No abstract

This research describes the preparation of membranes with chitosan (CS) as the polymeric matrix and cellulose nanocrystals (CNC) as reinforcement. The aim was to evaluate their physical, mechanical and biological properties, and to determine their potential for biomedical use. Membranes were prepared via casting CNC suspensions in CS solution, at CNC concentrations of 0.5%, 1.0% and 2.0% (w/w) with pure chitosan as a reference. Analysis of membrane properties was performed using several techniques, such as ATR-FTIR, SEM, swelling test, maximum water absorption, dynamical mechanical analysis and in vivo (Winstar rats) biocompatibility and biodegradability assays for biological evaluation. Experimental results established that CNC reduced swelling rates and increased the maximum water absorption when CNC concentration was higher. Therefore, the presence of CNC in the matrix reduced Young's modulus by approximately 50% in comparison with pure chitosan. All formulations demonstrated biocompatibility and biodegradability values ranged between 4% and 21% in the 30 days after implantation. Based on these results, these membranes may be of use for biomedical applications.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Kevlar is an organic fiber (poly-para-phenylene-terephthalamide) in the aromatic polyamide family, produced by DuPont de Nemours [1]. It has a unique combination of high strength, high modulus, toughness and thermal stability. Its remarkable properties are mainly related to the structure developed during the spinning process [2]. It was developed for demanding industrial and advancedtechnology applications. In this work, single Kevlar-29 fibers have been subjected to tensile tests at room temperature using a Universal Fiber Tester developed originally by Bunsell et al. [3], equipped with a load cell of 250 g calibrated from 0 to 100 g, with a precision of 0.01 g. The specimen gauge length was 30 mm and the fiber was gripped between two sets of jaws. Data acquisition used a PC linked to the fiber tester via a National Instrument interface card and WinATS 6.2 software from Sysma. In order to normalize the stress, the diameter of each fiber was systematically measured before each test by using a Mitutoyo LSM-500S laser apparatus, with an accuracy of 0.01 μm.The tensile stress-strain curves of Kevlar-29 fibers were acquired, which remain almost linear up to failure. The mechanical properties were calculated from these curves, whose values are shown in Table 1 together with the diameter; 30 fibers were broken and mean properties were determined. The standard deviation indicates there is considerable variability in the strength of these fibers. The surface appearance of the untested Kevlar-29 fibers in the scanning electron microscope is smooth, although some striations have been observed (Fig. 1a). The tensile fracture morphology was similar for all the fibers tested (Figs. 1b-e); this complex morphology is due to the structure of Kevlar-29, being highly anisotropic with a low transverse strength leading to splitting upon failure under tensile loading condition [2]; the origin of the splitting may be as a result of stress concentrations at flaws or inside the fibers. Fig. 1e shows the presence of a fibrillar structure. Due to the severe rupture during the solicitation, local heating has been seen in some parts of the fibers, which suggests that melting can occur (Fig. 1f). Fig. 1g displays a micrograph that reveals the existence of a "pleated" structure shown as a series of transverse bands. This radial-sheet structure consists of alternated bands in each sheet arranged at approximately equal but opposite angles to form pleats, as reported elsewhere [2].