Molar mass segregation

SBS is a three-block copolymer of styrene and butadiene. Incompatibility of PS and poly-butadiene (BR) causes segregation of the chain parts a two-phase system results in which PS and BR each have their own individuality and their own Tg. The fact that the highest Tg is lower than the Tg of normal PS, is a result of the relatively low molar mass of the PS chain ends. In general the effect of M on Tg is insignificant Tg= - K/ M, but when K is, e.g. 10 and M = 10,000, Tg can be 10°... 

Abstract The morphology of polyethylene has been an important theme in polymer science for more than 50 years. This review provides an historical background and presents the important findings on five specialised topics the crystal thickness, the nature of the fold surface, the lateral habit of the crystals, how the spherulite develops from the crystal lamellae, and multi-component crystallisation and segregation of low molar mass and branched species. 

The morphology of a polyethylene blend (a homopolymer prepared from ethylene is a blend of species with different molar mass) after crystallisation is dependent on the blend morphology of the molten system before crystallisation and on the relative tendencies for the different molecular species to crystallise at different temperatures. The latter may lead to phase separation (segregation) of low molar mass species at a relatively fine scale within spherulites this is typical of linear polyethylene. Highly branched polyethylene may show segregation on a larger scale, so-called cellulation. Phase separation in the melt results in spherical domain structures on a large scale. 

At low temperatures, partial co-crystallisation was indicated by transmission electron microscopy and differential scanning calorimetry [156, 157]. Both electron microscopy of stained sections and optical microscopy showed that the segregated low molar mass material was present as small domains between the stacks of dominant lamellae within the spherulites/ax-ialites [115, 157, 158],... 

The critical molar mass, above which segregation occurs, Mcr, is 2.5 to 5 times greater then for equivalent blends. This means a better miscibility of block copolymers, as a result of the entropy restrictions in the domain structure. 

Recent studies [111,214] indicate that Th-FFF can even be used to determine the relative chemical composition of two components in random copolymer and linear block copolymers whose monomers do not segregate due to solvent effects. However, this application is limited by the unpredictable nature of thermal diffusion. Nevertheless, combining information from Th-FFF with those derived on fractions by independent detectors selective to composition (such as an IR spectrometer) can yield further insight into the dependence of DT on the chemical composition. Even more powerful is the combination of Th-FFF with SEC as, here, the chemical composition (from Th-FFF) can be studied as a function of the molar mass (from SEC). This was demonstrated by van Asten et al. by cross fractionating copolymers and polymer blends with SEC and Th-FFF [358]. 

Phase separation in mixtures of polymers especially occurs at high concentrations and for large molar mass. The separation can be of two kinds. If the two polymers show mutual affinity, associative phase separation or complex coacervation occurs, i.e., a separation into a solution high in both polymers (a complex coacervate) and a very dilute solution. In most other cases, segregative phase separation or incompatibility is observed, i.e., separation in a phase rich in polymer A but poor in polymer B, and vice versa. 

Diphasic and segregation phenomena [ 127 -131 ] at the nematic-to-isotropic transition reveal the multicomponent nature of the main-chain polymers as due to the wide molar mass distribution. The thermodynamic width of the biphasic gap was delineated by annealing the samples inside the apparent biphasic gap, giving a result... 

They demonstrated the surface eiuichment of the star in isotopic blend with a linear polystyrene counterpart by dynamic secondary ion mass spectroscopy. However, in this study the star polymer was the deuterated component and had a lower molar mass than the linear matrix. Therefore, one could question if these two factors aren t themselves driving the surface segregation. 

Hyperbranched and comb polymers have also been used as surface active additive. Ariura et al. synthesized by combination of anionic and cationic polymerization a monodispersed hyperbranched polystyrene [73]. The authors proved by combination of DSIMS and neutron reflectivity the preferential surface enrichment of the branched protonated macromolecules when blended with its deuterated linear polystyrene counterparts with the same molar mass. Other systems involving the segregation of the branched macromolecules in binary blends were demonstrated such as in polyamide [74] or poly (methylmethacrylate) [75]. 

There is a wide variety of compositional variables which can affect the degree of phase segregation, the organization of the HS and consequently the PUs mechanical properties. The symmetry of the diisocyanate is an important factor. For example, Sung et al. studied a series of polymers based on the diisocyanates 2,4-TDl or 2,6-TDI, extended with BDO, and polyether or polyester SS of molar masses 1 000 or 2 000 g/mol. In the 2,4-TDI polymers, the asymmetric placement of the isocyanate residued with respect to the methyl group resulted in some head-to-tail... 

There was also evidence that the hydrogen bonding interaction can interfere with the SS crystallization, as long as the SS crystallization did not occur in polymers like those based on 2,4-TDI-PTMO (molar mass 2 000 g/mol), whereas a slight improvement in phase segregation occured in the polymers with 2,6-TDI-PTMO of molar mass 2 000 g/mol [134]. The SS segment crystallized freely as indicated by a further decrease in the glass transition temperature. 

As indicated in previous chapters, low molar mass materials can segregate to surfaces and hence the molar mass dependence of the surface tension is an important factor in considering the performance of a polymer material. Wu has proposed that... 

Most commercial polymer systems are a complex mixture of one or more polymers, plasticizers, antioxidants and processing aids. The surface of such a system will therefore not necessarily be determined by the dominant polymer but will often be influenced by the segregation of low molar mass and low... 

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