**4. Confined crystallization of PEG inside the preformed PLLA spherulite**

In this section, we focus on the confined crystallization of PEG inside the preformed PLLA spherulite. The PLLA sample used in this study is the product of NatureWorks (code 4032D, D-content = 1.4%). The PEG sample was purchased from Wako Pure Chemical Industries, Ltd., of which *Mw* is 20,000. The PLLA/PEG (50/50) blend specimen was prepared by the solution casting method, using DCM as a solvent to obtain a solution with ca. 5 wt% of the total polymer concentration. The polymer solution was then poured into a Petri dish for complete evaporation of DCM.

The PLLA/PEG (50/50) blend specimen was heated up to 180.0°C and kept at this temperature for 5 min to obtain complete melt without liquid–liquid phase separation. Then, a two-step temperature-jump was conducted as 180.0°C ! 127.0° C ! 45.0°C. The isothermal crystallization time at 127.0 °C was controlled as 0, 5, 10, and 15 min where the PLLA spherulite grew. After that, the specimen was quenched to 45.0 °C and kept at this temperature for 40 min to induce the crystallization of PEG, as shown in **Figure 26**.

As can be seen from the POM micrographs in **Figure 27(a)**, PEG does not crystallize at 45.0 °C upon the direct quench from melt at 180.0 °C, however at 41°C PEG crystallization was clearly observed. This is due to the freezing temperature (*T*f) depression of PEG in the mixture of the PLLA/PEG (50/50) blend specimen by noting that *T*<sup>f</sup> for the neat PEG specimen is 52.0 °C. It should be noted that here we prefer to use the terminology "freezing temperature" instead of "crystallization temperature" to avoid any confusion with the "isothermal crystallization temperature" at which the isothermal crystallization experiment was conducted. Furthermore, the *Tg* of the PLLA/PEG (50/50) blend is approximately estimated as �8.2 °C by using the Fox equation with *T*g,PEG = � 53.0 °C [41] and *T*g,PLLA = 59.6 °C [8]. Therefore, the homogeneous mixture of PLLA and PEG is in the rubbery state at 41°C. However, PLLA crystallization was not observed at 41°C which may be due to the worse crystallizability of PLLA as compared to that of PEG.

The direct evidence of the confined crystallization of PEG inside the preformed PLLA spherulite was observed by the bright-field optical microscopic observation

#### **Figure 26.**

*Temperature protocol for the POM observations and the DSC measurements for the PLLA/PEG (50/50) blend specimens isothermally crystallized by the two-step T-jump (adapted from reference [13] with a permission).*

*Recent Developments in the Crystallization of PLLA-Based Blends, Block Copolymers… DOI: http://dx.doi.org/10.5772/intechopen.97088*

#### **Figure 27.**

*POM micrographs showing the crystallization processes of PLLA/PEG (50/50) blend specimens at 45.0 °C (40 min) after the temperature jump from 127.0 °C, where PLLA was allowed to crystallize for (a) 0, (b) 5, (c) 10, and (d) 15 min. Subsequently, the specimens were subjected to reheating with 50°C min<sup>1</sup> up to 67.0°C. POM images shown in the right column were taken after 10 s of the temperature equilibration at 67.0°C (adapted from reference [13] with a permission).*

which is shown in **Figure 28**. Actually, polarizer and analyzer plates were removed after first-step of T-jump at 127.0°C for 600 s. Afterwards, the specimen was quenched to 45.0°C. Around 484–486 s elapsed at 45.0°C, the dark spokes were observed inside the PLLA spherulite which were disappeared when temperature was increased up to 67.0°C. Thus, the confined crystallization of PEG in the preformed PLLA spherulite was evident. Upon further quenching from 67.0°C to 45.0°C, the confined crystallization of PEG again occured inside the PLLA spherulite, as shown in the bottom row of **Figure 28**. It is interesting to observe that the crystallization of PEG did not start from the center of the preformed PLLA spherulite. It rather seems that the initiation of the PEG crystallization was at random. Also, interesting to note no memory effect, i.e., the trajectories of the second-time PEG crystallization were completely different from the first-time ones. Furthermore, there observed a bridging PEG crystalline region which continuously strides over two-neighboring PLLA spherulites being contacted to each other with a straight boundary.

**Figure 29(a)** shows the change in the WAXS profiles during the isothermal crystallization at 45.0°C after the PLLA crystallization for 15 min at 127.0°C. Initially at *t* = 315 s, only the PLLA crystalline reflection peaks at *q* = 11.95, 13.43, and 15.45 nm<sup>1</sup> were observed. As time goes on, the PEG crystalline reflection peaks also appear at *q* = 13.56, 15.94, 16.12, 16.30 nm<sup>1</sup> with increasing their intensity. **Figure 29(b)**. shows the plots of peak area as a function of time. The peak area for

#### **Figure 28.**

*POM image obtained at 127.0°C (600 s isothermal crystallization; shown at the top-left corner) and brightfield micrographs showing the confined crystallization of PEG in the PLLA spherulites for the PLLA/PEG (50/ 50) blend specimen. The specimen was quickly cooled from the melt state (180.0°C) to the crystallization state of PLLA at 127.0°C. after 600 s elapsed (isothermal crystallization for 600 s at 127.0°C), the specimen was again quenched to 45.0°C for the isothermal crystallization of PEG. Then, the specimen was subjected to reheating up to 67.0°C to melt the PEG crystalline phase. Subsequently (after 6 s elapsed), the specimen was quenched to 45.0°C for the isothermal crystallization at 45.0°C for the second time by keeping the same PLLA spherulites in which the confined crystallization of PEG took place again (adapted from reference [13] with a permission).*

the PLLA crystalline peak was unchanged with the time however, that of PEG peaks showed the increasing tendency and then leveled off. The onset time of the peak evolution can be considered as the induction period which was about 5 min for this particular case. Although the peak positions of the PEG reflections in the secondstep of T-jump of the blend specimen are the same as those for the neat PEG, it is specific to recognize the tremendous suppression of the (124)/(124) reflection peak for the blend specimen as compared to that for the neat PEG [13]. This may indicate the effect of the space confinement that the direction of the PEG crystal growth was suppressed, which is the [124]/[124] direction almost parallel to the *c*-axis, in turn, the polymer chain direction. On the other hand, the (120) reflection peak was not suppressed, indicating that the PEG crystal growth in the direction perpendicular to the polymer chains was not affected. Assuming the folded-chain crystal of the PEG crystalline lamella, these results suggest the suppression of the lamellar thickening due to the space confinement in the amorphous phase sandwiched by the preformed PLLA crystalline lamellae. This further suggests the orientation of the PEG lamellae parallel to those of PLLA. The parallel orientation of the PEG lamellae (parallel to the preformed PLLA crystalline lamellae) as a consequence of the space confinement can be explained by the previous work of Huang et al. [42].

Such a space confinement effect results in the formation of extraordinarily thin PEG lamellae, in turn the lowering of the melting temperature according to the Gibbs–Thomson equation. To check this speculation, the DSC measurements were conducted. The specimens were first quenched from 180.0°C to 127.0°C to allow isothermal crystallization of PLLA for X min (X = 5, 10, 15, and 20) in prior to the second-step T-jump to 45.0°C to allow isothermal PEG crystallization at 45.0°C for 30 min. After the isothermal PEG crystallization at 45.0°C for 30 min, the specimen was then heated with the rate of 10°C/min where the DSC measurement was conducted. **Figure 30** shows the change in *T*<sup>m</sup> of PEG as a function of the PLLA crystallization time at 127.0°C. It is clear that the *T*m<sup>0</sup> s of PEG in PLLA/PEG(50/50) blend are much lower than that for the neat PEG crystallized, suggesting the

*Recent Developments in the Crystallization of PLLA-Based Blends, Block Copolymers… DOI: http://dx.doi.org/10.5772/intechopen.97088*

#### **Figure 29.**

*(a) Time-resolved 1 d-WAXS profiles along the PEG crystallization at 45.0°C in the PLLA/PEG (50/50) blend specimen after PLLA crystallized at 127.0°C for 15 min. (b) Plots of the area of crystallization peaks as a function of time, which was evaluated from WAXS results shown in Figure 29(a) after the peak decomposition (adapted from reference [13] with a permission).*

formation of thinner PEG lamellae in case of the confined crystallization. It is noteworthy to observe that *T*<sup>m</sup> of PEG is monotonically increased with an increase of the PLLA crystallization time, which might imply that the space confinement effect becomes lesser with the growth of the PLLA spherulite and eventually reaching no space confinement effect for the PLLA crystallization time larger than 20 min. Although this tendency seems to be reasonable, it should be noted that thickening of the PLLA lamellae with an increase in the PLLA crystallization time results in more significant confinement to the PEG crystallization taking place in the amorphous region sandwiched by two PLLA crystalline lamellae. Therefore, the result shown in **Figure 30** rather implies the effect of the increase in the weight fraction of PEG (*w*PEG) in the amorphous region comprising the homogeneous mixture of PEG and PLLA. Furthermore, there might be still a weaker confinement, as the density of crystalline PLLA (1.29 g/cm3) [43] is higher compared to that of amorphous PLLA (1.25 g/cm3) [43]. Consequently, at the same time as PLLA

**Figure 30.**

*Tm of PEG as a function of the PLLA crystallization time at 127.0°C (adapted from reference [13] with a permission).*

lamellae grow in size, the amorphous phase will deplete in PLLA and due to the difference in densities there might be an overall gain in space, which enhances the growth of the PEG lamellae. As this effect becomes larger with the progress in the PLLA crystallization, the PEG lamellae can grow more so that the *T*<sup>m</sup> of PEG increases with an increase in the PLLA crystallization time at 127°C, as shown in **Figure 30**. Thus, the DSC confirmation of the above-mentioned speculation of the formation of the PEG lamellae oriented parallel to the preformed PLLA lamellae is not satisfactory. We will report results of detailed DSC experiments using several PEG/PLLA blend specimens with different *w*PEG in our future publication.

Furthermore, the *T*<sup>f</sup> of PEG in the melt mixture of PEG and PLLA amorphous phase were determined by conducting the DSC measurements. Here, it is important to avoid crystallization of PLLA. Therefore, the specimens were quickly cooled from 180°C to 80.0°C and then cooled to 10°C with cooling rate 1,2 5 and 10°C/min. We conducted cooling-rate dependencies to correct for the cooling-rate effect on this temperature and to evaluate *T*<sup>f</sup> of PEG by the extrapolation of the onset temperature of the exothermic peak to the zero-cooling rate [13]. Thus-evaluated *T*<sup>f</sup> of PEG is plotted as a function of *w*PEG for the PLLA/PEG blend specimens in **Figure 31(a)**. It can be seen that *T*<sup>f</sup> is increased when *w*PEG is increased. Based on this plot, the mechanism of the confined crystallization in the preformed PLLA spherulites is considered. Upon the crystallization of PLLA from the melt of the PLLA/PEG amorphous phase, the PEG content in the amorphous region inside the PLLA spherulite is increased so that *T*<sup>f</sup> is increased from its original one (*T*<sup>f</sup> = 45.3°C) at *w*PEG = 0.5. The fact that no PEG crystallization at 45.0°C (**Figure 27(a)**) was observed for this blend specimen with *w*PEG = 0.5 seems to conflict with the fact of *T*<sup>f</sup> = 45.3°C. Since this value (*T*<sup>f</sup> = 45.3°C) was estimated by the extrapolation of the onset temperature of the PEG crystallization to the zerocooling rate, the PEG crystallization at 45.0°C would take infinitely long time for this blend specimen (PLLA/PEG (50/50)). **Figure 27(a)** showing no PEG crystallization at 45.0°C implies that the PEG crystallization would take place more than 40 min. Consequently, it can be considered that when the PLLA crystallization time at 127.0°C is longer, the PEG content in the amorphous region becomes higher so that the PEG crystallizability becomes more sufficient. The experimental results definitely supported this speculation.

Next, the PEG fraction in the amorphous phase in the preformed PLLA spherulite was estimated by DSC measurements. The PLLA/PEG (50/50) blend specimens *Recent Developments in the Crystallization of PLLA-Based Blends, Block Copolymers… DOI: http://dx.doi.org/10.5772/intechopen.97088*

were annealed at 180.0°C for 5 min, and then quenched first to 127.0°C for 10 min to form the PLLA spherulites. Afterward, the specimens were quenched to 60.0°C and then cooled gradually down to room temperature and DSC scans were observed [13]. Thus-evaluated *T*<sup>f</sup> of PEG are plotted as a function of the crystallization time of PLLA at 127.0°C in **Figure 31(b)**. Based on the result shown in **Figure 31(b)** combined with **Figure 31(a)** it was possible to estimate *w*PEG, which is increased from *w*PEG = 0.5 with increasing of PLLA crystallization time at 127.0°C. **Figure 32** shows the Δ*w*PEG (= *w*PEG – 0.5) behavior as a function of the PLLA crystallization time. It is clearly observed that *w*PEG is increased with increasing of PLLA crystallization time at 127.0°C, which supports the above-mentioned discussion that *w*PEG in the amorphous region inside the larger PLLA spherulite is larger than that inside the

**Figure 31.**

*(a) Dependence of the freezing temperature of PEG (Tf) on the weight fraction of PEG (wPEG) in the PLLA/ PEG blend specimens. (b) Dependence of Tf of PEG on the PLLA crystallization time at 127.0°C (adapted from reference [13] with a permission).*

#### **Figure 32.**

*Plot of wPEG as a function of the PLLA crystallization time at 127.0°C (adapted from reference [13] with a permission).*

smaller PLLA spherulite because of the progress in the PLLA crystallization. It is noteworthy that it leveled off around 20 min, suggesting completeness of the PLLA crystallization at 127.0°C around 20 min.
