Polymer Characterization
M. D. Dadmun and B. Hu
 Mark D. Dadmun Associate Professor Department of Chemistry dad@utk.edu |
Overview: Mark Dadmun's research The research in Mark Dadmun’s group examines methods by which the properties of polymer mixtures can be optimized by control of morphology and dispersion size or by the selective migration of a polymeric additive to the surface. The group is currently examining systems that will provide an understanding of how manipulation and control of the structure of polymeric components can be utilized to efficiently and reproducibly produce nanoscale dispersions in a host polymer matrix, which can readily lead to nonlinear enhancement of material properties such as dimensional stability, flame resistance, and mechanical properties. They have also developed and utilize techniques to characterize the impact of specific branched architectures on the dynamics of a surface segregation process, their final surface structure and functionality, and the material properties of a surface-modified polymeric material. Thus, their research program is designed to provide specific fundamental information to enable the design and production of multicomponent |
Surface Segregation Engineering of Self-Healing Functional Polymeric Materials
For many properties of polymers, including anti-microbial behavior, flammability, and chemical resistance, the surface composition of a polymeric material will govern the response of that material to a change in its surroundings. A promising approach to control the chemical composition of the outer few layers of a polymeric material (and thus, these properties) is the surface segregation of a polymeric additive, in which the functional units are chemically bonded to a polymer that is compatible with the bulk material, as shown schematically in Figure 1. Modification of the surface functionality now occurs by the preferential segregation to the surface of one component in a multi-component polymer system.Selective migration of one component to the surface is, of course, common in all material classes and usually is driven by a reduction in the surface energy of the system.
However, in polymeric systems it has been shown that the higher surface energy species can preferentially segregate to the surface due to entropic factors. This may be realized when the segregating species has many chain ends and is therefore a highly branched polymer (i.e. a comb, graft, star, or randomly branched polymer). The surface segregation process can therefore make a material self-healing, since the loss of the additive from the surface would trigger the replenishment from a “reservoir” in the bulk.In order to realize the technological promise of this approach it is necessary to characterize the impact of specific branched architectures on the dynamics of the segregation process, the final surface structure and functionality, and the material properties of the surface-modified polymeric material. In a particular application, should an additive have many short branches to speed up the segregation process or fewer longer branches to improve final functionality? How many branches is enough? We are currently using neutron reflectivity and other surface sensitive techniques to investigate the microstructure and kinetics of the segregation processes in polymeric films for a variety of branched structures, such as those shown in Figure 2.
Reactive Processing to Form Optimal Polymeric Interfacial Modifiers
An understanding and control of the interactions that occur between two components across an interface in multicomponent polymer systems are critical to the development of novel polymeric materials.
The group recently presented a series of papers that indicate that the formation of loops at an interface offers an efficient mechanism by which a polymer matrix can interact with a surface/interface. We expect that this information can be utilized to enable the production of efficient and cost-effective polymeric interfacial modifiers, including those for organic/inorganic nanocomposites.Current and future work in this area involves developing novel reactive processing schemes by which these ‘loops’ at an interface can be synthesized in a cost-efficient manner. As is illustrated in Figure 3, a polymer chain that has two reactive end groups (a telechelic) is incorporated into a system with a surface or interface that contains moieties that are reactive to these end-groups (e.g., circles will react with stars in Fig. 3).
If the dynamics and thermodynamics of the nanoscale process of telechelic attachment and self-organization can be understood and controlled, it will be possible to rationally design and control the interfaces and morphology of multi-component polymer systems. Experiments utilizing self-assembled monolayers as model reactive interfaces and depth profiling techniques (SIMS, neutron reflectivity ellipsometry, etc…) are currently being completed to determine the conditions where loops are formed. Additionally, the impact of the presence of the loops on the strength of interfaces will be determined using asymmetric double cantilever beam and peel tests.Improved Dispersion in Nanoscale Composites
Recent results in our group provide guidelines by which the miscibility and dispersion of nanoscale fillers can be controlled by the optimization of preferred intermolecular interactions between the LCP and the matrix. More specifically, moieties that can undergo hydrogen bonding with functional groups present on a liquid crystalline polymer are introduced to the amorphous polymer. FTIR characterized the extent of intermolecular hydrogen bonding between the two polymers, and this parameter is correlated to the phase behavior of the blends. The results show that by controlling the spacing between the functional groups on the amorphous polymer chain that participate in hydrogen bonding, the extent of intermolecular interactions between the two polymers is optimized, and this induces miscibility in the systems studied. Moreover, the system that maximizes the extent of intermolecular hydrogen bonding is one where the hydrogen bonding moieties on the amorphous polymer are separated along the chain. These results therefore provide guidelines by which miscibility may be induced in polymer blends by the minor structural modification of one of the polymers.
Future and ongoing work in this area will include utilization of this concept to improve polymer nanocomposites, such as clays and nanotubes,(as shown in Figure 4) correlation of these experimental results to existing theory, determination of the effect of LCP structure (side-chain vs. main-chain, stiffness) on the relationship between miscibility and extent of intermolecular interactions, and the development of guidelines by which the feasibility of creating a true molecular composite can be estimated based on specific blend component structures.
Recent Publications
"Guidelines to Creating a True Molecular Composite: 2. Broadening Miscibility in Liquid Crystalline Polymer Blends," S. Viswanathan, M. D. Dadmun, Macromolecules (Submitted).
"Compatibilization of Poly(vinyl chloride) and Polyolefin Elastomer Blends with Multiblock/Blocky Chlorinated Polyethylenes," E. Eastwood, M. D. Dadmun, Polymer, 43 6707 6717 (2002).
"Neutron Reflectivity Studies On a Small Molecule Liquid Crystal/Polymer Interface," G. Lynn, M. D. Dadmun, E. K. Lin, W. E. Wallace, W. Wu, Liquid Crystals, 29, 551-557 (2002).
"Multi-Block Copolymers in the Compatibilization of Polystyrene and Poly(Methyl Methacrylate) Blends: Role of Polymer Architecture," E. Eastwood, M. D. Dadmun, Macromolecules, 35, 5069-5077 (2002).
"Guidelines to Creating a True Molecular Composite: Inducing Miscibility in Blends by Optimizing Intermolecular Hydrogen Bonding," S. Viswanathan, M. D. Dadmun, Macromolecules, 35, 5049-5060 (2002).
 Bin Hu Assistant Professor 608 Dougherty Engineering Building Phone: (865)974-3946; Fax: (865)974-4115 bhu@utk.edu |
Electronic and optical polymeric materials and devices, and laser spectroscopy Bin Hu's research program is focused on three areas. Materials science This research involves synthesizing and engineering functional polymeric materials including charge transport polymers, visible and infrared emitting polymers, lasing polymers, electroluminescent polymers, photorefractive polymers and polymer nano-composites. Investigations focus on structure/morphology – property relationship, quantum confinement effects of electrons, and coherent interactions of excited states in polymeric materials and nano-composites. Polymer optoelectronic devices Hu's research explores design and fabrication of polymer optoelectronic devices such as field-effect transistors, photodetectors, chemical sensors, optical amplifiers, light-emitting diodes, and laser diodes. Investigations focus on energy-band structure of polymer heterojunctions, charge injection, transport and recombination, energy transfer, and interfacial electronic states. |
This research includes characterizations of polymeric materials and devices by using photoluminescence, Raman, stimulated emission, 2nd and 3rd harmonic generation, and pump-probe spectroscopies. Investigations focus on dynamic processes of excited states in polymeric materials.
Conjugated polymers as a new class of semiconducting materials opened a new direction in optoelectronic devices due to the more facile tunabilities of electronic and optical properties based on the modifications of chemical and morphological structures. In the last decade, we have designed and synthesized a series of PPV type copolymers (shown in Fig.1) and studied the structure-property relationship.
Fig. 1 Chemical structure of conjugated-nonconjugated block copolymer
To achieve quantum confinement effects from polymeric materials, we proposed a new approach to obtaining polymer “quantum dots” by using the concept of phase separation in polymer blends. The proof of this concept has been demonstrated in the measurements of morphology, electroluminescence, and lasing action (stimulated emission) in the blend systems of the copolymers dispersed in Poly(N-vinyl carbazole) (PVK) matrix. These polymer “quantum dots” account for effective confinement of electrons and enhanced optical gain, leading to lasing actions (see Fig.2).
Fig. 2. Lasing spectra of the polymer “quantum dots”: blue, green and red PPV type copolymers dispersed in PVK, respectively
In the research of polymer optoelectronic devices, we extensively investigated energy band structures of polymer heterojunctions, and charge injection, transport and recombination processes. Through these investigations, we designed and fabricated advanced light-emitting diodes and demonstrated efficient blue, green, red and white electroluminescence from a polymer blend with three primary chromophoric polymers,
which presented a new display technology.Based on the success of polymer light-emitting diodes, we proposed an interdisciplinary research on coherent interactions of electrically pumped excited states with externally incoming photons in the polymer “quantum dots”. This research will lead to a next generation of optical amplifier as shown in Fig.3.
In summary, the goal of our research is to synthesize and engineer conjugated polymeric materials with advanced electronic and optical properties for the applications in optoelectronic devices.
Dr. Hu received the Ph.D in Chinese Academy of Sciences. He then worked in the department of polymer science and engineering at the university of Massachusetts/Amherst as a postdoctoral research scientist and in the R&D division at SICPA Product Security Inc as a senior research scientist. Dr. Hu joined UT as an assistant professor in August, 2002.
Recent Publications
“Pressure dependence of the photoluminescence of polyparaphenylene,” B. Hu, X. Zhang, Y. Zhou, C. Jin, and J. Zhang, Phys. Rev. B. 43, 14001-14008(1991)
"Solid-state tunable cavity lasing in a poly(para-phenylene vinylene) derivate alternating block co-polymer," N. D. Kumar, J. D. Bhawalkar, P. N. Prasad, F. E. Karasz, B. Hu, Appl. Phys. Lett. 71, 999-1001(1997)
"Electron and hole transport in a green-emitting alternating block copolymer: space-charge-limited conduction with traps," D. Ma, I. A. Hummelgen, B. Hu, F. E. Karasz, J. Phys. D: Appl. Phys. 32, 2568-2572(1999)
"Comparison of Electroluminescence Quantum Efficiency of PPV and a PPV-Related Alternating Block Copolymer," B. Hu and F. E. Karasz, Synth. Met. 92, 157(1998)
"Interfacial Effects in Polymer LEDs," B. Hu and F. E. Karasz, Chemical Physics, 227, 263(1998)
"A Highly Luminescent Poly[(m-phenylene vinylene)-alt(p-phenylene vinylene)] with Defined Conjugation Length and Improved Solubility," Y. Pang, J. Li, B. Hu, and F. E. Karasz, Macromolecules, 32, 3946(1999)
"Light-Emitting Copolymers of Cyano-containing PPV-based Chromophores and a Flexible Spacer," M. R. Pinto, B. Hu, F. E. Karasz, and L. Akcelrud, Polymer, 41, 2603(2000)
"Blue, Green and Red Lasing Output from Block Copolymers in Solution and Solid state under optical pumping," B. Hu, F. E. Karasz, and P. N. Prasad, in submission to J. Appl. Phys.