In Situ Formation and Gelation Mechanism of Thermoresponsive

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In Situ Formation and Gelation Mechanism of Thermoresponsive Stereocomplexed Hydrogels upon Mixing Diblock and Triblock Poly(lactic acid)/Poly(ethylene glycol) Copolymers Hailiang Mao, Pengju Pan, Guorong Shan, and Yongzhong Bao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03610 • Publication Date (Web): 01 May 2015 Downloaded from http://pubs.acs.org on May 4, 2015

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The Journal of Physical Chemistry

In Situ Formation and Gelation Mechanism of Thermoresponsive Stereocomplexed Hydrogels upon Mixing Diblock and Triblock Poly(lactic acid)/Poly(ethylene glycol) Copolymers

Hailiang Mao, Pengju Pan,* Guorong Shan, Yongzhong Bao

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

*To whom corresponding should be addressed. Tel.: +86-571-87951334; email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: A novel in situ formed gel system with potential biodegradability and biocompatibility is developed by mixing the diblock and triblock poly(lactic acid)/poly(ethylene glycol) (PLA/PEG) copolymers with opposite configurations of PLA blocks. In situ gelation of such system is extremely fast, which happens within 10 s after mixing. In situ gelation, gel-to-sol transition, crystalline structure, microstructures,

and

mechanical

properties

of

PLA-PEG/PLA-PEG-PLA

enantiomerically-mixed gels are significantly influenced by the mixing ratio, degree of polymerization for PEG block in triblock (DPPEG,tri) and diblock copolymers (DPPEG,di). It is found that in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixture just happen at relatively smaller PLA-PEG/PLA-PEG-PLA mass ratio and larger DPPEG,tri. Hydrodynamic diameters of PLA-PEG and PLA-PEG-PLA copolymers in dilute solution increase remarkably upon mixing, indicating the formation of bridging networks. Stereocomplexed crystallites are formed for the PLA hydrophobic domains in PLA-PEG/PLA-PEG-PLA enantiomeric mixtures. As indicated by synchrotron-radiation SAXS analysis, the enantiomeric mixture changes from a compactly to loosely aggregated structure and the intermicellar distance enhances with increasing DPPEG,tri, DPPEG,di, or PLA-PEG-PLA fraction. Gelation mechanism of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is proposed, in which part of PLA-PEG-PLA chains act as the connecting bridges between star and flower-like micelles and the stereocomplexed crystallites in micelle cores act as physically cross-linked points.

INTRODUCTION Amphiphilic block copolymers are able to self-assemble into the core-shell micelles having the hydrophobic cores surrounded by hydrophilic shells in aqueous 2 ACS Paragon Plus Environment

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solution. Colloid stability of micelle solution is strongly dependent on the nature of solvent, external stimuli, interactions between cores or shells of micelles, etc. When the micelles are destabilized and aggregated with the increased intermicellar interactions, they can form hydrogels through physical crosslinking. These physically cross-linked hydrogels usually show stimulus-responsive behavior and are highly attractive in the biomedical applications such as drug delivery and tissue engineering.1 Furthermore, the in situ gelation makes it more feasible to apply hydrogels for biomedical applications. A particularly interesting and important class of hydrogels is that formed by a simple mixing or sol-to-gel phase transition in water without any chemical reaction or external stimulation.2 Considering the structures of physically cross-linked networks, the micelles of amphiphilic block copolymers can form physical gels in situ through the bridging and non-bridging mechanisms, under the changes of concentration or environmental condition.3 On the one hand, amphiphilic ABA triblock copolymers consisting of hydrophobic lateral A blocks and hydrophilic central B block self-assemble into the flower-like micelles in aqueous solution. Two end blocks of the same chain may enter either into the same core forming loops or into different cores forming bridges between the multiplets.4,5 Thus, part of the central blocks can act as connecting bridges between the micelles; this would become more pronounced at high polymer concentration. In this way, a percolating network will be formed through the intermicellar bridging, which can be an alternative driving force for physical gelation. Numerical physical gels have been reported for ABA triblock copolymers such as poly(propylene glycol)-poly(ethylene glycol)-poly(propylene glycol) (PPG-PEGPPG),6 PLGA),7,8

poly(lactide-co-glycolide)-PEG-poly(lactide-co-glycolide) poly(caprolactone)-PEG-poly(caprolactone)


(PLGA-PEG-

(PCL-PEG-PCL),9,10


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poly(caprolactone-co-lactide)-PEG-poly(caprolactone-co-lactide) PCLA),11

poly(β-amino

ester)-PEG-poly(β-amino

(PCLA-PEG-

ester),12

poly(lactic

acid)-PEG-poly(lactic acid) (PLA-PEG-PLA),13−15 poly(L-lactic acid)-PEG-poly(Llactic acid) (PLLA-PEG-PLLA)/poly(D-lactic acid)-PEG-poly(D-lactic acid) (PDLAPEG-PDLA) enantiomeric mixture.16−19 On the other hand, the amphiphilic AB diblock and BAB triblock copolymers comprised of hydrophobic A block and hydrophilic B block assemble into the star-like micelles in aqueous solution. Even though no bridge is formed between these micelles, the micelle solution can undergo physical gelation at high concentration such as PEG-PPG-PEG,20 linear21 and star-shaped

PLA-PEG,22,23

PEG-PLA-PEG,21

PEG-PCL-PEG,24,25

PEG-PLGA-PEG,26,27 PLLA-PEG/PDLA-PEG and PEG-PLLA-PEG/PEG-PDLAPEG enantiomeric mixtures.28 In this case, the intermicellar hydrophobic interaction would be a main driving force of gelation. Crystallization of hydrophobic block has been a key factor influencing the gelation kinetics and physical proprieties in the physical gels of amphiphilic block copolymers.13,15 This effect is more significant for the case that the hydrophobic blocks can undergo stereocomplex (sc) crystallization. It is well known that equal-mass PLLA and PDLA are able to form stereocomplex.29,30 Typical examples of

stereocomplexation-induced

gelation

are

PLLA-PEG-PLLA/PDLA-PEG-

PDLA16−19 and star-shaped PLLA-PEG/PDLA-PEG enantiomeric mixtures.31,32 PLLA/PDLA racemic blend can also form organic gel in certain solvents.33 Several literatures have reported that the stereocomplexation of PLLA and PDLA blocks significantly decreases the critical gelation concentration, accelerates the gelation rate, and improves the modulus of resulted gels.2,31,32 This has been attributed to the enhanced intermicellar bridging induced by stereocomplexation. Therefore, we 4 ACS Paragon Plus Environment

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envision that the control over bridging density and bridging-related interactions between copolymer micelles would be a feasible way to construct the novel physical gels as well as to tune their gelation behavior and physical properties. Even though it has been proposed that the formation of intermicellar bridging networks is a driving force for the gelation of PLLA-PEG-PLLA/PDLA-PEG-PDLA enantiomeric mixtures,18 experimental evidences are still lacked for this proposal. Therefore, an in-depth study on the formation and structure of physically cross-linked networks would be essential to understand the gelation behavior and microstructure of stereocomplexed hydrogels. In this contribution, we report a novel approach to prepare the biodegradable, biocompatible, and in situ forming hydrogels by mixing the aqueous solutions of PLA-PEG diblock and PLA-PEG-PLA triblock copolymers with opposite PLA configurations (Scheme 1). This approach combines the non-bridging gelation mechanism of AB diblock copolymer and bridging gelation mechanism of ABA triblock copolymer. The physical gels are formed through the connecting bridges between AB and ABA block copolymers. In this enantiomerically-mixed gel, the bridging networks are formed through the stereocomplexation of PLLA and PDLA in the cores of PLA-PEG star-like micelles and PLA-PEG-PLA flower-like micelles. Thus, part of PLA-PEG-PLA chains act as the crosslinkers between star and flower-like micelles. Effects of PEG block length and mixing ratio of diblock and triblock copolymers on the in situ gelation and temperature-sensitive phase transition of enantiomerically-mixed gels were investigated. Crystalline state and microstructure of PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels and the role of PLLA/PDLA stereocomplexation in the in situ gelation were studied by synchrotron radiation wide and small angle X-ray scattering (WAXS, SAXS). Gelation



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mechanism

and

bridging

network

structure

of

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PLA-PEG/PLA-PEG-PLA

enantiomerically-mixed gels were proposed and discussed.

Scheme 1. Schematic illustration of stereocomplexation-driven in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixtures.

EXPERIMENTAL SECTION Synthesis

of

Diblock

and

Triblock

Copolymers.

PLA-PEG

and

PLA-PEG-PLA copolymers with either PLLA or PDLA blocks were synthesized according to the published methods (supporting information).16 The prepared diblock and triblock copolymers are marked as EzLy, EzDy, or LyExLy, where E, L, and D represent the PEG, PLLA, and PDLA blocks, respectively, and the subscripts x, y and z denote the degree of polymerization (DP) of each block. In Situ Gelation and Sol-to-Gel Transition. PLA-PEG and PLA-PEG-PLA copolymers were separately dissolved in deionized water and stirred vigorously at 20 °C for 15 min to prepare the copolymer solution (20 wt%). The solutions were then equilibrated at 4 °C for 24 h. The solutions of PLA-PEG and PLA-PEG-PLA were mixed at a predetermined mass ratio to examine the gelation behavior at 20 °C. The formation of gel and critical temperature in gel-to-sol transition (CGT) were measured via the test tube inverting method.9,18 2 g of gel or sol prepared from PLA-PEG/PLA-PEG-PLA enantiomeric mixtures was added into a vial (5 mL) and the vial was then immersed in a water bath. The temperature was increased from 4 to 6 ACS Paragon Plus Environment

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60 °C with a 2 °C interval. The sample was equilibrated at each temperature for 15 min. Physical state of sample was evaluated at each temperature interval by tilting the vial. The sample is recognized as a sol if it is flowed within 10 s and otherwise it is recognized as a gel.18 Measurements. Dynamic Light Scattering (DLS). DLS analysis was performed on a Zetasizer Nano ZS90 instrument with a 633 nm laser light set at a scattering angle of 90°. The copolymer solution (0.1 wt%) was equilibrated at 20 °C for 24 h before measurement. Synchrotron Radiation WAXS and SAXS. WAXS and SAXS experiments were performed at BL16B beamline of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of radiation source is 0.124 nm. The sample-to-detector distances were 0.11 and 5.0 m in WAXS and SAXS measurements, respectively. Scattering patterns were collected by a Rayonix SX-165 CCD detector (Rayonix, Illinois, USA), which had a resolution of 2048 × 2048 pixels and a pixel size of 80 × 80 µm2. The acquisition times were 300 and 150 s for WAXS and SAXS, respectively. All the data was corrected from the background and air scattering. 2D data was converted into 1D data by circularly averaging with a Fit2D software. Rheological Analysis. Rheological measurements were carried out on a RS6000 rheometer (Thermo Fisher, Germany). The solidlike gel was placed between two parallel plates with a diameter of 20 mm and a gap of 1.0 mm. The data was collected under a strain of 1% and a frequency of 1 Hz. Storage (G′) and loss moduli (G″) were recorded upon heating at 1 °C/min. Critical gelation temperature (CGT) was determined from the G′/G″ crossover point.34

RESULTS and DISCUSSION



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Synthesis and Characterization of Block Copolymers. Water solubility of amphiphilic block copolymers depends on the hydrophilic/hydrophobic balance. It has been reported that the mixed PLLA and PDLA oligomers cannot form stereocomplex when the DP of PLA is less than 7.35 Therefore, we used the PLA/PEG diblock and triblock copolymers with a shorter PLA block (DP = 10) to ensure that they can solubilize or disperse in water and can also form stereocomplex in the enantiomeric blend. PLA-PEG and PLA-PEG-PLA copolymers having expected DP = 10 for each PLA block, different PEG block lengths and PLA enantiomeric structures were synthesized by the ring-opening polymerization using mono- or di-hydroxyl PEG as the macroinitiator. Table 1. Molecular Characteristics of PLA/PEG Diblock and Triblock Copolymers samplea

designed Mn of each block (g/mol)

Mn,NMRb (g/mol)

Mn,GPCc (g/mol)

PDIc

E23D10

1000-720

1700

2200

1.15

E23L10

1000-720

1700

2150

1.13

E43D10

1900-720

2600

3360

1.09

E43L10

1900-720

2650

3400

1.10

E113D10

5000-720

5700

7380

1.06

E113L10

5000-720

5700

7200

1.07

L10E45L10

720-2000-720

3400

4300

1.08

L10E90L10

720-4000-720

5450

7000

1.06

L10E136L10

720-6000-720

7450

9640

1.05

L10E227L10

720-10000-720

11400

15700

1.04

L10E445L10

a

21350 26630 1.04 720-20000-720 E, D, and L denote PEG, PDLA, and PLLA blocks, respectively. Subscript numbers

represent DP of each block calculated from NMR data. For PLA block, DP is calculated based on the average number of lactic acid unit, rather than the lactide unit. b

Mn derived from 1H NMR. cMn and polydispersity index (PDI) measured by GPC.


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As shown in Table 1, the number-averaged molecular weight (Mn) of PEG block changes in a wide range in the diblock (1~5 kDa) and triblock (2~20 kDa). Mn of PLA block in copolymers was evaluated by comparing the intensity of methine proton signal for PLA block with the intensity of CH2 proton signal for PEG block in the

1

H nuclear magnetic resonance (NMR) spectrum (Figure S1). Mn,NMR,

representing the Mn derived from 1H NMR, was attained by summing the Mns of PEG and PLA blocks. Mns of copolymers measured by gel permeation chromatography (GPC) and NMR are consistent with each other, which increase with the PEG length. As indicated by GPC results, all block copolymers exhibit a narrow molecular weight distribution (PDI < 1.15). In Situ Gelation and Thermoresponsive Behavior. Aqueous solutions of PLA-PEG and PLA-PEG-PLA with a fixed concentration of 20 wt% were mixed to examine the gelation behavior. The individual diblock and triblock copolymer solutions (20 wt%) are all in sol state at a wide temperature region of 4~60 °C (Figures 1a, 1f). Interestingly, the mixed solutions of some diblock and triblock copolymers (e.g., E23D10/L10E227L10) turn into gels immediately upon just mixing and shaking at 20 °C (Figure 1c). The in situ sol-to-gel transition is very rapid and happens simultaneously with mixing. The gelation does not need other external stimuli such as the changes of temperature or pH value. The gelation times of all gel samples are less than 10 s and much faster than the typical gels formed from PLLA-PEG-PLLA/PDLA-PEG-PDLA16,19 and star-shaped PLLA-PEG/PDLA-PEG enantiomeric mixtures,31 in which the gelation time is around 10~30 min at elevated temperature or even longer at room temperature. Also, this in situ gelation behavior is different from those of PLLA-PEG-PLLA/PDLA-PEG-PDLA16,18,19 and star-shaped



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PLLA-PEG/PDLA-PEG enantiomeric mixtures,31 in which the gelation generally happens upon heating. (b)

(a)

E23D10 sol

(c)

(d)

(e)

(f)

E23D10/L10E227L10 E23D10/L10E227L10 E23D10/L10E90L10 E23L10/L10E227L10 L10E227L10, sol 70/30, sol

30/70, gel

30/70, sol

30/70, sol

Figure 1. Photographs of PLA/PEG diblock, triblock copolymers, and their mixtures in water (20 wt%) at 20 °C. The bottles were stuck on a blue background with a tilt angle of ~45°. Phase behavior of diblock/triblock enantiomeric mixtures with different mixing ratios and PEG block lengths were studied at various temperatures (Figure 2). Physical state of the enantiomeric mixtures in water strongly depends on the enantiomeric structure of PLA blocks, DP of PEG in diblock (DPPEG,di) and triblock (DPPEG,tri) copolymers, and PLA-PEG/PLA-PEG-PLA ratio. Several requirements are prerequisite for the in situ gelation of PLA-PEG/PLA-PEG-PLA mixtures. First, in situ gelation only takes place when DPPEG,tri is sufficiently larger than DPPEG,di. Larger DPPEG,tri and smaller DPPEG,di facilitate the gelation. As seen in Figures 1 and 2, E23D10 and E43D10 just form gels with L10E136L10, L10E227L10, or L10E445L10 possessing the longer PEG mid-block, while they cannot form gel with L10E90L10 having the shorter PEG mid-block. E113D10 possessing the long PEG block cannot

form

gel

with

any

triblock

copolymers

investigated.

Second,

PLA-PEG/PLA-PEG-PLA mixtures only form gel within a given mixing ratio. Except for the E43D10/L10E227L10 pair, the PLA-PEG/PLA-PEG-PLA mass ratio in mixtures is generally between 50/50~20/80, which suggests that adding more PLA-PEG-PLA than PLA-PEG is favorable for gelation. Third, the configuration of PLA segment in 10 ACS Paragon Plus Environment

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PLA-PEG and PLA-PEG-PLA should be opposite. When the PLA-PEG and PLA-PEG-PLA with the same PLA configuration are mixed, no gel is formed at various mixing ratio and temperatures investigated (Figure 1b). This demonstrates the in situ gelation is triggered by PLLA/PDLA stereocomplexation, as illustrated in the following sections. Interestingly, the enantiomerically-mixed gels are thermosensitive and turn into sols at elevated temperatures (Figure 2). Notably, the thermal responsivity of PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels is different from the PLLA-PEG-PLLA/PDLA-PEG-PLLA23 and star-shaped PLLA-PEG/PDLA-PEG31 stereocomplexed gels, which do not soluble but turn into opaque or syneresis at elevated temperatures. The critical temperature of gel-to-sol transition is defined as CGT.

CGTs

of

PLA-PEG/PLA-PEG-PLA

enantiomerically-mixed

gels

are

significantly influenced by the mixing ratio, DPPEG,tri, and DPPEG,di (Figure 2). First, the gels formed by mixing L10E227L10 with E23D10 or E43D10 exhibit higher CGTs and broader gelation regions with respect to the temperature and mixing ratio than other gels. The E23D10/L10E227L10 30/70 and E43D10/L10E227L10 30/70 gels have CGTs of 39 and 33 °C, respectively. Second, for the gels with the same diblock copolymer (e.g., E23D10/PLA-PEG-PLA series), CGT first increases and then decreases; meanwhile the gelation region first broadens and then narrows with increasing DPPEG,tri. This means that the enantiomerically-mixed gels are more thermally stable at an medium DPPEG,tri. Third, for the gels with the same triblock copolymer (e.g., PLA-PEG/L10E136L10 and PLA-PEG/L10E227L10 series), CGT decreases and gelation region narrows with increasing DPPEG,di, suggesting the reduction of gel stability with DPPEG,di. This would be correlated to the structures of physically cross-linked networks in gels. Finally, CGT generally increases first and then decreases with increasing PLA-PEG-PLA



fraction, indicating that adding too much diblock or triblock copolymers are not

Temperature (°C)

favorable to form thermally stable gels.

Temperature (°C)

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60

(a) E23D10/L10E90L10

(b) E23D10/L10E136L10

(c) E23D10/L10E227L10

(d) E23D10/L10E445L10

(e) E43D10/L10E136L10

(f) E43D10/L10E227L10

(g) E43D10/L10E445L10

(h) E113D10/L10E445L10

40 20 0 60 40 20 0

20

40

60

80

PLA-PEG-PLA%

20

40

60

80

PLA-PEG-PLA%

20

40

60

80

PLA-PEG-PLA%

20

40

60

80

PLA-PEG-PLA%

Figure 2. Physical state of PLA-PEG/PLA-PEG-PLA enantiomeric mixtures in water (20 wt%) at different mixing ratios and temperatures. Open square and solid triangle represent the sol and gel states, respectively. PLA-PEG-PLA% denotes the mass percentage of triblock copolymer in dried gel.

Micellation of Block Copolymer. The instantaneous sol-to-gel transition behavior of PLA-PEG/PLA-PEG-PLA hydrogels should be strongly correlated with their primary micellar formation and spontaneous secondary intermicellar aggregation properties in aqueous solution. As shown in Figure 3a and S2, E23D10 and L10E227L10 solutions show smaller hydrodynamic diameters (Dhs) of 32 and 40 nm with the unimodal distributions, respectively, which are similar to those of PLA/PEG copolymers with the comparable compositions.18,28 Dh of E23D10/L10E227L10 enantiomeric mixtures first increases and then decreases with increasing L10E227L10 content. E23D10/L10E227L10 30/70 enantiomeric mixture shows a large Dh of 98 nm and also exhibits a small scattering peak at 390 nm (Figure S2), attributable to the scattering of intermicellar aggregates. At a fixed PLA-PEG/PLA-PEG-PLA mass 12 ACS Paragon Plus Environment

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ratio of 30/70, Dh of E23D10/PLA-PEG-PLA enantiomeric mixture first increases and then decreases with increasing DPPEG,tri (Figures 3b, S3). It is notable that the E23D10/L10E227L10 mixture shows larger Dh than E23D10/L10E445L10, which may be attributable to the intermicellar aggregation. Because L10E227L10 possesses shorter PEG length and larger PLLA fraction than L10E445L10, E23D10/L10E227L10 mixture would have smaller intermicellar distance and higher extent to stereocomplex (as illustrated in following parts), leading to the stronger intermicellar interaction and aggregation. This can be identified by the appearance of additional scattering at large size (Figure S3). The variation trends of Dh for PLA-PEG/PLA-PEG-PLA mixtures with mixing ratio and DPPEG,tri are consistent with those for the thermal stability of gels (Figure 2). These DLS results indicate that the micelles of PLA-PEG and PLA-PEG-PLA with opposite configurations undergo the intermicellar aggregation, and specific interactions and intermicellar networks are formed upon mixing. 120

120

(a)

(b)

80

80

Dh (nm)

Dh (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


40

0

0

20

40

60

40

0

80 100

100

200

300 400 500

DPPEG,tri

L10E227L10%

Figure 3. Hydrodynamic diameter and its distribution of PLA-PEG/PLA-PEG-PLA enantiomeric mixtures in dilute water solution (0.1 wt%) at 20 °C: (a) E23D10/L10E227L10 mixtures with different mixing ratios; (b) mixtures of E23D10 with different triblock copolymers. Crystallization of PLA Hydrophobic Blocks in Gels. Crystalline structure of hydrophilic PLA block in PLA-PEG/PLA-PEG-PLA enantiomeric mixtures was analyzed by synchrotron radiation WAXS (Figure 4). As shown in Figure S4, the characteristic diffractions of PLA α-form homocrystallites (hc) at 2θ = 11.9, 13.5, 13 ACS Paragon Plus Environment


15.4, and 18.0°, corresponding to the diffractions of 010, 110/200, 203, and 015 planes, respectively,36 are only observed from the solutions of individual diblock or triblock copolymer, while the diffractions of PEG block are absent. This indicates that the PLA domains are homo-crystalline and PEG segments are solubilized in individual PLA-PEG and PLA-PEG-PLA solutions. It is notable that the X-ray used has a different wavelength (0.124 nm) from the traditional X-ray (0.154 nm). Even though the diffraction angles are different from those reported in literature,36 the lattice spacings calculated from Bragg equation are the same. Hc diffraction intensity of copolymer solution decreases significantly as the PEG length increases, due to the decreased PLA fraction (Figure S4).

sol

50/50

gel gel

40/60

8

L10E227L10

16

20

L10E445L10

8

2θ (°)

hc015 sc220

sc300/030

hc203

sc110

hc110/200

PLA-PEG/L10E227L10 30/70

E23D10 E43D10 E114D10

sol

8

12

12

2θ (°)

gel gel

hc015 sc200

L10E136L10

gel

gel

12

(c)

sc300/030

hc110/200

sc110

L10E90L10

sol

gel

gel gel

30/70 20/80

E23D10/PLA-PEG-PLA 30/70

hc203

hc015 sc220

Intensity (a.u.)

70/30

(b)

hc203 sc300/030

sc110 hc010

Intensity (a.u.)

E23D10/L10E227L10

hc110/200

(a)

Intensity (a.u.)

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

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16

20

2θ (°) 14 ACS Paragon Plus Environment

16

20

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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


Figure 4. WAXS patterns of PLA/PEG enantiomerically-mixed gels or sols with a copolymer concentration of 20 wt% at 20 °C: (a) E23D10/L10E227L10 mixtures with different mixing ratios; (b) enantiomeric mixtures of E23D10 with different triblock copolymers; (c) enantiomeric mixtures of L10E227L10 with different diblock copolymers. Hc and sc indicate the characteristic diffractions of PLA hc and sc crystallites, respectively. Subscript numbers of hc and sc denote the Miller indices of crystal lattice. Wavelength of used X-ray is 0.124 nm.

Upon mixing PLA-PEG and PLA-PEG-PLA solutions (20 wt%) with opposite PLA configurations, the intermicellar chain exchange and stereocomplexation between PLLA and PDLA segments take place. Both the characteristic diffractions of hc and sc (2θ = 9.7, 16.7, and 19.3°)37 crystallites are observed in the PLA-PEG/PLA-PEG-PLA enantiomerically-mixed sols and gels, as shown in Figure 4. Macromolecular exchange can be promoted by PLLA/PDLA stereocomplexation, because the stereocomplex is more thermodynamically stable and energy-favorable. Several literatures have reported that the stereocomplexed micelles are formed upon mixing the micelles with PLLA and PDLA cores and the stereocomplexed micelles have higher stability than their homo-crystalline counterparts.38,39 PLLA/PDLA stereocomplexation in PLA-PEG/PLA-PEG-PLA enantiomeric mixture is also confirmed by the WAXS patterns and FTIR spectra of freeze-dried samples. Both the diffractions of hc and sc crystallites of PLA, and PEG crystallites are observed in the WAXS patterns of freeze-dried enantiomeric mixtures (Figure S5).

As

compared

to

PLLA-PEG/PLLA-PEG-PLLA

mixture,

PDLA-PEG/PLLA-PEG- PLLA enantiomeric mixture shows a new band at 1751 cm−1 in FTIR spectra, characteristic of the carbonyl stretching vibration of sc crystallites (Figure S6).40



Relative fraction of sc crystallites (fsc) in the enantiomeric mixture was estimated by comparing the diffraction peak area of sc crystallites with the total areas of both sc and hc diffractions [fsc = Isc/(Isc + Ihc)] on basis of the WAXS results,41,42 as shown in Figure 5. Since the theoretical mass ratio of PLLA to PDLA segments (mPLLA/mPDLA) is 1:1 in sc crystallites, mPLLA/mPDLA is a key factor influencing the relative fractions of hc and sc crystallites in the enantiomeric mixtures. As compared to the hc crystallization, sc crystallization would be more predominant as the mPLLA/mPDLA is approaching 1:1. For the E23D10/L10E227L10 enantiomeric mixtures (Figures 4a, 5a), the hc diffraction intensity gradually decreases and fsc increases with increasing the PLA-PEG-PLA fraction from 30% to 80%, since the mPLLA/mPDLA ratio is approaching to 1:1. The increased fsc is favorable for the formation of stereocomplexed gel. 80

60

1.5

60

40

1.0

40

1.0

20 0

30 40 50 60 70 80

0.5

20

0.0

0

100

E23D10/PLA-PEG-PLA

200

fsc (%)

1.5

fsc (%)

60

mPLLA/mPDLA

(b)

0.4 0.3 0.2

20

0.0

0

DPPEG,tri

L10E227L10%

0.1 0.0 50 100 150 DPPEG,di

Figure 5. Relative fraction of sc crystallites (fsc) in PLA crystalline phase (left) and mass ratio of PLLA and PDLA block in PLA-PEG/PLA-PEG-PLA enantiomeric mixtures

(right)

plotted

as

function

of

(a)

PLA-PEG-PLA

fraction

in

E23D10/L10E227L10 mixtures, (b) DPPEG,tri for the mixtures of E23D10 with different triblock copolymers, (c) DPPEG,di for the mixtures of L10E227L10 with different diblock copolymers.

For the enantiomeric mixtures of E23D10 with different triblock copolymers (Figures 4b, 5b), the diffraction intensity of sc crystallites and fsc increase first with increasing DPPEG,tri from 90 to 136 and then decreases with further increasing 16 ACS Paragon Plus Environment

0.5

40

0.5

300 400500

(c) PLA-PEG/L10E227L10

mPLLA/mPDLA

2.0

80

2.0

(a) E23D10/L10E227L10

mPLLA/mPDLA

80

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

Page 16 of 33

Page 17 of 33

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


to 445. This may be ascribed that mPLLA/mPDLA is close to 1 at DPPEG,tri = 136. Notably, the E23D10/L10E90L10 enantiomeric mixture, which is a sol under the investigated conditions,

shows

strong

sc

diffractions,

implying

that

PLLA/PDLA

stereocomplexation indeed happens in the enantiomerically-mixed sols even though no gel is formed. For the enantiomeric mixtures of L10E227L10 with different diblock copolymers (Figures 4c, 5c), both the diffraction intensities of hc and sc crystallites decrease with increasing DPPEG,di, due to the decreased fraction of PLA segment in enantiomeric mixture. Long PEG block in PLA-PEG micelles impedes the PLLA/PDLA stereocomplexation in the micelle cores of diblock and triblock copolymers, because of the steric effect of thick corona layer. As DPPEG,di is increased from 23 to 113, fsc keeps nearly constant regardless of mPLLA/mPDLA (Figure 5c). Proposed Mechanism for Stereocomplexation-Driven In Situ Gelation. On basis of the aforementioned results, gelation mechanism of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is proposed. PLA-PEG forms the star-like micelles and PLA-PEG-PLA assemblies into the flower-like micelles in water (Scheme 1). Upon mixing these two kinds of micelles with opposite PLA configurations, the chain exchange

between

star

and

flower-like

micelles

and

the

PLLA/PDLA

stereocomplexation in micelle cores happen. Therefore, two PLA end blocks of PLA-PEG-PLA can enter either a same micelle or two adjacent micelles. However, the stereocomplexation is necessary but not sufficient for the in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixture. The gelation is also governed by the PEG block length in triblock copolymers (Scheme 2a). First, when the PEG block of PLA-PEG-PLA is short (i.e., DPPEG,tri/DPPEG,di is small) and smaller than the distances between micellar cores, two PLA blocks of PLA-PEG-PLA will



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

prefer to entering a same micelle. In this case, the stereocomplexed sols, rather than gels, are formed (Figures 1d, 2a, 2h). Second, when the PEG block length of PLA-PEG-PLA (or DPPEG,tri/DPPEG,di) is relatively large, two PLA blocks of PLA-PEG-PLA can enter the cores of different micelles, leading to form a bridging structure. In this case, PLA-PEG-PLA chains act as connecting bridges between PLA-PEG star-like micelles and a physically cross-linked network is formed. Giacomelli et al. have also suggested that the long middle block in ABA triblock copolymer can facilitate the formation of bridging structure between flower-like micelles in the B-selective solvent.5 Associated with the van der Walls and hydrophobic interactions, the stereocomplexation-induced bridging and physical crosslinking can cause the in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixtures, when DPPEG,tri/DPPEG,di is relatively large (Figure 1c, 2b-g). The stereocomplexation-driven formation of cross-linked network also lead to the increase of particle size in dilute solution (Figure 3). Third, as the DPPEG,tri/DPPEG,di is further increased, the PEG bridging chains between micelles are too long and the gels have too large content of hydrophilic PEG; this leads to the formation of loosely cross-linked gels. In this case, the enantiomeric mixtures show narrower gelation region and lower CGTs (Figures 2d, 2g).


Page 18 of 33

Page 19 of 33

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


Scheme 2. Schematic illustration on physically cross-linked structure in PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels with (a) different block lengths and (b) mixing ratio. Formation of bridging network in PLA-PEG/PLA-PEG-PLA enantiomeric mixture is also influenced by mixing ratio (Scheme 2b). At low PLA-PEG-PLA fraction, (i.e., PLA-PEG-PLA% < 40~50%), the bridges between star-like micelles are not dense enough and thus the enantiomeric mixture does not undergo sol-to-gel transition (Figure 2). When PLA-PEG-PLA fraction is too large (>80 %), flower-like micelles mainly exist in the enantiomeric mixture. Because of the high PEG and low PLA contents in enantiomeric mixture, the stereocomplexation-induced bridging is not sufficient for gelation or the formation of densely cross-linked gel. Therefore, the in situ gelation of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is favorable at a moderate mixing ratio [i.e., PLA-PEG-PLA% = 50~80 %] (Figure 2). Microstructures of Gels Investigated by Synchrotron Radiation SAXS. Microstructures of PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels were analyzed by synchrotron radiation SAXS. As shown in Figure 6, the scattering profiles of enantiomerically-mixed gels and sols strongly depend on mixing ratio, DPPEG,tri, and DPPEG,di. All samples display a power law scattering regime at low-q (q < 0.08 nm−1), indicative of the scattering from fractal structures. The solutions of PLA-PEG and PLA-PEG-PLA do not show any discernable scattering peak, while a broad scattering peak or shoulder is observed for the PLA-PEG/PLA-PEG-PLA enantiomeric mixtures at medium q (0.1~0.3 nm−1). As reported by Bhatia et al, the scattering peak at medium q is associated with the scattering of neighboring bridged or packed micelles and represents the average distance between scattering centers.15 However, these scattering peaks are quite broad, indicating the broad polydispersity of intermicellar distances in the enantiomerically-mixed gels. These SAXS results 19 ACS Paragon Plus Environment


also suggest that the intermicellar interactions increase significantly in the in situ gelation of enantiomeric mixtures.

(a)

5

100/0 50/50 20/80

10

4

I(q) (a.u.)

6

10

E23D10/L10E227L10

10

(b)


5

70/30 30/70 0/100

10

I(q) (a.u.)

6

10

3

10

2

10

4

10

L10E90L10

L10E136L10

L10E227L10

L10E445L10

3

10

2

10

1

1

10

0

10

10

0

10

0.1

1

-1

0.1

q (nm )

-1

1

q (nm )

6

10

5

(c) PLA-PEG/L10E227L10 30/70

10

4

I(q) (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 20 of 33

10

3

10

2

10

1

10

0

10

E23D10 E43D10 E114D10

0.1

1 -1

q (nm )

Figure 6. SAXS profiles of PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels or sols with a copolymer concentration of 20 wt% at 20 °C: (a) E23D10/L10E227L10 mixtures with different mixing ratios; (b) enantiomeric mixtures of E23D10 with different triblock copolymers; (c) enantiomeric mixtures of L10E227L10 with different diblock copolymers. The solid and dash lines in panel b indicate the fits by power law and Teubner-Strey model in the small and medium q regions, respectively. At low-q (0.03~0.08 nm−1), the scattering intensity I(q) does not obey Guinier law, due to the intermicellar interactions at high copolymer concentrations and the existence of largely aggregated structure. This is in consistent with the results of PLLA-PEG-PLLA copolymers.14 SAXS data in low-q was analyzed by a power law43 I ( q ) = I 0 q −α

(1)


Page 21 of 33

where I0 is the forward scattering intensity at q→0. The scaling exponent α is related to either mass (Dm) or surface fractal dimension (Ds) α = Dm (α < 3, mass fractal)

(2)

α = 6−Ds (3< α < 4, surface fractal)

(3)

Dm implies the “openness” of fractal structure, which varies from 1 for a loosely connected aggregates with ill-defined interfaces to 3 for a dense aggregate. Surface fractal is a compact structure and its fractal dimension Ds varies 3 for rough surface to 2 for smooth surface.14 E23D10 and L10E227L10 solutions (20 wt%) have the α values of 3.21 and 1.80, respectively. This indicates the existence of more compact aggregates in the copolymer solutions with high PLA content and loosely connected aggregates in the copolymer solution with low PLA content. Interestingly, all the enantiomerically-mixed gels are characteristic of α = 2~3 (Figure 7). 4

4

(a) E D /L E L 23 10 10 227 10

4

(b)

(c)


PLA-PEG/L10E227L10 30/70

3

3

3

α

α

α

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

2

1

30

40

50

60

L10E227L10%

70

80

1

2

sol gel

sol gel 100

sol gel 200

300 400 500

DPPEG,tri

1

50

100

DPPEG,di

Figure 7. Scaling exponents (α) of power law plotted as a function of (a) PLA-PEG-PLA fraction for E23D10/L10E227L10 mixtures; (b) DPPEG,tri for the enantiomeric mixtures of E23D10 with different triblock copolymers; (c) DPPEG,di for the enantiomeric mixtures of L10E227L10 with different diblock copolymers. As shown in Figure 7a, E23D10/L10E227L10 70/30 and 60/40 sols show the large α values of 3.57 and 3.38, respectively, indicating the relatively compact structure of


150


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

scattering objects in the PLA-PEG/PLA-PEG-PLA enantiomeric mixtures with low L10E227L10 fractions. With increasing L10E227L10 fraction from 30% to 80%, α decreases from 3.57 to 2.54, suggesting the change from surface to mass fractals (Figure 7a). This would be due to the formation of PEG bridges between micelles and the increased PEG fraction in enantiomeric mixture. At a fixed mixing ratio, α gradually decreases with increasing DPPEG,tri or DPPEG,di (Figures 7b,c). This could be attributed to the increased PEG chain length in the micelle shells or between the micelles, because more hydrophilic PEG segments would lead to the formation of loosely connected structures with ill-defined interfaces. SAXS data associated with the scattering peak or shoulder at medium q (0.1~0.4 nm−1), which reflects the averaged intermicellar distance,15 was analyzed by Teubner-Strey (T-S) model.44 This model has been widely used to describe the scattering of polymer colloids such as miniemulsion45 and copolymer micelles.46,47 Based on the T-S model, the scattering intensity can be expressed as

I ( q) =

1 a + c1q 2 + c2 q 4

(4)

 1  a 1/ 2 1 c  1  d = 2π    − 2 c 4 c   2  2    1  a 1/ 2 1 c  1  ξ=   + 2 c 4 c   2  2  

−1/ 2

(5)

−1/ 2

(6)

where d represents the periodic spacing between scattering domains and ξ is the correlation length. The variables a, c1, and c2 can be obtained by nonlinear least-squares fitting of SAXS data at medium q via eq. 4. Only the data associated with the scattering peak or shoulder was used for fitting. d and ξ can be calculated from eqs. 5 and 6, respectively. In the PLA-PEG/PLA-PEG-PLA enantiomeric


Page 22 of 33

Page 23 of 33

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


mixture, d would refer to the intermicellar distance. As shown in Figure 6b, the scattering peak can be well fitted by T-S model. As seen in Table 2, for the enantiomerically-mixed sols or gels of E23D10 with different triblock copolymers, d increases from 23.2 to 28.9 nm as DPPEG,tri is increased from 90 to 227, indicating the increase of intermicellar distance with the length of PEG bridging chains. In the case of E23D10/L10E227L10 enantiomeric mixtures, the intermicellar distance first decreases and then keeps nearly invariable as L10E227L10 fraction increases from 50% to 70%, suggesting that increasing the triblock copolymer fraction may enhance the bridging interactions between micelles and thus make the micelles pack more compactly. For the enantiomeric mixtures of L10E227L10 with different diblock copolymers, d increases with DPPEG,di, because of the enhanced shell thickness in star-like micelles of diblock copolymer.

Table 2. Structural Parameters Obtained by Fitting T-S Model from the SAXS of PLA-PEG/PLA-PEG-PLA Enantiomerically-Mixed Gels at Medium q Region sample

d (nm)

ξ (nm)

state

E23D10/L10E90L10 30/70

23.2

7.6

sol

E23D10/L10E136L10 30/70

25.0

8.2

gel

E23D10/L10E227L10 30/70

28.9

9.1

gel

E23D10/L10E227L10 50/50

35.1

7.4

gel

E23D10/L10E227L10 40/60

30.8

8.9

gel

E23D10/L10E227L10 20/80

30.1

10.2

gel

E43D10/L10E227L10 30/70

30.9

8.0

gel

E113D10/L10E227L10 30/70

36.5

6.6

sol

Rheological Properties. Mechanical properties of PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels were studied by rheological analysis. G′ and G″ of gels are less frequency-dependent and change little under different frequencies (Figure S7). Therefore, G′ and G″ were measured upon heating at a fixed frequency (1 Hz). As



shown in Figure S7 and Table 3, the initial higher value of G′ than G″ at 4 °C indicates the solid-like character of gel. Both G′ and G″ decrease with temperature and they cross upon heating, corresponding to the gel-to-sol transition. As elevated temperature, the higher G″ than G′ shows the fluid-like character of enantiomericallymixed gels. CGTs determined by rheological method are similar with those derived from the test tube inverting method. As shown in Table 3, E23D10/L10E227L10 30/70 gel shows a maximum G′ of 1190 Pa at 4 °C and its G′ is larger than the E23D10/L10E227L10 gels with other mixing ratios and different DPPEG,tri s of E23D10/L10ExL10, which is due to the denser bridging network in the former. This is consistent with the results illustrated in Figure 2, in which the E23D10/L10E227L10 30/70 gel shows the highest CGT of 39 °C. G′ of PLA-PEG/L10E227L10 30/70 gel decreases significantly as DPPEG,di is increased from 23 to 43, because of the decreased fraction of PLA hydrophobic domain. Therefore, it is concluded that the mechanical properties of PLA-PEG/PLA-PEG-PLA enantiomerically-mixed gels can be well tailored from the mixing ratio and copolymer composition.

3

(a)

E23D10/L10E227L10

10

3

10


(b)

2

10

2

G', G" (Pa)

G′, G″ (Pa)

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 33

1

10

50/50 G′ 40/60 G′ 30/70 G′ 20/80 G′

0

10

-1

10

10

20

G″ G″ G″ G″

30

10

1

10

0

10

-1

40

50

10

60

L10E136L10 G'

G"

L10E227L10 G'

G"

L10E445L10 G'

10

20

G"

30

40

50

60

Temperature (°C)

Temperature (°C)

Figure 8. Temperature-dependent storage (G′) and loss moduli (G′′) of PLA-PEG/PLA-PEG-PLA

enantiomerically-mixed

gels


with

a

copolymer

Page 25 of 33

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


concentration of 20 wt%: (a) E23D10/L10E227L10 mixtures with different mixing ratios; (b) enantiomeric mixtures of E23D10 with different triblock copolymers.

Table 3. Rheological Parameters of PLA-PEG/PLA-PEG-PLA Enantiomerically Mixed Gels

a

sample

G′ (Pa)a

G′′ (Pa)a

CGT (°C)

E23D10/L10E227L10 50/50

244

67

24.0

E23D10/L10E227L10 40/60

844

140

33.9

E23D10/L10E227L10 30/70

1190

186

40.2

E23D10/L10E227L10 20/80

290

106

31.3

E23D10/L10E136L10 30/70

221

96

27.8

E23D10/L10E445L10 30/70

217

57

25.4

E43D10/L10E227L10 30/70

425

112

34.1

G′ and G′′ were determined from the data collected at 4 °C.

CONCLUSION In situ formed physical gels are prepared by mixing the PLA-PEG and PLA-PEG-PLA copolymer solutions with opposite configurations of PLA blocks. This gelation process is quite fast and happens within 10 s after mixing. The in situ formed gels are thermoresponsive and turn into sols at elevated temperatures. The in situ gelation, thermal-triggered gel-to-sol transition, PLA crystalline structure, microstructures,

and

mechanical

properties

of

PLA-PEG/PLA-PEG-PLA

enantiomerically-mixed gels or sols depend strongly on the mixing ratio, DPPEG,tri, DPPEG,di. Generally, relatively larger PLA-PEG-PLA fraction and DPPEG,tri are favorable for the in situ gelation. Due to the formation of network structure, Dhs of PLA-PEG and PLA-PEG-PLA copolymers in dilute solution increase significantly upon mixing. The gelation mechanism of PLA-PEG/PLA-PEG-PLA enantiomeric mixture is proposed, in which part of PLA-PEG-PLA chains act as the connecting



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

bridges between PLA-PEG star-like micelles and PLA-PEG-PLA flower-like micelles. The formation of intermicellar bridges is highly promoted by the stereocomplexation of PLLA and PDLA blocks. This study provides a simple, yet effective method to prepare the in situ forming physical gels and tune the bridging network structures and physical properties of amphiphilic copolymer gels through the stereocomplex crystallization of hydrophobic blocks. This would be essential to shed light on the gelation mechanism and hierarchical self-assembled structures of amphiphilic block copolymers in aqueous media.

ASSOCIATED CONTENT Supporting Information Partial experimental section, NMR, DLS, WAXS and FTIR results of freeze-dried gels, and rheological plots of gels. 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 work was financially supported by the Natural Science Foundation of China (21274128, 21422406) and State Key Laboratory of Chemical Engineering (SKL-ChE-12D06). WAXS and SAXS experiments were performed at the BL16B beamline of SSRF, China.

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For “Table of Content” Use Only

In Situ Formation and Gelation Mechanism of Thermoresponsive Stereocomplexed Hydrogels upon Mixing Diblock and Triblock Poly(lactic acid)/Poly(ethylene glycol) Copolymers Hailiang Mao, Pengju Pan, Guorong Shan, Yongzhong Bao


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