Star Architectures

The salient feature of the experimental results is the observation of a pronounced minimum in the Q(Q)/Q3 vs. z plot. It occurs at the same position, where the static structure factor in its Kratky representation exhibits its maximum. Furthermore, the reduced line width scales with the scaling variable z in the same way that the static structure factor does. Thus, the occurrence of the minimum is directly related to peculiarities of the star architecture. 

As a signature of the star architecture the elastic scattering data of the completely labelled stars exhibit a pronounced peak at z = 1.5 in the scaled Kratky representation. In contrast to the PI systems, presented before, the Kratky plot of the measured scattering curve disagrees strongly with the prediction of Eq. (123). The experimental halfwidth of the peak is nearly only 50% of the theoretically predicted one. 

The term star-block copolymer is used for a star architecture in which each arm is a diblock. The influence of chain topology on mechanical and morphological properties was investigated for copolymers composed of PS and PB with a constant styrene content of = 0.74 by Michler s group (Fig. 32) [101,102], While hexagonally packed cylinders of PB in a PS matrix were observed in a symmetric PS-fo-PB-fr-PS triblock copolymer, an L phase... 

The calculation of g for Gaussian uniform star chains was carried out by Zimm and Kilb (ZK) [83]. They used a modified version of the dynamic Rouse theory including preaveraged HI (in the non-draining limit) that considers the particular connectivity of units consistently with the star architecture. This ap-... 

LAN Topologies. SYST1MAX SCS offers cabling architecture options for Fiber-to-the-Desktop installations tire traditional Hierarchical Star architecture and the new Single Point Administration architecture... 

The traditional Hierarchical Star architecture is designed for maximum flexibility. Cross-connect facilities are provided in both the telecommunications closets and the main equipment room. The riser backbone cables can be sized with low counts which allow only distributed active equipment, or for greatest flexibility, with high counts which permit both distributed and centralized active equipment. The horizontal cross-connect facility helps ensure the greatest life span for the system by allowing the... 

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. 

Many adhesives require special properties, for example, SBS polymers are often used for crosslinkable adhesives because the polybutadiene midblock is readily crosslinked by peroxides, UV light or electron-beam radiation. The vinyl content of the butadiene block of SBS polymers is often increased from the standard 8% to near 50% to make it more reactive to crosslinking. Star architectures are also used to reduce the number of crosslinks necessary to form a continuous network throughout the adhesive. For adhesives that require... 

Ethylene-propylene copolymers, acrylates and polyisobutylene are the most common competitors to block copolymers in these markets. The narrow molecular weight distributions and star architecture of the SBCs give them a better balance of properties at molecular weights of >100000 than the competing polymers. 

The star architecture effects are more important for I q 0) than for Dc because the ratio of the corresponding correction terms, k / k — k, is large when k k. Nevertheless, the experimental Dc c/c reveals a stronger speed-up of Dc with concentration in multiarm stars compared to the semidilute linear polymer solutions. The hard core contribution to the osmotic pressure is essentially hidden in the inhomogeneous density profile and the thermodynamic properties of the star solutions are primarily determined by their polymeric character. 

Star terpolymers of the ABC2, ABC4 and AB2C2 were synthesized, where A, B and C are PS, PI and poly(Q -methylstyrene)(PaMeS), respectively. As an example, the reactions used for the synthesis of one of the most complex star architectures, (PS)(PI)2(P(zMeS)2, are given in Scheme 96. 

Acrylate copolymers with complex linear or star architectures were prepared and characterized. Precise control over the sequence distribntion and overall composition of these materials was achieved by atom transfer radical polymerization. A strong correlation between the molecular stractme and composition of the copolymers and their thermomechanical behavior was foimd. This provides a new way for creating advanced materials with tailored properties. 

Despite these recent advances, step-growth polymerization does not offer many possibilities for the synthesis of complex star architectures, and the molecular characteristics cannot be controlled to the same degree as in the case of the living polymerization methods. 

S4 EOgg Cgo . SAXS experiments showed that all samples are miscible in the melt, and that the crystallization process produces microphase segregation, indicating that the star architecture induces a reduction of the Tqdt in comparison to this transition in sequential copolymers. The miscibility was also favoured by the low molecular weight of the PS block in the stars (i.e., 4.7 Kg/mol). 

The so-called palm tree-like copolymers consist in an arrangement of nB blocks with one A block of much larger size in a heteroarmed star architecture [8]. These architectures were obtained by sequential copolymerization of norbornenyl-PS macromonomers with a molecular comonomer, namely cyclooctadiene (COD), Scheme 6. Dumbbell-shaped copolymers can be viewed as double stars that are linked through a linear block. They are obtained upon using palm tree-like copolymers to trigger the polymerization of a new amount of PS macromonomers. Scheme 6. 

Polymer star architectures were realized when Mo-initiated polymers were end-capped with 1,3,5-benzenetricarboxaldehyde [78]./)-Bromomethylbenzaldehyde and /)-vinylbenzaldehyde were used to synthesize ROMP polymers with terminal ATRP initiator moieties [79,80]. 

Coca, S., and O Dwyer, J. B. (2002). Pigment dispersions containing dispersants having core and arm star architecture prepared by controlled radical polymerization. In US 6336966, PPG Industries Ohio, Inc., USA, 18 pp. 

Typical SAXS/WAXD spectra are shown in Figure 1 for the SEL-4,7/20/1.8 copolymers for a crystallization temperature of 313 K. The SAXS spectrum taken immediately before the T-jump indicated only a minor contribution from concentration fluctuations at 383 K. This is a result of the compatibility of PS and PCL in the melt state (8) and of the star architecture with PEO which further increases the intrinsic compatibility of the copolymer (9). After about 500 s the WAXD patterns develop peaks corresponding to the monoclinic unit cell of PEO. Notice that the crystallization of the shorter block is suppressed in the triarm star. At the same time the SAXS peaks develop signifying the formation of the PEO crystalline lamellar. 

The main molecular characteristics of the 4-arm comb star polystyrene are collected in Table 21.2. The Mw Ri(app)ls ratio, which is related to the volume contraction due to branching is 0.19 for the star comb versus 0.29 for the linear comb of same structure. These very low values agree with the highly branched chain architecture of the comb polymers on which is superposed the volume contraction related to the comb star architecture. The radius of gyration (/ g) and hydrodynamic radius (/ h) of the star comb on the one hand are very close to each other as are and of the corresponding one-branch linear comb polymer. They are equal to 45 nm, in THE at 25 °C for the star comb M = 9 000 000 g/mol) and 25 nm for the similar linear comb (M = 2 700 000 g/mol) in the same conditions. 

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