Mullins effect in a filled elastomer under uniaxial tension.

Modulus softening and permanent set in filled polymeric materials due to cyclic loading and unloading, commonly known as the Mullins effect, can have a significant impact on their use as support cushions. A quantitative analysis of such behavior is essential to ensure the effectiveness of such materials in long-term deployment. In this work we combine existing ideas of filler-induced modulus enhancement, strain amplification, and irreversible deformation within a simple non-Gaussian constitutive model to quantitatively interpret recent measurements on a relevant PDMS-based elastomeric cushion. We find that the experimental stress-strain data is consistent with the picture that during stretching (loading) two effects take place simultaneously: (1) the physical constraints (entanglements) initially present in the polymer network get disentangled, thus leading to a gradual decrease in the effective cross-link density, and (2) the effective filler volume fraction gradually decreases with increasing strain due to the irreversible pulling out of an initially occluded volume of the soft polymer domain.

Radiation-induced mechanical property changes in filled rubber.

In a recent paper we exposed a filled elastomer to controlled radiation dosages and explored changes in its cross-link density and molecular weight distribution between network junctions [A. Maiti et al., Phys. Rev. E 83, 031802 (2011)]. Here we report mechanical response measurements when the material is exposed to radiation while being under finite nonzero strain. We observe interesting hysteretic behavior and material softening representative of the Mullins effect, and materials hardening due to radiation. The net magnitude of the elastic modulus depends upon the radiation dosage, strain level, and strain-cycling history of the material. Using the framework of Tobolsky's two-stage independent network theory we develop a model that can quantitatively interpret the observed elastic modulus and its radiation and strain dependence.

Controlled manipulation of elastomers with radiation: Insights from multiquantum nuclear-magnetic-resonance data and mechanical measurements.

Filled and cross-linked elastomeric rubbers are versatile network materials with a multitude of applications ranging from artificial organs and biomedical devices to cushions, coatings, adhesives, interconnects, and seismic-isolation, thermal, and electrical barriers. External factors such as mechanical stress, temperature fluctuations, or radiation are known to create chemical changes in such materials that can directly affect the molecular weight distribution (MWD) of the polymer between cross-links and alter the structural and mechanical properties. From a materials science point of view it is highly desirable to understand, affect, and manipulate such property changes in a controlled manner. Unfortunately, that has not yet been possible due to the lack of experimental characterization of such networks under controlled environments. In this work we expose a known rubber material to controlled dosages of γ radiation and utilize a newly developed multiquantum nuclear-magnetic-resonance technique to characterize the MWD as a function of radiation. We show that such data along with mechanical stress-strain measurements are amenable to accurate analysis by simple network models and yield important insights into radiation-induced molecular-level processes.

Solvent screening for a hard-to-dissolve molecular crystal.

Materials with a high-degree of inter- and intra-molecular hydrogen bonding generally have limited solubility in conventional organic solvents. This presents a problem for the dissolution, manipulation and purification of these materials. Using a state-of-the-art density-functional-theory based quantum chemical solvation model we systematically evaluated solvents for a known hydrogen-bonded molecular crystal. This, coupled with direct solubility measurements, uncovered a class of ionic liquids involving fluoride anions that possess more than two orders of magnitude higher solvation power as compared with the best conventional solvents. The crystal structure of one such ionic liquid, determined by X-ray diffraction spectroscopy, indicates that F- ions are stabilized through H-bonded chains with water. The presence of coordinating water in such ionic liquids seems to facilitate the dissolution process by keeping the chemical activity of the F- ions in check.

Oestrogenic and androgenic activity of triclosan in breast cancer cells.

As a consequence of its widespread use as an antimicrobial agent in consumer goods, triclosan has become distributed ubiquitously across the ecosystem, and recent reports that it can cause endocrine disruption in aquatic species has increased concern. It is reported here that triclosan possesses intrinsic oestrogenic and androgenic activity in a range of assays in vitro which could provide some explanation for the endocrine disrupting properties described in aquatic populations. In terms of oestrogenic activity, triclosan displaced [(3)H]oestradiol from oestrogen receptors (ER) of MCF7 human breast cancer cells and from recombinant human ER alpha/ER beta. Triclosan at 10(-5) m completely inhibited the induction of the oestrogen-responsive ERE-CAT reporter gene in MCF7 cells by 10(-10) m 17beta-oestradiol and the stimulation of growth of MCF7 human breast cancer cells by 10(-10) m 17beta-oestradiol. On its own, 1 microm triclosan increased the growth of MCF7 cells over 21 days. Triclosan also had androgenic activity. It displaced [(3)H]testosterone from binding to the ligand binding domain of the rat androgen receptor (AR). Triclosan was able to inhibit the induction of the androgen-responsive LTR-CAT reporter gene in S115 mouse mammary tumour cells by 10(-9) m testosterone and in T47D human breast cancer cells by 10(-8) m testosterone at concentrations of 10(-7) m and 10(-6) m, respectively. Triclosan at 2 x 10(-5) m antagonized the stimulation of the growth of S115+A mouse mammary tumour cells by 10(-9) m testosterone. The finding that triclosan has oestrogenic and androgenic activity warrants further investigation in relation to both endocrine disruption of aquatic wildlife and any possible impact on human health.

Vapor pressure and sublimation rate of molecular crystals: role of internal degrees of freedom.

It is a common practice to approximate the desorption rate of atoms from crystal surfaces with an expression of the form nueff exp(-DeltaE/kBT), where DeltaE is an activation barrier to desorb and nueff is an effective vibrational frequency approximately 10(12) s(-1). For molecular solids, however, such an approximation can lead to a many orders of magnitude underestimation of vapor pressure and sublimation rates due to neglected contributions from molecular internal degrees of freedom. Here, we develop a simple working formula that yields good estimates for a general molecular (or atomic) solid and illustrate the approach by computing equilibrium vapor pressure of three different molecular solids and an atomic solid, as well as the desorption rate of a foreign (inhibitor) molecule from the surface of a molecular solid.