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Orthogonally Processable Carbazole-Based Polymer Thin Films by Nitroxide-Mediated Polymerization Owen Alfred Melville, Benjamin King, Christian John Imperiale, and Benoît H. Lessard Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03920 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Orthogonally Processable Carbazole-Based Polymer Thin Films by Nitroxide-Mediated Polymerization

Owen A. Melville1, Benjamin King,1 Christian Imperiale, 1 and Benoît H. Lessard1,*

*To whom correspondences should be addressed. E-mail: [email protected] 1

University of Ottawa, Department of Chemical and Biological Engineering,

161 Louis Pasteur, Ottawa, Ontario, K1N 6N5

Keywords : nitroxide mediated polymerization (NMP), orthogonal processing, carbazole, organic electronics, grafting, surface initiated polymerization.

ACS Paragon Plus Environment

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Abstract Crosslinking of hole-transporting polymer thin films in organic light emitting diodes (OLEDs) has been shown to increase device efficacy when subsequent layers are deposited from solution. This improvement, due to resistance of the films to dissolution, could also be achieved by covalently grafting the polymer film to the substrate. Using nitroxide mediated polymerization (NMP), we synthesized a novel poly(vinylbenzylcarbazole) (poly(VBK)) copolymer which can be crosslinked and also developed a simple method for the grafting-to or grafting-from also known as surface-initiated polymerization of poly(VBK) to indium tin oxide (ITO) substrates. All three of these methods produced thin films that could be orthogonally processed, that is they resisted dissolution when the spin-coating of a subsequent layer was simulated. Similar electrochemical behaviour for the poly(VBK) films was observed regardless of the technique used, suggesting that all 3 techniques could be used in the engineering of organic electronic devices. We expect that all three methods would be worth investigating in the solution-based assembly of OLEDs and other organic electronic devices.


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1. Introduction Organic electronic devices made with carbon-based small molecules and polymers promise to be useful in a wide variety of applications including organic light emitting diodes (OLEDs),1 organic thin film transistors (OTFTs)2–4 and organic photovoltaic devices (OPVs)5,6. Some potential advantages of organic electronic technology over traditional inorganic electronic technology include decreased manufacturing costs, mechanical flexibility, and ease of chemical modification in producing application-specific devices. As long as the organic components are soluble, solution processing techniques such as inkjet printing or spin-coating can be implemented rather than vacuum-based technology which tends to involve expensive manufacturing equipment and inefficient material utilization.7 In both OLEDs and OPVs a hole transport layer (HTL) and another layer, either the emissive layer or donor/acceptor heterojunction layer, can be deposited by sequential solution processing. However, without special care the deposition of the second layer can dissolve or degrade the first layer. The most common approach used to overcome this challenge is to use a material for the first layer that is not soluble in the solvent used for the second layer. For example, poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) can be deposited from an aqueous solution prior to the deposition of another layer in organic solvent.7,8 This approach, often referred to as orthogonal processing,9,10 can limit the choice of material combinations as most organic materials are not universally soluble. Indeed, the two materials desired as components of adjacent layers may only dissolve in the same solvents. An alternative approach is to crosslink the first layer prior to depositing the second in order to rend it generally insoluble. This avoids the need to select specific material-solvent combinations (Figure 1). For example, Kim et al found that crosslinking the HTL improved OLED device performance over


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those that did not have crosslinked HTL layers.11 The authors’ approach uses ultraviolet light to activate azide (-N3) groups to crosslink rather than a thermal (high-temperature) approach which requires more time and energy. The advantage of this technique is simplicity, however it does require the copolymerization of a monomer with a crosslinking group, which may decrease the resulting polymer film’s hole-transport mobility.

Figure 1 – Visual representation of orthogonal processing of two organic thin layers, where a) depicts the use of orthogonal solvents, b) the use of crosslinking and c) the use of grafting techniques. A third approach to orthogonal processing is to employ surface tethering of the polymer layer prior to the deposition of the second layer (Figure 1). The covalent binding of a polymer to a surface can be accomplished by reacting its end group to the surface, also called grafting-to. Another route is to first bind the initiator to the surface and grow the polymer directly from it, known either as grafting-from or surface initiated polymerization (SIP).12 These chemical bonds theoretically protect the thin polymer layer from dissolution in further processing steps. Holeconducting films of poly(triphenylamine acrylate) (poly(TPA)) made with SIP demonstrated superior conduction to spin-cast films,13 which was attributed to the vertical orientation and high density of films produced by this technique. SIP has also been used to produce HTLs for


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functional OLEDs14 and OPVs.15,16 Advincula et al. found that OPVs produced with surfaceinitiated films of poly(vinyl carbazole) as HTLs were comparable to those using PEDOT:PSS and postulated that they would be much more stable as PEDOT:PSS is sensitive to water and oxygen.15 Nitroxide-mediated polymerization (NMP) is a technique used to synthesize well defined polymers with narrow molecular weight distributions and controlled architectures.17–19 NMP does not rely on transition metal catalysts and the initiators are often benign resulting in polymers that can be implemented in sensitive electronic applications or biological applications without overly involved purification techniques.18,20 Initially, NMP was restricted to the homopolymerization of styrenics due to the high activation temperature associated to using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO).21 The development of second generation initiators based on N-tertbutyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (SG1)22 or 2,2,5-trimethyl-4-phenyl-3-azahexane

nitroxide

(TIPNO)23

has

facilitated

the

homopolymerization of acrylates,24–26 acrylamides,27,28 and, with a small amount of controlling comonomer, methacrylates29–33. 9-(4-Vinylbenzyl)-9H-carbazole (VBK), is a hole transport monomer that can be homopolymerized or copolymerized using NMP.34,35 OLEDs have even been fabricated from the resulting polymers either as a hole transport layer or as a host for phosphorescent emitters, illustrating the potential for polymers synthesized using VBK.11,36 NMP has also been used effectively for both grafting-to and grafting-from techniques.12 Generally, the NMP initiator is functionalized with a chlorosilane or alkoxysilane group to bind it to the substrate, usually silica nanoparticles or silicon wafer.12 However, to the best of our knowledge neither grafting-to nor grafting-from with the use of NMP has never been implemented in organic electronics or with poly(VBK).


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This study investigates three methods to produce films of poly(VBK) that can be further solution processed without dissolution. First, we used NMP to produce a crosslinkable copolymer of poly(VBK) to spin-cast and crosslink on an indium tin oxide (ITO) substrate. Second, we synthesize poly(VBK) homopolymers and graft them to ITO. Finally, we explore the use of surface initiated NMP to grow polymers from the ITO surface. In each case we demonstrate the presence of the poly(VBK) film and their ability to retain their physical properties when further solution processing steps are simulated.

2. Results and Discussion A method for obtaining crosslinked polymer thin films is to incorporate an azide functional monomer which can undergo crosslinking via UV-light irradiation or heat.11 Kim et al. utilized TIPNO to synthesize a series of azide containing copolymers by first synthesizing chloromethyl styrene (CMS) containing copolymers followed by a halide substitution using sodium azide (NaN3).11 The authors also found that this approach was not detrimental to the OLED performance, which is necessary for the technology to be useful.

2.1 Synthesis of poly(9-(4-vinylbenzyl)-9H-carbazole)) (poly(VBK)) homopolymers and copolymers. 9-(4-Vinylbenzyl)-9H-carbazole) (VBK) is a styrenic based carbazole which has received a great deal of attention over the years. VBK homopolymers have been utilized as both hole transport layers and even host polymers for phosphorescent emitting dopants in OLEDs.11,36 The VBK monomer itself has also found application as a controlling comonomer for the synthesis of


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mostly methacrylic copolymers (>97 mol % methacrylate) by NMP.37–39 VBK itself has been successfully homopolymerized and copolymerized using BlocBuilder-MA and several kinetic and reaction engineering studies have shown similar kinetic behaviour to styrene.33,37 In this study however, we employed the use of NHS-BlocBuilder when synthesizing a series of poly(VBK) homopolymers and poly(CMS-ran-VBK) copolymers so that the resulting polymers are chain end-functionalized with a succinimide group (Table S1). In all cases the resulting polymers were characterized by having relatively narrow molecular weight distributions, Mw /Mn < 1.1-1.6 and number average molecular weight (Mn) ranging from 8 to 40 kg·mol-1 depending on the final conversion and the commoner composition (Table 1). We should note that the final CMS molar composition, FCMS is smaller than the initial molar feed of CMS, fCMS,0 for the CMS/VBK copolymerization (Table 1). These results, suggest a slight preferential addition of VBK relative to CMS and while the reactivity ratios are not known for this system, the trend is similar to other VBK copolymer systems such as when copolymerized with methyl methacrylate (MMA) or other methacrylates.33 Electrochemical characterization of P(VBK)-NHS with cyclic voltammetry is shown in Figure S1 in the supporting information and summarized in Table 3. Two reversible oxidation peaks with formal potentials of 1.03 V and 1.26 V, which are estimated by averaging the anodic and cathodic peak potentials, are comparable to earlier studies of poly(VBK).40 The highest occupied molecular orbital (HOMO) of 5.26 eV was therefore estimated using an internal ferrocene reference.41

2.2 Cross-linking of azide containing copolymers


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As previously mentioned, cross-linking the thin film can be an effective and straight forward route to obtaining insoluble films and where a second thin layer can be deposited orthogonally without washing away the first layer. In order to facilitate this cross-linking, the chlorine groups in CMS containing copolymers can be easily substituted with a cross-linkable azide group. The azide groups have been shown to react with any carbon bound to hydrogen upon UV illumination or moderate heat, releasing nitrogen gas. Therefore following the procedure set out by Kim et al.11, P(VBK-ran-CMS)-10-NHS (Table S1), the poly(VBK-ran-CMS) copolymers with 5 % CMS (Table 1), were azide functionalized through the simple halide substitution explained in the experimental section. Although we could not observe differences in IR at this low CMS composition, the azide treatment of P(VBK-ran-CMS)-48-NHS clearly produced a peak at 2093 cm-1, corresponding to the azide stretch, shown in the supporting information (Figure S2). By 1H NMR, the peaks corresponding to the protons on the ethyl group next to the chlorine group (4.0-4.5 ppm) shift downfield upon reaction with sodium azide (3.8-4.2 ppm) further suggesting a reaction is taking place (Figure S6). Therefore, we surmise P(VBK-ranCMS)-10-NHS underwent similar reaction (as demonstrated in the following section) but the signal was too weak to detect by FTIR. Thin films of P(VBK-ran-CMS)-10-NHS were dynamically spin cast from THF on ITO substrates and crosslinked with UV irradiation. 1H NMR scans of P(VBK-ran-CMS)-NHS with and without azide substitution are reported in the supporting information (Figure S6). The presence of the polymer on the thin films was confirmed by assessing the hydrophobicity of the surface through contact angle measurements. After spin coating, the substrates each had an average contact angle measurement between 83 ° and 87 °, an increase from the bare untreated substrate (~55 °) as well as the bare plasma treated substrate (~15 °).


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Both films were relatively smooth and between 45 and 70 nm in thickness, as determined from linear profilometry scans (Supporting information Figure S7). Figure 2 illustrates the progression in contact angle as additional spins were performed with pure THF. These extra washes simulate sequential layer deposition that should dissolve a soluble polymer film revealing the native, hydrophilic substrate.

Clearly, the contact angle of the non-UV exposed films

decreased substantially due to dissolution while the UV-illuminated films showed no substantial change. The same retention of contact angle was not seen for UV illuminated P(VBK)-NHS and unsubstituted P(VBK-ran-CMS)-48-NHS films (supporting information Figure S5), suggesting crosslinking was due to the azide substitution and not another mechanism. These results illustrate that the cross-linked P(VBK-ran-CMS)-10-NHS films are no longer THF soluble and that additional layers can safely be deposited atop this first layer without detriment. These results also emphasize that narrow molecular weight distributed, azide UV-cross-linkable copolymers can be synthesized using NHS-BlocBuilder. Electrochemical measurements on the poly(VBK) films are shown in Figure 3 and summarized in Table 3. Repeated cycles clearly show the signal from the uncross-linked film decreasing more rapidly than that of the crosslinked film, about 12.2 % decay per cycle compared to about 2.6 % decay per cycle, respectively. This change corroborates the relative stability of the latter as the former is more easily dissolved in solution, making it unlikely that OLEDs fabricated with the former would function properly if subsequently processed in a non-orthogonal solvent. The formal potential of the observed oxidations are roughly 1 V and 1.25 V for both thin films, which is similar to the polymer in solution. For the crosslinked polymer, the difference between the anodic and cathodic peaks (Table 3) is about 0.25 V, which corresponds to an increase in ~0.1 V to uncross-linked thin films, suggesting a lower reversibility potentially due to the disruption of conjugation in the film by the crosslinking


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groups. The HOMO of both films was about 5.1 eV, which is shallower than for solution measurements of dissolved poly(VBK), which we attribute to the film having more pi-overlap and is thus easier to oxidize.

Figure 2. Contact angle of UV-exposed (purple) and non-UV exposed (red) P(VBK-ran-CMS)10-NHS films as they are subjected to additional spins with pure THF. Circles and triangles represent films on plasma treated and untreated ITO substrates, respectively. Each cycle involves a dynamic spin coat washing with 50 µL of pure THF at 1500 rpm followed by at least 5 min of drying. Contact angle images are for the plasma treated substrates. Polymer molecular characteristics can be found in Table 1.


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Figure 3. A: Typical cyclic voltammetry scan data of UV-exposed (orange, circles) and non-UV exposed (blue, squares) P(VBK-ran-CMS)-10-NHS films deposited on ITO/glass substrates. 20 scans from -0.2 V to 1.8 V were used for each film at 0.1 V/s with no control for electrode area. B: Decay of the maximum current over each cycle for each peak for crosslinked (orange) and non-crosslinked (blue) films.

2.3 Polymer Grafting Polymers synthesized using NHS-BlocBuilder can also undergo post polymerization reactions. For example, the succinimide group can react under mild conditions with amines resulting in the tethering of the resulting chains to a surface or to another polymer. Charleux et al. utilized NHSBlocBuilder to tether polymers to silica particles with good control over the grafting density and thickness.42,43 Nicholas and coworkers have reported the use of NHS-BlocBuilder for the pegelation of proteins through coupling of the succinimide group.44 Vinas et al. also synthesized block copolymers by coupling amino functional polymers to poly(styrene) chains which have been synthesized by NHS-BlocBuilder.45


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Scheme 1. Chemical scheme used to obtain poly(VBK) covered surfaces through grafting-to (top) or grafting from (bottom) techniques using 3-aminopropyltrimethoxysilane (APTMS). Typical reactions were i) 115 °C, 6 h, ii) room temperature, 24 h, iii) 50 °C, 1.5h, 0.005 mol/L in toluene, iv) 110 °C, 20 h, saturated in xylene v) 70 °C, 2 h, 2 mg/mL in toluene.

Grafting polymers to a surface, through chemical bonding, prevents film disruption during sequential processing steps. Aminopropyltrimethoxysilane (APTMS, Scheme 1) can be used to tether polymers to ITO: the amino group in the APTMS can easily couple to succinimide group present in the NHS group at the polymer chain end while the silane group of the APTMS can react with the hydroxyl groups present on the ITO surface. We explored two methods to graft poly(VBK) and poly(styrene) to ITO. In the first method, we attempted to first bind APTMS to the surface, and then react the NHS-functional poly(VBK) onto the APTMS-modified surface. In the second method we first bound APTMS to the polymer, then reacted the APTMS-functional


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polymer onto the surface. The APTMS treatment of ITO in the first route causes an increase in contact angle of the substrate from about 12 ° to about 54 ° and XPS data confirms the presence of silicon and nitrogen on the surface. This would suggest that APTMS did bind to the surface. The subsequent treatment of the APTMS-treated substrates with a polymer solution does increase the contact angle from about 54 ° to about 75 °, but not to the level of the spin-cast polymer (~94 °) which can be used for reference surface characteristics. The elemental composition both before and after polymer treatment is similar, with a C:Si ratio of 4-5 to 1. As the attachment of hydrophobic and carbon-dense polymers (PS or poly(VBK)) would increase the contact angle and carbon concentration on the surface relative to silicon, this indicates that the polymer chains were not reacting to a large extent with the surface. This was corroborated by a lack of oxidation peaks observed during cyclic voltammetry of the films. Elemental composition and contact angle information is displayed for comparison in Table 2. The second route requires the modification of pre-synthesized polymer (PS-NHS, Mn = 9.3 kg/mol and P(VBK)-NHS, Mn= 13.1 kg/mol, Table 1) with APTMS prior to reaction. The success of this reaction, between the chain end succinimide group and the amino group of APTMS, was evaluated using HNMR-spectroscopy on carefully dried samples. Post-reaction, an increase in the δ = 3.47 ppm peak can be attributed to the addition of methoxy groups from APTMS (Figure S3, supporting information). Two peaks between δ=2.87 ppm and δ=3.00 ppm decrease, which can be attributed to the loss of 4 succinimide protons, replaced by APTMS and washed away in methanol during precipitation. Using peak integration, we found the final polymer has approximately 50% of its end-groups silane functionalized. The apparent success of the subsequent surface reaction corroborates the silane functionalization of the polymers. Table 2 compares the elemental composition as determined by XPS and contact angle of PS-APTMS and


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P(VBK)-APTMS grafted to plasma treated ITO to spin-cast films of the PS-NHS and P(VBK)NHS. Characteristic XPS spectra for plasma treated ITO, PS-APTMS grafted ITO and P(VBK)APTMS grafted ITO can be found in the supporting information (Figure S10-12). The contact angle and elemental composition for PS that is spin-cast or grafted is similar, suggesting that in both PS is present in a continuous film. For P(VBK)-NHS, the contact angle is about 10 ° lower, although that difference disappears after leaving the grafted substrates out for a day or washing them with THF. The elemental composition differs, although there is still around 70 % carbon, much higher than a bare substrate, and a C:Si ratio of 22:1, much higher than in APTMS-treated substrates. As estimated from Figure S8 in the supporting information, the film’s thickness is around 50-60 nm but inconsistent, with much thinner areas exposed. This explains why the ITO substrate can still be detected by surface XPS measurements which typically have a penetrating depth of 10-15 nm and would occasionally see the surface. Further verification of the presence of the film was obtained using electrochemistry (Figure 4), with the characteristic oxidation formal potentials of poly(VBK) estimated on grafted films as around 1 V and 1.25 V (Table 3). The maximum current decayed at approximately 3.7 % per cycle, slightly higher than the crosslinked films, but still much lower than the spin-cast films. Overall, these findings indicate that the grafting-to approach using PS-NHS and P(VBK)-NHS was successful when first coupling the APTMS to the polymer followed by tethering to the surface.


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Figure 4. Typical cyclic voltammetry scan data of poly(VBK) grafted-to and grafted-from films compared to blank ITO (green). Scans from -0.2V to 1.8V were used for each film at 0.1 V/s with no control for electrode area.

Given the limited diffusion of large polymers, the formation of a dense polymer film could be kinetically limited. Figure S4 in the supporting information shows the contact angle of samples removed from a 2 mg/mL APTMS functionalized poly(VBK) solution in DMF at various time points. The contact angle increases to above 80 ° after only 10 min and appears to level after only about 60 min of treatment. After roughly 48 h removed from treatment the contact angle of these samples increases further to become comparable to spin coated poly(VBK), about 90-95 °. This suggests film formation is relatively fast, an advantage for any manufacturing process.


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To demonstrate orthogonal processing in the grafted films we washed the substrates by spin coating THF dynamically on both grafted and spin-coated poly(VBK) substrates to mimic another step in the manufacture of an organic electronic device. Figure 5 shows how the contact angle of the spin-coated film decreases quickly towards the bare substrate value (around 55 °) as it is soluble in THF and washes away. The grafted film, on the other hand, shows no decrease in contact angle throughout the cycles. The grafted samples even appear to experience an increase in contact angle which we believe could be due to a structural rearrangement of the grafted chains, however this has not been investigated. Most importantly, the grafted film does not wash away as it is chemically bound to the ITO surface, demonstrating orthogonal processability.

Figure 5. Contact angle of grafted (blue), surface-initiated (orange) and spin-coated (red) poly(VBK) films as they are subjected to additional spin coats with pure THF. Each cycle involves a dynamic spin coat washing of 50 µL of THF at 1500 rpm followed by at least 5 min of drying.

2.4 Surface Initiated Polymerization


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The major advantage of surface initiated polymerization (grafting-from) over grafting-to is the potential for higher polymer chain density, as only monomers must diffuse to the surface to react as opposed to entire polymer chains, which in solution are large coils.12 APTMS and a slight molar excess of NHS-BlocBuilder were mixed in attempt to get a silane modified initiator (bottom left, Scheme 1) and used in-situ. Treatment of plasma cleaned ITO with a toluene solution of the mixture increased the contact angle from ~15 ° to ~75 ° after 90 min of treatment, as shown in Table 2. This increase could simply be the attachment of unreacted APTMS or of the silane-modified initiator. However, 24-hour incubation at 110 ⁰C with a nitrogen-purged styrene-xylene mixture produced films with a ~90 ° contact angle and a carbon content measured using XPS as high as 91 %, much higher than prior to polymerization (26 %). This is a strong indication that polymerization from the substrate did occur. Incubation with saturated VBK solutions produced a contact angle of ~85 °, similar to grafted poly(VBK). Carbon concentration of these films was about 70 %, with indium and oxygen from the substrate showing through. As can be seen in Figure S9 in the supporting information, the smooth substrate transitions into an extremely rough film of inconsistent thickness. Despite this inconsistency, the polymer brush demonstrates similar resistance to dissolution (Figure 5) and electrochemical characteristics (Figure 4) as the grafted-to film. As seen in Table 3, the formal reduction potentials for the film were approximation 1 V and 1.25 V, consistent with poly(VBK), but the maximum current decayed faster (12.9 % per cycle) than the grafted-to film and crosslinked film, but slower than spin-cast poly(VBK) (16.4 % per cycle). Compared to the grafted-to film, the current above the apparent baseline was relatively low, indicating either an even thinner film, an undesirable higher surface coverage of APTMS, or both. Optimizing the reaction parameters, including the use of additional free initiator, might densify the poly(VBK) brush, but there is an innate


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limitation to the surface-initiated polymerization of VBK; NMP-based SIP occurs extremely slowly below 25 wt. % monomer concentration,46 and VBK has generally low solubility and may suffer from slow surface polymerization rates due to the effect of its high monomer weight on diffusion. We are currently exploring the use of more soluble monomers for future SIP in OLEDs and other organic electronic devices.

Experimental: Materials Styrene

(>

90

%),

4-(chloromethyl)styrene

(CMS,

>

90

%)

and

(3-

aminopropyl)trimethoxysilane (APTMS, > 97 %) were obtained from Sigma-Aldrich while sodium azide (NaN3, > 99 %) was obtained from Oakwood Chemicals. N,N-dimethyl formamyde (DMF, > 99.8 %) toluene and tetrahydrofuran (THF, > 99.9 %) were obtained from Fisher Scientific, while xylenes (> 98.5 %) was obtained from Anachemica Canada. BlocBuilder-MA™ initiator, graciously donated by Prof. Marc Dubé (Department of Chemical and Biological Engineering, University of Ottawa, originally obtained from Arkema). NHSBlocBuilder was synthesized from BlocBuilder-MA according to the literature.45 9-(4vinylbenzyl)-9H-carbazole) (VBK) was synthesized according to the literature.37


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VBK – CMS copolymerizations. The VBK homopolymerization and the VBK / CMS copolymerization were adapted from Lessard et al37. In a 50 ml 3-neck round bottom flask the monomer and initiator solution was bubbled using nitrogen for 15 min prior to heating. The experimental formulations can be found in Table S1 (supporting information) while the resulting polymers molecular characteristics can be found in Table 1. For example, the synthesis of P(VBK-ran-CMS)-48-NHS was performed by heating a solution of VBK (0.552 mol·L-1), CMS (0.516 mol·L-1), NHS-BlocBuilder (0.007 mol·L-1) and DMF (11.977 mol·L-1) at 115 °C, while stirring under a head of nitrogen. Conversion was monitored 1H NMR spectroscopy and the final product was precipitated in methanol, filtered and dried in a vacuum oven until constant weight was obtained (5 h to 24 h depending on sample size). P(VBK-ran-CMS)-48-NHS, was characterized as having a yield of 68%, (GPC, dn/dc

PS, THF, 25 °C

= 0.187) Mn = 34.6 kg mol-1 ,

Mw/Mn = 1.13 and molar composition, FCMS = 0.18, determined by 1H NMR comparing the ethyl groups of VBK: (Ar, 2H: 7.7-8.2 ppm, 6H: 6.9-7.5 ppm, 4H: 5.8-6.6 ppm, ethyl group at 2H: 4.8-5.2 ppm) and CMS: (Ar, 4H: 6.9-7.5 ppm and ethyl group at 2H: 4.0-4.5 ppm).

Azide substitution. As described by Park et al.,11 the synthesized chlorine-substituted polymer samples were dissolved in 10 mL of DMF. 1.2 molar equivalents of NaN3 compared to molar amount of –Cl substituents per polymer were dissolved in 5 mL of DMF. The two solutions were combined and the reaction mixture was left to stir vigorously for 15 h at room temperature. The final product was precipitated in methanol, filtered and dried in a vacuum oven until constant weight was obtained. Special care was employed to shield the sample from light and to avoid temperatures in excess of 40 °C


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Characterizations. NMR spectra were recorded using the Bruker© Avance 400 MHz and Bruker© Avance 300 MHz NMR spectrometers, in CDCl3 solvent with TMS (Cambridge Isotope Laboratories, 99.96 % purity) as the internal standard. Fourier transform infrared (FTIR) spectra were collected using Agilent Technologies© ATR-FTIR spectrometer. Spectra were measured from powder samples, therefore no solvent was used. Gel permeation chromatography (GPC) results were obtained from the Wyatt Optilab T-rEX©refractometer and were recorded and analyzed using Wyatt Astra 6.1© software. Using THF (1.0 mL/min / HPLC grade) two columns in series (MZ-Gel SD plus 105 Å 300 × 8.00 mm, particle size 5 µm, MZ-Gel SD plus 104 Å 300 × 8.00 mm, particle size 5 µm) were used in-line with a laser light scattering detector (Wyatt Dawn Heleos II, λ = 633 nm), a differential refractive index detector (Wyatt Optilab TrEX), and a differential viscometer (Wyatt ViscoStar ll). The refractive index increment (dn/dc) used was that for poly(styrene) (in THF, 25 °C) as 0.187 mL g−1. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Kratos Axis Ultra DLD instrument, using a monochromated Al X-ray source at 140 watts. Survey scans were obtained at pass energy 80 with the charge neutralizer active. Contact angle measurements were obtained using a VCA Optima goniometer produced by AST Products Inc. Deionized water droplets of 0.5 µL were deposited statically from a needle and imaged directly after using a 3-point curve fitting, while ignoring the shadow of the droplet on the reflective ITO surface. Cyclic voltammetry was performed on polymer solutions and films using a Versastat 3 by Ametek running at 0.1 V/s. The electrolyte used was 34.2 mg/mL tetrabutylammonium perchlorate in dichloromethane, with a platinum wire counterelectrode and an Ag/AgCl reference electrode. A platinum reference electrode was used for polymer solutions and the ITO substrate was used for the films. ITO electrode area was not controlled, meaning absolute current magnitudes are not comparable


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between samples. Spin-cast films were prepared for profilometry by wiping off some of the film with a kimwipe doused in methanol and by scratching a razor blade over the film. Grafted-to films were prepared with the razor blade method, while grafted-from films were prepared as to partially cover a substrate and the edge was analyzed. The Bruker Dektak Surface Profiler was used to scan the surface over 30 seconds with a stylus force of 3 mg, a vertical range of 6.5 µm and scan distance of 2-6 mm. Cleaning of ITO Substrates: ITO substrates obtained from Thin Film Devices Limited were cleaned by rinsing for 5 min in each of soapy water, distilled water, acetone, and methanol. The substrates were subjected to a UV-ozone treatment for 2 min in order to remove excess carbon and hydrolyze the surface. Spin coating of azide-treated CMS-VBK copolymer: Four ITO substrates were cleaned in an ultrasonic bath of soapy water, distilled water, acetone and isopropanol before being dried with compressed air, and two were subjected to 2 min of UV-ozone to further clean and hydrolyze the surfaces. An azide substituted 10 mg/mL P(VBK-ran-CMS)-10-NHS solution, covered in foil to prevent premature crosslinking, was filtered with a 0.2 µm PTFE membrane filter and 50 µL was deposited onto each substrate, already spinning at 1500 rpm. Two substrates, one plasma-treated and one not, were both placed under UV illumination (365nm, 11m W/cm2) for 5 min to crosslink the polymer. The other two substrates were covered to prevent crosslinking. Sequential treatment of ITO with APTMS and polymer: Clean ITO substrates were treated with a 1 vol % solution of APTMS in toluene at 70 °C for 45 min. These substrates were removed, rinsed with toluene, then placed in a 5 mg/mL DMF solution of polymer (PS-NHS or P(VBK)NHS) and left at room temperature. After 24 h, they were removed, cleaned with DMF and isopropanol, and then dried with compressed air.


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Silane functionalization of poly(VBK): 200 mg of P(VBK)-NHS was dissolved in 2 mL of THF with 50 µL of APTMS and left stirring at room temperature for 24 h. The resulting polymer, P(VBK)-APTMS was precipitated in 30 mL of stirred methanol, recovered, then dried in a vacuum oven overnight. Silane functionalization of PS: 200 mg of PS-NHS was dissolved in 5 mL of toluene and left stirring at room temperature for 24 h. The resulting polymer, PS-APTMS was precipitated in 50 mL of stirred methanol, recovered, then dried in a vacuum oven overnight. Grafting of polymers to indium tin oxide (ITO) substrates: A 2 mg/mL solution of P(VBK)APTMS in DMF or PS-APTMS in toluene was filtered with a 0.2 µm PTFE membrane and placed in a sealed beaker with the cleaned ITO substrates and heated to 70 °C for various lengths of time. When removed, samples were first rinsed with DMF/toluene and then isopropanol before drying with compressed air and subsequent characterization. Surface initiated polymerization of polymers on ITO: 100 mg of NHS-BlocBuilder was mixed with 32 µL of APTMS in 3 mL of toluene and left to react overnight then stored in the fridge. 80 µL of this solution was added to 920 µL of toluene in a closed scintillation vial with a clean ITO substrate and heated for 90 min at 50 °C. The substrate was then removed, rinsed in toluene and placed in a new vial. For poly(styrene), 250 µL of styrene and 750 µL of xylene were added to the vial, and it was purged with nitrogen using a needle and a balloon for roughly 5-10 min. For poly(VBK), 300 mg of VBK was mixed with 600 µL of pre-purged xylene. The sealed vial was heated at 110 °C for 24 h before the substrate was removed and cleaned with toluene and isopropanol before drying with compressed air.


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Spin coating of poly(styrene) and poly(VBK) controls: 10 mg/mL solutions of PS-NHS in toluene and P(VBK)-NHS in THF were prepared. Each solution was filtered with a 0.2 µm PTFE membrane filter and 50 µL was deposited onto each substrate, already spinning at 1500 rpm. These samples were left spinning for about 60 seconds then left to dry for at least 5 min.

Conclusion: We demonstrated the synthesis of a crosslinking poly(9-(4-vinylbenzyl)-9H-carbazole-ran-azidemethylstyrene)) (poly(VBK-ran-N3MS))) copolymer and produced crosslinked thin films that were resistant to solvent dissolution. While synthesizing these polymers we identified that the CMS can react with the initiator and therefore the use of NHS-BlocBuilder is required to avoid this potential side reaction. We also demonstrated the use of grafting-to and surface initiated polymerization (grafting-from), two methods for producing polymer thin films that are covalently linked to the substrate. These tethered films were also found to be resistant to solvent washings suggesting another route to obtaining orthogonally processable thin films. Finally, cyclic voltammetry was performed on the resulting polymer thin films and in all cases the electrochemical behaviour of poly(VBK) was preserved. A slight variation in relative electrochemical peak intensity stability and separation was observed between films made with the different techniques. These observations suggest that the formation method for poly(VBK) films plays a role in their electrochemical behaviour, a potential factor in OLED performance. These examples illustrate three unique routes which can be used to perform sequential depositions of thin films. In the fabrication of OPVs or OLEDs any of these methods could be used to fabricate a hole-transport layer or electron blocking layer between the ITO anode and the active layer, which would not be dissolved upon the addition of the active layer using spin-


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coating or another solution-based technique. In addition to organic electronics, these methods could be applied to any manufacturing of thin films, such as coatings for scratch resistant paints or electrode modification for improved batteries and fuel cell operation.

Acknowledgements: We thank Natural Science Engineering Research Council of Canada (NSERC) for the Discovery Grant. Thank you to Dr. Sander Mommers at the Centre for Catalysis Research and Innovation (CCRI) for performing X-Ray Photoelectron Spectroscopy measurements. We also thank Prof. Marc Dube (Chemical and Biological Engineering) for the use of the GPC.

Supporting Information. Electrochemical data for poly(VBK) formulations in solution. FTIR of azide substitution reaction. 1H-NMR data for P(VBK)-NHS and P(VBK)-APTMS. XPS and contact angle over time chart for grafting P(VBK)-APTMS. This supporting information is available free of charge via the Internet at http://pubs.acs.org/.


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Tables Table 1. Homopolymer and copolymer molecular weight characteristics. Experiment IDa)

fCMS,0b)

Mn c)

PDI c)

FCMS b)

(Kg·mol-1)

Final Yield ( %)

PS-NHS

0

9.3

1.10

0

48

P(VBK)-NHS

0

13.1

1.37

0

65

P(VBK-ran-CMS)-48-NHS

0.48

34.6

1.13

0.18

63


0.24

40.6

1.43

0.16

67


0.10

15.4

1.56

0.05

48

a) Experimental Identification is denoted by P(X)-NHS or P(VBK-ran-CMS)-Y-NHS where X is either S = styrene or VBK = 9-(4-vinylbenzyl)-9H-carbazole, CMS = chloromethyl styrene, Y is the feed weight ratio of CMS, and NHS is the type of initiator used: NHS = NHS-BlocBuilder. Experimental conditions can be found in the supporting information. b) fCMS,01 = initial molar feed ratio of chloromethyl styrene relative to VBK FCMS 1 = final molar ratio of chloromethyl styrene relative to VBK were determined by 1H NMR spectroscopy. c) Number average molecular weight (Mn) and the dispersity (Mw /Mn) were determined by gel permeation chromatography (GPC), run at 40 °C in THF.

Table 2. Contact Angle and XPS elemental composition of various samples on ITO substrates. The conditions used to obtain these values are described in the experimental section. Sample

Contact Angle ( °)

Elemental Composition (XPS)

± Standard Deviation


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Plasma treated

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12±5

26 %C, 55 %O, 16 %In, 2 %Sn

54±6

55 %C, 21 %O, 14 %Si, 9 %N

77±1

45 %C, 35 %O, 7 %N,

ITO APTMS treated ITO Silane-modified initiator on ITO

6 %Si, 5 %In, 1 %Sn Poly(styrene)a)

poly(VBK) a)

Poly(styrene) a)

poly(VBK) a)

85±3

75±3

66 %C, 16 %O,

59 %C, 21 %O,

APTMS-ITO

14 %Si, 7 %N

12 %Si, 8 %N

Silane-modified

94 %C, 3 %O, 1

73 %C, 15 %O,

%In,N,Si

5 %In, 3 %Si, 3

Polymer treated

polymer on ITO

97±3

85±2

%N, 1 %Sn SIP Polymer

90±2

85±2

from ITO

91 %C, 6 %O, 2

68 %C, 20 %O,

%In, 1 %Si

5 %N, 4 %Si, 3 %In,

Orthogonally Processable Carbazole-Based ... - ACS Publications

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