Dynamic mechanical analysis of the two glass transitions in a thermotropic polymer

A dynamic mechanical analysis has been performed on a thermotropic poly(ether ester) with biphenyl units as mesogens and spacers with methyl substituents. This polymer develops a smectic mesophase with a rather slow rate of formation, in such a way that the isotropic melt of this polymer can be easily quenched into the glassy amorphous state. A quenched amorphous sample and three specimens annealed above the glass transition for different times have been analysed. These annealed specimens exhibit different degrees of liquid crystal formation. The dynamic mechanical (and DSC) results show that the glass transition (α-relaxation) temperatures of the isotropic amorphous and anisotropic liquid crystalline states are clearly different, and when the mesophase transformation is not complete, as it happens in the two specimens annealed at intermediate times, the two glass transitions are simultaneously observed. The values of the storage modulus below the glass transition are dependent on the degree of liquid crystallinity, showing that the rigidity of the mesophase is significantly higher than the one for the amorphous component.