Dimitrios Papanagopoulos

Amphiphilic heteroarm star copolymers of polystyrene and poly(ethylene oxide)

Abstract Amphiphilic heteroarm star copolymers bearing polystyrene (PS) and poly(ethylene oxide) (PEO) branches have been synthesized by sequential anionic ‘living’ copolymerization. These samples have been characterized adequately and shown to exhibit rather well-defined structures. The functionality of the stars is influenced mainly by the molar ratio of divinylbenzene per living ends and the molecular weight of the linear PS precursor. These star copolymers exhibit association phenomena not only in water but also in tetrahydrofuran, which is not a selective solvent for the different arms. Finally, it has been shown that the PEO arms can be crystallized, forming well-defined spherulites. This ability is strongly affected by the PEO content and the thermal history of the samples.

Difference between the dynamic and static behaviour of polymers in dilute solutions: 2. The critical concentration c

The critical concentration, c * , at which overlapping between macromolecular chains occurs, has been studied using dynamic (viscometry) and static (light scattering) methods. With the same polymer-solvent system this critical concentration has a higher value when viscometry is used, compared to the concentration obtained when using a light scattering technique. This difference is attributed to an incipient decrease of the dimensions of the macromolecular coils above the critical concentration c ** , which is observed only under dynamic conditions. Relationship between the critical concentration c * and both the intrinsic viscosity and the molecular weight of the polymers are proposed

Three models for chain conformation of block copolymers in solution and in solid state

Solution properties of polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymer, polystyrene-poly(tertiobutyl methacrylate) (PS-PtBuMA) diblock copolymer, and poly(ethylene oxide)-polystyrene-poly(ethylene oxide) (PEO-PS-PEO) triblock copolymer have been measured by viscometry. The PEO-PS-PEO copolymer has been studied also in solid state by differential scanning calorimetry and by optical microscopy. All the block copolymers present a conformational transition in solution at a given temperature region which is relatively narrow. If below this transition temperature a copolymer adopts a segregated conformation (dumb-bell model), above this transition adopts a nonsegregated or pseudo-gaussian conformation, and vice versa. In the transition temperature region the copolymer adopts a compressed segregated conformation (core and shell model). If the passage from the solution to the solid state is performed in a given constant temperature in which the copolymer presents a segregated or nonsegregated conformation the same conformation is observed in the solid state (memory effect).

The influence of the molecular mass of two incompatible polymers on their miscibility in the solid state without compatibilizer after casting from solution

The influence of the molecular mass of two incompatible polymers on their miscibility in the solid state has been studied using Scanning Electron Microscopy and Optical Microscopy. Two pairs of incompatible polymers, polystyrene/poly(methyl methacrylate) and polystyrene/poly(oxyethylene) were cast from a nonselective solvent. When the two polymers are of comparable and relatively high molecular mass (M u , ∼ 60,000) we obtain films in which the domains of the two phases are reduced to 7 μm, without the use of any compatibilizer. On the contrary, when the two polymers present very different molecular masses, a lamellar structure is obtained due to the high repulsion between them. This repulsion has already been observed in solution (Refs. 4 and 5). In the case of polymers with comparable but very low molecular masses the repulsion between them, in solution, is also high, leading to phase separation in the films obtained after removal of the solvent.

Towards a better mixing of two incompatible polymers by casting from solution without compatibilizer

Blends of polystyrene/poly(oxyethylene) (PS/POE) and polystyrene/poly(methyl methacrylate) (PS/PMMA) have been obtained by casting from solution. Differential Scanning Calorimetry, Optical Microscopy, and Scanning Electron Microscopy showed that two incompatible polymers can present relatively good miscibility (formation of domains smaller than 5 μm) when the solvent from which the films are obtained does not present any noticeable selectivity towards the two polymers of the blends. An increase of the casting temperature increases the miscibility of PS and PMMA because the selectivity of the solvent used, towards these polymers decreases with increasing temperature. On the contrary, an increase of the casting temperature in the case of the PS and POE mixture decreases their miscibility because the selectivity of the solvent used increases with increasing temperature.

Influence of the shear rate, of the molecular architecture and of the molecular mass on the critical overlapping concentration c*

The critical overlapping concentration c * has been studied in the dynamic state (by viscometry) and in the static state (by light scattering). An increase of the shear rate of the solution provokes an increase of c * in the case of linear polymers. In the case of a highly branched polymer the shear rate has no influence on its critical concentration. The scaling law between c * and the molecular mass for a given polymer—solvent system presents two different exponents, depending on the molecular mass region of the polymer.

Relation between the repulsion of incompatible and compatible polymers in solution and their degree of mixing in the solid state: the memory effect

The incompatible polymer pairs polystyrene (PS)/poly(methyl methacrylate) (PMMA) and polystyrene (PS)/poly(oxyethylene) (POE) and the compatible polymer pair POE/PMMA have been studied in solution and in the solid state. A close relation is observed between the behaviour in solution and in the solid state for both incompatible and compatible polymer mixtures. When we use a selective solvent and when the molecular masses of the polymers of the mixture are different, even compatible polymers behave like incompatible polymers; the obtained films present an important demixing. The opposite is observed when the films are obtained by casting from a common solvent and when the two polymers possess similar molecular masses. In this case, even incompatible polymers behave like compatible polymers. These results confirm the already reported memory effect when we go from the solution to the solid state.

An approach to the theta temperature of star polymers based on the blob model

Abstract Using a relation derived from the blob model we obtain an equation giving the Θ temperature of the polymers. This equation predicts for the star shaped polymers a decrease or an increase in their Θ temperature compared with the Θ temperature of the corresponding linear polymers. The value of the Θ temperature of star polymers is tightly related to their degree of branching. The predictions of this model are compared with experimental results.

On the resistance to the interpenetration between macromolecular coils of different segment densities

Using viscometry techniques on polymer fractions, we determine the critical concentrationc* (separating the dilute and semi dilute solutions). The same measurements have been conducted with mixtures of these fractions (mixtures 1:1 by weight of fractions differing in molecular mass and chemical nature, or fractions differing only in molecular mass). The determined values of critical concentrationc* of the mixtures are higher than the values calculated based on the critical concentrations of the corresponding fractions. This deviation from the additivity rule is attributed to the resistance in the interpenetration (delay to the attainment of the homogeneous state) between macromolecular coils of different chemical nature or of the same chemical nature but of different molecular mass. Higher values of the reduced viscosities of the mixture of the fractions, compared to the values calculated using the reduced viscosities of the corresponding fractions, are observed above the critical concentrationc*. In this concentration region the interaction parameter between two different polymers is calculated.

Study of poly(2-vinyl pyridine)-poly(methyl methacrylate) graft copolymers in selective solvents: Micelle formation

Abstract A graft copolymer consisting of a poly(2-vinyl pyridine) backbone chain and of poly(methyl methacrylate) side chains, prepared anionically, has been studied in dilute solution, in selective solvents, using viscometric and light scattering techniques. In some solvent media in which the backbone chain is insoluble the copolymers remain soluble due to a “protective” effect of the side chains. In this solvent media the copolymer is found either as monomolecular or polymolecular micelles.