Membrane Formation and Modification - American Chemical Society

Chapter 17

Membranes from Modified Poly(1-trimethylsilyl-1-propyne) 1

2

1

1

C. Merano , M . Andreotti , A . Turturro , F. Vigo , and G . Costa

2

1

Dipartimento di Chimica e Chimica Industriale, University of Genova, via Dodecaneso 31-16146 Genova, Italy Istituto di Studi Chimico-Fisici di Macromolecole Sintetiche e Naturali, IMAG-CNR, via De Marini 6-16149 Genova, Italy

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0744.ch017

2

The application of poly(l-trimethylsilyl-1-propyne) [PTMSP] in membrane-based gas separation processes is limited by its low selectivity and very fast physical aging which negatively affect the performance of this extraordinary polymer. To overcome these drawbacks several methods have been proposed and among them copolymerization seems to be one of the most promising. Novel copolymers with different compositions, obtained from monomers of 1-trimethylsilyl-1-propyne [TMSP] and 1-phenyl-2trimethylsilylacetylene [PhC≡CSiMe ] or 1-trimethylsilyl-1pentyne [n-PrC≡CSiMe ] were synthesized, characterized, and isotropic films were made from solution by evaporation of the solvent under controlled conditions. Permeability and selectivity measurements, carried out with air, demonstrated that the permeation properties were stable even after a week of testing, indicating that the introduction of comonomer units to the PTMSP mainchain prevents physical aging to occur. When PhC≡CSiMe was used as a comonomer, the gas permeability was always lower than that of the PTMSP homopolymer and the selectivity depended on the copolymer composition. Membranes made from TMSP/nPrC≡CSiMe copolymers exhibited higher permeabilities than PTMSP and their selectivity was essentially independent of the composition. 3

3

3

3

Poly(1-trimethylsilyl-1-propyne) [PTMSP] has drawn great attention in the last decade due to its extremely high gas permeability, which, despite its glassy state, is an order of magnitude higher than that of polydimethylsiloxane, the most permeable rubber known (i). Moreover, the variation of its permeability as a function of

252

© 2000 American Chemical Society

In Membrane Formation and Modification; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


253 temperature shows an opposite trend as compared to that of conventional glassy polymers, such as polysulfone, that is, by decreasing the temperature, the permeability coefficient of PTMSP increases (2). A loose packing of the chains, due to the steric hindrance of the substituents, together with a rigid backbone induced by the presence of the alternating double bonds along the main chain with both cis- and trans-configuration seem to be responsible for an extremely high free volume and, consequently, the peculiar transport properties of PTMSP (3-6). However, when a PTMSP membrane is stored at ambient conditions or used continuously in gas separations with permanent gases or in pervaporation processes for the separation of liquids, its permeability decreases significantly over time (2,3,7-10). The physical aging in PTMSP and other substituted polyacetylene films, such as poly(tert-butylacetylene) (11), seems to be responsible for this reduction in permeability. Many efforts have been made to overcome this drawback and different approaches such as U V irradiation (12,13), copolymerization (14,16), chemical and physical modifications (17-21), plasma polymerization (22), and grafting of a proper monomer (23,24) have been suggested. In these cases, an improvement in selectivity and a reduction of the gas permeability combined with a stable long-term membrane performance were achieved. In the present work, the synthesis of some novel copolymers, obtained from TMSP and 1-trimethylsilyl-l-pentyne [«~PrCs€SiMe ] or l-phenyl-2trimethylsilylacetylene [PhOCSiMe ], and the transport properties of membranes composed of these polymers are reported. 3

3

Experimental Materials. The monomers TMSP, «-PrC=CSiMe , and P h O C S i M e (from Aldrich) were distilled over C a H and stored under argon. A l l solvents were dried under reflux over L1AIH4, distilled, and kept under argon atmosphere. The catalysts, TaCl and N b C l , and cocatalysts, Ph Bi (99%) and Bu Sn (98%), were purchased from Aldrich-Fluka and were used without further purification. 3

3

2

5

5

3

4

Polymer Synthesis. Homopolymers and copolymers were synthesized in toluene following the procedures described in the literature (25,26). The polymerization was carried out at 80°C over a period of 24 hours. Conversion was determined by measuring the initial and final monomer concentrations by gas chromatography (GC) using chlorobenzene as an internal standard. The polymer yield, as a methanolinsoluble fraction, was determined by gravimetry, after the polymers were dissolved in toluene and precipitated twice in a large excess of methanol. Polymer Characterization. Solubility tests in different solvents were made in order to ascertain the formation of copolymers. Molecular characterization of the


254 polymers was carried out by intrinsic viscosity measurements in toluene and by size-exclusion-chromatography (SEC; Perkin-Elmer Diode Array Detector), using CHCI3 as the eluent with a flow rate of lml/min and a set of μ-styragel columns of 10 , 10 , and 10 A . The thermal properties of all polymers were evaluated by thermal gravimetric analysis (Perkin Elmer TGA-7) and differential scanning calorimetry (DSC-7 Perkin Elmer). A heating rate of 10 K/min was always used. Raman spectra, both on monomers and polymers, were recorded with a Bruker RSS 100 spectrophotometer.


3

4

5

Membrane Preparation. Membranes were prepared from toluene solutions, typically 0.7-1.5% by weight of polymer, by evaporation at room temperature under controlled conditions over a period of 7 days and dried at 60°C under vacuum overnight. The thickness of the membranes ranged from 25 to 50 μιη. Only freshly prepared membranes were used for permeability measurements. Permeability and Selectivity Measurements. Membranes were tested by permeability measurements using gas chromatography-grade air. The apparatus used for the tests was assembled in our laboratories and described in a previous paper (24). Membranes were tested at a feed pressure of 1 M P a and a permeate pressure of 0.1 MPa. Each experiment was carried out for 140 to 180 hours in order to check the long-term stability of the membranes. Results and Discussion A preliminary investigation on the polymerization of «-PK>CSiMe and PhC=CSiMe (Figure 1) was carried out to verify the possibility to obtain homopolymers. The reactions were performed at 80°C in toluene for 24 hours by using different catalysts, such as N b C l and TaCl , with or without a cocatalyst (Ph Bi, B^Sn). None of the reactions resulted in high molecular weight polymers, that is, methanol insoluble products, although in some cases monomer conversions up to 70% were obtained. This result suggests that in our experimental conditions the two monomers were essentially unable to form homopolymers. 3

3

5

5

3

Copolymers of different compositions were obtained by polymerizing TMSP with various amounts of «-PrCsCSiMe or PhCsCSiMe . The results are summarized in Tables I and II, where the conversion, polymer yields, and inherent viscosities are given together with the molar ratios of the initial monomers; a molar ratio of [TMSP]/[Cat] = 50 was used in all experiments. The most efficient catalyst system was an equimolar mixture of TaCl and Ph Bi, which generally gave nearly 100% conversion of TMSP and lower values for the comonomers (between 30 and 80 wt%). This means that copolymer compositions differed from those of the corresponding monomer mixtures, which were used to identify the copolymers. Currently, characterizations by N M R 3

3

5

3


255 spectroscopy are in progress to determine the real compositions of the copolymers and their chemical structures. When only TaCl was used, conversions were fairly high, but the methanol insoluble fraction was generally rather low, indicating that high molecular weight polymers can only be obtained when a cocatalyst is present. Polymers made from N b C l always gave very low conversions and yields. 5

5

Table I. Copolymers of TMSP and n-PrCsCSiMe . [catalyst]/[co-catalyst] = 1; [ T M S P y i T a C y = 50; solvent: toluene; Τ = 80°C; t = 24 h. 3


[TMSP]o/[COM]

0

50:50 50:50 80:20 80:20 80:20 70:30 60:40 50:50 a) determined at c =

Catalyst

TaCl NbCl TaCl -Ph Bi TaCl -Bu Sn NbCl -Ph Bi TaCl -Ph Bi TaCl -Ph Bi TaCl -Ph Bi 0.1 g/dl in toluene at 5

5

5

3

5

4

5

3

5

3

5

3

5

3

Conversion (%) COM TMSP 28 84

Yield (wt%)

η inh"

24

(dl/g)

-

-

-

-

90 50 15 96 94 96 30°C.

71 5 10 33 10 33

80

7.3

-

-

-

6.8

63 51 53

4.3

Table Π. Copolymers of TMSP and P h O C S i M e . [catalyst]/[co-catalyst] = 1; [TMSPypraCy = 50; solvent: toluene; Τ = 80°C; t = 24 h. 3

[TMSP]o/[COM]

0

Catalyst

Conversion (%)

50:50 50:50 80:20 70:30 60:40 50:50 80:20

TaCl NbCl TaCl -Ph Bi TaCl -Ph Bi TaCl -Ph Bi TaCl -Ph Bi NbCl -Ph Bi 5

5

5

3

5

3

5

3

5

3

5

3

COM 36

TMSP 100 40 100 100 95 100 60

Yield (wt%)

η inh

51

-

a

(dl/g)

-

-

64 63 80 56 20

89 60 58 41

9.2 8.7 8.9

-

-

a) determined at c = 0.1 g/dl in toluene at 30°C.

Solubility tests and characterization by Raman spectroscopy, carried out on all polymers, confirmed that copolymers were obtained, although their composition was not the same as for the initial reaction mixture. The Raman spectrum of one of the copolymer samples [(TMSP/PhC=CSiMe (70/30)] together with that of the PTMSP homopolymer are shown in Figure 2; the presence of the aromatic substituent in the copolymer is confirmed by the absorption bands at 3,060 cm' . 3

1


256

Si(CH )

fi(CH )3

3

3

3

Si(CH ) 3


Ο

CH CH

l-phenyl-2-trimethylsilylacetylene (PhC=CSiMe ) 3

Figure 1.

3

3

3

1-trimethylsityl-l-pentyne

1-trimethylsilyl-l-propyne

(n-PrC=CSiMe )

(TMSP)

3

Structure of di-substituted acetylene-based monomers.

3000 2500 2000

500

1500 1000 Wavenumber cm -1

Figure 2. Raman spectra of (a) PTMSP homopolymer [catalyst: TaCl ], and (b) T M S P / P h O C S i M e 70/30 copolymer [catalyst:TaCl /Ph Bi]. 5

3

5

3


257 1

1

(aromatic CH), 1,600 cm" (anti-symmetric stretching of the ring), and 1,000 cm" (phenyl breathing). Furthermore, the thermal stability of the copolymers was always higher than that of PTMSP, as shown in Figure 3. Degradation temperatures, T , of 330, 370, and 390°C were found for PTMSP, TMSP/PhCsCSiMe , and T M S P / H P r O C S i M e (50/50 moles) copolymers, respectively. d

3

3

Molecular characterization was performed by SEC on TMSP/w-PrCsCSiMe and TMSP/PhCs=CSiMe copolymers obtained from 50/50 mixtures of the comonomers. Wide molecular weight distributions were found. The M k values, based on a polystyrene calibration, ranged from 200,000 g/mol for TMSP/«PrCsCSiMe copolymers to 500,000 g/mol for the TMSP/PhCsCSiMe systems. However, viscosity data and molecular weights cannot be related straightforwardly due to the limited number of samples examined so far. 3

3


pea

3

3

The DSC analysis did not show any glass transition temperature prior to polymer degradation or any exothermic phenomenon associated with chain packing, as previously observed for P T B A and PTMSP samples (11,27). The transport properties of the membranes were investigated by permeability measurements, reported in Barrer units (1 Barrer =10" cm (STP>cm/cm -s*cm«Hg). The membrane selectivity, a, was calculated from the ratio of the O2 and N2 permeabilities. Permeation tests carried out on membranes prepared from PTMSP and from TMSP/PhCsCSiMe copolymers are reported and compared as a function of time in Figures 4 and 5. The slight increase in permeability of copolymers which occurred after a few hours of testing (Figure 4) can be ascribed to moistening of the membranes which occurred after placing them into the test cell. This effect can hardly be detected in PTMSP due to the permeability decrease which occurred rapidly with time in this polymer. The permeability trend of copolymers is in accordance with the observed decrease in selectivity (Figure 5). However, after a few hours of operation, the permeability and selectivity of the copolymers did not change anymore until the end of the experiments (140 hours). On the contrary, as is well known (2,3,7-10), and shown in Figure 4, the performance of PTMSP changed dramatically during testing. Moreover, it appears that the characteristics of the membranes depended on the copolymer composition; the permeability increased and selectivity decreased by increasing the TMSP content in the copolymer and, after 140 hours, membranes made from TMSP/PhC=CSiMe (50/50) copolymer had practically the same permeability as PTMSP. 10

3

2

3

3

The introduction of a co-monomer unit bearing a phenyl group provides membranes with a lower but more stable permeability, which suggests that molecular motions might be restricted and/or the excess free volume of the copolymers did not decrease during aging. When the methyl group in PTMSP, which has a lower critical surface tension compared to a phenyl ring (28), was replaced by an aromatic moiety, an increase in chain packing occurred. In other words, the strong chain-chain interactions combined with the planar structure of the aromatic ring allowed a tight chain packing, and, thus, a reduction in the excess free volume (5).


258


120

0J

1

1

0

100

200

1

1

" " " 1 " 1

300 400 Temperature (°C)

500

600

' 700

Figure 3. T G A traces of PTMSP [catalyst:TaCl ] (—), TMSP/PhC=CSiMe 50/50 [catalyst: TaCl /Ph Bi] (—), and TMSP/«PrO=CSiMe 50/50 [catalyst: TaCl /Ph Bi] ( ) copolymers. 5

3

5

3

5

3

3

12,000

150

Figure 4. A i r permeability of membranes made from PTMSP [catalyst: TaCl ] and TMSP/PhG=CSiMe copolymers [catalyst: TaCl /Ph Bi] as a function of testing time. 5

3

5

3



259

30

60

90

120

150

Time (hours)

Figure 5. Oxygen/nitrogen selectivity of membranes made from PTMSP [catalyst TaCl ] and TMSP/PhCs€SiMe copolymers [catalyst TaCl /Ph Bi] as a function of testing time. 5

3

5


3

260 Similar results were recently published for membranes prepared from copolymers of TMSP and 1-phenyl-1-propyne [PP] and blends of the corresponding homopolymers (29). In this case, membranes with a high stability of gas permeability were obtained by using either copolymers of TMSP with small amounts of PP monomer or blends of PTMSP and PPP, although the permeability values were always lower than those of PTMSP. Very interesting results are shown in Figures 6 and 7 for TMSP/w-PrOCSiMe copolymer membranes. The air permeability of these copolymers was practically independent of the composition and was always much higher than that of PTMSP (13,000 and 3,000 Barrers, respectively, after 140 hours). On the other hand, the selectivity values of the copolymers were rather low, even lower than that of the PTMSP homopolymer. However, the most important feature is the constant value of the performance for all the copolymers membranes, at least for time period during which the measurements were carried out (0-140 hours).


3

The difference in the chemical structure of TMSP and «-PrfeCSiMe was only one substituent; the methyl group was replaced by a larger w-propyl side-group. It is known that longer w-alkyl side chains have higher flexibility and can increase the free volume ( J ) ; consequently, the high permeability of the copolymers of T M S P / A P r O C S i M e can be ascribed to a higher free volume content than that of PTMSP. 3

3

Conclusions The introduction of a phenyl- or a propyl-group can either inhibit the physical aging of PTMSP-based copolymer membranes or drastically reduce the kinetics of the packing phenomenon (5), due to the simultaneous presence of the co-monomers of the very bulky trimethylsilyl-group and a phenyl- or w-propyl-substituent. Spectroscopic investigations are currently in progress to obtain additional insights on the copolymer composition, in order to better correlate the performance of the samples to their structure. Further experimental work is under way to synthesize novel copolymers from TMSP and other silicon-containing comonomers bearing bulky substituents to improve our knowledge of structure-property relationships in this very interesting class of polyacetylenes. Acknowledgements. The authors greatfully acknowledge Mr. V . Trefiletti for carrying out the thermal characterizations. This work was partially supported by M U R S T (60%) and by Progetto Strategico CNR, Italy. Literature Cited 1. 2. 3.

Masuda, T.; Isobe, E.; Higashimura, T. J. Am.Chem. Soc. 1983, 105, 7473. Takada, K.; Matsuya, H.; Masuda, T.; Higashimura, T. J. Appl. Polym. Sci. 1985, 30, 1605. Masuda, T.; Higashimura, T. Adv. Polym. Sci. 1987, 81, 121.


261

TMSP/n-PrCsCSiMea 100/0 80/20 —50/50

18,000

14,000


10,000 \

6,000

2.000 0

1

1

1

20

40

60

H

80

h

1

1

1

100

120

140

160

180

Time (hours)

Figure 6. A i r permeability of membranes made from PTMSP [catalyst: TaCl ] and TMSP/«-PrC=CSiMe copolymers [catalyst: TaCl /Ph Bi] as a function of testing time. 5

3

1.1 -I 0

1

1

1

20

40

60

5

1

1

80 100 Time (hours)

3

1

1

1

!

120

140

160

180

Figure 7. Oxygen/nitrogen selectivity of membranes made from PTMSP [catalyst: TaCl ] and TMSP/«-PrC=CSiMe copolymers [catalyst: TaCl /Ph Bi] as a function of testing time. 5

5

3

3


262 4. 5. 6. 7. 8.


9.

10.

Clough, S.B.; Sun, X.F.; Tripathy, S.K.; Baker, G.L. Macromolecules 1991, 24, 4264. Savoca, A.C.; Surnamer, A.D.; Tien, C.-F. Macromolecules 1993, 26, 6211. Srinivasan, R.; Auvil, S.R.; Burban, P.M. J. Membr. Sci. 1994, 86, 67. Langsam, M.; Robeson, L.M. Polym. Eng. Sci. 1989, 29, 44. Tasaka, S.; Inagaki, N.; Igawa, M. J. Polym. Sci.,Part B, Polym. Phys. 1991, 29, 691. Yampol'skii, Yu.P.; Shishatskii, S.M.; Shantorovich, V.P.; Antipov, E . M . ; Zuzmin, N . N . ; Rykov, S.V.; Khodjaeva, V . L . ; Plate, N . A . J. Appl. Polym. Sci. 1993, 48, 1935. Camera-Roda, G.; Bottino, Α.; Capannelli, G.; Costa, G.; Sarti, G. Proceedings of 5 Int.Conference on "Pervaporation Processes in the Chemical Industry", R.Bakish (Ed.), Englewood NJ, 1991. Costa, G.; Chikhaoui, S.; Turturro, A . Carpaneto, L . Macromol. Chem. Phys. 1997, 198, 239. Hsu, K . K . ; Nataraj, S.; Thorogood, R . M . ; Puri, P.S. J. Membr. Sci. 1993, 79, 1. Chen, G.; Griesser, H.J.; Mau, A . H . J. Membr. Sci. 1993, 82, 99. Hamano, T.; Masuda, T.; Higashimura, T. J. Polym. Sci., Part A, Polym. Chem. 1988, 26, 2603. Goudong, Z.; Nakagawa, T.; Higuchi, Α.; Nagai, K. Chin. J. Polym. Sci. 1992, 10, 203. Nagai, K.; Higuchi, Α.; Nakagawa, T. J. Polym. Sci., Part B, Polym. Phys. 1995, 33, 289. Nagase, Y.; Ishihara, K.; Matsui, K. J. Polym. Sci., Part B, Polym. Phys. 1990, 28, 377. Nagase, Y.; Ueda, T.; Matsui, K.; Uchikura, M. J. Polym. Sci., Part B, Polym. Phys. 1991, 29, 171. Nagase, Y.; Sugimoto, K.; Takamura, Y.; Matsui, K. J. Appl. Polym. Sci. 1991, 43, 1227. Nagase, Y.; Takamura Y . ; Matsui, K. J. Appl. Polym. Sci. 1991, 42, 185. Nakagawa, T.; Fujisaki, S.; Nakano, H.; Higuchi, A. J. Membr. Sci. 1994 , 94, 183. Lin, X . ; Chen, J.; Xu, J. J. Membr. Sci. 1994, 90, 81. Yang, J.-S.; Hsiue, G.-H. J. Membr. Sci. 1996, 111, 27. Vigo, F.; Traverso, M . ; Uliana, C.; Costa, G. Sep. Sci. and Techn. 1996, 31, 349. Masuda, T.; Isobe, E.; Higashimura, T. Macromolecules 1985, 18, 841. Masuda, T.; Isobe, E.; Hamano, T.; Higashimura, T. Macromolecules 1986, 19, 2448. Costa, G. unpublished results. Burton, A . F . M . "Handbook of Solubility Parameters and other Cohesion Parameters", CRC Press Inc. Boca Raton (Florida) 1983. Nagai, K.; Mori, M . ; Watanabe, T.; Nakagawa, T. J. Polym. Sci., Part B, Polym. Phys. 1997, 35, 119. th

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.


Membrane Formation and Modification - American Chemical Society

Recommend Documents