Rubber materials from elastomers and nanocellulose powders: filler dispersion and mechanical reinforcement.

Rubber materials with well-dispersed fillers and large mechanical reinforcement have been obtained by melt-processing a diene elastomer matrix and tailored nanocellulose powders having both a high specific surface area and a modified interface. Such filler powders with a specific surface area of 180 m2 g-1 and 100 m2 g-1 have been obtained by freeze-drying suspensions of short needle-like cellulose nanocrystals (CNCs) and entangled networks of microfibrillated cellulose (MFC) in tert-butanol/water, respectively. A quantitative and toposelective filler surface esterification was performed using a gas-phase protocol either with palmitoyl chloride (PCl) to obtain a hydrophobic but non-reactive nanocellulose interface, or with 3,3'-dithiopropionic acid chloride (DTACl) to introduce reactive groups that can covalently bind the nanocellulose interface to the dienic matrix in a subsequent vulcanization process. A set of filled materials was prepared varying the filler morphology, interface and volume fraction. Transmission electron microscopy images of ultrathin cryo-sections showed that modified nanocellulose fillers presented a relatively homogeneous distribution up to a volume fraction of 20%. The materials also exhibited a significant modulus increase, while keeping an extensibility in the same range as that of the neat matrix. Strikingly, in the case of the reactive interface, a strong stress-stiffening behavior was evidenced from the upward curvature of the tensile curve, leading to a large increase of the ultimate stress (up to 7 times that of the neat matrix). Taken together, these properties, which have never been previously reported for nanocellulose-filled elastomers, match well the mechanical characteristics of industrial carbon black or silica-loaded elastomers.

Reinforcing mechanisms of starch nanocrystals in a nonvulcanized natural rubber matrix.

A phenomenological modeling approach was developed to try to understand the reinforcing mechanism of starch nanocrystals in a nonvulcanized natural rubber matrix. Natural rubber was not cross-linked to maintain the biodegradability of the biosourced materials. Nonlinear dynamic mechanical experiments highlighted the significant reinforcing effect of starch nanocrystals and the presence of the Mullins and Payne effects. Two models were used to predict the Payne effect considering that either filler-filler (Kraus model) or matrix-filler (Maier and Göritz model) interactions are preponderant. The use of the Maier and Goritz model demonstrated that phenomena of adsorption and desorption of NR chains on the filler surface governed nonlinear viscoelastic properties, even if the formation of a percolating network for filler contents >6.7 vol % was evidenced by the Kraus model.

Gas-phase surface esterification of cellulose microfibrils and whiskers.

A new and highly efficient synthetic method has been developed for the surface esterification of model cellulosic substrates of high crystallinity and accessibility, namely, freeze-dried tunicin whiskers and bacterial cellulose microfibrils dried by the critical point method. The reaction, which is based on the gas-phase action of palmitoyl chloride, was monitored by solid-state CP-MAS (13)C NMR. It was found that the grafting density not only depended on the experimental conditions, but also on the nature and conditioning of the cellulose samples. The structural and morphological modifications of the substrates at various degrees of grafting were revealed by scanning electron microscopy and X-ray diffraction analysis. These characterizations indicated that the esterification proceeded from the surface of the substrate to their crystalline core. Hence, for moderate degree of substitution, the surface was fully grafted whereas the cellulose core remained unmodified and the original fibrous morphology maintained. An almost total esterification could be achieved under certain conditions, leading to highly substituted cellulose esters, presenting characteristic X-ray diffraction patterns.

Processing and characterization of waxy maize starch films plasticized by sorbitol and reinforced with starch nanocrystals.

Nanocomposites films have been processed from a filler and a matrix having the same nature, i.e. waxy maize starch. The filler consists of nanoplatelet-like starch particles obtained as an aqueous suspension by acid hydrolysis of starch granules and the matrix was prepared by plasticization and disruption of starch granules with water and sorbitol. Nanocomposite films were obtained by casting and evaporating the mixture of the aqueous suspension of starch nanocrystals with the gelatinized starch. The resulting films were conditioned before testing and the effect of accelerated ageing in moist atmosphere was investigated. The thermal properties of the nanocomposite films were determined from DSC measurements and the mechanical characterization was performed in both the linear and nonlinear range.

Conversion of a non-water soluble potato starch waste into reducing sugars under non-conventional technologies.

In this exploratory work, the comparison of the utilisation of different non-conventional technologies (ultrasound and microwave irradiations) for the depolymerisation of a complex industrial starch-based waste into reducing sugars was investigated. Reducing sugars could then be converted into higher value-added compounds such as higher alcohols. The experiments were performed on three different starting materials named as 'Potato flour', 'Wet potato sludge' and 'Dry potato sludge'. The conversion of 'Potato flour' into reducing sugars reached in acidic conditions 61% within an hour under microwave irradiation, 70% and 84% within 120 min under low and high frequency ultrasonic irradiation, respectively.

The molecular structure of waxy maize starch nanocrystals.

The insoluble residues obtained by submitting amylopectin-rich native starch granules from waxy maize to a mild acid hydrolysis consist of polydisperse platelet nanocrystals that have retained the allomorphic type of the parent granules. The present investigation is a detailed characterization of their molecular composition. Two major groups of dextrins were found in the nanocrystals and were isolated. Each group was then structurally characterized using beta-amylase and debranching enzymes (isoamylase and pullulanase) in combination with anion-exchange chromatography. The chain lengths of the dextrins in both groups corresponded with the thickness of the crystalline lamellae in the starch granules. Only approximately 62 mol% of the group of smaller dextrins with an average degree of polymerization (DP) 12.2 was linear, whereas the rest consisted of branched dextrins. The group of larger dextrins (DP 31.7) apparently only consisted of branched dextrins, several of which were multiply branched molecules. It was shown that many of the branch linkages were resistant to the action of the debranching enzymes. The distribution of branched molecules in the two populations of dextrins suggested that the nanocrystals possessed a regular and principally homogeneous molecular structure.

Thermoplastic starch-waxy maize starch nanocrystals nanocomposites.

Waxy maize starch nanocrystals obtained by hydrolysis of native granules were used as a reinforcing agent in a thermoplastic waxy maize starch matrix plasticized with glycerol. Compared to our previous studies on starch nanocrystals reinforced natural rubber (NR) [Macromolecules 2005, 38, 3783; 2005, 38, 9161], the present system presents two particularities: (i) thermoplastic starch is a polar matrix, contrarily to NR, and (ii) the chemical structures of the matrix and the filler are similar. The influence of the glycerol content, filler content, and aging on the reinforcing properties of waxy maize starch nanocrystals (tensile tests, DMA) and crystalline structure (X-ray diffraction) of materials were studied. It was shown that the reinforcing effect of starch nanocrystals can be attributed to strong filler/filler and filler/matrix interactions due to the establishment of hydrogen bonding. The presence of starch nanocrystals leads to a slowing down of the recrystallization of the matrix during aging in humid atmosphere.

Surface chemical modification of waxy maize starch nanocrystals.

The surface of waxy maize starch nanocrystals obtained from sulfuric acid hydrolysis of native waxy maize starch granules was chemically modified using two different reagents, namely, alkenyl succinic anhydride and phenyl isocyanate. The occurrence of chemical modification was evaluated by FTIR and X-ray photoelectron spectroscopies. Contact angle measurements from which the surface energy of the materials under investigation was deduced showed that chemical modification led to more hydrophobic particles. Chemical modification altered the morphology of particles, as shown by observation by transmission electron microscopy, but not their crystallinity (X-ray diffraction analysis).

Platelet nanocrystals resulting from the disruption of waxy maize starch granules by acid hydrolysis.

Colloidal aqueous suspensions of starch nanocrystals were prepared by submitting native granules from A-type amylopectin-rich waxy maize to a hydrochloric acid hydrolysis. The insoluble residue contains polydisperse and more or less individualized platelet nanocrystals corresponding to the lamellae formed by the association of amylopectin side branches into parallel arrays of double helices. After 2 weeks of hydrolysis, 5-7 nm thick lamellae still connected by alpha(1-->6) linkages were seen edge-on using transmission electron microscopy. As the hydrolysis progressed up to 6 weeks, more alpha(1-->6) branching points located in the inter-lamellar areas were severed and the platelets were thus observed in planar view. Despite a variety of shapes, characteristic geometrical features of the nanocrystalsa were recognized, such as marked 60-65 degrees acute angles and constituting parallelepipedal blocks with a length of 20-40 nm and a width of 15-30 nm. X-ray and electron diffraction showed that these nanoplatelets retain the crystalline A-type of the parent granules.

Optimization of the preparation of aqueous suspensions of waxy maize starch nanocrystals using a response surface methodology.

Response surface methodology was used to investigate the effect of five selected factors on the selective H(2)SO(4) hydrolysis of waxy maize starch granules. These predictors were temperature, acid concentration, starch concentration, hydrolysis duration, and stirring speed. The goal of this study was to optimize the preparation of aqueous suspensions of starch nanocrystals, i.e., to determine the operative conditions leading to the smallest size of insoluble hydrolyzed residue within the shortest time and with the highest yield. Therefore empirical models were elaborated for the hydrolysis yield and the size of the insoluble residues using a central composite face design involving 31 trials. They allowed us to show that it was possible to obtain starch nanocrystals after only 5 days of H(2)SO(4) hydrolysis with a yield of 15 wt % and having the same shape as those obtained from the classical procedure after 40 days of HCl treatment, with a yield of 0.5 wt %.