Antiplasticization of a polyamide: a positron annihilation lifetime spectroscopy study

In order to modify the transport properties of the amorphous polyamide Trogamid-T™, several additives with structural characteristics that produce antiplasticization have been tested. Positron annihilation lifetime spectroscopy (PALS) has been used to determine both the size of free volume holes and the number of holes (free fractional free volume). Polyvinyl phenol (PVPh), 1,5 dihydroxy naphthalene (Ndiol) and a hexafluorinated Bisphenol A (HFBA) were chosen according to the results of a preliminary study based on density and PALS measurements with 15% of additive blends. Their blends with Trogamid show different behaviours: HFBA behaves clearly as a plasticizer while the polyvinyl phenol acts as an antiplasticizer. Ndiol has a more special effect: blend densities are greater than additive whereas the hole size decreases.     

Positron annihilation lifetime spectroscopy of molecularly imprinted hydroxyethyl methacrylate based polymers

Radiation-induced molecular imprinting of d-glucose onto poly(2-hydroxyethyl methacrylate) (HEMA) matrix was achieved to create three-dimensional cavities to recognize and bind glucose. Molecularly imprinted polymers (MIPs) were synthesized with different types of crosslinkers and varying amounts of template molecule in an attempt to elucidate the impact of imprint quantities on the effectiveness of imprinting technique. The crosslinking agents used in this study were diethylene glycol diacrylate (DEGDA), triethylene glycol dimethacrylate (TEGDMA) and polypropylene glycol dimethacrylate (PPGDMA) in the order of increasing chain length. Crosslinking agent concentration in the polymerization mixture (monomer, crosslinking agent and template) covered a range of 10, 20, 30, and 70 mol%. The mole ratio of template molecule, d-glucose to functional monomer, HEMA, was kept either as 1:3 or 1:6. The absorbed dose varied from 1 to 15 kGy. Control polymers were synthesized with exactly the same composition in the absence of d-glucose. Cavity sizes of MIPs were investigated by positron annihilation lifetime (PAL) measurements. A sandwich arrangement (sample-source-sample) was used. PAL experiments were carried out using a conventional fast-fast coincidence system having a time resolution (FWHM) of about 280 ps. Free-volume hole radii of samples were investigated in their dry and fully water swollen state.The results obtained from a systematic study of the effects of concentration and molecular size of the crosslinking agents, template to monomer ratio and irradiation dose experiments suggest that control of cavity size is feasible in nanometer scale by the optimization of these parameters revealed by means of (PAL) spectroscopy technique.     

Change of structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate) during thermal treatments by positron annihilation lifetime

The structure and free volume properties of semi-crystalline poly(3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV) were investigated in this study. The structure change and conformational motion of PHBV during melting and crystallization process were discussed by in situ FTIR. The free volume within the amorphous phase and its temperature dependence in the PHBV membrane, which was prepared by the compression molding method with isothermal crystallization processes, were characterized by using positron annihilation lifetime (PAL) spectroscopy. From the lifetime parameters, the temperature dependence of free volume size, amount, distribution, and fractional free volume, and the thermal expansion of free volume and/or polymer were discussed. Furthermore, the knee temperature was first observed in the melting process of the crystallized PHBV membranes. It indicated that there was structural transition of polymer chains during melting as the corresponding results observed with in situ FTIR measurement.     

Role of free volume characteristics of polymer matrix in bulk physical properties of polymer nanocomposites: A review of positron annihilation lifetime studies

Polymer nanocomposites which have one or more nano-dimensional phases dispersed in polymer matrix show enhancement in bulk physical properties. In order to achieve the desired properties, a large number of polymer nanocomposites have been prepared by choosing different polymers and nanofillers. These studies showed that interfacial interaction between polymer molecules and nanofillers is the most important factor to achieve the synergistic effect towards enhancement in the bulk physical properties. The strong interfacial interaction also promotes the fine dispersion of nanofillers in a polymer matrix which consequently enables the preparation of polymer nanocomposites with higher loading of nanofillers. The polymer matrix constitutes a large volume fraction of polymer nanocomposites and hence the molecular packing of the polymer matrix itself plays a deterministic role in governing the physical properties of the nanocomposites. The strong interfacial interaction brings severe changes in the original molecular packing. In order to establish the structure-property relationships for polymer nanocomposites, characterization of molecular packing of polymer matrix in its nanocomposites is essential. In this aspect positron annihilation lifetime spectroscopy (PALS) is a highly suitable technique for characterization of free volume holes in polymers or polymer nanocomposites. The present review briefly describes the positron annihilation lifetime spectroscopy technique and relevant models for calculations of free volume hole’s size, density and their size distribution in polymer nanocomposites. We present a summary of the recent studies focussed on investigation of free volume structure (molecular packing) of polymer nanocomposites using PALS and its impact on transport, thermal and mechanical properties of the nanocomposites.     

Free volume in PEP-silica nanocomposites with varying molecular weight

The influence of confinement in polymer-nanocomposites on free volume and glass transition of the polymer chains was studied. The molecular weight (Mw) of poly(ethylene-alt-propylene) (PEP) was varied from 3k to 200k and so the end-to end distance of the chains at fixed diameter (∼15 nm) and concentration (15%) of the silica nanoparticles increased. Thus the topological confinement increases with increasing Mw. Using hydrophobic PEP and particles functionalized with short organic molecules, we can rule out contributions of permanent adsorption of the chains. DSC showed no change in glass transition temperature. The decrease in the specific heat capacity could be explained by a simple mixing rule. By positron annihilation lifetime spectroscopy, taking properly into account contributions of the silica particles, we rule out an influence of the geometrical confinement on the free volume in the PEP nanocomposites studied here.     

Free volume in two differently plasticized poly(vinyl chloride)s: a positron lifetime and PVT study

The temperature dependence of the specific volume, V, and of the mean size of local free volumes (holes), v_h, of poly(vinyl chloride) containing 10 wt% (PVC-h) and 30 wt% (PVC-s) of the plasticizer di-n-butyl phthalate (DBP) were studied by pressure-volume-temperature (PVT) and positron annihilation lifetime spectroscopy (PALS) experiments. Using the Simha-Somcynsky equation-of-state, the hole fraction h was calculated from the PVT data. It was found that the thermal expansivity of the specific volume V, the specific free volume V_f=hV, and v_h increase with the content of plasticizer. From the comparison of V and V_f with v_h the hole number per gram N′_h is estimated. Other than v_h, N′_h does not depend on the temperature nor on the content of plasticizer.     

Characterization of free volume and density gradients of polystyrene surfaces by low-energy positron lifetime measurements

Free volume and density gradient widths of the free surface of high molecular mass polystyrene was characterized by use of a pulsed low-energy positron lifetime beam. A density gradient in terms of mass density (g/cm³) versus depth (nm) was obtained from the experimental beam data using a novel approach, yielding a width of the density gradient of approximately 3.5 nm. The procedure accounted for the broadening of the positron implantation probe as a function of energy as well as correcting for the increase in positron implantation depths due to the presence of a lower density at the surface region.Moreover, the spectra in the low implantation energy range were found to contain a long-lived lifetime that yielded very large estimates of the free volume effects at the surface. Accounting for this lifetime in the evaluation procedure, resulted in a much improved agreement with the results of other experimental and theoretical investigations in the literature.     

A variable energy positron annihilation lifetime spectroscopy study of physical aging in thin glassy polymer films

The influence of physical aging on the profile of free volume characteristics in thin polysulfone (PSF) films was investigated using variable energy positron annihilation lifetime spectroscopy. The PSF films exhibited decreasing o-Ps lifetime during physical aging, while o-Ps intensity remained constant. The o-Ps lifetime was reduced at lower implantation energies, indicating smaller free volume elements near the film surface (i.e., in the top ∼50 nm). These near-surface regions of the films age dramatically faster than bulk PSF. The accelerated aging is consistent with the notion of enhanced mobility near the film surface, which allows polymer near the surface to reach a lower free volume state more quickly than the bulk. No influence of the silicon wafer support on aging behavior was detected. Additionally, the impact of CO₂ conditioning on physical aging was briefly examined. The results from these studies were compared to aging behavior of ultrathin PSF films tracked by gas permeability measurements, and favorable agreement was found.     

Free volume distribution at the Teflon AF/silicon interfaces probed by a slow positron beam

We performed positron annihilation lifetime spectroscopy experiments at Teflon AF^®/silicon interfaces as function of the positron implantation energy to determine the free volume hole size distribution in the interfacial region and to investigate the width of the interphase. While no interphase was detected in very short chained solvent-free, thermally evaporated Teflon AF^®, an interphase of some tens of nm in extension was observed for high molecular weight spin-coated Teflon AF^® films. Influences of the native oxide layer on the data evaluation could be ruled out.     

Application of positron annihilation: Study of pervaporation dense membranes

Separation of liquid mixtures, particularly azeotropic mixtures, is being tried out by pervaporation process, which utilizes dense membranes. Such membranes, although are of non-porous type, still requires determination of free volume sizes which is crucial to the understanding of the process. Positron annihilation technique has been developed into a powerful characterization tool for the study of free volume and free volume fraction in polymers. In the present work, an attempt has been made to estimate the free volume sizes of commercial and laboratory made pervaporation membranes with the application of positron annihilation lifetime spectroscopy. The positron lifetime spectra were analyzed, assuming 3 or 4 exponential components. Long-lived components (lifetimes: in the range of 1.4-3 ns) were found, which were attributed to ortho-positronium (o-Ps) pick-off annihilations in free volumes. Accordingly, free volume size determinations were carried out, considering shapes for free volumes as spherical as well as cylindrical. Further, utilizing such free volume data, results of hydrazine hydrate separation by pervaporation were interpreted leading to a better understanding of the process.     

Study on the thermal behavior of a solution-cast liquid-crystalline polymer film by positron-annihilation lifetime spectroscopy

The thermal behavior of a solution-cast liquid-crystalline polymer (LCP) film was extensively studied by positron-annihilation lifetime spectroscopy (PALS). From the positronium (Ps) lifetimes of the first heating process from 40 to 250 °C at a heating rate of 2.5 °C/h, four characteristic temperatures (140, 170, 200, 235 °C) were observed. From a combined investigation with conventional differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), the first three characteristic temperatures were found to correspond to reorientation, glass-transition, and softening temperatures, respectively. The fourth temperature was related to the commencement of crystallization, which was observed above about 235 °C from a decrease in the Ps lifetime. A low-temperature PALS experiment exhibited the γ-transition due to rotation of the phenyl moiety at about −53 °C.     

Characterization of methyl‐substituted polyamides used for reverse osmosis membranes by positron annihilation lifetime spectroscopy and MD simulation

Three kinds of polyamides were synthesized from three diamines and 1,3,5‐benzenetricarbonyl trichloride (TMC). The diamines used were m‐phenylene diamine, N‐methyl‐m‐phenylenediamine, and N,N′‐dimethyl‐m‐phenylenediamine. The average free volume sizes of the polyamides were measured by positron annihilation lifetime spectroscopy (PALS), and the free volume fractions were evaluated by molecular dynamics (MD) simulations. The methyl substitution on amino groups of diamines brought about an increase in interstitial space of molecular chains of the polyamides. In addition, reverse osmosis (RO) membranes were prepared by interfacial polymerization from the three diamines and TMC. The increase in the degree of methyl‐substitution of diamines led to increased chlorine resistance and decreased salt rejections of the polyamide RO membranes. Thus, the methyl‐substitution of diamines significantly influenced membrane performance. The vacancy sizes and fractional volumes in polyamides evaluated by PALS measurement and MD simulation were well correlated with salt rejection of polyamide RO membranes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Free volume sizes in intercalated polyamide 6/clay nanocomposites

The effect of incorporating modified clay into a polyamide 6 (PA6) matrix, on the free volume cavity sizes and the thermal and viscoelastic properties of the resulting nanocomposite, was studied with positron annihilation lifetime spectroscopy, differential scanning calorimetry and dynamic mechanical analysis. At low concentrations of clay the fraction of PA6 crystals melting close to 212 °C was increased, while the fraction of the α-form PA6 crystals, melting close to 222 °C, was reduced. At higher concentrations of clay, a crystal phase with increased thermal stability emerged. Addition of more than 19 wt% clay caused a reduction of the heat of fusion of PA6. An unexpected reduction of the ΔC_p at the glass transition, contradicting the measured reduction of the heat of fusion, was detected, indicating an altered mobility in the non-crystalline regions. The viscoelastic response of PA6/clay nanocomposites, as compared to unfilled PA6, pointed towards a changed mobility in the non-crystalline regions. At high concentrations of clay (>19 wt%) an increase of the free volume cavity diameter was observed, indicating a lower chain packing efficiency in the PA6/clay nanocomposites. The increased free volume sizes were present both above and below the glass transition temperature of PA6.     

Positron/positronium annihilation as a probe for chemical environments of free volume holes in fluoropolymers

The sensitivity of the Doppler-broadening of positron annihilation radiation (DBAR) for the chemical environment of free volume holes was studied for fluorinated polymers. The free volume was characterized by positron annihilation lifetime spectroscopy (PALS). For this study, we prepared three series of copolymers of methacrylates with various semi-fluorinated methacrylates with increasing fractional number of fluorine atoms f_F. Moreover some commercial fluoroelastomers and semicrystalline fluoropolymers were investigated. After the subtraction of a narrow para-positronium DBAR component, calculated on a simplified way using the PALS data, from the conventional Doppler broadened annihilation energy line, the remaining broad distribution shows no response to the size of free volume holes or the positronium formation probability but can be related to the nature and density of the chemical species in the vicinity of the holes. Element specific information of the annihilation site can be expressed in terms of an integral parameter, e.g. the normalised curve shape parameter of the broad DBAR component. We observed that does not show the expected exponential decrease but goes down linearly with f_F in the range f_F<0.45 and levels-off for f_F≥0.5. This was explained assuming positronium localization and annihilation in two types of holes: holes, which have walls free of fluorine atoms and holes, which contain fluorine in their walls. A steeper, exponential-like decrease of with f_F seem to occur for the fluoroelastomeric and semicrystalline fluoropolymers which contain not less than one F atom per two backbone carbons. In this, the case ortho-positronium localized at a hole will always find at least one fluorine atom in which neighbourhood it can annihilate.     

Effect of adding feldspar on free volume properties of crosslinked polyester studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PAL) was applied to study the feldspar effect on the free volume properties of crosslinked polyester based on neopentyl glycol, succinic acid, phthalic anhydride, and maleic anhydride. The measurements have been carried on the polyester resin samples cured with three crosslinking agents namely styrene (SS) or styrene/methyl methacrylate (SM) or styrene/acrylonitrile (SA) comonomers mixtures in the ratios of 2 : 1 and loaded with different concentrations of feldspar in the range from 0 to 80 wt %. The free volume parameters (size and fractions of holes) depend on type of the crosslinking agent as well as the feldspar content added to the polyester. The results are supported by a significant variation in the nanoscale free volume hole size distributions. Moreover, the correlation between positron annihilation parameters and electrical parameters was discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Determination of the glass transition temperature of polymer/layered silicate nanocomposites from positron annihilation lifetime measurements

The glass transitions of acrylonitrile-butadiene rubber (NBR)/organoclay nanocomposites with various silicate contents were investigated using positron annihilation lifetime spectroscopy (PALS). The nanocomposites were prepared through melt intercalation of NBR with various concentrations of organoclay (OC30B) modified with the organic modifier, methyl tallow bis(2-hydroxyethyl) quaternary ammonium (MT2EtOH), i.e., Cloisite^® 30B. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) measurements of the NBR/OC30B nanocomposites showed that the NBR chains were intercalated between the silicate layers, thereby increasing the gallery heights of the organosilicates. The glass transition temperature of NBR was determined using differential scanning calorimetry (DSC). However, it seemed to be very difficult to clearly resolve the very small differences in T_gs caused from various loading of nanosized silicate in NBR/OC30B nanocomposites. Hence, we performed positron annihilation lifetime spectroscopy (PALS) on NBR/OC30B nanocomposites containing various amounts of OC30B (1-10 wt%). Significant changes in the temperature dependencies of free volume parameters (i.e., lifetimes and intensities) were observed at the transition temperature, T_g,PALS, and the T_g,PALS values were found to increase with increasing organoclay content in the samples. These observations are consistent with PALS having a higher sensitivity in the detection of very small changes in free volume properties. The present findings thus highlight the usefulness of PALS for studying phase transition phenomena in polymeric materials with nanoscale structural variations.     

Nanoscopic properties of silica filled polydimethylsiloxane by means of positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) was performed on a series of polydimethylsiloxane (PDMS)/fumed silicon dioxide (SiO₂) composites at temperatures between −185 and 100 °C to study the effect of filler content and filler particle size on the free volume properties and the positron annihilation characteristics. The glass transition behavior of the PDMS/SiO₂ composites was determined with differential scanning calorimetry. A clear influence on the o-Ps lifetime (τ₃) in the polymer upon addition of nano-sized fumed SiO₂ was observed at all temperatures. The observed o-Ps lifetime behavior was related to filler content and filler particle size. A transition in the temperature dependence of the o-Ps lifetime was observed close to −35 °C above which temperature PDMS exhibits long o-Ps lifetimes. A relationship between τ₃ and the surface tension, equivalent to the behavior of ordinary molecular liquids was observed in this temperature region. The o-Ps yield was strongly reduced in the crystallization region and by addition of SiO₂. The reduction due to filler addition did, however, in the case of nano-sized SiO₂ not follow a linear relationship with filler weight, which was observed for micron-sized fillers. The nonlinear relationship between filler weight and o-Ps yield could be due to out-diffusion of positrons and/or o-Ps from the filler particles to the matrix.     

Positron annihilation lifetime spectroscopy and diffusion studies of large molecule penetrants into alkyd resins

Alkyd resins can be used to create transparent films and their characteristics depend on the method used in their cure. Changing the catalysts can have profound effects on the physical properties of the films formed. Positron annihilation lifetime spectroscopy (PALS) measurements are used to explore how changes in catalyst influence the free volume for an alkyd resin system. Diffusion measurements are reported for large molecule permeants; dioctyl phthalate, 2-ethyl hexylbenzyl phthalate, nonyl phenol ethoxylate, isopropyl myristate and oleic acid into these alkyd resins. The diffusion behaviour changes as the temperature is raised through the glass transition temperature of the resin but is also sensitive to the chemical structure of the permeant. The diffusion deviates from simple Fickian behaviour in a number of cases and is indicative of plasticization influencing diffusion. Comparison of the average molecular size with the available free volume indicates that diffusion involves a complex co-operative motion in which the matrix plays an important role. Increasing the cross link density by post cure of the alkyd resin at elevated temperatures, reduces the free volume and decreases the ability for the large molecules to permeate in to the matrix. Oleic acid exhibits a two stage diffusion process; the second stage being associated with opening up of the resin structure as a result of acid catalysed degradation. The unusual behaviour of isopropyl myristate is attributed to a combination of its ability to adopt favourable conformations and a high degree of compatibility with the matrix. This study highlights the importance of the chemical structure of the permeant on its ability to enter and diffuse through the coating.     

Glass transition and free volume in the mobile (MAF) and rigid (RAF) amorphous fractions of semicrystalline PTFE: a positron lifetime and PVT study

The structure of the free volume and its temperature dependence in poly(tetrafluoroethylene) (PTFE) and of its copolymer with perfluoro(propyl vinyl ether) (PFA) was studied by pressure-volume-temperature (PVT) experiments (T=27-380 °C, P=0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS, T=−173-250 °C, P=10⁻⁵ Pa). From the analysis of these experiments we conclude on the volumetric properties of the mobile (MAF) and rigid amorphous fractions (RAF) in these semicrystalline polymers. The specific volumes of the MAF and RAF, V_MAF and V_RAF, were estimated assuming that V_MAF agrees with the specific volume of the melt extrapolated down to lower temperatures using the Simha-Somcynsky equation of state (S-S eos). V_RAF was then estimated from the specific volume of the entire amorphous phase, V_a, and the known V_MAF. The specific free volume V_f=V_a−V_occ was also estimated from V_a using the S-S eos hole fraction h, V_occ=(1−h)V_a. From the analysis of PALS data with the routine LT9.0 the mean volume, 〈v_h〉, and the width, σ_h, of the local free volume size distribution (holes of subnanometre size) were obtained. A comparison of 〈v_h〉 with V_f delivered the hole density N^′h. The volume parameters show that the RAF which is formed during crystallisation from the melt has a distinctly smaller specific free and total volume than the MAF. During cooling the contraction of the RAF slows down and finally, below room temperature, the RAF possesses a larger free volume than the MAF shows. Obviously, the restriction of the segmental mobility in the RAF by the crystals limits at high temperatures the free volume expansion and at low temperatures dense packing of the polymer chains. These conclusions from the analysis of the specific volume are confirmed by PALS experiments.     

Study on polystyrene thin film on glass substrate by scanning acoustic microscope

Acoustic micrograph and V(z) curves of polystyrene thin films on hydrophobic modified and unmodified alumino silicate glass substrates were studied in the frequency range from 170 to 450 MHz by a scanning acoustic microscope. The bright and dark parts in the acoustic micrograph of the unmodified glass samples appeared owing to permeation of water into the film. The blister was observed after about 20 s from dropping water. Sizes of the blister depended on the time and the thickness of thin films. On the other hand, the acoustic micrograph of the hydrophobic modified samples was a uniform image and the peeling of the thin film was not observed. V(z) curves of polystyrene thin film on the modified glass substrates had two oscillation periods in a certain frequency range that depended on the thickness of thin films. The short cycle and the long cycle components were assigned to a leaky surface acoustic wave (LSAW) and a leaky pseudo Sezawa wave, respectively. Velocities of the LSAW decreased linearly with an increase in film thickness.