Albert F. Yee

Toughening mechanisms in elastomer-modified epoxies

Some brittle epoxies can be toughened significantly by the addition of an elastomeric phase. A great deal of controversy still exists on the nature of the toughening mechanisms. In this work tensile dilatometry at constant displacement rates was used to determine whether voiding, crazing or shear banding are the deformation mechanisms. Diglycidyl ether-bisphenol A epoxies toughened by various levels of several types of carboxyl-terminated butadiene nitrile liquid rubber were studied. The results indicate that at low strain rates the rubber particles simply enhance shear deformation. At sufficiently high strain rates the rubber particles cavitate and subsequently promote further shear deformation. No indication of crazing as an important toughening mechanism is found. No significant effect of rubber particle size or type can be ascertained.

Influence of particle size and particle size distribution on toughening mechanisms in rubber-modified epoxies

The principal toughening mechanism of a substantially toughened, rubber-modified epoxy has again been shown to involve internal cavitation of the rubber particles and the subsequent formation of shear bands. Additional evidence supporting this sequence of events which provides a significant amount of toughness enhancement, is presented. However, in addition to this well-known mechanism, more subtle toughening mechanisms have been found in this work. Evidence for such mechanisms as crack deflection and particle bridging is shown under certain circumstances in rubber-modified epoxies. The occurrence of these toughening mechanisms appears to have a particle size dependence. Relatively large particles provide only a modest increase in fracture toughness by a particle bridging/crack deflection mechanism. In contrast, smaller particles provide a significant increase in toughness by cavitation-induced shear banding. A critical, minimum diameter for particles which act as bridging particles exists and this critical diameter appears to scale with the properties of the neat epoxy. Bimodal mixtures of epoxies containing small and large particles are also examined and no synergistic effects are observed.

Toughening mechanisms in thermoplastic-modified epoxies: 1. Modification using poly(phenylene oxide)

An epoxy based on the diglycidyl ether of bisphenol A (DGEBA) has been modified with poly(phenylene oxide) (PPO) and cured with piperidine. A two-phase alloy resulted, in which the DGEBA epoxy was the continuous phase. Several PPO loadings were investigated. The tensile yield strengths of these PPO-modified epoxies were found to be independent of PPO content. In contrast, the fracture toughness improved with PPO content in a linear fashion. The micromechanical mechanism responsible for the improvement in toughness was found to consist of crack bifurcation and microcracking. Some evidence of particle bridging was also observed, and it is thought that particle bridging may play an important role in the formation of a microcracked damage zone.

A Discussion of the Molecular Mechanisms of Moisture Transport in Epoxy Resins

A typical epoxy formulation can absorb several weight percent of water, seriously degrading the physical properties of the resin. In two preceding publications (Soles, C. L.; Chang, F. T.; Bolan, B. A.; Hristov, H. A.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 1998, 36, 3035; Soles, C. L.; Chang, F. T.; Gidley, D. W.; Yee, A. F. J Polym Sci Part B: Polym Phys 2000, 38, 776), the role of electron density heterogeneities, or nanovoids (as measured through positron annihilation lifetime spectroscopy), in the moisture-transport process is elucidated. In this article, the influence of these nanovoids is examined in light of both the specific epoxy-water interactions and the molecular motions of the glassy state to develop a plausible picture of the moisture-transport process in an amine-cured epoxy resin. In this description, the topology (nanopores), polarity, and molecular motions act in concert to control trans- port. Water traverses the epoxy through the network of nanopores, which are also coincident with the polar hydroxyls and amines. In this respect, the nanopores provide access to the polar interaction sites. Furthermore, the sub-Tg (glass-transition temper- ature) molecular motions coincident with the onset of the b-relaxation process incor- porate these polar sites and, hence, regulate the association of water with the epoxy. In effect, the kinetics of the transport mirror the dynamics associated with the local-scale motions of the b-relaxation process, and this appears to be the rate-limiting factor in transport. The volume fraction of the nanopores does not appear to be rate-limiting in the case of an amine-cured epoxy, contrary to popular theories of transport.

Constitutive modeling of polymeric foam material subjected to dynamic crash loading

This paper describes detail work on constitutive law modeling of low-density polymeric foam materials. Selected experimental results on low-density polyurethane (PU), polypropylene (PP), and polystyrene (PS) foams are presented. A rate-dependent hydrodynamic constitutive equation is presented for rigid polymeric foams. Focus has been placed on modeling of strain rate dependency and temperature effect on polymeric foams subjected to high rate impact loading. Numerical implementation procedure for the constitutive model is described. The constitutive model has been implemented into finite-element program as a user-defined material subroutine. Numerical examples are provided to validate the model under simple and complex loading conditions.

Toughening mechanisms in core-shell rubber modified polycarbonate

Abstract We have conducted a study of toughening mechanisms in rubber modified polycarbonate systems in order to evaluate the sequence of deformation events which improve fracture toughness. We conclude that cavitation of the rubber particles occurs first, followed by massive shear yielding of the matrix material. The size and shape of the deformation zone created in front of the crack is governed by the mechanical properties of the rubber particles and the stress state at the crack tip. The importance of using a variety of analytical techniques to characterize deformation mechanism is also illustrated.

Determination of pore-size distribution in low-dielectric thin films

Positronium annihilation lifetime spectroscopy is used to determine the pore-size distribution in low-dielectric thin films of mesoporous methylsilsesquioxane. A physical model of positronium trapping and annihilating in isolated pores is presented. The systematic dependence of the deduced pore-size distribution on pore shape/dimensionality and sample temperature is predicted using a simple quantum mechanical calculation of positronium annihilation in a rectangular pore. A comparison with an electron microscope image is presented.

Contributions of the nanovoid structure to the moisture absorption properties of epoxy resins

Epoxy resins absorb significant quantities of moisture, typically 1 to 7% by weight for various formulations, which can greatly compromise their physical properties. It is known that polarity of the epoxy is a significant factor in determining the ultimate moisture uptake. However, the contribution from molecular topology still remains vague. In this work, the effects of molecular topology are elucidated by synthesizing novel epoxies where the polarity is maintained constant but the topology is systematically altered. The molecular topology is quantified in part via Positron Annihilation Lifetime Spectroscopy (PALS) in terms of the nanometer-sized voids, or nanovoids, that are also commensurate with typical interchain distances. The nanovoids are separated into their absolute zero and thermally fluctuating fractions by performing PALS measurements over a wide range of temperatures. A strong correlation is observed between the absolute zero hole volume fraction and the ultimate moisture uptake. Although the correlation is clear, the absolute zero hole volume fraction alone is not sufficient to predict the ultimate moisture uptake, and network polarity must also be considered. It is surmised that the role of the nanovoids is to open the epoxy matrix and alleviate steric hindrances that may prevent a water molecule from associating with a polar group.

Reversal imprinting by transferring polymer from mold to substrate

A reversal imprinting technique was developed in this study. A polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. “Embossing” occurs at temperatures well above the glass transition temperature (Tg) of a polymer; “inking” occurs at temperatures around Tg with nonplanarized polymer coating surface on the mold; and “whole-layer transfer” occurs at temperatures around Tg but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the s...

Nanoimprinting over topography and multilayer three-dimensional printing

We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patte...