Renewable Sugar-Based Diols with Different Rigid Structure

Research Article pubs.acs.org/journal/ascecg

Renewable Sugar-Based Diols with Different Rigid Structure: Comparable Investigation on Improving Poly(butylene succinate) Performance Rong-Tao Duan, Qiu-Xia He, Xue Dong, De-Fu Li, Xiu-Li Wang,* and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, 29 Wangjiang Road, Chengdu 610064, China S Supporting Information *

ABSTRACT: In order to improve the mechanical and oxygen barrier properties of poly(butylene succinate) (PBS), two series of sugar-containing PBS copolyesters with different molecular stiffness were synthesized and comparatively investigated, in which a cyclic alditol, isosorbide (Is) or 2,3-O-isopropylidene-Lthreitol (ITh) was, respectively, used as a comonomer. Both cyclic alditols were easily available from biomass feedstock, such that Is is a bicyclic compound derived from D-glucose, and ITh is a monocyclic acetalized compound coming from naturally occurring L-tartaric acid. All these copolyesters containing up to 30 mol % of sugar-based units had satisfactory number-average molecular weights in the 16 000−83 000 g mol−1 range, and presented a random microstructure together with excellent thermal stability. The sugar-containing copolyesters were all semicrystalline and still possessed the monoclinic crystal structure of PBS. The incorporation of either Is or ITh units with rigid cyclic structure into PBS chain decreased the crystallinity and crystallization rate of PBS, while it enhanced its glass transition temperature. In addition, the partial replacement of 1,4butanediol by sugar-based units endowed PBS with steadily enhanced hydrophilicity. Depending on the type and content of sugar units, the copolyesters show different oxygen barriers and mechanical properties. It was found that copolyester with bicyclic Is sugar units exhibited a better oxygen barrier and mechanical property than that with monocyclic ITh units. KEYWORDS: Poly(butylene succinate), Isosorbide, 2,3-O-Isopropylidene-L-threitol, Thermal stability, Crystallization, Mechanical property, Oxygen barrier property

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INTRODUCTION

Aromatic polyesters such as poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) possess excellent thermal and mechanical properties. Therefore, aromatic terephthalate units with rigid structure are often introduced into the main chain of PBS to improve its thermal and mechanical performance.13−15 Nagata et al. synthesized poly(butylene succinate-co-butylene terephthalate) (PBST) random copolyesters,16 and found that the introduction of the aromatic component could enhance Tg and elongation at the break of PBS. Unfortunately, these aromatic terephthalate structures originate from fossil feedstock and are slow to degrade in the environment. In this regard, it would be highly desirable to utilize another rigid structure derived from renewable resources as a building block to improve PBS properties.

Driven by the increasing concerns of environmentally sustainable development, biobased monomers and polymers derived from renewable resources have been an expanding area with burgeoning scientific activity for their potential in promoting sustainable development of industry and reducing the utilization of petrochemicals.1−5 Up to now, some biobased polyesters, such as poly(L-lactide) (PLA), poly(butylene succinate) (PBS), and poly(butylene succinate adipate) (PBSA), are commercialized and show wide applications in biomedical, pharmaceutical, packaging, and agricultural fields.6−10 Among them, PBS is considered to be a promising substitute for polyethylene or polypropylene in many fields because of its balanced performance in mechanical properties and thermal stability as well as excellent processability.11,12 However, there are still some disadvantages existing in PBS, such as the low glass transition temperature (Tg) and slow degradation rate, etc., which will restrict its applications in some fields. © XXXX American Chemical Society

Received: October 20, 2015 Revised: November 27, 2015

A

DOI: 10.1021/acssuschemeng.5b01335 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

these cyclic sugar-based monomers can endow PBS with enhanced Tg and keep sustainability and biodegradability. Moreover, it is noted that bicyclic and monocyclic acetalized sugar units would able to endow PBS with different modified properties. Regarding this, in our work, the commercially available Is and more functionalized ITh are chosen as building blocks, respectively, and used in an effort to obtain a series of high molecular weight sugar-based PBS copolyesters with improved and adjustable properties. It is encouraging to comparatively explore the effect of these two renewable sugar-based diols with different rigid structure on improving PBS performance, including thermal, crystallization, oxygen barrier, and mechanical properties.

As the most abundant biomass feedstock, carbohydrates are widely used as polymer building blocks.17−19 Especially, cyclic sugar-derived monomers have received great attention for their capacity to improve the properties related to polymer chain stiffness. Therefore, Muñoz-Guerra’s group do a lot of work on preparation of aliphatic and aromatic copolyesters based on bicyclic sugar-based monomers with a diacetal constitution.20−22 Among bicyclic sugar monomers, the only one commercialized to date is 1,4:3,6-dianhydro-D-glucitol (isosorbide, Is), which is a bicyclic dianhydro alditol obtained by hydrogenation and subsequent dehydration of D-glucose coming from cereal starch. In the past decades, numerous attempts have been undertaken to utilize isosorbide as a monomer for various polyesters.23−28 Due to the low reactivity of secondary hydroxyl groups, it is difficult to achieve high molecular weight polyester directly using isosorbide and dicarboxylic acid as monomers via the classical melt polycondensation.27,28 Noordover et al. obtained the oligomers using Is, succinic acid, and other several diols as raw materials, which can only be used as renewable coating.29,30 Taking this into consideration, people substitute partial alkanediol with isosorbide, and in this way the isosorbide-containing copolyesters have improved properties.23 In Allais’s work, by judiciously substituting alkanediol segments associated to ferulic acid with isosorbide, the Tg of polymers can be easily improved and tuned to match those of fossile-based polyalkylene terephthalate.31−33 In addition, it was also found that only a small dosage of Is is needed in order to improve the Tg and thermomechanical resistance of aromatic polyesters (PBT and PET).33−36 This is ascribed to the fact that the stiff bicyclic structure of isosorbide restricts the internal rotatory degrees of freedom.36 2,3-O-Isopropylidene-L-threitol (ITh) is another cyclic sugar derivative. Unlike isosorbide, it only possesses a monocyclic structure. ITh can be easily synthesized from the acetalization of L-tartaric acid, an aldaric acid derived from L-threose, which mainly comes from a large variety of fruits. As a cyclic acetalized sugar-based diol, ITh has been employed in the preparation of polyurethanes and polyesters.37 Dhamaniya et al. synthesized a series of aliphatic polyesters and copolyesters made from isopropylidene acetalized L-tartaric acid derivatives. However, the molecular weight of these polymers was basically not more than 1 × 104 g mol−1.38,39 It is worth mentioning that, compared to the methylene acetalized protecting groups, the isopropylidene acetalized protecting groups in the copolyesters can be selectively hydrolyzed to generate functional polymers bearing pendant hydroxyl groups, which may have the potential to construct novel controlled drug delivery systems.38 More recently, the research focused on utilizing a cyclic sugar monomer to prepare PBS copolyesters is noteworthy. For example, Jacquel et al. synthesized on a pilot scale two series of PBS copolyesters with rigid biobased comonomers, namely, Is and 2,5-furandicarboxylic acid (FDCA), and found that Tg and elongation at break of these copolyesters were enhanced because of lower crystallinity.40 Muñoz-Guerra et al. reported that PBS copolyesters made of diacetalized bicyclic units derived from D-mannose (Manx-diol)24,41 exhibited highly increased Tg (68 °C for PManxS) and tensile strength.24 Additionally, when 2,3-di-O-methylene-L-threitol (Thx-diol) with acetalized monocyclic structure was introduced into the PBS main chain, the obtained PBS copolyesters displayed lower tensile strength but apparent increased elongation at break and Tg (13 °C for PThxS).42 Most exciting results have shown that

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EXPERIMENTAL SECTION

Materials. Succinic acid (SA, AR grade), 1,4-butanediol (BDO, AR grade), and zinc acetate (Zn(OAc)2, AR grade) were purchased from Kelong Chemical Corporation (Chengdu, China), and used without further purification. Tetrabutyl titanate (TBT) was also provided by Kelong Chemical Corporation, and dissolved in anhydrous toluene to prepare 0.2 g mL−1 solution. Isosorbide (Is, 98+%) was purchased from Alfa Aesar Chemical Co. (Tianjin, China). Dimethyl succinate (DMS, 99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. 2,3-O-Isopropylidene-L-threitol (ITh) was synthesized according to the literature.43,44 The synthetic procedures of ITh and its FT-IR, 1 H NMR, and HR-MS data are provided in SI (Figures S1 and S2). Other solvents (AR grade) were obtained from Kelong Chemical Corporation, and used without further purification. Characterization. Intrinsic viscosities of polyester were measured with an Ubbelohde viscometer at a concentration of 1.6 g dL−1 in chloroform at 25 ± 0.1 °C. Molecular weight determinations were performed by GPC, using a Waters apparatus equipped with a model 1515 pump, a Waters model 717 auto samplers, and a 2414 refractive index detector. Chloroform and monodisperse polystyrene were used as the eluent and standard, respectively. The concentration of sample and the flow rate of eluent were 2.5 mg mL−1 and 1.0 mL min−1, respectively. 1 H and 13C NMR spectra were recorded using a Bruker AC-P 400 MHz spectrometer at ambient temperature in CDCl3 solution (deuterated DMSO for PBThS16 via heating and its 1H NMR spectrum is shown in Figure S3) with tetramethylsilane as the internal reference. The thermal behavior of polyesters was examined by a TA Instrument (DSC-Q200). The samples of around 5 mg in aluminum pans were first heated to 140 °C at a heating rate of 10 °C min−1 (the first heating scan), and then held at 140 °C for 3 min to eliminate the thermal history. After that, it was cooled to −70 °C at a cooling rate of 10 °C min−1 (cooling scan), and finally reheated to 140 °C at the same heating rate (the second heating scan). Thermogravimetric analyses were performed on a TA Instrument (TGA-Q500). The thermograms were recorded under a nitrogen flow of 50 mL min−1 at a heating rate of 10 °C min−1, within a temperature range 40−700 °C. Sample weights of about 5 mg were used in the experiment. The isothermal crystallization kinetics of polyesters was also studied by DSC. The treatment on the samples was the following: the thermal history was removed by heating the samples up to 140 °C and held for 5 min, and then the samples were quickly cooled to the predetermined crystallization temperature, where they were left to crystallize until saturation. The samples annealed at 60 °C for 24 h were used for wide-angle Xray diffraction (WAXD) analysis. The patterns for the samples were recorded on the Philips X’Pert X-ray diffractometer using the Cu Kα radiation. The equipment was operated at room temperature with a scan rate of 2° min−1 scanning from 5° to 40°. The polarized optical microscope (POM) (NIKON ECLIPSE LV100POL) equipped with a temperature controller (HSC621V) was used to investigate the crystalline morphology of polyesters. For B


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ACS Sustainable Chemistry & Engineering Table 1. Composition and Molecular Weight of PBISx and PBThSx Copolyesters fa (mol %)

Fb (mol %)

mol wt

sample

yield (%)

fB

fI

f Th

FB

FI

FTh

[η]c (dL g−1)

Mne (g mol−1)

Mwe (g mol−1)

PDIe

PBS PBIS5 PBIS11 PBIS21 PBIS28 PBThS4 PBThS8 PBThS16 PBThS16* PBThS32

92 89 90 88 86 90 88 86 88 83

100 95 90 80 70 95 90 80 80 60

0 5 10 20 30

0

100 94.7 89.2 79.3 71.9 96.3 92.0 84.0 84.1 67.8

0 5.3 10.8 20.7 28.1

0

1.13 1.06 1.02 0.67 0.26 0.89 0.78 1.04d 0.53 0.50

69 500 65 800 83 000 31 600 15 600 37 400 30 900 f 24 400 16 000

204 100 133 600 169 200 58 700 21 100 124 100 116 100 f 59 000 40 400

2.93 2.03 2.04 1.86 1.35 3.32 3.76 f 2.41 2.53

5 10 20 20 40

3.7 8.0 16.0 15.9 32.2

Molar fraction of diols in the feed. bMolar fraction determined by 1H NMR spectra. cIntrinsic viscosity measured in chloroform at 25 °C. dIntrinsic viscosity measured in phenol:1,1,2,2-tetrachloroethane = 1:1 (v:v) at 25 °C. eObtained by GPC. fNot determined because of the insolubility of PBThS16 in chloroform. a

Scheme 1. Synthesis Route of PBISx and PBThSx Copolyesters.

measurement, the samples were first melted at 140 °C for 3 min to diminish any thermal history, and subsequently quenched to the selected temperature until crystallization finished. The water contact angles of the samples surfaces were tested by a contact angle analyzer (model JC2000). The sample was glued to a movable sample stage horizontally, and then about 3 μL of probe water was introduced on the sample sheet surface using a microsyringe. Dumbbell-shaped samples with thickness and width of 0.5 mm and 4 mm were prepared by hot pressing. The tensile strength, elongation at break, and Young’s modulus were measured at a stretching rate of 50 mm min−1 on a Sansi Universal Testing Machine (CMT, Shenzhen, China) at room temperature. At least five measurements were conducted for each sample, and the results were reported as averaged values. Oxygen permeability measurements were conducted on a gas permeation instrument (VAC-V1, Labthink Instrument Co, China). All testing samples were vacuum-dried at 60 °C for 24 h, and cut into circular discs with a thickness of 0.5 mm and a diameter of 50 mm. Each measurement was continuously monitored until a stable oxygen permeability rate was reached. Synthesis of Polymers. The polyesters with the selected composition were synthesized by melt polymerization in two steps, esterification or transesterification and polycondensation. Therefore, the more inexpensive succinic acid was used with 1,4-butanediol and Is for the synthesis of PBS copolyesters containing Is (PBISx). Since ITh is sensitive to the acid source, succinic acid had to be substituted by dimethyl succinate to polymerize with 1,4-butanediol and ITh to obtain the PBS copolyesters containing ITh (PBThSx). In the

abbreviations of PBISx and PBThSx, x refers to the molar percentage (mol %) of Is or ITh units relative to the total diols. An excess of diol with respect to diacid or diester was used in all cases in order to ensure the complete esterification or transesterification. For the two series of PBS copolyesters, the content of sugar-based units was in the range from 0 to 30 mol %. When the polymerization was completed, the product was cooled to room temperature. The obtained products were purified by dissolving in chloroform and then precipitating in an excess of methanol in order to remove oligomers and unreacted monomers. Finally, the filtered powder products were washed with methanol and dried under vacuum at 60 °C for 48 h. PBS Homopolyesters. Succinic acid and 1,4-butanediol with molar ratio of 1:1.06 were added into a three-necked round-bottom flask equipped with mechanical stirrer, water separator, and nitrogen inlet pipe. The esterification was carried out at 180 °C for 4 h, and then the catalyst tetrabutyltitanate (0.1 wt %, based on the total reactants) was introduced into the flask. Also, the polycondensation was conducted at 230 °C for 5 h under vacuum of 20−100 Pa. 1 H NMR (400 MHz, CDCl3), δ (ppm): 4.12 (t, 4H, J = 5.2 Hz, h H ), 2.63 (s, 4H, Hg), 1.71 (t, 4H, J = 5.2 Hz, Hi). 13C NMR (100.6 MHz, CDCl3), δ (ppm): 172.3 (CO), 64.2 (Ch), 29.0 (Cg), 25.2 (Ci). PBISx Copolyesters. The copolyesters were obtained by a similar procedure to that of PBS, while the polymerization conditions slightly differed for each composition. Succinic acid to diols (1,4-butanediol and Is mixture) molar ratio: 1:1.06. Esterification: 180 °C, 4 h. Polycondensation: 230 °C, 6 h (for x is 5 and 11 mol %); 220 °C, 7.5 h (for x is 21 and 28 mol %) . C


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Figure 1. 1H NMR spectra of (a) PBIS28 and (b) PBThS32. H NMR (400 MHz, CDCl3), δ (ppm): 5.23 (s, 1H, Hb), 5.18 (dt, 1H, J = 5.6 Hz, He), 4.84 (dd, 1H, J = 4.8 Hz, Hd), 4.50 (d, 1H, J = 4.4 Hz, Hc), 4.14 (t, 4H, J = 5.2 Hz, Hh), 3.99 (s, 2H, Haα, β), 3.95 (d, 1H, J = 6.0 Hz, Hfα), 3.82 (d, 1H, J = 5.2 Hz, Hfβ), 2.64 (s, 4H, Hg), 1.73 (t, 4H, J = 5.2 Hz, Hi). 13C NMR (100.6 MHz, CDCl3), δ (ppm): 172.3 (CO), 85.8 (Cc), 80.7 (Cd), 78.2 (Cb), 74.2 (Ce), 73.3 (Ca), 70.4 (Cf), 64.2 (Ch), 29.0 (Cg), 25.2 (Ci). PBThSx Copolyesters. Dimethyl succinate and diols (the compositions of 1,4-butanediol and ITh mixtures are shown in Table 1) with the molar ratio of 1:1.1 and transesterification catalyst zinc acetate (0.1 wt %, based on the total reactants) were added into a three-necked round-bottom flask equipped with mechanical stirrer, water separator, and nitrogen inlet pipe. The transesterification proceeded at 170 °C for 6 h (for x is 4, 8, and 16 mol %) or 160 °C for 7 h (for x is 16* and 32 mol %), and then the catalyst 1

tetrabutyltitanate (0.1 wt %, based on the total reactants) was introduced into the flask. The polycondensation was carried out at 220 °C for 6 h (for x is 4 and 8 mol %), 210 °C for 7 h (for x is 16 mol %), or 200 °C for 7 h (for x is 16* and 32 mol %) under vacuum of 20− 100 Pa. 1 H NMR (400 MHz, CDCl3), δ (ppm): 4.16−4.34 (m, 4H, Hj), 4.12 (t, 4H, J = 5.2 Hz, Hh), 4.05 (s, 2H, Hk), 2.62 (s, 4H, Hg), 1.73 (t, 4H, J = 5.2 Hz, Hi), 1.43 (s, 6H, Hl). 13C NMR (100.6 MHz, CDCl3), δ (ppm): 172.3 (CO), 110.4 (C(CH3)2), 75.7 (Ck), 64.0−64.2 (Ch+j), 29.0 (Cg), 26.9 (Cl), 25.2 (Ci).

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RESULTS AND DISCUSSION Synthesis and Chemical Structure of PBISx and PBThSx. Two sugar-containing copolyester series, PBISx and

D


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Figure 2. Four possible diads and 13C NMR spectra of the carbonyl carbons of succinate unit for (a) PBISx and (b) PBThSx.

37 000 g mol−1. For two copolyester series, Mn values steadily decrease with increasing Is or ITh content owing to their lower reactivity. It can be found that Mn values of the PBThSx series are lower than those of PBISx as a whole. The lower synthetic temperature conducted for PBThSx is the main reason for this difference. Besides, a few isopropylidene acetal ring openings probably occurring in the polycondensation may lead to the lower molecular weight. PBThSx and PBISx have good solubility in chloroform like PBS, except PBThS16, which can dissolve in phenol:1,1,2,2tetrachloroethane = 1:1 (v:v) or partially dissolve in dimethyl sulfoxide. The different solubility cannot be ascribed to the higher ITh content because PBTh32 can dissolve in chloroform. So, we deduce there are a few branching structures formed by the isopropylideneacetal ring opening during the selected reaction temperature (210 °C), and if the reaction temperature is decreased, this branching structure may be avoided. In order to verify this assumption, the same reaction condition as PBThS32 was used to synthesize PBThS16*. Undoubtedly, PBThS16* has good solubility in chloroform. However, with consideration of the low molecular weight of PBThS16*, in the following section, PBThS16 with higher molecular weight is chosen for further investigation. The chemical constitution of these copolyesters is ascertained by 1H NMR, and a detailed description of NMR data is given in the Experimental Section. Figure 1a,b shows the 1 H NMR spectra and shift assignments of two representative

PBThSx, were synthesized by a two-step melt polycondensation procedure which is usually applied in the industry for high scale production. The detailed chemical structures of these copolyesters are shown in Scheme 1. The essential difference between the two series is the type of sugar-based unit, which partially replaces the 1,4-butylene unit in PBS. For these two cases, the specific experimental conditions were selected to obtain sugar-containing PBS copolyesters with high molecular weight as much as possible. For PBS and PBISx, the esterification reaction occurred with the diacid/diol molar ratio of 1:1.06 at 180 °C without catalyst, and then TBT was added to catalyze the polycondensation reaction at 220−230 °C. For PBThSx, the transesterification reaction temperature was reduced to 160−170 °C with the diester/diol molar ratio of 1:1.1, and the subsequent polycondensation reaction was performed at 200−220 °C, where Zn(OAc)2 and TBT were used as the catalyst for transesterification and polycondensation, respectively. Lower reaction temperatures and longer reaction times were needed for the copolyesters with higher Is or ITh contents to minimize the thermo-oxidative degradation of these two sugar compounds. The obtained molecular weights for PBISx and PBThSx copolyesters are listed in Table 1. PBISx copolyesters exhibit satisfactory molecular weights; for example, their Mn values are in the range 16 000−83 000 g mol−1. Correspondingly, the Mn values of PBThSx copolyesters fall into the range 16 000− E


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ACS Sustainable Chemistry & Engineering samples (PBIS28 and PBThS32) for each series. As shown in Figure 1, the side peaks appearing at 2.6 ppm correspond to the Hg protons of the different diads. Integration of the proton signals arising from 1,4-butylene and sugar-based units leads to quantifying the composition of the copolyesters. The comonomeric composition of PBISx copolyesters was determined by comparing the integrated signal at 4.84 ppm (1Hd) of methine proton in Is to the signal at 1.73 ppm (4Hi) of methylene in 1,4-butylene units. For PBThSx copolyesters, the composition was determined by comparing the integrated methyl signals at 1.43 ppm (6Hl) deriving from ITh and the methylene signal at 1.73 ppm (4Hi) arising from 1,4-butylene units. The detailed results are also shown in Table 1. From the table we can see that the content of Is units in PBIS copolyesters is close to their respective feeds, whereas in the case of PBThSx copolyesters, the amount of ITh incorporated into the polymer is lower than their corresponding feeds. This discrepancy is attributed to the lower reactivity and higher volatility of ITh. The microstructure of PBISx and PBThSx copolyesters is determined by 13C NMR spectroscopy.45 As shown in Figure 2, the carbonyl carbon signals belonging to succinate units are split into multiplets due to the different chemical environments distributed between 1,4-butylene units and/or sugar-based units, which show enough resolution to analyze the sequence distributions of copolyesters. Regarding PBISx copolyesters, the 171.1−172.5 ppm region (shown from Figure 2a) produced by carbonyl carbons shows four resonance signal groups with the indication of B−B, B−I, I−B, and I−I diads. It can be found that the diad containing Is consists of multiple peaks as a consequence of the two possible orientations for Is units. Each diad molar fraction (N) was quantified by integrating the four diad-associated peaks. On the basis of such data, the probability (PBI, PIB) of finding an I(B) unit next to a B(I) unit, the number-average sequence lengths (LnB, LnI) and the degree of randomness (R) for PBISx copolyesters were calculated according to eq 1. The same method can be used to analyze the microstructure of PBThSx copolyesters. The carbonyl carbon signals appearing within the 171.8−172.4 ppm interval (shown from Figure 2b) correspond to four types of diads (B−B, B−Th, Th−B, and Th−Th). In this case, only one peak is observed for each diad. Unfortunately, the microstructure of PBThS16 could not be determined because of its low solubility in deuterated DMSO where much more dosage was needed. Equation 2 was used for calculating the LnB, LnTh, and R of PBThSx. PBI =

0.5(NB − I + NI − B) 1 = NB − B + 0.5(NB − I + NI − B) LaB

PIB =

0.5(NB − I + NI − B) 1 = R = PBI + PIB NI − I + 0.5(NB − I + NI − B) LnI

Table 2. Microstructure of PBISx and PBThSx Copolyesters.

0.5(NB − Th + NTh − B) 1 = NB − B + 0.5(NB − Th + NTh − B) LnB

PThB =

0.5(NB − Th + NTh − B) 1 R = NTh − Th + 0.5(NB − Th + NTh − B) LnTh

= PBTh + PThB

sample

NB−B

NB−I/I−B

NI−I

LnB

PBIS5 PBIS11 PBIS21 PBIS28

93.1 78.2 63.6 52.0

6.9 20.5 32.4 40.1

0 1.3 4.0 7.9

28.0 1.0 8.6 1.1 4.9 1.2 3.6 1.4 number-average sequence lengthsb

diad contenta (mol %) PBThS4 PBThS8 PBThS16* PBThS32

LnI

NB−B

NB−Th/Th−B

NTh−Th

LnB

LnTh

95.4 83.6 74.5 44.4

4.6 15.7 23.6 44.6

0 0.7 1.9 11.0

42.5 11.6 7.3 3.0

1.0 1.1 1.2 1.5

Rb 1.04 1.03 1.04 0.99

1.02 1.00 0.97 1.00

a Diad molar fraction (N) determined by 13C NMR. bNumber-average sequence lengths (n) and the degree of randomness (R) estimated by eqs 1 and 2.

sequence distribution in both two sugar-containing PBS copolyesters is essentially random. Besides, the NB‑B and LnB present a sustained downward trend with increasing the content of either Is or ITh units. This means the PBS chain has been isolated into shorter segments by the insertion of cyclic sugar units; thus, the regularity and continuity of PBS segment have been disturbed. Thermal Properties and Crystallization Behaviors of PBISx and PBThSx. The thermal stability of PBISx and PBThSx copolyesters was comparatively examined by thermogravimetric analysis (TGA). The TGA curves for two series as well as some illustrative derivative traces measured from 40 to 700 °C under a nitrogen atmosphere are shown in Figure S4. The thermal decomposition temperatures such as the temperature at 5% weight loss (T5%), the temperature at the maximum decomposition rate (Tmax), as well as the residue weight at 700 °C are listed in Table 3. The thermal decomposition of all these polyesters occurs in a single step leaving less than 4% of the initial weight remaining at 700 °C. For PBS, its T5% and Tmax are found at 324 and 400 °C, respectively, whereas, for the copolyesters, their thermal stability has a strong relationship with Is or ITh content. It was found that when the Is unit is 5 or 11 mol %, the thermal stability of PBISx copolyesters is slightly higher than PBS, reflected by its higher T5%. With a further increase in Is content, T5% of PBIS21 and PBIS28 are similar to that of PBS; however, their Tmax decreases, which can be ascribed to their poor molecular weight. Regarding PBThSx copolyesters, their Tmax values are lower than that of PBS and steadily decrease with increasing ITh unit content, although their T5% stays unchanged when the ITh content is lower than 16 mol %. For the two series, the thermal stability of copolyesters with sugar content up to about 30 mol % exhibits an exceptional decrease, which is mainly caused by their lower molecular weight. The above results show that copolyesters containing bicyclic Is units retain similar thermal stability as PBS, whereas those consisting of monocyclic ITh units exhibit lower thermal stability than PBS and PBISx, since the isopropylidene acetal ring of ITh is more sensitive to heat. The thermal transitions behaviors of these copolyesters were studied by DSC. The heating and cooling curves of the two series, measured from −70 to 140 °C, are shown in Figure 3,

(1)

PBTh =

number-average sequence lengthsb

diad contenta (mol %)

(2)

The results based on eqs 1 and 2 are listed in Table 2. It can be seen that R values are all very close to 1 indicating that the F


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61.0 54.5 50.4 68.2 53.1 47.6

Temperature at which 5% weight loss was observed. bTemperature for maximum degradation rate. cRemaining weight at 700 °C. dThe glass transition (Tg), melting (Tm), crystallization (Tc), and cold crystallization (Tcc) temperatures, and their respective enthalpies (ΔHm, ΔHc, and ΔHcc) of samples measured by DSC. eCrystallinity degree (Xc) calculated by the deconvolution of amorphous and crystalline peaks in WXRD patterns using the peak separation software.

50 −34 −26 −21 −19 −13 −29 −24 −22 −14 64.6 58.6 51.1 74 62 50

62.7 56.8 41.5 32.9 22.2 50.9 49.7 42.8 27.2 115 110 91/102 45/72/90 47/66/81 100/111 105 47/88/96 50.1 0.7 0.8 1.7 2.0 0.9 0.7 1.2 3.1 3.6 400 399 402 393 387 387 382 376 367 324 332 334 326 324 326 325 325 312 PBS PBIS5 PBIS11 PBIS21 PBIS28 PBThS4 PBThS8 PBThS16 PBThS32

while the thermal data are summarized in Table 3. PBISx and PBThSx copolyesters containing up to about 30 mol % of sugar-based units are semicrystalline, as evidenced by the melting endotherms recorded in the first heating traces. Nevertheless, Tm, ΔHm, Tc, and ΔHc of two series all decrease clearly with the introduction of either Is or ITh units. This reveals that the presence of rigid cyclic units in the polyester chains hinders the chain movement and packing, which depresses its crystallization. For PBISx and PBThSx copolyesters with low sugar unit contents, whose sugar comonomer content does not exceed 11 mol % or 16 mol %, show good crystallization ability and high melting enthalpy. When the content of Is or ITh is around 30 mol %, neither the crystallization peak in the cooling scan nor the melting peak in the second heating scan was observed. However, they can still crystallize and exhibit melting peaks in the first heating scans. The composition dependence of both Tm and ΔHm of two series of copolyesters can be revealed from their detailed values listed in Table 3. Although the Tm values and related enthalpies of these copolyesters containing sugar-based units all steadily decrease with the sugar content, the depressing effect of Is is higher than that of ITh. For PBIS21 and PBThS16, their Tm was observed to decrease from 113 °C of PBS to 88 and 95 °C, respectively. This means that the bicyclic sugar unit with higher stiffness will more seriously restrict the chain regularity and movement; therefore, more imperfect crystals form and exhibit lower Tm. The changes in T g of two copolyester series are comparatively represented in the plots depicted in Figure 4a. In both cases, Tg steadily increases with the increase in sugar content. In the case of the PBISx series, Tg increases from −34 °C of PBS up to −13 °C of PBIS28, while Tg of the PBThSx series also increases up to −14 °C for PBThS32, which is due to the replacement of the flexible butylene segments by the rigid sugar-based units. In addition, the enhancement effect of ITh is slightly weaker than that of Is. Such a difference results from the fact that, with respect to the isopropylidene acetal monocyclic structure of ITh, the bicyclic structure of Is confers higher stiffness to polymer chain and increasingly restricts its mobility. In order to make clear how these cyclic sugar-based units affect the crystal structure of PBS, WAXD is used, and the detailed WAXD patterns of copolyesters together with PBS are presented in Figure 4b. The crystallinity degree (Xc) of copolyesters can be calculated from the deconvolution of amorphous and crystalline peaks using the peak separation software (shown in Table 3).46 PBS shows three strong characteristic diffraction peaks at 2θ values of 19.7°, 21.9°, and 22.7° corresponding to (020), (021), and (110) planes, which are produced by the monoclinic crystal structure of PBS.47 Apparently, for either of the two copolyester series with contents of Is or ITh up to about 30 mol %, the similar diffraction peak positions are shared with PBS. This means that these sugar-containing PBS copolyesters retain the same monoclinic crystal structure of PBS. However, the Xc of these copolyesters steadily decreases with the increasing sugar-based units. For PBISx and PBThSx copolyesters, the values of Xc oscillate in the ranges 30.0−60.6% and 36.4−67.7%, respectively, which are below that of PBS (69.9%). Moreover, what is really remarkable that is observed from the WAXD patterns is that the decreasing effect on Xc is more pronounced for PBISx copolyesters. This result is in coincidence with the melting enthalpies measured by DSC. Such a reduction of

a

19.0

WXRD

69.9 60.6 53.9 45.1 30.0 67.7 66.6 63.0 36.4 63.5 55.2 44.8 14.4 2.3 59.8 50.6 43.5 113 108 100 88 83 109 104 95

ΔHcc (J g−1) ΔHc (J g−1) Tc (°C) ΔHm (J g−1) Tm (°C) Tmax (°C)

T5% (°C) sample

b a

TGA

c

RW (%)

first heating

d

Table 3. Thermal and Crystallization Properties of PBISx and PBThSx Copolyesters

cooling

d

DSC

Tg (°C)

Tcc (°C)

second heatingd

Tm (°C)

ΔHm (J g−1)

Xce (%)


G


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Figure 3. DSC curves of second (a, c) heating run and (b, d) cooling run at a rate of 10 °C min−1 of PBISx and PBThSx.

DSC in the 66−90 °C intervals. PBISx and PBThSx with sugarbased unit contents below 11 mol % and 16 mol %, respectively, are able to crystallize from the melt; therefore, they are suitable for realizing this study. Although the isothermal crystallization of homopolyester and copolyesters could not be studied at the common crystallization temperature (Tc) due to their large differences in crystallization rates, crystallization conditions were chosen as close as possible to draw a valuable conclusion. Actually, PBIS6, PBIS11, PBThS4, PBThS8, and PBThS16 except for PBS could be compared at the same Tc of 74 °C to ascertain the dependence of crystallizability on composition. The well-known Avrami equation is used to analyze the isothermal crystallization kinetics of the two copolyester series, which assumes that the relative degree of crystallinity develops with crystallization time as 1 − X t = exp( −kt n)

(3)

where Xt is the relative crystallinity at time t, n is the Avrami exponent which denotes the nature of the nucleation and growth process, and k is a rate constant depending on nucleation and crystalline growth rate.48 Equation 3 can be rewritten as log[− ln(1 − X t)] = log k + n log t

(4)

A plot of log[−ln(1 − Xt)] versus log t would give a straight line from which both the Avrami exponent and the rate constant can be determined. The Avrami parameters as well as the corresponding calculated half-crystallization time (t1/2) of these two copolyester series are listed in Table S1. It can be noted that more half-crystallization time was needed with higher temperature for each case. The crystallization rate reduced with the increase of Tc, which indicated that the process is controlled by nucleation. In the case of PBS and PBISx, the values of n are within the range 2.5−2.8, suggesting

Figure 4. (a) Tg values dependence of the content in sugar-based units and (b) WAXD patterns of PBISx and PBThSx copolyesters.

crystallinity caused by the cyclic sugar-based units can be interpreted as the decreased chain mobility and the difficulty in chain packing. The isothermal crystallization process of PBISx and PBThSx copolyesters as well as PBS was comparatively investigated by H


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replaced by the bicyclic Is units with higher stiffness rather than the monocyclic ITh units. Meanwhile, POM was used to further investigate the influence of sugar-based unit and crystallization temperature (Tc) on the spherulitic morphology of two series of PBS copolyesters. Figure S5 displays a series of POM images of PBIS5 at various isothermal crystallization temperatures. In general, the nucleation is more difficult at higher Tc, so the observed size of spherulites becomes larger with increasing Tc. Besides, all spherulites of PBIS5 show obvious banded structures at different Tc, and the band spacing increases with Tc. Banded spherulites are considered to form through the periodic twisting of lamellae along the radial growth direction. The unbalanced stress occurred at lamellar surface is regarded as its driving force.47,52−55 Higher Tc would lead to the increase of lamellae regularity and thickness, which makes the twist of lamellae more difficult, and thus results in the increase of band spacing. As shown in Figure 6, the effect of sugar-based unit content on the spherulite morphology is studied. Here, PBISx and PBThSx are all isothermally crystallized at 74 °C, and neat PBS is crystallized at 82 °C for a comparative study. Obviously, the size of spherulites increases with the increasing stiffness of polymer chain or sugar-based unit content regardless of PBISx or PBThSx. Such a result is reasonable since the higher stiffness or sugar-based unit content would make the supercooling and chain regularity of copolyesters decrease, which results in the reduction of nucleation density. In addition, for the two series of copolyesters, their band spacing increases with the content of sugar-based units, which also is ascribed to the decrease of supercooling. Further observing the spherulites’ morphology of PBISx and PBThSx, we can see that PBISx species show coarser bands and narrower band spacing. In the present system, the incorporation of Is with higher rigidity could increasingly disturb the chain mobility and thus would form the distorted lamellae and coarse bands. Bands with such defects were found here and there; however, its mechanism is still uncertain.56−58 Hydrophilicity and Mechanical Properties of PBISx and PBThSx. The increasing hydrophilicity is one of the factors that accelerate the hydrolytic degradation rate of polyesters. The water contact angle between water and polyester films was measured to characterize the surface hydrophilicity of PBISx and PBThSx, and the results are shown in Figure 7. In these two cases, the water contact angle of copolyesters presents a sustained downward trend with the increase of sugar-based units. The water contact angle of PBIS28 and PBThS32 is decreased from 72.1° of PBS to 57.0° and 63.4°, respectively. The results indicate that the incorporation of these two cyclic sugar-based units can enhance the hydrophilicity of PBS, which can be reasonably attributed to the greater hydrophilicity of the sugar-based moieties. Besides, from the Figure 7, a conclusion can be drawn is that the enhancing hydrophilicity effect is slightly weaker when ITh units replace the butylene units in the polyester. Also, this may be related to the lower hydrophilicity of ITh units due to the presence of the isopropylidene side group. It is well-known that the mechanical properties play an important role in the application of materials. In this study, the tensile properties of the copolyesters have been investigated, and the results are provided in Table 4. It can be observed that the influence of the sugar-based unit on the tensile properties of copolyesters is different. In PBISx cases, the tensile strength

that their crystallization is three-dimensional spherulitic growth with heterogeneous nucleation.49,50 Also, the incorporation of Is units did not change the isothermal crystallization mechanism of PBS. For PBThSx copolyesters containing 4 mol % and 8 mol % of ITh, the values of n are close to 2.5, presenting a similar crystallization mechanism to the above. Nevertheless, when the content of ITh units is up to 16 mol %, the values of n are lower and oscillate in the range 1.9−2.2, which illustrated that the crystallization mechanism of PBThS16 is changed to the heterogeneous nucleation with preferentially two-dimensional crystal growth.51 The decrease of spherulite growth dimension for PBThS16 is ascribed to the fact that the higher ITh content would lead to a serious dilution effect on crystallizable segment; thus, the regularity and continuity of PBS polymer chains is disturbed. In this situation, PBS chains tend to aggregate into two-dimensional axialites rather than three-dimensional spherulites.51 Besides, the relative crystallinity versus crystallization time plots and the corresponding Avrami double logarithmic plots are also comparatively depicted in Figure 5. From the figure, it

Figure 5. Isothermal crystallization of PBS, PBIS5, PBIS11, PBThS4, and PBThS8 at the indicated temperature: (a) relative crystallinity versus time plot and (b) log−log plot.

can be seen clearly that the depressing crystallization effect of PBISx cases is more pronounced with respect to that of the PBThSx series, which is in accordance with the previous DSC analyses. The t1/2 data (shown in Table S1) provide quantitative proof for the suppressing crystallization influence caused by the cyclic sugar-based units. The presence of the rigid cyclic structure with nonsymmetric configuration is responsible for the decrease of crystallization rate in PBISx and PBThSx copolyesters. Such an effect appears more obvious in the case of PBISx copolyesters, which should be attributed to the larger reduction in chain mobility when the butylene units are I


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Figure 6. Crystalline morphology of samples isothermally crystallized at the indicated temperatures: (a) PBS at 82 °C, (b) PBIS5 at 74 °C, (c) PBIS11 at 74 °C, (d) PBThS4 at 74 °C, and (e) PBThS8 at 74 °C.

break of PBThSx is comparable to that of PBS. This difference results from the lower rigid monocyclic structure of ITh. In the two series, Young’s modulus all continuously decreases with the increase of cyclic sugar-based unit content. The above results demonstrate that the insertion of bicyclic Is sugar unit into aliphatic polyesters exhibits a better mechanical property than that of monocyclic ITh units, especially for the samples with low Is contents (below 11 mol %). Oxygen Barrier Property of PBISx and PBThSx. Depending upon the application, a good oxygen barrier property is important for packaging material. Table 4 lists the oxygen permeability of PBISx and PBThSx. The oxygen permeability of neat PBS is 1037 mL μm m−2 day−1 KPa−1. Compared with that, the oxygen permeability of PBISx exhibits a trend of first decreasing and then increasing with the increasing content of Is. This consequence is also attributed to the combined action of both chain stiffness increasing and the crystallinity decreasing. According to Catalani or Jaffe’s reports,59,60 molecular chain orientation may play one of the important roles on the mechanical and oxygen barrier properties of copolyesters; however, it is not discussed as the representative effect here because the sugar-based units are random distributed in chain backbone, and under our processing conditions the molecular chain orientation is rarely occurred. As we all know, the gas permeability of the polymers depends on its diffusion and solubility process. Activated diffusion is described as the opening of a void space among a series of segments of polymer chain due to the oscillations of segments (“active state”), followed by the translational motion of permeant molecules within the void space before segments

Figure 7. Water contact angle of PBISx and PBThSx copolyesters as a function of the content in sugar-based units.

and elongation at break first increase with enhancing Is content, and subsequently decrease when the content of Is is further increased up to 21 mol %. It is deduced that the relatively lower molecular weight and degree of crystallinity of PBIS21 are the main reason leading to this phenomenon. It should be noticed that, though the crystallinity decreases steadily with Is content, PBIS5 and PBIS11 exhibit much higher tensile strength (45.3− 47.5 MPa) and elongation at break (731−925%) than PBS. This is a result of the competitive effect of chain stiffness increasing and crystallinity decreasing. However, for PBThSx copolyesters, the substantial reduction of tensile strength along with ITh content increase is observed, and the elongation at

Table 4. Mechanical and Oxygen Barrier Properties of PBISx and PBThSx mechanical property

oxygen barrier property

sample

tensile strength (MPa)

elongation at break (%)

Young’s modulus (MPa)

oxygen permeability (mL μm m−2 day−1 KPa−1)

PBS PBIS5 PBIS11 PBIS21 PBThS4 PBThS8 PBThS16

40.6 ± 1.7 45.3 ± 2.1 47.5 ± 1.7 23.5 ± 2.8 35.3 ± 0.6 29.9 ± 0.6 21.6 ± 1.2

382 ± 51 731 ± 57 925 ± 43 740 ± 51 327 ± 13 478 ± 40 379 ± 64

339 ± 20 300 ± 4 289 ± 7 235 ± 10 349 ± 43 302 ± 54 229 ± 55

1037 588 397 1987 726 657 734

J


Research Article

ACS Sustainable Chemistry & Engineering to their “normal state”.61 The growing chain stiffness resulting from the incorporation of cyclic sugar-based units will weaken the ability of a segment of polymer chain to relax and shift its structure. Thus, the permeant access becomes more difficult to newly form, which will result in a better oxygen barrier property. However, DSC and WAXD results have demonstrated that the incorporation of rigid cyclic sugar-based units can reduce the crystallinity of copolyesters and consequently enrich the amorphous phase, which will lead to the oxygen diffusion rising and worsen their oxygen barrier properties. When Is content is below 11 mol %, the effect coming from chain stiffness growing is dominant; thus, it will endow copolyester with increasing oxygen barrier properties. Especially, the oxygen permeability of PBIS11 is as low as 397 mL μm m−2 day−1 KPa−1. However, as Is content is further increased, the effect coming from crystallinity decreasing is prevailing, and the oxygen barrier property of PBS copolyesters turns to a diminished tendency. As far as PBThSx species are concerned, their effect on crystallinity decreasing and Tg increasing are not obvious compared with those of PBISx; therefore, their oxygen barrier property almost does not show composition dependence.

ers with sugar units also can improve PBS’s hydrophilicity and oxygen barrier properties, which will widen its application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01335. Experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone/fax: 86-28-85410755. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Science Foundation of China (51273123, 51421061, and J1103315), the Research Fund for the Doctoral Program of Ministry of Education of China (No. 20120181110049), and the Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026).

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CONCLUSIONS Two types of cyclic alditols, Is and ITh have been successfully used as comonomers to prepare sugar-containing PBS copolyesters by melting polycondensation. Although the secondary hydroxyl reactivity of Is is lower than that of the primary hydroxyl of ITh, the partial incorporation of Is leads to the random PBS copolyesters with pretty high molecular weights. Because the isopropylidene acetal ring of ITh is more sensitive to heat, the softer polycondensation conditions are needed for synthesis of PBThSx copolyesters. Although both Is and ITh possess cyclic structure, they endow PBS with different properties. Copolyesters containing Is units maintain excellent thermal stability of PBS, whereas those consisting of ITh units are less stable to heat than PBS and PBISx. The two copolyester series are semicrystalline for all the studied compositions and retain the same monoclinic crystal structure as PBS. The insertion of Is or ITh units into a PBS chain leads to a decrease of melting temperature, crystallinity, and crystallization rate; however, the depressing effect is more noticeable when bicyclic Is is used as the comonomer. One of the most remarkable results produced by the rigid nature of cyclic sugar units is the notable increase in Tg of PBS. Undoubtedly, Is with higher stiffness has a more noteworthy contribution to the increase of Tg. For example, the Tg of PBIS28 was −13 °C, which is apparently higher than that of poly(butylene succinate-co-butylene terephthalate) containing 30 mol % terephthalate units (PBST30, Tg ≈ −25 °C).16 The mechanical and oxygen barrier properties of PBS can be significantly improved when low Is content is incorporated. Also, the copolyesters containing ITh units also show a better oxygen barrier property than PBS, and almost do not change with their composition. In addition, the hydrophilicity of PBS steadily increases along with the augmentation of cyclic sugar unit content. Also, this increasing effect is slightly weaker for ITh units due to the presence of more hydrophobic isopropylidene side groups. This work demonstrates that when suitable cyclic alditols are chosen as comonomers, the thermal and mechanical performance of PBS can be improved and finely tuned to match those of fossile-based polyalkylene terephthalate. Besides, copolyest-

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