Journal of Materials Science

, Volume 48, Issue 11, pp 3877–3893

Synthesis and characterization of conjugated polymers for the obtainment of conductive patterns through laser tracing

Authors

    • Dipartimento di Chimica Industriale e dei MaterialiUniversità di Bologna
  • Francesco Paolo Di-Nicola
    • Dipartimento di Chimica Industriale e dei MaterialiUniversità di Bologna
  • Massimiliano Livi
    • Dipartimento di Chimica Industriale e dei MaterialiUniversità di Bologna
  • Luisa Paganin
    • Dipartimento di Chimica Industriale e dei MaterialiUniversità di Bologna
  • Fabio Cappelli
  • Filippo Pierini
    • Dipartimento di Chimica “G. Ciamician”Università di Bologna
Article

DOI: 10.1007/s10853-013-7204-1

Cite this article as:
Lanzi, M., Di-Nicola, F.P., Livi, M. et al. J Mater Sci (2013) 48: 3877. doi:10.1007/s10853-013-7204-1

Abstract

This article describes the preparation of thin films of conjugated polymers which can enhance their specific electrical conductivity by several orders of magnitude by changing their state from insulating to conducting materials. The examined polymers, i.e., a polyacetylenic and a polythiophenic derivative, are functionalized with thioalkylic side chains and are soluble in common organic solvents from which they lead to thick homogeneous films. The films can be deposited on different substrates, either rigid or flexible, and can be easily exposed to laser radiation to make them conductive. The process is irreversible, and the final conductivity is stable over time, even in the presence of high temperatures (up to 180 °C), moisture, and air. The high stability of treated samples, easy polymer synthesis and quick and inexpensive suitably tailored laser tracing procedure make these materials very promising for applications in organic electronics and in the development of new electronic circuitry.

Introduction

Conjugated polymers (ICPs) in their semiconducting or less conductive forms are widely used in the organic electronic field for applications such as light-emitting diodes [1], thin film field effect transistors [2], and bulk heterojunction solar cells [3]. Moreover, the lithographic patterning of conductive polymer films has been successfully applied to various conjugated polymers by means of different approaches such as photochemical-induced doping [4], photochemical polymerization [5], and reactive ion etching [6]. In all cases, the crucial step for obtaining acceptable results was the choosing of both the conjugated polymer and the right source of irradiation, generally Xe-lamps, Ar ion lasers, and KrF excimer lasers [7]. The conversion of polymers by laser irradiation from their non-conductive form to the conductive one has been reported for different conjugated and non-conjugated polymers such as poly(bis-alkylthioacetylene)s (PATACs) [8], polyimides (PIs) [9], and polyvinylchloride (PVC) [10]. Among the latter, disubstituted polyacetylenes are very promising materials for potential applications at the industrial and commercial level. Indeed, they are soluble in some solvents and can be prepared as thin and homogeneous films with a good thermal and environmental stability [11]. PATACs can be easily filmed from their concentrated solutions on several surfaces like glass, Si, Ge, ceramics, and metals by means of simple techniques such as spin coating, doctor blading, or drop casting.

The most studied PATAC is poly(bis-methylthioacetylene) (PATAC-Me); it can be easily synthesized with good yields starting from dimethylthioacetylene with a Ni(II) catalyst in tetrahydrofuran (THF) [12]. The low temperature polymerization of acetylene leads to the prevalence of the cis isomer which converts to the thermodynamically more stable trans isomer at high temperatures. This is also true for polyacetylenes bringing thioalkylic substituents when the side chain is not a sterically demanding group. Poly(terbuthylthioacetylene), for example, [13] leads to the predominance of the trans isomer already during the polymerization step [14].

Disubstituted polyacetylenes with thioalkylic groups are sensitive to visible and UV-radiation; when their thin films are exposed to a laser light of suitable wavelength and intensity, they can be easily and rapidly converted from the insulating to the conductive form, keeping a high electrical conductivity for long times also in presence of oxygen, moisture or aggressive atmosphere.

Today, many techniques are employed to cast active polymer layers (films) onto stiff or flexible substrates with the aim to cover large areas with low cost and high speed. The generated patterned polymeric structures can be the active components of organic field effect transistors (OFETs) [15], organic light-emitting diodes (OLEDs) [16], and organic photovoltaics (OPVs) [17] exploiting, in turn, the polymer semiconducting [18] or conducting properties (when nanoscaled composites are formed [19]), their transparency [20] or light-absorbing properties [21] and their thermal stability [22] or reactivity [23]. The most studied technique is called roll-to-roll (R2R) or reel to reel coating [24] which is compatible with many deposition systems, an important feature considering that each system has its strengths and weaknesses and can present some applicative restrictions.

The continuous deposition techniques which are compatible with R2R coating are: gravure printing, flexographic printing, rotary screen printing, knife/slot die coating, inkjet printing and spray coating.

A very exhaustive and recent review exploring in detail all of these systems is reported in Ref. [25]. A concise overview of polymer thin film devices obtained using R2R techniques is reported in Table 1.
Table 1

Overview of reported polymer thin film applications using R2R techniques

Processing method

Applications

Comments

Gravure printing

Active layers for OPV [26, 27], OTFT [28], PLED [29]

High speed (up to 15 m/s)

Different substrates (PC, PET, glass)

Less expensive

Requires a pre-patterned printing form

Flexographic printing

Loudspeakers [30]

Works well on paper

Expensive

Requires a pre-patterned printing form

Screen printing (flat bed)

Rotary screen printing (RSP)

Silver electrodes for OPV [31]

High speed (RSP)

Simple and less expensive

Requires a pre-patterned mesh

Stepwise process (flat bed)

Screen printing (flat bed)

Rotary screen printing (RSP)

OPV (all layers) [32], PLED [33], EC [34]

High speed (up to 200 m/min)

Less expensive

Wide applicability (premetered

coating system)

Inkjet printing

OPV [35, 36], PLED [37]

High resolution

High speed (up to 75 m/min)

No premanufactured printing form required

Expensive

Spray coating

Active layers and electrodes for OPV [38, 39], photodiodes [40], EC [41]

Simple and less expensive

Not very uniform coating

Light-induced thermocleavage

Active layers for OPV [42]

Simple and effective

High speed

Quite expensive

Requires a pre-patterned mask for the photonic sintering system and a suitable polymer

Laser-induced thermocleavage

Potential applications: OPV, TFT, PLED

High speed

Inexpensive

Simple and effective

No premanufactured printing form required

Requires a suitable functional polymer

This work mainly aims to find the best operating conditions for the laser-patterning of PATAC-Me films by means of commercial, lightweight, quite inexpensive diode lasers operating at low optical powers (lower than 1 W), with the aim of bringing these techniques one step forward toward the industrialization of the process. Prepared samples have been deeply characterized by using FT-IR, UV–Vis spectroscopy, optical microscopy, and SEM/EDS probe for in situ elemental analysis. Moreover, an attempt has been made to substitute PATAC-Me with a thiophenic polymer bearing a thioalkylic side chain. The properties and the applicability of the polythiophene derivative have been evaluated, showing that the latter is an easier obtainable polymer with better characteristics in terms of solubility, filmability, and optical transparence as compared to the acetylenic polymer.

Experimental

1H- and 13C-NMR were recorded on a Varian Mercury Plus spectrometer (400 MHz) using TMS as a reference. FT-IR spectra of the monomers (pure liquids) and polymers (films) were carried out on Ge disks using a Perkin Elmer Spectrum One spectrophotometer. Raman spectra were recorded using polymer films on glass slides and a Renishaw RM 1000 instrument with an excitation wavelength of 785 nm. Molecular weights were determined by gel permeation chromatography (GPC) using polystyrene standards and THF as an eluent on a HPLC Lab Flow 2000 apparatus equipped with a PL Gel MXL column and a Linear Instrument UV–Vis detector model UVIS-200 working at 263 nm. Elemental analyses were performed by Redox Laboratory, Monza, Italy. UV–Vis spectra were recorded using a Perkin Elmer Lambda 19 spectrophotometer. Polymer solutions on Hellma Suprasil quartz cuvettes were prepared using spectroquality solvents stored under molecular sieves, with a polymer concentration of about 7 × 10−5 mol × l−1, while films on quartz slides were cast from either chlorobenzene (PATAC-Me) or THF (PSBu) solutions (ca. 10−3 mol × l−1).

Thermal analyses were performed on a TA Instruments DSC 2920 in a nitrogen atmosphere and on a TGA 2050 in air at a heating rate of 10 °C/min. Electrical measurements were performed in air at room temperature using a Keithley 2101 electrometer (traced films) and an Alpha Lab teraohmeter (pristine films). The reported values were the means of some measurements performed on different parts of the same sample as well as on different samples. In all cases, the differences did not exceed 2–3 % of the final value. Electrical conductivity of the laser-traced samples was also examined in the 20–180 °C range by means of a hot-plate controlled by a Pt100 thermocouple. The laser sources were a Wicked Lasers Spyder III Arctic class 4 diode laser, operating at 445 nm and a Wicked Lasers Spyder III Krypton, class 4 diode laser, operating at 532 nm, both with a nominal power of 750 mW. The effective power of laser beams on the polymer surface was measured with a Coherent FieldMax II laser power meter. Samples were mounted on a computer-controlled positioning system (Thorlabs L490MZ) and moved on a plane perpendicular to the focused laser beam using two Thorlabs MTS TDC001 controllers. SEM analyses were performed on a Carl Zeiss EVO MA10 SEM apparatus, equipped with an EDS microanalysis probe. Optical 2D and 3D microscopy was performed using a Hirox KH-7700 Digital Microscope.

Monomer synthesis

3-(Buthylthio)thiophene (TSBu)

10.01 g (61.2 mmol) of 3-bromothiophene in 56 ml of anhydrous diethyl ether were stirred at −78 °C under Ar atmosphere and added dropwise with 40 ml of a 1.6 M buthyllithium solution in n-hexane. The mixture was stirred at −78 °C for 30 min, added with 2.06 g (64.2 mmol) of sulfur and stirred for 40 min at the same temperature. After heating to 0 °C, 16.77 g (0.122 mol) of bromobutane were added, and the mixture was stirred overnight at room temperature. 50 ml of aqueous 1 M HCl were then added to the reaction mixture and the organic phase washed with both a saturated solution of NaHCO3 (3 × 150 ml) and distilled water to neutrality, dried over MgSO4, and concentrated. The crude product was purified by column chromatography (silica gel, n-pentane/n-heptane 1:1) to give 4.13 g (24.0 mmol) of pure TSBu (40 % yield).

2,5-Dibromo-3-buthylthiothiophene (2,5BTSBu)

A solution of 1.22 g of N-bromosuccinimide (NBS, 6.88 mmol) in 7 ml of N,N-dimethylformamide (DMF) was added dropwise to a solution of 1.18 g (6.80 mmol) of TSBu in 7 ml of DMF. After stirring for 6 h at room temperature, 1.59 g (8.95 mmol) of NBS in 9 ml of DMF were added dropwise and the reaction mixture was stirred for 24 h at room temperature. The mixture was then poured into 450 ml of distilled water and extracted with n-pentane (3 × 150 ml). The collected organic phases were washed with 2× 300 ml of a 5 % KHCO3 solution, then with water to neutrality, dried with MgSO4 and concentrated. The crude 2,5BTSBu was purified by chromatography (silica gel, n-heptane) giving 1.59 g of pure 2,5BTSBu (70 % yield).

Polymer synthesis

Poly(bis-methylthioacetylene) (PATAC-Me)

Elemental analysis: [C4H6S2]n Calcd: 40.64 C %; 5.12 H %; 54.24 S %. Found: 41.22 C %; 4.98 H %; 53.80 S %.

Poly[(3-methylthio)thiophene] (PSMe)

1.6 ml of an aqueous solution of FeCl3 (7.5 × 10−3 M) was added dropwise to a solution of 0.622 g (2.31 mmol) of sodium dodecylsulfate (SDS) and 3-methylthiothiophene (TSMe, 0.200 g, 1.54 mmol) in 10 ml of distilled water and 3 ml of H2O2 (35 % vol). The mixture was stirred for 6 h at 50 °C under Ar atmosphere and filtered on a Teflon septum (0.45-μm pore size). The recovered polymer was diluted with 100 ml of distilled water and poured into a dialysis membrane (100 nm average pore size) which was immersed in a beaker and washed continuously with a gentle flow of distilled water. After two weeks the polymer solution was concentrated, giving 0.100 g (51 % yield) of PSMe.

Poly[(3-buthylthio)thiophene] PSBu—oxidative polymerization

A solution of 0.200 g (1.16 mmol) of TSBu in 12 ml of CHCl3 was added in 20 min to a suspension of 0.753 g (4.64 mmol) of FeCl3 in 10 ml of CHCl3. After stirring for 40 min at 20 °C under a gentle flux of Argon, 20 ml of freshly distilled THF were added to the reaction mixture. The resulting mixture was then poured into 150 ml of a 5 % HCl solution in methanol and filtered on a Teflon septum (0.20-μm pore size). The recovered polymer was washed several times with methanol and dried, giving 0.311 g of PSBu (40 % yield).

1H-NMR (CDCl3, 400 MHz, ppm) δ: 7.40 (s, 1H); 7.37 (s, 1H); 7.20 (s, 1H); 2.82 (t, 2H); 1.60 (m, 2H); 1.43 (m, 2H); 0.93 (t, 3H).

13C-NMR (CDCl3, 400 MHz, ppm) δ: 134.0, 132.3, 112.8, 110.9, 35.0, 31.5, 21.8, 13.6.

FT-IR (Ge disk, cm−1) 3101, 3057, 2956, 2927, 2869, 1521, 1463, 1377, 837, 746, 716.

Poly[(3-buthylthio)thiophene] PSBu—GRIM procedure

2.10 ml (2.10 mmol) of a 1 M solution of methylmagnesium bromide in di-n-buthylether was added to a solution of 0.688 g (2.08 mmol) of 2,5BTSBu in 12 ml of anhydrous THF. The reaction mixture was refluxed for 1 h under stirring and under a gentle flux of Argon. 11.85 mg (0.022 mmol) of [1,3-bis(diphenylphosphino)propane]nickel(II) chloride (Ni(dppp)Cl2) were then added and the mixture was refluxed again for 2 h. The mixture, cooled down to room temperature, was filtered on a Teflon septum (0.45-μm pore size). The recovered polymer was washed several times with methanol and dried, giving 0.200 g of regioregular PSBu (57 % yield).

Elemental analysis: [C8H10S2]n Calcd: 56.42 C %; 5.92 H %; 37.66 S %. Found: 57.03 C %; 5.55 H %; 37.42 S %.

Results and discussion

PATAC-Me and PATAC-Et (poly(bis-ethylthioacetylene)) films require only a laser exposure to become electrically conducting materials, without the need of doping or other treatment. In fact, if they are irradiated with an excimer laser (351 nm) or argon ion laser (488 nm), they can change their electrical conductivity by 16 orders of magnitude (from 10−14 to 102 S × cm−1) if the radiation is correctly focused on the film surface and of the suitable intensity [43]. In fact, on the one hand a too-intense laser radiation can lead to polymer pyrolysis (graphitization process), with the consequent formation of non-conductive carbon residues, while, on the other hand, if the radiation intensity is too low (or the tracing speed too high) no conversion toward the more conductive form is observed. The right conversion conditions reported in literature for the use of an Ar+ laser are well described: 200 mW of power and 13 mm/sec of scan speed for PATAC-Et, leading to a specific electrical conductivity (σ) of 2 S × cm−1, while for PATAC-Me it is possible to reach 4 S × cm−1 by using 300 mW and 15 mm/s [44]. During the laser exposure, the color and morphology of PATAC dramatically change, passing from the yellowish-brown of the untreated sample to the blue-black of the laser exposed films, also exhibiting an irregular and porous surface probably determined by the emission of gaseous reaction products. The conversion of PATAC samples starts with the generation of free radicals whose concentration reaches its peak and then remains almost constant [8]. This process can be considered a photopyrolysis of the polymer, leading to the formation of unsaturated structures with an extended π-system mainly made of sp2 hybridized carbon atoms partly linked with sulfur. The outflow of gaseous products during the conversion process (mainly sulfides and mercaptanes) and the insolubility of the final treated polymer suggest that the laser is able to both split-off thioalkylic side chains and determine a partial inter- or intra-chain cross-linking. This process can reduce the conformational mobility of the backbone, making the polymer more prone to assuming planar conformations with a more extended conjugation, thus leading to enhanced charge mobility and better electrical conductivity.

In view of the foregoing, the first examined polymer was the PATAC-Me, bought from Aldrich Chemical Co. This sample had a Mn of 4000 g × mol−1, a polydispersity index of 1.15 and a DPn of about 34 repeating units. Its 1H-NMR in CDCl3 is shown in Fig. 1.
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Fig. 1

1H-NMR spectrum of PATAC-Me

The presence of only one peak, fairly broad and centered at 2.50 ppm, is clearly evident.

This signal is ascribable to the thiomethylic protons, while its broadness is due to the presence of different proton chemical surroundings since commercial PATAC-Me is a mixture of stereoisomers. The elemental analysis of the polymer was in good agreement with the expected structure. In Fig. 2, the FT-IR spectrum of PATAC-Me in film cast from chloroform on Ge disk is shown before and after the laser tracing with blue laser (λ = 445 nm).
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Fig. 2

FT-IR spectra of PATAC-Me before (up) and after (down) laser treatment

In the spectrum of the pristine polymer, the signals ascribable to its chemical structure were clearly evident:
  • 2984, 2916 cm−1: CH3 antisymmetric and symmetric stretching;

  • 1664 cm−1: C=C stretching;

  • 1430 cm−1: CH3 antisymmetric deformation;

  • 1308 cm−1: CH3 symmetric deformation;

  • 645 cm−1: S–CH3 symmetric stretching.

The evolution in the intensity of the latter signal, which can range from 715 to 620 cm−1 [45], could be evidence of the effectiveness of the laser treatment. In fact, its transmittance sensibly increased after the exposure to the coherent light (Fig. 3).
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Fig. 3

Detail of the FT-IR spectra of PATAC-Me before (a) and after (b) laser exposure

Figure 4 shows the Raman spectra of unconverted and converted PATAC-Me films.
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Fig. 4

Raman spectra of PATAC-Me before (up) and after (down) laser exposure

Before laser exposure, the film spectrum showed a non-linear baseline and a high level of noise; this was due to the high reflective surface of the pristine polymer, giving a lot of scattering of the red laser radiation (785 nm) used to record the spectrum. In any case, two peaks could be found at 1554 and 708 cm−1; the first signal was ascribable to the in-phase vibration of the C=C double bonds, since polyenes usually show an intense absorption in the 1600–1500 cm−1 range, whose intensity may also be related to their extension of conjugation [46], while the signal around 700 cm−1 could be related to the C–S stretching of the –SMe groups. After laser exposure, the latter signal could not be found, while the peak around 1300 cm−1 could be ascribed to the in-phase vibration of the C–C bonds in polymeric chains.

The UV–Vis spectra of PATAC-Me in film on quartz slides are shown in Fig. 5.
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Fig. 5

UV–Vis spectra of PATAC-Me films

The polymer exhibited a non-structured profile, with a maximum absorption wavelength at 315 nm. The film was homogeneous and devoid of macroscopic aggregates and was obtained by dissolving PATAC-Me in chlorobenzene. The absence of any evident peak at the lower energies evidenced that this polymer had a high energy gap (Eg), probably determined by its reduced conjugation length, as a consequence of the steric hindrance due to the presence both of the thiomethylic substituent on each C atom of the backbone, and of the non-stereoregularity of the sample. The spectral behavior changed after laser conversion since the inhomogeneous broadening of the spectrum decreased slightly while the absorption at the lower energies slightly increased.

The solvatochromism of PATAC-Me was examined in CHCl3 (solvent)/CH3OH (non-solvent) mixtures. In Fig. 6, the spectra of the polymer in pure chloroform (methanol molar fraction: 0.00) and at the higher non-solvent molar fraction (methanol molar fraction: 0.99) are shown. A further increase in the non-solvent concentration inevitably caused the precipitation of the polymer.
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Fig. 6

Solvatochromism of PATAC-Me

PATAC-Me was completely insensitive to the non-solvent additions, since the presence of the thiomethylic substituents strongly reduced the rotational mobility of the polymeric backbone even when the polymer was almost completely devoid of the solvent molecules. Unfortunately, the solvatochromic behavior of the laser-converted PATAC-Me had not been examined since the exposed polymer is insoluble in common organic solvents.

PATAC-Me was then dissolved in chlorobenzene (5 mg in 2 ml) and the solution, sonicated for 30 min, was deposited by doctor blading on 10 × 2.5 cm glass slides. Samples were heated at 80 °C for 4 h and cooled down to room temperature. The conversion was made by means of Wicked Laser portable diode lasers, Artic (blue light, 1 W of peak power, λmax 445 nm) and Krypton (green light, 1 W of peak power, λmax 532 nm). The effective power of the laser radiation on the polymer surface was measured using a Coherent power meter while the laser beam was well focused at a lens-sample distance of 3.7 cm. The exposure of the polymer was performed line-by-line using a Thor Labs computer-controlled positioning system which allowed for both an accurate control of the samples’ position and their speed during irradiation. Samples were moved on the xy plane from left to right using different scan speeds, by tracing a series of parallel 10-cm-long traces. The system was set so that the laser irradiated the same trace only once. The polymer film thickness, measured by means of a Burleigh Vista AFM used as a profilometer, ranged from 1 to 10 μm and was influenced both by the adopted deposition technique (doctor blade, spin coating or drop casting, the latter giving thicker films) and by the possibility to make multiple depositions.

Under the same operating conditions (laser power, scan speed, and sample thickness), the Artic (blue) laser was more effective for the PATAC-Me conversion than the Krypton (green) one, being able to make the traces visible also from the opposite side of the sample. This fact is probably ascribable to the higher absorbance of PATAC-Me at 445 nm than at 532 nm.

The best sample conversion was obtained using the Artic laser at a scan speed of 5 cm × s−1 with an effective power of 600 mW. Figure 7 shows an image of a partially traced PATAC-Me film (optical microscope, magnification: 50×).
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Fig. 7

Optical microscope image of a partially traced film of PATAC-Me

In the lower part of the image, traces are clearly visible. The polymer morphology notably changed after the conversion, passing from a smooth surface, even if crossed by some cracks ascribable to the glassy nature of the film, to a rough, dull, and less homogeneous surface. Traces had a worm-like appearance and a porous structure, probably caused by the outflow of gaseous reaction products. Laser-converted paths had an average width of 50 μm and a mean depth of 3 μm. The laser-induced conversion of PATAC-Me using the blue laser was undoubtedly effective, since the pristine polymer conductivity was around 10−14 S × cm−1 and, after exposure, the value increased greatly, up to 80 S × cm−1.

Figure 8 shows the SEM micrograph of a traced sample of PATAC-Me. Paths are clearly evident on the left side of the image.
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Fig. 8

SEM micrograph of a PATAC-Me film after laser tracing

Figure 9 shows the topographic microanalysis of the latter sample in relation to the C and S elements recorded using an energy-dispersive spectrometer (EDS) probe.
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Fig. 9

EDS topographic microanalysis of the film reported in Fig. 8. Up carbon, down sulfur

Since the concentration of the element is proportional to the intensity of white, it is evident that in the converted pathways there was a higher amount of C and a lower amount of S than in the unexposed ones. In fact, C and S concentrations passed from 41 and 54 % of the unexposed area to 75 and 20 % of the laser treated portions, respectively, thus confirming the partial loss of side chains.

Even if PATAC-Me is a good candidate for electric and electronic applications, its non-complete solubility in common organic solvents is undoubtedly a great drawback. Concentrated solutions of this polymer can only be obtained in chlorinated or aromatic high-boiling solvents, such as chlorobenzene or o-dichlorobenzene. Films prepared from these solvents are usually quite homogeneous even though they sometimes appear spotted, dotted or with some cracks. Moreover, PATAC-Me solutions cannot be cast on transparent polymeric layers, such as PET, PMMA, or cellulose acetate, since the solvent corrodes the substrate and, in any case, the resulting film would be too fragile for a permanent adhesion to flexible and bendable surfaces.

The synthesis of a new polymer which, at the same time, has a conjugated backbone and photo-cleavable side chains while being capable of giving homogeneous and elastic films, could be a first attempt at circumventing these problems. Poly[(3-methylthio)thiophene] (PSMe) seemed to be a good substitute for PATAC-Me, since it belongs to the ICP class and bears thioalkylic substituents. 3-(Methylthio)thiophene was purchased from Atlantic Chemical Co. and subjected to an oxidative polymerization procedure in water, using H2O2 and FeCl3 as oxidizing agents and SDS as a surfactant [47]. This method should be particularly effective in molecular weight control, generally leading to low molecular weight polythiophenic fractions that are soluble in a wide range of organic solvents; it appeared particularly suitable for synthesizing PSMe, since short side chains usually lead to little-soluble polyalkylthiophenes (PATs)[48].

The polymerization reaction proceeded with an initial dark-brown color and the subsequent formation of a black oil, without any trace of precipitate. A first attempt to recover the polymer by multiple extractions of the reaction products with halogenated solvents did not give satisfying results, since the surfactant was found in both organic and aqueous phases. The polymer purification was then performed by means of a dialysis membrane (100 nm pore size), which allowed the recovery of the polymer without any trace of SDS and FeCl3 being detected. Contrary to expectations, PSMe was only partially soluble in organic solvents, leading to inhomogeneous and brittle films. Since its molecular weight was not particularly high, as evidenced by the 1H-NMR spectrum shown in Fig. 10, where the chain-end signals are clearly evident, its low solubility might be ascribed to the too-short substituent inserted in the 3-position of the aromatic ring.
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Fig. 10

1H-NMR spectrum of PSMe

The NMR spectrum shows only one signal in the aliphatic region (–SCH3 at 2.65 ppm), while the aromatic region is more complex, showing three signals ascribable to the protons of the ending thiophenes (H4 at 6.97 ppm, H2 at 7.03 ppm and H5 at 7.37 ppm [49]) and one signal at 7.25 ppm, belonging to the β-proton of the central thiophene units of polymeric chains. The FT-IR analysis of PSMe was performed on a thin film cast on a Ge disk (Fig. 11).
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Fig. 11

FT-IR spectrum of PSMe

The spectrum was in good agreement with the expected structure, showing the following peaks (in cm−1): 3059 (ν C–Hβ, thiophene); 2927 (νas –CH3); 2853 (νs –CH3); 1511 (νas C=C); 1462 (νs C=C); 1369 (–CH3 sym. deformation); 1014 (–CH3 rocking); 826 (γ C–H 2,3,5-trisubstituted thiophene); 648 (ν C–S).

The UV–Vis analysis of PSMe in solution (THF) and in film is shown in Fig. 12.
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Fig. 12

UV–Vis spectra of PSMe in solution and in film

The steric hindrance of the short side-chain, the presence of Head-to-Head (H–H) linkages and the low molecular weight negatively affected the conjugation length of the polymeric backbone. In fact, even if an evident bathochromic shift of the λmax was observable when passing from the solution to the solid state (Δλmax = 121 nm), the film spectrum showed a maximum absorption wavelength (332 nm) usually found in short thiophene oligomers [50]. Similar results were obtained using a different polymerization procedure which involved chloroform as a reaction solvent and an excess of iron trichloride as an oxidizing agent [51]. This time, the purification of the polymer was easier but the latter was, however, insoluble.

Taking into account the previous results, a PAT derivative with a longer thioalkylic side chain was synthesized, namely poly[(3-buthylthio)thiophene] (PSBu). The monomer (3-buthylthio)thiophene (TSBu) was synthesized according to Ref. [52], starting from commercial 3-bromothiophene and using a “one pot” reaction shown in Fig. 13.
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Fig. 13

Synthesis of the monomer TSBu

TSBu was recovered with a good yield after purification and was firstly subjected to an oxidative polymerization initially with FeCl3 in CH3NO2/CHCl3, a solvent mixture usually giving a good control of molecular weights as well as a low polidispersity index [53], and afterwards with FeCl3 in pure CHCl3, a method that leads to higher molecular weights [54]. In spite of this, in both cases, only short oligomers were recovered, probably because the low oxidation potential of TSBu promoted the formation of low molecular weight stable species [55].

TSBu was then selectively dibrominated in the 2,5-positions of the thiophenic ring by means of NBS in anhydrous N,N-dimethylformamide (DMF) by using an optimized reaction procedure involving the addition of the brominating agent in two distinct steps at room temperature. 2,5-Dibromo-3-(buthylthio)thiophene (2,5-BTSBu) was recovered in a good yield (70 %) after chromatography column purification and exploited for organometallic coupling reactions using the McCullough procedure [56]. This method is a useful and straightforward way to synthesize regioregular Head-to-Tail (H–T) linked poly(3-alkylthiophene)s through the magnesium-halogen exchange (Grignard Metathesis Reaction, GRIM) with a preformed organometallic derivative and subsequent Ni(II) catalyzed cross-coupling reaction (Fig. 14).
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Fig. 14

Synthesis of the polymer PSBu by using the GRIM reaction

PSBu was obtained with good yield (57 %) as a dark orange solid well soluble in common organic solvents. Despite its quite low molecular weight (Mn = 11.000, PDI = 1.4), it gave highly homogeneous and free-standing films. Its elemental analysis confirmed the expected chemical structure.

Table 2 show the characteristic FT-IR absorptions of the monomers TSBu and 2,5-BTSBu and of the polymer PSBu prepared with the GRIM procedure.
Table 2

FT-IR absorption bands (cm−1) and relative assignments for TSBu, 2,5-BTSBu and PSBu samples

Assignment

TSBu

2,5-BTSBu

PSBu

C–H stretching (thiophene, α-hydrogens)

3,103

C–H stretching (thiophene, β-hydrogen)

3,059

3,050

3,057

C–H stretching (antisymmetric, methyl)

2,953

2,958

2,956

C–H stretching (antisymmetric, methylenes)

2,928

2,927

2,927

C–H stretching (symmetric, methyl + methylenes)

2,871

2,874

2,870

C=C stretching (antisymmetric, thiophene)

1,492

1,501

1,501

C=C stretching (symmetric, thiophene)

1,464

1,464

1,463

CH3 deformation

1,350

1,387

1,378

C–Br stretching (aromatic)

992

C–H bending out-of-plane (2,3,5-trisubstituted thiophene)

839

826

C–H bending out-of-plane (3-substituted thiophene)

773

CH2 rocking

754

747

751

C–S stretching

686

647

714

The absorption at 3103 cm−1, ascribable to the stretching of the thiophene α-hydrogens, was absent in 2,5-BTSBu and PSBu samples, as well as the band at 773 cm−1, which was substituted by the new absorption around 830 cm−1. Moreover, aromatic C–Br stretching at 992 cm−1 in the 2,5-BTSBu spectrum was completely missing in the polymer spectrum, while the C–S stretching mode was evident in all the samples (Fig. 15).
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Fig. 15

Adopted atoms numbering for NMR analysis

Table 3 shows the 1H- and 13C-NMR data for both the examined monomers and the PSBu polymer, together with the corresponding assignments obtained by comparing some selected references [5761].
Table 3

1H- and 13C-NMR signals (ppm) and relative assignments for TSBu, 2,5-BTSBu, and PSBu samples

Atom numbera

TSBu

2,5-BTSBu

PSBu

δ1H

δ13C

δ1H

δ13C

δ1H

δ13C

2

7.11 (m)

122.8

111.6

132.3

3

132.3

134.7

129.0

4

7.00 (m)

129.7

6.91 (s)

133.0

7.37 (s)

130.3

5

7.32 (m)

126.0

113.5

134.0

6

2.82 (t)

35.0

2.81 (t)

35.7

2.81 (m)

35.0

7

1.60 (m)

31.5

1.56 (m)

32.3

1.56 (m)

31.6

8

1.43 (m)

21.8

1.44 (m)

22.4

1.43 (m)

21.7

9

0.93 (t)

13.6

0.93 (t)

14.3

0.93 (m)

13.6

aSee Fig. 15

The monomer 2,5-BTSBu was lacking in H atoms in the 2,5-positions of the thiophene therefore, its polymerization was only confirmed by the shift of the signal ascribable to the thiophene H-4 atom to a lower field. Moreover, the absence of the thiophene αCH2 group prevented the determination of the percentage of regioregularity of PSBu in terms of configurational dyads. However, the presence of only one signal in the aromatic region of the 1H-NMR spectrum of the polymer suggested that a high degree of regioregularity was achieved (Fig. 16).
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Fig. 16

1H-NMR of PSBu prepared using the McCullough procedure

This was also confirmed by the presence of only four aromatic carbon signals in the 13C-NMR spectrum (Fig. 17). The obtained results once again confirmed the easiness and versatility of the McCullough GRIM reaction for obtaining regioregular functionalized PATs.
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Fig. 17

13C-NMR of PSBu prepared using the McCullough procedure

The high solubility of PSBu in common organic solvents made it possible to perform its UV–Vis analysis in different solvent- and non-solvent mixtures. Figure 18 shows the solvatochromic behavior of PSBu in dimethylpropyleneurea (DMPU)/methanol and THF/methanol at increasing non-solvent (methanol) molar fractions.
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Fig. 18

Solvatochromism of PSBu in DMPU/MeOH (up) and THF/MeOH (down) at increasing methanol molar fractions

In both cases, the solvatochromic transition from the solvated to the less solvated conformation was clearly visible, resulting in the solution color change from dark red to violet, without any trace of macroscopic aggregation being detected even after many days. The non-solvent effect was more evident in the DMPU/MeOH system (Δλmax = 26 nm, from 501 to 527 nm) than in THF/MeOH (Δλmax = 17 nm, from 505 to 522 nm) while the polymer showed a clearly evident pure electronic transition (E0–0) around 604 nm, also visible in pure solvents. The moderate shifts of the λmax of the polymer spectra by progressive additions of the non-solvent as well as the presence of the E0–0 absorption without any addition of MeOH indicated the tendency of PSBu to assume well-ordered conformations even in good solvents, probably because the side chains possessed a high ability to self-assemble even in the solvated state. This can be a very important feature for the obtaining of concentrated solutions of preordered planar PAT chains; they are particularly useful for the preparation of homogeneous thick films which can be used either for the laser writing process or for the building up of bulk heterojunctions (BHJ) solar cells. PSBu spectrum in the film state is shown in Fig. 19.
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Fig. 19

UV–Vis spectrum of PSBu in film

The λmax of PSBu in film (538 nm) is very similar to that recorded in solution of pure solvents, thus indicating the presence of the same conformer in solution and in the solid state.

PSBu was then dissolved in THF (5 mg in 2 ml) and the solution was deposited on 10 × 2.5 cm preventively cleaned glass slides using the doctor blading technique. The obtained sample was annealed for 3 h at 80 °C in air, giving very homogeneous films with a thickness in the 5–10 μm range. The best tracing conditions were obtained using the Krypton green laser, operating at 532 nm, a wavelength very close to the λmax of PSBu in film, with a scan speed of 3 cm/s and an effective power of 600 mW, leading to a mean specific conductivity of 5 × 10−2 S × cm−1. The adopted tracing speed is slower than in PATAC-Me experiments, probably because this time the longer side chain is more difficult to eliminate from the substrate, and the final conductivity lower than for the polyacetylenic derivative. PSBu, however, is a very interesting material, since it is easy to synthesize and gives very homogeneous thick films which are completely devoid of macroscopic aggregates. Moreover, PSBu is able to increase its specific electrical conductivity by 8 orders of magnitude, reaching values comparable to those obtained in doped PATs [62] but without any detrimental effect of moisture and oxygen on its time performance.

PSBu films are essentially amorphous and more elastic than those obtained using PATAC-Me, so the former can be easily applied also on flexible substrates, such as PET, cellulose acetate, polyvinylalcohol, and PVC.

Figure 20 shows an optical microscope image of a traced sample of PSBu and the corresponding 3D elaboration.
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Fig. 20

Optical microscope images of traced PSBu films. Top left ×50, right ×400. Bottom: 3D elaboration, ×50

Moving from left to right, the untreated portion of PSBu film is clearly evident and is followed first by two traces obtained with the green laser operating at reduced power (60 mW), and then by two more traces produced with full-power laser (600 mW). In all cases, the tracing speed was 3 cm/s. The 3D image clearly shows homogeneous traces without any apparent pits or holes.

Figure 21 shows the SEM image of a traced PSBu film. The image covers an area of about 2 × 4 mm and clearly shows five parallel traces. The elemental microanalysis shows a sulfur content decreasing from 37 to 21 %.
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Fig. 21

SEM image of a traced PSBu film (left). EDS microanalysis relative to sulfur on the same film (right)

The IR spectrum of the traced PSBu sample showed the same main absorptions of the pristine polymer at 2927, 2853, 1515, 1462, 825, and 751 cm−1. The band at 714 cm−1 (ν C–S) is absent, thus confirming the partial loss of the side chains after the photopyrolysis procedure (Fig. 22).
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Fig. 22

FT-IR spectrum of a PSBu film. a Untraced and b traced

The thermal behavior of unexposed PATAC-Me and PSBu has been examined by means of DSC in inert atmosphere with a heating scan of 10 °C/min. Thermograms are shown in Fig. 23.
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Fig. 23

DSC thermograms of PATAC-Me (up) and PSBu (down)

Two second-order transitions (Tg) were found at 42 and 80 °C for PATAC-Me and PSBu respectively, and two first-order transitions (Tm) at 131 and 166 °C. After 200 °C, the two polymers started to decompose.

The TGA analyses of the two polymers were recorded from 25 to 600 °C with a heating scan of 10 °C/min in an oxidizing environment (air). PATAC-Me showed a two-phase weight loss: the first, starting around 200 °C, was compatible with the loss of a thiomethylic and methylic group while the second, over 400 °C, was due to the almost complete oxidation of the residual polymeric backbone (Fig. 24). The PSBu behavior was more complex, showing an initial weight loss at around 200 °C and ascribable to the loss of a methylic group, a second loss from 250 to 500 °C compatible with the loss of the –SCH2CH2CH2 residue and a third one over 500 °C due to the cracking of the polymeric backbone.
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Fig. 24

TGA thermograms of PATAC-Me (up) and PSBu (down)

The electrical conductivities of PATAC-Me and PSBu were also examined in the 293–453 K (20–180 °C) thermal range, far enough to arrive at their decomposition temperature.

The obtained results are shown in Table 4.
Table 4

Electrical conductivity (σ) of PATAC-Me and PSBu at different temperatures

Temperature (K)

σ PATAC-Me (S × cm−1)

σ PSBu (S × cm−1)

293

80

5.0 × 10−2

313

80

5.2 × 10−2

333

84

5.2 × 10−2

353

88

5.5 × 10−2

373

91

6.1 × 10−2

393

95

6.3 × 10−2

413

105

7.1 × 10−2

433

105

7.2 × 10−2

453

107

7.6 × 10−2

In the examined range the specific conductivity is subject to small changes, thus reflecting the polymers’ thermal stability in the examined temperature range, according to DSC measurements. The observed slight increase of σ with temperature may indicate that polymers followed the typical behavior of inorganic semiconductors [63].

Conclusion

In this work, we have successfully examined some conjugated polymers trying to find the best conditions for their laser tracing, with the aim of increasing their electrical conductivity. It is, in fact, well known that some ICPs can enhance their charge mobility, when irradiated with laser light at the suitable power and wavelength, by a photopyrolysis phenomenon which involves the partial loss of side-chain substituents. The laser-traced polymers are able to reach more planar conformations and then higher conjugation lengths since the macromolecular backbone becomes subjected to a lower degree of sterical crowding.

The first polymer examined belonged to the class of substituted polyacetylenes and was commercially available as dimethylthioderivative (PATAC-Me). After an accurate set-up of both the film deposition and the tracing conditions, we obtained pathways with an electrical conductivity similar to some metals by means of very simple procedures, involving the use of portable and inexpensive laser pointers. However, PATAC-Me films were rigid, fragile and brittle. In order to overcome this problem, we substituted the latter with the newly synthesized regioregular PSBu, a thiophenic polymer bearing a thiobutylic side chain, easily obtainable and able to give very homogeneous, flexible thick films. The high solubility of PSBu in common organic solvents, together with its flexibility, allowed for its deposition not only on glass or metal rigid substrates but also on plastic surfaces, thus leading, after laser exposure, to electrical conductivities comparable to those obtained with the best doped PATs films, but with a higher environmental and time stability.

PSBu films are thus very promising materials for the preparation of integrated circuits on rigid or flexible substrates by using the simple and rapid laser tracing technique.

Acknowledgements

This study was supported by University of Bologna-Funds for selected research topics 2012 and by Felsilab Srl, Bologna, Italy.

Copyright information

© Springer Science+Business Media New York 2013