Improved Electrical Conductivity via Polymer Addition - American

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Inverse Nanocomposites Based on Indium Tin Oxide for Display Applications: Improved Electrical Conductivity via Polymer Addition Ron Hoffmann, Valentin Baric, Hendrik Naatz, Sven o Schopf, Lutz Mädler, and Andreas Hartwig ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00191 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Nano Materials

Inverse Nanocomposites Based on Indium Tin Oxide for Display Applications: Improved Electrical Conductivity via Polymer Addition Ron Hoffmann,†‡ Valentin Baric, Hendrik Naatz, Sven O. Schopf, Lutz Mädler, Andreas Hartwig†‡* †

Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM), Wiener

Str. 12, 28359 Bremen, Germany ‡

University of Bremen, Department 2 Biology / Chemistry, Leobener Str. 3, 28359 Bremen,

Germany 

Leibniz Institute for Materials Engineering IWT, Badgasteiner Str. 3, 28359 Bremen, Germany



University of Bremen, Faculty of Production Engineering, Badgasteiner Str. 1, 28359 Bremen,

Germany

KEYWORDS Indium tin oxide (ITO), nanoparticle composite, inverse nanocomposite, flame-spray pyrolysis, electrical conductivity

*Corresponding

author e-mail (Andreas Hartwig): [email protected]

ACS Paragon Plus Environment

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ABSTRACT A new method for the preparation of electrically conductive nanoparticle-polymer-composite (NPC) films is shown in this work. These films are intended to be applied in displays. Thin layers of percolating nanoparticles in an organic polymer matrix are presented. First, a nanoparticle scaffold with high porosity is prepared. Subsequently, it is infiltrated with a monomer, which is finally polymerized. The formed composite is called inverse nanocomposite as the continuous percolating nanoparticle scaffold is formed first and mainly preserved during the whole process. Indium tin oxide (ITO) nanoparticles obtained from flame spray pyrolysis (FSP) are laminated onto a substrate. These porous scaffolds are infiltrated using liquid 1,6-hexanediol diacrylate (HDDA) as monomer. Restructuration of the particle network during the liquid imbibition caused by capillary forces leads to an increased electrical conductivity upon addition of the insulating organic monomer. A further and even stronger increase in the electrical conductivity was achieved after UV-curing of the HDDA filled nanoparticle films, which is explained by the shrinkage forces of the organic phase during polymerization. With this new method, electrically conductive thin films for opto-electronical applications with almost the conductivity of pure ITO coatings can be produced.


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INTRODUCTION Substituting bulk phase functional materials with mesoporous media increases resource efficiency and facilitates tuning the material’s physical and chemical properties by adjusting the porosity and internal structure. The reduced density of porous materials allows new developments in light-weight applications or enhanced composite materials by filling pores in a particle network with a second phase to be used as functional coatings and flexible electronics.1 Emergent capabilities of functional materials, e.g. enhanced strength, stability, electrical and thermal conductivity, reactivity or transmissivity can be achieved with multicomponent materials as well as functionalizing the interphase layer between both materials as in solar cells or sensors.2 By tuning the pores’ filling material, the continuous particle phase properties and morphology, new composite applications can be addressed, e.g. organic light emitting diodes.3 Herein, inorganic nanoparticles are used as porous particle scaffold serving as mesoporous phase. A common example for a metal oxide applied in a nanoparticle-polymer composite (NPC) is In1.9Sn0.1O3 (ITO), an optically transparent semi-conducting metal oxide applied in optoelectronics and solar cells. Conventionally, ITO layers for technical applications are produced in a sol-gel approach

4–7

or via sputter techniques.8,9 Alternatively, they might be applied as NPC

coating from liquid phase. To form a conductive NPC, these ITO nanoparticles are embedded into a polymer matrix, leading to highly filled NPC layers. Commonly, the particles are dispersed together with a thermoplastic polymer or a reactive binder in solution and subsequently casted or spin-coated on a support substrate forming a composite via solvent evaporation or polymerization of the binder.1,3,10–13 Different methods from mechanical engineering such as compression molding have been investigated also14,15 as well as layer-by-layer approaches.16–18


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Using vapor deposition polymerization,19,20 or annealing in a thermoset matrix

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21

are further

approaches to form these NPCs. All mentioned techniques involve particle dispersion with solvents, monomers or polymer melts resulting in an unspecified distribution of nanoparticles in the polymer or binder matrix. Often, a percolating particle structure can only be provided with a high filling level of particles.22 However, this results in a strong increase in viscosity and hence, a poor processability and, reduces the optical transparency as well.23 To overcome this limitation, several approaches of inverse preparation of such NPCs have been developed, where the nanoparticle network is formed in a first step and a capillary force driven imbibition process is used to synthesize the NPC.20,24–29 Here, capillary forces are used to infiltrate melted thermoplastics as shown by Huang et al.30 as well as grafting the particles with a sol-gel method and subsequent imbibition with a second monomer.13 Also in-situ reactivity of infiltrated particle layers with on-demand polymerization inside of meso-scaled cavities31 is a possible approach towards highly percolating NPCs with a low degree of particle filling. Preparation of the porous material can be achieved using flame spray pyrolysis (FSP) to form nanoparticles in an oxidative combustion.32 Even though flame aerosol synthesis has achieved a broad variety of industrial applications,17,33–35 percolating NPCs based on unmodified FSP generated particles are still emerging. In the case of optical and electrical applications, NPCs are the topic of current research.36 These NPCs can be versatilely tailored through the choice of particles and polymer matrix due to the combination of polymer functionalities with the unique nanoparticle properties.37 FSP particles based on indium tin oxide with the composition In1.9Sn0.1O3 are optically transparent semi-conducting metal oxide particles feasible for use in opto-electronics and solar cells.38–43 The use of FSP-generated ITO nanoparticles as a porous


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scaffold in opto-electronic devices like display units facilitates a reduction of the expensive material needed to reach a high conductivity combined with high transparency when transformed into a nanocomposite. Compared to conventional fabrication techniques like sputtering and dispersing particles in a polymer matrix to form ITO layers, the use of a nanoparticle scaffold improves the mass efficiency and makes the electrical device lighter. In the presented work, inverse nanocomposites are prepared. FSP is used to synthesize ITO nanoparticles, which are deposited on a collector (glass fiber filter) forming a highly porous filter cake layer. Afterwards this layer is transferred via a layer-to-layer lamination method to a suitable carrier substrate.44 The subsequent imbibition with a low viscous monomer shall mainly conserve the initial particle network scaffold, i.e. preserve percolation and yet existing particleparticle-contacts. Thin films of NPC are formed by UV-curing of the infiltrated acrylic monomer. Imbibition and the following polymerization using photo-latent initiators activated by UV-light resulted in a very fast and efficient thin-film-preparation route for inversely prepared NPCs. The rather low viscosity and surface tension of 1,6-hexanediol diacrylate (HDDA) combined with a fast reactivity and high cross-linking density due to double functionalization turn out to be very suitable for inverse thin-film composite preparation. The influence of the imbibition process as well as the curing on the composite’s morphology and network restructuration were investigated. Enhanced electrical conductivity was found after filling and curing. This work has the aim to present a fast and controllable technique for the inverse preparation of NPCs with a high mass ratio of polymer under permanent preservation of the nanoparticle percolation.


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EXPERIMENTAL SECTION Materials. Precursor materials were indium(III) acetylacetonate (98 % pure, Strem Chemicals, Germany) and tin(II) 2-ethylhexanoate (95 % pure, Sigma-Aldrich, Germany). As solvent, toluene (VWR Chemicals, Germany) and xylene (VWR Chemicals, Germany) were used. Infiltration was performed using industrial grade 1,6-hexanediol diacrylate (Laromer® HDDA, BASF,

Germany)

as

monomer

and

bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide

(Omnicure 819, IGM Resins, Netherlands) as photo-initiator. Adhesive was isotropically conductive PC3002 (epoxy with 70 wt.-% silver, Heraeus Electronics, Germany). Preparation. Particle synthesis. The ITO particles were synthesized using flame spray pyrolysis and collected on a glass fiber filter (Pall, Type A/E, 247 mm diameter) 60 cm above the nozzle. The precursor solutions were composed of 0.1 M indium(III) acetylacetonate dissolved in toluene and 0.1 M tin(II) 2-ethylhexanoate dissolved in xylene. Both solutions were mixed in a ratio of 1:19, fed to a nozzle with a flow rate of 5 mL/min and then dispersed with an O2 gas flow of 5 L/min. The spray was ignited with a support flame of CH4 (1.5 L/min) and O2 (3.2 L/min). Details of the process and setup were published previously.42 Electrode preparation. Following the synthesis, the particles were transferred from the glass fiber filter via low pressure roll-to-roll lamination (Hot Roll Laminator HL-101, Cheminstruments, Italy) to a glass substrate (Microscopic Glass Slides, VWR, Germany) with a 26 x 75 mm area via a lamination process according to Schopf et al.44 The nanoparticle layers were placed on top of microscopic glass slides compressed during lamination at a pressure of 3.4 MPa. The upper roll was made from chrome-plated steel and the drive roll was covered with 80-durometer silicone rubber. The lamination speed was set to a velocity of 7.4 mm s-1. The


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laboratory environment was at standard ambient temperature and pressure. After lamination, the remaining filter fibers from the glass filter were removed with pressurized air. The obtained ITO layers were dried at 120 °C in a vacuum furnace for 12 hours below 50 mbar to remove contaminations. Copper plates 25 x 25 mm in size serving as electrical connection were attached to the pure particle layer using 100 – 300 mg of the PC3002 silver filled adhesive. The adhesive joints were fixed with 500 g metal blocks and cured at 160 °C for 120 min in an air-circulated furnace. The layers were stored for 72 hours at standard atmosphere (21.5 °C, 44 % rel. humidity) for equilibration. For determination of the contact resistance, glass slides were sputtered with chromium and gold and similarly connected to copper plates with silver paste to measure the contact resistance of the joint between plate and sample. Filling of Pores. Liquid HDDA was mixed with 1 wt.-% of the radical photo-initiator to achieve photo-curing. After initial electrical testing (see below) of the particle electrodes, monomer with initiator was applied onto the particle layer using spray technique for homogenous and fast surface coverage. The monomer was applied with a spray nebulizer (spray bottles, pump of polypropylene, Carl Roth, Germany) perpendicular to the layer. After 5 min imbibition time, the electric conductivity was measured again. Photocuring. Curing was conducted in an UV system (UVACUBE Inert with UVAprint 100 HPV lamp, Hönle Group, Germany) under inert CO2 atmosphere (p*(O2) < 1.5 %) for 120 s at full lamp power and ambient temperature at a source distance of 150 mm. The composite films were stored for 12 hours at standard atmosphere (21.5 °C, 44 % rel. humidity) for equilibration prior to analysis. A particle-free HDDA film on glass substrate was prepared as well using dip coating and the same curing conditions to estimate resistance and transparency of the pure polymer.


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Characterization. Electron microscopy. Scanning electron microscopy imaging (SEM) was performed with a Leo1530 (Gemini, Zeiss, Germany) with an acceleration voltage of 5 kV after pre-sputtering of the samples using a platinum palladium layer to prevent electrical charging. Break edges were freshly created by cracking the glass support slide immediately before examination. Highresolution transmission electron microscopy (TEM) images were recorded using a transmission electron microscope (TEM, FEI Titan 80/300) operated with an acceleration voltage of 300 kV. BET/BJH. The specific surface area of ITO powder was obtained from 5-point nitrogen adsorption isotherms NOVA-4000 (Quantachrome) using the Brunauer-Emmet-Teller (BET) method45 at p/p0 from 0.1 to 0.3. The pore structure of the ITO layers was calculated from nitrogen desorption isotherms46 using the Barret-Joyner-Halender (BJH)47 method with the tcurve method48 to calculate the adsorption film thickness. Two segments of the particles deposited on the filter were laminated onto each other. This particle-filter combination was given to an Autosorb-1 (Quantachrome) for 24 h degassing in vacuum at 200 °C prior to recording a nitrogen adsorption and desorption isotherm (p/p0 from 0.025 to 0.995). The layer porosity was obtained from the total adsorbed nitrogen volume. All data were corrected to account for the surface area from the filter fibres. Thermomechanical testing. Dynamic mechanical analysis (DMA) of thin films was performed with a DMA 2980 Dynamic Mechanical Analyzer (TA-Instruments, New Castle, USA) with clamps in tension geometry. The films for DMA were prepared via particle lamination on polyvinyl alcohol foil (PVAL A200, MonoSol, Germany) and subsequent imbibition and curing. The support PVAL foil was then dissolved in deionized water for 24 hours. After washing, the composite film was dried and cut into proper sample dimensions.


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Sample size was 4.885 · 3.57 · 0.354 mm3 for the neat polymer and 8.301 · 6.42 · 0.031 mm3 for the composite, respectively. The oscillation frequency was 1 Hz and the heating rate 2 K/min up to 200 °C starting from ambient temperature. The response curves of moduli and loss angle were smoothened over 150 data points for better visibility of the curve shape. Glass transition temperatures were taken from the maxima of loss modulus TG''trans and loss angle Ttan(δ). The presented errors are derived from the repetition of the polymer film testing. Density measurement. The density of the bulk polymer was determined with a hydrostatic balance (XS3065 DeltaRange®, Mettler Toledo, USA) with a density-determination kit. The Archimedes principle was applied by weighing in ambient air and deionized water at ambient temperature. Thermal analysis. Thermogravimetric analysis (TGA) was performed with a thermal analyzer (Q5000, TA Instruments, New Castle, USA) from ambient temperature to 800 °C. Neat polymer and composite were filled into platinum crucibles and heated under ambient air flow as well as nitrogen atmosphere with a heating rate of 5 K/min. Evaluation was performed using the tangential method as described in DIN EN ISO 11358-1:2014. Differential scanning calorimetry. DSC was recorded with a Discovery DSC (TA Instruments, New Castle, USA) using standard non-sealed aluminum pans filled with either polymer or composite. Temperature profile started at -50 °C with 10 K/min heating up to 250 °C. For thermal equilibration, presented data corresponds to second heating cycle after cooling with the same rate as for the heating. Spectroscopic analysis. FT-Raman-spectroscopy was performed with milled samples of the NPC film in aluminum pans and liquid HDDA without photo-initiator in a quartz vial exposed to a VERTEX 70 FT-IR spectrometer coupled to a RAM II FT-Raman module (Bruker Optik,


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Germany) with 150 mW LASER power at 1064 nm in the Stokes regime. Spectra were normalized to maximum peak intensity. 1000 FT-scans were recorded for each, the liquid HDDA as well as the composite film. Optical analysis. UV/vis transparency of the material was measured using a Specord 210Plus spectral photometer (AnalytikJena, Jena, Germany). Measurements were performed of sample material coated on glass slides and referenced against cleaned substrate glass under ambient air atmosphere. Conductivity measurement. Electrical resistance was measured in 2-point geometry against internal reference with a digital multimeter (Keysight 34465A Truevolt, Santa Rosa, USA) with integration over 10 net power line cycles (minimum data point generation time). Resistivity was accessed with direct current supply. The power output was limited to approx. 5 µA to prevent the sample from heating and restructuration during conductivity testing. The resistance R was averaged over 30 s and normalized against the mean copper electrode spacing d to obtain a sample independent resistance/distance parameter R/d. This parameter was converted for each sample with the average cross sectional area of a width w of 2.6 cm and a mean film thickness ⟨t⟩ of 10 µm to obtain the average electrical conductivity σ. Mean values and standard deviations for resistivity were obtained from four independent samples which were varied in copper electrode distance for reduction of systematic errors. Distance d was varied between 25 and 60 mm. Exemplary representation is given in the Supporting Information.


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RESULTS AND DISCUSSION Preparation of inverse composite films. Nanoparticles of ITO are prepared via FSP with well-defined particle size and structure using the technique of Mädler et al.34 The collected ITO nanoparticle layers were transferred to a substrate using a layer-to-layer lamination process.44 The SEM image of such a porous nanoparticle scaffold on a glass substrate is shown in Figure 1. To prepare the inverse NPC, the nanoparticle scaffold was filled with a thermoset forming monomer via capillary force driven imbibition of the monomer into the particle network.27,49 After complete filling of the pores, the monomer was photo-chemically cured with UV irradiation to achieve conversion into a highly cross-linked polymer. Properties of nanoparticle scaffold. The layers were examined with scanning electron microscopy (SEM), nitrogen adsorption isotherms to evaluate the Brunauer-Emmet-Teller (BET) surface area and the theory of Barret-Joyner-Halender (BJH) to calculate the pore size distribution. The nitrogen adsorption isotherms revealed that the particle layer consists of primary particles with average size of 5.9 nm diameter (derived from BET surface area of 142 m²/g). X-ray diffraction shows expected cubic crystal structure of the primary particles and was used for the density calculation of particles.42 A diffraction pattern is available in the Supporting Information. The layer-to-layer-lamination led to an initial reduction of the porosity due to the compression. The median of the pore size distribution for the ITO layers is about 17.3 nm and the porosity was calculated to 85 % via BJH theory after transfer to the final substrate.


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(a)

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(b)

(c)

Figure 1. (a) SEM cross-section images of an ITO layer on glass substrate shown along the entire cross-section. (b) Magnification of top structure showing the fractal nature of the nanoparticle network with TEM. (c) TEM high resolution image of a single primary ITO particle.

A theoretical model of pore size distributions and channel geometries of FSP particle layers can be found elsewhere using the example of TiO2, where the fundamental structure of the scaffold is described by the fractal dimensionality of the tortuosity by Schopf et al.27 Figure 1a shows the cross section of the nanoparticle layer. The high resolution TEM image given in Figure 1b shows the hierarchical structure consisting of agglomerates, pores and channels in the particle network. The pore structure of aggregated nanoparticles is characterized by a broad distribution of pore diameters from a few nanometers up to 100 nm size, providing a very wide range of cavity volumes and shapes.27 Figure 1c depicts an image of a single ITO nanoparticle, where the crystallographic diffraction pattern verifies the internal crystalline particle structure. Vacuum treatment was performed at comparably low temperatures, slightly above 150 °C, as present during the production from FSP. The removal of residual contaminations may lead to higher capillary forces under liquid removal, shrinking the capillary bridges between the


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individual particles. However, if this had occurred after lamination, in SEM the imaging cracks would have been detectable in the particle layer due to volume reduction. As this was not seen for the layers after lamination, the influence of the vacuum processing should have a subordinate influence on the scaffold and, thus, on the conductivity increase. Furthermore, the ITO layers from sintering start reorganizing at temperatures of several hundred degrees centigrade. Conductivity measurements. The particle layers as well as the subsequently prepared composites were analyzed regarding their electrical conductivity σ. The ITO layers were connected with a conductive adhesive to copper plates serving as contact elements (Figure 2b). The adhesive is a highly viscous silver filled paste, which was immediately cured after attachment to the nanoparticle layer to prevent infiltration of the resin into the ITO nanoparticle layer. Figure 2a shows the schematic circuit diagram of the setup. Measurement of the total resistance R* was performed given by 𝑅 ∗ = 2𝑅′ +𝑅

(1)

Contact resistances R’ correspond to the resistance of the adhesive joints and their connection of the copper plates onto the sample. Contact of the adhesive to the ITO particles in the layer was not accessed separately. The resistance of the sample R is in the range of three to four orders of magnitude higher than the contact resistance R’. The contact resistance between clamps, copper plate and adhesive, was found to be below 3 Ω, and therefore, is neglected in the further discussions. The sketch in Figure 2b shows the electrode setup after the transfer to the glass substrate and attachment of the copper plates with the silver adhesive.


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(a)

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(b)

Figure 2. Electrical setup for resistance measurement of ITO layers and composites. (a) Schematic circuit diagram for 2point measurement. Resistance consists of contact resistance (saw tooth) and actual sample (box). (b) Sketch for electrode design showing the glass slide with polymer filled ITO films. Copper plates were connected to silver epoxy adhesive for electrical connection. Black scales indicate electrode spacing d, sample width w and film thickness t.

The measured resistance R* was converted into the electrical conductivity σ using the electrode spacing d, the mean film thickness ⟨t⟩ and width w with eq. (2). 1

[

d

i 4 σ = 4tw ∑i = 1𝑅𝑖∗

]

(2)

In a first approximation it is possible to consider the ITO-NPC-film shaped as a rectangular prism with a width w of 26 mm and a thickness t of 10 µm. Figure 3 shows the electrical conductivity for the pure polymer, the as prepared particle layer, the particle film infiltrated with the liquid monomer and the cured ITO-pHDDA-composite, respectively. The particle layer does not significantly change in particle concentration during the processing. The correlation between pressure-induced restructuration and conductivity enhancement as a function of pressure was carried out on doped SnO2 nanoparticles, which is not discussed in this work.2 The monomer is a low viscous fluid with a dynamic viscosity of 6.9 mPa·s and a surface tension of 33.2 mN/m at ambient temperature (as provided by the supplier) acting as an electrical insulator. The addition of the monomer to the percolating particle network results in an increase in electrical conductivity from 0.92·10-3 Scm-1 for the particle layer to 1.25·10-3 Scm-1 by approximately 36 %. This can only be the result from an increased number of particle-particle-


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contacts or particle-contacts with improved contact quality. High capillary forces lead to rearrangement of the mesoporous structure50,51. Rearrangement of such nanoparticle layers and the influence of packing density on electric conductivity has also been studied applying particle simulations.49,52,53 A very detailed study on the restructuration effects of liquids imbibing into TiO2 nanoparticle scaffolds analyzing the time and space dependency of the imbibition is presented by Schopf et al.27 After 5 minutes of imbibition time, the ITO layers were considered to have a homogeneous distribution of HDDA in the pores, since the maximum infiltration length was less than 20 µm for the imbibition front to migrate. Thereupon samples were polymerized. After curing and reacclimatizing the samples, a conductivity enhancement by a factor of 5 was detected compared to the pure ITO nanoparticle layer. Conventional methods of ITO layer production are usually characterized by electric conductivities in the range of 10-2 Scm-1 to 101 Scm-1.6,39,54 ITO-NPCs prepared on other routes reach conductivities in the region of 10-7 Scm-1 to 10-3 Scm-1.14,19 With the here presented inverse preparation route, the conductivity has been improved to reach 5.4·103

Scm-1, which is a high conductivity if one considers the high level of polymer filling and

network percolation and is in the range obtained with bulk ITO coatings. The conductivity is much higher compared to the one of 10-8 Scm-1 measured by Capozzi et al. embedding ITO nanoparticles in poly(methyl methacrylate).14 Comparable studies on forming conductive ITOpolymer composites use particle fillings in the order of 30 – 60 vol.-% of ITO material.1,11 A detailed overview of the techniques and corresponding conductivities is presented in the Supporting Information.


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Figure 3. Electrical conductivity of the layer for the different process steps during the imbibition with sketch for visualizing structural changes during processing and corresponding increase of particle-particle-contacts due to capillary rise driven scaffold rearrangement.

Acrylates are known to undergo a strong curing shrinkage during polymerization, and for the applied monomer HDDA this is up to 14 vol.-%.55 The shrinkage occurs for bulk polymerization due to a very high degree of cross-linking. This is hindered in presence of a filler, but nonetheless leads to a densification of the particle network by the vitrificated polymer. The distance between the nanoparticles is reduced due to the densification, which results in more electrical particle-particle-contacts and an increased contact region between existing particleparticle-contacts. Both effects contribute to an increased conductivity. Nevertheless, we are carrying out an ongoing study in which the conductivity change is examined during the polymerization of the monomer, while the degree of polymerization is also measured. Structure of the filled composite. A SEM image of the cross-section of the entire cured composite on the glass support slide is shown in Figure 4a. A thin polymer coating covers the homogeneous composite phase.


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Sufficient monomer filling prior to the curing is confirmed by the total coverage of the particle phase with a polymer coating, as shown in Figure 4a, because excessive liquid was removed after imbibition prior to photo-curing but was lacking entire detachment. Capillary forces are assumed high enough to soak the liquid into the pores completely.27 The polymer phase exceeds the particle network and covers the entire surface. As the electrical connection with the electrically conductive adhesive is prepared before monomer imbibition, this does not influence conductivity measurement. The mean composite film thickness obtained from multiple SEM cross-sections leads to a total thickness of approximately 10 µm after imbibition and curing (Figure 4a). The structure of the composite fracture surface in higher magnification is presented in Figure 4b. (a)

5 µm

(b)

0.5 µm

Figure 4. SEM images of the cross-section of a pHDDA-ITO-composite film. (a) Full cross-section with substrate at the bottom; particles are entirely covered with a polymer film. (b) Magnification of the composite film’s structure, the continuous polymer phase between the particles show filling of the pores with polymer.

This is a different imbibition procedure compared to the studies of Zhang et al.20, where a gas phase deposition of cyanoacrylates onto SnO2 nanoparticles leads to a polymerization on the particle surfaces resulting in a separation of the particles and a reduced number of particleparticle-contacts. Comparing the structural findings of Zhang with the pHDDA-ITO-composites in this study and considering similar physical properties of HDDA and cyanoacrylates (e.g. viscosity, surface tension, polarity), it can be assumed that the process of the polymer formation


17


itself primarily impacts the structural changes of the nanoparticle scaffold. Using the inverse filling process, the separating effect of spontaneous surface polymerization can be avoided. In fact, despite high capillary forces, the infiltration supports the formation of additional particleparticle-contacts. (a)

(b)

0.03 Raman intensity [a.u.]

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liquid HDDA composite

0.02

0.01

0.00

1800

1600

1400

1200

Wavenumber [cm-1]

Figure 5. (a) UV/vis transmission spectrum for HDDA (black dotted line) and HDDA-ITO-composite (red solid line). (b) FT-Raman spectrum in the Stokes regime for liquid HDDA (black dotted line) and HDDA-ITO-composite (red solid line).

Scans with a white light interferometry as well as SEM imaging did not reveal a significant change in the height of the sample in the range of instrument sensitivity. This will be further investigated with Temperature Modulated Optical Refractometry (TMOR) in-situ studies. In general, it is difficult to access the exact curing shrinkage even of pure bulk polymers, since the temperature also changes during the reaction. Optical transparency and spectroscopic properties. After liquid monomer imbibition, the opacity of the layer changed significantly. The pure nanoparticle layer is opaque and yellowish and the soaked sample turned to a transparent film with flavescent appearance, which disappeared after curing with UV-light. With the monomer's refractive index of n20 D = 1.456 (provided by the supplier) the refractive index of the polymer is estimated to be about 1.46. Due to densification, n of polymers is slightly higher compared to the corresponding monomer. To demonstrate its suitability for opto-electronic applications, UV/vis absorption spectra were


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recorded from the polymer film as well as the ITO-NPC (Figure 5a). Optical transparency is an important property of conductive coating applied e.g. in displays. The obtained transparency depends strongly on the film preparation method, and numerous examples are known from literature.13,40,56–58 The dotted line of the polymer shows only absorption in the UV area below 400 nm which implies almost complete optical transparency in the visible light regime. In presence of the ITO nanoparticles, the transmission is reduced to 80 - 90 % in the visible wavelength range, represented by the dashed curve. This is much higher compared to alternative routes for ITO thin film preparation. For example, Bu prepared 1 µm thick coating which showed a transmission of only 60 %.38 Films produced by Hasegawa et al. showed lower optical transmittance of less than 55 % for reduced thickness below 5 µm.56 A complying optical transparency of 80 - 90 % of the here presented inverse nanocomposites with a thickness of about 10 µm is fulfilled. The monomer conversion turnover was characterized by Raman spectroscopy. Figure 5b shows the Raman spectra of unconverted monomer (dotted line) and final NPC film (solid line). Vinyl groups induce resonance peaks at 1637 and 1620 cm-1. Remaining C=C bonds from unconverted HDDA-monomer are missing at 1637 and 1620 cm-1 in the composite, indicating quantitative consumption of the monomer. Characteristic resonances of the composite match the spectrum of polymerized HDDA of other studies in the region of 1300 to 1500 cm-1.59 As expected, the spectroscopic results confirm the full conversion of the monomer since signals vanished below the sensitivity of the Raman experiment. Thermal and thermomechanical behavior. Differential scanning calorimetry (DSC) was performed to characterize polymer and composite. The DSC curves have been reproduced 4 times from independently prepared samples revealing all the same characteristics. In the DSC


19


thermogram (shown in Supporting Information) the composite as well as the pHDDA film do not show any exothermic peak indicating again full polymerization. The course of the curves is very similar for both kind of samples and it is not possible to determine the glass transition temperature or any other transition temperature of polymer and composite by DSC. A small endothermal peak is visible in the thermogram of the composite at around 105 °C, which is attributed to evaporating water adsorbed at the nanoparticle surface always present on hydrophilic nanoparticle surfaces.50,60 In addition to DSC and to determine TG as well as the thermomechanical properties of polymer and composite, dynamic mechanical analysis (Figure 6) was applied. The storage (black curve) and loss moduli (grey curve) for pHDDA (dashed lines) and the ITO-composite (solid lines) were recorded under constant heating of the films. Table 1 lists the storage modulus at ambient temperature G'(RT), the storage modulus at 200 °C G'(200 °C) as well as the values of the maxima for the loss modulus G''trans and loss angle tan(δ), with their corresponding glass transition temperatures from the maxima positions TG''trans and Ttan (δ), respectively. The curves for the loss angle tan(δ) are shown in the Supporting Information. 1000

pHDDA composite

750

50

Loss modulus [MPa]

Storage modulus [MPa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20

500

10 250

0 40 60 80 100 120 140 160 180 Temperature [°C]

Figure 6. Temperature dependence of storage modulus and loss modulus for the pure HDDA polymer (pHDDA) and the ITO-pHDDA-composite measured by DMA (tension geometry at 1 Hz and heating rate 2 K/min). Solid lines represent the pHDDA, whereas dashed lines correspond to the composite, showing the storage moduli (black) and loss moduli (red).


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A decrease of the storage modulus by 70 % is detected for the composite compared to the polyacrylate (Figure 6, Table 1) at ambient temperature indicating that the nanocomposite is a material with decreased stiffness. Also, the loss modulus decreases by 74 % in comparison to the neat polymer. The strong decrease of the moduli is considered to be caused by the structure of the polymer phase. The polymeric HDDA consists of a highly and uniformly cross-linked polymer. If the HDDA is polymerized in the particle scaffold the situation is different. The monomer is distributed in the porous network of the particle layer and hence, in the confined space of pores from molecular size and a few nanometers to approximately 100 nm. When polymerization starts, the growing polymer chains often reach a particle surface before normal chain termination reaction occurs. Additionally, the further growth is limited by the diffusion of monomer which is restricted by the particle scaffold. This leads to lower molecular weight of the polymer or in our case of a cross-linking monomer to a lower degree of cross-linking and numerous dangling chain ends. Table 1. Storage moduli G' at 25 and 200 °C; loss moduli G'' at glass transition; tan(δ) at glass transition; and glass transition temperatures TG obtained from maximum of loss modulus TG''trans and loss angle Ttan(δ) for pHDDA and HDDAITO-composite gathered from DMA.

G' (RT) / (MPa)

G'(200 °C) / (MPa)

G''trans / (MPa)

tan(δ)

neat polymer 1080 ± 70

277 ± 5

50 ± 5

0.09±0.02

83±2

108±2

composite

128 ± 5

25 ± 5

0.14±0.02

116±2

117±2

292 ± 25

TG''trans / (°C)

Ttan(δ) / (°C)

The glass transition temperatures were determined from the maxima of the loss moduli and the loss angles obtained in the DMA measurements. For pHDDA, the glass transition temperature at the maximum loss angle was 108 °C. The loss modulus maximum gives a TG of 83 °C, which


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corresponds to the value of 87 °C reported from both, Ye et al.61 and Goswami et al.62 The composite reveals a slightly higher glass transition temperature from the loss modulus and tan(δ) maxima of 116 °C. The TG increase seems to be in conflict with the moduli decrease but it is known that TG of nanocomposites can be higher or lower compared to the pure polymer. On the one hand lower degree of polymerization63 or reduced crosslinking density64–66 leads to a decrease of TG. On the other hand an increase of TG was observed67 and is due to restriction of the molecular mobility by the particles surface. Due to the structure of the nanoparticle scaffold with narrower spaces between the particles compared to conventional nanocomposites, molecular mobility is more restricted as well as polymer growth during polymerization. This leads to the observed decrease of moduli and increase of glass transition temperature. Verification of composition. Thermogravimetric analysis (TGA) was applied to determine the total composition of the nanoparticle polymer films after preparing the NPC (Figure 7a). The results were examined according to standard DIN EN ISO 11358-1:2014 and the detailed results are shown in the Supporting Information. In the following the extrapolated start temperature of the decomposition TA according to the standard is taken for the discussion. The polymer as well as the nanocomposite show multi-step degradation in the case of ambient air atmosphere and single-step degradation in nitrogen gas. The pure polymer and the composite show higher thermal stability under oxygen exclusion compared to ambient air conditions during the heating process, since the decomposition start temperature TA decreases for pHDDA from 396 °C to 339 °C and for the composite from 360 °C to 330 °C upon exposure to oxygen. The composite degradation starts already at a 9 K lower temperature TA for air and 36 K lower for nitrogen compared to the pure polymer. A 2-step decomposition is observed for both kind of samples in air and a 1-step decomposition in nitrogen represented by the respective number of


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maxima in the first derivative of the TGA curves (Figure 7b). The maxima indicate the temperature with the highest decomposition rate. In addition to the temperature at which decomposition starts, also all maxima in DTG are lower for the composite compared to the polymer. The observed lower thermal stability of the composite is in accordance with the polymer’s reduced cross-linking density proposed from the thermomechanical properties. The observed decomposition reactions and the atmosphere’s influence are in line with differently cross-linked HDDA copolymers examined by Goswami et al. 68 Another influence on the decomposition reaction might result from the surface chemistry of the nanoparticles. This was e.g. examined by Liufu et al.69 for polyacrylates containing ZnO nanoparticles. Depending on the specific situation, stabilization or destabilization of the polymer by the nanoparticles could be observed. The resulting total mass loss is used to determine the volume fraction of the ITO scaffold after imbibition and curing. Pure pHDDA gives residual mass ratios of 0.5 % for nitrogen and 2.9 % for air. The composite shows a residual mass of 42.5 % for both atmospheres. The average loss of organic mass is given by subtracting the carbon residuum of the pure polymer from the composite total mass loss, since organic residue of 1.0 % is also apparent. This leads to mass ratio of mHDDA:mITO of 59:41. With the bulk density of the materials (ρ(ITObulk) = 7.11 g/cm³ and ρ(pHDDA) = 1.12 g/cm³) and the given mass ratios, the porosity of the particle network prior to filling can be calculated to approx. 90 %. Porosity measurements from gas adsorption isotherms revealing 85 % are consistent with this result from TGA examination. The gas adsorption measurements represent the exact physical porosity, whereas the degree of filling and corresponding volume fraction can be estimated from TGA. A minor overestimation of the porosity is attributed to the excess polymer coating on top of the composite phase (Figure 4a).


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Measuring the gas adsorption after annealing in the TGA experiment does not access the initial porosity, because during the combustion carbon residue remains in the product while the particles also undergo a sintering process. (a)

100

(b)

HDDA (air) HDDA (N2) composite (air) composite (N2)

80

Weight [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0

150

300

450

600

750

Temperature [°C]

Figure 7. (a) TGA curves under air and nitrogen atmosphere for HDDA and pHDDA-ITO-composite with a heating rate of 5 Kmin-1. (b) First derivatives of TGA curves in the decomposition region.

CONCLUSIONS It was shown, that percolating NPCs can be efficiently prepared from FSP-synthesized ITO nanoparticles in combination with imbibition and curing with HDDA monomer forming a polyacrylate thermoset composite. The imbibition and following polymerization resulted in an efficient and thin-film-preparation route for inversely prepared NPCs. This methodology revealed different composite characteristics compared to conventional preparation methods. Using the presented methodology, a significantly higher electrical conductivity of the ITO nanoparticle layers was found after imbibition and curing compared to common ITO containing nanocomposites. This effect results from an increased number and contact region of particleparticle-contacts from restructuration caused by capillary forces and densification by curing shrinkage of the acrylate. A high transparency was shown using UV/vis spectroscopy, which is crucial for applications in opto-electronics.


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A complete filling was evidenced by TGA experiments. As the filling proceeds through the pore structure, a complete filling must result in a continuous polymeric phase. Together with the percolating nanoparticle scaffold (confirmed by electrical conductivity measurements) a cocontinuous interpenetrating network of nanoparticles and cross-linked polymer forms. It seems that the polymer formed inside the particle scaffold has a lower modulus indicating lower crosslinking density. This is most likely caused by the strongly restricted space available for polymerization, which is at least partially in molecular dimensions. Due to this, the growing polymer chains reach the surface of the ITO particles, which leads to termination of chain growth or the next monomer is not present inside the cavities and there is no possibility for diffusion of monomer molecules to the growing polymer chain. Besides the application in flexible electronics and displays, further applications extending beyond classical functional materials towards opto-electronics are very likely due to the advantages of easy processing, scalability and cheap preparation route. This new method of inverse NPC preparation has been already applied for the electrochemical characterization of nanoparticle electrodes, which where infiltrated with a monomer followed by a plasma treatment to remove the polymer coating from the surface for proper electrical contact with the surrounding medium.70

ACKNOWLEDGEMENTS We gratefully acknowledge funding for this work by the Deutsche Forschungsgemeinschaft (DFG) under grant numbers HA 2420/16-1 for Ron Hoffmann and Andreas Hartwig and, MA 3333/10-1 for Valentin Baric, Hendrik Naatz, Sven O. Schopf and Lutz Mädler. Finally, the authors thank José Ahlering, University of Bremen, Faculty of Production Engineering, Germany for his measurement contributions.


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ABBREVIATIONS FSP: flame spray pyrolysis, HDDA: 1,6-hexanediol diacrylate, ITO: indium tin oxide, NPC: nanoparticle-polymer-composite

ASSOCIATED CONTENT Supporting Information Figure of DMA including loss angle, figure of DSC thermogram, photograph of nanocomposite electrode, table with evaluation of TGA data and literature overview on ITO coatings.

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Improved Electrical Conductivity via Polymer Addition - American

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