Partial crystallinity in alkyl side chain polymers dictates surface freezing

We have studied the structure of a novel crystalline surface monolayer on top of a disordered melt of the same material [poly(n-alkyl acrylate)s] using grazing incidence x-ray diffraction. The grazing incidence x-ray diffraction, surface tension, and bulk latent heat results show that side chains crystallize except the nine methylene units of the alkyl side chains closest to the polymer backbone. The partial crystallinity along with a thicker surface layer, due to the additional length of the linker group, explains why the difference between the surface order-to-disorder transition temperature and bulk melting temperature increases with a decrease in the length of the alkyl side chain.     

Adsorption of polymer chains at penetrable interfaces

We investigate the problem of adsorption (localization) of polymer chains in the system of two penetrable interfaces within the mean-field approximation. The saturation of the polymer system in the limit case of zero bulk concentration is studied. We find the exact solution of this mean-field polymer adsorption problem that opens the possibility to treat various localization problems for polymer chains in such environments using appropriate boundary conditions. The exact solution is controlled by a single scaling variable that describes the coupling between the interfaces due to the polymer chains. We obtain a nonmonotonic behavior of the amount of adsorbed polymers as a function of the distance between the interfaces. This leads to a high-energy and a low-energy phase for the double layer with respect to the amount of polymers localized. At the saturation point, we find the total energy of the system and determine the force acting between the interfaces to be strictly attractive and to monotonically decay to zero when the interface distance increases.     

Photo-orientation at liquid crystal—polymer interfaces

It is suggested that liquid crystal—polymer interfaces are coupled systems, in which the components mutually influence the orientational state of each other. The photo-orientation process at liquid crystal-polymer interfaces provides a striking example of such a coupling. Experiments show that the anisotropic structure generated by polarised light at a polymer surface is strongly affected by the phase of the liquid crystal covering the polymer. Photo-orientation is significantly more efficient when the liquid crystal is in the isotropic phase than when it exhibits orientational order. The observations are interpreted by assuming that in the smectic and nematic phases the liquid crystal stabilises to a large extent polymer chain-segments aligned parallel to the director, while it blocks the photo-induced formation of chain-segments in the perpendicular direction. Other situations, in which the coupling between the liquid crystal and the polymer can be important, are also discussed briefly.     

Adsorption of polymer chains at two impenetrable interfaces

We study the problem of adsorption of polymer chains in a system containing two impenetrable attracting interfaces within the mean-field approximation. We find the exact solution of this mean-field polymer adsorption problem that is controlled by a single scaling variable describing the coupling between the impenetrable interfaces due to the polymer chains. At the saturation point we obtain the total number of adsorbed monomers, the total energy of the system and the force acting between impenetrable interfaces that is turned to be strictly attractive and monotonously decaying towards zero for increasing distance between the interfaces.     

Dielectric properties of liquid-crystal azomethine polymer with alkyl-substituted side chain

Physics of the Solid State - Relaxation processes in side-chain liquid-crystal azomethine polymer have been studied using dielectric spectroscopy in wide ranges of temperatures and frequencies. Two...     

Deviations from liquidlike behavior in molten polymer films at interfaces

We have performed x-ray specular and diffuse scattering on liquid polymer films and analyzed the spectra as a function of film thickness and molecular weight. The results show that films whose molecular weight is close to the entanglement length behave as simple liquids except that the shortest wavelength is determined by the radius of gyration (R(g)) rather than the monomer-monomer distance. When the molecular weight is higher than the entanglement length, the strong deviations from liquidlike behavior are observed. We find that the long wavelength cutoff vector, q(l,c), scales with film thickness, d as d(-1.1+/-0.1) rather than the usual d(-2) expected for simple liquids. If we assume that these deviations are due to surface pinning of the polymer chains, then our results are consistent with the formalism developed by Fredrickson et al. to explain the capillary wave spectrum that can propagate in a polymer brush.     

Structure of polymer solutions at interfaces: a Monte Carlo simulation study

The canonical ensemble Monte Carlo method is applied to study the structure of polymer solutions confined between surfaces. The polymer molecules are modeled as fused-sphere freely rotating chains with fixed bond length and bond angles and the solvent as hard spheres. The simulation results for the configurational and conformational properties of the chains are presented with varying interfacial distances, chain concentrations, and chain lengths. The chains are depleted at the wall at lower density, which, however, becomes less at higher density. With an increase in the interfacial distance, the enhancement/depletion of the chains at the wall becomes more marked. At all interfacial distances and chain lengths, increasing the concentration of the solvent makes the oscillation in the density profile of the chains more pronounced. Conformational properties provide important indications regarding the behaviour of chains as they approach surfaces.     

Compact tunable three-dimensional polymer optical waveguide comb filter

A compact channel spacing tunable polymeric three-dimensional (3D) comb filter has been designed and fabricated. This comb filter is made of three waveguides laid in two different but parallel layers: one is in the lower layer and the other two are in the upper layer. These two waveguides in upper layer do not cross each other but instead each twice cross over the waveguide in the lower layer. At each crossing point an electrode heater is deposited on the top surface to adjust the coupling ratio between these guides. By supplying power to each heater pair, respectively, the channel spacing can be selected to be either 1.6 nm or 0.8 nm; in addition a flat-top spectral response may be obtained at the 1.6 nm channel spacing if power is supplied to both the heater pairs simultaneously.     

NMR investigation of interfaces in polymer composites

Nuclear magnetic resonance is used to study the interface between (i) a polymer matrix and silica particles and (ii) two polymer components in a polymer blend.     

Chains at interfaces

Simha, together with Frisch and Eirich, was among the first to realize that in energetic interaction with a surface a flexible polymer chain would be under a major conformational constraint which could be analyzed by solving a random walk (diffusional) problem near a barrier. These results stimulated more work, and today we have a fairly good knowledge of the factors which determine polymer adsorption from very dilute solution. Conformational problems arise also when more concentrated systems are used and it has become apparent that as concentration increases the surface phase will, more and more, be scaled by the root mean square diameter of the undeformed macromolecule. One can view the presence of an energetically attractive interface as creating a sink for macromolecules. The closer a polymer solution is to its Θ point, the more easily the polymer in solution can enter the sink and the stronger will be the extent of adsorption. The presence of an interface can thus be seen as a means to produce a phase separation in a polymer system which would otherwise be stable. Coatings of adsorbed polymers over colloidal particles create “macromolecular” solutions, where the “macromolecule” is the colloid particle with its adsorbed coat. This causes polymer segment/polymer segment contacts to dominate the encounter energetically. Hence, the solution behavior of the coated colloid becomes reminiscent of the solution behavior of the polymer in the coat. Stabilization by adsorbed polymer layers derives from converting the original solubility characteristics of the colloid, from what they were, essentially to those of the polymer being used.     

Gels at interfaces

We discuss the adsorption of polymer gels on flat surfaces. Even in cases of complete wetting where the spreading power S is positive and where an equivalent liquid would spread, the elastic stresses due to the gel deformation upon adsorption oppose the spreading. The competition between elasticity characterized by the bulk shear modulus G and capillarity characterized by the spreading power S defines a typical length scale ℓ = S/G for the deformation in the gel. For loose gels ℓ can be of the order of 1 μm. Macroscopic gels larger than ℓ deform only at their edges over a region of size ℓ. Microscopic gels smaller than ℓ show a finite deformation despite the elastic stresses. The elastic stresses limit the spreading of the polymer, but solvent can be sucked out of a swollen gel by wetting the surface. The thin solvent film can extend rather far from the gel edge and carry solvent. We calculate the kinetics of the solvent film formation and of the solvent transfer from a more swollen gel to a less swollen gel. Received 16 July 2001     

Stationary polaron motion in a polymer chain at high electric fields

We study the stationary motion of a polaron in a conducting polymer in the presence of a high electric field. Using the Su-Schrieffer-Heeger model plus an electric field, we find that at polaron velocities not exceeding the sound velocity, the dissipation of the electronic energy into the lattice occurs via emission of phonons with single selected wave vector. For this case the corresponding contribution to the polaron mobility can be calculated analytically. We discuss the issue of the polaron stability with respect to dissociation in a very high field at supersonic velocities.     

Dissociation of bipolaron in non-degenerate polymer chain at high electric fields

We investigate the dynamics of bipolaron in non-degenerate polymer (e.g. PPV) in an external electric field by using a nonadiabatic evolution method, which allows transition between instantaneous electronic states. When the applied electric field exceeds a critical value, a bipolaron is found to dissociate like the case of polaron due to the lattice distortion not being able to follow the fast moving electrons. The critical value is estimated to be of order ¹⁰⁶ V/cm${10}^6\text{ V}/\text{cm}$. This result is consistent with experiment in that a large increase in current in PPV occurs at high fields. At a given electric field, the saturation velocity of bipolaron will decrease with the increase of non-degenerate parameters te$t_e$.     

Dielectric properties of side chain liquid crystalline elastomers: influence of crosslinking on side chain dynamics

Dielectric response, from 1 Hz to 10 MHz, of liquid crystalline side chain elastomers has been compared to the one of analogous uncrosslinked materials. Results are discussed in terms of mobility of the mesogens. Two different systems have been investigated: smectic A elastomers allow to determine the influence of crosslinking on the relaxation (i.e. the reorientation of the whole mesogen around the chain), a S _C ^* elastomer shows the drastic influence of the polymer network on the Goldstone mode. Received 12 October 1998     

Superconductivity at disordered interfaces

The increase of the superconducting transition temperature T_c due to the tunneling of conduction electrons into negative-U centers at a disordered metal-semiconductor interface is calculated. The strong dependence of the experimental increase of T_c on the Fermi energy of the metal is accounted for by the polaronic reduction of the tunneling matrix elements. The latter reduction is dynamically suppressed by the decreasing lifetime of the localized state as E_F increases. The theoretical enhancement is sufficiently strong to explain the increase of T_c observed in eutectic alloys.     

Multiblock copolymers at interfaces

Summary We consider the configurations and the various regimes when an alternating, symmetric, multiblock copolymer made of successive sequences of A and B monomers with equal lengthZ is at the interface between two immiscible solvents that are respectively good for one of the sequences and poor for the other one. We find two semi-dilute regimes, when the copolymers overlap. For monomer concentrations belowГ ^**, we find a first range where the thickness of the copolymers is constant and equal to the radius of a sequence. For concentrations larger thanГ ^**, a second regime appears, where the seqeunces stretch our and adopt a conformation similar to what was reported for grafted polymers. Finally at a concentrationГ _sat, the interface is completely saturated, and the copolymers form micelles in the bulk: it becomes energetically more favourable to have copolymers in the bulk than at interface. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4‘8, 1994.     

Second-harmonic generation at interfaces

Second-harmonic light can be generated at dye coated surfaces of fused silica. By changing either the angle of incidence or the frequency of the fundamental, phase matching of signals produced at the two surfaces of a plane-parallel plate has been achieved. The interaction of light from multiple internal reflections has also been studied.