Nanostructure and mechanical properties of polybutylacrylate filled by grafted silica particles

International audienceWe investigate the nanostructure and the linear rheological properties of polybutylacrylate (PBA) filled with Sto¨bersilica particles grafted with PBA chains. The silica volume fractions range from 1.8 to 4.7%. The nanostructure ofthese suspensions is investigated by small-angle neutron scattering (SANS), and we determine their spectromechanicalbehavior in the linear region. SANS measurements performed on low volume fraction composites show that the graftedsilica particles are spherical, slightly polydisperse, and do not form aggregates during the synthesis process. Thesecomposites thus constitute model filled polymers. The rheological results show that introducing grafted silica particlesin a polymer matrix results in the appearance of a secondary process at low frequency: for the lowest volume fractions,we observe a secondary relaxation that we attribute to the diffusion of the particles in the polymeric matrix. Byincreasing the silica volume fraction up to a critical value, we obtain gellike behavior at low frequency as well asthe appearance of a structure factor on the scattering intensity curves obtained by SANS. Further increasing the silicaparticle concentration leads to composites exhibiting solidlike low-frequency behavior and to an enhanced structurepeak on the SANS diagrams. This quantitative correlation between the progressive appearance of a solidlike rheologicalbehavior, on one hand, and a structure factor, on the other hand, supports the idea that the viscoelastic behavior offilled polymers is governed by the spatial organization of the fillers in the matrix

Towards an elucidation of the three-dimensional structures of complex and flexible biological materials

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Structure of arabinogalactan-protein from Acacia gum: From porous ellipsoids to supramolecular architectures

International audienceThe structure of the arabinogalactan-protein (AGP) fraction of the gum exudate of Acacia senegal (gum Arabic) isolated from hydrophobic interaction chromatography was investigated using HPSEC-MALLS, small angle neutron scattering and TEM observations. Literature reported that the AGP structure of gum Arabic adopts a very compact conformation in solution due to the attachment of short arabinoside side chains and much larger blocks of carbohydrate to the polypeptidic backbone. The present study revealed that AGP in solution had a weight average molecular weight Mw of 1.86 × 106 g mol−1 and a radius of gyration Rg of 30 nm. In addition, two exponent values were identified in the Rg, [η], Rh and ρ vs. Mw relationships highlighting two types of conformations depending on the molecular weight range considered: a low molar mass population with long-chain branching and a compact conformation and a high molar mass population with short-chain branching and an elongated conformation. AGP would behave in solution as a branched or hyper-branched polymer with conformations ranging from globular to elongated shape depending on the size of the carbohydrate branches. Small angle scattering form factor revealed an elongated average conformation corresponding to a triaxial ellipsoid while inverse Fourier transform of the scattering form factor gave a maximum dimension for AGP of 64 nm. Transmission electron microscopy highlighted the existence of two types of flat objects with thicknesses below 3-5 nm, single particles with a more or less anisotropic spheroidal shape and aggregated structures with a more elongated shape. A remarkable feature of all particle morphologies was the presence of an outer structure combined to an inner more or less porous network of interspersed chains or interacting structural blocks, as previously found for the arabinogalactan (AG) main molecular fraction of Acacia gum. However, clear differences were observed in the density and morphology of the inner porous network, probably highlighting differences in the degree of branching. The existence of assembled AG as part of the AGP family was confirmed using TEM micrographs at high resolution. Fused AGP dimers, trimers, tetramers and multimers were also identified. These molecular assemblies questioned about the nature of interactions involved

Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: the self-similarity hypothesis of assembly from a common building block

Arabinogalactan (AG) and arabinogalactan-protein (AGP) fractions were treated enzymatically using several proteases in acidic (pH 4) and alkaline (pH 7) conditions in order to go deeper insight into the structure and conformations of the two main fractions of Acacia senegal gum. Endoproteinase Glu-C, pepsin and phosphatase acid were thus used in acidic conditions while subtilisin A, pronase, trypsin, papain and proteinase K were used in alkaline conditions to cleave protein moieties of the two fractions. Structures of AG and AGP were probed using HPSEC-MALLS, small angle neutron scattering and far-UV circular dichroism. Enzymes did not affect AG fraction structure whatever the pH conditions used, highlighting the inaccessibility of the peptide backbone and the remarkable stability of this fraction in acidic and alkaline conditions. This result was in agreement with the thin oblate ellipsoid model we previously identified for the AG fraction where the 43 amino-acid residues peptide sequence was supposed, based on spectroscopic methods, to be totally buried. Contrary to AG fraction, AGP protein component was therefore cleaved using enzymes in alkaline conditions, the absence of enzymatic efficiency in acidic conditions being probably ascribed to long range electrostatic repulsions occurring between negatively charged AGP and enzymes at pH 4. The decrease of AGP molecular weight after hydrolysis in alkaline conditions went from 1.79 x 10(6) gmol(-1) for control AGP to as low as 1.68 x 10(5) g mol(-1) for papain-treated AGP. The overall structure of the enzyme-treated AGPs was found to be surprisingly quite similar whatever the enzyme used and close, with however some subtle differences, to AG unit. A tri-axial ellipsoid conformation was found in enzyme-treated AGPs and the two main preferential distances identified in the pair distance distribution function would claim in favor of rod-like or elongated particles or alternatively would indicate the presence of two particles differing in dimensions. The secondary structures content of control and enzyme-treated AGPs were similar, highlighting both the high rigidity of the protein backbone and the overall symmetry of AGP. This conclusion was reinforced by the more compact structures found when AGP was intact compare to the more elongated structures found when AGP was enzymatically cleaved. Finally, the structural similarities found in enzyme-treated AGP together with the theoretical calculations to analytically probe the type of branching would suggest that AGP would be made of a self-similar assembly of two types of building blocks, the second being a five-fold repetition of the first one, for which palindromic amino acid sequence would ensure a self-ordering of carbohydrate moieties along the polypeptide chains. The cleavage would therefore lead to hydrolysed building blocks with similar secondary structures and conformations whatever the enzyme used

Flow-Induced Ordering in Cubic Gels Formed by P2VP-b-PEO-b-P(GME-co-EGE) Triblock Terpolymer Micelles: A Rheo-SANS Study

Reinicke S, Karg M, Lapp A, Heymann L, Hellweg T, Schmalz H. Flow-Induced Ordering in Cubic Gels Formed by P2VP-b-PEO-b-P(GME-co-EGE) Triblock Terpolymer Micelles: A Rheo-SANS Study. Macromolecules. 2010;43(23):10045-10054.Small-angle neutron scattering (SANS) measurements under steady shear were performed to determine the exact nature and degree of structural order within a hydrogel based on poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P2VP(56)-b-PEO410-b-P(GME(48)-co-EGE(48))) triblock terpolymer micelles. Previous static SANS measurements indicated the presence of a simple cubic (sc) or body centered cubic (bcc) packing. By exposing the sample to steady shear, different macroscopic structural transitions were induced, indicated by a stress plateau and a significant change of the 2D SANS patterns. A comparison of these 2D patterns with patterns from analogous systems reported in literature and theoretical predictions revealed the presence of a bcc structure. Furthermore, with increasing shear rate the structural alignment changes from a nonoriented state to an intermediate state consisting of polycrystalline bcc domains with weak preferential orientation, and finally to a highly aligned state in which twinned bcc domains exist. The [111] axis of the twinned bcc crystals is aligned in the direction of the shear flow, and the {110} slipping planes are preferentially aligned parallel to the shear plane (walls of the Couette cell). A minor fraction of twinned bcc domains, probably located in regions of lower shear velocity, i.e., close to the inner wall of the Couette cell (stator), is tilted by an angle of 90 degrees with respect to the shear plane

Formation and Self-Organization Kinetics of alpha-CD/PEO-Based Pseudo-Polyrotaxanes in Water. A Specific Behavior at 30 degrees C

alpha-Cyclodextrins (alpha-CDs) have the ability to form inclusion complexes with poly(ethylene oxide) (PEO) polymer chains. These pseudo-polyrotaxanes (PPRs) can be obtained by quenching an alpha-CD/PEO mixture in water from 70 degrees C down to a lower temperature (typically in the range from 5 to 30 degrees C) thanks to favorable interactions between alpha-CD cavities and PEO chains. Moreover, starting from a liquid alpha-CD/PEO mixture at a total mass fraction of 15% w/w at 70 degrees C, the formation of PPRs with time at a lower temperature induces a white physical gel with time, and phase separation is observed. We established that PPR molecules are exclusively found in the precipitated phase although unthreaded alpha-CD molecules and unthreaded PEO chains are in the liquid phase. At 30 degrees C, the physical gel formation is much slower than at 5 degrees C. At 30 degrees C, we established that, in a first step, alpha-CDs thread onto PEO chains, forming PPR molecules which are not in good solvent conditions in water. At a higher length scale, rapid aggregation of the PPR molecules occurs, and threaded alpha-CD-based nanocylinders form (cylinder length L 5.7 nm and cylinder radius R = 4.7 nm). At a higher length scale, alpha-CD-based nanocylinders associate in a Gaussian way, engendering the formation of precipitated domains which are responsible for the high turbidity of the studied system. At the end of this first step (i.e., after 20 min), the system still remains liquid and the PPRs are totally formed. Then, in a second step (i.e., after 150 min), the system undergoes its reorganization characterized by a compacity increase of the precipitated domains and forms a physical gel. We found that PPRs are totally formed after 20 min at 30 degrees C and that the system stays in a nongel state up to 150 min. This opens new perspectives regarding the PPR chemical modification: between these two characteristic times, we can easily envisage an efficient chemical modification of the PPR molecules in water, as for instance,in end-capping reaction leading to the synthesis of polyrotaxanes

Smart hydrogels based on double responsive triblock terpolymers

Reinicke S, Schmelz J, Lapp A, Karg M, Hellweg T, Schmalz H. Smart hydrogels based on double responsive triblock terpolymers. Soft Matter. 2009;5(13):2648-2657.In this work a new kind of "smart'' hydrogel is presented, which is composed of a poly(2-vinylpyridine)-block-poly(ethylene oxide)-block-poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P2VP-b-PEO-b-P( GME-co-EGE)) triblock terpolymer. The thermo-sensitive poly(glycidyl methyl ether-co-ethyl glycidyl ether) block exhibits a cloud point which is easily adjustable by the comonomer ratio. Copolymerization of GME and EGE produces nearly statistical copolymers with a weak preferential incorporation of GME, exhibiting a sharp coil-to-globule transition with almost no hysteresis. The triblock terpolymers aggregate in aqueous solution triggered by both pH and temperature. At sufficiently high concentration this stimuli-responsive behaviour leads to a reversible gel formation with gel strengths and transition points tuneable by pH, temperature, concentration, and block lengths. At pH 7, an unique gel-sol-gel transition accompanied with a strengthening of the gel is observed upon heating using rheology. Moreover, the structure of the low temperature gel phase is investigated by means of small-angle neutron scattering (SANS)