Scalable Fabrication of Thermally Insulating Mechanically Resilient

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Applications of Polymer, Composite, and Coating Materials

Scalable Fabrication of Thermally Insulating Mechanically Resilient Hierarchically Porous Polymer Foams Ali Rizvi, Raymond K.M. Chu, and Chul B Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11375 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

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Scalable Fabrication of Thermally Insulating Mechanically Resilient Hierarchically Porous Polymer Foams By Ali Rizvi, Raymond K.M. Chu, and Chul B. Park* Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8 (Canada) [*] C. B. Park E-mail: [email protected] Keywords: porous materials, polymers, hierarchical materials, thermal insulation, injection molding

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Abstract: The requirement of energy efficiency demands materials with superior thermal insulation properties. Inorganic aerogels are excellent thermal insulators but are difficult to produce on a large-scale, are mechanically brittle, and their structural properties depend strongly on their density. Here, we report the scalable generation of low-density, hierarchically porous, polypropylene (PP) foams using industrial-scale foam processing equipment, with thermal conductivity lower than commercially available high performance thermal insulators such as superinsulating Styrofoam. The reduction in thermal conductivity is attributed to the restriction of air flow caused by the porous nanostructure in the cell walls of the foam. In contrast to inorganic aerogels, the mechanical properties of the foams are less sensitive to density suggesting efficient load transfer through the skeletal structure. The scalable fabrication of hierarchically porous polymer foams opens up new perspectives for the scalable design and development of novel superinsulating materials.


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Introduction: Thermal insulators are employed in applications such as thermal protective skins, building insulation, 2 refrigeration, 3 clothing,

4

1

and thermoelectrics. 5 Commercially appealing

thermal insulators exhibit low thermal conductivity, mechanical resilience, and the ability to be fabricated from various natural and synthetic precursors in a high-throughput, sustainable and cost-effective process. Commercial thermal insulators such as expanded polystyrene (EPS) foams are noted for their ease of preparation using conventional industrial-scale foaming equipment, their mechanical stability, and recyclability. 6 However, these materials exhibit low thermal insulation performance with thermal conductivities typically > 0.035 W m-1 K-1 for air under ambient conditions. insulation performance

8, 9

7

On the other hand, inorganic aerogels show excellent thermal

with thermal conductivities < 0.02 W m-1 K-1 but are difficult to

fabricate, process, and recycle,

10

are highly brittle, and their structural properties depend

strongly on density. 11 Tailored biomaterials such as cellulose nanofibers/graphene oxide foams, 12 liquid crystal nanocellulose aerogels, 13 cellulose nanofiber biocomposite aerogels 14, 15 and nanowood, 16 are excellent thermal insulators, and have superior mechanical properties than inorganic aerogels but the hydrophilicity of these materials limits their applications to dry environments because moisture results in degradation of insulation performance. 17 Doubly crosslinked aerogels 18 show excellent flexibility, transparency, and superinsulating ability but the crosslinking renders the material non-recycle, posing an environmental disposal challenge. Organic-inorganic hybrid aerogels such as phenolic-silica aerogels

19

display mechanical resilience, low thermal

conductivities, and high service temperatures. However, fabrication of these materials using “top-down” approaches is difficult because inorganic fillers tend to aggregate and degenerate



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during dispersion and sintering in organic materials, resulting in poor thermal and structural properties,

20

and fabrication of these materials using “bottom up” strategies is cost- and

resource-intensive which makes scale-up challenging. 10, 21 The deficiencies of current insulating materials call for the need to develop alternative and effective thermal insulators that can be easily produced in a high-throughput, low-cost, and environmentally sustainable manner. Here, we prepare thermally insulating, low-density, hierarchically porous, thermoplastic polypropylene (PP) foams with macro-mesoporosity in the cell walls using industrial-scale foam injection molding (Fig S1), and a pressure/temperature regulated autoclave conventionally used for bead foaming. 7 This process allows the foams to be easily sintered into complex shapes of any dimensionality.

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The macro-mesopores in the cell walls cause the foams to exhibit an

overall thermal conductivity of 0.029 W m-1 K-1 for air at a density of 0.074 g cm-3, even when the average cell diameter is greater than 10 µm. This thermal conductivity value is lower than high-performance commercial insulators such as super-thermal insulating Styrofoam, and recently reported materials.

23, 24, 25, 26

The low thermal conductivity of these air-filled

hierarchically porous foams is primarily attributed to the reduction in air conductivity which can account for up to 70% of the overall thermal conductivity in cellular solids. 27 The nanoconfining geometry of the pores in the cell walls with diameters below the mean free path of air, subjects air particles to a Knudsen flow where the particles collide more frequently with the confining walls rather than with each other. By virtue of the insignificant particle-particle collisions, the energy conducted through air reduces, decreasing the overall thermal conductivity. The struts comprising the cell walls are mechanically connected and when subjected to a macroscopic load, respond by a linear-elastic bending deformation resulting in an elastic modulus, , which scales with density, , as

~ indicating efficient load transfer to the skeletal structure of the


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hierarchical foam.

21, 28

In contrast, thermally insulating inorganic aerogels exhibit a steeper

scaling with density, generally following ~ because of inefficient stress transfer between structural members. 29, 30 Recent research has focused on the development of low-density, nanocellular foams 31, 32, 33

with cell diameters below the mean free path of air to minimize the conductivity of air.

However, scalable fabrication of low-density, nanocellular foams remains a challenge. 34, 35 We believe that nanoconfinement through macro-mesoporous cell walls provides a pragmatic and scalable route to reducing thermal conductivity in low-density polymer foams.



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Experimental: Materials: Polypropylene

(Novatec-PP

FY4)

is

supplied

by

Japan

Polypropylene.

Polytetrafluoroethylene (MetablenTM A-3000) is supplied by Mitsubishi Rayon. High purity carbon dioxide and nitrogen are purchased from Linde Gas. All materials are used as received. Foaming process: The melt strength of PP is improved by blending 3 wt% PTFE using a literature method. 36, 37

Foams of PP/PTFE are prepared using a 50-ton injection molding machine (Arburg 270C).

A gas injection system (Trexel Mucell®) is used to deliver 10 wt% CO2 into the injection molding machine. The mold cavity temperature is maintained at 85°C. A full shot of the polymer + CO2 is injected into the mold cavity at an injection flow rate of 100 cm3 s-1, up to a cavity packing pressure of 18 MPa. The polymer/gas mixture is allowed to dwell in the cavity for 40 s. The mold is then opened at a mold opening rate of 20 mm s-1. The rapid depressurization from mold opening causes cell nucleation and growth. The mold opening distance is varied between 1.5 mm and 50 mm. Samples are extracted from the core of the injection molded foam for characterization. When the mold opening distance is 1.5 mm, the samples extracted from the core of the injection molded foam are subjected to a subsequent foaming process in a high pressure/ temperature regulated autoclave using CO2 at various temperatures and pressures for a saturation time of 1.5 hours. The foams are collected after rapid depressurization of the chamber for characterization.


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Measurements: The thermal conductivity of the foams is measured using a transient plane source hot disk thermal constant analyzer (Them Test Inc. TPS 2500). Prior to thermal conductivity measurements, the insulating gas in the samples is changed from CO2 to air by placing a foamed sample in a heated chamber at 80 °C for 24 hours. No change in the foam structure is observed. The thermal conductivity of the foams with air as the insulating gas is measured. The morphology of the foams is characterized using a scanning electron microscopy (JEOL 6060 and Hitachi S-5200). Foamed samples are cryogenically fractured and the exposed surface is coated with a thin layer of platinum using a sputter coater prior to observation. The SEM micrographs are used to estimate i) the cell density with the method described earlier, 36 ii) the cell wall porosity using the software Image Pro Plus ® (Fig S3), and iii) the average crosssectional diameter of the cells. The foam density is determined using ASTM-D792 and is used to calculate the volume expansion ratio and the void fraction. The pore volume distribution, the surface area, and the nitrogen sorption for the foams are characterized using Quantachrome Autosorb. The BJH method is used to estimate the pore size distribution. The BET method is used to estimate the specific surface area. The multicycle compression tests are conducted on Instron 5848. Ten compressive cycles are performed at 60% strain with a crosshead velocity of 1.5 mm min-1.



8

Results and Discussion: Preparation of the foams: ®

CO2 tank

MuCell pump

a

Mold opening distance

PP/PTFE

injection port

CO2 Injection tank pump Inlet valve

b

T and P regulated chamber

d

Dwelling

Mold opening distance: 1.5 mm

Mold opening

Foam-B

c

Injection

Foam-A

valve Mold opening distance: 50 mm

e Mold opening direction

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Figure 1. Foam preparation; a) Schematic of the foam injection molding process. The dwelling time is 40 s and the mold opening distance is varied; b) SEM micrograph of the foam obtained when the mold opening distance is 1.5 mm; c) Bead foaming setup used to subject the foam in b) to various temperatures and pressures; d) SEM micrograph of Foam-A; e) SEM micrograph of Foam-B produced with a mold opening distance of 50 mm showing anisotropic cells, elongated in the mold opening direction.

The polymer is injected into the mold cavity of an industrial-scale injection molding machine (Fig S1) with CO2 as the foam blowing agent and the process is schematically illustrated in Fig 1a. After a dwelling period of 40 s, the mold is quickly opened to a distance of 1.5 mm, so that foaming can be induced through rapid depressurization (Fig S1g). An electronic micrograph of the resultant foam is included in Fig 1b. This foam has an average cell diameter of 650 nm, a cell density of 1012 cells cm-3, an expansion ratio of 1.32 and a bulk density of 0.69 g cm-3. These foam characteristics are significantly different from the characteristics of foams typically obtained through mold opening injection molding,

38

and is ascribed to the limited

volume available for foam expansion due to the mold opening distance being small, severely restraining cell growth (Fig S2). Such geometric confinement favors cell nucleation in the competition between cell nucleation and cell growth, increasing the cell nucleation rate. 39 The high density of the foams is reduced in a subsequent batch foaming step using a bead foaming autoclave depicted in Fig 1c. Adjustment of the foaming temperature and the CO2 pressure provides control over the foam density, the cell diameter and the cell wall structure (Table S1). When a batch foaming temperature of 152 °C and a CO2 pressure of 1200 psi is employed, the foam shown in Fig 1d is produced with an average cell diameter of 13.8 µm, a cell density of 1010 cells cm-3, an expansion ratio of 12.4, a density of 0.074 g cm-3, and is referred to as Foam-A. The higher magnification micrograph reveals the presence of submicron pores in the cell walls of Foam-A. Pore formation in the cell walls of semicrystalline polymers is



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attributed to the crystalline growth during the dwelling/saturating stage of the foaming process leading to the formation of a structurally heterogeneous melt 40, 41, 42 creating stress concentration points 43, 44 in the cell walls which become origination points of pores as the cells grow and the cell walls become increasingly thinner. 45 For comparison purposes, foams are prepared with the same bulk density and expansion ratio as Foam-A using mold-opening injection molding where the mold opening distance employed is 50 mm. At this mold opening distance, geometric confinement becomes negligible and free foam expansion is permitted (Fig S2) resulting in foams with morphological characteristics commonly seen in previous studies. 38 The resultant foam is shown in Fig 1e and is referred to as Foam-B. Foam-B shows an average cross-sectional cell diameter of 60 µm, a cell density of 107 cells cm-3, an expansion ratio of 12, and a bulk density of 0.076 g cm-3. Further, the cells in Foam-B are elongated in the mold opening direction with average lengths of 800 µm corresponding to an aspect ratio of 13.3. Such anisotropy in cell structure is not observed in Foam-A and is expected to result in a higher axial thermal conductivity for Foam-B. 46 While both Foam-A and Foam-B have similar densities of about 0.07 g cm-3, the two-stage foaming process causes Foam-A to have drastically smaller cell diameters, submicron pores in the cell walls (Fig S3), and an isotropic cell structure compared to Foam-B which has a larger average cross-sectional cell diameter, and a highly anisotropic cell structure.


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Pore size analysis: 150

a

Foam-A Foam-B

0.4

b

Foam-A Foam-B

3

Volume Adsorbed (cm /g)

0.3

100

dV/ d logD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

0.2

0.1

0 0.0

0.0 0.2

0.4

0.6

0.8

1.0

1

10

100

D (nm)

P/P0

Figure 2 Pore size analysis. a) Nitrogen sorption isotherms for Foam-A and Foam-B; b) Pore size distribution for Foam-A and Foam-B.

Table 1. Physical characteristics of Foam-A and Foam-B.

a

b

c

d

e

f

(m2 g-1)

(cm3 g-1)

(nm)

(g cm-3)

(%)

(%)

Foam-A

31.7

0.2

25.2

0.074

92

14

Foam-B

15.6

0.09

-

0.077

91.6

32

Sample

a

Specific surface area determined with BET method using the desorption isotherm

b

Total pore volume determined by maximum nitrogen adsorption volume

c

Average pore diameter estimated using = 4 /

d

Foam density determined using ASTM D792

e

Foam void fraction calculated using = (1 − ! ⁄), where ! is the foam density and is the density of the bulk material

f

Cell wall porosity estimated from electronic micrographs of the cell walls (Fig S3)



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Porous characteristics of Foam-A and Foam-B are determined using nitrogen sorption analysis and the isotherms are presented in Fig 2a. Both foams show a type II isotherm with a hysteresis loop and a rapid nitrogen uptake in the high relative pressure region indicating the presence of mesopores and macropores. 47 Fig 2b shows the pore volume distribution calculated by the BJH model and the properties derived are included in Table 1. The average pore diameter in the cell walls of Foam-A is 25.2 nm. However, Foam-B shows an insignificant amount of pores with diameters < 100 nm. Thus, unlike Foam-A, most of the pores in the cell walls of Foam-B are macropores which are not likely to interfere with the mean-free path of air particles. This is further substantiated from the higher total pore volume, higher specific surface area, and larger maximum nitrogen adsorption capacity of Foam-A than Foam-B. The pore volume distribution of Foam-A reveals that some pores are larger than the mean free-path of air where the Knudsen diffusion of air particles ceases to be significant and results in an increase in the air conductivity. We believe that if the frequency of pores with diameters greater than the mean-free path of air particles can be reduced, superior thermal insulation performance could be achieved.


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Thermal transport properties:

0.212 0.210

a

0.206

0.06

Si-aerogels [59] Organic aerogels [58]

-1

-1

-1

Thermal conductivity (W m K )

0.208

0.204 0.202 0.200 0.040 0.035 0.030 0.025

EPS [54] Cellulose fiber [55] Wood fiber board [56] Wood wool [56] Foam glass [57]

0.07

-1

Thermal Conductivity (W m K )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Air

0.020 0.015 0.010 0.005 0.000 Foam-A

Foam-B

Bulk material

b

Foam-A (this work) Foam-B (this work) PP foams (this work) Scalable nanowood [26] Polyaniline/Pectin aerogels [24] Silk/Silica hybrid aerogels [23]

0.05

0.04

0.03

0.02

0.01 1

10

2

3

10

10 -3

Density (kg m )

c

Figure 3 Thermal transport properties a) Thermal conductivity of Foam-A, Foam-B, and the bulk material; b) Density dependence of thermal conductivity for: i) the prepared foams (Table S1), ii) traditional insulation materials, and iii) organic and inorganic aerogels. For comparison, the minimum thermal conductivity of recently reported scalable high performance insulators is also included; c) Mechanisms of thermal conductivity reduction in hierarchically porous foams. Black regions correspond to pores in the cell walls.



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The thermal conductivity for Foam-A, Foam-B, and the bulk material is tested at ambient conditions and the results are presented in Fig 3a. Foam-A shows a thermal conductivity of 0.029 W m-1 K-1 for air, and Foam-B exhibits a significantly higher thermal conductivity of 0.04 W m-1 K-1 which is similar to the thermal conductivity of PP-based foams reported in other studies.

48, 49, 50

The low thermal conductivity of Foam-A is ascribed to the average pore size of

the nanoporous cell walls (25.2 nm) being smaller than the mean free path of air (ca. 70 nm at ambient conditions), effectively reducing air conductivity contributions to the thermal conductivity which typically account for 60-70% of the thermal conductivity in foams.

27

The

conduction of the gas $% in the walls of the cells can be estimated by 51 &'( )

$% = *+,-.

(1)

where is the cell wall porosity (14% for Foam-A), $%/ is the thermal conductivity of gas in free space (0.025 W m-1 K-1 for air), 0 ≈ 2 for air in aerogels and Kn is the Knudsen number defined by Kn = 5/6 where 5 is the mean free path of a gas molecule (70 nm for air) and 6 is the pore diameter (25.2 nm for Foam-A). Thus, using eq. (1), the conductivity of air through the pores in the cell walls of Foam-A is calculated to be below 0.001 W m-1 K-1. Besides the conductivity of air, the porous cell walls also reduce the solid conductivity by creating a more tortuous path for phonons to transport heat (see Supplementary Discussion S1) resulting in scattering and diffusive thermal transport. 52 Further, the large cell density of Foam-A (1010 cells cm-3) improves reflectance of infrared light with many solid-air interfaces creating a “multiple reflective effect”. 53 Figure 3c schematically illustrates these mechanisms of thermal transport in hierarchically porous foams.


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Plotting the thermal conductivity of various insulation materials 54, 55, 56, 57, 58, 59 against their bulk density shows that an increase in density first causes the thermal conductivity to decrease, reach a minimum, and then increase (Fig 3b). The minima in thermal conductivity at an intermediate density occurs due to the competing requirements of radiative transport $7 and solid conductivity $8 : at low densities, the $7 contribution to thermal conductivity becomes important because of the inversely proportional relationship between

$7 and ,

58

and at high

densities, the $8 contribution to the thermal conductivity becomes important because it scales as $8 ~ 9 (where is greater than 1.2). 60 The lowest density of the foams prepared in this study is 0.074 g cm-3 (Foam-A) and approaches the apparent optimum density for traditional insulators. Other foams prepared here exhibit higher densities than Foam-A and show a monotonic increase in conductivity with density. The increase in conductivity at low densities is not observed because foams with densities < 0.07 g cm-3 could not be prepared to illustrate the effect of the enhanced $7 on thermal conductivity. The lowest thermal conductivity value of 0.029 W m-1 K-1 is achieved at a density of 0.074 g cm-3 (Foam-A) and compares favorably with the values reported for recent highperformance thermal insulators prepared with scalable technologies such as nanowood (0.032 W m-1 K-1 at 0.13 g cm-3),

26

silk/silica aerogels (0.033 W m-1 K-1 at 0.17 g cm-3),

polyaniline/pectin aerogels (0.033 W m-1 K-1 at 0.42 g cm-3) 24 included in Fig 3b.


23

and


16

Mechanical response under cyclic compression:

5

8x10

Cycle 1 5

6x10

5

4x10

Cycle 10 5

2x10

0

0

10

20

30

40

50

Compressive Strain (%)

60

0

1.0

Young's Modulus Energy Loss Coefficient Maximum Stress

7

10

0.9

0.8

0.7

0.6

6

10

5

9x10

5

7x10

-1

10

-2

10

-3

10

Foam-A (this work) PP foams (this work) Open-cell foam [66] Polycarolactone foam [67] Si Aerogels [68] Si Aerogels [11] CNT aerogel [69] CNT foam [70]

E/Es=(ρ/ρs)

-4

10

2

2

3

0.5

5

8x10

10

E/Es

a

Maximum Stress (Pa)

Compressive Stress (Pa)

Young's Modulus (Pa)

6

1x10

Energy Loss Coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5

10

b 1

3

E/Es=(ρ/ρs)

0.4

2

3

4

5

6

7

8

9

10

Cycle Number

-2

10

3

c

-1

10

ρ/ρs

Figure 4 Mechanical compression of Foam-A a) Stress-strain curves for 10 compression cycles at 60% strain; b) Young’s modulus, energy loss coefficient, and maximum stress for each cycle determined from a); c) Comparison of the relative compressive modulus of the prepared foams with select cellular solids. Multicompression test at 60% strain reveals that Foam-A is mechanically resilient and can be subjected to significant compression. In contrast, inorganic aerogels undergo catastrophic collapse at such strains. 61 The stress-strain response of Foam-A for ten compression cycles is shown in Fig 4a. For the first compression, Foam-A shows a linear elastic response at low strains. Deviation from elastic behavior begins at about 1.1 x 105 Pa. The transition between the elastic behavior and the plastic plateau is not clearly seen, contrary to the classical transition seen in cellular solids during uniaxial compression, and the concave shape of the curve suggest a nonlinear elastic behavior. 62 Further compression results in an increase in stress from the increased interaction between the cell struts, and stress generated by the air inside the foam. This increase in stress should not be confused with densification which is characterized as the impinging of the opposite sides of the cell, and occurs at higher strains than the maximum strain used here. 63 At the maximum compression of 60%, a peak stress of 9.3 x 105 Pa is seen. After unloading, the stress decreases rapidly. Foam-A undergoes a 7% residual strain after completion of the first


0

10

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17

cycle (Fig. S4). In comparison, plastic deformation of 10 to 20% is typical of polymer foams. 64, 65

Foam-A also shows hysteresis during compression cycles. For the first cycle, the work done in compression is estimated to be 245 mJ cm-3 and the energy dissipated is estimated to be 212 mJ cm-3 corresponding to an energy loss coefficient (∆;/;) of 0.86. This large value is attributed to structural damping from buckling, and breaking of cell walls and through mechanical and Coulombic friction between contacting cell walls (Fig S5). Furthermore, the formation of an oriented crystalline structure has been reported in the polymeric system, improving interfacial binding and hence, stress-transfer. 45 Subsequent compressive cycles show similar stress-strain curves but the residual strain decreases. Figure 4b shows the change with each compressive cycle, in the energy loss coefficient, the maximum stress, and the Young’s modulus. Both the energy loss coefficient and the maximum stress decrease with each cycle but the Young’s modulus shows a tendency to plateau after the third cycle. Figure 4c plots the relative compressive modulus /< against the relative density /< for Foam-A and other fabricated foams over a wide range of densities (Table S1). Select cellular solids are included in the figure, such as polymer foams, 66, 67 silica (Si) aerogels 68, 11 and carbon nanotube (CNT) aerogels. 69, 70 The relative compressive modulus of the foams prepared in this work scales with (/< ) which indicates that the deformation mechanism is bendingdominated. In contrast, highly porous cellular solids such as Si-aerogels and CNT-aerogels show a steeper power-law scaling relationship of (/< ) . The larger slope of Si-aeorgels suggests inefficient load transfer compared to polymer foams, and is attributed to the fact that highly porous Si-aerogels are not completely mechanically connected, so densification causes a progressively greater portion of the structure to become load bearing. 28



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Conclusions: Conventional industrial-scale foaming equipment is used to prepare recyclable and robust, thermal insulating foams in a high-throughput, sustainable, and cost-effective process. The method can form foams with complex geometries of any arbitrary size. Improvement in insulation properties is attributed to the nanoporous cell walls which reduce air conductivity by restricting the flow of air, reduce solid conductivity by creating a tortuous path for thermal phonon propagation, and reduce radiative transport by creating numerous air-solid interfaces. The method in principle can be applied to other polymers and their nanocomposites to create novel foam insulators for innovative applications in thermoelectric power generation, and energy conservation. Supporting Information: Photographs of i) the injection molding machine, ii) the mold, and iii) injection molded sample; shape and dimensions of the mold cavity; pressure profile during foam injection molding; effect of mold opening distance on foam morphology; processing/structure/properties relationship of variously prepared foams; effect of cell wall thickness on thermal conductivity of hierarchically porous foams; method for quantifying the cell wall porosity; residual strain of the foam after each compression cycle; SEM micrograph of foam after the first compression cycle. This material is available free of charge via the internet at http://pubs.acs.org/. Acknowledgements: The authors would like to thank the members of the Consortium of Cellular and Microcellular Plastics (CCMCP) for their financial support of this project. A.R. thanks the NSERC (Canada) for providing the CGS-D2 scholarship.


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Table of Contents Graphic:


Scalable Fabrication of Thermally Insulating Mechanically Resilient

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