Patent Application: US-201013266304-A

Abstract:
a film of a carboxylated polymer of formula : wherein the sum of x , y and z is an integer from 10 to 10 , 000 and degree of hydrolysis is 0 . 05 or greater provides gas separation materials in which the degree of hydrolysis may be used to tune the selectivity of the gases to an optimal required range . such films may be prepared by casting a film of a polymer of formula : wherein n is an integer from 10 to 10 , 000 , and hydrolyzing all or a portion of the — cn groups to form — cooh groups .

Description:
carboxylated pim membranes were prepared by in - situ hydrolysis of the nitrile groups of pim - 1 films . structural characterization was performed by fourier transform infrared spectroscopy ( ftir ) and proton nuclear magnetic resonance ( 1 h nmr ). the degree of hydrolysis was determined by carbon elemental analysis . the thermal properties were evaluated by differential scanning calorimetry ( dsc ) and thermogravimetric analysis ( tga ). compared with pim - 1 , carboxylated - pims with different degrees of hydrolysis have similar thermal and mechanical properties , but surprisingly show higher selectivity for gas pairs such as o 2 / n 2 , co 2 / n 2 , he / n 2 and h 2 / n 2 , with a corresponding decrease in permeability . selectivity coupled with high permeability combines to exceed the robeson upper - bound line for the o 2 / n 2 gas pair . the fully hydrolyzed pim - 1 film , dicarboxylated - pim , maintained good processibility since it could be dissolved in polar aprotic solvents such as dmf , dmac and nmp . the present invention demonstrates that significant improvements in gas separation properties may be obtained through post - modification of nitrile - based pim membranes , providing means to tune selectivity and permeability by varying hydrolysis reaction conditions , which can be feasibly applied to industrial application . the present invention improves the understanding of the relationship of structure / permeation properties , and also extends the pim spectrum beyond those reported previously . in addition , the incorporation of carboxylic acid sites has the potential for further modification reactions such as grafting and crosslinking . dimethylacetamide ( dmac , sigma - aldrich ), toluene ( sigma - aldrich ), methanol ( meoh , sigma - aldrich ), sodium hydroxide ( sigma - aldrich ), chloroform ( sigma - aldrich ) were used as received . 5 , 5 ′, 6 , 6 ′- tetrahydroxy - 3 , 3 , 3 ′, 3 ′- tetramethylspirobisindane ( ttsbi , sigma - aldrich ) was purified by crystallization from methanol . tetrafluoroterephthalonitrile ( tftpn , matrix scientific ) was purified by vacuum sublimation at 150 ° c . under inert atmosphere . the structures of the polymeric materials were fully characterized using nuclear magnetic resonance ( nmr ) spectroscopy at different temperature . nmr analyses were recorded on a varian unity inova ™ spectrometer at a resonance frequency of 399 . 961 mhz for 1 h and 376 . 276 mhz for 19 f . 1 h and 19 f nmr spectra were obtained from samples dissolved in cdcl 3 or dmso - d 6 using a 5 mm pulsed field gradient indirect detection probe . the solvent signals ( cdcl 3 1 h 7 . 25 ppm ; dmso - d 6 1 h 2 . 50 ppm ) were used as the internal references . an external reference was used for 19 f nmr : cfcl 3 0 ppm . molecular weight and molecular weight distributions were measured by gpc using ultrastyragel ™ columns and thf as the eluent at a flow rate of 1 ml / min . the values obtained were determined by comparison with a series of polystyrene standards . ftir ( fourier transformed infrared ) spectra were recorded on perkin - elmer ftir microscope with film samples at 8 cm − 1 resolution over the 400 - 4000 cm − 1 range . each sample was scanned 50 times . elemental analysis was carried out with a thermoquest chns — o elemental analyzer . polymer thermal degradation curves were obtained from thermogravimetric analysis ( tga ) ( ta instruments model 2950 ). polymer samples for tga were initially heated to 120 ° c . under nitrogen gas and maintained at that temperature for 1 h for moisture removal and then heated to 600 ° c . at 10 ° c ./ min for degradation temperature measurement . glass transition temperatures ( t g ) were observed from differential scanning calorimetry ( dsc ) ( ta instruments model 2920 ), and samples for dsc were heated at 10 ° c ./ min under a nitrogen flow of 50 ml / min , then quenched with liquid nitrogen and reheated at 10 ° c ./ min for the t g measurement . dense polymer films for gas permeability measurements were prepared from 1 - 2 wt % pim - 1 solutions in chloroform . pim - 1 solutions were filtered through 0 . 45 μm polypropylene filters and then cast into teflon ™ petri dishes in a glove box and allowed to evaporate slowly for 1 day . the membranes were soaked in methanol and dried in a vacuum oven at 100 ° c . for 24 h . the resulting membranes with thickness in the range of 70 - 90 μm were bright yellow and flexible . the absence of residual solvent in the membranes was confirmed by weight loss tests using tga . the pim - 1 membranes were soaked in 20 wt % sodium hydroxide solution ( h 2 o : methanol = 1 : 1 ). after hydrolyzing at different temperatures for different hydrolysis times , the membranes were boiled in water ( with a few drops hcl , ph = 4 - 5 ) for 2 h . following several washing cycles in water , the membranes were soaked in methanol for 1 h , then allowed to dry at ambient temperature . finally , the membranes were dried in a vacuum oven for 24 h by gradually increasing the temperature from ambient to 100 ° c . permeability coefficients ( p ) of n 2 , o 2 , he , h 2 , and co 2 were determined at 25 ° c . at a feed pressure of 50 psig and atmospheric permeate pressure using the constant - pressure / variable - volume method . the permeation flow was measured using a bubble flow meter , with the exception of co 2 , which was measured by a mass flow meter ( agilent adm 2000 ). p was calculated by using a following equation : where dv / dt is the permeate - side flow rate ( cm 3 / s ), t is the operation temperature ( k ) and δp is the gas pressure differential between the upstream and downstream sides of the membrane . the membrane effective area ( a ) was 9 . 6 cm 2 . in the present work , the pim - 1 starting material used for the hydrolysis experiments was gel - free and had high molecular weight ( mn = 50 , 000 , pdi = 2 . 0 ), which was obtained under an optimized polycondensation process ( du 2008 ). dense pim - 1 films were prepared from 1 - 2 wt % polymer solutions in chloroform , which were filtered through 0 . 45 μm polypropylene filters and then cast onto glass petri dishes in a glove box and allowed to evaporate slowly for 1 day . the resulting membranes with thicknesses in the range of 70 to 90 μm were soaked in 20 % sodium hydroxide solution ( h 2 o : methanol = 1 : 1 ) at different temperatures . scheme 1 shows possible resulting repeat units derived from different degrees of hydrolysis . polymer repeat units may contain zero , one or two nitrile groups and correspondingly have two , one or zero carboxylic acid groups . at low temperatures ( 25 ° c . and 65 ° c . ), the hydrolysis reaction occurred rapidly during the initial stage , but slowed down after 24 h , as demonstrated by ftir and 1h nmr measurements . at elevated temperature ( reflux at 120 ° c .) the hydrolysis reaction was complete within 5 h . the fully carboxylated - pim ( 120 ° c .- 5 h ) was characterized by 1 h and 19 f nmr spectroscopy . stacked 1 h nmr spectra of pim - 1 in cdcl 3 and carboxylated - pim in dmso - d 6 ( 120 ° c .- 5 h ) are displayed in fig1 along with peak assignments derived from 2d - nmr . the intensities and the shapes of the carboxylated - pim polymer 1 h nmr signals were monitored at different nmr probe temperatures : 50 ° c ., 85 ° c ., 100 ° c ., 120 ° c . the observed peak intensity ratio for the aromatic ( 5 . 75 - 8 . 50 ppm , h - 6 , 9 and cooh ) and aliphatic ( 0 . 25 - 2 . 4 ppm , h - 2 and ch 3 ) regions was exactly 6h : 16h as expected from the molecular structure . furthermore , the broad peaks in the 7 . 1 - 8 . 5 ppm area changed shape with increasing temperature . it is well known in nmr spectroscopy that changes in the sample temperature will affect the mobility of the molecules , and hence , the shape of the signals . this is particularly noticeable with protons involved in hydrogen bonding ( exchange rate , electron density around the h nuclei ) ( silverstein 1997a ) while other aromatic and aliphatic protons are often left unchanged . the spectra of fig1 are a good example of what can happen to the shape and shift of — cooh proton signals while h - bonding is affected by temperature changes . increased temperature disrupts the hydrogen bonding and gradually shifts the — cooh signals to lower frequencies ( less h - bonding ). a drop of d 2 o was added in the tube and its immediate effect observed in the 1 h nmr spectrum ( fig1 ). the — cooh protons exchanged with the deuterium nuclei of d 2 o ; hence , proving the presence of labile protons from the — cooh groups . it is worth mentioning that the full spectrum of carboxylated pim displayed in fig1 ( 120 ° c .) was acquired with a water suppression pulse sequence that resulted in the absence of a water peak at 3 . 7 ppm . finally , the polymers were scanned for 19 f nmr signals and no fluorine atoms were detected . the ftir spectra of the progress of hydrolysis at 120 ° c . at different reaction times to produce carboxylated - pim membranes are shown in fig2 . pim - 1 ( 0 h ) shows the characteristic nitrile absorption band at 2238 cm − 1 , while the absence of absorption bands in the range of 3000 to 3600 cm − 1 indicates no carboxylic acid group is present . after a one hour hydrolysis reaction time at 120 ° c ., the relative intensity of the nitrile absorption band decreased compared with other bands . broad strong absorptions comprising three bands are observed in the range of 3000 to 3600 cm − 1 , corresponding to o ˜ h stretching vibrations . a narrow intense absorption near 1700 cm − 1 arises due to stretching vibration of the c ═ o group . these combined bands imply that some of the nitrile groups were converted into carboxylic acid groups . it is notable that the three bands in the range of 3000 to 3600 cm − 1 represent three possible types of o — h stretching vibrations in carboxylated - pim membrane : free carboxylic acid structure ( 3500 cm − 1 ), hydrogen - bonded carboxylic acid dimers ( 3300 cm − 1 ) and o ˜ h hydrogen - bonded with dioxane ( 3100 cm − 1 ) ( silverstein 1997b ). this result is consistent with 1 h nmr results but more delicate . in addition , absorption near 1600 cm − 1 is observed in carboxylated - pims and pim - 1 , but is quite weak in pim - 1 . it is presumed that this band is comprised of the stretching vibrations of c ═ o and aromatic c — c . the c ═ o band is shifted to lower frequencies ( from 1700 cm − 1 to 1600 cm − 1 ) than those observed for free carboxylic acid due to strong hydrogen bonding . in addition , it is also considered that the intensity of the aromatic c — c band might increase because the center of symmetry in the aromatic ring is generally weaker in carboxylated benzene than in the one containing nitrile ( silverstein 1997 , clerc 1983 ). in order to prove that nitrile groups were converted into carboxylic acid groups , the hydrolysis reaction time was extended . the relative height of the c ═ o carboxylic acid absorption band increased obviously and the nitrile absorption band decreased until it disappeared after a 5 h reaction time , indicating that nitrile groups were completely hydrolyzed into carboxyl groups . generally , the intensities of — cn absorption bands can be used to calculate the approximate degree of hydrolysis . however , in the particular case of pim - 1 hydrolysis , where two symmetrical para - substituted — cn groups occur , symmetrical — cn groups have different intensities from asymmetrical — cn groups , which would occur after hydrolysis . therefore , the degree of hydrolysis cannot be conveniently determined quantitatively by ftir spectra . however , a relationship between degree of hydrolysis and carbon content of the polymer can be conveniently established using the following equations : c = 348 . 3 /( 460 . 48 + 38h ) or h =( 348 . 3 − 460 . 48c )/ 38c , where c is the carbon content and h is hydrolysis degree . a standard calibration line obtained by calculation is plotted in fig3 . the carbon content of the carboxylated - pims can be detected by elemental analysis , thus the degree of hydrolysis can be ascertained from the line . because carboxylated - pim is prone to water absorption from ambient air , the measured carbon content could be somewhat lower than the actual one , which would result in slightly lower degree of hydrolysis values . nevertheless , this method appears to be a feasible way to determine the degree of hydrolysis , and was much more effective than nitrogen analysis . as seen from fig3 and table 2 ( see below ), the hydrolysis reaction is very fast at elevated temperature . at 120 ° c ., 67 % — cn was hydrolyzed within 3 h . it is also shown that around 90 % — cn was converted into — cooh after 5 hours . however at low temperature , the reaction proceeds more slowly and the degree of hydrolysis is still quite low even after a prolonged reaction time . these results are in good agreement with 1 hnmr and ftir results . the reason that the hydrolysis reaction proceeds more slowly than the initial reaction can be explained plausibly by the ftir spectra . the hydrolysis of some of the nitrile groups to carboxylic acid groups provides the conditions for dimer formation due to strong hydrogen bonding of — cooh groups , or intermolecular hydrogen bonding of — oh with dioxane , which leads to the build up an impermanent network ( as shown in fig4 ). this could retard access of the hydrolysis reagent ( sodium hydroxide or other base and solvent ) into the polymer membrane or material . at elevated temperatures , the hydrogen or intermolecular bonds are weaker and the network is broken , such that the hydrolysis reagent can access the nitrile groups more easily , resulting in a faster reaction rate . after 5 h at 120 ° c ., the hydrolyzed membrane is still flexible and strong enough . as shown in table 1 ( see below ), the mechanical properties of carboxylated - pims are only slightly lower than those of pim - 1 . thermal analyses of carboxylated - pims and pim - 1 are summarized in table 1 . none of the polymers have a discernable t g in the measured range of 50 ° c . to 350 ° c . tga experiments showed that all the carboxylated - pims have good thermal stabilities and the actual onset temperature of decomposition in nitrogen is above 250 ° c . there is also a trend between t d and the degree of hydrolysis . generally , nitrile - containing polymers have high thermal stability , likely due to strong dipolar interactions . table 1 shows that with increasing degree of hydrolysis , the t d onset decreased . however , all carboxylated - pims still show very good thermal stability , even after complete hydrolysis of nitrile to carboxylic acid groups . pim - 1 is readily soluble in tetrahydrofuran ( thf ), dichloromethane ( ch 2 cl 2 ), chloroform ( chcl 3 ), but insoluble in polar aprotic solvents such as dimethylformamide ( dmf ), dimethylacetamide ( dmac ), and n - methylpyrrolidone ( nmp ). after partial hydrolysis at 120 ° c ., the hydrolyzed pim - 1 membrane was no longer soluble in ch 2 cl 2 and chcl 3 , but it was still partly soluble in thf . with further hydrolysis , thf was a non - solvent and dmf , dmac and nmp were good solvents for the hydrolyzed pim - 1 membrane , indicating that the carboxylated - pims still have good processability . gas permeabilities and selectivities of carboxylated - pims having different degrees of hydrolysis follow a trade - off relationship , similar to that observed for many glassy or rubbery polymers . in general , higher permeability is gained at the cost of lower selectivity and vice versa . pure - gas permeability coefficients ( p ) were measured on polymer dense films of pim - 1 and carboxylated - pims for o 2 , n 2 , h 2 , he , and co 2 . a summary of these p values and ideal selectivities for various gas pairs are shown in table 2 . gas permeability and selectivity of pim - 1 are known to be very sensitive to film preparation conditions and pre - treatment ( budd 2008 ). there is variation between the previously reported permeability data and the present data for pim - 1 as shown in fig5 . it is likely that this difference arises from the post - treatment protocol for the membranes . different from previous work , the hydrolyzed pim membranes were treated by first boiling in water ( with hcl , ph = 4 - 5 ), in order to remove sodium hydroxide and salts . after several washes in water , they were soaked in methanol , then allowed to dry naturally . finally , the membranes were dried in a vacuum oven for 24 h by gradually increasing the temperature from ambient to 100 ° c . for comparison , a pim - 1 membrane was treated identically . the permeabilities of the water - treated pim - 1 membrane for all the gases are higher than previous reports , while selectivities are lower , illustrating the strong effect of processing history on performance ( budd 2005b , du 2008 , budd 2008 , staiger 2008 ). the o 2 / n 2 selectivities for pim - 1 are above the robeson upper bound ( robeson 1991 ), with expected “ trade - off ” behavior between permeability and selectivity as shown in fig5 ( robeson 1991 , robeson 2008 ). surprisingly , all the carboxylated - pims exhibited higher o 2 / n 2 selectivity compared with pim - 1 ( no . 5 is pim - 1 which was fabricated and tested under the same conditions ), coupled with reductions in gas permeabilities . from the viewpoint of molecular modeling analyses by using hyperchem ™ 7 . 0 software , the interchain distance of the polymer is not extensively changed by introducing carboxylic acid groups into the pim . nitrile and carboxylic acid groups are similarly - sized small side groups , which do not have a large effect on interchain space filling . however , strong interchain hydrogen bonds may somewhat rearrange the chains , build up a network structure and enhance the rigidity of polymer chains , which would lead to lower permeability and higher selectivity . this hypothesis is in good agreement with the ftir spectral analysis . the intrinsic intermolecular force of these carboxylated - pims is expected to be independent of processing . the amount of hydrogen bonding network structures can be controlled by temperature and reaction time . thus , post - modification of pim - 1 by various hydrolysis conditions is a simple method to adjust or tune the gas permeability and selectivity . the contents of the entirety of each of which are incorporated by this reference . budd p m , ghanem b s , makhseed s , mckeown n b , msayib k j , tattershall c e . 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