Biomedical subjects

Alain Dufresne

Publications and source records attributed to Alain Dufresne.

13 recordsLinked to original sources

PubMed2006

Starch nanocrystals with large chain surface modifications.

Nanoscale monocrystalline starch particles were successfully modified using stearic acid chloride and poly(ethylene glycol) methyl ether. Surface modification was confirmed using FTIR, XPS spectroscopy, and contact angle measurements. X-ray diffraction and DSC analysis confirmed that there was no alteration of the starch crystalline structure due to the surface modification. The grafts at the starch surface were also found to crystallize on the surface. TEM showed the individualization of nanoparticles as a result of the reduction of polar and hydrogen bonding forces. These results show our ability to modify the starch nanocrystal surface with plasticizing chains. Modified nanoparticles can find applications as compatibilized polymer additives, surface-active particles, and co-continuous nanocomposite precursors.

Calorimetry, Differential Scanning↗

PubMed2006

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.

Crystallization↗

PubMed2006

Comparing the mechanical properties of high performances polymer nanocomposites from biological sources.

There are numerous examples where animals or plants synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by fibrous biopolymers. Cellulose and chitin are classical examples of these reinforcing elements, which occur as whisker-like microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, with the consequence of axial physical properties approaching those of perfect crystals. Starch is another example of natural semicrystalline polymer that is produced by many plants and occurs as microscopic granules. It acts as a storage polymer in cereals and tubers. These abundant and natural polymers can be used to create high performance nanocomposites presenting outstanding properties. Aqueous suspensions of crystallites can be prepared by acid hydrolysis of the purified substrates. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspension by subsequent vigorous mechanical shearing action. For cellulose and chitin, these monocrystals appear as rod-like nanoparticles which dimensions depend on the biological source of the substrate. In the case of starch they consist of platelet-like nanoparticles. High reinforcing capability was reported resulting from the intrinsic chemical nature of these polymers and from their hierarchical structure. During the last decade, many works have been devoted to mimic biocomposites by blending cellulose whiskers from different sources with polymer matrices.

Acids↗

PubMed2005

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).

Journal Article↗

PubMedDate not supplied

Morphological investigation of nanocomposites from sorbitol plasticized starch and tunicin whiskers.

Nanocomposites were prepared from waxy maize starch plasticized with sorbitol as the matrix and a stable aqueous suspension of tunicin whiskers-an animal cellulose-as the reinforcing phase. The composites were conditioned at different relative humidity levels. The conditioned films were characterized using scanning electron microscopy, differential scanning calorimetry, water uptake experiments, and wide-angle X-ray scattering studies. Contrarily to our previous report concerning tunicin whisker filled glycerol plasticized starch nanocomposites (Macromolecules 2000, 33, 8344), the present system exhibited a single glass-rubber transition, and no evidence of transcrystallization of amylopectin on cellulose whisker surfaces and resultant antiplasticizing effects were observed. It was found that the glass-rubber transition temperature of the plasticized amylopectin matrix first increases up a whiskers content around 10-15 wt % and then decreases. A significant increase in crystallinity was observed in the composites by increasing either moisture content or whiskers content.

Absorption↗

PubMedDate not supplied

Plasticized waxy maize starch: effect of polyols and relative humidity on material properties.

The plasticizing effect of different polyols such as glycerol, xylitol, sorbitol, and maltitol on waxy maize starch was investigated. The concentration of plasticizer was fixed at 33 wt % (dry basis of starch). The structure and mechanical performance of resulting films conditioned at different relative humidity levels were studied in detail. The effect of the plasticizer on the glass-rubber transition temperature (T(g)) and crystallinity was characterized using differential scanning calorimetry. It was found that T(g) decreases with increasing moisture content and decreasing molecular weight of the plasticizer. The water resistance of starch increased steadily with the molecular weight of the plasticizer and was directly proportional to the ratio of the end to total hydroxyl groups. As the molecular weight of the plasticizer increased, the brittleness of the dry system increased. However, the use of high molecular plasticizer allowed good mechanical properties of the moist material to be obtained in terms of both stiffness and elongation at break.

Humidity↗

PubMedDate not supplied

Crab shell chitin whisker reinforced natural rubber nanocomposites. 1. Processing and swelling behavior.

Nanocomposite materials were obtained from a colloidal suspension of chitin whiskers as the reinforcing phase and latex of both unvulcanized and prevulcanized natural rubber as the matrix. The chitin whiskers, prepared by acid hydrolysis of chitin from crab shell, consisted of slender parallelepiped rods with an aspect ratio close to 16. After the two aqueous suspensions were mixed and strirred, solid composite films were obtained either by freeze-drying and hot-pressing or by casting and evaporating the preparations. The processing and swelling behavior of composite films were evaluated. It was concluded that the whiskers form a rigid network assumed to be governed by a percolation mechanism in the evaporated samples only. Comparatively, better resistance of evaporated samples than hot-pressed ones against swelling in an organic solvent medium is good evidence for the existence of a rigid chitin network. The values of diffusion coefficient, bound rubber content, and relative weight loss also supported the presence of a three-dimensional chitin network within the evaporated samples. The mechanical behavior of the composites gives additional insight and evidence for this fact (part 2).

Animals↗

PubMedDate not supplied

Crab shell chitin whisker reinforced natural rubber nanocomposites. 2. Mechanical behavior.

In a previous work (part 1), nanocomposite materials were obtained using a latex of either unvulcanized or prevulcanized natural rubber as the matrix and a colloidal suspension of crab chitin whiskers as the reinforcing phase. The mechanical behavior of the resulting nanocomposite films was analyzed in both the linear and the nonlinear range in the present study. The effects of the filler and processing technique were evaluated, and the results are discussed based on the knowledge of the structural morphology and swelling behavior reported in our previous work. The reinforcing effect of chitin whiskers strongly depended on their ability to form a rigid three-dimensional network, resulting from strong interactions such as hydrogen bonds between the whiskers. The results emanating from the successive tensile test experiments give clear evidence for the presence of a three-dimensional chitin network within the evaporated samples. Cross-linking of the matrix was found to interfere with the formation of this network.

Animals↗

PubMedDate not supplied

Crab shell chitin whiskers reinforced natural rubber nanocomposites. 3. Effect of chemical modification of chitin whiskers.

The purpose of this study was to chemically modify the surface of chitin whiskers and to investigate the effect of the incorporation of these modified whiskers into a natural rubber (NR) matrix on the properties of the ensuing nanocomposite. Different chemical coupling agents were tested, namely, phenyl isocyanate (PI), alkenyl succinic anhydride (ASA) (Accosize 18 from American Cyanamid), and 3-isopropenyl-alpha,alpha'-dimethylbenzyl isocyanate (TMI). The extent of chemical modification was evaluated by Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and surface energy analysis. After chemical modification, nanocomposite films were obtained using a toluene natural rubber solution in which the whiskers were dispersed. Their mechanical properties were found to be inferior to those of unmodified chitin/NR composites presented in our previous study. In fact, even though there is an increase in filler-matrix interaction as a result of chemical modification of the chitin whiskers, this does not contribute to the improvement in the mechanical properties of the resulting nanocomposite. It is concluded that this loss of performance is due to the partial destruction of the three-dimensional network of chitin whiskers assumed to be present in the unmodified composites.

Animals↗

PubMedDate not supplied

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.

Crystallization↗

PubMedDate not supplied

Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field.

There are numerous examples where animals or plants synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by fibrous biopolymers. Cellulose, the world's most abundant natural, renewable, biodegradable polymer, is a classical example of these reinforcing elements, which occur as whisker-like microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, with the consequence of axial physical properties approaching those of perfect crystals. This quite "primitive" polymer can be used to create high performance nanocomposites presenting outstanding properties. This reinforcing capability results from the intrinsic chemical nature of cellulose and from its hierarchical structure. Aqueous suspensions of cellulose crystallites can be prepared by acid hydrolysis of cellulose. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspension by subsequent vigorous mechanical shearing action. During the past decade, many works have been devoted to mimic biocomposites by blending cellulose whiskers from different sources with polymer matrixes.

Cellulose↗

PubMedDate not supplied

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 %.

Biotechnology↗

PubMedDate not supplied

First evidence for the presence of weddellite crystallites in Opuntia ficus indica parenchyma.

Calcium oxalate crystallites occur very often in the plants tissues and their role is still poorly known. We report here the experimental protocol leading to the isolation of two forms of calcium oxalate crystallites differing in their hydration level in the parenchymal tissues of Opuntia ficus indica (Miller). Whereas the whewellite crystallites are habitual in all Opuntia species, the weddellite form has never been isolated from these species before, which is probably due to their small size (about 1 microm). We have identified these forms using X-ray diffraction and scanning electron microscopy.

Calcium Oxalate↗