Stereocomplex Crystallization of Linear Two-Armed Stereo Diblock

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Stereocomplex Crystallization of Linear Two-Armed Stereo Diblock Copolymers: Effects of Chain Directional Change, Coinitiator Moiety and Terminal Groups Hideto Tsuji, Michiaki Ogawa, and Yuki Arakawa J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00460 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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The Journal of Physical Chemistry

Stereocomplex Crystallization of Linear Two-Armed Stereo Diblock Copolymers: Effects of Chain Directional Change, Coinitiator Moiety and Terminal Groups

Hideto Tsuji,*Michiaki Ogawa, and Yuki Arakawa Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan E-mail: [email protected]

Abstract:

Two-armed poly(L-lactide) (PLLA)-b-poly(D-lactide) (PDLA) (2-LD) copolymers with a

wide-range of molecular weight were synthesized and the effect of coinitiator moiety, which functions as impurity and causes chain directional change in the middle of molecules (Effect A), and/or the additional effect of types of terminal groups (Effect B) on crystallization behavior of 2-LD copolymers were studied, in comparison with that reported for one-armed PLLA-b-PDLA (1-LD) copolymers.

Formation of only SC crystallites in 2-LD and 1-LD copolymers indicates that

neighboring PLLA and PDLA blocks facilitated SC crystallization and neither Effect A nor B affected the crystalline species.

Effect A and/or B (both hydroxyl terminal groups) disturbed cold SC

crystallization of 2-LD copolymers compared to that of 1-LD copolymers.

Crystalline growth

morphologies of 2-LD and 1-LD copolymers during cold SC crystallization were spherical and solid sheaf, respectively, exhibiting that crystalline growth morphology was influenced by Effects A and/or B.

The melting temperature or crystalline thickness of SC crystallites were determined by Mn per

one block and not affected by Effect A or B.

Maximum G values of 2-LD copolymers compared to

those of 1-LD copolymers were largely decreased by Effect A and/or B (both hydroxyl terminal groups).

1 ACS Paragon Plus Environment


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1. Introduction Poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] is a biobased polymer produced from renewable plant raw materials like corn starch and can be used in various applications such as commodity, industrial, biomedical, pharmaceutical and environmental applications.1

For enhancement of mechanical

performance and hydrolytic/thermal degradation resistance of poly(lactide) [i.e., poly(lactic acid) (PLA)]-based materials, stereocomplexation between PLLA and its enantiomer poly(D-lactide) [i.e., poly(D-lactic acid) (PDLA)] have been frequently utilized.2–6

Stereocomplex (SC) crystallization

between PLLA and PDLA is affected by the molecular architectures such as molecular weight, tacticity or optical purity of PLA polymers.

High molecular weight and low optical purity disturbs or

decelerates SC crystallization.2–6 Recently, various types of stereo block3,6–15 and star-shaped3,6,16–20 PLAs were synthesized, and the effects of stereo block and star-shaped or branching architectures on crystallization were extensively studied.

Stereo block architecture enhances predominant SC crystallization and suppresses

homo-crystallization in PLA polymers with weight-average molecular weight (Mw) values especially above 1×105 g mol-1.2–6

The SC crystallization rate of stereo block PLA polymers becomes higher

with increasing L-lactyl and D-lactyl unit sequence lengthes.12

However, even the stereo diblock

PLA having sufficiently long L-lactyl and D-lactyl unit sequences had the lower crystallization rate compared to that of linear one-armed PLLA/linear one-armed PDLA (1-L/1-D) blend, for PLA polymers with Mw values around 1×104 g mol-1.11,12

Also, for PLA polymers with Mw values around

1×104 g mol-1, the SC crystallization rate of linear PLLA-b-PDLA was lower than that of linear two-armed PLLA/linear two-armed PDLA (2-L/2-D) blends.14

Here, it should be noted that in linear

PLLA-b-PDLA chain directional change in the middle of molecules occurs at the connecting moiety between PLLA and PDLA blocks and its molecular structure is different from ordinary linear one-armed PLLA-b-PDLA (1-LD) with no chain directional change in a molecule.

These results

mean that the intramolecular and intermolecular SC crystallization rates in the stereo diblock copolymers were lower than the intermolecular SC crystallization rates in the enantiomeric polymer blends and indicate that stereo block architecture or coexistence of PLLA and PDLA segments in one


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molecule delays SC crystallization.11,12,14 As can be expected from the results for homo-crystallization of unblended star-shaped multi-armed PLLA homopolymers during thermal treatment or hydrolytic degradation,21,22 branching architecture delays SC crystallization of branched four-armed PLLA/branched four-armed PDLA (4-L/4-D) blends compared to that of non-branched linear 1-L/1-D and 2-L/2-D blends.20

The

melting temperature (Tm) or crystalline thickness of both one or multi-armed PDLA/PLLA (1-L/1-D, 2-L/2-D, and 4-L/4-D blends)42 or 1-L/4-D or six-armed PDLA (6-D) blends18 was determined by the molecular weight per one arm not by the overall molecular weight, irrespective of the arm number. The enthalpy of melting (ΔHm) of SC crystallites of 1-L/4-D or six-armed PDLA (6-D) blends can be optimized by selecting the molecular weight per one arm of branched PDLA.18

Also, the critical

molecular weight above which homo-crystallization occurs in addition to SC crystallization decreased in the following order: one-armed/one-armed > one-armed/three-armed > three-armed/three-armed, strongly suggesting that the molecular weight per one arm have crucial effects on phase-separation or simultaneous crystallization of SC and homo-crystallization.19

The branching architecture of PLLA

and PDLA facilitates the SC recrystallization of the multi-armed PLLA/PDLA blends during cooling from the melt compared to that of linear one-armed 1-L/1-D blends.16

Recently, 4-L and 4-D having

amorphous poly(DL-lactide) [i.e., poly(DL-lactic acid) (PDLLA)] blocks at the core and shell were synthesized and the effects of the position of amorphous PDLA blocks were investigated.23

The

PDLLA blocks at the shell strongly disturbed the homo-crystallization of unblended block copolymers compared to those at the core, whereas the position of PDLLA blocks had very weak disturbing effects on SC crystallization of enantiomeric block copolymer blends, irrespective of the position of amorphous PDLLA blocks. Moreover, multiple effects of stereo block and branching architectures were investigated.24-27 For the case of one-, three, and six-armed PLLA-b-PDLA stereo diblock copolymers (1-LD, 3-LD, and 6-LD, respectively), the higher arm number disturbed the SC crystallization of stereo diblock copolymers during cooling, when compared at the similar total molecular weight.24

For four-armed

PLLA-b-PDLA (4-LD) with different L-lactyl unit contents and molecular weights, the branching architecture rather than diblock architecture disturbed SC crystallization of equimolar 4-LD and



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simultaneous SC- and homo-crystallization of non-equimolar 4-LD was inhibited at intermediate L-lactyl

unit content between 0 and 50% or 50 and 100%.25,26

In addition to the effects of stereo diblock and branching architectures, SC crystallization behavior will be affected by the coinitiator moiety (Effect A), which present in linear two-armed PLLA-b-PDLA (2-LD) copolymers (Figure 1).

The coinitiator moiety of 2-LD copolymers functions

as an impurity and causes the chain directional change in the middle of the molecules.

As evident

from Figure 1, in terms of molecular structure, the coinitiator moiety of a 2-LD copolymer can be regarded a link between two linear one-armed 1-LD copolymers.

The coinitiator moiety of 2-LD

copolymers is expected to disturb SC crystallization compared to that of 1-LD copolymers.

Also, the

difference in the types of terminal groups will have additional effect on SC crystallization (Effect B). That is, 2-LD copolymers have two hydroxyl terminal groups, whereas 1-LD copolymers have one carboxyl and hydroxyl terminal groups.

Such difference in type of terminal groups affect the

intermolecular and intramolecular interaction of PLLA and PDLA blocks, causing the different crystallization behavior.

Effects A and/or B on SC crystallization can be investigated by the

comparative study of 2-LD and 1-LD copolymers.

Two-armed PLLA (2-L) is reported to have much

higher cold crystallization temperature compared to that of one-armed 1-L, indicating the cold homo-crystallization is disturbed by Effects A and/or B.28

Besides, Effect A and/or B of absence of

carboxyl terminal group and presence of another hydroxyl terminal group in two-armed PDLLA disturbed the hydrolytic degradation compared to one-armed PDLLA.29 However, as far as we are aware, there are few reports regarding the Effects A and B on SC crystallization of PLA-based materials.

The effects of terminal linear alkyl groups with different

lengths as coinitiator moieties were investigated for the homo-crystallization of one-armed 1-L28,30,31 and the SC crystallization of 1-L/1-D blends.32

It was found that cold homo-crystallization and

isothermal homo-crystallization of 1-L was accelerated with increasing the length of terminal alkyl group up to n-docosyl (n-C22H45-) group,28,31 whereas although the trend for cold SC crystallization of 1-L/1-D blends was similar to those reported for cold homo-crystallization of 1-L, isothermal SC crystallization became maximum for 1-L/1-D blend with n-hexyl (n-C6H13-) terminal group.32

In the

present study, we synthesized linear two-armed 2-LD copolymers and investigated their


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non-isothermal cold and isothermal SC crystallization behavior by the use of wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), polarized optical microscopy (POM), in comparison with the results reported for linear one-armed 1-LD copolymers.11

Based on the

obtained results, Effects A and/or B on SC crystallization behavior are discussed.

2-LD Effect A CH3

H H

(O

C C

Effect B PDLA

)(

O

)O

C C

m

O

H

H 3C

O

O CH2

n

CH2

CH2

( O

O

C C

H 3C

CH3

C

O

)

H

O n

m

H

PLLA

PLLA

) (C

H

Effect B PDLA

1-LD Effect B H 3C

O H

( O

O

C C

H 3C

m

H

PLLA

Figure 1.

) (C O

H C

) O

H

n

Effect B PDLA

Molecular structure of 2-LD and 1-LD copolymers.

Factor A (presence of coinitiator

moiety in the middle of moelcules), Factor B (types of terminal groups).

Arrows indicate chain

(-O-CO-) directions.

2. Experimental section 2.1. Materials The precursors for two-armed stereo diblock 2-LD copolymers, i.e., 2-L homopolymers with different molecular weights were synthesized by bulk ring-opening polymerization of L-lactide (3.5–4.5g) (PURASORB L®, Purac Biochem BV, Gorinchem, The Netherlands) initiated with 0.03 wt% of tin(II) 2-ethylhexanoate (Practical grade, Nacalai Tesque, Inc., Kyoto, Japan) in the presence of different amounts of 1,3-propanediol (Sigma-Aldrich Japan, K.K., Tokyo, Japan) as the coinitiator at 140°C for 10 h.20–22,31 Before polymerization, L-lactide was purified by repeated recrystallization using ethyl acetate (JIS special grade, Nacalai Tesque Inc.) as the solvent.


Also, tin(II) 2-ethylhexanoate


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was purified by distillation under reduced pressure and 1,3-propanediol was used as received. Synthesized 2-L homopolymers were purified by reprecipitation using chloroform (JIS special grade, Nacalai Tesque Inc.) and methanol (JIS special grade, Nacalai Tesque Inc.) as the solvent and nonsolvent, respectively, and then dried in vacuo for at least 6 days.

Two-armed stereo diblock

(2-LD) copolymers were synthesized by ring-opening polymerization of D-lactide (0.3g), in toluene (Nacalai special grade, H2O < 30ppm, Nacalai Tesque Inc.) (2 mL) initiated with 0.3 wt% of tin(II) 2-ethylhexanoate in the presence of 2-L (0.3g) as the coinitiator at 120°C for 36 h.11,26

Before

polymerization, D-lactide (PURASORB D®, Purac Biochem BV) was purified by repeated recrystallization using ethyl acetate as the solvent.

The synthesized 2-LD copolymers were obtained

as precipitates in toluene and were purified after removal of toluene by precipitation using a mixed solvent of chloroform (JIS special grade, Nacalai Tesque Inc.)/1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (HPLC grade, Nacalai Tesque Inc.) (vol/vol = 95/5) as the solvent and methanol as the nonsolvent, and then dried in vacuo for at least 6 days.

The isothermal melt-crystallization of the

polymers, which were packed in DSC aluminum cells and sealed in test tubes under reduced pressure, was performed at the crystallization temperature (Tc) for 3 h after melting at 240°C for 3 min.

The

samples after melt-crystallization were quenched at 0°C for at least 5 min to stop further crystallization.

2.2. Measurements and observation The Mw and number-average molecular weight (Mn) of the synthesized polymers were evaluated in chloroform at 40°C by a Tosoh (Tokyo, Japan) gel permeation chromatography (GPC) system with two TSK gel columns (GMHXL) using polystyrene standards. those relative to polystyrene.

Therefore, the molecular weights are

For GPC sample preparation, the mixed solvent of chloroform/HFIP

(vol/vol = 95/5) was used for 2-LD copolymers, whereas chloroform was used for 2-L homopolymers. Table 1 summarizes the molecular characteristics of polymers synthesized in the present study. number X after the abbreviations of polymers (2-LX and 2-LDX) means Mn/103.

The

The Mn values of

2-LD copolymers became 1.7–2.1 times those of precursors and Mw/Mn values were as low as 1.1–1.3,


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indicating the successful synthesis of 2-LD copolymers.

Table 1. Molecular characteristics of two-armed 2-L homopolymers and two-armed stereo diblock 2-LD copolymers synthesized in the present study. Code 2-L4 2-L8 2-L17 2-L33 2-LD9 2-LD14 2-LD29 2-LD56

Coinitiator 1,3-propanediol

2-L4 2-L8 2-L17 2-L33

Lactide/Coinitiator (mol/mol) or (w/w) 20.3/1c) 34.2/1c) 68.7/1c) 138.0/1c) 1/1 d)

Mna) (g mol-1)

Mw/Mna)

4.24×103 8.08×103 1.71×104 3.32×104 9.05×103 1.37×104 2.85×104 5.64×104

[α]25589 b) (deg dm-1 g-1 cm3)

1.35 1.18 1.15 1.15 1.33 1.24 1.19 1.11

-152.6 -157.4 -185.4 -172.0 -1.8 -3.5 -1.9 -6.7

L-lactyl

unit content (%)

50.5 50.9 50.5 51.8

a)

Mn and Mw are number- and weight-average molecular weights, respectively, estimated by GPC. Specific optical rotation measured in the mixed solvent of chloroform/HFIP (vol/vol = 95/5). c) In mol/mol. d) In w/w. b)

The specific optical rotation ([α]25589) values of the polymers were measured in the mixed solvent of chloroform/HFIP (vol/vol = 95/5) at a concentration of 1 g dL-1 and 25°C using a JASCO (Tokyo, Japan) P-2100 polarimeter at a wave length of 589 nm.

The L-lactyl unit contents of 2-LD

copolymers were estimated by the following equation: L-lactyl

unit content (%) = 100 × { [α]25589+ [α]25589(2-L) } / { 2 × [α]25589(2-L) }

(2),

where [α]25589(2-L) is the highest one for 2-L (–185.4 deg dm-1 g-1 cm3) measured in chloroform/HFIP (vol/vol = 95/5).

The L-lactyl unit contents evaluated for 2-LD copolymers are around 50% (Table 1),

indicating that synthesized polymers contain similar lengths of PLLA and PDLA blocks. The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) and the enthalpies of cold crystallization and melting (ΔHcc and ΔHm, respectively) were determined with a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter under a nitrogen gas flow at a rate of 50 mL min-1.

About 3 mg of sample was heated at a rate of 10°C min-1 from ambient

temperature to 240°C.

The transition temperatures and enthalpies were calibrated using tin, indium,

and benzophenone as standards.

The crystalline species and crystallinity (Xc) values of crystallized

samples were estimated by the use of WAXD.

The WAXD measurements were performed at 25°C



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using a Rigaku (Tokyo, Japan) RINT-2500 equipped with a Cu-Kα source (λ = 1.5418 Å), which was operated at 40 kV and 200 mA.

The isothermal spherulite growth of samples was observed using an

Olympus (Tokyo, Japan) polarized optical microscope (BX50) equipped with a heating-cooling stage and a temperature controller (LK-600PM, Linkam Scientific Instruments, Surrey, UK) under a constant nitrogen gas flow.

The samples were heated from room temperature to 240°C at 100°C

min-1, held at this temperature for 3 min, cooled at 100°C min-1 to an arbitrary Tc, and then held at the same temperature (spherulite growth was observed here).

3. Results and Discussion 3.1. Crystalline species and crystallinity To determine the crystalline species and crystallinity of 2-LD copolymers crystallized isothermally from the melt, WAXD measurements were performed. copolymers crystallized for 3 h from the melt.

Figure 2 shows the WAXD profiles of 2-LD

As seen in Figure 2, all 2-LD copolymers except for

those crystallized at the highest Tc values (amorphous), formed only SC crystallites with diffraction peaks at 12, 21, and 24°

2–6

but no homo-crystallites with diffraction peaks at 17 and 19° and so on.33

This result is attributable to the fact that stereo block architecture, wherein PLLA and PDLA blocks are neighboring each other, facilitated SC crystallization or to the fact that the molecular weight of each block of PLLA and PDLA should be too low to form homo-crystallites.3 here exhibits that neither Effects A nor B affects the crystalline species.

Moreover, the result

The crystallinity (Xc) values

estimated from the WAXD profiles in Figure 2 are plotted in Figure 3 as a function of Tc.

As seen,

the Xc values of 2-LD14, 2-LD29, and 2-LD 56 increased but those of lowest molecular weight 2-LD9 decreased with increasing Tc, and finally became nil when approached their Tm values. maximum Xc values increased with an increase in the Mn of 2-LD copolymers. consistent with those reported for 1-LD11 and 4-LD26 copolymers.


The

These trends are

(b) 2-LD14 180 (°C)

Intensity (arbitrary unit)

(a) 2-LD9 160 (°C)

140

120

100

160

140

120

100

80 5

80 10

15 20 2θ (°)

25

5

(c) 2-LD29

(d) 2-LD56 200 (°C)

180

180

160 140 120 100

15 20 2θ (°)

25

15 20 2θ (°)

25

160 140 120 100

80 5

Figure 2.

10

200 (°C)



80 10

15 20 2θ (°)

25

5

10

WAXD profiles of 2-LD9 (a), 2-LD14 (b), 2-LD29 (c), and 2-LD56 (d) copolymers

crystallized isothermally at shown different crystallization Tc values or quenched (Tc = 0°C) from the melt.

The broken lines show the 2θ values for SC crystallites.

60 50 40

Xc (%)

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30 20 2-LD9 2-LD14 2-LD29 2-LD56

10 0

80

100

120

140

160

180

200

Tc (°C)

Figure 3.

Crystallinity (Xc) values of 2-LD copolymers estimated by WAXD measurements, as a

function of Tc.



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3.2. Thermal properties To investigate the thermal properties of 2-LD copolymers crystallized isothermally or quenched from the melt, DSC measurements were carried out.

Figure 4 shows the DSC thermograms of 2-LD

copolymers crystallized for 3 h or quenched (Tc = 0°C) from the melt.

Thermal properties obtained

from DSC thermograms in Figure 4 are tabulated in Table S1.

The glass transition, cold

crystallization, and SC melting peaks are observed at 31–57°C, 84–144°C, and 94–217°C, respectively. The presence of cold crystallization peak for all 2-LD copolymers quenched or isothermally crystallized at the highest Tc values from melt and their Tm values similar to those of other isothermally crystallized 2-LD copolymers, which had only SC crystallites as crystalline species, indicate their SC crystallizability during DSC heating.

The observed double melting peaks for e.g.,

2-LD9 crystallized at Tc = 120 and 140°C and 2-LD14 crystallized at Tc = 140 and 160°C can be ascribed to melting original unstable SC crystallites and recrystallized more stable SC crystallites, respectively.

Recrystallization here should be thickening of SC crystallites or formation of more

perfect SC crystallites.

Such double melting peak observed for 2-LD copolymers with Mn below

1.4×104 g mol-1 was reported for 4-LD copolymers but not for 1-LD copolymers, suggesting that Effects A and/or B (two or four hydroxyl terminal groups) of 2-LD and 4-LD copolymers induced imperfect SC crystallization during isothermal condition.


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(a) 2-LD9

(b) 2-LD14

180 (°C)

140

Endo. ← → Exo.

Endo. ← → Exo.

160 (°C)

120

100

160 140 120 100

80 80 0

0

50

100 150 Temperature (°C)

200

50

(c) 2-LD29

200

200 (°C) 180

Endo. ← → Exo.

180 160 140 120

160 140 120

100

100

80

80

0

0

50

Figure 4.


(d) 2-LD56

200 (°C)

Endo. ← → Exo.

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



200

50


200

DSC thermograms of 2-LD9 (a), 2-LD14 (b), 2-LD29 (c), and 2-LD56 (d) copolymers


Figure 5 shows the Tg, Tcc, and Tm of SC crystallites [Tm(S)] values of 2-LD copolymers as a function of Mn and the Tm(S) values of 2-LD copolymers as a function of Mn per one block, together with those reported for 1-LD copolymers.11

The Tg values of 2-LD and 1-LD copolymers decreased

with decreasing Mn and overlap each other, indicating that segmental mobility was not altered by Effects A and B, in contrast with the result reported for 2-L and 1-L homopolymers,28 wherein Effects A and/or B (two hydroxyl groups) of 2-L increased segmental mobility and lowered Tg at Mn lower than 1×104 g mol-1.

The Tcc decreased with decreasing Mn from 90°C at Mn = 5.6×104 g mol-1 to 86°C



at Mn =1.4×104 g mol-1 but increased rapidly to 94°C at Mn = 9.1×103 g mol-1.

Such dependence of

Tcc of 2-LD copolymers on Mn indicates that cold SC crystallization of 2-LD copolymers was disturbed at Mn below 1×104 g mol-1, similar to the inclination reported for 1-LD copolymers at Mn below 1×104 g mol-1 11 and for 2-L homopolymers wherein cold homo-crystallization was completely prohibited for Mn below 3×103 g mol-1.28 of 1-LD copolymers.

The Tcc values of 2-LD copolymers were higher than those

Although Effects A and B did not affect Tg values, this result exhibits that

Effects A and/or B disturbed the cold SC crystallization of 2-LD copolymers compared to that of 1-LD copolymers. 60

100

(a) Tg against Mn 55

1-LD

90 Tcc (°C)

Tg (°C)

(b) Tcc against Mn 95

2-LD

50 45

85

40

80

35

75

30 2x103

1x104

70 2x103

1x105

1x104

Mn (g mol-1)

Mn (g mol ) 220

(c) Tm(S) against Mn 210

200

200 Tm(S) (°C)

210

190

180

170

170

1x104

1x105

(d) Tm(S) against Mn per one block

190

180

160 2x103

160 1x103

-1

1x104 Mn per one block (g mol-1)

Mn (g mol )

Figure 5.

1x105 -1

220

Tm(S) (°C)

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 12 of 25

Tg (a), Tcc (b), and Tm(S) (c) of melt-quenched 2-LD and 1-LD copolymers (Tc = 0°C) as

function of Mn and Tm(S) as a function of Mn per oble block.

Original data for 1-LD copolymers are

reported in ref. 11.

The Tm values of 2-LD copolymers were much lower than those of 1-LD copolymers when compared at the similar Mn values, whereas the Tm values of 2-LD and 1-LD are similar with each


Page 13 of 25

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other, when compared at the similar Mn values per one block, except for Mn per one block below 4×103 g mol-1.

This indicates that the Tm values of SC crystallites were mainly determined by the Mn

per one block but not by overall Mn, in agreement with the reported results for 4-LD and 1-LD copolymers54 and are not affected by Effect A or B.

The segments around the coinitiator moieties

cannot take part in crystallization, and such an effect will increase with decreasing molecular weight, resulting in lowered Tm values of 2-LD copolymers against Mn per one block at low Mn per one block below Mn = 4×103 g mol-1.

The estiamtion of Tg and Tm values of 2-LD copolymers at infinite

molecular weight (Tg∞ and Tm∞, respectively) and the excess free volume of the end groups of polymer chains (K), together with that of 1-LD is shown in Supporting Information (Figure S1). The relative crystallinity (Xr) of the melt-quenched 2-LD copolymers during heating was estimated from the DSC thermograms shown in Figure 4 using the following equation and is plotted in Figure S1 as a function of temperature: Xr (%) = 100 ∫0t (dHcc/dt)dt / ∫0∞(dHcc/dt)dt,

(1)

where dHcc denotes the measured enthalpy of cold crystallization during an infinitesimal time interval dt.

Overall crystallization kinetics traced by DSC in crystallization was analyzed according to the

Avrami theory,34-36 which is expressed by the following equation: 1 – Xr (%)/100 = exp(–ktcn),

(2)

where k is the crystallization rate constant. log [–ln (1– Xr/100)] = log k + n log tc.

Equation (2) can be transformed to equation (3): (3)

In the case of crystallization during heating or cooling, tc is the crystallization time and is defined by the following equation: tc = ⎥T – T0⎥/Φ,

(4)

where T0 is the temperature at which crystallization started and Φ (K min-1) is the heating rate.

T0

values were set to the lowest temperatures in the temperature ranges for Xr evaluation tabulated in Table 2.

The plots according to equation (3) for crystallization of 2-LD copolymers during heating



are given in Figure 6, together with those of 1-LD copolymers.

From these plots, the slope (n) and

the intercept (k) were obtained and the thus obtained n and k values are tabulated in Table 2.

To

avoid the effect of the noise level of Xr below 5% on the base line and avoid deviation from the theoretical curves for Xr exceeding 20%,37 we used the data for Xr in the range of 5–20% for estimating the n and k values.

Linear dependence of log [–ln (1 – Xr/100)] on log tc were observed for all plots.

In addition, the crystallization half time [tc(1/2)] was calculated using the following equation: tc(1/2)(cal) = [(ln 2)/k ]1/n.

(5)

The obtained values of n, k, and tc(1/2)(cal) and experimental tc(1/2) [tc(1/2)(exp)] are summarized in Table 2.

As seen in these tables, the tc(1/2)(cal) values for heating were very similar to the

tc(1/2)(exp) values.

Table 2.

Avrami exponent (n), crystallization rate constant (k), and crystallization half time [tc(1/2)] for 2-LD

and 1-LD copolymers. Arm number

Code

2

2-LD9 2-LD14 2-LD29 2-LD56 1-LD4 1-LD9 1-LD11

1

a)

Tcc Temperature range for Temperature range for (°C) Xr evaluation (°C) Avrami analysis (°C) 93.6 78.0–107.0 86.2–89.7 86.3 75.1–98.9 80.7–83.3 87.9 78.7–101.3 83.7–85.6 89.8 79.7–107.1 85.4–87.5 80.1 66.9–107.0 75.7–78.5 77.9 68.9–105.7 75.1–77.0 75.5 65.9–100.7 72.4–74.4

n 4.0 3.5 4.1 4.0 5.1 4.8 5.1

k tc(1/2)(exp) a) -n (min ) (min) 0.112 1.52 0.443 1.12 0.919 0.92 0.549 1.06 0.099 1.45 0.555 1.06 0.486 1.06

-0.4

-0.4 2-LD9 2-LD14 2-LD29 2-LD56

(a) 2-LD

Y = M0 + M1*X

(b) 1-LD -0.6

log [-ln (1- Xr /100)]

-0.6

-0.8

-1.0

-0.8

-1.0

M0

-0.94737

M1

3.952

R

0.9997

1-LD4 1-LD9 1-LD11

Y = M0 + M1*X M0

-1.2 -0.4

-1.2 -0.3

-0.2

-0.1 0.0 log tc

0.1

0.2

-0.4

5.0764

R

0.99964

M0

-0.35347

M1

3.5229

Y = M0 + M1*X

R

0.9999

M0

-0.25545

M1

4.7699

R

0.99975

M0 -0.036676 M1

4.0859

R

0.99982

-0.3

-0.2

Y = M0 + M1*X M0

14

-1.0032

M1

Y = M0 + M1*X

Y = M0 + M1*X

tc(1/2)(cal)a) (min) 1.58 1.14 0.93 1.06 1.46 1.05 1.07

tc(1/2)(exp) and tc(1/2)(cal) are experimental and calculated crystallization half times. The tc(1/2)(cal) values were calculated using following equation: tc(1/2)(cal) = [(ln 2)/k ]1/n.

log [-ln (1- Xr /100)]

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-0.26004

M1

3.9892

R

0.99992

ACS Paragon Plus Environment

Y = M0 + M1*X

-0.1 0.0 log tc

0.1

0.2

M0

-0.3133

M1

5.0658

R

0.99969

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Figure 6.

log[-ln(1-Xr/100)] of melt-quenched 2-LD (a) and 1-LD (b) copolymers during heating.

Part (b) was obtained from DSC thermograms reported in ref. 11.

As reported elsewhere, in the case of thermal nucleation with n values of 2, 3, 4, and ≥6 the crystalline growth morphologies are fibrillar (one dimensional), circular (two dimensional), spherical (three dimensional), and solid sheaf (three dimensional), respectively.38 The estimated n values are about 4 and 5 for 2-LD and 1-LD copolymers, irrespective of molecular weight of the polymers, indicating spherical and solid sheaf growth morphologies for 2-LD and 1-LD copolymers, respectively. The n values higher than 4 for 1-LD should have been caused by the presence of carboxyl terminal group.

This result strongly suggest that growth morphology depend on Effects A and/or B.

With

increasing Mn, the k values of 2-LD copolymers increased from 0.112 min-n at Mn = 9.05×103 g mol-1to 0.919 min-n at Mn = 2.85×104 g mol-1 and then decreased to 0.549 min-n at Mn = 5.64×104 g mol-1. Similarly, with increasing Mn, the k values of 1-LD copolymers increased from 0.099 min-n at Mn = 3.9×103 g mol-1to 0.555 min-n at Mn = 9.3×103 g mol-1 and then decreased to 0.486 min-n at Mn = 1.1×104 g mol-1.

3.3. Polarized Optical Microscopy To investigate the spherulitic morphology and growth behavior of 2-LD copolymers, polarized optical microscopy was performed.

Figure 7 shows the polarized optical photomicrographs of 2-LD

copolymers isothermally crystallized from the melt.

As seen in Figure 7, spherulites with a well

defined Maltese cross were observed for all 2-LD copolymers, irrespective of Mn, in agreement with SC spherulites of 1-LD copolymers,11 indicating the orientation of SC lamellae along the radial direction from the spherulite center and the lamellar orientation was not affected by Effects A and/or B.

In contrast, no Maltese cross was observed for the lowest molecular weight 4-LD,26 exhibiting the

branching architecture disturbs the lamella orientation. area increased with decreasing Tc.

As expected, the spherulite numbers per unit

The radial growth rate of spherulites (G) was estimated from the

photos obtained at different crystallization times and the thus obtained G values are plotted in Figure 8 for 2-LD copolymers, together with those reported for 1-LD copolymers.11


It should be noted the G


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Page 16 of 25

values could not be measured at lower Tc values, due to the rapid completion of SC crystallization. The maximum G [G(max)] and Tc which gives G(max) are summarized in Table 3.

With increasing

Mn, the maximum G [G(max)] values of 2-LD copolymers increased from 4.1 µm min-1 at Mn = 9.05×103 g mol-1 to 28.5 µm min-1 at Mn = 2.85×104 g mol-1 and then decreased to 13.5 µm min-1 at Mn = 5.64×104 g mol-1.

Similarly, with increasing Mn, the G(max) values of 1-LD copolymers increased

from 21.1 µm min-1 at Mn = 3.9×103 g mol-1 to 30.0 µm min-1 at Mn = 9.3×103 g mol-1 and then decreased to 25.5 µm min-1 at Mn = 1.1×104 g mol-1. 11

The decrease in G values at low and high Mn

values can be ascribed to the elevated Effect A and the effects of hydroxyl groups at both terminals and to the reduced segmental mobility, respectively.

At Mn = 9×103 g mol-1, G(max) was lower for

2-LD copolymer (4.1 µm min-1) than that for 1-LD copolymers (30.0 µm min-1).

This result

exhibits that Effects A and/or B (two hydroxyl terminal groups) decreased G(max) of 2-LD copolymer compared with that of 1-LD copolymer.


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Figure 7.


Polarized optical photomicrographs of 2-LD9 (a, b), 2-LD14 (c, d), 2-LD29 (e, f), and

2-LD56 (g, h) copolymers crystallized isothermally at shown Tc and crystallization time from the melt.



40

40

(a) 2-LD

(b) 1-LD

2-LD9 2-LD14 2-LD29 2-LD56

20

10

0 100

of Tc.

20

10

0

Figure 8.

1-LD4 1-LD9 1-LD11

30

G (µm . min-1)

30

G (µm . min-1)

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 25

120

140 160 Tc (°C)

180

200

100

120

140 160 Tc (°C)

180

200

Radial growth rate of spherulites (G) of 2-LD (a) and 1-LD (b) copolymers as a function

The data in part (b) are reproduced from ref. 11.

The nucleation constant (Kg) and the front constant (G0) of the samples were estimated using the nucleation theory established by Hoffman et al., which procedure and plots (Figure S3) are shown in the Supporting Information.

The estimated Kg and G0 values of 2-LD copolymers are shown in

Table S2, together with those reported for 1-LD copolymers.11

All 2-LD and 1-LD copolymers

except for 2-LD56 had only one Kg value, indicating that the crystalline growth mechanism was not varied by Tc or Tc range was too narrow to observe the change in the crystalline growth mechanism. The Kg values for lower and higher Tc range of 2-LD56 were 7.73×105 and 3.94×105 K2, respectively. The former Kg value was twice that of the latter, indicating that the former and latter Kg values are attributable to regimes III and II, respectively.

The transition Tc from regime III to regime II

[Tc(III-II)] was 170°C (Table 4), in agreement with the Tc(III-II) values of 180 and 170°C, reported for 1-armed and 2-armed PLLA/PDLA blends.20

Considering the Tc(III-II) values in the present and

reported studies, Kg values observed for other 2-LD and 1-LD copolymers in Tc range below 170°C can be ascribed to those of regime III.

4. Conclusions Two-armed PLLA-b-PDLA (2-LD) copolymers with different molecular weights (Mn = 9.05×103– 5.6×104 g mol-1) were synthesized.

Although it is difficult separate the Effects A and B, these effects 18

ACS Paragon Plus Environment

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on crystallization behavior were studied by comparing the present results for 2-LD copolymers (with coinitiator moiety of three sequential methylene units in the middle of molecules, and two hydroxyl terminal groups) with those reported for 1-LD copolymers (with no coinitiator moieties in the middle of molecules, and one hydroxyl and one carboxyl terminal groups).

The coinitiator moiety in the

middle of molecules functions as an impurity and causes the chain directional change in the middle of the molecules.

Only SC crystallization took place in 2-LD copolymers, irrespective of their Mn and

Tc, in agreement with those reported for 1-LD copolymers, confirming the neighboring PLLA and PDLA blocks facilitates SC crystallization and suppresses homo-crystallization and neither Effect A nor B affected the crystalline species.

The Tg values of 2-LD copolymers monotonously increased

with an increase in molecular weight and was determined only by Mn but were not affected by Effect A or B.

The Tcc dependence on Mn indicated that cold SC crystallization was disturbed for 2-LD

copolymers with Mn below 1×104 g mol-1, similar to the trends reported for SC crystallization of 1-LD copolymers with Mn below 1×104 g mol-1

22

homopolymer with Mn below 3×103 g mol-1.57

Higher Tcc values of 2-LD copolymers than those of

and reported for homo-crystallization of 2-L

1-LD copolymers exhibit that Effect A and/or B of both hydroxyl terminal groups disturbed cold SC crystallization.

Crystalline growth morphology of 2-LD copolymers during cold SC crystallization

estimated by Avrami analysis was spherical and the comparison of crystalline growth morphology between 2-LD and 1-LD copolymers revealed that crystalline growth morphology was influenced by Effects A and/or B.

The Tm or crystalline thickness of SC crystallites were determined by Mn per one

block not by overall Mn and not influenced by Effects A and B.

Also, G(max) values of 2-LD

copolymers compared to those of 1-LD copolymers were largely decreased by Effects A and/or B of both hydroxyl terminal groups.

The highest molecular weight 2-LD copolymer with Mn = 5.6×104 g

mol-1 showed two regimes (regime III and II), whereas other lower molecular weight 2-LD copolymer showed only regime III, in agreement with those reported for 1-LD copolymers.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.



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Thermal properties, Estimation with the Flory-Fox and Flory equations, and relative crystallinity, Estimation with the Hoffman theory (PDF)

Acknowledgments: This research was supported by JSPS KAKENHI Grant Number 16K05912 and MEXT KAKENHI Grant Number 24108005.


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Stereo Diblock Poly(lactide), Polymer 2016, 96, 167–181. 15. Han, L.; Xie, Q.; Jianna Bao, J.; Shan, G.; Bao, Y.; Pan, P. Click Chemistry Synthesis, Stereocomplex Formation, and Enhanced Thermal Properties of Well-Defined Poly(L-lactic acid)-b-poly(D-lactic acid) Stereo Diblock Copolymers. Polym. Chem., 2017, 8, 1006-1016. 16. Biela, T.; Duda, A.; Penczek, S. Enhanced Melt Stability of Star-Shaped Stereocomplexes As Compared with Linear Stereocomplexes. Macromolecules 2006, 39, 3710-3713. 17. Nagahama, K.; Nishimura, Y.; Ohya, Y.; Ouchi, T. Impacts of Stereoregularity and Stereocomplex Formation on Physicochemical, Protein Adsorption and Cell Adhesion Behaviors of Star-Shaped 8-Arms Poly(ethylene glycol)–Poly(lactide) Block Copolymer Films. Polymer 2007, 48, 2649–2658. 18. Inkinen, S.; Stolt, M.; Södergård, A. Effect of Blending Ratio and Oligomer Structure on the Thermal Transitions of Stereocomplexes Consisting of a D-lactic Acid Oligomer and Poly(L-lactide). Polym. Adv. Tech. 2011, 22, 1658–1664. 19. Shao, J.; Sun, J.; Bian, X.; Cui, Y.; Li, G.; Xuesi, C. Investigation of Poly(lactide) Stereocomplexes: 3-Armed Poly(L-lactide) Blended with Linear and 3-Armed Enantiomers. J. Phys. Chem. B, 2012, 116, 9983–9991. 20. Sakamoto, Y.; Tsuji, H. Stereocomplex Crystallization Behavior and Physical Properties of Linear 1-Arm, 2-Arm, and Branched 4-Arm Poly(L-lactide)/Poly(D-lactide) Blends: Effects of Chain Directional Change and Branching. Macromol. Chem. Phys. 2013, 214, 776–786. 21. Sakamoto, Y.; Tsuji, H. Crystallization Behavior and Physical Properties of Linear 2-Arm and Branched 4-Arm Poly(L-lactide)s: Effects of Branching. Polymer, 2013, 54, 2422–2434. 22. Tsuji, H.; Hayashi, T. Hydrolytic Degradation and Crystallization Behavior of Linear 2-Armed and Star-Shaped 4-Armed poly(L-lactide)s: Effects of Branching Architecture and Crystallinity. J. Appl. Polym. Sci., 2015, DOI: 10.1002/APP.41983. 23. Tsuji, H.; Ogawa, M.; Arakawa, Y., Homo- and Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo Diblock Copolymers of Crystalline and Amorphous Poly(lactide)s: Effects of Incorporation and Position of Amorphous Blocks. J. Phys. Chem. B, 2016, 120, 11052–11063. 24. Han, L.; Shan, G.; Bao, Y.; Pan, P. Exclusive Stereocomplex Crystallization of Linear and Multiarm Star-Shaped High-Molecular-Weight Stereo Diblock Poly(lactic acid)s. J. Phys. Chem. B 2015, 119, 14270–14279. 25. Tsuji, H.; Matsumura, N.; Arakawa, Y. Stereocomplex Crystallization and Homocrystallization of Star-Shaped Four-Armed Stereo Diblock Poly(lactide)s with Different L-Lactyl Unit Contents: Isothermal Crystallization from the Melt. J. Phys. Chem. B, 2016, 120, 1183−1193. 26. Tsuji, H.; Matsumura, N. Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo Diblock Poly(lactide)s with Different Molecular Weights: Isothermal Crystallization from the Melt. Macromol. Chem. Phys. 2016, 217, 1547−1557. 27. Tsuji, H.; Matsumura, N.; Arakawa, Y. Stereocomplex Crystallization and Homo-Crystallization


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of

Star-Shaped

Four-Armed

Stereo

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Poly(lactide)s

during Precipitation

and

Non-Isothermal Crystallization. Polym. J. 2016, 48, 1087–1093. 28. Tsuji, H.; Sugiura, Y. Crystallization Behavior of Linear 1-Arm and 2-Arm Poly(L-lactide)s: Effects of coinitiators. Polymer 2008, 49, 1385–1397. 29. Tsuji, H.; Yamamoto, J. Hydrolytic Degradation and Thermal Properties of Linear 1-Arm and 2-Arm Poly(DL-lactic acid)s: Effects of Coinitiator-Induced Molecular Structural Difference. Polym. Degrad. Stab. 2011, 96, 2229–2236. 30. Baez, J. E.; Marcos-Fernandez, A.; Galindo-Iranzo, P. Exploring the Effect of Alkyl End Group on Poly(L-lactide) Oligo-Esters. Synthesis and Characterization. J. Polym. Res. 2011, 18, 1137– 1146. 31. Tsuji, H.; Sugimoto, S. Long Terminal Linear Alkyl Group as Internal Crystallization Accelerating Moiety of Poly(L-lactide). Polymer 2014, 55, 4786–4798. 32. Tsuji,

H.;

Sugimoto,

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Accelerated

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Page 24 of 25

Figure captions Figure 1.

Molecular structure of 2-LD and 1-LD copolymers.

Factor A (presence of coinitiator

moiety in the middle of moelcules), Factor B (types of terminal groups).

Arrows indicate chain

(-O-CO-) directions. Figure 2.

WAXD profiles of 2-LD9 (a), 2-LD14 (b), 2-LD29 (c), and 2-LD56 (d) copolymers


The broken lines show the 2θ values for SC crystallites.

Figure 3.

Crystallinity (Xc) values of 2-LD9, 2-LD14, 2-LD29, and 2-LD56 copolymers estimated

by WAXD measurements, as a function of Tc. Figure 4.

DSC thermograms of 2-LD9 (a), 2-LD14 (b), 2-LD29 (c), and 2-LD56 (d) copolymers

crystallized isothermally at shown different crystallization Tc values or quenched (Tc = 0°C) from the melt. Figure 5.

Tg (a), Tcc (b), and Tm(S) (c) of melt-quenched 2-LD and 1-LD copolymers (Tc = 0°C) as

function of Mn and Tm(S) as a function of Mn per oble block.

Original data for 1-LD copolymers are

reported in ref. 22. Figure 6.

log[-ln(1-Xr/100)] of melt-quenched 2-LD (a) and 1-LD (b) copolymers during heating.

Part (b) was obtained from DSC thermograms reported in ref. 11. Figure 7.

Polarized optical photomicrographs of 2-LD9 (a, b), 2-LD14 (c, d), 2-LD29 (e, f), and

2-LD56 (g, h) copolymers crystallized isothermally at shown Tc and crystallization time from the melt. Figure 8. of Tc.

Radial growth rate of spherulites (G) of 2-LD (a) and 1-LD (b) copolymers as a function

The data in part (b) are reproduced from ref. 11.


Page 25 of 25

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TOC Graphic


Stereocomplex Crystallization of Linear Two-Armed Stereo Diblock

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