Non-isothermal crystallization
The non-isothermal crystallization and subsequent heating curves are presented in Fig. 1. No discernible crystallization peak can be found for neat PES and the Tc of neat TMC300 is about 100 ℃, as shown in Fig. 1a. Upon incorporation of the TMC300, a sharp crystallization peak appeared with the fTMC300 = 0.3%, indicating the TMC300 as an effective NA enhanced significantly the crystallizability. With a further increase in the fTMC300 (0.5% and 1%), the Tc of the PES changed little, probably ascribed with the saturated effect of the TMC300 NA [4]. In the subsequent heating process, neat PES showed a sharp cold crystallization (cc) peak at Tcc, a recrystallization (rc) peak at Trc, and two melting peaks at Tm1 and Tm2, respectively, as presented in Fig. 1b. The cold crystallization peak is an indicative of the rearrangement or adjustment of neat PES molecular chains in the heating process, that is, the insufficient crystallization occurred or the degree of the crystallinity is low for neat PES in the former cooling process. With the loading of the TMC300, no cold crystallization peak can be found, revealing that the TMC300 increased the orderness of the alignment of molecular chains to a certain extent in the cooling process. Multiple melting behaviors of the PES should be attributed to the melting-recrystallization-remelting mechanism, which is widely reported in the polymeric material. The TMC300 exhibited a Tm at about 160 ℃, substantially higher than that of the PES.
The parameters of the thermal property measured using the DSC are summarized in Table 1, where ∆Hc, ∆Hcc, ∆Hrc and Xc represent the crystallization enthalpy in the cooling process, cold crystallization enthalpy, recrystallization enthalpy in subsequent heating process and relative degree of the crystallinity, respectively. The Xc value refers to the ratio of ∆Hmʼ and ∆Hm0 ((∆Hmʼ/∆Hm0)×100%), and ∆Hmʼ = (∆Hm + ∆Hcc+ ∆Hrc) and ∆Hm0 is ∆Hm of an infinitely large crystal of the PES. It was documented that the ∆Hm0 value of the PES is 180 J/g [15]. The Xc value of the PES enhanced markedly in the presence of the TMC300, but a slight increase in the Xc value can be found for three PES/TMC300 composites (with the fTMC300 = 0.3-1%), confirming again that the nucleation of the PES tends to saturation with the higher fTMC300. To the best of our knowledge, the ∆Hm0 value of the TMC300 is unavailable.
The non-isothermal DSC curves at various rates are presented in Fig. S1 (Supplementary Material). With an increase in the cooling rate, the Tc of the PES decreased, suggesting that the lower cooling rate is favorable for the nucleation and crystallization of the PES. No crystallization peak could be found for neat PES when the cooling rate is 10 or 20 ℃/min, indicating that the difficulty in the crystallization for neat PES. The DSC data in the cooling process from the molten state at the rates of 2, 5 and 20 ℃/min, respectively, are shown in Table S1 (Supplementary Material). On the whole, both the Tc and the absolute value of the ∆Hc of the PES increased slightly with the fTMC300, at the same cooling rate.
Table 1
DSC data in non-isothermal cooling and subsequent heating process. Both cooling and heating rate are 10°C/min
Sample
|
Cooling
|
Heating
|
PES Xc/%
|
Tc (°C)
|
-∆Hc (J/g)
|
Tcc (°C)
|
-∆Hcc (J/g)
|
Trc (°C)
|
-∆Hrc (J/g)
|
Tm (°C)
|
∆Hm (J/g)
|
PES
|
N.P.
|
0
|
33.3
|
38.9
|
70.5
|
10.5
|
85.4, 99.1
|
79.8
|
16.9
|
PES/0.3%EBH
|
61.5
|
46.7
|
N.P.
|
0
|
81.4
|
8.2
|
89.8, 98.5
|
72.7
|
35.8
|
PES/0.5%EBH
|
62.8
|
49.2
|
N.P.
|
0
|
82.0
|
9.1
|
90.4, 97.9
|
75.3
|
36.7
|
PES/1%EBH
|
62.8
|
49.6
|
N.P.
|
0
|
82.9
|
8.6
|
90.7, 99.3
|
76.9
|
37.9
|
EBH
|
100.3
|
30.9
|
N.P.
|
0
|
N.P.
|
0
|
161.7
|
36.2
|
—
|
N.P., no peak is discernible; —, no data are available.
|
Isothermal crystallization and crystallization kinetics
Isothermal crystallization curves of neat PES and PES/TMC300 at different Tcs (60, 65, 70 and 75 ℃), are shown in Fig. 2. No obvious crystallization peak is discernible for neat PES. However, with addition of the TMC300, the sharp crystallization peak of the PES appeared and it narrowed much with an increase in the fTMC300. In addition, the crystallization time decreased largely in the presence of the TMC300, reflecting that the TMC300 is indeed an excellent NA on the PES.
For quantitative analysis of the crystallization kinetics of neat PES and PES/TMC300 at different Tcs, modified method and guideline based on Avrami Equation were adopted [45–48]. Figure 3 shows the Xt as a function of the crystallization time (t), in which the Xt represents the relative degree of the crystallinity at the time t. All these plots present the “S” shapes. Obviously, at the same t, the PES exhibited much higher crystallinity in the presence of the TMC300, and the TMC300 shortened substantially the crystallization time at which 100% crystallinity is achieved. The Avrami plots are shown in Fig. 4, and good linear fitting with correlation coefficient higher than 0.991 was obtained in each plot and their slopes are close to each other. The crystallization kinetics parameters of neat PES and PES/TMC300 at different Tcs, are listed in Table 2. t1/2 is the crystalliztion hafe-time at which the relative degree of the crystalinity is 50%. n is the Avrami exoponent ralated to both the nucleation manner (homogeneous or heterogeneous, n1) and crystal growth dimension (n2). n = n1 + n2 and n1 equals to 1 in the case of homogeneous nucleation or 0 in the case of heterogeneous nucleation. k is the crystallization rate constant which is influenced by both the nucleation and crystal growth. Clearly, the t1/2 depressed significantly and k increased substantially with the loading of the TMC300, indicating that the crystallization rate of the PES enhanced largely in the presence of the TMC300. In terms of n, neat PES is close to 3 (instead of 4), seemingly suggest that neat PES exhibits 2-dimensional crystal (n1 = 1 and n2 = 2). However, from the POM image in Fig. 5a, neat PES showed the typical 3-dimensinal spherulites not 2-dimensional crystal. It probably is ascribed with some impurities in the PES. With an increase in the fTMC300, n declined gradually to 2, suggesting that the morphology transition from the 3D spherulite in neat PES to 2D shape in the PES/TMC300. Due to some uncertainties, calculated n value is not an integer.
Table 2
Calculated crystallization kinetics parameters of PES and PES/TMC300 at various Tcs
Tc/oC
|
Sample
|
t1/2/min
|
n
|
k/min− n
|
60
|
PES
|
6.20
|
3.0
|
2.9×10− 3
|
PES/0.3%TMC300
|
0.34
|
2.9
|
15.8
|
PES/0.5%TMC300
|
0.32
|
2.8
|
16.8
|
PES/1%TMC300
|
0.28
|
2.6
|
19.0
|
65
|
PES
|
10.1
|
3.1
|
5.3×10− 4
|
PES/0.3%TMC300
|
0.61
|
2.8
|
2.8
|
PES/0.5%TMC300
|
0.46
|
2.4
|
4.5
|
PES/1%TMC300
|
0.25
|
2.2
|
14.6
|
70
|
PES
|
11.2
|
2.9
|
6.3×10− 4
|
PES/0.3%TMC300
|
1.88
|
2.8
|
0.12
|
PES/0.5%TMC300
|
1.09
|
2.6
|
0.55
|
PES/1%TMC300
|
0.57
|
2.2
|
2.39
|
75
|
PES
|
12.7
|
2.9
|
4.4×10− 4
|
PES/0.3%TMC300
|
5.40
|
2.7
|
1.1×10− 2
|
PES/0.5%TMC300
|
4.31
|
2.4
|
2.1×10− 2
|
PES/1%TMC300
|
1.51
|
2.1
|
0.30
|
Crystal morphology observation
65 ℃ as an example was chosen a Tc to observe the crystal morphology. Neat PES presented a large spherulite at 65 ℃ (Fig. 5a) and, the crystal size decreased markedly and crystal density increased significantly with the loading of the TMC300, especially in the PES/1%TMC300 (Fig. 5d). It indicates that the TMC300 showed the outstanding nucleation effect on the PES.
Hydrogen bond interaction measured by IR
Figure 6 shows the IR spectra (panels a and c) of neat PES, TMC300 and PES/TMC300 at Tc = 25 ℃ in two wavenumber regions. For clear distinguishing of subtle difference, the corresponding 2nd derivatives of the IR spectra are also presented in Fig. 6 (panels b and d). Neat PES showed the IR absorption peaks at 1730 and 1774 cm− 1, attributed to the carbonyl (C = O) group in the crystalline (denote as “C = O cry.” in Fig. 6a) and amorphous (denote as “C = O amo.” in Fig. 6a) phase [49–51], respectively. No IR peak could be found for neat TMC300 in the region of 1800 − 1700 cm− 1. With incorporation of the TMC300, the peak at 1774 cm− 1 shifted to 1770 cm− 1 (as presented in Fig. 6b) and that at 1730 cm− 1 changed little, suggesting that the TMC300 (‒NH‒ or ‒CH2‒ group) interacts with the C = O group of the amorphous phase of the PES. An IR peak at 1605 cm− 1 is assigned to the amide (O = C‒NH‒) group [52] of the TMC300 and it moved to a lower wavenumber region (1601 cm− 1), indicating that the amide group of the TMC300 interacts with the PES (‒CH2‒/‒CH3‒ group). The IR peak at 1116 cm− 1 is associated with the ester (O = C‒O‒C) group of the PES and it moved gradually to 1122 cm− 1 with an increase in the fTMC300, as shown in Figs. 6c and d, reflecting that the hydrogen bond interaction also exists between the ester of the PES and TMC300.
Possible nucleation mechanism
The nucleation mechanism of the polymer in the presence of the NA can mainly be attributed to the chemical nucleation and epitaxial nucleation [53, 54]. Chemical nucleation can be described as follows. In the sample preparation process, the chemical reaction between the polymer and NA occurs and newly formed substance can act as the NA of the polymer. In this case, there is an extremely low possibility that chemical reaction between the PES and TMC300 occurred. Epitaxial nucleation refers to good matching of the crystal lattice between the polymer and NA. To the best of our knowledge, no data on the crystal lattice sizes of the TMC300 is available at present. It is difficult that to confirm that whether or not there is good crystal lattice matching between the PES and TMC300.
It was documented that the uniform dispersion of the multiple wall carbon nanotube (MWNT) enhances greatly the nucleation and crystallizability of the poly(ɛ-caprolactone) (PCL) and the superfine MWNT NA exhibits the supernucleation effect on the PCL [55]. Authors speculates that the homogeneously/evenly distribution of the MWNT is an important factor leading to the nucleation of the PCL. In this work, the hydrogen bond interaction between the PES and TMC300 is favorable for good dispersion of the TMC300 in the PES matrix, which induced the nucleation of the PES.
Time-dependent in-situ FTIR
For real-time observation of spectra change, time-dependent FTIR spectra of neat PES and PES/1%TMC300 collected at 75 ℃, are shown in Fig. 7. From Fig. 7a, the intensity of most of IR peaks (at 1734, 1450, 1415, 1379, 1349, 1314, 1046, 971, 924 and 872 cm− 1) increased with the crystallization time and these peaks are considered as the crystalline peaks (denoted as the ↑ arrow). The intensity of an IR peak at 1146 cm− 1 decreased with the crystallization time and it is an amorphous peak (denoted as the ↓ arrow). With loading of the TMC300, two new IR absorption peaks between 1620 and 1550 cm− 1 appeared, related to the amide group of the TMC300. These two peaks are the crystalline one because they enhanced in their peak intensities. In addition, the peak shift occurred for 3 peaks in the presence of the TMC300, that is, 1415 cm− 1◊1422 cm− 1, 1146 cm− 1◊1152 cm− 1 and 1046 cm− 1◊1040 cm− 1, respectively. The intensity variation and peak shift should be ascribed with the reorganization/adjustment of the molecular chains of the PES in the crystallization process. Based on the previous literatures on the polyester [56–59], the IR peaks assignment of the PES is summarized as follows. The peak at 1734 cm− 1 is assigned to the carbonyl stretching mode, and those in 1500–1400 and 1400 − 1300 cm− 1 are ascribed with the CH2 bending and wagging mode, respectively. Peaks in the wavenumber of 1300 − 1100 cm− 1 are attributed to the ester stretching mode. Those in 1100 − 1000 cm− 1 are related to C–C backbone stretching mode. IR peaks in 1000 − 800 cm− 1 are assigned to CH2 rocking mode. It should be mentioned that the positions of the carbonyl and ester group in Fig. 7 differ from those in Fig. 6, probably resulted from the variation in the Tc.
With combination of IR results from Figs. 6 and 7, the hydrogen bond interaction formed between the PES and TMC300 is illustrated in Fig. 8. The TMC300 interacted with the carbonyl and ester group in the PES amorphous phase, and with the CH2 group in the PES crystalline phase.
For direct comparison of the crystallization rate, the normalized intensity of several IR characteristic peaks (assigned to the carbonyl, ester and C–C group, respectively) as the crystallization time, are plotted, as shown in Fig. 9. The peak intensity of the carbonyl and ester group increased with the time, but that of the ester group depressed. Clearly, by contrast, the intensity change of these 3 IR peaks accelerated substantially with loading of the TMC300, revealing that the TMC300 enhanced highly the crystallization rate of the PES. The variation rate of the carbonyl group was comparable to that of the C–C backbone, regardless of the presence or absence of the TMC300.
Crystal structure measurement by WAXD
The WAXD patterns of neat PES, TMC300 and PES/TMC300 at Tc = 25 ℃ are presented in Fig. 10. Neat PES showed two main peaks assigned to the diffraction planes (021) and (200) [14]. With loading of the TMC300, a weak should peak assigned to the diffraction planes (121) of the PES [20] could be found. Absence of this should peak in neat PES is probably attributed to its significantly large crystal (as presented in Fig. 5a), resulted in the orientation of neat PES spherulite on the film surface. Similar results have been reported in poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] [60], poly(3-hydroxybutyrate) [61] and poly(ethylene adipate) [62]. It is noteworthy that there was an obvious difference in the peak intensity ratio between (021) and (200), that is, I(021)/I(200), for neat PES and PES/TMC300, probably ascribed with different spatial arrangements in the same crystal lattice [63–65]. Hence, the TMC300 had little effect on the crystal structure of the PES.
Thermal stability by TGA
The mass and derivative mass as a function of the temperature are shown in Fig. 11. The mass loss of the sample occurred mainly in the temperature range of 300–455 ℃. From Fig. 11a (TGA curves), both neat PES and PES/TMC300 presented one-step decomposition behavior, but neat TMC300 showed two-step decomposition one, which could be clearly seen in the DTG curves in Fig. 11b (peaks 1 and 2). From 300 to 370 ℃, the order of the thermal degradation temperature is: neat PES > PES/TMC300 > neat TMC300. From 425 to 455 ℃, the thermal degradation temperature of neat TMC300 is higher than those of neat PES and PES/TMC300. In addition, the residue yield of neat PES is lower than those of neat TMC300 and PES/TMC300 at 600 ℃. The mass loss of neat TMC300 in the first stage (peak 1) is probably attributed to the evaporation of formed NH3 and H2O gases in a lower temperature region, and that in the second stage (peak 2) should be associated with the escape of the gases related to the decomposition of the benzene ring and C–C backbone in a relatively higher temperature region. The decomposition of neat PES and PES/TMC300 are mainly ascribed with the developed H2O, CO and CO2, and other low molecular weight gases containing the carbonyl and hydroxy groups. With comparison of neat PES, the enhanced carbon residue yield in neat TMC300 and PES/TMC300 are probably related to the slight flame retardance effect of the TMC300. It was reported that many organic substances with the N or P element (as the excellent flame retardant) increased the flame retardance or fire resistance of the polymeric composites, because these flame retardants migrate to the surface of the polymeric material and react with the polymer to form a thin carbon layer (with a lower heat conductivity) to prevent from the direct contact with the heat flow [66, 67].