The Segmental Motion and Gas Permeability of Glassy Polymer Poly(1

Chapter 22

The Segmental Motion and Gas Permeability of Glassy Polymer Poly(1-trimethylsilyl-1-propyne) Membranes

Downloaded by NORTH CAROLINA STATE UNIV on December 24, 2012 | http://pubs.acs.org Publication Date: January 28, 1999 | doi: 10.1021/bk-1998-0710.ch022

T. Nakagawa, T. Watanabe, and K . Nagai Department of Industrial Chemistry, Meiji University Higashi-mita, Tama-ku, Kawasaki 214, Japan

The permeability of poly[1-trimethylsilyl-1-propyne], PMSP, to light gases is higher than that of any other nonporous, synthetic polymer at ambient temperatures. PMSP is in the glassy polymer state at ambient temperatures. One problem with PMSP is a decrease of its gas permeability with age. During the aging process, C' decreased. The parameter C' represents the maximum concentration of penetrant gas in the unrelaxed (Langmuir) domains, that is microvoids, of glassy polymers. The spin-lattice relaxation time of C (T ) of the backbone chain carbons in the PMSP increased due to aging, i.e. molecular motion slowed, whereas molecular motion of the side-chain carbons did not change. A copolymer with 1-phenyl-1-propyne (PP) and a blend of PMSP and PPP showed smaller C ' values for CO and C H , and the decay of the permeability with aging was much improved for these polymers, especially for the blend PMSP/PPP 95/5 polymer membrane. T of the backbone carbons for the blend membrane were higher in the unaged state than for PMSP homopolymer but increased with aging to a similar value. The results suggest that the initial nonequilibrium state of the glassy PMSP, copolymer, and blend membranes was different and each glass approached a more stable state with age. H

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Poly[1-(trimethylsilyl)-1-propyne] (PMSP) is a typical glassy polymer at room temperature that was first synthesized by Masuda and Higashimura in the 1980's (1). Recently, membranologists have studied their gas permeation properties. The PMSP membrane has the highest gas permeability of all polymeric membranes. Therefore, this polymer is expected to have potential utility in industrial applications such as the separation of oxygen and nitrogen from air.

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©1998 American Chemical Society

In Structure and Properties of Glassy Polymers; Tant, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


327 An industrial gas separation system requires high gas permeability, a high separation factor, and durability. However, the biggest problem of PMSP polymer is a decrease in its gas permeability with age. Therefore, the authors have been studying the aging process and stabilization of the gas permeability of the PMSP membrane (2 - 7). It has been found that membrane contamination is a dominant factor causing the change in gas permeability and in the absence of contaminations, the aging is due solely to the relaxation of the unrelaxed volume (the unrelaxed free volume in the glassy state or excess free volume below the glass transition temperature). The volume relaxation can be attributed to physical aging, which is most likely related to a change in molecular motion. Normally the physical aging of glassy polymers depends on temperature, and the temperature effect on the physical aging of PMSP is remarkable (2). The high permeability of PMSP is due to the high diffusivity, which depends on a large excess free volume compared with other polymers. However, as this large excess free volume decreases, the diffusivity is reduced by physical aging. Controlling the free volume relaxation allows stabilization of the PMSP membrane through copolymerization with 1-phenyl-l-propyne (PP) (6,8) and blending with poly( 1-phenyl-l-propyne) (8). The objective of this investigation is to describe the effect of physical aging on the molecular motion of the membranes of PMSP and poly(l-trimethyl-lpropyne-co-1-phenyl-l-propyne) [poly(TMSP-co-PP] and blend polymer of PMSP with poly (1-phenyl-l-propyne) (PPP). The chemical structures of PMSP, poly(TMSP-co-PP), and PPP are shown in Figure 1.

PPP

PMSP

poly(TMSP-co-PP)

Figure 1. Chemical structures of PMSP, PPP, and poly(TMSP-co-PP). Experimental Materials The polymers used were the same PMSP, poly(TMSP-coPP), and PPP previously synthesized (6,8), having an average molecular weight between 80 χ 10 and 100 χ 10 . All membranes, including blends of PMSP and PPP, were cast on a horizontal glass plate from a solution of the polymer in toluene. They were immersed in methanol just before several measurements to prevent aging of the membranes. The drying conditions influenced the gas permeation properties of the PMSP membranes; therefore, the membranes were dried under vacuum according to the same method as previously described (6). 4

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Gas sorption and permeation measurements The gas sorption and permeation measurements were performed according to the same method described in previous In Structure and Properties of Glassy Polymers; Tant, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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studies (6,7). The gas sorption isotherms and the gas permeabilities were determined by the gravimetric method and the vacuum-pressure method, respectively. A silicone rubber packing that contained no blending agents was used for the gas permeation measurement. Aging condition The PMSP membranes were aged at 30°C in an evacuated vessel by an oil vacuum pump and a cold trap with liquid nitrogen which was placed between the pump and the vessel. The vacuum pressure was controlled between 10" and 10" mmHg. The vacuum pump oil used was Sanvac oil No. 160 (the Asahi Vacuum Chemical Industry Co., Ltd.), having a vapor pressure of 4.1 χ 10*mmHgatl0°C. 2

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NMR measurements The high resolution, solid state NMR measurements were performed at 30°C on a JEOL JNM 400 spectrometer. Sample membranes were cut into small pieces having an area of about 5 x 5 mm and packed into each sample tube. The spin lattice relaxation time (T ) of carbons and silicon were obtained by the CPT pulse sequence, operating at a spinning speed of 6 kHz. The measurements were carried out for more than three different membranes prepared at the sametimefrom the same cast solution. {

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Results and discussion Polymer Characterization The DSC curves of the PMSP, poly(TMSP-co-PP) showed no thermal change up to 280°C (6). The blends of the PMSP and PPP also had no thermal change during the DSC measurements over the same temperature range. The results of dynamic viscoelastic measurement by Masuda also showed no glass transition between -150 and 250°C. It is suggested that the Tg of these polymers is in excess of 280°C such that these polymers were glassy under the conditions used in the gas permeation, gas sorption, and solid-state NMR measurements in this study. Sorption isotherm for the aged PMSP Sorption isotherms for N , 0 , CH , and CO2 in PMSP membrane at 35°C have been reported by one of the authors (9). Sorption isotherms for QHg in the initial and aged PMSP at 35°C are shown graphically in Figure 2 in the form of plots of the concentration of c of the penetrant dissolved in the polymer versus the penetrant pressure p. Such isotherms are accurately described by the dual-mode sorption model (10): 2

,

c = c + c = k p + C bp/(l+bp) D

H

D

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(1)

H

where Cp is the concentration of the penetrant population in the Henry's domain and CH is the penetrant population dissolved in the unrelaxed domain or Langmuir's mode domain in the form of pre-existing microcavities; kp is a solubility coefficient in the Henry's mode domain, and C ' and b are a microcavities saturation constant and Langmuir affinity constant in the Langmuir's mode domain. C' can be taken as a measure of the excess free volume in this domain. As can be seen, with increasing the aging, the sorption of propane in the aged PMSP decreased. These isotherms were analyzed using the dual-mode sorption H

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Pressure [cmHg]

Figure 2. Sorption isotherms for propane in the initial and aged PMSP membranes at 35°C; initial (•), aged for 14 days (Δ), and for 30 days (O). Aging condition: stored in a vacuum vessel for 14 and 30 days. model. The dual-mode sorption parameter C' as calculated using equation 1 and decreased withtime:that is 54.4 in cm (STP)/cm (polymer) for one day, 40.0 for 14 days, and 22.3 for 30 days. A decrease in the C means that the membranes undergo excess volume relaxation with aging time. H

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Effect of aging on molecular motion of PMSP Figure 3 shows the C Tj in the initial and aged (14 days) PMSP membranes. The error-bar shows the maximum and the minimum values through the repeated measurements. The side chain protonated carbons of C and C have smaller T, values compared to the backbone chain non-protonated carbons of C and C (11), which means the molecular motion of carbon C, and Q was much faster than that of C and C . During aging, the change in the T values of the side chain carbons was smaller than those of the a

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Figure 3. C Tj in the initial (open) and aged (filled) PMSP. Aging condition: stored in a vacuum vessel for 14 days.



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backbone chain carbons. These results suggest that the molecular motions in methyl groups in side chains were not reduced by aging. Although relaxation of the unrelaxed volume occurred, the aged PMSP maintained a larger unrelaxed volume compared to other conventional glassy polymer. T value was reversible when the aged sample was re-cast. This NMR result may be due to a change in the degree of twisting of the backbone chains during aging. A schematic representation of the morphological change for molecular chain relaxation is shown in Figure 4. l

Effect of aging on the permeability and molecular motion of the membranes of PMSP, poly(TMSP-co-PP) and blend of PMSP/PPP Glassy polymers, such as PMSP, are nonequilibrium materials and their permeation and sorption properties drift over time as thermally driven, small-scale polymer segmental motions cause a relaxation of nonequilibrium excess free volume. The microcavities of large size which are present in PMSP membrane have been considered to be responsible for the decay of C and the gas permeability (4). Therefore, it is possible to stabilize the gas permeability by control the C' by copolymerization or blending with the other acetylene derivatives such as PP and PPP, respectively. Fig. 5 shows the effect of aging time on the permeability coefficient for oxygen in PMSP, poly(TMSP-co-PP), and PMSP/PPP blend membranes at 30°C. This figure also shows the effect of PP content in the copolymer and PPP content H

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Figure 5. Effect of aging on the permeability coefficient for in PMSP, copolymer, and blend membranes. Aging condition: stored in a vacuum vessel at 30°C. PMSP (•), poly(TMSP-co-PP) 95/5 (O), 80/20 ( · ) , blend PMSP/PPP 95/5 (Δ), and 80/20 (A). Table I Dual-mode sorption parameter, C , for C 0 and CjH in membranes homopolymer, copolymer, and blend polymer at 35°C Polymer C'„ [cmPÇSTPycm^polymer)] CjH, CQ> 151 50.1 PMSP 40.0 PMSP* 51.0 Poly(TMSP-co-PP)95/5 45.0 116 134 40.0 Blend PMSP/PPP 95/5 PPP 12.5 28.5 a: Aged under vacuum for 14 days. H

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in the blend polymer. Of course, the higher the PP or PPP content, the lower the permeability coefficient, because the gas permeability of homopolymer PPP is lower than that of PMSP (6,8). The most stable membrane in this study for the oxygen permeability was the blend 95/5 (PMSP/PPP). The phase separation was observed in the blend PMSP/PPP which contained the higher content of PPP than 5 wt%. The C' of these membranes for C 0 and C H are summarized in Table I. Interestingly, although the initial C value of C 0 in the blend polymer, which is the most stable polymer, is higher than that of copolymer, the opposite results appeared in the QHg case. However, C ' value is always total volume for the related microvoids. The microvoids of glassy polymers have a size distribution. A C0> molecule is small relative to a CjHg. If we compared with the C ' value for C H , C ' in the blend is smaller than that of the copolymer. This result suggests that there are a small number of large size microvoids in the blend, because QHg H

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Figure 6. Effect of aging on C T in poly(TMSP-co-PP) 95/5 and blend PMSP/PPP 95/5. (a) poly(TMSP-co-PP), (b) blend PMSP/PPP. t

molecules would not be able to sorb in the small size microvoids. Figure 6 shows the comparison of the spin-lattice relaxation time, T for the aged poly(TMSP-co-PP) and the aged blend PMSP/PPP with those of the corresponding fresh polymers. As can be seen, the T of the C and C in both the copolymer and the blend was not changed, similar to the result found for PMSP. The Tj of the C and C in the copolymer was not changed either. The volume relaxation may be prohibited by the added phenyl group, suggesting a stacking effect of the phenyl groups. It is considered that the copolymer still has the enough space for the molecular motion of the backbone carbons after aging. However, a distinct increase in T was observed at the C and C , backbone carbons, in the blend. Considering the results of C' , the very small morphological change due to aging in the blend, it is thought that the decrease of the microvoids of the larger size in the blend caused the slower molecular motion. The gas permeability of these membranes with rich PMSP structure depended on the larger free volume rather than on molecular motion in the T measurement. p

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Conclusions Poly(l-trimethylsilyl-l-peopyne), PMSP, contains an unusually large excess free volume, which may be also responsible for the high gas permeability and its decay with aging. Molecular motion, which is evaluated by the spin-lattice relaxation time, T,, of the backbone carbons was increased with age, but molecular motion of side chain was very stable after aging. This means that the larger molecular scale gap between polymer segments still exists in the aged PMSP membrane. Modification of PMSP for the stabilization of gas permeability by the copolymerization or blending with phenyl-containing acethylene derivative was successful. Especially, the blend PMSP/PPP 95/5 is very stable with high gas permeability. The change of molecular motion of the backbone carbons in the blend after aging is different than that of the copolymer. This suggests that the initial non-equilibrium state of the glassy PMSP, copolymer and blend membranes was different and approached a more stable state with age. Literature Cited 1. Masuda, T; Isobe, E.; Higashimura, T.; Takada, K. J. Amer. Chem. Soc. 1983, 105, 7473. 2. Nakagawa, T.; Saito,T.;Asakawa,S.; Saito, Y. Gas Separation and Purification 1988, 2, 3. 3. Asakawa, S.; Saito, Y.; Waragai, K.; Nakagawa, T. Gas Separation and Purification 1989, 3, 117. 4. Nakagawa, T.; Fujisaki, S.; Nakano, H.; Higuchi, A. J. Memb. Sci. 1994, 94, 183. 5. Nagai, K.; Higuchi, Α.; Nakagawa, T. J. App. Polym. Sci. 1994, 54, 1353. 6. Nagai, K.; Higuchi, Α.; Nakagawa, T. J. Polym. Sci., Polym. Phys. Ed. 1995, 33, 289. 7. Nagai, K.; Nakagawa, T. J. Memb. Sci. 1995, 105, 261. 8. Nagai, K.; Mori, M.; Watanabe, T.; Nakagawa, T. J. Polym. Sci., Polym. Phys. Ed. 1997, 35, 119. 9. Ichiraku, Y.; Stern, S. Α.; Nakagawa, T. J. Memb. Sci. 1987, 34, 5. 10. Vieth, W. R.; Howell, J. M.; Hsieh, J.J. Memb. Sci. 1976, 1, 177. 11. Costa, G.; Grosso, Α.; Sachi, M . C.; Stein, P. C.; Zetta, L. Macromolecules 1991, 24, 2858.


The Segmental Motion and Gas Permeability of Glassy Polymer Poly(1

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