Bruce M. Novak

Identification of the specific, shutter-like conformational reorientation in a chiroptical switching polycarbodiimide by VCD spectroscopy

The specific conformational states responsible for the unique, reversible temperature- and solvent-driven chiroptical switching process experienced by poly(N-1-naphthyl-N-octadecyl-carbodiimide) (PNOC) have been identified using VCD spectroscopy and DFT calculations. The distinct VCD spectra of PNOC corresponding to the two specific conformations were obtained for the polymer dissolved in DCM-d2 (state A) and CDCl3 (predominantly state B). To specifically assign the structures of both conformations, two simplified 7mer models were constructed and optimized using DFT calculations. The theoretical spectra associated with these model conformations show a high level of agreement when compared to the experimental VCD spectra. The two states consist of the naphthyl pendant groups aligned directionally opposing the helical rotation (model A) and aligned with the helicity of the backbone (model B). This pendant reorientation causes very large OR and ECD Cotton effect inversions upon modification to the temperature or solvent composition of dilute (+)-PNOC solutions in specific solvents. In addition, the pendant group equilibrium from state A to B causes a contraction of the helical pitch from the more expanded 5/1 pitch to the more contracted 7/2 pitch resulting in increased disorder of the solvation sphere surrounding the polymer chain.

A semiconducting liquid crystalline block copolymer containing regioregular poly(3-hexylthiophene) and nematic poly(n-hexyl isocyanate) and its application in bulk heterojunction solar cells

A liquid crystalline diblock copolymer containing regioregular poly(3-hexylthiophene) (P3HT) and poly(n-hexyl isocyanate) (PHIC) was synthesized by the combination of Grignard metathesis polymerization (GRIM) and titanium mediated coordination polymerization methods. The poly(3-hexylthiophene)-b-poly(n-hexyl isocyanate) (P3HT-b-PHIC) diblock copolymer used in this study contained ∼10 mol% of P3HT and ∼90 mol% of PHIC. The diblock copolymer displayed solvatochromism in THF–water and THF–methanol mixtures. The field-effect mobilities of the synthesized block copolymer were measured in bottom gate-bottom contact organic field-effect transistors (OFETs). The surface morphology of the polymer thin film was investigated in the channel region of the OFET devices by tapping mode atomic force microscopy (TMAFM). The diblock copolymer displayed nanostructured morphology in thin film and had good mobility despite the low content of the semiconducting P3HT block. The diblock copolymer was also used as an additive to improve the performance of P3HT/PCBM bulk heterojunction (BHJ) solar cells. Liquid crystalline characteristics of the diblock copolymer were examined by cross-polarizing microscopy and X-ray diffraction.

Utilization of a Meldrum's acid towards functionalized fluoropolymers possessing dual reactivity for thermal crosslinking and post-polymerization modification

New thermally cross-linkable and/or post-functionalizable perfluorocyclobutyl (PFCB) polymers containing Meldrums acid moieties have been successfully prepared via the thermal cyclopolymerization of a new Meldrums acid functionalized aromatic trifluorovinyl ether (TFVE) monomer.

Synthesis and characterization of a novel fluorinated bismaleimide via a nucleophilic addition–elimination reaction and its polymeric networks

The commercially available octafluorocyclopentene (OFCP) as both a fluorinated building block and a linker has been successfully utilized to prepare a new fluorinated bismaleimide monomer OFCP-BMI via a nucleophilic addition–elimination reaction. The resulting OFCP-BMI is characterized by 1H, 13C, 19F nuclear magnetic resonance (NMR) spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and high resolution mass spectrometry. The monomer OFCP-BMI reacts with a free radical initiator or self-cures to prepare its resins. No obvious glass transition temperature (Tg) below 260 °C was observed for the self-curing resin suggesting that a highly cross-linked polymeric network was formed. The self-curing resin exhibits excellent thermal stability with the on-set weight loss temperature (Ton-setd) of 445 °C and a 50% char yield at 800 °C under a nitrogen atmosphere. The resin obtained by a free radical polymerization process exhibits less thermal stability and a lower char yield than the self-curing resin due to its lower cross-linking density. The self-curing resin exhibits a low dielectric constant (Dk = 2.5).

The secondary structures of PEG-functionalized random copolymers derived from (R)- and (S)- families of alkyne polycarbodiimides

A series of helical rigid-rod (R)- and (S)-polycarbodiimides having PEG2K, 10K, and 20K groups attached to aromatic or aliphatic side chains have been successfully synthesized from the respective alkyne polycarbodiimide precursors using the CuI-catalyzed azide/alkyne cycloaddition (CuAAC) reaction. The AFM, TEM and SEM studies of this series revealed the formation of different types of aggregated morphologies, i.e., micro- and nanospheres, fiber-like crystallites, and porous aggregates, which can be tailored with the hydrophilic PEG segments in the polymer structure. In general, heavily PEGylated scaffolds comprising ∼44 ethylene oxide segments are prone to form round-shaped secondary structures, especially in the bulk, as evident by SEM measurements. AFM data suggested that spherical aggregates are the preferred motifs in a wide range of concentrations. Moreover, polycarbodiimide-graft-PEG(2K) random copolymers were shown to efficiently suspend SWCNTs in water, leading to novel neutral photoluminescent nanocomposite materials. Overall, these extensive self-assembly studies on various polycarbodiimide platforms featuring a hydrophobic rigid rod main chain and a flexible hydrophilic periphery may provide a basic layout for prospective biomedical applications, such as controlled drug delivery and enhanced dispersibility properties of SWCNTs.