Hristo A. Hristov

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.

Influence of cyclic fatigue on the mechanical properties of amorphous polycarbonate

Abstract Changes in the mechanical properties of amorphous polycarbonate during the fatigue failure initiation stage lead to overall embrittlement of the polymer. The results indicate that the fracture toughness of the material can be reduced by ∼ 35% without visible damage. The ‘static’ mechanical characteristics, such as yield stress and strain and absorbed energy, are influenced very modestly by the cyclic fatigue. The observations are explained by the presence of a small number of nanometre-sized voids or ‘protocrazes’, detected by transmission and scanning electron microscopy measurements. It is shown that physical ageing and fatigue loading affect the structural state of the polymer differently despite similar effects on the macroscopic mechanical properties.

Changes of the hole volume in model epoxy networks

Abstract Positron annihilation lifetime spectroscopy measurements were conducted on a series of epoxy specimens, prepared by systematically varying the network rigidity and crosslink density. The investigations demonstrate that changes in the molecular structure of the constituent monomers change the microscopic parameters of the epoxy networks. An increase of chain rigidity increases the size and the number density of the trapping sites where orthopositronium localizes, which leads to an increase of the hole volume fraction at T g . It was found that the hole volume fraction at T g approximates a linear relationship with the packing densities of the epoxies under investigation. The results are interpreted as a decrease of the efficiency of molecular packing with increasing network rigidity. As a result, macroscopic materials properties such as glass transition temperature, density and thermal expansion show considerable variations.

Positronium formation in semicrystalline poly(ethylene terephthalate)

Abstract The probability of positronium (Ps) formation in semicrystalline poly(ethylene terephthalate) (PET), has been investigated by means of positron annihilation lifetime spectroscopy (p.a.l.s.). The p.a.l.s. data are correlated with the structural characteristics of the samples, such as crystalline content, interlamellar spacing and lamella thickness. The results show an approximately linear decrease of the intensity ( I 3 ) of the long-lived orthopositronium ( o -Ps) component with an increase in the crystalline content up to 53%, while the lifetime remains practically constant. It seems that the other structural parameters do not affect the Ps formation. The decrease of I 3 cannot be completely accounted for if one assumes that o -Ps localizes in the amorphous phase only. It is suggested that the o -Ps also forms in the crystalline regions, due to the inhomogeneous electron density distribution along the c -axis of the PET lattice.

Small angle X-ray scattering (SAXS) studies of sol to gel transition in K2O-Al2O3-SiO2 system

The structural evolution during a sol to gel transition reaction in the K2O-Al2O3-SiO2 system was investigated by using in situ small angle X-ray scattering (SAXS) technique. The results are interpreted as evidence of the presence of at least two different gel phases. The analysis shows that the primary phase is composed of small particles with characteristic size of ∼3.0 nm, and considerable size and shape variation. The primary phase is identified as a random phase in the Debye sense. The primary particles aggregate into larger formations with spherical symmetry. The size of larger aggregates (second phase) increases continuously in the course of gelation and levels off after reaching ∼15.0 nm. The volume fraction of the second phase keeps increasing until the end of the measurement which is far after the gelation (3-fold of the gelation time).