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Blends of poly(lactic acid)s (PLAs) containing three different concentrations of d -isomer (i.e., 1, 4, and 10 mol%; referred to as PLA1, PLA4, and PLA10, respectively) with poly( dl -lactic acid) (PDLLA) were prepared via solution blending. The three PLAs were miscible with PDLLA, as determined by dynamic mechanical analysis. Drawn films of PLA1/PDLLA, PLA4/PDLLA, and PLA10/PDLLA blends exhibited endothermic peaks just above the glass transition temperature, which is evidence of strain-induced enthalpy relaxation. A thermodynamic model for the miscible blend system was applied to determine contributions of intramolecular and intermolecular energy changes to strain-induced enthalpy relaxation for PLA1, PLA4, and PLA10; the resultant values suggested that intramolecular energy changes are mainly responsible for strain-induced enthalpy relaxation.

The endothermic heat just above glass transition for the drawn poly(lactic acid)s (PLAs) films with four different d -isomer contents (1, 4, 10, and 20 mol%: named as PLA1, PLA4, PLA10, and PLA20) was observed on differential scanning calorimetry (DSC) heating scan. The amount of endothermic heat increases with increasing the draw ratio, which means that the endotherm is undoubtedly caused by stretching the PLAs films. The endotherm is clearly seen in the drawn films of amorphous PLAs (PLA10 and PLA20). In the crystalline PLA4, the endothermic heat decreases with increasing crystallinity. Hence, it is considered that the endotherm is closely related to some events occurring in the amorphous phase of the drawn PLAs films. In order to verify an association between endothermic event and macroscopic shrinking deformation, two types of dimensional constraints during DSC measurement, which are effective in restraining the sample shrinkage, were imposed on a drawn PLA10 monofilament. The endothermic temperature of PLA10 with draw ratio of 4 moves from 62 °C for a free filament to 67 °C for an ends-fixed one. For a filament ends-fixed and very tightly rolled up with aluminum sheet, a sharp endothermic peak does not appear. Instead, a very broad endothermic curve ranging from 90 °C to 172 °C shows up, suggesting that the stretched chains in a filament with severe dimensional constraint relax slowly toward isotropic state over broad range of temperature. This result implies that the heat-absorption is contemporaneous with the macroscopic shrinkage of drawn filament. It was also visually confirmed that a drawn sample is shrunk above the endothermic peak. The conformational energy change during macroscopic shrinking deformation, determined by using Raman spectroscopic method, is comparable to the endothermic heat. Hence, the main origin of endothermic heat seems to be the conformational energy change during shrinking deformation.

We have synthesized poly(octamethylene terephthalate) (POT), poly(octamethylene 2,6-naphthalate) (PON), and poly(octamethylene terephthalate- co -octamethylene 2,6-naphthalate)s [P(OT- co -ON)s] with various comonomer composition by melt-polycondensation reaction and investigated their chain structures, crystalline structures, melting and cocrystallization behavior by using 1 H NMR spectroscopy, wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC), respectively. It was observed that P(OT- co -ON)s exhibit clear melting and crystallization peaks in DSC thermograms and sharp diffraction peaks in WAXD patterns throughout the copolymer composition, resulting from the cocrystallization behavior of OT and ON units in copolymers. When the melting and crystallization temperatures of P(OT- co -ON)s are compared as a function of the copolymer composition, there exists an eutectic point at around 23 mol% ON, where the crystal transformation from POT-type to PON-type occurs. It was confirmed from WAXD patterns of the melt-crystallized samples that the crystal transformation from POT-type to PON α-type to PON β-type occurs with the increment of the comonomer ON content in copolymers, i.e., POT-type crystals for POT and P(OT- co -ON) with 11 mol% ON, PON α-type crystals for P(OT- co -ON)s with 23–48 mol% ON, and PON β-type crystals for PON and P(OT- co -ON)s with 62–87 mol% ON. Both DSC and WAXD results demonstrate the isodimorphic cocrystallization of P(OT- co -ON)s. Based on the Wendling–Suter model for cocrystallization thermodynamics, it was found that the average defect free energy for the inclusion of OT units into PON β-type crystals is much lower than the value of the incorporation of ON units into POT-type crystals.

It was revealed that poly(octamethylene 2,6-naphthalate) (PON) existed in two different crystal structures, α- and β-form, depending on crystallization process: The α-form crystal was dominantly developed from the cold-crystallization, whereas the β-form was from the melt-crystallization. The apparent melting temperatures of α- and β-form crystals were characterized to be 175 and 183 °C, respectively. On the basis of X-ray diffraction and molecular modeling studies, the crystal structure of β-form, developed dominantly from the melt-crystallization, was identified to be triclinic with dimensions of a = 0.601 nm, b = 1.069 nm, c = 2.068 nm, α = 155.68°, β = 123.25°, γ = 52.85°, and with the space group of P 1 ¯ . The calculated crystal density was 1.243 g/cm 3 , supporting that one repeating unit of PON exists in a unit cell. The octamethylene units in the PON backbone take largely all - trans conformation in the β-form unit cell.

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 δ versus temperature show that all PmN samples have three relaxation processes (β, β∗, and α) regardless of the number of methylene group in their backbone. It was found that both β∗- and α-relaxations are cooperative processes and that the activation energies of both relaxations as well as the glass transition temperature associated with the α-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 α- and β∗-relaxations, and initial modulus measured under a slow drawing rate exhibit odd–even fluctuations as the number of methylene group in PmN increases.

X-ray powder diffraction and molecular modeling are used to identify the crystal structure and chain conformation of poly(trimethylene 2,6-naphthalate) (PTN) β-form crystal. The unit cell of PTN β-form crystal was determined to be a triclinic with dimensions of a=0.4665 nm , b=0.7014 nm , c=2.2177 nm , α=100.85°, β=88.78° and γ=120.63°, and the space group of the crystal is identified as P 1 . The observed crystal density of 1.37 g cm−3 and the determined dimensions of unit cell indicate that the unit cell contains one polymer chain with two repeating units. In the unit cell, each trimethylene unit in PTN backbone is in gauche/gauche conformation and neighboring naphthalene units are in face-to-face type arrangement, forming π-stacks that lead to the lowest energy of the unit cell.

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting–recrystallization–remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman–Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen–Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.

The crystal structure of poly(pentamethylene 2,6-naphthalate) (PPN) was determined by using X-ray diffraction and molecular modeling. The unit cell of PPN was found to be triclinic ( P 1 space group) with dimensions of a=0.457 nm, b=0.635 nm, c=2.916 nm, α=121.6°, β=90.4°, γ=87.6°, and the calculated crystal density was 1.311 g cm−3. The unit cell contains one polymer chain with two repeating units. In the unit cell, the PPN backbone takes gauche/gauche conformation in the middle part of each pentamethylene unit, and two naphthalene rings are in face-to-face arrangement.

A series of poly(hexamethylene terephthalate- co -hexamethylene 2,6-naphthalate) (P(HT- co -HN)) random copolymers were synthesized by melt polycondensation and characterized using 1 H NMR spectroscopy and viscometry. Their cocrystallization behavior was investigated using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) method. Even though the P(HT- co -HN) copolymers synthesized are statistically random copolymers, they show a clear melting and a crystallization peak in DSC thermograms over the entire range of copolymer composition and have a minimum melting temperature in the plot of melting temperature versus copolymer composition. WAXD patterns of all the copolymer samples show sharp diffraction peaks and are largely divided into two groups, i.e. PHT type and PHN type crystals. In addition, WAXD patterns of the PHN type crystals are subdivided into two types of PHN α and PHN β according to the copolymer composition. These facts indicate that the P(HT- co -HN) copolymers show isodimorphic cocrystallization. The composition at which the crystal transition between PHT type and PHN type occurs is equivalent to the eutectic composition (22 mol% HN content) for the melting temperature. When the defect free energies were calculated by using the equilibrium inclusion model proposed by Wendling and Suter, the defect free energies in the case of incorporation of HT units in the PHN α and β crystals were higher than the case of incorporation of HN units in the PHT crystal lattice.

The melting behaviour of syndiotactic polystyrene (sPS) and its blend with poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) was examined by differential scanning calorimetry (d.s.c.), and the polymorphism of sPS was also investigated by X-ray diffraction. Both pure sPS and sPS/PPO blends showed three melting endotherms under our crystallization conditions. When the d.s.c. results are compared with the X-ray patterns, it is suggested that the lowest and middle melting endotherms are due to the melting of β- and α-forms of sPS, respectively, whereas the highest melting endotherm comes from the melting of recrystallized sPS crystals formed during the d.s.c. scan. It is also observed that the recrystallization of sPS in sPS/PPO blends is disturbed by the presence of PPO.

The effect of transesterification on the phase behaviour in poly(ethylene 2,6-naphthalate) and poly(ethylene terephthalate) blends was investigated by using differential scanning calorimetry. The transesterification between the two polymers was confirmed by 1 H nuclear magnetic resonance. The blend samples heat treated for up to 11 min at 280°C show two glass transitions, which approach closer with the increase of reaction time. After 13 min, a single glass transition is observed, and the glass transition range becomes narrow with the lapse of reaction time. From the glass transition behaviour, the phase compositions and the phase fractions were determined as functions of reaction time at 280°C. The composition difference between ethylene 2,6-naphthalate (EN)-rich and ethylene terephthalate (ET)-rich phases decreases as the transesterification proceeds. On the other hand, though the weight fractions of the EN-rich phase, the ET-rich one and the interfacial zone tend to change with the lapse of time, the changes are small in magnitude.

The extent of transesterification reaction, intrinsic viscosity, weight-average molecular weight and hydrodynamic radius of poly(ethylene 2,6-naphthalate) (PEN)/poly(ethylene terephthalate) (PET) (50/50, w/w) blend were measured as a function of reaction time at 280°C. In PEN/PET blend, the reactions between end groups as well as the transesterification should be considered since the molecular weight of the blend increases with reaction time.

The phase behaviour of ternary polymer blend of poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO) and poly(hydroxy ether of bisphenol-A) (phenoxy) was investigated. Although all three binary blends are separately miscible, there exists a closed immiscibility loop in the phase diagram of ternary blends. It may result from small differences in the interaction energy of binary systems, the so-called ‘Δχ effect’. The value of χ phenox/PMMA , by comparing the theoretically predicted phase diagram with experimental data, was determined to be −0.61.