Non-isothermal crystallization, melting behaviors and mechanical

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Nonisothermal Crystallization, Melting Behaviors, and Mechanical Properties of Isotactic Polypropylene Nucleated with a Liquid Crystalline Polymer Rong Yang,* Lv Ding, Xin Zhang, and Jinchun Li* Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, National Experimental Demonstration Center for Materials Science and Engineering (ChangzhouUniversity), School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, P. R. China S Supporting Information *

ABSTRACT: The nonisothermal crystallization and melting behaviors of isotactic polypropylene (iPP) nucleated by a mainchain liquid crystalline polyester (poly(4,4′-bis(6hydroxyhexyloxy)biphenyl phenylsuccinate), PBDPS) as a βnucleating agent have been investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarizing optical microscopy (POM). The effect of PBDPS on the mechanical properties of iPP was also studied. The results showed that nonisothermal crystallization kinetics of pure iPP and iPP/PBDPS blends can be described by the Jeziorny and Mo methods. PBDPS can induce both α and β crystals for iPP, with β-nucleation playing the more prominent role. The relative content of the β-crystal (kβ) increased as the PBDPS content increased and the cooling rate decreased. The kβ value reached 75%, even with a fast cooling rate of 40 °C/min. The elongation at break, flexural modulus, impact strength, and heat deformation temperature (HDT) are improved by introducing PBDPS, while the tensile strength remains approximately the same. agents. Inorganic β-nucleating agents such as calcium silicate, nanocalcium carbonate, and halloysite nanotubes have been used to induce β-iPP due to their low price and environmental friendliness.11−13 However, low nucleation efficiency, poor compatibility, and dispersion difficulties limit their application. Low molecular weight organic β-nucleating agents, including polycyclic aromatic hydrocarbons, certain IIA metal salts, mixtures with dicarboxylic acids, and aromatic amide crystalline compounds, are widely used due to their high efficiency.14−16 However, color, poor thermal stability, toxicity, and other shortcomings limit their application. Combining low cost, good compatibility, and thermal stability, macromolecular β-nucleating agents have attracted an increasing amount of attention. Recent reports suggested that commercial polymers including polystyrene, a polystyreneacrylonitrile copolymer, and liquid crystalline polymer (Vectra A950) could induce β-iPP crystal formation with kβ values of approximately 32%.17,18 Moreover, Hu et al. synthesized a series of side-chain liquid crystalline polymers with siloxane as the main-chain, and when blended with polypropylene, kβ

1. INTRODUCTION Polypropylene is one of the most widely used thermoplastics in the world due to its excellent thermal and mechanical properties. However, its poor impact strength, heat resistance, low temperature ductility, and dimensional stability of injection molded products limit the application of polypropylene. The crystallization behavior of semicrystalline polymers is one of the most important factors affecting its mechanical properties. Isotactic polypropylene (iPP) can form at least the following five crystal forms under different crystallization conditions: α, β, γ, ε, and smectic forms.1−5 The α-crystal is the most thermodynamically stable and forms easily under ordinary processing. The α-iPP crystal form has better tensile stress and flexural strength. While β-iPP has a lower crystal density and melting point and higher heat deflection temperature, it possesses especially excellent impact strength.6 The β-iPP crystal can form micropores to absorb energy during impact, which greatly improves its impact strength.7 However, the formation of the β-crystal requires special conditions, such as the temperature gradient, shear induction, and β-nucleating agent.8−10 Among them, adding a β-nucleating agent is the most economical and efficient method. So far, the β-nucleating agents of polypropylene are divided into the following three categories: inorganic β-nucleating agents, low molecular weight organic β-nucleating agents, and macromolecular β-nucleating © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

October 2, 2017 January 11, 2018 January 29, 2018 January 29, 2018 DOI: 10.1021/acs.iecr.7b04115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research values reached almost 70%.19,20 In our previous study, we reported a main-chain liquid crystalline polyester (poly(4,4′bis(6-hydroxyhexyloxy)biphenyl phenylsuccinate), PBDPS) with a benzene side group, which was used as an efficient βnucleating agent for iPP. By incorporating 4 wt % PBDPS into iPP, almost pure beta iPP was produced with k-values that exceeded 0.96, which is often higher than that for most commercial nucleating agents.21 However, iPP always underwent a nonisothermal crystallization during actual polymer processing. In that case, it is important to investigate the β nucleation effect of PBDPS under nonisothermal crystallization. In this paper, the nonisothermal crystallization, the melting behaviors of pure iPP and iPP/PBDPS blends, the nucleation activity of PBDPS, and the influence on the mechanical properties for iPP were investigated in detail.

radiation (λ = 1.54056 Å). The scanned 2θ range was from 5° to 50° at a scanning speed of 2° min−1. The relative content of the β-form crystal of iPP (kβ) was calculated according to the Turner−Jones eq 1:26 kβ =

Hβ(300) Hα(110) + Hα(040) + Hα(130) + Hβ(300)

(1)

where Hβ (300) is the height of β-crystal peak at the diffraction angle 2θ = 15.9°, while Hα (110), Hα (040), and Hα (130) are the heights of three α-crystal peaks at 2θ = 14.1°, 16.8°, and 18.5°, respectively. 2.3.3. Polarizing Optical Microscopy (POM). The nonisothermal crystallization morphologies of pure iPP and iPP/ PBDPS blends were measured with a polarizing optical microscopy instrument (Nikon 50I), which was equipped with a heating and cooling stage (Linkam THMS600). The samples were heated to 190 °C, held for 5 min, and then cooled to 25 °C at 2.5, 5, 10, 20, and 40 °C/min. 2.3.4. Mechanical Properties. The standard mechanical test bars of pure iPP and iPP/PBDPS blends were prepared by a miniature injection molding instrument (Thermo Scientific HAAKE MiniJet II). The tensile strength and flexural strength were measured by a universal testing machine (Shenzhen Kaiqiangli Testing Instruments Co., Ltd., WDT-10), according to ASTM D-638 and D-790. The impact strength was tested on the basis of ASTM D-256 by using an impact tester (Chengde testing machine limited liability company, XJU-22).

2. EXPERIMENTAL SECTION 2.1. Materials. Isotactic polypropylene (PPH-T03, MFR = 2.5 g 10 min−1, 96% isotacticity) was supplied by Sinopec Zhenhai Refining & Chemical Company. The macromolecular β-nucleating agent (PBDPS) was synthesized from 2-phenylsuccinic acid and 4,4′-bis(6-hydroxyhexyloxy)biphenyl by melting esterification reaction in our laboratory,22 with Mn = 42 300 and Mw/Mn = 2.10. The chemical structure of PBDPS is shown in Scheme 1. Scheme 1. Chemical Structure of PBDPS

3. RESULTS AND DISCUSSION 3.1. Nonisothermal Crystallization Behaviors of iPP/ PBDPS Blends. The nonisothermal crystallization behaviors of pure iPP and iPP/PBDPS blends are presented in Figure 1, and 2.2. Sample Preparation. The β-nucleating agent PBDPS and iPP were melted and blended by an internal mixer (Changzhou Suyan Science and Technology Co., Ltd., China, SU-70C) at 190 °C for 4 min, with a screw speed of 30 rpm. The concentrations of the β-nucleating agent were 0.5, 2, and 4 wt %. 2.3. Characterization. 2.3.1. Differential Scanning Calorimetry (DSC). The nonisothermal crystallization kinetics of pure iPP and iPP/PBDPS blends were studied by using differential scanning calorimetry (DSC, TA Q20) in a nitrogen atmosphere. The samples (5 mg) were initially heated to 190 °C, held constant for 5 min, and then cooled to 25 °C at cooling rates of 2.5, 5, 10, 20, and 40 °C/min. After cooling, the samples were reheated to 190 °C at a heating rate of 10 °C/ min. Considering the beta-to-alpha recrystallization process occurs in samples that are cooled below 100 °C, the melting memory effect of β-nucleated iPP can be eliminated by the following procedure published by Varga et al.23,24 Samples were initially heated to 190 °C, held constant for 5 min, then cooled to 25 °C, and reheated to 190 °C. Next, the sample was cooled to 100 °C and then heated to 190 °C. Both the heating and cooling rates were 10 °C/min (PerkinElmer Diamond DSC). The crystallinity of α-crystal and β-crystal and the relative content of the β-crystal (Φβ) were calculated according to the literature.13,25 2.3.2. Wide-Angle X-ray Diffraction (WAXD). Wide-angle Xray diffraction (WAXD) measurements were performed on a D/MAX 2500 (Rigaku) X-ray diffractometer with Cu Kα

Figure 1. DSC crystallization curves of iPP and iPP/PBDPS blends at different cooling rates in N2.

the relevant data including the onset crystallization temperature (To), the crystallization peak temperature (Tp), the half width of crystallization peak (ΔW), and the enthalpy of crystallization peak (ΔHc, including both α and β crystals) are summarized in Table S1. As shown in Figure 1, the crystallization peak temperature TP shifts to lower temperatures and becomes broader as the cooling rate increases, since polymer chains do not have time to crystallize at higher temperatures. Meanwhile, the To and TP of iPP shift to higher temperatures with a B

DOI: 10.1021/acs.iecr.7b04115 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 2. Plots of Xt versus T for nonisothermal crystallization of iPP with different concentrations of PBDPS.

decreased ΔW while incorporating PBDPS under the same cooling rate. This suggests that PBDPS accelerates the crystallization rate of iPP and exhibits an effective heterogeneous nucleation effect. The iPP molecular chains were adsorbed on the surface of PBDPS chains which reduced the crystal interface free energy of iPP. Therefore, iPP can crystallize at higher temperatures. 3.2. Nonisothermal Crystallization Kinetics. The practical production process is a nonisothermal crystallization process; in that case, it has a stronger guiding significance to investigate the nonisothermal crystallization kinetics of iPP/ PBDPS blends. The relative crystallinity (X t ) at any crystallization temperature in a DSC curve can be calculated according to the following equation (2):

function of crystallization time t by using the following equation (3; Figure 3): t=

Xt =

Q∞

=

1 − X t = exp(−Ztt n)

∫T (dHc/dT ) dT 0

T∞

∫T (dHc/dT ) dT 0

(3)

where T is the temperature at crystallization time t and ϕ is the cooling rate. The faster the cooling rate, the shorter the crystallization time span. The halftime of crystallization (t1/2) is used to characterize the crystallization rate. The t1/2 of nonisothermal crystallization of pure iPP and iPP/PBDPS blends can be estimated from Figure 4, and the results are listed in Table 1. 3.2.1. Jeziorny Method. The isothermal crystallization kinetics of polymers often obey the Avrami equation (4).27−29

T

Qt

|T0 − T | ϕ

(4)

where the Xt is the relative crystallinity at the crystallization time t, n is the Avrami exponent, and Zt is the crystallization rate constant. Taking the logarithm of eq 4 twice leads to the following relationship (5):

(2)

where To and T∞ are the onset and end of crystallization temperatures, respectively, Qt and Q∞ are the heat released at the crystallization temperatures To and T∞, respectively, and Hc is the enthalpy of crystallization. Figure 2 shows the relative crystallinity (Xt) as a function of temperature for pure iPP and iPP/PBDPS blends at different cooling rates. All of the curves show the same antisigmoidal shapes, indicating that the samples undergo a nucleation stage with a slower crystallization rate, a faster initial crystallization stage, and a relatively slower secondary crystallization stage. Xt can be transformed into the

log[−ln(1 − X t )] = log Zt + n log t

(5)

The plots of log[−ln(1 − Xt)] versus log t for pure iPP and iPP/PBDPS blends at different cooling rates are shown in Figure 4. In the later stage of crystallization, the curves deviated from the Avrami equation due to the collision between the spherulites during secondary crystallization. Therefore, the range from 10 to 80% of the curves was selected for linear C


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Figure 3. Plots of Xt versus t for nonisothermal crystallization of iPP with different concentrations of PBDPS.

fitting. The slopes and intercepts from these fits correspond to the values of n and ln Zt, respectively. Jeziorny modified the crystallization rate constant Zt with the cooling rate ϕ (6):30 log Zc = log Zt /ϕ

combined the Avrami equation with the Ozawa equation to derive a nonisothermal crystallization kinetics equation at a given crystallinity (7):32,33 ln Zt + n ln t = ln K (T ) − m ln ϕ

(7)

ln ϕ = ln F(T ) − α ln t

(8)

(6)

The nonisothermal crystallization rate constant Zc, Avrami exponent n, the halftime of crystallization t1/2, and the correlation coefficient R2 values are listed in Table 1. There are some values of the Avrami exponent above (4), which does not mean spherulitic growth of iPP is greater than threedimensional growth because there is no specific physical meaning for the Avrami exponent n under nonisothermal crystallization.31 In addition, it can be seen that the Avrami exponents of iPP and iPP/PBDPS blends are very similar, which suggested that incorporating PBDPS did not change the growth pattern of iPP. The t1/2 of each group of samples decreased as the cooling rate increased. Meanwhile, the t1/2 values of iPP/PBDPS blends were lower than those of pure iPP at the same cooling rate, which indicated that PBDPS accelerated the crystallization rate of iPP. The values of Zc increased as the cooling rate increased, which means that increasing the cooling rate also increased the crystallization rate. At cooling rates below 10 °C/min, the Zc of iPP/PBDPS blends was greater than that of pure iPP. At cooling rates above 10 °C/ min, the Zc of pure iPP and iPP/PBDPS blends were almost the same. This indicated that PBDPS are efficient at high crystallization temperatures. 3.2.2. Mo’s Method. Crystallinity is associated with the cooling rate and the crystallization temperature. Mo et al.

In eq 8, F(T) = [K(T)/Zt]1/m, where the F(T) is a function of the cooling rate, its physical meaning corresponds to the cooling rate necessary to reach a defined crystallinity in a specified crystallization time. F(T) characterizes the degree of difficulty of the sample to reach a certain crystallinity in a certain crystallization time. Smaller values of F(T) correspond to faster crystallization rates. The α is the ratio of the Avrami exponent n to the Ozawa exponent m (α = n/m). Figure 5 shows the plots of ln ϕ versus ln t for pure iPP and iPP/PBDPS blends at different relative crystallinities with a perfect linear relationship. The slopes and intercepts from these linear fits correspond to the values of −α and ln F(T), respectively. All of the values of α of iPP and iPP/PBDPS blends are 1.06−1.21, 1.08−1.31, 1.17−1.52, and 1.25−1.61. Values greater than one indicate that the Avrami exponent n is higher than the Ozawa exponent m. There is no specific physical meaning for the Avrami exponent; however, the Ozawa exponent is reliable, and it can predict the mechanism of nonisothermal crystallization of polymer. This explained why some Avrami exponents of iPP were greater than four. The values of α and F(T) are listed in Table 2. As the relative crystallinity increases from 20% to 80%, the values of F(T) also increase. This increase indicates that at a unit crystallization time, the high relative crystallinity comes D


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Figure 4. Plots of log[−ln(1 − Xt)] versus log t for pure iPP and iPP/PBDPS blends at different cooling rates.

3.3. Nucleation Activity. Dobreva and Gutzow proposed a simple method to calculate the nucleation activity φ of a foreign substance in a polymer melt.34,35 The nucleation activity φ can be calculated using eq 9:

Table 1. Nonisothermal Crystallization Kinetic Data of the Samples Calculated Using the Jeziorny Method samples pure iPP

iPP/PBDPS-0.5%

iPP/PBDPS-2%

iPP/PBDPS-4%

ϕ (°C/min)

n

Zc

t1/2 (min)

R2

2.5 5 10 20 40 2.5 5 10 20 40 2.5 5 10 20 40 2.5 5 10 20 40

7.41 5.62 4.57 3.66 3.02 7.69 5.93 4.65 3.75 2.84 6.26 4.93 4.07 3.47 2.57 6.09 4.54 3.88 3.10 2.56

0.16 0.77 1.04 1.06 1.04 0.16 0.78 1.05 1.06 1.04 0.26 0.85 1.06 1.06 1.04 0.34 0.91 1.05 1.06 1.03

4.0 1.6 0.8 0.4 0.3 3.8 1.5 0.7 0.4 0.3 3.2 1.4 0.7 0.4 0.2 2.6 1.2 0.7 0.4 0.2

0.999 0.999 0.998 0.996 0.994 0.999 0.999 0.998 0.994 0.994 0.999 0.999 0.997 0.993 0.994 0.997 0.997 0.996 0.996 0.994

φ = B* / B

(9)

where the φ is the nucleation activity and the B* and B correspond to the three-dimensional nucleation work of nucleated iPP and pure iPP, respectively. The stronger the nucleation activity of the foreign substance, φ decreases until it approaches 0. If the foreign substance has no nucleation activity, then φ = 1. The values of B* and B can be obtained from the slope of the lines from the following equations, (11): ln ϕ = const − B*/ΔTP 2

(10)

ln ϕ = const − B /ΔTP 2

(11)

where ΔTP is the degree of supercooling (ΔTP = Tm − Tc) of α or β crystals. Therefore, the nucleation activity of α and β nucleation can be calculated separately. The plots of ln ϕ versus 1/ΔTP2 for pure iPP and nucleated iPP are shown in Figure 6a,b and show the α and β nucleation activity of PBDPS, respectively. The nucleation activities of α crystallization for iPP/PBDPS-0.5%, iPP/PBDPS-2%, and iPP/ PBDPS-4% were 0.96, 0.86, and 0.79, respectively, and the activities of β crystallization were 0.40, 0.39, and 0.31, respectively. Values of φ decrease with the addition of PBDPS, while β-crystal activities are much smaller than those for α crystals. This finding indicates that PBDPS has a strong selective nucleation ability for the β crystal of iPP.

with a fast cooling rate. At the same relative crystallinity, the values of F(T) decrease with the addition of PBDPS. Meanwhile, the β-nucleating agent PBDPS accelerated the crystallization rate, which is consistent with the conclusion of the Jeziorny method. E


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Figure 5. Plots of ln ϕ versus ln t for pure iPP and iPP/PBDPS blends at different relative crystallinity.

to the melting of the iPP β crystal.11,36,37 This finding indicated that PBDPS induced the formation of the β crystal of iPP perfectly. Meanwhile, the intensity of β crystal melting peak increased with increased PBDPS and decreased cooling rates. At a 2 wt % addition of PBDPS, another shoulder peak appears (α′ crystal) on the α crystal melting peak at approximately 169 °C. The lower melting point corresponds to the α-iPP, which was formed during the crystallization of the sample under the constant cooling rate. However, the α′ crystal (with a slightly higher melting point) relates to the alpha modification formed due to the beta-to-alpha recrystallization during the heating process.17,21 The melting peak of α′ crystal shifted to higher temperatures with increased PBDPS addition and a decreased cooling rate. When the content of PBDPS reached 4 wt %, α crystals almost disappeared, indicating that amounts of β crystal formed. By incorporating PBDPS, the crystallinity of α crystal decreased, while β crystal increased. Faster cooling rates lowered the relative content of the β crystal, while the maximum Φβ value (92.4%) was reached with the addition of 4 wt % PBDPS and a cooling rate of 2.5 °C/min. Because of βα recrystallization, the β crystal content calculated by the melting enthalpy was no longer accurate. Varga et al.23,24 noted that βα recrystallization occurred if βnucleated iPP cooled below the critical temperature (TR = 100 °C). In that case, melting behaviors with different end cooling temperatures of PBDPS nucleated iPP were investigated. As shown in Figure S1, the crystallization curves of pure iPP and iPP/PBDPS blends with end cooling temperatures at TR= 25 and 100 °C are the same, indicating that βα recrystallization had no influence on the nucleation effect of PBDPS. As seen in Figure 8, iPP showed a single endothermic peak at 161.7 °C

Table 2. Nonisothermal Crystallization Kinetic Data of the Samples Calculated According to Mo’s Method samples pure iPP

iPP/PBDPS-0.5%

iPP/PBDPS-2%

iPP/PBDPS-4%

Xt (%)

α

F(T)

R2

20 40 60 80 20 40 60 80 20 40 60 80 20 40 60 80

1.06 1.08 1.14 1.21 1.08 1.15 1.22 1.31 1.17 1.28 1.36 1.52 1.25 1.39 1.49 1.61

7.14 8.30 9.44 11.04 6.88 8.00 9.15 10.98 6.49 7.42 8.55 10.25 4.75 5.52 6.54 8.51

0.997 0.998 0.999 0.999 0.999 0.999 0.998 0.998 0.997 0.998 0.998 0.999 0.997 0.997 0.995 0.995

3.4. Melting Behaviors of iPP/PBDPS Blends after Nonisothermal Crystallization. The nonisothermal melting curves of pure iPP and iPP/PBDPS blends are shown in Figure 7, and the calculated data are listed in Table S2. Pure iPP shows only one melting peak at approximately 163 °C, which belongs to the melting of the α crystal. While introducing the βnucleation agent PBDPS, two or three melting peaks appeared in the heating scan. The melting peak in the high-temperature region still corresponds to the α crystal, while another lowtemperature melting peak at approximately 150 °C is attributed F


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Figure 6. Plots of ln ϕ versus 1/ΔTP2 for pure iPP and iPP/PBDPS blends at different cooling rates.

Figure 7. DSC melting curves of iPP and iPP/PBDPS blends at different cooling rates in N2.

that belongs to the melting of α-iPP; subsequent heating curves with a recooled temperature at 100 °C (TR = 100 °C) behaved in a similar fashion. However, for the iPP/PBDPS blends, there were two obvious endothermic peaks at 149 (β-iPP) and 162 °C (α-iPP). When the recooled temperature was 25 °C (TR = 25 °C) for the iPP/PBDPS blends, the intensity of βmodification melting peaks decreased, while the α-modification remained the same. Moreover, an additional endothermic peak (α′-iPP) was observed at 168 °C and increased as the PBPDS content increased. These results suggested that βα recrystallization occurred during the heating process, while the β-nucleated

iPP cooled below the critical temperature (TR = 100 °C). Conversely, if the cooling temperature stayed above TR, no βα recrystallization occurs in the heating scan. This phenomenon is consistent with the findings demonstrated by Varga et al.23,24,38 that postcrystallization occurred during the cooling process (TR < 100 °C), crystallization nuclei consisting of α modification are formed within the β-spherulites, which induce the βα recrystallization of the molten β-spherulites in the α modification. Generally, there are melting, recrystallization and polymorphic transformations that occur during heating; therefore, G


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attributed to the (110), (040), (130), (111), and (−131) planes of α-crystal reflections, respectively.39 However, iPP/PBDPS blends show an additional diffraction peak at 15.9°, which is attributed to the (300) plane of the β-crystal. The diffraction intensity of the β-crystal peaks increases with lower cooling rates, while the α-crystal peak intensities decrease. The diffraction peaks of α crystals almost disappear when the addition of PBDPS reaches 4 wt % and a cooling rate of 2.5 °C/ min. Higher PBDPS contents in combination with slower cooling rates induced a higher β-crystal content. The highest relative content of the β crystal reached 97%. Values of kβ can reach a maximum value of 75% with a fast cooling rate (40 °C/ min). This finding is consistent with results from DSC experiments. The α and β crystals of polypropylene exhibit different optical properties. Polarized optical microscopy (POM) can determine crystalline morphologies and the growth of spherulites. Figure 10 shows the polarized optical microscope photographs of pure iPP and iPP/PBDPS blends that crystallized at a cooling rate of 2.5 °C/min. Under the nonisothermal crystallization condition, the α-crystal of pure iPP formed irregular shapes rather than spherulites. However, there are bright crystals observed in iPP/PBDPS blends that increased as the PDBPS content increased. These correspond to β-crystals due to higher birefringence.40 The spherulites of β crystal developed and the size decreased as the PBDPS content

Figure 8. Crystallization curves of pure iPP and iPP/PBPDS blends at two different recooling temperatures, TR = 25 (dotted line) and 100 °C (solid line).

the calculated crystallinity according to the melting enthalpy was not accurate. In that case, WAXD was utilized to further investigate the relative content of each crystal during nonisothermal crystallization. Figure 9 shows the WAXD curves of iPP and iPP/PBDPS blends prepared at different cooling rates. The WAXD curve of pure iPP comprises five peaks at 2θ values of 13.9°, 16.7°, 18.4°, 21°, and 21.7°

Figure 9. WAXD curves of iPP and iPP/PBDPS blends at different cooling rates. H


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Figure 10. Polarized optical microscope photographs of pure iPP and iPP/PBDPS blends crystallized at a cooling rate of 2.5 °C/min.

Figure 11. Polarized optical microscope photographs of iPP/PBDPS-0.5% blends crystallized at different cooling rates.

increased, which suggested that PBDPS is an efficient βnucleating agent for iPP. Figure 11 shows the POM photographs of iPP/PBDPS-0.5% crystallized at different cooling rates. The size of β crystals decreased as the cooling rates increased, which resulted from relatively low β-crystal growth rates and a high self-nucleation rate of iPP at lower temperatures. These conclusions are consistent with the results from DSC and WAXD experiments. 3.5. Effect of PBDPS on the Physicomechanical Properties of iPP. The crystal forms and crystallinity are two important factors affecting the mechanical properties of semicrystalline polymers. With respect to mechanical properties, α-iPP exhibits better tensile and flexural strength; however, β-iPP plays a major role in impact strength, heat deformation

temperature, and elongation at break.6,7,41 PBDPS can induce isotactic polypropylene to form a high content of the β-crystal, so it is necessary to study the influence of PBDPS loadings on the mechanical properties of iPP. Figure 12 shows the physicomechanical properties of pure iPP and iPP/PBDPS blends; detailed data are listed in Table S3. As the PBDPS content increased, the tensile strength and flexural strength initially increased but then decreased. Maximum values of tensile strength and flexural strength were obtained when the PBDPS content was 2 wt %. However, the elongation at break and the impact strength increased significantly as the PBDPS content increased. At a 4 wt % of PBDPS, the elongation at break increased by 1443%, which is 7.9 times greater than that of pure iPP. Meanwhile, the impact strength is approximately I


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Figure 12. Physico-mechanical properties of pure iPP and iPP/PBDPS blends (a) tensile properties, (b) flexural properties, (c) impact strength, and (d) heat deformation temperature (HDT).

impact strength of iPP increased from 183% to 1443% and 2.6 to 4.8 kJ/m2, respectively. Additionally, the flexural strength of iPP improved due to the dual nucleation behavior of PBDPS. The heat deformation temperature increased from 99.1 to 116.3 °C. Therefore, PBDPS is an efficient macromolecular βnucleating agent for iPP.

1.9 times greater than that of pure iPP, and the heat deformation temperature (HDT) of iPP improved from 99.1 up to 116.3 °C. According to the above analysis, PBDPS is a dual nucleating agent which can induce both α and β crystals in iPP. In general, the α-iPP shows better stiffness than β-iPP; however, β-iPP exhibits greater toughness than α-iPP. Therefore, the stiffness and toughness of iPP can be simultaneously enhanced by adjusting the relative content of α and β crystal of iPP.24,42 In this case, better comprehensive mechanical properties of iPP can be obtained by introducing 2 wt % PBDPS into iPP.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04115. Calculation method of the alpha and beta crystallinities from DSC; crystallization curves of pure iPP and iPP/ PBPDS blends with different recooling temperature; and relevant data obtained from Figures 1, 7, and 12. (PDF)

4. CONCLUSIONS In this paper, the nonisothermal crystallization, melting behaviors and mechanical properties of iPP nucleated with a main-chain liquid crystalline polyester (PBDPS) were investigated. PBDPS showed an efficient heterogeneous β-nucleation effect. The addition of PBDPS resulted in an increase in the crystallization temperature; an increase in the PBDPS content and a decrease in the cooling rate resulted in a higher crystallization temperature. The Jeziorny and Mo methods describe the nonisothermal crystallization kinetics very well. Meanwhile, PBDPS can induce both α and β-crystals for iPP; however, it exhibited a strong selective nucleation affinity for the β-crystal of iPP. The relative content of the β crystal (kβ) of iPP increased with the increasing concentration of PBDPS and the decreasing cooling rate, reaching a maximum value (97.1%) with an addition of 4 wt % PBDPS and a cooling rate of 2.5 °C/min. The introduction of PBDPS simultaneously increased the toughness and stiffness of iPP. Elongation at the break and the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rong Yang: 0000-0002-0193-6949 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Natural Science Foundation of Jiangsu Province (BK20150257), the National Natural Science Foundation of China (51473024), the Top-Notch Academic Programs Project J


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of Jiangsu Higher Education Institutions, and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Non-isothermal crystallization, melting behaviors and mechanical

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