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Dynamics in poly(dimethylsi1oxane) (PDMS) melts from Tg + 75 to T, + 175 K have been measured by the fluorescence anisotropy decay of a probe chromophore. The reorientational dynamics of the probe chromophore, 5-(dimethylamino)-1-naphthalenesulfonamide (dansylamide), attached to a trifunctional silane, are characterized in a small molecule solvent, cyclohexanol, and compared to its reorientation in the polymer system. In cyclohexanol, the orientational dynamics obey the Debye-Stokes-Einstein equation with a thermal activation energy equal to that of the cyclohexanol viscosity. In contrast, the rate of reorientation of the probe dispersed in PDMS polymer melts does not reflect the bulk properties of the samples. The local dynamics are exponentially activated, with activation energies that are higher than that of the viscosity of the bulk material. This result is different than conclusions of analogous studies made on carbon-based polymers. Two possible explanations are given based on the unique characteristics of the silicon-oxygen bonds in PDMS.

Methods to chemically passivate the surfaces of amorphous-carbon films (a-C) produced by dc magnetron sputtering were studied. The chemical composition of carbon surfaces produced via sputtering are dependent upon the environment to which the carbon is exposed immediately following deposition. When the sputtered film is vented to ambient conditions, free radicals produced at the surface during the deposition process are quenched by reaction with oxygen and/or water to form an oxidized, hydrophilic surface. If the sputtered carbon film is, however, exposed to a reactive gas prior to venting to ambient, the chemical nature of the resulting surface can be modified substantially. Specifically, a less highly oxidized and much more hydrophobic carbon surface is produced when the surface free radicals are quenched via either an addition reaction (demonstrated with a fluorinated olefin) or a hydrogen abstraction reaction (demonstrated with two alkyl amines). Chemical modification of amorphous-carbon films can also be accomplished by performing the sputtering in a reactive plasma formed from mixtures of argon with molecular hydrogen, amines, and perfluorocarbons. The elemental composition of these films, and the relative reactivity of the surfaces formed, were investigated via x-ray photoelectron spectroscopy and contact-angle goniometry, respectively. In the case of sputtering with a mixture of argon and hydrogen, increasing the hydrogen flow results in an increase in the amount of hydrogen incorporated into the carbon film and a decrease in the surface free energy. Sputtering in diethylamine produces an amorphous-carbon film into which nitrogen is incorporated. The free energies of the a-C:N surfaces produced in this process are similar to those of the a-C:H films. Sputtering in a fluorocarbon vapor results in the incorporation of fluorine into the film structure and the formation of very low free-energy surfaces. Increasing the concentration of the fluorocarbon in the sputtering plasma increases the amount of F incorporated into the film. At the highest fluorocarbon flow rates employed, a-C films were produced with stoichiometries and surface free energies comparable to those of bulk Teflon.

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Ultrathin fluoropolymer films were deposited by a plasma-enhanced chemical vapor deposition process, from mixtures of hexafluoropropylene (C3F6) and octafluoropropane (C3F8) precursors. Different monomer feed compositions result in different rates of deposition. The C3F6 is the primary polymerizing species and C3F8 acts to reduce the deposition rate. The structure and composition of these films were characterized using Fourier transform infrared and angle-resolved x-ray photoelectron spectroscopy. The bulk F:C ratio is 1.1 for the homopolymer film (deposited from C3F6 only) and 1.4 for the copolymer film (deposited from C3F6+C3F8 monomer feed). The copolymer film has more –CF3 groups and has a more highly branched structure. The surface F:C ratio is 1.5 for the homopolymer and 1.7 for the copolymer film. The water contact angle increased with film thickness and with addition of C3F8 to the monomer feed. A mechanism is presented to describe how the addition of C3F8 modifies the polymerization process, causes a high surface F:C ratio of the copolymer films, and leads to reduced surface energy.

A series of end-linked poly(dimethylsiloxane) (PDMS) networks were prepared with different cross-link functionalities and molecular weights. This was achieved by simultaneous end-linking and self-condensation of a trifunctional silane cross-link precursor. These networks had a nonpolar naphthalene chromophore covalently attached to a fraction of the cross-link junctions. We probe the time-dependent reorientation of the naphthalene, and infer reorientation of the cross-links, by determining the time-dependence of the fluorescence depolarization in the picosecond time domain. A two-step relaxation model describes the orientational dynamics. Fast, partial depolarization in a restricted geometry is superimposed on a slower relaxation that completely depolarizes the fluorescence. We determine the two rotational diffusion constants at temperatures varying from 235 to 298 K, while we vary network parameters such as cross-link density, molecular weight, and macroscopic strain. These diffusion constants have an Arrhenius activation energy of 11.4 ( 0.8 kJ/mol. The fast relaxation is driven by motions of a few chain segments; this process is dominated by the density of the network polymer around the labeled cross-links. The slower, complete reorientation is driven by cooperative motions of a larger number of chain segments connected to the cross-link that are insensitive to steric constraints in the immediate vicinity of the cross-links.

Poly(dimethylsi1oxane) (PDMS) networks were fluorescently labeled with the 1-(dimethylamino)-5-naphthalenesulfonyl (dansyl) chromophore. The networks were prepared by end-linking silanolterminated PDMS with a mixture of methyltriethoxysilane (MTES) and N-((triethoxysily1)propyl)-dansylamide (DTES). The selective sorption behavior of these networks swollen in binary mixtures of 1,Cdioxane and a series of water and linear alcohols was studied. Refractometry and equilibrium swelling measurements were used to determine the amounts of PDMS and solvent components inside the swollen networks. We find that the cosolvency behavior of the solvent mixture depends on the alkyl chain length of the amphiprotic solvent. Steady-state fluorescence spectra of the dansyl compound dissolved in binary solvent mixtures are compared to the fluorescence of dansyl-labeled networks swollen with the same solvent mixture. Differences in emission energy are related to differences in the polarity of the solvation shells surrounding the dansyl moieties. The composition around the labeled cross-link junctions is compared with the bulk composition to determine spatial heterogeneity in the distribution of solvent and polymer molecules. We find that while dioxane is preferentially sorbed, the alcohols preferentially solvate the fluorescent moieties. Water has a tendency to cluster inside the networks that is stronger than its tendency to preferentially solvate dansyl.

Abstract