Evolution of nanometer voids in polycarbonate under mechanical stress and thermal expansion using positron spectroscopy

Positron Annihilation Lifetime Spectroscopy (PALS) measurements were conducted on polycarbonate subjected to either thermal expansion or to tensile and compressive strains. It was found that thermal expansion affected both the nanometer hole size and the hole number density, whereas mechanical stress affected mainly the size of existing holes, and did not generate or eliminate holes in the quasielastic deformation region. The effect of stress on yield and postyield behavior of this glassy material was also investigated. The deduced hole volume fraction of this polymer at 25°C was 6.8 ± 0.5% from the thermal expansion experiment and 7.2 ± 1.2% from the mechanical loading experiment. When the specimen was under compression, the hole volume fraction was found to continuously decrease. This can be considered as evidence of the inability of the free volume concept in explaining the yield behavior of glassy polymers. ©1995 John Wiley & Sons, Inc.     

Effect of cyclic stress on structural changes in polycarbonate as probed by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is used to probe structural changes in glassy polycarbonate in terms of changes in the hole volume and the number density of holes during fatigue (cyclic stress) aging. The ortho-positronium (o-Ps) pickoff annihilation lifetime τ₃, as well as the intensity I₃, were measured as a function of cyclic stresses and various previous thermophysical aging histories. It is found that τ₃, the longest of the three lifetime components resolved in the PALS of glassy polycarbonate, increases when a cyclic stress is applied. These results indicate that there is a structural change during fatigue aging. The “holes” where o-Ps can localize become larger upon fatigue aging. These results also suggest that a significant distinction exists between structural changes induced by thermophysical aging and fatigue aging. The o-Ps annihilation intensity, which is a relative measure of the hole density in a material, showed a continuous decrease upon fatigue aging, indicating the possibility of hole coalescence, which could be a precursor of crazing. The interaction between thermophysical aging and fatigue aging corresponds very well with the enthalpy relaxation behavior as reported previously, viz., a well-aged sample is much more sensitive to cyclic stress. More importantly, it is hypothesized that fatigue failure initiation is probably closely related to hole size and density fluctuation.     

聚苯基单醚喹噁啉薄膜的性能与物理老化

研究了物理老化对聚苯基单醚喹啉薄膜的结构与力学性能的影响 .用差示扫描量热计 (DSC)及正电子湮没寿命谱 (PALS)方法表征了两种不同物理老化条件试样的凝聚结构以及自由体积的差别 .结果表明 ,物理老化使聚苯基单醚喹啉薄膜玻璃化转变温度移向高温 ,在其末端出现热焓吸收峰 ,分子链堆砌紧密使自由体积减小 ,分子可动性降低 .用动态力学分析 (DMTA)以及静态拉伸性能测试等方法研究了两类试样的力学性能 ,结果表明 ,物理老化后 ,试样的动态储能模量稍有增加 ,力学损耗降低 .而静态拉伸实验的断裂应变降低 ,屈服应力增加 ,断裂能降低 ,试样在宏观上由韧性断裂变为明显的脆性断裂 .     

On the measurement of structural relaxation in polymers using positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy has been identified as an effective means of characterizing the free volume content of amorphous polymers. The lifetime and intensity of the ortho-positronium (o-Ps) pick-off annihilation has been found to correlate with the average size and density of free volume sites, respectively. Recently, PALS has been used to evaluate and monitor the physical aging and structural relaxation of polymers in terms of both initial state and evolution in state with time. However, during extended PALS measurements in insulating materials, an electric field can build up due to positron-electron annihilation and can effectively reduce the probability of positronium formation. In this paper, an observed decrease in intensity associated with the o-Ps annihilation component in the glassy polymers polycarbonate and polystyrene is found to be unrelated to structural relaxation of the materials over the time periods examined as reported earlier by others, and, instead, to be more likely a result of electric charge build-up. © 1993 John Wiley & Sons, Inc.     

Free volume quantities and viscoelasticity of polymer glasses

We have investigated, in terms of the Cohen-Turnbull theory, a relationship for polycarbonate (PC) glasses between average stress relaxation times, <to, and average free volume sizes, 〈v_f〉, obtained from positron annihilation lifetime spectroscopy. This examination suggests that the minimum free volume required for stress relaxation, v*, decreases with decreasing temperature and that, near the glass transition temperature, only a subset of extremely large free volume elements contributes to the stress relaxation of PC glasses. This suggestion is consistent with the idea that near the glass transition temperature, the viscoelastic response is dominated by large-scale, main-chain motion, whereas at lower temperature it is controlled by local motion. Moreover, comparison with the v* value estimated from gas diffusivity through various PC species at room temperature shows that the required free volume size for stress relaxation in the glass transition region is much larger than that for gas diffusion. Previously we showed that the Doolittle equation fails to correlate viscoelastic relaxation times of polymer glasses with changing temperature; determining the free volume fraction, h, from theoretical analysis of volume recovery data and theory, the Doolittle equation is shown to be valid in PC above 135°C (T_g - 14°C) irrespective of temperature and physical aging times. This result supports the idea suggested in the previous article that, as glassy polymers approach the transition region, viscoelastic properties increasingly tend to be controlled by free volume. © 1996 John Wiley & Sons, Inc.     

Mechanisms of anelastic deformation in solid polymers: Solidlike and liquidlike processes

Several aspects of anelastic deformation of glassy polymers that cannot be explained in terms of existing theories are considered. Resemblance in the stress-strain response for solids of various natures and structures, including semicrystalline and glassy polymers, organic and inorganic solids, and low-molecular-mass and high-molecular-mass compounds, is analyzed. It was pointed out that the phenomena of the yield peak, strain softening, strain concentration (localization) in narrow shear bands, and transient effects are characteristic of the plastic deformation of any solid. The same is true for differences in the kinetics and mechanism of deformation at low (T _def < 0.7T _g) and high deformation temperatures (T _def > 0.7T _g). The mechanism of plastic deformation is discussed in detail for glassy polymers; at microscopic and nanoscale levels, plastic deformation proceeds via two stages: initial nucleation of small-scale shear transformations and their further coalescence. This coalescence leads to the advance of the shear front in the sample and to the nucleation and displacement of classical shear bands. The heat of plastic deformation is released out at the coalescence of shear transformations. It was assumed that shear transformations are responsible for the development and evolution of the yield peak in glassy polymers, strain softening, and other phenomena. The proposed mechanism of deformation in glasses fully agrees with the results of thermodynamic measurements and other experimental data reported in the literature. Computer simulation makes it possible to visualize the scenario of nucleation and evolution of shear transformations at the atomic level.     

Structural aspects of deformation of amorphous polymers

Experimental and theoretical data on the inelastic deformation of amorphous glassy polymers were analyzed. The decisive role of direct structural methods in determination of the deformation mechanism of glassy polymers was established. A new mechanism of deformation and thermally stimulated recovery of strained glassy polymers was considered on the basis of structural data analysis.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 1–6, January, 2005.     

Amorphous-amorphous transition in glassy polymers subjected to cold rolling studied by means of positron annihilation lifetime spectroscopy

In this study, polycarbonate (PC) and polystyrene (PS) are subjected to plastic deformation by means of cold rolling and the resulting variation of the free volume and its subsequent time evolution after rolling is investigated by means of positron annihilation lifetime spectroscopy (PALS). The value of the long lifetime component that is attributed to the decay of ortho-positronium (tau(o-Ps)) and its intensity (I(o-Ps)) are used to characterize, respectively, the size and the concentration of the free-volume holes. In addition to the PALS experiments, the effect of plastic deformation on the dynamic tensile modulus is investigated. The PALS results show that both for well-aged PC and PS an increase of tau(o-Ps) and a decrease of I(o-Ps) occur upon plastic deformation. During the subsequent aging, tau(o-Ps) tends to return to the value assumed before plastic deformation, while I(o-Ps) remains constant with time. These results corroborate the idea of an amorphous-amorphous transition, rather than that of a "mechanical rejuvenation" as proposed in the past to explain the ability of plastic deformation to reinitiate physical aging. Finally, a linear relation between the size of the free-volume holes and the dynamic tensile modulus is found, which suggests that the stiffness of amorphous glassy polymers is fully determined by their nanoscopic structure.     

Free volume and water uptake in a copolymer hydrogel series

A copolymer of 2-hydroxyethyl methacrylate (HEMA) with 2-ethoxy ethyl methacrylate (EEMA) was synthesized and the molecular mobility, free volume, and density properties examined as a function of composition. These properties were correlated with the equilibrium water uptake in order to determine which of the properties were most influential in causing high water sorption, as these materials are suitable candidates for hydrogel systems. It was found that the polar HEMA repeat unit results in a rigid, glassy sample at room temperature due to the high degree of hydrogen bonding between chains whereas high EEMA content leads to rubbery samples with subambient glass transition temperatures. The free volume properties on the molecular scale measured by positron annihilation lifetime spectroscopy (PALS) showed that higher HEMA content led to smaller, fewer holes and a lower free volume fraction than EEMA. Therefore the high water uptake of HEEMA-containing copolymers is largely related to the high polarity of the HEMA unit compared to EEMA, despite the low content of free volume into which the water can initially diffuse. Trends in density with copolymer composition, as measured on a macroscopic level, differs to that seen by PALS and indicates that the two techniques are measuring different scales of packing. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 463–471, 1998     

Kinetics of re‐embrittlement of (anti)plasticized glassy polymers after mechanical rejuvenation

Mechanical rejuvenation is known to dramatically alter the deformation behavior of amorphous polymers. Polystyrene (PS)—for example, typically known as a brittle polymer—can be rendered ductile by this treatment, while a ductile polymer like polycarbonate (PC) shows no necking anymore and deforms homogeneously in tensile deformation. The effects are only of temporary nature, as because of physical aging the increasing yield stress, accompanied by intrinsic strain softening, renders PS brittle after a few hours, while for PC necking in tensile testing returns in a few months after the mechanical rejuvenation treatment. In this study, it is found that physical aging upon rejuvenation in both PS and PC can be delayed in two different ways: (1) by reducing the molecular mobility through antiplasticization and (2) by applying toughening agents (rubbery core–shell particles). For the first route, even though progressive aging is found to decrease with increasing amounts of antiplasticizer added, dilution of the entanglement network results in enhanced brittleness. Besides antiplasticization effects, also some typical plasticization effects are observed, like a reduction in matrix T_g. For the second route, traditional rubber toughening using acrylate core–shell modifiers also results in a reduced yield stress recovery, and ductile tensile deformation behavior is observed even 42 months after mechanical rejuvenation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 134–147, 2008     

Effect of physical aging of poly(1‐trimethylsilyl‐1‐propyne) films synthesized with TaCl5 and NbCl5 on gas permeability,fractional free volume,and positron annihilation lifetime spectroscopy parameters

The effect of physical aging on the gas permeability, fractional free volume (FFV), and positron annihilation lifetime spectroscopy (PALS) parameters of dense, isotropic poly(1‐trimethylsilyl‐1‐propyne) (PTMSP) films synthesized with TaCl₅ and NbCl₅ was characterized. As‐cast films were soaked in methanol until an equilibrium amount of methanol was absorbed by the polymer. When the films were removed from methanol, film thickness initially decreased rapidly and was almost constant after 70 h in air for both catalysts. This timescale was much longer than the timescale for complete methanol desorption (ca. 5 h). From the film‐thickness data, the reduction in FFV with time was estimated. For samples prepared with either catalyst, the kinetics of FFV reduction were well‐described by a simple model based on the notion either that free‐volume elements diffuse to the surface of the polymer film and are subsequently eliminated from the sample or that lattice contraction controls polymer densification. Methane permeability decreased rapidly during the first 70 h, which was the same timescale for the thickness change. The decrease in methane permeability was smaller in films prepared with NbCl₅ than with TaCl₅. The logarithm of methane permeability decreased linearly as reciprocal FFV increased, in accordance with free‐volume theory. The PALS results indicate that the concentration of larger free‐volume elements (as indicated by the intensity I₄) decreased with aging time and that the other PALS parameters were not strongly influenced by aging. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1222–1239, 2000     

The Nonlinear Viscoelastic Response of Glassy Polymers Subjected to Physical Aging

Constitutive equations are developed for the nonlinear viscoelastic behavior of amorphous glassy polymers in the sub‐yield region. A polymeric glass is treated as an ensemble of cooperatively rearranging regions bridged by links. Stress‐strain relations are derived and verified by comparison with experimental data in static mechanical tests on polycarbonate and poly(methyl methacrylate). We analyze the effects of the straining state (tension, compression and torsion), strain intensity, temperature and time of annealing on stress relaxation. Fair agreement is demonstrated between observations and results of numerical simulation.     

Studies on the free volume and the volume expansion behavior of amorphous polymers

In order to estimate the free volume contribution on the volume change, we investigated the relationship between the volume expansion behavior by Pressure–Volume–Temperature measurement apparatus and the free volume behavior by Positron Annihilation Lifetime Spectroscopy for some amorphous polymers. From these results, the free volume fraction of the amorphous polymers was calculated by assuming that the core volume increases at a constant rate with temperature. It was found that the amount of free volume was not constant even in the glassy state and it played a very important role in the volume expansion.     

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     

Physical aging phenomena in silica and glass beads filled elastomers (EPDM)

The physical aging concept is generally used to explain the typical behavior of amorphous glassy materials such as amorphous polymers. It can be easily evidenced by measuring the effect of a static deformation on the dynamic mechanical properties. In this paper, an attempt is made to determine the “glassy” behavior of elastomeric EPDM chains when they are confined in the vicinity of the filler (glass beads, silicas) surface. It is demonstrated that glassy behavior and physical aging phenomena are detected even with a filled elastomer. Furthermore, the influence of the filler volume fraction, the filler nature and of filler surface treatments with silanes were studied. Finally, an original attempt is made to explain filler-rubber reinforcement by a kind of bimodal network created from linkages between a densely packed interfacial region and the outer loose matrix.     

Deformation induced evolution of mobility in PMMA

The stress relaxation response in the glassy state just below T_g 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     

Fundamental relationship between the nonequilibrium glassy state and yield stress of amorphous polymers

This paper reports the theoretical prediction and experimental verification of the connection between the yield stress of amorphous polymers and the physical aging phenomenon. The analysis reveals the existence of a fundamental relationship between the nonequilibrium glassy state and the thermally activated process controlling viscoelastic and plastic deformation. The results show that the volume relaxation and deformation kinetics share the same relaxation times, and that the activation energy for deformation below T_g is much smaller than previously mentioned in the literature. This indicates that the phenomenon of physical aging plays a very important role in the deformation and processing of polymers at low temperatures. The effect of quenching and annealing on the yield stress is described in terms of the mean energy of hole formation, the departure of volume from its equilibrium state, the distribution of hole energies, and lattice volume. The same set of molecular parameters obtained from the molecular kinetic theory of the glass transition and volume relaxation predicts the yield stress as a function of cooling rate, annealing time, temperature, and strain rate.     

Structure and plastic deformation of aromatic main-chain mesomorphic copolyesters

Thermodynamic characteristics of inelastic deformation (work W _def, heat Q _def, and stored energy ΔU _def) are studied for aromatic main-chain copolyesters (CPEs) based on p-hydroxybenzoic acid and poly(ethylene terephthalate) (Rodrun and SKB-1), p-hydroxybenzoic acid, naphthalene carboxylic acid, and terephthalic acid with hydroquinone and dioxyphenyl (HX-6000 and HX-7000). The samples are deformed under an active uniaxial compression by ?_def ≈ 50% at room temperature. All CPEs are semicrystalline polymers; their degree of crystallinity is (depending on their prehistory) 5–30%, and the melting temperature of crystallites is 275–350°C. Seemingly, the glassy component of CPEs includes two interpenetrating glassy structures, S-1 and S-2, with different glass-transition temperatures Tg: 90–120 and 250–270°C, respectively. During loading, all coexisting crystalline and glassy structures of CPEs store residual strain ?_res. The kinetics of the temperature-stimulated strain recovery of ?_res is measured. In component S-1, strain recovery occurs in the temperature interval ranging from T _room to 120°C. In the crystalline phase, this process occurs in the melting-temperature interval. In component S-2, strain recovery ?_res commences at T > 120°C. In CPEs, all structural components are involved in deformation at different ?_def. At small strains only component S-1 is deformed; then, at ?_def ≈ 10–15%, component S-2 is involved in the deformation. Crystallites join this process at ?_def > 20–25% (? _y = 8–10%). In CPE, two modes of deformation arise: reversible elastic (retarded elastic) and true plastic irreversible deformation. True plastic permanent strain always exists in the deformed CPEs. Deformation of all CPEs proceeds easier than that of all “common” glassy polymers (polystyrene, poly(methyl methacrylate), etc.). In CPEs, the yield stress and compressive modulus appear to be ≈40–50% lower than in “common” glassy polymers. It seems that the mesomorphic structure of LC CPEs enhances the elementary plastic processes in them. Thermodynamic characteristics of the S-1 phase plasticity are compared with the behavior of “common” glassy polymers. At the early stages of loading, nearly all mechanical work of deformation W _def spent is stored in phase S-1 in the form of δU _def, as in all “common” glassy polymers. This fact implies that the inelastic deformation of LC glasses commences with the nucleation of small-scale and localized intermolecular transformations of the nonconformational type. In both mesomorphic and “common” glassy polymers, the stage of nucleation of such transformations controls the overall kinetics of the inelastic and plastic deformation. Nucleation does not depend on chain rigidity, a circumstance that conflicts with the model of forced elasticity. It seems that crystallites in CPE are deformed according to crystallographic mechanisms. Currently, neither the structure nor the deformation mechanism of component S-2 is known.     

Thermophysical behaviour of solid polymers in simple elongation: A structural and molecular interpretation

Thermodynamic aspects of reversible simple extension of solid polymers have been considered in terms of the conventional equation of state and equations have been obtained for the thermodynamic functions. It is shown that simple deformation of solids is accompanied by inversion of internal energy which is controlled by coefficient of thermal expansion. Work, heat and internal energy as functions of strain have been determined by deformation calorimetry for the typical glass-like and crystalline polymers and it has been found that in uniaxially oriented crystalline polymers at aboveT _g the internal energy undergoes inversion due to the negative coefficient of thermal expansion. It has been demonstrated that the thermoelastic behaviour of two-phase crystalline polymers is controlled by the volume (irrotational) elasticity of amorphous regions rather than by shape elasticity typical of rubber elasticity. From this position, a thermophysical analysis of the deformation of the basic models of oriented crystalline polymers and combined investigation of the thermal phenomena and structural changes in oriented PE and PP have been carried out. It has been shown that the Peterlin-Prevorsek model which implies existence of both intra- and interfibrillar amorphous regions quite adequately account for the thermophysical and structural effect observed in tension of the oriented specimens in the original and annealed state. Thermoelastic properties of super-oriented crystalline polymers have also been discussed in brief.Dedicated to Professor Dr. F. H. Müller     

The impact of water and hydrocarbon concentration on the sensitivity of a polymer-based quartz crystal microbalance sensor for organic compounds

Long-term environmental monitoring of organic compounds in natural waters requires sensors that respond reproducibly and linearly over a wide concentration range, and do not degrade with time. Although polymer coated piezoelectric based sensors have been widely used to detect hydrocarbons in aqueous solution, very little information exists regarding their stability and suitability over extended periods in water. In this investigation, the influence of water aging on the response of various polymer membranes [polybutadiene (PB), polyisobutylene (PIB), polystyrene (PS), polystyrene-co-butadiene (PSB)] was studied using the quartz crystal microbalance (QCM). QCM measurements revealed a modest increase in sensitivity towards toluene for PB and PIB membranes at concentrations above 90 ppm after aging in water for 4 days. In contrast, the sensitivity of PS and PSB coated QCM sensors depended significantly on the toluene concentration and increased considerably at concentrations above 90 ppm after aging in water for 4 days. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR–FTIR) showed that there is a change in the sorption mechanism at higher toluene levels for PS and PSB. Positron annihilation lifetime spectroscopy (PALS) studies were performed to investigate the free volume properties of all polymers and to monitor any changes in the free volume size and distribution due to water and toluene exposure. The PALS did not detect any considerable variation in the free volume properties of the polymer films as a function of solution composition and soaking time, implying that viscoelastic and/or interfacial processes (i.e. surface area changes) are probably responsible for variations in the QCM sensitivity at high hydrocarbon concentrations. The results suggest that polymer membrane conditioning in water is an issue that needs to be considered when performing QCM measurements in the aqueous phase. In addition, the study shows that the hydrocarbon response is concentration dependant for polymers with a high glass transition temperature, and this feature is often neglected when comparing sensor sensitivity in the literature.