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Effects of catalytic transesterification and composition on the toughness of poly (lactic acid)/poly (propylene carbonate) blends Linyao Zhou, Guiyan Zhao, and Wei Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00315 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 21, 2016

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Industrial & Engineering Chemistry Research

Effects of catalytic transesterification and composition on the toughness of poly (lactic acid)/poly (propylene carbonate) blends Linyao Zhou, a, b Guiyan Zhao,*a Wei Jiang*a a

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China b

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

1

ACS Paragon Plus Environment


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ABSTRACT: Poly (lactic acid) (PLA) and maleic acid anhydride end-capped poly (propylene

carbonate)

transesterification

(PPC-MA)

catalyst

tetrabutyl

were

melt

titanate

blended (TBT).

with

The

gel

or

without

permeation

chromatography (GPC), dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) results indicated that the addition of TBT promoted the transesterification and the transesterification degree increased with increasing the TBT content. The scanning electron microscope (SEM) photographs showed that the PLA/PPC-MA/TBT could be plastic stretched deeply under tensile stress. Moreover, PPC-MA was more active than PLA in the catalytic transesterification and the weight percent of PPC-MA had obvious effects on the mechanical properties of the blends. The tensile toughness of PLA/PPC-MA blends was significantly improved with the addition of TBT when the PPC-MA content was higher than 50 wt%.

2


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1. INTRODUCTION Worldwide production and consumption of petroleum derived polymers have led to concerns of both economic and ecological sustainability.1 These crises can be alleviated by developing bioplastics. One of the most promising bioplastics that can substitute traditional petro-polymers for industrial application is poly (lactic acid) (PLA) because of its renewability, biodegradability and good mechanical properties.2-4 However, the inherent brittleness of PLA is a considerable drawback for its application in many cases.5-7 Blending with other biodegradable polymers is an efficient way to tailor the properties of PLA composites without sacrificing the biodegradability.8-10 Poly (propylene carbonate) (PPC), copolymer of carbon dioxide and propylene oxide, is another biodegradable polymer with big potential for commercialization.11 Due to the excellent tensile toughness of PPC, much attention both in academy and in industry have been attracted by this aliphatic polycarbonate since it was first reported in the pioneering work by Shohei Inoue.12, 13 The high elongation at break of PPC is a very favorable property for improving the brittleness of PLA. Meanwhile, the low stiffness of PPC can also be reinforced by PLA.14 In consideration of their complementary properties, PLA and PPC can be melt-compounded to prepare full biodegradable material with high tensile strength and elongation at break. However, PLA and PPC are partially miscible15 and direct melt blending of these two polymers usually leads to poor toughness. Therefore, compatibilization is very necessary for improving the mechanical properties of the PLA/PPC blends. Actually, many research efforts have been devoted to approach this

3



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goal and some interesting works were published in recent years. In Fu’s group, poly (vinyl acetate) (PVAc) and maleic anhydride (MA) were selected to improve the compatibility (or interfacial adhesion) and mechanical properties of PLA/PPC blends. Both PVAc and MA could enhance the mechanical properties dramatically.16, 17 Other low molecular weight reactive agents such as 2, 4-toluene diisocyanate (TDI),18 diphenylmethane 4,4 diisocyanate (MDI)19 were also used to promote the performance of PLA/PPC blends. Transesterification involving carbonate groups, ester groups and chain end groups could

occur

spontaneously

during

polycarbonate/polyester blends.20,

21

the

melt

blending

process

in

the

Copolymer of polycarbonate sequences and

polyester sequences generated at the interface through in situ transesterification. This copolymer acted as compatibilizer (or emulsifier) and the miscibility between phases was promoted subsequently.22 The degree of transesterification depends on the processing parameters (e.g. temperature, time etc). Under general melt processing condition, the transesterification proceeds in a low extent. By introducing catalyst, both the reaction rate and degree of transesterification can be promoted to a much higher level. Tetrabutyl titanate (TBT) was a widely used catalyst and had been proved to be efficient for catalyzing the transesterification between pairs of polyesters, such as：poly (ethylene terephthalate) and polycarbonate,23 poly (lactic acid) and poly (butylene adipate-co-terephthalate),24 poly (lactic acid) and polycarbonate,25 poly (lactic acid) and poly (ω-hydroxytetradecanoic acid)26 etc. This work concentrated on the application of catalytic transesterification upon the 4


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PLA/PPC system. The effects of catalyst amount and component weight ratio on the toughness, thermal properties and deformation mechanism were studied.

2. EXPERIMENTAL SECTION 2.1. Materials The poly (lactic acid) (PLA) (trade name 4032D) used in this study was obtained from Natureworks LLC (USA). Poly (propylene carbonate) (PPC) was kindly supplied by Taizhou Bangfeng Plastic Co., Ltd. (Wenling, Zhejiang, China). PLA and PPC pellets were dried at 50 oC in a vacuum oven for 12h to remove residual moisture. Maleic anhydride (MA) (chemical pure grade) was purchased from Beijing Chemical works, China. Tetrabutyl titanate (TBT) (chemical pure grade) was purchased from Tianjin Guangfu fine chemical research institute, China. 2.2 End-capping of PPC In order to minimize the thermal decomposition of PPC during the melt-processing at 180 oC, the end-capping of PPC was performed according to the method proposed by Fu et al27 : PPC and MA (weight ratio of 99:1) were reactive blended in HAAKE Torque Rheometer at 130 oC with a rotor speed of 50 rpm for 7 min. The product: PPC-MA was then cut into small pieces for the next procedure. Pure PPC and PPC-MA sheets for tensile test were compression molded at 130 oC with a holding pressure of 10 MPa for 4 min and cooled under 10 MPa for 10 min. 2.3 Titration test The grafting degree of PPC-MA (G) was tested according to the reported method 27, 5



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28

. Prior to titration test, PPC-MA was dissolved in chloroform at a concentration of 5

w/v% and then precipitated with excessive ethanol. This purification process was repeated again to remove the ungrafted MA. Pure PPC was also purified by this procedure to remove any impurity. The purified PPC-MA and PPC were dried in vacuum oven at 55 oC for 2 days. 0.5g well-dried PPC-MA or PPC was dissolved in 50 ml chloroform. This PPC-MA/chloroform (or PPC/chloroform) solution was titrated by 0.01 (mol L-1) potassium hydroxide/ethanol solution while phenolphthalein was used as the indicator. The grafting degree (G) of PPC-MA was defined as weight percent of grafted MA on the PPC chain and calculated from Eq. (1)

V 1 −V 0 G ( wt % ) = 1000

N

MMA

2W

×100% =

(V 1 − V 0) NMMA × 100% 2000W

(1)

Where, V1 (ml) is the base volume consumed by PPC-MA, V0 (ml) is the base volume consumed by pure PPC, N (mol L-1) is the base concentration, W (g) is the weight of PPC-MA (or PPC) and MMA (g mol-1) is the molecular weight of MA. 2.4 Thermo gravimetric analysis Thermo gravimetric analysis was performed using a Mettler Toledo instrument (TGA) under nitrogen atmosphere. The samples were heated from room temperature to 500 oC at 10 oC/min. 2.5 Blend preparation PLA/PPC and PLA/PPC-MA blends with or without TBT (formulations of the blends were shown in Table 1 and Table 2 respectively) were prepared in HAAKE Torque Rheometer at a barrel temperature of 180 oC and a rotation speed of 80 rpm for 5 min. The blends were compression molded at 200 oC with a holding pressure of 6


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10 MPa for 4 min. Thereafter, the mold was removed to another unheated compressor and cooled under 10 MPa for 10 min. Specimens for tensile test were cut into dumbbell-shape with a dimension of 50 mm (length-overall) ×4 mm (width of narrow section) ×1 mm (thickness). Specimens for impact test were cut with the dimension of 63.5mm×12.7mm×3.2mm. 2.6 Mechanical properties tests The tensile properties were measured at 20 oC according to GB/T 1040.1-2006 (this Chinese standard was equivalent to ISO 527-1:1993) with Instron 1121 material tester at a cross-head speed of 30mm/min. Five specimens were used for each composition to obtain a reliable mean value and standard deviation. Notched Izod impact test was carried out according to ASTM D-256 using a XJU-2.75 impact tester (Chengde Test Machine Company, China) at 20 oC. The specimens were V-notched (45o) and conditioned at 20 oC for 24 h prior to testing. Six specimens were tested for each batch and the average value was reported. 2.7 NMR measurement Quantitative

13

carbon

nuclear magnetic

resonance

(13C NMR) data of

PLA/PPC-MA/TBT (40/60/1) blend was recorded by a Bruker 400MHz spectrometer at room temperature with deuterated chloroform as solvent. 2.8 Gel permeation chromatography Gel permeation chromatography (GPC) tests were carried out at 25 oC on Waters 410 system equipped with Waters 2414 refractive index detector. Chloroform was used as the eluent at a flow rate of 1.0 ml/min. Calibration was performed with 7



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polystyrene standards. 2.9 Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was performed in tensile mode with a Mettler Toledo instrument (DMA/SDTA861e). The storage modulus (E΄), loss modulus (E΄΄) and dynamic loss factor (tanδ) were measured from 10 oC to 80 oC at a heating rate of 3oC/min. The frequency was fixed at 1 Hz. 2.10 Differential scanning calorimetry Differential scanning calorimetry (DSC) measurements were conducted using a Mettler Toledo instrument (DSC1) under nitrogen atmosphere. All the samples were heated from room temperature to 200 oC at 10 oC/min and kept isothermally at 200 oC for 5min to eliminate previous thermal history. They were then cooled to -20 oC at 5 o

C/min and immediately reheated from -20 oC to 200 oC at 5 oC/min. The glass

transition temperature (Tg), cold crystallization temperature (Tcc) and the melting temperature (Tm) were determined from second heating scans. 2.11 Microstructure characterization Microstructure was observed by field-emission-gun environmental scanning electron microscope (FEG ESEM) (XL30, FEI COMPANY) with an accelerating voltage of 10 kV. The sample surfaces were coated with a thin layer of gold before SEM observation.

8


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3. RESULTS AND DISCUSSION 3.1 Mechanical properties The tensile properties of PLA, PPC and their blends with or without transesterification catalyst TBT were exhibited in Table 1. Pure PLA was quite rigid with high tensile strength (~76 MPa), but fairly brittle with low elongation at break (~6%). In contrast, PPC was a ductile material with high elongation at break (~900%) and relatively soft with low tensile strength (~12 MPa). When they were blended, both tensile strength and elongation at break of the PLA/PPC blends were very low. Even though TBT was added, the toughness of PLA/PPC/TBT blends still could not be improved. Table 1. Tensile properties of PLA/PPC blends (20 °C) Sample

Tensile strength (MPa) Elongation at break (%)

PLA

76.4±1.2

5.7±0.6

PPC

12.1±1.2

901.1±73.7

PLA/PPC (30/70)

15.6±0.9

1.1±0.1

PLA/PPC/TBT (30/70/1)

14.9±3.7

1.4±0.2

PLA/PPC (40/60)

16.4±1.7

1.1±0.2

PLA/PPC/TBT (40/60/1)

14.0±2.1

1.1±0.1

Pure PPC suffered thermal degradation a lot during the melt processing at 180 oC and the tensile properties of PLA/PPC blends were dragged down by this degradation. In this situation, MA moieties was grafted onto PPC chain to end-cap the terminal hydroxyl groups (Scheme 1).

27, 28

As a result, chain unzipping degradation could be 9



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restrained effectively.

Scheme 1. Melt end-capping of PPC with MA The enhanced thermal stability of PPC-MA was proved by thermo gravimetric analysis (TGA) (Figure 1). Although the grafting degree of MA on PPC chain was only 0.12% (obtained from titration test), the onset thermal decomposition temperature increased from ~150 oC (pure PPC) to ~200 oC (PPC-MA). Namely, the thermal stability of PPC-MA was significantly improved.

Figure 1. TGA curves of PPC and PPC-MA The modified PPC (i.e. PPC-MA) was then blended with PLA at different weight ratio and the mechanical properties of PLA/PPC-MA blends with or without TBT 10


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were shown in Figure 2 and summarized in Table 2. For the PLA/PPC-MA (30/70) blend, the elongation at break was only 5.2±1.2 %, whereas it sharply increased up to 382.1±64.5 % for PLA/PPC-MA/TBT (30/70/1) blend. Similarly, the elongation at break of PLA/PPC-MA/TBT (40/60/1) blend was 253.1±36.2 % compared with 4.4±1.1 % of PLA/PPC-MA (40/60) blend. When the weight ratio of PLA to PPC-MA was 50:50, 2phr TBT was needed to toughen the blend. Further increasing the weight ratio to 70:30, the toughness could not be improved any longer by the addition of 2phr TBT. Comparing with the substantial increment of elongation at break, tensile strength decreased when TBT was added. For example, tensile strength decreased from 53.6±2.0MPa (PLA/PPC-MA, 40/60) to 40.1±2.5MPa (PLA/PPC-MA/TBT, 40/60/1). The reasons for these changes will be discussed in the following parts.

Figure 2. Elongation at break (a) and tensile strength (b) of PLA/PPC-MA blends with or without TBT (20 °C). The impact resistance of the blends was not improved whatever the range of weight ratio or catalyst amount was. This was because no toughener like rubber or elastomer formed during the reactive blending and the impact energy could not be dissipated within the specimen efficiently during the test. 11



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Table 2. Mechanical properties of PLA/PPC-MA blends (20 °C) Notched Izod PLA/PPC-MA TBT

Tensile strength

Elongation at

(MPa)

break (%)

impact strength (wt/wt)

(phr) -2

(kJ m ) 30/70

0

3.3±0.3

36.6±5.8

5.2±1.2

30/70

1

3.2±0.3

25.7±0.6

382.1±64.5

40/60

0

3.9±0.4

53.6±2.0

4.4±1.1

40/60

1

3.8±0.3

40.1±2.5

253.1±36.2

50/50

0

3.8±0.3

44.6±3.4

6.8±1.9

50/50

1

2.9±0.3

42.6±0.7

10.8±1.2

50/50

2

1.6±0.3

17.1±1.5

76.9±23.2

70/30

0

4.2±0.3

50.3±3.1

8.8±2.7

70/30

1

3.0±0.3

48.5±2.0

5.9±1.0

70/30

2

*

17.9±6.5

1.7±0.5

PLA

-

4.4±0.3

76.4±1.2

5.7±0.6

PPC-MA

-

3.7±0.6

19.6±1.7

599.6±60.7

*: The PLA/PPC-MA/TBT (70/30/2) blend was too brittle to be impact tested 3.2 Transesterification mechanism

12


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Figure 3. 13C NMR spectrum of PLA/PPC-MA/TBT (40/60/1) blend Since

13

carbon nuclear magnetic resonance spectroscopy was highly sensitive to

particular monomer sequences, quantitative

13

C NMR data of PLA/PPC-MA/TBT

(40/60/1) blend were recorded to study the transesterification mechanism in detail and the result was displayed in Figure 3 and Table 3. The signals of carboxyl groups for PLA homopolymer (carboxyl PLA) appeared at 169.59 ppm. From the selected partial enlargement of

13

C NMR spectrum, the signals of carbonate groups for PPC-MA

homopolymer (carbonate

PPC-MA)

at 154.69 ppm and triple peaks from 154.28 to

154.18 ppm were also clearly detected. The triple peaks should be assigned to the carbonate groups of PPC-MA that chemically bonded to PLA29 (i.e. the carbonate groups of PLA-co-PPC copolymer generated during the catalytic transesterification). Table 3. 13C NMR signals of PLA/PPC-MA/TBT (40/60/1) blend

13



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Type

Chemical shift (δ, ppm)

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Structural fragment O

Carboxyl PLA

169.59

CH C

O

CH3

n O

Carbonate PPC-MA

154.69

CH CH2 O

C

O m

CH3

Carbonate PLA-co-PPC

154.28, 154.23, 154.18

O

O

CH CH2 O C

O CH C

CH3

Deuterated chloroform (solvent)

O

CH3

77.38, 77.06, 76.74

CDCl3 O

Methine PPC-MA

73.44

CH

CH2

O

C

O m

CH3

O

Methine PLA

72.37

CH C

O

CH3

Methine PLA-co-PPC

72.12

n

O

O

CH CH2 O C

O CH C

CH3

O

CH3 O

Methylene PPC-MA

70.59

CH

CH2

O

C

O m

CH3

Methylene PLA-co-PPC

69.17, 68.99

O

O

CH CH2 O C

O CH C

CH3

O

CH3 O

Methyl PPC-MA

19.45

CH

CH2

O

C

O m

CH3

O

Methyl PLA

16.63

CH C

O

CH3

Methyl PLA-co-PPC

16.22, 16.18

n

O

O

CH CH2 O C

O CH C

CH3 14


CH3

O

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According to the transesterification reaction mechanism proposed in other polymer blending systems,

20, 25, 30-33

we assumed the transesterification mechanism between

PLA and PPC-MA as summarized in Scheme 2. There might be three types of transesterification reaction occurred during the melt mixing: alcoholysis, acidolysis and direct ester-carbonate exchange.

Scheme 2. Possible reaction mechanism between PLA and PPC-MA during the catalytic transesterification PLA and PPC-MA were fed nearly in equimolar ratio to prepare PLA/PPC-MA/TBT (40/60/1) blend. However, from the product, the integral area of carboxyl

PLA

13

C NMR spectrum of the

was much higher than that of

carbonate-PPC-MA. This result indicated that less PPC-MA homopolymer remained in the product compared with PLA homopolymer. Therefore, reaction 1 and reaction 3 occurred in little chance by which PPC segments generated. Reaction 2, reaction 4 and reaction 5 were probably the main mechanisms by which the transesterification 15



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took place. Namely, PPC-MA was more active than PLA to attack the other component. PLA-co-PPC copolymer, low molecular weight homo-PLA and homo-PPC segments generated through the rupture and reorganization of the polymer chains. Consequently, molecular weight of the PLA/PPC-MA/TBT samples would change as the reaction proceeded. Gel permeation chromatography (GPC) was employed in this study to measure the molecular weight (the data were listed in Table 4). For each composition of PLA/PPC-MA blends, both number average molecular weight (Mn) and weight average molecular weight (Mw) decreased with increasing the amount of TBT. But in the meantime, polydispersity index (PDI, i.e. width of molecular weight distribution) increased. These trends supported the assumed reaction mechanism (Scheme 2): in the presence of catalyst, the macromolecular backbone fractured heavily and the short fragment reconnected randomly. As more catalyst was used, the degree of transesterification deepened and the product also degraded more substantially. This degradation drove the tensile strength of PLA/PPC-MA/TBT blends declined with increasing the TBT amount.

16


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Table 4. Results of GPC analysis about PLA/PPC-MA/TBT blends Sample

Mn

Mw

PDI

PLA

73396

91147

1.24

PPC-MA

87740

126897

1.45

PLA/PPC-MA (30/70)

73896

115690

1.57

PLA/PPC-MA/TBT (30/70/1)

61961

112905

1.82

PLA/PPC-MA (40/60)

76076

112068

1.47


49507

107744

2.18

PLA/PPC-MA (50/50)

85872

111993

1.30


58041

100604

1.73


21904

70290

3.21

PLA/PPC-MA/TBT (70/30)

84612

110107

1.30

PLA/PPC-MA (70/30/1)

79371

104469

1.32


34691

74602

2.15

According to the proposed mechanism (Scheme 2), no cross-linking was expected to occur during the transesterification and this assumption was supported by the solubility test. Since chloroform was the good solvent for both PLA and PPC-MA, PLA/PPC-MA/TBT samples should be dissolved in chloroform easily if no cross-linked content existed. The pictures (Figure 4) showed that all the solutions were completely limpid without any visual gel and these phenomena demonstrated that no cross-linking occurred during the transesterification reaction. Therefore, the improvement of tensile properties had nothing to do with cross-linking. 17



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Figure 4. Solutions of PLA/PPC-MA/TBT samples in chloroform (10mg/10ml) (a): PLA/PPC-MA/TBT (30/70/1); (b): PLA/PPC-MA/TBT (40/60/1); (c): PLA/PPC-MA/TBT (50/50/1); (d): PLA/PPC-MA/TBT (50/50/2); (e): PLA/PPC-MA/TBT (70/30/1); (f): PLA/PPC-MA/TBT (70/30/2). 3.3 Thermal behavior characterization The effect of transesterification on the thermal behaviors of PLA and PPC-MA was evaluated through the dynamic mechanical analysis (DMA). In the DMA results (Figure 5), the plots of tan δ against temperature show prominent peaks which correspond to the glass transition of polymer and the maximum value for each peak is used to define the glass transition temperature (Tg) (Table 5). For the PLA/PPC-MA blends, two peaks appeared between 20 oC and 80 oC, the higher one corresponded to PLA phase and the lower one corresponded to PPC-MA phase. Usually, the glass transition peak of different polymer components approached each other if the compatibility was improved in some level.34, 35 In this study, after 1phr 18


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TBT was incorporated, the difference in the values of Tg between PLA and PPC-MA decreased slightly for PLA/PPC-MA (30/70) blends, remained nearly unchanged for PLA/PPC-MA (40/60 and 50/50) blends, or increased slightly for PLA/PPC-MA (70/30) blends. On the whole, the influence of 1phr TBT on the compatibility was not very obvious from the DMA results. However, in any of these cases, the Tg of both PLA and PPC-MA decreased after the addition of TBT. This decrement might be caused by the plasticization effect of low molecular weight homo-PLA and homo-PPC segments derived from the degradation of PLA and PPC-MA during the catalytic transesterification. When the addition amount of TBT increased to 2phr, the glass transition peaks of PLA and PPC-MA merged into one peak (Figure 5 c, d), meaning that the amount of PLA-co-PPC copolymer was produced in a much more extent to improve the compatibility between the two components more greatly.

19



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Figure 5. Dynamic mechanical analysis of PLA/PPC-MA/TBT blends: damping parameter. (a): PLA/PPC-MA (30/70) and PLA/PPC-MA/TBT (30/70/1); (b): PLA/PPC-MA (40/60) and PLA/PPC-MA/TBT (40/60/1); (c): PLA/PPC-MA (50/50), PLA/PPC-MA/TBT (50/50/1) and PLA/PPC-MA/TBT (50/50/2); (d): PLA/PPC-MA (70/30), PLA/PPC-MA/TBT (70/30/1) and PLA/PPC-MA/TBT (70/30/2); (e) PLA and PPC-MA

20


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Table 5. Thermal characteristics of PLA/PPC-MA/TBT blends Tg (oC),from DMA

Tg (oC),from DSC

Tcc (oC)

Tm (oC)

Sample PLA

PPC-MA

PLA

PPC-MA

PLA

PLA

Neat PLA

70.4

-

58.1

-

103.0

168.5

Neat PPC-MA

-

40.8

-

28.2

-

-

PLA/PPC-MA(30/70)

58.2

37.5

51.7

29.9

93.2

166.7

PLA/PPC-MA/TBT(30/70/1)

52.6

34.3

49.7

29.3

90.6

166.5

PLA/PPC-MA(40/60)

59.8

34.2

52.1

30.2

95.3

167.5


54.4

28.7

48.5

28.2

93.8

165.9

PLA/PPC-MA(50/50)

60.9

36.2

53.8

30.6

96.7

167.8


53.8

29.4

50.9

28.2

95.0

166.8

N.D.*

22.0

N.D.

158.8


One peak

PLA/PPC-MA(70/30)

61.3

36.6

54.8

30.7

97.3

168.3


59.7

32.8

53.9

29.8

95.4

167.0

N.D.

19.3

N.D.

161.2


One peak

*N.D. - not detected The molecular interaction between PLA and PPC-MA was also investigated by differential scanning calorimetry (DSC). Figure 6 depicts the thermal behaviors of PLA/PPC-MA/TBT blends during the second heating scans and the thermal properties are also summarized in Table 5. The glass transition process, the cold crystallization exothermic peak with its maximum at Tcc and the melting endothermic peak at Tm are clearly displayed in the thermograms. 21



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On the DSC trace of neat PPC-MA (Figure 6e), one can only observe the glass transition process and this result confirms that PPC-MA is amorphous polymer. In the PLA/PPC-MA blends, amorphous PPC-MA could act as “solvent”.36 Due to this solvent effect, the chain mobility of PLA was promoted as evidenced by the simultaneous decreasing of Tg, Tcc and Tm with increasing the PPC-MA content. After 1phr catalyst TBT was incorporated into the binary blends, the above mentioned thermal peaks of PLA and PPC-MA shifted to lower temperature compared with the correspondent non-catalyzed system. Reasonable explanation might be that the PLA and PPC-MA chains were easier to move under the plasticization effect of homo-PLA and homo-PPC oligomers. As 2phr catalyst TBT was added into the PLA/PPC-MA blends (50/50 and 70/30, wt/wt), the cold crystallization peak of PLA disappeared on the DSC curves while the melting endothermic peak of PLA was still visible and Tm decreased again (Figure 6c, d). These changes suggested that the chain movement of PLA was further promoted by more homo-PLA and homo-PPC oligomers.

22


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Figure 6. DSC thermograms of PLA/PPC-MA/TBT blends. (a): PLA/PPC-MA (30/70) and PLA/PPC-MA/TBT (30/70/1); (b): PLA/PPC-MA (40/60) and PLA/PPC-MA/TBT (40/60/1); (c): PLA/PPC-MA (50/50), PLA/PPC-MA/TBT (50/50/1) and PLA/PPC-MA/TBT (50/50/2); (d): PLA/PPC-MA (70/30), PLA/PPC-MA/TBT (70/30/1) and PLA/PPC-MA/TBT (70/30/2); (e) PLA and PPC-MA The DMA and DSC characterizations consistently pointed out that the degree of transesterification increased with increasing the TBT dose. The transesterification products: homo-PLA and homo-PPC oligomers plasticized and PLA-co-PPC 23



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Page 24 of 32

copolymer compatibilized the PLA/PPC-MA/TBT composites respectively. As the result, the elongation at break increased by dozens of times. The plasticization effect would also induce the decrement of tensile strength. However, when the PLA content was equal to or more than 50 wt%, the elongation at break could not be increased as easy as PPC-MA dominated in the blends although the miscibility was greatly improved by 2phr TBT. This might because the mechanical property was dominated by the rigidness of PLA in this situation. 3.4 Morphology In order to explore the effect of catalytic transesterification on the fracture behavior during the tensile test, scanning electron microscope (SEM) characterization of two representative blends were performed and the pictures are shown in Figure 7. Without catalyst TBT (Figure 7a), spherical PLA particles are distinguishable on the smooth brittle failure surface. This phenomenon denotes that the effect of transesterification can be almost ignored without catalyst. The interfacial adhesion between PLA and PPC-MA was very weak and the PLA/PPC-MA/TBT (40/60) composite was not well plasticized either. Thus, the material could not yield under the tensile stress, leading the high tensile strength while elongation at break was still very low. However, when 1phr TBT was added during the melt compounding process, transesterification was accelerated

a

lot.

Benefiting

from

this

catalytic

transesterification,

the

PLA/PPC-MA/TBT (40/60/1) blend was plasticized, namely, the free volume among the polymer chains was expanded by the homo-PLA and homo-PPC oligomers which would lead to the decrement of tensile strength. The softened material could be 24


Page 25 of 32

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stretched in pace with the tensile process and severe plastic deformation spread all over the ductile failure surface (Figure 7b). The tensile stress could be dissipated a lot within the PLA/PPC-MA/TBT (40/60/1) blend before ultimate fracture and the elongation at break was dramatically increased.

Figure 7. SEM photographs of tensile fractured surfaces (a): PLA/PPC-MA (40/60); (b): PLA/PPC-MA/TBT (40/60/1)

4. CONCLUSIONS Catalytic transesterification was successfully applied to the PLA/PPC-MA system. The elongation at break of PLA/PPC-MA/TBT blends was greatly increased comparing with PLA/PPC-MA blends when the PPC-MA content was higher than 50 wt%. The GPC results indicated that in the catalyzing condition, the PLA and PPC-MA macromolecular backbone fractured acutely. Homo-PLA oligomer, homo-PPC oligomer and PLA-co-PPC copolymer generated as the products of transesterification. The oligomers plasticized and copolymer compatibilized the PLA/PPC-MA/TBT composite respectively as demonstrated by the DMA curves. The DSC thermograms also implied that the chain mobility of PLA and PPC-MA was promoted by the homo-PLA and homo-PPC oligomers. As inferred from SEM 25



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pictures, the PLA/PPC-MA/TBT (40/60/1) blend plastic deformed under the tensile stress. In summary, for different compositions of the PLA/PPC-MA/TBT blends, the plasticization effect of homo-PLA, homo-PPC oligomers and the compatibilization effect of PLA-co-PPC copolymer were believed to be responsible for the improvement of toughness. However, if the PLA content was higher than 50 wt%, no rise in toughness could be observed even though 2phr catalyst was added. Besides, substantial

degradation

and

plasticization

effect

derived

from

catalytic

transesterification resulted in the decrement of the tensile strength.

AUTHOR INFORMATION *Corresponding Authors: Guiyan Zhao: Telephone: +86-43185262642; Fax: +86-43185262126; E-mail: [email protected] (G.Y. Zhao). Wei

Jiang:

Telephone:

+86-43185262151;

Fax:

+86-43185262126,

E-mail:

[email protected] (W. Jiang).

ACKNOWLEDGEMENTS The financial support of the National Natural Science Foundation of China for Youth Science Funds (No.51203156) is gratefully acknowledged.

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Effects of Catalytic Transesterification and ... - ACS Publications

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