Membrane Formation and Modification - American Chemical Society

Chapter 18

Pervaporation Properties of Surface-Modified Poly[(1-trimethylsilyl-1-propyne] Membranes T. Uragami, T. Doi, and Tadashi Miyata

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.ch018

Chemical Branch, Faculty of Engineering and High-Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan

Poly[l-(trimethylsilyl)-l-propyne] (PTMSP) membranes were surface-modified using polymer additives and their pervaporation properties for an aqueous ethanol solution were investigated. The polymer additives, PFA-g-PDMS and PMMA-g-PDMS, were synthesized by graft-co-polymerization of fluoroacrylate (PFA) and oligo dimethylsiloxane (DMS), and methylmethacrylate (MMA), respectively. The contact angle of water on the surface of the air-side of the surface-modified PTMSP membranes increased by the addition of PFA-g-PDMS and PMMA-g-PDMS. These results suggest that the surface-modified PTMSP membranes were more hydrophobic. Also, X-ray photoelectron spectroscopy (XPS) measurements supported that the polymer additives were concentrated at the surface of the air-side of the modified PTMSP membranes. The ethanol/water selectivity of the PTMSP membrane modified with PFA-g-PDMS increased significantly without a large decrease in flux compared with that of the PTMSP membrane. On the other hand, after modification of PTMSP with PMMA-g-PDMS both the flux and ethanol/water selectivity had a maximum at a certain amount of PMMA-g-PDMS. The ethanol/water selectivity of the modified membrane was higher than that of the PTMSP membrane. High ethanol/water selectivity of the surface-modified PTMSP membranes depended on the solubility selectivity of the permeating species.

It is well known that a poly[ 1 -(trimetylsily 1)-1 -propyne] (PTMSP) membrane has a high ethanol/water selectivity and a high flux for aqueous ethanol solutions in pervaporation (PV) (1,2). The high ethanol/water selectivity results from the high

© 2000 American Chemical Society

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

263


264 solubility of ethanol in the PTMSP membrane. Recent work has focused on improvement of the ethanol/water selectivity for aqueous ethanol solutions of PTMSP membranes in pervaporation applications. The ethanol/water selectivity of PTMSP membranes was improved by grafting dimethylsiloxane (3,4) and acrylic acid (5), introducing alkylsilyl (6) and fluoroalkyl groups on the PTMSP backbone (7), and blending dimethylsiloxane oligomer in PTMSP; however, the fluxes of the modified PTMSP membranes were low. A higher ethanol/water selectivity for a modified PTMSP membrane cannot be achieved on the basis of differences in diffusivity, because ethanol is larger than water. Therefore, modifications of the membrane surface to enhance the solubility of ethanol in the membrane relative to that of water are required. As the solubility of ethanol in the membrane is improved, however, the membrane surface swells, and consequently, the solubility of water in the membrane surface also increases. Accordingly, it is very important to lower the affinity of water molecules for the membrane. There are many techniques for surface modification of polymer membranes. One of these techniques is the addition of a polymer additive to the casting solution, which readily modifies the membrane surface. In this study, polymer additives consisting of a hydrophobic part to repel water at the membrane surface and an interacting part for the membrane matrix were synthesized for the purpose of surface modification of the PTMSP membrane. The relationship between the surface properties of the PTMSP membranes modified with the polymer additives and their permeation properties in pervaporation of aqueous ethanol solutions is discussed in detail. Experimental Materials. l-(Trimethylsilyl)-l-propyne, purchased from Huls America, Inc., was distilled twice over calcium hydride under nitrogen atmosphere. The polymerization catalyst, tantalum pentachloride (TaCl ) (Wako Pure Chemical Industries, Ltd.) was used without further purification. The polymerization solvent, toluene, was washed with 5 wt.% H S 0 , 1 0 wt.% NaOH solution, and water, dried over calcium chloride for 2 days, and then distilled twice over calcium hydride under nitrogen atmosphere. Heptadecafluoro decylacrylate (FA) and oligodimethylsiloxane (DMS) macro-monomer, which has 26 units of DMS, were supplied by Dainippon Ink Chemicals, and Toray Dow Corning Silicone Co. Ltd., respectively. Methyl methacrylate ( M M A ) was purified by distillation under reduced pressure under nitrogen atmosphere. 2,2 -Azobis(isobutyronitril) (AIBN) was recrystallized from benzene solution and used as an initiator. The other solvents and reagents were of analytical grade obtained from standard commercial sources and used without further purification. 5

2

4

,


265 Synthesis of PoIy[l-(trimethylsilyl)-l-propyne]. Poly[l-(trimethylsiryl)-lpropyne] (PTMSP) was prepared by the method developed by Masuda et al (8). 1-trimethylsilyl-l-propyne (TMSP) was polymerized in toluene with TaCl as a catalyst at 80°C for 3 h. The polymer was reprecipitated several times from a toluene solution into an excess amount of methanol, and was dried in vacuo at 80°C for 24 h before use. Number-average and weight-average molecular weights of PTMSP, determined by gel permeation chromatography (GPC) (Waters Associate Inc., R-400) equipped with a TSK-GEL column (Tosoh Co. Ltd; G2000HXL, G3000HXL, G5000HXL), and ultraviolet spectrophotometry (Shimadzu Co. Ltd; Spd-2A), were 6xl0 g/mol and 4xl0 g/mol, respectively. 5


5

5

Synthesis of Polymer Additives. F A and DMS macromonomer, and M M A and DMS were dissolved in toluene with A I B N (1.0 wt.% relative to the monomers) and were copolymerized at 60°C for 6 h under nitrogen atmosphere in a glass tube, respectively, as shown in Figure 1. The resulting PFA-g-PDMS and P M M A - g PDMS polymer additives were isolated by precipitation with ethanol and a 1:2 (w/w) mixture of w-hexane and ethanol, respectively. Thereafter, the additives were purified by reprecipitation from toluene solution into ethanol and a 1:2 (w/w) mixture of w-hexane and ethanol, respectively, and dried at 40°C in vacuo. Number-average and weight-average molecular weights of PFA-g-PDMS and PMMA-g-PDMS were determined by gel permeation chromatography (GPC) and ultraviolet spectrophotometry. Tetrahydrpfuran was used as an eluent and the calibration was made with a polystyrene standard. The compositions of PFA-gPDMS and PMMA-g-PDMS were determined from 400 M H z *H nuclear magnetic resonance (NMR) (JEOL; GSX-400) spectra by measuring the integrals of the peaks assigned to methylene protons (4.2 ppm) of the F A and D M S protons (0 ppm) of the DMS macromonomer, and methyl protons (3.5 ppm) of the M M A and DMS protons (0 ppm) of the DMS macromonomer, respectively, after the purified copolymer was dissolved in chloroform-rf containing 1 vol.% tetramethylsilane (TMS). Characterizations of the PFA-g-PDMS and PMMA-g-PDMS copolymers are summarized in Table I. Table I. Properties of polymer additives used for surface modification of PTMSP membranes. Polymer additive PFA-g-PDMS PMMA-s-PDMS

m:n 0.64:0.36 0.67:0.33

M n (g/mol) 2.0xl0 2.0xl0 5

5

M w (g/mol) 5.9xl0 7.5xl0 4

4

Mw/Mn 3.3 2.6

Preparation of Membranes. Desired amounts of polymer additive (PFA-gPDMS and PMMA-g-PDMS) were added to a 2 wt.% PTMSP solution in toluene at 25°C. PTMSP membranes modified with PFA-g-PDMS or PMMA-g-PDMS


266


were made by pouring the solutions onto a rimmed glass plate and allowing the casting solvent (toluene) to evaporate at 25°C. The modified PTMSP membranes were immersed in methanol for 3 days to remove the residual solvent from the membrane matrix and then kept in pure methanol. The modified PTMSP membranes were dried completely under reduced pressure and then tested in pervaporation experiments. The resulting membranes were transparent and their thickness was about 20 μιη. Contact Angle Measurements. The contact angles of water on the surface of the air- and glass-plate-sides of the PTMSP membrane and PTMSP membranes modified with PFA-g-PDMS and PMMA-g-PDMS were measured using a contact angle meter (Erma, Model G-I) at 25°C. The contact angle, Θ, was determined from equation 1, 1

θ = cos {(cosOa+cosOr) / 2}

(1)

where 9a and Or are the advancing contact angle and receding contact angle, respectively. X-ray Photoelectoron Spectroscopy (XPS). Elemental analyses of the surfaces on the air- and glass-plate-sides of the PTMSP and surface-modified PTMSP membranes were measured by X-ray photoelectoron spectroscopy (XPS, Shimadzu ESCA-750). Pervaporation Experiments. Pervaporation experiments were carried out using an apparatus reported in previous papers (9-12) at a temperature of 40°C and a permeate pressure of lxlO" Torr. The effective membrane area was 13.8 cm . A n aqueous solution of 10 wt.% ethanol was used as the feed solution. The compositions of the feed and permeate were determined by gas chromatography (Shimadzu GC-9A). The pervaporation fluxes were determined from the weight of the permeate collected in a cold trap, permeation time, and effective membrane area. 2

2

Characterization of Membranes. The contact angles of water on the surfaces of the air- and glass-plate-sides of surface-modified PTMSP membranes are shown in Figures 2 and 3 as a function of the amount of the polymer additive. Using PFA-gPDMS as a polymer additive, the contact angle of water on the surface of the airside increased with an increase of the PFA-g-PDMS content in the PTMSP membrane. However, the contact angle on the glass-plate-side did not change significantly, as shown in Figure 2. The increase in the contact angle to water suggests that PFA-g-PDMS was localized on the air-side of the membrane. Hence, the surface of the modified PTMSP membrane became more hydrophobic and,


267

CH=CH

CH3 I 2

AIBN, toluene

C=CH ç=o

+

C= 0 I

2

[çH-CH2]—4c-CH L2

60°C, 6h

ο

Ο I (CH2>3

ο (ÇH ) 2

(ÇH2)

CsFn

CH3-SÎ-CH3

2

CgFn FA

Ο I (CH2>3

2

CH3-SÎ-CH3

?

L C H 3 - ^ i - C H 3 25

CH3-SÏ-CH3


C4H9

C4H9

DMS PFA-g-PDMS

CH3

CH=CH

2

+

m

?

Ο I 3

-4 CH - C H 2 ] — 41c

60°C, 6h

C=0 ! CH

AIBN, toluene

C=CH2

CH

- C Hn2 L J

?

οι

(ÇH2)3 CH "Si-CH3

"

3

3

(ÇH2>3 CH3-^i-CH3

?

MMA LCH3-^i-CH3

CH3-Sji-CH

C4H9

25 3

C4H9

PMMA-g-PDMS

DMS

Figure 1. Synthesis of PFA-g-PDMS and PMMA-g-PDMS additives for modification of PTMSP membranes.

used as

§ 70 h

0

1.0

2.0

3.0

4.0

5.0

PFA-g-PDMS content (wt.%) Figure 2. Effect of the PFA-g-PDMS content on the contact angle to water on the surface of air-side ( Ο ) and glass-plate-side ( · ) of surface-modified PTMSP membranes.



268 consequently, the surface of the air-side of the modified PTMSP membranes was water-repellent. On the other hand, when a small amount of PMMA-g-PDMS was added to PTMSP, both contact angles of water on the surface of the air- and glass-plate-sides increased remarkably, as shown in Figure 3. The increases in contact angle demonstrate that both membrane surfaces became water-repellent. As the amount of PMMA-g-PDMS increased, the contact angles to water of both surfaces decreased and then became equal. To clarify the differences between PTMSP membranes modified with PFA-g-PDMS and PMMA-g-PDMS, elemental analyses of the surfaces of the air- and glass-plate-side of the modified membranes were performed by X P S . Table II summarizes the compositions on both surface sides of the PTMSP and PFA-g-PDMS- and PMMA-g-PDMS-modified PTMSP membranes. Table II also includes theoretical compositions of the PTMSP homopolymer and PFA-g-PDMS and PMMA-g-PDMS copolymers. Table II. Surface compositions of PTMSP and surface-modified PTMSP membranes determined by XPS. Sample

PTMSP

Additive content (wt.%) 0 0 0.1

Atomic ratio

Surface

Air Glass plate Air Glass plate Air Glass plate Air Glass plate Air Glass plate

F/C -

-

O/C

-

Si/C 0.164 0.177 0.205 0.170 0.184 0.199 0.200 0.179 0.184 0.161

PTMSP 0.087 0.107 modified 0.021 0.031 with PFA-g0.5 0.137 0.115 PDMS 0.071 0.025 PTMSP 0.1 0.085 modified 0.015 0.094 with 0.5 PMMA-g0.029 PDMS 0.167 PTMSP 0 PFA-gPDMS 0.302 0 0.332 0.351 PMMA-g0 0.355 0.397 PDMS" theoretical compositions were calculated from PTMSP homopolymer and PFA-gPDMS and PMMA-g-PDMS copolymers (Table I). a

8

For the PTMSP membranes surface-modified with PFA-g-PDMS, the ratio of F/C in the membrane surface on the air-side was considerably higher than that of the



269 membrane surface on the glass-plate-side. In addition, the F/C ratio on the membrane surface of the air-side increased with increasing PFA-g-PDMS content; however, the F/C ratio on the glass-plate-side did not change. A comparison of the F/C ratio of the surface-modified PTMSP with 0.1 wt.% PFA-g-PDMS and the theoretical ratio of F/C of PFA-g-PDMS suggests that PFA-g-PDMS occupies about 30 wt.% of the surface of PTMSP membranes. On the other hand, the ratio of O/C on the surface of the air-side of the PTMSP membrane modified with PMMA-g-PDMS was higher than that on the glass-plateside. Contact angle measurements to water on the surface of the glass-plate-side of the PTMSP membrane with additional PMMA-g-PDMS, however, showed that the surface of the glass-plate-side became water-repellent, as shown in Figure 3. Thus, the results of the XPS measurements are different from those of the contact angle measurements. This difference may be attributed to the fact that the contact angle measurement reflects only the surface properties of the membranes, whereas the XPS measurement provides information of the composition of the membrane for a depth of 0-100 Â. This difference has to be investigated in more detail in the future. From the results of the contact angle and XPS measurements, the structures of surface-modified PTMSP membranes with additional PFA-g-PDMS and P M M A g-PDMS are illustrated schematically in Figure 4. As shown in Figure 4(a), the PFA-g-PDMS molecules, which have a lower surface free energy than PTMSP, are localized on the surface of the PTMSP membrane on the air-side to thermodynamically stabilize the surface of the membrane. Consequently, the surface of the PTMSP membrane modified with PFA-g-PDMS became remarkably waterrepellent. The structure of the PTMSP membrane modified with PMMA-g-PDMS is shown in Figure 4(b). In this case, the PMMA-g-PDMS molecules were also localized on the surface of the PTMSP membrane on the air-side. Furthermore, because the P M M A component in the PMMA-g-PDMS molecule had a lower hydrophobicity than the PTMSP molecule, the PMMA-g-PDMS was also localized on the surface of the glass-plate-side. However, the degree of the localization was lower than that on the air-side. Consequently, the contact angles in both membrane surfaces on the air- and glass-plate-side increased remarkably with the addition of the PMMA-g-PDMS additive. Pervaporation Properties of Surface-Modified P T M S P Membranes. Figure 5 shows the flux and ethanol concentration in the permeate for an aqueous solution of 10 wt.% ethanol in pervaporation experiments of PTMSP membranes modified with PFA-g-PDMS. In all pervaporation experiments, the membrane surface of the air-side of the surface-modified PTMSP membranes was facing the feed solution. Figure 5 also includes the contact angles of the surface-modified PTMSP membranes. As can be seen from Figure 5, the flux decreased slightly but the ethanol



270

PMMA-g-PDMS content (wt.%) Figure 3. Effect of the PMMA-g-PDMS content on the contact angle to water on the surface of air-side (O) and glass-plate-side ( · ) of surfacemodified PTMSP membranes.

Figure 4. Structure of the surface-modified PTMSP membranes, (a) PTMSP membrane modified with PFA-g-PDMS: ψ is PFA-g-PDMS, · and I are PFA and PDMS, respectively, (b) PTMSP membrane modified with PMMA-g-PDMS:

Membrane Formation and Modification - American Chemical Society

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