† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 11575205, 11475197, 11675188, and 11475193).
Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol (PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.
In recent years, a slow positron beam technique with variable monoenergy has been successfully used to determine the free volume related atomic or nanoscale fine structure property in polymeric systems.[1–3] By tuning the incident energy of injected positrons, a variable energy positron beam with an adjustable energy and a narrow energy distribution allows depth-resolved measurements useful in thin film defect studies. Reported results using a variable monoenergetic slow positron probe show a great potential in studying bulk property correlated microstructure defects in polymer membrane materials, which is important for many industrial applications, such as coatings, gas separations, and pervaporation.[4–6] Doppler broadening energy spectroscopy (DBES), as a powerful technique to determine the fine layer structure in polymeric film system, has achieved great success in probing the relationship between the free volume related molecular chain packing density in polymers and macroscopic performance.[7–9] However, the analysis of the positron annihilation parameter obtained from DBES is a challenge, since the obtained positron annihilation spectroscopy signals was an integration of its dispersed depths in polymer samples and was contaminated in the beam chamber with the positrons reflected or back-diffuse to the surface.
There is a practical problem or drawback in using DBES for micro-structural analysis, which is low momentum resolution due to the limitation of energy resolution of detector and interference of the background events. Therefore, an application of a more sensitive physical method coupled with DBES could be developed to benefit a more polymeric fine structure analysis, and positron coincidence Doppler broadening (CDB) is such a complementary technique.[10] Compared with traditional Doppler broadening spectroscopy, the use of CDB measurements is principally more valuable because it is capable of eliminating the background events to a great extent in data analysis, which will also be important for the understanding of basic principles of positron annihilation spectroscopy in polymeric applications.
Combining DBES and CDB is a powerful tool to study in more depth the variation in the positron experimental data and for membrane fine structural analysis. In this paper, we report a combined study of the DBES and CDB methods coupled with variable monoenergy slow positron beams to study the positron annihilation characterization in polyethylene glycol (PEG) thin film formed with different average molecular weight and further do fine structural analysis.
Three kinds of PEG produced by the Sigma Aldrich with weight average molecular weight (Mw) of 20000, 35000, and 100000 were dissolved in the solvent mixture (ethanol and water ratio of 1:1), by using a magnetic stirrer to form a 15 wt% homogeneous solution. Then 16 wt% of maleic anhydride (MA) as crosslinking agent and 3 wt% trimethylamine as catalyst mixed with the solution. Next, the mixture was covered on a polyvinylidene fluoride (PVDF) supporting membrane, with standing at room temperature for 12 hours. A PVDF (PVDF-1015) supporting membrane was prepared using the non-solvent induced phase separation (NIPS) method.[11] Finally, the composite membranes were crosslinked at 80 °C temperature for 6 hours. All the reagents were of analytical reagent grade and were used as-received. The as-obtained membranes with average Mw of 20000, 35000, and 100000 are designated as PEG-2, PEG-3.5, and PEG-10, respectively.
The PEG/PVDF composite membrane samples were fractured in liquid nitrogen and coated with a conductive layer of sputtered gold. The cross-section morphologies of the PEG/PVDF membranes were investigated by a scanning electron microscope (SEM, HITACHI S-4800).
Positron annihilation measurements were conducted with a magnetically guided variable-energy positron beam (0.012– 20 keV) accelerated by the negative high voltage on samples. About 106 positrons/s are generated with a 50 mCi 22Na radiation source. Two positron annihilation spectrometers, DBES, and coincidence Doppler broadening energy spectroscopy were installed in the beam for this study. After an injecting positron is thermalized in matter, it annihilates with an electron and emits two 511 keV gamma quanta. The energy broadening of the gamma quanta pair recorded by using an HPGe detector is described as DBES. The signal of the detector is fed into a multichannel analyzer (MCA) after being amplified, which had 16384 channels of 48.6 eV/Ch. calibrated using 136Cs (661.7 keV) and 22Na (511 keV) isotopes. The shape of the DBES curve is expressed in S and W parameters. The S parameter was defined as a ratio of integrated counts in the central region (510.24–511.76 keV, as the area of A in Fig.
In measurement of CDB, coincidences between the two annihilation gamma rays (511 keV) were detected by two HPGe detectors placed at 180° to each other. Signals from the coincidence circuit are processed using double-analogue-to-digital converters (DADC), and the obtained data are fed into a two-dimensional MCA, which had 400 channels in both the X and Y directions, with focusing the peak of 511 keV at the center of a 400×400 matrix. It took 20–24 h to obtain a CDB spectrum for one sample with the integral number of total counts attained 1×107. The three-dimensional CDB spectrum is shown in Fig.
Different crosslinked polymer skin layers were formed by changing the molecular weight of PEG (20000, 35000, and 100000, respectively). The hierarchical structure of as-obtained membranes were measured using SEM. Figure
The sensitivity of positron probes to molecular dimensions, particularly the free volume related microstructure in a complex macromolecular system, has made it possible for them to be a unique and valuable examination tool to probe the subnanometer regions and defects in a wide variety of polymers.[12] Within the last few years, positron annihilation spectroscopy using low-energy positron beams has become a powerful tool in determining the layer structure in a hierarchical or multilayer polymeric system.[2,3,9,13] Therefore, the fine-microstructure of the above as-obtained three PEG membranes as a function of depth could be obtained with a variable monoenergy slow positron beam spectroscopy, and thus DBES experiments as a function of positron incident energy from 0– 20 keV were carried out.
DBES, which probes the energy broadening from electron motion of atoms or molecules in the annihilate site, represents the momentum density in the longitudinal direction of annihilation radiation.[14,15] Figure
There is a practical problem in using the S parameter for characterizing the fine structure of ultrathin composite membrane which is usually scarcely discussed, that is the so-called surface effect.[19–21] It is interesting to see that in Fig.
Positron implanted energy-dependent CDB spectra of PEG-2, PEG-3.5, and PEG-10 relative to the low-density polyethylene (LDPE) spectrum are shown in Fig.
To further explore the methodology of utilizing positron annihilation behavior in near surface and bulk layer of PEG membrane for microstructural difference illustrations, more positron annihilation information on representing the microstructural information could be extracted from the as-obtained CDB spectrum. Figure
It is found that simultaneous use of two positron techniques (DBES and CDB) based on available energy measurements can be efficiently used to extract microstructural and chemical information, where the positron annihilation parameters of the polymer film can be reliably determined. Experimental results show that the variation of positron annihilation parameter as a function of incident energy is interpreted in terms of the surface effect and differences in micro-structure or chemical environment of positron annihilation. Combined application of these two techniques to the analysis of positron annihilation data and correlated fine structure are promising in the near future.
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