Light Scattering Study of Poly (dimethyl siloxane) in Liquid and Supercritical CO 2.

Transcription

1 Supplemental Information. Light Scattering Study of Poly (dimethyl siloxane) in Liquid and Supercritical CO 2. Pascal André, Sarah L. Folk, Mireille Adam, Michael Rubinstein, and Joseph M. DeSimone Technical Information Static light scattering (SLS) and dynamic light scattering (DLS) experiments were performed with a setup provided by Brookhaven Instruments, including a spectrometer equipped with an argon ion laser (Coherent Innova 703) operating at a wavelength, λ=514 nm, a BI9000 correlator, and a computer controlled motordriven variable angle detection system (BI200SM). The scattering spectrum was measured through a bandpass filter (514.5 nm) and a photomultiplier tube (BIPMT9836). The experiments were conducted using several pinhole sizes (100, 200 and 400 µm), and the laser power was varied from 20 mw to 200 mw. For experiments in benzene, the Brookhaven setup was used without any modification. Decalin was used as a matching refractive index liquid. The temperature was regulated with a Science/Electronics LTD6 temperature bath which has an operating temperature range between 20 and 100 ºC with an error of ± 0.2 ºC. The window housing of the cell consists of a steel plug in series with a brass Oring, a thick sapphire window, and a Teflon Oring. Three windows located at 45, 90, and 135 relative to the incident beam can be used for scattering measurements, each over a 10 interval, while windows at 0 and 180 served as the entrance and exit windows for the light source. Each angular port was fitted with a sapphire window (thickness 8 mm, diameter 11 mm, Hemlux quality) from Crystal Systems of Salem, MA. To reduce optical reflection and improve the optical quality of the setup, an MgF 2 antireflective coating was applied by TwinStar Optics, Coatings & Crystals of Port Richey, FL. Toluene (Fisher) was used to correct the excess Rayleigh ratio of CO 2. Internal Angle Correction Because of the non cylindrical configuration of the cell, Snell s law must be used to account for the refractions of light at the interfaces between the compressed CO 2 and the sapphire window, and 1

2 between the sapphire window and the air. The internal angle of detection, θ, can thus be calculated as shown in Figure S1. Laser Beam θ ψ α 1 n 1 n 2 Sapphire Window n 3 α 3 θ ext Parallel to Laser Beam Figure S1 : Diagram for calculation of internal angle of detection, θ. In Figure S1, the sapphire window is tilted with respect to the laser beam. ψ is the angle of the sapphire window relative to perpendicular with respect to the laser beam and is equal to ± 45 or 0. The refractive indices n 1, n 2, and n 3 are for the CO 2 solution, the sapphire, and the air, respectively. θ is the internal angle of the scattered light which is used to calculate the wave vector, q, as defined earlier. θext stands for the external angle that is the position of the detector around the cell. Finally, α i are the angles of refraction of light on the surfaces of the sapphire windows which are related to θ through the following equation. ( + ψ ) + 90 o + θ 180 o α 1 = (1) The internal angle can be fully calculated with the use of Snell s law as illustrated by the following equation: n θ = 90 o ψ sin 1 3 sin 90 o ψ θ ext (2) n1 This correction was systematically applied to the calculation of the wave vector used in dynamic light scattering and enabled the determination of the diffusion coefficient. Experimental Procedure In benzene, a microbalance was used to determine the amounts of both polymer and solvent; the PDMS solution with the highest polymer concentration was prepared first. After the LS measurements, the initial solutions were diluted to obtain the lower concentrations. If dusty, the solution was filtered through a 0.2 µm pore size Teflon filter. 2

3 Prior to each measurement, the highpressure cell was thoroughly cleaned with trifluorotoluene and purged several times with highpressure CO 2. The scattering of the pure solvents was measured, and the cell was then loaded with polymer at room temperature. The amount of polymer was measured by weight difference with a syringe. At room temperature, after having sealed the cell, the CO 2 was injected into the cell. To insure the polymer was in solution, a stir bar was introduced into the cell and used prior to each measurement as well as after each concentration change. Each designated experimental condition was evidenced by stable pressure, temperature, scattered intensity, and intensity transmitted through the cell. The density was kept constant by closing the CO 2 inlet, and the temperature was increased to the highest temperature. The measurements were performed starting from the highest temperature down to 25ºC, after cell stability had been achieved. At 25ºC, the CO 2 pressure was then increased progressively and measurements were taken after having reached the equilibrium. After having completed a run in temperature and pressure, the volume of the cell was increased in order to decrease the polymer concentration. This increase in cell volume consequently induced a pressure drop. However, to prevent the polymer from precipitation, the pressure was always kept above the critical pressure by continuously adding CO 2 to compensate for the decrease in pressure. 3

4 Tables of Data ρ CO 2 (g/ml) T (± 0.2 o C) n co2 M w (K) co M 2 w M Benzene w (K) w co M 2 w M Benzene w Table S1 : Ratio of the molecular weights determined in benzene at 25 o C and in CO 2 at various temperatures and densities, T and ρco 2 respectively. nco 2 is the refractive index of the CO 2 calculated for each density. 4

5 ρ CO 2 (g/ml) T (± 0.2 o C) M w A 2 (ml/g) (M w : 31.3K) M w A 2 (ml/g) (M w : 24.3K) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.3 Table S2 : Measured second virial coefficient multiplied by the average molecular weight, M w A 2, for various M w, CO 2 densities, ρ CO 2, and temperature, T. 5

6 ρ CO 2 (g/ml) T (± 0.2 o C) η s (cp) k d (ml/g) (M w : 31.3K) k d (ml/g) (M w : 24.3K) k d (ml/g) (M w : 17K) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.1 Table S3 : Diffusional second virial coefficient, k d, for various molecular weights, M w, CO 2 densities, ρ CO 2, CO 2 viscosity, η s, and temperature, T. 6