Giuseppe R. Palmese

Covalent Incorporation of Aminated Nanodiamond into an Epoxy Polymer Network

Outstanding mechanical and optical properties of diamond nanoparticles in combination with their biocompatibility have recently attracted much attention. Modification of the surface chemistry and incorporation into a polymer is required in many applications of the nanodiamond. Nanodiamond powder with reactive amino groups (∼20% of the number of surface carbon atoms in each 5 nm particle) was produced in this work by covalent linking of ethylenediamine to the surface carboxyl groups via amide bonds. The synthesized material was reacted with epoxy resin, yielding a composite, in which nanodiamond particles are covalently incorporated into the polymer matrix. The effect of amino groups grafted on the nanodiamond on the curing chemistry of the epoxy resin was analyzed and taken into consideration. Covalently bonded nanodiamond-epoxy composites showed a three times higher hardness, 50% higher Youngs modulus, and two times lower creep compared to the composites in which the nanodiamond was not chemically linked to the matrix. Aminated nanodiamond produced and characterized in the present study may also find applications beyond the composites, for example, as a drug, protein, and gene delivery platform in biology and medicine, as a solid support in chromatography and separation science, and in solid state peptide synthesis.

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.

Effects of temperature on cure kinetics and mechanical properties of vinyl–ester resins

The relationships among cure temperature, chemical kinetics, microstructure, and mechanical performance have been investigated for vinyl–ester resins. Fourier transform infrared spectroscopy was used to follow the reactions of vinyl–ester and styrene during isothermal curing of Dow Derakane 411-C-50 at 30 and 90°C. Reactivity ratios of vinyl–ester and styrene vinyl groups were evaluated using the copolymer composition equation. The results indicate that the ratio of vinyl–ester to styrene double bonds incorporated into the network is greater for 30 than for 90°C cure. Mechanical properties were obtained for systems subjected to isothermal cures at 30 and 90°C and postcured above ultimate Tg. The results show that the initial cure temperature significantly affects the mechanical behavior of vinyl–ester resin systems. In particular, values of strength and fracture toughness for postcured samples initially cured isothermally at 30°C are significantly higher than those obtained for samples cured isothermally at 90°C. Examination of fracture surfaces using atomic force microscopy revealed the existence of a nodular microstructure possessing characteristic nodule dimensions that are affected by the temperature of cure. Such features suggest the existence of phase separation during cure. A binary interaction model in conjunction with chemical kinetic data and estimated solubility parameters was used to evaluate enthalpic interactions between the growing polymer network and monomers of the vinyl–ester system. The results indicate that the interaction energy becomes increasingly endothermic as cure progresses and that this energy is affected by the temperature of cure through differences in copolymerization behavior. Hence, in addition to entropic factors, the changes in enthalpic contribution to the Gibbs free energy suggest that the probability of phase separation increases with extent of cure and that its onset is potentially affected by cure temperature.

Room-Temperature Healing of a Thermosetting Polymer Using the Diels−Alder Reaction

Self-healing materials are particularly desirable for load-bearing applications because they offer the potential for increased safety and material lifetimes. A furan-functionalized polymer network was designed that can heal via covalent bonding across the crack surface with the use of a healing agent consisting of a bismaleimide in solution. Average healing efficiencies of approximately 70% were observed. The healing ability of fiber-reinforced composite specimens was investigated with flexural, short beam shear, and double cantilever beam specimens. It was found that solvent amount and maleimide concentration play key roles in determining healing efficiency.

Reversibly Cross-Linked Polymer Gels as Healing Agents for Epoxy−Amine Thermosets

The Diels-Alder reaction was used to develop a reversibly cross-linking gel as a healing agent for traditional epoxy-amine thermosets. Direct application of the reversibly cross-linking network to a crack surface in an epoxy-amine thermoset resulted in the recovery of 37% of the initial epoxy-amine networks strength. Composites in which the reversibly cross-linking gel was incorporated as a secondary particulate phase recovered 21% of the initial composite strength after the first healing cycle, with healing possible up to five times.

An investigation of vinyl-ester : Styrene bulk copolymerization cure kinetics using Fourier transform infrared spectroscopy

A Fourier transform infrared (FTIR) spectroscopy technique was developed to investigate the effects of reaction temperature and reactant composition on the isothermal curing kinetics of commercial vinyl nester resins comprised of vinyl–ester monomer (dimethacrylate of diglycidyl ether of bisphenol A DGEBA) and styrene. This technique enables a more complete evaluation of the bulk copolymerization reaction of vinyl–esterstyrene systems by monitoring the depletion of vinyl–ester and styrene double bonds independently. The results indicate that the rate of fractional conversion of styrene double bonds is initially less than that of vinyl–ester vinyl groups. However, styrene monomer continues to react after conversion of vinyl–ester double bonds has ceased. In addition, the overall extent of conversion was found to increase with increasing isothermal cure temperature, and it was observed that higher styrene concentration enhances final conversion of vinyl–ester double bonds and not styrene double bonds. Increasing styrene monomer concentration also resulted in lowering the apparent activation energy for the reaction of vinyl groups from both monomers as characterized by an empirical autocatalytic model used to fit the conversion results for styrene and vinyl–ester double bonds independently. The results of this work demonstrate that reaction temperature and resin composition significantly affect the cure behavior of vinyl–ester resins and provide insight into the development of the resulting network structure.

Relating elastic modulus to indentation response using atomic force microscopy

Abstracts are not published in this journal

Mechanical evaluation of poly(vinyl alcohol)-based fibrous composites as biomaterials for meniscal tissue replacement

In this study, poly(vinyl alcohol) (PVA) hydrogels were reinforced with ultrahigh molecular weight polyethylene (UHMWPE) and PP fibers and evaluated as potential nondegradable meniscal replacements. An investigation of hydrogel and composite mechanical properties indicates that fiber-reinforced PVA hydrogels could replicate the unique anisotropic modulus distribution present in the native meniscus; the most commonly damaged orthopedic tissue. More specifically, fibrous reinforcement successfully increased the tensile modulus of the biomaterial from 0.23±0.02MPa without any reinforcement to 258.1±40.1MPa at 29vol.% UHMWPE. Additionally, the molecular weight between cross-links, bound water and the microstructure of the PVA hydrogels were evaluated as a function of freeze-thaw cycles and polymer concentration to lend insight into the processes occurring during synthesis. These results suggest the presence of multiple mechanisms as causes for increasing hydrogel modulus with freeze-thaw cycling, including hydrogen bonding between amorphous and/or crystalline regions, and the formation of highly concentrated regions of mostly amorphous PVA chains. It is possible that the formation of regions with highly concentrated amounts of PVA increases the load-bearing ability of the hydrogels.

The role of crystallization and phase separation in the formation of physically cross-linked PVA hydrogels

The biocompatibility, processing ease, and mechanical properties of freeze-thawed poly(vinyl alcohol) (PVA)-based hydrogels have encouraged significant research toward developing this material for various biomedical applications. Crystallization that occurs during the freeze-thawing process is cited in the literature as the primary mechanism responsible for the resultant mechanical properties. Further analysis, however, shows the presence of two unique mechanisms that contribute to PVAs mechanical properties. During freeze–thaw cycling water freezes causing phase separation, which facilitates crystallization. The impact of phase separation during freeze–thaw cycling was investigated by comparing freeze-thawed and aged PVA hydrogels. Aged hydrogels were not prepared by freezing and, therefore, did not exhibit significant phase separation. The amount of phase separation was discerned using optical microscopy in the hydrated state. Crystallinity and mechanical properties were also evaluated as a function of the number of cycles (for freeze-thawed gels) and aging time (for aged gels). For freeze-thawed hydrogels, crystallinity deviated significantly from the trend observed in compressive modulus, indicating that crystallinity was not the only factor determining the hydrogels mechanical properties. Phase separation was found to occur during freeze–thaw cycling independently of crystallization, especially at later freeze–thaw cycles (after the third). The trends observed for both crystallinity and modulus for aged hydrogels, however, were in better agreement with each other. Further evaluation of the mechanical properties of aged and freeze-thawed hydrogels with similar crystallinities indicated that freeze-thawed hydrogels have significantly higher modulus values (p < 0.05). As a result, phase separation, independently of crystallization, was determined to have a significant effect on gelation during freeze–thaw cycling. In particular, PVA-rich regions that are formed during phase separation, without additional cross-linking, are believed to have a significant effect on the resultant mechanical properties.

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.