Free Volume Hole Size Research Articles

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 fF. 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 Sbn of the broad DBAR component. We observed that Sbn does not show the expected exponential decrease but goes down linearly with fF in the range fF<0.45 and levels-off for fF≥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 Sbn with fF 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.

Positronium annihilation lifetimes and dielectric spectroscopy studies on diethyl phthalate: Phenomenological correlations and microscopic analyses in terms of the extended free volume model by Cohen-Grest

A combined positronium annihilation lifetime spectroscopy (PALS) and dielectric spectroscopy (DS) study on a typical van der Waals glass-former diethyl phthalate (DEP) was performed and the results were compared. From phenomenological point of view, the mutual relationships between the characteristic PALS temperatures, the glass temperature TgPALS, and the crossover temperatures Tb1L and Tb2L on the ortho-positronium (o-Ps) lifetime versus the temperature plot, have been discussed with respect to the characteristic DS temperatures, the glass temperature TgDS and the dynamic crossover temperature TBST, concerning the crossover behavior of primary alpha-relaxation times. Next, simultaneous application of the extended free volume (EFV) model by Cohen-Grest on the temperature dependence of both the mean free volume hole size data as extracted from PALS and the dielectric alpha-relaxation time revealed a good agreement between the experimental Tb1L and the characteristic EFV temperatures T0DS and T0PALS at which a free volume percolation should occur. These results indicate the important role of free volume in control of the primary (alpha) dynamics of supercooled DEP.

Free volume and water vapor permeability of dense segmented polyurethane membrane

This paper presented micro-structure, free volume and water vapor permeability of dense segmented polyurethane (SPU) membrane. Wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques were employed to investigate the micro-structure. Free volume was measured by positron annihilation life spectroscopy (PALS). WAXD and DSC results indicated microcrystalline structure presents in the polymer membrane. Percent crystallinity of SPU would depend on the types of polyol used. The microphase-separated structure of copolymers was observed by transmission electron microscopy. Extent of phase separation was depended on the type and composition of polyol used. Free volume hole sizes are different for SPU samples due to the interaction between the polymer chains and packing of polymer chains due to the different compositions. The influences of temperature and chemical compositions on the free volume and water vapor permeability have been discussed. Experimental results revealed that the water vapor permeability can be controlled by free volume, which would depend on the chemical composition of SPU, and hydrophilicity of membranes.

Solid state structure and oxygen transport properties of copolyesters based on smectic poly(hexamethylene 4,4′-bibenzoate)

This study examined the solid state structure and oxygen barrier properties of copolyesters based on smectic poly(hexamethylene 4,4′-bibenzoate) (PHBB) and non-liquid crystalline poly(hexamethylene isophthalate) (PHI). The isophthalate content was varied from 10 to 75mol%. Differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), dynamic mechanical thermal analysis (DMTA), and atomic force microscopy (AFM) were employed to characterize the polymers. The strong ordering tendency of 4,4′-bibenzoate was demonstrated by the persistence of ordered PHBB structures in copolymers with large amounts of the kinked isophthalate comonomer. Copolymers with up to 50mol% isophthalate gave evidence of liquid crystalline (LC) character in the precursor melt. Copolymers with up to 75mol% isophthalate crystallized in the PHBB α-crystal form with only small perturbations of the unit cell. The copolymers provided insight into the low gas permeability of LC polymers. Changes in both solubility and diffusivity contributed to the lower oxygen permeability of the smectic glass compared to the amorphous glass. Smaller free volume hole size of the smectic glass gave rise to lower oxygen solubility and contributed to lower diffusivity. The extended chain conformation in the smectic glass, which reduced the fraction of glycol units in gauche conformations, was a second factor that contributed to lower diffusivity.

Amorphous Phase of Propylene/Ethylene Copolymers Characterized by Positron Annihilation Lifetime Spectroscopy

Recent advances in catalyst technology make it possible to synthesize high molecular weight propylene copolymers with a high degree of isotacticity and high levels of an α-olefin comonomer. The primary objective of this study is to systematically characterize the rubbery amorphous phase of propylene/ethylene (P/E) copolymers over a range in comonomer content. A series of new experimental P/E copolymers prepared with a group IV transition metal-based post-metallocene catalyst are compared with a series of P/E copolymers prepared with a conventional metallocene catalyst. Positron annihilation lifetime spectroscopy (PALS) is used to obtain the temperature dependence of the free volume hole size. The PALS measurements are supplemented with bulk volume−temperature measurements. It is found that the free volume hole size and the amorphous phase density at ambient temperature strongly depend on crystallinity. Densification of the amorphous phase is attributed to constraint imposed on rubbery amorphous chain segm...

Effects of the degree of polymerization on the free volume structure of linear amphiphilic poly(isobutylene)–poly(methyl vinyl ether) diblock copolymers

The free volume was studied in different poly(isobutylene)/poly(methyl vinyl ether) diblock copolymers by positron annihilation spectroscopy. The degree of polymerization of both constituents was varied in the work. The addition of further monomer units was found to increase the size of free volume holes continuously. However, the observed effect was much smaller than the size of monomer units. According to Doppler-broadening measurements, the structure of blocks changes with their size.

Compatibilizer‐induced microstructural changes in poly(trimethylene terephthalate)/EPDM blends studied by the positron annihilation lifetime technique and differential scanning calorimetry

AbstractBlends of poly(trimethylene terephthalate) (PTT) and ethylene propylene diene monomer (EPDM) with and without the compatibilizer poly (EPM‐graft‐MA) were investigated by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetric (DSC) measurements. The DSC results for the blend with 50/50 composition revealed two glass transition temperatures, indicating a two‐phase system. In the presence of compatibilizer, the two glass transition temperatures shifts towards each other, suggesting an increased interaction between the blend components. The PALS results for the blends without compatibilizer showed an increase in free volume hole size and concentration with increasing EPDM content in the blend, suggesting the coalescence of free volumes of EPDM with the PTT to some extent, but the phase‐separation behaviour continued. The free volume of these blends exhibited positive deviation from the known free volume linear additivity rule. However, poly(EPM‐graft‐MA) compatibilized blends of PTT/EPDM had a noticeable decrease in the free volume parameters, which was clearly due to the compatibilizing effect through increased interaction between blend components. Copyright © 2005 Society of Chemical Industry

Polymer Interphase Materials by Forced Assembly

The interphase between two immiscible glassy polymers was probed using nanolayer films with tens to thousands of alternating layers of two polymers. Various combinations of poly(methyl methacrylate), polycarbonate, and a series of styrene−acrylonitrile copolymers were brought together by forced assembly. Continuous nanolayers with thickness on the size scale of the interphase were observed directly using atomic force microscopy. Interphase thickness was extracted from the layer thickness dependence of oxygen permeability. The interphase thickness showed the predicted dependence on the χ interaction parameter and correlated with interphase strength as measured with the T-peel test. Interphase specific volume, as determined by density, exhibited both positive and negative deviations from constituent additivity. The deviations correlated with the change in free volume hole size from positron annihilation lifetime spectroscopy but did not correlate directly with the χ parameter. The origin of the deviations w...

Positron annihilation and differential scanning calorimetric study of poly(trimethylene terephthalate)/EPDM blends

The blends of poly(trimethylene terephthalate) (PTT) and ethylene propylene diene monomer (EPDM) with different composition have been studied by positron lifetime technique (PLT) and differential scanning calorimetric (DSC) measurements. The DSC results for the blends of 50/50 and 40/60 show clear two glass transition temperatures indicating two-phase system. No melting point depression was observed for the blend system, which strongly supports the incompatibility. From the positron results an increase in free volume hole size and its concentration has been observed with the increase in EPDM content of the blend which indicates further that there is coalescence of free volumes of EPDM with the PTT to some extent and phase separation behavior continues. Another interesting aspect is that the relative fractional free volume exhibits neither negative nor positive deviation from the log additivity rule. It agrees well with the log additivity rule. The interchain interaction parameter evaluated from these results show some complex behavior. XRD results show the decrease in crystallinity of the blend with the increase in fractional free volume with the increase in concentration of EPDM.

Correlation between the release characteristics of theophylline and the free volume of polyvinylpyrrolidone

The purpose of this work was to study the drug-release properties of an amorphous polymer, polyvinylpyrrolidone applied conventionally as a binder in tablets. In order to gain data on the wearing properties of tablets, they were stored for a 30 days period under different humidity conditions before the drug-release measurements. The active material chosen for the release study was theophylline. An exponential relationship was found, with good correlation, between the relative humidity of the storage medium and the mean dissolution time of theophylline from polyvinylpyrrolidone tablets and the size of free volume holes. Positron annihilation lifetime spectroscopy (PALS) measurements, performed parallel with the theophylline release study, showed that the main reason for this correlation is the rearrangement of the pore structure of PVP. The results suggest that the water-induced glassy to rubbery transition of the polymer plays a significant role in the drug-release characteristics.

Correlation Between Free-Volume Parameters and Physical Properties of Polyethylene-Nitrile Rubber Blend

Positron annihilation lifetime spectroscopy (PALS) was used to study the immiscibility of a polar nitrile rubber (NBR) that had been blended with pure and waste, low- and high-density polyethylene (PE). The effect of the weight percent of the rubber added to the PE was also investigated. It was found that a complicated variation (positive and negative) in both free-volume parameters (τ3 and I 3) from the values of the initial polymers forms an immiscible blend. These results are supported by a significant broadening in the free-volume hole size distributions. This has been interpreted in terms of interfacial spaces created between the boundaries of the two phases. Furthermore, a correlation was established between the free-volume parameters (τ3 and I 3) and the electrical and mechanical properties of the before mentioned polymer blends as a function of the weight percent of waste PE.

Gas transport properties of 6FDA-TAPOB hyperbranched polyimide membrane

Physical and gas transport properties of the hyperbranched polyimide prepared from a triamine, 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), and a dianhydride, 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), were investigated and compared with those of linear-type polyimides with similar chemical structures prepared from diamines, 1,4-bis(4-aminophenoxy)benzene (TPEQ) or 1,3-bis(4-aminophenoxy)benzene (TPER), and 6FDA. 6FDA-TAPOB hyperbranched polyimide exhibited a good thermal stability as well as linear-type analogues. Fractional free volume (FFV) value of 6FDA-TAPOB was higher than those of the linear-type analogues, indicating looser packing of molecular chains attributed to the characteristic hyperbranched structure. It was found that increased resistance to the segmental mobility decreases the gas diffusivity of 6FDA-TAPOB, in spite of the higher FFV value. However, 6FDA-TAPOB exhibited considerably high gas solubility, resulting in high gas permeability. It was suggested that low segmental mobility and unique size and distribution of free volume holes arising from the characteristic hyperbranched structure of 6FDA-TAPOB provide effective O 2/N 2 selectivity. It is concluded that the 6FDA-TAPOB hyperbranched polyimide has relatively high permeability and O 2/N 2 selectivity, and is expected to apply to a high-performance gas separation membrane.

Free volume and water vapor permeability properties in polyurethane membranes studied by positrons

The effects of the soft segment and temperature on the free volume and gas permeability have been studied by positron annihilation lifetime spectroscopy (PALS) in polyurethane membranes based on polybutylene adipate (PBA), polytetramethylene glycol (PTMG) and polycaprolactone (PCL) as soft segments. The wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) results indicated that PBA-PU and PCL-PU had crystalline region except for PTMG-PU. The PALS results indicated that the free-volume hole size increased rapidly as the crystals melted. The water vapor permeability (WVP) coefficients followed Arrhenius temperature dependence in the molten and rubbery state for all membranes. High free volume gave rise to the negative value of the apparent activation energy of permeation ( E p). Correlations between the free volume and gas transport properties have been discussed. We found that WVP was controlled by not only free volume but also hydrophilicity of polymer.

Hybrid organic inorganic nylon‐6/SiO2nanocomposites: Transport properties

AbstractOrganic‐inorganic hybrid membranes of nanosized SiO2‐filled polyamide composites were prepared via film casting and their transport properties were studied. Gas permeation measurements were performed at room temperature, and the membrane exhibited an increase in membrane permeability performance. In contrast to the performance of traditional dense filled polymer systems, the permeability increased with an increased number of nanosilica particles. The nanocomposites were studied using positron annihilation lifetime spectroscopy (PALS). From the ortho‐positronium (o–Ps) lifetime (τ3), the size of the local free volume (holes) was estimated. The increase in permeability is ascribed to the additional free volume obtained. This is created by the presence of nanoparticles that alter the PA chain packing. Furthermore, wide angle X‐ray diffraction (WAXD) patterns revealed that the incorporation of silica induced the structural modification of polymer chains by modifying the degree of crystallinity in comparison with the neat polymer. Polym. Eng. Sci. 44:1240–1246, 2004. © 2004 Society of Plastics Engineers.

Positronium at polymeric surface

AbstractAnnihilation of slow positrons at a polymeric surface has been discussed in terms of positron diffusion and trapping of positronium into free volume holes. The above model has been used to calculate the ortho‐positronium lifetime (τ3) in polystyrene (PS), epoxy resin film, polyurethane (PU) and polytetrafluroethylene (PTFE) as a function of temperature, incident positron energy and mean implantation depth. The results have been compared with the experimental observations of other authors. The variation in τ3 with respect to temperature clearly demonstrates a discontinuity in the curve at Tg corresponding to the glass transition temperature. The variation of calculated τ3 shows that the lifetime increases significantly above Tg however, below Tg it increases only slowly. This is a direct consequence of the change in the size of free volume holes. The Tg has been found to be dependent both on positron energy and density of the polymer. Large variation in τ3 has been observed at low energies suggesting a significant structure of free volume holes near the surface. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Oxygen Barrier Properties of Copolyesters Containing a Mesogenic Monomer

Thermal properties and solid-state structure of copolyesters based on smectic poly(diethylene glycol 4,4‘-bibenzoate) (PDEGBB) and non-liquid crystalline poly(diethylene glycol isophthalate) (PDEGI) were characterized previously. The present study examined the oxygen transport properties and free volume characteristics of quenched films with 10−50 mol % isophthalate. Isophthalate was not accommodated in the smectic lattice; as a result, copolymerization effectively disrupted the LC character. The estimated smectic volume fraction of PDEGBB was 0.84. Copolymerization with 10 and 20 mol % isophthalate decreased the smectic fraction to 0.59 and 0.16, respectively. The copolymer with 30 mol % isophthalate did not exhibit any LC character. Decreasing density and increasing free volume hole size accompanied the loss of LC features. Oxygen permeability also increased due to increases in both solubility and diffusivity. Oxygen solubility of LC glasses conformed to a two-phase model consisting of a less permeable ...

Oxygen permeability and free volume hole size in ethylene–vinyl alcohol copolymer film: temperature and humidity dependence

Relationship between oxygen permeability and free volume hole size evaluated from ortho-positronium lifetime in ethylene–vinyl alcohol copolymer (EVOH) was studied as functions of temperature and relative humidity. As the relative humidity rises at room temperature, the permeability first decreases and then increases. This behavior is attributed to the effects of pore filling and plasticization by water molecules. Linear correlations are obtained on the plots of logarithmic permeability versus reciprocal free volume hole size for EVOH at different temperatures and humidities as well as for different polymers including EVOH at room temperature, suggesting that the free volume hole size plays a crucial role in the oxygen permeation in EVOH and other polymers.

Chemical environment in halogenated styrene polymers studied by using positron annihilation lifetime spectroscopy

Polystyrene samples, incorporated with halogen elements (F, Cl, Br, I) on the para-position of the benzene ring, were studied using positron annihilation lifetime spectroscopy. It was found that the free-volume hole size is significantly affected by the internal Coulombic interaction of the halogen group, and is mainly related to the electronegativity of halogen–carbon bonds. In addition, it is found that the free-volume is secondarily modified by the steric effect of the side groups. The intensity of o-Ps has a linear relationship with the strength of the C–X bond and is strongly affected by the chemical environment in a halogenated styrene polymer system.

Polyamide, acrylic rubber and their blends studied by positron annihilation and other methods

Polyamide, acrylic rubber and their blends have been examined with use of PALS spectroscopy from 40 to 375 K. Free volume characteristics have been obtained from the PALS and PVT measurements. There is a great difference in the average size of free volume holes and the fractional free volume of PA and the rubber. The volume relaxation effects are seen at low temperature, especially in case of PA. The results of DSC and the impact strength measurements prove the compatibility of PA and acrylic rubber in the blends. The difference in the fractional free volume of the constituents of the blends found by us, supports the idea of the enhancement of mutual interaction of the blend constituents and the possibility of chemical reaction between them.