poly (propylene carbonate)

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Synthesis of an efficient processing modifier silica-g-poly (lactic acid)/poly (propylene carbonate) and its behavior for poly (lactic acid)/poly (propylene carbonate) blends. Zhao Wang, Xiangling Lai, Min Zhang, Wei Yang, and Ming-Bo Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03444 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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Industrial & Engineering Chemistry Research

Synthesis of an efficient processing modifier silica-g-poly (lactic acid)/poly (propylene carbonate) and its behavior for poly (lactic acid)/poly (propylene carbonate) blends Zhao Wang a, Xiangling Lai a, Min Zhang a, Wei Yang ab, Mingbo Yang ab.* a

College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, Sichuan, People’s Republic of China

b

State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, People’s Republic of China

ABSTRACT An efficient processing modifier silica-g-poly (lactic acid)/poly (propylene carbonate) (SiO2-g-PLA/PPC) was synthesized by grafting reaction and poly (lactic acid)/poly (propylene carbonate) (PLA/PPC) nanocomposites were prepared using a Haake torque rheometer. Fourier transforms infrared (FTIR) spectrometry, 1H nuclear magnetic resonance (1H NMR), thermogravimetric analysis (TGA) and rheological results showed together that the PLA and PPC molecular chains were successfully grafted on the surface of nano-silica (nano-SiO2). The scanning electron microscopy (SEM) photographs showed that SiO2-g-PLA/PPC could effectively improve the interface adhesion of PLA/PPC blends, so that the compatibility of the two improved. Moreover, SiO2-g-PLA/PPC could effectively toughen PLA/PPC blends, accelerate crystallization and increase the relaxation time of PLA/PPC blends which leads to an improvement of processing performance effectively. All the above improvements of PLA/PPC nanocomposites were attributed to the strong entanglements between polymer chains grafted on the nano-SiO2 and the polymer matrix.

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Key words: poly (lactic acid) (PLA); poly (propylene carbonate) (PPC); processing modifier; biodegradable; behavior

1. INTRODUCTION In recent years, environmental pollution and energy shortage problem1,2 make it imperative to develop biodegradable materials. Poly (lactic acid) (PLA) is one of the most promising biodegradable polymer materials, it comes from nature and returns to nature3,4. Because of some excellent properties such as good biocompatibility, mechanical strength and excellent transparency, it has been widely used as scaffold fabrication5,6, stretch-blown bottle7, tissue regeneration8 and so on. However, it still has some fatal flaws such as low crystallization rate, high brittleness, low melt strength and relatively high permeability toward gases and vapor, which still limit its use in the field of daily necessities such as packaging and plastic bag9-11. In recent years, researchers mainly use methods such as blending PLA with other polymers12,13, adding nucleation agent14,15, forming a stereogenic complex structure between PLLA and PDLA15-17 and so on to improve the toughness and crystallinity of PLA, while there are still some problems, such as the bad compatibility of the blends, the adverse influence of nucleating agent on the toughness of crystalline PLA and the quite harsh condition of forming PLLA/PDLA stereocomplex, all of which are very urgent to be solved. By grafting the PLA long chain molecules onto the surface of nanometer silica (nano-SiO2) and adding the grafted product to the PLA matrix, Wu et al.18,19


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effectively solved the problem of melt fracture during PLA blown film, the mechanism is that the PLA-g-SiO2 nanoparticles could construct the filler network in the PLA matrix, so the relaxation time of PLA molecular chains could be prolonged effectively, thus improved the melt strength of PLA significantly, while the toughness of nanocomposites still needs to be further improved. Poly(propylene carbonate) (PPC)

is a new biodegradable thermoplastic

synthesized by alternating copolymerization of carbon dioxide (CO2) and propylene oxide20-23. Due to the utilization of CO2 as one of the reaction components, the synthesis of PPC exhausts CO2, which can effectively reduce the environmental pollution. Simultaneously, because of weak molecular chain interaction and existence of many weak polar, flexible C–O–C bonds in the backbone, PPC is amorphous polymer with good melt flowability and better toughness, which is complementary to PLA, theoretically can be used to toughen the PLA. Extensive scientific work24-27 has been carried out about the simple blending of PLA and PPC, and the results manifested that the elongation at break increased but the tensile strength decreased of PLA/PPC blends with increasing of PPC content. Moreover, the addition of a certain amount of PPC into PLA matrix can also induces a faster growth of PLA spherulites which can increases the crystallization rate of PLA28. There is no doubt that the properties of a polymer blend are closely related to the compatibility of two components. Ma et al.29 reported that PLA and PPC were partially miscible because the blends appeared two Tg while manifested specific interactions due to the similar chemical structure of the two components. And in Cao’s



research30, PLA and PPC were more compatible at 30wt% PPC and below, phase separation occurred in the case of PPC content higher than 30wt%. Therefore, the compatibilization between PLA and PPC is still required to further improve its interfacial adhesion and then prepare blends with desirable properties. At present, the main measure to improve the compatibility of the PLA/PPC blends is to add compatibilizer or the third component which can reacts with PLA or PPC. Yao et al.31 reported that the toughness of PLA/PPC blends could be improved dramatically while the strength was almost kept constant by adding very low content of maleic anhydride (MA) into the blends, which resulted from the improvement of interfacial interaction between PLA and PPC phases. Gao et al.32 found that homopolymer poly (vinyl acetate) (PVAc) could be used to improve the compatibility of PLA/PPC blends. Because the interface-localized PVAc acted as not only a compatibilizer to improve the phase morphology remarkably but also as a bridge to enhance the interfacial bonding, the mechanical properties of blends with PVAc increased significantly, including tensile strength and elongation at break. Additionally, Wang et al.33 proved the compatibility of PLA/PPC blend fibers can be effectively improved by adding a small amount of ethylene-maleic anhydride copolymer (ZeMac) as a reactive compatibilizer in melt spinning. The tensile strength of blends with 0.7wt% ZeMac increased nearly double compared with the simple blends, while maintaining high elongation at break. On the other hand, some low molecular weight chemicals with reactivity can also be used to improve the compatibility of polymer blends by forming the block, grafted


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or branched copolymer during the blending process. As an example, Wang et al.34,35 reported that the interface between PLA and PPC phase became inconspicuous when the 1phr diphenylmethane-4, 4’-diisocyanate (MDI) was added, which leaded to an improvement of blends’ toughness. Zhao et al.36 added 2, 4-toluene diisocyanate (TDI) in the melt blending procedure of PLA/PPC blend as a reactive agent, and the results indicated that the interface interaction of blends was enhanced due to the existence of the situ produced chain-extended product, the tensile strength and the elongation at break increased simultaneously. Therefore, in order to improve the crystallization performance, toughness, and processability of PLA to broaden its application, a new, multi-functional, efficient processing modifier which was named as SiO2-g-PLA/PPC was synthesized in this work. The structure of the processing modifier was characterized by Fourier transforms infrared (FTIR) spectrometry, 1H nuclear magnetic resonance (1H NMR), thermogravimetric analysis (TGA) and rheological behavior analysis. The particle-particle interactions of nano-SiO2 before and after grafting reaction were studied by rheological behavior analysis of nanoparticle suspensions. The grafted nano-SiO2 was then added as a processing modifier to PLA / PPC blends, the phase morphology, crystallization behavior, crystal morphology, relaxation behavior and mechanical properties of PLA/PPC nanocomposites were characterized by scanning electron microscopic (SEM) observation, differential scanning calorimetry (DSC), polarizing optical microscopy(POM), the rheological behavior analysis and tensile tests.



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2. EXPERIMENTAL SECTION

2.1 Materials

PLA (trade name 4032D) was purchased from Natureworks LLC. USA, with an average molecular weight Mw=17.71×104g/mol, a molecular weight distribution index of about 1.91 as measured by gel permeation chromatography (GPC) and a D-unit content of 2% as determined by the supplier. PPC was produced by Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese

Academy

of

Sciences,

having

an

average

molecular

weight

Mw=98.87×104g/mol. and a molecular weight distribution index of about 6.36, as measured by GPC. Fumed Silica named Aerosil 200 (hydrophilic) was provided by Zhoushan–Mingri, China. The SiO2 showed spherical shape with a large surface area (>200 m2g-1) and an average particle size of ~12 nm. The silanol group content was 1.37 mmol/g by volumetrically measuring. Tetrabutyl titanate (TBT) with the purity of 98% was purchased from J&K Scientific Co., Ltd China and was used as accepted. Analysis of pure grade 2,4-toluene diisocyanate (TDI) was purchased from Sigma Aldrich Co., Ltd China. Toluene with the purity of 99% was purchased from Chengdu Haihong experiment instrument Co., Ltd and was dried over calcium hydride for more than 24 hours and distilled before use. Dichloromethane、chloroform and calcium hydride were purchased from Chengdu Haihong experiment instrument Co., Ltd and used without further purification.


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2.2 Synthesis of SiO2-g-PLA/PPC processing modifier

The synthesis of PLA and PPC grafted SiO2 nanoparticle via “grafting to” method. Before the experiment, PLA was dried in a vacuum oven at 60oC for 24 hours and PPC was dried in vacuum oven at 40oC for 24 hours, toluene were dried over CaH2 over-night and distilled before used. Then take 10g PLA and 15g PPC in a three-necked flask (the reason for this mass ratio of PLA and PPC is that the mechanism of chain scission reaction was different between PLA and PPC under the action of catalyst TBT, the scission reaction of PPC mainly started from the molecular chain of high molecular weight, which would lead to an inappropriate proportion of reactivity point ratio40, we found in our previous study that when the mass ratio of PLA and PPC was 2:3, the reactivity point ratio between PLA chains and PPC chains was more proportionable compared with other samples). After the experimental device was set up, it was evacuated and charged with nitrogen three times to ensure that the reaction was carried out in a nitrogen atmosphere. Then add 250ml toluene, heating up to the reflux temperature of toluene (110oC), when samples were completely dissolved, add 0.5wt% of TBT to the reaction system and start timing, TBT was used to catalyze the scission reaction of PLA and PPC molecular chain. After 2 hours of reaction, the temperature was lowered to 80 oC, after the temperature was stabilized, the toluene solution of the diisocyanate (1ml/30ml) was added to the reaction system. At the same time, 10g SiO2 nanoparticles was added to 500 ml dried toluene solution, and the mixture was stirred at room temperature for 30 minutes and then transferred to an ultrasonic cleaner for 2 hours. After the reaction was carried out



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for 2 hours, a toluene dispersion of silica was added to reaction system and start timing, the reaction stopped after 6 hours. The reaction solution was centrifuged (rotational speed 12000 rpm, time 8 min) and washed with chloroform to remove PLA and PPC molecular chains which is not grafted on nano-SiO2 as well as TBT and diisocyanate. The product was marked as SiO2-g-PLA/PPC and was dried in a 40 oC oven for 48 hours. The total synthetic route was shown in Scheme 1. O O

O

O O

PLA

O

O

PPC

O

O

O

0.5wt%TBT

0.5wt%TBT

O

+

O OH

O

O HO

+

O

C

+

N

N C O

+

OH

O

C

O

O NH

NH

NH C

NH C

C O

C O

N C

CH3

CH3

CH3 CH3

OH

O

OH

O

O

N

+

O

C

O

O O

O

O

Scheme 1. Illustration of the grafting of PLA and PPC onto SiO2 nanoparticles (SiO2-g-PLA/PPC) by “grafting to” method.

2.3 Preparation of PLA/PPC/ SiO2-g-PLA/PPC nanocomposites Prior to blending, PPC and SiO2-g-PLA/PPC were dried in vacuum oven at 40oC for 24h and PLA was dried in vacuum oven at 60oC for 24 h. In order to ensure that the nanoparticles are uniformly dispersed in the matrix of PLA/PPC blends, the raw materials are premixed in solution (dichloromethane) prior to melt blending.


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Afterwards, PLA/PPC/ SiO2-g-PLA/PPC nanocomposites were prepared by melt blending using a Haake torque rheometer (XSS-300, Shanghai Kechuang Rubber Plastics Machinery Set Ltd., China) at 165 oC, 10 rpm for 4 min and 50 rpm for 5 min. The samples with 0, 0.2, 0.5 and 0.8wt% SiO2-OH and SiO2-g-PLA/PPC nanoparticles were prepared and labeled as AC, S 0.2, S 0.5, S 0.8, G 0.2, G 0.5 and G 0.8, respectively. At the same time, as a contrast, pure PLA were also prepared. In this work, the mass ratio of PLA to PPC was 70:30. The composition and code of each group of blends were shown in Table 1. Table 1. Compositions and codes of the samples. Sample

PLA(g)

PPC-MA(g)

Nanoparticle(g)

PLA

50

-

-

AC

35

15

-

S 0.2

35

15

0.1

S 0.5

35

15

0.25

S 0.8

35

15

0.4

G 0.2

35

15

0.1

G 0.5

35

15

0.25

G 0.8

35

15

0.4

*A is PLA; C is PPC; S is SiO2-OH; G is SiO2-g-PLA/PPC.



2.4 Characterization of structure and properties

2.4.1 Fourier transforms infrared (FTIR) spectrometry

The synthesis of PLA/PPC-grafted nano-SiO2 (SiO2-g-PLA/PPC) processing modifier was confirmed by Fourier transforms infrared (FTIR) spectrometry. The testing was performed on a Nicolet 6700 FTIR spectrometer in the transmission mode at the wavelength range of 400-4000 cm-1 with the resolution of 4 cm-1. Samples of SiO2-OH and SiO2-g-PLA/PPC were respectively mixed with KBr powder and compressed into a disk for IR measurement.

2.4.2 1H nuclear magnetic resonance (1H NMR)

The further structural characterization and number-average molecular weight (Mn) of PLA and PPC molecular chains grafted on nano-SiO2 were characterized by 1H nuclear magnetic resonance (1H NMR). 1H NMR spectra were recorded in CDCl3 at 25oC on a Bruker AV 600 MPa instrument, and the peak positions were reported with respect to tetramethylsilane (TMS). The Mn of grafted PLA and PPC could be calculated through NMR43,44.

2.4.3 Thermogravimetry (TGA) analysis

In order to calculate the grafting rate (Gr) of PLA and PPC on nano-SiO2, the thermogravimetry (TGA) analysis was performed using a thermobalance (TGA Q600, TA Instruments, USA) apparatus from 30 to 800oC at a heating rate of 10oC/min under nitrogen atmosphere.


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2.4.4 Scanning electron microscopic (SEM) observation

Scanning electron microscopic (SEM) (JEOL JSM-5900LV, JEOL PTE Ltd., Tokyo, Japan) was used to observe the distribution, morphology and dispersability of different kinds of nano-SiO2 in PLA/PPC blends and the accelerating voltage was 10 kV. Samples were fractured in liquid nitrogen and the fracture surfaces were coated with gold before observation.

2.4.5 Differential scanning calorimetry (DSC)

The isothermal crystallization kinetics of PLA 、 PLA/PPC and PLA/PPC nanocomposites were studied by differential scanning calorimeter (DSC) (Q20, TA Instruments, New Castle, DE), which was calibrated with indium under a nitrogen gas flow of 50 ml min-1. Each sample (3~5mg) was firstly heated from 0 to 200oC at a heating rate of 10oC/min and kept at 200oC for 5min to eliminate the heat history. Then the sample was cooled down to the crystallization temperature (110, 120 and 130oC) with 40 oC/min and holding for 60 minutes to let the sample crystallize isothermally. The isothermal crystallization curve was recorded to describe the isothermal crystallization kinetics.

2.4.6 Rheological behavior analysis

Rheological behavior analysis of nanoparticle suspension was carried out on a stress-controlled rheometer (AR 2000ex, TA Instruments, USA) equipped with cone-plate geometry (diameter of 40mm and conicity of 2o). The gap was fixed at



53µm. Prior to measurements, the dried nano-SiO2 before and after grafting reaction were dispersed in chloroform respectively at a concentration of 2g/100ml. Then the continuous sweep was conducted in a time range from 0 to 300s at 25oC, and the applied strain and frequency were 0.1% and 1Hz respectively. The relaxation behavior analysis of PLA、PPC and PLA/PPC nanocomposites was studied with a stress-controlled rheometer (AR 2000ex, TA Instruments, New Castle, DE) equipped with parallel-plate geometry (diameter of 25mm and thickness of 1.5mm). The shear rheological test procedure was as follows: frequency sweep range was 0.01~100Hz, gap value was 1200µm and the applied strain was 0.1% (in the linear viscoelastic region of the composite melts), all the rheological experiments were conducted in a nitrogen atmosphere at 165oC.

2.4.7 Polarizing optical microscopy (POM)

The crystal morphology and nucleation effect of PLA、PPC and PLA/PPC nanocomposites were observed by an Olympus (Tokyo, Japan) polarizing optical microscope (POM, BX50) equipped with a heating stage (LK-600PM, Linkman Scientific Instruments, Surrey, UK) under nitrogen atmosphere. The test procedure was as follows: Samples were firstly heated to 200oC with the heating rate of 50oC/min and holding for 5 minutes to eliminate the heat history, then cooled down to 130oC at a rate of 50oC/min and holding for 10 minutes, snapping photos every 2 minutes.


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2.4.8 Tensile tests

Stress-strain measurements of the film samples were performed on a universal testing machine (5967, Instron, USA) using a 500N load cell with a stretch speed 5mm/min under ambient conditions. The solvent cast film samples from dichloromethane were cut to dumbbell-shaped with a thickness about 0.3mm. At least five specimens were measured for each sample.

3. RESULTS AND DISCUSSION

3.1 Structure characterization

The FTIR characterization could effectively provide the information about the structures appended to the surface of the nano-SiO2. As PLA and PPC molecular chains which were not grafted to nano-SiO2 have been removed by chloroform in centrifugal process, the information of FTIR should reflect the chemical structure of the grafted nanoparticles. The structure of nanoparticles before and after the grafting reaction was showed in Figure 1. Compared with FTIR result of SiO2-OH, the peaks at 1750cm-1、1490cm-1 and 1220cm-1 ascribed to the typical stretching vibration of carbonyl (-C=O)、the bending vibration of –CH bond and the stretching vibration of O-C-O bond29 were evident after the grafting reaction. Since both PLA and PPC molecular chains contain carbonyl and –CH bonds while O-C-O bonds are peculiar to PPC molecular chain, it can be concluded that the PPC molecular chain has been successfully grafted on the surface of nano-SiO2 but it is uncertain whether or not



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PLA molecular chain is grafted on nano-SiO2 successfully. It is worth noting that in the research of Ma et al.29, the C=O peak of pure PLA and pure PPC were in 1749cm-1 and 1738cm-1, respectively. But for PLA/PPC blends, there is only one C=O peak between the two, this is mainly due to the interaction between C-H and O=C-, C=O…O=C or C=O…O-C dipole-dipole interactions. So there is only one C=O peak in the grafted nanoparticles’ FTIR spectrum. On the other hand, only when the content of PPC is greater than 60%, the peak of O-C-O bond becomes obvious29, thus there is only a weak peak in the grafted nanoparticles’ FTIR spectrum.

Figure 1. FTIR spectrum of SiO2-g-PLA/PPC and SiO2 nanoparticles. b SiO2

O

O

CH 3

C

CH

a

O

O

CH3

C m

CH

a'

OH

c O SiO2

O

C

O

CH3 O

CH

a

CH2

b

O

* C n

CH3 O

CH


CH2

b'

OH

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Figure 2. H1 NMR spectra of PLA、PPC (a) and SiO2-g-PLA/PPC graft copolymer (b).

Since the molecular structure of PLA and PPC is similar, FTIR results did not directly prove that the PLA molecular chain is successfully grafted on the surface of nano-SiO2. In order to further characterize the structure of the grafted nanoparticles, 1

H NMR has been employed and the results were shown in Figure 2. The methine (a)、

methyl protons (b) and methine protons next to the terminal hydroxy group (a’) resonances of PLA were seen at 5.16 (a) ppm、1.57 (b) ppm and 4.36 (a’) ppm, respectively37. Peaks at 5.0 ppm (a)、4.2 ppm (b)、1.3ppm (c) and 3.7 (b’) in PPC molecular chain were contributed to the methine (a)、methylene (b)、methyl protons (c) and methylene protons next to the terminal hydroxy group (b’) respectively38, which was shown in Figure 2(a). As shown in Figure 2(b), the characteristic peaks corresponding to PLA and PPC molecular chains were all appeared in the 1H NMR spectroscopy of the grafted nanoparticles, indicating that PLA and PPC molecular chains were grafted on the surface of nano-SiO2 successfully. The ratio of grafted PLA to grafted PPC (mol/mol) as well as the molecular weights of PLA and PPC which were grafted on nano-SiO2 could be calculated from the H1NMR spectra by equation (1) and (2)19, the ratio of grafted PLA to grafted PPC (mol/mol) was about



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1/9.17 and the molecular weight of PLA and PPC grafted on the surface of nano-SiO2 was 8705g/mol. and 2657g/mol., respectively. Therefore, the average molecular weight of grafted chains calculated using the ratio above and the molecular weights of grated PLA and PPC was 3252g/mol. nPLA A = a' nPPC Ab ' / 2

(1)

A  PLA : Mn =  a + 1 * 72 + 209  Aa '  A  PPC：Mn =  b + 1 *102 + 209  Ab ' 

(2)

Where Aa and Aa ' stand for the peak areas of methine groups connected with the ester group and the terminal hydroxyl groups in PLA molecular chain, Ab and Ab ' stand for the peak areas of methylene groups connected with the ester group and the terminal hydroxyl groups in PPC molecular chain. The grafting ratios (Gr) and density of PLA and PPC molecular chains on nano-SiO2 were determined by thermo-gravimetric analysis (TGA). The polymer chains on the surface of nano-SiO2 were selectively eliminated by heating (TGA) at 10 oC/min under nitrogen from 30 oC to 800 oC and the grafting ratios and densities were calculated using equation (3)39 and (4)19, respectively.


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Figure 3. TGA curves of different samples.

Gr =

SiO2 Wgg−−PLA / PPC SiO2 1 − Wgg−−PLA / PPC

Grafting density=

Where

SiO2 Wgg−−PLA / PPC

reference

(3)

N AGr M gr S spe

(4)

is the difference between weight loss of the grafted samples and

samples

(for

SiO2-g-PLA/PPC), and

example,

SiO2 1 - Wgg−−PLA / PPC

specific surface (nm2/g) of the silica,

SiO2-OH

is

the

reference

represents the mass of SiO2. M gr

sample

S spe

of

is the

is the average molecular weight of PLA

and PPC which was grafted on nano-SiO2. N A is the Avogadro number. The TGA curves of PLA、PPC、SiO2-OH and SiO2-g-PLA/PPC nanoparticles were shown in Figure 3. We can see from Figure 3(a) that the grafted nano-SiO2 showed a significant weightlessness in the temperature range of 200oC to 600oC compared with SiO2-OH, which was mainly attributed to PLA and PPC molecular chains grafted onto nano-SiO2. Therefore, the TGA curves of SiO2-OH and grafted nano-SiO2 proved the success of the grafting reaction of PLA and PPC on nano-SiO2 particles to a certain degree. It had been calculated that the grafting ratios of PLA and PPC is 16.55% and the grafting density of PLA and PPC is 0.09 chains/nm2. Figure 3(b) showed that PLA



and PPC produced thermal degradation in the temperature range of 250oC to 389oC, while the grafted nano-SiO2 showed weight loss in the temperature range of 199oC to 598oC, which was significantly wider than that of PLA and PPC. The reason for the decrease of the temperature in initial thermal degradation might be that the catalyst TBT had not been removed completely during the post-treatment of the grafted nano-SiO2. Even so, the initial degradation temperature (199oC) was still higher than the subsequent processing temperature (165oC), thus the residual catalyst had little influence on the properties of subsequent PLA/PPC blends. The significant increase in the temperature of terminated thermal degradation proved the successful grafting of PLA and PPC molecular chains on the surface of nano-SiO2 from the side aspect, the existence of chemical bonds between PLA (PPC) molecular chain and nano-SiO2 made it difficult to degrade.

3.2 The rheological behavior of different suspensions

Figure 4. G’ (a) and viscosity (b) as functions of time for suspensions SiO2-OH and SiO2-g-PLA/PPC nanoparticles.

The particle-particle interaction could be investigated by studying the dynamic


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rheological behavior of nanoparticle suspension. Here, chloroform was used as the dispersing medium. As shown in Figure 4, the G’ (a) and viscosity (b) of SiO2-g-PLA/PPC nanoparticles were significantly lower than SiO2-OH nanoparticles, which confirmed that the particle-particle interaction was significantly decreased. The reduction of the particle-particle interaction is quite essential to the good dispersion of nanoparticles in PLA/PPC blends. At the later stage of scanning, the G’ and viscosity of SiO2 nanoparticles increased rapidly, which was caused by solvent volatilization. While for SiO2-g-PLA/PPC nanoparticles, particle-particle interaction is weak and particle-solvent interaction had been enhanced, solvent volatilization is slower than that of SiO2-OH, so its’ G 'and viscosity could be stabilized within a certain range.

3.3 Properties of PLA/ SiO2-g-PLA/PPC/ PPC nanocomposites

3.3.1 Morphology and dispersion of nanoparticles in the composites

The dispersed states of different kinds of nano-SiO2 in PLA/PPC blends studied by the SEM micrograph were shown in Figure 5. The results showed that in the system of PLA/PPC blends where the mass fraction of PPC is 30wt%, PPC was islands scattered in the PLA matrix; Both the grafted and ungrafted nano-SiO2 could be uniformly dispersed in PLA/PPC blends; Nano-SiO2 without any treatment tend to distribute in the dispersed phase PPC, while the grafted nano-SiO2 (processing modifier) tend to distribute in the interface of matrix phase PLA and dispersed phase PPC as well as dispersed phase PPC, it has the trend of phase migration from PPC to the interface of PLA and PPC as marked by the red dotted line in Figure 5; At the



same time we can find that the interface of the two phase in adding ungrafted nanocomposites was clear and there were many distinct gaps at the interface of matrix phase PLA and dispersed phase PPC as marked by the red arrow in Figure 5. However, the interface of PLA and PPC became vague and the continuity of the two phase improved significantly after adding the processing modifier SiO2-g-PLA/PPC. This phenomenon was more obvious when the content of processing modifier was 0.2wt%. With the increasing content of the processing modifier, the phenomenon of interface weakening was getting inconspicuous, the reason was that as the content of nanoparticle increases, the interaction between particles and particles increased and the interaction between the particles and the two-phase matrix became more and more weaker. The increase in the continuity of the matrix phase PLA and dispersed phase PPC indicated that the processing modifier SiO2-g-PLA/PPC could act on the interface of PLA and PPC and thus improved the compatibility of the two effectively, which was very advantageous for the improvement of the subsequent properties.


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Figure 5. SEM micrograph of PLA/PPC nanocomposites.

3.3.2 The isothermal crystallization kinetics of PLA/PPC nanocomposites

The isothermal crystallization kinetics of PLA/PPC nanocomposites was studied by DSC and the isothermal crystallization exothermic curves of different samples were shown in Figure 6. Figure 6(a) was the exothermic curves of all samples at 120oC, The results showed that by adding 30wt% content of PPC to PLA matrix, the time corresponding to the maximum exothermic peak was greatly extended and the exothermic peak was widened, which indicated that the crystallization rate of PLA/PPC blends (70:30) was slower than that of pure PLA. When adding nanoparticles with the same content (0.2wt% and 0.8wt%), the area of the exothermic peak of nanocomposites with processing modifier was larger than that of PLA / PPC blends, while the area of the exothermic peaks of nanocomposites with nano-SiO2 was less than PLA / PPC blends as shown in magnified Figure 6(a), indicating that the addition of the processing modifier could improve the crystallization ability of PLA / PPC blends to a certain degree. In particular, when the content of nanoparticles was 0.5wt%, the time corresponding to the maximum exothermic peak was greatly shortened (even closed to pure PLA) and the exothermic peak was narrowed obviously, indicating that the addition of 0.5wt% processing modifier could effectively improve the crystallization rate of PLA / PPC blends. Actually, the processing modifier SiO2-g-PLA/PPC influenced the crystallization behavior of PLA/PPC blends at two aspects. Firstly, the addition of processing modifier



SiO2-g-PLA/PPC improves the compatibility of PLA and PPC thus PPC with excellent toughness could effectively improve the mobility of molecular chain to discharge into the crystal lattice. On the other hand, SiO2-g-PLA/PPC could serve as the heterogeneous nucleating agent. Although Figure 5 showed that the compatibility of PLA and PPC in sample of G 0.2 is better than that of G 0.5, the accelerating crystallization of G 0.2 for PLA/PPC blends is not obvious due to the low content of nucleating agent.

Figure 6. Crystallization exotherms of neat PLA、PLA/PPC and PLA/PPC nanocomposites with different kind of nano-SiO2 from 110oC to 130oC (it is worth noting that the curves were not normalized).

In order to investigate the isothermal crystallization kinetics of the PLA/PPC nanocomposites at different temperatures, four specimens (PLA, AC, S 0.5 and G 0.5)


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were isothermal crystallized at 110oC, 120oC and 130oC respectively. The exothermic curves were shown in Figure 6(b), (c) and (d). The results showed that the crystallization rate at lower temperatures for PLA system was obviously higher than the crystallization rate at higher temperatures, indicating that the crystallization of PLA-based polymers is a nucleation control process, only the nucleation point increases, the crystallization rate would be accelerated, so the addition of nucleating agents is often considered to be an effective means of promoting the crystallization of PLA-based polymers. Compared with nanocomposites with nano-SiO2, the time corresponding to the maximum exothermic peak of nanocomposites with processing modifier was shortened and the exothermic peak was narrowed, this phenomenon was more obvious at high temperature, indicating that the addition of the processing modifier could effectively improve the crystallization rate of PLA / PPC, especially in the high temperature region. Relative crystallinity development was directly proportional to the evolution of heat released during the crystallization process. This relationship was depicted as equation (5). The relative crystallinity vers with time of different samples was shown in Figure 7. The results showed that it is still clear that the processing modifier could effectively improve the crystallization rate of PLA/PPC blends, especially in high temperature regions with a processing modifier content of 0.5 wt%. It is worth noting that due to the overlapping of curves in Figure 7(a) corresponding to Figure 6(a), Figure 7(a) was amplified in the time range of 6~8min.



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t

xt

∫ (dH / dt )dt = ∫ (dH / dt )dt 0 ∞

(5)

0

For all samples in isothermal crystallization at 120oC, t1/2 read from Figure 7(a) were summarized in Table 2. The results showed that compared to samples with ungrafted nanoparticles, t1/2 of samples with 0.2wt%, 0.5wt% and 0.8wt% content of processing modifier was reduced by 9.41%, 30.85% and 3.05%, respectively, indicating that the addition of the processing modifier could further improve the crystallization ability of PLA / PPC blends. In addition, t1/2 read from Figure 7(b), (c) and (d) were shown in Table 3. The results showed that the crystallization half-time was greatly shortened with the addition of processing modifier, especially at high temperature like 130oC, whose t1/2 was reduced by about 33.40%.

Figure 7. Evolution of relative crystallinity verses time during isothermal crystallization of neat


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PLA、PLA/PPC and PLA/PPC nanocomposites with different kind of nano-SiO2 from 110oC to 130oC.

Isothermal crystallization kinetics of PLA/PPC nanocomposites could be described by the classical Avrami equation as equation (6).

1 - xt = exp(−kt n )

(6)

Where xt is the relative crystallinity, n is the Avrami exponent, k is the overall crystallization rate constant, and t is the crystallization time. Table 2. The t1/2 read from Figure 7(a) for all samples in isothermal crystallization at 120oC. Sample

t1/2(min)

PLA

4.91

AC

7.79

S 0.2

8.08

G 0.2

7.32(*9.41%)

S 0.5

7.78

G 0.5

5.38(*30.85%)

S 0.8

7.87

G 0.8

7.63(*3.05%)

(* A), A is the decreased percentage of t1/2 of samples with processing modifiers compared to samples without the modifiers.

Applying logarithmic properties to both sides of equation (6), equation (7) could be obtained as follow: log[− ln(1 − xt )] = n log t + log k

(7)

The values of k and n could be calculated from the linear fitting of log[− ln(1 − xt )]

versus log t , and a linear portion of about 5-20% of relative



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crystallinity was used to obtain n and k. The relative n and k were listed in Table 3 too, the results showed that the nucleation index n of all the samples is higher than 3 at different temperatures, so the crystallization of pure PLA、PLA/PPC and its nanocomposites was heterogeneous nucleation. The higher the K value, the faster the crystallization rate is. With the addition of processing modifier, the crystallization rate of PLA/PPC blends increased, especially in the high temperature region, which was consistent with the trend of crystallization half-time. Table 3. The results from the isothermal crystallization kinetic study for different samples. Temperature(oC)

Sample

n

Log(K)(min-n)

t1/2(min)

110

PLA

5.53

-7.46

3.86

AC

5.21

-8.47

4.92

S 0.5

5.75

-9.41

5.16

G 0.5

5.24

-8.04

4.61

PLA

4.08

-6.86

4.91

AC

4.15

-8.75

7.79

S 0.5

3.97

-8.55

7.78

G 0.5

3.66

-6.40

5.38

PLA

4.02

-9.76

10.40

AC

4.19

-11.30

14.13

S 0.5

4.08

-10.89

13.54

G 0.5

4.38

-9.99

9.41

120

130

3.3.3 Crystallization morphology of PLA/PPC nanocomposites

The results of isothermal crystallization kinetics of PLA / PPC nanocomposites showed that the crystallization rate of PLA / PPC blends could be effectively ACS Paragon Plus Environment

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accelerated when the content of processing modifier was 0.5wt%. The influence of the addition of nano-SiO2 on the crystallization rate and crystal morphology of PLA / PPC blends could be observed directly by using polarizing microscope (POM) technique. The previous results showed that the nucleation effect of nano-SiO2 was mainly in the high temperature region, so the crystal morphology and crystallization rate of PLA, PPC, S 0.5 and G 0.5 samples were observed at 130oC, the results were shown in Figure 8. It could be seen from samples of PLA and AC that the amount of spherulites in the field of view of PLA / PPC blends was less than that of pure PLA, indicating that the crystallization rate of PLA/PPC (70:30) blends was lower than that of pure PLA; The addition of SiO2-OH did not have much effect on the crystallization rate and crystal morphology of PLA / PPC blends; Compared to the sample of S 0.5, the time at which the sample of G 0.5 began to crystallize was greatly reduced, when the crystallization time was 2 min, the sample of G 0.5 had formed a large amount of grains in the whole field of view, while the sample of S 0.5 came to this when the crystallization time was 4 min, it could be seen that the processing modifier did greatly in improving the nucleation efficiency of PLA/PPC blends, thereby greatly speeded up its crystallization rate, which was consistent with the previous discussion of crystallization kinetics and nucleation constants. Simultaneously, by contrasting S 0.5 and G 0.5 we could find out that the larger the nucleation effect of SiO2-OH was, the smaller spherical grain formed, so one can control the morphology of grain by adding nucleating agent, the properties of target materials would be improved, such as mechanical properties and so on.



Figure 8. The time evolution of crystallization morphology of PLA, AC, S 0.5 and G 0.5 at 130oC, the corresponding sample names were given at the left of each image (The scale bar was 200µm for all samples).

3.3.4 Relaxation behavior of PLA/PPC nanocomposites

The relaxation behavior of the polymer is the core of the overall performance of the material, including the processing performance, the relaxation time, which determines the effect of processing parameters on the properties of the material, including the shear induced orientation41 and so on. The continuous relaxation time spectra of PLA/PPC nanocomposites could be fitted according to the storage modulus (G’) and the loss modulus (G’’) values using formula (8) and (9).


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+∞

G ' (ω ) = ∫ H (τ ) −∞

ω 2τ 2 d ln τ 1 + ω 2τ 2

(8)

ωτ d ln τ 1 + ω 2τ 2

(9)

+∞

G ' ' (ω ) = ∫ H (τ ) −∞

Where τ

is the relaxation time, ω is the angular frequency, H (τ ) is the

relaxation spectrum42. According to the results of frequency sweep of different samples, the continuous relaxation spectra of PLA and PLA / PPC nanocomposites could be obtained as shown in Figure 9 (a). The relaxation time of pure PLA was very short (just about 0.02s), it would be relaxed in a short period of time, so that the ability to resist tensile deformation for PLA was very poor. After adding PPC with the content of 30wt% to the PLA matrix, the characteristic relaxation time of PLA/PPC blends moves toward the long time region and the intensity of the relaxation peak increased significantly. The addition of nano-SiO2 did not have much effect on the relaxation spectrum of PLA / PPC blends. While the addition of processing modifier would further develop long time relaxation behavior, the characteristic relaxation time of PLA / PPC nanocomposites will move to the long time region (shifted from 0.147s to 0.172s as shown in Figure 9(a)) and the peaks would become stronger further, this was because the introduction of the processing modifier could limit the movement of PLA molecular chain and make the molecular chain motion relaxation behavior becomes difficult, then the characteristic relaxation time moved to the long time region. The prolongation of the relaxation time ensured that the entanglement network of the PLA and PPC molecular chains were preserved during its relaxation time, these frozen



entangled network provided the effect of melt support during the film blown process of the PLA, making the film bubble grow steadily in the complex processing fields.

Figure 9. (a) The continuous enhanced relaxation spectra of PLA and PLA/PPC nanocomposites obtained from the frequency scanning data and (b) the weighted relaxation spectra of PLA and PLA/PPC nanocomposites obtained from the creep data.

The continuous relaxation spectra results in Figure 9 (a) also showed that for PLA/PPC nanocomposites, there was also a relaxation behavior in the long time region, the stronger relaxation peak in long time region indicated that in addition to the relaxation of PLA molecular chain, there were several relaxations for other units. In order to describe the overall relaxation behavior of PLA/PPC nanocomposites, it is necessary to broaden its’ relaxation spectra. The relaxation spectra of PLA/PPC nanocomposites were broadened by creep testing in this study and the results were shown in Figure 9 (b). We could see that pure PLA only has one relaxation spectrum over the entire test time range, which belongs to the relaxation of the PLA backbone. Both PLA/PPC blend and PLA/PPC nanocomposites have a quadruple relaxation peaks in the long time region, which belonged to PLA backbone, PPC backbone, restricted chain and macro tangled chain, respectively. After the addition of grafted


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nanoparticles, the four relaxation spectra of PLA / PPC nanocomposites were significantly shifted to the long time region and the intensity of the relaxation peaks was increased obviously, which was crucial in improving the processing performance of PLA / PPC nanocomposites, especially for the film blow molding.

3.3.5 Mechanical properties characterization

Mechanical test was performed to investigate the mechanical performance of PLA、 PPC and PLA/PPC nanocomposites. The tensile strength and elongation at break of PLA, PPC and PLA/PPC nanocomposites were exhibited in Table 4 and the corresponding histogram was shown in Figure 10. It was worth noting that nanocomposites with SiO2-OH (samples of S 0.2、S 0.5 and S 0.8) would be broken under a little tensile stress, thus their elongation at break was very low (similar to that of PLA pure sample), so the results of nanocomposites with SiO2-OH were no longer given here. It was predictable that brittle fracture occurred for pure PLA from Table 4. It had a low elongation at break of 2.75% and a tensile strength of 31.23MPa which was lower than the higher tensile strength about 58.4MPa in other reports45. Actually, it might due to the different methods to obtain samples for tensile measurement and rather slower tensile rate in our study. Pure PPC was a typical toughness material, with elongation at break of 350% while tensile strength of 10.12MPa. PLA and PPC were complementary to each other, both the brittleness of PLA and the low strength of PPC was improved by simple blending. However, because of the compatibility of the blend



was not very good, resulting in the promotion of mechanical performance was greatly limited, therefore, the elongation at break of sample of AC was only 7.18%. It was worth noting that the addition of the processing modifier could improve the tensile strength and elongation at break of PLA/PPC blends significantly, and with the content of the processing modifier increased, the tensile strength and elongation at break decreased slightly but still remained at a high level. For example, compared to the simple blending sample AC, the tensile strength and elongation at break of G 0.2 increased by 18.93% and 1309.47%, respectively. The strengthening and toughening action of the processing modifier could also be explained by the interfacial action mentioned before.

Figure 10. Tensile strength (a) and elongation at break (b) of PLA、PPC and PLA/PPC nanocomposites. Table 4. Tensile strength and elongation at break of PLA、PPC and PLA/PPC nanocomposites. Tensile strength(MPa) PLA PPC AC G 0.2 G 0.5 G 0.8

31.23 10.12 25.15 29.91 28.27 27.40

± ± ± ± ± ±

0.81 0.37 0.74 0.50 (*18.93%) 0.66 (*12.41%) 0.37 (*8.95%)

Elongation at break(%) 2.75 ± 0.07 350.18 ± 19.84 7.18 ± 0.76 101.2 ± 11.52 (*1309.47%) 66.77 ± 3.71 (*829.94%) 54.8 ± 7.26 (*663.23%)

(* A), A is the increased percentage of tensile strength and elongation at break of PLA/PPC


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nanocomposites compared to the simple blending sample AC.

3.4 The acting mechanism of the processing modifier SiO2-g-PLA/PPC

Scheme 2. Schematic depiction of the acting mechanism of the processing modifier SiO2-g-PLA/PPC.

As shown in Scheme 2. The surface of the processing modifier SiO2-g-PLA/PPC is simultaneously grafted with both PLA and PPC molecular chains, in the process of subsequent melt blending, the molecular chains grafted on the nano-SiO2 could penetrate into the corresponding matrix, and entangled with its’ free molecular chains to a certain degree, and then induced the effective interfacial action.

4. CONCULSION

The processing modifier SiO2-g-PLA/PPC was synthesized first and its structure was characterized by FTIR、TGA and 1H NMR analysis. The results showed that PLA and PPC molecular chains were successfully grafted on the surface of nano-SiO2, the grafting rate of PLA and PPC calculated from TGA curves was 16.55% and the



average grafting molecular weight of PLA and PPC was about 3252g/mol. The results of rheological studies on suspensions of the two nano-SiO2 showed that the particle-particle interactions of the grafted nano-SiO2 was significantly lower than that of the ungrafted nano-SiO2, which was quite essential to the good dispersion of nanoparticles in PLA/PPC blends. SiO2-g-PLA/PPC was introduced into PLA/PPC(70:30) blends to enhance the performance of PLA material. SEM results showed that the continuity of PLA and PPC phase improved obviously after adding the processing modifier SiO2-g-PLA/PPC, the compatibility of PLA and PPC was improved obviously. The crystallization behavior investigation revealed that processing modifier SiO2-g-PLA/PPC could effectively accelerate the crystallization of PLA / PPC blends, especially in the high temperature region. The crystal morphology was observed online by POM and the results showed that the processing modifier SiO2-g-PLA/PPC did greatly in improving the nucleation efficiency of PLA/PPC blends, thereby greatly speeded up its crystallization rate. The rheological studies showed that the relaxation spectrum of PLA / PPC nanocomposites with grafted nano-SiO2 was significantly shifted to the long time region and the intensity of the relaxation peaks increased obviously, which was crucial in improving the processing performance of PLA / PPC nanocomposites. The results of mechanical properties showed that PPC could effectively toughen the PLA, and the processing modifier SiO2-g-PLA/PPC could effectively amplify this result. The acting mechanism of this processing modifier was the molecular chains grafted on the nano-SiO2 could penetrate into the corresponding matrix, and entangled


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with its’ free molecular chains to a certain degree, so the interfacial action was very strong.

AUTHOR INFORMATION

Corresponding Authors * Mingbo Yang. Tel: +86-28-8546-0127; Fax: +86-28-8546-0127. Email: (M. B. Yang) [email protected].

ORCID Mingbo Yang: 0000-0001-7950-7645

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Grant No. 51421061).

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