3.1. Effect of Monomer Ratio
The crystal transition behavior of microcapsules determines the thermal properties and energy storage capacities.
Figure 2 shows the DSC curves of MC(BeA-
co-MMA) microcapsules with an
n-behenyl side-chain in the cooling process, measured at a cooling rate of 5 °C·min
–1. The phase transition temperatures and phase change enthalpy are listed in
Table 2. MC(BeA) had the strongest sharp exothermic peak with the highest phase change enthalpy. It indicated a quick transition from the isotropic phase to the crystal phase, and the great potential to be used as a thermal-energy-storage material. MC(MMA) was amorphous so it did not show any exothermic peaks in the cooling process. Compared to MC(MMA), MC(BeA-
co-MMA) copolymer microcapsules showed an exothermic peak thanks to the introduction of the phase change component BeA. The peak was corresponding to crystallization of the
n-behenyl side-chains. In the comb-like copolymer, eight or nine methylene units of the side-chain in the vicinity of the main-chain are amorphous. It is confirmed that the polymers of octadecyl and hexadecyl methacrylates crystallize with the terminal parts of 9–10 units and 7–8 units. [
28,
29]. In the cooling process, the segment motion of the crystalline
n-behenyl side-chain was restricted by the rigid copolymer skeleton. Crystalline side-chains did not have enough mobility to adjust the steric conformation to allow all methylene units to arrange in crystals. Thus, only the terminal parts of the
n-behenyl side-chain could form crystal regions, which led to a weaker and broader exothermic peak for MC(BeA-
co-MMA) copolymer microcapsules.
When the monomer ratio of BeA and MMA was 5:1, the crystallization temperature was only about 1 °C lower than that of MC(BeA). Additionally, the average phase change enthalpy Δ
H of Δ
Hc and Δ
Hm was 97.6 J·g
–1 for MC(BeA-
co-MMA)5 and 105.1 J·g
–1 for MC(BeA-
co-MMA)3, respectively. This indicated that the
n-behenyl side-chain of MC(BeA-
co-MMA) microcapsules could form a relatively perfect crystal region ensuring a high energy storage capacity. When the monomer ratio of BeA to MMA was 1:1, there existed two exothermic peaks. Many investigations on the crystallization behavior of
n-alkane indicate that there are metastable rotator phases before the isotropic liquid completely converted into a crystalline solid [
30,
31]. Rotator phases pass through one or more than one rotator phases between the isotropic and crystal phase, due to the gradual breakdown of orientation order. As the amount of MMA was increasing, a stronger restriction was caused by the copolymer skeleton, which contributed to the peak appearing at 43 °C.
SAXS were carried out at room temperature for MC(BeA-
co-MMA) microcapsules and MC(BeA), and the SAXS patterns are shown in
Figure 3. The locations of diffraction peaks along with their corresponding interplanar crystal spacing
d are listed in
Table 3. MC(BeA-
co-MMA) microcapsules could provide an energy storage capacity because of the crystal transition of the
n-behenyl side-chain. Three peaks were observed at diffraction angles 2
θ = 2.72°, 4.58° and 21.95° on the SAXS pattern for MC(BeA). The peak I at the small angle region was assigned to the layered structure of the alternating crystalline side-chain regions and amorphous regions. The interplanar crystal spacing
d of Peak I and Peak II corresponded to the distance between copolymer chains separated by the side-chains. Researchers indicate that the long
n-alkane chain compounds tend to crystallize into a lamellar structure, as the monolayer assemblies piled on one another, to arrange the functional groups regularly in each lamellar plane [
28,
32]. The peak I at 2.72° indicated the long spacing of 32.45 Å, which was nearly equal to the length of the fully extended BeA with a
n-behenyl chain. It suggested that the side-chains of the comb-like copolymer seemed to be aligned perpendicularly to the basal plane. The intense diffraction Peak III at 2
θ = 21.95° corresponded to the spacing of 4.05Å, which was considered the characteristic peak of the hexagonal packing of
n-behenyl side-chains [
14,
32,
33,
34].
For MC(BeA-
co-MMA) microcapsules, it was noted that with the increasing amount of BeA, diffraction Peak I and Peak II at small angles shift to higher angles, indicating a smaller interplanar crystal spacing
d. This suggested that the peaking of the repeating units of the
n-behenyl side-chain structure became denser with the decreasing amount of the amorphous component MMA. The PMMA segments with a pendent methyl group (–CH
3) were hard, which prevented the copolymer chains from rotating freely around the carbon–carbon bond and packing closely in a crystal pattern. Whereas the interplanar crystal spacing (Bragg spacing) of Peak III at wide angles was almost the same and the value was close to that of MC(BeA), which revealed that the packing type of the
n-behenyl side-chains was not affected by the monomer ratios. Besides, the addition of BeA caused the increasing intensity of Peak III. The increasing amount of BeA led to an increase in the number of methylene units in the crystals, which promised a better thermal energy storage capacity. For sample MC(BeA-
co-MMA)1, there could be seen an amorphous halo around 2
θ = 14° (indicated by the arrow in
Figure 3), which was considered to originate from the correlation between the segregate side-chains and main-chains [
14]. All the above results discovered that the increasing amount of BeA had a strong effect on the long spacing structure and the thermal-energy-storage capacity.
3.2. Effect of Temperature Changing
MC(BeA-
co-MMA) microcapsules exhibit a complicated transition behavior between the isotropic phase and crystal phase. A temperature variable SAXS of MC(BeA-
co-MMA) microcapsules and MC(BeA) were carried out in the cooling process at a slow rate of 0.5 °C·min
–1 from 70 to 35 °C shown in
Figure 4. In order to give a clear understanding and a direct comparison, the scale of intensity of each diagram is set to the same. The higher intensity and smaller half-peak width indicates a better crystal pattern with fewer disturbances from the main-chain skeleton [
17,
35]. For MC(BeA-
co-MMA)5 and MC(BeA-
co-MMA)3, the strong diffraction peaks suggested good crystallization properties, which ensured the high phase change enthalpy and good energy storage capacity. In our previous work, an infrared thermal testing was used to evaluate the thermoregulation property [
18,
20]. Owing to the good crystallization property, MC(BeA-
co-MMA)5 and MC(BeA-
co-MMA)3 effectively prolonged the time to reach the setting temperature, showing the huge capacity to store thermal energy.
MC(BeA-co-MMA) microcapsules showed a broad diffraction peak of the isotropic phase at 2θ = 19.82° for MC(BeA), 19.32° for MC(BeA-co-MMA)5, 19.31° for MC(BeA-co-MMA)3 and 18.38° for MC(BeA-co-MMA)1) caused by the disorder accumulation of the n-behenyl side-chains. Those strong peaks at the smaller angle of 2θ = 3.30° for MC(BeA), 2.84° for MC(BeA-co-MMA)5, 2.72° for MC(BeA-co-MMA)3 and 2.70° for MC(BeA-co-MMA)1) were also observed. Due to the tough and rigid property of PMMA segments, the interplanar spacing reduced with the decreasing amount of MMA. Therefore, the n-behenyl side-chains were found to transit into a crystal state at around 60 °C for MC(BeA-co-MMA)5, 55 °C for MC(BeA-co-MMA)3 and 50 °C for MC(BeA-co-MMA)1. These temperatures were close to the starting point of the corresponding exothermic peak of microcapsules according to DSC results. They showed that MC(BeA-co-MMA) microcapsules presented the exothermic peaks in the temperature range of 58 to 41 °C in the cooling process.
It is noted that within about 5 °C lower than the side-chain transition point, these peaks at the small angle area near 2
θ = 4.5 to 5.0 appeared, which were close to the Peak II mentioned above in
Figure 3. The peak corresponded to the long spacing order of copolymer chains separated by the side-chains. The copolymer main-chain structure could only transit into the solid phase at a lower temperature. In case of MC(BeA-
co-MMA)1, the broad weak peak around 2
θ = 14° appeared when the temperature cools down to 45 °C, which was similar to the temperature where the weak shoulder peak appeared on its DSC curve (
Figure 2). It indicated that the correlation between the segregate side-chains and main-chains contributes to the phase transition. MMA units worked like defects in the side-chain crystal structure causing irregularity and reducing the flexibility of copolymer as a hard segment. As the amount of MMA decreases, less restriction was caused by the main-chain skeleton, and more methylene units of the
n-behenyl side-chain could be arranged into crystals freely. This contributed to a higher phase change enthalpy and crystallinity of MC(BeA-
co-MMA) microcapsules, so that provided a better energy storage capacity and thermoregulation property.
3.3. Effect of Temperature Changing Rate
The regularity of the repeating units strongly affects the crystal transition behavior of MC(BeA-
co-MMA) microcapsules. DSC tests of MC(BeA-
co-MMA)3 measured at different cooling/heating rates, including 1 °C·min
–1, 3 °C·min
–1, 5 °C·min
–1 and 10 °C·min
–1, were used to evaluate the crystal transition behavior. The relative DSC curves are shown in
Figure 5, and the detailed data of the phase change temperature and phase transition enthalpy are listed in
Table 4. In the temperature range of 40 to 60 °C, MC(BeA-
co-MMA)3 displayed a different crystal transition behavior at various temperature changing rates. With the temperature changing rate increasing from 1 °C to 10 °C, the crystallization temperature tended to decrease; the same trend also for phase change enthalpy. On the cooling curve of MC(BeA-
co-MMA)3 at the temperature changing rate of 1 °C·min
–1, a small peak at 55 °C was detected, along with the corresponding endothermic peak at 60 °C in the heating process (indicated by the green arrows in
Figure 5 (left) and
Figure 5 (right), respectively). These peaks were not detected any more under the condition of a faster temperature changing rate. According to the XRD result of MC(BeA-
co-MMA)3 shown in
Figure 4c, the initial phase transition temperature of the crystalline
n-behenyl side-chains was the same around 55 °C. The introduction of a tough and rigid PMMA segments reduced the flexibility of the copolymer chains and the mobility of the crystalline side-chains, and made the copolymer harder to form a crystal. On the heating curves of MC(BeA-co-MMA)3 at a higher temperature changing rate, a small shoulder endothermic peak was observed below the melting point (indicated by the black arrow in
Figure 5 (right)). It revealed the existence of a metastable crystal forming below melting point with a faster cooling process. Melting of a remaining metastable crystal form occurred at a temperature near 48 °C, and then melting of the most stable form occurred.
As discussed in the previous section, the comb-like copolymer with a crystalline long
n-alkane side-chain attached to the main-chain skeleton packed into a layered structure. The side-chains of the comb-like MC(BeA-
co-MMA) microcapsules were aligned perpendicularly to the basal plane on the crystal pattern according to XRD results. The crystal structure of the MC(BeA-
co-MMA) copolymer microcapsules with a crystalline
n-behenyl side-chain are shown in
Figure 6. Their crystal transition behavior can be interpreted as follows. At a temperature above the melting point, the copolymer was in the isotropic phase. As the temperature was going down,
n-behenyl side-chains arranged into a crystal packing form, starting semi-crystalline and reaching a stable crystal pattern. Due to the restriction of the rigid copolymer skeleton, only the terminal parts of the
n-behenyl side-chain could pack into crystals. The increasing amount of BeA allowed methylene units of the
n-behenyl side-chain to arrange into crystals more freely, and contributed to a higher phase change enthalpy and crystallinity.
3.4. Effect of Synthesis Method
By adjusting the monomer ratio of the copolymer, microcapsules with varying phase transition behavior and thermal properties can be prepared. Using copolymer microcapsules and sheets with the same ingredients as samples, we investigated the effects of synthesis method on the thermal and crystallization performance of the copolymer materials. Monomer BeA and MMA can form copolymer sheets under UV irradiation through free-radical light-induced polymerization. The DSC curves of ST(BeA-
co-MMA) copolymer sheets are shown in
Figure 7, and the detailed data are listed in
Table 5.
Like microcapsules, the phase change enthalpy and phase transition temperature of the copolymer sheets increased with the increasing amount of phase change component BeA. However, the phase transition behavior of copolymer sheets were different from that of microcapsules. Under the same monomer ratio, the copolymer sheet sample exhibited a lower phase change enthalpy, a lower phase transition temperature and a higher degree of supercooling. The microcapsule samples were synthesized through suspension polymerization, and the yield was around 60%. This indicated a loss of monomers during the polymerization process. Owing to the slight solubility of MMA in water, part of the MMA might dissolve in the water phase. In addition, the emulsion system in suspension polymerization was a little bit unstable, which might also cause a loss. On the other hand, the sheet samples were synthesized through light-induced polymerization. The mixture of monomer BeA, monomer MMA, crosslinker MBAA and initiator BP in a homogeneous phase was injected into a mold. The sheet samples were prepared after 48 h UV irradiation. There was no loss because the added resin was included in the sheet one hundred percent. Clearly, the content of MMA in sheet samples were higher. The larger amount of MMA caused a stronger restriction on the mobility of the crystalline
n-behenyl side-chain, and formed more disturbances in the crystals. This reason explained the above DSC results in that the ST(BeA-
co-MMA) copolymer sheets presented weak shoulder peaks and longer melting ranges (
Figure 7).
The chemical composition and structure of MC(BeA-
co-MMA) and ST(BeA-
co-MMA) copolymer materials were evaluated by FTIR spectroscopy, presented in
Figure 8. Thermal-energy-storage copolymers synthesized from different methods showed the similar spectrum profile containing a series of characteristic absorption. The copolymers showed strong peaks with double intensive absorption appearing at 2920 cm
–1 and 2850 cm
–1 owing to the alkyl C–H stretching vibrations of the methylene group. The C=O stretching vibration of the ester group triggered an absorption band near 1734 cm
–1. The peaks near 1260, 1160 and 1130 cm
–1, which could be assigned to the C–O stretching vibration of the ester group, were typical of acrylic ester. The absorption band at 1444 cm
–1, which was attributed to the C–H bending vibration of the methyl group on the polymer backbone peak derived from MMA, were detected. Compared to microcapsules, these peaks of sheets at 1444 cm
–1 were stronger, and the intensity became weaker with the increasing amount of BeA. It indicated the content of MMA in sheets was higher than that of microcapsules. It matched the DSC results.
The crystal structure of ST(BeA-
co-MMA) copolymer sheets were further investigated by WAXS analysis. WAXS patterns of these copolymer sheets and homopolymer ST(BeA) are shown in
Figure 9. The values of
Wc and the location of the strongest peak with the corresponding interplanar crystal spacing
d for them are listed in
Table 6. Noted that despite synthesis methods were different, the diffraction peaks of the crystal packing of the
n-behenyl side-chains were around 2
θ = 21.86°, close to that of copolymer microcapsules. When the monomer ratio of BeA to MMA was 1:1,
Wc was only 13.39% (
Table 6), and the average Δ
H of Δ
Hc and Δ
Hm was only 24.5 J·g
–1 (
Table 5). ST(BeA-
co-MMA)1 could barely provide energy storage capacity because of relatively low Δ
H. With the increasing amount of BeA, the crystallinity of copolymer sheets increased and the thermal energy storage capacity improved. The highest
Wc and Δ
H reached 38.23% and 91.0 J·g
–1 under a monomer ratio of BeA to MMA at 5:1. The effect of synthesis method was mainly on the copolymer chemical component, but lightly on the crystal packing of the
n-behenyl side-chains. ST(BeA-
co-MMA) copolymer sheets exhibited complex crystal transition behavior, and the crystalline
n-behenyl side-chains suffered a stronger restriction from the rigid copolymer skeleton during the crystallization process.