Polymer dynamics at the melt/solid interface: experimental evidence of reduced center of mass mobility

Investigation of Diblock Copolymer thin film Morphology for Nanolithography

The microphase separated morphology of diblock copolymers can be used to generate well-ordered nanometer scale patterns over a large area. To achieve this goal, it is important to understand and control the behavior of diblock copolymer thin films on substrates, which can differ from the bulk behavior. We have investigated the morphologies and ordering in thin polystyrene-polybutadiene (PS-PB) diblock copolymer films on bare silicon and silicon nitride substrates, and also on polymethylmethacrylate (PMMA) coated substrates. The PS-PB copolymers are synthesized to form, in bulk, PB cylinders or spheres in a PS matrix. In thin films (10–60 nm thick), prepared by spin-coating, we observe that the morphology and ordering of the microdomains are affected by strong wetting constraints and a reduced chain mobility on the substrate. The thinnest self-assembled layer of the copolymer films shows no in-plane microphase separation on both types of substrates. The PS blocks wet the PMMA substrates whereas the PB blocks wet the bare substrates as well as the air interface. Hence, different film thicknesses are necessary on the two types of substrates to obtain a uniform film of the first self-assembled cylindrical or spherical microdomain layer. The first layer of the cylindrical copolymer can vary from cylindrical to spherical morphology with a few nanometer decrease in film thickness. In the case of spherical PS-PB diblock copolymer films, we observe that the ordering of the microdomains is improved in the films on the PMMA substrates, compared to those on the bare substrates. We also demonstrate a successful transfer of the microdomain patterns to silicon nitride substrates by a reactive ion etching technique.

Modeling diffusion in miscible polymer blend films

Recent experiments designed to probe polymer transport in the bulk and in the vicinity of surfaces have examined the interdiffusion of multilayer sandwiches of isotopically labeled polymers. The measured time dependent concentration profiles normal to the surface are typically fit to Fick's law, with a single fitting parameter, the mutual binary diffusion coefficient (MBDC). The resulting MBDCs are found to vary over a broad range of film thicknesses and time, with the time dependence being viewed as a unique signature of the reptation mechanism of long chain motion, and the thickness dependence being attributed to the slowing down of chain dynamics near surfaces. Since the experiments are conducted at finite concentration, the MBDC, which is a product of the bare mobility and the concentration derivative of the chemical potential, could be dominated by the time and thickness dependence of this second term (which is ignored in Fick's law). To quantify this conjecture we consider the more rigorous Cahn formulation of the diffusion problem in terms of chemical potential gradients. We use square gradient theory to evaluate chemical potentials, and fit the resulting time dependent concentration profiles to the analytical solution of Fick's law. By thus mimicking the experimental analysis we find that the apparent MBDCs vary with time as t(-1/2) at short times, in good agreement with existing experiments. We show that this time dependence reflects the system's desire to minimize concentration gradients, a fact ignored in Fick's law. Since these arguments make no reference to the mechanism of chain motion, we argue that the time dependence of MBDC derived from interdiffusion experiments does not provide unequivocal support for the reptation mechanism of long chain transport. The MBDC values, which also vary with the degree of confinement, are predicted to increase with decreasing thickness for model parameters corresponding to experimental systems. In contrast, since the experimental fits yield an opposite trend, we suggest that the bare mobility of the chains decreases strongly with decreasing thickness. These findings strongly support the idea that the chains are "pinned" irreversibly to the surfaces, in good agreement with other, independent experiments.

New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. II. Study of the transition temperatures of poly(methyl methacrylate) at various tacticities and of poly(methyl methacrylate) adsorbed on alumina and silica

The determination of the temperature transitions in polymers and more particularly when polymers are adsorbed on oxides is very important in many industrial processes. In this second part, we used inverse gas chromatography (IGC) at infinite dilution to determine the second order transition temperatures of poly(methyl methacrylate) (PMMA) adsorbed (or not) on alumina or on silica. Three types of PMMA were used: atactic (a), syndiotactic (syndio) and isotactic (iso). The IGC technique allowed to obtain the net retention volume Vn and the dispersive component of the surface energy gamma(s)d for various theoretical models of molecules, using the results of Part I. By plotting RT ln Vn as a function of (1/T) or gamma(s)d as a function of the temperature T, we proved the presence of three particular temperatures that correspond respectively to the transition temperature relative to beta-relaxation (Tbeta), the glass transition temperature (Tg), and the liquid-liquid transition temperature or order-disorder transition (T1.1). Results obtained in this part allowed us to show the effect of the tacticity of PMMA and the recovery fraction of polymer adsorbed on alumina or silica on the transition temperatures.

Study of the Second Order Transitions and Acid-Base Properties of Polymers Adsorbed on Oxides by Using Inverse Gas Chromatography at Infinite Dilution

In Part I, we gave the details concerning inverse gas chromatography (IGC) at infinite dilution and the methods and models that will be used to characterize solid substrates. This technique proved to be an excellent technique to determine not only the glass transitions, but also beta-transition and liquid-liquid transitions of polymers adsorbed on solid substrates. In this second part, we used the IGC technique to determine the second order transitions of the systems' polymethyl methacrylate (PMMA)/SiO(2) and PMMA/Al(2)O(3), at various covered surface fractions and for various tacticities of the polymer. The maxima of the dispersive component of the surface energy gamma(s)(d) of our two systems, obtained by IGC at infinite dilution, indicated clearly the presence of transition temperatures (glass or local transitions). In general, we observed with PMMA three principal maxima that reflect the changes in motions leading to reorganization and rearrangement of the various groups or chain segments of the polymer. The change in the retention mechanism of the probes at the transition temperatures is attributed to an increased molecular mobility of the polymer segments, allowing for the penetration of the probes into the polymer layer. The study of the chemical physical properties of PMMA/SiO(2) and PMMA/Al(2)O(3) revealed an important difference in the acidic and basic behavior, in Lewis terms, of oxide covered by various concentrations of PMMA. This study also highlighted an important effect of the tacticity of the polymer on the acidic basic character of PMMA adsorbed on oxides. Copyright 2001 Academic Press.