Isotactic Polystyrene

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Precursor of Shish-Kebab in Atactic Polystyrene/Isotactic Polystyrene Blend above Nominal Melting Temperature Cong Deng,†,‡,* Tetsuaki Fujiwara,† Inga Polec,† Go Matsuba,§ Ling Jin,† Rintaro Inoue,† Koji Nishida,† and Toshiji Kanaya†,* †

Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan Analytical and Testing Center, Sichuan University, Chengdu 610065, P. R. China § Department of Polymer Science and Engineering, Faculty of Science and Engineering, Yamagata University, Yonezawa, Yamagata-ken 992-8510, Japan ‡

S Supporting Information *

ABSTRACT: In a previous paper [Polymer 2009, 50, 2095] we found that string-like objects were formed on the micrometer scale, after applying shear flow on melt of isotactic polystyrene (iPS) above its nominal melting temperature Tm, and assigned to precursors of shishkebab (shear-induced precursor). In this paper we studied effects of a small amount of noncrystalline high molecular weight atactic polystyrene (aPS) in iPS on the shear-induced precursor formation above Tm (=223 °C). Polarized optical microscope (POM) measurements were mainly used to study the structure formation process of iPS and aPS/iPS blends under the chosen experimental conditions. It was found that there were more shear-induced precursors in aPS/iPS blend than in pure iPS at some shear conditions. This suggests that aPS can enhance the shear-induced precursor formation of iPS. The shearinduced precursors formed in the blends were as stable as those in pure iPS. Both could survive for more than 60 min at shear temperatures above Tm and melted at around 270 °C. An influence of aPS on the shear-induced precursor formation was also examined as functions of shear rate, shear strain and concentration of aPS. It was found that aPS enhanced the formation of the precursor at around 4 wt % of aPS most effectively in any shear conditions. These experimental findings suggest that the orientation and relaxation of noncrystalline high molecular weight aPS might have significant impact on the enhancement of precursor formation under shear flow. The corresponding mechanism presents new insights into the shear-induced crystallization.

1. INTRODUCTION Crystallization of polymers into oriented structures, caused by various kinds of flow fields in the melt, has been a subject of extensive study for many years.1−5 The so-called shish-kebab structure, which consists of an extended chain crystal (shish) in the central core and periodically grown folded chain lamella crystals (kebabs) on the shish, is often formed under shear flow.6−9 Although the formation process and formation mechanism of the shish-kebab have been illustrated by many researchers in past several decades,10−12 the formation mechanism of the shish-kebab is still not well understood regardless the considerable efforts. Recent development of advanced characterization techniques such as synchrotron radiation (SR) X-ray scattering, neutron scattering and light scattering has shed light on the substantial nature of the polymer crystallization under flow. Some of these works have focused on the structural formation in the early stage of the crystallization under flow using short-term shearing technique because it often governs or at least affects the final structure deeply.13−17 One of the most important issues in these studies is the precursor of shish-kebab. Because of the importance of the initial precursor structure in dominating the final structure © 2012 American Chemical Society

or the morphology, some methods have been applied to improve the shear-induced precursor formation in past researches.18−23 To date, a large number of studies on enhancing the precursor formation have dealt with the crystallization of high molecular weight crystalline polymer and the nucleation induced by inorganic materials like carbon nanotube (CNT) and whisker. Kornfield et al.18 have studied effects of the high molecular weight (HMW) component using model blends of HMW isotactic polypropylene (Mw = 923 000 g/mol) and low molecular weight (LMW) one (Mw = 186 000 g/mol), both of which had rather narrow molecular weight distributions, and found that the role of the HMW component in shear-induced crystallization is cooperative, enhanced by the entanglements among the long chains. Similar precursors studies above the nominal Tm were also reported for a blend of ultrahigh molecular weight polyethylene (PE) and low molecular weight PE and for some polymers even near the equilibrium melting temperature.19,20 Hsiao et al.21 used a high Received: January 28, 2012 Revised: May 14, 2012 Published: May 31, 2012 4630

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molecular weight polyethylene homopolymer with Mw of 250 000 g/mol and polydispersity of about 2 as the crystalline minor component and low molecular weight polyethylene as the noncrystalline matrices and then found the high molecular weight component dominated the formation of precursors in the blend under shear, which can act as a template for further crystallization. Martinez-Salazar et al.22 prepared the HDPE/ multiwall nanotube nanocomposite samples by melt mixing. The crystallization, probed by rheometry, differential scanning calorimetry and scanning electron microscopy, suggests that first formation of PE extended-chain shish directly on CNTs surface might occur after shear in the formation of hybrid shishkebab structures. Fu et al.23 also found the similar formation mechanism of PE extended-chain shish in injection-molded HDPE/multiwall nanotube nanocomposite bars. These results mean that CNTs induced the precursor formation before fullscale crystallization. In these researches, the positive enhancement influence of noncrystalline polymer in shear-induced crystallization was almost neglected although a pioneering work was reported on a blend of isotactic polypropylene and atactic polypropylene by Somani et al.24 However, it is well-known that under flow conditions all the polymer chains are submitted to shear and extension, resulting in oriented microstructures. So the positive effect of noncrystalline component is worth exploring in shearinduced crystallization. In some investigations the precursor with ∼10 nm diameter was observed by SAXS as streak-like scattering normal to the flow direction.25,26 But in others27−29 the precursor was observed by birefringence, light scattering, and polarized optical microscope (POM), suggesting that the precursor had a micrometer scale in diameter. In our previous measurements,5 on a blend of ultrahigh molecular weight protonated polyethylene (PE) and low molecular weight deuterated one by small-angle neutron scattering (SANS) a very large oriented structure in a micrometer scale including a small shish (extended chain crystal) with ∼10 nm diameter was found. Thus, experimentally shish or shish-like structure in different length scales (∼10 nm and ∼several tens of micrometers in diameter) were reported. Regardless the intensive studies we have no final conclusion on which size in precursor is the most important for the shish-kebab formation. It is therefore evident that the structure study in a wide length scale of nm to μm is absolutely important to elucidate the precursor. In previous papers,30,31 we studied structure formation process of isotactic polystyrene (iPS) after applying a pulse shear above the nominal Tm using depolarized light scattering (DPLS) and polarized optical microscope (POM) and found that precursors were formed in a micrometer scale during an annealing process above the nominal Tm. We speculate that this unexpected micrometer scale precursor is formed from a stretched tentative polymer network due to entanglements and stabilized by formation of small number of large crystallites although we have no direct evidence. In such events with the wide distributions, POM observation is more appropriate than DPLS one because the former has an advantage to see individual structure one by one while the latter analyses the average. For this reason, POM measurements were mainly performed on the structure formation of iPS and aPS/iPS blends after applying a pulse shear for various temperatures and shear conditions. In this article, we present our POM results showing that the formation of the precursor was enhanced by noncrystalline

polymer under shear conditions. This gives significant new insights into the enhancement of shear-induced precursor formation due to the addition of high molecular weight component.

2. EXPERIMENTAL SECTION In this experiment, we used isotactic polystyrene (iPS) with molecular weight (Mw) of 600 000 g/mol and atactic polystyrene (aPS) with Mw of 2 430 000 g/mol, and the polydispersities (Mw/Mn) were 2.0 (iPS) and 1.06 (aPS), respectively, where Mw and Mn are the weight-average and number-average molecular weights, respectively. The concentration of aPS (CaPS) was in a range from 0 to 10 wt %. These blends were prepared with a coprecipitation method to ensure that the two species were intimately mixed at the molecular level. Both PSs were dissolved in cyclohexane solution at 147 °C and stirred for 1 h. In order to prevent the sample degradation during mixing, 3 wt % of antioxidants was added. Then the solution was poured into methanol to precipitate the blend. After filtering, the obtained blend was washed with clean methanol and dried in a vacuum oven for 3 days before use. The blends were hot-pressed at 250 °C to form films about 500 μm thick and then quenched to room temperature. The DSC measurements at a heating rate of 5 °C/min were carried out to characterize the thermal properties of the samples using PerkinElmer Diamond DSC. The melting temperature of iPS was 223 °C. POM measurements were performed using Olympus BX50 with a video attachment. A Linkam CSS-450 high temperature shear cell was used to control the temperature of the samples and the shear conditions. The sample was placed between two quartz plates for the POM measurements. The sample thickness in the cell was 300 μm. In the beginning of our study we checked surface effect for the formation of the shear-induced precursor by changing the focus point in POM and found that the formation was enhanced near the surface, especially 20−30 μm near the surface. In order to reduce the surface effect we selected rather large sample thickness (300 μm) and focused on the structure formation in the middle of sample in POM measurements.

Figure 1. Temperature protocol for the shear experiments in iPS and aPS/iPS blends.

The temperature protocol for the shear experiments is shown in Figure 1. The iPS sample was heated up to 290 °C from room temperature at a rate of 30 °C/min, (b) held at 290 °C for 5 min, and (c) cooled to a given annealing temperature (Ta) at a rate of 30 °C/ min. (d) During the cooling process, a pulse shear was applied to the melt at 250 °C, which was (e) held at Ta for 60 min for the measurements during the isothermal annealing process and then (f) again heated up to 290 °C at a rate of 5 °C/min for the measurements. The range of the shear rates was from 7.5 to 90 s−1 and the shear strain was from 6000 to 24 000%. 4631

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then totally disappeared at about 270 °C for iPS and aPS/iPS blend. The disappearance temperature depended slightly on the size of the precursors although no differences were observed between the precursors of iPS and iPS blend. These results suggest that the precursors of iPS and aPS/iPS blend are almost identical in the stability. In our previous research,32 it has been demonstrated that the shear-induced precursors included small amount of large crystals. Similar stability of the precursor of iPS and aPS/iPS suggests that the large crystals do not include noncrystalline aPS. It is, however, very clear in Figure 2 that the shear-induced precursors were formed in aPS/iPS blend more than in iPS at the same shear condition, indicating that the formation of the shear-induced precursors was enhanced by addition of noncrystalline aPS. The enhancement effects were studied as functions of shear rate, shear strain and concentration of aPS in aPS/iPS blends in the following. 3.2. Evolution of the Shear-Induced Precursors Created in iPS and aPS/iPS Blends below the Nominal Tm. The previous paper31 has demonstrated that the shearinduced precursors created above the nominal Tm in iPS acted as a nucleating reagent when it was annealed below the nominal Tm. Do the shear-induced precursors formed at the same condition in aPS/iPS blends still act as nucleating reagents when annealed below Tm? In order to confirm it, the following experiments were performed. The shear-induced precursors were created by applying shear at 250 °C, and then cooled to 218 °C to observe the crystallization process. After 60 min of annealing at 218 °C, the sample was heated up to 290 °C at a heating rate of 5 °C/min. The resultant structures are presented in Figure 3: the left side shows the structures developed during the 60 min annealing period, and the right side illustrates a response of the structures on the temperature increase. The shear-induced precursors were created at 250 °C and began to grow after reaching at 218 °C. The structure behavior was different from those described in section 3.1 for iPS and aPS/iPS blend above Tm. The length, the width and the number increased with annealing time due to additional crystallization on the precursors, finally leading to the “colorful picture” due to the retardance. During the heating process, the crystals formed later on the shear-induced precursors melted at about the nominal Tm, and then the shear-induced precursors formed during shear could survive up to about 265 °C (the right part of Figure 3) in iPS and aPS/iPS blend. This result clearly indicates that the shear-induced precursors formed in both iPS and aPS/iPS blend can act as nucleating reagents to produce oriented crystals inside the precursor which might be a pseudonetwork with crystallites as cross-links.

3. RESULTS AND DISCUSSION 3.1. Stability of Shear-Induced Precursors in iPS and aPS/iPS Blends above the Nominal Tm. Because in the current experiment the shear was applied at 250 °C, which is 27 °C above its Tm, it is generally expected that both thermal motion and relaxation of the shear-induced precursor can have a significant impact on the crystallization. Therefore, before our further investigation, it is necessary to explore the stability of precursors formed under this condition. In this section, two typical POM results for pure iPS and aPS/iPS blend with 4 wt % aPS were chosen to demonstrate the stability of shearinduced precursors. The POM pictures are shown in Figure 2.

Figure 2. Time evolution of the shear-induced precursors formed in iPS and aPS/iPS blend during the annealing process at the following condition: γ = 60 s−1, ε = 18 000%, CaPS = 4 wt %, Ta = 250 °C (>Tm).

Figure 2 shows the time evolution of the shear-induced precursors after applying pulse shear with the shear rate of 60 s−1 and the shear strain of 18 000% at 250 °C for iPS and aPS/ iPS. The corresponding temperature dependence of the formed precursors while heating up to various temperatures after annealing is also shown in the right part of Figure 2, where t = 0 s was set to a time just after cessation of the shear. As can be seen at 250 °C, above the nominal melting temperature Tm, many shear-induced precursors created during the shear were observed 1 min after the cessation of the shear for both iPS and aPS/iPS blend. It can be noticed that the initial shear-induced precursors did not change both their length and width during the annealing time of 60 min. Moreover, number of shearinduced precursors did not grow with annealing time nor relax during this period for both iPS and its blend. The shearinduced precursors could survive for 60 min, suggesting that the precursors were dimensionally stable even above the nominal melting temperature Tm. Note that a few small objects were occasionally observed and disappeared during the observation time in both iPS and aPS/iPS blend.32 However, most of all precursors are very stable for both iPS and aPS/iPS blend as seen above. When the temperature was increasing after the annealing for 60 min, the precursors gradually melted and changed the size and shape (the right part of Figure 2), and

Figure 3. Time evolution of the shear-induced precursors formed in iPS and aPS/iPS blend with 4 wt % of aPS during the annealing process at the following condition: γ = 60 s−1, ε = 18 000%, CaPS = 4 wt %, Ta = 250 °C (>Tm). 4632

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3.3. Shear Strain and Shear Rate Dependence of Shear-Induced Precursors above the Nominal Tm. Figure 4 shows selected typical POM pictures of iPS and aPS/iPS

against the shear strain for pure iPS and aPS/iPS blends with aPS of 0.5, 1, 4, and 10 wt %. The error bars in the figure show the standard deviation. The integrated brightness increased with the shear strain for iPS and aPS/iPS blends, and depended on the aPS concentration, especially in the rather high shear strain region about 12 000%. The dependence on the aPS concentration is very complicated: it sometimes accelerated the precursor formation however it sometimes decelerated. This problem will be discussed later. We also examined the shear rate effects on the precursor formation. The selected POM pictures of iPS and aPS/iPS blends collected after shearing at various shear rates are shown in Figure 6. The following shear rates of 7.5, 15, 30, 60, and 90

Figure 4. POM pictures of shear-induced precursors formed at different shear strains during shear flow at 250 °C in iPS and aPS/iPS blends.

blends with aPS of 0.5, 1, 4, and 10 wt % created for the shear strains of 6000, 12 000, 18 000, and 24 000%. The shear rate was 60 s−1 for all the measurements. The POM pictures were taken at 1 min after cessation of shear at 250 °C above the nominal Tm. As mentioned above, the structure development was hardly observed in the annealing process above the nominal melting temperature Tm, so that the picture does not depend on the annealing time. All sheared melts of iPS and aPS/iPS blends exhibited the shear-induced precursors in the whole range of the shear strains utilized. It should be noted that the number of shear-induced precursors increased with shear strain and the degree of the increase depends on the concentrations of aPS. In order to see the shear strain dependence of amount of the precursors we have evaluated the integrated brightness in these POM pictures because the brightness in a given area is approximately proportional to the amount of the precursors. This approximation is held as long as the POM mage is in gray as in Figure 4, but not when it is colorful. As seen in the POM pictures in Figure 4, the precursors were rather scattered in size and the brightness. In order to see average brightness in a sample, we took 10 POM pictures at different positions in the sample and averaged them. In Figure 5 the integrated brightness thus obtained is plotted

s−1 were applied and the shear strain was 18 000% for all the measurements. All of the pictures were taken 1 min after the cessation of the shear flow and confirmed that no development of the precursors were observed during the annealing at 250 °C for 60 min. In order to see the shear rate dependence of the amount of precursors quantitatively, we evaluated the integrated brightness of the POM pictures in the same way as the case of the shear strain dependence, and plotted it as a function of the shear rate for various aPS concentrations in Figure 7. The integrated brightness of the POM pictures increased with the shear rate for iPS and aPS/iPS blends. At the shear

Figure 5. Shear strain dependence of the integrated brightness of shear-induced precursor at 250 °C in iPS and aPS/iPS blends.

Figure 7. Shear rate dependence of the integrated brightness of shearinduced precursor at 250 °C in iPS and aPS/iPS blends.

Figure 6. POM pictures of shear-induced precursors formed at different shear rates during shear flow at 250 °C in iPS and aPS/iPS blends.

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rate of 7.5 s−1 the brightness seems almost independent of the aPS concentration within the experimental error. Above the shear rate of ∼10 s−1, the brightness of aPS/iPS blend depends on the aPS concentration very much and shows the largest value at around 4 wt %, suggesting that the 4% blend accelerates the precursor formation most effectively. The dependence on aPS concentration is again very complicated while the 4% blend always shows the largest value. We have to consider effects of melt instability33 on shearinduced precursor formation, especially at high shear rate and for high viscous samples, which sometimes produce inhomogeneous flows and bubbles in the flow. In the experiments we did not observe inhomogeneous flows or bubbles before the formation of the shear-induced precursors, suggesting the effects of instability are not serious. However, we occasionally observed small amount of bubbles at the highest shear rate of 90 s−1, which was limited in a small area, implying a possibility of the instability effect on the precursor formation although the effects were not serious, and hence we were careful with quantitative discussion on the results at 90 s−1. 3.4. aPS Concentration Dependence of Shear-Induced Precursor Formation. As mentioned above, the brightness of the POM pictures, which is approximately proportional to the amount of the precursors, shows a very complicated aPS concentration dependence. In this section, we discuss the aPS concentration dependence of the precursors. In Figures 8 we

Figure 9. aPS concentration dependence of the integrated brightness at different shear rates.

strain region above ∼18 000% and shear rate region above ∼15 s−1. 4.5. Course for Enhancement of Shear-Induced Precursor Formation. In the following, we will consider why aPS, which is a noncrystalline polymer, can enhance the formation of the shear-induced precursors above the nominal Tm (=223 °C) and why it is enhanced at around 4% of aPS most effectively. In order to understand the cause of the enhancement, we first have to recall our previous experimental results on the relaxation of precursors in iPS above the nominal melting temperature.32 We prepared the precursors by applying a pulse shear flow on molten iPS at 250 °C and observed the relaxation in shape (length and width of the precursor). We clearly observed two-step decay of the shape: the fast decaying component in the very early stage after the pulse shear and the almost nondecaying component in the late stage. The latter suggests that there are a small number of very large crystals in the precursor, which have a higher melting temperature than the nominal melting temperature. The finding gives us a clue to understand the course of the enhancement of the precursor by aPS. The enhancement of the precursor must be caused by the enhancement of the large crystal formation. As reported in our previous paper on polyethylene (PE),34 the high molecular weight (HMW) component enhanced the formation of the shish-like structure, and there was a critical HMW concentration for anisotropic structure formation. In this case the added HMW component was the same crystalline polymers species although in the present experiment noncrystalline aPS was added into crystalline iPS. It is known that noncrystalline polymers exhibit usually negative effects on quiescent crystallization35,36 because they cannot be involved in the crystals and work as defects. In our shear experiments, we clearly observed the enhancement effects of the noncrystalline aPS on shear-induced precursor formation of iPS, suggesting that aPS is not included in the crystals in the precursor. This is supported by the fact that the melting temperature (or the disappearing temperature) of the precursor is almost identical between the aPS/iPS blend and pure iPS. Therefore, we have to consider the enhancement mechanism, in which aPS is not included in the crystals in the precursor. First we focus on the aPS concentration dependence of the integrated brightness of POM pictures, which almost corresponds to the amount of the precursors. As seen in Figures 8 and 9, in the low concentration below about 1% the integrated brightness is hardly enhanced by aPS while it increases with aPS concentration up to around 4% very much,

Figure 8. aPS concentration dependence of the integrated brightness at different shear strains.

plotted the brightness of the POM pictures as a function of aPS concentration for various shear strains at shear rate of 60 s−1. At the low shear strains of 6000 and 12 000%, the brightness seems almost independent of aPS concentration. On the other hand, with increasing the shear stain the brightness depends on aPS concentration very much. It increases with aPS concentration up to ∼4%, and after showing a maximum, it decreases with increasing the concentration. In Figure 9, we also plotted the brightness as a function of aPS concentration for various shear rates at a constant shear strain of 18 000%. At low shear rates such as 7.5 and 15 s−1 the aPS concentration dependence of the brightness is very weak; however, above ∼30 s−1, it increases with aPS concentration up to ∼4% and begins to decrease with increasing the aPS concentration. The aPS concentration dependence of the brightness in Figure 9 is very similar to that in Figure 8. It should be noted that the formation of precursor was enhanced at around CaPS of 4 wt % most effectively regardless the shear strain or the shear rate, especially in the high shear 4634

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Next, we consider the relaxation process of polymer chains because if the stretched polymer chains relax before the precursor formation the above-mentioned mechanism does not work. In the flow-induced crystallization the relaxation of polymer chains is important in two aspects. First, polymer chains must be stretched by applied shear flow. For this purpose, the shear rate must be faster than the relaxation rate (or inverse of the relaxation time). Second, stretched and oriented polymer chains must crystallize before relaxation to form the oriented crystals. For this purpose the crystallization rate must be faster than the relaxation rate. To check such situations, we evaluated relaxation times of HMW aPS and iPS. The corresponding relaxation processes are the segmental motion (Rouse motion) and the reptation motion40 whose relaxation times are respectively given by

especially in the high shear strain and shear rate regions. The result is similar to that for our previous experiment on HMW and LMW component blend of PE.34 In the experiment we found that there was a critical concentration C* of HMW component, below which the shish formation was scarcely enhanced by the HMW component, but above which it was very much enhanced. It was also found the critical concentration C* was about three times larger than the chain overlap concentration. This result could be understood as follows. The HMW chains are not well stretched below C* by the shear flow because they are isolated and not entangled, but they are stretched very much through the entanglements above C*, causing orientation of the HMW chains. This must enhance orientation-induced crystallization. Similar results in Figures 8 and 9 imply an important role of stretching of aPS. We have therefore calculated the overlap concentration C*Rg of aPS to confirm their stretch by the shear flow. The overlap concentration C*Rg, which is defined as a concentration above which random coils of aPS with radius of gyration Rg begin to overlap,37 is given by the following eq 1. C *R g =

MW (4/3)π ⟨R g 2⟩3/2 NA

(1)

where is the mean-square radius of gyration of aPS which is given by eq 2 under the Gaussian chain approximation with molecular weight distribution U = Mm/Mn − 138 bL(2U + 1) 3(U + 1)

(3)

⎛ M ⎞3.4 τr = τez 3.4 = τe⎜ W ⎟ ⎝ ME ⎠

(4)

where τe and ME are relaxation time of bond orientation and molecular weight between molecular entanglements. Taking the values of ME = 19 500 and 13 300 for iPS and aPS,41 respectively, τr = 2.63 × 10−3 s for aPS with Mw = 350 000 at 270 °C and τr = 1.26 × 10−3 s for iPS with Mw = 400 000 at 270 °C, we calculated the segmental relaxation time τR and the reptation time τr for aPS with Mw = 2 430 000 and iPS with Mw = 600 000 at 270 °C. The calculated values are as follows: τR = 1.30 × 10−3 and 4.13 × 10−5 s and τr = 1.91 and 5.00 × 10−3 s, for aPS and iPS, respectively. The concentration of aPS was rather low in the aPS/iPS blends and we also took into account the dilution effect42 in the calculation of τr of aPS, resulting in τr = 7.61 × 10−3, 4.01 × 10−2, and 1.21 × 10−1 s for the 1, 4, and 10 wt % blends at 270 °C, respectively. As was shown in the previous paper,34 the critical shear rate for the shear-induced precursor formation was ∼4 s−1 for iPS with Mw = 400 000 at 230−260 °C.43 Mw of iPS used in the present experiment is 600 000, and hence it is expected that the critical shear rate for pure iPS must be lower than 4 s−1. This shear rate is slower than the relaxation rates of the reptation motion (2.00 × 102 s−1) and the segmental motion (2.42 × 104 s−1) of iPS with Mw = 600 000, but we observed the shearinduced precursor. The results suggest that higher molecular weight component in the distribution works effectively for the shear-induced precursor formation. In fact, we could observe the shear-induced precursor even in the lowest shear rate (= 7.5 s−1) in this experiment. As for aPS, the relaxation rates of the reptation motions are 2.49 × 101 and 8.30 × 100 s−1 for the 4 and 10 wt % aPS/iPS blends, respectively, and that of the segmental motion is 7.68 × 102 s−1. These relaxation rates of reptation motion are slower than or comparable with the shear rates employed in the experiment (7.5−90 s−1). These results suggest that the aPS chains are stretched by the pulse shear flow before the relaxation due to the reptation motion but not due to the segmental motion. It is expected that higher molecular weight components in aPS/iPS blends are more effectively stretched because they have slower relaxation rate than the average one. As mentioned above, polymer chains must crystallize before the relaxation to form oriented crystals in the precursor. It is

⟨Rg2⟩

⟨R g 2⟩ =

⎛ M ⎞2 τR = τez 2 = τe⎜ W ⎟ ⎝ ME ⎠

(2)

where b and L are the persistence length and the contour length. Taking the literature data Rg/M1/2 = 3 × 10−2,39 the value of C*Rg has been calculated to be 1.0 wt %. As seen in Figures 8 and 9, the largest enhancement for the shear-induced precursor formation at 250 °C was observed at CaPS = 4 wt % in this experiment, which was four times larger than C*Rg. This suggests that the stretch of aPS chain plays an important role for the enhancement. However, aPS is a noncrystalline polymer and cannot act as a template for the successive crystallization even above C*Rg. Therefore, it is considered in the case of aPS/ iPS blends that the stretch of aPS induces the stretch of iPS chains to produce a domain of oriented iPS chains, resulting in the orientation-induced crystallization of iPS, which must not include aPS. The higher degree of shear-induced nucleation density facilitated the successive precursor formation and crystallization. Thus, the enhancement mechanism of precursor formation in aPS/iPS blend must be different from the previous one for blends of HMW PE and low molecular weight (LMW) PE, which are both crystalline polymers. It would be better to point out another possibility for the enhancement of the shearinduced precursor with aPS. As the concentration of aPS increases the shear stress increases at a given shear rate, which induces the oriented structure, resulting in the enhancement of the shear-induced precursor. It is impossible to deny the possibility at the moment. As the aPS concentration further increases from 4%, the integrated brightness decreases with the concentration. This must be an effect of too many entanglements of aPS in the blend. When there are too many entanglements in the blend, they hinder the stretch of chains during a given shear flow duration. 4635

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hard to say how fast the crystallization rate was at the moment. However, just after cessation of the shear flow, we observed the precursors, suggesting that the precursor formation (or the crystal formation) was finished during the shear flow. This must mean that the crystallization rate is very fast compared with the relaxation rate.

CONCLUSION In this work effects of high molecular weight noncrystalline aPS on the shear-induced precursor formation of crystalline iPS have been studied above the nominal Tm of 223 °C using polarized optical microscopy. New insights into the enhancement of precursor formation in shear-induced crystallization were presented. 1 Applying a pulse shear flow, the shear-induced precursor formation in crystalline iPS was enhanced by adding a small amount of high molecular weight noncrystalline aPS above the nominal Tm. This indicates that, like crystalline polymer, noncrystalline high molecular weight polymer can also facilitate the precursor formation under shear flow. It suggests that there might be a new approach to improve the polymer crystallization besides using crystalline polymer or some other agents in shearinduced crystallization. 2 The shear strain, shear rate and aPS concentration were responsible for the enhancement of the shear-induced precursor formation in aPS/iPS blends. It was found that the enhancement of the precursor formation occurred at aPS concentration of ∼4 wt % which was about 4 time larger than the chain overlap concentration of aPS. This result suggests that stretch and orientation of aPS chains through the entanglements is very important for the enhancement. 3 The mechanism for the enhancement by high molecular weight noncrystallizable aPS is different from that by high molecular weight crystallizable iPS. The stretching and orientation of aPS chains by shear flow are sustained for long time due to the long relaxation time. The restraints due to the high molecular weight aPS lead iPS chains remain oriented for a longer time, so the high molecular weight aPS chains facilitate the precursor formation. This mechanism is similar to that proposed by Somani et at.24 for shear-induced crystallization of blends of noncrystallizable atactic polypropylene and crystallizable isotactic polypropylene. ASSOCIATED CONTENT

S Supporting Information *

Discussion of the the melt instability problem. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.D.); [email protected]. jp (T.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Professor Hiroshi Watanabe for the discussion on the effects of flow instability. 4636

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(39) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D. R., Polymer handbook; Wiley: New York: 1999; Vol. 1999. (40) Doi, M.; Edwards, S. F. The theory of polymer dynamics; Oxford University Press: New York: 1988; Vol. 73. (41) Fetters, L. J.; Lohse, D. J.; Graessley, W. W. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1023. (42) Watanabe, H. Prog. Polym. Sci. 1999, 24, 1253. (43) Probably the critical shear rate depends on temperature, but the experimental error in the experiments is rather large because the shearinduced precursor phenomena are not frequent events. Hence, it was probably impossible to distinguish the small difference.

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dx.doi.org/10.1021/ma300207f | Macromolecules 2012, 45, 4630−4637