Star Linear viscoelastic behavior

Generally PSAs are well known for their very viscoelastic behavior, which is necessary for them to function properly. It was therefore important to characterize first the effect of the presence of diblocks on the linear viscoelastic behavior. Since a comprehensive study on the effect of the triblock/diblock ratio on the linear viscoelastic properties of block copolymer blends has recently been reported [46], we characterized the linear viscoelastic properties of our PSA only at room temperature and down to frequencies of about 0.01 Hz. Within this frequency range all adhesives have a very similar behavior in terms of elasticity, as can be seen in Fig. 22.10. The differences appear at low frequency, a regime where the free iso-prene end of the diblock chain is able to relax. This relaxation process is analogous to the relaxation of an arm of a star-like polymer [47], and causes G to drop to a lower plateau modulus, the level of which is only controlled by the density of triblock chains actually bridging two styrene domains [46]. 

Figure 10 Linear viscoelastic behavior of monodisperse four-arm star PI at 25 °C. The numbers indicate 10" /I4rm- Data taken from Milner, S. T. McLeish, T. C. B. Macromolecules 9S8,31,7479 and Fetters, L. J. ...

Investigation of the linear viscoelastic properties of SDIBS with branch MWs exceeding the critical entanglement MW of PIB (about -7000 g/mol ) revealed that both the viscosity and the length of the entanglement plateau scaled with B rather than with the length of the branches, a distinctively different behavior than that of star-branched PIBs. However, the magnitude of the plateau modulus and the temperature dependence of the terminal zone shift factors were found to... 

Only a few studies have been devoted to the bulk properties of asymmetric homopolymer stars. The main issue under investigation was, up to now, the selfdiffusion and viscoelastic behavior of three-arm stars where the molecular weight of the third arm was varied in order to observe the transition in the diffusion from linear polymer to a polymer with a star architecture. 

It is not clear why this transition should occur at such a higher level of arm entanglement for polystyrene stars than for other star polymers. This observation is in direct conflict with the standard assumption that through a proper scaling of plateau modulus (Go) and monomeric friction coefficient (0 that rheological behavior should be dependent only on molecular topology and be independent of molecular chemical structure. This standard assumption was demonstrated to hold fairly well for the linear viscoelastic response of well-entangled monodisperse linear polyisoprene, polybutadiene, and polystyrene melts by McLeish and Milner [24]. 

Fig. 23 (a) Frequency-dependent linear viscoelastic moduli (G closed symbols, G" open symbols) of a colloidal star with nominal values f = 12Sarms and = SOkgmol at a concentration 5 wt% in -tetradecane and different temperatures (circles 40°C, squares. 5O C. triangles 55°C). A liquid-lo-solid transition is marked between 50 and 55°C. Lines with slopes 1 and 2 indicate terminal behavior of G" and G, respectively. Inset The temperature dependence of the hydrodynamic radius /fh of the same star, indicating swelling, (b) Respective moduli for the same system at 4() C in two different solvents, n-decane (circles, solid-like behavior) and n-tetracane (triangles, Uquid-Uke behavior) [26]... 

The entire subject of the viscoelasticity of branched polymers is an active area of research at present. The linear viscoelastic properties of nonsymmetric stars, H-shaped polymers, polymeric combs, and randomly branched species are being investigated both theoretically and experimentally [78-82], and new ideas about their nonlinear responses both during shear and during extension are being considered [83]. With these and other initiatives, the molecular understanding of flow behavior in entangled-polymer liquids will surely expand rapidly in the next few years. 

The presented results show that star polymers in shear flow show a very rich sdnctural and dynamical behavior. With increasing functionality, stars in flow change from linear-polymer-like to capsnle-like behavior. These macromolecules are therefore interesting candidates to tune the viscoelastic properties of complex fluids. 

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