Free Volume Data Research Articles

Purification of Turkish Bentonites and Investigation of the Contact Angle, Surface Free Energy and Zeta Potential Profiles of Organo-Bentonites as a Function of CTAB Concentration

AbstractPurification of raw bentonites and organo-bentonite preparations is sometimes required for industrial use. Zeta (electrokinetic) potential (ζ), contact angle (wettability/hydrophobicity), and surface free energy (SFE) are important surface characteristics and vary significantly according to the applied surfactant concentration when preparing organo-bentonite. Changes in these characteristics determine the stability, behavior, and efficiency of organo-bentonites in various applications such as adsorption, composite materials, and drug-delivery systems. Knowing how much surfactant should be used to prepare organo-bentonite is, therefore, critical. The purpose of the present study was to determine the effect of concentration of the cationic surfactant cetyltrimethylammonium bromide (CTAB) adsorbed in organo-bentonite (prepared from two local and commercial raw bentonites with potential for use in adsorbent and composite materials) on the ζ potential, contact angle, and SFE profiles. The raw bentonites were purified using sedimentation and centrifugation techniques prior to preparation of the organo-bentonite. The purification results were evaluated in light of X-ray diffraction (XRD), cation exchange capacity (CEC), free swelling volume (FSV), X-ray fluorescence (XRF), and particle-size analysis data. Most of the gangue minerals (feldspar, calcite, clinoptilolite, opal, quartz, and mica) having particle size >5 μm were removed from the raw bentonites by using a one-stage sedimentation or a Falcon gravity separator (FGS). Higher yields (68.8% and 81.3% for two bentonites) were obtained with the FGS compared to sedimentation while purification levels were almost the same. ζ changed greatly from –35 mV (and –40 mV) toward 38 mV (and 40 mV) with increasing CTAB concentrations. Similar profiles were also obtained for wettability; maximum contact angles for organo-bentonites were measured as ~72–73o, while they were 12.65 and 14.1o for two purified and unmodified bentonites. SFEs were calculated using contact-angle data, and decreased to minimum values of 41.5–43.6 mJ/m2 from 78.6–78.2 mJ/m2 upon treatment of raw bentonites with CTAB. 100–130% CEC concentration was sufficient to prepare organo-bentonites with maximum hydrophobicity and positively charged surfaces.

Effect of Moisture Sorption on Free Volume and Relaxation of Spray Dried Dispersions: Relation to Drug Recrystallization

The effect of vapor sorption on the free volume of drug-polymer spray-dried dispersions (SDDs) was investigated, along with the crystallization propensity of drug molecules in SDDs after exposure to humidity. Subsequently, the correlation of free volume change and relaxation time with drug recrystallization was examined. Four polymers, including polyvinylpyrrolidone, polyvinylpyrrolidone vinyl acetate copolymer, hydroxypropyl cellulose, and hydroxypropyl methylcellulose acetate succinate, and 2 drugs (indomethacin and ketoconazole) were selected for preparing SDDs. Free volume data of the exposed SDDs were obtained with positron annihilation lifetime spectroscopy, while the relaxation time was measured using a TA rheometer. Additionally, the crystallization propensity of active pharmaceutical ingredients (APIs) in the exposed SDDs was assessed using both polarized light microscopy and powder X-ray diffraction, followed by relating API crystallization inclination with expansion of holes and relaxation time. Finally, Cohen and Turnbull molecular transport model, along with its extensions by Vrentas and Duda, was qualitatively utilized for interpreting the recrystallization propensity of API molecules. In conclusion, API recrystallization is closely related to free volume change upon moisture sorption and relaxation time, but system dependent; overall, drug-hydroxypropyl methylcellulose acetate succinate SDDs appear physically stable against recrystallization due to less increase in free volume.

Effect of shock on transition metal carbides and nitrides {MC/N (M = Zr, Nb, Ta, Ti)}

We investigated the effect of shock impact on some transition metal carbides and nitrides {MC/N (M=Zr, Nb, Ta, Ti)} within the Debye-Gruneisen theory. Parameters required for the model were obtained from first principles by fitting the total free energy vs. volume data obtained from DFT to the equation of state (EOS) proposed by Li. The EOS model was used to predict shock Hugoniot curves for these compounds to verify the effect of increasing pressures and temperatures on the EOS. Reasonably good agreement with experimental data was noted for NbC, ZrC, TaC and ZrN and TiN up to ∼200GPa considering the metals to exist in B1 (NaCl) phase. However, in case of ZrC, TaC and ZrN, single Hugoniot data points were available at higher pressures (∼400GPa). In all three cases, the predicted Hugoniot corresponding to B1 phase did not match with the experimental data point at that pressure. This was attributed to a structural phase transition occurring in the metals. We found all the compounds to be stable in the rock salt structure (B1) and transform to CsCl (B2) structure at high pressures (beyond ∼200GPa) and 0K. Since experiments are performed at finite temperatures, we investigated this transition at finite temperatures. From our analysis, we concluded that B1→B2 transition does not occur at high pressures under shock loading. Experimental evidence of the transition has also not been reported for these compounds as per our knowledge. The only other possibility of a structural phase transition was solid-liquid transition i.e. melting. We found that all the compounds melt under shock loading at high pressures beyond ∼200GPa.

Free volume characterization of sulfonated styrenic pentablock copolymers using positron annihilation lifetime spectroscopy

A series of sulfonated styrenic pentablock copolymers was characterized using positron annihilation lifetime spectroscopy (PALS). Ortho-positronium (o-Ps) lifetime was measured as a function of the polymer's degree of sulfonation and the environment in which the polymer was equilibrated. Samples ranged from an un-sulfonated styrenic pentablock copolymer to one with an ion exchange capacity (IEC) of 2.0meq/g(dry polymer), and the samples were equilibrated in either air having 50% relative humidity, de-ionized (DI) water, or 2000mgL−1 aqueous NaCl solution environments. The o-Ps lifetime, and thus, the average free volume element size, decreased as the polymer's degree of sulfonation increased and as the amount of water sorbed into the polymer increased, similar to reported decreases in o-Ps lifetime accompanying increased water sorption in uncharged cross-linked poly(ethylene glycol) hydrogels. The o-Ps lifetimes of the DI water equilibrated acid counter-ion form polymers were quite similar to those of the salt solution equilibrated sodium counter-ion form polymers, suggesting that the PALS data are not particularly sensitive to the counter-ion form of these hydrated polymers. Previously reported water and salt permeability data in polymers were correlated with the PALS data. Correlating water and salt permeability with free volume data, estimated using o-Ps lifetime, was complicated by the micro-phase separated nature of the block copolymers. The water permeability of polymers whose hydrophilic micro-domains are believed to be well connected correlated with free volume data, based on o-Ps lifetime. Salt permeability, however, did not correlate with free volume data estimated from o-Ps lifetime, according to free volume theory. In general, contributions of the hydrophobic micro-domain free volume to the PALS measurement frustrate the correlation of water and salt transport properties with PALS parameters in a manner that is not observed in other sulfonated and uncharged polymers.

Positron annihilation lifetime spectroscopy for miscibility investigations of styrene–butadiene–styrene copolymer/polystyrene blends

The free volume parameters of styrene–butadiene–styrene copolymer/polystyrene (SBS/PS) blends were investigated with positron annihilation lifetime spectroscopy (PALS) in this study. The behaviors of free volume distribution, average free volume, and relative fractional free volume revealed the difference of interfacial miscibility. Based on different models, inter‐chain interaction parameter β, geometric factor γ, and hydrodynamic interaction parameter α obtained from free volume data were employed to further determine the effect of molecular architecture and styrene content on the miscibility. The results suggest the better miscibility in star‐shaped SBS/PS blends than that of corresponding linear SBS/PS systems, even than that of systems containing more styrene unit. In addition, differential scanning calorimetry, dynamic mechanical analysis, and scanning electron microscopy, which are sensitive to heterogeneities in larger domain size, give different results of miscibility from free volume data. It should be attributed the difference of characterization scale. The mechanical property corroborates the results of miscibility. POLYM. ENG. SCI., 54:785–793, 2014. © 2013 Society of Plastics Engineers

Interface profile studies in immiscible and partially miscible binary polymer blends from free volume measurement

The diffused interface widths in an immiscible and a partially miscible polymer blend namely Polyvinyl chloride/Ethylene vinyl acetate (PVC/EVA) and Polystyrene/Polymethylmethacrylate (PS/PMMA) are experimentally measured and reported here. A new empirical relation found between hydrodynamic interaction parameter α derived from free volume data and the Flory-Huggins interaction parameter χ is used to construct density profile across the interface to derive the interface width in above two binary blends.

Microwave irradiation induced modifications on the interfaces in SAN/EVA/PVC and PVAc/BPA/PVP ternary polymer blends: Positron lifetime study

Ternary polymer blends of poly(styrene-co-acrylonitrile)/poly(ethylene-co-vinylacetate)/poly(vinyl chloride) (SAN/EVA/PVC) and poly(vinyl acetate)/bisphenol A/polyvinylpyrrolidone (PVAc/BPA/PVP) with different compositions have been prepared by solvent casting method and characterized by positron lifetime spectroscopy and differential scanning calorimetry DSC. Phase modifications have been induced by irradiating the blends with microwave radiation. These changes have been monitored by measuring the free-volume content in the blends. The results clearly show improved interactions between the constituent polymers of the blends upon microwave irradiation. However, the free-volume data and DSC measurements are found to be inadequate to reveal the changes at the interfaces and the interfaces determine the final properties of the blend. For this we have used hydrodynamic interaction (αij) approach developed by us to measure strength of hydrodynamic interaction at the interfaces. These results show that microwave irradiation stabilizes the interfaces if the blend contains strong polar groups. SAN/EVA/PVC blend shows an increased effective hydrodynamic interaction from −3.18 to −4.85 at composition 50/35/15 upon microwave irradiation and PVAc/BPA/PVP blend shows an increased effective hydrodynamic interaction from −3.81 to −7.57 at composition 20/50/30 after irradiation.

Experimental determination of interface widths in binary polymer blends from free volume measurements

Interfaces in binary polymer blends play a pivotal role in moulding their physical properties. We observe a diffused interface in immiscible and partially miscible binary polymer blends for which the methods of preparation characterize the interface widths. The interface widths for three binary blends of PVC/PS, PS/PMMA, and PVC/SAN are in the maximum range of 2.15, 5.04 and 6.24 nm respectively. We establish a correlation between hydrodynamic interaction and the Flory–Huggins interaction parameter which is consistent with self-consistent mean field theory and the usual constructed density profile across the interface. Our approach is based on the hydrodynamic interaction which is derived from free volume data measured by positron lifetime spectroscopy.

Multilayered Poly(vinylidene fluoride) Composite Membranes with Improved Interfacial Compatibility: Correlating Pervaporation Performance with Free Volume Properties

A spin-coating process integrated with an ozone-induced graft polymerization technique was applied in this study. The purpose was to improve the poor interfacial compatibility between a selective layer of poly(2-hydroxyethyl methacrylate) (PHEMA) and the surface of a poly(vinylidene fluoride) (PVDF) substrate. The composite membranes thus fabricated were tested for their pervaporation performance in dehydrating an ethyl acetate/water mixture. Furthermore, the composite membranes were characterized by field emission scanning electron microscopy (FE-SEM) for morphological change observation and by Fourier transform infrared spectroscopy equipped with attenuated total reflectance (ATR-FTIR) for surface chemical composition analysis. Effects of grafting density and spin-coating speed on pervaporation performance were examined. The composite membrane pervaporation performance was elucidated by means of free volume and depth profile data obtained with the use of a variable monoenergy slow positron beam (VMSPB). Results indicated that a smaller free volume was correlated with a higher pervaporation performance of a composite membrane consisting of a selective layer of spin-coated PHEMA on a PHEMA-grafted PVDF substrate (S-PHEMA/PHEMA-g-PVDF). The composite membrane depth profile illustrated that an S-PHEMA layer spin-coated at a higher revolutions per minute (rpm) was thinner and denser than that at a lower rpm.

An effective tool to characterize adhesion at the interface of binary polymer blends: a free volume study

Abstract In this work, an effective method of characterizing the adhesion at the interface of partially miscible and immiscible binary polymer is used. Three polymer blends namely PS/PMMA, PP/NBR and PVC/SAN were irradiated with e-beam and microwaves, in order to bring about interfacial modification, and were characterized in terms of free volume measured by Positron Lifetime technique. The changes in free volume parameters upon e-beam and microwave irradiation on the two systems clearly shows improved interaction between the blend constituents as a result of cross-linking through hydrogen bond formation. However, the extent of interfacial modification brought in at different weight compositions is not forthcoming. Therefore, the hydrodynamic interaction parameter α, which accounts for friction at the interface between the dissimilar chains of the blend constituents, is evaluated from the very same free volume data. Since α represents the strength of hydrodynamic interaction, it could be taken as an indicator of the adhesion strength at the interface. Present results showed that both e-beam and microwave irradiations can be used to stabilize the interfaces in binary polymer blends as evidenced by hydrodynamic interaction.

Characterization of interfacial adhesion in binary polymer blends: a new approach based on positron lifetime study

AbstractA new method of characterizing the adhesion at the interface of partially miscible and immiscible binary polymer blends based on free volume concept is described here. The efficacy of the method is tested in four binary polymer blend systems namely PS/PMMA, PVC/EVA, PP/NBR and PVC/SAN. The usefulness of hydrodynamic interaction is exploited which is derived from free volume data to understand the changes at the interface. We understand that if the adhesion at the interface is good it exhibits large negative values for hydrodynamic interaction parameter α, a measure of excess friction at the interface. The thermal properties (indicated by Tg) also improve for this condition. To show that the method is sensitive to the changes at the interface, we modified the interface through microwave and e-beam irradiation and found from α characterization, that blends with polar groups exhibit interface stabilization for microwave irradiation while e-beam is good for blends with no polar or weak polar groups.

Effect of Free Volume on Water and Salt Transport Properties in Directly Copolymerized Disulfonated Poly(arylene ether sulfone) Random Copolymers

The influence of cation form and degree of sulfonation on free volume, as probed via positron annihilation lifetime spectroscopy (PALS), and water and salt transport properties was determined in a systematic series of directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers. Polymer samples were studied in both the dry and hydrated states. PALS-based estimates of free volume in the dry polymers were compared with those estimated using density and the Bondi group contribution method, and PALS-based free volume data for hydrated polymers were correlated with water and salt transport properties. The transport properties depend strongly on free volume cavity size. Samples with larger free volume elements have higher water and salt solubility, diffusivity, and permeability and lower water/salt diffusivity and permeability selectivity. Sorption of water alters the characteristic free volume of the polymer matrix by two competing mechanisms: water molecules partially occupy the original free volume in the initially dry polymer, thereby reducing free volume cavity size, and water swells the polymer matrix therefore increasing the mean free volume size as a result of increased polymer chain plasticization. The importance of the second effect increases as water uptake and, thus, plasticization increases.

Characterization of ACS modified epoxy resin composites with fly ash and cenospheres as fillers: Mechanical and microstructural properties

AbstractComposites of diglycidyl ether of bisphenol A resin modified with Amine Containing Silicone as matrix and fly ash and cenospheres as fillers have been examined for their mechanical and microstructural properties. The microstructural features of the composites in terms of free volume size and content, and its influence on the mechanical properties has been investigated. The results indicate that amine containing silicones works as a good coupling agent between the filler and the resin. Composites with 5 phr fly ash and 30 phr cenospheres showed improved thermal stability as indicated by the increased Tg of the composites, possibly because of increased SiOSi linkages. The tensile strength of cenospheres‐filled composites showed good improvement compared with that of the fly ash‐filled composites. Low density (0.6 g/cc) and higher silica content (60%) of cenospheres seems to be the reason for this observation, which is well corroborated by the free volume data. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers

Molecular dynamics simulation based evaluation of glass transition temperatures of polystyrene in the presence of carbon dioxide

This paper develops a detailed molecular dynamics (MD) simulation model to study the glass transition temperature (Tg) of a finite polystyrene (PS) system in the presence of high pressure carbon dioxide (CO2). Validated inter-atomic potentials for pure PS and CO2 are used for these simulations. Specific parameters and combinations rules are introduced to accurately model the intermolecular interactions between PS and CO2. The intermolecular interaction model has a strong effect on the Tg of the PS–CO2 system. The MD model comprises PS and CO2 molecules confined in a finite walled system to manifest the effects of high pressure CO2. The effectiveness of the simulation model is established by comparison with experimental free-volume data from positronium annihilation lifetime spectroscopy (PALS). An important outcome of this study is the identification of clearly demarcated regions for bulk and surface analysis. Physical properties such as density, free volume, segmental motion across the thickness and end group mobility are also studied to gain insight into the polymer dynamics. As is expected, the simulations show that the presence of high pressure CO2 reduces the Tg of PS significantly due to an increase in chain mobility. Additionally, the simulation data show a remarkable effect of CO2 on the extent and characteristics of the surface layer.

Full Three-Dimensional Strain Measurements on Wood Exposed to Three-Point Bending: Analysis by Use of Digital Volume Correlation Applied to Synchrotron Radiation Micro-Computed Tomography Image Data

Abstract: A microscale three-point bend experiment on wood has been carried out. The full 3D strain field of the microscale wood structure has been determined by use of digital volume correlation, based on reconstructed 3D image data acquired with synchrotron radiation micro-computed tomography. The wood specimen, which measures 1.57 × 3.42 × 0.75 mm3, was scanned in different load states along the three-point bend load cycle, from unloaded state to failure. The correlation algorithm is based on a Chebyshev polynomial description of the displacements, which allows a continuous representation of the 3D deformation fields. The methodology of the correlation algorithm is described thoroughly and its performance is tested for a 3D structure that is exposed to a virtual pre-defined deformation. The performance is tested both for noise free volume data as well as for structures with additive noise content. The performance test shows that the correlation algorithm resolves the applied deformation satisfyingly well. In the real experiment, on wood microstructure, the displacement fields show a structural behaviour that is consistent with what is expected for a specimen exposed to three-point bend. However, there are also anomalous effects present in the displacement fields that can be coupled to characteristic features in the cellular structure of the wood. Furthermore, 3D strain calculations based on the obtained displacement data shows a concentration of tensile strain in the region where the specimen eventually collapses. The experimental results show that the use of X-ray-based tomography with high spatial resolution in combination with digital volume correlation can successfully be used to perform 3D strain measurements on wood, at the microscale.

Rheology, Morphology, Mechanical Properties and Free Volume of Poly(trimethylene terephthalate)/Polycarbonate Blends

The phase morphology, rheology, mechanical properties, and free volume of poly(trimethylene terephthalate)/polycarbonate (PTT/PC) blends have been investigated as a function of composition. The morphology indicated a two-phase structure, and the blends showed co continuous phase morphology between 30−60 wt % of poly carbonate. Due to the high viscosity of PC, PTT is more finely dispersed in the PC matrix than PC in the PTT matrix. The rheological measurements of the blends revealed that the complex viscosity increased with increase in PC content. Relatively low interfacial tension values of the blends determined using Palierne and Choi−Schowalter methods indicated that there is considerable interaction between the blend components (PTT and PC) due to the transesterification reactions. A random copolyester formed as a result of the transesterification acted as a compatibilizer in the initial stages of reactions which is the main factor for the change in miscibility. The phase morphology and the interfacial adhesion influence the mechanical properties such that addition of the PC phase decreases the tensile strength and Young’s modulus of the system. The free volume data from PALS results showed a slight positive deviation from the known linear additivity rule with increase in PC content, suggesting the blends are partially miscible. Our results show a good correlation among the phase morphology, rheology, mechanical, and free volume parameters.

Interfacial stabilization of binary polymer blends through radiation treatment: A free volume approach

AbstractPartially miscible blends of PS/PMMA and PVC/SAN with different compositions have been prepared by solvent casting technique and characterized by Positron Annihilation Lifetime Technique. Phase modification in these systems has been brought out using e‐ beam and microwave irradiation. These changes have been studied through free volume monitoring. The changes in free volume parameters upon e‐beam and microwave irradiation on the two systems clearly shows improved interaction between the blend constituents as a result of cross‐linking through hydrogen bond formations. The free volume data indicates the overall changes in the blend with respect to free volume, but the extent of interfacial modification occurred for different weight compositions are not forthcoming upon irradiation. Therefore, the hydrodynamic interaction parameter α which accounts for excess friction thus dissipation of energy at the interface between the constituents of polymer blend system is evaluated for this purpose from the very same free volume data. The parameter α represents the strength of hydrodynamic interaction or it is a measure of the adhesion strength at the interface. Present data based on hydrodynamic interaction parameter indicated that both e‐beam and microwave irradiation can be used to stabilize the interfaces in binary polymer blends but the better interface modifier on the two blend systems studied. But which route that is e‐beam or microwave depends on the presence of non‐polar/polar groups respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microwave assisted improvement in physico-mechanical properties of poly(vinyl alcohol)/poly(ethylene imine)/gelatin blends

Blends of poly (vinyl alcohol) (PVA), poly-N-acyl ethyleneimine (PEI) and gelatin (PVA/ PEI/Gel) in the weight ratios 100/0/2, 95/5/2, 90/10/2, 85/15/2, 80/20/2 and 70/30/2 were prepared by solution casting method. The effects of poly (ethylene imine) component in the blend and microwave irradiation on the mechanical and microstructural properties have been investigated using various techniques along with Positron Annihilation Lifetime Spectroscopy (PALS). We observed tremendous improvement in mechanical properties after microwave irradiation for 60 s in 5 wt% of PEI component blend. These results are supported by the SEM data as well the optical data. The free volume data from positron results showed minimum free volume at 5 wt% of PEI after 60 s of irradiation due to cross linking through hydrogen bond formation and possible estrification among the blend components. These results correlate well the mechanical properties like tensile strength and modulus.

A new method of stabilization and characterization of the interface in binary polymer blends by irradiation: A positron annihilation study

AbstractNew methods for stabilizing the interface of partially miscible and immiscible binary polymer blends and characterizing such an interface are described here. Interfacial modifications in four binary polymer blend systems namely PS/PMMA, PVC/EVA, PP/NBR, and PVC/SAN were induced by e‐beam and microwave irradiation. These changes have been characterized in terms of free volume measured by Positron lifetime technique and DSC as supplementary to free volume data. The changes observed in free volume parameters upon irradiation could not be connected to the changes at the interface and also not specific to composition of the blend. Owing to this limitation, we exploited the usefulness of hydrodynamic interaction parameter α derived from the very same free volume data to monitor the changes at the interface. The present results demonstrated that α is effective in revealing the changes at the interface and can be used to characterize the interfacial properties in partially miscible and immiscible polymer blends. Further, the results clearly show that microwave irradiation is a better route to stabilize the interface of a partially miscible or immiscible blend if its component polymers contain polar groups. E‐beam irradiation seems to be better route if the component polymers of the blend contain no polar groups. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 619–632, 2009

A free-volume study on the phase modifications brought out by e-beam and microwave irradiations in PP/NBR and PVC/SAN blends

Partially miscible blends of PP/NBR and PVC/SAN with different compositions have been prepared by solvent casting and characterized by positron annihilation lifetime and differential scanning calorimetry. Phase modification in these systems has been brought about using e-beam and microwave irradiations. These changes have been studied through free-volume monitoring. The changes in free-volume parameters on e-beam and microwave irradiations on the two systems clearly show improved interaction between the blend constituents as a result of cross-linking through hydrogen bond formation. However, the free-volume data do not indicate whether the improvement in miscibility is due to changes at the interface or in the constituents of the blend. We have extended free-volume data to evaluate the hydrodynamic interaction parameter α, which is the measure of excess friction at the interface of the blend. Hydrodynamic interaction parameter shows that e-beam is a better interface stabilizer if the blends contain no polar group/groups and microwave irradiation is a better route to stabilize the interface in the case where the blend contains a polar group.