ELASTOCALORIC EFFECTS IN POLYMERS

Elastocaloric (eC) materials undergo large thermal changes when subjected to changes in uniaxial mechanical stress. These elastocaloric effects represent an attractive solid-state alternative to current vapour compression cooling technologies, which are harmful to the environment and energy inefficient. Elastic polymers (elastomers) are a new group of elastocaloric materials that promise giant elastocaloric effects at room temperature near conformational phase transitions, due to large changes in their degree of crystallinity that can be driven reversibly using small mechanical stresses. Moreover, elastomers are typically non-toxic, recyclable, and inexpensive to produce, and therefore have potential for commercialisation in environmentally friendly solid-state coolers. In this research project, the elastocaloric performance of the prototypical elastomer natural rubber was studied. A complex investigation of mechanical and elastocaloric properties was conducted using both experimental and computational methods. A computational model for elastocaloric effects in natural rubber was created and compared with experimental data in order to gain full insights into the physics behind the elastocaloric properties of this technologically useful material. Further, a first of its kind elastocaloric prototype was built and tested. Indirect elastocaloric measurements based on one of the Maxwell relations were performed via the thermodynamical analysis of stress-strain curves that were taken at several temperatures. Direct elastocaloric measurements were performed using (i) an infra-red camera with high spatial resolution and time resolution, and (ii) a commercial mechanical testing machine that permits finite loading/unloading rates of 0.093 – 0.833 s-1. Large adiabatic changes in temperature of ~4.68 K on loading, and of ~ -5.49 K on unloading, at strain of 5 and the fastest available loading/unloading rate of 0.833 s-1, were observed near room temperature. The unexpected thermal asymmetry observed on loading and unloading was ascribed to an increase in the degree of crystallinity at constant strain, which was later confirmed with a computational model built using finite-element methods, and with Raman-spectroscopy. Further direct measurements were conducted using a new bespoke device that allowed stretching the natural rubber samples much faster than the commercial mechanical testing machine, and thus operating near the adiabatic limit of interest. The bespoke device permitted recording temperature changes of ~7.5 K and ~ -9.2 K for loading and unloading respectively, at strain of 5, and these values are in good agreement with the corresponding extrapolated adiabatic values of ~7.5 K and ~ -8.9 K that were obtained using the data taken with the commercial mechanical testing machine. The impact of pre-straining on natural rubber and its elastocaloric response was also investigated. It was established that pre-straining the sample by 100% prior to any elastocaloric cycling improved the mechanical properties of natural rubber without compromising their elastocaloric response.A complete finite-element model was developed in order to understand the mechanisms behind the large elastocaloric effects in natural rubber. The heat generated during a regular elastocaloric cycle was calculated by combining the entropic spring model and the Flory’s model of crystallisation. The combination of these two models provided a good representation of the thermal changes seen in natural rubber, and showed good agreement with data obtained from direct measurements. However, the Flory’s model of crystallisation assumes adiabatic conditions only and it is time independent, and thus it is unable to provide information on how the degree of crystallisation changes with time (the model provides information only at the start and the end of crystallisation). Therefore improvements to the model were done here via the addition of a mathematical function that reflects the changes in crystallisation with time. This improvement led to a better reproduction of the relevant processes behind the large elastocaloric effects seen in natural rubber. The newly modified model showed much better agreement with direct measurements and allowed investigating the influence of loading history on elastocaloric response. After establishing, using finite-element modelling, that large elastocaloric effects in natural rubber are primarily due to the crystallisation of polymer chains, further investigation of elastocaloric effects in samples of natural rubber filled with carbon black were conducted. Carbon black was identified as crystallisation enhancer in natural rubber compounds, and enhanced crystallisation in the carbon-black filled samples led to 300% improvements in elastocaloric response, yielding changes in adiabatic temperature of ~8.6 K and ~5.9 K for loading and unloading respectively, at strain of just 3. Finally, after having understood the complex elastocaloric behaviour of natural rubber, an elastocaloric cooling prototype based on this inexpensive material was built and tested. Most of the components for the prototype were produced in-house using 3D printing technology. The prototype was tested at every step, assembled and filled with water. During this process, the design was optimised several times to address issues that were identified during initial testing.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Kaminski, Michal

Advisor dc:contributor.advisor

• Moya Raposo, Xavier

• elastocaloric effects
• natural rubber

Licence

• https://www.repository.cam.ac.uk/bitstreams/10f1a39e-0a5f-4a3c-a01c-56547650925d/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.127257

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/398402