Critical Content of Ultrahigh-Molecular-Weight ... - ACS Publications

Nov 29, 2012 - Republic of China. §. Chemistry Division, Patent Examination Cooperation Center of State Intellectual Property Office of China, Beijin...
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Critical Content of Ultrahigh-Molecular-Weight Polyethylene To Induce the Highest Nucleation Rate for Isotactic Polypropylene in Blends Wei Shao,†,‡ Yaqiong Zhang,† Zhigang Wang,*,† Yanhua Niu,*,‡ Ruijuan Yue,§ and Wenping Hu‡ †

CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui Province 230026, People's Republic of China ‡ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China § Chemistry Division, Patent Examination Cooperation Center of State Intellectual Property Office of China, Beijing 100083, People's Republic of China S Supporting Information *

ABSTRACT: The influence of the addition of low amounts of ultrahigh-molecular-weight polyethylene (UHMWPE) on the crystallization kinetics of isotactic polypropylene (iPP) in iPP/UHMWPE blends has been investigated by means of differential scanning calorimetry (DSC) and polarized optical microscopy. During the nonisothermal crystallization process, the primarily formed UHMWPE crystals serve as heterogeneous nucleating agents for iPP nucleation, whereas during the isothermal crystallization process, UHMWPE is in the molten state, iPP nucleation preferentially occurs at the UHMWPE and iPP phase interfaces, and the spherulitic growth rates are not obviously affected. It is particularly interesting to find a critical UHMWPE content (2.5 wt %) in the blends to induce the highest iPP nucleation rate; however, above the critical UHMWPE content, the iPP nucleation rate slows because of aggregation of the UHMWPE component. A delicately designed DSC measurement provides insight into the nucleation mechanism of iPP at the interfaces between the UHMWPE and iPP phase domains. It is proposed that the concentration fluctuations generated from the unstable inhomogeneous phase interfaces in the iPP/UHMWPE blends promote the formation of nuclei, which eventually enhances the nucleation and overall crystallization rates of the iPP component.

1. INTRODUCTION Isotactic polypropylene (iPP) is currently one of the fastestgrowing polymers because of its high stiffness, excellent water and chemical resistance, low density, ease of processability, and, most importantly, superior performance-to-cost ratio, which makes it the highest-consuming polymer. However, some shortcomings, i.e., low heat distortion resistance and poor impact behavior, limit its applications. To overcome these deficiencies, blending with other polymers is a feasible method to modify the physical and mechanical properties of iPP products, which nowadays attracts much more attention. From the application viewpoint, iPP is conveniently blended with polyethylene (PE),1 in which the compatibility and interfered crystallization behavior play essential roles to determine the ultimate properties because failure of the materials takes place at the microscopic level. Despite the fact that the chemical structures of iPP and PE are similar, extensive studies prove that the miscibility of this blend system depends on the topology of the PE chains, i.e., the type and distribution of the comonomer, long chain branching, etc. iPP is miscible with linear low-density polyethylene if the iPP content is less than 20 wt % but immiscible with high-density polyethylene (HDPE) and low-density polyethylene (LDPE).2−9 The miscibility between iPP and PE has a significant influence on the crystallization behavior.2−4,6,7,10 For iPP/HDPE blends, the © 2012 American Chemical Society

number of nuclei of iPP increases with increasing HDPE content at the relatively lower temperatures, whereas it decreases significantly to a large extent above a certain crystallization temperature.3 For iPP/LDPE blends, the crystallization half-time (t1/2), Avrami exponent (n), and chain folding energy (σe) for iPP crystallization increase markedly above 10 wt % LDPE contents because of aggregation of molten LDPE.2 For iPP/ethylene-propylene-diene monomer rubber (EPDM) blends, the crystallization rate of iPP markedly increases when the EPDM content is low and reaches a maximum at 25 wt % EPDM content, which is ascribed to the increased nucleation rate; beyond that content, the crystallization rate decreases gradually because of the impingement effect of the rubber phase on the growth of spherulites.4 For structural material applications, the mechanical properties of polymer blends are inherently related to their morphologies. Previous reports clearly show the kind of relationship.11,12 The investigations on the mechanical properties of semicrystalline polymer blends show that even small changes of crystalline Received: Revised: Accepted: Published: 15953

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and a delicately designed DSC experiment, insight into the nucleation mechanism of iPP with the addition of UHMWPE is provided. This fundamental study helps us to well understand the influences of the addition of a second minor UHMWPE component on the crystallization kinetics of iPP. This, in turn, may have an influence on the mechanical properties of particular blends.

morphologies can obviously influence the mechanical properties.13−16 Recently, blends of polyolefins mixed with ultrahighmolecular-weight polyethylene (UHMWPE) have drawn increasing attention because mixing of just a few percent of UHMWPE can result in significant improvements of the physical and mechanical properties of the blends over the polyolefin matrixes.17,18 However, most works focus on the mechanical or processing properties, and rather limited publications are available relating to the crystallization behavior. The high-molecular-weight PE component is widely considered to have the capability of inducing PE nucleation. Whether UHMWPE has particular influences on the crystallization behavior of other types of semicrystalline polymers is an interesting topic and has not been well understood yet. Song et al. investigated the isothermal and nonisothermal crystallization kinetics of HDPE/UHMWPE blends and found that the addition of UHMWPE could greatly enhance the crystallization rate of HDPE.19 It is generally considered that UHMWPE is miscible with a HDPE matrix at low UHMWPE contents and the upper limit content ranges from 3 to 6% with respect to different types of PE.20 A low amount of UHMWPE in a HDPE matrix can disperse quite well at the molecular level. UHMWPE is demonstrated to be immiscible with a iPP matrix for iPP/ UHMWPE blends; thus, the dispersion state of the minor UHMWPE component is considered to have significant influence on iPP crystallization. A series of investigations focused on the shear-induced crystallization of iPP with the addition of UHMWPE.21−23 The UHMWPE component (5 and 10 wt %) played a key role for iPP nucleation, and the crystallization rate for the 5 wt % sample is even higher than that for the 10 wt % sample, which is different from the miscible HDPE/UHMWPE case. This result was pertinently ascribed to a phase-separation effect; however, the detailed crystallization mechanism is not well understood because the above studies mainly focused on the shish-kebab formation mechanism in the system. Recently, Xin et al. reported an investigation on the nonisothermal crystallization kinetics of iPP/UHMWPE blends,24 which revealed that the addition of UHMWPE led to increases in the crystallization temperature and relative crystallinity. Owing to the UHMWPE extremely high molecular weight, a question is raised relating to the mixing technique. It should be realized that the mixing conditions can exert profound effects on the miscibility and crystallization behavior of the blends. Virtual observation of the crystalline morphologies in real space during crystallization is certainly desired but was missed in the above studies. In addition, only iPP/UHMWPE blends with UHMWPE contents higher than 5 wt % were studied in the above publications. The effects of the lower UHMWPE contents were not taken into account; thus, the critical UHMWPE content to affect the iPP crystallization kinetics has not been revealed. In this study, we prepared a series of iPP/UHMWPE blends with a wide range of UHMWPE contents (0.5, 1.0, 2.5, 5.0, and 10.0 wt %) to systematically examine the influence of low contents of the long-chain species and interfacial phase area in the blends on iPP crystallization kinetics. Nonisothermal and isothermal crystallization kinetics for neat iPP and iPP/ UHMWPE blends were studied by differential scanning calorimetry (DSC). We find that at a critical UHMWPE content (2.5 wt %) UHMWPE induces the highest iPP nucleation rate. The result is further confirmed by polarized optical microscopy (POM) observation. Through data analyses

2. EXPERIMENTAL SECTION 2.1. Materials. The polymers employed in this study were iPP (bought from Aldrich Company, Mw = 340 K, Mn = 97 K, melt flow index = 4.00 g/10 min) and UHMWPE (kindly supplied by the Beijing Second Subsidiary Additive Factory, Mw = 3360 K). A solution mixing method was used to obtain the homogeneous blends. iPP/UHMWPE blends with different UHMWPE contents were prepared by coprecipitating from hot xylene solutions (ca. 125 °C) into chilled methanol and then filtering and drying under vacuum at 60 °C for 3 days. The iPP blends containing 0.5, 1.0, 2.5, 5.0, and 10.0 wt % UHMWPE were denoted as UH-0.5, UH-1.0, UH-2.0, UH-5.0, and UH10.0, respectively. Neat iPP was subjected to an identical preparation procedure for comparison purposes. 2.2. DSC Measurements. Nonisothermal and isothermal crystallization scans on the iPP/UHMWPE blends were carried out using a TA Q200 differential scanning calorimeter (TA Instruments, USA). Indium was used as a reference material to calibrate both the temperature and enthalpy before the blends were tested. The blend weights were about 4 mg. For the nonisothermal crystallization procedure, the blends were first heated to 200 °C at 10 °C/min, kept there for 10 min to eliminate thermal histories, and then cooled to −50 °C at a cooling rate of 10 °C/min. Afterward, the second heating scans at a heating rate of 10 °C/min were carried out. Isothermal crystallization scans were performed according to the following procedure: the blends were first heated to 200 °C, kept for 10 min to eliminate thermal histories, and then quenched with a cooling rate of 50 °C/min to the target crystallization temperatures of 130, 132, 134, and 136 °C. Note that at the above selected temperatures the UHMWPE component maintains the molten state without any crystallization traces during the isothermal crystallization and subsequent heating scans, as evidenced from the DSC measurements (see Figure S1 in the Supporting Information). A temperature higher than 136 °C was not chosen because of the much slower crystallization rates for the blends. 2.3. POM Observation. Morphologies of neat iPP and iPP/UHMWPE blends during isothermal crystallization at different temperatures of 130, 132, 134, and 136 °C on a homemade hot stage were observed using an Olympus BX-51 polarized optical microscope (Olympus, Japan). Blends were first hot-pressed between two cover glasses at 200 °C to form films with thicknesses of ca. 50 μm. The sandwiched film samples were placed on the hot stage and heated to 200 °C for 10 min followed by quenching to the isothermal crystallization temperatures of 130, 132, 134, and 136 °C. The optical micrographs were recorded using a CCD camera (HV1301UC; Da Heng Co., China). 3. RESULTS AND DISCUSSION 3.1. Crystallization Kinetics of iPP/UHMWPE Blends. Our previous studies demonstrate that the phase-separated 15954

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Table 1. Tcp and Tonset during Cooling and Tmp during Heating for iPP, UHMWPE, and iPP/UHMWPE Blends (Cooling and Heating Rates Are 10 °C/min)

blends and homogeneous counterparts with different internal phase structures, respectively, can induce obvious changes in the overall crystallization kinetics.25,26 In order to understand the effects of added UHMWPE on the crystallization kinetics of iPP in the iPP/UHMWPE blends, the miscibility of the UHMWPE and iPP components must be addressed. In fact, scanning electron microscopy and dynamic rheological measurements have been employed to examine the phase morphologies of the iPP/UHMWPE blends (see Figures S2 and S3 in the Supporting Information). UH-5.0 and UH-10.0 samples show remarkable heterogeneous morphologies, indicating the existence of phase separation. When the UHMWPE content decreases to 2.5 wt % or less, the above applied techniques unfortunately have difficulty picking up the UHMWPE component as a noticeable minor phase. Nevertheless, iPP/UHMWPE blends are considered as a thermodynamically immiscible blend system, which forms heterogeneous crystalline structures according to previous reports.3,23,27 3.1.1. Nonisothermal Crystallization Kinetics. The nonisothermal crystallization kinetics of iPP containing different UHMWPE contents is presented. Parts a−c of Figure 1 show

Tmp (°C) sample neat iPP UH-0.5 UH-1.0 UH-2.5 UH-5.0 UH-10.0 UHMWPE

Tcp (°C)

Tonset (°C)

iPP phase

113 115 116 116 116 117 120

117 117 118 119 118 118 123

162 162 161 161 161 161

UHMWPE phase 129 130 130 131 132 135

density in the blends with respect to neat iPP at the same thermal conditions. Here, it is reasonable to conclude that UHMWPE crystals can act as nucleating agents, greatly increasing the heterogeneous nucleation for iPP. Even though the added UHMWPE contents are low, the crystallization process of iPP during cooling can be enhanced remarkably with the assistance of UHMWPE crystals. The existence of UHMWPE crystals is inferred from the melting peak of UHMWPE crystals (peak 1) during heating shown in Figure 1b,c. Note that for all of the chosen UHMWPE contents the blends only show single exothermic peaks, which might be ascribed to the overlap of the crystallization peaks for the two components.30 For neat iPP, a single broad melting peak (peak 2) with a right-side shoulder during heating is observed, with the peak shoulder caused possibly by a melting−recrystallization− remelting (mrr) event (see Figure S5 in the Supporting Information). 31 With the addition of the UHMWPE component, the single melting endothermic peak of the iPP component (peak 2) located at about 161 °C becomes sharper and meanwhile a low-temperature endothermic peak located at about 130 °C (peak 1) appears corresponding to the melting of UHMWPE crystals. The deconvolution of peaks 1 and 2 can be well accomplished through a peak-fitting procedure of the OriginPro8 software (see Figure S6 in the Supporting Information). The peak positions of iPP and UHMWPE and the ΔH values for both endothermic peaks slightly increase with increasing UHMWPE content up to 5.0 wt %, as can be seen from Tables 1 and 2. When more UHMWPE is blended in iPP (e.g., 10.0 wt %), the immiscibility between UHMWPE and iPP causes aggregation of UHMWPE, which results in the reduction of ΔH(peak 2) and the crystallinity of the iPP phase. The results demonstrate that UHMWPE crystallizes and acts as a heterogeneous nucleating agent to assist iPP crystallization (see Figure 1a), which are consistent with the results for iPP/ LDPE blends previously reported by Varga et al.8,9 The UHMWPE component forms crystal-phase domains, which are separated from the iPP crystal-phase domains (see Figure 1b,c). The crystallinities listed in Table 2 are normalized by taking into account the different enthalpy values of 100% crystalline iPP (148 J/g) and UHMWPE (293 J/g).32 The fractions of the peak 1 enthalpy in the total enthalpy are correlated to mass fractions of UHMWPE in iPP/UHMWPE blends, as seen from Table 2. The blends with lower UHMWPE contents clearly show the existence of peak 1 but with lower fractions, possibly because of the lower enthalpy values approaching the DSC measurement limit. Note that the above crystallization behaviors are specific to the nonisothermal crystallization process for the iPP/UHMWPE blends, which are different from

Figure 1. Heat-flow curves of iPP/UHMWPE blends with different UHMWPE contents during cooling scans (a), heating scans for all samples (b), and heating scans for neat iPP, UH-0.5, and UH-1.0 (c). The heating and cooling rates are 10 °C/min. For clarity, the heat-flow curves are shifted vertically to avoid overlap.

the DSC heat flow curves for iPP/UHMWPE blends during cooling and subsequent heating scans with a rate of 10 °C/min. The onset temperature (Tonset) and peak temperature (Tcp) for crystallization during cooling, melting temperature (Tmp) during heating, and enthalpy (ΔH) during both cooling and heating can be obtained from the thermograms (data listed in Tables 1 and 2).28 During cooling, with increasing UHMWPE content, the sharpness of the crystallization peak is strengthened, Tonset is elevated, and Tcp shifts to higher values. An exothermic peak starts from Tonset of 116.7 °C on the heat flow curve of neat iPP during cooling, and an exothermic peak starts from Tonset of 122.7 °C for neat UHMWPE. This infers that iPP begins to crystallize when UHMWPE has about finished its primary crystallization stage (see Figure S4 in the Supporting Information). It was reported that iPP crystals nucleated on the surfaces of PE crystals during crystallization of the iPP/PE blends.29 This causes an increase of the iPP nucleus 15955

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Table 2. Enthalpy (ΔH, with Units of J/g), Crystallinity (%), and Fraction (%) of Peak 1 Enthalpy for iPP, UHMWPE, and iPP/ UHMWPE Blends during Cooling and Heating Scans at a Rate of 10 °C/min ΔH (J/g) sample neat iPP UH-0.5 UH-1.0 UH-2.5 UH-5.0 UH-10.0 UHMWPE

cooling

heating 74 86 94 95 113 110 147

crystallinity (%) peak 1

72 83 93 96 114 109 144

peak 2

UHMWPE 72 83 91 94 109 99

0.3 0.3 1.8 5.0 11 144

0.1 0.1 0.6 1.7 3.6 49

iPP

fraction (%) 48 56 62 64 74 67

0.2 0.2 0.9 2.3 5.4

more or less iPP nuclei. The real-space morphological observation by POM confirms the crystallization mechanism, which will be presented in a later section. The properties of a semicrystalline polymer depend on the structure and morphology that evolve from the melt.33 Studies of the crystallization kinetics can lead to an understanding of the corresponding mechanisms. To obtain more crystallization kinetics information, the heat-flow curves in Figure 2 are further analyzed using the Avrami equation as follows:34

the isothermal crystallization behaviors at higher temperatures, as will be discussed in the next section. 3.1.2. Isothermal Crystallization Kinetics. Figure 2 shows the heat flow curves of iPP/UHMWPE blends with different

X t = 1 − exp[−K n(T ) t n]

(1)

where the relative crystallinity Xt is expressed as a function of time t, n is the Avrami exponent relating to the nucleation type and geometry of growing crystals, and Kn(T) is the isothermal crystallization rate constant. Figure 3 shows the time evolution

Figure 2. Heat-flow curves for iPP/UHMWPE blends with different UHMWPE contents during the isothermal crystallization process at different temperatures of (a) 130, (b) 132, (c) 134, and (d) 136 °C.

UHMWPE contents during isothermal crystallization at different temperatures of 130, 132, 134, and 136 °C. Note that the UHMWPE component in the blends is in the amorphous molten state and only the iPP component is crystallizable at these temperatures (see Figure S1 in the Supporting Information). The heat-flow curves of neat UHMWPE during isothermal crystallization at the chosen temperatures (see Figure S1a in the Supporting Information) and subsequent melting processes (see Figure S1b in the Supporting Information) do not show any marked changes, which indicates that crystallization does not occur for neat UHMWPE and UHMWPE components in the blends during the isothermal process. In Figure 2, at each crystallization temperature, the times at the exothermic peak position (tp) for the iPP/UHMWPE blends are shorter than those of neat iPP, implying that the isothermal crystallization of iPP is enhanced through the addition of UHMWPE. Unexpectedly, tp does not show a monotonic decrease with increasing UHMWPE content but decreases and reaches the lowest value at UH-2.5 and then increases for UH-5.0 and UH-10.0. The nucleation rate is dominant for crystallization at these relatively high crystallization temperatures. Changes of the iPP crystallization rate in the presence of UHMWPE can be ascribed to the formation of

Figure 3. Evolution of the relative crystallinity Xt for iPP/UHMWPE blends with different UHMWPE contents during the isothermal crystallization process at different temperatures of (a) 130, (b) 132, (c) 134, and (d) 136 °C.

of Xt for iPP/UHMWPE blends with different UHMWPE contents at different isothermal crystallization temperatures. Evidently, the crystallization rates of iPP/UHMWPE blends are enhanced by the addition of UHMWPE at these crystallization temperatures. With increasing UHMWPE content, the enhancement effect becomes more prominent, while the highest crystallization rate is achieved at 2.5 wt % UHMWPE content. With the addition of 5.0 and 10.0 wt % UHMWPE (samples UH-5.0 and UH-10.0), the overall crystallization rates slow compared with that of sample UH-2.5. This result implies the existence of a critical UHMWPE content for the highest iPP crystallization rate, beyond which the iPP crystallization rate 15956

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even deteriorates. This observation is consistent with a previous report for a similar blend system by Avila-Orta et al.,23 however, in which only iPP/UHMWPE blends with UHMWPE contents of 5 and 10 wt % were studied. The critical UHMWPE content of 2.5 wt % was not revealed. Avila-Orta et al. ascribed the higher crystallization rate for the 5 wt % UHMWPE content sample than the 10 wt% UHMWPE content sample to the phase-separation-induced interfacial area decrease. In our study, UHMWPE can well disperse in the iPP matrix when the UHMWPE content is 2.5 wt % or less and the increased interfacial area can promote iPP crystallization. Phase coalescence driven by the thermodynamic interaction of UHMWPE chains begins to occur above the critical UHMWPE content of 2.5 wt %. Thus, the reduced interfacial area between the UHMWPE and iPP phases leads to the crystallization rate depression. It is thought that the unstable inhomogeneous phase interfaces generate the concentration fluctuations, which could assist in overcoming the usual energy barrier required for the formation of crystal nuclei and promote the conformation of iPP chains to favor subsequent chain folding for lamella growth. Nuclei are predicted to preferentially form at the phase interfaces, which will be demonstrated by a delicately designed DSC measurement in a later section. Figure 4 shows the Avrami plots for neat iPP and iPP/ UHMWPE blends. Theoretically, the Avrami plot should be

ln[− ln(1 − X t )] = ln K n(T ) + n ln t

(2)

By means of the above equations, the values of n, t1/2, and Kn(T) for iPP/UHMWPE blends during isothermal crystallization at different temperatures are obtained and listed in Table S1 in the Supporting Information. The Avrami exponent n of 2.7−2.9 is obtained for neat iPP, which is lower than the theoretical value of 3.35 The addition of UHMWPE seems not to change the n values much, although the addition of a second component may change the nucleation mechanism for iPP. Unlike n values, t1/2 shows obvious changes with variations of both the UHMWPE content and crystallization temperature, as shown in Figure 5, which are constant with the results shown in Figure 2.

Figure 5. Changes of the crystallization half-time, t1/2, as a function of the UHMWPE content for iPP/UHMWPE blends at different isothermal crystallization temperatures.

3.2. Evolution of Morphology Observed by POM. To provide morphological evidence, POM observation for neat iPP and iPP/UHMWPE blends at different isothermal crystallization temperatures ranging from 130 to 136 °C was conducted. Figure 6 displays typical optical micrographs collected at 134 °C. For neat iPP, several crystal nuclei appear after an isothermal crystallization time of 10 min. With the addition of UHMWPE, the number of iPP crystal nuclei increases and becomes more prominent for UH-2.5, UH-5.0, and UH-10.0. Compared with UH-2.5, the crystal nuclei of UH-5.0 and UH10.0 appear less at the same crystallization time, which is consistent with DSC measurements. We count the numbers of crystal nuclei per unit area for the blend samples to provide a quantitative comparison (see Figure S7 in the Supporting Information). Obviously, rapid increases of the number of crystal nuclei at the early stages can be observed for the iPP/ UHMWPE blends. The number of crystal nuclei for the UH2.5 sample is estimated to be 10 times higher than that for neat iPP at 134 °C, indicating the much higher nucleation rate for the UH-2.5 sample. It is clear that isothermal crystallization is a nucleation-controlled process, the crystallization rate mainly depends on the nucleation rate and, thus, we may infer that the UHMWPE component is likely to induce the formation of more iPP crystal nuclei, which enhances the overall crystallization rate. At the late crystallization stage, impingements of iPP spherulites are seen for the blend samples with high UHMWPE contents. The radial growth rates (G) of iPP spherulites in the iPP/ UHMWPE blends during isothermal crystallization at different temperatures are obtained by analysis on the optical micrographs.33 The result is shown in Figure 7. G remains constant with time during isothermal crystallization. The addition of the UHMWPE component in the supercooled iPP melt does not obviously influence the spherulitic growth rates of iPP at each

Figure 4. Avrami plots of ln[−ln(1 − Xt)] versus ln t for (a) neat iPP, (b) UH-0.5, (c) UH-1.0, (d) UH-2.5, (e) UH-5.0, and (f) UH-10.0 at different isothermal crystallization temperatures.

linear and the Avrami exponent n should be an integer. In a practical situation, the Avrami plot is not always linear and n varies as well because of the complexity of polymer systems. In our case, the Avrami plots almost follow a linear tendency except for a slight deviation at the late crystallization stage due to the secondary crystallization effect.33 From the slope and intercept of the initial linear part (eq 2), the Avrami exponent n and crystallization rate constant Kn(T) can both be obtained. 15957

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Figure 6. Morphological evolution from top to bottom for iPP/UHMWPE blends with different UHMWPE contents at an isothermal crystallization temperature of 134 °C. The white scale bar in the right bottom micrograph represents 500 μm and is applied to all of the micrographs.

decreases evidently with increasing crystallization temperature, indicating a thermodynamic dominant crystallization process. According to the Hoffman and Lauritzen theory,36 the spherulitic growth rate in binary blends is a complex function of the parameters that characterize the system, such as the crystallization temperature, composition, glass transition and melting temperatures, and molecular mass. Galeski et al.37 pointed out that, even with loading of 50 wt % LDPE in iPP, the growth rate of iPP is only decreased by 5%; thus, the iPP blends added with various PEs are typical examples showing a nucleation-controlled crystallization, which agrees well with our experimental results. 3.3. Examination of the Interfacial Effect on the iPP Nucleation Mechanism. UHMWPE is immiscible with iPP and keeps an amorphous molten state during the isothermal crystallization process as mentioned above. The interface between the dispersed minor UHMWPE phase and the iPP matrix plays a prominent role in affecting the crystallization

Figure 7. Changes of the spherulitic radial growth rate as a function of the UHMWPE content for iPP/UHMWPE blends at different isothermal crystallization temperatures. The solid lines are guides to the eyes.

temperature. This indicates that UHMWPE is not able to effectively penetrate the iPP phase because of immiscibility between iPP and UHMWPE components. Meanwhile, G

Figure 8. Schematic illustration of the sample preparation for the study of the interfacial effect on nucleation of iPP by DSC (a), DSC heat-flow curves during isothermal crystallization at 134 °C for neat iPP and iPP + UHMWPE samples (b), and schematic phase domain sizes for iPP/ UHMWPE blends with increasing UHMWPE content (c). 15958

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nuclei should have an intimate relationship with the interfacial area. Obviously, the nucleation rate is proportional to the area of the interfacial region. With increasing UHMWPE content, the enlarged interfacial area results in a more pronounced effect on the nucleation of iPP, and the most significant effect is achieved for UH-2.5. However, beyond the critical UHMWPE content of 2.5 wt %, aggregation of UHMWPE leads to a reduced interfacial area; thus, the number of nuclei decreases again (schematically shown in Figure 8c). The prediction is completely consistent with our experimental observation. Thus, we could propose the nucleation mechanism for crystallization in this study. The concentration fluctuations in the iPP/ UHMWPE blends caused by the unstable inhomogeneous phase interface can induce anisotropic alignment of the local polymer chain segments, promote the formation of nuclei, and eventually enhance the nucleation and overall crystallization rates for the iPP component. We note here that the highdensity chain entanglements of the UHMWPE component in iPP/UHMWPE blends are restricted in its own phase domains, which might not be able to significantly affect the nucleation and crystal growth of the iPP component in the blends. Therefore, the significant effects of chain entanglements on polymer crystallization45,46 cannot obviously be reflected in this study.

kinetics. Recently, extensive studies on the interplay between liquid−liquid-phase separation (LLPS) and crystallization for polyolefin blends by applying various advanced techniques have been reported, in which the LLPS-assisted nucleation for crystallization has been illustrated to occur mostly at the phaseseparated domain interfaces.26,38 In order to examine the interfacial effect, a delicately designed DSC experiment was performed on an iPP + UHMWPE sample with clear interfaces between UHMWPE and iPP. UHMWPE powders of 3.5 mg were covered on the surface of an iPP thin film of 3.6 mg in a DSC sample pan (Figure 8a). Figure 8b compares the heat-flow curves for neat iPP and the above iPP + UHMWPE sample during isothermal crystallization at 134 °C. As can be seen, the tp value (38.0 min) of the iPP + UHMWPE sample is approximately 2.3 min shorter than that of neat iPP (35.7 min). It is noticeable that the iPP + UHMWPE sample shows a higher overall crystallization rate compared with neat iPP, implying that isothermal crystallization of iPP can indeed be enhanced by the addition of UHMWPE through the nucleation effect at the iPP and UHMWPE interfaces. The enthalpy of the exothermic peak for the iPP + UHMWPE sample is 42.8 J/g, and that for neat iPP is 85.2 J/g. Considering normalization of the iPP mass fraction of 0.51 in the iPP + UHMWPE sample, the above enthalpy values of the exothermic peaks infer that UHMWPE melts above the iPP melts do not crystallize and only the iPP component crystallizes at the experimental temperature of 134 °C. Interestingly, this is contrary to a former conclusion that the high interfacial surface tension and sharp interface between immiscible blends can rarely induce crystal nuclei.39−41 The nucleation of iPP induced at the phase interface between UHMWPE and iPP in the blends strongly supports the recent findings about the interface-induced polymer nucleation for crystallization.26,38,42−44 In these studies, the polyolefin blends with quench show rapid nucleation everywhere because of the spontaneously occurring concentration fluctuations, which assist nucleation for crystallization at the interfacial regions. The “concentration-fluctuation-assisted nucleation” mechanism for crystallization of the polyolefin blends can be borrowed to explain the observed crystallization behavior for iPP/ UHMWPE blends. At the phase interfaces, the spatial inhomogeneous environment due to the existence of the UHMWPE component generates the concentration fluctuations, which may help the interfacial surface formation and overcome the usual energy barrier required for the formation of crystal nuclei. Specifically, in our study, the existence of a UHMWPE molten phase may assist in overcoming the surface energy of nuclei and the reptation of UHMWPE chains induces some chain segmental alignment and orientation during diffusion. This coupling effect could induce the iPP nuclei to appear preferentially at the interfaces and promote the conformation of iPP chains to favor the subsequent chain folding for crystal lamella growth. Therefore, it is reasonable to conclude that the concentration fluctuations caused by the unstable inhomogeneous UHMWPE molten phase can promote the formation of nuclei without having to overcome the usual activation energy barrier of nucleation and induce the anisotropic alignment of the local polymer segments to assist the formation of crystal nuclei. Consequently, the nucleation rate of the iPP component can be enhanced, and the crystallization process proceeds at a higher rate. In particular, on the basis of the concentration fluctuation theory, the nuclei appear at the interfaces, indicating that the number of crystal

4. CONCLUSIONS In this study, the crystallization kinetics of the iPP/UHMWPE blends added with a wide range of UHMWPE contents were studied by DSC measurements and POM observation. During isothermal and nonisothermal crystallization processes, the overall crystallization rates of iPP with the addition of UHMWPE become higher. The most interesting results are the different nucleation mechanisms due to the addition of the UHMWPE component in the blends during the two crystallization processes. In nonisothermal crystallization process, the primarily formed UHMWPE crystals act as nucleating agents, which greatly increases the number of heterogeneous nuclei for iPP crystallization, while in the isothermal crystallization process, UHMWPE in the blends is in an amorphous molten state. It is proposed that the interfaces between iPP and UHMWPE phase domains induce iPP crystal nuclei preferentially, which effectively enhances the isothermal crystallization rate. The concentration fluctuations caused by the unstable inhomogeneous phase interfaces in the iPP/ UHMWPE blends can induce the formation of nuclei through anisotropic alignment of the local polymer chain segments, eventually enhancing the nucleation and overall crystallization rates of the iPP component. In particular, the enhancement is most prominent at the critical UHMWPE content of 2.5 wt %, whereas at above the critical content, the reduced interfacial area due to aggregation of the UHMWPE component leads to a retarded crystallization rate. The enhanced crystallization of iPP is mainly ascribed to the enhanced nucleation rate because the spherulitic growth rate is not influenced much through the addition of UHMWPE. Different measurements show consistent results, indicating the reliability of the conclusions.

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ASSOCIATED CONTENT

* Supporting Information S

Supplementary figures and table showing the isothermal process at 130 °C and subsequent heating for UHMWPE, phase morphologies of iPP/UHMWPE blends, the nonisothermal process for the iPP/UHMWPE sample with a 15959

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clear interface, the mrr process of iPP, an example of the deconvolution procedure of peaks 1 and 2 for the blends, nucleation density changes for iPP/UHMWPE blends during the isothermal crystallization process, and values of the Avrami parameters for iPP and iPP/UHMWPE blends at different isothermal crystallization temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0551-3607703. Fax: +86 0551-3607703. E-mail: [email protected] (Z.W.), [email protected] (Y.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. acknowledges financial support from the National Science Foundation of China (Grant 51073145) and the National Basic Research Program of China (Grant 2012CB025901). Y.N. acknowledges financial support from the National Science Foundation of China (Grant 50803073).

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