Glass transition temperature compositional variation

Fig. 6. The variation of the heat capacity jumps at the respective glass transition temperatures versus inclusion-volume contents of iron-epoxy particulates of different diameters of inclusions. In the same figure is presented the variation of the coefficients X for the same composites and volume contents...

Fig. 14. The variation of the specific heat jumps at glass-transition temperatures of elacc-epoxy composites, versus the fiber volume content, uf. The values for the factor X and the mesophase, (uj and matrix, (nm) volume fractions, versus uf, as derived from the values of the respective AC, s are also plotted...

Mechanical properties of these linear polymers can be varied within wide limits by proper selection of the diol or mixture of diols. This is illustrated in Fig. 2 which shows variation in the glass transition temperature of the polymer as a function of diol composition... 

As expected, the terminal functional groups mainly determine the reactivity of these siloxane oligomers towards other reactants. The variations in the backbone composition have critical effects on the glass transition temperature, solubility parameter, thermal stability and surface behavior of the resulting oligomers(12,13). 

Since 1985, a major effort has been devoted to incorporating heterocyclic units within the backbone of poly(arylene etherjs (PAE). Heterocyclic units within PAE generally improve certain properties such as strength, modulus and the glass transition temperature. Nucleophilic and electrophilic aromatic substitution have been successfully used to prepare a variety of PAE containing heteorcyclic units. Many different heterocyclic families have been incorporated within PAE The synthetic approaches and the chemistry, mechanical and physical properties of PAE containing different families of heterocyclic units are discussed. Emphasis is placed on the effect variations in chemical structure (composition) have upon polymer properties. 

The Teflon AF family consists of copolymers of tetrafluoroethylene, (TFE) and 2,2-bis-trifluoromethyl-4,5-difluoro-l,3-dioxole, (PDD), whose structure is shown in Figure 2.1. The properties of these amorphous copolymers vary with the relative amounts of the comonomers. At present the two commercial grades of Teflon AF are AF-1600 and AF-2400 with glass transition temperatures of 160 and 240°C respectively. The variation of glass transition temperature with composition is shown in Figure 2.2. Thus AF-1600 and AF-2400 contain 64 and 83 mol % PDD, respectively. 

The homopolymers poly(methyl methacrylate) and poly-(ethyl methacrylate) are compatible with poly(vinylidene fluoride) when blended in the melt. True molecular com-patibility is indicated by their transparency and a single, intermediate glass transition temperature for the blends. The Tg results indicate plasticization of the glassy methacrylate polymers by amorphous poly(vinylidene fluoride). The Tg of PVdF is consistent with the variation of Tg with composition in both the PMMA-PVdF and PEMA-PVdF blends when Tg is plotted vs. volume fraction of each component. PEMA/PVdF blends are stable, amorphous systems up to at least 1 PVdF/I PEMA on a weight basis. PMMA/ blends are subject to crystallization of the PVdF component with more than 0.5 PVdF/1 PMMA by weight. This is an unexpected result. 

Couchman PR (1987) Thermodynamics and the compositional variation of glass-transition temperatures. Macromolecules 11 1156-1161... 

Dynamic thermal mechanical analysis indicates significant broadening of glass transition temperature at or around PU/EP composition of 70/30. This is true for all three groups studied. At this composition, storage modulus showed much less steep variations with temperature during the transition from glassy to rubbery state. 

It is known [53] that introduction of fillers can alter the glass transition temperature Tg of the composition as compared to the basic polymer Tg. Variation of the composition viscosity can be described by the equation [54] ... 

FIGURE 3.18 The variation of the glass transition temperature of binary systems with composition, presented in terms of the Gordon-Taylor relationship (a) phenobarbital-salicin and (b) antipirin-phenobarbital. (Reproduced from Fukuoka, E., Makita, M., and Yamamura, S., Chem. Pharm. Bull., 37, 1047, 1989. With permission.)... 

Fig. 3. Variation of the glass transition temperature of ethylceUulose/hydroxypropylmethylcellulose blends with blend composition and diethyl phthalate concentration (A) 20/80, (B) 40/60, (Q 60/40, and (D) 80/20 w/w ethylcellulose/hydroxypropylmethylcellulose. ( ) T, of hydroxypropylmethyl-cellulose-rich phase (O) Tg of ethylcellulose-rich phase. Full lines represent the behavior of the individual polymers [65]...

From Table 2, it is clear that the variations of the glass transition temperature as a function of the composition for amorphous Pd-Ni-P alloys is small. The weak composition dependence of Tg in Pd-Ni-P alloys is consistent with previously reported results [24,25], where Tg of glassy Pdgo-xNixP2o alloys were found to be between 585 and 602 K when x varies from 0.15 to 0.8. In contrast, Tg for bulk amorphous PdxCugo-xPzo alloys increases gradually as the palladium concentration increases, as illustrated in Figure 6. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

Figure 4.6 Variation in glass transition temperature with copolymer composition (schematic).

Probably the three most important variations to be avoided in sample preparation of polymers and their composites are the degree of crystallization, the glass transition temperature, and the void content of the polymers. To this extent, all three are separately discussed below. 

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