Diffuse Reflectance Infrared Spectroscopic Identification of Dispersant/Particle Bonding Mechanisms in Functional Inks

L. Jay Deiner; Elaheh Farjami

doi:10.3791/52744

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Summary

Formulation of stable, functional inks is critical to expanding the applications of additive manufacturing. In turn, knowledge of the mechanisms of dispersant/particle bonding is required for effective ink formulation. Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) is presented as a simple, inexpensive way to gain insight into these mechanisms.

Abstract

In additive manufacturing, or 3D printing, material is deposited drop by drop, to create micron to macroscale layers. A typical inkjet ink is a colloidal dispersion containing approximately ten components including solvent, the nano to micron scale particles which will comprise the printed layer, polymeric dispersants to stabilize the particles, and polymers to tune layer strength, surface tension and viscosity. To rationally and efficiently formulate such an ink, it is crucial to know how the components interact. Specifically, which polymers bond to the particle surfaces and how are they attached? Answering this question requires an experimental procedure that discriminates between polymer adsorbed on the particles and free polymer. Further, the method must provide details about how the functional groups of the polymer interact with the particle. In this protocol, we show how to employ centrifugation to separate particles with adsorbed polymer from the rest of the ink, prepare the separated samples for spectroscopic measurement, and use Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) for accurate determination of dispersant/particle bonding mechanisms. A significant advantage of this methodology is that it provides high level mechanistic detail using only simple, commonly available laboratory equipment. This makes crucial data available to almost any formulation laboratory. The method is most useful for inks composed of metal, ceramic, and metal oxide particles in the range of 100 nm or greater. Because of the density and particle size of these inks, they are readily separable with centrifugation. Further, the spectroscopic signatures of such particles are easy to distinguish from absorbed polymer. The primary limitation of this technique is that the spectroscopy is performed ex-situ on the separated and dried particles as opposed to the particles in dispersion. However, results from attenuated total reflectance spectra of the wet separated particles provide evidence for the validity of the DRIFTS measurement.

Introduction

Additive manufacturing has recently emerged as a promising technique for fabrication of everything from ceramics to semiconductors to medical devices¹. As the applications of additive manufacturing expand to printed ceramic, metal oxide, and metal parts, the need to formulate specialized functional inks arises. The question of how to formulate the required functional inks relates to a fundamental issue in surface and colloid science: what are the mechanisms by which particles in colloidal dispersion are stabilized against aggregation? Broadly, stabilization requires modification of the particle surfaces such that close approach of particles (and hence aggregation) is prevented either by Coulombic repulsion (electrostatic stabilization), by the entropic penalty of polymer entanglement (steric stabilization), or by a combination of the Coulombic and entropic forces (electrosteric stabilization)². In order to achieve any of these mechanisms of stabilization, it is usually necessary to modify the particle surface chemistry through attachment of polymers or shorter chain functional groups. Thus, the rational formulation of stable functional inks demands that we know whether a given chemical additive attaches to the particle surface and what chemical group attaches to the particle surface.

The goal of the method presented in this protocol is to demonstrate rapid characterization of chemical species adsorbed on particle surfaces in functional inks. This goal is particularly important as functional ink formulation transitions from a specialized task for surface and colloid scientists to an activity broadly practiced by the range of scientists and engineers interested in printing ceramic, metal oxide, and metal devices. Achieving this goal requires designing an experiment that overcomes the challenges of characterizing opaque, high solids loadings dispersions. It also requires discriminating between chemical species that are present in the dispersion but not adsorbed on the particles from those that are actually adsorbed. It further requires distinguishing between those species that are chemically adsorbed on the particles from those that are weakly physisorbed. In this experimental protocol, we present the use of diffuse reflectance infrared spectroscopy for characterization of dispersant attachment in functional inks. The diffuse reflectance infrared spectroscopy measurement follows a pre-analysis sample preparation technique necessary to distinguish adsorbed species from those merely present in the dispersion.

A variety of methods are currently used to gain insight into the nature of the interactions between chemical ink components and colloidally dispersed particles. Some of these methods are indirect probes in which measured properties are presumed to correlate with surface functionalization. For example, changes in slurry rheology or sedimentation rates are presumed to correlate with adsorption of surface modifiers³. Particle size distribution, as characterized by dynamic light scattering (DLS), and zeta potential, as characterized by electrophoretic mobility, provide insight into the adsorption of polymers or species with surface charge^4,5. Similarly, sample mass loss as probed by thermogravimetric analysis (TGA) relates to the presence of desorbing species and the strength of interaction between the adsorbate and the particle⁶. The information from the above mentioned indirect probes suggest changes in surface chemistry, but they do not provide direct insight into the identity of the adsorbing species or the mechanism of its adsorption. Direct insight is particularly important for functional inks in which a large number of components are present in the dispersion. To provide detailed molecular level information, X-ray photoelectron spectroscopy (XPS)⁷, ¹³C nuclear magnetic resonance (NMR) ^4,6, and infrared spectroscopy^8-12 have been explored. Of these three options, infrared spectroscopy is particularly promising. In comparison to ¹³C NMR, infrared spectroscopy does not require that inks be formulated with analytically pure solvents to prevent interferences during measurement¹³. In comparison to X-ray photoelectron spectroscopy, standard infrared spectroscopy can be conducted at ambient pressure, avoiding the need for ultrahigh vacuum conditions during measurement.

There is literature precedent for the use of infrared spectroscopy to probe the interaction between colloidally dispersed ceramic, metal oxide, and metal nanoparticles. These works can be separated into attempts to measure interfacial chemistry in situ using attenuated total reflectance infrared (ATR-IR)⁹, and attempts to measure interfacial chemistry ex situ using solid sampling⁸. While there are advantages to in situ measurements, the uncertainties that arise due to the necessity for spectral manipulation make the method difficult for multi-component inks in which there are solvents and multiple polymeric components. Therefore, this protocol focuses on solid sampling and ex situ measurement. All of the solid sampling methods entail a pre-treatment step where a solid is obtained by separating the particles from the solvent, and an analysis step where infrared measurements are performed on the solid particles. The difference between methods arises in the choice of sample pre-treatment and in the choice of experimental technique used for infrared analysis of the solid. Historically, the traditional way to use infrared spectroscopy to analyze solids was to grind small quantities (< 1%) of the solid sample with potassium bromide (KBr) powder, and then subject the mixture to high pressure sintering. The result is a transparent KBr pellet. This procedure has been attempted successfully with powders derived from aqueous suspensions of zirconia nanoparticles functionalized with polyethyleneamine¹⁰, with fatty acid monolayers on cobalt nanoparticles⁷, and with catechol-derived dispersants on Fe₃O₄ nanoparticles¹⁴. Despite these successful applications of the KBr pelleting technique for detection of adsorbed dispersants, diffuse reflectance infrared spectroscopy provides several advantages. One advantage is simplified sample preparation. In contrast to KBr pelleting, the solid sample in diffuse reflectance can be simply ground by hand. There is no sintering step as the powder itself is loaded into the sample cup and the diffusely scattered infrared light is measured. The other advantage of diffuse reflectance over KBr pelleting is the increased surface sensitivity¹⁵. The increase in surface sensitivity is especially useful for the present application in which the critical questions are the presence and nature of adsorbates on the nanoparticle surfaces.

Among works that have used the diffuse reflectance sampling technique to probe the adsorption of chemical species on colloidally dispersed samples, the primary differences arise in the method of separating the nanoparticles from the liquid medium. This step is critical because, without the separation, it would be impossible to distinguish specifically adsorbed dispersants from dispersants simply dissolved in the liquid medium. In several examples, the method of separation is not obvious from the experimental protocol^12,16,17. When specified, the most frequently practiced method involves gravitational separation. The rationale is that the ceramic, metal oxide, and metal nanoparticles are all more dense than the surrounding media. When they settle, they will drag down with them only the specifically adsorbed species. Chemical species not interacting with the particles will remain in solution. While dispersions may readily settle under normal gravitational force¹⁸, a stable inkjet ink should not observably settle over a time period of less than a year. As such, the method of employing centrifugation for pre-analysis separation is preferred. This has been demonstrated in several studies of dispersant adsorption on glass particles^19,20, dispersant binder adsorption on alumina⁸, and anionic dispersant functionalization of CuO¹¹. Most recently, we have used it to evaluate mechanisms of fatty acid binding in non-aqueous NiO dispersions used for inkjet and aerosol jet printing of solid oxide fuel cell layers²¹.