No abstract

Existing positron annihilation lifetime spectroscopy (PALS) uses the orthopositronium components, lifetimes, and intensities observed in molecular substrates, such as in polymers, to determine free-volume properties based on an infinitive potential spherical model originally proposed by Tao in 1972. However, in many molecular systems, positronium is either quenched or inhibited by interacting with chemical functional groups and leads to no or nearly no orthopositronium component in PALS. In this Article, a newly modified equation is developed by following the Tao’s quantum model using the positron component (not orthopositronium) of PALS. This modified equation is examined by fitting free-volume results obtained from o-Ps lifetimes with the positron lifetimes in pressure- and temperature-dependent data in polymers and calibrated with known or calculated cavity sizes in zeolite materials. A newly modified positron lifetime-free volume correlation equation is established for the determination of free volumes up to the mean radius of 5 Å in polymeric systems, where no orthopositronium component is observed in PALS.

The free-volume properties in a system of zinc oxide (ZnO) nanoparticles (20 nm) dispersed in waterborne polyurethane (WBPU) were measured using positron annihilation lifetime spectroscopy. Two glass-transition temperatures (T g), lower T g ∼ 220 K and higher T g ∼ 380 K of the ZnO/WBPU nanocomposites, were found and both increase with increasing zinc oxide content from 0% to 5%. These two glass transitions are interpreted from two segmental domains of WBPU; the lower T g is due to soft aliphatic chains and high T g is due to polar hard microdomains, respectively. The increase in T g with the addition of ZnO fillers is mainly attributed to interfacial interactions through hydrogen bonding, van der Walls forces, and electrostatic forces between the polymer matrix and zinc oxide nanoparticles. These results are supported by the data from the dynamic mechanical thermal analysis (DMTA). The relationship between the free volume obtained from nanoscopic positron method and the physical cross-link density from macroscopic DMTA method as a result of microphase separation of hard and soft segments in polyurethane is found to follow an exponential function. Chemical properties and surface morphology of nanocomposites were examined by Fourier transform infrared spectroscopy (FTIR) and by atomic force microscopy (AFM).

Polyblend and nanocomposite films of sodium salt of carboxymethylcellulose (Na‐CMC)/polyacrylamide (PAM) and Na‐CMC/PAM modified with carbon nanotubes (CNT) were synthesized by the solution casting technique. The effect of PAM and CNT loading on the structural, optical, and nanoscale free volume properties of Na‐CMC was studied. X‐ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy exhibited the existence of strong interactions between Na‐CMC and PAM and the non‐destructive effect of CNT on Na‐CMC/PAM structure. The HR‐TEM revealed the multi‐walled structure of CNT with a 7.06‐nm wall thickness and a 6.92‐nm wall inner diameter. Positron annihilation lifetime spectroscopy (PALS) was done, in a vacuum and at 30°C to 200°C, to investigate the nanoscale free volume properties by using a conventional fast‐fast coincidence spectrometer. It was found that the o‐Ps lifetime (τ3) and free volume (Vh) increase with increasing CNT percentage in the Na‐CMC/PAM blend. The distribution of the o‐Ps lifetime was broadened with increasing CNT ratios. Furthermore, the glass transition temperature (Tg) increases with increasing loads of CNT. For the first time, a correlation was done between Urbach energy (EU) and Vh. Finally, the results were represented and discussed in the frame of free volume properties. Optical measurements showed that the transmittance T% of Na‐CMC/PAM was 91.12% and decreased to 68.42% and 36.45% after loading with 1.0 and 2.0 wt % CNT. In addition, the blend shows higher insulating properties compared with the individual polymers. The CNT incorporation reduces the band gap significantly and increases the EU in the films.