Tuning the thermoreversible temperature domain of PTMC-based networks with thermosensitive links concentration.

In this work, thermoreversible poly(trimethylene carbonate) (PTMC) based networks with different crosslinking densities were obtained by Diels-Alder (DA) reaction between furan-functionalized PTMC precursors and a bismaleimide. Furan-grafted PTMC with various functionalities determined by 1H-NMR analyses were prepared from telechelic PTMC oligomer, glycerol, 4,4'-methylenebis(cyclohexyl isocyanate) (H12MDI) and furfuryl alcohol. The formation of network structures by DA reaction between furan and maleimide groups were proved by Fourier-transform infrared spectroscopy (FT-IR). Although both exo and endo DA adduct forms exist, the thermally more stable exo form dominates. The thermoreversibility of networks was evidenced by FT-IR, solubility, differential scanning calorimetry (DSC) and rheology experiments at different temperatures. By increasing furan functionality or node concentration, denser and stiffer networks could be formed with higher Young's modulus and true stress at break in tensile tests, as well as higher crossover temperature, which indicates a nominal transition from elastic behavior to viscous state. The disruption of networks was found to occur in high temperature ranges from 130 to 160 °C, depending on their crosslinking density.

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.

Viscoelastic behaviour of cellulose acetate/triacetin blends by rheology in the melt state.

The viscoelastic behaviour of cellulose acetate with a degree of substitution (DS) of 245 plasticized by triacetin was studied at short times by dynamic oscillatory measurements. Two distinct regimes and unexpected scaling behaviour according to plasticizer content were highlighted. The dynamics of chains and their structural organization are not modified up to 35 wt% of triacetin. The rheological behaviour is led by a constant correlation length corresponding to the distance between strong intermolecular interactions subsisting in the melt state at high temperature even in the presence of plasticizer. This particular structure involves the apparition of strain hardening effects during uniaxial extensional flow tests and an important elasticity corresponding to the apparition of a Weissenberg effect at really low shear rates during shear sweeps. Intramolecular hydrogen bonds are responsible of the high rigidity of cellulose acetate chains. Plasticized cellulose acetate in the melt state belongs to the class of associating polymers and its rheological behaviour is mainly led by stickers.

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.

Mediating gel formation from structurally controlled poly(electrolytes) through multiple "head-to-body" electrostatic interactions.

Tuning the chain-end functionality of a short-chain cationic homopolymer, owing to the nature of the initiator used in the atom transfer radical polymerization (ATRP) polymerization step, can be used to mediate the formation of a gel of this poly(electrolyte) in water. While a neutral end group gives a solution of low viscosity, a highly homogeneous gel is obtained with a phosphonate anionic moiety, as characterized by rheometry and diffusion nuclear magnetic resonance (NMR). This novel type of supramolecular control over poly(electrolytic) gel formation could find potential use in a variety of applications in the field of electro-active materials.