Polystyrenes polyblends

The valuable characteristics of polyblends, two-phase mixtures of polymers in different states of aggregation, were also discussed in the previous chapter. This technique has been widely used to improve the toughness of rigid amorphous polymers such as PVC, polystyrene, and styrene-acrylonitrile copolymers. 

The mechanical properties of two-phase polymeric systems, such as block and graft polymers and polyblends, are discussed in detail in Chapter 7. However, the creep and stress-relaxation behavior of these materials will be examined at this point. Most of the systems of practical interest consist of a combination of a rubbery phase and a rigid phase. In many cases the rigid phase is polystyrene since such materials are tough, yet low in price. 

Polyblends of impact-resistant polystyrene and PPO (NORYL, General Electric). ... 

Typical physical properties for an injection-molded transparent acrylic polyblend resin are given in Table II. The injection molding conditions used are given in Table III. Tensile, flexural, and impact properties are within the range reported for typical ABS and high impact polystyrene resins. Optical properties approach those of the acrylics [i.e., poly (methyl methacrylate)]. The strength properties are on the low side of those reported in the first paper for the transparent diene... 

Preparation of Electron-micrographs. The polystyrene-rubber polyblend sample was etched by solvent according to the technique developed by Traylor. A double replica technique was used to prepare the sample. The first replica was methylcellulose, the second platinum and carbon, 800 A. thick. 

It is well known that systems like polystyrene or polystyrene-acrylonitrile—generally considered brittle materials—have a remarkable increase in toughness and resistance to impact when polyblended with finely dispersed, crosslinked, but partly compatible, rubber particles. These particles are generally 0.1-10 fi in size and frequently consist of butadiene which has been grafted with monomers of similar composition to the matrix or continuous phase. 

Craze formation is a dominant mechanism in the toughening of glassy polymers by elastomers in polyblends. Examples are high-impact polystyrene (HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene) polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at strains of 10 , whereas rubber-modified grades of these polymers (e.g., HIPS and ABS) form many crazes before breaking at strains around 0.5. Rubbery particles in... 

Because no termination step exists, styrene/butadiene/styrene (SBS) triblock polymers can easily be made by charging styrene, butadiene, and again styrene, in succession, to the catalyst. Because the polystyrene blocks behave like styrene homopolymer, these triblock polymers are not suitable for use in automobile tires, but they lend themselves well in polyblends and as the backbone in the graft polymerization of Impact polystyrene and ABS... 

Compatible Polyblends. When the polymeric materials are compatible in all ratios, and/or all are soluble in each other, they are generally termed polyalloys. Very few pairs of polymers are completely compatible. The best known example is the polyblend of polyCphenylene oxide) (poly-2,6-dimethyl-l,4-phenylene oxide) with high-impact polystyrene (41). which is sold under the trade name of Noryl. It is believed that the two polymers have essentially identical solubility parameters. Other examples include blends of amorphous polycaprolactone with poly(vinyl chloride) (PVC) and butadiene/acrylonitrile rubber with PVC the compatibility is a result of the "acid-base" interaction between the polar substituents (1 ). These compatible blends exhibit physical properties that are intermediate to those of the components. 

Polyblends in which both phases are rigid are frequently called poly alloys. Poly (phenyl oxide) is blended with impact polystyrene to improve melt flow. Complete compatibility between the two phases is rare and was observed between poly (methyl methacrylate) and poly(vinylidene fluoride) by D. R. Paul and J. O. Altamirano. Thermoplastics are added to polyesters to reduce mold shrinkage. 

On the basis of a systematic study of the emulsifying effect of block copolymers in PS-PI and in polystyrene-poly (methyl methacrylate) (PS-PMM) polyblends (3), it was possible to represent schematically the appearance of the films for different blend compositions as functions of molecular weight and composition of the block copolymer (Cop), as well as of molecular weight of the homopolymers (see Figure 2). Thus in a polyblend containing PS and PI of practically the same molecular weight — M2), the best... 

In isomorphous systems, the eomponent monomers oceupy similar volumes and are capable of replacing each other in the erystal system The resulting eopolymer, irrespective of its geometry, is necessarily homogeneous, and polyblends of the individual homopolymers or copolymers have similar transition properties. Copolymerization merely shifts the Tg to the position intermediate between those of the two homopolymers it does not alter the temperature range or the modulus within the transition r on (Figure 4.4). This shift is illustrated in Figure 4.4, which shows the modulus temperature eurves for polybuladiene (100/0) and polystyrene (0/100) and for various compositions of butadiene-styrene eopolymer. 

Block and graft copolymers (incompatible copolymers) — For block or graft copolymers in which the component monomers are incompatible, phase separation will occur. Depending on a number of factors — for example, the method of preparation — one phase will be dispersed in a continuous matrix of the other. In this case, two separate glass transition values will be observed, each corresponding to the Tg of the homopolymer. Figure 4.6 shows this behavior for polyblends of polystyrene (100) and 30/70 butadiene-styrene copolymer (0). 

Figure 4.6 E/10) vs. temperature for polyblends of polystyrene and a 30/70 butadiene-styrene copolymer. Numbers on the curves are the weight percent of polystyrene in the blend. (From Toboisky A. /., Properties and Structure of Polymers, John Wiley Sons, New York, 1960. With permission of Dorothy Toboisky.)... 

The first commercial blend of two dissimilar polymers was Noryl, a miscible polyblend of poly(phenylene oxide) and polystyrene, introduced by General Electric in the 1960s. Since that time a large number of different blends have been introduced. A number of technologies have been devised to prepare polyblends these are summarized in Table 4.34. For economic reasons, however, mechanical blending predominates. 

Noryl, for example, is composed of polystyrene, an inexpensive polymer, and polyfphenylene oxide) or PPO, a relatively expensive polyether. For the most, the properties of Noryl are additive. For example, Noryl has poorer thermal stability than the polyether alone, but is easier to process. Its single glass transition temperature increases with increasing polyether content. In terms of tensile strength, however, the polyblend is synergistic. 

For example, a 50 50 blend of polystyrene (a hard, glassy polymer at ordinary temperature) and polybutadiene (an elastomer) will be hard if polystyrene is the continuous phase, but soft if polystyrene is the dispersed phase. In some cases, however, an immiscible polyblend may have both components dispersed as continuous phases. Evidently, a proper control of phase morphology is of utmost importance with immiscible blends. The size of the dispersed phase should be optimized considering the final performance of the blend. 

Commercial poly(butadiene), which is mainly the 1,4 isomer, is also used to improve the impact resistance of polystyrene (Chapter 1). Polydienes also increase the rate of physical disintegration of polyblend containing them. The addition of a styrene-butadiene block copolymer e.g. SBS, page 9 et seq.) to polyethylene also accelerates the peroxidation of the latter. However, this system also requires a polymer-soluble transition metal ion catalyst e.g. an iron or manganese carboxylate) to increase the rate of photooxidation in the environment by the reactions shown in Scheme 5.3. The products formed by breakdown of alkoxyl radicals (PO ) (Scheme 3.4) are then rapidly biodegradable in compost (page 107 et seq.). 

A shift in the glass transition temperature results from copolymerization of plastic A with B (A-co-B), in our example of butadiene with polystyrene. This process is also known as internal plasticization. By contrast, in block and graft copolymerization and in polyblends of two plastics C and D, their glass transition temperatures remain essentially unchanged. 

The simplest method of polyblending involves equipment such as rolls or extruders, which can effect the mechanical blending of the two polymeric components in the molten state (Matsuo, 1968). High-impact polystyrene (HiPS) is an important example of a polyblend made by this technique. Such materials commonly contain 5-20 % of rubber, usually polybutadiene, dispersed in a polystyrene matrix. As shown in Figure 3.1, electron microscopy studies on specimens stained with osmium tetroxide reveal well-defined, irregular rubber particles (1-10 fim in diameter) dispersed in the polystyrene matrix. The elastomer domains appear dark because the osmium tetroxide stains the elastomer preferentially (see Section 2.4). 

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