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

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Asymmetric Star Polymers Synthesis and Properties 123 PPO/PS phase. Scanning electron microscopy experiments showed that the different domains were smaller in the blends containing miktoarm material. The size of the domains decreased and their uniformity increased with increasing miktoarm content in the blends. Miktoarm stars with short PS arms were less effective in compatibilizing PPO and Nylon-6. Ternary blends were found to have higher modulus, strength and elongation than binary PPO/miktoarm blends. They also showed yield behavior. These improvements were attributed to the increased adhesion at the PPO/Nylon-6 interface due to the presence of polystyrene-containing copolymer. 4 Applications Although the synthesis of asymmetric star polymers was rather recently achieved, several efforts have been made to use these polymers for specific applications. An increased number of patents have appeared in recent years. Sometimes ill-defined and not well characterized polymers were used in these studies. Conventional star block copolymers bearing PS-b-PB or PS-b-PI arms have been employed in adhesive compositions. These copolymers have the disadvantage that the polydiene fraction degrades in processing and over time. The unsaturation is also susceptible to possible grafting and crosslinking reactions which lead to increased molecular weights, thus making the polymer ineffective in applications such as pressure sensitive adhesives and removable tapes. These applications require an adhesive that possesses a fourfold balance of tack, cohesive strength and the proper balance between adhesion and resistance to low stress peel. Asymmetric stars have been useful in the formulation of adhesive compositions for pressure sensitive adhesives, removable tape applications etc., showing improved properties compared to conventional star block copolymers. Miktoarm stars of the type (PS-b-PI) n (PI) m prepared using DVB as linking agent were employed in adhesive compositions, showing a good balance of tack, peel strength and hold power near room temperature due to their reduced cohesive strength and viscosity without any reduction in the service temperature [94]. Stars of the type [(PS-co-PI)-b-PI] n (PI-b-PS) m were also used in adhesives showing high resistance to low-stress peel while maintaining moderate adhesion [95]. This was achieved by controlling the T g of the end-block to relatively low levels. Tapes made from adhesives containing asymmetric star block copolymers of the type (PS-b-PI) n does not represent an asymmetric starblock copolymer [96]. By using asymmetric rather than the conventional symmetric star block copolymers the preparation of tapes with moderate adhesion and good resistance to low stress peel which are easily removed from the substrate without leaving adhesive residue was achieved. High adhesive performance at low molecular weight and viscosity was achieved using [PS-b-P(ethylene/butylene)] 2PI 2 and [PS-b-P(ethylene/buty-
124 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos lene)] 2 PI 4 miktoarm stars [97]. This behavior was probably obtained due to the superior phase separation between the PS end blocks and P(ethylene/butylene) rubber blocks compared to PS and PI blocks. (PS-b-PB) n PB m miktoarm stars having n+m=4 proved to be able to produce good adhesives after crosslinking by radiation [98]. Improved high temperature resistance was obtained without chemical crosslinking agents, such as dimethacrylates or acrylates that have been proven to be dangerous for the human health and the environment. Asymmetric stars have also been added to a variety of oils including crude oils, lubricating and fuel oils to produce oil compositions, generally having improved viscosity index characteristics. The newer engines place increased demands on the engine lubricants. Asymmetric stars are effective viscosity index improvers. Miktoarm stars of the type (PI) n(PtBuMA) n provided lower viscosity at low temperatures and better engine oil pumpability compared with common dispersant viscosity index improvers [99]. Asymmetric stars possessing PI arms of different molecular weights have been shown to produce oil films with increased thickness [100]. This is a result of the increased elasticity of the polymer due to the combined longer and shorter arms. Stars having the higher ratio of molecular weights between long and short arms (~2:1 to 3:1) and >12% long arms had lower viscosity values, thus providing improved fuel efficiency. Improved low temperature pumpability was also achieved using hydrogenated (PS-b-PI) nPI m stars [101]. The thickening efficiency in this case varies mainly with the MW of the hydrogenated PI block of the copolymer arm and secondly with the PS molecular weight. Asymmetric stars have also been used or have the potential to be used in many other applications as compatibilizers [50], impact modifiers and in sealant [100] and molding compositions etc. 5 Concluding Remarks Living polymerization methods have been proven very efficient in preparing asymmetric star polymers. These materials include stars having arms of different molecular weight, chemical nature or topology. Most of these products are well defined and molecularly and compositionally homogeneous, thus providing the ability to correlate the molecular structure with the properties. Several theories exist that try to predict the properties of the asymmetric stars. However in most cases there are no experimental results to evaluate these theories. Partly deuterated miktoarm stars and new architectures have to be designed for this purpose. The growing interest of the industry for products involved in special applications will expand the use of asymmetric stars as adhesives, melt viscosity index modifiers, compatibilizers, impact modifiers etc.
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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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Poly(macromonomers), Homo- and Copo
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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 101-142 235 Og
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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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