Carbon Dioxide Induced Crystallization for Toughening Polypropylene

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Carbon Dioxide Induced Crystallization for Toughening Polypropylene Jin-Biao Bao,†,‡ Tao Liu,† Ling Zhao,†,* and Guo-Hua Hu‡,§ †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Laboratory Reactions and Process Engineering, CNRS-Nancy Universite-INPL-ENSIC, 1 rue Grandville, BP 20451, 54001 Nancy, France § Institut Universitaire de France, Maison des Universites, 103 Boulevard Saint-Michel, 75005 Paris, France ‡

ABSTRACT: This study proposes a novel process for significantly toughening isotactic polypropylene (iPP) by finely tuning and controlling the structure and morphology of iPP. The toughness of injection-molded iPP specimens can be significantly improved by controlled shearing, CO2-induced recrystallization, and adequate cooling without loss of strength. The distribution and structure of the iPP samples before and after toughening were characterized by polarized optical microscopy, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry, to investigate the structureproperty relationship. Under shear, a high degree of orientation can be obtained with “shish-kebab” crystals formed in the shear zone. During the subsequent CO2 treatment, a crystal network morphology can be formed as a result of an increase in the numbers of primary lamellae and crosshatched subsidiary lamellae, which leads to an increase in toughness. Wide-angle X-ray diffraction patterns indicate that quenching in icewater of scCO2-treated iPP promotes the formation of nanosized mesomorphic phase domains in the shear zone, which further toughens the iPP. The impact strength of the best toughened iPP sample was found to be over 12 times that of the original sample without loss in tensile strength and modulus.

1. INTRODUCTION For semicrystalline polymers such as isotactic polypropylene (iPP), their final properties are dictated by their crystallinity and crystalline morphology, which, in turn, are affected by the thermomechanical history (flow and temperature) they experience during processing.17 Injection molding is one of the most widely employed processes for manufacturing polymer products and consists of three main steps: filling, packing/holding, and cooling. Typically, multiple layers or a skincore structure of iPP samples are formed during injection molding.3,6,7 A shear zone with a high level of orientation is mainly responsible for shearinduced properties.4,6 Oriented crystals can lead to significant improvement in tensile strength,8 modulus, and stiffness.9 However, improvement in the performance of polymers seems to be available only in the oriented direction.4,6,7 In the perpendicular direction, the performance properties such as impact strength are often more or less reduced. Toughening of iPP has been a subject of intense study.10 Toughening methods include copolymerization of propylene with other types of olefin monomers,11,12 blending of iPP with particles or a phase-separated second polymer,1315 and addition of a β-nucleating agent.1618 Unfortunately, the enhancement of toughness is often at the expense of other properties such as strength and heat resistance.19 There are very few successful examples of simultaneously and efficiently reinforcing and toughening iPP.6,7,20 Thus, it is a great challenge to enhance the performance of iPP in both the oriented and perpendicular directions in an injection molding process. Supercritical carbon dioxide (scCO2) has been increasingly used as a promising alternative to organic solvents for applications in polymer processing, such as grafting,2123 foaming,24,25 and impregnation of additives.2628 The effect of CO2 on the r 2011 American Chemical Society

crystallization behavior of iPP has also been widely studied. The dissolution of CO2 in the polymer increases the free volume between molecular chains and chain mobility and, consequently, affects the crystallization behavior.2933 Moreover, there is an interesting phenomenon that nonisothermal crystallization of iPP melt at a high cooling rate can yield nanosized nodular mesomorphic crystals.3439 This work aims at improving the toughness of injectionmolded iPP specimens by controlling their crystal structure. This is done by three successive steps as depicted in Figure 1. The first step consists of controlling the degree of orientation of the crystal structure in the shear zone by tuning the injection molding conditions (Figure 1a). Highly oriented crystals can significantly improve the tensile strength of iPP samples. In the second step, the injection-molded iPP specimens are subjected to recrystallization in the solid state under scCO2 (Figure 1b). Additional fine lamellae can be formed in the shear zone (see Figure 1b). They can improve the toughness of iPP samples while retaining the high tensile strength. The third step is to further tune the crystal structure by controlling the cooling rate of the scCO2-treated injection-molded iPP specimens. Nonisothermal crystallization of iPP melt at a high cooling rate can yield nanosized nodular mesomorphic crystals (see Figure 1c). These nanosized mesomorphic crystals in the shear zone would further improve the toughness of the scCO2-treated injection-molded iPP specimens. The originality of this work lies in these last two steps. Received: March 1, 2011 Accepted: June 30, 2011 Revised: June 30, 2011 Published: June 30, 2011 9632

dx.doi.org/10.1021/ie200407p | Ind. Eng. Chem. Res. 2011, 50, 9632–9641

Industrial & Engineering Chemistry Research

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Figure 2. Schematic of the positions of iPP samples for optical light microscopy. Figure 1. Schematic of crystal morphology development.

Table 1. iPP Specimens Used in This Work sample

preparation technique

PP1

Injection-molded at the melt temperature of 200 C, mold temperature of 100 C, and injection speed of 70% and

PP2

Injection-molded at the melt temperature of 180 C,

then annealed at the temperature of 110 C for 5 h mold temperature of 40 C, and injection speed of 30% and then annealed at the temperature of 110 C for 5 h PP3

PP1 annealed under scCO2 (100 C, 15 MPa, 5 h) and then

PP4

PP2 annealed under scCO2 (100 C, 15 MPa, 5 h) and then quenched in ambient water

PP5


PP6


quenched in ambient water

quenched in icewater quenched in icewater

2. EXPERIMENTAL SECTION Materials and Sample Preparation. iPP pellets (RS1684) were purchased from Basell Co., Elkton, MD. Its mass-average molar mass and polydispersity index were 231800 g/mol and 14.8, respectively. Its crystallinity measured by differential scanning calorimetry (DSC) was 45.5%. CO2 [purity 99.9% (w/w)] was purchased from Air Products Co., Shanghai, China. The iPP was injection-molded on an 80-ton Chen Hsong injection molding machine equipped with an instrumented mold to make ASTM-D256 impact bars and ASTM-D638 tensile bars. Two iPP specimens were injection-molded under two different sets of conditions: PP1 under standard conditions and PP2 under conditions that were more favorable for orienting the iPP (see Table 1). In both cases, the holding pressure and holding time were 200 bar and 10 s, respectively. Prior to the CO2 treatment, both PP1 and PP2 specimens were annealed in a vacuum oven at 110 C for 5 h to remove their previous thermal history. CO2 Treatment. PP1 and PP2 specimens obtained by injection molding were treated under CO2 in a high-pressure vessel placed in a homemade oil bath with a temperature controller.21 The temperature of the oil bath was controlled at a desired temperature with an accuracy of (0.2 C. PP1 and PP2 were placed in the vessel. The latter was sealed and carefully washed

with low-pressure CO2. Thereafter, it was charged with a given amount of CO2 using a syringe pump with an accuracy of 0.01 cm3 (DZB-1A, Beijing Satellite Instrument Co., Beijing, China). The vessel pressure was measured with a type P31 pressure transducer from Beijing Endress & Hauser Ripenss Instrumentation Co., Beijing, China. Its accuracy was (0.01 MPa. The vessel was immersed in the oil bath and heated to the desired temperature. The specimens were saturated with CO2 under prescribed pressure and temperature for at least 5 h so that the saturation was surely reached. Then, the specimens were cooled in an icewater bath or in ambient water before the CO2 was released. The cooling rates for the icewater bath and ambient water were about 30 and 12 K/min, respectively. As a result, four additional iPP specimens were obtained. Characterization of the iPP Specimens. The iPP specimens were cut into slices with thicknesses of of about 25 μm using a Leica slit microtome from their central positions (see Figure 2). They were analyzed by polarized optical microscopy (Leika Metalographic Aristomet model) to visualize crystal orientation. DSC on a Netzsch DSC 204 HP instrument was used to characterize the melting behavior of the iPP specimens under ambient N2. For each DSC run, about 510 mg of the iPP was heated from 40 to 200 C at a rate of 10 C/min. The crystallinity of the iPP in mass fraction, Xc, was calculated using the equation Xc ¼

ΔH 100% ΔHm0

ð1Þ

where ΔH is the enthalpy of crystallization per gram of sample and ΔH0m is the enthalpy of crystallization per gram of 100% crystalline iPP, which is equal to 209 J/g.40 The crystalline morphology of the iPP samples after foaming was characterized with a JSM-6360LV scanning electron microscopy (SEM). Wide-angle X-ray diffraction (WAXD) on a Rigaku D/max 2550 VB/VC X-ray diffractometer (Cu KR Nifiltered radiation) was used to study their crystalline structures. The scan rate was 1(θ)/min, and the diffraction angle (2θ) ranged from 3 to 50. Subsequently, through deconvolution of the peaks of the WAXD profiles, the overall crystallinity Xc was calculated as Xc ¼

∑

∑

Acryst Acryst þ Aamorph

∑

ð2Þ

where Acryst and Aamorph are the fitted areas of crystalline and amorphous regions, respectively. The relative amount of the 9633

dx.doi.org/10.1021/ie200407p |Ind. Eng. Chem. Res. 2011, 50, 9632–9641


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Figure 3. Comparison between iPP samples in terms of impact strength, tensile strength, elongation at break, and tensile modulus.

β-form crystal, Kβ, was evaluated by the method of Turner-Jones et al.41 as Aβ ð300Þ Kβ ¼ Aβ ð300Þ þ AR ð040Þ þ AR ð110Þ þ AR ð130Þ

ð3Þ

where Aβ(300) is the area of the (300) reflection peak and AR(040), AR(110), and AR(130) are the areas of the (040), (110) and (130) reflection peaks, respectively. Meanwhile, the crystallinity of the β-form crystal, Xβ, was calculated as Xβ ¼ Kβ Xc

ð4Þ

Small-angle X-ray scattering (SAXS) measurements were conducted to monitor the formation of lamellar crystals and were carried out on the BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF). Two-dimensional diffraction patterns were recorded using an image-intensified charge-coupled device (CCD) detector. All experiments were carried out at room temperature with the radiation wavelength λ = 0.154 nm. The scattering intensities were corrected for absorption, background scattering, and incident X-ray fluctuations of the samples. The tensile test was performed at 23 C according to standard method ASTM D-638 at a cross-head speed of 5 mm/min, whereas to evaluate the tensile modulus, a draw speed of 1 mm/min was applied. The notched impact test was carried out at 23 C according to standard method ASTM D-256. Five samples from each molding trial were tested. The orientation of the amorphous and crystalline phases of the iPP samples from the skin to the core was measured by Fourier transform infrared (FTIR) spectroscopy. The orientation function was calculated as3 D1 f ¼ ð5Þ D þ 2

)

where D is the dichroic ratio (i.e., the ratio between the absorbance of the infrared radiation polarized along the directions parallel and perpendicular to the orientation). For the orientation function of the crystal phase, fc, the band at 998 cm1 was considered. D is thus the ratio between the absorbance in the parallel direction, A , at 998 cm1 and the corresponding absorbance in the perpendicular direction, A^. As for the amorphous-phase bands, the peak at 973 cm1 is the most commonly used.3

3. RESULTS AND DISCUSSION Effect of CO2 Treatment on the Mechanical Properties of iPP. Compared with PP1 and PP2, the iPP samples after CO2

treatment, namely, PP3PP6, exhibited superior toughness, as shown in Figure 3a. The impact strength of PP2 was 6.1 ( 0.2 kJ/m2, slightly lower than that of PP1, 7.2 ( 0.1 kJ/m2, because of the higher orientation level of PP2 compared to PP1 along the flow direction. The impact strengths of PP3 (10 ( 0.1 kJ/m2) and PP5 (43.6 ( 0.1 kJ/m2) were about 1.4 and 6 times that of PP1, respectively, whereas those of PP4 (18.5 ( 0.2 kJ/m2) and PP6 (75.7 ( 0.2 kJ/m2) were 3 and 12 times that of PP2, respectively. These results indicate that CO2 treatment indeed toughened the iPP samples. Moreover, when subjected to the same annealing and subsequent quenching conditions, an iPP sample with more highly oriented crystals (PP2) exhibited a significantly higher toughness than an iPP sample with less oriented crystals (PP1). On the other hand, quenching in icewater after annealing resulted in much higher toughness than quenching in water. This is true for both highly and weakly oriented crystal samples. Therefore, the key process parameters to enhancing toughness are a high degree of orientation of the crystals, adequate scCO2induced recrystallization in the solid state, and rapid cooling of scCO2-treated samples. Figure 3b compares PP6 with PP1 and PP2 in terms of tensile strength, elongation at break, and tensile modulus. Because of its higher crystal orientation, PP2 had a higher tensile strength and modulus and a lower elongation at break than PP1. The elongation at break of PP2 after the CO2 treatment (PP6) was significantly higher than that of PP2, whereas the tensile strength and modulus remained unchanged. These results indicate that highly oriented iPP can be significantly toughened without loss in tensile strength and modulus by scCO2-induced recrystallization and subsequent fast cooling in icewater. Additionally, the elongation of all of the samples led to necking. Next, the morphologies and structures of PP1PP6 were investigated using DSC, polarized optical microscopy (POM), FTIR spectroscopy, SEM, SAXS, and WAXD to better understand the structureproperty relationship. It should be noted that the volume of the iPP samples did not change much after the CO2 swelling. As such, changes in the geometry and density of the samples are ignored in the subsequent discussion. Distribution of Molecular Orientation. Figures 4 and 5 show polarized optical micrographs of cross sections of PP1 and PP2. Figure 4 reveals a typical morphological pattern of an injection9634



Figure 4. Polarized optical micrographs of the cross sections of injection-molded iPP samples at lower magnification: (a) PP1 and (b) PP2. Distinct layers are (A) skin layer, (B) intermediate layer, and (C) core layer.

Figure 5. Polarized optical micrographs of the cross sections of injectionmolded iPP samples at higher magnification: (a) PP1 and (b) PP2. The images were obtained from the different zones shown in Figure 4.

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molded semicrystalline sample, namely, a skin layer, an intermediate layer, and a core layer. Figure 5 shows the morphologies of different zones (zones AC) in Figure 4 at higher magnification. The obvious striped birefringence and colorful interference in zone A indicate an oriented and closely crystalline structure. In zone B, the interference color of PP1 becomes light, whereas that of PP2 is still dark. This indicates that the structure of PP2 was more highly oriented and more closely crystalline than that of PP1. Especially in zone C, PP2 still exhibited a slight orientation, whereas PP1 formed isotropic spherulites. Thus, the orientation level of PP2 at all positions (from zone A to zone C) was significantly higher than that of PP1, and PP2 had a closer structure. The orientation degrees of PP1PP6, in both the flow direction and the perpendicular direction, are discussed in detail in the following section. Figure 6 shows the degrees of orientation in both the flow and perpendicular directions for PP1PP6 at different positions from the skin to the core. The degree of orientation was indeed higher for PP2 than for PP1 in all layers. The highest degree of orientation was, as expected, in the skin layer. Figure 6a,b also shows that the CO2 treatment slightly altered the orientation of the iPP in flow direction. However, the change is obvious in the perpendicular direction after CO2 treatment. as shown in Figure 6c,d. Moreover, comparison of PP1 with PP3 and PP5 and of PP2 with PP4 and PP6 indicates that the increase in the orientation degree in the perpendicular direction was slight from the distance of about 800 μm to the core, for both the crystalline phase and the amorphous phase. It seems that the orientation degree in the perpendicular direction after CO2-induced recrystallization increased only when highly oriented crystal existed. The slight change in orientation in the flow direction might be

Figure 6. Orientation distribution along the thickness determined by IR spectra for iPP: (a) crystalline phase in the flow direction, (b) amorphous phase in the flow direction, (c) crystalline phase in the perpendicular direction, and (d) amorphous phase in the perpendicular direction. The lines only indicate the trends. 9635



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Figure 7. DSC thermograms of PP1-PP6 in the (a) skin, (b) intermediate, and (c) core layers.

due to the presence of the highly oriented crystalline phase. This is consistent with our assumption in Figure 1b. However, it does not mean that the CO2 treatment has no effect on the isotropic zone. Thermal Properties. It is stated above that a highly oriented crystal structure is a prerequisite for the formation of an interconnected network structure. The POM results confirmed the formation of such an oriented structure in the skin layer of the iPP samples, and the FTIR results revealed changes in both the crystalline and amorphous phases after CO2 treatment. Parts ac of Figure 7 show the DSC thermograms of samples PP1 PP6 in the skin, intermediate, and core layers, respectively. For each condition, DSC measurements were conducted at least three times, and the average melting parameters and corresponding maximum deviations were obtained. The percentages of crystallinity are also shown in the patterns. Comparing Figure 7a with Figure 7b,c, the DSC traces of PP1 and PP2 in the skin layer show two obvious endothermic peaks corresponding to lamellar and stretched crystals. Even after CO2 treatment, the two endothermic peaks still exist, as shown in Figure 7a. On the other hand, after the CO2 treatment, the corresponding samples (PP3PP6) showed higher degrees of crystallinity, and their main melting peaks were higher and broader than those of PP1 or PP2. Moreover, a very broad endotherm peak appeared between 100 and 130 C. It is well-known that a broad melting peak can be attributed to a broad distribution of lamellar thicknesses due to crosshatched structures consisting of thick primary lamellae with a higher melting temperature and thin subsidiary lamellae with a lower melting temperature.30,32 Therefore, an increase in the number of lamellae, especially for the subsidiary lamellae perpendicular to the primary lamellae, is considered to be the reason for the broader peak. In other words, scCO2-induced recrystallization might allow the development of a crystalline network similar to the one depicted in Figure 1 as a result of an increase in the

Figure 8. Two-dimensional SAXS images of (a) PP2 before CO2 treatment, (b) PP2 after CO2 treatment, (c) PP1 before CO2 treatment, and (d) PP1 after CO2 treatment.

numbers of the primary lamellae and crosshatched subsidiary lamellae. In addition, there is not much difference in terms of the DSC traces between PP4 and PP6 (or PP3 and PP5). Nevertheless, the difference between the pairs of samples is significant in terms of the impact strength as a result of the difference in cooling rates. Crystalline Phase Morphology in the Skin Layer. Figure 8 shows SAXS patterns of both PP1 and PP2 before and after annealing under CO2. The flow direction of the samples is shown in Figure 8, and the measurement direction is vertical. Typically, the equatorial streak in the SAXS pattern can be attributed to the formation of an oriented structure parallel to the flow direction. The meridional maxima are attributed to the layerlike oriented structure perpendicular to the flow direction.42 Compared with Figure 8c, Figure 8a exhibits stronger reflections along the equator, which indicates the higher orientation of PP2 compared to PP1. 9636



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Figure 9. SEM images of iPP samples in the shear zone foamed at 145 C and a CO2 saturation pressure of 15 MPa: (a) PP2, (b) PP6. The white bar at the bottom of each image is 1 μm. The arrows indicate the flow direction.

Figure 10. SEM images of the impact-fractured surfaces of PP1PP6 at lower magnification: (a) PP1, (b) PP2, (c) PP3, (d) PP4, (e) PP5, and (f) PP6. AC indicate the crack initiation zone, crack propagation zone, and later stage of crack propagation, respectively. The rectangular zone shown by dashed lines represents the nonfractured part of the specimen during the impact fracture process.

The significant increase in the meridional maxima of PP2 after CO2 treatment, as shown in Figure 8b, indicates that the CO2 treatment brought about a significant increase in the oriented structure perpendicular to the flow direction. On the other hand, compared with those of PP2, the change in the meridional maxima of PP1 was not obvious, as shown in Figure 8c,d. This is consistent with the FTIR results in suggesting that the orientation degree in the perpendicular direction increased after CO2-induced recrystallization only when highly oriented crystal was present. In addition, the strong scattered intensity of Figure 8b due to the increase in crystallinity further corroborates the conclusion that annealing under CO2 induces iPP to recrystallize. As a result, the initially formed oriented and unoriented crystals grew in size and approached perfection. Moreover, Figure 8b shows strong diffused scattering indicative of the unoriented crystals, which corroborates the foregoing DSC results that the broad endotherm between 100 and 130 C represents the components oriented in the perpendicular direction, in addition to the main skeleton of the shish-kebab structure that is generated during the CO2 annealing. It is common practice to observe crystal structures of polyolefins such as iPP by etching or pigmentation. However, it is not

always easy to obtain distinct images. A previous work showed that foaming in the solid state could be a good alternative for observing the crystal structure of a semicrystalline polymer in an indirect manner. The amorphous regions should foam, whereas the crystalline ones will remain intact.25 To observe the crystal morphologies of PP2 and PP6, the samples were foamed at 145 C and a CO2 saturation pressure of 15 MPa. A certain proportion of crystals could be molten under these conditions. Figure 9 shows SEM images of PP2 and PP6 foams in the skin layer. It confirms the formation of the crystal network structure as a result of the CO2-induced crystallization in which structures oriented parallel and perpendicular to the flow direction were connected with each other. The closely connected shish-kebab lamellar crystals improved the strength perpendicular to the orientation direction. Moreover, the existence of crosshatched subsidiary crystals among the primary lamellae led to a more uniform distribution of the rigid crystalline phase domains in the soft amorphous phase domains. These unique nanosize structures could promote the absorption of the impact energy perpendicular to the orientation direction and, hence, enhance the toughness of the samples. 9637



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Figure 12. WAXD profiles of iPP samples.

Figure 11. SEM images of the impact-fractured surfaces of PP1PP6 at higher magnification. The images were obtained from the different zones shown in Figure 10. (a) PP1, (b) PP2, (c) PP3, (d) PP4, (e) PP5, and (f) PP6.

Impact-Fractured Surface Morphologies. Studying the impact-fractured surface morphology is favorable for understanding the toughening mechanism of PP samples. Figures 10 and 11 show representative impact-fractured surface morphologies of PP1PP6. It is well-known that the fracture process of a specimen under the load of impact includes at least two steps, namely, crack initiation and crack propagation. Thus, to provide more detailed information about the impact fracture process of the specimens, the overall impact-fractured surface was subclassified into the crack initiation zone (zone A), the early stage of the crack propagation zone (zone B), and the later stage of the crack propagation zone (zone C), as shown in Figure 10a, according to the methodology developed by Misra and co-workers.43,44 An interesting characteristic feature of the fracture surface of PP before CO2 treatment (PP1 and PP2) is that a rough surface is observed in the crack initiation zone (zone A), followed by a flat and smooth surface in crack propagation zone (zones B and C). This indicates that strain energy absorption mainly occurred in the crack initiation zone, leading to the rapid breakdown of the crack propagation zone. Furthermore, one can observe a small zone with a parabolic ridge or a thin curved band at relatively

lower magnification (Figure 10a), related to the change in the stress state. For PP3 and PP4, the typical morphology of brittlefracture mode was observed without any considerable plastic deformation of matrix. Moreover, the fractured surface in the crack propagation zone (zones B and C) became coarser, and some microvoids (see Figure 11c,d) appeared in the impactfractured surface because of the interconnected network crystalline structure after CO2 treatment. This observation agrees well with the enhancement of the impact strength as shown in Figure 3. For PP5 and PP6, completely different surface morphologies were observed in comparison with those of PP1PP4. First, an obvious plastic deformation appeared in the later crack propagation zone (zone C in Figure 10e,f). Second, one can notice that the samples were not fractured completely because of the relatively higher fracture resistance, as shown in Figure 3. PP6 exhibited a larger nonfractured part (see the rectangular zone in dashed lines in Figure 10) than PP5, suggesting a higher fracture resistance of PP6. In the later stage of the crack propagation process (zone C in Figure 11e,f), one can observe the local deformation of the matrix and the formation of a large amount of striations running perpendicular to the crack propagation direction. These plastic deformation zones absorbed a significant amount of impact energy during the fracture process and made the sample more ductile.45 This result is attributed to the more uniform distribution of the rigid crystalline phase domains in the soft amorphous phase domains after the third step, as shown in Figure 3c. In other words, the crack propagation resistance was dramatically improved with the interconnected network crystalline structure and the uniform distribution of crystal and amorphous phases after CO2 treatment of the oriented iPP specimens. Effect of Quenching on the Crystal Structure. It is stated above that the increased number of lamellar crystals as a result of scCO2-induced recrystallization is a reason for the improved toughness of the iPP samples (PP2 versus PP4). The cooling stage that follows the scCO2 treatment is another important reason for the improvement in toughness (PP4 versus PP6). What structural changes occur during the cooling stage? Figure 12 shows the WAXD patterns of PP1PP6 in the skin layer. The WAXD patterns of PP1 and PP2 exhibit five major peaks located at 14.1, 16.1, 16.9, 18.6, and 20.3, corresponding to the diffraction of crystal reflections from the R(110), β(300), R(040), R(130), and γ(117) planes, respectively. The percentages of crystallinity are shown in the patterns. For PP1 and PP2, there is no obvious peak in the range from 20 to 22, possibly because of the high degree of orientation in the skin layer. It is interesting that the amounts of β and γ crystals in PP1 9638



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Figure 13. (a) Impact strength and (b) WAXD pattern of PP6 after reannealing for 5 h at various temperatures.

are significantly higher than those in PP2. Normally, shear can induce crystallization leading to β and γ crystals.46,47 However, very high shear might prevent β and γ crystals from forming.20 This is because shear accelerates the nucleation and growth of R-row nuclei, which tend to induce the formation of R crystals rather than β and γ crystals. In other words, the shear rate for PP1 in the injection molding process was able to promote the formation of β and γ crystals, whereas it was too high in the case of PP2. On the other hand, the WAXD pattern of PP3 or PP4 is not different from that of PP1 or PP2. That of PP6 shows that the crystallinity of the β-form crystal decreased, the γ-form peak disappeared, and a new peak at 21.8 appeared. This indicates that, under scCO2 treatment followed by quenching in ice water, the β- and γ-form crystals were transformed to R-form ones, and a new type of crystal might have formed. It has been reported that, when iPP is quenched from melt to low temperature, an intermediate phase called a mesomorphic phase can be formed.3439 The WAXD pattern of the mesomorphic phase exhibits only two peaks at about 14.8 and 21.8. Thus, during the cooling process in an icewater bath, the iPP that had already been subjected to scCO2 treatment might produce mesomorphic phase domains of several dozen nanometers in size. Such nanosize phase domains distributed in the existing crystalline network structure would significantly enhance the impact strength. Effect of the Instability of Nanosize Mesomorphic Phase. It is well-known that the mesomorphic phase is unstable. Chains in the mesomorphic phase domains can reorganize or “melt” at an elevated temperature and are transformed into the R-monoclinic phase.34 PP6 was reannealed in a vacuum oven for 5 h at 30, 40, 50, or 80 C. Parts a and b of Figure 13 show the effects of this reannealing on the impact strength and WAXD pattern, respectively. As shown in Figure 13b, the areas of all of the reflection peaks normalized to that at 21.8 decreased with increasing reannealing temperature. The impact strength decreased accordingly. The impact strength of PP6 was 75.7 kJ/m2 and was reduced to 18 kJ/m2 when it is reannealed at 80 C for 5 h, during which the mesomorphic phase domains almost completely disappeared (no reflection peak at 21.8). This value is close to that of PP4, as expected. The above results further confirm the formation of the mesomorphic phase of the scCO2-treated iPP during quenching in icewater.

4. CONCLUSIONS This study proposes a novel process for significantly toughening isotactic polypropylene by finely tuning and controlling the structure and morphology of the iPP. More specifically, the toughness of injection-molded iPP specimens can be significantly improved by controlled shearing, CO2 induces recrystallization and adequate cooling without loss of strength. Under shear, a high degree of orientation can be obtained with “shish-kebab” crystals formed in the shear zone. During the subsequent CO2 treatment, a crystal network morphology can be formed as a result of an increase in the numbers of primary and crosshatched subsidiary lamellae. Two-dimensional SAXS images and SEM micrographs of iPP foamed in the solid state confirm the formation of a crystal network structure in the shear zone in which structures oriented parallel and perpendicular to the flow direction can be connected with each other. WAXD patterns indicate that quenching in icewater of scCO2-treated iPP promotes the formation of nanosized mesomorphic phase domains in the shear zone, which further toughens the iPP. The impact strength of the best toughened iPP sample was over 12 times that of the original sample without any loss in tensile strength and modulus. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +86-21-64253175. Fax: +86-21-64253528. E-mail: zhaoling@ ecust.edu.cn.

’ ACKNOWLEDGMENT The authors are grateful to the National Natural Science Foundation of China (Grants 20976045, 20976046), Shanghai Shuguang Project (08SG28), Program for New Century Excellent Talents in University (NCET-09-0348), Innovation Program of Shanghai Municipal Education Commission (11CXY23), Program for Changjiang Scholars and Innovative Research Team in University (IRT0721), 111 Project (B08021), and “the Fundamental Research Funds for the Central Universities” for support. ’ REFERENCES (1) Meijer, H. E. H.; Govaert, L. E. Mechanical performance of polymer systems: The relationship between structure and properties. Prog. Polym. Sci. 2005, 30 (89), 915–938. 9639


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Carbon Dioxide Induced Crystallization for Toughening Polypropylene

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