Glass transition temperature Reduced

Note that subtracting an amount log a from the coordinate values along the abscissa is equivalent to dividing each of the t s by the appropriate a-p value. This means that times are represented by the reduced variable t/a in which t is expressed as a multiple or fraction of a-p which is called the shift factor. The temperature at which the master curve is constructed is an arbitrary choice, although the glass transition temperature is widely used. When some value other than Tg is used as a reference temperature, we shall designate it by the symbol To. 

The glass-transition temperature, T, of dry polyester is approximately 70°C and is slightly reduced ia water. The glass-transitioa temperatures of copolyesters are affected by both the amouat and chemical nature of the comonomer (32,47). Other thermal properties, including heat capacity and thermal conductivity, depend on the state of the polymer and are summarized ia Table 2. 

More recently, test products were created of a blend of PMMA with a phenyl-substituted methacrylate these products have a glass-transition temperature of around 125°C, a significantly reduced water absorption compared to pure PMMA of about 0.32%, but also a higher birefringence (a stress-optic coefficient of 5.2 X 10 , compared with 0.3 X 10 for PMMA and 6.8 x 10 for BPA-PC). 

Substituted nonheat-reactive resins do not form a film and are not reactive by themselves, but are exceUent modifier resins for oleoresinous varnishes and alkyds. Thein high glass-transition temperature and molecular weight provide initial hardness and reduce tack oxygen-initiated cross-linking reactions take place with the unsaturated oils. 

Oxyhalide Glasses. Many glasses contain both oxide and haUde anions. The introduction of haUdes into an oxide glass typically serves to reduce the glass-transition temperature, T, and to increase the coefficient of thermal expansion. Oxyfluorophosphates have been investigated as laser host... 

Gel Dyeing. Continuous methods exist for dyeing wet-spun acryhc yams while they are stiU in thek swollen nonaligned state. The advantage of these methods is that they are extremely rapid, and the rate of uptake of dye is not dependent on glass-transition temperature so reducing the constraints on dyestuff selection. 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

Inclusion of double bonds will stiffen the chain at the point of inclusion but at the same time may increase the flexibility of adjacent bonds. The net effect may therefore be to reduce the glass transition temperature and this appears to occur in 1,4-polybutadiene when compared with polyethylene. 

Before providing such an explanation it should first be noted that progressive addition of a plasticiser causes a reduction in the glass transition temperature of the polymer-plasticiser blend which eventually will be rubbery at room temperature. This suggests that plasticiser molecules insert themselves between polymer molecules, reducing but not eliminating polymer-polymer contacts and generating additional free volume. With traditional hydrocarbon softeners as used in diene rubbers this is probably almost all that happens. However, in the... 

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