Poly(amide-6-b-ethylene oxide)/silica의 기체투과특성

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

이영무, 김재훈, 박호범

한양대학교 공과대학 응용화학공학부

Gas Permeation Properties of Poly(amide-6-b-ethylene oxide)/Silica Hybrids

Young Moo Lee, Jae Hoon Kim, Ho Bum Park

National Research Laboratory for Membrane, School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Korea

Introduction

In the last two decades, incorporation of highly crosslinked inorganic network into a polymer matrix has been extensively studied because it exhibits a wide range of multifunctional properties. Hybrid materials can be easily prepared by using the sol-gel process, which offers several advantages over other techniques. The micro- and macrostructure of the host polymer matrix can be controlled through the optimization of several synthetic parameters, for example, pH, concentration, water-to-alkoxide ratio, temperature, pressure, type of catalyst, and solvent at low temperature [1]. The presence of finely dispersed silica network provides good mechanical resistance, extraordinary thermal stability, and amorphous character. The porosity can be controlled and the hydrophilic/hydrophobic balance can be adjusted. This chemical synthesis route is widely known as potentially attractive ways for the preparation of novel materials in many scientific fields such as coatings [2,3], optical and electronic materials [4], adsorbents, catalysts, biochemical sensors, and more recently membranes.

Recently, a number of groups have reported organic-inorganic hybrids as one of candidates to improve the permeation characteristics. At least, two effects of inorganic particles in polymer matrix on permeation properties can be described. First, the strong interaction between polymer and inorganic particle reduces segmental and subsegmental mobility and inhibits segmental packing of polymer. A change in polymer structures contributes to increase diffusivity (by disrupting interchain packing) and diffusivity selectivity (by increasing polymer backbone stiffness), which plays more important role than solubility in determining the productivity of given polymers. The fact that a polymer with a high glass transition temperature and a large interchain spacing shows high permeability/selectivity properties is consistent with the conclusion of Robeson and many other structure/property studies. Second, the interaction between the residual OH groups on the inorganic component and polar gases (such as CO2 and SO2) and the morphological changes can give rise to the increase of solubility associated with an increase of the Henry's contribution.

In the present study, we explore gas transport properties of PEBAX□-silica hybrids.  These composites have been prepared via in situ polymerization of tetraethoxysilane (TEOS) using the sol-gel process. The permeability and permselectivity at various temperatures were obtained for He, CO2, O2 and N2. It is our intention to investigate and discuss the effect of silica incorporation into PEBAX□on the structural changes and corresponding permeation properties.

Experimental

Materials

PEBAX 1657 was purchased from ELF Atochem Inc. (Philadelphia, PA, U.S.A.).  PA is a polyamide block and PE is a polyether block. PEBAX used in this study is composed of polytethyleneoxide (PEO) and Nylon 6. As an inorganic precursor, tetraethoxysilane was purchased from Aldrich Chem. Co. (Milwaukee, WI, U.S.A.) and used as received.  Deionized water (18M□) was used for the hydrolysis. Methanol, propanol and n-butanol were received from Aldrich Chem. Co. and used without further purification.

Preparation of silica particles and hybrids

For comparison, amorphous silica particles were synthesized by the addition, under vigorous stirring, of TEOS to a solution of methanol and the stoichiometric amount of water in the form of 1M NH4OH. The solution was mixed at room temperature for 10hr before drying.

PEBAX free-standing film was prepared by the solvent casting method. PEBAX solutions were prepared using alcohol mixture (propanol:n-butanol = 3:1 in weight ratio) as a solvent with reflux at 80oC and vigorous stirring. Then 3wt% PEBAX□solution was poured into a Petri dish and dried at 40oC for 24hr. Finally, PEBAX film was dried at 50oC for another 24hr in a vacuum oven to remove any residual solvent.

For the preparation of PEBAX-silica hybrids, the measured amount of TEOS (previously diluted in propanol) was carefully added to a 3wt% PEBAX□solution. A 0.15M HCl solution, also diluted with propanol, dropped into the PEBAX-TEOS solution mixture at 80oC under reflux with fast agitation, keeping 4:1 water to TEOS hydrolysis ratio. After 30hr reaction, the homogeneous solution was poured into a covered Teflon-coated Petri dish. After 2 weeks at room temperature, the cover was removed and the solvent was evaporated for at least another 48hr. To remove any residual solvent, the film was further dried under vacuum for 24hr. Table 1 summaries the chemical composition of PEBAX-silica hybrids and silica content in the final hybrid materials.

Table 1. Preparation scheme of PEBAX-silica hybrids and silica content in the final composites.

	PEBAX (wt%)	TEOS (wt%)	Silica content (wt%)		Observation
			theoretical	expt'l
PEBAX	3	0	0	0	Transparent
P9S1	2.69	1.08	10	10	Transparent
P8S2	2.64	2.38	20	19	Transparent
P7S3	2.58	3.96	30	27	Transparent
P6S4	2.51	5.86	40	35	Transparent, brittle
P5S5	2.42	8.46	50	43	Transparent, brittle

Results and Discussion

Permeation experiments were carried out with PEBAX and hybrids at 3 atm and 25oC.  The results obtained for various gases are summarized in Table 2. PEBAX□copolymer consisted of two distinct regions, the impermeable crystalline PA phase and the permeable amorphous PE phase. Thus, the permeability was a function of PE block content in PEBAX.  The crystallinity of PA phase was significantly reduced by the introduction of silica particles.  This may increase the total volume of amorphous region available for a gas flow. The PE phase is also greatly modified by the incorporation of silica in PEBAX Moreover, PA block-silicate and PE block-silicate interface regions may have contributed to the increased gas permeability. In addition, if the silica domains are microporous, a gas can flow through these silica particles in hybrid materials. However, this is not the case here, because the permeabilities of microporous silica and the hybrids are very different. Even with high content of silica (27%), the N2 permeability is much lower than that of microporous silica (10.5 Barrer). Thus, the increase of gas permeability in PEBAX-silica hybrids attributes to the total contribution of permeability of the modified PE block, of the amorphous PA block and of the interphase.

Sample	He	CO2	O2	N2
Permeability coefficient
PEBAX	18.6	122	5.84	1.71
P9S1	21.4	154	6.82	2.13
P8S2	25.5	205	8.74	2.73
P7S3	32.2	277	11.3	3.52
Diffusion coefficient
PEBAX	7.25	1.52	0.92	0.55
P9S1	6.89	1.41	0.85	0.54
P8S2	6.24	1.30	0.77	0.51
P7S3	5.94	1.15	0.71	0.49
Solubility coefficient
PEBAX	2.56	80.2	6.34	3.11
P9S1	3.11	109	8.02	3.94
P8S2	4.09	157	11.4	5.35
P7S3	5.42	240	15.9	7.18

Table 2. Gas permeability, diffusion and solubility coefficients of PEBAX-silica hybrids at 3atm and 25oC.

The diffusion coefficient decreased after the incorporation of silica domains.  By analogy to the expressions for semicrytalline polymers, a consideration of tortuosity effects in particulate filled systems can be expressed by

(1)

where is the diffusivity for a polymer,  is the volume fraction of a amorphous polymer, and m is the power factor for tortuosity. Using the volume fraction obtained from the results that the densities of PEBAX and SiO2 were 1.14 g/cm3 and 1.6 g/cm3, respectively. These values are greater than those for He (0.59) and CO2 (1.09) permeation through pure PEBAX□film, indicating that the presence of silica domains in polymer matrix significantly modifies the diffusivity by creating more sinuous and/or restricted pathways for diffusing molecules. In spite of the decrease of diffusivity, the permeability of hybrids increased in comparison with that of PEBAX□film. This might be due to the increase of the solubility of the hybrid. Two possible factors can explain the increased sorption ability of the hybrid. First, the morphological changes occur at the interface between the silica and the PA block and between the silica and PE block, resulting in the increased amorphous region of the hybrid, as confirmed by X-ray diffraction and DSC analysis, respectively. Second, the residual OH groups on the silica domain and the polar CO2 molecules interact each other, as confirmed by 29Si solid-state NMR and FT-IR analysis. Thus, it is reasonable to conclude that the increase of the permeability of the hybrid membrane arises from the increased amorphous polymer regions together with an additional contribution from the adsorption in silica domains and interphase.

Figure 1 shows the Robesons trade-off relation of CO2 and N2 for various PEO containing polymeric membranes, the organic-inorganic hybrids and the other polymeric films at 25oC.  PEO containing polymers exhibit 2~3 times higher permeselectivity with a similar CO2 permeability than non-PEO based polymers. Although the other hybrid membranes did not exceed the upper bound, they displayed improved performances after an incorporation of inorganic domains. The PEBAX-silica hybrids had higher permeation performance than the other hybrids and the similar performance to PEO-based poly(ether imide) segmented copolymers and crosslinked PEO containing films prepared by plasma irradiation.

Conclusions

The organic-inorganic hybrids, based on poly(amide-6-b-ethylene oxide) (PEBAX) and silica, were successively prepared via in situ polymerization of tetraethoxysilane (TEOS) using the sol-gel process. The hybrid membranes exhibited higher gas permeability coefficients and permselectivities than those of PEBAX, particularly at an elevated temperature: = 72 at 25oC with PCO2 = 205 Barrer and = 44 at 85oC with PCO2 = 507 Barrer for P8S2 obtained. The high permeability and permselectivity of the hybrid membranes are attributed to the strong interaction between CO2 molecules and SiO2 domains, additional sorption sites in PA block of PEBAX□and the organic/inorganic interphase.

References

[1] C.J. Brinker, G.W. Scherer, Sol-Gel Science; the Physics and Chemistry of Sol-Gel	Processing, Academic Press, San Diego 1999.

화학공학의 이론과 응용 제8권 제1호 2002년

[2] H. Schmidt, H. Walter, Organically modified ceramics and their applications, J. Non-Cryst. Solids, 121 (1990) 428.

[3] B. Wang, G.L. Wilkes, Novel hybrid inorganic-organic abrasion-resistant coatings prepared by a sol-gel process, J. Macromol. Sci. Pure Appl. Chem, A31 (1994) 249.

[4] H. Drug, H. Schmidt, Organic-inorganic nanocomposites for micro-optical applications, New J. Chem, 18 (1994) 1125.