The contact angle of thin-uncured epoxy films: thickness effect

Abstract

The contact angle of thin-uncured epoxy films spin casted on oxidized silica wafers has been investigated as a function of their thickness in the range of 5–75 nm. Based on the contact angle measurements with polar and apolar liquids, the surface free energy of various films were analyzed and compared using Lifshitz van der Walls/acid–base and harmonic mean (Wu’s) approaches. The results show that the nature of the surface of epoxy films change to more hydrophobic state as its thickness increases. Furthermore, time monitoring of the contact angle of the surface of epoxy films reveals a more rapid conformational change or flip–flop of surface epoxy chains for thicker layers.

On the other hand, epoxy layer could screen out the substrate interaction for both polar and apolar liquids with a pseudo-exponential functionality on film thickness. A pseudo-exponential trend of the contact angle and surface free energy components of the films towards the values of thick films was observed.

Introduction

Contact angle measurement using sessile drop technique is widely used for investigation of the surface characteristics of various materials. On the other hand, surface free energy of polymers control their adhesion, adsorption, lubrication, steric stabilization, wetability and their similar surface related properties [1], [2]. In some theories of self-stratification of coatings [3] surface tension is a determinant factor. The macroscopic Young–Dupre equation correlates the contact angle to the surface and interfacial tensions, all of which maybe further decomposed into Lifshitz–van der Waals (LW) and the polar components of surface energies [4]. The Young–Dupre equation together with theories of polar and apolar surface tension components, remains the mainstay of measurements of surface properties by sessile drop measurement. Different routes of surface free energy analyzing like Zisman, Harmonic mean and Lifshitz–van der Waals/acid–base method yield different results and meanings.

Sessile drop approach suffers from some limitations. Most interactions between surfaces are less characterized, so some researchers argue that this method is unreliable, and the results of experiments largely depend on the sample preparation conditions and also test conditions [5].

As different researchers have mentioned, the most important parameters which affect the contact angle are temperature, material transitions such as glass and crystalline transitions, contaminants and adsorbed materials, polar and apolar interactions, drop dimension, surface crystallinity, molecular weight and conformation of chains [6]. Thus, for proper measurement of the contact angle, such limitations must be considered. But yet, there are some new or old experimental problems, which reveal the role of some unconsidered parameters, which needs further investigation.

There are a few reports on thin film thickness effect on the contact angle [7], [8], [9]. It is suggested that the surface free energy of thin films depends not only on the molecular forces of film material but also it is affected from the interactions or attractive forces of substrates. Mate and Wu [8] have investigated the dependence of the contact angles on the thickness of perflouropolyether lubricants on silicon wafers and disks over coated with amorphous hydrogenated carbon. They characterize a screening length in which, substrate interaction effect decreases exponentially, and then the contact angle reach to a steady state condition but this length for alkenes droplets are just a few angstroms. On the other hand, for water droplets screening length is almost one order of magnitude higher. They attributed this difference to the replacement of film molecules with water droplet, which was mentioned also by others [10].

Some researchers [9], [11], [12], [13] try to relate this effect with long-range macroscopic interactions between hydrophobic substances that refer to attractive forces between two or more apolar surfaces or solutes that separate with a liquid. Hato [9] mentioned that the range of the long-range interactions is about 5–20 nm. Molecular structuring of the neighboring liquid molecules or interaction of liquid molecules between two surfaces [14] has mentioned as the result of this attraction. In addition, it has been argued that the origin of these forces is electrostatic and did not relate it to long-range hydrophobic interactions. Others think that it originated from an unknown reason and is statistical in nature [15]. Most of the investigators have studied the hydrophobic interactions between two surfaces, so yet there is a vast field for investigation about other type of forces or attractions.

In another field of polymer science, researchers who work on T_g of thin films have expressed that chain mobility or glass transition temperature of polymeric thin films are affected until 15R_g (according to molecular weight of chains and substrate) thickness from substrate. These effects in length scale are observed about 10–100 nm [16], [17], [18], [19], [20], [21], [22]. Decreasing of chain mobility may have some reflections in the contact angle of some liquid drops on the surface. Rafailovich et al. [21] argued that mechanical properties of polymeric films are affected by interactions with the solid substrate, the elasticity of the film layer and hence the restoring force can be thickness dependent. This effect is interpreted with dynamical properties of polymeric chains near an attractive interface. The interface composed of chains that are pinned to the surface by one or more contacts, the rest of the film, which have been defined as the chains that do not have physical contacts with the surface, are partially entangled with the first layer. Consequently, an interfacial regime can exist where chain motion is possible but the dynamics are more sluggish than the bulk. He proposes that far from the surface, a factor of three radii of gyration or greater, bulk dynamics is restored. In other words, he claims that viscous energy dissipation is increased markedly at distances below 100 Å [22], thus elasticity of surface against drop is decreased.

Also in IR experiments it has been that observed as the thickness of SiO₂ layer on silicone wafers increases, frequency of bond stretching vibration (v) increases [23]. In addition, as the oxide thickness approaches 700 Å, the values of v begin to approach values characteristics of thick oxides (>1000 Å). It has also been recognized that in X-ray and photoelectron spectra (XPS) and Auger electron spectra (AES) from thin SiO₂ films grown on Si substrates show differences compared to those from similarly grown thick oxide films [24].

Obviously, a better understanding of the physical and chemical factors that influence surface free energy would have a great value for coating industry, which use the contact angle measurement to study their products. In this paper, we concentrate on polar, apolar interactions and the effect of substrate on the contact angle measurements in thin epoxy films. So, when a solution of a polymer poured on a substrate, a thin layer of resin adsorbs on the attractive substrate, and changes the surface free energy. In this respect, different liquids with different polarities on thin films with different thicknesses were examined to investigate their relationships.