Short Chain Mixed Cellulose Esters: Effects ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Long/Short Chain Mixed Cellulose Esters: Effects of Long Acyl Chain Structures on Mechanical and Thermal Properties Shukichi Tanaka,†,‡ Tadahisa Iwata,*,† and Masatoshi Iji*,‡ †

Graduate School of Agricultural and Life Sciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-8657, Japan IoT Device Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan

‡

S Supporting Information *

ABSTRACT: Long/short chain mixed cellulose esters (MCE) are practical, promising polymers with interesting properties. In the molecular design of MCE, using long acyl chains made from renewable resources is important and enhances the value of MCE as sustainable materials. In this study, we focused on two types of renewable long acyl chains for MCE: the aromatic 3-pentadecylphenoxy acetyl (PA) group derived from cardanol extracted from cashew nutshells and the aliphatic stearoyl (St) group made from vegetable oils. Using these long acyl chains and the acetyl (Ac) group as a short acyl chain, we synthesized PA/Ac MCE (P-series) and St/Ac MCE (S-series) in LiCl/DMAc medium. The thermal and mechanical analyses revealed that a mixed substitution of long and short acyl chains prevented the crystallization of the long acyl chain moieties in MCE. The P-series had slightly higher bending strength and glass transition temperature than those of the S-series but showed low impact strength because of the existence of the aromatic ring in the PA group, which caused an increase in the stiffness of the cellulose backbone and the extra intermolecular interaction. However, the S-series without aromatic rings showed remarkably improved impact strength with sufficient balanced mechanical properties for use in durable products due to its composition of low crystalline long acyl chain moieties. KEYWORDS: Bioplastic, Cellulose ester, Long acyl chain, Cardanol, Stearic acid

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INTRODUCTION Progress in biomass-based plastics (bioplastics) from renewable resources has been accelerating in the face of petroleum resource depletion and global climate change.1,2 Moreover, due to the possibility of future food shortages, the use of nonedible plant resources to produce bioplastics has been promoted. Cellulose is a very abundant renewable polymer.3,4 It therefore should be thermoplasticized to broaden the present range of its applications. Cellulose esters (CE) represent a class of commercially important thermoplastic polymers in the fiber, film, and filter industries.5 However, classical short-chain CE such as cellulose acetate and cellulose acetate propionate require the addition of a large amount of external plasticizers on melt processing to widen the processing window between the melting and degradation temperatures. These external plasticizers frequently give rise to extraction or volatilization. Confronted by this problem, many researchers have attempted to thermoplasticize cellulose by bonding long side chains, namely, fatty acyl groups, as internal plasticizers.6−8 The careful investigations revealed that thermal properties of these long-chain CE can be controlled by changing the acyl chain length.9,10 Especially in fully substituted long-chain CE, the melting point declined sharply with increasing the acyl chain © XXXX American Chemical Society

length from 2 (acetate) to 6 carbons (hexanoate) and increased slightly in the fatty acyl range from 8 (octanoate) to 18 carbons (stearate).11,12 Moreover, about mechanical properties, the previous literatures reported the fatty acyl CE has some low strength and brittleness even if the acyl chain length changed.13−15 These results have indicated that the crystallinity of long acyl chains dominated the thermal and mechanical properties of long-chain CE. In contrast, long/short chain mixed cellulose esters (MCE) are practical, promising polymers with interesting properties because a wide range of the properties can be achieved by a limited degree of substitution (DS < ca. 1.0) of the long-chain. Vaca-Garcia et al. have synthesized MCE including fatty acyl and acetyl groups in a lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) medium and showed that MCE were highly hydrophobic and more mechanically resistant than the corresponding simple CE including only the fatty acyl group.16 Edgar et al. have investigated practical mechanical properties of MCE such as cellulose acetate-hexanoate and cellulose acetate-nonanoate.17 From these results, we have Received: August 28, 2016 Revised: December 5, 2016 Published: December 19, 2016 A

DOI: 10.1021/acssuschemeng.6b02066 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Synthesis of MCE with Mixture of Anhydrides: (a)PA/Ac MCE (P-series) and (b)St/Ac MCE (S-series)

Industry Co., Ltd., and 2-propanol was purchased from Tokuyama Corp. N,N-Dimethylaminopyridine (DMAP) was purchased from Sigma-Aldrich Co. LLC. All were used as received without any further purification. Synthesis of PA/Ac MCE (P-series). The design for the synthesis process is shown in Scheme 1a. Before esterifying cellulose, mixed anhydrides, which were used as esterification reagents in this study, were prepared as follows. PAA (13.4−80.4 g, 37−222 mmol) and acetic anhydride (21.0 mL, 222 mmol) were mixed at 100 °C for 1 h under a dry nitrogen atmosphere. As shown in Scheme S2 of the Supporting Information, the liquid product consisted of three acid anhydrides (acetic anhydride, PA/Ac mixed anhydride, and anhydride of PAA) and two acids (acetic acid and PAA). The molar compositions of these mixtures calculated by NMR are listed in Table S1 of the Supporting Information. Cellulose powder with ca. 6% adsorbed water (6.0 g by dry weight and 37 mmol per anhydroglucose unit (AGU)) was stirred in deionized water (90 mL) overnight, and the resulting dispersion liquid was filtered off by vacuum filtration. The swollen substrate was stirred twice in DMAc (90 mL) overnight and isolated by vacuum filtration each time. The pretreated cellulose with ca. 8 g of the adsorbed DMAc was dispersed in 194 g of 8.0 wt % LiCl/DMAc solution and dissolved by being stirred overnight at ambient temperature after being heated at 100 °C for 1 h. After the solution became clear, 150 mL of DMAc, 3 g of DMAP22,23 dissolved in DMAc (120 mL), and the mixed anhydride prepared above dissolved in DMAc (150 mL) were slowly added to the solution. The resulting solution was stirred at 100 °C for 6 h under a dry nitrogen atmosphere. During stirring, the solution tended to be gelatinized when infeed of PAA was over 2 eq per AGU. After methanol (2.5L) was added to precipitate the products, synthesized MCE was separated by vacuum filtration. The massive gelatinized parts were pulverized by freeze-crushing. The obtained products were stirred three or four times in 2-propanol (200 mL) at 60 °C for 0.5 h to elute unreacted components and isolated by vacuum filtration each time. The purified products named P1−P4 were obtained by drying in

hypothesized that a limited DS of long chains and appropriate DS of short chains improve the mechanical properties of MCE by decreasing the crystallinity of the long chains. Additionally, the influence of various structures of the long acyl chains on the properties of MCE must be investigated further to promote their practical use. To enhance the value of MCE as sustainable materials with a high biomass content, using long acyl chains made from renewable resources is very important in the molecular design of MCE. In this study, we focused on two types of renewable long acyl chains for MCE: the aromatic 3-pentadecylphenoxy acetyl (PA) group derived from cardanol extracted from discarded cashew nut shells and the aliphatic stearoyl (St) group made from vegetable oils. Cardanol is made of inedible plant resources and is a phenol derivative with a long hydrocarbon chain.18,19 Using these two long acyl chains and the acetyl (Ac) group as a short acyl chain, we synthesized PA/ Ac MCE (P-series) and St/Ac MCE (S-series) with various ratios of the long/short chains in the LiCl/DMAc medium and investigated structural effects of the long acyl chains on the thermal and mechanical properties of MCE.

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EXPERIMENTAL SECTION

Materials. Cellulose powder (KC FLOCK, W50GK, DP: 1300, purity: 95%−97%) made from wood pulp was supplied by Nippon Paper Industries Co., Ltd. DP was determined by viscometry.20 The cellulose powder was 10−50 μm in diameter and 100−500 μm long. We synthesized 3-pentadecylphenoxy acetic acid (PAA), which is hydrogenated and carboxymethylated cardanol, as previously reported.21 The scheme for the synthesis of PAA is shown in Scheme S1 of the Supporting Information. Acetic anhydride, DMAc, and LiCl were used as supplied by Kanto Chemical Co., Inc. without any further purification. Stearic acid (SA) was purchased from Tokyo Chemical B


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ACS Sustainable Chemistry & Engineering Table 1. Infeed and Compositions of P-series and S-series 1

Infeed/AGU

a

no.

PAA

SA

P1 P2 P3 P4 S1 S2 S3 S4

1 eq 2 eq 3 eq 6 eq    

    3 eq 6 eq 9 eq 12 eq

H NMR (CDCl3)

Ac2O 6 6 6 6 6 6 6 6

eq eq eq eq eq eq eq eq

DSPA

DSSt

DSAc

DSTotal

conversion ratio of long chain (%)a

weight ratio of long chain (wt %)

0.4 0.8 1.0 1.5    

    0.4 0.7 l.0 l.l

2.6 2.2 2.0 1.2 2.6 2.3 2.0 1.9

3.0 3.0 3.0 2.7 3.0 3.0 3.0 3.0

40 40 33 25 13 12 11 9.2

34 52 59 71 28 42 52 55

note partly gelatinized gelatinized gelatinized

Ratio of long-chain DS in the infeed amount.

a vacuum for 5 h at 105 °C. The DS of the P-series analyzed by nuclear magnetic resonance (NMR) is listed in Table 1. Synthesis of St/Ac MCE (S-series). The design for the synthesis process is shown in Scheme 1b. As with the P-series, before esterifying cellulose, mixed anhydrides, which were used as esterification reagents in this study, were prepared as follows. SA (31.6−126.3 g, 111−444 mmol, 3−12 eq/AGU) and acetic anhydride (21.0 mL, 222 mmol, and 6 eq/AGU) were mixed at 100 °C for 1 h under a dry nitrogen atmosphere. As shown in Scheme S3 of the Supporting Information, the liquid product consisted of three acid anhydrides (acetic anhydride, St/Ac mixed anhydride, and anhydride of SA) and two acids (acetic acid and SA). The molar compositions of these mixtures calculated by NMR are listed in Table S2 of the Supporting Information. Cellulose powder with ca. 6% adsorbed water (6.0 g by dry weight and 37 mmol/AGU) was pretreated and dissolved by the same method as the P-series. Each sample of the S-series (S1−S3) was synthesized with the mixed anhydride prepared from SA and acetic anhydride in the same reaction condition and the resultant products’ recovery method as the P-series. No gelation occurred in the synthesis of the S-series. The DS of the S-series analyzed by NMR is listed in Table 1. Measurements. NMR spectroscopy was performed in CDCl3 using a JEOL EX 400 MHz NMR spectrometer at ambient temperature to determine the DS values of synthesized MCE. The integration method is described in the DS Calculation section. The molecular weights were measured by GPC on a Shimadzu 10AVP system. All the measurements were carried out at 40 °C using polystyrene gel columns (Shimadzu Simpack GPC-80MC × 2 and GPC-8025C), and chloroform was used as an eluent at a flow rate of 1.0 mL/min. Thermogravimetric analysis (TGA) was performed by a Seiko TGA6200/EXSTAR6000 apparatus using a temperature range from 50 to 600 °C at a heating rate of 10 °C/min in 200 mL/min N2. A sample of about 10 mg was used with predrying at 120 °C for 0.5h. The thermal decomposition temperature (Td1%) was taken as the onset of significant (≥1%) weight loss. Dynamic mechanical analysis (DMA) measurements were carried out on a Seiko DMS6200/EXSTAR6000 apparatus operating in tensile mode at a frequency of 0.1, 1, 2, 5, and 10 Hz in the temperature range from −100 to 150 °C. The temperatures of Tα and Tβ were taken at their respective tan δ peak maxima. Hot-press films (0.3 mm × 5 mm × 40 mm) for DMA measurements were molded using a tabletop test press machine (Tester Sangyo Co. Ltd., SA-303−II-S) operating at 210 °C. Differential scanning calorimetry (DSC) was performed using a Seiko DSC6200/EXSTAR6000 apparatus. The samples were first cooled to −100 °C and then heated at a scanning rate of 10 °C/min to +230 °C, which was maintained at 230 °C for 3 min (first heating scan) and immediately quenched to −100 °C at a rate of 50 °C/min. The second heating scans were run from −100 to +250 °C at a scanning rate of 10 °C/min to record stable thermograms. Wide-angle X-ray diffraction (WAXD) patterns of the hot-press films were recorded with a Rigaku RINT 2000 with monochromator-

treated Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA by using the diffraction method. Bar-type specimens (2.4 mm × 80 mm × 12.4 mm) for measurements of the mechanical properties were molded using an injection molding machine (Thermoficher Scientific, HAAKE MinijetII) operating at 210 °C. The bending properties were measured in accordance with the American Society for Testing and Materials (ASTM) D790 standards by using an INSTRON5567 testing system. The notched Izod impact test was conducted in accordance with JIS K 7110 using a notching tool (Toyo Seiki Co., A-4E) and an impact test instrument (Toyo Seiki Co., Universal Impact Tester C1). The bending strength and impact strength were taken as the average of four samples. DS Calculation. According to previous reports,14,24 the DS values of the P-series were calculated from 1H NMR spectra as

DSPA =

7 × Iδ 0.86 (3 × Iδ 3.0 − 5.5 − 2 × Iδ 0.86)

(1)

DSAc =

7 × Iδ1.8 − 2.2 (3 × Iδ 3.0 − 5.5 − 2 × Iδ 0.86)

(2)

where Iδ0.86 denotes the integrals of the triplet peak at δ0.86 assigned to the terminal methyl protons in the PA group, Iδ3.0−5.5 denotes the integrals of the multiple peaks from δ3.0 to δ5.5 including the AGU protons and the methylene protons next to the carbonyl group in the PA group, and Iδ1.8−2.2 denotes the integrals of the multiple peaks from δ1.8 to δ2.2 assigned to the acetyl protons. Similarly, the DS values of S-series were calculated from 1H NMR spectra as

DSSt =

7 × Iδ 0.86 3 × Iδ 3.0 − 5.5

(3)

DSAc =

7 × Iδ1.8 − 2.2 3 × Iδ 3.0 − 5.5

(4)

where Iδ0.86 denotes the integrals of the triplet peak at δ0.86 assigned to the terminal methyl protons in the St group, Iδ3.0−5.5 denotes the integrals of the multiple peaks from δ3.0 to δ5.5 including the AGU protons, and Iδ1.8−2.2 denotes the integrals of the multiple peaks from δ1.8 to δ2.2 assigned to the acetyl protons. Weight Ratio of Long Acyl Chain Moieties in MCE. The mechanical and thermal properties of MCE are often affected by the weight ratios of long acyl chain moieties in MCE. The weight ratios of long acyl chain moieties in the P-series and S-series are different even in the same DS of the PA group and St group because the two long acyl chain moieties used in this study have different molecular weights. The weight ratios of the long acyl chain moieties in MCE were calculated as follows: WR PA (wt %) =

345.55 × DSPA × 100 159.12 + 43.05 × DSAc + 345.55 × DSPA

(5) C


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Figure 1. 1H NMR spectra of sample P3 (a) and sample S2 (b). WR St (wt %) =

267.48 × DSSt × 100 159.12 + 43.05 × DSAc + 267.48 × DSSt

esterified in a LiCl/DMAc medium with DMAP by using mixed anhydride, an important esterified reagent to simultaneously bond long and short acyl chains to cellulose. Synthesizing mixed anhydrides from acetic anhydride and long-chain carboxylic acid is a particularly efficient procedure because it is not necessary to prepare long-chain carboxylic acid anhydrides.23,25,26 DMAP, which survived protonation in DMAc, catalyzed nucleophilically the esterification of cellulose with the intermediate N-acyl derivative of DMAP, though an auxiliary base was not added in this system. The DS values of the P-series and S-series were calculated from 1H NMR spectra. Figure 1 illustrates the 1H NMR spectra

(6) where WRPA denotes the weight ratio of PA group in P-series, WRSt denotes the weight ratio of St group in S-series, DSAc denotes the DS of Ac group in P-series or S-series, DSPA denotes the DS of PA group in P-series, and DSSt denotes the DS of St group in S-series.

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RESULTS AND DISCUSSION Synthesis of P-series and S-series. On the synthesis of the P-series and S-series, cellulose was homogeneously D


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ACS Sustainable Chemistry & Engineering Table 2. Thermal Properties of P-series and S-seriesa TGA

a

DSC

DMA

no.

DSPA

DSSt

DSAc

Td1% [°C]

Tm1% [°C]

ΔHm1 [J/g]

Tm2% [°C]

ΔHm2 [J/g]

Tβ [°C]

Tα [°C]

P1 P2 P3 P4 S1 S2 S3 S4

0.4 0.8 1.0 1.5    

    0.4 0.7 1.0 1.1

2.6 2.2 2.0 1.2 2.6 2.3 2.0 1.9

286 315 311 302 324 327 2S1 290

 −22 −9 12  −17 −1 7

 7.5(14.4) 10.2(17.2) 25.6 (36.1)  5.9 (14.0) 15.6 (30.0) 20.9 (38.0)

 144 150 121  156 168 165

 2.1 3.3 2.5  4.1 7.5 7.5

−34 −29 −24 −13 −41 −34 −28 −23

142 135 125 99 138 126 123 119

Between blankets: melting enthalpy by weight of long acyl chain moieties.

Figure 2. DSC thermograms of P-series and S-series (second run). WR: weight ratio of long acyl chain moieties.

of the representative samples of MCE. As listed in Table 1, almost all the hydroxyl groups of every MCE sample were substituted to long/short acyl chains, and the ratio of these chains was able to be controlled by varying the infeed of PAA and SA. While the reaction solution in the synthesis of the Pseries tended to be gelatinized as the PAA infeed increased, the reaction solution in the synthesis of the S-series was not gelatinized even in much SA infeed. This gelation of the Pseries was due to the intermolecular interaction that is probably π−π stacking between the aromatic rings in the PAA moiety or CH−π interaction between the aromatic ring and the hydrophobic plane of cellulosic glucopyranose ring. The gelatinized and nongelatinized ingredients of each of the samples had similar thermal behaviors because DSC analysis of both ingredients showed similar thermograms. The molecular weight of the cellulose main chain therefore would influence the gelation. In fact, the gelatinized ingredient showed a higher molecular weight (Mn = 13.1 × 104) than the whole sample (Mn = 4.3 × 104) during measurement of the GPC of sample P2. By comparing the conversion ratio which is the ratio of the grafted amount to the infeed amount of long-chain moieties, the PA group was revealed to have higher reactivity than the St group. While the conversion ratio of the PA group was 25%− 40%, that of the St group was limited to 9.2%−13%. SA needed much more infeed than PAA to obtain the same level DS of the long chain. The high reactivity of the PA group was attributed to the oxygen atom at the β position of the carbonyl carbon. The inductive effect of the β-oxygen atom increased the

reactivity of PA by activation of the acylium cation of the PA group. When long and short acyl chains are bonded to cellulose simultaneously, bulky long chains are expected to be bonded at the C6 carbon of glucopylanose.27 But by analyzing 13C NMR, we found that long acyl chains in the P-series and S-series were bonded to each C2, C3, and C6 positions evenly. A couple of triplets indicating the acyl group and the long-chain moieties were observed in the carbonyl region of the 13C NMR spectra of both series (Figure S1, Supporting Information). Sample P3 showed a signal at 168.7 ppm assigned to the carbonyl carbon of the PA group at the C6 carbon of glucopyranose, 168.1 ppm assigned to that at C3, and 167.6 ppm assigned to that at C2. Similarly, sample S2 showed the signal at 173.1 ppm assigned to the carbonyl carbon of the PA group at the C6 carbon of glucopyranose, 172.5 ppm assigned to that at C3, and 171.9 ppm assigned to that at C2. The appearance of the PA carbonyl carbon peak in a higher magnetic field than that of the St peak supported the inductive effect of the PA-β-oxygen atom mentioned above. These results indicated that there is no regioselectivity of long-chain moieties in homogeneously synthesized P-series or S-series. While it is well known that regioselective etherification of cellulose is obtained easily by using the trityl group,28,29 cellulose esters were not possible to be obtained with perfect regioselectivity in direct acylation with acid chlorides even when bulky substituents were used such as the adamantoyl group and trimethylbenzoyl group.30 Both the P-series and S-series had no regioselectivity of long-chain E


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Figure 3. Tα (a) and Tβ (b) of MCE from DMA.

chains in MCE.34 This relaxation probably originated from motions of the amorphous part of the long acyl chains. Figure 3a compares Tβ between both the P-series and S-series against the weight ratios of long acyl chain moieties in MCE. As with the Tm1 peak, the Tβ peak showed a shift to high temperatures as DSPA or DSSt increased. This is mainly due to increasing crystallinity of long acyl chain moieties as revealed byΔHm1 in DSC thermograms. The crystallization should impede the mobility of the long acyl chain moieties.10 Additionally, the Tβ peaks of the P-series and S-series were similar against weight ratios of long acyl chain moieties in MCE. This result was also confirmed by the activation energy of the mechanical relaxations calculated from Arrhenius-type plots (details in Supporting Information). Accordingly, the mechanical relaxations of the long acyl chains were revealed to be influenced not the existence of the aromatic ring but the aliphatic chains because two long acyl chain moieties in this study have the aliphatic chains that are almost the same length: 15 carbons in the PA group and 17 carbons in the St group. The high temperature tan δ peaks (Tα) were assigned to the glass transition (Tg) of MCE. In the DSC thermograms, sample P1 and S1 showed Tg with a baseline shift at 136 °C (P1) and 135 °C (S1), supporting this assignment. The Tg around 140 °C was also observed in previous MCE with PA group (DS ca. 0.5) and Ac group (DS 2.1).19 Figure 3b shows the Tα of the Pseries and S-series against the weight ratios of long acyl chain moieties in MCE. The Tg of both the P-series and S-series decreased as DSPA or DSSt increased due to the effect of internal plasticizing, and Tα was about 10 °C higher in the P-series than in the S-series. Consequently, the aromatic ring of the PA group disposed near the cellulose main chain increased the glass transition temperature of MCE in the P-series. Structural Analysis of P-series and S-series. P-series and S-series showed very similar tendencies of WAXD patterns, and the results are illustrated in Figure 4. Two conspicuous diffraction peaks were observed in all samples: a broad peak at 2θ ≈ 20° and a sharp peak at 2θ ≈ 3° (marked with dotted lines in Figure 4). The broad peaks probably mainly originated from an amorphous halo. In addition to the two major peaks mentioned above, some small diffraction peaks at 2θ ≈ 5° and 2° were observed (marked with arrows in Figure 4). These peaks probably originated from the anisotropic phase observed in Figure S3 of the Supporting Information. The peaks of the linear aliphatic structure of long acyl chains in the other cellulose esters totally substituted by the fatty acyl groups (over 14 carbons) were observed similarly in a previous study.10 The single sharp peaks at 2θ ≈ 3° must have originated from the periodicity of the cellulose backbone chain, which is in line with other cellulose esters including long acyl chain moieties.33

moieties in the glucopyanose ring because of a lack of steric hindrance of the PA and St groups. Thermal Properties of P-series and S-series. The thermal properties of MCE measured by TGA, DMA, and DSC are summarized in Table 2. By TGA, the thermal decomposition temperatures (Td1%) of the P-series and S-series were almost the same for all the samples around 300 °C, typical of cellulose esterified with long acyl chain moieties.31,32 In measuring DMA and DSC, similar patterns were observed in the S-series and P-series. As shown in DSC thermograms (Figure 2), both series had two major endothermic peaks at low temperature (−20−10 °C: Tm1) and high temperature (120− 160 °C: Tm2). Moreover, two relaxation peaks of tan δ in a high and low temperature (Tα and Tβ) with a drop of E′ were observed in DMA of both series (Figure S2, Supporting Information). The endothermic peak in the DSC thermogram at low temperature (Tm1), which became larger and shifted to high temperatures as DSPA or DSSt increased, related to the thermal transitions of the long acyl chains. The Tm1 peak originated from the order−disorder transition of the long acyl chains, namely, melting crystals.9 As stated in Crépy’s literature, the growth and high temperature shift of the Tm1 peak indicated an increase in crystallinity and broadening of crystal thickness.10 Moreover, the melting enthalpy by weight of long acyl chain moieties (ΔHm1) in both series correlated with DSPA or DSSt. Especially, even in sample S4 which had a maximum DSSt in the S-series, ΔHm1 by weight of long acyl chain moieties (38.0 J/g) was about half of the cellulose tristearate (67 J/g of ΔHm by weight of long acyl chain moieties, reported by Crépy et al.10). This result suggests limited DSPA or DSSt and a mixed substitution of long and short acyl chains prevented the crystallization of the long acyl chain moieties. Another endothermic peak in the DSC thermogram at high temperature (Tm2) was also observed in cellulose acetatehexanoate.31 The enthalpy (ΔHm2) was too small (ca. 3−5 J/g) to identify Tm2 as the melting points of MCE. As a result of polarized optical micrograph (POM) observations of the Pseries (Figure S3, Supporting Information), birefringence or theanisotropic phase, which increased with DSPA, mostly disappeared together with overheating to Tm2, indicating the occurrence of a transition from the anisotropic phase to an isotropic one. Hence, Tm2 assigned to a liquid crystalline phase−isotropic phase transition temperature because of the previous report that long-chain cellulose esters have highly viscous thermotropic liquid crystalline behavior, which is initiated by flexible long acyl chain moieties.33 The tan δ peak in low temperatures (Tβ) with a drop of E′ is a mechanical relaxation assigned to motions of the long acyl F


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Mechanical Properties of P-series and S-series. The bending strength and impact strength of synthesized MCE are summarized in Table 3. The bending strength and modulus in Table 3. Mechanical properties of P-series and S-series bending test no.

DSPA

DSSt

DSAc

strength [MPa]

P1 P2 P3 P4 S1 S2 S3 S4

0.4 0.8 1.0 1.5    

    0.4 0.7 1.0 1.1

2.6 2.2 2.0 1.2 2.6 2.3 2.0 1.9

84 48 34 14 85 44 26 22

modulus [GPa]

Izod impact [kJ/m2]

2.6 1.3 0.90 0.33 2.5 1.3 0.71 0.58

3.7 4,4 2.0 1.9 5.9 6.7 24.8 27.9

both series decreased as DSPA or DSSt increased due to internal plasticizing with the long-chain moieties. These bending properties were able to vary widely from 14 to 85 MPa with DSPA or DSSt variation. The effect of the internal plasticizing is closely related to the weight ratios of long acyl chain moieties in MCE. Plotting the bending strength against the weight ratios of long acyl chain moieties in Figure 6a, we found that the P-series had higher bending strengths than the S-series. This tendency probably resulted from the stiffness of the cellulose main chains in MCE, which was increased by the aromatic ring of the PA groups. Additionally, the internal plasticizer kept a higher bending strength than the external plasticizer because the cellulose diacetate (short chain cellulose ester) with 41 wt % of external plasticizer (triethylcitrate) showed 34 MPa of bending strength.19 In contrast, the Izod impact strength of the P-series and Sseries showed different tendencies as shown in Figure 6b. Although the impact strength of the P-series stayed at a low level or slightly decreased as the weight ratio of the PA group increased, the impact strength of the S-series remarkably increased to 27.9 kJ/m2 as the weight ratio of the St group increased. This is a level above petroleum-based acrylonitrile− butadiene−styrene (ABS) resin (ca. 20 kJ/m2) and the highest level in cellulose esters without external plasticizers. While Winkler et al. recently reported starch stearate (DS 2.2, 78 wt % of stearate moieties) was a waxy and brittle polymer with highly crystallized stearate moieties,35 the crystallization of the long acyl chain moieties in MCE was prevented by a mixed substitution of long and short acyl chains, and the melting point (Tm1) is below room temperature as mentioned in thermal properties section. The long acyl chain moieties in MCE were therefore kept flexible at room temperature. The high impact strength of samples S3 and S4 attributed to a composition of more than 50 wt % of the flexible long acyl chain moieties. On the other hand, the low impact strength of the P-series mainly was caused by the influence of the aromatic ring of the PA groups as mentioned above. As indicated by the gelation in synthesizing, the P-series has the extra intermolecular interaction that the S-series does not. This interaction by the aromatic ring of the PA groups likely inhibits a stress relaxation on the impact deformation of the P-series, which results in their low impact strength. The balanced mechanical properties (e.g., more than ca. 40 MPa of bending strength and 5 kJ/m2 of Izod impact strength) are important for use in durable products such as electronic

Figure 4. WAXD patterns of P-series (a) and S-series (b). WR: weight ratio of long acyl chain moieties.

The intensity of these sharp peaks in both series became larger as DSPA or DSSt increased. This result indicated that the layered structure region in MCE extended as the long-chain DS (weight ratio) increased. The full width at half-maximum (fwhm) of these sharp peaks in both series was decreased as the weight ratios of long acyl chain moieties increased (Figure 5)

Figure 5. Full width at half-maximum (fwhm) of the sharp peaks (2θ = 2.8° in P-series, 2θ = 2.9° in S-series).

and showed almost similar tendencies especially in high longchain DS (weight ratio). Therefore, the existence of the aromatic rings in long acyl chains did not influence the tendency to form the periodic structures. This tendency is probably influenced by the aliphatic chain length of long acyl chain moieties. On the other hand, the diffraction angles of these sharp peaks (2θ = 2.8° (d = 3.15 nm) in P-series and 2θ = 2.9° (d = 3.05 nm) in S-series) did not change as the long-chain DS (weight ratio) varied. Contrary to our expectations, this result means that the interlayer distance, which is the distance between the cellulose backbones of MCE, is not influenced by the long-chain DS (weight ratio). G


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Figure 6. Bending strength (a) and impact strength (b) of P-series and S-series.

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devices. Samples S1 and S2 in the S-series had sufficient balanced mechanical properties. Consequently, it was revealed that using the aliphatic long acyl chain and controlling longchain DS to 0.4−0.7 (weight ratio to 28−42 wt %) were appropriate for using MCE in durable products.

Corresponding Authors

*E-mail: [email protected]. Tel.: +81 3 5841 5266. Fax: +81 3 5841 1304 (Tadahisa Iwata). *E-mail [email protected]. Tel.: +81 29 850 1512. Fax: +81 29 856 6136 (Masatoshi Iji).

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CONCLUSIONS Effects of the long acyl chain structure and DS ratio of long/ short chains on the several properties of MCE were investigated by synthesis under homogeneous conditions of the two types of MCE: PA/Ac MCE (P-series) and St/Ac MCE (S-series). In terms of the reactivity of long-chain moieties with cellulose, the PA group was more reactive than the St group due to the inductive effect of the β-oxygen atom in PA group. Thermal analysis revealed the presence of two transition (Tm1 and Tm2) and two relaxation temperatures (Tα and Tβ). The lower Tm1 and Tβ related to the thermal transition and mechanical relaxation of long acyl chain moieties. The higher Tm2 and Tα related to the cellulosic backbone’s transition and relaxation. The P-series had a higher bending strength and glass transition temperature but lower impact strength than the S-series. These results were attributed to the existence of an aromatic ring in the PA group, which caused an increase in the stiffness of the cellulose backbone and extra intermolecular interaction. On the other hand, the S-series showed practical, promising properties with the highest impact strength in CE without external plasticizers or balanced mechanical properties for durable products. Although long-chain CE was previously reported as a brittle material with highly crystallized long acyl chain moieties, the crystallization in MCE was prevented by a mixed bonding of long and short acyl chains. The high impact strength of the S-series was attributed its composition of low crystalline (flexible) long acyl chain moieties. These findings on the effects of the long acyl chain structure should make important contributions to promote the practical use of MCE. We are currently investigating the more efficient synthesis process of MCE.

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AUTHOR INFORMATION

ORCID

Tadahisa Iwata: 0000-0002-7249-6469 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ms. Toshie Miyamoto for her support in the experiments and the measurements. This work was supported by the advanced low carbon technology research and development program (ALCA) of the Japan Science and Technology Agency (JST).

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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.6b02066. Schemes and compositions of mixed anhydrides, 13C NMR spectra, DMA charts, polarized optical micrographs of MCE, and calculation of activation energy of the long side chain’s mechanical relaxations from Arrhenius-type plots. (PDF) H


Research Article


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