Polymorphic Crystalline Structure and Crystal Morphology of

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Polymorphic Crystalline Structure and Crystal Morphology of Enantiomeric Poly(lactic acid) Blends Tailored by A Self-Assemblable Aryl Amide Nucleator Qing Xie, Lili Han, Guorong Shan, Yongzhong Bao, and Pengju Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00191 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 8, 2016

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ACS Sustainable Chemistry & Engineering

Polymorphic Crystalline Structure and Crystal Morphology of Enantiomeric Poly(lactic acid) Blends Tailored by A Self-Assemblable Aryl Amide Nucleator

Qing Xie, Lili Han, Guorong Shan, Yongzhong Bao, Pengju Pan*

State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China.

*Corresponding author. Tel.: +86-571-87951334; email: [email protected]

ABSTRACT: Stereocomplex (SC) crystallization has been an effective method to improve the heat resistance of poly(lactic acid) (PLA). However, the preparation of SC-type PLA material is still challenge because SC crystallization is much less prevailing

than

homocrystallization

in

the

high-molecular-weight

(HMW)

poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) racemic blends. In this study, we have successfully promoted SC formation and controlled crystal morphology of HMW PLLA/PDLA

blend

by

using

a

self-assemblable

N,N’,N’’-tricyclohexyl-1,3,5-benzenetricarboxylamide

aryl (BTCA).

amide

nucleator,

Crystallization

kinetics, polymorphic crystalline structure, crystal morphology and superstructure of BTCA-nucleated PLLA/PDLA blends were investigated. During the nonisothermal melt crystallization and isothermal crystallization at different temperatures (80~170 °C), the crystallization rate of PLLA/PDLA blend is significantly promoted and the

1

ACS Paragon Plus Environment


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fraction of SCs is enhanced with incorporating small amount of BTCA. SCs are exclusively formed in the BTCA-nucleated PLLA/PDLA blends at a high crystallization temperature (e.g., 170 °C). Due to the diversified self-assembled structure of BTCA in polymer melts, SCs with short, long shish-kebab-like, and granular

structures

are

attained

with

varying

the

BTCA

concentration.

BTCA-promoted SC formation of PLLA/PDLA blend is ascribed to the hydrogen bonding interactions between BTCA and PLLA, PDLA chains. Key words: poly(lactic acid), stereocomplex, crystalline structure, crystal morphology, nucleator, hydrogen bonding

INTRODUCTION Poly(lactic acid) (PLA) is a representative biobased and biodegradable thermoplastic, which can be produced from the renewable sources and is fully biodegradable after use. PLA has the good physical properties including biocompatibility, versatile processability, high tensile strength and elastic modulus; this makes it a promising alternative to the traditional petroleum-based plastics.1−3 However, the crystallization rate of PLA is much slower than most of the semicrystalline polymers. Therefore, its articles prepared in practical processing (e.g., injection molding) usually show low thermal stability and poor mechanical properties, because of the low degree of crystallinity. Moreover, the long-term behavior of less-crystallized PLA is poor due to the pronounced relaxation of amorphous chains in physical aging.4 Interestingly, the complementary enantiomers of PLA, i.e., poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA), are able to form stereocomplexes (SCs) in 1:1 ratio in the common crystallization process.5,6 PLLA and PDLA chains interact with each other through intermolecular hydrogen bonds in the crystal lattice of SCs; 2


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this causes the denser chain packing of SCs than the homocrystallites (HCs) formed by enantiopure PLLA or PDLA.7-9 Because of such unique crystalline structure, SC-type PLA exhibits many improved physical properties such as high melting point (Tm = ~ 220 °C, ca. 50 °C than HCs),5 better thermomechanical properties10−13 and better hydrolytic resistance.14−16 Therefore, SC crystallization has been regarded as an effective method to improve the thermal-resistant and long-term performances of PLA. In the crystallization of PLLA/PDLA racemic blends, the formations of HCs and SCs are competing; not all of the racemic blends can exclusively crystallize in SC polymorph. In the conventional melt and cold crystallizations, SCs are preferentially formed in the low-molecular-weight (LMW) PLLA/PDLA blends (typically < 20 kg/mol);17−20 yet the homocrystallization of PLLA or PDLA becomes prevailing with their molecular weights increase. Since just the high-molecular-weight (HMW) PLA (> ~80 kg/mol) has good mechanical properties and processability, a key issue to prepare the heat-resistant SC-type PLA is to suppress HC formation and facilitate SC formation in the processing and crystallization processes. Moreover, because of the competing formations of SCs and HCs, the crystallization kinetics and polymorphic structure of HMW PLLA/PDLA blends are more complicated than the LMW ones; this, however, remains far less well understood. Many approaches have been reported in literatures to promote the SC formation of

HMW

PLLA/PDLA

blends;

these

methods

include

the

stereoblock

copolymerization,21,22control over chain structure and topology,23−28 polymer blending, 11,29,30

supercritical fluid treatment,31 thermal annealing or processing at specified

temperature,32,33 processing under stretching34,35 and shear,36 additions of nanofiller 37−39

and SC-type nucleator.40−44 Most of these methods need the chemical modification

of polymer chains or complicated processing procedures. In contrast, the use of 3



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nucleator is a simple and efficient way to accelerate the crystallization rate and control the crystal modification, superstructure of semicrystalline polymers. Although the researchers have reported several SC-type nucleators of PLA, most of these nucleators cannot be dissolved in the polymer matrix.40−44 Some organic nucleators (e.g., low-molecular-weight amide,44,45 hydrazide,46,47and oxalamide48) have been used to direct the crystal growth and induce the formation of unique crystal morphology (e.g., shish-kebab-like crystals) in semicrystalline polymers such as PLA. These nucleators are partially or completely dissolved in polymer melt and can recrystallize, self-assemble during the melt processing. Crystal morphology and superstructure of these nucleator-modified polymers usually change with varying the nucleator concentration and crystallization condition. Due to the larger surface area and unique interconnected structure, the shish-kebab-like crystals usually have better physical properties than the common spherulitic crystals. Therefore, the control over crystal morphology enables the feasible tune of physical performances of materials in practical processing.45,49 In this article, we use one of the 1, 3, 5-benzenetricarboxylamide derivatives, N, N’, N’’-tricyclohexyl-1,3,5-benzenetricarboxylamide (BTCA), as a nucleator to tailor the crystal modification and superstructure of HMW PLLA/PDLA racemic blend. 1,3,5-Benzenetricarboxylamide

derivatives

are

a

family

of

nucleators

for

polypropylene;50 BTCA has been used as the nucleator of enantiopure PLLA in homocrystallization.45,51 In this study, the crystallization kinetics, polymorphic crystalline structure, crystal superstructure and morphology of BTCA-nucleated PLLA/PDLA blends were investigated. The HMW PLLA/PDLA blends with enhanced SC content and diversified crystal superstructures including the cone-like, shish-kebab-like, and needle-like crystals were obtained. Furthermore, the mechanism

4


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for promoted SC formation in the BTCA-nucleated PLLA/PDLA blends were discussed and proposed.

EXPERIMENTAL Materials. PLLA (Mw = 143 kg/mol, Mw/Mn = 1.71) was obtained from Shimadzu co. (Kyoto, Japan). D-lactide (> 99%) was purchased from Purac Co. (Gorinchem, the Netherlands) and purified by recrystallization from ethyl acetate. PDLA (Mw = 191 kg/mol, Mw/Mn = 1.44) was synthesized via the bulk ring-opening polymerization of D-lactide at 130 °C for 5 h with stannous octoate as the catalyst and dodecanol as the initiator.22 BTCA with a mean particle size of 20 µm was kindly supplied by Seemore New Material Technology Co., Ltd (Hangzhou, China). The chemical structure of BTCA is shown in Figure S1. Preparation of Blend Samples. PLLA, PDLA, and BTCA were mixed by solution blending to avoid the possible thermal degradation of polymer matrix. Equal mass of PLLA and PDLA were separately dissolved in methylene chloride (50 g·L−1) and then mixed. BTCA with a predetermined weight was then added into the mixed solution and the suspension was rigorously stirred at 25 °C for 30 min. Subsequently, the mixture was cast onto a polytetrafluoroethylene (PTFE) petri dish and the solvent was removed under evaporation at 25 °C for 24 h. The sample was finally dried in a vacuum oven at 80 °C for 8 h to remove the residual solvent. Measurements. Differential Scanning Calorimeter (DSC). Crystallization and melting behavior were analyzed on a NETZSCH DSC 214 Polyma instrument (NETZSCH, Germany) equipped with an IC70 intracooler under the protection of nitrogen gas flow (40 mL·min-1). For investigating the nonisothermal melt crystallization, the sample (8~10 mg), encapsulated in an aluminum pan, was first melted at 250 °C for 3 min to erase the thermal history. It was then cooled to 0 °C at a 5



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cooling rate of 10 °C·min−1, followed by a reheating to 250 °C at 10 °C·min−1. For the isothermal melt crystallization, after melting at 250 °C for 3 min, the sample was fast cooled to the desired crystallization temperature (Tc = 80~170 °C) at 100 °C·min−1 and then held at this temperature for crystallization. After crystallization, the sample was reheated to 250 °C at 10 °C·min−1 to examine the melt behavior. Wide-Angle X-ray Diffraction (WAXD). Conventional WAXD patterns were measured on a Rigaku RU-200 (Rigaku Co., Japan) with a Ni-filtered Cu Ka radiation (λ = 0.154 nm). The instrument was operated at 40 kV and 200 mA. The WAXD patterns were recorded in 2θ = 7 ~ 40° at a scanning rate of 2°⋅min−1. The samples were crystallized according to the thermal procedure that was the same as that used in DSC isothermal crystallization. On basis of the WAXD data, the relative fraction of SCs (fSC,WAXD) was calculated by comparing the diffraction peak area of SCs with the total areas of both SCs and HCs [fSC,WAXD = ISC/(ISC + IHC)], where ISC and IHC are the total diffraction peak areas of SCs and HCs, respectively. In-situ WAXD measurements were performed on the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of radiation source is 0.124 nm and the sample-to-detector distance is 160 mm. The diffraction patterns were recorded on a Rayonix SX-165 CCD detector (Rayonix, Illinois, USA) having a resolution of 2048 × 2048 pixels and a pixel size of 80 × 80 µm2. The melt-quenched film sample (thickness ~0.3 mm) was prepared by quenching into the liquid nitrogen after hot-pressing at 250 °C for 3 min. The sample sandwiched by polyimide films was heated from 40 to 260 °C at 10 °C·min−1 on a Linkam THMS600 hot stage. Diffraction patterns were recorded every 30 s during heating. The collection time of each pattern was 15 s. Two-dimensional WAXD patterns were converted to the one-dimensional data by integration via a Fit2D software. 6


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Polarized Optical Microscopy (POM). Crystal morphology and superstructure were observed on an Olympus BX51 (Tokyo, Japan) equipped with an Instec HCS402 hot stage (Instec Inc., Colorado, USA). The sample, sandwiched by two glass slides, was first melted at 250 °C for 3 min and then fast cooled (100 °C·min-1) to the desired Tc for isothermal crystallization.

RESULTS AND DISCUSSION Kinetics and Crystalline Structure in Nonisothermal Crystallization. Crystallization kinetics and polymorphic crystalline structure of the BTCA-nucleated PLLA/PDLA blends in nonisothermal melt crystallization were investigated via DSC and WAXD. Figure 1 shows the DSC curves of neat and BTCA-containing (0.1~1wt%) PLLA/PDLA blends recorded in the nonisothermal melt crystallization and subsequent heating. Based on these DSC data, the crystallization temperature (Tc) and enthalpy (∆Hc), Tm, melting enthalpy (∆Hm), and degree of crystallinity (Xc) of HCs (Xc,HC) and SCs (Xc,SC) were calculated (Table 1). Xc,HC and Xc,SC were calculated by comparing ∆Hm to the corresponding value of an infinitely large crystal (∆Hm0), in which ∆Hm0s of HCs and SCs are 93 and 142 J/g , respectively.52,53 The crystallization of neat PLLA/PDLA blend is slow and no discernible crystallization peak is observed upon cooling at 10 °C/min (Figure 1a). Meanwhile, a cold crystallization peak is seen for the neat blend in the subsequent heating scan. This is in agreement with the WAXD data of sample crystallized under the same conditions, in which the very weak diffractions are observed besides the broad halo peak of amorphous phase (Figure 2). However, the crystallization of PLLA/PDLA blend is drastically accelerated with an incorporation of BTCA. When the weight concentration of BTCA (CBTCA) is larger than 0.3%, the crystallization of PLLA/PDLA blend is completed during cooling at 10 °C/min and no cold crystallization is observed in the 7



subsequent heating scans. Tc and ∆Hc of PLLA/PDLA blend increase as the CBTCA increases from 0 to 0.5% (Table 1). Notably, when CBTCA ≥ 0.3%, PLLA/PDLA blends exhibit the fractional crystallization behavior and two crystallization exotherms are observed upon cooling; this is attributable to the sequential crystallizations of SCs and

endo up

(a)

CBTCA 1% 0.5%

Heat flow

endo up

HCs.

Heat flow

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0.3%

Tm,SC

CBTCA 1% 0.5% 0.3%

0

0

100

Tm,HC

0.1%

0.1%

50

(b)

150

200 50 100 150 200 250 Temperature (°C) Temperature (°C) Figure 1. DSC curves recorded in (a) nonisothermal melt crystallization and (b) subsequent heating scans for PLLA/PDLA blends with different CBTCAs. Both the cooling and heating rates are 10 °C/min.

Table 1. Thermal properties of PLLA/PDLA blends with different CBTCAs obtained in nonisothermal melt crystallization and subsequent heating scan

CBTCA (wt%) 0 0.1 0.3 0.5 1

10 °C/min cooling Tc ∆Hc (J/gPLA) (°C) NP 0.0 93.4 14.2 121.9 44.4 125.0, 155.0 63.8 129.6, 149.0 65.9

10 °C/min heating ∆Hm,HC Tm,SC ∆Hm,SC (J/gPLA) (°C) (J/gPLA) 32.7 219.5 8.3 26.9 216.7 11.9 26.7 215.8 17.9 28.4 218.1 31.3 28.5 220.3 45.4

Tm,HC (°C) 165.6 167.5 163.6 164.2 160.5

8


Xc,HC (%) 35.2 28.9 28.7 30.6 20.7

Xc,SC (%) 5.8 8.4 12.6 22.0 32.0

10°C/min SC220

SC300/030

HC203

HC110/200

SC110

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity(a.u.)

Page 9 of 30

CBTCA 1% 0.5% 0.3% 0.1% 0

10

15

20 2θ (°)

25

Figure 2. WAXD patterns of neat and BTCA-containing PLLA/PDLA blends crystallized upon cooling at 10 °C/min. The samples were melted at 250 °C for 3 min before cooling.

BTCA also influences the polymorphic crystalline structure of PLLA/PDLA blend. As shown in Figure 1b, all samples show dual melting regions at 160~170 and 210~230 °C, attributed to the melts of HCs and SCs, respectively. As seen in Table 1, ∆Hm and Xc of SCs (∆Hm,SC, Xc,SC) in neat PLLA/PDLA blend are much smaller than those of HCs (∆Hm,HC, Xc,HC), indicating that HCs, rather than SCs, are mainly formed. However, ∆Hm,SC and Xc,SC of PLLA/PDLA blend gradually increase with an increase in CBTCA; Xc,SC increases from 5.8% to 32.0% as the CBTCA increases from 0 to 1.0%. Similar results are seen from the WAXD patterns of samples crystallized upon cooling. As shown in Figure 2, the characteristic diffractions of SCs at 2θ = 12.0, 19.1, and 24.0° (corresponding to the 110, 300/030 and 220 crystalline planes, respectively18) enhance drastically with the CBTCA increases. At the same time, the intensities of HC 110/200 (2θ = 16.6°) and 203 (2θ = 18.9°) diffractions enhance with increasing CBTCA. These DSC and WAXD results demonstrate that BTCA not only enhances the crystallization rate but also facilitates the SC formation in HMW PLLA/PDLA blend.

9



Kinetics and Crystalline Structure in Isothermal Crystallization. Effects of BTCA on the crystallization kinetics and polymorphic crystalline structure of PLLA/PDLA blends were investigated in isothermal crystallization within a wide Tc range (80~170 °C). Figures 3a and 3b show the representative DSC curves of neat and BTCA-containing PLLA/PDLA blends collected in the isothermal crystallization at 140 °C and subsequent heating scans. As shown in Figure 3a, the crystallization time reduces and the crystallization peak becomes sharper with the presence of BTCA in PLLA/PDLA blend. Based on these DSC data, the isothermal crystallization kinetics were analyzed by Avrami equation;54 the kinetic parameters such as crystallization half-time (t1/2), Avrami index (n), and overall crystallization rate constant (k) were

(a)

endo up

endo up

attained.

CBTCA 1%

Tm,HC

Tm,SC

CBTCA 1% 0.5%

Heat flow

Heat flow

0.5%

(b)

0.3% 0.1%

0.3% 0.1%

0

0

10

20

30

40

0

50

150

Time (min)

(c)

225 Temperature (°C)

0.4

40 0.3 30

∆Hm,HC

fSC,DSC 0.2

∆Hm,SC

20 10

SC,DSC

∆Hm(J/gPLA)

50

175

f

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Page 10 of 30

0.0

0.2

0.4

0.6

0.8

1.0

0.1

CBTCA (%) 10


200

250

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Figure 3. DSC curves recorded in (a) isothermal crystallization at 140 °C and (b) subsequent heating scans at 10 °C /min for the neat and BTCA-containing PLLA/PDLA blends. (c) Plots of ∆Hm,HC, ∆Hm,SC, and fSC,DSC as a function of CBTCA derived from the results shown in panel b. As shown in Table 2, t1/2 decreases and k increases drastically with incorporating BTCA in PLLA/PDLA blend, indicating the enhanced crystallization rate. In the melt crystallization at Tc = 140 and 170 °C, t1/2s of neat PLLA/PDLA blend are 15.2 and 117.4 min, respectively; they are shortened to 0.9 and 5.8 min with an addition of 0.5% BTCA, respectively. Meanwhile, k of PLLA/PDLA blend increases by 100~1000 times with an addition of 0.5% BTCA. n of neat and BTCA-containing PLLA/PDLA blends is generally in 2~3; it decreases to < 2 for the blends (CBTCA ≥ 0.5%) crystallized at Tc = 140 °C. The decreased n is due to the incomplete DSC thermograms collected in isothermal process (Figure 3a), because of the too fast crystallization under this condition.54 Table 2. Kinetic parameters of PLLA/PDLA blends with different CBTCAs crystallized at 140 and 170 °C. CBTCA (wt%) 0 0.1 0.3 0.5 1.0

Tc = 140 °C t1/2 (min) 15.2 11.4 3.2 0.9 2.1

n 2.69 2.41 2.05 1.32 1.10

Tc = 170 °C

k (min-n) 7.62×10-4 1.78×10-3 0.118 0.961 0.194

t1/2 (min) 117.4 57.6 16.9 5.8 6.2

n 2.17 2.03 2.01 2.00 2.00

k (min-n) 2.20×10-5 1.90×10-4 2.36×10-3 0.022 0.018

As shown in Figure 3b, all samples crystallized at 140 °C exhibit two melting regions during subsequent heating. ∆Hm,HC, ∆Hm,SC, Xc,HC, and Xc,SC are calculated based on these DSC results. Relative fraction of SCs in the crystalline phase of PLLA/PDLA blend is further estimated by fSC,DSC = Xc,SC/(Xc,HC + Xc,HC). As shown in Figure 3c, ∆Hm,SC and Xc,SC of PLLA/PDLA blend increase and its ∆Hm,HC decreases 11



with the CBTCA increases. Effect of BTCA on the crystal modifications of PLLA/PDLA blend in isothermal crystallization was further confirmed by WAXD. The diffraction intensity of SCs enhances gradually with an increase of CBTCA (Figure 4a). The peak areas of SC and HC diffractions were compared to attain the relative fraction of SCs in crystalline phase (fSC,WAXD). As seen in Figure 4b, fSC,WAXD increases gradually with the CBTCA increases from 0 to 0.5% because of the facilitated SC formation with the presence of BTCA. However, fSC,WAXD keeps nearly unchanged with the further

(b)

Tc=140°C

Tc = 140 °C

0.4 CBTCA 1% 0.5% 0.3%

fSC,WAXD

SC220

HC010

SC110

(a)

HC203 SC300/030

HC110/200

increase of CBTCA to 1.0%.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

0.1% 0

0.2 10

15

20

25

0.0 0.2 0.4 0.6 0.8 1.0

CBTCA (%)

2θ (°)

Figure 4. (a) WAXD patterns of neat and BTCA-containing PLLA/PDLA blends crystallized at 140 °C. (b) Plots of relative fraction of SCs (fSC,WAXD) as a function of CBTCA. Because SCs and HCs have different Tms and thermal stabilities, Tc would be a critical factor influencing the competing formations of these two polymorphs. We further

investigate

the

Tc-dependent

crystalline

structures

of

neat

and

BTCA-containing PLLA/PDLA blends within a wide Tc range (80~170 °C). After isothermal crystallization at Tc = 80~160 °C, the neat and BTCA-containing (0.5%) blends show two melting regions that correspond to HCs and SCs (Figure S2); both the diffraction peaks of HCs and SCs are clearly observed in WAXD patterns (Figure 5a,b).

12


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The HC diffractions (e.g., 16.6°, 19.1°) are stronger and SC diffractions (11.9°, 20.1°, 24.0°) are relatively weaker for the neat blends crystallized at different Tcs (Figure 5a). However, the SC diffractions become much stronger and sharper with an addition of

160 140 120

140 120 100 80

80

15

20

25

10

15

2θ (°) 1.0

Tc(°C) 170 160

100

10

SC220

HC110/200

SC110

Intensity (a.u.)

SC220

SC300/030

Tc(°C) 170

CBTCA=0.5%

HC203 SC300/030

(b)

neat blend HC203

SC110

Intensity (a.u.)

(a)

HC110/200

BTCA, which is more obvious for the samples crystallized high Tcs (e.g., 160~170 °C).

20

25

2θ (°)

(c)

0.8

fSC,WAXD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


neat blend CBTCA=0.5%

0.6 0.4 0.2 0.0

80

100 120 140 160 180

Temperature (°C) Figure 5. WAXD patterns of (a) neat and (b) BTCA-containing (0.5%) PLLA/PDLA blends crystallized at different Tcs. (c) Plot of relative fraction of SCs (fSC) as a function of Tc for neat and BTCA-containing (0.5%) PLLA/PDLA blends. The polymorphic crystalline structure of PLLA/PDLA blend is strongly influenced by Tc. Intensity of SC diffractions increases remarkably for both the neat and BTCA-containing PLLA/PDLA blends with increasing Tc. After crystallization at Tc = 170 °C, the melting peak of HCs disappears in the BTCA-containing (0.5%) blend; 13



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while it can be clearly observed for neat blend. This indicates that the selective nucleation effect of BTCA to SC crystallization is more significant at high Tc. As shown in Figure 5c, fSC,WAXD enhances with an increase of Tc, because the higher Tc is favorable for the formation of more thermodynamically stable crystallites. SCs are nearly exclusively formed when Tc is close to the Tm of HCs. Polymorphic Crystallization Investigated by In Situ WAXD. Crystallization, melting, and structural organization of the melt-quenched PLLA/PDLA blends with and without BTCA were further investigated by synchrotron radiation WAXD in the heating process. According to the temperature-dependent WAXD profiles (Figure 6a,b), the intensity changes of SC 110 and HC 110/200 diffractions are evaluated and plotted against the temperature in panels c and d of Figure 6. For comparison, the diffraction intensities of sample were normalized by the maximum value of its SC 110 diffraction. The neat and BTCA-containing (0.5%) PLLA/PDLA blends prepared by melt quenching do not show any discernable diffractions before heating, confirming the amorphous structure of melt-quenched samples. HCs and SCs start to form at 100 °C in the neat PLLA/PDLA blends upon heating (Figure 6a,c); while the onset temperature of crystallization decreases to 85 °C for the BTCA-containing (0.5%) sample because of the enhanced crystallization rate. For neat blend, the intensity of HC 110/200 diffraction increases at 100~130 °C and decreases significantly at 160~200 °C in heating process, which are attributed to the crystallization and melt of HCs, respectively. The intensity of SC 110 diffraction increases remarkably at 100~120 and 150~220 °C; these low and high temperature ranges correspond to the primary crystallization of SCs and the HC-to-SC melt recrystallization, respectively. Similar two-step increase of SC diffraction is also observed for the BTCA-containing sample in heating process. For the neat 14


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PLLA/PDLA blend, the intensity of HC 110/200 diffraction is nearly 5 times that of the SC 110 diffraction after crystallization at 100~140 °C (Figure 6a,c), meaning that HCs are mainly formed in the primary crystallization. However, in the case of BTCA-nucleated sample, the intensity of SC 110 diffraction is similar to that of HC 110/200 diffraction after crystallization at 85~140 °C; the maximum intensity of SC 110 diffraction is larger than that of HC 110/200 diffraction (Figure 6b,d), These demonstrate that BTCA promotes the primary crystallization of SCs in HMW PLLA/PDLA blend.

(a)

(b)

SC110 HC110/200

SC110

CBTCA=0.5%

neat blend SC300/030

HC110/200 SC220

SC300/030

SC220 260°C

260°C

8

12

16

20

50°C 40°C

8

12 16 2θ (°)

2θ (°) SC100 HC110/200

neat blend

1.0

0.5

0.0

50

(d) 1.5 Normalized peak intensity

(c) 1.5 Normalized peak intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SC110 HC110/200

20

50°C 40°C

CBTCA=0.5%

1.0

0.5

0.0

50

100 150 200 250 Temperature (°C)

100 150 200 250 Temperature (°C)

Figure 6. Temperature-dependent WAXD patterns for melt-quenched (a) neat and (b) BTCA-containing (0.5%) PLLA/PDLA blends collected upon heating from 40 to 260 °C. Wavelength of X-ray is 0.124 nm. Plots of diffraction peak area as a function 15



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of temperature for (c) neat and (d) BTCA-containing (0.5%) PLLA/PDLA blends. Peak areas are normalized by the maximum value of SC 110 diffraction.

Crystal

Superstructure

of

BTCA-Nucleated

PLLA/PDLA

Blend.

Interestingly, BTCA-nucleated PLLA/PDLA blends exhibit diversified crystal morphology and superstructure with varying the CBTCA and crystallization condition. Figures 7, 8 show the POM images of neat and BTCA-containing PLLA/PDLA blends collected at Tc = 170 °C before and after partial crystallizations of polymers. WAXD and DSC results have confirmed that SCs are mainly formed in the crystallization of BTCA-nucleated (CBTCA ≥ 0.3%) PLLA/PDLA blends at Tc = 170 °C (Figures 5b, S2b). At CBTCA ≤ 0.3wt%, no obvious birefringence of BTCA is observed within the resolution scale of POM before the crystallization of polymer (Figure 7a), indicating that BTCA is dissolved in polymer matrix and cannot self-organize into large assembles at such low concentration. As expected, neat blend exhibits regular Maltese-cross spherulites (Figure 8a). However, with an addition of 0.1% BTCA, short shish-kebab-like crystals with a length of several tens of micrometers are formed between the spherulites in the latter state of crystallization, as indicated by arrows in Figure 8b. In the shish-kebab-like crystals, the disk-shaped kebab-like SCs are uniformly distributed on the surface of BTCA fibrils (i.e., shish), as described below.

Figure 7. Optical micrographs of PLLA/PDLA blends with different CBTCAs recorded at 170 °C prior to the crystallization of polymer. Micrographs are shown in gray mode for clarity. 16


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Figure 8. POM micrographs of neat and BTCA-containing PLLA/PDLA blends at 170 °C after partial crystallization of polymer. Arrows in panels b, c, and d indicate the shish-kebab-like crystals.

At CBTCA = 0.3%, the short or cone-shaped shish-kebab-like structure is formed besides the Maltese-cross spherulites (Figure 8c). To clarify the formation mechanism of such unique structure, we investigated the evolution of crystal superstructure for BTCA-containing (0.3%) PLLA/PDLA blend during crystallization at 170 °C. Maltese-cross spherulites are first formed in early stage of crystallization (Figure 9a-c). In the crystallization process of polymer, BTCA would be expelled from the PLLA/PDLA spherulites, leading to the locally increased concentration of BTCA around the growth front of spherulites. Therefore, the accumulated BTCA self-assembles into fibril structure near the spherulites with as the crystallization progresses, as indicated by the arrow in Figure 9d. These self-assembled fibrils of BTCA can serve as shish and nucleate the SC crystallization of PLLA and PDLA (Figure 9e,f), leading to form the short shish-kebab-like crystals around spherulites. Notably, BTCA fibrils, shish-kebab-like crystals, PLLA/PDLA spherulites and kebab-like crystals grow simultaneously in the latter stage of crystallization. Expect for 17



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the increase of fibril length, BTCA fibrils also form branches in self-assembling. Therefore, the length and branching number of shish-kebab-like crystals increases with the progress of crystallization (Figure 9e-h). Because the growth time of kebabs on fibril surface decrease from the fibril root (initially formed fibril) to fibril tip (growth front of BTCA fibril), the unique cone-shaped shish-kebab crystals with gradiently decreased kebab size are typically formed in the BTCA-containing (0.3%) blend (Figures 8c).

Figure 9. POM micrographs of BTCA-containing (0.3%) PLLA/PDLA blend after isothermal crystallization at 170 °C for different periods. Arrows indicate the formed short shish-kebab-like crystals at the latter stage of crystallization.

At CBTCA = 0.5%, BTCA is dissolved in the polymer melt and it self-assembles into the highly ordered dendritic and branching fibrils in polymer melt immediately after cooling to 170 °C (Figures 7b); the fibrils have the branch length of several hundreds of micrometers. This is similar to the self-assembled structure of hydrazide-type nucleator in enantiopure PLLA melt.45,47 Such branched fibrils are generated through the self-organization of BTCA in polymer melt through intermolecular hydrogen bonding of amide groups upon cooling.45 The self-assembling rate of BTCA in polymer melt is strongly dependent on CBTCA; it increases 18


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significantly with CBTCA from 0.1% to 0.5%. A similar concentration dependence of self-assembled structure of nucleator in polymer melt has been reported for BTCA-nucleated enantiopure PLLA45 and acryl amide derivative nucleated PP.55 At CBTCA = 0.5%, the highly ordered BTCA fibrils with large surface can nucleate SCs of PLLA and PDLA to form the long shish-kebab-like crystals with well-organized structure (Figure 8d). As shown in Figure S3, the disk-shaped kebab-like crystals gradually grow and they pack more compactly with the crystallization progresses. However, with a further increase of CBTCA to 1.0%, BTCA cannot be completely dissolved and large amount of undissolved needle-like crystals is observed even at 250 °C (Figure S4a). In this case, the self-assembling ability of BTCA is destroyed severely upon cooling and just the needle-like crystals a shorter length (typically < 30 µm) and less-defined structure are observed with cooling to 170 °C. The needle-like crystals of BTCA cannot induce the formation of fine shish-kebab-like SCs in the following crystallization of PLLA and PDLA (Figure S4); the granular crystals or small spherulites are mainly formed in PLLA/PDLA blends with high CBTCA (Figure 8e). The CBTCA-dependent crystallization kinetics of PLLA/PDLA blends can be explained from the diversified self-assembled structures of BTCA. The fastest crystallization of PLLA/PDLA blend at CBTCA = 0.5% (Table 2) is attributable to the fine self-assembled structure, uniform dispersion, and high content of BTCA fibrils. Proposed Mechanism for BTCA-Promoted SC Crystallization. Several mechanisms have been proposed to explain the accelerated crystallization of nucleator-modified polymers, which include the epitaxial growth of polymer crystals on nucleator surface,56 chemical reaction57 and hydrogen bonding interactions46,58 between nucleator and polymer. Xing et al. have proposed that the hydrogen bonding interactions between −NH groups of hydrazide-type nucleator and −C=O groups of

19



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PLLA could promote the migration and nucleation of PLLA chains to nucleator crystals.46 On the other hand, previous studies have reported that BTCA is an effective nucleator for the homocrystallization of enantiopure PLLA;45,51 this suggests that PLLA and PDLA chains would have good interactions with BTCA. As verified by FTIR (Figure S5), the carbonyl stretching band [ν(C=O)] of PLA shifts from 1758 to 1755 cm−1 and N−H stretching band [ν(N−H)] of BTCA shifts from 1635 to 1632 cm−1 in the PLA/BTCA mixture with a high CBTCA. These low-frequency shifts of ν(C=O) and ν(N−H) would suggest the existence of weak N−H···O=C hydrogen bonding interactions between BTCA and PLA chains (Figure 10a).

Figure 10. Schematic illustration for (a) hydrogen bond formation and (b) crystallization process of BTCA-nucleated PLLA/PDLA blend.

Based on the aforementioned results, mechanism for the promoted SC formation in BTCA-nucleated PLLA/PDLA blend is proposed, as illustrated in Figure 10b. In the cooling process of BTCA-containing PLLA/PDLA blend, BTCA crystallizes first and self-assembles in branched fibrils prior to the crystallization of polymer. N−H···O=C hydrogen bonds would promote the migration of both PLLA and PDLA chains to BTCA fibril surfaces. Furthermore, due to the hydrogen bonds between BTCA and

20


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PLA chains, the intermolecular attractions between PLLA and PDLA enantiomeric chains would be enhanced with the presence of BTCA. Previous studies have reported that the intermolecular ordering occurs59 and enantiomeric chains form the racemic helical pairs with 31 chain conformation60 in the initial stage of crystallization of PLLA/PDLA blend. Migration of PLLA and PDLA chains to BTCA fibril surfaces would speed up the conformational ordering; the attractive interactions between enantiomeric chains in BTCA-nucleated PLLA/PDLA blend would facilitate the formation of precursor racemic helical pairs (i.e., embryo of SC nuclei), rather than the homo helical pairs (i.e., embryo of HC nuclei). This would promote the nucleation and epitaxial growth of SCs on BTCA surfaces compared to those of HCs.

CONCLUSIONS In summary, a SC-type nucleator of PLA (i.e., BTCA) has been found, which can not only promote the formation of SCs but also control the crystal morphology in HMW PLLA/PDLA blends. In nonisothermal melt crystallization and isothermal crystallization at Tc = 80~170 °C, HCs are predominantly generated in neat PLLA/PDLA blend; while the SC content increases remarkably with an addition of BTCA. Crystallization half-time of PLLA/PDLA blend decreases drastically with an incorporation of BTCA. Interestingly, BTCA exerts a significant influence on the crystal morphology and superstructure of SC crystallization in PLLA/PDLA blend. SCs with short, long shish-kebab-like, and granular structures are attained with increasing CBTCA in PLLA/PDLA blend. BTCA-promoted SC formation in PLLA/PDLA blend is proposed to be attributable to the hydrogen bonding interactions between BTCA and PLLA or PDLA chains. This study has provided a simple, yet efficient method to promote the formation and tailor the crystal morphology of SCs in

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HMW PLLA/PDLA blends, which would be helpful for preparing the heat-resistant SC-type PLA materials with controllable physical properties.

ACKNOWLEDGEMENT This research was financially supported by Natural Science Foundation of China (21422406) and Fundamental Research Funds for the Central Universities (2015XZZX004-08). In situ WAXD was measured on the beamline BL16B1 of SSRF.

SUPPORTING INFORMATION Chemical structure of BTCA, DSC and POM data of neat and BTCA-containing PLLA/PDLA blends, FTIR data of BTCA, PLLA, and PLLA/BTCA mixture. This material is available free of charge via the Internet at http://pubs.acs.org.

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For Table of Content Use Only Polymorphic Crystalline Structure and Crystal Morphology of Enantiomeric Poly(lactic acid) Blends Tailored by A Self-Assemblable Aryl Amide Nucleator Qing Xie, Lili Han, Guorong Shan, Yongzhong Bao, Pengju Pan

Synopsis Polymorphic crystalline structure and morphology of biodegradable and biobased PLLA/PDLA blends were successfully tailored by a self-assemblage aryl amide nucleator.

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Polymorphic Crystalline Structure and Crystal Morphology of

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