Pendant Chain Effect on the Synthesis, Characterization, and

May 4, 2017 - Glass transition temperature, dielectric properties, and thermal stability of the PDAIIs were studied and these properties were tunable ...
0 downloads 12 Views 4MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Pendant Chain Effect on the Synthesis, Characterization, and Structure−Property Relations of Poly(di‑n‑alkyl itaconate-coisoprene) Biobased Elastomers Weiwei Lei,†,‡ Thomas P. Russell,*,§,∥ Lei Hu,†,‡ Xinxin Zhou,†,‡ He Qiao,†,‡ Wencai Wang,†,‡ Runguo Wang,*,†,⊥ and Liqun Zhang*,†,‡ †

State Key Laboratory for Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Engineering Research Center of Ministry of Education on Energy and Resource Saved Elastomers, Beijing University of Chemical Technology, Beijing 100029, China § Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ∥ Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States ⊥ Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China ‡

S Supporting Information *

ABSTRACT: A series of biobased elastomers, poly(di-n-alkyl itaconateco-isoprene)s (PDAIIs), were prepared by using itaconic acid as a renewable starting material. It is revealed that pendent chain profoundly affected the structure and properties of PDAIIs. The molecular structures of the PDAIIs were analyzed by 1H NMR and FTIR. Dialkyl itaconate to isoprene reactivity ratios measured by Kelen−Tüdös method demonstrated that PDAIIs with short side chains tended to alternate copolymerized and the long side chain ones tented to gradient copolymerized. Glass transition temperature, dielectric properties, and thermal stability of the PDAIIs were studied and these properties were tunable by varying the side chain length. Tensile tests revealed that crosslinked PDAIIs are elastic with a high elongation at break. With renewable resources and the side chain adjustability, PDAII elastomers potentially have a positive economic and ecological impact. KEYWORDS: Renewable, Biobased elastomers, Itaconic acid, Alkyl side chain, Monoalcohol, Polymer structure design

■

INTRODUCTION Elastomers are used extensively. The basic building blocks of synthetic elastomers are derived from petrochemical resources, such as butadiene, styrene, isoprene, ethene, and propene. The limited supply of fossil fuel resources and environmental concerns stimulate demand for polymers from renewable resources.1,2 In this context, many renewable chemicals such as plant oils,3 lactic acid,4,5 and terpenes,6,7 have been used to synthesize biobased polymers. Renewable chemicals with specific chemical structures, for example, mono- or bicyclic structures, oxygen atoms, chirality, or olefinic moieties have drawn much attention with degradable, hydrophilic, and biocompatible properties. However, biobased elastomers from renewable chemicals, in large quantities, having properties similar or superior to those of petroleum-derived analogs for engineering applications have rarely been reported. In this context, we introduced the concept of biobased engineering elastomer to be applied in engineering field.8 Among the renewable chemicals, itaconic acid (IA), listed as one of the “top 12” biobased building block molecules by the © 2017 American Chemical Society

U.S. Department of Energy, is obtained by the fermentation of glucose.9,10 With an active 1,1-disubstituted vinyl group, IA can be polymerized by a radical method to form vinyl polymers,11−15 or polymerized with diols to form polyesters.8,16−18 Hence, IA has been used widely in hydrogels,19 fibers,20 superabsorbent polymers,21 and coatings.22 But in general, itaconic acid act as polymer matrix modifier, was used less than 10% in these cases. In this study, we chose IA as a starting material to design elastomers for potential engineering applications. Dialkyl itaconate was copolymerized with isoprene as the flexible unit providing the cross-link points. In addition, it is feasible to replace petrochemical isoprene with bioisoprene for material and energy efficiency, though the cost of the biotechnological pathway is slightly higher than the petrochemical route.23 IA was esterified with monoalcohols before the copolymerization. Received: February 23, 2017 Revised: April 23, 2017 Published: May 4, 2017 5214

DOI: 10.1021/acssuschemeng.7b00574 ACS Sustainable Chem. Eng. 2017, 5, 5214−5223

Research Article

ACS Sustainable Chemistry & Engineering

pentanol, n-hexanol, n-heptanol, n-octanol, n-nonanol, and n-decanol were obtained from Alfa Aesar Co. Isoprene (purity of 95%) was obtained from Alfa Asear Co. and purified by distilling to remove the stabilizer prior to usage. A redox initiator system composed of tertbutyl hydroperoxide (TBH), hydroxymethane sulfinate (SHS) and ferric ethylenediaminetetraacetic acid salt (Fe-EDTA), the emulsifier of sodium dodecyl benzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS), the buffer agent of potassium phosphate tribasic, the salt of potassium chloride, and the inhibitor of hydroxylamine to stop further reaction were obtained from Sigma-Aldrich Co. and directly used. Dimethyl itaconate and diethyl itaconate were obtained from Sigma-Aldrich Co. and used as received. The monomers di-n-alkyl itaconates were synthesized in our laboratory as described in the following section. All the other chemicals were obtained commercially and used as received. Synthesis of Di-n-alkyl Itaconate Monomers. Itaconic acid (130.1 g, 1 mol), different amounts of straight-chain monoalcohols as listed in Table 1, cyclohexane (50 mL), and sulfuric acid (1.38 g, 14

In a shift from fossil fuels to renewable fuels, monoalcohols make up most of the biofuel market today and in the foreseeable future. The feedstock of biomethanol is widely available.24 Bioethanol, the most extensively produced and used at present,25 has been added to gasoline in Brazil, North America, and Europe. Much biofuel research has also focused on the development of advanced biofuels and higher alcohols (≥C3). In particular, biobutanol offers advantages as gasoline substitutes because of its high energy density and low hygroscopicity.26 With synthetic biology, organisms can be constructed to express specifically engineered proteins for the generation of advanced biofuels including 1-pentanol, 1hexanol, 1-heptanol, and 1-octanol.27−29 In a previous study, we reported poly(diisoamyl itaconate-coisoprenes) synthesized from itaconic acid, isoamyl alcohol, and isoprene.12 Recently, we designed biobased elastomers from dialkyl itaconates with isoprene and butadiene, and focused on silica reinforced elastomers for low hysteresis tread.15 But the side chain length was not studied and its effect on the synthesis and properties of obtained polymers was still unclear. Previously, poly(di-n-alkyl itaconates) (PDAIs) with different side chains were synthesized by radical polymerization.30,31 The glass transition temperature (Tg) of PDAI, which is associated with the cooperative motions of different polymer units, including the backbone carbons, decreases with increasing n for n < 8 due to the internal plasticization effect.30,31 For n = 7 to 11, the DSC thermograms exhibit two relaxations: the upper and lower glass transition temperatures, T gU and T gL , respectively. Both TUg and TLg increase with increasing number of carbon atoms between 7 and 11, reflecting an increasing tendency of the side chains to align. At n ≥ 12, the PDAIs crystallize.31 TLg presents a second major relaxation and this phenomenon has been ascribed to independent motions of the long side chains, as confirmed by DSC,30,31 dynamic mechanical analysis,31 X-ray diffraction,32 dielectric spectroscopy,11,33 solution NMR,34 and molecular simulations.34 Therefore, dialkyl itaconates are versatile precursors to tune side chain structures and properties of the relevant polymers. To elucidate the side chains responsible for the reactivity, structure, and properties of poly(di-n-itaconate-co-isoprene) (PDAII), PDAIIs were synthesized via radical emulsion polymerization of biobased bulk monomer itaconic acid, which has polymerizable double bonds. The acid functional group of itaconic acid was esterified with monoalcohols CnH2n+1OH with n from 1 to 10. Then copolymerized with isoprene and cross-linkable functional groups was executed through radical emulsion polymerization. PDAIIs structures and properties were measured by gel permeation chromatography, infrared spectroscopy, NMR, DSC, dielectric spectroscopy, and thermogravimetric analysis. DAIs and isoprene reactivity ratios were determined by Kelen−Tüdös method. Tensile testing revealed the elastomeric behavior of sulfurcross-linked PDAIIs, with high elongation at break. Our study revealed inherent properties of PDAIIs and fundamental for PDAIIs used as engineering elastomers. With adjustable sidechains, PDAII possessed various potential applications including tire tread,15 dielectric elastomer,35 and oil resistance elastomer.36 We have also been expanding our studies to gas barrier elastomer and shape memory elastomer.

■

Table 1. Recipe for Preparing DAIs Ingredients n-propanol n-butanol n-pentanol n-hexanol

Amount/g (mol) 300 296 334 361

(5.00) (4.00) (3.75) (3.50)

Ingredients n-heptanol n-octanol n-nonanol n-decanol

Amount/g (mol) 381 394 399 398

(3.25) (3.00) (2.75) (2.50)

mmol) were added into a 1000 mL round-bottomed glass flask fitted with a mechanical agitator, a Dean−Stark trap, a condenser, and a thermometer. The weights of straight-chain monoalcohols in the feed decrease as the alkyl chains increase in length, as shown in Table 1, because the boiling points increase as the alkyl chains increase in length. The mixture was refluxed under elevated temperature (115− 130 °C, except n-propanol) until no more water was collected (∼3.5 h). Because n-propanol is partly soluble in water, which has a boiling point of 97 °C, the reaction temperature was lower than 100 °C, and more than 36 mL of water was obtained. Then, the product was cooled down to room temperature and washed by deionized water for three times in a separating funnel and concentrated in vacuum to obtain DAI. Synthesis of PDAII by Redox-Initiated Emulsion Polymerization. All the aqueous solutions were prepared according to the recipe in Table 2 prior to synthesis. Deionized water, SDBS or SDS

Table 2. Recipe for Synthesis of PDAIIs Ingredients (concentration/%)

Amount (g)

di-n-alkyl itaconate isoprene deionized water SDS (10%)c SDBS (10%)d K3PO3 (10%) KCl (10%) SHS (10%) Fe-EDTA (1%) TBH (10%) hydroxylamine (50%)

variablea 27.3 variableb 50 wt % of monomer 50 wt % of monomer 2 5 2 4 0.5 0.4

a The molar ratio of di-n-alkyl itaconate to isoprene was 2:3. b30 wt % of the solid content of the latex compound. cSDS was used when the number of repeating CH2 units on the side chain was 0, 1, 2, 3, or 4. d SDBS was used when the number of repeating CH2 units on the side chain was 5, 6, 7, 8, or 9. SDS, sodium dodecyl sulfonate; SDBS, sodium dodecyl benzenesulfonate; K3PO4, potassium phosphate; KCl, potassium chloride; SHS, sodium hydroxymethanesulfonate; FeEDTA, ferric ethylenediaminetetraacetic acid; TBH, tertbutyl hydroperoxide. Polyisoprene (PIP) was also synthesized for comparison.

EXPERIMENTAL SECTION

Raw Materials. Itaconic acid was supplied by Qingdao Langyatai Group Co., Ltd. and used as received. n-Propanol, n-butanol, n5215

DOI: 10.1021/acssuschemeng.7b00574 ACS Sustainable Chem. Eng. 2017, 5, 5214−5223

Research Article

ACS Sustainable Chemistry & Engineering solution, potassium chloride solution, and potassium phosphate tribasic solution were sequentially transferred into a 1000 mL round-bottomed glass flask fitted with a sampling device, a nitrogen inlet, a mechanical agitator, and a reflux condenser under a nitrogen atmosphere to keep away the air. Subsequently, preblended di-n-alkyl itaconate and isoprene was transferred into the aqueous solution and then mixed vigorously at 20 °C for 30 min under atmospheric pressure. Afterward, Fe-EDTA solution, SHS solution, and TBH solution were injected into the flask, and the polymerization was continued for a certain hour under moderate agitation. The reaction was terminated by the hydroxylamine solution. The target latex was coagulated by using an excess of calcium chloride solution to obtain off-white solid then washed with ethanol and deionized water alternately for removal of water and surfactant. Finally, the off-white solid was dried at 60 °C in an air-circulating oven to a constant weight prior to calculate the yield referred to the weight ratio of PDAIIs to the monomers, and carry on other tests. The samples at different reaction time were taken for calculation time dependence of yield of PDAIIs. Preparation of Cross-Linked PDAII. The PDAII and additional agents were mixed in a 6 in. two-roll mill according to the recipe listed in Table 3. The mixture was cured in an XLB-D 350 × 350 hot press

Figure 1. 1H NMR spectra of DAIs ranging from methyl to decyl. The full names and abbreviations of the DAI samples are as follows: dimethyl itaconate (DMI), diethyl itaconate (DEI), di-n-propyl itaconate (DPrI), di-n-butyl itaconate (DBI), di-n-pentyl itaconate (DPeI), di-n-hexyl itaconate (DHxI), di-n-heptyl itaconate (DHpI), din-octyl itaconate (DOI), di-n-nonyl itaconate (DNI), and di-n-decyl itaconate (DDI).

Table 3. Recipe for Cross-Linking PDAII by Sulfur

a

Ingredients

Loading (phr)

PDAII zinc oxide (ZnO) stearic acid antioxidant (4010NA) accelerator Ma accelerator CZb sulfur

100 3.0 1.0 1.0 0.6 1.0 1.0

M means 2-mercaptobenzothiazole. benzothiazole.

b

the same chemical environment before and after esterification. As the deshielding effect of the carboxyl groups decreases with increasing methylene number, the chemical shifts of −CH3 at the end of the alkyl chain move to higher fields. And the intensity of peak f increases from DBI to DDI because the methylene number increases. The spectra of dimethyl itaconate and diethyl itaconate are also shown in Figure 1 for comparison. The residual acid groups and the isomerized methyl maleic (di)esters were not detected. Synthesis of PDAII. PDAIIs with different side chain lengths were prepared by redox-initiated emulsion polymerization.12,37 According to the preliminary studies in this work, the surfactant played a major role in the formation and stabilization of the emulsion. SDS, SDBS, and potassium oleate were tried in the syntheses of PDAIIs with different side chain lengths. We found that SDBS was an excellent surfactant for the syntheses of PDAIIs with long side chains, all three surfactants worked for the syntheses of PDBII and PDPeII, but SDS was much more effective than the other two surfactants for the syntheses of PDAIIs with fewer than four carbon atoms on a side chain, because these surfactants have different hydrophile− lipophile balance constants, which are crucial for selecting the surfactant. The PDAIIs with short side chains are more hydrophilic than the PDAIIs with long side chains. Figure 2 shows that the reaction rate decreases as the side chain length increases because the activity of DAI decreases significantly as the steric hindrance of DAI increases. With more than eight side chain carbon atoms, the steric hindrance decreases the rate of the polymerization reaction. For PDAIIs with one to seven side chain carbon atoms, the reaction time was defined as the time at which the yield is 85%. For PDOII, PDNII, and PDDII, the reaction time was 36 h. The overall polymerization rate coefficient decrease with increasing the chain length of di-n-alkyl itaconates from methyl to pentyl, and from hexyl to decyl, respectively (Figure S2). The GPC traces of PDAII showed roughly monomodal distributions, corresponding to the high-molecular weight molecules (Figure 3). All fractions possessed a rather broad molecular weight distribution with polydispersity index between 2.0 and 3.9. The broad molecular weight distribution especially for high molecular weight polymers like elastomer

CZ means N-cyclohexyl-2-

(Huzhou Eastmachinery Co., China) under 15 MPa and 150 °C with its optimum cure time, which was measured by a disk oscillating rheometer. Characterization Methods. The FTIR measurements of DAIs and PDAIIs were performed using a Bruker Tensor 27 spectrometer. The spectra have been collected in the 4000−400 cm−1 range at an accumulated resolution of 4 cm−1 and over 32 scans. 1H NMR spectroscopy of DAIs and PDAIIs was performed on a Bruker AV400 spectrometer. The solvent was CDCl3 with traces of tetramethylsilane as an internal reference. The molecular weight of PDAIIs was measured by a Waters Breeze gel permeation chromatograph (GPC) using three water columns (Styragel HT3_HT5_HT6E) and a Waters 2410 refractive index detector. Tetrahydrofuran was used as the eluent, pumped at 1 mL/min. Linear polystyrene standard was used for calibration. The glass transition temperatures (Tg values) of PDAIIs and polyisoprene were determined by Mettler-Toledo differential scanning calorimeter (DSC). The samples were first heated to 150 °C, kept for 3 min, and then cooled to −100 °C and reheated to 150 °C under heating and cooling rate of 10 °C/min and a nitrogen rich atmosphere. The Tg was defined as the midpoint of the trace change at the glass transition process during the second heating scan. Wideband dielectric spectroscopy of PDAIIs was carried out on and E4980A impedance analyzer at room temperature in the frequency range of 102 to 106 Hz under a voltage of 1 V. Tensile tests of cured PDAIIs were performed using a LRX Plus tensile apparatus (LRX Plus, Lloyd Instruments, Ltd., UK) according to ASTM D412. Samples were prepared via compression molding and cut into dumbbell-shaped.

■

RESULT AND DISCUSSION Synthesis and Structure of DAIs. Figure 1 shows the 1H NMR spectra of the DAIs obtained by the esterification reaction. Peaks a, b, and c appear at the same chemical shift positions in all spectra because every hydrogen atom has almost 5216

DOI: 10.1021/acssuschemeng.7b00574 ACS Sustainable Chem. Eng. 2017, 5, 5214−5223

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. Time dependence of yield of PDAIIs with constant DAI to isoprene feed molar ratio of 2 to 3, but different surfactants: (a) SDS and (b) SDBS.

Figure 3. Gel permeation chromatography traces for PDAII and the blank sample containing only tetrahydrofuran eluent. The inset part, Mn means number-average molecular weight, and PDI means polydispersity index.

Figure 4. FTIR spectra of PDAIIs with DAI to isoprene feed molar ratio of 2 to 3.

units, but the 1,2- and 3,4-units are too ill-defined to be detected. The enlarged region from 4.5 to 5.0 ppm is illustrated in Figure 5b. Weak peaks can be detected from 4.60 to 4.75 ppm and from 4.80 to 4.95 ppm, attributed to the =CH2 groups of 3,4 addition and 1,2 addition, respectively. The intensities of the peaks of 3,4- and 1,2-units decrease with the number of side chain carbon atoms n increasing from 1 to 5, but sharply increase with n increasing from 8 to 10. The sharp peak at 2.44 ppm and the broad shoulder from 2.30 to 2.90 ppm are attributed to the −CH2− (itaconate) units between the main chain and carbonyl group and on the main chain, respectively. PDMII, with the shortest side chain, shows the peaks at 3.64 and 3.68 ppm, representing the −CH3 group on the side chain. For PDEII, the chemical shifts of the two −CH3 groups on the side chain move to the low fields of 1.24 and 1.26 ppm because of the deshielding effect of the ester groups. For the other PDAIIs, the chemical shifts of the −CH3 groups on the side chainsPDPrII (4, 0.94 ppm), PDBII (4, 0.93 ppm), PDPeII (3, 0.90 ppm), PDHxII (3, 0.88 ppm), PDHpII (3, 0.88 ppm), PDOII (3, 0.88 ppm), PDNII (3, 0.88 ppm), and PDDII (3, 0.88 ppm)are shown in the enlarged part of Figure 5c. According to the chemical shifts of the −CH3 groups, the deshielding effects of the ester groups on −CH3 almost vanish with 5 or more methylene groups. The decrease of the chemical shift peak position of δ from 1.38 to 1.26 ppm is also attributed

materials means good nature for processing. Macromolecular parameters of PDAII derived from the GPC traces were collected in the inserted part. The number-average molecular weights of PDAIIs are all above 130 000, ensuring an adequate mechanical strength and elasticity of the polymers for them to be used as elastomers after being cross-linked. FTIR and NMR Spectra of PDAII. The double bond stretching and bending absorptions of DAIs at 1640 and 816 cm−1 in Figure S1 disappear after the copolymerization, confirming that the DAIs have been successfully copolymerized (Figure 4). The COC dual absorption peaks in Figure S1 merge into a single one at 1172 cm−1 due to the vanishing of the conjugate effect between CO and CC. The vibration band at 1381 cm−1 is attributed to the cis-1,4- and trans-1,4units in isoprene.38 The broad bands between 2850 and 2957 cm−1 are attributed to the stretching vibrations of the −CH, −CH2, and −CH3 groups of PDAII. The intensities of these broad bands increase from PDMII to PDOII and sharply increase from PDNII to PDDII because the isoprene portion increases, exhibiting the same trend as that of the absorption band at 1380 cm−1. Figure 5 shows the 1H NMR spectra and the general formula of PDAII elastomers. In Figure 5a, the left peak at about 5.10 ppm is attributed to the −CH= group in cis-1,4- and trans-1,45217

DOI: 10.1021/acssuschemeng.7b00574 ACS Sustainable Chem. Eng. 2017, 5, 5214−5223

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. 1H NMR spectra and general formula of PDAIIs: (a) 1H NMR spectra of PDAIIs, (b) enlarged figure of chemical shift interval between 4.5 and 5.0 ppm, with the spectrum of polyisoprene included for comparison, (c) enlarged figure of chemical shift interval between 0.8 and 1.0 ppm, and (d) general formula of PDAIIs. n stands for the number of C atoms per side chain (in panels a to c).

using both the Fineman−Ross (FR) method and the Kelen− Tüdös (KT) method in ascending and descending order (Tables 5 and 6). The 1H NMR spectroscopy has proven to be a convenient method to determine the compositions of constituent monomer units in PDAII. With PDBII, for example, the initial feed ratios and the polymer compositions of PDBII as well as the KT and FR parameters are presented in Table 5. In Figure 6, both the FR and KT methods show acceptable correlation coefficients, but the KT method is better than the FR method because the positive order and reverse order results of the KT method are much more consistent than FR method. So, the KT method was employed for further analyses. Table 7 shows that the values of rDBI and rIP calculated by both KT method and FR method. The results show that the copolymerization of DBI and IP is a nonideal azeotropic copolymerization with an azeotropic point of 0.43. That is, if the feed ratio of DBI to IP is 0.43, the copolymer has an identical composition as the feed. The initial feed ratios and the polymer compositions of PDAIIs as well as the KT method parameters are presented in Table S1 to Table S6 (see Supporting Information). The reactivity ratios of DMI, DEI, DPrI, DBI, DHxI, DOI, or DDI to IP were measured by the KT method, and the results are listed in Table 8. When there are fewer than 8 carbon atoms on a side chain, r DAI and r IP are less than 1 and the copolymerization is nonideal azeotropic. However, when the number of side chain carbon atoms is 8 or more, rDAI < 1 and rIP > 1, and the reaction is a nonideal copolymerization with no azeotropic point. The results of rDAI and rIP suggest strong potential of alternating capacity of itaconates with short side

to the weakening of the deshielding effect of carboxyl group from PDBII to PDDII. The general formula of PDAIIs is deduced from FTIR and 1H NMR analyses and given in Figure 5(d). The PDAII compositions obtained from the chemical shift peak areas are listed in Table 4 and Table 5. In brief, the Table 4. Compositions of PDAIIs Isoprene molar ratio (%) PDAII

1,4

3,4 and 1,2

Itaconate molar ratio (%)

PDMII PDEII PDPrII PDBII PDPeII PDHxII PDHpII PDOII PDNII PDDII

55.9 56.6 56.8 56.5 56.4 56.7 60.0 72.6 89.9 89.4

0.5 0.6 0.5 0.4 0.1 0.2 0.2 1.4 2.5 4.0

43.6 42.8 42.6 43.1 43.5 43.1 39.8 26.1 7.6 6.6

peak area d and h, r and n, and γ represent double 3,4- and 1,2isoprene units, 1,4-trans and 1,4-cis isoprene units, and six times itaconate units, respectively (Figure S3). Reactivity Ratios of DAIs and Isoprene. An understanding of the monomer reactivity ratio is useful for tailoring polymers with the desired compositions for specific end-use applications.39 The reactivity ratios of DAIs and isoprene were determined in redox-initiated polymerizations with various contents of DAIs in the feed under a low conversion (