Polyesters thermal transitions

A different type of flexible substituent are phenylalkyl substituents [19]. Monosubsti-tuted polyesters with phenylalkyl substituents with different lengths of the alkyl linkage are characterized by high melting temperatures (Table 1). The thermal transitions can be lowered signifieantly if the terephthalic moieties and the phenylene moieties are substituted by phenylalkyl moieties. The replacement of the phenylalkyl substituent at the hydroquinone moiety by f-butylhy-droquinone results in an increase of the glass transition temperature. The polycondensa-... 

A series of studies based on the combination of the difuran-dicarboxylic derivatives 11 and a wide choice of diols 12 were undertaken recently using all the classical procedures for the synthesis of polyesters 13, namely transesterification, condensation between the acid chloride moiety and the OH function and direct condensation. This comprehensive investigation included the structural characterization of all the polymers obtained, the determination of their molecular weight and molecular weight distribution, as well as of the thermal transitions and stability. From this broad set of results, interesting structure properties relationships could be drawn [4e]. 

Liquid crystalline textures were investigated by POM and thermal transitions in a glass-forming polydomain nematic network (liquid crystalline polyester, P5tB) showing an SME were determined [54], Two transitions, both nematic-isotropic could be detected from experiments with hot-stage POM. 

Dynamic mechanical analysis (DMA) measures a compound s modulus (stiffness) as its temperature is raised. This instrument has provided interesting insights into the properties of phenolics as well as those of DAPs, thermoset polyesters, silicones, and epoxies, by indicating the ability of thermosets to retain their modulus at elevated temperatures. DMA can also be used to determine thermal transition temperatures of the sample. 

Schmaljohann and co-workers [7] showed that in aromatic hyperbranched polyesters with an increasing length of the alkyl chain, the Tg increases due to intermolecular hydrogen bonding and a low temperature thermal transition and base separation occur. 

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. 

In conclusion, the different thermal histories imposed to PTEB have a minor effect on the /3 and y relaxations, while the a. transition is greatly dependent on the annealing of the samples, being considerably more intense and narrower for the specimen freshly quenched from the melt, which exhibits only a liquid crystalline order. The increase of the storage modulus produced by the aging process confirms the dynamic mechanical results obtained for PDEB [24], a polyester of the same series, as well as the micro-hardness increase [22] (a direct consequence of the modulus rise) with the aging time. 

All the thermoplastic hyperbranched polyesters are transparent, slightly yellow solids below and viscous liquids above the glass transition temperature, T. Ther-mogravimetric (TGA) studies on a hydroxyfunctional hyperbranched polyester, H6, show that the thermal stability is good. The weight loss was only 3.5 wt% up to 340 °C, where the thermal degradation started (Figure 10). 

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