We have studied the thickness dependence of the spinodal decomposition in thin films of a binary polymeric mixture. Nuclear reaction analysis (NRA) and time-of-flight forward recoil spectrometry (TOF-FRES) have been used to monitor the phase separation in thin films of poly(ethylenepropy1ene) (PEP) and perdeuterated poly(ethylenepropy1ene) (dPEP) after a quench into the two-phase region. The composition profiles are modeled by two distinct spinodal waves originating from the two surfaces of the films, and interference effects are observed as the film thickness is varied. Below a critical film thickness the coarsening of the phase structure is found to be severely altered with respect to the thick film behavior. Cell-dynamical simulations have been performed which closely resemble the experimental findings.The spinodal decomposition of binary mixtures has been a matter of considerable interest in the past, both experimentally and the~retically.'-~ Following a rapid quench from the one-phase region into their miscibility gap, bulk fluids phase separate through an exponential growth of concentration fluctuations with a characteristic, time-independent wavenumber qm. At later times, the wavenumber shifts toward zero, leading to an increasingly coarsened phase structure. This process is characterized by an isotropic structure factor, reflecting the translational and rotational symmetry of the bulk fluid. However, a quite different behavior can be expected when the symmetry is broken, e.g., in the vicinity of a surface.69 This has first been suggested by Ball and Essery on theoretical grounds6 and was confirmed shortly after by Jones et al.,l0 who observed a strongly anisotropic structure factor in the spinodal decomposition of thin films of an isotopic polymer blend. The film surface was found to be enriched in one of the components, and an oscillatory composition profile was observed perpendicular to the surface. The characteristic wavelength of this layered structure was found to grow considerably slower with respect to the bulk behavior.'l In a similar kind of experiment, Bruder and Brenn concentrated on the equilibrium phase structure at late times;12 they could show that a stable layered structure of binodal compositions can only be established when both interfaces are completely wet by one of the components. Otherwise, the competing growth of bulk domains was found to eventually destroy the layered structure.In the present study, we shall investigate the influence of the film thickness on the spinodal decomposition of a binary polymer mixture. Assuming an attraction of either of the two components to both the polymerlvacuum and the polymer/substrate interfaces, respectively, one expects two separate spinodal waves to extend into the film which eventually will interact with each other as the film thickness becomes comparable to the decay length of the oscillations. We present the first experimental evidence for both constructive and destructive interference of these waves by varying the film thickness. In additio...

We have studied the growth of the wetting layer formed at the surface of a critical mixture of poly (ethylene propylene) and perdeuterated poly (ethylene propylene) during spinodal decomposition. The wavelength of the surface spinodal wave is found to grow as t 1/3 over the entire range of times accessible in the experiment. The composition profiles exhibit universal scaling behavior in the near-surface region. These results are in excellent agreement with a recent numerical study on surface-directed spinodal decomposition.PACS numbers: 61.25.Hq, 05.70.Fh, 64.60.Ht, When a binary mixture is quenched from a single homogeneous phase to a point inside the spinodal curve, spontaneous phase separation takes place. For bulk systems, this process has been extensively studied in the past both by theory and experiment [1][2][3][4][5][6]. Several distinguishable time regimes during the evolution of bulk spinodal decomposition are now well established. The initial stage is described by the linear theory of Cahn, Hillard, and Cook, while at later times nonlinear effects become dominant and a scaling description is applicable, where the average size of the domains R(t) is the only relevant length scale in the system. In this regime, R(t) is found to grow like R(t) -t ,/3 . For fluid mixtures, however, hydrodynamic effects become important at later times and the growth law is found to change to R(t)~~t in the socalled final stage of spinodal decomposition.Only recently, it has been recognized that in the vicinity of a symmetry breaking surface the process of spinodal decomposition may be severely altered as compared to the bulk behavior [7], If one of the two components of the mixture is preferentially attracted to the surface, a quite anisotropic evolution of the phase separation is expected to occur in the near-surface region. This was first observed experimentally by Jones et al. [8] who found an oscillatory composition profile near the surface of an isotopic polymer mixture after a quench into the two-phase region. Meanwhile, different theoretical studies have focused on this effect [9-11], aiming toward a detailed understanding of the time dependence of the spinodal decomposition close to an attractive wall. For sufficiently deep quenches, both simple scaling arguments [10] and coarse-grained order parameter simulations [10,11] predict a / 1/3 growth of the wetting layer thickness at later times, closely resembling the bulk behavior. In addition, the composition profiles perpendicular to the wall are expected to show scaling behavior; i.e., they should collapse onto a unique master curve when rescaled with the actual wetting layer thickness. In contrast to these clear and unanimous theoretical results, the experimental data on surface-directed spinodal decomposition are still rare and an unambiguous analysis of the time dependence of the "surface spinodal wave" is still lacking. In this Letter, we present the first experimental evidence for both a power law growth of the wetting layer thickness /(/) during spinodal decomp...

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ABSTRACT:The thermal properties of amorphous gelatin films and gelatin films with renatured structural order were measured by using conventional and temperature modulated differential scanning calorimetry (DSC). Different amounts of gelatin structural order associated with a melting enthalpic change in the DSC measurement were prepared based on different gelatin drying conditions. Two consecutive heating and cooling DSC measurements on the gelatin films showed that there was no change in the glasstransition temperature (T g ) for the amorphous gelatin but there was a decrease in the T g for the structural gelatin on the second DSC scan. This decrease was attributed to the plasticizing effect from the release of originally hydrogenbonded water associated with the structural gelatin. In addition, a reversing endotherm observed upon melting of the structural gelatin during a temperature modulated DSC measurement indicated that the transition of bound water to free water occurred as the partial triple-helix gelatin melted.

An all-conjugated diblock copolymer, poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-hexylthiophene) (PPP-b-P3HT), was synthesized and applied as a hole transport material (HTM) for the fabrication of solid-state dye-sensitized solar cells (ss-DSCs). This copolymer is characterized by an enhanced crystallinity, enabling its P3HT component to self-organize into interpenetrated and long-range-ordered crystalline fibrils upon spin-drying and ultimately endowing itself to have a faster hole mobility than that of the parent P3HT homopolymer. Transient photovoltage measurements indicate that the photovoltaic cell based on PPP-b-P3HT as the HTM has a longer electron lifetime than that of the reference device based on P3HT homopolymer. Moreover, comparing the two ss-DSCs in terms of the electrochemical impedance spectra reveals that the electron density in the TiO2 conduction band is substantially higher in the PPP-b-P3HT device than in the P3HT cell. Above observations suggest that the PPP block facilitates an intimate contact between the copolymer and dye molecules absorbed on the nanoporous TiO2 layer, which significantly enhances the performance of the resulting device. Consequently, the PPP-b-P3HT ss-DSC exhibits a promising power conversion efficiency of 4.65%. This study demonstrates that conjugated block copolymers can function as superior HTMs of highly efficient ss-DSCs.