Geometric control of rippling in supported polymer nanolines.

We study the swelling behavior of finlike polymer line gratings supported on a rigid substrate and show that the edge-supported polymer laminae undergo a rippling instability with a well-defined ripple wavelength λ transverse to the plane of the solid supporting substrate and a ripple amplitude that monotonically decreases from its maximum at the free-edge. These ripple patterns develop due to inhomogeneous compressive strains that arise from the geometric constraints that progressively suppress swelling near the supporting substrate where the laminae are clamped. By experimentally examining the influence of swelling strain and pattern geometry on the observed rippling instability, we find that the ripple wavelength λ scales with line width w for sufficiently long gratings, which is consistent with a simple theory. These trends were validated for polymer nanoline test patterns having w between (50 to 250) nm and a height-to-width aspect-ratio in the range 0.5 to 5. Our results suggest that line geometry, rather than material properties, governs the onset of rippling and suggest simple rules for their control.

Toxicological evaluation of lung responses after intratracheal exposure to non-dispersed titanium dioxide nanorods.

Fine- and coarse-sized titanium dioxide (TiO2) particles are considered to be relatively inert when inhaled. The goal of this study was to assess potential lung toxicity associated with well-characterized, non-dispersed rutile TiO2 nanorods (10 × 40 nm). In vitro bioreactivity of TiO2 nanorods was determined by electron spin resonance (ESR) to measure free radical production. To assess pulmonary effects in vivo, Sprague-Dawley rats were intratracheally instilled with saline, silica, or TiO2 nanorods (10 µg, 100 µg, or 1 mg/rat). On d 1, 3, and 6 posttreatment, left lungs were preserved for microscopy and histopathology, and lung lavage was performed on right lungs. Additional rats were treated with saline or TiO2 nanorods (100 µg or 1 mg/rat) on d 0, intratracheally inoculated with 5 × 10(5) Listeria monocytogenes on d 3, and bacterial clearance was assessed. ESR showed a significant concentration-dependent generation of hydroxyl radicals by TiO2 nanorods in the presence and absence of macrophages; however, the hydroxyl radical signals from TiO2 samples were low compared to silica. Rats exposed to 1 mg of TiO2 nanorods had significantly elevated levels of lung injury, inflammation, and lavage fluid monocyte chemoattractant protein (MCP)-1 and macrophage inflammatory protein (MIP)-2 on d 1 and 3 that subsided by d 6, unlike the silica response that persisted. Immune cytokine secretion in the lung and bacterial clearance were not affected by preexposure to TiO2 nanorods. To summarize, non-dispersed TiO2 nanorods were found to induce radical formation and cellular oxidant production, and to generate transient and reversible pneumotoxic effects, and to not markedly alter pulmonary immune function.

Thermodynamic interactions in double-network hydrogels.

Double-network hydrogels (DN-gels) prepared from the combination of a moderately cross-linked anionic polyelectrolyte (PE) and an uncross-linked linear polymer solution (NP) exhibit mechanical properties such as fracture toughness that are intriguingly superior to that of their individual constituents. The scheme of double-network preparation, however, is not equally successful for all polyelectrolyte/neutral polymer pairs. A successful example is the combination of poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS) cross-linked network and linear polyacrylamide (PAAm), which results in DN-gels with fracture strength under compression approaching that of articular cartilage ( approximately 20 MPa). Small-angle neutron scattering was used to determine the thermodynamic interaction parameters for PAMPS and PAAm in water as a first step to elucidate the molecular origin responsible for this superior property. Measurements on PAMPS/PAAm DN-gels and their solution blend counterparts indicate that the two polymers interact favorably with each other while in water. This favorable PAMPS/PAAm interaction given by the condition chi(PE-NP) < chi(PE-water)

Molecular model for toughening in double-network hydrogels.

A molecular mechanism is proposed for the toughness enhancement observed in double-network (DN) hydrogels prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) polyelectrolyte network and poly(acrylamide) (PAAm) linear polymer. It is an extension of the phenomenological model set forth recently by Gong et al. ( Macromolecules 2007, 40, 6658- 6664 ). This mechanism rationalizes the changes in molecular structure of the DN gel constituents observed via in situ neutron scattering measurements, the composition dependence of the solution viscosity, and the thermodynamic interaction parameters of PAMPS and PAAm molecules obtained previously from neutron scattering studies. More specifically, this proposed mechanism provides an explanation for the observed periodic compositional fluctuations in the micrometer range induced by large strain deformation.

Effect of chemical structure on the volume-phase transition in neutral and weakly charged poly(N-alkyl(meth)acrylamide) hydrogels studied by ultrasmall-angle x-ray scattering.

Neutral poly(N-isopropylacrylamide) (PIPAAm), poly(N,N-diethylacrylamide) (PDEAAm), and poly(N-isopropylmethacrylamide) (PIPMAm) hydrogels and their weakly charged counterparts prepared by copolymerizing with sodium methacrylate (x(MNa)=0,0.025,0.05) were studied using ultrasmall-angle x-ray scattering. The volume-phase transition in hydrogels was observed as an increase in the inhomogeneity correlation length of the networks. The change in inhomogeneity correlation length was abrupt in neutral PIPAAm and PIPMAm gels with increase in temperature but was continuous in neutral PDEAAm gels. Addition of ionic comonomer to the network backbone suppressed the volume-phase transition in poly(N-alkylacrylamide)s but not in PIPMAm. The observed differences in temperature-induced volume change of these three polymers in water cannot be rationalized based on their relative hydrophobicity and are instead explained by considering the hydrogen-bonding constraints on their thermal fluctuations. Both PIPAAm and PDEAAm undergo volume collapse since their thermal fluctuations are constrained by hydrogen bonding with water to an extent that beyond a critical temperature they seek entropic compensation. Although thermal fluctuations in both PIPAAm and PIPMAm are equally constrained, thermal energy of the latter can be relaxed via the rotation of alpha-methyl groups allowing it greater flexibility. Compared to N-alkylacrylamides, N-alkylmethacrylamide can thus sustain hydrogen bonding to relatively higher temperatures before seeking entropic compensation by undergoing volume collapse.