New dielectric relaxation process reveals mesomorphic ordering in rapidly cooled poly(ethylene naphthalate)

A curious, strong dielectric relaxation process (delta) was found in rapidl y cooled poly(ethylene naphthalate). This process, which is located between two known beta and beta* relaxations of PEN, appears predominantly after r apid cooling and remains present even after heating above the glass transit ion temperature. In view of its very low activation energy of similar to 10 kJ/mol, its markedly high relaxation strength of up to Delta epsilon = 5, and its Debye-like peak shape, a collective relaxation mechanism is propose d, which involves collective crankshaft motions of the -O-CH2-CH2-O- sequen ces in a regular arrangement of the main chains. The analogy between this d elta -relaxation and an ultra-slow relaxation recently found in the smectic E phase of a side-chain liquid crystalline polymer suggests a (close-to) h exagonal smectic ordering in PEN. The very existence of liquid-crystalline order in PEN is corroborated by the observation of a thermo-reversible disc ontinuity in the relaxation parameters around -90 degreesC, which resembles a broadened LC-LC phase transition. Re-evaluation of experimental data of the beta* relaxation, which occurs in the non-crystalline fraction of PEN. suggests that this relaxation is sensitive to the local orientational order , which extends from nematic to isotropic. The shift in the temperature of the beta* peak and even the splitting of this peak found by other authors c an be ascribed to the lowering of the activation energy by the local ordere d packing of the PEN chains in line with a lower activation energy in the n ematic order. The coexistence of isotropic and nematic regions in PEN is pu t in the context of orientational order fluctuations during the induction p eriod of cold crystallisation of semi-flexible polymers.