Static Dissipative Biopolymer Composites Prepared by In Situ Polymerization of Polypyrrole on Poly(lactic acid) Surfaces

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1 Journal of Metals, Materials and Minerals, Vol.20 No.3 pp.81-85, 2010 Static Dissipative Biopolymer Composites Prepared by In Situ Polymerization Walaiporn PRISSANAROOM-OUAJAI*, Aksorn RIENGSILCHAI, Nuttawan ANUWAREEPHONG and Sirisart OUAJAI Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut s University of Technology North Bangkok, Bangkok 10800, Thailand Abstract Static Dissipative Biopolymer Composites were successfully prepared via a simple in situ chemical polymerization of PPy on PLA surface. Prior to coating, the PLA surface was pre-treated by plasma modification, resulting in the formation of the oxygenated groups, hence increasing surface hydrophilicity of the PLA. The effect of the PPy polymerization time and oxidant concentration, FeCl 3, on the R s, surface and interface properties were investigated. The proposed biopolymer composites exhibited the R s in the range of 10 6 ohm/sq with a good adhesion between the PPy coating and PLA sheet, offering a potential in ESD protection materials. Key words: Polypyrrole, Poly(lactic acid), Static dissipative Introduction Polypyrrole (PPy), one of the most promising conducting polymers, is known to possess high conductivity, easy synthesis, good environmental stability and less toxicological problem. (1) For these reasons, PPy can be incorporated with insulating polymer matrix in different ways, resulting in the improved electrical properties of the composites. Static dissipative materials, having surface resistivity (R s ) between 10 6 and ohm/sq, are required for electronic packaging in order to minimize the effects of electrostatic discharge (ESD) of the sensitive electronic devices during handling and transportation. (2) Poly(lactic acid) (PLA), a linear aliphatic polyester, is polymerized economically from lactic acid obtained form corn and sugar beet. PLA becomes an environmentally-friendly commodity polymer due to facile degradation. (3) The main purpose of this research is to prepare PPy/PLA composites via a simple in situ chemical polymerization of PPy on PLA surface. Influences of PPy polymerization time and concentration of FeCl 3, the selected oxidant, on the R s, surface and interface properties were investigated. Materials and Experimental Procedures Materials Pyrrole (Fluka) was doubly distilled under reduced pressure before use. FeCl 3, (Fluka), a oxidizing agent, was used as received. 1 M HCl was used as a solvent for all solutions. PLA (NatureWork 4042D) was processed using a cast film extruder to form a PLA sheet at the T-die temperature of 175 C and used as a supported matrix. Surface Modification of PLA Sheets by Plasma Treatment PLA sheets were plasma-treated individually in a compact cylindrical glow discharge cell (Model PCD-32G, Harrick Scientific Corp., USA) using a plasma power of 18W. The cell was initially evacuated before it was filled with argon gas. PLA sheets were subjected to glow discharge for a period of time ranging from zero to 5 minutes. The plasma-treated PLA sheets were then exposed to the atmosphere at room temperature for at least 10 minutes before further coating and characterization. *Corresponding author Tel: ext. 4808; Fax: ext. 4824; wpr@kmutnb.ac.th

2 82 PRISSANAROON-OUAJAI, W. et al. Chemical Coating of PPy on Plasma-Treated PLA Sheet The plasma-treated PLA sheet was horizontally placed in a Petri-dish. Pyrrole solution (30 mm, 25 ml) was gently poured into the above Petri-dish and left for 5 minutes before pouring FeCl 3 solution (25 ml). The pyrrole-absorbed PLA sheet was chemically polymerized by FeCl 3. Polymerization time varied from 0.5 hour to 24 hours and the concentration of FeCl 3 was ranged from 5 mm to 50 mm. After polymerization, the PPy-coated PLA sheets (hereafter referred to as PPy/PLA ) were rinsed thoroughly with distilled water in order to remove the unreacted components, dried with a stream of nitrogen gas and then kept in a desiccator for at least 24 hours before following testing. Characterization Water contact angle measurement was performed by the sessile drop method at room temperature (25 o C) using an optical contact-angle measuring instrument (Dataphysics OCA20, Germany). X-ray photoelectron spectroscopy (XPS) experiments were performed using a Kratos Axis Ultra DLD spectrometer with a monochromatised Al Kα radiation source (hν = ev) operating at 150 W. Survey and high-resolution region spectra were recorded at analyzer pass energies of 160 ev and 20 ev, respectively. To follow ASTM D257, surface resistivity (R s ) was determined using an Electrometer (6517A, KEITHLEY) equipped with a standard Test Fixture by applying a certain voltage and measuring current. surface after plasma treatment. Since the 1 minute of plasma treatment was long enough to produce the proper modification, all the PLA sheets were plasma-treated for 1 minute before subjected into the next PPy coating. Contac angle (degree) Plasma treatment time (min) Figure 1. Contact angle of Ar-PLA treated with different periods of time. (a) (c) (b) Results and Discussion Plasma Treatment of PLA Surfaces Change in the water contact angle of the PLA sheet after plasma treatment is shown in Figure 1. The contact angle of the untreated PLA was 73 and reduced to 54 after 1 minute of plasma treatment, indicating increased surface hydrophilicity of the PLA. This may be caused by the formation of new polar species, such as hydroxyl groups. The assumption was supported by the change in the surface composition, investigated by XPS, of the PLA after plasma treatment. As seen in Figure 2, the C1s spectra of the untreated PLA and Ar-PLA were similar fitted, consisting of the C-C (285.0 ev), C-O (287.0 ev) and O-C=O (289.1 ev). (4) After treated with plasma, the intensities of C-O and O-C=O peaks became more pronounced, suggesting the introduction of oxygenated species on the PLA Figure 2. C1s XPS spectra of (a) untreated PLA and PLA treated with plasma for (b) 1 minute and (b) 5 minutes. Characterization of PPy/PLA Sheets Effect of PPy Polymerization Time The plasma-treated PLA sheet was coated by PPy layer via a chemical polymerization of PPy, resulting in dark gray PPy/PLA sheet. Effect of PPy polymerization time was investigated at the fixed pyrrole and FeCl 3 concentrations of 30 mm and 5 mm, respectively. As presented in Figure 3,

3 Static Dissipative Biopolymer Composites Prepared by In Situ Polymerization 83 a rapid reduction of the R s for the PPy/PLA by 10 5 times compared to the plasma-treated PLA sheet was observed after only 1 hour of PPy polymerization. The R s of 10 9 ohm/sq was achieved at 8 hours of PPy polymerization. The longer polymerization time resulted in a little increase in the R s due to overoxidation of PPy. (5) (a) (b) Surface resistivity (ohm/sq) 1.0E E E E E E Polymerization time (h) (c) (d) Figure 3. R s of PPy/PLA as a function of PPy polymerization time. [Pyrrole] = 30 mm and [FeCl 3 ] = 5 mm. (e) (f) Survey XPS spectra of the PPy/PLA sheets (not shown) revealed C, N, O and Cl signals, as expected. The presence of Cl signal indicated the doping reaction of the PPy structure, leading to the conducting state. The C1s spectra of the PPy/PLA sheets with different polymerization times showed the similar peak fit and peak assignments including C-C (285.0 ev), C-N (285.9 ev), C-O (287.0 ev), C=O (288.3 ev) and O-C=O (289.1 ev), as illustrated in Figures 4(a), (c) and (e). Reduction of the oxygenated carbon components with the increased PPy polymerization time suggested the more coverage of PPy layer on the PLA surfaces. In addition, the N1s spectra for the PPy/PLA sheets, presented in Figures 4(b), (d) and (f), were deconvoluted into four components corresponding to N= (398.3 ev), NH (400.3 ev), N+ (401.5 ev) and N++ (402.7 ev). The positively charged nitrogen components arise from delocalization of electron density from the PPy ring, indicating that the PPy exists in the doped state. (5) The decrease in N= peak of the PPy/PLA sheet as increase in PPy polymerization time implied the higher doping level of PPy. This result corresponds well with the R s mentioned previously. Figure 4. High resolution C1s and N1s XPS spectra of PPy/PLA polymerized at (a)-(b) 0.5 hour, (c)-(d) 4 hours and (e)-(f) 8 hours. Effect of FeCl 3 Concentration Effect of FeCl 3 concentration was investigated at the fixed pyrrole of 30 mm and PPy polymerization time of 8 hours. Figure 5 shows that the R s of the PPy/PLA sheets decreased to 10 6 ohm/sq as increased FeCl 3 concentration up to 10 mm. In this case, FeCl 3 acts not only as the oxidant, creating chemically active cation-radicals of pyrrole monomer and starting the polymerization, but also as dopant, removing the electron from PPy chain and compensating the positive charge with Cl -. However, no significant change in R s was observed when the FeCl 3 concentration grater than 10 mm was used.

4 84 PRISSANAROON-OUAJAI, W. et al. Surface resistivity (ohm/sq) 1.0E E E E E E E Concentration of FeCl3 (mm) oxygenated groups, hence increasing surface hydrophilicity of the PLA. The PPy/PLA sheet polymerized using the concentration of pyrrole and FeCl 3 of 30 mm and 10 mm for 8 hours exhibited the R s of 10 6 ohm/sq with the good adhesion between the PPy coating and PLA sheet. The proposed biopolymer composites offer a potential in electronic packaging application since they achieve the static dissipative properties, exhibiting excellent ESD protection, as well as they are largely biodegradable. Figure 5. R s of PPy/PLA sheets as a function of FeCl 3 concentration. [Pyrrole] = 30 mm and polymerization time = 8 hours. Adhesive tape after peeling PPy/PLA after peeling PPy/PLA before peeling 5 mm 10 mm 20 mm 30 mm 50 mm Figure 6. Surfaces of PPy/PLA sheets and adhesive tapes after peel tests performed on the PPy/PLA sheets polymerized at different FeCl 3 concentrations. Figure 6 shows the results of the peel tests conducted on the PPy/PLA sheets. Good adhesion was observed when the FeCl 3 concentration was not greater than 10 mm. Above this point, the amount of PPy coating peeled off by the adhesive tape (seen as dark patches) increased as the FeCl 3 concentration increased. Since the interaction between the PPy coating and the PLA sheets is expected to be the strongest at the point of contact, as the PPy coating increased in thickness due to faster polymerization, the topmost layer was the most easiest peeled off. Conclusions The PPy thin layer has easily been deposited onto the biodegradable PLA surface by in situ chemical polymerization using FeCl 3 as the oxidant. The PLA surface was pretreated by plasma modification, resulting in the formation of the Acknowledgements The authors are grateful to the financial support from National Research Council of Thailand (KMUTNB s Capital Budget 2010). Assoc. Prof. Paul J. Pigram and Dr. Robert, Jones, La Trobe University, Australia, are acknowledged for the assistance in XPS experiments. References 1. Wang, L.X., Li, X.G. & Yang, Y.L. (2001). Preparation, properties and applications of polypyrroles. Reactive & Functional Polymers. 47: Koul, S., Chandra, R. & Dhawan, S.K. (2000). Conducting polyaniline composite for ESD and EMI at 101 GHz. Polymer. 41 :

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