Chapter 4. Tunable thermal insulator with micro/nanoporous shape memory polymer
4.3 Results and Discussion
Figure 4.1a shows the structure of human hairs on the skin, where the hairs stand up or lie down according to the environmental temperature. The surface morphology of hairs on human or mammal skin, and feathers of birds are controlled by the muscle, which contracts or relaxes in response to the temperature change.196 When the hairs stand up, the air layer between hairs acts as a thermal insulating layer. According to the environmental temperature, the degree of thermal insulation is automatically adjusted through the modulation of hair shape to maintain the body temperature.
Inspired by the tunable thermal insulation of human skin through the shape-changing hair structure, we fabricated a hair-like patterned porous SMP by using the vapor-induced phase separation method.197 The shape of SMP hairs can be programmed between standing and lying states based on its shape memory property (Figure 4.1b), inducing the switchable thermal insulation between high and low degree of insulation.
The porous SMP composite is a mixture of PLA and TPU polymers. Its shape memory characteristic is resulted from the considerable modulus change of the semicrystalline PLA around its glass transition temperature (Tg) at ~60 °C (Figure 4.2). Above the Tg of PLA phase, the porous SMP becomes softer due to the rubbery PLA, which enables the deformation of the composite into the desired shape. When the SMP composite is cooled down below the Tg of PLA while maintaining the applied force, the shape of SMP composite can be fixed. In this step, the elastic TPU stores the applied stress during the deformation. Simple heating the SMP composite over Tg makes the PLA phase rubbery again, leading to the recovery of its original shape.
Based on the simple deformation and recovery of SMP composites, the heat transport through the hair-patterned porous SMP can be controlled and held at the desired level. In terms of the thermal insulation performance, two types of shape can be changed simultaneously, which are hair- standing/hair-lying and pore-open/pore-closed states (Figure 4.1b). At the original state, the porous SMP exhibits the high degree of thermal insulation because heat transfer is greatly disturbed.
Heat transfer can be explained by three mechanisms; thermal convection, conduction, and radiation.
In the micro/nanoporous structure, all of these heat transfer factors are restricted, resulting in high thermal insulation performance (Figure 4.1b). First, the thermal convection is highly reduced as the air flow is blocked by the micropores. If the pore size is far less than 3 mm, the thermal convection term can be neglected because the difference in air density is not large enough to induce the bulk movement of air molecules.198, 199 Second, the thermal conduction of the porous PLA/TPU composite is the combination of the thermal conduction through PLA, TPU, and air, which can be decreased because of the high air occupation between the pillars and in the micro/nanopores. Additionally, as shown in Figure 4.3 nanopores with an average size of 58.5 nm contribute to low thermal conduction
101
because they are less than the mean free path (≈70 nm) of the gas molecules.184 The randomly distributed hierarchical micro/nanopores also suppress the heat transfer by radiation due to the lower radiation transmittance.185, 186, 200
When the hairy SMP is deformed by the compressive force, the morphology is transformed into the hair-lying and pore-closed structure, decreasing the degree of thermal insulation through the reduced amount of air. In addition, when the nanoporous cellular walls are deformed, the heat-transfer pathways are shortened, reducing the thermal resistance.201 Figure 4.1c exhibits the cross-sectional micro/nanoporous morphology of the flat porous SMP at the original state, which can be greatly deformed into the half of original thickness after compressing the film as shown in Figure 4.1d. The micropores were significantly deformed into the flat structure, and the nanopores were also deformed (Figure 4.4).
Porous PLA/TPU composite has a hierarchical micro/nanoporous structure as shown in Figure 4.1c because of the partial miscibility of PLA and TPU that are linked only through the hydrogen bonding.202 The pore walls of the large microporous structure (Ø, several tens of μm) is mainly composed of TPU, and the small microporous structure (Ø, a few μm) is mainly made of PLA.
Especially, the small microporous structure consists of many nanoporous walls, which is similar to the flake-like morphology of PLA foams made by phase separation method.193
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Figure 4.1. Tunable thermal insulation with porous SMP. (a) Schematic illustration of human skin and hair structure in cool or warm air environment. (b) Scheme of the pillar-patterned porous SMP before and after shape deformation. λPMR and λair mean thermal conduction through polymer and air.
λrad is thermal radiation through the porous SMP. Cross-sectional SEM images of the porous SMP in (c) original and (d) deformed states for (i) microporous and (ii) nanoporous regions.
a b
Deform/Recovery Heat
M icropore Nanopore
λrad
λair λPMR
High thermal insulation Pore-open state
SM P composite Heat Air
Low thermal insulation
Pore-closed state Warm/Cool air
Hair
Skin
Contracted muscle
Skin
Relaxed muscle
c
d
10 µm
10 µm
2 µm
2 µm 20 µ m
20 µm
i
ii
i
ii
i ii
i ii
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Figure 4.2. Storage modulus and loss modulus of PLA film as temperature increased.
Figure 4.3. Size distribution of nanopores in the porous SMP.
Pore diameter : 58.5 ±13.8 nm 020 30 40 50 60 70 80 90 5
10 15 20
Count
Pore Diameter (nm)
2 μm Nanopore
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Figure 4.4. Cross-section SEM images of nanoporous region (a) before and (b) after deformation.
1 µ m 1 µ m
a b
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The distribution of PLA and TPU was analyzed by Raman mapping of the aromatic band of TPU and the C-COO band of PLA, respectively (Figure 4.5). To compare the position of PLA and TPU, representative Raman peaks of each component were analyzed. The band at 877 cm-1 from the C- COO stretching in PLA,203 and the band at 1618 cm-1 from aromatic ring chain vibrations in TPU as shown in Figure 4.5a.204 From the Raman mapping image of aromatic band (TPU-dominant), the Raman intensity at the wall of the large microporous structure is higher than that of the smaller microporous structure (Figure 4.5b), indicating the higher portion of TPU in the walls of large microporous structure. In contrast, inside the large microporous structure at the same region, high Raman intensity of C-COO band (PLA-dominant) is shown on the small microporous structure containing the nanoporous walls (Figure 4.5c). Therefore, in the hierarchical porous structure, PLA consists of a larger portion in the nanoporous region.
Figure 4.5. Raman spectra and mapping images of the porous SMP. (a) Raman spectra of microporous and nanoporous walls in the same sample Optical and mapping images about (b) aromatic and (c) C-COO bands. ‘str’ means the stretching mode in Raman spectra.
0 1000 2000 3000
Position 2
Raman Intensity
Raman Shift (cm-1) Position 1
CH3str / C-H str -CH2str C-COO
str aromatic
Microporous wall
Nanoporous wall
10 µm 10 µm 3 µm
10 µm 10 µm 3 µm
a b ‘aromatic’ band ‘C-COO’ band
Nanoporous walls
c
Micropore walls
Small micropores
Small micropores
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Figure 4.6a exhibits the thermal insulation performance of porous SMPs as the plate temperature- dependent change of ‘T/thickness’ value, here, the T value is the temperature difference between the heating plate and the sample surface. The T values of the porous SMP are stable for 30 min, which is attributed to the stable thermal insulation performance of the porous SMP regardless of the PLA concentration (Figure 4.7). As increasing the PLA concentration, the thermal insulation performance is enhanced, which can be attributed to the increased portion of nanoporous structures (Figure 4.8). Figure 4.6b shows the porous SMPs with different concentration of PLA with respect to TPU placed on the heating plate at approximately 60 °C. The corresponding infrared (IR) thermometer image of the porous SMPs is shown in Figure 4.6c. As increasing the PLA concentration, the porous SMP has lower surface temperature, which means higher thermal insulation performance.
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Figure 4.6. Thermal insulation property of the porous SMP. (a) Plate temperature-dependent ΔT/thickness graph, (b) Photograph and (c) IR thermometer image of the porous SMP with different PLA concentrations.
Figure 4.7. Time-dependent ΔT graph of the porous SMP with different PLA concentrations.
b c
a
PLA 20% PLA 40%
PLA 50%
PLA 60% PLA 80%
50 50.5 51 51.5 52 52.5 53 53.5 54 54.5 55 55.5 56 56.5 57 57.5 58 58.5 59 59.5 60 61
°C
50 5 0 .5 5 1 5 1 .5 5 2 5 2 .5 5 3 5 3 .5 5 4 5 4 .5 5 5 5 5 .5 5 6 5 6 .5 5 7 5 7 .5 5 8 5 8 .5 5 9 5 9 .5 6 0 61
°C
61 °C
50 °C
1 cm 1 cm
53.8 °C 53.3 °C
52.6 °C
51.5 °C 50.5 °C
20 40 60 80
0 3 6 9 12 15
ΔT / thickness (°C/mm)
Plate temperature (°C) PLA 20%
PLA 40%
PLA 50%
PLA 60%
PLA 80%
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Figure 4.8. Cross-section SEM images of porous SMP with different amount of PLA.
PLA 50 wt%
PLA 20 wt% PLA 40 wt% PLA 60 wt% PLA 80 wt%
30 µm
10 µm
30 µm
10 µm
30 µm
10 µm
30 µm
10 µm 10 µm
30 µm
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The porous SMP is flexible and its mechanical properties can be controlled by changing the concentration of PLA compared to TPU due to the different modulus of PLA and TPU. Figure 4.9a shows the photographs of knotted and twisted porous SMPs, indicating high flexibility and shape reconfigurability of the porous SMP. Further, because of its shape memory characteristic, the knotted or twisted shapes are fixed without continuous external force until heating over Tg. From Figure 4.10, the PLA has a much higher elastic modulus of ~4.6 GPa than the TPU (~69.7 kPa), therefore, the modulus of porous SMP increased as increasing the PLA concentration. As a result, the compressive modulus of the porous SMP increases with increasing concentration of PLA compared to TPU as shown in Figure 4.9b.
Because the porous SMP becomes more rigid with the increase of PLA concentration, the stronger power is required to deform the porous SMP. When the same compressive force is applied to deform the porous SMP, the deformation ratio increases as decreasing the PLA concentration. On the other hand, the recovery ratios are over 90% regardless of the PLA concentration (Figure 4.9c). Even though the PLA 20% sample has bigger deformation ratio, it exhibited lower thermal insulation than the samples with more PLA as shown in Figure 4.9d. Therefore, the PLA 50% with both of high thermal insulation and shape memory performances was chosen as the optimum concentration for the further experiments. During the cyclic test with the PLA 50% sample, the deformation ratio shows the similar value about 60% in relative thickness, and the recovery ratio maintained the high level over 99%
of its original thickness without any noticeable fluctuation (Figure 4.9d). This result indicates that the mechanical stability and the shape memory performance of the porous SMP are high enough to make the repetitive deformation and recovery.
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Figure 4.9. Mechanical and shape memory properties of the porous SMP. (a) Photographs of the shape-programmed porous SMP films with knotted and twisted shape. (b) Compressive modulus of the porous SMP with different PLA concentrations. (c) Deformation and recovery ratios of the porous SMP with different PLA concentrations. (d) Cyclic shape memory test of the porous SMP with 50 wt%
PLA.
a b
c d
3 mm
1 cm
0 2 4 6 8 10
60 70 80 90 100
Relative Thickness (%)
Shape Memory Cycle
20 40 60 80
0 100 200 300 400 500
Compressive Modulus (kPa)
PLA Concentration (wt%)
20 40 50 60 80
0 30 60 90 120
PLA Concentration (wt%)
Deformation / Recovery Ratio (%)
Deformation ratio Recovery ratio
111 Figure 4.10. Stress-strain curves of PLA and TPU films.
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Mimicking the human skin hair structure is an effective way to enhance the tunable thermal insulation performance. The skin-hair structure serves as a heat insulator by trapping air between the hairs. The hairs usually lie down because the erector pili muscles connected to the hairs are relaxed at the warm environmental condition. In the cold environment, the muscle contracts and makes hairs standing, making higher thermal insulation to conserve body heat. Inspired by the tunable insulating mechanism of human hairs, mammal fur, and bird feathers, hair-like pillar-pattern was fabricated with the porous SMP.
When fabricating the hair-patterned porous SMP, graphene oxide (GO) was added to the SMP composite to improve the mechanical property and the shape memory performance. From the SEM images of cryo-fractured samples in Figure 4.11., the micro/nanoporous structure exists in both of the pillars and the substrate. Comparing the Figures 4.11a and b, the PLA/TPU sample has many cracks in junctions between the pillars and the substrate, while the PLA/TPU/GO sample has sturdy junctions without large cracks. Therefore, the PLA/TPU/GO sample has more number of straight pillars (94%) compared to the PLA/TPU sample (69%) (Figure 4.12a). In addition, the pillar-patterned porous PLA/TPU/GO exhibits slightly good shape memory performance (Figure 4.12b), and further improves the tunability of thermal insulation performance through the switching between the pillar-lying and the pillar-standing states (Figure 4.13a). The IR images show tunable thermal insulation performance as changing the pillar shape. The thermal insulation performance becomes low at the pillar-lying state and high with the pillar-standing state.
Figure 4.13b shows the relative temperature difference between the heating plate and the sample surface of the flat and the pillar-patterned porous SMP during the shape memory step. Thanks to the high deformability of the pillar structure, the relative ΔT decreased down to ~40% when deformed, which is ~1.85 times bigger change compared to the flat porous SMP (the compressive deformation down to ~70% of the original thickness). The dimension of pillar is diameter of 500 µm and aspect ratio of 5 (diameter/aspect ratio, 500/5), which was optimized in terms of the thermal insulation performance during the shape memory test. As shown in Figure 4.14, the pillar pattern with 500/5 shows the biggest change and the stable recovery in relative ΔT values. In addition, the pillar- patterned porous PLA/TPU/GO exhibits great cyclic shape memory property, and the thermal insulation performance maintained after 10th cyclic shape memory test (Figure 4.15).
The thermal insulation performance as time goes by is shown in Figure 4.13c. For original, deformed, and recovered states, the thermal insulation performance is highly stable for 30 min without large fluctuation. When comparing the ΔT value with commercial textiles, our hairy sample shows 1.4‒1.76 times higher thermal insulation performance due to the hierarchical micro/nanoporous structure (Figure 4.16). Furthermore, Figure 4.13d exhibits the time-dependent plot
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of ΔT value for the pillar-patterned porous SMP at the original and deformed states as changing the plate temperature. When comparing the temperature of hot plate, original and deformed samples at 60 °C, the temperature difference between hot plate and sample is 16.4 °C and 9.2 °C for the original and the deformed sample, respectively, indicating the great tunability of the thermal insulation performance.
Our SMP-based thermal insulator provides unique characteristics of tunable and fixable degree of thermal insulation, which can be favorably compared with previous porous thermal insulating materials. As shown in Table 4.1, our pillar-patterned porous SMP has not only great thermal insulation performance but it also achieved the tunable degree of thermal insulation due to the shape memory property.205-209 Besides, our phase separation method is easier to fabricate a hierarchical micro/nanoporous structure compared to freeze-drying or supercritical drying methods of porous structures, which have disadvantages of expensive specialized equipment and limited sample size.
Even though the graphene foam and the cellulose nano-fibrillar network-based porous films showed high thermal insulation properties, the porous foam cannot be reshaped and the degree of thermal insulation is not fixable.184, 201 To the best of our knowledge, the tunable and fixable thermal insulator has not been reported.
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Figure 4.11. SEM images of the pillar-patterned porous (a) PLA/TPU and (b) PLA/TPU/GO.
Figure 4.12. Shape memory test of pillar-patterned porous PLA/TPU and PLA/TPU/GO.
500 μm
100 μm 50 μm
500 μm
100 μm 50 μm
b a
300 μm 300 μm
PLA/TPU
PLA/TPU/GO
Original Deformed Recovered
3 mm
Top view
3 mm 3 mm 3 mm
3 mm 3 mm 3 mm 3 mm
Side view
Top view Side view
b
94% straight pillars 69% straight pillars
1 cm 1 cm
a PLA/TPU PLA/TPU/GO
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Figure 4.13. Shape memory-related thermal insulation properties of the pillar-patterned porous SMP. (a) Photographs and IR thermometer images of the pillar-patterned porous PLA/TPU/GO after each step of the shape memory test. Relative ΔT of flat and pillar-patterned porous PLA/TPU/GO composites at original, deformed, and recovered states (b) 30 min after heating, and (c) ΔT as time goes by. (d) Time-dependent temperature changes of the pillar-patterned porous PLA/TPU/GO under original and deformed states as changing the hot plate temperature.
a
c d
b
original deformed recovered 40
60 80 100 120
Relative ΔT (%)
Shape Memory Step
Flat Pillar
0 10 20 30
6 9 12 15 18
ΔT (°C)
Time (min)
Original Deformed Recovered
L1L1 L1L1L1L1L1L1L1 Min:
Max:Min:
Max:Min:
Max:
Min:
Max:Min:
Max:
Min:
Max:Min:
Max:Min:
Max:
Min:
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44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C 44.7 °C 60.9 °C A: 54.5 °C
A: 54.5 °C
A: 54.5 °CA: 54.5 °CA: 54.5 °CA: 54.5 °CA: 54.5 °CA: 54.5 °CA: 54.5 °C B: 45.5 °CB: 45.5 °CB: 45.5 °CB: 45.5 °CB: 45.5 °CB: 45.5 °CB: 45.5 °CB: 45.5 °CB: 45.5 °C
C: 54.0 °C C: 54.0 °C C: 54.0 °CC: 54.0 °CC: 54.0 °CC: 54.0 °CC: 54.0 °CC: 54.0 °CC: 54.0 °C
42 44 46 48 50 52 54 56 58 60 62
°C
L1L1 L1L1 L1L1L1L1 L1 Min:
Max:
Min:
Max:Min:
Max:
Min:
Max:Min:
Max:
Min:
Max:
Min:
Max:Min:
Max:
Min:
Max: 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C 52.3 °C 60.4 °C A: 55.3 °C
A: 55.3 °C
A: 55.3 °CA: 55.3 °CA: 55.3 °CA: 55.3 °CA: 55.3 °CA: 55.3 °CA: 55.3 °C B: 54.0 °CB: 54.0 °CB: 54.0 °CB: 54.0 °CB: 54.0 °CB: 54.0 °CB: 54.0 °CB: 54.0 °CB: 54.0 °C
C: 54.1 °C C: 54.1 °C C: 54.1 °CC: 54.1 °CC: 54.1 °CC: 54.1 °CC: 54.1 °CC: 54.1 °CC: 54.1 °C
42 44 46 48 50 52 54 56 58 60 62
°C
L1L1 L1L1L1L1L1L1L1 Min:
Max:
Min:
Max:Min:
Max:
Min:
Max:Min:
Max:
Min:
Max:
Min:
Max:Min:
Max:
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43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C 43.2 °C 60.9 °C A: 55.0 °C
A: 55.0 °C
A: 55.0 °CA: 55.0 °CA: 55.0 °CA: 55.0 °CA: 55.0 °CA: 55.0 °CA: 55.0 °C B: 43.2 °CB: 43.2 °CB: 43.2 °CB: 43.2 °CB: 43.2 °CB: 43.2 °CB: 43.2 °CB: 43.2 °CB: 43.2 °C
C: 53.9 °C C: 53.9 °C C: 53.9 °CC: 53.9 °CC: 53.9 °CC: 53.9 °CC: 53.9 °CC: 53.9 °CC: 53.9 °C
42 44 46 48 50 52 54 56 58 60 62
°C
Original Deformed Recovered
4 4 4 6 4 8 5 0 5 2 5 4 5 6 5 8 6 0 6 2 6 4
°C
62 °C
42 °C 5 mm 5 mm 5 mm
5 mm 5 mm
5 mm
0 100 200 300 400 500 20
30 40 50 60
Temperature (°C)
Time (sec)
Joule heater Original Deformed
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Figure 4.14. Relative ΔT of pillar-patterned porous PLA/TPU/GO with different pillar dimeter (μm) and aspect ratio (diameter / aspect ratio).
Figure 4.15. Repetitive SMP test of pillar-patterned PLA/TPU/GO.
0 2 4 6 8 10
20 40 60 80 100
Shape Memory Cycle
Relative Thickness (%)
Relative ΔT (%)
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Figure 4.16. Comparison of thermal insulation property with commercial textiles.
118 Table 4.1. Previous researches about thermal insulation.
Materials Pore size Thermal insulation (Temperature difference)
Tunability of thermal insulation
Shape
fixability Ref.
Fluoroalklsilane/PDMS/
AgNWs/Nylon cloth <300 nm
ΔT = 4.6 °C (37 °C plate) ΔT = ~10 °C (50 °C plate)
x x 205
3D graphene foam ~35 µm N/A O x 201
Silicone carbide foam 5~50 µm N/A x x 206
PS/SiO2
NPs/AgNWs/Glass 15 nm ΔT = ~10 °C
(60 °C plate) x x 207
Polyacrylonitrile/CNT/
Fe3O4 aerogel
70~130 µm &
4 nm
ΔT = 14.42 °C
(50 °C plate) x x 186
Cellulose nanofibrillar network/mesoporous polymethylsilsesquioxane
~20 nm
ΔT = 8.8 °C (33 °C plate) ΔT = 28.7 °C (55.2 °C plate)
x x 184
Porous silk fiber & textile ~30 µm ΔT = 15 °C
(60 °C plate) x x 208
Cellulose nanofibers/GO/
Sephiolite nanorods foam ~20 µm & 3 nm N/A x x 209
CNT-coated triacetate- cellulose bimorph textile
A few hundreds
of µ m N/A x x 187
Pillar-patterned porous PLA/TPU
Hierarchical micro/nanopores
ΔT = 18.6 °C
(63.5 °C plate) O O This
work
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In nature, many creatures such as chameleon,22 octopus, and cuttlefish210 dynamically change their skin color in response to the changes of surrounding environments. Those adaptive camouflage in the visible region can be also realized in the infrared region with the dynamically adaptive thermal insulation. The adaptive IR camouflage or stealth technology can find many applications in automobile industry, housing, military, and aerospace. thermal-insulating porous SMP can be used as a wearable thermal insulator or a tunable thermal encryption film. The pillar-patterned porous SMP with high flexibility can be deformed into a user-defined shape, demonstrating its unique ability as an adaptable thermal insulator (Figure 4.17a). With its reconfigurable property, the pillar-patterned porous SMP can be wrapped around the human finger or configured into a wavy structure. The dynamically adaptive thermal insulation property is shown in Figure 4.17b, where a hairy SMP on human finger induces the selective IR invisibility on the covered area due to the same temperature of the hairy SMP as the environment when the hairs are at the standing state. Then, the locally IR invisible area becomes IR visible when the hairs are at a lying state.
The adaptive thermal camouflage also can be used for the tunable thermal encryption, as schematically illustrated in Figure 4.17c. Here, when a voltage is applied on a nickel fabric with a letter shape of “UNIST”, the nickel letter can be heated up by the Joule heating principle. The heating element can store the specific information. For example, when 3 V was applied for 20 s, the letter- shaped Ni film was heated up to ~53 °C, exhibiting IR visible information (Figure 4.17d). The letter information could be encrypted by the thermal-insulating pillar-patterned porous SMP. Then, the encrypted information can be decoded by deforming the pillars into lying shape even without removing the porous SMP (Figure 4.17e). Therefore, the tunable thermal encryption was successfully achieved by the pillar-patterned porous SMP.
In addition to the passive insulating, the active heating can be included in our thermal insulating film for more effective heat management. As shown in Figure 4.18, the Joule-heating film is attached on the flat side of the pillar-patterned porous SMP. The temperature of both Joule heater side can be controlled by applying different voltage, and the temperature of both sides were measured by using the IR images. The temperature difference between heater and insulator sides of the flat porous SMP and the pillar-patterned porous SMP exhibits the higher thermal insulation performance of the pillar- patterned sample than the flat one. With this integrated device, the smart heat management can be achieved in both of active and passive way at the same time.