Temperature-Controlled Diffusion in PNIPAM-Modified Silica Inverse Opals

We report a new strategy for the preparation of well-defined and mechanically stable porous nanostructures with tunable porosity. Silica inverse opals, which are known as a model system for a porous periodic nanostructure, were grafted with brushes of the thermoresponsive poly­(<i>N</i>-isopropylacrylamide) grown via atom transfer radical polymerization. By tuning the temperature, the swelling state of the brush layer is reversibly altered, and with this we were able to control the overall porosity of the system and, thus, the mobility of small penetrants. Fluorescence correlation spectroscopy, a method combining single molecule sensitivity with small probing volume (<1 μm³), was used to directly monitor and quantify in situ the changes in the penetrants’ mobility

Media 1: Effect of the domain shape on noncollinear second-harmonic emission in disordered quadratic media

Originally published in Optics Express on 16 December 2013 (oe-21-25-31462

Soft Elastomers via Introduction of Poly(butyl acrylate) “Diluent” to Poly(hydroxyethyl acrylate)-Based Gel Networks

We report a new strategy for the synthesis of stable and well-defined supersoft elastomers. First, four-arm star-like polymers, poly­(trimethylsilyloxyethyl acrylate), were synthesized and cross-linked to form a relatively uniform polymer gel network. Second, short poly­(<i>n</i>-butyl acrylate) side chains were grown from the initiating sites along the network backbone via atom transfer radical polymerization. These soft side chains act as low molecular weight “diluent” that “swells” the cross-linked polyHEA network, but cannot be leached from it. Using this strategy, materials with shear modulus less than 5 kPa were prepared and the effect of grafted side chain fraction on their mechanical properties was explored

From Single Chains to Aggregates, How Conjugated Polymers Behave in Dilute Solutions

Conjugated polymers offer unique combination of easily tailored mechanical, electrical and optical properties that makes them perfect materials for the preparation of various devices such as light-emitting diodes, photovoltaic cells or field-effect transistors. However, the design and fabrication of such devices in a controlled and reproducible way are possible only if the behavior and the properties of individual polymer chains are well understood. One major problem in this respect is that aggregation often occurs even in dilute solutions and prevents the single polymer chain studies. To address this issue, in this work we employed fluorescence correlation spectroscopy (FCS) to study the behavior of a model conjugated polymer, poly­(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in several commonly used solvents. The very high sensitivity of FCS allowed measurements in ultradilute solutions and thus unambiguous determination of the hydrodynamic radius of single polymer chains. The solvent quality for MEH-PPV was then quantitatively evaluated from the measured logarithmic scaling of the single chain hydrodynamic radius versus the polymer molecular weight. Scaling exponents of 0.40, 0.41, and 0.43 were found in toluene, chloroform and 1,2-dichlorobenzene, respectively. These values are well below the θ-condition, emphasizing poor solvent quality for MEH-PPV, despite the fact that all studied solvents are commonly regarded as “good” solvents. In addition, by investigating the aggregation behavior of MEH-PPV at higher polymer concentrations, we found a clear relation between aggregates size and solvatochromism that indicates more extended chain conformation in larger aggregates.

Aggregation Behavior of Amphiphilic p(HPMA)-<i>co</i>-p(LMA) Copolymers Studied by FCS and EPR Spectroscopy

A combined study of fluorescence correlation spectroscopy and electron paramagnetic resonance spectroscopy gave a unique picture of p­(HPMA)-<i>co</i>-p­(LMA) copolymers in aqueous solutions, ranging from the size of micelles and aggregates to the composition of the interior of these self-assembled systems. P­(HPMA)-<i>co</i>-p­(LMA) copolymers have shown high potential as brain drug delivery systems, and a detailed study of their physicochemical properties can help to elucidate their mechanism of action. Applying two complementary techniques, we found that the self-assembly behavior as well as the strength of hydrophobic attraction of the amphiphilic copolymers can be tuned by the hydrophobic LMA content or the presence of hydrophobic molecules or domains. Studies on the dependence of the hydrophobic lauryl side chain content on the aggregation behavior revealed that above 5 mol % laury side-chain copolymers self-assemble into intrachain micelles and larger aggregates. Above this critical alkyl chain content, p­(HPMA)-<i>co</i>-p­(LMA) copolymers can solubilize the model drug domperidone and exhibit the tendency to interact with model cell membranes

Synergistic Growth of Giant Wormlike Micelles in Ternary Mixed Surfactant Solutions: Effect of Octanoic Acid

The synergistic growth of giant wormlike micelles in ternary mixed solutions composed of an anionic surfactant (sodium laurylethersulfate, SLES), a zwitterionic surfactant (cocamidopropyl betaine, CAPB), and octanoic acid (HC8) is studied. Rheological data and their analysis in terms of Cole–Cole plots and micellar characteristic times are presented, and the micellar structures behind the observed rheological behavior are revealed by cryo-TEM micrographs. The surfactant composition is fixed near the maximal micelle size of the binary SLES + CAPB system, whereas the concentration of HC8 is varied. At a given HC8 concentration, the viscosity of the ternary micellar solutions exhibits a very high and sharp peak. Polarized-light optical microscopy indicates that all investigated solutions are isotropic rather than liquid-crystalline. The cryo-TEM imaging shows complex phase behavior: wormlike micelles to the left of the peak, giant entangled wormlike micelles at the peak, and long wormlike micelles coexisting with multiconnected micellar aggregates to the right of the peak. The formation of multiconnected micelles leads to a drop in viscosity at the higher concentrations. The results contribute to a better understanding of the structure–rheology relations in micellar surfactant solutions and could be useful for controlling the properties of formulations in personal-care and house-hold detergency

Molecular Probe Diffusion in Thin Polymer Films: Evidence for a Layer with Enhanced Mobility Far above the Glass Temperature

We studied experimentally the influence of interfaces on the dynamics in thin polymer films at temperatures far above the glass temperature (<i>T</i>_g + 80 °C). Polyisoprene (PI) was employed as a model system. We examined glass substrate supported films with thicknesses (<i>d</i>) spanning the range from 10 μm to 10 nm that correspond to <i>d</i>/<i>R</i>_g from 400 to 1, where <i>R</i>_g is the polymer radius of gyration. We employed fluorescence correlation spectroscopy (FCS) to monitor the translational diffusion of small fluorescent tracer molecules, dispersed at nanomolar concentrations in the PI matrix. In thick films, a single diffusion process correlated to the bulk segmental dynamics of the matrix polymer was present. However, when the film thickness was smaller than the normal dimension of the FCS observation volume, a second, faster diffusion process appeared, reflecting enhanced segmental dynamics near the free surface. Our results provide direct experimental evidence for the existence of a layer with enhanced mobility near the free surface of supported PI films at temperatures as high as 80 °C above the bulk <i>T</i>_g

Dynamics in Stimuli-Responsive Poly(<i>N</i>‑isopropylacrylamide) Hydrogel Layers As Revealed by Fluorescence Correlation Spectroscopy

We employ fluorescence correlation spectroscopy (FCS) to study the translational mobility of molecular tracers in stimuli-responsive grafted poly­(<i>N</i>-isopropylacrylamide) (PNiPAAm) hydrogels, under variable solvency conditions. Tracer–matrix interactions were tuned by selecting three different molecular tracers. In contrast to a noninteracting tracer (Alexa 647), the mobility of a weakly (Alexa 488) and a strongly interacting (Rhodamine 6G) tracer deviates from a simple single Fickian diffusion. In addition to pure crowding effects, the mobility of both Alexa488 and Rhodamine 6G is influenced by tracer–polymer interactions. We interpret the observed trends in tracer mobility in terms of the interplay between Coulombic repulsions and short-range attractions. Although tracer dynamics and hydrogel swelling ratio are interdependent properties, their relation turns out to be nontrivial and does not allow predictions of tracer dynamics on the basis of polymer structural information. Hence, a universal scaling behavior is not possible, due to tracer–polymer interactions

Fluorescence Correlation Spectroscopy Monitors the Hydrophobic Collapse of pH-Responsive Hairy Nanoparticles at the Individual Particle Level

Fluorescence correlation spectroscopy (FCS) was applied to directly monitor the hydrophobic collapse of pH-responsive hairy nanoparticles at the individual particle level. To this end, fluorescent nanoparticles (hydrodynamic radius 20 nm) with polystyrene core and poly­(<i><i>N</i></i>,<i><i>N</i></i>-diethylaminoethyl methacrylate) (PDEA) shell were prepared and used as a model system. Dynamic light scattering and turbidity measurements showed that the hydrophobic collapse of the hairs at high pH values is associated with strong interparticle aggregation that hinders determination of individual particles size. However, at the ultralow concentrations assessable by FCS (less than one particle per femtoliter) the aggregation was prevented. Thus, the pH-induced change in the particles size caused by the swelling or the collapse of the PDEA hairs was systematically measured and compared with that of individual freely diffusing PDEA chains under similar conditions

Selective Interfacial Olefin Cross Metathesis for the Preparation of Hollow Nanocapsules

The first synthesis of hollow nanocapsules with an aqueous core via olefin cross metathesis is presented. The reaction was tailored such that it proceeds selectively at the oil–water interface of aqueous nanodroplets in an inverse miniemulsion. The cross metathesis takes place between an acrylated polysaccharide and unsaturated organophosphates under mild conditions. This general protocol allows the synthesis of biocompatible and polyfunctional nanocapsules via the bioorthogonal olefin metathesis, thus generating a highly versatile methodology for the design of future materials for biomedical applications but also for materials science. Functionalization of the nanocapsules was demonstrated with fluorescent labels, which can be attached to the pendant phosphoester either within the cross-linker, exploiting the versatility of the phosphorus chemistry, or via coupling to the capsules’ surface