A novel experimental approach involving fluorescence nonradiative energy transfer (NRET) is employed to study the Fickian diffusion of small molecules in rubbery polymer films near the glass transition. A theoretical formalism has been developed which directly relates the small molecule translational diffusion coefficient, 𝒟, to changes in the energy transfer efficiency, E. Values of 𝒟 as low as 5 × 10-16 cm/s have been measured. In this approach, two thin polymer films are sandwiched together, one labeled with either NRET donor or acceptor chromophores and the second doped with the complementary chromophore. Upon annealing for a time t, dopant chromophore diffusion occurs in which E is proportional to (𝒟t)1/2/w, where w is the donor film thickness. Values of 𝒟 for pyrene, N-(2-hydroxyethyl)-N-ethyl-4-(tricyanovinyl)aniline (TC1), bis(phenylethynylanthracene) (BPEA), and decacyclene in poly(isobutyl methacrylate) (PiBMA) and for BPEA in poly(ethyl methacrylate) (PEMA) have been measured over temperatures ranging from ca. T g to T g + 20 °C. Among these chromophores, significant differences in both the magnitude and temperature dependence of 𝒟 were observed and are attributed to differences in molecule size, shape, and flexibility. Two anomalous effects are observed from a comparison of translational diffusion and rotational reorientation dynamics of TC1 in PiBMA near T g. The first is an apparent enhancement in translational diffusion relative to rotational reorientation dynamics, with the average translational displacement of a chromophore during an average rotational relaxation time, 〈τrot〉, being a couple orders of magnitude larger than the length of the molecule. This behavior may be explained by significant local-scale heterogeneity in the polymer, i.e., the broad distribution of polymer α-relaxation times. The second regards the different temperature dependencies of 〈τrot〉 and 𝒟 near T g. This may be explained qualitatively by a strong temperature dependence of the breadth of the distribution of α-relaxation times, an effect known to be present in the TC1−PiBMA system employed in this comparison as well as a variety of other polymer systems near T g.
Atomistically detailed models of free-standing thin films and the bulk structure of amorphous atactic polystyrene have been produced by reverse mapping from equilibrated coarse-grained models. The bridging technique employed in the simulation allows the generation of a moderate sized atomistic system (six independent parent chains of C 400 H 402 , 4812 atoms) with a more reasonable computational effort than is required when all of the construction is performed on chains expressed with atomistic detail. Reverse mapping from the coarsegrained model to the atomistically detailed model is found to be straightforward, without ring piercing or concatenation. The calculated surface energy (38 ( 10 erg/cm 2 ) is in reasonable agreement with prior experimental findings. The surface of the thin films is enriched in phenyl rings. The rings at the surface tend to be oriented so that they are pointing outward, but rings in the middle of the thin film show no preferred orientation. In contrast with the phenyl rings, the bisectors for the methylene groups show little tendency for orientation, even when the methylene groups are close to the surface. These observations in the simulation are in qualitative agreement with conclusions reported recently (Gautam et al. Phys. ReV. Lett. 2000, 30, 3854, and Briggman et al. J. Phys. Chem. B 2001, 105, 2785, based on the application of new spectroscopic techniques to the characterization of polymer surfaces.
We present a systematic study of how adsorption history affects the thickness, surface forces, and interfacial rheology of a model cationic polymer. The polymer was quaternized poly-4-vinylpyridine, QPVP (weight-average degree of polymerization n w = 325 and 98% quaternized with ethyl bromide). The main comparisons concerned one-step adsorption from solution at a variable salt concentration up to 0.5 M NaCl, versus two-step adsorption (initial adsorption from buffer solution without added salt, then NaCl added later). The aqueous solutions were buffered at pH = 9.2 such that the surfaces (mica in the case of surfaces forces (SFA) experiments, oxidized silicon in the case of in situ infrared (FTIR-ATR) experiments) in each case carried a large negative charge. The SFA and FTIR-ATR experiments gave consistent estimates of the amount of polymer adsorbed, confirming the expectation that adsorption should be driven by electrostatic attraction to the surface of large opposite charge. The adsorbed amount showed little dependence on path, validating the common assumption of equilibration in this respect. However the layer thickness measured by surface forces, the shear nanorheology response at a given surface force, and the dichroism of pendant side groups of the polymer all showed a pronounced dependence on the path to reach the adsorbed state. We interpret the measurements to suggest that two-step adsorption produces an inhomogeneous layer comprised of a dense layer of segments closest to the solid surface and a sparse outer layer. In particular, two-step adsorption produced thicker layers and a greater tendency to decouple shear forces from those that resist compression in the normal direction, thereby lessening the shear forces at a given level of normal force.
Fluids of mesoscopic thickness can be sheared and their molecular orientation probed concurrently with the new instrument described in this paper. The fluid is confined between parallel optically flat windows whose spacing is controlled, using piezoelectric inchworms, from submicrometer thickness to ∼500 µm, with no essential lower limit apart from surface roughness. Capacitance sensors or optical interferometry is used to monitor spacing between the windows with submicrometer accuracy. Piezoelectric bimorphs are used to apply periodic shear displacements with amplitude 0.l-10 µm and frequency 0.1-700 Hz. Shear-induced molecular alignment during sinusoidal shear cycles is determined, with up to 5 µs time resolution, using step-scan time-resolved infrared spectroscopy. To demonstrate capabilities of this new instrument, we describe an experiment in which shear and electric fields were applied in orthogonal directions to 5-cyanobiphenyl (5CB), a simple nematic liquid crystal. Provided that the molecule lacked the time to relax during the period of oscillation, the molecule tilted back and forth around the equilibrium orientation under the action of small-amplitude oscillating shear. The shear alignment appeared to be proportional to the shear displacement, not to the effective shear rate.
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