Impact of Backbone Composition on Homopolymer Dynamics and Brush Block Copolymer Self-Assembly.

Four series of brush block copolymers (BBCP), with near identical side chain compositions but varying backbone structures, were synthesized to investigate the effect of backbone structure on the process of thermal BBCP self-assembly to photonic crystals (PCs). Each of the self-assembled PC films were examined by reflection measurements, small angle X-ray scattering measurements, and scanning electron microscopy to compare the resulting properties of the polymeric photonic crystal and the nanostructured morphology impacted by the backbone structure. It was found that the composition of the brush backbone within a BBCP has a dramatic effect on the ability of the BBCP to self-assemble into ordered nanostructures and on the local ordering of the nanostructure morphology accessed with higher molecular weight (MW) BBCPs (> 1,500 kg/mol). BBCPs with a norbornene imide-based backbone were able to thermally self-assemble to longer wavelength reflecting PCs and had higher fidelity ordering of lamellar nanostructures with higher MW polymers. By analyzing the melt rheological responses of the backbone compositions, both as linear polymers and homobrush polymers, it was concluded that the inherent fragility of the backbone promotes enhanced local ordering in the lamellar nanostructure morphology as well as access to larger domain sizes.

Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-Opening Polymerization of Vinylcyclopropanes.

Although radical polymerizations are among the most prevalent methodologies for the synthesis of polymers with diverse compositions and properties, the intrinsic reactivity and selectivity of radical addition challenge the ability to impart control over the polymerization propagation and produce polymers with defined microstructure. Vinylcyclopropanes (VCPs) can be polymerized through radical ring-opening polymerization to produce polymers possessing linear (l) or cyclic (c) repeat units, providing the opportunity to control polymer structure and modify the polymer properties. Herein, we report the first organocatalyzed photoredox radical ring-opening polymerization of a variety of functionalized VCP monomers, where high monomer conversions and spatial and temporal control were achieved to produce poly(VCPs) with predictable molecular weight and low dispersity. Through manipulating polymerization concentration and temperature, tunable l or c content was realized, allowing further investigation of thermal and viscoelastic materials properties associated with these two distinct compositions. Unexpectedly, the photoredox catalysis enables a postpolymerization modification that converts l content into the c content. Combined experimental and computational studies suggested an intramolecular radical cyclization pathway, where cyclopentane and cyclohexane repeat units are likely formed.

3D printing using powder melt extrusion.

Additive manufacturing promises to revolutionize manufacturing industries. However, 3D printing of novel build materials is currently limited by constraints inherent to printer designs. In this work, a bench-top powder melt extrusion (PME) 3D printer head was designed and fabricated to print parts directly from powder-based materials rather than filament. The final design of the PME printer head evolved from the Rich Rap Universal Pellet Extruder (RRUPE) design and was realized through an iterative approach. The PME printer was made possible by modifications to the funnel shape, pressure applied to the extrudate by the auger, and hot end structure. Through comparison of parts printed with the PME printer with those from a commercially available fused filament fabrication (FFF) 3D printer using common thermoplastics poly(lactide) (PLA), high impact poly (styrene) (HIPS), and acrylonitrile butadiene styrene (ABS) powders (< 1 mm in diameter), evaluation of the printer performance was performed. For each build material, the PME printed objects show comparable viscoelastic properties by dynamic mechanical analysis (DMA) to those of the FFF objects. However, due to a significant difference in printer resolution between PME (X-Y resolution of 0.8 mm and a Z-layer height calibrated to 0.1 mm) and FFF (X-Y resolution of 0.4 mm and a Z-layer height of 0.18 mm), as well as, an inherently more inconsistent feed of build material for PME than FFF, the resulting print quality, determined by a dimensional analysis and surface roughness comparisons, of the PME printed objects was lower than that of the FFF printed parts based on the print layer uniformity and structure. Further, due to the poorer print resolution and inherent inconsistent build material feed of the PME, the bulk tensile strength and Young's moduli of the objects printed by PME were lower and more inconsistent (49.2 ± 10.7 MPa and 1620 ± 375 MPa, respectively) than those of FFF printed objects (57.7 ± 2.31 MPa and 2160 ± 179 MPa, respectively). Nevertheless, PME print methods promise an opportunity to provide a platform on which it is possible to rapidly prototype a myriad of thermoplastic materials for 3D printing.

Impact of the Pendant Group on the Chain Conformation and Bulk Properties of Norbornene Imide-Based Polymers.

Three series of well-defined norbornene imide-based polymers with different pendant groups were synthesized to investigate the effect of the pendant group on the polymer conformation in solution and bulk melt properties. Each of these three series was examined by analyzing the polymers' bulk z-average radius of gyration via static light scattering and the polymers' melt viscoelastic properties via oscillatory measurements and differential scanning calorimetry. Sterically bulky pendant wedge groups modestly increase the rodlike conformation of the norbornene-imide polymer, however, the inherent rigidity of the polymer main-chain can still be observed with less bulky substituents. In stark contrast, the different side groups significantly impacted the bulk viscoelastic and thermal properties. By increasing the pendant group size, the chain diameter of the polymer increases and lowers the entanglement modulus. Finally, as the wedge pendant group size increases, the segmental relaxation time and the fragility index of these norbornene-based polymers are decreased.

Structural Color for Additive Manufacturing: 3D-Printed Photonic Crystals from Block Copolymers.

The incorporation of structural color into 3D printed parts is reported, presenting an alternative to the need for pigments or dyes for colored parts produced through additive manufacturing. Thermoplastic build materials composed of dendritic block copolymers were designed, synthesized, and used to additively manufacture plastic parts exhibiting structural color. The reflection properties of the photonic crystals arise from the periodic nanostructure formed through block copolymer self-assembly during polymer processing. The wavelength of reflected light could be tuned across the visible spectrum by synthetically controlling the block copolymer molecular weight and manufacture parts that reflected violet, green, or orange light with the capacity to serve as selective optical filters and light guides.

Responses of neurons in the caudal medullary lateral tegmental field to visceral inputs and vestibular stimulation in vertical planes.

The dorsolateral reticular formation of the caudal medulla, or the lateral tegmental field (LTF), has been classified as the brain's "vomiting center", as well as an important region in regulating sympathetic outflow. We examined the responses of LTF neurons in cats to rotations of the body that activate vestibular receptors, as well as to stimulation of baroreceptors (through mechanical stretch of the carotid sinus) and gastrointestinal receptors (through the intragastric administration of the emetic compound copper sulfate). Approximately half of the LTF neurons exhibited graviceptive responses to vestibular stimulation, similar to primary afferents innervating otolith organs. The other half of the neurons had complex responses, including spatiotemporal convergence behavior, suggesting that they received convergent inputs from a variety of vestibular receptors. Neurons that received gastrointestinal and baroreceptor inputs had similar complex responses to vestibular stimulation; such responses are expected for neurons that contribute to the generation of motion sickness. LTF units with convergent baroreceptor and vestibular inputs may participate in producing the cardiovascular system components of motion sickness, such as the changes in skin blood flow that result in pallor. The administration of copper sulfate often modulated the gain of responses of LTF neurons to vestibular stimulation, particularly for units whose spontaneous firing rate was altered by infusion of drug (median of 459%). The present results raise the prospect that emetic signals from the gastrointestinal tract modify the processing of vestibular inputs by LTF neurons, thereby affecting the probability that vomiting will occur as a consequence of motion sickness.