Bio-based multifunctional macroglycol containing smart thermoplastic

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 4370−4381

pubs.acs.org/journal/ascecg

Biobased Multifunctional Macroglycol Containing Smart Thermoplastic Hyperbranched Polyurethane Elastomer with Intrinsic Self-Healing Attribute Tuhin Ghosh and Niranjan Karak* Advanced Polymer and Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur 784028, India S Supporting Information *

ABSTRACT: Smart biobased self-healable polymers are advanced sustainable materials. Thus, a thermoplastic hyperbranched polyurethane (HBPUR) elastomer was synthesized by using a biobased multifunctional macroglycol along with other required components, for the first time. This biobased macroglycol was obtained by stoichiometric controlled esterification reaction of dimer acid with glycerol. Fourier transform infrared, nuclear magnetic resonance spectroscopic, gel permeation chromatography, and X-ray diffraction studies confirmed the physicochemical structure of the synthesized macroglycol and HBPURs. The degree of branching value (0.78−0.91) of HBPURs varies with this macroglycol content. These thermoplastic PUR elastomers exhibited outstanding toughness (205 MJ.m−3), unprecedented high elongation at break (2810−3160%), good tensile strength (8.2−9.5 MPa), impact resistance (>17.3 kJ/m), scratch resistance (4.0−4.5 kg), durometer hardness (52−60 Shore A), adhesive strength (3.3−6.7 kPa), thermostability (241−249 °C), and chemical and UV-resistance. Notably, HBPURs also showed excellent repeatable intrinsic self-healing efficiency (100%) under exposure of microwave (450 W). Moreover, HBPURs exhibited outstanding thermoresponsive shape recovery (100%) within 50−56 s at 60 °C. In addition, HBPURs showed acceptable biodegradation under the exposure of Pseudomonus aeruginosa and Bacillus subtilus bacterial strains. Again, HBPUR demonstrated superior performance over linear analogous PUR. Thus, HBPUR has great potential as a smart sustainable self-healing thermoplastic elastomer for different applications. KEYWORDS: Biobased polyurethane, Self-healing material, Tough sustainable material, Hyperbranched thermoplastic elastomer, Macroglycol branching unit

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INTRODUCTION Development of smart materials from renewable resource is the order of the day, and the significance of such material is enormus.1 Thus, introduction of renewable resource based raw materials as a substitute for fossil fuel based feed stocks in the production of various materials including polymers is essential. This approach can address shortcomings like nonrenewability, nonbiodegradability, toxicity, and health issues of petroleum based resources. Again, over the last few decades constant efforts have been made to develop stimuli-response smart materials that are able to respond to the internal and external stimuli, as desired.2 Among various smart properties, selfhealing is a crucial feature to prolong the durability, requires less maintenance, and minimizes the cost of the material.3 The concept of self-healing material is derived from biological systems but replication of this process in the synthetic materials is a daunting challenge. This is because of its requirement of a highly coordinated and complex structure akin to the living systems.4 However, to mimic biological systems, scientists are attempting many tireless approaches including microencapsulation, reversible bond formation, vascular network formation, © 2018 American Chemical Society

incorporation of nanomaterials, etc. in various polymeric materials.5 In this milieu, among different self-healing polymers (SHPs), polyurethanes (PURs) have attracted significant interest to the research enthusiasts due to their inborn versatility and tunable properties with manipulation of structure and composition of the components. Again, three-dimensional globular, nonentanglement architectural hyperbranched PURs (HBPURs) demand special attention because of their high solubility, compatibility, surface groups, and low viscosity over the analogous linear PURs.6 Thus, efforts can be made to develop renewable resource based smart HBPUR with selfhealing abilities. Literature advocates a few attempts. White et al. synthesized an autonomous healing material by incorporation of microcapsules in a polyester matrix containing dicyclopentadiene (DCPD) and Grubbs catalyst.7 These microcapsules release the healing agent through capillary effect in the fractured surface. This healing agent is then polymerized Received: January 2, 2018 Revised: January 26, 2018 Published: January 30, 2018 4370

DOI: 10.1021/acssuschemeng.8b00001 ACS Sustainable Chem. Eng. 2018, 6, 4370−4381

Research Article

ACS Sustainable Chemistry & Engineering

Synthesis of Hyperbranched Polyurethanes (HBPURs) Using Prepared Macroglycol. A three neck round-bottom flask equipped with a nitrogen gas inlet, a magnetic stirrer, a Teflon septum, and a thermometer was used for the polymerization process. PCL (3 mM, 6 g) and DAGP (0.3 mM, 0.226 g) were placed in the flask with 2.3 mL of xylene, and the solid content was about 75 wt %. After dissolving PCL, TDI (4.5 mM, 0.783 g) was added dropwise to the above reaction mixture by using a syringe at room temperature under nitrogen atmosphere. Then the reaction mixture was heated at 85−90 °C for 4.5 h to achieve the desired viscosity. This viscous mass was considered as prepolymer. Then MG (6.9 mM, 2.442 g) and IPDI (6 mM, 1.337 g) were added at room temperature with the required amount of xylene with solid 40 wt % content. The reaction was continued for about 5 h, at a stepwise increase of the temperature from 90 to 120 °C until high viscosity was achieved without gel formation. The completion of the reaction was confirmed by butyl amine test and absence of isocyanate band at 2270 cm−1 in the FTIR spectrum. By following the exactly same procedure other HBPURs with different compositions were synthesized (Table 1). The linear PUR (LPUR) with same composition was also prepared with an equivalent amount of BD in place of branch generating components.

by ring opening metathesis reaction by embedded Grubbs catalyst. However, they can heal only once at the same location, due to exhaustion of the healing agent. Further, this approach is well-known for healing microscopic fracture but is not very efficient for macroscopic damage. To overcome these shortcomings, material scientist designed a three-dimensional microvascular network in the polymer matrix with healing agent, which can deliver in fractured surface and heal repeatedly.8 However, designing such a network is very expensive, and the poor mechanical property of the resulting polymer minimizes their applications. Chung et al., on the contrary, reported a photo induced self-healing mechanism through photochemical [2 + 2] cyclo-addition reaction.9 However, the reported healing efficiency (14−26%) is too poor. The author’s group also synthesized a castor oil based HBPUR self-healing nanocomposite, which needs special nanomaterials.10 Ghosh and Urban reported oxetane substituted chitosan PUR networks with self-repairing ability, but designing such PUR requires special attention.11 Despite the great advantages of the reported self-healing PURs, some serious complications in design and fabrication limit their practical applicability. Again, it is noticed that no researcher uses biobased multifunctional macroglycol in such PURs, although biobased chain extenders are often used.5,6 The authors, therefore, wish to report the synthesis of thermoplastic HBPUR elastomers from biobased macroglycol as the branch generating moiety with varying compositions. The structural characterizations of the synthesized HBPURs were performed by using different instrumental tools. The performance including mechanical, thermal, chemical, etc. was evaluated along with self-healing, shape recovery, and biodegradability in order to highlight its potential as a safe and sustainable material.

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Table 1. Composition of the Reactants for the Synthesized HBPURs and LSPUR with Other Important Parameters reagent(mM)/ parameter PCL DAGP PE BD MG TDI IPDI NCO/OH ratio soft segment (wt %) hard segment (wt %) branching unit (wt %) biobased content (wt %)

EXPERIMENTAL SECTION

Materials. Dimer acid (DA, Mn ≈ 570 g/mol, Sigma-Aldrich, U.S.A.), glycerol (Merck, India), sunflower oil (Sigma-Aldrich, U.S.A.), poly(ε-caprolactone)diol (PCL, Mn = 2000 g/mol, Sigma-Aldrich, U.S.A.), and 1,4-butanediol (BD, Merck, India) were dried in an oven for 12 h prior to use. Isophorone diisocyanate (IPDI, Sigma-Aldrich, U.S.A.), 2,4/2,6-toluene diisocyanate (TDI, Merck, Germany), pentaerythritol (PE, Sigma, India), oxalic acid (Rankem, India), potassium hydroxide (Rankem, India), butylamine (Sigma-Aldrich, U.S.A.), isopropyl alcohol (SRL, Mumbai, India), hydrochloric acid (36%; Rankem, India), bromophenol blue indicator at pH 3.5−4.6 (Merck, India), para-toluene sulfonic acid (p-TSA SRL, Mumbai, India), CaO (Merck, India), and toluene (Merck, India) were used as received. Xylene (Merck, India) was used after distillation. N,NDimethylacetamide (DMAc, Merck, India) was dried overnight using CaO, distilled under reduced pressure, and kept in 4A type molecular sieve before use. Biobased Macroglycol. A biobased macroglycol of DA and glycerol was synthesized by stoichiometric controlled esterification reaction, and the product is encoded as DAGP (dimer acid-glycerol modified polyol). Briefly, 17.1 g (30 mM) of DA and 5.526 g (60 mM) of glycerol were taken in a three neck round-bottomed flask fitted with a nitrogen gas inlet, a magnetic stirrer, and a Teflon septum. An amount of 0.0855 g (0.5 wt % of DA) of p-TSA was used as the catalyst. The reaction was carried out at a temperature of 160 °C for 2 h in a controlled temperature oil bath. The progress of the reaction was monitored by determination of free acid value. Preparation of the Monoglyceride (MG) of Sunflower Oil. The MG of sunflower oil was prepared by an earlier reported method and a brief description is provided in the Supporting Information (SI).12

HBPUR1 HBPUR2 3 0.3

HBPUR3 HBPUR4

LPUR

3 0.15 0.15

3 0.075 0.225

3

3

6.9 4.5 6 1 57.75

6.9 4.5 6 1 57.35

6.9 4.5 6 1 56.88

6.9 4.5 6 1 56.60

0.6 6.9 4.5 6 1 57.04

42.24

42.65

43.12

43.4

42.96

2.09

1.24

0.81

0.38

-

24.75

23.90

23.47

23.04

23.01

0.3

All “as prepared” PUR solutions were cast on steel plate (150 mm × 50 mm × 0.61 mm) for impact and glass plates (10 mm × 60 mm × 5.9 mm) for gloss, scratch resistance, and chemical resistance and kept under ambient conditions for 48 h to facilitate the removal of volatile (solvent). Further these PUR films were dried in a forced convection oven at 70 °C to remove the remaining solvent along with any entrapped volatile. Finally the films were kept inside a desiccator for further testing and analyses after peeling from the plates. Characterization. Structural Analysis. The FT-IR spectra of DAGP and HBPURs were recorded over the range of wavenumber, 4000−400 cm−1 by a Nicolet (Madison, U.S.A.) FT-IR impact 410 spectrophotometer using KBr pellets. The NMR spectra of DAGP and HBPURs were taken by a 400 MHz NMR spectrometer (JEOL, Japan) using CDCl3 and DMSO-d6 as solvents with TMS as the internal standard. The X-ray diffraction (XRD) study was carried out at room temperature by using a D8 Focus XRD machine (Bruker AXS, Germany) equipped with a Cu Kα radiation source (λ = 1.54 Å) over the range of 2θ = 10−70°. Bragg’s equation was used to evaluate interplanner distance (d) of PCL moiety by the following equation.

d = nλ /2 sin θ

(1)

where n = integer (1), λ = wavelength of incident X-ray beam, and θ = angel of incident X-ray beam. Determination of Physical Properties. The specific gravity of HBPURs was measured as per the standard ASTM D7932 method. 4371


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ACS Sustainable Chemistry & Engineering Scheme 1. Synthetic Routes of (a) DAGP and (b) HBPUR1

The specific viscosity was measured by Ubbelohde viscometer as per the standard ASTM D4020 method. The gloss of the polymer films was measured by using mini-gloss meter (Sheen Instrument Ltd., U.K.). The number-average molar mass (Mn), weight-average molar mass (Mw), and polydispersity index (PDI) of all PURs were evaluated by gel permeation chromatography using linear polystyrene as the standard (GPC, Waters, U.K.). Mechanical Properties Testing. The tensile strength and elongation at break of the PUR films were measured as per the ASTM D 657 with the help of a Universal Testing Machine (UTM), model WDW-10 (JINAN, CHINA), with a 1.0 kN load cell and crosshead speed of 50 mm/min. The scratch hardness of the film was measured by using a scratch hardness tester, model number 705 (Sheen Instrument Ltd., U.K.) with stylus accessory and a travel speed of 50 mm/s. The impact strength was measured by an impact tester (S.C. Dey & Co, Kolkata, 1 m is the maximum height) using the standard ASTM D1037 falling weight method. The energy per unit thickness corresponding to the maximum height was taken as the impact strength. Adhesion test of the prepared PURs was also measured by lap shearing test between different surfaces like wood−

wood (W−W), plastic−plastic (P−P), and metal−metal (M-M) substrates. Thermal Properties Testing. The thermo gravimetric analysis (TGA) was done by a TGA-4000 (PerkinElmer, U.S.A.) in a temperature range of 32−720 °C under ultrapure nitrogen gas with flow rate of 30 mL min−1 at heating rate of 5 °C min−1. Similarly, a differential scanning calorimetry (DSC) study was performed by DSC 6000 instrument (PerkinElmer, U.S.A.) equipped with a refrigerator cooling system at a heating rate of 10 °C min−1 with the ultrapure nitrogen gas flow rate of 30 mL min−1 from −70 to +120 °C by following heating, cooling, and heating cycles. Chemical Resistance Test. The chemical resistance was performed by the standard ASTM D 543-67 method, and a brief description is provided in the SI. Self-Healing Ability Test. The self-healing ability of PUR films was evaluated by cutting the rectangular strips with a razor blade in a transverse direction and then individually healing them under the exposure of microwave (at 450 W). The healing efficiency was calculated as the ratio of the tensile strength of each sample before and after healing as measured by UTM under the same conditions, both before and after healing. Microwave healing was done by a domestic 4372


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Figure 1. FT-IR spectra of (a) DA and DAGP and (b) HBPUR1, HBPUR2, HBPUR3, HBPUR4, and LPUR. micro-oven (800 W) operating at a frequency of 2.45 GHz. Healing efficiency of the films was evaluate by using of the following equation.

presence of carbonyl stretching frequency for carboxylic acid group. However, for DAGP the carbonyl stretching frequency appears at 1741 cm−1 which corresponds to the ester group indicating the conversion of the acid group to an ester group. The presence of a broad peak near 3425 cm−1 of DAGP indicates the O−H stertching frequency, but a broad stretching band appears at 3425 cm−1 indicating the O−H stretching frequency of the carboxylic acid group of DA. In addition the sharp peak at 936 cm−1 represents the out of plane vibration of the O−H group of carboxylic acid of DA, whereas this bending vibration is totally absent for DAGP because the carboxylic acid of DA converted to ester group. Other characteristic bands of DA and glycerol moieties were also observed in FT-IR spectrum of DAGP (Figure 1a). Thus the spectral analyses provide sufficient evidence in favor of formation of DAGP with a proposed structure, which is further confirmed from NMR spectral analyses. 1 H and 13C NMR spectral anlyses confirmed the chemical structure of the synthesized macroglycol, DAGP. In the 1H NMR spectrum (Figure 2a), the peak at 0.84 (a) ppm is attributed to the terminal methyl (−CH3) protons and the peak at 1.23 (b) ppm is due to internal methylene (−CH2) protons of DAGP molecule. The peaks at 2.32 (c) and 1.59 (d) ppm are due to the α and β methylene protons with repect to the ester group (OCO). The peak at 2.06 (e) ppm is assigned to the protons attached to the carbon α to the double bond. The peak for unsaturated moiety of DAGP appears at around 5.04− 5.13 (f) ppm. The low intensity may be due to the partial hydrogenation of the dimer acid. The peaks at 4.13 (g) and 3.56 (h) ppm are attributed to the methylene protons, and 3.90 (i) ppm is due to the methine (−CH) protons of the glycerol unit.15 The peak that appears at 2.51 (j) ppm is assigned to hydroxyl group (−OH) of the glycerol moiety. In the 13C NMR spectrum (Figure 2b), the carbon of the terminal methyl group of DAGP appears at 14.21 (a) ppm. The peaks in between 22.74 and 32.01 (b) ppm represent the internal methylene carbons. The methylene carbon, α, to ester appears at 34.22 (c) ppm. The methlene carbon of the glycerol moiety attached to the ester linkage appears at 65.24 (d) ppm, and another methylene appears at 63.36 (e) ppm. The peak at 70.34 (f) ppm is assigned to methine proton of the glycerol moiety.15 Double bonded carbons were found at 129.22 (g) ppm. The carbonyl carbon peak is attributed to the peak at 174.44 (h)

healing efficiency = 100 × [tensile strength(healed) /tensile strength(initial)]

(2)

UV-Aging Test. Artificial aging of the PUR strips (5 cm × 1 cm) was carried out in an accelerated aging chamber for a total duration of 168 h. A UV-light containing chamber (Labtech, India) with a wavelength of irradiation and power of 256 nm and 8 W, respectively, was used for artificial aging. The sample to lamp distance was maintained at 25 cm. The tensile test was used to study the influence of UV-aging on the mechanical properties of PUR strips and was studied by the measurement of retention (%) of tensile strength, as calculated by the following equation: retention,

R(%) = 100 − [(P0 − P1)/P0] × 100

(3)

where P0 and P1 represent the values of tensile strength before and after UV-aging, respectively. Biodegradation Test. Biodegradation of the PURs was studied by using the McFarland turbidity method upon exposure of P. aeruginosa and B. subtilus bacterial strains the details of which are provided in the SI.13,14

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RESULTS AND DISCUSSION Synthesis and Characterization of DAGP. In this current work, to synthesize a biobased polyol DAGP, DA was esterified by glycerol at an elevated temperature of 160 °C as described in Scheme 1a. The progress of the reaction was checked by determination of free acid value. The acid values of DA 224.44 mg KOH/g while for DAGP it was 6.95 mg KOH/g, which confirmed the formation of desired ester with high yield (96.9%). The DAGP was obtained as a yellowish brown liquid and was found to be soluble in organic solvents like tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, acetone, dichloromethane, chloroform, xylene, etc. but insoluble in hexane and water. This result further supports the presence of polar moieties like −OH and OCO as well as long chain nonpolar hydrocarbons of fatty acids, which was further supported by FT-IR spectral analysis. The structural evidence for the formation of DAGP was displayed by a comparative study between DAGP and DA through Fourier transform infrared (FT-IR) spectra, as shown in Figure 1a. A sharp band at 1715 cm−1 of DA represents the 4373


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Figure 2. (a) 1H and (b) 13C NMR spectra of DAGP.

chain with MG and by maintaining the overall NCO/OH ratio unity by addition of another isocyanate, IPDI. The second diisocyanate, IPDI, is aliphatic and hence less reactive, and thus, controlling the reaction is easy. Further, the reactivity of two isocyanate groups of IPDI is different, and hence, the reaction occurred in a step by step manner, which helps to avoid the gel formation. Similarly, other PURs were also synthesized, where DAGP and PE were used as the branch generating units for HBPUR2 and HBPUR3, while only PE was used for HBPUR4. The reason behind the use of the PE branching unit with equal number of functionality as DAGP is to judge the effect of biobased polyol unit on performance of HBPUR. On the other

ppm. Thus, the resulting NMR spectrum confirmed the proposed structure of the DAGP molecule. Synthesis and Characterization of HBPURs. HBPUR1 was synthesized by using PCL as the main macroglycol, the branch generating unit, DAGP, as a biobased macroglycol, MG of sunflower oil as the chain extender, TDI as an aromatic diisocyanate, and IPDI as an aliphatic diisocyanate, through two-step one-pot prepolymerization technique, as shown in Scheme 1b. The isocyanate terminated prepolymer was produced by the reaction of PCL and DAGP with excess TDI, in the first step. However, the desired HBPUR1 was formed by the reaction of this prepolymer by extending the 4374


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Figure 3. (a) 1H NMR and (b) 13C NMR spectra of HBPUR1 and (c) dendritic, terminal, and linear units of HBPUR2, HBPUR3, and HBPUR4.

content was kept at 42−43.5 wt %, while to obtain the biodegradability as well as to replace the synthetic petrochemical, about 23−25 wt % appropriate biobased content was used (Table 1). Chemical Structure. The FT-IR spectra of the prepolymer proposed in Scheme 1 and synthesized PURs are shown in Figure S1 (SI) and Figure 1b. The absence of a band for the free NCO group near 2270−2250 cm−1 clearly indicates the completion of the reaction. The band appearing near 3436 cm−1 is due to the N−H stretching vibration of the urethane

hand, LPUR was prepared without using any branching unit, and thus, the chain extender BD was used in place of the branching moiety. Here it is pertinent to mention that the branch generating component, DAGP, was incorporated in the first step for HBPUR1 and PE was introduced to replace the DAGP for the rest of the HBPURs. This is due to the fact that DAGP is a macroglycol, while PE is a chain extender. The compositions of the components of PURs with their codes are given in Table 1. Further, as per the literature reported data, in order to achieve the optimum performance, the hard segment 4375


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Figure 4. (a) X-ray diffractograms of PURs and (b) stress−strain profiles of PURs.

DAGP, and TDI moieties. Methyl carbons of IPDI appear at 22.51 (c), 28.3 (d), and 33.8 (e) ppm. Methylene carbons of DAGP and long chain fatty acids of MG and PCL moieties were found at 24.60 (f), 27.13 (i), 29.27, 29.65 (g), and 31.4 (h) ppm. Peaks at 63.98 (j) and 68.74 (k) ppm are associated with the methylene carbons. The methine carbon of glycerol moiety appears at 69.82 (l) ppm. The peaks for the aromatic carbons appear at 136.9−137.7 (n) ppm, and double bonded carbons appear in between the region 128.38−130.8 (m) ppm. Peaks at 154.02−154.81 (o) and 173.19 (p) ppm are due to the urethane carbon and ester carbon, respectively. Peaks at 63.44 (D1), 63.17 (D2), 64.46 (L1), and 66 (T1) ppm were observed in the 13C NMR spectrum of HBPUR1 and represented the carbon atoms of tetra-, tri-, di-, and mono-substituted DAGP moieties. The degree of branching (DB) of HBPUR1 was calculated by the following equation by determining the integration of the peaks (D1, D2, L1, and T1).

bond coupled with the O−H stretching vibration. A small band near 1735 cm−1 is assigned to the carbonyl stretching of the ester group, while the sharp band at 1633 cm−1 is attributed to the carbonyl stretching of the amide group of the urethane bond. The small band near 1496 cm−1 corresponds to the N−H bending vibration. Again, the band in the range of 980−1208 cm−1 indicates the presence of OCO and N−C stretching frequency. These results clearly indicate the presence of urethane linkage in the structure of PURs. Furthermore, the bands at 2925 and 2852 cm−1 indicate the asymmetric and symmetric stretching of the sp3 hybridized C−H bond. The small band in the region of 1400−1460 cm−1 represents the C− H bending vibration. Further the proposed structure of HBPUR is confirmed from NMR spectral analyses, as discussed below. In general, it is very difficult to predict the exact structure of the prepolymer and final polymer because of high reactivity and high molar mass. However, 1H and 13C NMR spectral analyses support the proposed structure of the prepolymer and HBPUR1 as shown in Scheme 1b. 1H and 13C NMR spectra of the prepolymer are displayed in Figure S2 (SI) and confirmed the presence of DAGP, PCL, and TDI moieties in the structure. In the 1H NMR spectrum (Figure 3a), peaks at 0.75 (a), 0.80 (b), 0.88 (c), 0.94 (d), and 2.46 (e) ppm for the methyl protons of MG, DAGP, IPDI, and TDI moieties, respectively. Methylene protons of DAGP, PCL, and MG were found in the range from 1.19 to 1.27 (f) ppm. The peaks at 2.22 (h) and 1.49 (g) ppm are due to α and β-methylene protons of the DAGP moiety with respect to the ester group. The peak at 1.96 (i) ppm is due to the protons of the DAGP ring adjacent to the double bond while the protons attached to the carbon, α, to the double bond was found at 2.06 (j) ppm. The peak at 2.68 (k) ppm is due to methylene protons present in between two double bonds of the MG moiety. The peaks at 3.56 ppm (m) are due to the proton adjacent to the amide bond in the IPDI ring.14 Protons linked with α and β carbons with respect to oxygen of the ester moiety appear at 4.07 (n) and 3.94 (o) ppm. Methylene protons of the glycerol moiety were observed at 4.56 (p) and 4.79 (q) ppm.15 Aromatic protons of TDI and CC attached protons of MG and DAGP appear at 6.9−7.1 (s) and 5.26 (r) ppm. The −NH urethane peaks of IPDI and TDI were assigned at 8.66−8.77 (t) and 9.42 ppm (u). In the 13 C NMR spectrum (Figure 3b), peaks appearing at 14.47 (a) and 17.60 (b) ppm are due to the methyl carbon of MG,

DB = (D1 + D2 + T)/(D 1 1 + D2 + T1 + L1)

(4)

DBs of other synthesized HBPURs with another branching unit PE, were evaluated by taking integration of the additional peaks for tetra (D4), tri (D3), di (L2), and mono (T2) substituted PE moieties in the following equation:15 DB = (D1 + D2 + D3 + D4 + T1 + T2) /(D1 + D2 + D3 + D4 + T1 + T2 + L1 + L 2) (5)

Physical Structure. The degree of crystallinity of all PURs was measured from XRD, and the values vary from 18.37 to 20.15% (Table S2 of the SI) even though they have almost the same PCL content.16 These results clearly indicate the influence of other moieties on the crystallinity of PURs. However, the crystallinity nature of the PCL moiety remained intact in the polymer. Further, the study revealed that the presence of two diffraction peaks of this crystalline PCL moiety at 2θ = 21.2° (calculated d spacing of 4.18 Å) and 2θ = 23.5° (calculated d spacing of 3.78 Å) correspond to (110) and (200) planes in all PURs displayed in Figure 4a. Furthermore, although the weight percentage of PCL in HBPURs and LPUR was the same, a minute shift in diffraction peaks of HBPURs from LPUR was observed. This may be due to the fact that the branching architecture resulted in less crystallinity in HBPURs than LPUR. 4376


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ACS Sustainable Chemistry & Engineering Table 2. Mechanical Properties and Adhesive Strength of PURs property

HBPUR1

HBPUR2

HBPUR3

HBPUR4

LPUR

tensile strength (MPa) elongation at break (%) scratch hardness (kg) impact resistance (kJ m−1) toughness (MJ m−3) hardness (Shore A) adhesive strength (W−W) (kPa)

8.7 ± 0.3 2549 ± 50 4.5 ± 0.1 15.7 ± 0.3 132.7 ± 1.3 59 ± 0.5 5.8 ± 0.4

8.2 ± 0.5 2400 ± 100 4.0 ± 0.03 14.6 ± 0.2 102.4 ± 1.1 54 ± 0.3 4.2 ± 0.3

9.5 ± 0.2 3496 ± 60 4.0 ± 0.02 17.3 ± 0.6 205.5 ± 1.6 57 ± 0.6 6.7 ± 0.5

8.4 ± 0.5 3160 ± 50 4.5 ± 0.05 16.6 ± 0.4 189.84 ± 2.4 57 ± 0.4 3.3 ± 0.2

7.7 ± 2 2449 ± 100 3.6 ± 0.01 13.22 ± 0.3 128.2 ± 1.5 53 ± 0.8 2 ± 0.6

Figure 5. (a) TGA themograms of PURs (b) 1st derivative of TG thermograms of PURs and (c) DSC curves showing glass transition and melting temperatures of PURs

Physical Properties. The physical properties of HBPURs are provided in Table S1 of the SI. HBPURs were soluble in polar aprotic solvents such as tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, etc. as well as halogenated solvents such as chloroform, dichloromethane, etc. and even in acetone but insoluble in hexane, ethanol, etc. This high solubility in different types of solvents can be accredited to the globular structure of HBPURs and the presence of polar surface groups like hydroxyl, ester, and urethane along with long hydrocarbon moiety in them. Specific gravity of the HBPURs varies in the range of 1.19−1.47 which is similar to the reported other biobased PURs.17 The average molar mass was found to be on the order of 105, which seems to be very high and thus HBPURs may exhibit elastomeric character. This high molar mass is reflected in their solution viscosity values, which increase with the increase of molar mass of PURs. However, the values were found to be a little less, compared to their molar mass which may be due to the globular structure of HBPURs. Most significantly, the polydispersity index (PDI) of the synthesized HBPURs lies within 1.32−1.57 which indicates low dispersity and hence is expected to show well-defined property (Table S1 of the SI).14 Mechanical Properties. The molar mass of PUR and its soft segment, flexibility and rigidity in the chain, physical crosslinking, chain entanglement, orientation of segments, amount of rigid moiety, hydrogen bonding, and other van der Waals force, etc. strongly influence the mechanical properties.18 The mechanical properties of all HBPURs along with LPUR are documented in Table 2. The typical stress strain profiles of PURs are shown in Figure 4b. All HBPURs and LPUR exhibited excellent elongation as well as high tensile strength. The high flexibility is attributed to the high molar mass of the polymer chains, chain entanglement, amorphousness, and flexibility of DAGP, PCL moiety, ester linkages, and long hydrocarbon chains in monoglyceride of sunflower oil. Higher

the molar mass, the higher the chain entanglement will be. In addition secondary bonds are also helping in full extension of the molecular chains and thereby increasing the elongation at break value of PURs. This also depends on the rate of applied external forces. The lower the rate of applied external forces, the higher the elongation will be as the polymer chain gets sufficient time for relaxation. The rigidity of all PURs is assigned by chain entanglement, intermolecular attractions, crystallinity, and the presence of aromatic moiety in TDI and fused ring in IPDI moiety. Good strength and exceptional elongation result in very high toughness and makes it mechanically tough (Table 2). Biobased HBPUR reported by other researchers also showed similar high toughness.13 However, the elongation at break values of these HBPURs were found to be much higher than the reported any PURs. The impact resistance of all HBPURs and LPUR was also found to be very good (greater than the maximum limit of the used instrument) for the same reason, i.e., high flexibility and good strength of the synthesized PURs. In exactly the same manner, the value of scratch hardness, i.e., resistance to deformation under dynamic condition, increases with the increasing strength and flexibility of PURs. While static hardness as measured by durometer is the measure of the degree of rigidity or softness, the values were found to be moderately high for all PURs. Moreover HBPURs displayed (Table 2) better mechanical properties compared to LPUR due to the presence of a high degree of secondary interactions in HBPURs. Large numbers of polar groups (ester, urethane, and hydroxyl groups) present in the structure of the synthesized PURs are responsible for adhesive strength with polar substrates. Thus the adhesive strength values for wood−wood substrates were found to be higher than those for plastic and metal substrates (Table 2). The adhesive strength of PURs with nonpolar substrates arises from the mechanical interlocking due to penetration of polymer chains in the pores of the substrates. 4377


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Figure 6. (a) Plausible healing mechanism of the PURs and (b) optical images of mechanically damaged films; HBPUR1 (a−c after exposure of 0, 30, and 90 s of microwave radiation), HBPUR2 (d−f after exposure of 0, 30, and 90 s of microwave radiation), HBPUR3 (g−i after exposure of 0, 120, and 245 s of soft microwave radiation), HBPUR4 (j−l after exposure of 0, 60, and 168 s of microwave radiation), and LPUR (m−o after exposure of 0, 75, and 205 s of microwave radiation).

due to its high molar mass compare to others. Again LPUR demonstrated the least thermal stability compare to HBPURs. The glass transition (Tg) and melting temperature (Tm) for the soft segments of the synthesized PURs were obtained from the DSC curves (Figure 5c), and the values are given in Table S2 of the SI. The Tg was observed in between the range from −55 to −50 °C due to presence of the PCL moiety as a soft segment of PURs, while the Tm was found in the range between −3.2 and +4.2 °C. Low Tm of the synthesized PUR is attributed to the plasticization effect of MG and low degree of crystallinity (Table S2 of the SI).12 Self-Healing Properties. The healing process of PURs at the fractured surface involves five stages such as segmental surface rearrangements, surface approach, wetting, diffusion, and randomization (Figure 6a).20,21 Applying MW (450 W) on the fractured surface, some polar groups absorb energy and started to oscillate their dipoles. The heat generated from the oscillation is high, and thus the temperature reached is greater than the Tg and Tm of the soft segment. Hence a rapid Brownian motion occurred in the soft segments and lead to rearrangement of molecular chains in soft segments to repair the crack surface. This rearrangement process results in quick healing of the damaged surface. At the same time, a hard segment of PURs helps to retain its original shape. As selfhealing is achieved by rearrangement and diffusion of the soft segment of PURs thus healing of the polymer matrix repeated itself again and again upon fracture. Typically optical images of fractured, intermediate, and healed films are displayed in Figure 6b. A comparison between optical images before and after healing clearly demonstrates the completion of the healing process. In fact, the healing efficiency of PURs remains almost the same even after the fourth cycle of testing (Figure 7a). From the healing study, it was observed that the healing time increases with the decrease in amount of DAGP. This clearly indicates the role of DAGP in the healing process. Thus HBPUR1 shows a minimum healing time compares to PE based HBPUR4. The minimum healing time with 100% retention in mechanical strength makes the PURs more

However, as the physicochemical interactions such as Hbonding, polar−polar interaction, polar induced−polar interactions, etc. (as observed in FTIR spectrum) with the hydroxyl group of the wood substrate are higher, so the adhesive strength for the cellulosic substance is also much higher than the same for plastic and metal substrates. Similar results are also observed in the vegetable-oil-based HBPUR system.18 Thermal Properties. Thermal properties of the synthesized PURs were studied from TGA and DSC analyses (Figure 5a−c and Table S2 of the SI). Generally the thermal stability of thermoplastic PURs depends on the hard to soft segment ratio, degree of urethane linkages, presence of aromatic moiety, extent of physical cross-linking, and molar mass with its distribution. Three step degradation patterns for all PURs were clearly demonstrated by their TGA thermograms and corresponding DTG curves (Figure 5a,b). A similar degradation pattern is supported by literature reports.19 The first step of degradation corresponds to the degradation of thermolabile moieties such as long aliphatic chains of MG and DAGP. On the other hand the second step of degradation is due to the presence of relatively more thermostable moieties like urethane and ester linkages, cycloaliphatic ring of DAGP, etc. The final step of degradation corresponds to the degradation of most thermostable moiety, i.e., the aromatic unit of TDI. Due to the presence of a large number of different secondary interactions such as H-bonding, polar−polar interaction, etc. in the chains, the second step of degradation has started before completion of the first step, as observed in DTG curves (Figure 5b). Similarly, due to the presence of a large number of polar linkages as well as cycloaliphatic moiety, there is overlap of second and third degradation steps. Further, the initial degradation temperature (Ton) of PURs increases with increasing such interactions as well as with their molar mass (Table S1 of the SI). However, as the difference in molar mass is not significantly different so the variation of Ton is only marginal. It is clearly understood from the thermograms that with the decrease of biobased content in the synthesized PURs the degradation temperature increases. HBPUR3 showed maximum degradation temperature (TMAX) 4378


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Figure 7. (a) Healing efficiency (%) of PURs for repeated cycles under microwave. (b) Growth curve of P. aeruginosa on PURs. (c) Mechanism of biodegradation. (d and e) SEM images of HBPUR1 and LPUR of the biodegraded surface.

efficient than other reported self-healing polymers.11 Thus the excellent self-healing ability is due to favorable Brownian motion of the molecular chain of PUR. High elongations at break with low tensile strength as well as low glass transition and melting temperature and high molecular mass of the soft segment are the favorable factors resulting in easy diffusion of the polymer chain in the cracks upon exposure of the stimulus. In addition the presence of a large number of polar functional groups results in a high amount of secondary interactions once the stimulus is removed. Thus thermoplastic adhesive bonding is really helpful in the repeatable self-healing ability of PUR Chemical Resistance. All synthesized PURs exhibited good chemical resistance in the tested chemical environments of 10% hydrochloric acid (v/v), 20% ethanol (v/v), 10% sodium chloride (w/v), and tap water except that of 1% sodium hydroxide (w/v; Table S3 of the SI). The results are supported by reported data of similar biobased PURs.22 Good chemical resistance is due to strong interactions through H-bonding, polar−polar interactions, etc. in the structure of PRUs. Poor alkali resistance accredited to the presence of hydrolyzable ester

linkages in the MG and PCL moieties of PURs. HBPURs showed better chemical resistance than their linear analogue due to their confined geometry and larger extent of secondary interactions. UV-Aging Study. Synthesized PURs were subjected to UV aging tests to evaluate the weather resistance property. However, natural weathering tests are more time-consuming, and thus artificial weathering conditions are adopted to stimulate the natural weathering process.23 UV-aged films showed a slight decrease in elongation at break but the strength of the strips remains almost constant (Table S4 of the SI). It indicates that long time UV exposure creates additional crosslinking density or molecular recombination as well as chain scission from the oxidation reaction which reduces the flexibility of the material. As these two effects on mechanical strength are canceling each other, the resulting strength remains almost same. HBPURs possess better UV resisting power than LPUR due to the hyperbranched structure with confined geometry. However, in all HBPURs as the UV-active component was same, so the effect was almost same. 4379



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Biodegradation Study. Biodegradation of PURs by P. aeruginosa and Bacillus sp. was studied by plotting optical density (OD) against time of exposure (Figure 7b). Kay et al. reported that P. aeruginosa act as effective bacteria for biodegradation of PURs due to the difference in cell wall structure by which they can easily attach with the chain of the polymer.24 It was observed that the PUR films were degraded to a considerable amount after 8 weeks of incubation by the used P. aeruginosa bacterial strain. It indicates that PURs with long repeating units and hydrolytic groups (ester moiety) are more accessible to biodegradation. Actually, the biodegradation process is a combination of physical, chemical, and biological actions as shown in Figure 7c. Huang and Robay observed that the degradation proceeds in amorphous region preferentially than the crystalline region.25 The structure of HBPUR1 contains less crystalline region (18.37%), resulting in more biodegradation than other HBPURs. All HBPURs undergo more degradation compare to LPUR having a high crystalline region (Table S2 of the SI). Also the well expanded hyperbranched architecture of HBPURs is responsible for attachment of the bacterial strain in high extent compare to LPUR. From the biodegradation study it is also evident that degradation decreases with decrease in wt % of biobased content from HBPUR1 to LPUR (Figure 7b). LPUR showed the least degradation which is clearly displayed in the SEM images as it contains the lowest biobased content (Figure 7c,d).

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CONCLUSION

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ASSOCIATED CONTENT

Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel: + 91-3712-267009. Fax: +91-3712-267006. E-mail: [email protected]. ORCID

Niranjan Karak: 0000-0002-3402-9536 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to the research project assistance given by DST, India through the Grant No. EMR/ 2016/001598, dated 04-Jan-2017.

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REFERENCES

(1) Yang, Y.; Urban, M. W. Self-healing polymeric materials. Chem. Soc. Rev. 2013, 42, 7446−7467. (2) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging applications of stimuli-responsive polymer materials. Nat. Mater. 2010, 9, 101−113. (3) Liu, F.; Urban, M. W. Recent advances and challenges in designing stimuli-responsive polymers. Prog. Polym. Sci. 2010, 35, 3− 23. (4) Han, R.; Campbell, K. P. Dysferlin and muscle membrane repair. Curr. Opin. Cell Biol. 2007, 19, 409−416. (5) Karak, N. Biobased Smart Polyurethane Nanocomposites: From Synthesis to Applications; Royal Society of Chemistry: Cambridge, U.K., 2017. (6) Karak, N. Bio-based hyperbranched polyurethane nanocomposites. In Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc.: Hoboken, NJ, 2015; 10.1002/0471440264.pst638. (7) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Autonomic healing of polymer composites. Nature 2001, 409, 794−797. (8) Olugebefola, S. C.; et al. Polymer microvascular network composites. J. Compos. Mater. 2010, 44, 2587−2603. (9) Chung, C. M.; Roh, Y. S.; Cho, S. Y.; Kim, J. G. Crack healing in polymeric materials via photochemical [2 + 2] cycloaddition. Chem. Mater. 2004, 16, 3982−3984. (10) Thakur, S.; Karak, N. A tough, smart elastomeric bio-based hyperbranched polyurethane nanocomposite. New J. Chem. 2015, 39, 2146−2154. (11) Ghosh, B.; Urban, M. W. Self-repairing oxetane-substituted chitosan polyurethane networks. Science 2009, 323, 1458−1460. (12) Kalita, H.; Karak, N. Bio-based hyperbranched shape-memory polyurethanes effect of different vegetable oils. J. Appl. Polym. Sci. 2014, 131, 1−8. (13) Duarah, R.; Singh, Y. P.; Mandal, B. B.; Karak, N. Sustainable starch modified polyol based tough, biocompatible, hyperbranched polyurethane with a shape memory attribute. New J. Chem. 2016, 40, 5152−5163. (14) Bayan, R.; Karak, N. Renewable resource modified polyol derived unprecedented aliphatic hyperbranched polyurethane as a biodegradable and UV-resistant smart material. Polym. Int. 2017, 66, 839−850. (15) Das, B.; Konwar, U.; Mandal, M.; Karak, N. Sunflower oil based biodegradable hyperbranched polyurethane as a thin film material. Ind. Crops Prod. 2013, 44, 396−404. (16) Thakur, S.; Karak, N. Castor oil-based hyperbranched polyurethanes as advanced surface coating materials. Prog. Org. Coat. 2013, 76, 157−164.

A high molar mass, extremely flexible thermoplastic hyperbranched polyurethane (HBPUR) elastomer with desired biodegradability can be successfully synthesized from biobased multifunctional macroglycol by the conventional prepolymerization technique using an Ax + By (x, y ≥ 2) approach, as a sustainable material. The NMR spectral studies support the hyperbranched structure of HBPUR. Synthesized HBPURs not only exhibited high mechanical strength but also provide high thermostability along with a significant amount of UVresistance ability. Most interestingly the flexibility of the synthesized PURs was found to be extremely high, which was not reported so far for any polymers. In addition this biobased PURs showed very good microwave responsive repeatable selfhealing ability. The biobased macroglycol containing a branching unit has much more influence on the performance of HBPUR than petroleum based chain extender containing a branching unit. Thus studied biobased, biodegradable HBPURs have remarkable potential as a sustainable material.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00001. Experimental section for preparation of monoglyceride, chemical resistance test, and biodegradation test; FT-IR and NMR spectra of prepolymer, NMR spectra for HBPUR2, HBPUR3, HBPUR4, and LPUR, stress−strain recovery curve; tables for physical properties, thermal properties, weight changes in chemical resistance test, and retention of properties by UV aging. (PDF) 4380


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ACS Sustainable Chemistry & Engineering (17) Gogoi, S.; Karak, N. Bio-based biodegradable waterborne hyperbranched polyurethane as an ecofriendly sustainable material. ACS Sustainable Chem. Eng. 2014, 2, 2730−2738. (18) Deka, H.; Karak, N. Bio-based hyperbranched polyurethane/ clay nanocomposites: adhesive, mechanical, and thermal properties. Polym. Adv. Technol. 2011, 22, 973−980. (19) Trovati, G.; Sanches, E. A.; Neto, S. C.; Mascarenhas, Y. P.; Chierice, G. O. Characterization of polyurethane resins by FTIR, TGA, and XRD. J. Appl. Polym. Sci. 2010, 115, 263−268. (20) Burattini, S.; Greenland, B. W.; Chappell, D.; Colquhoun, H. M.; Hayes, W. Healable polymeric materials: A tutorial review. Chem. Soc. Rev. 2010, 39, 1973−1985. (21) Wool, R. P.; O'Connor, K. M. A theory of crack healing inpolymers. J. Appl. Phys. 1981, 52, 5953−5963. (22) Deka, H.; Karak, N. Bio-based hyperbranched polyurethanes for surface coating applications. Prog. Org. Coat. 2009, 66, 192−198. (23) Boubakri, A.; Guermazi, N.; Elleuch, K.; Ayedi, H. F. Study of UV-aging of thermoplastic polyurethane material. Mater. Sci. Eng. Mater. Sci. Eng., A 2010, 527, 1649−1654. (24) Kay, M. J.; Morton, L. H. G.; Prince, E. L. Bacterial degradation of polyester polyurethane. Int. Biodeterior. 1991, 27, 205−222. (25) Huang, S. J.; Roby, M. S. Biodegradable polymers poly (amideurethanes). J. Bioact. Compat. Polym. 1986, 1, 61−71.

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Bio-based multifunctional macroglycol containing smart thermoplastic

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