Bench-Scale Synthesis and Characterization of Biodegradable

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Bench-Scale Synthesis and Characterization of Biodegradable Aliphatic– Aromatic Random Copolymers with 1,4-Cyclohexanedimethanol Units Towards Sustainable Packaging Applications Seokmin Hahm, Jin-Seong Kim, Hongseok Yun, Ji Hae Park, Rachel A. Letteri, and Bumjoon J. Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04720 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Sustainable Chemistry & Engineering

Bench-Scale Synthesis and Characterization of Biodegradable Aliphatic– Aromatic Random Copolymers with 1,4-Cyclohexanedimethanol Units Towards Sustainable Packaging Applications Seokmin Hahm,1,2 Jin-Seong Kim,1 Hongseok Yun,1 Ji Hae Park,2 Rachel A. Letteri3 and Bumjoon J. Kim*,1 1

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science

and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea 2

R&D Center, LOTTE Chemical, 115 Gajeingbuk-ro, Yuseong-gu, Daejeon 34110, Korea

3

Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904,

United States

KEYWORDS biodegradable, blown film, packaging applications, dimethyl carbonate, 1,4cyclohexanedimethanol, bench-scale synthesis

*E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT The development of biodegradable packaging films can resolve environmental issues caused by plastic waste, but it still remains a great challenge to develop economically-feasible polymers that simultaneously balance robust mechanical properties, biodegradability and transparency. In this work, we describe the bench-scale synthesis (~1.5 kg) and blown film characterization of new biodegradable aliphatic–aromatic copolymers, poly(1,4-butylene-1,4-cyclohexanedimethylene carbonate-terephthalate)s (PBCCTs) with different molar ratios of two diol monomers, 1,4cyclohexanedimethanol (CHDM) and 1,4-butandiol (BD), from 0:1 to 5:5 (CHDM:BD) to optimize the mechanical, optical, thermal properties, and biodegradability. The incorporation of CHDM units significantly impacted the thermal properties of the blown films from these copolymers; PBCCT films with 50 mol% CHDM content had a more amorphous and glassy character compared with the films with 0 mol% CHDM. And, PBCCT films with 30-50 % CHDM content exhibited superior mechanical properties (tear strength: 11.5 kgf /mm and tensile strength: 369 kgf/cm2) and comparable transparency (haze: 16%) to those of non-degradable polyethylenes (PEs), the most commonly employed materials for packaging film applications. Taken together, the bench-scale synthesis of biodegradable polymers with suitable thermomechanical, optical, and permeability properties presented here showcases the potential of these materials as sustainable packaging materials.


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INTRODUCTION During the past several decades, a significant amount of research on the synthesis and application of biodegradable polymeric materials has been directed to resolve environmental issues caused by plastic waste.1-3 Non-degradable polymers used in conventional packaging account for ca. 40% of the plastic industry and have a short shelf life of less than one year; replacing these materials with biodegradable polymers that can be decomposed by microorganisms has tremendous potential to decrease the environmental impact of packaging.4-6 Packaging films require a certain level of mechanical properties, and, in particular, tensile and tear strengths of polymers are important properties for packing applications.7,

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In addition to the mechanical

properties, transparency and oxygen permeability can be important characteristics, depending on the use of the package.9, 10 For example, transparency is important in applications where the packaged items should be visible and oxygen permeability is an essential factor in the case of food storage. However, the development of new biodegradable packaging film remains a great challenge, since the most widely used biodegradable polymers do not often provide effective film properties, such as tensile strength, tear strength, and transparency at the same time. Aliphatic and aliphatic-aromatic polycarbonates (APCs) with functional structures, including aromatic and cyclic units, are considered strong potential candidates for replacing conventional film materials owing to their nontoxic and biodegradable properties.11-14 Commonly, APCs are prepared using copolymerization of carbon dioxide (CO2) and epoxides15-17 or ringopening polymerization.18-20 However, synthesis of APCs from CO2 and epoxide requires improved catalytic technologies to generate polymers that provide suitable materials properties15 and ring-opening polymerizations are not yet amenable to large-scale production due to low yields of the cyclic monomers.20, 21 To address this issue, high molecular weight poly(butylene carbonate)


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(PBC) was synthesized from dimethyl carbonate (DMC).22, 23 DMC is an inexpensive, sustainable monomer that is mass-produced commercially from carbon monoxide (CO) or CO2,24, 25 and is highly biodegradable and nontoxic.26, 27 However, the melting temperature (Tm) of PBC is quite low (around 60 °C) and it has a slow crystallization rate, which presents problems for processing PBC. To overcome these problems, a terephthalate unit was introduced in PBC to yield a high molecular weight copolymer, poly(1,4-butylene carbonate-co-terephthalate) (PBCT).28, 29 Notably, the Zheng group reported the terephthalate composition-dependent properties of PBCT and obtained the crystallization, thermal, and mechanical properties.30 Despite the enhanced thermal and mechanical properties of the PBCT copolymers relative to those of PBCs, the tear strength (0.1 kgf/mm) and transparency (haze: 39 %) of the PBCT copolymers remains lower than the tear strength (6.0 kgf/mm) and transparency (haze: 12 %) of linear low-density polyethylene (LLDPE) materials currently used for packaging applications. Herein, we present the bench-scale synthesis and characterization of a series of poly(1,4butylene-1,4-cyclohexanedimethylene carbonate-terephthalate) (PBCCT) copolymers with different contents of 1,4-cyclohexanedimethanol (CHDM) and 1,4-butanediol (BD), from 0 to 50% CHDM to tailor their materials properties for packaging film applications. We chose the CHDM unit because it is anticipated that the introduction of CHDM units into the polymer backbone allows for tuning the polymer properties such as polymer chain rigidity, thermomechanical properties, and transparency.31-35 While prior reports describe an increase in the mechanical properties of the polymers upon incorporation of bulky and rigid CHDM units,30, 31 here we report the improvement of both mechanical properties (i.e., tear strength) and transparency upon incorporation CHDM into PBCT to improve both mechanical properties (i.e, tear strength) and transparency. These observations are mainly attributed to the enhanced rigidity provided by


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CHDM units, the low crystallinity of PBCCT, and the increase of the glass transition temperature with CHDM content. In addition, CHDM units are inexpensive and commercially mass-produced, resulting in cost-effective polymers. The PBCCTs were synthesized by a two-step melt polycondensation on a large scale (1.5 kg) using sodium hydroxide (NaOH) as an inexpensive catalyst to yield copolymers with comparable molecular weights and molecular weight distributions. The synthesized PBCCTs were processed into the blown films, which is one of the most important industrial processes in the production of polymeric films. The film properties of the PBCCTs were evaluated, including tensile strength, elongation at break, tear strength, haze, oxygen permeability, and biodegradability to determine the suitability of the PBCCTs for biodegradable film applications. (Scheme 1).

Scheme 1. Schematic illustration of the bench-scale synthesis and blown film extrusion of PBCCT.

EXPERIMENTAL SECTION


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General. For the synthesis of PBCCTs, BD (BASF), a cis- and trans- mixture CHDM (99.0%, Sigma-Aldrich), DMC (LOTTE Chemical), and dimethyl terephthalate (DMT, Sigma-Aldrich) were used as co-monomers for producing PBCT and PBCCTs. Glycerol propoxylate (MW ~266 g/mol, Sigma-Aldrich) was used as a branching agent and sodium hydroxide (NaOH 98%, Samchun) was used as a catalyst. For blown film processing of PBCT and PBCCTs, primary and secondary antioxidants (Irganox 1010, 0.1 wt%, Alkanox 240, 0.1 wt%), a slip agent (Erucamide, 0.15 wt%) and an anti-blocking agent (calcium carbonate, 5 wt%) were added. Preparation of PBCCTs using a bench reactor. Bench-scale polymerizations were carried out using facilities at LOTTE Chemical R&D center, which comprised two reactors (12 L) equipped with a mechanical stirrer for transesterification and polycondensation reactions, preheaters, temperature controllers, a nitrogen cylinder, gas flowmeters, pressure gauges, a vacuum pump and a cold trap cooled using dry ice and liquid nitrogen. All reactor equipment was connected with stainless steel pipes and plastic tubing. BD (12.5 - 25 mol), CHDM (0 - 12.5 mol), DMC (27.5 mol), DMT (11.25 mol), glycerol propoxylate (0.05 mol), and NaOH (0.05 mol) were added to a reactor equipped with a mechanical stirrer and distillation apparatus under a nitrogen atmosphere. As the temperature was increased, the mixed materials began to melt. The solution temperature was maintained at 120 °C while distilling off the volatiles at atmospheric pressure for 2 h. The temperature was then increased to 200 °C and maintained at this temperature for an additional 2 h while continuously distilling off the volatiles at atmospheric pressure to promote transesterification. At 200 °C, polycondensations were conducted by gradually reducing the pressure as follows: from 760 to 200 mmHg for 10 min, from 200 to 100 mmHg for 15 min, from 100 to 50 mmHg for 15 min, from 50 to 10 mmHg for 15 min and from 10 to 0.5 mmHg for 60 min. The reaction was terminated at predetermined melt viscosities measured by the torque of the mechanical stirrer,


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which was used to estimate the molecular weight of the polymer. When the torque had reached the desired value, the pressure was returned to ambient pressure by injecting nitrogen. All melt polymers were cooled in a water bath (~ 15 °C) and pelletized into small chips. Characterization. The structure and sequence distribution of PBCT and PBCCTs were analyzed using 1H nuclear magnetic resonance (NMR) and 13C NMR spectroscopy (Agilent DD2 500 MHz). Deuterated chloroform was used as the solvent, and tetramethylsilane (TMS) was used as the chemical shift reference. Sixteen scans with 16k data points each were acquired for each 1H NMR spectrum. The relaxation delay was 1 s. Each 13C NMR spectrum comprised 5,000 scans with 32k data points each. The relaxation delay was 10 s. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and dispersity (Ð) of the polymers were measured by gel permeation chromatography (GPC, Schambeck RI2012A) using tetrahydrofuran (THF) as the eluent with a flow rate of 1.0 mL∙min-1. The GPC was calibrated using standard grade polystyrene. The melting index (MI) of the polymers was measured (190 oC and 2.16 kg load) by a melt indexer (TOYOSEIKI) following American Society for Testing and Materials (ASTM) D1238 standards. The thermal properties were measured using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). DSC was performed on a TA instruments Q200 system under a N2 atmosphere calibrated with indium and zinc for temperature and heat capacity, respectively. The samples were heated from - 20 to 200 °C at 10 °C/min. Thermal stability was analyzed using TGA (TA Instruments Q500), heating from 30 to 800 °C at a rate of 10 °C/min under a N2 atmosphere. Diffraction patterns were obtained with a PANalytical Empyrean using Ni-filtered Cu-K α radiation (λ = 1.5406 Å, 40 kV, 30 mA) in the range from 5o to 90o at a scanning rate of 0.0056 o

/sec.


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Blown film extrusion. Before blown film extrusion, all polymers were dried in an oven at 80 °C for 12 h. Polymers and additives, i.e., primary and secondary antioxidants (Irganox 1010, 0.1 wt%, Alkanox 240, 0.1 wt%), a slip agent (Erucamide, 0.15 wt%), and an anti-blocking agent (calcium carbonate, 5 wt%), were blended in a lab-scale extruder (HAAKE Rheomex). The temperature was set at 170 °C, and the screw speed was maintained at 20 rpm. After being cooled in a water bath, the blended samples were pelletized into small chips. The small chips were dried in an oven at 80 °C for 12 h. Blown film extrusion was performed using an annular 25 mm diameter blow die coupled to a labscale extruder (Brabender, PL 2200). The temperature of the feed zone to the die was set at 170 °C, and the screw speed was maintained at 60 rpm. Upon blowing air through the die head, the tube inflated into a thin tubular bubble, which was then cooled. The blown film was flattened between nip rolls and pulled up by a winder. In this process, the molten polymer was extruded at a constant rate through an annular die. Then the thin films were deformed axially and cooled by the introduction of air into the polymer bubble.36 The average film thickness was 60 μm, as measured by an electronic thickness micrometer. Measurement of mechanical properties, haze and oxygen permeability. The tensile properties were measured on a stress-strain tester (Instron 4466, load cell 100 N) according to ASTM D638 standards, with a drawing rate of 200 mm/min. The tear strength was measured according to ASTM D1004. The haze of the films was measured by a haze meter (NDH-5000) in accordance with ASTM D1003. All tensile strength, tear strength and haze measurements were reported as the average of five samples. The oxygen permeability of the films was measured by an oxygen transmission rate tester (OX-TRAN Model 2/21) in accordance with ASTM D3985. The gas


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permeability rate was calculated by measuring the oxygen passing through the film at 23 °C and 760 mmHg. Enzymatic degradation test. Sample films, 1 cm × 1 cm × 60 μm, were prepared and tested in duplicate. The samples were placed in vials containing phosphate buffer solution (pH = 6.9) and the lipase from Pseudomonas sp., with an activity = 34 units/mg, as reported by the manufacturer (Sigma-Aldrich). The initial concentration of enzyme in the buffer solution was 10 units/mL. The enzyme was added to the samples at 10 units/mg sample. Each sample was incubated in buffer solution containing a lipase, with shaking (100 rpm) at 37 oC for 40 days and the media was replaced every 5 days in order to maintain the activity of the enzyme. The films were washed with distilled water and methanol several times, and dried under vacuum at room temperature until no additional mass loss was observed. The degree of biodegradation was estimated by the sample weight loss, according to the following equation: 𝑊𝑊𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 =

𝑊𝑊0 − 𝑊𝑊1 𝑊𝑊0

(1)

The film surface topography was imaged using scanning electron microscopy (SEM, Hitachi SU8200, accelerating voltage: 40 kV) after coating with osmium tetroxide.


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RESULTS AND DISCUSSION

Scheme 2. Chemical structure of (a) the diol and dimethoxy monomers and, and (b) the resulting PBCT and (c) PBCCT copolymers. The molar feed ratio of the diol monomers was adjusted from 0.0:1.0 to 0.5:0.5 CHDM:BD.

Synthesis and characterization of the polymers. The PBCCT copolymers were prepared by a two-step melt polymerization method involving transesterification, followed by polycondensation in a bench scale reactor (Scheme 1). As shown in Scheme 2a, diol (BD and CHDM) and dimethoxy monomers (DMC and DMT) were added to the reactor in a molar ratio of 1: 1.55, targeting the molar ratio of the final product to be 1:1 between the diols and dimethoxy monomers due to the volatility of DMC. In order to investigate the effects of CHDM content on the properties of the resulting copolymers, the feed molar ratio between the two diol monomers (CHDM:BD) was adjusted from 0:1.0 (poly(1,4-butylene carbonate-co-terephthalate), PBCT, Scheme 2b) to 0.5:0.5

(poly(1,4-butylene-1,4-cyclohexanedimethylene

carbonate-terephthalate),

PBCCT,


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Scheme 2c), while the molar ratio of DMC:DMT was fixed at 0.5:0.5. For convenience, the polymers are denoted as PBCCTX, where X is the mole percentage of CHDM relative to the total diol monomer content. In addition, a small amount (~0.002 mol) of glycerol propoxylate was used as a branching agent for broadening the molecular weight distribution. Such branching agents are known to reduce the reaction time of bulk polycondensations and enhance the processability and mechanical properties of the resulting polymers.29 The initial temperature of the transesterification reaction was set at 120 °C and increased to 200 °C at a rate of 2 °C min-1. Then, the polycondensation reaction was initiated by reducing the pressure of the reactor from atmospheric pressure to 10 mmHg for 55 min, followed by further pressure reduction below 10 mmHg for 60 min.29 Finally, molten PBCT and PBCCTs having 4 repeating units, BD-carbonate (BC*), BDterephthalate (BT), CHDM-carbonate (CC*) and CHDM-terephthalate (CT) were cooled in a water bath and pelletized into small chips.

Table 1. Compositions and characteristics of PBCT and PBCCTs Mn (kg mol-1)

Mw (kg mol-1)

Ð

100:0

45

119

2.66

90:10

88:12

49

124

2.53

PBCCT20

80:20

78:22

49

122

2.51

PBCCT30

70:30

65:35

40

111

2.76

PBCCT40

60:40

53:48

40

114

2.85

PBCCT50

50:50

45:55

51

119

2.36

Sample

[BD]:[CHDM] in feed

PBCT

100:0

PBCCT10

[BD]:[CHDM] in polymer


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Figure 1. 1H NMR spectra of PBCT and PBCCT copolymers acquired in CDCl3.

The Mn, Mw, and Ð were measured by GPC eluting in THF (Table 1) and molecular weight distribution curves of PBCT and PBCCTs were shown in Figure S1. All of the PBCT and PBCCT polymers had very similar Mw values between 110 and 125 kg mol-1 and comparable Ð values (2.4-2.8). The chemical structures of the copolymers were determined by 1H NMR spectroscopy as shown in Figure 1, and the calculated CHDM contents are provided in Table 1. The mole percentages of CHDM in the copolymer chains were similar to the feed ratios. The sequence distribution of the PBCCTs was also analyzed using 13C NMR spectroscopy, as shown in Figures S2 and S3, to determine the microstructure of the backbone, such as the degree of randomness and number-average sequence length.37, 38 As shown in Table S1, the molar fractions (𝑓𝑓) of each dyad were obtained by integrating the relative peak areas. The number-average sequence length (L) and the degree of randomness (R) of these dyads were calculated based on the amounts of carbonate


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and terephthalate according to previously published equations.39,

40

The microstructure of the

resulting polymers, i.e., whether they are random, block-like or alternating structures, influences the materials properties significantly.41, 42 As such, the degrees of randomness based on carbonate (RC*) were calculated and found to be 1.09 (PBCCT10), 0.92 (PBCCT20), 0.96 (PBCCT30), 0.96 (PBCCT40), 0.98 (PBCCT50) and the degrees of randomness values based on terephthalate (RT) were 1.16 (PBCCT10), 1.01 (PBCCT20), 0.99 (PBCCT30), 0.98 (PBCCT40), 0.95 (PBCCT50). In the case of a random copolymer, R takes a value of 1, while alternate copolymer and block copolymer take values of 2 and near 0, respectively.43,

44

Therefore, the R values of all the

synthesized PBCCTs close to 1 suggests that they were random copolymers with a uniformly distributed sequence of monomers. The melting index (MI) was measured (190 oC and 2.16 kg load) in order to determine the suitability of the polymers for melt-blowing processes used to manufacture packaging film.45 All PBCCT polymers displayed MI values between 2.2 and 6.4 g/10 min, which are in an appropriate range for blown film extrusion (Table S2). The MIs of PBCT and PBCCTs decreased with increasing CHDM content. This finding is attributed to the bulky and rigid CHDM units disturbing melt-flow behavior of the polymers at the processing temperature.

Blown film extrusion. Blown film extrusion is one of the most important industrial processes in the production of polymeric films due to economic feasibility and productivity. As shown in Scheme 1, molten polymer is extruded at a constant flow rate through an annular die followed by drawing of the extruded polymer vertically via nip rolls into tube shapes. At the same time, cold air is injected into the polymer tube through the center of the die mandrel to form a bubble. Films


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of PBCT and PBCCTs, with an average thickness of 60 μm, were prepared using this blown film process for evaluation of their properties as packaging materials.

Figure 2. DSC heating curves of the PBCT and PBCCT blown films.

Table 2. Thermal characteristics of blown films of PBCT and PBCCTs. Polymer

Tg (oC)

Tm,1 (oC)

Tm,2 (oC)

ΔHm,1 (J g-1)

ΔHm,2 (J g-1)

ΔHm,total (J g-1)

Td,5% (oC)

Td,10% (oC)

PBCT

-0.2

57.7

124.9

5.7

16.9

22.6

283

296

PBCCT10

7.6

52.5

106.4

6.1

9.0

15.1

294

310

PBCCT20

16.4

54.5

93.0

9.9

3.3

13.2

296

312

PBCCT30

21.8

-

-

-

-

-

298

315

PBCCT40

24.5

-

-

-

-

-

307

320

PBCCT50

34.0

-

-

-

-

-

309

326


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Thermal properties. First, DSC of the blown films was conducted to understand the influence of CHDM content on their thermal properties. The first heating cycles of the blown films are shown in Figure 2 and their thermal properties are summarized in Table 2. In the low temperature range, glass transitions (Tg) of PBCT, PBCCT10, PBCCT20, PBCCT30, PBCCT40, and PBCCT50 films were observed at -0.2, 7.6, 16.4, 21.8, 24.5, and 34.0 °C, respectively. The increasing Tg with CHDM content may be due to hindered rotation of the bulky CHDM units reducing the mobility of the copolymer chains. In the higher temperature range (> 90 ºC), the PBCT film showed two separated melting peaks (Tm), which are attributed to the two distinct crystalline structures of the “1” segment (composed of BC* units, Tm,1 = 57.7 °C) and the “2” segment (composed of BC*B and BTB units, Tm,2 = 124.9 °C) in accordance with previous results.29 Similarly, PBCCT10 and PBCCT20 films exhibited two distinct Tms. The Tm,1 values of PBCCT10 and PBCCT20 were 52.5 and 54.5 °C, and the Tm,2 values were 106.4 and 93.0 °C, respectively. For polymers with higher CHDM content, i.e., PBCCT30, PBCCT40 and PBCCT50, no Tm was discernible, which can be attributed to suppressed crystallization owing to the bulky CHDM structures and the increased sequence length related to CHDM.46-50 For example, the number-average sequence length of CC* unit (LCC*) of PBCCT10 and PBCCT50 were found to be 1.00 and 2.13, respectively (Table S1). The melting enthalpies (ΔHm) of PBCT, PBCCT10, and PBCCT20 were also compared. The enthalpy of the first melting peak (ΔHm,1) increased from 5.7 (PBCT) to 9.9 J g-1 (PBCCT20) as the CHDM content increased, whereas that of second melting peak (ΔHm,2) decreased significantly from 16.9 (PBCT) to 3.3 J g-1 (PBCCT20). Considering the chemical structure of PBCCT, the population of BC*B and BTB sequences relative to BC* segments decreased markedly with increasing CHDM content (Table S1). Thus, the total melting enthalpy (ΔHm,total = ΔHm,1 + ΔHm,2) of PBCT, PBCCT10, and PBCCT20 decreased progressively from 22.6, 15.1, to 13.2 J g-1,


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respectively, which indicated that the incorporation of CHDM units suppressed the overall crystallization behavior. In order to identify the crystalline phases, XRD patterns of PBCT and PBCCT30 are shown in Figure S4. The crystal structure of the BT groups of PBCT shows diffraction peaks at 16.0 °, 17.2 °, 20.6 ° and 23.3 °. By contrast, PBCCT30 showed no obvious diffraction peaks, presumably due to its amorphous character. Further, calcium carbonate additives of blown film yield strong diffraction peaks at 21.5°, 29.5°, 36.1°, 39.5°, 43.2°, 47.6° and 48.6°. This result supports our DSC observations that the PBCT films have crystalline structures and PBCCT30 is more amorphous due to the bulky CHDM structures. In addition, the thermal stability of PBCT and PBCCT was measured by TGA (Figure S5). The degradation temperature at 5% weight loss (Td,5%) of PBCT, PBCCT10, PBCCT20, PBCCT30, PBCCT40, and PBCCT50 increased gradually as a function of CHDM content, from 283, 294, 296, 298, 307 to 309 °C, respectively. Also, the degradation temperature at 10% weight loss (Td,10%) increased gradually from 296 to 326 °C as the CHDM content increased from 0 to 50 mol% (Table 2). Therefore, we concluded that the copolymerization with CHDM enhanced the thermal stability of PBCCT copolymers. Additionally, TGA measurements showed a two-step degradation of PBCT and PBCCTs, consistent with previous reports on carbonate copolymers.30 This two-step degradation is attributed to the distinct thermal degradation mechanisms of BC* and BT units. In the first degradation step, the BC* units begin to degrade, accompanied by intramolecular transesterification. Then, the second degradation step of BT units occurs at higher temperature. Similar two-step degradation behaviors were also reported in other carbonate copolymers such as poly(butylene carbonate-co-butylene succinate) and poly(ω-pentadecalactone-co-trimethylene carbonate) copolymers.51, 52


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Figure 3. Mechanical properties of the PBCT and PBCCT copolymers: (a) tensile strength and elongation at break and (b) tear strength as a function of CHDM content.

Table 3. Properties of films formed from PBCT, PBCCTs, LDPE, LLDPE and HDPE.

PBCT

Tensile Strength (kgf/cm2) 267

PBCCT10

300

435

4.9

36

14.7

PBCCT20

305

337

5.1

31

19.1

PBCCT30

322

289

6.7

17

11.5

PBCCT40

357

232

8.6

16

11.9

PBCCT50

369

157

11.5

16

9.0

LDPE

200a

250a

9.5a

7a

189.5d

LLDPE

430b

740b

6.0b

12b

-

HDPE

320c

500>c

1.5c

70c

43.3d

Polymer

Oxygen Permeability

Elongation at break (%)

Tear Strength (kgf/mm)

Haze (%)

454

0.1

39

6.5

(cc∙mm/㎡∙day∙atm)

Obtained from the LOTTE Chemical Titan LDF260GG data sheet (thickness: 30 μm). bObtained from the LOTTE Chemical UF315 data sheet (thickness: 40 μm). cObtained from the LOTTE Chemical 7000F data sheet and experimental results (thickness: 30 μm). dObtained from ref. 9. a


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Mechanical Properties. Mechanical properties of the blown PBCT and PBCCT films were measured in accordance with ASTM standards (Figure 3). The film properties were compared with those of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE), and the results are summarized in Table 3. The mechanical properties of the PEs were obtained from LOTTE Chemical data sheets and previous reports.9 The tensile strength is one of the most important characteristics dictating polymer performance in blown film applications. However, PBCT displayed relatively lower tensile strength (267 kgf/cm2) compared with HDPE (320 kgf/cm2) and LLDPE (430 kgf/cm2). The mechanical properties of PBCT and PBCCT films changed significantly as a function of CHDM content (Figure 3), with the tensile strengths of PBCT, PBCCT10, PBCCT20, PBCCT30, PBCCT40, and PBCCT50 measured to be 267 ± 12, 300 ± 12, 305 ± 6, 322 ± 8, 357 ± 3, and 369 ± 9 kgf/cm2, respectively. Elongation at break of PBCCT, PBCCT20, PBCCT30, PBCCT40, and PBCCT50 was measured as 454 ± 24, 435 ± 14, 337 ± 15, 289 ± 11, 232 ± 18, and 157 ± 14%, respectively (Figure 3a). These results can be explained by the transition of the amorphous regions from rubbery to glassy states at room temperature with increasing CHDM content. As described above, the incorporation of CHDM increased the Tg of PBCCT, thereby polymers with higher CHDM contents are more glassy at room temperature. As a result, the tensile strength increased and elongation at break decreased with increasing CHDM content. Figure 3b shows the tear strengths of the blown films, which provides information related to the resistance to the propagation of defects or cracks in a film.53 The tear strengths of PBCCT increased remarkably with CHDM content, and were 0.1 (PBCT), 4.9 (PBCCT10), 5.1 (PBCCT20), 6.7 (PBCCT30), 8.6 (PBCCT40), and 11.5 kgf/mm (PBCCT50). It should be noted that the tear strength of PBCCT10 was ca. 50 times that of PBCT. The significant differences


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among the tear strengths of the blown films are attributed to the bulky and rigid CHDM units disrupting the one-directional alignment of polymer chains, which commonly occurs during the extrusion process.54 As such, the polymer chains inside the film tend to be entangled, which prevents the propagation of cracks typically caused by physical dissociation of the polymer chains.7 Comparing the results with the tear strength of LLDPE (6.0 kgf/mm), which is one of the most commonly used materials for packaging films, PBCCT30, PBCCT40 and PBCCT50 provide superior tear strengths and, thus, they can be considered excellent candidates for packaging films.

Figure 4. (a) Haze value of PBCT and PBCCTs and (b) images of PBCT, PBCCT20, PBCCT30 and PBCCT50 blown films. Optical properties and oxygen permeability. Transparency is a very important feature of packaging films to provide content information to consumers. Often, the transparency of films is evaluated using the quantitative parameter haze, which is defined as the percentage of scattered light distributed at an angle >2.5° relative to incident light. In the case of the blown films, haze is affected by light scattering and reflection/refraction due to both internal and surface roughness


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effects.10 Further, it is well known that structural fluctuations in polymer films cause lightscattering that promotes turbidity.55 The haze of the films from PBCT, PBCCT10, PBCCT20, PBCCT30, PBCCT40 and PBCCT50 was measured to be 39 ± 2, 36 ± 1, 31 ± 1, 17 ± 1, 16 ± 1, and 16 ± 1 %, respectively (Figure 4a), indicating the blown films were more transparent at higher CHDM contents, as shown in Figure 4b. The low haze values of PBCCT30, PBCCT40 and PBCCT50, notably comparable to that of LLDPE (12%), is attributed to the greater amorphous character in the polymers with higher CHDM contents (Table 3). In addition, the oxygen permeability of the PBCT and PBCCT copolymers was measured by an oxygen transmission rate tester and compared with that of LDPE, and HDPE; the results are summarized in Table 3. The oxygen permeability of PBCT and PBCCT films ranged from 6.5 and 19.1 cc∙mm/m2∙day∙atm, and was notably lower than that of non-degradable PEs, with oxygen permeabilities ranging from 43.3 to 189.5 cc∙mm/m2∙day∙atm. Several polymer properties are known to influence oxygen permeability, including chemical structure, polarity, free volume, crystallinity, and processing conditions.9,

56, 57

The low oxygen permeability of the PBCCT

copolymers is attributed to the polar carbonate units that reduce the free volume of the polymer film (Figure S6), since the high cohesive energy density imparted by the polarity is known to hinder polymer chain motion, thereby preventing passage of permeants.9 However, as the CHDM content increased by 20%, the oxygen permeability increased from 6.5 to 19.1 cc∙mm/m2∙day∙atm. This can be attributed to the decrease of crystallinity upon the addition of CHDM, as evidenced by the decrease of ∆Hm,total in Table 3. When the CHDM content was further increased to 50%, the polymer films became not only more amorphous but also more rigid as can be inferred from the increase of Tg. In particular, since the Tg values of PBCCT30, 40, and 50 are close to or over room temperature, the chain mobility of PBCCTs is limited due to the rigid structure of CHDM


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and thereby the oxygen permeability of the films decreases. As a result, the polymers showed a decreasing trend in oxygen permeability from 19.1 to 9.0 cc∙mm/m2∙day∙atm (Figure S7). The PBCCT30-50 films, which were found to exhibit optimal thermomechanical and optical properties, were also measured to have low oxygen permeability, between 9.0 and 11.5 cc∙mm/m2∙day∙atm. Thus, these results suggest that these degradable PBCCT copolymers provide comparable and even superior packaging properties as the non-degradable LDPE and HDPE conventionally used for sustainable food packaging applications.

Figure 5. (a) Weight loss of PBCT and PBCCT copolymer films measured as a function of time in the presence of enzyme, and (b) SEM images of PBCCT30 before, after 20 days, and after 40 days of film immersion in a solution with the degrading lipase.


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Enzymatic degradation. In general, the enzymatic degradation rate of polymers is related to different factors, such as the degree of crystallinity, molecular weight, side chain composition, melting temperature, and the molecular mobility of the amorphous phase.38,

58, 59

In order to

evaluate the biodegradability of the synthesized polymers, enzymatic degradation was monitored over 40 days. The weight loss of the samples was measured as a function of time during enzymatic degradation, and these data are presented in Figure 5a. After 40 days in the presence of the Pseudomonas sp. lipase, PBCT showed 37% weight loss, whereas PBCCT30 showed 52% weight loss over the same period of time, consistent with its lower degree of crystallinity, as shown in the DSC results. As the CHDM content increased, from PBCT to PBCCT30, the amorphous content also increased. The molecules in the amorphous region are loosely packed, and thus more susceptible access by degrading enzymes.60, 61 However, further increases of the CHDM content resulted in rigid polymer films with limited mobility of CHDM units, which presumably hindered biodegradation, explaining the lower weight loss of PBCCT40 and PBCCT50 relative to that of PBCCT30. The SEM images of PBCCT30 after different degradation times in Figure 5b clearly showcase the degradation of the material, by the increase in porosity and significant structural changes.


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CONCLUSIONS In this study, we successfully synthesized a series of PBCCT copolymers with different CHDM and BD contents (from 0 to 50%) on a large scale (1.5 kg). The various PBCCTs were extruded to produce blown films in order to explore their potential as materials for eco-friendly biodegradable packaging applications. The incorporation of CHDM units greatly impacted the thermal and mechanical properties, transparency, oxygen permeability and biodegradability of the PBCCT films. As the CHDM content increased in the copolymers, the PBCCT films became increasingly amorphous and glassy. PBCCT30-50 polymers were identified as particularly promising candidates as PE replacements for packaging films. For example, PBCCT50 demonstrated comparable and superior tensile strength (tear strength: 11.5 kgf mm-1 and tensile strength: 369 kgf cm-2) and comparable transparency (haze: 16%) relative to LDPE (tear strength: 9.5 kgf mm-1 and tensile strength: 200 kgf cm-2). In addition, the oxygen permeability of PBCCT50 (9.0 cc∙mm/m2∙day∙atm) was substantially lower than that of LDPE (189.5 cc∙mm/m2∙day∙atm), which is anticipated to provide significant advantages in food packaging applications. The incorporation of CHDM units into the biodegradable PBCT random copolymers presented produced materials with an optimal combination of the thermomechanical properties, transparency, permeability and biodegradability, offering a sustainable alternative to conventional nondegradable packaging films.


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ASSOCIATED CONTENT Supporting Information. Additional characterization data and results including GPC, 13C NMR, sequence distribution, Melting indexes, XRD, TGA, and molecular structures. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation (NRF) of Korea (2017M3D1A1039553,

2017K2A9A2A12000315,

and

2017R1D1A1B03036034).

We

acknowledge additional support for this work from the Research Projects of the KAIST-KUSTAR and Basic Research Program (NK211B) funded by the Korea Institute of Machinery and Materials.


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(61) Jia, Z.; Wang, J.; Sun, L.; Zhu, J.; Liu, X. Fully bio‐based polyesters derived from 2, 5‐ furandicarboxylic acid (2, 5‐FDCA) and dodecanedioic acid (DDCA): From semicrystalline thermoplastic to amorphous elastomer. J. Appl. Polym. Sci. 2018, 135 (14), 46076, DOI 10.1002/app.46076.


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TABLE OF CONTENTS

Synopsis Bench-scale synthesis of biodegradable PBCCTs that are mechanically robust and transparent offers a sustainable alternative to conventional non-degradable packaging films.


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Bench-Scale Synthesis and Characterization of Biodegradable

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