3.1 Molecular weight and chemical structure
FT-IR spectra for ISB-PC and BPA-PC are depicted in Figure 2. The ether bond’s C-O-C stretching is measured at 1250 cm-1. The vibration absorption peaks of the benzene ring are 1510 cm-1 and 1600 cm-1. All samples exhibited absorption bands at 1770 cm-1 corresponding to the C=O stretching of carbonyl bonds. The peaks for the asymmetric and symmetric stretching of aliphatic -CH2 and -CH are two absorptions near 2980 cm-1 and 2870 cm-1. The weight average molecular weight (Mw), number average molecular weight (Mn) and the polydispersity index (PDI) of ISB-PC measured from the GPC analysis were 49987 g/mol, 27095 g/mol and 1.84 , respectively. This indicates that good mechanical properties plausible possessed by ISB-PC.
The structural propertie of ISB-PC and BPA-PC were also studied by liquid phase 1H NMR spectroscopy with CDCl3 as the solvent. As examples, the typical 1 H NMR spectra of ISB-PC and BPA-PC are shown in Fig. 3, respectively. A characteristic peak at δ = 3.8-4.1 ppm represented the protons attached to primary carbons in the furan ring of ISB-PC, (i.e., peak 1 and 6 in Fig. 3), and the proton attached to the secondary carbon in the furan ring of ISB-PC was identfied at 4.55 ppm, (i.e., peak 4 in Fig. 3). The chemical shift at δ = 4.8 ppm can also be assigned to the proton attached to the secondary carbon in the furan ring of ISB-PC (i.e., peak 3 in Fig. 3). An intense characteristic peak at δ = 5-5.1 ppm resulted from the proton attached to two opposite secondary carbons in the furan ring of ISB-PC(i.e., peak 2 and 5 in Fig. 3). Furthermore, the intensity of the proton on the benzene ring in the terminal phenoxy structure in the ISB-PC is 7.2 and 7.42, (i.e., peak c and d in Fig. 3). Therefore, the results show that ISB and DPC were polymerized to generate the target polymer ISB-PC.
3.2 Thermal properties
The thermal properties of BPA-PC and ISB-PC polymers were tested by DSC and TG, and the results are shown in Figure 4 and Figure 5. As shown in Figure 4, the Tg value of the synthesized ISB-PC polymer measured by DSC is 145.8 °C and that of the BPA-PC polymer is 146.1°C, which shows that the homemade ISB-PC has reached commercial demand and means that ISB-PC has more application scenarios, such as liquid crystal display fields. As shown in Figure 5, the synthesized ISB-PC sample was degraded in a N2 atmosphere in the next step, and the weight loss rate reached 100%. Before 350 ℃, there was no obvious thermal grain loss in ISB-PC samples, which was due to the presence of adjacent furan rings in ISB-PC molecules, which increased the rigidity of the chain structure in the macromolecular structure. BPA-PC began to lose weight at approximately 500 ℃. This is because BPA-PC contains benzene rings, which do not move easily and therefore have better thermal stability than ISB-PC. The temperature of the copolymer with 5% weight loss (Td,5%) was determined by thermogravimetric analysis. The bond energy of C=C is higher than that of the fat bond, so the Td,5% of BPA is 496°C, higher than that of ISB-PC 350 ℃.
3.3 Mechanical properties
The mechanical properties of BPA-PC and ISB-PC were compared by tensile, bending and impact tests. The tensile strength, tensile modulus and elongation at break of BPA-PC are clearly shown in Figure 6, and their values are repeated in the table. The tensile strengths of ISB-PC and BPA-PC were 73.3 MPa and 57.8 MPa, and the bending strength was 103.1 MPa and 86.9 MPa, respectively. In general, the elasticity of polymers is influenced by a combination of rigidity and molecular weight [18]. The ISB-PC has good stability and high tensile strength, which is the result of the high molecular weight, rigidity and low polarity of the homoperic polymer. Moreover, the stereochemically guided connection between the long chain molecules and the internal hydrogen bond network of the sugar group in ISB also makes ISB its own characteristic compound with high elasticity resilience.
Table 1 shows that the impact strength of ISB-PC is 7.4 kJ/m2, which is smaller than that of BPA-PC (14 kJ/m2). This is because the methyl groups in the structure of bisphenol A give the PC-BPA macromolecular chains greater flexibility, so the impact strength is high. By testing the pencils hardness of two kinds of PCsamples, the ISB-PC hardness is 3 H, and the BPA-PC hardness is 2 H. This indicates that the hardness of ISB-PC is higher. Experiments show that with the increase in the content of the rigid molecule ISB, the hardness level of the pencil is higher, indicating that the rigidity of ISB is the main component of hardness. At the same time, the general hardness is higher, the better the scratch resistance, so it also shows that ISB-PC scratch resistance is better than BPA-PC.
3.4 Transmittance and yellowness
The ultraviolet spectrophotometer showed that both ISB-PC and BPA-PC had better light transmittance because polycarbonate, as an amorphous polymer, had better light transmittance. However, the transmittance of ISB-PC is 91.3%, which is better than BPA-PC's transmittance of 88.6%. Generally, polymers containing benzene rings have a higher refractive index than aliphatic polymers, but the presence of benzene rings makes their dispersion more serious. The Abbe number is inversely proportional to the degree of dispersion, and most resins with high Abbe numbers are aliphatic resins. Therefore, the comprehensive optical characteristics of ISB-PC are better.
The two kinds of PC were irradiated under 254 nm ultraviolet light for the same time, and the yellowness test was carried out by colorimetry. The results showed that with increasing irradiation time, the yellowness of both kinds of PC increased from negative to positive. The growth rate of ISB-PC was slower and finally stabilized at 0.44, while the growth rate of BPA-PC was faster. It stabilized at 4.42 to 4.43. Additionally, during the test, it was found that under ultraviolet light, ISB-PC appeared blue and BPA-PC appeared yellow. After 192 h of exposure, ISB-PC showed little change, while BPA-PC showed more serious discoloration, from colorless and transparent at the beginning to yellow.