PDLA Blends

Feb 14, 2014 - Stereocomplex Crystallite Network in Asymmetric PLLA/PDLA Blends: Formation, Structure, and Confining Effect on the Crystallization Rat...
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Stereocomplex Crystallite Network in Asymmetric PLLA/PDLA Blends: Formation, Structure, and Confining Effect on the Crystallization Rate of Homocrystallites Xin-Feng Wei, Rui-Ying Bao, Zhi-Qiang Cao, Wei Yang,* Bang-Hu Xie, and Ming-Bo Yang State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, Sichuan, China

ABSTRACT: Stereocomplex (SC) crystallites, formed between enantiomeric poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), show a melting point 50 °C higher than that of PLLA or PDLA homocrystallites, which makes it possible for SC crystallites to be reserved in the melt of PLLA in asymmetric PLLA/PDLA blends and to act as a rheological modifier and a nucleation agent for PLLA. Herein, by a rheological approach, a transition from the liquid-like to solid-like viscoelastic behavior was observed for the SC crystallites reserved melt, and a frequency-independent loss tangent at low frequencies appeared at a PDLA concentration of 2.0 wt %, revealing the formation of SC crystallite network. By a delicately designed dissolution experiment, the structure of the formed network was explored. The results indicate that the network are not formed by SC crystallites connected directly with each other or by bridging molecules, but by the interparticle polymer chains which are significantly restrained by the cross-linking effect of SC crystallites. Nonisothermal and isothermal crystallization show that the reserved SC crystallites can accelerate remarkably the crystallization rate of PLLA due to heterogeneous nucleation effect. Besides, a special PDLA concentration dependence, e.g., the overall crystallization rate is almost independent of PDLA content for the blends with PDLA content higher than PDLA percolation concentration (2.0 wt %), was also observed. The increase of nuclei density for the blends containing PDLA from 2 to 5 wt % was estimated from POM observations. The result of an enhanced nucleation but an unchanged overall crystallization rate reveals the confining effect of the SC crystallite network on PLLA crystallization. This confining effect can be ascribed to the restrained diffusion ability of PLLA chains owing to the SC crystallite network.

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rate of PLLA.13−16 SC crystallites show a melting point 50 °C higher than that of PLA homocrystallites.17 As a result, in asymmetric PLLA/PDLA blends only homocrystallites are melted, and SC crystallites are reserved in the melt when the blend is processed at temperatures between the melting temperature of homocrystallites and that of SC crystallites. It means that the reserved SC crystallites can play the role of heterogeneous nucleation sites for subsequent PLLA crystallization. In fact, the addition of only a small amount of PDLA can enhance the nucleation process of PLLA remarkably and thus accelerate the overall crystallization rate of PLLA greatly when the crystallization conditions are scrupulously selected.18

INTRODUCTION Polylactide (PLA), as one of the most promising biopolymers, has exhibited vast appeal in the past decades due to its excellent performance in renewability, biodegradability, biocompatibility, and mechanical properties.1−5 Unfortunately, the application of PLA has been limited because of its poor thermal stability and thermal resistance, and great efforts have been made to improve these properties. Formation of stereocomplex (SC) crystallites between enantiomeric poly(L-lactide) (PLLA) and poly(Dlactide) (PDLA) has been proven to be one of the most effective and promising methods to enhance the properties of PLA, including heating resistance, thermal stability, mechanical performance, and hydrolysis resistance.6−12 Moreover, SC crystallite has been evidenced to be an efficient and biodegradable nucleating agent for PLLA; thus, SC crystallites can be employed to increase the poor crystallization © 2014 American Chemical Society

Received: December 30, 2013 Revised: January 27, 2014 Published: February 14, 2014 1439

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ential scanning calorimetery (DSC) measurements. In addition, films with a thickness of about 30 μm were also prepared by hot-pressing at 190 °C for optical microscopic observation. For brevity, PLLA/PDLA blend with X wt % PDLA was labeled as LD-X. For instance, the blend containing 0.5 wt % PDLA was named as LD-0.5. WAXD Measurements. WAXD measurements of the compression-molded specimen were carried out with a DX-1000 X-ray diffractometer (Dandong Fanyuan Instrument Co. LTD, China) using a Cu Kα radiation source (λ = 0.154 056 nm, 40 kV, 25 mA) in the scanning angle range of 2θ = 5°−50° at a scan speed of 3°/min. Rheological Measurements. Rheological measurements were performed on a stress-controlled dynamic rheometer AR2000ex (TA Instruments) with a parallel plate geometry (diameter: 25 mm) under the protection of a nitrogen atmosphere. The samples were dried in a vacuum at 60 °C for 6 h before measurement. Oscillatory frequency sweeps were performed from 0.01 to 100 Hz at 180 °C with a strain of 1% (within the linear viscoelasticity regime) for neat PLLA and its blends with different PDLA concentrations. Dynamic Light Scattering (DLS). The solutions of the blends for light scattering were prepared with dichloromethane (1 mg/mL) and filtered through a 0.45 μm filter prior to use. DLS measurement was performed as that in ref 36 and was conducted with a BI-200SM (Brookhaven Instruments) at 20 °C. The scattering angle and He−Ne laser operating were set at 90° and 532 nm. The results were analyzed by the regularized CONTIN method. DSC Measurements. The melting behaviors of neat PLLA and the blends were investigated with DSC at a heating rate of 10 °C/min from 40 to 250 °C using a DSC Q20 (TA Instruments). The nonisothermal and isothermal crystallization behaviors of neat PLLA and the blends were also recorded. Samples of around 5 mg were first heated to 190 °C at a rate of 100 °C/min and held at 190 °C for 5 min to melt the homocrystallites completely, then cooled to 30 °C at a rate of 2.5 °C/min, and finally reheated to 190 °C (second heating) at a rate of 10 °C/min to record the nonisothermal crystallization behavior and corresponding melting behavior. For isothermal crystallization, samples were also first held at 190 °C for 5 min and then cooled to 137, 140, 143, and 146 °C at a rate of 100 °C/min to monitor the isothermal crystallization process. All these processes were carried out under a nitrogen atmosphere. Nucleation Efficiency Test by Self-Nucleation. The selfnucleation procedure was adopted to investigate the nucleation efficiency of the formed SC crystallites. Details of the experimental method can be found in the literature.14,19,36−39 The procedure employed was as follows: (i) Neat PLLA was heated to 190 °C, a temperature high enough to erase the thermal history, and held at 190 °C for 5 min. Then, the melt was cooled to 80 °C at 2.5 °C/min, the same rate as the selected cooling rate in the nonisothermal crystallization of PLLA/PDLA blends. This procedure provides a standard thermal history. (ii) Neat PLLA was heated to a temperature (Ts, self-nucleation temperature) in the partial melting zone and isothermally kept for 5 min to create self-nucleated seeds. After that, the samples were cooled to 80 °C at 2.5 °C/min. The crystallization peak temperatures (Tp) of the self-nucleated PLLA were determined from the cooling curves. In addition, the samples after melting at Ts for 5 min were also immediately heated to 190 °C at 10 °C/min to evaluate whether all the crystallites had been sufficiently melted at Ts. Optical Microscopic Observation. The nucleation and crystalline morphologies of neat PLLA and PLLA/PDLA blends were observed using an Olympus BX51 polarizing optical microscopy (Olympus Co., Tokyo, Japan) equipped with a hot-stage (LINKAM THMS 600). The hot-pressed films of neat PLLA and PLLA/PDLA blends with a thickness of about 30 μm, sandwiched between two cover glasses, were melted at 190 °C for 5 min and then quickly cooled to 145 °C at a cooling rate of 50 °C/min for isothermal crystallization. The morphologies of growing spherulites were monitored by taking micrographs at an appropriate time interval.

For instance, with the introduction of PDLA at a content as low as 0.25 wt %, the nuclei density was over 150 times higher than that of neat PLLA.14 In addition, SC crystallites show much higher nucleation efficiency than a commonly used PLA nucleating agent, talc.14,19 Many attempts have been made to identify the influence and the mechanism of PDLA concentration, molecular weight of homopolymers, optical purity, fine structure of SC crystallites, initial melt states, preparation methods, and crystallization conditions on the crystallization promoting effect of SC crystallites.14−25 In our previous works, we have revealed that the promoting effect of SC crystallites for PLLA crystallization can be further improved with the help of shear flow or plasticizer.26,27 Besides, Yamane et al.28 reported that in the presence of the previously formed SC crystallites the melt can be significantly reinforced and show a strong strain hardening feature, indicating that the reserved SC crystallites can also be a rheological modifier to improve the low melt strength of PLLA. That SC crystallites can act as a kind of nucleation agent and rheological modifier for PLLA is primarily due to the particular structure that SC crystallites are reserved in the melt of asymmetric PLLA/PDLA blends. This structure is similar to that of polymer melt at the early stage of crystallization in which a small amount of crystals has been formed and embedded in the melt. Winter and co-workers29−32 pointed out that polymer crystallization can be viewed as a physical gelation process, and the transition between liquid-like and solid-like behavior takes place at a critical gel point. Similarly, with increasing content of SC crystallites, such a physical gelation process is also probable to be observed in the melt embedded with SC crystallites; e.g., the embedded SC crystallites are probable to form a network in the melt of the asymmetric PLLA/PDLA blends. For this issue, Rahman et al.16 once guessed that SC crystallites were not isolated in PLLA melt but connected like a physical gel from their crystallization results, and recently this network was witnessed directly by Saeidlou et al.33 using optical microscopy. However, the transition from liquid-like to solid-like behavior, the critical gel point, and the structure of the network are to be revealed. In this work, the existence of such a network and the corresponding transition were successfully probed by a rheological approach which is sensitive to the microstructure evolution of polymer melt. The structure of this network and its effect on PLLA crystallization were investigated.

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EXPERIMENTAL SECTION

Materials. A commercial PLA (trade name 4032D) was used as a PLLA source. It is a semicrystalline grade from NatureWorks LLC that comprises about 2% D-units, and its weight-averaged molecular weight (Mw) is 2.1 × 105 g mol−1 and the polydispersity (PDI) is 1.7. PDLA, synthesized by ring-opening polymerization of D-lactide,25,34,35 was kindly supplied by Professor Chen Xue-Si at State Key Laboratory of Polymer Physics and Chemistry, China. The Mw and PDI of PDLA were about 1.0 × 105 g mol−1 and 1.7, respectively. Sample Preparation. PLLA was melt blended with 0.5, 1, 2, 3, 4, and 5 wt % PDLA in a Haake internal mixer (Rheomix 600, Germany). Prior to blending, the resins were vacuum-dried at 60 °C for 24 h. The blending was carried out at 180 °C for 6 min at a rotation speed of 40 rpm. For comparison, neat PLLA was also processed under the same conditions. The prepared samples were again dried at 60 °C for 6 h and then compression molded into disks with a diameter of 25 mm and a thickness of 1.5 mm on a hot-press at 190 °C and 10 MPa and then cooled on a cold-press at 18 °C and 10 MPa. These compression-molded samples were used for the rheological tests, wide-angle X-ray diffraction (WAXD), and differ1440

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Figure 1. WAXD (a) and DSC melting (b) profiles of the compression-molded samples.

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RESULTS AND DISCUSSION Stereocomplex Formation. In our previous work, stereocomplex crystallites were successfully and efficiently introduced in both symmetric PLLA/PDLA blends and asymmetric PLLA/ PDLA blends by a low-temperature approach.40,41 Here, the asymmetric blends were also prepared by this approach at 180 °C. Figure 1 shows the WAXD patterns and DSC melting curves of these prepared samples. In Figure 1a, all WAXD patterns do not exhibit diffraction peaks of PLA homocrystallites at 2θ values of 16°, 18.4°, and 21.8°,42 indicating no homocrystallites existed, and neat PLLA was amorphous. With the addition of 0.5 wt % PDLA, a weak but still visible diffraction peak appears at 2θ value of 11.8°, corresponding to the (110) planes of SC crystallites. 43 Increasing the concentration of PDLA to be above 1 wt %, other two diffraction peaks of SC crystallites also appear at 2θ value of 20.6° and 23.8°, assigned to the (300) and/or (030) planes and (220) planes of SC crystallites.43 These peaks become stronger with increasing PDLA concentration. By dividing the sum of SC diffraction peak area to the total peak area, SC crystallinity was calculated and listed in Table 1. Ideally, a crystallinity of 2X

also be observed for neat PLLA and the blends during the heating scan. The glass transition temperatures and the melting temperatures of homocrystallites seem to be not affected by the addition of PDLA or the formed SC crystallites. The cold crystallization peak temperature decreases with increasing concentration of PDLA, indicating exactly the same effect of the formed SC crystallites on the crystallization of PLA that will be observed in the nonisothermal and isothermal crystallization behaviors later. Rheological Behaviors and SC Crystallite Network. To explore the effect of formed SC crystallites on the melt rheological properties of the blends, oscillatory shear rheological measurements were carried out at 180 °C. Here, the melting point of homocrystallites is around 165 °C and that of SC crystallites is around 210 °C (as shown in Figure 1b); thus, at 180 °C only homocrystallites were melted, and SC crystallites were reserved in the melt of the blends. Moreover, at 180 °C the formed SC crystallites would not grow further because of the imbalanced composition of the components at a low content of PDLA.28 Figure 2 shows the frequency dependences of storage modulus (G′), loss modulus (G″), loss tangent (tan δ), and complex viscosity (|η*|) for neat PLLA and its blends with different concentrations of PDLA. In Figure 2a,b, at low frequencies, G′ and G″ increase with increasing PDLA concentration, and the change of G′ is more significant than that of G″. Neat PLLA chains relax fully and exhibit the typical terminal behavior with the scaling law of approximate G′ ∝ ω2 and G″ ∝ ω1. With increasing PDLA concentration, the slopes of the modulus curves of the blends at low frequencies decrease. The more obvious nonterminal behavior for the blends than that of neat PLLA suggests that a slower relaxation behavior in the melt of the asymmetric blends, which can be ascribed to the formed SC crystallites that restrain the long-range motions of polymer chains.47,48 The loss tangent (tan δ = G″/G′) is an essential parameter characterizing the relaxation behavior of the viscoelastic materials and is regarded more sensitive to the relaxation changes than G′ and G″.49 It can be seen in Figure 2c that tan δ of neat PLLA decreases with increasing frequency, which is a typical behavior for viscoelastic liquid. With increasing PDLA concentration, tan δ decreases gradually, reflecting that the elastic response of the melt becomes more significant when the formed SC crystallites increase. When PDLA concentration reaches 2.0 wt %, a frequency-independent loss tangent appears for sample LD-2.0 at low frequencies, while when PDLA concentration is above 2.0 wt %, a tan δ peak, an indicator of a dominant elastic response of the melt,50 can be observed. These results indicate that a network has been formed in the melt of

Table 1. Degree of Crystallinity (Xc) of SC Crystallites Obtained from WAXD and SC Formation Efficiency (FE) for the PLLA/PDLA Blendsa

a

samples

LD-1.0

LD-2.0

LD-3.0

LD-4.0

LD-5.0

Xc (%) FE (%)

1.2 60

2.6 65

3.2 53.3

5.4 67.5

6.0 60

The diffraction peak of LD-0.5 is too weak to be calculated.

(2 times of PDLA concentration (X wt %)) was expected;33 thus, a stereocomplex formation efficiency for the blends around 60% was obtained for our melt-prepared asymmetric blends. The stereocomplex formation efficiency depends on the optical purity and the molecular weight of PLLA and PDLA. 60% is a relatively high stereocomplex formation efficiency compared with the reported results.33,44−46 These results clearly indicate that the blends with various SC crystallite concentrations have been achieved. The melting peaks of SC crystallites at around 210 °C can be observed in Figure 1b for the asymmetric blends except for sample LD-0.5, in which indeed a low content of SC crystallites existed (see Figure 1a). It agrees well with the results of WAXD. Besides the melting of SC crystallites, three other transitions including glass transition at around 60 °C, cold crystallization at around 110 °C, and melting−recrystallization−remelting of PLA homocrystallites at around 165 °C can 1441

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Figure 2. Variation of (a) storage modulus (G′), (b) loss modulus (G″), (c) loss tangent (tan δ), and (d) complex viscosity (|η*|) as functions of frequency for asymmetric PLLA/PDLA blends with different PDLA concentrations. The measurements were performed with a strain of 1.0% at 180 °C.

Figure 3. Schematic diagram (a) for the probable structures of the formed network and the photos (b) of the solutions of the prepared samples for neat PLLA, LD-2.0, and LD-5.0 after dissolving in dichloromethane with a concentration of 0.3 g/dL without stirring.

wt %. However, for the blends with higher concentration of PDLA, the Newtonian plateau disappears and shear thinning can be observed at low frequencies. Furthermore, for sample LD-0.5 and sample LD-1.0, the viscosity increases slightly compared with neat PLLA, but for sample LD-2.0 a sharp increase of viscosity can be observed. These results reveal that the formed network significantly reinforces the melt. It is essential to discuss how the SC crystallites influence the melt rheological behaviors. For PLA melt embedded with SC crystallites, SC crystallites have two distinct effects on the rheological responses of the melt. First, SC crystallites, possessing a high modulus, can act as dispersed solid particles in the melt and reinforce the melt, referred to as filler effect here. Second, SC crystallites formed from the cocrystallization of PLLA and PDLA chains can act as cross-linking points of

the blends with increasing content of the reserved SC crystallites. Based on the approach proposed by Winter et al.,29,51 it is clear that a critical physical gel is formed at a PDLA concentration of 2.0 wt %. An SC crystallite concentration of 2.6% has been reached in this case (see Table 1). Thus, the SC crystallite percolation concentration for the network formation is 2.6%. The transition from the liquid-like to solid-like viscoelastic behaviors at low frequencies also demonstrates that the long-range polymer chains motion is restrained by the formed SC crystallite network significantly. The complex viscosity (|η*|) was also employed to reveal the effect of the reserved SC crystallites on the melt rheological behaviors. It can be seen in Figure 2d that the typical Newtonian plateau at low frequencies is observed for neat PLLA and the blends with a PDLA concentration lower than 2 1442

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Figure 4. DSC cooling curves (2.5 °C/min) (a) and heating curves (10 °C/min) (b).

bridging molecules which have segments in neighboring SC crystallites for the cases of D and E. While for the cases of A− C, neighboring SC crystallites are connected by the amorphous chains, which can be dissolved in dichloromethane. So after these chains dissolved in dichloromethane, the neighboring SC crystallites will be separated with each other in the solvent for the cases of A−C. It can be seen from Figure 3b that the solution is transparent for neat PLLA while becoming opaque for LD-2.0 and LD-5.0 due to the undissolved SC crystallites. In addition, the SC crystallites are not connected together as a whole but are separated from each other and suspended homogeneously in the solvent for the network containing sample LD-2.0 and LD5.0, thus excluding the possibilities (i) and (ii). This result is in agreement with the conclusions of Coppola et al.32 What should be pointed out is that the structures of (i) and (ii) may still exist locally, especially for the blend melt containing high content of SC crystallites, but from the view of threedimensional spaces, SC crystallites are connected by the amorphous chains through entanglements. As aforementioned, SC can act as a cross-linking point of PLA chains, which make these interactional chains relax in a much slower way and take part in the formation of network. Based on the discussion above, it can be concluded that the existed network should be formed by the rigid SC particles and the interparticle polymer chains which are significantly restrained by the cross-linking effect of SC crystallites. Moreover, the size of the SC crystallites dispersed in dichloromethane was determined by DLS. A hydrodynamic diameter (Dh) of 553 nm was obtained for LD-2.0 and 808 nm for LD-5.0. There exist more PDLA chains for SC crystallites to grow to a larger size in LD-5.0 compared to that of LD-2.0. Narita et al.23 have investigated the lamellar thickness (lc) of SC crystallites in the asymmetric PLLA/PDLA blends by smallangle X-ray scattering, and an lc of around 20 nm was found. There is almost 1 order of magnitude different between Dh and reported lc, which seems to reveal that the SC crystallites in dichloromethane are agglomerated together. However, the contribution of the amorphous segments of the complexed chains (as shown in Figure 3a) to the increase of Dh should be stressed. These amorphous segments surround the SC crystallites and can expand in the dichloromethane, resulting in the increase of Dh. Crystallization Behaviors of the Asymmetric PLLA/ PDLA Blends. From the discussion above, it is clear that the long-range motion of polymer chains is restrained by the formed SC crystallites, and this restraining effect shows itself more remarkably after the network forms. As is known, the

these participant chains, resulting in an increase in the apparent molecular weight.28,33 Furthermore, these physically crosslinking connected chains are transformed from linear to branched since the pending segments of the chains that have not participated in stereocomplex formation will act as branches on the opposite chain.27,28,33 This is referred to as the crosslinking effect here. As solid fillers, SC crystallites can reinforce the melt and increase the elastic response of the melt. Owing to the cross-linking effect of SC crystallites, a slower relaxation structure, no matter increased apparent molecular weight or the branched molecular structure, is introduced into the melt and can also reinforce the melt. The reinforcement has been reflected clearly by the increase of G′ and |η*| and the decrease of tan δ. Besides these two effects, the contribution of the threedimensional network, formed in the melt when the PDLA concentration reaches 2.0 wt %, should be addressed. The formation of such a network can enlarge sharply these reinforcing effects and results in the transition from the liquid-like to solid-like viscoelastic behaviors. Structure of the Formed Network. As proved above, there exists a network in the PLA melt embedded with SC crystallites. However, its structure is still unknown. Similar to the structures of the gel formed during polymer crystallization process proposed by Horst and Winter,29,32 three probabilities may exist for the structure of the formed network here: (i) immediate contact between structural units (i.e., SC crystallite impingement), (ii) a network of bridging molecules which have segments in neighboring SC crystallites, or (iii) impingement of amorphous chains, immobilized by their segment attachment within a crystalline structure, with similarly immobilized chains from adjacent structures. To give a clearer illustration of the probable structures of the formed network, a schematic diagram was presented in Figure 3a. The structures of A−C, D, and E stand for the probabilities of (iii), (ii), and (i), respectively. As is shown, with the structure evolution from A to E the distance between neighboring SC crystallites should decrease. Coppola and co-workers32 have ruled out the possibilities of (i) and (ii) by the proof that a relatively low crystallinity, of the order of 1%, was obtained at the gel transition. Here, a dissolution experiment was delicately designed to reveal the structure of the formed network. It is based on the fact that SC crystallites cannot be dissolved in dichloromethane while dichloromethane is a good solvent for PLA. For the structures of D and E, neighboring SC crystallites are immediately connected by each other and by the bridging molecules, respectively. So, after the uncomplexed PLLA and PDLA chains are dissolved in dichloromethane, SC crystallites are not dissolved and still connected by each other by these 1443

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PDLA concentrations lower than 2.0 wt %, Tp is 112.6 °C for neat PLLA and sharply increases to 133.0 °C, showing a shift of 20.4 °C, for sample LD-2.0 with a PDLA concentration of 2 wt %. While for PLLA/PDLA blends with PDLA concentrations higher than 2.0 wt %, Tp just slightly further increases to 135.7 °C for sample LD-5.0 with a PDLA concentration of 5 wt %, only showing a 2.7 °C shift compared with that of LD-2.0. Compared with the 20.4 °C shift of Tp, a change of 2.7 °C is so small that it can be roughly regarded that Tp almost keeps constant with increasing PDLA concentration for PLLA/PDLA blends with PDLA concentration higher than 2.0 wt %. Interestingly, 2 wt % is exactly the critical concentration of PDLA for the network formation. These results indicate that the crystallization rate of PLLA/PDLA blends increases sharply below the critical concentration of PDLA for the network formation but almost keeps constant above the critical concentration of PDLA. To further confirm this peculiar PDLA concentration dependence for the effect of the formed SC crystallites on the crystallization of PLLA, the isothermal crystallization behaviors of PLLA/PDLA blends were monitored under various crystallization temperatures, and the development of heat flow and relative degree of crystallinity X c of homocrystallites as functions of time are displayed in Figures 6 and 7, respectively. For brevity, the data of isothermal crystallization at 143 and 140 °C are not shown. Xc is a relative parameter defined as52

diffusion ability of polymer chains influences the process of polymer crystallization notably. Thus, the crystallization behaviors of PLLA in the presence of SC crystallites and/or the network are attractive and worth to be investigated. Here, DSC was adopted to study crystallization kinetics of the asymmetric PLLA/PDLA blends with various PDLA concentrations. The nonisothermal crystallization behaviors of the blends were investigated, and the DSC cooling curves and subsequent heating curves are presented in Figure 4. As shown in Figure 4b, cold crystallization peaks are not observed for both neat PLLA and its blends, indicating that all samples can crystallize completely in the previous cooling cycle at such a low cooling rate of 2.5 °C/min. From the cooling curves in Figure 4a, with the addition of PDLA, the crystallization peak temperatures of the blends shift to higher temperature, and these peaks become sharper compared with that of neat PLLA. The results manifest evidently that the formed SC crystallites improve the crystallization rate of PLLA remarkably due to the heterogeneous nucleating effect of SC crystallites. Interestingly, a special PDLA concentration dependence, e.g., an initially rapid increase of Tp and subsequently modest increase of Tp (almost constant) with increasing PDLA concentration, is observed in the meltprepared PLLA/PDLA blends. In order to compare the crystallization kinetics of PLLA/ PDLA blends with different PDLA concentrations quantitatively, the onset crystallization temperature (To), peak crystallization temperature (Tp), and crystallization enthalpy (ΔH), which are key parameters that relate to the nucleation characteristics of the system, the overall crystallization kinetics, and the degree of crystallinity, respectively, have been displayed in Figure 5. Based on the method proposed in refs 14, 18, 21,

Xc = Xc(t )/Xc(t∞) = = ΔHt /ΔH∞

t

d H (t ) dt / dt

∫0

∞

d H (t ) dt dt (2)

where dH/dt is the rate of heat evolution, ΔHt is the heat generated at time t, and ΔH∞ is the total heat. As shown in Figure 6a, at Tc = 146 °C no heat flow peak is observed for neat PLLA in a period as long as 90 min, indicating that no crystallization occurred at such a high temperature. However, obvious heat flow peaks are seen for the PLLA/PDLA blends in the presence of SC crystallites. Moreover, at Tc = 137 °C the peak position of the exothermic heat flow curve (standing for the time when the maximum crystallization rate appears, tmax)49 shifts to a much shorter time region when only 0.5 wt % PDLA was introduced. These results attested the remarkable promoting effect of SC crystallites on the overall crystallization rate of PLA. In addition, the special concentration dependence can be also observed in Figures 6 and 7. At the same time, the half-crystallization time, t0.5, often chosen as a key parameter to characterize the overall crystallization kinetics, was obtained from the curves of Xc(t) at Xc(t) = 50% and displayed in Figure 8. A similar phenomenon of an initially rapid decrease of t0.5 and subsequently a much slow decrease of t0.5 (almost constant) at each isothermal crystallization temperature is observed, confirming the existence of peculiar PDLA concentration dependence. Based on the results above, a special PDLA concentration dependence for the promoting effect of the formed SC crystallites on the crystallization rate of PLLA was observed during the nonisothermal crystallization process and was verified during the isothermal crystallization process. There have been lots of work focusing on the effect of PDLA (with various molecular weights and optical purity) concentration on the crystallization promoting effect of SC,14−16,18,19 and

Figure 5. Crystallization peak temperature (Tp), onset temperature (To), and enthalpy (ΔH) obtained from Figure 4 against the concentration of PDLA.

and 22, ΔH was normalized by eq 1, which assumes that all of the PDLA molecules have formed SC crystallites with the same amount of PLLA (“2” in this equation is based on this assumption and X (wt %) is the PDLA concentration) and only the remaining uncomplexed PLLA molecules can form PLLA homocrystallites. ΔH = ΔHc(H)/(1 − 2 × X (wt %)/100)

∫0

(1)

As shown in Figure 5, these three parameters show the same trend with the increase of PDLA concentration, and the special PDLA concentration dependence is clearly demonstrated. Taking Tp as an example, for PLLA/PDLA blends with 1444

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Figure 6. DSC heat flow as a function of time at isothermal crystallization temperatures of (a) 146 and (b) 137 °C.

Figure 7. Relative crystallinity from the DSC test as a function of time for the samples isothermally crystallized at (a) 146 and (b) 137 °C.

NE (%) =

Tp − Tpmin Tpmax − Tpmin

× 100 (3)

where Tmin is the crystallization peak temperature of pure p polymer and Tpmax is the maximum crystallization peak temperature of polymer after it has been self-nucleated from a self-nucleation experiment. Self-nucleation is considered to be the ideal case for homopolymer crystallization due to an optimum dispersion of crystallites and the favorable interactions between polymer melt and polymer crystal fragments; thus, a highest crystallization temperature (Tmax p ), corresponding to the 100% NE, can be obtained.36 The results of selfnucleation experiment are presented in Figure 9. As is shown, at a self-nucleation temperature (Ts) of 190 °C, Tmin = 112.8 °C. Because of the self-nucleation seeds created by melting in the partial melting zone, the crystallization peak temperature increases sharply with the decrease of Ts. Tmax should occurs at the lowest Ts of the partial melting zone which is defined as the temperature at which stable crystal fragments reach a state of saturation and insufficient melting occurs at temperatures lower than it. Here, the heating curves of samples that have been already melted at Ts for 5 min are employed to decide whether the sample is melted sufficiently at Ts. As shown in Figure 9b, the melting peak was not observed at Ts equal to or higher than 168 °C, while it appeared when Ts was lower than 168 °C, indicating that 168 °C is the lowest Ts of the partial melting zone. Thus, Tmax = 142.7 °C was obtained. Using the Tp obtained from the nonisothermal crystallization, the nucleation efficiency can be calculated and were displayed in Figure 10.

Figure 8. Half-crystallization time (t0.5) obtained from Xc(t) = 50% against the concentration of PDLA.

Saeidlou et al. summarized the crystallization temperatures (Tc) of these works in ref 2. From the summary, it is clear that most work showed that Tc increased with PDLA concentration up to a plateau, in agreement with our results. Unfortunately, the reason for the appearance of the plateau was thought to be caused by the saturation of the nucleation effect of SC crystallites. To check if the nucleation effect of SC crystallites has been saturated at a PDLA concentration of 2 wt % for the asymmetric blends, the nucleation efficiency of the formed SC crystallites was employed and obtained by following the procedure originally outlined by Wittmann et al.36,39 The nucleation efficiency (NE) can be calculated from the crystallization temperature (Tp) by using eq 336,39 1445

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Figure 9. (a) Selected DSC cooling scans at a rate of 2.5 °C/min after self-nucleation of neat PLLA at different self-nucleation temperatures (Ts). The peak values Tp (°C; left) along with ΔHc values (J/g; right) are shown below each exotherm. (b) Selected DSC heating curves at a rate of 10 °C/min for the samples after being annealed at different Ts for 5 min.

behave as a nearly ideal nucleating agent for PLLA. Compared with their results, a NE of 67.8% is far from the highest NE of SC crystallites, and there still exists a relatively large space for further increase of NE with increasing PDLA concentration above 2 wt %. Thus, it can be concluded that the highest crystallization promoting effect of SC was not achieved at a concentration of 2 wt % PDLA yet. Nucleation and Crystalline Morphologies during Isothermal Crystallization of PLLA/PDLA Blends. In order to further reveal the crystallization behaviors of the blends, the nucleation and crystalline morphologies of the blends were investigated by in-situ POM observation. Typical POM micrographs of spherulites imaged during the crystallization process of neat PLLA and PLLA/PDLA blends at 145 °C for different time are shown in Figure 11. The last row presented the eventual morphologies of these crystallized samples. As expected, with the addition of PDLA (the formed SC crystallites), much more spherulites appear at the beginning of crystallization and result in a mass of small-size spherulites in the final morphologies. The spherulites seem to be uniform in size. Considering that SC crystallites provide nucleation sites during the crystallization of PLLA, it suggests that the spatial distribution of SC crystallites is uniform in micrometer scale in the PLLA/PDLA blends. In order to describe quantitatively the

Figure 10. Nucleation efficiency (NE) of SC crystallites to PLLA at various PDLA concentrations.

As shown in Figure 10, the nucleation efficiency is rapidly increased with increasing concentration of PDLA for the blends with a concentration of PDLA lower than 2 wt %, and a NE = 67.8% is obtained for sample LD-2.0. Hillmyer et al.19 discussed the nucleation efficiency of the SC crystallites acting as a nucleating agent for PLLA. They observed a high NE of 94%, very close to 100%, for their melt-blended asymmetric blend with 3 wt % of 18 kg mol−1 PDLA and concluded that SC can

Figure 11. Selected POM micrographs during isothermal crystallization at Tc of 145 °C for the samples of (A) neat PLLA, (B) LD-0.5, (C) LD-1.0, (D) LD-2.0, and (E) LD-5.0. The scale bar in the bottom micrograph of panel A represents 100 μm and applies to all the micrographs. The crystallization time is indicated in the micrographs. 1446

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increasing content of PDLA or SC crystallites and increases prominently when the network forms on the account of that PLLA chains in the molten state are mostly confined by the network structure. As a result, for the blends with a PDLA concentration higher than the critical PDLA concentration (2 wt % here), at which the network forms, the confining effect is so high that further increased crystallization promoting effect is offset, and thus the overall crystallization rate keeps independent of the PDLA concentration.

effect of formed SC crystallites on the nucleation density of PLLA, the number of nuclei was counted and presented in Table 2. It can be seen that the addition of PDLA accelerates Table 2. Numbers of Nuclei Counted for Neat PLA and PLLA/PDLA Blends with Various PDLA Concentrations at Tc = 145 °C neat PLLA 3

LD-0.5 101

LD-1.0 231

LD-2.0 to LD-5.0 too many to be counted

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CONCLUSIONS This work has shown the existence of a SC crystallite network in the melt of asymmetric PLLA/PDLA blends and the confining effect of the formed SC crystallites and the SC crystallite network on the crystallization of PLLA. First, with the incorporation of PDLA, the SC crystallites were successfully introduced to asymmetric PLLA/PDLA blends through a low temperature approach. Investigation on the rheological properties of the melts embedded by SC crystallites showed that the presence of the formed SC crystallites can reinforce the PLLA melt due to the filler effect and cross-linking effect of SC. Moreover, a transition from the liquid-like to solid-like viscoelastic behaviors was observed, which exactly reveals the existence of the SC crystallite network in the melt. With the formation of the network, the reinforcing effect of SC crystallite was sharply increased. From our designed dissolution experiment, it can be concluded that the formed network should be formed by the rigid SC particles and the interparticle polymer chains which are significantly restrained by the cross-linking effect of SC crystallites. Moreover, the nonisothermal and isothermal crystallization behaviors of PLLA/PDLA blend with various PDLA concentrations were studied by DSC and POM. A special PDLA concentration dependence for the crystallization promoting effect of the formed SC crystallite on the crystallization rate of PLLA; e.g., with increasing PDLA concentration, the crystallization rate of PLLA/PDLA blends increases sharply below the critical corresponding concentration of PDLA for network formation, but almost keeps constant above the critical corresponding concentration of PDLA, was observed during the nonisothermal crystallization process and was verified during the isothermal crystallization process. By a selfnucleation experiment, the possibility that the highest nucleation promoting effect has been reached at a PDLA concentration of 2 wt %, was excluded. By the POM observation, the higher nucleation density of PLLA/PDLA blends with a PDLA concentration of 5 wt % was higher than that with 2 wt % concentration. With more formed SC crystallites, the nucleation was further improved but the overall crystallization rate almost kept constant, which reveal that there exists a confining effect on the crystallization of PLA. This confining effect can be ascribed to that the diffusion ability of PLLA chains is restrained by SC crystallites and the SC crystallite network. Our work provides some guiding information on using SC as a rheological modifier or a nucleation agent in actual industrial applications.

the nucleation of PLLA dramatically; the nucleation density increases to 77 times that for neat PLLA by adding 1 wt % PDLA. For samples LD-2.0 and LD-5.0, too many spherulites crowd together, making it difficult to count their numbers. However, their nucleation densities can be still compared qualitatively from the POM micrographs. As shown, a larger density of spherulites at the beginning of crystallization and a smaller spherulites size at the final morphologies were observed for sample LD-5.0 compared with that of sample LD-2.0, indicating a larger nuclei density in sample LD-5.0. As is known, SC crystallites increase the crystallization rate of PLLA by acting as heterogeneous nucleation sites, so with increasing concentration of PDLA, more SC crystallites were formed (as shown in Table 2); thus, more nucleation sites or surface can be provided. With more heterogeneous nucleation sites, the overall crystallization rate of PLLA should be further increased. It is for sure that the nucleation density was further enhanced by more SC crystallites when increasing the PDLA concentration from 2 to 5 wt % (seen in Figure 11). Okada et al.53 have reported that the induction time of nucleation does not depend on the concentration of the nucleating agent. The acceleration of polymer crystallization is, thus, achieved by the increases in nucleation density by adding more nucleating agent. However, the overall crystallization rate of the blends almost keeps constant with further increasing the concentration of PDLA (see Figures 4 and 7). An enhanced nucleation density but an almost unchanged overall crystallization rate for sample LD-5.0 compared to sample LD-2.0 indicates that the expected increase of the crystallization promoting effect from more SC crystallites is offset by some confining effect. Based on the rheological results (seen in Figure 2), it is clear that the motion of polymer chains is restrained by the previous formed SC crystallites due to their filler effect and cross-linking effect. Moreover, this restraining effect shows itself more remarkably after the SC crystallite network forms as the transition from the liquid-like to solid-like viscoelastic behaviors occurs. Typical crystallization of polymers is a process in which polymer chains orderly fold into crystal lattices (crystal growth) after stable nuclei form (nucleation), so the chain mobility can influence the crystallization process and its rate prominently. The presence of the formed SC crystallites restrains the motion of PLLA chains in the melt (for example, by the increase of viscosity), which exactly results in the confinement for crystallization process, principally crystal growth. Thus, the effect of SC crystallites on the crystallization rate of PLLA is twofold: (i) enhancing the nucleation rate by providing heterogeneous nucleation sites or surface; (ii) confining the crystallization by its restraining effect on the mobility of PLLA chains. At a low concentration of PDLA (or SC crystallites), this confining effect is quite weak, and thus, the crystallization promoting effect is dominant, resulting in a sharp increase of the crystallization rate. The confining effect increases with

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*Tel/Fax + 86 28 8546 0130; e-mail [email protected] (W.Y.). 1447

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NNSFC Grants 21374065 and 51121001), the MOST (Grants 2011CB606006 and 2012CB025902), and the Fundamental Research Funds for the Central Universities (Grant 2011SCU04A03).

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