Non-Isothermal Cold-Crystallization Behavior and Kinetics of Poly(l-Lactic Acid)/WS₂ Inorganic Nanotube Nanocomposites

Abstract

In order to accelerate the crystallization of poly(l-lactic acid) (PLLA) biopolymer and enhance its crystallizability, biocompatible and environmentally friendly tungsten disulphide inorganic nanotubes (INT-WS₂) were introduced into the polymer matrix. The non-isothermal cold-crystallization and subsequent melting behaviour of pure PLLA and PLLA/INT-WS₂ nanocomposites were investigated in detail by varying both the heating rate and INT-WS₂ loading. The kinetic parameters of the cold-crystallization process of PLLA chains under confined conditions, successfully described using Liu model, shows that the addition of INT-WS₂ significantly increased the crystallization rate and reduced the total cold-crystallinity of PLLA, while the crystallization mechanism and crystal structure of PLLA remained unchanged in spite of the INT-WS₂ loading. Similarly, the final crystallinity and melting behaviour of PLLA were controlled by both the incorporation INT-WS₂ and variation of the heating rate. The differential isoconversional method of Friedman was applied to estimate the dependence of the effective activation energy on the relative crystallinity and temperature for PLLA and PLLA/INT-WS₂. On the other hand, the double-melting peaks, mainly derived from melting-recrystallization-melting processes upon heating, and their dynamic behaviour is coherent with a remarkable nucleation-promoting effect of INT-WS₂ involved in accelerating the cold-crystallization of PLLA. These observations have considerable practical significance for the future sustainable, economic and effective technological utilisation of PLLA, as it will enable the development of novel melt-processable biopolymer nanocomposite materials.

1. Introduction

Poly(l-lactide acid) (PLLA) is a well-known thermoplastic considered among the family of environmentally friendly polymers that has attracted increasing attention in recent years because it can be produced from renewable resources, such as corn, and is non-toxic to the human body [1,2,3]. It exhibits multifunctional properties such as biodegradability (hydrolytic) and biocompatibility, good transparency, excellent tensile strength, and tensile modulus. Thus, PLLA offers great promise as one of the select few candidates for use in biomedical applications such as surgical sutures, bone fixation devices, and controlled drug-delivery systems. Moreover, due to its favorable biodegradability, good mechanical properties, and versatile fabrication processes, it has excellent potential for substitution of petroleum-based polymers [4,5,6]. Unfortunately, PLLA has several drawbacks, in terms of the processability and mechanical properties. The crystallization rate of PLLA is very slow, resulting in the long processing cycle time and low production efficiency of products in the melt processing and molding. Therefore, the improvement of the thermal properties and crystallizability PLLA would be necessary in order to produce PLLA composites with desired performance [7].

In recent years, the incorporation of layered metal dichalcogenide (tungsten disulfide, WS₂ and molybdenum disulfide, MoS₂) nanophases in the polymer systems has become a field of recent interest as it opens opportunities for many new applications [8,9]. Since the discovery of such nanophases in 1992 and 1993 [10,11] many new inorganic nanotubes (INT) and inorganic fullerene-like (IF) nanoparticles have been synthesized from variety of compounds with layered (2D) nanostructures (e.g., transition metal disulfides: MoS₂, NbS₂, TaS₂, TiS₂, and ReS₂); transition metal oxides: Cs₂O, Tl₂O; halides: CdI₂; BN nanotubes, etc.). Diverse synthetic strategies have been developed or adapted for the synthesis of these nanostructures, including laser ablation, self-assembly, template synthesis, hydrothermal processing, metal–organic chemical vapor deposition (MOCVD), spray pyrolysis, microwave induced plasmas, gas–solid reactions and fluidized bed reactors) [12,13,14,15]. In the case of IF- and INT-WS2, careful investigation of the growth mechanism resulted in the synthesis of a pure phase, and the breakthrough development of a one-pot reaction, i.e., via a sequence of reactions taking place in a single vertical reactor [16,17], paved the way to large-scale production for a broad range of applications. These nanotubes possess a highly perfect crystalline structure and consequently exhibit very desirable mechanical properties. They are ultra-strong impact-resistant materials with potential uses in personal protection (bullet-proof vests, helmets, etc.) and other safety equipment, car bumpers, high strength adhesives and binders, amongst others [18]. In addition to protective materials, WS₂ nanotubes can also be employed in nanoelectronics, fuel cells, ultra-filtration membranes, and catalysts [14,15]. Their optical properties allow various other applications in fields such as nanolithography or photocatalysis [19]. Equally important, the cytotoxicity of the INT-WS₂ was evaluated positively in comparison to standard environmental particulate matter, demonstrating much lower cytotoxicity than other nanoparticles, such as silica or carbon black [20]. Promising results have also been recently found with respect to the biocompatibility of INT(IF)-WS₂ with salivary gland cells [21]. With such excellent properties, and relatively simple and inexpensive fabrication, the incorporation of INT-WS₂ in polymer systems has become increasingly important [22,23,24,25].

In this regard, inorganic nanotubes (INT-WS₂) were well-dispersed into a PLLA matrix through simple melt blending, and the thermomechanical properties were investigated in detail with various techniques such as ultra-high field emission scanning microscopy (FE-SEM), time-resolved synchrotron X-ray, diffraction differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) [26]. The excellent dispersion of INT-WS₂ confirmed by FE-SEM lead to composites with substantially enhanced thermal and mechanical properties. However, to date the influence of the INT-WS₂ on the cold-crystallization behaviour of PLLA has not been investigated. The present research continues work in this field and focuses on the use of INT-WS₂ to improve the cold-crystallizability of PLLA. In this study, we analyzed the role of the INT-WS₂ concentration on the non-isothermal cold-crystallization and melting behaviour of PLLA polymer chains under confined conditions. This is because the knowledge of the cold-crystallization behaviour is valuable for the processing and use of PLLA.

2. Experimental Section

2.1. Materials and Processing

The biopolymer matrix was poly(l-lactic acid) (PLLA) purchased from Goodfellow Ltd. (density = 1.25 g/cm³, M_w ≈ 1.5 × 10⁵ g/mol, Cambridge, UK). Multiwall WS₂ nanotubes (INT-WS₂) with diameters of 30–150 nm and lengths of 1–20 μm were obtained from NanoMaterials Ltd (Yavne, Israel). Each mixture of PLLA and INT-WS₂ (0.1, 0.5 and 1.0 wt %) was dispersed in a small volume of ethanol and homogenized by mechanical stirring and bath ultrasonication for approximately 10 min. Subsequently, the dispersion was partially dried in vacuum at 60 °C under a pressure of about 70 mbar for 24 h. Melt-mixing of the resulting dispersions was performed using a micro-extruder (Thermo-Haake Minilab system, Madison, WI, USA) operated at 190 °C and a rotor speed of 100 rpm for 10 min. This strategy yields finer dispersion, with INT-WS₂ almost fully debundled into individual tubes, which are randomly oriented in the PLLA matrix [26]. In no case was severe aggregation of nonmodified INT-WS₂ observed in the PLLA matrix despite the INTs loadings, indicating that the variation of INT-WS₂ contents from 0.1 to 1.0 wt % does not significantly affect the dispersion and distribution of INT-WS₂ in the polymer matrix.

2.2. Characterization Techniques

The morphology and structure of PLLA/INT-WS₂ nanocomposites were first characterized by ultra-high field-emission scanning microscopy (FE-SEM, SU8000, Hitachi Co., Tokyo, Japan) and wide angle X-ray scattering (WAXS, Desy-Hasylab, Hamburg, Germany) experiments using synchrotron radiation [26]. The processability and dispersion of INT-WS₂ was considered when evaluating thermal and mechanical response and performance of these new formulations in comparison with those previously reported in other PLLA nanocomposite systems. Successful dispersions of INT-WS₂ have been demonstrated by a remarkable improvement in the melt-crystallization processes and dynamic-mechanical properties of PLLA.In particular, it was found that 0.5 wt % loading led to the highest property improvements.

DSC cold-crystallization experiments were performed on a Perkin Elmer DSC7/Pyris differential scanning calorimeter (Perkin-Elmer España SL, Madrid, Spain), operating under nitrogen flow. Samples of 10 mg sealed in aluminum pans were initially held at 225 °C for 5 min, cooled to 40 °C at a rate of 40 °C/min and then reheated to 225 °C at a rate of 1, 2, 5, 10 and 20 °C/min, respectively. The melting temperature (T_m) and the cold-crystallization peak temperature (T_cc) were determined from the maximum of the melting endotherm and the minimum of the crystallization exotherm observed during the heating scan, respectively. The crystallinity (1-λ) was calculated as follows:

$$\left(1-\lambda \right)=\frac{\text{Δ}{H}_c}{\text{Δ}{H}_m^0}$$

(1)

where ΔH_c is the crystallization enthalpy and ΔH⁰_m is the enthalpy of melting for perfect crystals: 93 J/g [27].

3. Results and Discussion

3.1. Non-Isothermal Cold-Crystallization Behavior

The crystallization of isotropic polymers by heating above the glass transition temperature (T_g) is denominated cold-crystallization. Unlike melt crystallization, in which the motion of polymer chains can be carried out entirely via molecular reptation [28], the polymer chains in the rubbery state complete the corresponding conformational rearrangements via cooperative segmental movements [29]. As a result, the crystal structure and morphology obtained from cold-crystallization may be expected to differ from that obtained by melt-crystallization. In addition, it well-known that the molecular motion of semicrystalline polymers above T_g involves discrete interaction between amorphous and crystalline regions. The formation and development of crystalline regions inevitably limits the motion of polymer chains in the amorphous region. This is especially important when the low heating rate is used. Thus, investigation of the cold-crystallization behaviour of amorphous samples contributes to further understanding of the kinetic behaviour of polymer chains under confined conditions.

PLLA manifests slow crystallization on cooling from the melt. It does not apparently crystallize if it is cooled at a rate of 20 °C/min or faster [26]. However, on subsequent heating after rapid cooling from the melt, the cold-crystallization phenomenon occurs. For the study of the cold-crystallization process and the melting of non-isothermally cold-crystallized PLLA/INT-WS₂ nanocomposites, samples with different INT-WS₂ content were examined. Figure 1 shows the DSC melting thermograms for neat PLLA and PLLA/INT-WS₂ nanocomposites recorded at heating rates of 1, 2, 5, 10 and 20 °C/min after rapid cooling from 225 °C at 40 °C/min, and the specific values of the crystalline parameters of all samples are listed in Table 1.

Figure 1. DSC melting thermograms of (a) PLLA; and PLLA/INT-WS₂ nanocomposites of (b) 0.1; (c) 0.5; and (d) 1.0 wt % of INT-WS₂ recorded at indicated heating rates after rapid cooling at 40 °C/min.

Figure 1. DSC melting thermograms of (a) PLLA; and PLLA/INT-WS₂ nanocomposites of (b) 0.1; (c) 0.5; and (d) 1.0 wt % of INT-WS₂ recorded at indicated heating rates after rapid cooling at 40 °C/min.

Table 1. Cold-crystallization and melting parameters for pure poly(l-lactic acid) (PLLA) and PLLA/inorganic nanotubes (INT-WS₂) nanocomposites.

INT-WS2 content (wt %)	ϕh (°C/min)	Tcc (°C)	(1-λ)cc (%)	Tm1 (°C)	Tm2 (°C)	(1-λ)m (%)
0.0	1	78.5	33.7	133.8	152.0	38.9
	2	83.7	37.7	135.3	151.6	41.1
	5	91.5	44.5	137.1	151.3	45.3
	10	99.9	45.7	139.1	152.1	46.8
	20	109.8	45.4	143.4	153.9	46.1
0.1	1	77.6	32.2	134.9	152.9	39.7
	2	83.0	34.4	136.0	152.3	40.8
	5	91.1	42.9	137.8	151.6	46.2
	10	98.6	44.8	140.0	152.7	47.1
	20	108.4	44.6	143.6	154.8	46.4
0.5	1	75.9	28.1	138.5	154.2	40.8
	2	80.5	32.4	139.8	154.2	42.7
	5	88.3	40.6	135.8	151.0	47.0
	10	94.7	42.6	137.8	151.3	47.9
	20	103.5	44.4	140.9	153.0	47.5
1.0	1	76.0	25.0	140.4	155.1	43.0
	2	80.7	30.5	141.6	155.0	46.1
	5	88.2	33.1	143.2	155.1	48.2
	10	95.6	40.5	145.0	156.0	49.7
	20	103.6	43.0	147.4	157.8	45.5

Table 1. Cold-crystallization and melting parameters for pure poly(l-lactic acid) (PLLA) and PLLA/inorganic nanotubes (INT-WS₂) nanocomposites.

INT-WS2 content (wt %)	ϕh (°C/min)	Tcc (°C)	(1-λ)cc (%)	Tm1 (°C)	Tm2 (°C)	(1-λ)m (%)
0.0	1	78.5	33.7	133.8	152.0	38.9
	2	83.7	37.7	135.3	151.6	41.1
	5	91.5	44.5	137.1	151.3	45.3
	10	99.9	45.7	139.1	152.1	46.8
	20	109.8	45.4	143.4	153.9	46.1
0.1	1	77.6	32.2	134.9	152.9	39.7
	2	83.0	34.4	136.0	152.3	40.8
	5	91.1	42.9	137.8	151.6	46.2
	10	98.6	44.8	140.0	152.7	47.1
	20	108.4	44.6	143.6	154.8	46.4
0.5	1	75.9	28.1	138.5	154.2	40.8
	2	80.5	32.4	139.8	154.2	42.7
	5	88.3	40.6	135.8	151.0	47.0
	10	94.7	42.6	137.8	151.3	47.9
	20	103.5	44.4	140.9	153.0	47.5
1.0	1	76.0	25.0	140.4	155.1	43.0
	2	80.7	30.5	141.6	155.0	46.1
	5	88.2	33.1	143.2	155.1	48.2
	10	95.6	40.5	145.0	156.0	49.7
	20	103.6	43.0	147.4	157.8	45.5

During heating, the exothermic peaks attributed to the cold-crystallization process appear for the samples crystallized at higher cooling rates, indicating that the melt-crystallization process during the prior cooling step was incomplete. The double melting peak was widely observed and will be discussed later. Both heating rate and INT-WS₂ loading were the two main factors that affected the non-isothermal cold-crystallization behaviour of PLLA in the nanocomposites. With increasing heating rate, the crystallization exotherms became broader, and the cold-crystallization peak temperature (T_cc) shifted to higher temperature. Moreover, at a given heating rate (for example, 10 °C/min), T_cc for neat PLLA was 99.9 °C, whereas T_cc for 0.1, 0.5 and 1.0 wt % were found to be 98.6, 94.7, and 95.6 °C, respectively. In particular, at a concentration of 0.5 wt % of INT-WS₂ the value of T_cc for PLLA continued to decrease and tended to stabilize, or to slightly increase, for the highest concentration of 1.0 wt %, Table 1. This evolution of the nucleation effect with the concentration of INTs could be related to the variation of the level of the dispersion of the INTs with increasing INT-WS₂ content. The T_cc values of neat PLLA and its nanocomposites at various heating rates were plotted in Figure 2. It is clear from Figure 2a that T_cc shifted towards higher temperatures with increasing heating rate for all the samples. Moreover, the values of T_cc also apparently decreased with increasing INT-WS₂ content, relative to neat PLLA, at all heating rates (Figure 2b). Such results indicate that the incorporation of INT-WS₂ significantly enhanced the non-isothermal cold-crystallization of PLLA matrix, to a degree that was strongly dependent on the INT-WS₂ content. Further, the crystallinity value (1-λ)_cc calculated for PLLA/INT-WS₂ samples (Figure 3) was affected not only by the heating rate but also by the INT-WS₂ loading, increasing with increasing heating rate (Figure 3a), being more pronounced in the case of the nanocomposites. That is to say, whereas the crystallinity of PLLA was lower at slower heating rate (33.7% at 1 °C/min) and became increasingly higher as the heating rate was increased (45.4% at 20 °C/min), in the case of the nanocomposites with 1.0 wt % of INT-WS₂, the increment in crystallinity observed varied from 25% to 43% at the same heating rates. The influence of INT-WS₂ content on the crystallinity of PLLA is illustrated in Figure 3b, where the (1-λ)_cc values are represented at different INT-WS₂ loadings. Similar to the results shown in Figure 2b, the presence of INT-WS₂ induces a remarkable decrease in the crystallinity of PLLA in the nanocomposites with respect to neat PLLA at a relatively slow heating rate of 1 °C/min, becoming smaller as the heating rate was increased. This is probably due to the presence of a higher percentage of crystals formed during cooling that reduce the chain mobility. This reduction negatively affects the cold-crystallization process. The remarkable decrease in the crystallinity and T_cc of PLLA obtained at low inorganic nanotubes loadings (≤0.5 wt %) indicates that INT-WS₂ act as efficient nucleating agents in the PLLA matrix under non-isothermal conditions.

Figure 2. Variation of the cold-crystallization peak temperature (T_cc) of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration.

Figure 2. Variation of the cold-crystallization peak temperature (T_cc) of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration.

Figure 3. Variation of the cold-crystallinity (1-λ)_cc of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration.

Figure 3. Variation of the cold-crystallinity (1-λ)_cc of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration.

In the case of polymer crystallization, the measured rate of heat release is assumed to be proportional to the macroscopic rate of crystallization:

$$\frac{dQ}{dt}=Q_c\frac{dx}{dt}$$

(2)

where Q_c is the measured heat of crystallization calculated by integration of a DSC peak. The values of Q_c can further be used to determine the crystallization rate, dx/dt, as well as the extent of the melt conversion:

$$x\left(t\right)=\frac{1}{Q_c}\underset{0}{\overset{t}{{\displaystyle \int }}}\frac{dQ}{dt}dt$$

(3)

The value of x(t) varies from 0 to 1 and represents the degree of conversion. The transformation from temperature to time is performed using a constant heating rate ϕ:

$$t=\frac{T_0-T}{\varphi }$$

(4)

where T is the temperature at time t and T_i is the temperature at the beginning of crystallization. Figure 4 shows typical conversion curves at various heating rates for PLLA/INT-WS₂ nanocomposites. The conversion curves shift over a longer time with decreasing heating rate, suggesting that the diffusion of PLLA becomes very difficult for cold-crystallization.

Figure 4. Conversion curves for the cold-crystallization of (a) PLLA and (b) PLLA/INT-WS₂ (1.0 wt %).

Figure 4. Conversion curves for the cold-crystallization of (a) PLLA and (b) PLLA/INT-WS₂ (1.0 wt %).

3.2. Lui Model

To quantitatively describe the evolution of crystallinity during non-isothermal crystallization, a number of models have been proposed in the literature [30]. The most common approach is that proposed by Lui et al. [31] who combined the Avrami and Ozawa models [32,33,34,35] to analyze the non-isothermal crystallization kinetics. Since the degree of crystallinity is related to the heating rate (ϕ) and the crystallization time t, or temperature T, and the relation between ϕ and t can be established for a particular degree of crystallinity, the final expression of their model can be written as follows:

$$ln\varphi =lnf\left(T\right)-\alpha lnt$$

(5)

where f(T) = [k´(T)/k]1/m, corresponds to the value of heating rate chosen at unit crystallization time, when the system has a certain degree of crystallinity, α is the ratio of the Avrami exponents to Ozawa exponents (i.e., α = n/m), and ϕ is the heating rate. Plotting ln ϕ vs. ln t at a given degree of conversion yields a linear representation, as shown in Figure 5. This indicates that the Lui model provides a satisfactory description for the cold-crystallization of PLLA/INT-WS₂ nanocomposites. The kinetic parameters, ln f(T) and α, which are derived from the slope and the intercept of those lines, are listed in Table 2. These data show that f(T) for PLLA increases rapidly with the crystallinity from 83.58 for x = 10% to 114.42 for x = 90%. Noticeably, f(T) for PLLA is much greater than that for PLLA/INT-WS₂ for the same conversion x and the effect becomes less pronounced with increasing nanoparticle loading (≥0.5 wt %). This means that to approach an identical degree of crystallinity, the nanocomposites required a higher crystallization rate than that of the pure PLLA. That is to say, the PLLA/INT-WS₂ nanocomposites crystallize easier than PLLA. In addition, the values of the parameter α are nearly constant (1.2 to 1.3), indicating that the mechanism of nucleation and growth is more or less the same for both PLLA and PLLA/INT-WS₂.

Figure 5. Liu plots for the cold-crystallization of (a) PLLA and (b) PLLA/INT-WS₂ (1.0 wt %).

Figure 5. Liu plots for the cold-crystallization of (a) PLLA and (b) PLLA/INT-WS₂ (1.0 wt %).

Table 2. Values of α and f(T) vs. conversion (x) based on the Liu model for pure PLLA and PLLA/INT-WS₂.

INT-WS2 content (wt %)	x (%)	α	F (T)	ΔEa (kJ/mol)
0.0	10	1.24	83.58	100.8
	30	1.25	90.89
	50	1.26	95.76
	70	1.27	101.73
	90	1.29	114.42
0.1	10	1.24	80.29	102.7
	30	1.24	87.02
	50	1.26	93.30
	70	1.27	99.31
	90	1.30	111.33
0.5	10	1.23	73.09	112.8
	30	1.24	79.65
	50	1.24	83.31
	70	1.25	87.24
	90	1.26	95.24
1.0	10	1.29	62.41	111.9
	30	1.22	74.47
	50	1.24	81.18
	70	1.24	85.88
	90	1.26	95.10

Table 2. Values of α and f(T) vs. conversion (x) based on the Liu model for pure PLLA and PLLA/INT-WS₂.

INT-WS2 content (wt %)	x (%)	α	F (T)	ΔEa (kJ/mol)
0.0	10	1.24	83.58	100.8
	30	1.25	90.89
	50	1.26	95.76
	70	1.27	101.73
	90	1.29	114.42
0.1	10	1.24	80.29	102.7
	30	1.24	87.02
	50	1.26	93.30
	70	1.27	99.31
	90	1.30	111.33
0.5	10	1.23	73.09	112.8
	30	1.24	79.65
	50	1.24	83.31
	70	1.25	87.24
	90	1.26	95.24
1.0	10	1.29	62.41	111.9
	30	1.22	74.47
	50	1.24	81.18
	70	1.24	85.88
	90	1.26	95.10

3.3. Effective Energy Barrier

Activation energy is an important parameter for characterizing the non-isothermal crystallization of polymers and can be used to estimate the growth ability of the chain segments. The higher the crystallization activation energy ΔE, the more difficult is the transport of macromolecular segments to the growing surface. Considering the variation of the peak temperature with the heating rate ϕ, ΔE could be derived from the Kissinger equation [36]:

$$ln\left(\frac{\varphi }{T_{\text{cc}}^2}\right)=Constant-\frac{\text{Δ}E}{R{T}_{\text{cc}}}$$

(6)

where R is the universal gas constant. The activation energies were calculated using the slopes of the lines obtained from plots of log ϕ/T_cc² against 1/T_cc (Figure 6). Thus the values of ΔE for neat PLLA and its nanocomposite containing 0.1 wt % of INT-WS₂ were calculated to be 100.8 and 102.7 kJ/mol, respectively (Table 2). At higher INT-WS₂ concentrations, the value of ΔE for PLLA continues to rise and tends to stabilize for the highest concentrations of 0.5% and 1% (i.e., ΔE_0.5 = 112.8 kJ/mol and ΔE_1.0 = 111.9 kJ/mol.

It was also noticed that the primary crystallization is an interface-controlled process with a decreasing nucleation rate. Thus, it is evident that the crystallization process is complex (and not isokinetic) during the entire crystallization event. The complexity of the crystallization mechanism can be judged from the dependence of ΔE on crystalline conversion using the so-called differential isoconversional method of Friedman [37], according to the following expression:

$$ln{(\frac{dx}{dt})}_{x,i}=Constant-\frac{\text{Δ}{E}_x}{R{T}_{x,i}}$$

(7)

where dx/dt is the instantaneous crystallization rate as a function of time at a given conversion x. By selecting the appropriate conversion (i.e., from 10% to 90%) the values of dx/dt at a specific x are correlated to the corresponding crystallization temperature at this x (i.e., T_x). Then, from the slopes of these representations for the PLLA and the nanocomposites, the ΔE can be calculated. As an example, Figure 7 shows the dependence of the effective energy barrier on conversion x for neat PLLA and its nanocomposites with 0.1, 0.5 and 1.0 wt % of INT-WS₂. As can be seen, the effective energy barrier decreases as the conversion rises and the temperature increases. Thus, isoconversional methods not only provide accurate value of activation energy, but it also hints towards the degree of complexity of the crystallization mechanism from the knowledge of the dependence of ∆E on x. According to this Figure, at a given x, the effective energy barrier is dependent on the INT-WS₂ concentration. More importantly, the apparent ∆E values of the all nanocomposites are higher than that of neat PLLA, indicating that the restriction of molecular mobility of PLLA chains does not appear to be a limiting factor in the crystallization rate, and demonstrating that the nucleation activity of INTs plays a dominant role in accelerating the crystallization of PLLA.

Figure 6. Kissinger plots for the cold-crystallization of PLLA/INT-WS₂ nanocomposites.

Figure 6. Kissinger plots for the cold-crystallization of PLLA/INT-WS₂ nanocomposites.

Figure 7. Dependence of the effective energy barrier (∆E) on the degree of conversion (x) of PLLA/INT-WS₂ nanocomposites using differential isoconversional methods of Friedman.

Figure 7. Dependence of the effective energy barrier (∆E) on the degree of conversion (x) of PLLA/INT-WS₂ nanocomposites using differential isoconversional methods of Friedman.

3.4. Melting Behaviour

It is of interest to evaluate the effect of heating rate on the melting behaviour of PLLA in the nanocomposites. As shown in Figure 1, the heating rate effect is evident in all samples, which led to the appearance of complex double-melting peaks. The first peak was observed at 134–143 °C and the other at 152–154 °C (Table 1). The analogous data of double-melting peaks versus heating rate with INT-WS₂ concentration as a parameter are also shown in Table 1. In general, it is accepted that the double endothermic peaks are attributed to melting-recrystallization-melting processes of PLLA lamellae. Obviously, the first endothermic peak is attributed to the fusion of thin lamellae formed during the DSC heating process (e.g., cold-crystallization), and the second to the fusion of lamellae that are newly formed through the melting-recrystallization of primary thin lamellae occurring at relatively higher temperature [38,39]. In particular, an increasing heating rate allows less time for the crystals to reorganize and re-melting occurs over a lower temperature range. Ideally, the melting of reorganized crystals should completely vanish over a certain heating rate where recrystallization is totally inhibited. Therefore, the first endothermic peak can be used to reflect the lamellar structure of PLLA in the nanocomposites. Figure 8a compares the evolution of melting temperatures for pure PLLA and PLLA/INT-WS₂ as a function of the cooling rate. The low and high-temperature melting endotherms were labelled T_m1 and T_m2, respectively. T_m1 gradually shifted to higher temperature with increasing heating rate, whereas T_m2 showed much lower dependence on the heating rate. On the other hand, increasing amounts of INT-WS₂ in the PLLA/INT-WS₂ nanocomposites progressively reduces the apparent area under the first endotherm. These results confirm that the double-melting behaviour of the samples is mainly derived from the melting-recrystallization-remelting processes upon heating. However, the most relevant result was the reduction of the difference between the melting temperature peaks of the nanocomposite and that of the neat matrix (T_m2–T_m1) with increasing heating rate. This phenomenon was less pronounced for the nanocomposites with high INT-WS₂ contents, which also exhibited lower recrystallization at a given heating rate (Figure 8b). Therefore, the crystals in the nanocomposites are more perfect and stable than those in neat PLLA. This is related to the strong nucleation effect of INT-WS₂ involved in the acceleration of the crystallization of PLLA in the nanocomposites and is also in agreement with the aforementioned shift of the crystallization ranges of PLLA along the crystallization temperature axis.

In tune with these observations, both heating rate and (the INT-WS₂ loading are the two main factors that influence the crystallinity of PLLA in the nanocomposites Table 1). On the one hand, with increasing heating rate, (1-λ)_m shifts towards higher values, and the melting process is enhanced for both neat PLLA and its nanocomposites. On the other hand, the addition of INT-WS₂ apparently enhances (1-λ)_m of PLLA when a low cooling rate is used. For more clarity, Figure 9 summarizes the variation of (1-λ)_m with heating rate and composition. The data presented also shows the difference between the overall crystallinity change derived from the difference between endotherm and exotherm, Δ(1-λ) =(1-λ)_m−(1-λ)_cc, for neat PLLA and its nanocomposites, which can be used to highlight the recrystallization ability of the PLLA crystals. At lower ∆(1-λ), the reorganization of the PLLA crystals is more difficult. It can be seen that increasing the heating rate allows less time for the molten materials to reorganize into new crystals, thus lowering Δ(1-λ) (Figure 9a) and hence, the reorganization process is largely inhibited. On the other hand, the INT-WS₂ have shown an important capacity for accelerating the cold-crystallization process (discussed earlier) and reducing the crystallinity (1-λ)_cc due to a remarkable heterogeneous nucleation role in the nanocomposites. However, the role of INT-WS₂ on the variation of the Δ(1-λ) values of PLLA appears to be only relevant at a low heating rate. These results confirm that the variation of ∆(1-λ) is mainly derived from the melting-recrystallization-remelting processes upon heating. However, in our case, we cannot exclude the nucleation capacity of INT-WS₂ during cooling, which may induce the generation of new crystals of PLLA in the nanocomposites. That is, the difference ∆(1-λ) may involve both nucleation during cooling as well as reorganization ability of the PLLA crystals during subsequent heating.

Figure 8. Variation of the melting temperatures (T_m1 and T_m2) of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration.

Figure 8. Variation of the melting temperatures (T_m1 and T_m2) of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration.

Figure 9. Variation of the melting crystallinity (1-λ)_m of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration; inset is the difference between the overall crystallinity change derived from the difference between endotherm and exotherm, Δ(1-λ)=(1-λ)_m-(1-λ)_cc, for PLLA/INT-WS₂ nanocomposites.

Figure 9. Variation of the melting crystallinity (1-λ)_m of PLLA/INT-WS₂ nanocomposites with (a) heating rate and (b) INT-WS₂ concentration; inset is the difference between the overall crystallinity change derived from the difference between endotherm and exotherm, Δ(1-λ)=(1-λ)_m-(1-λ)_cc, for PLLA/INT-WS₂ nanocomposites.

4. Conclusions

The results of our study demonstrate the effectiveness of the use of small amounts of INT-WS₂ to improve the cold-crystallizability of PLLA biopolymer. The results obtained from the DSC analysis show that the heating rate and INT-WS₂ loading are very efficient factors for the control of the cold-crystallization rate and crystallinity of PLLA. In particular, it was shown that INT-WS₂ enhanced the crystallization rate of PLLA significantly, and the degree of enhancement is strongly dependent on the INT-WS₂ content. The introduction of INT-WS₂ to the PLLA matrix induces a remarkable decrease in the (1-λ)_cc value of PLLA at a relatively slow heating rate of 1 °C/min, becoming smaller as the heating rate was increased. The Lui model was found to be effective in describing the crystallization of PLLA/INT-WS₂ nanocomposites. All rate parameters demonstrated that the PLLA/INT-WS₂ crystallized easier than pure PLLA due to excellent nucleating effect of INT-WS₂. Further, the energy barrier governing the crystallization, calculated based on isokinetic and differential (Friedman) isoconversional methods, confirmed that the nucleation activity of INTs plays a dominant role in accelerating the crystallization of PLLA. However, the nucleation mechanism of PLLA remains unchanged. On subsequent heating, double-melting peaks for PLLA and its nanocomposites can be attributed to a melt-recrystallization mechanism. It was found that increasing the heating rates reduced the difference between the melting temperature peaks of PLLA/INT-WS₂ and that of the neat PLLA (i.e., impeded the recrystallization of PLLA in the nanocomposites). This phenomenon was less pronounced for the nanocomposites with high INT-WS₂ content. In addition, the difference in the apparent crystallinity ∆(1-λ) was found to be an effective method to highlight the recrystallization ability of the PLLA crystals. The quantitative analysis showed that increasing the heating rate can effectively inhibit the reorganization process of PLLA, and a further increase of the INT-WS₂ concentration led to a marked influence on the recrystallization of PLLA, especially when at low heating rates. These results have considerable practical significance for technological processing of PLLA, where premature crystallization hinders the forming stage, and offers new prospects for the preparation of PLLA biopolymer materials for possible use in many eco-friendly (e.g., sustainable packaging) and biomedical (e.g., surgical sutures, bone fixation devices, etc.) applications.