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February 13, 2004 Tuning Substrate Surface Energies for Blends of Polystyrene and Poly(methyl methacrylate)D.A. Winesett1, S. Story1, J. Luning2, and H. Ade1 We have developed and characterized three simple and inexpensive ways to make substrate surfaces that are neutral, i.e. that have the same interfacial energy, for polystyrene (PS), a common hard plastic, and poly(methyl methacrylate) (PMMA), a commercial plastic. In the first two of these surface treatment methods, two different high molecular-weight copolymers made of styrene and methyl methacrylate were absorbed onto hydroxylated silicon oxide. In another method, octyltrichlorosilane (OTS), a highly hydrophobic material, was covalently bonded onto hydroxylated silicon, a highly hydrophilic surface. A fourth method covalently attached hydroxyl-terminated, miscible, low molecular-weight homopolymers of PS and PMMA to the substrate. We found that the OTS treatment is advantageous because a gradient of continuous interaction can be created. Nonetheless, the principles of each method should be applicable to other polymer systems.
Homopolymer blends of polystyrene (PS) and poly(methyl methacrylate) (PMMA) of relatively low molecular weight (10,000 to 200,000 g/mol) make excellent model systems for a number of studies. Both polymers exhibit comparable glass transition temperatures (temperatures below which a polymer becomes hard and brittle) of approximately102 to 120 degrees Celsius (°C) and are readily codissolved in a common solvent. Moreover, for typical annealing temperatures (140°C to 180°C), the interfacial tension between PS and PMMA is larger than the absolute value of the difference between the surface energies of PS and PMMA. Hence, neither polymer can spread onto the other at an air interface. A polymer-air interface might thus be one of two “neutral” interfaces that can be used to produce a two-dimensional system. A “neutral” substrate surface will provide the second defining interface.
Thin polymer films are often cast on silicon substrates. At elevated temperatures, it is well established that the PMMA in a PS/PMMA blend will quickly segregate to the native silicon oxide (SiOx) interface to form a wetting layer. In order to avoid and/or control this segregation, the effectiveness of several simple surface energy tuning methods has been tested. Using a standard UV/O3 process, in which ultraviolet light is used to produce ozone and oxygen radicals in the air that removes carbon from surfaces, silicon substrates were cleaned and simultaneously activated to produce –OH groups on their surfaces. They were subsequently treated with either a commercial random copolymer, P(S-ran-MMA), an alternation copolymer, P(S-alt-MMA), a partial or gradient n-octyltrichlorosilane (OTS) self-assembled monolayer, or a brush of a hydroxyl (–OH)-terminated low molecular-weight PS/PMMA blend with varying PS/PMMA composition. The relative effectiveness of all four methods was characterized using optical microscopy, Near Edge X-ray Absorption Fine Structure (NEXAFS) microscopy and spectroscopy, atomic force microscopy, and contact angle measurements. The optical micrographs in Figure 1 clearly show that very different morphologies develop, depending on the surface treatment. The P(S-ran-MMA)-treated surface shows a semi-bicontinuous morphology, wherein the two material phases are partially continuous across the whole sample. However, the untreated SiOx surface and the P(S-alt-MMA)-treated surface show large droplets. Therefore, the P(S-alt-MMA) treatment clearly failed. To verify that the P(S-ran-MMA) treatment was successful, NEXAFS microscopy (performed with the Stony Brook Scanning Transmission X-ray Microscope (STXM) at beamline X1A) was used to quantitatively map the thickness of the PS and PMMA phases on P(S-ran-MMA)-treated silicon nitride (Si3N4) membranes (Fig. 2). The results show that the morphology is essentially two-dimensional in nature, with a relatively uniform film thickness and domains of PS and PMMA that span the entire film thickness.
Our specific interests and studies on “neutral” surfaces were restricted to surfaces that would have the same interfacial energy for PS and PMMA. Hydroxy-terminated PS and PMMA is the most novel approach investigated. Except for the P(S-alt-MMA) treatment, all of the methods yielded neutral or near-neutral surfaces. The OTS treatment has the advantage that a gradient of continuous interaction ranging from preferential PMMA to preferential PS interactions can be created. However, the principles of all the methods developed should be applicable to other polymer systems. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
The ability to adjust the interactions between a material or a
molecule and a substrate is desirable for a number of applications.
This includes nano-patterning, sensing, and the dewetting and phase
segregation in two-dimensional systems of polymer blends at elevated
temperatures. In polymer blends, differences in interfacial
energies, which reflect a preferential interaction, generally cause
one polymer to segregate to the substrate or air interface. This
segregation has to be avoided in a number of applications, and is
prevented by tuning the polymer/substrate interfacial properties.
