Enhancement of Growth and Adhesion of Electroactive Polymer

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Langmuir 1998, 14, 2820-2826

Enhancement of Growth and Adhesion of Electroactive Polymer Coatings on Polyolefin Substrates K. G. Neoh,* H. W. Teo, and E. T. Kang Department of Chemical Engineering, National University of Singapore, Kent Ridge 119260, Singapore

K. L. Tan Department of Physics, National University of Singapore, Kent Ridge 119260, Singapore Received December 16, 1997. In Final Form: February 23, 1998 The coating of polyaniline (PAN) and polypyrrole (PPY) films on low-density polyethylene (LDPE) substrates was carried out via immersion of the substrate in a reaction mixture of the monomer, an oxidizing agent, and a doping agent or counterion. The effects of the immersion time in the reaction mixture, the doping agent, and the nature of the substrate’s surface were investigated. The coatings were characterized using ultraviolet (UV)-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and conductivity measurements. The substrate’s surface plays a major role in determining the growth and adhesion of the electroactive film. LDPE grafted with acrylic acid (AAc) greatly enhances the growth of the electroactive films on the substrate surface. The N,N-dimethylacrylamide (DMAA) graft copolymerized LDPE also affects the growth of the PAN film but not that of the PPY film. Both the AAc and DMAA graft copolymerized substrates promote the adhesion of the electroactive films.

Introduction Research efforts on electroactive polymers are increasingly being focused on potential applications of such materials. Some of the factors currently limiting the applications of such materials include the problems of processability of the polymers, low mechanical strength, and cost. To overcome these problems, composites of electroactive polymers and other functional polymersl-4 or thin film coatings of electroactive polymers on such substrates5-7 are being developed. One of the problems which may arise in the coating of electroactive polymers on conventional polymeric substrates is that of adhesion. The bonding of polypyrrole to a thermoplastic material can be enhanced by heating the coated substrate to its softening point, followed by cooling.5 Another method to enhance adhesion of the conductive coating to the substrate is through the pretreatment of the substrate by initiators which undergo polymerization with the conjugated monomer.6 Work has also been carried out on the adhesion between conducting polymers and monolayerderivatized substrates, in which the adhesion of the polymer films can be controlled by the monolayer terminal group.8 We have recently reported on the surface functionalization of polymeric substrates via graft copolymerization.9,10 The surface functionalized groups may * Corresponding author. Telephone: (65) 8742186. Fax: (65) 7791936. E-mail: [email protected]. (1) Im, S. S.; Byun, S. W. J. Appl. Polym. Sci. 1994, 51, 1221. (2) Bao, J.; Xu, C.; Cai, W.; Bi, X. T. J. Appl. Polym. Sci. 1994, 52, 1489. (3) Wan, M.; Li, M.; Li, J.; Liu, Z. Thin Solid Films 1995, 259, 188. (4) Benseddik, E.; Makhlouki, M.; Benerde, J. L.; Lefrant, S.; Pron, A. Synth. Met. 1995, 72, 237. (5) Ratcliffe, N. M. U.S. Patent 5,089,294, 1992. (6) Han, C. C.; Baughman, R. H.; Elsenbaumer, R. L. U.S. Patent 5,225,495, 1993. (7) Schoch, K. F., Jr.; Byers, W. A.; Buckley, L. J. Synth. Met. 1995, 72, 13. (8) Rozsnyai, L. F.; Wrighton, M. S. Chem. Mater. 1996, 8, 309. (9) Kang, E. T.; Neoh, K. G.; Tan, K. L.; Loh, F. C.; Liaw, D. J. Polym. Adv. Technol. 1994, 5, 837.

undergo charge-transfer interactions with polyaniline cast from N-methylpyrrolidinone (NMP) solution onto the substrate and promote adhesion.10 In this paper, we report on how the growth (polymerization) and characteristics of electroactive thin films on low-density polyethylene (LDPE) substrates are affected by the nature of the surfaces of the substrates. Two electroactive polymers, polyaniline (PAN) and polypyrrole (PPY), were chosen. These polymers have relatively high conductivity and stability and hence are good potential candidates for use in commercial applications. Two organic acid dopants, p-toluenesulfonic acid (TSA) and sulfosalicylic acid (SSA), were used. Organic acid dopants were chosen, since both PAN and PPY films can be easily prepared with such dopants, and the films may be more stable, since these organic acids are less volatile than inorganic acids such as HCl.7,11 The LDPE substrates were used either in the pristine state or after surface graft copolymerization with an anionic monomer, acrylic acid (AAc), or a cationic monomer, N,N-dimethylacrylamide (DMAA). The electroactive polymer-coated substrates were characterized by UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrical conductivity (σ) measurements. Experimental Section Preparation of Thin Film Coatings. Low-density polyethylene (LDPE) films of 0.125-mm thickness (obtained from Goodfellow Cambridge Ltd) were cleaned by Soxhlet extraction with methanol for 6 h and then dried under vacuum. The precleaned LDPE films were then cut into strips of 2 cm × 5 cm and either used for the coating of electroactive polymers or subjected to surface graft copolymerization with AAc (Aldrich Chemical Co.) or DMAA (Aldrich Chemical Co.) prior to the coating process. The grafting process has been described in detail (10) Pun, M. Y.; Neoh, K. G.; Kang, E. T.; Loh, F. C.; Tan, K. L. J. Appl. Polym. Sci. 1995, 56, 355. (11) Gregory, R. V.; Kimbrell, W. C.; Kuhn, H. H. Synth. Met. 1989, 28, C835.

S0743-7463(97)01380-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/21/1998

Electroactive Polymer Coatings on Polyolefin Substrates elsewhere,12 and a brief description is provided here: O3 pretreatment of the precleaned LDPE strips was carried out in a Fisher Model 500 ozone generator with a pure O2 input flow rate of 100 L h-1 and an O3 production rate of 3 g h-1 for 2-10 min. The pretreated films were immersed in an aqueous monomer (AAc or DMAA) solution (monomer concentration ranging from 2 to 25 wt %) which was thoroughly degassed and sealed off under a N2 atmosphere. Graft copolymerization was thermally induced at 65 °C for 45 min, after which the films were removed from the viscous homopolymer solution and washed thoroughly with deionized water and pumped dry under vacuum. The polyaniline (PAN)-coated films were prepared by suspending the LDPE films in a beaker containing a reaction mixture of 0.025 M (NH4)2S2O8 and 0.05 M aniline in either 0.5 M p-toluenesulfonic acid (TSA) or 0.5 M sulfosalicylic acid (SSA). The films were removed from the reaction mixture after varying periods of time and rinsed with either 0.05 M TSA or SSA and pumped dry under reduced pressure. The PAN samples will be denoted as PAN-TSA or PAN-SSA depending on whether TSA or SSA was used in the polymerization process. In the case of polypyrrole (PPY)-coated films, a reaction mixture of 0.042 M FeCl2‚6H2O, 0.02 M pyrrole, and either 0.015 M TSA or 0.015 M SSA was used. The procedure was identical to that of PAN except that deionized water was used to rinse the films. Similarly, depending or whether TSA or SSA was used, the PPY samples will be denoted as PPY-TSA or PPY-SSA. Sample Characterization. The PAN- and PPY-coated films after varying periods of immersion in the polymerization mixture (tp) were subjected to analysis by UV-visible absorption spectroscopy, XPS, and electrical conductivity (σ) measurements. For each condition, a minimum of two films was made and all measurements were repeated at least twice. The UV-visible absorption measurements were carried out using a Perkin-Elmer Model Lambda 20 scanning spectrophotometer. A VG ESCALAB MKII spectrometer with a Mg KR X-ray source (1253.6 eV) was used for the XPS measurements. The X-ray source was run at a reduced power of 120 W (12 kV and 10 mA), and the pressure in the analysis chamber was maintained at 10-8 mbar or lower during the measurements. The core-level spectra were obtained at a photoelectron takeoff angle of 75°, measured with respect to the film surface. Surface elemental stoichiometries were determined from the peak area ratios, after correcting with experimentally determined sensitivity factors and are accurate to within (10%. All binding energies (BEs) were referenced to the C(1s) neutral carbon peak at 284.6 eV to compensate for surface charging effects. In peak synthesis, the line widths (full width at half-maximum, fwhm) of the Gaussian peaks were maintained constant for all components in a particular spectrum. The σ’s of the films were measured using the two-probe technique. To get a qualitative indication of how well the electroactive films adhere to the LDPE substrates, peel tests were conducted by means of pressing a piece of Scotch tape firmly onto the film surface and peeling it off with a perpendicular motion.

Results and Discussion Density of Grafting. The density of the surface graftcopolymerized AAc and DMAA polymers is calculated from the XPS studies. The C(1s) core-level spectrum of pristine LDPE (Figure 1a) shows a major peak component at a binding energy (BE) of 284.6 eV, consistent with the structure of LDPE, which consists entirely of -CH2bonds. After surface modification of the LDPE via graft copolymerization with AAc, the C(1s) spectrum indicates the presence of another major peak component at 288.9 eV in addition to the 284.6 eV component (Figure 1b). The former is attributed to the carboxylic group (OsCdO) of the surface-grafted AAc.13,14 A small peak component (12) Chen, W. Surface Structures and Adhesion Characteristics of Polymer Films after Modification by Graft Copolymerization. MEng Thesis, National University of Singapore, 1996, p 19. (13) Dwight, D. W.; McGrath, J. E.; Wightman, J. P. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1978, 34, 35. (14) Chastain, J., Ed. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992; p 217.

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Figure 1. XPS C(1s) core-level spectra of (a) pristine LDPE and (b) LDPE graft copolymerized with AAc. Table 1. Density of Grafted AAc Polymer on LDPE Substrate

sample

ozone pretreatment time (min)

monomer concentration (wt %)

grafting densitya

A2 A10 A25

2 10 10

2 10 25

0.06 1.88 2.61

a

Expressed as number of COO groups per LDPE repeating unit.

at 286.0 eV attributable to CsO is also indicated in Figure 1b. The O3 pretreatment does not result in any significant COO groups on the LDPE surface. Hence, the density of the grafted AAc polymer on the surface of the LDPE film expressed as the number of COO groups per LDPE repeating unit can be calculated as follows:

MAAc MLDPE

)1

peak area of (OsCdO) /2[total C(1s) area - 3(peak area of OsCdO)]

The stoichiometric factors of 3 and 1/2 are introduced to account for the three carbon atoms in each AAc unit and the two carbon atoms per repeating unit of the LDPE substrate. The grafting densities of AAc graft-copolymerized LDPE obtained at three different conditions are compared in Table 1. In the subsequent sections of this paper, the results are based on the LDPE substrate graft copolymerized in the 10 wt % AAc monomer solution (A10), unless otherwise indicated. In the case of the LDPE film graft copolymerized with DMAA, the presence of the DMAA polymer is indicated by peak components at 285.6 and 287.2 eV in the C(1s) core level spectrum attributed to the CsN and NsCdO groups, respectively,14 and also a peak at 399.4 eV in the N(1s) spectrum attributable to the NsC group. The density of the DMAA polymer on the surface of the LDPE film can be calculated from a comparison of the area of the NsCdO component peak and the total C(1s) area, taking into account the fact that each DMAA monomer unit also contributes five C atoms to the latter. This calculation shows that, for an O3 pretreatment time of 10 min and 10 wt % DMAA monomer solution, the graft copolymerization results in over 95% of the total C(1s) area being contributed by the grafted DMAA polymer giving a grafting density of 8.2. Alternatively, the grafting density can be calculated by comparing the NsC peak area in the N(1s) spectrum with the total C(1s) area, taking into account the stoi-

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Figure 3. UV-visible absorption spectra of PAN-TSA coated on pristine LDPE (s), AAc graft-copolymerized LDPE (- ‚ - ‚ -), and DMAA graft-copolymerized LDPE (- ‚ - ‚ -).

Figure 2. UV-visible absorption spectra of (a) PAN coated on pristine LDPE and (b) PPY coated on pristine LDPE. Acids used: (s) TSA; (- ‚ - ‚ -) SSA. Time on spectrum indicates time in reaction mixture (tp).

chiometry of the DMAA monomer and the sensitivity factors. This calculation shows complete coverage of the LDPE substrate by the grafted DMAA polymer. Thus, both sets of calculations are consistent and show that graft copolymerization of LDPE with DMAA is more readily achievable than that with AAc under the experimental conditions tested. Polyaniline Coatings. The UV-visible absorption spectra of PAN coated on pristine LDPE films after varying periods in the reaction mixture of aniline, ammonium persulfate, and TSA (tp) are shown in Figure 2a. This figure shows that the shape and intensity of the spectra are not significantly affected by the change in tp from 0.5 h to 3 h. The absence of a distinct π-π* transition band at 320 nm, and the absorption in the near-IR region in these spectra are features of protonated polyaniline.15 However, the band in the 900 nm region instead of an extended charge-transfer absorption tail is indicative that interchain effects rather than intrachain charge effects are dominant, which may be a result of the relatively short polymer chains in the coatings.15,16 The absorbance of the spectra also provides an indirect qualitative measure of the thickness of the film. The weak dependence of absorbance on tp is consistent with the autoacceleration or self-catalysis polymerization mechanism of aniline, in which the initial reaction rate is slow but then increases rapidly before finally leveling off.17,18 In the polymerization of aniline, the rate-limiting step is the initial oxidation of aniline. The dimer and succeeding oligomers have lower oxidation potentials than aniline. The rapid increase in reaction rate is achieved by incorporating monomeric aniline onto the oligomeric species.19,20 The use of SSA instead of TSA in the polymerization medium results in a coating with a higher absorbance (15) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263. (16) Cao, Y.; Li, S.; Xue, Z.; Guo, D. Synth. Met. 1986, 16, 305. (17) Wei, Y.; Sun, Y.; Tan, X. J. Phys. Chem. 1989, 93, 4878. (18) Neoh, K. G.; Kang, E. T.; Tan, K. L. Polymer 1993, 34, 3921.

under the same experimental conditions (Figure 2a). The shape of the absorption spectrum is essentially the same except for a distinct band in the 300 nm region which is present in all the films synthesized in SSA. This band is attributed to the absorption by SSA, which may be present in substantial amounts in the film. XPS analysis of the PAN-SSA films indicates that the amount of SSA incorporated into the films is substantially higher than that required to protonate the PAN, unlike the results obtained with TSA (see below). Thus, the excess SSA anions remain as “free” anions associated with H+ rather than the positively charged nitrogens of PAN. The higher absorbance of the PAN-SSA films compared to the PANTSA films implies that the polymerization and coating process of the PAN on LDPE is dependent on the acid dopant. However, even though a thicker coating (as indicated by the higher absorbance) is obtained with SSA as the dopant, the conductivity of the films is only about half of that of the TSA protonated films (σ of PAN-TSA films is on the order of 104 Ω/sq). The use of LDPE graft copolymerized with AAc as the substrate results in a significant increase in the absorbance of the coatings, as shown in Figure 3 for the TSA protonated PAN. As in the case with pristine LDPE as substrate, the absorbance is only weakly dependent on tp. Similar results are obtained with SSA as the protonic acid in the reaction medium. With LDPE graft copolymerized with DMAA as the substrate, an increase in absorbance of the coating is also observed (Figure 3) but not to the same extent as that observed with LDPE graft copolymerized with AAc even though the graft density of the DMAA copolymer is much higher. Both the AAc and DMAA graft-copolymerized LDPE substrates are more hydrophilic compared with the pristine LDPE and thus enhance wetting by the reaction mixture. We postulate that as the polymerization of aniline proceeds, chain entanglement between the PAN and the graft copolymer chains occurs. O3-pretreated LDPE films (without graft copolymerization) do not enhance the growth of the PAN films. In the AAc graft-copolymerized LDPE, the presence of COO- groups further enhances electrostatic interactions with the positively charged nitrogen (N+) in PAN. In the case of DMAA graft-copolymerized LDPE, any N+H groups will attract the SO3- anions, which may also serve as counterions for the PAN. (19) Breitenbach, M.; Heckner, K. H. J. Electroanal. Chem. 1971, 29, 306. (20) Hand, R. L.; Nelson, R. F. J. Electrochem. Soc. 1978, 125, 1059.

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Table 2. Comparison of Surface Composition of PAN-TSA on Pristine LDPE and AAc Graft-Copolymerized LDPE substrate

time in polymerization mixture, tp (h)

sNd/N

NH/N

N+/N

SO3-/N

(N/C)correcteda

pristine LDPE pristine LDPE AAc graft-copolymerized LDPE AAc graft-copolymerized LDPE AAc graft-copolymerized LDPE

0.5 3.0 0.25 0.5 2.0

0 0 0 0 0

0.55 0.54 0.47 0.44 0.51

0.45 0.46 0.53 0.56 0.49

0.42 0.40 0.68 0.84 0.52

0.12 0.14 0.09 0.09 0.15

a

(N/C)corrected refers to N/C after correction for C in the TSA anions.

Figure 4. XPS C(1s) and N(1s) core-level spectra of (a and b) PAN-TSA on pristine LDPE, tp ) 0.5 h; (c and d) PAN-TSA on AAc graft-copolymerized LDPE, tp ) 0.5 h; and (e and f) PAN-TSA on AAc graft-copolymerized LDPE, tp ) 2 h.

While the UV-visible absorption results presented above have provided qualitative information on the thickness and electronic structure of the PAN coating, XPS analysis was used to determine the film composition. The XPS C(1s) and N(1s) core-level spectra of PAN coated on pristine LDPE using a tp of 0.5 h are given in parts a and b of Figure 4, respectively. The C(1s) spectrum indicates the presence of small amounts of oxidized C (CsO and CdO groups) in the sample. The N(1s) spectrum can be deconvoluted into a peak component at 399.4 eV attributed to the NH (amine) groups and a high BE tail comprised of two peak components at 1.45 and 2.95 eV from the amine peak, respectively.21 The high BE tail is attributed to positively charged nitrogen (N+), and although the use of the fixed fwhm approach in peak deconvolution results in the high BE tail being fitted with two peak components, it is possible that the N+ species possess a continuous distribution of charges. The N+/N ratio is close to 0.5, which is the value expected of PAN in the emeraldine (50% oxidized) state with all sNd (imine) units being fully protonated. The C(1s) and N(1s) spectra of the PAN coating for tp ) 3 h are similar to those in Figure 4a and b. The detailed results are given in Table 2. The TSA counterions give rise to a SO3- peak in the S(2p) core-level spectrum at 168 eV,22 and the SO3-/N ratio is close to the N+/N ratio for the PAN films on the pristine LDPE substrate (Table 2), as expected for charge neutrality. An N/C ratio can be calculated with the C contributed by the TSA anions subtracted from the total (21) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. Phys. Rev. B 1989, 39, 8070. (22) Chastain, J., Ed. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992; p 236.

C ((N/C)corrected in Table 2). An increase in this ratio is seen when tp is increased from 0.5 h to 3 h. However, the value after 3 h is still about 18% lower than 0.17, which is expected of PAN. This may be due to some surface contamination, or the PAN film may be sufficiently thin that the underlying LDPE substrate contributes to the C signal. The C(1s) and N(1s) core-level spectra of PAN-TSA coated on AAc graft-copolymerized LDPE for tp ) 0.5 h are given in Figure 4c and d, respectively. A comparison of parts c and a of Figure 4 reveals that the former indicates an additional peak at 288.9 eV attributable to the OsCdO groups of the AAc graft copolymer. The intensity of this component is much reduced compared to that of the AAc graft-copolymerized LDPE before coating with PAN (Figure 1b). After tp ) 2 h, the OsCdO component is barely discernible (Figure 4e). In Figure 4d, the N(1s) spectrum is deconvoluted into the NH peak at 399.4 eV and an N+ component at 1.95 eV from the NH peak. This spectrum is distinctly different from that in Figure 4b. We have earlier shown that the spectrum in Figure 4b is typically obtained when only sNd units in PAN are protonated whereas the spectrum in Figure 4d is typical when a substantial amount of NH units are protonated due to the availability of a high concentration of anions.23,24 For the PAN-TSA coated on the AAc graft-copolymerized LDPE, both TSA and COO- anions are available to serve as counterions. From Table 2, it can be seen that, for such samples with tp e 0.5 h, the N+/N ratio is greater than 0.5 and the SO3-/N ratio is significantly higher, unlike the case when pristine LDPE is used as a substrate. When tp is increased to 2 h, the COO- groups are no longer detected by the XPS analysis and the N(1s) spectrum (Figure 4f) and the N+/N and SO3-/N ratios are similar to those discussed earlier for the PAN coating on a pristine LDPE substrate. Since XPS is a surface-sensitive technique and the sampling depth is typically limited to the top 5 nm of the material, it can be interpreted from the XPS results that, with tp ) 2 h, the PAN film has become sufficiently thick to cover the AAc graft copolymer chains and the surface of the PAN film is protonated predominantly by the TSA anions from the polymerization medium. The results of the peel test performed on the PANTSA coatings are presented in Figure 5. The PAN-TSA film is readily removed from the pristine LDPE substrate when a piece of Scotch tape is applied and then peeled off (Figure 5a-c). On the other hand, the PAN films on the AAc graft-copolymerized LDPE show no sign of peeling off (Figure 5d-5f) even though these films are much thicker than those on the pristine LDPE substrate (as indicated by the higher absorbances in Figure 3). Similar peel tests performed on the corresponding DMAA graftcopolymerized LDPE substrates showed that a small amount of PAN-TSA film was removed by the Scotch (23) Neoh, K. G.; Kang, E. T.; Tan, K. L. Polymer 1993, 34, 1630. (24) Neoh, K. G.; Kang, E. T.; Tan, K. L. J. Phys. Chem. 1997, 101, 726.

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Figure 5. Peel test results of PAN-TSA coating on pristine LDPE and AAc graft-copolymerized LDPE for various tp. Each figure shows the amount of coating peeled off from the LDPE substrate.

tape. Thus, the adhesion strength of the PAN coatings on the DMAA graft-copolymerized LDPE are judged to be lower than those on the AAc graft-copolymerized LDPE but still significantly higher than those on pristine LDPE. The enhancement of the adhesion strength of the PAN coating by the presence of surface graft copolymers on the LDPE substrates is attributed to the chain entanglement between PAN and the graft copolymers as well as the electrostatic interactions between groups of different polarities. The difference in effectiveness in promoting growth and adhesion of the PAN films between the AAc and DMAA graft-copolymerized LDPE substrates results from the difference in the functional groups, OdCsOH in the AAc copolymer and OdCsN(CH3)2 in the DMAA copolymer, which acquire opposite polarities. The results presented thus far for the surface-functionalized LDPE substrates were those obtained with the graft copolymerization carried out in either 10 wt % AAc monomer or 10 wt % DMAA monomer solution. The effect of AAc grafting density on the growth and adhesion of the PAN coatings was also investigated. Figure 6a shows that the AAc grafting density (given in Table 1) has a significant effect on the thickness of the PAN film. With a grafting density of 0.06 (A2 sample in Table 1) there is no significant increase in absorbance over that obtained with the pristine LDPE substrate and there is also no significant improvement in adhesion. With the A25 substrate, the coating obtained is very much thicker than that obtained with the A10 substrate (Figure 6). However, the nature of the A25 substrate (before PAN coating) can be visually seen to be different from those of the other substrates. The A25 substrate has lost much of the flexibility of the pristine LDPE film and appears opaque. We postulate that the AAc graft copolymer has penetrated deeply into the LDPE film. Polypyrrole Coatings. The UV-visible absorption spectra of PPY-TSA coated on pristine LDPE for various tp’s are shown in Figure 2b. In contrast to the results obtained for the PAN coating (Figure 2a), the absorbance of the PPY coating is strongly dependent on tp, consistent with the results of an earlier study.7 The PPY coatings are also thicker, as shown by the higher absorbances. The absorption spectra in Figure 2b are characteristic of that of doped PPY with the presence of the π-π* transition at 470 nm and the absorption tail extending into the nearIR region.25 The variation of the thickness of the PPY (25) Street, G. B.; Clarke, T. C.; Krounbi, M.; Kanazawa, K.; Lee, V.; Pfluger, P.; Scott, J. C.; Weiser, G. Mol. Cryst. Liq. Cryst. 1982, 83, 253.

Figure 6. Effect of grafting densities on the (a) PAN-TSA and (b) PPY-TSA coatings: (---) pristine LDPE; (- - -), A2 substrate; (s) A10 substrate; (- ‚ - ‚ -) A25 substrate. Grafting densities of the substrates are given in Table 1.

Figure 7. UV-visible absorption spectra of (s) PPY-TSA coated on pristine LDPE, (- ‚ - ‚ -) AAc graft-copolymerized LDPE, (- - -) and DMAA graft-copolymerized LDPE.

coating with time also results in an increase in σ with time. There is an order of magnitude increase in σ when tp is increased from 0.5 to 3 h. The resistance of the coating after 3 h is about 103 Ω/sq. The use of SSA as a dopant results in an absorption band at 300 nm and a thicker coating, similar to the results obtained with the PAN coatings. The PPY-SSA coatings are also less conductive than the PPY-TSA coatings and show a larger variation of σ with time (∼103 variation from tp ) 0.5 to 3 h). The effects of using AAc and DMAA graft-copolymerized substrates are shown in Figure 7. There appears to be no substantial differences in the absorbance of the PPY coatings on the pristine and DMAA graft-copolymerized LDPE substrates, unlike the case of the PAN coatings. The AAc graft-copolymerized LDPE substrates again

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Table 3. Comparison of Surface Composition of PPY-TSA on Pristine LDPE and AAc Graft-Copolymerized LDPE substrate

time in polymerization mixture, tp (h)

sNd/N

NH/N

N+/N

SO3-/N

Cl-/N

(N/C)correcteda

pristine LDPE pristine LDPE AAc graft-copolymerized LDPE AAc graft-copolymerized LDPE AAc graft-copolymerized LDPE

0.5 3.0 0.25 0.5 2.0

0.06 0 0 0 0

0.67 0.73 0.73 0.76 0.71

0.27 0.27 0.27 0.24 0.29

0.11 0.18 0.16 0.15 0.16

0.06 0.13 0.05 0.07 0.11

0.20 0.18 0.09 0.11 0.19

a

(N/C)corrected refers to N/C after correction for C in the TSA anions.

Figure 8. Peel test results of PPY-SSA coating on pristine LDPE and AAc graft-copolymerized LDPE for various tp. Each figure shows the amount of coating peeled off from the LDPE substrate.

resulted in significantly thicker PPY coatings, and the higher the graft density, the greater is the increase over the pristine LDPE substrate (Figure 6b). An improvement in the adhesion strength of the PPY coating is also achieved with the AAc graft copolymerized substrate (Figure 8). However, it can be seen that as the coating becomes thicker, it becomes easier to remove part of the PPY coating. This can be envisioned as a lessening of the extent of chain entanglement and electrostatic interactions between the PPY and the AAc graft copolymer for the PPY deposited furthest away from the substrate. The XPS C(1s) and N(1s) core-level spectra of PPYTSA coated on pristine LDPE for tp ) 0.5 h are shown in parts a and b of Figure 9, respectively. Comparing Figure 9a with Figure 4a, it can be seen that PPY contains a higher proportion of oxidized C species including OsCdO groups. The N(1s) spectrum in Figure 9b can be deconvoluted into peak components at 397.7 and 399.7 eV and a high BE tail above 401 eV. The peak components at 397.7 and 399.7 eV are assigned to the sNd and NH nitrogens, respectively, while the BE tail, which is further deconvoluted into two components, is again assigned to N+.26 The small amount of unprotonated sNd shown in Figure 9b is probably due to a loss of some anions from the film surface when the film was rinsed with de-ionized water after the coating process. The XPS analysis of the thicker film obtained for tp ) 3 h shows that the extent of undoping is reduced and no unprotonated sNd units are detected (Figure 9d), but the C(1s) spectrum still indicates the presence of a substantial amount of C oxidized species (Figure 9c). In these PPY-TSA and PPY-SSA films, the counterions are the SO3- anions from the acid as well as Cl- anions. The Cl(2p) core-level spectrum shows that the Cl species in the film exist mainly as Cl- anions (peak component at ∼197 eV)21 and that there is only a small amount (