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PVAc blends during heat treatment under strain
Polymer 52 (2011) 2964e2969
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Micro-phase separation and crystallization behavior of amorphous oriented PLLA/ PVAc blends during heat treatment under strain Yongjin Li*, Jichun You College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2011 Received in revised form 7 April 2011 Accepted 1 May 2011 Available online 6 May 2011
Amorphous oriented poly(L-lactide) (PLLA)/poly(vinyl acetate) (PVAc) 50/50 films were prepared by uniaxial drawing of melt-mixed blends at 65 C. The morphology development and crystal organization of the blends during heat treatment under strain were investigated using small angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Equatorial scattering maxima in the SAXS patterns for samples annealed at 75 C were observed before the appearance of crystal reflections. Further annealing of the samples at higher temperature induced two further discrete meridian scattering maxima. The observations indicated that homogenous oriented PLLA/PVAc film undergoes micro-phase separation first, followed by crystallization of PLLA in the PLLA-rich phase. The micro-phase separated PVAc nanodomains are aligned parallel to the stretching direction, whereas the crystallized PLLA lamellae are oriented perpendicular to the stretching direction (crystal c-axis along the stretching direction). Microphase separation was not observed when films were annealed at 120 C, at which temperature the high crystallization rate of PLLA overwhelmed the micro-phase separation process. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Oriented crystallization Micro-phase separation Miscible blends
1. Introduction Hierarchical structure control from manipulating chain alignment to controlling the phase structure of polymer blends has been one of the major research themes for improving the macroscopic properties of blends. Oriented crystallization (crystallization under strain) is an effective strategy for producing novel textures of polymer blends. Extensive investigations have been carried out on oriented crystallization of immiscible crystalline/crystalline polymer blends such as polypropylene (PP)/polyethylene (PE) [1e3], PP/ polybutene (PB) [4], PP/nylon 11 [5], poly(vinylidene fluoride) (PVDF)/nylon 11 [6e8], and poly(3ecaprolactone) (PCL)/PE [9]. When the component with lower melting temperature (Tm) is crystallized in the oriented matrix of the other component, unusual orientation textures can be induced in the dispersed phase. The phase structure of these immiscible blends is usually micrometer scale, and the component with lower Tm crystallizes within the dispersed phase without molecular chain segregation from the other component. Oriented crystallization has recently been applied to some miscible crystalline/crystalline polymer blends such as the poly(1,4-butylene succinate) (PBSU)/PVDF [10,11] and
* Corresponding author. Tel.: þ86 571 28868879; fax: þ86 571 28867899. E-mail address: [email protected] (Y. Li). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.05.003
poly(3-hydroxybutyrate) (PHB)/PVDF [12] systems. Various types of orientation textures can be obtained for PBSU/PVDF blends by carefully controlling the crystalline morphology of the blend [10,11]. Orthogonal orientation textures in which the crystal axes of the two components align in mutually opposite directions, have been produced by oriented crystallization of PHB in its blend with PVDF [12]. It should be noted that these blends are miscible only in the amorphous region, but the two components crystallize separately and form independent lamellar structures. Thus oriented crystallization of these blends occurs locally and the crystallization space is confined within a much smaller size depending on the lamellae arrangements in the blend, compared with oriented crystallization of immiscible blends. The size is usually several tens of nanometers for interlamellar inclusion structures, and 100e200 nm for interlamellar exclusion structures. Miscible crystalline/amorphous polymer blends have been widely investigated [13e17] and oriented crystallization has also been applied to some miscible crystalline/amorphous polymer blends [18e23]. The crystallization of PCL in miscible blends with poly(vinyl chloride) under strain leads to crystalline orientation perpendicular to the strain direction under most conditions, whereas a parallel chain orientation is observed under conditions where the draw rate is rapid and the draw ratio is high [18,19]. The crystal orientation has also been found to be highly dependent on the molecular weight of PCL [20]. The oriented crystallization of iPS
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Fig. 1. Typical SAXS patterns and corresponding WAXD patterns of oriented PLLA/PVAc blend annealed at 75 C under strain with the indicated time (The arrow indicates the stretching direction).
in oriented iPS/PPO blends results in a texture containing highly oriented iPS crystals and nearly isotropic PPO chains [21]. Recently, Iwata et al. found that an orthogonal orientation texture was developed by oriented crystallization of PHB in PHB/cellulose propionate (CP) blends, in the CP-rich composition range [22,23]. Obviously, the inevitable chain segregation process during crystallization occurs for the oriented crystallization of the miscible crystalline/amorphous polymer blends. Although the literature reported various orientation textures formed from the miscible crystalline/amorphous blends by oriented crystallization, few investigations have focused on the details of the structure development process for the oriented crystallization of such blends. Poly(L-lactide) (PLLA)/poly(vinyl acetate) (PVAc) is a miscible blend with c parameter 0.12, indicating relatively weak interactions between PLLA and PVAc [24,25]. In the present study, the oriented crystallization behavior of PLLA/PVAc has been investigated with emphasis on the structure development during annealing under strain at different temperatures. It was found that the homogenous oriented PLLA/PVAc blends first undergo microphase separation, followed by crystallization of PLLA in the PLLA
rich phase during annealing at low temperature. By contrast, micro-phase separation was not observed during annealing at high temperature. 2. Experimental section 2.1. Materials and sample preparation The PLLA sample used was commercially available material with molecular weight 170,000 g mol1. The sample included 1.2% of Dlactide. PVAc with molecular weight approximately 260,000 g mol1 was purchased from Sowa Science Co. (Japan). The polymers were vacuum dried at 60 C for at least 12 h before processing. Blends with PLLA/PVAc in 1:1 weight ratio were prepared using a Brabender-type plastic mixer (Toyoseiki Co. KF70V) with two rotors at rotation speed 100 rpm at 190 C for 10 min. After blending, all samples were hot pressed at 200 C for 5 min into film with thickness 500 mm, followed by quenching in ice water. Oriented films were prepared by uniaxially stretching the meltquenched films using an automatic stretching machine setting in a circulating oven at a draw ratio of 4.0 at 65 C. The stretching speed was 5 mm/min. The oriented films were rapidly cooled to room temperature under strain, then heat-treated at fixed length at different temperatures for various times. 2.2. Characterization
Fig. 2. Radically corrected SAXS profiles for PLLA/PVAc blends annealed at 75 C under strain with the indicated time.
Small-angle X-ray scattering (SAXS) patterns were obtained using microfocused Cu Ka radiation (45 kV, 60 mA) generated by an X-ray diffractometer (Rigaku Ultrax 4153A 172B), and an imaging plate detector. Wide-angle X-ray diffraction (WAXD) patterns were obtained using CuKa radiation (40 kV, 120 mA) generated by an Xray diffractometer (Rigaku, Ultrax 8000) with an imaging plate detector. Miscibility was characterized by dynamic mechanical analysis (DMA), carried out with a RHEOVIBRON DDV-25FP (Orientec Corp.) instrument in tensile mode. The dynamic storage and loss moduli were determined at frequency 1 Hz and heating rate 3 C min1 as a function of temperature from 150 to 150 C. Blend morphologies were observed by transmission electron microscopy (TEM) (Hitachi
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Fig. 3. The SAXS pattern (a) and corresponding WAXD pattern (b) of the PLLA/PVAc blend annealed at 120 C for 30 min after annealed at 75 C for 120 min.
H7000) operating at acceleration voltage 75 kV. The blend samples were ultra-microtomed at 120 C to about 70 nm thickness. The sections were then stained with RuO4 for 20 min. 3. Results Fig. 1 shows typical SAXS patterns and corresponding WAXD patterns of oriented PLLA/PVAc blends annealed at 75 C under strain for the indicated time. The SAXS and WAXD patterns of neat PLLA subjected to oriented crystallization with the same draw ratio are also shown for comparison. Oriented crystallization of neat PLLA induced a highly oriented crystal structure. The two meridian maxima in the SAXS pattern can be assigned to an aligned lamellae structure perpendicular to the stretching direction (crystal c-axis along the stretching direction). The sharp WAXD reflections for the neat PLLA indicate that highly oriented PLLA crystals and the molecular chains (c-axis of crystals) were aligned along the stretching direction. No discernable scatterings and diffractions were observed from the SAXS and WAXD patterns for the oriented PLLA/PVAc blends before annealing, indicating that the stretched PLLA/PVAc samples were homogeneous (see the first pattern for SAXS and WAXD in Fig. 1). In other words, PLLA and PVAc are miscible at the molecular chain level and stretching at 65 C did not induce detectable heterogeneity. The PLLA in the oriented blend was also amorphous because no reflections from PLLA crystals were observed before annealing. Almost the same WAXD patterns with only the amorphous halo were found when the sample was annealed at 75 C for 15 min, but the SAXS pattern showed significant differences from
a
the pattern obtained before annealing. Two equatorial scattering peaks were observed in the SAXS pattern for the sample annealed at 75 C for 15 min. These peaks suggest that annealing leads to a new structure parallel to the stretching direction. From the peak positions, the long period in the equatorial direction estimated from the Bragg equation was about 20 nm. Upon increasing the annealing time to more than 30 min, discrete crystal reflections were observed in WAXD patterns, suggesting formation of PLLA crystals in the oriented blend. The major reflections of a-PLLA are 020, 110 and 210, which are expected at 2q ¼ 14.7, 19.1 and 22.3 , respectively. The appearance of the hk0 reflections of PLLA crystals in the equatorial direction indicates that the crystal c-axis (molecular chain axis) of PLLA was oriented in the stretching direction. Moreover, the crystallinity of PLLA increased gradually because the diffraction intensity increased with annealing time. However, the SAXS patterns for all annealed samples at 75 C exhibited only equatorial scattering, but the scattering peak shifted to lower angle with increasing annealing time, as shown in the radically corrected SAXS analysis profiles in Fig. 2 from the 2dimensional SAXS patterns. All reflections of PLLA crystals in the blends annealed at 75 C were relatively weak and broad, suggesting low crystallinity and irregular PLLA crystals. To make highly ordered and perfect PLLA crystals in the oriented blend, the sample was further annealed under strain at 120 C for 30 min after first heat-treatment at 75 C for 120 min. The SAXS and WAXD patterns of the two-steps annealed sample are shown in Fig. 3. As expected, the WAXD pattern shows much stronger and sharper reflections after
b
Fig. 4. DMA results of (a) PVAc, (b) PLLA, (c) oriented amorphous PLLA/PVAc blend, and (d) PLLA/PVAc blend annealed at 75 C for 120 min (dash lines indicate the Tgs of neat PLLA and neat PVAc).
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Fig. 5. TEM images of for the oriented PLLA/PVAc blends after annealed at 75 C for (a) 0 min, (b) 15 min, (c) 30 min, and (d) 120 min.
annealing, indicating increased crystallinity and improved crystal perfection. The crystal orientation was unchanged with the c-axis oriented along the stretching direction. Surprisingly, from the SAXS pattern two new meridian maxima were observed after annealing at 120 C, in addition to the original equatorial scattering. Both the SAXS and the WAXD results show that the two meridian maxima originate from PLLA lamellae that are correlated and oriented perpendicular to the stretching direction. For the samples annealed only at 75 C, the PLLA crystals formed were imperfect due to the low crystallization temperature, and the density difference between the amorphous region and the crystalline region was small in those cases. Consequently, no scattering from the crystal lamellae was observed in the meridian direction. It was very interesting to find unique equatorial SAXS scattering for all of the annealed samples. Obviously the scattering cannot be
assigned to PLLA crystals, because PLLA in the blend is amorphous in the early stages with 15 min annealing time. Moreover, the oriented PLLA crystals resulted in the meridian scattering shown in Fig. 3. We concluded that micro-phase separation between PLLA and PVAc occurred in oriented homogenous blends upon annealing under strain. The densities of PLLA and PVAc are 1.26 and 1.15 g cm3, respectively. Therefore, an SAXS peak can be expected if micro-phase separation between PLLA and PVAc occurs. The phase separation under strain leads to aligned nanodomains parallel to the stretching direction; consequently the SAXS equatorial scattering maxima were observed. To confirm the microphase separated structure of the blend, DMA was used to evaluate the glass transition of the sample annealed at 75 C. Fig. 4 shows the storage modulus and tan d of the oriented PLLA/PVAc blend as a function of temperature before annealing, and after
Fig. 6. The SAXS pattern (a) and corresponding WAXD pattern (b) of the PLLA/PVAc blend directly annealed at 120 C for 30 min.
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Fig. 7. Schematic diagram for the structure development of the oriented amorphous PVAc/PLLA blends during annealing at 75 C.
annealing at 75 C for 120 min, together with corresponding data for neat PLLA and PVAc. The glass transition temperatures of neat PLLA and PVAc were 62 and 37 C from the peak positions of tand curves, respectively. The oriented PLLA/PVAc blend before annealing exhibited only one strong tand peak at 52 C, locating at the temperature between the Tgs of neat PLLA and neat PVAc, suggesting that a miscible (single phase) state for PLLA and PVAc in the oriented blend. However, a very clear tand shoulder at 38 C was observed in addition to a peak at 61.5 C for the blend after annealing at 75 C. Those relaxation temperatures are very close to the glass transition temperatures of neat PVAc and PLLA, indicating a two-phase structure for the annealed, oriented PLLA/PVAc blend. Two corresponding step changes in storage modulus were also clearly observed for the annealed sample. The morphological development with the annealing time for the PLLA/PVAc blends annealed at 75 C was characterized using TEM, as shown in Fig. 5. PLLA appears as the white phase and PVAc as the dark phase because PVAc was more readily stained than PLLA. It is apparent that the oriented blend before annealing is homogeneous, as seen in Fig. 5 (a). The prolonged annealing time induced the phase separation between PVAc and PLLA, which may originate from the phase decomposition by concentration fluctuation during annealing. When the sample annealed for 120 min, it is seen that numerous elongated PVAc domains were precisely dispersed in the PLLA phase in Fig. 5(d). The domains were highly oriented parallel to the stretching direction, and the long period of the domains was in the range 10e30 nm, which is consistent with the corresponding distance estimated from the equatorial scattering in SAXS. Close examination of the image of the PLLA matrix showed many very small crystals that were aligned perpendicular to the stretching direction. The effects of annealing temperature on structure development of the same blend samples were also investigated. The WAXD and SAXS patterns in Fig. 6, for the sample annealed at 120 C for 30 min, show that the PLLA crystalline orientation direction had the same caxis orientation when crystallized at high temperature. However, by contrast with the SAXS patterns for the sample annealed at 75 C, only two strong meridian scattering maxima from the PLLA crystals were observed. This result suggests that micro-phase separation did not occur during annealing at the higher temperature. 4. Discussion Structure development for the oriented amorphous PVAc/PLLA blends during heat treatment at 75 C is depicted schematically in Fig. 7. In the first step, the quenched blend before drawing is a homogeneous mixture of PLLA and PVAc. Both PLLA and PVAc are amorphous and are miscible at the molecular chain level as suggested by previous miscibility investigations [24,25]. When the homogeneous mixture of PLLA and PVAc is stretched uniaxially at
65 C, there is no doubt that the molecular chains of the two components are oriented along the drawing direction. In this state the oriented blend is a homogeneous single phase, as revealed by the single Tg from DMA analysis and no detectable scattering in the SAXS pattern. Subsequent annealing under strain at low temperature results in apparent phase separation, with PVAc nanodomains aligned parallel to the stretching direction. The interaction between PVAc and PLLA is relatively weak. The reduced entropy of both PLLA and PVAc resulting from stretching may lead to the decreased miscibility between PLLA and PVAc. When the sample is heated to a temperature above the glass transition temperature of PLLA the molecular chains of PLLA and PVAc become flexible and phase separation occurs, as indicated by the SAXS, DMA, and TEM measurements. The subsequent crystallization of PLLA drives the further segregation of PLLA chain from PLLA/PVAc mixing state. The increased crystallinity during annealing induces not only the enlarged density difference between the two phases, but also the increased long periods (as shown in Fig. 2). The details of the microphase separation process and mechanism are still under investigation using in-situ SAXS measurements. It is worthy of note that the annealing temperature at which micro-phase separation occurs is only slightly higher than Tg for PLLA, and the crystallization rate at the annealing temperature is very low. Consequently PLLA is still in the amorphous state, and prolonged annealing time is required to induce its crystallization. Crystallization under strain generally leads to segmental chain orientation along the strain direction, which originates from linear nuclei produced by row nucleation of extended molecular chains. Thus PLLA crystals are oriented with the molecular chain (c-axis) along the stretching direction. However, the crystallinity of PLLA is relatively low and the boundary between the crystalline and amorphous layers is not very clear, so that no meridian SAXS peaks are observed at this stage. Further annealing at 120 C results in not only increased crystallinity but also well-defined crystalline and amorphous layers, so very sharp SAXS peaks are observed in the meridian direction in addition to the equatorial scattering peaks. However, the microphase separation process is not observed for samples annealed at 120 C. The high temperature annealing results in the high PLLA crystallization rate and the mobility of PLLA chains is restricted by the formation of the crystals. Therefore, no micro-phase separation of the blends was observed at the high annealing temperatures. It should be noted that no micro-phase separation phenomenon was observed for the samples with the draw ratio of less than 2.5. 5. Conclusion In summary, the structure development of oriented amorphous PLLA/PVAc blends during annealing was investigated. We observed micro-phase separation behavior before crystallization from
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a miscible polymer blend, which provides a new avenue to fabrication of nanostructured polymer materials by a bottom-up process. Acknowledgment This study was partially supported by the National Science Foundation (21074029) and Zhejiang Provincial Natural Science Foundation of China (R4110021). References [1] [2] [3] [4] [5]
Nishio Y, Yamane T, Takahashi T. J Macromol Sci.ePhys 1984;B23:17. Kojima M, Satake H. J Polym Sci Polym Phys 1984;22:285. Gross B, Petermanm J. J Mater Sci 1984;19:105. Takahashi T, Nishio Y, Mizuno H. J Appl Polym Sci 1987;34:2757. Seth KK, Kempster CJE. J Polym Sci. Polym Symp 1977;58:297.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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Li Y, Kaito A. Macromol Rapid Commun 2003;24:255. Li Y, Kaito A. Macromol Rapid Commun 2003;24:603. Li Y, Kaito A. Polymer 2003;44:8167. Takahashi T, Inamura M, Tsujimoto I. Polym Lett 1970;8:651. Li Y, Kaito A, Horiuchi S. Macromolecules 2004;37:2119. Kaito A, Iwakura Y, Hatakeyama K, Li Y. Macromolecules 2007;40:2751. Kaito A. Polymer 2006;47:3548. Hu YS, Kamdar AR, Ansems P, Chum SP, Hitner A, Baer E. Polymer 2006;47:6387. Rostami SD. Eur Polym J 2000;36:2285. Dreezen G, Koch MH, Reynaers H, Groeninckx G. Polymer 1999;40:6451. Dreeezen G, Fang Z, Groeninckx G. Polymer 1999;40:5907. Rostami SD. Polymer 1990;31:899. Zhao Y, Keroack D, Prud’homme R. Macromolecules 1999;32:1218. Morin D, Zhao Y, Prud’homme R. J Appl Polym Sci 2001;81:1683. Zhang Y, Prud’homme R. Macromol Rapid Commun 2006;27:1565. Dikshit A, Kaito A. Polymer 2003;44:6647. Park JW, Doi Y, Iwata T. Macromolecules 2005;38:2345. Park JW, Tanaka T, Doi Y, Iwata T. Macromol Biosci 2005;5:840. Gajria AM, Davé V, Gross RA, McCarthy SC. Polymer 1996;37:437. Yoon JS, Oh SH, Kim MN, Chin IJ, Kim YH. Polymer 1999;40:2303.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Micro-phase separation and crystallization behavior of amorphous oriented PLLA/ PVAc blends during heat treatment under strain Yongjin Li*, Jichun You College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 February 2011 Received in revised form 7 April 2011 Accepted 1 May 2011 Available online 6 May 2011
Amorphous oriented poly(L-lactide) (PLLA)/poly(vinyl acetate) (PVAc) 50/50 films were prepared by uniaxial drawing of melt-mixed blends at 65 C. The morphology development and crystal organization of the blends during heat treatment under strain were investigated using small angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD). Equatorial scattering maxima in the SAXS patterns for samples annealed at 75 C were observed before the appearance of crystal reflections. Further annealing of the samples at higher temperature induced two further discrete meridian scattering maxima. The observations indicated that homogenous oriented PLLA/PVAc film undergoes micro-phase separation first, followed by crystallization of PLLA in the PLLA-rich phase. The micro-phase separated PVAc nanodomains are aligned parallel to the stretching direction, whereas the crystallized PLLA lamellae are oriented perpendicular to the stretching direction (crystal c-axis along the stretching direction). Microphase separation was not observed when films were annealed at 120 C, at which temperature the high crystallization rate of PLLA overwhelmed the micro-phase separation process. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Oriented crystallization Micro-phase separation Miscible blends
1. Introduction Hierarchical structure control from manipulating chain alignment to controlling the phase structure of polymer blends has been one of the major research themes for improving the macroscopic properties of blends. Oriented crystallization (crystallization under strain) is an effective strategy for producing novel textures of polymer blends. Extensive investigations have been carried out on oriented crystallization of immiscible crystalline/crystalline polymer blends such as polypropylene (PP)/polyethylene (PE) [1e3], PP/ polybutene (PB) [4], PP/nylon 11 [5], poly(vinylidene fluoride) (PVDF)/nylon 11 [6e8], and poly(3ecaprolactone) (PCL)/PE [9]. When the component with lower melting temperature (Tm) is crystallized in the oriented matrix of the other component, unusual orientation textures can be induced in the dispersed phase. The phase structure of these immiscible blends is usually micrometer scale, and the component with lower Tm crystallizes within the dispersed phase without molecular chain segregation from the other component. Oriented crystallization has recently been applied to some miscible crystalline/crystalline polymer blends such as the poly(1,4-butylene succinate) (PBSU)/PVDF [10,11] and
* Corresponding author. Tel.: þ86 571 28868879; fax: þ86 571 28867899. E-mail address: [email protected] (Y. Li). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.05.003
poly(3-hydroxybutyrate) (PHB)/PVDF [12] systems. Various types of orientation textures can be obtained for PBSU/PVDF blends by carefully controlling the crystalline morphology of the blend [10,11]. Orthogonal orientation textures in which the crystal axes of the two components align in mutually opposite directions, have been produced by oriented crystallization of PHB in its blend with PVDF [12]. It should be noted that these blends are miscible only in the amorphous region, but the two components crystallize separately and form independent lamellar structures. Thus oriented crystallization of these blends occurs locally and the crystallization space is confined within a much smaller size depending on the lamellae arrangements in the blend, compared with oriented crystallization of immiscible blends. The size is usually several tens of nanometers for interlamellar inclusion structures, and 100e200 nm for interlamellar exclusion structures. Miscible crystalline/amorphous polymer blends have been widely investigated [13e17] and oriented crystallization has also been applied to some miscible crystalline/amorphous polymer blends [18e23]. The crystallization of PCL in miscible blends with poly(vinyl chloride) under strain leads to crystalline orientation perpendicular to the strain direction under most conditions, whereas a parallel chain orientation is observed under conditions where the draw rate is rapid and the draw ratio is high [18,19]. The crystal orientation has also been found to be highly dependent on the molecular weight of PCL [20]. The oriented crystallization of iPS
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Fig. 1. Typical SAXS patterns and corresponding WAXD patterns of oriented PLLA/PVAc blend annealed at 75 C under strain with the indicated time (The arrow indicates the stretching direction).
in oriented iPS/PPO blends results in a texture containing highly oriented iPS crystals and nearly isotropic PPO chains [21]. Recently, Iwata et al. found that an orthogonal orientation texture was developed by oriented crystallization of PHB in PHB/cellulose propionate (CP) blends, in the CP-rich composition range [22,23]. Obviously, the inevitable chain segregation process during crystallization occurs for the oriented crystallization of the miscible crystalline/amorphous polymer blends. Although the literature reported various orientation textures formed from the miscible crystalline/amorphous blends by oriented crystallization, few investigations have focused on the details of the structure development process for the oriented crystallization of such blends. Poly(L-lactide) (PLLA)/poly(vinyl acetate) (PVAc) is a miscible blend with c parameter 0.12, indicating relatively weak interactions between PLLA and PVAc [24,25]. In the present study, the oriented crystallization behavior of PLLA/PVAc has been investigated with emphasis on the structure development during annealing under strain at different temperatures. It was found that the homogenous oriented PLLA/PVAc blends first undergo microphase separation, followed by crystallization of PLLA in the PLLA
rich phase during annealing at low temperature. By contrast, micro-phase separation was not observed during annealing at high temperature. 2. Experimental section 2.1. Materials and sample preparation The PLLA sample used was commercially available material with molecular weight 170,000 g mol1. The sample included 1.2% of Dlactide. PVAc with molecular weight approximately 260,000 g mol1 was purchased from Sowa Science Co. (Japan). The polymers were vacuum dried at 60 C for at least 12 h before processing. Blends with PLLA/PVAc in 1:1 weight ratio were prepared using a Brabender-type plastic mixer (Toyoseiki Co. KF70V) with two rotors at rotation speed 100 rpm at 190 C for 10 min. After blending, all samples were hot pressed at 200 C for 5 min into film with thickness 500 mm, followed by quenching in ice water. Oriented films were prepared by uniaxially stretching the meltquenched films using an automatic stretching machine setting in a circulating oven at a draw ratio of 4.0 at 65 C. The stretching speed was 5 mm/min. The oriented films were rapidly cooled to room temperature under strain, then heat-treated at fixed length at different temperatures for various times. 2.2. Characterization
Fig. 2. Radically corrected SAXS profiles for PLLA/PVAc blends annealed at 75 C under strain with the indicated time.
Small-angle X-ray scattering (SAXS) patterns were obtained using microfocused Cu Ka radiation (45 kV, 60 mA) generated by an X-ray diffractometer (Rigaku Ultrax 4153A 172B), and an imaging plate detector. Wide-angle X-ray diffraction (WAXD) patterns were obtained using CuKa radiation (40 kV, 120 mA) generated by an Xray diffractometer (Rigaku, Ultrax 8000) with an imaging plate detector. Miscibility was characterized by dynamic mechanical analysis (DMA), carried out with a RHEOVIBRON DDV-25FP (Orientec Corp.) instrument in tensile mode. The dynamic storage and loss moduli were determined at frequency 1 Hz and heating rate 3 C min1 as a function of temperature from 150 to 150 C. Blend morphologies were observed by transmission electron microscopy (TEM) (Hitachi
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Fig. 3. The SAXS pattern (a) and corresponding WAXD pattern (b) of the PLLA/PVAc blend annealed at 120 C for 30 min after annealed at 75 C for 120 min.
H7000) operating at acceleration voltage 75 kV. The blend samples were ultra-microtomed at 120 C to about 70 nm thickness. The sections were then stained with RuO4 for 20 min. 3. Results Fig. 1 shows typical SAXS patterns and corresponding WAXD patterns of oriented PLLA/PVAc blends annealed at 75 C under strain for the indicated time. The SAXS and WAXD patterns of neat PLLA subjected to oriented crystallization with the same draw ratio are also shown for comparison. Oriented crystallization of neat PLLA induced a highly oriented crystal structure. The two meridian maxima in the SAXS pattern can be assigned to an aligned lamellae structure perpendicular to the stretching direction (crystal c-axis along the stretching direction). The sharp WAXD reflections for the neat PLLA indicate that highly oriented PLLA crystals and the molecular chains (c-axis of crystals) were aligned along the stretching direction. No discernable scatterings and diffractions were observed from the SAXS and WAXD patterns for the oriented PLLA/PVAc blends before annealing, indicating that the stretched PLLA/PVAc samples were homogeneous (see the first pattern for SAXS and WAXD in Fig. 1). In other words, PLLA and PVAc are miscible at the molecular chain level and stretching at 65 C did not induce detectable heterogeneity. The PLLA in the oriented blend was also amorphous because no reflections from PLLA crystals were observed before annealing. Almost the same WAXD patterns with only the amorphous halo were found when the sample was annealed at 75 C for 15 min, but the SAXS pattern showed significant differences from
a
the pattern obtained before annealing. Two equatorial scattering peaks were observed in the SAXS pattern for the sample annealed at 75 C for 15 min. These peaks suggest that annealing leads to a new structure parallel to the stretching direction. From the peak positions, the long period in the equatorial direction estimated from the Bragg equation was about 20 nm. Upon increasing the annealing time to more than 30 min, discrete crystal reflections were observed in WAXD patterns, suggesting formation of PLLA crystals in the oriented blend. The major reflections of a-PLLA are 020, 110 and 210, which are expected at 2q ¼ 14.7, 19.1 and 22.3 , respectively. The appearance of the hk0 reflections of PLLA crystals in the equatorial direction indicates that the crystal c-axis (molecular chain axis) of PLLA was oriented in the stretching direction. Moreover, the crystallinity of PLLA increased gradually because the diffraction intensity increased with annealing time. However, the SAXS patterns for all annealed samples at 75 C exhibited only equatorial scattering, but the scattering peak shifted to lower angle with increasing annealing time, as shown in the radically corrected SAXS analysis profiles in Fig. 2 from the 2dimensional SAXS patterns. All reflections of PLLA crystals in the blends annealed at 75 C were relatively weak and broad, suggesting low crystallinity and irregular PLLA crystals. To make highly ordered and perfect PLLA crystals in the oriented blend, the sample was further annealed under strain at 120 C for 30 min after first heat-treatment at 75 C for 120 min. The SAXS and WAXD patterns of the two-steps annealed sample are shown in Fig. 3. As expected, the WAXD pattern shows much stronger and sharper reflections after
b
Fig. 4. DMA results of (a) PVAc, (b) PLLA, (c) oriented amorphous PLLA/PVAc blend, and (d) PLLA/PVAc blend annealed at 75 C for 120 min (dash lines indicate the Tgs of neat PLLA and neat PVAc).
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Fig. 5. TEM images of for the oriented PLLA/PVAc blends after annealed at 75 C for (a) 0 min, (b) 15 min, (c) 30 min, and (d) 120 min.
annealing, indicating increased crystallinity and improved crystal perfection. The crystal orientation was unchanged with the c-axis oriented along the stretching direction. Surprisingly, from the SAXS pattern two new meridian maxima were observed after annealing at 120 C, in addition to the original equatorial scattering. Both the SAXS and the WAXD results show that the two meridian maxima originate from PLLA lamellae that are correlated and oriented perpendicular to the stretching direction. For the samples annealed only at 75 C, the PLLA crystals formed were imperfect due to the low crystallization temperature, and the density difference between the amorphous region and the crystalline region was small in those cases. Consequently, no scattering from the crystal lamellae was observed in the meridian direction. It was very interesting to find unique equatorial SAXS scattering for all of the annealed samples. Obviously the scattering cannot be
assigned to PLLA crystals, because PLLA in the blend is amorphous in the early stages with 15 min annealing time. Moreover, the oriented PLLA crystals resulted in the meridian scattering shown in Fig. 3. We concluded that micro-phase separation between PLLA and PVAc occurred in oriented homogenous blends upon annealing under strain. The densities of PLLA and PVAc are 1.26 and 1.15 g cm3, respectively. Therefore, an SAXS peak can be expected if micro-phase separation between PLLA and PVAc occurs. The phase separation under strain leads to aligned nanodomains parallel to the stretching direction; consequently the SAXS equatorial scattering maxima were observed. To confirm the microphase separated structure of the blend, DMA was used to evaluate the glass transition of the sample annealed at 75 C. Fig. 4 shows the storage modulus and tan d of the oriented PLLA/PVAc blend as a function of temperature before annealing, and after
Fig. 6. The SAXS pattern (a) and corresponding WAXD pattern (b) of the PLLA/PVAc blend directly annealed at 120 C for 30 min.
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Fig. 7. Schematic diagram for the structure development of the oriented amorphous PVAc/PLLA blends during annealing at 75 C.
annealing at 75 C for 120 min, together with corresponding data for neat PLLA and PVAc. The glass transition temperatures of neat PLLA and PVAc were 62 and 37 C from the peak positions of tand curves, respectively. The oriented PLLA/PVAc blend before annealing exhibited only one strong tand peak at 52 C, locating at the temperature between the Tgs of neat PLLA and neat PVAc, suggesting that a miscible (single phase) state for PLLA and PVAc in the oriented blend. However, a very clear tand shoulder at 38 C was observed in addition to a peak at 61.5 C for the blend after annealing at 75 C. Those relaxation temperatures are very close to the glass transition temperatures of neat PVAc and PLLA, indicating a two-phase structure for the annealed, oriented PLLA/PVAc blend. Two corresponding step changes in storage modulus were also clearly observed for the annealed sample. The morphological development with the annealing time for the PLLA/PVAc blends annealed at 75 C was characterized using TEM, as shown in Fig. 5. PLLA appears as the white phase and PVAc as the dark phase because PVAc was more readily stained than PLLA. It is apparent that the oriented blend before annealing is homogeneous, as seen in Fig. 5 (a). The prolonged annealing time induced the phase separation between PVAc and PLLA, which may originate from the phase decomposition by concentration fluctuation during annealing. When the sample annealed for 120 min, it is seen that numerous elongated PVAc domains were precisely dispersed in the PLLA phase in Fig. 5(d). The domains were highly oriented parallel to the stretching direction, and the long period of the domains was in the range 10e30 nm, which is consistent with the corresponding distance estimated from the equatorial scattering in SAXS. Close examination of the image of the PLLA matrix showed many very small crystals that were aligned perpendicular to the stretching direction. The effects of annealing temperature on structure development of the same blend samples were also investigated. The WAXD and SAXS patterns in Fig. 6, for the sample annealed at 120 C for 30 min, show that the PLLA crystalline orientation direction had the same caxis orientation when crystallized at high temperature. However, by contrast with the SAXS patterns for the sample annealed at 75 C, only two strong meridian scattering maxima from the PLLA crystals were observed. This result suggests that micro-phase separation did not occur during annealing at the higher temperature. 4. Discussion Structure development for the oriented amorphous PVAc/PLLA blends during heat treatment at 75 C is depicted schematically in Fig. 7. In the first step, the quenched blend before drawing is a homogeneous mixture of PLLA and PVAc. Both PLLA and PVAc are amorphous and are miscible at the molecular chain level as suggested by previous miscibility investigations [24,25]. When the homogeneous mixture of PLLA and PVAc is stretched uniaxially at
65 C, there is no doubt that the molecular chains of the two components are oriented along the drawing direction. In this state the oriented blend is a homogeneous single phase, as revealed by the single Tg from DMA analysis and no detectable scattering in the SAXS pattern. Subsequent annealing under strain at low temperature results in apparent phase separation, with PVAc nanodomains aligned parallel to the stretching direction. The interaction between PVAc and PLLA is relatively weak. The reduced entropy of both PLLA and PVAc resulting from stretching may lead to the decreased miscibility between PLLA and PVAc. When the sample is heated to a temperature above the glass transition temperature of PLLA the molecular chains of PLLA and PVAc become flexible and phase separation occurs, as indicated by the SAXS, DMA, and TEM measurements. The subsequent crystallization of PLLA drives the further segregation of PLLA chain from PLLA/PVAc mixing state. The increased crystallinity during annealing induces not only the enlarged density difference between the two phases, but also the increased long periods (as shown in Fig. 2). The details of the microphase separation process and mechanism are still under investigation using in-situ SAXS measurements. It is worthy of note that the annealing temperature at which micro-phase separation occurs is only slightly higher than Tg for PLLA, and the crystallization rate at the annealing temperature is very low. Consequently PLLA is still in the amorphous state, and prolonged annealing time is required to induce its crystallization. Crystallization under strain generally leads to segmental chain orientation along the strain direction, which originates from linear nuclei produced by row nucleation of extended molecular chains. Thus PLLA crystals are oriented with the molecular chain (c-axis) along the stretching direction. However, the crystallinity of PLLA is relatively low and the boundary between the crystalline and amorphous layers is not very clear, so that no meridian SAXS peaks are observed at this stage. Further annealing at 120 C results in not only increased crystallinity but also well-defined crystalline and amorphous layers, so very sharp SAXS peaks are observed in the meridian direction in addition to the equatorial scattering peaks. However, the microphase separation process is not observed for samples annealed at 120 C. The high temperature annealing results in the high PLLA crystallization rate and the mobility of PLLA chains is restricted by the formation of the crystals. Therefore, no micro-phase separation of the blends was observed at the high annealing temperatures. It should be noted that no micro-phase separation phenomenon was observed for the samples with the draw ratio of less than 2.5. 5. Conclusion In summary, the structure development of oriented amorphous PLLA/PVAc blends during annealing was investigated. We observed micro-phase separation behavior before crystallization from
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a miscible polymer blend, which provides a new avenue to fabrication of nanostructured polymer materials by a bottom-up process. Acknowledgment This study was partially supported by the National Science Foundation (21074029) and Zhejiang Provincial Natural Science Foundation of China (R4110021). References [1] [2] [3] [4] [5]
Nishio Y, Yamane T, Takahashi T. J Macromol Sci.ePhys 1984;B23:17. Kojima M, Satake H. J Polym Sci Polym Phys 1984;22:285. Gross B, Petermanm J. J Mater Sci 1984;19:105. Takahashi T, Nishio Y, Mizuno H. J Appl Polym Sci 1987;34:2757. Seth KK, Kempster CJE. J Polym Sci. Polym Symp 1977;58:297.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
2969
Li Y, Kaito A. Macromol Rapid Commun 2003;24:255. Li Y, Kaito A. Macromol Rapid Commun 2003;24:603. Li Y, Kaito A. Polymer 2003;44:8167. Takahashi T, Inamura M, Tsujimoto I. Polym Lett 1970;8:651. Li Y, Kaito A, Horiuchi S. Macromolecules 2004;37:2119. Kaito A, Iwakura Y, Hatakeyama K, Li Y. Macromolecules 2007;40:2751. Kaito A. Polymer 2006;47:3548. Hu YS, Kamdar AR, Ansems P, Chum SP, Hitner A, Baer E. Polymer 2006;47:6387. Rostami SD. Eur Polym J 2000;36:2285. Dreezen G, Koch MH, Reynaers H, Groeninckx G. Polymer 1999;40:6451. Dreeezen G, Fang Z, Groeninckx G. Polymer 1999;40:5907. Rostami SD. Polymer 1990;31:899. Zhao Y, Keroack D, Prud’homme R. Macromolecules 1999;32:1218. Morin D, Zhao Y, Prud’homme R. J Appl Polym Sci 2001;81:1683. Zhang Y, Prud’homme R. Macromol Rapid Commun 2006;27:1565. Dikshit A, Kaito A. Polymer 2003;44:6647. Park JW, Doi Y, Iwata T. Macromolecules 2005;38:2345. Park JW, Tanaka T, Doi Y, Iwata T. Macromol Biosci 2005;5:840. Gajria AM, Davé V, Gross RA, McCarthy SC. Polymer 1996;37:437. Yoon JS, Oh SH, Kim MN, Chin IJ, Kim YH. Polymer 1999;40:2303.