European Polymer Journal

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1 European Polymer Journal 44 (2008) Contents lists available at ScienceDirect European Polymer Journal journal homepage: Microstructure and properties of polyamideimide/silica hybrids compatibilized with 3-aminopropyltriethoxysilane Minsoo Son a, Youri Ha a, Myeon-Cheon Choi a, Taesung Lee a, Donghee Han b, Sewon Han b, Chang-Sik Ha a, * a Department of Polymer Science and Engineering, Pusan National University, Busan , South Korea b Advanced Materials and Application Research Laboratory, Korea Electrotechnology Research Institute, P.O. Box 20, Changwon , South Korea article info abstract Article history: Received 23 February 2008 Received in revised form 19 April 2008 Accepted 22 April 2008 Available online 29 April 2008 Keywords: Polyamideimide Silica Hybrid Compatibilized Microstructure In this work, we prepared and characterized polyamideimide (PAI)/silica hybrids compatibilized with 3-aminopropyltriethoxysilane (APTES). PAI/silica nanohybrid thin films were prepared using an in situ sol gel process, followed by thermal imidization. We have investigated the microstructures and properties of the PAI/silica hybrids using FT-IR spectroscopy, X-ray diffraction, small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). We also measured their tensile properties, thermal properties, refractive indices, and dielectric constants. In general, the properties of the PAI/silica hybrids were optimized when the silica content was 6 wt.%. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Organic/inorganic hybrid composites that utilize polymers as a matrix have been recognized as one of the new classes of advanced materials. This is because they benefit from the advantageous properties of both the inorganic materials (e.g. rigidity and thermal stability) and the organic polymers (e.g. flexibility, ductility, and processability) [1 10]. Among the various polymers used for hybrid composites, aromatic polyimides (PI) have been particularly popular. This is due to their excellent thermal, mechanical, and electrical properties [11 12]. However, these polymers are difficult to process because of their high softening temperatures and poor solubility in organic solvents. Several approaches to improve their solubility and processability have been proposed. These were based on the incorporation of flexible segments bearing sulfone, amide, and ether * Corresponding author. Tel.: ; fax: address: csha@pnu.edu (C.-S. Ha). moieties on the polymer backbone, without sacrificing the material s heat resistance [13]. Thus, several copolymers have been developed, such as polyamideimides (PAIs), which are known to have high thermal stability and good solubility in polar amide-type solvents. PAIs provide a favorable balance between processability and performance and are useful in numerous applications, such as in electrical wire enamel, adhesives, and injection-molding and extrusion products. So far, various approaches have been devised to prepare PAIs [14 17]. Silica has been commonly used as the inorganic component because it has proved to be effective in enhancing the mechanical and thermal properties of the polymers. Various studies focused on the preparation of polymer/silica composite films have been reported, most of them having employed a sol gel process [18 22]. A typical sol gel process involves the in situ generation of silica nanoparticles that are three-dimensionally crosslinked by successive reactions of hydrolysis and polycondensation of their corresponding precursor molecules. The most widely used precursor of silica is tetraethoxysilane (TEOS) [19,20]. The /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.eurpolymj

2 M. Son et al. / European Polymer Journal 44 (2008) properties of the resulting polymer/silica hybrid composites are significantly influenced by various factors, such as ph, temperature, the nature of the solvent, the type of polymer etc. When these factors are not optimally controlled, most of the resulting polymer/silica hybrid films exhibit a large scale phase segregation that reduces the combined advantages of the organic and inorganic constituents. In fact, sometimes the resulting composites have much poorer properties than their individual constituents [9]. Therefore, minimizing the degree of phase segregation is of utmost importance. This can be achieved by the following different methods: (1) introducing desirable functional groups at both ends of the polymer chains, (2) selecting a silica precursor having an organic group, (3) incorporating comonomers that have suitable functional groups, and (4) adding a coupling agent to chemically bind the polymer chains and silica. 3-Aminopropyltriethoxysilane (APTES), in particular, has been widely used as a coupling agent [18,21,22]. Other examples include (aminophenyl)trimethoxysilane and 3-glycidyloxypropylmethoxysilane [23]. It has been reported that the alkoxy ends of APTES and other silanes undergo hydrolysis and polycondensation, along with TEOS, to form silica or polymeric silicates, while the other end groups (such as amino groups) provide primary or secondary bonds with the PI chains. Another advantage of introducing the coupling agent to bond the PI with the silica is the possibility of achieving improved reinforcement of the resultant hybrid materials [23]. Recently, there have been a few studies on the preparation and characterization of PAI/silica hybrids, although each focused on a different PAI. Chang et al. studied the degradation behavior of PAI/silica hybrids [24,25] and also used solid state NMR to reveal the structure and relaxation behavior of the hybrids [26]. Babooram et al. reported the microstructures and various other properties of PAI/silica hybrids from the use of scanning electron microscopy, X-ray diffraction, and a thermal analyzer [27]. In this study, we prepared PAI/silica hybrid composite films were prepared using an in situ sol gel process, followed by thermal imidization. Trimellitic anhydride chloride (TAC) and 4,4 0 -oxydianiline (ODA) were used to prepare PAI, while tetraethoxysilane (TEOS) was used as a silica precursor. In addition, we attempted to reduce the degree of phase segregation in the hybrids by using 3-aminopropyltriethoxy silane (APTES) as a coupling agent. The resulting composite films were characterized using multiple techniques and thermal/mechanical measurements Preparation of the polyamideimide/silica hybrid films First, ODA ( g ; mmol) was added to NMP (28.97 ml) in a two-necked round-bottom flask under a nitrogen atmosphere, and stirred for 2 h until complete dissolution. Next, TAC ( g ; mmol) was added to the solution of ODA in NMP. The reaction (polymerization) was carried out for 12 h at room temperature, to give a poly(4,4 0 -oxydianiline trimellitamic acid) (TAC ODA PAA) solution. To this PAA solution, a corresponding amount of silica precursors was added and stirred for 30 min to obtain a homogeneous solution. Next, deionized water was dropped into the solution of TAC ODA PAA and silica precursor (in the cases of T2, T4, T6, T8, and T10, we added APTES as a coupling agent), and then stirred for 12 h in order to complete the sol gel reaction. The solution mixture (after the sol gel reaction) was spin-coated onto a glass slide and then soft baked at 80 C for 6 h. The solid concentration was fixed to 20 wt.% for all precursor solutions, meaning that the ratio of the solid mixture (TAC ODA) to NMP was 1:4 by weight. The soft-baked films (thickness = 100 lm) were thermally imidized by a stepwise imidization process under a nitrogen atmosphere for 30 min at 140 C, 200 C, and 250 C. The heating rate employed was 2 C/min, which resulted in the formation of the hybrid composite films of poly(4,4 0 - oxydianiline trimellitimide) (TAC ODA PAI) and silica. Fig. 2 summarizes the whole preparation process of the hybrid films. The chemical structures of TAC ODA PAA, TAC ODA PAI, and the silica precursors are shown in Fig. 1. Table 1 summarizes the detailed specifications for the preparation of the hybrid precursor films and the corresponding names of the samples Measurements Fourier transform infrared (FT-IR) spectra of the films were measured using a spectrometer (React IR TM 1000, Applied System, ASi). Interferogram spectra were collected between 400 and 4000 cm 1, at a resolution of 4 cm scans were taken. The morphology of the fractured surfaces of the hybrid films were measured using field 2. Experimental part 2.1. Materials 4,4 0 -Oxydianline (ODA, 97%), trimellitic anhydride chloride (TAC, 97%), 1-methyl-2-pyrrolidinone (NMP, 97%), tetraethoxysilane (TEOS, 98%), and 3-aminopropyltriethoxy silane (APTES, 98%) were purchased from Aldrich and used as received. Fig. 1. Chemical structure of (a) TAC ODA PAA, (b) TAC ODA PAI, (c) TEOS, (d) APTES.

3 2238 M. Son et al. / European Polymer Journal 44 (2008) sample dimensions were as follows: 4 cm (long) 5mm (wide) 100 lm (thick). The reported tensile testing results are the average values calculated from five specimens in each case. We determined the in-plane and out-of plane refractive indices using a prism coupler (PC-2000, Metricon). The dielectric constants were determined using a LCR meter (4284A, Agilent) with a dielectric kit (16451B, Agilent) at 1 khz. 3. Results and discussion 3.1. Examination of imidization and silica formation Fig. 2. Schematic diagram for the preparation process of TAC ODA PAI/silica hybrid films. Table 1 Preparation of TAC ODA PAI/silica hybrid films Sample code PAA + NMP a (g) TEOS (ll) APTES (ll) H 2 O + NMP b (ll) TEOS contents c (wt%) T T T T T T T T T T T T T2W T T6W O T10W O a b c Film state d Weight of PAA NMP (=20:80) solution. Volume of H 2 O NMP (=20:80) solution. TEOS contents (wt%) in PAA/silica hybrids. d Film state of PAI and PAI/silica hybrid films: T; transparent, O; opaque. emission-scanning electron microscopy (FE-SEM, JSM- 6700F) with an acceleration voltage of 15 kv. Wide-angle X-ray diffraction measurements (WAXD, Miniflex, Rigaku) were performed for pure PAI and the hybrid films. Smallangle X-ray scattering (SAXS) patterns were measured using a synchrotron X-ray source (located at the Pohang Accelerator Laboratory, South Korea), using CoKa (k = Å) radiation with an energy range of 4 16 kev (energy resolution (DE/E)= ; photon flux = ph./s; beam size = 1 mm 2 ). The SAXS scan range was fixed at nm 1 < q < nm 1. The glass transition temperatures (T g s) of the fully cured PI and its hybrid films were determined using a differential scanning calorimeter (DSC, Q100, TA Instrument) in a nitrogen atmosphere with a heating rate of 10 C/min. All of the T g s were taken as the half-height point of the heat capacity increase in the second scan. Tensile properties of the hybrid films were determined from the stress strain curves obtained from the use of a universal testing machine (UTM, Instron) at a uniform strain rate of 10 mm/min, using a 1 kn load cell, at room temperature. For these tests, the Prior to undertaking detailed studies of the hybrid films, we examined the FT-IR spectra of pure TAC ODA PAA and TAC ODA PAI films in order to confirm the synthesis of the precursor polymer and resulting polyimide (see Fig. 3). The TAC ODA PAA solution shows characteristic peaks at around 1650 cm 1 (C@O vibration, due to the amide group and the forming amic acid) and 3410 cm 1 (N H vibration of the amide group). The TAC ODA PAI film shows characteristic peaks at 1710 and 1770 cm 1, which correspond to both the symmetric and asymmetric C@O stretching of the imide group, respectively. Additionally, the bands at 1370 and 737 cm 1 are attributed to the C N stretching vibration and imide ring deformation, while the band at 1497 cm 1 is assigned to the vibration of the benzene ring in the polyimide. The band at 1173 cm 1 is attributed to the aromatic C 6 H 4 or C 6 H 2. The peak at 677 cm 1 is assumed to originate from the CAC@O inplane swing vibration. The extent of thermal imidization in the films was verified from both the decrease of the band at 1650 cm 1 and the increase of the band at 1710 cm 1, the latter of which corresponds to the growth of the imide group. Fig. 4 shows the IR spectra of the TAC ODA PAI/silica hybrid films. In the inset of Fig. 4, normalized (around the wavenumbers between 700 cm 1 and 1200 cm 1 ) IR spectra of the TAC ODA PAI/silica hybrid films are shown. The weak band at 1019 cm 1, which corresponds to the vibration of silica networks (Si O Si), gradually increases in intensity with Fig. 3. IR spectra of pristine TAC ODA PAA and PAI, where they are designated as T-PAA and PAI, respectively.

4 M. Son et al. / European Polymer Journal 44 (2008) adhesion. However, in the case of T10, the silica particles have good interaction with the matrix. In addition, the images show that the silica particles of T10 are smaller than those of T10W (with average diameters of around lm). In Fig. 6, the sizes of the silica particles are: several tens of nm for T2, nm for T4, nm for T6, lm for T8, and lm for T10. This indicates that the particle size increases with an increasing content of silica precursors. The hybrid films containing a coupling agent, however, exhibited good adhesion between the PAI matrix and silica domains, as expected [10,33] Periodicity of PI in the hybrid films Fig. 4. IR spectra of TAC ODA PAI/silica hybrid films using APTES. In the inset is shown their normalized (around the wavenumbers between 700 cm 1 and 1200 cm 1 ) IR spectra. For sample notations, see Table 1. increasing silica content. This clearly indicates the formation of a three-dimensional Si O Si network in the PAI/silica hybrid films. This result is in good agreement with earlier studies on PI/silica and PAI/silica hybrids [8,10,21,23 33] Morphology of the hybrid films As shown in the FE-SEM images (Fig. 5), in the case of T10W, the dispersed silica particles appear as small beads with average diameters of around lm. This micrograph also reveals that the silica particles are completely detached from the PAI matrix, indicating poor interfacial Fig. 7 shows the XRD patterns of the pure PAI and the hybrid films. The pattern of the pure PAI displays a small peak (d-spacing value is 30 Å) at around 2h = 3.2. In this study, we confirmed that this small peak disappeared by adding a silica precursor. This means that the pure PAI exhibits a small periodicity due to the chain packing in the backbone of the polyimides and this periodicity is collapsed by the silica moiety. Fig. 8 shows the Lorentz-corrected SAXS patterns [34 36]. All of the films show a broad SAXS peak at 0.03 < q < Å 1, which is attributed to the mean long period of TAC ODA PAI (ca. 117 Å for the pure PAI film). The mean long period, showing a presence of a certain microstructure, was estimated from the peak maximum using the wave vector, q =(4p/k) sinh, with the wavelength k of the radiation source and the scattering angle 2h [34]. Although the size and distribution of the silica particles formed in the hybrid films were not quantitatively Fig. 5. Fractured surface morphology of hybrid composite films (a), (b) T10W, (c), (d) T10. For sample notations, see Table 1; (a) is in low magnification (2000); (b) and (c) are in middle magnification (5000); (d) is in high magnification (20,000).

5 2240 M. Son et al. / European Polymer Journal 44 (2008) Fig. 6. Fractured surface morphology of hybrid composite films using APTES (a) T, (b) T2, (c) T4, (d) T6, (e) T8, (f) T10. For sample notations, see Table 1; (a) (f) are in same magnification (50,000). Fig. 7. XRD graphs of pristine TAC ODA PAI and hybrid films using APTES (a) T, (b) T2, (c) T4, (d) T6, (e) T8, (f) T10. For sample notations, see Table 1. examined, these SAXS results indicate that various sizes of silica particles were made during the sol gel process and they had an influence on the mean long period of the polyimide [37]. Here, the long period of the PAI/silica hybrid Fig. 8. Lorentz-corrected SAXS patterns of pristine TAC ODA PAI and hybrid films using APTES (a) T, (b) T2, (c) T4, (d) T6, (e) T8, (f) T10. For sample notations, see Table 1. films is larger than that of the pure PAI. Basically, the increased long period can be attributed to the size of the grown crystal or phase domains. In these SAXS data, we

6 M. Son et al. / European Polymer Journal 44 (2008) can see that the long period of the hybrid films increased with increasing silica precursor content, but only up to 6 wt.%. Above this silica precursor content, the long period decreased Thermo-mechanical properties The glass transition temperatures (T g s) of the hybrid films are shown in Fig. 9. These were taken from the DSC measurements. The T g s of the hybrid films with APTES increased with increasing silica precursor content (again up to 6 wt.%). The results suggest that the mobility of the PAI chains was restricted by the interaction of the fairly dispersed silica domains with the matrix PAI. They also indicate that the degree of crosslinking increased with increasing amounts of the silica precursors [28]. In the case of the composites having high silica contents, the T g s of the hybrids decreased due to phase separation. However, in the case of the hybrid without APTES, we could find no change of T g s. This result is due to the fact that the PAI/silica hybrids compatibilized with APTES have better interfacial interactions between the PAI and the silica. It is interesting to note that the change of T g, according to the silica content, is analogous to that of the long periodicity Fig. 9. Glass transition temperatures (T g s) of pristine TAC ODA PAI and hybrid films. Fig. 11. Tensile properties of pristine TAC ODA PAI and hybrid films. from the SAXS pattern. Fig. 10 shows comparative graphs of the two sets of data. These results imply that the T g behavior, as a function of silica content, is directly related to the effect of the silica moiety on the mean long periodicity of the chain packing in PAI. Fig. 11 shows the tensile properties of pure PAI and the two types of hybrid films, Tx and TxW (where the x means silica precursor content). The tensile moduli of the films for both types of composite system are generally found to increase with increasing inorganic phase content [28,33]. The tensile strengths of the hybrid films without APTES, however, gradually decrease with increasing silica content, while pure PAI has a tensile strength of 98.9 MPa. In the case of the PAI/silica hybrid films with APTES, the tensile strengths generally increase with increasing silica precursor content, but again only up to 6 wt.%. These results are due to the coupling agent resulting in enhanced interactions between the PAI matrix and the silica domains in the hybrid system. However, when the silica precursor content exceeds 6 wt.%, a small reduction in tensile strength was found (see Fig. 11), which is indicative of the large domains of the silica, due to the poor interfacial adhesion. It is worth noting, however, that the tensile strengths of the hybrids are still higher than that of the pure PAI. Fig. 10. Comparison of changes of glass transition temperatures (T g s) with those of long periodicity of pristine TAC ODA PAI and hybrid films using APTES. Fig. 12. TGA curves of TAC ODA PAI and hybrid films. For sample notations, see Table 1.

7 2242 M. Son et al. / European Polymer Journal 44 (2008) Fig. 12 shows the TGA curves of pure PAI and its hybrid films. As expected, the thermal stability of PAI, in terms of the initial thermal degradation temperature, was enhanced with an increasing silica contents Refractive indices and dielectric constants Fig. 13 displays the refractive indices of pure PI and the hybrid composite films having various silica contents. n xy and n z represent the in-plane and out-of-plane refractive indices, respectively. All of the hybrid films, as well as PAI, showed a larger in-plane refractive index than outof-plane refractive index, indicating molecular chain orientation. This phenomenon is due to the anisotropy of the samples due to the film-casting process [35]. We calculated the dielectric constants both from the use of the LCR meter at 1 khz and from the average refractive index, using the following two relations (Fig. 14): Capacitance-dielectric constant relations; e r ¼ðCdÞ=ðe 0 sþ and Maxwell relations; e r ¼ n 2 where e r, C, d, e 0, s, and n represent the dielectric constant (relative permittivity), the capacitance, the distance between the two electrodes, permittivity of free space the Fig. 13. Refractive index of pristine TAC ODA PAI and hybrid films using APTES by a prism coupler. area of the electrode, and the refractive index, respectively. As shown in Fig. 14, at first, we find that the dielectric constants of PAI and its hybrid films, measured by the LCR meter, are larger than those calculated from the refractive index. This is because the frequency was very low in the measurement by LCR meter (1 khz). (Generally, the higher the frequency is, the lower the dielectric constant is.) The dielectric constants obtained from the LCR meter decreased with increasing silica content. This phenomenon can be explained by the increasing microvoid between the PAI matrix and the silica domains. However, this does not mean a gross phase separation. Some parts of the silica particles connect with pure PAI (like a cocoon, see Fig. 6), while other parts of the silica particles have gaps between them and the matrix. When large amounts of silica precursors were introduced into the PAI, the phase was rapidly separated and the dielectric constants became lower. 4. Conclusions We have discussed the microstructures and properties of polyamideimide/silica hybrids compatibilized with APTES. The FT-IR spectra confirmed successful synthesis and preparation of PAI and its hybrid films. In the SAXS data, we could see that the long period of the hybrid films increased with increasing silica precursor content up to 6 wt.%. Above this silica precursor content, the long period decreased. This change in the mean long periodicity directly affected the T g. When the coupling agent, APTES, was used, the silica particles have good interaction with the matrix, and the mechanical properties were improved. The tensile strengths of the hybrid films without the coupling agent gradually decrease with increasing silica content. In the case of the PAI/silica hybrid films with the coupling agent, the tensile strengths generally increase with increasing silica content. These results are due to the enhanced interactions between the polyamideimide matrix and the silica domains. However, when the silica precursor content exceeds 6 wt.%, we confirmed a reduction in tensile strength. This is indicative of large domains of silica due to poor interfacial adhesion. The thermal stability of PAI, in terms of the initial thermal degradation temperature, was enhanced with an increasing silica contents. The dielectric constants (measured using a LCR meter) decreased with increasing silica content. When large amounts of silica precursors were introduced into the PAI, the dielectric constants became lower. In this work, in general, optimal thermo-mechanical properties of hybrid films were observed when the silica content was 6 wt.%. Acknowledgements Fig. 14. Dielectric constants, measured by two different methods (LCR meter and a prism coupler), of pristine TAC ODA PAI and hybrid films using APTES. The work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Program funded by the Ministry of Science and Technology (MOST; M J ), the SRC/ERC Program of MOST/KOSEF (Grant # R ), the Korea Electrotechnology Research Institute, and the Brain Korea 21 Project. We also thank

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