Star polymers viscoelasticity

The deliberate introduction of multifunctional branching into anionically prepared polydiene and poly (diene-co-styrene) polymers produces materials with unique morphological and viscoelastic properties (1-3). Work has included synthesis of symmetric star polymers produced by reaction of living polyanionic "arms" with multi-functional chlorosilane (4-9),... 

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]. 

The similarity between viscoelastic properties of lipopolymer monolayers and star polymers suggests that the gelation transition in lipopolymer monolayers might be caused by a jamming transition of micelles as well. Such a model is attractive because the ability to form surface micelles should be strongly connected to the... 

The viscoelastic behavior in the melt state of end-fiinctionalized polyisoprenes was also investigated (30). The results can be compared with predictions based on the star model for the aggregates. It is well known that the viscoelastic properties of star polymers in the melt state depend on arm molecular weight and they are insensitive to their functionality (31). 

Vega, D. A., Sebastian, J. M., Russel, W. B., and Register, R. A., Viscoelastic properties of entangled star polymer melts comparison of theory and experiment, Macromolecules, 35, 169-177 (2002). 

An early study by Meijer and co-workers used poly(propy-lene oxide-co-ethylene oxide) three-arm star polymers with hydroxyl end-group functionality that could be converted into UPy functional termini in two steps using bis-isocyanate and methylisocytosine. The materials were determined to be viscoelastic in nature and were easily degraded through the addition of a small monovalent molecule. Moreover, the addition of a small amount of water led to large decreases in mechanical properties through competitive H-bonding with the UPy units. 

V.R. Raju, E.V. Menezes, G. Marin, W.W. Graessley, and L.J. Fetters, "Concentration and Molecular Weight Dependence of Viscoelastic Properties in Linear and Star Polymers," Macromolecules, 1, 1668 (1981). 

Ilie anomalous behaviour in the linear viscoelasticity has been explained by the tube model.Figure 7.26 shows schematically how the stress relaxation takes place in star polymers. In the crude theory, it is assumed that the centre of the star is fixed during the viscoelastic relaxation time and that the relaxation takes place only by the contour length fluctuation, i.e., by the process that the polymer retracts its arm down the tube and evacuates from the deformed tube as shown in Fig. 7.26. 

All these observed characteristics of viscoelasticity for star polymers are natural consequences of the tube model. As suggested by the sketch in Fig. 3.49, the presence of even one long branch would surely suppress reptation [53]. There is no longer any direction for the star to move freely into new positions and conformations, and accordingly relaxation and diffusion must occur by some other motion. The Pearson-Helfand theory for stars based on tube-length fluctuations alone [72]... 

With a consideration on these existing research results, this chapter approaches to the viscoelasticity of gelation with several topics those are theoretical and experimental works on power laws observed for various properties in sol-gel transition region, delay of gel peint and the ring formation for the gelation of RA/n + R lyp polymerisation, network formation by the end-linking of star polymer and the viscoelastic behaviour. 

As for the end-linking, an experimental work has already been published for the viscoelastic relaxation of poly(propylene sulfide) star by Nicol et al., they discussed the relation of p and viscoelasticity in detail. (Nicol et al, 2001) It is a 3-arm star polymer and can be end-linked by the reaction with hexamethyl diisocyanate(HMDI) in the presence of a small amount of catalyst of dibutyl tin dilaurate. Also, it is regarded as a network formation of RA3 + R B2 type reaction.(The values are functionalities.) The characteristic features to remark for poly(propylene sulfide) star, abbreviated as PPS star, is low Tg( -37°C). 

The plateau modulus. Go, was calculated using the equation written below, then viscoelasticity curve was calculated by BOB rheology for three arms star polymer, which results were also shown in Figure 20. 

The effect of coupling reaction was considered in the viscoelasticity calculation of three arms star polymer. As mentioned above, the prep>aration of the star polymer accompanies the coupling reaction which mainly generates the dimer. It is obvious that the dimer is in the architecture of H-polymer, therefore the calculations were carried out with mixing three... 

Nicol, E. Nicolai, T. Durand, D. (2001) Effect of Random End-Linking on the Viscoelastic Relaxation of Entangled Star Polymers, Macromolecules, Vol.34, 5205-5214. 

Tanaka, Y. (2009) Viscoelastic Behaviour for End-Linking of Entangled Star Polymer Application of the Calculation for Branched Polymer to End-Linked Polymer, Nihon Reoroji Gakkaishi, Vol.37,89-95. 

Correlations between a,G o, and G20 are very nearly the same for linear and for star polymers. The polymer s geometry does not play a major role in these aspects of the viscoelastic functions. If the modes of motion of linear and star polymers differ, the differences are not evident in the comparisons made here. 

Time and frequency do not enter the above calculations. However, the solutionlike-meltlike transition suggested a structure for fixed points of the Altenberger-Dahler renormalization group. An ansatz extending the structure from a single concentration variable to a two-variable concentration-time plane indicated a possible form for the complex viscosity(14). Chapter 13 successfully compares the ansatz predictions with experiment. This two-parameter temporal scaling approach has since been applied successfully to describe viscoelastic functions of linear polymers and soft-sphere melts(15), of star polymers(16), and of hard-sphere colloids(17). 

G. D. J. Phillies. Temporal scaling analysis Viscoelastic properties of star polymers. J. Chem. Phys., Ill (1999), 8144-8150. 

Cell, C. B., Graessley, W. W., Efstratiadis, V., Pitsikalis, M., Hadjichristidis, N. Viscoelasticity and self-diffusion in melts of entangled asymmetric star polymers./. Polym Sci., B, Polym Phys. (1997) 35, pp. 1943-1954... 

Raju, V. R., Menezes, E. V., Marin, G., Graessley, W. W. Concentration and molecular weight dependence of viscoelastic properties in linear and star polymers. Macromol (1981) 14, pp. 1668-1676... 

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