Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

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‐T_g 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 T_g, 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     

热历史对已固化环氧树脂松弛行为的影响

<正> 环氧树脂是一种性能优良且应用广泛的高分子材料,近年来,人们对已固化环氧试样在玻璃化温度(T_g)以下贮存时,物理性能随时间而变化的物理老化现象进行了广泛的研究.Ophir等发现已固化环氧试样经淬火处理后,其T_g随贮存时间升高,认为是由于自由体积随贮存时间减小所致.Mijovic用不同的方法进行过类似的研究,发现淬火环氧试样的T_g随贮存时间降低,认为是由于试样中的内应力松弛所致.本文试图进一步搞     

Structure and property relationships in model diglycidyl ether of bisphenol‐a and diglycidyl ether of tetramethyl bisphenol‐a epoxy systems. I. Mechanical property characterizations

Model epoxy networks, with variations in crosslink density and in epoxy monomer rigidity, were prepared to study how the network structure affects modulus, T_g, and toughness/toughenability of epoxy resins. Diglycidyl ether of bisphenol‐A and diglycidyl ether of tetramethyl‐bisphenol‐A, along with the corresponding chain extenders, were chosen to study how monomer backbone rigidity and crosslink density affect physical and mechanical properties of epoxies. The present study indicates that, as expected, the backbone rigidity of the epoxy network, not the crosslink density alone, will strongly influence modulus and T_g of epoxy resins. Upon rubber toughening, it is found that the rigidity of the epoxy backbone and/or the nature of the crosslinking agent utilized are most critical to the toughenability of the epoxy. That is, the well‐known correlation between toughenability and the average molecular weight between crosslinks (M_c) does not necessarily hold true when the nature of epoxy backbone molecular mobility is altered. The potential significance of the present findings for a better design of toughened thermosets for structural applications is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2137–2149, 1999     

Physical aging in polycarbonate nanocomposites containing grafted nanosilica particles: A comparison between enthalpy and yield stress evolution

Understanding and controlling physical aging below the glass transition temperature (T_g) is very important for the long‐term performance of plastic parts. In this article, the effect of grafted silica nanoparticles on the physical aging of polycarbonate (PC) below the T_g is studied by using the evolution of the enthalpy relaxation and the yield stress. The nanocomposites were found to reach a thermodynamic equilibrium faster than unfilled PC, implying that physical aging is accelerated in presence of grafted nanosilica particles. The Tool‐Narayanaswamy‐Moynihan model shows that the aging is accelerated by the grafted silica nanoparticles, but the molecular mechanism responsible for physical aging remains unaltered. Furthermore, dynamic mechanical analysis shows that the kinetics of physical aging can be related to a free volume distribution or a local attraction‐energy distribution as a result of the change in mobility of the polymer chain. Finally, a qualitative equivalence is observed in the physical aging followed by both the enthalpy relaxation and yield stress. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2069–2081     

Modeling mechanical aging shift factors in glassy polymers during nonisothermal physical aging. I. Experiments and KAHR‐ate model prediction

This article presents experimental results and model predictions of the mechanical response of polymers during nonisothermal physical aging. The nonisothermal temperature history leads to a complex evolution in the aging behavior of the material. To characterize this response, sequential creep tests of polyether‐ether‐ketone (PEEK) and polyphenylene sulfide (PPS) films are performed at various aging times using a dynamic mechanical analyzer. The resulting strain histories are analyzed to determine discrete aging shift factors (a_te) for each of the creep tests. The nonisothermal aging response is then predicted using the KAHR‐a_te model, which combines the KAHR model of volume recovery with a suitable linear relationship between aging shift factors and specific volume. The KAHR‐a_te model can be utilized to both predict aging response or to determine necessary model parameters from a set of aging shift factor data. For the PEEK and PPS materials considered in the current study, predictions of mechanical response are demonstrated to be in good agreement with the experimental results for several thermal histories. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 340–352, 2009     

Contributions of side groups to the water diffusion in cured epoxy resins (2) study on physical aging

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 T_g‐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     

Synthesis of aniline‐terminated polyethers and resulting polyurethane/polyurea elastomers

Functionalization of polyols with aromatic amines offers a potential route to modify properties of polyurethanes, polyamides, and epoxies. Additionally, aniline termination of polyether backbones provides the opportunity to speed up reactions with isocyanates relative to hydroxyl functionalization and slow down epoxy reactions compared to reactions with primary and secondary amines. In this article, the synthesis, characterization, and physical properties of aniline‐terminated polyols with varying molecular weight, monomer type, and functionality is described. Numerous analytical techniques are employed to track the chemical modification kinetics and the resulting aniline functionalized polyol properties. In addition, synthesis and properties of poly(urethane‐urea) elastomers from several of the modified polyols are presented. The effect of hard segment composition and process temperature on tensile properties, dynamic mechanical properties, phase morphology, and chemical resistance is explored. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1730–1742     

Micellar structure and mechanical properties of block copolymer‐modified epoxies

Amphiphilic block copolymers provide a unique means for toughening epoxy resins because they can self‐assemble into different inclusion shapes before epoxy curing. The two examples reported here are spherical micelles and vesicles, which form in blends containing epoxy and symmetric or asymmetric poly(ethylene oxide)–poly(ethylene‐alt‐propylene) (PEO–PEP) block copolymer with PEO volume fractions of 0.5 and 0.26, respectively. The vesicles and spherical micelles were characterized by transmission electron microscopy and small‐angle X‐ray scattering (SAXS), respectively. SAXS data from the spherical micelles were fit to the Percus–Yevick model for a liquid‐like packing of spheres with hard‐core interactions. Mechanical properties of spherical‐micelle‐modified and vesicle‐modified epoxies in the dilute limit are compared. The glass‐transition temperature and Young's (storage) modulus were tested with dynamic mechanical spectroscopy, and compact‐tension experiments were performed to determine the critical plane‐strain energy release rate for fracture. Vesicles were most effective in improving the epoxy fracture resistance. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2996–3010, 2001     

Aging and cold crystallization of melt‐extruded poly(trimethylene terephthalate) films

We analyzed the thermal crystallization, glass‐transition behavior, and mechanical properties of melt‐extruded poly(trimethylene terephthalate) (PTT) films to investigate their physical aging and annealing effects. The physical aging and annealing of PTT films had an influence on the glass‐transition temperature, recrystallization behavior, and mechanical properties. When samples were aged at an ambient temperature, the crystallization temperature decreased largely within 5 h, the heat of crystallization increased, and the breaking stress and breaking elongation increased. The glass‐transition temperature of annealed samples, which was obtained from differential scanning calorimetry and dynamic mechanical measurements, increased with increasing annealing temperature below 80 °C but decreased above that temperature. In addition, the glass‐transition temperature and modulus of annealed samples were largely affected by the annealing time; in particular, they increased sharply within 1 h on annealing at 50 °C. Consequently, the change in the glass‐transition temperature on annealing was ascribed to the fact that the molecular constraint due to recrystallization and the mobility of rigid amorphous PTT chains competed with each other, being dependent on the annealing temperature. The mechanical properties of aged samples were closely related to their cold‐crystallization behavior. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1920–1927, 2001     

Effect of chemical activity jumps on the viscoelastic behavior of an epoxy resin: Physical aging response in carbon dioxide pressure jumps

We report the results from tensile creep tests performed on an epoxy resin in the presence of carbon dioxide at different pressures (Pco₂) and at a constant temperature below the glass‐transition temperature. Time‐Pco₂ superposition was applied to the data to account for the plasticization effect because of the interaction between the carbon dioxide molecules and the polymer. In addition, physical aging of the epoxy films was investigated with sequential creep tests after carbon dioxide pressure down‐jumps at constant temperature and after temperature down‐jumps at constant carbon dioxide pressure. The isothermal pressure down‐jump experiments showed physical aging responses similar to the isobaric temperature down‐jump experiments. However, the aging rate for the CO₂ jump was slightly lower than that for the temperature‐jump (T‐jump) experiments, and the retardation time for the Pco₂‐jump experiments was up to 6.3 times longer than for the T‐jump conditions. The results are discussed in terms of classical physical aging and structural recovery frameworks, and speculation about the differences in the energy landscape resulting from the Pco₂‐jump and T‐jump experiments is also made. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2050–2064, 2002     

Reinforcement effect of carbon nanofillers in an epoxy resin system: Rheology,molecular dynamics,and mechanical studies

The reinforcing effect of carbon nanoparticles in an epoxy resin has been estimated with different approaches based on rheology, molecular dynamics (evaluated by differential scanning calorimetry, dielectric relaxation spectroscopy, and thermally stimulated depolarization current), and dynamic mechanical analysis. Carbon particles aggregate as the volume increases and form a fractal structure in the matrix polymer. The dispersion microstructure has been characterized by its viscoelastic properties and relaxation time spectrum. The scaling of the storage modulus and yield stress with the volume fraction of carbon shows two distinct exponents and has thus been used to determine the critical carbon volume fraction of the network formation (Φ*) for the carbon/epoxy dispersions. At nanofiller concentrations greater than Φ*, the overall mobility of the polymer chains is restricted in both dispersions and solid nanocomposites. Therefore, (1) the relaxation spectrum of the dispersions is strongly shifted toward longer times, (2) the glass‐transition temperature is increased and (3) the relaxation strength of both the secondary (β) and primary (α) relaxations increases in the nanocomposites, with respect to the pure polymer matrix. The dispersion microstructure, consisting of fractal flocs and formed above Φ*, is proposed to play the main role in the reinforcement of nanocomposites. Moreover, the network structure and the interface polymer layer (bond layer), surrounding nanoparticles, increases the relaxation strength and slows the cooperative α relaxation, and this results in an improvement of the mechanical properties. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 522–533, 2005     

Monitoring real-time curing of epoxies in situ using single-sided NMR

Adhesively bonded joints using epoxy are widely used in aircraft and aerospace structures. Quality control and defect detection during epoxy curing in such applications is critical. We used single-sided nuclear magnetic resonance (NMR) to nondestructively probe and spatially resolve the change in the characteristic NMR relaxation time (T₂) of epoxies during curing on a substrate. Time-dependent T₂ values were fit to a Weibull function to model temporal changes in the NMR measurables. Our results demonstrate that the reduction in molecular mobility of various epoxy/curing agent mixtures occurs more rapidly at the interface than in the bulk. Further use of single-sided NMR to acquire spatially resolved T₂ data will provide a route for elucidatory epoxy curing studies. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 616–623     

Glass‐formation kinetics of miscible blends of atactic polystyrene and poly(2,6‐dimethyl‐1,4‐phenylene oxide)

We present a detailed investigation of the kinetics associated with the glass transitions of miscible blends composed of atactic polystyrene (a‐PS) and poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO). According to both dynamic mechanical analysis and differential scanning calorimetry, relaxation times displayed an enhanced temperature dependence (i.e., more fragile or more cooperative behavior) for the blends compared with additive behavior based on the responses of neat a‐PS and PPO. This is consistent with the notion that specific interactions between the blend components heighten the intermolecular cooperativity. The compositional dependence of fragility provided insight into physical aging results for the properties of volume and enthalpy. The combination of our research and a previously reported pressure–volume–temperature study by Zoller and Hoehn (J Polym Sci Polym Phys Ed 1982, 20, 1385) provided evidence that the observation of increased glassy densities for the blends compared with those of the pure polymers was kinetic in origin and was not a feature of the thermodynamics of miscibility. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2118–2129, 2001     

Influence of nanoparticle surface chemistry on the thermomechanical and magnetic properties of ferromagnetic nanocomposites

The effect of nanoparticle surface chemistry on the thermal, mechanical, and magnetic properties of poly(methyl methacrylate) (PMMA) nanocomposites with cobalt ferrite nanofillers was studied by comparing nanofillers coated with oleic acid (OA; which does not covalently bond to the PMMA matrix) and 3‐methacryloxypropyltrimethoxysilane (MPS, which covalently bonds to the PMMA matrix). Thermogravimetric analysis revealed an increase in the thermal degradation temperature of the nanocomposites compared with the neat polymer. The effect of cobalt ferrite nanofiller on the glass transition temperature (T_g) of the nanocomposite was evaluated by differential scanning calorimetry. The T_g value of the material increased when the particles were introduced. Dynamic mechanical analysis indicated an increase in the storage modulus of the nanocomposite because of the presence of nanofiller and a shift in the peak of loss tangent toward higher temperature. Magnetic measurements indicated that both nanocomposites had a small hysteresis loop at 300 K and no hysteresis at 400 K. However, estimates of the nanofiller's rotational relaxation times and measurements of the zero field cooled temperature‐dependent magnetization indicate that the observed lack of hysteresis at 400 K is likely because of particle rotation in the polymer matrix. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

A new conduction phenomenon observed in polyethylene and epoxy resin: Ultra‐fast soliton conduction

A new conduction mechanism in polyethylene and epoxy resin is presented and discussed in this article. This mechanism is based on the presence of charge pulses that can be seen as solitons (solitary waves) crossing dielectrics with mobility 4–5 orders of magnitude larger than that of conventional charge carriers. The nature of this new process that is characterized by charge pulses with such high mobility requires a completely different mechanism for transport to be theorized with respect to that, mediated by trap sites, of conventional charge carriers. It is speculated in this article that injection and transport of positive and negative solitons occurs through the coupling of space charge and relaxation processes involving molecular chains, but of different nature for negative or positive solitons. Observation of space charge shows the existence of such solitons for at least two families of materials, polyethylene, and epoxy resin. In addition, it has been observed that nanostructuration, which is able to modify mechanical properties, affects also the presence and size of the solitons. In this article, we not only seek to demonstrate the existence of this new phenomenon, but attempt to provide an explanation and a kind of qualitative–quantitative model, which shows that the assumption of a pulsive conduction mechanism mediated by chain relaxation processes, transport in free volume (for negative solitons), and reverse‐tunneling between macromolecular chains (positive solitons) seems to fit quite well with the experimental observations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Effects of annealing above Tg on the physical aging of quenched PLLA studied by modulated temperature FTIR

The effect of the annealing few degrees above the glass transition temperature (T_da = 62 °C) on the physical aging (T_pa = 51 °C) of amorphous quenched poly(l ‐lactide) is investigated by an implementation of variable temperature Fourier transform infrared (FTIR). By using a temperature program composed of a linear heating ramp superimposed to a temperature modulation (modulated temperature FTIR), the reversing and nonreversing intensity variation of selected bands, related to high‐energy gg and low‐energy gt conformers, is investigated. It is observed that the annealing above T _g changes irreversibly the conformation distribution of the liquid polymer. The glasses obtained from annealed and nonannealed liquids behave differently, evolving in the physical aging toward their own liquid state and retaining the memory of their original condition before the vitrification. The recovery through T _g of the relaxation occurred in the physical aging depends not only from aging conditions but also by the thermal history of the sample above the T_g. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 174–181     

The effect of different hardeners on the ageing of crosslinked epoxy resins

Polymers based on epoxies are widely used in the industrial production of materials for different applications. The crosslinking is often done with hardeners like aliphatic or aromatic amines, with dicarboxylic acid anhydride or anionically. The resins containing mineral fillers possess similar thermal mechanical properties. On thermal oxidative ageing, however, the systems behave very differently. An account is given of the thermal mechanical behaviour and of the weight loss on ageing above the glass transition temperature. By using semiempirical molecular orbital calculations we have been able to identify a possible reason for the different thermal oxidative stability of epoxy resins crosslinked with different hardeners.     

Anisotropy in the thermal shrinkage of polyimide film

Anisotropy in a polyimide film was investigated in a quantitative manner by a nonlinear regression of the thermal shrinkage data obtained from thermomechanical analysis. The thermally induced shrinkage of this pyromellitic dianhydride–oxydianiline polyimide film at 573 K was directionally anisotropic in the film plane by as much as about 0.4%. The direction of maximum thermal shrinkage was inclined by about π/6 rad from the machine direction of the film. The thermal shrinkage behavior of the polyimide films in the vicinity of the glass‐transition temperature (T_g) showed an unusual anisotropic response. On the basis of a correlation between the anisotropy in the thermal shrinkage of the films and the molecular orientation of the polyimide, this characteristic thermal shrinkage behavior around T_g is suggested to be due to a recovery of the free volume lost by a physical aging process. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3222–3229, 2000     

Further Study of Sub‐Tg Heat Flow Transition of a Cured Epoxy Resin

To further study the sub‐T_g heat flow transition of a cured epoxy resin, cured samples with different thermal history were investigated using torsion pendulum analysis (TPA) and thermal mechanical analysis (TMA). The results indicate that sub‐T_g heat flow transition could be related to the molecular relaxation from 20°C to the α‐peak, and that frozen‐in extra free volume is necessary for the appearance of sub‐T_g heat flow transition.