Glass transition tacticity

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

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. 

Polymerization of norbornene using chiral metallocenes results in insoluble polymers exhibiting a glass transition temperature of about 210 °C. Although they have been shown by oligomerization to be tactic, no melting up to 500 °C... 

Differences in tacticity were also reflected by thermal data. While the iso-tropization temperature of (-)-poly-(IV-ll) and ( )-poly-(IV-ll),synthesized using initiator 1, stayed approximately unchanged, the isotropization temperatures for the chiral liquid crystalline polymers shifted to higher values when initiators 2 or 3 were used. The difference was up to 7 °C. If the decreased glass transition temperatures (Tg) for the chiral analogues were taken into account, the temperature range of the liquid crystalline phase was broadened by up to 12 °C. This means that a certain diad must be responsible for this behavior. The authors assumed that the diad cmHT was most suitable one for the formation of stable liquid crystalline phases in poly(norbornene) main chains. 

A great number of initiators and monomers are now available, allowing almost perfect control over most of the important parameters of LCPs main chain stiffness tacticity glass transition temperatures processabiUty from solution or melt mesogen density along the main chain combination with... 

Although the anionic polymerization of MMA in THF leads to highly syndiotactic PMMA (Table 1), that one of tBuMA results in an equimolar mixture of syndiotactic and heterotactic triads (Table 1). For both monomers an increase in temperature results in higher heterotacticity at the expense of the syndiotacticity . The chain tacticity has a direct influence on the macroscopic properties, as illustrated in Table 2 for both the glass transition (Tg) and the melting (T a) temperatures of PMMA . 

TABLE 2. Influence of tacticity on the glass transition (Tg) and melting (Ea) temperatures of PMMA. Reproduced from Reference 56 by permission of John Wiley Sons, Inc. 

The glass transition temperature of amorphous polymers is a function of the chemical structure of the polymer chain. It varies widely with the types of skeletal atoms present, with the t T)es of side groups, and with the tacticity of side groups along the polymer backbone. Table 14.11 demonstrates the effects of structural variations on the crystalline melting temperature and glass transition temperature for several polymers. 

Table 1 Dependence on Tacticity of the Glass Transition Temperature Tg of Poly(methylmethacrylate)...

Soldera, A. Comparison between the glass transition temperatures of the two pmma tacticities A molecular dynamics simulation point of view, Macromol. Symp. 133, 21-23 (1998)... 

It is apparent that the conjunct roles of ester group and 8 "Backbone substituents are important in determination of the extent to which backbone segments experience mobility as the glass transition is approached. In this respect the influence of tacticity in 1,1-disubstituted alkyl methacrylate polymers requires investigation. 

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.

Thus, in this case, flexibility is not related to tacticity defects, as happens for polystyrene. The glass transition temperature is lower than — 100°C and the polymer in bulk is known to have good elastic properties. 

However, as shown in Table 12, the glass transition temperature of poly 6-[4 -(4"-methoxyphenoxycarbonyl)phenoxy]hexyl methacrylate] is essentially independent of tacticity. With the exception of the polymer prepared by anionic polymerization [45], the nematic-isotropic transition also appears to be independent of tacticity. However, as demonstrated by the data in Fig. 8, the extrapolated transition temperatures (G 44 N 105 I) of an infinite molecular weight polymer prepared by anionic polymerization are nearly identical to those of the rest of the polymers presented in Table 12. 

Grohens, Y., Hamon, L., Carriere, R, HoU, Y., and Schultz, J., Tacticity and surface chemistry effects on the glass transition temperature of thin supported PMMA films. Mater. Res. Soc. Symp., 629, FF171-FF177 (2000). 

Methods for determining the presence, kind, and amount of configurational base units can be classified as relative or absolute. Absolute methods do not require calibration with polymers of known tacticity. Relative methods, on the other hand, require comparison with standard substances. X-ray crystallography, nuclear magnetic resonance, infrared spectroscopy, and optical activity measurements are all absolute methods. Relative methods include crystallinity, solubility, glass transition temperature, and melting temperature measurements as well as chemical reactions (Table 3-2). 

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