Comparison Study on the Heterogeneous Nucleation of Isotactic

Article pubs.acs.org/IECR

Comparison Study on the Heterogeneous Nucleation of Isotactic Polypropylene by Its Own Fiber and α Nucleating Agents Yannan Quan, Huihui Li, and Shouke Yan* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: The crystallization of isotactic polypropylene (iPP) in the presence of its own fiber or/and α nucleating agents, i.e., DBS and NA11, was studied. The results show that both nucleating agents exhibit nucleation ability toward iPP and accelerate iPP melt crystallization. This is manifested by an upward shift of crystallization temperature by up to 10 °C compared with its bulk crystallization. The iPP fiber exhibits also high nucleation ability toward the iPP matrix as demonstrated by the formation of transcrystalline layers. The crystallization of iPP in the nucleated iPP matrix/fiber system shows that the nucleating agents do not affect the transcrystallization of iPP surrounding its fiber. This indicates that the fiber exhibits higher nucleation ability toward iPP than the nucleating agents. On the basis of the exact same chemical and crystallographic structures of the iPP fiber and matrix, the results demonstrate the importance of structure similarity and matching between the crystallographic structures in the heterogeneous nucleation of polymers.

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INTRODUCTION It is well known that many polymeric materials exhibit pronounced polymorphisms,1−9 which show a strong influence on their properties or even functionality. For example, the isotactic polypropylene (iPP) exhibits at least three different crystal modifications, designated as α, β, and γ forms, depending on the thermal and mechanical treatments.10−15 The properties of iPP are closely related to its crystal modification. It was reported that β-iPP, distinguished from its other counterparts, exhibits some excellent performance characteristics such as improved elongation at break and impact strength.16,17 For the issue of functionality, the poly(vinylidene fluoride) (PVDF) provides an excellent example, which exhibits at least four different crystalline modifications (designated as α, β, γ, and δ).18−24 It was reported that β-phase PVDF is the one that has attracted the widest interest due to its extensive piezoelectric and pyroelectric applications, while α-PVDF is the most common phase. Taking all this into account, the crystal structure control of polymorphic polymers during processing is of great significance to obtain material with desired properties and functionality. To control the crystal structure of polymeric materials, application of a special nucleating agent (NA) is the most frequently and widely used method, e.g., the βcrystallization of iPP.25−31 This has led to tremendous studies on heterogeneous nucleation of polymers concerning efficiency and mechanism. For polyolefin, the most widely used nucleation agents are organic compounds, such as, organic phosphate salts and sorbitol derivatives for iPP. Lotz and co-workers have performed excellent work on the heterogeneous nucleation mechanism of polymers on organic compounds.32−35 Systematic studies devoted to the heterogeneous nucleation of polymers with organic compounds show that some kind of geometric matching between organic compounds and polymeric materials plays an important role in the nucleation efficiency. For polymer−polymer heteroepitaxy, geometric matching is also frequently found.36,37 However, investigations on the fiber-induced heterogeneous nucleation of polymers © 2013 American Chemical Society

show different results. For example, through systematic studies on fiber-induced polymer crystallization, Chatterjee et al.38−41 have found that the strong nucleating action of the interacting polymer pairs is not linked to (i) similarity in chemical structure, (ii) similarity in crystallographic unit cell geometries, or (iii) similarity of the chain conformation. Therefore, they concluded that some kind of geometric matching is not necessary for heterogeneous nucleation of polymers. From these results, it seems that the heterogeneous nucleation mechanism of polymers at the fiber surface is different from the organic NA-triggered crystallization of polymers. Here in this work, we have chosen iPP as a model system. The crystallization behavior of iPP triggered by organic NAs and its single polymer fibers are compared. The results show that while the NA with closer matching exhibits evident nucleation ability toward iPP, the iPP at its single polymer fiber has the highest nucleation density. This may indicate that the existence of some matching is very important for the substance to show high nucleation ability toward a polymer.

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EXPERIMENTAL SECTION The matrix polymer used in this work was commercial grade isotactic polypropylene (iPP), GB-2401, with a melt flow index of 2.5 g/10 min, Mw ≈ 437900, and melting temperature of 170 °C, produced by Yanshan Petroleum and Chemical Corp., China. The iPP fibers were produced with a homemade meltspinning device. The spinning temperature was 200−240 °C. The resultant iPP fibers have been subjected to a cold-draw procedure at 110 °C with a draw ratio of 6. The material used for fiber spinning is chemically degraded iPP GB-2401 with a melt flow index of 15 g/10 min, Mw ≈ 245100, Mn ≈ 73600, Received: Revised: Accepted: Published: 4772

November 20, 2012 February 1, 2013 March 8, 2013 March 8, 2013 dx.doi.org/10.1021/ie303200z | Ind. Eng. Chem. Res. 2013, 52, 4772−4778

Industrial & Engineering Chemistry Research

Article

Scheme 1. Schematic Representation of Chemical Structure of DBS and Rigid 3D Hydrogen-Bonded Network It Formed in the iPP Matrix

To study the iPP fiber induced iPP crystallization, thin iPP matrix films either with or without NAs were first heated to 200 °C for 5 min to erase possible effects of thermal history and then moved to a preheated heat plate at 160 °C where the iPP fiber was introduced into the supercooled iPP melt. Finally, the prepared iPP fiber/matrix composites were cooled to selected temperatures, e.g., 137 and 141 °C, for isothermal crystallization. An Axioskop 40A Pol Optical Microscope (Carl Zeiss) was used for optical microscopy observation. All optical micrographs presented in this paper were taken under crossed polarizers. For scanning electron microscopy (SEM) observation, the obtained composites were etched according to a procedure introduced by Bassett et al.44 The etching agent was made of two parts of sulfuric acid (98%), one part of phosphoric acid (85%), and 1 wt % potassium permanganate, which was dissolved into the above mixed solvent. The samples were etched for 3−4 h at room temperature. After etching, the specimens were washed first with a mixture of sulfuric acid, water, and hydrogen peroxide, with a ratio of 2:7:1, and then rinsed with distilled water and acetone, respectively. The samples were finally sputtered with a thin layer of gold after being dried in a vacuum oven. SEM images were taken on a JEOL JSM-6701F scanning electron microscope, with an acceleration voltage of 20 kV. Differential scanning calorimetry (DSC, Perkin-Elmer) was employed to measure the crystallization peak temperature. DSC measurements were performed with 3−5 mg samples at a standard heating and cooling rate of 10 °C/min under a nitrogen atmosphere within the temperature window from 50 to 200 °C.

and melting temperature of ca. 170 °C. The average diameter of thus-prepared iPP fiber is ca. 20 μm. To remove the sizing agents on the fiber surface, the fibers were treated for 4 h with refluxing acetone and then dried in a vacuum oven at 40 °C for 5 h. The α nucleation agents with trade names of DBS (1,3:2,4dibenzylidene sorbitol, a typical sorbitol based clarifier) and NA11 (sodium 2,2′-methylene-bis-(4,6-di-t-butylphenylene) phosphate) are the products of Shanxi Provincial Institute of Chemical Industry and Luoyang Zhongda Chemical CO., LTD, respectively. Their chemical structures are presented in Schemes 1 and 2, respectively. It should be noted that the Scheme 2. Chemical Structure of Used NA11 nucleation agent

nucleation mechanisms of DBS and NA11 toward iPP are different. DBS has a threshold concentration around 800−1000 ppm, which is related to its solubility in iPP and the formation of a sorbitol network.42,43 The nucleation of iPP occurs only at locations where the sorbitol network forms. This means that the concentration of DBS should be at least 800 ppm. Considering that the nucleus density will be too large to produce individual iPP spherulite with overloaded DBS, the iPP with 800 ppm DBS was prepared in this work for convenience of optical microscopy observation. On the other hand, the heterogeneous nucleating agent NA11 does not dissolve in iPP. It starts to nucleate iPP at any dosage level. In our research, 400 ppm of NA11 was adopted as the optimum amount for convenient optical microscopy observation. Because in most cases we deal with individual iPP spherulite, the different concentrations of the NAs do not influence the analyses of nucleation ability. The mixing of nucleating agents and iPP matrix was carried out in a melt blending in an internal mixer. Thin iPP films with or without NAs of ca. 40 μm in thickness for optical microscopy observation were prepared by compression molding the iPP/NA mixtures or iPP granules at 200 °C under 75 kg/cm2.

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RESULTS AND DISCUSSION Crystallization of iPP with and without NAs. Figure 1 shows the optical micrographs of the iPP samples with and without NAs, which were taken during the process of isothermal crystallization at 137 °C. The time shown in the images indicates the period after the samples were cooled to 137 °C. It is attested by selective melting experiments that the iPP crystals shown in Figure 1 are all in the α-form. This is expected because both NAs are α nucleation agents for iPP. From Figure 1, we can see that the crystallization of all iPP samples either with or without NAs takes place 10 min after the samples were cooled from 200 to 137 °C, see left column of Figure 1. The nucleation densities of the iPP/NA systems, as revealed in Figure 1b and c for the iPP/DBS and iPP/NA11 systems, respectively, are clearly higher than the pure iPP system. This indicates an efficient heterogeneous nucleation ability of both DBS and NA11 toward iPP. Because of the 4773

dx.doi.org/10.1021/ie303200z | Ind. Eng. Chem. Res. 2013, 52, 4772−4778


Article

Figure 1. Polarized optical micrographs of (a) neat iPP, (b) iPP/DBS (800 ppm), and (c) iPP/NA11 (400 ppm) crystallized isothermally at 137 °C for various times as indicated in the images.

different contents of DBS and NA11 used here, a comparison of the nucleation densities cannot reflect the nucleation ability of DBS and NA11 toward iPP directly. However, the onset of crystallization judged from the diameters of the grown iPP spherulites can tell us which NA, i.e., DBS or NA11, exhibits higher nucleation ability toward iPP. We can see that the average size of spherulites in Figure 1c is the largest, while that in Figure 1a is the smallest. As shown in the second column in Figure 1, after samples were cooled to 137 °C for 20 min, the diameters of the iPP spherulites were estimated to be around 45, 60, and 70 μm for the iPP, iPP/DBS, and iPP/NA11 systems, respectively. This clearly indicates that the crystallization of iPP starts first in the iPP/NA11 system, then in the iPP/DBS system, and finally in the pure iPP. In other words, both DBS and NA11 are effective α nucleation agents for iPP with the nucleation efficiency of NA11 somewhat higher than that of DBS. The above conclusion has been confirmed by DSC experiments. Figure 2 shows the DSC cooling scans of iPP with and without NAs. From Figure 2, one can see that the crystallization of pure iPP during the cooling process at a rate of 10 °C/min takes place at 108 °C. The crystallization peaks of iPP in the nucleated samples, either with DBS or with NA11, are shifted upward by up to 10 °C. The characteristic data of the different samples are listed in Table 1. From Table 1, we can see that the crystallization of iPP in the nucleated systems is earlier than the pure iPP, indicating the effective nucleation of both DBS and NA11 toward iPP. However, both the onset and peak crystallization temperatures of the iPP in the iPP/NA11 are higher than those of the iPP in the iPP/DBS sample. This again demonstrates the higher nucleation ability of NA11 toward iPP compared to DBS. The effective heterogeneous nucleation of NA11 and DBS toward iPP can be explained as follows. As reported in the

Figure 2. Crystallization exotherms of iPP, iPP/DBS, and iPP/NA11 with a cooling rate of 10 °C/min after erasing the thermal history.

Table 1. Characteristic Data Concerning Onset and Peak Crystallization Temperatures and Corresponding Crystallization Enthalpy (ΔHc) of Pure and Nucleated iPPs Recorded during the Nonisothermal Crystallization Process

iPP DBS/ iPP NA11/ iPP

4774

onset crystallization temperature (°C)

peak crystallization temperature (°C)

ΔH (J/g)

115.21 123.35

108.54 119.71

98.19 96.34

125.15

121.02

98.08



Article

oriented along the fiber axis direction, the above observed parallel alignment of iPP transcrystalline lamellae reflects the occurrence of homoepitaxy induced by its single polymer fiber owing to the existence of perfect geometric matching. The homoepitaxy is different from the heteroepitaxy of iPP on other polymeric substrates, which never produces a parallel alignment of both polymers involved, such as the epitaxial crystallization of iPP on a oriented polyethylene surface resulting in the molecular chains of both polymers 50° apart from each other. There are some short lamellae that are observed between the dominant parallel aligned lamellae, illustrating the characteristic and unique wide angle lamellar branching of the α-iPP. The SEM result indicates that in the iPP fiber/matrix single polymer composite the iPP fiber serves as nucleation substance and induces a vast number of heterogeneous nuclei of iPP matrix that produces a quantity of laterally grown edge-on lamellae. The high nucleation efficiency of the iPP fiber toward its single polymer matrix is unambiguously related to the same chemical structure and perfect crystallographic matching required for the occurrence of epitaxy. Crystallization of iPP in Nucleated Fiber/Matrix Systems. From the above discussions, it is concluded that the existence of crystallographic matching results in a higher nucleation ability of NA11 toward iPP than DBS, while the same chemical structure and perfect crystallographic matching lead to an efficient nucleation of the iPP fiber toward its single polymer matrix. To determine the importance of similarities in chemical structure and crystallographic unit cell geometries for the heterogeneous nucleation of iPP, the crystallization of iPP in the nucleated fiber/matrix systems was followed. Figure 4

literature, the sorbitol derivatives will be solved in the iPP matrix and self-assemble into the fibrous structures of a rigid 3D hydrogen-bonded network.45,46 Therefore, no geometric matching will be found between DBS and iPP. However, as illustrated in Scheme 1, there are cavities in the rigid 3D hydrogen-bonded network. The cavity for the dimer is reported to be 1.5 nm long and encompasses 2.5 helix turns.47 From the structure feature of the DBS network, it was suggested that the formed network helps to stabilize the helical chain segments of polypropylene and therefore promotes the formation of more nuclei.47 For the heterogeneous nucleating agent NA11, the monoclinic unit cell has parameters a = 2.644, b = 0.608, and c = 3.717 nm and β = 93.65°.48 From this structure feature, a 0.608 nm periodicity was found to match both the a- and c-axis periodicity of iPP in its monoclinic α-form with unit cell parameters a = 0.665, b = 2.096, c = 0.65 nm and β = 99.2°. The mismatching between the interchain distance of iPP in the a-axis direction and the NA11 along the b-axis direction is only 7.9%, well within the upper limit for the occurrence of epitaxy. The discrepancy between the periodicities of iPP and NA11 along their c-axis directions, which is only 6.9%, is even smaller. The close matching leads to the occurrence of heteroepitaxial crystallization of iPP on the NA11 particles. On the basis of the above analysis, a higher nucleation efficiency of NA11 toward iPP with respect to DBS could be associated to the existence of geometric matching between the NA11 and iPP. Crystallization of iPP Induced by Its Single Polymer Fiber. Figure 3a shows the optical micrograph of an iPP fiber/

Figure 3. (a) Optical micrograph and (b) SEM image of an iPP fiber/ matrix composite prepared by introducing the iPP fibers into their supercooled homogeneous matrix at 160 °C and subsequently isothermally crystallized at 137 °C for 1.5h.

matrix single polymer composite, which was prepared by introducing the iPP fiber into its supercooled homogeneous matrix at 160 °C, then crystallized isothermally at 137 °C for 1.5 h, and finally cooled to room temperature in air. It can be seen that column cylindrites (or in other words, transcrystalline layers) of iPP are produced surrounding its single polymer fiber, while spherulites of iPP are observed away from the fiber. It is confirmed that the iPP grown both in the matrix and surrounding its single polymer fibers is its α-form. It is reasonable because the iPP fibers are also composed of α-iPP crystals. From Figure 3a, in the transcrystalline layers, the density of the iPP nucleus generated on its own fiber surface was too high to identify each individual one by an optical microscopy resolution. Fine structures revealed by scanning electron microscopy, as seen in Figure 3b, show clearly edge-on lamellae grown perpendicular to the fiber axis direction. These parallel aligned crystalline lamellae construct the transcrystalline layers of the iPP. Considering that the iPP molecular chains are

Figure 4. Optical micrographs of (a) DBS-nucleated and (b) NA11nucleated iPP fiber/matrix systems taken during the isothermal crystallization at 137 °C. The iPP fibers were introduced into the nucleated iPP matrix at 160 °C. Crystallization times are 5 min for the left column and 30 min for the right column.

shows the optical micrographs of the iPP fiber/matrix single polymer systems with the matrix iPP nucleated by DBS (Figure 4a) and NA11 (Figure 4b), respectively. The crystallization temperature was set at 137 °C. It can be clearly seen that the crystallization of iPP in all cases, i.e., the iPP fiber induced transcrystallization and NAs (DBS and NA11) induced spherulite growths, starts within 5 min after the sample was 4775



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cooled to 137 °C. It should be noted that in both systems the transcrystalline layers of iPP surrounding its single polymer fibers can be clearly observed. This demonstrates the higher nucleation ability of the iPP fiber compared to DBS and NA11. Otherwise, the NA induced crystallization will break the transcrystalline layer when the NAs are located at the fiber place. The higher nucleation ability of the iPP fiber toward the iPP matrix is further manifested by an early start of fiberinduced iPP transcrystallization. This has been clearly revealed by the time dependent dimension change of the transcrystalline layer and the biggest iPP spherulites in the nucleated matrix. As shown in Figure 5, the crystal dimensions of the transcrystalline

Figure 6. Polarized optical micrographs of the samples taken in the same area at varied crystallization time. The iPP fiber was introduced at 160 °C into (a) iPP/DBS, (b) iPP/NA11, and the composites crystallized isothermally at 141 °C for 5 and 30 min.

isothermal crystallization at 137 °C as shown in Figure 4, here the crystallization of iPP surrounding its fiber starts ca. 5 min after the sample was cooled to 141 °C. At that time point, the NAs (both DBS and NA11) induced iPP crystallization has not started yet. This shows more clearly that the iPP fiber exhibits a higher nucleation ability toward its single polymer matrix than the used DBS- and NA11-nucleating agents. Therefore, the formation of transcrystalline layers is not stopped by the presence of nucleating agents. The dimensions of the fiberinduced transcrystals and NA-induced spherulites against crystallization time are also plotted in Figure 7. It shows again a linear relationship between the crystal sizes with crystallization time as expected for isothermal crystallization. Now, the slopes of the lines are smaller than those shown in Figure 5, indicating a slower crystal growth rate of iPP at 141 °C. Moreover, the crystallization induction times are somewhat

Figure 5. Time-dependent dimension change of the transcrystalline iPP layer and the biggest iPP spherulite in the nucleated matrix. Isothermal crystallization temperature was 137 °C.

crystals as well as the DBS- and NA11-induced spherulites are all proportional to the crystallization time, which corresponds to the time lapse after the sample was cooled to the crystallization temperature. It should be noted that all of the dimension vs time plots are parallel to each other, indicating that the crystal growth rates of the transcrystalline iPP crystals and NA-induced iPP spherulites are the same. This is reasonable because all these crystals grew under exactly the same thermal condition. Furthermore, it can be clearly seen that the thicknesses of the iPP fiber-induced transcrystalline layer in the DBS- and NA11-nucleated systems at each time point are located exactly at the same line. This indicates that the fiber-induced crystallization is not stopped by the existence of nucleation agents. From Figure 5, the crystallization induction time, the time used to initiate the crystallization after the sample was cooled to the crystallization temperature, can be obtained through extrapolating the crystal size vs time plots to the horizontal axis. The results indicate that the iPP fiberinduced transcrystallization needs the shortest induction time, reflecting the highest nucleation ability. The induction time of NA11-induced iPP crystallization is somewhat shorter than that of the DBS-induced iPP crystallization. This implies a higher nucleation ability of NA11 toward iPP compared with DBS. The above results have been more evidently demonstrated at elevated crystallization temperatures. For example, Figure 6 shows the optical micrographs of the DBS- (Figure 6a) and NA11- (Figure 6b) nucleated iPP single polymer fiber/matrix systems crystallized isothermally at 141 °C. Unlike the case of

Figure 7. Plots of fiber-induced transcrystals and NA-induced spherulites dimensions against crystallization time during isothermal crystallization at 141 °C. 4776



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longer than those obtained from Figure 5. This demonstrates a decrease in nucleation efficiency at elevated temperatures in all cases. Nevertheless, the iPP fiber shows a still higher nucleation ability toward its homogeneous matrix than the used DBS- and NA11-nucleating agents. Taking the exact same chemical and crystallographic structures of the iPP fiber and matrix into account, the obtained results may suggest that similarity in chemical structure and matching between the crystallographic structure play very important roles in nucleating a polymer. Of course, other factors such as the better surface wet ability may also be important for an excellent nucleating agent.

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CONCLUSIONS The crystallization behavior of iPP in the fiber/matrix single polymer composites with and without nucleating agents was studied. The results showed that the used α-nucleating agents, i.e., DBS and NA11, exhibit evident nucleation ability toward iPP as manifested by the upward shift of the crystallization temperatures by up to 10 °C. It is further found that the NA11 shows relatively higher nucleation ability toward iPP when compared with DBS, which may originate from the existence of geometric matching between the NA11 and iPP crystallographic unit cells. This is supported by the even higher nucleation ability of the iPP fiber toward its single polymer matrix as demonstrated by the formation of transcrystal layers regardless of whether and what kind of nucleating agents were used. Moreover, the shorter crystallization induction time of the iPP fiber-induced transcrystal growth as compared with that of the NA-induced iPP spherulite growth also reveals a higher nucleation ability of the iPP fiber toward its homogeneous matrix than the used DBS- and NA11-nucleating agents. On the basis of the exact same chemical and crystallographic structures of the iPP fiber and its single polymer matrix, the obtained results demonstrate very important roles of similarity in chemical structure and matching between the crystallographic structures in heterogeneous nucleation of polymers.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0086-010-64455928. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China under Grant Nos. 50833006, 21274009, 51221002, and 50973008 is gratefully acknowledged.

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REFERENCES

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Comparison Study on the Heterogeneous Nucleation of Isotactic

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