Characterisation of vinylphosphonic acid-based block copolymers

  • Michael I. Horgan

Western Sydney University thesis: Master's thesis

Abstract

Block copolymers incorporate the properties of multiple homopolymers into discrete chemical domains. A wide variety of physical and electrochemical behaviours are exhibited by these 'smart' polymers. Changes in environment effect the confirmation and structures of these materials, such stimuli-responsivity includes pH, temperature or ion concentration. This potential for controlled behaviour in solution and surface chemistry has enabled the production and investigation of block copolymers in many fields. Notably these materials are applied as flocculants and facilitate ion transport in water filtration systems without the need for power generators. Therefore block copolymers present a valuable construction material for use at industrial facilities. Where each block exhibits like-attractions with hydrogen-bonding environments, these species of copolymers are called double hydrophilic block copolymers (DHBC). These particular groups of block copolymers are of primary interest in the development of PEM batteries and drug delivery systems. DHBCs with vinylphosphonic acid (VPA) moieties display strong research potency in these investigations. Elucidation of the chemical structure for such materials has been attempted using a number of analytical techniques. However, at current there is no established method that evaluates the purity of the block copolymers in terms of the parent polymers and the distribution of compositions. Techniques for the analysis of these properties in polymers generally involve separation chemistry. Size exclusion chromatography (SEC) is the most common method within research and industry for determining these properties. This method separates analyte in solution by hydrodynamic volume, polymer volumes are highly dependent on molar mass as well as composition and branching, as such incomplete separation by one of these factors occurs in SEC leading to errors in molecular weight calculations. Furthermore 'smart' polymers that exhibit charges are susceptible to aggregation and adsorption onto the stationary phase. To evaluate DHBCs with charged and neutral blocks alternative methods were investigated in this work. Block copolymers of PVPA have been produced by RAFT/MADIX polymerization, this synthetic scheme for aqueous production of high charge polymers can result in parent homopolymers remaining in sample. In order to assess the purity of PVPA-based DHBCs made via RAFT/MADIX this project presents a method for their quantification via free solution CE-CC. Free solution capillary electrophoresis (CE) is a separation method for the characterization of ions based on their charge to friction ratio. Sample analyte is placed in a fluid medium with buffer and subjected to an electric field. This causes ionization of analyte components while migration is initiated by attraction of these ions to the poles of the electric field. It has previously been observed that hydrodynamic volume in approaches a maximum with an increase in size of polyelectrolytes. Above a certain threshold these polymers exhibit negligible increases in volume; in these cases separation by electrostatic friction outweighs molar mass dependent hydrodynamic friction at these chain lengths. This state of CE separation for large polyelectrolytes is termed the 'critical conditions' (CE-CC) where polymers can be characterised in terms of their structure or end-groups. The charged-uncharged DHBCs were examined in this work by CE-CC and the complete separation and detection of the parent homopolymers was achieved. Polymers constructed from these charged species, though valuable for industrial and bio-technologies, can be difficult to synthesize with narrow chemical distributions and few techniques are available that can accurately quantify all species present in these samples. In the separation of block copolymers by CE we observed that short chain di-block copolymers of charged PVPA and neutral block species PAM, PEG and PVP displayed large distributions in block structure as well as a number of impurities. For RAFT/MADIX synthesised PVPA-based DHBCs the dispersity of different structures was found to be dependent on the degree of polymerisation of the first charged block species more so than the second neutral block species. The heterogeneity of these distributions was examined using CE-CC and related to the compositions of the block copolymers using a novel conversion of electrophoretic mobility to distribution of charge density. This work builds a robust and efficient method for assessing the structures and species of interesting DHBCs by separation in CE. To summarise, the purity and heterogeneities of structures in smart charged-uncharged DHBCs were investigated by CE-CC. Properties such as structural distribution, purity and charge density were assessed within a single separation using this technique. Further investigation of other difficult to characterise copolymers especially smart DHBCs by CE-CC could be used to study the synthesis of these materials and aid the development of more complex copolymers for commercial application.
Date of Award2017
Original languageEnglish

Keywords

  • block copolymers

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