Free Volume Sites Research Articles

The effect of crystallinity on chain mobility and free volume in the amorphous regions of a miscible polycarbonate/polyester blend

AbstractHomopolymers and blends of polycarbonate/ThermX have been investigated by differential scanning calorimetry, dynamic mechanical analysis, density measurements, and positron annihilation lifetime spectroscopy. The study focuses on the chain mobility and free volume in the amorphous miscible regions of the blends and how this mobility and free volume are affected by increasing crystallinity. It is proposed that judicious annealing, which results in increasing crystallinity (while avoiding ageing or transesterification), induces a constraint on the amorphous region leading to an increase in Tg and coalescence of free volume sites. © 1994 John Wiley & Sons, Inc.

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

AbstractPositron 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.

Positron lifetime study of the free-volume properties in the polymer polyacrylonitrile

Positron lifetime measurements have been made in the polymer polyacrylonitrile, in the iodine doped state as well as the undoped state. The measured lifetime spectra were well resolved into three components. The behaviour of the intermediate component and its intensity seems to indicate an initial positron trapping at the interfaces between the amorphous and the crystalline regions and a later trapping by the kinks of the crystalline region. From the behaviour of the average free-volume size and its density we interpret that the iodine atoms initially occupy the free-volume sites in the amorphous region and at larger concentrations they form complexes with the nitrile group of the polymer.

An investigation of the yield and postyield behavior and corresponding structure of poly(methyl methacrylate)

AbstractThe mechanical behavior of glassy polymers is time and temperature dependent as evidenced by their viscoelastic and viscoplastic response to loading. The behavior is also known to depend strongly on the prior history of the material, changing with time and temperature without chemical intervention. In this investigation, we examine the effects of this process of physical aging on the yield and postyield behavior and corresponding evolution in the structural state of glassy polymers. This has been achieved through a systematic program of uniaxial, isothermal, constant strain–rate tests on poly(methyl methacrylate) (PMMA) specimens of different thermal histories and by performing positron annihilation lifetime spectroscopy (PALS) measurements prior to and after mechanical deformation. PALS is an indicator of the free volume content, probing size and density of free volume sites and can be considered to be a measurement of structural state. The results of the mechanical tests show that aging acts to increase both the initial yield stress and the amount of strain softening which occurs subsequent to yield. Moreover, the amount of strain softening was found to be independent of strain rate indicating that softening is related to an evolution in structure as opposed to deformation kinetics. Furthermore, after sufficient inelastic straining, the initial thermal history is completely erased as evidenced by identical values of flow stress following strain softening, for both annealed and quenched polymer. Strong confirmation of the structural state or free volume related nature of the strain softening process is obtained by our companion PALS measurements. PALS detects an increase in the size of free volume sites following inelastic deformation and finds the initially annealed and quenched specimens to posses the same post‐deformation distribution. The size of sites is found to evolve steadily with inelastic strain until it attains a steady‐state value. This evolution of free volume with strain follows the observed softening of the flow stress to a steady‐state value. These results provide experimental evidence that an increase in free volume with inelastic straining accompanies the strain softening phenomenon in glassy polymers and that strain softening is indeed a de‐aging process. Based on our experimental results a mechanistically based constitutive model has been formulated to describe the effects of thermal history on the yield and postyield deformation behavior of glassy polymers up to moderate strains. The model is found to successfully capture the effects of physical aging, strain softening, strain rate, and temperature on the inelastic behavior of glassy polymers when compared with experimental results. © 1993 John Wiley & Sons, Inc.

Solid‐State Nuclear Magnetic Resonance Studies of Ions in Protective Coatings III : Sodium, Lithium, and Cesium Ions in Polyimide and Epoxy‐Polyamide Coatings

Sodium, lithium, and cesium ions were cathodically implanted into polybutadiene coatings previously and their chemical environments were examined by solid‐state nuclear magnetic resonance (NMR) techniques.1,2 Herein, these ions were implanted into polyimide and epoxy‐polyamide coatings in order to extend earlier findings. The ions were present in two types of environments when exposed to water in the polyimide coating. The cesium ions in polyimide generally yielded two peaks in the 133Cs NMR spectrum, while two species of sodium and lithium ions were inferred from relaxation data. The majority of the ions was associated with those present in the free volume sites of the polymer, while the remainder was associated with those present in pathways (defects). The NMR data showed that lithium, sodium, and cesium ions were present as a single species when implanted into the epoxy‐polyamide coating. In all three cases, a single peak occurred in the NMR spectra and a single component exponential decay curve was determined from the spin‐spin relaxation data. The most probable location of these ions is in pathways (defects) as inferred from the relaxation data. Lithium and sodium ions associated with the carboxyl groups present in the matrix when the coatings were dried.

Thermally induced microstructural changes in undoped and doped polyacrylonitrile: A positron-annihilation study.

Positron-lifetime measurements have been made in undoped and polymers 0.8 wt %--iodine-doped polyacrylonitrile (PAN) under annealing from room temperature to 160 \ifmmode^\circ\else\textdegree\fi{}C. From the measured lifetimes, the free-volume size is calculated following the treatment of Nakanishi, Jean, and collaborators. The variation in free-volume size as function of temperature seems to suggest molecular-chain-folding processes in amorphous regions of the polymer, positron trapping at kink-type defects, and at the interface between crystalline and amorphous regions at lower temperatures. This study indicates that in the undoped state, the polymer has a glass transition temperature at 90 \ifmmode^\circ\else\textdegree\fi{}C which is shifted toward lower temperature in the doped state by 30 \ifmmode^\circ\else\textdegree\fi{}C. The results also suggest the degradation of the polymer in the two states after 100 \ifmmode^\circ\else\textdegree\fi{}C and the effect of annealing is only on the size but not on the density of free-volume sites.

Characterizing viscoelastic materials using the free volume microprobe

A great deal of effort has been directed toward the development of new polymer blends and interpenetrating polymer networks in recent years. This approach has emerged as an important means of developing new polymeric materials and improving properties such as sound damping. The thermodynamic models that best describe polymer‐polymer interactions have an important free volume contribution. Until recently, however, there was no technique that could make a direct, nondestructive measurement of the molecular free volume. Using the free volume microprobe (FVM), it is now possible to characterize both the average free volume site size and the relative number of free volume sites. Thus it is possible to make some assessment of the total free volume as well as the free volume distribution. The basic theory of the FVM technique will be presented as well as some supporting data from a series of miscible, immiscible, and partially miscible polymer blends.

Ductility and swelling of neutron-irradiated amorphous Fe40Ni40B20

Neutron-irradiation of amorphous Fe40Ni40B20 at 70°C leads to a swelling which saturates at about 1% beyond fluences > 1 × 1019 n/cm2. The ductility of thermally embrittled specimens is restored by the irradiation once the radiation-induced excess free volume exceeds a certain amount, indicating an interrelation to exist between the excess free volume sites and the mechanical properties. At high damage levels all specimens become brittle because of the concurrent radiation-enhanced formation of amorphous (Fe,Ni)3B clusters.

Frottement intérieur dans les solides vitreux vers la transition vitreuse

Several mechanical relaxation phenomena are observed on glasses. The characteristics of these phenomena vary from one type of glass to another but some features seem to be common to all solids in the vitreous state. Hence, results about: 1) oxide glasses; 2) organic glasses; 3) vitreous semi-conductors; 4) metallic glasses are recalled. Then, it is shown, that there are a relaxation process about the glass transition T g (primary or α relaxation) and several relaxation processes more or less differenciated at lower temperature (secondary relaxations): if one is restricted to the temperature range not too lower than T g , a peak or an anomaly in the spectrum internal friction versus temperature more or less resolved from the increase of damping due to the α relaxation, is often observed. This feature seems to be associated to delayed elasticity. We propose that such a behavior corresponds to a second relaxation phenomenon (β relaxation) which is, as α relaxation, characteristic of the vitreous state. Most models describing such a behaviour remain very phenomenological. Hence, it seems interesting to propose an interpretation following more physical basis: after a short review about ideas previously put forward and concerning molecular mobility in vitreous solids near T g , a new description of elementary processes of deformation in glass induced by an applied stress, is presented. Such a model takes again into account the assumption about localized processes (high free volume sites) but structural relaxation is considered in order to explain that internal stresses associated to such processes can be relaxed, thus allowing new processes to occur. Then the deformation of vitreous solids near T g might be essentially controlled by recovery phenomena. From such a model of deformation, dynamic modulus (internal friction and modulus effects) can be calculated and the results are discussed by comparison to experimental data Bilan des resultats concernant les proprietes micromecaniques des solides vitreux (oxydes, semi-conducteurs, composes organiques, alliages metalliques), qui permet d'illustrer l'existence des relaxations α (viscoplasticite) et β (viscoelasticite). Discussion des modeles utilises jusqu'ici pour interpreter ces relaxations. Proposition d'un nouveau modele reprenant l'idee que ces processus sont localises (sites a volume libre eleve), mais en tenant compte de l'effet de la relaxation structurale qui supprimerait les contraintes internes associees a ces processus, qui pourraient alors etre reactives