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Preferential Stereocomplex Crystallization in Enantiomeric Blends of Cellulose Acetate-g-Poly(lactic acid)s with Comblike Topology Jianna Bao, Lili Han, Guorong Shan, Yongzhong Bao, and Pengju Pan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b05398 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 15, 2015

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The Journal of Physical Chemistry

Preferential Stereocomplex Crystallization in Enantiomeric Blends of Cellulose Acetate-g-Poly(lactic acid)s with Comblike Topology

Jianna Bao, 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]

1

ACS Paragon Plus Environment


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ABSTRACT: Although stereocomplex (sc) crystallization is highly effective to improve the thermal resistance of poly(lactic acid) (PLA), it is much less predominant than homocrystallization in high-molecular-weight (HMW) poly(L-lactic acid)/ poly(D-lactic acid) (PLLA/PDLA) racemic blends. In this contribution, the sc crystallization of HMW PLLA/PDLA racemic blends was facilitated by using the comblike PLAs with cellulose acetate as the backbone. Competing crystallization kinetics, polymorphic crystalline structure, and structural transition of comblike PLLA/PDLA blends with a wide range of MWs were investigated and compared with the corresponding linear/comblike and linear blends. HMW comblike blend is preferentially crystallized in sc polymorph and exhibits faster crystallization rate than the corresponding linear blend. Sc crystallites are predominantly formed in nonisothermal cold crystallization and isothermal crystallization at above 120 °C for the comblike blends. Except for the facilitated sc formation in primary crystallization, synchrotron radiation WAXD analysis indicates that the presence of comblike component also facilitates the transition or recrystallization from homocrystallite (hc) to sc crystallite upon heating. Preferential sc formation of comblike blends is probably attributable to the favorable interdigitation between enantiomeric branches and increased mobility of polymer segments. After crystallization under the same temperature, the comblike blends, which mainly contain sc crystallites, show smaller long period and thinner crystalline lamellae than the corresponding PLLA with homocrystalline structure. Key word: Poly(lactic acid), comblike, stereocomplex, crystalline structure

INTRODUCTION Poly(lactic acid) (PLA) is a well-known biobased and biodegradable thermoplastic and has many advantages such as versatile processing ability and high 2


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mechanical properties. Except for biomedical applications, PLA has been considered as a candidate to substitute the conventional petroleum-derived thermoplastics.1 Because lactic acid is a chiral molecule, PLA has two isotactic stereoisomers, i.e., poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). As the semicrystalline polymers, PLLA and PDLA can crystallize in different polymorphs under different conditions.2 One of the most interesting polymorphs is the stereocomplex (sc) formed between PLLA and PDLA.3 Melting point (Tm) of sc crystallites (210~230 °C) is 50 °C higher than that of homocrystallites (hc). Therefore, sc crystallization has been regarded as one of the most efficient methods to improve thermal resistance and long-lasting durability of PLA material.4,5 Because of the presence of intermolecular hydrogen bonds between enantiomers,6−8 sc crystallites possess denser chain packing and slower molecular relaxation than their hc analogs.8 Such unique structure renders sc-type

PLA

highly

improved

physical

properties

such

as

superior

(thermo)mechanical strength and modulus,9,10 better resistances to thermal and hydrolytic degradation.11−13 Because sc and hc crystallizations share the similar crystallization temperature windows, polymorphic crystalline structure of PLLA/PDLA blends is quite complicated. Since PLLA and PDLA enantiomers must arrange in a combined manner during sc crystallization, sc formation has longer diffusion path and suffers from a larger kinetic barrier than homocrystallization, despite that the former is more thermodynamic favorable. Therefore, sc crystallization of PLLA/PDLA racemic blends is usually accompanied by homocrystallization of individual enantiomer. It has been well recognized that molecular weight (MW) is a critical factor for the competing crystallization of sc and hc polymorphs in PLLA/PDLA blends. Sc formation is just favorable for the racemic blends containing one or two low-MW 3



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components (MW < ~20 k).14−19 But the sc crystallization is significantly depressed and hc crystallization becomes prevailing for the PLLA/PDLA racemic blends with medium and high MWs.19−22 Because just the high MW (HMW) PLAs are commercial available and suitable for use as thermoplastics, a critical issue to prepare sc-type PLA is how to facilitate the sc crystallization and depress the homocrystallization of HMW PLLA/PDLA blends. Up to date, many approaches have been reported to promote the sc crystallization of HMW PLAs such as stereoblock copolymerization,23,24 control over polymer topology,25−29 supercritical fluid treatment,30 thermal stretching,31,32 incorporation of heterogeneous nucleator,33,34 and use of specific crystallization conditions.35−37 Macromolecular architecture has been a key factor determining the crystallization kinetics, polymorphic crystalline structures, and crystalline transition of semicrystalline polymers, which plays a much more important role in the sc crystallization of enantiomeric polymers. It has been reported that sc formation of HMW PLLA/PDLA blends is promoted by the presence of multi-arm star-shaped or nanofiller-grafted polymers such as star-shaped PLAs with ≥ 13 arms,25 PLAs grafted on carbon nanotubes26,27 and graphene oxide.28 Compared to the multi-arm star-shaped structure, comblike topology can significantly decrease the length of each segment, promote the interdigitation and interactions between branches, and help the directional arrangement of branches along polymer backbone. Therefore, it is envisioned that comblike topology would facilitate the sc crystallization between HMW PLLA and PDLA. On the other hand, it has been reported that, upon heating PLLA/PDLA racemic blends, hc is able to transform or recrystallize into the more thermally

stable

sc

polymorph

through

structural

reorganization.38−41

The

thermal-induced hc-to-sc crystalline reorganization of branched PLLA/PDLA blends 4


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may be different from that of typical linear blends; this, however, remains unexplored up to date. In this study, comblike HMW PLLAs and PDLAs (MW > 300 k) with different MWs were synthesized via ring opening polymerization (ROP) using cellulose acetate (CA) as the backbone.42 Crystallization kinetics, polymorphic crystalline structure, crystalline transition, and lamellae structure of comblike PLLA/PDLA racemic blends were systematically investigated and compared with those of HMW linear/comblike and linear blends. It is found that sc formation is highly favored in the crystallization of HMW comblike blends. Mechanisms for the topology-dependent crystallization kinetics and polymorphic structure of HMW racemic blends were discussed and proposed.

EXPERIMENTAL Materials. L- and D-lactide ( > 99%) were purchased from Purac Co. (Gorinchem, the Netherlands) and purified by recrystallization from ethyl acetate. Cellulose acetate (CA, Mn = 42.7 kg/mol, Mw/Mn = 2.26) with an acetyl content of 39.8 wt%, a degree of acetyl substitution of 2.2, and an average degree of polymerization (DP) of 176 was purchased from Sigma-Aldrich. Tin(II) 2-ethylhexanoate [Sn(Oct)2, > 98 %, Aldrich-Sigma] was distilled before use. Linear PLLA (l-PLLA, Mw = 143 kg/mol, Mw/Mn = 1.71) was obtained from Shimadzu co. (Kyoto, Japan). Linear PDLA (l-PDLA, Mw = 191 kg/mol, Mw/Mn = 1.44) was prepared by bulk ROP of D-lactide at 130 °C using Sn(Oct)2 as the catalyst and dodecanol as the initiator. Synthesis of Comblike PLLA and PDLA with CA Backbone. Comblike PLLA and PDLA were synthesized via the ROP of lactide using CA as the macroinitiator and Sn(Oct)2 as the catalyst, according to a published method.42 MW 5



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and PLA graft length of comblike polymers were controlled by changing the lactide/CA molar ratio. A typical procedure to synthesize comblike PLLA with an expected Mn of 700 kg/mol is shown as follow. Dried L-lactide (30.0 g, 0.21 mol), CA (1.92 g, 0.045 mmol), and Sn(Oct)2 (0.24 g, 0.59 mmol) were added into a vigorously dried Schlenk tube and further dried at 70 °C for 1.0 h under reduced pressure to remove the trace of water in lactide, CA, and catalyst. The reaction mixture was heated to 130 °C and allowed to polymerize at this temperature for 5 h under the argon atmosphere. After reaction, the crude product was dissolved in chloroform, washed by HCl solution (1.0 mol/L) three times, and then washed by deionized water to neutral pH to remove the catalyst. The product was then precipitated from chloroform into ethyl alcohol to remove the unreacted monomer. The polymer was finally dried at 80 °C in vacuo for 6 h. Comblike PLA is marked as c-PLA-xk, in which “c” represents “comblike” and x denotes the Mw (in kg/mol). Preparation of Blend Sample. Equivalent mass of PLLA and PDLA with comblike or linear topology were separately dissolved in chloroform (50 g/L). The solutions of PLLA and PDLA were mixed and stirred rigorously for 30 min. The mixed solution was then cast onto a PTFE dish and the solvent was allowed to evaporate at 25 °C for 24 h. Residual solvent was removed by further drying in vacuo at 80 °C for 6 h. The c-PLLA/c-PDLA, l-PLLA/c-PDLA, and l-PLLA/l-PDLA blends are denoted as comblike, linear/comblike, and linear blends, respectively. Measurements. Nuclear Magnetic Resonance (NMR). 1H NMR spectra of polymers were obtained by a 400MHz Bruker AVANCE II NMR spectrometer (Bruker BioSpin Co., Switzerland) with CDCl3 as the solvent. Gel Permeation Chromatography (GPC). Molecular weights of polymers were measured by a Waters gel permeation chromatography (Waters Co., Milford, 6


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MA, USA) consisting of a Waters 1525 isocratic HPLC pump, a Waters 2414 refractive index (RI) detector, a Waters 717 autosampler, and a PL-gel 10 µm MIXED-BLS column at 30 °C. Tetrahydrofuran (THF) was used as the mobile phase and polystyrene was used as the standard. Differential Scanning Calorimetry (DSC). Crystallization and melting behavior of PLLA/PDLA blends were determined on a NETZSCH 214 Polyma DSC (NETZSCH, Germany) equipped with an IC70 intracooler under a nitrogen gas flow (40 mL/min). To investigate nonisothermal cold crystallization, the sample (8~10 mg), encapsulated in an aluminum pan, was first heated to 250 °C and kept for 2 min to erase the thermal history. It was then quenched to 0 °C at a cooling rate of 100 °C/min and reheated to 250 °C at 10 °C/min to examine the crystallization and melting behavior. For isothermal melt crystallization, after melting at 250 °C for 2 min, the sample was cooled to desired crystallization temperatures (Tc = 80~180°C) at a cooling rate of 100 °C/min and then held at this temperature for a sufficient period to crystallize. It was then reheated to 250 °C at 10 °C/min to detect the melt behavior. Wide-Angle X-ray Diffraction (WAXD) and Small-Angle X-ray Scattering (SAXS). Conventional WAXD patterns of PLLA/PDLA blends crystallized at different Tcs were recorded on a Rigaku RU-200 (Rigaku Co., Japan) using the Ni-filtered Cu Kα radiation (λ = 0.154 nm) at 25 °C. The instrument was worked at 40 kV and 200 mA. Film sample with a thickness of ~0.3 mm was prepared on a Linkam THMS600 hot stage (Linkam, Surrey, UK) using the same thermal program as that used in DSC isothermal crystallization. The sample was step-scanned from 7 to 40° at a 2θ scanning rate of 2°/min. SAXS and in situ WAXD were measured on the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF) under an X-ray wavelength of 0.124 nm. 7



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Scattering patterns were collected by a Rayonix SX-165 CCD detector (Rayonix, Illinois, USA) with a resolution of 2048 × 2048 pixels and a pixel size of 80 × 80 µm2. In SAXS analysis, the film samples (thickness ~0.3 mm) isothermally crystallized at different Tcs were prepared on a Linkam THMS600 hot stage using the same thermal program as that used in DSC isothermal crystallization. For in situ WAXD analysis, amorphous film sample with a thickness of ~0.2 mm was prepared by quenching the melted (250 °C, 3 min) sample into liquid nitrogen. The sample was placed between two pieces of polyimide films and heated from 50 to 260 °C at 10 °C/min on a Linkam THMS600 hot stage. WAXD pattern were collected by a time interval of 30 s during heating. Sample-to-detector distances were 0.12 and 2.5 m in WAXD and SAXS analyses, respectively. Acquisition times in WAXD and SAXS measurements were 15 and 90 s, respectively. Two-dimensional (2D) data was converted into one-dimensional profile by circularly averaging with a Fit2D software.

RESULTS and DISCUSSION Synthesis of Comblike PLLA and PDLA with CA Backbone. Chemical structure and MW of comblike polymers were characterized by 1H NMR and GPC. As shown in Figure 1, resonance peaks corresponding to the methyl, methine, and terminal methine protons of PLA are observed at 1.5 (peak a), 5.1 (peak b), and 4.3 ppm (peak b′), respectively.43 The resonance peaks for methyl proton of acetyl group in CA appears at 1.8~2.1 ppm (peak c).42 GPC curve of comblike PLLA and PDLA shows a single peak (Figure S1). On basis of NMR data, the degree of lactyl substitution (DS) in glucose unit and average degree of polymerization (DP) of PLA grafts (DPPLA) of comblike PLAs were calculated DS = 2.2 × 3 × Ab′/Ac DPPLA = (Ab + Ab′)/Ab′ 8


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where A is the area of corresponding NMR peak and 2.2 is the degree of acetyl substitution in CA. As shown in Table 1, lactyl DS of comblike PLA increases with increasing the lactide/CA feed ratio and it is close to 0.8 when the molar feed ratio of lactide to glucose unit is larger than 26, suggesting that almost all hydroxyl groups of CA initiate the ROP of lactide and the synthesized comblike PLAs have similar grafting densities. As calculated from the Mn (42.7 kg/mol) and degree of acetyl substitution (2.2) of CA, a CA macroinitiator approximately contains 140 hydroxyl groups. Therefore, it is estimated that the averaged graft number in comblike PLA is 104~140. Averaged DP of PLA branches (DPPLA) ranges 38~103, corresponding to the MW for each branch of 2.7~7.4 kg/mol. MW and DPPLA increase with the lactide/CA feed ratio, indicating that the MW and graft length of comblike PLAs can be well tailored from the feed ratio. DPPLA calculated from NMR agrees with that calculated from the feed molar ratio of lactide to glucose (or unsubstituted hydroxyl group in CA). These results can demonstrate the successful synthesis of comblike PLLA and PDLA. Table 1. Feed ratio and chemical characteristics of comblike PLLA and PDLAs with CA backbone samplea

LA/glucoseb (mol/mol)

Mn,GPCc (kg/mol)

Mw,GPCc (kg/mol)

PDIc

c-PLLA-341k c-PDLA-365k c-PLLA-501k

15.1/1 15.1/1 26.3/1

217.8 231.5 330.9

340.7 364.6 500.5

1.56 1.57 1.51

degree of lactyl substitution 0.65 0.59 0.75

c-PDLA-467k

26.3/1

312.2

467.0

1.50

0.8

51

c-PLLA-684k

43.1/1

440.6

684.0

1.55

0.8

103

DPPLAd 42 38 68

c-PDLA-611k 43.1/1 423.5 610.7 1.44 0.8 91 Numerals in sample code denote Mw (kg/mol) of comblike PLLAs and PDLAs

a

measured by GPC. bFeed molar ratio of lactide (LA) to glucose unit in CA, which is 0.8 time of feed molar ratio of LA to unsubstituted hydroxyl group in CA. cMn, Mw

9



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and polydispersity index (PDI) measured by GPC. dAveraged DP for PLA branches with respect to the lactic acid unit. c-PLLA-341k RO

b

a OR

O

O

n

OR O

b O

CH 3 or

R=

a'

O

c

a

b'

6

5

b' OH

y O

Glucose unit

4

c

3

2

a'

1

Chemical shift (ppm) Figure 1. 1H NMR spectrum of comblike PLLA with CA backbone. Crystallization Kinetics Crystallization kinetics of comblike PLLA/PDLA racemic blends with different MWs were studied via DSC and also compared with the linear/comblike and linear blends. Based on the DSC heating curves of melt-quenched samples (Figure 2), glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm) and enthalpy (∆Hm) of hc and sc crystallites were calculated (Table 2). As shown in Figure 2 and Table 2, no distinct change in Tg is observed for the blends with different MWs and topologies. Interestingly, except for the cold crystallization at 90~120 °C, all comblike blends (i.e., c-PLLA/c-PDLA blends) exhibit single melting region at high temperature (210~230 °C), suggesting the predominant formation of high-Tm sc crystallites in nonisothermal cold crystallization. Tcc of comblike blend decreases and its ∆Hm,sc increases with increasing MW, suggesting that the comblike blends with longer PLA branches have faster crystallization rate and higher ability to stereocomplex.

10


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However, for the linear/comblike and linear blends (i.e., l-PLLA/c-PDLA and l-PLLA/l-PDLA blends), two melting peaks corresponding to the hc and sc crystallites are present at 150~180 and 210~230 °C, respectively (Figure 2). ∆Hm,hcs of linear/comblike and linear blends are much larger than their ∆Hm,scs (Table 2), indicating the predominant formation of hc. Notably, the linear/comblike blend shows smaller ∆Hc,hc and larger ∆Hc,sc than its linear analog, demonstrating that the presence of comblike component facilitates sc crystallization of PLLA and PDLA. Table 2. Thermal properties of PLLA/PDLA racemic blends with different topologies collected in nonisothermal cold crystallization of melt-quenched samples. Tcc (°C) 111.7

Tm,hc (°C) 178.4

∆Hm,hc (J/g)

l-PLLA/l-PDLA

Tg (°C) 61.0

l-PLLA/c-PDLA-611k

59.4

107.9

166.8

blend

19.9

34.8

219.1

24.9

a

59.4

115.0

NP

0

222.2

21.3

c-PLLA-501k/c-PDLA-467k

61.6

109.0

NP

0

210.5

22.6


60.5

102.3

NP

0

213.0

30.6

No peak is observed.

endo up

a

∆Hm,sc (J/g)

40.8

Tm,sc (°C) 221.4




Heat flow

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



l-PLLA/c-PDLA-611k l-PLLA/l-PDLA

50

100

150

200

250

Temperature (°C) Figure 2. DSC curves recorded in heating of melt-quenched PLLA/PDLA racemic blends with different topologies.

11



Isothermal crystallization kinetics of PLLA/PDLA racemic blends were further investigated in a wide temperature range (Tc = 80~180 °C). On basis of the DSC curves collected in isothermal melt crystallization (Figure S2), kinetic parameters including crystallization half-time (t1/2) and overall crystallization rate constant (k) of PLLA/PDLA blends crystallized at different Tcs were analyzed by Avrami equation,44 which are shown in Figures 3a and 3b, respectively. Because the crystallization was controlled by nucleation and chain diffusion at low and high supercooling, respectively, t1/2s of all racemic blends exhibit minimums (i.e., maximums of crystallization rate) and their k values show maximums under the medium Tc (100~140 °C). t1/2 increases and k decreases as Tc is increased toward Tm or decreased toward Tg (Figure 3). Normal spherulites with typical Maltese-cross patterns are observed at different Tcs for all PLLA/PDLA blends in the measurement of polarized optical microscopy (POM). No discernible difference is seen between the spherulitic morphologies of hc and sc crystallites (Figure S3).

10

10

10

(a)

2

-n

10

l-PLLA/l-PDLA l-PLLA/c-PDLA-611k c-PLLA-341k/c-PDLA-365k c-PLLA-501k/c-PDLA-467k c-PLLA-684k/c-PDLA-611k

3

k (min )

10

t1/2 (min)

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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1

0

80

100 120

140

160 Temperature (° C)

180

0

10

-1

10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

10

-8

(b)

l-PLLA/l-PDLA l-PLLA/c-PDLA-611k c-PLLA-341k/c-PDLA-365k c-PLLA-501k/c-PDLA-467k c-PLLA-684k/c-PDLA-611k

80

100 120

140

160

180

Temperature (° C)

Figure 3. Kinetic parameters of PLLA/PDLA racemic blends with various topologies crystallized at different Tcs: (a) crystallization half-time, (b) overall crystallization rate constant.

12


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Polymorphic crystallization kinetics of PLLA/PDLA blends are strongly influenced by the macromolecular topologies and graft length. As shown in Figure 3, under the same Tc, t1/2 of comblike blend reduces and its k increases remarkably with increasing the MW and graft length. POM analysis also indicates that the spherulitic growth rate of comblike blend increases with the MW and graft length (Figure S4). All these results suggest that smaller graft length decrease the crystallization rate. Branching and grafting topologies usually delay and disturb the crystallization of polymers, due to the lower degree of molecular symmetry.45 For the comblike polymers, each graft chain is tethered on the backbone and its diffusion ability is retarded due to the branching and steric effects. In the case of sc crystallization, the diffusion of polymer chains is a more critical factor because sc crystallization only proceeds when both enantiomers are diffused to the crystalline growth faces. Branching effect would become more severe for the comblike polymers with shorter grafts, because of the elevated number of branching points per unit mass. Besides, the change of chain direction and incorporation of initiator moiety in comblike polymers can also decrease the crystallization rate.46 Even though the branching topology of PLA decreases its homocrystallization rate,43,45 the comblike blends exhibit faster crystallization (i.e., smaller t1/2 and larger k) than the linear and linear/comblike blends with the same comblike component (Figure 3). This can be attributed to the effect of crystalline polymorph, as illustrated in the following parts. Polymorphic Crystalline Structure Since the hc and sc crystallites have distinct Tms and WAXD patterns, polymorphic crystalline structures of PLLA/PDLA racemic blends crystallized at different Tcs (100~180 °C) are analyzed by DSC and WAXD (Figures 4, 5). Macromolecular topology and Tc influence the crystalline structure, sc and hc

13



contents remarkably. As shown in Figures 4a, 4b, 5a, comblike blends merely exhibit the melting peaks and characteristic diffractions of sc crystallites (2θ = 12.0, 20.8, 24.0°),47 while the linear/comblike and linear blends show the melting peaks and characteristic diffractions of both hc (2θ = 16.8, 19.0°)48 and sc crystallites after

611 c-PLLA-684k/c-PDLAc-PLLA-501k/c-PDLA-

Heat flow

endo up

(a) Tc = 160 °C k

467k

(b) c-PLLA-684k/ c-PDLA-611k

Heat flow

endo up

crystallization at Tc = 160 °C.

65k c-PLLA-341k/c-PDLA-3

Tc (°C) 180 160

l-PLLA/c-PDLA-611k

140 120

l-PLLA/l-PDLA

100 160

200

240

120

endo up

Temperature (°C)

(c)

Tc

l-PLLA/ c-PDLA-611k

(°C) 180

160

200 Temperature (°C)

240

160

Heat flow

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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140 120 100 120

160

200 Temperature (°C)

240

Figure 4. DSC heating curves of PLLA/PDLA racemic blends: (a) racemic blends with different topologies after isothermal melt crystallization at Tc = 160 °C; (b) c-PLLA-684k/c-PDLA-611k and (c) l-PLLA/c-PDLA-611k blends after isothermal melt crystallization at different Tcs. 14


D C B A

10

15

(c)

20 2θ ( ° )

25

10

15

20 2θ (°)

sc220

sc300/030


hc110/200

E

(b) Intensity (a.u.)

sc300/030

hc110/200

sc110

Intensity (a.u.)

sc220

Tc = 160 °C

(a)

Tc (°C) 180 160 140 120 100

25

sc220

sc300/030

hc110/200

l-PLLA/c-PDLA-611k

sc110

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


sc110

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Tc (°C) 180 160 140 120 100

10

15

20

25

2θ (°) Figure 5. (a) WAXD patterns of PLLA/PDLA racemic blends melt-crystallized at Tc = 160 °C: A) l-PLLA/l-PDLA; B) l-PLLA/c-PDLA-611k; C) c-PLLA-341k/c-PDLA365k; D) c-PLLA-501k/c-PDLA-467k; E) c-PLLA-684k/c-PDLA-611k. (b, c) WAXD patterns of (b) c-PLLA-684k/c-PDLA-611k and (c) l-PLLA/c-PDLA-611k blends after melt crystallization at different Tcs. The wavelength of X-ray is 0.154 nm. On basis of the WAXD patterns, relative fraction of sc crystallites (fsc) in the blends was estimated by comparing the diffraction peak area of sc crystallites with the total areas of both sc and hc diffractions, i.e., fsc = Isc/(Isc + Ihc), where Isc and Ihc are the total peak areas of sc and hc diffractions, respectively.22,35 We have tried to

15



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

calculate the absolute crystallinities of sc and hc crystallites from WAXD. But the broad halo peak of PLA amorphous phase is not obvious in the WAXD patterns of PLLA and PLLA/PDLA blends,19,48 leading that the crystallinity estimated from WAXD is much larger than that estimated from DSC. Therefore, fsc was just estimated here. As shown in Figures 5b, 5c and 6, sc diffraction intensity and sc fraction steadily increase with Tc for all the PLLA/PDLA blends, because sc crystallites are more thermally stable and favored at high Tcs. fsc of c-PLLA-684k/c-PDLA-611k blend is about 40% at Tc = 100 °C and it increases to 100% at Tc = 140 °C. All the racemic blends crystallize in sc polymorph at Tc = 180 °C, because this Tc has exceeded the Tm of hc. After crystallization at the same Tc (100~160 °C), the comblike blends show less obvious melting of hc (Figures 4b, 4c), more predominant sc diffraction (Figures 5b, 5c), and larger sc fraction (Figure 6) than the corresponding linear/comblike and linear blends. Moreover, the linear/comblike blend has higher sc fraction than its linear analog after crystallization at Tc = 100~160 °C (Figure 6). Crystalline structures of solution-crystallized comblike blends are consistent with those in nonisothermal and isothermal crystallizations. The melting peaks and WAXD diffractions of sc crystallites are mainly observed, while those of hc are much less obvious (Figure S5). Therefore, it is rational to conclude that the comblike topology of PLLA and PDLA promotes their sc formation in racemic blends. The presence of one or two comblike enantiomers facilitates the sc crystallization of HMW PLLA/PDLA blends. Interestingly, the degree of sc crystallization in comblike blends also depends on MW and graft length. After crystallization at Tc = 100 and 120 °C, sc fraction of comblike blend first increases and then decreases with increasing MW and graft length (Figure 6). First, the branching effect, which impedes chain diffusion45 and 16


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rearrangement in sc crystallization, is more predominant in the comblike polymer with shorter graft chains. Second, it has been well documented that the competing formations of hc and sc crystallites in PLLA/PDLA blends are MW-dependent and the sc crystallization is facilitated in low-MW blends. As the MW and graft length of comblike polymer increase, the viscosity and entanglement between branches enhance, impeding the polymer segment inter-diffusion and sc formation. As reported by Sakamoto and Tsuji, when the Mn of each branch in 2- and 4-arm PLLA/PDLA blends exceeds 4.5 kg/mol, the homocrystallization also takes place in addition to sc crystallization.49 Mn of each branch in our comblike PLLA and PDLA is ranging 2.7~7.4 kg/mol, leading to the occurrence of homocrystallization with increasing graft length. Due to the synergetic effects of branching, MW, and graft length, comblike blend with moderate graft length (i.e., c-PLLA-501k/c-PDLA-467k) shows the largest sc fraction and the best ability to stereocomplex (Figure 6). By comparing the results shown in Figures 4b and 5b, it can be seen that the polymorphic structural information derived from DSC and WAXD is contradictory. It is notable that DSC and WAXD samples have the same thermal history in melt crystallization, both of which are quenched directly to Tc after melting. In the case of c-PLLA-684k/c-PDLA-611k blend, the melting of sc crystallites is solely observed at different Tcs (100~180 °C). However, hc diffractions are clearly detected for this sample at Tc = 100~120 °C, indicating the existence of both hc and sc crystallites. The difference between DSC and WAXD results, which has also been observed in linear PLLA/PDLA racemic blend,50 is ascribed to the heating-induced hc-to-sc crystalline transition. It has been reported that hc reorganizes into its sc counterpart upon heating or annealing at elevated temperatures.38−41 This hc-to-sc transition is also influenced by the initial state of hc. The hc, which is formed at low Tcs (e.g., 80 °C) and 17



possesses a less ordered structure, can reorganize or melt-recrystallize into sc more easily than those formed at high Tcs (e.g., >120 °C).41 As illustrated in the following in situ WAXD results, the endothermic melting of hc and exothermic recrystallization of sc take place simultaneously upon heating, especially for the racemic blends crystallized at low Tcs. Offset of hc melting and sc recrystallization would lead to the disappearance of melting peak of hc in the temperature range 140~170 °C.

1.0 0.8

sc

0.6 D

f

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

Page 18 of 36

0.4 E 0.2 0.0

C B A

100

120

140

160

180

Temperature (°C) Figure 6. Fraction of sc crystallites (fsc) for PLLA/PDLA racemic blends melt-crystallized

at

different

temperatures:

A)

l-PLLA/l-PDLA;

B)

l-PLLA/c-PDLA-611k; C) c-PLLA-341k/c-PDLA-365k; D) c-PLLA-501k/c-PDLA467k; E) c-PLLA-684k/c-PDLA-611k.

Polymorphic Crystallization and Crystalline Transition Investigated by In Situ

WAXD.

Crystallization

and

structural

reorganization

of

amorphous

PLLA/PDLA racemic blends upon heating were investigated by synchrotron radiation WAXD (Figures 7, S6). On basis of the temperature-dependent WAXD profiles, intensity changes for sc (110) diffraction and hc (110)/(200) diffraction were evaluated and plotted as a function of temperature in Figure 8. No discernable diffraction is observed in all the melt-quenched PLLA/PDLA blends before heating, confirming their amorphous structures. For the comblike blends, sc starts to form 18


Page 19 of 36

upon heating to 100 °C and its diffraction intensity continuously increases with heating to 190 °C, indicating the gradual sc crystallization (Figures 7a, 8a). Upon further heating to above 190 °C, sc diffraction intensities decrease due to the sc melting. It is notable that hc diffraction is much less obvious throughout the heating process. A tiny hc (110)/(200) diffraction appears at 120 °C and disappears

8

sc220

sc300/030

(b) 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 50

Intensity (a.u.)

250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 50

hc110/200

sc110

(a) sc220

sc300/030

hc110/200

sc110

completely upon further heating to 170 °C.

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


10 12 14 16 18 20

8

10 12 14 16 18 20

2θ (°)

2θ (°)

Figure 7. Temperature-dependent WAXD patterns for melt-quenched PLLA/PDLA racemic blends collected upon heating: (a) c-PLLA-501k/c-PDLA-467k, (b) l-PLLA/ c-PDLA-467k. The wavelength of X-ray is 0.124 nm.

In the case of linear/comblike blends, the variation trend of sc diffraction intensity is similar to that observed for the comblike blend, but the hc diffraction is obviously shown upon heating to 110 °C (Figures 7b, 8b). This demonstrates that hc can form more easily when one of the components in comblike blend is replaced by the linear polymer. As shown in Figure 8b, the sc diffraction of linear/comblike blend increases drastically upon heating in the temperature ranges of 90~140 and 160~210 19



°C, which are, respectively, attributed to the primary crystallization of sc and the hc-to-sc crystalline transition (or recrystallization of sc during hc melting). Notably, upon heating in 160~210 °C, the increased ratio of sc (110) diffraction is similar to the decreased ratio of hc (110)/(200) diffraction, implying that most of hc transforms

(a)

1.0

c-PLLA-501k/ c-PDLA-467k

Normalized peak area (a.u.)


into its sc analog.

sc110

0.8

hc110/200 0.6 0.4 0.2 0.0 50

100

150

200

250

1.0

(b) l-PLLA/c-PDLA-467k

0.8

sc110 hc110/200

0.6 0.4 0.2 0.0 50

Temperature (°C)

100

150

200

250

Temperature (°C)

6


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

Page 20 of 36

(c)

l-PLLA/l-PDLA sc110

4

hc110/200

2

0 50

100

150

200

250

Temperature (°C)

Figure 8. Temperature-dependent peak areas of sc (110) diffraction and hc (110)/(200) diffraction collected in heating of melt-quenched PLLA/PDLA racemic blends: (a) c-PLLA-501k/c-PDLA-467k; (b) l-PLLA/c-PDLA-467k; (c) l-PLLA/l-PDLA. The data was obtained from in situ WAXD patterns shown in Figures 7 and S6.

Upon heating the linear blend, the sc diffractions are much less predominant than those of hc (Figure S6), demonstrating that hc polymorph is mainly formed. In the 20


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heating process, the ratio of maximum intensity for sc (110) diffraction to that of hc (110)/(200) diffraction are about 1/5 for linear blend (Figure 8c), which, however, are around 3/1 for comblike blend (Figure 8a) and 2/1 for linear/comblike blend (Figure 8b). Upon heating the linear blend to 170~220 °C, the sc diffraction intensity gradually increases, with accompanied by the significant decrease in hc diffraction intensity. However, the increase of sc diffraction intensity is much less obvious than the decreased intensity of hc diffraction, demonstrating that just a minority of hc transforms or recrystallizes into its sc analog upon melting. By comparing the in situ WAXD results shown in Figures 8b,c, it can be concluded that the presence of comblike PDLA not only facilitates the sc formation in primary crystallization but also the hc-to-sc crystalline transition upon heating. On the other hand, for all three kinds of racemic blends with different topologies, sc diffraction appears upon heating to around 100 °C, which is prior to that of hc diffraction (110~120 °C). This demonstrates that sc formation is preceding that of hc in the early stage of crystallization. All the DSC and WAXD results have demonstrated that the PLLA/PDLA racemic blend with comblike topology is favorable to form stereocomplex. Therefore, the topology-dependent crystallization kinetics of racemic blends can be explained by different growth kinetics of hc and sc polymorphs. In situ WAXD results have illustrated that the sc formation between PLLA and PDLA in racemic blends is preceding that of hc; this may imply that sc crystallization is faster than hc crystallization

under

the

same

conditions.

Therefore,

the

comblike

and

linear/comblike blends exhibit faster crystallization rates than the linear blend at various Tcs, as shown in Figure 2.

21



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

Plausible Mechanism for Preferential Sc Crystallization of Comblike PLLA/PDLA Blends. Preferential sc formation of comblike PLLA/PDLA blends might be attributable to the favorable interdigitation between enantiomeric branches. Biela et al. have reported a similar phenomenon for the multiarm star-shaped PLLA/PDLA blends, which exclusively form the sc crystallites when the arm number is larger than 13.25 They have proposed that the enhanced sc crystallization of star-shaped blends is ascribed to the predominant interactions of enantiomeric branches packed in the antiparallel manner.25 Since the synthesized comblike PLAs have similar multi-branched geometry as the star-shaped ones, it is considered that the comblike geometry may enable PLLA and PDLA branches extending from the backbone to confront and interdigitate with each other. This would facilitate the reassembling and packing of enantiomeric branches coming from adjacent macromolecules in an antiparallel manner. Such packing would allow for the interchain interactions between enantiomeric branches, favoring the nucleation and crystallization of sc crystallites. However, the HMW linear PLLA/PDLA blend lacks the geometrical advantage for sc crystallization. Furthermore, it has been demonstrated that the sc crystallites generated in initial state of crystallization can act as the physical crosslinks and lead to form the network structure,51,52 which would significantly restrain the diffusion of bridged and surrounded chains. On the other hand, the existed sc crystallites, which are formed prior to homocrystallization, can promote the heterogeneous nucleation of homocrystallization.52 Therefore, sc crystallization is significantly depressed compared to homocrystallization in the HMW linear blends. Because of the unique grafting topology and short PLA segments, the formation of network structures may

22


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be less predominant in comblike blends; this would decrease the kinetic barrier of sc crystallization. Notably, the investigated comblike blends with CA as the backbone show different polymorphic crystalline structure from the reported racemic blends of poly(norbornene)-g-PLAs.53,54 It has been found that the sc crystallization in poly(norbornene)-g-PLLA/poly(norbornene)-g-PDLA blend is restricted compared to the comblike stereoblock PLAs54 or the linear/brush enantiomeric blends.53 Even though the exact reasons for the different stereocomplexation ability between our comblike blends and the poly(norbornene)-g-PLA racemic blends are still unclear, it is conjectured that this difference may be originated from the effects of graft density, uniformity of graft length, and the backbone/graft compatibility (or miscibility). In the studies of Grubbs et al.53 and Satoh et al.,54 the poly(norbornene)-g-PLAs were prepared

by

the

ring-opening

metathesis

polymerization

of

norbornene

end-functionalized PLAs. Both the prepared norbornene-functionalized PLA macromonomer and poly(norbornene)-g-PLAs

have the very narrow MW

distributions with PDI < ~1.1. However, in our study the comblike PLAs were synthesized by ROP using CA as the macroinitiator. The DS of PLA grafts is 0.59~0.8 relative to the glucose unit. Considering the relatively larger size of glucose unit and smaller DS value, our synthesized comblike PLAs would have relatively smaller graft density than the poly(norbornene)-g-PLAs reported in literatures.53,54 Furthermore, because the reaction and initiation activities of the unsubstituted hydroxyl (including primary and tertiary hydroxyl) groups in CA macroinitiator are different, the graft length would be less uniform and show wide distribution in the synthesized comblike PLAs. The small graft density and low uniformity of graft length could result in the larger spacings between the grafts, which may decrease the

23



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

conformationally steric effects on sc crystallization of our comblike PLLA/PDLA blends, compared to the racemic blends of poly(norbornene)-g-PLAs. On the other hand, previous studies have reported that the PLA grafts have good compatibility with CA backbone42 while poly(norbornene) is immiscible with PLA in their graft copolymers.55 It has been found that adding a miscible component in PLLA/PDLA racemic blend56,57 or incorporating a few soft segments in PLLA or PDLA29 can significantly enhance their stereocomplexation ability, because of the improved diffusion ability and conformational freedom. Therefore, the better compatibility between CA backbone and PLA grafts is also a possible reason for the preferential stereocomplexation of the comblike PLLA/PDLA blends. Crystalline Lamellae Structure Crystalline lamellae structure of comblike racemic blends crystallized at different Tcs was investigated via SAXS and also compared with that of comblike PLLA. On basis of the original SAXS profiles, Lorenz-corrected SAXS profile (Iq2~q plot) and one-dimensional correlation function γ(z) were evaluated (Figure 9). γ(z) is expressed as58 ∞

∫ I ( q ) q cos ( qz ) dq r ( z) = ∫ I ( q ) q dq 2

0

∞

2

0

where I(q) or I is the scattering intensity, q is the scattering vector (q = 4πsinθ/λ, 2θ is the scattering angle), and r(z) is the electron density correlation length. Of note is that the correlation function is valid for two-phase structure. The correlation function was used to estimate the average values of lamellar repeat distance (or long period, dac), crystalline (dc) and amorphous (da) layer thicknesses (Figure 9b), even though the comblike blends crystallized at low Tcs (≤ 120 °C) contain both hc and sc crystallites. Lorenz-corrected SAXS profiles of comblike blend exhibit single scattering peak 24


Page 24 of 36

Page 25 of 36

(Figure 9a). Long spacing (LP) was calculated from the Bragg equation LP = 2π/qmax, where qmax corresponds to the q value of scattering peak top in Lorenz-corrected SAXS profiles. The scattering peak shifts toward low q with increasing Tc, indicating the formation of more prefect crystallites with larger LP (Table 3). da=dac-dc dac

r(z)

(a) c-PLLA-684k/c-PDLA-611k

0

5

10

r(z)

2

160

140 120 100

1.2

Tc (°C) 180

20

140 120 100

160

0.8

15

z (nm)

Tc(°C) 180

0.4

(b)

dc

0

Iq (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


1.6

0

10

-1

q (nm )

20

30

z (nm)

Figure 9. SAXS data of c-PLLA-684k/c-PDLA-611k blend after isothermal melt crystallization at different Tcs: (a) Lorentz-corrected SAXS, (b) correlation function. Inset of panel b shows the evaluation of lamellae parameter from correlation function.

Table 3. Lattice parameters of comblike PLLA/PDLA racemic blend and comblike PLLA melt-crystallized at various temperatures. c-PLLA-684k/c-PDLA-611k Tc (°C)

c-PLLA-684k

100

LP (nm) 15.3

dac (nm) 14.4

dc (nm) 6.3

da (nm) 8.0

LP (nm) 28.2

dac (nm) 25.5

dc (nm) 11.0

da (nm) 14.5

120

16.3

15.5

6.7

8.8

23.2

22.7

9.7

13.0

140

17.4

16.1

7.0

9.1

28.5

26.1

11.2

14.8

160

19.3

18.9

8.1

10.8

30.6

26.5

11.4

15.0

180

23.9

22.8

9.0

13.8

--

--

--

--

As shown in Table 3, LP and dac calculated from the Lorenz-corrected SAXS profiles and correlation function are similar and both of them increase with Tc. LP and dac of comblike blends (14~24 nm) are larger than those reported for sc crystallites 25



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

formed in linear racemic blends19,59 and stereoblock copolymers,60 which are generally in the range of 11~17 nm. Larger LP and dac of comblike blends may be attributed to the large amorphous layer thickness, because the comblike polymer has low structural symmetry and generally shows smaller degree of crystallinity than linear polymer. Besides, the presence of hc in comblike blends crystallized at Tc ≤ 120 °C (as indicated in Figure 6) may also lead to the increased long period. dc and da of comblike blends increase with Tc. However, LP, dac, dc, and da of comblike PLLA with homocrystalline structure all show minimums at Tc = 120 °C and increase for both higher and lower Tc, agreeing with the unique temperature-dependent lamellar structure of linear PLLA.61 As shown in Table 3, the comblike blend has smaller LP and dac than neat PLLA after crystallization under the same Tc, demonstrating the smaller lamellar repeat distance of sc crystallites than their hc analogs.40 The long diffusion path of enantiomeric chains and large kinetic barrier of sc crystallization may restrict the formation of thick lamellae.

CONCLUSIONS In conclusion, macromolecular topology is a crucial factor influencing the competing sc and hc crystallizations, polymorphic crystalline structure, and structural transition of PLLA/PDLA racemic blends. Sc crystallization is preferential in the HMW racemic blends with comblike topology and CA backbones. Compared to the linear and linear/comblike blends, the comblike blends have faster crystallization rate and are prefer to sc crystallization. HMW comblike blends nearly exclusively crystallize in the sc polymorph during nonisothermal cold crystallization or isothermal crystallization at high Tc ( > 120 °C). Crystallization rate of comblike blend increases with increasing the graft length, due to the decreased branching effect. Furthermore, the presence of comblike component in HMW PLLA/PDLA blend also facilitates the 26


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hc-to-sc crystalline transition or recrystallization of sc crystallites upon heating. Mechanism for the facilitated sc crystallization of comblike blends is proposed, which might be attributable to the favored interdigitation between enantiomeric branches and increased mobility of polymer segment. Due to the large fraction of sc crystallites, the comblike blend exhibits smaller long period and thinner crystalline lamellae compared to the corresponding PLLA with homocrystalline structure. This study has provided an effective method to facilitate the sc crystallization of HMW PLLA/PDLA blends and also shed light on the polymorphic crystallization mechanism and crystalline transition of enantiomeric polymer blends with complicated topologies.

ASSOCIATED CONTENT Supporting Information GPC curves, isothermal crystallization DSC curves, POM results, DSC and WAXD data of solvent-cast blends, and in situ WAXD patterns of linear blends. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel +86-571-87951334; e-mail [email protected] ACKNOWLEDGMENT. This study was financially supported by the Natural Science Foundation of China (21274128, 21422406) and the Fundamental Research Funds for the Central Universities (2015XZZX004-08). SAXS and in situ WAXD were measured on beamline BL16B1 of SSRF, China.

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