Influence of PVDF / Pebax TFC Casting Temperature towards CO 2 / N 2 Gas Separation

Objectives: A Thin Film Composite (TFC) membranes has been successfully synthesized via introducing casting temperature adjustment to counter the permeability-selectivity conflict. Methods: 15 wt% Polyvinylidene Fluoride (PVDF) was diluted in 85 wt% N-Methyl-2-Pyrrolidone (NMP) and cast on a glass plate at 40°C-80°C. To form the TFC, 3 wt% of Polyether block amides (Pebax 1657) was used as a coating layer on top of the PVDF substrate. Findings: The surface roughness of the samples cast at 60°C was measured to be 102.6 nm while the other is less than 100 nm and the CO2 and N2 flux was measured to be 749 and 13 barrer respectively with CO2/N2 selectivity of 58.54. Application: The result has been plot on Robeson 1991 and it fall beyond the upper boundaries, indicate that the future of coating material and casting techniques can be further explored to maximize the limit of material capabilities. Influence of PVDF/Pebax TFC Casting Temperature towards CO2/N2 Gas Separation M. S. Abdul Wahab and A. R. Sunarti* Faculty of Chemical Engineering and Natural Resources, Universiti Malaysia Pahang, Gambang 26300, Pahang, Malaysia; sunarti@ump.edu.my


Introduction
Polymeric membrane system possesses several criteria including low capital cost, low energy requirement, high mechanical resistance, energy efficient and ease of operation over conventional separation techniques such as cryogenic distillation and pressure swing adsorption 1 .However, every good in polymeric membranes come with it prices when there is a slight weakness in them which discover for ages and popular said as a trade-off between permeability and selectivity by Robeson's upper bound curves.Figure 1 shown in the upper bound that was plot in 1991 and 2008 where Robeson analysed all available permeation data of polymeric membranes for CO 2 /N 2 gas pairs and plotted each membrane's permeability against selectivity for the gas 2 .
From the plot, a visible scattered data revealed that there is an upper bound which is the limit of gas polymeric membranes performance for the selectivity-permeability relationship, such that the selectivity decreases linearly with increasing permeability.Since then, the development of polymeric membrane materials that can surpass Robeson's upper bound has been the central topic in polymeric membrane development for gas separation.
Recently, work on membranes matrix modification to enhance the separation efficiency by having several polymeric materials combines with an inorganic porous substance such as Zeolite, Carbon Nano Tube (CNT) and Zeolitic Imidazolate Framework (ZIF) or often called Mixed Matrix Membranes (MMM) and Thin Film Composite (TFC)-multiple layer combination, show a significant improvement, but still have a room for an improvement.
The focus of this work is to study how the surface roughness as a result of different casting temperature influence the separation efficiency of the PVDF/Pebax 1657 TFC and did the involve parameter manage to surpass the Robeson trade-off limits.Few works on casting techniques which involve temperature iteration has shown a significant improvement towards the membranes efficiency but none are specific for CO 2 /N 2 Separation.In 3 his work reported that membranes cast at 100°C perform better than that cast at low temperature due to having strong mechanical stability.But, by increasing the casting temperature, the membranes might turn out to be brittle.To encounter with the high and low temperature study, Pebax 1657 is chose as a selective layer because of its good CO 2 adsorbent, flexible and high mechanical strength due to nylon 6 contents in the polymer matrix 4 .

Materials and Methods
Pallet of analytical grade Polyvinylidene Fluoride (PVDF) was purchased from Sigma Aldrich.Pebax 1657 (60% Polyether and 40% polyamide) was purchased from Arkema France.Two solvent used in this study which is ethanol and N-Methyl-2-pyrrolidone for dope preparation was supplied by Fisher Scientific.There are two gases used, 99.99% purified CO 2 and N 2 which will be used for TFC performance study was supplied by Azam Synergy.
Preparation of PVDF substrate was started by diluting 15 wt% PVDF pellet in 85% NMP and uniformly stirred at 80°C for about 7 hours until all the pellets dissolved.The PVDF dope solution was then degassed by leaving at room temperature for 24 hours to achieve bubbles free solution.The solution was cast on a glass plate at room temperature, 40°C, 60°C and 80°C by using automatic casting machine with an adjustable temperature control with thickness of 0.03 mm before submerging in a water bath for 2 hours and then dried at room temperature for 24 hours.3 wt% of Pebax 1657 was dissolved in the combination of 70/30 ethanol/ water at 80 °C for 2 hours until a homogeneous, clear and dilute solution was obtained.This Pebax 1657 coating solution was left at room temperature for 24 hours before it was coated on the PVDF substrate.The dip coating method was used where 5 ml of the Pebax 1657 solution was poured into a glass dish only to damp the dish surface then PVDF substrate was dip one side for 5 seconds.The film dimension was fixed to be 5 cm diameter.Pebax 1657 was coated on the PVDF substrate by 3 times, where every coating the TFC was dried at 60 °C for 15 minutes to let the solvent vaporize.For extra vaporization of the solvent, TFC was further dried at 60 °C for another 24 hours.

Characterization
The TFC was characterized chemically and physically to detect the presence of each functional group of molecule in the sample.Fourier transform infrared spectroscopy, FTIR Perkin Elmer-Spectrum 100, which allow the samples to absorb all different wavelength characteristic produced from a beam that later will be computerized to obtain the desired data in the form of wave numbers, cm -1 .The cross-sectional area of the sample was examined by Scanning electron microscope, SEMJSM-7800F.All samples were freeze and fractured in liquid nitrogen to get the smooth cutting without any part of the membrane blocking the cutting surface for SEM analysis.The sample was then flush with nickel coating to avoid any charge build up by the electron beam which often called space charge effect during surface imaging.The surface topography of the TFC was examined by atomic force microscopy, AFM JPK-NW-Nano-opticsfor any visible peak of roughness due to temperature exposure during film casting.

Single Gas Permeation Test
Each gas CO 2 and N 2 was tested individually by allowing them to flow at 2 bar toward the membrane cell and the flow rate of the permeate was measured by a bubble flow meter.Equation (1) was used to calculated the gas permeability and it was expressed in Barrer by taking the conversion factor into account [1 Barrer = 1 x 10 -10 cm 3 (STP) cm/(cm 2 scmHg)]. ( Where P is the permeability, l is the thickness of the membrane in cm, A is the effective membrane area in cm 2 , V is the volume in cm 3 displaced in time t(s) and ∆p is the trans-membrane pressure expressed in cmHg.The membrane selectivity is the membrane ability to separate the two gases (A and B).It is the ratio of the Permeance A and B -Equation (2).The TFC used have a diameter of 5 cm with 3 times data replication for each sample.

FTIR Characterization
The chemical formula and the spectra of PVDF, Pebax 1657 and PVDF/Pebax 1657 TFC was showed in  (C-O) and nylon-6 with stretching of N-H at 1000-1300 cm -1 and 3278 cm -1 respectively 6 .The amide segment of the elastomer was transmitted with the stretching of C=O at 1640 cm -17 .Other peaks in the spectrum of the composite membrane are in range of 2869-2939 cm -1 can be attributed to the asymmetric and symmetric stretching of the C-H bond 7 .The spectra proved that no additional chemical interfere in the TFC development and no chemical disappear during solvent vaporization and heating procedure.

Surface Morphology
Figure 4 shows the cross-sectional view of the TFC.The membranes exhibit an asymmetric structure with a top side of Pebax 1657 layer, followed with a finger-like macro voids region and a spongy porous bottom.The presence of finger-like structure was believed as an effect from the rapid liquid-liquid demixing during the wet phase inversion in the water bath.The void occurs when the solvent rapidly escaped from the top part of the film and leave the large and long finger-like structure.As we all know, porosity and void might influence the membrane flux 8 .The film was left for 2 hours in the water to let the rest of the solvent slowly come out from the bottom and the spongy porous might be the result from the instant solidify of the top side and the slow pace of solvent demixing 9 .From the cross-sectional view, it is observed that changes in casting temperature have not influenced the pore and void formation of the film.

Surface Roughness
The used of atomic force microscopy is to view any visible changes of surface roughness of the PVDF/Pebax 1657 TFC as a result of the different substrate casting temperature.Both plane and three-dimensional view of the surface topography was shown in Figure 5 and its respective average roughness parameter, RA and the root mean square, RMS of each sample tabulated in Table 1.Based on the image, the samples which cast at 60°C is the best among all with a rougher surface and a small bump compared to the nanoscale roughness on the other samples.The dark region of the image attribute to the valley and the bright white is the rough peak.The change in temperature might disturb the arrangement of the polymeric chain during dry/wet phase inversion of the film and from the lamellar

Separation Performance Study
The separation performance of the TFC has been studied via single gas permeance test and the result is tabulated in Table 2.For all samples, CO 2 permeability record a higher figure compared to N 2 as normal gas permeability always follow the kinetic diameter of the gas which CO 2 <N 2 , 3.33 Å and 3.68 Å respectively.Even though gas permeability decreases with increasing of casting temperature but still a significant improvement was observed for the gas selectivity.The data agree with the surface roughness of the samples which the roughness order from smooth to rough is room>40>80>60.The change in casting temperature has disturbed the polymeric chain and result in surface topography of the film.
PVDF/Pebax 1657 is a TFC with a combination of both dense and porous membranes, so the gas transport of the membranes follows a three steps transport behaviour which is the feed side penetrant sorption, penetrant diffusion through the film, and the desorption of the permeate 11 .The rough surface of the TFC provides large adsorption site compared to the smooth one and selectively transport the gas molecules.By having such nanoscale rough surface, the film possibly has a less free volume compared to a rough surface of the one cast at 60°C, so the ability to transport and separate the gas molecules decrease.To answer the objective of the work, the data has been plot on a Robeson curve as in Figure 6.The samples which the PVDF substrate cast at 60°C manage to surpass the trade-off limits with CO 2 permeability and gas selectivity of 749 barrer and 58.54 respectively.As discussed before, the top-notch performance is a result of the surface topography of the film.

Conclusion
PVDF/Pebax 1657 TFC was successfully fabricated by dry/wet phase inversion technique.The variations of casting temperature has influence the surface roughness of the film and more or less play a big role towards gas separation.The suitable temperature condition was found to be at 60°C when the gas selectivity able to cross the Robeson trade-off limits.It can be concluded that, casting temperature is one of the factors to be consider for membranes development as the exposure of the film during solvent vaporization could dramatically increase the film roughness and further increase the separation efficiency.The novelty of this work is on the casting techniques itself where many researchers' disregards that exposing

Figure 1 .
Figure 1.Robeson upper bound of gas selectivity vs permeability Figure 2 and 3 respectively.(a) The TFC cast at room temperature, (b) 40 o C, (c) 60 o C and (d) 80 o C exhibit the combination of both PVDF and Pebax 1657 wave numbers.CH 2 deformation of PVDF and Pebax 1657 molecules for all samples was detected as strong bending vibration in range of 1350-1470 cm -1 .In 5 the strong stretching vibration of C-F bonding of the dual fluoride found in PVDF was spot around 1000-1400 cm -1 .Pebax 1657 was detected through several molecules bonding starting with ether stretching

Figure 6 .
Figure 6.Robeson's plot of the data

Table 1 .
The roughness parameters of the cast film

Table 2 .
Gas permeability of each of the samples