**2. Improvement of PLLA crystallizability by biobased additives**

### **2.1 Solid state additives (nucleation agents)**

In this section, we report the effect of solid-state additives (SFN and CNC) on the isothermal crystallization of PLLA from the melt (200°C).

#### *2.1.1 Silk fibroin nanodisc (SFN)*

The SFN used in this study was extracted from wastes of the muga silk (*Antheraea assama*) cocoon [21]. The crystalline portion (the β sheets) of the silk fibroin was isolated by using the acid-hydrolysis method. The obtained extract comprises 83.8% L-alanine, and the well-defined disc-like nano particles were obtained (see **Figure 1** for the chemical structure of poly(L-alanine)). Such morphology and dimensions have been reported as the average diameter and thickness of 45 nm and 3 nm, respectively [21]. The detailed information about the preparation of SFN can be found in Ref. [21]. PLLA/ SFN specimens were prepared by the solution-casting method, using dichloromethane (DCM) as a solvent [8]. The specimens are labeled as D1.4/SFN(*x*) or D0.5/SFN(x), where the numbers after D denote the % of D moiety in PLLA, and *x* stands for % of SFN inclusion.

POM observations were conducted to observe spherulites and to evaluate the growth rate of spherulites and the nucleation density as a function of time. The specimens were melted on the hot stage at 200°C for 3 min, then quickly cooled (cooling rate = 150°C/min) to the isothermal crystallization temperature (*T*iso) of 120°C, and then kept isothermally until the completion of the crystallization process. The representative images of the evolution of spherulites for the D1.4 neat and D1.4/SFN(1.0) specimens at 120°C as a function of time are shown in **Figure 2(a)** and **(b)**. First, negative spherulites were observed for both of the D1.4 neat and D1.4/SFN(1.0) specimens. As shown in **Figure 2(d)** the total number of spherulites increased 4.7 times (from 9 to 42). In addition, the induction period was shortened from 101 to 39 s. However, the growth rate of the spherulites was unchanged (5 μm/min) by the addition of SFN. These results clearly show that SFN can enhance nucleation of PLLA. SFN is considered to provide sites for easy formation of PLLA nuclei.

#### **Figure 1.**

*Chemical structures of poly(L-lactic acid) and poly(L-alanine). L-alanine is the main component (83.8%) of the silk fibroin nanodisc.*

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

**Figure 2.**

*POM images as a function of time for isothermal crystallization at 120*°*C (a) D1.4 neat (b) D1.4/SFN(1.0) specimens. (c) Spherulite diameter and (d) the number of spherulites formed during the isothermal crystallization at 120*°*C (adapted from reference [8] with a permission).*

**Figure 3** shows the DSC results of the isothermal crystallization of neat and 1% SFN included specimens at 110°C. the degree of PLLA crystallinity (*ϕDSC*) as evaluated based on the heat flow results using the following equation.

$$\phi\_{\rm DSC}(t) = \frac{\int\_0^t H(t)dt}{\Delta H\_m^o} \tag{1}$$

where *t* denotes time and Δ*H<sup>o</sup> <sup>m</sup>* is the enthalpy of fusion for the 100% crystal of PLLA. The value of Δ*H<sup>o</sup> <sup>m</sup>* is taken as 93 J/g, following reference [34]. **Figure 3(b)** clearly indicates that the induction period was reduced and the final degree of crystallinity was increased by the presence of SFN. Maximum achievable crystallinity was found in the case of D0.5/SFN(1.0).

The inverse of crystallization half-time (*t*0.5) can be used for the discussion of the crystallization rate, which is plotted as a function of the crystallization temperature in **Figure 4**. The graph shows a parabolic curve thereby producing maximum crystallization rate (1/*t*0.5,max). As shown in **Figure 4**, the overall crystallization growth rate was significantly increased by the inclusion of SFN, the maximum crystallization rate, 1/*t*0.5,max, was observed at 107.2°C. It should be noted that for both specimens D1.4 and D0.5, the crystallization rate showed the same tendency, as the most effective temperature is �107°C, although the *<sup>T</sup><sup>O</sup> <sup>m</sup>* differs (The *T<sup>O</sup> m*

**Figure 3.**

*(a) Heat flow as a function of time during isothermal crystallization at 110°C and (b) degree of crystallinity (ϕDSC) as a function of time, which was evaluated based on the heat flow result (adapted from reference [8] with a permission).*

**Figure 4.**

*Inverse of crystallization half time as a function of crystallization temperature, calculated from DSC results (adapted from reference [8] with a permission).*

values for the D1.4 neat and D0.5 neat specimens were 180.7°C and 193.3°C, respectively [8]). These results indicate that the PLLA crystallization is predominantly governed by the kinetic driving force (*T T*g) rather than the thermodynamic driving force (*T<sup>O</sup> <sup>m</sup> T*). It is important to note here that the spherulite growth rate does not change in the presence of SFN (**Figure 2**), while 1/*t*0.5 is increased. The reason for the enhancement of 1/*t*0.5 can be attributed to the increased number of nuclei due to the addition of SFN (**Figure 2**). The enhanced nucleation of PLLA by SFN may be ascribed to the plausible formation of hydrogen bonding between the C=O group in PLLA and the N–H group of poly(L-alanine) (see the chemical structure in **Figure 1**).

**Figure 5(a)** and **(b)** show the time-resolved WAXS profiles for the D1.4 neat and D1.4/SFN(1.0) specimens as a function of time for isothermal crystallization at 110°C. Here, the magnitude of the scattering vector *q* is defined as, | *q* | = *q* = (4π/*λ*)

~

~

sin(*θ*/2) with *λ* and *θ* being the wavelength of X-ray and the scattering angle, respectively. As shown in **Figure 5(a)** and **(b)**, there is no crystalline peak initially, which shows the presence of 100% amorphous phase in the early stage. As time

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

#### **Figure 5.**

*Time-resolved (a)-(b) WAXS and (c)-(d) Lorentz-corrected SAXS profiles of D1.4 neat and D1.4/SFN(1.0) specimens. The red arrow indicates the first detection of the peak (adapted from reference [9] with a permission).*

proceeds, a crystalline peak appears at *q* = 11.96 nm�<sup>1</sup> (which has been shown by the red arrow). The induction period (*t*0) of the crystallization is evaluated from the first detection of the crystalline peak. It was found that loading of 1% SFN decreased the *t*<sup>0</sup> from 90 s to 40 s, which shows that SFN enhanced the nucleation of PLLA.

The time evolution of the degree of crystallinity was calculated from the WAXS profiles by using the following equation

$$\phi\_{\text{WAXS}} = \frac{\Sigma A\_c}{\Sigma A\_c + A\_a} \tag{2}$$

Here, Σ*Ac* is the summation of the peak area of the crystalline peaks, and *Aa* is the peak area of the amorphous halo. The peak decomposition was conducted, and

#### **Figure 6.**

*Degree of crystallinity as a function of time for the isothermal crystallization at 110*°*C (adapted from reference [9] with a permission).*

the degree of crystallinity *ϕ*WAXS was calculated, which is plotted as a function of crystallization time in **Figure 6**.

As can be seen in **Figure 6**, the final degree of the crystallinity has been increased and *t*0.5 is decreased by the inclusion of 1% SFN, indicating the acceleration of the crystallization rate.

**Figure 5(c)** and **(d)** show the changes in the Lorentz-corrected SAXS profiles as a function of time during the isothermal crystallization at 110°C for the D1.4 neat and D1.4/SFN(1.0) specimens. Here, the scattering intensity, *I*(*q*), is corrected as *q*2 *I*(*q*) by multiplying *q*<sup>2</sup> . There was no SAXS peak observed in the early stage of crystallization. As time goes on, a clear scattering peak was observed at *t* = 180 s for the D1.4 neat or *t* = 90 s for the D1.4/SFN(1.0) specimen, which indicates the development of the lamellar stacking with sandwiching the amorphous layers. It is significant to observe that the SAXS peak appears later than the WAXS peak (**Figure 5**) which indicates that single lamellae (without stacking) are generated in the initial state of the PLLA crystallization from the melt.

As seen in **Figure 5(c)** and **(d)**, the SAXS peak moves towards the higher *q* as the crystallization proceeds. The long period (*D*) of the lamellar stacks was evaluated from the peak position (*q*\*) as *D* = 2π/*q*\*. As shown in **Figure 7(a)**, the *D* decreases as a function of the crystallization time which seemed to be conflicting to

#### **Figure 7.**

*SAXS results for isothermal crystallization at 110*°*C (a) long period (D), (b) average lamellar thickness (L) as a function of (t-t0), where t0 is the induction period (adapted from reference [9] with a permission).*

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

the process of crystallization. To understand this behavior, the average thickness of the crystalline lamella (*L*) was calculated from the SAXS profile through the correlation function [35]. The correlation function is expressed as:

$$\gamma(r) = \frac{\stackrel{\text{'''}}{\int} I(q)q^2 \cos(qr) dq}{\stackrel{\text{'''}}{\int} I(q)q^2 dq} \tag{3}$$

Here, *γ*(*r*) is the correlation function and *r* is the distance in the real space. *γ*(*r*) function was obtained from the comprehensive *I*(*q*) from *q* = 0 to *q* = ∞ by conducting the extrapolation of SAXS data for *q* ! ∞ according to Porod's law and for *q* ! 0, Guinier's law is used. The detailed procedure is reported in Ref. [7]. **Figure 7(b)** shows thus-evaluated *L* as a function of time. As a result, the average lamellar thickness, *L* increases with time, which is reasonable as a crystallization behavior. Therefore, the decreasing behavior of *D* (as shown in in **Figure** *7***(a)**) is also reasonable, as schematically shown in **Figure 8**. Upon crystallization, shrinkage takes place. Since the lamella thickens with time, this results in the decrease of *D* (**Figure 8(b)** and **(c)**), as the amorphous layer thickness is decreased to a greater extent as compared to the increasing extent of *L* (lamellar thickness).

#### *2.1.2 Cellulose nanocrystal (CNC)*

In this section, we discuss the enhancement in PLLA crystallizability by the inclusion of marine green algae biomass residue (ABR) based additives, i.e., washed ABR (WABR) and the ABR extracted cellulose nanocrystal (CNC). The CNC was extracted from the waste of ABR by using acid hydrolysis method. The complete extraction and characterization process is reported in Ref. [14]. Apart from effect of CNC on the crystallization behavior of PLLA, we also compare the utility of waste ABR after washing, i.e., WABR (washed algae biomass residue) as a filler for PLLA. As reported

#### **Figure 8.**

*Schematic illustrations showing the change in the nanostructure upon crystallization of PLLA. (a) At the amorphous state before crystallization of the polymer melt, (b) in an early stage of crystallization, (c) lamellar thickening in the subsequent stage of the crystallization (adapted from reference [9] with a permission).*

in Ref. [14] it was found that WABR had irregular morphology (micron size), while the CNC had needle-like morphology with an average diameter of 30–35 nm, and average length of 520–700 nm. [14]. PLLA/WABR and PLLA/CNC composites were prepared by solution casting method using chloroform as a solvent. The loading amount of the additives were 0.5%, 1%, and 2% by weight. The effects of WABR and CNC on isothermal crystallization of PLLA are discussed by DSC and POM.

**Figure 9** shows the degree of crystallinity as a function of time based on DSC results for the isothermal crystallization of neat PLA, PLA/WABR and PLA/CNC nanocomposites at 110°C. The degree of crystallinity (*ϕ*) was calculated by using Eq. (1). As shown in **Figure 9**, the addition of WABR, and CNC can improve the crystallizability of PLLA by reducing induction period, crystallization half-time, and by increasing the ultimate degree of crystallinity. It was observed that the CNCs were more effective as a crystallizing agent in comparison to WABR. Based on the DSC measurement in heating scan, it was found that WABR and CNC does not change the *T*<sup>g</sup> and *T*<sup>m</sup> of PLLA [14]. These results are similar to the case of loading SFN which is also a solid state additive [8].

**Figure 10** shows the representative POM images for the isothermal crystallization at 125 °C for the neat PLA, PLA/WABR(1.0), and PLA/CNC(1.0) specimens at *t* = 12 min. First, negative spherulites were observed for all the specimens which shows that WABR and CNC had no effect on the structure of the PLA spherulites.

#### **Figure 9.**

*Degree of crystallinity as a function of time based on DSC results for the isothermal crystallization of neat PLA, PLA/WABR and PLA/CNC nanocomposites at 110*°*C. the degree of crystallinity (ϕ) was calculated by using Eq.(1) (adapted from reference [14] with a permission).*

#### **Figure 10.**

*The representative POM images for the (a) neat PLA, (b) PLA/WABR(1.0), and (c) PLA/CNC(1.0) specimens for the isothermal crystallization at 125*°*C for t = 12 min (adapted from reference [14] with a permission). The figure has been slightly modified.*

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

**Figure 11.**

*(a) Spherulite diameter, (b) number of spherulites in the PLA/CNC bio-nanocomposites, and (c) spherulite diameter, (d) number of spherulites in the PLA/WABR bio-composites as a function of time at an isothermally crystallized temperature of 125°C (adapted from reference [14] with a permission).*

Furthermore, **Figure 11** shows the spherulite diameter, and number of spherulites as a function of time, which were calculated from the POM images. It was observed that the incorporation of WABR, and CNC into the PLA matrix accelerated the rate of nucleation, however, the growth rate of the PLA spherulite was almost unchanged. CNC was found to be more effective than the WABR. The needle-like morphology and high aspect ratio of CNC were mainly accountable for the better effectiveness on improvement in the crystallization of PLA. On the contrary, the larger particle size of WABR might be the possible reason for its less effectiveness.

The results shown in this section suggest that even the low loading amount of solid state additives can enhance the crystallization of PLLA by providing more sites for nucleation without altering *T*g,*T*m*,* and spherulite growth during the isothermal crystallization from the melt.

#### **2.2 Liquid state additive (plasticizer)**

In this section, we will focus on the enhancement in crystallizability of PLLA by using a special plasticizer (organic acid monoglyceride; OMG). The chemical structure of OMG is shown in **Figure 12**. OMG is a product of Taiyo Kagaku Co., Ltd. The commercial name of OMG is Chirabazol D, which is a biobased plasticizer. The OMG has a molecular weight of 500 and a melting temperature of *T*<sup>m</sup> = 67°C. PLLA/ OMG specimens were prepared by the solution casting method, using chloroform as a solvent. The specimens are labeled as D1.4/OMG(*x*) or D0.5/OMG(x), where the numbers after D denote the % of D moiety in PLLA, and *x* stands for % of OMG inclusion.

**Figure 13** shows the effect of OMG on the glass transition temperature (*T*g) of PLLA. It is noticeably observed that *T*<sup>g</sup> of PLLA decreases with the OMG content. More rigorously, we compare the result with the estimation by the plasticizing effect. The *T*<sup>g</sup> can be simply estimated as.

$$T\_{\mathfrak{g}}\,(\text{PLLA/plasticizer})(\mathbf{K}) = \left(1 - \frac{\text{wt\text{\textquotedblleft}of\text{\textquotedblright}plasticizer}}{100}\right) \times T\_{\mathfrak{g}}(\text{heat})(\mathbf{K})\tag{4}$$

**Figure 13** shows the experimental *T*<sup>g</sup> and the estimated *T*<sup>g</sup> (with two straight lines) as a function of OMG content. The data points are almost in accord with the lines up to 1.0%, and then deviated from the lines above the OMG content of 1.0%. These results suggest that the OMG acts as a plasticizer for PLLA when the OMG loading is <1.0%. **Figure 14(a)** shows the DSC results of the isothermal crystallization of neat and 1% OMG loaded specimens at 110°C. The reduction in *t*0.5 in **Figure 14(b)** confirmed the enhancement in crystallization rate. **Figure 14(c)**

$$\begin{aligned} &\text{CH}\_2-\text{OCO}-\text{R} \\ &\mid \\ &\text{CH}\_2-\text{OH} \\ &\mid \\ &\text{CH}\_2-\text{OH} \end{aligned}$$

**Figure 12.** *Chemical structure of OMG.*

#### **Figure 13.**

*Glass transition temperature (Tg) as evaluated from DSC curves. The lines show estimated Tg (adapted from reference [10] with a permission).*

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

#### **Figure 14.**

*(a) DSC results for isothermal crystallization at 110*°*C (b) crystallization half-time as a function of OMG loading (c) apparent crystallinity based on WAXS results, and (d) d spacing as a function of time (adapted from reference [7] with a permission).*

shows the apparent degree of crystallinity which was evaluated using Eq. (2), based on the WAXS results as a function of the crystallization time for all specimens. The effect of the OMG is very clear for the acceleration of crystallization.

**Figure 15(a)** shows the change in long period, *D* as a function of time during the isothermal crystallization at 110°C for neat and 1% OMG loaded specimens. The long period decreases as the crystallization proceeds which can be explained by **Figure 8**. Furthermore, **Figure 15(b)** shows changes in the lamellar thickness (calculated from SAXS profile through the correlation function, *γ*(*r*)) as a function of

#### **Figure 15.**

*SAXS results for isothermal crystallization at 110*°*C (a) long period (D), (b) lamellar thickness (L) as a function of time (adapted from reference [7] with a permission).*

#### *Crystallization and Applications*

time. As time proceeded, the lamellar thickness increased in the early stage and then leveled off in the later stage. These results suggest that the lamellar thickness increased quickly in the early stage of crystallization due to a decrease in the Dcontent and the addition of OMG. The ultimate lamellar thicknesses (at 3000 s) for all specimens are relatively similar, although the value for the D0.5/OMG is 0.94 times those of the others.

The time-resolved Lorentz-corrected SAXS profiles during isothermal crystallization at 100°C form the melt (200°C) for D1.4/OMG and D0.5/OMG specimens are shown in **Figure 16**. There was no SAXS peak observed in the early stage. As the crystallization proceeds, the SAXS peak appears which gradually shifts towards the higher *q* range. As can be seen in **Figure 16(b)**, there was observed a clear second peak in the higher *q* range for the D0.5/OMG(1.0) specimen. However, the position of the new peak is not twice of the position of the first-order peak which means that the new peak is independent of the first-order peak. This result correspond to the newly formed lamellar stacking. There are three possible models to account for the appearance of a new peak in the higher *q* range. One is the formation of new lamellar stacks in the amorphous region with much shorter long period, as schematically shown in **Figure 17(a)**. The second one is the new lamellar stacks formed perpendicular to the original lamellar stacks, as schematically shown in the **Figure 17(b)**. This model is referred to as the cross-hatched lamellae [36–38]. The third one is the insertion of a new lamella into the amorphous phase, which is sandwiched by the neighboring two preceding lamellae, as schematically shown in the **Figure 17(c)**. This kind of insertion of a new lamella has been considered by

#### **Figure 16.**

*Temporal changes in the Lorentz-corrected SAXS profiles upon T-jump from 200 °C to 100 °C for the specimen (a) D0.5 neat, (b) D0.5/OMG(1.0), (c) D0.5/OMG(2.0), (d) D1.4 neat, (e) D1.4/OMG(1.0), and (f) D1.4/OMG(2.0) specimens (adapted from reference [10] with a permission).*

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

#### **Figure 17.**

*Schematic representation for the formation of a new lamellar stack. (a) Independent stacking, (b) formation of a new stack in the amorphous phase in between the original lamellae with the stacking direction perpendicular to each other (cross-hatched lamellae) and (c) insertion of a new lamella in between the original lamellae (adapted from reference [10] with a permission).*

Hama et al. [39]. At present, it is difficult to specify which model is appropriate. Although there was no such peak observed for the D0.5/OMG(2.0) specimen, the shoulders are clearly observed for this specimen (**Figure 16(c)**). Therefore, even for this specimen the effect of OMG to induce such a new lamellar stack can be recognized.

The POM observations were conducted to count the number of the spherulites as a function of time during the isothermal crystallization at 130 °C. **Figure 18(a)** and **(b)** show the representative POM images for the isothermal crystallization at 130 °C for the D1.4 neat and D1.4/OMG(1.0) specimens at *t* = 29 min. Since, the negative spherulites were observed which indicates that there is no effect of OMG on the structure of the spherulites. **Figure 18(b)** shows enhanced number of spherulites by loading of OMG which indicates that OMG can enhance the nucleation process of PLLA. From **Figure 18(c)**, it can be seen that the OMG enhances the spherulite growth of PLLA which clearly shows the effect of OMG to improve the crystallizability of PLLA. We speculate that the lowering of the activation energy for the PLLA crystallization may be the main effect of the OMG [10].

## **3. Enhancement in stereocomplex crystallization of PLLA/PDLA blend**

In this section, the PLLA/PDLA (50/50) blends were prepared by solution casting method. Firstly, the PLLA and PDLA solutions were separately prepared with a concentration of 5% (w/v), using dichloromethane (DCM) as a solvent. The SFN was dispersed in DCM by using the ultrasonication method as discussed in the reference [8]. The PLLA, PDLA solutions, and the SFN dispersion, all together were mixed in one glass vessel and stirred for 12 h. The loading of SFN was 1% with the weight ratio of PLLA, PDLA, and SFN as 49.5/49.5/1.0. After the mixing, the solution was poured into a Petri dish for solvent evaporation at RT. After complete evaporation of the solvent, the as-cast films were obtained which were further dried in a vacuum oven at 50°C for 24 h. The specimens are labeled as LD neat and

#### **Figure 18.**

*(a), (b) POM images for the D1.4 neat and D1.4/OMG(1.0) specimens for the isothermal crystallization at 130*°*C for t = 29 min. (c) Plots of the radius of spherulite vs. time evaluated from the results of POM (adapted from reference [10] with a permission). The figure has been slightly modified.*

LD/SFN(x), where LD denotes the blend of PLLA/PDLA(50/50), and x denotes the % loading of SFN.

Prior to the study of the effect of SFN on the crystallization of PLLA/PDLA (50/50) blend, we checked the effect of SFN on PDLA crystallization as SFN was known to improve the crystallization of PLLA (see Section 2.1.1). **Figure 19** shows the comparison of degree of crystallinity during isothermal crystallization of PLLA neat, PLLA/SFN(1.0), PDLA neat, and PDLA/SFN(1.0) specimens at 110°C. It can be seen from this figure that the ultimate degree of crystallinity (*ϕ*∞) at the isothermal crystallization temperature of 110°C is increased by adding 1% SFN in PLLA or PDLA specimen. As shown in **Figure 19** the induction period, *t*<sup>0</sup> and the crystallization half-time, *t*0.5 of the PDLA neat specimen are shorter than those of the PLLA neat specimen. This may be because the optical purity of the PDLA sample (D-content >99.5%) is higher than that of PLLA sample (L-content = 99.5%), as we know that the nucleation and crystallization of PLA (PLLA or PDLA) becomes quicker with the increasing optical purity. By adding SFN, the *t*<sup>0</sup> was almost unchanged for the case of the PDLA/SFN(1.0) specimen, while it was significantly decreased for the case of PLLA/SFN(1.0), furthermore, the *t*0.5 is decreased for both cases. It was much decreased for the case of PLLA than that of PDLA, ensuring the

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

#### **Figure 19.**

*Degree of crystallinity (ϕDSC) as a function of time, which was evaluated based on the heat flow results (adapted from reference [15] with a permission).*

superior SFN effect due to its similarity of the chemical structure to PLLA. These results indicate the enhancement in the crystallizability of PDLA by adding 1% SFN, although SFN is much effective for the improvement of crystallizability of PLLA.

For the isothermal crystallization from melt, we set the melt temperature at 260° C for 5 min and then immediately quench to 110°C or 170°C and hold it isothermally until the crystallization completes. The reason why we selected 110°C is that it was found in **Figure 4** that the rate of crystallization of PLLA HC crystal is maximum at 110°C. This is the best temperature to achieve the maximum amount of crystallinity of PLLA which is desirable for industrial purposes.

Furthermore, since at 110°C the formation of HC and SC occurs simultaneously so to see the effect of SFN on the formation of SC crystals solely, we conducted the isothermal crystallization at 170°C because at this temperature HC crystals cannot form (*T*m,HC = 170 180°C) due to the shallow quench depth (Δ*<sup>T</sup>* <sup>=</sup> *<sup>T</sup><sup>O</sup> <sup>m</sup>* – *T*c).

The effect of SFN on the isothermal crystallization behavior of the PLLA/PDLA blend specimen was investigated at *T*iso = 110°C. **Figure 20** shows the DSC results of the isothermal crystallization of LD neat and LD/SFN(1.0) specimens at 110°C from the melt (260°C). In **Figure 20(a)**, the heat flow as a function of time at the isothermal crystallization temperature is plotted. Adding 1% SFN, the crystallization exothermic peak shifts to the shorter time, showing an enhancement in the crystallization speed. However, it was not possible to distinguish the evolution of HC and SC phases from the plots of **Figure 20(a)**. To see the crystallites formed in the isothermal crystallization, the subsequent heating is conducted after the complete crystallization at 110°C. **Figure 20(b)** shows the results of the DSC heating scan. It is seen that by adding 1% SFN the Δ*H*m,HC and *T*m,HC decreased and the Δ*H*m,SC and *T*m,SC increased. These results indicate that the SFN can enhance the formation of SC and can suppress the formation of HC. The change in the melting temperature indicates that the presence of SFN may increase the lamellar thickness of the SC crystals while the lamellar thickness of HC crystals may be decreased due to the suppression of the HC crystallization. The increase in the melting point of SC is helpful to increase the thermal stability of PLA.

The effect of SFN on the isothermal crystallization behavior of the PLLA/PDLA blend specimen at 170°C was studied by the DSC measurement as shown in **Figure 20(c)**. Since the temperature 170°C is too high for the formation of HC (*T*m,HC = 170 180°C), only the formation of SC can be considered at this

**Figure 20.**

*(a) Heat flow as a function of time during the isothermal crystallization at 110*°*C from the melt (260*°*C), and (b) the subsequent heating scan from 110–260°C with the rate of 20*°*C/min. (c) Heat flow curves as a function of time during the isothermal crystallization at 170*°*C from the melt (260*°*C), and (d) changes in the degree of crystallinity (ϕDSC) as a function of time at 170*°*C, (adapted from reference [15] with a permission).*

temperature (see later WAXS results in **Figure 21(c)** and **(d)**). From **Figure 20(c)** and **(d)**, it is clearly seen that the ultimate degree of crystallinity at the isothermal crystallization temperature of 170°C is increased by the presence of SFN. The *t*<sup>0</sup> was decreased from 13 min to 7.4 min and the *t*0.5 was also decreased from 32.3 min to 19.1 min. These results indicate the enhancement in the stereocomplex crystallization of PLLA/PDLA (50/50) blend specimens by adding 1% SFN.

To clearly distinguish the evolution of HC and SC during the isothermal crystallization, we conducted the time-resolved WAXS measurements at 110°C upon Tjump from 260°C. **Figure 21(a)** and **(b)** show the change in WAXS profiles for the LD neat and LD/SFN(1.0) specimens as a function of time at 110°C. The peaks located at *q* = 8.75, 14.6, and 16.6 nm<sup>1</sup> belong to the SC crystals while the other reflection peaks belong to the HC. As shown in **Figure 21(a)**, even at the very early stage the SC(110) refection peak was observed for both the specimens, while the peak area of SC(110) was much larger for the case of LD/SFN(1.0) specimen. The shorter induction period of SC than HC is due to the fact that the nucleation of SC is faster than that of HC in PLLA/PDLA (50/50) blend by the difference in the thermodynamic driving force of the crystallization. (Δ*T*SC > Δ*T*HC where Δ*T*SC = *T*m, SC– T and Δ*T*HC = *T*m, HC– T). As time goes on, the HC peak appears at 125 s. It is noteworthy to observe here that the induction period of HC is unchanged by adding SFN. The time evolution of the degree of crystallinity was calculated from the WAXS profiles which is plotted as a function of time in **Figure 22**. As can be seen from **Figure 22** for the case of LD neat specimen, the HC peak appears later than the SC peak while it keeps on increasing and finally, *ϕ*HC overcomes *ϕ*SC. For the case of LD/SFN(1.0) specimen, the SC crystallization is much accelerated

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

#### **Figure 21.**

*Time-resolved WAXS profiles after the T-jump from 260–110°C for (a) LD neat (b) LD/SFN(1.0) specimens. (the red arrow indicates the first detection of the peak for the HC). (c) Time-resolved WAXS profiles after the T-jump from 260–170°C for (c) LD neat (d) LD/SFN(1.0) specimens. (the red arrow indicates the first detection of the SC peak) (adapted from reference [15] with a permission).*

in the very early stage with the almost zero induction period and in the final stage *ϕ*HC < < *ϕ*SC. The fraction of SC (*f*SC) is increased after loading of 1% SFN while the total degree of crystallinity (*ϕ*HC + *ϕ*SC) is unchanged at 30 min.

The average crystallite size (*Dhkl*) in the direction normal to the (*hkl*) plane was evaluated by Scherrer's Equation [40].

$$D\_{hkl} = \frac{K\lambda}{\beta\_{hkl}\cos\left(\frac{\theta}{2}\right)}\tag{5}$$

where *K* is a constant (0.9) and *λ* is the wavelength of the incident X-ray. *βhkl* is a full-width at half maximum (FWHM) in the unit of radian, and *θ* is the scattering

**Figure 22.**

*Degree of crystallinity calculated from the results of Figure 21 for (a) LD neat and (b) LD/SFN(1.0) specimens. (c) Total (HC + SC) degree of crystallinity and (d) fraction of SC as a function of time. Plots of average crystallite size as a function of time evaluated by Scherrer's equation for (e) HC and (f) SC (adapted from reference [15] with a permission).*

angle. Note here that the raw data were used as the *βhkl* values without correction for the peak broadening due to the collimation error of the WAXS setup, if any.

As seen in **Figure 22(e)** and **(f)**, the crystallite size is initially increasing as a function of time and it levels off after 5 min elapsed from the onset of crystallization. The slope of the plots in **Figure 22(e)** and **(f)** can be considered as the crystallite growth rate. Then, it can be stated that the growth rate of the HC crystallites is unchanged by the addition of SFN. The final value of the size of the HC crystallite for LD/SFN(1.0) specimens is slightly smaller than that in the LD neat specimen due to the effect of the SFN loading. As can be seen from **Figure 22(f)**, the size of the SC crystallite in the LD/SFN(1.0) specimen is much smaller than those of the LD neat specimen. Furthermore, it is interesting to notice that the initial size of the SC crystallite is the same for both the LD neat and the LD/SFN(1.0) specimens (**Figure 22(f)**).

To check the effect of SFN loading on the formation of SC crystals solely, we conducted the time-resolved WAXS measurements at 170°C. The changes in the WAXS profile were measured in the isothermal crystallization process at 170°C from the melt (260°C). **Figure 21(c)** and **(d)** show the WAXS profile for the LD *Recent Developments in the Crystallization of PLLA-Based Blends, Block Copolymers… DOI: http://dx.doi.org/10.5772/intechopen.97088*

neat and LD/SFN(1.0) specimens as a function of time. It is also clearly shown that there is no peak for HC crystals which is due to such a high temperature, i.e. 170°C. It can be said that at 170°C only SC crystal formation takes place.

**Figure 23(a)** and **(b)** show the changes in Lorentz corrected SAXS profiles as a function of time during isothermal crystallization at 110°C for the LD neat and LD/ SFN(1.0) specimens. The SAXS profiles in **Figure 23(a)** and **(b)** show the scattering from both the HC and SC crystal (as evidenced by WAXS results in **Figure 21**). To distinguish the contribution of HC and SC crystal, we conducted the peak decomposition of the SAXS profiles. The detailed procedure about the peak

#### **Figure 23.**

*Changes in the Lorentz-corrected SAXS profiles as a function of time for LD neat and LD/SFN(1.0) specimens during the isothermal crystallization at (a)-(b) 110*° *C and (c)-(d) 170*°*C from the melt (260*°*C) (adapted from reference [15] with a permission).*

decomposition is mentioned in Ref. [15]. For *t* < 6 min, the SAXS profiles are symmetric which belong to SC crystals. As time goes on, after *t* = 6 min, the second peak appears which shows the scattering from HC crystals. From the peak position (*q*\*), the long period (D) of the lamellar stacks was evaluated as *D* = 2π/*q*\*. **Figure 24(a)** shows the plot of *D* as a function of time for the LD neat and LD/SFN (1.0) specimens which show the contribution of HC and SC separately. As seen in **Figure 24(a)**, *D* decreases as a function of time for HC crystals in the LD neat specimen. After loading 1% SFN, a similar trend was observed while the value of *D* of the HC lamellar stack was smaller than that of the LD neat specimen. The *D* of the SC lamellar stack in the LD neat specimen first increases up to *t* = 6 min and then decreases after *t* > 6 min. Considering **Figure 22(a)**, *t* = 6 min can be taken as *t*0.5 (crystallization half-time). Then, the crystallization was quick in the stage *t* < *t*0.5. Namely, *D* increased with time during the rapid crystallization while *D* decreased with time during the subsequent slow crystallization. **Figure 22(f)**, also suggests that the lateral size of SC lamellae was very rapidly increased for t < 6 min. Therefore, it can be considered that the SC lamellae grow in their lateral direction by folding the polymer chains in the amorphous region outside of the lamellar stacks. In the meantime, thickening of the lamellae can be considered during this stage (*t* < *t*0.5). Namely, the lamellar thickening may be considered to take place by including the amorphous chains from outside of the lamellar stack. This situation quite differs from **Figure 8**, for which *D* is explained to be decreasing with time because of shrinkage in volume upon crystallization. As for the current case, no change in the amorphous layer with increasing of the thickness of the crystalline lamellae result in increasing the long period, *D*. For the LD/SFN(1.0) specimen *D* of the SC lamellar stack decreases from the beginning however the decreasing tendency became more evident for *t* > 6 min.

**Figure 23(c)** and **(d)** show the changes in Lorentz corrected SAXS profiles as a function of time during isothermal crystallization at 170°C for the LD neat and LD/ SFN(1.0) specimens. The intensity of the peak observed at *q* = 0.29 nm<sup>1</sup> increases as a function of time. As seen in **Figure 23(c)** and **(d)**, the SAXS peak moves towards the higher q as the crystallization proceeds. Increase in *q* suggests the decrease in *D* as shown in **Figure 24(b)**. It can be seen that the long period, *D* decreases by the loading of SFN.

POM observations were conducted to evaluate the spherulite growth rate and the nucleation density as a function of time. The POM images of the evolution of spherulites for the LD neat and LD/SFN(1.0) specimens at 170°C as a function of

#### **Figure 24.**

*Plots of long period (D) as a function of time during the isothermal crystallization at (a) 110*°*C and (b) 170*°*C from the melt (260*°*C) (adapted from reference [15] with a permission).*

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

#### **Figure 25.**

*POM images as a function of time for the isothermal crystallization at 170*°*C for (a) LD neat (b) LD/SFN (1.0) specimens. (c) Changes in the number of spherulites as a function of time, (d) the plots of spherulite diameter as a function of time during isothermal crystallization at 170*°*C, evaluated from the POM images (adapted from reference [15] with a permission).*

time are shown in **Figure 25(a)** and **(b)**. First, negative spherulites were observed with the typical Maltese-cross patterns for both of the LD neat and LD/SFN(1.0) specimens. The number of spherulites and the spherulite diameter as a function of time are plotted in **Figure 25(c)** and **(d)**. As shown in **Figure 25(c)** the number of spherulites increases as a function of time for the LD neat specimen below 4 min, suggesting homogeneous nucleation. In contrast, for the case of LD/SFN(1.0), the number of spherulites significantly increases and kept constant as a function of time (**Figure 25(d)**), suggesting heterogeneous nucleation due to the nucleation effect of SFN. The final number of spherulites increased approximately 3.6 times (from 21 to 73) upon the addition of SFN. Based on these results, SFN is considered as a nucleation agent for SC nuclei. The induction period calculated from **Figure 25(c)** looks unchanged. Furthermore, as seen in **Figure 25(d)** the growth rate (8.6 μm/ min) of the spherulites in the LD/SFN(1.0) specimen is smaller than that of the spherulites in the LD neat specimen (10.7 μm/min). Although the growth rate of the SC crystals is decreased by the loading of SFN, the ultimate degree of crystallinity at 170°C (see **Figure 20(d)**) is increased by the loading of SFN. The slower growth of SC spherulites by adding SFN seems conflicting with the larger nucleation effects of SFN. These two conflicting results (as shown in **Figure 25(c)** and **(d)**) induced by the SFN loading are worthy of future research.
