Search Results
Abstract
To obtain a biodegradable polymer material with satisfactory thermal properties, higher elongation and modulus of elasticity, a new copolyester, poly(hexylene terephthalate-co-lactide) (PHTL), was synthesized via direct polycondensation from terephthaloyl dichloride, 1,6-hexanediol and oligo(lactic acid). The resulting copolyesters were characterized by proton nuclear magnetic resonance (1H NMR), differential scanning calorimetry (DSC), thermogravimetry (TG) and wide-angle X-ray scattering (WAXS). By using the relative integral areas of the dyad peaks in 1H NMR spectrum of copolyesters PHTL, the sequence lengths of the hexylene terephthalate and lactide units in the resultant copolyesters are 3.5 and 1.5, respectively. Compared to poly(hexylene terephthalate) (PHT), PHTL has lower T m but higher T g due to the incorporation of lactide unit into the main chains of copolyesters. The degradation test of copolyesters under a physiological condition shows that the degradability of PHTL is sped up due to incorporation of lactide segments.
Abstract
Poly(2-hydroxyethoxybenzoate), poly(ε-caprolactone), and random poly(2-hydroxyethoxybenzoate/e-caprolactone) copolymers were synthesized and characterized in terms of chemical structure and molecular mass. The thermal behavior was examined by DSC. All the samples appear as semicrystalline materials; the main effect of copolymerization was lowering in the amount of crystallinity and a decrease of melting temperature with respect to homopolymers. Flory's equation described well the T m-composition data. Amorphous samples (in the 20–100%2-hydroxyethoxybenzoate unit concentration range) obtained by quenching showed amonotonic decrease of the glass transition temperature T g as the content of caprolactone units is increased. The Wood's equation described the T g-composition data well.
Abstract
The toughness of amorphous copolyester sheets was assessed by the essential work of fracture (EWF) concept. While the yielding-related work of fracture terms did not change significantly, the necking-related parameters strongly decreased with decreasing entanglement density of the copolyesters having different amounts of cyclohexylenedimethylene (CHDM) units in their backbones. Furthermore, copolyesters with high CHDM content and thus less entanglement density showed full recovery of the necked region beyond the glass transition temperature, i.e. the ‘plastic’ zone in the related specimens formed by cold drawing and not by true plastic deformation. By contrast, the copolyester with negligible amount of CHDM did not show this shape recovery. Modulated differential scanning calorimetry (MDSC) revealed that the necking in the latter system was accompanied by strain-induced crystallization. The superior work hardening in the necking stage of the respective poly(ethylene terephthalate) (PET) specimens can thus be ascribed to stretching of the entanglement network with superimposed crystallization.
Abstract
The tensile loading-induced necking in notched specimens of an amorphous copolyester (aCOP) was studied by modulated differential scanning calorimetry (MDSC). It was shown that necking occurred by cold drawing since the enthalpy of cold crystallization and that of the subsequent melting agreed fairly with each other. Increasing deformation in the necking zone and increasing deformation rate of the specimens shifted the onset of cold crystallization toward lower temperatures and yielded a slightly higher glass transition temperature (Tg). This was attributed to the molecular orientation caused by mechanical loading. The finding that the melting contained a non-reversing part was considered as appearance of possible microcrystallinity. The Tg range was strongly influenced by the deformation rate and reflects the thermomechanical history of the samples accordingly.
Abstract
The fracture toughness of blends of polypropylene terephthalate (PPT) with polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) were investigated. Binary blends were prepared comprising 10:90, 30:70, 50:50, 70:30 and 90:10 mass/mass%. The fracture toughness was determined for each blend using the essential work of fracture (EWF) method and thin film double edge notched tension (DENT) specimens. The specific essential work of fracture, w e, values obtained for blends of PET/PPT ranged from 27.33 to 37.38 kJ m–2 whilst PBT/PPT blends yielded values ranging from 41.78 to 64.23 kJ m–2. Differential scanning calorimetry (DSC) was employed to assess whether or not crystallinity levels influence the mechanical properties evaluated. The fracture toughness of PPT deteriorated with PET incorporation. However, high we values exceeding that of pure PPT were obtained for PBT/PPT blends across the composition range studied.
Abstract
A calorimetric study of blends of poly(ethylene terephthalate-co-p-oxybenzoate), PET/PHB, with poly(butylene terephthalate), PBT has been carried out in the form of as-spun and drawn fibres. DSC melting and crystallization results show that PBT is compatible with LCP and the crystallization of PBT decreases by the addition of LCP in the matrix. The crystallization behaviour of blend fibres is investigated as a function of temperature of crystallization. A detailed analysis of the crystallization course has been made utilizing the Avrami expression. The isothermal calorimetric measurements provide evidence of decrease of rate of crystallization of PBT on addition of the liquid crystalline component up to about 50% by weight. The values of the Avrami exponents change in the temperature range from 200° to 215°C. Dimensionality changes in crystallization could be due to LCP mesophase-transition.
Abstract
The poly(1,4-butylene terephthalate-co-DL-lactide) (BLA) copolymers were successfully prepared by the melt reaction between poly(1,4-butylene terephthalate) (PBT) and DL-oligo(lactic acid) (OLA) in the presence of 1,4-butanediol (BDO) without any catalysts. The transesterification between butylenes terephthalate (BT), 1,4-butanediol and lactide (LA) segments during the reaction was confirmed by the 1H NMR analysis. The chemical structure of the copolymers was further investigated by the 13C NMR and two-dimensional 1H–13C HMQC (heteronuclear multiple quantum correlation) technique. The effect of reaction temperatures and the starting feed ratios on the molecular microstructures, molecular weights, solubility and thermal stability of the copolyesters was extensively studied. The sequence length of BT (NBT) was found to play a vital role on the solubility and thermal behaviors of the resulting copolyesters. The copolyesters with NBT in the range of 2.8 and 7.3 were soluble in chloroform. The B10LA40 copolyester with the shortest NBT of 2.8 exhibited almost the lowest glass-transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm), crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) as compared with the other copolyesters. The copolyester of B10LA40 was able to hydrolytically degrade and the fabricated scaffold that showed good biocompatibility towards the human bone marrow stromal cells.
Abstract
Polymer blends of poly(β-hydroxybutyrate-co-b-hydroxyvalerate) (Biopol) with polyamide 11, possessing copolyester continuous phase, were degraded during 25 weeks in compost. The biodegradation was followed by mass loss and melting enthalpy measurements. The degradability was primary dependent on the hydroxyvalerate content in the blend.
