Thermal behavior of co[poly(ethylene terephthalate)-p-oxybenzoate]

In a previous investigation of a co[poly(ethylene terephthalate)-p-oxybenzoate] containing 30 mole % oxybenzoate units (referred to as T2/30), Krigbaum and Salaris identified the endotherm at t_III = 244°C as a nematic → isotropic transition. Subsequent investigation revealed that the t_III endotherm disappeared if the polymer is heated to the isotropic melt or dissolved and reprecipitated. The loss of the t_III transition might be due to molecular weight reduction, sequence randomization, or erroneous identification of the transition. Viscosity and high-field NMR data eliminate the first two explanations. Annealing studies at temperatures higher than those of the earlier work demonstrate that the t_III transition, in fact, represents the melting of crystallites formed during a high-temperature annealing operation. Moreover, we cannot determine the thermodynamic melting temperature of the copolymer from solid-state annealing studies. Hence, the principal objective of the study of Krigbaum and Salaris, to compare the enthalpies and entropies of the nematic → isotropic and crystal → isotropic transitions, must be fulfilled in future work.     

Synthesis and characterization of poly[4-(4-hydroxyphenoxy) benzoic acid]

Poly[4-(4-hydroxyphenoxy) benzoic acid] was prepared by the bulk polycondensation of 4-(4-acetoxyphenoxy) benzoic acid. Polycondensation was conducted at 350°C for 3 h under a reduced pressure of 0.1 mmHg and gave a polymer with X?n of 255. The polymer was characterized by elemental analysis, IR spectroscopy, differential scanning calorimetry, and wide-angle X-ray measurement. The crystal/nematic and nematic/isotropic phase transition temperatures of polymer, which depend on the molecular weight, were observed at about 300°C and 410°C, respectively. The polymers with low molecular weights showed nematic textures above 300°C. This nematic/isotropic phase transition temperature is lower than that of poly (4-hydroxybenzoic acid). This thermal behavior of polymer comes from ether units, which increase the flexibility (the rotation or torsion of skeletal bonds) of the polymer chain. © 1994 John Wiley & Sons, Inc.     

How did we find the rigid amorphous phase?

The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich [1]: when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers [poly(ethylene terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] [2], the absence of the hysteresis peak at the lower temperature glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled. Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity was confirmed [1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC _p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses of polymers. When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC _p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min⁻¹ rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF, Nylon 66 and polycaprolactone [1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate [3] were added to this list.     

Fluorescence study on a thermotropic liquid-crystalline polyester at high temperatures

A thermotropic liquid-crystalline (LC) polyester, poly[(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %), is investigated by fluorescence technique using two model compounds: dimethyl terephthalate (DMT) and methyl methoxybenzoate (MMB) and is demonstrated to form an intermolecular ground-state complex between the terephthalate and OBA moieties. The change in fluorescence of PET40/OBA60 film is studied from 25°C to 450°C. The peak wavelength change for fluorescence of the intermolecular ground-state complex from 394 to 430 nm was observed in the temperature range between T_g and the LC transition temperature (115～ 250°C). This is attributed to the electronic distribution change between terephthalate and OBA moieties in the excited state, which play roles of acceptor and donor, respectively. The increase in the fluorescence intensity from the temperature near the annealing temperature to the temperature near the isotropic temperature (287～370°C) is suggested to be the increase in LC configuration and the formation of a more stable excited state due to the electronic distribution change between terephthalate and OBA moieties. The lifetime of PET40/OBA60 film quenched from LC temperature (300°C) to room temperature is in agreement with that of the nonannealed one, which is due to the fact that the deactivation process of the sample quenched from LC temperature is in accord with that of the nonannealed one. © 1995 John Wiley & Sons, Inc.     

Relations between dynamic mechanical properties and melting behavior of nylon 66 and poly(ethylene terephthalate)

Nylon 66 films exhibiting form I melting behavior show the γ mechanical relaxation at ?140°C. Samples which have form II melting behavior do not show this relaxation. The γ relaxation disappears when material having form I behavior is converted to material having form II behavior by annealing or by cold drawing. The form I and form II types of melting behavior are also found in poly(ethylene terephthalate); the interconversions and thermal behavior of the forms are analogous to the nylon 66 case. In poly(ethylene terephthalate), the β relaxation at ?40 to ?60°C is present only when form I melting behavior is found. Conversion to form II melting behavior by annealing or drawing (80°C) again causes the relaxation to disappear. No β relaxation was found in amorphous polymer. The γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) can therefore be associated with the crystalline structure responsible for form I melting behavior. Form I melting behavior has been associated with foldedchain crystals based on previous work. It is therefore postulated that the γ dispersion in nylon 66 and the β dispersion in poly(ethylene terephthalate) are associated with motions in the chain folds. This assignment is not inconsistent with the change in the γ dispersion of nylon 66 with the number of backbone CH₂ units, since these will affect the fold structure.     

Fiber spinning from the nematic melt. II. Effect of thermal history on spinning of the copolyester of polyethylene terephthalate and p-oxybenzoate

Fibers were spun from the nematic phase of the copolymer of polyethylene terephthalate having 60 mol % of p-oxybenzoate units. A capillary rheometer was used for spinning with a shear rate at the wall of 6.4 sec^?1, and capillary (length/diameter) ratio of 14.1. The spinning temperature was varied from 250° to 300°C and, at each temperature, the spin-draw ratio was examined as a variable. Spinning was performed under two conditions. When spinning from the melt without preheating, the initial modulus of the fibers increased with spin-draw ratio and increased with increasing spinning temperature for a fixed spin-draw ratio. In the second case, the melt was preheated and then cooled to the desired temperature before spinning the fibers. The preheating temperature was 280°C for spinning at 250°C, and 300°C for spinning at 280°C. Preheating increased the fiber modulus to the value obtained by spinning at the preheating temperature. A reduction of the viscosity due to the melting of poly(p-hydroxybenzoic acid) (PHB) crystallites produces better orientation and higher modulus. However, with increasing spin-draw ratio, the modulus of the preheated fibers decreased to the values expected for the spinning temperature. This decrease in modulus is due to recrystallization of PHB in the threadline.     

Depression of the crystal-nematic phase transition in thermotropic liquid crystal copolyesters

The crystal-nematic phase transition of a copolyester consisting of 20 mol% poly(ethylene terephthalate) and 80 mol% p-hydroxybenzoic acid (PHB) was characterized by depression of the crystal-nematic transition by the addition of a liquid crystal diluent. This copolyester contains blocks of crystalline PHB. Its transition behavior was compared with thatrandom copolyester with diluent of the same composition. From the extrapolated transition temperature depression data, the heat of transition per mole of p-oxybenzoate was calculated as about 1.3 kcal/mol, with an entropy of about 2 cal/deg mol. This assumes that only the p-oxybenzoate unit crystallized from the nematic state. The validity of the Flory-Huggins model for this transition point depression was confirmed graphically by comparison with two different thermotropic-liquid crystal polyesters. These results may represent the first reported crystal-nematic temperatures and heats generated by the dilution method for liquid crystal copolyesters of this type.     

Calorimetric studies of poly (2-phenoxyethylacrylate)/5CB mixtures

The thermophysical properties of mixtures of poly (2-phenoxyethylacrylate) and 4-cyano-4′-pentyl-biphenyl, 5CB, are investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The polymer has a molar mass M _w = 181 000 g mol ^-1 the low molecular mass liquid crystal exhibits a nematic to isotropic transition at 35.3°C and crystallizes below 23°C. The phase diagram exhibits miscibility gaps in certain regions of temperature and composition where coexisting nematic and isotropic phases are found. From a practical point of view when considering the electro-optical applications of these systems, it proves to be useful to know precisely the amount of small liquid crystal molecules dissolved in the polymer matrix and the concentration of polymer in the nematic phase. The former quantity has a mechanical impact due to a plasticizing effect, an optical impact since it changes the polymer refractive index, while the polymer in the nematic phase shifts the transition temperatures influencing the electro-optical response of the liquid crystal. The present work addresses these important aspects using POM and DSC.     

Melting behavior of ethylene oxide–propylene oxide (sym-PEP) block copolymers

Melting points and lamellar thicknesses have been measured for ethylene oxide–propylene oxide block copolymers (sym-PEP) with central poly(ethylene oxide) block lengths of 70–100 chain units and end poly(propylene oxide) block lengths of 0–30 chain units. Melting points of the block copolymers are lower than those of the corresponding poly(ethylene oxide) homopolymer by an amount (up to 15°C) which increases as the poly(propylene oxide) block length increases. Most samples have more than one melting transition, which can be assigned to variously folded chain crystals. End interfacial free energies σ_e for the various crystals have been estimated by use of Flory's theory of melting of block copolymers. For a given crystal type (e.g., once-folded-chain) σ_e is higher the longer the chain length of the end poly(propylene oxide) blocks. For a given copolymer σ_e is lower, the more highly folded the poly(ethylene oxide) chain.     

Crystalline transitions in powders of nylon 66 crystallized from solution

Powders of nylon 66 were crystallized from solution in methanol and in other solvents. These powders exhibit a latent heat of about 4.5 cal/g at the Brill transition near 200°C where the unit cell changes from a triclinic to a pseudohexagonal form. The dimensions of the hydrogen-bonded sheets are almost unchanged up to 240°C, but the separation between the sheets increases with increasing temperature. Above 245°C, the interchain separation increases rapidly, and permits the powder to be extruded in an essentially plastic manner to form coherent extrudates. As the temperature of treatment is increased above 245°C, the latent heat of the Brill transition is reduced toward zero, the heat of fusion is reduced from about 30 cal/g to about 20 cal/g, and an endotherm at 261°C is replaced by one at 267°C.     

LC-polyimides XXXIV. Noncrystalline thermotropic copoly(ester-imide)s based on PET

Four series of copoly(ester-imide)s (co-PEIs) were prepared by transesterification of poly(ethylene terephthalate), PET, with N-(4-carboxyphenyl)trimellitimide and an acetylated diphenol. Methylhydroquinone, tert. butylhydroquinone, phenylhydroquinone, and 2,7-dihydroxynaphthalene were used as diphenols. The chemical structures of these co-PEIs were characterized by chemical analyses, ¹H-, and ¹³C-NMR spectra. A low degree of crystallinity was observed when the PET content was above 85% mol %. Between 60 and 80 mol % PET all co-PEIs are biphasic, whereas below 60 mol % the co-PEIs form a homogeneous nematic melt and below the glass transition temperature (T_g) a nematic glass. The T_gs vary continously with the molar composition but the mechanical properties drop sharply when the nematic phase changes to an isotropic one. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1813–1820, 1998     

Thermotropic liquid-crystalline copolyesters containing substituted phenylene terephthalate and ethylene terephthalate units

Three series of aromatic, thermotropic copolyesters, based on terephthalic acid (TA), ethylene glycol (EG), and another diol were prepared. The third monomer was selected from three different hydroquinones including, ethoxyhydroquinone (EHQ), phenylhydroquinone (PHQ), and hydroquinone itself (HQ). The amounts of the different hydroquinone terephthalate units were varied while the amounts of ethylene terephthalate units remained constant. The copolymers and terpolymers were characterized for solubility, for morphology by polarized light microscopy (PLM), for molecular weight by solution viscometry, and by NMR, DSC, and TGA. At elevated temperatures all samples, when observed by PLM, displayed the characteristic texture of a nematic phase. The melting transition temperatures, T_m, were found to vary from 255 to 325°C, while the 5 wt % loss temperatures, T_d, were found to vary from 330 to 440°C. The inherent viscosities varied from 0.6 to 1.9 dL/g. Increases in the HQ monomer content caused a decreased solubility and an increase in melting point. Copolymer compositions determined by NMR showed that only about one-half of the EG added was incorporated into the copolymers. © 1997 John Wiley & Sons, Inc.     

Poly(ethylene terephthalate) reinforced by N,N′‐diphenyl biphenyl‐3,3′,4,4′‐tetracarboxydiimide moieties

Starting with 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and methyl aminobenzoate, we synthesized a novel rodlike imide‐containing monomer, N,N′‐bis[p‐(methoxy carbonyl) phenyl]‐biphenyl‐3,3′,4,4′‐tetracarboxydiimide (BMBI). The polycondensation of BMBI with dimethyl terephthalate and ethylene glycol yielded a series of copoly(ester imide)s based on the BMBI‐modified poly(ethylene terephthalate) (PET) backbone. Compared with PET, these BMBI‐modified polyesters had higher glass‐transition temperatures and higher stiffness and strength. In particular, the poly(ethylene terephthalate imide) PETI‐5, which contained 5 mol % of the imide moieties, had a glass‐transition temperature of 89.9 °C (11 °C higher than the glass‐transition temperature of PET), a tensile modulus of 869.4 MPa (20.2 % higher than that of PET), and a tensile strength of 80.8 MPa (38.8 % higher than that of PET). Therefore, a significant reinforcing effect was observed in these imide‐modified polyesters, and a new approach to higher property polyesters was suggested. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 852–863, 2002; DOI 10.1002/pola.10169     

Model phase diagrams of binary nematic mixtures. A comparative study between linear and crosslinked polymers

Model calculations of phase diagrams of side chain liquid crystal polymers (SCLCP) and low molecular weight liquid crystals (LMWLC) are presented. The polymer is assumed to have grafted side chain units characterized by a nematic‐isotropic transition temperature T_{NI 2}, and the LMWLC presents also a similar transition at a temperature T_{NI 1} . The model calculations can accommodate for the cases where the latter two temperatures are comparable or widely different. For the sake of illustration, the case T_{NI 1} = 60°C and T_{NI 2} = 80°C is adopted here. The main point of interest here is to perform a comparative study of the equilibrium phase diagrams of SCLCP made either of linear free chains or crosslinked chains forming a single network. To our knowledge this is the first comparative study of the phase behavior of binary nematic mixtures involving linear and crosslinked polymer matrices which permits to clearly identify the effects of crosslinks present in the polymer matrix. The crosslinks attribute elasticity to the polymer constituent which induces important distortions in the phase diagram. To highlight these distortions, examples of hypothetical binary nematic mixtures are chosen involving both linear and crosslinked polymers with side chain mesogen units. The quadrupole interaction parameter between the two nematogens is related to individual parameters via a geometric average ν²₁₂ = κν₁₁ν₂₂ with a coupling parameter κ. Different values of this parameter are considered and the impact of coupling strength on the phase diagram is discussed for crosslinked and linear polymers.     

A Fourier transform infrared study of the phase transition in aqueous solutions of Ethylene oxide–propylene oxide triblock copolymer

The phase transition between unimer and micellar phases of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer Pluronic P105 in aqueous solution has been investigated as a function of temperature using Fourier transform infrared spectroscopy. The transition of 8 wt% Pluronic P105 in aqueous solution was found to occur at 25 °C. As temperature increases, PO blocks appear to be stretched conformers with strong interchain interaction, and the formation of a hydrophobic core in the micellar phase. The EO chains are found to change to a more disordered structure with low-chain packing density from the unimer phase to the micellar phase. Both the EO and PO blocks exhibit dehydration during the phase transition. Received: 17 September 1998 Accepted in revised form: 10 December 1998     

Free volumes in liquid-crystalline main-chain polymer probed by positron annihilation

Free volumes in a copolymer consisting of p-hydroxybenzoic acid (HBA) and poly(ethylene terephthalate) (PET) were probed by positron annihilation technique. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured in the temperature range between 30 and 230°C. Above the glass transition temperature (ca. 60°C), the lifetime of ortho-positronium (o-Ps) annihilated in the free volumes and its annihilation probability were found to increase with increasing specimen temperature. These facts were attributed to the increase both in the size of the free volumes and in the concentration of such regions due to rearrangements of molecules. From the observed lifetime of o-Ps, it was found that the size of the free volumes increases from 0.05 nm³ to 0.1 nm³ in the temperature range between 30 and 230°C. At solid-mesophase transition temperature (150–220°C), the size of the free volumes monotonously increases, while the increase in the concentration of such regions saturates at 174°C. The clear relationship between the data obtained by the positron annihilation and those obtained by differential scanning calorimetry was confirmed. ©1995 John Wiley & Sons, Inc.     

Poly(ethylene terephthalate) terpolyesters containing isophthalic and 5‐tert‐butylisophthalic units

Poly(ethylene terephthalate‐co‐isophthalate‐co‐5‐tert‐butylisophthalate) (PETI^tBI) terpolymers were investigated with reference to poly(ethylene terephthalate) (PET) homopolymer and poly(ethylene terephthalate‐co‐isophthalate) (PETI) copolymers. Three series of PETI^tBI terpolyesters, characterized by terephthalate contents of 90, 80, and 60 mol %, respectively, with different isophthalate/5‐tert‐butylisophthalate molar ratios, were prepared from ethylene glycol and mixtures of dimethyl terephthalate, dimethyl isophthalate, and 5‐tert‐butylisophthalic acid. The composition of the terpolymers and the composition of the feed agreed. All terpolymers had a random microstructure and number‐average molecular weights ranging from 10,000 to 20,000. The PETI^tBI terpolyesters displayed a higher glass‐transition temperature and a lower melting temperature than the PETI copolymers having the same content of terephthalic units. Thermal stability appeared essentially unchanged upon the incorporation of the 5‐tert‐butylisophthalic units. The PETI^tBIs were crystalline for terephthalate contents higher than 80 mol %, and they crystallized at lower rates than PETI. The crystal structure of the crystalline terpolymers was the same as that of PET with the 1,3‐phenylene units being excluded from the crystalline phase. Incorporation of isophthalate comonomers barely affected the tensile modulus and strength of PET, but the brittleness of the terpolymers decreased for higher contents in 5‐tert‐butylisophthalic units. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 124–134, 2003     

Poly(ethylene terephthalate) copolymers containing nitroterephthalic units. I. Synthesis and characterization

The synthesis, microstructure, and thermal behavior of a series of poly(ethylene terephthalate) (PET) copolymers containing nitroterephthalic units are described. These novel copolyesters were synthesized by transesterification followed by melt copolycondensation of dimethyl terephthalate and dimethyl nitroterephthalate mixtures with ethylene glycol. The molar ratio of the two comonomers in the feed varied from 95/5 to 25/75. Furthermore, PET and poly(ethylene nitroterephthalate) homopolymers were synthesized with the same method and comparatively studied. Copolyester compositions were practically the same as in the feed, and weight‐average molecular weights ranged from 10,000 to 60,000. The two monomeric units were randomly distributed along the polymer chain, and the experimentally determined average sequence lengths were in accordance with ideal copolycondensation statistics. Melting temperatures and enthalpies of the copolyesters decreased with increasing content in nitroterephthalic units, and they all showed a single glass‐transition temperature superior to that of PET. They appeared to be stable up to 300 °C, and thermal degradation occurred in two well‐differentiated steps. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3761–3770, 2000     

Structure of ethylene–chlorotrifluoroethylene copolymers

The structures of copolymers of ethylene and chlorotrifluoroethylene have been studied by infrared, nuclear magnetic resonance, and x-ray diffraction techniques. Copolymers varying in ethylene composition from 80 to 50 mole-% were prepared at a number of different temperatures with a peroxide catalyst system. Compositions of 50/50 mole ratio were found to be semicrystalline and to have melting points as high as 241°C. These materials were found to be copolymers with a high degree of one-to-one alternation. They were similar in structure to 1:1 copolymers which had been reported previously by other workers who used a triethylboron catalyst system. The x-ray evidence indicated that the copolymers prepared with the peroxide catalysts were not stereoregular. A hexagonal unit cell with a theoretical density of 1.70 g/cc was determined for the alternating one-to-one copolymer by x-ray techniques. A value of 262°C was determined for the melting point of the theoretical 100% alternating one-to-one copolymer. Values of ΔH_? = 4500 cal/mole and ΔS_? = 8.4 cal/deg-mole were also calculated for the alternating 1:1 copolymer. The preferred conformation of the material appears to be a “kinked” structure with the crystalline phase having ethylene units in one chain lining up opposite chlorotrifluoroethylene units in the adjacent chain. Polar association which can occur between fluorine and hydrogen atoms in this arrangement may account in part for the relatively high melting point of the alternating one-to-one copolymers.     

Oxygen‐barrier properties of oriented and heat‐set poly(ethylene terephthalate)

The improvement in the oxygen‐barrier properties of poly(ethylene terephthalate) (PET) by orientation and heat setting was examined. Orientation was carried out at 65 °C by constrained uniaxial stretching to a draw ratio of about 4. Heat setting was performed at temperatures from 90 to 160 °C with the specimen taut. Orientation decreased the permeability of PET to almost one‐third that of the unoriented, amorphous polymer because of decreases in both the diffusion coefficient and the solubility coefficient. The proposed two‐phase model for oriented PET consisted of a permeable isotropic amorphous phase (density = 1.335 g/cm³) with ethylene linkages predominately in the gauche conformation and an impermeable oriented phase (density = 1.38 g/cm³) with ethylene linkages that had transformed from the gauche conformation to the trans conformation during stretching. Chain segments in the trans conformation did not possess crystalline order; instead, they were viewed as forming an ordered amorphous phase. Crystallization by heat setting above the glass‐transition temperature did not dramatically affect the permeability. However, a decrease in the diffusion coefficient, offset by an increase in the solubility coefficient, indicated that crystallization affected the barrier properties of the permeable amorphous phase. Analysis of the barrier data, assuming a two‐phase model with variable density for both the permeable and impermeable phases, revealed that the impermeable phase density increased during crystallization, approaching a value of 1.476 g/cm³. This value is consistent with previous measurements of the density of the defective crystalline phase in PET. The density of the permeable amorphous phase decreased concurrently to about 1.325 g/cm³, indicating the appearance of additional free volume. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1679–1686, 2000