blends under shear flow

research papers Journal of

Applied Crystallography ISSN 1600-5767

Received 20 May 2013 Accepted 22 November 2013

Precursors in stereo-complex crystals of poly(L-lactic acid)/poly(D-lactic acid) blends under shear flow1 Kota Hemmi,a Go Matsuba,a* Hideto Tsuji,b Takahiko Kawai,c Toshiji Kanaya,d Kiyotsuna Toyohara,e Akimichi Odae and Kou Endoue a

Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan, bDepartment of Environmental and Life Sciences, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan, cDivision of Production Science and Technology, Gunma University, Ota, Gunma 373-0057, Japan, dInstitute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan, and eTeijin Limited, Iwakuni, Yamaguchi 7408511, Japan. Correspondence e-mail: [email protected]

# 2014 International Union of Crystallography

To improve the mechanical and the thermal performance of poly(lactic acid) materials, this work focuses on the formation of stereo-complex crystals by blending poly(l-lactic acid) (PLLA) with poly(d-lactic acid) (PDLA). The resulting structure was analyzed using time-resolved in situ X-ray scattering, optical microscopy, differential scanning calorimetry and viscoelastic measurements. The objective of this study is to investigate the effect of shear flow imposed prior to crystallization on higher-order structure formation and acceleration of stereo-complex crystal growth of PLLA and PDLA blends using a wide spatial scale analysis and viscoelastic measurements. Density fluctuations of 100 nm scale were observed prior to nucleation by in situ simultaneous wideand small-angle X-ray scattering measurements. These density fluctuations grew with time and the intensity increased with increasing shear rate. Furthermore, the results revealed that the PLLA and PDLA chains were only partially interpenetrated; consequently, stereo-complex crystals could grow only in the mixed PLLA/PDLA phase. The correlation length of density fluctuation prior to nucleation was strongly dependent on the mixed phases.

1. Introduction Plant-derived poly(l-lactic acid) (PLLA) has been widely used as an alternative to petroleum-based polymers (Ikada & Tsuji, 2000; Auras et al., 2011; Iwata & Doi, 1999; Kimura, 2009; Urayama et al., 2003; Pantani et al., 2010; Kawai et al., 2007). PLLA materials are preferred for practical use and are now manufactured on a commercial scale. However, the application of PLLA materials is limited because of their higher cost and inferior thermal stability and mechanical properties, which impede their processibility, compared with those of conventional petroleum-based polymers. To improve the mechanical and thermal performance of these polymers, we pursued the formation of stereo-complex (Sc) crystals by blending PLLA with poly(d-lactic acid) (PDLA) (Tsuji & Ikada, 1993; Ikada et al., 1987; Okihara et al., 1991; Tsuji et al., 2012; Fujita et al., 2008; Yang et al., 2012; Rahman et al., 2009). A decrease in the rate of hydrolytic degradation is one reported improvement afforded by Sc crystal formation (Tsuji, 2000; Tsuji & Miyauchi, 2001; Ishii et al., 2009). The melting 1 This article will form part of a virtual special issue of the journal, presenting some highlights of the 15th International Small-Angle Scattering Conference (SAS2012). This special issue will be available in early 2014.

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temperature of Sc crystals increases (to 483–503 K) compared to that of the -crystalline form of PLLA or PDLA (Tm = 433– 453 K) because the structures of Sc crystals are composed of equimolar l- and d-lactide unit sequences packed side-by-side within the crystals and the interaction between PLLA and PDLA chains is stronger than that between chains of PLLA or between chains of PDLA. The growth rate of Sc crystals of PLLA and PDLA is much faster than that of the -crystalline form at the same crystallization temperature. Investigation of polymer melt crystallization under an external field, such as shear flow, has suggested the possibility of controlling the final morphology and the mechanical and functional properties of semi-crystallizable polymers (Strobl, 2007). In the case of crystallization under shear flow, the socalled shish-kebab structure may be observed. The shishkebab structure consists of a long central fiber core (shish) which is surrounded by lamella crystals (kebabs) periodically attached along the shish. In previous studies, Sc crystal fibers were prepared from mixed solutions of PLLA and PDLA by wet and dry spinning processes (Tsuji et al., 1994; Furuhashi et al., 2006). In these cases, differential scanning calorimetry (DSC) revealed that the complex fibers contained both Sc crystals and crystals of the single polymers, PDLA and PLLA J. Appl. Cryst. (2014). 47, 14–21

research papers (homo-crystal), regardless of the spinning methods. The ratio of racemic crystals to homo-crystals increased with the draw ratio of the complex fiber. Furuhashi et al. (2006) described the higher-order structures and mechanical properties of melt spun fibers from blends of PLLA and PDLA. The drawn fibers exhibited amorphous highly oriented homo-crystals of PLLA or PDLA or a mixture of homo- and Sc crystals with a fairly low orientation. Using polarized optical microscopy (POM), Yamazaki et al. (2010) observed the formation of shish-like fibril crystals of PLLA from a sheared melt. The critical molecular weight and crystallization temperature were evaluated from polarized optical micrographs of the fibril crystals. To improve the mechanical and thermal properties of poly(lactic acid), it is very important to enhance the overall crystallization rate. In previous investigations, we studied the structure formation process of isotactic polypropylene (Ogino, Fukushima, Takahashi et al., 2006), isotactic polystyrene (Hayashi et al., 2009; Zhao et al., 2011) and polyethylene using small- and wide-angle X-ray scattering (SAXS and WAXS, respectively), depolarized light scattering and optical microscopy techniques (Matsuba et al., 2007; Kanaya et al., 2007; Ogino, Fukushima, Matsuba et al., 2006). We evaluated the crystallization process under shear flow and found an acceleration of the overall crystal growth rate and/or morphological change from spherulites to shish-kebabs. In particular, in the case of isotactic polystyrene, we observed the precursor on a micrometre scale during the annealing process. We found that the precursor was formed from a stretched polymer network owing to entanglements and small crystallites (Zhao, Matsuba, Moriwaki et al., 2012; Deng et al., 2012). Even in the small amount of amorphous polystyrene with a high molecular weight, precursor formation was enhanced during annealing just after the application of shear flow. In this study, we investigated the nucleation and crystal growth processes under shear flow using blends of PLLA and PDLA with in situ X-ray scattering, optical microscopy, DSC and viscoelastic measurements. The objective of this study is to investigate the effect of shear flow on the higher-order structure and acceleration of Sc crystal growth.

pans were subjected to heating and cooling ramps from 303 to 573 K at 5 K min1. The Tm and Tc values were obtained from the peaks of the endotherms and exotherms acquired during programmed heating and cooling regimes, respectively. Time-resolved SAXS and WAXS measurements were performed on beamline BL15A at the Photon Factory, KEK, Tsukuba, Japan (Amemiya et al., 1983), and on beamline BL40B2 at SPring-8, Japan (Goto et al., 1998). The wave˚, lengths of the incident X-ray beams were 1.5 and 1.0 A respectively. CCD cameras (model C4880 from Hamamatsu Photonics KK, Hamamatsu, Japan) were used for timeresolved SAXS and an image intensifier (C9728DK-10, Hamamatsu Photonics KK) was also used for time-resolved WAXS measurements. Time-resolved POM was performed on a BX-3500T microscope (Wraymer Inc., Osaka, Japan) with a video attachment. The ranges for the length of the scattering vector q in the SAXS and WAXS measurements were 0.07–0.8 and 5–22 nm1, respectively, where q is given by q = (4/ ) sin (2 is the scattering angle). Viscoelastic behavior was measured using an ARES-G2 torsional parallel disc type rheometer (TA Instruments, Tokyo, Japan). The pellets were pressed into discs 25 mm in diameter and 2 mm in thickness in the molten state. Samples were measured under the oscillation mode with a constant strain of 10% to study the linear viscoelastic behavior of the melt, such as the storage and loss modulus, as a function of frequency. A nitrogen atmosphere was applied to the samples to prevent thermal degradation. The blend was maintained at 513 K for 5 min to erase the thermal history and then quenched to the desired temperature (493–513 K). To control the temperature and shear conditions, a Linkam CSS-450 high-temperature cell (Linkam, UK) was used. The sample thickness in the cell was 300 mm for all experiments. The specimen was placed between two quartz plates for optical microscopy and between two stainless steel plates with 50 mm-thick Kapton windows for X-ray measurements. The temperature protocol (see Fig. 1) for the shear experiments of time-resolved SAXS and WAXS measurements can be divided into five stages. (a) The PLLA/PDLA

2. Experimental Samples of PLLA and PDLA, both with weight-average molecular masses Mw of 170 000 and polydispersities Mw /Mn of 2.0, were provided by Teijin Ltd (Tokyo, Japan). A PLLA/ PDLA blend in a 1:1 weight ratio was extruded for 3 min at 513 K and pelletized. The PLLA/PDLA-blend pellets were melted in a vacuum oven at 513 K for 2 min and then quenched to 273 K with a cooling gas to obtain amorphous blend films. Subsequently, the amorphous films were dried for one day in a vacuum oven at room temperature to remove water. DSC was used to determine the melting and the crystallization temperatures (Tm and Tc, respectively) of the sample. DSC measurements were performed with a TA Instruments Q100 calorimeter calibrated using Tm values of indium and cyclohexane standards. Samples of mass 5–10 mg in aluminium J. Appl. Cryst. (2014). 47, 14–21

Figure 1 Temperature protocol for the shear experiment with the PLLA/PDLA blend. The crystallization time, t, starts just after the temperature reaches 473 K. Kota Hemmi et al.

Precursors crystals of poly(lactic acid) blends

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research papers blend sample was heated from room temperature up to 513 K, which was above the Tm of the Sc crystals (494 K from DSC measurements), at a rate of 30 K min1; (b) the sample was held at 513 K for 4 min to erase its thermal history; (c) the sample was cooled to the shear temperature (473 K), which is between the Tm of the Sc crystals and that of the -crystalline form of PLLA or PDLA, at a rate of 30 K min1; (d) the sample was subjected to pulse shear immediately after reaching 473 K, and then (e) annealed at 473 K. The shear rate is from 0 and 75 s1 and the strain is from 0 to 75 000%, respectively.

below Tm of Sc and/or homo-crystals. This transition from melt to glass was observed on poly(ethylene terephthalate) (PET) melt crystallization (Nishida et al., 2004). In order to effect a viscoelastic characterization below the Tm of Sc crystals (494.0 K), we then calculated the relaxation time by fitting the complex modulus, the storage modulus (G0 ) and loss modulus (G00 ), with a multimode Maxwell mode: G0 ð!Þ ¼

N X Gi ð!i Þ2 2 i¼1 ½1 þ ð!i Þ

ð1Þ

G00 ð!Þ ¼

N X Gi ð!i Þ 2 ; i¼1 ½1 þ ð!i Þ

ð2Þ

and

3. Results and discussion From the DSC measurements, we noted that glass transition was observed at around 333 K, and the Tm values of the homoand Sc crystals were 449.1 and 494.0 K, respectively. Fig. 2 shows the DSC curve during the cooling process (30 K min1). Two exothermic peaks due to Sc and homo crystallization were observed at 440 and 380 K. We observed acceleration of the nucleation process (nucleation rate/density) and transformation of the crystal morphology by shear flow. Zhao and co-workers reported a correlation between the shear rate and the relaxation time (Zhao, Matsuba & Ito, 2012; Zhao et al., 2011). To interpret the mode of the chain segments in the entangled polymer melt, it was difficult to apply the pure reptation theory introduced by de Gennes (1971, 1981) and developed by Doi and Edwards (Doi, 1983; Doi & Edwards, 1988). Supplementary models such as contour length fluctuation (Doi, 1983) and convective constraint release (CCR) (Marrucci, 1996) are necessary for a polydisperse system. Fig. 3 shows the storage modulus (G0 ) and the loss modulus (G00 ) dependence on the frequency (!) for molten PLLA/PDLA blends. The data were collected at every 5 K from 493 to 513 K and were reduced at 493 K. We observed a typical response of the oscillatory shear of the polymers with a linear structure and a relatively narrow molecular weight distribution in the terminal region (G0 ’ !2, G00 ’ !) at low frequencies. The storage modulus curve is very similar to the isotropic molten state of liquid crystals (Rubin et al., 1995). This suggests that the PLLA/PDLA blends could become the liquid–crystalline-type phase when cooled down

where Gi and i are the Maxwell parameters. The viscosityaveraged relaxation time, , involves individual discrete relaxation times in all modes and is given by .P N N P ¼ i i i ; ð3Þ i¼1

i¼1

where i is the viscosity of the Maxwell parameters (= Gi!i). The Rouse time, R, at 473 K was estimated by fitting the viscoelastic data (Fig. 3) with a multimode Maxwell mode as explained in our previous study (Zhao, Hayasaka et al., 2012). R is given by R ¼ N 2 b2 =32 kT ( , N, b and k are the friction coefficient, the number of chain segments, the Kuhn step length and the Boltzmann constant, respectively). For the PLLA/PDLA blend used in this work, the reptation time, rep, that is the longest relaxation, is given by rep = Z R ’ 16 R (Z is the number of entanglements per chain) (Grijpma et al., 1994). The data suggested that rep ’ 0.018 s and R ’ 0.29 s at 493 K. The fitted viscoelastic data suggested that rep ’ 9.0 103 s and R ’ 0.15 s at 473 K. Under this experimental condition, the imposed shear rate is 25 or 75 s1; therefore, the shear rate is larger than the reciprocal of the reptation time and smaller than the reciprocal of the Rouse time. In that case,

Figure 2

Figure 3

DSC curves acquired during cooling of the molten PLLA/PDLA blend at 30 K min1.

Master curves of G0 (storage modulus) and G00 (loss modulus) for the PLLA/PDLA blend at 493 K.

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Figure 4 WAXS profiles acquired during the crystallization process at 473 K after shear flow under the shear rates/shear strains of (a) 0 s1/0%, (b) 25 s1/ 50 000% and (c) 75 s1/75 000%.

CCR (Marrucci, 1996) becomes the dominant mode, which indicates disentanglement of the polymeric topological network. By applying shear flow, the polymer chains are deformed and release the constraint of entanglements (Graham et al., 2003). 3.1. Evaluation of the incubation time

We determined the incubation time, tinc, as the starting point of crystallization to perform in situ WAXS measurements. Fig. 4 shows the time evolution of the two-dimensional WAXS profiles of the PLLA/PDLA blend samples during crystallization at 473 K for the quiescent state (_ = 0 s1) (Fig. 4a) and under the shear rates (_ )/strains () of 25 s1/50 000% (Fig. 4b) and 75 s1/75 000% (Fig. 4c). We observe the same three diffraction peaks under all conditions; hence the profiles are assigned as triclinic racemic crystals ( form), referred to as Sc crystals. The peaks at q = 8.3, 14.4 and 16.6 nm1 are due to the crystal planes (110), (300)/(030) and (220), respectively. By deconvolution of the crystal diffraction peaks and the amorphous halo from these WAXS profiles, we determined that isothermal crystallization began after 240, 120 and 96 s at 473 K in the quiescent state and under the shear rates/shear

strains of 25 s1/50 000%, 25 s1/75 000% and 75 s1/75 000%, respectively. More quantitative information was obtained by calculating the time evolution of the crystalline diffraction and amorphous halo. As shown in Fig. 5, the azimuthally averaged WAXS profile was divided into crystalline peaks and an amorphous halo. The Ic and Ia fractions of the Sc crystals and the amorphous components, respectively, were obtained as ratios of the integrated intensity of each component to the total integrated intensity. Fig. 6 shows the crystallinity, Xc = 100Ic /(Ic + Ia), as a function of crystallization time. The arrows show the starting point of increasing Xc, that is the incubation time, tinc. tinc decreases with increasing shear rate/strain, showing the large effects of the shear conditions on the nucleation rate (Fukushima et al., 2005). Furthermore, Xc increases with time and after application of shear flow. The dependences of tinc and Xc on the shear conditions after crystallization for 1800 s are listed in Table 1. Application of shear flow has substantial effects on crystal nucleation and crystallinity. 3.2. Small-angle X-ray scattering measurements

For structural analysis on a scale of several tens of nanometres, we performed time-resolved two-dimensional SAXS measurements of the crystallization process of PLLA/PDLA

Figure 6 Figure 5 Example of peak separation of the crystal diffraction and amorphous halo from azimuthally averaged WAXS profiles. J. Appl. Cryst. (2014). 47, 14–21

Evolution of Xc over the period of crystallization under various shear rate conditions. The arrows show the starting point of increasing crystallinity corresponding to the incubation time of crystallization. Kota Hemmi et al.


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research papers Table 1 Incubation time (tinc) prior to crystallization under various shear rate conditions. Shear rate (s1)

Shear strain (%)

0 25 75

0 50 000 75 000

tinc (s) 240 120 96

Xc (%) 1.7 18 22

blends. In Fig. 7, the time evolutions of the two-dimensional SAXS profiles during the crystallization process at 473 K in the quiescent state and after shear flow of the shear rate/strain of 25 s1/50 000% are shown. In these measurements, we could cover the scattering vector magnitude range from 0.07 to 0.8 nm1. In the quiescent condition, only the isotropic profiles were observed and the onset time of the scattering intensity increase was 300 s after reaching the crystallization temperature (473 K). Isotropic lamellar crystal growth was observed in this q range. In the case of the shear condition, weak anisotropic patterns appeared along the shear direction at approximately 60 s after applying the pulse shear. The anisotropic pattern along the flow direction must correspond to the distance between the stacked lamellar crystals (kebabs) periodically aligned along the shear direction. The observed anisotropic pattern is covered by the isotropic patterns; therefore, the anisotropic patterns were gradually weakened by growth of the isotropic pattern corresponding to isotropic crystal growth. Furthermore, we did not observe any scattering from the shish structure in the direction normal to the shear flow. These results suggest that the shear flow is strong enough to orient the chain segments; therefore, the lamellar crystals were stacked along the shear direction. Fig. 8 shows the time evolution of the scattering intensity in the directions parallel and normal to the shear flow in the q range of 0.07–0.3 nm1 under the shear condition of shear rate/strain of 25 s1/50 000%. The scattering peak in the direction parallel to the shear flow exemplifies the long spacing between the stacked lamellar crystals along the shear direction and the isotropic lamellar crystals. The peak positions qm of the scattering intensity in the directions parallel

and normal to the shear indicate the spacing between the lamellae aligned and non-aligned by shear flow, respectively. The peak position qm of the scattering intensity parallel to the shear flow indicates the spacing between the aligned and isotropic lamellar crystals. The scattering peak positions for both directions increase with crystallization time. Such increases were observed in the case of crystallization of isotactic polypropylene under shear flow. After a crystallization period of 1800 s, the spacing values evaluated from 2/ qm were 42 nm in both directions. The morphology of the lamellar crystals is independent of the shear conditions. 3.3. Structure-forming process during incubation

To analyze the structure-forming process under various shear rate conditions from the SAXS intensity, we subtracted the intensity of the sample immediately after quenching (crystallization time = 0 s) from those of the annealed samples. Fig. 9 shows the resulting difference scattering intensity profiles of the normal direction in the early stage of crystallization in the quiescent state and under the shear rates/strains of 0 s1/0%, 25 s1/50 000% and 75 s1/75 000%. It is noted that, during the incubation time (open symbols), the scattering intensity increases as the scattering vector magnitude q decreases and this trend is enhanced gradually during the incubation time. However, we could not observe any peaks in this q window during the incubation time. Fig. 10 shows the difference intensity profiles of the normal direction during the crystallization period after nucleation. The so-called longspacing period peak appeared and grew at around q = 0.15 nm1. However, the increment in the low-q region was observed only as a shoulder of the long-spacing period peak at q = 0.15 nm1. The intensity increment was observed even in the quiescent state and the intensity increased rapidly with increasing shear rate and strain. These findings suggest that the intensity increment is strongly correlated with structure formation before nucleation of the Sc crystals. The density fluctuations, which grow during the incubation time tinc prior to crystallization, are assigned as ‘precursors’ of crystal nuclei. A similar time evolution of the density fluctuations prior to

Figure 7

Figure 8

Time evolution of two-dimensional SAXS profiles during the crystallization process at 473 K in (a) the quiescent state and (b) under the shear rate/shear strain of 25 s1/50 000%.

Time evolution of SAXS profiles in the directions parallel and normal to the shear flow. The shear rate and shear strain are 25 s1 and 50 000%, respectively.

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Figure 9 Difference intensity of normal direction versus q under various shear conditions in the early stage of crystallization in the quiescent state (a) and under the shear rates/shear strains of 25 s1/50 000% (b) and 75 s1/75 000% (c), respectively. Open and filled symbols: the incubation and crystallization period, respectively.

Figure 10 Difference intensity of normal direction versus q under various shear conditions after the incubation time in the quiescent state (a) and under the shear rates/shear strains of 25 s1/50 000% (b) and 75 s1/75 000% (c).

the crystallization process has been observed and described as a spinodal decomposition-type phase separation (Kaji et al., 2005) in PET (Nishida et al., 2004), polyethylene naphthalate (Matsuba et al., 2000) and isotactic polypropylene (Heeley et al., 2003) crystallization. However, in the present case, it is very difficult to fully understand the evolution of the growth

process of precursors over time, because we could observe only the tail of the density fluctuation peak. Nishida et al. (2004) found that micrometre-scaled density fluctuations could be observed immediately after fast quenching below Tm. Fig. 11 shows the time-evolved POM images acquired in the quiescent state and under the shear flow. We observed several crystal nuclei because of the acceleration of crystal nucleation by shear flow. During the incubation time of crystal nucleation, no density and/or orientation fluctuations were observed in the POM images. This result suggests that the scale of the density fluctuations might be between several tens and hundreds of nanometres.

3.4. Schematic drawing of density fluctuations

Figure 11 Polarized optical micrographs acquired during crystallization (473 K) of the PLLA/PDLA blend in (a) the quiescent state and (b) under the shear rate/shear strain of 25 s1/50 000%. J. Appl. Cryst. (2014). 47, 14–21

In the PLLA/PDLA crystallization case, we especially focused on the component fluctuations between PLLA and PDLA. The radial growth rates of spherulites were approximately 1.7 mm min1 in the quiescent state and under the shear rate/shear strain of 25 s1/50 000%. The growth rate is independent of shear flow, while the crystallinity increases strongly with increasing shear conditions. The nucleation density appears to increase to more than ten times that in the quiescent state after applying shear flow. These results suggest Kota Hemmi et al.


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research papers well mixed PLLA/PDLA phase, which was promoted by the mixing effect of shear flow.

4. Conclusions

Figure 12 Schematic drawing of PLLA/PDLA phase structures (a) in the quiescent state and (b) after applying shear flow. l and d indicate the PLLA- and PDLA-rich phase, respectively, and Sc indicates the mixed phase of PLLA and PDLA.

that applying shear flow strongly influences the crystal nucleation rate and density. Next, we investigated the effect of shear flow on crystal nucleation. In Fig. 2, the DSC curve of the cooling process shows two peaks, one due to Sc crystal growth and another due to homo-crystal growth. This result suggests that the PLLA and the PDLA chains were not mixed completely even in the molten state of the blended sample above 513 K in the case of extrusion for 3 min at 513 K. Even in the case of a cast film sample mixed in dilute solution, two melting temperatures were observed for high molecular weight PLLA and PDLA from the DSC measurements (Tsuji et al., 1991). The results suggest that PLLA and PDLA is also mixed inhomogeneously even in the dilute solution. From these results, PLLA and PDLA blends could not be mixed entirely homogeneously. Therefore, the molten state of the blended sample is a partially mixed state composed of a mixed PLLA/PDLA phase and a relatively dense PLLA (or PDLA) phase, as shown in Fig. 12(a). Polymer chains in the mixed phase crystallized below approximately 494 K and those in the dense PLLA (or PDLA) phase crystallized below approximately 449 K. On annealing at 473 K, Sc crystal growth occurred only in the mixed PLLA/PDLA phase. Next we focused on the effect of shear flow on nucleation. The application of shear flow has two possible outcomes in blended polymers: phase separation and phase mixing. From the results of the WAXS measurements, we observed acceleration of the Sc crystal nucleation rate and increasing Sc crystallinity after applying shear flow. These results suggest that the fraction of the possible region for Sc crystal growth increases by applying shear flow, that is, phase mixing occurs (Fig. 12b). In the PLLA/PDLA blend crystallization, we observed density fluctuations prior to crystal nucleation larger than the long-spacing period. The density fluctuations grew with time and depended on the shear rate and strain. It is conceived that the density fluctuations occurring prior to crystal nucleation corresponded to the structure in the molten state, that is, a structure partially mixed with PLLA and PDLA. The correlation length between the well mixed PLLA/PDLA, homo PLLA and homo PDLA phases is above 100 nm as determined by the lack of peaks outside the q window in the SAXS measurements. These density fluctuations could grow only in the mixed PLLA/PDLA phase. Sc crystal nucleation processes were accelerated by increasing the fraction of the region of the

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We have investigated the Sc crystallization process of blended PLLA and PDLA samples under various shear conditions using time-resolved WAXS, SAXS and POM techniques. The time required for nucleation decreased with increasing shear rate and strain, and we could not observe an oriented crystal lattice and shish-kebab structure; instead, only a weak lamellar crystal orientation parallel to the shear direction was noted. The shear conditions were larger than the inverse of estimated reptation time; therefore, CCR is the dominant mode, which indicates disentanglement of the PLLA/PDLA chain network, and then these stretched chains enhanced crystal nucleation. Furthermore, we observed density fluctuations prior to crystal nucleation above 100 nm. The density fluctuations grew over time during the incubation period. The intensity increased with the shear rate and strain. These results suggest that the density fluctuations might grow only in the mixed PLLA/PDLA phase. Sc crystal nucleation processes were accelerated by increasing the region of the mixed PLLA/ PDLA phase, which was promoted by the mixing effect of shear flow. The authors would like to thank Dr Noboru Ohta (SPring-8, JASRI) for the synchrotron radiation X-ray scattering measurements. This work was supported by a KAKENHI grant, No. 22750197, and by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University (grant No. 2012-37).

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