Carbon Membranes from Polyamideimide and Polyetherimide
CARBON MEMBRANES FROM POLYAMIDEIMIDE AND POLYETHERIMIDE FOR NITROGEN AND METHANE SEPARATION AND ITS PARAMETER STUDY
ABSTRACT
Carbon membranes prepared from polyamideimide and polyetherimide were studied to find out the effects of different parameters on membrane characteristics. Their properties were analyzed to understand its scope in methane purification. Different compositions were tried to find out the optimum polymer composition as well as the optimum blend composition. They were pyrolyzed at various temperatures to study the effect of pyrolysis temperature on the morphology of the membrane. Efforts have been made for obtaining a narrow pore distribution as nitrogen and methane have comparable sizes. Analysis of the microstructure of membranes using XRD confirms the variations in chain packing density and d-spacing of polymer chains as a result of a change in pyrolysis temperature. CHN analysis revealed the percentage composition of different elements in the membrane as it was important to control amount of oxygen in the membrane. Glass transition temperature is found using DSC to confirm perfect blending of polymers. TGA has been done to find out how the polymer composition affects degradation temperature and to study the chemical changes occurring during pyrolysis. SEM images, both cross section, and surface have been taken to analyze pore structure of the membrane.
Keywords: Polyetherimide, Polyamideimide, Pyrolysis, XRD, SEM, CHN.
INTRODUCTION
Membranes have been widely received as a promising technology in gas separation processes. This is due to its high reliability, low capital investment and low energy usage which overshadow conventional gas separation techniques such as cryogenic distillation, liquid absorption, pressure swing adsorption etc. These established conventional techniques are known for their complexity in processes and high energy intensity. For this reason, researchers are making efforts for an alternative way that promises to give high-cost effectiveness and easy handling (1).
Polymer membranes have been widely studied for their application in gas separating units, but they always faced drawbacks like low thermal and chemical stability. Henceforth studies have been focused on carbon membranes, which are the novel and advanced type of membranes. They have been giving promising results ever since and marked a new benchmark for the selectivity of gasses. Excellent ability to withstand high temperature and chemicals made carbon membranes a new branch of study for the researchers to follow. (2)
Carbon membranes are prepared by pyrolysing polymer membranes. Pyrolysis is the most significant step in the preparation of carbon membrane. There are different pyrolysis factors that affect membrane properties. During pyrolysis the parameters like pyrolysis temperature, heating rate, atmospheric condition, soaking time etc. can influence the transport properties of the membrane. These factors are chosen wisely based on the application of the membrane. (3)
Material selection is the prime factor concerning in preparation of carbon membrane. There are wide varieties of precursor polymers available for preparation of carbon membrane (4). Selection of polymer is the important factor as pyrolysis of different polymer gives carbon membrane with entirely different properties. The precursor polymer should satisfy minimum criteria such as it should not soften nor liquefies during pyrolysis. It should be able to withstand high temperature (5).
Polyamidimides and polyetherimides are being recently studied and are found to be giving satisfactory results due to its high melting point and thermal stability (6).
Fuertes and Centeno used polyetherimide to prepare supported carbon membrane for gas separation (7). They conducted gas permeability studies for individual gasses as well as for many mixtures of gasses. They also used phenolic resins for their studies (8).
Different polymers are always blended to effectively combine the desirable properties of the blending polymers. Gas separation properties of carbon membranes are enhanced while combining different materials. New studies are going on investigating the blending techniques for an optimized result (9).
Pore size control is one of the most challenging factors in carbon membrane preparation. Nitrogen and methane is having a kinetic diameter of 3.6°A and 3.8°A respectively (10). So in order to separate them by molecular sieving very narrow pore distribution has to be obtained. To get very specific pore size, pyrolysis temperature was changed accordingly.
Two types of polymers were chosen so as to understand the effect of individual polymer on membrane properties. Membrane parameters such as polymer concentration, pyrolysis temperature were varied to understand the morphological as well as chemical changes taking place in the membrane. Comparison of carbon membrane with polymer membrane was carried out, which showed interesting results that can be further used to study gas permeation properties of the membranes. Results revealed that these membranes can be used for the separation process of nitrogen from methane.
EXPERIMENTAL
Material
Polyamideimide (PAI) polymer and Polyetherimide (PEI) polymer which are required for the preparation of polymer membrane were purchased from UTM, Malaysia. N-methyl 2-pyrrolidone (NMP) was the solvent used and it was purchased from Merck Life Science Private Limited, Mumbai, India. Methanol used for washing was bought from Titan Biotech Limited, Rajasthan, India. Acetone was purchased from Merck Specialities Private Limited, Mumbai, India. All reagents were used without any further purification.
Polymer Membrane Preparation
Polymers that were chosen for preparation were polyamideimide and polyetherimide. The solvent used was N-Methyl-2- pyrolidone. The polymer concentration in NMP was varied from 2- 20 wt.% as shown in the Table 1. Different batches of polymer solution were prepared by dissolving the corresponding amount of polymer in 25ml NMP. Polyamideimide-polyetherimide ratio in the solution was varied as 25/75%, 50/50%, 75/25%, 0/100% and prepared different batches accordingly. For complete dissolution, the solution was kept for magnetic stirring for 3 hrs. The temperature was increased up to 80oC occasionally for 10 minutes, to avoid undissolved particles in the solution. Later the solution was sonicated in a sonication bath. The solution was then casted on a glass plate using a casting rod. The casting rode that was used has a dimension of 450μm. The glass plate was kept in the atmosphere for two days for evaporation of the solvent. Later it was kept inside a vacuum oven at 60oC for complete removal of NMP.
Table 1: Polymer Batches
|
Sample Code |
Total Polymer (%) |
PAI/PEI wt percentage (%) |
|
PM-01 |
2 |
25/75 |
|
PM-02 |
9 |
25/75 |
|
PM-03 |
12 |
25/75 |
|
PM-04 |
15 |
25/75 |
|
PM-05 |
18 |
25/75 |
|
PM-06 |
20 |
25/75 |
Carbon Membrane Preparation
Polymer membrane was cut into a rectangular piece and transferred into an alumina crucible. It was then kept inside a tubular furnace by VBCC for pyrolysis. It was a horizontal furnace with alumina tube. Nitrogen environment was chosen for pyrolysis. The flow rate of nitrogen was kept at 25ml/min. Polymer membranes with PAI/PEI composition of 25/75% was only further used for making carbon membrane as they showed good structural stability compared to other batches.
For pyrolysis different protocols were followed. The protocols are as shown below,
1) 50 to 250 oC at a ramp rate of 13.3 Co/min.
(2) 250 oC to (Tmax-15) oC at a ramp rate of 3.85 Co/min.
(3) (Tmax-15) oC to TmaxoC at a ramp rate of 0.25 Co/min.
(4) Soak at Tmax for 2 h. (11)
After pyrolysis, it was kept for natural cooling before taking out. The different batches of carbon membranes based on different pyrolysis protocol were prepared as given in the Table 2. Polymer membrane with 9% polymer concentration was the membrane used for studying different pyrolysis protocols.
Table 2: Carbon Membrane Batches
|
Sample Code |
Total Polymer (%) |
Pyrolysis Temperature(oC) |
|
CM-01 |
2 |
600 |
|
CM-600 |
9 |
600 |
|
CM-450 |
9 |
450 |
|
CM-800 |
9 |
800 |
|
CM-03 |
12 |
600 |
|
CM-04 |
15 |
600 |
|
CM-05 |
18 |
600 |
|
CM-06 |
20 |
600 |
Characterization of Synthesized Membranes
Various techniques were employed for the characterization of polymer as well as their derivative carbon membranes. Differential Scanning Calorimetry (DSC) was used to find out glass transition temperature of polymer membranes and to study the blending of polymers. Under nitrogen atmosphere, scans from 50 to 450oC at the heating rate of 10oC/min were performed on a DSC Perkin Elmer model 6000.
Thermogravimetric analysis (TGA) was used to study the thermal degradation of the polymer membranes. Degradation temperature of the membrane, as well as the weight loss during the process, is analyzed. It was carried out on a TGA Perkin Elmer 4000 model, in nitrogen atmosphere at a flow rate of 30 mL min-1. The temperature range was from 50 up to 750 oC.
X-ray Diffractometer (XRD) was used to study the structural changes in membrane due to variation in pyrolysis temperature. Perkin Elmer 1621 wide-angle X-ray diffractometer was the instrument used to study the microstructural changes in membrane. Using Bragg’s law the average d-spacing of the membrane was evaluated. It is as shown below,
nλ = 2d sin θ, where n is an integral number, λ is the X-ray wavelength, d is for the inter-layer spacing between the polymer chains and θ is the diffraction angle.
CHN analysis was conducted to study the variation in the elemental composition with pyrolysis temperature. Elementar Vario EL was the equipment used here for analysis. Compostion of oxygen was found using separate apparatus.
Scanning Electron Microscopy (SEM) was used to get pore size of the membrane as well as get a closer image of the membrane. JEOL Model JSM – 6390LV is the model used for the analysis.
RESULTS AND DISCUSSION
Physical properties
Both PAI and PEI met the requirements for preparing carbon membrane with promising results. While contents of PAI in membrane made the membrane more brittle, which is due to its aromatic rings, (10) PEI gave structural support to the membrane. So PAI/PEI content was fixed at 25/75% for all the batches later on.
As two polymers are used, the miscibility of the precursors has to be tested and glass transition temperature of the membrane was taken as the criteria for the evaluation. For the polymer blends, Tg was found to be in between of that of individual polymers. Tg of PEI and PAI are 217°C and 280°C respectively. And from the Table 3 it is clear that both the polymers are completely miscible and was perfectly blended together.
Table 3: Glass Transition Temperature
|
Sample Code |
PAI/PEI wt Percentage (%) |
Glass Transition Temperature (oC) |
|
PM-25 |
25/75 |
233 |
|
PM-50 |
50/50 |
249 |
|
PM-75 |
75/25 |
265 |
|
PM-100 |
0/100 |
219 |
Effect of polymer concentration on thermal characteristics
Unlike polymer membrane, carbon membrane was thermally and chemically stable. Thermal stability is analyzed by TGA. TGA analysis of all polymer membranes is shown in Figure 1. It shows that concentration does not have much effect on degradation temperature of membranes. But it gives an insight into chose the pyrolysis protocol that has to be followed. There are different pyrolysis protocols for preparation of carbon membranes, based on type of precursors used, one protocol is fixed.
Figure 1: Thermal analysis of Polymer Membranes
Figure 1 illustrates the weight variation of polymer membranes during the heating process up to 800°C. According to the TGA thermo-diagram, the degradation temperature (Td) is 550°C and was defined as the temperature corresponding to 15% weight loss. This indicates the high thermal resistance of the membrane. The total weight loss at 800°C, with 10°/min heating rate, was approximately 60%. The enhanced thermal stability of the membrane highlights its quality as a precursor for the preparation of carbon molecular sieve membranes.
Effect of polymer concentration on structure of membrane
Figure 2: XRD of Carbon Membrane of different concentrations
Effect of polymer concentration on structure of carbon membrane is shown in Figure 2. As the concentration of polymer was increased from 2% to 20%, peak intensification took place, resulting in a more compact structure. Pore size was reduced and membrane with higher packing density is formed.
Effect of pyrolysis temperature on microstructure of membrane
Structural analysis on carbon membranes was done by obtaining XRD spectra as a function of pyrolysis temperature. As shown in Figure 3, the XRD spectrum for membrane prepared at 450oC is a merger of two peaks at 22.7o and 18.8o. With increase in pyrolysis temperature, the peaks joined into a single but intensified peak with a shift toward the smaller pore sizes; indicating a decrement in average d-spacing.
Figure 3: XRD of Carbon Membrane prepared at different temperatures
d-spacing in carbon membrane refers to interlayer distances, with increase in temperature more compact structures are formed. 800 °C was found to be the optimum pyrolysis temperature as membrane having high density and packing efficiency are formed at this temperature. (1)
Effect of pyrolysis temperature on membrane composition
In order to study how pyrolysis temperature changed the chemical composition of the membrane, elemental analysis has been conducted for both polymers as well as carbon membrane. In Figure 4, the point zero in x-axis corresponds to precursor membrane and it has the lowest carbon content and they started to increase with increase in pyrolysis temperature. However other elemental contents like oxygen, nitrogen and hydrogen was reduced with increase in temperature.
Presence of oxygen compounds in the carbon membrane surface can make the membrane more hydrophilic and can cause swelling of membranes (11). For the better performance of carbon membrane oxygen content has to be less than 4% (12).When pyrolysis temperature in increased, oxygen content is reduced. Membrane prepared at 800oC has oxygen content less than 4%.
So the effect of pyrolysis temperature on hydrophilictiy of the carbon membrane was tested to study the influence of oxygen in the membrane. From the Table 4 it is clear that as pyrolysis temperature is increased, amount of water absorbed is decreased, thus increasing the hydrophobic nature of carbon membranes. This can be explained by the reduction in oxygen containing group in membrane with pyrolysis temperature.
Figure 4: Elemental analysis of membranes
Table 4: Hydrophilicity of carbon membranes
|
Sample-Code |
Pyrolysis Temperature (°C) |
Wet weight of the membrane (g) |
Dry weight of the membrane (g) |
Amount of water absorbed (g) |
|
CM-450 |
450 |
0.0588 |
0.0553 |
0.0035 |
|
CM-600 |
600 |
0.0523 |
0.0503 |
0.0020 |
|
CM-800 |
800 |
0.0687 |
0.0607 |
0.0008 |
Morphology of Membranes
The surface and cross-section morphologies of carbon membranes as well as polymer membranes were investigated by SEM (Scanning Electron Microscopy) techniques.
Surface image of polymer membrane is shown in Figure 6. It shows a smooth and defect free surface without any deformation. Cross section of the polymer membrane is also shown below. Compared to carbon membrane it does not have uniform pore distribution. All the pores are elongated pores and the membrane formed is very dense. Thickness of polymer membrane was found to be 250μm.
The honey-comb structure of carbon membrane shows the pore structure in the membrane (Figure 5). It is clear from the image that the membrane is rich in pores and has quite a uniform pore distribution. The membrane was having a sponge-like matrix unlike polymer membrane. This uniform pore distribution allows carbon membrane to have high selectivity over polymer membrane.
- (b)
Figure 5: SEM images: (a) Top surface and (b) Cross section of Carbon Membrane
- (b)
Figure 6: SEM images: (a) Top surface and (b) Cross section of PolymerMembrane
CONCLUSION
Different polymer blends were tried for the preparation of polymer membrane and membranes derived from Polyamideimde/Polyetherimide (25/75wt.%) exhibited more attractive performance than the other blends. Polymer concentration was varied from 2-20% and at 9% it was found to be exhibiting best results. Polymer membranes prepared from PAI/PEI were transparent hydrophilic membranes. Polymer membranes were thermally unstable compared to carbon membrane and were found to have degradation temperature around 450oC. Complete miscibility of both the polymers was confirmed and the glass transition temperature of the polymer membrane was also found.
For the preparation of carbon membrane different protocols were followed for pyrolysis, and 800oC was found to be the optimum temperature for pyrolysis. Thermal analysis of membrane had conducted, which proved the higher thermal stability of the carbon membrane. Morphological studies shows that carbon membrane prepared at 800oC have desirable pore size compared to other membranes prepared at lower temperatures. XRD studies of the carbon membrane showed that as temperature increases, more compact membranes are obtained, which decreases the permeability of the membrane. All the membranes have shown promising results that can be further investigated for gas separation studies.
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