Stereocomplex Crystallization and Homocrystallization of Star-Shaped

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Stereocomplex Crystallization and Homo-Crystallization of StarShaped 4-Armed Stereo Diblock Poly(lactide)s with Different LLactyl Unit Contents: Isothermal Crystallization from the Melt Hideto Tsuji, Nobutsugu Matsumura, and Yuki Arakawa J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b11813 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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The Journal of Physical Chemistry

Stereocomplex Crystallization and Homo-Crystallization of Star-Shaped 4-Armed Stereo Diblock Poly(lactide)s with Different L-Lactyl Unit Contents: Isothermal Crystallization from the Melt Hideto Tsuji,* Nobutsugu Matsumura, 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:

The effects of L-lactyl unit content on star-shaped four-armed stereo diblock poly(lactide)

(4-LD) polymers and star-shaped four-armed poly(L-lactide) (4-L) on the isothermal crystallization from the melt were investigated. Solely stereocomplex (SC) crystallites were formed in equimolar 4-LD polymer with L-lactyl unit content of about 50%, irrespective of crystallization temperature (Tc) values.

4-L and 4-LD polymers with L-lactyl unit contents of 100 and 93% formed only

homo-crystallites, regardless of Tc, whereas only SC crystallites with traceable amounts were formed in 4-LD polymers with L-lactyl unit contents of 72 and 31% at a limited narrow Tc range of 110– 120°C, when crystallization was continued for as long as 24 h. About 20% deviation of L-lactyl unit content from 50% dramatically decreased the spherulite growth rate (G) values of SC crystallites, whereas a 7% decrease of L-lactyl unit content from 100% significantly decreased the G values of homo-crystallites and largely degreased the overall homo-crystallization rates.

Branching

architecture rather than diblock architecture hindered the simultaneous formation of SC and homo-crystallites of non-equimolar 4-LD polymers.

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1. Introduction The stereocomplexed materials prepared from enantiomeric poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e., poly(D-lactic acid) (PDLA)] are attracting much attention as advanced poly(lactide) (PLA)-based materials, because of their higher mechanical performance and hydrolytic/thermal degradation resistance compared to conventional PLA-based materials.1-5

In

addition to the investigations regarding the stereocomplex (SC) crystallization and properties of both linear one-armed PLLA (1-L)/one-armed PDLA (1-D) blends,6-28 those regarding the synthesis, crystallization, and properties of various types of linear one-armed stereo block PLA polymers composed of PLLA and PDLA blocks, including stereo diblock, triblock, and multiblock PLA polymers have been performed.29-45

In the linear one-armed stereo block PLA polymers, facile SC

crystallization can occur compared to that in 1-L/1-D blends when the polymers have weight-averaged molecular weight (Mw) values exceeding 1×105 g mol-1, due to the fact that PLLA and PDLA blocks are neighboring each other.2 On the other hand, star-shaped multi-armed PLLA and PDLA were intensively synthesized and the properties and crystallization behavior of the blends from both multi-armed PLLA and PDLA46–49 and from 1-L and multi-armed PDLA (or multi-armed PLLA and 1-D)50–53 were investigated. Star-shaped or branching architecture disturbed SC crystallization as estimated by spherulitic growth rate and final crystallinity of both star-shaped four-armed PLLA/PDLA (4-L/4-D) blends compared to the SC crystallization of both linear one- or two-armed PLLA/PDLA (1-L/1-D or 2-L/2-D) blends,49 which can be expected from the result of comparative study on the homo-crystallization of neat 2-L and 4-L.54

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) was determined by the molecular weight per one arm, irrespective of the arm number.49

Normally, SC crystallites are main crystalline species as

far as a polymer or system contains equimolar PLLA and PDLA chains and the molecular weight of at least one PLA, or its arm or block is lower than the order of 105 g mol-1. 49

Branching architecture of

PLLA and PDLA (arm number ≥ 13) was reported to facilitate the recrystallization of SC crystallites in the equimolar both multi-armed PDLA/PLLA blend during cooling from the melt compared to both one-armed 1-L/1-D blends.47 2 ACS Paragon Plus Environment

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For one-armed 1-L/four-armed 4-D or six-armed PDLA (6-D) blends, the Tm and enthalpy of melting (∆Hm) of SC crystallites were reported to depend strongly on blending ratio and the arm-length of PDLA.46

In the case of crystallization of three-armed PLLA (3-L)/three-armed PDLA

(3-D) or 1-D blends, the maximum Tm values were obtained for the middle molecular weight binary blends, and for similar molecular weight blends (or the molecular weight of linear one-armed PLA was similar to that of each branch of three-armed PLA enantiomers), with an increase in composition of three-armed PLA, the phase separation molecular weight decreased gradually (1-armed/1-armed > 1-armed/3-armed > 3-armed/3-armed).51

The effects of incorporated oligomeric 1-D, 4-D, or 6-D on

the hydrolytic degradation of high molecular weight 1-L were explored.50

It was found that

increasing the fraction of added oligomeric PDLA with a linear structure or with four alcoholic end-groups resulted in slower mass loss, due to higher degree of stereocomplexation, whereas addition of oligomeric PDLA with carboxylic chain-ends lowered degree of stereocomplexation and larger mass and molar mass loss, and also the release of degradation products increased and that increasing the number of alcoholic chain-ends from four to six decreased the degree of stereocomplexation, leading to faster mass loss.

Furthermore, miktoarm PLA having both PLLA and PDLA chains were

synthesized and their stereocomplxationability were investigated.55 In a previous study, two types of equimolar star-shaped 4-armed stereo diblock PLA polymers with PLLA and PDLA cores (4-LD and 4-DL, respectively) and L-lactyl unit (a half of L-lactide unit) content at about 50% were synthesized and their crystallization behavior was investigated.56

It was

found that solely SC crystallites were formed in 4-LD and 4-DL and their 50/50 blend, irrespective of crystallization temperature (Tc) in the Tc range of 100–150°C, and overall crystallization rate was highly elevated by blending 4-LD with 4-DL.

For the crystallization of equimolar 4-LD/1-L and

4-LD/1-D blends during solvent evaporation, SC crystallites were the main crystalline species, although the crystallinity was lower than those of 1-L/1-D blends, and 1-L or 1-D/one-armed stereo diblock PLA (1-LD) blends.57

Also, the incorporated 4-LD can act as a crystallization accelerating

agents for homo-crystallization of high molecular weight linear one-armed 1-L or 1-D and such an effect was higher for 4-LD added to 1-L than for 4-LD added to 1-D, which difference should be caused by the configurational disagreement and agreement of the shell of 4-LD with 1-L and 1-D, 3 ACS Paragon Plus Environment


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respectively.58

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However, as far as we are aware, the crystallization behavior of star-shaped stereo

diblock PLA polymers having different L-lactyl unit contents has not been reported in detail.

As is

well-known, the parameter of L-lactyl unit content is basic and crucial to determine the crystalline species (SC and homo-crystallites) and crystallization behavior of PLLA/PDLA blends or stereo block copolymers.2,4 To explore the detailed effects of L-lactyl unit content on isothermal crystallization of 4-LD and 4-L polymers from the melt for the first time, 4-L polymers with different molecular weights were synthesized and then 4-LD polymers with similar overall molecular weights and different L-lactyl unit contents were synthesized by ring-opening polymerization of D-lactide in the presence of 4-L polymers with different molecular weights. The isothermal crystallization behavior of the synthesized 4-LD and 4-L polymers at different crystallization temperature (Tc) values from the melt was investigated using wide-angle X-ray diffractometry (WAXD), differential scanning calorimetry (DSC), polarized optical microscopy (POM). Based on the comparison between the results obtained in the present study for 4-LD polymers and those reported for 1-LD polymers33,38,43,45 4-L/4-D49 and 1-L/1-D59-61 blends, the effects of branching and stereo diblock architectures on the isothermal crystallization are discussed.

2. Experimental section 2.1. Materials The precursors of 4-LD polymers, i.e., 4-L polymers were synthesized by bulk ring-opening polymerization of L-lactides (3g) (PURASORB L®, Purac Biochem BV, Gorinchem, The Netherlands) initiated with 0.03 wt% of tin(II) 2-ethylhexanoate (Nacalai Tesque, Inc., Kyoto, Japan) in the presence of different amounts of pentaerythritol (SigmaAldrich Japan, K.K., Tokyo, Japan) as the coinitiator (Table 1) at 140°C for 10 h.38,49,54,56 Synthesized 4-L polymers were purified by reprecipitation using chloroform and methanol as the solvent and non-solvent, respectively, and then dried in vacuo for at least 6 days. 4-LD polymers with different molecular weights or L-lactyl unit contents were synthesized by ring-opening polymerization of D-lactide (PURASORB D®, Purac Biochem BV) in toluene (6 and 3 mL for the synthesis of equimolar 4-LD polymer and non-equimolar 4-LD polymers, respectively) initiated with 0.3 wt% of tin(II) 2-ethylhexanoate in the presence of the 4 ACS Paragon Plus Environment

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purified and dried 4-L polymers as the coinitiator at 120°C for 36 h. 38,49,54,56

The total weight of 4-L

polymer and D-lactide in the feed was 2 and 1 g for the synthesis of equimolar 4-LD polymers and non-equimolar 4-LD polymers, respectively, and the weight ratios of precursor (4-L polymer) to D-lactide

mixed

are shown in Table 1. Synthesized 4-LD polymers were purified by precipitation using a solvent

of

chloroform

(Guaranteed

grade,

Nacalai

Tesque

Inc.)/1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (HPLC grade, Nacalai Tesque Inc.) (vol/vol = 95/5) and methanol as the solvent and non-solvent, respectively, and then dried in vacuo for at least 6 days. HFIP was added to chloroform to increase the solubility of SC crystallites. For preparation of isothermally crystallized samples, each sample was sealed in a test tube under reduced pressure, melted at 240°C for 3 min, crystallized at different crystallization temperature (Tc) values of 100– 200°C for 3 h, and quenched at 0°C for at least 5 min to stop further crystallization.

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

For GPC sample preparation, the mixed solvent of

chloroform (Guaranteed grade, Nacalai Tesque Inc.)/HFIP (vol/vol = 95/5) was used for 4-LD polymers, whereas chloroform was used for 4-L polymers. The estimated molecular characteristics of the synthesized polymers are summarized in Table 1. The Mn(GPC) values of 4-LD polymers were higher than that of precursors 4-L polymers, indicating the successful synthesis of 4-armed stereo diblock copolymers. Also, the Mn(NMR) values of the synthesized polymers were determined from the 400 MHz 1H NMR spectra obtained in deuterated chloroform (50 mg mL-1) by a Bruker BioSpin (Kanagawa, Japan) AVANCE III 400 using tetramethylsilane as the internal standard. Due to the overlapping of peaks for the methine protons of lactyl unit at the chain terminal and HFIP, we used only deuterated chloroform as the solvent even for 4-LD polymers. The Mn(NMR) values were estimated according to the following equation using the peak intensities for methine protons of lactyl units inside the chain 5 ACS Paragon Plus Environment


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(I1) and at the hydroxyl terminal (I2), observed at around 5.2 and 4.4 ppm, respectively:68,80 Mn(NMR) = 136.2 + (144.1 / 2) × 4[(I1 + I2) / I2],

(1)

where 136.2 g mol-1 is the molecular weight of the coinitiator, 144.1 g mol-1 is the molecular weight of L-

and D-lactides, and 4 is the arm number. The increases were observed for the Mn(NMR) values of

4-LD polymers after the second-step polymerization, in agreement with the result for Mn(GPC) values (Table 1).

Table 1. Characteristics and thermal properties of 4-L homopolymers and 4-LD copolymers synthesized in the present study.

Code a) 4-L8 4-L11 4-L14 4-L16 4-L14D2 4-L11D5 4-L8D8 4-L4D11

Coinitiator L-lactic acid

4-L14 4-L11 4-L8 4-L4

Lactide/Coinitiator (mol/mol) or (w/w) 68.4 d) 82.3 d) 117.1 d) 137.0 d) 7/93 e) 30/70 e) 50/50 e) 70/30 e)

Mn(NMR)b) (g mol-1) 8.37×103 1.05×104 1.43×104 1.63×104 1.60×104 1.62×104 1.62×104 1.53×104

Mn(GPC)b) (g mol-1) 1.66×104 2.19×104 2.73×104 3.07×104 2.90×104 3.12×104 3.52×104 2.64×104

Polymer Mw(GPC)/ [α]25589 c) b) Mn(GPC) (deg dm-1 g-1 cm3) 1.28 -173.5 1.12 -163.5 1.07 -172.1 1.12 -175.1 1.26 -151.9 1.08 -76.0 1.08 -3.4 1.24 68.1

L-lactyl

unit content (%) 93.3 71.7 51.0 30.6

a)

The figure following the letters indicates the Mn value/103. Mn and Mw are number- and weight-average molecular weights, respectively. c) Specific optical rotation measured in mixed solvent of chloroform/HFIP (vol/vol = 95/5). d) In mol/mol. e) In w/w. f) Mn(NMR) could not be obtained due to low fraction of hydroxyl terminal group and the molecular weight of PDLA segments for the code 4-L16D15 was estimated from L-lactyl unit content.

b)

The specific optical rotation ([α]25589) values of 4-LD and 4-L 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 D-lactyl unit contents of 4-LD polymers were estimated by the following equation: L-lactyl

unit content (%) = 100 × {[α]25589(PLLA) + [α]58925} / {2 × [α]25589(PLLA)}

(2),

where [α]25589(PLLA) is that of 4-L16 (–175.1 deg dm-1 g-1 cm3) measured in the mixed solvent of chloroform/HFIP (vol/vol = 95/5). The L-lactyl unit contents evaluated for 4-LD polymers varied depending on the L-lactyl unit contents in the feed. Table 1 summarizes the molecular characteristics of 4-L and 4-LD polymers used in the present study.

Here, the 4-L and 4-LD polymers are


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abbreviated as 4-LX and 4-LXDY, respectively, and the X and Y values in the codes of 4-LX and 4-LXDY indicate the total Mn(NMR)/103 values of PLLA and PDLA blocks, correspondingly.

In

addition to the increased Mn(GPC) and Mn(NMR) values of 4-LD after the second-step polymerization, their very narrow molecular weight distribution values below 1.3 and decreased L-lactyl unit contents indicate the successful synthesis of star-shaped 4-armed stereo diblock PLA polymers. The thermal properties of samples were measured 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 using a Rigaku (Tokyo, Japan)

RINT-2500 equipped with a Cu-K source (λ = 1.5418 Å), which was operated at 40 kV and 200 mA. The isothermal spherulite growth of the 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 1 min, cooled at 100°C min-1 to an arbitrary Tc, and then held at the same temperature (spherulite growth was observed here). With the same heating and cooling program utilized for the observation of isothermal spherulite growth, the overall crystallization behavior of samples during isothermal crystallization was monitored by measuring the light intensity transmitted through the samples using BX-50 equipped with LK-600PM and an As One (Osaka, Japan) LM-332 photometer.

3. Results and Discussion 3.1. Crystalline species and crystallinity To determine the crystalline species and crystallinity of 4-LD and 4-L polymers after isothermal crystallization for 3 h form the melt, WAXD measurements were performed. Figure 1 shows the WAXD profiles of the 4-LD and 4-L polymers with the different L-lactyl unit contents. In this figure, 7 ACS Paragon Plus Environment


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the values in the parentheses are those of L-lactyl unit contents. As seen in Figure 1, the equimolar 4-LD polymer [4-L8D8(51)] was crystallizable for the Tc ranges from 100 to 160°C and only crystalline diffraction peaks at 12, 21, and 24° ascribed to SC crystallites2 were observed for all Tc values, indicating the sole formation of SC crystallites without formation of homo-crystallites, irrespective of Tc. On the other hand, 4-L16(100) and 4-L14D2(93) were crystallizable in the Tc range from 100 to 120°C. 4-L16(100) and 4-L14D2(93) had only α- or δ-form homo-crystalline peaks at 15, 17, and 19° and 4-L16(100) had α-form homo-crystalline peaks at 22.5°.62–64 The 4-L11D5(72) and 4-L4D11(31) were amorphous for the Tc range from 100 to 200°C and crystallization time (tc) of 3 h. This is marked contrast with polymer blends of linear one-armed 1-L/1-D blends59 or linear one-armed 1-LD polymers43,45 having an L-lactyl unit content around 30 and 70%, which were crystallizable and formed both SC and homo-crystallites.

This suggests that

branching architecture rather than diblock architecture disturbs crystallization and the formation of both crystalline species. The crystallinity (Xc) values were estimated from the WAXD profiles in Figure 1 and are plotted in Figure 2 as a function of Tc. The Xc values increased with an increase in Tc and started to decrease when the Tc values of polymers approached to their Tm values, in agreement with the reported results for neat 1-L, 2-L, three-armed PLLA (3-L),65 4-L polymers,54, 1-L/1-D, 2-L/2-D, 4-L/4-D blends,49 and 1-LD polymers.38

The maximum Xc values (ca. 50%) for 4-LD polymers were comparable with

those for 1-LD polymers38 but lower than those for 1-L/1-D, 2-L/2-D, and 4-L/4-D blends (85, 70, and 60 %, respectively)49 and 2-L and 4-L polymers (80 and 70%).54

However, we do not discuss

maximum Xc values further due to the difference in tc between the present and reported studies.

The

Xc values of 4-L14D2 with L-lactyl unit content of 93% were slightly lower than those of 4-L16 with L-lactyl

unit content of 100%, indicating the incorporation of short lengths of PDLA blocks had a very

small effect on the final crystalline fraction.


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Figure 1.

WAXD profiles of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl

unit contents: (a) 4-L16 (100), (b) 4-L14D2 (93), (c) 4-L11D5 (72), (d) 4-L8D8 (51), and (e) 4-L4D11 (31). The values in the parentheses are those of L-lactyl unit content.

Figure 2. Crystallinity (Xc) of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl unit contents. The values in the parentheses are those of L-lactyl unit content.



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Figure 3. Crystalline species in 4-LD and 4-L polymers (a), 1-armed PLLA-b-PDLA (1-LD) polymers,33 and low molecular weight 1-armed PLLA/1-armed PDLA (1-L/1-D) blends59 (c) having different L-lactyl unit contents crystallized at different Tc values from the melt.

The crystalline species formed in the 4-LD and 4-L polymers having the different L-lactyl unit contents are summarized in Figure 3(a), together with those of 1-LD [Figure 3(b)]33 and low molecular weight 1-L/1-D blends [Figure 3(c)].59

As seen, considering that 1-LD should be

crystallizable at L-lactyl unit content of 100%, the crystallizable L-lactyl unit content range of 4-LD and 4-L polymers was narrower than that of 1-LD polymers and 1-L/1-D blends.

In the case of

1-L/1-D blends, crystalline species changed from only SC crystallites to SC and homo-crystallites, and then to only homo-crystallites, with the deviation of L-lactyl unit content from 50%. In the case of 1-LD polymers, for the relatively low Tc range below 100 °C, crystalline species changed from only SC crystallites to SC and homo-crystallites, and then to only homo-crystallites, with deviation from 50% of L-lactyl unit content, whereas for the relatively high Tc range above 110°C, crystalline species changed directly from only SC crystallites to only homo-crystallites, with the deviation of L-lactyl unit content from 50%.33

Also, the crystalline species change similar to 1-L/1-D blends and 1-LD

polymers at relatively low Tc range was observed for 1-LD polymers when 1-LD polymers were crystallized during slow cooling from the melt.43,45 However, in the case of 4-LD and 4-L polymers, crystalline species changed from only SC crystallites to amorphous, and then to only homo-crystallites, with the deviation of L-lactyl unit content from 50%, reflecting both crystalline species were not formed in 4-LD polymers at the intermediate L-lactyl unit contents for tc up to 3 h. These findings are indicative of the fact that the branching architecture rather than the diblock architecture disturb


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crystallization and the simultaneous formation of SC and homo-crystallites at the intermediate L-lactyl unit contents between 0–50% or 50–100%.

3.2. Thermal properties To investigate the thermal properties of 4-LD and 4-L polymers after isothermal crystallization from the melt, DSC measurements were carried out. Figure 4 shows the DSC thermograms of the 4-LD and 4-L polymers with the different L-lactyl unit contents. The glass transition, cold crystallization, and melting peaks were observed in the ranges of 34–55°C, 90–119°C, and 119–195°C, respectively, except for 4-L11D5 and 4-L4D11, wherein only glass transition appeared.

Thermal properties

estimated from the DSC thermograms in Figure 4 are tabulated in Table S1. Absence of melting peak or non-crystallizability of 4-L11D5 and 4-L4D11 agrees well with the WAXD results. The cold crystallization was observed for the crystallizable polymers at relatively high Tc values, reflecting imperfect crystallization or no crystallization of the polymers during isothermal crystallization.

For

the equimolar 4-LD polymer (4-L8D8), the melting peaks of SC crystallites had a small shoulder at a lower temperature (shown with arrows), which should be ascribed to the melting of original SC crystallites formed during isothermal crystallization from the melt, whereas the main high peak at a higher temperature is attributable to the SC crystallites formed by recrystallization of original SC crystallites.

This is evidenced by Tc dependent and independent Tm values of low and high

temperature melting peaks, respectively.

Similar to the equimolar 4-LD polymer, 4-L16 and

4-L14D2 had two melting peaks, which should be ascribed to melting of original homo-crystallites formed during isothermal crystallization from the melt and the homo-crystallites formed by recrystallization of original crystallites.



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Figure 4. L-lactyl

DSC thermograms of melt-crystallized 4-LD and 4-L polymers having the different

unit contents: (a) 4-L16 (100), (b) 4-L14D2 (93), (c) 4-L11D5 (72), (d) 4-L8D8 (50), and (e)

4-L4D11 (31). Arrows indicate the melting temperature of SC crystallites formed during isothermal crystallization from the melt. The values in the parentheses are those of L-lactyl unit content.

The lower and higher Tm, abbreviated Tm(L) and Tm(H), respectively, are plotted in Figure 5 as a function of Tc. As can be expected from the ascription of each Tm, Tm(L) values depended on Tc, except for the samples having a cold crystallization peak in the DSC thermograms, whereas most Tm(H) values were independent of Tc. The linear parts of plots for 4-L8D8 and 4-L16 are extracted from Figure 5 and replotted in Figure S1 as Hoffman-Weeks plots. The thus obtained equilibrium Tm (Tm0) were 266.0 and 163.9°C for 4-L8D8 and 4-L16, respectively.

The Tm0 value (266.0°C) of SC

crystallites for the 4-L8D8 were comparable with or lower than 279°C reported linear one-armed equimolar 1-L/1-D blends.60

Also, 163.9°C of homo-crystallites for 4-L16 was much lower than

those reported for linear one-armed 1-L (212 and 215°C).66,67

The low Tm0 values of SC of equimolar

4-LD and 4-L polymers are attributable to the fact that the low molecular weights of PLLA and PDLA 12 ACS Paragon Plus Environment

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blocks compared to 1-L/1-D blends and 1-L polymers and the branching architecture and/or diblock architecture disturbed the growth of SC and homo-crystallites. However, due to the lack of reported Tm0 data for 1-LD polymers, we could not discuss which architectural parameter is dominant to determine Tm0 of 4-LD polymers.

Figure 5.

Lower and higher Tm [Tm(L) (a) and Tm (H) (b), respectively] of melt-crystallized 4-LD

and 4-L polymers having the different L-lactyl unit contents as a function of Tc. The values in the parentheses are those of L-lactyl unit content.

3.3. Polarized Optical Microscopy To inquire the spherulitic morphology and growth behavior of 4-LD and 4-L polymers, POM observation was performed. Figure 6 shows the polarized optical photomicrographs of the 4-LD and 4-L polymers with the different L-lactyl unit contents. The equimolar 4-LD polymer (4-L8D8) had Maltase crosses and the spherulitic radii increased with increasing Tc. Maltese crosses were also observed for 4-L and non-quimolar 4-LD polymers similar to those of equimolar 4-LD polymers and the spherulitic radii became higher with an increase in Tc, except for 4-L14D2.

The overall

brightness and the contrast between the bright and dark regions were low and the dark lines along the radius direction were observed for 4-L11D5 and 4-L4D11. These findings reflect the disturbed orientation of the lamellae and low fractions of crystalline regions in the spherulites. It is surprising to note that 4-L11D5 and 4-L4D11 were amorphous after crystallization for 3 h when monitored by WAXD and DSC, despite the fact the spherulites were formed in both polymers.

For further

confirmation of crystalline species of 4-L11D5 and 4-L4D11, prolonged crystallization was carried out 13 ACS Paragon Plus Environment


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for as long as 24 h at 115°C from the melt (Figure S2). From Figure S2, it is evident that only SC crystallites were formed in 4-L11D5 and 4-L4D11 with L-lactyl unit contents of 72 and 31% after crystallization for 24 h.

This means that 4-LD could form a WAXD traceable amount of SC

crystallites for the wide L-lactyl unit content range of at least 31–72 %, although the crystallization rates were very low at L-lactyl unit contents of 31 and 72 %. As shown in Figure 3(b), in the case of 1-LD polymers, only homo-crystallites were formed at L-lactyl unit content of 80%, irrespective of Tc, whereas both SC and homo-crystallites were formed at L-lactyl unit content of 66% for Tc below 100°C.33

However, in the case of 4-L11D5 and 4-L4D11, although the formation of SC crystallites

was confirmed, crystallizable Tc range was too narrow to monitor the effects of Tc on crystalline species. The radial growth rate of spherulites (G) was estimated from the photos obtained at different tc values and the thus obtained G is plotted in 7(a). The G values of 4-LD and 4-L polymers having the different L-lactyl unit contents decreased in the following order: 4-L8D8(51) > 4-L16(100) > 4-L14D2(93) > 4-L11D5(72) > 4-L4D11(31). Interestingly, the spherulites of SC crystallites in 4-L8D8 had G values higher than those of spherulites of homo-crystallites in 4-L16, although the crystallizable PLLA and PDLA block lengths of 4-L8D8 were half of PLLA arm length of 4-L16. This result confirms the higher crystallization rate of SC crystallites compared to that of homo-crystallites.61

However, the G values of four-armed 4-LD polymers could not be compared

with those of one-armed 1-LD polymers because the G values of 1-LD polymers could not be measured at a low Tc below 135°C due to the high numbers of spherulites per unit area, which caused the rapid completion of crystallization.38


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4-L16(100)　(100°C )　

4-L16(100)　(120°C )　

4-L14D2(93) (100°C )

4-L14D2(93) (110°C )

4-L11D5(72)　(115°C, 220 min)　 4-L8D8(51) (120°C )

4-L8D8(51) (160°C )

4-L4D1１(31)　(115°C, 342 min)　

1 0 0 µm

Figure 6. Polarized optical photomicrographs of 4-LD and 4-L polymers having the different L-lactyl

unit contents after the completion of crystallization (except for 4-L11D5 and 4-L4D11, for

which crystallization times are shown) at shown Tc from the melt. The values in the parentheses in are those of L-lactyl unit content.



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Figure 7. Radial growth rate of spherulites (G) (a) and ln G + 1500/R(Tc–T∞) (b) of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl unit contents. The values in the parentheses are those of L-lactyl unit content.

The nucleation constant (Kg) and the front constant (G0) of the samples were estimated using the nucleation theory established by Hoffman et al.,68,69 in which G can be expressed by the following equation: G = G0 exp [–U*/ R(Tc–T∞)] exp [– Kg / (Tc∆Tf)],

(3)

where ∆T is supercooling Tm0–Tc when Tm0 is equilibrium Tm, f is the factor expressed by 2Tc/(Tm0+Tc) which accounts for the changes in the heat of fusion as the temperature is decreased below Tm0, U* is the activation energy for the transportation of segments to the crystallization site, R is the gas constant, and T∞ is the hypothetical temperature where all motion associated with viscous flow ceases. The ln G + 1500/R(Tc-T∞) of the samples are plotted in Figure 7(b) as a function of 1/(Tc∆Tf), using Tm0 = 279°C60 of SC crystallites for equimilar 4-LD polymers and 4-L11D5 and 4-L4D11 and Tm0 = 212°C66 of homo-crystallites for equimilar 4-L16 and 4-L14D2, U*=1500 cal mol-1 and T∞ = Tg – 30 K.

Also,

as Tg values, those of melt-quenched samples were used (56, 43, 40, 46, and 46°C for 4-L16, 4-L14D2, 4-L11D5, 4-L8D8, and 4-L4D11, respectively). The plot in Figure 7(b) gives Kg as a slope and the intercept ln G0 and thus estimated Kg and G0 values are tabulated in Table 2. All samples had only one Kg value, indicating that the crystalline growth mechanism was not altered by Tc. The Kg value of equmimolar 4-DL polymer (4-L8D8) [Mn(GPC) = 3.52×104 g mol-1] was 9.65×105 K2, which were lower than those reported for equimilar 4-LD (1.51×106 K2) and 4-DL (1.44×106 K2) having Mn(GPC) 16 ACS Paragon Plus Environment

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values of 1.30 and 1.35×104 g mol-1, respectively,56 and those of non-equimolar 4-LD polymers (4-L11D5 and 4-L4D11) (1.99 and 1.89×106 K2, respectively), but higher than those of 4-L16 and 4-L14D2 (2.74 and 3.01×105 K2, respectively).

The regime change was not observed for 4-LD and

4-L polymers, in agreement with equimolar 1-LD polymers,38 equimolar 4-L/4-D49 and 1-L/1-D61 blends. The dependence of G0 values on crystalline form and L-lactyl unit content were the similar to that of Kg values.

Table 2.

Tc which gives maximum G, G(max) [Tc(max)], front constant (G0), and nucleation

constant (Kg). Code

Tc(max)

G(max) (µm min-1) 6.3 1.5 0.057 12.3 0.030

(°C) 4-L16 4-L14D2 4-L11D5 4-L8D8 4-L4D11

130 110 115 130 115

G0 (µm min-1) 7.52×107 1.88×107 2.30×1018 1.66×1012 2.36×1017

Kg (K2) 2.74×105 3.01×105 1.99×106 9.65×105 1.89×106

3.3. Overall crystallization behavior The overall isothermal crystallization behavior of the samples at different Tc values were estimated by the time change of light intensity transmitted through a sample (I) using polarized optical microscope. The I increases with an increase in crystallinity and finally levels off when crystallization completes. We used the I defined by the following equation as an index of relative crystallinity (Xr):40,70,71 Xr (%) = 100(It – I0)/(I∞– I0)

(4),

where It and I0 are the I values at crystallization time (tc) = t and 0, respectively, and I∞is the I value when it leveled off. Thus obtained typical Xr values are plotted in Figure 8(a) as a function of tc (all plots are shown in Figure S3). As seen in Figure 8(a), although the Xr values of equimolar 4-LD polymer (4-L8D8) and pure L-polymer (4-L16) increased rapidly, whereas rather slow increase of Xr values were observed for 4-L14D2. The latter result indicates the incorporation of short lengths of PDLA blocks in 4-L largely decelerate homo-crystallization.



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Figure 8. Relative crystallinity (Xr ) (a) and –ln(1 – Xr/100) (b) at shown Tc of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl unit contents. The values in the parentheses are those of L-lactyl unit content.

Isothermal crystallization kinetics traced by light intensity measurements was analyzed with the Avrami theory,72–74 which is expressed by the following equation: 1-Xr (%)/100 = exp (–ktcn)

(5),

where k is the crystallization rate constant. Equation (5) can be transformed to equation (6): ln [–ln (1-Xr /100)] = ln k + n ln tc

(6).

To avoid deviation from the theoretical curves, as suggested by Mandelkern et al. and Lorenzo et al.75,76 we used Xr in the range of 5-20% for estimating n and k. Typical plots with equation (6) are shown in Figure 8(b) (all plots are shown in Figures S4). The plots with equation (6) give n as slope and intercept ln k. Thus obtained n and k values are listed in Table 3. The n values of equimolar 4-LD (4-L8D8) are around 2, except for that at Tc = 130°C (4.10), whereas those of 4-L16 and 4-L14D2 varied from 2.3–3.9 depending on the type of sample and Tc. Assuming the thermal nucleation,77 the n values around 2 of 4-L8D8 reflect the line growth geometry, respectively, whereas the n values of 2.3–3.9 of 4-L16 and 4-L14D2 mean the change from line growth geometry to spherical growth geometry, depending on the type of sample and Tc. 18 ACS Paragon Plus Environment

In other words, the crystallite

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growth geometries of 4-LD and 4-L polymers depended on the L-lactyl unit content and Tc. In addition, crystallization half time [tc(1/2)] was calculated using the following equation: tc(1/2)(Cal.) = [(ln 2)/k ]1/n (7) The values of tc(1/2)(Cal.) thus obtained are tabulated in Table 3. The tc(1/2)(Cal.) values were very similar to the experimental tc(1/2) [tc(1/2)(Exp.)] values. The tc(1/2)(Exp.) values of the samples are plotted in Figure S5. As, expected, the tc(1/2)(Exp.) values increased upon approaching Tm values.

Table 3. Avrami exponent (n), crystallization rate constant (k), and crystallization half time [tc(1/2)] for isothermal crystallization. Code

Tc

n

4-L16

100 110 120 100 110 130 140 150 160

2.85 3.89 2.26 3.72 2.56 4.10 2.00 2.48 2.09

4-L14D2 4-L8D8

a)

k (min-n) 5.66×10-3 7.64×10-3 4.59×10-4 2.57×10-5 3.59×10-5 7.50×10-1 3.14×10-1 1.98×10-1 2.61×10-4

tc(1/2)(Exp.) a) (min) 6.1 3.5 24.8 16.4 48.3 1.1 1.7 1.9 43.9

tc(1/2)(Cal.)a) (min) 5.4 3.2 25.4 15.5 46.9 0.98 1.5 1.7 43.4

tc(1/2)(Exp.) and tc(1/2)(Cal.) are experimental and calculated crystallization half times.

4. Conclusions The isothermal crystallization behavior of 4-LD and 4-L polymers with different L-lactyl unit contents from the melt was investigated for the first time. Solely SC crystallites were formed for equimolar 4-LD polymer (4-L8D8) with L-lactyl unit contents of about 50%, irrespective of Tc values. 4-L and 4-LD polymers with L-lactyl units of 100 and 93% formed only homo-crystallites, regardless of Tc, whereas only SC crystallites were formed in 4-LD polymers with L-lactyl unit content of 72 and 31% at a limited narrow Tc range of 110–120°C. WAXD traceable amounts of SC crystallites were formed in 4-LD polymers for L-lactyl unit contents of 31 and 72%, when crystallization was continued for as long as 24 h. About 20% deviation of L-lactyl unit content from 50% dramatically decreased



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the G values of SC crystallites (0.03–0.06 µm min-1 at L-lactyl unit contents of 72 and 31%) compared to 2–15 µm min-1 at L-lactyl unit contents of 51%, and a 7% decrease of L-lactyl unit content from 100% or incorporation of 7% of D-lactyl units significantly decreased the G values of homo-crystallites (0.3-2 µm min-1 at L-lactyl unit content of 93%) compared to 3–6 µm min-1 at L-lactyl

unit content of 100%. The G values of 4-LD and 4-L polymers having the different L-lactyl

unit contents decreased in the following order: 4-L8D8(51) > 4-L16(100) > 4-L14D2(93) > 4-L11D5(72) > 4-L4D11(31). The 7% decrease of L-lactyl unit content from 100% largely degreased the overall homo-crystallization rates. The Tm0 value of equimolar 4-LD polymer was 266.0°C, which was comparable with or lower than the value reported earlier (279°C). The WAXD, DSC, and POM results for crystalline species, crystallinity, and maximum radial growth rate of spherulites values indicate that branching architecture rather than diblock architecture hindered the simultaneous formation of SC crystallites and homo-crystallites of non-equimolar 4-LD polymers at intermediate L-lactyl

unit contents of 31 and 72%. The crystallite growth geometries of 4-LD and 4-L polymers

depended on the L-lactyl unit content and Tc.

Supporting Information. Thermal properties of 4-LD and 4-L polymers (Table S1, Figure S1), WAXD profiles of 4-L11D5(72) and 4-L4D11(31) after long time crystallization (Figure S2), relative crystallinity of 4-LD and 4-L polymers (Figure S3), –ln(1 – Xr/100) of 4-LD and 4-L polymers (Figure S4), and experimental crystallization half time of 4-LD and 4-L polymers (Figure S5).

Acknowledgments: This research was supported by JSPS KAKENHI Grant Number 24550251 and MEXT KAKENHI Grant Number 24108005


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Figure captions

Figure 1.

WAXD profiles of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl

unit contents: (a) 4-L16 (100), (b) 4-L14D2 (93), (c) 4-L11D5 (72), (d) 4-L8D8 (51), and (e) 4-L4D11 (31). The values in the parentheses are those of L-lactyl unit content.

Figure 2. Crystallinity (Xc) of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl unit contents. The values in the parentheses are those of L-lactyl unit content.

Figure 3. Crystalline species in 4-LD and 4-L polymers (a), 1-armed PLLA-b-PDLA (1-LD) polymers,33 and low molecular weight 1-armed PLLA/1-armed PDLA (1-L/1-D) blends59 (c) having different L-lactyl unit contents crystallized at different Tc values from the melt. Figure 4. DSC thermograms of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl

unit contents: (a) 4-L16 (100), (b) 4-L14D2 (93), (c) 4-L11D5 (72), (d) 4-L8D8 (50), and (e)

4-L4D11 (31). Arrows indicate the melting temperature of SC crystallites formed during isothermal crystallization from the melt. The values in the parentheses are those of L-lactyl unit content.

Figure 5.

Lower and higher Tm [Tm(L) (a) and Tm (H) (b), respectively] of melt-crystallized 4-LD

and 4-L polymers having the different L-lactyl unit contents as a function of Tc. The values in the parentheses are those of L-lactyl unit content.

Figure 6. Polarized optical photomicrographs of 4-LD and 4-L polymers having the different L-lactyl

unit contents after the completion of crystallization (except for 4-L11D5 and 4-L4D11, for

which crystallization times are shown) at shown Tc from the melt. The values in the parentheses in are those of L-lactyl unit content.

Figure 7. Radial growth rate of spherulites (G) (a) and ln G + 1500/R(Tc–T∞) (b) of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl unit contents. The values in the parentheses are those of L-lactyl unit content.

Figure 8. Relative crystallinity (Xr ) (a) and –ln(1 – Xr/100) (b) at shown Tc of melt-crystallized 4-LD and 4-L polymers having the different L-lactyl unit contents. The values in the parentheses are those of L-lactyl unit content.



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Graphical Abstract


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Stereocomplex Crystallization and Homocrystallization of Star-Shaped

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