Showing papers by "Christoph Weder published in 2010"

Review: current international research into cellulose nanofibres and nanocomposites

Abstract: This paper provides an overview of recent progress made in the area of cellulose nanofibre-based nanocomposites. An introduction into the methods used to isolate cellulose nanofibres (nanowhiskers, nanofibrils) is given, with details of their structure. Following this, the article is split into sections dealing with processing and characterisation of cellulose nanocomposites and new developments in the area, with particular emphasis on applications. The types of cellulose nanofibres covered are those extracted from plants by acid hydrolysis (nanowhiskers), mechanical treatment and those that occur naturally (tunicate nanowhiskers) or under culturing conditions (bacterial cellulose nanofibrils). Research highlighted in the article are the use of cellulose nanowhiskers for shape memory nanocomposites, analysis of the interfacial properties of cellulose nanowhisker and nanofibril-based composites using Raman spectroscopy, switchable interfaces that mimic sea cucumbers, polymerisation from the surface of cellulose nanowhiskers by atom transfer radical polymerisation and ring opening polymerisation, and methods to analyse the dispersion of nanowhiskers. The applications and new advances covered in this review are the use of cellulose nanofibres to reinforce adhesives, to make optically transparent paper for electronic displays, to create DNA-hybrid materials, to generate hierarchical composites and for use in foams, aerogels and starch nanocomposites and the use of all-cellulose nanocomposites for enhanced coupling between matrix and fibre. A comprehensive coverage of the literature is given and some suggestions on where the field is likely to advance in the future are discussed.

Cellulose Whisker/Epoxy Resin Nanocomposites

Abstract: New nanocomposites composed of cellulose nanofibers or “whiskers” and an epoxy resin were prepared. Cellulose whiskers with aspect ratios of ∼10 and ∼84 were isolated from cotton and sea animals called tunicates, respectively. Suspensions of these whiskers in dimethylformamide were combined with an oligomeric difunctional diglycidyl ether of bisphenol A with an epoxide equivalent weight of 185−192 and a diethyl toluenediamine-based curing agent. Thin films were produced by casting these mixtures and subsequent curing. The whisker content was systematically varied between 4 and 24% v/v. Electron microscopy studies suggest that the whiskers are evenly dispersed within the epoxy matrix. Dynamic mechanical thermoanalysis revealed that the glass transition temperature (Tg) of the materials was not significantly influenced by the incorporation of the cellulose filler. Between room temperature and 150 °C, i.e., below Tg, the tensile storage moduli (E′) of the nanocomposites increased modestly, for example from 1...

Natural Biopolymers: Novel Templates for the Synthesis of Nanostructures

Abstract: Biological systems such as proteins, viruses, and DNA have been most often reported to be used as templates for the synthesis of functional nanomaterials, but the properties of widely available biopolymers, such as cellulose, have been much less exploited for this purpose. Here, we report for the first time that cellulose nanocrystals (CNC) have the capacity to assist in the synthesis of metallic nanoparticle chains. A cationic surfactant, cetyltrimethylammonium bromide (CTAB), was critical to nanoparticle stabilization and CNC surface modification. Silver, gold, copper, and platinum nanoparticles were synthesized on CNCs, and the nanoparticle density and particle size were controlled by varying the concentration of CTAB, the pH of the salt solution, and the reduction time.

Bio-inspired mechanically-adaptive nanocomposites derived from cotton cellulose whiskers

Abstract: A new series of biomimetic, stimuli-responsive nanocomposites, which change their mechanical properties upon exposure to physiological conditions, was investigated. The materials were produced by introducing percolating networks of cellulose whiskers isolated from cotton into poly(vinyl acetate). Below the glass-transition temperature (Tg ∼ 63 °C), the tensile storage moduli (E′) of the dry nanocomposites increased two fold, from 2 GPa for the neat polymer to 4 GPa for a nanocomposite with 16.5% v/v whiskers. The relative reinforcement was more significant above Tg, where E′ was increased nearly 40 fold, from ∼1.2 MPa to ∼45 MPa. Upon exposure to emulated physiological conditions (immersion in artificial cerebrospinal fluid at 37 °C) all nanocomposites showed a pronounced decrease in E′, for example to 5 MPa for the 16.5% v/v whisker nanocomposites with only about 28% w/w swelling. This is a significant reduction in the amount of swelling required to decrease the E′, compared to earlier material versions based on cellulose whiskers with higher surface charge density; the decreased swelling may be a considerable advantage for the intended use of these materials as adaptive substrates for intracortical electrodes and other biomedical applications.

Stimuli-responsive mechanically adaptive polymer nanocomposites.

Abstract: A new series of biomimetic stimuli-responsive nanocomposites, which change their mechanical properties upon exposure to physiological conditions, was prepared and investigated. The materials were produced by introducing percolating networks of cellulose nanofibers or "whiskers" derived from tunicates into poly(vinyl acetate) (PVAc), poly(butyl methacrylate) (PBMA), and blends of these polymers, with the objective of determining how the hydrophobicity and glass-transition temperature (Tg) of the polymer matrix affect the water-induced mechanically dynamic behavior. Below the Tg (approximately 60-70 degrees C), the incorporation of whiskers (15.1-16.5% v/v) modestly increased the tensile storage moduli (E') of the neat polymers from 0.6 to 3.8 GPa (PBMA) and from 2 to 5.2 GPa (PVAc). The reinforcement was much more dramatic above Tg, where E' increased from 1.2 to 690 MPa (PVAc) and approximately 1 MPa to 1.1 GPa (PBMA). Upon exposure to physiological conditions (immersion in artificial cerebrospinal fluid, ACSF, at 37 degrees C) all materials displayed a decrease in E'. The most significant contrast was seen in PVAc; for example, the E' of a 16.5% v/v PVAc/whisker nanocomposite decreased from 5.2 GPa to 12.7 MPa. Only a modest modulus decrease was measured for PBMA/whisker nanocomposite; here the E' of a 15.1% v/v PBMA/whisker nanocomposite decreased from 3.8 to 1.2 GPa. A systematic investigation revealed that the magnitude of the mechanical contrast was related to the degree of swelling with ACSF, which was shown to increase with whisker content, temperature, and polarity of the matrix (PVAc>PBMA). The mechanical morphing of the new materials can be described in the framework of both the percolation and Halpin-Kardos models for nanocomposite reinforcement, and is the result of changing interactions among the nanoparticles and plasticization of the matrix upon swelling.

Luminescent Mechanochromic Sensors Based on Poly(vinylidene fluoride) and Excimer‐Forming p‐Phenylene Vinylene Dyes

Abstract: Cyano-substituted excimer-forming oligo(phenylene vinylene) dyes (cyano-OPVs) with terminal alkyl tails of different length were blended with two fluorinated host polymers with similar chemical composition but differing crystallinity. These blends were used to fabricate luminogenic mechanochromic thin films, which change their emission color upon deformation. The alkyl tails affect the solubility of the chromophores in the polymer matrix and lead to different aggregation properties; this is of importance because the mechanochromic fluorescence color change of the blends is related to the self-assembly of the excimer-forming dye in the unperturbed polymer matrix, and the dispersion of the dye aggregates upon deformation. Besides the length of the solubilizing tails, the dye concentration has an important influence on the aggregate size, which is crucial to creating a mechanochromic response, since the dye aggregates must be small enough to be dispersed during the deformation process. Insitu opto-mechanical measurements have shown that the mechanochromic effect occurs primarily during plastic deformation and that the mechanically induced dispersion of the dye aggregates becomes more pronounced as the crystallinity of the matrix polymer increases. http://doc.rero.ch

Photonic crystal devices with multiple dyes by consecutive local infiltration of single pores.

Abstract: [ ∗] P. W Nolte , . Dr. D. Pergande , Dr. S. L. Schweizer , Prof. R. B. Wehrspohn Martin-Luther-Univesity Halle-Wittenberg Heinrich Damerow Str. 4, 06120 Halle (Germany) E-mail: peter.nolte@physik.uni-halle.de M. Geuss , B. Makowski , Prof. C. Weder Adolphe Merkle Institute and Fribourg Center for NanomaterialsUniversity of Fribourg P.O. Box 209, CH-1723 Marly (Switzerland) B. Makowski , Prof. C. Weder Case Western Reserve University Department of Macromolecular Science and Engineering 2100 Adelbert Rd., Cleveland, OH 44107–7202 (USA) R. Salzer , Prof. R. B. Wehrspohn Fraunhofer Institute for Mechanics of Materials Walter-Hulse-Strase 1, 06120 Halle (Germany) P. Mack Institut fur Nanotechnologie Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz-Platz 176344 Eggenstein-Leopoldshafen (Germany) Dr. D. Hermann , Prof. K. Busch Institut fur Theoretische Festkorperphysik and DFG-Center for Functional Nanostructures (CFN) Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe (Germany) M. Geuss , Prof. M. Steinhart Max Planck Institute of Microstructure Physics Weinberg 2, 06114 Halle (Germany) Prof. M. Steinhart Institute for ChemistryUniversity of Osnabruck 49069 Osnabruck (Germany)

Coextruded Multilayer All-Polymer Dye Lasers

Abstract: Coextrusion has been used to prepare multilayered polymeric films that display a photonic band gap whose position and shape can be controlled by changing layer thickness, which is achieved by adjusting processing conditions or by post processing biaxial stretching. In combination with organic fluorescent dyes, these films have been used to fabricate both distributed Bragg reflector (DFB) and distributed feedback (DFB) lasers. Both architectures displayed low lasing thresholds and high efficiencies, with DBR lasers outperforming the DFB lasers.

Polymer Light‐Emitting Diodes

Abstract: Electroluminescent devices based on polymeric materials have been a primary subject of research interest because of their potential for display and solid-state lighting applications. These types of devices, termed polymer light-emitting diodes, or PLEDs, offer the efficiency and lifetime benefits associated with solid-state semiconductor technology, in addition to cost effective solution processing. Due to these inherent advantages, the field has grown rapidly over the past two decades, beyond the point of commercial exploitation. This article summarizes the exciting progress of the technology, addressing a broad range of topics including operating principles, device architecture, materials selection, and application requirements. Keywords: PLED; polymer light emitting diode; polymer LED; light emitting polymer; OLED; organic light emitting diode; organic electronics; semiconductor; electroluminescence; solid state lighting