S. J. Wang

Interaction of two phases in PP/EPDM polymer blend probed by positron annihilation : CONTIN analysis

Positron annihilation lifetime measurements were performed on pure polypropylene (PP), ethylene-propylene-diene monomer (EPDM) rubber, and their blends PP/EPDM with a series of EPDM volume fraction ϕ (= 10–40%). A numerical Laplace inversion technique (i.e., CONTIN algorithm), was employed to obtain the probability distribution functions (PDF) of free-volume radius. We observed that, first, the average free-volume radius in PP/EPDM blends is generally same as that in PP and is much smaller than that in EPDM. Second, the standard deviation σR or the width of the free-volume radius PDF in the blend decreases with ϕ in the region of ϕ = 10ndash;30%, and it increases when ϕ increases from 30% to 40%. The difference in the σR of the blend and the calculated value σcR according to the simple-mixing rule of PP and EPDM is interpreted by the existence of the two-phase interaction (i.e., the residual thermal pressure and shear stress between PP and EPDM phases in the PP/EPDM blends). The correlation between σR, which indicates the interaction of two phases, and the impact strength of PP/EPDM blends was found and discussed.

Excited-state enhancement of the third-order optical nonlinearity of a square planar complex

Abstract Excited-state enhancement of the third-order nonlinear optical susceptibility of a square planar complex, (2,2′-bipyridine)Pt(1,2-dicyano-1,2-ethylene-dithiolate), was observed when the sample was optically pumped by either a picosecond or a nanosecond pump beam of 355 nm wavelength. For the picosecond case, the temporal behavior of the enhancement was investigated, and found to be characterized by a slow and a fast components, which are attributed to populations of the first excited state S 1 and the second excited state S 2 , respectively. For the nanosecond case, the enhancement of the nonlinearity is attributed predominantly to the population of the first excited state S 1 . The molecular hyperpolariabilities of the S 1 state and the S 2 state, γ S1 and, γ S2 , were determined to be 1.9 × 10 −31 and 1.0 × 10 −30 esu, respectively, which are much larger than the γ 8 of the ground state ( γ g −33 esu).

Free volume and ionic conductivity of poly(ether urethane)‐LiClO4 polymeric electrolyte studied by positron annihilation

The positron lifetime spectra and ionic conductivity have been measured for poly(ether urethane)‐LiClO4 polymeric electrolyte as a function of temperature. The glass transition temperature Tg, free‐volume Vf, and fractional free‐volume f were derived from the positron annihilation parameters. A correlation between fractional free‐volume f(T) and conductivity σ above Tg, log[σ/σ(Tg)]=C1[f(T)−f(Tg)]/f[T], was first experimentally confirmed using measured positron annihilation results. The comparison of the value of the obtained constant C1 with the universal values for the segmental diffusion of amorphous polymers indicated that the critical free volume required for the ion transport is much smaller than that required for polymer chain segment mobility. Carrier transport and the segmental motion are discussed in terms of the free‐volume theory.

Study on the microstructure and miscibility of dynamically vulcanized EPDM/PP blend by positron annihilation

Abstract Positron annihilation lifetime spectroscopy (PALS) was employed to investigate the relationship between the free volume hole properties and miscibility of dynamically vulcanized ethylene propylene diene monomer (EPDM)/polypropylene (PP) blend. The free volume hole concentration and the relative fractional free volume show negative deviation from linear additivity only when the weight percent of EPDM is over 50%. Combined with the results of dynamical mechanical thermal analysis measurements and mechanical properties tests, we found that the noncrystalline region of PP and EPDM are partially miscible and the miscibility of the blend became better when the weight percent of EPDM is over 50%.

Microstructure and mechanical properties of polymers studied by positron annihilation

The recent progress of positron studies on polymers are briefly reviewed. The correlation between free-volume holes and mechanical properties are discussed. The results indicate that the positron spectroscopy is a potential tool to characterize the microstructure and mechanical properties of polymeric materials.

Study of polyaniline by positron annihilation technique

Abstract Positron lifetime, Doppler broadening of the annihilation γ energy and conductivity have been measured for polyaniline as a function of protonation level (pH from 7 to −0.8). Positron lifetime spectra have been resolved into three exponentials. We observed that (1) the short lifetime τ 1 decreased with the protonation level and (2) the intermediate lifetime, τ 2 ∼ 360 ps, almost remained constant, whereas its intensity I 2 increased with increasing protonation level which was related to the conductivity of the material. These observations, combined with the measurement of the lineshape parameter S , are consistent with the conducting island model.

Positron annihilation in ionic conductivity polymers

Positron annihilation lifetime spectra and ionic conductivity have been measured for poly(etherurethane)-LiClO4 as a function of temperature. The effects of Li salt on glas transition free volume and ionic conductivity have been discussed. A correlation between fractional free volume and ionic conductivity was first experimentally confirmed by using the free volume theory.

Defect evolution and its impact on the ferromagnetism of Cu-doped ZnO nanocrystals upon thermal treatment: A positron annihilation study

CuO/ZnO nanocomposites with 4 at. % CuO were annealed in air at various temperatures between 100 and 1200 °C to produce Cu-doped ZnO nanocrystals. X-ray diffraction shows that a CuO phase can be observed in the CuO/ZnO nanocomposites annealed at different temperatures, and the Cu-doped ZnO nanocrystals are identified to be of wurtzite structure. The main peak (101) appears at slightly lower diffraction angles with increasing annealing temperature from 400 up to 1200 °C, which confirms the successful doping of Cu into the ZnO lattice above 400 °C. Scanning electron microscopy indicates that most particles in the CuO/ZnO nanocomposites are isolated when annealing at 100–400 °C, but these particles have a tendency to form clusters or aggregates as the annealing temperature increases from 700 to 1000 °C. Positron annihilation measurements reveal a large number of vacancy defects in the interface region of the nanocomposites, and they are gradually recovered with increasing annealing temperature up to 1000 °C....

Defect Properties in Plastically Deformed p‐GaAs Studied by Positron Lifetime Measurements

Positron lifetime spectra have been measured on plastically deformed Zn-doped p-type GaAs with strain ranging from 0 to 15% at room temperature. The lifetime spectra were analyzed using PATFIT and MELT programs. Two lifetime components were observed in deformed samples. The second lifetime τ 2 ∼ 400 to 450 ps indicated that vacancy clusters were formed during deformation. From the temperature behavior of positron lifetime in 15% deformed samples, the positron detected negative ions as shallow positron traps, and the dominant shallow traps were attributed to acceptor impurity Zn ions with negative charge. The positron trapping rate of vacancy clusters was also estimated.

Study of the ionic transport in polymer electrolyte using positron lifetime distribution method

Positron annihilation spectroscopy has been applied to measure the free-volume hole distribution in poly(ether urethane) as a function of temperature. The hole radius distribution determined from orthopositronium lifetime distribution is found to shift to a larger values with increasing temperature. This result, combined with the variation of ionic conductivity, suggests that carrier ions do not migrate naked but are bound to polymer segments through ion-dipole interaction forces, and the ion migration is controlled primarily by segmental motion of the polymer.