Abstract
The objective of this study is to make an electroactive, relatively fast (moderate speed), durable, and self-contained polymer gel actuator at the centimetre scale using redox polymers. There are many prototype electric field active polymeric actuators and Chapter one discusses some of the main kinds.
Previous actuator studies in the group, using quinone gel actuators showed that, to some extent, the above mentioned aims can be achievable. However, there were many problems found with those systems that require improvements mainly in the cycle life of the actuator. Limited long term stability mostly results from rapid solvent degradation. Hence, first part of this study was focused on overcoming this main problem of the reported actuator.
The quinone based gel actuator was re-prepared using a new cross linking agent of eight arm poly (ethylene oxide) macro crosslinker and also re-prepared with two new monomers of acrylamide-functionalised quinone, idebenone acrylate monomers (Chapter three). Unfortunately, all approaches did not improve the performance of the quinone based actuator and this led to a search for a new redox polymer material.
A class of conductive polymer material that is known as a radical polymer was selected to be used in actuator studies. The bulk of this study was based on the specific type of radical polymer material of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or TEMPO.
The chemistry and applications of TEMPO based polymers are described in Chapter two. Details of the preparation of the radical polymers and construction of their actuators are described in Chapter four. Three main routes were followed to prepare TEMPO polymer gels: (01) polymerisation of an oxoammonium monomer, (02) polymerisation of piperidine precursor monomers and subsequent chemical oxidation, and (03) post-modification of active-ester polymer gels with an amine functionalised radical molecule (amino-TEMPO).
The post-modification approach appears the most promising route, leading to fewer side-reactions and degradation of the radical gel. A combination of graphene and multiwall carbon nanotubes gave high conductivity and improved actuation in the gels, with at least 32% linear actuation. This also allowed the construction of dual actuating systems, where both anode and cathode are responsive. A dual actuator system showed good stability over 10 cycles, showing its promise.
Chapter five presents an approach towards obtaining tough gel materials using ionic salt compounds, known as ion-pair-comonomers (IPC). The synthesis of eight new IPC salts is discussed and copolymerisation of one particular IPC salt with non-ionic monomer of n-Butyl methacrylate showed improved mechanical properties. However, not all monomer and solvent combinations were successful, showing that if IPC salts were to be used in the actuators the system would have to be tailored carefully to maximise any benefits.
In addition, IPC salts were also tested as a convenient and fast approach to achieve, polyampholyte gels by homopolymerisation. Hydrogels with excellent mechanical properties were synthesized by radical photo-polymerization of three different types of ion-pair comonomers (IPC), without requiring any chemical cross-linking agent. Insoluble gels formed only at a specific solution concentration range, which was unique to the particular salt. The gels changed properties after one days soaking in water, becoming less stiff and more extendible, but remained stable after that. Strain of up to 4000% was measured for one salt pair and ultimate stress of up to 1.7 MPa for another. Self-healing properties arise through a combination of electrostatic and hydrophobic interactions of the polymer chains. Immersion of the gels in salt solution screened the electrostatic interactions, resulting in dissolution of the gel.
Prepared monomer materials (quinone, radical, and IPC) were characterised with many techniques and a short description on each of the instruments that were used in the characterisation are included in the experimental sections at the end of each chapter.