Commercial polymer INDEX

Poly(vinyl acetate) is too soft and shows excessive cold flow for use in moulded plastics. This is no doubt associated with the fact that the glass transition temperature of 28°C is little above the usual ambient temperatures and in fact in many places at various times the glass temperature may be the lower. It has a density of 1.19 g/cm and a refractive index of 1.47. Commercial polymers are atactic and, since they do not crystallise, transparent (if free from emulsifier). They are successfully used in emulsion paints, as adhesives for textiles, paper and wood, as a sizing material and as a permanent starch . A number of grades are supplied by manufacturers which differ in molecular weight and in the nature of comonomers (e.g. vinyl maleate) which are commonly used (see Section 14.4.4)... 

Chemically, the simplest class of commercial polymers in the polyhydrocarbons. The most important commercial hydrocarbon polymer is polyethylene. The consumption of monomers from which polyethylene is derived can be used as an index for comparing the relative living standard of nations. 

The branch dispersion index (BDI) was developed to quantify blocking, while simultaneously correcting for the normal statistical influence of branch content on blocking. The measured number of paired branches is normalized by the number expected from a purely statistical distribution of branches. A BDI of 100% indicates that the branch distribution is random. Most commercial polymers have a BDI of less than 100% [441], indicating more than a statistical amount of blocking. 

The resistance of commercial polymer grades to thermooxidation is commonly indicated by their UL temperature index (Table 15.1). It shows at which continuous temperature the polymer will serve for 100,000 h, that is, 11.4 years. Will serve in this context means until its impact strength, or strain at break, or other chosen mechanical property is reduced by 50%. 

The following index of trade names and suppliers is based on a choice of commercial polymer blends and alloys. No claim is made for completeness. Detailed lists can be found in the source cited. 

Unlike polyethylene, which crystallizes in the planar zigzag form, isotactic polypropylene crgtaUizes in a helical form because of the presence of the methyl groups on the chain. Commercial polymers are about 90 to 95 percent isotactic. The amount of isotac-ticity present in the chain will influence the properties. As the amount of isotactic material (often quantified by an isotactic index) increases, the amount of crystallinity will also increase, resulting in increased modulus, softening poinL and hardness. 

CAS states (2) that specific polymers are named on the basis of the monomers from which they are formed and/or on the basis of their structure, as represented by an SRU. Since original documents do not always provide sufficient structural information to allow generation of the SRU name, the method most frequently used for describing polymeric substances is by citation of the component monomers. A few commercial polymers, each of which accounts for a large number of index entries, are indexed only at the SRU-based systematic polymer name. Systematic (SRU) nomenclature for polymers has been adopted from the system developed by the Committee on Nomenclature of the Division of Polymer Chemistry of the American Chemical Society (ACS) (14). Note the lUPAC recommendations (PureAppl. Chem. 48,373-385 (1976)) are in full agreement with CAS practice. Names derived by this system, in addition to monomer-based entries, are cited for polymers whose structural repeating imits are well-documented or can confidently be assumed. 

Table 5.7 Theoretical and experimental refractive index for standard commercial polymers...

The chemical and therefore structural nature of the polymer determines Tg. For most commercial polymers, values lie in the range — 100 °C to 250 °C as illustrated in Table 1.1. The value can be >250°C (e.g. in thermosets) but decomposition often occurs before it is reached. Tg can be determined by any technique which shows a change in a particular property of the polymer with temperature, e.g. density, modulus, heat capacity, refractive index, dielectric loss, X- and j8-ray adsorption, gas permeability, proton and NMR. The value of Tg can be obtained from plots of the magnitude of this property against temperature and is indicated by a break in linearity. Figure 1.4 shows modulus (i.e. strength) v. temperature and Figure 1.5 specific volume v. temperature for a typical polymer. 

It may be shown that My, > M (x > x ). The averages are equal only for a monodisperse polymer, where aU of the chains are exactly the same length. The ratio My,IM = Xy/x is known as the polydispersity index, PI. PI is a measure of the breadth of the molecular weight distribution. A monodisperse sample would have a PI of 1.0, but typical values range from about 1.02 for carefully fractionated polymers synthesized by anionic addition reactions to over 50 for some commercial polymers. 

Finally, sample (C) shows the relaxation modulus for a polydisperse material having a polydispersity index (M /M ) of about four, withM Mq. The broadening of the molecular weight distribution results in the loss of a true plateau, because there is now abroad range of times over which relaxation occurs only via the slow process of escape from entanglements. Larson [28 ] noted that the relaxation moduli of commercial polymers having broad molecular weight distributions can sometimes be approximated by a power-law expression ... 

The polydispersity index is a measure of the breadth of mole fraction (or molecular weight) distribution. For a monodisperse polymer, Q is unity commercial polymers may have a value of Q lying anywhere between 2 and 20. 

---