Читать книгу Poly(lactic acid) - Группа авторов - Страница 138

The stereocomplex of PLLA and PDLA was discovered in 1987 by Ikada et al., in which the blend samples of various L/D ratios were prepared from solution [27]. They reported the formation of stereocomplex occurs apparently over a comparatively wide range of L/D ratio. But their conclusion was that the complexation occurs stoichiometrically at the 1 : 1 ratio only, and the excess amount of the component is crystallized into the α form or remains in the amorphous region.

The crystal structure of PLLA/PDLA stereocomplex was proposed by Okihara et al. by analyzing the WAXD and electron diffraction data [28, 33]. The unit cell dimensions were a = 9.16 Å, b = 9.16 Å, and c (chain axis) = 8.70 Å, α = 109.2°, β = 109.2°, and γ = 109.8°. As shown in Figure 6.16b, a pair of PLLA and PDLA chain stems of 3/1 helical conformation is packed in the triclinic unit cell of space group P1 with the statistically disordered mode: the upward and downward chains of the same handedness locate at 50% probability at a lattice site. On the basis of the X‐ray powder diffraction data, the electron diffraction data of a single crystal and also the packing energy calculations, Brizzolara et al. suggested the importance of the van der Waals interactions between the PLLA and PDLA chains [74]. By analyzing the electron diffraction data of a stereocomplex single crystal, Cartier et al. proposed a model of the trigonal unit cell with a = b = 14.98 Å, c (chain axis) = 8.70 Å, α = β = 90° and γ = 120° [75]. As shown in Figure 6.16a, the three couples of PLLA and PDLA pairs are packed in this large cell. They proposed the two candidates of the space group: R3c and . In the former case, all the chains are directed toward the same direction concerning the upward and downward orientation along the chain axis. In the latter case, the upward and downward chains are positioned at 50% probability at one lattice site.

FIGURE 6.16 (a) and (b) Various chain packing modes of PLA stereocomplex with L/D 50/50 ratio. (c) Illustrated structures of the R and L chain stems projected along the chain axis.

Source: Reproduced from Tashiro et al., Macromolecules 2017, 50, 8048−8065.

At this point, we have to remember one important experimental data, which was reported by the several researchers [27, 37, 46, 76]: the PLLA/PDLA stereocomplex is formed at an L/D mass ratio in the region of 7/3–4/6, not only at 5/5. The X‐ray diffraction data are shown in Figure 6.17a and b. The sample films were blends of L/D 50/50 and 70/30 ratio, which were cast from the chloroform solution. The film was heated up continuously, during which the X‐ray diffraction profile was measured stepwise. The starting samples showed the weak peaks of the α form in addition to the main amorphous halo. By heating, the α form peaks increased in intensity and disappeared at the melting point (~190°C). In parallel, the diffraction peaks of the stereocomplex started to appear and increased. These peaks disappeared at 230°C, the melting point of the stereocomplex. After being melted, the sample was cooled slowly toward the room temperature. During cooling, only the X‐ray peaks of the stereocomplex appeared and increased in intensity. On the second heating, only these peaks were observed, and no trace of the α peaks was detected up to the melting. This finding was observed not only for the L/D = 50/50 sample but also for the L/D = 70/30 sample. Beyond the ratio 70/30, the blend sample showed a mixture of the X‐ray diffraction peaks of the stereocomplex and the α form. The relative intensity for the stereocomplex and α form peaks is plotted against the L/D content as shown in Figure 6.17c. The pure stereocomplex was detected in the L/D ratio of 70/30–40/60. By combining the various experimental data, the most reasonable structure is the co‐crystallization model of PLLA and PDLA chains at the various ratios.

Figure 6.18 shows the typical 2D‐WAXD patterns of stereocomplex with different enantiomer ratios. The unit cell parameters are almost common to these stereocomplexes; a = b = 14.94 Å, c (chain axis) = 8.624 Å, α = β = 90° and γ = 120°. Although the R3c model is not necessarily reasonable in such a point that only R : L = 1 : 1 ratio is accepted, the X‐ray equatorial line profile reproduces the observed data quite well, while the layer line profiles are not very well reproduced. In the case of the model, the upward and downward chains of the same handedness are located at 50% at one lattice site. The oppositely‐handed chains are positioned at the neighboring sites. As a result, this model gives the R : L = 1 : 1 ratio only. The model did not reproduce the observed diffraction profiles very well, which can be deleted here from the preferential candidates. A new possible model of P3 space group was proposed by Tashiro et al., which can cover the stereocomplex produced in the range of L/D 7/3–4/6 [46, 47]. In the P3 model, the packing mode of the chains is similar to those of the above‐mentioned two models. But, the two neighboring sites are symmetrically independent. This means that the pair of the neighboring sites (left and right sides) can be (R and R), (R and L) or (L and L) at an arbitrary ratio. However, the packing structure must be stereochemically reasonable. As mentioned above, the R3c model can reproduce the X‐ray equatorial line profile well, suggesting the projected structure of R and L chains should be the same as that predicted for the R3c model (see Figure 6.16a). It is noticed that the right‐handed upward (Ru) chain and the left‐handed downward (Ld) chain show the same projected structure perfectly (Figure 6.16c). The Lu and Rd chains are also in the same situation. Then, the plausible pair of the two neighboring sites is (Ru/Ld and Lu/Rd, case 3) or (Ru/Ld and Ru/Ld, case 4), as illustrated in Figure 6.16c. Finally, the model of case 3 was found as the best candidate to reproduce the observed X‐ray diffraction data. The crystal structure is shown in Figure 6.19. Different from the case of the R3c model, the P3 model gives better agreement for all the layer‐line profiles up to the higher angle regions as seen in Figure 6.20a. Besides, as already mentioned, the R and L chains can be located at one site with various ratios, satisfying well the experimental data shown in Figure 6.20.

FIGURE 6.17 Temperature dependence of X‐ray diffraction profiles of the solution‐cast films, (a) PLLA/PDLA 50/50 and (b) PLLA/PDLA 70/30 sample measured in the heating, cooling, and reheating processes. (c) The relative content of the α form and SC phase measured for the PLLA/PDLA blend samples with the various L/D contents on the basis of the X‐ray diffraction data measured after cooling from the melt.

Source: Reproduced from Tashiro et al., Macromolecules 2017, 50, 8048−8065.

FIGURE 6.18 2‐Dimensional X‐ray diffraction diagrams of the uniaxially oriented SC phase of the PLLA/PDLA blend samples with the various L/D ratios. The diffraction diagram of the uniaxially oriented PLLA α form is shown here also for comparison.

Source: Reproduced from Tashiro et al., Macromolecules 2017, 50, 8048−8065.