Viscoelasticity-Induced Onset of Slip at the Wall for Polymer Fluids.

The progressive onset of slip at the wall, which corresponds to a slip length increasing with the solicitation time before reaching a plateau, has been investigated for model viscoelastic polymer solutions, allowing one to vary the longest relaxation time while keeping constant solid-fluid interactions. A hydrodynamic model based on a Maxwell fluid and the classical Navier's hypothesis of a linear response for the friction stress at the interface fully accounts for the data. In the limit of the linear viscoelasticity of the fluid, we could postulate a Newtonian response for the interfacial friction coefficient, reflecting the local character of solid-liquid friction mechanisms. Deviations between the experiments and our model are observed when the fluid is far from its linear viscoelastic behavior.

Temperature-Controlled Slip of Polymer Melts on Ideal Substrates.

The temperature dependence of the hydrodynamic boundary condition between a polydimethylsiloxane melt and two different nonattractive surfaces made of either an octadecyltrichlorosilane self-assembled monolayer or a grafted layer of short polydimethylsiloxane chains has been characterized. We observe a slip length proportional to the fluid viscosity. The temperature dependence is deeply influenced by the surfaces. The viscous stress exerted by the polymer liquid on the surface is observed to follow exactly the same temperature dependences as the friction stress of a cross-linked elastomer sliding on the same surfaces. Far above the glass transition temperature, these observations are rationalized in the framework of a molecular model based on activation energies: increase or decrease of the slip length with increasing temperatures can be observed depending on how the activation energy of the bulk viscosity compares to that of the interfacial Navier's friction coefficient.

Supramolecular Hydrogel Based on pNIPAm Microgels Connected via Host⁻Guest Interactions.

In this work, host⁻guest supramolecular hydrogels were prepared from poly(N-isopropylacrylamide) (pNIPAm) microgels utilizing electrostatic and host/guest self-assembly. First, pNIPAm microgels bearing a poly(acrylic acid) (pAAc) shell were coated with positively charged ß-cyclodextrin polymers. Addition of adamantane-substituted dextrans (Dex-Ada) allowed us to establish interparticle connections through ß-cyclodextrin-adamantane (ßCD-Ada) inclusion complex formation, and thus to prepare hierarchical hydrogels. Under the conditions of hydrogel formation, close contact between the microgels was ensured. To the best of our knowledge, this is the first example of doubly crosslinked microgels prepared by noncovalent crosslinking via host⁻guest interactions. The prepared macrogels were studied with rheology, and fast mechanical response to temperature variation was found. Furthermore, the hydrogels exhibit fully reversible temperature-induced gel⁻sol transition at the physiological temperature range (37⁻41 °C), due to the synergetic effect between shrinking of the microgels and dissociation of ßCD-Ada crosslinks at higher temperatures. This opens up attractive prospects of their potential use in biomedical applications.

Host-guest interaction and structural ordering in polymeric nanoassemblies: Influence of molecular design.

Host-guest nanoassemblies made from spontaneous self-association of host and guest polymers in aqueous solutions have been studied. The specific motivation behind this work was to clarify the impact of the molecular design of the polymers on the interactions between them and on the inner structure of the resulting nanoassemblies. The polymers were composed of a dextran backbone, functionalized with either pendant ß-cyclodextrin (CD) or adamantyl (Ada). Those groups were connected to the backbone either directly or with hydrophilic polyethylene glycol (PEG) spacers. To study the impact of those spacers we have proposed a synthetic pathway to new guest polymers. The latter relied on the use of thiol-substituted dextrans as a scaffold, which is subsequently transformed into PEG-Ada grafted guest polymers via nucleophile-mediated thiol-click reaction. Surface plasmon resonance (SPR) studies evidenced strong mutual affinities between the host and guest polymers and showed that the stoichiometry was close to the ideal one (CD/Ada = 1/1) when PEG spacers were introduced. The structure of the nanoassemblies was studied by a combination of dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). The nature of the individual host or guest polymers has a strong impact on the size and internal structure of the resulting nanoassemblies. The presence of PEG spacers in the polymers led to smaller and less compact nanoassemblies, as evidenced by their large correlation length values (4-20nm compared to 2nm without PEG spacers). At the same time, all types of nanoassemblies appear to have radial density distribution with denser cores and pending polymer chains at the periphery. This study, centered on the influence of the molecular design on the host-guest interactions and structural ordering in polymeric nanoassemblies, will help to tailor host-guest nanoassemblies with attractive drug delivery profiles.

Cyclodextrin-Mediated Hierarchical Self-Assembly and Its Potential in Drug Delivery Applications.

Hierarchical self-assembly exploits various non-covalent interactions to manufacture sophisticated organized systems at multiple length scales with interesting properties for pharmaceutical industry such as possibility of spatially controlled drug loading and multiresponsiveness to external stimuli. Cyclodextrin (CD)-mediated host-guest interactions proved to be an efficient tool to construct hierarchical architectures primarily due to the high specificity and reversibility of the inclusion complexation of CDs with a number of hydrophobic guest molecules, their excellent bioavailability, and easiness of chemical modification. In this review, we will outline the recent progress in the development of CD-based hierarchical architectures such as nanoscale drug and gene delivery carriers and physically cross-linked supramolecular hydrogels designed for a sustained release of actives.

Bifunctionalized dextrans for surface PEGylation via multivalent host-guest interactions.

The main goal of this work was to develop a supramolecular chemistry strategy to decorate interfaces with polyethylene glycol (PEG) grafts. A series of novel bifunctionalized dextrans, bearing 40-60 PEG pending chains and 12-24 hydrophobic adamantyl groups, have been prepared by copper(I)-catalyzed azide-alkyne cycloaddition. Their binding properties toward native ßCD and ßCD polymers were characterized both in solution and at interface using isothermal titration microcalorimetry and surface plasmon resonance. The polymers were found to form stable layers onto neutral and positively charged ßCD-polymer films pre-adsorbed on gold surfaces, due to multivalent interpolymer host-guest interactions. The thickness and stability of the layers could be tuned by varying the ratio between the degrees of substitution by PEG and adamantyl groups.