*e-mail: whjpoly@plaza.snu.ac.kr 1598-5032/10/459-07 ©2004 Polymer Society of Korea 459 Macromolecular Research, Vol. 12, No. 5, pp 459-465 (2004) Cocrystallization of Poly(1,4-cyclohexylenedimethylene terephthalate-co-hexamethylene terephthalate) Copolymers Young Gyu Jeong and Won Ho Jo* Hyperstructured Organic Materials Research Center and School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea Sang Cheol Lee School of Advanced Materials and Systems Engineering, Kumoh National Institute of Technology, Gumi 730-701, Korea Received May 14, 2004; Revised August 26, 2004 Abstract: We have synthesized poly(1,4-cyclohexylenedimethylene terephthalate-co-hexamethylene terephthalate) [P(CT-co-HT)] random copolymers having various comonomer contents, from 0 to 100 mol% HT, by melt- condensation and have investigated their crystallization behavior by using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). We observed that P(CT-co-HT)s exhibit clear melting and crystallization peaks in their DSC thermograms and sharp diffraction peaks in their WAXD patterns for all of their copolymer compositions as a result of cocrystallization of the CT and HT units, even though the copolymers are statistically random copolymers. When we plotted the melting and crystallization temperatures of P(CT-co-HT)s and the d-spacings of all the reflections against the copolymer composition, we observed a eutectic point at ca. 80 mol% HT, which suggests that a crystal transition occured from a PCT-type crystal to a PHT-type crystal. Both the DSC and WAXD results support the notion that P(CT-co-HT) copolymers undergo an isodimorphic cocrystallization. Keywords: poly(1,4-cyclohexylenedimethylene terephthalate-co-hexamethylene terephthalate), isodimorphic cocrystallization, eutectic melting point, crystal transition. Introduction Poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) has been commercially produced and widely used for fiber, film, a component of dental composites, etc. 1-3 Owing to incorporation of 1,4-cyclohexylenedimethyl group instead of linear alkylene groups in poly(alkylene terephthalate), PCT has high melting temperatures of 251~318 o C, depend- ing on the trans/cis isomer ratio of 1,4-cyclohexylenedime- thyl units. 2 It is interesting that, despite its high melting temperature, PCT exhibits ductile mechanical property as compared to poly(ethylene terephthalate). 4-6 It has recently been demonstrated by Yee et al. 4,5 that the ductile mechanical property of PCT originates from the cooperative secondary relaxation due to the conformation transition of cyclohexyl- ene rings from chair, twisted boat to chair. However, the poor thermal stability above the melting temperature of PCT often makes it difficult to process in the melt for practical applications. Consequently, the synthesis of PCT-based copolymers or reactive blending with other polyesters have been studied to improve their processibility as well as thermal and mechanical properties through variation of the copoly- mer composition. 7-11 It is well-known that, in most copolyesters where both A and B components are crystallizable, the degree of crystal- linity decreases with increasing the composition of minor component, and as a result the copolymers often become totally amorphous even at low comonomer content. If two crystallizable components of the copolymers are compatible in their crystal lattices (i.e., cocrystallization), thermal and mechanical properties of the copolymers can be controlled without significant loss of crystalline properties. 10-15 Yoo et al. 9 reported that poly(1,4-cyclohexylenedimethylene terephthalate-co-ethylene terephthalate) copolymers, in which ethylene terephthalate (ET) units can crystallize with complete rejection of 1,4-cyclohexyleneterephthalate (CT) units from the crystal whereas CT units can cocrystallize with ET units to some extent, exhibits cocrystallization. However, copolymers with the middle range of comonomer content are completely amorphous. It has been recently reported that poly(1,4-cyclohexylenedimethylene terephtha-