Mechanical properties and superstructure of poly(ethylene terephthalate) fibers zone-drawn and zone-annealed by CO2 laser heating

A laser-heating zone-drawing and zone-annealing method using a continuous-wave carbon dioxide laser was applied to poly(ethylene terephthalate) (PET) fiber to improve its mechanical properties. The as-spun fiber was zone-drawn under a applied tension (σ_a) of 4.44 MPa at a laser power density (PD) of 6.08 W cm⁻², and then the laser-heated zone-drawn fiber was zone-annealed. The laser-heating zone-annealing was carried out in three steps: the first annealing was carried out under σ_a = 139 MPa at 4.83 W cm⁻²; the second annealing was carried out under σ_a = 283 MPa at 4.83 W cm⁻², and the third annealing was carried out under σ_a = 432 MPa at 3.45 W cm⁻². The surface temperature distribution of the fiber irradiated with the CO₂ laser was measured by using an infrared thermographic camera equipped with a magnifying lens. The relation between the laser power and the surface temperature of the fiber became clear in the laser-heating zone-drawing and the laser-heating zone-annealing. The fiber obtained finally had a birefringence of 0.239, a degree of crystallinity of 55%, a tensile modulus of 19.8 GPa, and a storage modulus of 25.7 GPa at 25°C. In FTIR measurements, a trans conformation increased with the processing, but a gauche one decreased. The laser-heating zone-drawing and zone-annealing method was found to be effective in producing the PET fiber with high modulus and high strength. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2775–2783, 2001     

Blends of poly(ethylene terephthalate) and a polyarylate before and after transesterification

Blends of poly(ethylene terephthalate) (PET) and a copolyester of bisphenol A–terephthaloylisophthaloyl (PAr) (2:1:1) have been studied both before and after transesterification. The physical blends exhibit phase separation in their amorphous states: a pure PET phase and a mixed PAr-rich phase. In spite of this phase separation, PET crystallinity in blends, normalized to PET fraction, surprisingly goes through a maximum at 25% PAr content. The transesterfied copolymers are noncrystallizable and exhibit a single T_g between those of starting polymers, PET and PAr.     

Crystallization kinetics of poly(1,4‐butylene‐co‐ethylene terephthalate)

The crystallization kinetics of poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET), and their copolymers poly(1,4‐butylene‐co‐ethylene terephthalate) (PBET) containing 70/30, 65/35 and 60/40 molar ratios of 1,4‐butanediol/ethylene glycol were investigated using differential scanning calorimetry (DSC) at crystallization temperatures (T_c) which were 35–90 °C below equilibrium melting temperature . Although these copolymers contain both monomers in high proportion, DSC data revealed for copolymer crystallization behaviour. The reason for such copolymers being able to crystallize could be due to the similar chemical structures of 1,4‐butanediol and ethylene glycol. DSC results for isothermal crystallization revealed that random copolymers had a lower degree of crystallinity and lower crystallite growth rate than those of homopolymers. DSC heating scans, after completion of isothermal crystallization, showed triple melting endotherms for all these polyesters, similar to those of other polymers as reported in the literature. The crystallization isotherms followed the Avrami equation with an exponent n of 2–2.5 for PET and 2.5–3.0 for PBT and PBETs. Analyses of the Lauritzen–Hoffman equation for DSC isothermal crystallization data revealed that PBT and PET had higher growth rate constant G_o, and nucleation constant K_g than those of PBET copolymers. © 2001 Society of Chemical Industry     

Biodegradability of poly(ethylene terephthalate) copolymers with poly(ethylene glycol)s and poly(tetramethylene glycol)

Poly(ethylene terephthalate) copolymers were prepared by melt polycondensation of dimethyl terephthalate and excess ethylene glycol with 10–40mol% (in feed) of poly(ethylene glycol) (E) and poly(tetramethylene glycol) (B), with molecular weight (MW) of E and B 200–7500 and 1000, respectively. The reduced specific viscosity of copolymers increased with increasing MW and content of polyglycol comonomer. The temperature of melting (T_m), cold crystallization and glass transition (T_g) decreased with the copolymerization. T_m depression of copolymers suggested that the E series copolymers are the block type at higher content of the comonomer. T_g was decreased below room temperature by the copolymerization, which affected the crystallinity and the density of copolymer films. Water absorption increased with increasing content of comonomer, and the increase was much higher for E1000 series films than B1000 series films. The biodegradability was estimated by weight loss of copolymer films in buffer solution with and without a lipase at 37°C. The weight loss was enhanced a little by the presence of a lipase, and increased abruptly at higher comonomer content, which was correlated to the water absorption and the concentration of ester linkages between PET and PEG segments. The weight loss of B series films was much lower than that of E series films. The abrupt increase of the weight loss by alkaline hydrolysis is almost consistent with that by biodegradation.     

Synthesis,characterization, crystalline morphologies and hydrophilicity of A4BA4 nonlinear block copolymers

Novel, well‐defined A₄BA₄ nonlinear block copolymers [poly(?‐caprolactone)]₄‐block‐poly(propylene oxide)‐block‐[poly(?‐caprolactone)]₄ (PCL₄‐b‐PPO‐b‐PCL₄) with eight arms were synthesized by ring‐opening polymerization. An investigation of melting and crystallization demonstrated that the values of crystallization temperature, melting temperature and degree of crystallinity of PCL₄‐b‐PPO‐b‐PCL₄ were enhanced with an increase of PCL chain length. At the same time, the crystallizability of PCL segments was influenced by the star‐shaped structure of the copolymers and the amorphous PPO segments in the copolymers. Furthermore, PCL₄‐b‐PPO‐b‐PCL₄ showed crystalline morphologies that were different from that of linear PCL according to polarized optical microscopy. Moreover, the hydrophilicity of the copolymers could be improved and adjusted by the star‐shaped structure and the alteration of the relative content of the PCL and PPO segments in the copolymers.© 2013 Society of Chemical Industry     

Improving oxygen barrier properties of poly(ethylene terephthalate) by incorporating isophthalate. II. Effect of crystallization

The present study examined crystallization of poly(ethylene terephthalate) (PET) and a series of random and blocky copolymers in which up to 30% of the terephthalate was replaced with isophthalate. Isothermal crystallization kinetics and direct observation of the spherulitic morphology revealed that the blocky copolymers crystallized more rapidly than PET, at least in part, as the result of enhanced spherulite nucleation. The statistical copolymers with 10 and 20% isophthalate achieved almost the same level of crystallinity as that of the blocky copolymers. The statistical copolymers with 10% isophthalate crystallized almost as fast as PET, although the statistical copolymer with 20% isophthalate crystallized much more slowly. Crystallization substantially reduced the oxygen permeability. Analysis of oxygen‐transport parameters in terms of a two‐phase structural model that considered a dispersion of lower‐permeability spherulites in an amorphous matrix of higher permeability revealed that dedensification of the PET interlamellar amorphous regions was responsible for the unexpectedly high oxygen solubility of crystallized PET. In contrast, copolymerization with isophthalate prevented dedensification of the interlamellar amorphous regions. As a result, crystallization was more effective in reducing the oxygen permeability. It was speculated that segregation of kinked isophthalate units to the amorphous regions of the spherulite relieved constraint on the interlamellar amorphous chain segments. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1629–1642, 2005     

Improving oxygen barrier properties of poly(ethylene terephthalate) by incorporating isophthalate. I. Effect of orientation

The present study examined poly(ethylene terephthalate) (PET) and a series of statistical and blocky copolymers in which up to 30% of the terephthalate was replaced with isophthalate by copolymerization and melt blending, respectively. Some level of transesterification during processing of melt blends resulted in blocky copolymers, as confirmed by NMR. Random and blocky copolymers exhibited similar properties in the glassy state, including a single glass transition, due to miscibility of the blocks. However, random copolymerization effectively retarded cold‐crystallization from the glass whereas blocky copolymers readily cold‐crystallized to a crystallinity level close to that of PET. The polymers were oriented at four temperatures in the vicinity of the T_g and characterized by oxygen transport, wide‐angle X‐ray diffraction, positron annihilation lifetime spectroscopy, and infrared spectroscopy. Orientation of all the copolymers resulted in property changes consistent with strain‐induced crystallization. However, blocky copolymers oriented more easily than random copolymers of the same composition and after orientation exhibited slightly lower oxygen permeability, higher density, and higher fraction trans conformers. Analysis of oxygen solubility based on free volume concepts led to a two‐phase model from which the amount of crystallinity and the amorphous phase density were extracted. Dedensification of the amorphous phase correlated with the draw temperature. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1615–1628, 2005     

Activity of substrates in the catalyzed nucleation of poly(ethylene terephthalate) melts

The activity, Φ of AgBr, AgI, PbF₂, Ag₂S, LiF, and CaF₂ in the catalyzed nucleation of poly(ethylene terephthalate) (PET) melts was determined using a nonisothermal differential scanning calorimetry (DSC) technique. A comparison with existing experimental data was made. It is established that the higher the melting temperature of the substrate the lower its activity as a crystallization core in the heterogeneous nucleation of PET. The lateral surface energy, σ, the end surface energy, σ_e, the adhesion energy, β, and the difference between the surface energies at the substrate/melt, σ_sf, substrate/deposit, σ^*, and the total energy of misfit dislocations, E_d [i.e., σ_sf - (σ^* - E_d)] were calculated. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 349–353, 1997     

Chain confinement in electrospun nanocomposites: using thermal analysis to investigate polymer-filler interactions

We investigate the interaction of the polymer matrix and filler in electrospun nanofibers using advanced thermal analysis methods. In particular, we study the ability of silicon dioxide nanoparticles to affect the phase structure of poly(ethylene terephthalate), PET. SiO₂ nanoparticles (either unmodified or modified with silane) ranging from 0 to 2.0 wt% in PET were electrospun from hexafluoro-2-propanol solutions. The morphologies of both the electrospun (ES) nanofibers and the SiO₂ powders were observed by scanning and transmission electron microscopy, while the amorphous or crystalline nature of the fibers was determined by real-time wide-angle X-ray scattering. The fractions of the crystal, mobile amorphous, and rigid amorphous phases of the non-woven, nanofibrous composite mats were quantified by using heat capacity measurements. The amount of the immobilized polymer layer, the rigid amorphous fraction, was obtained from the specific reversing heat capacity for both as-spun amorphous fibers and isothermally crystallized fibers. Existence of the rigid amorphous phase in the absence of crystallinity was verified in nanocomposite fibers, and two origins for confinement of the rigid amorphous fraction are proposed. Thermal analysis of electrospun fibers, including quasi-isothermal methods, provides new insights to quantitatively characterize the polymer matrix phase structure and thermal transitions, such as devitrification of the rigid amorphous fraction.     

Macromolecular design of novel sulfur‐containing copolyesters with promising mechanical properties

The miscibility of poly(butylene succinate) (PBS)/poly(butylene thiodiglycolate) (PBTDG) blends was investigated by DSC technique. PBS and PBTDG were completely immiscible in as blended‐state, as evidenced by the presence of two T_gs at ?34 and ?48°C, respectively. The miscibility changes upon mixing at elevated temperature: the original two phases merged into a single one because of transesterification reactions. Poly(butylene succinate/thiodiglycolate) block copolymers, prepared by reactive blending of the parent homopolymers, were studied to investigate the effects of transesterification reactions on the molecular structure and solid‐state properties. ¹³C‐NMR analysis evidenced the formation of copolymers whose degree of randomness increased with mixing time. Thermal characterization results showed that all the samples were semicrystalline, with a soft rubbery amorphous phase and a rigid crystal phase whose amount decreased by introducing BTDG units into the PBS chain (20 ≤ χ_c ≤ 41). Lastly, the mechanical properties were found strictly related to crystallinity degree (χ_c), the random copolymer, exhibiting the lowest elastic modulus (E = 61 MPa) and the highest deformation at break (ε_b (%) = 713). © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Melt polymerization of copoly(ethylene terephthalate–imide)s and thermal properties

Copoly(ethylene terephthalate–imide)s (PETI) were prepared by melt polycondensation of bis(2-hydroxyethyl)terephthalate (BHET) and imide containing oligomer, i.e., 4,4′-bis[(4-carbo-2-hydroxyethoxy)phthalimido]diphenylmethane(BHEI). The apparent rate of poly-condensation reaction was faster than that of homo poly(ethylene terephthalate) (PET) due to the presence of imide units. The PETI copolymers with up to 10 mol % of BHEI unit in the copolymer showed about the same molecular weight and carboxyl end group content as homo PET prepared under similar reaction conditions. The increase in T_g of copolymer was more dependent on molar substitution of BHEI than on substitution of BHEN, reaching 91°C with 8 mol % BHEI units in the copolymer from T_g = 78.9°C of homo PET. In the case of PETN copolymer, 32 mol % of bis(2-Hydroxyethyl)naphthalate (BHEN) units gave T_g of 90°C. The maximum decomposition temperature of PETI copolymer was about the same as that of homo PET by TGA analysis. The char yield at 800°C was higher than that of homo PET. © 1996 John Wiley & Sons, Inc.     

Transitions and relaxations of linear polyesters related to poly(ethylene terephthalate). II. Glassy and gamma

The β-relaxation at T_g of the terephthalate and isophthalate series is due to molecular motions of the backbone chains in the amorphous region, but differs for the isophthalate series in that the p-phenylene group does not exhibit the free rotation possible for the m-phenylene group. Consequently, the relaxation times of the terephthalate series are longer than for the isophthalate series. The γ-relaxation (T_γ) for the higher homologues of the terephthalate series cannot be explained in terms of the poly(ethylene terephthalate) analogy. For poly(ethylene terephthalate) and poly(tetramethylene terephthalate), an induced cooperative type of motion of all the moieties is possible, whereby overlapping processes caused by “rocking vibrations” are observed as one γ-peak. The resolution of the γ-loss peaks for the above-mentioned polyesters into components is not possible at the experimental frequency of 110 Hz. For poly(hexamethylene terephthalate) and poly(decamethylene terephthalate), the “rocking vibrations” between the moieties of the skeletal chain are reduced so that even at a test frequency of 110 Hz, the γ-loss peaks could be resolved into two or three components. In the case of poly(decamethylene terephthalate), three components are resolved; the lowest temperature peak γ₁ is attributed to hindered motions of the methylene portions, the γ₂ peak is attributed to motions of the carbonyl group in the gauche conformation, and the γ₃ peak is attributed to the carbonyl group in the trans conformation of the skeletal chain in the amorphous region. The general observations obtained by other techniques were confirmed by the forced vibration analyses. As the length of the methylene chain increased, T_g decreased. As crystallinity increased, the β-relaxation moved to higher temperatures and the damping peak was smaller and broader. The damping peak moved to lower temperatures and increased in size as the length of the methylene chain increased. The damping peak was larger for the isophthalate homologue than for the corresponding terephthalate polyester.     

Polyester-Polyether Block Copolymers II. Thermal and Mechanical Properties of Poly(hexamethylene terephthalate)/Poly(oxytetramethylene) Block Copolymers

The melting and crystallisation behaviour of crystalline phases in poly (hexamethylene terephthalate)/poly(oxytetramethylene) block copolymers have been investigated in relation to copolymer composition and polyether block molecular weight (m.w.). In contrast to that in corresponding homopolymer blends, the polyester crystallinity in the block polymers is greatly reduced by incorporation of polyether units, though some persists even at low polyester contents. Concomitant changes in the glass transition temperatures show part of the polyester component to form a homogeneous component of the amorphous phase. The mechanical properties change with composition in parallel with the changes in copolymer crystallinity and T_g. Copolymers with 20-60 w % of poly(oxytetramethylene) units of m.w. 2000 are highly extensible elastomers. Those with higher m. w. polyether blocks have higher modulus and strength but suffer a serious loss of properties at 60d?C. The observations are interpreted in terms of a model in which polyester crystallites (and polyether crystallites also, for the higher m. w. polyether blocks) are supported within an amorphous matrix by tie-molecules whose nature changes with the copolymer compositions. The results are compared with those for analogous polyester-polyethers having different structural components.     

Effect of nano ribbons formed by the amide segment on the solvent resistivity of segmented block copolymers based on polystyrene

A segmented block copolymer is synthesized using dihydroxy terminated polystyrene (PSt) (M_W = 2,500 g/mol) and crystallizable amide segments. PSt length in the copolymer is varied from 2,500 to 10,000 g/mol using dimethylterephthalate (T). Amide segment is synthesized in situ using diamine‐diamide 6X6 (X = A or T) (synthesized by dimethylterepthalate [T], adipic acid [A], and hexamethylenediamine [6]) and T. This work is to modify the high T_g amorphous PSt to a semicrystalline copolymers (‐(PSt‐T)_y‐6X6‐T‐)‐_n). These copolymers have a very high inherent viscosity and depending on the amide concentration, the melting temperature of the polymers was ranged between 129°C and 248°C. The crystallinity of the amide segments is up to 75%. The AFM analysis showed the presence of crystalline ribbons with a high aspect ratio. All the polymers show single stage decomposition temperature centered around 420°C. The solvent resistivity of these materials is very high even at a low concentration of (5 wt%) amide content. POLYM. ENG. SCI., 58:361–368, 2018. © 2017 Society of Plastics Engineers     

Segmented copolymers of uniform tetra‐amide units and poly(phenylene oxide) by direct coupling

Segmented copolymers with telechelic poly(2,6‐dimethyl‐1,4‐phenylene ether) (PPE) segments and crystallizable bisester tetra‐amide units (two‐and‐a‐half repeating unit of nylon‐6,T) were studied. The copolymers were synthesized by reacting bifunctional PPE with hydroxylic end groups with an average molecular weight of 3500 g/mol and bisester tetra‐amide units via an ester polycondensation reaction. The bisester tetra‐amide units had phenolic ester groups. By replacing part of the bisester tetra‐amide units with diphenyl terephthalate units (DPT), the concentration of tetra‐amide units in the copolymer was varied from 0 to 11 wt%. Polymers were also prepared from bifunctional PPE, DPT, and a diaminediamide (6T6‐diamine). The thermal and thermal mechanical properties were studied by DSC and DMA and compared with a copolymer with flexible spacer groups between the PPE and the T6T6T. The copolymers had a high T_g of 180–200°C and a melting temperature that increased with amide content of 220–265°C. The melting temperature was sharp with monodisperse amide segments. The T_m – T_c was 39°C, which suggests a fast, but not very fast, crystallization. The crystallinity of the amide was ～ 20%. The copolymers are semicrystalline materials with a high T_g and a high T_g/T_m ratio (> 0.8). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 512–518, 2007     

Segmented copolymers of uniform tetra-amide units and poly(phenylene oxide). Part 4. Influence of the extender

Copolymers of telechelic poly(2,6-dimethyl-1,4-phenylene ether) (PPE) segments, uniform crystallisable tetra-amide units (T6T6T, 6-15 wt%) and different diols (C2-C36, polytetramethylene oxide) as an extender were synthesised. The telechelic PPE segment was end-functionalised with terephthalic ester groups and had a molecular weight of 3100 g/mol. The coupling between the PPE segment and the T6T6T unit was made with diols. The influence of the length and flexibility of the diol-extender and the concentration of the T6T6T units were studied on the thermal (DSC) and thermal-mechanical (DMA) properties of the copolymers. A crystalline T6T6T phase in the copolymers was evident from 9 wt% onwards. The length of diol extender had an effect on the glass transition temperature of the PPE phase, the crystallinity of the T6T6T segments and modulus above the glass transition temperature. With ethylene glycol the T_g of the copolymer was high but the crystallinity of the T6T6T rather low. With dodecanediol or hexanediol as an extender the T_gs of the PPE phase were somewhat lower, but the crystallinities of the T6T6T segments higher. With C36 and polytetramethylene oxide diols, the T_g were strongly decreased and broad and the modulus above the glass transition temperature not so high.     

Compatibilization effects of styrenic/rubber block copolymers in polypropylene/polystyrene blends

Compatibilizing effects of styrene/rubber block copolymers poly(styrene‐b‐butadiene‐b‐styrene) (SBS), poly(styrene‐b‐ethylene‐co‐propylene) (SEP), and two types of poly(styrene‐b‐ethylene‐co‐butylene‐b‐styrene) (SEBS), which differ in their molecular weights on morphology and selected mechanical properties of immiscible polypropylene/polystyrene (PP/PS) 70/30 blend were investigated. Three different concentrations of styrene/rubber block copolymers were used (2.5, 5, and 10 wt %). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the phase morphology of blends. The SEM analysis revealed that the size of the dispersed particles decreases as the content of the compatibilizer increases. Reduction of the dispersed particles sizes of blends compatibilized with SEP, SBS, and low‐molecular weight SEBS agrees well with the theoretical predictions based on interaction energy densities determined by the binary interaction model of Paul and Barlow. The SEM analysis confirmed improved interfacial adhesion between matrix and dispersed phase. The TEM micrographs showed that SBS, SEP, and low‐molecular weight SEBS enveloped and joined pure PS particles into complex dispersed aggregates. Bimodal particle size distribution was observed in the case of SEP and low‐molecular weight SEBS addition. Notched impact strength (a_k), elongation at yield (ε_y), and Young's modulus (E) were measured as a function of weight percent of different types of styrene/rubber block copolymers. The a_k and ε_y were improved whereas E gradually decreased with increasing amount of the compatibilizer. The a_k was improved significantly by the addition of SEP. It was found that the compatibilizing efficiency of block copolymer used is strongly dependent on the chemical structure of rubber block, molecular weight of block copolymer molecule, and its concentration. The SEP diblock copolymer proved to be a superior compatibilizer over SBS and SEBS triblock copolymers. Low‐molecular weight SEBS appeared to be a more efficient compatibilizer in PP/PS blend than high‐molecular weight SEBS. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 291–307, 1999     

Electrospun fiber membranes of novel thermoplastic polyester elastomers: Preparation and characterization

Electrospinning nanotechnology has recently attracted lots of attention in different kinds of applications. Poly(butylene terephthalate) random‐segment copolymers, named poly[(butylene terephthalate)‐co‐(1,4‐cyclohexanedimethanol terephthalate)]‐b‐poly(tetramethylene glycol) (P(BT‐co‐CT)‐b‐PTMG), were synthesized in this study. On the basis of the new thermoplastic polyester elastomers (TPEEs), the fiber membranes were subsequently electrospun. With the aid of a cosolvent of trifluoroacetic acid and dichloromethane, the resulting solutions with a concentration between 24 and 32% w/v were electrospun into fibers without beads. The results also show a good spinnability for the copolymer solution in a range of voltages from 16 to 24 kV. When the molar ratio of 1,4‐cyclohexanedimethanol to 1,4‐butanediol was 10 : 90, the electrospun membrane prepared by the corresponding copolymers had a higher elastic modulus than the commercial TPEE (Hytrel 4056, 4.51 ± 0.35 MPa). Differential scanning calorimetry and X‐ray diffraction showed that a crystalline phase existed in the electrospun poly[(butylene terephthalate)‐co‐(1,4‐cyclohexanedimethanol terephthalate)]‐b‐poly(tetramethylene glycol) (P(BT‐co‐CT)‐b‐PTMG) copolymer fiber membranes. The melting point of the electrospun fibers was approximately less than that of the corresponding copolymers © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Segmented copolymers of poly(phenylene ether) and polyester

Copolymers of telechelic poly(2,6-dimethyl-1,4-phenylene ether) (PPE) segments with terephthalic methyl ester end groups (PPE-2T, 3500 g/mol) and poly(dodecane terephthalate) (PDDT) were made via a polycondensation reaction in the melt. The inherent viscosities of the segmented copolymers were high. The thermal properties of the copolymers were studied by DMA. The segmented block copolymers had a transparent melt at low (12 wt%) PDDT contents. The segmented block copolymers had at higher PDDT contents a non-transparent melt and two glass transition temperatures. The glass transition temperature of the PPE phase decreased strongly with PDDT content in the copolymer. The glass transition temperature of the PDDT phase increased moderately with PPE content. At low PPE contents the modulus of the PDDT increased strongly with increasing PPE content.     

Effects of water sorption on the physical properties of PET,PBT, and their long fibers composites

The behavior of poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) on water aging has been studied above and below the glass transition temperature (T_g). The aging process is caused by: degradation of the matrix and an increase in crystallinity above T_g, and microcavitation at the amorphous/crystalline interface below T_g. Such behavior well explains the deviation of the sorption kinetics from the Fickian model. The apparent water diffusion coefficients and the transport activation energies of PET and PBT have been calculated at temperatures above and below T_g. The mechanical behavior of the two polymers on water aging has been measured by means of fracture mechanics and Izod impact tests at different stress concentration factors. An increase of toughness of PET at short aging times has been shown by mechanical tests and SEM analysis fracture surfaces of differently aged samples. Izod tests of PET and PBT composites reinforced by long glass fibers have shown the contribution of fibers to the total fracture energy.