Dynamic mechanical analysis of ethylene tetrafluoroethylene (ETFE) foils in use for transparent membrane buildings

Glass transition temperature and tan delta (the ratio of loss modulus to storage modulus) are indispensable parameters for determining appropriate application range of ETFE foils. In this study, ETFE foils in terms of specimen number, material direction and thickness were investigated with dynamic mechanical analysis (DMA) over a temperature range of -70-100 °C at frequencies of 0.1, 1, and 10 Hz. Glass transition temperatures were obtained with storage modulus, loss modulus and tan delta curves. It is found that frequency effect on glass transition temperature was proportional and that frequency effect was more significant than material direction effect. Moreover, a comparison study showed that elastic modulus determined with quasi-static experiments was greater than storage modulus calculated with dynamic mechanical experiments. To propose suitable glass transition temperature ranges for engineering application, an approach to determine confidence interval based on statistical analysis was employed. The resulting intervals with confidence coefficient of 95% were 31.2–32.7 °C, 60.5–66.4 °C and 79.6–83.3 °C for storage modulus, loss modulus and tan delta, respectively. In general, this study could provide useful observations and values for evaluating dynamic mechanical properties of ETFE foils.     

Study of the thermomechanical behavior of UHMWPE yarns under different loading paths

UHMWPE viscoelastic fibers show great interest as reinforcement within composites and especially when used in SRPs (Self-Reinforced Polymers). They provide ductility, lightness and recyclability, benefits that glass or carbon fibers cannot provide. It is, therefore, necessary to increase knowledge about the behavior of UHMWPE fibers. Before the thermomechanical characterization of these yarns, an experimental protocol is proposed, validated and it supplements the existing standard. Monotonous, load-unload and creep tensile tests were carried out on Doyentrontex® yarns. Temperature and strain rate dependencies were observed. A time-temperature superposition is used to reconstruct the evolutions of modulus at 0.5%, maximum strength, and strain at break at 23 °C over a wide range of strain rates. The behavior of the yarns studied appears to be complex. Indeed, at low temperatures, a hyperelastic type of behavior, combined with plasticity, predominates whereas a more elasto-viscoplastic one emerges at 100 °C. From creep tests, a time-temperature-stress level superposition leads to the reconstruction of the yarns creep behavior over a long period at the reference temperature 23 °C and the reference stress level, which is 40% of the stress at break in tensile tests at any given test temperature.     

Mechanical properties of fluorinated ethylene propylene (FEP) foils in use for neutrino detector project

The use of fluorinated ethylene propylene (FEP) foils as engineering materials for aerospace, solar thermal collector and neutrino detector applications has attracted considerable attention in recent decades. Mechanical properties are indispensable for analyzing corresponding structural behavior to meet the demands of safety and serviceability. In this paper, uniaxial tensile tests taking into account loading speeds, uniaxial tensile cyclic tests in terms of stress amplitude and loading cycles and creep tests considering loading stress and time were carried out to characterize mechanical properties. For uniaxial tensile properties, elastic modulus, yield stress, breaking strength and elongation were analyzed in detail. It is found that these mechanical properties except breaking elongation increased with loading speeds and that mechanical properties obtained in transverse direction were more sensitive than those obtained in machine direction. For cyclic properties, elastic modulus and ratcheting strain tended to be stable after certain cycles, demonstrating that cyclic elastic moduli were more suitable for analyzing structural behavior than those obtained in uniaxial tensile experiments. For creep properties, apparent strain at 6 MPa suggested that special attention was necessary for analyzing structural behavior if maximum stress was larger than 6 MPa. In general, this study could provide useful observations and values for understanding mechanical properties of FEP foils.     

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     

Temperature-dependent ratcheting of PTFE gaskets under cyclic compressive loads with small stress amplitude

Uniaxial stress-controlled ratcheting experiments on PTFE gaskets under cyclic compressive loads with small stress amplitude were performed. The effect of temperature on the deformation behavior was considered. Results showed that the compressive modulus decreases rapidly when the temperature increases from 100 °C to 200 °C. Compressive ratcheting deformation with cycles increase significantly with the increases of temperature. The ratcheting deformations at 100 °C, 150 °C and 200 °C are nearly two, three and five times that at room temperature, respectively. Most of ratcheting deformation mainly occurs during the first 20 cycles because the subsequent ratcheting rate and strain range are small and much less than those in the previous cycles. The accumulated deformation under cyclic loads with small stress amplitude is relatively approach to the static compressive creep with the same peak stress. Therefore, the accumulated deformation with time of PTFE gaskets obtained by cyclic compression with small stress amplitude can be estimated by the corresponding static creep deformation with good accuracy under the approximate stress rate and the same temperature, especially at room temperature.     

Anomalous aging in two-phase systems: Creep and stress relaxation differences in rubber-toughened epoxies

From time–aging time superposition principles, similar to time–temperature superposition, one would expect similar shifting or superposition behaviors for both creep and stress relaxation responses. In particular, for isotropic homogeneous systems, in the linear viscoelastic regime, consideration of superposition in rheology by Markowitz¹ or the discussion by Ferry² from the Kramers–Kronig relation would seem to demand that creep and stress relaxation shift in the same way. Here we report on results from creep and stress relaxation measurements in two-phase, rubber-toughened epoxies that exhibit Boltzman additivity of creep or relaxation behaviors and follow the time–aging time superposition behavior in creep, but not in stress relaxation. While the lack of superposition in stress relaxation is, perhaps, not surprising, the finding that the creep responses at different aging times superimpose while the stress relaxation responses do not, presents an anomalous behavior that has not been previously reported. In addition, our findings show that the stress relaxation responses show short time “softening” upon aging. Possible reasons for the anomalous behaviors are briefly considered. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1167–1174, 1997     

Creep-ratcheting behavior of PTFE gaskets under various temperatures

Compressive creep-fatigue experiments on PTFE gaskets with small stress amplitude were tested. The effects of peak holding times and temperatures on creep-ratcheting behaviors were further discussed. Results showed that at room temperature, the accumulated creep-ratcheting strain corresponding to 400 cycles for peak holding 1min is nearly 1.06 times that for peak holding time 0.5min and 1.3 times that without peak holding. While, at 100 °C, 150 °C and 200 °C, they are almost 1.1 times and 1.6 times, 1.1 times and 1.7 times, 1.05 times and 1.9 times, respectively. This means although the accumulated creep-ratcheting strains for peak holding 0.5min and 1min are very close to each other, but short peak holding time still has obvious effect on the accumulated deformation comparing that without peak holding. Moreover, the creep-ratcheting deformation with short peak holding time and small stress amplitude for PTFE gaskets can be simply estimated by the corresponding compressive static creep strain at the same peak stress in practical engineering with relatively good accuracy.     

Viscoelastic phase diagram of fluorinated and grafted polymer films and proton‐exchange membranes for fuel cell applications

The influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)‐based proton‐conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion^® NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 °C for 10% < RH < 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion^®. Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power‐law behavior with exponent equal to ?0.04 ± 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power‐law behavior at RH < 60% was also observed. However, a markedly different behavior was evident at RH > 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH ≥ 70% and 30 °C. The general behavior of grafted films observed at 30 °C was also obtained at 50 °C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 °C when compared with 30 °C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time‐dependent behavior, from power‐law scaling to exponential scaling. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1139–1148     

Compressive ratcheting effect of expanded PTFE considering multiple load paths

Uniaxial stress-controlled ratcheting behaviors of expanded PTFE (ePTFE) under cyclic compressive loads were tested. The effects of temperature, stress rate and mean stress on the ratcheting behaviors of ePTFE considering multiple load paths were discussed in detail. Results present that the steady ratcheting strain is rate-independent when the stress rate is less than about 0.1 MPa/s, while it approximately linearly decreases with increasing the stress rate for greater stress rate. Additionally, the steady ratcheting is temperature-independent when the temperature is greater than about 150 °C, but it nearly linearly increases with enhancing the temperature for lower temperature. Especially, the stress rate almost has little effect on the ratcheting strain of ePTFE at 200 °C. Moreover, the accumulated ratcheting strain enhances rapidly in about the first 80 cycles, and subsequently tends to shakedown in the subsequent cycles for each load path. Furthermore, if a higher stress is used in the prior cycling, the greater ratcheting strain may be produced, and a negative ratcheting strain rate can be obtained in the subsequent cycling with lower mean stress due to the greater strain hardening and deformation resistance produced by the previous higher stress.     

Mechanical properties of anion exchange membranes by combination of tensile stress–strain tests and dynamic mechanical analysis

The thermomechanical properties of anion exchange polymers based on polysulfone (PSU) quaternized with trimethylamine (TMA) or 1,4‐diazabicyclo[2.2.2]octane (DABCO) and containing hydroxide or chloride anions by tensile stress–strain tests and dynamic mechanical analysis (DMA) have been determined. The reported mechanical properties included the Young's modulus, tensile strength, and elongation at break from tensile tests and the storage and loss modulus and glass transition temperature from DMA. The anion exchange membranes behaved as stiff polymers with Young's modulus in the order of 1 GPa, relatively with high strength (about 30 MPa) and low elongation at break (around 10%) was observed. Tensile tests were also made with membranes exchanged with hydrogen‐carbonate and carbonate anions to control the absence of important carbonation of the OH form. The glass transition temperatures were of the order of 150 °C (PSU‐TMA) or 200 °C (PSU‐DABCO) for the hydroxide form, confirmed by differential scanning calorimetry; they increase further by about 50 K, when hydroxide ions are replaced by chloride. This result and the increase of the storage modulus could be interpreted by the higher hydration of hydroxide ions and the plasticizing effect of water, which reduced the Van der Waals interactions between the macromolecular chains. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1180–1187     

Strain rate and temperature dependence of a nanoparticle‐filled poly(dimethylsiloxane) undergoing shear deformation

The mechanical properties in shear of unfilled and nanoparticle‐filled polydimethylsiloxane (PDMS) networks are reported. The effect of silicate‐based nanoparticles on the mechanical response was studied as functions of rate and temperature using the time–temperature superposition principle. An apparent yielding phenomenon was observed in the filled polymer in spite of the more typical elastomeric behavior exhibited by the pure PDMS network. The time–temperature superposition principle was applied to capture the shear strain rate (10^?4–10^?1 s^?1) and temperature (?40 to 60°C) dependence of the stress response of the filled PDMS at different strains and at the yield point. A power‐law relationship was found to adequately describe the resulting master curves for yield stress in shear. Using a triangular shear displacement profile at 10^?2 s^?1, the effect of temperature (?20 to 80°C) on the recovery from a particularly pronounced Mullins effect was investigated as a function of rest time. Given adequate rest time (between 10 and 10² min), recovery was observed for the temperature range studied. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion‐based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long‐time creep of ～40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ～0.55 MPa in 125‐μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ～15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006     

Experimental test and curve fitting of creep recovery characteristics of modified graphene oxide natural rubber and its relationship with temperature

The creep recovery properties of different graphene-doped rubber and the effect of temperature on them were studied. Doping graphene, especially with the surface functional group or surface microstructure, can significantly improve the creep resistance of natural rubber (NR). The permanent creep of each composite tested under the same conditions for 20 min. Graphene oxide, hydrazine hydrate reduced graphene oxide, and 3-aminopropyltriethoxysilane (APTS) grafted graphene oxide was 33%, 16%, and 51% lower than those filled with carbon black respectively. Four parameter model and Weibull distribution function used to analyze and evaluate the creep and recovery test results of composite rubber. These curve fitting results can adequately describe the influence of different types of nanofillers on the creep and recovery properties of composite rubber. The long-term creep of composites forecasted by the time-temperature superposition principle (TTSP). The results show that graphene doping can improve the creep resistance of the rubber. Besides, graphene oxide and surface-modified graphene oxide had better creep resistance than reduced graphene oxide filled natural rubber. It can see that the interfacial properties between the graphene sheet and the natural rubber matrix play an essential role in the creep and recovery properties of graphene/natural rubber composites.     

Shape memory effect and recovery stress property of carbon nanotube/waterborne epoxy nanocomposites investigated via TMA

Shape memory polymers (SMPs) have received great attention and scientific interest in widespread technological development during last few decades. Besides the development of novel SMPs, various techniques have been practiced for characterization of shape memory effect (SME) of SMPs. In this study, the shape memory effect and recovery stress property of the carbon nanotube (CNT)/waterborne epoxy (WEP) nanocomposites below and above the glass transition temperature (T_g) of the nanocomposites and under isostrain and isostress were systematically investigated via thermal mechanical analysis (TMA), respectively. The experimental results showed that the nanocomposites exhibit excellent shape memory effect. The shape memory fixity and recovery ratios were approximately 100% even below glass transition temperature (T_g). A remarkable point is that the strain of the nanocomposites suddenly increased with the temperature decreasing in a certain period of the heating-cooling cycles under isostress condition and the strain increment increased with temperature in general. Especially at low temperature, the recovery stress was very sensitive to temperature under isostrain condition of ±0.25 °C temperature with differential of 25.5 °C developed pressure difference of 0.20 MPa. Moreover, TMA is a practical method for quantifying the SME and recovery stress properties of SMPs and their composites.     

Viscoelastic properties of Nafion at elevated temperature and humidity

Tensile stress–strain and stress relaxation properties of 1100 equivalent weight Nafion have been measured from 23 to 120 °C at 0–100% relative humidity. At room temperature, the elastic modulus of Nafion decreases with water activity. At 90 °C, the elastic modulus goes through a maximum at a water activity of ～ 0.3. At temperatures ≥90 °C, hydrated membranes are stiffer than dry membranes. Stress‐relaxation was found to have two very different rates depending on strain, temperature, and water content. At high temperature, low water activity, and small strain, the stress relaxation displays a maximum relaxation time with stress approaching zero after 10³–10⁴ s. Water absorption slows down stress‐relaxation rates. At high water activity, the maximum stress relaxation time was >10⁵ s at all temperatures. No maximum relaxation time was seen at T ≤ 50 °C. Increasing the applied strain also resulted in no observed upper limit to the stress relaxation time. The results suggest that temperature, absorbed water, and imposed strain alter the microstructure of Nafion inducing ordering transitions; ordered microstructure increases the elastic modulus and results in a stress relaxation time of >10⁵ s. Loss of microphase order reduces the elastic modulus and results in a maximum stress relaxation time of 10³–10⁴ s. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 11–24, 2009.     

Post-irradiation time effects on the graft of poly(ethylene-alt-tetrafluoroethylene) (ETFE) films for ion exchange membrane application

Grafting of styrene followed by sulfonation onto poly(ethylene-alt-tetrafluoroethylene) (ETFE) was studied for synthesis of ion exchange membranes. Radiation-induced grafting of styrene onto ETFE films was investigated after simultaneous irradiation (in post-irradiation condition) using a ⁶⁰Co source. The ETFE films were irradiated at 20 kGy dose at room temperature and chemical changes were monitored after contact with styrene for grafting. The post-irradiation time was established at 14 days when the films were remained in styrene/toluene 1:1 v/v. After this period the grafting degree was evaluated in the samples. The grafted films were sulfonated using chlorosulfonic acid and 1, 2-dichloroethane 20:80 (v/v) at room temperature for 5 h. The membranes were analyzed by infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), thermogravimetric measurements (TG) and degree of grafting (DOG). The ion exchange capacity (IEC) of membranes was determined by acid–base titration and the values for ETFE membranes were achieved higher than Nafion^® films. Preliminary single cell performance was made using pure H₂ and O₂ as reactants at a cell temperature of 80 °C and atmospheric gas pressure. The fuel cell performance of ETFE films was satisfactory when compared to state-of-art Nafion^® membranes.     

Creep behavior of amorphous ethylene–styrene interpolymers in the glass transition region

The viscoelastic behavior of amorphous ethylene–styrene interpolymers (ESIs) was studied in the glass transition region. The creep behavior at temperatures from 15°C below the glass transition temperature (T_g) to T_g was determined for three amorphous ESIs. These three copolymers with 62, 69, and 72 wt % styrene had glass transition temperatures of 11, 23, and 33°C, respectively, as determined by DMTA at 1 Hz. Time–temperature superposition master curves were constructed from creep curves for each polymer. The temperature dependence of the shift factors was well described by the WLF equation. Using the T_g determined by DMTA at 1 Hz as a reference temperature, C₁ and C₂ constants for the Williams, Landel, and Ferry (WLF) equation were calculated as approximately 7 and 40 K, respectively. The master curves were used to obtain the retardation time spectrum and the plateau compliance. The entanglement molecular weight obtained from the plateau compliance increased with increasing styrene content as 1,600, 1,870, and 2,040, respectively. The entanglement molecular weight of the ESIs was much closer to that of polyethylene (1,390) than to that of polystyrene (18,700); this was attributed to the unique chain microstructure of these ESIs with no styrene–styrene dyads. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2373–2382, 1999     

Serum albumin ligand binding volumes using high pressure denaturation

The pressure shift assay (PSA, also termed either PressureFluor or differential pressure fluorimetry) was used to study the thermodynamics of decanoate and dodecanoate lipid binding to human serum albumin (HSA) in the temperature range from 25 °C to 80 °C and the pressure range from 0.1 MPa to 400 MPa. The ligands stabilized HSA against both pressure and temperature denaturation. The P–T phase diagram for HSA bound to saturated fatty acids is shown. Pressure induced HSA denaturation reversibility is demonstrated via either intrinsic tryptophan or extrinsic probe 1,8-anilinonaphthalene sulfonate (ANS) fluorescence. The effect of guanidinium in a PSA was studied. PSA provides information on ligand binding volumes. The volume changes from protein–ligand binding are thermodynamically important and could be used in designing compounds with specific volumetric binding properties.     

Thermal–mechanical studies on a heterocyclic polymer (BBB). I. Tensile creep and recovery

Creep and recovery measurements have been used to study the thermal–mechanical properties of polybisbenzimidazobenzophenanthroline-dione (BBB) over the temperature interval 30–500°C (in vacuo). The creep measurements were augmented by x-ray diffraction, isothermal contraction, and solubility measurements. It is found that extensive intermolecular association giving a supramolecular structure with (nearly) planar polymeric repeat units stacked in a “graphite-like” array dominates the properties of BBB, and that this structure is maintained at temperatures as high as 500°C. The principal mode of creep gives rise to fully recoverable Andrade creep for which the strain depends on the cube-root of time.     

Prediction of polyurethane behaviour via time-temperature superposition: Meanings and limitations

Two main issues are essential nowadays for practitioners in the field of polymeric materials: how a polymer will behave under dynamic loading conditions and for how long a polymer is reliable. In this sense, the time-temperature superposition principle was applied to the main viscoelastic properties (E′, E″ and tan δ) of a series of polyurethane coatings (PU-DEG-TMP) tested for mechatronic devices. Polyurethanes are derived from an ester glycol (poly(ethylene adipate) glycol), an aromatic diisocyanate (4,4′-dibenzyldiisocyanate) and di/trifunctional chain extenders - diethylene glycol (DEG) and trimethylol propane (TMP). Despite polyurethane intrinsic rheologic complexity, the moduli/loss factor curves superimpose well over several decades of reduced frequency at the glass transition temperature (T_g), 0 °C and 15 °C, the last temperature being considered the midpoint of the practical testing range. Three criteria were for checking the applicability of the time-temperature superposition: the Cole-Cole plot, the similarity between the a_T calculated from both moduli (E′, E″) and the visual appearance of the final master curve. The presence of both hydrogen bonding and chemical joint points, along with some dangling chains put in a broader context the discussion of the microstructural features resulted from the application of the William-Landell-Ferry (WLF) equation.