Reduced viscosity, hyperbranched

A combination of enhanced reactivity and reduced viscosity for alkyd resins has been achieved by using hyperbranched polyester structures as discussed in Sect. 4.2.3 [ 123]. This study clearly showed the benefits of using highly branched structures in coating applications to obtain improved properties. 

Figure 4, Reduced viscosity (Tjg/c) of hyperbranched copolyesters as a function of concentration in chloroform at 20°C...

As compatibilizers, dendritic poly(ester) resins are useful. This covers both dendrimers and hyperbranched polymers (51). Compatibilizers are needed, because conventional poly(arylene ether)/PA blends exhibit inadequate flow properties at the processing temperatures that are needed to minimize the thermal degradation of the resins. However, increasing the processing temperature beyond these temperatures in order to reduce viscosity of the blends results in brittle parts and surface imperfections in the final part. 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene... 

The lack of mechanical strength for thermoplastic hyperbranched polymers makes them more suitable as additives in thermoplast applications. Hyperbranched polyphenylenes have been shown to act successfully as rheology modifiers when processing linear thermoplastics. A small amount added to polystyrene resulted in reduced melt viscosity [31]. (Sect> 3.1). 

Hyperbranched polymers differ greatly from linear or moderately branched polymers. For example, the solubility is much higher for hyperbranched polymers but not as high as for dendrimers. Hyperbranched polymers normally exhibit an amorphous, nonentangled behavior, i.e., a Newtonian behavior in the melt. The nonentangled state also makes hyperbranched polymers rather brittle. Several thermoset resin materials have been described where the hyperbranched polymer exhibits a low resin viscosity, thereby reducing the need for solvents to attain the application viscosity. At the same time. 

Processing Aids. The use of hyperbranched polyphenylenes as processing aid for polystyrenes has been reported (4). The melt viscosity of polystyrene was reduced while not affecting the final properties to any larger extent. The addition of the polyphenylenes also improved the thermal stability of the system. 

Fig. 5.5 Typical plots of polymer concentration dependence of reduced specific viscosity (rjs C) of hyperbranched polystyrenes (PSt-8.8k) made of one macromonomer in toluene at T=25 °C...

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