Miscibility and Double Glass Transition Temperature Depression of

Jul 11, 2013 - The poly(l-lactic acid)/poly(oxymethylene) (PLLA/POM) blends have been prepared by simply melt blending. The phase diagram, miscibility...
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Miscibility and Double Glass Transition Temperature Depression of Poly(L‑lactic acid) (PLLA)/Poly(oxymethylene) (POM) Blends Jishan Qiu, Chenyang Xing, Xiaojun Cao, Hengti Wang, Lian Wang,* Liping Zhao, and Yongjin Li* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China S Supporting Information *

ABSTRACT: The poly(L-lactic acid)/poly(oxymethylene) (PLLA/POM) blends have been prepared by simply melt blending. The phase diagram, miscibility, glass transition temperatures, and physical properties have been investigated systematically. The PLLA/POM blends exhibit typical lower critical solution temperature (LCST) behaviors. PLLA and POM are miscible in the melt state at low temperature and become phase-separated at elevated temperatures. It was found that the weak interactions between the carboxyl groups of PLLA and methylene groups of POM (weak C−H...O hydrogen bonding) account for the miscibility of the two components. Although the PLLA/POM blends are homogeneous at the melt state in the miscible temperature region, two distinct glass transition temperatures are observed for the all blends when quenched from the homogeneous state. More surprisingly, both POM and PLLA exhibit the apparent glass transition temperature (Tg) depression in the blends, compared with Tgs of the neat polymers. The behaviors are totally different from other reported miscible or partially miscible polymer blends, in which Tgs shift to each other or merge into one glass transition temperature. The investigation indicates that the crystallization of POM in the blend induces the phase separation of PLLA/POM blends and forms novel morphologies with the interpenetrated (cocontinuous) PLLA and POM phases. The double glass transition temperature depression of both PLLA and POM in the blends originates from the mismatch thermal shrinkage during cooling down from the high temperature. Moreover, we observed the improved ductility of the PLLA/POM blends as compared with the neat PLLA and POM, which has been attributed to higher molecular mobility due to the glass transition temperature depression for both PLLA and POM in the blends.

1. INTRODUCTION Polymer blending has been investigated as one of the most important ways to develop new high polymeric materials.1 Polymer blends with different physical properties often exhibit the possibility of enhancing the overall properties of a material through a synergistic combination of the desirable properties of the original polymers. While some polymer pairs are completely miscible, most blends are immiscible because of the high molecular weight, resulting in small mixing entropy contribution.2 Immiscible polymer blends present separated phases and clear interfaces. The morphology and the interfacial properties of immiscible polymer blends affect the physical properties significantly.3,4 Miscible polymer blends are homogeneous (a single-phase structure) and show the average properties depending on the compositions of the blends. Some of miscible polymers show lower critical solution temperature (LCST) behavior. The components of the mixture are homogeneous (miscible) at low temperatures and tend to phase separate at elevated temperatures.2 The glass transition temperature (Tg) values of polymer blends as a function of composition are of particular interest. The existence of a single or two Tgs has been usually used as a criterion to judge whether a binary polymer is miscible or not.5,6 Full miscibility is characterized by a single glass © 2013 American Chemical Society

transition temperature for all the blends. The binary immiscible polymer blends show their two Tgs at the same temperature as the neat material. Compatible polymer blends is generally reflected by the presence of two Tgs in the blend, which are shifted toward each other; i.e., elevation of the temperature of the low Tg and depression of the high Tg. Such blends usually have very fine phase structure. It is argued that molecular chain interpenetrating at or near the interface changes the average free volumes, which leads to the inward shifted Tgs.1,2 Poly(L-lactide) (PLLA) has attracted great interest in recent years because it is produced from renewable recourses and is biodegradable. It has been widely used for biomedical applications because of its reasonable mechanical properties and good biocompatibility.7−9 Moreover, PLLA has also become an alternative to traditional commodity plastics for everyday applications as an environmental friendly polymer due to its reasonable price and unique properties such as high strength, high stiffness, resistance to fats and oil.10−12 However, the inherent brittleness and low heat distortion temperature have restricted its practical application.13,14 To obtain Received: May 24, 2013 Revised: July 1, 2013 Published: July 11, 2013 5806

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followed by quenched (cool-pressing) to room temperature. The obtained films were used for the following characterization. The sample films with thickness about 10 μm for SALS and FTIR testing were prepared using the same process. 2.2. Structural Characterization. The time-resolved small-angle light scattering (SALS) apparatus was the same as that Zheng et al.32 reported in previous paper. The blend samples were heated from room temperature to 260 °C at a heating rate of 2 °C/min. Phase contrast photographs were obtained by using an Olympus BX51 microscope equipped with a Linkam LTS350 hot stage. The samples were heated from room temperature to 260 °C at a heating rate of 2 °C/min. Infrared spectra of blends were recorded on a Nicolet 6700 timeresolved Fourier transform infrared spectrophotometer (FTIR). The sample was sandwiched between a pair of KBr plates. Thermal treatments were performed in a temperature cell accessory controlled with an Omega Temperature Controller within an accuracy of ±1 °C. The sample was heated at 10 °C/min to 190 °C, held for 5 min to melt the polymer and completely erase the thermal history, and cooled at 1 °C/min to room temperature. IR spectra of the specimen were collected with a 5 min interval during the cooling process. In all cases spectra were recorded by coadding 32 scans at a 2 cm−1 resolution. Morphology of the blends was observed by field-emission scanning electron microscope (FESEM). A Hitachi S-4800 SEM system was used for SEM measurements at an accelerating voltage of 10 kV. All the samples were fractured after immersion in liquid nitrogen for about 15 min. The phase structure of the blends was also observed directly using a transmission electron microscope (TEM) (Hitachi HT7700) operating at an acceleration voltage of 80 kV. The blend samples were ultramicrotomed at −120 °C to a section with a thickness of about 70 nm. The sections were then stained by ruthenium tetroxide (RuO4) for one night. Differential scanning calorimeter (DSC) was carried out under nitrogen flow at a heating rate of 10 °C/min from 30 to 200 °C and then cooled down to 30 °C at the cooling rate of 10 °C/min. It measured with a differential scanning calorimeter (DSC) system (TA Instruments DSC Q2000). The heating and cooling DSC traces were recorded. Before sample scan, the heat flow and temperature of the instrument were calibrated with sapphires and pure indium, respectively. Dynamic mechanical analysis (DMA) was carried out with a TA Instruments Model Q800 apparatus in the tensile mode. All the measurements were performed in the linear region with the strain of 0.03%. Dynamic loss (tan δ) was determined at a frequency of 5 Hz and a heating rate of 3 °C/min, as a function of temperature (from −100 to +160 °C). 2.3. Physical Property Measurements. Tensile tests were carried out according to the ASTM D 412−80 test method, using dumbbell-shaped samples punched out from the molded sheets. The tests were performed using a tensile testing machine (Instron, Model 5966) at a crosshead speed of 10 mm/min at 20 °C and 50% relative humidity. At least three specimens were tested for each sample.

toughened products, extensive studies have been carried on PLLA blends with other soft polymers because it is more versatile and economic to blend PLLA with other polymers than chemical modification.15,16 Previously, several blend systems containing PLLA have been investigated, such as PLLA/poly(ε-caprolactone) (PCL),17−20 PLLA/poly(ethylene oxide) (PEO),21 PLLA/poly(butylenes succinate) (PBS),22 PLLA/poly(butadiene-co-acrylonitrile) (NBR),23 PLLA/polyurethane.24 However, most of these blends are immiscible and compatibilizers are needed to improve their compatibility and reinforce the interface. In addition, the toughening of PLLA by rubbers inevitablely leads to the significant decreasing in the strength and modulus. Poly(oxymethylene) (POM) is an important engineering plastic and has been widely used in highly demanding applications, such as mechanical, electrical, and automotive engineering, construction of household appliances, and biomedical materials.25−27 It is superior in chemical resistance, mechanical properties, abrasion resistance, fatigue resistance, and mold ability. We consider that the PLLA/POM blends are of both scientific and technological interest. PLLA has been reported to be thermodynamically miscible with polyoxyethylene (PEO) and PLLA/PEO is a weakly interacting blend system.21 POM has similar molecular chain structure with PEO, but the methylene groups in POM rather than ethylene groups in PEO connected by ether bonds. It is therefore interesting to investigate the miscibility of the PLLA/POM blends. In addition, we are also interested in the physical properties of the PLLA/POM blends even though both PLLA and POM are rigid polymers. The blends of hard/ hard polymer systems have attracted much attention recently as a novel strategy to new high performance polymeric materials.28−30 In this work, the miscibility and phase diagram of PLLA/ POM blends have been investigated. It was found that the PLLA/POM blends exhibit typical lower critical solution temperature (LCST) behaviors and the phase separation induced by the crystallization of POM occurs upon cooling down from the homogeneous melt state. In addition, we have observed the novel phenomena that both PLLA and POM show apparent glass transition temperature depression in the blends, as compared with the neat PLLA and POM. Furthermore, the blending of PLLA and POM leads to the synergistic effects on the toughness of both POM and PLLA. PLLA/POM blends exhibits much higher elongation at break than both PLLA and POM with excellent tensile strength and modulus.

3. RESULTS 3.1. Phase Diagram of PLLA/POM Blends. The phase behavior of PLLA/POM blends has been investigated by means of SALS and optical microscopy. SALS has been used to determine the weak concentration fluctuation and fine domain size at the early stage of phase-separation for binary polymer blends with the different refractive index of the components.32−34 Therefore, SALS is a powerful method to investigate miscibility and phase behavior of polymer blends. The temperature dependence of scattering intensity for PLLA/ POM blends at various compositions with the heating rate of 2 °C/min is demonstrated in Figure 1. It is shown that a drastic increase in the scattering intensity was observed for the all blends during heating. The onset temperature of the change in the scattering intensity has been considered to be the cloud point. The blends are in homogeneous state at the temperature

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The PLLA sample used was purchased from Nature Works Co. LLC (USA), under the trade name of 3001 D. The Mn and Mw/Mn are reported to be 89300 ± 1000 g/mol and 1.77 ± 0.02, respectively.31 The sample included 1.6% of D-lactide content. The POM (MC 90) samples used in this work were kindly provided by Shenhua Co., Ltd., China. Melt flow index is 9.23 g/10 min. The weight-average molar mass and molecular polydispersity of the POM sample are Mw = 174300 g/mol and Mw/ Mn = 2.19. PLLA and POM were dried in a vacuum oven at 80 °C for 12 h prior to use. The blends with PLLA/POM weight composition varying from 90/10 to 10/90 were prepared using a batch mixer (Haake Polylab QC) with a twin screw at a rotation speed of 20 rpm at 190 °C for 1 min and then with rotation speed raised to 50 rpm for 5 min. After blending, all the samples were then hot-pressed at 190 °C under a 14 MPa pressure for 3 min to a film with a thickness of 300 μm, 5807

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Figure 1. Scattering intensity vs temperature for PLLA/POM blends with a heating rate of 2 °C/min.

Figure 3. LCST phase diagram for PLLA/POM blends as a function of composition.

below cloud point and phase separation occurs at the elevated temperature. On the other hand, it is obvious that the cloud point depends intensely on the blend compositions, and the cloud point moves toward higher temperature with the larger component ratios gap. This behavior is similar to most of the partially miscible polymer blends.34 Furthermore, the morphological evolution of the typical PLLA/POM (60/40) blend has been observed using optical microscopy during heating from 180 to 230 °C at the heat rate of 2 °C/min, as shown in Figure 2. It can be seen that the PLLA/POM (60/40) blend is homogeneous at 185 °C, indicating the miscible state. The phase separation can be observed when the sample was heated to about 201 °C, where the obvious cocontinuous morphology appears. Moreover, with further increasing temperature to 210 °C, the cocontinuous structure transforms into the typical sea-island structures. Such phase behaviors indicate that the sample follows the typical spinodal decomposition.34,35 The blends with other compositions present similar results that homogeneous state is observed at low temperature and phase separation occurs at the elevated temperatures. The optical microscopy results indicate the LCST phase behaviors of the PLLA/POM blends, which is consistent with those obtained by SALS. Combining the melting temperatures of POM and PLLA measured using DSC and the cloud points for the blends; the phase diagram of PLLA/POM blends is shown in Figure 3. Note that melting temperatures of both POM and PLLA decrease with addition of the other component, which refers to interactions between POM and PLLA. 3.2. Glass Transition Temperatures of PLLA and POM in the PLLA/POM Blends. Glass transition temperature of polymer blend is critically important to evaluate the miscibility of the components.2 Only one Tg is observed for the miscible

polymer blend where the segments of components are mixed. The two each other shifted Tgs of polymer blends indicate the molecular chain interpenetrating at the interface, which is considered to be a partially miscible blend. The immiscible polymer blends present two independent Tgs, same with the neat component material. No interactions between the components can be detected for the immiscible polymer blend and these blends usually show very big phase size and sharp interface. The glass transition temperatures of the quenched PLLA/POM blends have been evaluated by dynamic mechanical analysis. Note that the blends used for the DMA measurements are the samples quenched from 190 °C at which temperature the blends with the all compositions are miscible. In addition, PLLA is in the amorphous state and POM is wellcrystallized for all quenched samples due to the high crystallization speed of POM and the low crystallization rate of PLLA (as confirmed by wide-angle X-ray diffraction measurements in Figure S1, Supporting Information). Figure 4 shows plots of the dynamic loss (tan δ) by DMA as a function of temperature for neat PLLA, neat POM, and PLLA/POM blends with various compositions. Although all the samples are quenched from the miscible region, two distinct relaxation peaks were observed for the all blend samples in Figure 4, which means that two Tgs exist in the all quenched blend samples. The fact that two Tgs are observed in the blends indicates the phase-separated structures in the samples. It is therefore considered that the two Tgs correspond to the POM and PLLA phases, respectively. The dependence of the two Tgs on the compositions is shown in Figure 5. The Tgs of neat PLLA and neat POM are +75 and −56 °C, respectively. Surprisingly, the Tgs corresponding to both POM phase and PLLA phase in the blends show unique composition dependence. For the high Tg corresponding to the PLLA phase, the addition of POM leads to the decreasing of the glass transition

Figure 2. Optical micrographs for PLLA/POM (60/40) blend during heating at the indicated temperatures. 5808

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temperature. At the same time, for the Tg at low temperature, corresponding to the Tg of POM phase, the addition of PLLA (high Tg component) does not increases Tg of POM, but leads to the Tg depression of POM. In other words, the double Tg depression for both PLLA and POM was observed in the blends. 3.3. Crystallization of POM-Induced Phase Separation of POM/PLLA blends. POM and PLLA are miscible in the melt state at 190 °C. However, two Tgs have been observed for the melt-quenched POM/PLLA blends, which means that the POM/PLLA blends are phase-separated when quenched from the miscible region. The PLLA crystallization rate is very slow and PLLA keeps amorphous state for the quenched blends. In contrast, the crystallization speed of POM is fast, so POM is well crystallized in the quenched blends (as evidenced by Figure S1, Supporting Information). Therefore, we considered that the POM crystallization induces the phase separation of PLLA and POM when cooling down from melt miscible region. In fact, Woo et al. have reported the crystallization-induced phase separation behavior in the PLLA/PBA blends.36 Figure 6 shows the SEM images of the fracture surface of the quenched blends with the indicated compositions. The corresponding SEM images after etching using chloroform are also shown in Figure 6. Almost no detectable structure was observed for fractured surface before etching for the all samples. However, the irregular holes are clearly seen for the all samples after etching by chloroform. These holes correspond to the PLLA (rich) phase because PLLA could be easily dissolved in chloroform and POM could not. Moreover, two interesting results should be addressed during the Soxhlet etching experiments by chloroform. On the one hand, almost all the PLLA (>95 wt %) were extracted by the Soxhlet etching experiments by comparing the weight of the blend samples before and after the etching. On the other hand, not only the samples with POM as the major component but also the samples with PLLA as the major component can self-support (or maintain the original shape) after the Soxhlet extraction. Such experiment results indicate that both PLLA and POM are

Figure 4. Dynamic loss for the PLLA/POM blends as a function of temperature: (a) neat POM, (b) PLLA/POM = 20/80, (c) PLLA/ POM = 40/60, (d) PLLA/POM = 50/50, (e) PLLA/POM = 60/40, (f) PLLA/POM = 80/20, and (g) neat PLLA.

Figure 5. Glass transition temperatures of PLLA and POM from DMA analysis versus contents in PLLA/POM blend.

Figure 6. SEM images for the fractured surface, (a) PLLA/POM = 80/20, (b) PLLA/POM = 50/50, and (c) PLLA/POM = 20/80, and for the fractured surface after solvent etching, (a′) PLLA/POM = 80/20, (b′) PLLA/POM = 50/50, and (c′) PLLA/POM = 20/80. 5809

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Figure 7. TEM images of the PLLA/POM blends: (a) PLLA/POM = 80/20, (b) PLLA/POM = 50/50, and (c) PLLA/POM = 20/80.

continuous structure in 100% continuity in the quenched samples, independent of the component ratio of the blend. To elucidate the phase structure of quenched PLLA/POM blends, the morphologies of quenched samples have been directly observed using TEM, which are shown in Figure 7. In these figures, PLLA is observed as a white phase and POM is observed as a dark phase because POM is more readily stained by RuO4 than PLLA. POM crystallizes into crystal stacks and these stacks connect to each other and then form a POM crystal stack network. The POM crystal stacks network and amorphous PLLA are interpenetrated to form a bicontinuous structure. This unique bicontinuous structure originates from the crystallization of POM from the miscible PLLA/POM blends and simultaneously expelling out the PLLA chains from the front of POM crystals. The miscibility and the POM crystallization-induced phase separation have also been investigated by time-resolved FTIR during cooling from the 190 to 40 °C, as shown in Figure 8. POM and PLLA are miscible in the melt state at 190 °C. The miscibility can be attributed to the interactions between −C O of PLLA with C−H of POM, as evidenced in Figure 8. The absorption at 1760 cm−1 has been assigned to the stretching vibration of carbonyl group of PLLA, denoted as v(CO).37,38 v(CO) locates at 1756 cm−1 for the 80/20 blend at 190 °C, where PLLA and POM are in a miscible state and shifts to higher wavenumber during cooling. For POM, the peak at 2980 cm−1 arises from asymmetric stretching vibration band of C−H, vas(C−H), and the peak at 2923 cm−1 is due to the symmetric stretching vibration band of C−H, vs(C−H) at 40 °C. These three peaks move toward lower wavenumber at high temperature, suggesting reduction of the intrachain dipolar interaction of these two groups at molten stage. Furthermore, the lowfrequency shift of v(CO), vas(C−H) and vas(C−H) indicating the formation of weak C−H...OC hydrogen bonds between PLLA and POM chains in the melt.39 Upon cooling down, it is observed that v(CO) shifts to higher wavenumber (1760 cm−1) region. In addition, the halfwidth of this signal increases and a small shoulder can be observed at low temperature, which means that part of carbonyl groups become released from interchain interaction (or weak hydrogen bonding). The decreasing of the temperature (crystallization of POM) induces the higher wavenumber shifted vas(C−H) and vas(C−H), which indicates the phase separation between POM and PLLA due to the POM crystallization-induced phase separation. 3.4. Mechanical Properties. Strain−stress curves of neat PLLA, POM and PLLA/POM blends are shown in Figure 9. Both POM and PLLA are very rigid and shows pretty high tensile strength. On the other hand, the elongation at break values of neat POM and PLLA are 21% and about 7%,

Figure 8. FTIR spectra of PLLA/POM = 20/80 blend as a function of temperature during cooling from 190 °C with the interval of 10 °C in the range (a) 2800−3100 cm−1, (b) 1600−1850 cm−1, and (c) schematic diagram for the interaction between CO groups with CH2 groups.

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observed by many researchers. It is generally accepted that the high Tg depression originates either from the interfacial zone of the blend or the partially the molecular entanglement with the molecular chains of the low Tg component. On the other hand, the depression of the low Tg has mainly been observed in rubber-toughened plastics such as acrylonitrile−butadiene− styrene copolymer (ABS), 44,45 high-impact polystyrene (HIPS),46,47 PP toughened PS,48 and rubber-toughened PLLA,15,49 where the low Tg components were all domains dispersed in the rigid matrix. All these systems consist of the microspherical inclusions of a rubber in a rigid plastic matrix. In other words, the rubber phase is finely dispersed in the rigid plastic matrix for all these multiphase systems. Depression of the rubber Tg has been explained based on negative pressure resulting from differential contraction due to the thermal expansion mismatch upon cooling from the liquid state. However, in the present PLLA/POM systems, we see that the glass transition temperatures of both PLLA and POM in blends are lower than those of neat PLLA and POM, respectively, over the whole component ratio. Thus, it is the double phases that exhibit the Tg depression in the PLLA/ POM blends. Very recently, del Valle−Carrandi et al. reported the glass transition depression of poly(dimethylsiloxane) (PDMS) blocks in polystyrene (PS)-b-PDMS diblock copolymers.50 They found that the glass transition depression was associated with the poor segment packing (changing in the PDMS chain conformation) of PDMS in the block copolymers, as detected by FTIR. We have also checked the FTIR results of PLLA/POM blends and no evidence of the conformation change was observed for both PLLA and POM. By considering the phase structure of PLLA/POM blends quenched from the miscible region, the double Tg depressions have been attributed to the negative pressure resulting from differential contraction due to the thermal expansion mismatch between PLLA and POM phases, as shown in Figure 10. The PLLA/POM blends are homogeneous at 190 °C and phase separation occurs when cooling down from the melt state by the crystallization of POM. POM crystallizes into the small crystal stacks and these crystal stacks connect each other to form a three-dimensional POM rigid network. At the same time, the PLLA molecular chains are expelled out from the POM crystals. Therefore, a fine cocontinuous phase structure is formed with the POM as the rigid frame and PLLA as the soft phase between the Tc of POM and the Tg of PLLA. The further decreasing in the temperature leads to the large shrinkage of PLLA phase while POM exhibits less thermal shrinkage with temperature due to the crystallized nature of POM. The different contraction due to the thermal shrinkage mismatch induces apparent negative pressure at the interface of POM and PLLA. Note that the nice adhesion or partially entangled molecular chains at the interface transfer the negative pressure effectively. Therefore, the negative pressure not only constraints the shrinkage of PLLA but also exerts the tensile stress on the POM. The two effects lead to the larger free volume for both PLLA and POM; therefore, the double Tg depression was observed. Note that the phase structure of the PLLA/POM blends is bicontinuous, so the thermal shrinkage mismatch was large enough over the whole composition of the blend, whether PLLA or POM is majority. The higher content of PLLA, the stronger negative pressure on the POM and the lower Tg was observed. The exact pressure dependence of the Tg for POM was not known, but O’Reilly reported the value is dTg/dP = 0.24 °C

Figure 9. Strain−stress curves for neat PLLA, neat POM, PLLA/POM = 30/70 and PLLA/POM = 40/60.

respectively. Surprisingly, the quenched PLLA/POM blend film exhibits significantly improved elongation at break. The elongation at break of the blend reaches 57%, which is 8 times higher than that of the neat PLLA and about 3 times higher than that of the neat POM. Moreover, one can see that the blends maintain very high tensile strength and modulus, as shown in Table 1. It is very interesting that synergistic effects in the elongation at break were observed for blending of the two rigid/rigid polymers. Table 1. Mechanical Properties of PLLA/POM Blends sample PLLA POM PLLA/ POM = 30/70 PLLA/ POM = 40/60

tensile modulus/ GPa

tensile strength at yielding/MPa

tensile strength at break/MPa

elongation at break/%

2.30 1.72 2.10

64.83 68.02 66.04

51.88 61.99 50.92

7.31 21.32 37.15

2.22

66.60

48.32

50.07

4. DISCUSSION The PLLA/POM blends exhibit the LCST phase behavior and the miscibility between PLLA and POM originates from the specific interactions −CO of PLLA with C−H of POM. It is interesting to find that a cocontinuous phase structure consisting crystallized POM and amorphous PLLA phase was obtained by the cooling down from the miscible region, which was attributed to the crystallization-induced phase separation.36,40 It is more interesting to find the Tg depressions for both PLLA and POM with the addition of the other component. In other words, the Tg of PLLA decreases with addition of the POM and the Tg of POM decreases with the addition of PLLA. This phenomenon has been observed over the whole blend composition investigated. To our best knowledge, this is the first time to report the double glass transition temperature depression in a mechanical mixed polymer blend over the whole blend compositions. In fact, many literatures reported the Tg depression behaviors in multiphase polymer systems containing the hard and soft components.15,41−49 The depression of the high Tg of hardcomponent in a partially miscible polymer blend has been 5811

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Figure 10. Schematic diagram of the developing process of negative pressure caused by thermal shrinkage mismatch in the PLLA/POM blends.

MPa−1 for polyisoprene.51 Assuming all polymers have similar Tg pressure dependence, it can be calculated that the level of the negative pressure in PLLA/POM blends is about 40−50 MPa. This value is close to the thermal stresses of 78 MPa due to the asymmetric shrinkage for polystyrene/polybutadiene block copolymers.43 Moreover, it can be expected that such negative pressure may be affected when applying mechanical deformation. We carried out the hot rolling experiments for the PLLA/POM = 20/80 sample at 80 °C and the draw ratio is 2.0, followed by the Tgs measurements by DMA. It was found the Tgs of both PLLA and POM increase after the deformation in both parallel and vertical direction (Figure S2 and Table S1, Supporting Information), which means that the negative pressure was partially compensated by the mechanical deformation. It should be noted that the large mechanical deformation at 80 °C simultaneously induces the crystallization of PLLA and molecular chain orientation of components and all these parameters affect the Tgs greatly. The detail effects of mechanical deformation on the thermal stress are currently in progress. To investigate the phase structure effects on the T g depression, another experiment has been carried out. We heated the PLLA/POM = 20/80 to 230 °C and annealed there for 30 min to complete phase separation because the temperature is above LCST. The sample was then quenched to room temperature. Figure 11 shows the TEM image of the sample quenched from phase separation region. As expected, a typical sea-island structure was observed with PLLA domains dispersed within POM matrix in the size of 1−2 μm. The glass transition temperatures of this sample were also characterized by DMA and compared with the sample quenched from the miscible region (190 °C), as seen in Figure 12. It is seen that the Tgs of both PLLA and POM of melt-phase-separated sample are higher than those of the sample quenched from

Figure 11. TEM image for PLLA/POM = 20/80 after annealing at 230 °C for 30 min.

miscible region, as seen in Table 2. The melt-phase-separated sample has the structure with PLLA domains dispersed in POM matrix. In contrast, the sample quenched from the miscible melt is continuous structure. Therefore, the stronger negative pressure for the sample quenched from the miscible state is expected much larger than that for the annealed sample. Moreover, the miscible quenched sample show stronger interfacial interaction than the annealed sample, which means that the negative pressure could be more effectively transferred cross the interface. Therefore, the melt miscible quenched sample exhibit higher Tg depression for both PLLA and POM than the 230 °C annealed sample. The decreased Tg of both PLLA and POM in the blends indicates the increased local segmental motions of both PLLA and POM. Therefore, a novel tensile behavior has been 5812

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Y.L.) [email protected]; (L.W.) wanglinhznu@ gmail.com. Fax: +86 571 28867899. Telephone: +86 571 28867026. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (21074029, 51173036), Zhejiang Provincial Natural Science Foundation of China (R4110021), the Project of Zhejiang Key Scientific and Technological Innovation Team (2010R50017), and PCSIRT (IRT 1231). The authors thank the reviewers for the valuable suggestions.

Figure 12. Dynamic loss for the PLLA/POM blends as a function of temperature.

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Table 2. Glass Transition Temperatures of PLLA/POM Blends Quenched from Miscible Region and PhaseSeparation Region

sample Tg of PLLA phase/°C Tg of POM phase/°C

neat PLLA

neat POM

PLLA/POM = 20/80 quenched from miscible region

75.1

−

50.9

55.1

−

−57.9

−67.2

−64.9

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PLLA/POM = 20/80 quenched from phaseseparated region

observed. The PLLA/POM blends show ductile deformation even though both neat PLLA and neat POM are brittle.

5. CONCLUSION The binary PLLA/POM blend system shows typical LCST phase diagram. PLLA and POM are fully miscible in the melt at the low temperature as a result of the formation of weak hydrogen bonding between the carbonyl group of PLLA and the methylene groups of POM. However, the POM crystallization for the quenched sample from the miscible melt state induces the phase separation, as evidence by the two separated Tgs of the quenched sample. The morphological investigation indicates the cocontinuous structure of the quenched blends. Both PLLA and POM show significant Tg depression in the blends with the cooperation of the other component, as compared with the neat samples. It has concluded that the double Tg depression may be due to the negative pressure resulting from differential contraction due to the thermal shrinkage mismatch from the cocontinuous structures. It is further found that such rigid/rigid PLLA/ POM blends show improved ductility than the neat PLLA and neat POM. The improved ductile properties may be originated from the decreased Tgs and improved mobility of POM and PLLA. This paves a new strategy leading to high performance polymeric alloys with good modulus−toughness balance.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

WAXD diffraction patterns of PLLA/POM blends quenched from 190 °C and the Tgs of the mechanical deformed PLLA/ POM blends. This material is available free of charge via the Internet at http://pubs.acs.org. 5813

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