Chitosan-based hydrogels: Influence of crosslinking strategy on rheological properties.

The effect of two crosslink strategies on the preparation of chitosan-based covalent hydrogels was investigated employing the widely used thiol-ene reaction. This versatile "click" chemistry can be activated either photochemically or thermochemically. Initially, well-purified chitosan (CS, DA ∼4 %, Mw ∼580 kg mol-1) was separately functionalized with vinyl (CS-ene) or thiol (CS-SH) groups in aqueous media. Subsequently, two strategies were compared where thiol-ene reaction occurs respectively between: (S1) modified chitosans CS-ene and CS-SH, in a polymer - polymer strategy, and (S2) CS-ene and di(ethylene glycol) dithiol (dEG-(SH)2), in a polymer - molecule strategy. Both crosslinking strategies were evaluated through rheological measurements, starting with entangled chitosan solutions. The difference in diffusion of functional groups, whether attached to polymer chains or to free molecules, leads to faster gelation kinetics with S2. Consequently, stronger gels were obtained with S2, where the modulus was connected with the degree of functionalization, while S1 produced weaker gels closer to the percolation point, where crosslinked density was associated with the entanglement number derived from the initial concentration. Nevertheless, networks formed by both strategies were homogenous with minimal dissipative contributions to their rheological properties, indicating that structural defects are negligible.

Isostructural softening of vulcanized nanocomposites.

Following previous work evidencing that short poly-propylene glycol (PPG) chains incorporated into crude SBR/silica nanocomposites act as filler-network softeners without changing their structure, we propose in the present report to examine more operative cross-linked materials. We first evidence that the adsorption of PPG onto silica deactivates progressively the particle's catalytic effect on vulcanization, without perturbing however the cross-link density distribution that we investigate through multiple-quantum NMR. In addition, electron microscopy confirms that the silica structure is conserved after vulcanization and that it does not depend on the PPG content either. Composites containing various amounts of PPG can thus be seen as structurally identical, both from a matrix and filler point of view - which is confirmed by small and medium amplitude oscillation shear rheology showing strikingly identical viscoelastic properties. The PPG signature only appears above 100% in tensile deformation where it is observed to soften dramatically the filler network. Our discovery makes it consequently possible to decorrelate the mechanical behavior of reinforced rubbers under normal conditions of use and urgent needs of energy dissipation.

Conformation of Tunable Nanocylinders: Up to Sixth-Generation Dendronized Polymers via Graft-Through Approach by ROMP.

Well-defined dendronized polymers (denpols) bearing high-generation dendron are attractive nano-objects as high persistency provides distinct properties, contrast to the random coiled linear polymers However, their syntheses via graft-through approach have been very challenging due to their structural complexity and steric hindrance retarding polymerization. Here, we report the first example of the synthesis of poly(norbornene) (PNB) containing ester dendrons up to the sixth generation (G6) by ring-opening metathesis polymerization. This is the highest generation ever polymerized among dendronized polymers prepared by graft-through approach, producing denpols with molecular weight up to 1960 kg/mol. Combination of size-exclusion chromatography, light scattering, and neutron scattering allowed a thorough structural study of these large denpols in dilute solution. A semiflexible cylinder model was successfully applied to represent both the static and dynamic experimental quantities yielding persistent length (l p), cross-sectional radius (R cs), and contour length (L). The denpol persistency seemed to increase with generation, with l p reaching 27 nm (Kuhn length 54 nm) for PNB-G6, demonstrating a rod-like conformation. Poly(endo-tricycle[4.2.2.0]deca-3,9-diene) (PTD) denpols exhibited larger persistency than the PNB analogues of the same generation presumably due to the higher grafting density of the PTD denpols. As the dendritic side chains introduce shape anisotropy into the denpol backbone, future work will entail a study of these systems in the concentrated solutions and melts.

Isostructural softening of the filler network in SBR/silica nanocomposites.

A new formulation of the widely used nanocomposites based on SBR (ca. 250 kg mol-1) and fractal silica fillers is proposed by substituting the usual covering and coupling agents with short chains (4 kg mol-1) of polypropylene glycol (PPG). We study in a systematic way the structural evolution and the changes in the linear and non-linear mechanical properties of two series of samples varying: (i) the silica volume fraction (Φsi = 0, 5, 10 and 15 vol%) in PPG-free samples and (ii) the amount of PPG for a given silica content Φsi = 15 vol%. While the first series is used as a reference, showing expected trends (e.g. the enhancement of the plateau modulus), the second series reveals in contrast, a surprising PPG insensitivity, both in terms of the filler structure (investigated by means of SAXS, SEM and TEM) and properties "at rest" (linear rheology). However, increasing the strain amplitude (both in shear and tensile tests) discloses the great effect of the oligomers, opening possibly the way to a fruitful decorrelation between the low and high deformation performances of tires. Although this study is limited to the investigation of uncrosslinked materials, it will be extended to more operative industrial formulations in due course.

Facile Synthesis of Chitosan-Based Hydrogels and Microgels through Thiol-Ene Photoclick Cross-Linking.

Polysaccharide-based microgels are effective vectors for the delivery of biopharmaceutics and functional components in tissue engineering due to their bioactivity and biocompatibility. Currently, the synthesis of chemically cross-linked microgels typically requires long reaction times and a high-energy input and are low yielding due to low volumes of the water phase used. Herein, we report the synthesis of norbornene-derived chitosan (CS-nbn-COOH), which can undergo rapid gelation in the presence of a thiolated cross-linker through the highly efficient thiol-ene photoclick reaction. This water-soluble photo-cross-linkable derivative, synthesized on scale via a single step from native chitosan and commercially available carbic anhydride, represents the first example of a norbornene-functionalized CS to the best of our knowledge. Microgels with controlled cross-linking densities and diameters varying between 100 and 400 nm were obtained via a low-energy water-in-oil nanoemulsion templating method at room temperature, with photo-cross-linking initiated in a flow reactor powered with a domestic UV-A lamp, a method that is suitable for the scale-up synthesis of the microgels. We also demonstrate that the resulting microgels were nontoxic to human dermofibroblasts (HDF) cell lines and that residual norbornene groups could be reacted in a late stage through tetrazine ligation, highlighting the potential of these microgels as scaffolds for functional nanomaterials with biomedical applications.

Transforming frozen self-assemblies of amphiphilic block copolymers into dynamic pH-sensitive micelles.

The self-assembly in water of an amphiphilic P(nBMA(50%) -stat-DMAEMA(50%) )(100)-b-PDMAEMA(235) diblock copolymer based on hydrophilic dimethylaminoethylmethacrylate (DMAEMA) units and hydrophobic n-butylmethacrylate (nBMA) ones is reported. DMAEMA units have been incorporated into the hydrophobic block of this copolymer to moderate its hydrophobic character. Light scattering experiments revealed the formation of micelles whose apparent aggregation number varied reversibly with the ionization degree of the DMAEMA units. Incorporating hydrophilic units into the hydrophobic block of an amphiphilic block copolymer is thus a way to generate dynamic aggregates in aqueous medium. As this strategy was also successful using other types of hydrophilic units, we believe it to be universal.