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High Melt Strength and High Toughness PLLA/PBS Blends by Copolymerization and In Situ Reactive Compatibilization Bao Zhang, Bin Sun, Xinchao Bian, Gao Li, and Xuesi Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03151 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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Industrial & Engineering Chemistry Research

High Melt Strength and High Toughness PLLA/PBS Blends by Copolymerization and In Situ Reactive Compatibilization Bao Zhang, Bin Sun, Xinchao Bian*, Gao Li, Xuesi Chen* （Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022）

ABSTRACT: Poly(L-lactide)/poly(butylene succinate) (PLLA/PBS) blends were prepared by melt mixing with PLLA-based compatibilizer (PBS-PLLA) and chain extender tri-arm block copolymer (PLLA-block-poly(glycidyl methacrylates))3 (PLLA-b-PGMA)3. The tensile testing showed significant improvement in mechanical properties, and remarkably maintained high strength. Rheological investigation of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 indicated that the viscosity and storage modulus was improved greatly compared with neat PLLA. Elongational viscosity measurements exhibited strong strain-hardening behavior. The increase of the torque indicated the occurrence of chain branching and chain extension reaction. The imperfect crystallization of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends was demonstrated by the lowered melt point of PLLA. SEM showed that the PBS-PLLA and (PLLA-b-PGMA)3 significantly improved the compatibility of the PLLA/PBS blends. It was indicated that the synergistic effects of PBS-PLLA and (PLLA-b-PGMA)3 in PLLA/PBS blends played a key role in properties enhancement. With

copolymerization

and

in

situ

reactive

compatibilization,

PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends not only improved the toughness, but also improved the melt strength. INTRODUCTION Poly(L-lactide) (PLLA) was one of the biodegradable polyesters which was 1

ACS Paragon Plus Environment


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prepared from annually renewable natural resources.1 With high strength, thermal stability, and processability, PLLA had great potential to replace petroleum-based plastic products from both environmental and economic perspectives.2-4 However, the inherent drawbacks of PLLA including its poor toughness and low melt strength substantially limited its application in more fields. Many methods were used to improve its toughness and melt strength, including blending, copolymerization, composites and nanocomposites.5-10 PLLA blending was an economic approach to improve the toughness of PLLA.6,11 However, the blends were usually phase separated when mixing immiscible component, which eventually led to a deterioration in properties. In order to improve the miscibility of PLLA with dispersed phase, one way would be to use suitable block or graft copolymers containing blocks that were miscible with both the dispersed phase and PLLA as interfacial compatibilizers.12-13 Interfacial compatibilizers were introduced during the mixing process to improve the interfacial interaction of the blends. For example, Pepels et.al synthesized poly(pentadecalactone)-PLLA block copolymers.12 The application of these block copolymers as compatibilizers for high density polyethylene (HDPE)/PLLA blends obviously improved the interfacial adhesion, and significantly influenced the interfacial adhesion of LLDPE/PLLA blends. Tsuji et.al prepared PLA/PCL blends film in the presence of their random copolymer PLLA-CL.13 The blends film showed significant improvement in its mechanical properties, which indicated that the compatibility of PLA/PCL blends was improved after additon of PLLA-CL. Reactive blending was an economic and effective method to prepared compatibilizers during mixing, and it was widely utilized to improve the compatibility of conventional polymer blends.14-18 In the mixing process, the reactants can react 2


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with the blended components to form copolymers, thereby enhancing the compatibility and interfacial interactions of the blends. Wang et.al. prepared PLA/PBS blends with a small amount of dicumyl peroxide (DCP), which decreased the size of dipersed phase and improved the interfacial interaction of the blends. 17 However, the structure and molecular weight of the materials were difficult to be controlled. Xu et.al demonstrated that the compatibility of PLA/polypropylene (PP) can be improved greatly in the presence of a copolymer with epoxy group, which effectively improved the mechanical properties of the blends.18 Many researchers investiaged the improvement of the compatibility and mechanical properties

of

PLLA blends

by

adding

premade

copolymer

or

reactive

compatibilization, however, the melt strength was not improved effectively. To improve the stability of PLLA in the blown film, considerable research efforts were committed to improve the rheological properties of PLLA. It was proved that polymers with branching structures and high molecular weight can effectively solve this problem.19-21 Various approaches were used to prepared PLLA with high molecular weight and branching structures. Copolymerization was used to synthesize branched polymer with multifunctional comonomers in solution. Many multifunctional monomers were used, including isocyanates,22 acid anhydride,23 phenyl phoshites,24 epoxies25. However, chain extending was usually predominant during the copolymerization, and extended chains did not effectively improve the melt strength of PLLA. In addition, considering the environment protection and economy, solution polymerization should be avoided as much as possible. The branching reactions can be carried out by reactive processing in bulk, which was easier and more economical. The free radical reaction was one of the branching reactions, and it was very convenient and easy to carry out.26,27 The 3



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branched structure was obtained in this process, however, the structure and molecular weight of the blends were not easy to control. More importantly, the reaction was often accompanied by excessive cross-linking and degradation Compared with free radical branching, it was proved that the functional group reactions of the polyester with chain extenders can effectively get branched structures. Also, chain extenders was able to increase viscosity of polyester, improving melt strength. 28-30 In addition, chain extenders was also an effective reactive compatibilizer. The epoxy polymers were usually used in the chain extension of polyesters due to their high reactivity with hydroxyl and carboxyl of PLLA, and the molecular weight was increased. Branched PLLA was produced when the content of multifunctional epoxide exceeded 1.5 wt%.28 In the past work, many researchers investigated the improvement of the melt strength and processing properties of PLLA by the addition of chain extender.29,30 However, the chain extenders was usually composed of an acrylate monomer or a vinyl monomer with an epoxy-based monomer, and it was difficult to disperse uniformly in the PLLA, leading to unstable performance of the product. To solve this problem, an novel chain extender composed of PLLA and epoxy-based monomer was synthesized in this work. In summary, research on the modification of PLLA had made great progress, and developed many methods. However, the performance of the product was difficult to meet the requirements by the single modification method, and little work was reported to achieve the improvement of these properties by the combination methods. These motivated us to improve multiple properties of PLLA resins by the combination of copolymerization and in situ reactive blending. In this work, high toughness PBS-PLLA block copolymer and a chain extender (PLLA-b-PGMA)3 were synthesized and used for the modification of PLLA. PBS-PLLA effectively improved 4


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the compatibility of PLLA/PBS blends. (PLLA-b-PGMA)3 not only improved compatibility of PLLA/PBS blends but also improved the melt strength of the PLLA. The elongation at break of resulting PLLA blends increased by 80 times, and the tensile strength was also remarkably maintained. The viscosity and storage modulus were increased greatly, and the strain-hardening behavior was observed obviously from the elongational viscosity. The interface adhesion of blends was increased. In this work, a novel method was proposed for simultaneously improving the toughness, melt strength and compatibility of PLLA blends, and remarkably maintained high strength.

EXPERIMENTAL SECTION Materials. Succinic acid (SA), 1,4-butanediol (1,4-BD), and 2, 2´-Bipyridine (bpy) were purchased from Beijing Chemical Company. Tetra-n-butyl-titanate (Ti(OBu)4), α-bromoisobutyryl bromide, and Stannous octoate (Sn(Oct)2) were purchased from Sigma-Aldrich. L-lactide (LLA, Purac) was recrystallized three times from ethyl acetate. Glycerol (Sigma-Aldrich) was dried under reduced pressure before use. Glycidyl methacrylates (GMA, Sigma-Aldrich) were distilled in the presence of calcium hydride under reduced pressure. Copper(I) chloride (CuCl) was purchased from Beijing Chemical Company and purified with acetic acid; the CuCl powder was obtained by filtration, then washed with ethanol, and finally placed into oven under reduced pressure. Dichloromethane was purchased from Tianjin Chemical Company and purified by the distillation in CaH2. Triethylamine was purified by the distillation with CaH2 after refluxing for 12h. N,N`-dimethylformamide (DMF) was purified by the distillation with CaH2 under reduced pressure. PLLA (Mn=8.02 104 g/mol, D=1.33) was semicrystalline, and purchased from Zhe-jiang Hisun Biomaterials Co., 5



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Ltd (China). Poly(butylene succinate) (PBS) was purchased from Aldrich Chemical. Mw was 200,000 g/mol, and melting point was 114 °C. All chemicals were analytical-grade. General process for poly(butylene succinate) (PBS) synthesis. The PBS was synthesized by the esterification and polycondensation in sequence.31 The monomer succinic acid (118 g, 1 moL) and 1,4-butanediol (99 g, 1.1 moL) were added into a 500 mL four-necked flask with a condenser, thermometer, gas inlet and a stirrer, and then placed into an oil bath at 160 oC with stirring at a constant speed, and water produced during the reaction was removed by distillation. When the water was not distilled at atmospheric pressure, the catalyst Ti(OBu)4 (1.36 mL, 0.5 g/mL) was added, and the reaction was performed under high vacuum. The reaction was increased to 230 oC and maintained for another 3 h. The product was cooled and dissolved in CHCl3. The PBS was obtained by the precipitation of solution in cold alcohol, and dried under reduced pressure at 50 oC for 12 h. Mn,GPC=17000, Mw/Mn=1.7.

1

H

NMR

(CDCl3,

δ):

4.17

(t,

COOCH2

in

PBS),

2.63

(s,COOCH2CH2COO in PBS), 1.70 (m, COOCH2CH2O). Synthesis of PBS-PLLA block copolymer. All the PBS-PLLA was synthesized in analogous way but with different PBS contents.31 144 g (1 mol) monomer LLA and 48 g (2.4 10-3 mol) initiator PBS were placed into a 500 ml flask which was dried in oven. Water and oxygen was removed by freeze-pump-thaw process and repeated 3 times, and the reaction was carried out in an oil bath at 130 oC with magnetic stirring. After LLA and PBS was melted homogeneously, 0.1 g Sn(Oct)2 was consecutively added into the flask with an argon (Ar)-protected syringe. The reaction was terminated after 24 h and then cooled. The CHCl3 was added into the flask to dissolve the reaction mixture, and PBS(25%)-PLLA block copolymer was obtained by the 6


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precipitation in cold alcohol. The product was filtrated and dried at 50 oC for 2 days in vacuum (Yield=94%). Mn,GPC=62000, Mw/Mn=1.5. 1H NMR (CDCl3, δ): 4.17 (t, COOCH2 in PBS), 2.63 (s, COOCH2CH2COO in PBS), 1.70 (m, COOCH2CH2O), 5.2 (m, CH in PLLA), 1.4 (d, -CH3 in PLLA). Synthesis of 3-arm PLLA. The synthesis process was according to previous reports.32,33 The monomer LLA (144 g, 1 mol) and initiator glycerol (1.84 g, 0.015 mol) were placed into a 500 ml flask which was dried in oven, and then the stopper was added. After the LLA was melted with magnetic stirring at 120 oC, 0.144 g Sn(Oct)2 was consecutively added into the flask with a Ar-protected syringe. The reaction was terminated after 24 h and then cooled. The CHCl3 was added into the flask to dissolve the reaction mixture, and 3-arm PLLA was obtained by the precipitation in cold alcohol. The product was filtrated and dried at 60 oC for 2 days in vacuum (Yield=97%). Mn,GPC=9200, Mw/Mn=1.51. 1H NMR (CDCl3, δ): 5.2 (m, CH in PLLA), 1.4 (d, -CH3 in PLLA). Synthesis of macroinitiator. 3-arm PLLA (50 g, 5.3 mmol) was added to CH2Cl2 (200 mL) and then added to an ice bath for cooling. Triethylamine (10 mL, 100 mmol) was consecutively added to the reaction mixture with magnetic stirring for 5 min.α-bromoisobutyryl bromide (4.6 g, 20 mmol) was dissolved in CH2Cl2 (10 mL), and then added dropwise to the reaction mixture in 1 h. The mixture was stirred continuously for 2 h at 0 °C and then for 22 h at room temperature. The produced quaternary ammonium halide (CH3CH2)3NH+Br- was removed by the filtration, and the filtrate was concentrated to precipitate in alcohol (Yield=95%). Mn,GPC=9500, Mw/Mn=1.42. 1H NMR (CDCl3, δ): 5.2 (m, CH in PLLA), 4.36 (m, -CHBr-), 1.80 (d, -CH3CHBr), 1.4 (d, -CH3 in PLLA). Synthesis of the 3-arm block copolymer (PLLA-b-PGMA)3. CuCl (0.9 g, 0.027 7



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mol), bpy (4.2 g, 0.08 mol), and macroinitiator 3-arm PLLA (25 g, 0.0026 mol) were placed into a flask which was dried in oven. The reactants were degassed three times by performing freeze-pump-thaw cycles. 100.0 mL DMF and 28 g GMA was degassed by Ar and then added in the reactants, which were stirred continually for 12 h in an oil bath at 80 oC, and then cooled. The product was purified by passing the reaction mixture through Al2O3 column, and obtained by the precipitation in cold alcohol. The (PLLA-b-PGMA)3 was dried for 12 h in vacuum (Yield=50%). Mn,GPC=15000, Mw/Mn=1.38. 1H NMR (CDCl3, δ): 5.2 (m, CH in PLLA), 4.33, 3.79 (m, CH2), 3.21 (s, CH), 2.63, 2.83 (t, CH2), 1.85 (m, CH2 in PGMA), 1.4 (d, -CH3 in PLLA), 0.96, 1.13, 1.28 (t, CH3). Sample Preparation. Before processing, PLLA, PBS, PBS-PLLA, and (PLLA-b-PGMA)3 were dried under reduced pressure at 60 oC overnight. The structure was shown in Figure 1. The PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 composites with different composition were obtained by melt blending with a Torque Rheometer (XSS-300) at the rotary speed of 40 rpm for 8 min at 180 oC. The PLLA blends were mixed in a weight ratio, and manually mixed before melt blending. The blends were further placed on a polyimide film, which can prevent adherence to the press plates. The sheets were prepared by placing the blends in a hydraulic press with a mold at 180 °C and preheated for 10 min, and then increased the pressure to 10 MPa for an additional 3 min to make sure that the material fully filled the mold cavity. The samples were then cooled to room temperature in another compression molding machine, and removed from the press plates. The sheets obtained were about 1 mm thick and void-free for further characterizations.

8


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O

O O

O m

n

O

a

O O m

O

O

O

m

O

O O

p

O

n

O

O

O O

O

O

p

O

b

n O

O

O

O

m O

O

O

O

O

O

O

m

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O

O

O

O O

m

c d Figure 1. The structure of (a) PLLA, (b) PBS, (c) PBS-PLLA, and (d)

(PLLA-b-PGMA)3. Characterization. Nuclear magnetic resonance (NMR) spectra were obtained on a Bruker AV 400 NMR spectrometer with deuterated chloroform (CDCl3) as solvent. Infrared spectra were obtained on a Bruker Vertex 70 FTIR spectrophotometer with a spectral resolution of 1 cm−1 and scanned for 50 times. The samples was dissolved in chloroform and cast onto the potassium bromide sheet. The sheet was firstly dried in the air, and then heated to 50 °C under reduced pressure. Molecular weights (number average (Mn), weight average (Mw) and dispersity (Mw/Mn)) were measured by the gel permeation chromatography (GPC) at 25°C using a Waters 410 HPLC pump with a Waters 2414 RI Detector. HT2 and HT4 were linear Styragel columns and used in the measurement. The chloroform was used as mobile phase and the flow rate was 1.0 mL min-1. The monodispersed polystyrene standards were used as conventional calibrations. The thermal properties of the samples were measured by differential scanning calorimetry (DSC-Q10, TA Instruments). The heating and cooling rate was 10 oC/min (nitrogen flow=200 mL/min), and the temperature range of the test was 25 to 200 °C. Rheological measurements were carried out on a Physica MCR301 rheometer (Anton-Paar). The parallel plate mode was used with the diameter of 25 mm. A dynamic frequency sweep was carried out at 180oC under nitrogen at a gap of 1 mm. 9



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The strain rate was 1% and the frequency range was 0.1-100 rad/s. Uniaxial extension experiments were performed on a Physica MCR301 rheometer (Anton-Paar) at 178.5 °C with an extensional viscosity fixture. In order to fix the sample, pre-elongation was set for 6 s before the measurements. The strain rate was set 3 s−1. The dimension of sample sheets were 20 5 mm2. The tensile measurement was performed on an Instron 1211 machine (Instron Co., UK). The samples was prepared as dumbbell-shaped, and the dimension of the samples was 20 4 1 mm3. The crosshead speed was 20 mm/min, and five measurements were performed for each sample. The phase morphologies of the PLLA blends were characterized by field-emission scanning electron microscope (FE-SEM, XL30). The accelerated voltage was 10 kV. Before SEM test, the samples were immersed into liquid nitrogen for 10 min. Subsequently, the samples were fractured, and the fractured surfaces were sputtered with gold.

RESULTS AND DISCUSSION Mechanical Properties. Tensile strength of PLLA was usually more than 60 MPa and the magnitude order of Young’s modulus was GPa. However, the elongation at break was only 4.2%. In contrast, poly(butylene succinate) (PBS) possessed higher elongation at break with more than 700%, and it was a biodegradable polymer. So it was a good choice for the modification of PLLA. However, PLLA/PBS blends showed sever phase separation, which increased the defect of the sample and lowered their mechanical properties. Hence, it was necessary to design and synthesize the compatibilizer for the PLLA/PBS blends. Base on this reason, the PBS-PLLA block copolymer was synthesized in this work. 10


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Table 1. Character and mechanical properties of PLLA and PBS-PLLA. Sample

PBS/PLLA

Monomerb) conversion

Mn,GPCa) (g/mol)

Mw/Mna)

εb (%)

TS (MPa)

TM (MPa)

1

0/100

95.1%

73150

1.32

4.2

70.2

2110

2

5/95

96.2%

72500

1.32

9.1

54.8

2020

3

15/85

96.2%

60100

1.32

25.4

45.2

1870

4

25/75

96.7%

47000

1.33

379.2

40.9

1670

35/65

97.1%

41000

1.36

401.1

35.7

1530

5 a)

Determined by GPC measurements.

b)

The conversion was determined gravimetrically.

The compositions of PBS-PLLA block copolymer significantly influenced their mechanical properties, including the tensile modulus (TM), tensile strength (TS) and elongation at break (εb). As shown in Table 1, the elongation at break increased with increasing PBS weight fraction, and reached to 379.2% when the PBS contents were reached 25 wt%, and the tensile strength was still more than 40 MPa. With the weight fraction of PBS increased to 35%, the elongation at break was increased to 401%, but the tensile strength was lower than that of PE (40 MPa), which is not beneficial for the application. Hence, PBS(25%)-PLLA was chose to be compatibilizer of the PLLA/PBS blends. The effect of PBS(25%)-PLLA block copolymer on mechanical properties of PLLA/PBS (70/30) blends system was investigated firstly. The tensile properties of PLLA/PBS blends versus PBS(25%)-PLLA contents was shown in Figure 2. The neat PLLA underwent a brittle failure without necking. The binary PLLA/PBS blends displayed a slightly increase in elongation at break (26%) when PBS contents was 30%. In contrast, the elongation at break increased greatly after addition of PBS(25%)-PLLA block copolymer as a compatibilizer, and achieved to 75% for the PLLA/PBS/PBS(25%)-PLLA (70/30/20) blend, indicating a very effective toughening behavior. The compatibilized samples displayed plastic deformation, and the necking 11



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and cold-drawing was obviously, which was not typically the case for the neat PLLA. The result indicated that PBS(25%)-PLLA was an effective compatibilizer for the PLLA/PBS blends.

Figure 2. Stress/strain curves for PLLA/PBS/PBS(25%)-PLLA with different ratios: a. 70/30/0; b. 70/30/10; c. 70/30/20; d. 70/30/30. To investigate the influence of PBS-PLLA on morphology of immiscible PLLA/PBS blends, cryogenically fractured blend samples were analyzed by SEM. As shown in Figure S1(A), the fracture surface of PLLA/PBS (70/30) blends showed many cavities, and had distinct boundaries between the two phases, indicating incompatibility between the phases. After addition of PBS-PLLA, the size of PBS phase was decreased, and the voids was disappeared. PBS was dispersed homogeneously in the PLLA. The interface became unclear when the contents of PBS-PLLA were more than 20 wt%, which may be the result of the low interfacial tension between the two phases. The results charified an improvement in the compatibilization of the PLLA/PBS blend. The good compatibility of PLLA/PBS blends decrease the particle size of the dispersed phase, and increased the interfacial interaction of the two phases. Figure 3 showed the complex viscosity (η*), storage modulus (G'), and loss modulus (G") against frequency. The η* of neat PLLA and its blends displayed 12


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obvious non-Newtonian behavior at high frequencies. The incorporation of 30 wt% PBS decreased the complex viscosity due to its low viscosity and week interaction between PLLA and PBS. In contrast with PLLA/PBS blends, the complex viscosity was increased after addition of 10 wt% PBS-PLLA, which indicated the longer relaxation due to the improved interface interaction between PBS and PLLA. However, the η* of PLLA/PBS/PBS-PLLA blends decreased with increasing amount of PBS-PLLA, suggesting that the addition of small amount of PBS-PLLA can improve the interaction of the blends, but excessive addition of PBS-PLLA resulted in the reduction of η* of blends due to their native low viscosity. The G' and G" also showed similar change.

Figure 3. Frequency dependence of (A) complex viscosity η*, (B) storage modulus G' and (C) loss modulus G" of PLLA/PBS/PBS-PLLA blends: ■neat PLLA; □70/30/0; ▲70/30/10; △70/30/20; ●70/30/30. 13



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To

increase

branched

structure

of

the

blends,

a

novel

Page 14 of 34

chain-extender

(PLLA-b-PGMA)3 was synthesized. The (PLLA-b-PGMA)3 was composed of lactic acid unit and GMA unit, making it had good compatibility with PLLA and can be uniformly dispersed. Figure 4 showed the stress-strain curves of the PLLA/PBS blends containing various amounts of (PLLA-b-PGMA)3. The strain increased with increasing (PLLA-b-PGMA)3 content. When the (PLLA-b-PGMA)3 content was less than 1 wt%, the strain increased slightly. However, the strain became much higher when the (PLLA-b-PGMA)3 content was increased to 2 wt%. Finally the strain showed a slow but gradual increase with further increase of the (PLLA-b-PGMA)3 content. The increase in strain of the sample was probably due to some compatibilization effect. The compatibilizers PLLA-PBS formed after addition of (PLLA-b-PGMA)3, which increased the interfacial interaction between the blends. However, beyond 2 wt% (PLLA-b-PGMA)3 concentration, the modulus and tensile strength was decreased, but the strain was not increased correspondingly. It was believed that a certain quantity of (PLLA-b-PGMA)3 would participate the chain extending or branching reaction by reacting with hydroxyl and carboxyl groups of the polymers. However, the excess (PLLA-b-PGMA)3 would plasticize the polymer blend due to the limited number of reactive groups.

Figure 4. Stress/strain curves of PLLA/PBS/PGMA blends: (a) 70/30/1; (b) 70/30/2; 14


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(c) 70/30/5. SEM images of the PLLA/PBS blends containing different contents of the (PLLA-b-PGMA)3 was shown in Figure S2. The morphology was changed greatly with addition of (PLLA-b-PGMA)3. A 1 wt% (PLLA-b-PGMA)3 concentration led to a sharp decrease of dispersed-phase sizes, and few cavities were observed in the blends (Figure S2A). The size of dispersed phase was further decreased when the (PLLA-b-PGMA)3 content was increased to 2 wt%, and the blend surface was more uniform; the cavities was almost disappeared (Figure S2B). It was indicated that the interface adhesion of PLLA/PBS blend was improved dramatically. The morphology had no obviously change when the (PLLA-b-PGMA)3 content was 5 wt% (Figure S2C), which was in agreement with the result of tensile experiment.

Figure 5. Frequency dependence of (A) complex viscosity η*, (B) storage modulus G', and (C) loss modulus G" of PLLA/PBS/PGMA blends: ■ 70/30/0, □ 70/30/1, ● 15



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70/30/2, ○ 70/30/5. The η*, G', and G" versus frequency for the PLLA/PBS/(PLLA-b-PGMA)3 blend were given in Figure 5. The η* of PLLA/PBS/(PLLA-b-PGMA)3 increased with increasing (PLLA-b-PGMA)3 contents in the entire frequency range, which indicated that branching and chain extension reaction was occurred during the processing. The branched structure formed an effective entanglement with the blends, which in turn increased the viscosity of the blend. The G' showed a distinct rubbery plateau in the low frequency for the PLLA/PBS/(PLLA-b-PGMA)3 blends. The rubbery plateau was a result of the entanglement of the blend with the branched structure. The increase of G" also supported this conclusion. Base on above results, the modification of PLLA/PBS blends by the copolymerization improved the compatibility greatly, but the melt strength was still low. Reactive blending of (PLLA-b-PGMA)3 in the PLLA/PBS system improved the melt strength and compatibility, but the compatibility still needed to be improved further. Subsequently, the modification of PLLA/PBS was investigated by the combination of copolymerization and reactive blending.

Figure 6. Stress/strain curves for PLLA/PBS/PBS(25%)-PLLA/(PLLA-b-PGMA)3 blends with different ratios: A (a) 90/10/20/2; (b) 80/20/20/2; (c) 70/30/20/2; (d) 60/40/20/2; (e) 50/50/20/2, and PLLA/PBS: B (a) 90/10; (b) 80/20; (c) 70/30; (d) 16


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60/40; (e) 50/50. The tensile properties of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends with different compositions of PLLA/PBS were presented in Figure 6A. As a contrast, the corresponding tensile properties of PLLA/PBS blends were also investigated (Figure 6B). The strain increased with increasing PBS content until the PBS content reached 30 wt%, and decreased at the content of 40 wt%. However, the strain increased again when the PBS content was 50 wt%. The addition of high tough PBS can improve the strain of PLLA (Figure 6B(a,b,c)). However, the poor compatibility between PLLA and PBS would increase the defect of the sample and lowered their mechanical properties, as shown in Figure 6B(d). With the increase of the PBS contents to 50wt%, PLLA/PBS blends formed a co-continuous structure, and the strain was increased again, as shown in Figure 6B(e). When PBS-PLLA and (PLLA-b-PGMA)3 were added to the PLLA/PBS blends, the strain were monotonically increased with the increase of the PBS content, and increased to 340% when the PBS contents was 50 parts, nearly 80 times improvement than neat PLLA, as clearly shown in Figure 6A(e). The stress-strain curve showed considerable plastic deformation platform. The samples had obvious stress whitening phenomenon and rough fracture surface, indicated a brittle to ductile transition. In this system, PBS-PLLA was consists of the polylactide segment and PBS segment, which played a very good compatibilization. Also, the in situ reactive blending of (PLLA-b-PGMA)3 in the PLLA/PBS matrix produced a new PBS-PLLA copolymer at the PLLA/PBS interface, which further improved compatibility between the two phases. Both of them played the role of compatibilization, decreasing the size of dispersed phase and enhancing the interfacial adhesion. In addition, the branched structure was formed after addition of 17



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(PLLA-b-PGMA)3, and it increased the entanglement of the blends. In compare with PLLA/PBS/PBS-PLLA and PLLA/PBS/(PLLA-b-PGMA)3, the drastic improvements of mechanical properties indicated that PBS-PLLA, and (PLLA-b-PGMA)3 synergistic improved the toughness of PLLA/PBS blends. All these factors contributed to the improvement of interfacial interaction between the blends, and improved their mechanical properties. The mechanical properties were closely related to the morphology of the blends. SEM images of the PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends containing different mass ratios of the PLLA/PBS were shown in Figure 7. The addition of PBS-PLLA and (PLLA-b-PGMA)3 in the PLLA/PBS blend significantly influenced their morphologies. The entire fracture surface was uneven, and appeared complex plastic deformation, indicating there were strong interfacial adhesions in the PLLA blends. The surface has no obvious phase separation, which clarified the improved compatibility of PLLA and PBS. As well as we known, the copolymer composed of two components of the blends has good compatibilization for the blends. In this system, the copolymer PBS-PLLA was premade, and a new compatibilizer PLLA-PBS mixed chains was formed by the in situ melt compounding and likely located at the PLLA/PBS interface. The synergistic effect of PBS-PLLA and (PLLA-b-PGMA)3 on the compatibility of the blends was intuitively observed from the SEM characterizations, leading to the dramaticically plastic deformation. In addition, the branching structure was also formed in the blends, which formed an effective entanglement in the blends, leading to the improvement of the interface binding. The deformation was more pronounced with the increase in PBS content, and it was the relults of the good compatibility and the entanglement between the PLLA and PBS phase. The SEM result was further supported the change of mechanical 18


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properties.

Figure 7. SEM images of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends: (A) 90/10/20/2; (B) 80/20/20/2; (C) 70/30/20/2; (D) 60/40/20/2; (E) 50/50/20/2. Linear viscoelastic properties of the polymers were closely related to their structure. So it was necessary to compare the viscoelastic properties between PLLA and its blends. The η*, G', and G" versus frequency for the PLLA/PBS/PBS-PLLA/ (PLLA-b-PGMA)3 blends were given in Figure 8. In contrast with pure PLLA, the η* of blends was improved sharply, and increased gradually with the increase of PBS 19



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contents, indicating the occurrence of chain branching and chain extension in the blends, as shown in Figure 9 (D). The molecular weight was increased and the motion of the molecular chain was restricted, and branched structure produced more entanglements, both of which contributed to the improvement of viscosity. Besides the effect

of

branching and

chain

extension

reaction

in

PLLA/PBS

blend,

compatibilization was also contributed to improvement of the viscosity. During the melt blending, the new compatibilizers were formed, as shown in Figure 9 (A, B, C). The interfacial adhesion was also increased, which increased the viscosity of the blends. The result was in agreement with the reports.17 With the increase of PBS contents, the groups that participated in chain extension and branching reaction were increased, and increased the branching structure of the blends. In contrast with neat PLLA, the complex viscosity of the PLLA blends was increased in the entire range of the frequency. The remarkable improvements of η* for PLLA/PBS/PBS-PLLA/ (PLLA-b-PGMA)3 indicated that PBS-PLLA, and (PLLA-b-PGMA)3 had synergistic effects on improving the rheological properties of PLLA/PBS blends. The synergistic effects was due to the branching and chain extension reaction from epoxy groups of (PLLA-b-PGMA)3 with end groups of PLLA, PBS-PLLA and PBS, as shown in Figure 9. The addition of PBS-PLLA introduced more end groups that can react with (PLLA-b-PGMA)3, and a large amount of copolymer with branching structure was formed, which contributed to the improvement of the rheological properties. The G' showed a distinct rubbery plateau in the low frequency for the PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends unlike that of neat PLLA. The rubbery plateau was a result of the entanglement of blending components with the branched structure. It was found that the highest value of plateau modulus was increased with the increasing of the PBS contents, which was caused by the increase 20


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of entanglement in the melting of the blends. On the other hand, the in situ reaction of PLLA, PBS, PBS-PLLA with (PLLA-b-PGMA)3 caused the formation of branching or entangled structure, which contributed to the formation of entanglement in the blends. The terminal slope of the curve approached zero for the PLLA blends, demonstrating a solid-like feature. The relaxation of the blend chains significantly influenced the rheological properties of the materials, and only long relaxation contributed to improve the viscoelasticity of the material. The decrease in the slope of the blend curve indicated a high elasticity of PLLA blends. The results indicated the increase of the entanglements due to the formation of a branching structure in the blends. The increase of Gʺ further proved the formation of the branching structure in the blends.

Figure 8. Frequency dependence of (A) complex viscosity η*, (B) storage modulus Gʹ and (C) loss modulus G" of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends: ■ neat PLLA; □90/20/10/2; ▲80/20/20/2;▼ 70/20/30/2; △60/20/40/2;▽50/20/50/2. 21



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Figure 9. Schematic illustrations of in situ reactive blending of PBS-PLLA, PLLA, PBS in the melt in the presence of (PLLA-b-PGMA)3: (A) represents PBS-g-(PLLA-b-PGMA)3-g-PLLA

copolymer;

(PBS-PLLA)-g-(PLLA-b-PGMA)3-g-PLLA (PBS-PLLA)-g-(PLLA-b-PGMA)3-g-PBS

(B)

copolymer; copolymer;

represents

(C)

represents

(D)

represents

PBS-g-(PLLA-b-PGMA)3-g-(PBS-PLLA)-g-(PLLA-b-PGMA)3-g-PLLA copolymer. PLLA-b-PBS-b-PLLA represents PBS-PLLA, g represents graft.

Figure 10. Cole-Cole plot for PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 series: (a) neat PLLA; (b) 70/30/0/0; (c) 70/30/20; (d) 70/30/20/2; (e) 60/40/20/2;.(f) 50/50/20/2. 22


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To better understand the viscoelastic properties of PLLA/PBS/PBS-PLLA/ (PLLA-b-PGMA)3 blends, Cole-Cole plots (Gʹ-Gʺ) were investiagted (Figure 10). The intersection (Gʹ=Gʺ) determined the transition of the material viscoelasticity. Compared with neat PLLA, PLLA blends showed greatly increase in the elastic part after addition of PLLA-b-PBS-b-PLLA and (PLLA-b-PGMA)3, and increased with increasing PBS content (Figure 10(d, e, f)). The slopes in the terminal regime of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends were decreased compared with other blends (Figure 10(a, b, c)), indicating that the existence of branched structure and a long relaxation mechanism occurred in blends samples.

Figure 11. Elongational viscosity of the PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends at strain rate of 3 s-1: (a) neat PLLA; (b) 70/30/0/0; (c) 70/30/20; (d) 70/30/20/2; (e) 60/40/20/2; (f) 50/50/20/2. Uniaxial extension experiment was very important for evaluating the structure of materials, and it can determine the strain hardening behavior of the materials.34 In the molding process of materials, especially for film blowing, strain hardening behavior was very important.35 The elongational viscosities versus time for the PLLA and its blends were shown in Figure 11. For the neat PLLA, PLLA/PBS, and PLLA/PBS/PBS-PLLA blend, the elongational viscosities increased firstly, and then 23



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decreased, exhibiting obvious strain softening behavior. In these systems, the chain slipping of the polymer was not prevented because the blends did not contain branched structure. In the PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends, the branching or cross linking structure can be formed by the branching/cross linking reaction of PLLA, PBS, PBS-PLLA with (PLLA-b-PGMA)3, and formed an effective entanglement with the blend chains, which resulted in a lower chain disentanglement rate than chain deformation rate. The elongational viscosity increased to resist chain slipping before the sample broken, causing strain hardening behavior. The elongational viscosities increased with increasing PBS content, which indicated that the increase in PBS content increased chain extension and/or branching reactions in the PLLA blends.

Figure 12. Torque and temperature evolutions for PLLA/PBS/PBS-PLLA/ (PLLA-b-PGMA)3 reaction courses: (A) neat PLLA; (B) 70/30/0/0; (C) 70/30/20/0; (D) 70/30/20/2; (E) 60/40/20/2;.(F) 50/50/20/2. Figure 12 showed the torque-time evolutions of neat PLLA and its blends. All samples were blending at the same temperature to ensure similar thermodynamic condition. The change of the torque value during the blending process was closely related to viscosity of the materials, which indirectly reflected the change of 24


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molecular weight and chain structure of the blends. When the torque increased, the molecular chains in the blends would become longer and/or the branched structures would be formed. As can be seen in Figure 12, the torque of the linear PLLA, PLLA/PBS blend, and PLLA/PBS/PBS-PLLA blends continuously decreased during processing, indicating the possible thermal degradation. However, the torque showed opposite trend after addition of (PLLA-b-PGMA)3. The torque decreased at first and then increased continuously until it reached a plateau. In this work, the influence of temperature on the blending was negligible because of the same temperature. The increase of the torque was ascribed to the increase of the branched structure and molecular weight because of the chain extension and chain branching in the blends.

Figure 13. DSC thermogram of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 of first heating: (A) neat PLLA; (B) 70/30/0/0; (C) 70/30/20/0; (D) 90/10/20/2; (E) 80/20/20/2; (F) 70/30/20/2; (G) 60/40/20/2;(H) 50/50/20/2. DSC scans of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 during the first heating were shown in Figure 13, and the crystallization and melting parameters were listed in Table S1. The melting temperature (Tm) of the PLLA component decreased from 177.36 °C for the neat PLLA to 176.01 °C for PLLA/PBS/PBS-PLLA blends, and further decreased after additon of (PLLA-b-PGMA)3. The reduction of the Tm of 25



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PLLA was ascribed to its imperfect crystal after addition of PBS-PLLA and/or (PLLA-b-PGMA)3. The premade copolymer and the copolymer formed during the blending were stayed at the interface of two phases, prohibiting the crystal formation, and resulting in imperfect crystals. The results was in agreement with the report.36 In addition, the branched structure were formed by the branching/cross linking reaction of (PLLA-b-PGMA)3 with other blending components. The branched structure entangled with blends effectively, which restricted the molecular segmental mobility of PLLA and suppressed the increase of the lamella thickness. Imperfect and thinner crystallization resulted in a decrease in the Tm of the PLLA. As a contrast, we studied the influence of PLLA content on its Tm, as shown in Figure S3. Tm of PLLA did not change in the PLLA/PBS blends with the content of PLLA in our study. The Tm of PLLA was closed related with its crystalline structure. If the blending component was not compatible with PLLA, the blending components cannot be inserted into the lamellar structure of PLLA. Hence, the crystallization structure of PLLA did not change, and the Tm also did not change. In our work, because the PLLA and PBS were incompatible, the Tm of PLLA did not change. In contrast, the PLLA had good compatibility with compatibilizer, resulting in the intercalation of part of the compatibilizer segment into the lamellar structure of the PLLA. Therefore, the crystallinity of PLLA was also imperfect after addition of compatibilizer, and the Tm was decreased. The Tm of the PLLA component decreased from 177.36 °C for the neat PLLA to 176.01 °C for PLLA/PBS/PBS-PLLA blends proved this conclusion.

CONCLUSIONS The purpose of this work was to support an simple and effectively method to prepare 26


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high toughness and high melt strength PLLA blends. In this work, the copolymerization and in situ reactive blending were combined to improve the comprehensive performance of PLLA. The triblock copoloymer PBS-PLLA and chain extenders

was

synthesized

successfully,

and

PLLA/PBS/PBS-PLLA/

(PLLA-b-PGMA)3 blends were prepared by melting blending. The mechanical properties, rheological properties, thermal stability, and compatibility were investigated in detail. The blends showed significant improvements in mechanical properties, and remarkably maintained strength. The complex viscosity and storage modulus were increased greatly compare with neat PLLA, and the elongational viscosity analysis showed that the blends exhibited strong strain-hardening behavior, indicating a great increase of melt strength. The morphological studies indicated that the PBS-PLLA and (PLLA-b-PGMA)3 improved the compatibility of PLLA/PBS blend greatly. The lowered melt point of PLLA indicated the imperfect crystallization of PLLA/PBS/PBS-PLLA/(PLLA-b-PGMA)3 blends. There was a clear synergistic effect on improving the mechanical properties, compatibility and rheological properties because of the chain extension and chain branching from epoxy groups of (PLLA-b-PGMA)3

with

end

groups

of

PBS-PLLA.

As

a

whole,

the

PLLA/PBS-PLLA/PBS/(PLLA-b-PGMA)3 blends was thought to be a promising material for a packaging application because of improved melt strength and tensile properties.

AUTHOR INFORMATION Corresponding author * Xinchao Bian, Xuesi Chen. Email: [email protected]; [email protected]

27



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ACKOWLEDGEMENTS This work was supported by two grants. Author Bin Sun received funding from National Natural Science Foundation of China (Key Project 51403199). Author Gao Li received funding from National High Technology Research and Development Program of China (863 Program Project 2015AA034004).

Supporting Information The SEM images of the PLLA/PBS-PLLA/PBS and PLLA/PBS/(PLLA-b-PGMA)3, and DSC curves of PLLA/PBS blends were available in the Supporting Information.

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Schematic illustrations of in situ reactive blending of PBS-PLLA, PLLA, PBS in the melt in the presence of (PLLA-b-PGMA)3: (A) represents PBS-g-(PLLA-b-PGMA)3-g-PLLA copolymer; (B) represents (PBS-PLLA)-g(PLLA-b-PGMA)3-g-PLLA copolymer; (C) represents (PBS-PLLA)-g-(PLLA-b-PGMA)3-g-PBS copolymer; (D) represents PBS-g-(PLLA-b-PGMA)3-g-(PBS-PLLA)-g-(PLLA-b-PGMA)3-g-PLLA copolymer. PLLA-b-PBS-b-PLLA represents PBS-PLLA, g represents graft. 84x47mm (300 x 300 DPI)


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High Melt Strength and High Toughness PLLA ... - ACS Publications

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