Davide S.A. De Focatiis

Time-dependent mechanical behavior of human amnion: macroscopic and microscopic characterization.

Characterizing the mechanical response of the human amnion is essential to understand and to eventually prevent premature rupture of fetal membranes. In this study, a large set of macroscopic and microscopic mechanical tests have been carried out on fresh unfixed amnion to gain insight into the time-dependent material response and the underlying mechanisms. Creep and relaxation responses of amnion were characterized in macroscopic uniaxial tension, biaxial tension and inflation configurations. For the first time, these experiments were complemented by microstructural information from nonlinear laser scanning microscopy performed during in situ uniaxial relaxation tests. The amnion showed large tension reduction during relaxation and small inelastic strain accumulation in creep. The short-term relaxation response was related to a concomitant in-plane and out-of-plane contraction, and was dependent on the testing configuration. The microscopic investigation revealed a large volume reduction at the beginning, but no change of volume was measured long-term during relaxation. Tension-strain curves normalized with respect to the maximum strain were highly repeatable in all configurations and allowed the quantification of corresponding characteristic parameters. The present data indicate that dissipative behavior of human amnion is related to two mechanisms: (i) volume reduction due to water outflow (up to ∼20 s) and (ii) long-term dissipative behavior without macroscopic deformation and no systematic global reorientation of collagen fibers.

Viscoelastic melt rheology and time-temperature superposition of polycarbonate-multi-walled carbon nanotube nanocomposites

This work investigates the linear and non-linear viscoelastic melt rheology of four grades of polycarbonate melt compounded with 3 wt% Nanocyl NC7000 multi-walled carbon nanotubes and of the matching matrix polymers. Amplitude sweeps reveal an earlier onset of non-linearity and a strain overshoot in the nanocomposites. Mastercurves are constructed from isothermal frequency sweeps using vertical and horizontal shifting. Although all nanocomposites exhibit a second plateau at ∼105 Pa, the relaxation times estimated from the peak in loss tangent are not statistically different from those of pure melts estimated from cross-over frequencies: all relaxation timescales scale with molar mass in the same way, evidence that the relaxation of the polymer network is the dominant mechanism in both filled and unfilled materials. Non-linear rheology is also measured in large amplitude oscillatory shear. A comparison of the responses from frequency and amplitude sweep experiments reveals the importance of strain and temperature history on the response of such nanocomposites.

A model for the compressible, viscoelastic behavior of human amnion addressing tissue variability through a single parameter

A viscoelastic, compressible model is proposed to rationalize the recently reported response of human amnion in multiaxial relaxation and creep experiments. The theory includes two viscoelastic contributions responsible for the short- and long-term time-dependent response of the material. These two contributions can be related to physical processes: water flow through the tissue and dissipative characteristics of the collagen fibers, respectively. An accurate agreement of the model with the mean tension and kinematic response of amnion in uniaxial relaxation tests was achieved. By variation of a single linear factor that accounts for the variability among tissue samples, the model provides very sound predictions not only of the uniaxial relaxation but also of the uniaxial creep and strip-biaxial relaxation behavior of individual samples. This suggests that a wide range of viscoelastic behaviors due to patient-specific variations in tissue composition can be represented by the model without the need of recalibration and parameter identification.

Roles of prestrain and hysteresis on piezoresistance in conductive elastomers for strain sensor applications

Abstract Simultaneous experimental measurements of stress, strain and electrical resistivity were carried out by exposing two carbon black filled cross-linked elastomers and two nanocomposites of thermoplastic polyurethane and multiwalled carbon nanotubes to a series of cyclic strain histories. It was found that the resistivity–strain relationships of the materials exhibited different hysteresis and dependence on prestrain. The resistivity of the nanotube filled elastomers changed dramatically with prestrain, making them suitable for memory sensor applications. During cyclic loading, the carbon black filled elastomers exhibit a lower resistivity during the loading part of the cycles than during the unloading part; the opposite effect was seen in the nanotube filled elastomers. The phenomena can be explained in terms of plastic flow processes in the thermoplastics, and of cohesive forces between carbon black particles in the cross-linked elastomers. Bending and buckling of the nanotubes give rise to a region of strain at constant resistance, making them unsuitable for real time sensing.

Solid‐State Constitutive Modelling of Glassy Polymers: Coupling the Rolie‐Poly Equations for Melts with Anisotropic Viscoplastic Flow

This paper investigates the behaviour of a well‐characterised monodisperse grade of entangled atactic polystyrene across a very wide temperature and strain rate range through linear and non‐linear melt rheology and solid‐state deformation. In an effort to construct a constitutive model for large deformations able to describe rheological response right across this wide timescale, two well‐established rheological models are combined: the well known RoliePoly (RP) conformational melt model and the Oxford glass‐rubber constitutive model for glassy polymers. Comparisons between experimental data and simulations from a numerical implementation of the model illustrate that the model can cope well with the range of deformations in which orientation is limited to length‐scales longer than an entanglement length. One approach in which the model can be expanded to incorporate the effects of orientation on shorter length scales using anisotropic viscoplastic flow is briefly discussed.

Reversibility of the Mullins effect for extending the life of rubber components

ABSTRACT This paper investigates the recovery of the Mullins effect by heat-treatment of predeformed (300% tensile strain) carbon-black filled ethylene-propylene-diene rubber (EPDM) in an air-circulating oven. The tensile response was measured, and the secant modulus at 100% strain, energy to deform to 300% strain, ultimate tensile strength, strain to failure and permanent deformation were compared to undeformed specimens subjected to the same heat-treatments, 3–24 h at temperatures of 60–80°C. Recovery of properties is dependent on time and temperature of treatment, increasing with longer times and higher temperatures. Recovery of secant modulus follows time-temperature superposition (TTS) with an activation enthalpy of 111 kJ mol−1; TTS is shown to be applicable to the other measures, probably because of additional cure in the rubber resulting from the heat-treatment. Treatments under vacuum led to similar effects as in an air-circulating oven.

Influence of processing history on the mechanical properties and electrical resistivity of polycarbonate – multi-walled carbon nanotubes nanocomposites

In this work we investigate the effects of compounding temperature and secondary melt processing on the mechanical response and electrical behaviour of polycarbonate filled with 3 wt% carbon nanotubes. The nanocomposites were melt compounded in an industrial setting at a range of temperatures, and subsequently injection moulded or compression moulded. The surface hardness, uniaxial tensile properties and electrical resistivity were measured. Secondary melt processing is found to be the dominant process in determining the final mechanical properties and resistivity of these materials.

Rheological techniques for determining degradation of polylactic acid in bioresorbable medical polymer systems

A method developed in the 1980s for the conversion of linear rheological data to molar mass distribution is revisited in the context of degradable polymers. The method is first applied using linear rheology for a linear polystyrene, for which all conversion parameters are known. A proof of principle is then carried out on four polycarbonate grades. Finally, preliminary results are shown on degradable polylactides. The application of this method to degrading polymer systems, and to systems containing nanofillers, is also discussed. This work forms part of a wider study of bioresorbable nanocomposites using polylactides, novel hydroxyapatite nanoparticles and tailored dispersants for medical applications.

A Novel Method of Extraction of Blend Component Structure from SANS Measurements of Homopolymer Bimodal Blends.

A new method is presented for the extraction of single-chain form factors and interchain interference functions from a range of small-angle neutron scattering (SANS) experiments on bimodal homopolymer blends. The method requires a minimum of three blends, made up of hydrogenated and deuterated components with matched degree of polymerization at two different chain lengths, but with carefully varying deuteration levels. The method is validated through an experimental study on polystyrene homopolymer bimodal blends with . By fitting Debye functions to the structure factors, it is shown that there is good agreement between the molar mass of the components obtained from SANS and from chromatography. The extraction method also enables, for the first time, interchain scattering functions to be produced for scattering between chains of different lengths.