POLYIMIDE-CO-POLYBENZOXAZOLE COPOLYMER, PREPARATION METHOD THEREOF, AND GAS SEPARATION MEMBRANE COMPRISING THE SAME

[Technical Field]

The present invention relates to a polyimide-polybenzoxazole copolymer, a method for preparing thereof and a gas separation membrane comprising the same. More specifically, the present invention relates to a polyimide-polybenzoxazole copolymer suitable for use in the preparation of gas separation membranes applicable to various types of gases in a variety of forms such as films, fibers and hollow fibers due to its superior gas permeability and gas selectivity, a method for preparing the copolymer and a gas separation membrane comprising the copolymer.

[Background Art]

Polyimides are high-performance macromolecules which are generally obtained via polycondensation of aromatic and/or alicylic dianhydride and diamine structures [E. Pinel, D. Brown, C. Bas, R. Mercier, N. D. Alberola, S. Neyertz. Chemical Influence of the dianhydride and the diamine structure on a series of copolyimides studied by molecular dynamics simulations. Macromolecules. 2002;35:10198-209].

These aromatic polyimides have been used in many high technology fields due to their excellent thermal, mechanical and electrical properties [Y. Li, X.

Wang, M. Ding, J. Xu. Effects of molecular structure on the permeability and permselectivity of aromatic polyimides. JAppl Polym Sci. 1996;61 :741-8].

Among those applications, gas separation using polyimides has attracted great interest, because polyimides have significantly better permselective performance than typical glassy polymers such as cellulose acetate and polysulfone [A. Bos, I. G. M. Punt, M. Wessling, H. Strathmann. Plasticization-resistant glassy polyimide membranes for CO2/CH ₄ separations. Sep Purif Technol. 1998;14:27-39].

In addition, high temperature polymers (e.g., polybenzimidazole, polybenzoxazole and polybenzothiazole) have drawn a great deal of attention due to their potential of obtaining superior gas separation performance under harsh conditions. In order to use the polymers for membrane materials, mild fabrication processes are required instead of using acidic solvents.

For example, fluorinated polybenzoxazole membranes can be synthesized by solution cyclization techniques using mild solvents [W. D. Joseph,

J. C. Abed, R. Mercier, J. E. McGrath. Synthesis and characterization of fluorinated polybenzoxazoles via solution cyclization techniques. Polymer.

1994;35:5046-50]. Their gas permeability increases according to the degree of cyclization of benzoxazole rings because increases in solubility and diffusivity coefficient are observed after cyclization [K. Okamoto, K. Tanaka, M. Muraoka, H.

Kita, Y. Maruyama. Gas permeability and permselectivity of fluorinated polybenzoxazoles. J Polym Sci Pol Phys. 1992;30:1215-21].

Meanwhile, Burns and Koros proposed a polymeric molecular sieve

concept using ultrarigid polymers which exhibited entropic selectivity capabilities [R. L. Burns, W. J. Koros. Structure-property relationships for poly(pyrrolone-imide) gas separation membranes. Macromolecules. 2003;36:2374-81]. Poly(pyrrolone-imides) composed of open regions and bottleneck selective regions can mimic molecular sieves by tuning the polymeric matrix through the use of different monomer stoichiometry.

In an attempt to find the ways to improve gas permeability, the inventors of the present invention have conducted research based upon the fact that copolymerization of high temperature polymers and polyimides results in higher gas separation performance. As a result, the present inventors have disclosed polymer structures acting as permeable sites and considered incorporating the polymer structures into polyimide backbones.

Consequently, the present inventors ascertained that aromatic polymers interconnected with heterocyclic rings (e.g., benzoxazole, benzothiazole and benzopyrrolone) showed higher gas permeation performance due to their well-controlled free volume element formation by thermal rearrangement in the glassy phase. In addition, these materials have a flat and rigid rod structure with high torsional energy barriers to rotation between respective rings. An increase in rigidity of polymer backbones with high microporosity showed positive effects in improving gas separation performance.

[Disclosure] [Technical Problem]

Therefore, it is one object of the present invention to provide a polyimide-polybenzoxazole copolymer that has microcavities, exhibits increased polymer backbone rigidity and improved fractional free volume, and shows superior gas permeability and gas selectivity, and a method for preparing the copolymer.

It is another object of the present invention to provide a gas separation membrane comprising the polyimide-polybenzoxazole copolymer, suitable for application to various types of gases, and a method for preparing the gas separation membrane.

It is yet another object of the present invention to provide a precursor used for the preparation of the polyimide-polybenzoxazole copolymer.

[Technical Solution]

In accordance with one aspect of the present invention for achieving the above object, there is provided a polyimide-polybenzoxazole copolymer having repeating units represented by Formula 1 below: Formula 1

wherein An, Ar ₂ ^' and Ar ₃ ^' are identical or different, are each independently a bivalent C ₅ -C ₂₄ arylene group or a bivalent C ₅ -C ₂₄ heterocyclic

ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C1-C10 alkyl, C1-C10 alkoxy, CrC ₁₀ haloalkyl and C1-C1 ₀ haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ )p (in which 1<p<10), (CF ₂ ) _q (in which 1<q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ and C(=O)NH;

Ar ₂ and Ar ₃ are identical or different, are each independently a trivalent C ₅ -C ₂₄ arylene group or a trivalent C ₅ -C ₂₄ heterocyclic ring, which is substituted or unsubstituted with at least one substituent selected from the group consisting of C ₁ -C10 alkyl, C ₁ -C-1 ₀ alkoxy, C ₁ -C1 ₀ haloalkyl and C ₁ -C10 haloalkoxy, or two or more of which are fused together to form a condensation ring or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ ) _P (in which 1<p<10), (CF ₂ ) _q (in which 1 <q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ and C(=O)NH;

Q is O, S, C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ ) _P (in which 1<p<10), (CF ₂ ) _q (in which 1<q<10), C(CHs) ₂ , C(CF ₃ ) ₂ , C(=O)NH, C(CH ₃ )(CF ₃ ), CrC ₆ alkyl-substituted phenyl or C _r C ₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; m is an integer of 10 to 400; and n is an integer of 10 to 400.

In accordance with another aspect of the present invention, there is

provided a method for preparing a polyimide-polybenzoxazole copolymer of Formula 1 by thermally treating a polyimide-poly (hydroxyimide) copolymer of Formula 2, as depicted in Reaction Scheme 1 below: Reaction Scheme 1

wherein An, Ar2, Ar ₂ ^' , Ar ₃ , Ar ₃ ^' , Q, m and n are defined as above.

In accordance with another aspect of the present invention, there is provided a gas separation membrane comprising the polyimide-polybenzoxazole copolymer of Formula 1.

In accordance with another aspect of the present invention, there is provided a method for preparing a gas separation membrane comprising the polyimide-poly (hydroxyimide) copolymer of Formula 1 , comprising casting the polyimide-polybenzoxazole copolymer of Formula 2, followed by thermal

treatment.

In accordance with another aspect of the present invention, there is provided a polyimide-poly (hydroxyimide) copolymer as an intermediate used for the preparation of the polyimide-polybenzoxazole copolymer.

[Advantageous Effects]

The polyimide-polybenzoxazole copolymer according to the present invention is simply prepared through thermal-rearrangement performed by thermally treating the polyimide-poly (hydroxyimide) copolymer as a precursor. The polyimide-polybenzoxazole copolymer thus prepared exhibits increased polymer backbone rigidity and improved fractional free volume.

The present copolymer shows superior gas permeability and gas selectivity, thus being suitable for use in gas separation membranes in various forms such as films, fibers or hollow fibers. It is advantageous that the gas separation membrane thus prepared can endure harsh conditions such as long operation temperature, acidic conditions and high humidity due to the rigid polymer backbone present in the copolymer.

[Description of Drawings] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is FT-IR spectra of HPI-PI precursor membranes prepared at

various copolymerization ratios;

FIG. 2 is FT-IR spectra of PBO-PI copolymer membranes prepared at various copolymerization ratios;

FIG. 3 is a TGA thermogram of HPI-PI precursor membranes prepared at various copolymerization ratios;

FIG. 4 is a TGA-MS thermogram of an HPI-PI (10:0) precursor membrane;

FIG. 5 is a TGA-MS thermogram of an HPI-PI (5:5) precursor membrane; FIG. 6 is a TGA-MS thermogram of an HPI-PI (0:10) precursor membrane;

FIG. 7 is UV/VIS spectra of HPI-PI precursor membranes according to various copolymerization ratios;

FIG. 8 is UV/VIS spectra of PBO-PI copolymer membranes prepared at various copolymerization ratios; FIG. 9 is X-ray diffraction patterns of HPI-PI precursor membranes prepared at various copolymerization ratios;

FIG. 10 is X-ray diffraction patterns of PBO-PI copolymer membranes prepared at various copolymerization ratios;

FIG. 11 is N ₂ adsorption/desorption isotherms of PBO-PI copolymer membranes prepared at various copolymerization ratios;

FIG. 12 is a graph showing a diffusion coefficient of PBO-PI copolymer membranes for O2, CO2, N2 and CH ₄ as single gases;

FIG. 13 is a graph showing O ₂ /N ₂ permselectivity of the PBO-PI

copolymer membrane and common polymers as a function of O ₂ permeability; and

FIG. 14 is a graph showing CO2/CH4 permselectivity of the PBO-PI copolymer membrane and common polymers as a function of CO ₂ permeability.

[Best Mode]

Hereinafter, the present invention will be illustrated in more detail. In one aspect, the present invention is directed to a polyimide-polybenzoxazole copolymer (hereinafter, referred to as a 'PBO-PI copolymer ¹ ) having repeating units represented by Formula 1 below: Formula 1

wherein Ar-i, Ar ₂ ^' and Ar ₃ ^' are identical or different, are each independently a bivalent C ₅ -C ₂₄ arylene group or a bivalent C ₅ -C ₂₄ heterocyclic ring which is substituted or unsubstituted with at least one substituent selected from the group consisting of C _r Ci ₀ alkyl, C1-C10 alkoxy, CrCi ₀ haloalkyl and C1-C10 haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ )p (in which 1<p<10), (CF ₂ ) _q (in which 1<q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ and

C(=O)NH;

Ar ₂ and Ar ₃ are identical or different, are each independently a trivalent C ₅ -C24 arylene group or a trivalent C ₅ -C ₂₄ heterocyclic ring which is substituted or unsubstituted with at least one substituent selected from the group consisting of C1-C10 alkyl, C _r Ci ₀ alkoxy, C-1-C10 haloalkyl and C1-C10 haloalkoxy, or two or more of which are fused together to form a condensation ring, or covalently bonded to each other via a functional group selected from the group consisting of O, S, C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ ) _P (in which 1<p<10), (CF ₂ ) _q (in which 1≤q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ and C(=O)NH; Q is O, S, C(=O), CH(OH), S(O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ ) _P (in which 1<p<10),

(CF ₂ ) _q (in which 1<q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ , C(=O)NH, C(CH ₃ )(CF ₃ ), or C ₁ -C ₆ alkyl-substituted phenyl or CrC ₆ haloalkyl-substituted phenyl in which Q is linked to opposite both phenyl rings in the position of m-m, m-p, p-m or p-p; m is an integer of 10 to 400; and n is an integer of 10 to 400.

In Formula 1 , An, Ar ₂ , Ar ₂ ^' , Ar ₃ , and Ar ₃ ^' may be the same arylene group or heterocyclic ring.

Preferably, Ar-i, Ar ₂ ^' and Ar ₃ ^' are selected from the following compounds and the linkage position thereof includes all of o-, m- and p- positions.

wherein X is O, S ₁ C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ ) _P (in which 1<p<10), (CF ₂ ) _q (in which 1<q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ , or C(=O)NH; Y is O, S or C(=O); and Z ₁ , Z ₂ and Z ₃ are identical or different and are O, N or S.

More preferably, Ar-i, Ar ₂ ^' and Ar ₃ ^' are selected from the following compounds:

Preferably, Ar ₂ and Ar ₃ are selected from the following compounds and the linkage position thereof includes all of o-, m- and p- positions.

wherein X is O, S, C(=O), CH(OH), S(=O) ₂ , Si(CH ₃ ) ₂ , (CH ₂ ) _P (in which 1<p<10), (CF ₂ ) _q (in which 1<q<10), C(CH ₃ ) ₂ , C(CF ₃ ) ₂ , or C(=O)NH; Y is O, S or C(=O); and Z ₂ and Z ₃ are identical or different and are O, N or S.

More preferably, Ar ₂ and Ar ₃ are selected from the following compounds.

Preferably, Q is C(CH ₃ ) ₂ , C(CF ₃ ) ₂ , C(=O)NH, C(CH ₃ )(CF ₃ ) or

More preferably, Ari is , A Arr ₂ -/ ^' a anndd A AirV ₃ ^' a arrpe

Ar ₂ and Ar ₃ are and Q is C(CF ₃ ) ₂ . The physical properties of the PBO-PI copolymer represented by Formula

1 can be controlled by controlling a copolymerization ratio of PBO to Pl blocks. The copolymerization ratio of PBO to Pl (n:m) is adjusted to 1 :9 to 9:1 , more preferably, 2:8 to 8:2, most preferably, 3:7 to 7:3. This copolymerization ratio affects the morphology of the membrane for gas separation applications, as illustrated in the following. This morphological change is closely related to gas permeability and gas selectivity. For this reason, control over the copolymerization ratio is considerably important.

Preferably, the PBO-PI copolymer has a density of 1.10 to 1.37 g/cin ³ , a fractional free volume (FFV) of 0.10 to 0.30, and a d-spacing of 0.55 to 0.70 nm. In another aspect, the present invention is directed to a method for preparing a polyimide-polybenzoxazole copolymer of Formula 1 by thermally treating a polyimide-poly (hydroxyimide) copolymer of Formula 2, as depicted in Reaction Scheme 1 below:

Reaction Scheme 1

wherein An, Ar ₂ , Ar ₂ ^' , Ar ₃ , Ar ₃ ^' , Q, m and n are defined as above.

As shown in Reaction Scheme 1 , the poly (hydroxyimide)-polyimide copolymer (hereinafter, referred to as an ¹ HPI-PI ¹ ) 2 as the precursor is converted into the PBO-PI copolymer 1_ by thermal treatment. The conversion from the HPI-PI copolymer 2 to the PBO-PI copolymer I ₁ is carried out by removing CO ₂ present in the poly (hydroxyimide).

After the thermal rearrangement through the thermal treatment, the PBO-PI copolymer 1 undergoes morphological change including reduced density, considerably increased fractional free volume (FFV) due to increased microcavity size and increased d-spacing, as compared to the precursor 2. As a result, the PBO-PI copolymer 1 exhibits considerably high gas permeability, as compared to

the precursor 2.

Such a morphological property can be readily controlled by the design taking into consideration the characteristics (e.g., steric hindrance) of the functional groups, An, Ar ₂ , Ar ₂ ^' , Ar ₃ , Ar ₃ ^' and Q, present in the molecular structure and permeability and selectivity to various types of gases can thus be controlled.

According to the present invention, the thermal treatment is carried out at 150 to 500 ⁰ C, preferably 350 to 450 ⁰ C for 5 minutes to 12 hours, preferably for 10 minutes to 2 hours under an inert atmosphere. When the thermal treatment temperature is less than the level as defined the above, the thermal rearrangement is incomplete, thus remaining unreacted precursors, causing deterioration of purity. Increasing the thermal treatment temperature above the level defined above provides no particular advantage, thus being economically impractical. Accordingly, the thermal treatment is properly carried out within the temperature range as defined above.

At this time, the reaction conditions are properly designed by depending on Ar-i, Ar ₂ , Ar ₂ ^' , Ar ₃ , Ar ₃ ^' and Q, the functional groups of the precursor and specific conditions can be adequately selected and modified by those skilled in the art. Preferably, the PBO-PI copolymer 1 is designed in the preparation process such that it has a desired molecular weight. Preferably, the weight average molecular weight of the PBO-PI copolymer 1 is adjusted to 10,000 to 50,000 Da. When the weight average molecular weight is less than 10,000 Da,

physical properties of the copolymer are poor. When the weight average molecular weight exceeds 50,000 Da, the copolymer is poorly soluble in the solvent, thus making it difficult to cast the polymeric membrane.

As depicted in the following Reaction Scheme 2, the polyimide-poly(hydroxyimide) copolymer of Formula 2 is prepared by reacting the compounds of Formulae 3, 4 and 5 as monomers with one another, to prepare polyimide of Formula 6 and poly(hydroxyimide) of Formula 7, and copolymerizing the polyimide of Formula 6 with the poly(hydroxyimide) of Formula 7. Reaction Scheme 2

wherein Aη, Ar ₂ , Ar3, Q, m and n are defined as above.

More specifically, first, a diamine compound 3 and a hydroxy diamine compound 5 as monomers are reacted with an anhydride compound 4 to prepare polyimide 6 and poly(hydroxyimide) 7.

Then, the poiyimide 6 is copolymerized with the poly(hydroxyimide) 7 to prepare an HPI-PI copolymer 2 as a precursor.

The polymerization and copolymerization are carried out through a two-step process employing two reactors, or a one-step process employing controlled reaction conditions in one reactor. For example, the polymerization and copolymerization are carried out at 0 to 80 ⁰ C for 30 minutes to 12 hours and reaction conditions may be properly controlled by those skilled in the art, depending on the type of the functional groups, i.e., Aη, Ar ₂ , Ar ₂ ', Ar ₃ , Ar ₃ ' and Q.

In addition, the level of copolymerization can be adequately controlled depending on the molar ratio of respective monomers used.

In one embodiment of the present invention, the PBO-PI copolymer of Formula 8 is prepared through the process, as depicted in Reaction Scheme 3 below:

Reaction Scheme 3

12 13

14

wherein m and n are defined as above.

That is, 4,4'-oxydianiline(ODA) of Formula 9 is reacted with ,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) of Formula 10 to polymerize

polyimide (Pl) of Formula 12, and

2,2'-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF) of Formula 11 is reacted with 3, 3", 4, 4'- biphenyltetracarboxylic dianhydride (BPDA) of Formula 10 to polymerize poly(hydroxyimide)(HPI) of Formula 13. Subsequently, polymers of Formulae 12 and 13 are copolymerized to prepare a poly(hydroxyimide)-polyimide copolymer (HPI-PI) precursor of Formula 14, and the precursor 14 is thermally treated to prepare a PBO-PI copolymer of Formula 8.

In another aspect, the present invention is directed to a gas separation membrane comprising the polyimide-polybenzoxazole copolymer of Formula 1 below: Formula 1

wherein An, Ar ₂ , Ar ₂ ^' , Ar ₃ , Ar ₃ ^' , Q, m and n are defined as above.

The PBO-PI copolymer contains a plurality of aromatic rings in the molecular structure thereof. For this reason, the PBO-PI copolymer has a structure in which copolymer chains are packed such that they are spaced from one another by a predetermined distance and has a rigid-rod structure due to its limited mobility.

Accordingly, the gas separation membrane prepared from the copolymer

can endure not only mild conditions, but also harsh conditions, e.g., long operation time, acidic conditions and high humidity.

In addition, the PBO-PI copolymer has a specific surface area of over 0.1 and under 480 liiVg, a total pore volume of 0.0004 to 0.25 m ³ and a pore size of 21 to 40A. In addition, the PBO-PI copolymer exhibits excellent permeability of CO2, O2, N2 and CH ₄ and superior selectivity for mixed gas pair of O2/N2, CO ₂ /CH ₄ , CO2/N2 and N ₂ /CH ₄ .

In preferred embodiments of the present invention, the PBO-PI copolymer membrane has well-connected microcavities and shows linear increases in volume, FFV and d-spacing with an increase of the copolymerization ratio of PBO present therein. In addition, for permselectivity of O2/N2 and CO2/CH ₄₎ the PBO-PI copolymer membrane surpasses the upper bound line of common polymers for gas separation membrane applications.

In another aspect, the present invention is directed to a method for preparing a gas separation membrane comprising the PBO-PI copolymer of Formula 1 , by casting the HPI-PI copolymer of Formula 2, followed by thermal treatment. Formula 1

wherein An, Ar ₂ , Ar ₂ ^' , Ar ₃ , Ar ₃ ', Q, m and n are defined as above.

Formula 2

wherein Ar-i, Ar ₂ , Ar ₃ , Q, m and n are defined as above.

More specifically, the HPI-PI copolymer precursor of Formula 2, is prepared as a solution, is coated or cast into films or fibers (in particular, hollow fibers), and is then subjected to thermal treatment to prepare the gas separation membrane comprising the PBO-PI copolymer of Formula 1.

That is, the gas separation membrane has advantages in that the gas separation membrane can be directly prepared from the precursor without using any additional dissolving process to prepare the separation membrane and can thus be readily prepared in various forms. Another advantage of the gas separation membrane is that physical properties can be controlled by addition of other additives, if necessary.

[Mode for Invention]

Hereinafter, preferred examples will be provided for a further understanding of the invention. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

(Example 1 )

A polyimide-polybenzoxazole(PBO-PI) copolymer represented by Formula 8 was prepared in the manner as depicted in Reaction Scheme 3 above.

Formula 8

5 mmol of 4,4'-oxydianiline (ODA, Aldrich, Milwaukee, Wl, USA) and 5 mmol of 2,2'-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF, Tokyo Kasei Co., Inc., Tokyo, Japan)) as diamine monomers were dissolved in NMP in a 100 ml flask under nitrogen purging. To the resulting diamine solution 10 mmol of S^'A^-biphenyltetracarboxylic dianhydride (BPDA, Aldrich, Milwaukee, Wl, USA) was added to and the mixture was then homogeneously mixed.

The mixture solution was allowed to react at 25°C for 3 hours to polymerize polyimide (Pl) and polyhydroxyimide (HPI) and was then further allowed to react at 250 ⁰ C for 24 hours to prepare a poly(hydroxyimide)-polyimide (HPI-PI) copolymer precursor.

Subsequently, the poly(hydroxyimide)-polyimide copolymer (HPI-PI) solution precursor was cast onto a glass substrate and then dried at 250 ⁰ C to obtain a precursor membrane. The membrane was subjected to thermal treatment at 45O ⁰ C for 60 minutes to prepare a polyimide-polybenzoxazole

copolymer (PBO-PI (5:5)) membrane.

(Example 2)

An HPI-PI precursor membrane and a PBO-PI copolymer membrane were prepared in the same manner as in Example 1 except that a molar ratio of

ODA to APAF was controlled such that a copolymerization ratio of PBO to Pl was adjusted to 2:8, to obtain the HPI-PI (2:8) precursor membrane and the PBO-PI

(2:8) copolymer membrane.

(Example 3)

An HPI-PI precursor membrane and a PBO-PI copolymer membrane were prepared in the same manner as in Example 1 except that a molar ratio of ODA to APAF was controlled such that a copolymerization ratio of PBO to Pl was adjusted to 8:2, to obtain the HPI-PI (8:2) precursor membrane and the PBO-PI (8:2) copolymer membrane.

(Comparative Example 1 )

A polyimide membrane with a ratio of PBO to Pl of 0:10 was prepared in the same manner as in Example 1 except that only ODA was used as the diamine monomer.

(Comparative Example 2)

A polybenzoxazole membrane with a ratio of PBO to Pl of 10:0 was prepared in the same manner as in Example 1 except that only APAF was used

as the diamine monomer.

The membranes were prepared as set forth in Table 1 below in accordance with the methods disclosed in the Examples and Comparative Examples. For these membranes, component analysis, physical properties and gas permeability were characterized.

TABLE 1

(Experimental Example 1 ) FT-IR analysis In order to characterize precursor and polymer membranes, ATR-FTIR spectra were obtained using an Infrared Microspectrometer (IHuminatlR, SenslR

Technologies, Danbury, CT, USA).

FIG. 1 is FT-IR spectra of HPI-PI precursor membranes prepared at various copolymerization ratios. FIG. 2 is FT-IR spectra of PBO-PI copolymer membranes prepared at various copolymerization ratios.

As can be seen from FIG. 1 , imide peaks were observed at 1 ,778 cm ^"1

(u(C=O), in-phase, imide) and 1 ,705 cm ^"1 (u(C=O), out-of-phase, imide).

Characteristic peaks of C-F bonds, APAF functional groups, stretched at 1 ,246,

1,196 and 1,151 cm ^"1 , as the HPI content increases. In addition, C-H vibrations,

out of the plane of the aromatic ring, from APAF functional group were observed at 986 cm ^"1 and 963 cm ^"1 corresponding to typical 1 ,2,4-tri substituted aromatic ring structure in the case of HPI-PI(10:0) and HPI-PI(5:5).

As shown in FIG. 2, characteristic benzoxazole peaks are observed at 1 ,550, 1 ,480 and 1 ,054 cm ^"1 . In addition, the difference in C-H stretching bands due to the change in molar ratio of ODA and APAF was observed from the disappearance of stretching bands at 1 ,498 and 1,367 cm ^"1 from ODA and appearance of the band at 1 ,474 cm ^"1 from APAF.

(Experimental Example 2) Thermogravimetric Analysis/Mass Spectroscopy (TGA-MS)

The precursor membranes as set forth in Table 1 above were subjected to thermogravimetric analysis/mass spectroscopy (TGA-MS) to confirm CO ₂ evolution. The TGA-MS was carried out using TG 209 F1 Iris and QMS 403C Aeolos (NETZSCH, Germany), while injecting Ar into each precursor membrane.

The results thus obtained are shown in FIG. 3.

FIG. 3 is a TGA thermogram of HPI-PI precursor membranes produced at various copolymerization ratios.

As can be seen from FIG. 3, the precursor membrane began to decompose at a thermal conversion temperature of 350 to 500 ⁰ C. The decomposition product was subjected to MS to confirm the presence of CO ₂ .

FIGs. 4 to 6 are TGA-MS thermograms of HPI-PI precursor membranes prepared at various copolymerization ratios. Specifically, FIG. 4 is a TGA-MS

thermogram of an HPI-PI (10:0) precursor membrane, FIG. 5 is a TGA-MS thermogram of an HPI-PI (5:5) precursor membrane, and FIG. 6 is a TGA-MS thermogram of an HPI-PI (0:10) precursor membrane.

It can be seen from FIGs. 4 to 6 that as the amount of Pl present in the copolymer increases, the thermal conversion temperature increases.

(Experimental Example 3) Ultraviolet-Visible (UV-VIS) spectroscopy

The precursor and polymer membranes as set forth in Table 1 were subjected to UV-VIS spectroscopy using an S-3100 (Seoul, Korea) diode array type spectrophotometer to obtain UV-VIS spectra. At this time, UV irradiation was carried out using a mercury lamp without using any filter. The sample was allowed to cool in air during the UV irradiation.

FIG. 7 is UV/VIS spectra of HPI-PI precursor membranes prepared at various copolymerization ratios. FIG. 8 is UV/VIS spectra of PBO-PI copolymer membranes prepared at various copolymerization ratios.

As shown in FIGs. 7 and 8, the precursor and copolymer membranes absorb intense visible light due to their conjugated aromatic structure and intramolecular or intermolecular charge-transfer complexes (CTCs) created between or within the polymer chains, thus rendering polymer colors from pale yellow to dark brown.

It can be seen from FIG. 7 that the Pl domain increases, i.e., a Pl copolymerization ratio increases, the cut-off wavelength increases, but the transmittance (%) decreases. This behavior can be explained from the fact that

the electron-donating ether groups present in ODA diamine and the electron-withdrawing CF ₃ group present in APAF diamine were presumably effective in decreasing charge transfer complexes between polymer chains through steric hindrance and inductive effects. It can be confirmed from FIG. 8 that after PBO conversion was completed by thermal rearrangement reaction in PBO-PI copolymers, the cutoff wavelength of PBO-PI copolymers shifted to a higher wavelength than that in the HPI-PI state, while the transmittance was severely reduced.

(Experimental Example 4) X-ray diffraction (XRD) analysis

The precursor and polymer membranes as set forth in Table 1 above were subjected to X-ray diffraction using a wide angle X-ray diffractometer (D/MAX-2500, Rigaku, Japan) operating with a scanning rate 5°/min at 2θ of 5 to 60°, to obtain X-ray diffraction patterns. FIG. 9 is X-ray diffraction patterns of HPI-PI precursor membranes prepared at various copolymerization ratios. FIG. 10 is X-ray diffraction patterns of PBO-PI copolymer membranes prepared at various copolymerization ratios.

As apparent from FIGs. 9 and 10, the precursor and copolymer membranes are amorphous, and as the APAF content increases, the peak center of 2θ shifts to lower values. In the case where the precursor and copolymer membranes are copolymehzed in the same amount, 2θ of the copolymer membrane shifts to lower values.

This shift behavior means that thermal conversion of HPI results in

rearrangement of PBO molecules, causing an increase in d-spacing between the PBO molecules. The increase in d-spacing is attributed to the fact that bulky groups such as hexafluoroisopropylidene linkages present in APAF diamine affect this morphological change because of reduced intra- and interpolymeric chain interactions, resulting in loose polymer chain packaging and aggregates.

(Experimental Example 5) Physical Properties

The physical properties of the precursor and polymer membranes shown in Table 1 above were measured. First, density of the membranes was measured by a buoyancy method using a Sartorius LA 120S analytical balance. The fractional free volume (FFV,

V _f ) was calculated from the data in accordance with Equation 1 below [W. M. Lee.

Selection of barrier materials from molecular structure. Polym Eng Sci.

1980;20:65-9]. Equation 1

wherein V is the polymer specific volume and V _w is the specific Van der Waals volume. The Van der Waals volume was estimated by a Cerius 4.2 program using a synthia module based on the work of J. Bicerano [J. Bicerano. Prediction of polymer properties, Third Edition. Marcel Dekker Inc. 2002].

The d-spacing was calculated in accordance with Bragg's equation from X-ray diffraction pattern results of Experimental Example 4.

TABLE 2

As can be seen from Table 2, all of the precursor and polymer membranes showed increased d-spacing and FFV due to bulky CF ₃ groups of an APAF moiety, as the APAF content increases.

In particular, after conversion to PBO, FFV of PBO-PI (10:0) was two or more times higher than that of HPI-PI (10:0) due to the decrease in density of the PBO-PI series after thermal treatment. In addition, HPI-PI (0:10) and PBO-PI (0:10) do not show any significant difference in physical properties before and after the thermal treatment, because there is no space in which rearrangement of polymer structures induced by heterocyclic ring modification occurs.

On the other hand, the PBO-PI (5:5) copolymer membrane according to the present invention has an FFV of 0.1559 and a d-spacing of 0.6447 nm. The FFV and d-spacing of the PBO-PI (5:5) copolymer membrane are in the range from those of PBO-PI (0:10) (Ae., PBO homopolymer) to those of PBO-PI (10:0) (Ae., Pl homopolymer), and affect permeability and selectivity of the gas separation membrane.

(Experimental Example 6) Adsorption and desorption isotherm analysis

This experiment was performed to determine N ₂ adsorption/desorption characteristics of the PBO-PI copolymer membranes shown in Table 1. N ₂ adsorption isotherms of the PBO-PI polymer membranes were measured by a BET method. The results thus obtained are shown in FIG. 11.

FIG. 11 is N ₂ adsorption/desorption isotherms of PBO-PI copolymer membranes prepared at various copolymerization ratios.

As can be seen from FIG. 11 , no micropores are observed in the PBO-PI (0:10) (Ae., Pl homopolymer) membrane, which means non-occurrence of N ₂ adsorption. In addition, the amount of adsorbed nitrogen increases, as the amount of PBO present in the PBO-PI copolymer membrane increases. This means that the number of micropores created is proportional to the amount of thermal conversion from HPI to PBO. In order to realize more precise characterization, the pore volume of

PBO-PI copolymer membranes was measured using a specific surface area and pore analyzer (ASAP2020, Micromeritics, GA, USA). At this time, the copolymer membranes were transferred to pre-weighed analytic tubes which were capped with Transeal™ to prevent permeation of oxygen and atmospheric moisture during transfers and weighing. The copolymer membranes were evacuated under dynamic vacuum up to 300 ⁰ C until an outgas rate was less than 2 mTorr/min. The results are shown in Table 3 below.

TABLE 3

As can be seen from Table 3, the specific surface area and pore volume of the PBO-PI copolymer membranes were gradually increased from 0.1 to 480 U) ² Ig and from 0.004 to 0.25 m ³ , respectively, as the amount of PBO present in the PBO-PI copolymer membrane increases. These values are higher than those of common polymers for separation membrane material applications, and comparable to conventional adsorbents such as activated carbon, zeolites and microporous alumina.

Meanwhile, the PBO-PI (10:0) (i.e., PBO homopolymer) membrane has a large specific surface area and a small pore size, as compared to PBO-PI (8:2),

PBO-PI (5:5) and BO-PI (4:8) membranes. These results demonstrate that the size of PBO-PI copolymers can be adjusted to a desired level by controlling the copolymerization ratio between PBO and Pl blocks of the PBO-PI copolymers.

(Experimental Example 7) Measurement of Permeability and Permselectivity

This experiment was carried out in the following manner to determine gas permeability and gas permselectivity of the PBO-PI copolymer membranes.

The gas permeability of single gases such as CO ₂ , O ₂ , N ₂ and CH ₄ was

measured by a time-lag method, which was carried out at various temperatures under a pressure of 760 Torr. Permselectivity of gas pair such as O2/N2 and CO ₂ /N ₂ was calculated from the ratio of single gas permeability. The results are shown in FIG. 12 and Table 4. FIG. 12 is a graph showing a diffusion coefficient of PBO-PI copolymer membranes for O2, CO ₂ , N ₂ and CH ₄ as single gases.

It can be seen from FIG. 12 that the diffusion coefficient of the PBO-PI copolymer membrane is proportional to the amount of PBO present in the PBO-PI copolymer. This behavior indicates that improved gas permeability is attributed to the micropores created by PBO.

Table 4

As can be seen from Table 4, all gas species showed considerably increased gas permeability according to a ratio of a thermally converted domain to a stable domain. After being fully converted from HPI to PBO (PBO-PI (10:0)),

permeabilities of all tested gases were around 1 ,500 times higher than annealed pure polyimide (PBO-PI (0:10)) without any significant selectivity loss. The increase in gas permeability corresponds to the FFV values (See Table 2) and is caused by microcavities created during thermal modification in a solid state. FIG. 13 is a graph showing O ₂ /N ₂ permselectivity of the PBO-PI copolymer membrane and conventional polymers as a function of O ₂ permeability. FIG. 14 is a graph showing CO ₂ /CH ₄ permselectivity of the PBO-PI copolymer membrane and conventional polymers as a function of CO ₂ permeability. In FIGs. 13 and 14, PET indicates poly( ethylene terephthalate), PSf indicates polysulfone, CA indicates cellulose acetate, PC indicates polycarbonate, PS indicates polystyrene, PPO indicates poly(phenylene oxide), PTMSP indicates poly(1-trimethylsilyl-1-propyne), PA indicates polyamide, Pl indicates polyimide, PMP indicates poly(4-methyl-2-pentyne), and PDMS indicates polydimethylsiloxane. As apparent from FIGs. 13 and 14, the PBO-PI copolymer membrane according to the present invention has well-connected microcavities, which are linearly increased, as the amount of PBO present in the copolymer increases.

Although PTMSP shows still higher O ₂ and CO ₂ permeability, it does not surpass the upper bound line owing to low gas selectivity. However, selectivity for important gas pair (e.g., O ₂ /N ₂ and CO ₂ /CH ₄ ) of the PBI-PI copolymer membranes according to the present invention is much higher than that of PTMSP.

[Industrial Applicability]

As apparent from the foregoing, the PBO-PI copolymer according to the present invention can readily rendered into gas separation membranes in various forms including flat-sheets, hollow fibers and organic-inorganic complexes from highly soluble precursors. The gas separation membrane thus prepared can endure harsh conditions such as long operation time, acidic conditions and high humidity due to the rigid polymer backbone thereof.