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Polymer Degradation and Stability 98 (2013) 844e852

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

Melt stereocomplexation from poly(L-lactic acid) and poly(D-lactic acid)with different optical purity

Yanlong Liu a,b, Jingru Sun a, Xinchao Bian a,b, Lidong Feng a,b, Sheng Xiang a,b, Bin Sun b, Zhiming Chen b,Gao Li a, Xuesi Chen a,b,*

aKey Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Chinab Zhejiang Hisun Biomaterials Co., Ltd, Taizhou 31800, Zhejiang, China

a r t i c l e i n f o

Article history:Received 31 October 2012Received in revised form14 December 2012Accepted 21 December 2012Available online 31 December 2012

Keywords:Poly(lactic acid)Melt stereocomplexationOptical purityHeat stability

* Corresponding author. Key Laboratory of PolymInstitute of Applied Chemistry, Chinese Academy of SChangchun 130022, China. Tel.: þ86 431 85262112.

E-mail address: xschen@ciac.jl.cn (X. Chen).

0141-3910/$ e see front matter Crown Copyright � 2http://dx.doi.org/10.1016/j.polymdegradstab.2012.12.0

a b s t r a c t

Poly(L-lactic acid) (PLLA) and Poly(D-lactic acid) (PDLA) with different optical purity were blended in aninternal mixer and their crystallization behavior were investigated. Nearly complete stereocomplexcrystallites could be obtained at specific condition between PLLA and PDLA. The melting temperature ofstereocomplex was related with the optical purity of PLLA and PDLA, and also with the mixing tem-perature and time. When the mixing temperature was set dozens of degrees above the melting tem-perature of pure PLLA and PDLA at a weight ratio 1:1, the stereocomplex could be formed within a shortperiod of time, and no homocrystallization or trace homocrystallization in some samples could be found.The possible mechanism of stereocomplex formation was achieved in this article and the shear force playan important role during the melt mixing process.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Poly(lactic acid) (PLA) or polylactide is considerate as a promis-ing eco-friendly polymer material to resolve many environmentalissues owing to its biorenewability and biodegradability [1e3]. PLAor modified PLA is a good candidate to substitute conventional oilbased thermoplastics because of its good mechanical strength,toughness, processability etc. [4e8]. PLA has two enantiomericisomers, poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA).Blending PLLA with PDLA leads to the formation of stereocomplex,which has a melting temperature around 230 �C, 50 �C higher thanhomocrystallized PLLA or PDLA [9e13]. This character gives anopportunity to increase the heat distortion temperature (HDT) ofPLA, which is around only 55 �C for normal PLA [14,15]. The highestHDT for well crystalline PLA reached over 100 �C and HDT forstereocomplex PLA is predicted over 200 �C.

The stereocomplex of PLLA and PDLAwas first reported by Ikadaet al. [16], who mixed low molecular weight PLLA with PDLAthrough solvent evaporation. After that, many efforts were taken toprepare full stereocomplex without existence of homocrystallized

er Ecomaterials, Changchunciences, 5625 Renmin Street,

013 Published by Elsevier Ltd. All24

PLLA or PDLA. Until now, most experiments were done in solvents,and themost favorableweight ormolar ratio of PLLA to PDLA is from60/40 to 40/60 with low molecular weight. In general melt mixingprocess, the co-existence of homo crystallization and stereo-complex was not helpful to increase the HDT of blends; thus pre-paring stereocomplex by melt mixing of PLLA and PDLA becamea challenge because it was valuable in applied field. Annealing ofmelted blends from PDLA and PLLA resulted in formation of largeamounts of racemic crystallites only when the mixing ratioapproached 1:1 and the molecular weight of both polymers was aslow as 103 [17]. Both racemic crystallites and homocrystallites weresimultaneously formed if themixing ratio of themelt deviated fromequimolar blending or the molecular weight of the polymer washigher than 104 [18,19]. Annealing PLLA/PDLA above Tm of PLLA orPDLA homopolymer was helpful to form stereocomplex [20], but itneeds long time isothermal condition.

High molecular weight PLLA/PDLA stereo block with high ster-eocomplex content can be synthesized by direct polycondensationinvolving bothmelt polycondensation (MPC) of lactic acid and solid-state post polycondensation (SSP) [21e23]. The Tmof stereocomplexmadebySSPwas around215 �Cwith amolecularweight higher than100kDa, and the coexistence of stereocomplex andhomocrystallitesoccurred even when a 50/50 PLLA/PDLA ratio was achieved.

Blending PLLA, PDLA with relatively lower optical purity (lessthan 99%) had attracted little attention in recent decades [24,25].

rights reserved.

Table 1Primary data of PLLA and PDLA raw materials.

Tm (�C) L (%) Mn (�104) Mw (�104) Mw/Mn

L149 149 95.0 7.2 11.6 1.61L155 155 95.9 7.6 12.3 1.63L162 162 97.8 7.4 11.7 1.58L172 172 99.4 9.3 14.8 1.59L177 177 99.6 4.7 9.3 1.99D155 155 4.5 6.9 11.6 1.67D174 174 0.5 7.3 11.7 1.62

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852 845

For homopolymers, there was some relationship with the Tm andoptical purity of PLLA or/and PDLA [26,27], but for stereocomplex,the optical purity of PLLA or/and PDLA also had some effects on themelting behavior of PLLA and PDLA homopolymers with moderateoptical purity [24,28].

In this study, the stereocomplex formation of PLLA and PDLAwith different optical purity was achieved by general melt mixingtechnique. The mixing conditions were adjusted to test the ster-eocomplex formation ability of PLLA and PDLA with different op-tical purity. At last, the possible mechanism was proposed todepict the molecular motion during the melt mixing of PLLA andPDLA.

Fig. 1. Torqueetime curves of PLLA/PDLA 50/50 blends with different optical purity at the teb, L155/D155; c, L162/D155; d, L172/D155; e, L177/D155; f, L177/D174.

2. Experimental

2.1. Materials

Poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) withdifferent optical purity were supplied by Zhejiang Hisun Bio-materials Co. Ltd., and the detailed information was shown inTable 1.

2.2. Preparation of PLLA/PDLA blends

PLLA and PDLA with different optical purity were weightedbefore being blended in an internal mixer for 1e16 min (XSS300,Shanghai Kechuang Rubber & Plastic Machinery Co., Ltd.), thetemperature was set between several to dozens degree above theTm of PLLA or PDLA, and the rotation speed was set at 30 rpm.During the mixing process, the torqueetime curves were recor-ded to evaluate the interaction strength between PLLA and PDLA.

2.3. Characterization

Thermal properties of the blends were examined by differentialscanning calorimetry (DSC Q100, TA Instruments, USA) under N2

mperature several to dozens degrees above the Tm of PLLA or/and PDLA. a, L149/D155;

Fig. 2. Torqueetime curves of PLLA/PDLA blends with different composition at a specific temperature (a, L155D155 blended at 160 �C; b, L177/D155 blended at 180 �C).

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852846

atmosphere. For analysis of the samples, the non-isothermal ex-periments were done from 40 �C to 250 �C at a speed of 10 �C/min,the glass transition temperature, cold crystallization temperature,homo and stereocomplex melting temperature, the melting

Fig. 3. The DSC curves of PLLA/PDLA blends with different optical purity at different mixing50/50. a, L149/D155; b, L155/D155; c, L162/D155; d, L172/D155; e, L177/D155; f, L177/D174

enthalpy of homo and stereocomplex, as well as the cold crystal-lization enthalpy were recorded.

The wide-angle X-ray diffraction (WAXD) measurement wascarried out on a Bruker D8 Advance X-ray diffractometer, using Cu

temperature (First heating run from 40 �C to 250 �C with PLLA/PDLA blending ratio at. The figure after the symbol @ is the blending temperature).

Fig. 4. WAXD profiles of PLLA/PDLA blends with different optical purity that are mixed at different temperature with 50/50 weight ratio (a, L155D155; b, L177D155).

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852 847

Ka radiation; the scattering angle ranged from 2q ¼ 5� to 35� ata scan speed of 3�/min at room temperature.

Thermogravimetric analysis (TGA) of stereocomplex PLA sam-ples were conducted in a thermal gravimetric analyzer (TGA Q500,TA Instruments, USA) in aluminum pans under a constant nitrogenflow (100 mL/min). About 4e5 mg of each sample was heated fromroom temperature to 600 �C at 10 �C/min, and the data was col-lected at the same time. For isothermal pyrolysis, the samples inaluminum pans were heated to a certain isothermal temperature at100 �C/min, and the temperature were kept for some specific timeand then the TA curves were recorded.

3. Results and discussion

3.1. Torque evolution during the blending process

The torque-time curves during the blending process can be anindication of interaction strength between polymers. If there isa strong interaction between two polymers, for example reactiveblending, a higher torque value can be seen in the torqueetimecurves [29,30]. The torqueetime curves of binary blends of PLLAwith different optical purity and PDLA (D155) with 4.5 L% (L-unitcontent) are shown in Fig. 1(aee).

Table 2The thermal properties of PLLA/PDLA blends of different optical purity blended at differe

Sample Tg (�C) Tcc (�C) DHcc (J/g)

L149D155@160 54.2 e e

L149D155@170 56.7 e e

L149D155@180 56.2 e e

L149D155@190 57.4 e e

L155D155@160 56.8 e e

L155D155@170 56.8 e e

L155D155@180 58 e e

L155D155@190 57.4 e e

L162D155@170 57.2 111 1.0L162D155@180 56.9 122 1.4L162D155@190 56.1 e e

L162D155@200 58.9 118 3.6L172D155@180 60.3 120 2L172D155@190 59.8 e e

L172D155@200 55.5 e e

L172D155@210 59.4 127 2.2L177D155@180 59.3 e e

L177D155@190 59.4 e e

L177D155@200 59 e e

L177D155@210 56.1 100 1.2L177D174@180 61.8 e e

L177D174@190 61.1 e e

L177D174@200 62.8 e e

L177D174@210 60.2 e e

L177D174@220 55.5 e e

The Tm(SC) with asterisk means that there are two melting points, here showed is the m

The first peak in the picture (Fig.1aee) is themelting peak of theblend, after that, the situation changes. For L149/D155 (Fig. 1a),when blended at 160 �C, 5 �C higher than the Tm of PDLA, there isa strong peak after the melting peak. This second peak should bethe interaction between PLLA and PDLA. After the second peak, thetorque value goes down to a platform. With the increase ofblending temperature, the second peak becomes lower, and theplatforms are comparable until the mixing temperature reaches190 �C, which has a much lower platform and no obvious secondpeak is found, seems that there is no obvious PLLA/PDLA stereo-complex formed at that temperature.

For PLLA with other optical purity, the torqueetime curvesduring the blending process show the similar phenomena: forexample, lower blending temperature helps the appearance of thesecond peaks and a higher torque platform, while relatively higherblending temperature will give no second peak, nor higher torquevalue. There exists a temperature range that is most suitable toproduce the second torque peak, only several to dozens degreesabove the Tm of PLLA or PDLA.

When PLLA and PDLAwith high optical purity are blended (Fig.1(f)), the torqueetime curves show a similar result that is mentionedabove. But within the temperature range of 180 �Ce220 �C, thesamples all show second peaks in the torque curves, and the

nt temperature with weight ratio 50:50.

Tm(H) (�C) DH(H) (J/g) Tm(SC) (�C) DH(SC) (J/g)

e e 196 43.4e e 206 20.9e e 202 21.2e e 206/190 21.6152 0.6 199 45152 0.3 200 46e e 201 35153 0.1 208* 15.7155 0.6 203 49.1158 0.8 205 23.1e e 213 28.4157 4.2 213 22.2166 0.8 205 42.5e e 208 40.6166 0.3 213 33.5170 1.7 220* 28.5170 1.1 210 55.2169 0.9 213 57.7170 0.3 219 55.7173 5.9 221* 34.1e e 217 56.2e e 218 57.2e e 228 41.1e e 225 68.1e e 235 43.9

ajor one.

Fig. 5. The Tm (a) and DH(SC) (b) vs mixing temperature.

Fig. 6. The DSC curves of PLLA/PDLA with different blending ratio at a specific temperature (First heating run from 40 �C to 250 �C with PLLA/PDLA blending ratio from 80/20 to50/50. a, L155/D155 blended at 160 �C; b, L177/D155 blended at 180 �C.)

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852848

intense of the peaks becomes weak with the increase oftemperature.

When blending PLLA and PDLA at different ratio, the torqueetime curves are quite different, comparing with samples of PLLA/PDLA 50/50 blending ratio, as can be seen in Fig. 2, where thetorqueetime curves of L155D155 blended at 160 �C and L175D155blended at 180 �C with different weight ratio are shown. The sec-ond peak intense becomes weak when the blending ratio deviates50/50, which means that the interaction between PLLA and PDLAbecomes weak, and the torque platform lowers to some extent. It isassumed that the strong interaction between PLLA and PDLA in thetorqueetime curves confirms the emergency of stereocomplex, andthe further analysis should be done by the DSC and XRD method.

3.2. Thermal properties of PLLA/PDLA blends

The DSC curves of first heating run are shown in Fig. 3, togetherwith the WAXD curves of L155D155 and L172D155 in Fig. 4. It can

Table 3The thermal property of PLLA/PDLA blends of different optical purity with different blen

Sample Tg (�C) Tcc (�C) DHcc (J/g)

L15580D15520@160 56.8 113 11.7L15570D15530@160 60.1 118 3.6L15560D15540@160 60.1 e e

L15550D15550@160 56.8 e e

L17780D15520@180 57.1 e e

L17770D15530@180 55.6 e e

L17760D15540@180 57.5 e e

L17750D15550@180 59.3 e e

be seen from Fig. 3 that, in most cases, the PLLA/PDLA blends havehigh stereocomplex contents except the blends mixed at relativelyhigher temperature, and no homocrystallites or only small amountof homocrystallites can be detected by this method. For L149/D155blend, there exist strong stereocomplex melting peaks when themixing temperature below 180 �C. When the blending temperatureis higher than 180 �C, the coexistence of homocrystallites andstereocomplex appears, and the strength of melting peak of ster-eocomplex becomes lower with the increase of blending temper-ature. For other PLLA/PDLA samples with different optical purity,the variation is similar with L149/D155, and the only difference isthe upper limit temperature, which increases with the optical pu-rity of PLLA or PDLA in the blends. Especially, the blend L177/D174has no obvious homocrystallite even blended at temperature ashigh as 220 �C. Furthermore, for all blends, the relatively lowermixing temperature is helpful to form stereocomplex.

From the WAXD profiles (Fig. 4), only the characteristic dif-fraction peaks of PLLA/PDLA stereocomplex at 2q around 12�, 21�,

ding ratio.

Tm(H) (�C) DH(H) (J/g) Tm(SC) (�C) DH(SC) (J/g)

154 10.9 198 10.1151 3.1 203 12.3e e 205 20.3152 0.6 199 45180 23.5 209 8179 12.6 210 14.9172 6.3 216 30.3170 1.1 210 55.2

Fig. 7. The relationship of Tm (a) and DH(SC) (b) with the PDLA content.

Fig. 8. The possible stereocomplex formation mechanism of PLLA/PDLA by meltingblending.

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852 849

and 24� can be seen in L155D155 (Fig. 4(a)) and L177D155 samples(Fig. 4(b)), which is also the testimony of the existence of stereo-complex crystallites [16].

Table 2 gives detailed information about the DSC results in Fig. 3.In most cases, the homocrystallization of PLLA or PDLA, as well ascold crystallization is very weak. On the contrary, nearly all thesamples have strong stereocomplex melting enthalpy, meaningthat a highly stereocomplex amount is obtained. The Tm (SC) of themost samples with identified PLA optical purity become higherwith the increase of themixing temperature to some extent, but thehigher mixing temperature can also change the melting peak fromsingle, narrow peak to multi, broad peaks in some cases, as can beseen in Fig. 5(a). For example, the Tm (SC) of blend L172/D155 in-creases with the mixing temperature from 205 �C to 220 �C whenthe blending temperature increases from 180 �C to 210 �C. But forthe melting enthalpy of stereocomplex, the variation is not asregular as Tm (SC) of stereocomplex, partly because of the emer-gency of multi melting peaks during the heating process. But onerule can be determined that blending of PLLA/PDLA with a higheroptical purity will give a higher Tm (SC) and a larger DH(SC).

In Fig. 6 and Table 3, the thermal properties of PLLA/PDLA blendswith different blending ratio are shown. In the blend L155/D155@160 with different ratio, almost complete stereocomplex canbe obtained in the 50/50 and 60/40 samples, so that only stereo-complex melting peaks are found in the DSC charts. With theincrease of PLLA content in the blend, the homocrystallites meltingpeaks appear, as can be seen in the blends with PLLA/PDLA ratio of70/30 and 80/20 (Fig. 6(a)). These results are consistent with thosereported in the literature, nothing to do with the molecular weight[18,29]. For the sample L172/D155 blended at 180 �C with differentblending ratio, the similar results are observed, and the largeamount of stereocomplex can be formed when blending ratio ap-proaches to 1:1. The occurrence of homo-crystallization in theblend PLLA/PDLA 60/40 is conflict with the literature [31], whichmight be caused by the different optical purity between L177 andD155. The value of Tm(SC) and DH(SC) increase with the PDLAcontent for L155D155, L177D155 blends with different blendingratio, and the samplewith higher optical purity has a higher Tm (SC)value and a larger DH(SC) (L177D155 blend), as can be seen in Fig. 7.The Tm (SC) of PLLA/PDLA 60/40 is higher than that with 50/50weight ratio, which is not consistent with the results in literature[16,17]. This might be caused by different blending method and/orblending temperature. In this study, the low temperature meltblending method is used, and the low melting temperature ofstereocomplex at 50/50 in Fig. 6a is probably caused by the lowblending temperature, which was only several centigrade higherthan themelting temperature of neat PLLA or neat PDLA. Under thiscondition, the rapidly formed stereocomplex were excluded from

the melts because of their higher Tm. This process is faster for PLLA/PDLA 50/50 than 60/40 blend, so the crystalline structure of PLLA/PDLA 50/50 stereocomplex might not be so perfect compared withPLLA/PDLA 60/40, causing lower Tm of stereocomplex with weightratio PLLA/PDLA 50/50 to 60/40.

3.3. Possible stereocomplex formation mechanism

The formation of stereocomplex cannot be obtained by generalblending PLLA and PDLA, except an external force that affects thearrangement of PLLA and PDLA molecules. The stereocomplex hasa triclinic form with 3/1 helix structure, and the strong interactionbetween PLLA and PDLA is supposed to be hydrogen bond [32] orvan der Waals forces [16,33]. A complete stereocomplex could notbe obtained even by the method of casting film from PLLA/PDLAsolvent, if they have the molecular weight higher than 105, becausethe rate of racemic crystallization is quite low compared to that ofthe solvent evaporation, although racemic crystallization proceedsmore quickly than homopolymer crystallization in microscopicallywell mixed solutions [34]. On the other hand, by pouring PLLA/PDLA solvent into a non-solvent under strong stirring, only ster-eocomplex could be found in the blend, independent of the mo-lecular weight [35]. Thus, the impact of external force may work onthe formation of stereocomplex during the blending process.

Fig. 8 gives the possible stereocomplex formation mechanism ofPLLA/PDLA bymelt blending. After being melted (stage 1), the PLLAand PDLAmolecules can be stretched from random coil to extended

Fig. 9. The DSC curves of L172/D155 blended for different time at 180 �C, 190 �C and 200 �C (First heating run from 40 �C to 250 �C with PLLA/PDLA blending ratio at 50/50. a, L172/D155 blended at 180 �C for 1e16 min; b, L172/D155 blended at 190 for 1e16 min; c, L172/D155 blended for 1e16 min. The figure after the symbol @ is the blending temperature.)

Fig. 10. Non-iso and isothermal degradation TGA curves of PLLA177, PDLA174 and their blend at different temperature (a, Non-isothermal of L177, D174 and their blends at differenttemperature with a heating speed of 10 �C/min from room temperature to 600 �C; b, c, d, isothermal at 260 �C, 280 �C and 300 �C for L177, D174 and their blends that mixed atdifferent temperature.)

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852850

Y. Liu et al. / Polymer Degradation and Stability 98 (2013) 844e852 851

state (stage 2) under strong shearing effect in the internal mixer.When the molecules are stretched into the parallel state (stage 3),there is a great chance to form stereocomplex by strong interactionbetween PLLA and PDLA molecules (stage 4). During the blendingprocess, the stereocomplex could be formed and excluded from themelt once after the parallel stage is achieved by shearing (as can beseen in the DSC chart in Fig. 9 about PLLA/PDLA blends mixing fordifferent time. The stereocomplex formation speed is much fasterwhen blending L172/D155 at 180 �C than 200 �C. Also, the sup-pressed homo-crystallization at longer blending time in Fig. 9(aec)may be partly due to the lowered molecular weight caused bythermal degradation.), so a relatively higher shearing strength(higher torque value in torqueetime curves in Fig. 1), i.e. lowerblending temperature is helpful for stereocomplex formation.

3.4. The thermal stability of PLLA/PDLA blends

The non-isothermal and isothermal TGA experiments weredone for PLLA with Tm 177 �C, PDLA with Tm 174 �C and their 1:1blends that mixed between 180 �C and 220 �C (Fig. 10aed). Forreference, the neat PLLA and PDLA were melt processed under thesame condition to PLLA/PDLA blends at 180 �C. The non-isothermalTGA results (Fig. 10a) show that the L177/D174 samples blended ata temperature range between 180 �C and 220 �C have meanweightloss behavior between those of the pure PLLA and PDLA samples atthe temperature below 360 �C, which is consistent with the liter-ature [11,36]. But different TGA results of PLLA/PDLA blends showa different behavior under isothermal conditions of 260 �C, 280 �Cand 300 �C (Fig. 10bed). During the isothermal degradation pro-cess, PDLA shows faster degradation speed than pure PLLA andPLLA/PDLA blends, and PLLA/PDLA blends have less weight lossthan pure PLLA especially at extended isothermal time, meaningthat the thermal stability of the PLLA/PDLA blends is increased byblending. The blending temperature has some effect on the heatstability of PLLA/PDLA, generally, blending at a relatively lowertemperature is helpful for heat stability. This is probably because ofthe smaller amount degradation effect when blending PLLA andPDLA at a lower temperature.

4. Conclusions

In this article, the PLLA and PDLA with different optical purityare blended in an internal mixer below the melting temperature oftheir stereocomplex. At the blending temperature a few degrees todozens degrees Celsius higher than the Tm of PLLA or PDLA, a nearlycomplete stereocomplex can be formed, irrespective of opticalpurity of components, and the Tm of stereocomplex formed by thismethod increases with the optical purity of PLLA or PDLA. For thespecific PLLA and PDLA blends, the Tm of stereocomplex increaseswith the blending temperature, while the change of meltingenthalpy of stereocomplex is irregular, partly because of theappearance of amulti melting peak or a side peak of stereocomplex.The possible mechanism of stereocomplex formation could bedepicted as follows: the PLLA and PDLA molecules are stretchedfrom a random coil to an extended parallel state during the meltingprocess, followed by the appearance of stereocomplex that isexcluded from the melts, because the Tm of stereocomplex is higherthan the mixing temperature. TGA results show that the heat sta-bility of stereocomplex produced by this method is higher thanpure PLLA and PDLA.

Acknowledgments

The authors acknowledge the financial support from NationalNatural Science Foundation of China (21004066, 51073155,

51073154, 51033003) and Innovative Research Group (51021003),and The National High-tech R&D Program of China (863 program)(No. 2011AA02A202).

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