Effect of orientation and mobility of polymer molecules at surfaces on contact angle and its hysteresis

The contact angle of a water droplet on the surface of a solid polymer or hydrogel (water-swollen three-dimensional network) depends on whether a hydrophilic moiety of the polymer molecule is oriented towards the air interface or towards the bulk of the solid, but not on the hydrophilicity of the molecule. Therefore, the short-range rotational mobility of a polymer molecule has a major influence on the apparent hydrophilicity of a polymer surface as measured by the contact angle of water. By the came principle, the abnormally large hysteresis effect observed in advancing and receding contact angles of water on some polymer surfaces can be attributed to the reorientation of hydrophilic moieties of polymer molecules at the surface. These factors are demonstrated by selected polymer surfaces with different degrees of mobility at the polymer-air interface.     

Activation volumes associated with chromophore reorientation in corona poled guest–host and side-chain polymers

The decay of the second-order optical susceptibility χ⁽²⁾ as a function of temperature and pressure has been studied in a variety of corona poled guest–host and side-chain polymeric materials using second harmonic generation (SHG). The specific systems studied include the side-chain copolymer poly(disperse red 1 methacrylate-co-methyl methacrylate) (DR1-MMA) as well as the series of guest–host materials formed by individually dissolving the dyes Disperse Red 1 (DR1), Disperse Orange 3 (DO3), and N,N dimethyl-p-nitroaniline (DpNA) in poly(methyl methacrylate) (PMMA), polycarbonate (PC), and polystyrene (PS). In each of these systems, the observed relaxation of χ⁽²⁾ can be represented by a Kohlrausch-Williams-Watts stretched exponential, from which the decay time τ and decay distribution width β are determined. For pressures up to approximately 1000 atm, the natural log of the pressure shift factor is seen to vary linearly with applied pressure, yielding the activation volume for rotational reorientation of the chromophores in each system. The activation volumes are loosely correlated with dopant size in a given polymer host, but are not the same for a given dopant in different hosts. Modeling the chromophores as rotating cylinders, we show that the measured activation volumes do not correspond to the average volume swept out by the dye molecules as they reorient. On the other hand, the activation volumes for each of the three dyes dissolved in PS are seen to be in agreement with the measured activation volumes for the molecular motions associated with volume recovery in neat PS. Moreover, the activation volumes for DR1 and DpNA dissolved in PS are seen to correlate with the proposed couplings between the rotational reorientation of DR1 and the α-relaxation dynamics of PS and the slight decoupling of DpNA from the α-transition motion of PS. This correlation suggests a possible relationship between the activation volumes for chromophore reorientation and the size of the components of the host polymer or the volume swept through by the polymer components during structural reconfiguration. We demonstrate that assuming activation volumes for chromophore reorientation to be related to the size or motion of the polymer host constituents yields a consistent interpretation of the observed trends in the measured activation volumes. © 1995 John Wiley & Sons, Inc.     

Single molecule probing of dynamics in supercooled polymers

Fluorescence experiments with single BODIPY molecules embedded in a poly(methyl acrylate) matrix have been performed at various temperatures in the supercooled regime. By using pulsed excitation, fluorescence lifetime and linear dichroism time trajectories were accessible at the same time. Both observables have been analyzed without data binning. While the linear dichroism solely reflects single particle dynamics, the fluorescence lifetime observable depends on the molecular environment, so that the dynamics from the polymer host surrounding a chromophore contributes to this quantity. We observe that the lifetime correlation decays slightly faster than polarization correlation, indicating the occurrence of large angular reorientations. Additionally, dichroism time trajectories have been adducted to reveal directly the geometry of rotational dynamics. We identify small but also significantly larger rotational jumps being responsible for the overall molecular reorientation.     

Time-resolved fluorescence spectroscopy for illuminating complex systems

Over the years, the emissive characteristics (spectral, temporal, and polarization) of fluorophores have been widely used to probe a wide variety of systems. Fluorescence lifetime and rotational reorientation time measurements, in particular, offer a means to elucidate key details about complex systems. Further, because fluorescence occurs on the nanosecond (10⁻⁹ s) timescale, competing or perturbing kinetic processes like collisional quenching, solvent relaxation, energy transfer, and rotational reorientation can affect the fluorescence and hence be quantified. Thus, a carefully chosen and “placed” fluorophore can serve as an reporter on a wide range of nanosecond or faster events. This contribution is divided into three sections. The Theory section discusses time-resolved anisotropy and intensity decay kinetics (time and frequency domains), pump–probe spectroscopy, and up-conversion. The second section describes time-correlated single photon counting (TCSPC) and multifrequency phase-modulation fluorescence instruments. The final section is divided into subsections on the use of time-resolved fluorescence: (1) to study solvation dynamics, biochemical systems, polymer photophysics, and organized media; (2) as a tool in the separation sciences, microscopy, and sensing; and (3) coupled with multiphoton excitation strategies.     

Reorientation of a liquid-crystalline side chain polymer

The reorientation times of a side chain nematic polymer have been measured. The polymer samples were oriented in magnetic fields of 2·1 T and the reorientation experiments were carried out using a NMR spectrometer operating at 1·5 T. Temperature and twist angle dependencies of the proton NMR spectra were studied in detail and discussed with regard to the alignment of the mesogenic molecule groups in the polymer medium. An additional result concerns the homogeneous or inhomogeneous reorientation.     

Single-molecule microscopy studies of electric-field poling in chromophore-polymer composite materials

One strategy for increasing the efficiency of organic electrooptic devices based on chromophore-polymer composite materials is to improve chromophore ordering. In these materials, ordering is induced through the interaction of the chromophore dipole moment with an external electric field, applied at temperatures near the Tg of the polymer host, a process referred to as "poling". To provide insight into the molecular details of the poling process under conditions representative of device construction, the rotational dynamics of single 4-dicyano-methylene-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM) molecules in poly(methyl acrylate) at T = Tg + 11 degrees C in the presence and absence of an electric field are investigated using single-molecule confocal fluorescence microscopy. Single-molecule rotational dynamics are monitored through the time evolution of the fluorescence anisotropy. The anisotropy correlation function demonstrates nonexponential decay, with beta values derived from fits using the Kohlrausch-Williams-Watts law ranging from 0.7 to 1 with beta(KWW) = 0.83. This observation is consistent with previous studies of molecular rotation dynamics in polymer melts and reflects the dynamical heterogeneity provided by the polymer host. The rotational dynamics of DCM are weakly perturbed in the presence of a 50 V/microm electric field, typical of the field strength employed in device construction. The expected perturbation of the rotational dynamics is determined and found to be consistent with the alignment potential created by the electric field relative to the amount of thermal energy available. The relevance of these findings with respect to current models of the poling process is discussed. This work demonstrates the utility of polarization-sensitive single-molecule microscopy in elucidating the details of molecular reorientation during poling.     

Anisotropy time dependence of photoexcited C60 and C70 in transient grating experiments. Solvent effect

Anisotropy time dependence of photoexcited C₅₀ and C₇₀ has been measured by picosecond transient grating techniques at room temperature in various solvents. The monoexponential anisotropy decay was observed for the C₆₀ molecule and biexponential anisotropy decay was observed for the C₇₀ molecule. Both for the C₆₀ and for the C₇₀ molecules anisotropy time decay is very fast. The data obtained were analyzed in terms of the rotational reorientation of fullerene molecules in solvents. Hynes-Kapral-Weinberg theory reasonably explains the observed reorientation times of fullerenes molecules. The dielectric friction effect on the C₇₀ rotational reorientation on the short axis is reported.     

Is molecular rotation really influenced by subtle changes in molecular shape?

In an attempt to seek out whether the reorientation time of a solute molecule is influenced by marginal changes to its shape, rotational relaxation of four coumarin solutes that are almost identical in size but subtly distinct in shape has been investigated in a viscous nonpolar solvent as a function of temperature. It has been observed that the reorientation times of the four coumarins differ significantly from one another. The four solutes have been treated as asymmetric ellipsoids and Stokes-Einstein-Debye hydrodynamic theory has been employed to calculate the shape factors and boundary condition parameters. The measured reorientation times when normalized by respective shape factors and boundary condition parameters can be scaled on a common curve, which is an indication that ellipsoid based hydrodynamic theory is adequate to model the reorientation times even when the differences in the shapes of the solute molecules are minimal.     

Thermally enhanced photoinduced molecular reorientation in a photo-crosslinkable polymer liquid crystal based on polarization triplet energy transfer

Polarization-selective photosensitization and molecular reorientation in photo-crosslinkable polymer liquid crystal (PMCB6M) films, based on polarization-selective triplet energy transfer using LP-365?nm and LP-405?nm light with different photosensitizers, are described. A dramatic improvement in the photoreactivity of the film was observed with a small amount of photosensitizer; and a high degree of molecular reorientation and slantwise reorientation were generated when annealing the resulting films at elevated temperatures. The efficiency of reorientation depended on the photoinduced dichroism of the film.     

剪切场作用下环形二嵌段共聚物微相形态变化的耗散粒子动力学研究

采用耗散粒子动力学(Dissipative particle dynamics, DPD)方法研究了在剪切场作用下, 环形二嵌段共聚物微观相分离过程中的形态变化. 在层状(lamellae, LAM)体系中发生了微相的平行重取向和平行-垂直转变以及剪切导致的波动不稳定现象. 对于穿孔层状(Perforated lamellae, PL)体系, 强剪切导致了穿孔层状-柱状(Hexagonal cylinder, HEX)微相转变. 在剪切场作用下, 柱状体系中同样也有平行重取向发生. 可以用相区破坏-相区重生的两步机理描述微相的平行重取向、平行-垂直转变以及PL-HEX转变现象. 在球状相(Body centered cubic, BCC)体系中发现了剪切诱导相融合.     

Thermally enhanced photoinduced molecular reorientation in a photo-crosslinkable polymer liquid crystal based on polarization triplet energy transfer

Polarization-selective photosensitization and molecular reorientation in photo-crosslinkable polymer liquid crystal (PMCB6M) films, based on polarization-selective triplet energy transfer using LP-365 nm and LP-405 nm light with different photosensitizers, are described. A dramatic improvement in the photoreactivity of the film was observed with a small amount of photosensitizer; and a high degree of molecular reorientation and slantwise reorientation were generated when annealing the resulting films at elevated temperatures. The efficiency of reorientation depended on the photoinduced dichroism of the film.     

Effects of salt and nanoparticles on the segmental motion of poly(ethylene oxide) in its crystalline and amorphous phases: 2H and 7Li NMR studies

We use (2)H NMR to investigate the segmental motion of poly(ethylene oxide) (PEO) in neat and nanocomposite materials that do and do not contain salt. Specifically, in addition to a neat low-molecular-weight PEO, we study mixtures of this polymer with TiO 2 nanoparticles and LiClO 4. To characterize the polymer dynamics over a wide range of time scales, we combine (2)H NMR spin-lattice relaxation, line-shape, and stimulated-echo analyses. The results consistently show that the presence of nanoparticles hardly affects the behavior of the polymer, while addition of salt leads to substantial changes; e.g., it reduces the crystallinity. For neat PEO and a PEO-TiO 2 mixture, stimulated-echo spectroscopy enables measurement of rotational correlation functions for the crystalline phase. Analysis of the decays allows us to determine correlation times, to demonstrate the existence of a nonexponential relaxation, which implies a high complexity of the polymer dynamics in the crystal, and to show that the reorientation can be described as a large-angle jump. For a PEO-TiO 2-LiClO 4 mixture, we use (2)H and (7)Li NMR to study the polymer and the lithium dynamics, respectively. Analysis of the (7)Li spin-lattice relaxation reveals a high lithium ionic mobility in this nanocomposite polymer electrolyte. The (7)Li stimulated-echo decay is well described by a stretched exponential extending over about 6 orders of magnitude, indicating that a broad and continuous distribution of correlation times characterizes the fluctuations of the local lithium ionic environments.     

Rotational relaxation of neutral red in alkanes: effect of solvent size on probe rotation

Rotational reorientation times of a polar molecule neutral red (NR) have been measured in n-alkanes using steady-state fluorescence depolarization technique. The rotational dynamics of NR in alkanes is described by the Stokes-Einstein-Debye hydrodynamic theory with slip boundary condition. However, we have observed that as the size of the solvent molecule becomes bigger than the size of the solute molecule, the probe molecule experiences reduced friction and the experimentally measured reorientation times are shorter than those predicted by the hydrodynamic theory. These size effects have been analyzed using quasihydrodynamic theories.     

Photo-orientation of axial molecules

We examine the photo-orientation of molecules in a linearly polarized field and the ensuing optical anisotropy of a sample. We propose a theoretical model that considers both photoinduced reorientation and rotational diffusion, for the case of linear or axial molecules not interacting among them, as in dilute solutions in viscous media. We perform numerical simulations to highlight the dependence on the parameters of the molecular reorientation processes, on the intensity of the exciting light, and on the use of cross polarized pulses. As a realistic example we simulate the photo-orientation of azobenzene in ethylene glycol.     

Collective rotational dynamics in ionic liquids: a computational and experimental study of 1-butyl-3-methyl-imidazolium tetrafluoroborate

The aim of this study is the analysis of the rotational motion in ionic liquids, in particular, 1-butyl-3-methyl-imidazolium tetrafluoroborate. By comparing single-particle and collective motion it is found that the Madden-Kivelson relation is fairly fulfilled in long-term simulation studies (>100 ns), i.e., the collective reorientation can be predicted by the corresponding single-particle property and the static dipolar correlation factor, GK. Furthermore, simulated reorientation is in accordance with hydrodynamic theories yielding hydrodynamic radii comparable to van der Waals radii. Since viscosity is the central quantity entering hydrodynamic formulas, we calculated and measured the viscosity of our system in order to have two independent cycles of hydrodynamic evaluation, a computational and an experimental one. While the static dielectric constant agrees with dielectric reflectance experiment, the hydrodynamic radii derived from the experiments are much lower as a consequence of enhanced rotational motion. Even more, a considerable dynamic broadening is observed in the experiments.     

Nematic reorientation in electric and magnetic fields

Homogeneous reorientation processes of two nematic liquid crystals in electric and magnetic fields have been observed using proton nuclear magnetic resonance spectroscopy (NMR). Using a recently developed experimental set-up, it is possible to study reorientation processes in liquid crystals by means of NMR experiments in a very flexible way. The time constant τ describing these processes has been determined as a function of the applied electric field. It emerges that the electric field cannot only be used to increase the reorientation time but also to slow the director reorientation by approximately one order of magnitude. Experimental data for 5CB and a fluorinated liquid crystal (BCH-5 FFF) are presented. The reorientation time measured as a function of the electric field can be used to calculate the rotational viscosity γ ₁. By repeating these experiments at different temperatures it was possible to investigate the temperature behaviour of γ ₁.     

Rotational motions of biradical metal chelates in polar solvents

Models of motion continuously varying from random jumps to rotational diffusion have been tested to simulate the ESR lineshape of biradical metal chelates spin probes in the slow-motion region. The evidence of a diffusional regime for the molecular reorientation contrasts with the anomalous temperature dependence of the process.     

Fourier-transform infrared study of the switching process in a ferroelectric liquid crystalline polymer

The implementation of the step-scan technique on our FT-IR spectrometer enabled us to follow the electric-field induced reorientation dynamics of different molecular segments of a ferroelectric liquid crystalline polymer on a sub-millisecond time scale. It was detected that not only the mesogen but also the spacer and at least part of the backbone take part in the reorientation process.     

Fluorescence anisotropy of ionic probes in AOT reverse micelles: influence of water droplet size and electrostatic interactions on probe dynamics

Fluorescence anisotropies of two structurally similar ionic probes, rhodamine 110 and fluorescein, were measured in di(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles as a function of the mole ratio of water to surfactant W. This study was undertaken to explore the influence of water droplet size and electrostatic interactions on the rotational diffusion of the probe molecules. It was noticed that at W = 1 and 2, the anisotropy decays of both the probes display single-exponential behavior and for a particular value of W, the time constants sensed by rhodamine 110 and fluorescein are identical. Moreover, an increase in the reorientation time was observed from W = 1 to 2. These observations indicate that, at W = 1 and 2, it is the overall rotation of micelle which is responsible for the decay of the anisotropy and also rule out the possibility of internal rotation of the probes within the reverse micelles. However, from W = 4 to 20, the anisotropy decays of the probes could only be described by a biexponential function with two time constants. The rotational diffusion of rhodamine 110 and fluorescein in the above-mentioned range of W was rationalized using the two-step model. The average reorientation time decreases with an increase in W for both the probes, and this decrease is pronounced in the case of fluorescein compared to that in rhodamine 110. The decrease in the average reorientation time with W is due to the change in the micellar packing within the core. The significant reduction in the average reorientation time of fluorescein is a consequence of repulsive electrostatic interactions between the negatively charged probe and the anionic head groups of the surfactant AOT.     

Evaluating the role of chromophore side group identity in mediating solution-phase rotational motion

We report on the rotational diffusion dynamics of the chromophore 7-nitrobenz-2-oxa-1,3-diazole (NBD) in a series of protic and polar aprotic solvents, as a function of the identity of the side group appended to the chromophore amine functionality. The central issue we address is whether or not the side groups play a role in mediating the anisotropic reorientation dynamics of the chromophore. To understand the motional properties of the chromophores in detail, we use both one-photon and two-photon excited fluorescence anisotropy decay measurements, and from these complementary excitation methods, we extract two of the Cartesian components of the rotational diffusion constant, D. The experimental data indicate that, regardless of the functionality of the pendant side group, the reorienting moieties exhibit ratios of Dz/Dx in the range 1.8-2.0. There is a small but discernible difference between the substituted chromophores. For all of the substituted NBD chromophores, dielectric friction plays a discernible role in determining their reorientation dynamics.