142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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Asymmetric Star Polymers Synthesis and Properties 107 and PEO branches in THF. Although THF is not very selective for PS increased values of apparent weight average molecular weights were obtained. The micelles formed in THF had molecular weights two orders of magnitude larger than the M w of the unimer. The authors observed an increase in the depolarization ratio by increasing polymer concentration indicative of the formation of large multimolecular micelles with PEO cores. Micelle formation was also observed, by the same authors [72], in the case of miktoarm stars with equal number of polystyrene and poly(tert-butyl acrylate) branches in methanol a good solvent for PtBuA and a precipitant for PS. For a sample of the type PS 15 PtBuA 15 of molecular weight M w =218,000 and 70 wt% tBuA, the association number was estimated to be 27 by light scattering. It was assumed that the multimolecular micelles were spherical in shape with PtBuA chains in the corona and PS in the core. The DVB cores of the unimers were assumed to be located at the core-corona interface forming a third domain with high segment density. In the same study viscometric experiments in acetone indicated the existence of a conformational transition around 35 ˚C. In that temperature region a sudden decrease in the intrinsic viscosity and a maximum for the Huggins constant, k H, were observed. Although static light scattering envelopes were curved downwards at low angles, indicating the existence of a small number of high molecular weight associated species, extrapolation from higher angles gave molecular weights very close to the unimer molecular weight. The authors concluded that these phenomena could only be interpreted by a transition from a segregated to a nonsegregated conformation. The micellar structures formed in solutions of well defined (PE) 2 (PEP) 2 , PE(PEP) 2 and (PE) 2 PEP type polyethylene (PE)-poly(ethylene-alt-propylene) (PEP) miktoarm stars in n-decane were investigated by Richter et al. [73] by small angle neutron scattering using the contrast variation technique. The solvent is selective for the PEP branches. The formation of multimolecular structures was also influenced by the ability of PE to crystallize. The PE chains were 75% deuterated to enhance scattering contrast and their MW was kept constant and the MW of PEP was varied in order to study the influence of composition and architecture on the micellar properties. The miktoarm stars formed multimolecular micelles with flat dense, disk-like, crystalline PE cores and soft coronas of PEP “hairs” sticking out on both sides of the core. Using model fitting procedures structural parameters like core and corona thickness were estimated. It was confirmed that the molecular architecture does not modify the global characteristics (shape) of the micelles but only its geometrical parameters. The average thickness of the core was nearly constant and independent of the MW of the PEP brush for the PE 2PEP 2 case. For PE(PEP) 2 and (PE) 2PEP the thickness decreased with increasing MW of PEP. It was found that for all PEP molecular weights the chains are more extended if the PE chains are connected to two PEP arms instead of one. The difference was more pronounced when the PEP molecular weights were higher. On the other hand the thicknesses of the cores were higher for the system (PE) 2PEP. In comparing the systems (PE) 2(PEP) 2 and
108 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos PE(PEP) 2 it was suggested that the average extension of the brush arms was higher for the latter architecture except for the samples with medium molecular weight (M PEP ~9000). At the same time the thickness of the core and the blob size were lower for the PE(PEP) 2 samples. From measurements at different temperatures a critical micelle temperature of about 60 ˚C was found. At room temperature the equilibrium was found to be shifted completely in favor of the micelles. The authors also developed a thermodynamic model to describe the system at low temperature, based on the free energy of a simple micelle. Although this model predicts scaling relations dependent on the star architecture and was found to agree well with previous data on PE-b-PEP diblocks it could not describe the core thickness variation correctly for the miktoarm systems. The solution properties of umbrella star poly(styrene-u-butadiene) copolymers, with PS cores in solvents with decreasing solvating power for polystyrene, were studied by viscometry and light scattering [33]. Intrinsic viscosities decreased in the order toluene (common good solvent)>cyclohexane (q for PS at 35 ˚C)>methylcyclohexane (q solvent for PS at 70 ˚C). Decreasing the temperature from the q point produced only a slight contraction of the size of the molecule most likely due to the contraction of the PS core. Intrinsic viscosities of a sample in n-decane are about two times lower than in the q solvents and the umbrella star was found, by light scattering, to be molecularly dissolved in the same concentration range, forming monomolecular micelles with collapsed PS star cores and marginally solvated PB coronas. The association behavior of three-arm polybutadiene stars with one or two dimethylamino or sulfozwitterionic groups (see Scheme 24 for structures) was studied by Hadjichristidis and coworkers [58, 74] in dilute solutions and compared with the behavior of monofunctional linear and regular three-arm star polybutadienes with all ends functionalized. The amine-capped samples showed no evidence of association in cyclohexane, a good nonpolar solvent for polybutadiene. Strong association was observed for the zwitterionic materials. The weight average aggregation number, N w , determined by low angle laser light scattering and the number average aggregation number, N n , by membrane osmometry, were found to decrease by increasing the number of functional groups in the star polymers, at the same total molecular weight, whereas N w and N n were decreasing function of molecular weight for the same number of end groups. The difunctional and trifunctional stars formed gels even at low concentrations. Taking into account the low aggregation numbers for these samples at concentrations lower than c gel the authors postulated that in very dilute solutions intramolecular association dominates. The association numbers obtained for the monofunctional stars were lower than those determined for the linear monofunctional species due to the steric hindrance of the unfunctionalized arms. Studies on the hydrodynamic properties of the same samples by dynamic light scattering and viscometry confirmed the conclusions of the static methods. Decreased diffusion coefficients and k D values, where k D is the coefficient of the concentration dependence of D (k D=2A 2M–k fv), were observed for the zwitteri-
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142 Advances in Polymer Science Edi
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Branched Polymers I Volume Editor:
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Volume Editor Dr. Jacques Roovers I
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Preface While books have been writt
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Contents Synthesis of Branched Poly
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Synthesis of Branched Polymers by C
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Dendrimers and Dendrimer-Polymer Hy
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Author Index Volumes 101-142 Author
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Author IndexVolumes 101-142 231 Dun
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Author Index Volumes 101-142 233 Ka
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Author Index Volumes 101-142 237 Su
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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