The hierarchical microstructure responsible for the unique energy-absorbing properties of natural materials, like native spider silk and wood, motivated the development of segmented polyurethanes ...with soft segments containing multiple levels of order. As a first step in correlating the effects of crystallinity in the soft segment phase to the hard segment phase, we chose to examine the morphology and mechanical behavior of polyurethanes containing polyether soft blocks with varying tendencies to crystallize and phase segregate and the evolution of the microstructure with deformation. A series of high molecular weight polyurethanes containing poly(ethylene oxide) (PEO) (1000 and 4600
g/mol) and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) (1900
g/mol) soft segments with varying hard segment content were synthesized using a two-step solution polymerization method. The presence of soft segment crystallinity (PEO 1000
g/mol) was shown to improve the storage modulus of the segmented polyurethanes below the
T
m of the soft block and to enhance toughness compared to the PEO–PPO–PEO soft segment polyurethanes. We postulate that this enhancement in mechanical behavior is the result of crystalline soft regions that serve as an additional load-bearing component during deformation. Morphological characterization also revealed that the microstructure of the segmented polyurethanes shifts from soft segment continuous to interconnected and/or hard domain continuous with increasing hard segment size, resulting in diminished ultimate elongation, but enhanced initial moduli and tensile strengths. Tuning the soft segment phase crystallinity may ultimately lead to tougher polyurethane fibers.
This work demonstrates the production of a well-controlled, chemical gradient on the surface of graphene. By inducing a gradient of oxygen functional groups, drops of water and ...dimethyl-methylphosphonate (a nerve agent simulant) are “pulled” in the direction of increasing oxygen content, while fluorine gradients “push” the droplet motion in the direction of decreasing fluorine content. The direction of motion is broadly attributed to increasing/decreasing hydrophilicity, which is correlated to high/low adhesion and binding energy. Such tunability in surface chemistry provides additional capabilities in device design for applications ranging from microfluidics to chemical sensing.
The phase-segregated nature of polyurethanes allows meaningful connections to be made between morphological and physical properties. We have taken advantage of this behavior by synthesizing a series ...of polyurethanes with varying extents of crystallinity and studying their morphologies in both the unstrained and deformed states, going from a completely amorphous soft segment to one with similar chemistry that displays a high extent of soft domain crystallization, thus enhancing phase segregation. The presence of dispersed semicrystalline regions within the continuous soft domain has been shown to provide a reinforcing effect when compared to that of a non-crystalline soft segment polyurethane. Incorporating a semicrystalline soft segment (PEO, 1000 g/mol) has been shown to improve overall sample toughness; however, if higher molecular weight PEO soft segments are employed (4600 g/mol), extensibility and, consequently, toughness are adversely affected due to an increased continuous domain modulus. In-situ deformation experiments demonstrate two very different deformation responses. In the copolymer-containing polyurethane (PEO−PPO−PEO, 1900 g/mol), the hard domains retain a tilted configuration up to strains of ∼450%, with only a small fraction of the hard segments undergoing reshuffling. The PEO1000-containing polyurethane, on the other hand, begins to demonstrate meridional scattering at strains of 200%, with it being the dominant peak by a strain of 300%. These two deformation behaviors are indicative of the two primary responses to deformation, which are shear and tensile, respectively. Frequently, a tensile mechanism points to decreased polyurethane mechanical properties, though this phenomenon is not seen in the series of interest.
Materials which induce molecular motion without external input offer unique opportunities for spatial manipulation of molecules. Here, we present the use of polyacrylamide hydrogel films containing ...built-in chemical gradients (enthalpic gradients) to direct molecular transport. Using a cationic tertiary amine gradient, anionic molecules were directionally transported up to several millimeters. A 40-fold concentration of anionic molecules dosed in aerosol form on a substrate to a small region at the center of a radially symmetric cationic gradient was observed. The separation of mixtures of charged dye molecules was demonstrated using a boronic acid-to-cationic gradient where one molecule was attracted to the boronic acid end of the gradient, and the other to the cationic end of the gradient. Theoretical and computational analysis provides a quantitative description of such anisotropic molecular transport, and reveals that the gradient-imposed drift velocity is in the range of hundreds of nanometers per second, comparable to the transport velocities of biomolecular motors. This general concept of enthalpy gradient-directed molecular transport should enable the autonomous processing of a diversity of chemical species.
Disagreements persist regarding the rate constants for the initial reduction of perfluorooctanoate (PFOA) by the hydrated electron, eaq –, with published values differing by nearly 2 orders of ...magnitude (between ∼1 × 107 and ∼1 × 109 M–1 s–1). In this letter, we demonstrate a nonlinear dependence of the eaq – decay rate on the PFOA which we attribute as being a result of aggregation of PFOA at concentrations below the critical micellar concentration. We invoke a kinetic model for the reaction between PFOA and eaq – at concentrations below 1 mM involving dimerization of the PFOA; higher-order aggregates are acknowledged but not modeled. Our kinetic model adequately fits the nanosecond transient absorption data at PFOA ≤ 1 mM providing apparent rate constants for the reduction of PFOA monomers (k 1 = 1.4 × 109 M–1 s–1) and dimers (k 2 = 1.1 × 107 M–1 s–1) by eaq –. For reasons that are only partially understood, the aggregation of PFOA decreases the probability of the reduction of PFOA by eaq –. These results reconcile the reports of differing rate constants for the initial reduction of PFOA by eaq – suggesting its origin to be related to the aggregation of PFOA even under dilute conditions.
The negative thermal expansion showing contraction of unit cell dimensions on heating is an unusual phenomenon which is rarely observed in organic materials. Here we report that a highly aligned ...hexagonal columnar liquid crystalline phase of discotic liquid crystal, cobalt octakis(n-decylthio)porphyrazine (CoS10), in thin films shows negative thermal expansion in the transverse plane and positive thermal expansion along the column axis. This exceptional thermal expansion behavior of CoS10 at the hexagonal columnar liquid crystalline phase was understood as the synergetic effect of the increased vibrational motion of n-alkyl side chains perpendicular to the chain axes and the increased tilting of the cores with temperature.
Display omitted
•We calculate the energy band gap of gradient copolymers of thiophene and pyrrol.•The optimal thiophene fraction range between zero and 40% gives the maximum band gap gradient.•It is ...suggested a route for enhanced charge separation in gradient copolymers.•Photovoltaic properties can be tuned by changing the composition of gradient copolymers.
The electronic properties of copolymers can be tuned by controlling their monomer ratio, and therefore can potentially be used to improve charge separation in organic photovoltaic devices. Here we show evidence based on density functional theory calculations that it is possible to control the electronic structure of π-conjugated copolymers of thiophene and pyrrole to obtain a gradient in the band gap and both conduction and valence crystal orbital band levels by controlling their composition. Our calculations predict and optimal thiophene monomer fraction range between zero and 40% is needed in order to obtain the largest electronic structure gradients.
We report the observation of a cubic phase consistent with the double gyroid structure in strongly segregated diblock copolymers of PS-b-PDMS over a volume fraction (φPDMS) range of ~0.39 to 0.45. ...The samples have respective molecular weights of 127 kg/mol and 73 kg/mol and degree of segregation Nχ equal to 187 and 106, respectively, at annealing temperature of 130 °C. It is important to highlight that two out of the total four samples investigated, exhibited hexagonally close packed cylindrical domains of PDMS and alternating lamellae at φPDMS = 0.39 and 0.45, respectively, indicating the possible narrow range of the DG morphology for the specific diblock copolymers.
Uniaxially oriented and highly ordered columnar superstructure of cobalt octa‐(n‐decylthio)porphyrazine (CoS10) on a large area of substrate has been fabricated, simultaneously utilizing an applied ...magnetic field and the interaction of CoS10 with an OTS‐functionalized substrate. This very simple and straightforward alignment technique can be instrumental for practical applications of discotic supramolecular systems including various organoelectronic devices.
We describe herein a polymeric material that prefers to align perpendicular to a stretch-aligned polymer host in the solid state. Poly(iptycene) poly-1 was synthesized from monomer 1 under hyperbaric ...techniques via a Diels−Alder polymerization. Polarized excitation spectra of the anthracene end groups in this material in a stretch-aligned, solution-cast poly(vinyl chloride) (PVC) film showed that the poly(iptycene) prefers to align normal (counter aspect ratio) to the stretching direction of the PVC. This is explained by a “threading” mechanism, whereby the PVC intercalates through the internal free volume presented by poly-1, similar to effects observed in small molecule iptycenes under similar conditions.
PDF
