Glass transition temperature property changes

Most plastics undergo dramatic property changes above their glass transition temperature. Property changes of less than 20-25 percent are usually considered minimal, although the acceptable or allowable property changes are determined by the specifications for the individual system in which the plastic is used. 

In the case of statistic copolymers of two monomers (binary copolymers) the glass transition temperature steadily changes with the molar amounts of the two monomers. In many cases, a similar behavior is observed with some mechanical properties (tensile strength, impact strength, stiffness, and hardness) (see Chap. 1). Deviations can occur in copolymers, which contain only a few percent of one comonomer. 

In this context, it is instructive to have a look at some previous studies on spin-cast thin polymer films [10-13, 15-18,47]. Being able to prepare smooth thin polymer films of variable thickness allowed to study questions related to the chain-like nature of macromolecules. In particular, it has been speculated that polymer properties like viscosity, chain orientation, interdiffusion rates, or mechanical properties and the glass transition temperature may change once the thickness of the film decreased below the diameter which Gaussian polymer coils have in bulk samples. 

Polymeric materials are unique owing to the presence of a glass-transition temperature. At the glass-transition temperatures, the specific volume of the material and its rate of change changes, thus, affecting a multitude of physical properties. Numerous types of devices could be developed based on this type of stimuli—response behavior however, this technology is beyond the scope of this article. 

T and are the glass-transition temperatures in K of the homopolymers and are the weight fractions of the comonomers (49). Because the glass-transition temperature is directly related to many other material properties, changes in T by copolymerization cause changes in other properties too. Polymer properties that depend on the glass-transition temperature include physical state, rate of thermal expansion, thermal properties, torsional modulus, refractive index, dissipation factor, brittle impact resistance, flow and heat distortion properties, and minimum film-forming temperature of polymer latex... 

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. 

The insertion of ISP monomeric units also results in significant changes in the thermomechanical properties of the copolymers. The glass-transition temperature... 

This variation in the properties of polymers along their interfaces with inclusions is extended to layers of a sometimes significant thickness. This follows from the fact that, if only a thin surface-layer of the polymer was affected by its contact with the other phase, then the change in Tg should be insignificant, since the level of the glass transition temperature is associated with the bulk of the polymer, or, at least, with a large portion of it. 

Although each of these cyclic siloxane monomers can be polymerized separately to synthesize the respective homopolymers, in practice they are primarily used to modify and further improve some specific properties of polydimethylsiloxanes. The properties that can be changed or modified by the variations in the siloxane backbone include the low temperature flexibility (glass transition temperature, crystallization and melting behavior), thermal, oxidation, and radiation stability, solubility characteristics and chemical reactivity. Table 9 summarizes the effect of various substituents on the physical properties of resulting siloxane homopolymers. The... 

This difference in spatial characteristics has a profound effect upon the polymer s physical and chemical properties. In thermoplastic polymers, application of heat causes a change from a solid or glassy (amorphous) state to a flowable liquid. In thermosetting polymers, the change of state occurs from a rigid solid to a soft, rubbery composition. The glass transition temperature, Tg, ... 

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