Solubility of CO2 in Polymers

The presence of dissolved CO2 molecules in a polymer results in the plasticization of the amorphous component of the matrix. In this respect CO2 mimics the effect of heat but with the important distinction that the Tg is depressed. The extent of the Tg depression is dependent on the wt% of CO2 in the matrix. As previously mentioned, one of the characteristics of plasticization is the enhancement of segmental motion, which has been observed spectroscopically for the ester groups of PMMA [20] and the phenyl rings of polystyrene [21]. The consequential increase in free volume of the matrix has been studied by methods such as laser dilatometry [22], in situ FTIR spectroscopy [20], high-pressure partition chromatography [23], and inverse gas chromatography [24]. 

This Tg depression enables the processing of polymeric systems that were previously not possible by conventional methods. If one wanted to impregnate a polymer with an active component, the selection of active components would be dictated by the Tg of the host matrix in terms of a requirement for thermal sta-bihty of the component at temperatures of at least the Tg. With the use of high-pressure CO2 and the consequential reduction of the Tg, the selection of active components is broadened to include components that would have suffered from thermal degradation via traditional melt processing. This ability of high-pressure or supercritical CO2 to plasticize the matrix is at the heart of aU applications described in the following sections. 

High pressure vessel containing a semi-crystalline polymer with a relatively low degree of crystallinity 

Poly(ethylene terephthalate) (PET) X-ray diffraction, infrared spectroscopy and density measurements [25] 

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The investigated spray process is based on the dissolution of supercritical carbon dioxide (SCCO2) into the liquid which shall be atomized. Therefore, CO2 is one of the central substances within this project. The carbon dioxide is provided by YARA with a purity of 99.9% (v/v). The critical temperature of CO2 is 304.13 K the critical pressure amounts 7375 MPa. The solubility of CO2 in water is in the order of 2-5 wt.% at 313 K and a pressure range of 2-15 MPa [5]. The solubility of CO2 in polymers is in the range of 10 wt.% at 353 K and 14 MPa [6]. 

Several authors [3-9] studied the solubility of polymers in supercritical fluids due to research on fractionation of polymers. For solubility of SCF in polymers only limited number of experimental data are available till now [e.g. 4,5,10-12], Few data (for PEG S with molar mass up to 1000 g/mol) are available on the vapour-liquid phase equilibrium PEG -CO2 [13]. No data can be found on phase equilibrium solid-liquid for the binary PEG S -CO2. Experimental equipment and procedure for determination of phase equilibrium (vapour -liquid and solid -liquid) in the binary system PEG s -C02 are presented in [14]. It was found that the solubility of C02 in PEG is practically independent from the molecular mass of PEG and is influenced only by pressure and temperature of the system. 

At pressures above these limits, the solubility of CO2 in the polymer phase remains relatively constant as seen on the right hand branches of Figures 4 and 5. This condition affects the distribution of CO2 and polymer between the two phases. When the composition of the polymer phase is almost constant, a preferential partitioning of CO2 into the SCF phase drives a certain amount of polymer from the polymer phase into the SCF phase based on the criterion of phase equilibria. This effect together with the solvent density increase due to pressure cause the enhancement of the solubility of polymer in the SCF phase as observed on the left hand branches of Figures 4 and 5. 

It is evident from Figures 4 and 5 that, the solubility of CO2 in the polymer... 

Sorption equilibria. Equilibrium solubilities of CO2 in a variety of different polymers at 25 C have been determined by our gravimetric desexption method. At higher pressures, the solubilities are quite substantial and vary markedly with the polymer type. Equilibrium uptakes of liquid 0X 2 range from about 3 g/100 g in polyethylene to over 50 g/lOOg in poly(vinyl acetate) other high values have been found for PMMA (27 g/100), ethyl cellulose (30 g/100), and cellulose acetate (27 g/100). 

Fig. 1 Solubility of CO2 in several polymers (at 200°C, a gravimetric method). (From Ref 1)...

Fig. 1 shows the solubility of CO2 in LDPE, HDPE, PP, ethylene-ethylacrylate copolymer (EEA), and PS, which were measured at temperatures from 150°C to 200° C and pressure up to 12 MPa using a magnetic suspension balance (MSB). As shown in Fig. 1, the solubility of gases in polymers increases as the saturation... 

The low reaction rate due to the low solubility of CO2 in aqueous media can be enhanced by elevation of the pressure and utilization of 3-dimensional electrodes, such as gas diffusion electrodes, solid polymer electrolytes, and packed bed electrodes. 

Contrary to other atmospheric gases like N2, O2 or CO2, the permeability of polymer films for SO2 is very high (Table III). Obviously the molecular size of SO2 is not the dominating factor for its permeation rate. As the permeability coefficient P is defined by the product of the diffusion coefficient, D, and the solubility, S, the SO2 solubility of the polymer films plays an Important role. In fact the few data published on solubility of SO2 in polymers corroborate this expectation. Equilibrium solubility of SO2 in polyacrylate was found to be as high as 21.5% by weight at 760 mmHgl. In case of a bisphenol A polycarbonate. 

As a further example of the applicability of the NET-GP results, we consider the solubility of CO2 in PMMA at 33° C and in Teflon AF1600 (poly[2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole(87%)-co-tetrafluoroethylene]) at 35°C[ . For both systems, the polymer dilation data are available from independent experimental measurements, enabling us to calculate the value A s. exp of the experimental swelling coefficient. The swelling coefficient has also been estimated from the solubility data by... 

Carbon dioxide is very soluble in some of these polymers (Figure 3). For example, in PDMS-308 000, the CO2 content of the condensed phase reaches 40% mass fraction at only 26 MPa and 50 °C [31], The solubility of CO2 in PEG is slightly lower than in PDMS. Weidner et al. [16] found only 29 wt.% CO2 in PEG-1500 at those conditions. Similar values have been found for PEG-400, PEG-600, and PEG-4000 [23, 32]. Daneshvar et al. [28] obtained much higher values but it has been suggested that equilibrium had not been reached in those experiments [16]. The solubility of CO2 in PEG and PDMS is significantly lower at higher temperatures. Unfortunately there seem to be few data for other liquid polymers. 

It should be emphasized that permeation through a polymer involves three steps, as shown in Fig. 14.1 (1) the permeant dissolves at the polymer interface, (2) the permeant diffuses within the polymer film from the side of high concentration toward the low concentration side, and (3) the permeant diffuses out from the opposite polymer interface. These steps are always present in any system regardless of whether D and S follow Pick s and Henry s laws, respectively, or not. For instance, the solubility of CO2 in PET follows the Langmuir-Henry s law model, Equation 14.7, and P is given by,... 

SHI Shieh, Y.-T. and Lin, Y.-G., Equilibrium solubility of CO2 in robbery EVA over a wide pressure range Effects of carbonyl group content and crystallinity. Polymer, 43, 1849, 2002. 

Siripurapu et al. [60] studied the continuous foaming of highly crystalHne poly (vinyUdene fluoride) (PVDF) at 175 °C with 2 wt% of SCCO2, which resulted in a foam structure with a heterogeneous cell size distribution and a low cell density, attributed to the low solubility of CO2 in the polymer and therefore tittle CO2 available for bubble formation. A further restriction is the semi-crystaUine structure of PVDE. As the temperature falls below the crystallization occurs, which expels CO2 into the amorphous domains and may lead to cracking of the cell walls and therefore formation of discrete particles. Improvements in the foam characteris-... 

The conundrum in designing membranes for high CO2 permeability is that the separation is enhanced by the solubility of CO2 in the polymer and that the polymer needs to be dissolved in solvents in order for the phase inversion process to occur and form the asymmetric structure. Properties that enhance CO2 performance also can make the structure sensitive to contaminants that over the long term can hurt performance and in worst case scenarios completely shut down performance. 

The solubility of gas in polymers decreases with an increase in temperatiu-e. The solubility of N2 is considerably less than that of CO2. Since the amoimt of gas that can be dissolved is a function of the saturation pressiu-e and since the gas diffusion rate is the rate-limiting process, we can use supercritical CO2 to enhance the solubility and diffusion rate. CO2 is supercritical at pressures and temperatiu-es greater than 7.4 MPa and 31.1°C. 

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