Thermoplastic matrices polystyrene

Shear yield behaviour of polymer melts containing plate-like filler particles is also prevalent and is clearly shown in Fig. 8 for talc-filled polystyrene. In this system an estimate was made of shear yield values, which were found to increase with increasing particle loading and decreasing particle size. These results are compared with reported yield values for other particulate-filled polymers in Table 2. It is evident that shear yield values also depend on the particle type and thermoplastic matrix used. 

The second concept for the generation of monolithic polymers is based on diblock copolymers which were prepared by Hillmyer and coworkers [27]. These copolymers contain oriented nanoscopic cylinders of the degradable polymer polylactide (PLA) which were embedded in polystyrene. The latter served as an inert thermoplastic matrix, while PLA could be selectively removed under well-defined conditions using sodium hydroxide in aqueous methanol. The resulting mesoporous monolithic polystyrene contains nanochannels with defined pore size. The major drawback of this material free of any cross-linker is associated with reduced mechanical and chemical stability. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Thermoplastic Elastomers. These represent a whole class of synthetic elastomers, developed siace the 1960s, that ate permanently and reversibly thermoplastic, but behave as cross-linked networks at ambient temperature. One of the first was the triblock copolymer of the polystyrene—polybutadiene—polystyrene type (SheU s Kraton) prepared by anionic polymerization with organoHthium initiator. The stmcture and morphology is shown schematically in Figure 3. The incompatibiHty of the polystyrene and polybutadiene blocks leads to a dispersion of the spherical polystyrene domains (ca 20—30 nm) in the mbbery matrix of polybutadiene. Since each polybutadiene chain is anchored at both ends to a polystyrene domain, a network results. However, at elevated temperatures where the polystyrene softens, the elastomer can be molded like any thermoplastic, yet behaves much like a vulcanized mbber on cooling (see Elastomers, synthetic-thermoplastic elastomers). 

ABA triblock copolymers of the styrene-diene type are well known, and owe their unique properties to their heterophase morphology. This arises from the incompatibility between the polystyrene A blocks and the polydiene B blocks, leading to the formation of a dispersion of very small polystyrene domains within the polydiene matrix. This type of elastic network, held together by the polystyrene "junctions", results in thermoplastic elastomer properties. 

The preferred morphology of these rubber modified amorphous thermoplastics is the distribution of distinct rubber particles unfilled or filled in an isotropic matrix of the basic polymer. This was shown to be the case for rubber modified polystyrene and for ABS-type polymers. 

Type 3 Hard matrix 4- soft dispersed phase High impact thermoplastics High impact polystyrene and ABS High impact PVC... 

The situation is quite different with block copolymers. As an example we again take a copolymer of styrene and butadiene, but now as a three-block copolymer, SBS. The incompatibility of polystyrene and polybutadiene now results in a phase separation, which is enabled by the circumstance that the blocks can live their own life . The polystyrene chain ends clog together into PS domains, which lie embedded in a polybutadiene matrix. These glassy domains act as physical cross-links, so that the polymer has the nature of a thermoplastic rubber. The glass-rubber transitions of PS and BR both remain present in between these two temperatures the polymer is in a, somewhat stiffened, rubbery condition (see Figure 3.8). This behaviour is dealt... 

The matrix is considered to be the binder for the microspheres. Typical matrix materials include (a) thermosetting resins such as epoxy resins, unsaturated polyesters, vinyl esters, phenolics, polyurethanes, and silicones (b) thermoplastic resins such as polyethylene, polystyrene, polyvinyl chloride (c) asphalt and (d) gypsiun and cement. 

When the PEELS measurement was conducted, an abrupt drop in density was observed at the interface between the matrix and the craze bands (Figure 4). In addition, a drop of approximately 50% in density was found at the base of the already unloaded craze band. This observation implies that an extension ratio of at least 2 exists for the craze fibrils. This phenomenon is not uncommon for thermoplastic crazes (5, 10). To ensure that the PEELS method gives reasonable results, the density of the craze band inside a polystyrene tensile specimen was measured (Figure 5) using the same sample-preparation procedures described in the section Experimental Details. The measured density of the craze band in the unloaded polystyrene was found to be about 0.62 g/cm3, which is in good agreement with the number reported in the literature (5,10, 24). 

The styrene-diene triblock copolymer consists of individual chains of three blocks, an elastomeric diene block in the center and a thermoplastic styrene block on each end. This polymer is called a thermoplastic elastomer. It exhibits some of the physical properties of elastomers at use temperature and is as pro-cessable as conventional plastics (5). The styrene/diene triblock copolymer has the unique morphology of glassy polystyrene domains in the rubbery diene matrix. Therefore, such an elastomer does not require conventional vulcanization since the glassy polystyrene domains act as physical crosslinks. 

Reinforced plastics are composites in which a resin is combined with a reinforcing agent to improve one or more properties of the resin matrix. The resin may be either thermosetting or thermoplastic. Typical thermosetting resins used in RPs include unsaturated polyester, epoxy, phenolic, melamine, silicone, alkyd, and diallyl phthalate. In the field of reinforced thermoplastics (RTFs), virtually every type of thermoplastic material can be, and has been, reinforced and commercially molded. The more popular grades include nylon, polystyrene, polycarbonate, polyporpylene, polyethylene, acetal, PVC, ABS, styrene-acrylonitrile, polysulfone, polyphenylene sulfide, and thermoplastic polyesters. 

Among thermoplastic resins used as the matrix in reinforced plastics, the largest tonnage group is the polyolefins, followed by nylon, polystyrene, thermoplastic polyesters, acetal, polycarbonate, and polysulfone. The choice of any thermoplastic is dictated by the type of application, the service environment, and the cost. 

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