CNT microcellular foams

Journal Pre-proof Ultra-lightweight, super thermal-insulation and strong PP/CNT microcellular foams Jinchuan Zhao, Guilong Wang, Chongda Wang, Chul B. Park PII:

S0266-3538(19)32771-X

DOI:

https://doi.org/10.1016/j.compscitech.2020.108084

Reference:

CSTE 108084

To appear in:

Composites Science and Technology

Received Date: 5 October 2019 Revised Date:

14 February 2020

Accepted Date: 20 February 2020

Please cite this article as: Zhao J, Wang G, Wang C, Park CB, Ultra-lightweight, super thermal-insulation and strong PP/CNT microcellular foams, Composites Science and Technology (2020), doi: https:// doi.org/10.1016/j.compscitech.2020.108084. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

CRediT authorship contribution statement Jinchuan Zhao: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing, Funding acquisition. Guilong Wang: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - review & editing, Project administration, Funding acquisition. Chongda Wang: Methodology, Software, Formal analysis, Investigation. Chul B. Park: Methodology, Investigation, Supervision, Writing review & editing.

Ultra-lightweight, super thermal-insulation and strong PP/CNT microcellular foams

Jinchuan Zhao a,b, Guilong Wang a,∗∗, Chongda Wang b, Chul B. Park b

a

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education),

Shandong University, Jinan, Shandong 250061, China b

Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering,

University of Toronto, Toronto, Ontario M5S 3G8, Canada

∗

Corresponding author. E-mail address: [email protected] (G. Wang) 1

Abstract The global energy crisis has been widely concerned by the public due to the massive energy requirement caused by rapid-developing economy and society. Ultra-lightweight, super-insulating, and strong polymer foams exhibit a promising prospect, in terms of saving materials and resources, and reducing energy consumption. However, there exists a great challenge to achieve highly expanded microcellular polymer with satisfactory thermal insulation performance. Herein, polypropylene (PP) with carbon nanotubes (CNTs) was used to prepare multifunctional foams by using batch foaming process with carbon dioxide. This cost-efficient and facile process endowed PP/CNT composite a variety of unprecedented advantages, including over 50-fold expansion ratio, a rather low thermal conductivity of 28.69 mW/m·K, and remarkably improved compressive strength. Acting as both crystal and cell nucleating agents, CNTs contributed to the fabrication of such ultralight foams with refined cell structure, which led to significantly reduced solid thermal conduction and enhanced mechanical properties. Moreover, thanks to CNTs’ outstanding infrared radiation shielding capacity, the thermal radiation through the ultra-lightweight PP/CNT foam was significantly suppressed. Thus, ultra-lightweight, super thermal-insulation, and strong PP/CNT composite foams were achieved by using microcellular foaming technology, paving a way for designing and synthesizing multifunctional polymer-based composite foams for high-performance thermally insulating applications.

Key words Polypropylene; Carbon nano tube; Composite; Microcellular foam; Thermal insulation

2

1. Introduction Rapid economic and social development is always associated with a massive requirement for energy, while the generation of energy is often accompanied by a large amount of emissions, which poses serious environmental and health challenges [1,2]. In dealing with environmental and energy hurdles, many new strategies for production and utilization energy have been attempted, such as developing renewable energy to reduce the strong dependence on petroleum-based fuels [3,4], but few of these innovations have achieved the beneficial results as expected [5]. In this context, the demand for innovative materials with super-high thermal insulation and ultralight performance is imminent and global, which has presented economic and environmental benefits by reducing material and energy loss. Among the current thermal insulation materials, polymer foams are believed to play an increasingly significant role in reducing emissions, preserving energy, and saving materials [6-8]. Polymer foams show attractive combined functional properties including lightweight, thermal and acoustic insulation, and energy damping, which contribute to their wide applications in automobile, transportation, construction, and aerospace industries [9-12]. Polypropylene (PP), as one of the most cost-efficient thermoplastic polymers, has already been widely applied in many fields including construction, textile, packaging, transportation, and automotive industries, due to its low price, excellent mechanical properties, outstanding processability, good chemical resistance, and high recyclability [13,14]. Thus, PP is determined as a promising candidate for fabricating ultralight, super-insulating, and strong polymer foams. However, it is still challenging to manufacture extremely lightweight microcellular PP foams with outstanding thermally-insulating and mechanical properties. First, regular PP shows rather poor foaming ability due to the low melt strength caused by its linear chain structure, and hence it is very difficult to achieve microcellular PP foams with very large expansion ratios [15]. Second, for superhigh void-fraction microcellular polymer, the significantly decreased cell wall thickness severely increases the transmittance of thermal radiation, and dramatically decays the rigidity of cells, leading to the considerable attenuation of thermal insulation and mechanical performance [16,17]. Additionally, compared with other polymers such as polystyrene (PS) [6,7,18] and polymethyl methacrylate (PMMA) [19,20], PP has much higher solid thermal conductivity, much poorer infrared radiation (IR) absorbing ability, and lower strength and stiffness, which further increases the challenge of fabricating high-performance PP foams for thermal insulation applications. Many efforts have been devoted to promoting the foaming ability of regular PP over the past three decades. Basically, the strategies used for improving the foaming ability of PP include enhancing melt strength, improving molecular crystallization, and heterogeneous nucleation [13,15]. Polymer modification by incorporating inorganic or organic phases has been demonstrated to be a cost-effective way in improving PP’s foaming ability [21,22]. The commonly used inorganic fillers include carbon nanotube (CNT) [11,23], cellular nanofiber (CNF) [24], clay [25,26], talc [15,27], silica [28]. The organic fillers include polyethylene terephthalate (PET) [29,30], polyethylene (PE) [31], and polytetrafluoroethylene (PTFE) [13,32-34]. Among them, CNTs have exhibited outstanding capability in 3

improving PP’s foaming behavior, which has been identified by many researchers. Firstly, CNTs are very effective heterogeneous nucleating agents due to their highly-ordered molecular structure and extremely large specific surface areas, which can significantly accelerate crystallization and refine crystal sizes [23,35]. Secondly, CNTs can generate numerous physical entanglement points that contribute to increased melt viscoelasticity and enhanced melt strength [11,12,36]. Thirdly, CNTs themselves can act as heterogonous cell nucleating agents to increase cell density and decrease cell size [37]. Overall, it is believed that the addition of CNTs would efficiently contribute to the fabrication of ultra-lightweight PP foams with refined cell structure, by dramatically improving the foaming ability of PP. Gas and solid conduction, and thermal radiation are considered as the main heat transfer modes for microcellular polymers [7]. For polymer foams with high void fractions, gas conduction is the major contribution of heat transfer. Thus, replacing air with insulation gases is very effective in improving the total thermal insulation performance of polymer foams. However, the insulation gases usually suffer from high cost, environmental damage, flammability, toxicity, and other issues. Moreover, as insulating gases are gradually replaced by air, the thermal insulation performance of foam deteriorates as time being [38,39]. Due to the so-called Knudson effect, reducing cell sizes into nanoscale is another effective way to substantially enhance the thermal insulation performance of polymer foams. However, the current foaming technologies restrict the fabrication of nanofoams with high expansion ratios, leading to the relatively high thermal conductivity contributed by solid conduction [40,41]. For highly expanded foams, solid conduction and thermal radiation are two conflicting variables related to the expansion ratio of polymer foam. Increasing expansion ratio leads to reduced solid conduction but increased thermal radiation [6,7,42]. Many studies have identified that carbonaceous materials such as graphite and CNTs show great significance to pronouncedly reduce the thermal radiation of largely expanded polymers, by converting radiative energy to thermal energy [43,44]. Moreover, researches have demonstrated that the incorporation of CNTs into polymer matrix led to significantly enhanced mechanical performance of polymer foams due to the following reasons. Firstly, CNTs could pronouncedly refine the cellular structure, which effectively contributes to the reinforcement of foam strength according to Gibson and Ashby’s model (Eq. S7-S11) [17]. Secondly, it is also identified that CNTs are able to improve the Young’s modulus and yield strength of the matrix, and further to promote the mechanical properties of foams [45,46]. Herein, this study reported an economic, flexible, easy-to-control, and non-polluted foaming process to fabricate ultra-lightweight, super thermal-insulation and strong PP/CNT composite foams. Firstly, extruded PP and PP/CNT composites were prepared using a micro compounder, and then, the CNT morphology and distribution were observed using a transmission electron microscope (TEM). Afterwards, the heat change, crystal evolution, and crystal-structure difference in crystallization process of PP and PP/CNT composites were characterized by a differential scanning calorimeter (DSC), a in-situ crystallization visualization system, and an X-ray diffractometer, 4

respectively. The frequency and temperature dependence as well as extensional rheological properties of all materials were measured by an oscillatory rheometer. Subsequently, batch foaming experiments were carried out to prepare microcellular samples with supercritical carbon dioxide (CO2), and the foam structure was characterized, followed by the analysis of the CNTs’ impact on the foaming behavior based on the materials’ crystallization and rheological behavior. Finally, the mechanical and thermal insulation properties of foams were evaluated, and the results demonstrated that microcellular PP/CNT composites with a 30 µm cell size and an over 50-fold expansion ratio were successfully prepared, presenting a super thermal insulation performance, as low as 28.69 mW/m·K thermal conductivity. The multifunctional ultralight microcellular material provides a promising solution to satisfy advanced thermal-insulation requirements.

2. Experimental 2.1. Preparation and characterization of PP/CNT composites A commercial homopolymer, polypropylene (PP), was provided by LyondellBasell Industries N.V., USA, with a grade of Moplen HP 400R. Its density and melt flow rate are 0.9 g/cm3 and 25 g/10 min (230°C/2.16 kg), respectively. Commercial PP/multi-walled carbon nanotube (CNT) 10 wt.% were supplied by Nanocyl SA, Germany. As for the CNTs, with a grade of NC 7000, their density is 1.75 g/cm3 and carbon purity is 90%. Additionally, the surface area, mean diameter and mean length of CNTs are 250-300 m2/g, 9.5 nm, and 1.5 µm, respectively. Carbon dioxide (CO2) with a purity of 99.9%, was selected as the physical blowing agent, purchased from Linde Gas Inc. A DSM micro compounder, MC 15 HT, made by Xplore Instrument B.V., Netherlands, was employed to prepare PP composites with four different CNT contents (0.0 wt.%, 0.5 wt.%, 2.5 wt.%, and 5 wt.%). Before compounding, both the pure PP and PP/CNT masterbatch in pellet form were dried at 85°C for 5 h using a vacuum oven. 14 g total weight of each dry blend were compounded at 190°C and 50 rpm for 10 min, which was contributed to the uniform dispersion of CNTs in PP matrix. An environmental scanning electron microscope (ESEM, Quanta FEG 250) made by FEI company, USA, and a transmission electron microscope (TEM, H7650) manufactured by Hitachi Group, Japan, were employed characterized the dispersion of CNTs in PP matrix. As for the ESEM sample, the blend was first cryogenically fractured in liquid nitrogen, and then the cross-section was coated with a nano-scale layer of Pt for ESEM observation using a sputter coater (SC7620), manufactured by Quorum Technologies Ltd., UK. To prepare the TEM sample, a Leica microtome (EM UC6) was used to cut an ultrathin section (around 100 nm) from the PP/CNT 0.5 wt.% composite at -100°C. Furthermore, a hot compression (Model CH 4386) made by CARVER Inc., USA, was employed to prepare the samples for rheological properties, crystal structures, and spectral properties. Samples were hot compressed at 190°C for 8 min, using three types of molds, a round one (Φ25 mm × 1.2 mm), a thick rectangle one (18 mm × 10 mm × 0.7 mm), and a thin rectangle one (25 mm × 25 mm × 0.05 mm). The round samples were used to in the dynamic 5

oscillatory shearing testing. The thick rectangle samples were used in the extensional viscosity and wide angle X-ray diffraction (WAXD) measurements, and the thin samples were applied to measure the spectral properties of all materials.

2.2. Crystallization behavior characterization A differential scanning calorimeter (DSC Q2000), made by TA instruments Inc., USA, was employed to study the non-isothermal crystallization behaviors of PP composites with different contents of CNTs. Around 10 mg material of each composite, packaged by an aluminum cap and pan, was prepared for DSC testing. In measurement, the testing sample, placed in a chamber filled with nitrogen, was heating and cooling twice between 30ºC and 240ºC with a rate of 10ºC/min, from which the exothermic peak and endothermic peak were acquired resulting from the formation and dissolution of crystals, respectively. Notably, the temperature of the chamber was kept at 240ºC for 10 min after the first heating scan, to remove the material’s thermal history, and then, the first cooling scan and the second heating scan were selected to determine the crystallinity calculated by Eq. S(1) and transition temperatures. Additionally, to ensure the reliability of the characterization, all measurements for each composite were repeat three times, and the mean values were reported as the results. An in-situ crystallization visualization system illustrated by Fig. S1, mainly consisting of a transparent glass assembled vessel, an imaging system, and a temperature control system, was employed to investigate the influence of CNTs on the isothermal crystallization behavior of PP matrix. This system could directly embody the difference of the crystallization speed and crystal size between PP and PP/CNT 0.5 wt.% composites. In measurement, a thin layer of material was first sandwiched by two microscope slides, and then was put in the middle of the visualized vessel. Thereafter, the vessel was heated to 210ºC at a rate of 20ºC/min, and was kept isothermally for 10 min. Subsequently, the vessel was sharply cooled to 140ºC at a rate of 30ºC/min, to avoid the formation of crystals in the cooling stage, and then maintained the temperature to observe the isothermal crystallization behaviors of PP and PP/CNT composite. An X-ray diffractometer (Generator-PW 1830 HT), made by Philips Analytical X-ray B.V., Netherlands, was used to collect the X-ray diffraction patterns of all materials, which could exhibit the crystal structure. The samples were scanned with an angle range of 10º-30º (2θ) and a step size of 0.02º/s under a high-resolution Cu Kα radiation source. Then, an X’Pert HighSore software was used to analyze the obtained diffraction patterns, especially for the peak intensities and their corresponding 2θ.

2.3. Rheological characterization A rotational rheometer (ARES-G2) with a Φ25 mm parallel plate geometry, manufactured by TA instruments Inc., USA, was used to conduct two different types of oscillatory shearing testing for all materials, including dynamic frequency sweep testing (DFST) and dynamic temperature step testing (DTST). To investigate the dependence of blends’ viscoelastic performance on shear frequency at a constant temperature, DFST was performed 6

at 190ºC as shear frequency varying between 0.1 rad/s and 100 rad/s. After that, DTST was carried out at a constant frequency of 0.63 rad/s when temperature decreased from 220ºC to 120ºC with a rate of 2ºC/min. For both of the DFST and DTST, the strain rate was set as 2%. The same rheometer with an extensional viscosity fixture was employed to demonstrate the influence of CNTs on the uniaxial elongational flow of PP matrix. In measurement, samples of PP and PP/CNT composites were used in the testing. Moreover, three extension rates, 0.01 s-1, 0.1 s-1, and 1 s-1, were taken, and the testing temperature was set as 170ºC.

2.4. Foaming process and cell morphology characterization A high-pressure autoclave specially designed with a capacity of 10 cm3, was used to perform the batch foaming experiments. A high-pressure syringe pump (260D), made by Teledyne ISCO Inc., USA, and a temperature control system illustrated by Fig. S2 were employed to previously regulate the saturation pressure and temperature. In foaming procedure, first the autoclave was heated to predetermined saturation temperature, followed by putting the rod sample of each composite into the autoclave, which is prepared by the pristine extrudate from the DSM extruder with 15 mm in length and around 3 mm in diameter. Then, CO2 was pressurized to a specific value, and the saturation time was recorded after purging the autoclave twice. After the saturation time, the pressure was rapidly depressurized by leasing the CO2 in the autoclave, which induced the foaming behavior. The saturation pressure, temperature, and time was set as 13.79 MPa, 161.5ºC, and 10 min, respectively. The water displacement method according to ASTM D792-00, was used to calculate the expansion ratio with Eq. S(2). Noteworthily, the solid skin layers of the foamed specimens were cut off using an edge blade prior to measurement to remove their influence on the results. Afterwards, the same ESEM mentioned above was applied to observe the cellular morphology of the foamed samples. In prior to the characterization, the prepared foams of all materials were frozen in liquid nitrogen, and then were cryogenically fractured to protect the initial cell structure. Subsequently, the same coater was used to coat the fracture surface with a thin layer of Pt before ESEM observation. Then, three of the achieved ESEM micrographs for each material were further analyzed using a commercial software, ImageJ, to measure cell size and count cell number in a specific area. Based on the results, the average cell size and cell density as well as their standard deviation were calculated using the Eq. S(4)-(5).

2.5. Compressive property characterization An electromechanical testing system (INSTRON 2710-102), made by Instron Engineering Corporation, USA, was employed to investigate the influence of the cellular structure on the compressive properties of the foamed specimens. Before measurement, a thermo cut hot wire cutter (37080), purchased from Proxxon Inc., was applied to prepare samples for compressive testing by cutting the foams into 6 mm-length cylindrical portion. Further, the cylindrical portion was polished after being immersed into liquid nitrogen for 20 min, to remove the solid skin and meanwhile avoid damage the cell structure. It’s worth noting that the polished cylindrical samples were fully dried in 7

a vacuum oven at 60ºC for 3 days, and then placed at room temperature for 5 days, which are used to eliminate the impact of environmental conditions including CO2, N2 and moisture in foams, on the testing results.

2.6. Thermal insulation property characterization A transient plane source (TPS 2500) Hot Disk thermal constant analyzer, manufactured by Thermtest Inc., Sweden, was applied to investigate the cellular structure and CNTs on the thermal conductivity of PP foams. In a normal measurement, a planar sensor is sandwiched with two pieces of samples, which simultaneously act as a power supplier and a temperature sensor shown as Fig. S3. When the sensor supplies step-wise power, it can record its own corresponding temperature response over heating time. Then, the thermal analyser software built-in Hot Disk was used to deduce the sample’s thermal conductivity by analyzing the temperature versus time response, and the test mechanism is illustrated by Eq. S(12)-(13). Herein, the thermal conductivity measurement was performed at room temperature using two cylindrical specimens (Φ 10 mm × 5 mm), which had been dried completely in a vacuum oven and placed at test conditions for 3 weeks. The diameter of the sensor employed in the measurement was 4 mm. The power output and test duration for foamed samples were set as 5 mW and 10 s, and these for solid samples were 10 mW and 5 s. A Fourier transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum One), made by PerkinElmer Inc., USA, was applied to characterize the spectral properties of the prepared films and foams. The spectral range of wavenumber used in measurement is from 400 cm-1 to 4000 cm-1, or wavelength range from 25 µm to 2.5 µm, and 8 scans for each sample were implemented to obtain the stabilized spectral data. Additionally, the background noise caused by the airborne CO2 and H2O was registered before measuring the infrared radiation (IR) transmittance of the prepared samples. First, the reflectance and transmittance of PP and PP/CNT films with a thickness of around 45 µm were recorded to achieve the transmittance and the IR absorption of the four PP/CNT composites, according to Eq. S(16)-(18). Then, the transmittance of the foamed specimens with different thickness cut from the achieved foams were measured to estimate the foams’ IR radiative energy using Eq. S(19)-(23). Noteworthily, the foamed samples used in the FTIR testing were filled with air at standard conditions only by the process for preparing the compressive samples. More than 5 specimens for each case, with a thickness range from 0.3 mm to 1.5 mm, were applied to calculate the radiative thermal conductivity, which helps to remove the effect of foam thickness on the IR transmittance.

3. Results and discussion 3.1. Composite morphology Fig. 1 presents an ESEM image and a TEM image of PP/CNT 0.5% composite. From Fig. 1a, together with Fig. S4 which shows the ESEM images of PP/CNT 5.0 wt.% and 2.5 wt.% composites, it is observed that the dispersion of CNTs in PP matrix was uniform, and no wide-range aggregation occurred in the composites. However, the mean length of CNTs shown by the TEM image was much shorter than 1.5 µm described in the material specification. The 8

reason for this phenomenon is that the large shear force during compounding helps to disentangle the CNTs, and thus their tendency of massive aggregation in PP matrix is effectively inhibited. Conversely, the large compounding force also leads to severe reduction of CNTs’ initial length. It is noteworthy that uniformly dispersed CNTs can not only pronouncedly improve PP’s foaming ability by enhancing viscoelasticity and crystallization as well as acting as a heterogeneous cell nucleating agent, but also effectively absorb thermal radiation by acting as a black body [7,11,23,47]. Consequently, it is expected that CNTs could help to achieve super-lightweight PP foams with high thermal insulation and excellent compressive strength.

Fig. 1. (a) ESEM image and (b) TEM image of PP/CNT 0.5% composite.

3.2. Crystallization behavior of PP/CNT composites Fig.2 a-b plot the DSC curves of all PP/CNT composites in the non-isothermal crystallization process at a cooling/heating rate of 10°C/min. It can be found from Fig. 2a that the presence of CNTs effectively contributes to the shift of crystallization peak and onset crystallization temperature to higher temperature, compared with that of pure PP, and the higher the CNT content, the greater the effect on the shift. It demonstrates that the CNTs, act as crystal nucleating agents, can reduce the crystal nucleation energy barrier of PP matrix, and thus, low degree of subcooling can lead to the occurrence of crystallization, on which high CNT contents have a more pronounced impact [23,32]. As Fig. 2b plots, the melting temperature of PP matrix is obviously increased after adding CNTs, manifesting that influence of CNTs on the crystal morphology as well as the crystallinity of PP. Table 1 quantitatively summarized the key parameters about the crystallization behaviors of PP and PP/CNT composites, calculated using the DSC data. Obviously, the impact of CNTs on PP’s crystallization was mainly concentrated on the crystal morpholgy and amount, detailed as the following aspects. First, the onset crystallization temperature (Tonset), crystallization temperature (Tc), and melting temperature (Tm) of PP/CNT composites were increased up to 9.1ºC, 8.3ºC and 2.1ºC respectively, compared with that of pure PP, which has been confirmed that the phenomenon of crystallization process shifting to higher temperature benefits the improvement of PP’s faming ability. It demonstrates that the significant effect of CNTs as crystal nucleating agents on crystal morphology once again. Second, the crystallinity (Xc) and entropy (Hc) of PP/CNT composites both were smaller than that of pure PP, showing that CNTs decreased the total amount of PP’s crystals. It is inferred that the presence of CNTs might

9

increase the amount of molecular entanglements, and thus suppress the movement of PP chains to a certain extent, leading to the reduction of crystallinity and entropy [32]. 4

a

Heat flow Endo. up (J/g)

Heat flow Endo. up (J/g)

0

0.0 wt.% CNT

-4 0.5 wt.% CNT

-8

2.5 wt.% CNT 5.0 wt.% CNT

-12 110

120

130

b

3

0.0 wt.% CNT

2

0.5 wt.% CNT 2.5 wt.% CNT

1 5.0 wt.% CNT

0 140

140

150

160

170

Temperature (°C)

Temperature (°C)

Fig. 2. Non-isothermal crystallization behaviors of PP/CNT composites in (a) the cooling and (b) the reheating processes. Table 1. Key parameters for the non-isothermal crystallization kinetics of PP and PP/CNT composites CNT content

Tonset (ºC)

Tp (ºC)

Hc (J/g)

Tm (ºC)

Xc (%)

0.0 wt.%

127.4±0.7

122.6±0.5

101.8±0.2

164.2±0.1

49.2±0.3

0.5 wt.%

132.4±0.4

125.7±0.1

99.1±0.3

164.9±0.2

48.1±0.5

2.5 wt.%

135.8±0.3

128.3±0.6

97.4±0.1

166.1±0.2

48.3±0.1

5.0 wt.%

136.5±0.1

130.9±0.2

96.0±0.4

166.3±0.5

48.8±0.6

According to the DSC results, it can be speculated that the CNTs could influence the crystallization behavior of PP matrix, but it is difficult to know exactly how the crystal morphology has been changed. Therefore, a in-situ visualization system was employed to investigate the crystal evolution process during isothermal crystallization, and both the optical and polarized photos were shown in Fig. 3. By comparing these photos, it can be found that the introduced CNTs pronouncedly improved the number of nucleated crystals and greatly reduced the crystal size. Meanwhile, the uniformity of the crystal nucleating was also significantly promoted. After analyzing the crystallization process, both the onset and total crystallization time were dramatically decreased by the presence of CNTs, up to 48% and 64%, respectively. It manifests that the crystallization rate of PP matrix was obviously increased by adding CNTs [48]. These phenomena all are the verification and refinement of DSC results, and they further detailed the effect of CNTs on PP’s crystallization process. The refined crystal and increased crystallization rate have been proved to significantly promote the cell morphology of PP matrix, by providing more cell nucleation sites and improving PP’s melt strength especially at higher temperature [11]. To further determine whether the added CNTs have an effect on PP’s crystal structure, the WAXD diffraction patterns and the α-crystal diffraction peaks’ intensities of four PP/CNT composites were collected and illustrated by Fig.4. Obviously, all WAXD profiles from Fig. 4a showed the same diffraction peaks mainly at 2θ=14.2º, 16.9º, 18.7º, 31.3º, and 20.2º. These peaks are consistent with the α-crystal of PP, attributed to the reflection of (110), (040), (130), (111), and (131/041) planes, respectively, which indicates that CNTs do not alter the crystal structure of PP matrix. Comparable behavior has been reported by other studies [46,49]. Nevertheless, there is a difference in the 10

intensity of these diffraction peaks by comparing with PP and PP/CNT composites, and thus the relative intensities of the α-crystal diffraction peaks for each composite were calculated and plotted in Fig. 4b. Remarkably, the relative intensities of all diffraction peaks for PP/CNT composites exhibited the similar value with that for PP, excluding the PP/CNT 0.5 wt.% and 5 wt.% composites. When CNT content was 0.5 wt.%, the relative intensity of the peak at 2θ=14.2º decreased, and that at 2θ=16.9º increased, while at the CNT content of 5 wt.%, the relative intensity of the peak at 2θ=16.9º decreased, and that at 2θ=21.2º increased. It is speculated that the unit cell structure parameters of PP/CNT composites might be slightly different from pure PP, leading to the difference of diffraction peaks’ intensities [50, 51]. As expected, the WAXD results confirmed that the crystal structure of PP matrix, α-crystal phase was not impacted by the introduction of CNTs, and these slight changes of relative intensities would have little influence on the properties of PP matrix.

Fig. 3. Crystal evolution process of PP and PP/CNT 0.5 wt.% composite in isothermal crystallization at 140ºC

Fig. 4. (a) WAXD diffraction patterns and (b) relative intensities of different α-crystal diffraction peaks collected from all PP composites.

3.3. Rheological behavior Fig.5 plots the oscillatory shearing responses of all PP/CNT composites in the dynamic frequency sweep testing (DFST) as frequency (ω) varying between 0.1 rad/s and 100 rad/s at 190ºC. From Fig. 5a-b, it is obvious that both the storage (G′) and loss (G′′) modulus were pronouncedly improved by introducing CNTs in comparison with neat PP, especially for high CNT contents. Moreover, the increasement of G′ was more significant than that of G′′ for the 11

PP composite with same CNT contents, and the impact of CNTs on both G′ and G′′ at low frequency were much greater than that at high frequency. It demonstrates that CNTs could effectively promote the viscoelasticity of PP matrix, and the higher the CNT content, the more significant the effect. It is attributed to the increased PP’s inter-molecular force in presence of CNTs, leading to the increased entanglements of molecular chains [52]. In turn, this restricts the relative motion of molecular segments, and thus manifests the improvement of PP’s melt strength. Therefore, both the elastic and viscous responses of PP matrix increase by adding CNTs, which is more remarkable with an increase of the CNT content. It has already been proved that enhanced melt strength can effectively contribute to the refinement of PP’s cellular morphology [11,32,33]. Fig. 5c-d depicts the loss tangent (tan δ) descripted as tan δ= G′/G′′, showing the material’s elastic response, against frequency and CNT content, respectively. It is observed from Fig. 5c that tan δ of pure PP matrix decreased considerably with an increase in ω, while tan δ of PP composites with higher CNT contents (>2.5 wt.%) became nearly independent of ω, showing typical rheological features of liquid-like and solid-like matrices, respectively. This phenomenon can be understood by analyzing the effect of shear frequency on the molecular entanglements of the composites. For the pure PP, the high shear stress resulted from the increasing frequency leads to its liquid-like behavior by breaking PP’s molecular entanglements, while PP composites with high CNT contents are able to effectively resist the impact of high shear stress, exhibiting solid-like behavior, attributed to the considerably increased CNT-PP chain entanglements. Similar behaviors following Winter-Chambon criteria were observed by previous studies, suggesting the formation of physical gel [53,54]. To further achieve the gel point which describes the rheological property transition mentioned above, a multi-frequency (0.1 rad/s to 10 rad/s) graph of tan δ versus the CNT content was plotted as Fig. 5d. From this figure, tan δ decayed gradually as CNT content increased, and an intersection of tan δ was occurred at 1.6 wt.%, marked as gel point (Cg). It is also seen that tan δ decreased as increasing the shear frequency when the CNT content was smaller than Cg, but the tendency reserved after the CNT content was larger than Cg. This phenomenon already has been reported in several polymer matrices with nanofillers [53-55]. To investigate the temperature dependence of oscillatory viscoelasticity for all PP/CNT composites, their complex viscosity (η*) and loss tangent (tan δ) were obtained via dynamic temperature step testing (DTST), illustrated by Fig. 6. Noticeably, a critical temperature for each composite emerged in the cooling process. Before reducing to the critical values, both η* and tan δ changed gradually, while there was sharply increase of η* and tan δ after crossing their critical values. Additionally, the critical temperatures shifted to higher values as CNT content increased. The presence of critical temperatures is caused by the generation of crystals, which forming an interconnected spatial network in melt matrix, leading to a dramatical increase of viscoelasticity [23,54]. The added CNTs act as crystal nucleating agents to facilitate the crystallization, resulting in the increase of critical temperature, which is already identified by the DSC and visualization results shown as Fig. 2a and Fig. 3. Moreover, it is also 12

found that when the testing temperature was higher than the critical value, there was a slow and regular change of η* and tan δ for pure PP in a decrease of temperature, but nearly none for PP/CNT composites. Similar phenomenon is reported by previous studies [32,34], and it is consistent with the DFST results as Fig. 5c plots. 105

a

104

104

103

103

G" (Pa)

G' (Pa)

105

102

102 101

101 100 10-1 120

100

ω (rad/s)

101

100 10-1

102

103

c

110

0.0 wt.% CNT 0.5 wt.% CNT 2.5 wt.% CNT 5.0 wt.% CNT

100

ω (rad/s)

101

102

d

102 Gel point

tan δ

100

tan δ

b

101

90 20

Increasing frequency

100

10

Cg = 1.6 wt.%

0 10-1

-1

100

ω (rad/s)

101

102

10

0

1

2

3

4

5

CNT content (wt.%)

Fig. 5. DFST rheological properties of all PP/CNT composites: (a) G′(ω); (b) G"(ω); (c) tan δ (ω); (d) tan δ (CNT content). 107

60

a

106

45

+

104

tan δ

η (Pa·s)

105

103 102

b

0.0 wt.% CNT 0.5 wt.% CNT 2.5 wt.% CNT 5.0 wt.% CNT

30 15

101 100 120

140

160

180

200

220

0 120

140

160

180

200

220

Tempeature (°C)

Temperature (°C)

Fig. 6. DTST rheological properties of all PP/CNT composites: (a) η+(ω); (b) tan δ.

Elongational viscoelastic properties play an important role for the matrix to subject uniaxial or biaxial stretch caused by the behavior of cell nucleation and growth, severely impacting its foaming ability. Hence, extensional testing was applied with three strain rates, 0.01 s-1, 0.1 s-1, 1 s-1, at 170ºC, and the results were shown in Fig.7 and Fig. S5. The extensional viscosity (

) of all PP/CNT composites was increased by one order of magnitude

comparing with that of pure PP. Further, CNTs also exhibited excellent features to pronouncedly accelerate the strain hardening behaviors of PP matrix. This promotion in extension flow again indicates that there is a physical network formed by the enhanced interconnection of PP’s molecular chains after introducing CNTs. This phenomenon is agreed with the crystallization behaviors and oscillatory rheological response plotted by Fig. 2-3 and 13

Fig. 4-5, respectively. In addition, the degree of strain hardening for PP/CNT composite showed strong dependence on the applied strain rate, and its growth tendency increased with increasing strain rate. The reason for this trend is that the disentanglement rate of the polymer macromolecule nearly determined the strain hardening behavior [32,56]. Therefore, when increasing the strain rate, there will be less time left for molecules to disentangle and to follow the deformation, resulting in a higher degree of strain hardening behavior [57]. Overall, it is believed that both the increased extensional viscosity and enhanced strain hardening behavior could improve the cell nucleation and contribute to the stable growth of cells [8,32,56]. 106 5 wt.% CNT

η+Ε (Pa·s)

105 104 0 wt.% CNT 1.00 s-1 0.10 s-1 0.01 s-1

103 102 10-2

10-1

100

101

102

Time (s)

Fig. 7. Extensional rheological properties of PP and PP/CNT 5 wt.% composite.

3.4. Foam morphology Fig. 8 displays the representative ESEM images of PP and PP/CNT composite foams fabricated under 13.79 MPa at 161ºC. Overall, uniform microscale cells were achieved for all the PP/CNT composites, which is further conformed by the low magnification ESEM images of cellular morphology in Fig.S4. It is also observed that as increasing the CNT content, the cell size reduced, and cell number increased, indicating that the evenly dispersed CNTs effectively enhanced PP’s foaming ability. To quantitatively investigate the impact of CNTs on the foaming behaviors of PP matrix, the foam expansion ratio, cell density, and cell size were measured and illustrated as a function of CNT content in Fig.9. With a presence of CNTs, foam expansion ratio slightly decreased from 55.5-fold to 50.1-fold, while the cell density significantly increased more than 1 orders, and cell size considerably reduced by 49.7%, compared with that of pure PP foam. It indicates that ultra-lightweight microscale PP composite foams were successfully fabricated, whose weight reduction was up to 98.01 wt.%, and cell size was smaller than 30 µm. The significantly increased cell density and decreased cell size by CNTs can be attributed to three main reasons as follows. First, CNTs can play a role of heterogenous nucleating agent to effectively promote cell nucleation. Second, CNTs can significantly improve PP’s crystallization behavior by acting as crystal nucleating agents, such as higher crystallization and melting temperature (Fig. 2), and smaller crystals (Fig.3), and further benefit to cell nucleation. It can also be demonstrated by the changes of the two melting peaks for PP foams with different CNT concentrations in Fig. S7 [51]. Normally, for the DSC curves with two melting peaks, the high-temperature peak is attributed to the isothermal crystallization behavior, and the low-temperature peak is attributed to the cooling stage 14

after foaming, which could directly show the molecular structure of the achieved foams and further reveal the crystallization behavior in the real foaming condition [51]. Third, after introducing CNTs, the melt strength of PP matrix was pronouncedly increased owning to the restriction of CNTs on molecular chain slip, expressing as the enhanced viscoelasticity (Fig. 5), complex viscosity (Fig. 6), and strain hardening behavior (Fig. 7). It is believed that improved melt strength could supress cell coalescence and collapse during foaming [6,11,12,58]. The slightly reduced expansion ratio (less than 10%) is because of the increased entanglements caused by CNTs between molecular chains, which increased the resistance of cell growth to some extent. Furthermore, the superhighly expanded microcellular PP foam achieved by batch foaming demonstrates that foaming technology exhibiting a promising future to fabricate ultra-lightweight polymer materials.

Fig. 8. ESEM images of PP and PP/CNT composite foams.

Fig. 9. Foam structure under different CNT contents: (a) expansion ratio, (b) cell density, and (c) cell size.

3.5. Compressive properties Elastic response during compression deformation is of great significance for ultra-lightweight materials. Hence, compressive strength of PP and PP/CNT composite foams were measured to estimate the correlation between foam structure and compressive performance. Fig.10a depicts the stress-strain curves of PP and PP/CNT composite foams. Generally, there are four typical stages in all deformation curves, including elastic behavior up to peak stress, post-peak softening, plateau, and densification. For cellular materials, the first step is controlled by the elastic bending of cell walls, from which the compressive strength and modulus were achieved, as Fig.10b plots. Obviously, 15

with an increase of the CNT content, the compressive strength increased from 0.18 MPa to 0.57 MPa, and the compressive modulus increased from 0.52 MPa to 1.74 MPa. By comparing with the commercially available EPP foams, summarized in Table S1, it is obvious that the compressive strength of the achieved pure PP foams is 2 times higher than that of the EPP foams with the same density, which is further pronouncedly promoted by introducing CNTs. It manifests that both compressive strength and modulus increases with a decrease in the expansion ratio, combining the connection of CNT content and foam structure. Moreover, by comparison the PP/CNT 2.5 wt.% and 5 wt.% composite foams, the compressive strength and modulus were further improved through reducing the cell size and increasing cell density, which helps to decrease the bending moment of cell walls, leading to the improvement of foams’ stiffness and strength [32,40,59]. This phenomenon can be further studied by using a theoretical model established by Gibson and Ashby, expressed by Fig. S8 and Eq. S(7)-(11). 2.0

b

0.8

0.8 0.4

1.5

0.6 1.0 0.4 0.5

0.2

0.0

0.0

0

2

4

6

0.0 0 0.0

8

Strain

Modulus (MPa)

1.2

1.0

a

0.0 wt.% CNT 0.5 wt.% CNT 2.5 wt.% CNT 5.0 wt.% CNT

Strength (MPa)

Compressive stress (MPa)

1.6

50 0.5

100 2.5

150 5.0

CNT content (wt.%)

Fig. 10. Compressive properties of PP and PP/CNT composite foams: (a) stress-strain curves, and (b) strength and modulus.

3.6. Thermal insulation performance To further analyze the impact of CNTs and foam structure on the thermal insulation properties of the ultra-lightweight PP/CNT composite foams, the thermal conductivity was measured using the thermal constant analyzer, which is reported by Table 2. Overall, the total thermal conductivities of the PP and PP/CNT composite foams were all lower than 36.05 mW/m·K, and the best thermal insulation performance was achieved by PP/CNT 5 wt.% composite foams, which is as low as 28.69 mW/m·K. As far as the reported studies, it is the minimum record of the ultra lightweight PP foams’ thermal conductivity without any insulation gas. Additionally, the thermal conductivity of the prepared foams decreased with an increase in CNT content, indicating that both the introduced CNTs and foam structure could remarkably influence the thermal insulation performance. Table 2. Thermal conductivity of PP and PP/CNT composites measured using TPS analyzer. CNT content

0.0 wt.%

0.5 wt.%

2.5 wt.%

5.0 wt.%

Thermal conductivity (mW/m·K)

36.05

32.47

30.12

28.69

To clarify the mechanism of this enhanced thermal insulation performance by CNTs, this study tried to theoretically analyze the heat transfer through microcellular polymer/carbonaceous material composite using a mathematical model included in the Supplementary Material. According to the three fundamental modes of heat 16

transfer: convention, conduction, and radiation, heat transfer through cellular polymers is considered to compose of four contributions, including thermal convention, solid and gas conduction, and thermal radiation, expressed by Eq. S(14). Owning to its minimal effect compared to thermal conduction and radiation, thermal convection is usually disregarded for micro-scale cellular polymers [7,13]. Then, the thermal conduction contributed by solid phase and gas phase can be estimated by Eq. S(15), which showing that the thermal conduction is determined by the foam porosity, the fraction of cell struts, thermal conductivity of solid and gas phases. Noticeably, the cell struts are defined as the thick zones of the polygonal cell edges, which can be calculated by Eq. S(6). The foam porosity can be calculated with expansion ratio by Eq. S(3). The thermal conductivity of solid phase was measured by the TPS analyzer, and the thermal conductivity of gas was considered as round 25.5 mW/m·K at the standard condition. Considering the contradiction between the increased thermal radiation by ultra-high foam expansion ratio and the CNTs’ black-body effect for infrared radiation (IR), the following mainly focuses on the mechanism of CNTs to reduce thermal radiation and the calculation of thermal radiation of PP and PP/CNT composite foams. First, since IR is the main component of the thermal radiation, especially that with long wavelength [6,7], the IR transmittance and absorption of PP and PP/CNT composite film were measured to investigate the blocking capability of CNTs on the thermal radiation, and the results were plotted by Fig. 11. It is observed from Fig. 11a that the IR absorption of pure PP was very weak, shown as its over 90% IR transmittance in the whole wavelength ranges (2.5-25 µm). The introduce of CNTs in PP films remarkably reduced the IR transmittance, and the higher the CNT content was, the more dramatically the IR transmittance was reduced. For instance, the IR transmittance was decreased from 90% to 60% after adding 0.5 wt.% CNTs, but it was sharply dropped to 2% when the CNT content was 5 wt.%. Furthermore, the IR absorption indexes of all PP/CNT composites were calculated by Eq. S(16), which were plotted in Fig. 11b and summarized in Table 3. Obviously, the IR absorption index of pure PP was nearly 0 in the whole applied wavelength despite several absorption peaks. In the presence of CNTs, both the IR absorption and the baseline of the index curves were pronouncedly improved, and they were further enhanced with an increase in CNT content, which is also exhibited by the significant increase of wavelength-average IR absorption index in Table 3. It is worth noting that the increase of IR absorption index is almost linear with CNT content, which was achieved 0.061 at 5 wt.% CNT content, up to 22 times larger than that of pure PP. Overall, the strong IR absorption of CNTs as well as their ability to effectively enhance PP’s IR absorption has been verified, which is confirmed that the excellent IR-absorption performance of CNTs could effectively reduce the thermal radiation.

17

Wavenumber (cm-1) 2000

1000

667

500

400

0.20

a

80

IR absorption index

IR transmittance (%)

100

Wavenumber (cm-1)

60 40 20 0

2000

1000

667

500

b

0.0 wt.% CNT 0.5 wt.% CNT 2.5 wt.% CNT 5.0 wt.% CNT

0.15

400

0.10 0.05 0.00

5

10

15

20

25

5

Wavelength (µm)

10

15

20

25

Wavelength (µm)

Fig. 11. (a) Spectral transmittance, and (b) IR absorption index of PP and PP/CNT composite film with a thickness of about 45 µm. Table 3. Wavelength-average IR absorption index of PP and PP/CNT composites

Wavelength-average IR absorption index

0.0 wt.% CNT

0.5 wt.% CNT

2.5 wt.% CNT

5.0 wt.% CNT

2.79×103

10.45×103

34.09×103

61.32×103

To quantitatively study thermal radiation of PP and PP/CNT composite foams, the IR transmittance of all prepared foams were measured and plotted by Fig. 12a. It is found that the IR transmittance of pure PP foam was over 40 % in the long wavelength range (20-25 µm), due to the weak IR absorption and ultra-high expansion ratio. As for the PP/CNT 0.5 wt.% composite foam, the IR transmittance was decayed to 30 %, and that of PP/CNT 5 wt.% was significantly decreased to less than 5%. Then, based on the FTIR results, the IR extinction coefficient was calculated by Eq. S(19)-(22), whose results was depicted by Fig. 12b. Compared with the IR absorption data shown by Fig. 11b, similar trend of IR extinction coefficient to IR absorption index appeared, that is CNTs contributed to the enhancement of IR extinction coefficient, especially for high CNT content. The IR extinction coefficient of PP/CNT 5 wt.% composite foam was higher than 4 mm-1 at the wavelength range of 20-25 µm, while that of pure PP was almost 0 mm-1. Further, the thermal radiation was achieved using the Rosseland Equation, which is detailed in the Supplementary Material, by calculating the transmitted radiative energy based on the obtained IR extinction coefficient. Table 4 summarized the calculated radiative thermal conductivities of PP and PP/CNT composite foams. Obviously, the radiative thermal conductivity of PP/CNT 5 wt.% composite foam was as low as 0.73 mW/m·K, which was decayed over 10 times compared with that of pure PP foam. It indicates that CNTs can help PP foam pronouncedly decrease radiative thermal conductivity. In summary, CNTs showed outstanding properties to block IR transmission, including low IR transmittance, high IR absorption index and large IR extinction coefficient. Thus, the radiative thermal conductivity of PP/CNT composite foam was reduced significantly, which clarified that CNTs helped to decrease the thermal conductivity of PP composite foam by decaying the thermal radiation.

18

Wavenumber (cm-1)

Wavenumber (cm-1) 1000

667

500

400

IR extinction coefficient (mm-1)

IR transmittance (%)

50

2000

a

40 30 20 10 0 5

10

15

20

15

2000

500

667

b

400

0.0 wt.% CNT 0.5 wt.% CNT 2.5 wt.% CNT 5.0 wt.% CNT

10

5

0 5

25

1000

10

15

20

25

Wavelength (µm) Wavelength (µm) Fig. 12. (a) Spectral transmittance, and (b) IR extinction coefficient of PP and PP/CNT composite foams with a thickness of around 200 µm. Table 4. Radiative thermal conductivity of PP and PP/CNT composites CNT content

0.0 wt.%

0.5 wt.%

2.5 wt.%

5.0 wt.%

Radiative thermal conductivity (mW/m·K)

8.38

4.98

2.63

1.38

Based on the achieved strut fraction, foam porosity, and thermal conductivity of all PP/CNT composites summarized in Table 5, the thermal conductivity contributed by gas and solid phase for each composite was calculated using Eq. S(23). The calculations together with the achieved thermal conductivities contributed by radiation were plotted in Fig. 13, which also compared the calculated and the measured total thermal conductivities of PP composite foams with different CNT contents. It is observed that the calculated thermal conductivities were in good agreement with the measured results, indicating that the mathematical model use in this study is available to estimate the thermal conductivity of porous polymer/carbonaceous material composite. Noticeably, the calculated values were a little higher than the measured values, and the difference was gradually increased as an increase of the CNT content. This is because there were some micro holes or micro/nano fibrils formed in the cell walls shown by Fig. 8. It is believed these structures could increase the tortuosity of the heat transfer path by breaking the PP matrix’s continuity, and simultaneously could enhance the phonon scattering at the boundaries of the micro/nano structures, leading to the reduction of thermal conductivity contributed by solid phase [32,33,60]. Moreover, it is also identified that these micro/nanoscale porous cell walls could reduce the thermal conductivity contributed by gas phase [61]. Thus, both the decreased solid and gas thermal conductivity results in the difference of the calculated and measured thermal conductivities, and the increased difference might be caused by the increasing tendency of cell walls with micro/nano structures as CNT content increases. Furthermore, it is worth noting that the thermal conductivity contributed by the gas and solid phase is almost the same for all the prepared foams, and the dramatically decreased radiative conductivity leaded to the lowest thermal conductivity of PP/CNT 5 wt.% composite foam, 28.69 mW/m·K. It demonstrates that decreasing thermal radiation is a very efficient method to achieve ultra-lightweight polymer foams with low thermal conductivity, and CNTs show excellent capacity to reduce thermal radiation by blocking IR transmission. Therefore, the remarkably decreased solid conductivity due to 19

the ultra-high expansion ratio and the formed micro/nanoscale structures in cell walls, the significant reduced radiative thermal conductivity due to the strong IR blocking capacity of CNTs are together response for the super high thermal insulation performance of the microcellular PP/CNT composite. Table 5. Strut faction, porosity and thermal conductivity of pure PP foam and PP/CNT composite foams CNT content

0.0 wt.%

0.5 wt.%

2.5 wt.%

5.0 wt.%

Strut fraction (%)

10.3±2.1

11.7±3.1

9.1±2.3

13.5±1.4

Foam porosity (%)

98.20±1.68

98.15±2.63

98.03±1.45

97.99±1.36

Composite thermal conductivity (mW/m·K)

227±7

232±15

268±11

309±19

Fig. 13. Comparation of the calculated and measured total thermal conductivities of PP and PP/CNT composite foams.

4. Conclusion In this study, we reported a facile, environmentally friendly, and cost-effective method for preparing the multifunctional PP/CNT composite foams with microscale cell structures. The fabricated cellular PP/CNT composite has an extremely high expansion ratio, larger than 50-fold, with an average cell size of smaller than30 µm. The easy-to-control foaming process, combining with the refined crystal structures and the enhanced viscoelasticity by the introduced CNTs, promotes the cell nucleation and pronouncedly facilitates the rapid and stable cell growth by inhibiting large-scope cell coalescence and collapse. The fabricated ultralight microcellular PP/CNT composite presents a super-insulating and excellent mechanical performance, a thermal conductivity of as low as 28.69 mW/m·K and a compressive strength of 0.57 MPa. So far, this is the first time for the reported results to achieve such super thermal-insulation properties with ultra-lightweight polymer foams. The severely reduced solid thermal conductivity by the extremely low foam density and formed micro/nano structures in cell walls, associated with CNTs’ outstanding blocking capability of thermal radiation, leads to such a dramatically decrease in thermal conductivity. The successfully achieved multifunctional ultralight porous polymer provides a promising solution for improving the insulating performance using a novel, simple and flexible foaming process, which also offers a good prospect to expand the application fields of polymer foams, such as aerospace with high weight requirements and modern construction industries. 20

Acknowledgement The authors are grateful to the National Natural Science Foundation of China (NSFC, Grant No. 51875318, 51905308, 51905307), the Major Science and Technology Innovation Project of Shandong Province (Grant No. 2019JZZY020205), and the Qilu Outstanding Scholar Program of Shandong University.

Appendix A. Supplementary Material Supplementary Material related to this article can be found online.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: