Promoted Stereocomplex Crystallization in Supramolecular

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Promoted Stereocomplex Crystallization in Supramolecular Stereoblock Copolymers of Enantiomeric Poly(lactic acid)s Jianna Bao, Ruoxing Chang, Guorong Shan, Yongzhong Bao, and Pengju Pan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01627 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016

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Crystal Growth & Design

Promoted Stereocomplex Crystallization in Supramolecular Stereoblock Copolymers of Enantiomeric Poly(lactic acid)s

Jianna Bao, Ruoxing Chang, 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: Telechelic and 3-arm star-shaped poly(L-lactic acid)s (PLLAs) and poly(D-lactic acid)s (PDLAs) end-functionalized by 2-ureido-4[1H]-pyrimidione (UPy) self-complementary quadruple hydrogen-bonding motifs were synthesized by terminally modifying the corresponding hydroxyl-terminated polymers. The PLLA/PDLA stereoblock supramolecular polymers (sb-SMPs) with linear and branched architectures were further prepared by mixing the UPy end-functionalized telechelic, 3-arm star-shaped PLLA and PDLA through the dimerization of UPy groups, as verified by viscometry. Crystallization kinetics, polymorphic crystalline structure, and lamellae morphology of the PLLA/PDLA sb-SMPs were investigated and compared with the corresponding blends without UPy end functionalities. Compared to the PLLA/PDLA blends, the sb-SMPs exhibit higher crystallization rate and prefer to crystallizing into the high-melting-point stereocomplex (sc), rather than the low-melting-point homocrystallite in both the nonisothermal and isothermal crystallization processes. The sc crystallization ability and crystallization rate of sb-SMPs are further enhanced with increasing the concentration of terminal UPy groups or decreasing the block lengths of PLLA and PDLA. The enhanced sc crystallization ability and faster crystallization rate of sb-SMPs are ascribed to the hydrogen bonding interactions formed between the terminal UPy groups of PLLA and PDLA. Due to the preferential sc crystallization, the sb-SMPs exhibit smaller long periods than the corresponding PLLA/PDLA blends.

INTRODUCTION Stereocomplex (sc) crystallization is a unique cocrystallization manner of the polymers with opposite configurations or chiralities, in which the enantiomeric polymer pair cocrystallize in 1:1 ratio in the crystal unit cell.1−3 Sc crystallization has been found in many polymers such as poly(methyl methacrylate),4 poly(lactic acid) 2


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(PLA),5 epoxide/CO2 polycarbonate,6,7 and poly(propylene succinate).8 In the sc crystallization, the complementary polymer enantiomers can interact with each other through the specific interactions (e.g., intermolecular hydrogen bonding), leading to the more compact chain packing in the sc polymorph.6,7,9−11 Because of this structural characteristic, the stereocomplexed materials usually have higher melting temperature (Tm),1 better heat resistance, larger strength and modulus,12,13 and better chemical and hydrolytic resistance14,15 than the homocrystalline materials with the similar chemical structure. A representative polymer that can undergo sc crystallization is PLA. The two stereoisomers of PLA, i.e., poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA), can cocrystallize in the sc polymorph in their enantiomeric blends and stereoblock copolymers.1,2,16 The stereocomplexed PLA has a high Tm of 230 °C, about 50 °C larger than that of their homocrystalline parent polymers. Therefore, the sc crystallization of PLLA and PDLA has been an effective way to improve their physical properties such as the thermal and hydrolytic resistance. Nevertheless, not all PLLA/PDLA blends are able to completely cocrystallize in sc form during the traditional crystallizations upon heating, cooling, and thermal annealing. For example, the

sc

crystallization

between

complementary

enantiomers

and

the

homocrystallization of individual enantiomer are competing in the PLLA/PDLA blends.17,18 Because of the larger kinetic barrier, the sc crystallization is less favorable than homocrystallization in the PLLA/PDLA blends with the relatively high molecular weight (MW > 40 kDa).17,18,19−23 Because only the high-MW PLA has good mechanical property and processing ability, promoting the sc crystallization of high-MW PLLA/PDLA blends is essential to prepare the sc-PLA materials with better physical performances.

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The intermolecular interactions between complementary polymer enantiomers has been a key factor influencing their sc crystallization behavior.24−29 It has been reported that the rate and extent of PLLA/PDLA sc crystallization are significantly promoted by covalently bonding the opposite enantiomers (i.e., stereoblock copolymerization).24−28 The covalently-bonded PLLA/PDLA stereoblock copolymers exclusively crystallize in sc polymorph in the conventional crystallization processes. Furthermore, previous study has found that the racemic helical pairs (i.e., embryo of sc nuclei) are generated prior to the nucleation of sc polymorph in the crystallization of PLLA/PDLA blends.30 Therefore, enhancing the intermolecular interactions between complementary enantiomers would facilitate the nucleation and growth of sc crystallites. Except for the covalent bonding, constructing the intermolecular noncovalent interactions would be an alternative method to enhance the interactions and linkages between

different

polymer

segments.

Noncovalent

interactions

including

hydrogen-binding, π−π stacking, electrostatic, and metal-ion coordination interactions have been widely used to prepare the supramolecular polymers (SMPs).31 A well-studied group of SMPs are those bonded by the 2-ureido-4[1H]-pyrimidinone (UPy) self-complementary quadruple hydrogen bonding unit, which has a high dimerization constant and bonding strength close to those of the covalent bonds.32,33 The UPy-bonded SMPs can be feasibly prepared by the end functionalization of polymers or their oligomers, which is often achieved by reacting the terminal amine and hydroxyl groups with a specific UPy-synthon (e.g., UPy derivative containing an active isocyanate linker).34 This method is particularly straightforward and effective to prepare the UPy-bonded SMPs based on the poly(lactone)s, because the poly(lactone)s such as PLA and poly(ε-caprolactone) (PCL) prepared by ring-opening

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polymerization (ROP) are typically terminated by the hydroxyl functionality.35−38 Despite the recent progress on the preparation and functions of UPy-bonded SMPs, the polymorphic crystallization and its correlation with the involved noncovalent interactions for such SMPs are still unclear and remain to be elucidated. In this work, we have selected the UPy self-complementary quadruple hydrogen bonding unit as a model supramolecular motif and introduced it to the chain terminals of telechelic and 3-arm star-shaped PLLAs and PDLAs. The dimerization of UPy groups can randomly link the PLLA and PDLA blocks or segments, causing to form the supramolecular (or pseudo) stereo multiblock copolymers or networks that have the similar structures as the conventional covalently-bonded stereoblock copolymers or networks. This would effectively increase the intermolecular interactions between the complementary enantiomers, which is envisioned to promote the sc crystallization and depress the homocrystallization of PLLA and PDLA. Effects of the UPy end functionalization and formation of stereoblock SMP (sb-SMP) on the competing crystallization kinetics, polymorphic crystalline structure, and lamellar morphology of PLLA/PDLA blends were investigated. Roles of UPy concentration and PLLA, PDLA block lengths on the polymorphic crystallization behavior of sb-SMPs were elucidated. Mechanism for the favored sc crystallization of PLLA/PDLA sb-SMPs was also proposed.

EXPERIMENTAL SECTION Materials. L- and D-lactide ( > 99%, Purac Co., Gorinchem, the Netherlands) were purified by recrystallization from ethyl acetate. Tin(II) 2-ethylhexanoate [Sn(Oct)2, Aldrich-Sigma] was purified by distillation under reduced pressure. 1,6-Hexanediol

(1,6-HDO,

Aldrich-Sigma)

and

trimethylolpropane

(TMP,

Aldrich-Sigma), 1-methyl-2-pyrrolidinone (NMP, J&K), 2-amino-4-hydroxy-65



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methylpyrimidine (J&K), 1,6-hexyldiisocyanate (HDI, Wanhua Industrial Group Co., Ltd.), and dibutyltin dilaurate (DBTDL, J&K) were used without further purification. 2(6-Isocyanatohexylaminocarbonyl-amino)-6-methyl-4[1H]

pyrimidinone,

abbreviated as UPy-NCO, was prepared by a reported procedure.34 Toluene was dried by sodium and distilled after being refluxed for 48 h. Other chemicals were all reagent grade and used as received. Synthesis of Hydroxyl-Terminated Telechelic and 3-Arm Star-Shaped PLLAs and PDLAs. A typical procedure to synthesize the 3-arm star-shaped PLLA with an expected Mn of 60.0 kg/mol is shown as below. Dried L-lactide (30.0 g, 0.21 mol), TMP (0.067 g, 0.50 mmol), and Sn(Oct)2 (0.09 g, 0.22 mmol) were added into a Schlenk flask and further dried at 65 °C for 1 h under the reduced pressure. The flask was heated to 130 °C and the reaction was proceeded at this temperature for 5 h under the protection of argon. After reaction, the crude product was dissolved into chloroform and the resulting solution was then precipitated into ethanol to remove the unreacted monomer. The polymer was finally dried at 80 °C in vacuo for 6 h. Other PLLAs and PDLAs were synthesized by a similar method. 1,6-HDO was used as the initiator for synthesizing the hydroxyl-terminated telechelic PLLAs and PDLAs. The as-prepared telechelic and 3-arm star-shaped PLLAs and PDLAs are marked as 2L(D)-xk or 3L(D)-xk, where the prefixed numerals “2” and “3” denote the telechelic and 3-arm star-shaped architectures, respectively, L and D denote PLLA and PDLA, respectively, and x represents the Mw (in kg/mol) of polymer measured by gel permeation chromatography (GPC). Synthesis of UPy-Terminated Telechelic and 3-Arm Star-Shaped PLLAs and PDLAs. 3.0 g of hydroxyl-terminated PLLA or PDLA and 10-fold molar excess of UPy-NCO with respect to the terminal hydroxyl groups were added into a dried

6


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Schlenk flask, which was further dried at 65 °C for 1 h under a reduced pressure. The flask was purged with dry argon, followed by the addition of toluene (150 ml) and Sn(Oct)2 (0.09 g, 0.22 mmol). The reaction mixture was stirred at 110 °C for 12 h. After reaction, the solvent was removed by a rotary evaporator. The obtained product was dissolved in chloroform (200 ml) and treated by silica (5.0 g) and dibutyltin dilaurate (20.0 mg) at 60 °C for 1 h.34 Then, the silica-bound UPy was removed by filtration and the filtrate was concentrated under reduced pressure. Residual solvent in the product was finally removed by drying at 80 °C in vacuo for 6 h. The as-prepared UPy-terminated telechelic and 3-arm star-shaped PLLAs and PDLAs are, respectively, marked as 2L(D)-U-xk and 3L(D)-U-xk, in which “U” represents the end functionalization by UPy groups. Preparation of Sb-SMPs and PLLA/PDLA Blends. For preparing PLLA/PDLA blends, equivalent mass of hydroxyl-terminated PLLA and PDLA were separately dissolved in chloroform (30 g/L). The solutions were mixed and stirred rigorously at room temperature for 0.5 h. Because the UPy end-functionalized PLLA and PDLA are less soluble in organic solvent such as chloroform, equivalent mass of UPy-terminated PLLA and PDLA were separately dissolved in chloroform at a lower concentration (10 g/L) and a higher temperature of 45 °C. The solutions were then mixed and kept stirring at 45 °C for 2.0 h. 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. Sb-SMPs prepared from the UPy-terminated telechelic and 3-arm star-shaped PLLA and PDLA are, respectively, denoted as 2L-U-xk/2D-U-xk and 3L-U-xk/3D-U-xk, and the corresponding PLLA/PDLA blends are marked as 2L-xk/2D-xk and 3L-xk/3D-xk.

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Measurements. 1H NMR spectra were collected on a 400MHz Bruker AVANCE II NMR spectrometer (Bruker BioSpin Co., Switzerland) with CDCl3 as the solvent. Molecular weights were analyzed by a Waters GPC (Waters Co., Milford, MA, USA) consisting of a PL-gel 10 µm MIXED-BLS column and a Waters 2414 refractive index (RI) detector at 30 °C. Tetrahydrofuran (THF) was used as the mobile phase and polystyrene was used as the standard. Viscosities of hydroxyl and UPy-terminated PLLAs in CHCl3 solution at different concentrations were measured by an Ubbelohde viscometer at 30 °C. Thermogravimetric analysis (TGA) was conducted on a Pyris 1 TGA instrument (PerkinElmer, Waltham, MA, USA). The sample was heated from 50 to 600 °C at a heating rate of 10 °C under a nitrogen atmosphere. Crystallization and melting behavior of PLLA/PDLA sb-SMPs and blends were analyzed on a NETZSCH 214 Polyma differential scanning calorimetry (DSC, NETZSCH, Germany) equipped with an IC70 intracooler under a nitrogen gas flow. To investigate the nonisothermal cold crystallization, the sample 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. For the isothermal melt crystallization, after melting at 250 °C for 2 min, the sample was cooled to a desired temperature (Tc = 80 ~ 160°C) at a cooling rate of 100 °C/min and then maintained at this temperature for sufficient time to crystallize. It was then reheated to 250 °C at 10 °C/min to examine the melt behavior. The spherulitic morphology and spherulite growth rate (G) were measured on an Olympus BX51 polarized optical microscopy (POM, Olympus Co., Tokyo, Japan) equipped with a Instec HCS402 hot stage (Instec Inc., Colorado, USA). The sample was sandwiched between two glass slides and melted at 250 °C for 2 min. In the

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melting process, the sample was slightly pressed to spread into a thin film. The sample was then fast cooled to the desired Tc at 100 °C/min for isothermal crystallization. During crystallization, the spherulite morphology was recorded by taking photo at a fixed time interval. The average radial growth rate (G) was estimated from the slope of spherulite radius (R) vs crystallization time (t) plot. Conventional wide-angle X-ray diffraction (WAXD) patterns were measured by a RU-200 diffractometer (Rigaku, Japan) using a Ni-filtered Cu Kα radiation (λ = 0.154 nm) at room temperature. The film samples of sb-SMPs and PLLA/PDLA blends with a thickness of ~0.3 mm were prepared by hot pressing using the same thermal program as that used in the DSC isothermal crystallization. The measured 2θ region was 7~40° and the scanning rate was 2°/min. Small-angle X-ray scattering (SAXS) and temperature-variable WAXD were analyzed on the BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF) with an X-ray wavelength of 0.124 nm. The scattering profiles were collected by Rayonix SX-165 CCD detector (Rayonix, USA) having the resolution of 2048 × 2048 pixels and pixel size of 80 × 80 µm2. The distances between sample and detector were 0.12 and 2.5 m for WAXD and SAXS, respectively. For the SAXS analysis, the samples (thickness ~0.2 mm) crystallized at different Tcs were prepared on a Instec HCS402 hot stage using the same thermal programs as those used in DSC isothermal crystallization. For the in situ WAXD analysis, the amorphous sample with a thickness of ~0.2 mm was obtained by quenching the melted (250 °C, 2 min) sample into liquid nitrogen. The sample was sandwiched by polyimide films and was heated from 50 to 260 °C at 10 °C/min on a Linkam THMS600 hot stage. WAXD patterns were collected by a time interval of 30 s during heating. The acquisition times of

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WAXD and SAXS were 15 and 90 s, respectively. One-dimensional profile was attained from the two-dimensional (2D) data by integrating with a Fit2D software.

RESULTS AND DISCUSSION Synthesis of UPy-Bonded PLLA/PDLA Sb-SMPs. Hydroxyl-terminated telechelic and 3-arm star-shaped PLLAs or PDLAs with different MWs were prepared by the bulk ROP of L- or D-lactide using the 1,6-HDO or TMP as the multifunctional initiators and Sn(Oct)2 as the catalyst (Scheme 1a,b). MWs of PLLAs and PDLAs were controlled by changing the lactide/initiator feed ratio (Table 1). As shown in Table 1, high yields ( ≥ 88%) are achieved in the ROP of L- and D-lactide. The chemical structure and MWs of synthesized PLLAs and PDLAs were characterized by means of GPC and 1H NMR. The synthesized PLLAs and PDLAs show single elution peaks in the GPC curves (Figure S1), corresponding to the relatively low polydispersity (PDI < ~1.30). The elution peak shifts toward the smaller retention time side with increasing the lactide/initiator feed ratio. Table 1. Molecular characteristics of UPy end-functionalized telechelic and 3-arm star-shaped PLLAs and PDLAs. samplea

lactide/[OH] (mol/mol)

Yield (%)

Mn,thb (kg/mol)

Mn,NMRc (kg/mol)

Mn,GPCd (kg/mol)

Mw,GPCd (kg/mol)

PDId

2L-64k

104/1

92

27.6

26.8

49.3

64.1

1.30

2L-93k

208/1

9891

53.4

663.6.9

75.9

92.9

1.22

3L-55k

69/1

90

27.0

28.0

44.5

55.1

1.24

3L-91k

138/1

90

54.0

68.2

75.5

90.5

1.20

2D-64k

104/1

88

26.4

30.0

51.3

63.6

1.24

2D-110k

208/1

92

55.2

53.9

83.0

109.6

1.32

3D-52k

69/1

91

27.3

21.0

45.6

52.0

1.14

3D-85k 138/1 91 54.6 46.4 69.2 85.4 1.23 a b Numerals in sample codes denote the Mw (kg/mol) measured by GPC. Mn calculated theoretically, M n,th = M n,initiator +

[lactide] × 144.13 × yield , where Mn,initiator is the [initiator] 10


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molecular weight of initiator and [lactide]/[initiator] is the feed molar ratio of lactide to initiator. cMn derived from 1H NMR. dMn, Mw, and polydispersity index (PDI) measured by GPC.

O

(b)

O O

O

O

OH

O O

HO

O

HO Sn(Oct)2

OH

N

O

OCN

6N

H

N H

N H

O

n-1

O

Sn(Oct)2

O

3PLA

TMP

O

N

H N

H N

O

H N

O

6

NH

O

O

3PLA-UPy

O n

O

O

2L-U/2D-U sb-SMP

(c)

3L-U/3D-U sb-SMP

Scheme 1. (a, b) Synthetic route of the UPy end-functionalized (a) telechelic and (b) 3-arm star-shaped PLLAs and PDLAs. (c) Schematic illustration of sb-SMP formation in the enantiomeric mixture of UPy end-functionalized PLLA and PDLA.

As shown in Figures 1 and S2, the hydroxyl-terminated PLLAs and PDLAs show the characteristic resonance peaks at 1.6 (peak b), 5.1 (peak a), and 4.3 ppm (peak a′), assigned to the methyl, methine, and terminal methine protons of lactic acid 11



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units, respectively. Besides, the characteristic resonance peaks of initiators such as the terminal methylene protons of 1,6-hexanediol (peak m, Figure S2) and the methylene protons of TMP (peak m, Figure 1) are clearly seen in the 1H NMR spectra. Based on the NMR results, Mns of PLLA and PDLA (denoted as Mn,NMR) were estimated by comparing the resonance peak area of terminal methine protons to methine protons. The theoretical Mn (Mn,th) was calculated from the lactide/initiator feed ratio and yield (Table 1). As shown in Table 1, Mn,NMRs of PLLA and PDLA are consistent with Mn,ths, both of which are smaller than the relative Mns measured by GPC (Mn,GPC). All these results demonstrate the successful syntheses of the hydroxyl-terminated telechelic and 3-arm star-shaped PLLAs and PDLAs with controlled MWs and macromolecular architectures.

3L-U-55k j

i

h

b a

14

12 l

g

f m

10

4.5 4.0 3.5 3.0

5.8 5.9

d k

o b n

3L-55k

a

b' a' m

7

6

5 4 3 2 Chemical shift (ppm)

o

1

Figure 1. 1H NMR spectrum of 3-arm star-shaped PLLAs with and without the UPy end groups.

The UPy end-functionalized PLLAs and PDLAs are prepared by the post-polymerization functionalization of hydroxyl-terminated PLLAs and PDLAs

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through reacting with the excess of UPy-NCO (Scheme 1a,b).34 Successful terminal functionalization of PLLAs and PDLAs by UPy motifs are demonstrated by the appearance of characteristic resonance peaks of UPy N–H groups at 10.2, 11.9, and 13.1 ppm in NMR spectra39 (Figures 1, S2) and the characteristic vibrational bands of UPy at 1663, 1587, and 1522 cm−1 in the FTIR spectra (Figure S3).40 Resonance peaks for the terminal methine protons of hydroxyl-terminated PLLAs and PDLAs at 4.3 ppm are absent after reaction with UPy-NCO, confirming that the terminal hydroxyls are converted to UPy nearly quantitatively. As shown in the GPC curves (Figure S4), the UPy end-functionalized PLLAs show the similar molecular weights with their precursors, indicating that polymers are not degraded during terminal modification. Formation of SMPs in the UPy end-functionalized PLLAs and PDLAs could be observed by viscometry.32,41 The enantiopure PLLA, rather than the PLLA/PDLA enantiomeric mixtures were used in the viscometry analysis, in order to avoid the precipitation of PLLA/PDLA sc crystallites in solution. Figure 2 compares the specific viscosity (ηsp) of hydroxyl and UPy end-functionalized PLLAs at various concentrations in CHCl3. The UPy end-functionalized PLLAs display viscosity with a complete different concentration dependence from their hydroxyl-terminated precursors. The UPy end-functionalized PLLAs have higher viscosities at all concentrations compared to the hydroxyl-terminated precursors. The plots of viscosity vs concentration for the hydroxyl-terminated PLLAs are straight lines over the entire concentration range, having the small slopes of 0.08 for 2L-64k and 0.07 for 3L-55k. However, the viscosities of UPy end-functionalized PLLAs show a strong concentration dependence. At low concentrations, their viscosities are low and the plots of viscosity vs concentration have the slopes of approximately 0.23 for

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2L-U-64k and 0.27 for 3L-U-55k. The viscosities of UPy end-functionalized PLLAs increase remarkably with concentrations and the slopes of viscosity vs concentration plots are upward to 0.70 for 2L-U-64k and 1.04 for 3L-U-55k at the concentrations above 2.0 g/L. This concentration dependence is attributed to the strong hydrogen bonding interactions between UPy units and formation of supramolecular structure through the dimerization of UPy terminals in solution above a critical concentration.32,41 However, at the same concentration, even though the hydroxyl terminated 3-arm star-shaped PLLAs have the smaller viscosity than the linear ones, the UPy-terminated 3-arm star-shaped PLLAs display larger viscosities than their linear counterparts, as well as the more eminent concentration dependence. This is attributed to the higher concentration of UPy groups and more denser hydrogen bonding networks formed in the UPy-terminated 3-arm star-shaped polymers. The literature has documented that the UPy dimerization is dynamic and reversible in solution.32,35,41−43 Meijer et al. have reported that the supramolecular polymers formed by UPy motifs are reversibly breakable in chloroform at 20 °C and the equilibrium between different hydrogen-bonded UPy motifs are quite fast in solution at room temperature.32,42 Viscosity data in Figure 2 also shows that the UPy dimerization in solution just happens above a critical concentration (ca. 2.0 g/L). This also suggests that the UPy-bonded supramolecular system is dynamic and exchangeable in solution. Furthermore, many researchers35,41,43 have prepared the supramolecular multiblock or segmented copolymers by mixing the UPy end-functionalized homopolymers A and B in solution, because of the dynamic and reversible exchange of UPy motifs between A and B segments or blocks. For these reasons, it is concluded that the UPy-terminated linear telechelic and 3-arm star-shaped PLLA/PDLA blends would form the linear and branched stereoblock

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SMPs (sb-SMPs) through the dynamic, reversible dissociation/association of UPy-UPy hydrogen bonds (as illustrated in Scheme 1c), rather than the blends of supramolecular homopolymers. 5 2L-U-64k 2L-64k 3L-U-55k 3L-55k

4 3

ηsp

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 1 0

0

2

4

6

8

Concentration (g/L)

Figure 2. Relationship between the specific viscosity (ηsp) and concentration of linear and 3-arm star-shaped PLLAs with and without the UPy end groups.

Crystallization Kinetics. To confirm that the prepared sb-SMPs are thermally stable in the thermal treatment and crystallization processes, the thermal stabilities of sb-SMPs and UPy end-functionalized PLLAs are investigated. TGA results indicate that the sb-SMP has the similar onset temperature of thermal degradation (~270 °C) as the PLLA/PDLA blend (Figure S5). After the thermal treatment at 250 °C for 2 min and subsequent cooling, which is similar to the thermal procedure used in DSC and WAXD measurements, the UPy-terminated PLLA displays almost the same 1H NMR spectrum as the untreated sample with respect to the peak position and relative intensity (Figure S6). These results demonstrate that the thermal degradation and decomposition of PLLA, PDLA, and terminal UPy units could be negligible in the thermal procedures used for crystallization studies. On the other hand, because UPy is a high polar group, water absorption of PLLA/PDLA blend increases after the end functionalization by UPy motifs. After equilibrating at 20 °C and a relative humidity 15



of 70% for 48 h, water absorption of 2L-64k/2D-64k is 0.66% and it increases to 0.96% for the 2L-U-64k/2D-U-64k sb-SMP. Therefore, all the samples are thoroughly dried before measurement. Isothermal crystallization and melting behavior of sb-SMPs formed from the UPy-bonded linear and 3-arm star-shaped PLLAs and PDLAs with different MWs are first studied via DSC and WAXD, which are also compared with the corresponding PLLA/PDLA blends. DSC curves recorded in isothermal crystallization (Figure S7) are fitted by the Avrami model to attain the kinetic parameters.44 Crystallization half-time (t1/2) is plotted as a function of Tc for the PLLA/PDLA sb-SMPs and blends in Figure 3. As shown in this figure, the t1/2~Tc curves of all PLLA/PDLA blends without UPy functionalities exhibit minima at Tc = 100 °C, corresponding to the fastest crystallization rate. This agrees with the Tc-dependent crystallization rate of enantiopure PLLA, which shows a minimum at Tc =100~110 °C.45 However, for the PLLA/PDLA sb-SMPs, the Tc corresponding to the minimal t1/2 and fastest crystallization rate shifts toward to the higher temperature and is observed at 120~140 °C, attributed to the favorable formation of more thermally stable sc polymorph. 2

2L-U-64k/2D-U-64k 2L-64k/2D-64k 2L-U-93k/2D-U-110k 2L-93k/2D-110k

(a)

10

1

10

0

80

100

10

t1/2 (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

Page 16 of 35

120

140 160 Temperature (°C)

2


(b)

10

1

10

0

80

100

120

140

160

Temperature (°C)

Figure 3. Crystallization half-times of PLLA/PDLA sb-SMPs and blends made from (a) linear and (b) 3-arm star-shaped PLLAs and PDLAs at different crystallization temperatures. 16


Page 17 of 35

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


Crystallization kinetics of the PLLA/PDLA sb-SMPs and blends are strongly influenced by the MW and UPy end functionalization. As shown in Figure 3, except for the linear PLLA/PDLA sb-SMPs and blends with high MW (i.e., 2L-93k/2D-110k, 2L-U-93k/2D-U-110k), the crystallization rates of PLLA/PDLA blends at different Tcs are generally increased with the end-functionalization of UPy motifs and the formation of sb-SMPs. POM analysis indicates that both the PLLA/PDLA sb-SMPs and blends form the spherulites with typical Maltese-cross patterns (Figure 4a,b). The spherulites of sb-SMPs grow much faster than the PLLA/PDLA blends (Figure 4c,d). Spherulitic radius growth rate (G) of sb-SMP made from the UPy-terminated 3-arm star-shaped PLLA and PDLA is nearly twice of those for the corresponding blend (Figure 4d). The acceleration effect of UPy terminals on the crystallization of sb-SMPs can be also observed in the nonisothermal cold crystallization (Figure S8, Table S1). The melt-quenched sb-SMPs generally have lower crystallization temperatures than the corresponding blends upon heating, indicating the faster crystallization of sb-SMPs. The faster crystallization of sb-SMPs than the PLLA/PDLA blends contradicts with the crystallization kinetics of conventional polymers. For most of the semicrystalline polymers, the crystallization rate is generally decreased with the incorporation of large terminal groups to the chain ends or with increasing the interactions and physical crosslinking between different chains, because of the decreased structural symmetry and chain diffusion ability. The decreased crystallization rate induced by the UPy end functionalization has been observed for PCL36 and enantiopure PLLA.37,46 The unusual acceleration effect of terminal UPy moieties on the crystallization of PLLA/PDLA mixture is ascribed to the improved sc crystallization ability, as described below. Moreover, the crystallization rate of 17



PLLA/PDLA sb-SMPs and blends are affected by the MWs of polymers or blocks. The sb-SMPs and PLLA/PDLA blends made from low-MW PLLA and PDLA exhibit the smaller t1/2s and faster crystallization rates, due to the higher diffusion ability of polymer segments. With the similar MWs of PLLA and PDLA blocks, the 2L-U/2D-U and 3L-U/3D-U sb-SMPs have the comparable crystallization rates, as demonstrated by the similar t1/2 and G values.

(a)

(b)

100µm 3L-U-91k/3D-U-85k

3L-91k/3D-85k

15

10


7.47 dR/dt = 7.76 µm/min

15

(c)

5.17

R (µm)

R (µm)

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 35

5

10

(d)

3L-U-55k/3D-U-52k 3L-55k/3D-52k 3L-U-91k/3D-U-85k 3L-91k/3D-85k dR/dt= 7.84 µm/min

7.65 4.32

5 3.56

2.25

0 0.0

0.5

1.0

0 0.0

1.5

Time (min)

0.5

1.0

1.5

Time (min)

Figure 4. (a, b) Representative POM micrographs of (a) sb-SMP and (b) blend made from 3-arm star-shaped PLLAs and PDLAs. (c, d) Spherulitic growth rate plots for PLLA/PDLA sb-SMPs and blends made from the (c) linear and (d) 3-armed star-shaped PLLAs and PDLAs.

Polymorphic Crystalline Structure. The end functionalization of UPy motifs significantly influences the polymorphic crystalline structure of sb-SMPs, as evidenced by DSC and WAXD. The PLLA/PDLA sb-SMPs and blends generally 18


Page 19 of 35

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


show dual melt regions in 150~180 and 200~250 °C after the isothermal crystallization at Tc = 80~160 °C (Figures 5, S9) and nonisothermal cold crystallization upon heating (Figure S8). These two melt regions at low and high temperatures correspond to the melts of homocrystallite (hc) and sc, respectively. In the case of PLLA/PDLA blends, the melt of hc is much more remarkable than that of sc, indicating the predominant formation of hc (Figures 5b,d, S8). However, in the case of sb-SMPs, the melt of sc is overwhelming and that of hc is nearly disappeared, suggesting the prevailing crystallization of sc in the isothermal and nonisothermal crystallizations (Figures 5a,c, S8, S9). Similar results can be clearly seen from the WAXD data (Figures 6, S10). After the isothermal crystallization at different Tcs (100~160 °C), the PLLA/PDLA blends exhibit strong diffractions of α-form hc at 2θ = 16.7 and 19.0° (characteristic of the diffractions of (110)/200 and (203) planes, respectively45) but much weak diffractions of sc at 2θ = 12.0, 20.8 and 24.0°, which correspond to the diffractions of sc (110), (300)/(030), and (220) planes, respectively (Figure 6b,d).47 However, in the case of sb-SMPs, the sc diffractions become sharper and their intensities enhance remarkably compared to the PLLA/PDLA blends, regardless of the MWs of PLLA and PDLA blocks (Figures 6a,c and S10). Meanwhile, the characteristic diffractions of hc decrease with increasing Tc and disappear completely at Tc = 160 °C for all sb-SMPs.

19


140 120

endo up

2L-U-93k/ 2D-U-110k

(°C) 160

Page 20 of 35

(b) 2L-93k/ 2D-110k

Tc(°C) 160

Heat flow

Tc

Heat flow

(a)

140 120

100

100

80 150

200 250 Temperature (°C) Tc

3L-U-91k/ 3D-U-85k

(°C)

160 140 120

100

150

200

250

Temperature (°C) endo up

(c)

100

80

(d) Tc(°C)

3L-91k/ 3D-85k

160

Heat flow

endo up

100

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


140 120

100

100

80

80

150


100

150


Figure 5. DSC heating curves of (a) 2L-U-93k/2D-U-110k sb-SMP, (b) 2L-93k/2D-110k blend, (c) 3L-U-91k/3D-U-85k sb-SMP, and (d) 3L-91k/3D-85k blend after isothermal melt crystallizations at different Tcs.

The DSC and WAXD results were further analyzed quantitatively. Based on the DSC results, the degree of crystallinities for hc and sc (Xc,hc, Xc,sc) are calculated by comparing the melting enthalpy (∆Hm) to that of an infinitely large crystal (∆Hm0); ∆Hm0 is taken as 93 J/g48 for hc and 142 J/g49 for sc. Relative sc fraction in the crystalline phase (fsc) was calculated from the WAXD patterns by comparing the peak area of sc diffractions with the total areas of sc and hc diffractions, i.e., fsc = Isc/(Isc + Ihc), where Ihc and Isc are the peak areas of hc and sc diffractions, respectively21,50. 20


Page 21 of 35

Xc,hc, Xc,sc, and fsc of sb-SMPs and the corresponding blends are plotted against Tc in panels a−c of Figure 7. For the PLLA/PDLA blends with high MWs (i.e., 2L-93k/2D-110k and 3L-91k/3D-85k blends), their Xc,hcs are larger than 40% while their Xc,scs are smaller than 12 % under all the investigated Tcs. Their Xc,sc and fsc increase and Xc,hc decreases drastically after the UPy end functionalization. After isothermal crystallization at Tc = 160 °C, the Xc,sc of high-MW 3-arm star-shaped PLLA/PDLA blends (i.e., 3L-91k/3D-85k blend) increases from 3.4% to 36.5% with the end functionalization by UPy motifs. On the other hand, because the sc is more thermodynamically stable than hc, the higher Tc is favorable for sc crystallization and thus fsc is generally enhanced with increasing Tc. All the sb-SMPs exclusively crystallize in sc polymorph at a high Tc of 160 °C. Therefore, it is concluded that the end functionalization by UPy motifs and formation of sb-SMPs improve the sc crystallization ability of PLLA and PDLA.

140 120

15

20 2θ (°)

(°C) 160 140 120

100

10

Tc sc220

sc300/030

sc110

(°C) 160

Intensity (a.u.)

sc220

sc300/030

sc110

hc110/200

Tc

hc110/200

(b) 2L-93k/2D-110k

(a) 2L-U-93k/2D-U-110k 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


100

10

25

15

21


20 2θ (°)

25


140 120

15

20

25

Tc (°C) 160 140 120 100

100

10

sc220

sc110

sc300/030

(°C) 160

Intensity (a.u.)

sc220

sc300/030

sc110 hc110/200

Tc

hc110/200

(d) 3L-91k/3D-85k

(c) 3L-U-91k/3D-U-85k 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

Page 22 of 35

10

15

2θ (°)

20

25

2θ (°)

Figure 6. WAXD patterns of (a) 2L-U-93k/2D-U-110k sb-SMP, (b) 2L-93k/2D-110k blend, (c) 3L-U-91k/3D-U-85k sb-SMP, and (d) 3L-91k/3D-85k blend after isothermal melt crystallizations at different Tcs. The X-ray wavelength is 0.154 nm.

Additionally, the extent of sc crystallization also depends on the MWs of PLLA, PDLA blocks and UPy concentration in sb-SMP. Increasing the MW or block lengths of PLLA and PDLA would decrease the concentration and relative contribution of UPy end groups. The Xc,hc increases and Xc,sc, fsc decrease as the MWs or block lengths of PLLA and PDLA increase for the sb-SMPs and blends, indicating that the high-MW PLLA and PDLA are less favorable to form sc (Figure 7). This agrees with the results reported for linear PLLA/PDLA blends.17,18 On the other hand, Xc,sc and fsc of the 3-arm star-shaped PLLA/PDLA blends (i.e., 3L-91k/3D-85k blend) are comparable to the linear blends with the similar MWs (i.e., 2L-93k/2D-110k blend). However, the sb-SMPs based on the UPy-terminated 3-arm star-shaped PLLA and PDLA (i.e., 3L-U-91k/3D-U-85k sb-SMP) have much larger Xc,sc and fsc than the sb-SMP made from the linear polymers (Figure 7), which would be ascribed to the higher UPy concentration and stronger interactions between enantiomeric blocks in the former.

22


Page 23 of 35

0.8

0.5

(a)

E

C

Xc,sc

Xc,hc

F D

0.4

C 0.2 0.0

0.3 B

0.2

D E

0.1

A B 80

100

120

140

160

0.0

F 80

Temperature (°C) 1.0

(b)

A

0.4

0.6

100

120

140

160

Temperature (°C)

(c)

0.8 0.6

fsc

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


0.4 0.2

C D A B

F E

0.0 100

120

140

160

Temperature (°C) Figure 7. Plots of (a) Xc,hc, (b) Xc,sc, and (c) fsc against crystallization temperature: A) 2L-U-64k/2D-U-64k

sb-SMP;

B)

2L-U-93k/

2D-U-110k

sb-SMP;

C)

3L-U-91k/3D-U-85k sb-SMP; D) 2L-64k/2D-64k blend; E) 2L-93k/2D-110k blend; F) 3L-91k/3D-85k blend.

Structural Changes Investigated by In Situ WAXD. The structural reorganization and crystallization processes of melt-quenched PLLA/PDLA sb-SMPs and blends upon heating are investigated by the synchrotron radiation WAXD (Figures 8, S11). The melt-quenched samples do not show any discernable diffraction before heating, confirming their amorphous structures. On basis of the temperature-dependent WAXD profiles, the intensity changes of sc (110) and hc (110)/(200) diffractions are calculated and plotted against temperature in Figure 9. For

23



the sb-SMPs, the sc starts to form upon heating to 80~90 °C, which precedes the formation of hc (Figure 8a,b). For the 2L-U-93k/2D-U-110k sb-SMP, a tiny hc (110)/(200) diffraction appears at 110 °C and disappears completely upon further heating to 180 °C. The intensity of sc diffraction enhances remarkably upon heating in and after the melting region of hc (150~210 °C). The increase of sc diffraction intensity is much more obvious than the decreased intensity of hc diffraction and the sc diffraction intensity continues increasing until 210 °C (Figures 8a, 9a). This demonstrates the melt recrystallization of hc to sc and the recrystallization of sc for the amorphous chains in the heating process. In the case of 3L-U-91k/3D-U-85k sb-SMPs, the sc is solely formed upon heating (Figure 8b), due to the high

220 180

200 180

160

160

140

sc300/030

hc110/200

140 120 100 80 50

100 80 50

80 50

20

160

120

100

16

260 240 220 200 180

140

120

12

sc110 250 240 220

200

8

(c) sc220

250 240

(b) sc220

sc300/030

sc110

(a) sc220

sc300/030

hc110/200

sc110

concentration of UPy end groups.

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

Page 24 of 35

8

12

2θ (°)

16

20

8

2θ (°)

12

16

20

2 θ (° )

Figure 8. Temperature-dependent WAXD patterns of the melt-quenched (a) 2L-U-93k/2D-U-110k

sb-SMP,

(b)

3L-U-91k/3D-U-85k

sb-SMP,

and

(c)

3L-91k/3D-85k blend collected upon heating. The X-ray wavelength is 0.124 nm.

Upon heating the PLLA/PDLA blend (Figures 8c, S11), hc is predominantly formed and the hc intensity is much stronger than that of sc before the melt of hc. As

24


Page 25 of 35

shown in Figures 9c and S12, the ratios of maximum intensities for sc (110) to hc (110)/(200) diffractions are about 1/2.5 and 1/3.5 for the 2L-93k/2D-110k and 3L-91k/3D-85k blends, respectively. Upon heating to the melting region of hc, the increase of sc diffraction is much less obvious than the remarkable decrease of hc diffraction, manifesting that only a small portion of hc melt-crystallizes into its sc analogue upon heating. These in-situ WAXD results further demonstrate that the sb-SMPs have much better sc crystallization ability than the corresponding blends

Normalized peak area (a.u.)

1.2


without UPy functionalities.

(a) 2L-U-93k/2D-U-110k

0.9

sc110 hc110/200

0.6 0.3 0.0 50


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


4

100

150

200 Temperature (°C)

250

(b) 3L-U-91k/3D-U-85k

1.0 0.8 0.6 0.4

sc110

0.2

hc110/200

0.0 50

100

150


250

(c) 3L-91k/3D-85k sc110

3

hc110/200

2 1 0 50

100

150


250

Figure 9. Changes of diffraction peak areas of sc (110) plane and hc (110)/(200) plane upon

heating

the

melt-quenched

(a)

2L-U-93k/2D-U-110k

sb-SMP,

(b)

3L-U-91k/3D-U-85k sb-SMP, and (c) 3L-91k/3D-85k blend. The data was derived from in-situ WAXD patterns shown in Figure 8. 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

Page 26 of 35

Crystalline Lamellae Structure. Crystalline lamellae structures of sb-SMP and the corresponding blends crystallized at different Tcs were investigated via SAXS. On basis of the original SAXS data, the Lorentz-corrected SAXS profile (Iq2~q plot) was evaluated (Figure 10), where q and I are scattering vector and intensity, respectively. Long spacing (LP) was evaluated from the Bragg equation (LP = 2π/qmax), in which qmax corresponds to the q value of scattering peak top in the Lorentz-corrected SAXS profile. As seen in Table 2, the sb-SMP exhibits smaller LP than the corresponding blend after crystallization under the same Tc, demonstrating the smaller lamellar repeat distance of sc than hc. The unique alternative packing of enantiomeric chains in sc crystallization may prohibit from forming the thick lamellae. Table 2. Lattice parameters of PLLA/PDLA sb-SMPs and blends melt-crystallized at various temperatures. 3L-U-91k/3D-U-85k Tc (°C)

−1

3L-91k/3D-85k

q (nm )

LP (nm)

q (nm−1)

LP (nm)

80

0.81

7.7

0.38

16.4

100

0.71

8.8

0.46

13.8

120 140 160

0.64 0.53 0.45

9.8 11.8 14.0

0.47 0.41 0.28

13.3 15.4 22.1

LPs of sb-SMPs are ranging from 8 to 14 nm, which are smaller than those reported for the sc crystallites of linear PLLA/PDLA blends18,51 and stereoblock copolymers,52 which generally range in 11~17 nm. Smaller LPs of sb-SMPs may be attributed to the strong interactions between UPy motifs, which would depress the mobility and diffusion ability of polymer chains and thus restrict to form the more perfect crystallites. As shown in Figure 10 and Table 2, the scattering peaks of sb-SMPs shift toward low-q side with increasing Tc, indicating the formation of more prefect crystallites with larger LPs. However, LP of PLLA/PDLA blend, which is 26


Page 27 of 35

mainly crystallized in hc polymorph, shows minimum at Tc = 120 °C and increases for both higher and lower Tc, consistent with the unique temperature-dependent lamellar thickness of homocrystalline PLLA.53

(a) 3L-U-91k/3D-U-85k

(b) 3L-91k/3D-85k

0.6

0.9

−1

1.2

2

2

0.3

80 100 120 140 160

Iq (a.u.)

80 100 120 140 160

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


0.3

1.5

0.6

0.9

1.2

1.5

−1

q (nm )

q (nm )

Figure 10. Lorentz-corrected SAXS profiles of 3L-U-91k/3D-U-85k sb-SMP and 3L-91k/3D-85k blend after isothermal melt crystallization at different Tcs. Proposed Mechanism for Favored Sc Crystallization of Sb-SMP. On basis of the aforementioned DSC, WAXD, and SAXS data, it is concluded that the incorporation of UPy motifs and formation of sb-SMP significantly improve the sc crystallization ability of PLLA and PDLA. Mechanism for the favored sc formation of sb-SMPs could be due to the enhanced intermolecular interactions between complementary enantiomers (Figure 11). Due to the high strength of quadruple hydrogen bonds formed between UPy motifs, the terminal UPy groups of PLLA and PDLA would random dimerize upon mixing, causing to form the noncovalent crosslinks between the complementary enantiomeric chains and thus assemble into the linear and branched (or networked) supramolecular structures. Therefore, the sb-SMPs would have similar topological structures as the PLLA-b-PDLA stereoblock copolymers or networks. It has been well recognized that the stereoblock PLLA-b-PDLAs have significantly enhanced sc crystallization ability than the 27



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

PLLA/PDLA racemic blends, due to the improved interactions between the opposite enantiomers. Although the hydrogen bonds are partly destroyed and UPy dimers are partly melted upon heating,54 the interactions between UPy groups and some UPy dimers can persist at a high temperature.55 Furthermore, it has been found that the PLLA and PDLA form racemic helical pairs with 31 chain conformation in the initial stage of nucleation in their enantiomeric blends.30 These remained hydrogen bonding interactions between the complementary enantiomeric chains in PLLA/PDLA sb-SMPs would facilitate to form the precursor racemic helical pairs (embryo of sc nuclei), rather than the homo helical pairs (embryo of hc nuclei). This would enhance the sc nucleation of sb-SMP (Figure 11). It is notable that the hydrogen bonding interactions between UPy groups are thermal responsive.

Figure 11. Schematic illustration of enhanced sc crystallization in PLLA/PDLA sb-SMP. On the other hand, the alternative folding and packing of complementary enantiomers are required in sc crystallization. The sc crystallization only proceeds when both the complementary enantiomer chains are diffused to the growth fronts of sc crystallites, leading to the long diffusion pathway of polymer chains in sc crystallization. Therefore, the sc crystallization would be more diffusion-controlled and less kinetically favorable than the homocrystallization. Due to the presence of strong terminal interactions between PLLA and PDLA, the complementary

28


Page 28 of 35

Page 29 of 35

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


enantiomer chains would have high level of mixing and distribute in a more homogenous, alternative, and contacted manner in the melt and glass of sb-SMPs. This will decrease the averaged distance between complementary enantiomers and thus shorten the diffusion pathway of enantiomeric chains for the alternative folding in sc crystallization. Therefore, the sb-SMP would have a lowered kinetic barrier to stereocomplex, as compared to the PLLA/PDLA blend. Additionally, the reduced amount of free chain ends and limited chain folding within the UPy-bonded homopolymer segments may be another reason for the depressed homocrystallization and improved sc crystallization in sb-SMPs.

CONCLUSIONS In conclusion, the UPy end-functionalized telechelic and 3-arm star-shaped PLLA and PDLA with different MWs were synthesized by the terminal modification of hydroxyl-terminated polymers. Sb-SMPs with linear and branched architectures were further prepared by mixing the UPy end-functionalized PLLA and PDLA. The crystallization kinetics, polymorphic crystalline structure, and lamellae morphology of PLLA/PDLA mixtures are strongly influenced by the self-complementary hydrogen bonding interactions between terminal UPy groups and the formation of sb-SMPs. The sb-SMPs have faster crystallization rate and are preferential to form sc polymorph than the corresponding PLLA/PDLA blends in both the nonisothermal and isothermal crystallizations, ascribing to the hydrogen bonding interactions between UPy groups. Furthermore, increasing the UPy concentration in chain terminals and decreasing the block lengths of PLLA and PDLA further enhance the sc crystallization ability and crystallization rate of sb-SMPs. Due to the enhanced sc crystallization, the sb-SMPs show smaller LPs than the corresponding PLLA/PDLA blends. This study has illustrated the effects of noncovalent intermolecular

29



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

interactions on the cocrystallization and sc crystallization of complementary enantiomeric polymers and also provided an effective and feasible method to promote the sc formation between enantiomeric polymers.

ASSOCIATED CONTENT Supporting Information GPC curves of hydroxyl and UPy-terminated PLLAs, 1H NMR spectra of hydroxyl and UPy-terminated PLLAs with and without thermal treatment, FTIR spectra, TGA curves, partial DSC and WAXD data of sb-SMP and blend. 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] (P.P.).

ACKNOWLEDGMENTS We acknowledge the financial supports of Natural Science Foundation of China (21274128, 21422406) and Fundamental Research Funds for the Central Universities (2015XZZX004-08). SAXS and in situ WAXD were measured on the beamline BL16B1 of SSRF, China.

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For Table of Content Use Only

Promoted Stereocomplex Crystallization in Supramolecular Stereoblock Copolymers of Enantiomeric Poly(lactic acid)s

Jianna Bao, Ruoxing Chang, Guorong Shan, Yongzhong Bao, Pengju Pan*

Synopsis: A new method is reported to promote the stereocomplex crystallization of enantiomeric polymers through constructing the stereoblock supramolecular polymers (sb-SMPs). Sb-SMPs were prepared from the telechelic and star-shaped poly(L-lactic acid)s and poly(D-lactic acid)s end-functionalized by 2-ureido-4[1H]-pyrimidione self-complementary hydrogen-bonding motifs. These sb-SMPs show higher crystallization rates and prefer to crystallizing in high-melting-point stereocomplex crystallites.

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Promoted Stereocomplex Crystallization in Supramolecular

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