Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams

Abstract

•Anisotropic cellulose nanofibril foams are super-insulating also at high humidity•Moisture-induced swelling offsets thermal conductivity increase due to water uptake•Thermal boundary conductance decreases 6-fold by increasing inter-fibrillar gap•Thinner fibrils enhance phonon scattering and reduce the thermal conductivity Efficient thermal insulation using biobased materials could reduce the energy consumption and minimize the carbon footprint of buildings. Biobased materials are sensitive to moisture, and there is a need to better understand how moisture controls the thermal conductivity. Here, we show that anisotropic cellulose nanofibril foams can be super-insulating also at high humidity. The relative humidity dependence of the thermal conductivity of super-insulating nanocellulose foams is controlled by moisture-induced phonon scattering and the replacement of air with water. The moisture-induced swelling and increase of the inter-fibrillar separation distance results in a reduction of the thermal boundary conductance that exceeds the thermal conductivity increase due to water uptake up to high relative humidity. Humidity-dependent phonon engineering could be used to tailor the heat transfer properties of biobased nanofibrillar materials for packaging and thermal management in buildings. Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications. Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications. 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Braun P.V. Effects of chemical bonding on heat transport across interfaces.Nat. Mater. 2012; 11: 502-506Crossref PubMed Scopus (469) Google Scholar can have a strong influence on the thermal boundary conductance of nanomaterials. Hence, it would be important to determine the contribution and importance of phonon scattering to the moisture-dependent thermal conductivity of anisotropic foams. However, the understanding of how RH and moisture uptake controls the thermal conductivity of hygroscopic anisotropic nanofibrillar-based foams is poor and the thermal boundary conductance of CNM-based materials has not been investigated previously. Here, we have combined thermal conductivity measurements and molecular simulations to quantify the effect of RH on the anisotropic heat transfer and thermal boundary conductance of ice-templated CNF foams with highly aligned nanofibrils of different diameters in the foam walls. The RH dependence of the thermal conductivity of hygroscopic nanocellulose foams was shown to be controlled by moisture-induced phonon scattering and the replacement of air with water. The moisture-induced swelling and increase of the inter-fibrillar separation distance can result in a 6-fold reduction of the thermal boundary conductance that exceeds the thermal conductivity increase due to water uptake up to high RH. Foams made from thinner fibrils display lower thermal conductivities due to enhanced phonon scattering. Understanding how heat transport of biobased nanofibrillar foams can be tuned by moisture uptake and release could enable novel ways to engineer hygroscopic super-insulating nanomaterials in packaging and building applications. Low-density foams were produced by ice templating aqueous suspensions of CNFs. We used three different CNFs that differed primarily in the degree of carboxylation at the C6 positions in the anhydroglucose units and the diameter (d) and length (L) of the fibrils (Figure 1A). X-ray diffraction (XRD) and atomic force microcopy (AFM) image analysis (Figure S1), together with conductometric titration and sedimentation measurements38Varanasi S. He R. Batchelor W. Estimation of cellulose nanofibre aspect ratio from measurements of fibre suspension gel point.Cellulose. 2013; 20: 1885-1896Crossref Scopus (102) Google Scholar of non-oxidized CNFs with an average diameter of 19 nm (CNF19), medium-charge 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized CNFs with an average diameter of 4.4 nm (CNF4.4), and high-charge TEMPO-oxidized CNFs with an average diameter of 2.3 nm (CNF2.3) showed that more intensive oxidation led to smaller fibril diameter, higher aspect ratio (L/d; Equation S1 and S2), and lower crystallinity index (Equation S3) (Table 1).Table 1Physical Properties of CNF19, CNF4.4, and CNF2.3 at 295 KCDaCD was measured by conductometric titration. (mmol COO–g−1)Crystallinity indexbCrystallinity was measured by XRD.(%)Diametercd was measured by AFM image analysis. d (nm)Aspect ratiodAspect ratio was measured by sedimentation.L/dCNF190.02 ± 0.0045419 ± 7.9110 ± 26CNF4.40.30 ± 0.020524.4 ± 1.8140 ± 4CNF2.31.60 ± 0.010452.3 ± 0.7200 ± 16a CD was measured by conductometric titration.b Crystallinity was measured by XRD.c d was measured by AFM image analysis.d Aspect ratio was measured by sedimentation. Open table in a new tab Directional growth of the ice crystals during freeze-casting resulted in strongly anisotropic foams with the CNF particles aligned in the growth direction of the ice crystals (Figure 1B). The nanofibril alignment was confirmed by 2D XRD patterns and azimuthal integration (Figure 1C, Equations S4 and S5), and is consistent with previous studies on CNM-based freeze-cast foams.39Munier P. Gordeyeva K. Bergström L. Fall A.B. Directional freezing of nanocellulose dispersions aligns the rod-like particles and produces low-density and robust particle networks.Biomacromolecules. 2016; 17: 1875-1881Crossref PubMed Scopus (104) Google Scholar,40Deville S. The lure of ice-templating Recent trends and opportunities for porous materials.Scr. Mater. 2018; 147: 119-124Crossref Scopus (58) Google Scholar The SEM images of the cross section along the radial direction (Figure 1D) of the CNF foams showed a porous honeycomb-like architecture with a narrower pore size distribution in the CNF2.3 foams (Figure 1E) than in the CNF19 (Figure S2A) or CNF4.4 (Figure S2B) foams. The macroporous structure of freeze-cast materials is determined predominantly by the confined growth of the ice crystals, and the nanoporous structure within the pore walls is strongly influenced by the ability of the particles to be transported and assembled as the freezing front moves through the dispersion.40Deville S. The lure of ice-templating Recent trends and opportunities for porous materials.Scr. Mater. 2018; 147: 119-124Crossref Scopus (58) Google Scholar The nanoporosity, Πnp, of the foams corresponds to pores with sizes between 2 and 100 nm. CNFs with low charge density (CD) display electrostatic repulsions that may be of insufficient magnitude to prohibit the CNF particles from aggregating during ice templating and therefore result in foams with disordered structures.41Anderson A.M. Worster M.G. Periodic ice banding in freezing colloidal dispersions.Langmuir. 2012; 28: 16512-16523Crossref PubMed Scopus (42) Google Scholar CNFs with high CD (i.e., 1.6 mmol COO–g−1) display strong interparticle repulsion41Anderson A.M. Worster M.G. Periodic ice banding in freezing colloidal dispersions.Langmuir. 2012; 28: 16512-16523Crossref PubMed Scopus (42) Google Scholar and produce ordered ice-templated structures (Figure 1D). High-resolution scanning electron microscopy (HRSEM) showed that the foam walls of the highly charged CNF2.3 foam were compact and thin, with a thickness of about 200 nm (Figure 1E). The difference in the Brunauer-Emmett-Teller (BET) surface area between the CNF2.3 foam (15 m2/g) and the CNF19 foam (8.7 m2/g; Figure S3) is much smaller than the >60-fold difference predicted based upon the fibril diameters, which suggests that the fibrils in the walls are tightly packed. Nitrogen sorption experiments (Figure S3) performed at dry conditions show that the foam walls contain a low fraction (3.5%–5%) (Equation S6 and S7, Table S1) of pores up to 12 nm (Figure S3C) in diameter. The radial (λr) and axial (λa) thermal conductivities of the CNF19, CNF4.4, and CNF2.3 ice-templated foams (Figure 2A) with a dry density between 5.9 and 6.4 kg m−3 were measured using the anisotropic mode of the Hot Disk at controlled temperature and RH (Figures S4A and S4B). The hot disk records the time-dependent temperature increase in response to a transient power pulse and determines the radial thermal diffusivity (αr, Figure S4C). The radial thermal conductivity, λr is calculated by Equation 142Lagüela S. Bison P. Peron F. Romagnoni P. Thermal conductivity measurements on wood materials with transient plane source technique.Thermochim. Acta. 2015; 600: 45-51Crossref Scopus (46) Google Scholar:λr=αrρCP(Equation 1) where CP is the specific heat capacity of the foam, and ρ is the density of the foam. The “dry” CP (RH = 0) is determined from differential scanning calorimetry (DSC) measurements (Figure S5), and the RH-dependent CP at RH > 0 is estimated by the rule of mixtures (Equation S12) taking into account the water uptake (Figure 3A) of the foam and the CP of water. The CP at dry (RH = 0) conditions decreased with increasing CD (and decreasing diameter) of the CNFs, from 1,180 to 753 J kg−1 K−1 for CNF19 and CNF2.3, respectively (Figure S5). The axial thermal conductivity is measured in the direction of the ice crystal growth and thus the direction of the columnar macropores, while the radial thermal conductivity is measured perpendicularly to the macropores and the aligned CNFs.Figure 3Experimental and Hybrid GCMC/MD Simulations of Moisture Uptake and Foam-Wall-Sorption-Induced SwellingShow full caption(A) Experimental moisture content (H2Ow) by mass of ice-templated foams prepared from CNF2.3, CNF4.4, and CNF19 compared with the moisture content obtained from hybrid GCMC/MD simulations for the CNF2.3 (CNF2.3-S) as a function of RH% at 295 K.(B) The estimated swelling (continuous line-○) and average inter-fibril gap (dashed line-Δ) of a CNF2.3 foam as a function of RH%, and the inter-fibril gap calculated by hybrid GCMC/MD (▲) as a function of RH%.(C–E) (C) Initial arrangement of four individually equilibrated fibrils before drying. Snapshots of cellulose bundle of (D) aligned CNF2.3 fibril after the drying when they come close to each other, and (E) the same fibrils subjected to RH that have swelled as water molecules have entered their interstitial sites. Cellulose chains are colored in orange, COO− groups in green, counterions in pink, and water in blue.View Large Image Figure ViewerDownload (PPT) (A) Experimental moisture content (H2Ow) by mass of ice-templated foams prepared from CNF2.3, CNF4.4, and CNF19 compared with the moisture content obtained from hybrid GCMC/MD simulations for the CNF2.3 (CNF2.3-S) as a function of RH% at 295 K. (B) The estimated swelling (continuous line-○) and average inter-fibril gap (dashed line-Δ) of a CNF2.3 foam as a function of RH%, and the inter-fibril gap calculated by hybrid GCMC/MD (▲) as a function of RH%. (C–E) (C) Initial arrangement of four individually equilibrated fibrils before drying. Snapshots of cellulose bundle of (D) aligned CNF2.3 fibril after the drying when they come close to each other, and (E) the same fibrils subjected to RH that have swelled as water molecules have entered their interstitial sites. Cellulose chains are colored in orange, COO− groups in green, counterions in pink, and water in blue. The thermal conductivities of the ice-templated CNF foams were anisotropic and depended strongly on the RH. The λr (Figure 2B) of the CNF foams were 3–10 times lower than λa (Figures S4D and S4E) depending on the RH and the diameter of the CNFs. The anisotropy of the thermal conductivity of the ice-templated foams is related to the alignment of the nanofibrils in the freezing direction (Figures 1C and S6) and the intrinsic anisotropy of the thermal conductivity of cellulose. The λr of the ice-templated foams decreased with an increase of RH up to 35%–50% RH and increased as the RH increased from 65% to 80% RH (Figure 2B). Freeze-cast foams prepared from CNFs with the smallest diameter (CNF2.3) displayed a lower λr compared with foams prepared from CNFs with larger diameter and lower CD (Figure 2B), and the lowest thermal conductivity (14 mW m−1 K−1) was attained at 35% RH for the CNF2.3 foam. This value of the radial thermal conductivity is significantly below the value for air (λ = 25.7 mW m−1 K−1), which is surprising considering that the majority of the pores of ice-templated foams are much larger than the mean free path of air (about 70 nm in open space).19Sakai K. Kobayashi Y. Saito T. Isogai A. Partitioned airs at microscale and nanoscale: thermal diffusivity in ultrahigh porosity solids of nanocellulose.Sci. Rep. 2016; 6: 20434Crossref PubMed Scopus (72) Google Scholar,43Zeng S.Q. Hunt A. Greif R. Transport properties of gas in silica aerogel.J. Non. Cryst. Sol. 1995; 186: 264-270Crossref Scopus (85) Google Scholar,44Ruckdeschel P. Philipp A. Retsch M. Understanding thermal insulation in porous, particulate materials.Adv. Funct. Mater. 2017; 27: 1-11Google Scholar The convection contribution should be negligible because the macropores are sufficiently small to minimize gas transport over large distances and the radiation contribution is also small at RT (295 K).44Ruckdeschel P. Philipp A. Retsch M. Understanding thermal insulation in porous, particulate materials.Adv. Funct. Mater. 2017; 27: 1-11Google Scholar,45Collishaw P.G. Evans J.R.G. An assessment of expressions for the apparent thermal-conductivity of cellular materials.J. Mater. Sci. 1994; 29: 2261-2273Crossref Scopus (99) Google Scholar The presence of nanopores in the foam walls is expected to reduce the gas conduction contribution, but it is clear that additional effects, such as a substantial decrease of the thermal boundary conductance by phonon scattering, need to be invoked to explain the very low thermal conductivities. The λr of the anisotropic freeze-cast CNF foams remained below the thermal conductivity of air between 10% and 70% RH (Figure 2B) and was always several times lower than λa (Figures S4D and S4E). In contrast, λa increased with increasing RH over the entire measured range (7%–80% RH) for all of the CNF foams (Figures S4D and S4E), similar to isotropic CNF- and polyoxamer-based foams.26Apostolopoulou-Kalkavoura V. Gordeyeva K. Lavoine N. Bergström L. Thermal conductivity of hygroscopic foams based on cellulose nanofibrils and a nonionic polyoxamer.Cellulose. 2018; 25: 1117-1126Crossref Scopus (23) Google Scholar Figure 2C shows that the effect of RH on λr was reversible between 7% and 50% RH, which suggests that the moisture uptake is reversible29Kulasinski K. Effects of water adsorption in hydrophilic polymers.Polym. Sci. Res. Adv. Pract. Appl. Educ. Asp. 2016; : 217-223Google Scholar and that the structure of the foam walls is not irreversibly affected by the moisture uptake within the investiga