Amorphous thermoplastics polystyrene

The regular syndiotactic and isotactic structures are capable of crystallisation whereas the atactic polymer carmot normally do so. In the case of polypropylene the isotactic material is a crystalline fibre-forming material. It is also an important thermoplastic which can withstand boiling water for prolonged periods. Atactic polypropylene is a dead amorphous material. Polystyrene as commonly encountered is atactic and glass-like but the syndiotactic material... 

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. 

Amorphous thermoplastics These are made from polymers which have a sufficiently irregular molecular structure to prevent them from crystallising in any way. Examples of such materials are polystyrene, poly methyl methacrylate and polyvinyl chloride. 

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. 

Figure 3.14 Left Shift factors aT from time-temperature superposition of two amorphous thermoplastics [91. Right Arrhenius plot a(T)=f(1/T). Lines WLF Eq. 3.15 with c pc = 4.2 and c2 pc=88 K for polycarbonate (PC) and with c,. PS=5.3 and c2 P = 113.4 K for polystyrene (PS)...

Transparent Polymers. Amorphous thermoplastics, like poly (methyl methacrylate), polystyrene, SAN, PVC, or the cellulose esters are transparent and used for glazing, photographic film, blown bottles, or clear packaging containers. Only a few crystalline thermoplastics, like poly (4-methyl-l-pentane), where the crystalline and the amorphous phases have almost identical refractive indexes, or polycarbonate, which has smaller crystals than the wavelength of light, are also transparent. R. Kosfeld and co-workers analyzed the mobility of methyl groups in polycarbonate, poly (methyl methacrylate) and poly( -methyl styrene) by NMR spectroscopy. 

Most thermoplastic foams can be solvent cemented. However, some solvent cements will collapse thermoplastic foams. The best way to determine if such a problem exists is to try it. In cases where the foam collapses due to softening of the foam cell walls it is desirable to use water-based adhesives based on SBR or polyvinyl acetate, or 100%-solids adhesives. In general, the relatively amorphous thermoplastics, such as the cellulosics, polycarbonate, and polystyrene are easier to solvent cement than the crystalline materials, but there are exceptions. 

Both Tg and Tm are important parameters that serve to characterize a given polymer. While Tg sets an upper temperature limit for the use of amorphous thermoplastics like poly(methyl methacrylate) or polystyrene and a lower temperature limit for rubbery behavior of an elastomer-like SBR rubber or 1,4-cij-polybutadiene, Tm or the onset of the melting... 

In neat polymers the fatigue failure process is quite similar, but in certain amorphous thermoplastics, such as polystyrene, phase crazes are formed during the initiation [13] with the subsequent fatigue crack propagation phase to final failure. (Crazes resemble regions with many cracks bridged by oriented fibrils). 

SAN, styrene I acrylonitrile plastics. Copolymer of styrene and acrylonitrile. An amorphous thermoplastic, transparent in the original form, which is tougher, less brittle, somewhat more heat stable than the standard polystyrene. Applications transparent covers of car and signal lamps, machinery and instrument parts, and various consumer goods. Trade names Kostil (I), Luran (FRG), Lustran (USA), Tyril (USA). 

Amorphous polymers of commercial importance include polymers which are glassy or rubbery at room temperature. Many amorphous thermoplastics, such as atactic polystyrene and poly (methyl methacrylate), form brittle glasses when cooled from the melt. The glass transition temperature, Tg or glass-rubber transition, is the temperature above which the polymer is rubbery and can be elongated and below which the polymer behaves as a glass. Thermal analysis of amorphous polymers shows only a glass transition temperature whereas crystalline poly-... 

Linear amorphous polymers exist in a number of characteristic physical states depending on the timescale of the measurement and temperature. These are illustrated in Fig. 5.2 in terms of an arbitrary modulus function and are classified as glassy, leathery, rubbery, rubbery flow, and viscous (Tobolsky 1960 Collins et al. 1973). All linear amorphous polymers exhibit these five physical states when they are observed over a wide range of time or temperature. Materials of this type are typical of amorphous thermoplastics, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), or polycarbonate (P(i ) polymers. Polymers that are either crosslinked or crystalline do not exhibit the rubbery flow and viscous liquid responses as illustrated. Crystalline polymers, however, will exhibit a viscous response at temperatures above the melting transition. 

Serit < 0.8% for amorphous thermoplastics (with the exception of polystyrene),... 

Acrylonitrile butadiene styrene (ABS) is an amorphous thermoplastic terpolymer. It has been molded in the past commercially and has been the subject of several major research projects at university level. Studies have shown that ABS (along with polystyrene and acrylic) is a material that is sensitive to degradation and one that can sinter to produce a porous and friable structure with low tensile strength and high surface porosity. Increased rubber content in ABS leads to increased difficulty in densification. 

Amorphous thermoplastics were some of the earliest plastics, excepting polycarbonate, to find general acceptance indeed, cellulose plastics were the first thermoplastics available commercially, and poly(methyl methacrylate), (PMMA), was a commercial product in the 1930s. Their continued application some 50 years later is a comment on their usefulness, and on their properties compared with those of polystyrene, since PMMA is almost twice the price of PS, and the cellulose plastics are significantly more expensive than PMMA. Polycarbonate, a development of the late 1950s, is some three times more expensive than PS, and so finds use in critical applications where performance rather than cost is the criterion of acceptability. There are other amorphous thermoplastics with yet more advantageous properties which have not reached the status of commodity materials a selection of these with elevated service temperatures is reviewed in a later chapter. 

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