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Biodegradable Compatibilizers for Poly(hydroxyalkanoate)/ Poly(#-caprolactone) Blends Through Click Reactions With End-Functionalized Microbial Poly(hydroxyalkanoate)s Takafumi Oyama, Shingo Kobayashi, Tetsuo Okura, Shunsuke Sato, Kenji Tajima, Takuya Isono, and Toshifumi Satoh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00897 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Sustainable Chemistry & Engineering

Biodegradable Compatibilizers for Poly(hydroxyalkanoate)/Poly(ε-caprolactone) Blends Through Click Reactions With End-Functionalized Microbial Poly(hydroxyalkanoate)s

Takafumi Oyama,† Shingo Kobayashi,‡ Tetsuo Okura,§ Shunsuke Sato,‡ Kenji Tajima,¶ Takuya Isono,¶,* and Toshifumi Satoh¶,*

†Graduate

School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan

‡Bioproducts

Research Group, Biotechnology Research Laboratories, KANEKA, Takasago 676-8688, Japan

§Polymer

Fundamental Research Team, BDP Technology Laboratories, KANEKA, Takasago 676-8688, Japan

¶Division

of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

*Corresponding

authors: [email protected] (T.I.); [email protected] (T.S.)

1 ACS Paragon Plus Environment

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ABSTRACT Poly(hydroxyalkanoate)s (PHAs) have gained significant attention because they readily biodegrade even under environmental conditions; however, expanding the applications of these polymers to a number of fields is hindered by their brittle nature. Blending with a soft biodegradable polymer, such as poly(ɛ-caprolactone) (PCL), is currently the best solution for improving the toughness of a PHA material without sacrificing biodegradability; consequently, the development of biodegradable compatibilizers is strongly desired. This article describes a systematic investigation into the compatibilizing abilities of a number of architecturally varied block copolymers (BCPs) composed of microbially derived poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(3HB-co-3HHx); PHBH as the A block) and PCL (as the B block), including AB-, A2B-, AB2-, ABA-, and A2BA2-type BCPs. These BCPs were precisely synthesized through the click reactions of azido-functionalized PCLs with microbially produced propargyl-end-functionalized PHBH. Transmission electron microscopy and tensile-testing experiments reveal that the mechanical properties of PHBH/PCL/BCP (= 67/23/10 wt%) ternary blends are highly dependent on the choice of BCP architecture, and are closely related to their morphologies A remarkable five-to-ten-fold increase in elongation at break compared to that of the BCP-free blend (PHBH/PCL = 75/25) was achieved by the addition of the ABA- or A2BA2-type BCP as the compatibilizer.

KEYWORDS: Biodegradable plastic; Block copolymer; Aliphatic polyester; Microbial polyester; Macromolecular architectures, Click chemistry 2 ACS Paragon Plus Environment

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INTRODUCTION Marine plastic pollution has recently received tremendous societal attention and is currently one of the most serious worldwide environmental concerns.1 Many conventional plastic products are not biodegradable and therefore persist as microplastics for many years in the ocean. Hence, ocean-biodegradable plastic materials are of high demand as alternatives to conventional non-degradable plastics. Poly(lactic acid) (PLA) is a well-known biodegradable polymer and is recognized to be one of the most promising substitutes for non-biodegradable plastics. However, PLA degrades slowly or negligibly both in soil and water under environmental conditions.2−5 On the other hand, poly(hydroxyalkanoate)s (PHAs), which are isotactic microbial polyesters composed of hydroxy fatty acid monomer units, have gained particular attention due to their good biodegradabilities even under environmental conditions.6−9 The fact that PHAs can be synthesized from biomass feedstock, such as beet/cane molasses and plant oils, through bacterial fermentation makes them even more attractive from sustainability and renewability perspectives. Importantly, the physical properties of a PHA can be controlled by adjusting the structure and composition of the comonomer.10,11 Indeed, PHAs containing over 150 types of monomer have been synthesized by choosing either the microorganism or fermentation conditions.12 For example, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(3HB-co-3HHx); PHBH) displays a broad range of thermal and mechanical properties that depend on the comonomer unit composition.13−15 Since PHBH can be tailored to produce materials that range from semi-crystalline plastics to amorphous elastomers,13,16,17 it is expected to be useful in a variety of applications, such as disposable plastic bags, food packaging, agricultural mulch films, and medical materials.13,18 However, most PHAs suffer from secondary crystallization that 3 ACS Paragon Plus Environment


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engenders them with brittleness and limits their practical applications.19 Blending a PHA with a soft polymer is an effective approach for overcoming the above-mentioned drawback. For this purpose, poly(ɛ-caprolactone), which is a biodegradable and semi-crystalline flexible polymer with a low glass-transition temperature of −60 °C, is a good blend-partner candidate for PHAs.20 Inoue et al. reported that the blending of a small amount of PCL (2.5−5.0 wt%) with PHBH produced a material with improved elongation at break due to the effective delocalization of applied stress. However, the blend exhibited heterogeneous macrophase separation when more than 5.0% PCL was added, which resulted in poor mechanical properties.21,22 To maximize synergism through polymer blending, it is important that the immiscible blend components are compatibilized. An immiscible polymer blend can be compatibilized by the addition of a block copolymer (BCP) composed of polymer units miscible with each blend component. Despite the strong need to improve the physical properties of PHAs, compatibilizers composed of PHA segments have rarely been reported. Based on a ring-opening-polymerization approach, Doi et al. successfully synthesized a diblock copolymer consisting of poly[(R,S)-3-hydroxybutyrate] (atactic P[(R,S)-3HB]) and PCL segments as the compatibilizer for a poly[(R)-3-hydroxybutyrate] (isotactic P[(R)-3HB])/PCL blend.23 However, the atactic P[(R,S)-3HB] segment was only miscible with the amorphous region of the isotactic P[(R)-3HB]; hence a BCP with isotactic PHA segments is essential in order to further improve compatibilizing performance.22,24 Another approach that improves compatibilization involves the judicious design of the BCP architecture,25,26 which has been reported to have a significant impact on the interfacial environment;27 hence, mechanical properties are adjustable by optimizing the BCP architecture. Consequently, a systematic investigation of a 4 ACS Paragon Plus Environment

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series of isotactic PHA-containing BCPs with various compositions and architectures is of particular interest when developing a high performance compatibilizer. However, although significant effort had been directed toward the synthesis of PHA-containing block and graft copolymers, challenges associated with the limited availability of structurally well-defined isotactic PHAs still remain. Recently, PHAs bearing functional groups at the carboxy-terminal, such as thiol, allyl, and propargyl, have been prepared through microbial fermentation approaches, in which the corresponding functional alcohol is added to the cultivation medium as the chain-transfer agent.28 Among various end-functionalities, the propargyl group is highly attractive from the synthetic point of view because of its high reactivity toward azido-functionalized polymers in copper-catalyzed azido-alkyne click chemistry, leading to a variety of PHA-containing BCPs.29 A click-reaction strategy based on end-functionalized microbial PHAs will benefit the systematic investigation and development of high performance biodegradable compatibilizers. Herein, we report the efficient synthesis of a series of BCPs composed of isotactic PHBH and PCL segments with varied architectures, namely PHBH-b-PCL, (PHBH)2-b-PCL, PHBH-b-(PCL)2, PHBH-b-PCL-b-PHBH, and (PHBH)2-b-PCL-b-(PHBH)2, through the click reactions of azido-functionalized PCLs and propargyl-end-functionalized microbial PHBH (Scheme 1). The obtained BCPs were used as compatibilizers in PHBH/PCL blends, after which their thermal, morphological, and mechanical properties were investigated. Through BCP-architecture screening, a remarkable improvement in the elongation at break was discovered when an ABA- or A2BA2-type BCP was used as the compatibilizer. Our results, which are based on well-defined model BCPs, reveal that adjusting the BCP architecture is a promising means of fine-tuning the mechanical properties of a PHBH/PCL blend. 5 ACS Paragon Plus Environment


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Scheme 1. Pathways for the syntheses of AB-, A2B-, AB2-, ABA-, and A2BA2-type BCPs, where A and B represent PHBH (red) and PCL (blue) segments respectively


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RESULTS AND DISCUSSION Preparing and Characterizing Propargyl-End-Functionalized PHBH The propargyl-end-functionalized PHBH (PHBH-C≡CH) was prepared by fermentation with a genetically engineered PHBH-producing microorganism in the presence of 2-propyne-1-ol as the chain-transfer agent. The details of PHBH production have been reported elsewhere.28 The microstructure of PHBH-C≡CH was initially investigated by NMR spectroscopy (Figure 1a), which revealed a signal due to the methylene proton of the propargyl group at 4.67 ppm (proton a) along with signals attributable to the PHBH backbone (protons A – H). The signal due to the methine proton of the propargyl group overlapped with the signals of the PHBH backbone. The mole fractions of the 3HB and 3HHx units were calculated to be 88.5 and 11.5 mol%, respectively, from the integrated ratios of the signals derived from each side chain at around 1.58−0.86 ppm. The number-average molecular weight (Mn,NMR) of PHBH-C≡CH was calculated to be 29 200 g mol−1 by 1H NMR integration, assuming perfect incorporation of the terminal propargyl group. The

13C

NMR spectrum of PHBH-C≡CH (Figure S1) exhibits signals derived from each carbon atom of PHBH

(carbons a – j) that reasonably correspond to the expected chemical structure. Two distinct signals are observed at 169.2 and 169.4 ppm when the carbonyl signal is examined in more detail; these signals are assignable to the 3HB-3HB (BB) and 3HHx-3HB (HB; or 3HB-3HHx (BH)) diad sequences, respectively.30 From the integration ratio, the BB and HB (or BH) diad contents were calculated to be 69.0 and 31.0%, respectively. The prepared PHBH-C≡CH was then subjected to size exclusion chromatography (SEC), which revealed a unimodal molecular weight distribution with a Ð value of 1.76; Mn,SEC was determined to be 33 900 g mol−1 when calibrated 7 ACS Paragon Plus Environment


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against polystyrene.

C, H

CHCl3

A E, F

D

(a) B, G

D A B

F C

G

E

a H

a

PHBH-C≡CH

4.70

4.68

4.66

4.64

a

(b)

8

6

PHBH-pyrene

b

2

4

2

 / ppm

c a’

4

aromatic

a’ aromatic

b

c

8

6

 / ppm

Figure 1. 1H NMR spectra of (a) PHBH-C≡CH and (b) PHBH-pyrene in CDCl3 (400 MHz).

We next click reacted the prepared PHBH-C≡CH with 1-azidomethylpyrene (AMP) to confirm the presence of the terminal propargyl group and to demonstrate its click reactivity. The click reaction was carried out in CH2Cl2 at r.t. for 1 d with CuBr/N,N′,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) as the catalytic system. After completion, the crude mixture was purified by alumina column chromatography and subsequent reprecipitation in cold MeOH to give the pyrene-terminated PHBH (PHBH-pyrene) as a white powder. The 1H NMR spectrum of PHBH-pyrene (Figure 1b) showed signals due to the aromatic pyrene rings at around 8.30−7.98 ppm, the triazole ring at 7.62 ppm (proton c), and 8 ACS Paragon Plus Environment

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the methylene adjacent to the pyrene ring at 6.28 ppm (proton b), along with major signals that correspond to the PHBH backbone (protons A – H). Importantly, a signal due to the methylene proton of the propargyl group at 4.67 ppm (proton a) was not detected, suggesting that the click reaction proceeded quantitatively to form a triazole junction between the PHBH chain and the pyrene. The obtained PHBH-pyrene was further investigated by SEC equipped with reflective index and UV (λ = 370 nm) detectors (Figure S2). No significant change was observed in the SEC trace before and after the click reaction. While pristine PHBH-C≡CH is itself undetectable by UV, PHBH-pyrene is detectable due to the terminal pyrene moiety, which is a UV chromophore. Overlaying the SEC traces for PHBH-pyrene acquired by RI and UV detection reveals that the elution peaks have coincidental shapes, which provides strong evidence that terminal propargyl groups are introduced over the entire PHBH molecular-weight range. These results collectively show that the microbially synthesized PHBH-C≡CH is sufficiently reactive toward azido-functionalized compounds.

Synthesis of Block Copolymers Composed of PHBH and PCL Segments Block copolymers (BCPs) composed of PHBH and PCL segments (i.e., the compatibilizers for the PHBH/PCL blend) were synthesized as shown in Scheme 1. Here, we targeted a series of BCPs with different polymer chain architectures that included AB-, A2B-, AB2, ABA-, and A2BA2-type BCPs, where “A” and “B” represent PHBH and PCL segments, respectively. These BCPs provide the opportunity to investigate the parameters that affect compatibilizer performance. BCP synthesis was achieved in two steps, which involved the synthesis of the



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azido-functionalized PCL followed by the click reaction with PHBH-C≡CH. We previously reported that the diphenyl-phosphate-catalyzed (DPP-catalyzed) ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) with a functional alcohol as the initiator proceeded in a living manner to afford a variety of end-functionalized PCLs with narrow Ð values.31 For example, an azido-functionalized PCL (N3-PCL25k; where the subscript represents the molecular weight) was prepared by DPP-catalyzed ROP with 6-azido-1-hexanol (N3-OH)32 as the initiator. Polymerization was performed in dry toluene at an [ε-CL]0/[N3-OH]0/[DPP] ratio of 250/1/1 at r.t. for 71 h. A lower molecular weight N3-PCL11k was also prepared by changing the monomer-to-initiator ratio. After reprecipitation, the N3-PCLs were obtained as white powders in yields of 66.4−69.0%. The 1H NMR spectrum of N3-PCL25k (Figure 2a) exhibits a signal at 3.26 ppm due to the methylene proton adjacent to the azido group (proton a), along with major signals that correspond to the PCL backbone (protons α – ε). The Mn.NMR values of N3-PCL25k and N3-PCL11k were determined to be 24 700 and 11 200 g mol−1, respectively, which are close to their nominal molecular weights. The FT-IR spectrum of N3-PCL25k (Figure S3) exhibits an absorption band at around 2100 cm−1, confirming the presence of the azido group. The SEC traces of N3-PCL25k and N3-PCL11k (Figures 3a and b) display unimodal molecular weight distributions with significantly narrow Ð values of 1.05−1.08.


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b,  g

a

e

CHCl3

(a)

b

a

a

 g

e

e’

e’

N3-PCL25k

3.8

3.6

a 3.4

3.2

3.0

e’ a C, H

A

(b)

E, F B, G

D

A B C

F

E

G H

b

c c a’

PHBH30k-b-PCL27k

b

7.64 7.60 7.56

a’

c

8

6

4

 / ppm

Figure 2. 1H NMR spectra of (a) N3-PCL25k and (b) PHBH30k-b-PCL27k in CDCl3 (400 MHz).


2

D


(a) PHBH30k-b-PCL27k

(b) PHBH30k-b-PCL13k

(c) (PHBH27k)2-b-PCL29k (N3)2-PCL

N3-PCL

N3-PCL PHBH-C≡CH

(d) PHBH30k-b-(PCL13.5k)2

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PHBH-C≡CH

PHBH-C≡CH

(e) PHBH28k-b-PCL26k-b-PHBH28k (f) (PHBH30k)2-b-PCL27k-b-(PHBH30k)2

N3-PCL -N3

N3-(PCL)2

N3-PCL

PHBH-C≡CH

PHBH-C≡CH

PHBH-C≡CH

Figure 3. SEC traces of (a) PHBH30k-b-PCL27k, (b) PHBH30k-b-PCL13k, (c) (PHBH27k)2-b-PCL29k, (d) PHBH30k-b-(PCL13.5k)2, (e) PHBH28k-b-PCL26k-b-PHBH28k, and (f) (PHBH30k)2-b-PCL27k-b-(PHBH30k)2 (eluent, THF; flow rate, 1.0 ml min−1). The black, blue, and red traces correspond to the BCP, azido-functionalized PCL, and PHBH-C≡CH, respectively.

Similarly,

(N3)2-PCL28k

and

N3-(PCL12.5k)2

were

synthesized

by

the

DPP-catalyzed

ROP

with

2,2-bis((6-azidohexyloxy)methyl)propan-1-ol ((N3)2-OH)33 and 2-[(6-azidohexyloxy)methyl]-2-methylpropane-1,3-diol


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(N3-(OH)2)33

as

initiators,

respectively.

The

α,ω-diazido-functionalized

PCL

(N3-PCL25k-N3)

and

the

α,α,ω,ω-tetraazido-functionalized PCL ((N3)2-PCL26k-(N3)2) were produced by the acylation of the PCL-diol (HO-PCL24k-OH) with 6-azidohexanoic acid (N3-COOH)32 and 3,5-bis((6-azidohexyl)oxy))benzoic acid ((N3)2-COOH), respectively. The obtained azido-functionalized PCLs were fully characterized by 1H NMR and FT-IR spectroscopies and SEC. The Mn values calculated from the 1H NMR spectra (Figures S4–S7) were 27 500 g mol−1 for (N3)2-PCL28k, 25 200 g mol−1 for N3-(PCL12.5k)2, 25 100 g mol−1 for N3-PCL25k-N3, and 25 900 g mol−1 for (N3)2-PCL26k-(N3)2. The SEC traces of the obtained PCLs (Figures 3c–f) showed unimodal molecular-weight distributions with significantly narrow Ð values of 1.05−1.10. AB-, A2B-, AB2, ABA-, and A2BA2-type BCPs were next synthesized by the click reactions of PHBH-C≡CH and the azido-functionalized PCLs, namely N3-PCL25k, N3-PCL11k, (N3)2-PCL28k, N3-(PCL12.5k)2, N3-PCL25k-N3, and (N3)2-PCL26k-(N3)2, respectively. The click reactions were performed in CH2Cl2 with 1.1 equiv. of PHBH-C≡CH with respect to the azido group, using the CuBr/PMDETA catalytic system at r.t. for 2 d. After completion, excess PHBH-C≡CH was removed by reaction with azido-functionalized polystyrene resin.34 The crude mixture was purified by alumina column chromatography and subsequent reprecipitation in cold MeOH to remove the lower-molecular weight fraction, to give the target BCPs, namely PHBH30k-b-PCL27k (symmetric AB-type; PHBH30k-b-PCL13k (asymmetric AB-type;

AsymAB-type),

SymAB-type),

(PHBH27k)2-b-PCL29k (A2B-type), PHBH30k-b-(PCL13.5k)2

(AB2-type), PHBH28k-b-PCL26k-b-PHBH28k (ABA-type), and (PHBH30k)2-b-PCL27k-b-(PHBH30k)2 (A2BA2-type), as white powders in yields of 66.4−83.8%. The prepared BCPs were then characterized by SEC, and IR and NMR 13 ACS Paragon Plus Environment


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spectroscopies. The SEC traces of the prepared BCPs are shown in Figure 3 along with those of the corresponding starting materials. An increase in molecular weight from the precursor to the corresponding BCP was observed following each click reaction, while the SEC traces reveal that unimodal molecular-weight distribution were maintained. For example, the Mn,SEC value of PHBH30k-b-PCL27k was determined to be 58 800 g mol−1, which is clearly higher than those of both PHBH-C≡CH (33 900 g mol−1) and N3-PCL25k (35 800 g mol−1). The Ð values of the obtained BCPs are in the 1.31−1.52 range, and the FT-IR spectrum of PHBH30k-b-PCL27k no longer exhibits an absorption peak at around 2100 cm−1 (Figure S3), which strongly suggests that that azido group in N3-PCL25k has been quantitatively consumed. Furthermore, 1H NMR spectroscopy confirmed the expected chemical structure of the BCPs. For example, the 1H NMR spectrum of PHBH30k-b-PCL27k showed signals assignable to both the PHBH and PCL backbones (protons A – H and α – ɛ, respectively) along with the characteristic signal due to the triazole ring (proton c) at 7.62 ppm, while signals at 4.67 and 3.26 ppm due to the methylene proton of the propargyl group in PHBH-C≡CH and the methylene proton adjacent to the azido group in N3-PCL25k (proton a) were not detected (Figure 2). These results confirm that the click reaction proceeded quantitatively to form a triazole junction between the PHBH and PCL blocks to give the target PHBH30k-b-PCL27k (SymAB-type). In a similar manner, the other BCPs, namely PHBH30k-b-PCL13k (AsymAB-type), (PHBH27k)2-b-PCL29k (A2B-type), PHBH30k-b-(PCL13.5k)2 (AB2-type), PHBH28k-b-PCL26k-b-PHBH28k (ABA-type), and (PHBH30k)2-b-PCL27k-b-(PHBH30k)2 (A2BA2-type) were also fully characterized (Figures 3b–f and S3–S7). The mole fraction of the 3HHx unit in the PHBH segment (n3HHx) and the weight fraction of PHBH in the BCP (ΦPHBH) were calculated by 1H NMR integration, the results of which are summarized in Table 1 together with the molecular weight 14 ACS Paragon Plus Environment

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parameters.


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Table 1. Molecular characteristics of the prepared BCPs composed of PHBH and PCL segments Mn,Total

Mn,PHBH

Mn,PCL

(g mol−1) a

(g mol−1) a

(g mol−1) a

PHBH30k-b-PCL27k

57 000

29 500

PHBH30k-b-PCL13k

43 400

(PHBH27k)2-b-PCL29k

27 300

11.7

0.52

58 800

1.35

76.5

30 500

12 700

11.2

0.71

39 800

1.46

78.2

83 500

54 300

28 800

11.7

0.65

105 000

1.35

74.4

PHBH30k-b-(PCL13.5k)2

57 200

29 600

27 300

11.0

0.52

76 900

1.31

66.4

PHBH28k-b-PCL26k-b-PHBH28k

82 900

56 500

25 900

11.3

0.69

105 000

1.35

74.4

143 000

115 500

27 000

11.4

0.81

126 000

1.52

76.5

(g mol−1) d

Ðd

Yield

ΦPHBH c

(PHBH30k)2-b-PCL27k-b-(PHBH30k)2 a

Mn,SEC

n3HHx (%) b

BCP

(%)

Determined by 1H NMR spectroscopy in CDCl3. b Mole fraction of the 3HHx unit in the PHBH segment. c Weight fraction of PHBH in the BCP. d Determined by

SEC in THF using polystyrene standards.


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Preparing Blended Films We next investigated the properties of the BCPs as compatibilizers for the PHBH (Mw = 530 000 g mol−1, 6 mol% of 3HHx unit)/PCL (Mw = 150 000 g mol−1) blend. To obtain film samples of PHBH and PCL with different BCPs, all components were dissolved in CHCl3 and then cast onto a Teflon Petri dish. The solvent-cast films were then heat pressed at 150 °C to produce films samples of uniform thickness (~20 µm), which were stored in a desiccator for 1 week prior to testing to allow them to achieve crystallization-state equilibria. The weight ratio of PHBH, PCL, and the compatibilizer (BCP) in each blended film was fixed at 67/23/10. In addition to PHBH/PCL/BCP ternary blends, a neat PHBH (= 100 wt%) film as well as a PHBH/PCL (= 75/25 wt%) binary blend film were also prepared for comparison purposes. Each blended sample was labeled in the following way: “neat PHBH” for PHBH = 100 wt%, “BCP-free blend” for PHBH/PCL = 75/25 wt%, “SymAB-blend” for PHBH/PCL/PHBH30k-b-PCL27k,

“AsymAB-blend”

for

PHBH/PCL/PHBH30k-b-PCL13k,

“A2B-blend”

for

PHBH/PCL/(PHBH27k)2-b-PCL29k, “AB2-blend” for PHBH/PCL/PHBH30k-b-(PCL13.5k)2, “ABA-blend” for PHBH/PCL/PHBH28k-b-PCL26k-b-PHBH28k,

and

“A2BA2-blend”

for

PHBH/PCL/(PHBH30k)2-b-PCL27k-b-(PHBH30k)2. The composition of each blended sample is summarized in Table 2. Figure S8A shows photographic images of neat PHBH, the BCP-free blend, and

SymAB-blended

films after

solvent casting. The neat PHBH film exhibits a semitransparent appearance, while the BCP-free blend and the SymAB-blended

films appear white due to PCL crystallization and phase separation. A spider-web-like pattern was

observed in the BCP-free blend film, which is possibly due to the heterogeneous phase separation of PHBH and PCL. On the other hand, the SymAB-blend has a uniform white appearance, which indicates that the PCL domains in 17 ACS Paragon Plus Environment


Page 18 of 36

the PHBH matrix are homogeneously dispersed. As shown in Figure S8B, all blended films that contain neat PHBH are semitransparent following heating-press treatment and 1 week of aging.


Page 19 of 36 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


Table 2. Compositions and thermal properties of the PHBH/PCL/BCP blended films Sample ID a

Added BCP

PHBH/PCL/BCP

Tm,PCL

Hm,PCL

XPCL

Hm,PHBH

XPHBH

Tg,PHBH

(wt%)

(°C) b

(J g−1) b

(%)

(J g−1) b

(%)

(°C) c

(a) neat PHBH

-

100/0/0

-

-

-

48.6

33.3

−2.5

(b) BCP-free blend

-

75/25/0

56.9

45.4

31.9

46.8

32.0

2.5

(c) SymAB-blend

PHBH30k-b-PCL27k

67/23/10

56.9

52.8

37.2

49.7

34.0

1.0

(d) AsymAB-blend

PHBH30k-b-PCL13k

67/23/10

56.1

47.5

33.4

51.7

35.4

−4.5

(e) A2B-blend

(PHBH27k)2-b-PCL29k

67/23/10

57.6

53.7

37.8

53.9

36.9

−4.0

(f) AB2-blend

PHBH30k-b-(PCL13.5k)2

67/23/10

57.7

44.2

31.1

48.3

33.1

1.0

(g) ABA-blend

PHBH28k-b-PCL26k-b-PHBH28k

67/23/10

57.3

43.6

30.7

48.5

33.2

−0.5

(h) A2BA2-blend

(PHBH30k)2-b-PCL27k-b-(PHBH30k)2

67/23/10

57.1

49.9

35.1

50.9

34.9

−4.0

a

Prepared by heating pressing at 150 °C and subsequent aging for 1 week. b Determined by DSC during the first heating at a rate of 10 °C min−1 c Determined by

DSC during the second heating at a rate of 10 °C min−1 before cooling from 180 °C to −20 °C at a rate of 20 °C min−1.



Page 20 of 36

Differential Scanning Calorimetry To provide insight into the crystallization properties of the PCL and PHBH in the blended films, the heat-pressed blend films (after 1 week of aging) were subjected to differential scanning calorimetry (DSC), with the first and second heating DSC thermograms of the neat PHBH, the BCP-free blend, and PHBH/PCL/BCP ternary blended films shown in Figures S9A and B, respectively. The thermal properties of the blended films are summarized in Table 2. Each blended film exhibited a PCL melting peak at around 60 °C (Tm,PCL) and two PHBH melting peaks at around 120–130 (Tm1,PHBH) and 140–150 °C (Tm2,PHBH) during the first heating process. The double melting behavior of PHBH is commonly observed in a variety of PHAs.35,36 Importantly, PCL and PHBH melting peaks were observed independently, which indicates that the two components are phase separated. The first melting peak Tm1,PHBH is attributed to the melting of crystals formed during solvent casting and subsequent aging. The second melting peak Tm2,PHBH is attributed to the melting of crystals formed by recrystallization during the heating process. The crystallinity percentages of PCL (XPCL) and PHBH (XPHBH) were calculated to be 30.7−37.8% and 32.0−36.9%, respectively. These results suggest that the addition of the compatibilizer has a negligible impact on the crystallization behavior of PCL and PHBH. Each blended film exhibited a PHBH glass transition at around 0 °C (Tg,PHBH) during the second heating process.

Morphologies The blended films were subjected to transmission electron microscopy (TEM) in order to investigate the effect of the compatibilizer on film morphology. Figure 4 shows TEM micrographs of the BCP-free blend and the PHBH/PCL/BCP ternary blended films prepared by heat pressing at 150 °C and subsequent aging for 1 week. The 20 ACS Paragon Plus Environment

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darker regions are assignable to PCL domains that are selectively stained with RuO4. The average diameter of the PCL domains (DPCL) was calculated by a reported method;37 irregularly shaped large PCL domains with a DPCL value of 4.9 µm were observed in the PHBH matrix of the BCP-free blend (Figure 4a). On the other hand, the PHBH/PCL/BCP ternary blends (Figures 4b–g) exhibit finely dispersed PCL domains in their PHBH matrices, with DPCL values in the 2.1−0.51 µm range, which are clearly smaller than that of the BCP-free blend. The results confirm that the BCPs successfully act as compatibilizers in the PHBH/PCL blend; the BCP reduces the interfacial tension between the PHBH and PCL phases. For comparison purpose, the precursors of PHBH30k-b-PCL27k (i.e., PHBH-C≡CH and N3-PCL25k) were added to PHBH/PCL to produce a PHBH/PCL/PHBH-C≡CH/N3-PCL25k (67/23/5/5 wt%) quaternary blend; this blend did not exhibit compatibilization, with the TEM micrographs revealing large PCL domains with a DPCL value of 3.3 µm (Figure S10). This result provides strong evidence that the covalently bonded block architecture is responsible for compatibilization of the PHBH/PCL blend.



(a) BCP-free blend

DPCL (µm)

(c) AsymAB-blend

(b) ABA-blend

(d) AB2-blend

5 µm

5 µm

5 µm

Page 22 of 36

5 µm

2.1

1.4

1.1

(e) SymAB-blend

(f) A2BA2-blend

(g) A2B-blend

4.9

5 µm

5 µm

5 µm

0.97

0.51

0.72

Figure 4. TEM micrographs of: (a) PHBH/PCL (75/25 wt%), (b) PHBH/PCL/PHBH28k-b-PCL26k-b-PHBH28k (67/23/10 wt%), (c) PHBH/PCL/PHBH30k-b-PCL13k (67/23/10 wt%), (d) PHBH/PCL/PHBH30k-b-(PCL13.5k)2 (67/23/10

wt%),

(e)

PHBH/PCL/PHBH30k-b-PCL27k

(67/23/10

wt%),

(f)

PHBH/PCL/(PHBH30k)2-b-PCL27k-b-(PHBH30k)2 (67/23/10 wt%), and (g) PHBH/PCL/(PHBH27k)2-b-PCL29k (67/23/10 wt%). All blended films were prepared by heating pressing at 150 °C and subsequent aging for 1 week. The darker regions correspond to PCL domain, which were selectively stained with RuO4.

Interestingly, the DPCL value was found to depend strongly on the BCP architecture, and decreased in the order: ABA-blend (2.1 µm) >

AsymAB-blend

(1.4 µm) > AB2-blend (1.1 µm) >

SymAB-blend

(0.97 µm) > A2BA2-blend

(0.72 µm) > A2B-blend (0.51 µm). This trend can be explained in terms of the conformation of the BCP molecule at the interface between the PHBH and PCL phases. In the case of the A2B-blend, local crowding of the PHBH chains 22 ACS Paragon Plus Environment

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of BCP was higher on the matrix side of the interface, which results in an increase in interfacial curvature that leads to small PCL domains. Conversely, local crowding of the PCL chains of the BCP in the AB2-blend is higher on the domain side; hence interfacial curvature decreases, which leads to larger PCL domains. The

SymAB-blend

can be

regarded to be intermediate between these two extremes. As was observed for the A2B-blend, local crowding of the PHBH chains of the BCP is higher at the PHBH matrix side of the A2BA2-blend, which results in an increase in interfacial curvature and small PCL domains. When the linear BCPs are compared, the SymAB-blend, rather than the AsymAB-blend,

was found to finely disperse the PCL domain, which is ascribable to the symmetric linear BCP

preferring to present at the interface between the PHBH and PCL phases, which leads to a large reduction in interfacial tension and domain size. Conversely, the asymmetric linear BCP tends to form micelles in the PHBH matrix, which does not improve interfacial properties.38 A similar discussion can be applied to compare the DPCL values for the A2B- and A2BA2-blends. Although the A2B- and A2BA2-type BCPs are relevant each other in terms of the branched architecture, the DPCL for the A2BA2-blend was greater than that for the A2B-blend. The A2B-type BCP having rather symmetric block ratio (ΦPHBH = 0.65) preferred to present at the interface between the PHBH and PCL, which leads to a larger reduction in domain size rather than the case of highly asymmetric A2BA2-type BCP (ΦPHBH = 0.81). Overall, the newly synthesized BCPs successfully worked as compatibilizers in the PHBH/PCL blend. In addition, the DPCL value can be controlled through appropriate design of the BCP architecture.



Page 24 of 36

Mechanical Properties Figure 5 displays stress-strain curves of the blended films prepared by heating pressing at 150 °C and subsequent aging for 1 week; the mechanical properties of these blended films are summarized in Table 3. The neat PHBH film was very brittle and fractured when 10% strain was applied (Figure 5a). Cracks and/or voids formed during secondary crystallization may have resulted in poor toughness.39 Blending PCL into the PHBH matrix did not contribute to improved toughness (Figure 5b), which is attributable to inhomogeneous delocalization of stress due to macrophase separation of the PCL and PHBH phases.21 On the other hand, the SymAB-, A2B-, AB2-, ABA-, and A2BA2-blends exhibited enhanced toughnesses and elongations at break (Figures 5 (c), (e), (f), (g), and (h), respectively), demonstrating that the BCPs indeed act as compatibilizers. However, the

AsymAB-blend

showed no

improvement in elongation at break due to the interfacial instability of PHBH30k-b-PCL13k (AsymAB-type) (Figure 5d).


Page 25 of 36

35 35

30

30

25

20 15

(a) neat PHBH (b) BCP-free blend neat PHBH (c) (a) SymAB-blend (b) BCP-free blend (d) (c) AsymAB-blend SymAB-type (e) (d) A2B-blend AsymAB-type (f) AB (e) 2A-blend 2B-type (g) (f) ABA-blend AB2-type (h) (g) A2BA 2-blend ABA-type



20



Stress / MPa

25

Stress / MPa

15 10

10

5

(h) A2BA2-type

0 0

20

0

Figure

5.

40 Strain / %

20

Stress-strain

curves

0

20

60

40 Strain / %

for:

(a)

80

PHBH

(100

40

wt%),



5



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


60 Strain / % 100

60

(b)

PHBH/PCL

80

100

80

(75/25

wt%),

(c)

PHBH/PCL/PHBH30k-b-PCL27k (67/23/10 wt%), (d) PHBH/PCL/PHBH30k-b-PCL13k (67/23/10 wt%), (e) PHBH/PCL/(PHBH27k)2-b-PCL29k (67/23/10 wt%), (f) PHBH/PCL/PHBH30k-b-(PCL13.5k)2 (67/23/10 wt%), (g) PHBH/PCL/PHBH28k-b-PCL26k-b-PHBH28k

(67/23/10

wt%),

and

(h)

PHBH/PCL/(PHBH30k)2-b-PCL27k-b-(PHBH30k)2 (67/23/10 wt%). All blended films were prepared by heating pressing at 150 °C and subsequent aging for 1 week.


100

ACS Sustainable Chemistry & Engineering 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

Page 26 of 36

Table 3. Mechanical properties of the PHBH/PCL/BCP blended films Sample ID a

Elongation at break (%)

Maximum stress (MPa)

Young’s modulus (MPa)

Toughness (MJ m−3)

DPCL (µm)

b

(a) neat PHBH

10.0 ± 0.3

33.3 ± 0.2

545 ± 2

2.2 ± 0.1

-

(b) BCP-free blend

11.2 ± 0.2

27.2 ± 0.6

534 ± 8

2.2 ± 0.1

4.9

(c) SymAB-blend

28.3 ± 3.3

22.2 ± 0.1

(d) AsymAB-blend

6.6 ± 0.6

23.5 ± 0.8

(e) A2B-blend

32.8 ± 1.8

22.2 ± 1.2

181 ± 27

(f) AB2-blend

16.8 ± 2.6

24.2 ± 0.9

367 ± 30

(g) ABA-blend

102.2 ± 2.5

25.2 ± 0.8

533 ± 34

22.0 ± 2.6

50.6 ± 5.5

19.5 ± 0.9

417 ± 14

7.0 ± 0.8

(h) A2BA2-blend a

220 ± 24 473 ± 9

3.6 ± 0.4 1.5 ± 0.1 5.5 ± 0.5 3.0 ± 0.02

0.97 1.4 0.51 1.1 2.1 0.72

Prepared by heating-press at 150 °C and subsequent aging for 1 week, (a) PHBH (100 wt%), (b) PHBH/PCL (75/25 wt%), (c) PHBH/PCL/PHBH30k-b-PCL27k

(67/23/10 wt%), (d) PHBH/PCL/PHBH30k-b-PCL13k (67/23/10 wt%), (e) PHBH/PCL/(PHBH27k)2-b-PCL29k (67/23/10 wt%), (f) PHBH/PCL/PHBH30k-b-(PCL13.5k)2 (67/23/10 wt%), (g) PHBH/PCL/PHBH28k-b-PCL26k-b-PHBH28k (67/23/10 wt%), and (h) PHBH/PCL/(PHBH30k)2-b-PCL27k-b-(PHBH30k)2 (67/23/10 wt%). Calculated by stress-strain curve in the region under 30% strain except for ABA- and A2BA2-blends.


b

Page 27 of 36 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


When the A2B-,

SymAB-,

and AB2-blends are compared (Figures 5 (e), (c), and (f), respectively), the toughness

and elongation at break were found to correlate with DPCL; the elongation at break and toughness were found to increase with decreasing DPCL (Figures S11a and b), which is possibly due to the effective delocalization of applied stress with increasing numbers of PCL domains.40 In addition, the Young’s modulus tended to decrease with decreasing DPCL (Figure S11c). The elongations at break of the ABA- and A2BA2-blends were more effectively improved than those of the A2B-,

SymAB-,

and AB2-blends (Figures 5g and h). In addition, a yield point can be

clearly observed in the ABA- and A2BA2-blends. Thus, the “PHBH-PCL-PHBH” triblock sequence was found to be a suitable compatibilizer architecture for the present blend system. We presume that since the PCL chains of the BCP form loops in the PCL domains, BCP exists stably at the interface and is difficult to remove due to strong entanglement with the domain components.41 The maximum value of the elongation at break increased by a factor of about 10 compared to that of the BCP-free blended film. These results suggested that the mechanical properties of a PHBH/PCL blend can be adjusted by the addition of BCPs with different architectures as compatibilizers.

CONCLUSION We successfully synthesized architecturally varied BCPs composed of PHBH and PCL segments, namely PHBH-b-PCL (AB-type), (PHBH)2-b-PCL (A2B-type), PHBH-b-(PCL)2 (AB2-type), PHBH-b-PCL-b-PHBH (ABA-type),

and

(PHBH)2-b-PCL-b-(PHBH)2

(A2BA2-type),

through

click

chemistry

involving

azido-functionalized PCLs and propargyl-end-functionalized microbial PHBH. TEM and tensile testing experiments reveal that the prepared BCPs indeed act as compatibilizers for PHBH and PCL blends. The 27 ACS Paragon Plus Environment


Page 28 of 36

mechanical properties, such as elongation at break, tensile strength, Young’s modulus, and toughness of the PHBH/PCL/BCP ternary blend was found to depend significantly on the choice of BCP architecture, and are closely related to their morphologies. A remarkable five-to-ten-fold increase in elongation at break, compared with that of the BCP-free blend, was achieved by the addition of an ABA- or A2BA2-type BCP as the compatibilizer. These results provide new guidance for the creation of high performance biodegradable compatibilizers that expand the potential applications of sustainable PHA-based materials. Given its simplicity and efficiency, our synthetic approach based on end-functionalized microbial PHAs is applicable to the synthesis of a wide variety of PHA-based compatibilizers, and provides new avenues for the development of high-performance and high-value-added biodegradable plastics.


Page 29 of 36 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


ASSOCIATED CONTENT Supporting Information Experimental details, photographic images of the blend films, and additional SEC, NMR, FT-IR, DSC, TEM, and tensile results. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx.

AUTHOR INFORMATION Corresponding Authors (T.I.) E-mail: [email protected] (T.S.) E-mail: [email protected]

Funding This work was financially supported, in part, by a JSPS Grant-in-Aid for Scientific Research (B) (No. 16H04152) and JSPS KAKENHI (No. 18H04639, Hybrid Catalysis for Enabling Molecular Synthesis on Demand).

Note The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors thank Prof. T. Nakajima (Hokkaido University, Japan) and Mr. S. Seno (Hokkaido Research Organization) for their assistance with tensile testing and heating-press experiments, respectively. 29 ACS Paragon Plus Environment


Page 30 of 36

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Polymer

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FOR TABLE CONTENTS USE ONLY “Click Reaction”

PHA-C≡CH

N3-PCL

PHA-b-PCL

Compatibilization

5 µm

5 µm

PHA/PCL blend

PHA/PCL/PHA-b-PCL blend

Synopsis text: Architecturally varied block copolymers composed of microbially derived poly(hydroxyalkanoate) and

biodegradable

poly(ɛ-caprolactone)

segments

were

synthesized

poly(hydroxyalkanoate)/poly(ε-caprolactone) blended polymers.


as

compatibilizers

for

Biodegradable Compatibilizers for Poly ... - ACS Publications

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