Poly(l-lactide) Materials with Balanced Mechanical Properties

Article pubs.acs.org/IECR

Poly(L‑lactide) Materials with Balanced Mechanical Properties Prepared by Blending with PEG-mb-PPA Multiblock Copolymers Ruoyun Li, Linbo Wu,* and Bo-Geng Li State Key Laboratory of Chemical Engineering at ZJU, College of Chemical & Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: In this work, PEG-mb-PPA (abbr.: PEGPA) copolymers were synthesized via chain extension/coupling of poly(ethylene glycol) (PEG) and poly(1,2-propylene adipate) (PPA) diols, and blends of commercial poly(L-lactide) (PLLA) and PEGPA were prepared via melt blending. The multiblock copolymers were characterized by 1H NMR and DSC, and the PLLA/PEGPA blends were characterized by DSC, SEM, tensile, and impact testing. Plasticization-dominant (elongation ∼250%), toughening-dominant (impact strength 33 kJ/m2), and plasticized-toughened (250%, 14 kJ/m2) blends were obtained by varying the composition and loading of PEGPA. The PLLA/ PEGPA blends exhibited much more balanced mechanical properties in comparison with PEG-plasticized and PPAtoughened PLLAs, and the plasticization-dominant blends had superior performance stability in comparison with the PEGplasticized PLLA. The SEM micrographs indicate that the immiscible PPA segment formed elongated microphases and acted as a toughener, whereas the miscible PEG segment acted as a plasticizer of PLLA and a compatibilizer of PPA. Such a synergistic effect of both segments was responsible for the balanced mechanical properties. The improved performance stability was attributed to the high molecular weight of the multiblock copolymers and the depressed crystallization of the PEG segment.

1. INTRODUCTION Biodegradable polymeric materials derived from renewable resources have attracted more and more attention because of the continuous reduction of petroleum resources and daily increasing concerns about the environment.1 Among the various biodegradable materials, polylactide (PLA) is considered to be an excellent substitute for traditional petroleumbased polymeric materials. Poly(L-lactide) (PLLA) is a semicrystalline polyester having excellent biocompatibility, biodegradability, transparency,2−4 and high stiffness and has been extensively utilized for biomedical materials, packaging materials, and other disposal products.5−8 However, PLLA also has inherent drawbacks in some properties including slow crystallization, poor heat resistance, poor ductility, and impact toughness. Typically, semicrystalline PLLA has an elongation at break lower than 10%9 and izod impact strength less than 5 kJ/ m2.10,11 These shortcomings severely limit its applications in materials with specific end-use performance.12 Blending a rigid polymer with elastomeric modifiers is widely used for polymer toughening in industries. In order to achieve satisfactory toughness of PLA and to maintain its biodegradability meanwhile, various flexible biodegradable polymers, especially aliphatic or aliphatic-aromatic (co)polyesters, have been employed to blend with PLA10,11,13−17 in the presence of a premade or in situ formed compatibilizer which is necessary to achieve satisfactory toughness. However, it is often difficult to achieve excellent ductility and toughness at the same time via blending with only elastomers.18−20 © XXXX American Chemical Society

Incorporating a plasticizer into a rigid polymer is an effective way to improve the ductility or tensile toughness, and sometimes the impact toughness can also be improved. Extensive attempts have been made to improve the ductility of PLA, using various plasticizers including epoxy soybean oil,21 citrate esters and its derivatives,22−24 poly(ethylene glycol) (PEG), and poly(propylene glycol).12 However, low molecular weight plasticizers have a strong tendency to migrate from the PLA matrix, gradually reducing the ductility of PLA-based materials with aging time9,25−27 and causing food contamination problems in packaging applications. Therefore, oligomeric or high molecular weight plasticizers are more preferred to overcome the problem of plasticizer migration.28−30 Another problem is that plasticization is often achieved at the expense of sacrificing material stiffness. Furthermore, although excellent ductility or tensile toughness was achieved, the impact toughness of plasticized PLA is still poor in some reports.27,31,32 Therefore, it is quite desired to modify PLA to reach a balanced mechanical performance such as ductility, impact toughness, stiffness, and performance stability. Among various plasticizers, PEG is widely used as it is miscible with PLA, is commercially available in a wide molecular weight range, and has a low glass transition Received: December 29, 2016 Revised: February 16, 2017 Accepted: February 17, 2017

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DOI: 10.1021/acs.iecr.6b05046 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research temperature (Tg, −70 to −55 °C)33 due to its excellent chain flexibility. PEGs with different molecular weights have been assessed as plasticizers of PLA.23 However, low molecular weight PEGs are easy to migrate to the surface of PLA materials. With regard to high molecular weight PEGs, phase separation between PEG and PLA occurs due to the crystallization of PEG, leading to the embrittlement of PLA materials.34,35 To avoid PEG migration in PLA, PEG-based graft, block, and random copolymers have been reported as PLA plasticizers in some literature. PEG of low molecular weight ranging from 300 to 500 g/mol was grafted onto PLA chains via a free radical reaction, and well plasticized PLA materials were obtained.25−27 PEGs of relatively high molecular weight were chain-extended, and the products were used to plasticize PLA by melt blending.36,37 Amorphous copolymer poly(ethylene glycol)-succinate-L-lactide (PESL) with high molecular weight was synthesized and blended with PLA; the elongation of the modified PLA materials remained stable after aging for three months.38 In the present work, multiblock copolymers composed of a miscible PEG segment and an immiscible flexible poly(1,2propylene adipate) (PPA) polyester were designed to act as modifiers for PLLA so as to prepare PLLA materials with balanced mechanical properties and improve performance stability. When PLLA is blended with such a copolymer, it is plasticized by the miscible PEG segment and toughened by the immiscible PPA segment which tends to aggregate to form dispersed elastomeric microphases. In the meantime, the PEG segment also acts as a compatibilizer for the PPA−PLLA interface. The multiblock copolymers (PEG-mb-PPA) were synthesized from PEG and PPA diol prepolymers via chain extension/coupling. The effects of PEGPA composition and content on crystallization/melting and mechanical properties as well as microphase morphology of the blends were investigated. The aging stability of the blends was also assessed.

the resulting multiblock copolymer PEG-mb-PPA was taken out from the flask. For simplicity, the copolymers are denoted as PEGPA30, PEGPA50, and PEGPA80, respectively. 2.4. Preparation of PLLA/PEGPA Blends. PLLA and a modifier (PEGPA, PEG, or PPA) were melt blended in a Brabender Torque Rheometer (Brabender Technology, Germany) with a rotor speed of 60 rpm at 180 °C for 10 min. PLLA was dried in a vacuum oven at 80 °C, and the modifiers were dried at 40 °C for at least 24 h before blending. The obtained blends were indicated as “modifier-Y”, where Y represents the weight percentage of the modifier in the blend. The obtained blends were injection molded using HAAKE MiniJet II (Thermo Electron, Germany) to prepare dumbbellshaped (narrow section dimension: 20 × 4 × 2 mm3) and rectangular specimens (80 × 10 × 4 mm3) for tensile and impact testing, respectively. The blends were first melted in the cylinder at 180 °C and then injected into a mold preheated to 35 °C at an inject pressure of 850 bar for 30 s. Neat PLLA was also processed under the same conditions and used as a blank sample to make a reference. 2.5. Characterization. 1H NMR spectra of the PPA prepolymer and the multiblock copolymers were recorded with a Bruker AC-80 400 MHz NMR spectrometer using CDCl3 as solvent and tetramethylsilane as internal standard. Intrinsic viscosity was measured at 25 °C with an automatic viscosity tester (ZONWON IVS300, China) equipped with an Ubbelohde viscometer (inner diameter 0.36 mm), using CHCl3 as solvent. Thermal transition behaviors were recorded with differential scanning calorimetry TA-Q200 (TA Instrument, America). The PLLA blend samples were first heated to 200 °C at 40 °C/min, kept for 3 min, and then cooled to −70 °C at 20 °C/min. After having been kept at −70 °C for 8 min, the samples were reheated to 200 °C at 10 °C/min. The modifiers were scanned at the same heating and cooling rates in the temperature range of −70 to 90 °C. Tensile properties of the modified PLLAs were measured with a universal testing machine Zwick/Roell Z020 (Zwick, Germany) at 25 °C according to ASTM D638. The test was carried out at a crosshead speed of 10 mm/min with a gauge length of 25 mm. Notched Izod impact testing was carried out according to ASTM D256 using a CEΛST Resil impact tester (CEΛST, Italy) with a pendulum of 4 J. All samples for mechanical testing were conditioned at room temperature for at least 48 h before testing. Five replicates were tested for each sample. The microphase morphology of the blends was observed with an Utral 55 (Carl zeiss, Germany) scanning electron microscope at an acceleration voltage of 1.5 kV. The blends were immersed in liquid nitrogen for 3 min and then fractured. Cryo-fracture surfaces were coated with a thin gold layer prior to observation. The particle size of the dispersed phase in the PLLA matrix was determined using Image Pro Plus software. For each sample, at least 200 particles were measured to calculate the average size.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(L-lactide) (PLLA, 4032D, NatureWorks), hexamethylene diisocyanate (HDI, Aladdin), polyethylene glycol (PEG6000, Mn 6000 g/mol, intrinsic viscosity 0.23 dL/g, Sinopharm, China), 1,2-propylene glycol (1,2-PG, Sinopharm, China), adipic acid (AA, kindly provided by Shandong Haili Chem. Co.), and tetrabutoxide titanium (TBT, J&K) were all used as received. 2.2. Synthesis of PPA Diol Prepolymer. 1,2-PG and AA (1.2:1 molar ratio) were mixed in a 1000 mL four-necked round bottomed flask at 140 °C under stirring until AA was dissolved in 1,2-PG. Then TBT (0.1 mol % based on AA) was added as catalyst. The esterification reaction was carried out under nitrogen atmosphere at 170−190 °C until no water was distilled out. Finally, the pressure was reduced to below 100 Pa, the temperature was raised to 200 °C, and melt polycondensation was conducted under these conditions for 10 h. The product obtained was cooled to room temperature under N2 protection. 2.3. Synthesis of Multiblock Copolymers. PEG (70, 50, 20 wt %) and PPA (30, 50, 80 wt %) diol prepolymers were mixed in a 500 mL three-necked flask, heated to 120 °C and kept at the temperature under stirring and pressure below 100 Pa for at least 5 h in order to remove the moisture in them. Then, HDI (NCO/OH molar ratio 1.05:1) was added into the mixture to start the chain extension/coupling reaction under nitrogen atmosphere at 140 °C for approximately 1 h. Finally,

3. RESULTS AND DISCUSSION 3.1. Design, Synthesis, and Characterization of PEGPA Multiblock Copolymers. Multiblock copolymers were designed aiming at modifiers to plasticize and toughen PLLA simultaneously. The copolymers are composed of two types of segments. Segment A is PEG which is miscible with PLLA, while segment B is flexible and immiscible with both PEG and B


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hydroxyl end groups exist in PPA because of the asymmetric structure of 1,2-PG. The peaks at 3.73−3.55 ppm are attributed to the PG methylene protons (c′) next to the primary hydroxyl. For the PG methylene protons (c″) next to the secondary hydroxyl, the peaks appear at 4.13 and 3.93 ppm. Based on the areas of these peaks (c, c′, and c″), the Mn of PPA was determined to be 4000 g/mol. After chain extension/coupling, the characteristic signals of PPA (a, b, c, d, e) and PEG methylene (g) remained, and a new peak attributed to the methylene protons (f) linked to the newly formed urethane linkages appeared at 3.15 ppm. Meanwhile, the signals of the methylene protons (c″) next to the secondary hydroxyl in PPA disappeared in the spectrum of PEGPA50, as shown in the locally inserted spectra at a higher magnification range of 4.2− 3.9 ppm. Due to overlapping with the peak g attributed to methylene protons in PEG, the disappearance of c′ protons was unable to be observed in the spectra. 3.2. Thermal Transition Behavior of PEGPA and PLLA/ PEGPA Blends. Figure 2 shows DSC thermograms of the two prepolymers and the three PEGPA copolymers. The thermal transition properties of these modifiers are given in Table 1. As expected, PEG6000 is a highly crystallized prepolymer displaying a sharp and strong exothermic melt crystallization peak around 30 °C and an endothermic melting peak around 60 °C, and PPA4000 is an amorphous prepolymer with a glass transition around −40 °C. The structural asymmetry of the 1,2propylene glycol unit accounts for the amorphous state of PPA. As the PPA segment was amorphous, only the PEG segment in PEGPA exhibits crystallization and melting transitions in the DSC thermograms. However, the crystallization of the PEG segment was inhibited dramatically as compared with the original PEG prepolymer. The enthalpies of melt crystallization and melting decreased, and the peaks shifted gradually to lower temperatures with an increase in the content of the PPA segment. When the PPA content in PEGPA was 80 wt %, the PEG segment in PEGPA was not able to crystallize from melt at the cooling rate of 20 °C/min; instead, cold crystallization occurs during the subsequent heating at 10 °C/min. The results indicate that the motion of the PEG segment is limited by the presence of the PPA segment, and therefore PEG crystallization is depressed. It is well-known that crystallization of PEG not only weakens its plasticizing effect but also results in leakage of PEG and deterioration of material properties after aging for several months.34 So, such a crystallization depression is desirable for PEGPA as a plasticizer for PLLA. Figure 3 shows the DSC thermograms of neat PLLA (4032D) and its blends. The thermal transition properties in the second heating scan are summarized in Table 2. As a typical slowly crystallized polymer, neat PLLA did not melt-crystallize during cooling but displayed a weak and broad cold crystallization and a small melting peak (7.3 J/g) around 166 °C. The glass transition appeared around 59 °C. The DSC traces of the two PLLA/PEG blends, PEG-20 and PEG-30, show wide melt crystallization (80−83 °C, 20.4−24.5 J/g) and sharp melting peaks (167 °C, 34.4−36.4 J/g) attributed to PLLA, revealing that PLLA chain mobility was enhanced after blending with PEG. This implies the miscibility between PEG and PLLA and the plasticizing effect of PEG on PLLA. Unfortunately, the decrease in glass transition temperature was not detected because it is rightly overlapped with the melting peak of PEG. Excellent crystallizability of PEG can be certificated as PEG displayed a big melt-crystallization peak around 25 °C and meanwhile a big melting peak at 54 °C in

PLLA. When PLLA is blended with such a copolymer, segment B will aggregate to form dispersed microphases compatibilized and stabilized by the PEG segment which tends to locate in the PLLA matrix and at the phase interface. Therefore, PLLA will be plasticized by the miscible PEG segment and in the meantime toughened by the dispersed elastomeric microphases. In this study, poly(1,2-propylene adipate) (PPA) was tried as a model polymer for segment B. A hydroxyl-terminated poly(1,2-propylene adipate) (PPA) prepolymer was synthesized via melt polycondensation of 1,2propylene glycol and adipic acid. It appears to be a transparent and very viscous liquid. The intrinsic viscosity (IV) of the PPA prepolymer is of 0.21 dL/g. The number-average molecular weight (Mn) determined from the 1H NMR spectrum is of 4000 g/mol. Then, PEG-mb-PPA (or PEGPA, for simplicity) multiblock copolymers were synthesized by chain extension/ coupling of the PPA and PEG (Mn 6000 g/mol, IV 0.23 dL/g) diols using hexamethylene diisocyanate as an extension/ coupling agent. The reactions are given in Scheme 1. The Scheme 1. Synthesis of PEGPA Multiblock Copolymers and Achievement of PLLA/PEGPA Blends

products appeared to be translucent elastic polymers. The IV reached 1.08 dL/g, 0.71 dL/g, and 0.89 dL/g for PEGPA30, PEGPA50, and PEGPA80, respectively, demonstrating successful synthesis of high molecular weight PEGPAs. The 1H NMR spectra of PPA and PEGPA50 are given in Figure 1, together with the corresponding chemical shift

Figure 1. 1H NMR spectra of the prepolymer PPA (Mn 4000 g/mol) and the multiblock copolymer PEGPA50.

assignments. For PPA, all the chemical shift assignments are consistent with those reported by Hamdani et al.39 The signals at 1.65 and 2.35 ppm are attributed to the chemical shifts of the protons in the two middle methylene groups (e) and the two methylene groups linked to the ester bonds (d) in the adipic acid moiety, respectively. The resonance peaks of the methine (a), methyl (b), and methylene (c) protons in the PG unit appear at 5.19 ppm, 1.26−1.23 ppm, and 4.23−4.00 ppm, respectively. Interestingly, both primary and secondary C


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Figure 2. DSC thermograms of PEG6000, PPA4000, and PEGPA multiblock copolymers: (A) cooling scan at 20 °C/min and (B) second heating scan at 10 °C/min.

Table 1. Thermal Transition Properties of PEG6000, PPA4000, and PEGPA Multiblock Copolymersa cooling at 20 °C/min

second heating at 10 °C/min

sample

Tc (°C)

ΔHc (J/g)

PEG6000 PEGPA30 PEGPA50 PEGPA80 PPA4000

32.5 23.1 9.51

166.9 81.6 60.9

b

c

d

Tg (°C) −43.1 −38.0 −43.8 −39.3

e

Tcc (°C)

−8.2

ΔHccf (J/g)

Tmg (°C)

ΔHmh (J/g)

ΔHmi (J/gPEG)

16.4

60.9 51.0 50.0 41.2

173.6 82.2 59.9 19.7

173.6 117.4 119.8 98.5

a

The crystallization and melting transitions were attributed to PEG or the PEG segment in PEGPA, and the glass transition was attributed to PPA or the PPA segment in PEGPA. bTemperature of melt crystallization. cEnthalpy of melt crystallization. dGlass transition temperature. eTemperature of cold crystallization. fEnthalpy of cold crystallization. gTemperature of melting. hEnthalpy of melting. iMelting enthalpy per gram PEG segment.

Figure 3. DSC thermograms of PLLA/PEG, PLLA/PEGPA, and PLLA/PPA blends: (A) cooling scan at 20 °C/min and (B) second heating scan at 10 °C/min.

Table 2. Thermal Transition Properties of Neat PLLA and Its Blends in the Second Heating DSC Scan at 10 °C/mina sample PEG-20 PEG-30 PEGPA30-20 PEGPA30-30 PEGPA50-20 PEGPA50-30 PEGPA80-20 PEGPA80-30 PPA-20 PPA-30 neat PLLA

Tg,PPA (°C)

−42.7 −44.2 −41.0 −40.3

Tg,PLLA (°C)

52.7 55.3 52.2 50.0 58.5

Tcc,PEG (°C)

ΔHcc,PEG (J/g)

−29.6 −27.6

1.3 1.3

6.6

2.0

Tcc,PLLA (°C)

78.1 75.5 82.1 77.1 89.2 95.2 89.9 89.2 135

ΔHcc,PLLA (J/g)

18.0 12.3 18.9 14.1 20.2 18.4 19.7 18.6 7.2

Tm,PEG (°C)

ΔHm,PEG (J/g)

Tm,PLLA (°C)

ΔHm,PLLA (J/g)

48.8 54.2 48.8 50.1 47.7 46.7

8.8 28.7 4.6 14.0 4.3 9.8

44.1

2.0

167.3 167.2 166.9 166.6 166.6 166.1 166.6 167.2 167.0 166.9 166.3

34.4 36.4 32.0 28.6 30.5 29.1 33.0 24.7 30.5 27.2 7.3

“T” and “ΔH” mean the temperature and enthalpy of thermal transition, respectively. The subscripts “g”, “cc”, and “m” mean glass transition, cold crystallization, and melting, respectively. The subscripts “PEG”, “PPA”, and “PLLA” mean that the thermal property is attributed to the PEG segment, the PPA segment, and the PLLA matrix, respectively.

a

D


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PEG blends show PLLA-like homogeneous phase with visible plastic deformation. Similar results had been reported by Park et al.36 On the contrary, the PLLA/PPA blends show a typical two-phase structure with spherical PPA particles/cavities dispersed in continuous PLLA phase. The diameters of these particles varied from 1 to 3 μm. The microphase structure and morphology indicate the PLLA−PEG miscibility and the PLLA−PPA immiscibility. The immiscible PPA aggravated during blending to form PPA particles. Some particles were debonded from the matrix during the cryo-fracture process, leaving the cavities. The PLLA/PEGPA blends also show a two-phase structure. PEGPA30-20 appears like the PEG-plasticized PLLA as it contains minimum PPA content (6 wt %), and the particle size is very small (5.4 kJ/m2) and meanwhile retaining high tensile modulus (>1.4 GPa), yield, and breaking strengths (∼30 MPa and >20 MPa). PEGPA50-30 is a moderately toughened PLLA material, while PEGPA80-30 behaves like a highly toughened (impact strength 33 kJ/m2) PLLA material with satisfactory modulus (1.24 GPa) and strengths (24.6 and 18.5 MPa), and PEGPA30-30 exhibits more balanced properties in ductility (254%), toughness (14.4 kJ/m2), modulus (1.07 GPa), and strengths (22 and 19 MPa). The relative mechanical properties of typical blends are summarized in Figure 11.

Figure 11. Relative mechanical properties of neat PLLA and four typical blends: PEGPA30-20, PEGPA30-30, PEGPA50-30, and PEGPA80-30.

3.5. Aging Behaviors. To investigate the performance stability of the plasticization-dominant blends, the tensile specimens of PEG-20, PEG-30, and PEGPA30-20 were aged under room temperature for 6 months and tested again. The results are given in Figure 12. A ductility-to-brittleness change was unavoidable for PEG-20 and PEG-30 because clear leakage and crystallization of PEG significantly weakened the plasticization effect. Severe leakage and clear reduction in transparency were observed for PEG-30 so that it became too brittle to be tested. PEG-20 kept transparent, but its elongation at break decreased dramatically from 200% to 0.7%, its breaking strength decreased from 25.6 to 12.3 MPa, and its tensile modulus increased from 610 MPa to 1.8 GPa. So, the PLLA/ PEG blends lost ductility completely after 6 months of storage. As expected, no observable leakage was found for the PLLA/ PEGPA blends owing to the much higher molecular weight of PEGPA. PEGPA30-20 still exhibited ductile tensile behavior though there was clear decline in elongation at break from 236% to 107%, a slight decrease in breaking strength, and a slight increase in tensile modulus and yield stress. The property change was attributed to further crystallization of the PEG6000

Figure 10. Tensile modulus and yield strength of PLLA blends as a function of the PEG content in the blends. H


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (2016YFB0302402), the National Nature Science Foundation of China (51373152), State Key Laboratory of Chemical Engineering (SKL-ChE-15D01), and 151 Talents Project of Zhejiang Province.

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Figure 12. Comparison of the tensile properties of the as-prepared (blue) and aged (for 6 months, red) PEG-20 and PEGPA30-20 blends.

segment which was not completely depressed by chain extension/coupling. Although the PEGPA30-20 blend lost partial ductility, its ductility retention (defined as the retention of the elongation at break) was much higher than that of PEG20 (45% vs 0.35%), and the stiffness only showed a slight change. Therefore, PEGPA30-20 has much better performance stability than PEG-20. The performance stability could be further improved if the crystallization of the PEG segment in PEGPA is better depressed. Further study is still under way.

4. CONCLUSIONS Multiblock copolymers PEGPAs with various compositions were synthesized via chain extension/coupling of OHterminated PEG and PPA prepolymers and used as plasticizing-toughening modifiers for PLLA. In PLLA/PEGPA blends, a microphase separation structure was formed: the PPA segments in PEGPA aggregated to form dispersed phases, and the PEG segments in PEGPA diffused into the PLLA matrix to plasticize PLLA and to compatibilize the PPA microphase and the PLLA matrix. Therefore, PLLA was plasticized by PEG in the continuous phase and was toughened by the dispersed PPA elastomeric particles. The microphase morphology was determined by the PPA content and the PEG-modulated compatibilization and had a significant influence on the mechanical properties. As the multiblock copolymers had high molecular weight as well as a less crystallized PEG segment and elongated PPA particles were formed in the blends at appropriated PEGPA composition and loading, PEGPA behaved as a superior plasticizer versus PEG and a superior toughener versus PPA. By varying the composition and loading of PEGPA, modified PLLA materials with sufficient stiffness and excellent ductility or toughness or both of them can be achieved. As there is a broad spectrum in designing the structure and composition of the dispersed phase, this study presents a general approach to prepare modified PLLA materials with balanced mechanical properties and good performance stability.

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Linbo Wu: 0000-0001-9964-6140 I


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Poly(l-lactide) Materials with Balanced Mechanical Properties

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