An experimental study was made of the effects of prior molecular orientation on large tensile deformations of polystyrene in the glassy state. A new hybrid glass-melt constitutive model is proposed for describing and understanding the results, achieved by parallel coupling of the ROLIEPOLY molecularly-based melt model with a model previously proposed for polymer glasses. Monodisperse and polydisperse grades of polystyrene are considered. Comparisons between experimental results and simulations illustrate that the model captures characteristic features of both the melt and glassy states. Polystyrene was stretched in the melt state and quenched to below T g , and then tensile tested parallel to the orientation direction near the glass transition. The degree of strain-hardening was observed to increase with increasing prior stretch of molecules within their entanglement tubes, as predicted by the constitutive model. This was explored for varying temperature of stretching, degree of stretching, and dwell time before quenching. The model in its current form, however, lacks awareness of processes of subentanglement chain orientation. Therefore, it underpredicts the orientation-direction strain hardening and yield stress increase, when stretching occurs at the lowest temperatures and shortest times, where it is dominated by subentanglement orientation. INTRODUCTIONAs demonstrated by a wide range of polymer products, molecular orientation is one of the fundamental parameters that determine the mechanical response of a melt processed, thermoplastic polymer.1 The degree of orientation present within a particular product is a complex function of its rheology and the process parameters employed. The rheological behavior itself is intrinsically linked to the molecular weight and its distribution, and the chain architecture, as well as the presence of any additives. Consequently, as industrial polymer processing engineers have long known, mechanical properties of practical importance such as Young's modulus, yield stress, and fracture toughness in any given direction are highly sensitive to the grade of polymer employed and the flow history encountered during processing.2-9 Therefore, it is of great practical interest to understand better the development of frozen-in molecular orientation during melt processing, and the relationships between this orientation and resulting solid-state properties. In particular, there is an engineering need to achieve a predictive capability, for use in optimizing polymer products.

Edoardo (2015) Time-dependent mechanical behavior of human amnion: Macroscopic and microscopic characterization. Acta Biomaterialia, 11 . pp. 314-323. ISSN 1878-7568 Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/45411/1/mauri_actabiomaterialia2014_preprint.pdf Copyright and reuse:The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions. This article is made available under the Creative Commons Attribution Non-commercial No Derivatives licence and may be reused according to the conditions of the licence. For more details see: http://creativecommons.org/licenses/by-nc-nd/2.5/ A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. AbstractCharacterizing 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 has 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 2 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 seconds) and (ii) long-term dissipative behavior without macroscopic deformation and no systematic global reorientation of collagen fibers. IntroductionThe fetal membrane (FM) surrounds the growing fetus and ensures its environment during gestation. Preterm premature rupture of the membrane affects about 3% of all pregnancies and increases the risk of morbidity in the newborn [1]. The etiology of preterm premature rupture of the membrane is complex and not completely understood. Repeated mechanical loading, such as that occurring as a result of fetal movement and labor, was recen...

A simple model of deploying tree leaves is assembled in different arrangements to produce polygonal foldable membranes for use as deployable structures. One family of folding patterns exhibits a small strain mechanism, which is investigated. Variations on the basic arrangements can be used to fold membranes with a discretized curvature.

Employing a test method with coupled application and peel phases, tack was characterised for a UD prepreg tape. Different aspects of tack were explored by varying test parameters and material condition. In addition, different surface combinations were studied. In general, the test parameters, feed rate and temperature, affect the balance between cohesion within the resin and adhesion between resin and substrate. Exploring a range of parameters is required to understand the effect of viscoelastic resin properties on tack. The application pressure determines the true contact area between prepreg and substrate and hence affects tack. Changes in molecular mobility in the resin related to specimen conditioning, i.e. ageing or moisture uptake, result in maximum tack to occur at lower or higher feed rates, respectively. Differences in tack for different material combinations can be attributed to different molecular interactions at the contact interfaces and different resin distributions on the prepreg surfaces.

(2015) High-pressure rheological analysis of CO2-induced melting point depression and viscosity reduction of poly(ε-caprolactone). Polymer, Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/45413/1/curia_polymer2015_preprint.pdf Copyright and reuse:The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions. This article is made available under the Creative Commons Attribution Non-commercial No Derivatives licence and may be reused according to the conditions of the licence. For more details see: http://creativecommons.org/licenses/by-nc-nd/2.5/ A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Furthermore, a significant decrease in the viscosity of two PCL grades with different molecular weight (M n~1 0 kDa and 80 kDa) was also detected upon increasing the CO 2 pressure to 300 bar.Experimental viscosity data were fitted to the Carreau model to quantify the extent of the plasticising effects on the zero-shear viscosity and relaxation time under different conditions. Similar analyses were conducted under high-pressure nitrogen, to compare the effects obtained in the presence of a non-plasticising gas.

No abstract

Visible craze initiation stress has been measured for a wide range of linear and branched monodisperse polystyrenes (PS) soaked in diethylene glycol. Results show that, for a given time under stress, craze initiation in linear PS is disentanglement-dominated below a critical molar mass and chain scission-dominated above it. Branched monodisperse PS behaves similarly, with the relevant molar mass in this case being the span molar mass. Disentanglement craze initiation stress is found to vary linearly with log molar mass and log time. These observations can be explained in terms of Eyring-type stress acceleration of the process of chain retraction, required to achieve the entanglement loss necessary for creation of craze fibril surfaces. A single effective activation volume of 1.8 nm3 accounts for the dependence of crazing stress on molar mass, time, and temperature under uniaxial tensile stress, both as observed in the present data and in a previous study of rate/temperature dependence.

No abstract