Pressure dependence of the demixing behaviour in polymer blends: I. Experimental procedure

The most common way to influence the liquid-liquid phase behaviour in partially miscible (co-)polymer blends is changing the blending temperature. Since most extruders can handle pressures, up to 300 bar, pressure may also be used to influence the miscibility of polymers during blending. We have developed equipment and an experimental procedure to study the pressure dependence of the liquid-liquid demixing behaviour of high-viscous polymer blends under equilibrium conditions. Small amounts (1–4 grams) of specially made polymers are blended in the ‘DSM MINI EXTRUDER’. After a chosen mixing time, a small portion of the blend is injected into a small capillary tube and kept at the blending temperature. The phase behaviour of the blends as a function of temperature and pressure is studied via laser light scattering (at a scattering angle of 90°) in a specially made 400 bar/250°C window autoclave, where the capillary cell is placed in a high temperature grade silicon oil.     

Phase behavior of polymer blends under equilibrium and nonequilibrium conditions

This contribution embraces two topics related to phase behavior of polymer blends under equilibrium and nonequilibrium. 1. Polymer blends can undergo different phase changes as liquid-liquid phase transition and crystallization. Coupling of demixing and crystallization may occur at the kinetic stage. This is illustrated by blends of poly(ϵ-caprolactone)(PCL) and poly(styrene-co-acrylonitrile)(SAN). 2. Extension of studies to blend systems under flow is necessary for the better understanding of structure formation in polymer blends outside equilibrium. Polymer molecules will be oriented and stretched when subjected to flow. This may result in flow-induced phenomena. Effects of flow on the phase behavior have been studied only for a few blends, as yet. The primary observation was flow-induced miscibility. Apparent shifts of the phase transition temperatures will be discussed qualitatively in terms of a decoupled mode theory.     

LIQUID-LIQUID PHASE EQUILIBRIUM OF POLYMER SOLUTIONS AND POLYMER BLENDS UNDER POSITIVE AND NEGATIVE PRESSURE

In this paper we would like to give a brief review about the extensibility of the liquid-liquid locus into the negativepressure region. Negative pressure states are hardly explored; most researchers believe that the pressure scale ends at p = 0.We would like to show that this is not true, the p = 0 point is not a special point for liquids, it can be "easily" crossed. We aregoing to give a few example, where the extension of liquid-liquid locus for polymer blends and solutions below p = 0 givesus some interesting results, like the merging of UCST and LCST branches in weakly interacting polymer solutions or thereason why most UCST blends exhibit pressure induced immiscibility. Also, we will see what happens with the immiscibilityisland of aqueous polymer solutions when -- reaching the critical molar mass -- it "disappears".     

Isotope and pressure effects in polymer solutions. Demixing of polystyrene(h/d) poly-2-chlorostyrene blends

Liquid-liquid demixing, following spinodal quenches of poly-2-chlorostyrene/polystyrene blends, was followed by light scattering at 632.8 nm. The dependences of demixing on H/D substitution and molecular weight of the polystyrene, and on pressure, are reported. In the region of interest, the phase diagram is of the lower critical solution (LCS) type, and demixing is induced by raising the temperature. The transition temperature is lowered by deuterium substitution. At constant quench depth the transition proceeds more rapidly at elevated pressure. © 1995 John Wiley & Sons, Inc.     

Calculation of phase equilibria in random copolymer systems

Synthesis and application of copolymers are not seldom connected with different phase equilibria. Their precise knowledge is of importance for industrial processing as well as it is a profound basis for a better understanding of the nature and thermodynamics of such systems. As a common situation today, enough experimental information is seldom available in the necessary or desired amount, and a lot of model calculation is, therefore, more or less unavoidable to cover the desired ranges of application. Different equations-of-state as well as lattice models are discussed with respect to their applicability for calculating liquid-liquid and gas-liquid phase equilibria in copolymer solutions and blends. Examples for high-pressure phase equilibria in monomer/copolymer mixtures, liquid-liquid demixing in copolymer blends and for the isotropicnematic phase equilibrium in systems with rigid rod-like copolymers characterized by distributions of rigid and flexible chain parts are given. The effects of copolymer polydispersity are included by means of continuous thermodynamics. Literature references for original sources, earlier reviews and further applications round up this paper.     

Thermodynamics of polymer blends

The general principles of thermodynamic equilibrium in binary liquid systems are reviewed briefly, and extended to quasi-binary mixtures of polydisperse polymers. Molecular models allowing actual phase behaviour to be discussed in terms of molecular parameters are exposed to data on the system polystyrene/polyvinylmethylether. Disparity in size and share between the repeating units must be introduced to obtain reasonable agreement between theory and experiment. The neccessary introduction of the molar-mass distribution detracts from this agreement which makes clear that other aspects exist that must be taken into account. For example, cross association between repeating units has a marked effect on phase behaviour. Blends are subject to two kinds of thermodynamic aging which lead either to considerable mutual solubility in supposedly immiscible blends, or to metastable equilibria transforming into states of lower Gibbs energy. In both cases physical proerties of the blend will change with time.     

Phase equilibria in ternary polymer blends

The phase states and rheological properties of blends of three polymers??polystyrene, poly(methyl methacrylate), and the styrene-acrylonitrile copolymer??in the common solvent chloroform are studied. The phase diagrams are constructed and the positions of spinodals are determined via the method of turbidity points. The effect of the third polymer on the compatibility of the binary blend obeys Prigogine??s rule; that is, it is determined by the solubility of the added polymer in the first two components. The extremum composition dependence of rheological properties of ternary polymer systems in the vicinity of the separation point (the metastable region) is found. Through the method of convex-shell construction, the phase diagrams are calculated.     

Shear influences on polymer blends: experimental,theoretical approaches and technical implications

We examine the effects of shear on polymer blends consisting of partially miscible components, i.e. systems close to the phase boundary. The eminent phenomenon is the shift of the phase boundary, either extending the homogeneous area (flow‐induced mixing) or the opposite effect (flow‐induced demixing). The kinetics of the demixing process and concentration fluctuations are also influenced by flow fields, inducing anisotropy due to the flow direction. Experiments (scattering, rheology, in‐situ flow‐scattering, microscopy, DSC) are carried out with the academic model blend polystyrene/poly(vinyl methyl ether) and the industrial poly(styrene‐co‐maleic anhydride)/poly (methyl methacrylate) blend. The experimental results are rationalised in terms of a generalised Gibbs energy of mixing by including the energy which is stored in the sheared fluids.     

Shear flow effect on phase behavior and morphology in polymer blends

The effect of simple shear flow on the phase behavior and morphology was investigated for both polystyrene/poly(vinyl methyl ether) (PS/PVME) and poly(methyl methacrylate)/poly(styrene‐co‐acrylonitrile) (PMMA /SAN‐29.5) blends, which have LCST (lower critical solution temperature)‐type phase diagram. The measurements were carried out using a special shear apparatus of two parallel glass plates type. The PS/PVME blends showed shear‐induced demixing and shear‐induced mixing at low and high shear rate values, respectively. In addition, the rotation speed and the sample thickness were found to have a pronounced effect on the phase behavior under shear flow. On the‐other hand, PMMA/SAN blend showed only shear‐induced mixing and the magnitudes of the elevation of the cloud points were found to be composition and molecular weight dependent. The morphology of the PMMA/SAN=75/25 blend indicated that shear‐induced mixing occurred at a critical shear rate value, below which the two phases were highly oriented and elongated in the flow direction.     

Shear induced mixing/demixing: blends of homopolymers,of homopolymers plus copolymers,and blends in solution

Shear may shift the phase boundary towards the homogeneous state (shear induced mixing, SIM), or in the opposite direction (shear induced demixing, SID). SIM is the typical behavior of mixtures of components of low molar mass and polymer solutions, SID can be observed with solutions of high molar mass polymers and polymer blends at higher shear rates. The typical sequence with increasing shear rate is SIM, then occurrence of an isolated additional immiscible area (SID), melting of this island into the main miscibility gap, and finally SIM again. A three phase line originates and ends in two critical end points. Raising pressure increases the shear effects. For copolymer containing systems SID is sometimes observed at very low shear rates, preceding the just mentioned sequence of shear influences.     

扩散致相转化法制备结晶性聚合物多孔膜

介绍了扩散致相转化法制备结晶性聚合物多孔膜的研究现状。其三元等温成膜体系的相图包含液-液分相和固-液分相两种相分离方式,是理解成膜过程的重要工具,总结了成膜机理和膜的结构形貌：单纯S-L相分离生成粒子状对称膜结构;单纯L-L相分离生成蜂窝状非对称膜结构;两种相分离方式竞争发生将生成多样的混合膜结构。铸膜液浓度、非溶剂种类、铸膜溶剂组成、凝胶浴组成、制膜温度是影响膜结构形貌的主要因素。     

Homogeneous crystal nucleation triggered by spinodal decomposition in polymer solutions

We report dynamic Monte Carlo simulations of polymer crystal nucleation initiated by prior spinodal decomposition in polymer solutions. We observed that the kinetic phase diagrams of homogeneous crystal nucleation appear horizontal in the concentration region below their crossovers with the theoretical liquid-liquid spinodal. When the solution was quenched into the temperature beneath this horizontal boundary, the time evolution of structure factors demonstrated the spinodal decomposition at the early stage of crystal nucleation. In comparison with the case without a prior liquid-liquid demixing, we found that the prior spinodal decomposition can regulate the nanoscale small polymer crystallites toward a larger population, more uniform sizes, and a better spatial homogeneity, whereas chain folding in the crystallites seems little affected.     

An AFM, XPS and wettability study of the surface heterogeneity of PS/PMMA-r-PMAA demixed thin films

A series of homopolymer/random copolymer blends was used to produce heterogeneous surfaces by demixing in thin films. The chosen homopolymer is polystyrene (PS) and the random copolymer is poly(methyl methacrylate)-r-poly(methacrylic acid) (PMMA-r-PMAA), whose acidic functions could be used as reactive sites in view of further surface functionalization. The proportion of each polymer at the interface was deduced from X-ray photoelectron spectroscopy (XPS) data using, on the one hand, the O/C ratio, and on the other hand, decomposition of the carbon peak of the blends in two components corresponding to the carbon peaks of PS and PMMA-r-PMAA. Combining the information from XPS with atomic force microscopy (AFM) images, water contact angle measurements and PS selective dissolution, it appears that the surfaces obtained from blends with a high PS content (90/10 to 70/30) display pits with a bottom made of PMMA-r-PMAA, randomly distributed in a PS matrix. On the other hand, the surfaces obtained from blends with a low PS content (30/70 to 10/90) display randomly distributed PS islands surrounded by a PMMA-r-PMAA matrix. The characteristics of the heterogeneous films are thought to be governed by the higher affinity of PMMA-r-PMAA for the solvent (dioxane), which leads to the elevation of the PS phase compared to the PMMA-r-PMAA phase, and to surface enrichment in PMMA-r-PMAA.     

On the nature of phase separation of polymer solutions at high extension rates

Experiments with stretching moderately concentrated polymer solutions have been carried out. Model experiments were carried out for poly(acrylonitrile) solutions in dimethyl siloxane. Just the choice of concentrated solutions allowed for a clear demonstration of a demixing effect with the formation of two separate phases—an oriented polymer fiber and solvent drops sitting on its surface. An original experimental device for following all subsequent stages in the demixing process was built. It combined two light beams, one transverse to the fiber and a second directed along (inside) the fiber, the latter played the role of an optical line. This gives a unique opportunity to observe processes occurring inside a fiber. The process of demixing starts from the volume phase separation across the whole cross section of a fiber at some critical deformation and the propagation of the front of demixing along the fiber. Then a solvent cylindrical skin appears which transforms into a system of separate droplets. New experimental data are discussed based on a comparison of the current different points of view on the phenomenon of deformation‐induced phase separation: thermodynamic shift of the equilibrium phase transition temperature, growth of stress‐induced concentration fluctuations in two‐component fluids, and mechanically pressing a solvent out from a polymer network. The general belief is that a rather specific (so‐called “beads‐on‐a‐string”) structure of a filament is realized in stretching dilute solutions: beads of a polymer solution connected by oriented polymer bridges forming a single object. The situation in stretching moderately concentrated solutions appears quite different: real phase separation was observed. So, the alternative phenomenon to the formation of the “beads‐on‐a‐string” structure has been experimentally proven. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 559–565     

溶剂对PVDF铸膜液相转化和微孔膜皮-亚两层结构的不同影响

采用皮-亚分步凝固成膜机理分析了3种不同溶剂对聚偏氟乙烯(PVDF)铸膜液相转化和膜结构的影响,采用浊度法测定铸膜液体系的热力学性质,沉淀速度采用光透射仪测定.结果显示,3种膜的皮层分相主要由热力学性质控制,均发生延时液固分相,生成了相互融合的球粒组成的致密皮层.3体系的亚层分相行为由动力学扩散过程控制;对于二甲基亚砜(DMSO)、N,N-二甲基乙酰胺(DMAc)体系亚层发生瞬时液液分相,结晶化对动力学过程影响小,表现为光透射曲线上分相时间t2短,生成了大孔结构为主的亚层,膜厚度、孔隙率和气通量均高、结晶度低;N,N-二甲基甲酰胺(DMF)体系亚层发生延时液液分相,结晶化对动力学过程影响大,t2长,生成蜂窝状孔结构亚层,其膜厚度、孔隙率和气通量较低,但膜的结晶度高.     

Polymerization-induced phase separation III. Morphologies and contrast ratios of polymer dispersed liquid crystals

Polymerization induced phase separation in mixtures of liquid crystals (LCs) and acrylates (Merck TL205/PN393) proceeds by liquid-gel demixing, in most cases of practical interest. At high LC content or low temperature of polymerization liquid-liquid separation cannot be excluded. Depending on the elasticity and homogeneity of the polymer network at the onset of phase separation, spherical or non-spherical LC domains are observed; non-spherical domains reflect an inhomogeneous gel structure. The change from spherical to non-spherical occurs in a very narrow range of LC concentrations and curing temperatures. The transition between these two morphologies can be explained using conversion phase diagrams obtained from the Flory-Huggins-Dusek theory. The contrast ratio of PDLCs made from the Merck mixture passes through a maximum when the droplet shape at the onset of phase separation changes from spherical to non-spherical. Lowering the LC content or increasing the temperature leads to smaller LC domains which scatter less efficiently. The reverse changes lead to early phase separation and large LC domains which also scatter inefficiently. It is speculated that the maximum of the contrast ratio is related to secondary phase separation, leading to subdomains of an appropriate size.     

SOME ASPECTS OF MORPHOLOGIES AND INTERFACES IN COPOLYMER/HOMOPOLYMER BLENDS

Based on a series of morphological studies of blends of homopolymer (Homo) and a variety of block and graft copolymers (Cop), the nature of phase separation, interface, emulsification and inner morphology of copolymer-dispersed phase etc. in the blends are discussed. In the cases of Cop AB/Homo A/Homo B systems, in which one homopolymer forms matrix, it is observed that the dispersed homopolymer phase is exclusively associated with Cop AB, i.e. no Homo A-Homo B interface exists. This phenomenon is believed to be caused by minimizing the interfacial energy of the systems. Meanwhile, preferential solubilization or anchoring of the like chains of copolymer into homopolymer matrix leads to stabilization of the dispersed phase in the matrix. In addition, regular variation of the inner morphology of the dispersed copolymer phase with the composition and molecular parameters of the component polymers is observed. When the two components have comparable proportions, alternating concentric shells are the most common feature which is associated with minimizing the interfacial energy in the Cop/Homo systems.     

Thermoreversible gelation of solutions of vinyl polymers

The thermoreversible gelation of solutions of isotactic poly(methyl methacrylate) is investigated. Amorphous gels can be prepared in l-butanol by a combination of a liquid-liquid demixing with an upper critical demixing temperature and a glass transition. Annealing of the demixed solutions above their glass transition temperature T_G, results in the formation of a crystalline gel. In oxylene, crystalline gels are formed by a liquid-liquid demixing with an lower critical demixing temperature and an annealing at room temperature. Very fast gelation is observed to occur far below room temperature in solvents like 2-butanone.     

Demixing Time and Temperature Influence on Porosity and Interconnection of PLLA Scaffolds Prepared via TIPS

Scaffolds suitable for tissue engineering applications were prepared by Thermally Induced Phase Separation (TIPS) starting from a ternary solution PLLA/dioxane/water. The experimental protocol consisted of three consecutive steps, a first quench from the homogeneous solution to an appropriate demixing temperature (within the binodal region), a liquid-liquid demixing stage for a given time and a final quench from the demixing temperature to a low temperature (within the spinodal region). A large variety of morphologies, in terms of average pore size and interconnection were obtained upon modifying the demixing time and temperature, owing to the interplay of nucleation and growth processes during the residence in the metastable state. An interesting combination of micro and macro-porosity was observed for longer demixing times (above 30 min at 35 °C).     

Ginzburg number and phase behavior of binary polymer blends in pressure fields

In some polymer blends the temperature and pressure dependence of thermal composition fluctuations have been measured with small angle neutron scattering. The Ginzburg number Gi, the Flory‐Huggins parameter Γ, and the phase boundaries were determined for pressure fields up to 150 MPa. In polymer blends the compressibility leads to a strongly increased Gi which could be appreciably larger than in low molecular liquids and which decreases with increasing pressure fields. Usually, the phase boundaries of UCST as well as of LCST blends shift with pressure to higher temperatures. One blend having PDMS as one component, however, shows an abnormal decrease of the phase boundaries with increasing pressure. The Clausius‐Clapeyron equation correctly predict from the experimentally determined Γ and Gi the observed pressure dependence of the phase boundaries.