Microstructure of free volume in SMA copolymers I. Free volume from Simha-Somcynsky analysis of PVT experiments

The structure of the free volume and its temperature dependence between 25 and 200 °C of copolymers of styrene with maleic anhydride, SMA (0-35 mol% MA), is studied by pressure-volume-temperature (PVT) experiments and positron annihilation lifetime spectroscopy (PALS). In this first part of the work, PVT data are reported which were analysed with the Simha-Somcynsky equation of state to estimate the volume fraction of holes, h, which constitute the excess free volume. The temperature and pressure dependence of the specific volume V, the specific occupied and free volume, V_occ=(1−h)V and V_f=hV, and the corresponding isobaric expansivities and isothermal compressibilities for both the rubbery and glassy state are estimated. We obtained the unexpected results that (i) the occupied volume changes its coefficient of thermal expansion at T_g from α_occ,g≈0.5α_g≈1×10⁻⁴ K⁻¹ below T_g to almost zero (≈0.2×10⁻⁴ K⁻¹) above T_g and (ii) the isothermal compressibility of the occupied volume at zero pressure below T_g is rather high, κ_occ≈2.5×10⁻⁴ MPa⁻¹, and decreases only slightly at T_g to about 2×10⁻⁴ MPa⁻¹ above T_g. The variation of total, occupied, and free volume parameters with the composition of the SMA copolymers is discussed.     

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.     

Effect of polymer structure on the molecular dynamics and thermal behavior of poly(allyl acetoacetate) and copolymers

In this paper, dielectric and calorimetric studies of the small-molecule glass former allyl acetoacetate monomers as well as its newly synthetized homopolymer and copolymers with different styrene composition were performed in both the liquid and glassy states. The molecular dynamics studies by the broadband dielectric spectroscopy and the stochastic temperature modulated differential scanning calorimetry enabled us to explore relaxation processes of examined materials in the wide frequency range. We found that the copolymers reveal two co-existing glass transitions characterized by the glass transition temperatures, which are very close to those of the corresponding homopolymers. These results suggest that the copolymers exhibited some sequences of acetoacetate units with a microphase-separated morphology in agreement with the value of reactivity ratio previously determined. We investigated effects of copolymerization compositions on the glass transition temperature, the isobaric fragility index, the dielectric and calorimetric intensity, and the dynamic heterogeneity on the glass transitions of the materials.     

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.     

Influence of fillers on free volume and gas barrier properties in styrene-butadiene rubber studied by positrons

The measurements of free-volume hole property by positron annihilation lifetime spectroscopy (PALS) have been carried out for polymer-clay nanocomposite materials that consist of styrene-butadiene rubber (SBR) and layered silicate clay of rectorite and conventional composite materials N326 (carbon black) /SBR. The PALS and differential scanning calorimeter (DSC) results show layered rectorite has a stronger effect on restraining polymer chain mobility which results in the decrease of fraction free volume and gas permeation than carbon black. The dispersion of nanoscale rectorite clay in SBR largely enhances gas barrier property in contrast to results obtained in N326/SBR system. Experimental results reveal that gas permeability in rectorite/SBR nanocomposite is mainly influenced by fractional free volume and tortuous diffusional path effects attributed to the clay plateletlike morphology.     

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.     

Using X-ray tomography,PALS and Raman spectroscopy for characterization of inhibitors in epoxy coatings

Model paint materials were generated by adding a range of inorganic materials into an epoxy. The inorganic materials included inhibitors (Zn₃(PO₄)₂ and SrCrO₄) and a filler (rutile TiO₂).The SrCrO₄ system was characterized using SEM, TEM, PALS and Raman spectroscopy and found to have an even distribution of inhibitor in the polymer matrix. X-ray tomography was performed on the mixed SrCrO₄/TiO₂ and Zn₃(PO₄)₂/TiO₂ systems. A new technique called data constrained modelling was combined with the tomographic technique to produce a 3D distribution of the inorganic phases within the polymer matrix.     

Molecular-level free volume as a crucial complementary factor affecting miscibility and nanoscopic homogeneity of polyarylate/poly(vinyl chloride) blends

Molecular-level miscibility and scale of mixing were characterized and evaluated for three model blends of polyarylates having structural variants with PVC through cross polarization/magic-angle spinning (CP/MAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy. From this study, we found that the polyarylate with sulfonyl central connector and tetramethyl substitutions on bisphenol rings (shortly, TMBPS-PAr) was nanoscopically miscible with PVC as homogeneously mixed down to a scale of ∼2.3 nm. In contrast, the structurally similar polyarylate, just in absence of tetramethyl substitutions from TMBPS-PAr, (BPS-PAr) was partially mixed with PVC at a homogenization scale of at least ∼27-32 nm, regardless of almost identical specific intermolecular interaction relative to TMBPS-PAr. In the case of polyarylate with isopropyl central connector other than the sulfonyl (TMBPA-PAr) was found to be completely immiscible with PVC, despite the presence of tetramethyl substitutions, due to the lack of polar sulfonyl groups that is known to promote mixing. These observations, in conjunction with the structural combinations of polyarylates, suggest the need to seek another complementary key parameter to promote miscibility and nanoscopic homogeneity in addition to the specific interaction. From the results of the positron annihilation analysis (PALS) on the intermolecular free volume through the entire composition ratio, the TMBPS-PAr/PVC blend resulted in a larger negative deviation of the ortho-positronium (o-Ps) pick-off lifetimes (τ₃) and the free cavity sizes (R) from linear additivity of those parameters of component polymers compared to the BPS-PAr/PVC blend. The larger negative deviation was interpreted to be a higher/greater contraction of free space and more tightly packing of molecular chains in TMBPS-PAr/PVC upon mixing irrespective of similar specific intermolecular interaction between two blend systems, which shows good agreement with the results of densities with blend compositions. Recognizing that the only structural variation between TMBPS-PAr and BPS-PAr was tetramethyl substitutions and that TMBPA-PAr/PVC was completely immiscible despite the tetramethyl substitutions, the combined results obtained from NMR and PALS lead to rationalize additional effects of the molecular free volume space created by the tetramethyl groups on the good miscibility of the blends under the circumstance when enough specific intermolecular interaction existed upon mixing.     

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.     

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.     

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 and crystallinity of poly(ethylene naphthalate) treated in pressurized carbon dioxide

The free volume and crystallization behaviour of poly(ethylene naphthalate) (PEN) treated in pressurized carbon dioxide was studied, using Positron Annihilation Lifetime Spectroscopy (PALS) and Differential Scanning Calorimetry (DSC). PALS probes the values of free volume cavity sizes in materials, thus making it possible to investigate the effect of pressurized carbon dioxide treatment on free volume hole sizes in PEN. The crystallinity and melting behaviour of PEN was analyzed with temperature modulated DSC. We found that the carbon dioxide pressure during treatment of PEN was the prime parameter affecting the value of free volume after the treatment. The free volume sizes were observed to be insensitive to the other two parameters, temperature and time. Increasing the time of the treatment however, increased the crystallinity of PEN. Interestingly, this was not coupled to a decrease in ortho-Positronium intensity as was expected, indicating that positronium may form in the crystalline fraction of the polymer as well as in the amorphous fraction.     

Effect of OPS Dispersion Method on the Free Volume of Polyurethane by Positron Annihilation Lifetime Spectroscopy (PALS)

The free volume of octaphenyl polyhedral oligomeric silsesquioxane (OPS) modified polyurethane prepared from different dispersion method was investigated using Positron annihilation lifetime spectroscopy (PALS). The relationships of free volume and mechanical property were analyzed. Our results showed that OPS decreased the original free volume hole size and fraction, but varied with the dispersion method. For the sonication dispersion samples, the tensile was conform to the free volume fraction, while the elongation at break was against the free volume hole size. In the high shear dispersion process, the tensile increased and no apparent change was seen on the elongation at break.     

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.     

Free volume and conductivity in polymer electrolytes

Positron annihilation lifetime spectroscopy (PALS) and impedance spectroscopy (IS) have been employed to study the effect of temperature and pressure on the DC conductivity (σ_DC) and the mean hole volume (V_h) of a NaPF₆ ethylene oxide based polyurethane electrolyte. The DC conductivity of the polymer electrolyte displayed a characteristic non-Arrhenius temperature dependence yielding acceptable values for both the “pseudo-activation energy” (B) and the “zero mobility temperature” (T₀) from a VTF fit. V_h(T) showed a linear increase of 0.53 cm³ (mol K)⁻¹. When extrapolating V_h(T) to 0 K a temperature very close to T₀ from the VTF fit was obtained, which suggests a free volume mediated conductivity mechanism. This suggestion is further supported by the linear dependence of ln(σ_DC(T)) on . Conductivity was measured as a function of pressure (σ_DC(P)) with ln(σ_DC(P)) showing a characteristic decrease with increasing pressure. The activation volumes (V_A) calculated from these measurements ranged from 45 to 20 cm³ mol⁻¹ over a temperature from 304 to 365 K. Critical volumes calculated from two current free-volume models were found to be unrealistic. Combining the extra volume required for ionic motion (V_A) with the available free volume (V_h) at the same temperature poses a realistic and ‘model-free’ figure of 117 cm³ mol⁻¹ for the critical volume at 304 K. This equates roughly to the volume of 3-4 EO units. The pressure dependence of free volume (V_h(P)) for a polymer electrolyte has been measured for the first time, and yielded a linear decrease in V_h with increasing pressure. A linear dependence of σ_DC(P) on was also found. A comparison of the isothermal and isobaric dependence of σ_DC on illustrates the contribution of factors other than free volume have on charge carrier number and mobility. This comparison shows that the variation of V_h with temperature and the variation of V_h with pressure affect the conductivity in very different ways. These results clearly show that free volume cannot be considered the sole factor responsible for conductivity in polymer electrolytes.     

PALS free volume and mechanical properties in dimethacrylate‐based thermosets

Dynamic mechanical thermal analysis, impact strength, bulk density and double‐bond conversion are reported for a series of dimethacrylate‐based networks in which the addition of styrene or/and divinylbenzene co‐monomer leads to variations in the morphology and crosslink density. The glass transition region was very broad for DVB‐rich networks and this was interpreted in terms of network heterogeneity. The mean size and the size distribution of free‐volume holes were estimated via analyses of positron annihilation lifetime spectroscopy (PALS) measurements at 22 °C and were found to be inversely correlated with the bulk density and dependent on the network heterogeneity. The PALS free volume was raised for DVB‐rich systems, due to the poor molecular packing in these highly crosslinked systems and because some regions in this heterogeneous network were in a rubbery state. The impact strength was found to be lower for the more highly crosslinked systems but was inversely correlated with the level of molecular mobility as measured by DMTA. Copyright © 2004 Society of Chemical Industry     

Free volume behavior in spincast thin film of polystyrene by energy variable positron annihilation lifetime spectroscopy

Free volume behavior in polystyrene thin films with thickness ranging from 22 to 1200 nm on silicon substrates was studied by energy variable positron annihilation lifetime spectroscopy (EVPALS). The films were prepared by spincasting from toluene solutions of 0.5-5.0 wt% polystyrene with M_w = 1?090?000 g/mol. Distinct deviations from bulk polystyrene in thermal expansion of the free volume holes and the glass transition temperature associated with free volume behavior were observed for the thinnest film with 22 nm thickness, indicating its exclusively high chain mobility. Comparison of the polystyrene concentration in the precursor solution around the overlap concentration suggests that the high chain mobility is due to less entangled chains caused by rapid removal of the solvent from the diluted solution in order to prepare very thin film.     

Microstructure evolution and magnetic properties of Eu doped CuFeO2 multiferroic ceramics studied by positron annihilation

A series of multiferroic ceramics CuFe_1-xEu_xO₂ (x?=?0–0.10) are prepared by traditional solid-state reaction. The effects of Eu doping on the microstructure, vacancy-type defects, and magnetic properties of CuFeO₂ ceramics are investigated systematically by means of X-ray diffraction, Raman spectroscopy, scanning electron microscope, positron annihilation lifetime and physical property measurement system. The results show that no phase transition occurs in the entire range of doping content (x?=?0–0.10), but the single phase structure is damaged by high Eu content (x?=?0.04–0.10). Positron annihilation measurements indicate that the local electron density and the vacancy-type defect concentration increase gradually with the increase in Eu content from 0 to 0.08. Furthermore, abnormal changes in lifetime parameters can be found in x?=?0.10 sample induced by the existence of impurity phase in the system. The magnetic measurements reveal that all the samples exhibit two successive magnetic transitions at T?=?15 and 11?K. In x?=?0.02 sample, the coexistence of ferromagnetism and antiferromagnetism can be found, and a maximum saturation magnetization of 11.548?emu/g at 5?kOe is achieved. The possible reasons for the above observations are discussed in detail.     

Correlation between molecular structure,free volume,and physical properties of a wide range of main chain thermotropic liquid crystalline polymers

Physical phenomena such as glass transition temperatures, melting points, and relaxational behavior have been determined for a wide range of thermotropic liquid crystalline polymers (TLCPs) and polycarbonate (PC). Damping intensities of rigid TLCPs during glass transition are greater than those of semirigid and semiflexible TLCPs. Positron annihilation lifetime spectroscopy was also employed to evaluate free volume parameters of polymers. In general, the positron annihilation lifetime spectroscopy (PALS) free volume parameters show that all TLCPs have much smaller, fewer free volume sites compared with PC. Correlations were found between PALS free volume parameters and glass transition temperature, dynamic mechanical damping strength, and bulk density where greater free volume parameters corresponded to higher glass transition temperatures, greater damping strength, and lower density. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2252–2267, 2001