The aggregation properties of a standard conjugated polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), in two distinct solvents (chloroform and toluene) and a range of polymer concentrations (c = 0.1-3 mg/mL) have been unequivocally resolved using combined dynamic and static light scatterings (DLS/SLS). The prime challenges for analyzing this peculiar, practically important, solution system arise from the wide size distribution and unknown aggregate morphology, as well as pronounced interferences between translational and internal motions of aggregate clusters of considerably varying size. To cope with these central difficulties, we propose a self-consistent formulation for analyzing the dynamic structure factor in DLS experiment by extending an existing theory on free-draining bead-spring chains that explicitly accounts for internal fluctuations, along with two candidate form factors on Gaussian coil and rigid sphere, respectively, serving as two limiting cases to be discriminated in combined DLS and SLS measurements. Given that no accessibility to ultrasmall angular resolutions is a prerequisite, the suggested protocol can readily be carried out in conventional light-scattering apparatus. The present analyses unanimously support the rigid-sphere form factor in describing the entire set of light-scattering data on MEH-PPV solutions, differing from early small-angle neutron/X-ray scattering interpretations suggesting certain 2D fractal structures for the aggregation network. Scrutiny into the interior dynamics of aggregate clusters further disclosed that the segmental motions are noticeably more suppressed than for usual, nonaggregated polymer solutions, and no existing theories based on the bead-spring picture can yet capture the observed scaling behavior as manifested by the present data. Accordingly, we report several first-revealed properties of MEH-PPV solutions on the aggregate morphology, the size distribution (and mean size), mean aggregation number, and interior segmental dynamics, which serve as valuable information for linking solution properties with those for dried thin films in contemporary applications with conducting conjugated polymers.

The linear viscoelastic (LVE) spectrum of a soft colloidal glass is accessed with the aid of a time−concentration superposition (TCS) principle, which unveils the glassy particle dynamics from in-cage rattling motion to out-of-cage relaxations over a broad frequency range 10 −13 rad/s < ω < 10 1 rad/s. Progressive dilution of a suspension of hairy nanoparticles leading to increased intercenter distances is demonstrated to enable continuous mapping of the structural relaxation for colloidal glasses. In contrast to existing empirical approaches proposed to extend the rheological map of soft glassy materials, i.e., time−strain superposition (TSS) and strain−rate frequency superposition (SRFS), TCS yields a LVE master curve that satisfies the Kramers−Kronig relations which interrelate the dynamic moduli for materials at equilibrium. The soft glassy rheology (SGR) model and literature data further support the general validity of the TCS concept for soft glassy materials.

We report on the dynamics of entangled polymers in polymer–nanoparticle composites (PNCs) with moderate and high particle loadings. Using composites comprised of poly­(ethylene glycol) (PEG)-tethered silica (SiO2) nanoparticles and poly­(methyl methacrylate) (PMMA), we show by means of small-angle X-ray scattering (SAXS) analysis that the nanoparticles are uniformly dispersed over a range of particle concentrations. From oscillatory shear rheology measurements, we find that the time–temperature superposition (TTS) principle holds for these SiO2-PEG/PMMA PNCs up to a particle concentration ϕG ≈ 11% where the materials begin to exhibit soft glassy rheological properties. At particle concentrations below ϕG, we take advantage of TTS to create dynamic maps for PNCs that span time scales ranging from fast segmental motions to slow terminal relaxation of polymer chains. These maps reveal that at low ϕ and depending on the host polymer (PMMA) molecular weight (M w), hairy nanoparticles may either speed up or slow down terminal relaxation of the host. In contrast, at high ϕ, nanoparticles slow down polymer dynamics on intermediate and long time scales, irrespective of the host polymer M w. The slowdown coincides with particle concentrations at which the mean interparticle spacing lies below the equilibrium tube diameter of the entangled PMMA host and is thought to reflect the onset of confinement dynamics of polymer chains in PNCs. Studies of PNCs in the confinement regime reveal important analogies between glassy attributes of 3D-confined polymer chains in nanocomposites with those of polymer thin films supported on attractive substrates.

Above a critical surface chemistry-dependent particle loading associated with nanoscale interparticle spacing, ligand−ligand interactionsboth electrostatic and stericcome into play and govern the structure and dynamics of charged oligomer-functionalized nanoparticle suspensions. We report in particular on the structure, ion transport, and rheology of suspensions of nanoparticle salts created by cofunctionalization of silica particles with tethered sulfonate salts and oligomers. Dispersion of the hairy ionic particles into medium and high dielectric constant liquids yields electrolytes with unique structure and transport properties. We find that electrostatic repulsion imparted by ion dissociation can be tuned to control the dispersion state and rheology through counterion size (i.e., Li + , Na + , and K + ) and dielectric properties of the dispersing medium. Analysis of small-angle X-ray scattering (SAXS) structure factors and the mechanical modulus shows that when the interparticle spacing approaches nanometer dimensions, weakly entangled anchored ligands experience strong and long-lived topological constraints analogous to those normally found in well-entangled polymeric fluids. This finding provides insight into the molecular origins of the surprisingly similar rubbery plateau moduli observed in hairy nanoparticle suspensions and entangled polymers of the same chemistry as the tethered ligands. Additionally, we find that a time−composition superposition (TCS) principle exists for the suspensions, which can be used to substantially extend the observation time over which dynamics are observed in jammed, soft glassy suspensions. Application of TCS reveals dynamical similarities between the suspensions and entangled solutions of linear polymer chains; i.e., a hairy particle trapped in a cage appears to exhibit analogous dynamics to a long polymer chain confined to a tube.

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