C. Ranganathaiah

Free‐volume microprobe study of iodine diffusion in polymers

The effect of iodine sorption on the free-volume of three polymers, namely, acrylonitrile-butadiene-styrene, poly(ethylene terephthalate) and polycarbonate have been investigated by the positron annihilation Lifetime technique. Diffusion coefficients have been evaluated from the sorption curves using Cranks solution for Ficks second law of diffusion. The positron results are explained in terms of iodine filling the free-volume holes, swelling and conformational changes. It has also been found that the diffusion process in acrylonitrile-butadiene-styrene and poly(ethylene terephthalate) deviates from Ficks law of diffusion whereas in polycarbonate, the diffusion process obeys Ficks law. Further, for polymers in which diffusion deviates from Ficks law, the sorption process has been separated into diffusion-controlled (Fickian) and relaxation-controlled (non-Fickian) parts using the dual-mode sorption model. Interestingly, the present results show that the polymers which deviate from Fickian diffusion also deviate from Fujitas free-volume theory of diffusion and vice versa. The positron results also indicate that sorbed iodine in the polymer acts as an inhibitor of orthopositronium

Degradation of acrylonitrile-butadiene-styrene and polycarbonate by UV irradiation

The effects of ultra-violet irradiation on the microstructure of an oxygen free polymer viz., acrylonitrile-butadiene-styrene and a polymer containing oxygen viz., polycarbonate have been investigated using positron annihilation lifetime measurement. Lifetime results in both polymers in the early stages of irradiation indicate that chain-scission leading to free radical formation is the predominant process. Radiation induced cross-linking becomes dominant in the later stages of irradiation. An interesting feature observed in the present studies is that an oxygen containing polymer attains free volume stability which is attributed as due to the formation of photostabilisers whereas no such stability is observed in the case of an oxygen free polymer. UV absorption spectroscopy results supplement the positron lifetime results that oxygen containing polymer becomes photostabilised.

Positron lifetime study of diffusion kinetics in electron irradiated polycarbonate

Positron lifetime and gravimetric measurements of iodine sorption and diffusion behaviour in electron-irradiated polycarbonate and un-irradiated polycarbonate are investigated. The increase in ortho-positronium lifetime on electron-irradiation shows that scission of carbonate groups in the main-chain and formation of free radicals is the predominant process. The UV absorption measurement supports this. The free volume in polycarbonate after electron-irradiation seems to be not composed of independent pores but of interconnected channels. The positron results on iodine diffusion in un-irradiated polycarbonate shows a slower diffusion rate, whereas in electron-irradiated polycarbonate faster diffusion is observed. The gravimetric measurement of iodine diffusion attains an early saturation in electron-irradiated polycarbonate as compared to un-irradiated polycarbonate although in both cases the diffusion is nearly Fickian. An exponential type of correlation has been observed between fractional free volume and diffusion coefficient validating Fujitas free volume theory for un-irradiated and electron-irradiated polycarbonate.

Physical ageing of poly(chlorotrifluoroethylene): A positron annihilation study

Isothermal annealing of poly(chlorotrifluoroethylene) (PCTFE) was carried out at a temperature of 70 °C (slightly above Tg) using the positron annihilation lifetime (PAL) technique. Both the o-Ps lifetime and intensity shows systematic variation with ageing time, and its intensity can be represented by two additive decay exponentials. The free volume which is estimated using the Nakanishi et al. equation developed from Taos model, exhibits Doolittle type free volume relaxation. Physical ageing seems to yield close packing of polymer chains without crystallization. The relaxation time, and the nonexponentiality parameter, were estimated using a Kolrausch-Williams-Watt (KWW) function.

Microstructure of polycarbonate seen by positrons as an in-situ probe

Positron Annihilation Lifetime Spectroscopy (PALS) is used to probe the micro-structural changes in the polymer polycarbonate in terms of the changes in free volume hole size and their content as a function of temperature. The measured spectra are best fitted to three lifetime component analysis. The average hole size in the amorphous regions is determined from the measured Ortho Positronium (O-Ps) lifetime τ3 by following the treatment of Nakanishi et al. On the other hand, information about defects in the crystalline regions is revealed by trapped positrons lifetime τ2. In this polymer the average hole radius varies from 2.71 Å to 2.77 Å. The present study indicates that this polymer has a glass transition temperature of 152°C. Further, we have calculated the trapping rates in the ordered and disordered regions of the polymer based on Goldanskiis kinetic equations and an attempt is made for the first time to estimate the activation energy in the amorphous and crystalline regions separately.

A positron annihilation study of the tensile behaviour of bivoltine silk fibers

Abstract The influence of longitudinal stress on the microstructure of bivoltine silk fibers has been investigated by the Positron Lifetime Technique (PLT). The variation of o–Ps (ortho–positronium atom) pick-off lifetime and intensity as a function of stress (0–240 kPa) reveals that the free volume size and its number density remains unchanged within the elastic limit (60 kPa). Thereafter, the free volume size decreases from 67 to 57 A 3 . Owing to the loose helix structure, the chain alignment in the crystalline regions starts prior to that in the amorphous regions, leading to a more ordered silk-II type. The estimated low activation energy in the disordered regions suggests a folded chain conformation for the more ordered silk-II type.

A free volume microprobe study of water sorption in a contact lens polymer.

The kinetics of moisture uptake in a rigid gas-permeable contact lens material, Fluoroperm60, has been investigated. The free volume changes accompanied by the sorption of water were monitored using the positron lifetime spectroscopy (PLS) technique. The positron results indicated swelling in the initial stages of sorption and the filling of vacant free volume sites with water in the later stages of sorption. The sorption curve suggests that the diffusion process in this polymer follows the dual-mode sorption model. Using this model, it was possible to separate Fickian-controlled diffusion and relaxation-controlled diffusion. Furthermore, the positron results combined with the sorption results indicated that Fujitas free volume theory does not seem to be valid for the diffusion process in the present sample under study. The variation of the permeability as a function of the free volume suggests that the amount of water in a contact lens is a measure of its permeability.

UV DEGRADATION OF BIVOLTINE SILK FIBER: A POSITRON ANNIHILATION STUDY

The effects of ultra-violet (UV) irradiation on the microstructure of semicrystalline bivoltine silk fibers have been investigated using the positron annihilation lifetime (PAL) technique. Lifetime results indicate that in the early stages of UV irradiation, chain scission leads to free radicals formation. Cross-linking is the main process at longer radiation time to UV. Present results also show that silk-I type with a low degree of molecular orientation changes to silk-II type on UV irradiation.

Mechanical Properties of Modified Epoxies as Related to Free Volume Parameters

Epoxy resins need modifications by rubbery additives, fillers, etc. for enhancement of their usage in various applications. This study aims to determine the mechanical properties of silicone (i.e., amine-containing poly dimethyl siloxane)-modified resins in combinations with fly ash. The mechanical properties, especially tensile/flexural strength and moduli, were determined. Amine-containing silicone (ACS) was found to weaken the epoxy resin. However, ACS-modified epoxy resin was found to tolerate the addition of a small amount of fly ash filler as it had better tensile and flexural strengths compared with the unfilled, modified resin. In contrast to this, the unmodified resin was found to have its tensile and flexural strengths deteriorated by the filler. The reason for this behaviour was speculated to be due to the ACS acting as an additional coupling agent between the resin and the filler. These properties were sought to be correlated with free volume parameters measured from Positron Annihilation Lifetime Spectroscopy. Free volume measurements appear to indicate enhanced epoxy-fly ash interactions by ACS. Scanning Electron Microscopy indicates better filler distribution in the ACS-modified resin compared with the unmodified resin.

Correlation between physico-mechanical and free volume properties of Gaur-Gum filled polyurethane/polymethyl methacrylate biodegradable composites

This study reports characterization of polyurethane/polymethyl methacrylate (PU/PMMA, 50/50) Semi-Interpenetrating Polymer Network (SIPN) prepared by in-situ polymerization, and filled with different weight percent (wt%) of natural polymer, Gaur-Gum (GG). These are characterized for density, tensile strength and percentage elongation at break. Positron annihilation lifetime measurements have also been carried out to measure the free volume of these composites. The results show good correlation between free volume content of the composites with the mechanical properties. These natural polymer-filled composites were subjected to biodegradation using a specific microorganism, Aspergillus niger. The influence of A. niger on composites has also been studied. The results show that the weight loss of the specimens exposed to A. niger media increases with increase in gaur gum in the composite. Interestingly the weight loss of GG-filled composites seems to be more than the quantum of GG incorporated which is a clear indication that degradation of PU/PMMA system is responsible for the weight loss.