The storage stability and miscibility of PLA/TBBC blends are related to TBBC content and their thermal histories. Miscibility is generally stated when only one glass transition temperature is recorded on the DSC traces and also yields some changes in crystallization and melting behaviors . The thermal properties of PLA/TBBC blends are shown in Fig. 3 and their DSC data are summarized in Table 4. The melting and cold crystallization behaviors of PLA/TBBC blends in the first heating thermogram are presented in Fig. 3a. All samples exhibit only one glass transition temperature, so they can be considered as miscible ones and no phase separation is expected. Tg of PLA/TBBC blends shifts to lower temperature with an increase in TBBC content, around 54, 36, 28, and 24°C for neat PLA, PLA/TBBC-15, PLA/TBBC-20, and PLA/TBBC-25, respectively. Therefore, the flexibility of their films is great improved by adding TBBC due to Tg reduction. It should also be emphasized that the experimental Tg values obtained for PLA/TBBC blends is comparable to the results obtained for PLA plasticized with oligomeric lactic acid (OLA) and PEG [29, 32], that show a downward tendency and the similar Tg value and in all cases for the same plasticizer concentrations. Nevertheless, the crystallinity of PLA might influence the miscibility of this system. A cold crystallization and a subsequent melting peak are observed for PLA/TBBC blends. The similar enthalpy values for the cold crystallization and subsequent melting is an indication of the amorphous nature of the virgin samples. The neat PLA exhibits two melting points (Tm), respectively, at 141.8 and 151.6°C, and the cold crystallization temperature (Tcc) at 105.4°C, which are consistent with previous studies . The lower melting peak corresponds to the melts of the original crystalline grains, while the higher melting peak corresponds to the melts of more stable grains because recrystallization occurs during thermogram scan. With an increase in TBBC content, Tm and Tcc shift toward the lower temperature. Table 4 shows the first peak of Tm is located at 131.7, 126.9, and 128.6°C and the second peak of Tm is located at 144.8, 141.5, and 139.2°C, respectively, whereas Tcc is located at 68.5, 61.9, and 60.1°C. TBBC effectively reduces Tg and promotes the PLA chains mobility, which induces cold crystallization to start at an earlier temperature in the first heating thermogram . Since the cold crystallization reduces the transparence of the blends and enhances the surface migration of plasticizer, so it is expected that Tcc should much higher than the storage temperature. From Table 4 and Fig. 3a, it concludes that the PLA/TBBC blends can keep good transparence during shelf storage, because the onset of cold crystallization of them is at least 20°C above the room temperature. PLA/TBBC-15 is more stable than PLA/TBBC-20 and PLA/TBBC-25, which is consistent to the results of Fig. 2.
Figure 3b shows that PLA does not exhibit the hot crystallization temperature (Thc) in the cooling thermogram, whereas PLA/TBBC blends exhibit Thc appearing at around 90°C with an addition of TBBC due to the promotion of crystallization of PLA. The peak width of the hot crystallization becomes broader by adding TBBC. It is in agreement with previous studies and similar results about other plasticizers such as acetyl tributyl citrate, and PEGs for plasticizing PLA .
Figure 3c shows the melting behaviors of PLA/TBBC blends in the second heating thermogram. The neat PLA exhibits Tm at 153.6°C and Tcc at 121°C. The PLA/TBBC blends do not exhibit any crystallization peak because they are well crystallized during the previous cooling process. PLA/TBBC blends shows double endothermic peaks. The melting points (Tm) decrease correspondingly with an increase in TBBC content. The first peak is located between 128 and 132°C while the second peak is located between 145 and 148°C. The double melting peaks of plasticized PLA could attributed to the melt of coexistence of disorder (α′) and order (α) crystalline phase of PLA, which are promoted by adding TBBC [30, 34].
The melting enthalpy (ΔHm) of neat PLA is about 30 J g−1, which corresponds to the crystallinity of 32.2%. Even though the melting enthalpies (ΔHm) of PLA/TBBC blends are in the range of 30–32 J g−1, after deducting the weight percentage of TBBC, these values correspond to the crystallinity of 39, 48, and 52%, respectively, with an increasing of TBBC content. The increase in crystallinity of PLA in the blends should be attributed to the plasticizing effect of TBBC which increases the chain mobility and promotes the formation of a higher crystallinity.