Enhanced Barrier Performance of SiOx-Modified Polymer Substrates: Some Morphological Considerations

Diffusive transport of environmental gases through the free volume inherent in commodity polymers renders these materials highly ineffective for use in a variety of packaging applications. Attempts to overcome this drawback have sought to either control polymer free volume using novel microstructured polymers [1, 2] polymer blends [3, 4], or to modify surface properties through the addition of thin inorganic (ceramic or metal) coatings [5, 6]. As an example of the latter, plasma-deposited SiOx ®lms on polymer substrates have been of considerable interest in the food packaging industry for the past decade [7]. Only recently have similar needs for high-barrier polymers emerged from the medical community, which is in the process of replacing brittle glass with tough polymeric materials in biomedical devices. Glass devices requiring a sustained vacuum exhibit excellent barrier properties but readily shatter upon impact, thereby endangering the safety of medical personnel. If glass is to be replaced by a commodity polymer in such devices, optical transparency (required for visual and spectroscopic analyses [8]) and impermeability cannot be compromised. Tranasparency is ensured by using amorphous (glassy) polymers or clari®ed (nucleated) semi-crystalline polymers [9, 10], while enhanced gas impermeability is most ef®ciently achieved through plasma deposition of a thin SiOx surface ®lm. Recent advances in plasma-enhanced chemical vapor deposition (PECVD) technology [11, 12] have greatly facilitated the surface modi®cation of polymers, especially those processed into complex shapes. Previous attempts to correlate the morphological characteristics of thin surface ®lms with corresponding barrier properties have relied extensively on thickness measurements acquired from interferometry [13] or scanning electron microscopy [14]. More recent efforts in this endeavor used [14±16] scanned probe microscopy techniques, such as atomic force microscopy (AFM). According to these previous studies, two morphology=barrier property correlations have been identi®ed: (1) gas permeability decreases with a reduction in ®lm roughness [17], and (2) surface roughness increases as the ®lm thickness increases [18]. It has been pointed out [16], however, that the presence of inhomogeneities in surface morphology over large areas severely hinders the establishment of relationships between morphology (e.g., surface roughness) and permeability. Moreover, while AFM is well-suited for highresolution topographical analysis, little (if any) information regarding sub-surface features in multilayered coatings have been extracted from AFM images. For this reason, planar transmission electron microscopy (TEM), which relies on electrons that penetrate through a specimen for image formation, constitutes an ideal complement to AFM in the characterization of such coatings. In this work, we employ both AFM and TEM to examine the morphological features of thin SiOx ®lms deposited on two polymeric substrates and provide preliminary evidence for additional morphology=permeability relationships. A series of SiOx ®lms was deposited onto extruded polystyrene (PS) and polycarbonate (PC) ®lms measuring ca. 75 im thick using Radio Frequency (RF) capacity-coupled glow discharge in conjunction with feed mixtures of oxygen and hexamethyldisiloxane (HMDSO). During the deposition process, conditions such as deposition time, power, pressure and O2=HMDSO ow rate were systematically varied to produce coatings differing in permeance, surface roughness, and ®lm thickness. Oxygen permeance was measured at 30 8C with an Oxtran 220 permeability testing instrument. Specimens examined by energy-®ltered TEM were prepared by arranging small pieces of the coated polymers on 400-mesh copper TEM grids. The grids were placed on a graphite screen in contact with a solvent capable of dissolving the polymer substrate (toluene for PS and chloroform for PC) so that after substrate dissolution, unsupported SiOx ®lms adhered to the grids. Planar, rather than cross-sectional [11] images of the ®lms were obtained with a Zeiss EM902 electron spectroscopic microscope operated at 80 kV and 0 eV energy-loss. Zero-loss imaging selectively removed inelastically scattered electrons