Segregation chain end

If the chain ends are segregated on the surface at the start of the welding, then the same general scaling law applies but with different values of r and s for some of the number properties. The average properties remain unaffected [1]. 

The ripple experiment works as follows In Fig. 6, HDH and DHD are depicted by open and filled circles where the filled circles represent the deuterium labeled portions of the molecule and the open circles are the normal (protonated) portions of the chains. Initially, the average concentration vs. depth of the labeled portions of the molecules is 0.5, as seen along the normal to the interface, unless chain-end segregation exists at / = 0. If the chains reptate, the chain ends diffuse across the interface before the chain centers. This will lead to a ripple or an excess of deuterium on the HDH side and a depletion on the DHD side of the interface as indicated in the concentration profile shown at the right in Fig. 6. However, when the molecules have diffused distances comparable to Rg, the ripple will vanish and a constant concentration profile at 0.5 will again be found. 

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°... 

Another noteworthy observation is the segregation, or localization, of chain ends at the film surface, which was found in both nonfunctional and functional chain ends [172], The endbead density obtained from the MC simulations is... 

Fig. 10 Possible model for depth dependence of Tg as a function of Mn, showing the mobility gradient in the surface region. Within the depth region in which chain ends are segregated, the mobility gradient should be strongly dependent on M. This would be not the case far beyond the segregated surface layer of chain ends...

Figure 9. Schematic models for the cores of half integral lines in polyester 1. Chain ends segregate (a) in the core (5=-t-l/2) or (b) along three quasi walls (S=-l/2) [57].

P. Doruker andW. L. Mattice, Segregation of chain ends is a weak contributor to increased mobility at freepolymersurfaces,/.P//pi. Chem.B 103 178(1999). 

In all these cases is reported the importance of the end-chain on the superficial migration of the additive. On the contrary, diffusion of cyclic polymers having no chain ends would be of great interest in the study of polymer surface segregation. In 2006, Tanaka compared time evolution of isotopic interfaces in two bilayers films, one made of cyclic polystyrene (c-hPS/c-dPS) and the linear counterpart (1-ZiPS/l-dPS) [76]. He showed that interdiffusion of the cyclic macromolecules was faster than the linear polymers one. Therefore, it would be very interesting to examine the surface segregation behavior of a binary film composed of a cyclic additive in a linear matrix. 

Tanaka, K., Kawaguchi, D., Yokoe, Y., Kajiyama, T., Takahara, A., Tasaki, S. Surface segregation of chain ends in a,CD-fluoroaIkyl-tenninated polystyrenes films. Polymer 44, 4171 177 (2003)... 

Finally we should consider the location of the chain ends. On physical groimds it can be argued that a chain with its end near a surface suffers a smaller loss of entropy by virtue of the surface than does a chain with a middle segment at the surface, so that the density of chain ends at the surface should be somewhat enhanced. This picture is supported by simulations for example in the variable density self-consistent mean field study of PDMS of relative molecular mass 3270 already cited, more than 50% of the population of the outermost layer of the polymer consisted of chain ends, even though chain ends represented only 17% of the total mass. On the other hand experimental evidence for chain-end segregation is much more equivocal. The reason for this is that, in order to detect such segregation, it is necessary experimentally to label the end of the chain in some way so that a surface analysis technique may be used to detect an excess of the ends. However, as we shall see in chapter 6, even a very small substitution at the end of a polymer chain may lead it to be... 

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