3.1. Preparation of MC/PLA Membrane
To successfully prepare a MC/PLA membrane, several solvents, including dimethyl sulfoxide, 1-methylpyrrolidine, 1,4-dioxane, tetrahydrofuran, dimethylacetamide, dimethylformamide, and HFIP, were tested so as to screen out a solvent which can simultaneously dissolve MC and PLA. Fortunately, it was found that HFIP could simultaneously dissolve MC and PLA, but not the other solvents. Therefore, HFIP was selected as a solvent for MC and PLA dissolution. The well-blended homogeneous MC/PLA/HFIP solutions with arbitrary mass ratio of MC to PLA were gained by dissolving MC and PLA in HFIP. Subsequently, the MC/PLA membrane was successfully obtained by pouring a MC/PLA/HFIP solution into a mold, followed by evaporating HFIP. Moreover, low boiling point (59 °C) of HFIP makes it easy to recycle and reuse. At the same time, MC and PLA can feasibly be dissolved in HFIP at ambient temperatures, which is obviously energy-saving as extra heating or cooling is not required. Therefore, this indicates a scale-up manufacturing possibility of MC/PLA membranes.
3.2. SEM, Solid-State 13C NMR, XRD and TGA Analysis
SEM was used to investigate the morphology of the MC/PLA and MC membranes, and the SEM homographs of the cryo-fractured surfaces of these membrane are shown in Figure 3
. As observed from Figure 3
, the morphological structures are strongly affected by MC/PLA mass ratio. Neat MC is conglomerated into a homogeneous and dense membrane. MC/PLA (99:1) and MC/PLA (97:3) membranes show a similar morphology to MC membranes. MC/PLA (95:5), MC/PLA (9:1), and MC/PLA (7:3) membranes display a significant phase separation morphological structure, in which PLA beads are distributed in a continuous MC matrix phase. MC/PLA (1:1), MC/PLA (3:7), and MC/PLA (1:9) membranes show porous morphological characteristics. This can be ascribed to the fact that when the content of PLA is less than or equal to 3 wt.%, PLA is homogenously hybridized with MC, thus exhibiting a homogeneous and dense morphology like MC/PLA (99:1) and MC/PLA (97:3) membranes. With increasing PLA (e.g., MC/PLA (95:5), MC/PLA (9:1), and MC/PLA (7:3) membranes), PLA is conglomerated into PLA beads due to the poor compatibility of MC and PLA. The further increase in PLA content can lead to a severe phase separation and generate porous morphological characteristics like MC/PLA (3:7) and MC/PLA (1:9) membranes. In addition, it was found that, at the same MC/PLA mass ratios (1:1, 3:7, and 1:9), the morphological structures of the MC/PLA composites in this study were different from those of the MC/PLA composites reported by Guan in which thin strips formed by MC were observed [45
]. In the studies from Guan, MC and PLA were blended in chloroform. This suggests that the solvent dissolving MC and PLA affects the morphological structures of the MC/PLA composites.
represents the solid-state 13
C NMR spectra of PLA, MC/PLA (1:1) membrane, and MC. Successful hybridization of MC with PLA was confirmed by the solid-state 13
C NMR spectra, as shown in Figure 4
. The MC/PLA (1:1) membrane only exhibited the 13
C NMR spectra of PLA and MC. Moreover, no new peaks appeared in the 13
C NMR spectra of the MC/PLA (1:1) membrane, suggesting that the hybridization of MC with PLA was a physical process.
The X-ray diffraction patterns were evaluated to define the crystal structures of original MC, MC/PLA membranes, and RPLA (regenerated PLA from 8 wt.% PLA solution in HFIP), which are given in Figure 5
. Three typical X-ray diffraction peaks situated at 14.7°, 16.7°, and 19.1° were assigned to the (010), (200/110), and (203) crystal planes, respectively [27
]. MC shows a semi-crystalline structure, confirmed by the two X-ray diffraction peaks situated at 8.9° and 19° [45
]. In all MC/PLA membranes, the X-ray diffraction peak at 8.9° attributed to MC cannot be observed, and meanwhile the diffraction peak intensity at 19.7° becomes weak as the content of MC increases. At the same time, the diffraction peak intensities at 14.7°, 16.7°, and 19.1°, attributed to PLA, noticeably become weak as well. The results suggest that the crystallinity of MC and PLA in the MC/PLA membranes are reduced because of the hybrid of MC with PLA. It was also observed that, in spite of being reduced for the diffraction peak intensity of MC and PLA in the membranes, the diffraction peaks were still observable. This suggests that MC heterogeneously hybridizes with PLA, and the MC/PLA membranes show agglomeration and phase separation. This is in agreement with the SEM result discussed above. The observable diffraction peaks of MC and PLA in the membranes suggest weak or almost no interaction between MC and PLA.
Thermogravimetric analysis was used to estimate the thermostability of the MC/PLA membranes (Figure 6
). As a comparison, the thermogravimetric curves of the original MC and PLA are also included in Figure 4
. The thermal decomposition temperature of MC is 343 °C, and slightly higher than that of PLA (332 °C). The thermal decomposition temperatures of the MC/PLA (9:1), MC/PLA (7:3), MC/PLA (1:1), MC/PLA (3:7), and MC/PLA (1:9) membranes are 301, 308, 335, 338, and 341 °C, respectively. The decrease in the thermal decomposition temperature is mainly due to the decreased crystallinity of MC and PLA in the MC/PLA membranes, which is similar to results reported by Spiridon et al. [13
]. Although the thermal decomposition temperatures of these MC/PLA membranes are less than those of MC and PLA, they still possess good thermostability and can be used at temperatures less than 300 °C.
3.3. Analysis of Mechanical Properties
and Figure 8
show MC/PLA mass ratio (RMC:PLA
) dependent on tensile strength, and breaking elongation of neat MC membrane and MC/PLA membranes, respectively. Mass ratio remarkably impacts tensile strength and breaking elongation. As shown in Figure 7
and Figure 8
, the tensile strength and breaking elongation show continuous improvements with increasing PLA content and reaches the maximum value at RMC:PLA
= 97:3. The tensile strength and breaking elongation of MC are 31.6 MPa and 17%, respectively. It was interesting to find that the maximum tensile strengths and breaking elongation at RMC:PLA
= 97:3 were higher than those of neat MC by about 30% and 35%, respectively. This was mainly due to decreased interfacial adhesion between MC and PLA. However, as PLA content further increases, the tensile strength and breaking elongation obviously decrease. This was mainly attributed to the obvious phase seperation and/or pore structures which result in decreased mechanical properties. This was consistent with the SEM observation, in which when MC/PLA mass ratio was equal to or greater than 9:1, apparent aggregation and phase separation were observed, producing interspaces and stress concentration. In addition, it was found that the tensile strengths of some MC/PLA membranes, such as MC/PLA (97:1; 33.1 MPa), MC/PLA (97:3; 38.8 MPa), MC/PLA (97:5; 36.3 MPa), MC/PLA (9:1; 33.3 MPa), and MC/PLA (7:3; 20.1 MPa), were higher than that of printing paper (17.2 MPa) [50
3.4. Thermocompression Effect on Mechanical Properties
As a representative, the tensile strength and elongation at break of the MC/PLA (1:1) membrane, before and after thermocompression, are presented in Figure 9
. It can be clearly seen from Figure 9
that the thermally compressed MC/PLA (1:1) membrane at 10 MPa and 140 °C displays improved tensile strength and elongation at break, compared with a non-thermally compressed one. Additionally, we also determined the tensile strength and elongation at break of the MC/PLA (97:3) membrane after thermocompression at 10 MPa and 140 °C. It was found that the tensile strength and elongation at break of the MC/PLA (97:3) membrane increased by about 2 MPa and 1%. It was known that thermocompression can result in the redistribution of MC and PLA macromolecules and decrease interspaces and stress concentration. This makes the membrane more compact, and thus increase its tensile strength and elongation at break. It was also found, that after thermocompression at the same conditions, the tensile strength of the MC/PLA (1:1) membrane (increase of 5 MPa) is markedly increased, compared with that of the MC/PLA (97:3) membrane (increase of 2 MPa). This finding was easily understood, considering the fact that there were more interspaces in the MC/PLA (1:1) membrane than in the MC/PLA (97:3) membrane, and thermocompression can decrease interspaces and stress concentration. This was also an indication that thermocompression is especially beneficial to improving the tensile strength of composites with interspaces, compared with dense ones. In addition, the elongations at break of MC/PLA (97:3) and MC/PLA (1:1) membranes were increased by around 1%, suggesting that the elongations at break were hardly affected by thermocompression.