Rigid amorphous

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. 

Rubbery materials are often incorporated into rigid amorphous thermoplastics to improve their toughness but it is a moot point whether or not they should be... 

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. 

When the temperamre is lowered, rubbers become stiff and brittle. All rubbers eventually stiffen to a rigid, amorphous glass at the glass transition temperature (Tg). This temperature also indicates the low-temperature service limit of the rubber. Tg values are dependent on the structure, degree of cross-linking (vulcanization) and isomeric composition of the rubber. 

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... 

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... 

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 this report we will only consider type 3, i.e. mixtures of a rigid amorphous thermoplast with small amounts of an elastomer which is the underlying principle for all rubber toughened plastics to improve impact behavior. 

Cheng, S. Z. D., Pan, R. and Wunderlich, B Thermal analysis of polybutylene terephthalate for heat capacity, rigid-amorphous content, and transition behavior, Makromol. Chem., 189, 2443-2458 (1988). 

Crystalline polymers with high melting temperatures and a very narrow melting range are generally difScult to weld ultrasonically, whereas the rigid amorphous plastics (e. g. polycarbonate or polystyrene) are best. 

Polymers can exist as liquids, as elastomers or as solids but can be transferred into the gaseous state only under very special conditions as are realized in, for example, MALDI mass spectrometry. This is because their molecular weight is so high that thermal degradation sets in before they start to evaporate. Only a few polymers are technically applied in the liquid state (silicon oils, specidty rubbers) but most polymers are applied either as elastomers, or as rigid amorphous or semicrystalline solids. 

Beyond flexible polymers rigid amorphous fraction. 189... 

The discussion of the influence of the interphase need not be limited to just linear polyethylenes. Interphases of several nm have been reported in polyesters and poly-hydroxy alkanoates. One major difference between the interphase of a flexible polymer like polyethylene and semi-flexible polymers like PET, PEN and PBT is the absence of regular chain folding in the latter materials. The interphase in these semi-flexible polymers is often defined as the rigid amorphous phase (or rigid amorphous fraction, RAF) existing between the crystalline and amorphous phases. The presence of the interphase is more easily discerned in these semi-flexible polymers containing phenylene groups, such as polyesters. 

Fig. 17 Representation of the volume-temperature relationship for rigid amorphous and mobile amorphous phases of PET...

Tg (mobile amorphous) Tg (rigid amorphous) Tm (rigid crystalline)- Although this polymeric view... 

Figure 21 Changes in component ratio for metallocene (filled symbols) and Ziegler films (open symbols) (A) Mobile, (B) intermediate and (C) rigid amorphous components. The shaded area indicates the plateau stress region.

Komolprasert V, McNeal TP, Begley TH. Effects of gamma- and electron beam irradiation on semi-rigid amorphous polyethylene terephthalate copolymers. hood Addit Contain 2003 20(5) 505 17. 

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