Macromolecular Liquids Research Articles

Liquid crystals 100 years later. What are the new concepts used in the design of molecular, macromolecular and supramolecular liquid crystals?

AbstractA brief discussion of selected synthetic strategies pursued in the field of molecular, macromolecular and supramolecular liquid crystals is made by reviewing some tandem historical events of the past 100 years of activity in this field. This survey will try to suggest the rational for the discovery, evolution and investigation of some of these new concepts. Also a brief and speculative forecast of the future of this field will be presented.

Analytic solutions of the PRISM equations for Gaussian polymers

The polymer reference interaction site model (PRISM) equations for the collective structure of macromolecular liquids are simplified using the Wiener-Hopf factorization technique. This approach is expected to be useful in systems exhibiting strongly different length scales. As an application, melts and solutions of Gaussian flexible polymers are considered. It is shown that PRISM theory in agreement with experiments predicts liquid like packing for high polymer concentrations and mean-field like scaling laws for the dilute to semidilute crossover at the Θ-temperature. In a systematic expansion the range of validity and the leading corrections to the low density scaling law description are calculated.

Randomly crosslinked macromolecular systems: Vulcanization transition to and properties of the amorphous solid state

As Charles Goodyear discovered in 1839, when he first vulcanised rubber, a macromolecular liquid is transformed into a solid when a sufficient density of permanent crosslinks is introduced at random. At this continuous equi- librium phase transition, the liquid state, in which all macromolecules are delocalised, is transformed into a solid state, in which a nonzero fraction of macromolecules have spontaneously become localised. This solid state is a most unusual one: localisation occurs about mean positions that are distributed homogeneously and randomly, and to an extent that varies randomly from monomer to monomer. Thus, the solid state emerging at the vulcanisation transition is an equilibrium amorphous solid state: it is properly viewed as a solid state that bears the same relationship to the liquid and crystalline states as the spin glass state of certain magnetic systems bears to the paramagnetic and ferromagnetic states, in the sense that, like the spin glass state, it is diagnosed by a subtle order parameter. In this review we give a detailed exposition of a theoretical approach to the physical properties of systems of randomly, permanently crosslinked macromolecules. Our primary focus is on the equilibrium properties of such systems, especially in the regime of Goodyear's vulcanisation transition.

Direct Force and Friction Measurements Reflecting Structural Changes in Confined Diblock Copolymer Liquids

We have measured the normal and shear forces for a symmetric polystyrene-polyisoprene (PS-PI) diblock copolymer liquid in toluene, a good solvent for both blocks, between two mica surfaces. The aim was to investigate the structure of the macromolecular liquid when confined between surfaces separated by molecular-sized distances. The surface forces apparatus was used for these studies, and the microstructure was manipulated by varying the fraction of diblock in solution. Measurements of the equilibrium normal force profiles (intermolecular force versus separation distance between the confining surfaces) were made. All of these normal force profiles exhibited monotonic repulsive behavior. We found that in the disordered regime, as the fraction of diblock was increased, the intermolecular forces became much longer-ranged. As the fraction of diblock was further increased to concentrations near the bulk order-disorder transition (ODT), the ranges of the force profiles unexpectedly collapsed to shorter distances. In a series of dynamic surface forces experiments, the frictional force required to slide steadily one surface past the other with the macromolecular liquid intervening was measured as a function of shear rate. The shearing results reflected the same pattern of behavior exhibited in the normal force experiments. We determined that the frictional force of the most concentrated liquid was distinctly lower than that of the disordered samples at all shear rates and attribute both the equilibrium and dynamic results to be reflective to the inherent structure of the liquid. We believe that the reduction in the range of the normal force profiles and the reduction in frictional forces at the highest concentrations reflect pretransitional ordering in the confined sample. Our results are discussed in terms of surface-induced ordering and composition fluctuations, which are not accounted for in the mean-field description of polymer phase behavior.

Time-transient process of magnetically induced growth of nematic domains in a biological macromolecular liquid crystal

A lyotropic nematic liquid crystal of biological macromolecules was investigated at a nonequilibrium state induced by a magnetic field. By using synchrotron radiation, an x-ray scattering method, and a specially designed magnetic circuit, we have succeeded in observing a time-transient process of a drastic change of the scattering profile from the macromolecular assembly. We can describe this process by a magnetically induced orientation of nematic liquid crystal domains along the field which is equivalent to a growth of nematic domain from polycrystalline to a single liquid crystal. We can estimate the growth rate constant, the longitudinal relaxation time, and the dimension of the nematic microdomains.

Mode-coupling theory of macromolecular liquids

A mode-coupling theory of entangled polymer liquids has been developed and extensively applied to dense melts of long chains. The theory predicts the emergence of a plateau shear modulus due to dynamical correlations induced by chain connectivity and excluded volume interactions. Mode-coupling predictions for the dependence of transport coefficients on degree of polymerization agree with the phenomenological reptation/tube theory. Anomalous early and intermediate time diffusion also occurs due to power-law decay of the fluctuating force memory functions. The mode-coupling renormalizations are found to depend sensitively on spatial and polymer fractal dimensions in analogy with dynamic critical phenomena. An alternative, self-consistent approximation for the memory functions results in the prediction of a nonreptation viscosity scaling law for chains, and an ideal "entanglement-induced" glass transition at very large molecular weights.

Low-voltage high-resolution SEM investigations of novel macromolecular liquid crystals

The ability to visualize detailed 3-dimensional surface topology with SEM at low voltage and high resolution holds profound promise for analyzing liquid crystal textures, both in polymers and other macromolecular forms. Director textures, domain boundaries, and defects such as inversion walls, disclinations and dislocations can now be easily visualized with this technique. Studies concerning the effects of shear flow and magnetic fields on these defects are currently under way.Resolution of 4.0 nm at 1.0 keV is now possible with commercial SEM's, which incorporate the latest advances in lens design optimized for low voltage operation, and use high brightness, low energy spread field emission electron guns. The low energy spread of the field emission gun reduces chromatic aberration effects and facilitates successful operation at low keV. Low voltage operation provides dramatically improved image contrast due to the smaller beam/sample interaction volume and also greatly reduces sample charging artifacts. By operating at near crossover conditions, the need for coating nonconducting specimens with a conducting layer of metal or carbon is greatly reduced or eliminated.

Interfacial structure and dynamics of macromolecular liquids: a Monte Carlo simulation approach

Etude de la structure a l'equilibre et du comportement dynamique de polymeres fondus a l'interface avec un solide, en appliquant des simulations de Monte Carlo a un liquide dense constitue de chaines flexibles de 20 chainons dans un reseau cubique au voisinage de parois solides de pouvoir attracteur fort ou faible avec ou sans barriere de potentiel

What can we Learn from Molecular Dynamics Simulations of Macromolecular Liquids?

ABSTRACTWe describe how molecular dynamics simulations for a relatively simple coarse grained model can be very useful for investigating the static and dynamic properties of polymers and other macromolecular liquids. We show that it is important to use a simplified coarse grained model instead of a detailed microscopic model if one is interested in studying on modern supercomputers large systems which also relax slowly. As examples we present results for isolated star polymers with f-arms and diluted gelation/percolation clusters. We find in agreement with recent neutron scattering experiments that diluted percolation clusters swell and that their fractal dimension is reduced from 2.5 to 2. We also discuss our results for a dense melt of entangled linear polymers to show that the method is effective at high density. Our results for the entangled melt cover the crossover from Rouse to reptation and strongly support the concept of reptation.

Macromolecular structure and liquid crystallinity.

The relationship between low-molar-mass or monomeric liquid crystals and polymeric liquid crystals is explored by examining the historical development of experimental and theoretical aspects of the latter materials. Polymeric liquid crystals are initially contrasted with conventional, non-mesogenic, flexible polymer chains (random coils), and then semiflexible and rigid-rod polymers are examined. Both thermotropic and lyotropic phases are described, with emphasis placed on the former; in particular, the relationship between dimers and polymers belonging to the semi-flexible category: [graphic omitted](mesogenic core)—(flexible spacer)[graphic omitted]n. The review concludes by drawing attention to the curious behaviour of polypeptide liquid crystals (cholesteric compensation). The underlying theme of the lecture is the need to recognize the synergism and affinity that characterizes research on monomeric and polymeric liquid crystals.

Hydrostatic Pressure Effects on Helical Pitches in Lyotropic Macromolecular Cholesteric Liquid Crystals: Ethyl Acetate Solutions of Poly-γ-Ethyl-D-Glutamate

A pressure bomb with two straight-through optical windows has been constructed; one of the windows has an angular aperture as wide as ±24°C. It facilitates the obtaining of laser diffraction patterns from which we can determine the helical pitches of cholesteric liquid crystals. Pitches of 16 and 19 wt.% ethyl acetate solutions of poly-γ-ethyl-D-glutamate have been measured by applying pressure up to 200 MPa at several temperatures between 18 and 29°C. The pitch increases with pressure, though no tendency is observed to diverge at a certain pressure.

Cytomorphologic results of preparation experiments for monolayer deposition of cervical material.

For automated prescreening methods by high resolution analysis serving as a detecting method in gynecologic mass screening programs a new monolayer deposition method of cervical material has been used. This method will be outlines briefly and the mode of evaluation as well as current cytomorphological findings will be presented. With regard to measurability of the slides prepared according to the new method a number of cytologic criteria were thought to be of particular importance. These criteria are delineated and compiled in a table. A form in which these criteria were listed was filled in by cytopathologists for each slide evaluated. When performing isolation and centrifugation procedures several new morphologic questions arose to the cytopathologist which can only partly be answered by now. If taking into account all criteria of evaluation it may be followed from the present experiences that slides of cervical material are much more suited for automated prescreening methods by high resolution analysis if prepared after isolation and centrifugation in macromolecular liquids than are conventional Papanicolaou smears or slides from suspensions with isolated cells that were not subjected to centrifugation procedures.

Statistical Thermodynamics of Macromolecular Liquids and Solutions

AbstractThe separability of the configuration partition function for long chain molecules and their solutions into intermolecular and intramolecular factors is discussed. Two consequences thereof are emphasized: (i) the thermodynamic properties of a polymer solution are virtually independent of the structure of the macromolecule and especially of its flexibility, and (ii) the chain configuration does not depend appreciably on concentration, even at the limit of the undiluted polymer. This separation of the partition function holds, however, only if the molecular chains are sufficiently flexible. Above a critical degree of stiffness that depends on the concentration, a state of partial order (e. g., liquid crystalline) is preferred. Theory for the limiting case of rigid, rod‐like particles accounts satisfactorily for the biphasic equilibrium between an isotropic and an anisotropic (nematic) phase above a concentration limit that depends on the axis ratio x. The theory can be adapted to polydisperse systems in which the solute comprises species that differ in x. Applied to a distribution of such particles which is asymptotically of the form fx0 = α2 x exp (—αx), it predicts separation of an anisotropic phase at volume fractions v2 ⩾ α exp α. The largest members of the distribution are partitioned preferentially into the anisotropic phase.

Dynamic viscosity of macromolecular liquids for a superposition of static and oscillating velocity gradients

Two coupled, inhomogeneous relaxation equations for the friction pressure tensor and the alignment tensor are derived within the framework of irreversible thermodynamics. These equations are solved for a specific geometry, viz. flow between flat plates, and for a velocity gradient of the form Γ 0 + Γ 1 cos Ωt with small Γ 1. From the resulting relation between the (time-dependent) friction pressure tensor and the velocity gradient, the dynamic viscosity and the normal pressure can be inferred. The frequency dependence of the relevant viscosity coefficients is discussed. If Г 0, the magnitude of the static part of the velocity gradient is large enough, a type of resonance behavior is found with the resonance frequency Ω res≈(1 + ξ) -1Γ 0 where ξ is the ratio between the relation times of the friction pressure tensor and of the alignment tensor.

Non-newtonian viscosity and normal pressure associated with the flow alignment in macromolecular liquids

Non-linear constitutive laws previously obtained within the framework of irreversible thermodynamics are applied to the calculation of the flow alignment and the friction pressure tensor of macromolecular liquids. A non-newtonian viscosity is obtained which decreases with increasing magnitude of the velocity gradient. For rigid molecules it reaches a finite asymptotic value, viz. the second newtonian viscosity. The friction pressure tensor contains normal components. The magnitude of this normal pressure is closely related to the change of the viscosity caused by the velocity gradient. Flow alignment and the pressure tensor are also studied for time-dependent (alternating) velocity gradients.

Characterization of macromolecules in liquid solutions by thermal diffusion. II. Dependence of the soret coefficient on the molecular mass of the solute

AbstractExperiments have been carried out, in connection with those mentioned in Part I of this series, with the general aim of checking some predictions of the radiation‐pressure theory of thermal diffusion in the case of macromolecular liquid solutions and in particular with the intent of establishing the dependence of the Soret coefficient on molecular mass of the dispersed particles.The results confirm the dependence of Soret coefficient on the shape and on the mass of the particles. The analytical expressions obtained by the radiation‐pressure theory of the phenomenon constitute a satisfactory description of the thermodiffusive behavior of such complex systems. The inner constitution of the particles determines the intensity of the radiation pressures exherted by thermal waves; the molecular cross section determines the total force acting on each particle, while the spatial conformation determines the ratio between mass and cross section and at the same time also the hydrodynamical behavior of a particle within the liquid. All these parameters concur in the phenomenon of thermal diffusion, in the way specified by the radiation‐pressure theory of thermal waves, as proved by our experiments.The possibility of a characterization of macromolecules and supermolecular particles dispersed in a liquid on the basis of their thermodiffusive behavior is demonstrated.

Diffusio birefringence in colloidal suspensions and macromolecular liquids

The constitutive law for diffusio birefringence is introduced. Theoretical results are reported for the characteristic coefficient determining the size of this new effect in a colloidal suspension or a macromolecular liquid.

Problems of molecular adsorption chromatography

In this review we have tried to show that molecular gas and liquid chromatography on adsorbents have a number of advantages for analytical and preparative applications. The vast amount of experimental data obtained on adsorbents with known and reproducible chemical and geometrical structure, together with the combined use of static and chromatographic methods, makes the development of a molecular theory of gas adsorption possible on the basis of molecular-statistics and semi-empirical, or effective, potential functions of intermolecular interactions. Advances made in this field so far warrant further applications of this theory to gas adsorption chromatography. Of practical interest is the fact that forecasts of certain parameters and phenomena can be made on the basis of this theory. The theory of adsorption from solutions and liquid chromatography has been developed to a lesser degree. However, here, too, the main qualitative aspects of this theory have been established. Further efforts should be made to develop the theory of adsorption from liquid solutions and apply this theory to equilibrium, non-equilibrium, molecular and macromolecular sieve liquid chromatography.