All the films prepared by compression molding were transparent. The light transmittance was above 90% in the visible wavelength region, as exemplified in Fig. 1. Since the surface refraction was estimated to be approximately 8–9% [6, 36], the results demonstrated that the films did not scatter or absorb visible light.
Because TEC has a hydroxyl group, it was potentially thermally degradable by hydrolysis during mixing and compression molding at 180°C [37–39]. Therefore, we checked the molecular weights of the sample films prepared by compression molding. Figure 2 shows the size exclusion chromatography curves of the films. The films all had the same molecular weight and molecular weight distribution within the experimental error. Moreover, the values of the blended films were similar to those of the original ISB-PC before mixing. These results demonstrated that no degradation occurred during processing.
Figure 3 shows the temperature dependence of the tensile storage moduli E′ and loss moduli E″ of the sample films. The peaks in the E″ curve were clearly attributable to both a-dispersion, i.e., glass-to-rubber transition, and b-dispersion, as reported previously for pure ISB-PC [6, 35]. The b-dispersion, located in the temperature range − 100 to -50°C, was ascribed to the local motion of chain segments, and has been intensively studied in bisphenol-A polycarbonate [40]. Owing to b-dispersion, the E′ exhibited a stepwise decrease at approximately − 75°C, which was obvious in the film without TEC. The magnitude of the E″ peak decreased as the TEC content decreased, suggesting that there was less b-dispersion.
The E″ peak temperature of the a-dispersion is regarded as the glass transition temperature Tg, which was 124.8°C for pure ISB-PC. As the TEC content increased, the Tg shifted to a lower temperature as single peak, as shown in Fig. 4. In the present study, the miscibility was further evaluated by the full-width at half maximum (FWHM) of the E″ peak ascribed to the a-dispersion [41, 42]. The narrow FWHM indicated good miscibility with ISB-PC. The results are also shown in Fig. 4. The film with 5 wt% TEC had the smallest FWHM value. When the TEC content was 15 wt%, the FWHM value was larger than that of pure ISB-PC.
Although not clearly detected, Fig. 3 reveals that the E′ values in the glassy region were enhanced by the addition of TEC. In Fig. 5, the E′ values at 30°C are plotted on a linear scale as a function of the TEC content. The film containing 10 wt% TEC had the highest E′ value, which was more than 10% higher than that of the pure ISB-PC film.
This phenomenon has been called antiplasticization, and originates from a decrease in the free volume fraction [43–48]. Owing to the reduction in free volume, an antiplasticized system has good gas barrier properties [48] and reduced thermal expansion [46]. Recently, it was discovered that the stress-optical coefficient in the glassy region was reduced by antiplasticization [47].
Tensile tests were performed to evaluate the antiplasticization. The stress–strain curves are shown in Fig. 6. In the figure, both stress and strain are the engineering values. The blend comprising 10 wt% TEC had a high initial modulus (Young’s modulus), which corresponded with Fig. 5. Moreover, the yield strain of the blend was lower than that of the polymer without TEC. The average values with standard deviations are summarized in Table 1. The table confirms that TEC acted as an antiplasticizer for ISB-PC.
Table 1
Tensile properties of the films.
| TEC content (wt%) | Young's modulus (GPa) | Yield stress (MPa) | Yield strain |
| 0 | 0.85 (± 0.02) | 67.8 (± 1.3) | 0.13 (± 0.02) |
| 10 | 0.95 (± 0.02) | 69.0 (± 1.8) | 0.09 (± 0.01) |
This antiplasticization behavior indicated a possible reduction in the water contents of the films. Besides thermal degradation [37–39], the water content is known to greatly affect mechanical [6, 49] and optical properties [6, 50]. Therefore, we evaluated the water contents of the films with/without TEC. Figure 7 shows the water contents of films kept under two conditions: temperature and humidity control at 50% RH and 25°C; and immersion in distilled water at 25°C. In both cases, the water contents of the films decreased following the addition of TEC. This must be attributed to antiplasticization, i.e., reduction of free volume.
The oscillatory shear moduli, i.e., the storage modulus G′ and loss modulus G″ at 200°C, are plotted against the angular frequency w in Fig. 8.
The rheological terminal region was detected in all samples including the pure ISB-PC. Therefore, the rheological parameters, i.e., the zero-shear viscosity\({\eta _0}\)and the steady-state shear compliance \(J_{e}^{0}\), were obtained using the following equations [13]:
$${\eta _0}=\mathop {\lim }\limits_{{\omega \to 0}} \frac{{G^{\prime\prime}}}{\omega }$$
1
,
$$J_{e}^{0}=\mathop {\lim }\limits_{{\omega \to 0}} \frac{{G^{\prime}}}{{{{G^{\prime\prime}}^2}}}$$
2
.
The values are summarized in Table 2 with the weight-average relaxation time tw calculated from the product of \({\eta _0}\) and \(J_{e}^{0}\).
Table 2
Zero-shear viscosity h0, steady-state shear compliance Je0, and weight-average relaxation time τw at 200°C of the ISB-PC/TEC blends.
| TEC content (wt%) | \({\eta _0}\) (Pa s) | \(J_{e}^{0}\) (Pa− 1) | τw (s) |
| 0 | 6.2 × 103 | 4.0 × 10− 6 | 2.5 × 10− 2 |
| 5 | 2.9 × 103 | 4.8 × 10− 6 | 1.4 × 10− 2 |
| 10 | 1.2 × 103 | 5.7 × 10− 6 | 6.8 × 10− 3 |
| 15 | 6.6 × 102 | 7.4 × 10− 6 | 4.9 × 10− 3 |
Because the system is miscible, the zero-shear viscosity h0 can be predicted from the volume fraction of the polymer using the Berry–Fox formula [10, 51]:
$${\eta _0}\left( \phi \right)={\zeta _0}{\phi ^{3.6}}$$
3
where z0 is the monomeric frictional coefficient and f is the volume fraction of the polymer, i.e., ISB-PC.
Figure 9 shows the h0 values as a function of the volume content of ISB-PC f. The figure demonstrates that the viscosity drop was much greater than those predicted by Eq. (3). It indicates that the monomeric frictional coefficient was reduced by the addition of TEC. To the best of our knowledge, such results have not been reported before.
Figure 10 shows the complex shear viscosity η* calculated from G′ and G″ and the steady-state shear viscosity η measured using a capillary rheometer at 200°C for samples containing various amounts of TEC. As shown in the figure, the Cox–Merz empirical rule [52–54] was applicable to all the samples, indicating that wall slippage did not occur during capillary extrusion. Therefore, there was also a large reduction in shear viscosity during capillary extrusion, i.e., in the non-linear region. This result demonstrated that the addition of TEC significantly improves flowability during processing.