Morphologies and Crystallization Behaviors in Melt-Miscible

Nov 17, 2015 - Morphologies and Crystallization Behaviors in Melt-Miscible Crystalline/Crystalline Blends with Close Melting Temperatures but Differen...
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Morphologies and Crystallization Behaviors in Melt-Miscible Crystalline/Crystalline Blends with Close Melting Temperatures but Different Crystallization Kinetics Lijun Ye, Cuicui Ye, Kangyuan Xie, Xianchun Shi, Jichun You, and Yongjin Li* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, People’s Republic of China S Supporting Information *

ABSTRACT: Poly(L-lactic acid) (PLLA) and poly(oxymethylene) (POM), with very close melting temperatures (Tm), can crystallize simultaneously or separately in their blends depending on composition and crystallization temperature (Tc), resulting in various types of morphology. It is mainly attributable to the greatly different crystallization kinetics of PLLA and POM. At a content of POM (φPOM), 3 wt % < φPOM < 20 wt %, PLLA crystallization kinetics are comparable to POM, and therefore two type spherulites exhibit “side-byside” simultaneous growth with the penetration of PLLA spherulites into POM crystals. Although crystal growth rate (vc) of POM is still a bit faster than that of PLLA, for φPOM = 3 wt %, the nucleation of POM is restrained and POM spherulites can only develop on the propagating PLLA growth fronts with the generation of novel “core−shell” blended spherulites. For 20 wt % ≤ φPOM < 80 wt %, interspherulitic growth of PLLA inside the pre-existing matrix of POM spherulites causes the formation of interpenetrated blended spherulites, owing to the large discrepancy in kinetics. At φPOM ≥ 80 wt %, PLLA molecular chains are redistributed into the interlamellar level regimes within the POM spherulites and can only crystallize into tiny crystals (owing to strong confinement). PLLA/POM blends provide a perfect example and new insights for understanding the crystallization of miscible crystalline/crystalline polymer blends (with very similar Tm’s), in which kinetic factors could play a significant role in crystallization behaviors and morphology. (ethylene oxide) (PEO),33 poly(ethylene succinate) (PES)/ PEO,34,35 PES/poly(p-dioxanone) (PPDO),36 and poly(ethylene succinate-co-ethylene adipate) (PESA)/PEO,38 have been reported to crystallize simultaneously from their homogeneous melts. When ΔTm is intermediate (∼50 °C), two crystalline components in a binary miscible polymer blend, i.e., PVDF/ PBSU14,16,17 and PBSU/PEO,18−20,23−26 generally crystallize separately, not simultaneously. The high-Tm component crystallizes first and fills the whole melt volume, and then, with prolonged crystallization time, the low-Tm component nucleates and grows inside the pre-existing matrix of high-Tm components spherulites. The original spherulitic shape of highTm components is maintained.18 If ΔTm is quite large (>50 °C), the temperature at which the high-Tm component crystallizes is generally high enough that the low-Tm component cannot nucleate and stays in an amorphous state, causing it to act as a diluent, similar to binary crystalline/amorphous systems.5,6,8,10−13 In the amorphous state, the low-Tm component is distributed into the interlamellar, interfibrillar, or interspher-

1. INTRODUCTION Over the past two decades, the morphology and crystallization behavior of binary miscible crystalline/crystalline polymer blends have received increasing attention.1−46 From both academic and industrial points of view, the study of polymer blends composed of two miscible crystalline components is of great interest and importance. However, the presence of two crystallizable components and the interplay between the two components make it more difficult to understand the morphology and crystallization behavior of such special binary polymer blends. Generally, the difference between the melting temperatures (ΔTm) of the two components is assumed to play a significant role in determining the morphology and crystallization behavior of binary miscible crystalline/crystalline polymer blends. When ΔTm is small, the two components can crystallize simultaneously to form interpenetrated spherulites. In fact, miscible crystalline/crystalline polymer blends in which the two components have distinct chemical structures and exhibit simultaneous crystallization are quite rare. So far, only a few pairs of crystalline polymers, including poly(3-hydroxybutyrate) (PHB)/poly(L-lactide) (PLLA),28 poly(butylene succinate) (PBSU)/poly(vinylidene chloride-co-vinyl chloride) (PVDC-VC),29,30 poly(ester carbonate) (PEC)/PLLA,31,32 poly(butylene succinate-co-butylene adipate) (PBSA)/poly© XXXX American Chemical Society

Received: August 30, 2015 Revised: October 31, 2015

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crystallization of PLLA. Tm’s of PLLA (166.3 °C) and POM (165.4 °C) are actually very close. ΔTm was generally considered to play a dominant role in morphology and crystallization behavior of binary miscible crystalline/crystalline polymer blends. However, Nishi et al. reported that the morphology and crystallization behaviors of PBSU/P(VDCVC) blends were strongly dependent upon the crystallization kinetics.29,30 Most recently, fractional crystallization behaviors has been observed in poly(ethylene suberate) (PESub)/PEO blends with similar Tm components by Qiu et al.42 Therefore, it is of great interest to investigate and understand the morphology and crystallization behaviors of PLLA/POM blends, which undoubtedly represent the most appropriate example of a pair of components with very similar Tm but large discrepancy in crystallization kinetics. Interestingly, in this work, PLLA/POM blends are found to exhibit very rich morphologies that are generally observed in several polymer systems with different ΔTm, depending on composition and Tc. It is moreover, to find that at a given Tc, the discrepancy in crystallization kinetics (nucleation and crystal (spherulitic) growth) between the two components strongly determine the crystallization behaviors of PLLA and POM (side-by-side simultaneous/stepwise crystallization). Furthermore, the crystallization kinetics of PLLA is dependently influenced by confinement (narrow/wide space) within the POM scaffolds.

ulitic space of the high-Tm component, depending on the crystallization temperature (Tc) and composition. Therefore, the low-Tm component might nucleate and grow in the remaining melt or inside or on the interspherulitic borders of the spherulites of the high-Tm component when Tc is lowered. Several morphologies can occur in binary blends containing two crystalline components, i.e., side-by-side, interpenetrating, interspherulitic, interlocking, and interfilling spherulitic growth, as has been well summarized by Liu and Jungnickel.1 Although much attention has already been paid to the crystallization of high-Tm components, the crystallization of low-Tm components is much more complicated and somewhat still a mystery. Interestingly, Nishi et al. have reported that the growth of PBSU spherulites changed slightly with increasing PVDF content in PVDF/PBSU blends and that the crystal structure of the PBSU was mainly controlled by the crystallization conditions. The actual PBSU concentration was higher than the initial volume fraction because PVDF was already crystallized to compensate for the reduction in the growth rate of PBSU spherulites.14 Yan et al. also investigated the morphology and crystallization behavior of PVDF/PBSU blends. They found that the nucleation and growth of PBSU components were confined in the spherulitic PVDF matrix and that the interconnectedness of the molten pockets of amorphous material within the PVDF spherulites strongly determined the growth kinetics of PBSU.16,17 The proposal that the growth kinetics of low-Tm components was determined by interconnectedness has also been shared by Han et al. for PBSU/PEO blends.26 Qiu et al. showed that the nucleating ability of PBSA in PVDF/PBSA blends was enhanced, while the crystal growth was reduced within the pre-existing PVDF spherulites.10 Ikehara et al. found that PEO crystallized primarily within the interfibrillar regimes of PBSU spherulites in PBSU/PEO blends and that the molecular mobility differed little from that in a free melt.23,25 In fact, PEO could be excluded into the interlamellar, interfibrillar, or interspherulitic space of PBSU spherulites, and crystallization of PEO in the nanoscaled interlamellar regimes is far more difficult than in the wider interfibrillar regions. Pan and He et al. demonstrated that the distribution of PEO in PBSU/PEO blends was controlled by Tc of PBSU. Confined and fractional crystallization of PEO were observed, as a result of the various locations (interlamellar and/or interfibrillar regions) of PEO component within the PBSU spherulites.19−22 Similar fractional crystallization behavior of PEO in PBSU/PEO blends has been reported by Schultz24 and Han.26 Various morphologies and crystallization behaviors in miscible polymer blends containing crystallizable components have been reviewed by Schultz.2 We have previously systematically investigated the miscibility, crystallization behavior, and morphology of PLLA/POM blends, which are binary crystalline/crystalline polymer blends. In our previous work, we observed that PLLA/POM blends exhibit typical lower critical solution temperature (LCST) phase behaviors.47,48 Although PLLA and POM are both crystallizable polymers, the crystallization kinetics/overall crystallization rate of PLLA is much lower than that of POM. Over a wide range of compositions (from 20/80 to 80/20), POM crystallizes first to occupy the whole volume of the initial homogeneous melt, accompanied by the exclusion of PLLA chains still in an amorphous state into interlamellar regimes within the POM spherulites.49,50 In these previous works, we focused on the morphology and crystallization behavior of POM, and therefore less attention was paid to the

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The PLLA (3001 D, Mn = 89 300 g/mol, Mw/Mn = 1.77, Tm = 166.3 °C) used in this work was purchased from Nature Works Co., LLC (USA). The sample included 1.6% D-lactide content. The POM (MC 90, Mw = 174 300 g/ mol, Mw/Mn = 2.19, Tm = 165.4 °C) used in this work was kindly provided by Shenhua Co., Ltd. (China). The melt flow index of POM was 9.23 g/10 min. Prior to use, PLLA and POM were dried in a vacuum oven at 80 °C for 12 h. PLLA/POM blends were prepared at 190 °C in a batch mixer (Haake Polylab QC) with a twin screw at an initial rotation speed of 20 rpm for 2 min, which was subsequently raised to 50 rpm for another 5 min. All the obtained samples were then hot-pressed at 14 MPa and 190 °C for 3 min into thin films with a thickness of 300 μm, followed by rapid cooling to room temperature. 2.2. Sample Characterization. The morphologies of PLLA/ POM blends were imaged with a polarizing light microscope (PLM, Olympus BX51) equipped with a digital camera. All PLLA/POM samples were sandwiched between two plates of glass, and the temperature was controlled with a Linkam LTS 350 hot stage. The PLLA/POM samples were prepared by heating to 190 °C for 10 min and then quenching to the desired Tc for crystallization (and reheated to 163 °C). The crystallization behaviors of the PLLA/POM samples were investigated using a differential scanning calorimeter (DSC Q2000) under a nitrogen flow at a heating rate of 40 °C/min from 20 to 200 °C, at which they were held for 5 min to erase the thermal history before being cooled at a rate of 3 °C/min to 20 °C again (or rapidly cooled to the desired Tc for isothermal crystallization or cooled at a rate of 10 °C/min to 120 °C and then reheated to 200 °C). All the DSC traces were recorded. Before sample testing, the heat flow and temperature of the instrument were calibrated with sapphire and pure indium references, respectively.

3. RESULTS 3.1. Effect of Composition on Morphologies in PLLA/ POM Blends. 3.1.1. “Side-by-Side” Simultaneous Growth and Interpenetrated Blended Spherulites (3 wt % < φPOM < 20 wt %). PLLA/POM blends are binary melt miscible crystalline/crystalline polymer blends. Although Tm’s of PLLA and POM are almost the same, we have observed a large B

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Moreover, the growth fronts of POM spherulites became distorted (blue arrow), as shown in Figure 1b, after they engulfed the whole PLLA spherulites. Because PLLA spherulites are quite compact and regular, we consider that it was difficult for POM spherulites to penetrate into the PLLA ones in this case. This can also be confirmed by the unchanged sign and brightness of the birefringence within the PLLA spherulites. Therefore, new growth fronts of POM spherulites were generated on the boundaries of the growing PLLA spherulites, as we considered, and were always ahead of the initial ones, as shown in Figure 1b. This might be ascribed to the nucleating effect of PLLA spherulitic boundaries, and it could also be observed in elsewhere, as marked by B1 and B2 in Figure 1. It is clear from Figure 1b,c that a spherulite of POM nucleated and grew on the border of a developing PLLA one. This is a very interesting phenomenon that has never been observed before, and the detailed nucleating mechanism is still unclear. Furthermore, PLLA spherulites continued to grow inside POM ones and formed interpenetrated blended spherulites of the two components after being engulfed by the latter, as marked by C1 and C2 in Figure 1c,d. This is distinctly evidenced by the increased brightness part of POM spherulites penetrated by the PLLA ones and the areas of unchanged brightness where PLLA spherulites had not yet reached.30,33,35,36,38 The formation of interpenetrated blended spherulites is generally attributable to the discrepancy in density of lamellae between two types spherulites, as proposed by Qiu et al.35 In this case, the density of lamellae within the POM spherulites is considered to have been much lower than that of the PLLA ones, so PLLA spherulites could continue to penetrate into the POM ones on contact. Further, it was found that the spherulitic growth rate of PLLA, after penetrating, differed little from that developed in the free melt (Figure S1), implying that the actual PLLA concentration changed only slightly due to the inclusion of large quantities of PLLA chains within the POM spherulites. The lamellae of PLLA spherulites, most interestingly, which were not observed to form banded spherulites in the free melt, may have twisted along the POM lamellae, as revealed by the enhanced contrast between the bright and dark rings within the POM spherulites.23,25 This could also be further evidence of the formation of interpenetrated blended spherulites and indicates the “templating” effect of POM spherulites on the PLLA ones. Furthermore, PLLA spherulites were observed to nucleate and grow from the inside of the spherulitic POM crystals, as marked by D1 and D2 in Figure 1d. Figure 2 shows the morphology of a PLLA/POM (95/5) sample isothermally crystallized at 145 °C. Similar to the morphology of the 90/10 sample, both types of PLLA and POM spherulites crystallized simultaneously, as shown in Figure 2. Although the crystallization kinetics of POM still appeared to be faster than those of PLLA, it is clear that POM crystals were unable to maintain the original banded spherulitic shape as PLLA content (φPLLA) was further increased. Moreover, POM spherulites exhibited hexagonal dendritic growth and acted as single crystal packing, as seen in Figure 2a. This behavior may have originated from the “diluting” effect of large amount PLLA molecular chains in the homogeneous melt. Actually, the single crystal form growth of polymers in their miscible blends has been reported in PLLA/PBA and PES/PVP blends before.41,54 Furthermore, the penetrating growth of PLLA spherulites into the POM ones on contact, the nucleation of POM spherulites on the developing boundaries of

discrepancy in crystallization kinetics between the two polymers in previous works.47−50 The nucleation and crystal (spherulitic) growth of POM are very rapid, whereas those of PLLA are relatively slow under the same conditions. At a φPOM of 20 wt % and higher in binary blends, POM component crystallized first into banded spherulites and filled the whole melt volume, expelling the poorly crystallizable PLLA chains that were still in amorphous state into the interlamellar regions of POM crystals.49,50 It is expected that the difference in crystallization kinetics between PLLA and POM might be reduced and simultaneous crystallization of the two components would occur if φPOM were decreased. Figure 1 shows the

Figure 1. Polarized light images of the PLLA/POM (90/10) sample isothermally crystallized at 145 °C for (a) 45, (b) 50, (c) 85, and (d) 165 min.

morphology of a PLLA/POM (90/10) blend sample crystallized at 145 °C. Interestingly, two distinct types of spherulites are observable in this blend. The large and banded spherulites correspond to the spherulites of POM, while the small and compact ones are the spherulites of PLLA. As shown in Figure 1a, PLLA and POM crystallized simultaneously, leading to “side-by-side” spherulitic growth of the two components, though crystallization kinetics of the latter is still much faster than that of the former. POM crystallizes into regular banded spherulites in the presence of PLLA. The formation of banded spherulites is commonly ascribed to the cooperative twisting of POM lamellae. This twisting of POM lamellae can be regarded as a response to a “self-generated compositional field”, resulting from the exclusion of poorly crystallizable moieties (PLLA-rich melt) from POM lamellae, as reported by Schultz.51,52 It also implies that large quantities of the amorphous PLLA molecular chains were included within the POM spherulites. The inclusion of PLLA chains within the POM spherulites may have originated from the rapid spherulitic growth rate of POM crystals and poor diffusibility of PLLA amorphous chains near the growth fronts of POM spherulites.1,2,53 For PLLA spherulites, it is just the opposite: the relatively slow spread of PLLA growth fronts and high rate of motion of POM molecular chains caused PLLA spherulites to become compact and regular. On further extending the crystallization time, as shown in Figure 1a,b, PLLA and POM spherulites came into contact with each other. It seems that POM spherulites continued to grow around the PLLA ones, as marked by A1 and A2 in Figure 1. C

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Figure 3. Polarized light images of the PLLA/POM (97/3) sample isothermally crystallized at 145 °C for (a) 30, (b) 130, and (c, d) 180 min.

increasing of the actual POM concentration in the remainder of the homogeneous melt during the crystallization of PLLA. At this stage, hierarchical spherulites of PLLA and POM with a “core−shell” structure formed. As the crystallization time was further prolonged, the “core” PLLA spherulites continued to grow along the lamellae of loose “shell” POM spherulites, as revealed by the increasing brightness of POM spherulites. Thus, triple-layered hierarchical blended spherulites of PLLA and POM were observed, as marked by arrows 1, 2, and 3 in Figure 3c. The marked hierarchical layers in the blended spherulites, from the inner to the outer, correspond to a compact and regular PLLA spherulite, interpenetrating spherulites of PLLA and POM, and loosely propagating POM spherulite, respectively. Obviously, this novel triple-layered structure can only occur when the PLLA first crystallizes into compact spherulites followed by the crystallization of POM starting from the boundary of the PLLA spherulites. In this case, the crystallization kinetics of POM is mainly controlled by the nucleation stage. In fact, the behavior that POM spherulites develop on the propagating growth front of the PLLA ones and then template the subsequent growth of PLLA crystals within the POM scaffolds can be termed as “cooperative crystallization”,2 as also confirmed by the kinetics in Figure 8c. 3.1.3. Interspherulitic Growth and Interpenetrated Blended Spherulites (20 wt % ≤ φPOM < 80 wt %). In our previous works, the POM component in PLLA/POM crystallized first into banded spherulites and volume-filled with PLLA molecular chains when φPOM ≥ 20 wt %.49,50 In this case, the crystallization kinetics of PLLA in binary blends was much lower than that of POM, even ΔTm < 1 °C. PLLA/POM blends with φPOM ≥ 20 wt % exhibit stepwise crystallization behaviors similar to those of blends with a large ΔTm between the two components, i.e., PVDF/PBSU and PBSU/PEO blends.16,17,19,23−26 Much attention has been paid to the crystallization of POM in PLLA/POM blends in previous works, and therefore we focus on the crystallization behaviors and morphology of PLLA components within the pre-existing POM spherulites in this section. Figure 4 shows the morphology of PLLA/POM blend samples crystallized first at 145 °C with the compositions of 80/20, 60/40, 40/60, and 20/ 80 and then reheated to 163 °C. POM spherulites crystallized and occupied the whole volume of the initial homogeneous

Figure 2. Polarized light images of the PLLA/POM (95/5) sample isothermally crystallized at 145 °C for (a) 40 and (b) 100 min.

the PLLA ones (while arrow), and the nucleating of PLLA spherulites inside the POM ones can also be seen in 95/5 sample, as shown in Figure 2. On prolonging the crystallization time, however, no banded PLLA spherulites were observed in this case, again indicating a “templating” effect of pre-existing POM matrix on PLLA crystals. 3.1.2. Cooperative Growth and “Core−Shell” Structural Blended Spherulites (for φPOM = 3 wt %). As shown above, simultaneous crystallization and various morphologies were observed in PLLA/POM blends when φPOM was lowered. This may be mainly ascribed to the competition between the crystallization of the two components in PLLA/POM blends. It is of great interest to investigate the crystallization behavior and morphology of PLLA/POM blends with even lower φPOM, in which PLLA spherulites might appear first. Figure 3 shows the morphology of the PLLA/POM (97/3) sample crystallized at 145 °C. As expected, only PLLA spherulites were observed in the homogeneous melt within the first 30 min of crystallization, as shown in Figure 3a. Upon prolonging the crystallization time, interestingly, POM spherulites (white arrow) can only loosely develop on the boundaries of the PLLA ones, as seen in Figure 3b. In this case, the POM concentration was very low and POM molecular chains were strongly diluted by the PLLA ones. The PLLA spherulites appeared compact and regular as mentioned above, and POM molecular chains were mainly rejected from PLLA spherulites. As we considered, it was very difficult for POM to nucleate in the homogeneous melt, and therefore the local accumulation of POM on the growing boundaries of PLLA spherulites (Figure S1) would have effectively promoted POM chains to nucleate on the compact and regular “liquid−solid” interfaces between PLLA crystals and the remaining melt. Once POM crystals nucleated on the borders of PLLA spherulites, as shown in Figure 3c, the radial growth of POM spherulites appeared to be faster than that of the PLLA ones. This can also be observed in binary blends with 3 wt % < φPOM < 20 wt % and might be mainly attributed to the D

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Figure 4. Polarized light images of PLLA/POM samples isothermally crystallized at 145 °C and then reheated to 163 °C. (a) PLLA/POM = 80/20; (b) PLLA/POM = 60/40; (c) PLLA/POM = 40/60; (d) PLLA/POM = 20/80.

melt, and PLLA spherulites develop with the matrix of POM crystals. As the isothermal time was increased, for 80/20 and 60/40 cases, several PLLA spherulites with strong birefringence nucleated and developed continuously within the matrix of POM crystals, as seen in Figure 4a,b. Similar to the morphologies exhibited in section 3.1.1, the brightness of POM spherulites increased in the regimes where PLLA crystals appeared and remained unchanged in the areas that had not yet been occupied by the PLLA growth fronts. Moreover, the extinction feature of the pre-existing POM spherulites was not altered by the subsequent PLLA crystallization, indicating that PLLA lamellae had the same orientation as the POM ones. In other words, the growth of PLLA spherulites was templated by

the pre-existing matrix of POM crystals. Most interestingly, POM spherulites gradually disappeared and PLLA crystals with a reverse replication of the POM scaffolds were distinctly revealed, upon the process of reheating to 163 °C. It is for the first time that we can have a clear seeing of the crystals confined within the spherulitic frameworks in binary blends. And this can also be a straightforward evidence for the formation of interpenetrated blended spherulites of PLLA and POM. Figure 5 shows the morphology of PLLA/POM (70/30) crystallized at 145 °C and then reheated to 163 °C. For 70/30 blend, interestingly, most of PLLA component nucleated preferentially at the centers of POM spherulites to form homocentric blended spherulites of the two components, as E

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increase long space (L) of POM crystals has been observed, upon the addition of PLLA in binary blends (Figure S2), implying that PLLA chains were mainly included in the interlamellar regions of POM crystals.50 However, the interlamellar regions can be wide (hundreds of nanometers) or narrow (few tens of nanometers). L of POM in binary blends with φPLLA > 20 wt % appears quite wide (more than 120 nm). However, L of POM in binary blends with φPLLA ≤ 20 wt % is no more than 50 nm, indicating that PLLA molecular chains are redistributed into narrow interlamellar level regions of POM crystals. Moreover, the interconnectedness of PLLA-rich melt within the POM spherulites is distinctly depressed (Figure S3). In this case, PLLA molecular chains were strongly confined in narrow interlamellar level regimes of POM spherulites, and therefore only tiny PLLA crystals were sporadically generated and interfilled within the POM scaffolds (interfilling crystallization). The extensive space confinement on PLLA crystallization in the 20/80 blend was also convinced by nonisothermal crystallization behaviors (Figure 6). The tiny PLLA crystals would remelt first before the POM frameworks “collapse”, as we considered, upon the reheating process (Figure 4d). Figure 6a shows melting crystallization behaviors of PLLA/ POM blends cooled from the melt state in a 3 °C/min rate. Only a weak exothermic peak assigned to PLLA crystallization was observable for the blend with a 50 wt % of φPLLA, and no exothermic peaks were observed upon lowering φPLLA, as shown in Figure 6a. The crystallization kinetics/overall crystallization rate of PLLA confined within the POM scaffolds was gradually depressed, with the decreasing of φPLLA. Figure 6b shows cold-crystallization behaviors of PLLA/POM blends during the heating process in a 10 °C/min rate. It was clear from Figure 6b that a distinct exothermic peak assigning to cold-crystallization of PLLA was observed, when φPLLA decreased to 50 wt %. In this case, the crystallization kinetics/overall crystallization rate of PLLA within the POM spherulites was strongly suppressed. Therefore, the total crystallization of PLLA was not completely finished during the cooling process from the melt state, as convinced by a “sudden” drop of crystallinity for melting crystallization (Xmc) in Figure 6c and an evident exothermic peak for cold crystallization in Figure 6b. Upon further lowering φPLLA to 20 wt %, only a very weak exothermic peak which could be ignorable and neglectable for PLLA cold crystallization was found. In this case, the crystallization of PLLA was extensively restrained, owing to the interlamellar level inclusion of PLLA (with low interconnectedness) within the POM spherulites. The dependence of Tc, Xmc, and total crystallinity (Xc) on composition was summarized in Figure 6c. With increasing φPOM, Tc (or Tcc) of PLLA in binary blends appeared less sensitive to composition, as seen in Figure 6c, though a decreasing tendency was found. As for Xc (the sum of Xmc and Xcc) of PLLA, it dropped gradually at first and then quickly with the increase of φPOM. This might be ascribed to the decrease of crystallization kinetics/overall crystallization rate of PLLA, owing to the increasing space confinement within the POM scaffolds. The crossover was seen at a φPLLA of about 50 wt % where Xmc was almost equal to Xcc. The Xc of PLLA was mainly attributable to melting-crystallization (Xmc) for the blends with φPLLA > 50 wt %, whereas that was mainly assignable to coldcrystallization (Xcc) for the blends with φPLLA < 50 wt %. However, no distinct crystallization of PLLA was observable in binary blends with φPLLA ≤ 20 wt % during nonisothermal

Figure 5. Polarized light images of the PLLA/POM (30/70) sample isothermally crystallized at 145 °C for (a) 120 min and (b) reheated to 163 °C.

marked by H1 and H2 in Figure 5a, while a few spherulites of PLLA were found to randomly develop inside the POM matrix (W1 and W2). Although the nucleation of low-Tm components at the center of high-Tm component spherulites has been previously reported, the detailed mechanism has not yet been made clear.17,35 The nucleation sites at the centers of POM spherulites might not have been expended and therefore may have been still active for PLLA components.17 Such novel homocentric blended spherulites can also be confirmed by the reheating process, as shown in Figure 5b. The spherulitic growth of PLLA within the POM spherulites can be distinctly observed by the subsequent reheating process, for the 40/60 case, though POM spherulites were originally colorful, and the growth of PLLA crystals was uneasiness to trace by the changes of birefringence, as shown in Figure 4c. In other words, the interspherulitic growth of PLLA and interpenetrated blended spherulites of PLLA and POM can still be found, when φPLLA decreased to 40 wt %. However, no PLLA spherulites can be seen in the 20/80 case, as shown in Figure 4d, after the remelting of POM scaffolds. In this case, the crystallization of PLLA within the POM spherulites were strongly confined due to the interlamellar level (several tens of nanometers) inclusion of PLLA chains, and the details will be discussed in section 3.1.4. 3.1.4. Interlamellar Level Incorporation and Interfilling Growth (φPOM ≥ 80 wt %). Section 3.1.3 shows that interspherulitic growth, the spherulitic growth of PLLA crystals within POM spherulites, is observed in PLLA/POM blends, similar to PVDF/PBSU and PBSU/PEO blends,.16,17,19,23−26 In contrast, no spherulitic growth of PLLA crystals was found in the matrix of POM spherulites, when φPOM ≥ 80 wt %. It is clear that the large and banded spherulites of POM in PLLA/ POM (20/80) blend served as a framework, as shown in Figure 4d, and no distinct spherulitic growth of PLLA was found after remelting of POM scaffolds. In our previous work, a continuing F

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Figure 7. Polarized light images of the PLLA/POM (90/10) sample isothermally crystallized at (a) 140 °C (10 min), (b) 135 °C (5 min), and (c) 130 °C (2 min).

Figure 6. (a) DSC cooling curves of PLLA/POM blends at 3 °C/min rate; (b) second heating curves of PLLA/POM blends at 10 °C/min rate; (c) crystallization temperature and crystallinity of PLLA (from the DSC cooling curves) as a function of compositions.

of both PLLA and POM could be observed in the homogeneous melt, as shown in Figure 7, POM components exhibited a much faster spherulitic growth rate than PLLA. In this case, POM components always nucleated and grew ahead of PLLA. PLLA spherulites with strong birefringence developed either in the rest of the homogeneous melt or inside of the developing POM spherulites. This behavior originated from the discrepancy between the crystallization kinetics of PLLA and POM. The crystallization kinetics of PLLA is much lower than that of POM. The time difference in nucleation of the two components decreased with Tc, as shown in Figure 7. It was also evidenced by the two mutually approaching but never merging exothermic peaks in the isothermal crystallization curves shown in Figure 8a. The above results distinctly indicate that crystallization kinetics played a significant role in controlling the crystallization behaviors and morphology of PLLA/POM blends, even with the very close Tm of the two components. 3.2.2. Kinetics. As described above, the morphology and crystallization behaviors of PLLA/POM blends were found to be controlled by the crystallization kinetics of the two components, even though ΔTm < 1 °C. Figure 8 shows the isothermal crystallization behaviors of PLLA/POM blends with

crystallization, again indicating the strong confinement of the interlamellar level regions on PLLA. 3.2. Effects of Crystallization Temperature on Crystallization in PLLA/POM Blends. 3.2.1. Morphology. In section 3.1, various morphologies in PLLA/POM blends were demonstrated depending on composition. PLLA/POM blends with φPOM ≥ 20 wt % exhibited typical stepwise crystallization behaviors. POM components crystallized first and filled the whole melt volume. Then, PLLA components developed inside or on the boundaries of POM spherulites as the crystallization time was increased. When φPOM < 20 wt %, the two components of PLLA/POM blends could crystallize simultaneously. In this dection, we mainly focus on the effects of Tc on the morphology and crystallization behavior in PLLA/POM blends with φPOM < 20 wt %. Kinetic factors are expected to play a dominant role in crystallization behaviors and morphology of miscible crystalline/crystalline polymer blends, and thus Tc should have a critical effect on the crystallization kinetics of both PLLA and POM, which in turn will affect their morphology in binary blends. Figure 7 shows the morphology of a PLLA/POM (90/10) blend crystallized at 140, 135, and 130 °C. Although spherulites G

DOI: 10.1021/acs.macromol.5b01904 Macromolecules XXXX, XXX, XXX−XXX

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less distinct kinetic differences, and has been termed “cooperative crystallization” by Schultz.2 For the PLLA/POM (97/3) case, quite interestingly, only a wide exothermic peak for crystallization could be observed in the isothermal crystallization curves above 135 °C. With decreasing Tc, the wide exothermic peak was found to divide into two peaks at 135 °C and then merge again into a single one at 125 °C, as shown in Figure 8c. Clearly there is some form of cooperation between the crystallization processes of the components, as convinced by Figure 3, owing to their comparable and equivalent crystallization kinetics in PLLA/ POM (97/3) blend. As for the blends with φPOM ≥ 20 wt %, PLLA crystals developed within the matrix of POM spherulites, similarly to that in PVDF/PBSU and PBSU/PEO blends. Confined crystallization of low-Tm components within the matrix of high-Tm component crystals has been reported by Yan,17 Ikehara,25 and Han26 et al. The T c of high-T m components strongly affects the morphology and crystallization behaviors of low-Tm components. The confined crystallization of PLLA within the POM spherulites and the effects of Tc of POM on PLLA crystallization can also be observed in PLLA/ POM blends and will be discussed in detail in a subsequent paper. 3.3. Miscibility of PLLA and POM in Binary Blends. In our previous work, PLLA/POM blends were found to exhibit typical LCST phase behaviors, as confirmed by small-angle light scattering and optical microscopy.47 The simultaneous crystallization and the formation of interpenetrated blended spherulites described above again indicated the miscibility of PLLA and POM in the melt state. Moreover, this can also be confirmed by the depression of Tc and Tm of POM. Figure 9

Figure 8. Heat flow as a function of isothermal crystallization time of PLLA/POM blends. (a) PLLA/POM = 90/10; (b) PLLA/POM = 95/5; (c) PLLA/POM = 97/3.

compositions of 90/10, 95/5, and 97/3. Two exothermic peaks were distinctly observed in the isothermal crystallization curves of the PLLA/POM (90/10) blend. These two peaks corresponded to the crystallization of POM and PLLA, with the first one assignable to POM crystallization, as confirmed by Figure 7. It is clearly seen from Figure 8a that the two peaks approached each other but never merged, implying that in this case the crystallization of POM was always ahead of that of PLLA. For PLLA/POM (95/5) blend, as shown in Figure 8b, the two exothermic peaks merged into a single one at a temperature of about 110 °C. The POM component exhibited faster crystallization kinetics (overall crystallization rate) at temperatures of about 130−150 °C, whereas the fastest crystallization kinetics of PLLA occurred at about 100−120 °C. Thus, the crystallization kinetics of PLLA was more sensitive to Tc than that of POM for crystallization at low temperatures. In the PLLA/POM (95/5) case, the difference in crystallization kinetics between PLLA and POM was negligible at low temperatures, and therefore a “real” simultaneous crystallization from the kinetic point of view was observed in Figure 8b. In fact, this phenomenon has been previously observed in several other polymer systems,43−46,55 mostly with

Figure 9. (a) Crystallization temperatures of POM (from DSC cooling scans at 10 °C/min rate) versus the POM content in PLLA/POM blends. (b) DSC curves of second heating scans at 10 °C/min rate for PLLA/POM blends (in the range from 140 to 180 °C), following the cooling process from the melt state to 120 °C at 10 °C/min rate. H

DOI: 10.1021/acs.macromol.5b01904 Macromolecules XXXX, XXX, XXX−XXX

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Figure 10. Schematic illustration of crystallization behaviors of PLLA/POM blends at a given Tc, depending on the discrepancy in crystallization kinetics between the two components.

over a very wide range of compositions (φPOM ≥ 20 wt %), though ΔTm is relatively small (