FTIR analysis
It is well known that FTIR could be used to detect the molecular interactions between two components of a blend. The FTIR spectra of PPC/PBS and PPC/PBS/EHBP blends are shown in Fig. 2. The wide absorption band near 3447 cm-1 corresponded to the stretching vibration of the -OH group. The bands near 2961 cm-1 and 1747 cm-1 corresponded to C-H and C=O stretching vibrations. In addition, the peaks at 1458 cm-1 and 1230 cm-1 were attributed to -C-H bending and -CO vibration. The C-C of methyl had a peak at 980 cm-1, while the methylene deformation had a peak at 788 cm-1. The sharp bands at 1070 cm-1 were attributed to C-O-C vibrations. There was a peak at 651 cm-1, which was attributed to the -COO bending zone. After adding EHBP, one can obviously observe that the vibration absorption peak of methyl in PPC/PBS becomes more sharps. The C=O band of ester carbonyl was at 1747 cm-1, while the absorption peak of C=O in PPC/PBS/EHBP composites was shifted to 1744 cm-1 after the addition of EHBP. The vibration absorption peak of C-O-C was shifted from 1070 cm-1 to 1070 cm-1. The offset of the three different groups proved that the original hydrogen bonding in the blend system was increased. The results showed that the hydrogen bond strength and interaction of the composites were strengthened after adding EHBP.
Gel content of the PPC/PBS/EHBP blends
The gel content of the PPC/PB/EHBP blends was measured to demonstrate the presence of the chemical micro-crosslinking effect. During the dissolving process, molecular chains with physical micro-crosslinking are completely dissolved due to the rupture of hydrogen bonds. However, those molecular chains with chemical micro-crosslinking can only be swelled. The pure PPC, PBS, and EHBP can be completely dissolved in chloroform. However, after adding EHBP, the modified products cannot be completely dissolved. The specific gel content measurement results were shown in Fig.3, the addition of different amounts of EHBP will cause different degrees of chemical micro-crosslinking between the blends. The gel content of the pure blends was 1.23%, and when the content of EHBP was 2.0phr, the degree of chemical micro-crosslinking was about 6.63%. The gel content analysis confirmed that there was not only physical micro-crosslinking but also chemical micro-crosslinking between PPC/PBS/EHBP blends upon addition of EHBP.
Rheological analysis
It can be seen from Fig.4 that with the incorporation of the EHBP, the storage modulus (G’), the loss modulus (G”) and the viscosity of the blends showed a gradual decrease trend. On the one hand, EHBP was of spherical structure and low viscous nature. Consequently, it can lubricate the PPC/PBS molecular chains. With the increase of EHBP content, such a lubrication effect was also increased. On the other hand, there were hydrogen bonding and chemical reactions with the terminal hydroxyl groups and carboxyl groups of PPC/PBS and the epoxy-terminated groups of EHBP, resulting in a small amount of micro-crosslinkings, which gradually made the structure change from a linear state to a micro cross-linked state. However, the lubricating effect of the system was dominant, so G’, G” and the viscosity gradually decreased.
Thermal properties
To investigate the miscibility between PPC and PBS, DMA and DSC of the blends were performed. The DMA and DSC curves and the relative parameters of PPC/PBS blends containing different content of EHBP were shown in Fig.5 and Tab.1. The glass transition temperatures (Tα) values of PBS and PPC were denoted as Tα1 and Tα2, respectively, and the difference between Tα1 and Tα2 was denoted as ΔTα. It can be seen that two glass transitions appeared in the PPC/PBS blends, indicating that the two moieties were partially compatible systems. After adding EHBP, the ΔTα reduced from 76.76ºC to 71.13ºC, suggesting the improved miscibility between the two moieties.
As shown in Fig.5 (d), the glass transition temperature (Tg) values of PBS and PPC were denoted as Tg1 and Tg2, respectively, the difference between Tg1 and Tg2 was denoted as ΔTg. Similar with ΔTα, ΔTg was reduced from 76.76ºC to 71.13ºC. The reason for this phenomenon was that after adding EHBP, the active oxygen atom on the epoxy group at the end of EHBP easily formed intermolecular hydrogen bonding with carboxyl groups and ester groups in the PPC molecular chains and PBS molecular chains, and even chemical reactions occurred. Physical and chemical micro-crosslinking points enhanced the entanglement between the PPC and PBS molecular chains, therefore the compatibility between the two-phase interfaces in the PPC/PBS blends was improved. The storage modulus(G') of PPC/PBS/EHBP blends with respect to the temperature was shown in Fig.5 (c). It can be seen that with EHBP incorporated, the storage modulus of the blends reduced. The reason would be similar with those the storage modulus in Rheological analysis.
Tab. 1 Thermal parameters of the PPC/PBS/EHBP blends
EHBP content/ phr
|
Tα1/ºC
|
Tα2/ºC
|
ΔTα /ºC
|
Tg1/ºC
|
Tg2/ºC
|
ΔTg /ºC
|
0
|
-41.95
|
34.81
|
76.76
|
-42.53
|
26.98
|
69.51
|
0.1
|
-38.02
|
34.51
|
72.53
|
-41.99
|
27.72
|
69.71
|
0.5
|
-37.50
|
33.78
|
71.28
|
-40.65
|
27.56
|
68.21
|
1.0
|
-39.97
|
33.26
|
73.23
|
-41.19
|
27.70
|
68.89
|
1.5
|
-39.70
|
32.66
|
72.66
|
-39.56
|
27.84
|
67.40
|
2.0
|
-39.20
|
31.93
|
71.13
|
-38.73
|
27.27
|
66.00
|
Thermal stability
The thermal stability of the PPC/PBS/EHBP blends was studied using TGA and shown in Fig. 3. The thermal degradation temperatures for T5% (5% weight loss), T50% (50% weight loss), Tmax (maximum degradation temperature) and the residual carbon content under 500℃ (Rd%) are summarized in Tab.2. Compared with pure PPC/PBS, the stability of PPC/PBS/EHBP blends showed a gradually decreasing trend with the increase of EHBP.
Two major weight loss steps were observed for all samples. The difference in thermal decomposition behavior of the samples can be seen more clearly from the DTG curves. As can be seen, the DTG curves showed double peaks for all samples, which indicated thermal degradation consisted of two major weight loss steps. The peak temperatures, which were the mid-points of the degradation at each major step, were a measure of thermal stability. The incorporation of EHBP shifted the DTG peak to a lower temperature compared to the pure PPC/PBS blends.
The reason for this phenomenon was that the synthesized EHBP was liquid and had lower thermal stability compared with the matrix. After the blending with the PPC/PBS blends, the thermal stability of the blends was decreased. However, the initial decomposition temperature was still much higher than the processing temperature, so it had little effect on the process as well as the applications.
Tab.2 TGA data for the PPC/PBS/EHBP blends
EHBP content/phr
|
T5%/℃
|
T50%/℃
|
Tmax1/℃
|
Tmax2/℃
|
Rd/%
|
0
|
277.00
|
310.23
|
302.27
|
399.11
|
3.34
|
0.1
|
276.89
|
311.78
|
298.74
|
395.58
|
3.52
|
0.5
|
272.40
|
307.35
|
292.69
|
385.49
|
4.14
|
1.0
|
271.61
|
303.36
|
287.14
|
374.90
|
4.05
|
1.5
|
270.96
|
302.76
|
286.63
|
370.86
|
4.10
|
2.0
|
270.84
|
302.76
|
285.12
|
370.36
|
3.40
|
Mechanical properties
Fig.7 showed the impact strength, the elongation at break and the tensile strength of the PPC/PBS blends with different content of EHBP. It can be seen from Fig.7(a) that the impact strength of the PPC/PBS/EHBP blends increased firstly and then decreased upon addition of EHBP. The impact strength of the PPC/EHBP blends with 0.5 phr content of EHBP was increased by 81.20%, from 9.55 kJ/m-2 of the pure PPC/PBS blends to 17.3 kJ/m-2.
As shown in Fig.7(b), the tensile strength of the PPC/PBS/EHBP blends increased gradually with the increase of EHBP, and then decreased with the maximum value appeared at 0.5phr EHBP. In this case, the tensile strength of the blends reached a maximum value of 16.85 MPa, a 68.7% increase over the tensile strength of pure PPC/PBS (9.99 MPa). Meanwhile, the elongation at break of the PPC/PBS/EHBP blends were increased by 74.90%, from 136.29% to 238.38%, when the content of EHBP was 1.0 phr. The above results showed that the mechanical strength as well as the toughness of the blends could be dramatically improved by certain amount of EHBP.
SEM
Fig.8 showed the SEM images of the impact fracture morphology of the pure PPC/PBS and PPC/PBS/EHBP blends. It can be seen that the surface of the blends became rougher after adding EHBP, indicating that the toughness of the blends increased. However, with further increase of EHBP (higher than 1.0%), the surface of the blend presented a fuzzy and mushy appearance, indicating excessive EHBP was wrapped on the surface of the blends and certain agglomeration appeared, which would damage the mechanical performances.
The modification mechanism
The strengthening and toughening effect of EHBP on PPC/PBS blends can be divided into the following three aspects: (1) in the blending process, the carboxyl groups and hydroxyl groups in the PPC molecular chain and PBS molecular chain can easily interact with many active oxygen atoms on the epoxy group at the end of EHBP. At the same time, a large number of active epoxy groups at the end of EHBP can react with the hydroxyl groups in the PPC and PBS molecular chain. Therefore, the addition of EHBP improves the compatibility between PPC and PBS. (2) EHBP itself is a highly branched three-dimensional spherical structure, and there are a large number of cavities in the molecule. With the addition of EHBP, these cavities can quickly absorb part of the energy under external force, thus improving the toughness of the blends. (3) due to the chemical interaction and intermolecular hydrogen bond interaction between matrix molecular chain and EHBP, chemical and physical micro-crosslinking points were formed, which increased the molecular entanglement. The blends would change from the initial linear to the later reticular micro-crosslinking structure, and when the external force acted on the surface of the sample, it can be transmitted to the interior of the polymer. When the stress concentration point is encountered, a crack will occur, and when the crack continues to conduct and encounter the "rivet" structure, the crack will bend or turn to a certain extent, and the "rivet" structure will remain stable in the same place. When the crack turns or bends, a new second-order crack occurs, which absorbs the impact energy. Therefore, the increased entanglement limits the movement of molecular chains to a certain extent, resulting in the increased strength and toughness of the system. With low content of EHBP (below 0.5phr-1.0phr), the tensile strength, elongation at break, and impact strength of the blends increase significantly; but when the content of EHBP is more than 1.0phr, excessive "rivets" as well as the agglomeration resulted in the decreased mechanical properties.