Thicker Lamellae and Higher Crystallinity of Poly(lactic acid) via

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Thicker Lamellae and Higher Crystallinity of Poly(lactic Acid) via Applying Shear Flow and Pressure and Adding Poly(ethylene glycol) Jia-Feng Ru, Shu-Gui Yang, Jun Lei, and Zhong-Ming Li J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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The Journal of Physical Chemistry

Thicker Lamellae and Higher Crystallinity of Poly(lactic acid) via Applying Shear Flow and Pressure and Adding Poly(ethylene glycol) Jia-Feng Ru, Shu-Gui Yang, Jun Lei,* and Zhong-Ming Li*

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China *corresponding authors e-mail: [email protected] and [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: In this work, we explored the crystallization of Poly(lactic acid) (PLA) blended with poly(ethylene glycol) (PEG) under two inevitable processing fields (i.e., flow and pressure) that coexist in almost all processing for the first time. Here, the PEG was incorporated into PLA as a molecular chain activity promoter to induce PLA crystallization. A home-made pressuring and shearing device (PSD) was utilized to prepare samples and necessary characterization methods, such as differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and synchrotron radiation, were used to investigated the joint effects of PEG, pressure and shear flow on the crystallization behaviors and morphologies of PLA/PEG samples. The results reveal that adding 3 - 5 wt% PEG into PLA can significantly increase the PLA crystallinity due to the efficient plasticization effect of PEG, while the PEG content reaches 10 wt%, the PLA crystallinity decreases drastically as the phase separation between PEG and PLA occurs. We also find that applying a higher pressure (~ 100 MPa) can facilitate the formation of thicker lamellae with fewer defects as well as higher crystallinity under an equal degree of supercooling compared to normal pressure or a low pressure condition because the slip of molecular chains during crystallization makes the lamellae thicker under higher pressures. The PLA crystalline structure in PLA/PEG sample is not influenced by the shear flow, yet the crystallinity is largely enhanced by applying a shear flow with an appropriate intensity (0 – 3.5 s-1). It is worth noting that pressure and shear flow show a synergetic effect to fabricate PLA/PEG samples with high crystallinity. These meaningful results could beyond doubt help comprehending the relationship between crystallization conditions and 2


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crystallization behaviors of PLA/PEG samples and thus provide guidance to obtain high-performance PLA/PEG products via controlling crystallization conditions.

1. INTRODUCTION

Biodegradable poly(lactic acid) (PLA), an increasingly important bio-based polymer derived from renewable resources, has attracted great interest due to its remarkable advantages such as good biocompatibility, thermal processability, and favorable mechanical properties, and hence is widely applied in the commercial fields of packaging, medicine, and tissue engineering.1-14 Although PLA is an eco-friendly bioplastic with excellent biocompatibility and processibility, it has drawbacks as well, especially its poor toughness and low crystallization ability, which largely restrict its broader applications.15-18 In order to overcome the aforementioned disadvantages and widen the application field of PLA, a powerful means is copolymerizing PLA with other monomers or macromolecules to produce

PLA-based

random

or

block

copolymers.19-22

Compared

with

copolymerization, blending with various plasticizers, such as poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG) and citrate esters, is a simpler, more economical and practical method for industrialized production to improve the toughness and to accelerate the crystallization kinetics of PLA as plasticizers help promoting the mobility of molecular chain.23-31 As one of the most efficient plasticizers for PLA, PEG is a crystallizable thermoplastic polyether possessing excellent biocompatibility, hydrophilicity, lubrication, dispersibility, high mobility, and nontoxicity. Moreover, 3



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PEG has very good miscibility with PLA because the terminal hydroxyl groups in PEG molecules can react with the carboxyl groups in PLA molecules.23,24,32 Recently, many studies have focused on the miscibility, mechanical, thermal, and rheological properties of plasticized PEG/PLA blends. Jacobsen and Fritz24 studied the influences of the content and type of plasticizers (PEG, glucose monoesters, and partial fatty acid esters) on the mechanical properties of PLA. Results show that the introduction of PEG led to the largest decline in the glass-transition temperature (Tg, nearly 30 K), elastic modulus, and tensile strength, while the biggest increases in the elongation at break (up to 180%) and impact strength when compared with the two other plasticizers. Sheth, Baiardo and their coworkers25,26 found that the degree of miscibility of PEG with PLA depended on the content (WPEG) and molecular weight of PEG. Additionally, it was reported that PEG phase would separate from PLA/PEG samples when the WPEG was higher than 10 wt%. The phase separation could deplete the amorphous phase of the plasticizer, and thus the blends became brittle at a relatively high WPEG value.27 Apart from the effect of PEG, it is well known that the crystallinity and the crystalline morphology and structure of PLA play a decisive role for the properties of final PLA/PEG products. Therefore, fully understanding the crystallization behaviors of PLA/PEG samples under processing fields (e.g., pressure and flow) is of vital importance for improving the properties of products.1,33 As we all know, pressure and flow fields are the two most vital external factors to the crystallization behaviors of PLA. Yuan et al.34 discovered that α-form formed by annealing PLA under 100 MPa 4


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at 85-95 °C but was not found under 200 MPa at 105-145 °C, demonstrating that pressure field notably influenced the crystalline structure of PLA, such as the size of crystallites, the inverse spacing, the long periods and lamellae thicknesses. Besides pressure having effects on PLA crystallization, it was also verified that shear field dramatically impacted the crystal growth rate and crystal structure of PLA 18, 35-39. The aforementioned work mainly focused on the crystallization of PLA melt under pressure or flow field alone. However, during practical processing, such as injection molding and extrusion molding, pressure and flow fields always coexist. Therefore, the above research results deviate more or less from the real situations. In order to reveal what happens for PLA during processing, we investigated PLA crystallization under the coexistence of external flow and pressure in our group’s previous work, and obtained interesting results.40 Almost exclusive β-form crystals of PLA were generated directly from melt when crystallizing PLA at a given condition (shear 13.6 s-1, pressure 100 MPa and crystallization temperature 160 °C). This charming phenomenon enlightened us to carry out further research on pressure and flow jointly induced crystallization of PLA/PEG samples. In this study, we used a homemade pressurizing and shearing device (PSD, introduced in detail in our group’s previous work) as a tool to prepare samples.40-42 The effects of the PEG content, pressure and shear rate on the crystallization behaviors and morphologies of PLA/PEG samples were investigated by means of differential scanning calorimeter (DSC), two-dimensional wide-angle X-ray diffraction (2D-WAXD), two-dimensional small-angle X-ray scattering (2D-SAXS), 5



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and scanning electron microscope (SEM). The results will theoretically reveal the relationship between the structure and properties, and provide guidance for optimizing the processing conditions to enhance the performances of the PLA/PEG products in real processing.

2. EXPERIMENTAL SECTION

Materials. The PLA used in this work comprises ~2% D-LA and was purchased from Nature Works (trade name 4032D). Its weight-average molecular weight and number-average molecular weight are 2.23 × 105 g/mol and 1.06 × 105 g/mol, respectively. The nominal melting point measured by DSC at a heating rate of 10 °C /min is 167 °C. The plasticizer PEG was obtained from Dow Chemical Company under the trade name Carbowax and has a nominal weight-average molecular weight of 3.35 × 103 g/mol.

Figure 1. (a) Model diagram of the PSD. (b) Radial distribution of the shear rate in disk specimen. (c) Schematic diagram of the measured positions.

Sample Preparation. PLA and PEG were first dried under vacuum overnight to avoid hydrolysis degradation during processing, such as extrusion or injection 6


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molding. PLA was blended with various PEG contents (0, 3, 5, and 10 wt%) by a Mini-Lab twin-screw micro-extrusion machine (Thermo Electron Corp.) at 200 °C for 5 min, and then a disc sample with a 20 mm diameter and a 1 mm thickness for rheology

testing

was

prepared

by

compression molding

(plate

vulcanizer

XLB-D400×400×2-2, Qingdao Yadong Rubber Machinery Group Co. Ltd. China) at 200 °C for 10 min under a fixed pressure of 10 MPa. For convenience, the plasticized PLA samples containing 0, 3, 5, and 10 wt% PEG are hereafter referred to as PLA0, PLA3, PLA5, and PLA10, respectively. The homemade PSD, used to prepare PLA/PEG samples under pressure and flow fields, is shown in Figure 1a. To apply designed shears on PLA/PEG melt, a rotator with a same radius as the sample was rotated at an angular speed π/12 for every sample. The sample has a radius r of 10 mm and a thickness d of 0.5 mm as Figure 1b •

•

shows. As the γ imposed on the PLA/PEG melt increases linearly with radius ( γ =

ωr/d, where ω is the angular velocity of the rotator, r the radial position on the disk, •

and d the thickness of disk), the largest shear rate ( γ max ) is 5.2 s-1 (at the edge of the disk) in this work. Figure 1c illustrates the shear rate of measuring position for sample in this work. The temperature, pressure and shear protocols were set as Figure 2 shows: (1) heating PLA/PEG blend from room temperature to 200 °C at a rate of 10 °C/min; (2) holding temperature at 200 °C for 5 min to eliminate the thermal and mechanical histories; 7



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(3) cooling to the Tc at a rate of 10 °C/min; (4) pressurizing to Pc within 5 s; (5) applying shear immediately after pressurization for 12 s; (6) isothermally crystallizing for 30 min at a temperature (Tc) and a pressure (Pc) under nitrogen atmosphere to prevent degradation; (7) cooling to room temperature at 20 °C/min under Pc. For simplicity, the disk sample which crystallizes at a given Pc and Tc is defined as PLA-Pc-Tc. For instance, PLA10-100-165 stands for a PLA/PEG sample containing 10 wt% PEG crystallizing under 100 MPa and at 165 °C. For the pressure-shearing samples, it must be noted that the specimens for all the measurements mentioned in the article were obtained from the radial positions 9.5 mm away from the disc sample •

center with γ of 5.0 s−1. Particularly, we characterized the PLA5-100-165 and •

PLA5-0-150 samples at multiple radial positions 0.5, 1.5, 2.5…9.5 mm (with γ = 0.5, 1.0, 1.5…5.0 s−1, respectively) away from the center to explore the effect of shear rate on the PLA crystallite structure and crystallinity.

Figure 2. Temperature, pressure and shear protocol applied to conduct shear-induced 8


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PLA crystallization under pressures.

Differential Scanning Calorimetry (DSC). DSC measurement was carried out using a differential scanning calorimeter (TA DSC Q2000) at a heating rate of 10 °C/min from 40 to 200 °C to detect the thermal behavior of pressure-shearing sample. The sample was cut from the vicinity of edge and the weight is about 5 mg. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD). 2D-WAXD measurements were implemented to characterize the intensive structure information of PLA/PEG samples developing during shear-induced crystallization under pressures. The 2D-WAXD measurements were performed at the assigned positions of the disk at the beamline BL15U1 of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China), equipped with an X-ray CCD detector (Model SX165, a resolution of 2048 × 2048 pixels of 80 µm × 80 µm, Rayonix Co. Ltd., America) to collect the 2D-WAXD images. The X-ray beam had a wavelength of 0.124 nm and the distance from sample to the detector was held at 137 mm. The two-dimensional patterns were azimuthally integrated to a one-dimensional profile of intensity versus scattering vector, q = 4π sinθ/λ (2θ is the scattering angle and λ is the wavelength of X-ray). To quantify the specimen crystallinity, one-dimensional WAXD intensity profiles were fitted using the origin software, assuming Gaussian profiles for the amorphous halo and all crystalline peaks. The crystallinity (Xc) is defined as follows: Xc =

A(010)α + A(200,110)α + A(203)α + A(015)α A(010)α + A(200,110)α + A(203)α + A(015)α + Aamorphous

Where A(010)α, A(200, 110)α, A(203)α and A(015)α are the areas of corresponding 9



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resolved Gaussian crystalline diffraction peaks of α crystal, and Aamorphous is the area of amorphous part.

Two-Dimensional Small-Angle X-ray Scattering (2D-SAXS). 2D-SAXS measurements were carried out at the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) to examine the superstructure of samples. The sample-to-detector distance was held at 1890 mm and the wavelength of the X-ray radiation is 0.124 nm. The 2D-SAXS images were collected with an X-ray CCD detector (Model Mar165, 2048 pixels × 2048 pixels of 80 µm × 80 µm). One-dimensional scattering intensity distribution Iq2 (q) was obtained by integrating the two-dimensional scattering patterns. Additionally, electron density correlation function analysis also has been used to the one-dimensional result analysis to describe the lamellar thicknesses of samples as introduced by Strobl.43,44 The electron density correlation function K(z) can be derived from the inverse Fourier transformation of the experimentally intensity distribution I(q) as follows:

∫ K ( z) =

∞

0

I (q )q 2 cos(qz ) dq

∫

∞

0

I ( q )q 2 dq

where z represents the location measured along a trajectory normal to the lamellar surfaces, and the multiplication of I(q) with q2 (Lorentz correction) was performed because of isotropically distributed stacks of parallel lamellae in the sample. Besides, crystalline lamella thickness (dc), amorphous layer thickness (da), and the long period (dac) were calculated according to the correlation function and provided in the 10


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Supporting Information (SI).

Scanning Electron Microscopy (SEM). The samples cut from the edge of disk were etched in a water−methanol (1:2, v:v) solution containing 0.025 mol/L of sodium hydroxide for 24 h at 25 °C. A field-emission SEM (Inspect F, FEI, Finland) was introduced to observe the crystalline morphology operating at 5 kV to avoid surface damages. Prior to observation, the etched surface was covered with a thin layer of gold to enhance electrical conductivity.

3.

RESULTS AND DISCUSSION

Melting Behaviors of Pressure-Shearing Samples. Figure 3a shows the DSC first heating curves for PLA/PEG samples crystallizing at 100 MPa and 165 °C. In order to obtain the influences of pressure on the flow-induced crystallization PLA/PEG samples, we also performed an experiment under normal pressure for comparison purpose. The PLA/PEG samples crystallizing at 150 °C and under atmospheric pressure are shown in Figure 3b, which possess the same degree of supercooling as the corresponding ones crystallizing at 165 °C and under 100 MPa.45 (i.e., the supercooling of sample PLA3-100-165 is equal to sample PLA3-0-150, the supercooling of sample PLA5-100-165 is equal to sample PLA5-0-150, and the supercooling of sample PLA10-100-165 is equal to sample PLA10-0-150.). For convenience of comparison, we set same crystallization temperature for atmospheric pressure and high pressure samples. Besides, the quantitative thermal characteristic data for atmospheric pressure and high pressure samples are listed in Table 1. From 11



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Figure 3a and Table 1, one can see the glass transition temperature (Tg) decreases from 62.7 to 61.8 °C as the PEG fraction rises from 0 to 5 wt%. The Tg of atmospheric pressure sample also decreases from 61.8 to 59.5 °C as the PEG fraction rises from 0 to 5 wt% (Figure 3b and Table 1). Besides, it is interesting to see minor endothermic peaks at 56.6 and 56.7

o

C for samples PLA10-100-165 and

PLA10-0-150, respectively. We consider the minor endothermic peaks for samples PLA10-100-165 and PLA10-0-150 should result from the melting of PEG as Sungsanit et al. reproted.27 This result demonstrates that appropriate content of PEG can reduce the Tg of PLA/PEG samples by enhancing the segmental mobility of PLA, but phase-separation of PEG occurs for both atmospheric pressure and high pressure PLA/PEG samples when adding 10 wt% PEG. In addition, as shown in Figure 3a, sample PLA10-100-165 displays an obvious cool crystallization peak at ca. 89 °C. So does for sample PLA10-0-150 (Figure 3b). The unusual cold crystallization peaks of samples PLA10-100-165 and PLA10-0-150 possibly result from the PEG phase separation as previously introduced, which could also be observed in the Cole-Cole plots as illustrated in Figure S1 in the SI. It was reported that the PLA crystallization usually could lead to a PEG phase separation when the PEG content was high (＞10 wt%) in PLA/PEG sample, and the phase separation has a confinement effect on the PLA molecule mobility.27,46 Thus, only a small fraction of PLA crystallized and just formed some thinner crystals during the isothermal crystallization in this work. The presence of these thinner crystals will lead to an increase in cold-crystallization ability during the heating process, and the PLA 12


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crystals with thinner lamellae became thicker ones through rearranging the molecules, i.e., cold-crystallization. Besides, a weak exothermal peak at about 100 °C for PLA0-100-165 can be seen, which should result from the creation of new crystals during heating.

Figure 3. DSC heating traces of pure PLA and PLA/PEG samples from 20 °C to 200 °C at 10 °C/min: (a) crystallized at 165 °C and 100 MPa and (b) at 150 °C and ambient pressure. The shear rate of measured position is 5.0 s-1.

Table 1. DSC and WAXD Data of the Pure PLA and PLA/PEG Samples Crystallizing under Ambient Pressure and 100 MPa

High pressure samples

ambient pressure samples

Samples

Tg (°C)

Tm (°C)

∆Hm (J/g)

Xc1 (%)

Xc2 (%)

PLA0-100-165

62.7

169.1

48.2

51.5

25.1

PLA3-100-165

60.3

166.7

56.0

61.7

30.7

PLA5-100-165

61.8

166.5

59.8

67.3

31.9

PLA10-100-165

—

165.4

41.0

44.9

19.5

PLA0-0-150

61.8

169.1

46.1

49.2

23.6

PLA3-0-150 PLA5-0-150

59.9 59.5

165.8 165.2

54.7 56.1

60.2 63.1

28.2 30.2

PLA10-0-150

—

164.8

37.5

40.1

14.1

Notes: Tg and Tm are the peak temperatures. Xc1 and Xc2 are obtained from DSC and WAXD data, respectively. 13



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According to Table 1, for high pressure samples, the crystallinity (Xc) significantly increases from 51.5% of sample PLA0-100-165 to 67.3% of sample PLA5-100-165, and for atmospheric pressure samples, the Xc also increases from 49.2% of sample PLA0-0-150 to 63.1% of sample PLA5-0-150. This increase in Xc is associated with the plasticizing effect of PEG. When the PEG is added their molecular chains are inserted between the PLA chains, which weakens the interactions between the PLA molecular chains and forms additional free volumes, that is, the incorporation of PEG enhances the mobility ability of PLA molecules and thus improve the crystallization ability of PLA. This result can be further verified by the rheology characterization in the SI. However, as the PEG content reaches 10 wt%, the Xc of samples PLA10-100-165 and PLA10-0-150 decline to 44.9% and 40.1%, respectively. For the reason of the abrupt drop in Xc of samples adding of 10 wt% PEG, one possible explanation is that PEG phase separation occurs during the crystallization of PLA owing to its high content, and suppressed the plasticization effect and weakened the movability of PLA molecules compared to other samples with less PEG contents.27,28,47, On basis of the DSC analysis, one can conclude that adding an appropriate amount of PEG (≤ 5 wt%) is beneficial to promote the PLA crystallization. It is unexpected that the atmospheric pressure samples possess a faint melting peak prior to the major melting peak (i.e., double melting behavior), while the high pressure samples exhibit single melting peak. About this double melting phenomenon, so far, there are three main mechanisms proposed to explain it, namely, 14


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melting-recrystallization, dual lamellae population and dual crystal structures.48,49 The melting-recrystallization mechanism suggests that the low- and high- melting peaks can

be

assigned

to

the

original

crystals

and

those

formed

through

melting-recrystallization process, respectively. The dual lamellae population mechanism associates the multiple melting of crystals with different morphologies. In the case of polymorphic polymer, the existence of different crystal structures possibly also leads to the appearance of double melting peaks. The WAXD results (Figure 5) reveal that only PLA α-form was developed for the PLA/PEG samples. Thus, we speculate that the double melting behavior for the low-pressure samples resulted from dual lamellae population (thinner or thicker PLA crystals) or from the melting of thinner crystals and recrystallization to form thicker crystals or from both. However, it is beyond doubt that the double melting behavior demonstrates the existence of thinner PLA crystals under atmospheric pressure with the equivalent supercooling. Moreover, as listed in Table 1, the melting temperature (Tm) of the PLA0-100-165 sample is 169.1 °C, while it is only 166.7 °C when adding 3 wt% PEG, and even lower for the PLA10-100-165 sample (165.4 °C). Of course, the Tm of the corresponding atmospheric pressure samples also decrease from 169.1 °C to 164.8 °C as the PEG concentration increases from 0 to 10 wt%. The reduction in the Tm should be ascribed to the thinner lamella caused by the addition of the plasticizer PEG. The incorporation of PEG may disturb the arrangement of PLA lamella. In other word, the PLA crystals may have more defects with increase in the PEG fraction.46 Therefore, the PLA melting point drops with increasing the PEG content. 15



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Besides, Table 1 also reveals that the melting points of the atmospheric pressure samples are slightly lower than those of the corresponding high pressure ones, except that the pure PLA samples prepared under the two pressures possess the same melting point of 169.1 °C. From the viewpoint of molecular relaxation, the free energy for molecular mobility is identical under an identical degree of undercooling (in other words, the mobility and relaxation of PLA molecular chains are basically similar under different pressures as long as the degree of undercooling is the same).40,41,50,51 As previous literature introduced, polymers crystallizing under a higher pressure tend to form thicker lamellae with higher melting points due to the different way of molecular chain arrangement and crystal growth from that under a lower pressure.52-55 Therefore, we speculate that molecular chain slip during crystallization makes the crystals thicker under a high pressure, and hence, under a higher pressure and a same supercooling, the lamellae become thicker than in the case of a lower pressure. Therefore, the samples crystallized under atmospheric pressure possess double melting behavior and their melting points are lower than those of corresponding high pressure ones under the same supercooling. That is to say, under a same supercooling, applying a higher pressure is beneficial to form PLA crystals with fewer defects as well as thicker lamellae. This provides an effective way to improve the performance of PLA products through regulating the crystallization behavior of PLA.

Crystalline Structure of Pressure-Shearing Samples. The WAXD results of PLA/PEG samples crystallizing under 100 MPa and ambient pressure are shown in Figures 4 and 5, respectively. One can see a series of Debye-Scherrer rings of two 16


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groups samples in the 2D-WAXD patterns from isotropic α crystals and the strongest ones are (200, 110)α and (203)α. However, for the samples containing 10 wt% PEG, the diffraction peaks become weaker. Besides, the peaks of (200, 110)α and (203)α diffraction shift to smaller q, which represents a looser crystalline structure for the higher PEG content samples. The phase separation of high-content PEG sample restrained closely-stacked PLA lamella formation, thus the PLA crystals of samples containing high-content PEG possess a looser crystalline structure. In addition, for the high pressure samples, the crystallinity fitted from the 1D-WAXD curves (Table 1) increases from 25.1 to 31.9% as the PEG fraction goes up from 0 to 5 wt%. However, it suddenly drops to 19.5% when the PEG fraction reaches 10 wt%. For the ambient pressure samples, their crystallinity also increases from 23.6 to 30.2% with the PEG concentration increasing from 0 to 5 wt% and decreases to 14.1% when there is 10 wt% PEG. This is in good agreement with the DSC data.

Figure 4. 2D-WAXD patterns and corresponding 1D-WAXD curves of pure PLA and PLA/PEG samples crystallized at 165 °C and 100 MPa. Here, the shear rate in the measuring position is 5.0 s-1. 17



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Figure 5. 2D-WAXD patterns and corresponding 1D-WAXD curves of pure PLA and PLA/PEG samples crystallizing at 150 °C and ambient pressure. Here, the shear rate in the measuring position is 5.0 s-1.

Effect of Shear Rate and Pressure on Crystallization. Figure 6 shows the DSC thermograms at various shear rates for samples (a) PLA5-100-165 and (b) PLA5-0-150. Besides, the quantitative thermal characteristic data for these two samples are listed in Table 2. First, the DSC curves of static samples (without shear) both exhibit distinct cold-crystallization peak and the Xc is 26.1% and 23.3%, respectively. Second, for sample PLA5-100-165, the cold-crystallization behavior disappears and the Xc increases to 65.2% and 67.3% when applying shear rate of 2.5 and 5.0 s-1, respectively. Besides, the Tm becomes higher as shear rate increases. Applying stronger shear flow field facilitates forming more locally oriented nuclei, so crystallization becomes faster and the lamellae are thicker. For sample PLA5-0-150, when applying a shear rate of 2.5 s-1, the DSC curve still reveals a tiny cold-crystallization peak and the Xc is 56.3%; while at shear rate 5.0 s-1, the DSC curve shows no cold-crystallization behavior and the Xc increases to 63.1%. Besides, 18


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the three DSC curves of sample PLA5-0-150 all reveals a weak low temperature melting peak prior to the high temperature main melting peak. The low temperature melting peak should result from the thinner crystals formed under atmospheric pressure.

Figure 6. DSC heating traces of PLA/PEG samples at various shear rates: (a) sample PLA5-100-165 and (b) sample PLA5-0-150. The three shear rates in measuring position are 0 (black curve), 2.5 (red curve) and 5.0 s-1 (blue curve).

Table 2. DSC Data of Samples PLA5-100-165 and PLA5-0-150 at Various Shear Rates Samples PLA5-100-165

PLA5-0-150

Shear rate (s-1)

Tm (°C)

∆Hm (J/g)

Xc (%)

0

165.0

23.2

26.1

2.5

165.3

58.0

65.2

5.0

166.5

59.8

67.3

0 2.5

164.4 164.8

20.7 50.1

23.3 56.3

5.0

165.2

56.1

63.1

Notes: Tm is the peak temperature. Figures 7 and 8 display the 2D-WAXD images and 1D-WAXD profiles as a function of shear rate for the PLA5-100-165 and PLA5-0-150 samples, respectively. As revealed in Figures 7 and 8, under the quiescent condition (0 s-1), the 2D-WAXD 19



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patterns of the two samples all show amorphous diffraction characteristics. Once applying shear, the 2D-WAXD patterns of two samples show a series of Debye-Scherrer rings representing randomly arranged α crystals. The representative rings are (200, 110)α and (203)α composite reflections. The corresponding 1D-WAXD profiles of two samples exhibit four distinct diffraction peaks at q = 10.5, 11.8, 13.5, and 15.8 nm-1, which are ascribed to the (010)α, (200, 110)α, (203)α, and (015)α crystalline planes, respectively.56,57 The appearance of Debye-Scherrer rings after applying shear suggests that shear is favorable for the PLA crystallization. Nevertheless, for both samples under our crystallization condition, the PLA molecular chains can hardly be oriented because of the plasticization effect of PEG and the faintness of shear flow. Hence, the PLA molecules are inclined to develop into typical isotropic α-form but not oriented one.

Figure 7. 2D-WAXD patterns and corresponding 1D-WAXD curves of sample PLA5-100-165 at various shear rates (radial positions). 20


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Figure 8. 2D-WAXD patterns and corresponding 1D-WAXD curves of sample PLA5-0-150 at various shear rates (radial positions).

The Xc as a function of shear rate for PLA5-100-165 and PLA5-0-150 samples were also estimated from the WAXD data, as shown in Figure 9. As a comparison, the Xc of their static samples is also shown in Figure 9. It is seen that the Xc of the two static samples is ca. 0, implying that PEG can hardly promote the PLA crystallization under quiescence. Note that the Xc of static sample measured by DSC is larger than 0, as cold crystallization always occurs during the DSC heating process. For PLA5-100-165 sample, once applying a weak shear (0.5 s-1), the Xc increases notably from 0 to over 30%. Also, we can see a gradual growth in Xc from 31.1% to 42.2% with increasing the shear rate from 0.5 to 3.5 s−1, respectively. The notable augment in Xc originates from a combined effect of the plasticizer PEG and shear flow. On one hand, applying flow field can facilitate forming a large quantity of locally oriented nuclei; on the other hand, introducing PEG into PLA can significantly promote the 21



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mobility of PLA chains, that is, once the nuclei form, crystallization is easier. However, as the shear rate further increases to 5.0 s−1, the Xc declines to 31.9% at the edge of the sample. The reason for this unexpected drop is that the cavity wall weakens the shear flow field and even makes the shear flow disordered near the cavity edge.50 For the PLA5-0-150 sample Xc also increases from 27.4% to 39.9% with increasing the shear rate from 0.5 to 3.5 s−1. Further increasing the shear rate, the Xc begins to decline, and the Xc is 30.2% at the edge of the sample. Through comparing the two samples, we can see that the crystallinity of PLA5-100-165 is higher than that of PLA5-0-150 when applying the same shear rate. The crystallinity difference between the two samples should be attributed to the synergetic effect of shear flow and pressure. As previously introduced, under a higher pressure and a same supercooling, the molecular chains step into crystal lattice more easily because of the acceleration of high pressure. Besides, the shear flow field facilitates forming locally oriented chain segments, which are easier to step into PLA crystal lattices than random segments. Above all, the PLA molecules step into crystal lattice more rapidly upon applying pressure and shear flow. Therefore, PLA5-100-165 and PLA5-0-150 samples exhibit same crystallinity under static, and once applying a shear rate, the sample crystallizing under a higher pressure possesses higher crystallinity than the corresponding atmospheric pressure one under the same supercooling. In summary, the crystallinity of plasticized sample will be enhanced when applying an appropriate range of shear rate. Besides, under a same supercooling, pressure and shear flow possess a synergetic effect to fabricate plasticized samples with higher crystallinity, 22


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which provides an effective approach to regulate sample crystallization behavior and to obtain desired structures and performances.

Figure 9. PLA crystallinity for PLA-0-150 and PLA5-100-165 samples at various shear rates (radial positions).

Periodic Crystalline Structure of Pressure-Shearing Samples. Figure 10 presents the SAXS results of the high pressure samples containing various PEG fractions. For the sake of comparison, the SAXS results are derived from the radial position 9.5 mm away from the sample center. The 2D-SAXS pattern of sample PLA0-100-165 seems to be weakly oriented while other three samples exhibit a whole isotropic scattering ring (Figure 10a). When adding PEG to PLA, PEG molecular chains can promote the mobility of PLA molecules, so the oriented structure formed during the shear would easily to relax, resulting in random lamellae for PLA/PEG samples and weakly oriented lamellae for pure PLA sample. Particularly, the 2D-SAXS pattern of PLA10-100-165 exhibits a very weak intensity, suggesting a lower crystallinity. Figure 10b presents the resultant correlation functions derived from one-dimensional scattering intensity distribution profiles (Figure S2a in the SI) 23



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for high-pressure samples with various PEG fractions. The smaller value presented in the correlation function curves should be ascribed to the thin lamellae as their crystallinity fitted from the WAXD profiles (Table 1) is smaller than 50%. Afterwards, the dc, da, and dac are calculated according to the correlation function (Figure S3 in the SI). The result is demonstrated in Figure 10c. Figure 10c displays that the long spacing and the average thickness of the lamellae both decrease with increasing the PEG concentration up to 5 wt%, indicating the closely-packed crystalline structure. When the PEG content reaches 10 wt%, the lamellar thickness continues to decrease. The decrease of PLA lamellae thickness with increasing the PEG concentration is consistent with the DSC results, that is, the PLA melting point declines with the PEG content (Figure 3a). However, the thickness of amorphous layers and the long spacing both rise slightly. This is because the phase separation largely restricts the PLA crystallization as previously discussed, which agrees with the WAXD and DSC results. The SAXS data of corresponding atmospheric pressure samples are also presented in Figure 11. One can clearly see that the results exhibit nearly the same trend as the high pressure samples with increasing the PEG fraction, which suggests that the influence of plasticizer PEG on the crystalline periodic structure of the PLA/PEG samples under atmospheric pressure is similar to that under 100 MPa with a same degree of supercooling in the range of our experiment condition. Besides, under an equal PEG concentration, the dc values of high pressure samples are larger than those of corresponding atmospheric pressure samples as shown in Figure S4 in the SI, 24


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suggesting that higher pressures facilitate the formation of thicker lamellae. This conclusion is well in agreement with the DSC results.

Figure 10. SAXS data of the pure PLA and PLA/PEG samples crystallizing at temperature of 165 °C and pressure of 100 MPa: (a) 2D-SAXS patterns, One-dimensional scattering intensity distribution profiles Iq2 versus q(b) resultant correlation function curves, and (c) evolution of the dc, da and dac as a function of the PEG fraction. Here, the shear rate in the measuring position is 5.0 s-1.

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Figure 11. SAXS data of the pure PLA and PLA/PEG samples crystallizing at temperature of 150 °C and ambient pressure: (a) 2D-SAXS patterns, (b) resultant correlation function curves, and (c) evolution of the dc, da, dac as a function of the PEG fraction. Here, the shear rate in the measuring position is 5.0 s-1.

Crystalline Morphology. To intuitively verify the relationship between crystalline structure and PEG content, SEM characterization was performed. The SEM micrographs for etched high pressure samples at shear rate of 5.0 s-1 (PLA0-100-165, PLA5-100-165 and PLA10-100-165) are shown in Figure 12 (parts a-c). The photographs in the right list (Figure 12, parts a2-e2) show the enlarged observation of the dotted rectangles of corresponding left ones (a1-e1). First, we can see that the PLA0-100-165 sample (Figure 12a1) displays typical randomly distributed lamellae morphology which can be more clearly observed from the enlarged image in Figure 12a2. When adding 5 wt% PEG (Figure 12, parts b1 and b2), the sample (PLA5-100-165) also shows randomly distributed lamellae, but the lamellae are more 26


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closely-packed than the PLA0-100-165 sample. Therefore, the dc and dac both decrease slightly as the WAXD and SAXS results present. Further aggrandizing the PEG fraction to 10 wt%, the PLA10-100-165 sample (Figure 12c1) demonstrates spherulitic crystals with larger sizes compared to the ones containing lower PEG contents. The corresponding enlarged figure (Figure 12c2) exhibits a large clusters of PLA lamellae, which should result from the PEG phase separation from the PLA matrix. The phase separation restrains the continuous and compact stacking of PLA lamellae, so we can see independent clusters of PLA crystals, which can be reflected in the smaller deviation of diffraction peaks for higher PEG content samples as WAXD data show. In addition, the SEM photographs of the PLA5-0-150 sample possessing a same degree of supercooling as PLA5-100-165 are also displayed in Figures 12d and 12e. To explore the influence of shear rate on crystalline structure, Figures 12d and 12e exhibit the crystalline morphology of sample PLA5-0-150 at shear rate of 2.5 s-1 and 5.0 s-1, respectively. Figure 12d1 and 12d2 exhibits randomly distributed lamellae morphology. Figure 12e1 also exhibits randomly distributed lamellae morphology, but the enlarged observation (Figure 12e2) shows that the lamellae are more dense than those of sample PLA5-0-150 at shear rate of 2.5 s-1 (Figure 12d2), as higher shear rate facilitates the formation of more locally oriented nuclei, and thus induces more dense lamellae. Besides, the enlarged observation of sample PLA5-0-150 at shear rate of 5.0 s-1 (Figure 12e2) displays that the lamellae are thinner than those of PLA5-100-165 (Figure 12b2), illustrating that higher pressures facilitate the formation of thicker PLA 27



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lamellae under the same degree of supercooling. From Figure 12, we can see the difference in PLA crystal morphology between samples obtained under various high pressures, which should be attributed to the molecular chain activity promoter PEG because it largely influences the mobility ability of PLA chains depending on its content. Moreover, the morphology images of PLA5-100-165 display thicker lamellae than the sample PLA5-0-150 as the higher pressure leads to the stronger ability of PLA chains slip during crystallization under an equivalent supercooling. The result is consistent with the DSC and SAXS data. The difference in crystal morphology between samples could help to further comprehend the relationship between crystallization condition and inner crystal structure, and thus afford guidance for practical processing to obtain good-performance PLA products via controlling crystalline structure.

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Figure 12. SEM photographs of samples: (a) PLA0-100-165 (5.0 s-1), (b) PLA5-100-165 (5.0 s-1), (c) PLA10-100-165 (5.0 s-1), (d) PLA5-0-150 (2.5 s-1) and (e) PLA5-0-150 (5.0 s-1). The four photographs in the right (a2 - e2) represent the enlarged observation of the dotted rectangles of corresponding left ones (a1 - e1).

4. CONCLUSIONS

To improve the toughness of PLA, PLA was blended with various content of PEG. Besides, in order to establish relationship between theoretical study and practical processing, the effects of pressure and shear rate on the crystallization behavior of PLA/PEG samples were also systematically investigated by home-made PSD. The WAXD, DSC and SEM results revealed that blending appropriate amount of PEG could not only effectively decrease the glass-transition temperature of PLA/PEG samples but also significantly enhance the crystallization capacity of PLA. This was because the introduction of PEG reduced the intermolecular force and enhanced the mobility of PLA chains. However, as the PEG content reached 10 wt%, the crystallinity of PLA decreased drastically due to the phase separation inevitably occurred, which suppressed the crystallization of PLA. When crystallizing the blends under an equal degree of supercooling, the pressure possesses a non-negligible effect on the thickness and perfection of lamellae as well as sample crystallinity due to the difference in the capacity for PLA chains slip during crystallization under various pressures. We also explored the role of shear rate for PLA5-100-165 and PLA5-0-150 30


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samples and discovered that the PLA crystalline structure of PLA/PEG sample was not influenced by the shear rate, yet the crystallinity was largely enhanced by applying an appropriate range of shear rate (0 – 3.5 s-1). Besides, under a same supercooling, pressure and shear flow possess a synergetic effect to fabricate PLA/PEG samples with higher crystallinity. Through this study, the pressure and shear rate dependent crystallization behaviors of PLA/PEG samples with various composition ratios could be comprehended. Meanwhile, the results are quite important because we can achieve high physical properties of final PLA/PEG products by optimizing the content of plasticizer PEG and the crystallization conditions.

ASSOCIATED CONTENT Supporting Information Available:

Additional results, such as rheological

behaviors, SAXS results and an explanation for the choice of isothermal crystallization temperature.

AUTHOR INFORMATION Corresponding Author *E-mail: (J.L.) [email protected]; (Z.M.-L.) [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants

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51573119 and 51533004), Science & Technology Department of Sichuan Province (Grants 2014TD0002) and the Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University, sklpme2015-1-01 and sklpme2016-2-02). The authors thank Shanghai Synchrotron Radiation Facility (SSRF) for supporting the X-ray measurement.

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Xu,

H.;

Xie,

L.;

Hakkarainen,

M.

Beyond

a

model

of

polymer

processing-triggered shear: reconciling shish-kebab formation and control of chain degradation in sheared poly(L-lactic acid). ACS Sustainable Chem. Eng. 2015, 3, 1443-1452.

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Thicker Lamellae and Higher Crystallinity of Poly(lactic acid) via

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