Effect of 1,4-Naphthalenedicarboxylic Acid Derivative on Crystallization and Performances of Poly(L-lactide)

In this work, biodegradable Poly(L-lactide) (PLLA) was modified through adding a new organic additive N, N'-bis(benzoyl) 1, 4-naphthalenedicarboxylic acid dihydrazide (NABH). A comparison on crystallization of the pure PLLA and PLLA/NABH revealed that the NABH as effective heterogeneous nucleation sites enhanced PLLA’s crystallization, and an increase of NABH loading was able to further accelerate crystallization rate of PLLA; whereas a faster cooling rate was not conducive to PLLA’s crystallization, but the appearance of obvious crystallization peak upon cooling at 30oC/min confirmed the advanced enhancing role of NABH for PLLA crystallization again. The investigation on influence of the final melting temperature on the crystallization behavior of PLLA showed that the 170 oC was optimum final melting temperature for enhancing crystallization, even the onset crystallization temperature of PLLA/NABH were higher than 150oC. The melting processes of PLLA/NABH after different crystallization not only could reflect the previous crystallization, but also depended on crystallization temperature and heating rate. Thermal decomposition results showed that the existence of NABH slightly weakened thermal stability of PLLA, and the maximum difference in onset thermal decomposition temperature was only 9.4oC comparing with the pure PLLA. However, the presence of NABH in PLLA matrix seriously weakened optical property.


1.Introduction
Solid waste pollution from the fossil-based and non-degradable polymeric products has led to a severe crisis for promoting social sustainable development, what is worse, up to now, the large-scale usage of non-degradable polymeric products has not been still reduced. Inevitably, developing bio-based and degradable polymeric materials as fossil-based polymers substitutions has become an important researching hotspot [1]. Poly(L-lactide) (PLLA) produced from renewable sources is widely known as a commercially available thermoplastic [2] with excellent degradability and compatibility [3,4], easy processing [1,5], appropriate mechanical performances [6,7], etc. These advantages endow PLLA with promising applications in different fields, mostly for packaging materials [8][9][10], biomedical materials [11][12][13] and 3D printing materials [14][15][16]. For instance, to develop degradable carrier for controlled drug delivery, Li et al [17] prepared the polylactic acid composite membranes containing zeolitic imidazolate framework-8 via secondary growth method, the relative results indicated that the zeolitic imidazolate framework-8/polylactic acid composite membranes possessed excellent drug release and pH response capacity, and over 90% of loaded drug could be released in the buffer of pH value of 7.4.
However, the inherent drawbacks of high cost, slow crystallization rate, poor heat resistance and low crystallinity [2,18] seriously restrict the practical demanding applications of PLLA. Among aforementioned drawbacks, both poor heat resistance and low crystallinity result from the slow crystallization rate, additionally, the slow crystallization rate also affects PLLA's mechanical properties via forming amorphous products with low strength and modulus as well as poor dimensional stability [6]. Therefore, accelerating crystallization of PLLA is extremely important to remedy the performance defects of PLLA and thoroughly realize the replacement of the conventional fossil-based thermoplastic. Adding physical nucleating agent is thought to be an easy industrialization way to accelerate crystallihttps://doi.org/10.37358/MP.21.1.5445 zation rate of semi-crystalline polymers, which is also suitable for PLLA. In the early research, the crystallization accelerating agent for PLLA focused on the commercial nucleating agents for other polymers such as polypropylene and polyethylene, these typical additives included clay [19][20][21], ethylene bis stearamide [20], calcium carbonate [22,23], etc. After that, with the in-depth research, more inorganic or organic compounds including commercial or newly-synthesized compounds were employed to estimate their roles in enhancing PLLA's crystallization, and the relative results showed that metal phosphonates [24,25], inorganic whiskers [26,27], nano-inorganic matter [28,29], benzhydrazide derivatives [30,31], sorbitol derivatives [32,33] and 1H-benzotriazole derivatives [34] were highly effective nucleating agents for crystallization.
Through analysis, it is found that, in comparison to inorganic nucleator, the organic nucleator has attracted increasing attentions due to the better compatibility with PLLA and flexible designability of molecular structure. However, it is still unclear which key organic groups can affect the crystallization nucleation effect according to a finite category and quantity of the current organic nucleating agents. Thus, in this work, a new organic compound named N, N'-bis(benzoyl) 1, 4-naphthalenedicarboxylic acid dihydrazide (NABH) was developed to estimate its influences on the crystallization process, melting process, thermal decomposition behavior and optical property of PLLA. This work will be helpful to enrich the amount of organic nucleating agent and explore nucleation mechanism according to the relative key structure.

Materials and reagents
A commercial grade 4032D PLLA produced by Nature-Works LLC was adopted. All reagents used to synthesis NABH were required to be analytically pure and directly used as received without purification before usage; these relative reagents included 1, 4-naphthalenedicarboxylic acid, benzoic hydrazide, N, N′-dimethylformamide (DMF), thionyl chloride and pyridine.

Synthesis of NABH
The synthetic pathway of NABH is shown in Figure 1. A detailed synthetic route is following: 9 g 1, 4-naphthalenedicarboxylic acid and 1mL DMF as catalyst were added into a three-necked flask with 60 mL thionyl chloride, respectively. And then with increasing of temperature to 80ºC, the solid-state 1, 4-naphthalenedicarboxylic acid was gradually dissolved in thionyl chloride with stirring, the aforementioned mixed solution were stirred at 80°C for 36 h, after that, 1, 4-naphthalenedicarboxylic acid dichloride was obtained via vacuum distillation. The 1, 4-naphthalenedicarboxylic acid dichloride was subsequently added into a solution including 0.006 mol benzoic hydrazide, 100 mL DMF and 3 mL pyridine, the mixture was stirred at ice bath for 1.5 h, heated up to 60°C to keep stirring for 3 h, the resulting solution was poured into 300 mL distilled water, the crude product was obtained by filtration method, and the crude product was further washed by distilled water for three times to obtain the final white product NABH. Fourier Transform Infrared Spectrometer (FT-IR) υ: 3448.

Preparation of PLLA/NABH
First, PLLA and NABH were dried in vacuum at 45°C for 24 h to avoid the influence of water on the high temperature blending of PLLA with NABH. Second, The PLLA containing various NABH concentrations (0, 0.5, 1, 2 and 3 wt%) was blended on a torque rheometer, and the melt-blending temperature was set at 190°C; the blending rotation speed was set to be 32 rpm for 7 min, and then further increased to 64 rpm for 7 min. After that, the aforementioned mixture was hot pressed by 20 MPa pressure at 180°C, followed by cool-pressing at room temperature to obtain sample with a thickness of 0.4 mm to perform the relevant tests.

Test
Both crystallization and melting processes of the pure PLLA and modified PLLA were recorded by Q2000 DSC with 50 mL/min nitrogen. Although the testing conditions of crystallization and melting behaviors were different, the temperature and heat flow firstly needed to be calibrated using an indium standard before testing, and the thermal history also needed to be eliminated to ensure the relative tests at the same level. Under atmospheric condition, a Q500 TGA was used to perform a comparison test on thermal decomposition of the pure PLLA and modified PLLA, the testing temperature was from 40°C to 650°C with a heating rate of 5°C/min. The light transmittance was determined by the average value of the 5 times measurements using a DR82 transmittance instrument.

3.Results and discussions Crystallization process
Upon cooling at 1 °C/min, the DSC thermograms of the pure PLLA and modified PLLA were displayed in Figure 2, and the relative crystallization parameters were listed in Table 1. As shown in Figure 2, DSC thermograms of the pure PLLA is clearly indicative of no melt-crystallization peak detected, indicating that, although that the cooling rate of 1°C/min is very slow, the pure PLLA cannot still form crystal in cooling via homogeneous nucleation, this finding confirms PLLA's poor crystallization ability as reported [35,36]. In contrast with the pure PLLA, the melt-crystallization peaks of all PLLA/NABH samples can be easily detected in DSC thermograms, furthermore, the meltcrystallization peaks observed in DSC thermograms are very sharp, suggesting that the presence of NABH as effective heterogeneous nucleation sites is able to enhance crystallization ability and accelerate crystallization rate of PLLA. Additionally, Figure 2 displays the effect of NABH loading on the crystallization process of PLLA. Compared to the other three NABH loading, with the addition of 1 wt% NABH, the PLLA/1%NABH has the highest onset crystallization temperature (Toc) of 142.3°C, crystallization peak temperature (Tmc) of 138.2°C, and the melting temperature (Tm) of 169.4°C, as well as the largest crystallization enthalpy (ΔHc) of 48.9 J/g, showing that 1 wt% NABH is the optimal loading for inducing crystallization of PLLA. However, the temperature difference (ΔTc) between Toc and Tmc is https://doi.org/10.37358/MP.21.1.5445 not the minimum value, which implies that the crystallization rate of PLLA/1%NABH is not the fastest in cooling, because a samller ΔTc often means the faster crystallization rate. For the other three NABH loading of 0.5 wt%, 2 wt% and 3 wt%, an increase of NABH loading leads to a shift of crystallization peak to higher temperature side because of an increase of heterogeneous nucleation density in PLLA matrix; as seen in Table.1, the Toc and Tmc increase from 137.7°C and 133.1°C to 139.4°C and 136.2°C, respectively; meanwhile the ΔTc decreases from 4.6°C to 3.2°C, this drop in ΔTc confirms that the NABH can accelerate PLLA's crystallization rate again. Overall, the addition of NABH can significantly promote the PLLA's crystallization in cooling, but the effect of NABH loading on the crystallization may depend not only on the NABH loading, but also the migration of PLLA chain segment.  Evaluating crystallization ability of the modified PLLA at a higher cooling rate is necessary to practical production, because an increase of cooling rate often weakens the ability of additives for enhancing PLLA crystallization [37][38][39]. Figure 3 depicts the crystallization processes of PLLA/NABH from 190°C at various cooling rates (5, 10, 20 and 30°C/min). For a given PLLA/NABH sample, it is observed that a wider melt-crystallization peak located at lower temperature side appears in the DSC curve with an increase of cooling rate, suggesting a relative weaker crystallization ability and a relative slower crystallization rate. Even then, all PLLA/NABH still have obvious crystallization peaks at all cooing rates, even the NABH is still able to accelerate the PLLA's crystallization rate upon cooling at 30°C/min, which further confirms the advanced crystallization accelerating role of NABH. Additionally, upon cooling at a higher cooling rate, it is noteworthy that a larger NABH loading have a greater inhibition for preventing the weakness of crystallization ability as shown in Figure 3, the reason is that a larger amount of NABH can provide a higher nucleation density in PLLA matrix and cause meltcrystallization to occur at a relative higher temperature, which be proved by the crystallization peak shift toward higher temperature side with the increase in NABH loading in PLLA matrix.  The final melting temperature (Tf) determines solubility of NABH in PLLA resin according to the principle of "like dissolves like", correspondingly the Tf must affect the crystallization processes of PLLA/NABH in cooling. Figure 4 shows the DSC thermograms of PLLA/NABH from different Tf at a cooling rate of 1°C/min, and the relative crystallization parameters are listed in Table 2. As shown in Figure 4, the crystallization processes are significantly affected by Tf. When the Tf is 170°C, the crystallization peak moves toward the higher temperature side with the increase in NABH concentration, which is different from the results about the influence of NABH loading on the crystallization behaviors from other Tf at a cooling rate of 1°C/min. Moreover, Table.1 indicates that the Tf of 170°C is conducive to crystallize because of the presence of the highest Toc and Tmc and the largest ΔHc, the probable reason is that, on the hand, the relative low Tf in this study can provide the PLLA chain segment with excellent motility to ensure crystal growth; on the other hand, the relative low Tf can avoid the destruction of crystal stability resulting from the high temperature, as well as Tf of 170°C can ensure a larger amount of undissolved NABH as heterogeneous nucleus in PLLA matrix. Under this circumstance, PLLA/NABH have fast nucleation rate and crystal growth rate. As a result, the crystallization is completed in the higher temperature region. Additionally, it is noted that the Toc of PLLA/2%NABH and PLLA/3%NABH are higher than 150°C, which is rather close to the melting range of the pure 4032D PLLA (about 160～175°C [40]), this result confirms the powerful crystallization ability of PLLA/NABH again. When the Tf is increased to 190°C, a given PLLA/NABH sample has the minimum Toc, Tmc and ΔHc, but the Toc, Tmc and ΔHc gradually increase when further increasing Tf to 200°C , this effect may depend on the interaction of PLLA with undissolved NABH, because an increase of Tf must dissolve more NABH in PLLA matrix, and these dissolved NABH can enhance the compatibility and interaction of PLLA with undissolved NABH, resulting in the fast and ordered arrangement of PLLA molecular chains.

Melting behavior
Usually, the melting behavior of semi-crystalline polymers depends on their crystallinity and crystal perfection. Figure 5 is DSC thermograms of PLLA/NABH at various heating rates (1, 2, 5 and 10°C/min) after melt-crystallization at 1°C/min. It can be observed from Figure 5 that, upon heating at 1°C/min, the PLLA/0.5%NABH has double melting peaks, the reason is that the rather slow heating rate of 1°C/min can further promote crystallization to occur, when the previous melt-crystallization cannot be thoroughly completed. However, it should be note that the high-temperature side melting peak is in the form of the shoulder peak, indicating that the amount of recrystallized crystals is very low. Upon heating at other rates, all PLLA/NABH only have the single melting peak, but the melting peak of a given PLLA/NABH becomes wider with an increase of heating rate due to the thermal hysteresis. In melt-crystallization section, the Tmc of all PLLA/NABH are about 130 °C～140°C, meaning the crystallization rate is the fastest in this temperature region. Thus, the melting behaviors of PLLA/NABH after isothermal crystallization in the temperature range of 130°C～140 °C were further studied by DSC, Figure.6 is the heating processes of PLLA/NABH at 10°C/min after isothermal crystallization at various crystallization temperatures (130, 132, 134, 136, 138 and 14°C) for 180 min. As isothermal crystallization temperature increases from 130°C to 140 °C, there are two typical features in DSC curves, one feature is that all PLLA/NABH exhibit the single melting peak, showing that the crystallization has been completed after sufficient crystallization for 180 min; another feature is that the single melting peak moves toward the higher temperature side with increasing of crystallization temperature, because a higher crystallization temperature can cause the crystal to grow more perfect.

Thermal stability and optical property
For practical application, investigating on the thermal decomposition behavior in air is very necessary to adapt for various applications. Hence, Figure 7 displays the TGA curves of the pure PLA and four PLA/NABH samples which exhibit only one mass loss stage of decomposition from TGA curves, and this mass loss stage appears in the temperature range of 300°C～400°C, which results from chain scissions and combustion of ester groups. Whereas the onset thermal decomposition temperature is affected by NABH, and the onset thermal decomposition temperatures are following: PLLA 341.3°C, PLLA/0.5%NABH 334.8°C, PLLA/1%NABH 334.4°C, PLLA/2%NABH 333.9°C and PLLA/3% NABH 331.9°C. Through data analysis, it is a fact that the presence of NABH weakens the thermal stability of PLLA, but there is no significant decrease, and the maximum difference in onset thermal https://doi.org/10.37358/MP.21.1.5445 decomposition temperature is less than 10°C compared with the pure PLLA. Furthermore, although the onset thermal decomposition temperature decreases with increasing of NABH loading from 0.5 wt% to 3 wt%, the difference between all PLLA/NABH samples is only 2.9°C, indicating that the influence of NABH loading on the onset thermal decomposition temperature of PLLA/NABH is very slight.

Figure 7. TGA curves of PLLA with and without NABH
Transparency is another important characteristic of PLLA itself, but the addition of a color additive often decreases the light transmittance, affecting its application in some fields. Figure 8 is the effect of NABH and its loading on PLLA's light transmittance, unfortunately, the introduction of NABH seriously destroys PLLA's light transmittance, when the NABH loading is 0.5 wt%, the light transmittance is decreased to 9.7% comparing with the 78.2% of the pure PLLA, what is worse, when the NABH loading is higher than 1 wt%, the light transmittance is almost zero, the primary reason is thought to be the white NABH in PLLA matrix. In addition, the crystallization promoting role of NABH can also make PLLA become opaque to some extent because of crystal formation.

4.Conclusions
In conclusion, compared to the pure PLLA, the sharp crystallization peaks of all PLLA/NABH samples indicated that PLLA's crystallization ability was effectively improved by the introduction of NABH as a new organic heterogeneous nucleating agent. DSC measurements further showed that, upon cooling at 30ºC/min, PLLA/NABH could still crystallize resulting from the powerful promoting effect of NABH for PLLA's crystallization, although an increase of cooling rate was seriously negative to https://doi.org/10.37358/MP.21.1.5445 PLLA/NABH's crystallization. Additionally, in melt-crystallization section, the influence of Tf on crystallization behavior showed that, when the Tf was 170ºC, PLLA/NABH exhibited the highest Toc, Tmc and the largest ΔHc. The heating rate significantly affected the melting behaviors of NABHnucleated PLLA after melt-crystallization, and a decrease of heating rate could cause the melting peak to become sharper and promote the recrystallization to occur in heating. When the crystallization time was long enough, the melting behavior after isothermal crystallization depended on crystallization temperature. The incorporation of NABH could almost not change thermal decomposition profile of PLLA in air, moreover, the influence of NABH on the onset thermal decomposition temperature of PLLA was also not distinct, which is helpful to satisfy demanding applications. Unfortunately, the light transmittance of PLLA was seriously weakened by the NABH, when the NABH concentration was higher 1 wt%, the light transmission of PLLA/NABH was almost disappeared.