Twist-glass transition

K. Yazawa, Y. Inoue, T. Yamamoto and N. Asakawa. Twist glass transition in regioregulated poly(3-alkylthiophene)s. Los Alamos Nat. Lab. Prepr. Arch. Condens. Matter, 1-12 (2006). 

The size of the group attached to the main chain carbon atom can influence the glass transition point. For example, in polytetrafluoroethylene, which differs from polyethylene in having fluorine instead of hydrogen atoms attached to the backbone, the size of the fluorine atoms requires the molecule to take up a twisted zigzag configuration with the fluorine atoms packed tightly around the chain. In this case steric factors affect the inherent flexibility of the chain. 

A close connection exists between the presence of a flexible polymer skeleton and the flexibility of the bulk material. Macromolecular flexibility is often defined in terms of the glass-transition temperature, Tg. Below this temperature, the polymer is a glass, and the backbone bonds have insufficient thermal energy to undergo significant torsional motions. As the temperature is raised above the Y g, an onset of torsional motion occurs, such that individual molecules can now twist and yield to stress and strain. In this state the polymer is a quasi-liquid (an elastomer) unless the bulk material is stiffened by microcrystalfite formation. Thus, a polymer with a high Tt is believed to have a backbone that offers more resistance to bond torsion than a polymer with a low 7 g. 

Angell CA (1995) The old problems of glass and the glass transition, and the many new twists. Proc Natl Acad Sci USA 92(15) 6675-6682... 

Two or three relaxation processes occur in semicrystalline polymers. The low-temperature (y or P) process is a subglass process occurring in the amorphous phase. The medium or high temperature process (p or a,) is associated with the glass—rubber transition of the amorphous component. The glass transition is very weak, and in many cases difficult to find, in highly crystalline polymers like linear polyethylene. A certain class of polymers shows a high-temperature relaxation process denoted a, which is a combined crystalline and amorphous process. Reorientation of the chain by a 180° twist of the molecule in the crystals and a certain axial... 

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