Temperature-Dependent Alternating α-or β-Transcrystalline Layers in

Jun 23, 2017 - 1, Yihuan Road, Chengdu, Sichuan 610065, China. ABSTRACT: Continuous and orderly α- or β-transcrystallin- ity plays a significant rol...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Temperature-Dependent Alternating α- or β‑Transcrystalline Layers in Coextruded Isotactic Polypropylene Multilayered Films Yibo Yu, Shuo Yang, Huaning Yu, Jiang Li,* and Shaoyun Guo The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu, Sichuan 610065, China ABSTRACT: Continuous and orderly α- or β-transcrystallinity plays a significant role in achieving excellent performance for isotactic polypropylene (iPP). However, this special structure is difficult to obtain through conventional methods. Here, continuous and orderly α- or β-transcrystallinity was constructed in an iPP layer of an eight-layer iPP/α-iPP sample using a layered design and temperature control. The crystalline structure and morphology of the iPP layer were greatly influenced by the cooling rate. The iPP layer contained a large amount of β-transcrystallinity and a small amount of αtranscrystallinity when the sample was cooled to 130 °C for isothermal crystallization at a cooling rate of 50 °C/min. However, only α-transcrystallinity was formed in the iPP layer when the cooling rate was 1 °C/min. The different crystallization mechanisms are discussed in this paper. In addition, the experimental results showed that β-transcrystallinity could simultaneously improve both the tensile and impact properties. coated glass fibers to produce a β-transcrystalline layer,30−32 and Fu and co-workers33 observed a β-transcrystalline structure with an ordered lamellae arrangement using the molding method to transform β-NA into a needle-like distribution. The formation of the β-nuclei in these cases underwent a heterogeneous nucleation mechanism. In addition, it was found that the crystallization of iPP under a temperature gradient encouraged the formation of a β-transcrystalline layer even in the absence of fibers or plates.34 It has been reported that during crystallization under a temperature gradient a kinetically favored α-to-β growth transition occurred due to either the presence of latent β-prenuclei or the influence of local stresses. A common way to obtain a transcrystalline layer with β-iPP as a rich phase was by melt shearing, such as fiber pulling over a temperature interval with a lower (373 K) and an upper (413 K) threshold.16,35−38 Varga et al. suggested that this β-transcrystalline layer formed on the preoriented α-row nuclei induced by melt shearing as an α-to-β transition during the crystalline modification of iPP.16,35,39 They defined the shearinduced β-transcrystalline layer as cylindrite to indicate its distinct homogeneous nucleation from the transcrystalline structure triggered by surface-induced heterogeneous nucleation. Yan et al. further proved that the oriented chains of iPP crystallize easily and act as β-nucleants for the transcrystalline layer by introducing highly oriented iPP fibers into the homogeneous molten matrix.40−42 In conclusion, the first essential condition to achieving βtranscrystallinity was the formation of β-nuclei in iPP melts for

1. INTRODUCTION Semicrystalline isotactic polypropylene (iPP), as the first representative of industrially manufactured stereoregular polymers, can crystallize into several crystalline forms, i.e., α, β, γ, and δ forms.1−6 Among these crystalline forms of iPP, the β-form, which usually forms together with the α-form, has been the subject of a number of investigations in the past few decades because β-iPP has exhibited improved toughness and ductility compared to traditional α-iPP.7−10 However, the mechanism of the formation of the β-form is still not fully understood. IPP also displayed a diverse crystalline morphology on a larger scale (i.e., at the level of a polarizing microscope), such as spherulite and transcrystals (sometimes cylindritic or fanshaped). In recent years, many studies have examined transcrystallization occurring at the fiber/iPP matrix interface of fiber-reinforced composites, as the transcrystalline layer plays an important role in interfacial adhesion and mechanical properties.11−18 Similar transcrystallization behavior has been observed at the layered interface between iPP and solid plates.19−22 Under certain conditions, the fibers or plates induced a relatively high crystal nucleation density on their surface. The closely packed nuclei hindered the lateral extension of spherulites, which were then forced to grow only perpendicular to the interface.23−26 It has been suggested that during the quiescent crystallization the α-transcrystalline layer is heterogeneously induced by the majority of fibers or plates in accordance with the proposed mechanism. Recently, significant interest had increasingly centered on the formation of the β-transcrystalline layer, as both the β-form and transcrystalline layer can improve the mechanical properties of iPP.27−29 For example, some researchers have used β-nucleant © XXXX American Chemical Society

Received: May 15, 2017 Revised: June 14, 2017

A

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules either homogeneous or heterogeneous nucleation. In addition to the methods stated above, it is also possible to induce βnuclei by rapidly cooling iPP melts to a specific temperature range and then performing isothermal crystallization. Padden and Keith et al. obtained β-crystals for the first time in 1959 when they rapidly cooled an iPP film from the melt state to a temperature range between 110 and 148 °C. They found that a new spherulite formed when the crystallization was conducted over temperature ranges from 128 to 132 °C, which they named β-crystal.43,44 Jozsef Varge and Leugering et al. later found that a fast cooling rate could help the formation of βcrystals.45,46 However, the content of β-crystals was very small, and since then, this approach had not received much attention. The second requirement for obtaining β-transcrystallinity is that the resulting β-nuclei must have enough room to develop into transcrystallinity. For the pure or α-nucleating agents evenly dispersed in iPP melts, the formation of β-nuclei was simultaneous or lagged behind the formation of α-nuclei when the iPP melts were cooled down rapidly to the crystallization temperature. In this case, the β-crystals could not grow any more, as the surrounding space was occupied by α-crystals. For iPP melts where the β-nucleating agents were dispersed uniformly, β-nuclei could not form β-transcrystallinity, as the amount was excessive. Consequently, although rapidly cooling the iPP melts is a simple method to induce β-nuclei, it cannot cause the formation of β-transcrystallinity in iPP where β-nuclei are dispersed uniformly. In our previous work,47 we employed a multilayered coextrusion technology to prepare a layered iPP material where pure iPP and β-nucleating agent/iPP layers were alternatively arranged. During the crystallization of the layered samples, the asymmetrical growth of β-nuclei at the twodimensional layered interfaces caused the β-lamellae to freely grow perpendicular to the interface and finally form βtranscrystallinity in the pure iPP layer. In this paper, we used the same technology to obtain a layered iPP material where pure iPP and α-nucleating agent/ iPP layers were alternatively arranged. Because of the existence of the α-nucleating agents, the crystallization of the αnucleating agent/iPP layer occurred prior to that of the pure iPP layer. When the β-nuclei induced by the rapid cooling started to appear in the α-nucleating agent/iPP layer, the pure iPP layer was still in the melt state. In this case, the β-nuclei at the layered interface had enough space in the pure iPP layer to grow into β-transcrystallinity. In addition, the effects of the cooling rate on the crystallization behavior and mechanical properties were investigated.

Scheme 1. Molecular Formula of NA21

a temperature-controlled chamber. The temperature was increased to 250 °C for 30 min to erase their thermal history completely and subsequently cooled to 130 °C at cooling rates of 50 and 1 °C/min. Then, the samples were held at this temperature for 120 min before finally cooling them to room temperature. The iPP and α-iPP samples with the same size as the eight-layer sample were thermally treated using the above method as a control. These treated samples were prepared for differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), polarized optical microscopy (POM), scanning electron microscopy (SEM), and mechanical property measurements. 2.3. Characterization and Testing. 2.3.1. Differential Scanning Calorimetry (DSC) Measurement. The crystallization behavior of the samples was studied under a nitrogen atmosphere using the DSC instrument (Q20, TA Instruments). For each test, the samples, which ranged in weight from 7 to 8 mg, were heated from 40 to 250 °C at a rate of 10 °C/min and then maintained at 250 °C for a period of 10 min to ensure an identical thermal history. The specimens were subsequently cooled to 130 °C at different cooling rates of 50 and 1 °C/min and then maintained at 130 °C for 120 min. Afterward, the specimens were reheated to 250 °C at a scanning rate of 10 °C/min, and the melting process was recorded. 2.3.2. Wide-Angle X-ray Diffraction (WAXD) Measurement. To characterize the content of the β-crystals in the specimens, WAXD patterns of the thermally treated samples were recorded with a DX2500 X-ray diffractometer (Cu Kα, λ = 1.54 Å; 40 kV; 25 mA; reflection mode). The experiments were performed over a 2θ range of 5°−45° at a scanning rate of 0.03°/s and a scanning step of 0.02°. The content of the β-crystal (Kβ) was calculated from2

Kβ =

Iβ1 Iβ1 + Iα1 + Iα 2 + Iα3

(1)

where Iα1, Iα2, and Iα3 are the intensities of the (110), (040), and (130) reflections from the α-crystal at 2θ = 14.1°, 16.8°, and 18.7°, respectively, and Iβ1 is the intensity of the (300) reflection of the βcrystal at 2θ = 16.1°. 2.3.3. Polarized Optical Microscopy (POM) Observation. First, an approximately 15 μm think slice was obtained from the sample using a microtome for morphology observation. Then, the isothermal crystallization process and selective melting at 165 °C of the samples were observed on a POM (BX51, OLYMPUS) equipped with a crossed polarizer, a video camera, and a hot stage (HCS302, INSTEC). For the isothermal crystallization process, the slice was first heated to 250 °C for 30 min and subsequently cooled to 130 °C at cooling rates of 50 and 1 °C/min before holding at this temperature for a period of 120 min and finally cooling to 40 °C at a cooling rate of 10 °C/min. For the selective melting, the slice was heated to 165 °C at a heating rate of 10 °C/min and held at this temperature for a period of 10 min. 2.3.4. Scanning Electron Microscopy (SEM) Observation. The microscopic structure of layered iPP/α-iPP samples was characterized using a scanning electron microscope (JSM-5900LV, Japan) with an accelerating voltage of 20 kV. The surfaces of the samples were etched for 24 h with an etchant (approximately 100 mL) containing 1.3 wt % potassium permanganate (KMnO4), 32.9 wt % concentrated sulfuric acid (H2SO4), and 65.8 wt % concentrated phosphoric acid (H3PO4), according to the procedure described by Olley.48 Prior to SEM

2. EXPERIMENTAL SECTION 2.1. Materials. The polymer matrix was commercial-grade iPP (F401) with a melting flow index of 2.9 g/min (230 °C, 2.16 kg), a density of 0.89 g/cm3, and an isotacticity index of 98%. The material was produced by Yangzi Petroleum and Chemical Co. (Nanjing, China). The organic phosphate α-nucleating agent of NA21, the molecular formula of which is shown in Scheme 1, was supplied by Luoyang Zhongda Chemical Co. (Luoyang, Henan, China). 2.2. Sample Preparation. Pellets of α-nucleating agent filled iPP (α-iPP with 0.5 wt % α-nucleating agent) were prepared using a twinscrew extruder. The mixing temperature gradient was varied between 160 and 210 °C from hopper to die. The eight-layer samples consisting of alternating layers of virgin iPP and iPP with the αnucleating agent were prepared as described in our previous work.47 The eight-layer samples (length 80 mm; width 10 mm; thickness 2 mm; parallel to the direction of the extruder in length) were placed in B

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules characterizations, the surfaces of all the samples were coated with a thin layer of gold by ion sputtering. 2.3.5. Mechanical Properties Measurement. Tensile tests were performed on a SANS CMT4104 testing machine (Shenzhen, China) at a strain rate of 50 mm/min according to ASTM D638. The overall length and width of the samples were 80 mm and 10 mm, respectively, and the length and width of the gage section were 30 mm and 4 mm, respectively. The thickness of the samples was approximately 2 mm. The tensile force was applied parallel to the extrusion direction. The notched Izod impact strength was tested with an XJU-22 impact test machine (Chengde, China). The sample dimensions were 80 mm × 10 mm × 4 mm (thickness). The load direction for the impact tests was vertical to the extrusion direction. The samples for impact testing were prepared through compression molding of two sheets at 210 °C.

3. RESULTS AND DISCUSSION 3.1. Effect of Cooling Rate on the Crystallization Behaviors of α-iPP and iPP. First, we examine the effect of

Figure 2. POM micrograph of the crystalline morphology in the α-iPP sample that was isothermally crystallized at 130 °C for 60 min followed by cooling at a rate of 50 °C/min and was then heated to 165 °C. The crystals marked by red circles belonged to the β-form.

Figure 3. POM micrographs of the eight-layer iPP/α-iPP sample. The black regions are α-iPP layers, whereas the white regions correspond to iPP layers.

iPP and α-iPP began to crystallize at 130 °C when the cooling rate was 50 °C/min. The difference was that the α-iPP sample showed a sharp and obvious crystallization peak at 34.0 min, whereas the iPP sample showed a weak crystallization peak at 53.4 min. This demonstrated that the addition of 0.5 wt % αnucleating agent not only caused the crystallization peak to appear 19.4 min earlier but also increased the rate of crystallization significantly. Additionally, the α-iPP began to crystallize at 145 °C for a cooling rate of 1 °C/min. According to the literature, no β-crystals can form at this temperature.16 Compared to the α-iPP, iPP began to crystallize at 130 °C when the cooling rate was 1 °C/min, crystal formation lagged behind, and the rate of crystallization decreased. As indicated in the Introduction, a necessary condition for the formation of β-transcrystallinity is the establishment of βnuclei in the iPP melts. It has been widely reported that a small amount of β-crystals would be obtained when the pure iPP melts were cooled rapidly.43,44 To confirm if β-crystals were formed in the α-iPP melts under rapid cooling, a polarizing hot stage experiment was conducted in which the α-iPP sample was heated to 250 °C and held at this temperature for 10 min before cooling to 130 °C at a rapid cooling rate of 50 °C/min. This time was set as 0 s. The sample’s isothermal crystallization temperature was kept at 130 °C for 60 min and later increased

Figure 1. Isothermal crystallization curves for the iPP and α-iPP samples at 130 °C at a cooling rate of 50 °C/min (a) or 1 °C/min (b) from 250 to 130 °C.

the cooling rate on the iPP and α-iPP melt crystallization behavior. Here, the cooling rate refers to the cooling rate for the iPP and α-iPP melts from 250 to 130 °C. A temperature of 130 °C was found to be most conducive to the formation and growth of β-nucleation and was selected for the isothermal crystallization.35 Figures 1a and 1b show the isothermal crystallization DSC spectra at 130 °C when the cooling rates of the α-iPP and iPP were 50 and 1 °C/min, respectively. Both C

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. POM micrographs of eight-layer iPP/α-iPP samples: isothermal crystallization at 130 °C for 120 min with different cooling rates of 1 and 50 °C/min (8L-1 and 8L-50, respectively, in parts a and c) and the selective melting of 8L-1 and 8L-50 at 165 °C (parts b and d). The white arrows indicate the nucleation sites for β-transcrystallinity.

Figure 5. SEM micrograph of sample 8L-50 etched for 24 h.

Figure 6. WAXD spectra for samples 8L-1 and 8L-50 isothermally crystallized at 130 °C for 120 min at cooling rates of 1 and 50 °C/min, respectively.

to 165 °C with a heading rate of 10 °C/min, followed by a 10 min holding time. The process was recorded using polarized microscopy, with the results shown in Figure 2. Figure 2a shows that crystallization does not occur in melts when the temperature reaches 130 °C. After 10 s of isothermal crystallization, a large number of small and dark crystal grains and a small amount of large and bright crystals, marked by red circles, arose almost simultaneously (Figure 2b). As the isothermal crystallization time increased, these two kinds of crystals continued to grow and improve, but the difference in the structures and morphology still existed (60 min/130 °C). When the temperature increased to 165 °C, the large and bright crystals disappeared, and only the smaller and dark crystals remained. The result of the selective melting of iPP crystals showed that the large and bright crystals were β-

Table 1. Content of the β-Crystals Obtained from the WAXD Spectra in Figure 6 sample

Kβ/%

8L-50 8L-1

24.6 0

crystals. The amount of the β-crystals, which were obtained by rapidly cooling the α-iPP melts, was very low, meaning that they could not be detected by DSC and WAXD testing. However, the following experiments show that a small amount of β-nuclei is the key to inducing continuous and orderly βtranscrystallinity in the layered iPP/α-iPP sample. D

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 7. Some typical isothermal crystallization photographs of the eight-layer specimen at 130 °C obtained from POM, with a cooling rate of 50 °C/min.

3.2. Crystallization Behavior of Alternating Layered iPP/α-iPP. We prepared eight-layer iPP/α-iPP material using the multilayered coextrusion machine in our laboratory, as described in our previous work.47 As shown in Figure 3, the black regions are the α-iPP layer, and the white regions correspond to the iPP layer. Compared to the iPP layer, the αiPP layer exhibited no birefringence due to the presence of nucleating agents, leading to the formation of small spherulites in the α-iPP layer. The iPP and α-iPP layers were alternately arranged to form a smooth interface with good continuity and consistency. The initial eight-layer iPP/α-iPP sample was recrystallized in two ways. In the first way, heating was applied to 250 °C and held for 30 min to eliminate the thermal history, and then cooling to 130 °C using cooling rates of 1 or 50 °C/min to induce isothermal crystallization was applied. The recrystallized samples are denoted as 8L-1 and 8L-50, respectively. Figure 4 shows the POM photographs of the 8L-1 and 8L-50 samples before and after the selective melting at 165 °C. Although the component and layer structures of 8L-1 and 8L-50 were the same, their iPP layers exhibited completely different crystal structures and morphologies due to the different cooling rates. Figure 4a shows that the two transcrystalline layers grew perpendicularly to the layered interface and converged at the middle of the iPP layer of sample 8L-1. Compared with Figure 4c, Figure 4a shows that all of the transcrystallinity in the iPP layer of sample 8L-1 did not melt at 165 °C, which is a temperature between the two melting points for the α- and β-

crystals. This comparison indicates that the iPP layer of sample 8L-1 is only composed of α-transcrystallinity. However, a change in the crystallization procedure led to the formation of β-crystals. In Figure 4c, sample 8L-50 exhibited two kinds of different crystalline structures and morphologies in the iPP layer: a thin column layer for the iPP close to the layered interface and a fan-shaped structure in the iPP exhibiting a bright appearance due to its strong negative birefringence. As shown in Figure 4d, the selective melting at 165 °C revealed that the column layers were composed of αcrystals, whereas the melting of the fan-shaped structure showed β-crystal characteristics. In Figure 4d, the point-like nuclei of the β-fan-shaped transcrystallinity, indicated by arrows, are located at the layered interface. This indicates that the β-nuclei at the layered interface induced by the fast cooling grew to β-fan-shaped transcrystallinity in the iPP layer. As discussed, the amount of the β-crystals in the α-iPP layer is very low, meaning that the number of nuclei of αtranscrystallinity is much larger than that of the point-like nuclei of β-fan-shaped transcrystallinity. However, the latter prevailed over the former and eventually covered a majority of the iPP layer due to a faster overgrowth rate of the β-phase than the α-phase. Moreover, the growth rate ratio of the β- to αcrystals, Gβ/Gα = 1.5, at 130 °C is obtained from Figure 4d, according to the method derived by Lovinger.34 This result was nearly consistent with that of Lovinger (Gβ/Gα = 1.4 at 130 °C). The small amount of β-nuclei at the interface and their greater growth rate caused the fan-shaped growth of the βE

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 8. Some typical isothermal crystallization photographs of the eight-layer specimen at 130 °C obtained from POM, with a cooling rate of 1 °C/min.

from the interface. At this time, β-nuclei at the interface were too small and difficult to observe by POM. With the increase of crystallization time, the α-transcrystalline layer became thick, and the thickness remained uniform. After 30 s of isothermal crystallization, three bright spots belonging to the β-form, marked by red circles, appeared in the α-transcrystalline layer. With a further increase in the crystallization time, these three βcrystals continued to grow in fan shape like individual flowers and occupied the vast majority of the iPP layer, exhibiting a typically strong negative birefringence characteristic when exposed to a polarizing microscope. Meanwhile, the growth of the α-transcrystalline layer was suppressed and stopped after 80 s because it was wrapped by fan-shaped β-crystals, resulting in the jagged morphology of the α-transcrystalline layer being formed. It is noted that an α-nucleus was induced after 10 s in the lower right corner of the picture and eventually grew into a large size α-spherical crystal due to a lack of competitive nuclei in its vicinity. As shown in Figure 8, when the cooling rate was 1 °C/min, the α-transcrystallinity formed at the interface of the iPP and αiPP layers grew uniformly in the direction perpendicular to the layered interface until meeting another α-transcrystallinity from the opposite direction or α-spherical crystals in the center of the iPP layer. During the isothermal crystallization, no βcrystals were formed. According to the above results, the isothermal crystallization mechanism of the iPP layer of the eight-layer sample at different cooling rates was summarized. As shown in Figure 9, the cooling rate had a significant impact on the crystal structure and morphology of the iPP layer. When cooling the eight-layer sample rapidly with a cooling rate of 50 °C/min, sporadic β-

crystals in the iPP layer. Therefore, the thin α-transcrystallinity layer exhibited a jagged shape around the interface, as shown in the SEM image. Figure 5 shows an SEM image of the cross section of sample 8L-50 where the amorphous phase has been etched away. Compared to the α-crystal, the structures of the βcrystals are looser, causing them to be more likely to be etched and become brighter in the SEM image. As seen from Figure 5, the iPP layer of sample 8L-50 can be divided into two areas according to the crystal form: two jagged α-transcrystalline layers close to the layered interface and a β-fan-shaped transcrystalline layer sandwiched between the α-transcrystalline layers. The nucleation sites of the β-fan-shaped transcrystallinity are sparse and located at the interface. To confirm some of these conclusions, WAXD experiments were conducted on samples 8L-1 and 8L-50, with the patterns shown in Figure 6. Among them, sample 8L-1 only exhibited αform characteristic peaks at approximately 2θ = 14° (110), 16.9° (040), and 18.4° (130). In addition to α diffraction peaks, sample 8L-50 exhibited an obvious β (300) crystal diffraction peak at 2θ = 16.0°. From the data in Table 1, which were calculated using eq 1, the amounts of the β-crystals in samples 8L-1 and 8L-50 are 0% and 24.6%, respectively. The previous POM and SEM exhibited crystalline structure and morphology after crystallization was completed. Figures 7 and 8 show a dynamic process of isothermal crystallization for the eight-layer samples at different cooling rates. Figure 7 shows that the iPP and α-iPP layers of the eight-layer sample underwent isothermal crystallization after the sample’s temperature decreased to 130 °C at a cooling rate 50 °C/min. After 10 s of isothermal crystallization, the α-iPP layer crystallized, and a thin α-transcrystalline layer appeared in the iPP layer and grew F

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 9. Schematic diagram describing the formation mechanism of α- and β-transcrystallinity in the layered sample when the cooling rates were 1 or 50 °C/min.

of isothermal crystallization of the iPP layer, both α- and βlamellae grew along the direction perpendicular to the interface. Subsequently, β-crystals grew quickly following the fan-shaped growth model and finally occupied the majority of the iPP layer due to its faster growth rate at 130 °C compared to α-crystal. As a result, two thin α-transcrystalline layers attached to the interface and fan-shaped β-transcrystallinity were formed in the iPP layer. The right schematic in Figure 9 shows that when the cooling rate was 1 °C/min, there was no β-nucleus induced at the interface and in the α-iPP layer; therefore, only an αtranscrystalline layer existed in the iPP layer. It should be noted that for either a rapid or slow cooling rate, the large individualistic α-spherulite appeared in the center of the iPP layer and terminated the growth of β- or α-transcrystallinity. 3.3. Mechanical Properties. By regulating the cooling rate, the continuous and orderly β- or α-transcrystallinity in the

Table 2. Mechanical Properties of the 8L-50 and 8L-1 Samples sample

yield strength (MPa)

elongation at break (%)

impact strength (kJ/m2)

8L-50 8L-1

33.6 ± 0.6 31.4 ± 0.4

84.3 ± 2.7 39.8 ± 1.9

2.78 ± 0.24 1.96 ± 0.13

nuclei and closely packed α-nuclei could form at the interface and then grow asymmetrically. The growth of α- and β-nuclei at the interface was suppressed in the α-iPP layer by other αspherulites because a large number of α-nuclei appeared simultaneously in the α-iPP layer. In contrast, the iPP layer provided enough space for the growth of α- and β-nuclei at the interface since the crystallization of the iPP layer lagged obviously behind that of the α-iPP layer. During the early stage G

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(6) Addink, E. J.; Beintema, J. Polymorphism of crystalline polypropylene. Polymer 1961, 2, 185−193. (7) Papageorgiou, D. G.; Chrissafis, K.; Bikiaris, D. N. β-Nucleated Polypropylene: Processing, Properties and Nanocomposites. Polym. Rev. 2015, 55, 596−629. (8) Sun, X. L.; Li, H. H.; Zhang, X. Q.; Wang, D. J.; Schultz, J. M.; Yan, S. Effect of Matrix Molecular Mass on the Crystallization of βForm Isotactic Polypropylene around an Oriented Polypropylene Fiber. Macromolecules 2010, 43, 561−564. (9) Labour, T.; Vigier, G.; Séguéla, R.; Gauthier, C.; Orange, G.; Bomal, Y. Influence of the β-crystalline phase on the mechanical properties of unfilled and calcium carbonate-filled polypropylene: Ductile cracking and impact behavior. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 31−42. (10) Kotek, J.; Raab, M.; Baldrian, J.; Grellmann, W. The effect of specific β-nucleation on morphology and mechanical behavior of isotactic polypropylene. J. Appl. Polym. Sci. 2002, 85, 1174−1184. (11) Miltner, H. E.; Grossiord, N.; Lu, K. K.; Loos, J.; Koning, C. E.; Van Mele, B. Isotactic Polypropylene/Carbon Nanotube Composites Prepared by Latex Technology. Thermal Analysis of Carbon Nanotube-Induced Nucleation. Macromolecules 2008, 41, 5753−5762. (12) Wang, C.; Liu, F. H.; Huang, W. H. Electrospun-fiber induced transcrystallization of isotactic polypropylene matrix. Polymer 2011, 52, 1326−1336. (13) Sun, X. L.; Li, H. H.; Lieberwirth, I.; Yan, S. K. α and β Interfacial Structures of the iPP/PET Matrix/Fiber Systems. Macromolecules 2007, 40, 8244−8249. (14) Gao, Y.; Xie, M. Y.; Liu, L. Q.; Zhang, Z. Effect of supramolecular microstructures on the adhesion of SWCNT fiber/iPP interface. Polymer 2013, 54, 456−463. (15) Li, H. H.; Yan, S. K. Surface-Induced Polymer Crystallization and the Resultant Structures and Morphologies. Macromolecules 2011, 44, 417−428. (16) Varga, J.; Karger-Kocsis, J. Rules of supermolecular structure formation in sheared isotactic polypropylene melts. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 657−670. (17) Somnuk, U.; Eder, G.; Phinyocheep, P.; Suppakarn, N.; Sutapun, W.; Ruksakulpiwat, Y. Quiescent crystallization of natural fibers−polypropylene composites. J. Appl. Polym. Sci. 2007, 106, 2997−3006. (18) Assouline, E.; Grigull, S.; Marom, G.; Wachtel, E.; Wagner, H. D. Morphology of α-transcrystalline isotactic polypropylene under tensile stress studied with synchrotron microbeam X-ray diffraction. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 2016−2021. (19) Fernandez, M. R.; Merino, J. C.; Gobernado-Mitre; Pastor, J. M. Molecular and Lamellar Orientation of α- and β-Transcrystalline Layers in Polypropylene Composites by Polarized Confocal MicroRaman Spectroscopy: Raman Imaging by Static Point Illumination. Appl. Spectrosc. 2000, 54, 1105−1113. (20) Yan, B. W.; Wu, H.; Jiang, G. J.; Guo, S. Y.; Huang, J. Interfacial Crystalline Structures in Injection Over-Molded Polypropylene and Bond Strength. ACS Appl. Mater. Interfaces 2010, 2, 3023−3036. (21) Cho, K.; Kim, D.; Yoon, S. Effect of Substrate Surface Energy on Transcrystalline Growth and Its Effect on Interfacial Adhesion of Semicrystalline Polymers. Macromolecules 2003, 36, 7652−7660. (22) Menyhard, A.; Varga, J.; Liber, A.; Belina, G. Polymer blends based on the β-modification of polypropylene. Eur. Polym. J. 2005, 41, 669−677. (23) Ye, J.; Fang, J. P.; Zhang, L.; Li, C. Z. Transcrystalline induced by MWCNTs and organic nucleating agents at the interface of glass fiber/polypropylene. Polym. Compos. 2017.10.1002/pc.24360. (24) Huang, J.; Xu, C. J.; Wu, D. F.; Lv, Q. L. Transcrystallization of polypropylene in the presence of polyester/cellulose nanocrystal composite fibers. Carbohydr. Polym. 2017, 167, 105−114. (25) Sanadi, A. R.; Caulfield, D. F. Transcrystalline interphases in natural fiber-PP composites: effect of coupling agent. Compos. Interfaces 2000, 7, 31−43.

iPP layer of the eight-layer sample could be obtained. The different crystalline structures and morphologies may greatly affect the mechanical properties of the materials. The tensile and impact properties of the 8L-1 and 8L-50 samples are listed in Table 2, with the results showing that all the mechanical properties of the 8L-50 sample are better than those of the 8L-1 sample. In comparison, the elongation at break and impact strength of the 8L-50 sample, which characterize the roughness of the material, exhibited more obvious increases of 112% and 42%, respectively. According to our preliminary studies, the βtranscrystallinity in the 8L-50 sample had a significant impact on the mechanical properties of the materials.49

4. CONCLUSION In this work, we found that β-nuclei can be observed in α-iPP melts by simply rapidly cooling the α-iPP melts from 250 to 130 °C. Although the amount of these β-crystals is very low, they played a key role in inducing continuous and orderly βtranscrystallinity in the iPP layer of an eight-layer iPP/α-iPP sample. By changing the cooling rate of the layered sample, the crystalline structure and morphology in the iPP layer exhibited a large change. When the cooling rate was 50 °C/min, two thin α-transcrystalline layers attached to the interface and fanshaped β-transcrystallinity formed in the iPP layer. In this case, the content of the β-crystals was 24.6%. When the cooling rate was 1 °C/min, the iPP layer of the layered sample only contained α-transcrystallinity. Because of the existence of βtranscrystallinity, sample 8L-50 exhibited better mechanical properties than the 8L-1 sample, especially in terms of toughness.

■

AUTHOR INFORMATION

Corresponding Author

*Tel +86-28-85466077; Fax +86-28-85466077; e-mail li_ [email protected] (J.L.). ORCID

Jiang Li: 0000-0002-5159-2320 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (51227802, 51473103, 51421061, and 51273132), the Sichuan Province Youth Science Fund (2014JQ0001), and the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant 2014TD0002) for financially supporting this work.

■

REFERENCES

(1) Brucükner, S.; Meille, S. V.; Petraccone, V.; Pirozzi, B. Polymorphism in isotactic polypropylene. Prog. Polym. Sci. 1991, 16, 361−404. (2) Turner-Jones, A.; Aizlewood, J. M.; Beckett, D. R. Crystalline forms of isotactic polypropylene. Makromol. Chem. 1964, 75, 134− 158. (3) Turner-Jones, A.; Cobbold, A. J. The β crystalline form of isotactic polypropylene. J. Polym. Sci., Part B: Polym. Lett. 1968, 6, 539−546. (4) Morrow, D. R.; Newman, B. A. Crystallization of Low-MolecularWeight Polypropylene Fractions. J. Appl. Phys. 1968, 39, 4944−4950. (5) Sauer, J. A.; Pae, K. D. Structure and Thermal Behavior of Pressure-Crystallized Polypropylene. J. Appl. Phys. 1968, 39, 4959− 4968. H

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Induced by Two-Dimensional Layered Interface. Macromolecules 2015, 48, 3965−3973. (48) Olley, R. H.; Bassett, D. C. An improved permanganic etchant for polyolefines. Polymer 1982, 23, 1707−1710. (49) Yang, S.; Yu, H. N.; Li, J.; Shen, J. B.; Guo, S. Y. Structural Evolution and Toughening Mechanism of β-Transcrystallinity of Polypropylene Induced by the Two-Dimensional Layered Interface during Uniaxial Stretching. ACS Omega 2017, 2, 814−827.

(26) Quan, H.; Li, Z. M.; Yang, M. B.; Huang, R. On transcrystallinity in semi-crystalline polymer composites. Compos. Sci. Technol. 2005, 65, 999−1021. (27) Grein, C. Toughness of Neat, Rubber Modified and Filled βnucleated Polypropylene: from Fundamentals to Applications. Adv. Polym. Sci. 2005, 188, 43−104. (28) Tjong, S. C.; Shen, J. S.; Li, R. K. Y. Impact fracture toughness of β-form polypropylene. Scr. Metall. Mater. 1995, 33, 503−508. (29) Karger-Kocsis, J.; Varga, J.; Ehrenstein, G. W. J. Comparison of the fracture and failure behavior of injection-molded α- and βpolypropylene in high-speed three-point bending tests. J. Appl. Polym. Sci. 1997, 64, 2057−2066. (30) Liu, Q.; Li, H. H.; Yan, S. K. Structure and properties of βpolypropylene reinforced by polypropylene fiber and polyamide fiber. Chin. J. Polym. Sci. 2014, 32, 509−518. (31) Luo, G. J.; Li, W. Z.; Liang, W. B.; Li, G. X. Coupling effects of glass fiber treatment and matrix modification on the interfacial microstructures and the enhanced mechanical properties of glass fiber/ polypropylene composites. Composites, Part B 2017, 111, 190−199. (32) Santoro, G.; Yu, S.; Krywka, C.; Ellis, G. Microfocus X-ray scattering and micro-Raman spectroscopy: Transcrystallinity in isotactic polypropylene. Rapid Res. Lett. 2014, 8, 724−727. (33) Luo, F.; Geng, C.; Wang, K.; Deng, H.; Chen, F.; Fu, Q.; Na, B. New Understanding in Tuning Toughness of β-Polypropylene: The Role of β-Nucleated Crystalline Morphology. Macromolecules 2009, 42, 9325−9331. (34) Lovinger, A. J.; Chua, J. O.; Gryte, C. C. Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient. J. Polym. Sci., Polym. Phys. Ed. 1977, 15, 641−656. (35) Varga, J.; Karger-Kocsis, J. Interfacial morphologies in carbon fibre-reinforced polypropylene microcomposites. Polymer 1995, 36, 4877−4881. (36) Sun, B. B.; Qin, Y. J.; Xu, Y. H.; Chen, J. B. Study of shearinduced interfacial crystallization in polymer-based composite through in situ monitoring interfacial shear stress. J. Mater. Sci. 2013, 48, 5354−5360. (37) Sun, X.; Li, H.; Wang, J.; Yan, S. Shear-Induced Interfacial Structure of Isotactic Polypropylene (iPP) in iPP/Fiber Composites. Macromolecules 2006, 39, 8720−8726. (38) Somani, R. H.; Hsiao, B. S.; Nogales, A.; Tsou, A. H. Structure Development during Shear Flow Induced Crystallization of i-PP: In Situ Wide-Angle X-ray Diffraction Study. Macromolecules 2001, 34, 5902−5909. (39) Varga, J.; Ehrenstein, G. W. Formation of β-modification of isotactic polypropylene in its late stage of crystallization. Polymer 1996, 37, 5959−5963. (40) Li, H. H.; Zhang, X.; Kuang, X.; Yan, S. A Scanning Electron Microscopy Study on the Morphologies of Isotactic Polypropylene Induced by Its Own Fibers. Macromolecules 2004, 37, 2847−2853. (41) Li, H. H.; Jiang, S. D.; Wang, J. J.; Wang, D. J.; Yan, S. K. Optical Microscopic Study on the Morphologies of Isotactic Polypropylene Induced by Its Homogeneity Fibers. Macromolecules 2003, 36, 2802− 2807. (42) Sun, X.; Li, H. H.; Zhang, X.; Wang, J.; Wang, D.; Yan, S. K. Effect of Fiber Molecular Weight on the Interfacial Morphology of iPP Fiber/Matrix Single Polymer Composites. Macromolecules 2006, 39, 1087−1092. (43) Padden, F. J.; Keith, H. D. Spherulitic Crystallization in Polypropylene. J. Appl. Phys. 1959, 30, 1479−1484. (44) Keith, H. D.; Padden, F. J.; Walter, N. M.; Wyckoff, H. W. Evidence for a Second Crystal Form of Polypropylene. J. Appl. Phys. 1959, 30, 1485−1488. (45) Leugering, V. H. J.; Kirsch, G. Beeinflussung der kristallstruktur von isotaktischem polypropylen durch kristallisation aus orientierten schmelzen. Angew. Makromol. Chem. 1973, 33, 17−23. (46) Varga, J. Modification change during spherulitic growth of polypropylene. Angew. Makromol. Chem. 1982, 104, 79−87. (47) Yang, S.; Yu, H. N.; Lei, F.; Li, J.; Wu, H.; Guo, S. Y. Formation Mechanism and Morphology of β-Transcrystallinity of Polypropylene I

DOI: 10.1021/acs.macromol.7b01012 Macromolecules XXXX, XXX, XXX−XXX