**Abstract**

Despite the extensive studies of poly(L-lactic acid)(PLLA), the crystallization of PLLA-based materials is still not completely understood. This chapter presents recent developments of crystallization of PLLA-based blends, block copolymers and nanocomposites. The first section of the chapter discusses the acceleration of PLLA crystallization by the inclusion of biobased (solid and liquid state) additives. It was found that the solid state additives work as a nucleating agent while the liquid-state additive works as a plasticizer. Both type of the additives can significantly enhance the crystallization of PLLA, as indicated by crystallization half-time (*t*0.5) values. Such composites are of great interest as they are 100% based on renewable resources. The second section talks about the enhanced formation of stereocomplex (SC) crystals in the PLLA/PDLA (50/50) blends by adding 1% SFN. It was found that the loading of SFN enhances the formation of SC crystals and it suppresses the formation of HC (homocrystal). The third section deals with confined crystallization of poly(ethylene glycol) (PEG) in a PLLA/PEG blend. The PLLA/PEG (50/50) blend specimen was heated up to 180.0°C and kept at this temperature for 5 min. Then, a two-step temperature-jump was conducted as 180.0°C ! 127.0°C ! 45.0° C. For this particular condition, it was found that PEG can crystallize only in the preformed spherulites of PLLA, as no crystallization of PEG was found in the matrix of the mixed PLLA/PEG amorphous phase. The last section describes the confined crystallization of PCL in the diblock and triblock copolymers of PLA-PCL. Furthermore, enantiomeric blends of PLLA-PCL and PDLA-PCL or PLLA-PCL-PLLA and PDLA-PCL-PDLA have been examined for the purpose of the improvement of the poor mechanical property of PLLA to which the SC formation of PLLA with PDLA components are relevant.

**Keywords:** Crystallization, poly(lactic acid), stereocomplex crystallization, poly (ethylene glycol), poly(caprolactone), biobased additives, improvement of crystallizability, X-ray scattering, crystalline block copolymer, crystalline polymer blend, confined crystallization

## **1. Introduction**

Biobased polymers are gaining great popularity recently due to the increasing environmental concerns associated with conventional polymers. One such polymer is poly(lactic acid)(PLA), which is obtained from 100% natural resources such as corn starch and sugar cane. PLA has a good advantage of mechanical strength and modulus (comparable to PET), however, it has slow crystallization rate, low elongation at break, and processing difficulties due to the low thermal stability which significantly restricts its practical applications. PLA exists in three optical isomeric forms poly(L-lactic acid) (PLLA), poly(D-lactic acid)(PDLA), and poly(D, L-lactic acid) (PDLLA). The PLLA and PDLA both can be partially crystallized with a melting temperature of 170–180 °C. However, a racemic blend (50% L and 50% D) gives an amorphous polymer. Generally, commercial PLA grades are comprised of L-lactic acid in majority with small amount of D moiety. The thermal and mechanical properties of PLLA are significantly affected by the presence of D units in PLLA [1].

The study of crystallization behavior of PLLA is very important to control its thermal, mechanical, and gas-barrier properties. The crystalline structure of PLLA has been studied by many researchers [2–5]. It has been reported that the crystallization of PLLA leads to several crystal forms (*α*, *α*', *β*, and *γ*). The *α* form is the most stable polymorph which is developed from the melt or solution. The crystalline structure of PLLA *α* form is pseudo-orthorhombic with dimensions of *a* = 1.0683 nm, *b* = 0.617 nm, *c* (chain axis) = 2.78 nm, where the molecules adopt a 103 helical conformation. Aleman et al. [6] proposed the space group of P212121 as the most plausible packing mode of 103 helices.

In this chapter, we review the recent developments [7–15] of crystallization of PLLAbased blends, block copolymers and nanocomposites. This chapter contains four sections. The first section deals with the enhancement in the crystallization of PLLA by adding biobased additives. Over the years, there have been several strategies employed by researchers to improve the crystallizability of PLLA [1, 16–19]. One of the most common and effective method is the addition of a nucleating agent. The nucleating agents are known to provide the sites for nucleation in polymers which results in the enhancement of overall crystallization process. Most of the nucleating agents reported for PLLA (talc, carbon nanotubes, graphene, clay) are inorganic materials that are nonbiodegradable in nature [1, 20]. Recently, it is an emerging trend to utilize renewable resources for the improvement of crystallizability of PLLA. In this regard, we used solidstate biobased additives like silk fibroin nanodisc (SFN) and cellulose nanocrystal (CNC) with the aim of improving the crystallization of PLLA. The SFN is a biobased and environmentally benign material which was extracted from the waste of muga silk cocoons, which is composed of 83.8% poly(L-alanine) [21]. The CNC is also a biobased material which was extracted from the waste of marine green algae biomass residue (ABR). Further, we used liquid-state biobased additive, i.e., organic acid monoglyceride (OMG) for the sake of improvement of crystallizability of PLLA. The differential scanning calorimetry (DSC), polarizing optical microscopy (POM), synchrotron small-angle X-ray scattering (SAXS), and wide-angle X-ray scattering (WAXS) measurements were used for the study of crystallization of PLLA. It is worthy to mention here that the timeresolved SWAXS (simultaneous measurements of SAXS andWAXS) technique is one of the most promising technique to detect the initiation of nucleation and follow the change in the structure of growing crystals during the crystallization from the melt.

The second section talks about the stereocomplex crystallization of PLA. When PLLA (left-handed helix) and PDLA (right-handed helix) are mixed, the resultant mixture is known to form a complex so-called "stereocomplex (SC)". The SC is known to improve the thermal stability of PLA [22, 23]. This is due to the approximately 50°C higher melting temperature of the SC crystals compared to the PLLA or PDLA homopolymer crystal (HC). While pure PLLA and PDLA crystallize in pseudo-orthorhombic form with a 103 helix conformation, the SC has a 31 helix form [24]. The crystalline structure of PLA stereocomplex is triclinic with dimension of *a* = *b* = 0.916 nm, *c* (chain axis) = 0.87 nm, *α* = *β* = 109.2°, and *γ* = 109.8°, in

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

which PLLA and PDLA chains are packed parallel taking 31 helical conformation [25]. The formation of SC is influenced by the molecular weights of PLLA and PDLA. It is very challenging to get SC crystals exclusively in the high molecular weight PLLA/PDLA blend due to the competing formation of SC and HC during the crystallization [26]. In view of this, we added 1% SFN in PLLA/PDLA (50/50) blend, aiming to enhance the SC crystallization.

The third section deals with the blend of PLLA and poly(ethylene glycol), PEG. PEG is a biocompatible polymer which is known for improving the toughness of PLLA [20, 27, 28]. The crystallization study of the PLLA/PEG blend is important from the aspect of the structural development, due to the fact that both the component (PLLA and PEG) are crystallizable having different *T*<sup>g</sup> and *T*m. Since PLLA and PEG are known to be miscible with each other, PLLA/PEG blend has attracted many researchers for the studies of structure control. Although there have been extensive research on the crystallization of PLLA part in the dual crystalline PLLA/PEG blend [28–32], the effect of PLLA spherulites on the PEG crystallization is not well-known. In this regard, we studied the effects of space confinement when the PEG crystallizes from the molten mixed amorphous phase sandwiched by the crystalline lamellae of the PLLA.

The final section of this chapter deals with the block copolymers of PLA (PLLA or PDLA) and poly (*ε*-caprolactone), PCL. PCL is a biodegradable polymer which is also known for improving the toughness of PLLA [28, 33]. Since it is known that PLA and PCL are immiscible, copolymerization is a better route in comparison with the blending to avoid the macro-phase separation. In view of this, we studied the crystallization behavior of dual crystalline PLLA-*b*-PCL, PDLA-*b*-PCL diblock copolymers by changing the block length of PLLA or PDLA, however, the block length of PCL was fixed. Furthermore, the blend of PLLA-*b*-PCL and PDLA-*b*-PCL is also studied for the study of formation of SC crystal by changing the block length of PLLA and PDLA components.

The PLLA samples were obtained from NatureWorks and the PDLA was obtained from Purac. The sample characteristics are summarized in **Table 1**. The specimens preparation method is mentioned in the respective section.

The DSC measurements for the isothermal crystallization were performed by DSC214 *Polyma* (NETZSCH, Germany). The specimens were first melted at 200°C (or 260°C) for 5 min and immediately cooled to *T*iso with the cooling rate of 300°C/min, and kept isothermally until the completion of the crystallization process. POM observations were conducted by using a Nikon Eclipse C*i*-POL polarizing optical microscope equipped with the Linkam THMS600 hot stage (Linkam Scientific, UK). The specimens were sandwiched between two coverslips. Next, the specimens were melted on the hot stage, then quickly cooled (cooling rate = 150°C/min) to the isothermal crystallization temperature, and then kept isothermally until the completion of the crystallization process. The POM images were taken under crossed polarizers with a 530 nm optical retardation plate inserted in the optical path. The time-resolved SWAXS measurements were carried out at the beamline BL-6A of Photon Factory at the KEK (High-Energy Accelerator Research Organization) in Tsukuba, Japan. The wavelength of the incident X-ray beam was 0.150 nm. The T-jump experiments were


**Table 1.** *Sample characterization.* conducted using a sample holder designed to allow for a quick T-jump (385°C/min). The details of the experimental set-up are reported elsewhere [7].
