Moisture absorption into ultrathin hydrophilic polymer films on different substrate surfaces
Introduction
Moisture absorption in polymeric films is important for a variety of industries ranging from microelectronics to adhesives and coatings. In many applications, water absorption leads to reliability problems such as the degradation of dielectric properties, corrosion or delamination [1]. A significant number of studies covering many polymer systems have focused on characterizing the absorption and diffusion properties of water in polymer films. However, many of the significant problems observed are due to interfacial effects and have not been fully examined. For instance, the water concentration within a supported polymer film may not necessarily be uniform through the thickness of the film; concentration gradients are sometimes observed near the interfaces [2], [3], [4]. For moderately hydrophobic polymer films supported on silicon oxide substrates, there can be a 30 Å thick water-rich layer near the substrate [3], [5]. Neutron reflectivity (NR) measurements show that the concentration of deuterium labeled water (D2O) at this polymer/silicon interface is noticeably greater than the bulk, approaching 30% by volume. We recently demonstrated that the total amount of water in this excess layer can be inferred, albeit not as quantitatively as from NR, from simple swelling experiments as a function of film thickness using X-ray reflectivity (XR) [5]. The total swelling is a linear combination of equilibrium or bulk-like swelling and a thickness independent excess swelling near the interface. Combined, these effects lead to an increase in the degree of swelling with decreasing film thickness. It is possible to reduce the concentration of interfacial water by modifying the surface with a silane coupling agent [2], [6]. However, even with the silane coupling agents, the interfacial concentrations were larger than the water concentration in the bulk polymer.
It is also important to understand the kinetics of moisture absorption into thin polymer films. The moisture absorption mechanisms into polymers can be complex and numerous models have been formulated to describe these processes [7], [8]. Recently, thin film confinement effects have been explored by several groups [9], [10], [11]. In one study, the diffusivity of water decreased by five orders of magnitude when the film thickness decreased from 200 to 3 nm [10]. The decreased diffusivity is thought to reflect a coupling of the water transport with the local chain dynamics, which also become strongly suppressed in thin films [12]. However, in another study, there was no change in the swelling kinetics for films ranging from 45 to 20 nm thick [9]. The qualitatively different thin film confinement effects on the moisture absorption kinetics suggest that the effect is not universal across all polymers and substrates. This observation is consistent with changes in other thermophysical properties of thin films, such as the glass transition temperature (Tg), that are also polymer thickness and substrate dependent. For example, in thin films the Tg has been found to increase, decrease or remain constant as the film thickness approaches the radius of gyration (Rg) of the polymer chain, depending on the polymer and the substrate surface energy [13], [14].
Understanding the interfacial and confinement effects on the moisture absorption properties of polymers is becoming increasingly important. Polymers are frequently used in thin film applications where the total film thickness continues to decrease. A clear example is lithography where the advent of immersion processing places a liquid in direct contact with a thin polymeric photoresist. As the film thickness decreases, the interfacial properties can dominate the material response. Likewise, highly filled or multi-layer polymer structures can be modeled by a thin film interface. For example, multi-layers of nanometer thick polymers and inorganic desiccant materials are utilized as barrier coatings to prevent moisture absorption into microelectronics packaging or organic electronic devices. These structures are composed almost entirely of interfacial material. Here, we measure the equilibrium uptake and diffusion coefficient of water into hydrophilic, uncharged ultrathin polymer films with varying thickness. The moisture absorption into poly(vinyl pyrrolidone) (PVP) films was measured using X-ray reflectivity (XR) and quartz crystal microbalance (QCM) on both hydrophilic (silicon oxide) and moderately hydrophobic (hexamethyldisilazane (HMDS)) substrates.
Section snippets
Materials and film preparation
PVP with a relative molecular mass (Mn,r) of approximately 10,000 g/mol was purchased from KAF1. All films were prepared by spin-coating from
Substrate effect on the total equilibrium moisture absorption
The moisture absorption from saturated vapor was measured as a function of film thickness on both HMDS and silicon oxide surfaces. The average volume fraction of water from the total amount absorbed determined from the degree of swelling (relative thickness change) using XR is shown in Fig. 1. The water concentration can also be determined from the shift of the critical angle of the film with water absorption. In this case the swelling was determined from the thickness change due instrumental
Conclusions
The influence of film thickness and the substrate surface on the moisture absorption in PVP was examined. The equilibrium moisture concentration absorbed from saturated vapor was independent of PVP film thickness for films supported on silicon oxide surfaces with film thickness as small as 5.6 nm. This thickness independence of swelling is consistent with previous measurements for a polyelectrolyte on silicon oxide [10] and suggests that physical confinement of the polymer chains does not
Acknowledgements
BDV acknowledges financial support from the NIST/NRC postdoctoral fellowship program. The authors thank B.J. Bauer for graciously providing the poly(vinyl pyrrolidone).
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