Aging studies on flame retardant treated lignocellulosic fibers

The study deals with chemical and flame retardant (FR) treatment of flax fabric. Sheets of flax fabric were subjected to chemical treatments using NaOH and silane coupling agents. A phosphate‐based flame retardant (DAP) was also applied to improve the flammability of the fabric. The effects of the chemical treatments and FR treatments on flax fabric were investigated using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and vertical flame resistance test. Aging studies were carried out by exposing the samples in an environmental chamber at specified conditions for two weeks. The mechanical properties of the fabric, before and after environmental aging, were investigated. Flammability of flax fabric was improved after FR treatment. Thermal studies revealed a shift of decomposition temperature to lower temperatures and an increase in char residue after FR treatment. Despite treatment of the fabric with NaOH and silane, the tensile strength of FR‐treated flax fabric declined by more than 90% after aging for two weeks at 90 °C and 50% RH. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44175.     

Development and characterization of flax fiber reinforced biocomposite using flaxseed oil‐based bio‐resin

In this study, acrylated epoxidized flaxseed oil (AEFO) resin is synthesized from flaxseed oil, and flax fiber reinforced AEFO biocomposites is produced via a vacuum‐assisted resin transfer molding technique. Different amounts of flax fiber and styrene are added to the resin to improve its mechanical and physical properties. Both flax fiber and styrene improve the mechanical properties of these biocomposites, but the flexural strength decreases with an increase in styrene content. The mass increase during water absorption testing is less than 1.5% (w/w) for all of the AEFO‐based biocomposites. The density of the AEFO resin is 1.166 g/cm³, which increases to 1.191 g/cm³ when reinforced with 10% (w/w) flax fiber. The flax fiber reinforced AEFO‐based biocomposites have a maximum tensile strength of 31.4 ± 1.2 MPa and Young's modulus of 520 ± 31 MPa. These biocomposites also have a maximum flexural strength of 64.5 ± 2.3 MPa and a flexural modulus of 2.98 ± 0.12 GPa. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41807.     

Influence of alkali treatment on flax fiber for use as reinforcements in polylactide stereocomplex composites

The flax and equivalent proportion of poly(l ‐lactic acid)/poly(d ‐lactic acid) (PLLA/PDLA) were melt compounded and injection molded to prepare flax‐reinforced polylactide stereocomplex (sc‐PLA) bio‐composite, and the effect of alkali treatment on the structure and properties of flax as well as the flax/sc‐PLA composite was investigated. SEM and FTIR results showed hemicellulose in flax was almost completely removed after alkali treatment and the treated flax (ALK‐flax) bundles were more separated with a cleaner surface than untreated flax (UN‐flax). DSC results showed homo‐crystallites (hc, T_m = 160–170°C) and stereocomplex crystallites (sc, T_m ～210°C) coexisted in sc‐PLA and flax/sc‐PLA composites. Compared with sc‐PLA, the total crystallinity and sc‐crystallinity of flax/sc‐PLA composite increased regardless of whether the flax were treated with alkali, whereas ALK‐flax/sc‐PLA composite showed a little higher crystallinity than UN‐flax/sc‐PLA composite. TGA results confirmed ALK‐flax/sc‐PLA composite had a higher thermal degradation temperature than UN‐flax/sc‐PLA composite. The mechanical tests indicated although the mechanical properties of sc‐PLA increased significantly by reinforcing with flax, the ALK‐flax/sc‐PLA composite showed little lower mechanical properties than UN‐flax/sc‐PLA composite. The alkali treatment of flax had no obvious influence on the Vicat softening temperature (VST) of flax/sc‐PLA composites, a higher heat resistance with VST at ～155°C could be obtained for flax/sc‐PLA composite. POLYM. ENG. SCI., 55:2553–2558, 2015. © 2015 Society of Plastics Engineers     

Effect of isothermal aging on the imidization of PMR-15

The imidization of polymerizable reactive mixtures, PMR-15 has been performed in a vacuum oven at isothermal aging temperatures ranging from 65 to 200°C for aging periods of 0.5 to 2.5 h. The weight loss of the resin and chemical changes that occurred as a result of aging were monitored gravimetrically and by FT-IR spectroscopy. Differential scanning calorimetry was used to determine the temperature at which imidization took place. Imidization was observed to commence at 65°C after long aging times, t ≥ 2.5 h and at ∼95°C at a shorter time, t ∼0.5 h. At higher aging temperatures of 135 to 165°C, extensive imidization occurred. This was shown by the dramatic increase in imide absorption bands at 1780 and 1380 cm⁻¹. Beyond 165°C, there were no significant changes in the imide absorption bands, suggesting that imidization was nearly complete. The activation energy for isothermal aging was determined from the slope of the log of the rate of weight loss vs 1/T curve to be ∼4.5 kJ/mol and is lower than the average activation energy for imidization ∼43 kJ/mol obtained from the plot of the log of the rate of increase of the imide carbonyl peak absorption at 1780 cm⁻¹ vs 1/T.     

Microstructure evolution of ammonia‐catalyzed phenolic resin during thermooxidative aging

The thermooxidative aging of ammonia‐catalyzed phenolic resin for 30 days at 60–170°C was investigated in this article. The aging mechanism and thermal properties of the phenolic resin during thermooxidative aging were described by thermogravimetry (TG)–Fourier transform infrared (FTIR) spectroscopy, attenuated total reflectance (ATR)–FTIR spectroscopy, and dynamic mechanical thermal analysis. The results show that the C? N bond decomposed into ammonia and the dehydration condensation between the residual hydroxyl groups occurred during the thermooxidative aging. Because of the presence of oxygen, the methylene bridges were oxidized into carbonyl groups. After aging for 30 days, the mass loss ratio reached 4.50%. The results of weight change at high temperatures coincided with the results of TG–FTIR spectroscopy and ATR–FTIR spectroscopy. The glass‐transition temperature (T_g) increased from 240 to 312°C after thermooxidative aging for 30 days, which revealed the postcuring of phenolic resins. In addition, an empirical equation between the weight change ratio and T_g was obtained. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Versatile bio-based epoxy resin: From banana waste to applied materials

Bio-based epoxy resin is a promising candidate for petroleum-based resins due to its abundant reserves and low-cost products, which mainly use polyphenolic composites as precursors. Liquefied banana pseudo stem (LBPS), a highly active compound obtained by liquefaction with 7.5% of sulfuric acid as catalyst at 160°C, was used to synthesize bio-based epoxy resin. FTIR and SEM demonstrated the synthetic process of LBPS-based epoxy resin (LBPSER) from waste banana pseudo stem (BPS). Mechanical test, reagent resistance, dynamic mechanical analysis, and thermogravimetric analysis were utilized to evaluate the mechanical and chemical properties of LBPSER. With polyamide (PA) as hardener, LBPSER-PA adhesive exhibited an optimum shear strength that is comparable with that of commercial diglycidyl ether of bisphenol A (DGEBA), corresponding to 11.86 vs 11.89 MPa. Interestingly, the shear strength of this adhesive curing at 40°C could get 9.52 MPa for wood materials. The adhesive also performed excellent resistance to organic agents, adverse acidic, and alkaline environments. Notably, as the content of LBPSER in adhesive increased, an increase of glass transition temperature could be verified from 42 to 100°C. The LBPSER-PA adhesive presented good thermal and physicochemical performances, thereby suggesting the potential of utilizing liquefied product from BPS as alternative to toxic bisphenol A in synthesizing bio-based epoxy resin. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47135.     

Mechanical properties and aging behaviour of Y-TZP with 64S bioglass additions for dental restorations

Yttria-partially stabilised zirconia (Y-TZP) of 3?mol-% with 5.4, 10.5 and 19.9 vol.-% 64S bioglass compacts was sintered at 1300–1500°C. The influence of 64S content and sintering temperature on the mechanical properties and aging behaviour of Y-TZP ceramics were studied. Among Y-TZP ceramics with 64S additions, maximum hardness and flexural strength values were found for Y-TZP with 10.5 vol.-% 64S at 1400°C. Y-TZP with 19.9 vol.-% 64S at 1500°C presented the highest fracture toughness; crack deflection and pinning by ZrSiO₄ particles combined with zirconia microcracking contributed to the fracture toughness. Y-TZP at 1500°C was extremely susceptible to hydrothermal degradation and its flexural strength markedly decreased after aging. On the contrary, Y-TZP with 10.5 vol.-% 64S at 1400°C remained almost unaltered; it maintained its flexural strength at a high level during aging, becoming the most promising ceramic in terms of mechanical properties and aging behaviour.     

Effect of isothermal aging on post-imidization and glass transition temperature of LaRC-IA polyimide resin

The isothermal aging of partilly imidized NASA Langley Research Center, LaRC-IA polymide resin containing 70 wt% N-methyl pyrrolidone, NMP was performed in a vacuum oven at 65, 95, 135, 165 and 200°C for 0.5, 1.0, 1.5 and 2 h. The weight loss and chemical changes that occurred during aging was determined gravimetrically and by FTIR spectroscopy, respectively. The imide absorption peak at 1778 cm⁻¹, increased in intensity as the aging temperature was increased from 65 to 200°C. The expulsion of NMP (70 wt%) was completed after ∼2.5 h of aging at 135°C. Additional weight loss ≤4 wt%, after the expulsion of NMP, was attributed to post-imidization. The imide carbonyl peak absorption at 1721 and 1778 cm⁻¹, respectively, were broadened after aging at 200°C. The broadening of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm⁻¹ and is attributable to post-imidization of the partially imidized polyamic-acid. Dissolution of the polyimide aged at T ≥ 165°C in dimethyl formamide, DMF, was unsuccessful even after long times of stirring (∼12 h) at elevated temperature (T ∼ 85°C). The DSC thermogram for the LaRC-IA resin showed a series of broad endothermic peaks between 150–180°C and narrow endothermic peaks at 210°C. The low temperature endotherm disappeared after aging at T ≥ 135°C for t ≥ 1 h. The high temperature endotherm decreased with increased aging temperature and time. The glass transition temperature of the polyimide increased with increased aging temperature and time.     

Catalytic effect of 2,2′‐diallyl bisphenol A on thermal curing of cyanate esters

Cyanate esters are a class of thermal resistant polymers widely used as thermal resistant and electrical insulating materials for electric devices and structural composite applications. In this article, the effect of 2,2′‐diallyl bisphenol A (DBA) on catalyzing the thermal curing of cyanate ester resins was studied. The curing behavior, thermal resistance, and thermal mechanical properties of these DBA catalyzed cyanate ester resins were characterized. The results show that DBA is especially suitable for catalyzing the polymerization of the novolac cyanate ester resin (HF‐5), as it acts as both the curing catalyst through depressing the exothermic peak temperature (T_exo) by nearly 100°C and the toughening agent of the novolac cyanate ester resin by slightly reducing the elastic modulus at the glassy state. The thermogravimetric analysis and dynamic mechanical thermal analysis show that the 5 wt % DBA‐catalyzed novolac cyanate ester resin exhibits good thermal resistance with T_d⁵ of 410°C and the char yield at 900°C of 58% and can retain its mechanical strength up to 250°C. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1775–1786, 2006     

Mechanical and thermal properties of biphenyldiol formaldehyde resin/gallic acid epoxy composites enhanced by graphene oxide

In this study, the gallic acid‐based epoxy resin (GA‐ER) and alkali‐catalysed biphenyl‐4,4′‐diol formaldehyde resin (BPFR) are synthesized. Glass fibre‐reinforced GA‐ER/BPFR composites are prepared. Graphene oxide (GO) is used to improve the mechanical and thermal properties of GA‐ER/BPFR composites. Dynamic mechanical properties and thermal, mechanical, and electrical properties of the composites with different GO content are characterized. The results demonstrate that GO can enhance the mechanical and thermal properties of the composites. The glass transition temperature, T_g, of the BPFR/GA‐ER/GO composites is 20.7°C higher than the pure resin system, and the 5% weight loss temperature, T_d5, is enhanced approximately 56.6°C. When the BPFR: GA‐ER mass ratio is at 4 : 6 and GO content is 1.0–1.2 wt %, the tensile and impact strengths of composites are 60.97 MPa and 32.08 kJ/m² higher than the pure resin composites, respectively. BPFR/GA‐ER composites have better mechanical properties, and can replace common BPA epoxy resins in the fabrication of composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42637.     

Effects of hygrothermal aging on the thermal–mechanical properties of vinylester resin and its pultruded carbon fiber composites

Hygrothermal aging was carried out on vinyl ester (VE) resin cast and its pultruded carbon fiber reinforced composite (CF/VE) by immersing them in distilled water at 65 and 95°C. Hygrothermal aging effects on the samples were studied in terms of thermal–mechanical properties, as well as moisture absorption behavior, interfacial adhesion, and transverse mechanical properties. Moisture absorption behaviors of the VE casts and the CF/VE composites were characterized as Fickian behavior. Dynamic mechanical thermal analysis (DMTA) tests showed that the tan δ peak temperatures of the VE casts and CF/VE composites decreased with immersion time at 65 and 95°C. Moreover, there existed a splitting in the tan δ peaks at 95°C, which was reversible and could be recovered by dehydration. Three‐point flexural test indicated that flexural strengths of both the VE casts and the composites decreased by hygrothermal aging with a trend related to their moisture absorption behaviors, while flexural modulus of the composites was less affected. The ILSS of the CF/VE composites was also depressed by deterioration in interfacial adhesion, which was proved by the interfacial adhesion parameters, A and α. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Effect of lactic acid chain length on thermomechanical properties of star‐LA‐xylitol resins and jute reinforced biocomposites

Star‐shaped bio‐based resins were synthesized by direct condensation of lactic acid (LA) with xylitol followed by end‐functionalizing of branches by methacrylic anhydride with three different LA chain lengths (3, 5 and 7). The thermomechanical and structural properties of the resins were characterized by ¹³C NMR, Fourier transform IR spectroscopy, rheometry, DSC, dynamic mechanical analysis (DMA), TGA and flexural and tensile tests. An evaluation of the effect of chain length on the synthesized resins showed that the resin with five LAs exhibited the most favorable thermomechanical properties. Also, the resin's glass transition temperature (103 °C) was substantially higher than that of the thermoplast PLA (ca 55 °C). The resin had low viscosity at its processing temperature (80 °C). The compatibility of the resin with natural fibers was investigated for biocomposite manufacturing. Finally, composites were produced from the n5‐resin (80 wt% fiber content) using jute fiber. The thermomechanical and morphological properties of the biocomposites were compared with jute‐PLA composites and a hybrid composite made of the impregnated jute fibers with n5 resin and PLA. SEM and DMA showed that the n5‐jute composites had better mechanical properties than the other composites produced. Inexpensive monomers, good thermomechanical properties and good processability of the n5 resin make the resin comparable with commercial unsaturated polyester resins. © 2017 Society of Chemical Industry     

Bis[4‐(4‐maleimidephen‐oxy)phenyl]propane/N,N‐4,4′‐bismaleimidodiphenylmethyene blend modified with diallyl bisphenol A

In this article, 2,2′‐bis[4‐(4‐maleimidephen‐oxy)phenyl)]propane (BMPP) resin and N,N‐4,4′‐bismaleimidodiphenylmethyene (BDM) resin blends were modified by diallyl bisphenol A (DABPA). The effects of the mole concentration of BMPP on mechanical properties, fracture toughness, and heat resistance of the modified resins were investigated. Scanning electron microscopy was used to study the microstructure of the fractured modified resins. The introduction of BMPP resin improves the fracture toughness and impact strength of the cured resins, whose thermal stabilities are hardly affected. Dynamic mechanical analysis shows that the modified resins can maintain good mechanical properties at 270.0°C, and their glass transition temperatures (T_g) are above 280.0°C. When the mole ratio of BDM : BMPP is 2 : 1(Code 3), the cured resin performs excellent thermal stability and mechanical property. Its T_g is 298°C, and the Charpy impact strength is 20.46 KJ/m². The plane strain critical stress intensity factor (K_IC) is 1.21 MPa·m^0.5 and the plane strain critical strain energy release rate (G_IC) is 295.64 J/m². Compared with that of BDM/DABPA system, the K_IC and G_IC values of Code 3 are improved by 34.07% and 68.10%, respectively, which show that the modified resin presented good fracture toughness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40395.     

Synthesis of a Sustainable and Bisphenol A-Free Epoxy Resin Based on Sorbic Acid and Characterization of the Cured Thermoset

In the present study, an epoxy compound, 1,2-epoxy-6-methyl-triglycidyl-3,4,5-cyclohexanetricarboxylate (EGCHC) synthesized from sorbic acid, maleic anhydride, and allyl alcohol is proposed. Using commodity chemicals, a bio-based carbon content of 68.4 % for the EGCHC resin is achieved. When cured with amine hardeners, the high oxirane content of EGCHC forms stiff cross-linked networks with strong mechanical and thermal properties. The characterization of the epoxy specimens showed that EGCHC can compete with conventional epoxy resins such as DGEBA. A maximum stiffness of 3965 MPa, tensile strength of 76 MPa, and T_g of 130 °C can be obtained by curing EGCHC with isophorone diamine (IPD). The cured resin showed to be decomposable under mild conditions due to the ester bonds. The solid material properties of EGCHC expose its potential as a promising bisphenol A, and epichlorohydrine free alternative to conventional petroleum-based epoxies with an overall high bio-based carbon content.     

Chemistry and properties of a phenylethynyl-terminated polyimide

As part of an ongoing effort to develop processable, high-performance resins for aerospace applications, a phenylethynyl-terminated imide (PETI) oligomer designated LaRC^MTPETI-1 was developed. This reactive oligomer has a number-average molecular weight of 6300 g/mol and a T_g of 218°C. Upon curing the reactive oligomer at 350°C for 1 h, a tough material with a T_g of 249°C was obtained. The properties of cured PETI-1 in the form of composites, adhesive specimens, thin films, and neat resin moldings are excellent. The synthesis, characterization, and mechanical properties of this polyimide are discussed. © 1996 John Wiley & Sons, Inc.     

A dielectric study of water uptake in epoxy resin systems

The influence of epoxy resin formulation on the nature and extent of moisture is studied using dielectric and gravimetric analysis for samples cured at 40, 50, 60, and 70 °C and aged at 50 °C and 70 °C. The equilibrium moisture uptake depends on the difference between the glass transition and aging temperature. Dielectric relaxation data measured from 1 to 3 × 10¹⁰ Hz indicates the presence of water in at least two different environments. The high frequency relaxation ca. 1 × 10¹⁰ Hz is associated with water clustered in nano‐voids, whereas the relaxation at 10⁵ Hz arises from a combination of OH pendant group reorientation motion coupled with that of molecularly dispersed water molecules. A correlation exist between the dielectric permittivity and the amount of moisture absorbed. Water initially resident in voids is re‐dispersed with aging into the resin matrix aiding plasticization and allowing densification of the matrix. The extent to which changes occur depends on the chemical functions forming the matrix and densification leads to a drop in the amount of water absorbed. In the complex resin systems, water interacts with both OH groups and polyether of the amine curing agent which is not possible with the aliphatic diamine in the simple system.] © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44717.     

Effect of crosslink structures on dynamic mechanical properties of natural rubber vulcanizates under different aging conditions

The effects of crosslink structures on the dynamic mechanical properties (DMPs) of unfilled and carbon black N330‐filled natural rubber (NR) vulcanizates cured with conventional (CV), semiefficient (SEV), and efficient (EV) cure systems and having about the same total crosslink densities were investigated before and after aerobic and anaerobic aging at 100°C. The three unfilled NR vulcanizates cured with the CV, SEV, and EV systems had about the same mechanical loss factor (tan δ) values at about 0°C but showed some apparent differences in the tan δ values in the order EV > SEV > CV at relatively high temperatures of 40–80°C before aging. However, N330‐filled NR vulcanizates gave higher tan δ values than the unfilled vulcanizates and showed little effect of the crosslink types on the tan δ at different temperatures over the glass‐transition temperature (T_g) before aging. Aerobic heat aging increased the T_g and tan δ values of the vulcanizates over a wide range of temperatures from ?80 to 90°C that was mainly due to the changes in the total density and types of crosslinks. The unfilled vulcanizates cured with the CV system showed the greatest change in DMP because of their poor resistance to heat aging. Aerobic heat aging of NR vulcanizates caused a more significant change in the DMP than anaerobic heat aging because of the dominant effect of the oxidative degradation during aerobic heat aging on the main‐chain structure, crosslink structures, and DMPs of the vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 710–718, 2001     

Synthesis and characterization of bis(4‐cyanato 3,5‐dimethylphenyl) naphthyl methane/epoxy/glass fiber composites

Bis(4‐cyanato 3,5‐dimethylphenyl) naphthylmethane was prepared by treating CNBr with bis(4‐hydroxy 3,5‐dimethylphenyl) naphthylmethane in the presence of triethylamine at −5 to 5°C. The dicyanate was characterized by FT‐IR and NMR techniques. The prepared dicyanate was blended with commercial epoxy resin in different ratios and cured at 120°C for 1 hr, 180°C for 1 hr, and post cured at 220°C for 1 hr using diamino diphenyl methane (DDM) as curing agent. Castings of neat resin and blends were prepared and characterized by FT‐IR technique. The morphology of the blends was evaluated by SEM analysis. The composite laminates were also fabricated from the same composition using glass fiber. The mechanical properties like tensile strength, flexural strength, and fracture toughness were measured as per ASTMD 3039, D 790, and D 5528, respectively. The tensile strength increased with increase in cyanate content (3, 6, and 9%) from 322 to 355 MPa. The fracture toughness values also increased from 0.7671 kJ/m² for neat epoxy resin to 0.8615 kJ/m² for 9% cyanate ester epoxy modified system. The thermal properties were also studied. The 10% weight loss temperature of pure epoxy is 358°C and it increased to 398°C with incorporation of cyanate ester resin. The incorporation of cyanate ester up to 9% loading level does not affect the T_g to a very great extent. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Bio-resourced eugenol derived phthalonitrile resin for high temperature composite

Eugenol (EG) is an abundant renewable compound that has been widely used in the synthesis of bio-based thermosetting resin, but there are few reports on the phthalonitrile (PN) resin derived from EG. In this study, a new kind of bio-based PN resin (MEG-PN) derived from EG derivative was successfully synthesized. PN is a traditional class of high-performance thermosets with poor processability for its ultra-high melting point and curing temperature. The MEG-PN resin possesses excellent processability: its melting temperature is much lower (77°C), and it can be cured at a moderate temperature (281°C) in the absence of curing agents. The cured MEG-PN resin exhibited great heat resistance according to its 5% weight loss temperature at 448°C and its char yield percentage as high as 75.6% at 800°C under nitrogen. The properties of the carbon-fiber reinforced MEG-PN composite were comparable to those of petroleum-based PN resins: the glass transition temperature was around 397°C; the flexural strength and modulus were as high as 756 MPa and 119 GPa, respectively. Overall, a bio-based PN thermoset with great comprehensive performance was synthesized possessing the potential in the application of advanced composite.     

Physical aging time scales and rates for poly(vinyl acetate) stimulated mechanically in the Tg-region

The physical aging behavior of poly(vinyl acetate), PVAc, in the T_g-region has been investigated using dynamic mechanical analysis (DMA). Tests in shear and flexure modes were performed for samples quenched from a stabilization temperature of 40°C to aging temperatures 35.0°C and 32.5°C (down-jumps), and from equilibrium at a stabilization temperature of 32.5°C to aging temperature 36.2°C (up-jump). Volume recovery was investigated for the same thermal histories using mercury-in-glass dilatometry. The time scales for the evolution of the storage and loss moduli during physical aging have been compared with the volumetric time scales. Aging rates, µ, have been calculated in the frequency and time domains. The evolution of dynamic mechanical properties during aging is found to be different from the volumetric behavior. In the case of the temperature down-jump, the evolution of the volume ceases earlier than that for the storage and loss moduli. In the up-jump experiment, the opposite is found. Aging rates for the storage and loss moduli are analyzed. A suggestion is offered that the evolution of dynamic mechanical properties during physical aging is retarded by the cyclic mechanical stimulus, and that this effect (rejuvenation) is more noticeable the higher is the damping; consequently rejuvenation in the T_g-region can be caused by small strains in dynamic mechanical analysis.