Steven H. McKnight

Electrospinning of polymer nanofibers with specific surface chemistry

Electrospinning is a process by which sub-micron polymer fibers can be produced using an electrostatically driven jet of polymer solution (or polymer melt). Electrospun textiles are of interest in a wide variety of applications including semi-permeable membranes, filters, composite applications, and as scaffolding for tissue engineering. The goal of the research presented here is to demonstrate that it is possible to produce sub-micron fibers with a specific surface chemistry through electrospinning. This has been accomplished by electrospinning a series of random copolymers of PMMA-r-TAN from a mixed solvent of toluene and dimethyl formamide. X-ray Photoelectron Spectroscopy (XPS) analysis shows that the atomic percentage of fluorine in the near surface region of the electrospun fibers is about double the atomic percentage of fluorine found in a bulk sample of the random copolymer, as determined by elemental analysis. These results are in good agreement with XPS and water contact angle results obtained from thin films of the same copolymer materials.

Nanoscale Indentation of Polymer Systems Using the Atomic Force Microscope

Abstract The use of the atomic force microscope (AFM) to measure surface forces has been developed to optimize its operation as a surface imaging tool. This capability can potentially be extended to evaluate nanoscale material response to indentation and would be ideal for the evaluation of multi-component polymer systems, such as adhesives and composites. In this paper, previous work related to the development of the AFM as a nanoindentation device is reviewed, and a technique is proposed which allows the AFM to be used to probe local stiffness changes in polymer systems. Cantilever probes with spring constants ranging from 0.4–150 N m were used to investigate a number of polymer systems, including an elastomer, several polyurethane systems, thermally cured epoxies, a thermoplastic polymer-thermosetting polymer adhesive system, and a thermoplastic matrix composite.

Relating elastic modulus to indentation response using atomic force microscopy

Abstracts are not published in this journal

The effects of glass-fiber sizings on the strength and energy absorption of the fiber/matrix interphase under high loading rates

The interphases of various sized E-glass-fiber/epoxy-amine systems were tested at displacement rates in the range 230–2450 μm/s by a new experimental technique (dynamic micro-debonding technique). By this method, the rate-dependent interphase properties, apparent shear strength and absorbed energies due to debonding and frictional sliding, were quantified. The systems include unsized, epoxy-amine compatible, and epoxy-amine incompatible glass fibers. The high displacement rates that induce high-strain-rate interphase loading were obtained by using the rapid expansion capability of piezoelectric actuators (PZT). The results of dynamic micro-debonding experiments showed that the values of interphase strength and specific absorbed energies varied in a manner that is dependent on the sizing and exhibited significant sensitivity to loading rates. The unsized fibers exhibit greater frictional sliding energies that could provide better ballistic resistance, while the compatible sized fibers show higher strength values that improve the structural integrity of the polymeric composites. In addition, significantly higher amounts of energy are absorbed within the frictional sliding regime compared to debonding. By using the experimental data obtained, a case study was performed to reveal the importance of the interphase related micro damage modes on energy absorption (and therefore ballistic performance) of glass/epoxy composite armor.

Molecular characterization of glass fiber surface coatings for thermosetting polymer matrix/glass fiber composites

Model multi-component glass fiber sizings, with formulations based upon current patent disclosures, were prepared to model the full coating packages used in commercial glass fiber manufacture. The sizings consisted of silane coupling agent, film former, and emulsifying surfactant in water and were applied to glass fibers prepared directly from molten glass. Fibers were analyzed before and after acetone extraction. The analyses of the extract solutions, with the fiber analysis, were used to determine the quantity and quality of the physically and chemically adsorbed layers. It was found that all three species remain on the fiber after extraction and that both coupling agent and surfactant concentrations in the coatings are higher than in the applied sizing. The impact of these species on the polymer composite/glass fiber interphase is discussed.

USE OF SILANE COUPLING AGENTS TO ENHANCE THE PERFORMANCE OF ADHESIVELY BONDED ALUMINA TO RESIN HYBRID COMPOSITES

Abstract Silane coupling agents were employed to improve the adhesion of vinyl-ester to alumina (Al2O3). Shear test by compression loading (ASTM D905) was used to study dry and wet adhesion. Scanning electron microscopy (SEM) was used to study the uniformity of silane coatings and the fracture modes after shear testing. Results showed that the adhesion and durability of the sandwiched alumina/vinyl-ester systems were significantly improved by certain silane surface treatment for most of the systems.

Investigation of properties of fiber/matrix interphase formed due to the glass fiber sizings

Sizings on glass fibers consist of a silane-based network that is chemically bound to the fiber and other compounds that are adsorbed onto the glass surface. Formation of interphase involves dissolution of adsorbed species and inter-diffusion of these compounds and resin monomers into the interphase region and chemical reaction of available functional groups. All these phenomena occur at the presence of the silane-based network. In this study, the effects of the silane-based network on the properties of the interphase region are investigated for an epoxy/amine resin system and compatible sized glass fibers. The composition of the sizing material bound to glass was determined using nuclear magnetic resonance (NMR) spectroscopy. Based on this information, model interphase materials were synthesized that were a blend of an epoxy/amine matrix and inclusions. The inclusions consist of an interpenetrating network of silane-based polymer and epoxy/amine thermoset that represents the interphase material formed during processing. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques were used to characterize the glass transition temperature and flexural modulus of the model materials. The properties of the model interphase material were obtained using the DMA results and established micromechanics models. The results show that the glass transition temperature of the model interphase is about −5°C, and its flexural storage modulus at room temperature is about 50% of that of the bulk matrix. This work has also shown that a reduction in the cross-link density of the bound network might significantly reduce the modulus within the interphase region by a factor of 5 to 8.

Effects of thermoplastic preforming binder on the properties of S2-glass fabric reinforced epoxy composites

Abstract The effect of a thermoplastic polyester binder on the thermophysical and mechanical properties of an S2-glass/epoxy–amine system was investigated. The purpose of the polymeric binder is to bond the individual fabric layers together during preforming prior to composite fabrication. This paper will address the significance of the binder chemistry, i.e., the compatibility of the binder with the matrix polymer, on the composite properties. The peel strength of preforms consolidated with various concentrations of binder was evaluated using the T-peel test. The highest peel resistance was obtained from preforms that have full coverage of the binder on the glass fabric. Further increase of the concentration of the binder does not change the peel strength. Scanning electron microscopy (SEM) on peel test fracture surfaces revealed mostly adhesive-type failure between binder and fiber. Double cantilever beam (DCB) and short beam shear (SBS) test results of the composite showed that the presence of about 2.6 wt% of the polyester binder reduces the Mode I interlaminar fracture toughness and apparent interlaminar shear strength of the S2-glass/SC-15 epoxy-amine system by about 60% and 25%, respectively. Moreover, the Tg of the matrix polymer within the interlaminar region decreases about 6°C due to the presence of the binder. The dissolution of the polyester binder within the reacting matrix resin is limited for the standard cure cycle.

In situ examination of water diffusion to the polypropylene-silane interface using FTIR-ATR

This study investigated the effect of moisture on a model silane coupling agent modified adhesive bond. Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy was used to characterize the transport of moisture to a polypropylene-silane interphase and monitor the resulting chemical changes. The FTIR-ATR method offers the advantage of in-situ examination of the diffusion process, as well as the ability to characterize chemical changes that occur due to the presence of moisture. Experiments were conducted at ambient and elevated temperatures. The results of the real-time measurements demonstrated that moisture will migrate through the polypropylene to the silane interphase. The diffusion behavior was described well by a Fickian model. The apparent diffusion coefficients for water in the polypropylene-silane bilayer were on the order of the diffusion coefficients for water in polypropylene at both test temperatures. Furthermore, changes in the spectra were observed during the diffusion experiments. These changes were indicative of hydrolysis of the siloxane backbone in the silane layer while buried beneath the polypropylene film. This finding is significant as it presents direct evidence of a degradation mechanism in silane-modified adhesive bonds.

Diffusion of reacting epoxy and amine monomers in polysulfone: a diffusivity model

Abstract In this work, a diffusivity model based on free volume theory is presented for the simultaneous diffusion of diglycidyl ether of bisphenol A (DGEBA) epoxy and bis( p -aminocyclohexyl) methane (PACM 20) amine monomers into amorphous polysulfone (PSU). This model is expected to predict and explain the diffusion behavior of the epoxy and amine monomers into PSU during the initial time periods. The overall free volume of the polymer system is estimated using a Kelley–Bueche approximation for free volume in a binary mixture consisting of a non-reacting thermoplastic and the reacting thermoset. The fractional free volume of the thermoset is estimated by the DiBenedetto equation. The model is valid only for low epoxy–amine concentrations and degrees of cure. The diffusivity model developed here suggests that reaction reduces the species diffusivity with increasing cure from a loss of the overall fractional free volume for diffusion. Further, a model for increased epoxy diffusivity from amine-induced PSU swelling is presented and validated using data from previous studies on the single-component diffusion of epoxy into amine-swollen PSU. By combining the reaction and swelling terms with the Arrhenius epoxy diffusivity, the epoxy diffusivity expression during the simultaneous diffusion and reaction of epoxy and amine into PSU for small times, is determined. Parametric studies on the nature of diffusivity are performed to determine the influence of the various free volume parameters on thermoset diffusion, and these studies show that the thermoset diffusivity, in general, decreases with time from reaction.