Christina Scheffler

Cure Kinetics of Epoxy Nanocomposites Affected by MWCNTs Functionalization: A Review

The current paper provides an overview to emphasize the role of functionalization of multiwalled carbon nanotubes (MWCNTs) in manipulating cure kinetics of epoxy nanocomposites, which itself determines ultimate properties of the resulting compound. In this regard, the most commonly used functionalization schemes, that is, carboxylation and amidation, are thoroughly surveyed to highlight the role of functionalized nanotubes in controlling the rate of autocatalytic and vitrification kinetics. The current literature elucidates that the mechanism of curing in epoxy/MWCNTs nanocomposites remains almost unaffected by the functionalization of carbon nanotubes. On the other hand, early stage facilitation of autocatalytic reactions in the presence of MWCNTs bearing amine groups has been addressed by several researchers. When carboxylated nanotubes were used to modify MWCNTs, the rate of such reactions diminished as a consequence of heterogeneous dispersion within the epoxy matrix. At later stages of curing, however, the prolonged vitrification was seen to be dominant. Thus, the type of functional groups covalently located on the surface of MWCNTs directly affects the degree of polymer-nanotube interaction followed by enhancement of curing reaction. Our survey demonstrated that most widespread efforts ever made to represent multifarious surface-treated MWCNTs have not been directed towards preparation of epoxy nanocomposites, but they could result in property synergism.

Poly(ethylene succinate)/single-walled carbon nanotube composites: a study on crystallization

An investigation on the crystallization of composites based on poly(ethylene succinate) and unmodified single-walled carbon nanotube was made in this report. Both isothermal and non-isothermal modes were studied along with subsequent melting behavior using differential scanning calorimetry. Crystal morphology was then explored using X-ray scattering and infrared spectroscopy. It was observed during isothermal crystallization that carbon nanotube (CNT) could contribute to the crystallization rate through heterogeneous nucleation. Furthermore, nanotubes enhanced the crystallinity within low and high undercooling rather than medium undercooling. Similar findings were obtained in non-isothermal crystallization mode. At lower cooling rates, the crystallization rate was more strongly influenced by the nanotubes, while at higher cooling rates the crystallinity was affected to the greater extent. The onset of the cold crystallization of polymer remained unaffected in presence of the nanotube, while its extent was reduced. X-ray diffraction together with infrared spectroscopy found that the polymer crystalline morphology was of α type, and no transition from α to β occurred in presence of the CNT.

A Single Glass Fiber with Ultrathin Layer of Carbon Nanotube Networks Beneficial to In-Situ Monitoring of Polymer Properties in Composite Interphases

Electrophoretic deposition (EPD) is used to deposit multiwalled carbon nanotube networks (CNTs) onto electrically insulating glass fiber surfaces. We found that the thin networks on a single glass fiber surface exhibit semiconducting properties. This enables us to realize a single CNT-glass fiber as a probe with novel multifunctional capabilities for in-situ monitoring of various chemical/physical transitions, particularly in the interphase region between polymer and glass fiber. Because of the intimate interaction between CNTs and polymers in the vicinity of a glass fiber, our CNT probe can rapidly sense the local changes of fundamental polymer properties, such as glass transition, reaction activation energy, cross-linking reaction, and crystallization.

Poly (vinyl alcohol) fiber reinforced concrete: investigation of strain rate dependent interphase behavior with single fiber pullout test under quasi-static and high rate loading

A new single fiber pullout device was used to investigate interfacial debonding in the poly (vinyl alcohol) (PVA) fiber–cementitious matrix system under impact loading. For comparison, PVA fibers with two different surface states were used – as-received and after fiber finish removal by extraction in solvents. The indirect method was employed to determine the interfacial strength parameters, namely, the local interfacial shear strength, , and the critical energy release rate, G ic, from the peak load values reached in the pullout test. For both fiber surface states, the and G ic values for high loading rates appeared to be considerably greater than the corresponding parameters determined by means of a quasi-static pullout test. This can be explained using a model based on Zhurkov’s kinetic (thermal fluctuation) theory of the strength of solids, which also enabled to estimate the apparent activation energy for interfacial debonding. Its values (1.3 eV for both as-received and treated fibers) evidenced formation of strong bonds between PVA fiber and the concrete matrix and the identity of debonding mechanisms for both fiber types. Finish removal decreased interfacial adhesion due to smoothing the fiber surface and reducing interfacial mechanical bonding.

Influence of microwave plasma treatment on the surface properties of carbon fibers and their adhesion in a polypropylene matrix

A commercially available carbon fiber (CF) with an epoxy-based sizing (EP-sized CF) and an unsized CF have been plasma treated to study the effect on the fiber-matrix adhesion towards a polypropylene matrix. The EP-sized fiber was chosen because of its predictable low adhesion in a polypropylene (PP) matrix. The fibers have been modified using a microwave low-pressure O2/CO2/N2-gas plasma source (Cyrannus®) developed at IWS in a batch process. One aim of this study was the evaluation of parameters using high energies and short time periods in the plasma chamber to see the effect on mechanical performance of CF. These results will be the fundamental work for a planned continuous plasma modification line. The CF surface was characterized by determining the surface energies, single fiber tensile strength and XPS analysis. The adhesion behavior before and after plasma treatment was studied by single fiber pull-out test (SFPO) and scanning electron microscopy (SEM). It was shown that the CO2- and O2-plasma increases the number of functional groups on the fiber surface during short time plasma treatment of 30 s. Carboxylic groups on the unsized CF surface resulting from O2-plasma treatment lead to an enhanced fiber-matrix adhesion, whereas the fiber strength was merely reduced.

Synergistic interfacial effects of ionic liquids as sizing agents and surface modified carbon fibers

This paper presents investigations into the use of ionic liquids as sizing agents for carbon fibers in epoxy matrices. A variety of aqueous ionic liquid solutions was employed as the sizing bath, at concentrations relevant to industry standards. Of the ionic liquids used, the widely available and inexpensive BmimCl was found to give the optimal fiber-to-matrix adhesion improvements of 250% relative to control fibers in epoxy, and 66% improvement in polypropylene. Molecular dynamics simulations suggest that the diffusion of the ionic liquids into the polymer phase accompanied by self-agglomeration generates a plasticised interphase resulting in high shear tolerant materials.

Interphases in Cementitious Matrix: Effect of Fibers, Sizings, and Loading Rates

Single fiber model composites of polyvinyl alcohol (PVA) fibers as well as alkali resistant (AR-) glass fibers and a cementitious matrix were used to investigate the adhesion strength under quasi-static and high-rate loading in a pull-out test. Differently sized AR-glass fibers were spun for fundamental understanding the effect of the fiber/matrix interphase on the strain hardening. As a first approach, the fiber surface was modified in oppositional ways using the following sizings: one based on a polypropylene (PP, weak) film former and another one based on a styrene-butadiene (SB, strong) film former. This unique set of data allows the comparison of the force-displacement curves during pull-out depending on the applied sizing, the embedded length and the loading rate. A significant influence of the sizing on the frictional behavior after debonding was found. In the case of PVA fibers, the interfacial behavior of as-received (finished) fibers and fibers from which the finish was removed by extraction in solvents (ethanol and n-hexane) was compared. It was shown that PVA fibers in concrete are highly strain rate dependent, whereby the fiber surface treatment controls the ratio between completely pulled out fibers and sheared-off fibers.

Surface, interphase and tensile properties of unsized, sized and heat treated basalt fibres

Recycling of fibre reinforced polymers is in the focus of several investigations. Chemical and thermal treatments of composites are the common ways to separate the reinforcing fibres from the polymer matrices. However, most sizings on glass and basalt fibre are not designed to resist high temperatures. Hence, a heat treatment might also lead to a sizing removal, a decrease of mechanical performance and deterioration in fibre-matrix adhesion. Different basalt fibres were investigated using surface analysis methods as well as single fibre tensile tests and single fibre pull-out tests in order to reveal the possible causes of these issues. Heat treatment in air reduced the fibre tensile strength in the same level like heat treatment in nitrogen atmosphere, but it influenced the wetting capability. Re-sizing by a coupling agent slightly increased the adhesion strength and reflected a decreased post-debonding friction.