Rigid amorphous polymer

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. 

Little is known about the variation of the critical stress ", with structure and temperature. For the polyethylene discussed abovedecreased from 620 psi at 22X to 39general trend with all polymers. Turner (84) found that the value of (r(. for polyethylenes increased by a factor of about 5 in going from a polymer with a density of 0.920 to a highly crystalline one with a density of 0.980. Reid (80,81) has suggested that for rigid amorphous polymers. ", should be proportional t° (Tt - T) For brittle polymers, the value of ", may be related to the onset of crazing. 

In the semi-crystaUine polymer solid, most of polymers trespass the crystalline interfaces, and a restriction occurs to the mobility of non-crystalline part of polymers near the crystalline-amorphous interfaces. The restricted portion of polymers displays glass transition at the temperature higher than those noncrystalline free polymers. Wunderlich named this non-crystalline part of polymer near the crystalline surfaces as the rigid amorphous polymer (Wunderlich 2(X)3). Treating semi-crystalline polymers as formed by three parts (flexible amorphous far... 

The commercial PMMA-based acrylic resins are rigid, amorphous polymers (Tg s ranging from 85 °C to 105 °C) particularly noted for their exceptional clarity and UV resistance. They are therefore widely used for glazing, extruded sheet and thermoforming applications, as well as in several molding applications in which these properties are well utilized. 

The fragile character of rigid amorphous polymers is an important limitation in many potential applications. The problem is circumvented by blending with an elastomer phase. This can produce materials capable of resisting impact. Hence, car rear lights are made of poly(methyl methacrylate) strengthened by addition of a polyacrylate with Tg around —40°C. 

The glass transition is described in Sect 3.43 with respect to its basic thermodynamics, and in Sect 4.7.3 with respect to its microscopic theory, kinetics, and hysteresis behavior. The behavior of copolymers and solutions with respect to the broadening of the glass transition is illustrated in Fig. 436. A final remark is made here about block copolymers and the discovery of rigid amorphous polymers, the amorphous portions of molecules that are restrained by crystals. 

Irregularities such as branch points, comonomer units, and cross-links lead to amorphous polymers. They do not have true melting points but instead have glass transition temperatures at which the rigid and glasslike material becomes a viscous liquid as the temperature is raised. 

Figure 9.3. Stress-strain curves for (a) rigid amorphous plastics material showing brittle fracture and (b) rubbery polymer. The area under the curve gives a measure of the energy required to break the...

As with other rigid amorphous thermoplastic polymers such as PVC and polystyrene (see the next chapter) poly(methyl methacrylate) is somewhat brittle and, as with PVC and polystrene, efforts have been made to improve the toughness by molecular modification. Two main approaches have been used, both of which have achieved a measure of success. They are copolymerisation of methyl methacrylate with a second monomer and the blending of poly(methyl methacrylate) with a rubber. The latter approach may also involve some graft copolymerisation. 

Although it is not difficult to make injection mouldings from polystyrene which appear to be satisfactory on visual examination it is another matter to produce mouldings free from internal stresses. This problem is common to injection mouldings of all polymers but is particularly serious with such rigid amorphous thermoplastics as polystyrene. 

The mechanical and thermal behaviors depend partly on the degree of crystallinity. For example, highly disordered (dominantly amorphous) polymers make good elastomeric materials, while highly crystalline polymers, such as polyamides, have the rigidity needed for fibers. Crystallinity of polymers correlates with their melting points. 

Below Tg tensile strengths are usually at least as strong for crystalline polymers as for amorphous polymers. Between and T, the strength and rigidity will be very dependent on the degree of crystallinity and... 

Polyvinyl chloride (p.v.c.) P.V.C. is one of the two most important plastics in terms of tonnage and shows many properties typical of rigid amorphous thermoplastics. More individually, it softens at about 70°C, burns only with difficulty and is thermally unstable. To reduce this instability, stabilisers are invariably compounded into the polymer. 

Not only do the creep properties of crystalline polymers change rapidly with temperature, but in some cases at a given temperature a crystalline type will creep more with time than will the rigid amorphous or cross-linked (TS) types. However, a crystalline type above its Tg creeps very little, compared to the others. Thus, crystalline types tend to... 

The glass transition is a phenomenon observed in linear amorphous polymers, such as poly(styrene) or poly(methyl methacrylate). It occurs at a fairly well-defined temperature when the bulk material ceases to be brittle and glassy in character and becomes less rigid and more rubbery. 

The free volume is considered to represent the difference between the actual volume of the liquid (or the amorphous polymer) and the minimum volume which it would occupy if its molecules were packed firmly in contact with each other. Incompressible molecules with rigid dimensions are implied in this definition of a free volume. The unrealistic nature of this implication undermines precise determination, or even an exact definition, of the free volume. The concept has proved useful nevertheless. 

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

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