Anionic polymerization and controlled radical polymerization enabled the synthesis of novel block copolymers with non-covalent interactions (electrostatic interaction and/or hydrogen bonding) to examine the relationships between mesoscale phenomenon and macroscopic physical properties. Non-covalent interactions offer extra intra- and inter-molecular interactions to achieve stimuli-responsive materials in various applications, such as artificial muscles, thermoplastic elastomers, and reversible biomacromolecule binding. The relationship between non-covalent interaction promoted mesoscale phenomenon (such as morphology) and consequent macroscopic physical properties is the key to optimize material design and improve end-use performance for emerging applications.
Pendant hydrogen bonding in ABA block copolymers promoted microphase separation and delayed the order-disorder transition, resulting in tunable morphologies (through composition changes) and extended rubbery plateaus. Reversible addition-fragmentation chain transfer (RAFT) polymerization afforded a facile synthesis of ABA triblock copolymers with hydrogen bonding (urea sites) and electrostatic interactions (pyridinium groups). Pyridine groups facilitated hydrogen bonding through a preorganization effect, leading to highly ordered, long-range lamellar morphology and a significant increase of flow temperature (Tf) 80 °C above the hard block Tg. After quaternization of pyridine groups, electrostatic interaction, as a second physical crosslinking mechanism, disrupted ordered lamellar morphology and decreased Tf. Yet, extra physical crosslinking from electrostatic interactions pertained ordered hydrogen bonding at high temperature and exhibited improved stress-relaxation properties.
Both conventional free radical polymerization and RAFT polymerization generated a library of poly(ionic liquid) (PIL) homopolymers with imidazolium groups as bond charge moieties. A long chain alkyl spacer between imidazolium groups and the polymer backbones ensured a low glass transition temperature (Tg), which is beneficial to ion conductivity. Four different counter anions enabled readily tunable Tgs all below room temperature and showed promising ion conductivities as high as 2.45 × 10⁻⁵ S/cm at 30 °C. For the first time, the influence of counter anions on radical polymerization kinetics was observed and investigated thoroughly using in situ FTIR, NMR diffusometry, and simulation. Monomer diffusion and aggregation barely contributed to the kinetic differences, and the Marcus theory was applied to explain the polymerization kinetic differences which showed promising simulation results. RAFT polymerization readily prepared AB diblock, ABA triblock and (AB)3 3-arm diblock copolymers using the ionic liquid (IL) monomers discussed above and deuterated/hydrogenated styrene. We demonstrated the first example of in situ morphology studies during an actuation process, and counter anions with varied electrostatic interactions showed different mesoscale mechanisms, which accounted for macroscopic actuation. The long chain alkyl spacer between imidazolium groups and polymer backbones decoupled ion dynamics and structural relaxation. For the first time, composition changes of block copolymers achieved tunable viscoelastic properties without altering ion conductivity, which provided an ideal example for actuation materials, solid electrolytes, and ion exchange membranes. / Ph. D. / My research focuses on the synthesis of novel soft materials with a special interest in responsive polymers. The incorporation of responsive chemistry, such as hydrogen bonding and ionic interactions, enables soft materials with complex responsive behavior were achieved. Polymers with ion pairs promise great potential as solid-state electrolytes (which transfer ions to generate current) to eliminate potential fire hazard in batteries, which has been an arising concern for modern cellphone and electric car industry. The introduction of strong dipoles into polymers allows the fabrication of actuators, which convert electric signals to physical movement. Under applied voltage, polymers bend within seconds while holding physical loads. Actuator studies in polymers paves the way towards artificial muscles as well as soft robotics. Temperature responsive hydrogen bonding in polymers offers drastically different viscoelastic properties at different temperature and serves as the key mechanism in holt-melt adhesives, controlled drug release, and high performance materials.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/84525 |
| Date | 08 August 2018 |
| Creators | Chen, Mingtao |
| Contributors | Chemistry, Long, Timothy E., Schulz, Michael, Moore, Robert Bowen, Cheng, Shengfeng |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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