Blends of Linear and Long-Chain Branched Poly(l-lactide)s

Article pubs.acs.org/IECR

Blends of Linear and Long-Chain Branched Poly(L‑lactide)s with High Melt Strength and Fast Crystallization Rate Liangyan Wang,†,‡ Xiabin Jing,† Haibo Cheng,†,‡ Xiuli Hu,† Lixin Yang,†,‡ and Yubin Huang†,* †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China ‡ Graduate School of Chinese Academy of Sciences, Beijing, 100039, P. R. China S Supporting Information *

ABSTRACT: The long-chain branched polylactides (LCB-PLAs) prepared by coupling the hydroxyl-terminated two-arm (linear) and triarm PLA prepolymers of identical arm length with hexamethylenediacianate (HDI) were used to improve the melt rheological and crystallization properties of linear polylactide resin, PLA 4032D from NatureWorks. The blends containing LCBPLA displayed higher zero shear viscosities, more significant shear shinning, more melt elasticity, and much longer relaxation times together with significant strain hardening in elongational deformation. Tg, Tm and crystallinity (Xc) of linear PLA remained virtually unaffected, but the crystallization rate increased obviously, since the branch points of LCB-PLAs could play a role of nucleating agent. High melt strength, fast crystallization, and favorable miscibility improved the foaming ability of the linear/ LCB-PLA blends, substantially.

1. INTRODUCTION Polylactide (PLA)1−3 with relatively good mechanical properties, thermal plasticity, and totally produced from renewable agriculture resources has been intensively studied for its large potential applications in the field of biomedical and various packaging products. However, relatively bad processing ability due to slow crystallization rate, low melt strength, and melt elasticity also limited its wide application in foaming and blowing.4−6 The rheological properties of PLA can be improved through several approaches including copolymerization,7−10 cross-linking,11−13 and blending. Long-chain branching (LCB)14 means that the molecular chain between branch points is long enough to entangle with other chains in the melt and thus to result in high melt strength. To overcome the shortcoming of low melt strength, long-chain branches were introduced into the polyester backbone. Reactive extrusion and high-energy irradiation are usually used to get LCB for polyesters such as poly(ethylene terephthalate) (PET),15 poly(butylene succinate) (PBS),16 and PLA.17 Branching reactions by reactive processing18 in bulk through the functional group reactions or the free radical course are evidently more convenient and cheaper, and can produce large volume of polymers. Kinds of multifunctional monomers such as epoxy,19,20 phenyl phosphites,21 anhydride20 have been used effectively to produce LCBs of PLA with high molecular weight and high melt strength. However, it is eventually hard to get control on the molecular weight and topological structure of the final products due to the randomness of branching, which would be proven to have a substantial effect on the foaming properties. Numerous studies have been carried out to improve the crystallization properties of PLA by adding a nucleating agent such as talc,22,23 or by stereocomplexing with poly (D-lactic acid).24,25 However, there still are some problems. On the one hand, blending with low molecular weight compounds and © 2012 American Chemical Society

oligomers always leads to migration of the plasticizer to the surface, which would cause embrittlement in long-term use. On the other hand, by blending with relatively high molecular weight polymers, the aforementioned problem could be prevented, but the desired properties are not obtained due to the thermo-dynamical immiscibility between the different constitutes with different structures.5,26−29 Hence, it is intriguing to study the effect of blending branched PLA with linear PLA, because they both have identical chemical structure. In 1992, Kim and Webster first reported the blending of hyperbranched polymers with a commercial linear polymer.30 And from then, blends of linear materials and dendritic materials were obtained with enhanced miscibility and additional positive effects on the mechanical, rheological, and thermal properties of the polymer blends.31−33 For example, M. Zuideveld et al.16 investigated the miscibility of PLLA/hbPLLA blends. Many methods were employed, including differential scanning calorimetry (DSC), dynamic thermomechanical analysis (DMA), and atomic force microscopy (AFM), to demonstrate complete miscibility between linear PLA and dendritic PLLAs. Another negative factor for PLA foaming is the low crystallinity due to the slow crystallization kinetics of PLA. Crystallinity has a profound influence on the mechanical properties of polymer products.34 One way to partially resolve the problem is to anneal PLA materials at temperatures higher than Tg and below the melting point.35 Another strategy to increase the crystallinity of PLA is by incorporating a nucleating agent in the polymer during extrusion.16 Developing a Received: Revised: Accepted: Published: 10088

February 28, 2012 July 9, 2012 July 11, 2012 July 11, 2012 dx.doi.org/10.1021/ie300526u | Ind. Eng. Chem. Res. 2012, 51, 10088−10099

Industrial & Engineering Chemistry Research

Article

16% for 75-TN). Prior to blending, 4032D and LCB-PLAs were dried in vacuo at 60 °C for 24 h. Melt blends were prepared on a Haake batch intensive mixer (Haake Rheomix 600, Germany) with a batch volume of 50 mL. Polymers were mixed at a screw speed of 15 rpm for 4 min and then 30 rpm for 3 min at 180 °C. The torque was continuously monitored during the whole mixing process. Also, the neat PLA 4032D was subjected to the mixing treatment so as to have the same thermal history as the blends. 2.4. Foaming by Supercritical CO2. Sample of PLA were cut into small sheets and put into a high-pressure mold. Then CO2 was injected into the mold to the pressure of 13 MPa and the mold was heated to foaming temperature at 160 °C. After 8 h, the pressure was released at an average speed of about 2.3 MPas−1. Finally, the products were taken out quickly. The foamed samples were fractured in liquid nitrogen, coated with a thin layer of gold on the fracture surface, and observed with scanning electron microscope (JEOL JSM-7401F). The accelerating voltage was 7.0 kv. 2.5. Analytical Methods. 1H NMR spectra were recorded on a Bruker AV 400 M in CDCl3 at 25 °C and the peak positions are reported with respect to tetramethyl silicane (TMS). Absolute molecular weight, molecular weight distribution, and intrinsic viscosity of all LCB-PLAs and 4032D were obtained by size exclusion chromatography (SEC) with triple detectors. The chromatographic system consisted of a Waters 1515 isocratic HPLC pump, a multiangle laser scattering system (MALLS) detector, a viscosity detector, and a differential refractive index detector. A value of 0.020 mL/g was used for the refractive index increment (dn/dc) of all PLA samples. This dn/dc value was validated using Astra software to determine the integration of the entire elution peak using a calibrated refractive index detector and assuming 100% sample elution. Polymer solutions were prepared with amounts of about 10 mg of polymer in 5 mL of chloroform and were eluted at 35 °C with a flow rate of 1 mL/min. Rheological measurements. Dynamic shear tests were carried out on a Physica MCR300 instrument (Stuttgart, Germany). For the study of rheological behaviors, the samples were pressed into 1.2 mm thick plates at 180 °C. Frequency sweep was carried out under nitrogen at 180 °C, using a 25 mm plate/ plate geometry, and the sample gap was set as 1 mm. The strain and angular frequency range used during testing were 2% and 0.1−100 rad/s, respectively. Isothermal oscillatory tests at a frequency of 10 rad/s and a strain amplitude of 2% were performed to test the melt stability of the PLA samples at 180 °C for up to 30 min. The temperature was controlled by feeding heated N2 gas into the forced convection oven (sample chamber) of the rheometer. The viscoelastic parameters, namely, storage modulus (G′), loss modulus (G″), complex viscosity (η*) and loss angle (δ) were calculated by using MCR300 software. The creep measurement was conducted on a Physica MCR300 instrument (Stuttgart, Germany) and a shear stress of about 20 Pa used for creep test was within the range of linear viscoelastic behavior for the materials. The sample sheets were cut into pieces with a width of 10 mm and a length of 17 mm for the uniaxial elongational viscosity measurements, which were carried out on an ARES rheometer (TA Instrument, USA) with the extensional viscosity fixture (EVF) at constant strain rate of 0.05, 0.1, 0.3, and 0.5 s−1 at 170 °C. Polar optical microscopy (POM) studies were carried out on Microphoto (Linkam TM600) equipped with a hot stage. The

significant crystalline structure in the PLA during processing is also one way to partially circumvent this problem.36 In the present study, long-chain branched polylactide (LCBPLA) prepared in our previous work was blended into linear PLA. It is supposed that the LCB-PLA has the ability to entangle with different chains and show high melt elasticity and melt strength. At the same time, the molecular chain was long enough for LCB-PLA to fold, and more branch points can help nucleation, both of the two factors can be beneficial to crystallization of blends. The chemical composition of LCBPLAs and linear PLA was identical, and they could mix with each other without differentiation and phase separation. Therefore, miscibility in the LCB-PLA/linear PLA 4032D system is surely better than that in the traditional polymer/PLA system. To illustrate the influences of LCB-PLA on the linear PLA, melt rheology and crystallization behaviors were examined in detail. To our knowledge, few reports on this subject have been seen in the literature.

2. EXPERIMENTAL SECTION 2.1. Chemicals. L-lactide (LA) was prepared in our own laboratory and recrystallized from ethyl acetate three times before use. Sn(Oct)2 was purchased from Sigma Aldrich (USA) and used as received. 1,4-Butanediol (BDO) was purchased from Aladdin Reagent Co. Ltd. (Shanghai, China) and used after vacuum distillation for three times. 1,1,1-Trimethylolpropane (TMP) was purchased from Sigma Aldrich (USA) and used without further purification. Hexamethylene diisocyanate (HDI) was purchased from Aladdin Reagent Co. Ltd., and used without further purification. Toluene was dried and distilled with sodium under a nitrogen atmosphere before use. Other reagents were commercially available and used as received. A linear poly-L-lactide sample, PLA 4032D, was purchased from NatureWorks LLC (USA) with a melt flow index of 7.75 g/10 min at 190 °C and under 2.16 kg. 2.2. Synthesis of Long-Chain Branched Polylactide (LCB-PLA). The polymerization of L-lactide was performed in a flame-dried and argon-purged glass reactor with magnetic stirring. First, initiator TMP (179 mg, 1.33 mmol) or BDO (180 mg, 2 mmol) was weighed into different reactors, and then, L-lactide monomer (43.2 g, 0.3 mol) was added. After repeated evacuation and purge with argon, the reactors were immersed into an oil bath at 120 °C. The polymerization was started by injection of Sn(Oct)2/toluene solution (3 mL, 10−4 mol/mL). After 24 h, the reaction was terminated by cooling the reactor to room temperature. The polymers were dissolved in chloroform and precipitated into a 10-fold excess of cold methanol, isolated by filtration, and dried under vacuum at 50 °C for 48 h. Then the polylactide prepolymer was first dissolved in dry toluene at 95 °C, to which HDI and Sn(Oct)2 solutions in toluene were added via an argon-purged glass syringe. The reaction was allowed to proceed for 1 h, and then the hot solution was precipitated directly into a 10-fold excess of cold methanol. Polymer samples were isolated by filtration and then dried at 50 °C under vacuum for 48 h. These LCBPLAs were coded as 75-TN, 75-BTN, and 75-B2TN, respectively, where “75” is the degree of polymerization of each arm, “B” stands for two-arm PLA from BDO and “T” stands for three-arm PLA from TMP, “B2T” means the molar ratio of B to T is 2, and “N” denotes coupling of these pre-PLA with HDI in an NCO/OH ratio of 1.0. 2.3. Preparation of Blends. LCB-PLA was mechanically mixed with linear PLA (4032D) at a mass ratio of 2 to 8% (to 10089

dx.doi.org/10.1021/ie300526u | Ind. Eng. Chem. Res. 2012, 51, 10088−10099


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Scheme 1. Synthetic Scheme of Polylactide Prepoymers (T and B) and Long-Chain Branched Polylactides (LCB-PLAs)

3. RESULTS AND DISCUSSION As we all know, long-chain branching (LCB) can increase entanglements between different chains in melt or in concentrated solution, thereby drastically altering the rheological behavior. The detail for synthesis of LCB-PLA can be seen in our previous article,37 the structures of LCB-PLAs were shown in Scheme 1, and basic information about these materials was listed in Table 1.

samples were dissolved in chloroform at a concentration of 1 wt %. Then a 50-μL aliquot of the solution was dripped on a glass slide, and was vacuum-dried under 45 °C overnight to remove the solvent. The specimens were heated to 180 °C quickly and held at that temperature for 5 min, and then quickly quenched to 110 or 120 °C at a rate of 50 °C/min. The observation started as soon as the temperature was cooled to the setting temperatures. Differential scanning calorimetry (DSC) measurements were performed on a Perkin-Elmer Pyris 1 DSC instrument under a N2 atmosphere. The samples were heated from 30 to 180 °C and cooled to 30 °C at a rate of 10 °C/min and then reheated to 180 °C. The T g (glass transition temperature), T cc (temperature of cold crystallization), Tm (melting temperature), and ΔHm (fusion heat) were taken from the second heating curve to eliminate the effect of thermal history. Two crystallinities were determined. The crystallinity after cold crystallization (Xacc) was calculated using Xacc (%) = ΔHm/ ΔHm0 × 100, where ΔHm is the fusion enthalpy and ΔHm0 is the fusion enthalpy of 100% crystalline PLA (93.7 J/g). The crystallinity before second heating (Xc) was calculated using Xc (%) = (ΔHm − ΔHcc)/ΔHm0 × 100, where ΔHcc is the cold crystallization enthalpy. Isothermal crystallization behaviors of PLA 4032D/LCB-PLA blends were also evaluated using DSC by premelting the sample at 180 °C for 5 min to completely eliminate any possible crystals or residual stresses in the sample and then rapidly cooling the melt to 110 °C and holding this temperature for 30 min to allow crystallization from the quiescent melt. And then, the temperature was increased back up to 180 °C with a heating rate of 10 °C/min to measure the melting point and enthalpy of fusion after isothermal crystallization. For isothermal kinetic analysis, the exothermic curves of heat flow as a function of time were recorded, and relative crystallinity was expressed as the ratio of the area under curve up to time t to that up to the end of crystallization

Table 1. SEC−MALLS, 1H NMR, and Rheological Data of PLA 4032D and LCB-PLAs Prepared sample 4032D 75-TN 75-BTN 75B2TN

Mwa (104 g/mol) 11.0 17.8 17.8 18.0

PDI

Mbb (104 g/mol)

Bnb

η0c (103 Pa.s)

η*@0.1 rad/s (103 Pa.s)

nd

1.44 1.67 1.77 1.37

2.16 2.68 5.04

6.5 2.8 1.3

4.32 375 48.0 15.8

3.7 61.8 27.0 11.4

0.64 0.37 0.49 0.58

a

a

Determined by SEC−MALLS. bDetermined by 1H NMR, Mb = average molecular weight between branch points, Bn = average number of branches per chain. cZero shear viscosity, determined by creep test. d Shear thinning index defined in eq 1, determined by small-amplitude dynamic oscillatory shear measurement and curve-fitting.

A series of LCB-PLAs based on PLA prepolymers with DP = 75 were synthesized and well-characterized in our present work, shown in Table 1. All the LCB-PLAs have almost identical absolute molecular weight, but different branch points and branch densities. From the results in Table 1, we can conclude that the branch density was antiparallel of the content of B. LCB-PLA with more B will show some properties similar to linear PLA, while with more T units LCB-PLA will be similar to a branched polymer. This was proven by size exclusion chromatography−multiple-angle laser light scattering (SEC− MALLS). The melt rheological results in our previous article 10090



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Figure 1. Complex viscosities η* as a function of angular frequency (a) and Cole−Cole plots (b) for blends at 180 °C: (A) 4032D, (B) 4032D/2%75-TN, (C) 4032D/4%-75-TN, (D) 4032D/8%-75-TN, (E) 4032D/10%-75-TN, (F) 4032D/16%-75-TN.

Figure 2. η0 and η* at 0.1 rad/s (a), and the slope of log G′ to log G″ (b) as a function of the content of 75-TN in the blends.

creep test increased with the content of 75-TN, and the η0 of blend with 16% 75-TN was 3.3 times of 4032D. Also shown in Figure 2a, when the content of branched 75-B2TN, 75-BTN, and 75-TN was kept at 8%, the η0 values increased with the branch density of LCB-PLAs, which were 5.12, 5.54, and 7.47 kPa·s. From the Mb in Table 1, we know that for all the three LCB-PLAs, branch length between branch points is long enough to entangle with other chains in the melt, and the mobility of polymers is restricted. The branch density of 75-TN is greater than 75-BTN and 75-B2TN, and the increase of content of 75-TN could increase the numbers of LCB, so the η0 values of blends increase with the content of 75-TN and the branch density of LCB-PLA. 3.1.2. Small-Amplitude Dynamic Oscillatory Shear Measurement (SADOS). SADOS was used to study the rheological properties of blends, such as complex viscosity (η*), storage modulus (G′), loss modulus (G″), and shear thinning index (n). Figure 1a summarizes the complex viscosities (η*) of PLA 4032D and blends as a function of shear frequency (ω). As shown in Figure 1a, blends with 75-TN at different contents have almost identical η* values at 100 rad/s but their η* values at 0.1 rad/s are different. 4032D/16%-75-TN showed the highest frequency dependence, while PLA 4032D showed the least. At 0.1 rad/s, their η* values are 3.70, 4.63, 5.10, 6.22, 7.12, and 9.56 kPa·s, respectively, in the order from PLA 4032D to 4032D/16%-75-TN and in parallel to the content of 75-TN. From Figure 2, we can see η* at 0.1 rad/s from shear

showed that the LCB structures had obvious influence on the rheological properties (data not shown). When branch density was increased, zero shear viscosity η0, storage modulus G′ at low frequency, shear-thinning degree and relaxation time would all increase, while loss angle decreased. In other words, longchain branching can increase the melt elasticity and melt strength of PLA. Meanwhile, the branched structure resulted in a short induction period and more rapid crystallization speed of isothermal crystallization. To endow the industrial product PLA 4032D with strong melt strength for foaming, PLA 4032D was blended with the above synthesized LCB-PLAs. Zuideveld M, et al.23 has investigated the miscibility of PLLA/hb-PLLA blends using different methods, including DSC, DMA, and AFM, to evidence complete miscibility between linear PLA and dendritic PLLAs. We also proved the good miscibility between 4032D and LCB-PLAs through the smooth interface in SEM, as shown in Supporting Information, Figure S1. This is attributed to the same chemical composition of 4032D and LCB-PLA. And now we will focus on the melt rheological and crystallization properties of the blends. 3.1. Rheological Behavior. All rheological measurements were carried out at 180 °C and to ensure high temperature stability, special measures were taken to get rid of the influence of the residual moisture from the heating gas. 3.1.1. Creep Measurements. Creep measurements were performed on the industrial sample PLA 4032D and blends. As shown in Figure 2a, the zero shear viscosities (η0) from the 10091



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Figure 3. Storage modulus (a), loss modulus (b), and Han-plot (c) for 4032D/75-TN blends as a function of frequency at 180 °C: (A) 4032D, (B) 4032D/2%-75-TN, (C) 4032D/4%-75-TN, (D) 4032D/8%-75-TN, (E) 4032D/10%-75-TN, (F) 4032D/16%-75-TN.

rheological measurements was close to η0 from the creep test for PLA 4032D, but when 75-TN increased, the deviation increased. For example, η0 from the creep test for 4032D and 4032D/16%-75-TN was 4.32 and 14.23 kPa·s, but η* at 0.1 rad/s was 3.7 and 9.56 kPa·s. All these results implied that the extent of shear thinning increases with 75-TN content. There are more chain entanglements in blends than in PLA 4032D, and the chain disentanglement or chain alignment in blends is much more difficult (much longer relaxation time) than that in neat PLA 4032D due to the long-chain branching effect. Moreover, the viscosity data at high shear frequencies was fitted by eq 1 to quantify the shear-thinning behavior according to the power law model,38

η* = mω(n − 1)

responsible for this deviation. First, branched polylactide 75TN can give rise to more free volume due to its abundant terminal groups. With the increase of free volume, the internal friction of molecules is decreased, and thus the melt viscosity is markedly decreasing. Second, the 75-TN molecules themselves have smaller hydrodynamic volume. Their entanglement with linear PLA 4032D chains in the blends leads to reduction of the hydrodynamic volume of these entangled PLA 4032D chains. Therefore, the apparent viscosity of the blends decreases, when compared to the simple additivity. However, as shown in the Supporting Information, Table S1, for a given content of 8%, the blends containing 75-BTN and 75-B2TN exhibit positive deviations. In other words, the B-containing blends display the opposite effect. This may be related to different structures of these LCB-PLAs. In 75-BTN and 75-B2TN, the molar ratios of B to T are 1:1 and 2:1, respectively. They are basically linear with a limited number of branches. The linear long chains of them make no difference with linear PLA 4032 and obey the additivity, while the branches play a role of cross-linker and lead to increased apparent viscosity. When the imaginary part (η″) of the complex viscosity is plotted against the real part (η′) of it, a Cole−Cole plot38 is obtained. As shown in Figure 1b, PLA 4032D gives an arcshaped plot, and all blends display significant deviations on the high η′ side, and η″ of which keeps increasing monotropically with increasing η′ caused by its elastic-solid-like feature. This is due to the contribution from the 75-TN. 3.1.3. Storage Modulus and Loss Modulus. It is well-known that based on the complex viscosity measurements, dynamic mechanical properties, storage modulus G′, and the loss modulus G″ of the melt can be calculated:

(1)

where m is the consistency index and n is the power-law index which indicates the degree of non-Newtonian behavior. The more pronounced the shear-thinning phenomenon is, the smaller the value of n is. Therefore it is called shear-thinning index for simplicity. The values of n for the samples at 180 °C decreased from 0.64 to 0.58, corresponding to the 75-TN content from 0 to 16% in the blends. It is assumed that both dynamic and creep measurements give the same η0 value for a given sample.38 Under this condition, η0 could be considered approximately as the viscosity at the inflection point of the log(η*) vs log(ω) curve. From the η0 values obtained from creep experiments and the slopes in Figure 1a, the onset frequencies of 4032D, 4032D/4%-75-TN, 4032D/8%-75-TN, and 4032D/16%-75-TN were estimated to be 25, 1, 0.2, and 0.08 rad/s, respectively. These results were in antiparallel to the content, indicating that the addition of 75TN could increase relax time through more entanglements. It has been assumed that two polymer samples of the same kind often exhibit viscosity additivity illustrated by eq 2,39

η1,2* = φ1η1* + φ2η2*

G′ =

(2)

G″ =

where φ1 and φ2 are the weight fractions of the components 1 and 2, respectively, and η1* and η2* are the complex viscosities of the two components. In the present study, η* (4032D) at 1 rad/s is 3.64 and η* (75-TN) at 1 rad/s is 55.4 kPa·s. By comparing the η1,2* determined experimentally with that predicted by eq 2, it is found that the measured complex viscosities of 4032D/75-TN blends were significantly lower than predicted ones, as shown in Supporting Information, Table S1. For the blends with 75-TN, the deviation increases with increasing 75-TN content. Several factors might be

ω 2η02Je 0 1 + (ωη0Je 0 )

(3)

ωη0 1 + (ωη0Je 0 )2

(4)

where η0 and are the zero shear viscosity and steady state compliance, respectively. Usually, the storage modulus G′ reflects the elastic part of polymer melt and loss modulus G″ reflects the viscous part of polymer melt. From eqs 3 and 4, it is seen that G′ is proportional to ω2 and G″ is proportional to ω to the first approximation. Therefore, log G′ is proportional to 2 log ω, while log G″ is proportional to log ω for an ideal polymer melt. Je0

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Figure 4. Elongational viscosity as a function of time at an elongational rate of 0.05 s−1 (a) for the 4032D/75-TN blends and at different elongational rates (b) for the 4032D/16%-75-TN at 170 °C: (A) 4032D, (B) 4032D/2%-75-TN, (C) 4032D/4%-75-TN, (D) 4032D/8%-75-TN, (E) 4032D/ 10%-75-TN, (F) 4032D/16%-75-TN.

Therefore, log G′ for monodispersive flexible polymers should be proportional to log G″ with a slope of 2. We can observe from Figure 3c and Figure 2b that the slope of the plot decreases with increasing 75-TN content, from 1.78 for PLA 4032D to 1.30 for 4032D/16%-75-TN. It implies that PLA 4032D is closer to a viscous polymer while 4032D/16%-75-TN behaves more elastic (closer to the straight line log G′ = log G″) due to the contribution from the LCB-PLA. This elasticsolid-like feature implied the improvement on melt elasticity, which was due to the abundant entanglements rooting from the long chain branches. Meanwhile, the melt properties of blends with 8% of 75-TN, 75-BTN, and 75-B2TN were compared in Supporting Information, Figures S2 and S3. All data lines of 4032D/8%75-BTN and 4032D/8%-75-B2TN are between those of PLA 4032D and 4032D/8%-75-TN, and close to those of 4032D/ 4%-75-TN in Figures 1 and 3. It indicates that in order to obtain blends with similar melt elasticity, one can choose LCBPLAs with a greater branch density at a low content, or LCBs with a lower branch density but at a high content. This may be due to the different structures of the three LCBPLAs. All the LCB-PLAs had similar total molecular weight, but different number of branch points. The length of molecular chain between branch points increased with the content of B units, thus increased the linear structure within the polymer. The linear structure is free to diffuse to disentangle, while the mobility of the long-chain branched polymer is restricted. Therefore, it is not surprising that long-chain branched polymers could exhibit very different properties, since the chain entanglements play the important role in this case. In the three LCB-PLAs, 75-TN has more branch points in its structure than the other two, which can be more benefical for chain entanglement to increase the melt elasticity. 3.1.4. Uniaxial Elongation. Elongational viscosity is a viscosity measured under tensile mode. When elongational viscosity is recorded as a function of specimen strain, a sudden increase in viscosity happening at a certain strain is called strain-hardening. By this method, strain-hardening behaviors have been reported for branched polypropylene,38,45,46 low density polyethylene,46,47 and PS.48 The onset-strains of strainhardening are dependent on the branching degree or on the fraction of the high molecular weight components. Because strain-hardening is responsible for foaming of polymeric

In Figure 3a,b, storage modulus G′ and loss modulus G″ of PLA 4032D and blends are plotted against ω in a doublelogarithmic way in a ω range from 0.1 to 100 rad/s, and the increase trend is found to be similar to that for η*. It is interesting to notice that all samples have almost identical G′ at ∼100 rad/s and G″ somewhere above 100 rad/s, respectively. With decreasing measuring frequency, both G′ and G″ go down, with PLA 4032D being the fastest and 4032D/16%-75TN being the slowest. Therefore, the G′ and G″ values of the samples follow the order of 75-TN content increasing at a given measurement frequency. Among all samples tested, PLA 4032D had the highest slopes, and the value of blends decreased with the content of 75-TN. The slope of 4032D is 1.65 (Figure 3a) for G′ and 0.98 (Figure 3b) for G″, respectively. Meanwhile, the slope of 4032D/16%-75-TN is 0.94 for G′ and 0.78 for G″, respectively. It implies that PLA 4032D is an ideal Newtonian melt, but blends are not ideal Newtonian melts. The degrees of deviation from ideal melt are in parallel to the 75-TN content. In the low frequency zone, where only the longest relaxation times contribute to the viscoelastic behavior, the nonterminal behavior of blends suggests that there is a longer relaxation mechanism, which can be ascribed to the long chain branches formed from these LCB-PLAs. The Han-plot, that is, plot of log G′ vs log G″, has been thought to be a useful tool to investigate order−disorder transitions in block copolymers,40,41heterogeneity of thermotropic copolyesters,42 the effect of polydispersity in polyethylenes,43 and miscibility of polymer blends.44 Moreover, Han-plot can provide information about elastic and viscous properties of polymers in low frequency region, where the linear viscoelastic data are most sensitive to differences in structure. The intersection (G′ = G″ determines the transition from more elastic behavior (G′ > G″) to more viscous behavior (G′ < G″). The slope of log G′ vs log G″ is also informative. The application of the general viscoelastic model leads to the following equation:44 G′ =

⎛6⎞ 2 ⎜ ⎟(Me / ρRT )(G″) ⎝5⎠

(5)

where ρ is the density, R is the universal gas constant, T is the absolute temperature, and Me is the entanglement molecular weight. Taking logarithms of both sides of eq 5, one obtains log G′ = 2 log G″ + log(Me/ρRT ) + log(6/5)

(6) 10093



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Figure 5. DSC curves of 4032D and 4032D/75-TN blends during the first cooling scan (a) and the second heating scan (b): (A) 4032D, (B) 4032D/2%-75-TN, (C) 4032D/4%-75-TN, (D) 4032D/8%-75-TN, (E) 4032D/10%-75-TN, (F) 4032D/16%-75-TN.

Table 2. The Values of Tg, Tm and Xc from DSC Measurements heating−cooling−heating

after isothermal crystallization

sample

Tg (°C)

Tm1 (°C)

Tm2 (°C)

Xacc (%)

Xc (%)

Tm1 (°C)

Tm2 (°C)

Xc (%)

4032D 4032D/2%-75-TN 4032D/4%-75-TN 4032D/8%-75-TN 4032D/10%-75-TN 4032D/16%-75-TN 4032D/8%-75-BTN 4032D/8%-75-B2TN

61.5 61.6 61.1 62.0 61.7 61.4 60.7 61.4

162.5 162.2 162.2 164.2 163.5 164.0 161.2 161.9

168.5 168.1 167.8 167.5 167.2

31.9 32.3 33.0 33.1 33.8 34.2 32.2 32.6

1.9 21.2 21.7 33.1 33.8 34.2 5.8 2.9

161.4 161.0 161.4 161.0 161.8 161.3 161.2 161.3

168.5 168.2 168.6 168.8 168.2 168.3 168.1 168.2

34.2 33.6 33.0 33.2 33.9 34.0 34.1 33.6

167.1 168.2

average chain length between branch points of 21.6 kg/mol (Table 1). It is really a long-chain branched polylactide. Each branch is long enough to entangle with linear PLA chains. In addition, the interchain interactions would be strong due to the polar carboxyl groups and urethane. These factors lead to effective contribution of the 75-TN to the strain-hardening of the blends. 3.2. Thermal Properties. The thermal behavior of 4032D/ LCB-PLA blends was studied by DSC. The samples were first heated to 180 °C and kept there for 5 min to eliminate the thermal history, then cooled to 30 °C to observe their cooling crystallization, and finally reheated to 180 °C to observe their cold crystallization and melting. Two crystallinities were calculated: crystallinity after cold crystallization (Xacc (%) = ΔHm/ΔHm0 × 100, assuming the fusion enthalpy of 100% crystalline PLA (ΔHm0) is 93.7 J/g) and crystallinity before second heating (Xc (%) = (ΔHm − ΔHcc)/ΔHm0 × 100, where ΔHcc is the cold crystallization enthalpy during the second heating. The DSC curves of 4032D/75-TN blends of various 75-TN contents are shown in Figure 5. The thermal parameters, such as glass transition temperature (Tg), melting temperature (Tm), and crystallinity (Xc, Xacc) obtained from the DSC curves are listed in Table 2. During the first cooling, no crystallization is observed for PLA 4032D and 4032D/2%-75-TN, but a sharp peak for 4032D/16%-75-TN at ∼120 °C and broad crystallization peaks centered at ∼110 °C are observed for the other three blends (Figure 5a). In Figure 5b, cold crystallization peaks (Tcc) are observed for the blends with 75-TN content lower than 8%, but no Tcc for content of 8% and higher. The crystallinity (Xc) increases from 1.9% for 4032D to 34.2% for 4032D/16%-75-TN, indicating that the

materials, it has attracted more and more research interest in recent years. It is well-known that neat linear PLA is not suitable for foaming because of its low melt strength and little strainhardening.32,49 Figure 4a shows the elongational viscosities under different times for the samples of 4032D and blends at the elongational rate of 0.05 s−1. It is clear that the elongational viscosity of 4032D and 4032D/2%-75-TN increases with time at the early time of stretching, and then decreases, which is called strain softening and generally exists in linear polymers. For other blends, the elongational viscosity rapidly increases until a strain of 3.4, implying the occurrence of strain hardening. In addition, the elongational viscosity increased with the content of 75-TN. The maximal elongation viscosity is 3 × 105 Pa.s for 4032D/4%-75-TN and 1.5 × 106 Pa.s for 4032D/16%-75-TN, respectively. And from the results in Figure 4b, we can find that quick elongation can make strain hardening appear earlier, but the maximum elongational viscosity kept almost the same, regardless of the elongational rates. This result confirms that rapid foaming is possible for blends with only 4% of 75-TN. Higher entanglements for the blends containing LCB-PLA could make the linear 4032D form stable foams. It is noticed that strain-hardening was observed only for the polymers with sufficient LCBs.38,50 For example, blends of linear PP and LCB-PP displayed little strain hardening when they contain less than 50 wt % of LCB-PP.38 This was attributed to less entanglement of diluted LCB molecules. But in our case, strain-hardening was observed for the blend containing more than 4 wt % of LCB-PLA. A probable reason for this low threshold is the unique structure of 75-TN. It has a Bn (the average number of branches per chain) of 6.5 and an 10094



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Figure 6. The relative crystallinity vs time at temperatures 110 °C (a) and 120 °C (b): (A) 4032D, (B) 4032D/2%-75-TN, (C)4032D/4%-75-TN, (D) 4032D/8%-75-TN, (F) 4032D/16%-75-TN.

crystallization ability of the blends increases with 75-TN content. The total fusion enthalpy is believed to include the contributions from the crystals formed during the first cooling and the second heating. From Table 2 we can see that Xc increased with the content of 75-TN, but Xacc of PLA 4032D was equal to or a little bit higher than those of the blends. In conclusion, the blends can crystallize more easily or more quickly than PLA 4032D, but their maximal crystallinities have no difference. It is noticed that in Figure 5b, all samples displayed double melting peaks, coded as Tm1 and Tm2, respectively, except 4032D/16%-75-TN. Multiple melting peaks are a common phenomena for a semicrystalline polymer. Many authors have explained double-melting behavior with a melt-recrystallization model.51 According to this model, the low-temperature and high-temperature peaks in the differential scanning calorimetry (DSC) curve are attributed to the melting of some amount of the original crystals and to the melting of crystals formed through a melt-recrystallization process during a heating scan, respectively.35 The bimodal peak also may be a result of the polymorphic crystalline transition, since PLA can display three different kinds of crystal modification, i.e., α, β, and γ. However, it was reported from the literature that the β and γ crystal forms can only appear for PLA upon hot drawing to high draw ratios and stroking, respectively. The crystal state we obtained in our conditions could only be α crystal. To demonstrate it, the WAXD patterns of PLA and its LCB-PLA blends are given in Supporting Information, Figure S4. The characteristic reflection peaks of α crystal of PLA kept almost the same position regardless of the composition variations. It is believed that only α form existed in the present samples. Therefore, the bimodal melting peaks in our work should result from meltrecrystallization process. Supporting Information, Figure S5 gives the DSC curves of the 4032D/LCB-PLA blends with identical content (8%) but different branch density (75-TN, 75-BTN and 75-B2TN). As shown in Table 2, 4032D/8%-75-TN displays more cooling crystallization while the other two display more cold crystallization. After a cooling−heating circle, the total crystallinities of 4032D/8%-75-BTN and 4032D/8%-75B2TN are almost the same as 4032D/2%-75-TN. This may be due to the different structures of the three LCB-PLAs. All the LCB-PLAs had similar total molecular weight, but with different number of branch points. The existence of denser branching can promote the self-nucleation in the amorphous

phase, and indolent branching structures act as nucleation sites for linear segments with a higher local order. So the crystallization ability increased with the number of branch points in these LCB-PLAs, and then the blend with 8% 75-TN has the most crystallinities. After isothermal crystallization at 110 °C, the DSC measurement is repeated. The thermal parameters obtained are also listed in Table 2 for comparison. It is seen that all the samples look identical after isothermal crystallization. Therefore, a conclusion can be drawn: blending with 75-TN accelerates crystallization of the blends but does not increase the crystallinity or improve the crystal perfection. The acceleration effects of 75-B2TN and 75-BTN are not as good as 75-TN because of their less degrees of branching, but their crystallization parameters after isothermal crystallization are similar to those of 75-TN-containing blends. This is because all components in these blends are polylactide in nature and thus they display identical crystallization properties under the optimum conditions. 3.3. Isothermal Crystallization. Most PLAs are finished to their final products by melt extrusion or by injection molding; therefore, fast crystallization is a desired or even required property of PLA. It was addressed above that 75-TN improved the crystallization ability of PLA. To further demonstrate it, isothermal crystallizations of neat linear PLA 4032D and the blends were investigated at 110 and 120 °C. The heat flow (dH/dt) was recorded as a function of time. From the isothermal crystallization curve, relative crystallinity Xt was calculated by dividing the under-curve area up to time t by that up to the end of crystallization, that is, t

Xt =

∫o (dH /dt ) dt ∞

∫o (dH /dt ) dt

(7)

To analyze the crystallization kinetics, the classical Avrami equations in exponential and logarithmic forms are used:52

X t = 1 − exp( −kt n)

(8)

log[− ln(1 − X t )] = log k + n log t

(9)

where n is the Avrami exponent depending on the geometry of crystal growing and k is an Avrami parameter related to crystallization rate. By plotting log(−ln(1 − Xt)) against log t, a straight line would be obtained and n and log k could be taken 10095



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Table 3. Kinetic Parameters of Isothermal Crystallization for 4032D/LCB-PLA Blends at 110 and 120 °C crystallization at 110 °C

crystallization at 120 °C

sample

t0 (min)

t1/2 (min)

log k

n

t0 (min)

t1/2 (min)

log k

n

4032D 4032D/2%-75-TN 4032D/4%-75-TN 4032D/8%-75-TN 4032D/10%-75-TN 4032D/16%-75-TN 4032D/8%-75-BTN 4032D/8%-75-B2TN

7.20 4.84 2.15 1.18 1.39 0.75 4.02 4.36

3.44 1.59 1.26 0.78 0.76 0.44 1.39 1.90

−2.09 −1.18 −0.88 −0.37 −0.37 0.21 −1.01 −1.27

2.20 2.48 2.36 2.32 2.38 2.32 2.29 2.17

10.8 4.57 1.96 1.19 1.09 0.55 4.38 6.29

6.11 4.56 1.38 0.86 0.80 0.45 2.26 3.23

−2.14 −1.26 −0.87 −0.48 −0.44 0.21 −1.46 −1.70

2.26 2.45 2.40 2.33 2.44 2.43 2.30 2.06

from the straight line as its slope and its intersection with the coordinate. Figure 6 and Supporting Information, Figure S6 collected the Avrami curves of 4032D/LCB-PLA blends in exponential and logarithmic forms, respectively. It is seen in Figure 6 that both induction period (t0) and half crystallization time (t1/2) are composition-dependent. In the group of 4032D/75-TN blends (Figures 6a,b), the sample containing more 75-TN has less t0 and less t1/2. In short, crystallization rate increases with increasing 75-TN content. Similarly in the group of 4032D/ 8%-LCB-PLA blends (Supporting Information, Figure S7), the crystallization rate increases with increasing branching degree. The straight lines obtained in Supporting Information, Figure S6 indicate that eqs 8 and 9 can describe the experimental data very well. Related kinetic parameters calculated are collected in Table 3. The Avrami exponent n varies between 2.17 and 2.48 at 110 °C, and 2.06 to 2.45 at 120 °C, indicating that the crystallization mode is of three-dimensional growth. As shown in Table 3, at 110 °C and from 4032D to 4032D/16%-75-TN, t0 is shortened from 7.20 to 0.75 min, t1/2 is shortened from 3.44 to 0.44 min, and log k increased from −2.09 to 0.21. In the group of 4032D/8%-LCB-PLA blends from 75-B2TN to 75TN, t0 is shortened to from 4.4 to 1.2 min, t1/2 is shortened to from 1.9 to 0.8 min, and log k increased from −1.27 to −0.37. The kinetic parameters at 120 °C exhibit similar composition and branching dependence, but the absolute values are different, because the system has a slower nucleation speed at 120 °C than at 110 °C. The temperature dependence of nucleation of PLA and iPP has been observed by other groups. Yasuniwa et al.35 showed in their work that the nucleation speed of PLA was dependent on the temperature, and high degree of supercooling was advantageous to nucleation. Tian et al.53 proved that LCB-PP can improve the ability of nucleation compared to linear ones, for the branch point can serve as effective nucleation agent. Homogeneous nucleation starts spontaneously by chain aggregation below the melting point, which requires a longer time, whereas heterogeneous nucleation forms simultaneously as soon as the sample reaches the crystallization temperature. Considering the above-mentioned thermal analysis result, it can be concluded that blends with LCB-PLA mainly crystallize via heterogeneous nucleation. The total branch points of 75-B2TN and 75-BTN were less than 75-TN, when the blending weight of three LCB-PLAs was kept the same. From the results in Table 3, with a more branched chain structure, the time for complete crystallization more dramatically decreased. On the one hand, the existence of denser branching can promote the self-nucleation in the amorphous phase; on the other hand, indolent branching structures act as nucleation sites for linear segments with a

higher local order. This was in accordance of the results, reported by Nofar Mohammadreza, etc.54 To illustrate the reason for the accelerated crystallization of the 4032D/75-TN blends observed above, POM was used to monitor the isothermal crystallization of 4032D/8%-75-TN at 120 °C. POM can reveal the process of spherulite formation (nucleation) and growth. As shown in Figure 7, at 120 °C and

Figure 7. POM images of 4032D (a,c,e), and 4032D/8%-75-TN (b,d,f) isothermally crystallized from quiescent melt at 120 °C for 10 min (a,b), 20 min (c,d), and 30 min (e,f).

10 min, much less spherulites are formed in 4032D than in 4032D/8%-75-TN, indicating higher nucleation speed in the blend. The two PLA samples showed well-defined spherulites with a ‘‘Maltese-cross’’ structure, whereas the blend showed more nucleation sites, indicating higher nucleation speed in the blend. This is ascribed to the incorporation of the 75-TN as a heterogeneous nucleating agent. From 10 to 30 min, the small spherulites grow bigger and bigger and new small spherulites are seldom formed during this period (Figure 7c−f). The average spherulite diameters in 4032D and in 4032D/8%-75TN are almost identical at 20 min or at 30 min. The introduction of LCB did accelerate the nucleation, but the 10096



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Figure 8. SEM micrographs of (a) 4032D and (b) 4032D/8%-75-TN foams and (c) photograph of 4032D/8%-75-TN before and after foaming.

The success of blends foaming by supercritical CO 2 demonstrated that blends of 4032D with more than 8% 75TN have the potential to be used in PLA foaming or high speed spinning.

radial growth rate of the spherulites was almost the same, which agrees with the analysis on melt-isothermal kinetic parameters. This is because the spherulite growth speed depends mainly on nucleation in our system. And PLA chain mobility and package were kept almost the same, for the linear PLA segment is the predominant component in both 4032D and in 4032D/8%-75TN. 3.4. Foaming by Supercritical CO2. In our work, we further studied the foaming properties of the linear and LCBPLA blended PLA under the help of the supercritical CO2 technique. The SEM images of foamed 4032D and PLA blends are shown in Figure 8a,b. Under the same foaming condition, the walls of 4032D foams are thick and the cell size is small and inhomogenous. However, the foams of blends exhibit a nicely interconnected, thin-cell structure, and the cell size of the PLA blend foam is very large (d ≈ 100 μm). Thanks to long-chain branching structures in blends, the melt strength is improved and the structures of foams could be notably well-developed and uniform. Figure 8c shows a photograph of 4032D/8%-75TN before and after foaming with a high expansion ratio of approximate 30 times. It is concluded that the blend samples have better foaming ability than only 4032D, and such foaming ability is strongly related to the long-chain branched chains structures.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental data about the difference of different branched PLA on the physical properties of linear PLA but not shown. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: + 86-431-85262769. Fax: +86-431-85262769. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The project was financially supported by the National Natural Science Foundation of China (project No: 51021003 and 51003101), by “100 Talents Program” of the Chinese Academy of Sciences (No. KGCX2-YW-802), and by the Ministry of Science and Technology (the ‘‘863’’ Project No: 2007AA03Z535).

4. CONCLUSIONS Blends of linear PLA 4032D and long-chain branched PLAs (LCB-PLAs) exhibited increased melt elasticity and more pronounced shear thinning behavior, and produced obvious strain hardening, as indicated by dynamic oscillatory shear measurement and elongational rheological measurement. These effects depended on the content and branch density of LCBPLAs. 75-TN was more effective than 75-BTN or 75-B2TN and in the group of 4032D/75-TN blends, 8% of 75-TN was enough to achieve significant melt elasticity and strain hardening. Although Tg, Tm, and maximal crystallinities remained virtually unaffected by LCB-PLAs, the crystallization speed was accelerated by LCB-PLAs because the branch points played a role of heterogeneous nucleating agent in the blends.

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REFERENCES

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