Improving Structural Homogeneity, Hydraulic Permeability, and Mechanical Performance of Asymmetric Monophasic Cellulose Acetate/Silica Membranes: Spinodal Decomposition Mix

Abstract

In this paper, we propose an optimized protocol to synthesize reproducible, accurate, sustainable integrally skinned monophasic hybrid cellulose acetate/silica membranes for ultrafiltration. Eight different membrane compositions were studied, divided into two series, one and two, each composed of four membranes. The amount of silica increased from 0 wt.% up to 30 wt.% (with increments of 10 wt.%) in each series, while the solvent composition was kept constant within each series (formamide/acetone ratio equals 0.57 wt.% in series one and 0.73 wt.% in series two). The morphology of the membranes was analyzed by scanning electron microscopy and the chemical composition by Fourier transform infrared spectroscopy, in attenuated total reflection mode (FTIR-ATR). Mechanical tensile properties were determined using tensile tests, and a retest trial was performed to assess mechanical properties variability over different membrane batches. The hydraulic permeability of the membranes was evaluated by measuring pure water fluxes following membrane compaction. The membranes in series two produced with a higher formamide/acetone solvent ratio led to thicker membranes with higher hydraulic permeability values (47.2-26.39 kg·h^-1·m^-2·bar^-1) than for the membranes in series one (40.01-19.4 kg·h^-1·m^-2·bar^-1). Results obtained from the FTIR-ATR spectra suggest the presence of micro/nano-silica clusters in the hybrid membranes of series one, also exhibiting higher Young's modulus values than the hybrid membranes in series two.

Keywords: CA/SiO2 membrane; hydraulic permeability; integral asymmetric membranes; mechanical tensile properties; monophasic hybrid membrane; spinodal decompositions mix.

Figure 1

Polymer-solvent–nonsolvent system: (A). Classical Loeb–Sourirajan method setup; (B). Ternary phase diagram.

Figure 2

Silica sol–gel system: (A) ternary TEOS-EtOH-H₂O system; (B) chemical miscibility/immiscibility between solvents and reactants used in polymer and sol–gel silica systems; (C) spinodal phase separation: phase diagram.

Figure 3

Flowchart of hybrid membranes preparation.

Figure 4

Top dense surface and cross-sectional images for the series one and series two membranes at 5000× and 2500× magnification.

Figure 4

Top dense surface and cross-sectional images for the series one and series two membranes at 5000× and 2500× magnification.

Figure 5

Total membrane thickness in (a) series one and (b) series two.

Figure 6

FTIR-ATR wide spectra (3750–700 cm⁻¹) of the membranes in series one (a) and series two (b).

Figure 7

FTIR-ATR absorption spectra of the membranes from series one: CA1/SiO₂ 100/0, 90/10, 80/20, and 70/30 membranes: (a) in the region 950–1190 cm⁻¹ and (b–e) curve-fitting decomposition of the bands ν(C-O), ν_δ(Si-O-Si), and ν_δ(Si-O-C), present in the spectra obtained for: (b) CA1/SiO₂ 100/0 membrane, (c) CA1/SiO₂ 90/10 membrane, (d) CA1/SiO₂ 80/20 membrane, and (e) CA1/SiO₂ 70/30 membrane. Experimental results are shown by the thick grey line and the simulated band by the black dashed line.

Figure 7

FTIR-ATR absorption spectra of the membranes from series one: CA1/SiO₂ 100/0, 90/10, 80/20, and 70/30 membranes: (a) in the region 950–1190 cm⁻¹ and (b–e) curve-fitting decomposition of the bands ν(C-O), ν_δ(Si-O-Si), and ν_δ(Si-O-C), present in the spectra obtained for: (b) CA1/SiO₂ 100/0 membrane, (c) CA1/SiO₂ 90/10 membrane, (d) CA1/SiO₂ 80/20 membrane, and (e) CA1/SiO₂ 70/30 membrane. Experimental results are shown by the thick grey line and the simulated band by the black dashed line.

Figure 8

FTIR-ATR absorption spectra of series two membranes: CA2/SiO₂ 100/0, CA2/SiO₂ 90/10, 80/20, and 70/30: (a) In the region 950–1190 cm⁻¹ and (b–d) curve-fitting decomposition of the ν_δ(C-O), ν_δ(Si-O-Si) and ν(Si-O-C) bands, present in the spectra obtained for: (b) CA2/SiO₂ 100/0 membrane, (c) CA2/SiO₂ 90/10 membrane, (d) CA2/SiO₂ 80/20 membrane, and (e) CA2/SiO₂ 70/30 membrane. Experimental results are shown by the thick grey line and the simulated band by the black dashed line.

Figure 8

FTIR-ATR absorption spectra of series two membranes: CA2/SiO₂ 100/0, CA2/SiO₂ 90/10, 80/20, and 70/30: (a) In the region 950–1190 cm⁻¹ and (b–d) curve-fitting decomposition of the ν_δ(C-O), ν_δ(Si-O-Si) and ν(Si-O-C) bands, present in the spectra obtained for: (b) CA2/SiO₂ 100/0 membrane, (c) CA2/SiO₂ 90/10 membrane, (d) CA2/SiO₂ 80/20 membrane, and (e) CA2/SiO₂ 70/30 membrane. Experimental results are shown by the thick grey line and the simulated band by the black dashed line.

Figure 9

Hydraulic permeability Lp values (kg·h⁻¹·m⁻² bar⁻¹) in (a) series one and (b) series two (* p < 0.01; ** p < 0.05).

Figure 10

Mechanical properties for series one (left) and series two (right): (a) Young’s modulus (top); (b) stress at yield point (middle); and (c) strain at yield point (bottom) (* p < 0.01; ** p < 0.05; *** p < 0.1).