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Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and Matrimid
Accepted Manuscript Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and Matrimid Wai Fen Yong, Tai-Shung Chung PII:
S0032-3861(15)00057-9
DOI:
10.1016/j.polymer.2015.01.013
Reference:
JPOL 17544
To appear in:
Polymer
Received Date: 13 August 2014 Revised Date:
8 January 2015
Accepted Date: 9 January 2015
Please cite this article as: Yong WF, Chung T-S, Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and Matrimid, Polymer (2015), doi: 10.1016/j.polymer.2015.01.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Transparent with enhanced plasticization resistance
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cPIM-1/Matrimid (10:90) 15.0
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cPIM-1/Matrimid (5:95) Matrimid
10.0
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Permeability (Barrer)
20.0
5.0 0
5
10
15
Pressure (atm)
20
25
30
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Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and
Wai Fen Yonga, Tai-Shung Chunga,*
a
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Matrimid
Department of Chemical & Biomolecular Engineering,
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National University of Singapore, 4 Engineering Drive 4,
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Singapore 117585, Singapore
*Correspondence author. Tel: +65 6516 6645; fax: +65 67791936.
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E-mail address: [email protected] (T.-S. Chung).
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Abstract
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Miscible blends of Matrimid and carboxylated polymers of intrinsic microporosity (cPIM-1) at the molecular level have been discovered. Their miscibility has been confirmed by polarized light microscopy (PLM), atomic force microscopy (AFM), differential scanning calorimetry
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(DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The existence of hydrogen bonding promotes compatibility between these two polymers. A good
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agreement between experimental and predicted data on gas permeability and selectivity is observed. The addition of cPIM-1 in Matrimid significantly enhances the plasticization pressure for all blended membranes. A small loading of 5-10 wt% of cPIM-1 in Matrimid improves the plasticization pressure from less than 10 atm to 15 atm while a higher loading of cPIM-1 shifts
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the plasticization pressure to 20 atm.
Keywords: miscibility; polyimides; hydrogen bonding
Introduction
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1.
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Matrimid is a commercially available polyimide with a glass transition temperature of 320 oC. It possesses not only high thermal resistance, chemical resistance, mechanical properties but also good processibility [1-5]. These superior properties have made it as a potential material for various applications [6-20], especially for gas separation because of its high gas-pair selectivity. However, Matrimid has a relatively low permeability. Various strategies have been attempted to enhance its permeability. One of them is polymer blending, through which the strengths of two different materials can be combined into a new compound with unique and synergetic properties 2
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that can hardly be obtained by synthesis routes [21, 22]. Nevertheless, the main challenge in polymer blends is the miscibility. Due to different physical and chemical properties; most polymer blends do not mix at the molecular level, but appear dispersed and continuous phases.
form a single phase when blending them together [1, 6-12].
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Up to date, there are only few pairs of polymers which are compatible in the molecular level and
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Regarding to Matrimid blends, Grobelny et al. were the pioneers in discovering miscibility between Matrimid and polybenzimidazole (PBI) [6]. Thereafter, numerous studies have found
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the miscibility between Matrimid and other polymers such as oligomer Thermid FA-700 [1], polyaryletherketone (PAEK) [7], P84 (copolyimide of 3, 3’ 4, 4’-benzophenone tetracarboxylic dianhydride and 80% methylphenylene-diamine + 20% methylene diamine) [8], PBI [9], Ultem [10], Torlon 4000T [11] and sulfonated aromatic poly(ether ether ketone) (S-PEEK) [12]. To
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prove the miscibility, various analytical tools have been employed. Grobelny et al. used solidstate 13C nuclear magnetic resonance spectroscopy (NMR) to study the proton rotating frame spin-lattice relaxation. Their results suggested the miscible blend is due to existence of hydrogen
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bonding. Besides, optical inspection and glass transition temperature (Tg) have also been utilized
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to assess miscibility. A miscible blend generally shows a transparent film with a single Tg.
Partially miscible blends with Matrimid have also been reported such as polysulfone (PSF)/Matrimid [13] and Matrimid/polyethersulfone (PES) blends [14]. These blends showed two Tgs in the DSC curves. Blends made from Matrimid and polymers of intrinsic microporosity, specifically PIM-1 [23] were studied by our group for gas separation. PIM-1 was selected due to its high fractional free volume and high gas permeability [23-25]. The results showed that the
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blends at the Matrimid:PIM-1 ratios of 5:95, 10:90, 30:70, 50:50, 70:30 and 80:20 were partially miscible and exhibited dual phases and two Tgs. Their miscible blends only existed for (90:10)
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and (95:5) Matrimid/PIM-1 blends.
To promote a better compatibility, we thereafter modified PIM-1 to carboxylated PIM-1 (cPIM1) [26, 27] and studied its blends with Torlon [26]. The cPIM-1 blends reveal better miscibility
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than PIM-1 blends owing to the formation of hydrogen bonding and charge transfer complexes among polymer chains. However, the miscibility was still not in molecular level. Therefore, we
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would like to examine the miscibility behavior between Matrimid and cPIM-1 in this work. The chemical structures of cPIM-1 and Matrimid are depicted in Fig. 1. In addition to miscibility, we aim to investigate the possible interaction between them and study their gas transport properties
2. Experimental
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2.1. Materials
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in comparison with predicted values.
The monomers 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI, 97 %)
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and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN, 99 %) were purchased from Alfa Aesar and Matrix Scientific, respectively. TTSBI was purified by re-crystallization in methanol, while TFTPN was sublimated under vacuum prior to use. Matrimid powder was supplied by Huntsman Advanced Materials. It was dried overnight at 120 oC under vacuum before use. N-methyl-2pyrrolidone (NMP, > 99.5%) from Merck was further purified via vacuum distillation before usage. Anhydrous potassium carbonate (K2CO3, > 99.5%) and sodium hydroxide (NaOH, ≥ 98%) from Sigma Aldrich were used as received. Methanol (MEOH, ≥ 99.9%) and N, N4
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dimethylformamide (DMF, > 99.5%) from Merck were utilized without further purification. Hydrochloric acid (HCl, 37.5%), ethanol (EtOH, ≥ 99.9%), dichloromethane (DCM, 99.99%)
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and tetrahydrofuran (THF, 99.99%) from Fisher Scientific were used as received.
2.2. Modification of PIM-1 to cPIM-1 and fabrication of polymer blend membranes
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PIM-1 was synthesized by polycondensation of TFTPN with TTSBI [24, 25]. The detailed synthesis procedures could be found in our earlier publication [23]. The synthesis of cPIM-1 was
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carried out by hydrolyzing the nitrile group of PIM-1 [26, 27]. Dense films with different ratios of cPIM-1/Matrimid were prepared via the solution casting method. Matrimid was first dissolved in NMP and stirred overnight at 65 °C. Subsequently, cPIM-1 was added into the solution and stirred overnight. The final solution containing 2 wt% cPIM-1/Matrimid was then filtered
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through a 1 µm PTFE filter and cast onto a silicon wafer at ambient temperature. The polymer solution was heated under vacuum at 40 °C for 12 h and then increased to 75 °C for 24 h. The formed dense films were peeled off from the silicon wafer and dried under vacuum with a
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temperature ramp of 25 °C/ 30 min to 250 °C and hold for 12 h. The resultant dense films were labeled as “cPIM-1/Matrimid (weight composition ratio)”, for example, cPIM-1/Matrimid (5:95).
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The average thickness of the membranes was measured from 10 different points using a Digimatic indicator (IDC-112B-5) with an accuracy of 1 µm. The thicknesses of the cast films were about 50 ± 5 µm.
2.3. Characterizations
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The surface morphology of membranes was investigated using an Olympus BX50 polarized light microscope (PLM). The PLM micrographs were further analyzed by Image Pro Plus 3.0 software. The roughness of membranes was characterized by atomic force microscopy (AFM,
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Bruker Dimention ICON) and then analyzed with NanoScope Analysis V1.4. In the AFM analysis, the membrane surface with a dimension of 5 x 5 µm2 was tested.
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The glass transition temperature, Tg of membranes was characterized by using differential scanning calorimetry (DSC, DSC822e, Mettler Toledo). The samples were tested with two
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consecutive scans at a heating rate of 10 oC min-1 from 40 to 450 oC. The first cycle of ramping and cooling was to eliminate any thermal history of the samples. The Tg of each sample was determined based on the mid-point transition temperature of the second heating curve.
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The chemical changes of polymeric membranes before and after the hydrolysis modification were analyzed by an attenuated total reflectance (ATR) mode using a Shimadzu Fourier transform infrared spectroscopy (FTIR) 8400 spectrometer in the range of 600-4000 cm-1. The
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spectra were obtained with an average of 16 scans at a resolution of 4 cm-1. The characteristic band appeared at 2350 cm-1 was the difference between the background and samples contributed
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from CO2.
The changes in inter-chain spacing of polymeric membranes were investigated by X-ray diffraction (XRD) using an X-ray diffractor, Bruker D8 series, General area detector diffraction system (GADDS) and a Cu X-ray source with a wavelength of 1.54 Å. The X-ray diffraction
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angles between 5° to 30° were studied. The average d-space was evaluated according to the Bragg’s law as follows: nλ = 2d sin θ
(1)
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where n is an integer (1, 2, 3,…), λ represents the X-ray wavelength, d represents the intersegmental spacing between two polymer chains and θ denotes the X-ray diffraction angle.
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The mechanical strength of the membranes was measured using an Instron 5542 Universal tensile tester at ambient temperature. The dense membranes were tested under a constant
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elongation rate of 10 mm/min. Each membrane has an initial gauge length of 15 mm and a width of 5 mm. The test was repeated at least three times for each blend ratio and an average value was obtained.
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2.4. Measurements of gas transport properties
Both pure gas and binary gas were employed to study the gas transport properties. The pure gas
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permeability was measured using a variable-pressure constant-volume gas permeation cell. Pure gas permeability was tested in the sequence of H2, O2, N2, CH4, and CO2 at 35 oC and 3.5 atm.
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The rate of pressure increase (dp/dt) at steady state was used to calculate gas permeability as follows:
P=
273 × 1010 760
dp 76 dt AT p2 × 14.7 Vl
(2)
where P is the gas permeability of the membrane in Barrer (1 Barrer = 1 x 10-10 cm3 (STP) cm/cm2s.cmHg), V is the volume of the downstream chamber (cm3), A refers to the effective
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membrane area (cm2), l is the membrane thickness (cm), T is the operating temperature (K), p2 is defined as the upstream operating pressure (psia). The permeability tests were repeated at least
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three times with different membranes and the average deviation obtained was less than 5 %.
The ideal selectivity is the ratio of pure-gas permeability of a gas pair across the membrane as described in Eq. (3): PA PB
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α=
(3)
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where PA and PB are the gas permeability of gases A and B, respectively.
The mixed gas permeation properties were evaluated at 35 oC and 7 atm using a binary mixture containing 50 mole% CO2 and 50 mole% CH4. The gas permeability of CO2 and CH4 are
PCH 4
273 × 1010 = 760
dp 76 dt AT X CO2 P2 14.7 yCO2 Vl
(
)
(1 − y )Vl dp 76 dt AT [(1 − x )P ] 14.7 CO2
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PCO 2
273 × 1010 = 760
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expressed by following equations:
CO2
(5)
2
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(4)
where PCO2 and PCH4 are the permeability (Barrer) of CO2 and CH4, respectively. P2 denotes the upstream feed gas pressure (psia), x refers to the mole fraction in the feed gas and y is the mole fraction in the permeate. The others symbols remain the same meanings as described earlier.
3. 3.1.
Results and Discussion Interaction between cPIM-1 and Matrimid 8
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Fig. 2 displays Tg as a function of blend ratio. The Matrimid exhibits a characteristic Tg at about 320 oC. The Tg of the cPIM-1, cPIM-1/Matrimid (90:10) and cPIM-1/Matrimid (95:5) could not
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be detected in the range of 50 to 450 oC due to the high stiffness and low rotational freedom of cPIM-1 chains [26, 28]. A single and distinct Tg can be observed for all other blend ratios. It
temperature with a higher cPIM-1 loading in the blends.
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confirms the miscibility of cPIM-1 and Matrimid blends. Besides, Tg shifts to a higher
components as follows [29]:
1 W1 W2 = + Tg Tg1 Tg 2
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For homogeneous blends, Tg could be predicted from the Fox equation using the Tg of pure
(6)
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where W1 and W2 are the mass fractions of components 1 and 2 in the blend, and Tg1 and Tg2 are the respective glass transition temperatures of the components 1 and 2. Fig. 3 displays a comparison of Tg as a function of composition between the experimental and theoretical values
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assuming cPIM-1 has a Tg of 442 oC. The experimental Tg is well correlated with the predicted
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value if the cPIM-1 percentage is at and below 30 wt%.
The interaction between cPIM-1 and Matrimid was examined by using FTIR-ATR. Fig. 4 depicts the FTIR spectra of the cPIM-1, Matrimid and cPIM-1/Matrimid membranes. The representative peaks of imide from Matrimid appear at 1712 cm-1 (C=O stretching), 1361 cm-1 (C-N stretching) and 705 cm-1 (C=O stretching). On the other hand, the C=O absorption band of carboxylic group from cPIM-1 appears at 1700 cm-1. It can be seen that the intensity of imide bands of Matrimid decreases when cPIM-1 loading increases. Fig. 5 enlarges the 3600-3000 cm-1 region which 9
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represents the carboxylic group of the cPIM-1/Matrimid membranes. The absorption band of carboxylic group shifts to a lower band with an increase in Matrimid loading. This shift and the decreases in imide band intensity suggest the formation of hydrogen bonding between these two
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polymers. In particular, the hydrogen bonds may be formed between different group pairs such as (i) -OH group of CPIM-1 and C=O group of Matrimid; (ii) C=O group of CPIM-1 and -H
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group of Matrimid; and (iii) C-O-C group of CPIM-1 and -H group of Matrimid.
Fig. 6 illustrates the interstitial space of the blend membranes measured by XRD. The Matrimid
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shows a sharp peak at 5.7 Å [30]. On the other hand, the Bragg peaks of cPIM-1 are similar with PIM-1 which has the d-spacing values between 3.8 Å to 11.9 Å [31-34] due to their amorphous nature. The d-spacing of 6.5 Å refers to the micropores structure of cPIM-1 because of its contorted polymeric backbone. The broad bands at 4.7 Å and 3.8 Å represent the chain-chain
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distances of polymers and aromatic systems, respectively. Interestingly, the broad band at 6.5 Å for cPIM-1/Matrimid blends move to a lower d-spacing value when there is an increment in Matrimid loading. Clearly, there is an interaction among polymer chains between cPIM-1 and
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Matrimid with a reduction in interstitial space and an enhancement in chain packing.
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Table 1 displays the mechanical properties of the membranes. The Young’s modulus of the blends decreases by 16-43 % as compared to that of the Matrimid. This is likely due to the fact that the introduction of cPIM-1 disturbs the chain stiffness in the original polymer chains. Interestingly, the extension at break is improved 3-48% by adding 5-30 wt% cPIM-1 in Matrimid but decreases 14-38% when the presence of cPIM-1 in Matrimid is more than 50 wt%. In short,
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the blend membranes have a greater extension when Matrimid is in the dominant phase and vice
3.2.
Experimental and predicted gas transport properties
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versa.
Table 1 presents the gas transport properties of the cPIM-1/Matrimid membranes. Compared to
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the Matrimid, the addition of cPIM-1 increases the permeability in the resultant blends. On the other hand, the incorporation of Matrimid into the blends enhances the membrane selectivity as
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compared to the cPIM-1. The CO2 permeability of cPIM-1/Matrimid (5:95) and (10:90) increases from the original value of 9.4 to 13.6 (i.e., an increment of 45%) and 17.9 Barrers (i.e., an increment of 90%), respectively, without much compromising their CO2/CH4 and CO2/N2 selectivity. The enhancement in permeability is attributed to the incorporation of cPIM-1 into the
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Matrimid matrix. The high free volume of cPIM-1 allows more gases to pass through the membrane, while the aforementioned reduction in d-spacing helps sustain the high gas-pair
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selectivity.
Since the blend membranes display miscibility, their gas permeability and selectivity may be
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estimated by using the rule of semi-logarithmic addition as follows [35-38]: ln Pb = φ1 ln P1 + φ 2 ln P2
P P P ln 1 = φ1 ln 1 + φ 2 ln 1 P2 P2 1 P2 2
(9)
(10)
where Pb is the permeability of the polymer blend, P1 and P2 are the permeability of components 1 and 2, φ1 and φ2 are the respective volume fractions of components 1 and 2. Fig. 7 shows a
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comparison between the experimental and predicted permeability for cPIM-1/Matrimid membranes. Interestingly, all the predicted data are comparable with the experimental data possibly due to the formation of hydrogen bonding between cPIM-1 and Matrimid that promotes
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homogeneity. Fig. 8 reveals the experimental and predicted O2/N2 and CO2/CH4 selectivity for the blend system. Compared to the predicted selectivity, the experimental data displays higher O2/N2 and CO2/CH4 selectivity. These results are consistent with our expectation as strong
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intermolecular interaction due to hydrogen bonds would induce dense packing and result in a
3.3.
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higher experimental selectivity [39-40].
CO2-induced plasticization behavior
Fig. 9 shows the permeability as a function of CO2 pressure for cPIM-1/Matrimid blend
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membranes. The Matrimid has a low plasticization pressure of less than 10 atm. The addition of cPIM-1 in Matrimid enhances the plasticization pressure for all membranes impressively. A small loading of 5-10 wt% of cPIM-1 in Matrimid improves the plasticization pressure to 15 atm
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while a higher loading of cPIM-1 shifts the plasticization pressure to 20 atm. It is clear that the suppression of plasticization is not due to the densification that normally observed in cross-
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linked membranes because there is no cross-linking involved in this work [41-43]. The enhancement of the plasticization resistance in this cPIM-1/Matrimid system is likely attributed to the integration of the rigid polymer backbone of cPIM-1 in Matrimid and their newly formed hydrogen bonds.
3.4.
Robeson Upper Bound and binary gas tests
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Fig. 10 shows the gas transport properties of cPIM-1/Matrimid membranes against the Robeson upper bound for (a) H2/N2, (b) O2/N2, (c) CO2/CH4, and (d) CO2/N2 separations [44]. Compared
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to the Matrimid, the incorporation of cPIM-1 in the system moves the gas transport properties towards the upper bound. The binary CO2/CH4 (50%/50%) gas tests for membranes consist of 10, 30, 50, 70 and 90 wt% cPIM-1 in Matrimid are compared with the pure gas tests in Table 2.
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of sorption competition between gases [45, 46].
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In general, the mixed gas data are slightly lower than the pure gas ones because of the presence
4. Conclusions
We have found that cPIM-1 and Matrimid are miscible for all the compositions as confirmed by
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PLM, AFM, DSC, FTIR and XRD. The miscibility between the two polymers is due to the formation of hydrogen bonding. As a result, a good agreement was observed between experimental and predicted gas permeability and selectivity. The addition of cPIM-1 in Matrimid
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also significantly enhances the plasticization pressure for all membranes. To our best knowledge,
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this is the first work discovering molecular miscibility that involves cPIM-1.
Acknowledgements
This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP Award No. NRF-CRP 5-2009-5 (NUS grant number R-279-000-311-281)).
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List of Figures
Chemical structures of (a) cPIM-1 and (b) Matrimid.
Fig. 2.
DSC curves of cPIM-1, Matrimid and cPIM-1/Matrimid blend films.
Fig. 3.
Comparison of Tg of the blend films obtained from DSC and the Fox equation.
Fig. 4.
FTIR spectra of cPIM-1/Matrimid blend systems.
Fig. 5.
FTIR spectra in the region of 3600-3000 cm-1 region of the cPIM-1/Matrimid
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membranes.
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Fig. 1.
Fig. 6.
XRD spectra of the cPIM-1/Matrimid membranes.
Fig. 7.
Correlations between experimental and predicted data of cPIM-1/Matrimid blends on O2 (□), N2 (∆), CH4 (○) and CO2 (◊) permeability.
Comparison between experimental and predicted data for: (a) O2/N2 selectivity; (b)
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Fig. 8.
CO2/CH4 selectivity. Fig. 9.
Permeability as a function of CO2 pressure and blend composition for cPIM-1/Matrimid
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membranes.
Fig. 10. Comparison with the Robeson upper bound: (a) H2/N2; (b) O2/N2; (c) CO2/CH4; (d)
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CO2/N2.
List of Tables
Table 1. Mechanical properties of cPIM-1/Matrimid membranes.
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Table 2. Pure gas permeability and selectivity of the cPIM-1/Matrimid membranes tested at 35 °C and 3.5 atm.
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Table 3. Binary gas permeability and selectivity of cPIM-1/Matrimid membranes.
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[26] W.F. Yong, F.Y. Li, T.S. Chung, Y.W. Tong, Plasticization and enhanced gas transport properties of cPIM-1/Torlon blend membranes for CO2 capture, J. Membr. Sci. 462 (2014)
SC
119–130.
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polymers of intrinsic microporosity (PIMs) with tunable gas transport properties, Macromolecules 42 (2009) 6038–6043.
[28] N. Du, H.B. Park, G.P. Robertson, M.M. Dal-Cin, T. Visser, L. Scoles, M.D. Guiver, Polymer nanosieve membranes for CO2-capture applications, Nat. Mater. 10 (2011) 372–
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375.
[29] T.G. Fox, Influence of diluent and of copolymer composition on the glass temperature of a polymer system, Bull. Am. Phys. Soc. 1 (1956) 123–125.
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[30] S. Sridhar, R.S. Veerapur, M.B. Patil, K.B. Gudasi, T.M. Aminabhavi, Matrimid polyimide membranes for the separation of carbon dioxide from methane, J. Appl. Poly. Sci. 106
AC C
(2007) 1585–1594.
[31] U.H.F. Bunz, V. Enkelmann, L. Kloppenburg, D. Jones, K.D. Shimizu, J.B. Claridge, H.-C. zur Loye, G. Lieser, Solid-state structures of phenyleneethynylenes: comparison of monomers and polymers, Chem. Mater. 11 (1999) 1416–1424.
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[32] J. Weber, Q. Su, M. Antonietti, A. Thomas, Exploring polymers of intrinsic microporosity –microporous, soluble polyamide and polyimidea, Macromol. Rapid Commun. 28 (2007) 1871–1876.
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[33] F.Y. Li, Y.C. Xiao, Y.K. Ong, T.S. Chung, UV-rearranged PIM-1 polymeric membranes for advanced hydrogen purification and production, Adv. Energy Mater. 2 (2012) 1456– 1466.
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[34] W.F. Yong, F.Y. Li, T.S. Chung, Y.W. Tong, Highly permeable chemically modified PIM1/Matrimid membranes for green hydrogen purification, J. Mater. Chem. A 1 (2013)
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13914–13925.
[35] M. Hasegawa, I. Mita, M. Kochi, R. Yokota, Miscibility of polyimide/polyimide blends and charge-transfer fluorescence spectra, Polymer 32 (1991) 3225–3232. [36] A.E. Barnabeo, W.S. Creasy, L.M. Robeson, Gas permeability characteristics of nitrile-
1979–1986.
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containing block and random copolymers, J. Polym. Sci.: Polym. Chem. Ed. 13 (1975)
[37] S.L. Liu, R. Wang, T.S. Chung, M.L. Chng, Y. Liu, R.H. Vora, Effect of diamine
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composition on the gas transport properties in 6FDA-durene/3,3’-diaminodiphenyl sulfone copolyimides, J. Membr. Sci. 202 (2002) 165–176.
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[38] M.L. Chng, Y.C. Xiao, T.S. Chung, M. Toriida, S. Tamai, The effects of chemical structure on gas transport properties of poly (aryl ether ketone) random copolymers polymer, Polymer 48 (2007) 311–317. [39] Z. Wang, T. Chen, J. Xu, Novel poly(aryl ether ketone)s containing various pendant groups. II. gas-transport properties, J. Appl. Poly. Sci. 64 (1997) 1725–1732.
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[40] W.H. Lin, R.H. Vora, T.S. Chung, Gas transport properties of 6FDA-Durene/1,4phenylenediamine (pPDA) copolyimides, J. Polym. Sci. B: Polym. Phys. 38 (2000) 2703– 2713.
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[41] P.S. Tin, T.S. Chung, Novel approach to fabricate carbon molecular sieve membranes based on consideration of interpenetrating network, Macromol. Rapid Commun. 25 (2004) 1247–1250.
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[42] T.S. Chung, L. Shao, P.S. Tin, Surface modification of polyimide membranes by diamines for H2 and CO2 Separation, Macromol. Rapid Commun. 27 (2006) 998–1003.
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[43] N. Du, M.M. Dal- Cin, I. Pinnau, A. Nicalek, G.P. Robertson, M.D. Guiver, Azide-based cross-linking of polymers of intrinsic microporosity (PIMs) for condensable gas separation, Macromol. Rapid Commun. 32 (2011) 631–636.
[44] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400.
2010.
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[45] Y. Yampolskii, B. Freeman, Membrane Gas Separation, John Wiley & Sons, Chichester,
[46] M.R. Coleman, W.J. Koros, Conditioning of fluorine-containing polyimides. 2. Effect of
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conditioning protocol at 8% volume dilation on gas-transport properties, Macromolecules
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32 (1999) 3106–3113.
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Maximum tensile stress (MPa)
3.24 ± 0.22 2.60 ± 0.34 2.71 ± 0.11 2.05 ± 0.99 1.84 ± 0.94 1.95 ± 0.79
6.04 ± 1.01 6.27 ± 1.96 7.83 ± 2.19 8.91 ± 0.70 6.92 ± 1.00 3.75 ± 1.15
114.07 ± 8.77 108.27 ± 3.51 111.39 ± 2.01 92.37 ± 1.49 78.25 ± 1.64 60.52 ± 3.52
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Extension at break (%)
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Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30)
Modulus (GPa)
AC C
Membranes ID
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Table 1 Mechanical properties of cPIM-1/Matrimid membranes.
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CH4
2.03 3.09 3.91 9.01 27 88 177 445 462
0.32 0.48 0.63 1.72 5.4 21 47 121 143
0.26 0.39 0.55 1.74 6.1 25 57 170 209
1 Barrer = 1 × 10-10 cm3 (STP)cm/cm2s cmHg.
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a
N2
CO2
9.4 13.6 17.9 48.7 145 486 982 2268 2654
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Matrimid 31.9 cPIM-1/Matrimid (5:95) 41.8 cPIM-1/Matrimid (10:90) 48.8 cPIM-1/Matrimid (30:70) 86.5 cPIM-1/Matrimid (50:50) 199 cPIM-1/Matrimid (70:30) 481 cPIM-1/Matrimid (90:10) 867 cPIM-1/Matrimid (95:5) 1568 cPIM-1 1619
O2
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H2
EP
Membranes ID
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Permeability (Barrera)
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Table 2 Pure gas permeability and selectivity of the cPIM-1/Matrimid membranes tested at 35 °C and 3.5 atm. Ideal Selectivity
H2/N2 O2/N2 CO2/N2 CO2/CH4 99.7 87.1 77.5 50.3 36.9 22.9 18.4 13.0 11.3
6.3 6.4 6.2 5.2 5.0 4.2 3.8 3.7 3.2
29.4 28.3 28.4 28.3 26.9 23.1 20.9 18.7 18.6
36.2 34.9 32.5 28.0 23.8 19.4 17.2 13.3 12.7
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Table 3 Binary gas permeability and selectivity of cPIM-1/Matrimid membranes. Permeability (Barrer)a CO2 CH4
Membranes ID
Selectivity CO2/CH4
cPIM-1/Matrimid (10:90)
13.9 (17.9)b
cPIM-1/Matrimid (30:70)
43.2 (48.7)
cPIM-1/Matrimid (50:50)
131 (145)
6.7 (7.1)
19.6 (20.4)
cPIM-1/Matrimid (70:30)
417 (486)
22.8 (25)
18.3 (19.4)
cPIM-1/Matrimid (90:10)
905 (982)
53.5 (57)
16.9 (17.2)
1.78 (1.88)
24.3 (25.9)
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28.4 (32.5)
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a
0.49 (0.55)
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CO2/CH4 (50:50 mole %) at 35 °C and 7 atm. b Number in parentheses is the permeability and ideal selectivity obtained from pure gas tests.
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O
H 3C CH3
(a)
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COOH O
O
O
H 3C
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CH3 COOH
(b)
O
O
O
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n
CH3
O
H3C
CH3
n
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O
N
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N
Fig. 1. Chemical structures of (a) cPIM-1 and (b) Matrimid.
250
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300
350
400
Temperature (oC)
Fig. 2. DSC curves of cPIM-1, Matrimid and cPIM-1/Matrimid blend films.
450
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440
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420
380
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360 340 320 300
0.2
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0
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Tg (oC)
400
0.4
Fox equation Experimental 0.6
0.8
1
cPIM-1 mass fraction
Fig. 3. Comparison of Tg of the blend films obtained from DSC and the Fox equation.
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1700 (carboxylic C=O)
cPIM-1 cPIM-1/M (95:5)
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cPIM-1/M (90:10) cPIM-1/M (70:30) cPIM-1/M (50:50) cPIM-1/M (30:70) cPIM-1/M (10:90) cPIM-1/M (5:95)
AC C
EP
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Matrimid
1712 (imide C=O)
2000
1800
1600
1361 (imide C-N)
1400
1080 (imide C-N-C)
1200
Wavenumbers (cm-1)
Fig. 4. FTIR spectra of the cPIM-1/Matrimid blend system.
1000
705 (imide C=O)
800
600
3000-3600 (-COOH)
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cPIM-1
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cPIM-1/Matrimid (95:5)
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cPIM-1/Matrimid (90:10)
AC C
EP
cPIM-1/Matrimid (70:30)
3600
3400
cPIM-1/Matrimid (50:50)
3200
3000
Wavenumbers (cm-1)
Fig. 5. FTIR spectra in the 3600-3000 cm-1 region of the cPIM-1/Matrimid membranes.
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~5.7 Å
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Matrimid cPIM-1/Matrimid (5:95)
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cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30)
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cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5)
AC C
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cPIM-1
~4.7 Å
~6.5 Å
5
10
15
~3.8 Å 20
2 Theta (o)
Fig. 6. XRD spectra of the cPIM-1/Matrimid membranes.
25
30
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100
O2
1
N2
EP
10
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CO2
CH4 0.1 0.0
AC C
Permeability (Barrer)
1000
0.2
0.4
Rule of semi-logarithmic
0.6
0.8
1.0
Mass fraction of cPIM-1
Fig. 7. Correlations between experimental and predicted data of cPIM-1/Matrimid blends on O2 (□), N2 (∆), CH4 (○) and CO2 (◊) permeability.
35
4 3 2 selectivity
1
0.2 0.4 0.6 Mass fraction of cPIM-1
0.8
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0
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selectivity (prediction)
0
30 25
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5
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6
CO2/CH4 selectivity
(b) 40
O2/N2 selectivity
(a) 7
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1
20 15 10 5
selectivity selectivity (prediction)
0 0
0.2 0.4 0.6 Mass fraction of cPIM-1
0.8
Fig. 8. Comparison between experimental and predicted data for: (a) O2/N2 selectivity; (b) CO2/CH4 selectivity.
1
cPIM-1/Matrimid (95:5)
2000 1500
cPIM-1/Matrimid (90:10)
1000
500 400 300 200
cPIM-1/Matrimid (70:30)
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cPIM-1
2500
600
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3000
(b)
100
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Permeability (Barrer)
(a) 3500
Permeability (Barrer)
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cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (30:70)
0
500 0
5
10
15
20
25
20
25
30
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10 15 Pressure (atm)
15.0
EP
cPIM-1/Matrimid (10:90)
cPIM-1/Matrimid (5:95)
AC C
Permeability (Barrer)
20.0
5
30
Pressure (atm) (c)
0
Matrimid
10.0
5.0 0
5
10
15
20
25
30
Pressure (atm)
Fig. 9. Permeability as a function of CO2 pressure and blend composition for cPIM-1/Matrimid membranes.
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(b) 100
1
10 100 1000 H2 permeability (Barrer)
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10000
(c)1000
1 10 100 O2 permeability (Barrer)
1000
100 Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
10 1
EP
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(d)1000
AC C
CO2/CH4 selectivity
Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
0.1
0.1 1
1
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Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
10
10
SC
O2/N2 selectivity
100
1 10 100 1000 CO2 permeability (Barrer)
100 Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
10 1 0.1
0.1 0.1
CO2/N2 selectivity
H2/N2 selectivity
(a)1000
10000
0.1
10 1000 CO2 permeability (Barrer)
Fig. 10. Comparison with the Robeson upper bound: (a) H2/N2; (b) O2/N2; (c) CO2/CH4; (d) CO2/N2.
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Miscible blends of Matrimid and cPIM-1 at the molecular level have been discovered.
•
Hydrogen bonding promotes compatibility between these two polymers.
•
All blend membranes has a single Tg.
•
The inclusion of cPIM-1 in Matrimid enhances the plasticization pressure.
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•
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Supplementary data Morphology of cPIM-1/Matrimid blend films
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Fig. S1 shows the photos of cPIM-1/Matrimid dense films while Fig. S2 shows the homogeneity of dense blend films examined by PLM. All films cast from different cPIM-1/Matrimid ratios appear to be homogenous and transparent which indicate good miscibility. Fig. S3 depicts their surface roughness and root mean square value, Rq, measured by AFM. All cPIM-1/Matrimid films have Rq less than 4 nm which suggest very smooth surfaces in these blends.
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List of Figures
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Fig. S1. Photos of cPIM-1/Matrimid blend films. Fig. S2. PLM images of dense blend films: (a) Matrimid; (b) cPIM-1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM-1/Matrimid (50:50); (f) cPIM1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM-1/Matrimid (95:5); (i) cPIM-1. Fig. S3. AFM images on the surfaces of dense blend films: (a) Matrimid; (b) cPIM1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM1/Matrimid (50:50); (f) cPIM-1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM1/Matrimid (95:5); (i) cPIM-1.
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Fig. S1. Photos of cPIM-1/Matrimid blend films.
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(e) cPIM-1/Matrimid (50:50)
(f) cPIM-1/Matrimid (70:30)
(c) cPIM-1/Matrimid (10:90)
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(b) cPIM-1/Matrimid (5:95)
(g) cPIM-1/Matrimid (90:10)
(d) cPIM-1/Matrimid (30:70)
(h) cPIM-1/Matrimid (95:5)
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EP
(i) cPIM-1
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(a) Matrimid
100µm
Fig. S2. PLM images of dense blend films: (a) Matrimid; (b) cPIM-1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM1/Matrimid (50:50); (f) cPIM-1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM-1/Matrimid (95:5); (i) cPIM-1.
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Rq = 1.4
(b) cPIM-1/Matrimid (5:95) Rq = 1.2
(c) cPIM-1/Matrimid (10:90) Rq = 1.2
(e) cPIM-1/Matrimid (50:50) Rq = 2.7
(f) cPIM-1/Matrimid (70:30) Rq = 2.6
(g) cPIM-1/Matrimid (90:10) Rq = 2.5
(d) cPIM-1/Matrimid (30:70) Rq = 3.2
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(h) cPIM-1/Matrimid (95:5) Rq = 1.1
AC C
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(i) cPIM-1 Rq = 0.9
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(a) Matrimid
Fig. S3. AFM images on the surfaces of dense blend films: (a) Matrimid; (b) cPIM1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM1/Matrimid (50:50); (f) cPIM-1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM1/Matrimid (95:5); (i) cPIM-1.
S0032-3861(15)00057-9
DOI:
10.1016/j.polymer.2015.01.013
Reference:
JPOL 17544
To appear in:
Polymer
Received Date: 13 August 2014 Revised Date:
8 January 2015
Accepted Date: 9 January 2015
Please cite this article as: Yong WF, Chung T-S, Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and Matrimid, Polymer (2015), doi: 10.1016/j.polymer.2015.01.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Transparent with enhanced plasticization resistance
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cPIM-1/Matrimid (10:90) 15.0
EP
cPIM-1/Matrimid (5:95) Matrimid
10.0
AC C
Permeability (Barrer)
20.0
5.0 0
5
10
15
Pressure (atm)
20
25
30
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Miscible blends of carboxylated polymers of intrinsic microporosity (cPIM-1) and
Wai Fen Yonga, Tai-Shung Chunga,*
a
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Matrimid
Department of Chemical & Biomolecular Engineering,
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National University of Singapore, 4 Engineering Drive 4,
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Singapore 117585, Singapore
*Correspondence author. Tel: +65 6516 6645; fax: +65 67791936.
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E-mail address: [email protected] (T.-S. Chung).
1
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Abstract
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Miscible blends of Matrimid and carboxylated polymers of intrinsic microporosity (cPIM-1) at the molecular level have been discovered. Their miscibility has been confirmed by polarized light microscopy (PLM), atomic force microscopy (AFM), differential scanning calorimetry
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(DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The existence of hydrogen bonding promotes compatibility between these two polymers. A good
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agreement between experimental and predicted data on gas permeability and selectivity is observed. The addition of cPIM-1 in Matrimid significantly enhances the plasticization pressure for all blended membranes. A small loading of 5-10 wt% of cPIM-1 in Matrimid improves the plasticization pressure from less than 10 atm to 15 atm while a higher loading of cPIM-1 shifts
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the plasticization pressure to 20 atm.
Keywords: miscibility; polyimides; hydrogen bonding
Introduction
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1.
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Matrimid is a commercially available polyimide with a glass transition temperature of 320 oC. It possesses not only high thermal resistance, chemical resistance, mechanical properties but also good processibility [1-5]. These superior properties have made it as a potential material for various applications [6-20], especially for gas separation because of its high gas-pair selectivity. However, Matrimid has a relatively low permeability. Various strategies have been attempted to enhance its permeability. One of them is polymer blending, through which the strengths of two different materials can be combined into a new compound with unique and synergetic properties 2
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that can hardly be obtained by synthesis routes [21, 22]. Nevertheless, the main challenge in polymer blends is the miscibility. Due to different physical and chemical properties; most polymer blends do not mix at the molecular level, but appear dispersed and continuous phases.
form a single phase when blending them together [1, 6-12].
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Up to date, there are only few pairs of polymers which are compatible in the molecular level and
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Regarding to Matrimid blends, Grobelny et al. were the pioneers in discovering miscibility between Matrimid and polybenzimidazole (PBI) [6]. Thereafter, numerous studies have found
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the miscibility between Matrimid and other polymers such as oligomer Thermid FA-700 [1], polyaryletherketone (PAEK) [7], P84 (copolyimide of 3, 3’ 4, 4’-benzophenone tetracarboxylic dianhydride and 80% methylphenylene-diamine + 20% methylene diamine) [8], PBI [9], Ultem [10], Torlon 4000T [11] and sulfonated aromatic poly(ether ether ketone) (S-PEEK) [12]. To
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prove the miscibility, various analytical tools have been employed. Grobelny et al. used solidstate 13C nuclear magnetic resonance spectroscopy (NMR) to study the proton rotating frame spin-lattice relaxation. Their results suggested the miscible blend is due to existence of hydrogen
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bonding. Besides, optical inspection and glass transition temperature (Tg) have also been utilized
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to assess miscibility. A miscible blend generally shows a transparent film with a single Tg.
Partially miscible blends with Matrimid have also been reported such as polysulfone (PSF)/Matrimid [13] and Matrimid/polyethersulfone (PES) blends [14]. These blends showed two Tgs in the DSC curves. Blends made from Matrimid and polymers of intrinsic microporosity, specifically PIM-1 [23] were studied by our group for gas separation. PIM-1 was selected due to its high fractional free volume and high gas permeability [23-25]. The results showed that the
3
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blends at the Matrimid:PIM-1 ratios of 5:95, 10:90, 30:70, 50:50, 70:30 and 80:20 were partially miscible and exhibited dual phases and two Tgs. Their miscible blends only existed for (90:10)
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and (95:5) Matrimid/PIM-1 blends.
To promote a better compatibility, we thereafter modified PIM-1 to carboxylated PIM-1 (cPIM1) [26, 27] and studied its blends with Torlon [26]. The cPIM-1 blends reveal better miscibility
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than PIM-1 blends owing to the formation of hydrogen bonding and charge transfer complexes among polymer chains. However, the miscibility was still not in molecular level. Therefore, we
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would like to examine the miscibility behavior between Matrimid and cPIM-1 in this work. The chemical structures of cPIM-1 and Matrimid are depicted in Fig. 1. In addition to miscibility, we aim to investigate the possible interaction between them and study their gas transport properties
2. Experimental
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2.1. Materials
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in comparison with predicted values.
The monomers 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI, 97 %)
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and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN, 99 %) were purchased from Alfa Aesar and Matrix Scientific, respectively. TTSBI was purified by re-crystallization in methanol, while TFTPN was sublimated under vacuum prior to use. Matrimid powder was supplied by Huntsman Advanced Materials. It was dried overnight at 120 oC under vacuum before use. N-methyl-2pyrrolidone (NMP, > 99.5%) from Merck was further purified via vacuum distillation before usage. Anhydrous potassium carbonate (K2CO3, > 99.5%) and sodium hydroxide (NaOH, ≥ 98%) from Sigma Aldrich were used as received. Methanol (MEOH, ≥ 99.9%) and N, N4
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dimethylformamide (DMF, > 99.5%) from Merck were utilized without further purification. Hydrochloric acid (HCl, 37.5%), ethanol (EtOH, ≥ 99.9%), dichloromethane (DCM, 99.99%)
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and tetrahydrofuran (THF, 99.99%) from Fisher Scientific were used as received.
2.2. Modification of PIM-1 to cPIM-1 and fabrication of polymer blend membranes
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PIM-1 was synthesized by polycondensation of TFTPN with TTSBI [24, 25]. The detailed synthesis procedures could be found in our earlier publication [23]. The synthesis of cPIM-1 was
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carried out by hydrolyzing the nitrile group of PIM-1 [26, 27]. Dense films with different ratios of cPIM-1/Matrimid were prepared via the solution casting method. Matrimid was first dissolved in NMP and stirred overnight at 65 °C. Subsequently, cPIM-1 was added into the solution and stirred overnight. The final solution containing 2 wt% cPIM-1/Matrimid was then filtered
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through a 1 µm PTFE filter and cast onto a silicon wafer at ambient temperature. The polymer solution was heated under vacuum at 40 °C for 12 h and then increased to 75 °C for 24 h. The formed dense films were peeled off from the silicon wafer and dried under vacuum with a
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temperature ramp of 25 °C/ 30 min to 250 °C and hold for 12 h. The resultant dense films were labeled as “cPIM-1/Matrimid (weight composition ratio)”, for example, cPIM-1/Matrimid (5:95).
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The average thickness of the membranes was measured from 10 different points using a Digimatic indicator (IDC-112B-5) with an accuracy of 1 µm. The thicknesses of the cast films were about 50 ± 5 µm.
2.3. Characterizations
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The surface morphology of membranes was investigated using an Olympus BX50 polarized light microscope (PLM). The PLM micrographs were further analyzed by Image Pro Plus 3.0 software. The roughness of membranes was characterized by atomic force microscopy (AFM,
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Bruker Dimention ICON) and then analyzed with NanoScope Analysis V1.4. In the AFM analysis, the membrane surface with a dimension of 5 x 5 µm2 was tested.
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The glass transition temperature, Tg of membranes was characterized by using differential scanning calorimetry (DSC, DSC822e, Mettler Toledo). The samples were tested with two
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consecutive scans at a heating rate of 10 oC min-1 from 40 to 450 oC. The first cycle of ramping and cooling was to eliminate any thermal history of the samples. The Tg of each sample was determined based on the mid-point transition temperature of the second heating curve.
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The chemical changes of polymeric membranes before and after the hydrolysis modification were analyzed by an attenuated total reflectance (ATR) mode using a Shimadzu Fourier transform infrared spectroscopy (FTIR) 8400 spectrometer in the range of 600-4000 cm-1. The
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spectra were obtained with an average of 16 scans at a resolution of 4 cm-1. The characteristic band appeared at 2350 cm-1 was the difference between the background and samples contributed
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from CO2.
The changes in inter-chain spacing of polymeric membranes were investigated by X-ray diffraction (XRD) using an X-ray diffractor, Bruker D8 series, General area detector diffraction system (GADDS) and a Cu X-ray source with a wavelength of 1.54 Å. The X-ray diffraction
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angles between 5° to 30° were studied. The average d-space was evaluated according to the Bragg’s law as follows: nλ = 2d sin θ
(1)
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where n is an integer (1, 2, 3,…), λ represents the X-ray wavelength, d represents the intersegmental spacing between two polymer chains and θ denotes the X-ray diffraction angle.
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The mechanical strength of the membranes was measured using an Instron 5542 Universal tensile tester at ambient temperature. The dense membranes were tested under a constant
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elongation rate of 10 mm/min. Each membrane has an initial gauge length of 15 mm and a width of 5 mm. The test was repeated at least three times for each blend ratio and an average value was obtained.
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2.4. Measurements of gas transport properties
Both pure gas and binary gas were employed to study the gas transport properties. The pure gas
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permeability was measured using a variable-pressure constant-volume gas permeation cell. Pure gas permeability was tested in the sequence of H2, O2, N2, CH4, and CO2 at 35 oC and 3.5 atm.
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The rate of pressure increase (dp/dt) at steady state was used to calculate gas permeability as follows:
P=
273 × 1010 760
dp 76 dt AT p2 × 14.7 Vl
(2)
where P is the gas permeability of the membrane in Barrer (1 Barrer = 1 x 10-10 cm3 (STP) cm/cm2s.cmHg), V is the volume of the downstream chamber (cm3), A refers to the effective
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membrane area (cm2), l is the membrane thickness (cm), T is the operating temperature (K), p2 is defined as the upstream operating pressure (psia). The permeability tests were repeated at least
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three times with different membranes and the average deviation obtained was less than 5 %.
The ideal selectivity is the ratio of pure-gas permeability of a gas pair across the membrane as described in Eq. (3): PA PB
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α=
(3)
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where PA and PB are the gas permeability of gases A and B, respectively.
The mixed gas permeation properties were evaluated at 35 oC and 7 atm using a binary mixture containing 50 mole% CO2 and 50 mole% CH4. The gas permeability of CO2 and CH4 are
PCH 4
273 × 1010 = 760
dp 76 dt AT X CO2 P2 14.7 yCO2 Vl
(
)
(1 − y )Vl dp 76 dt AT [(1 − x )P ] 14.7 CO2
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PCO 2
273 × 1010 = 760
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expressed by following equations:
CO2
(5)
2
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(4)
where PCO2 and PCH4 are the permeability (Barrer) of CO2 and CH4, respectively. P2 denotes the upstream feed gas pressure (psia), x refers to the mole fraction in the feed gas and y is the mole fraction in the permeate. The others symbols remain the same meanings as described earlier.
3. 3.1.
Results and Discussion Interaction between cPIM-1 and Matrimid 8
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Fig. 2 displays Tg as a function of blend ratio. The Matrimid exhibits a characteristic Tg at about 320 oC. The Tg of the cPIM-1, cPIM-1/Matrimid (90:10) and cPIM-1/Matrimid (95:5) could not
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be detected in the range of 50 to 450 oC due to the high stiffness and low rotational freedom of cPIM-1 chains [26, 28]. A single and distinct Tg can be observed for all other blend ratios. It
temperature with a higher cPIM-1 loading in the blends.
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confirms the miscibility of cPIM-1 and Matrimid blends. Besides, Tg shifts to a higher
components as follows [29]:
1 W1 W2 = + Tg Tg1 Tg 2
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For homogeneous blends, Tg could be predicted from the Fox equation using the Tg of pure
(6)
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where W1 and W2 are the mass fractions of components 1 and 2 in the blend, and Tg1 and Tg2 are the respective glass transition temperatures of the components 1 and 2. Fig. 3 displays a comparison of Tg as a function of composition between the experimental and theoretical values
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assuming cPIM-1 has a Tg of 442 oC. The experimental Tg is well correlated with the predicted
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value if the cPIM-1 percentage is at and below 30 wt%.
The interaction between cPIM-1 and Matrimid was examined by using FTIR-ATR. Fig. 4 depicts the FTIR spectra of the cPIM-1, Matrimid and cPIM-1/Matrimid membranes. The representative peaks of imide from Matrimid appear at 1712 cm-1 (C=O stretching), 1361 cm-1 (C-N stretching) and 705 cm-1 (C=O stretching). On the other hand, the C=O absorption band of carboxylic group from cPIM-1 appears at 1700 cm-1. It can be seen that the intensity of imide bands of Matrimid decreases when cPIM-1 loading increases. Fig. 5 enlarges the 3600-3000 cm-1 region which 9
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represents the carboxylic group of the cPIM-1/Matrimid membranes. The absorption band of carboxylic group shifts to a lower band with an increase in Matrimid loading. This shift and the decreases in imide band intensity suggest the formation of hydrogen bonding between these two
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polymers. In particular, the hydrogen bonds may be formed between different group pairs such as (i) -OH group of CPIM-1 and C=O group of Matrimid; (ii) C=O group of CPIM-1 and -H
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group of Matrimid; and (iii) C-O-C group of CPIM-1 and -H group of Matrimid.
Fig. 6 illustrates the interstitial space of the blend membranes measured by XRD. The Matrimid
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shows a sharp peak at 5.7 Å [30]. On the other hand, the Bragg peaks of cPIM-1 are similar with PIM-1 which has the d-spacing values between 3.8 Å to 11.9 Å [31-34] due to their amorphous nature. The d-spacing of 6.5 Å refers to the micropores structure of cPIM-1 because of its contorted polymeric backbone. The broad bands at 4.7 Å and 3.8 Å represent the chain-chain
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distances of polymers and aromatic systems, respectively. Interestingly, the broad band at 6.5 Å for cPIM-1/Matrimid blends move to a lower d-spacing value when there is an increment in Matrimid loading. Clearly, there is an interaction among polymer chains between cPIM-1 and
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Matrimid with a reduction in interstitial space and an enhancement in chain packing.
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Table 1 displays the mechanical properties of the membranes. The Young’s modulus of the blends decreases by 16-43 % as compared to that of the Matrimid. This is likely due to the fact that the introduction of cPIM-1 disturbs the chain stiffness in the original polymer chains. Interestingly, the extension at break is improved 3-48% by adding 5-30 wt% cPIM-1 in Matrimid but decreases 14-38% when the presence of cPIM-1 in Matrimid is more than 50 wt%. In short,
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the blend membranes have a greater extension when Matrimid is in the dominant phase and vice
3.2.
Experimental and predicted gas transport properties
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versa.
Table 1 presents the gas transport properties of the cPIM-1/Matrimid membranes. Compared to
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the Matrimid, the addition of cPIM-1 increases the permeability in the resultant blends. On the other hand, the incorporation of Matrimid into the blends enhances the membrane selectivity as
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compared to the cPIM-1. The CO2 permeability of cPIM-1/Matrimid (5:95) and (10:90) increases from the original value of 9.4 to 13.6 (i.e., an increment of 45%) and 17.9 Barrers (i.e., an increment of 90%), respectively, without much compromising their CO2/CH4 and CO2/N2 selectivity. The enhancement in permeability is attributed to the incorporation of cPIM-1 into the
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Matrimid matrix. The high free volume of cPIM-1 allows more gases to pass through the membrane, while the aforementioned reduction in d-spacing helps sustain the high gas-pair
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selectivity.
Since the blend membranes display miscibility, their gas permeability and selectivity may be
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estimated by using the rule of semi-logarithmic addition as follows [35-38]: ln Pb = φ1 ln P1 + φ 2 ln P2
P P P ln 1 = φ1 ln 1 + φ 2 ln 1 P2 P2 1 P2 2
(9)
(10)
where Pb is the permeability of the polymer blend, P1 and P2 are the permeability of components 1 and 2, φ1 and φ2 are the respective volume fractions of components 1 and 2. Fig. 7 shows a
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comparison between the experimental and predicted permeability for cPIM-1/Matrimid membranes. Interestingly, all the predicted data are comparable with the experimental data possibly due to the formation of hydrogen bonding between cPIM-1 and Matrimid that promotes
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homogeneity. Fig. 8 reveals the experimental and predicted O2/N2 and CO2/CH4 selectivity for the blend system. Compared to the predicted selectivity, the experimental data displays higher O2/N2 and CO2/CH4 selectivity. These results are consistent with our expectation as strong
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intermolecular interaction due to hydrogen bonds would induce dense packing and result in a
3.3.
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higher experimental selectivity [39-40].
CO2-induced plasticization behavior
Fig. 9 shows the permeability as a function of CO2 pressure for cPIM-1/Matrimid blend
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membranes. The Matrimid has a low plasticization pressure of less than 10 atm. The addition of cPIM-1 in Matrimid enhances the plasticization pressure for all membranes impressively. A small loading of 5-10 wt% of cPIM-1 in Matrimid improves the plasticization pressure to 15 atm
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while a higher loading of cPIM-1 shifts the plasticization pressure to 20 atm. It is clear that the suppression of plasticization is not due to the densification that normally observed in cross-
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linked membranes because there is no cross-linking involved in this work [41-43]. The enhancement of the plasticization resistance in this cPIM-1/Matrimid system is likely attributed to the integration of the rigid polymer backbone of cPIM-1 in Matrimid and their newly formed hydrogen bonds.
3.4.
Robeson Upper Bound and binary gas tests
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Fig. 10 shows the gas transport properties of cPIM-1/Matrimid membranes against the Robeson upper bound for (a) H2/N2, (b) O2/N2, (c) CO2/CH4, and (d) CO2/N2 separations [44]. Compared
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to the Matrimid, the incorporation of cPIM-1 in the system moves the gas transport properties towards the upper bound. The binary CO2/CH4 (50%/50%) gas tests for membranes consist of 10, 30, 50, 70 and 90 wt% cPIM-1 in Matrimid are compared with the pure gas tests in Table 2.
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of sorption competition between gases [45, 46].
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In general, the mixed gas data are slightly lower than the pure gas ones because of the presence
4. Conclusions
We have found that cPIM-1 and Matrimid are miscible for all the compositions as confirmed by
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PLM, AFM, DSC, FTIR and XRD. The miscibility between the two polymers is due to the formation of hydrogen bonding. As a result, a good agreement was observed between experimental and predicted gas permeability and selectivity. The addition of cPIM-1 in Matrimid
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also significantly enhances the plasticization pressure for all membranes. To our best knowledge,
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this is the first work discovering molecular miscibility that involves cPIM-1.
Acknowledgements
This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP Award No. NRF-CRP 5-2009-5 (NUS grant number R-279-000-311-281)).
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List of Figures
Chemical structures of (a) cPIM-1 and (b) Matrimid.
Fig. 2.
DSC curves of cPIM-1, Matrimid and cPIM-1/Matrimid blend films.
Fig. 3.
Comparison of Tg of the blend films obtained from DSC and the Fox equation.
Fig. 4.
FTIR spectra of cPIM-1/Matrimid blend systems.
Fig. 5.
FTIR spectra in the region of 3600-3000 cm-1 region of the cPIM-1/Matrimid
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membranes.
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Fig. 1.
Fig. 6.
XRD spectra of the cPIM-1/Matrimid membranes.
Fig. 7.
Correlations between experimental and predicted data of cPIM-1/Matrimid blends on O2 (□), N2 (∆), CH4 (○) and CO2 (◊) permeability.
Comparison between experimental and predicted data for: (a) O2/N2 selectivity; (b)
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Fig. 8.
CO2/CH4 selectivity. Fig. 9.
Permeability as a function of CO2 pressure and blend composition for cPIM-1/Matrimid
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membranes.
Fig. 10. Comparison with the Robeson upper bound: (a) H2/N2; (b) O2/N2; (c) CO2/CH4; (d)
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CO2/N2.
List of Tables
Table 1. Mechanical properties of cPIM-1/Matrimid membranes.
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Table 2. Pure gas permeability and selectivity of the cPIM-1/Matrimid membranes tested at 35 °C and 3.5 atm.
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Table 3. Binary gas permeability and selectivity of cPIM-1/Matrimid membranes.
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Maximum tensile stress (MPa)
3.24 ± 0.22 2.60 ± 0.34 2.71 ± 0.11 2.05 ± 0.99 1.84 ± 0.94 1.95 ± 0.79
6.04 ± 1.01 6.27 ± 1.96 7.83 ± 2.19 8.91 ± 0.70 6.92 ± 1.00 3.75 ± 1.15
114.07 ± 8.77 108.27 ± 3.51 111.39 ± 2.01 92.37 ± 1.49 78.25 ± 1.64 60.52 ± 3.52
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Extension at break (%)
EP
Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30)
Modulus (GPa)
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Membranes ID
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Table 1 Mechanical properties of cPIM-1/Matrimid membranes.
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CH4
2.03 3.09 3.91 9.01 27 88 177 445 462
0.32 0.48 0.63 1.72 5.4 21 47 121 143
0.26 0.39 0.55 1.74 6.1 25 57 170 209
1 Barrer = 1 × 10-10 cm3 (STP)cm/cm2s cmHg.
AC C
a
N2
CO2
9.4 13.6 17.9 48.7 145 486 982 2268 2654
M AN U
Matrimid 31.9 cPIM-1/Matrimid (5:95) 41.8 cPIM-1/Matrimid (10:90) 48.8 cPIM-1/Matrimid (30:70) 86.5 cPIM-1/Matrimid (50:50) 199 cPIM-1/Matrimid (70:30) 481 cPIM-1/Matrimid (90:10) 867 cPIM-1/Matrimid (95:5) 1568 cPIM-1 1619
O2
TE D
H2
EP
Membranes ID
SC
Permeability (Barrera)
RI PT
Table 2 Pure gas permeability and selectivity of the cPIM-1/Matrimid membranes tested at 35 °C and 3.5 atm. Ideal Selectivity
H2/N2 O2/N2 CO2/N2 CO2/CH4 99.7 87.1 77.5 50.3 36.9 22.9 18.4 13.0 11.3
6.3 6.4 6.2 5.2 5.0 4.2 3.8 3.7 3.2
29.4 28.3 28.4 28.3 26.9 23.1 20.9 18.7 18.6
36.2 34.9 32.5 28.0 23.8 19.4 17.2 13.3 12.7
ACCEPTED MANUSCRIPT
RI PT
Table 3 Binary gas permeability and selectivity of cPIM-1/Matrimid membranes. Permeability (Barrer)a CO2 CH4
Membranes ID
Selectivity CO2/CH4
cPIM-1/Matrimid (10:90)
13.9 (17.9)b
cPIM-1/Matrimid (30:70)
43.2 (48.7)
cPIM-1/Matrimid (50:50)
131 (145)
6.7 (7.1)
19.6 (20.4)
cPIM-1/Matrimid (70:30)
417 (486)
22.8 (25)
18.3 (19.4)
cPIM-1/Matrimid (90:10)
905 (982)
53.5 (57)
16.9 (17.2)
1.78 (1.88)
24.3 (25.9)
SC
28.4 (32.5)
M AN U
TE D
a
0.49 (0.55)
AC C
EP
CO2/CH4 (50:50 mole %) at 35 °C and 7 atm. b Number in parentheses is the permeability and ideal selectivity obtained from pure gas tests.
ACCEPTED MANUSCRIPT
O
H 3C CH3
(a)
RI PT
COOH O
O
O
H 3C
SC
CH3 COOH
(b)
O
O
O
M AN U
n
CH3
O
H3C
CH3
n
AC C
EP
O
N
TE D
N
Fig. 1. Chemical structures of (a) cPIM-1 and (b) Matrimid.
250
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
300
350
400
Temperature (oC)
Fig. 2. DSC curves of cPIM-1, Matrimid and cPIM-1/Matrimid blend films.
450
RI PT
ACCEPTED MANUSCRIPT
SC
440
M AN U
420
380
TE D
360 340 320 300
0.2
AC C
0
EP
Tg (oC)
400
0.4
Fox equation Experimental 0.6
0.8
1
cPIM-1 mass fraction
Fig. 3. Comparison of Tg of the blend films obtained from DSC and the Fox equation.
ACCEPTED MANUSCRIPT
RI PT
1700 (carboxylic C=O)
cPIM-1 cPIM-1/M (95:5)
M AN U
SC
cPIM-1/M (90:10) cPIM-1/M (70:30) cPIM-1/M (50:50) cPIM-1/M (30:70) cPIM-1/M (10:90) cPIM-1/M (5:95)
AC C
EP
TE D
Matrimid
1712 (imide C=O)
2000
1800
1600
1361 (imide C-N)
1400
1080 (imide C-N-C)
1200
Wavenumbers (cm-1)
Fig. 4. FTIR spectra of the cPIM-1/Matrimid blend system.
1000
705 (imide C=O)
800
600
3000-3600 (-COOH)
M AN U
SC
cPIM-1
RI PT
ACCEPTED MANUSCRIPT
cPIM-1/Matrimid (95:5)
TE D
cPIM-1/Matrimid (90:10)
AC C
EP
cPIM-1/Matrimid (70:30)
3600
3400
cPIM-1/Matrimid (50:50)
3200
3000
Wavenumbers (cm-1)
Fig. 5. FTIR spectra in the 3600-3000 cm-1 region of the cPIM-1/Matrimid membranes.
RI PT
ACCEPTED MANUSCRIPT
~5.7 Å
SC
Matrimid cPIM-1/Matrimid (5:95)
M AN U
cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30)
TE D
cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5)
AC C
EP
cPIM-1
~4.7 Å
~6.5 Å
5
10
15
~3.8 Å 20
2 Theta (o)
Fig. 6. XRD spectra of the cPIM-1/Matrimid membranes.
25
30
RI PT
ACCEPTED MANUSCRIPT
SC M AN U
100
O2
1
N2
EP
10
TE D
CO2
CH4 0.1 0.0
AC C
Permeability (Barrer)
1000
0.2
0.4
Rule of semi-logarithmic
0.6
0.8
1.0
Mass fraction of cPIM-1
Fig. 7. Correlations between experimental and predicted data of cPIM-1/Matrimid blends on O2 (□), N2 (∆), CH4 (○) and CO2 (◊) permeability.
35
4 3 2 selectivity
1
0.2 0.4 0.6 Mass fraction of cPIM-1
0.8
AC C
EP
0
TE D
selectivity (prediction)
0
30 25
M AN U
5
SC
6
CO2/CH4 selectivity
(b) 40
O2/N2 selectivity
(a) 7
RI PT
ACCEPTED MANUSCRIPT
1
20 15 10 5
selectivity selectivity (prediction)
0 0
0.2 0.4 0.6 Mass fraction of cPIM-1
0.8
Fig. 8. Comparison between experimental and predicted data for: (a) O2/N2 selectivity; (b) CO2/CH4 selectivity.
1
cPIM-1/Matrimid (95:5)
2000 1500
cPIM-1/Matrimid (90:10)
1000
500 400 300 200
cPIM-1/Matrimid (70:30)
RI PT
cPIM-1
2500
600
SC
3000
(b)
100
M AN U
Permeability (Barrer)
(a) 3500
Permeability (Barrer)
ACCEPTED MANUSCRIPT
cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (30:70)
0
500 0
5
10
15
20
25
20
25
30
TE D
10 15 Pressure (atm)
15.0
EP
cPIM-1/Matrimid (10:90)
cPIM-1/Matrimid (5:95)
AC C
Permeability (Barrer)
20.0
5
30
Pressure (atm) (c)
0
Matrimid
10.0
5.0 0
5
10
15
20
25
30
Pressure (atm)
Fig. 9. Permeability as a function of CO2 pressure and blend composition for cPIM-1/Matrimid membranes.
ACCEPTED MANUSCRIPT
(b) 100
1
10 100 1000 H2 permeability (Barrer)
RI PT 0.1
10000
(c)1000
1 10 100 O2 permeability (Barrer)
1000
100 Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
10 1
EP
TE D
(d)1000
AC C
CO2/CH4 selectivity
Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
0.1
0.1 1
1
M AN U
Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
10
10
SC
O2/N2 selectivity
100
1 10 100 1000 CO2 permeability (Barrer)
100 Matrimid cPIM-1/Matrimid (5:95) cPIM-1/Matrimid (10:90) cPIM-1/Matrimid (30:70) cPIM-1/Matrimid (50:50) cPIM-1/Matrimid (70:30) cPIM-1/Matrimid (90:10) cPIM-1/Matrimid (95:5) cPIM-1
10 1 0.1
0.1 0.1
CO2/N2 selectivity
H2/N2 selectivity
(a)1000
10000
0.1
10 1000 CO2 permeability (Barrer)
Fig. 10. Comparison with the Robeson upper bound: (a) H2/N2; (b) O2/N2; (c) CO2/CH4; (d) CO2/N2.
ACCEPTED MANUSCRIPT
Miscible blends of Matrimid and cPIM-1 at the molecular level have been discovered.
•
Hydrogen bonding promotes compatibility between these two polymers.
•
All blend membranes has a single Tg.
•
The inclusion of cPIM-1 in Matrimid enhances the plasticization pressure.
AC C
EP
TE D
M AN U
SC
RI PT
•
ACCEPTED MANUSCRIPT
RI PT
Supplementary data Morphology of cPIM-1/Matrimid blend films
SC
Fig. S1 shows the photos of cPIM-1/Matrimid dense films while Fig. S2 shows the homogeneity of dense blend films examined by PLM. All films cast from different cPIM-1/Matrimid ratios appear to be homogenous and transparent which indicate good miscibility. Fig. S3 depicts their surface roughness and root mean square value, Rq, measured by AFM. All cPIM-1/Matrimid films have Rq less than 4 nm which suggest very smooth surfaces in these blends.
M AN U
List of Figures
AC C
EP
TE D
Fig. S1. Photos of cPIM-1/Matrimid blend films. Fig. S2. PLM images of dense blend films: (a) Matrimid; (b) cPIM-1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM-1/Matrimid (50:50); (f) cPIM1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM-1/Matrimid (95:5); (i) cPIM-1. Fig. S3. AFM images on the surfaces of dense blend films: (a) Matrimid; (b) cPIM1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM1/Matrimid (50:50); (f) cPIM-1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM1/Matrimid (95:5); (i) cPIM-1.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. S1. Photos of cPIM-1/Matrimid blend films.
ACCEPTED MANUSCRIPT
(e) cPIM-1/Matrimid (50:50)
(f) cPIM-1/Matrimid (70:30)
(c) cPIM-1/Matrimid (10:90)
RI PT
(b) cPIM-1/Matrimid (5:95)
(g) cPIM-1/Matrimid (90:10)
(d) cPIM-1/Matrimid (30:70)
(h) cPIM-1/Matrimid (95:5)
TE D AC C
EP
(i) cPIM-1
M AN U
SC
(a) Matrimid
100µm
Fig. S2. PLM images of dense blend films: (a) Matrimid; (b) cPIM-1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM1/Matrimid (50:50); (f) cPIM-1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM-1/Matrimid (95:5); (i) cPIM-1.
ACCEPTED MANUSCRIPT
Rq = 1.4
(b) cPIM-1/Matrimid (5:95) Rq = 1.2
(c) cPIM-1/Matrimid (10:90) Rq = 1.2
(e) cPIM-1/Matrimid (50:50) Rq = 2.7
(f) cPIM-1/Matrimid (70:30) Rq = 2.6
(g) cPIM-1/Matrimid (90:10) Rq = 2.5
(d) cPIM-1/Matrimid (30:70) Rq = 3.2
M AN U
TE D
(h) cPIM-1/Matrimid (95:5) Rq = 1.1
AC C
EP
(i) cPIM-1 Rq = 0.9
SC
RI PT
(a) Matrimid
Fig. S3. AFM images on the surfaces of dense blend films: (a) Matrimid; (b) cPIM1/Matrimid (5:95); (c) cPIM-1/Matrimid (10:90); (d) cPIM-1/Matrimid (30:70); (e) cPIM1/Matrimid (50:50); (f) cPIM-1/Matrimid (70:30); (g) cPIM-1/Matrimid (90:10); (h) cPIM1/Matrimid (95:5); (i) cPIM-1.