Philosophiae Doctor - PhD (Chemistry)

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Browsing by Author "Adeniyi, Olushola Rotimi"

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    Swift heavy ion irradiation of polyester and polyolefin polymeric film for gas separation application
    (University of the Western Cape, 2015) Adeniyi, Olushola Rotimi; Petrik, Leslie; Nechaev, A.N.; Teplyakov, V.V.
    The combination of ion track technology and chemical etching as a tool to enhance polymer gas properties such as permeability and selectivity is regarded as an avenue to establish technology commercialization and enhance applicability. Traditionally, permeability and selectivity of polymers have been major challenges especially for gas applications. However, it is important to understand the intrinsic polymer properties in order to be able to predict or identify their possible ion-polymer interactions thus facilitate the reorientation of existing polymer structural configurations. This in turn can enhance the gas permeability and selectivity properties of the polymers. Therefore, the choice of polymer is an important prerequisite. Polyethylene terephthalate (PET) belongs to the polyester group of polymers and has been extensively studied within the context of post-synthesis modification techniques using swift heavy ion irradiation and chemical treatment which is generally referred to as ‘track-etching’. The use of track-etched polymers in the form of symmetrical membranes structures to investigate gas permeability and selectivity properties has proved successful. However, the previous studies on track-etched polymers films have been mainly focused on the preparation of symmetrical membrane structure, especially in the case of polyesters such as PET polymer films. Also, polyolefins such as polymethyl pentene (PMP) have not been investigated using swift heavy ions and chemical etching procedures. In addition, the use of ‘shielded’ material on PET and PMP polymer films prior to swift heavy ion irradiation and chemical etching to prepare asymmetrical membrane structure have not been investigated. The gas permeability and selectivity of the asymmetrical membrane prepared from swift heavy ion irradiated etched 'shielded' PET and PMP polymer films have not been determined. These highlighted limitations will be addressed in this study. The overall objective of this study was to prepare asymmetric polymeric membranes with porous surface on dense layer from two classes of polymers; (PET and PMP) in order to improve their gas permeability and selectivity properties. The research approach in this study was to use a simple and novel method to prepare an asymmetric PET and PMP polymer membrane with porous surface and dense layer by mechanical attachment of ‘shielded’ material on the polymer film before swift heavy ion irradiation. This irradiation approach allowed for the control of swift heavy ion penetration depth into the PET and PMP polymer film during irradiation. The procedure used in this study is briefly described. Commercial PET and PMP polymer films were mechanically ‘shielded’ with aluminium and PET foils respectively. The ‘shielded’ PET polymer films were then irradiated with swift heavy ions of Xe source while ‘shielded’ PMP polymer films were irradiated with swift heavy ions Kr. The ion energy and fluence of Xe ions was 1.3 MeV and 106 respectively while the Kr ion energy was 3.57 MeV and ion fluence of 109. After swift heavy ion irradiation of ‘shielded’ PET and PMP polymer films, the attached ‘shielded’ materials were removed from PET and PMP polymer film and the irradiated PET and PMP polymer films were chemically etched in sodium hydroxide (NaOH) and acidified chromium trioxide (H2SO4 + CrO3) respectively. The chemical etching conditions of swift heavy ion irradiated ‘shielded’ PET was performed with 1 M NaOH at 80 ˚C under various etching times of 3, 6, 9 and 12 minutes. As for the swift heavy ion irradiated ‘shielded’ PMP polymer film, the chemical etching was performed with 7 M H2SO4 + 3 M CrO3 solution, etching temperature was varied between 40 ˚C and 80 ˚C while the etching time was between 40 minutes to 150 minutes. The SEM (surface and cross-section micrograph) morphology results of the swift heavy ion irradiated ‘shielded’ etched PET and PMP films showed that asymmetric membranes with a single-sided porous surface and dense layer was prepared and remained unchanged even after 12 minutes of etching with 1 M NaOH solution as in the case of PET and 2 hours 30 minutes of etching with 7 M H2SO4 + 3 M CrO3 as observed for PMP polymer film. Also, the swift heavy ion irradiated ‘shielded’ etched PET polymer film showed the presence of pores on the polymer film surface within 3 minutes of etching. After 12 minutes chemical etching with 1 M NaOH solution, the dense layer of swift heavy ion irradiated ‘shielded’ etched PET polymer film experienced significant reduction in thickness of about 40 % of the original thickness of as-received PET polymer film. The surface morphology of swift heavy ion irradiated ‘shielded’ etched PET polymer film by SEM analysis revealed finely distributed pores with spherical shapes for the swift heavy ion irradiated ‘shielded’ etched PET polymer film within 6 minutes of etching with 1 M NaOH solution. Also, after 9 minutes and 12 minutes of etching with 1 M NaOH solution of the swift heavy ion irradiated ‘shielded’ etched PET polymer film, the pore walls experienced complete collapse with intense surface roughness. Interestingly, the 12 minutes etched swift heavy ion ‘shielded’ irradiated PET did not lose its asymmetrical membrane structure despite the collapse of the pore walls. In the case of swift heavy ion irradiated ‘shielded’ etched PMP polymer film, SEM morphology analysis showed that the pores retained their shape with the presence of defined pores without intense surface roughness even after extended etching with 7 M H2SO4 + 3 M CrO3 for 2 hours 30 minutes. Also, the pores of swift heavy ion irradiated ‘shielded’ etched PMP polymer films were observed to be mono dispersed and not agglomerated or overlapped. The SEM cross-section morphology of the swift heavy ion irradiated ‘shielded’ etched PMP polymer film showed radially oriented pores with increased pore diameters in the PMP polymer film which indicated that etching was radial instead of lateral, and no through pores were observed showing that the dense asymmetrical structure was retained. The SEM results revealed that the pore morphology i.e. size and shape could be accurately controlled during chemical etching of swift heavy ion ‘shielded’ irradiated PET and PMP polymer films. The XRD results of swift heavy ion irradiated ‘shielded’ etched PET revealed a single diffraction peak for various times of chemical etching in 1 M NaOH solution at 3, 6, 9 and 12 minutes. The diffraction peak of swift heavy ion irradiated ‘shielded’ etched PET was observed to reduce in intensity and marginally shifted to lower angles from 25.95˚ 2 theta to 25.89˚ 2 theta and also became broad in shape. It was considered that the continuous broadening of diffraction peaks due to an increase in etching times could be attributed to disorderliness of the ordered region within the polymer matrix and thus decreases in crystallinity of the swift heavy ion irradiated ‘shielded’ etched PET polymer film. The XRD analysis of swift heavy ion irradiated ‘shielded’ etched PMP polymer films indicated the presence of the diffraction peak at 9.75˚ 2 theta with decrease in intensity while the diffraction peaks located at 13.34˚, 16.42˚, 18.54˚ and 21.46˚ 2 theta disappeared after chemical etching in acidified chromium trioxide (H2SO4 + CrO3) after 2 hours 30 minutes. The TGA thermal profile analysis of swift heavy ion irradiated ‘shielded’ etched PET did not show the evolution of volatile species or moisture at lower temperatures even after 12 minutes of etching in 1 M NaOH solution in comparison with commercial PET polymer film. Also, it was observed that the swift heavy ion irradiated layered’ etched PET polymer film started to undergo degradation at a higher temperature than untreated PET which resulted in an approximate increase of 50 ˚C in comparison with the commercial PET polymer film. The TGA results of swift heavy ion irradiated ‘shielded’ etched PMP polymer film revealed an improvement of about 50 ˚C in thermal stability before thermal degradation even after etching in acidified chromium trioxide for 2 hours 30 minutes at 80 ˚C. Spectroscopy (IR) analysis of the swift heavy ion irradiated ‘shielded’ etched PET and PMP polymer films showed the presence of characteristic functional groups associated with either PET or PMP structures. The variations of irradiation and chemical etching conditions revealed that the swift heavy ion ‘shielded’ irradiated etched PET polymer film experienced continuous degradation of available functional groups as a function of etching time and also with complete disappearance of some functional groups such as 1105 cm-1 and 1129 cm-1 compared with the as-received PET polymer film which are both associated with the para-substituted position of benzene rings. In the case of swift heavy ion irradiated ‘shielded’ etched PMP polymer film, spectroscopic (IR) analysis showed significant variations in the susceptibility of associated functional groups within the PMP polymer film with selective attack and emergence of some specific functional groups such as at 1478 cm-1, 1810 cm-1 and 2115 cm-1 which were assigned to methylene, CH3 (asymmetry deformation), CH3 and CH2 respectively Also, the IR results for swift heavy ion irradiated ‘shielded’ etched PMP polymer showed that unsaturated olefinic groups were the dominant functional groups that were being attacked by during etching with acidified chromium trioxide (H2SO4+CrO3) which is an aggressive chemical etchant. The gas permeability analysis of swift heavy ion irradiated ‘shielded’ etched PET and PMP polymer films showed that the gas permeability was improved in comparison with the as-received PET and as-received PMP polymer films. The gas permeability of swift heavy ion irradiated ‘shielded’ etched PET increased as a function of etching time and was found to be highest after 12 minutes of chemical etching in 1 M NaOH at 80 ˚C. In the case of swift heavy ion irradiated ‘shielded’ etched PMP, the gas permeability was observed to show the highest gas permeability after 2 hours 30 minutes of etching in H2SO4 + CrO3 solution. The gas permeability analysis for swift heavy ion irradiated ‘shielded’ PET and PMP polymer films was tested for He, CO2 and CH4 and the permeability results showed that helium was most permeable compared with CO2 and CH4 gases. In comparison, the selectivity analysis was performed for He/CO2 and CH4/He and the results showed that the selectivity decreased with increasing in etching time as expected. This study identified some important findings. Firstly, it was observed that the use of ‘shielded’ material on PET and PMP polymer films prior to swift heavy ion irradiation proved successful in the creation of asymmetrical polymer membrane structure. Also, it was also observed that the chemical etching of the ‘shielded’ swift heavy ion irradiated PET and PMP polymer films resulted in the presence of pores on the swift heavy ion irradiated side while the unirradiated sides of the PET and PMP polymer films were unaffected during chemical etching hence the pore depth could be controlled. In addition, the etching experiment showed that the pores geometry can be controlled as well as the gas permeability and selectivity properties of swift heavy ion ‘shielded’ irradiated etched PET and PMP polymer films. The process of polymer bulk and surface properties modification using ion-track technology i.e. swift heavy ion irradiation and subsequent chemical treatment of the irradiated polymer serves to reveal characteristic pore profiles unique to the prevailing ion-polymer interaction and ultimately results in alteration of the polymer characteristics.