Thermal Conductivity of Cellulose Fibers in Different Size Scales and Densities.

Mathis Antlauf,Linn Berglund,Ove Andersson,Nicolas Boulanger,Kristiina Oksman

doi:10.1021/acs.biomac.1c00643

Mathis Antlauf, Linn Berglund + Show 3 more

Open Access

https://doi.org/10.1021/acs.biomac.1c00643

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Abstract

Considering the growing use of cellulose in various applications, knowledge and understanding of its physical properties become increasingly important. Thermal conductivity is a key property, but its variation with porosity and density is unknown, and it is not known if such a variation is affected by fiber size and temperature. Here, we determine the relationships by measurements of the thermal conductivity of cellulose fibers (CFs) and cellulose nanofibers (CNFs) derived from commercial birch pulp as a function of pressure and temperature. The results show that the thermal conductivity varies relatively weakly with density (ρsample = 1340–1560 kg m–3) and that its temperature dependence is independent of density, porosity, and fiber size for temperatures in the range 80–380 K. The universal temperature and density dependencies of the thermal conductivity of a random network of CNFs are described by a third-order polynomial function (SI-units): κCNF = (0.0787 + 2.73 × 10–3·T – 7.6749 × 10–6·T2 + 8.4637 × 10–9·T3)·(ρsample/ρ0)2, where ρ0 = 1340 kg m–3 and κCF = 1.065·κCNF. Despite a relatively high degree of crystallinity, both CF and CNF samples show amorphous-like thermal conductivity, that is, it increases with increasing temperature. This appears to be due to the nano-sized elementary fibrils of cellulose, which explains that the thermal conductivity of CNFs and CFs shows identical behavior and differs by only ca. 6%. The nano-sized fibrils effectively limit the phonon mean free path to a few nanometers for heat conduction across fibers, and it is only significantly longer for highly directed heat conduction along fibers. This feature of cellulose makes it easier to apply in applications that require low thermal conductivity combined with high strength; the weak density dependence of the thermal conductivity is a particularly useful property when the material is subjected to high loads. The results for thermal conductivity also suggest that the crystalline structures of cellulose remain stable up to at least 0.7 GPa.

Highlights

Cellulose is the structural component of the plant cell wall and the perfect choice of polymer for real green highstrength applications and other environmentally benign applications associated with structures based on macromolecules or nanoparticles.[1,2] In its nano-structured form, nanocellulose, it shows similar properties as other nanomaterials such as nanotubes with the potential of producing cheap, light-weight, strong constructions and/or functional materials while conforming to the demands of a sustainable society
cellulose fibers (CFs) and cellulose nanofibers (CNFs) samples were initially pressurized from atmospheric pressure to 0.06 GPa at room temperature
The sample porosity ε, or void content for CFs at 0.06 GPa, was calculated from eq 3, which gave a porosity of ε = 0.11 and a density of 1340 kg m−3; this should be a good estimate for the CNF sample

Summary

Introduction

Cellulose is the structural component of the plant cell wall and the perfect choice of polymer for real green highstrength applications and other environmentally benign applications associated with structures based on macromolecules or nanoparticles.[1,2] In its nano-structured form, nanocellulose, it shows similar properties as other nanomaterials such as nanotubes (e.g., high-strength and large aspect ratio) with the potential of producing cheap, light-weight, strong constructions and/or functional materials while conforming to the demands of a sustainable society. Because of its abundance and potential use in a wide range of applications, cellulose and nanocellulose microstructure−property relationships are important, and one key property is the thermal conductivity κ. The temperature variation of κ, and the effect of fiber size, is scarcely studied down to low temperatures; the results will help in understanding the origin of thermal resistivity in cellulose. We solve the issues by using pressure as a variable to determine the effect of porosity and density of κ and to reliably measure the thermal conductivity of cellulose and nanocellulose, as a function of both temperature and pressure. The results show the stability of the crystalline structures of cellulose up to high pressure and high density

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