Nanolignin as Nucleating Agents Promoting Crystallization of Isotactic Polypropylene

Lignin is the second most abundant pollution-free biomass material. However, most of lignin is discarded as waste in river or burned as fuel, which results in serious environmental pollution problems and low utilization efciency of lignin at present. Tus, high-value utilization of lignin has become a hot research feld. Herein, nanolignin (nano-Lig) is prepared successfully by the self-assembly method, and then nano-Lig and isotactic polypropylene (iPP) are mixed to prepare a series of nano-Lig/iPP composites by the solution blending method. Nano-Lig not only enhance thermostability of iPP but also improve crystallization properties of iPP. When 1.0 wt% nano-Lig was added in iPP, the crystallinity of iPP increased by 6.39% compared to iPP. Nano-Lig can increase the crystallization rate of iPP through investigation of kinetics of nonisothermal crystallization, suggesting nano-Lig can be used as a nucleating agent for iPP.


Introduction
Isotactic polypropylene (iPP) is one of the most used, large volume, low price polymorphic semicrystalline thermoplastic polymers [1,2]. It has several unique properties such as high toughness, elasticity, ease of processability, excellent water, and chemical resistance. Terefore, it can be found in various applications, such as the automobile industry, food packaging, and the manufacture of toys and furniture [3][4][5][6].
Te fabrication of iPP products involves heating and cooling during processing. It melts and crystallizes upon heating and cooling, respectively. Te ultimate properties of iPP products are mainly controlled by the polymer structure, which can be afected by these phenomena of melts and crystallizes during processing. iPP crystallizes into four forms, namely, monoclinic α, trigonal β, orthorhombic c, and a mesomorphic form, under diferent thermal and mechanical conditions and the usage of nucleating agents (NAs) [7][8][9][10][11][12]. Diferent kinds of nucleating agents can afect not only the crystallization rate of iPP in the forming process but also the crystalline structure.
NAs can be divided into natural NAs, synthetic NAs, and inorganic NAs. Te natural NAs mainly include rosin, sorbitol, and sorbitol derivatives such as dibenzylidene sorbitol and 3, 4-dimethyldibenzylidene sorbitol [13][14][15][16][17][18][19][20][21][22][23][24]. Te synthetic NAs include polycyclic aromatic hydrocarbons, organic acids/salts, amides, [25] benzoic acid salts, [26] hydrazide compounds, [27] and various organic pigments. Inorganic NAs have various salts oxides, such as CaCO 3 [28,29], talc [30,31], and Al 2 O 3 [32]. Tree kinds of NAs can increase the rate of PP crystallization, and the temperature at which the maximum rate of crystallization occurs upon cooling from the molten state. However, the disadvantage of synthetic NAs is that the technology of their production is not environmentally friendly, which usually include several stages of organic syntheses using specialty compounds, as well as toxic solvents. Talc and other inorganic fllers are also suitable NAs; they are inexpensive and serve as reinforcing agents. However, high concentrations of these materials are needed.
Lignin is the second most abundant biopolymer on the earth next to cellulose and is obtained as a major industrial waste material from pulp and paper industries [33]. Whereas 98% of the lignin is burned or discharged into rivers, thereby causing serious environmental pollution and grievous waste of renewable resources [34]. It has attracted much interest in scientifc and industrial feld because of its unique properties, such as environmentally friendly, sustainable, biodegradable, thermal stability, and low cost, which can contribute to the reduction of the environmental problem [35][36][37][38][39][40][41]. Tus, lignin has been applied to many felds, such as binder, carbon materials, fre retardant, and biomedical application [42][43][44][45]. In the past decades, nano-Lignin (nano-Lig) had been developed to design high-value materials. Nano-Lig has many merits, such as more uniform size, shape, and a high surface area to volume ratio, which helps to reinforce and strengthen polymer matrices and improve polymer thermal stability [46][47][48]. In addition, nano-Lig also shows superior dispersion and miscibility in the hydrophobic matrices [46]. Te study of Nano-Lig mainly focus on lignin-based hydrogels [47] and enhancement mechanical properties of polyvinyl alcohol and polylactic acid [48]. However, to the best of our knowledge, there is still no report on using nano-Lig as NA for iPP, even though there was report on lignin could act as NA for iPP [49]. It is expected that nano-Lig can act as an efective NA for iPP; in other words, a smaller amount of nano-Lig can promote crystallization of iPP.
Herein, we report that nano-Lignin (nano-Lig) can serve as a NA for iPP. Nano-Lig is prepared by a simple selfassembly method using tetrahydrofuran as good solvent and H 2 O as poor solvent. First, nano-Lig/iPP composites are prepared by the solution blending method. Second, the efect of nano-Lig on the melting properties, crystallization structure, and thermal stability of iPP are investigated. Finally, nonisothermal kinetics of crystallization of iPP and nano-Lig/iPP composite are demonstrated.

Preparation of Nano-Lig.
Nano-Lig aqueous solution was prepared by a simple self-assemble method according to a previous report [50]. Te preparation process of nano-Lig was as follows (Scheme 1): enzyme-hydrolyzed lignin of 0.50 g was dissolved in 50 ml THF at room temperature. Ten, deionized water of 200 mL was slowly dropped to the above lignin/THF solution under stirring. After dropping, the fnal solution was stirred for 4 h until the suspension was formed. Finally, the suspension was transferred into a dialysis bag (M w : 14000) and dialyzed in deionized water. Te water was changed every 6 h and repeated for 5 times to obtain nano-Lig aqueous solution. Nano-Lig powder was obtained after the vacuum freeze drying process of nano-Lig aqueous solution.

Preparation of Nano-Lig/iPP Composites.
A series of nano-Lig/iPP composites (0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt% nano-Lig) were prepared by the solution blend method. Here, we just show the preparation process of 1 wt% nano-Lig/iPP composite. 4.95 g iPP was dissolved in xylene at 130°C to obtain iPP/xylene solution. Ten, nano-Lig of 0.05 g was added into iPP/xylene solution. After stirring and heating the solution at 80°C for 4 h, then the white nano-Lig/ iPP composite was obtained after drying the solid product at 75°C under vacuum for 12 h.

Dynamic Light Scattering (DLS) Analysis.
Te diameter of nano-Lig in H 2 O was determined by DLS analyzer (BIC-90plus PALS, Marven, England). About 1 mL nano-Lig aqueous solution was balanced for 5 min at room temperature before measurement, and the measurement result was the average of the 10 collected data.

Fourier Transform Attenuated Total Refectance Infrared Spectroscopy (FTIR-ATR).
Te surface of the samples was scanned with FTIR-ATR. Each sample was scanned 32 times in the wavenumber range from 4000 cm −1 to 400 cm −1 .

X-Ray Difraction (XRD) Analysis.
Te iPP and nano-Lig/iPP composites were crystallized at 120°C for 2 h, and then those samples were measured by X-ray difractometer (D/Max 2500, Japan Science Co., Ltd.) to determine the crystal structure of iPP. Te current of the instrument was 200 mA, the acceleration voltage of the instrument was 40 kV, λ � 0.154 nm, the scanning step was 0.02°, the range was 10°∼25°, and the scanning step frequency was 2°/min.

Diferential Scanning Calorimeter (DSC) Analysis.
Te crystallization and melting behavior of iPP and nano-Lig/iPP composites were studied by DSC (NETZSCHTG 214). Te test atmosphere was N 2 and the sample was weighed about 3 mg. Te testing process is as follows: Heating up to 200°C at a heating rate of 30.0°C min −1 , eliminating thermal history at 200°C for 1 min, cooling to 30°C at a cooling speed of 20°C min −1 at 30°C for 1 min, and then heating up (second heating) to 200°C at a heating rate of 20.0°C min −1 . Te enthalpy of crystallization during the cooling process and the melting enthalpy during the second heating process were recorded. Te crystallization peak temperature and melting temperature of the sample were obtained from the crystallization and melting curve, respectively. According to the assumption of the two-phase model, when the polymer is melted, only the crystal part of the polymer changes; so, the melting heat is essentially the heat needed to destroy the crystal structure. Tus, the higher the crystallinity is, the larger the melting heat is. In other words, the melting heat of the polymer is proportional to its crystallinity. Using this principle, the crystallinity X c of the polymer can be calculated by equation (1): where ∆H and ∆H 0 (207 J·g −1 ) [51] are the heats of fusion of the experimental sample and the perfect (defect-free) iPP crystal, respectively. Te w f is the weight fraction of nano-Lig in the composite. Nonisothermal kinetic test: the sample was heated to 200°C at a heating rate of 30.0°C min −1 . After the heat history was eliminated at 200°C for 1 min, it was cooled to 50°C at the cooling rates of 2.5°C min −1 , 5.0°C min −1 , 10.0°C min −1 , and 20.0°C min −1 , respectively. Te crystallization curve and crystallization peak temperature at each cooling rate were recorded.

Termogravimetric (TG) Analysis.
Under N 2 atmosphere, the sample of 3 mg was weighed, the temperature increased from room temperature to 600°C at a heating rate of 20.0°C min −1 , and the mass change of the sample during the test was recorded.

Characterization of Nano-Lig and Nano-Lig/iPP
Composites. Figure 1(a) shows the nano-Lig particle size distribution in H 2 O at room temperature. Te average particle size (hydrodynamic radius) of nano-Lig in H 2 O was 153.2 nm, the particle size distribution (PDI) was 0.200, and the PDI was relatively narrow. Such size of nano-Lig is comparable with that of the previous report [50]. Tis result indicated that nano-Lig was prepared successfully. Te FTIR spectra of iPP, nano-Lig, and nano-Lig/iPP composites are shown in Figure 1(b). Te peaks from 2837 cm −1 to 2953 cm −1 and the peaks from 1375 cm −1 to 1454 cm −1 were assigned to stretching vibration and bending vibration of methylene of iPP, respectively. Te wide peak from 1500 cm −1 to 1800 cm −1 was the stretching vibration of the aromatic ring skeleton of nano-Lig. A new wide peak from 1500 cm −1 to 1800 cm −1 was shown in all nano-Lig/iPP composites, which could be assigned to the characteristic peaks of the aromatic ring skeleton of nano-Lig. Te results demonstrated that nano-Lig/iPP composites were prepared successfully. Figure 2 shows the TG curves of iPP, nano-Lig, and nano-Lig/iPP. Te TG results of iPP, nano-Lig, and nano-Lig/iPP are shown in Table 1. For nano-Lig, a mass loss occurred at around 100°C, possibly because of water evaporation in nano-Lig. Te T d (the temperature at which the initial mass of the material has been lost) of nano-Lig was 210.0°C. For iPP, the T d and T i (the temperature at which 5 wt% of the initial mass of the material has been lost) of iPP were 354.8°C and 400.0°C, respectively. For nano-Lig/iPP composites, the T d and T i of nano-Lig/iPP composites were larger than those of iPP compared with iPP; furthermore, the T d and T i of nano-Lig/iPP composites increased with the increase of the nano-Lig content in nano-Lig/iPP composites, suggesting all nano-Lig/iPP composites had higher thermostability compared to iPP. Te T i of nano-Lig/iPP composite increased by 34.4°C when the nano-Lig content was 2 wt% in nano-Lig/ iPP composites. While a previous report showed that T i of lignin/iPP composite prepared by melt mixing lignin (not nano-Lignin) and iPP increased by 33°C when the lignin content was 5 wt% in lignin/iPP composites [49]. Also, another previous report showed that T i of lignin/iPP composite decreased by 7°C under N 2 atmosphere and increased by 23°C when the lignin content was 5 wt% in lignin/ iPP composites [52]. Tis result demonstrated that nano-Lig had a more efective promotion in the thermostability of iPP than lignin. Tese results indicated that nano-Lig could improve the thermostability of iPP. Figures 3(a) and 3(b) show the DSC heating and cooling curves of iPP and nano-Lig/iPP composites, respectively. Te DSC results data are listed in Table 2. Te melting temperature (T m ) of the nano-Lig/iPP composites were comparable with that of iPP, while the melting enthalpy (∆H) of the nano-Lig/iPP composites were higher than that of iPP. Tus, the crystallinity (X c ) of all nano-Lig/iPP composites were higher that of iPP. Compared with iPP, the highest X c of nano-Lig/iPP composites increased by 6.39% when the nano-Lig content was 1 wt% in the nano-Lig/iPP composite. Te crystallization temperature (T c ) of the nano-Lig/iPP composites shifted to a higher temperature compared to iPP. Te results suggested that nano-Lig could serve as a nucleating agent for iPP because 1 wt% nano-Lig/iPP composite showed the highest X c ; thus, the characterization of 1 wt% nano-Lig/iPP composite are shown in the following parts.

Nonisothermal Crystallization Behavior of iPP and
Nano-Lig/iPP Composites. Figure 4(a) shows the DSC crystallization curve of iPP at cooling rates of 2.5°C min −1 , 5.0°C min −1 , 10.0°C min −1 , and 20.0°C min −1 . Te relative crystallinity (X t ) varying with temperature can be expressed by equation (2): where ∆H ∞ is the maximum enthalpy value reached at the end of the nonisothermal crystallization process and ∆H T is the enthalpy evolved as a function of the crystallization temperature (T). T 0 and T ∞ represent the crystallization start and completion temperatures, respectively. Both H ∞ and ∆H T can be acquired through the software of a standard DSC. Using equation (2), the relationship between X t and crystallization temperature for iPP at diferent cooling rates can be obtained and is shown in Figure 4(b). Te relationship between T and the crystallization time (t) is shown in equation (3), where R is the cooling rate.
By using equation (3), the relationship between X t and T (Figure 4(b)) can be converted to the relationship between X t and t for iPP (Figure 4(c)). Figure 4(d) shows the DSC crystallization curve of nano-Lig/iPP at cooling rates of 2.5°C min −1 , 5.0°C min −1 , 10.0°C min −1 , and 20.0°C min −1 . Figures 4(e) and 4(f) show the relationship between X t and Tand the relationship between X t and t of nano-Lig/PP, respectively. When X t � 50%, the half time of crystallization (t 1/2 ) can be obtained from Figures 4(c) and 4(f). Te value of the inverse of t 1/2 refects the relative crystallization rate of iPP and nano-Lig/iPP, i.e., the smaller t 1/2 is, the larger the inverse of t 1/2 is and the faster relative crystallization rate is. Te crystallization kinetic iPP Nano-Lig 0.5wt% nano-Lig/iPP 1.0wt% nano-Lig/iPP 1.5wt% nano-Lig/iPP 2.0wt% nano-Lig/iPP  parameters of iPP and nano-Lig/iPP are obtained from Figure 4 and shown in Table 3. It was found that T c of iPP and nano-Lig/ iPP decreased with the increase of the cooling rate due to the thermal hysteresis phenomenon of iPP. At the same cooling rate, T c of nano-Lig/iPP was higher than that of iPP, and t 1/2 of nano-Lig/iPP was smaller than that of iPP, suggesting that nano-Lig could promote the crystallization of iPP. Te result is consistent with our expectation that a smaller amount of nano-Lig (only 1 wt%) can accelerate the crystallization kinetics because it acts as a nucleating agent. However, a previous report showed that lignin could promote iPP crystallization when the lignin content was 5 wt% in lignin/iPP composites [49].

Analysis of Dynamic Parameters of Avrami.
Using the abovementioned X t and t data, the isothermal crystallization of iPP and nano-Lig/iPP can be described by the modifed Avrami equation (4) [53,54].
where Z t is the crystallization rate constant; R is the cooling rate; and n is the Avrami exponent. To obtain Z t and n values, equations (4) can be converted to (5).  Figure 5, respectively, and shown in Table 4.
Z t Values of iPP and nano-Lig/iPP were comparable at the same cooling rate, at the cooling rate of 10.0°C min −1 and 20.0°C min −1 , and the n values of both iPP and nano-Lig/iPP were larger than 3. At the cooling rate of 2.5°C min −1 and 5.0°C min −1 , the n values of nano-Lig/iPP were larger than 3 and the n values of iPP were smaller than 3. Te results indicated the three-dimensional growth of iPP crystalline units is induced by heterogeneous nucleation. Figure 6 shows the XRD spectra of iPP and 1 wt% nano-Lig/iPP composite. iPP showed typical α-crystal difraction peaks at 2θ � 14.1°, 16.8°, 18.5°, 21.1°, and 21.8°, which was related to (110), (040), (130), (131), and (111) crystalline planes, respectively [55][56][57]. Te characteristic peaks of 1 wt% nano-Lig/iPP composite were similar to those of iPP, indicating that the nano-Lig's efects on the crystallization behavior of iPP were negligible, and the crystalline structure of iPP remained unchanged in the composite.   ) and (e) are relative crystallinity (X t ) verse temperature during the nonisothermal crystallization process at diferent cooling rates for iPP and nano-Lig/ iPP, respectively. (c) and (f ) are the relationship of X t and time during the nonisothermal crystallization process at diferent cooling rates for iPP and nano-Lig/iPP, respectively. Table 3: Nonisothermal crystallization kinetic parameters of iPP and nano-Lig/iPP.

Conclusion
In this work, nano-Lignin (nano-Lig) was prepared successfully by the self-assembly method. Nano-Lig could improve thermostability and crystallization properties of iPP. Te T i of nano-Lig/iPP composite increased by 34.4°C when the nano-Lig content was 2 wt% in nano-Lig/iPP composites. Te crystallinity of iPP increased by 6.39% when 1.0 wt% nano-Lig was added in iPP. Te study of kinetics with nonisothermal crystallization found that nano-Lig can increase the crystallization rate of iPP.

Data Availability
Te research data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that there are no conficts of interest.