Block copolymers toughness enhancement

Thermoplastic rubber is a relatively new class of polymer. It has the solubility and thermoplasticity of polystyrene, while at ambient temperatures it has the toughness and resilience of vulcanized natural rubber or polybutadiene. These rubbers are actually block copolymers. The simplest form consists of a rubbery mid-block with two plastic end blocks (A-B-A), as shown in Figure 5.7. Examples of commercial products are Kraton and Solprene . These materials are often compounded with plasticizers to decrease hardness and modulus, eliminate drawing, enhance pressure-sensitive tack, improve low-temperature flexibility, reduce melt and solution viscosity, decrease cohesive strength or increase plasticity if desired, and substantially lower material costs. Low levels of thermoplastic rubbers are sometimes added to other rubber adhesives. These materials are used as components in the following applications PSAs, hot-melt adhesives, heat-activated-assembly adhesives, contact adhesives, reactive contact adhesives, building construction adhesives, sealants, and binders. Two common varieties of thermoplastic rubber adhesives are styrene-butadiene-styrene (S-B-S) and styrene-isoprene-styrene (S-I-S). ... 

The conttolled bottom-up fabrication of nanomaterials with well-defined but complex architectures [167-173] can be achieved by preparing the samples as colloidal stable entities. Cylindrical micelles [174, 175] obtained from the solution self-assembly of block copolymers have found use as additives for the enhancement of the toughness of epoxy resins [176], as templates for the mineralization of hydroxyapatite [177], the formation of metal nanoparticles [178-181], as... 

To extend the use of polyethylene, it is desirable to enhance polyethylene s polarity, toughness, adhesion and compatibility with other materials. One approach is by incorporating polyethylene in block copolymer structures (Hong et al, 2002). Polyethylene block copolymers can maintain some of the superior properties of polyethylene while introducing the desired new properties from the other copolymer segments. In this way, the utility of polyethylene can be expanded to higher value areas, especially in polymer blends or composites, the preparation of micelles and the fabrication of nanoporous membranes (Wang and Hillmyer, 2001 Chen et al, 2009 Uehara et al, 2006 Uehara et al, 2009). 

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

Enhancement of mechanical properties is of interest only if it is not accompanied by a loss of other important properties of the blend. Of particular concern for such polymer blends is stiffness, because most means of increasing impact strength also reduce stiffness (14-19). But this is not the case for the iPS-fc-iPP-iPS-iPP blends studied here as seen in Table II. It is clear that the enhancement in toughness just described is not accompanied by a loss of stiffness, but it is essentially unaffected by the compatibilizer. And the stiffness of iPS-fc-iPP-iPS-iPP is higher than that of iPP and HIPS. The impact-modulus behavior seems to be due to the tough (or rigid) characteristics, morphologies of phases, and semicrystalline isotactic structure of each block in the iPS-b-iPP diblock copolymer. 

The urethane methacrylate polymer which contains no soft-block and the ethoxylated bisphenol A/MMA copolymer both have relatively low values of K q and their composites have fracture toughnesses approximately double that of the respective polymer. The urethane methacrylate polymer which contains a polyol soft-block, on the other hand, has a fracture toughness which is approximately twice that of the other polymers, but for the composite Kiq is increased only modestly. Thus, the higher the fracture toughness of the polymer then the less is the fracture toughness of the particulate composite enhanced relative to it. 

---