Understanding the mechanisms involved in shape memory starch: Macromolecular orientation, stress recovery and molecular mobility

cited By 20International audienceThe effect of a deformation near and above the glass transition on macromolecular orientation, residual stress, and molecular mobility was investigated on amorphous stimuli-responsive starch. The recovery stress, linked to the residual stress in the temporary shape, evolves linearly with the macromolecular orientation. Increasing the deformation temperature decreases the residual stress and the macromolecular orientation. This also leads to a higher glass transition temperature induced by a decrease in molecular mobility. The local structures potentially involved in orientation and stress recovery and their behavior during deformation at different temperatures are discussed. © 2011 American Chemical Society

Linear Rheology of Bis-Urea Functionalized Supramolecular Poly(butylacrylate)s: Part I – Weak Stickers

International audienceWe investigated the linear viscoelastic properties in the melt of a series of nearly monodisperse poly(n-butyl acrylate)(PnBA) chains center-functionalized with a bis-urea sticker group, able to self-associate by quadruple hydrogen bonding. All materials are viscoelastic liquids at 40°C and their Newtonian viscosity varies from 100 to 5000Pa.s. However we show clearly that the viscosity changes non monotonously going through a minimum for a molecular weight (Mn) of 20 kg/mol and a sticker density of 1.1 wt%. We found two regimes: For Mn 20 kg/mol, the viscosity increases with Mn, the rheology is controlled by the polymer dynamics and the stickers simply increase the terminal relaxation time relative to unfunctionalized PnBA of the same Mn, as expected from sticky reptation theory. Despite clear evidence of sticker-sticker interactions by FTIR, none of the materials self-assemble at 20°C into structures with a long range order detectable by SAXS

Effects of Multifunctional Cross-linkers on Rheology and Adhesion of Soft Nanostructured Materials

International audienceWe investigate the nanostructure, the rheology and the adhesion of soft supramolecular materials elaborated by blending monofunctional and multifunctional poly(isobutene) (PIB) chains. Monofunctional PIB chains (PIBUT) are linear and unentangled polymer chains (Mn ≈3kg/mol) functionalized in the middle by a bis-urea interacting moiety, able to self-associate by four hydrogen bonds. Covalent coupling of monofunctional PIB allows us to synthesize longer chains bearing two or three interacting moieties. These chains are then added to monofunctional PIB to prepare blends containing up to 10% of multifunctional PIB (M-PIBUT). The influence of M-PIBUT on the supramolecular nanostructure, which results from the self-assembly of stickers, is studied by Atomic Force Microscopy and Small Angle X-ray Scattering at room temperature. Multifunctional and monofunctional chains are shown to interact with each other to form bundles of rod-like aggregates. The consequences of these interactions on the rheology of the blends were studied by shear tests in the linear and non linear regimes, below and above the order-disorder transition temperature. A pronounced strengthening effect of M-PIBUT is observed at room temperature: the supramolecular blends become more elastic and are more resistant to creep with increasing concentration of M-PIBUT. The effects of M-PIBUT on the nanostructure and the rheology suggest that M-PIBUT, which can link with more than one supramolecular aggregate, plays the role of a physical cross-linker. The impact of these supramolecular cross-linkers on the adhesion of the blends is studied by probe-tack tests and discussed by analyzing the in-situ deformation through the debonding images

Modeling of segmented pure polyurethane electrostriction behaviors based on their nanostructural properties

International audiencePolyurethane (PU) exhibit very high electromechanical activity, and has a great interest for a wide range of transducer and actuator applications. It has been recently pointed out that this strong electrostriction may result from the phase separation. In the present work, a model taking account the PU nanostructure is presented. In order to validate this model, three PUs of similar compositions but different soft segment fractions have been characterized by DSC, TEM, AFM and SAXS and their electrostriction properties have been measured. When taking into account the diameter of the hard domains (HD) and the HD-HD distance, it is found that the model proposed well reproduces the experimental electrostriction properties of these phase-separated PUs