In this work, a series of poly(1,4-cyclohexylenedimethylene terephthalate-co-1,4-cyclohexylene dimethylene 2,6-naphthalenedicarboxylate) (PCTN) copolymers with different compositions of two diacids: terephthalic acid (TPA) and 2,6-naphthalenedicarboxylic acid (NDA), and stoichiometry matched 1,4-cyclohexanedimethanol (CHDM) were synthesized directly by a melt polymerization technique. Thermal properties of synthesized copolymers were measured by a differential scanning calorimetry (DSC). The incorporation of either NDA or CHDM improved the glass transition temperature (T_g) of PCTN compared to poly(ethylene terephthalate) (PET). Thermal degradation behaviors were analyzed using a thermogravimetric analysis (TGA). Chemical structures of the obtained copolyesters were investigated by a nuclear magnetic resonance spectroscopy (¹H NMR). Our results showed that the thermal and optical properties (determined by UV-Vis spectrophotometer) of the PCTN copolyesters were directly dependent on the ratio of TPA and NDA. It was also found that PCTN films have superior optical, thermal, and barrier (reduced water absorption) properties than PET and poly(ethylene naphthalate) (PEN) films.

Terephthalic acid(TPA), 2,6-naphthalenedicarboxylic acid(NDA), 1,4-cyclohexanedimethanol(CHDM)을 사용하여 poly(1,4-cyclohexylenedimethylene terephthalate-co-1,4-cyclohexylene dimethylene 2,6-naphthalenedicarboxylate) (PCTN)를 용융중합하였다. DSC를 통해 열특성을 분석하였고, NDA와 CHDM이 첨가된 PCTN 공중합체의 유리전이온도는 poly(ethylene terephthlatae)(PET) 대비 크게 향상됨을 확인하였다. 열분해 거동은 TGA를 사용하여 확인하고 ¹H NMR을 통해 화학구조를 규명하였다. PCTN 공중합 폴리에스터는 TPA와 NDA의 함량에 따라 변화되는 광학특성을 보여주었고, 또한 PCTN 필름이 기존의 PET와 poly(ethylene naphthalate)(PEN) 필름과 비교하여 뛰어난 열특성 및 차단 특성을 보여주었다.

Keywords: copolyester, 1,4-cyclohexanedimethanol, 2,6-naphthalenedicarboxylic acid, melt polymerization, optical and barrier properties

Polymers have gained the attention of scientists and researchers because of their many desirable properties. Among numerous polymers, poly(ethylene terephthalate) (PET) is one of the most well-known polymer because of its wide range of applications. It is used worldwide to produce textiles, electronics substrates, food packagings, beverage bottles, and molded plastic parts.^1-5 Because of the intensive industrial applications of polyester, researchers have keen interest to synthesize newly developed polyesters having enhanced thermal stability and barrier properties compared to original PET.
Owing to the growing demand of the advanced polyester, many efforts have been attempted to improve the thermal and barrier properties of PET. Recently, there have been many studies for using PET as flexible film substrate for electronic devices,³ textile fibers,⁶ thermoplastic resin,⁷ transparent and shrinkable films, and elastomers.^8,9 However, the tendency of PET to crystallize at a relatively low glass transition temperature (T_g) exerts as an obstacle to use this polymer at an elevated temperature. The moisture barrier property of PET drops rapidly above its T_g. Because of this limitation, PET is unsuitable to be used for the products that require moisture barrier at a temperature above 100 ℃.
Inferior moisture barrier property of PET above its T_g can be improved by introducing some fillers like graphene,⁸ silica nanoparticles, or nano tubes.^10,11 Another, most widely used approach is by controlling the chemical structures of polyester themselves. Some new polyester-based materials have been synthesized with proper molecular design. 1,4-cyclohexanedimethanol (CHDM) is a commercially available cycloaliphatic diol with a reasonable price and is used for the synthesis of commercial polyester, poly(cyclohexane 1,4-dimethylene terephthalate) (PCT). CHDM strongly influences the physical and thermal properties of resulting polyester. Compared to PET, the cyclic repeating unit present in this polyester imparts better thermal (T_g and T_m), hydrolysis resistance, and barrier properties. Due to enhanced thermal properties of PCT, it is principally used as injection molded polymer for electronic and automotive parts.12 However, PCT has a high Tm (about 295 ℃) so the final esterification reaction during synthesis must be carried out at a temperature higher than 300 ºC which is very close to its decomposition temperature, making the processing difficult. Due to the high crystallinity and limited processing window, PCT homopolymer is not suitable to be fabricated into film. For the film application PCT should be modified with different level of comonomer to decrease the melting temperature (Tm).
Another type of polyester having enhanced thermal property is poly(ethylene naphthalene 2,6-dicarboxylate) (PEN). Terephthalic acid (TPA) of PET is replaced by a naphthalenedicarboxylic acid and the resulting polymer has much higher Tg (about 120 ℃). This polymer rapidly finds applications in the industries as performance materials for printing and embossing films, electrical devices, and packaging materials. Due to thermal stability along with its good barrier, electrical, and chemical characteristics, PEN has earned a strong position in the high temperature applications.¹³ But, PEN exhibits high birefringence phenomenon which limits its applications as optical films. In addition, the high cost of a monomer, 2,6-naphthalenedicarboylic acid (NDA) used for the synthesis of PEN renders an obstacle for the applications of this polymer in versatile areas.
It would be highly appreciable to introduce a synthesis of new polyester having enhanced thermal and barrier properties by combining three polyesters mentioned so far, that is, PET, PCT, and PEN. In this study, the synthesis of poly(1,4-cyclohexylenedimethylene terephthalate-co-1,4-cyclohexylenedimethylene 2,6-naphthalenedicarboxylate) (PCTN) copolymers was attempted. To the best of our knowledge, it is the first time that thermal and optical properties of PCTN films are reported. The main objective of this work was to synthesize a series of PCTN copolymers and investigate the thermal and barrier (especially barrier to water absorption) properties of films fabricated from them. We focused on 1,4-cyclohexanedimethanol (CHDM) as a single component for diol and various compositions of TPA and NDA as a diacid component for the constituents of resulting copolyester. The copolymers were synthesized directly by a melt polymerization technique. The thermal stability and barrier properties of obtained copolyester were compared to those of PET and PEN.