Wetting transitions of asymmetric binary polymer mixtures

We examine the effect on surface-wetting phase transitions in polymer mixtures when the degrees of polymerization of the two components are different. It has been demonstrated by Schmidt and Binder (J. Phys. (Paris) 46 , 1631 (1985)) that in a symmetric polymer mixture a second-order wetting transition occurs if the wall-polymer interaction f(ϕ₀) has a negative curvature f″(ϕ₀), where ϕ₀ is the surface volume fraction of the polymer component preferred by the wall. We found that in an asymmetric mixture this is not necessarily the case.     

Liquid-liquid phase equilibria in polymer solutions and polymer mixtures

The pressure dependence of liquid-liquid equilibria in weakly interacting binary macromolecular systems (homopolymer solutions and blends) will be discussed. The common origin of the separate high-temperature/low-temperature and high-pressure/low-pressure branches of demixing curves will be demonstrated by extending the study into the region of metastable liquid states including the undercooled, overheated and stretched states (i.e. states at negative pressures). The seemingly different response of the UCST-branch of solutions and blends when pressurized (pressure induced mixing for most polymer solutions, pressure induced demixing for most blends) will be explained in terms of the location of a hypercritical point found either at positive (most solutions) or negative pressure (most blends). Further, it is shown that the pressure dependence of demixing of homopolymer solutions and blends may be described using a ‘master-curve’ which, however, is sometimes partly masked by degradation or by vapour-liquid and/or solid-liquid phase transitions. Experimental results demonstrating the extension of liquid-liquid phase boundary curves into the metastable regions will be presented, and the existence of solubility islands in the vicinity of the hypercritical points discussed.     

Simulation of first- and second-order transitions in asymmetric polymer mixtures

The critical properties of dense asymmetric binary polymer mixtures are studied by grand canonical simulations within the framework of the 3-dimensional bond fluctuation lattice model. The monomers interact with each other via a potential ranging over the entire first peak of the pair distribution. An asymmetry is realized by giving the ratio of interactions λ = ∈_AA/∈_BB between monomers of the A-species and of the B-species a value different from 1. Using multiple histogram extrapolation techniques for the data analysis, the two phase region, which is a line of first-order transitions driven by the chemical potential difference, and the critical point are determined for a mixture of chains with 32 monomers each. At a critical potential difference Δμ_c unmixing occurs below a critical temperature T_c. It is found that Δμ_c is proportional to the asymmetry (1 - λ) and that the quantity 4k_BT_c/(3 + λ)∈ is independent of the asymmetry, consistent with the prediction of the Flory theory.     

Chirality-induced phase transitions in some liquid crystalline binary mixtures

Novel binary mixtures have been prepared between an optically active antiferroelectric liquid crystal, (S)-4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxy-biphenyl-4-carboxylate, and an optically active twin liquid crystal, (R)-3-methyladipic acid bis[4-(5-octyl-2-(pyrimidinyl)phenyl] ester, and the liquid crystalline properties investigated. The stability of each liquid crystal phase was found to decrease by mixing these two liquid crystalline materials. Furthermore, a phase diagram between these compounds showed a clear discontinuity in phase sequences. These results indicate that the liquid crystal phases are different in nature between these materials. The mixture consisting of the antiferroelectric material (40 per cent) and the twin material (60 per cent) shows an unusual liquid crystal phase, where the texture is similar to that reported for the twist grain boundary (TGB) phase. Related binary mixtures have been prepared between optically active or racemic materials, where the chirality of the system is expected to be altered systematically. The TGB phase was found to be induced only in the mixture between the optically active materials. Two kinds of effect on the appearance of the TGB phase, i.e. a strong helical structure induced by the optically active twin liquid crystal and a decrease of the smectic layer strength achieved by mixing between two types of liquid crystalline materials, are discussed.     

Chemical transformations and phase transitions in polymer rheology and technology

Fundamental rheokinetic effects observed in processes of linear polymerization and three-dimensional oligomer curing are discussed. It was shown that changes in rheological properties during linear polymerization can be described by relationships based on treating a reactive mass as a polymer solution, if a newly formed polymer is soluble in reactive medium. Phase separation limits this approach and leads to some new rheological consequences such as change in the rate of viscosity growth (in some cases viscosity can even decrease in the course of polymerization). Shearing influences temperature of a phase transition and kinetics of chemical reaction if it proceeds in heterogeneous conditions. The same phenomena were also observed for oligomer curing which can occur in heterogeneous manner with microgelation and formation of colloid particles of a new phase before the gelation of a system as a whole.     

Structure and phase transitions of a liquid crystalline polymer

Summary The structures, textures as well as thermodynamic properties of a side chain polymer exhibiting a liquid crystalline phase in addition to a partially crystalline state and the isotropic fluid state were investigated. Furthermore the kinetics of phase transitions between these states were analyzed. It was found that the properties of this polymer are intermediate between that of low molecular weight liquid crystals and common polymers. In particular it was observed that the relation between the liquid crystalline texture and the structure is different from that of low molecular weight liquid crystals and that the properties of the crystalline and liquid crystalline state depend strongly on the tacticity of the polymer.

---

Zusammenfassung Für ein Seitenkettenpolymeres, das zusätzlich zum teilkristallinen Zustand und zur isotropen Schmelze einen flüssig-kristallinen Zustand aufweist, wurden die Strukturen und Texturen untersucht sowie thermodynamische Eigenschaften. Außerdem wurde die Kinetik der Phasenumwandlungen analysiert. Es zeigt sich, daß die Eigenschaften zwischen denen üblicher Polymerer und denen von niedrigmolekularen flüssigen Kristallen liegen. Insbesondere zeigte sich, daß die Beziehung zwischen molekularer Struktur und Textur von der bei niedrigmolekularen flüssigen Kristallen abweicht, und daß die Taktizität der Kette einen starken Einfluß auf die Eigenschaften nicht nur der kristallinen Phase sondern auch der flüssig-kristallinen Phase ausübt.

     

Dynamics of phase separation and critical phenomena in polymer mixtures

The phenomenological mean-field theory for statics and dynamics of polymer mixtures is described, generalizing the approaches of Flory-Huggins, Cahn-Hilliard and de Gennes. Predictions are made for critical behavior, spinodal decomposition and homogeneous nucleation. The validity of the mean-field approximations is discussed with Ginzburg criteria. The results of the theory are compared to computer simulations and recent experiments.Invited talk delivered at the Tagung der Deutschen Physikalischen Gesellschaft, Fachausschu\ Polymerphysik, Kaiserslautern, 12–14 March 1986, and at the Gordon Research Conference on Polymer Physics, Andover, New Hampshire, 14–18 July 1986.The author is grateful to Dieter W. Heermann, Alla Sariban, Harry L. Frisch, Josef JÄckie and Thomas Schichtel for their fruitful collaboration on this research described in this review. He thanks them and Arthur BaumgÄrtner for allowing to present partially unpublished material, and for stimulating discussions. Furthermore the author has benefitted from discussions with P. G. de Gennes, P. Pincus, H. Sillescu and G. R. Strobl. This research is supported in part by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 41.     

Inverse gas chromatography for the study of one phase and two phase polymer mixtures

Experiment finds that, for a chlorinated polyethylene (chlorine content 62.1% by weight)/poly(ethyl methacrylate) blend, a negative value of χ′_{2 3} is obtained, which indicates compatibility. With increasing temperature, χ′_{2 3} increases towards zero as required by the lower critical solution temperature behaviour of polymer blends. For chlorinated polyethylene/poly(butyl acrylate) blends however the specific retention volume is a linear function of composition and a positive χ′_{2 3} results if calculated by the conventional theory. The magnitude of χ′_{2 3} is determined by the difference between the retention volumes of the pure polymers and decreases with temperature. This effect is assumed to be a result of phase separation during coating the blend onto the support. A theoretical treatment is developed to explain this behaviour.     

Thermal induced phase transitions and structural relaxation in apoferritin encapsulated copper nanoparticles

Nanocrystalline metals display interesting basic and technological properties related to their chemical and structural properties. Among other properties, they display a richer phase diagram due to the additional degree of freedom introduced by the nanoparticles surface. Metal nanoparticles encapsulated within biological macromolecules have the additional advantage of biocompatibility. In this paper we investigate the thermal evolution of the structure and dynamics of apoferritin encapsulated nanocrystalline copper. We determined the occurrence of a yet unexpected phase transition from a low temperature FCC to a complex high temperature phase including a (putative) amorphous precursor. The occurrence of a FCC-icosahedral transition is also discussed as a possible explanation to our results. The lattice dynamics of the FCC phase (monitored by its Debye temperature) differs from the behaviour expected for nanosized structures.     

Nonmonotonic composition dependence of vibrational phase relaxation rate in binary mixtures

We present here isothermal-isobaric N-P-T ensemble molecular dynamics simulations of vibrational phase relaxation in a model system to explore the unusual features arising due to concentration fluctuations which are absent in one component systems. The model studied consider strong attractive interaction between the dissimilar species to discourage phase separation. The model reproduces the experimentally observed nonmonotonic, nearly symmetric, composition dependence of the dephasing rate. In addition, several other experimentally observed features, such as the maximum of the frequency modulation correlation time tau(c) at mole fraction near 0.5 and the maximum rate enhancement by a factor of about 3 above the pure component value, are also reproduced. The product of mean square frequency modulation [] with tau(c) indicates that the present model is in the intermediate regime of inhomogeneous broadening. The nonmonotonic composition chi(A) dependence of the dephasing time tau(v) is found to be primarily due to the nonmonotonic chi dependence of tau(c), rather than due to a similar dependence in the amplitude of Delta omega(2)(0). The probability distribution of Delta omega shows a markedly non-Gaussian behavior at intermediate composition (chi(A) approximately =0.5). We have also calculated the composition dependence of the viscosity in order to explore the correlation between the composition dependence of viscosity eta(*) with that of tau(v) and tau(c). It is found that both the correlation time essentially follow the composition dependence of the viscosity. A mode coupling theory is presented to include the effects of composition fluctuations in binary mixture.     

Tethered polymer layers: phase transitions and reduction of protein adsorption

The structural and thermodynamic properties of tethered polymer layers formed by spreading diblock copolymers at a solid surface or at a fluid‐fluid interface are studied using a molecular mean‐field theory. The role of the anchoring block in determining the properties of the tethered polymer layer is studied in detail. It is found that both the anchoring and the tethered blocks are very important in determining the phase behavior of the polymer layer. The structures of the coexisting phases, the phase boundaries and the stability of the layer are found to depend on the ratio of molecular weight between the two blocks, the polymer‐interface (surface) interactions and the strength of the interactions between the two blocks. The different phase transitions found are related to experimental observations. The properties of the polymer layers at coexistence reflect the block that is the dominant driving force for phase separation. The ability of the tethered polymer layers, under different conditions, to control protein adsorption to surfaces is also studied. It is found that the most important factors determining the ability of a polymer layer to reduce the equilibrium amount of proteins adsorbed to a surface are the surface coverage of polymer and the surface‐polymer interactions. The polymer chain length plays only a secondary role. For the kinetic control, however, it is found that the potential of mean‐force, and thus the early stages of adsorption, depends strongly on polymer molecular weight. Further, it is found that the molecular factors determining the ability of the tethered polymer layer to reduce the equilibrium amount of protein adsorption are different than those that control the kinetic behavior. Comparisons with experimental observations are presented. The predictions of the theory are in very good agreement with the measured adsorption isotherms. Guidelines for building optimal surface protection for protein adsorption, both kinetic and thermodynamic, are discussed.     

Investigation of the global phase behavior of polymer mixtures in the shield region

This paper is a contribution of our systematic investigation of the global phase behaviors of the chain molecules mixtures, i.e., polymer mixture solutions. The phase behavior of fluid mixtures is understood by the critical lines in fluid-gas diagrams. The critical lines of binary fluid system may, under circumstances, exhibit closed loops in the critical lines. A distinction is made between free critical loops, as described by type VI in the Scott and van Konynenburg classification, and "rooted" critical loops, as found in the shield region. We define rooted loops as closed critical lines that are attached to the critical line structure by means of unstable critical line. We obtain the rooted loops in the global phase diagrams of the polymer mixture solutions within the framework of a model that combines the lattice gas model of Schouten, ten Seldam and Trappeniers with the Flory-Huggins theory, and we present the influence of the chain length of long molecules on the rooted critical loops. We present the results in the density-density and the temperature (T)-pressure (P) planes in detail.     

Nuclear magnetic relaxation and phase transitions of diethylene glycol in Vycor porous glasses

The times of longitudinal T ₁ and transverse T ₂ magnetic relaxation of protons of diethylene glycol in the bulk phase and in Vycor porous glasses with average pore radii of 4, 11, and 32 nm are measured by the pulse NMR method in the 172–350 K temperature range. It is found that, for all samples of porous glasses, the crystallization of diethylene glycol is not observed if its content corresponds to the monolayer surface filling. The minimum content of diethylene glycol, which makes it possible to cause its crystallization in porous glass, is determined. By analyzing the dependence of the characteristics of a component with T _2b = 20−40 μs in the transverse magnetization decay on the content of diethylene glycol in porous glasses, the volume of nanopores comparable in size with diethylene glycol molecule is estimated.     

Coupling scheme applied to solvent relaxation modified by dissolved polymer molecules

A coupling scheme which has been successful in explaining relaxation properties of neat solvents can also predict their modifications by dissolved polymer molecules.     

Scattering function and the dynamics of phase separation in polymer mixtures under shear flow

The phenomenological mean-field theory describing concentration fluctuations and spinodal decomposition of binary mixtures of long flexible macromolecules is generalized to mixtures under steady shear flow. This shear flow leads to a partial orientation and stretching of the coils, as well as to an anisotropic deformation of concentration fluctuations. Generalizing the approach of Onuki and Kawasaki, we obtain the collective scattering function describing these concentration fluctuations in the mixture under shear flow. Both the steady-state situation in the one-phase region and the initial stages of spinodal decomposition for concentrations inside of the spinodal curve are considered.Contributed paper delivered at the Tagung der Deutschen Physikalischen Gesellschaft, Fachausschuß Polymerphysik, Berlin, March 30–April 3, 1987.     

Systematic investigation of global phase behavior of polymer mixtures in the pressure-temperature plane

We investigate the critical lines of polymer mixtures in the presence of their vapor phase at the mathematical double point, where two critical lines meet and exchange branches, and its environment. The model used combines the lattice gas model of Schouten, ten Seldam and Trappeniers with the Flory-Huggins theory. The critical line structure is displayed for various combinations of the chain length and system parameters in the pressure (P)-temperature (T) plane, as is usually done with experimental results. This type of work sheds light on the essential transition mechanism involved in the phase diagram's change of character, such as multi-critical points and mathematical double points, which are of great practical importance in supercritical fluid extraction processes. The P, T diagrams are discussed in accordance with the Scott and van Konynenburg binary phase diagram classification. We found that our P, T plots were in agreement with type II, type III, or type IV phase diagram behaviors. We also found that some of our phase diagrams represent the liquid-liquid equilibria in polymer solutions and mixtures.     

Aqueous polymeric sensors based on temperature-induced polymer phase transitions and solvatochromic dyes

This feature article provides, for the first time, an overview of the research that guided the way from fundamental studies of the thermo-responsive phase separation of aqueous polymer solutions to polymeric sensor systems. The incorporation of solvatochromic dyes into thermoresponsive polymers as well as the concepts of polymeric sensors are presented and discussed in detail.     

FTIR of liquid crystals. Organization and phase transitions in binary mixtures of nonylcyanobiphenyl and phenyl-cyclohexanecarboxylate

The FTIR spectra of the isotropic and mesomorphic N, S_Ad, S_A1, S_B and S_G phases of 4-n-nonyl-4'-cyanobiphenyl (9CB), 4-n-pentylphenyl-trans-4'-n-pentylcyclohexane-1-carboxylate (5H5) and their binary mixtures have been recorded and analysed. Changes in absorbance related to spontaneous reorientation of the molecules at the phase transitions have been observed and assigned to the orientation of the transition dipoles of typical vibrations. The results are discussed in terms of the phase diagram obtained by D.S.C. and by optical microscopy.