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APPLICATION Of NOVEL dOPAMINE-POLyPyRROLE NANOfIbERS fOR ELECTRICALLy CONdUCTIVE AdhESIVES

Wei Zhang, behnam Meschi Amoli, Jeffrey d’Eon, and boxin ZhaoDepartment of Chemical Engineering, Waterloo Institute for Nanotechnology

University of WaterlooWaterloo, Ontario, Canada

Alex ChenCelestica Inc.

Toronto, Ontario, Canada

AbSTRACTElectrically conductive adhesives (ECAs) offer many advantages

such as lower processing temperature, simpler processing steps and finer-pitch interconnection in comparison with the traditional soldering technology. However, due to the poor metallurgical connections of silver flakes in the ECA network, it requires a large amount of silver or other metallic fillers to achieve high conductivity, which is not cost-effective for industrial applications. In this work, we present the utilization of a novel type of dopamine-functionalized polypyrrole (DA-PPy) nanofibers to develop a hybrid nanocomposite adhesive with lower metallic contents. Compared to conventional ECAs, the hybrid ECAs displayed significantly higher electrical conductivity. In particular, the introduction of 3 wt% nanofibers into a conventional ECA with 57.9 wt% silver flakes resulted in a significant electrical conductivity enhancement from 110 S/cm to 2400 S/cm, which was comparable to that of ECA filled with 80 wt% silver flakes.

Key words: Polypyrrole nanofiber, Electrically Conductive Adhesives, Dopamine, Conductivity.

INTROdUCTIONWith the rapidly growing demand of advanced electronic devices,

interconnection technologies have been extensively used to provide fine conductive pathways between circuit elements in electronic manufacturing and packaging industries.1,2 In the past decades, tin/lead solders have dominated the interconnect technologies.3 However, they also raised a major concern on environment and human health due to the high toxicity.4,5 As a result, great efforts have been made to develop novel interconnection materials that can potentially replace the tin/lead solders.6,7 Electrically conductive adhesives (ECAs) have attracted considerable attentions as a promising alternative to provide robust and environmental-friendly bonding solutions in interconnection technologies.8-10 Compared to traditional soldering technology, ECAs offer many advantages such as lower processing temperature, simpler processing steps and finer-pitch interconnection.11-13 The application of ECAs can also enable the new technology development such as wearable electronics and biological sensor devices.

The conventional ECAs typically consist of a polymer matrix that provides mechanical strength and a conductive component that offers electrical conductivity.14 The polymer resins are either thermosets or thermoplastics, and the conductive fillers are normally metallic materials including gold, silver, copper, and nickel in various shapes and sizes.15 Currently, commercialized ECAs mainly consist of epoxy resin and silver micro flakes; but due to the lack of metallurgical connections among silver flakes in the polymer network, their electrical conductivities are relatively low and cannot completely replace the traditional solders.16,17 Moreover, limited current-carrying capability, unstable contact resistance and poor mechanical properties are also major obstacles, which have hindered the wide applications of ECAs as electronic interconnection material.18

Although incorporating sliver based materials (nanoparticles, nanobelts and nanowires) has a synergetic effect on the performance of ECAs, their high cost and complicated synthetic procedures are main drawbacks for wide industrial usages.19-21 In this work, we present our newly-developed dopamine-functionalized polypyrrole (DA-PPy) nanofibers and their dispersion in solvents and their application as a co-filler in the system of epoxy and silver micro flakes to develop a hybrid nanocomposite adhesive.

Polypyrrole (PPy) is a unique conducting polymer, which has attracted many attentions because of its easy preparation, long-term stability and excellent biocompatibility.22 Nevertheless, pristine PPy is in the form of solid precipitate that is not soluble or dispersible in water and in most organic solvents.23,24 This situation makes it difficult to process. Inspired by the chemistry of the adhesive proteins of marine mussels,25,26 we have utilized the biological neurotransmitter dopamine (DA) to functionalize PPy.27 It was found that after being functionalized by DA, the morphology of PPy changed from globular to fibrous; the dispersibility of PPy was enhanced due to the hydrophilic nature of DA; and proper DA/pyrrole (Py) reacting ratios also led to PPy conductivity improvement.27 These new features can significantly improve PPy processability and broaden its potential application in ECA composites. In this work, DA-PPy nanofibers were implemented as co-fillers into the epoxy network; we found that the introduction of a small amount of DA-PPy nanofibers (3 wt%) resulted in a significant conductivity improvement by 20 times higher than that of conventional ECAs filled with the same amount of silver flakes.

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copper plate were removed. The above procedures were also applied to prepare other pastes at different DA-PPy weight ratios.

Characterization MethodsThe morphologies of the DA-PPy nanofibers and cured ECA

nanocomposites were investigated by scanning electron microscope (SEM, Zeiss LEO 1550) operating at 5 kV. The electrical resistivity of DA-PPy nanofibers and cured nanocomposites were measured by a four-point probe setup that consists of a four-point probe (Cascade microtech Inc.) and a source meter (Keithley 2440 5A Source Meter, Keithley Instru- ments Inc.).

figure 2. (a) digital photo of the electrical conductivity measurement setup including a four-point probe and source meter; (b) schematic illustration of the bulk resistivity measurement of ECA nanocomposites.

The conductivity can be calculated by the following equation:28k = 1/ρ = (1/F) × (ln2/πt) × (I/V) (S/cm)where t represents the sample thickness; I, V, and F are the applied

current, measured voltage and dimensionless correction factor, respectively.

RESULTS ANd dISCUSSIONMorphological and Structural Characteristics of DA-PPy nanofibers

The powder structures of both unmodified PPy and DA functionalized PPy were characterized by scanning electron microscope (SEM) and shown in Figure 3. The hydrophobic PPy polymer chains tend to entangle and twist to form a sphere morphology with a diameter of about 300 nm as shown in Figure 3a. However, after DA modification, DA-PPy exhibited fibrous morphology (Figure 3b) and each nanofiber had a diameter ranging from 40 nm to 70 nm, and a length of up to several micrometer. Although fabrication of fibrillar PPy has already been achieved by

EXPERIMENTAL SECTIONSynthesis of dopamine functionalized polypyrrole (DA-PPy) nanofibers

Pyrrole (Py, 98%), ferric chloride (FeCl3), and dopamine hydrochloride (DA) were purchased from Sigma-Aldrich. The DA-PPy nanofibers was prepared by the previously-established procedures.27 The typical experimental setup for DA-PPy preparation is illustrated in Figure 1, where 0.12 ml or 0.116 gram pyrrole monomer and 0.02 g dopamine hydrochloride were dissolved in 20 ml 1 M HCl and cooled down to 8 °C on a cooling plate. FeCl3/HCl solution (0.4 g/5 ml) was introduced to the DA/Py solution dropwisely under vigorous stirring for 12 hr at a reaction temperature maintained at 8 °C. The precipitates were filtered and collected by centrifuging the solution and sequentially washing with deionized water for 5 times. The washed precipitates were then freeze-dried for 24 hrs to produce a powdered material.

figure 1. In-situ polymerization setup for the synthesis of dA-PPy nanofibers (the pyrrole solution was in the flask; the oxidant solution was in the dripping funnel).

Nanocomposite PreparationSilver micro flake (10 μm, resistivity 1.59 μΩ/cm, density 10.49 g/

cm3) was purchased from Sigma-Aldrich as a main conductive filler in epoxy resin (DER 322; Dow Chemical). Triethylenetetramine (TETA) containing crosslinking agent (DEH 24; Dow Chemical) was used for curing the epoxy. Following our previous work,17 a small amount of acetone (2 drops, or 6 wt% of the epoxy) was added to dilute the epoxy, then 5 mg of synthesized DA-PPy nanofibers along with 200 mg of silver flakes were added into the epoxy, directly. The composite was mixed for 45 min with a vortex mixer and then sonication for 45 min. After mixing, the curing agent TETA was added to the composite and the final mixture was added into a 7 × 7 × 0.5 mm3 (L × W × D) mode made by two layers of adhesive tapes on a microscope glass slide. To ensure a smooth ECA surface with controlled thickness for electrical conductivity measurement, a clean copper plate was pressed on top of the paste to squeeze out extra epoxy. The sample was pre-cured for 30 min at 60 °C and then cured at 150 °C for 2 hrs. After curing, the adhesive tapes and the

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many researchers using different templates, their approaches all suffer from using extra steps to remove the sacrificial templates.29-

31 Our one-step, in-situ, and template-free method provides an alternative strategy to the synthesis of PPy nanofibers.27

figure 3. SEM images of (a) pristione PPy with sphere nanostructure; (b) fibrous PPy morphology obtained from dA modification.

Dispersion of DA-PPy in various types of solventThe modified DA-PPy nanofibers contain catechol groups,

which are hydrophilic and can improve their dispersion in aqueous solutions. To investigate the dispersion of DA–PPy particles, 0.005 g DA-PPy powder was dispersed into 3 ml of different solvents and mixed in an ultrasonic bath for 15 min. Figure 4 shows the visual observation of pristine PPy and functionalized PPy aqueous dispersions at different times. For pristine PPy, it quickly destabilized and precipitated out of the aqueous dispersion within 5 hr in all solvents including tetrahydrofuran, acetonitrile, water, chloroform, acetone, dimethylformamide and dimethyl sulfoxide. On the other hand, DA-PPy exhibited excellent stability in acetonitrile, water, chloroform, acetone and dimethyl sulfoxide after 24 hr, but precipitated in tetrahydrofuran and dimethylformamide. The results further indicate that PPy nanofibers were successfully functionalized by DA, in which abundant amine and hydroxyl groups contributed to its good dispersibility.

figure 4. digital photo of dA-PPy and pristine PPy dispersed in various solvents after 1 hr and 24 hr.

Electrical Properties of DA-PPy and the Hybrid ECA Com-posite

The electrical conductivity of DA–PPy and pristine PPy were characterized by a standard four-point probe setup in both powder compressed pellets and solution cast films. It was found that the electrical conductivity of pristine PPy was about 0.04 S/cm for the pellet and 0.01 S/cm for the film. For DA modified PPy, the pellet conductivity of 7.4 S/cm and the film conductivity of 1.5 S/cm were observed, which are about two orders of magnitude better than that of the unmodified PPy. We attributed the electrical conductivity improvement to the fact that PPy morphology shifted from globular to fibrous after DA modification, the fiber structure is more effective to establish a complete electrical network for electron transportation.

The improved dispersibility and electrical conductivity of DA-PPy nanofiber make it an ideal candidate as a co-filler inside the conventional formulation of ECAs. In order to investigate how DA-PPy nanofibers influence the overall electrical conductivity of their polymer composites, we used conventional ECAs containing only silver flakes at various weight fractions as a control. Figure 5a presents the electrical conductivity of conventional ECA versus the weight fractions of silver flake. When the sliver flake fillers loading level exceeded the percolation threshold (40 wt%), the composite became conductive, adding more sliver flakes into epoxy after percolation threshold did not dramatically improve the electrical conductivity. For all hybrid ECA samples with DA-PPy nanofiber co-fillers, the absolute amount of sliver flakes was kept constant, whereas the amount of DA-PPy was increased.

Table 1. Electrically conductive adhesive samples and the weight fraction of the silver flakes and dA-PPy cofillers.

Samples Composition Silver flakes wt%

DA-PPy wt%

Sample-1Epoxy + 200 mg

silver flakes + 5 mg DA-PPy

58.8 1.5

Sample-2Epoxy + 200 mg

silver flakes + 10 mg DA-PPy

57.9 3

Sample-3Epoxy + 200 mg

silver flakes + 15 mg DA-PPy

57.1 4.3

Sample-4Epoxy + 200 mg

silver flakes + 20 mg DA-PPy

56.3 5.6

Table 1 presents the changes of overall concentrations of both silver flakes and DA-PPy nanofibers in the composite samples. The addition of DA-PPy nanofibers decreased the sliver flakes weight fraction from 58.2% to 56.4%, which is comparatively not significant to affect the overall conductivity of the composites. The electrical conductivities of the hybrid ECAs with a constant total amount of silver flakes at various DA-PPy weight concentrations were plotted in Figure 5b. It can be found that adding a small

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amount of DA-PPy nanofibers (1.5 wt%) remarkably improved the electrical conductivity to 1077 S/cm, which is 10 times higher than that of the conventional ECA with 60 wt% sliver flakes. The maximum conductivity of 2400 S/cm was achieved at 3 wt% DA-PPy nanofiber concentration. When more DA-PPy was added into the system, the electrical conductivity began to decrease, but still one order of magnitude higher than that of the conventional ECA with the same amount of silver flake. The electrical conductivity decrease at higher DA-PPy concentration can be attributed to the fact that a large amount of nanofibers dramatically increased the number of contact points (contact resistance), which may counter-balance the electrical conductive pathways that DA-PPy nanofibers added to the ECA network.

figure 5. Electrical conductivity as a function of filler fraction for both of the conventional adhesives filled only with silver flakes and the hybrid composites.

Implementing hybrid filler systems, which composed of two or more filler components is one of the most commonly used strategy to develop novel ECAs with desired properties.32,33 For examples, Jiang et al. incorporated surface functionalized silver nanoparticles into the silver flakes/epoxy ECA, and low-temperature sintering of silver nanoparticles decreased the number of contact points in the polymer matrix and improved the electrical conductivity of the ECA.19 Amoli et al. prepared a hybrid ECA which consisted of high aspect-ratio silver nanobelts and silver flakes.21 This hybrid

system displayed a superior electrical conductivity similar to that of a eutectic solder but with only a small amount of silver fillers. Zhang et al. reported the integration of the silver nanowires and the silver flakes into the epoxy blender and found a significant increase in electrical conductivity of the resulted ECA.20 Compared to their works, we used a conducting polymer instead of silver-based materials as a co-filler to reduce the total amount of silver as well as the final cost. Moreover, the nanocomposite conductivity at 3 wt% DA-PPy nanofiber and 57.9 wt% sliver flakes concentration was measured to be 2400 S/cm, which was comparable to the electrical conductivity of the conventional ECA with 80 wt% silver flake fillers.

To obtain insights of how DA-PPy nanofibers interact with other conductive fillers within the conductive composite, SEM images of a conventional ECA and a hybrid ECA containing 3% DA-PPy nanofibers were examined. Figure 6a shows silver flakes in the conventional ECA were separated by epoxy, indicating a non-continuous electrical network. In contrast, in the hybrid ECA, DA-PPy nanofibers were distributed evenly throughout the epoxy network without noticeable aggregation as illustrated in Figure 6b, c and d. Due to the homogenous dispersion of the nanofibers in the nanocomposite, the separated silver flakes were effectively connected, which led to the overall electrical conductivity improvement.

figure 6. SEM images of (a) a conventional ECA with only silver flakes fillers and (b, c and d) a hybrid ECA containing 3% dA-PPy nanofibers.

CONCLUSIONSOur previous investigations have demonstrated that after being

functionalized by DA, the PPy morphology changed from globular to fibrous; the dispersibility and electrical conductivity of PPy were both improved. In this work, the modified DA-PPy nanofibers were successfully incorporated into the conventional ECA system consisting of silver flakes and epoxy to fabricate hybrid ECAs. Due to the bridging effect of DA-PPy nanofibers among the silver flakes in epoxy network, hybrid ECAs displayed much higher electrical conductivity compared to the conventional ECAs. In particular, the introduction of 3 wt% nanofibers into a conventional ECA with 57.9 wt% silver flakes resulted in an electrical conductivity enhancement of 2400 S/cm, which was comparable to the electrical

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conductivity of an ECA filled with 80 wt% silver flakes. This functional inorganic/organic hybrid ECA significantly reduced the usage of sliver-based nanofillers and may open the possibility of developing the next generation innovative bio-nano electronic devices.

fUTURE WORKThe incorporation of DA-PPy nanofiber as a cofiller has shown

great potential in ECA applications, whereas the unique properties of DA-PPy including improved solvent dispersibility and electrical conductivity have proven to be extremely beneficial. Further work would be focused on the mechanism of how DA-PPy fibers influence the electrical conductivity of hybrid ECAs. Moreover, the mechanical properties (Young’s modulus and binding strength) of this new ECA system would also be investigated for its effective transfer into practical engineering.

ACKNOLEdgEMENTThe authors would like to thank the Refined Manufacturing

Acceleration Process Network (ReMAP), Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ontario Centre of Excellent (OCE) for the financial support.

REfERENCES(1) Li, L.; Lizzul, C.; Kim, H.; Sacolick, I.; Morris, J. E. IEEE

Trans. Compon., Hybrids, Manufact. Technol. 1993, 16, 843-851.(2) Jagt, J. C.; Beris, P. J. M.; Lijten, G. IEEE Trans. Compon.,

Packag, Manufact. Technol. 1995, 18, 292-298.(3) Mir, I.; Kumar, D. Int. J. Adhes. Adhes. 2008, 28, 362-371.(4) Yim, M. J.; Li, Y.; Moon, K.; Paik, K. W.; Wong, C. P. J.

Adhes. Sci. TechnoL. 2008, 22, 1593-1630.(5) Lu, D.; Wong, C. P. IEEE Trans. Compon. Packag. Technol.

2000, 23, 620-626.(6) Jagt, J. C. IEEE Trans. Compon., Packag, Manufact. Technol.

1998, 21, 215-225.(7) Kang, S. K.; Rai, R. S.; Purushothaman, S. IEEE Trans.

Compon., Packag, Manufact. Technol. 1998, 21, 18-22.(8) Klosterman, D.; Li, L.; Morris, J. E. IEEE Trans. Compon.,

Packag, Manufact. Technol. 1998, 21, 23-31.(9) Lu, D.; Tong, Q. K.; Wong, C. P. IEEE Trans. Compon.

Packag. Technol. 1999, 22, 365-371.(10) Kang, S. K.; Purushothaman, S. J. Electron. Mater. 1999,

28, 1314-1318.(11) Kim, H. K.; Shi, F. G. Microelectron. J. 2001, 32, 315-

321.(12) Murray, C. T.; Rudman, R. L.; Sabade, M. B.; Pocius, A. V.

MRS Bull. 2003, 28, 449-454.(13) Novák, I.; Krupa, I.; Chodák, I. Syn. Met. 2004, 144, 13-

19.(14) Li, Y.; Moon, K. S.; Wong, C. P. Science 2005, 308, 1419-

1420.(15) Tan, F.; Qiao, X.; Chen, J.; Wang, H. Int. J. Adhes. Adhes.

2006, 26, 406-413.

(16) Li, Y.; Wong, C. P. Mater. Sci. Eng., R. 2006, 51, 1-35.(17) Amoli, B. M.; Gumfekar, S.; Hu, A.; Zhou, Y. N.; Zhao, B.

J. Mater. Chem. 2012, 22, 20048-20056.(18) Rao, Y.; Lu, D.; Wong, C. P. Int. J. Adhes. Adhes. 2004,

24, 449-453.(19) Jiang, H.; Moon, K. -S.; Li, Y.; Wong, C. P. Chem. Mater.

2006, 18, 2969-2973.(20) Zhang, R.; Moon, K.; Lin, W.; Agar, J. C.; Wong, C.

Compos. Sci. Technol. 2011, 71, 528-534.(21) Amoli, B. M.; Marzbanrad, E.; Hu, A.; Zhou, Y. N.; Zhao,

B. Macromol. Mater. Eng. 2013, 299, 739-747.(22) Voss, D. Nature 2000, 407, 442-444.(23) Lendlein, A.; Langer, R. Science 2002, 296, 1673-1676.(24) Faverolle, F.; Attias, A. J.; Bloch, B.; Audebert, P.; Andrieux,

C. P. Chem. Mater. 1998, 10, 740-752.(25) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B.

Science 2007, 318, 426-430.(26) Zhang, W.; Yang, F. K.; Han, Y.; Gaikwad, R.; Leonenko, Z.;

Zhao, B. Biomacromolecules 2013, 14, 394-405.(27) Zhang, W.; Yang, F. K.; Pan, Z.; Zhang, J.; Zhao, B.

Macromol. Rapid Commun. 2014, 35, 350-354.(28) Khan, Z.; Khannam, M.; Vinothkumar, N.; De, M.;

Qureshi, M. J. Mater. Chem. 2012, 22, 12090-12094.(29) Xing, S.; Zhao, G. J. Appl. Polym. Sci. 2007, 104, 1987-

1996.(30) Zhang, X.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126,

12714-12715.(31) Xing, S.; Zhao, G. Polym. Bull. 2006, 57, 933-943.(32) Gumfekar, S. P.; Amoli, B. M.; Chen, A.; Zhao, B. J. Polym.

Sci., Part B: Polym. Phys. 2013, 51, 1448-1455.(33) Amoli, B. M.; Trinidad, J.; Hu, A.; Zhou, Y. N.; Zhao, B. J.

Mater. Sci: Mater Electron. 2015, 26, 590-600.

bIOgRAPhIESWei Zhang is a Ph.D student in the Surface

Science and Bionanomaterials Laboratory at the University of Waterloo. He obtained his MASc in Chemistry Science in 2011 from the University of Waterloo in Canada and his BSc in Material Science in 2009 from Huaqiao University in China. His research interests include synthesis and functionalization of bio-inspired materials,

chemical modification of electrically conductive polymers and adhesives, contact mechanics studies (adhesion, friction, and wetting) of synthetic biomimetic surfaces, and interfacial phenomena at nanoscales.

Dr. Behnam Meschi Amoli obtained his PhD (Chemical Engineering-Nanotechnology) in 2015 from University of Waterloo, Canada. He also received his BSc (Chemical Engineering) at 2007 and MSc (Polymer Engineering) at 2010 from Sharif University of Technology, Iran. He is currently working as Research Assistant Intern under Connect-Canada program at

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the University of Waterloo, colaborating with Celestica Inc and Microbonds Inc companies. Dr. Meschi Amoli’s research in novel electronic packaging materials focuses on synthesis and surface functionalization of conductive nanomaterials to fabricate advanced electrically conductive adhesive (ECA) composites.

Jeffrey d’Eon is currently pursuing his Bachelor’s in Applied Science, Honors in Nanotechnology Engineering at the university of Waterloo. His research interests include polymer synthesis and characterization. He hopes to continue research in the field and one day pursue a PhD.

Alex Chen is a technology development engineer with Celestica, an Electronics Manufacturing Service provider. His primary responsibilities include process development, technology implementation and manufacturing site qualification. He has also been actively involved in electronics failure analysis in the past decade. He recently worked with customers on

developing reliability performance evaluation methodologies. Alex holds Master’s Degree of Engineering from McMaster University. He has been working in IBM and then Celestica for over 20 years.

Dr. Boxin Zhao is an associate professor in Chemical Engineering at the University of Waterloo. Dr. Zhao obtained his PhD in Chemical Engineering from McMaster University in 2004. He had worked as a postdoctoral researcher in the department of chemical engineering at the University of California at Santa Barbara during 2005-2008. Since joining the University

of Waterloo in 2008, he has established the Surface Science and Bionanomaterials Laboratory, working on both fundamental and applied research to meet the growing need of bionanotechnologies in advanced manufacturing, e.g. multifunctional green and smart materials and processes, additive manufacture or 3D printing. He was awarded the Early Researcher Award from the Province of Ontario in 2012. His research work has been well recognized in the field and has been invited to deliver talks and lectures to Celestica, Xerox and Magna and other national and international conferences.