Thermally induced microstructural changes in cotton fibers: A free‐volume study

I measured positron lifetime in natural polymer–cotton fibers as a function of isochronal annealing temperature in the range 27–290°C. The variations in the positron results indicated structural changes occurring in the cotton fibers and determined the glass‐transition temperature as 80°C. Activation energies were measured separately for the crystalline and amorphous regions, indicating the versatility of the technique. These values were close to the O? H bond dissociation energy, suggesting O? H bond dissociation, the most probable process occurring under thermal treatment. As an extension of the positron results, the molecular weight of the cotton fibers was determined to be 1,200,000 based on free volume, which was within the range suggested for cotton. There seemed to be an indication that crosslinking changed the spiral structure of cotton fibers to the network type. However, this needs to be validated by other measurements. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3336–3345, 2002     

Differential scanning calorimetric and free volume study of reactive compatibilization by EPM‐g‐MA of poly(trimethylene terephthalate)/EPDM blends

Differential scanning calorimetry (DSC) and positron annihilation lifetime measurements have been carried out to study the effect of the compatibilizer maleic anhydride grafted ethylene propylene copolymer (EPM‐g‐MA) in poly trimethylene terephthalate and ethylene propylene diene monomer (PTT/EPDM) immiscible blends. The DSC results for the blends of 50/50 and 30/70 compositions show two clear glass transition temperatures, indicating that the blends are two‐phase systems. With the addition of compatibilizer, the separation between the two glass transitions decreased, suggesting an increased interaction between the blend components with compatibilizer. At 5 wt % of compatibilizer, the separation between the T_gs reduced in both 50/50 and 30/70 blends. The positron results for the blends without compatibilizer showed an increase in relative fractional free volume, as the EPDM content in the blend is increased. This suggests the coalescence of free volume of EPDM with the free volumes of PTT due to phase separation. However, the effect of compatibilizer in the blends was clearly seen with the observed minimum in free volume parameters at 5% of the compatibilizer, further suggesting that this percent of compatibilizer seems to be the optimum value for these blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 740–747, 2006     

Synthesis and characterization of methacrylic acid‐based molecular imprinted polymers by positron lifetime spectroscopy

Three molecular imprinted polymers (MIPs) were prepared with different amounts of crosslinker (ethylene glycol dimethacrylate) in methacrylic acid functional monomer. Positron lifetime spectroscopy has been used to study the variation in free volume cavities (nanosize pores) in these MIPs of different monomer‐crosslinker ratio. The changes in free volume cavities after the template molecule 4‐chlorophenol extracted from the polymer matrix were measured. The results indicate that the optimum monomer‐crosslinker ratio for this MIP is 1:5. The free volume distribution derived from CONTIN program further reveals the presence and absence of template molecules in the system. The Fourier transform infrared spectra results have been used to compliment this through the absence of stretching frequencies at 648 cm^?1 and 825 cm^?1 of C? Cl suggesting the successful extraction of the template from the MIPs. Scanning electron microscope (SEM) scans indicate the grains are of heterogeneous shape and size in agreement with earlier studies. POLYM. ENG. SCI., 54:667–674, 2014. © 2013 Society of Plastics Engineers     

Photocatalytic degradation of indigo carmine dye using TiO<Subscript>2</Subscript> impregnated activated carbon

The photocatalytic degradation of indigo carmine dye was studied using hydrothermally prepared TiO₂ impregnated activated carbon (TiO₂: AC). A comparison between the degradation of the indigo carmine dye using commercial TiO₂ and TiO₂: AC revealed the efficiency of the title compound. The degradation reaction was optimized with respect to the dye concentration and catalyst amount. The reduction in the chemical oxygen demand (COD) revealed the mineralization of dye along with colour removal. The active compound like TiO₂ was impregnated onto the activated carbon surface under mild hydrothermal conditions (<250°C, P ∼ 40 bars). The impregnated activated carbon samples were characterized using powder X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Water diffusion in a soft contact lens polymer and its tolerance to UV radiation studied by positron lifetime technique

The kinetics of water sorption, the topology or the free‐volume changes due to the presence of sorbed water in a soft contact lens polymer, poly(2‐hydroxyethyl methacrylate), were investigated by using the positron lifetime technique. It was found that the ortho‐positronium lifetime increases in the beginning of sorption because of microstructural swelling of the polymer matrix. After reaching a maximum, the lifetime decreases and becomes constant, maybe because of the filling of the free‐volume holes with water molecules. The diffusion process is found to be non‐Fickian. By using the dual‐mode sorption model, the Fickian‐controlled part and the relaxation‐controlled part of diffusion were separated. Further, the positron results seem to indicate the existence of water clusters in the sorbed lens material. The tolerance or stability of the soft lens material to UV radiation seems to be satisfactory as revealed by positron results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1355–1366, 2004     

Structural modifications in bivoltine silk fiber under thermal treatment

The microstructural changes of semicrystalline bivoltine silk fibers annealed between 27 to 260°C were measured using positron annihilation lifetime spectroscopy (PALS). Measured lifetime and intensity as a function of temperature reveals the removal of water molecules until 60°C. Present results indicate a T_g of 140°C, which is further confirmed by differential scanning calorimetry (DSC). The structural changes like random-coil to alpha and then to beta form are observed in the crystalline and amorphous domains at 100 and 205°C, respectively. Degradation of the fibers occur around 240°C. As an extension of the present measurements, we have estimated the activation energy of 8.0 KJ/mol in the disordered domain of the fiber, based on Goldanskii's kinetic relations. © 1997 John Wiley & Sons, Inc.     

Positron Lifetime Spectroscopy and Differential Scanning Calorimetric study of polystyrene‐based composites with fly ash,cenospheres, and calcium aluminosilicate as fillers

Polystyrene‐based composites with Fly ash, Cenospheres, and Calcium aluminosilicate as fillers have been examined for their mechanical and microstructure properties. Free volume measurements have been carried out using Positron Lifetime Spectroscopy to probe the microstructural features of the composites. Incorporation of fly ash, cenospheres, and calcium aluminosilicate improved the thermal properties of the composites due to increased T_g of the composites as revealed by the DSC data. Also, it was observed that the mechanical properties of calcium aluminosilicate filled composites improved more compared to fly ash and cenospheres filled composites. We attribute these favorable changes to the higher silica content of calcium aluminosilicate on account of possible interactions between the polymer matrix and silica and to some extent from particulate size. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Experimental determination of interface widths in binary polymer blends from free volume measurements

Interfaces in binary polymer blends play a pivotal role in moulding their physical properties. We observe a diffused interface in immiscible and partially miscible binary polymer blends for which the methods of preparation characterize the interface widths. The interface widths for three binary blends of PVC/PS, PS/PMMA, and PVC/SAN are in the maximum range of 2.15, 5.04 and 6.24 nm respectively. We establish a correlation between hydrodynamic interaction and the Flory–Huggins interaction parameter which is consistent with self-consistent mean field theory and the usual constructed density profile across the interface. Our approach is based on the hydrodynamic interaction which is derived from free volume data measured by positron lifetime spectroscopy.     

Influence of gamma irradiation on the formation of methanol induced micro-cracks in polycarbonate

The effects of -irradiation on the methanol diffusion behaviour in polycarbonate have been investigated by UV-visible, Fourier Transform Infra-red (FTIR) Spectroscopy, Positron Annihilation Lifetime Spectroscopy (PALS), Differential Scanning Calorimetry (DSC), Sorption, and Optical Microscopy. Positron results show that the free volume size and its content decreases after -irradiation which is due to cross-linking of chains. The cross-linking process is further confirmed by DSC measurements, which indicate an increase in glass transition temperature (T _g) after -irradiation. Sorption results reveal that methanol diffuses faster in virgin polycarbonate as compared to cross-linked polycarbonate (after gamma irradiation) in agreement with the results of supplementary techniques. Interestingly, the optical microscopic examination of the sample surfaces provides clear evidence of decreased micro-crack formation in -irradiated polycarbonate as compared to the virgin sample. This decrease in crack formation can be related to the decrease in the rate of methanol diffusion.