Competing Stereocomplexation and Homocrystallization of Poly(l

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Competing Stereocomplexation and Homocrystallization of Poly(L-lactic acid)/Poly(D-lactic acid) Racemic Mixture: Effects of Miscible Blending with Other Polymer Jianna Bao, Xiaojia Xue, Kai Li, Xiaohua Chang, Qing Xie, Chengtao Yu, and Pengju Pan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b03287 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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The Journal of Physical Chemistry

Competing Stereocomplexation and Homocrystallization of Poly(L-lactic acid)/Poly(D-lactic acid) Racemic Mixture: Effects of Miscible Blending with Other Polymer

Jianna Bao,1 Xiaojia Xue,2 Kai Li,2 Xiaohua Chang,1 Qing Xie,1 Chengtao Yu,1 Pengju Pan*,1

1

State Key Laboratory of Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China 2

The Institute of Oil and Gas Technology of Changqing Oilfield Company, Xi'an 710018,

China

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

ACS Paragon Plus Environment

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ABSTRACT: Promoting the stereocomplexation ability of high-molecular-weight poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) is an efficient way to improve the thermal resistance of resulted materials. Herein, we studied the competing crystallization kinetics, polymorphic crystalline structure, and lamellae structure of PLLA/PDLA component in its miscible blends with poly(vinyl acetate) (PVAc) and proposed a method to improve the stereocomplexation ability of PLLA and PDLA through miscible blending with the other polymer. Crystallization of PLLA/PDLA component is suppressed after the addition PVAc, due to the dilution effect. Stereocomplexation ability of PLLA and PDLA is enhanced by blending with PVAc; this becomes more obvious at a high PVAc content (≥ 50 wt%) but less significant with the further increase of PLLA, PDLA molecular weights. Almost exclusive formation of SCs is achieved for PLLA and PDLA after blending with a large proportion of PVAc (e.g., 75 wt%). Incorporation of PVAc also facilitates the HC-to-SC structural reorganization upon heating. The increased chain mobility, decreased equilibrium melting point, and enhanced intermolecular interactions may account for the preferential stereocomplexation in PLLA/PDLA/PVAc blends.


2

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INTRODUCTION Poly(lactic acid) (PLA) is a representative ecofriendly thermoplastic that can be derived from the renewable resources and is fully degradable under composting conditions after use. These attributes make PLA an extremely attractive material, due to the increasing environmental concerns

and resource crisis

associated with the

traditional

petroleum-based polymers. Commercially-available PLA exhibits good processing ability, high strength and modulus; it has been widely used in many fields such as textile industry, agriculture, and medical devices.1,2 However, the poor thermal property of PLA is still not fulfilling the criteria needed for ordinary materials, which impedes its broader commercial exploitation.3 PLA exists in typical three isomeric forms including poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA), and racemic PLA, which display a wide variety of properties. It has been demonstrated that blending PLLA with PDLA results in the formation of stereocomplex crystallites (SCs)4 with denser chain packing and stronger interchain interactions.5 Chemical and physical properties of SC-PLA highly surpass those of their parent enantiopure, homocrystalline polymers, such as the higher mechanical strength and modulus,6,7 better thermal stability,8 lower thermal degradation as well as improved hydrolysis resistance. One of the most notable features of SCs is their high melting temperature (Tm) of 230 °C, which is significantly higher than that of PLLA or PDLA homocrystallites (HCs) (approximately 170 °C).4 However, formations of HCs and SCs are competing in the crystallization and processing of PLLA/PDLA blends. Generally, full stereocomplexation between PLLA and PDLA (i.e., without homocrystallization of enantiopure polymers) can be achieved in the PLLA/PDLA blends containing at least one low-molecular-weight component ( 98%, Sigma-Aldrich] was distilled before use. 1-dodecanol [Sigma-Aldrich] and other reagents were used without further purification. Two PLLAs (Mw = 81.9 kg/mol, PDI = 1.40; Mw = 192.2 kg/mol, PDI= 1.44) and PDLAs (Mw = 91.3 kg/mol, PDI = 1.34; Mw = 191.2 kg/mol, PDI= 1.44) with different MWs were synthesized by the bulk ring-opening polymerization using


6

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1-dodecanol as the initiator and Sn(Oct)2 as the catalyst, according to a published method.18 Preparation of Blend Samples. PLLA/PDLA and PLLA/PDLA/PVAc blends were prepared from solution. Briefly, PLLA, PDLA, and PVAc were separately dissolved in dichloromethane with a concentration of ~20 g/L at 40 °C and the solutions were then mixed under vigorous stirring. The mixed solution was cast onto a Petri dish, followed by the evaporation of solvent at room temperature and further vacuum-drying to remove the residual solvent. Weight ratio of PLLA to PDLA was maintained at 1:1 in all blends and the mass fraction of PVAc (mPVAc) was ranged from 0 to 75 wt % based on the total weight of blend. The as-prepared blend was referred as L82/D91/PVAc-x or L192/D191/PVAc-x, where L, D, and x represent PLLA, PDLA, and mPVAc, respectively; the numerals behind L or D denote the Mw (in kg/mol) of PLLA or PDLA. Measurements. Differential Scanning Calorimeter (DSC). Crystallization and melting behavior of the blends were characterized by a NETZSCH DSC 214 Polyma instrument (NETZSCH, Germany) equipped with an IC70 intracooler. All the experiments were performed under ultrapure nitrogen flow and the instrument was calibrated with indium and stannum standards. The samples (~10 mg) were quenched to 0 °C under a cooling rate of 100 °C/min after melting at 250 °C for 2 min to erase any previous thermal history. Then, they were heated to 250 °C at 10 °C/min for nonisothermal cold crystallization or fast heated (at 100 °C/min) to the desired temperature (Tc = 80 ~ 180 °C) for isothermal crystallization. After cold crystallization, the sample was reheated to 250 °C at 10 °C/min to examine the melting behavior. Polarized Optical Microscopy (POM). The investigation on spherulitic morphology and spherulite growth rate was carried out on an Olympus BX51 polarized optical microscopy (POM, Olympus Co., Tokyo, Japan) equipped with an Instec


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HCS402 hot and cooling stage (Instec Inc., Colorado, USA). The sample was melted at 250 °C for 2 min and then fast cooled to the preset Tc at 100 °C/min for isothermal melt crystallization. Spherulite morphology was recorded by taking photo at a fixed time interval during crystallization. The values of spherulite growth rate were estimated from the slope of spherulite radius vs crystallization time plot. Wide-angle X-ray Diffraction (WAXD) and Small-angle X-ray Scattering (SAXS). WAXD and SAXS measurements were carried out on the beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of radiation source is 1.24 Å and the sample-to-detector distances were 140 and 1850 mm for WAXD and SAXS, respectively. An X-ray CCD detector (Model SX165, a resolution of 2048 × 2048 pixels of 80 µm × 80 µm, Rayonix Co. Ltd., USA) was utilized to collect the scattering images. 2D data was converted into 1D profile by circularly averaging with the Fit2D software. SAXS patterns were corrected from the background and air scattering. For the room-temperature WAXD and SAXS measurements, the samples (thickness ~0.3 mm) crystallized at different Tc’s were prepared on an Instec HCS402 hot stage using the same thermal program as that used in DSC isothermal crystallization. The acquisition times of each WAXD and SAXS pattern were 60 and 90 s, respectively. For the temperature-variable WAXD analysis, the amorphous sample with a thickness of ~0.3 mm was obtained by quenching the melted (250 °C, 2 min) sample into liquid nitrogen. The sample was placed between two pieces of polyimide films and heated from 50 to 260 °C at 10 °C/min on a Linkam THMS600 hot stage. WAXD pattern was collected every 5 °C with an acquisition time of 15 s.

RESULTS and DISCUSSION


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Nonisothermal Crystallization. Figure 1A shows the DSC heating curves of neat PVAc and PLLA/PDLA/PVAc blends with various PVAc contents after they were quenched from the melt. Based on the DSC results, glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm) and enthalpy (∆Hm) of HCs and SCs were calculated, as shown in Table 1. Degree of crystallinities for HCs (Xc,HC) and SCs (Xc,SC) were estimated by comparing ∆Hm to the corresponding value of an infinitely large crystal (∆Hm0), which were 93 and 142 J/g52,53 for HCs and SCs, respectively. It is worth noting that the recorded ∆Hm were normalized according to the mass fraction of PLLA/PDLA component in the blend. Relative fraction of SCs in the crystalline phase of PLLA/PDLA blends was calculated by fSC,DSC = Xc,SC/(Xc,SC+Xc,HC)

endo up

(Table 1).

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


SC

HC

mPVAc

1.0

0.75 0.5 0.25 0.1 0

50

100

150

200

250

Temperature (°C)

Figure 1. DSC curves collected upon heating (10 °C/min) for the melt-quenched L82/D91/PVAc blends with different PVAc contents.


9


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Table 1. Thermal Parameters of PLLA/PDLA and PLLA/PDLA/PVAc Blends Obtained in Nonisothermal Cold Crystallization of Melt-Quenched Samples Tg Tcc Tm,HC Tm,SC ∆Hm,HC ∆Hm,SC Xc,HC Xc,SC mPVAc (J/g) (J/g) (J/g) (J/g) (°C) (°C) (°C) (°C) 0

60.5

106.2

177.8

226.6

38.5

10.4

41.4

7.3

0.1

58.0

116.0

178.2

227.6

41.5

14.6

44.6

10.3

0.25

55.1

130.1

177.3

226.6

32.6

29.8

35.1

21.0

0.5

50.0

138.1

175.4

224.8

9.9

47.0

10.6

33.1

0.75

44.8

161.7

--

224.3

0

39.4

0

27.7

1

40.0

--

--

--

0

0

0

0

As shown in Figure 1 and Table 1 all the blends with different PVAc contents exhibit a single Tg between the Tg values of PVAc and PLLA/PDLA components, as expected for the thermodynamically miscible blends. The measured Tg values agree well with the predicted values using the Flory-Fox equation (data not shown). Neat PVAc exhibits a Tg around 40 °C and that of L82/D91 blend is about 60 °C. Tg gradually shifts to lower temperature with increasing mPVAc, indicating that the chain mobility of PLLA and PDLA is enhanced with the presence of PVAc. DSC heating curves of all blends show the obvious cold crystallization peak and two melting peaks corresponding to HCs and SCs, respectively (Figure 1). In addition, no crystallization or melting peak is identified for PVAc, confirming its amorphous nature. The presence of PVAc has an obvious effect on the crystallization behavior of PLLA/PDLA component. Tcc of PLLA/PDLA component shifts gradually to higher temperature (from 106 to 162 °C) and the crystallization peak becomes much broader with an increase of mPVAc to 75% (Figure 1, Table 1). All these results indicate the depressed crystallization rate of PLLA/PDLA component upon blending with non-crystallizable PVAc. PVAc shows a remarkable influence on the melting behavior of PLLA/PDLA component, indicating the change of crystalline structure upon blending. For the


10

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PLLA/PDLA blend, two melting endotherms in the vicinity of 180 and 225 °C were observed upon heating, indicating the coincident crystallization of SCs and HCs. As shown in Figure 1 and Table 1, the intensity of HC melting peak is much stronger than that of SCs; Xc,HC (41%) is more than five times of Xc,SC (7%) and fSC,DSC is only about 15%. However, the content of SCs increases and that of HCs decreases remarkably as mPVAc increases. There is significant difference between the PLLA/PDLA/PVAc-0.25 and PLLA/PDLA/PVAc-0.5 blends, in which Xc,HC drops from 35% to 11% and fSC,DSC increases from 37% to 76%. As the mPVAc increases to 0.75, the melting peak of HCs disappears; accordingly, a large decrease in Xc,HC and considerable increases in Xc,SC and fSC

are

observed

with

increasing

mPVAc from

0

to

75

wt%.

Xc,SC

of

PLLA/PDLA/PVAc-0.75 blend is 27.7% and its fSC,DSC is 100%. Spherulitic Morphology. To demonstrate the effect of PVAc on crystal morphology and spherulitic growth rate of PLLA/PDLA component, isothermal melt crystallization of the blends was performed at 140 °C from the melt. Figure 2 shows the typical POM micrographs of PLLA/PDLA/PVAc blends with various PVAc contents during the crystallization process. As expected, normal spherulites with smooth boundaries were observed for the PLLA/PDLA blend. The boundaries become rough and the spherical symmetry becomes incomplete with the addition of PVAc. The asymmetry appears more remarkable with the further increase of mPVAc. POM graphs of PLLA/PDLA/PVAc-0.75 blend show feather-like dendrites due to the presence of PVAc, which acts as the amorphous diluent to disintegrate the originally well-rounded spherulites into dendrites in the crystallization of PLLA/PDLA component (Figure 2A).11 The distribution of PVAc at the crystal growth front will lead to unbalanced lamellar extension that no longer sustains well-round spherulites. On the other hand, the size of spherulites seems to be similar for all blends, suggesting that there is little


11


dilution effect by residual PVAc chains on the nucleation process during the crystallization.

A

B

25

Spherulitic radius (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

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mPVAc 0 0.1 0.25 0.5 0.75

G=8.65

20 3.95 1.77

15

1.22 0.48

10 0

2

4

6

8

10

Time (min)

Figure 2. (A) POM images and (B) determination of spherulitic radius growth rate for L82/D91/PVAc blends melt-crystallized at 140 °C.

Figure 2B shows the spherulitic radius growth rate (G, slopes of the linear fitted lines) of blends with various PVAc contents, as evaluated by the plots of spherulitic radius vs time. G value of PLLA/PDLA spherulites progressively decreases with increasing mPVAc. G values of PLLA/PDLA component in the blends with mPVAc = 0, 0.1, 0.25, 0.5, and 0.75 are calculated to be 8.65, 3.95, 1.77, 1.22, and 0.48 µm/min, respectively. It should be noted that the G value of PLLA/PDLA blend is approximately twenty times of that for the PLLA/PDLA/PVAc-0.75 blend. All these results indicate that the growth of PLLA/PDLA spherulite is severely restricted upon blending with the non-crystallizable PVAc, which is in line with the aforementioned DSC results. It has been reported that blending with the third polymer has diversified effects on the crystallization kinetics of PLLA/PDLA component. Crystallization rate of PLLA/PDLA component was highly increased after the incorporation of PEG, because of the highly increased diffusion ability of PLLA (or PDLA) chians.39 However, when the high-Tg components such as PVPh35 and PMMA40 were miscibly blended with PLLA/PDLA


12

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component, crystallization rate of PLLA/PDLA component was highly depressed, due to the dilution effect and redistricted chain mobility. The decreased crystallization rate of PLLA/PDLA component in PLLA/PDLA/PVAc blend is ascribed to the dilution effect of PVAc. Crystalline Structure. Because the crystallization temperature varies in nonisothermal process, isothermal crystallization under various Tc’s is conducted to study the peculiar dependences of HC, SC crystallization on PVAc content. Since HCs and SCs have distinct Tm, both the melting behavior and WAXD patterns of PLLA/PDLA and PLLA/PDLA/PVAc blends were examined to elucidate the crystalline structure. Figures 3 and 4 show the DSC heating curves and WAXD profiles of isothermally-crystallized PLLA/PDLA and PLLA/PDLA/PVAc blends with mPVAc = 0.5 and 0.75. As shown in Figure 3A, for the PLLA/PDLA blend crystallized at different Tc’s (80~170 °C), a majority of HCs melt at 160~190 °C, followed up with the melting of little SCs at around 210~240 °C in the heating process. When crystallized at a Tc above 180 °C, at which the temperature is higher than the Tm of HCs, only the melting peak of SCs is seen in the heating process. As presented in Figure 4A, in the whole Tc range of 110~160 °C, the diffraction peaks at 2θ = 13.4 and 15.3°, corresponding to (110/200) and (203) planes of HCs of PLLA or PDLA,54 and the diffraction peaks at 2θ = 9.6, 16.6, and 19.2°, corresponding to (110), (300/030) and (220) planes,55 are observed. The polymorphic structures revealed by WAXD are consistent with those by DSC, indicating that the formation of HCs is overwhelming to the formation of SCs in the PLLA/PDLA blend.


13

HC

180 170

Heat flow

160 140 120

endo up

mPVAc=0

SC

B

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SC

mPVAc=0.5

180

HC

170

Heat flow

A

160 140 120 100

100

80

80

100

150

200

250

endo up

Temperature (°C)

C

100

150

200

250

Temperature (°C)

SC

mPVAc=0.75

180 170 160

Heat flow

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

endo up


140 120 100 80

100

150

200

250

Temperature (°C) Figure 3. DSC heating curves of blend samples after cold crystallization at different temperatures: (A) L82/D91/PVAc-0; (B) L82/D91/PVAc-0.5; (C) L82/D91/PVAc-0.75.

As shown in Figures 3 and 4, incorporation of PVAc in PLLA/PDLA mixture enhances the intensities of melting and diffraction peaks associated with SCs, and correspondingly decreases those of HCs. In addition, because SCs are more thermally stable than HCs, the relative peak intensities of SC diffractions enhance with increasing Tc. Especially, for the L82/D91/PVAc-0.75 blend, the melting peak of HCs disappears and only the peak at around 225 °C is clearly observed in the DSC heating curves, suggesting the predominant formation of SCs (Figure 3C). Three characteristic diffraction peaks of SCs presented on the WAXD profiles of L82/D91/PVAc-0.75 blend


14

Page 15 of 35

(Figure 4C) also indicate that SCs are predominantly formed over the entire Tc range of

120 100

SC220

SC300/030

Intensity (a.u.)

140

mPVAc=0.5 HC203

SC220

160

HC110/200

B

SC110

mPVAc=0 SC300/030

Intensity (a.u.)

SC110

HC110/200

A

HC203

80~160 °C.

120 100

16 2θ (°, λ=1.24Å)

20

8

12

16

20

2θ (°, λ=1.24Å)

mPVAc=0.75

D 1.2

160 140 120

0.8

fSC,WAXD

SC300/030

SC110

C

12

80

SC220

8

160 140

80

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


mPVAc = 0

mPVAc = 0.1

mPVAc = 0.25

mPVAc = 0.5

mPVAc = 0.75

0.4

100 80

8

12

16 2θ (°, λ=1.24Å)

20

0.0 80

100

120

140 160 Temperature (°C)

Figure 4. WAXD results of blend samples after cold crystallization at different temperatures: (A) WAXD patterns of L82/D91/PVAc-0 blend, (B) WAXD patterns of L82/D91/PVAc-0.5 blend, (C) WAXD patterns of L82/D91/PVAc-0.75 blend, (D) plot of SC fraction vs crystallization temperature.

Degree of crystallinity and fraction of SCs were calculated from both WAXD and DSC results, which are shown as a function of Tc in Figures 4D and 5. On basis of the WAXD results, the relative fractions of SCs and HCs (fSC, fHC) in PLLA/PDLA crystalline phase were estimated from the area of diffraction peaks for each polymorph (2ߠ of 9.6, 16.6, 19.2° for SCs and 2ߠ of 13.4, 15.3° for HCs). In order to compare with


15


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Page 16 of 35

the fSC derived from DSC (i.e., fSC,DSC), fSC attained from WAXD is denoted as fSC,WAXD, as shown in Figure 4D. As shown in Figure 5A,B, Xc,SC of L82/D91 blend is less than 15% after crystallization at various Tc’s and its Xc,HC is larger than 40% (except for Tc=180°C, where the temperature is higher than the Tm of HCs). In contrast to Xc,HC, a considerable increase in Xc,SC is observed in Figure 5B with an increase of mPVAc to 0.75. At Tc = 80–180 °C, Xc,SC’s of PLLA/PDLA/PVAc blends with mPVAc =0, 0.1, 0.25, 0.5 are around 10%, 15%, 25%, and 35%, respectively (Figure 5B). It seems that Xc,SC increases by about 10% by adding every 25 wt% PVAc. Especially, a fSC,DSC value of 100% is achieved for the L82/D91/PVAc-0.5 blend crystallized at Tc of 170, 180 °C and L82/D91/PVAc-0.75 blend crystallized at the entire Tc range of 80–180 °C (Figure 5C). In addition, fSC,DSC rises up gradually from 20 to 100% as mPVAc increases to 0.75.Similar variation trends are also seen from fSC,WAXD (Figure 4D). These results clearly show that PVAc plays an important role in promoting the SC formation of PLLA/PDLA component and SCs can be predominantly achieved for PLLA/PDLA component at a high PVAc content. As reported by Yang et al.,39 dominant formation of SCs can be achieved in the PLLA/PDLA blend after incorporation of 10 wt% of PEG (Mn = 1 or 2 kg/mol).39 The promotion effect of low-molecular-weight PEG39 on PLLA/PDLA SC formation is better than the PVAc studied in this work; this may due to the better plasticizing effect of PEG. By comparing the results shown in Figures 4D and 5C, it can be seen that fSC,WAXD is generally smaller than fSC,DSC. In the case of PLLA/PDLA/PVAc-0.5 blend crystallized at Tc = 80~160 °C, fSC,WAXD ranges in 12~65%; whereas fSC,DSC ranges in 51~90%. The difference between fSC,DSC and fSC,WAXD results is ascribed to the heating-induced HC-to-SC crystalline transition. Previous study has demonstrated that HCs melt and reorganize into SCs upon heating or annealing at elevated


16

Page 17 of 35

temperatures;16,22 this can lead to the overestimation of fSC,DSC. MWs of PLLA and PDLA play key roles in the competing formations of HCs and SCs.9-16 To investigate whether PVAc is efficient to facilitate the SC formation for the PLLA/PDLA mixture with further higher MW, we also compared the crystallization behavior of L192/D191/PVAc-0.5 blend with that of L192/D191 blend. DSC heating curves and WAXD profiles of these blends are shown in Figure 6. The melting peaks of both HCs and SCs are observed in the DSC heating curves of L192/D191 and L192/D191/PVAc-0.5 blends crystallized at Tc = 80~160 °C (Figure 6A). In addition to the diffraction peaks of SCs at 2θ values of 9.6, 16.6, and 19.2°, the diffraction peaks of HCs at 13.4 and 15.3° appear in the WAXD profiles of L192/D191/PVAc-0.5 blend crystallized at Tc = 80~160 °C (Figure 6B). All the DSC and WAXD results show that the isothermal crystallization of L192/D191 and L192/D191/PVAc-0.5 blends exhibits similarities with those of L82/D91 and L82/D91/PVAc-0.5 blends, which indicates the coexistence of both HCs and SCs due to the rather higher MWs of parent PLLA and PDLA.

A

B

60

Xc, SC (%)

60

Xc, HC (%)

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 mPVAc=0

20

0.1 0.25 0.5 0.75

120

0.1

0.25 0.75

0.5

40

20

0 90

mPVAc=0


0

90



120

17


C

fSC,DSC

0.8

mPVAc=0

0.1

0.25 0.75

0.5

0.4

0.0

90

120

150

180

Temperature (°C) Figure 5. Xc,HC, Xc,SC, and SC fraction calculated from the DSC results shown in Figure

Heat flow

140 120 100

SC220

160

SC300/030

180

HC203

SC110

HC

HC110/200

B

SC

Intensity (a.u.)

A

endo up

3 for the L82/D91/PVAc blends after cold crystallization at different temperatures.

120 100

80 100

150

200

Temperature (°C)

250

160 140

80 8

12

16

Å

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

20

2θ (°, λ=1.24 )

Figure 6. (A) DSC heating curves and (B) WAXD patterns of L192/D191/PVAc-0.5 blend after isothermal cold crystallization at different temperatures.

Figure 7 shows the degree of crystallinity and fraction of SCs for the L192/D191 and L192/D191/PVAc-0.5 blends crystallized at various Tc’s. In comparison with the L192/D191 blend crystallized at the same Tc, Xc,SC improves slightly from about 5% to 12% after the incorporation of 50 wt% of PVAc (Figure 7A). The values of fSC,DSC and fSC,WAXD slightly increase with an addition of 50wt% of PVAc in L192/D191 blend; but this increase is much less significant than the case of L82/D91 blend (Figure 7B). For the L82/D91/PVAc-0.5 and L192/D191/PVAc-0.5 blends having the same PVAc


18

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contents, the PVAc-induced enhancements of Xc,SC and fSC are much larger for the former, with an increment of approximately 22% and 30%, respectively. However, the values of Xc,SC and fSC for L192/D191/PVAc-0.5 blends just show slight increment compared with the L192/D191 blend. This suggests that the promotion effect of PVAc on SC formation depends strongly on the MWs of PLLA and PDLA. Compared with the L192/D191 mixture, PVAc shows a better promotion role in the SC formation of

A

60

40

Xc,HC, mPVAc = 0, Xc,HC, mPVAc = 0.5 Xc,SC, mPVAc = 0

20

0

Xc,SC, mPVAc = 0.5

90

120

150

180

B

100

fSC, DSC, fSC,WAXD(%)

L82/D91 mixture.

Xc, HC Xc, SC(%)

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


80 fSC,DSC, mPVAc = 0,

60

fSC,DSC, mPVAc = 0.5

40

fSC,WAXD, mPVAc = 0 fSC,WAXD, mPVAc = 0.5

20

0

Temperature (°C)

90

120

150

180

Temperature (°C)

Figure 7. (A) Xc,HC, Xc,SC, (B) SC fraction of L192/D191 and L192/D191/PVAc-0.5 blends after cold crystallization at different temperatures.

Heating-Induced Crystalline Phase Reorganization. Crystallization and structural reorganization of PLLA/PDLA/PVAc blends in the heating process were investigated via in situ WAXD. Figure 8 displays the temperature-dependent WAXD patterns collected during heating from 40 to 260 °C at 10 °C/min for the melt-quenched L81/D91 and L81/D91/PVAc-0.5 blends. Intensity changes for SC (110) and HC (110/200) diffractions were evaluated and plotted as a function of temperature in Figure 9. No discernable diffraction is observed in all the melt-quenched blends before heating, confirming their amorphous structure.


19

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SC220

SC300/030

HC110/200

SC220

SC110

B

SC300/030

SC110

A

260

260 220 200 180 160 140

8

12 16 2θ (°, λ=1.24Å)

240

Intensity (a.u.)

Intensity (a.u.)

240

220 200 180 160 140

120

120

100 90 80 40

100 90 80 40

20

8

12 16 2θ (°, λ=1.24Å)

20

Figure 8. Temperature-dependent WAXD patterns of melt-quenched blends collected upon heating: (A) L82/D91, (B) L82/D91/PVAc-0.5.

A

B L82/D91

1.0

SC110

0.8

HC110/200

0.6 0.4 0.2 0.0 80

120

160

200

Temperature (°C)

240

Normalized peak area

Normalized peak area

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

HC110/200


L82/D91/PVAc-0.5

1.5

SC110 HC110/200

1.0

0.5

0.0 80

120

160

200

240

Temperature (°C)

Figure 9. Temperature-dependent diffraction peak areas in the heating process of melt-quenched (A) L82/D91 and (B) L82/D91/PVAc-0.5 blends. Peak area was normalized by the maximum value of HC (110/200) diffraction.

For the PLLA/PDLA/PVAc blend (mPVAc = 0.5), it is obvious that SC diffractions are much stronger than those of HCs throughout the heating process (Figure 8B). A tiny peak corresponding to the HC (110/200) diffraction appears at 110 °C; its diffraction intensity continuously goes up with heating to 150 °C, followed by the weakening of diffraction upon further heating to 200 °C due to the completely melts of HCs. There are


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three different stages of intensity changes for SCs: SC diffraction intensity first increases slowly in the temperature region of 100~150 °C and dramatically increases in the temperature region of 150 to 210 °C; it then decreases upon further heating to 250 °C because of the melts of SCs. Coincidentally, the temperature region (150~210 °C) of the second stage featured with rapidly forming SCs almost overlaps with the melting region of HCs. Therefore, we conclude that the drastically rises of SCs at 150~210 °C is attributed to the thermal induced HC-to-SC crystalline reorganization or the recrystallization of SCs in the amorphous region during the melts of HCs. It should be inferred that the increased ratio of SC (110) diffraction reaches half of the decreased ratio of HC (110/200) diffraction, implying that almost half of HCs melt and reorganize into SCs. For the L81/D91/PVAc-0.5 blend, the HC content reaches maximum at 150 °C; then it begins to reduce while the SC content increases upon further heating. In the end, the ratio of maximum intensity for SC (110) diffraction to that of HC (110/200) diffraction is about 3/2 (Figure 9B). In the case of L81/D91 blend, the variation trends of HC and SC diffraction intensities are similar to those observed for the PLLA/PDLA/PVAc blend. However, comparing the increased SC intensity with the decreased HC intensity, it is estimated that just a small portion (~15%) of HCs melt-reorganize into SCs; consequently, the ratio of maximum intensity for SC (110) diffraction to that of HC (110/200) diffraction is only about 1/4 for neat L82/D91 blend (Figures 8A, 9A). According to these results, it is confirmed that the presence of PVAc not only efficiently facilitates the SC formation in primary crystallization but also the HC-to-SC structural reorganization upon heating. Crystalline Lamellae Structure. Crystalline lamellae structure of PLLA/PDLA blends with various PVAc contents was investigated via SAXS and compared with that of PLLA/PDLA blend. Figure 10 shows the Lorentz-corrected SAXS profiles (Iq2~q


21


plot) of PLLA/PDLA and PLLA/PDLA/PVAc blends crystallized at various Tc’s. Long spacing (LP) was calculated from the Bragg equation (LP = 2π/qmax), in which qmax corresponds to the q value of scattering peak top in Lorenz-corrected SAXS profiles. LPs of all blends crystallized at different Tcs are listed in Table 2.

A

B

L82/D91

0.2

0.4

0.6

0.8

L82/D91/PVAc-0.25 80 100 120 140 160

2

Iq (a.u.)

2

Iq (a.u.)

80 100 120 140 160

1.0

0.2

0.4

0.8

1.0

q (nm )

q (nm )

C

0.6 −1

−1

L82/D91/PVAc-0.75 80 100 120 140 160

2

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

Page 22 of 35

0.2

0.4

0.6

0.8

1.0

−1

q (nm ) Figure 10. Lorentz-corrected SAXS profiles of (A) L82/D91, (B) L82/D91/PVAc-0.25, and (C) L82/D91/PVAc-0.75 blends after cold crystallization at different temperatures.


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Table 2. Long Spacing (in nm) of HCs or SCs in PLLA/PDLA/PVAc Blends Cold-Crystallized at Different Temperatures Tc (°C)

L82/D91

L82/D91/PVAc-0.25 L82/D91/PVAc-0.75

80

--

24.0

--

100

--

22.5

--

120

17.2

21.2

--

140

17.4

23.0

17.2

160

18.1

24.4

17.5

For the PLLA/PDLA blend, no discernable scattering peak of HCs is observed at Tc ≤ 100 °C in the SAXS profiles, which may be due to the less regular crystalline lamellae structure and wide distribution of LP for the HCs formed at low Tc (Figure 10A). At Tc = 120~140 °C, a single scattering peak corresponding to HCs appears at low q. At Tc = 160 °C, a new broad scattering peak corresponding to SCs is observed at higher q, in addition to the sharp peak at low q. The peak corresponding to HCs shifts to low q with increasing Tc, indicating the enhancement of LP of HCs (Table 2). Generally, the longer diffusion path of enantiomeric chains and larger kinetic barrier of SC crystallization may restrict the formation of thick lamellae. Consequently, SCs show smaller LPs than those of HCs after crystallization under the same Tc. Hence, it is reasonable to conjecture that the scattering peaks of L82/D91/PVAc-0.75 are inclined to locate at larger q because of the predominant formation of SCs. However, it is clearly shown in Table 2 that the LPs of SCs formed in L82/D91/PVAc-0.75 blend are similar as those of HCs formed in the L82/D91 blend and those reported for the SCs formed in other PLLA/PDLA racemic blends56 and stereoblock copolymers,57 which are generally in the range of 11~17 nm (Figure 10C). This may be arisen from the increased thickness of amorphous layer, as induced by the segregation of PVAc in the amorphous layer. It has been reported that most PVAc component segregates in the interlamellar


23


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regions of PLLA after the crystallization of PLLA component in PLLA/PVAc blend.47 Moreover, the L82/D91/PVAc-0.25 blend with a large fraction of HCs exhibits LPs in the range of 21~24.5 nm due to the enlarged amorphous layer thickness induced by PVAc, which are much larger than those of HCs formed in L82/D91 blend (Figure 10B). Besides, the depression of Tm0 by miscible blending may also lead to the lamellae thickening, which has been reported for several systems such as PVDF/PBA58 and poly(ε-caprolactone) (PCL)/PVPh blends.59 Proposed Mechanism for the Improved Stereocomplexation Ability in PLLA/PDLA/PVAc Blend. It is proposed that the enhanced stereocomplexation ability of PLLA and PDLA after blending with PVAc is ascribed to the (i) increased chain mobility, (ii) decreased Tm0, (iii) increased intermolecular interactions between PLLA and PDLA chains. First, it has been demonstrated that the chain mobility plays a crucial role in the competing formation of HCs and SCs due to a high level of mixing required between the PLLA and PDLA chains in SC crystallization. However, the HMW PLLA and PDLA used in this study have higher viscosity and lower diffusion ability. Previous studies have found that the SC formation of HMW PLLA/PDLA blend can be promoted by incorporating flexible segments or polymers (e.g., PCL28, PEG29,30,39) due to the increased chain mobility. In our systems, Tg of PLLA/PDLA component gradually decreases after miscible blending with PVAc, due to the plasticizing and dilution effects. The decrease of Tg implies the increased chain mobility of PLLA/PDLA component; this would facilitate the SC formation in crystallization. Second, as shown in Figure 1A, both the Tm’s of HCs and SCs decrease with increasing the PVAc content; this is more obvious for the blends with mPVAc = 0.5 and 0.75. Therefore, it is conjectured that the Tm0 of SCs would decrease after miscible blending with PVAc. The decrease of Tm0 for crystalline component is a general


24

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behavior for the miscible polymer blends.42,43,60 The decrease of PLLA/PDLA Tm0 with miscible blending would shift the critical formation temperature of SCs and HCs to lower temperature and thus widen the crystallization temperature region of SCs, which is a more thermal stable polymorph. The similar proposal has been used to explain the facilitated formation of more thermally-stable polymorph for s-PS,42 PBA,43 and PLLA60 in the miscible blend systems. Third, previous report has proposed that hydrogen bonding interactions are formed between PLA and PVAc due to the proton accepting and proton donating properties of carboxyl groups of PLA and α-hydrogens of PVAc.45 The interactions between PVAc and PLLA/PDLA components may be also evidenced by the morphology transition from well-rounded spherulites to dendritic crystals based on the POM images. Several literatures have reported that the enhanced interactions between enantiomeric PLLA and PDLA chains would facilitate the formation of precursor racemic helical pairs (i.e., embryo of SC nuclei), rather than the homo helical pairs (i.e., embryo of HC nuclei), which would lead to the preferred nucleation and crystal growth of SCs than HCs.32−35 The enhanced interactions and chain mobility between PLLA and PDLA caused by PVAc would facilitate the PLLA and PDLA to confront with each other, leading to the preferential formation of SCs.

CONCLUSIONS In summary, ternary miscible blends of HMW PLLA, PDLA and amorphous PVAc were developed to study the competing crystallizations of HCs and SCs of PLLA/PDLA component. PLLA/PDLA/PVAc blend shows a single composition-dependent Tg at various PVAc contents, indicating miscibility and interaction between PVAc and PLLA/PDLA components. Crystallization rate of PLLA/PDLA component is hindered;


25


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Page 26 of 35

the final spherulitic morphology and lamellae structure morphology of PLLA/PDLA component are altered by presence of PVAc. Stereocomplexation ability, crystalline structural reorganization, and lamellae structure of PLLA/PDLA component are strongly influenced by the presence of PVAc and the PVAc content in the blends. Miscible blending with PVAc not only facilitates the SC formation in the nonisothermal and isothermal crystallizations, but the HC-to-SC structural reorganization in the heating process. When the PVAc content reaches to 75 wt% in PLLA/PDLA/PVAc blend, PLLA and PDLA can predominantly cocrystallize in SCs without the formation of HCs. It is proposed that the facilitated formation of SCs than that of HCs induced by miscible blending are attributable to the enhanced segmental mobility, decreased Tm0 of PLLA/PDLA component, and the increased intermolecular interactions between PLLA and PDLA enantiomeric chains. This study would help to further understand the stereocomplexation mechanism and competing crystallization behavior of PLLA and PDLA in the complicated polymer blend systems; thus afford guidance in the structural manipulation and practical processing of SC-PLA materials.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Tel +86-571-87951334; e-mail [email protected] ACKNOWLEDGMENT. This work was financially supported the National Key Research and Development Program (2016YFB0302402), National Natural Science Foundation of China (21422406), and the second level of Zhejiang Province 151 Talent Project. WAXD and SAXS experiments were performed at the BL16B beamline of SSRF.


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and

Morphology

of

Poly(L-lactide-b-D-lactide)

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Diblock

Copolymers.

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Competing Stereocomplexation and Homocrystallization of Poly(l

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