As the macromolecular version of mechanically interlocked molecules, mechanically interlocked polymers are promising candidates for the creation of sophisticated molecular machines and smart soft materials. Poly[]catenanes, where the molecular chains consist solely of interlocked macrocycles, contain one of the highest concentrations of topological bonds. We report, herein, a synthetic approach toward this distinctive polymer architecture in high yield (~75%) via efficient ring closing of rationally designed metallosupramolecular polymers. Light-scattering, mass spectrometric, and nuclear magnetic resonance characterization of fractionated samples support assignment of the high-molar mass product (number-average molar mass ~21.4 kilograms per mole) to a mixture of linear poly[7-26]catenanes, branched poly[13-130]catenanes, and cyclic poly[4-7]catenanes. Increased hydrodynamic radius (in solution) and glass transition temperature (in bulk materials) were observed upon metallation with Zn.

Gels formed via metal–ligand coordination typically have very low branch functionality, f, as they consist of ∼2–3 polymer chains linked to single metal ions that serve as junctions. Thus, these materials are very soft and unable to withstand network defects such as dangling ends and loops. We report here a new class of gels assembled from polymeric ligands and metal-organic cages (MOCs) as junctions. The resulting ‘polyMOC’ gels are precisely tunable and may feature increased branch functionality. We show two examples of such polyMOCs: a gel with a low f based on a M2L4 paddlewheel cluster junction and a compositionally isomeric one of higher f based on a M12L24 cage. The latter features large shear moduli, but also a very large number of elastically inactive loop defects that we subsequently exchanged for functional ligands, with no impact on the gel's shear modulus. Such a ligand substitution is not possible in gels of low f, including the M2L4-based polyMOC.

Metallic nanoparticles that absorb and concentrate light are leading to greater efficiencies in nanophotonic devices. By confining gold nanorods (Au NRs) in a polymer film, we can control their spacing and orientation and, in turn, the absorption and polarization characteristics of the nanocomposite. In this study, we systematically increase the volume fraction of Au NRs (φrod) (aspect ratio v=3.3) while maintaining a uniform dispersion. As φrod increases from 1 to 16 vol %, the spacing between rods decreases from 120 to 20 nm and scales as φrod φ0.4. Simultaneously, the local 2D orientational order parameter increases linearly with φrod, although the rods are globally isotropic. The Au NR dispersion is found to depend on the enthalpic interactions between poly(ethylene glycol) brush grafted to the Au NRs and the poly(methyl methacrylate) matrix chains. Furthermore, the plasmon resonance exhibits a red shift with increasing φrod, and coupling is observed for separations up to 70 nm. Because NR spacing and orientation can be finely controlled using polymer matrix, these films are ideally suited for understanding fundamental behavior (e.g., plasmon coupling) as well as practical devices (e.g., solar cells).

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An understanding of the dispersion of nanoparticles into polymer melts is needed in order to control material properties of polymer nanocomposites. Here we study the dispersion of polymer-grafted nanorods in homopolymer melts of the same chemistry, using both experiment and theory. The theoretical calculations are performed over the range of experimental system parameters. Polymer-grafted gold nanorods (Au NRs) were found to be dispersed when the matrix chain lengths were small relative to the brush chain lengths, and aggregated at higher matrix chain lengths. Both classical density functional theory (DFT) and self-consistent field theory (SCFT) are used to calculate the structure of a polymer brush around an isolated NR in a polymer melt. Both theories predict a gradual transition from a "wet" to a "dry" brush as the grafting density, the NR radius, and/or the ratio of matrix to brush chain lengths is increased. DFT calculations of the interaction free energy between two NRs find an attractive well at intermediate NR separations, with a repulsive barrier at closer NR separations. The strength of the attraction increases as the brushes become more dry. Including the van der Waals attractions between the NRs gives an estimate of their total interaction free energy, which can be used to predict at which values of the system parameters the NRs are dispersed or aggregated. A dispersion map shows good agreement between DFT calculations and experimental observations of dispersed and aggregated nanorods.

Nanoparticles are new and valuable additives that can favorably tune thermomechanical, electric, optical, and magnetic properties of polymeric materials. The addition of nanoparticles can also enhance or slow down polymer dynamics depending on the mixture thermodynamics and key length scales, namely, nanoparticle size, interparticle spacing (ID), and the polymer radius of gyration (R g ). Presently, a framework for understanding how nanoparticles affect polymer dynamics is not available, in part, because of a lack of wide-ranging experimental studies. Here, tracer diffusion is studied in model nanocomposites containing silica nanoparticles grafted with either polymer brushes (soft nanoparticles) or short ligands (hard nanoparticles). Over a wide range of tracer molecular weights and nanoparticle loadings, the normalized diffusion coefficient collapses onto a universal curve for both soft and hard nanoparticles when plotted against a confinement parameter, defined as ID/R g , which accounts for tracer penetration into the brush. These experimental results provide new insights into the fundamental principles required to construct predictive models of polymer dynamics in nanocomposites.

Because of their shape anisotropy, nanorods are attractive components for creating functional polymer nanocomposites. In many cases, this anisotropy is the basis of physical properties that are distinct from those obtained from isotropic particles, such as nanospheres. For instance, the shape of gold nanorods makes them ideal candidates for applications involving the manipulation of incident light and sensitive molecular spectroscopy due to enhanced polarizability of the particle. On the other hand, semiconducting nanorods, such as those composed of CdSe, have shown promise in polymer-based photovoltaic devices as sites for electron transport and charge transfer. In this Perspective, we motivate the fabrication of functional polymer nanocomposites based specifically upon the inclusion of nanorods over other nanoparticle shapes and discuss ways in which the anisotropy of the individual nanoparticles enhances assembly in and the properties of polymer nanocomposites in comparison to spherical nanoparticles. We briefly summarize methods to successfully disperse and organize nanorods within polymers and discuss applications of polymer nanocomposites involving sensing, energy harnessing, and mechanical enhancement. Finally, we comment on unresolved issues for fabricating nanorod-based polymer composites and suggest topics warranting future investigation.

Small-angle neutron scattering was used to investigate poly(N-isopropylacrylamide) (PNIPAM) polymer solutions in d-water/d-ethanol mixtures. A wide poor-solvent region was observed for mixtures near 60% d-water/40% d-ethanol mixture. Spinodal lines were determined, permitting a mapping of the mixing/demixing regions of the phase diagram which comprises two main branches: the left branch (with mostly d-ethanol) where phase separation occurs upon cooling (UCST) and the right branch (with mostly d-water) where phase separation occurs upon heating (LCST). The ternary random phase approximation model was used to analyze SANS data. Three Flory–Huggins interaction parameters (PNIPAM/d-water, PNIPAM/d-ethanol and d-water/d-ethanol) were obtained. These display the reassuring 1/T behavior but show strong dependence on d-water/d-ethanol fraction. The conformation of polymer chains was determined by monitoring of the radius of gyration. Chains tend to swell with increasing temperature except close to the boundary of the left branch of the phase diagram (40% d-water) where they are observed to shrink.