Star polymers first-order

In order to illustrate the potential applications of rheo-NMR five examples have been chosen. The first example deals with wormlike micelles [22] in which NMR velocim-etry is used to profile anomalous deformational flow and deuterium NMR spectroscopy is used to determine micellar ordering in the flow. The second example concerns flow in a soft glassy material comprising a solution of intermittently jammed star polymers [23], a system in which flow fluctuations are apparent. The third... 

Relatively few theoretical studies have been devoted to the conformational characteristics of asymmetric star polymers in solution. Vlahos et al. [63] studied the conformational properties of AnBm miktoarm copolymers in different solvents. Analytical expressions of various conformational averages were obtained from renormalization group calculations at the critical dimensionality d=4 up to the first order of the interaction parameters uA> uB> and uAB between segments of the same or different kind, among them the radii of gyration of the two homopolymer parts < S > (k=An or Bm) and the whole miktoarm chain < /im > > the mean square distance between the centers of mass of the two homopolymer parts A and B < > and the mean square distance between the center of... 

The theory of the collapse transition in star-like PEs was developed by Borisov et al. [155] and Ross and Pincus [156] on the basis of a box-like model, which assumes a fairly uniform concentration of the monomers within the star (see Sect. 5). This analysis suggested that, in contrast to a neutral polymer star, which collapses gradually upon a decrease in the solvent strength [141], the collapse of a PE star has a first-order nature and involves coexistence of the coUapsed and swollen states. 

In a well-controlled radical system, the monomer conversion is first order, molar mass increases linearly with monomer conversion, and the molar mass distribution MJM is below 1.5. In addition, chain end functionalization and subsequent monomer addition allow the preparation of well-controlled polymer architectures, for example, block copolymers and star polymers by a radical mechanism, which had been up to now reserved for ionic chain growth polymerization techniques. 

Relatively few theoretical studies have been devoted to the conformational characteristics of asymmetric star polymers in solution. The conformational properties of A B miktoarm star copolymers in different solvents were studied by renormalization group calculations. Analytical expressions of various conformational averages were obtained at the critical dimensionality d = 4 up to the first order of the interaction parameters... 

This method is referred to as the core-first or arm-out or divergent approach. According to this procedure, multifunctional compounds capable of simultaneously initiating the polymerization of several arms are used. There are several requirements a multifunctional initiator has to fulfill in order to produce star polymers with controllable molecular weights, uniform arm lengths, and low molecular-weight distribution. All initiating sites must be equally... 

First-order perturbation calculations of the mixed second virial coefficients A have been carried out for chains differing in molecular weight but having the same architecture and chemical structure (so that the segment-segment excluded volume parameter is the same for all polymer species), and for some combinations of topologically different chains, e,g. linear chains with rings, stars or combs, stars with stars of different functionality, stars with combs, The simplest result is that for... 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

Similar results are obtained for linear, four and six-branched polyisoprenes at a concentration of 0.145 g/ml. In this case, however, at hi r concentrations more serious deviations occur from theory. The higher molecular weight samples can have zero shear viscosities higher than linear polymers of the same molecular wei t Such behavior was first noted in a study of melt viscosity of regular star-branched polybutadienesViscosities of the order of one hundred times that of a linear equivalent could be observed, but the effect decreased rapidly on dilution with solvents i.e. the viscosities of branched polymers were more sensitive to concentration than those of linear polymers. Star-branched polyisoprenes show viscosity enhance-... 

As mentioned above, some difficulties arose in the synthesis of dendrimer-like star-branched polymers by the two aforementioned methodologies based on the core-first divergent and arm-first convergent approaches described in Sections 5.2.1 and 5.2.2. In fact, most of the polymers are limited to 4G stages and a few 10 g/mol orders in molecular weight except for two cases reported by Gnanou et al. (Tables 5.1 and 5.2). Moreover, the structures of the resulting polymers could not be well characterized and structural imperfections were indicated in several cases. 

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