Atactic polymers, glass transition temperatures

The value of the glass-transition temperature, T, is dependent on the stereoregularity of the polymer, its molecular weight, and the measurement techniques used. Transition temperatures from —13 to 0°C ate reported for isotactic polypropylene, and —18 to 5°C for atactic (39,40). 

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)... 

The approximately random placement of side groups in atactic polymers prevents them from developing regular structures. For this reason, atactic polymers are non-crystalline and behave as rubbers or glasses, depending on whether they are above or below their glass transition temperature. 

Atactic polymethacrylate esters, glass transition temperatures of, 16 273t Atactic polypropene, 16 104 20 524 Atactic polystyrene, 10 180, 182 Atactic propylene polymers, 17 704, 705 Atactic PSSA, 20 468 Atenolol, 5 102, 160... 

Polymerization. Poly (methyl methacrylate) was obtained commercially. The polymers of other methacrylates and their copolymers were prepared in toluene with 2,2 -azobisisobutyronitrile (AIBN) at 60 °C. All the polymers prepared free radically were syndiotactic or atactic. Isotactic poly(a,a-dimethylbenzyl methacrylate) was obtained using C6H5MgBr as the initiator in toluene at 0°C. Poly(methacrylic acid) was prepared in water using potassium persulfate at as the initiator 60 °C. The molecular weights, glass transition temperatures and tacticities of the polymethacrylates are summarized in Table I. 

Commercial polystyrene, one of the most representative general-purpose resins, is an amorphous atactic polymer with no melting point (softening temperature ca 100 °C, glass transition temperature ca 70-100 °C). Since the discovery of Ziegler-Natta catalysts, many efforts to produce stereoregular polystyrenes have been made. 

A common example of a plasticized polymer is poly(vinyl chloride). The common atactic form has a Tg of about 80 °C, well above room temperature. Without a plasticizer, vinyl is stiff and brittle. Dibutyl phthalate (see the structure at left) is added to the polymer to lower its glass transition temperature to about 0 °C. This plasticized material is the flexible, somewhat stretchy film we think of as vinyl raincoats, shoes, and even inflatable boats. Dibutyl phthalate is slightly volatile, however, and it gradually evaporates. The soft, plasticized vinyl gradually loses its plasticizer and becomes hard and brittle. 

Simultaneously, thermomechanical investigations were performed [28], obtained under continuous impact of compressing stress (100 g/cm3) on the sample in accordance with the technique [62], Glass transition temperatures, shown in the Tables, are determined from the primary deviation of thermomechanical curve run in the area of positive deformations. Depending on the spatial structure of the cycle, synthesized atactic and syndiotactic polymers are characterized by almost equal glass transition temperatures, Tg. 

PS, in either its atactic or syndiotactic form, is a polymer which shows no segmental mobility of chain segments below its glass transition temperature. Secondary relaxation processes which can be attributed to mobility in the main chain are missing. Therefore, these materials do not exhibit long-range energy... 

Study the chain dynamics in the amorphous polymer atactic polypropylene. " The arrest of the structural relaxation above the glass-transition temperature Tg was investigated using ID and 2D deuteron NMR spectra. 

Table 6.6. Glass transition temperatures of syndiotactic, isotactic and atactic polymers, in degrees Kelvin. Many of the values listed for the syndiotactic and isotactic polymers are extrapolations from measurements on series of polymers with differing tacticities. Some of the listed Tg values are averages of two or more published values. Some of the Tg values listed for atactic polymers differ from those listed in Table 6.2 because they are from different sources.

Since the homogeneous nucleation probability cannot be measured for glass-forming liquids, it has not been possible to either prove or disprove this denial of an in-principle ground state for the hquid state of simple substances. The plausibility of Kauzmann s resolution, however, has suffered from the identity of behavior of crystallizable and atactic (noncrystalliz-able) polymers, and by the experimental contrasts in the composition dependencies of homogeneous nucleation temperatures and glass transition temperatures (7 and T ) observed in binary solutions. 

Examples of these three types of structural arrangements are known in general, stereoregular polymers are synthesized by the use of coordination catalysts, whereas atactic polymers are formed by uncoordinated catalysts such as free radicals or free ions. Stereoregular polymers are often partially crystalline, and usually, even the isotactic and syndiotactic isomers have different properties. For example, isotactic poly(methyl methacrylate) (PMMA) has a glass-transition temperature of 35 °C, while that of the syndiotactic polymer is 105 °C. 

Only atactic or amorphous poly(vinyl acetate) is currently commercially available. It has a glass transition temperature, Tg, of 29°C. Consequently, the polymer becomes sticky at temperatures slightly above ambient. The low-molecular polymers, which are normally brittle, become gumlike when masticated (used in chewing gums). Its adhesive strength is dictated by its water sensitivity. 

FIGURE 12.6 Plot of the glass transition temperature as a function of log r, where x is the number of chain atoms or bonds in the backbone. Data for (-0-) poly(a-methyl styrene), (-A-) poly(methyl methacrylate) (- -) polystyrene, (- -) poly(vinylchloride), (-A-) isotactic poly-propylene, (- -) atactic polypropylene, and (- -) poly(dimethylsiloxane). (From Cowie, J.M.G., Eur. Polym. J., 11, 297, 1975. With permission of Pergamon Press.)... 

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