Glass transition temperature generalities

Polymers will be elastic at temperatures that are above the glass-transition temperature and below the liquiflcation temperature. Elasticity is generally improved by the light cross linking of chains. This increases the liquiflcation temperature. It also keeps the material from being permanently deformed when stretched, which is due to chains sliding past one another. Computational techniques can be used to predict the glass-transition and liquiflcation temperatures as described below. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

This type of adhesive is generally useful in the temperature range where the material is either leathery or mbbery, ie, between the glass-transition temperature and the melt temperature. Hot-melt adhesives are based on thermoplastic polymers that may be compounded or uncompounded ethylene—vinyl acetate copolymers, paraffin waxes, polypropylene, phenoxy resins, styrene—butadiene copolymers, ethylene—ethyl acrylate copolymers, and low, and low density polypropylene are used in the compounded state polyesters, polyamides, and polyurethanes are used in the mosdy uncompounded state. 

As appHed to hydrocarbon resins, dsc is mainly used for the determination of glass-transition temperatures (7p. Information can also be gained as to the physical state of a material, ie, amorphous vs crystalline. As a general rule of thumb, the T of a hydrocarbon resin is approximately 50°C below the softening point. Oxidative induction times, which are also deterrnined by dsc, are used to predict the relative oxidative stabiHty of a hydrocarbon resin. 

A reexamination of polycarbonate chemistry was carried out about 50 years after the first aromatic polycarbonates of resorcinol and hydroquinone were discovered. In independent investigations at Bayer AG and General Electric, it was discovered that the polycarbonates of BPA could be prepared (eq. 2). Unlike the ahphatic polycarbonates prepared earlier, which were either hquids or low melting sohds, the aromatic polycarbonates were amorphous sohds having elevated glass-transition temperatures. 

The use of elastomeric modifiers for toughening thermoset resias generally results ia lowering the glass transition temperature, modulus, and strength of the modified system. More recendy, ductile engineering thermoplastics and functional thermoplastic oligomers have been used as modifiers for epoxy matrix resias and other thermosets (12). 

The glass-transition temperature in amorphous polymers is also sensitive to copolymerization. Generally, T of a random copolymer falls between the glass-transition temperatures of the respective homopolymers. For example, T for solution-polymerized polybutadiene is —that for solution-polymerized polystyrene is -HlOO°C. A commercial solution random copolymer of butadiene and styrene (Firestone s Stereon) shows an intermediate T of —(48). The glass-transition temperature of the random copolymer can sometimes be related simply as follows ... 

Silicones. Polydimethylsiloxanes, polydiphenylsiloxanes, and polymethylphenylsHoxanes are generally called siUcones (see Silicon COMPOUNDS, silicones). With a repeating unit of alternating siUcon-oxygen, the siloxane chemical backbone stmcture, siUcone possesses excellent thermal stabiUty and flexibility that are superior to most other materials. Polydimethjisiloxane provides a very low glass-transition temperature T material but is suitable for use... 

The glass transition temperatures of the nylons appear to be below room temperature so that the materials have a measure of flexibility in spite of their high crystallinity under general conditions of service. The polymers have fairly sharply defined melting points and above this temperature the homopolymers have low melt viscosities. Some thermal properties of the nylons are given in Table 18.4. 

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