The effect of flexible chain length on thermal and mechanical properties of poly(m-methylene 2,6-naphthalate)s Young Gyu Jeong a , Won Ho Jo a, * , Sang Cheol Lee b a Hyperstructured Organic Materials Research Center and School of Materials Science and Engineering, Seoul National University, Sun 56-1, Shinlim-Dong, Kwanak-ku, Seoul 151-742, South Korea b School of Advanced Materials and Systems Engineering, Kumoh National University of Technology, Kumi 730-701, South Korea Received 28 October 2003; received in revised form 5 February 2004; accepted 2 March 2004 Abstract The effect of flexible chain length on the thermal and mechanical properties such as melting temperature, glass transition temperature, dynamic mechanical relaxation behavior, and uniaxial tensile deformation for melt-quenched poly(m-methylene 2,6-naphthalate) (PmN) films was investigated using differential scanning calorimeter (DSC), dynamic mechanical thermal analyzer, and universal tensile machine. It was found from DSC thermograms that PmNs with even number of methylene group have higher melting temperatures and faster crystallization rates than PmNs with odd number of methylene group, showing an odd –even fluctuation. The plots of tan d versus temperature show that all PmN samples have three relaxation processes (b, b p , and a) regardless of the number of methylene group in their backbone. It was found that both b p - and a-relaxations are cooperative processes and that the activation energies of both relaxations as well as the glass transition temperature associated with the a-relaxation show odd – even fluctuations as a function of the number of methylene group. The initial tensile modulus at the low drawing rate of 0.15 cm/min also shows an odd – even fluctuation. In summary, the macroscopic thermal and mechanical properties of PmN such as melting temperature, glass transition temperature, crystallization rate, activation energies of a- and b p -relaxations, and initial modulus measured under a slow drawing rate exhibit odd – even fluctuations as the number of methylene group in PmN increases. q 2004 Elsevier Ltd. All rights reserved. Keywords: Poly(m-methylene 2,6-naphthalate); Odd–even fluctuation; Physical properties 1. Introduction The members belonging to poly(m-methylene 2,6- naphthalate) (PmN, where m denotes the number of methylene group) are semicrystalline polymers whose preparation was first reported in 1969 [1]. The chemical structure of PmN is equivalent to that of poly(m-methylene terephthalate) (PmT) except that a naphthalene ring replaces a benzene ring in PmT, as shown in Fig. 1. Since 2,6- naphthalenedicarboxylic acid, a monomer of PmN, has been recently produced in large scale, the PmN family has been recognized to have high potential as commercial engineer- ing thermoplastics. Poly(ethylene 2,6-naphthalate) (PEN, m ¼ 2) is the most well known polymer among the PmN family, since it has superior properties such as chemical resistance, flame resistance, gas barrier property, and high mechanical strength. Its glass transition temperature, melting tempera- ture, and mechanical properties such as tensile modulus and creep resistance are higher than those of poly(ethylene terephthalate) (PET), since the naphthalene ring imparts greater rigidity to the polymer backbone than the benzene ring does in PET. Therefore, numerous studies have been undertaken to investigate the crystalline structure [2–4], morphology [5–8], crystallization and melting behavior [9–15], and mechanical properties [16–22] of PEN. Poly(butylene 2,6-naphthalate) (PBN, m ¼ 4) has also several interesting and useful properties as an engineering plastic, viz. thermostability, hydrolytic stability, and gas barrier property. However, there are only a few studies on its crystal structure [23,24], crystallization and melting behavior [25–28]. While polymerization kinetics, rheo- logical, and thermal properties of poly(trimethylene 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.03.016 Polymer 45 (2004) 3321–3328 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 82-2-880-7192; fax: þ82-2-885-1748. E-mail address: whjpoly@plaza.snu.ac.kr (W.H. Jo).