Polymers have many desirable properties for engineering systems–e.g., low mass density, chemical stability, and high strength-to-mass ratio–but applications of polymers in situations where heat ...transfer is critical are often limited by low thermal conductivity. Here, we leverage the enormous research and development efforts that have been invested in the production of high-modulus polymer fibers to advance understanding of the mechanisms for thermal transport in this class of materials. Time-domain thermoreflectance (TDTR) enables direct measurements of the axial thermal conductivity of a single polymer fiber over a wide temperature range, 80 < T < 600 K. Relaxation of thermoelastic stress in the Al film transducer has to be taken into account in the analysis of the TDTR data when the laser spot size is small because the radial modulus of the fiber is small. This stress relaxation is controlled by the velocity of the zero-order symmetric Lamb mode of a thin Al plate. We find similarly high thermal conductivities of Λ ≈ 20 W m–1 K–1 in crystalline polyethylene and liquid crystalline poly(p-phenylene benzobisoxazole). For both fiber types, Λ(T) ∝ 1/T near room temperature, suggesting an intrinsic limit to the thermal conductivity governed by anharmonicity, not structural disorder. Because of the high degree of elastic anisotropy, longitudinal acoustic phonons with group velocities directed along fiber axis are likely to be the dominate carriers of heat.
Patterning with block copolymer thin films Segalman, Rachel A.
Materials science & engineering. R, Reports : a review journal,
02/2005, Volume:
48, Issue:
6
Journal Article
Peer reviewed
Open access
The nanometer-scale architectures in thin films of self-assembling block copolymers have inspired a variety of new applications. For example, the uniformly sized and shaped nanodomains formed in the ...films have been used for nanolithography, nanoparticle synthesis, and high-density information storage media. Imperative to all of these applications, however, is a high degree of control over orientation of the nanodomains relative to the surface of the film as well as control over order in the plane of the film. Induced fields including electric, shear, and surface fields have been demonstrated to influence orientation. Both heteroepitaxy and graphoepitaxy can induce positional order on the nanodomains in the plane of the film. This article will briefly review a variety of mechanisms for gaining control over block copolymer order as well as many of the applications of these materials. Particular attention is paid to the potential of perfecting long-range two-dimensional order over a broader range of length scales and the extension of these concepts to functional materials and more complex architectures.
Conjugated polymers and related processing techniques have been developed for organic electronic devices ranging from lightweight photovoltaics to flexible displays. These breakthroughs have recently ...been used to create organic thermoelectric materials, which have potential for wearable heating and cooling devices, and near-room-temperature energy generation. So far, the best thermoelectric materials have been inorganic compounds (such as Bi2Te3) that have relatively low Earth abundance and are fabricated through highly complex vacuum processing routes. Molecular materials and hybrid organic–inorganic materials now demonstrate figures of merit approaching those of these inorganic materials, while also exhibiting unique transport behaviours that are suggestive of optimization pathways and device geometries that were not previously possible. In this Review, we discuss recent breakthroughs for organic materials with high thermoelectric figures of merit and indicate how these materials may be incorporated into new module designs that take advantage of their mechanical and thermoelectric properties.Thermoelectrics can be used to harvest energy and control temperature. Organic semiconducting materials have thermoelectric performance comparable to many inorganic materials near room temperature. Better understanding of their performance will provide a pathway to new types of conformal thermoelectric modules.
Block copolymer network morphologies have been proven interesting for applications ranging from mechanical to transport and optical properties. The shape of the polymer chain and its ability to ...stretch across an interface can be used as handles to target these network morphologies. Here, we show that the double gyroid phase window is broadened when there is a flexible segment near the interface and narrowed when a more constrained segment is placed there by using a series of poly(styrene-b-peptoid) block copolymers in which the polypeptoid block chain conformation can be tuned to adopt either a helical or a coil conformation (NRpe6 vs Npe6). The double gyroid phase is accessed in both block copolymer series, while the phase boundaries are shifted toward larger polypeptoid volume fractions in the helix-forming PS–(NRpe6Nme y ) series, due to the more compact helix segment (i.e., the helix segment has the same chain volume as its random coil counterpart but occupies less space). The space-filling difference between the helix and coil segment is further confirmed by the smaller domain spacing of PS–(NRpe6Nme39) compared to PS–(Npe6Nme36) (13.9 nm vs 14.3 nm) despite the former having a longer polypeptoid block. Furthermore, a broadened double gyroid phase window is accessed in the PS–(Npe6Nme y ) series that has a flexible coil segment near the interface. These results demonstrate the possibility of tuning the double gyroid phase in linear block copolymers by chain conformation near the interface alone, highlighting chain conformation as a versatile handle in block copolymer design.
While polymers hold significant potential as low cost, mechanically flexible, lightweight large area photovoltaics and light emitting devices (OLEDs), their performance relies crucially on ...understanding and controlling the morphology on the nanometer scale. The ca. 10 nm length scale of exciton diffusion sets the patterning length scale necessary to affect charge separation and overall efficiency in photovoltaics. Moreover, the imbalance of electron and hole mobilities in most organic materials necessitates the use of multiple components in many device architectures. These requirements for 10 nm length scale patterning in large area, solution processed devices suggest that block copolymer strategies previously employed for more classical, insulating polymer systems may be very useful in organic electronics. This Perspective seeks to describe both the synthesis and self-assembly of block copolymers for organic optoelectronics. Device characterization of these inherently complex active layers remains a significant challenge and is also discussed.
Thermoelectricity in Molecular Junctions Reddy, Pramod; Jang, Sung-Yeon; Segalman, Rachel A. ...
Science (American Association for the Advancement of Science),
03/2007, Volume:
315, Issue:
5818
Journal Article
Peer reviewed
By trapping molecules between two gold electrodes with a temperature difference across them, the junction Seebeck coefficients of 1,4-benzenedithiol (BDT), 4,4′-dibenzenedithiol, and ...4,4"-tribenzenedithiol in contact with gold were measured at room temperature to be +8.7 ± 2.1 microvolts per kelvin (μV/K), +12.9 ± 2.2 μV/K, and +14.2 ± 3.2 μV/K, respectively (where the error is the full width half maximum of the statistical distributions). The positive sign unambiguously indicates p-type (hole) conduction in these heterojunctions, whereas the Au Fermi level position for Au-BDT-Au junctions was identified to be 1.2 eV above the highest occupied molecular orbital level of BDT. The ability to study thermoelectricity in molecular junctions provides the opportunity to address these fundamental unanswered questions about their electronic structure and to begin exploring molecular thermoelectric energy conversion.
Self-assembly of rod–coil block copolymers Olsen, Bradley D.; Segalman, Rachel A.
Materials science & engineering. R, Reports : a review journal,
07/2008, Volume:
62, Issue:
2
Journal Article
Peer reviewed
Rod–coil block copolymers are an increasingly important class of molecules for the self-assembly of functional polymer systems, many of which have rodlike chain conformations due to rigid secondary ...structures, extended π-conjugation, or aromatic groups along the polymer backbone. Examples of these polymers are helical proteins, polyisocyanates, main-chain semiconducting polymers, and aromatic polyesters, polyamides, or polyimines. Many hindered or liquid crystalline systems self-assembled from block copolymers, including dendronized polymers and mesogen-jacketed liquid crystalline polymers, also form rod–coil block copolymers. In these cases, steric crowding of the bulky or mesogenic side chains causes the polymer backbone to become quasi-linear. The incorporation of rigid rod polymers into the block copolymers results in extremely rich self-assembly behavior that differs markedly from that of traditional block copolymers due to the interplay between microphase separation of the rod and coil components and liquid crystalline alignment. The combination of these effects results in novel structures both in solution and melts. This review discusses in detail the self-assembly and thermodynamics of rod–coil diblock and triblock copolymers. After summarizing the applications of these materials, their aggregation and gelation in solution is discussed. Our knowledge of their bulk phase behavior is thoroughly reviewed both from the experimental and theoretical perspectives, and the self-assembly of these materials in thin film geometries that are critical to many applications in organic electronics and functional surface patterning is treated. Finally, the outlook for the future of these systems is summarized along with current knowledge gaps and exciting areas for the advancement of the field.
In 1999, Freeman published a visionary article that proposed a molecular basis for the trade-off between permeability and selectivity for polymeric gas separation membranes, which is often codified ...as an “upper bound”. This work has had major impacts in the gas separation membrane community and is paralleled by developments in the water purification membrane literature. A common theme between both communities is optimizing free volume or pore size distributions in polymer membranes to maximize separation performance. In guiding future development in the field, we identify the need to develop isoporous ultrafiltration membranes and highlight the potential for block copolymer self-assembly to achieve direct access to “structure by design” without requiring complex optimization of phase inversion processes.
The chain shape of polymers affects many aspects of their behavior and is governed by their intramolecular interactions. Delocalization of electrons along the backbone of conjugated polymers has been ...shown to lead to increased chain rigidity by encouraging a planar conformation. Poly(3-hexylthiophene) and other poly(3-alkylthiophenes) (P3ATs) are interesting for organic electronics applications, and it is clear that a hierarchy of structural features in these polymers controls charge transport. While other conjugated polymers are very rigid, the molecular structure of P3AT allows for two different planar conformations and a significant degree of torsion at room temperature. It is unclear, however, how their chain shape depends on variables such as side chain chemistry or regioregularity, both of which are key aspects in the molecular design of organic electronics. Small-angle neutron scattering from dilute polymer solutions indicates that the chains adopt a random coil geometry with a semiflexible backbone. The measured persistence length is shorter than the estimated conjugation length due to the two planar conformations that preserve conjugation but not backbone correlations. The persistence length of regioregular P3HT has been measured to be 3 nm at room temperature and decreases at higher temperatures. Changes in the regioregularity, side chain chemistry, or synthetic defects decrease the persistence length by 60–70%.
Anhydrous proton transport has been investigated in a series of proton conducting polystyrene-block-polymerized ionic liquid (PS-b-PIL) copolymers spanning a range of molecular weights and ...compositions. The PIL is a macromolecular analogue of imidazolium bis(trifluoromethane sulfonimide) (ImTFSI), a well-known proton conducting ionic liquid, and consists of imidazole linked to a polymer backbone via the 5-carbon. In contrast to prior work on nitrogen-linked imidazolium PILs, carbon-linked imidazolium has two nitrogens which can both function as proton donor/acceptors and participate in Grotthus mechanism conduction. The conductivity of the PIL block is shown to be dramatically impacted upon confinement by a PS block and can exceed the conductivity of the homopolymer in the range of 30–130 °C for PIL-rich block copolymer composition. At high temperature the conductivities track with ionic content while at room temperature the conductivities are nonmonotonic. X-ray scattering reveals a suppression of the peak associated with ionic aggregation in all block copolymers relative to the homopolymer consistent with the higher conductivities observed at room temperature. The dependence of ionic conductivity on temperature, as quantified by the VFT strength parameter D, decreases with decreasing PIL block length corresponding to a change in the packing efficiency of the conductive block. These changes in packing are hypothesized to lead to the different temperature dependences of conductivity which cause the nonmonotonic block copolymer conductivities observed at room temperature. Finally, we demonstrate that the fraction of PIL in the block copolymer is the main factor governing the high temperature ionic conductivity of these materials while confinement effects become important at room temperature.
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