Effect of the Inorganic Filler Contents on Polymer

Effect of the Inorganic Filler Contents on Polymer ANALYSIS OF ZIF 8/PAI AND CMS/PAI MEMBRANES FOR CO2/CH4 GAS SEPARATION Yohannan Subin Sabilon Department of Chemical Engineering, National Institute of Technology Tiruchirappalli, India Zeolitic Imidazolate Frameworks 8 (ZIF 8) nanocrystals and Carbon Molecular Sieves (CMS) particles were prepared by using standard procedures. UV visible spectroscopy and XRD tests were done for the confirmation of the particles prepared and scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) analysis were done to study the morphology of the particles prepared. ZIF 8/PAI and CMS/PAI MMMs were successfully synthesized by using ZIF 8 and CMS inorganic fillers and Polyamideimide (PAI) polymer using phase inversion technique. Various weight contents (1%, 2% and 3%) of the inorganic fillers were incorporated in the polymer matrix. Reinforcing of the polymer matrix with inorganic fillers was done in the form of nano and micro particles respectively. The effect of the inorganic filler contents on the mechanical properties of the polymer was investigated. Hydrophilic nature and porosity determination test, Fourier transform infrared spectroscopy (F TIR) and SEM were done to study the hydrophilicity and morphology of the composite system. Keywords: Carbon dioxide, Methane, Mixed Matrix Membranes, Carbon Molecular Sieves,Zeolitic Imidazolate Frameworks INTRODUCTION Carbon dioxide (CO2) is one of the components of landfill gas, natural gas and biogas. It is also the main combustion product of fossil fuels and a leading contributor to global warming as its a greenhouse gas. In order to obtain fuel with enhanced energy content, to prevent corrosion problems in the gas transportation system and to reduce the climatic impact of CO2 gas it is quite essential to remove CO2 from those gas streams. This has driven the development of different technologies for CO2 gas separation. Among the different types of technologies being used membrane technology has experienced substantial growth, breakthroughs and advances during past decades [10]. Membrane technology offers high energy efficiency, simplicity in design and construction of membrane modules and environment compatibility. Although there are different types of membranes being used the combination of the superior performance of inorganic materials with the handling properties of the polymers is offered by Mixed Matrix Membranes (MMMs). Therefore in our study we will be using MMMs for CO2/CH4 gas separation. In the MMMs the inorganic fillers are added to the polymer matrix. Over the years different inorganic fillers have been used for preparing MMMs for CO2/CH4 gas separation out of which Zeolitic Imidazolate Framework 8 (ZIF 8) is known to show maximum selectivity while Carbon Molecular Sieves (CMS) is known to show maximum permeability [19]. In this study the preparation and characterization of these inorganic fillers is shown. These inorganic fillers were successfully incorporated in the Polyamideimide (PAI) polymer matrix and MMMs were prepared. The characterization and analysis of the ZIF 8/PAI and CMS/PAI MMMs have been done with different loading of inorganic fillers in order to choose the best possible membrane combination for CO2/CH4 gas separation. EXPERIMENTAL SECTION Materials Zinc hydrate crystals and N-methyl 2-pyrrolidone (NMP) required in the preparation of ZIF 8 nanocrystals were purchased from Merck Life Science Private Limited, Mumbai, India. Methanol used for washing during centrifugation was also bought from Titan Biotech Limited, Rajasthan, India. 2-methylimidazole and n-butylamine also required for the preparation of ZIF 8 nanocrystals were bought from Otto Group Hamburg, Germany, Polyamidieimide polymer was also purchased from UTM, Malaysia. Acetone was purchased from Merck Specialities Private Limited, Mumbai, India. All reagents were used without any further purification. Synthesis of ZIF 8 nanocrystals ZIF 8 nanoparticles were synthesized based on the procedure reported by Cravillon et al[3]. The ZIF-8 nanocrystals so formed was sent for UV spectroscopy, XRD, HRTEM and SEM analysis. Synthesis of CMS particles CMS particles were synthesized based on the procedure reported by De. Q. Vu et al[8] The CMS particles were then sent for XRD analysis. Synthesis of ZIF 8/PAI membranes Membranes of 3 different concentrations i.e., 1%, 2% and 3% of ZIF 8 nanocrystals were prepared by solution casting method. 17wt% of polyamideimide polymer solution was prepared by dissolving exactly 5.274g mixture of polyamideimide polymer i.e., Torlon and ZIF 8 nanocrystals in 25ml of NMP solvent in a beaker. A magnetic bead was cleaned and dried using acetone and was placed in the beaker. The 3 beakers containing the 3 different concentration solutions were kept on 3 different magnetic stirrer for complete dissolution. The exact amount of polymer and inorganic filler taken for respective concentrations is given in the table below: Table 1 Composition of ZIF 8/PAI membranes Concentration of ZIF 8/PAI Amount of PAI (g) Amount of ZIF 8 membranes (wt %) nanocrystals (g) 1 5.116 0.158 2 5.169 0.105 3 5.221 0.053 Now 3 glass plates and casting rods were washed and kept for drying. After drying the glass plates and the casting rods were cleaned and dried by using acetone. After complete dissolution the polymer solution in the 3 beakers were casted on 3 different glass plates using casting rods of 750 ÂÂ µm thickness. The glass plates after casting were allowed to dry at room temperature overnight for all the NMP solvent to evaporate. After drying the polymer membrane so formed was peeled off the glass plate. The membrane samples were sent for SEM analysis. Synthesis of CMS/PAI membranes Membranes of 3 different concentrations i.e., 1%, 2% and 3% of CMS particles were prepared by solution casting method. 17wt% of polyamideimide polymer solution was prepared by dissolving exactly 5.274g mixture of polyamideimide polymer i.e., Torlon and CMS particles in 25ml of NMP solvent in a beaker. The exact amount of polymer and inorganic filler taken for respective concentrations is given in the table below: Table 2 Composition of CMS/PAI membranes Concentration of ZIF 8/PAI Amount of PAI (g) Amount of CMS particles membranes (wt %) (g) 1 5.116 0.158 2 5.169 0.105 3 5.221 0.053 A magnetic bead was cleaned and dried using acetone and was placed in the beaker. The 3 beakers containing the 3 different concentration solutions were kept on 3 different magnetic stirrer for complete dissolution. Now 3 glass plates and casting rods were washed and kept for drying. After drying the glass plates and the casting rods were cleaned and dried by using acetone. After complete dissolution the polymer solution in the 3 beakers were casted on 3 different glass plates using casting rods of 750 ÂÂ µm thickness. The glass plates after casting were allowed to dry at room temperature overnight for all the NMP solvent to evaporate. After drying the polymer membrane so formed was peeled off the glass plate. The membrane samples were sent for SEM analysis. TESTING AND CHARACTERIZATION Confirmation tests for inorganic filers UV visible spectroscopy analysis. The ultraviolet-visible spectroscopy (UV-Vis)utilizes light to determine the absorbance or transmission of a chemical species in either solid or aqueous state. The UV Visible Spectroscopy analysis was done for the confirmation of ZIF 8 nanocrystals. XRD analysis. XRD can be done on a number of different kinds of samples. Smallvolume of sample was tapped on microscope slide glass. The intensity of the beam used was 40 kV and 40 mA. The XRD analysis was done for the confirmation of ZIF 8 nanocrystals and CMS particles. Morphological studies of Inorganic fillers and MMMs SEM with EDX analysis. The surface morphology of PAI polymer was observed usingthe JSM-6701F with high resolution field-emission scanning electron microscopy (FE-SEM) with the magnification of 5000ÃÆ'-. For EDX analysis the acceleration voltage was set to 20kV and the working distance was set to 14mm. The detector was moved down to 45mm. The sample was scanned by X-rays for a time of 200s. The elemental analysis of film in order to confirm the presence of carbon was done using an energy dispersive X-ray spectrometer (EDX) with magnification of 3000ÃÆ'- and acceleration voltage of 15 kV. After the scan was completed the spectrum was plotted using the data obtained from the scan. SEM with EDX was done for the confirmation of the CMS polymer film. TEM analysis. The sample preparation was done by sputtering the same with gold.Then the chamber door was opened and the sample was placed in the sample holder. The chamber door was closed and the required input like voltage, acceleration and time for scan were given to the system connected to the TEM analyzer. The scan was started and the results were recorded. TEM analysis was done for the size determination of the ZIF 8 nanocrsytals. FTIR analysis. Fourier transform infrared spectroscopy (FTIR) is a technique whichis used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high spectral resolution data over a wide spectral range. Sulfonic acid group functionality of membrane was studied using attenuated -total-reflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Thermo scientific Nicolet iS5 FTIR spectrometer). The spectra for all dried membranes were observed from the range from 4000 to 400 cm-1 wavelength. Mechanical strength test The material strength of the membranes prepared were studied by the performing Stress-Strain tests. The Universal Testing Machine was used to perform the tests. The samples of the membranes were cut into dimensions of height 30mm, width 10mm and thickness 0.45mm. The initial gauge length was set at 20mm. The samples were placed in a sample holder one at a time and the tests were performed. The data was recorded and the graphs were plotted for respective samples. Hydrophilic nature and Porosity determination test The hydrophilic or hydrophobic nature of the membranes were studied by immersing a 1cmx1cm membrane samples in different beakers each containing 20ml water. The beakers were kept on a rotary shaker for continuous mixing overnight. After 24 hours the membrane samples were taken out and the weight of the wet membranes were measured using a digital weighing balance. After that the membranes samples were dried in a vacuum oven at 60oC for 6 hours and then the weight of the dry membranes were measured similarly. The amount of water absorbed and the average porosity of the membranes were determined and the results were tabulated. RESULTS AND DISCUSSION Confirmation of ZIF 8 Nanocrystals The UV Visible Spectroscopy analysis was done for the primary confirmation of ZIF 8 nanocrystals. UV of ZIF 8 nanocrystals 12 10 Absorbance 8 6 4 2 0 200 212 224 236 248 260 272 284 296 308 320 332 344 356 368 380 392 404 416 428 440 452 464 476 488 500 512 524 536 548 560 572 584 596 Wavelength Series1 Figure 1 UV visible spectroscopy result of ZIF 8 nanocrystals The penetration depth was found to be directly proportional to the exciting wavelength i.e., 325nm because of decreased absorbance which is in accordance with the reference paper, Liu et aL, (2013)[1]. Therefore we can confirm that its ZIF 8 nanocrystals. The XRD analysis was done for the secondary confirmation of ZIF 8 nanocrystals. Figure 2 XRD result of ZIF 8 nanocrystals When n-butylamine is added as the modulating ligand, nearly instantaneous formation of a solid is observed upon combining the component solutions, and pure-phase ZIF-8 nanocrystals are recovered after 24 h (see XRD pattern in Figure 2). An average size of 18 nm is estimated from the broadening of the Bragg reflections. The XRD results were also in accordance with the reference paper Cravillon et aL, (2011). Hence we can confirm that the particles synthesized were ZIF 8 nanocrystals. Morphology of ZIF 8 Nanocrystals ZIF materials constitute a new distinctive, rapidly developing subclass of crystalline porous coordination polymers (PCPs) or metal organic frameworks (MOFs). The tetrahedral framework structures of ZIFs are constructed from bivalent metal cations and bridging substituted imidazolate anions and frequently possess a zeolite topology. Numerous ZIFs combine the attractive features of MOFs (diversity of framework structures and pore systems, large surface areas, post-synthetically modifiable organic bridging ligands) with high thermal and chemical stability. Figure 3 SEM image of ZIF 8 nanocrystals It is this combination of properties which makes ZIFs very promising candidate materials for many technological applications. Properties and performance of porous materials rely much on their supply as nano and microcrystals of well-defined size and shape, as is well-known for zeolites. SEM images (Figure 3) reveal that the well-defined nanocrystals have a rhombic dodecahedral shape crystal structure. Figure 4 TEM image of ZIF 8 nanocrystals TEM images (Figure 4) show roughly spherical particles being Confirmation of CMS Particles It is not possible to directly measure permeation properties of CMS particles as with CMS films, replicate mixed matrix films prepared with the two different sieves give very similar effective mixed matrix film permeation properties using powder-pyrolyzed sieves versus the film-pyrolyzed sieves. XRD was performed on the CMS films and powder, as shown in Fig. 5. The XRD diffractograms show very similar peaks and d-spacings, suggesting similar planar dimensions between the film-pyrolyzed CMS and the powder-pyrolyzed CMS, thereby confirming that the particles produced were CMS particles. CMS particles Polymer film Figure 5 XRD results of CMS particles and CMS polymer film Surface Morphology of CMS Polymer Film The CMS membrane films have an intrinsic CO2/CH4 selectivity of 200 with a CO2 permeability of 44 Barrers at 35oC. For UltemÂÂ ®-CMS mixed matrix membrane films, pure gas permeation tests show enhancements by as much as 40% in CO2/CH4 selectivity over the intrinsic CO2/CH4 selectivity of the pure UltemÂÂ ® polymer matrix. Likewise, for MatrimidÂÂ ®- CMS mixed matrix films, enhancements by as much as 45% in CO2/CH4 selectivity were observed. Effective permeabilities of the fast-gas penetrants (O2 and CO2) through the mixed matrix membranes were also significantly enhanced over the intrinsic permeabilities of the UltemÂÂ ® and MatrimidÂÂ ® polymer matrices. These encouraging selectivity and permeability enhancements confirm that mixed matrix membrane behaviour is achievable with CMS particles. Figure 6 SEM image of CMS polymer film Fig. 6 shows top surface SEM micrographs of a CMS polymer film. These micrographs show a smooth surface without any defects. Figure 7 EDX result of CMS polymer film The table below shows the EDX analysis of the CMS polymer film. The sharp Silicon peak is present due to the Silicon detector used during the EDX analysis. Table 3 EDX result of CMS polymer film Element Series unn. C norm. C Atom. C Error (3 [wt.%] [wt.%] [at.%] Sigma) [wt.%] Carbon K-series 8.50 23.61 36.45 4.40 Oxygen K-series 9.89 27.46 31.82 4.26 Sodium K-series 1.16 3.22 2.60 0.31 Aluminium K-series 4.56 12.67 8.70 0.74 Silicon K-series 9.37 26.03 17.19 1.28 Calcium K-series 2.52 7.01 3.24 0.31 Total: 36.01 100.00 100.00 The Oxygen peak is due to the oxygen present in the atmosphere during EDX analysis. The Carbon peak denotes the confirmation of the CMS polymer film prepared. As expected it shows a maximum wt % of 23.61. Cross Sectional Morphology of CMS/PAI Membranes Scanning electron micrographs of the CMS fibers are shown in figures 8, 9 and 10 Figure 4.8 SEM image of 1% CMS/PAI membrane Although asymmetry appeared to be present in the CMS fiber morphology, the thicknesses of the layers were markedly different from each other and from those of the precursor fibers (compare with those of the precursor fibers in Figure 6). The original polymeric precursor fibers consisted of a very thin dense skin (1000-2000 Ã…) on a porous core. This skin layer in polymeric fibers has been observed at very high resolution under SEM. In figure 8, high magnification of the wall in the cross section of the PAI CMS fiber reveals a gradual transition from the porous inner core to the denser outer micropore structure. In contrast, high magnification of the PAI CMS fiber shows a uniform dense micropore structure in figure 9. Figure 9 SEM image of 2% CMS/PAI membrane Figures 8, 9 and 10 show SEM micrographs of a mixed matrix film after these modifications. These micrographs demonstrate smaller CMS particles (mostly Figure 10 SEM image of 3% CMS/PAI membrane Cross Sectional Morphology of ZIF 8/PAI Membranes Figures 11, 12 and 13 shows SEM images of ZIF-8/PAI mixed matrix dense films, which indicates good contact of bare ZIF-8 to the PAI matrix without sieve-in-a-cage morphology at each loading. It is noteworthy that the good contact was achieved without any surface treatment of the sieve. This should be due to the hydrophobic nature of ZIF-8, proved by TGA measurements in reference paper Zhang et. al. (2012). Interestingly, in the SEM images of ZIF-8/PAI mixed matrix dense films, as shown in figures 11, 12 and 13, we observe a morphology that has not been previously reported in mixed matrix membranes prepared with other molecular sieves. Other than well-dispersed 10 nm ZIF-8 particles, there also exist many non-ideal large clusters of ZIF-8 with size ranging from 50 nm to several microns, which is more than an order of magnitude larger than single ZIF-8 particles. Also, volume fraction of large ZIF-8 clusters in the matrix increases with increasing ZIF-8 loading. Figure 11 SEM image of 1% ZIF 8/PAI membrane Unlike agglomerations of molecular sieve particles that have been previously reported in mixed matrix membranes prepared with other molecular sieves, the surface of these large ZIF-8 clusters as revealed in figures 11, 12 and 13 looks fairly smooth. Also, almost no defects were observed for these clusters among all the ZIF-8/PAI dense film samples. Since film samples were randomly fractured for SEM analysis, we believe that the mostly non-defective feature of these large ZIF-8 clusters shown in figures 11, 12 and 13 is representative of their interior structures. It is important to understand the formation mechanism of these large ZIF-8 clusters and their impacts on gas transport properties of the mixed matrix membrane to allow extension to practical asymmetric structures. By achieving the desired uniform distribution of individual ZIF-8 particles with the PAI matrix we can achieve outstanding gas separation results. Figure 12 SEM image of 2% ZIF 8/PAI membrane Figure 13 SEM image of 3% ZIF 8/PAI membrane The cross sectional view of the ZIF 8/PAI membranes shows good adhesion between the inorganic filler ZIF 8 and the polymeric membrane PAI. The figures show the SEM images of 1%, 2% and 3% ZIF 8/PAI membranes prepared respectively. FTIR Analysis of ZIF 8/PAI membranes The FTIR results shows that the aluminosilicates are present in the ZIF 8/PAI membranes prepared. The aluminosilicates are present due to the presence of ZIF 8 nanocrystals. FTIR Results Conjugated cyclic Aluminosilicates 120 100 %T 80 Unsaturated aromatic 60 40 20 ketoaldehydes or enols dimer esters and lactones 0 3691 2970 4000 3897 3794 3588 3485 3382 3279 3176 3073 2867 2764 2661 2558 2455 2352 2249 2146 2043 1940 1837 1734 1631 1528 1425 1322 1219 1116 1013 910 807 704 601 498 cm-1 Series1 Series2 Series3 FTIR Analysis of CMS/PAI membranes The FTIR results shows that the carbon bonds are present in the CMS/PAI membranes prepared. The carbon bonds are present due to the presence of CMS particles. FTIR Results Carbon bonds 120 100 %T 80 60 unsaturated aromatic 40 dimer ketoaldehydes or enols 20 0 Conjugated cyclic esters and lactones 3691 2970 4000 3897 3794 3588 3485 3382 3279 3176 3073 2867 2764 2661 2558 2455 2352 2249 2146 2043 1940 1837 1734 1631 1528 1425 1322 1219 1116