Glass transition temperature effect

Various viscosity models have implicitly included the effects of gelation on the chemoviscosity, and these were reviewed in Table 4.2 incorporating gelation-conversion and glass-transition-temperature effects implicitly in the cure effects on chemoviscosity. Explicit models for the expression of gel time versus temperature and time are sparse, with empirical measurements mainly being used. 

Priestley RD, Broadbelt LJ, Torkelson JM (2005) Physical aging of ultrathin polymer films above and below the bulk glass transition temperature effects of attractive vs neutral polymersubstrate interactions measured by fluorescence. Macromolecules 38 654—657... 

In Figure 2.4 the pressure-volume isotherm is shown at 25°C for polystyrene using the FOV equation of state. The characteristic parameters were obtained from Table 2.2. The EOS does not explicitly account for molecular weight and glass transition temperature effects. 

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. 

T is the glass-transition temperature at infinite molecular weight and is the number average molecular weight. The value of k for poly(methyl methacrylate) is about 2 x 10 the value for acrylate polymers is approximately the same (9). A detailed discussion on the effect of molecular weight on the properties of a polymer may be found in Reference 17. 

Qiana, introduced by Du Pont in 1968 but later withdrawn from the market, was made from bis(4-aminocyclohexyl)methane and dodecanedioic acid. This diamine exists in several cis—trans and trans—trans isomeric forms that influence fiber properties such as shrinkage. The product offered silk-like hand and luster, dimensional stabiUty, and wrinkle resistance similar to polyester. The yam melted at 280°C, had a high wet glass-transition temperature of - 85° C and a density of 1.03 g/cm, the last was lower than that of nylon-6 and nylon-6,6. Qiana requited a carrier for effective dyeing (see Dye carriers). 

Since successful commercialization of Kapton by Du Pont Company in the 1960s (10), numerous compositions of polyimide and various new methods of syntheses have been described in the Hterature (1—5). A successful result for each method depends on the nature of the chemical components involved in the system, including monomers, intermediates, solvents, and the polyimide products, as well as on physical conditions during the synthesis. Properties such as monomer reactivity and solubiHty, and the glass-transition temperature,T, crystallinity, T, and melt viscosity of the polyimide products ultimately determine the effectiveness of each process. Accordingly, proper selection of synthetic method is often critical for preparation of polyimides of a given chemical composition. 

Fig. 3. Effect of comonomer stmcture on the glass-transition temperature of VDC copolymers (72), where A represents acrylonitrile B, methyl acrylate ...

The dynamic mechanical properties of VDC—VC copolymers have been studied in detail. The incorporation of VC units in the polymer results in a drop in dynamic modulus because of the reduction in crystallinity. However, the glass-transition temperature is raised therefore, the softening effect observed at room temperature is accompanied by increased brittleness at lower temperatures. These copolymers are normally plasticized in order to avoid this. Small amounts of plasticizer (2—10 wt %) depress T significantly without loss of strength at room temperature. At higher levels of VC, the T of the copolymer is above room temperature and the modulus rises again. A minimum in modulus or maximum in softness is usually observed in copolymers in which T is above room temperature. A thermomechanical analysis of VDC—AN (acrylonitrile) and VDC—MMA (methyl methacrylate) copolymer systems shows a minimum in softening point at 79.4 and 68.1 mol % VDC, respectively (86). 

The highly polar nature of the TGMDA—DDS system results in high moisture absorption. The plasticization of epoxy matrices by absorbed water and its effect on composite properties have been well documented. As can be seen from Table 4, the TGMDA system can absorb as much as 6.5% (by weight) water (4). This absorbed water results in a dramatic drop in both the glass transition temperature and hot—wet flexural modulus (4—6). 

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. 

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