Liquid – liquid phase separation morphologies in ultra-white beetle scales and a synthetic equivalent

Cyphochilus beetle scales are amongst the brightest structural whites in nature, being highly opacifying whilst extremely thin. However, the formation mechanism for the voided intra- scale structure is unknown. Here we report 3D x-ray nanotomography data for the voided chitin networks of intact white scales of Cyphochilus and Lepidiota stigma. Chitin-filling frac- tions are found to be 31 ± 2% for Cyphochilus and 34 ± 1% for Lepidiota stigma, indicating previous measurements overestimated their density. Optical simulations using finite- difference time domain for the chitin morphologies and simulated Cahn-Hilliard spinodal structures show excellent agreement. Reflectance curves spanning filling fraction of 5-95% for simulated spinodal structures, pinpoint optimal whiteness for 25% chitin filling. We make a simulacrum from a polymer undergoing a strong solvent quench, resulting in highly reflective ( 94%) white films. In-situ X-ray scattering confirms the nanostructure is formed through spinodal decomposition phase separation. We conclude that the ultra-white beetle scale nanostructure is made via liquid–liquid phase separation

Fast Polymer Diffusion through Nanocomposites with Anisotropic Particles

Polymer nanocomposites (PNCs) have characteristic length scales associated with both the nanoparticles (i.e., size and interparticle distance) and the polymer molecules (i.e., tube diameter of entanglement and radius of gyration; <i>R</i>_g). When the nanoparticle (NP) and polymer length scales are comparable, the polymer dynamics exhibit contrasting behavior for NPs differing only in size and shape. For spherical NPs and short anisotropic NPs, the polymer diffusion coefficient decreases monotonically with NP concentration. However, for long anisotropic NPs, polymer diffusion slows down at low NP concentration and recovers for NP concentrations above the critical concentration for network formation. By spanning intermediate ranges of nanoparticle size and shape, the role of the NP geometric parameters on the polymer dynamics is substantially advanced, thereby providing new routes toward controlling polymer dynamics and viscoelasticity of PNCs

Temperature Dependence of Polymer Diffusion in MWCNT/PS Nanocomposites

Polymer tracer diffusion in multiwalled carbon nanotubes (MWCNT)/polystyrene (PS) nanocomposites as a function of MWCNT concentration was measured from 152 to 214 °C. At 170 °C, we previously reported a minimum in the tracer diffusion coefficient that occurs at ∼2 wt % MWCNT; this concentration corresponds to the onset of solidlike behavior as measured by linear viscoelasticity. The minimum in the tracer diffusion coefficient in the nanocomposites (<i>D</i>) normalized by the tracer diffusion coefficient without MWCNTs (<i>D</i>₀) is shallower at higher temperatures, namely, (<i>D</i>/<i>D</i>₀)_min = 0.9 at 214 °C and (<i>D/D</i>₀)_min = 0.6 at 152 °C. Using our trap model, this implies that the difference between the tracer diffusion parallel and perpendicular to the MWCNT is smaller at higher temperatures. Finally, at fixed MWCNT concentrations (0.5, 2, and 6 wt %) the temperature dependence of the tracer diffusion coefficient follows the Williams–Landel–Ferry (WLF) equation, indicating that polymer dynamics in nanocomposites is captured by changes in free volume with temperature

Anisotropic Polymer Conformations in Aligned SWCNT/PS Nanocomposites

Polymer radii of gyration in isotropic single-walled carbon nanotube (SWCNT)/polymer nanocomposites were previously found to increase with increasing SWCNT concentration. Here, the SWCNTs in nanocomposites were aligned by melt fiber spinning, and the polymer chain conformations were found to be anisotropic. Using SAXS and SANS, the anisotropic SWCNT meshes were found to have smaller mesh sizes in the direction perpendicular to the alignment direction than along the alignment direction. At fixed SWCNT orientation, the radius of gyration was probed parallel and perpendicular to the alignment direction, <i>R</i>_g^par and <i>R</i>_g^per, respectively, using SANS. With increasing SWCNT concentration, <i>R</i>_g^per increases significantly more than <i>R</i>_g^par, such that the extent of anisotropy increases with SWCNT concentration. The anisotropic polymer conformation is larger in the direction perpendicular to the alignment direction, which corresponds to a smaller SWCNT mesh size. Thus, when the SWCNT concentration and alignment combine to produce a SWCNT mesh size that is smaller than the unperturbed <i>R</i>_g, the polymer conformation circumvents the SWCNTs by adopting a larger <i>R</i>_g. Changes in the polymer conformation in nanocomposites with rod-like nanoparticles has important ramifications for entanglement density, polymer dynamics, and mechanical properties

Macromolecular Diffusion through a Polymer Matrix with Polymer-Grafted Chained Nanoparticles

Diffusion of deuterated polystyrene (dPS) is probed in PS matrices containing stringlike chained nanoparticles (cNP) grafted with PS. This investigation connects prior diffusion studies in model spherical and cylindrical NP systems and provides insight for technological applications, which typically involve irregularly shaped NPs such as carbon black. We report that the presence of chained NPs in PS matrices induces a minimum in the diffusion coefficient (<i>D</i>) with increasing cNP concentration when the key length scale, 2<i>R</i>_g/<i>L</i> ≤ 1.5, where <i>R</i>_g is the gyration radius of dPS and <i>L</i> is the mean length of the impenetrable core of the chained NPs. When 2<i>R</i>_g/<i>L</i> > 1.5, <i>D</i> decreases monotonically as the NP concentration increases. Note that in all cases 2<i>R</i>_g is larger than the diameter of these short-stringy NPs. The diffusion minimum is attributed to anisotropic diffusion in the vicinity of the chained NPs and requires that the long dimension of the cNP be comparable to or longer than the tracer molecule. Two normalizations are explored to provide insight about the diffusion mechanism: <i>D</i>/<i>D</i>₀ where <i>D</i>₀ is the diffusion coefficient in a pure homopolymer matrix and <i>D</i>/<i>D</i>_e where <i>D</i>_e is an effective diffusion coefficient that accounts for the distinct dynamics in the PS matrix and PS brush regions. For <i>D</i>/<i>D</i>₀, a sharp transition from a diffusion minimum to a monotonic decrease is observed as dPS molecular weight increases, while for <i>D</i>/<i>D</i>_e the transition is more gradual. These studies show not only that the NPs act as impenetrable obstacles for polymer diffusion but that the polymer brush grafted to the cNP provides an alternative pathway to control polymer dynamics

Polymer Chain Conformations in CNT/PS Nanocomposites from Small Angle Neutron Scattering

Chain conformations dictate many of the important physical properties of polymers including their dynamics. Using small angle neutron scattering, we probed chain conformations, specifically the radius of gyration (<i>R</i>_g), in both SWCNT/polystyrene (<i>r</i>_SWCNT/<i>R</i>_g ∼ 0.4) and MWCNT/polystyrene (<i>r</i>_MWCNT/<i>R</i>_g ∼ 1) nanocomposites. We fit the scattering data using a model that includes an ideal Gaussian chain to describe the polymer conformation and a rod network to describe the carbon nanotube (CNT) network. The scattering contribution from the rod network increases with the CNT concentration in both SWCNT and MWCNT nanocomposites, and the contribution is higher for SWCNT nanocomposites due to the smaller mesh size and higher mesh density. When the SWCNT and MWCNT concentrations are below 2 wt %, there is no significant change in <i>R</i>_g. Above 2 wt %, <i>R</i>_g in the MWCNT nanocomposites decreases slightly, while <i>R</i>_g in the SWCNT nanocomposites increases monotonically as a function of CNT concentration, showing a ∼30% increase at 10 wt % SWCNT loading. Although we previously found a minimum in the tracer diffusion coefficient near the critical nanotube concentration, this trend is absent in the concentration-dependent polymer conformation

Nanoparticle Brush Architecture Controls Polymer Diffusion in Nanocomposites

We show that polymer diffusion in polymer nanocomposites (PNCs) is controlled by the architecture of polymer brushes grafted to hard spherical nanoparticles (NPs). At high grafting density, diffusing chains are excluded from the polymer brush leading to greater confinement. However, at lower grafting density, these chains penetrate the brush and diffusion is similar to the hard NP case, compared at the same NP loading. We calculate the effective interparticle spacing (ID_eff) by modeling polymer penetration into the grafted brush using self-consistent field theory. When plotted against a confinement parameter (ID_eff/2<i>R</i>_g, where <i>R</i>_g is the radius of gyration of the diffusing polymer), reduced diffusion coefficients (<i>D</i>/<i>D</i>_<i>o</i>) fall on a master curve independent of brush architecture. These findings show that brush architecture provides a new route toward controlling polymer dynamics and viscoelasticity of PNCs

Fast Nanorod Diffusion through Entangled Polymer Melts

Nanorod diffusion in polymer melts is faster than predicted by the continuum model (CM). Rutherford backscattering spectrometry is used to measure the concentration profile of titanium dioxide (TiO₂) nanorods (<i>L</i> = 43 nm, <i>d</i> = 5 nm) in a polystyrene (PS) matrix having molecular weights (<i>M</i>) from 9 to 2000 kDa. In the entangled regime, the tracer diffusion coefficients (<i>D</i>) of TiO₂ decrease as the <i>M</i>^–1.4, whereas the CM predicts <i>D</i>_CM ∼ <i>M</i>^–3.0 using the measured zero-shear viscosity of TiO₂(1 vol %): PS­(<i>M</i>) blends. By plotting <i>D</i>/<i>D</i>_CM versus <i>M</i>/<i>M</i>_e, where <i>M</i>_e is the entanglement molecular weight, diffusion is enhanced by a factor of 10–10³ as <i>M</i>/<i>M</i>_e increases. The faster diffusion is attributed to decoupling of nanorod diffusion from polymer relaxations in the surrounding matrix, which is facilitated by the nanorod dimensions (i.e., <i>L</i> greater than and <i>d</i> less than the entanglement mesh size, 8 nm)

Simultaneous Measurement of Thirteen Steroid Hormones in Women with Polycystic Ovary Syndrome and Control Women Using Liquid Chromatography-Tandem Mass Spectrometry

<div><p>Background</p><p>The measurement of adrenal and ovarian androgens in women with PCOS has been difficult based on poor specificity and sensitivity of assays in the female range.</p><p>Methods</p><p>Women with PCOS (NIH criteria; n = 52) and control subjects with 25–35 day menstrual cycles, no evidence of hyperandrogenism and matched for BMI (n = 42) underwent morning blood sampling. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to simultaneously measure 13 steroids from a single blood sample to measure adrenal and ovarian steroids. Androgen and progesterone results were compared in the same samples using RIA.</p><p>Results</p><p>Testosterone, androstenedione, progesterone and 17OH progesterone levels were higher when measured using RIA compared to LC-MS/MS, although the testosterone RIA demonstrated the best agreement with the LC-MS/MS using a Bland-Altman analysis. Results using LC-MS/MS demonstrated that the concentration of androgens and their precursors were higher in women with PCOS than controls [median (2.5, 97.5th %ile); 1607 (638, 3085) vs. 1143 (511, 4784) ng/dL; p = 0.03]. Women with PCOS had higher testosterone [49 (16, 125) vs. 24 (10, 59) ng/dL], androstenedione [203 (98, 476) vs. 106 (69, 223) ng/dL] and 17OH progesterone levels [80 (17, 176) vs. 44 (17, 142) ng/dL] compared to controls (all P<0.02), but no differences in serum concentrations of the adrenal steroids DHEAS, cortisol, corticosterone and their 11 deoxy precursors. Women with PCOS also had an increase in the product:precursor ratio for 3β-hydroxysteroid dehydrogenase [22% (6, 92) vs. 20% (4, 43); p = 0.009].</p><p>Conclusion</p><p>LC-MS/MS was superior to RIA in measuring androstenedione, progesterone and 17OH progesterone levels, while testosterone measurements were better matched in the two assays. Androgen levels were higher in women with PCOS in the absence of a difference in adrenal-predominant steroids. These data support previous findings that the ovary is an important source for the androgen excess in women with PCOS.</p></div

Do Attractive Polymer–Nanoparticle Interactions Retard Polymer Diffusion in Nanocomposites?

Diffusion of deuterated poly­(methyl methacrylate) (dPMMA) is slowed down in a PMMA matrix filled with hydroxyl-capped spherical silica nanoparticles, from 13 to 50 nm in diameter and at loadings up to 40 vol %. At constant <i>T</i> – <i>T</i>_g = 75 K, the normalized diffusion coefficients (<i>D</i>/<i>D</i>₀) collapse onto a master curve, when plotted against the confinement parameter, <i>ID</i>/2<i>R</i>_g, where <i>ID</i> is interparticle distance and 2<i>R</i>_g is probe size. This result suggests that the confinement parameter captures the effect of nanoparticle size, size polydispersity, and volume fraction on polymer dynamics for the PMMA composite. For <i>ID</i> < 2<i>R</i>_g, the master curve exhibits a strongly confined region where <i>D</i>/<i>D</i>₀ decreases by up to 80%, whereas for <i>ID</i> > 2<i>R</i>_g, the curve falls in a weakly confined region where <i>D</i>/<i>D</i>₀<i> </i>decreases only moderately by up to 15%. Surprisingly, <i>D</i>/<i>D</i>₀ is reduced even when <i>ID</i> is 8 times larger than 2<i>R</i>_g. A comparison between the master curves for PMMA and polystyrene nanocomposites indicates that attractive interactions in the PMMA system do not significantly alter the center-of-mass diffusion of macromolecules in polymer nanocomposites