**5. Confined crystallization of PCL in the block copolymer of PLA and PCL**

In this section, we focus on block copolymers of PLA with PCL, namely, PLLA-PCL diblock copolymers and PLLA-PCL-PDLA triblock copolymers with respect to the confined crystallization within the microphase-separated domain of PCL which is sandwiched by the glassy PLLA microphase. In this study, PLLA, PDLA, PCL and their block copolymers were synthesized. The polymer synthesis method is described in Ref. [12]. The molecular weight information is reported in **Table 2**. The diblock copolymers are represented by XCL-YL and XCL-YD where X and Y denote the block length or the number-average molecular weights (*M*n) in kDa of the component PCL (CL), PLLA (L), and PDLA (D) blocks. Whereas, the tristereoblock (trisb) copolymers are represented by XCL-YL-ZD where X, Y, and Z denote the block lengths or *M*<sup>n</sup> values in kDa of its following block sequences shown by the abbreviated symbols. Further, the synthesized diblock copolymers were blended with their corresponding enantiomers, which are abbreviated as B\_X-Y where X and Y denote the block length. For example, B\_10–10 shows the blend of 10CL-10 L and 10CL-10D. Furthermore, all the prepared specimens were hotpressed followed by quenching in ice-water, to obtain polymer films.

The DSC thermograms of the block copolymers compared with that of neat PCL and neat PDLA are shown in **Figure 33**. The enthalpy of melting (Δ*H*m) of PCL decreases from 42.4 to 20.2 J/g with increasing the block length of PDLA, whereas the Δ*H*<sup>m</sup> of PDLA is increased from 36.8 to 43.4 J/g. These changes in enthalpy correspond to the decreasing content of PCL and increasing content of PDLA in the copolymer system. Furthermore, different melting peaks of PCL and PDLA confirms separate crystallization of PCL and PDLA blocks of varying length. **Figure 33(b)** shows the DSC results of the enantiomeric diblock copolymer blends and the trisb copolymer in the heating scan (10°C/min). As can be seen in the figure, the specimen B\_10–10 form perfect SC crystal upon blending, without generating the HC. This is because of the low molecular weight of the PDLA/PLLA blocks. Furthermore, double melting peaks were observed for SC crystal at 221.8°C and 235.4°C which may be due to the formation of relatively thinner and thicker lamellae of SC crystal. It should be noted that more formation of HC in B\_10–20 and B\_10\_30 specimen is due to the increasing molecular weight of PLLA/PDLA blocks. Therefore, it can be concluded that the higher molecular weight of the block sequences increases the formation of HC instead of SC.


**Table 2.** *Molecular weight of the synthesized block copolymers (analyzed by GPC).* *Recent Developments in the Crystallization of PLLA-Based Blends, Block Copolymers… DOI: http://dx.doi.org/10.5772/intechopen.97088*

**Figure 33.**

*DSC thermograms of (a) neat PCL, neat PDLA, and PCL-PDLA diblock copolymers and (b) enantiomeric diblock copolymer blends and trisb copolymer (adapted from reference [12] with a permission).*

The stress–strain (SS) curves of the polymer films of the block copolymers and their enantiomeric blends as well as that of the trisb copolymer are shown in **Figure 34**. As can be seen that the elongation of 30PCL specimen is higher than that of 30D specimen. The higher elongation of PCL is due to its soft and flexible nature as the stress–strain test is performed at 25°C (rubbery region of PCL) while PDLA is in glassy state. The tensile strength, modulus, and the toughness of the diblock copolymers are enhanced with increasing the block length of PDLA. The highest elongation at break was found for the specimen 10CL-30D. To understand such an

**Figure 34.**

*Representative data for stress vs. (%) Strain of (a) homopolymers and diblock copolymers; (b) diblock blends and trisb copolymer (adapted from reference [12] with a permission).*

unusual increase in elongation at break for 10CL-30D, it is important to consider the structure at the amorphous state at higher temperature. Here, it is noted that PCL/PLLA polymer blends exhibit LCST (lower critical solution temperature) phase behavior [44] so that PCL-PLLA blends are subjected to microphase separation at higher temperature. As a matter of fact, the SAXS results (**Figure 35**) indicated the microphase separation at higher temperature (210°C) for the PCL-PDLA diblock copolymers. The judgment of morphology was uncertain due to the presence of only a first order peak. For the 10CL-30D specimen, there may be the

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

**Figure 35.**

*Lorentz-corrected scattering intensity vs. q of (a) homopolymers and diblock copolymers and (b) enantiomeric diblock copolymer blends and trisb copolymer (adapted from reference [12] with a permission).*

possibility of the formation of PCL lamellar or cylindrical microdomains due to the PCL fraction being 25%. It may be perceived that upon rapid quenching from 210 to 0°C, the PDLA matrix is vitrified due to which the PCL block chains would only crystallize in the interior of the cylindrical microdomains surrounded by the glassy matrix. Because of the confined crystallization, the crystallite may be considered as tiny and dispersed in the interior of PCL microdomains. In such a situation, the crack propagation of glassy PDLA matrix is terminated at the PCL microdomains when the specimen is drawn which could be attributed to amorphous PCL phase (rubbery domain) at room temperature. Also, the PCL block chains are much easier to be unfolded from the tiny crystalline lamellae as compared to larger (thicker) lamellae in other specimens. Therefore, the 10CL-30D specimen is found to exhibit the most stretchable character which may be ascribed to its structural origin.
