Mater. Res. Soc. Symp. Proc. Vol. 1312 2011 Materials Research Society DOI: 10.1557/opl.2011. 906 Influence of the ph on the Surface and Optical Properties of the Thin Film of Polyaniline / Polyethylene Terephthalate Composite. The AFM and Spectroscopies Studies. Rafaella T. Paschoalin 1,2,Clarice Steffens 1,2, Alexandra Manzoli 1, Mhbuti R. Hlophe 3, Paulo S. P. Herrmann 1,2 1 National Nanotechnology Laboratory for Agribusiness (LNNA), Embrapa Agricultural Instrumentation, P. O. Box 741, 13560-970, São Carlos, SP, Brazil 2 Biotechnology Graduate Program-UFSCar, São Paulo, Brazil 3 North West University, Mafikeng Campus, South Africa ABSTRACT The effect of ph was investigated on the morphological and spectroscopic properties of PANI thin films on PET. PANI/PET strips were prepared by a patternprinting technique and coated with a thin film of polyaniline in situ emeraldine oxidation state, doped with HCl (PANI-HCl), obtained by in situ polymerization. AFM, UV-VIS-NIR and FTIR/ATR spectroscopic results showed that ph has a great influence on the polymer layer morphology of the coating layers, while the intensity of vibrational bands decreases with rising ph, increase due to changes in the H-bonded structure in the polymer chain. Keywords: Polyaniline, in situ polymerization, roughness, and conductivity. 1. INTRODUCTION In the last few years, the interest in the development of new sensors for use in industry, agriculture, environment, medicine and other areas, has increased significantly. Among conducting polymers, polyaniline (PANI) has been studied more than any other, owing to its easy chemical synthesis and high environmental stability. PANI has electrical and optical properties suitable for use in optical sensors for monitoring ph [1], since its electronic absorption band sensitive to changes in ph is very broad, and optical sensors using PANI as the active layer are suitable for ph measurements in the range of 2 to 12 [2]. Given the advantages of low cost and short response time, ph optical sensors based on PANI thin film as the sensitive layer represent an interesting alternative to other sensors [3]. This paper reports the characterization of a low-cost optical sensor material assessing the influence of ph on a PANI film coated on PET. A printing technique was used to manufacture the coated
pattern. PET (polyethylene terephthalate) and PANI thin film in the emeraldine oxidation state, deposited by in situ polymerization, were used as substrate and sensitive layer, respectively [4]. The morphology and roughness of the films were analyzed by atomic force microscopy (AFM). The degree of doping of the PANI was investigated by UV-VIS-NIR and FTIR/ATR spectrophotometry. 2. EXPERIMENTAL 2.1. Materials Aniline (Aldrich) was purified by distillation. Ammonium persulfate ((NH4) 2 S 2 O 8 ), Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were used as received (Merck). Water was purified to18 MΩ*cm -1 resistivity by the Milli-Q system (Millipore Inc.). The substrate PET (polyethylene terephthalate) was purchased from Nashua (XF-20, Nashua-USA). 2.2. Stripes The stripes were developed by a pattern-printing technique [4,5] on a PET substrate. Briefly, a negative template was laser-printed, coated with PANI thin film by in situ polymerization and, thereafter, the toner was removed with toluene and M.E.K leaving the desired pattern. 2.3. In situ polymerization of PANI All directions for formation of the PANI film in situ by polymerization in the emeraldine oxidation state on a surface have been published elsewhere [5,6]. 2.4. UV-Visible-NIR Spectra The UV-Visible-NIR spectra of PANI thin films were collected at various phs in the range from 2 to 12 in a Shimadzu UV-1601PC model Spectrophotometer at wavelengths from 400 to 2000 nm. 2.5. Fourier transform infrared spectroscopy/ attenuated total reflectance (FTIR/ATR) The FTIR / ATR spectra of the PANI films were collected at various phs in the range from 2 to 12, in a Perkin-Elmer (Spectrum 1000) in the spectral region between 4000 and 650 cm -1, with 16 scans per spectrum and 2 cm -1 resolution. A ZnSe crystal
was used as internal reflection element, with an input face at 45. For each ph solution a single fresh strip of PET/PANI was used. 2.5. Atomic Force Microscope (AFM) The morphology and roughness of PANI thin films on the PET substrate were analyzed with a Dimension V (Veeco) AFM, using a pyramidal silicon nitride tip attached to a cantilever with a spring constant of 0.03 N m -1, in contact mode at a scan rate of 1 Hz. The images were processed with the aid of the data analysis package Gwydion 2.1 data analysis software. 3. RESULTS AND DISCUSSION 3.1. Characterization of PANI thin films on PET in different phs solutions. 3.1.a. UV-Visible-NIR Spectra 1.0 Absorbance (a.u) 0.8 0.6 0.4 0.2 ph2 ph3 ph4 ph5 ph6 ph7 ph8 ph9 ph10 ph11 ph12 0.0 400 600 800 1000 1200 1400 1600 1800 2000 wavelength (nm) Figure 1. Spectra of UV-Vis-NIR of PANI thin films at various phs. In Figure 1 it is seen that of the band located at 600 nm increases in absorbance with ph increasing (basic solution). This band corresponds to an n-π * transition in the quinoid rings, characteristic of the dedoped form of PANI [7]. The other two bands located around 420 nm and 800 nm are characteristic of doped PANI and an these show an increasing interest with decreasing ph (acid phs). With increasing ph, there was also a steady fall in the localized polaron peak and of the free carrier tail between 1000-2000 nm. An increase of the free carrier tail this is associated with higher conductivity [8].
3.1.b. Fourier transform infrared spectroscopy/ attenuated total reflectance (FTIR/ATR) ph 2 Transmitance (%) 1716 1558 1456 1506 1408 ph 2 ph 7 ph 12 1340 Transmitance (%) 2916 2968 2850 2850 ph 7 ph 12 1018 1120 1242 872 1098 720 2500 2200 2000 1800 1600 1400 1200 1000 800 λ (cm -1 ) 2972 2956 2914 3400 3200 3000 2800 2600 λ (cm -1 ) Figure 2. FTIR / ATR of PANI thin film in different phs The film of PET / PANI in acid conditions (ph = 2) in Figure 2 showed transmittance peaks at 2968, 2916, 2850, 1716, 1558, 1506, 1456, 1408, 1340, 1242, 1120, 1098, 1018, 872 and 720 cm -1, which are consistent with values reported in the literature for PANI[9]. At phs higher than 7 was observed two additional bands were seen at 2972 and 2956 cm -1. The bands at 872 and 720 cm -1 are characteristic on p- substitution of the aromatic ring and they are assigned as C-H out-of- plane deformation vibration of the benzoic groups. The broad band at 720 cm -1 can be also attributed to N- H bond wagging of the secondary amine. The rings are mostly coupled at C4 and N positions, the corresponding C-N stretching modes are observed in the film of PET / PANI form at 1242 and 1098 cm -1. The band around 1506 cm -1 results from the C-C ring-stretching mode and the band at about 1558 cm -1 (medium intensity) arises from a disorder-induced C-C ring stretching mode. The vibrational mode in the range of 1120 cm -1 is assigned as either C-H bending or ring amine bending vibrations. The bands at about 1408 and 1340 cm -1 have three contributions, coming from C-C ring deformation, C-H bending and C-N bending modes. The vibrational absorption at 1716 cm -1 is assigned as a C = N vibrational mode. These results are in agreement with an earlier report by Epstein and collaborators and Ping and collaborators [10,11]. Around 3446 cm -1, two bands of different intensities appear (broad vibrational bands in the range of 3400 to 3350 cm -1 and 3300 to 3200 cm -1 ), which are attributed to H bonding, due to OH stretching and N-H 2 asymmetric stretching vibrations. These bands can also be called overtones, which appear at half the intensity of that at 1716 cm -1. The differences in the intensities of bands at 2968, 2916 (ph = 2), 2972, 2956 and 2914 cm -1 (ph = 12) are caused by the differences in the H-bonding between the doped and dedoped forms of PANI thin film on PET. Thus, the transmittance of vibrational bands decreases as ph rises owing to changing the H-bond pattern in the polymer chain. At basic ph, the bands
also showed a broadening due to interaction of the OH group. Crosslinking of the chains is caused by hydrogen bonds formed by the amine and imine units. This generates a molecular conformation with is a twist between adjacent rings, causing a less planar structure between the rings and benzoid [12]. 3.1.c. Roughness ph 2 ph 3 ph 4 ph 5 ph 6 ph 7 ph 8 ph 9 ph 10 ph 11 ph 12 PET Figure 3. Topographic images of PANI thin films at different phs. The area of each image is 100µm 2. Table I. Average (ra) and root mean square (rms) roughness of PANI thin films at different phs. ph 2 3 4 5 6 7 8 9 10 11 12 ra (nm) 37.9 ±9.28 51.0 ±9.46 62.0 ±3.30 29.1 ±0.82 31.8 ±0.45 43.7 ±0.72 44.5 ±2.64 31.2 ±0.54 25.8 ±1.03 28.4 ±0.57 20.4 ±0.85 rms (nm) 52.6 ±24.0 74.0 ±15.5 90.0 ±4.30 38.2 ±1.12 45.7 ±1.00 66.8 ±1.39 58.1 ±3.99 42.0 ±0.62 33.6 ±2.77 39.2 ±2.33 27.0 ±0.42 An increase in roughness can be discerned from ph 2 to 4. In figure 3 and table I. At acid ph, the amine groups of PANI are more protonated, which favoring H bond interactions. Comparing ph 4 with ph 2, there are more interactions by H-bonding at ph 4, as there is a better balance equilibrium between protonated and deprotonated amines. As the folds and coils in polymer chains are formed by H-bonding, it is be expected that at ph 4 the polymer chains are more convoluted, that the PANI films are rougher. Thin films of PANI have a thickness around 140 nm, so the degree of roughness between ph 5 and 12 can be attributed to a solvent effect. At intermediate ph
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