Isotopic polymer mixtures

Bates, F. S., Muthukumar, M., Wignall, G. D., and Fetters, L. J., Thermodynamics of isotopic polymer mixtures - significance of local structural symmetry. J. Chem. Phys. 89, 535-544 (1988). 

For a polymer pair that would undergo phase separation at high relative molecular masses, such as an isotopic polymer mixture like polystyrene/ deuterated polystyrene, this criterion means that one would have to approach closer to the critical point than about 0.05 °C for these fluctuation effects to be important. Even for much more strongly immiscible pairs, that undergo phase separation at much lower relative molecular masses, the non-classical region will be within only a few degrees of the critical point (Schwahn et al. 1987, Bates et al. 1990). This is in sharp contrast to the situation for small-molecule liquids, for which fluctuations must always be taken into account in order to predict the phase behaviour. Mean-field theories - neglecting bulk concentration fluctuations - often can be expected to work quite well for polymers. 

Diffusion in Block Copolymers and Isotopic Polymer Mixtures 319... 

DIFFUSION IN BLOCK COPOLYMERS AND ISOTOPIC POLYMER MIXTURES... 

We present here a forward recoil spectrometry (FRES) study of thermodynamic slowing down" of mutual diffusion in isotopic polymer mixtures and of the diffusion of homopolymers into symmetric diblock copolymer structures. The measurements of "thermodynamic slowing down" were performed on binary mixtures of normal and deuterated polystyrene (PS). Both the Flory interaction parameter, the upper critical... 

Presented polymer mixtures are composed of amorphous macromolecules with different molecular architecture homopolymers and random copolymers, with different segments distributed statistically along the chain, form partly miscible isotopic and isomeric model binary blends. The mixing of incompatible polymers is enforced by two different polymers covalently bonded forming diblock copolymers. Here only homopolymers admixed by copolymers are considered. The diblock copolymer melts have been described recently in a separate review by Krausch [17]. 

The staining of individual molecules, obtained by a replacement of hydrogen for deuterium (used to create a contrast required in experiments with condensed matter) may lead to drastic changes in the phase behavior of the studied systems. It has been widely recognized that this effect influences bulk interactions in polymer mixtures, as is discussed in Sect. 2.2.3. Here we describe first experimental results [145] on the role of deuterium labeling on surface interactions in blends which are not isotopic mixtures. 

Mixtures of polymer chain belonging to the same chemical species but with different isotopic compositions (deuterated and non-deuterated) have been widely used for experimental studies of polymer structures, since good neutron beams became available. This technique, combining the preparation of adequate samples and neutron scattering experiments, enabled the experimentalists to determine the size of polymer chains (polystyrene or polydimethylsiloxane), in all kinds of polymer mixtures or concentrated polymer solutions. However, the technique relies on the fact that the deuterated and non-deuterated isotopic varieties of a same polymer are compatible with one another. It is admitted that under the experimental conditions described above, the mixture constitutes a unique phase. In fact, the mixing energy of deuterated and non-deuterated chains is probably very small. However, it is non-zero, in particular, because of differences in atomic volumes and polarizabilities. Thus, there is no doubt that demixtion may occur in mixtures of deuterated and undeuterated chains of very high molecular masses. 

Surface-directed spinodal decomposition was first observed in an isotopic polymer blend (Jones et al. 1991) thin films of a mixture of poly(ethylene-propylene) and its deuterated analogue were annealed below the upper critical solution temperature and the depth profiles measured using forward recoil spectrometry, to reveal oscillatory profiles similar to those sketched in figure 5.30. Similar results have now been obtained for a number of other polymer blends, including polystyrene with partially brominated polyst)u-ene (Bruder and Brenn 1992), polystyrene with poly(a-methyl styrene) (Geoghegan et al. 1995) and polystyrene with tetramethylbisphenol-A polycarbonate (Kim et al. 1994), suggesting that the phenomenon is rather general. 

Contrast variation methods can sometimes be used to remove a component of the scattering by matching its scattering power with that of the medium in which it is dispersed. This principle can be used in SANS experiments via isotopic solvent mixtures (e.g. H2O/D2O) to adjust the scattering power of the medium, as for example in studies of polymer latexes. Grancio and Williams [35] postulated a polymer-rich spherical core surrounded by a monomer-rich shell which serves as the major locus of polymerization, thus giving rise to core-shell morphology. Thus, the first formed... 

Equation (5.1) includes only the ideal, combinatorial entropy of mixing and the simplest conceivable regular solution type estimate of the enthalpy of mixing based on completely random mixing of monomers mm ( ) = 1 in the liquid state language i referred to as the bare chi parameter since it ignores all aspects of polymer architecture and Interchain nonrandom correlations. For these reasons, the model blend for which Eq. (5.1) is thought to be most appropriate for is an interaction and structurally symmetric polymer mixture. The latter is defined such that the only difference between A and B chains is a v (r) tall potential, which favors phase separation at low temperatures. The closest real system to this idealized mixture is an isotopic blend, where the A and B... 

The data of Zink et al. (1998) illustrate the measurement by NRA of near-surface composition profiles in isotopically labelled polymer blends. If a mixture of polymers is adjacent to a phase interface (e.g. a solid or an air surface), often one of the components is preferentially attracted to the surface and will segregate to it, and this phenomenon will influence the tribological behaviour the interface (lubrication, wear and adhesion). 

In most of the examples, the isotopic methods are used essentially for analytical purposes. In this connection, their great value lies in extreme sensitivity and specificity. Many of the problems are concerned with the accurate determination of low concentrations of particular groups in polymer molecules. Another type of problem involves the determination of small weights of substances of low molecular weight formed simultaneously with polymer, or formed from polymer by chemical reactions for this purpose, the technique of isotope dilution analysis is of great value. The substances are usually present as minor components in rather complex mixtures so that determination by conventional methods would be extremely difficult. The method of isotope dilution analysis involves the addition to the mixture of a substantial quantity of the substance to... 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

The mechanism of benzene and higher polymer formation remains uncertain with further work on isotopic mixtures needed to help determine the processes which occur. It is interesting to note that, in the P- and X-ray radiolysis of C2H2-C2D2 mixtures, Mains et observe all benzenes do to in the products and conclude that a C-H rupture must occur in the radiolysis. Dorfman and Wahl have shown that in the helium-sensitized radiolysis of acetylene, where only ionized states of acetylene are formed, there is no formation of benzene. The strong pressure-dependence of the benzene formation in the direct photolysis still provides the strongest evidence for a molecular mechanism such as given in the reaction scheme. 

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