Synthesis and structure of the various g-PBCT/m-PPZn nanocomposites
The various compositions of PBCT copolyesters were clarified via 1H-NMR spectroscopy. Figure 2 reveals the 1H-NMR data of the PBCT-50 copolymers. The signal of phenylene (-C6H4-) was obtained at 8.1 ppm as a singlet while the signals of -CH2O- and -CH2CH2O- groups were obtained within 4.5–4.1 and 2.1–1.7 ppm, respectively. The -CH2O- signal was divided into two signals at 4.5–4.3 and 4.3–4.1 ppm; the former is designated to -CH2O(CO)C6H4- (ester) and the latter to -CH2O(CO)O- (carbonate). The calculated peak areas of these two signals allowed the determination of the mole fraction of the terephthalate units; F[TPA] = (A4.5–4.3)/(A4.5–4.3 + A4.3–4.1) where A is the calculated peak area signified by the subscript [5, 6]. Table 1 shows the chemical composition of PBCT-50 copolyester determined using F[TPA]. The molar ratio of carbonate to terephthalate is very close to the feed ratio of [DMC] to [DMT], recommending that the composition of the fabricated PBCT is approximately equal to the calculated composition on the basis of the feed ratio.
Related results for the synthesized PBCT copolymers with various [DMC]/[DMT] compositions are also given in Table 1. The molecular weights of the numerous PBCT copolymers clarified using GPC are also recorded in Table 1. The weight-average molecular weight (Mw) and polydisperse indices (PDI) of the fabricated PBCT copolymers were in the range of 26600–35300 g/mol and 1.70–1.77, respectively. The glass-transition temperatures and the melting temperatures of the numerous PBCT copolymers verified via DSC are also listed in Table 1. The melting temperatures of PBCT-30, PBCT-50, and PBCT-90 are 180.4, 155.4, and 42.4 ℃, respectively.
Figure 3 displays the FTIR spectra of PBCT-50 and g-PBCT-50. The absorption peaks of PBCT-50 and g-PBCT-50 around 1102 and 1233 cm−1 are assigned to the stretching of the –COC- bonds in the ester group [18, 19]. The characteristic absorption peaks at 1027 and 1729 cm−1 are attributed to the stretching vibration of the O-C-C bonds in the polymer backbone and the –C=O bonds of the carbonyl group, respectively . An additional peak at about 1716 cm−1 corresponded to the O-C=O bond was observed in the modiﬁed polymer, which reveals the existence of free acid in the modiﬁed PBCT. This result reveals the acrylic acid group is successfully grafted onto PBCT. Similar results have been reported in previous literature [21, 22]. Analysis of the 13C-NMR spectra presents additional support for the successful grafting of AA as presented in Figure 4. The 13C-NMR spectra of g-PBCT-50 contain an additional small peak at δ = 173.3 ppm relative to that of ungrafted PBCT-50. This peak is attributed to the O-C=O bond of the AA, which also conﬁrms the grafting of AA into PBCT [20, 21].
The WAXD diﬀraction profiles of diﬀerent compositions of g-PBCT copolyesters were represented in Figure 5. Five strong diﬀraction peaks at 2θ = 16.1°, 17.4°, 20.7°, 23.3°, and 25.2° were obtained for the g-PBCT-30 and g-PBCT-50 specimens, which indicate the crystalline form of polybutylene terephthalate (PBT) [6, 19]. These results demonstrated that the crystal structure of g-PBCT-30 and g-PBCT-50 are dominated by the crystalline PBT. As presented in this ﬁgure, the diﬀraction peaks of the g-PBCT-90 copolymers at 2θ = 21.2° and 21.7° are comparable with those of crystalline polybutylene carbonate (PBC) . This finding discloses that the structure of the fabricated g-PBCT-90 copolymer was transformed from the crystal structure of PBT to the crystal structure of PBC. As presented in Table 2, the melting temperatures of g-PBCT-3, g-PBCT-50, and g-PBCT-90 determined by DSC were 177.6, 159.2, and 41.4 °C, respectively.
Figure 6 exhibits the XPS data of the g-PBCT-50 copolyester and g-PBCT/m-PPZn nanocomposite. XPS analysis is an effective instrument that was used to illustrate the formation of amide linkages in the g-PBCT-50/m-PPZn nanocomposites. It is evident from Figure 6 that an extra peak of nitrogen at the binding energy of 400 eV was observed for the g-PBCT/m-PPZn nanocomposite . This result recommends that the structural change from the O-C=O formation of g-PBCT-50 to N-C=O formation of g-PBCT-50/m-PPZn nanocomposites.
Figure 7 shows the WAXD diﬀraction curves of the g-PBCT-50/m-PPZn nanocomposites. For comparison, the X-ray diﬀraction data of m-PPZn is also shown in this ﬁgure. A small trace of diﬀraction peak at 2θ = 5.9° was clearly observed in the experimental results of the specimens of high m-PPZn content, which contributes to the stacking layers of m-PPZn. These findings indicate that the intercalated conformation was obtained for the g-PBCT-50/m-PPZn nanocomposites. Similar findings were also observed for the g-PBCT-30/m-PPZn and g-PBCT-90/m-PPZn nanocomposites.
Physical properties of the various g-PBCT/m-PPZn nanocomposites
TGA analysis was operated to investigate the thermal behaviors of the various g-PBCT/m-PPZn nanocomposites. Figure 8 presents the TGA curves of the g-PBCT-50/m-PPZn nanocomposites. Similar findings were also observed for the g-PBCT-30/m-PPZn and g-PBCT-90/m-PPZn nanocomposites. The slight increase in the initial degradation temperature and the temperature of maximum degradation rate illustrated in these patterns is recorded in Table 2. As presented in this table, the temperature of maximum degradation rate for the g-PBCT-30 and g-PBCT-50 is higher than that of g-PBCT-90. These experimental observations reveal that the thermal stability of g-PBCT-90 is relative lower as compared to these synthesized copolyesters, analogous to previously reported results of PBCT copolymers without grafting reaction . Nevertheless, the slight increase in the temperature of maximum degradation rate of the g-PBCT/m-PPZn nanocomposites is higher compared to those of the g-PBCT copolymers. This finding is assigned to the presence of m-PPZn in the g-PBCT matrix, which can cause the covalent bond between g-PBCT and m-PPZn, thus, increasing the thermal stability.
The change of storage modulus (E’) against temperature of g-PBCT-50/m-PPZn nanocomposites in a temperature ranging from -70 to 80 °C is presented in Figure 9. The E’ of g-PBCT-50 at -70 °C is around 1570 MPa and decreases as the temperature increases. This result reveals that the molecular motion of g-PBCT in the glassy state is not enough for a molecular transition. While the temperature is larger than the glass-transition temperature, the thermal energy ends up to be equivalent to the potential energy barriers of the molecular motions. The E’ of the g-PBCT-50/m-PPZn nanocomposites at −70 °C increases as the content of m-PPZn increases. The E’ values of the g-PBCT-50/m-PPZn nanocomposites were about 1710, 1930 and 2080 MPa for 1, 3, and 5 wt% intercalation of m-PPZn into the g-PBCT-50 polymer matrix, respectively. The improvement of E’ may be ascribed to the addition of inorganic and stiﬀ m-PPZn which results in covalent linkages with the g-PBCT and induces a reinforcement effect, thus, enhancing the rigidity of the g-PBCT polymer matrix. Similar results were also found for the g-PBCT-30/m-PPZn and g-PBCT-90/m-PPZn nanocomposites. Detailed E’ values for all the nanocomposites are also presented in Table 2.