New dielectric relaxation process reveals mesomorphic ordering in rapidly cooled poly(ethylene naphthalate) |
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Authors: | M Wübbenhorst A L de Rooij J van Turnhout J Tacx V Mathot |
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Institution: | (1) Department of Polymer Materials & Engineering, Delft University of Technology, Faculty of Applied Sciences, Julianalaan 136, 2628 BL Delft The Netherlands e-mail: wuebbenhorst@tnw.tudelft.nl Tel.: +31-15-278 6940; Fax: +31-15-278 7415, NL;(2) DSM Research, Departments of `Material Properties' and `Performance Materials R & D', P.O. Box 18, 6160 MD Geleen The Netherlands, NL |
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Abstract: | A curious, strong dielectric relaxation process (δ) was found in rapidly cooled poly(ethylene naphthalate). This process,
which is located between two known β and β* relaxations of PEN, appears predominantly after rapid cooling and remains present
even after heating above the glass transition temperature. In view of its very low activation energy of ∼10 kJ/mol, its markedly
high relaxation strength of up to Δɛ=5, and its Debye-like peak shape, a collective relaxation mechanism is proposed, which
involves collective crankshaft motions of the -O-CH2-CH2-O- sequences in a regular arrangement of the main chains. The analogy between this δ-relaxation and an ultra-slow relaxation
recently found in the smectic E phase of a side-chain liquid crystalline polymer suggests a (close-to) hexagonal smectic ordering
in PEN. The very existence of liquid-crystalline order in PEN is corroborated by the observation of a thermo-reversible discontinuity
in the relaxation parameters around −90 °C, which resembles a broadened LC-LC phase transition. Re-evaluation of experimental
data of the β* 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 β* peak and
even the splitting of this peak found by other authors can be ascribed to the lowering of the activation energy by the local
ordered packing of the PEN chains in line with a lower activation energy in the nematic order. The coexistence of isotropic
and nematic regions in PEN is put in the context of orientational order fluctuations during the induction period of cold crystallisation
of semi-flexible polymers.
Received: 31 August 2000 Accepted: 30 October 2000 |
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Keywords: | Poly(ethylene naphthalate) (PEN) Dielectric relaxation spectroscopy Low temperature cooperativity Hexagonal smectic order Liquid crystallinity Quenching |
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