Stereocomplex Crystallite-Assisted Shear-Induced Crystallization

Dec 1, 2015 - A series of asymmetric biodegradable poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blends with low PDLA compositions was prepared using ...
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Stereocomplex Crystallite-Assisted Shear-Induced Crystallization Kinetics at a High Temperature for Asymmetric Biodegradable PLLA/ PDLA Blends Jing Bai,† Junyang Wang,† Wentao Wang,† Huagao Fang,‡ Zhaohua Xu,§ Xuesi Chen,∥ and Zhigang Wang*,† †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China ‡ Provincial Key Laboratory of Advanced Functional Materials and Devices, Institute of Polymer Materials and Chemical Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui Province 230009, P. R. China § Department of Material Technology, Jiangmen Polytechnic, Jiangmen, Guangdong Province 529090, P. R. China ∥ CAS Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China S Supporting Information *

ABSTRACT: A series of asymmetric biodegradable poly(Llactide) (PLLA)/poly(D-lactide) (PDLA) blends with low PDLA compositions was prepared using a solution blending method. The formation of stereocomplex (SC) crystallites in PLLA/PDLA blends was evidenced by differential scanning calorimetry (DSC), as indicated by the melting point of SC crystallites being about 50 °C higher than that of PLLA homocrystallites. Isothermal crystallization kinetics under shear conditions at the specific high temperature of 160 °C for the PLLA/PDLA blends was investigated using polarized optical microscopy (POM) and rheometry. It was found that the crystallization process of PLLA in the blends was greatly accelerated under shear conditions due to the existence of SC crystallites and the crystallization kinetics of PLLA was promoted with increasing shear rate or shear time. The crystalline morphology remained spherulitic with the spherulitic growth rates unaltered at the applied shear conditions, and the accelerated crystallization kinetics could be attributed to the significantly enhanced nucleation density, for which the extra number of activated nuclei was correlated to shear as a kinetic model to assess the effects of shear on isothermal crystallization kinetics of PLLA/PDLA blends containing SC crystallites. The discrete Maxwell relaxation time spectra at the applied isothermal crystallization temperature of 160 °C were used to obtain the reptation and Rouse times of PLLA chains with high molecular masses. Even though the PLLA chains might be orientated under the applied shear, the relaxation time of the blends was still too short to induce any orientated crystal nuclei. KEYWORDS: Stereocomplex, Shear, Spherulite, Nucleation, Crystallization, Rheology

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INTRODUCTION Polylactide (PLA) shows excellent performance in the terms of biocompatibility, biodegradability, renewability, and applicable mechanical properties. Thus, PLA has attracted extensive attention in recent decades as a promising biopolymer.1−3 However, PLA also shows limited applications due to its low crystallization rate, poor thermal stability, and low thermal distortion temperature. Many studies to improve these properties have been carried out. Ikada et al. reported for the first time that stereocomplex (SC) crystallites could form in poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blends, and SC crystallites showed a melting point about 50 °C higher than that of PLA homocrystallites.4−6 The SC crystallites formed between enantiomeric PLLA and PDLA could enhance the © 2015 American Chemical Society

properties of PLA-based materials, such as thermal stability, thermal resistance, hydrolysis resistance, and mechanical performance, which was thought to result from a peculiar interaction between L-lactyl and D-lactyl unit sequences.5,7−11 For example, Tsuji et al. reported that the enantiomeric blending successfully enhanced thermal stability of PLLA/ PDLA films as compared with neat PLLA and PDLA films.7 Tsuji also reported that hydrolysis resistance of PLLA could be improved by blending PLLA with PDLA.12 Rathi et al. found that blending PLLA with a triblock copolymer (PDLA-b-PEGReceived: September 17, 2015 Revised: November 18, 2015 Published: December 1, 2015 273

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b-PDLA) with a stereocomplex formation could toughen PLLA.13 People can pursue the utility of PLA through the property improvements of this biomass-based polymer. The above-mentioned various property enhancements are certainly related to the enhancements of crystallization kinetics for asymmetric PLLA/PDLA blends. Brochu et al. pointed out that the stereocomplexation in asymmetric PLLA/PDLA blends could occur with an addition of as low as 10 wt % PDLA.14 Yamane and Sakai showed that the SC crystallites behaved as a nucleating agent for PLLA crystallization when blended with PDLA of low amounts (1−5 wt %).15 Tsuji et al. also reported the effects of SC crystallites on isothermal and nonisothermal crystallization of PLLA from melt blending with PDLA in a composition range from 0.1 to 10 wt %.16 On the other hand, when semicrystalline polymers experience the processing and shaping operations, such as injection molding, film blowing, and fiber spinning, etc., an intensive shear and/or elongation flow field can make the polymer chains highly oriented,17−21 which often accelerates the crystallization kinetics of polymers by orders of magnitude and meanwhile obviously changes the crystalline morphologies of the products. Therefore, shear-induced crystallization of polymers has been extensively investigated considering both scientific and industrial values from the experimental22−24 and theoretical studies25−27 to computational studies,28−30 which affects the product properties and performance. For examples, Fang et al. demonstrated that shear could greatly enhance the crystallization kinetics of PLLA as compared with quiescent conditions,22 and Wei et al. demonstrated that shear and SC crystallites could provide a notable synergistic effect on an increase of PLLA crystallization rate at the relatively low crystallization temperatures.23 Because both the sterecomplex formation and shear flow field make dramatic contributions to crystallization kinetics enhancement of PLLA/PDLA blends, it is necessary to investigate the combined effects of SC crystallites and shear flow field. In this study, a series of asymmetric PLLA/PDLA blends with different PDLA compositions was prepared by using solution blending method. Polarized optical microscopy (POM), rotational rheometry, and differential scanning calorimetry (DSC) were used to evaluate the effects of shear on isothermal crystallization kinetics at a high temperature for these asymmetric PLLA/PDLA blends with the initially formed SC crystallites. The changes in both nucleation density and spherulitic growth rate for PLLA/PDLA blends were obtained to study the effects of SC crystallites on shear-induced isothermal crystallization kinetics in these asymmetric PLLA/ PDLA blends. We emphasize that the shear-induced crystallization kinetics for these asymmetric PLLA/PDLA blends at the high temperature close to the nominal melting point of PLLA was rarely reported,22−24 which is considered to be more crucial for the industrial processing of PLLA materials because the processing and manufacture are normally performed starting from the high temperature and PLLA usually does not crystallize during the subsequent quench even under the shear conditions due to the much slow shear-induced crystallization rate. We further intend to examine the possible formation of any oriented crystalline morphology at the high temperature under the shear conditions for these asymmetric PLLA/PDLA blends by using POM.

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EXPERIMENTAL SECTION

Materials and Sample Preparation. Poly(L-lactide) (PLLA, grade 4032D) purchased from NatureWorks (China) had weight- and number-average molecular masses, Mw and Mn, of 160 and 96 kg/mol, respectively, and polydispersity, Mw/Mn of 1.7. Poly(D-lactide) (PDLA) synthesized through ring-opening polymerization of D-lactide at the Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (China), had Mw and Mn values of 74 and 46 kg/mol, respectively, and Mw/Mn of 1.6. Before dissolved in chloroform separately PLLA and PDLA were dried in a vacuum oven at 60 °C for 12 h. For preparation of PLLA/ PDLA blends, PLLA and PDLA solutions with certain volume ratios were mixed with vigorous stirring for 3 h. Then, the mixed solutions were poured into Petri dishes to allow solvent evaporation. The prepared PLLA/PDLA blends were further dried in a vacuum oven at 40 °C for 72 h. The obtained PLLA/PDLA blends with 0.5, 2.0, 5.0, and 10.0 wt % PDLA compositions were abbreviated as LD-0.5, LD-2, LD-5, and LD-10, respectively. Note that neat PLLA was subjected to the same preparation procedure for the comparison purpose. Differential Scanning Calorimetry (DSC). A differential scanning calorimeter (DSC TA Q2000, TA Instruments) was used to measure the melting temperatures of neat PLLA and PLLA/PDLA blends. For all the DSC measurements 5.0−8.0 mg of the samples were taken. The samples were heated from room temperature to 200 °C at a heating rate of 50 °C/min. After being held at 200 °C for 5 min, the samples were cooled to 25 °C at a cooling rate of 50 °C/min; then, the samples were heated again to 240 °C at a heating rate of 10 °C/min. Polarized Optical Microscopy (POM). POM was used to observe the nucleation and spherulitic growth for PLLA/PDLA blends during isothermal crystallization after application of shear. The polarized optical microscope (Olympus BX51) was equipped with an optical shearing system (Linkam CSS-450, Linkam Scientific Instruments). In the Linkam CSS-450 the gap between the two quartz windows was set at 20 μm. PLLA/PDLA blend films were melted at 200 °C for 5 min to hold the comparable thermal histories and then cooled to 160 °C at a cooling rate of 30 °C/min. Step shears at different shear rates or shear times were applied to PLLA/PDLA blend films when the sample temperature reached 160 °C. Just after the shear cessation, the polarized optical micrographs were taken at appropriate time intervals. Therefore, the changes of nucleation density and spherulitic growth rate of PLLA with shear rate or shear time could be well followed. The micrograph taking could be stopped when impingements of the growing spherulites were observed. Rotational Rheometry. A rotational rheometer (AR2000EX, TA Instruments) was used to measure the rheological properties and isothermal crystallization kinetics for neat PLLA and PLLA/PDLA blend samples after shear application. The samples were compression molded at 200 °C into circular discs of 25 mm in diameter and 1 mm in thickness by using a homemade vacuum laminator. For the rheological property measurements, dynamic frequency sweeps in the linear viscoelastic regime were performed at 180 °C with a frequency range from 500 to 0.05 rad/s. Prior to the dynamic frequency sweeps, the disk-shaped samples were kept at 200 °C for 5 min to remove previous thermal histories. The isothermal crystallization processes for PLLA/PDLA blends after shear were studied by using the rotational rheometer in an oscillatory time sweep mode. A disk-shaped sample was kept at 200 °C for 5 min to remove previous thermal histories. Then, the sample was cooled to 160 °C at a cooling rate of 15 °C/min and, subsequently, was subjected to a step shear pulse. Thereafter, the shear-induced isothermal crystallization process of the sample was followed by a time sweep test, for which an angular frequency of 5 rad/ s and a strain of 0.5% were chosen. Basically the time sweep test provided the changes in storage modulus of the sample with time during the crystallization process. Note that the strain sweep test from 0.1 to 200% was performed at an angular frequency of 5 rad/s to determine the linear viscoelastic regime of the blend samples. Furthermore, the blend samples were subjected to a 30% strain in 274

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RESULTS AND DISCUSSION Stereocomplex Formation in PLLA/PDLA Blends Evidenced by DSC. The DSC heat flow curves for PLLA/ PDLA blends with different PDLA compositions are displayed in Figure 1. Neat PLLA shows typical three physical transitions,

Figure 1. DSC heat flow curves during heating scan at a heating rate of 10 °C/min for neat PLLA and asymmetric PLLA/PDLA blends with different PDLA compositions. Prior to the heating scan, the samples were rapidly cooled from the molten state at 200 °C to 40 °C at a cooling rate of 50 °C/min.

that are glass transition (Tg), cold crystallization (Tc), and melting of crystals (Tm), consistent with our previous reports.31,32 The DSC heat flow curves of PLLA/PDLA blends exhibit three specific features. The most interesting feature is that a melting peak appears around 216 °C, which illustrates the existence of SC crystallites due to the formation of stereocomplex.4 The amount of SC crystallites of PLLA/PDLA blends increases with increasing PDLA composition, which can be reflected by the increasing melting peak area. The second feature is that when SC crystallites preexist in PLLA/PDLA blends, the cold crystallization of PLLA becomes much more obvious, which indicates that SC crystallites serve as an effective nucleating agent to enhance PLLA crystallization. Accordingly, a third feature is that the melting of PLLA/PDLA blends gives out double melting peaks, which is similar to the case when a foreign nucleating agent is added in PLLA.32 We further note that the glass transition temperature, Tg at about 60 °C does not change obviously with increasing PDLA composition, indicating that the stereocomplex formation does not affect much the chain segmental motions in the blends. Stereocomplex Formation in PLLA/PDLA Blends Evidenced by Rheology. Figure 2 shows the frequency dependences of storage modulus, G′, loss modulus, G″, and loss tangent, tan δ, for the studied samples as measured at 180 °C. G′ and G″ of PLLA/PDLA blends at low frequencies increase with increasing PDLA composition and the changes of G′ with frequency are more obvious than that of G″. Note that the data points of G″ for neat PLLA and LD-0.5 are almost overlapping, which is shown in Figure S1 for clarification. Neat PLLA shows the frequency dependences of G′ ∝ ω2 and G″ ∝ ω, indicating that the viscoelastic behavior results from the longest relaxation times. The slopes of the modulus curves at low frequencies decrease with increasing PDLA composition. The more obvious nonterminal behaviors suggest slower relaxation in melt of the asymmetric blends, which evidence the formation of SC

Figure 2. Changes of (a) storage modulus, G′, (b) loss modulus, G″, and (c) loss tangent, tan δ, as functions of frequency for neat PLLA and asymmetric PLLA/PDLA blends with different PDLA compositions as measured at 180 °C.

crystallites because the existing SC crystallites restrain the long-range motions of PLLA chains. Loss tangent, tan δ (= G″/G′) characterizes the relaxation behavior, which behaves more sensitive than G′ and G″ to relaxation. Figure 2c shows that tan δ of neat PLLA decreases with increasing frequency, illustrating that neat PLLA behaves as a typical viscous liquid. For PLLA/PDLA blends, tan δ values at low frequencies decrease gradually with increasing PDLA composition, reflecting the enhanced elastic response of the melts with increasing content of SC crystallites. When the PDLA composition reaches 0.5 wt %, a plateau of tan δ in the low frequency region can be seen where the change of tan δ is independent of frequency, indicating that the elastic response in the melt starts to be dominant.33 This result also demonstrates that a physical network of SC crystallites forms in PLLA/PDLA blends. Wei et al. reported that a similar event appeared at the PDLA composition of 2.0 wt % for their PLLA/PDLA blends, disclosing the network formation of SC crystallites.34 Thus, the 275

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Figure 3. Selected POM micrographs taken during isothermal crystallization at Tc of 160 °C for LD-0.5 after sheared at the shear rate of 1 s−1 with different shear times of (A) 0, (B) 10, (C) 20, (D) 30, and (E) 40 s. Crystallization time is indicated in each micrograph. The scale bar represents 100 μm for all the micrographs. The olive arrow indicates the shear direction.

significant, which is attributed to the relatively low shear intensity, insufficient for strong macromolecular stretching even if with a long shear time of 40 s, and to the low content of SC crystallites in LD-0.5, producing a weak crystallite network for deformation. The POM micrographs taken for LD-2, LD-5, and LD-10 are shown in Figures S2−S4. As same as for LD-0.5, all the PLLA spherulites show the typical Maltese cross pattern, and the nucleation density increases gradually with increasing shear time. With increasing PDLA composition, much more point-like nuclei form under POM, especially for LD-5 and LD10. However, no row-like nuclei appear with increasing shear time, which may result from the relatively low shear intensity. The nucleation densities formed in LD-0.5 can be obtained from the micrographs as shown in Figure 3. The number of spherulites, NA, in the micrograph taken from POM can be counted and, then, the volume nucleation density, N, can be calculated using a simple approximation according to eq 1, where A is the observed sample area in the micrograph and H is the sample thickness, which is 20 μm according to the experimental setup.

formation of SC crystallite network is verified for our blend system, which is expected to have dramatic influences on quiescent and shear-induced crystallization of PLLA/PDLA blends, which are presented in the following sections. Shear Rate and Time Dependences of Isothermal Crystallization Kinetics for PLLA/PDLA Blends. The effects of shear on the crystallization kinetics for PLLA/PDLA blends as studied by using POM equipped with a shear hot stage are presented in this section. In order to gain insight into the effects of shear on the crystallization kinetics of the asymmetric PLLA/ PDLA blends, different shear conditions were applied. The shear rates were chosen as high as to guarantee Wis > 1, where Wis was the Weissenberg number,35 at which the stretch of the high molecular mass chains of PLLA was expected.36 The detailed discussion about this can be found in the last section. Because the SC crystallites start to form the network in LD-0.5 as evidenced by rheological measurements, the results of shearinduced crystallization for LD-0.5 are presented at first. The morphological evolution for LD-0.5 during isothermal crystallization at Tc of 160 °C after sheared at the shear rate of 1 s−1 with different shear times is shown in Figure 3. In our previous work, we found that it was difficult for neat PLLA to crystallize isothermally under quiescent condition at the temperatures higher than 140 °C.37 However, in this study the much higher temperature of 160 °C was selected to apply for clarifying the nucleating effect of SC crystallites on crystallization kinetics for PLLA/PDLA blends. We note that the temperature of 160 °C is close to the nominal melting point of PLLA (Tm = 168 °C). It is interesting to find from Figure 3 that at such a high crystallization temperature one PLLA spherulite appears in LD0.5 without any shear conditions, indicating that SC crystallites can induce nucleation for PLLA crystallization at the high temperature.34 Furthermore, it is evident that the PLLA spherulites formed in LD-0.5 with or without an assistance of shear show the typical Maltese cross pattern. At the applied shear rate of 1 s−1 the nucleation density increases with increasing shear time. However, the increase of nucleation density with increasing shear time for LD-0.5 is not much

N = NA /(AH )

(1)

To further assess the shear contribution with different shear intensity to the nucleation density enhancement in PLLA/ PDLA blends, it is necessary to introduce a criterion, the specific work, w, which has been demonstrated to be effective in the literature.38,39 The w values at different shear conditions can be calculated as follows:40 w = η̅

∫0

ts

γ 2̇ (t ) dt

(2)

where η̅ is the viscosity, averaged over the experienced shear rates, the value of which can be obtained by a peak hold step before a time sweep procedure, γ̇ is the time dependent shear rate, and ts is the total shear time. Shear can enhance the nucleation rate for crystallization as seen from Figure 3. Figure 4a displays the change of nucleation density, N, as a function of specific work, w, for LD-0.5 276

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displays the radial growths of spherulites with crystallization time for LD-0.5 during crystallization at 160 °C at different shear conditions. The slopes of the fitted lines are the spherulitic growth rates, G. The G values for LD-0.5 under shear with the shear times of 0, 10, 20, 30, and 40 s are around 0.1 μm/min. The result indicates that the shear time does not give obvious effect on the spherulitic growth rate for LD-0.5 at such a high crystallization temperature. For LD-0.5 the crystallization kinetics with applications of shear at different shear rates with the constant shear time of 10 s is presented in this section. Figure 5 shows the selected POM micrographs, which indicates that with increasing shear rate the nucleation density in LD-0.5 is significantly improved and the growing spherulites show dramatically shortened impingement time. Plenty of point-like nuclei appear at the shear rates of 10 and 20 s−1, consistent with the result in our previous work.24 POM micrographs for LD-2, LD-5, and LD-10 during isothermal crystallization at Tc of 160 °C with the shear time of 10 s at different shear rates are shown in Figures S5−S7. The nucleation density increases rapidly with increasing shear rate for LD-2, LD-5, and LD-10. The point-like nuclei are much easier to form in PLLA/PDLA blends with higher PDLA compositions. Different from our forecast, row-like nuclei cannot be seen with increasing shear rate. The reason will be provided in details in the last section. The results about the nucleation rate and spherulitic growth rate in LD-0.5 for various shear rates are shown in Figure 6. Shear can strongly enhance the formation of nuclei. The nucleation density for LD-0.5 with no shear is about 3.9 × 1011 m−3, while the nucleation density for LD-0.5 with the shear rate of 20 s−1 is about 2.0 × 1014 m−3, which increases by a factor of about 513. Therefore, the dramatic enhancement of nucleation density through shear contributes mainly to the crystallization kinetics of LD-0.5. Figure 6b shows that the spherulites grow linearly with crystallization time and the fitted lines are almost parallel. The G values for LD-0.5 with the shear rates of 0, 1, 5, 10, and 20 s−1 are around 0.1 μm/min. The result indicates that the

Figure 4. (a) Change of nucleation density, N, as a function of specific work, w, and (b) changes of spherulite radius as functions of time for LD-0.5 during isothermal crystallization at Tc of 160 °C after sheared at the shear rate of 1 s−1 with different shear times, ts.

crystallized at Tc of 160 °C. The nucleation density of LD-0.5 in quiescent crystallization is about 3.9 × 1011 m−3, while the nucleation density with the shear time of 40 s becomes 1.3 × 1013 m−3, which increases by a factor of about 33. The changes in the spherulitic growth rate are further evaluated for LD-0.5 at different shear conditions. Figure 4b

Figure 5. Selected POM micrographs taken during isothermal crystallization at Tc of 160 °C for LD-0.5 after sheared with the constant shear time of 10 s at different shear rates of (A) 0, (B) 1, (C) 5, (D) 10, and (E) 20 s−1. Crystallization time is indicated in each micrograph. The scale bar represents 100 μm for all the micrographs. The olive arrow indicates the shear direction. 277

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this study. The POM observation is presented at first. Figure 7 shows the nucleation and spherulitic growth during isothermal crystallization at Tc of 160 °C for neat PLLA and PLLA/PDLA blends with different PDLA compositions. These samples were sheared at the shear rate of 10 s−1 for 10 s prior to the crystallization. The typical Maltese cross pattern for spherulites is visible in the micrographs for neat PLLA after the shear application and crystallization. While at the applied shear condition, PLLA/PDLA blends show much high nucleation density, with the nuclei densely filling up the sample volume. Therefore, to count the numbers of nuclei in the blend samples does not sound reasonable, especially for the blends with relatively higher PDLA compositions. However, the crystallization times shown in these micrographs indicate that the completion of crystallization takes shorter times for the blends with relatively higher PDLA compositions. The result shown in Figure 7 infers some measurement limitation of the POM observation because the nucleation for PLLA/PDLA blends with high PDLA compositions is enhanced so strongly that it is difficult to count the nucleus numbers precisely. Therefore, rheological measurements were applied to resolve this problem. This widely employed rheological technique was proved to be effective in investigating the shear-induced crystallization for semicrystalline polymers due to its important advantages.41−43 In general, the rapid raise in storage modulus, G′ can be well documented by the nucleation and growth of crystals, for which the filling effects of the growing crystals can be considered as a suspension of particles in an amorphous polymer matrix. Isothermal crystallization processes at 160 °C for neat PLLA and PLLA/ PDLA blends after sheared at the shear rate of 10 s−1 for 10 s were tracked by using a rotational rheometer and the results are shown in Figure 8. A sigmoidal shape represents the changes of storage modulus, G′ with time during crystallization (Figure 8a). G′ keeps about constant values prior to the beginning of crystallization, rapidly rises when crystallization starts and then

Figure 6. (a) Change of nucleation density, N, as a function of specific work, w, for LD-0.5 crystallized at Tc of 160 °C with different shear conditions. (b) Changes of spherulite radius as functions of time for LD-0.5 crystallized at Tc of 160 °C after sheared at different shear rates for the shear time of 10 s.

spherulitic growth rates for LD-0.5 at different shear rates are almost same, indicating that shear does not give much effects on spherulitic growth rate for PLLA/PDLA blends. Shear-Induced Crystallization Kinetics for PLLA/PDLA Blends with Different PDLA Compositions. It is certainly necessary to investigate the effect of PDLA composition on shear-induced crystallization kinetics for PLLA/PDLA blends in

Figure 7. Selected POM micrographs taken during isothermal crystallization at Tc of 160 °C for (A) PLLA, (B) LD-0.5, (C) LD-2, (D) LD-5, and (E) LD-10 after sheared at the shear rate of 10 s−1 for the shear time of 10 s. Crystallization time is indicated in each micrograph. The scale bar represents 100 μm for all the micrographs. The olive arrow indicates the shear direction. 278

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moduli prior to the beginning of crystallization increase with increasing PDLA composition for the blends, which is consistent with the increasing content of SC crystallites formed in the blends because these SC crystallites form crystallite network, contributing to the storage modulus enhancement. To remove the effect of initial modulus difference, the relative crystallinity, x(t), can be obtained by logarithmic normalization of the G′ values, which was proposed by Pogodina et al. as shown in eq 3:45 x(t ) =

′ log G′(t ) − log Gmin ′ ′ log Gmax − log Gmin

(3)

where G′min and G′max are the starting storage modulus and ending plateau storage modulus, respectively. Figure 8b displays the changes of relative crystallinity with time for neat PLLA and PLLA/PDLA blends during crystallization at Tc of 160 °C after sheared at the shear rate of 10 s−1 for 10 s. Much long time is needed for neat PLLA to finish the primary crystallization, while the times needed for PLLA/PDLA blends decrease obviously with increasing PDLA composition. The crystallization half-time, t1/2, at which the relative crystallinity reaches a value of 0.5 can be used to indicate the crystallization rate. The change of crystallization half-time, t1/2, as a function of PDLA composition for PLLA/PDLA blends during isothermal crystallization at Tc of 160 °C after sheared at the shear rate of 10 s−1 for 10 s is shown in Figure 8c. The blend LD-10 with the highest PDLA composition apparently possesses the lowest t1/2 value, accordingly, possessing the fastest crystallization rate. The obvious enhancement in crystallization kinetics for the blends is attributed to the increase of nucleation density because of the combined effect of shear and SC crystallite network, which becomes stronger with increasing PDLA composition. In general, the space-filling effect by growing spherulites in quiescent isothermal crystallization can be described by the Avrami eq (eq 4). All nuclei, N, are assumed to appear at the same time, t0, and the spherulitic growth keeps a constant rate, G(t). ⎤ ⎡ 3 ϕ(t ) = 1 − exp⎢ − πNG(t )3 (t − t0)3 ⎥ ⎦ ⎣ 4

Figure 8. (a) Changes of storage modulus, G′, with time, (b) changes of relative crystallinity, x(t), with time, and (c) changes of induction time, t0, and crystallization half-time, t1/2, as functions of PDLA composition for PLLA/PDLA blends during isothermal crystallization at Tc of 160 °C after shearing at the shear rate of 10 s−1 for the shear time of 10 s.

(4)

In our POM measurements, nucleation occurs during shear and then the spherulites start to grow after shear cessation. Moreover, our POM observations on the crystalline morphologies of PLLA/PDLA blends demonstrate that shear does not change the nature of point-like nucleation feature, that is to say, the shish-like nucleation does not exist. This is true even for the lower crystallization temperatures that we selected to use (see Figure S8 in the Supporting Information). From Figures 4 and 6, it can be seen that the shear conditions do not have influence on the spherulitic growth rates of PLLA at Tc of 160 °C. In order to be able to measure the radii of spherulites, a shear at the shear rate of 1 s−1 for 10 s was applied to PLLA/PDLA blends and the result is shown in Figure S9 in the Supporting Information. The spherulitic growth rates at 160 °C for PLLA/ PDLA blends with different PDLA compositions are all about 0.1 μm/min (0.17 × 10 −8 m/s). We conclude that the shear conditions applied in this study do not bring out effects on spherulitic growth rate at the high temperature of 160 °C for PLLA in the blends. The empirical equation to decide the relative crystallinity can be used to obtain the degree of space filling. Under the

reaches the plateau values at the late stage of primary crystallization.22,24,44 The sigmoidal curves in Figure 8a show different initial induction times for the modulus upturn and different initial constant moduli prior to the beginning of crystallization. The induction time for the modulus upturn defined as the induction time, t0, for crystallization can be obtained by taking the intersection time of the two dashed lines drawn on the storage modulus−time curve for neat PLLA as an example. The storage modulus−time curves shift obviously to the left side, demonstrating obviously reduced induction time with increasing PDLA composition for the blends. At the applied temperature of 160 °C the induction times for neat PLLA and LD-10 of about 4444 and 236 s, respectively, are needed for the onset of crystallization, which infers an induction period shortening by 16 times when PDLA of 10 wt % is mixed with PLLA. In addition, the initial constant 279

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ACS Sustainable Chemistry & Engineering

increased by more than 3 orders of magnitude. The change of nucleation density as a function of specific work for PLLA and PLLA/PDLA blends is shown in Figure 9c. It is seen that at the same shear condition, more specific work is needed for PLLA/ PDLA blends when the PDLA composition increases because of the strengthened SC crystallite network. Relationship between Shear Conditions and Crystalline Morphology for PLLA/PDLA Blends. To understand the shear-induced crystallization of semicrystalline polymers gaining an insight of the macromolecular conformations in the shear flow field is crucial. In the above sections, shear-induced point-like nuclei appear in PLLA/PDLA blends without the row-like nuclei or shish-like morphology be observed. This section provides a reason why the row-like nuclei or shish-like morphology do not form. Before any discussion it is necessary to introduce two characteristic Wessenberg numbers, which discriminate the shear flow regimes correlating with the macromolecular conformations by van Meerveld: Wirep = τrepγ̇ and Wis = τRγ̇, with γ̇ being the shear rate and τrep and τR being the reptation time and the Rouse time of the high molecular mass chains, respectively.35 The regime transitions are briefly described as follows. For Wirep < 1 and Wis < 1, polymer chains stay in their equilibrium state and shear does not affect crystallization (Regime I). For Wirep > 1 and Wis < 1, polymer chains are oriented (Regime II). For Wis > 1, polymer chains are either stretched (Regime III) or strongly stretched for sufficient time to make the macromolecular stretch ratio, λ, larger than a critical value, λ*(T) (Regime IV). Enhancement of the number density of activated nuclei occurs in Regime II and sufficient stretch of polymer chains into a conformation ideal for the formation of oriented nuclei is ensured in Regime III. A minimum shear rate, 1/τR is obvious, below which the oriented nuclei cannot form and the shish-kebab morphology is unlikely. In order to obtain the Rouse time, τR for PLLA, dynamic frequency sweep measurements had been performed and the results are shown in Figure 2. The dynamic moduli, G′ and G″ are expressed in terms of discrete Maxwell relaxation time spectra (Gi, τi):46

assumption that all nuclei have formed after shear cession, the nucleation density can be estimated from the space filling value by using the Avrami eq (eq 5).38 The results for PLLA and PLLA/PDLA blends after sheared at the shear rate of 10 s−1 for the shear time of 10 s are shown in Figure 9. N=

−3ln[1 − ϕ(t )] 4πG(t )3 t 3

(5)

N

G′(ϖ) =

∑ i=1 N

G ″ (ϖ ) =

∑ i=1

Giω 2τi 2 1 + ω 2τi 2

(6)

Giωτi 1 + ω 2τi 2

(7)

where Gi and τi are the Maxwell parameters. The standard nonlinear regression method was used to obtain Gi and τi through calculating the G′ and G″ data (Trios Software, TA Instruments). A viscosity-averaged relaxation time, τ ̅ (= τrep) including individual discrete relaxation times is given in eq 8:47

Figure 9. (a) Changes of nucleation density as functions of space filling, (b) change of nucleation density as a function of PDLA composition, and (c) change of nucleation density as a function of specific work at Tc of 160 °C for PLLA/PDLA blends after sheared at the shear rate of 10 s−1 for shear time of 10 s.

N

τ̅ = Figure 9a indicates that for the space filling, ϕ, between 0.2 and 0.8 the nucleation density, N, keeps almost constant. The value at ϕ of 0.5 can be taken as the nucleation density for a reasonable approximation. The change of nucleation density as a function of PDLA composition at Tc of 160 °C for PLLA and PLLA/PDLA blends after sheared at the shear rate of 10 s−1 for 10 s is shown in Figure 9b. The nucleation density increases obviously with increasing PDLA composition. For an instance, the nucleation density for neat PLLA is about 1.0 × 1015 m−3, whereas the value for LD-10 becomes 1.7 × 1019 m−3, which is

∑i = 1 ητ i i N

∑i = 1 ηi

(8)

The Rouse time, τR, of PLLA can be obtained by using eq 9 according to the Tube model of Doi and Edwards:46 τrep τR = (9) 3Z where Z is the average entanglement number (Z = Mw/Me, Me is molecular mass between entanglements). The Z value is 20 by using Me of 8000 g/mol for PLLA.48 Table 1 lists the reptation time together with the Rouse time for PLLA at 180 280

DOI: 10.1021/acssuschemeng.5b01110 ACS Sustainable Chem. Eng. 2016, 4, 273−283

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Reptation Time, τrep, Rouse Time, τR, and Corresponding Critical Shear Rates for Regime Transitions of PLLA T (°C)

τrep (s)

τR (s)

γ̇I→IIa (s−1)

γ̇II→IIIa (s−1)

180 160

1.48 3.95

0.025 0.066

0.68 0.25

40.7 15.2

γ̇I→II and γ̇II→III are the critical shear rates for shear flow regime transitions between Regimes I and II and Regimes II and III, respectively. a

and 160 °C, respectively. The data at 160 °C were calculated by considering the temperature dependence of the Arrhenius type (Ea = 80 kJ/mol24). It must be noted that it is quite complicated to measure the Rouse time for PLLA/PDLA blends because of the existence of SC crystallite network in the system, which controls the deformation of PLLA/PDLA blends (Figure 2c). It is generally considered that the macromolecular chains are stretched and aligned along the shear direction when the melts are sheared. The aligned chains assemble to form the fibril bundles, which serve as nucleating sites for polymer crystallization. However, in our study even if the applied shear rate (such as 20 s−1) was raised above 1/τR (15.2 s−1 in Table 1), the POM results did not show the row-like nuclei in PLLA/PDLA blends at 160 °C. Although the lower crystallization temperatures were applied too, only much more point-like nuclei could be found and the row-like nuclei did not appear (Figure S8). We here provide an explanation to the above phenomenon. Different from the polymers with flexible chains such as polyethylene and polypropylene,49,50 PLLA chains are semirigid with relatively short chain lengths. Therefore, the longest shear time of 40 s we chose in this study might not be sufficiently long to stretch the PLLA molecular chains above the critical λ*(T), only leading to the nucleation density enhancement. For PLLA/PDLA blends the complex viscosity increases with increasing PDLA composition (Figure S10) because of the formation of SC crystallite network, which hinders the macromolecular chains to be stretched and aligned along the shear direction.51 Therefore, the strong SC crystallite network might shield the orientation of macromolecular chains if applying a weak shear pulse.23 On the other hand, at the relatively high isothermal crystallization temperature the macromolecular chains might relax much fast after the shear orientation, leading to the absence of row-like nuclei as confirmed by POM observations. We further performed the stress relaxation test to clarify our above explanation. Figure 10 shows the typical stress relaxation modulus curves (G(t, γ) versus time) for PLLA and LD-2 as measured at 160 °C. It can be seen that neat PLLA samples with or with no shear all present a rapid one-step relaxation behavior and can relax within 10 s, reflecting a relaxation mechanism for linear polymers and the shear conditions have no obvious effects on the relaxation of linear PLLA. On the contrary, the stress relaxation modulus curve for LD-2 with no shear is characterized by three relaxation stages: the first fast relaxation stage apparently due to the relaxation of neat PLLA, the second plateau region with about 31 s relaxation time, reflecting the relaxation of SC crystallite network, and the third terminal relaxation region finally appearing at the longer time.52,53 Note that when the crystallite network has been established, the internal structures might be effectively

Figure 10. Changes of stress relaxation modulus, G(t, γ), as functions of time for PLLA and LD-2 at 160 °C.

maintained after an applied shear. Therefore, it might still need a long time to relax.54 The stress relaxation modulus curve for LD-2 with the same shear condition as in the crystallization procedure shows that a relaxation time of 809 s is apparent. Although the crystallite network structure might be deformed at this shear condition, indicated by absence of the plateau relaxation region, the relaxation still takes a long time as compared with neat PLLA. The POM micrographs shown in Figure 7 for LD-2 after sheared at the shear rate of 10 s−1 for 10 s indicate that no nuclei appear within the initial induction time of at least 15 min (900 s). This comparison infers that even though the macromolecular chains in LD-2 have been orientated after applied shear, the relaxation time of the chains is still too short to induce any orientated crystal nuclei.

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CONCLUSIONS Shear-induced crystallization kinetics at the high temperature of 160 °C for asymmetric biodegradable poly(L-lactide) (PLLA)/ poly(D-lactide) (PDLA) blends with low PDLA compositions (no more than 10 wt %) were investigated by using polarized optical microscope and rotational rheometer. The introduction of low amounts of PDLA into PLLA led to the formation of SC crystallites during the blend preparation. The rheological measurements indicated the formation of SC crystallite networks in the blends even at the PDLA composition of 0.5 wt %. SC crystallites could act as a heterogeneous nucleating agent, accelerating the crystallization kinetics of PLLA through enhancing the nucleation density. With the application of various shear conditions, the crystallization kinetics of PLLA/ PDLA blends were further accelerated with increasing shear rate, shear time, and PDLA composition. With the combined effects of shear and SC crystallites preformed in the blends, the crystallization kinetics of PLLA/PDLA blends could be significantly enhanced mainly through the nucleation density enhancements at the chosen high crystallization temperature, which inferred that the introduction of SC crystallites at low amounts would be helpful for PLLA processing. Under the experimental shear conditions in this work, the enhanced pointlike nuclei were observed, while the shish-like or row-like nuclei were not seen at all. The stress relaxation test results demonstrated that for PLLA/PDLA blends even though the macromolecular chains might have been orientated after the applied shear, the relaxation time of the chains was still too short to induce any orientated crystal nuclei. 281

DOI: 10.1021/acssuschemeng.5b01110 ACS Sustainable Chem. Eng. 2016, 4, 273−283

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ACS Sustainable Chemistry & Engineering

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01110. Additional experimental data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0551-63607703. Fax: +86 0551-63607703. E-mail: [email protected] (Z.G.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.G.W. acknowledges financial support from the National Basic Research Program of China with Grant No. 2012CB025901 and National Science Foundation of China with Grant No. 21174139. The project is also supported by the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

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