Blend PALS measurements

As aging tends to be accompanied by changes in the packing density of the system, PALS can provide a qualitative estimate of the free volume in the system and its changes due to blending. For PS/PPO blends, PALS measurements were found to be in qualitative with stress-relaxation data. As discussed in the previous sections, Chang et al. (1997) found that PS/PPO and PS/PVME blends were less dense than PS, while PMMA/PEO was denser than PMMA, thereby making chain relaxation easier in the former and more difficult in the latter, a result that could explain the stress-relaxation behavior. 

Recent developments have been in the area of microthermal analysis using thermal conductivity with thermal diffiisivity signals or AFM to visualize specific areas or domains in the material and perform localized thermal analysis studies (183,184). Relaxational behavior over time and temperature is related to changes in free volume of the material. Positron annihilation lifetime spectroscopy (PALS) measurements of positron lifetimes and intensities are used to estimate both hole sizes and free volume within primarily amorphous phases of polymers. These data are used in measurement of thermal transitions (185,186) structural relaxation including molecular motions (187-189), and effects of additives (190), molecular weight variation (191), and degree of crystallinity (192). It has been used in combination with DSC to analyze the range of miscibility of polymethyl methacrylate poly(ethylene oxide) blends (193). 

The first positron annihilation lifetime (PAL) measurements on polymer blends were conducted by Jean et al. [9], who investigated the free volume properties of (i) a miscible blend, namely tetramethyl-bisphenol A polycarbonate (TMPC) and polystyrene (PS) and (ii) an immiscible blend, namely bisphenol A polycarbonate (PC) and PS. It was observed that TMPC formed a miscible blend with PS as it had larger fractional free volume cavities than PC. For the miscible blend, the free volume showed negative deviation from the linear additivity rule (Eq. (27.19)), whereas for immiscible blend it was observed that the free volume, as detected by o-Ps lifetime as a function of composition, was complicated due to the presence of... 

Three blends have been studied by Chang et al. [1997] using stress relaxation measurements and positron annihilation lifetime spectroscopy (PALS). It was observed that for blends of PS with PPE and PS with PVME the stress relaxation rates were faster for the blends in comparison with PS alone, whereas the opposite was true for a PMMA/PEG blend when compared with neat PMMA. 

Positron annihilation lifetime spectroscopy (PALS) can be used to measure the free volume in various materials. Jean et al. discussed the application of positron annihilation spectroscopy (PAS) in the detailed study of polymers and polymers with fillers. The primary experimental PAS technique used in this research is PALS, one of the three techniques in the PAS family and a powerful tool for measuring the free volume in various materials. The free volume has a great role in polymer research and is widely used to explain the behaviour of physical properties such as glass transition temperature, viscosity and physical ageing. Free volume is affected by the blending of polymers, ageing and addition... 

Positron annihilation spectroscopy (PALS) is a technique of free volume determination in polymers that involves the injection of subatomic particles and the measurement of their decay times. This technique can be very sensitive to the degree of miscibihty and free volume behavior of polymer blends. The concept of free volume is important to understand polymer characteristics in the glassy state. For instance, PALS was used to evaluate the free volume sites of thermotropic hquid crystalline polymer blends. These blends presented smaller and fewer free volume sites than expected from a weighted average due to their intrinsic affinity. This is interesting because in contrast to thermoplastic blend results, the degree of blend miscibility alters free volume behavior as a function of blend composition [106]. In addition, the order and the dynamics in the mesophase can be accessed by nuclear magnetic resonance [107]. 

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