Thermal conductivity of amorphous polymers and its dependence on molecular weight

Thermal conductivity is an important transport property governing the performance of polymers in non-isothermal conditions. Nevertheless, its dependence on molecular weight M has not been the subject of as much attention as other properties of polymeric materials. We determine the thermal conductivity of polystyrene and polyisobutylene for a wide range of molecular weight by measuring the density, heat capacity and thermal diffusivity. Using coarse-graining and reverse mapping methods, we were able to produce molecular melts to study the thermal conductivity of polystyrene using molecular dynamics simulations over a similar range of molecular weight. We find satisfactory agreement between the experimental and simulation results. However, all of our results show that thermal conductivity depends only slightly on molecular weight up the entanglement limit and it is independent thereafter. Our results put into question the few previous experimental studies on this topic by showing that the previously accepted proportionality to $\sqrt{M}$ does not hold. Our findings could have significant implications for the understanding of complex phenomena such as anisotropic thermal conductivity in polymers subjected to flow.Marie Skłodowska-Curie IF MTCIATTP 750985

Experimental investigation of anomalous molecular probe diffusion in entangled polymer melts

Investigations on the diffusion of small molecules or particles in polymeric materials are important to numerous technologies and can also be used to gain insight on polymer chain dynamics. Systems where the probe size is comparable to (or smaller than) a characteristic length of the polymer chain, the tube diameter for example, are of particular interest because the diffusion coefficient of the probe can be orders of magnitude larger than the value predicted by the Stokes-Einstein relation. In the present study, we employ the optical technique known as forced Rayleigh scattering to study the diffusion of a molecular probe (dye) in several entangled polymer melts over a wide range of length and time scales. The probe size is much smaller than the tube diameter for the systems studied. We find the diffusion coefficient is larger by four to five orders of magnitude than the Stokes-Einstein prediction. More interestingly, we observe anomalous, non-Fickian, diffusion where the value of the measured diffusion coefficient can abruptly change by as much as 50%. We suggest that this unexpected behavior occurs when the time scale for diffusion is larger than the relaxation time associated with the constraint release mechanism for polymer chain dynamics

Nanoparticle-Assisted Water-Flooding in Berea Sandstones

The use of nanoparticles to improve reservoir characterization or to enhance oil recovery (EOR) has recently received intensive interest; however, there are still many unresolved questions. This work reports a systematic study of the effect of rutile TiO2 nanoparticle-assisted brine flooding. Rutile ellipsoid TiO2 nanoparticles were synthesized and stabilized by trisodium citrate dihydrate for brine flooding of water-wet Berea sandstone cores. Careful characterization of the rock samples and nanomaterials before and after the flooding was conducted, and the relative contributions to the modified flooding results from the stabilizer and the nanoparticles of different concentrations were examined. The oil recovery performance was evaluated both at the breakthrough (BT) point and at the end of flooding (∼3.2 pore volumes). Nanoparticle migration behavior was also investigated in order to understand the potential mechanisms for oil recovery. The results showed that both nanoparticle transport rate and EOR effect were strongly dependent on the particle concentration. The oil recovery efficiency at the BT point was found to increase at low nanoparticle concentrations but decrease at higher values. A maximum 33% increase of the recovery factor was observed at the BT point for a TiO2 concentration of 20 ppm, but higher nanoparticle concentrations usually had higher ultimate recovery factors. The presence of an oil phase was found to accelerate the particle migration though the core. The discussion of various mechanisms suggested that the improvement in the mobility ratio, possible wettability change, and log-jamming effect were responsible for the observed phenomena

Predictions of anisotropic thermal transport in non-linear-non-isothermal polymeric flows

Trabajo presentado en: 90th Annual Meeting of The Society of Rheology, 14 a 18 de ocubre de 2018, HoustonOver the last decades, significant efforts have been dedicated to include more complete rheological constitutive models into finite elements methods to simulate the complex flows in polymer manufacturing. However, while a remarkable portion of these processes are intrinsically non-isothermal, the study and implementation of non-isothermal flows has been very limited. The degree of complexity of such calculations is considerably increased by: 1) the addition to the problem of the energy equation; 2) a strong coupling to the momentum balance due to a highly temperature-dependent rheological behavior and 3) the strong influence that deformation-induced molecular orientation has on the thermo-physical properties of polymeric materials. Experimental evidence has shown that thermal conductivity becomes anisotropic in polymers subjected to deformation. Furthermore, a linear relationship between the thermal conductivity and stress tensors has been found to be universal (i.e. independent of polymer chemistry) and to extend beyond the finite extensibility limit. We make use of molecular simulation techniques to gain insights into the transport mechanisms behind these surprising results. On a more practical level, our work combines the thermal conductivity/stress response with two recent constitutive equations proposed for linear (Rolie Poly) and branched (eXtended Pom-Pom) polymers to venture predictions for the anisotropy in thermal conductivity in a number of interesting flows. These two constitutive models provide accurate descriptions of the available non-linear rheology and thermal transport data. Remarkably, our approach allows implementation of anisotropy in thermal conductivity into finite elements simulations without adding any adjusting parameters to those of the viscoelastic model. Our work represents a first step towards a molecular-to-continuum methodology for the simulation of industrially relevant non-isothermal flows to predict flow characteristics and the material final properties after processingMolecular to Continuum Investigation of Anisotropic Thermal Transport in Polymers “MCIATTP” Project # 750985Horizon 2020, “MCIATTP” Project # 75098

Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review

Nanofluids, i.e., well-dispersed (metallic) nanoparticles at low- volume fractions in liquids, may enhance the mixture's thermal conductivity, knf, over the base-fluid values. Thus, they are potentially useful for advanced cooling of micro-systems. Focusing mainly on dilute suspensions of well-dispersed spherical nanoparticles in water or ethylene glycol, recent experimental observations, associated measurement techniques, and new theories as well as useful correlations have been reviewed

First Assessment of the Impacts of the COVID-19 Pandemic on Global Marine Recreational Fisheries

This work is the result of an international research effort to determine the main impacts of the COVID-19 pandemic on marine recreational fishing. Changes were assessed on (1) access to fishing, derived from lockdowns and other mobility restrictions; (2) ecosystems, because of alterations in fishing intensity and human presence; (3) the blue economy, derived from alterations in the investments and expenses of the fishers; and (4) society, in relation to variations in fishers’ health and well-being. For this, a consultation with experts from 16 countries was carried out, as well as an international online survey aimed at recreational fishers, that included specific questions designed to capture fishers’ heterogeneity in relation to behavior, skills and know-how, and vital involvement. Fishers’ participation in the online survey (5,998 recreational fishers in 15 countries) was promoted through a marketing campaign. The sensitivity of the fishers’ clustering procedure, based on the captured heterogeneity, was evaluated by SIMPER analysis and by generalized linear models. Results from the expert consultation highlighted a worldwide reduction in marine recreational fishing activity. Lower human-driven pressures are expected to generate some benefits for marine ecosystems. However, experts also identified high negative impacts on the blue economy, as well as on fisher health and well-being because of the loss of recreational fishing opportunities. Most (98%) of the fishers who participated in the online survey were identified as advanced, showing a much higher degree of commitment to recreational fishing than basic fishers (2%). Advanced fishers were, in general, more pessimistic about the impacts of COVID-19, reporting higher reductions in physical activity and fish consumption, as well as poorer quality of night rest, foul mood, and raised more concerns about their health status. Controlled and safe access to marine recreational fisheries during pandemics would provide benefits to the health and well-being of people and reduce negative socioeconomic impacts, especially for vulnerable social groups.Versión del edito