clay nanocomposite materials with advanced anticorrosive properties prepared from solution dispersion technique

Acta Materialia 52 (2004) 475–486 www.actamat-journals.com

Organo-soluble polyimide (TBAPP–OPDA)/clay nanocomposite materials with advanced anticorrosive properties prepared from solution dispersion technique Yuan-Hsiang Yu a

a,b

, Jui-Ming Yeh

a,*

, Shir-Joe Liou a, Yen-Po Chang

a

Department of Chemistry and Center for Nanotechnology at CYCU, Chung-Yuan Christian University, Chung Li 320, Taiwan, ROC b Department of Electronic Engineering, Lan-Yan Institute of Technology, I-Lan 261, Taiwan, ROC Received 18 September 2002; received in revised form 12 September 2003; accepted 23 September 2003

Abstract A series of organic soluble polymer–clay nanocomposite (PCN) materials that consist of organic soluble polyimide (SPI) and layered montmorillonite (MMT) clay are successfully prepared by the solution dispersion technique. The as-synthesized PCN materials are characterized by infrared spectroscopy, wide-angle powder X-ray diffraction, transmission electron microscopy and scanning electron microscopy. Polymer–clay nanocomposite materials, in the form of coating, incorporating with low clay loading on cold-rolled steel (CRS) are found much superior in anticorrosion over those of bulk SPI on the basis of a series of electrochemical measurements of corrosion potential, polarization resistance and corrosion current in 5 wt% aqueous NaCl electrolyte. Effects of the material composition on the O2 /H2 O molecular permeability, optical clarity and thermal stability of SPI along with PCN materials, in the form of both membrane and fine powder, are also studied and compared to insoluble polyimide system prepared from the thermal imidization by molecular permeability analysis, UV–vis transmission spectra, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Corrosion; Polyimide; Nanocomposite

1. Introduction Some of the most effective anticorrosive undercoatings in use today are based on chromium-containing compounds, which may need to be replaced with alternative materials due to the environmental and health concerns [1–6]. Therefore, many novel coating materials with effective anticorrosion properties attract considerable research attention in the recent decades. Recently, we found that incorporating the inorganic nanolayers of montmorillonite (MMT) clay into the organic polymeric matrix can effective enhance the corrosion protection effect of pristine polymers (e.g., polyaniline [7,8], poly (methyl methacrylate) [9]) on cold-rolled steel (CRS) *

Corresponding author. Tel.: +886-326-533-40; fax: +886-326-533-

99. E-mail address: [email protected] (J.-M. Yeh).

coupon based on a series of electrochemical corrosion measurements in saline. In general, the primary effect of a polymeric coating is to function as a physical barrier against aggressive species such as O2 and Hþ . As a second line of defense for corrosion, the inorganic materials, such as clay, with a plate-like shape, are usually employed to effectively increase the length of the diffusion pathways for oxygen and water as well as decrease the permeability of the coating [10]. The chemical structures of MMT consist of two fused silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either magnesium or aluminum hydroxide. The Naþ and Caþ2 existing in the interlayer regions can be replaced by organic cations such as alkylammonium ions through a cationic-exchange reaction to render the hydrophilic layer silicate organophilic, leading to the striking development of polymer–clay nanocomposite (PCN) materials. The earlier historical

1359-6454/$30.00 Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.09.031

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study of polymer–clay nanocomposite can be traced back to the work of polyamide–clay nanocomposite reported by ToyotaÕs research group in 1990 [11]. Up to now, the dispersion of nanolayers of mineral clay was reported to boost the thermal stability [12], mechanical strength [13], molecular barrier [14] and flame retardant [15] properties of polymers. Among many engineering polymers, aromatic polyimides (PIs) are regarded as thermally stable polymers that exhibit excellent mechanical strength and thermal stability. During the past decade, interest in these polymers has increased in response to increasing technological applications in a variety of fields such as aerospace, automobile and microelectronics. Recently, significant synthetic efforts in the research field of hightemperature aromatic PIs have focused on improving processability and solubility through the synthesis of new diamine or dianhydride monomers. Several approaches (e.g., introduction of flexible bridging linkages into the polymer backbone [16,17], incorporation of bulky substituents along the polymer backbone [18,19]) have led to significant success to prepare many organic soluble polyimides (SPIs). Lately, there are considerable amount of publications associated with the preparation and properties of polyimide–clay nanocomposite [20–24] and organic soluble polyimide–clay nanocomposite materials [25– 28]. However, organic soluble polyimide–clay (SPI–clay) nanocomposite materials in the form of coating for the corrosion protection application on metallic surface have never been reported. In this paper, SPI–clay nanocomposite materials are first prepared by chemical imidization and followed by dispersion the clay platelets into the polymer matrix via the solution dispersion technique. We present the first evaluation of anticorrosive effect of organic soluble polyimide–clay nanocomposite materials which show excellent anticorrosive properties. The as-synthesized PCN materials are characterized by wide-angle powder X-ray diffraction, transmission electron microscopy and infrared spectroscopy. PCN materials in the form of coatings with low clay loading on cold-rolled steel (CRS) are found much superior in anticorrosion over those of SPI based on the series of electrochemical measurements of corrosion potential, polarization resistance and corrosion current in 5 wt% aqueous NaCl electrolyte. Furthermore, we find that a further increase of clay loading results in further enhanced corrosion protection properties of PCN materials. Effect of material composition on the molecular barrier, optical clarity and thermal stability are also studied and compared to insoluble polyimide system prepared from the thermal imidization by the molecular permeability analysis, UV–vis transmission spectra, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively.

2. Experimental section 2.1. Chemicals and instrumentations 4,40 -Isopropylidenebis(2,6-dimethylphenol) (Aldrich, 98%), p-chloronitrobenzene (Fluka, 98%), anhydrous potassium carbonate (SHIMAKYUÕS PURE CHEMICALS), N,N-dimethylformamide (Mallinckrodt), hydrazine monohydrate (SIGMA, 64% hydrazine), 10% Pd on active carbon (Lancaster), ethanol (Riedel– deHa€en), 4,40 -oxydiphthalic anhydride (OPDA) (Aldrich, 97%), N,N-dimethylacetamide (DMAc) (Riedel– deHa€en), pyridine (Riedel–deHa€en), acetic anhydride (Fisher Chemical), 1-methyl-2-pyrrolidinone (NMP) (Tedia, 99.97%), methanol (Riedel–deHa€en) and tetrahydrofuran (THF) are used as received without further purification. (4-carboxybutyl)-Triphenylphosphosphonium bromide (Lancaster, 98%) is used as intercalating agent. The montmorillonite clay, supplied by Industrial Technology Research Institute (ITRI), is employed and had a unit cell formula of Naþ 0:31 [Al1: 67 Mg0: 33 ]Si4 O10 (OH)2
5.8H2 O and a CEC value of 122 mequiv/100 g. The NMR spectrum of 3,30 ,5,50 -tetramethyl-2,2-bis [4-(4-aminophenoxy) phenyl]propane (TBAPP) in solution is recorded on a Bruker Advanced 300 FTNMR spectrometer operating at 300 MHz for proton resonance. Sample is usually dissolved in deuterated dimethyl sulfoxide (DMSO-d6 ) and tetramethylsilane (TMS) is employed as an internal standard. FTIR spectra are measured on pressed KBr pellets using a JASCO FT/IR-460 plus spectrometer. Wide-angle X-ray diffraction study of the samples is performed on a Rigaku D/MAX-3C OD-2988N X-ray diffractometer with a copper target and Ni filter at a scanning rate of 4°/min. The samples for transmission electron microscopy (TEM) study is first prepared by putting the membrane of PCN materials into epoxy resin capsules followed by curing the epoxy resin at 100 °C for 24 h in a vacuum oven. Then the cured epoxy resin containing PCN materials are microtomed with a Reichert–Jumg Ultracut-E into 60–90 nm thick slices. Subsequently, one layer of carbon about 10 nm thick is deposited on these silices on mesh 100 copper nets for TEM observations on a JEOL200FX with an acceleration voltage of 120 kV. The surface morphology of as-synthesized materials is evaluated by a scanning electron microscopy with a model Hitachi S-4100 FE-SEM. Electrochemical measurements of corrosion potential, polarization resistance and corrosion current on sample-coated CRS coupons are performed on VoltaLab 21 Potentiostat/Galvanostat in a standard corrosion cell equipped with two graphite rod counter electrodes and a saturated calomel electrode (SCE) as well as the working electrode. A Yanagimoto Co. Ltd. Gas permeability analyzer (model GTR 10) is employed to perform the permeation experiment of oxygen gas

Y.-H. Yu et al. / Acta Materialia 52 (2004) 475–486

and water vapor. UV–vis transmission spectra are obtained using a Hitachi U-2000 UV–vis spectrometer. Perkin–Elmer Thermal Analysis System equipped with model 7 DSC and model 7/DX TGA are employed for the thermal analyses under air flow. The programmed heating rate is 20 °C/min. The molecular weight of SPI is determined on a Waters GPC model 2 II equipped with a model 590 programmable solvent delivery module, a differential refractometer detector and a Styragel HT column with THF as eluant and monodispersed polystyrenes as calibration standards. 2.2. Synthesis of diamine, TBAPP 3,30 ,5,50 -Tetramethyl-2,2-bis[4-(4-aminophenoxy)phenyl] propane is synthesized similar to the procedure reported by Liaw et al. [22]. As a representative procedure to prepare TBAPP, a solution of 4,40 -isopropylidenebis(2,6-dimethylphenol) (2.884 g, 0.01 mol), pchloronitrobenzene (3.47 g, 0.022 mol), potassium carbonate (3.31 g, 0.024 mol) and N,N-dimethylformamide (DMF, 10 mL) is refluxed for 8 h. The mixture is precipitated in ethanol–water mixture (1:1 volume ratio); and the precipitated crude dinitro intermediate is collected on a Buchner funnel and then recrystallized from DMF. The crude product is obtained by filtering and then drying the compound under vacuum at room temperature. The typical yield of dinitro compound is about 83%. A solution of obtained dinitro compound (3.156 g, 6 mmol), 0.05 g of 10% Pd on active carbon (Pd–C) and 60 mL ethanol is added dropwise of hydrazine monohydrate (20 mL) at 85 °C. and then is refluxed for 24 h. The mixture is filtered to remove Pd–C. Solvent and hydrazine are removed by rotavapor. The crude diamine monomer (TBAPP) is dissolved in a small quantity of ethanol, and then distilled water is added to recrystallize the compound. The product is obtained by filtering and followed by drying under vacuum. The typical yield of TBAPP in the form of fine powder is ca. 82%. 2.3. Synthesis of polyimide derived from TBAPP/OPDA 2.3.1. Preparation of organo-soluble polyimide powder through chemical imidization As a representative step to synthesize soluble PI powder through chemial imidization is given as follows: A solution of 4,40 -oxydiphthalic anhydride (OPDA) (0.930 g, 3 mmol) in DMAc (9 g) is gradually added to a stirred solution of TBAPP (1.398 g, 3 mmol) in DMAc (9 g). The mixture is stirred at room temperature for 24 h to form the poly(amic acid) with a weight-average molecular weight 65,000 g/mol. Then an equimolor mixture of acetic anhydride (0.306 g, 3 mmol) and pyridine (0.24 g, 3 mmol) is added into the prepared poly(amic acid) solution. The mixture is stirred at room

477

temperature for 1 h and then heated at 100 °C for 2 h. It is subsequently poured into methanol and the solid precipitate is filtered off, washed with hot water and methanol, and dried under vacuum (at 40 °C) to afford SPI with a weight-average molecular weight of 80,300 g/mol. The weight-average molecular weights of the poly(amic acid) and SPI should high enough to prepare a flexible and tough membrane of polyimide. 2.3.2. Preparation of insoluble PI membrane through thermal imidization As a typical procedure to prepare the PI by thermal imidization is given as follows: 4,40 -oxydiphthalic anhydride (OPDA) (0.310 g, 1 mmol) in DMAc (3 g) is gradually added to a stirred solution of TBAPP (0.466 g, 1 mmol) in DMAc (3 g). The mixture is stirred at room temperature for 24 h to form the poly(amic acid). A casting is then cast from the poly(amic acid) solution onto a glass plate (5 cm  5 cm, 3 c.c./pcs) and subsequently heated 6 h at 80 °C, 6 h at 150 °C, 2 h at 200 °C, 3 h at 250 °C and 2 h at 300 °C to convert the poly(amic acid) into a PI membrane. 2.4. Preparation of organophilic clay The organophilic clay is prepared by a cationicexchange reaction between the sodium cations of MMT clay and quaternary alkylphosphonium cations of (4-carboxybutyl)-triphenylphosphosphonium bromide. The procedure is similar to methods described in [7–9]. 2.5. Preparation of soluble PCN materials by solution dispersion technique As a representative procedure, the soluble PCN materials can be prepared by the solution dispersion technique as shown in Scheme 1: First, organophilic clay content calculated by 1, 3 and 5 wt% with respect to SPI are introduced into NMP under magnetically stirring 24 h at room temperature. SPI prepared by chemical imidization is subsequently added into the previous organophilic clay solution of NMP to form a 6 wt% of PCNs solution. Under magnetically stirring for additional 12 h at room temperature, the as-prepared PCN solutions are subsequently filtered by a 0.45-lm hydrophilic PVDF filter (Millpore Millev-HV) and cast onto a glass plate (5 cm  5 cm, 3 c.c./pcs) followed by drying at 100 °C for 6 h in an oven to give the membranes of soluble PCN materials. For example, a 6 wt% of PCNs solution containing 1 wt% organophilic clay can be prepared as follows: 0.0084 g of organophilic clay is dispersed in 13.16 g of NMP under magnetically stirring for 24 h at room temperature. 0.8316 g of SPI solid is subsequently introduced into organophilic clay solution and further stirred for 12 h at room temperature to give a PCNs solution. This solution is filtered by a 0.45-lm

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H3C H2N

O

O

O

CH3

CH3

O

O

+

NH2

O

O

CH3 H3C

CH3

TBAPP

O

polymerization

H3C

CH3

CH3

O H3C

chemical imidization

n

OH O

O

pyridine acetic anhydride 100 0C, 2hrs O

O

CH3

O

O

O

N

N

CH3 H3C

N

HO polyamic acid

CH3

O

N

CH3

H3C

O H

H O

O CH3

O

OPDA

solvent: DMAc r.t.

CH3

O

n

O

polyimide

organophilic montmorillonite

dispersion of organophilic montmorillonite

interchalated agent:

polyimide-clay nanocomposite solvent: DMAc stirred at r.t. casting on glass r.t., 24hrs 12hrs 0 curring at 100 C

O P+ Br -

OH

Scheme 1.

hydrophilic PVDF filter (Millpore Millev-HV) and could be cast onto 4 pcs of glass plate (5 cm  5 cm, 3 c.c./pcs) for preparation of free-standing membranes or coating onto cold-rolled steel (CRS) coupons for electrochemical measurements. 2.6. In situ polymerization of insoluble PCN membranes by thermal imidization A typical procedure to prepare PCN materials by thermal imidization shown in Scheme 2 is given as follows: An appropriate amount of organophilic clay with respect to PI by 1, 3 and 5 wt% is introduced into 6 g of DMAc under magnetically stirring 24 h at room temperature. Diamine monomer TBAPP (0.932 g, 2 mmol) is subsequently added to the solution, which is stirred for another 24 h. Then a separate solution containing 4,40 -oxydiphthalic anhydride (OPDA) (0.620 g, 2 mmol) in DMAc (6 g) under magnetically stirring is gradually added to the stirred TBAPP solution. The mixture is further stirred at room temperature for 24 h to give a poly(amic acid)–clay solution. The as-prepared poly(amic acid)–clay solution were subsequently filtered by a 0.45-lm hydrophilic PVDF filter (Millpore Millev-HV) and cast onto 4 pcs of glass plate (5 cm  5 cm, 3 c.c./ pcs). Thermal imidization is performed under the con-

dition of 6 h at 80 °C, 6 h at 150 °C, 2 h at 200 °C, 3 h at 250 °C and 2 h at 300 °C to convert the poly(amic acid)– clay solution into an insoluble PCN membrane. 2.7. Preparation of coatings and electrochemical measurements Soluble polyimide and soluble PCN fine powders are found to be soluble in NMP, DMAc, m-cresol and THF. As a dissolving media, NMP is found to exhibit much better film formation quality relative to other organic solvents. Typically 6 wt% solutions of SPI and soluble PCN materials for coating formation are prepared for electrochemical measurement. The asprepared solutions are cast dropwise onto the CRS coupons (1.0 cm  1.0 cm) followed by drying in air for 24 h at 100 °C to give coatings of ca. 20 lm, measured by digimatic micrometer (Mitutoyo), in thickness. The coating ability of soluble PCN materials on CRS are found similar to that of SPI. The coated and uncoated coupons are then mounted to the working electrode so that only the coated side of the coupon is in direct contact with the electrolyte. The edges of the coupons are sealed with super fast epoxy cement (SPARÒ ). All the electrochemical measurements of corrosion potential, polarization resistance and corrosion current are

Y.-H. Yu et al. / Acta Materialia 52 (2004) 475–486

479

organophilic montmorillonite interchalated agent:

O P+ OH

Br -

dispersion of organophilic montmorillonite

solvent: DMAc r.t., 24hrs H3C

diamine monomer was induced into the layers of clay

H 2N

CH3

CH3

O

O

NH2

CH3 H3C O

H3C

P+

CH3

CH3

O

N

Br -

CH3

TBAPP

O

NH2

CH3

H H3C

O

CH3

O

O

polymerization

O

O OPDA

O H3C

CH3

CH3

O

O H

H O

O

O

N

CH3 H3C

O

N

n

OH

HO

CH3

O

O polyamic acid-clay nanocomposite

thermal imidization

H3C

casting on glass curing at 300 0C

CH3

O

O

O

CH3

O

O

N

N

CH3 H3C

CH3

O

n

O

polyimide-clay nanocomposite

Scheme 2.

performed on a VoltaLab 21 Potentiostat/Galvanostat and repeated at least three times. The electrolyte is NaCl (5 wt%) aqueous solution. The open circuit potential (OCP) at the equilibrium state of the system is recorded as the corrosion potential ðEcorr in V vs. SCE). The polarization resistance ðRp in X/cm2 ) is measured by sweeping the applied potential from 20 mV below to 20 mV above the Ecorr at a scan rate of 500 mV/min and recording the corresponding current change. The Rp value is obtained from the slope of the potential–current plot. The Tafel plots are obtained by scanning potential from 250 mV below to 250 mV above the Ecorr at a scan rate of 500 mV/min. Corrosion current ðicorr Þ is determined by superimposing a straight line along the linear portion of the cathodic or anodic curve and extrapolating it through Ecorr . The corrosion rate, ðRcorr , in milli-inches per year, MPY) is calculated from the following equation: Rcorr ðMPYÞ ¼ ½0:13 icorr ðE:W:Þ=½Ad; where E.W. is the equivalent weight (in g/eq), A is the area (in cm2 ) and d is the density (in g/cm3 ).

2.8. Preparation of membranes and barrier property measurements As-prepared sample-coated glass substrates are immersed into boiling water for 2 h to give PI and PCN membranes. Oxygen permeability of membrane is determined by using the Yanco GTR-10 gas permeability analyzer. Gas permeability is calculated by the following equation: q=t ; A where P is the gas permeability [cm3 (STP) cm/cm2 s cm Hg], q=t is the volumetric flow rate of gas permeate [cm3 (STP)/s], l is the membrane thickness (cm), A is the effective membrane area (cm2 ), and p1 and p2 are the pressures (cm Hg) on the high pressure and low pressure sides of the membrane, respectively. The rate of transmission of O2 is obtained by gas chromatography, from which the air permeability was calculated. On the other hand, experiment of H2 O permeability performed by apparatus similar to our previous published paper [9], that the feed P ¼ l=ðp1  p2 Þ 

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solution is not in contact with the membrane. The feed solution is vaporized first and subsequently permeated through the membrane with an effective area of 10.2 cm2 . The permeation rate is determined by measuring weight of permeate.

(C31 H34 N2 O2 ): IR (KBr) 3404, 3367 and 3335 cm1 (N–H stretching); 1623 cm1 (N–H bending); 1 HNMR (DMSO-d6, 300 MHz) d/ppm 1.52 (6H, s, 2 CH3 ), 1.88 (12H, s, 4 CH3 ), 4.35 (4H, s, 2 NH2 ), 6.04 (8H, dd, 8 CH), 6.51 (4H, s, 4 CH). 3.2. Characterization

3. Results and discussions Montmorillonite has been reported in literatures that it is a clay mineral containing a high aspect ratio and a platey morphology, its structure consists of stacked sil
in thickness icate sheets measuring approaching to 10 A
and 2200 A in length [29]. MMT has a high swelling capacity, which is important for efficient intercalation of the polymer, and is composed of stacked silicate sheets that boost many properties of bulk polymers. In this study, the preparation of composites can be classified into the following two steps. First, organic soluble polyimide (SPI) is first prepared by chemical imidization process. Secondly, the organophilic clay is dispersing into the SPI matrix by the solution dispersion technique. The preparation flowchart of soluble PCN materials by chemical imidization is given in Scheme 1. The composition of the PCN materials is varied from 0 to 5 wt% of clay with respect to SPI content as summarized in Table 1.

Typical FTIR spectra of poly(amic acid) and SPI prepared from chemical imidization are shown in Fig. 1. FTIR spectroscopy confirms the formation of poly(amic acid). As shown in Fig. 1(a), the characteristic absorption bands of the amic acid [30] appear near 3436 (N–H and O–H stretching), 1720 (acid, C@O stretching), 1663 (amide, C@O stretching) and 1506 cm1 (N–H bending). Next, the chemical conversion to SPI is carried out through the incorporation of pyridine and acetic anhydride into the poly(amic acid) solution and followed by heating the mixture up to 100 °C. FTIR spectra of SPI prepared by chemical imidization are shown in Fig. 2(b). The characteristic absorption bands of the imide ring [30] appear near 1780 (asymmetric C@O stretching), 1720 (symmetric C@O stretching), 1390 (C–N stretching) and 745 cm1 (imide ring deformation). Furthermore, the representative FTIR spectra of the organophilic clay, SPI and PCN materials are shown in Fig. 2. Characteristic vibration bands of MMT clay are shown at 1044 cm1 (Si–O), 515 cm1 (Al–O) and

3.1. Monomer synthesis In this study, two steps are employed to synthesize 3,30 ,5,50 -tetramethyl-2,2-bis[4-(4-aminophenoxy)phenyl] propane (TBAPP). First, 4,40 -isopropylidenebis (2,6-dimethylphenol) react with p-chloronitrobenzene in DMF in the presence of potassium carbonate as an acid acceptor to generate dinitro intermediate, which is hydrogenated to generate diamine monomer, TBAPP. Identify of the dinitro compound (C31 H30 N2 O6 ): IR (KBr) 1587 and 1349 cm1 (–NO2 asymmetric stretching); 1 H NMR (DMSO-d6 , 300 MHz) d/ppm 1.64 (6H, s, 2 CH3 ), 1.99 (12H, s, 4 CH3 ), 6.90 (4H, d, 4 CH), 7.07 (4H, s, 4 CH), 8.22 (4H,d, 4 CH). Identify of TBAPP

Fig. 1. Representative FTIR spectra of: (a) poly(amic acid) and (b) soluble polyimide.

Table 1 Relations of the composition of soluble polyimide (SPI)–MMT clay nonocomposite materials with the Ecorr , Rp , icorr and Rcorr measured from electrochemical methods Compound code

Bare SPI CLSPI-1 CLSPI-3 CLSPI-5 a b

Electrochemical corrosion measurementsa

Composition Polymer (wt%)

MMT (wt%)

Ecorr (mV)

Rp (KX cm2 )

– 100 99 97 95

– 0 1 3 5

)604 )494 )390 )241 )146

2.7 3.6  102 2.8  103 3.2  103 5.0  103

Saturated calomel electrode was employed as reference electrode. As measured by digimatic micrometer.

icorr (nA/cm2 ) 1.9  104 1.9  102 9.9 8.1 2.7

Thicknessb (lm)

Rcorr (MPY) 36.7 3.7  1.9  1.6  5.2 

101 102 102 103

– 21 20 20 20

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Fig. 2. The FTIR spectra of: (a) SPI, (b) CLSPI-1, (c) CLSPI-3, (d) CLSPI-5 and (e) organophilic clay.

465 cm1 (Mg–O) [7–9]. As the loading of MMT clay is increased, the intensities of MMT clay bands become stronger in the FTIR spectra of PCN materials. Fig. 3(a) shows the wide-angle powder X-ray diffraction patterns of organophilic clay, SPI and a series of PCN materials. For CLSPI-1, there is a lack of any diffraction peak in 2h ¼ 2–10° as opposed to the diffraction peak at 2h ¼ 5:15° (d spacing ¼ 1.71 nm) for organophilic clay, indicating the possibility of having exfoliated silicate nanolayers of organophilic clay dis-

Fig. 3. Wide-angle powder X-ray diffraction patterns of organophilic clay, SPI and a series of soluble PCN materials prepared by: (a) solution dispersion and (b) in situ polymerization.

481

persed in SPI matrix. When the amount of organoclay increase up to 5%, there was a small peak appearing at 2h ¼ 5:0°, corresponding a d spacing of 1.76 nm. This implies that there is a small amount of organoclay that cannot be exfoliated in SPI and existed in the form of an intercalated layer structure. Similarly, the situation is also observed for the insoluble polyimide systems prepared by the in situ polymerization through thermal imidization up to 300 °C of processing temperature, as shown in Fig. 3(b). As a comparison, the inorganic clay platelets is found to show a better dispersing capability in polymeric matrix through the in situ polymerization (e.g., 5 wt%, d ¼ 1:80 nm) instead of the solution dispersion technique (e.g., 5 wt%, d ¼ 1:76 nm) based on the powder X-ray diffraction pattern studies. This result can be further reconfirmed by visual observation from the transmission electron microscopy (TEM). In Fig. 4, the TEM of PCN materials incorporated with 3 wt% clay reveals that both the nanocomposites (prepared by chemical and thermal imidization) display a mixed nano-morphology. Individual silicate layers, along with two and three layer stacks, are exfoliating in the polymer matrix. Besides, some larger intercalated tactoids can also be identified. Furthermore, Figs. 4(c) and (d) show that the clay platelets present a better dispersion capability in polymer matrix than that in Figs. 4(a) and (b), which is consistent with the previous result obtained from the powder X-ray diffraction studies. The surface morphology of the membranes prepared from both soluble and insoluble PCN materials are compared by SEM analyses. Fig. 5 shows the morphological images of CLSPI-3 and TCLSPI-3 at magnification 50,000. The SEM images show that the insoluble PCN membrane has a smooth morphology, whereas the soluble PCN membrane has a nanoporous morphology. The difference in surface morphology between soluble and insoluble PCN membranes may be dependent on the preparation process. Better surface performance of insoluble PCN membranes may be attributed to a relatively moderate preparing condition by in situ imidization at a high temperature programmed curing process. On the other hand, the soluble PCN membranes prepared by chemical imidization will have a relative disordered orientation of polymer because the membrane formation is just using a condition of solvent evaporation. Solvent evaporation during the preparation of soluble PCN membranes may cause the nanoporous morphology with 50 nm cracking gap. The presence of cracking on top view of soluble PCN membranes may also lead to poor barrier properties as compared to insoluble PCN membranes. This is further evidenced by the molecular permeability studies. However, the in situ polymerization to prepare polyimide by thermal imidization requires high process temperature up to 300 °C, which is not suitable for the

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Fig. 4. TEM images of CLSPI-3 (solution dispersion) and TCLSPI-3 (in situ polymerization): (a) CLSPI-3 at magnification 10,000, (b) CLSPI-3 at magnification 50,000, (c) TCLSPI-3 at magnification 10,000 and (d) TCLSPI-3 at magnification 50,000.

anticorrosion application. Therefore, we solely discuss the anticorrosion performance of SPI systems in the following section. 3.3. Advanced anticorrosive performance of coatings Polyimide–clay nanocomposite materials prepared by thermal imidization are insoluble in all organic solvent and not suitable for anticorrosive property studies, so we develop organic soluble polyimide–clay nanocomposite materials. As the results, organic soluble PCN materials seem to extremely suitable for anticorrosive application. Excellent anticorrosive performance of sample-coated CRS coupons can be examined from the values of corrosion potential ðEcorr Þ, polarization resistance ðRp Þ, corrosion current ðicorr Þ and corrosion rate ðRcorr Þ, as listed in Table 1. The CRS coupon coated with SPI shows a higher Ecorr value than the uncoated

CRS. However, it exhibits a lower Ecorr value than the specimen-coated with PCN materials. For example, the CLSPI-1-coated CRS has a high corrosion potential of ca. )390 mV at 30 min. Even after 5-h measurement, the potential remains at ca. )400 mV. Such a Ecorr value implies that the CLSPI-1-coated CRS is more noble towards the electrochemical corrosion compared to the SPI. The CLSPI-1-coated CRS shows a polarization resistance ðRp Þ value of 2.8  103 kX/cm2 in 5 wt% NaCl, which is about three orders of magnitude greater than the uncoated CRS. The Tafel plots for (a) uncoated, (b) SPI-coated, (c) CLSPI-1-coated, (d) CLSPI3-coated and (e) CLSPI-5-coated CRS are shown in Fig. 6. For example, the corrosion current ðicorr Þ of CLSPI-1-coated CRS is ca. 9.9 nA/cm2 , which is correspondent to a corrosion rate ðRcorr Þ of ca. 1.9  102 MPY (Table 1). Corrosion current of PCN materials as coatings on CRS was found to decrease gradually with

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further increase in clay loading. This novel advanced anticorrosive property for PCN materials compared to bulk SPI might arise from dispersing silicate nanolayers of clay in SPI matrix to increase the tortousity of diffusion pathway of oxygen and water [7–9]. This is further evidenced by the studies of the O2 and H2 O molecular barrier effect as discussed in the following section. 3.4. Molecular permeability of membranes

Fig. 5. SEM images of CLSPI-3 (solution dispersion) and TCLSPI-3 (in situ polymerization) at magnification 50,000: (a) CLSPI-3 and (b) TCLSPI-3.

Fig. 6. Tafel plots for: (a) uncoated, (b) SPI-coated, (c) CLSPI1-coated, (d) CLSPI-3-coated and (e) CLSPI-5-coated CRS measured in 5 wt% NaCl aqueous solution.

The membranes used for the molecular permeability measurements are prepared to have thickness of 50 lm. The O2 molecular permeability property of PCN membranes shows that the dispersing of clay platelets promoting the molecular barrier of O2 gas. Furthermore, it should be noted that a further increase of clay loading results in a further enhanced molecular barrier property of PCN materials, as illustrated in Fig. 7. This is attributed to the plate-like clays effectively increase the length of the diffusion pathways as well as decrease the permeability of PCN membranes. Furthermore, the O2 permeability of insoluble PI membrane prepared from thermal imidization is found to be lower than that of SPI prepared from chemical imidization. This may be attributed to the much highly dense structure and smooth surface morphology of insoluble PI membrane as evidence by the SEM analyses. Moreover, for the H2 O vapor permeability studies, we also find that the incorporation of clay platelets into PI matrix results in a decrease of H2 O vapor permeability in both the soluble and insoluble PCN membranes, as shown in Fig. 8. For example, soluble PCN membrane at low clay loading (e.g., 1 wt%) shows about 56% reduction of H2 O permeability compared to SPI membrane. Insoluble PCN membranes at 1 wt% clay loading exhibits similar trend (60% reduction of H2 O permeability compared to

Fig. 7. Gas permeability of O2 as a function of the MMT clay content in the soluble PI–clay nanocomposite materials. (a) Solution dispersion and (b) in situ polymerization.

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that the optical clarity of insoluble PI system is significantly lower than that of SPI system, as shown in Figs. 9(e)–(h). 3.6. Thermal properties of membranes

Fig. 8. Permeability of H2 O vapor as a function of the MMT clay content in the soluble PI–clay nanocomposite materials. (a) Solution dispersion and (b) in situ polymerization.

insoluble PI). The insoluble PCN membranes have better barrier properties than the soluble PCN membranes may be attributed to the better dispersing of nanoclay and more smooth surface structure. 3.5. Optical clarity of membranes In this study, optical clarity of membranes remains high at low clay contents (e.g., CLSPI-1) because of the nanoscale dispersion of the clay platelets into the soluble polyimide matrix, which yield primarily exfoliated composites [9]. Figs. 9(a)–(d) show the UV–vis transmission spectra of pure PI and PCNs with 1 and 5 wt% MMT. These membranes have thickness of 50 lm. The spectra of CLSPI-1 show that the visible region (400–700 nm) is nearly not affected by the presence of the clay and retains the high transparency of the PI. However, the spectra of CLSPI-5 exhibiting low transparency reflect the primarily intercalated composites [9]. For the ultraviolet wavelength, there is strong scattering and/or absorption, resulting in very low transmission of the UV light. Moreover, we find

Fig. 9. UV–vis transmission spectra of: (a) SPI, (b) CLSPI-1, (c) CLSPI-3, (d) CLSPI-5 prepared by solution, (e) TSPI, (f) TCLSPI-1, (g) TCLSPI-3 and (h) TCLSPI-5 prepared by in situ polymerization.

Fig. 10 shows typical TGA thermograms of weight loss as a function of temperature for SPI and soluble PCN membranes prepared from chemical imidization, as measured under an air atmosphere. The TGA curves of SPI membrane as illustrated in Fig. 10(a), two major weight losses are observed in the range of 100–300 °C and at 500 °C for the NMP-containing SPI membrane. The weight loss at 110–300 °C can be assigned to the evaporation of the NMP solvent (b.p. ¼ 202 °C). The amount of NMP existed in the SPI membrane can be easily estimated from the TGA thermogram to be 15 wt%, this value is similar to those obtained from the NMP-containing polyaniline membrane reported by Wei et al. [31]. The major weight loss at 500–700 °C is attributed to the structural decomposition of the polymer. Furthermore, the thermal decomposition of those soluble PCN membranes shift slightly toward the higher temperature range than that of SPI, which confirms the enhancement of thermal stability of intercalated soluble polyimide, as shown in Figs. 10(b) and (c) [32]. Similarly, the TGA thermograms of insoluble PI system also show that the incorporation of clay platelets results in an enhancement of thermal stability of bulk polyimide, as shown in Fig. 11. It should be noted that the char yield of insoluble PI ð> 40%) is found significantly higher than that of SPI, indicating that the insoluble PI may show enhanced flame resistant property relative to SPI. DSC traces of the SPI and soluble PCN materials are shown in Fig. 12. SPI exhibits an endotherm at 167.3 °C, corresponding to the Tg of SPI. All the soluble PCN materials show an increased Tg compared to pure SPI.

Fig. 10. TGA curves of SPI–clay nanocomposite materials prepared by solution dispersion. (a) SPI, (b) CLSPI-1 and (c) CLSPI-3.

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This is tentatively attributed to the confinement of the intercalated polymer chains within the clay galleries that prevents the segmental motions of the polymer chains. Moreover, the Tg of insoluble polyimide (Tg ¼ 251 °C, shown in Fig. 13) is found higher than of soluble polyimide up to 84 °C. This can be explained as follows: In SPI system, 15 wt% NMP exists in the membrane and functions as plasticizer, leading to a lower Tg of SPI compared to the NMP-free insoluble PI system.

4. Concluding remarks

Fig. 11. TGA curves of PI–clay nanocomposite materials prepared by in situ polymerization. (a) TSPI, (b) TCLSPI-1 and (c) TCLSPI-3.

Fig. 12. DSC curves of SPI and a series of soluble PCN materials prepared by solution dispersion. (a) SPI, (b) CLSPI-1, (c) CLSPI-3 and (d) CLSPI-5.

Fig. 13. DSC curves of PI and a series of insoluble PCN materials prepared by in situ polymerization. (a) TSPI, (b) TCLSPI-1 and (c) TCLSPI-3.

A series of organo-soluble polymer–clay nanocomposite (PCN) materials consist of organic soluble polyimide (SPI) and inorganic MMT clay platelets are prepared by effectively dispersing the inorganic nanolayers of MMT clay in SPI matrix by the solution dispersion technique. Soluble PCN materials are first prepared by chemical polymerization and followed by solution dispersing the clay platelets into the SPI matrix. The as-synthesized PCN materials are characterized by infrared spectroscopy, wide-angle powder X-ray diffraction, transmission electron microscopy and scanning electron microscopy. Soluble PCN materials, in the form of coating, with low clay loading (e.g., 1 wt%) on cold-rolled steel (CRS) are found much superior in anticorrosion over those of SPI based on a series of electrochemical measurements of corrosion potential, polarization resistance and corrosion current in 5 wt% aqueous NaCl electrolyte. The molecular weight of SPI was determined by gel permeation chromatography (GPC) with THF as eluant. Effects of the material composition on the molecular barrier, optical clarity and thermal stability of SPI along with PCN materials, in the form of membrane, are also studied and compared to insoluble PI system prepared from thermal imidization by molecular permeability analysis, UV–vis transmission spectra, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. The incorporation of clay platelets into SPI membrane results in an enhancement of O2 and H2 O molecular barrier properties based on the gas permeability analyses. Since the better dispersing of nanoclay and more smooth surface morphology, the insoluble PCN membranes have better barrier properties than the soluble PCN membranes. Higher clay loading in SPI membrane leads to a significant decrease of optical clarity based on the UV–vis transmission spectra studies. Dispersing of MMT clay platelets into SPI matrix is found to boost the thermal stability such as the enhancement of thermal decomposition temperature (Td ) and glass transition temperature (Tg ) of SPI based on the TGA and DSC studies, respectively.

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Acknowledgements The financial support of this research by the NSC 90-2113-M-033-010 is gratefully acknowledged.

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