High-quality positron lifetime measurements (70 million total counts) are reported for polyethylenes (PEs) of different crystallinities (X c ϭ 3-82%). The specific volumes of the crystalline and amorphous phases (V c and V a , respectively) were estimated from density and wide-angle X-ray scattering (WAXS) experiments. Some samples (those with low values of X c ) were branched PEs, and those with high values of X c were linear PEs for which X c was varied with changes in the crystallization temperature. Both V c and V a increase with decreasing X c in the range 0% Յ X c Յ 56% (the branched PEs) but are constant for X c Ն 56% (the linear PEs). The lifetime spectra were analyzed with the MELT and LIFSPECFIT routines. Artifacts that can appear in the spectrum analysis were checked via an analysis of computer-generated spectra. Four lifetime components appeared in all of the PEs; the two long-lived ones are attributed to pick-off annihilation of ortho-positronium (o-Ps) in crystalline regions ( 3 ) and in holes of the amorphous phase ( 4 ). With increasing X c , 3 decreases from about 1.2 to 1 ns, 4 decreases from 3.0 to 2.5 ns, and the intensity I 4 decreases from 29 to 0%. An increase in I 3 from 6 to 12% was observed. A comparison with simulations shows that the true I 3 value approaches 0 for X c 3 0%. The decrease in I 4 is weaker than the increase in X c ; this leads to the conclusion that the apparent specific o-Ps yield in the amorphous phase I 4 Xc increases with X c . Possible reasons for this surprising results are discussed. The fractional free hole volume [h ϭ (V a Ϫ V occ )/V a , where V occ is the crystalline occupied volume] was estimated from density and WAXS results. Between X c ϭ 0 and 56%, h decreases from 0.151 to 0.090, but it does not change further above X c ϭ 56%. The mean size (v) of the local free volumes (holes) estimated from 4 decreases from 200 to 150 Å 3 . The number density of holes (N h ) calculated from these values (N h ϭ h/v) also decreases from 0.8 to 0.6 nm Ϫ3 in the range 0% Յ X c Յ 56%. The values of V a , V c , h, and N h increase with an increasing degree of branching but do not vary for linear PEs. The possible influence of a crystalline-amorphous interfacial phase (three-phase model) on the observed lifetime parameters is also discussed.

Positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) were used to study the interdiffusion in a particle-matrix system consisting of the miscible polymers poly(vinyl chloride) (particles) and poly(n-butyl methacrylate) (matrix, 30:70 and 50:50 w/w). Starting from the demixed state of the blends, the transition to the mixed state during annealing at 110 °C was investigated. We deliver further evidence that PALS parameters such as the orthopositronium intensity I 3 and the average positron lifetime τav respond to chemical inhomogeneities in blends and their disappearance due to mutual diffusion of molecules of both phases. The response is due to inhibition of Ps formation by one of the constituents when its molecules migrate into the phase made from the second one. A core-shell model for the description of the o-Ps response to a local chemical inhomogeneity is developed which makes use of a calculated concentration-distance profile. The comparison with the experiment allows the estimation of the interface width and of kinetic parameters, such as the exponent of the time dependence of the interface width and the coefficient of mutual diffusion. The potential and the limitation of this new technique are discussed.

Positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) were used to study the interdiffusion in a particle matrix system consisting of (styrene-maleic anhydride) copolymer containing 24% maleic anhydride (SMA) and poly(methyl methacrylate) (50:50). Starting from the phase separated state of the miscible components, the transition to the mixed state due to interdiffusion during annealing at 160 and 180°C was investigated. The diffused mass fractions of both components were estimated by DSC; they increase linearly with the logarithm of the annealing time. Both components show the same behavior, suggesting an average interphase composition of 50:50. For subsequent annealing, the kinetic behavior differs from the earlier behavior, probably because of the different diffusion coefficients of the two components. Experimental evidence is presented that the orthopositronium (o-Ps) intensity correlates to the interdiffusion process. The response is due to the diffusion of inhibitors for Ps formation from one phase into the other. A semiquantitative model is developed that allows the estimation of the interphase volume fraction from the relative change of the annihilation parameters. The PALS results are discussed in relation to those obtained by DSC. The results show that PALS is a useful tool to study mixing and demixing processes in polymer blends with a three-dimensional morphology.

No abstract

Positron lifetime and Doppler broadening spectroscopy were applied for a study of defect properties of semi-insulating GaAs after diffusion of copper. A 30 nm layer of Cu was deposited by evaporation to the undoped GaAs samples. The diffusion of Cu was performed during an annealing step at 1100 °C at different arsenic vapor pressures. The samples were quenched into room temperature water. The initial semi-insulating (SI) undoped GaAs sample shows no positron traps. After annealing, a vacancy-type complex and a shallow positron trap were observed to be efficient positron traps. Due to the Cu contamination during the annealing process, the shallow trap is believed to be the CuGa double acceptor. The nature of the vacancy-type defects could not be determined unambiguously. The concentration of these vacancies shows inverse relationship to the As vapor pressure that refers to the arsenic vacancy as a part of this complex. Moreover, Doppler-coincidence spectroscopy shows clearly that Cu atoms are not bound in the direct vicinity to the observed vacancies.