共查询到20条相似文献,搜索用时 31 毫秒
1.
Christopher G. Robertson Jeremy E. Monat Garth L. Wilkes 《Journal of Polymer Science.Polymer Physics》1999,37(15):1931-1946
The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐Tg annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to Tg, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999 相似文献
2.
Hilmar Koerner Elizabeth Opsitnick Christopher A. Grabowski Larry F. Drummy Ming-Siao Hsiao Justin Che Megan Pike Vernecia Person Michael R. Bockstaller Jeff S. Meth Richard A. Vaia 《Journal of polymer science. Part A, Polymer chemistry》2016,54(2):319-330
Matrix free assemblies of polymer-grafted, “hairy” nanoparticles (aHNP) exhibit novel morphology, dielectric, and mechanical properties, as well as providing means to overcome dispersion challenges ubiquitous to conventional polymer-inorganic nanocomposite blends. Physical aging of the amorphous polymer glass between the close-packed nanoparticles (NPs) will dominate long-term stability; however, the energetics of volume recovery within the aHNPs is unknown. Herein, we compare glass transition temperature (Tg) and enthalpy recovery of aHNPs to NP-polymer blends, across different nano-silica loadings (0–50 v/v%) and canopy architecture of polystyrene (PS) grafted silica. For aHNPs, the grafting of PS to silica imposes an additional design constraint between silica volume fraction, graft density, and graft molecular weight. At low and intermediate silica volume fraction, the Tg of blended nanocomposites is independent of silica content, reflecting a neutral polymer-NP interface. For aHNPs, the Tg decreases with silica content, implying that chain tethering decreases local segment density more than the effect of molecular weight or polymer-NP interactions. Additionally, the Tg of the aHNPs is higher than a linear matrix of comparable molecular weight, implying a complementary effect to local segment density that constrains cooperativity. In contrast, enthalpy recovery rate in the blend or aHNP glass is retarded comparably. In addition, a cross-over temperature, Tx, emerges deep within the glass where the enthalpy recovery process of all nanocomposites becomes similar to linear unfilled matrices. Differences between structural recovery in aHNP and blended nanocomposites occur only at the highest silica loadings (∼ 50 v/v%), where enthalpy recovery for aHNPs is substantially suppressed relative to the blended counterparts. The absence of physical aging at these loadings is independent of brush architecture (graft density or molecular weight of tethered chains) and indicates that the impact of chain tethering on effective bulk structural relaxation starts to appear at particle-particle surface separations on the order of the Kuhn length. Overall, these observations can be understood within the context of how three separate structural characteristics impact local segment density and relaxation processes: the dimension and architecture of the tethered polymer chains, the separation between NP surfaces, and the confinement imposed by chain tethering and space filling within the aHNP. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 319–330 相似文献
3.
Yung P. Koh Sindee L. Simon 《Journal of polymer science. Part A, Polymer chemistry》2008,46(24):2741-2753
The Tg depression and kinetic behavior of stacked polystyrene ultrathin films is investigated by differential scanning calorimetry (DSC) and compared with the behavior of bulk polystyrene. The fictive temperature (Tf) was measured as a function of cooling rate and as a function of aging time for aging temperatures below the nominal glass transition temperature (Tg). The stacked ultrathin films show enthalpy overshoots in DSC heating scans which are reduced in height but occur over a broader temperature range relative to the bulk response for a given change in fictive temperature. The cooling rate dependence of the limiting fictive temperature, Tf′, is also found to be higher for the stacked ultrathin film samples; the result is that the magnitude of the Tg depression between the ultrathin film sample and the bulk is inversely related to the cooling rate. We also find that the rate of physical aging of the stacked ultrathin films is comparable with the bulk when aging is performed at the same distance from Tg; however, when conducted at the same aging temperature, the ultrathin film samples show accelerated physical aging, that is, a shorter time is required to reach equilibrium for the thin films due to their depressed Tg values. The smaller distance from Tg also results in a reduced logarithmic aging rate for the thin films compared with the bulk, although this is not indicative of longer relaxation times. The DSC heating curves obtained as a function of cooling rate and aging history are modeled using the Tool-Narayanaswamy-Moynihan model of structural recovery; the stacked ultrathin film samples show lower β values than the bulk, consistent with a broader distribution of relaxation times. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2741–2753, 2008 相似文献
4.
Summary Volume and enthalpy relaxation in polycarbonate subjected to double temperature jumps in the Tg region has been analysed. It concerns both initial Tdown-jump from equilibrium above Tg to consolidation temperature below Tg and fina1 Tup-jump to relaxation temperature, also below Tg. The measured H and V data after Tup-jump were compared with respect to aging time calculating (dH/dV) ratio denoted as aging bulk modulus, Ka. According this new methodology H and V relaxation response after Tup-jump demonstrates differences in relaxation responses. 相似文献
5.
Exposure to extended periods of sub‐Tg temperatures causes physical changes in the molecular structure of epoxy resins and epoxy‐based materials to occur. These physical aging mechanisms include the reduction in free volume and changes to the molecular configuration. As a result, mechanical, thermodynamical, and physical properties are affected in ways that can compromise the reliability of epoxy‐based engineering components and structures. In this review, the physical changes in the molecular structure of epoxies are described, and the influence of these changes on the bulk‐level response is detailed. Specifically, the influence of physical aging on the quasistatic mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, volume relaxation, enthalpy relaxation, endothermic peak temperature, fictive temperature, and moisture/solvent absorption capability is reviewed. Also discussed are relationships between relaxation functions, crosslink density, composite reinforcement, and epoxy/copolymer blending and the physical aging response of epoxies. Finally, the concepts of thermal and mechanical rejuvenation are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
6.
The physical aging process of 4,4′-diaminodiphenylsulfone (DDS) cured diglycidyl ether bisphenol-A (DGEBA) blended with poly(ether sulfone) (PES) was studied by differential scanning calorimetry (DSC) at four aging temperatures between Tg-50°C and Tg-10°C. At aging temperatures between Tg-50 and Tg-30°C, the experimental results of epoxy resin blended with 20 wt% of PES showed two enthalpy relaxation processes. One relaxation process was due to the physical aging of PES, the other relaxation process was due to the physical aging of epoxy resin. The distribution of enthalpy relaxation process due to physical aging of epoxy resin in the blend was broader and the characteristic relaxation time shorter than those of pure epoxy resin at the above aging temperatures (between Tg-50 and Tg-30°C). At an aging temperature between Tg-30 and Tg-10°C, only one enthalpy relaxation process was found for the epoxy resin blended with PES, the relaxation process was similar to that of pure epoxy resin. The enthalpy relaxation process due to the physical aging of PES in the epoxy matrix was similar to that of pure PES at aging temperatures between Tg-50 and Tg-10°C. © 1997 John Wiley & Sons, Inc. 相似文献
7.
Z. Y. Tan S. L. Sun X. F. Xu C. Zhou Y. H. Ao H. X. Zhang 《Journal of Polymer Science.Polymer Physics》2005,43(19):2715-2724
Polycarbonate (PC) was blended with acrylic impact modifiers (AIMs). The effects of modifiers weight fraction on the Izod impact strength and yield strength of PC/AIM blends were investigated. The samples with 4% modifiers were aged under the Tg of PC in an air‐circulating oven, and the effects of aging time on impact strength, yield strength, modulus, elongation at break, post yield stress drop (PYSD) values, and morphology of fracture surface were investigated. The effects of aging time on the shape of stress–strain curve were also investigated. The aged samples were heat‐treated over the Tg of PC to erase the effects of physical aging. It was found that the drop of impact strength caused by physical aging can be recovered, the increment of yield strength and PYSD value caused by physical aging can only be partly recovered, and the heat‐treatment over the Tg of PC caused further increment of modulus. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2715–2724, 2005 相似文献
8.
9.
S. Montserrat 《Journal of Polymer Science.Polymer Physics》1994,32(3):509-522
The physical aging of an epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of Tg-10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τeff. The rate of relaxation of the system given by 1/τeff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = ?333, EH = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub-Tg peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc. 相似文献
10.
Perla Rittigstein John M. Torkelson 《Journal of polymer science. Part A, Polymer chemistry》2006,44(20):2935-2943
The effects of confinement on glass transition temperature (Tg) and physical aging are measured in polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinyl pyridine) (P2VP) nanocomposites containing 10- to 15-nm-diameter silica nanospheres or 47-nm-diameter alumina nanospheres. Nanocomposites are made by spin coating films from sonicated solutions of polymer, nanofiller, and dye. The Tgs and physical aging rates are measured by fluorescence of trace levels of dye in the films. At 0.1–10 vol % nanofiller, Tg values can be enhanced or depressed relative to neat, bulk Tg (Tg,bulk) or invariant with nanofiller content. For alumina nanocomposites, Tg increases relative to Tg,bulk by as much as 16 K in P2VP, decreases by as much as 5 K in PMMA, and is invariant in PS. By analogy with thin polymer films, these results are explained by wetted P2VP–nanofiller interfaces with attractive interactions, nonwetted PMMA–nanofiller interfaces (free space at the interface), and wetted PS–nanofiller interfaces lacking attractive interactions, respectively. The presence of wetted or nonwetted interfaces is controlled by choice of solvent. For example, 0.1–0.6 vol % silica/PMMA nanocomposites exhibit Tg enhancements as large as 5 K or Tg reductions as large as 17 K relative to Tg,bulk when films are made from methyl ethyl ketone or acetic acid solutions, respectively. A factor of 17 reduction of physical aging rate relative to that of neat, bulk P2VP is demonstrated in a 4 vol % alumina/P2VP nanocomposite. This suggests that a strategy for achieving nonequilibrium, glassy polymeric systems that are stable or nearly stable to physical aging is to incorporate well-dispersed nanoparticles possessing attractive interfacial interactions with the polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2935–2943, 2006 相似文献
11.
Broadband dielectric spectroscopy was used to study the segmental (α) and secondary (β) relaxations in hydrogen‐bonded poly(4‐vinylphenol)/poly(methyl methacrylate) (PVPh/PMMA) blends with PVPh concentrations of 20–80% and at temperatures from ?30 to approximately glass‐transition temperature (Tg) + 80 °C. Miscible blends were obtained by solution casting from methyl ethyl ketone solution, as confirmed by single differential scanning calorimetry Tg and single segmental relaxation process for each blend. The β relaxation of PMMA maintains similar characteristics in blends with PVPh, compared with neat PMMA. Its relaxation time and activation energy are nearly the same in all blends. Furthermore, the dielectric relaxation strength of PMMA β process in the blends is proportional to the concentration of PMMA, suggesting that blending and intermolecular hydrogen bonding do not modify the local intramolecular motion. The α process, however, represents the segmental motions of both components and becomes slower with increasing PVPh concentration because of the higher Tg. This leads to well‐defined α and β relaxations in the blends above the corresponding Tg, which cannot be reliably resolved in neat PMMA without ambiguous curve deconvolution. The PMMA β process still follows an Arrhenius temperature dependence above Tg, but with an activation energy larger than that observed below Tg because of increased relaxation amplitude. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3405–3415, 2004 相似文献
12.
Summary The present work is an extension of an earlier study that compared the stress relaxation between two molecular masses of a bisphenol-A polycarbonate due to thermal aging. The enthalpy relaxation of the same materials has been characterized. First, by measuring the change in enthalpy loss (ΔHa) and fictive temperature (Tf) as a function of aging temperature (Ta) ranging from -25 to 120°C, using differential scanning calorimetry. For the limited aging time of 120 h, ΔHa and Tf changes were only appreciable for (Tg -70 K)<Ta<Tg . While the influence of molecular mass was somewhat discernible, enthalpy measurements were not as sensitive as stress relaxation tests in differentiating molecular mass effects. In a second investigation, the kinetics of enthalpy relaxation upon isothermal aging at 130°C was evaluated using the peak shift method and found to be comparable to literature values. The plot of ΔHa as a function of log (aging time) showed two distinct regions: a brief non-linear portion (less than 1 h aging) which is followed by a linear relationship as typically reported in the literature. In contrast to the linear region, the non-linear relaxation behaviour of the poorly aged state does not appear to be dependent on molecular mass. 相似文献
13.
J. J. Fontanella D. A. Boyles T. S. Filipova S. Awwad C. A. Edmondson J. T. Bendler M. C. Wintersgill J. F. Lomax M. J. Schroeder 《Journal of Polymer Science.Polymer Physics》2012,50(4):289-304
The relative permittivity, loss, and breakdown strength are reported for a commercial sample of bisphenol A‐polycarbonate (comm‐BPA‐PC) and a purified sample of the same polymer (rp‐BPA‐PC) as well as for two new polycarbonates having low molecular cross‐sectional areas, namely a copolymer of tetraaryl polycarbonate and BPA‐PC (TABPA‐BPA‐PC) and a triaryl polycarbonate homopolymer (TriBPA‐PC). The glass transition temperatures of the new polymers are higher than the Tg of BPA‐PC (187 and 191 °C vs. 148 °C). Relative permittivity and loss measurements were carried out from 10 to 105 Hz over a wide temperature range, and results for the α‐ and γ‐relaxation regions are discussed in detail. For the α‐relaxation, the isochronal peak position, Tα, scales approximately with Tg. On the other hand, the peak temperature for the γ‐relaxation is approximately constant, independent of Tg. Also, in contrast to what is observed for α, γ exhibits a strong increase in peak height as temperature/frequency increases and a significant difference is found between Arrhenius plots determined from isochronal and isothermal data analyses. Next, the γ‐relaxation region for comm‐BPA‐PC and associated activation parameters show strong history/purity effects. The activation parameters also depend on the method of data analysis. The results shed light on discrepancies that exist in the literature for BPA‐PC. The shapes of the γ loss peaks and hence glassy‐state motions for all the polymers are very similar. However, the intensities of the TriBPA‐PC and TABPA‐BPA‐PC γ peaks are reduced by an amount that closely matches the reduced volume fraction of carbonate units in the two new polymers. Finally, for comm‐BPA‐PC, the breakdown strength is strongly affected by sample history and this is assumed to be related to volatile components in the material. It is found that the breakdown strengths for TriBPA‐PC and TABPA‐BPA‐PC are relatively close to that for rp‐BPA‐PC with the value for TriBPA‐PC being slightly larger than that for rp‐BPA‐PC or the value usually reported for typical capacitor grade polycarbonate. Finally, it is shown that the real part of the relative permittivity remains relatively constant from low temperatures to Tg. Consequently, based on the dielectric properties, TriBPA‐PC and TABPA‐BPA‐PC should be usable in capacitors to at least 50 °C higher than BPA‐PC. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
14.
Mojun Liu Yifu Ding Minghai Wang Shanjun Li Weihua Liu Bo Wang 《Journal of Polymer Science.Polymer Physics》2003,41(11):1135-1142
Novolac epoxy resins cured with novolac resin, novolac acetate resin, novolac butyrate resin, and novolac phenylacetate resin named as EP, EPA, EPB, and EPP, respectively, were prepared. Their physical aging behavior at a Tg‐30 °C (30 °C below glass‐transition temperature) was examined by positron annihilation lifetime spectroscopy and differential scanning calorimetry. The ortho‐positronium annihilation lifetime τ3 variation extent of EP is less apparent than that of the other three esterified samples during physical aging. The time dependence of ops intensity I3 agreed with the Kohlrausch‐Williams‐Watts (KWW) equation. The relaxation time (τ0) and nonexponential parameter were calculated. The free volume and enthalpy relaxation rate characterized by the reciprocal of τ0 and ?ΔH/?logt, respectively, exhibit the same order—EPP > EPB > EPA > EP. These results suggest that the extend and rate of relaxation are not only related to the frozen free volume produced by quenching but also significantly influenced by segmental mobility of the network that attributed to the side‐group flexibility and their interaction with networks. This work also supports the fact that side‐group flexibility and the free‐volume fraction and distribution act in concert to control the water‐diffusion behavior in epoxy networks. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1135–1142, 2003 相似文献
15.
Above,below, and in‐between the two glass transitions of ultrathin free‐standing polystyrene films: Thermal expansion coefficient and physical aging
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In previous work we observed two simultaneous transitions in high molecular weight (MW) free‐standing polystyrene films that were interpreted as two thickness‐dependent reduced glass transition temperatures (Tgs). The weaker lower transition agreed well with the MW‐dependent Tg(h) previously reported, while the much stronger upper transition matched the MW‐independent Tg(h) previously observed in low‐MW free‐standing films. Here, we investigate the nature of these two transitions by inspecting the temperature dependence of the films' thermal coefficient of expansion (TCE) and present physical aging measurements using ellipsometry both below and in‐between the two transitions. TCE values indicate approximately 80 to 90% of the film solidifies at the upper transition, while only 10 to 20% remains mobile to lower temperatures, freezing out at the lower transition. Physical aging is observed at a temperature below the upper transition, but above the lower transition, indicative of the upper transition being an actual glass transition associated with the α‐relaxation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 64–75 相似文献
16.
Eun‐Woong Lee Grigori A. Medvedev James M. Caruthers 《Journal of Polymer Science.Polymer Physics》2010,48(22):2399-2401
The stress relaxation response in the glassy state just below Tg was measured for poly(methylmethacrylate) following application of constant strain rate uniaxial tensile deformation at various locations on the stress–strain curve, including the yield and post‐yield region. The macroscopic mobility was determined from analysis of the relaxation response. Up to a factor of 3 decrease in relaxation time was observed with the fastest relaxation occurring in the post‐yield softening region. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010 相似文献
17.
Osamu Araki Masayasu Horie Toshiro Masuda 《Journal of Polymer Science.Polymer Physics》2001,39(3):337-341
The physical aging of polycarbonate was investigated with dynamic viscoelastic measurements. Physical aging was observed for samples aged at 110 °C (QA) and room temperature (QP) after being quenched from the molten state. The shapes of the temperature dispersion curves of the dynamic viscoelastic functions (E′, E″, and tan δ) of the QA and QP samples changed with aging time in a temperature range below the glass‐transition temperature (Tg). However, at temperatures close to but below Tg, the curves for the aged samples merged into the curve of the quenched sample at a temperature denoted TH. TH increased with aging time. The experimental results suggest that the aged sample has a memory of having been quenched and that as the sample approaches the equilibrium state, this memory is lost. Differential scanning calorimetry thermograms showed an endothermic peak below Tg for the QA samples. The peak temperature (Tp) also increased with aging time. TH and Tp of the QA samples were approximately the same. The increase of both TH and Tp with aging time indicates that the structure of the polymeric chain in the glassy state relaxes over larger segment scale lengths because the scale of the movable segments is related to temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 337–341, 2001 相似文献
18.
Yunil Hwang T. Inoue Paul A. Wagner M. D. Ediger 《Journal of Polymer Science.Polymer Physics》2000,38(1):68-79
A photobleaching method has been used to observe the reorientation of tetracene and rubrene in polystyrene during physical aging. Rotation times change more than an order of magnitude during isothermal aging after a temperature quench from above Tg. Down‐ and up‐jumps of the temperature show the expected asymmetry due to the nonlinearity of the aging process. The rotation times of tetracene and rubrene require the same amount of time to reach equilibrium after a temperature change (103 − 105 s in the range 93–99 °C). These equilibration times are the same order of magnitude as equilibration times for volume and enthalpy relaxation, but have a somewhat weaker temperature dependence. Very near equilibrium, the rates of aging are different for the two probes, with rubrene approaching equilibrium more rapidly at very long times. This may be understood if the aging process is spatially heterogeneous, that is, if aging occurs more rapidly in some small regions of the sample than in others. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 68–79, 2000 相似文献
19.
K. Pathmanathan Jean-yves Cavaill G. P. Johari 《Journal of Polymer Science.Polymer Physics》1989,27(7):1519-1527
The dielectric permittivity and loss of Bisphenol-A-polycarbonate (PC) was measured over the frequency range 100 Hz to 200 kHz and temperature range 77–383 K. One sub-Tg relaxation peak is observed which rapidly broadens with a decrease in temperature. This is attributed to a progressive separation of the γ and β peaks, which at high temperatures are merged to form one peak of high strength. The strength of the sub-Tg relaxations decreases on physical aging of PC but is increased if the sample is quenched from a temperature above its Tg. Slowly cooled PC has a lower strength of its sub-Tg relaxation than a quenched specimen. The thermal history of PC affects the magnitude of its sub-Tg relaxation. 相似文献
20.
The enthalpy relaxation of a series of linear amorphous polyesters (poly(propylene isophthalate) (PPIP), poly(propylene terephthalate) (PPTP), poly(ethylene terephthalate) (PETP), and poly(dipropylene terephthalate) (PDPT)) has been investigated by differential scanning calorimetry (DSC). These polyesters have been annealed at equal undercooling below their respective glass transition temperatures, Tg, (Tg − 27°C, Tg − 15°C, and Tg − 9°C) for periods of time from 15 min to 480 h. The key parameters of structural relaxation, namely the apparent activation energy (Δh*), the nonlinearity parameter (x) and the nonexponentiality parameter (β), have been determined for each polyester and related to an effective relaxation rate (1/τeff) and to the chemical structure. We observe that the variation of the structural relaxation parameters shows a trend that is common to other polymeric systems, whereby an increase of x and β corresponds a decrease in Δh*. The comparison of these parameters in PETP and in PPTP gives information about the effect of the introduction of a methyl group pendant from the main chain; the x parameter increases (i.e., a reduced contribution of the structure to the relaxation times), β increases (i.e., a narrow distribution of relaxation times), and Δh* decreases. Additionally, enthalpy relaxation experiments show that a decrease of Δh* correlates with an increase of 1/τeff, when they are measured at a fixed value of the excess enthalpy, δH. The introduction of an isopropyl ether group in PDPT with respect to PPTP decreases both x and β, but increases Δh*, which the rate of relaxation decreases. The ring substitution in PPTP and PPIP originates less significant changes in the structural parameters. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 113–126, 1998 相似文献