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© 2003 American Chemical Society

In Synthesis and Properties of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Solubility in C 0

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Because liquid and supercritical C 0 (liq, scC0 ) have low dielectric constants, low polarizability per volume, and minimal Van der Waals interactions, many highly polar species (i.e. water, salts) and most polymeric species are insoluble in C0 . Only two main classes of polymers were shown to be appreciably soluble in C 0 : amorphous fluoropolymers and polysiloxanes (2). Poly(ether-carbonates) (J) and oligomeric poly(propylene oxide) (PPO) (4) were also found to be soluble in C 0 but to a lesser extent. Although polysiloxanes are less soluble than most fluoropolymers, they are easier to characterize and significantly less expensive to use, especially on a large scale. Their relatively low cost combined with characteristics such as low glass transition temperature, low surface tension, good thermal and oxidative stability, and optical transparency have resulted in widespread attention in the field. In order for polysiloxanes to be successfully employed in compressed C 0 processes, basic research on the solution properties of the most elementary system, poly(dimethylsiloxane)/C0 , is necessary. 2

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Macroscopic Solubility The most commonly used method to determine the macroscopic solubility of poly(dimethylsiloxane) (PDMS) in liq and scC0 is with cloud point measurements. PDMS forms a clear, colorless solution in C 0 . The transition from a one-phase to a two-phase solution occurs by varying the temperature or pressure. The point of precipitation is determined visually, or through more advanced techniques such as turbidimetry or light scattering. The characteristic upper and lower critical solution behavior of PDMS homopolymer in C 0 , including the upper and lower critical solution temperatures, was demonstrated (5). The addition of a polar group to PDMS homopolymers usually results in a distinct decrease in C 0 solubility (Figure 1). However, an increased solubility of PDMS with an addition of propyl acetate side chains was shown to be due to interactions between the side chain carbonyl and C 0 (6). The solubilities of PDMS analogs of conventional hydrocarbon surfactants were also determined in C0 (7). Determination of solubility properties of PDMS in liq and scC0 is important for processes such asfractionationof high polydispersity samples which depend upon differences in solubility (8). The rate of pressure quench on the pressure-induced phase separation (PIPS) of PDMS in C 0 (9) was investigated along with the mechanistic change in phase separation from 2

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nucleation and growth to spinodal decomposition as a function of the depth of pressure quench (10). The ability to determine the binodal and spinodal envelopes, and hence the change in growth mechanism, is beneficial for determining the final particle morphology. PIPS is an important step in many SCF-based processes such as spray coating, textile dyeing, and the rapid expansion of supercritical solutions (RESS). A recent paper exploiting the RESS process explored spraying PDMS from a solution to coat a chemical sensor (//). The chemical sensor coated with uniform microspheres of PDMS showed an increased sensitivity and a rapid and reversible response.

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Figure 1. Solubility curves before (a: +, •, o) and after (b: ·) polar functionalization at 5 w/w% and 0.5 w/w%, respectively, of 2 kg/mol (+, +), 3 kg/mol (u, m), and 5 kg/mol(o m) PDMS. f

Microscopic Solubility Although the solubilities of homopolymers in C 0 were found to correlate strongly with the cohesive energy density and surface tension of the polymers in the bulk phase (4), the extent of solubility of individual polymer chains in any solvent is dependent upon the relative strengths of the polymer-polymer, solvent-solvent, and polymer-solvent interactions. The net interaction between two polymer chains serves to classify the solvent as a "good," "theta (Θ)," or "poor" solvent for a particular polymer at specified conditions. For systems 2


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where the net interaction is repulsive, the chains are swollen (larger than the ideal Flory size) and the solvent is deemed a "good solvent" for the polymer. Conversely, if the net interaction is attractive, then the polymer is in a "poor solvent" environment and either insoluble, or soluble and smaller than ideal size. When the net interaction is zero (or very nearly zero), the polymer is in "Θ solvent" conditions and assumes an ideal conformation. Small angle neutron scattering (SANS) studies probed these interactions for solutions of PDMS in C 0 . Although PDMS was reasonably soluble in C 0 , C 0 was designated as a poor solvent for PDMS, according to the above solvent definitions, based on experimental results wherein the individual chains experienced aggregation except in very dilute solutions (72). Additional work with this system at a constant C 0 density of 0.95 g/cm showed the existence of a theoretically predicted theta temperature (Τ ) of 65 °C at which the chains exhibited ideal behavior (75, 14). Perhaps more interesting was that the compressible nature of C 0 permitted the researchers to determine a theta pressure (Ρ ) of 52MPa (Figure 2) along with the more traditional Τ (75). This is believed to be thefirstΡ on record.


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Figure 2. Variation of the radius ofgyration (Rg) for PDMS with M = 22,500 g/mol in a solution of(h + d) PDMS in scCÛ2 vs pressure at Τ = 70 °C. Adaptedfromreference 13. Copyright 1999 American Chemical Society. w


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SANS in combination with dynamic light scattering more recently was used to measure the dynamic and static correlation lengths of PDMS chains in poor solvent conditions (Τ < Τ ) (75). Additionally, fluorescence techniques were used to determine the mean-free distance between termini of pyrene end-labeled PDMS as a function of density (16). θ


Swelling with C 0

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The swelling of both cross-linked and uncross-linked PDMS are areas of current research. The swollen cross-linked PDMS system is beneficial to modeling stationary phase behavior of SCF chromatography (17, 18). Uncrosslinked PDMS swollen with C 0 at ambient temperatures, on the other hand, is an analog to thermoplastic melts at their processing temperatures. The sorption of gases into polymer melts is important to polymer processing with SCFs, especially processes such as extrusion and injection molding in the manufacture of foams and composites. Dissolved C 0 modifies the rheological properties of the melt by lowering the viscosity. This decrease in viscosity is due primarily to the increase in free volume upon swelling although also due to the decreased concentration of polymer chains (19). The use of scC0 as an additive enables well-defined tuning of the processing systems as well as ease of additive removal. This is in contrast to traditional additives which present challenges in achieving desired properties of the system both during and after processing. 2

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The effects of pressure, temperature, and molecular weight on the extent and kinetics of swelling of the uncross-linked PDMS system were determined (20-22). Swelling, monitored in situ as a function of time, showed an initial region of dramatic increase in volume followed by a relatively small increase to reach an equilibrium value as seen in Figure 3 (21). The overall change in volume increased with an increase in pressure or molecular weight, but exhibited pressure-dependent temperature effects. In situ ATR (Attentuated Total Reflectance)-IR spectroscopy was used to simultaneously measure C 0 sorption and polymer swelling as well as to provide insight into the molecular interactions (22). The effect of C 0 swelling on the dynamics of a non-polar solute in PDMS was also determined using fluorescence measurements (23). Additionally, the first report of the swelling of thin films (

Polysiloxanes in Compressed Carbon Dioxide - American Chemical

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