Introduction

Recently, polymer nanocomposites became a subject of many studies and applications because it is believed that the tremendous interfacial area between nanoparticles and polymer helps to influence the composite’s properties to a great extent [1]. Many studies have shown that polypropylene (PP) nanocomposites exhibited enhanced strength, toughness, heat distortion temperatures, flame resistance, barrier properties and decreased thermal expansion coefficient [2–8]. Of these nanocomposites, PP/CaCO3 nanocomposites have attracted considerable interests because of the availability of nano-CaCO3 in readily usable form and low cost. Therefore, PP composites filled by nano-CaCO3 have been extensively studied [9–22]. Yang et al. [12] reported that nano-CaCO3 particles had better toughening effect on improving the impact strength of PP matrix than micro-CaCO3 particles. Weon et al. [13] reported that addition of nano-CaCO3 reduced the size of spherulites of PP and induced the formation of β-phase crystallites, which led to a more ductile PP matrix and improved the impact strength of PP. Similar result was observed by Mai et al. [11]. Chan et al. [14] also found that nano-CaCO3 particle was a very effective nucleating agent for PP.

However, nanoparticles tend to agglomerate in the polymer matrix due to the tremendous surface area and high surface free energy. Aggregation has a detrimental effect on the properties of polymer composites [10, 15]. In order to improve the dispersion of the nanoparticles, the coupling agents (e.g. silane and titanate) and macromolecular compatibilizers were used to modify the surface of the particles. Surface treatment changes the interfacial interaction between particles and polymer matrix, and then influences the crystallization and melting behavior of polymer composites. It is found that nano-CaCO3 particles treated with nonionic modifier increased the crystallization rate of PP matrix [11]. Similar result was obtained when surface modified with a coupling agent [22]. However, a contrary result was found by Avella et al. [16] that addition of nano-CaCO3 coated with fatty acids delays crystallization of PP. Avella et al. [16, 17] also demonstrated that addition of PP-g-MA enhanced the heterogeneous nucleation ability of nano-CaCO3 particles by improving the interfacial interaction between CaCO3 particles and PP. Mai et al. [20] found that the PP-g-AA could improve the heterogeneous nucleation effect of nano-CaCO3 and further increased the crystallization temperature of PP. Surface treatment also influence the mechanical properties of PP/nano-CaCO3 composites. Mai et al. [11] found the addition of a nonionic modifier greatly improved the Izod impact energy of PP/CaCO3 nanocomposites due to lowering the particle–polymer interaction. Ma et al. [21] demonstrated that an obvious synergistic effect between the grafted PBA and nano-CaCO3 led to a significant increment in notched impact strength duo to the chemical bonding between the elastomer grafted PBA and nano-CaCO3.

Many investigations indicated that the interfacial interaction among polymer composites plays an important role in determining the physical and mechanical properties of polymer composites. Thus, the study on the relationship between the interfacial interaction and the properties of polymer composite is of great importance. Although polymer composites with desired structures and properties can be obtained by selecting appropriate surface treatment agent or macromolecular compatibilizer to control the interfacial interaction between inorganic particles and polymer matrix, the surface treatment agent, including low molecule surfactants and macromolecular compatibilizer, was generally considered as a whole interface in many investigations. In fact, when modified by polar macromolecular compatibilizer, a core-shell structure with particles encapsulated by compatibilizer would be formed in polymer matrix, resulting in the formation of two interfaces: the interface between particles and compatibilizer as well as the interface between compatibilizer and polymer matrix. Few studies have done to investigate the effect of these two interfaces on the properties of PP composites [23].

In order to investigate the effect of interfacial interaction on the crystallization behavior of PP/nano-CaCO3 composites, three kinds of compatibilizers (PP-g-MA, POE-g-MA and EVA-g-MA) with the same polar groups (MA) but different backbones were used as compatibilizers to obtain the different interfacial interactions. It is believed that the MA polar group of compatibilizers would associate with the surface of nano-CaCO3 particles by polarity–polarity interaction, resulting in the formation of the similar interfacial interaction between nano-CaCO3 particles and compatibilizers. The different miscibility between the macromolecular chain of compatibilizer and PP matrix results in the formation of different interfacial interaction between PP matrix and compatibilizer. The effect of interfacial interaction between PP matrix and compatibilizer on the crystallization and melting behavior as well as morphology of PP/nano-CaCO3 composites was investigated in this paper.

Experimental

Materials

Polypropylene (EPS30R), ethylene content 2.87%, MFI = 2.1 g/10 min (2.16 kg at 230 °C), was supplied by Dushanzi Petroleum Chemical, China. Polypropylene grafted with maleic anhydride (PP-g-MA), grafting ratio 1.0%, MFI > 15 g/10 min (2.16 kg at 230 °C); Ethylene–octene copolymer grafted with maleic anhydride (POE-g-MA), grafting ratio 1.1%, MFI = 0.72 g/10 min (2.16 kg at 230 °C); Ethylene–vinyl acetate copolymer grafted with maleic anhydride (EVA-g-MA), grafting ratio 1.0%, MFI = 2.46 g/10 min (2.16 kg at 230 °C). The compatibilizers were provided by Guangzhou Lushan Chemical Materials Co. China. Nano-CaCO3 (CC), with particle size of 70–90 nm was obtained from Shiraishi Kogyo Kaisha LTD, Japan.

Sample preparation

All materials were dried in an oven at 60 °C for 12 h. PP/nano-CaCO3 composites with and without compatibilizers were prepared using a Berstoff ZE25A corotating twin-screw extruder. The blending temperature was set at 200 °C. All the materials were simultaneously added into the extruder after previous mixing. The compositions for the specimens are listed in Table 1.

Table 1 The compositions for PP/nano-CaCO3 composites
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Methods

The crystallization behavior of specimens was examined using Perkin–Elmer DSC-7 differential scanning calorimetry (DSC) under nitrogen atmosphere. The heat flow and temperatures of DSC were calibrated with standard materials, indium and zinc. The weight of specimens is about 5 mg. For non-isothermal crystallization, the specimens were rapidly heated to 220 °C at a rate of 200 °C/min and melted for 3 min to erase the thermal history, and then cooled down to 50 °C at a rate of 10 °C/min. The crystallized specimens were re-heated at a rate of 10 °C/min to investigate the melting behavior of PP. For isothermal crystallization, all the specimens were heated to 220 °C at a rate of 200 °C/min and melted for 3 min, and then rapidly cooled down at a rate of 200 °C/min to different designed crystallization temperatures (T c) for 30 min. The crystallized specimens were heated at rate of 10 °C/min to investigate the corresponding melting behavior of PP.

A Rigaku D/max-2200 VPC X-ray diffractometer with the Cu Kα radiation at a voltage of 40 kV and a current of 30 mA was used for Wide angle X-ray diffraction (WAXD) experiments. The scan speed was 4°/min in a range of 2θ = 5–40° at ambient temperature. The specimens were pre-treated on Perkin–Elmer DSC-7 thermal system, heating rapidly to 220 °C, holding for 3 min, and then cooling to 50 °C at rate of 10 °C/min.

The spherulitic morphology of specimens was observed with a LEIT2, Orthoplan Pol polarized optical microscope (POM) equipped with a crossed polarizer and a hot stage. The ~10 μm thick films cut perpendicularly to the injection molding direction of the rectangular bars and was melted at 200 °C for 3 min via the hot stage, and then the specimen was cooled down to room temperature at a rate of 10 °C/min.

Results and discussion

Non-isothermal crystallization and melting behavior of PP/nano-CaCO3 composites

Figure 1 shows non-isothermal crystallization and melting curves of PP and PP/nano-CaCO3 composites, the crystallization and melting parameters are listed in Table 2. It can be seen that a small amount of nano-CaCO3 can increase the crystallization temperature (T c) of PP about 3–4 °C due to the heterogeneous nucleation of nano-CaCO3. Many other nanopaticles were also found to have nucleation effect for PP crystallization [24, 25]. However, the T c of PP changed little with increasing nano-CaCO3 content. It is suggested that there would be a saturated content for the heterogeneous nucleation of nano-CaCO3 for PP crystallization in PP/nano-CaCO3 composites, just as demonstrated by Mai et al. [23, 26] in the study of the crystallization behavior of PP/Mg(OH)2 composite. Addition of nano-CaCO3 also causes a slight increase in the melting temperature of PP, which indicates the formation of more perfect crystal of PP. Mai et al. [18] found that nano-CaCO3 could induce PP to form β-crystal and there was a melting peak of β-crystal at about 150 °C. However, there are no melting peaks at about 150 °C in the melting curves of PP/nano-CaCO3 composites (Fig. 1b), indicating that nano-CaCO3 could not induce the formation of β-crystal of PP in this study.

Fig. 1
figure 1

DSC a crystallization and b melting curves of PP and PP/nano-CaCO3 composites

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Table 2 DSC results of non-isothermal crystallization and melting of PP/compatibilizer blends and PP/nano-CaCO3 compositesa
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Non-isothermal crystallization and melting behavior of PP/compatibilizer blends

Three compatibilizers (PP-g-MA, POE-g-MA and EVA-g-MA) with the same polar grafted groups (MA) but different backbones were used to modify PP/nano-CaCO3 composites. The crystallization and melting parameters of these compatibilizers are listed in Table 2. The crystallization temperature (T c), melting temperature (T m) and crystallization enthalpy (ΔH c) of PP-g-MA are close to those of PP matrix. POE-g-MA and EVA-g-MA can also crystallize, however, the T c, T m and ΔH c of POE-g-MA and EVA-g-MA are much lower than those of PP matrix.

Figure 2 shows the non-isothermal crystallization and melting curves of PP/compatibilizer blends, the crystallization and melting parameters are listed in Table 2. It can be observed from Fig. 2 and Table 2 that different compatibilizer has different influence on the crystallization behavior of PP matrix. Although the T c of PP-g-MA is lower than that of PP, addition of PP-g-MA slightly increases the T c of PP matrix. However, addition of POE-g-MA and EVA-g-MA with low T c decreases the T c of PP matrix. The increase of T c of PP by adding PP-g-MA indicates that the compatibilizer has a heterogeneous nucleation for PP crystallization [27]. However, these heterogeneous nuclei formed by the heterogeneous nucleation of compatibilizer are located in the phase of compatibilizer. As a result, the heterogeneous nucleation of these heterogeneous nuclei in the phase of compatibilizer depends on the miscibility between compatibilizer and PP matrix.

Fig. 2
figure 2

DSC a crystallization and b melting curves of PP and PP/compatibilizer blends

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Although the miscibility of PP-MA with PP generally decreases with the increase of MA content in the copolymer [28, 29], the PP chains in PP-g-MA are high miscible with PP matrix. Therefore the heterogeneous nucleation of the heterogeneous nuclei in PP-g-MA can induce the crystallization of PP matrix and increase the T c of PP matrix. However, the POE chain in POE-g-MA is partially miscible with PP matrix [30] and the EVA chain in the EVA-g-MA is considered immiscible with PP matrix due to the strong polar acetate group [31]. The decreased miscibility between compatibilizer and PP matrix restricts the heterogeneous nucleation of the heterogeneous nuclei in the phase of POE-g-MA or EVA-g-MA for PP crystallization. Furthermore, the partially miscible and/or immiscible components may disturb the crystallization of PP matrix [32]. Thus, the T c of PP matrix in PP/POE-g-MA or EVA-g-MA blends is lower than that of neat PP.

Non-isothermal crystallization and melting behavior of PP/nano-CaCO3 composites modified by compatibilizers

The non-isothermal crystallization and melting curves of PP in PP/nano-CaCO3 composites modified by different compatibilizer are shown in Fig. 3. Table 2 gives the crystallization and melting parameters of these modified PP/nano-CaCO3 composites. It can be observed from Fig. 3 and Table 2 that addition of PP-g-MA significantly increases the T c of PP in PP/nano-CaCO3 composites. The T c of PP in PP-10a is approximately 8 °C higher than that of PP and 4 °C higher than that in PP-10. Addition of POE-g-MA slightly increases the T c of PP in PP/nano-CaCO3 composite, the T c of PP-10b is about 5 °C higher than that of PP and 1 °C higher than that in PP-10. However, addition of EVA-g-MA reduces the T c of PP in PP/nano-CaCO3 composite and the T c of PP-10c is even lower than that of PP.

Fig. 3
figure 3

DSC a crystallization and b melting curves of PP and PP/nano-CaCO3 composites with and without compatibilizers

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When the compatibilizer is added into the PP/nano-CaCO3 composite, nano-CaCO3 particles will be encapsulated by compatibilizer due to the strong polar interaction between nano-CaCO3 particles and compatibilizer, resulting in the formation of compatibilizer phase between nano-CaCO3 and PP matrix. Two kinds of interface were formed: the interface between PP and compatibilizer, as well as the interface between particles and compatibilizer. Addition of PP-g-MA and POE-g-MA increases the T c of PP in PP/nano-CaCO3 composites, indicating that there exist a synergism of heterogeneous nucleation in the interface between compatibilizer and nano-CaCO3 for PP crystallization. It is suggested that the chemical reaction between the polar groups (MA) of compatibilizer and nano-CaCO3 particles in the interface between compatibilizer and nano-CaCO3 results in the formation of carboxylate salts [33]. The carboxylate salts is a more effective nucleating agent than nano-CaCO3 particle. Therefore, addition of compatibilizer increases the T c of PP in PP/nano-CaCO3 composites.

The heterogeneous nucleation of carboxylate salts not only increases the T c of PP, but also induces the formation of β-crystal of PP. The peak at about 150 °C in Fig. 3b is attributed to the melting of β-crystal of PP. The β-crystal of PP is obtained for PP-10a and PP-10b with higher T c of PP. However, no β-crystal of PP was formed in PP, PP-10 and PP-10c with lower T c. The formation of β-crystal of PP is proved by WAXD measurement in Fig. 4. For PP-10a and PP-10b, a peak at 2θ ≈ 16° corresponding to β-crystal of PP is observed except for the characteristic crystalline peaks of α-crystal of PP. The synergism of heterogeneous nucleation in the interface between compatibilizers and nano-CaCO3 for PP crystallization is also proved by optical micrographs in Fig. 5. Addition of nano-CaCO3 particles dramatically increases the number of spherulites and decreases the size of spherulites due to the heterogeneous nucleation of nano-CaCO3 particles. Addition of PP-g-MA or POE-g-MA further decreases the size of PP spherulites, and β-crystals of PP are visible in the optical micrographs of PP-10a and PP-10b (Fig. 5c, d). It is attributed to the heterogeneous nucleation of carboxylate salts between compatibilizers and nano-CaCO3 particles.

Fig. 4
figure 4

WAXD patterns of PP and PP/nano-CaCO3 composites with and without compatibilizers

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Fig. 5
figure 5

POM photographs of PP and PP/nano-CaCO3 composites with and without compatibilizers a PP, b PP-10, c PP-10a, d PP-10b and e PP-10c

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Because of compatibilizer with the same polar groups, it is considered the interfacial interaction is basically same between nano-CaCO3 and compatibilizer. Addition of EVA-g-MA decreases the T c of PP and no β-crystal of PP are formed in PP-10c, and the spherulite size of PP in PP-10c is larger than that in PP-10. It is suggested that the heterogeneous nucleation of carboxylate salts is dependent on the miscibility between nano-CaCO3 and compatibilizer. The carboxylate salts formed in the interface between nano-CaCO3 particles and compatibilizer do not play a direct nucleation for PP crystallization due to the presence of the compatibilizer layer in the PP/nano-CaCO3 composites. The heterogeneous nucleation of carboxylate salts may firstly nucleate the crystallization of compatibilizer layer and/or the PP matrix included in the compatibilizer phase, and then nucleates the PP matrix. Therefore, the heterogeneous nucleation of carboxylate salts depends on the crystallizability of compatibilizer and the miscibility between PP and compatibilizer.

It can be seen from Table 2 that addition of nano-CaCO3 particles rarely affect the crystallization of POE-g-MA and EVA-g-MA, and slightly increase the T c of PP-g-MA. The T c of these three compatibilizers in compatibilizer/nano-CaCO3 composites is lower than the T c of PP matrix in the corresponding PP/compatibilizer/nano-CaCO3 composites. That is to say, in PP/compatibilizer/nano-CaCO3 composites, the crystallization of PP matrix starts earlier than that of compatibilizer. Because the crystallization of compatibilizers does not affect the crystallization of PP matrix, it is suggested that the carboxylate salts mainly nucleate the crystallization of PP matrix included in compatibilizer phase, and then nucleates PP matrix. However, the content of PP matrix included in this phase is dependent on the miscibility between PP and compatibilizer. For PP-10a, PP-g-MA is highly miscible with PP matrix, and the heterogeneous nucleation of carboxylate salts can nucleate the PP matrix included in PP-g-MA phase, and then nucleates PP matrix, leading to a high T c of PP in PP-10a. For PP-10b, the partial miscible between PP and POE in POE-g-MA decreases the content of PP matrix included in POE-g-MA phase, which weakens the nucleation effect of carboxylate salts. For PP-10c, the immiscibility between PP and EVA-g-MA retards the heterogeneous nucleation of carboxylate salts due to the few content of PP matrix in EVA-g-MA phase. Therefore, addition of EVA-g-MA into PP/nano-CaCO3 composite decreased the T c of PP and the spherulite size of PP is larger than that of PP-10.

Isothermal crystallization and melting behavior of PP/nano-CaCO3 composites modified by compatibilizers

The synergism of heterogeneous nucleation of PP-g-MA and nano-CaCO3 for PP crystallization are proved by isothermal crystallization experiment. The relationship of reciprocal crystallization peak time (i.e. t −1p ) versus T c for PP and its composites is shown in Fig. 6. Generally, t −1p is proportional to the crystallization rate [34]. It can be observed that the crystallization rates of PP and its composites decrease with the increase of isothermal crystallization temperatures. When isothermally crystallized at the same temperature, the t −1p of the specimens ranges as: PP-10a > PP-10b > PP-10 >> PP ≥ PP-10c. That is to say, the crystallization rate of these specimens ranges as: PP-10a > PP-10b > PP-10 >> PP ≥ PP-10c. These results are in agreement with those of non-isothermal crystallization experiment and indicated the presence of the synergism of heterogeneous nucleation of PP-g-MA and nano-CaCO3 for PP crystallization increases the crystallization rate of PP.

Fig. 6
figure 6

The plots of reciprocal crystallization peak time (t −1P ) against crystallization temperature for PP and PP/nano-CaCO3 composites with and without compatibilizers

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Crystallization activation energy and fold surface free energy of PP crystallization in PP/nano-CaCO3 composites

It is well known that the crystallization of polymers is controlled by two factors: the dynamic factor, which is related to the effective activation energy for the transport of crystalline unites across the phase, and the static factor, which is related to the free energy barrier for nucleation.

The activation energy (ΔE) in non-isothermal crystallization process can be determined by Kissinger method through calculating the variation of the crystallization peak temperature (T p) with the cooling rate (λ) [35]:

$$ {\text{d ln}}(\lambda /T_{\text{p}}^{ 2} )/{\text{d}}\left( { 1/T_{\text{p}} } \right) = - \Updelta E/R $$
(1)

where R is the gas constant. Generally, high crystallization activation energy would hinder the crystallization and result in the decrease of crystallization rate.

The plots of ln(λ/T 2p ) against 1/T p give a straight line (Fig. 7). The ΔE values of specimens can be calculated form the slope of the line, tabulated in Table 3. It can be seen that addition of nano-CaCO3 increases the ΔE of PP. This result is similar to those reported for many other nucleating agents [36–38]. It is suggested that the addition of nano-CaCO3 restricts the transport of macromolecular segments from PP melts to the crystal growth surface. The addition of PP-g-MA further increases the ΔE of PP in PP/nano-CaCO3 composites due to the strong interaction between PP-g-MA and nano-CaCO3 as well as good miscibility between PP-g-MA and PP matrix to restrict the transport of macromolecular segments of PP. The ΔE of modified composites decreases with decreasing the miscibility between compatibilizer and PP matrix.

Fig. 7
figure 7

The plots of ln(λ/T 2p ) against 1/T p for PP and PP/nano-CaCO3 composites with and without compatibilizers

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Table 3 The crystallization activation energy ΔE and surface free energy σ e of PP and its composites
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According to the Hoffman theory, the growth rate G of crystals can be expressed as follows [39]:

$$ G = G_{0}\, {\exp}[{ - U^{*}/({R({T_{\text{c}} - T_{\infty } })})}]{\exp}[ - K_{\text{g}} /(T_{\text{c}} \Updelta Tf)] $$
(2)

Assuming the growth rate G is reciprocally proportional to the reciprocal half-crystallization time t −11/2 . Then it can be obtained [36]:

$$ { \ln }(t_{ 1/ 2} )^{ - 1} + U^{*} /( {R( {T_{\text{c}} - T_{\infty } })} ) = {\text{ A}} - K_{\text{g}} /(T_{\text{c}} \Updelta Tf) $$
(3)

where U* is the activation energy of polymer segments transporting to the crystal front through the subcooled melt, and U* is 6270 J/mol for PP, R is the gas constant, T ∞ is the temperature somewhat below the glass transition temperature T g and is given by T g − 30 K (T g is 263 K for PP), T c is the crystallization temperature, ΔT is the degree of supercooling (T m° − T c), where T m° is the equilibrium melting temperature (here T m° is 481 K for PP), f is a correcting factor for variation in the heat of fusion with temperature and is approximated by f = 2T c/(T m° + T c), and K g is the nucleation constant.

The plots of ln(t 1/2)−1 + U*/(R(T c − T ∞)) against 1/(T cΔTf) give a straight line (Fig. 8). The K g values of specimens can be calculated form the slope of the line, tabulated in Table 3. The fold surface free energy σ e can be obtained by the following equations [40]:

$$ K_{\text{g}} = 4\sigma \sigma_{\text{e}} b_{0} T_{\text{m}}^{^\circ } /k\Updelta H $$
(4)
$$ \sigma = \alpha b_{0} \Updelta H $$
(5)

where k is the Boltzman constant, b 0 is the thickness of the molecular layer and is determined by lattice parameters (b 0 is 6.56 × 10−10 m), ΔH is the theoretical heat of fusion (ΔH is 1.4 × 108 J/m3 for PP), σ and σ e are interfacial free energies per unit area parallel and perpendicular to the molecular chain direction, respectively. The value of σ for PP is 9.2 × 10−3 J/m2, so the fold surface free energy σ e can be directly obtained from K g is shown in Table 3. Generally, the smaller the σ e of crystallization surface, the more easy the formation of nuclei is. It can be seen from Table 3 that addition of nano-CaCO3 decreases the σ e and addition PP-g-MA further decreases the σ e. It is suggested that nano-CaCO3 and the carboxylate salts formed by reaction between nano-CaCO3 and compatibilizer can act as nucleating agents, reduce the free energy barrier for nucleation and increase the nucleation and crystallization rate of PP. However, the σ e increases with decreasing the miscibility between compatibilizer and PP, indicating the poor miscibility between compatibilizer and PP does not favor the formation of nuclei.

Fig. 8
figure 8

The plot of ln(t 1/2)−1 + U*/(R(T c − T ∞)) against 1/(T cΔTf) for PP and PP/nano-CaCO3 composites with and without compatibilizers

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Although addition of nano-CaCO3 increases the ΔE of PP, the crystallization rate of PP is controlled by nucleation rate. The addition of nano-CaCO3 and the formation of carboxylate salts acted as the heterogeneous nucleating agent favor the formation of nuclei. Therefore, the crystallization rate is increased in PP/nano-CaCO3 composites due to the existence of large amounts of heterogeneous nuclei. However, the heterogeneous nucleation of nano-CaCO3 and carboxylate salts depends on the miscibility between PP and compatibilizer. The decrease in the miscibility between compatibilizer and PP restricts the heterogeneous nucleation of nano-CaCO3 and carboxylate salts, resulted in the low crystallization rate in PP/nano-CaCO3 composites.

Conclusions

The crystallization and melting behavior of modified PP/nano-CaCO3 composites with different interfacial interaction between compatibilizer and PP were investigated using differential scanning calorimetry, X-ray diffraction and polarized optical microscope. The results indicate addition of different compatibilizer has a different effect on the crystallization behavior of PP. The addition of PP-g-MA increases the crystallization temperature of PP. However, the crystallization temperature of PP is decreased by adding of POE-g-MA and EVA-g-MA. The addition of nano-CaCO3 increases the crystallization temperature of PP due to the heterogeneous nucleation of nano-CaCO3. The higher crystallization temperature of PP in modified PP/nano-CaCO3 composite is attributed to the synergistic effect of heterogeneous nucleation of nano-CaCO3 and compatibilizer due to the formation of carboxylate salts by the chemical reaction between the compatibilizers and nano-CaCO3. The synergistic effect of heterogeneous nucleation of nano-CaCO3 and compatibilizer further increases the crystallization temperature, crystallization rate and decreases the fold surface free energy as well as favors the formation of β-crystal of PP. However, the synergistic effect of heterogeneous nucleation of nano-CaCO3 and compatibilizer for PP crystallization depends on the miscibility between PP and compatibilizer. The increased miscibility between compatibilizer and PP favors the synergistic effect of heterogeneous nucleation of nano-CaCO3 and compatibilizer.