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Supplementary Information

Mixed-Matrix Membranes of Zeolitic Imidazolate Framework

(ZIF-8)/Matrimid® Nanocomposite: Thermo-Mechanical Stability and

Viscoelasticity Underpinning Membrane Separation Performance

E. M. Mahdi and J.C. Tan*

Department of Engineering Science, University of Oxford, Parks Road, OX1 3PJ, Oxfordshire, United Kingdom

*Email: jin-chong.tan@eng.ox.ac.uk

This supplementary document contains:

1. High-magnification SEM images of MMMs

Figs. S1 – S2 p. 2–3

2. Schematic depicting interactions between ZIF-8 nanoparticles and Matrimid

Fig. S3 p. 4

3. XRD diffractograms and crystallinity determination

Fig. S4 – S5 p. 5–6

4. Nanoindentation load-displacement data, averaged elastic moduli and hardness as a function of surface penetration depths of 2000 nm

Fig. S6 – S8 p. 7–9

5. DMA results from the multi-strain frequency sweep tests

Fig. S9– S10 p. 10–11

6. Glass transition temperature as a function of ZIF-8 nanoparticle wt.% loading

Fig. S11 p. 12

7. Time-temperature superposition (TTS) plots of loss modulus 8. TGA plots for the unannealed and annealed ZIF-8/Matrimid® nanocomposite MMM 9. Characterisation of ZIF-8 nanoparticle size from electron micrographs 10. Membrane thickness determination  

Fig. S12 Fig. S13-S14 Fig. S16 Table S1 Fig. S17

p. 13 p. 14-15 p. 16-18 p. 19-20

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Figure S1. SEM cross-sectional microstructures of the 10 wt.% ZIF-8/Matrimid MMM unannealed samples (cured at 60 °C). Representative examples of the polymer-coated ZIF-8 nanoparticles are in the marked white circles.

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Figure S2. SEM cross-sectional microstructures of the 20 wt.% ZIF-8/Matrimid MMM unannealed sample (cured at 60 °C). Note the polymer-encapsulated ZIF-8 nanoparticles. Representative examples of the polymer-coated ZIF-8 nanoparticles are in the marked white circles.

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Figure S3. A model representing the interaction between ZIF-8 nanoparticles and Matrimid®, with a) coated ZIF-8 nanoparticles, which forms the majority of the interaction in the membrane, b) unattached ZIF-8 nanoparticles that are free within the Matrimid® matrix, and c) Matrimid®, on its own, not coating any ZIF-8 nanoparticles or have any free ZIF-8 particles attached to its surface. Both constituents are held together by hydrogen bonding between ZIF-8 (methyl-imidazole linkers) and Matrimid® (carbonyl and hydroxyl groups). Therefore there will be free volume (microscopic gap) between ZIF-8 and Matrimid®, which contributes to the formation of pores within the structure; although due to the relatively lower loadings of the nanoparticles, these pores or free volume have negligible influence upon the macroscale mechanical interaction of the membranes. However, at a localised, micro level, its influence is rather significant, as seen in the results from nanoindentation.

 

 

 

 

 

 

 

voids/pores

Coated ZIF8

Free ZIF8

Unbounded Matrimid®

Tensile

Tensile

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Figure S4. The X-Ray Diffraction (XRD) diffractograms of the a) unannealed and b) annealed ZIF-8/Matrimid® nanocomposite MMM, from which it was determined that crystallinity is directly proportional to both ZIF-8 nanoparticle loadings and annealing (see Fig. S5). Further evacuation of entrapped solvents amongst polyimide chains and inside ZIF-8 pores further substantiates the level of crystallinity (XRD intensity).

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Figure S5. The relationship between crystallinity and ZIF-8 nanoparticle loadings derived from XRD diffractograms (Fig. S4). The increase in crystallinity was significant from Matrimid® (i.e. 0 wt.%) to 10 wt.% ZIF-8, which kept increasing as wt.% is increased. Annealing does increase the crystallinity of the membranes, although not by a large margin.

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Figure S6. Nanoindentation load-displacement (representative) plots for Matrimid® and the unannealed and annealed nanocomposite MMMs, with a) Matrimid®, b) 10 wt.% ZIF-8, c) 20 wt.% ZIF-8, d) 30 wt.% ZIF-8, while e) Matrimid® (annealed), f) 10 wt.% ZIF-8 (annealed), g) 20 wt.% ZIF-8 (annealed), and h) 30 wt.% ZIF-8 (annealed). Note the relatively higher scatter as wt.% rises associated with greater local membrane inhomogeneity.

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Figure S7. Nanoindentation averaged Young’s Modulus (E) plots as a function of surface penetration depths (0 to 2,000 nm) for Matrimid® and the unannealed and annealed nanocomposite MMMs, with a) Matrimid®, b) 10 wt.% ZIF-8, c) 20 wt.% ZIF-8, and d) 30 wt.% ZIF-8. Data from 1,000 to 2,000 nm were used to derive the averaged values reported in manuscript. The larger scatter at below 500 nm can be linked to imperfect tip-to-surface contacts due to roughness (asperities).

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Figure S8. Nanoindentation averaged nanohardness (H) plots as a function of surface penetration depths for Matrimid® and the unannealed and annealed nanocomposite MMMs, with a) Matrimid®, b) 10 wt.% ZIF-8, c) 20 wt.% ZIF-8, and d) 30 wt.% ZIF-8. Data from 1,000 to 2,000 nm were used to derive the averaged values reported in manuscript. The larger scatter at below 500 nm can be linked to imperfect tip-to-surface contacts due to roughness (asperities).

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Figure S9. Additional data from the DMA multi-strain frequency sweep tests, classified as per its respective test frequencies for the unannealed Matrimid/ZIF-8 nanocomposite MMMs, with a) Storage Modulus, E’ (5 Hz), b) Loss Modulus, E” (5 Hz), c) Tan δ (5 Hz), d) Storage Modulus, E’ (15 Hz), e) Loss Modulus, E” (15 Hz), f) Tan δ (15 Hz), g) Storage Modulus, E’ (25 Hz), h) Loss Modulus, E” (25 Hz), and i) Tan δ (25 Hz).

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Figure S10. Additional data from the DMA multi-strain frequency sweep tests, classified as per its respective test frequencies for the annealed Matrimid/ZIF-8 nanocomposite MMMs, with a) Storage Modulus, E’ (5 Hz), b) Loss Modulus, E” (5 Hz), c) Tan δ (5 Hz), d) Storage Modulus, E’ (15 Hz), e) Loss Modulus, E” (15 Hz), f) Tan δ (15 Hz), g) Storage Modulus, E’ (25 Hz), h) Loss Modulus, E” (25 Hz), and i) Tan δ (25 Hz).

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Figure S11. A plot correlating the glass transition temperature (Tg) and ZIF-8 nanoparticle loadings (wt.%). Overall, it can be seen that the addition of ZIF-8 nanoparticles into Matrimid® does little to alter the Tg of the resulting nanocomposite MMM, where the fluctuation of Tg is less than 5% for both the unannealed and annealed nanocomposites. This implies that near the Tg, the phase change/mechanical/thermal response is primarily dominated by the Matrimid® matrix for both cases. Here the introduction of ZIF-8 fillers did not results in chain rigidification or other physical changes to the matrix, therefore the composite Tg will remain close to the Tg of the original matrix [1].

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Figure S12. Time-temperature-superposition (TTS) plots of the loss modulus (E”) constructed using Tref = Tg (345 °C), for the (a) unannealed and (b) annealed ZIF-8/Matrimid® nanocomposite MMMs.

 

 

 

 

 

 

 

 

 

 

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Figure S13. Compilation of the TGA plots of the unannealed and annealed ZIF-8/Matrimid® nanocomposite membranes, with a) unannealed, and b) annealed membranes. There is a clear distinct drop in the wt. % near 320 oC for the unannealed membranes, which is attributed to the occluded solvent during the glassy-to-rubbery phase change of the membranes. This slight decrease in the wt. % is absent from the annealed membranes, with decomposition initiated at ~450 oC, and it can be assumed from this that the occluded solvent is completely evacuated in the annealed membranes.

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Figure S14. Comparison of individual unannealed and annealed membranes’ TGA plots, with a) Matrimid®, b) 10 wt.%, c) 20 wt.%, and d) 30 wt.%. As mentioned previously, the wt. % decrease in the unannealed membranes is attributed to occluded solvents, corresponding to the ζ-phase seen in Fig. 7(c). Increased ZIF-8 nanoparticle loadings also increased the wt. % of the occluded solvents being removed from the membranes, suggesting that solvents might not only be occluded in Matrimid®, but in the pores of ZIF-8 as well.

The precise amount of the occluded solvent (in this case, CHCl3) is less than 7 wt.% of the membranes. For the unannealed samples, the loss of occluded solvent for Matrimid® was 5 wt. %; 10 wt.% sample was 6 wt.%; 20 wt.% sample was 6 wt.%; and 30 wt.% sample was 10 wt.%, averaging out to 6.75 wt.% of total occluded solvent in the unannealed membranes. This value (~6.75 wt.%) can be extrapolated across all unannealed membrane samples, regardless of ZIF-8 nanoparticle wt.%, as a way to gauge the amount of solvents that are still present within the membranes. For example, in a 100 g unannealed membrane, it can be surmised that there is ~6.75 g of CHCl3 in it. For Matrimid® on its own (unannealed), a total of 4 samples were tested to confirm the presence of occluded solvents (CHCl3), while for the other samples, two samples, taken from random parts of the membranes, were tested. The reason only two samples were tested is because the results are identical, which makes testing or confirmation redundant.

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Characterisation of ZIF-8 nanoparticle size from electron micrographs

(a)

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(b)

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(c)

Figure S15. Measured individual particles from the FEG-SEM images for ZIF-8 nanoparticles dispersed Chloroform (CHCl3), with 20 particles being measured in each image and averaged to a total of 60 particles from 3 different batches of synthesised ZIF-8. We established the averaged particle size is 147.26 ± 7.05 nm. (Note scale bar for middle image is 1 µm thus individual particles appear relatively bigger in this micrograph)

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Membrane thickness determination

Table S1: Compilation of the thicknesses of the membranes used for DMA and uniaxial tensile test (DIC) analyses

Samples DMA DIC S1 (µm) S2(µm) S1(µm) S2(µm) S3(µm) S4(µm) S5(µm)

Matrimid 145 155 160 150 159 159 170 10 wt.% 148 165 160 170 168 140 180 20 wt.% 145 150 138 125 120 120 130 30 wt.% 160 160 171 161 155 125 158

Matr._ann 154 145 135 135 160 162 160 10

wt.%_ann 145 137 140 135 150 132 140

20 wt.%_ann

135 137 145 130 145 142 140

30 wt.%_ann

139 139 162 162 145 145 130

Fig. S16 The thickness of the samples being tested, with (a) an average from 2 test coupons for DMA analysis, and (b) an average of 5 test coupons used for the uniaxial tensile test, with the results of each test described in the manuscript.

We produced a total of two full membranes per cycle, at multiple cycles with varying

thickness from 50-200 µm (intervals of 50 µm) to determine the suitable casting thickness for our

membrane. We concluded that 150 µm is the most suitable casting thickness due to the relative

stability of sample thickness, as sample integrity and decrease of thickness became issues at lower (50

µm) and higher (200 µm) casting thicknesses, respectively. We casted a total of two full membranes

at a casting thickness of 150 µm, and used these membranes for our subsequent tests (DMA, uniaxial

tensile test, nanoindentation). The samples being used for these analyses were taken from random

parts (top, bottom, sides, middle) of the membranes to be more representative of the membranes as a

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whole, and another batch of samples were annealed at 180 oC to produce the annealed membranes,

which helps guarantee complete and uniform annealing of the entire sample prior to the analyses. Fig.

S16 presents the thicknesses of the samples that were used for DMA and uniaxial tensile tests. Fig.

S16(a) is an average thickness taken from an average of two test coupons (DMA), while Fig. S16(b)

shows the average thickness of 5 test coupons that were used for the uniaxial tensile tests. All in all,

the thicknesses measured are consistent, and taking into account the standard deviation, it can be

concluded that the average thickness of the membranes falls within 145.14 ± 6.98 µm.

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References

[1] S. Sorribas, B. Zornoza, C. Téllez, J. Coronas, Mixed matrix membranes comprising silica-(ZIF-8) core–shell spheres with ordered meso–microporosity for natural- and bio-gas upgrading, J. Membr. Sci., 452 (2014) 184-192.