Synthesis of biocompatible surfaces by nanotechnology methods

SYNTHESIS OF BIOCOMPATIBLE SURFACES BY NANOTECHNOLOGY METHODS 701nm12001000800600400200(a)0 200 400 600 800 1000 1200nmnm2.22.01.81.61.41.21.00.80.60.40.2012001000800600400200(b)0 200 400 600 800 1000 1200nm87654nm3210Fig. 6. AFM images of PMMA films: (a) initial film on silicon substrate; (b) nanostructured surface upon treatment in an oxygencontainingRF plasma.pressure of 2 and 100 Pa for various periods of time(0.5, 1, 2, 5, 10, and 20 min) on a setup described indetail elsewhere [3].The photon energy at λ = 123.6 is about 10 eV,which is large enough to excite polymer molecules andbreak chemical bonds such as C–C, C–H, C–CH 2 ,and C–O [15]. Photolysis leads to the formation oflow-molecular-mass gaseous products such as CO,CO 2 , H 2 , H 2 O, and CH 4 , as well as to high-molecularmassgaseous products such as methyl formate(HCOOCH 3 ), formaldehyde (CH 2 =O), methanol(CH 3 –OH), and methyl methacrylate[CH 2 =C(CH 3 )–COOCH 3 ] [19]. In addition, exposureto VUV leads to the formation of intermolecularcross-links. Thus, the VUV-induced smoothing ofPMMA nanorelief is due to the joint action of severalprocesses, including the photoetching of the polymer,the redeposition of the gaseous products of photolysis,and the formation of intermolecular cross-links. Thedegree of smoothing (at a fixed radiation intensity) isdetermined primarily by the treatment duration.An analysis of our results showed that even 2-minVUV irradiation produced clearly detectable smoothingof the roughnesses on the nanostructured PMMAsurface. After 10-min exposure, the average roughnesssize decreased by a factor of 2.6–3; the average lateralsize of nanograins was reduced by half, and their averageheight decreased by a factor of 15–18. Thus, 10-min VUV treatment rendered the PMMA film surfacepractically smooth.MODIFICATION OF PMMA SURFACEBY SYNCHROTRON RADIATIONIt was also of interest to study the modification ofnear-surface layers of polymers by irradiation in thehard VUV (λ = 50–100 nm) and soft X-ray (λ = 10–50 nm) ranges. The experiments on polymer modificationin the short-wavelength spectral range wereperformed using a small synchrotron ring of the KurchatovSource of Synchrotron Radiation (Moscow).However, since it is still a difficult task to separate thecoherent collimated beams of various wavelengthsfrom the entire synchrotron spectrum, the samples ofPMMA films were exposed to a beam of “white” synchrotronradiation (SR). The white beam representedSR in the entire spectrum with λ = 5–400 nm.The irradiation of PMMA by high-energy photonsof the white SR beam leads to the formation of radicals(via the Norrish type I reaction) by rupturing chemicalbonds (C–C, C–H, C=O) with different energies andforming simple products such H 2 , CO, CO 2 , and CH 4 .At the same time, long-wavelength photons producethe reverse effect (cross linking). As a result, exposureto white SR is accompanied by the destruction ofPMMA and the subsequent cross linking of the products.Figure 7 shows AFM images of the surface relief ofPMMA films, which show evidence for self-organizationand a decrease in the size of nanostructured particles.An explanation of this phenomenon can bebased on the fact that polymers containing tertiarycarbon atoms and possessing low enthalpy of polymerizationare subject to depolymerization (thermodestruction).In addition, this is accompanied by UVradiation-inducedphotodestruction in the presence ofC=O, which decreases the strength of bonds in thepolymer backbone. Both processes lead to polymerdestruction with a quantitative yield of monomers[15]. Thus the exposure of PMMA to the white SRbeam leads to the appearance of structured monomerson the sample surface.NANOTECHNOLOGIES IN RUSSIA Vol. 5 Nos. 9–10 2010