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Exclusive Stereocomplex Crystallization of Linear and Multi-Arm Star-Shaped High-Molecular-Weight Stereo Diblock Poly(lactic acid)s Lili Han, Guorong Shan, Yongzhong Bao, and Pengju Pan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b06757 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015

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The Journal of Physical Chemistry

Exclusive Stereocomplex Crystallization of Linear and Multi-Arm Star-Shaped High-Molecular-Weight Stereo Diblock Poly(lactic acid)s

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

State Key Laboratory of Chemical Engineering, College of Biological and Chemical Engineering, Zhejiang University, Hangzhou 310027, China

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

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Abstract: Linear, 3 and 6-arm star-shaped stereo diblock copolymers of L- and D-lactic acid (PLLA-b-PDLA) with high molecular weights (MWs) were synthesized via the two-step ring-opening polymerization (ROP) with 1-dodechanol, glycerol, D-sorbitol as the initiators, respectively. Chemical structure, nonisothermal, isothermal crystallization kinetics, crystalline structure, lamellar morphology, and mechanical thermal property of PLLA-b-PDLAs with different macromolecular topologies

were

investigated.

Compared

to

the

high-MW

poly(L-lactic

acid)/poly(D-lactic acid) (PLLA/PDLA) racemic blends, PLLA-b-PDLAs exhibit faster crystallization rate upon cooling and isothermal melt crystallization; they crystallize exclusively in stereocomplex (sc) crystallites under all the conditions investigated. This is attributable to the enhanced interactions between enantiomeric blocks linked covalently. Macromolecular topology influences the crystallization kinetics and crystalline structure of PLLA-b-PDLAs significantly. The crystallization temperature in cooling, melting temperature, degree of crystallinity, spherulitic growth rate, crystallite size, long period, and crystalline layer thickness of PLLA-b-PDLA decrease with increasing the branching number, because of the retarding effect of branching on crystallization rate and crystallizability. Due to the formation of high-melting-point sc crystallites, both the linear and star-shaped PLLA-b-PDLAs exhibit better thermal resistance and higher storage modulus at high temperature than the homocrystalline PLLA. Keywords:

stereo

block;

crystallization

kinetics;

crystalline

structure;

macromolecular topology.

INTRODUCTION Poly(lactic acid) (PLA) is a representative plant-derived biodegradable thermoplastic with good biocompatibility, processibility, and mechanical properties1,2 2 ACS Paragon Plus Environment

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and it has been widely used in the biomedical fields and other commodities for substitution of the conventional oil-based thermoplastics. Due to the slow crystallization rate, relative low degree of crystallinity (Xc) and melting temperature (Tm), the homocrystalline PLA, e.g., poly(L-lactic acid) (PLLA), has relatively lower heat resistance and heat deformation temperature. Therefore, improvement of the heat resistance of PLA is an essential issue to widen the scope of industrial and commodity applications. An effective method to enhance the thermal stability and heat resistance of PLA is the stereocomplex (sc) crystallization between its two enantiomers, i.e., PLLA and poly (D-lactic acid) (PDLA).3,4 Tm of sc crystallites (210~240 °C) is nearly 50 °C higher than that of homocrystalline PLA.5 It has been demonstrated that the two enantiomeric chains in sc crystallites pack more tightly through the intermolecular H-bond interactions.6−8 This structural feature offers sc-type PLA many unique properties such as high mechanical strength, modulus,9 good resistances to solvent, thermal and hydrolytic degradations.10,11 A general approach to prepare sc-PLA materials is the sc crystallization of PLLA/PDLA racemic blend. However, not all of the PLLA/PDLA racemic blends can achieve full stereocomplexation, i.e., without any homocrystallization of individual PLLA or PDLA. Stereocomplexation and homocrystallization are competing in PLLA/PDLA racemic blends in the conventional crystallization processes such as cooling, heating, and annealing.12,13 Stereocomplexation is favorable for the PLLA/PDLA blend containing at least one low molecular weight (MW) component (MW < 20 kDa),12−18 while the homocrystallization of individual enantiomer is prevailing for the racemic blends with medium- or high-MW PLLA and PDLA.12,13,19−23 To overcome this issue, many methods have been reported to enhance the sc crystallization between PLLA and PDLA such as the stereoblock 3 ACS Paragon Plus Environment


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copolymerization,24−33 control over macromolecular topology,34,35 and addition of additives.36−38 Because of the enhanced interactions between neighboring enantiomeric blocks, sc crystallites can be preferentially generated in the PLLA-b-PDLA stereoblock copolymers. Synthesis of stereoblock PLA with high MW is effective to obtain the sc-type PLA material with high performance.4 As compared to the linear topology, the star-shaped polymers can exhibit many unique properties such as special crystallization kinetics, smaller hydrodynamic radius, and lower solution viscosity. Under the presence of multifunctional initiators, the star-shaped PLLAs with controlled branching length and number have been synthesized.39-41 Due to the branching effects, the star-shaped PLLAs show much different crystallization behavior from their linear counterparts.40−42 It has been revealed that the star-shaped PLLAs have lower Tm, Xc, and spherulite growth rate than those of the linear ones, due to the poor chain mobility caused by branching topology.40,41 Because of the synergetic effects of branching and covalent linking between different blocks, the star-shaped PLLA-b-PDLAs may possess unique stereocomplexation kinetics and crystalline

structure

compared

to

their

linear

analogs.

However,

the

stereocomplexation kinetics and crystalline structure of high-MW star-shaped PLLA-b-PDLAs still remain unexplored up to date, except for the low-MW ones.32,43 In this work, the linear, 3 and 6-arm star-shaped PLLA-b-PDLA stereoblock copolymers with high MWs were prepared via the sequential ring-opening polymerization (ROP) of L- and D-lactides. Molecular structure, nonisothermal and isothermal crystallization kinetics, crystalline structure, and crystalline lamellar morphologies of PLLA-b-PDLAs with various macromolecular topologies and MWs were studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction 4 ACS Paragon Plus Environment

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(WAXD), Fourier transform infrared spectroscopy (FTIR), small-angle X-ray scattering (SAXS), polarizing optical microscope (POM), and dynamic mechanical thermal analysis (DMTA). Effects of crystallization temperature (Tc) and macromolecular topologies on the crystallization kinetics and lamellar structures of PLLA-b-PDLAs were symmetrically investigated and discussed.

EXPERIMENTAL Materials. L- and D-lactide (> 99%) were purchased from Purac Co. (Gorinchem, the Netherlands) and purified by recrystallization from ethyl acetate. Tin(II) 2-ethylhexanoate [Sn(Oct)2, >98 %, Aldrich-Sigma] was purified by distillation under reduced pressure. 1-Dodecanol (99%, Amethyst Chemical), glycerol (99.5%, Sigma-Aldrich), and D-sorbitol (97%, Across Organics) were thoroughly dried under reduced pressure before use. Toluene was purified by distillation after drying with sodium. High-MW linear PLLA (Mn = 133.4 kDa, Mw = 192.2 kDa, Mw/Mn = 1.44) and PDLA (Mn = 132.5 kDa, Mw = 191.2 kDa, Mw/Mn = 1.44) were synthesized by the bulk ROP of L- or D-lactide at 130 °C using 1-dodecanol as the initiator and Sn(Oct)2 as the catalyst. Synthesis of Linear and Star-Shaped PLLAs. A typical polymerization procedure for 6-arm star-shaped PLLA with an expected Mn of 100 kDa is described as follows. L-Lactide (50.8 g, 0.35 mol), D-sorbitol (95.4 mg, 0.52 mmol), and Sn(Oct)2 (0.34 g, 0.83 mmol) were added into a vigorously dried Schlenk tube and further dried at 70 °C for 1.0 h under reduced pressure. The flask was then purged with dry argon and heated to 130 °C for 3 h for polymerization. After reaction, the polymer was dissolved in chloroform and then precipitated into excess ethanol to remove the unreacted monomer. The product was isolated and dried at 80 °C in vacuum for 8 h. The linear, 3, and 6-arm star-shaped PLLAs are marked as 1L-x, 3L-x, and 6L-x, 5 ACS Paragon Plus Environment


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respectively, in which x denotes the number-averaged molecular weight (Mn) measured by GPC. Synthesis of Linear and Star-Shaped PLLA-b-PDLAs. The polymerization procedure of 6-arm star-shaped PLLA-b-PDLA with an expected Mn of 200 kDa is described as follows. D-lactide (12.0 g, 0.083 mmol), 6-arm star-shaped PLLA (6L-109k, 10.0 g), and Sn(Oct)2 (0.12 g, 0.30 mmol) were added into a vigorously dried Schlenk flask and further dried at 70 °C for 1.0 h under reduced pressure. After the flask was purged with dry argon, 120 mL of dried toluene was injected. The reaction was allowed to proceed at 110 °C for 24 h. After the polymerization, the crude product was dissolved in the chloroform/hexafluoro-2-propanol (HFIP, 9/1, v/v) mixed solvent and precipitated into excess ethanol. The product was dried at 80 °C in vacuum for 8 h. The obtained linear, 3, and 6-arm copolymers are denoted as 1L-D-x, 3L-D-x, and 6L-D-x, respectively, where x represents Mn calculated from the yield and monomer/initiator ratio. Measurements. Specific Optical Rotation. Specific optical rotation, [α], was measured on a Hanon P810 automatic polarimeter with a wavelength of 589 nm at 25 °C. The polymer was dissolved in CHCl3/HFIP (9/1, v/v) mixed solution with a concentration of 10.0 g/L. Nuclear Magnetic Resonance (NMR). 1H NMR spectra of polymers were measured on a 400 MHz Bruker Advance2B spectrometer with the deuterated chloroform (CDCl3) or trifluoroacetic acid (d-TFA) as the solvent. Chemical shift was referred by the solvent signal. Gel Permeation Chromatography (GPC). MWs of PLLAs were analyzed by a Waters GPC (Waters Co., Milford, MA, USA) equipped with a Waters 1525 isocratic HPLC pump, a Waters 2414 refractive index (RI) detector, a Waters 717 autosampler, 6 ACS Paragon Plus Environment

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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. DSC. Crystallization and melting behavior of PLLA-b-PDLAs were measured on a NETZSCH 214 Polyma DSC (NETZSCH, Germany) equipped with an IC70 intracooler under a nitrogen gas flow (40 mL/min). The pre-weighted sample (8~10 mg) was sealed in an aluminum pan. For the nonisothermal melt crystallization, the sample was firstly melted at 250 °C for 3 min to erase the thermal history, and then it was cooled to 0 °C and reheated to 250 °C. Both the cooling and heating rates are 10 °C/min. In the isothermal melt crystallization, after melting at 250 °C for 3 min, the sample was cooled to desired crystallization temperature (Tc = 130~170 °C) at a cooling rate of 100 °C/min and held at this temperature for enough time to crystallize. For the nonisothermal cold crystallization, the sample was quenched into liquid nitrogen immediately after melting at 250 °C for 3 min and it was then reheated to 250 °C at 10 °C/min in DSC. WAXD. WAXD patterns were recorded on a Rigaku RU-200 (Rigaku Co., Japan) using the Ni-filtered Cu Kα radiation (λ = 0.154 nm). The instrument was worked at 40 kV and 200 mA. The film sample with a thickness of ~0.6 mm was prepared on an Instec HCS402 hot stage (Instec Co., Colorado, USA) using the same thermal program as that used in DSC. The sample was step-scanned from 7 to 35° at a 2θ scanning rate of 2°/min. FTIR. FTIR spectra of PLLA-b-PDLAs crystallized at different Tcs were measured on a Nicolet 5700 FTIR (ThermoFisher Co., USA) in the transmission mode. The spectra were registered with 64 scans and a resolution of 2 cm−1. SAXS. SAXS patterns of PLLA-b-PDLAs were measured on the beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF) with a wavelength of 7 ACS Paragon Plus Environment


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0.124 nm. The sample (~0.6 mm thickness) was prepared on an Instec HCS402 hot stage (Instec Co., Colorado, USA) using the same thermal program as that used in DSC. Scattering pattern was collected by using a Rayonix SX-165 CCD detector (Rayonix, Illinois, USA), which had a resolution of 2048 × 2048 pixels (pixel size = 80 × 80 µm2). The distance between sample and detector was 2.0 m and the acquisition time of each pattern was 300 s. 2D data was converted to the 1D data by circularly averaging with the Fit2D software. All data was corrected for the background scattering, air scattering, and beam fluctuation. POM. Spherulitic morphology and growth rate of PLLA-b-PDLAs were observed on an Olympus BX51 POM. The sample sandwiched between two glasses was first melted at 250 °C for 3 min and then was fast cooled under the liquid nitrogen flow to a desired Tc (130~180 °C) for crystallization. The size of growing spherulite was monitored by taking microphotograph at an appropriate time interval. The average radial growth rate of spherulite (G) was obtained by plotting the spherulite radius (R) against growth time (t). DMTA. DMTA analysis was performed on a DMTA Q800 (TA, USA) instrument at a frequency of 5 Hz. The solution-cast film sample with a dimension of 40 × 6 × 0.5 mm3 was heated from −30 to 250 °C at a heating rate of 3 °C/min.

RESULTS AND DISCUSSION Synthesis and Molecular Characteristics of PLLA-b-PDLAs. Linear, 3 and 6-arm star-shaped PLLAs with different MWs were first synthesized via ROP of L-lactide with 1-dodecanol, glycerol, and D-sorbitol as the initiators, respectively (Scheme 1). Yields of PLLAs with different topologies are larger than 90% (Tables 1, S1). Chemical structure and MW of PLLAs were characterized by 1H NMR and GPC. Resonance peaks in NMR spectra were assigned according to the literatures.39,40 As 8 ACS Paragon Plus Environment

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shown in the 1H NMR spectra (Figure 1), all PLLAs show the resonance peaks of methyl, methine, and terminal methine protons at 1.8 (peak a), 5.5 (peak b), and 4.6~4.8 ppm (peak b′) in d-TFA, respectively. Those chemical shifts are slightly larger than those collected in CDCl3 (Figure S1).44

Scheme 1. Synthesis of linear, 3, and 6-arm PLLA-b-PDLAs.

Table 1. Molecular characteristics of linear, 3, and 6-arm PLLAs

Sample 1L-54k 3L-48k 6L-50k 1L-108k 3L-100k 6L-109k a [OH]

[L-lactide]/ [α] Yield Mn,thb Mn,NMRc Mn,GPC a [OH] in feed PDI [deg⋅cm3 (%) (kDa) (kDa) (kDa) (mol/mol) /(dm⋅g)] 40.0 277/1 98.6 39.6 50.7 53.6 1.43 −184.3 40.0 275/1 94.6 37.6 39.7 47.6 1.14 −184.6 40.0 277/1 97.6 39.1 41.2 50.3 1.22 −184.1 d 100.0 694/1 96.9 97.1 -108.4 1.45 −188.9 100.0 695/1 94.6 94.9 -99.7 1.37 −186.7 100.0 694/1 96.9 97.1 -108.6 1.29 −183.7 represents the mole of hydroxyl initiating groups in 1-dodechanol, glycerol, Mn,design (kDa)

and D-sorbitol initiators.

b

M n,th =

[L-lactide] ×144.13 × Yield + M n,initiator , where [OH]

Mn,initiator is the MW of initiator. cMn,NMR derived from the 1H NMR spectra collected in d-TFA. dMWs of PLLAs with MW around 100 kDa were not determined from 1H NMR, because of the extremely small resonance peak of protons in initiator and chain terminal.



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In the 1H NMR spectra, the resonance peak (peak c) of methylene protons in initiator is overlapped with that of terminal methine protons of PLLA. MWs of PLLAs with different topologies were calculated for 1H NMR data by comparing the peak areas of methylene protons in initiator, terminal methine protons of PLLA, and methyl protons of PLLA. MWs of PLLAs measured by NMR and GPC (Mn,NMR, Mn,GPC) are consistent with the theoretical values (Mn,th) that are calculated from the yield and monomer/initiator ratio. MW of PLLA increases with increasing the monomer/initiator feed ratio. PLLAs show the [α] values of −184~−189 deg⋅cm3/(dm⋅g) in the chloroform/HFIP (9/1, v/v) mixed solvent at 25 °C, which are larger than the values measured in chloroform [ca. −150 deg⋅cm3/(dm⋅g)].12 Linear, 3, and 6-arm star-shaped PLLA-b-PDLAs with the designed MWs of 80 and 200 kDa were further prepared in the second-step ROP using the synthesized PLLAs as the macroinitiators. The mass feed ratio of D-lactide to PLLA macroinitiator was kept as 1.2:1. PLLA-b-PDLAs exhibit the similar NMR spectra as PLLA macroinitiator. However, compared to the resonance peaks of methyl and methine protons of PLLA (peaks a, b), the peaks of PLLA-b-PDLA terminal methine protons and initiator methylene protons (peak c, b′) become less obvious, suggesting the elongation of polymer chain. Because the synthesized PLLA-b-PDLAs are not soluble in the common solvents such as chloroform, THF, and DMF, their MWs were not measured by GPC but analyzed from

1

H NMR in d-TFA. Mn,NMRs of

PLLA-b-PDLAs were evaluated by a method similar to that used for PLLAs. As shown in Table 2, Mn,NMRs of PLLA-b-PDLAs are consistent with those calculated from the yield and D-lactide/macroinitiator feed ratio, both of which are similar to the designed MWs.


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(a)

(b)

a,a'

b

b,d

d

b' c 1L-D-84k

e

1L-54k 6

5

4

3

2

1

Chemical shift (ppm)

6

a,a'

c,b'

3L-D-80k

c,b'

3L-48k

5

4 3 2 Chemical shift (ppm)

1

(c)

a,a'

b,d

6

b',c

6L-D-84k

b',c

6L-50k

5 4 3 2 Chemical shift (ppm)

1

Figure 1. 1H NMR of (a) linear, (b) 3-arm, and (c) 6-arm PLLAs and PLLA-b-PDLAs in d-TFA. The [α] values of PLLA-b-PDLAs are close to zero (Table 2). L-lactyl unit content in PLLA-b-PDLAs were calculated from the [α] value by31 L-lactyl unit content = {[α](copolymer) + [α](PLLA)}/{2×[α](PLLA)} where [α](copolymer) and [α](PLLA) are the [α] values of PLLA-b-PDLA and the corresponding PLLA macroinitiator, respectively. It is calculated that the L-lactyl unit contents in PLLA-b-PDLAs are close to 50%, indicating the nearly equal amount of L- and D-lactyl units in the synthesized stereoblock copolymers. To better illustrate the copolymer structure, we also synthesized the linear, 3, and 6-arm star-shaped PLLA-b-PDLAs with a low MW of ~ 20 kDa, since their MWs can be more accurately measured from 1H NMR. As shown in Figure S1, Tables S1, S2, the 11 ACS Paragon Plus Environment


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low-MW PLLA-b-PDLAs show similar results and variation trends as those of the medium and high-MW samples. These results could demonstrate the successful synthesis of PLLA-b-PDLAs with different MWs and chain topologies. Table 2. Molecular characteristics of linear, 3, and 6-arm PLLA-b-PDLAs [D-lactide]/ L-lactyl [α] Yield Mn,thb Mn,armc Mn,NMRd a 3 Sample [OH] in feed [deg⋅cm unit content (%) (kDa) (kDa) (kDa) (mol/mol) (%) /(dm⋅g)] 1L-D-84k 80.0 330/1 92.2 83.5 83.5 101.2 10.8 47.1 3L-D-80k 80.0 313/1 94.1 80.1 26.7 85.5 50.4 −1.5 6L-D-84k 80.0 326/1 95.8 84.0 14.0 85.6 0 50.0 1L-D-202k 200.0 808/1 90.3 202.3 202.3 --e 5.1 48.7 3L-D-193k 200.0 790/1 86.4 193.3 64.4 -7.4 48.0 6L-D-201k 200.0 808/1 88.8 200.6 33.4 -7.9 47.8 a [OH] represents the mole of hydroxyl initiating groups in linear PLLA, 3-arm PLLA, Mn,design (kDa)

and 6-arm PLLA macroinitiators. b M n,th =

[D-lactide] × 144.13 × Yield + M n ,th ( P LLA ) , [I ]

where [I] and Mn,th(PLLA) are the mole and calculated Mn of PLLA macroinitiator. c

d

Mn of per arm calculated from Mn,th.

Mn calculated from 1H NMR spectra

collected in d-TFA. eMWs of PLLA-b-PDLAs with MW around 200 kDa were not determined from 1H NMR, because of the extremely small resonance peak of protons in initiator and chain terminal.

Crystallization Kinetics. Figure 2 shows the DSC curves of PLLA-b-PDLAs collected upon nonisothermal melt crystallization and subsequent heating, in which the high-MW PLLA/PDLA racemic blend is included for comparison. On basis of the DSC curves, thermal parameters of PLLA-b-PDLAs such as crystallization temperature (Tc), crystallization enthalpy (∆Hc), melting temperature (Tm), melting enthalpy (∆Hm), and Xc were determined, as shown in Table 3. Xc of sc crystallites is calculated by comparing ∆Hm with the ∆Hm value of sc crystallites with infinite crystalline thickness (∆Hm0 = 142 J/g for sc crystallites45), i.e., Xc =∆Hm/∆Hm0 × 100%. 12 ACS Paragon Plus Environment

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3L-D-193k 1L-D-202k 6L-D-84k 3L-D-80k 1L-D-84k

80

120

160

200

(b)

1L-D-202k

6L-D-201k 3L-D-193k

6L-D-84k 3L-D-80k 1L-D-84k

PLLA/PDLA

40

Endo up

6L-D-201k

Heat flow

(a)

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


Endo up

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PLLA/PDLA

240

40

80

Temperature (°C)

120

160

200

240

Temperature (°C)

Figure 2. DSC curves of linear, 3, and 6-arm PLLA-b-PDLAs and PLLA/PDLA racemic blend obtained upon (a) cooling and (b) subsequent heating at 10 °C/min. Table 3. Thermal properties of linear, 3, and 6-arm PLLA-b-PDLAs obtained upon nonisothermal melt crystallization and subsequent melting Sample

Tc (ºC)

∆Hc (J/g)

Tm (°C)

∆Hm (J/g)

Xc (%)

1L-D-84k 3L-D-80k 6L-D-84k 1L-D-202k 3L-D-193k 6L-D-201k

134.6 130.1 117.6 137.8 128.9 128.4

−56.0 −49.1 −46.4 −57.3 −49.4 −49.1

223.2 219.5 205.4 225.9 218.8 218.2

54.7 49.4 46.3 57.0 47.3 41.2

38.5 34.8 32.6 40.1 33.3 29.0

High-MW PLLA/PDLA racemic blend does not show obvious crystallization peak upon cooling at 10 °C/min. It exhibits a cold crystallization peak at 100~120 °C and a predominant melting endotherm at 160~180 °C upon subsequent heating, which are ascribed to the melting of hc. Melting endotherm of sc crystallites cannot be clearly observed in the heating of PLLA/PDLA racemic blend. This demonstrates that the high-MW PLLA/PDLA racemic blend has slow crystallization rate and predominantly crystallizes in the hc form upon cooling and heating.13 However, for all PLLA-b-PDLAs, a sharp crystallization peak is seen at 100~160 °C upon cooling and 13 ACS Paragon Plus Environment


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single melting region is observed at 190~240 °C in the following heating scan (Figure 2), suggesting that stereo block copolymerization significantly increases the crystallization rate and sc crystallization ability of PLA. Single melting region corresponding to the sc crystallites is also observed in the heating process of melt-quenched PLLA-b-PDLAs (Figure S2). Notably, the thermal parameters of PLLA-b-PDLAs strongly depend on the macromolecular topology. As shown in Figure 2 and Table 3, Tc, Tm, ∆Hm, and Xc of PLLA-b-PDLAs all decrease with increasing the branching number, suggesting that the lowering of macromolecular symmetry and regularity diminishes the crystallization rate, crystalline perfection, and crystallizability of PLLA-b-PDLAs. Tc and Tm of linear and 3-arm PLLA-b-PDLAs change little while those of 6-arm copolymers increase with increasing the MW. This indicates that the retarding effect of branching on sc crystallization is less serious in the high-MW copolymers, because of the increased branching length. Isothermal crystallization kinetics of PLLA-b-PDLAs with various topologies were further investigated via DSC in a wide temperature range (Tc = 130~170 °C). On basis of the DSC curves collected in isothermal melt crystallization (Figures S3a, S3b), the crystallization kinetics were analyzed by Avrami equation46

log[− ln(1 − X t )] = nlogt + logk

(1)

where Xt is the relative degree of crystallinity, k is the crystallization rate constant, t is the crystallization time, and n is the Avrami exponent, which is linked to the type of nucleation and the geometry of crystal growth. Theoretically, n equals to 4 for a polymer with spherulitic morphology that is nucleated sporadically or homogenously. n should be 3 if the nucleation is instantaneous or heterogeneous. In order to ensure the accuracy of Avrami plotting, the data within Xt = 3~20% was just used in the 14 ACS Paragon Plus Environment

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analysis.46 For all PLLA-b-PDLAs, the plot of log[−ln(1−Xt)] versus logt displays a straight line as expected (Figures S3c, S3d). The n values of PLLA-b-PDLAs are ranging in 2.1~2.9 and are almost not dependent on the macromolecular architecture, MW, and Tc, indicating that PLLA-b-PDLAs undergo heterogeneous nucleation under the investigated crystallization conditions. The crystallization half-time (t1/2), which is defined as the time spent from the onset of crystallization to the point with Xt = 50 %, is considered as a measure of the overall crystallization rate. As seen in Figures 3a and S4a, t1/2 shows a minimum at Tc = 140 °C and it increases to both side of lower and higher Tcs at Tc = 130~170 °C, because of the difficulty in chain diffusion and nucleation at low and high Tcs, respectively. t1/2 increases with increasing the MW, suggesting the lowered crystallization rate with MW. This is consistent with the MW-dependent crystallization rate of PLLA homopolymer.47 At the similar MW, t1/2 enhances with increasing the branching number, ascribed to the lowered crystallization rate induced by branching effect. Obviously, t1/2s of PLLA-b-PDLA are much shorter than those of high-MW PLLA/PDLA racemic blend after crystallization at the same Tc, due to the exclusive formation of sc crystallites in the former.

10

(a)

3 3L-D-80k 1L-D-202k 3L-D-193k 6L-D-201k

G (µm/min)

15

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


5

0

2

3L-D-80k 1L-D-202k 3L-D-193k 6L-D-201k

1

0

130 140 150 160 170 Temperature (°C)

(b)

130 140 150 160 170 180 Temperature (°C)

Figure 3. (a) Crystallization half-time (t1/2) and (b) radial growth rate of spherulite (G) 15 ACS Paragon Plus Environment


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

of linear, 3, and 6-arm PLLA-b-PDLAs isothermally crystallized at different temperatures. Spherulite morphology and spherulitic growth rate of PLLA-b-PDLAs with various topologies were analyzed by POM. All copolymers form the spherulites with typical Maltese cross patterns at Tc = 130~170 °C and no obvious difference in spherulitic morphology is observed for the copolymers with different macromolecular topologies (Figure S5). Similar as the Tc-dependent t1/2 results, the radial growth rate of spherulite (G) of PLLA-b-PDLAs is the largest at 140 °C and it generally decreases with increasing the branching number at the similar MWs. As shown in Figures 3b and S4b, G of PLLA-b-PDLA varies in 0.5~4.0 µm/min at Tc = 130~180 °C, which is lower than the low-MW PLLA-b-PDLAs crystallized under the similar conditions.31,43 Crystalline Structure. Since the hc and sc crystallites of PLA possess distinct Tms, polymorphic crystalline structure of PLLA-b-PDLAs can be attained from their melting behavior (Figures 4a, S6). Xcs of PLLA-b-PDLAs crystallized at different Tcs were calculated from the ∆Hm derived from the meting process. Tm shifts to the lower temperature with increasing the branching number or decreasing the MW (Figure 4a). However, Tms of linear and star-shaped PLLA-b-PDLAs hardly change with varying Tc (Figure S6). Tms of PLLA-b-PDLAs crystallized at different Tcs are also plotted as a function of the Mn per arm (Mn,arm), as shown in Figure S7. Tms of PLLA-b-PDLAs first increase and then keep nearly unchanged with increasing Mn,arm (Figure S7). As shown in Figure 4b, Xc increases with Tc because of the formation of more prefect and ordered crystallites at a smaller supercooling at the same Tc. Xcs of PLLA-b-PDLAs are much larger than those of sc crystallites formed in PLLA/PDLA racemic blend. At the same Tc, Xc of PLLA-b-PDLAs decreases with the increasing branching number and increases with increasing the MW. Xcs of linear PLLA-b-PDLA with high MW


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Page 17 of 34

are in the range of 0.44~0.50 after crystallization at Tc = 130~170 °C, in agreement

Endo up

with the results reported by Tsuji et al.31 for the low-MW copolymers.

(a)

0.5

(b)

6L-D-201k 3L-D-193k

0.4 1L-D-202k

Xc

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


6L-D-84k

PLLA/PDLA 3L-D-80k 1L-D-202k 6L-D-201k

0.3 0.1

3L-D-80k 1L-D-84k

1L-D-84k 6L-D-84k 3L-D-193k

PLLA/PDLA

0.0

160 180 200 220 240

Temperature (°C)

130

140

150

160

170

Temperature (°C)

Figure 4. (a) DSC heating curves of PLLA-b-PDLAs and PLLA/PDLA racemic blend after isothermal melt crystallization at 140 °C. (b) Degree of crystallinity (Xc) for sc crystallites formed in PLLA-b-PDLAs and PLLA/PDLA racemic blend after crystallization at different temperatures. Crystalline structures of linear and star-shaped PLLA-b-PDLAs were characterized by WAXD and FTIR. As shown in the WAXD patterns (Figure 5a, S8), all PLLA-b-PDLAs exhibit characteristic diffractions at 2θ = 12.0, 20.8, and 24.0°, corresponding to the diffractions of (110), (300)/(030) and (220) crystalline plane of sc crystallites, respectively;48 no characteristic reflection of hc is observed under the investigated Tcs. However, PLLA/PDLA racemic blend mainly shows the characteristic diffractions of α-form hc at 2θ = 16.7 and 19.2°. As seen from the FTIR spectra collected in the crystalline structure-sensitive region (1000~800 cm−1), all PLLA-b-PDLAs show the characteristic band of sc crystallites at 908 cm−1, associated with the combination of CH3 rocking vibration and C−COO stretching vibration modes of molecular chains with the 31 helical conformation in the sc crystalline phase.8 However, the corresponding band of α-form hc with 103 helical chain 17 ACS Paragon Plus Environment


conformation at 921 cm−1 is predominantly observed in the PLLA/PDLA racemic blend. All the WAXD and FTIR data confirm the exclusive formation of sc crystallites in PLLA-b-PDLAs crystallized under various conditions. The 869 and 955 cm−1 bands of FTIR spectrum are mainly associated with the amorphous phase of PLLA-b-PDLAs. As seen in Figure 5b, the intensity ratio of 908 to 869 cm−1 band decreases with increasing the branching number, which is more obvious for the high-MW copolymers. This suggests the smaller Xc of star-shaped PLLA-b-PDLAs

869 921 908

955

(b) (220)

(110)

Intensity (a.u.)

(a)

(300)/(030)

than their linear analogs, in agreement with the Xc data derived from DSC.

6L-D-201k

3L-D-193k 1L-D-202k

ABS

6L-D-201k

3L-D-193k 1L-D-202k

6L-D-84k

6L-D-84k

3L-D-80k

3L-D-80k

1L-D-84k PLLA/PDLA

10

15

20

25

1L-D-84k PLLA/ PDLA

30

960

2θ (°)

920 880 840 Wavenumber (cm−1)

20 (c) 18

D110 (nm)

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 34

16 1L-D-84k 3L-D-80k 6L-D-84k 1L-D-202k 3L-D-193k 6L-D-201k

14 12 130

140

150

160 170 Temperature (°C)

Figure 5. (a) WAXD profiles and (b) FTIR spectra of linear, 3, 6-arm PLLA-b-PDLAs and PLLA/PDLA racemic blend crystallized at 150 °C. (c) Crystallite size of (110) plane (D110) for linear, 3, and 6-arm PLLA-b-PDLAs crystallized at different temperatures. 18 ACS Paragon Plus Environment

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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


On basis of the WAXD results, the crystallite size perpendicular to hkl plane (Dhkl) was estimated by Scherrer’s equation49

Dhkl = K λ / (β cosθ )

(2)

where K is the Scherrer constant and is generally taken as 0.9, λ is the wavelength of X-ray (λ = 0.154 nm), β is the full width at half maximum (FWHM) of diffraction peak, and θ is the diffraction angle of peak top. As shown in Figures 5c, D110s of PLLA-b-PDLAs are in the range of 13~20 nm. Crystallite size increases with Tc, ascribed to the formation of crystallites with high perfection and orderness. Similar phenomenon has been observed in the crystallization of PLLA homopolymer.50,51 After crystallization at the same Tc, the star-shaped PLLA-b-PDLA exhibits smaller crystallite size than its linear counterpart and the crystallite size further decreases with increasing the branching number, indicating the branching architecture impedes forming the larger crystallites. Crystalline Lamellar Morphology. Crystalline lamellar morphology of PLLA-b-PDLAs is analyzed by SAXS. Figure 6a shows the one-dimensional SAXS curves of PLLA-b-PDLAs with different topologies after crystallization at 150 °C. The scattering peaks corresponding to long period (LP) are not clear enough in the original SAXS profiles of PLA. To determine LP, Lorentz-corrected SAXS profiles, i.e., Iq2~ q curve, were plotted (Figures 6b, c), where I is the scattering intensity and q is the scattering vector (q = 2πsin2θ/λ). The Lorentz-corrected SAXS profiles of PLLA-b-PDLAs show single broad peak at q = 0.2~0.5 nm−1. As seen in the Lorentz-corrected SAXS profiles, the scattering peak of PLLA-b-PDLAs shifts to high q with increasing the branching number or decreasing Tc, indicating the decrease of long period. LP of PLLA-b-PDLAs was calculated by Bragg equation (LP =



2π/qmax), in which qmax corresponds to the peak top of Lorentz-corrected SAXS profile.

(b)

(a)

6L-D-201k

I(q) (a.u.)

3L-D-193k 6L-D-201k 3L-D-193k

1L-D-202k

Iq2

1L-D-202k

6L-D-84k

6L-D-84k

3L-D-80k

3L-D-80k

1L-D-84k

1L-D-84k

PLLA/PDLA

PLLA/PDLA

0.5

1

−1

1.5 2

0.5

q (nm )

−1

1

1.5 2

q (nm )

(c) 170°C 2

160°C

Iq

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 34

150°C 140°C 130°C

0.5

−1

1

1.5 2

q (nm ) Figure 6. (a) Original and (b) Lorentz-corrected SAXS profiles of linear, 3, and 6-arm PLLA-b-PDLAs and PLLA/PDLA racemic blend crystallized at 150 °C. (c) Lorentz-corrected SAXS profiles of 6L-D-201k sample crystallized at different temperatures. To estimate the thicknesses of amorphous and crystalline layers, SAXS data was further analyzed by the one-dimension correlation function r(z)52 ∞

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

0

∞

2

(3)

0

The correlation function is valid with the assumption of two-phase model that consists of the alternately stacked structure of the crystalline and amorphous layers. To attain 20 ACS Paragon Plus Environment

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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


the correlation function, I(q) was corrected by subtracting the thermal diffuse scattering (ITDS), which was caused by the fluctuation of electron density in the sample. ITDS can be considered as a constant and I(q) at high q (q > 1.5 nm−1) is expressed by the modified Porod-Ruland equation53

I ( q ) = K p / q 4 + I TDS

(4)

where Kp is the Porod’s constant. ITDS and Kp can be obtained from the linear approximation of I(q)q4 vs q4. I(q) at low q (q < 0.2 nm−1) was replaced by the calculated intensity based on Guinier’s law,54 in which the parameters were calculated by the linear approximation of ln [I(q)] vs q2 plot. At high q (q > 1.5 nm−1), I(q) can be expressed by the Porod law55

I (q) = Aq−m

(5)

where m is the Porod exponent and A is the prefactor. According to the ln[I(q)] vs lnq plot, m and A can be obtained (Figure S9). 1L-D-202k, 3L-D-193k, and 6L-D-201k samples have the Porod exponents of 4.02, 3.97, and 3.60, respectively, which are all close to 4. This indicates that the scattering of PLLA-b-PDLAs at high q can be well described by the Porod model. The decrease of Porod exponent with branching number demonstrates that the surface of scattering objects (e.g., crystalline phase) becomes less smooth and defined in the star-shaped copolymers. This is consistent with the DSC and WAXD results and would be attributable to the less perfect and ordered crystallites formed in the star-shaped PLLA-b-PDLAs. The one-dimensional correlation function r(z) of PLLA-b-PDLAs with different topologies after crystallization at various Tc are depicted in Figure 7. The r(z) shows a number of minima and maxima and decreases to zero at large z, indicating the presence of structural periodicity. The lamellar repeat distance (i.e., long period, Lac), 21 ACS Paragon Plus Environment


thicknesses of crystalline and amorphous layers (Lc, La) were determined by analyzing r(z) using the method of Strobl and Schneider,52 as shown in the inset of Figure 7a. As shown in Figure 7, the position of first maximum in r(z) shifts to low z with increasing the branching number or decreasing Tc, reflecting the decrease of Lac. LP derived from Bragg equation, Lac, Lc, and La calculated from the one-dimensional correlation function are plotted as a function of Tc in Figures 8 for PLLA-b-PDLAs. la=L-lc

(a)

(b)

r(z)

L lc

170°C

10 15 20 25 30

z (nm)

6L-D-201k 3L-D-193k 1L-D-202k

r (z)

5

r (z)

0

160°C 150°C 140°C

6L-D-84k 3L-D-80k 1L-D-84k

0

10

20

130°C

30

0

10

z (nm)

20

30

z (nm)

Figure 7. One dimension correlation function curves for (a) linear, 3, and 6-arm PLLA-b-PDLAs crystallized at 150 °C and for (b) 3L-D-193k crystallized at different temperatures. 18

(a)

16

(b)

Lac (nm)

16

LP (nm)

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 22 of 34

14 12

1L-D-84k 3L-D-80k 6L-D-84k

10 130

140

1L-D-202k 3L-D-193k 6L-D-201k

14

12 1L-D-84k 3L-D-80k 6L-D-84k

10

150


130

140



150

1L-D-202k 3L-D-193k 6L-D-201k

Page 23 of 34

7

(c)

9

La (nm)

5

4

(d)

8

6

Lc (nm)

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


1L-D-84k 3L-D-80k 6L-D-84k

130

140

1L-D-202k 3L-D-193k 6L-D-201k

7 1L-D-84k 3L-D-80k 6L-D-84k

6 5

150


130

140

150

1L-D-202k 3L-D-193k 6L-D-201k

160

170

Temperature (°C)

Figure 8. Morphological parameters of linear, 3, and 6-arm PLLA-b-PDLAs crystallized at different temperatures: (a) long period (LP) calculated from Bragg equation, (b) Lac; (c) Lc, and (d) La derived from the one dimension correlation function curves.

Under the same Tcs, the long periods derived from these two methods are consistent with each other, which are ranging in 11~18 nm. Long period of PLLA-b-PDLA is consistent with that of sc crystallites formed in the low-MW stereo diblock copolymers25 and PLLA/PDLA racemic blends.13,56,57 However, the long period of PLLA-b-PDLA is much smaller than that of homocrystalline PLLA (~ 21.5 nm at Tc = 80 °C58). Similar phenomenon has been observed in the crystallization of PLLA/PDLA racemic blend.13,57 Besides, the larger qmax of PLLA-b-PDLAs than PLLA/PDLA racemic blend in the Lorentz-corrected SAXS profiles also demonstrate the smaller long period of stereocomplexed PLLA-b-PDLAs. Smaller long period of sc than hc may be ascribed to the long diffusion path in stereocomplexation. Obviously, LP, Lac, Lc, and La increase with increasing Tc or decreasing the branching number. The Tc-dependent lamellar thickness can be ascribed to the formation of perfect crystallites with thick crystalline lamellae at a small supercooling. The decrease of LP, Lac, and Lc in star-shaped PLLA-b-PDLAs stems from the disturbance



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

in forming regular lamellar structure caused by the incorporation of branching arms. Because the branching effects become less significant at a larger branching length, LP, Lac, and Lc values of PLLA-b-PDLAs increases with increasing MW (Figure 8). Lcs of PLLA-b-PDLAs crystallized at different Tcs are plotted as function of Mn,arm in Figure S10. Consistent with the Tm results shown in Figure S7, Lcs of PLLA-b-PDLAs first increase and then change little with increasing the Mn,arm, indicating that the crystalline perfection enhances with increasing the arm length of PLLA-b-PDLAs with relatively shorter arms. Thermal Mechanical Property. Because of the effect on crystalline structure, stereo block copolymerization exerts a significant influence on the thermal mechanical property of PLA. Figure 9 shows the temperature-dependent storage moduli of solvent-cast film for high-MW PLLA-b-PDLAs, in which the homocrystalline PLLA film prepared under the same conditions is included for comparison. Upon heating, the storage moduli of PLLA-b-PDLAs and PLLA drop in 50~80 °C and > 150 (or 200) °C, attributable to the glass transition and crystallite melting. PLLA-b-PDLAs show the much larger storage modulus than homocrystalline PLLA above the glass transition temperature (Tg), even though their storage moduli are similar below Tg. Homocrystalline PLLA starts to melt at around 150 °C, leading to the drastic decrease of storage modulus. However, due to the presence of high-Tm sc crystallites, PLLA-b-PDLAs still retain high storage moduli at 200 °C, which are 30.7, 14.2, and 12.2 MPa for the linear, 3, and 6-arm star-shaped copolymers, respectively. Higher storage modulus of linear than star-shaped PLLA-b-PDLAs at high Tc is attributable to the larger Xc of the former. These results suggest that the synthesized linear and star-shaped PLLA-b-PDLAs have much better thermal resistance than the conventional homocrystalline PLLA. 24 ACS Paragon Plus Environment

Page 24 of 34

Page 25 of 34

3

10

Storage modulus (MPa)

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


2

10

1

10

L192k 1L-D-202k 3L-D-193k 6L-D-201k

0

10

-1

10

-2

10

0

50

100 150 200 250

Temperature (°C) Figure 9. Storage modulus of PLLA-b-PDLAs and PLLA homopolymer plotted as a function of temperature.

CONCLUSION In conclusion, the linear and multi-arm star-shaped PLLA-b-PDLAs with high MWs were synthesized by two-step ROP. Nonisothermal, isothermal crystallization kinetics, crystalline structure, lamellar morphology, and mechanical thermal property of

PLLA-b-PDLAs

strongly

depend

on

the

macromolecular

topology.

PLLA-b-PDLAs exhibit faster crystallization rate than the corresponding high-MW PLLA/PDLA racemic blends and exclusively form sc crystallites under various crystallization conditions, ascribed to the covalent bonding between the enantiomeric blocks. Because of the branching effect on sc crystallization, star-shaped PLLA-b-PDLAs show lower Tc, Tm, Xc, G, D110, LP, Lac, and Lc than their linear counterparts. Both the linear and star-shaped PLLA-b-PDLAs have much better thermal resistance and higher storage modulus at high temperature than the conventional homocrystalline PLLA. Storage modulus of PLLA-b-PDLAs at high temperature decreases with increasing the branching number, because of the lowered Xc of sc crystallites. This study has clarified the topology-dependent sc crystallization 25 ACS Paragon Plus Environment


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 26 of 34

kinetics and crystalline structure of PLLA-b-PDLAs and may serve as the basis for preparing the high-MW sc-PLA materials with good thermal resistance.

ASSOCIATED CONTENT Supporting Information Partial NMR, DSC, POM, WAXD results of PLLAs and PLLA-b-PDLAs. 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), Fundamental Research Funds for the Central Universities (2015XZZX004-08), and Zhejiang Top Priority Discipline of Textile Science and Engineering (2015KF05). SAXS experiments were carried out on the beamline BL16B1 of SSRF, China.

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