Characterization of polymer films using a slow positron beam

A phenomenological model linking the macroscopic volume from dilatometry with the microscopic free volume from positron annihilation lifetime spectroscopy is presented. The temperature dependences of both quantities enable to obtain the so-called initial temperature Ti at which dynamic free volume change sets in, the corresponding initial volume Vi, and subsequently, all free volume characteristics. Application of this model on the literature data of a typical low-molecular weight glass former, o-terphenyl, leads to the findings that (i) Ti ≅ TK the Kauzmann temperature and (ii) a packing fraction of the extrapolated equilibrium liquid at Ti is close to the closest regular packing of hard spheres. The published viscosity−temperature data over 12 decades of magnitude are correlated with the free volume hole fraction fh, as determined from the PALS results, via the WLF−Doolittle type equation. These findings indicate that the orthopositronium (o-Ps) is able to detect the part of the total free volume distribution which is relevant for the viscosity over the measured temperature interval from Tg up to ∼ 1.5Tg. In addition, the change of the free volume hole behavior above Tr = 1.25Tg, lying in the vicinity of the crossover temperature Tc from the idealized version of mode-coupling theory, is critically analyzed within a modified bubble concept (the equilibrium interaction of o-Ps with the soft matrix).

Silicone rubber is widely used as an insulating material. In this article, silicone rubber samples were prepared by varying the content of crosslinker (2,5-bis(tert-butyl-peroxy)-2,5-dimethylhexane, DBPMH), and the free volume holes in the samples were investigated by means of positron annihilation lifetime spectroscopy (PALS) measurement. The surface chemical structure, surface micromorphology and water diffusion of the samples after corona discharge treatment were studied by FTIR, SEM and EIS measurements, respectively. As the crosslinker weight ratio increased from 0.2 wt.% to 1.5 wt.%, the mean free volume hole size first decreased and then remained unchanged. However, the concentration of free volume holes did not vary as the crosslinker weight ratio increased. SEM morphologies show that surface cracks were produced on samples having high crosslinking levels after corona treatment. The water diffusion coefficient of samples after corona treatment increased from 3.13 × 10−10 cm2 s−1 to 17.68 × 10−10 cm2 s−1 in the initial immersion period, as the crosslinker weight ratio increased from 0.2 wt.% to 3.0 wt.%. The results indicated that deterioration of samples with high crosslinking levels were more serious and water repellency more easily lost. The corona resistance ability of low crosslinking level silicone rubber stems from internal low molecular weight molecules.

Effects of montmorillonite (MMT) content on the free volume hole and miscibility were first investigated for polyurethane/epoxy resin (PU/EP) interpenetrating polymer network (IPN) nanocomposites by positron annihilation lifetime spectroscopy (PALS), X‐ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). An exfoliated (IPN/MMT1), a coexistence of exfoliated and intercalated (IPN/MMT3), and an intercalated (IPN/MMT5) structures were confirmed by XRD. Influences of two kinds of hydrogen bonding interactions including both between carbonyl group and N‐H group, and the other between urethane group and silicate layers have been discussed. PALS results reveal that the size of the free volume holes decreases after the MMT loaded due to the increase of chain packing efficiency. The large difference between IPN/MMT1 and IPN/MMT5 in the free volume distribution was observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Free volume evolution in polyoxymethylene studied by positron annihilation spectroscopy

Since the macroscopic properties of materials are relevant to their micro structure, the detection of microscopic defects or their changes in size and concentration is an interesting topic in materials science. The positron annihilation lifetime (PAL) method used here is established as a unique technique that can provide information about the size and concentration of free volume holes directly without significantly intervening in the bulk properties of the specimens. To our knowledge, this is the first time that PAL techniques have been used to study the wear mechanism of macromolecular tribomaterials. Here we present the results of a study in which the correlation between the fretting wear behaviour of polytetrafluorethylene (PTFE) and the changes in its free volume size and concentration in amorphous‐crystal interface regions was found to be δω =A.δτ3, where δω is the wear loss of PTFE under fretting wear conditions, A is a constant, and δτ3 represents the relative change in the third lifetime components after the friction test. The results indicate that fretting mainly influences the characteristics of atomic‐scale free volume holes in the amorphous‐crystal region in PTFE, and under high‐load fretting wear conditions the ductile polymer PTFE possesses a self‐curing function for its defect‐free micro volume holes.

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.

In this paper, a systematic study is provided of the chemical environment inside free volume holes in a series of halogenated polystyrenes (p-position), −[CH2CH C6H5X]n– (X = F, Cl, Br, I), by using positron annihilation spectroscopy. In such polymers it was determined that the chemical environment is the major effect on Doppler broadening of two 511 keV γ photons from positron–electron annihilation. Doppler broadening energy spectroscopy (DBES) and positron annihilation lifetime spectroscopy (PALS) were combined in a novel approach to study the chemical environment in a polymer system. A highly linear relationship between Doppler broadenings caused by electron kinetic energies of valence electrons and the ionization potentials of halogen elements was obtained, as well as a similar correlation with the results from ab initio molecular orbital (MO) calculations for monohalogenobenzenes, C6H5X (X = F, Cl, Br, I). The results demonstrate that the combination of DBES and PALS may provide an effective way to study the chemical environment inside free volume holes in polymers.

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

A phenomenological analysis of the ortho-positronium (o-Ps) annihilation from positron annihilation lifetime spectroscopy (PALS) and the dynamics from broadband dielectric spectroscopy (BDS) are reported on a small molecular glass former of intermediate H-bonding and fragility: salol. The dielectric spectra extend over a very broad frequency range of about 2 × 10(-2)-3.5 × 10(11) Hz, providing information on the α-relaxation, the secondary relaxation giving rise to the excess wing, and the shallow high-frequency minimum in the micro- to milli-meter wave range. A number of empirical correlations between the o-Ps lifetime, τ(3)(T), and the various spectral and relaxation features have been observed. Thus, the phenomenological evaluation of the τ(3)(T) dependence of the PALS response of the amorphous sample reveals three characteristic PALS temperatures: T(g)(PALS), T(b1)(L) = 1.15T(g)(PALS) and T(b2)(L) = 1.25T(g)(PALS), which are discussed in relation to similar findings for some typical small molecular vdW- and H-bonded glass formers. A slighter change of the slope at T(b1)(L) appears to be related to the transition from excess wing to the primary α-process-dominated behavior, with the secondary process dominating in the deeply supercooled liquid state below T(b1)(L). The high-temperature plateau effect in the τ (3)(T) plot occurs at T(b2)(L) and agrees with the characteristic Stickel temperature, T(B)(ST), marking a qualitative change of the primary α process, but it does not follow the relation T(b2)(L) < T(α) [τ(3)(T(b2)) < τ(α)]. Both effects at T(b1)(L) and T(b2)(L) correlate with two crossovers in the spectral shape and related non-exponentiality parameter of the structural relaxation, β (KWW). Finally, the application of the two-order parameter (TOP) model to the structural relaxation as represented by the primary α relaxation times from BDS leads to the characteristic TOP temperature, T(m)(c), close to T(b1) from PALS. Within this model the phenomenological interpretation is offered based on changes in the probability of occurrence of solid-like and liquid-like domains to explain the dynamic as well as PALS responses. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in small molecule glass formers.

Positron annihilation lifetime spectroscopy (PALS) is a notoriously difficult technique to apply on a slow positron beam because no natural “start signal” is available for marking the entry of the positron into the sample. A new variable energy positron annihilation lifetime spectroscopy (VEPALS) system (otherwise known as a lifetime beam) based on secondary electrons (SEs) emission in the forward geometry from a carbon foil (C-foil) was constructed. In this system, a C-foil was installed on a stainless steel supporting mesh and a tungsten needle with the length of 1 cm was connected at one of its end on the centre of the foil while the other end touches the target sample. The slow positron passes through the C-foil thus generating SE in the forward direction. The detected SE is treated as a “start signal” of an electronic clock. The clock is stopped by the detection of the annihilation gamma radiation originating from the annihilation of the positron with an electron inside the sample.&#13;\n&#13;\nSE emission in the forward geometry from the C-foil with thicknesses from 1.0 – 5.0 𝜇g/cm2 induced by the bombardment of 1 – 20 keV positron beam was studied with the aid of a micro-channel plate (MCP). The electronic stopping power, the material dependence parameter, the maximum scattering angle of the emitted SE and the energy distribution of SEs were presented. The results were analyzed with some calculations and Monte Carlo simulations. The SE yield drops to a constant value for positron beam energy above 15 keV. The maximum SE scattering angle was about 60°.&#13;\n&#13;\nIn order to test the efficiency of this needle-foil device without the target sample, the yield of SE and the transmission coefficient of positrons were studied as a function of the incident positron energy ranging from 1 – 20 keV, the electric potential of the annular electrode and three different column lengths of the annular electrode. The spot size of the transmitted positrons and the diameter of the SE ring were also investigated.&#13;\n&#13;\nA circular single crystal p-type silicon (Si) sample was used to test the performance of the VEPALS system. A conventional positron lifetime spectrum of Si was obtained and compared with results from the VEPALS system. The positron lifetime spectra of the Si were studied for annular electrode potential ranging from 1.5 – 3.0 kV. The 1.5 kV annular electrode provided the optimal positron lifetime of Si. The Si positron lifetime spectra were further studied as a function of positron beam energy for the 1.5 kV annular electrode. The analyzed spectra have a bulk defect-free lifetime component of 231 ps with a more than 66% intensity. This defect-free lifetime component intensity could be higher than 80% for a 15 keV positron beam.&#13;\n&#13;\nThe unwanted PALS events and the satellite peak of the PALS were examined and explained in terms of a few positrons being scattered to the extent that they miss the sample and strike the MCP directly. A positive bias potential at the MCP grid was provided for eliminating the satellite peak.

Free volume study of 70 MeV carbon induced modification in polymers through positron annihilation

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

The chemi‐crystallization and free volume changes of high‐density polyethylene (HDPE) exposed to subtropical humid climate of Guangzhou, China, were investigated using Fourier transform infrared spectroscopy, differential scanning calorimetry, wide‐angle X‐ray diffraction, dynamic mechanical analysis and positron annihilation lifetime spectroscopy. An increase in content of carbonyl groups and significant chemi‐crystallization were observed to occur during natural exposure. Chain scission accounted for the chemi‐crystallization and would lead to greater crystallizability of the molecules. The reheating DSC run indicated that the crystallizability of the degraded HDPE molecules increased initially with exposure time and then decreased. Positron data showed the new crystals induced by chemi‐crystallization indeed had more imperfect crystal structure in comparison with the pre‐existing parent crystals, and the free volume located in amorphous regions decreased involving a shrinking of the free volume holes. The shrinkage of free volume holes was correlated with the loss of mobility of HDPE molecules, which was confirmed by the increase of glass transition temperature. The formation of new imperfect crystals might increase the amount of rigid amorphous fraction of HDPE materials, as well as the occurrence of crosslinking reactions of molecules located in the interior of HDPE materials, consequently decreasing the molecular mobility. © 2016 Society of Chemical Industry

Investigation on the variation in the fine structure of plasma-polymerized composite membrane by positron annihilation spectroscopy

Accounting for the thickness dependence of the Tg in supported PS films via the volume holes diffusion model