Rotational diffusivity of rodlike molecules in amorphous polymeric matrices

In this note, scaling laws for rotational diffusivity of dilute monodisperse rigid-rod molecules (guest rods) in semidilute amorphous polymer solutions (host molecules) are derived. The coillike matrix molecules are modeled as a collection of flexibly connected rigid subunits. This allows an analogy with the Doi-Edwards theory for monodisperse rigid rods in semidilute solutions to be used in the analysis. Very strong dependencies are predicted for the rotational diffusivity of the rods on host polymer volume fraction and rod length. In semidilute polymer solutions the coils dramatically hinder the rotational freedom of the rods for r² ? ψ_p^?1, r being the rod aspect ratio and ψ_p the polymer volume fraction.     

The effect of restricted rotational diffusion on the dynamics of rodlike molecules in concentrated solution

The model of Doi for the rotational dynamics of rodlike molecules in solution is compared with that of Warchol and Vaughan and Wang and Pecora. The similarities in the results for dielectric relaxation are discussed and the application of the models to the behaviour of isotropic solutions, anisotropic (lyotropic-nematic) solutions, and mixtures of small molecules with polymers is briefly discussed.     

Inertial and bias effects in the rotational brownian motion of rodlike molecules in a uniaxial potential

Inertial effects in the rotational brownian motion in space of a rigid dipolar rotator (needle) in a uniaxial potential biased by an external field giving rise to asymmetry are treated via the infinite hierarchy of differential-recurrence relations for the statistical moments (orientational correlation functions) obtained by averaging the Euler-Langevin equation over its realizations in phase space. The solutions of this infinite hierarchy for the dipole correlation function and its characteristic times are obtained using matrix continued fractions showing that the model simultaneously predicts both slow overbarrier (or interwell) relaxation at low frequencies accompanied by intermediate frequency Debye relaxation due to fast near-degenerate motion in the wells of the potential (intrawell relaxation) as well as the high frequency resonance (Poley) absorption due to librations of the dipole moments. It is further shown that the escape rate of a brownian particle from a potential well as extended to the Kramers turnover problem via the depopulation factor yields a close approximation to the longest (overbarrier) relaxation time of the system. For zero and small values of the bias field parameter h, both the dipole moment correlation time and the longest relaxation time have Arrhenius behavior (exponential increase with increasing barrier height). While at values of h in excess of a critical value however far less than that required to achieve nucleation, the Arrhenius behavior of the correlation time disappears.     

Static and dynamic light scattering of a random coil polymer in rodlike polymer solution

We have studied ternary solutions of rodlike and random coil polymers in a common solvent. In these solutions, the rods scatter weakly, and almost all scattering comes from the coil component. From light scattering measurements, we gained information about the thermodynamic and hydrodynamic properties of the coil in rodlike polymer solutions.     

Electrochromism of indolinobenzospirans dissolved in rigid amorphous polymer matrices

The effect of a static electric field on the position of absorption bands of colored stereoisomers of two photochromic compounds of the indolinobenzospiropiran group dissolved in matrices of poly(methyl methacrylate), poly(n-butyl methacrylate), polystyrene, and polycarbonate at room temperature was studied. In the case of matrices with the polymethacrylates the effects observed show that applied electric field produces a shift of thermal equilibrium towards stablization of stereoisomers of larger dipole moment. On the basis of kinetic measurements of attainment of thermal equilibrium on switching the electric field off, the rate constants of their interconversion have been determined. In the case of matrices with polystyrene and polycarbonate, a small red shift only was observed.     

Photochromism of diarylethene single molecules in polymer matrices

Robust fluorescent photoswitching molecules, having perylene bisimide as the fluorescent unit and diarylethene as the switching unit, were prepared, and their photochromic reactions were measured at the single-molecule level in various polymer matrices. The histograms of the fluorescent on and off times were found to deviate from normal exponential distribution and showed a peak when the molecules are embedded in rigid polymer matrices, such as Zeonex or poly(methyl methacrylate) (PMMA). In soft polymer matrices, such as poly(n-buthyl methacrylate) (PnBMA), exponential distribution was observed for the on and off times. The abnormal distribution suggests that the quantum yields of the photoreactions are not constant and the molecules undergo the reactions after absorbing a certain number of photons. A multilocal minima model was proposed to explain the environmental effect.     

Internal rotation in molecules dispersed in polymer matrices

Studies of certain polar or photochromic molecules dispersed in polymer matrices show that internal motion may be profoundly affected by the matrix. Thus, in polystyrene, dielectric studies of relatively small molecules show that the inversion motion of the central ring of xanthene becomes possible at temperatures below the recognized β and glass transition temperatures of the polymer. On the other hand, movement of the central oxazole ring in 2,5-diphenyl oxazole becomes marked at temperatures corresponding to the onset of polymer β motion, but dipole reorientation of diphenyl ether is not appreciable below the main glass transition.Considering a slightly larger molecule, the bleaching rate of the coloured isomer of 6′-nitro-1,3,3, trimethylindolinobenzopyrylospiran also depends on the nature of the polymer matrix. The observations are best explained in terms of a matrix-controlled internal rotation in which the photochromic molecules exist in a distribution of environments in the glassy matrix.     

Characterizing polymer macrostructures by identifying and locating microstructures along their chains with the kerr effect

In this brief report, we demonstrate that Kerr effect measurements, which determine the excess birefringence contributed by polymer solutes in dilute solutions observed under a strong electric field, are highly sensitive to and capable of determining their microstructures, as well as their locations along the macromolecular backbone. Specifically, using atactic triblock copolymers with the same overall composition of styrene (S) and p-bromostyrene (pBrS) units, but with two different block arrangements, that is, pBrS₉₀-b-S₁₂₀-b-pBrS₉₀ (I) and S₆₀-b-pBrS₁₈₀-b-S₆₀ (II), which are indistinguishable by NMR, we detected a dramatic difference in their molar Kerr constants (_mK), in agreement with those previously estimated. Although similar in magnitude, their Kerr constants differ in sign, with _mK(II) positive and _mK(I) negative. In addition, S/pBrS random and gradient copolymers synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization exhibit a heretofore unexpected enhanced enchainment of racemic (r) pBrS-pBrS diads. Comparison of their observed and calculated _mKs suggests that the gradient S/pBrS copolymers possess an unanticipated additional gradient in stereosequence that parallels their comonomer gradient, that is, as the concentration of pBrs units decreases from one end of the copolymer chain to the other, so does the content of r diads. This conclusion could only be reached by comparison of observed and calculated Kerr effects, which access the global properties of macromolecules, and not NMR, which is only sensitive to local polymer structural environments, but not to their locations on the copolymer chains. Molar Kerr constants are characteristic of entire polymer chains and are highly sensitive to their constituent microstructures and their distribution along the chain. They may be used to both identify constituent microstructures and locate them along the polymer chain, thereby enabling, for the first time, characterization of their complete macrostructures. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Simulations of rodlike molecules represented by anisotropic Lennard-Jones potentials

In the interests of carrying out computer simulations efficiently for systems of rodlike molecules, the model proposed by Berne and coworkers, in which each rod is represented by an anisotropic Lennard-Jones potential, was extensively compared with a site-site model. Using this potential, Monte Carlo simulations were carried out for rodlike molecules with aspect ratio (R_a) ranging from 3 to 50. Short rods such as R_a < 10 exhibit an isotropic phase for essentially the entire range of volume fraction ϕ, while long ones (R_a > 20) show an isotropic phase at only very low ϕ < 0.07. The attractive part of the potential makes rodlike molecules associate as well as align even at low ϕ. At high ϕ around or above the transition, the repulsive potential dominates and the attractive potential becomes secondary. Based on simulation results, some implications for designing molecular composites are offered.     

Measurement of thermal diffusivity in polymer melts using forced Rayleigh light scattering

Thermal diffusivity measurements on three polymer melts were made using the Forced Rayleigh Light Scattering technique. The polymers, which were tested at room temperature where they are in the molten state, included a polydimethylsiloxane and two polyisobutylenes. The optical setup and procedures developed in this study to conduct thermal Forced Rayleigh Light Scattering experiments are shown to be capable of producing thermal diffusivity data with a high degree of accuracy and precision. From measurements on a reference fluid (ethanol), experimental error was estimated to be no greater than 2%, and could be reduced to less than 1% by appropriate design of a series of experiments. Discrepancies of 4 and 14% in thermal diffusivity data on the polymer samples between measured values and those found in the literature were observed. It is suggested that these deviations are attributable to either sample variations or to errors in the techniques used in previous investigations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1069–1078, 1999     

Measurement of thermal diffusivity by the photoacoustic method

Analysis of the photoacoustic signalvs. the modulation frequency allows the determination of thermal diffusivity. Computer simulations have been carried out to determine the optimal conditions, in the case of rear surface illumination, for which the thermal diffusivity is accurately measured. In accordance with these conditions, measurements were performed on a reference sample (silicon).     

In situ characterization of solution–grown polymer single crystals by light scattering and kerr effect

Simultaneous measurements of light scattering and electric birefringence on a crystallizing solution of poly(ethylene oxide) (PEO) are shown to lead to the determination of the radius, thickness, and number of crystals at the early stages of the crystallization process.     

Diffusion and segmental dynamics of rodlike molecules by fluorescence correlation spectroscopy

The dynamics of weakly bending polymers is analyzed on the basis of a Gaussian semiflexible chain model and the fluorescence correlation spectroscopy (FCS) correlation function is determined. Particular attention is paid to the influence of the rotational motion on the decay of the FCS correlation function. An analytical expression for the correlation function is derived, from which the averaged segmental mean square displacement can be determined independent of any specific model for the polymer dynamics. The theoretical analysis exhibits a strong dependence of the correlation function on the rotational motion for semiflexible polymers with typical lengths and persistence lengths of actin filaments or fd viruses. Hence, FCS allows for a measurement of the rotational motion of such semiflexible polymers. The theoretical results agree well with experimental measurements on actin filaments and confirm the importance of large relaxation times.     

Thermal diffusivity of polymer films by the flash radiometry method

We have developed a flash radiometry technique for the determination of the thermal diffusivity of polymer films that consists of flashing a laser pulse on the front surface of the sample and monitoring the thermal radiation from the rear surface as a function of time using an infrared detector. This method is applicable to polymer films of thickness 14–250 μm in the wide temperature range between 150 and 500 K. We have performed measurements on four polymers, including polycarbonate, polyimide, polyvinylidene fluoride, and polyethylene terephthalate. The temperature dependence of the thermal conductivity of polycarbonate can be understood by considering the phonon mean free path, and the crystallinity and orientation dependence of the thermal conductivity of polyethylene terephthalate can be explained in terms of the two-phase model proposed by Choy and Young.     

Excluded volume effect of linear polymer molecules

An approximate closed expression for the excluded volume effect of linear polymer molecules is developed with the aid of a uniform expansion model of perturbed chains. The linear expansion factor α for the end-to-end distance is given by (α³ ? 1) + (3/8) (α⁵ ? α³) = (5/2)z where z is the excluded volume parameter. This equation is numerically close to the Ptitsyn equation in the ordinary range of α; i.e., for 1 ≤ α ≤ 2.     

Thermal diffusivity of polymer films by pulsed photothermal radiometry

We have developed a pulsed photothermal radiometry technique for determining the thermal diffusivity parallel to the surface of a polymer film that involves flashing a line-shaped laser beam on the surface of the sample at right angle to its length, and monitoring the temperature change with time at a distance from the line source using an infrared detector. Combining this with our previous laser-flash radiometry method for thermal diffusivity measurement perpendicular to the film surface, we can now measure the thermal diffusivity of a polymer film along all directions. These two techniques have been used to study uniaxially and biaxially oriented poly(ethylene terephalate) and uniaxially drawn ultrahigh molecular weight polyethylene films. For uniaxially oriented poly(ethylene terephalate), the thermal diffusivity along the draw direction is substantially higher than that in the transverse direction, which in turn, is slightly higher than that in the thickness direction. For a polyethylene film with a draw ratio of 200, the axial thermal diffusivity is extremely high, being about five times that of stainless steel. The anisotropy of the thermal diffusivity of this film exceeds 90. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1621–1631, 1997     

The effect of transcrystallinity on the behavior of fibers in polymer matrices

Single copper fibers embedded in a polypropylene matrix along part of their length will, on application of a tensile load, break in the absence of transcrystallinity adjacent to the fiber, or pull out of the polymer matrix in the presence of a transcrystalline zone. It is shown that the transcrystalline zone allows thinning of the fiber in the matrix up to the point at which compressive forces created by the shrinkage of the polymer around the fiber on cooling are removed and fiber pullout occurs.     

Measurement of diffusion coefficients of additive molecules in colloidal polymer particles by electron paramagnetic resonance

Nonpolar paramagnetic additives mixed into the aqueous serum of colloidal polymer dispersions are absorbed by the polymer particles with a rate that depends on the diffusion coefficient of the additive in the polymer. The absorption leads to an immobilization of the additive which can be detected in the electron paramagnetic resonance spectrum. By fitting the time dependence of the immobilized fraction to the appropriate diffusion model, it is possible to determine the diffusion coefficient of the additive in the polymer if the polymer particles are approximately uniform in size. This opens up a new way to determine diffusion coefficients in the range between 10^-14 and 10^-17 cm²s^-1, as are expected for low-molecular-weight additives in polymers below their glass-transition temperature.     

Calculation of rotational barriers in amorphous polymers

The Monte Carlo method was used for generation of amorphous polyethylene configurations on a diamond lattice. Chain building was performed on the tetrahedral lattice of edges 36, 62 and 43 Å with periodic boundary conditions imposed.32 chains were generated, each with a length of 100 CH₂-groups (resulting density = 0.81 g·cm⁻³). Small spherical volumes with a radius of 10 Å were chosen at random from the total volume for the calculation of rotational barriers. The rotating bond was chosen to be close to the center of this sphere. We employed the method of molecular mechanics in order to calculate the rotational barriers. The calculation was made for 578 rotating bonds and the obtained distribution of rotational barriers is approximated by the corresponding Γ-distribution.     

Hofmeister effect on the interfacial dynamics of single polymer molecules

The Hofmeister effect on interfacial dynamics has been discovered for single charged polymer molecules (sodium polystyrene sulfonate) adsorbed on a hydrophobic surface from an aqueous solution. The presence of ions in the aqueous solution affects the surface diffusivity, and its amplitudes and the surface friction follow the Hofmeister series-the kosmotropic ions slowed down the surface diffusivity and the chaotropic ions speeded it up. The amplitude of the surface friction exhibits a good correlation with the surface tension increment, indicating the interfacial feature of the Hofmeister effect.