Emily Pentzer

Polymer Composites for Thermoelectric Applications

This review covers recently reported polymer composites that show a thermoelectric (TE) effect and thus have potential application as thermoelectric generators and Peltier coolers. The growing need for CO2-minimizing energy sources and thermal management systems makes the development of new TE materials a key challenge for researchers across many fields, particularly in light of the scarcity or toxicity of traditional inorganic TE materials based on Te and Pb. Recent reports of composites with inorganic and organic additives in conjugated and insulating polymer matrices are covered, as well as the techniques needed to fully characterize their TE properties.

Bioactive and Therapeutic ROMP Polymers

This review summarizes the developments in the area of ROMP‐based polymer carriers as well as side‐chain and main‐chain polymer therapeutics during the period of 1993–2007. The promising emergence of ROMP‐derived amphiphilic block copolymers containing therapeutic agents, their assembly into polymer nanoparticles, together with their modification for the targeting group attachment is outlined. Recent application of ROMP‐based side‐chain polymer therapeutics as multivalent scaffolds and the synthesis of DIVEMA‐type main‐chain polymers from ROMP‐active monomers are also discussed. Finally, an outlook on the opportunities and the future directions in the use of ROMP towards the development of the next generation of polymer therapeutics is provided.

Probing Inter- and Intrachain Exciton Coupling in Isolated Poly(3-hexylthiophene) Nanofibers: Effect of Solvation and Regioregularity.

We report wavelength and time-resolved photoluminescence studies of isolated extended (1-10 μm length) poly(3-hexylthiophene) (P3HT) nanofibers (xNFs) cast on glass from suspension. The PL spectra of xNFs show multiple vibronic replicas that appear to be associated with the existence of both H- and J-type aggregates. The PL spectra of xNFs made from regioregular (rr)- (93%) and highly regioregular (hrr)-P3HT (98%) both show similarities in PL spectra suggestive of common chain packing features, as well as subtle differences that can be attributed to higher long-range order in the hrr-xNFs. Specifically, PL spectral measurements on isolated xNFs made from highly regioregular (>98%) P3HT showed a red-shifted electronic origin (≈30 meV) and increased 0-0/0-1 PL intensity ratio for the J-type species, suggestive of enhanced structural coherence length and intrachain order.

Morphology-dependent electronic properties in cross-linked (P3HT-b-P3MT) block copolymer nanostructures.

Combined Kelvin probe force microscopy and wavelength-resolved photoluminescence measurements on individual pre- and post-cross-linked poly(3-hexylthiophene)-b-poly(3-methyl alcohol thiophene) (P3HT-b-P3MT) nanofibers have revealed striking differences in their optical and electronic properties driven by structural perturbation of the crystalline aggregate nanofiber structures after cross-linking. Chemical cross-linking from diblock copolymer P3HT-b-P3MT using a hexamethylene diisocyanate cross-linker produces a variety of morphologies including very small nanowires, nanofiber bundles, nanoribbons, and sheets, whose relative abundance can be controlled by reaction time and cross-linker concentration. While the different cross-linked morphologies have almost identical photophysical characteristics, KPFM measurements show that the surface potential contrast, related to the work function of the sample, depends sensitively on nanostructure morphology related to chain-packing disorder.

Cross-linked functionalized poly(3-hexylthiophene) nanofibers with tunable excitonic coupling.

We show that mechanically and chemically robust functionalized poly(3-hexylthiophene) (P3HT) nanofibers can be made via chemical cross-linking. Dramatically different photophysical properties are observed depending on the choice of functionalizing moiety and cross-linking strategy. Starting with two different nanofiber families formed from (a) P3HT-b-P3MT or (b) P3HT-b-P3ST diblock copolymers, cross-linking to form robust nanowire structures was readily achieved by either a third-party cross-linking agent (hexamethylene diisocyanate, HDI) which links methoxy side chains on the P3MT system, or direct disulfide cross-link for the P3ST system. Although the nanofiber families have similar gross structure (and almost identical pre-cross-linked absorption spectra), they have completely different photophysics as signaled by ensemble and single nanofiber wavelength- and time-resolved photoluminescence as well as transient absorption (visible and near-IR) probes. For the P3ST diblock nanofibers, excitonic coupling appears to be essentially unchanged before and after cross-linking. In contrast, cross-linked P3MT nanofibers show photoluminescence similar in electronic origin, vibronic structure, and lifetime to unaggregated P3HT molecules, e.g., dissolved in an inert polymer matrix, suggesting almost complete extinction of excitonic coupling. We hypothesize that the different photophysical properties can be understood from structural perturbations resulting from the cross-linking: For the P3MT system, the DIC linker induces a high degree of strain on the P3HT aggregate block, thus disrupting both intra- and interchain coupling. For the P3ST system, the spatial extent of the cross-linking is approximately commensurate with the interlamellar spacing, resulting in a minimally perturbed aggregate structure.

The Distribution of Fox Squirrel (Sciurus niger) Leaf Nests within Forest Fragments in Central Indiana

Abstract We examined the abundance and placement of leaf nests by fox squirrels in six urban woodlots in central Indiana ranging in size from 1.06 to 8.28 ha. Four of the woodlots were disturbed, or subject to extensive human impact, whereas the remaining two were nature preserves. We counted all leaf nests present in each woodlot and recorded nest tree characteristics. We then conducted a quantitative vegetation analysis of trees present and estimated percentages of herbaceous and shrub cover along a minimum of two 100 m transects at each site. Fox squirrels showed a preference to build nests in certain species of trees. However, preference for nest tree species was not consistent across sites. Fox squirrels preferred to build nests in large trees with vines in the canopy at all sites. Characteristics of nests and nest trees did not differ among sites, but nest density was greater in the disturbed sites compared to the nature preserve sites. The nature preserve sites differed from the disturbed sites only with regard to the amount of shrub and herbaceous cover; shrub cover was greater and herbaceous cover was less at the disturbed sites. Results of this study suggest that fox squirrels are flexible with regard to nest tree species used and that the choice of a nest tree is dependent, in part, on tree size and the presence of vines. Further, a higher density of leaf nests in disturbed woodlots suggests that habitat disturbance and fragmentation due to urbanization may not have detrimental effects on the abundance and persistence of fox squirrels.

Beyond binary: optical data storage with 0, 1, 2, and 3 in polymer films

The evergrowing amount of data created and collected is met with the increased need to store this data. In compliment to improving data storage capabilities using engineering controls such as decreased pixel size (i.e., Blu-ray) or 3-D pixels (i.e., voxels), chemistry-based approaches are required to move beyond current limitations and meet our future needs. Herein, we present a new methodology to optically store data in a quaternary code of 0, 1, 2, 3 in a commodity polymer containing a low loading of two small molecules, and using heat and UV light to write, and read fluorescence output. The as-prepared film is non-fluorescent (0), and can be written through a wooden or metal mask with thermal treatment (1), light treatment (2), or both (3), giving three different colours of fluorescence under UV irradiation. The flexible polymer film remains colourless and transparent under ambient light after patterning, retains the stored data after exfoliation with sandpaper, and can be removed from the substrate and mechanically deformed without detriment to the pattern. This straightforward and scalable system demonstrates the use of simple and robust chemical reactions to improve data storage capabilities and has the potential to exponentially increase information density.

Distinct Chemical and Physical Properties of Janus Nanosheets

Janus particles have recently garnered significant attention for their distinct properties compared to particles that are homogeneously functionalized. Moreover, high aspect ratio Janus particles that are rod-like or planar (i.e., nanosheets) are especially intriguing considering their interfacial properties as well as their ability to assemble into higher order and hybrid structures. To date, major challenges facing the exploration and utilization of 2D Janus particles are scalability of synthesis, characterization of tailored chemical functionalization, and ability to introduce a diverse set of functionalities. Herein, a facile method to access Janus 2D graphene oxide (GO) nanosheets by combining a Pickering-type emulsion and grafting-from polymerization via ATRP is reported. Janus GO nanosheets bearing PMMA on one face as well as the symmetrically functionalized analogue are prepared, and the chemical, thermal, structural, surface, and interfacial properties of these materials are characterized. Time-of-flight secondary ion mass spectrometry coupled with Langmuir-Blodgett films is shown to be an ideal route to conclusively establish asymmetric functionalization of 2D materials. This work not only provides a facile route for the preparation of Janus nanosheets but also demonstrates the direct visualization of polymer grown from the surface of GO.

Simultaneous Reduction and Functionalization of Graphene Oxide via Ritter Reaction

Graphene oxide, the oxidized form of graphite, is a common precursor to conductive nanosheets and used widely in the preparation of composite materials. GO has the benefits of easy exfoliation and handling, but it tends to aggregate and restack when reduced. One approach to overcoming this undesired aggregation is covalent modification of the nanosheets; however, this typically requires additional reagents and time. Herein, we report the simultaneous reduction and functionalization of graphene oxide using the Ritter reaction such that reduced nanosheets show good conductivity without the aggregation typical of unmodified material. GO reacts with nitriles in strongly acidic conditions to give highly reduced graphene oxide (C:O of 4.38:1) with covalently attached amides, which compatibilizes it to a number of organic solvents. This Ritter-type reaction produces carbocations on the basal plane of graphene oxide, which allows nucleophilic attack by the nitrogen of the nitrile and produces amides upon hydrolysis. The product has sheet resistance (57.60 ± 4.04 kΩ/sq) substantially lower than that of the starting graphene oxide (529.60 ± 10.04 kΩ/sq) and, more importantly, can easily be dispersed in various organic solvents and does not restack into graphite-like materials upon drying. This method yields individual conductive nanosheets that can be readily incorporated into a number of different systems.

Polythioether Particles Armored with Modifiable Graphene Oxide Nanosheets

Facile and scalable fabrication methods are attractive to prepare materials for diverse applications. Herein, a method is presented to prepare cross-linked polymeric nanoparticles with graphene oxide (GO) nanosheets covalently attached to the surface. Alkene-modified GO serves as a surfactant in a miniemulsion polymerization, and the alkene functionalities of GO exposed to the oil-phase are incorporated into the polymer particle through thiol-ene reactions, leaving the unreacted alkene functional groups of the other face of GO available for further functionalization. The surface of GO-armored polymer particles is then modified with a small molecule fluorophore or carboxylic acid functional groups that bind to Fe2 O3 and TiO2 nanoparticles. This methodology provides a facile route to preparing complex hybrid composite materials.