Exploring the Effects of MXene on Nonisothermal Crystallization and Melting Behavior of β-Nucleated Isotactic Polypropylene

In this study, one of the commonly used MXene (Ti3C2Tx) and β nucleated isotactic polypropylene (β-iPP)/MXene composites of different compositions were fabricated. The effects of MXene on non-isothermal crystallization and polymorphic behavior of β-iPP/MXene composites were comparatively studied. The non-isothermal crystallization kinetics indicates that for all samples, the lower cooling rates promote composites to crystallize at higher temperatures. When MXene and β-Nucleating agent (β-NA) are added separately, the crystallization temperature of composites shifts towards higher temperatures at all cooling rates. When MXene and β-NA are added simultaneously, the composite shows different cooling rate dependence, and the effects of improving crystallization temperatures is more obvious under rapid cooling. The activation energy of four samples iPP, iPP/MXene, iPP/β-NA, and iPP/MXene/β-NA were −167.5, −185.5, −233.8, and −218.1 kJ/mol respectively, which agree with the variation tendency of crystallization temperatures. The polymorphic behavior analysis obtained from Differential Scanning calorimetry (DSC) is affected by two factors: the ability to form β-crystals and the thermal stability of β-crystals. Because β-crystals tend to recrystallize to α-crystals below a critical temperature, to eliminate the effect of β-α recrystallization, the melting curves at end temperatures Tend = 50 °C and Tend = 100 °C are comparatively studied. The results show that more thermally unstable β-crystals would participate in β-α recrystallization with higher cooling rates. Moreover, thermal stability of β-crystals is improved by adding MXene. To further verify these findings, samples of three different thermal conditions were synthesized and analyzed by DSC, X-Ray Diffraction (XRD), and Polarized Light Optical Microscopy (PLOM), and the results were consistent with the above findings. New understandings of synthesizing β-iPP/MXene composites with adjustable morphologies and polymorphic behavior were proposed.


Introduction
Isotactic polypropylene (iPP), firstly synthesized by Natta et al. in the laboratory and industrialized in 1957 [1], has become one of the most widely applied thermoplastic polymers benefiting from its balance between excellent mechanical properties, versatile processability, and low manufacturing cost [2,3]. iPP is a semicrystalline polymer and shows interesting polymorphic behaviors [4] including monoclinic α-crystal, trigonal βcrystal, triclinic γ-crystal, and smectic forms exhibiting different properties [3,5]. α-crystal is the most stable and is commonly found in iPP, it can be obtained by practical processing conditions [6]. β-crystal is thermodynamically metastable and can only be formed under particular conditions such as crystallization in temperature gradient field [7,8], melt shear field [9], or in the presence of β-nucleating agent (β-NA) [2,10]. Adding β-NA is the investigated the crystallization behavior of Poly(ethylene oxide) (PEO)/Ti 3 C 2 T x composites at different concentrations. They found that the crystallization process is promoted below 0.5 wt% Ti 3 C 2 T x due to the heterogeneous nucleation effect, but slows down with further increasing Ti 3 C 2 T x content because of the rigid confinement network [33]. Similar results were also found in our previous work in studying the non-isothermal crystallization behavior of iPP/Ti 3 C 2 T x composites. When 0.5 wt% Ti 3 C 2 T x was added, the peak crystallization temperature and crystallization rate both increased. Once the concentration of Ti 3 C 2 T x reached 1 wt%, the crystallization process was retarded, which might be caused by the confined network [34].
It is well-known that crystallization behaviors can affect the physical and mechanical properties of composites. Moreover, the practical manufacturing process is usually proceeded under non-isothermal crystallization conditions, understanding the non-isothermal crystallization is of great importance. To our best knowledge, the roles of MXene in the polymorphic behavior, crystalline morphologies of β-iPP at varied cooling conditions had not been investigated yet. Thereby, this work chooses the common MXene Ti 3 C 2 T x and fabricates β-iPP/MXene composites to investigate the non-isothermal crystallization kinetics and polymorphic behavior of composites by differential scanning calorimetry (DSC), X-Ray Diffraction (XRD), and polarized light optical microscopy (PLOM). New understandings in preparing the β-iPP/MXene composites with tunable morphologies and polymorphic behavior were also proposed.
The isotactic polypropylene resin (tradename T38F) was obtained from Lanzhou PetroChemical Corp. (Lanzhou, China). The molecular weight was 347,000 gmol −1 and the average isotacticity was around 97.6%. The β-nucleating agent (β-NA) used in the work was WBG-II, which is a commercial nucleating gent. The general formula of WBG-II is Ca x La 1−x (LIG1) m (LIG2) n , where LIG1 and LIG2 are dicarboxylic acid and amide-type ligands [35,36].

Sample Preparation 2.2.1. Etching of MXene
To selectively etch the precursor Ti 3 AlC 2 , HCl and LiF were used to in situ form HF. The schematic diagram of etching Ti3AlC2 into Ti3C2Tx is indicated in Figure 1. In the first step, 34 mL HCl was slowly added to 33 mL distilled water. A total of 2 g LiF powder was dissolved in the diluted HCl under magnetic stirring for 10 min. Then, 3 g Ti 3 AlC 2 was immersed slowly into the solution and kept under magnetic stirring for 24 h at a temperature of 40 • C to allow full reaction. When the etching process was completed, the mixed solution was washed with distilled water and centrifuged at 8000 rpm for 10 min to separate the supernatant from the Ti 3 C 2 T x sediment. This washing-centrifugation cycle was repeated until the pH value of the supernatant reached around 6. The sediment was immersed in distilled water for ultrasonication in the ice bath water for 2 h. The mixture was then centrifuged again to collect the final supernatant and dried under vacuum.

Synthesis of β-iPP/MXene Composites
The composites were prepared by melt blending via a Mini-Lab Extruder (HAAKE MiniLab II, Thermo Fisher Scientific Corp., Waltham, MA, USA). The Ti3C2Tx and β-NA WBG-II were firstly mixed with iPP resin by the Mini-Lab extruder at a temperature of 200 °C and screw speed of 80 rpm to prepare two masterbatches with 2.5 wt% concentration. In the second step, the masterbatches were mixed again with iPP resin to prepare four different samples. The prepared samples were then pressed by a pressure molding machine under 190 °C and 8 MPa for further characterization. To benefit the following discussion, the prepared samples were named as neat iPP, iPP/MXene, iPP/β-NA, and iPP/MXene/β-NA. The concentrations of Ti3C2Tx and WBG-II were fixed at 0.5 wt% and 0.1 wt%, which were chosen based on some previous studies [2,34,37,38]. The schematic diagram of synthesizing the β-iPP/MXene composites is shown in Figure 1.

X-ray Diffraction (XRD)
XRD measurements were performed using a diffractor (Ultima IV, Rigaku, Japan) with a Cu Kα radiation (λ = 0.154 nm) with a voltage of 40 kV and filament current of 40 mA. To measure the spectra of Ti3AlC2 and Ti3C2Tx powders, the scanning range was set to 2θ = 2 − 80° and the scanning rate was 10°/min. Before measuring iPP composites, the samples were firstly hot molded into sheets with 1 mm thickness, and the scanning range was 2θ = 5 − 40° at a scanning rate of 2°/min. The relative content of the β phase (kβ) could be calculated from the XRD spectra via the following equation [12,39]: where Hβ(300) denotes the intensity of (300) reflection of β phase. Hα(110), Hα(040), and Hα(130) denote intensities of the three strongest reflections of α phase.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)
The morphology of Ti3AlC2 and Ti3C2Tx powders were observed by SEM (Apreo S HiVoc, Thermo Fisher Scientific Corp., Waltham, MA, USA) equipped with EDS. The voltage was 5 kV and working distance was 4.9 mm.

Synthesis of β-iPP/MXene Composites
The composites were prepared by melt blending via a Mini-Lab Extruder (HAAKE MiniLab II, Thermo Fisher Scientific Corp., Waltham, MA, USA). The Ti 3 C 2 T x and β-NA WBG-II were firstly mixed with iPP resin by the Mini-Lab extruder at a temperature of 200 • C and screw speed of 80 rpm to prepare two masterbatches with 2.5 wt% concentration. In the second step, the masterbatches were mixed again with iPP resin to prepare four different samples. The prepared samples were then pressed by a pressure molding machine under 190 • C and 8 MPa for further characterization. To benefit the following discussion, the prepared samples were named as neat iPP, iPP/MXene, iPP/β-NA, and iPP/MXene/β-NA. The concentrations of Ti 3 C 2 T x and WBG-II were fixed at 0.5 wt% and 0.1 wt%, which were chosen based on some previous studies [2,34,37,38]. The schematic diagram of synthesizing the β-iPP/MXene composites is shown in Figure 1 To measure the spectra of Ti 3 AlC 2 and Ti 3 C 2 T x powders, the scanning range was set to 2θ = 2 − 80 • and the scanning rate was 10 • /min. Before measuring iPP composites, the samples were firstly hot molded into sheets with 1 mm thickness, and the scanning range was 2θ = 5 − 40 • at a scanning rate of 2 • /min. The relative content of the β phase (k β ) could be calculated from the XRD spectra via the following equation [12,39]: where H β (300) denotes the intensity of (300) reflection of β phase. H α (110), H α (040), and H α (130) denote intensities of the three strongest reflections of α phase. The morphology of Ti 3 AlC 2 and Ti 3 C 2 T x powders were observed by SEM (Apreo S HiVoc, Thermo Fisher Scientific Corp., Waltham, MA, USA) equipped with EDS. The voltage was 5 kV and working distance was 4.9 mm.

Transmission Electron Microscope (TEM)
TEM characterization was conducted on a Tecnai G2 F20 S-TWIN (FEI Corp., Hillsboro, OR, USA) with an accelerating voltage of 200 kV. To observe the structure and dispersion of Ti 3 C 2 T x , the sample was dispersed in distilled water under ultrasonication for 10 min. Then, the solution was dropped on a copper grid for observation.

Differential Scanning Calorimetry (DSC)
A Mettler Toledo DSC3 (Mettler Tolado Corp., Zurich, Switzerland) differential scanning calorimetry was used to perform the nonisothermal crystallization experiments under a continuous nitrogen flow of 50 mL min −1 . For each experiment, the standard procedure was applied as follows: 3-5 mg sample was weighted and heated to 200 • C to erase the previous thermal history. Then, the sample was cooled to end temperature 50 • C at a cooling rate of 5, 10, 20, 30, and 40 • C/min, respectively, and reheated to 200 • C at 10 • C/min to analyze its crystallization and following melting behavior. To ensure the accuracy of the data, the sample was repeatedly tested five to eight times to obtain the average value.
The relative degree of crystallinity (X t ) as a function of temperature (T) can be calculated by the following equation [40,41]: where T 0 and T ∞ are the onset and end crystallization temperature, and dH/dT is the heat flow rate. In nonisothermal crystallization, the time scale t can be transformed from the temperature T by the equation: where ∅ is the cooling rate. Therefore, the graph of the relative degree of crystallinity X t versus time t can be plotted. Avrami equation is generally applied to analyze the isothermal crystallization kinetics by the equation: where X t is the relative degree of crystallinity, t is crystallization time, n is the Avrami exponent, and k is a crystallization rate constant. n usually varies between 1 and 4 and can be influenced by the combined effect of nucleation and growth [42]. The double logarithmic form of this equation is: By plotting the graph of ln[−ln(1 − X t )] vs. lnt, the values of n and lnk can be calculated by fitting the experimental data. Since the Avrami equation describes the isothermal crystallization process, Jeziorny suggested a method to modify the parameter k by introducing the cooling rate ∅ to describe the nonisothermal crystallization: The relative percentage crystallinity of β phase (β c ) was calculated by the following equation: Polymers 2021, 13, 3815 6 of 20 where (1 − λ) is the crystallinity of each phase and is calculated by ∆H/∆H u . ∆H and ∆H u are the apparent and complete crystalline heats of fusion respectively. The value of ∆H u for 100% crystalline iPP is 209 J/g [43][44][45].

Polarized Light Optical Microscopy (PLOM)
The crystalline morphologies of samples were investigated by PLOM (Eclipse LV100 POL, Nikon, Tokyo, Japan) coupled with a hot-stage (Linkam Scientific Instruments Ltd., Tad-worth, UK). A small piece of the sample was cut and placed between glass covers, melted at 200 • C [46] for 5 min, and cooled slowly to allow full crystallization. Then, the film samples were observed by PLOM.

Morphology and Structure of MXene
As shown in Figure 2a, the MXene particles dispersed in distilled water contain flakes with lateral sizes around a few hundred nanometers. In addition, the X-ray spectra in Figure 2b indicates that a sharp (002) diffraction peak has shifted from 2θ ≈ 9.6 • to 6.4 • after etching, suggesting an expanded interlayer distance. The disappearance of the most intense (104) diffraction peak at 2θ ≈ 39 • , which is representative of Ti 3 AlC 2 , further confirms the complete etching. The crystalline morphologies of samples were investigated by PLOM (Eclipse LV100 POL, Nikon, Tokyo, Japan) coupled with a hot-stage (Linkam Scientific Instruments Ltd., Tad-worth, UK). A small piece of the sample was cut and placed between glass covers, melted at 200 °C [46] for 5 min, and cooled slowly to allow full crystallization. Then, the film samples were observed by PLOM.

Morphology and Structure of MXene
As shown in Figure 2a, the MXene particles dispersed in distilled water contain flakes with lateral sizes around a few hundred nanometers. In addition, the X-ray spectra in Figure 2b indicates that a sharp (002) diffraction peak has shifted from 2θ ≈ 9.6° to 6.4° after etching, suggesting an expanded interlayer distance. The disappearance of the most intense (104) diffraction peak at 2θ ≈ 39°, which is representative of Ti3AlC2, further confirms the complete etching. SEM was conducted to observe the morphologies of bulk Ti3AlC2 and Ti3C2Tx. As shown in Figure 3a, the bulk MAX phase Ti3AlC2 exhibits a compact layered structure in which the flakes were closely stacked, and this particular structure can often be observed in ternary carbides [47]. After the selective etching process was completed, the flakes are weakly stacked and the interlayer distance increases. This morphology is also named accordion-like morphology. The expanded layered structure agrees well with the results of XRD and is possibly caused by escaped gas such as H2 during the etching process due to the exothermic reaction between HF and Al [48,49]. SEM was conducted to observe the morphologies of bulk Ti 3 AlC 2 and Ti 3 C 2 T x . As shown in Figure 3a, the bulk MAX phase Ti 3 AlC 2 exhibits a compact layered structure in which the flakes were closely stacked, and this particular structure can often be observed in ternary carbides [47]. After the selective etching process was completed, the flakes are weakly stacked and the interlayer distance increases. This morphology is also named accordion-like morphology. The expanded layered structure agrees well with the results of XRD and is possibly caused by escaped gas such as H 2 during the etching process due to the exothermic reaction between HF and Al [48,49].
shown in Figure 3a, the bulk MAX phase Ti3AlC2 exhibits a compact layered structure in which the flakes were closely stacked, and this particular structure can often be observed in ternary carbides [47]. After the selective etching process was completed, the flakes are weakly stacked and the interlayer distance increases. This morphology is also named accordion-like morphology. The expanded layered structure agrees well with the results of XRD and is possibly caused by escaped gas such as H2 during the etching process due to the exothermic reaction between HF and Al [48,49].

Nonisothermal Crystallization Behavior of β-iPP/MXene Composites
The cooling curves of the four samples are plotted in Figure 4, and crystallization parameters including peak crystallization temperature (T c ), onset and end crystallization temperatures (T conset , T cend ), and crystallization peak width (T conset -T cend ) are plotted in Figure 5. The larger the T conset -T cend , the greater the crystallization temperature range [43,50].

Nonisothermal Crystallization Behavior of β-iPP/MXene Composites
The cooling curves of the four samples are plotted in Figure 4, and crystallization parameters including peak crystallization temperature (Tc), onset and end crystallization temperatures (Tconset, Tcend), and crystallization peak width (Tconset-Tcend) are plotted in Figure  5. The larger the Tconset-Tcend, the greater the crystallization temperature range [43,50]. Firstly, it is found that for all samples, the lower the cooling rate is, the larger the value of Tc, Tconset, and Tcend. In other words, a lower cooling rate enables the sample to crystallize at a higher temperature. Moreover, the crystallization peak width Tconset-Tcend Interestingly, when MXene and β-NA are added simultaneously, the crystallization parameters of iPP/MXene/β-NA exhibit a different cooling rate dependence from other samples. At a lower cooling rate (≤10 °C/min), Tc and Tconset-Tcend are both similar to iPP/MXene, indicating that under the combined effect of MXene and β-NA, the crystallization ability is not further improved. However, when the cooling rate increases (>10 °C/min), Tc and Tcend are higher than iPP/MXene but lower than iPP/β-NA, and the crystallization peak width is lower than iPP/MXene. This finding suggests that adding MXene and β-NA improve the crystallization temperature while narrowing the crystallization peak, and this effect is found more obvious under rapid cooling. The cumulative relative degree of crystallinity (Xt) as a function of time curves at different cooling rates were plotted in Figure 6 from which the half crystallization time t1/2 is calculated when Xt is 50%. The value of t1/2 can be used as a measure of crystallization rate and ability. In general, the larger the value of t1/2 is, the longer the time needed for the crystallization process. The plotted curves in Figures 6 and 7 show that t1/2 of neat iPP is higher than the other three composites at all cooling rates, suggesting its crystallization rate is the lowest. t1/2 of iPP/MXene/β-NA at lower cooling rates is almost the same as that of iPP/β-NA, while increases significantly at higher cooling rates, proving that the crystallization ability of iPP/MXene/β-NA is not as good as iPP/β-NA. This result agrees with findings from DSC cooling curves. Firstly, it is found that for all samples, the lower the cooling rate is, the larger the value of T c , T conset, and T cend . In other words, a lower cooling rate enables the sample to crystallize at a higher temperature. Moreover, the crystallization peak width T conset -T cend decreases with a lower cooling rate, which indicates that the polymer chains get sufficient time to rearrange into crystalline phase. The variation trend for the four samples is slightly different. From Figure 5, it can be observed that the crystallization parameters T c , T conset, and T cend for neat iPP are the lowest, while the crystallization peak width is the largest. Neat iPP has the weakest crystallization ability among all samples, so it crystallizes at lower temperatures. When adding MXene, T c , T conset , and T cend increase, but the crystallization peak width T conset -T cend remains nearly unchanged, suggesting that the crystallization ability is improved at all cooling rates by adding MXene. The improved crystallization ability of iPP/MXene might be attributed to the heterogeneous nucleation effect of MXene [34]. When β-NA is added, T c , T conset, and T cend of iPP/β-NA are the highest while T conset -T cend is the smallest, indicating that the addition of β-NA can greatly improve the crystallization ability of iPP, and the effect is more significant than the addition of MXene. This finding is consistent with the previous research results [51].
Interestingly, when MXene and β-NA are added simultaneously, the crystallization parameters of iPP/MXene/β-NA exhibit a different cooling rate dependence from other samples. At a lower cooling rate (≤10 • C/min), T c and T conset -T cend are both similar to iPP/MXene, indicating that under the combined effect of MXene and β-NA, the crystallization ability is not further improved. However, when the cooling rate increases (>10 • C/min), T c and T cend are higher than iPP/MXene but lower than iPP/β-NA, and the crystallization peak width is lower than iPP/MXene. This finding suggests that adding MXene and β-NA improve the crystallization temperature while narrowing the crystallization peak, and this effect is found more obvious under rapid cooling. The cumulative relative degree of crystallinity (X t ) as a function of time curves at different cooling rates were plotted in Figure 6 from which the half crystallization time t 1/2 is calculated when X t is 50%. The value of t 1/2 can be used as a measure of crystallization rate and ability. In general, the larger the value of t 1/2 is, the longer the time needed for the crystallization process. The plotted curves in Figures 6 and 7 show that t 1/2 of neat iPP is higher than the other three composites at all cooling rates, suggesting its crystallization rate is the lowest. t 1/2 of iPP/MXene/β-NA at lower cooling rates is almost the same as that of iPP/β-NA, while increases significantly at higher cooling rates, proving that the crystallization ability of iPP/MXene/β-NA is not as good as iPP/β-NA. This result agrees with findings from DSC cooling curves.    The results from the Avrami method are reported in Table 1, where the value of n strongly depends on the mechanism of the crystal growth and k can be used as a measure The results from the Avrami method are reported in Table 1, where the value of n strongly depends on the mechanism of the crystal growth and k can be used as a measure of crystallization rate. n for samples at all cooling rates varies between 2.5 and 3.2. The values can be attributed to heterogeneous nucleation and three-dimensional growth in crystallites [52,53], suggesting that the addition of the above fillers does not alter the crystallization mechanism of iPP. In addition, it can be seen that the value of lnk increases after the addition of MXene and β-NA, indicating that the crystallization rate is improved. For iPP/MXene/β-NA, the value of lnk is higher than iPP/MXene, but lower than iPP/β-NA, and this variation trend is the same as t 1/2 .
The nonisothermal crystallization activation energy E c represents the energy barrier of crystallization. In general, the larger the E c is, the more difficult for the occurrence of the crystallization process [54,55]. The Kissinger plot of studied samples is shown in Figure 8, the slope can be used to calculate the activation energy of the sample. Table 1 revealed that after the addition of MXene and β-NA, the crystallization energy barrier is lowered and the crystallization becomes easier. However, when MXene and β-NA are added simultaneously, the energy barrier increases again suggesting that the crystallization process becomes more difficult. Combining the other crystallization parameters and previous study [51], the results suggest that there exists a competitive relationship between MXene and β-NA as fillers.

Melting and Polymorphic Behavior of β-iPP/MXene Composites
The subsequent melting curves at a rate of 10 °C/min of four samples are shown in Figure 9. It can be observed that both iPP and iPP/MXene show only one melting peak between 160-170 °C, therefore the addition of MXene promotes the crystallization of αcrystals. At the same time, when β-NA is added, iPP/β-NA and iPP/MXene/β-NA show β-crystals melting peaks between 140-155 °C. The melting behaviors of iPP/β-NA and iPP/MXene/β-NA show different dependence on cooling rates. For iPP/β-NA, as the cooling rate increases, the β peak gradually splits into two independent peaks, proving the formation of β-crystals with different crystalline perfections. On the contrary, this phenomenon is found weaker in iPP/MXene/β-NA, suggesting that the melting behavior of

Melting and Polymorphic Behavior of β-iPP/MXene Composites
The subsequent melting curves at a rate of 10 • C/min of four samples are shown in Figure 9. It can be observed that both iPP and iPP/MXene show only one melting peak between 160-170 • C, therefore the addition of MXene promotes the crystallization of α-crystals. At the same time, when β-NA is added, iPP/β-NA and iPP/MXene/β-NA show β-crystals melting peaks between 140-155 • C. The melting behaviors of iPP/β-NA and iPP/MXene/β-NA show different dependence on cooling rates. For iPP/β-NA, as the cooling rate increases, the β peak gradually splits into two independent peaks, proving the formation of β-crystals with different crystalline perfections. On the contrary, this phenomenon is found weaker in iPP/MXene/β-NA, suggesting that the melting behavior of iPP/MXene/β-NA might be less dependent on the cooling rates.
Ec by Kissinger method.

Melting and Polymorphic Behavior of β-iPP/MXene Composites
The subsequent melting curves at a rate of 10 °C/min of four samples are shown in Figure 9. It can be observed that both iPP and iPP/MXene show only one melting peak between 160-170 °C, therefore the addition of MXene promotes the crystallization of αcrystals. At the same time, when β-NA is added, iPP/β-NA and iPP/MXene/β-NA show β-crystals melting peaks between 140-155 °C. The melting behaviors of iPP/β-NA and iPP/MXene/β-NA show different dependence on cooling rates. For iPP/β-NA, as the cooling rate increases, the β peak gradually splits into two independent peaks, proving the formation of β-crystals with different crystalline perfections. On the contrary, this phenomenon is found weaker in iPP/MXene/β-NA, suggesting that the melting behavior of iPP/MXene/β-NA might be less dependent on the cooling rates. The relative percentage crystallinity of β-phase (βc) is calculated by Equation (7) and shown in Figure 11. Figure 11a shows that at Tend = 50 °C, βc gradually decreases with increasing cooling rate, which seems that a lower cooling rate is more beneficial for the formation of high content β-crystals. However, the βc calculated from DSC is affected by two factors. One is the ability to form β-crystals, i.e., the amount of β-crystals. The second factor is the thermal stability of β-crystals. According to the study of Varga et al. [56][57][58], during the partial melting of β-crystals, β-iPP cooled below a critical temperature (Tend = 100-105 °C) would recrystallize into α-iPP in the subsequent melting behavior. The exothermic recrystallization peak of β-crystals coincides with the endothermic melting peak, which would interfere with the accuracy of βc on DSC curves. To comparatively study the polymorphic behavior at different cooling rates, we compared the melting curves at two end crystallization temperatures Tend = 50 °C and Tend = 100 °C, the melting curves and calculated βc are plotted in Figures 10 and 11. The relative percentage crystallinity of β-phase (β c ) is calculated by Equation (7) and shown in Figure 11. Figure 11a shows that at T end = 50 • C, β c gradually decreases with increasing cooling rate, which seems that a lower cooling rate is more beneficial for the formation of high content β-crystals. However, the β c calculated from DSC is affected by two factors. One is the ability to form β-crystals, i.e., the amount of β-crystals. The second factor is the thermal stability of β-crystals. According to the study of Varga et al. [56][57][58], during the partial melting of βcrystals, β-iPP cooled below a critical temperature (T end = 100-105 • C) would recrystallize into α-iPP in the subsequent melting behavior. The exothermic recrystallization peak of β-crystals coincides with the endothermic melting peak, which would interfere with the accuracy of β c on DSC curves. To comparatively study the polymorphic behavior at different cooling rates, we compared the melting curves at two end crystallization temperatures T end = 50 • C and T end = 100 • C, the melting curves and calculated β c are plotted in Figures 10 and 11. during the partial melting of β-crystals, β-iPP cooled below a critical temperature (Tend = 100-105 °C) would recrystallize into α-iPP in the subsequent melting behavior. The exothermic recrystallization peak of β-crystals coincides with the endothermic melting peak, which would interfere with the accuracy of βc on DSC curves. To comparatively study the polymorphic behavior at different cooling rates, we compared the melting curves at two end crystallization temperatures Tend = 50 °C and Tend = 100 °C, the melting curves and calculated βc are plotted in Figures 10 and 11. At Tend = 50 °C, it is found in Figure 11a that when the cooling rate decreases, the βcrystals content would increase. To eliminate the effects of thermal stability of β-crystals, Tend is set to 100 °C so that no β-α recrystallization would occur. At Tend = 100 °C, the βcrystals melting peaks in Figure 10 are wider and larger, suggesting that the β-crystal content is also higher. The possible reason behind is at Tend = 100 °C the interference of recrystallization is eliminated, so the β-crystals content is much higher. Secondly, Figure 11b indicates that βc increases with higher cooling rates. After β-α recrystallization interference is removed, rapid cooling favors the formation of more β-crystals. When MXene is added, β crystallization is inhibited due to the competitive effect between MXene and β-NA, while the cooling rate dependence remains unchanged. Figure 11c shows the βc difference at Tend = 100 °C and Tend = 50 °C, when cooling rate increases, the difference between βc is higher. The possible explanation is proposed as the following, when the cooling rate increases more thermally unstable β-crystals tend to participate in β-α recrystallization. Moreover, when MXene is added, βc decreases indicating that the addition of MXene can decrease the content of β-crystals with low thermal stability. From melting curves, it can be summarized that the crystallization behavior and polymorphic behavior of MXene/β-iPP composites can be influenced by cooling rates and thermal conditions.

Effects of Thermal Conditions
In order to comprehensively understand the influences of thermal conditions on crystallization and polymorphic behavior of the composites, three groups of samples with different thermal histories were molded and named Fast Cooling Rate, Medium Cooling Rate, and Slow Cooling Rate.

PLOM Observation
The crystallization morphology of the samples was observed by PLOM and the photos are shown in Figure 12. It is found that iPP and iPP/MXene exhibit spherulites morphology that corresponds to the α-crystals. When the cooling rate increases, crystal nucleation is more favorable while crystal growth is confined. As a result, it is observed that the spherulite size decreases gradually with increasing cooling rates. At T end = 50 • C, it is found in Figure 11a that when the cooling rate decreases, the β-crystals content would increase. To eliminate the effects of thermal stability of β-crystals, T end is set to 100 • C so that no β-α recrystallization would occur. At T end = 100 • C, the β-crystals melting peaks in Figure 10 are wider and larger, suggesting that the β-crystal content is also higher. The possible reason behind is at T end = 100 • C the interference of recrystallization is eliminated, so the β-crystals content is much higher. Secondly, Figure 11b indicates that β c increases with higher cooling rates. After β-α recrystallization interference is removed, rapid cooling favors the formation of more β-crystals. When MXene is added, β crystallization is inhibited due to the competitive effect between MXene and β-NA, while the cooling rate dependence remains unchanged. Figure 11c shows the β c difference at T end = 100 • C and T end = 50 • C, when cooling rate increases, the difference between β c is higher. The possible explanation is proposed as the following, when the cooling rate increases more thermally unstable β-crystals tend to participate in β-α recrystallization. Moreover, when MXene is added, β c decreases indicating that the addition of MXene can decrease the content of β-crystals with low thermal stability. From melting curves, it can be summarized that the crystallization behavior and polymorphic behavior of MXene/β-iPP composites can be influenced by cooling rates and thermal conditions.

Effects of Thermal Conditions
In order to comprehensively understand the influences of thermal conditions on crystallization and polymorphic behavior of the composites, three groups of samples with different thermal histories were molded and named Fast Cooling Rate, Medium Cooling Rate, and Slow Cooling Rate.

PLOM Observation
The crystallization morphology of the samples was observed by PLOM and the photos are shown in Figure 12. It is found that iPP and iPP/MXene exhibit spherulites morphology that corresponds to the α-crystals. When the cooling rate increases, crystal nucleation is more favorable while crystal growth is confined. As a result, it is observed that the spherulite size decreases gradually with increasing cooling rates. Figure 12c,d indicate that after addition of β-NA, a large number of β-crystals appear. In iPP/β-NA, the β-crystals dominate, and crystal size decreases gradually with increasing cooling rates. At slow cooling rate, iPP/MXene/β-NA exhibits obvious and well-grown α-crystals, which decrease in size as the cooling rate increases. The PLOM results agree well with the previous findings.

DSC Analysis
To analyze the polymorphic behavior, the melting curves of iPP/β-NA and iPP/ MXene/β-NA at a cooling rate of 10 • C/min are shown in Figure 13 and the β c calculated from the curves as a function of cooling rate is plotted in Figure 14. It is found that as the cooling rate increases, the β-crystals peak becomes weaker and β c decreases accordingly. Furthermore, the β-crystal content in iPP/β-NA is higher than iPP/MXene/β-NA at all cooling rates. This variation trend in β c as a function of cooling rate is consistent with the results from nonisothermal crystallization analysis in the previous part.

DSC Analysis
To analyze the polymorphic behavior, the melting curves of iPP/β-NA and iPP/MXene/β-NA at a cooling rate of 10 °C/min are shown in Figure 13 and the βc calculated from the curves as a function of cooling rate is plotted in Figure 14. It is found that as the cooling rate increases, the β-crystals peak becomes weaker and βc decreases accordingly. Furthermore, the β-crystal content in iPP/β-NA is higher than iPP/MXene/β-NA at all cooling rates. This variation trend in βc as a function of cooling rate is consistent with the results from nonisothermal crystallization analysis in the previous part.  The XRD results of iPP/β-NA and iPP/MXene/β-NA were further compared in Figure  15. From the XRD patterns, four main peaks can be found at 2θ ≈ 14.1°, 16.1°, 16.9°, and 18.6° that correspond to α (110), β (300), α (040), and α (130) diffractions [45,59,60]. Since XRD is performed at room temperature, during which the effects of β-α recrystallization

X-ray Diffraction (XRD)
The XRD results of iPP/β-NA and iPP/MXene/β-NA were further compared in Figure 15. From the XRD patterns, four main peaks can be found at 2θ ≈ 14.1 • , 16.1 • , 16.9 • , and 18.6 • that correspond to α (110), β (300), α (040), and α (130) diffractions [45,59,60]. Since XRD is performed at room temperature, during which the effects of β-α recrystallization can be excluded. Therefore, the k β value calculated from XRD patterns at all cooling rates is higher than β c value calculated from DSC melting curves. Apart from that, when the cooling rate increases, the k β decreases from 91.3 to 82.8% in iPP/β-NA and from 90.0 to 79.1 % in iPP/MXene/β-NA. This trend also agrees with findings from DSC melting curves.

X-ray Diffraction (XRD)
The XRD results of iPP/β-NA and iPP/MXene/β-NA were further compared in Figure  15. From the XRD patterns, four main peaks can be found at 2θ ≈ 14.1°, 16.1°, 16.9°, and 18.6° that correspond to α (110), β (300), α (040), and α (130) diffractions [45,59,60]. Since XRD is performed at room temperature, during which the effects of β-α recrystallization can be excluded. Therefore, the kβ value calculated from XRD patterns at all cooling rates is higher than βc value calculated from DSC melting curves. Apart from that, when the cooling rate increases, the kβ decreases from 91.3 to 82.8% in iPP/β-NA and from 90.0 to 79.1 % in iPP/MXene/β-NA. This trend also agrees with findings from DSC melting curves.

Conclusions
The effects of MXene on nonisothermal crystallization behavior β-iPP/MXene composites were investigated. The DSC analysis shows that the crystallization ability of the samples follows the order iPP/β-NA, iPP/MXene/β-NA, iPP/MXene, iPP. When MXene and β-NA are added separately, the crystallization temperature of composites increases with lower cooling rates. However, when MXene and β-NA are added simultaneously, they tend to compete with each other. Due to the low thermal stability of β-crystals, the melting behaviors at two end temperatures T end = 50 • C and T end = 100 • C show different variation tendency. When the crystallization end temperature T end = 50 • C, a lower cooling rate leads to more formation β-crystals. But this trend is reversed at T end = 100 • C because more unstable β-crystals would participate in β-α recrystallization with higher cooling rates. Moreover, the experimental phenomenon that k β calculated from XRD is higher than β c calculated from DSC at all cooling rates further proves the β-α recrystallization. The study paves a method to prepare β-iPP/MXene composites with adjustable crystallization and polymorphic behavior, which can provide new insight to develop high performance iPP/MXene composites with excellent mechanical properties in future work.