Anastasios Dondos

Linear representation of viscosity data as a function of molecular weight

Abstract It is proposed to represent viscosity data by plotting 1 [η] versus 1 M 1 2 . This gives a straight line even when the exponent a in the Mark-Houwink-Sakurada plot is not constant. The inverse of the slope of the straight line is approximately equal to K ϑ . The intercept determined by extrapolating the straight line plot to infinite M increases with the quality of the solvent.

Viscometric study of aggregation phenomena in polymer dilute solutions and determination of the critical concentration c

Abstract A crossover is observed in the variation of the reduced viscosity, η sp c , as a function of the concentration, c , for the systems poly(methyl methyacrylate)/nitromethane and poly(vinyl-2 pyridine)/benzene. This crossover is correlated with the critical concentration of the polymer c∗∗ in which the macromolecular coils start their contacts. The critical concentration c∗∗ is related with the molecular weight and the intrinsic viscosity of the polymer through the scaling laws: c∗∗∝ M −α and c∗∗∝[η] −1 .

Unperturbed Dimensions of Polymers in Binary Solvent Mixtures

Abstract Intrinsic viscosity measurements were carried out on polystyrene and polyvinylpyridine solutions in various solvents and solvent mixtures. The Stockmayer-Fixman extrapolation was applied to the date: it yields the unperturbed dimensions K θ of the chain. A unique value of K θ was found for a given polymer in all pure solvents, as expected; in the case of binary mixtures, the K θ value is found higher or lower than this unique value, depending upon the thermodynamic parameter x 12 characterizing the solvent mixture. According to these results, the properties of θ-solutions (as defined classically for pure solvents) should not be extended to ternary systems, since K θ does not depend solely on temperature.

Viscometric determination of the two critical concentrations of polymers in solution

A viscometric study is presented using ternary systems polymer-1/polymer-2/solvent. The variation of the reduced viscosity of the polymer mixture with the total concentration shows two crossover points corresponding to the two critical concentrations of polymers C∗∗ and C∗. The influences of the temperature and of the hydrodynamic volume of the polymers on the critical concentrations are also presented.

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

Study on polymer solutions in solvent mixtures in the vicinity of the critical point of the solvents: 4. Saturation phenomena

Abstract Turbidimetric results have been obtained with the ternary system N,N-dimethylformamide/cyclohexane/polystyrene. The displacement of the heterogeneous region depends on the molecular weight of the polystyrene sample and for each polystyrene sample the displacement changes with the concentration of the polymer. Above a certain molecular weight of the polymer, with a given concentration, the displacement reaches a plateau region. A plateau is also obtained when we change the concentration of the polymer of a given molecular weight. The crossover region, which is determined by the appearance of the plateau region, obeys the scaling laws and is probably the region where the distance between the macromolecular coils is comparable to their diameters.

A new approach to the determination of the statistical segment length of wormlike polymers

The Stockmayer-Fixman-Burchard (SFB) and the Dondos-Benoit (DB) equations have been applied to determine the unperturbed dimensions parameterKχ of wormlike polymers. An empirical relation between the Florys constant Φ and the Mark-Houwink-Sakurada (MHS) exponenta has been proposed. The Φ values found by this equation are lower than the value 2.5×1023 used in the case of flexible polymers and this deviation is attributed to the influence of the draining effect. From theKχ value and the so calculated value of Φ, we calculate the Kuhn statistical segment length of wormlike polymers. The obtained — for a great number of wormlike polymers — statistical segment lengths are almost the same as these calculated by the Yamakawa-Fujii and the Bohdanecky methods. The molecular mass regions in which the SFB, the DB, and the MHS equations are valid are explored. A criterion for the distinction between flexible and wormlike polymers is proposed based on the way of approach to the power law.

Segregation and conformational transition in triblock copolymers: Part 2. Light-scattering studies

From light-scattering experiments carried out on a polystyrene (PS) sample, a poly(methyl methacrylate) (PMMA) sample, on a random copolymer of styrene and MMA and on an anionically prepared triblock copolymer PMMA-PS-PMMA, all of the same molecular weight, in various solvents over a range of temperature (18–65°C) it was shown that a conformational transition occurs for the triblock copolymer. At lower temperatures segregation occurs and the second virial coefficient A 2 is the weighted average between the A 2A and A 2B values for the corresponding homopolymers (PS and PMMA). At higher temperatures contacts between chemically unlike parts of the molecule become possible, and the additivity rules are no more valid for A 2 , as well as for the expansion coefficient. This change from a segregated to a ‘pseudo-gaussian’ conformation involves a change in specific volume of the molecule, and it there-fore brings with it an abnormal variation of the refractive index increment, which has been investigated on the same light-scattering diagrams.

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).

Influence of annealing and casting solvent on the morphology of poly(ethylene oxide)-b-polystyrene-b-poly(ethylene oxide) triblock copolymer : compatibility effects

Abstract The influence of annealing on the morphology of solvent-cast films of poly(ethylene oxide)-b-polystyrene-b-poly(ethylene oxide) (PEO-b-PS-b-PEO) triblock copolymer was studied. The annealing temperature was chosen between the melting temperature of PEO and the glass transition temperature of PS. If the casting solvent is selective for PEO, microphase separation occurs and the crystallinity of the PEO is not affected by annealing. On the other hand, if the casting solvent is a good solvent for both blocks, a dramatic decrease in crystallinity of PEO is observed. This phenomenon is due to compatibility effects. The T g region of the PS-rich phase is shifted towards lower values. This behaviour was elucidated using differential scanning calorimetry combined with enthalpy relaxation experiments.