Browsing by Author "Chen, Mingtao"

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    Bridging Mesoscale Phenomena and Macroscopic Properties in Block Copolymers Containing Ionic Interactions and Hydrogen Bonding
    Chen, Mingtao (Virginia Tech, 2018-08-08)
    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.
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    Non-isocyanate poly(amide-hydroxyurethane)s from sustainable resources
    Zhang, Keren; Nelson, Ashley M.; Talley, Samantha J.; Chen, Mingtao; Margaretta, Evan; Hudson, Amanda G.; Moore, Robert Bowen; Long, Timothy E. (Royal Society of Chemistry, 2016-05-19)
    A two-step synthesis of epoxidation and carbonation afforded a hetero-functional AB monomer with cyclic carbonate and methyl ester (CC-ME) using plant oil-based methyl 9-decenoate and CO2. A unprecedented one-pot synthetic platform of CC-ME with 1,12-diaminododecane and poly(tetramethylene oxide) (PTMO)-based polyether diamine allowed synthesis of both nonsegmented poly(amide-hydroxyurethane) (PA12HU) and segmented PA12HU-PTMOs with varying polyether contents. 1H NMR spectroscopy confirmed complete conversion of cyclic carbonates and methyl esters to hydroxyurethanes and amides, respectively. Thermal analysis revealed distinctive thermal stability and transitions of PA12HU and PA12HU-PTMOs compared to their precursors and model oligomers. PA12HU and PA12HU-PTMOs were melt compression molded into semicrystalline, free-standing films, except for PA12HU-PTMO100 with 100% polyether diamine. PA12HU-PTMO100 was a viscous liquid with a glass transition temperature (Tg) of −64 °C and zero-shear melt viscosity of 449 Pa s. PA12HU formed a semicrystalline, rigid film with Tg of 11 °C. Polyether incorporation afforded creasable PA12HU-PTMO films with broad glass transitions near −50 °C. Thermal and thermomechanical analysis revealed significant phase-mixing of the hard and soft segments from annealed PA12HU-PTMO films. Polyether soft segments mixed with the amorphous hard segments, forming a miscible soft phase; crystallizable hard segments with ordered hydrogen bonding formed a hard phase. Surface morphological analysis of each PA12HU-PTMO film displayed ribbon-like, hard domains with composition-dependent aspect ratios. PA12HU-PTMOs exhibited higher moisture uptake than traditional thermoplastic polyurethane (TPU) due to resultant hydroxyls. Variable temperature FTIR spectroscopy demonstrated that ordered hydrogen bonding in the crystalline domains was disrupted or dissociated as the crystallites melted. Although tensile strength of segmented PA12HU-PTMOs proved lower than traditional polyurethanes due to phase-mixing, these compositions represent the first examples of film-forming, linear isocyanate-free polyurethanes with mechanical integrity and processability.
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    Phosphonium-containing diblock copolymers from living anionic polymerization of 4-diphenylphosphino styrene
    Schultz, Alison R.; Fahs, Gregory B.; Jangu, Chainika; Chen, Mingtao; Moore, Robert Bowen; Long, Timothy E. (The Royal Society of Chemistry, 2015-11-20)
    Living anionic polymerization of 4-diphenylphosphino styrene (DPPS) achieved well-defined homopolymers, poly(DPPS-b-S) styrenic block copolymers, and poly(I-b-DPPS) diene-based diblock copolymers with predictable molecular weights and narrow polydispersities. In situ FTIR spectroscopy monitored the anionic polymerization of DPPS and tracked monomer consumption for kinetic analysis. Post-alkylation enabled controlled placement of phosphonium functionality in poly(I-b-DPPS) diblock copolymers, producing well-defined phosphonium-containing block copolymers with low degrees of compositional heterogeneity. Incorporating phosphonium charge disrupted the lamellar bulk morphology of the neutral diblock precursor and provided morphologies with interdigitated packing of alkyl chains on the phosphonium cation.
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    Ureido cytosine and cytosine-containing acrylic copolymers
    Zhang, Keren; Chen, Mingtao; Drummey, Kevin J.; Talley, Samantha J.; Anderson, Lindsey J.; Moore, Robert Bowen; Long, Timothy E. (Royal Society of Chemistry, 2016-10-12)
    Regioselective Michael addition afforded a novel N1-substituted cytosine acrylate monomer for the synthesis of acrylic random copolymers with cytosine pendant groups. Quantitative post-functionalization converted cytosine to ureido-cytosine (UCy) with an increased self-association strength due to quadruple hydrogen bond formation. Thermogravimetric analysis (TGA) revealed a lower onset temperature of weight loss (∼200 °C) for UCy-containing copolymers, however, they proved to be more thermally stable at ≤130 °C than the cytosine-containing precursors during isothermal rheological experiments. The incorporation of UCy into random copolymers resulted in higher Tgs, enhanced mechanical performance, and better microphase-separation than the cytosine-containing precursors. Both dynamic mechanical analysis and rheological analysis revealed a plateau regime for each UCy-containing copolymer as well as a tan delta transition that corresponded to hydrogen bond dissociation. In contrast, the viscoelastic behavior of cytosine-containing random copolymers resembled entangled, non-associating polymers with increasing Tg as the cytosine content increased. A solution-cast UCy-containing copolymer film exhibited a more well-defined surface morphology with nano-fibrillar hard domains compared to the cytosine control. Variable temperature FTIR spectroscopy verified the presence of hydrogen bonding, and thermogravimetric sorption analysis (TGA-SA) compared the water uptake of UCy and cytosine-containing copolymers. UCy-containing random copolymers showed various advantages for applications as adhesives and thermoplastic elastomers compared to the cytosine copolymers, including superior cohesive strength, higher thermal stability, wider service temperature window, and lower moisture uptake. Free radical copolymerization of a quadruple hydrogen bond containing acrylic monomers provides a versatile avenue to supramolecular polymers with a tunable composition and improved scalability compared to earlier telechelic oligomers. This report describes the first synthesis of an acrylic monomer family and complementary evidence for tunable association in random copolymers.