Transition temperature, second order

Stabilization of the Cellular State. The increase in surface area corresponding to the formation of many ceUs in the plastic phase is accompanied by an increase in the free energy of the system hence the foamed state is inherently unstable. Methods of stabilizing this foamed state can be classified as chemical, eg, the polymerization of a fluid resin into a three-dimensional thermoset polymer, or physical, eg, the cooling of an expanded thermoplastic polymer to a temperature below its second-order transition temperature or its crystalline melting point to prevent polymer flow. 

Poly(vinylchloride). Cellular poly(vinyl chloride) is prepared by many methods (108), some of which utili2e decompression processes. In all reported processes the stabili2ation process used for thermoplastics is to cool the cellular state to a temperature below its second-order transition temperature before the resia can flow and cause coUapse of the foam. 

Poly(vinyl chloride) has a good resistance to hydrocarbons but some plasticisers, particularly the less polar ones such as dibutyl sebacate, are extracted by materials such as iso-octane. The polymer is also resistant to most aqueous solutions, including those of alkalis and dilute mineral acids. Below the second order transition temperature, poly(vinyl chloride) compounds are reasonably good electrical insulators over a wide range of frequencies but above the second order transition temperature their value as an insulator is limited to low-frequency applications. The more plasticiser present, the lower the volume resistivity. 

A useful group of rubbers are the stereo specific poly(butadiene) rubbers formed by the polymerization of 1,3-butadiene. These rubbers have a ris-isomer content of more than 30%. They contain at least about 85% of poly(butadiene) formed by 1,4 addition. Further, the rubber should have a second order transition temperature of preferably not higher than -20°C (8). 

The Glass Temperature. Determining the glass temperature, Tgt is the most important of the methods that measure chain mobility. The glass temperature, also called the second-order transition temperature, is the temperature at which a polymeric substance turns into a glass on cooling. To put it quite simply, it is the temperature above which the substance is soft and below which it is hard. 

The thermal properties of grafted cellulose are interesting grafting was found to lower the second order transition temperature of the cellulose and Arthur et al. (146) have found some thermoplasticity to... 

Edgar, O. B., and E. Ellery Structure-property relationships in polyethylene terephthalate co-polyesters. Part. I. Melting points. Part II. Second-order transition temperatures, J. Chem. Soc. (London) 1952, 2633—2643 J. Polymer Sci. 8, 1—22 (1952). 

I. Second-order transition temperatures. J. Appl. Phys. 20, 564—571 (1949). 

The butadiene concentration was generally 3-10% by weight of PVC to limit the extent of grafting and to minimize changes in the basic polymer properties—most particularly, the second-order transition temperature. The preferred extent of grafting was less than 5%. 

Elastomers, prepared by free-radical initiated copolymerization of ethyl acrylate with cellulose to several hundred percent extent of grafting of poly (ethyl acrylate) onto cellulose, exhibited rubber-like behavior and second-order transition temperatures. Cellulose-poly (ethyl acrylate) elastomers had transition temperatures below —35°C, about — 20°C, and below 5°C when measured in ethyl acetate, dry air, and water, respectively (43, 44). 

Macromolecular properties of grafted cellulosic fibers usually measured are differential solubility in either polymeric or cellulosic solvents, mechanical or physical properties, and abrasion resistances. The molecular weights of the grafted or block polymers and of cellulose, both before and after formation of macrocellulosic radicals, have been determined. The number of grafted or block polymer molecules per cellulose molecule calculated has usually been much less than one. Grafted cellulosic fibers exhibit second order transition temperatures, dependent on the composition of the grafted polymer (3, 4). 

Fox, T. G., and P. J. Flory Second-order transition temperatures and related properties of polystyrene. I. Influence of molecular weight. J. Appl. Phys. 21, 581 (1950). 

FIG. 25.5 Correlation between the Izod Impact Strength and Tp (the temperature of the p-relaxation, i.e. the second order transition temperature, directly below the glass transition temperature). For reference numbers see Tables 13.12 and 25.4. The deviating polymers have a strongly developed crystallinity. 

The significance of the amorphous regions in relation to the second order transition is shown clearly in the curves in Fig. 7.11. Orion, the most crystalline of the three fibres shows the least reduction of stiffness with increase of temperature. Dynel, a copolymer, with the least crystallinity exhibits the greatest reduction, and lying between these two is an experimental acrylic fibre specially prepared with an intermediate degree of orientation. Second order transition temperatures are polyethylene tere-phthalate (partly crystalline) 81 "C, nylon 66 (partly crystalline) 47°C, and polyacrylonitrile 81 "C. 

The acrylics have a second-order transition temperature within the range... 

In reviewing equation (4-6), it is evident that temperatures other that Tg can be used as the reference temperature with corresponding linear changes in the values of C, and C2 (see Problem 4-2). One choice of particular interest is that suggested by the discussion above—the second-order transition temperature, T2 where log aT becomes unbounded. In this case... 

So, at the second-order transition temperature, the following discontinuities are observed ... 

Polymers can exhibit a number of different conformational changes with each change accompanied by differences in polymer properties. Two major transitions occur at Tg, which is associated with local, segmental chain mobility in the amorphous regions of a polymer, and the melting point (Tjj), which is associated with whole chain mobility. The Tn is called a first-order transition temperature, and Tg is often referred to as a second-order transition temperature. The values for Tjj are usually 33 to 100% greater than for Tg, and Tg values are typically low for elastomers and flexible polymers and nigher for hard amorphous plastics. 

---