No abstract

The structure and the thermodynamics of polymer blends made from two poly(styrene/ acrylonitrile) copolymers with a different ratio of styrene/acrylonitrile are investigated by neutron scattering. These systems are endothermic, exhibit a positive value for the Huggins parameter x and show phase separation at sufficiently large molecular weights. The results are in good agreement with the corresponding states theory of Prigogine and Patterson. Other polymer blends are exothermic, exhibit negative x-values and are compatible even for infinitely large molecular weights. In these cases one can adapt the theoretical equations to the experiments by the introduction of a suitable parameter for an exothermic contact energy. This is shown for blends from poly(methy1 methacrylate) and poly(styrene/acrylonitrile).

Provided a polymer is soluble, i. e., molecularly dispersed in another polymer irrespective of the molecular weight of the components, the solution is exothermic. By increasing the temperature two effects, both unfavourable to mixing become larger: (i) the excess entropy of mixing caused by contact interaction and (ii) the total effect from the difference of the free volumes of the pure components. So, an upper miscibility gap occurs. The thermodynamic properties of the mixture cannot be derived from the properties of the pure components. They can be described by the corresponding states theory of Prigogine, Flory, and Patterson with suitable values for the contact energy and contact entropy parameters X , , and QIz. The temperature of separation can be predicted from measurements on the mixture at low temperatures. Basic principlesBy using neutron scattering to investigate chemically heterogeneous polymer blends it is possible to obtain direct experimental evidence of molecular dispersion, to determine the conformation of the polymer chain and to quantify the thermodynamic interaction between segments of different polymers' -lo). It is possible to decide definitely and without precondition, whether the different polymers are homogeneously mixed also at the molecular level. In this paper, polymer blends are understood to be mixtures of two polymers without a lower molecular weight component. The Flory-Huggins equationThe thermodynamic properties of polymer mixtures may be described by the Flory-Huggins equation'] -I3).R and T have the usual meaning; PI and Pz are the degrees of polymerization of components 1 and 2, or more correctly the number of lattice sites occupied by the segments of a chain molecule. P, and Pz are derived from the partial molar volumes a) Present address: Institut Max v. Laue-Paul Langevin,

Polymeric materials with novel properties for new technological applications are increasingly obtained by combining existing polymers, while the synthesis of new monomers has receded into the background. These polymer combinations or "alloys" (polyblends) are characterized by their chemical composition, the conformation of the chain molecules, and the morphology, i. K . the state of order at supramolecular level. Multiphase constitution is a typical characteristic of these substances, with a decisive influence on their macroscopic properties. The morphology of multiphase polymer alloys can be controlled to a limited extent via the chemical composition of their components when homopolymers are mixed in the melt or as dispersions. Graft copolymerization, on the other hand, makes it possible to achieve the desired morphology at a given chemical composition. Furthermore, transparent two-phase polymer alloys can be obtained under certain conditions. In multiphase polymers the reduction of stress without fracture, caused by mechanical loading will be treated using models. Certain combinations of properties such as hardness and toughness are connected with the coexistence of disperse and continuous phases. Equilibrium thermodynamical criteria for liquid mixtures will be used to explain demixing phenomena in polymers. In the last few years it has become possible to determine the chain conformation experimentally using neutron scattering. Basic PhenomenaMacromolecular materials with new technical properties can be obtained by the synthesis of new monomers, by new methods of polymerization, and by the combination of existing polymersr'] having various properties. Preparative chemistry has produced about 1000 compounds that can be polymerized in chain or stepwise reactions["*], forming the basic repeating units of the resulting polymers. About 50 of these monomers are used on an industrial scale, and this number has not increased in the past few years. In applied polymer research the interest has instead been turned more and more toward methods of polymerization and the combination of existing polymers. The simple idea behind this second method is to build up required composite industrial materials by combining known p o l y m e r~ [~-~] .For example, it seemed obvious to combine the hard and stiff, but at the same time brittle homoand copolymers of styrene (repeating unit 3 C H ( C 6 H + CH2f) with the soft and resilient butadiene polymers (repeating units (*CH2-CH=CH-CH23 or +CH2-CH(CH=CH&) by polymerizing the former in the presence of prepolymerized butadiene to produce a material exhibiting hardness and stiffness as well as impact resistance. According to their composition, composites are to be regarded as polymer mixtures. They can be described as polymer alloys, because they often have qualitatively new characteristics not possessed by their constituents. Kunststomaboratorium der BASF-AG D-6700 Ludwigshafen (Germany) [**] The term 'polymer' denotes an assembly of macromolecules which are built up of a large number...

Mit Hilfe der Neutronenkleinwinkelstreuung wird der molekulardisperse Charakter einer glasklaren Mischung von zwei Polymeren bei Abwesenheit eines niedermolekularen Lösungsmittels in einem Fall nachgewiesen, im anderen widerlegt. Falls eine molekulardisperse Mischung vorliegt, können das chemische Potential und die Knäueldimensionen bestimmt werden. Polymethylmethacrylat und Styrol‐Acrylnitril‐Copolymere bilden für gewisse Zusammensetzungen des Copolymeren molekulardisperse Mischungen. Sie sind exotherm, besitzen eine negative Exzeßentropie und die Molekülknäuel sind gegenüber den ungestörten Knäueldimensionen expandiert. Die Theorie der Löslichkeitsparameter von Hildebrand und Scott ermöglichte zwar die Auffindung des mischbaren Systems, aber die gemessenen zweiten osmotischen Virialkoeffizienten sind in der Größenordnung verschieden von denen, die nach dieser Theorie vorausberechnet werden können.