Bio-based multifunctional macroglycol containing smart thermoplastic

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Bio-based multifunctional macroglycol containing smart thermoplastic hyperbranched polyurethane elastomer with intrinsic self-healing attribute Tuhin Ghosh, and Niranjan Karak ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00001 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Sustainable Chemistry & Engineering

Bio-based 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 *Corresponding author: Niranjan Karak, Tel: + 91-3712-267009; Fax: +91-3712-267006, E-mail: [email protected]. KEYWORDS: Bio-based polyurethane, self-healing material, tough sustainable material, hyperbranched thermoplastic elastomer, macroglycol branching unit.

ABSTRACT: Smart bio-based self-healable polymers are advanced sustainable materials. Thus, thermoplastic hyperbranched polyurethane (HBPUR) elastomer was synthesized by using a biobased multifunctional macroglycol along with other required components, for the first time. This bio-based 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 physico-chemical structure of the synthesized macroglycol and HBPURs. The degree of branching value (0.780.91) of HBPURs varies with this macroglycol content. These thermoplastic PUR elastomers

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exhibited outstanding toughness (205 MJ.m-3), unprecedented high elongation at break (28103160%), 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 thermo-responsive shape recovery (100%) within 50-56s 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.

INTRODUCTION Development of smart materials from renewable resource is the order of the day and significance of such material is enormus.1 Thus introduction of renewable resource based raw materials as the substitute of fossil fuel based feed stocks in production of various materials including polymers, is essential. This approach can address shortcomings like non-renewability, non-biodegradability, 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, self-healing 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 to akin the living systems.4 However, to mimic biological systems, scientists are attempting many


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tireless approaches including microencapsulation, reversible bond formation, vascular network formation, 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, non-entanglement 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 self-healing ability. 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 by ring opening metathesis reaction by embedded Grubbs catalyst. But they can healed only once at the same location, due to exhaustion of the healing agent. Further, this approach is well known for healing microscopic fracture but not very much efficient for macroscopic damage. To overcome this shortcomings material scientist designed a three dimensional microvascular network in polymer matrix with healing agent, which can deliver in fractured surface and heal repeatedly.8 However, designing of such network is very expensive and poor mechanical property of the resulting polymer minimize their applications. Chung et al., on contrary reported a photo induced self-healing mechanism through photochemical [2+2] cyclo-addition reaction.9 But reported healing efficiency (14-26%) is too poor. 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


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self-repairing ability but designing such PUR requires special attention.11 Despite of 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 bio-based multifunctional macroglycol in such PURs, although bio-based chain extenders are often used.5,6 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. were evaluated along with self-healing, shape recovery and biodegradability in order to highlight its potential as a safe and sustainable material. EXPERIMENTAL SECTION Materials. Dimer acid (DA, Mn~570 g/mol, Sigma-Aldrich, USA), glycerol (Merck, India), sunflower oil (Sigma-Aldrich, USA), poly(ε-caprolactone)diol (PCL, Mn=2000 g/mol, SigmaAldrich, USA) and 1,4-butanediol (BD, Merck, India) were dried in an oven for 12 h prior to use. Isophorone diisocyanate (IPDI, Sigma-Aldrich, USA), 2,4/2,6-toluene diisocyanate (TDI, Merck, Germany), pentaerythritol (PE, Sigma, India), oxalic acid (Rankem, India), potassium hydroxide (Rankem, India), butylamine (Sigma-Aldrich, USA), 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.


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Bio-based macroglycol. A bio-based 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) 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 monoglyceride (MG) of Sunflower oil. MG of sunflower oil was prepared by an earlier reported method and brief description is provided in Electronic Supplementary information (ESI).12 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 xylene and solid content was about 75 wt%. After dissolving PCL, TDI (4.5 mM, 0.783 g) was added drop wise 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 pre-polymer. Then MG (6.9 mM, 2.442 g) and IPDI (6 mM, 1.337 g) were added at room temperature with required amount of xylene with solid 40 wt% content. The reaction was continued for about 5 h, at step wise increase of temperature from 90-120°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


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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 equivalent amount of BD in place of branch generating components. All ‘as prepared’ PUR solutions were cast on steel plate (150 mm × 50 mm × 0.61 mm) for impact, glass plate (10 mm × 60 mm ×5.9mm) for gloss, scratch resistance, 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 rest amount of 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 wave number, 4000-400 cm-1 by a Nicolet (Madison, USA) 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 Xray 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 A°) over the range of 2θ = 10-70°. Bragg’s equation was used to evaluate inter-planner distance (d) of PCL moiety by the following equation. d= nλ/2sinθ--------------------------(1)

Where n= integer (1), λ= wavelength of incident X-ray beam and θ= angel of incident X-ray beam. Determination of physical properties


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The specific gravity of HBPURs was measured as per the standard ASTM D7932 method. 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, UK). 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, UK). 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 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, UK) 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 (Perkin Elmer, USA) in a temperature range 32-720°C under ultra-pure nitrogen gas with flow rate of 30 mL.min-1 at heating rate of 5°C.min-1. Similarly, differential scanning calorimetry (DSC) study was performed by DSC 6000 instrument (Perkin Elmer, USA) equipped with a refrigerator cooling


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system at heating rate of 10°C.min-1 with the ultra-pure 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 the brief description is provided in ESI. 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 healed 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 micro-oven (800W) operating at a frequency of 2.45 GHz. Healing efficiency of the films was evaluate by using of the following equation. Healing efficiency = 100 × [Tensile strength (healed) / Tensile strength (initial)]-------(2)

UV-aging test Artificial aging of the PUR strips (5cm ×1 cm) was carried out in an accelerated aging chamber for a total duration of 168 h. UV-light with wavelength of irradiation and power of 256 nm and 8 W respectively containing chamber (Labtech, India) 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 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.


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Bio-degradation test Biodegradation of the PURs was studied by using McFarland turbidity method upon exposure of P. aeruginosa and B. subtilus bacterial strains and details of which is provided in ESI.13,14 RESULTS AND DISCUSSION Synthesis and characterization of DAGP. In this current work to synthesize a bio-based polyol DAGP, DA was esterified by glycerol at 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 mgKOH/g while for DAGP it was 6.95 mgKOH/g, which confirmed the formation of desired ester with high yield (96.9%). The DAGP was obtained as 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 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 presence of carbonyl stretching frequency for carboxylic acid group. But,for DAGP the carbonyl stretching frequency appears at 1741 cm-1 which corresponds to ester group indicating the conversion of acid group to ester group. Presence of broad peak near 3425 cm-1 of DAGP indicates the O-H stertching frequency but a broad stretching band appears at 3425 cm-1 indicates the O-H stretching frequency of carboxylic acid group of DA. In addition the sharp peak at 936 cm-1 represents the out of plane vibration of


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O-H group of carboxylic acid of DA. While 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 favour of formation of DAGP with proposed structure, which is further confirmed from NMR spectral analyses. 1

H and

13

C NMR spectral anlyses confirmed the chemical structure of the synthesized

macroglycol, DAGP. In 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 attach to the carbon α to the double bond. The peak for unsaturated moiety of DAGP is appeared 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) ppmare attributed to the methylene protons and 3.90(i) ppm is due to the methine (-CH) protons of glycerol unit.15 The peak appeared at 2.51 (j) ppm is assigned to hydroxyl group (-OH) of glycerol moiety.In

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C

NMR spectrum (Figure 2b), the carbon of 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. Methlene carbon of glycerol moiety attached to 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 glycerol moiety.15 Double bonded carbons were found at 129.22 (g) ppm. The carbonyl carbon peak is attributed at 174.44 (h) ppm. Thus resulting NMR spectrum confirmed the proposed structure of DAGP molecule.


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Synthesis and characterization of HBPURs. HBPUR1 was synthesized by using PCL as the main macroglycol, the branch generating unit, DAGP, as a bio-based 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 pre-polymerization technique, as shown in Scheme 1b. The isocyanate terminated pre-polymer was produced by the reaction of PCL and DAGP with excess TDI, in the 1st step. However, the desired HBPUR1 was formed by the reaction of this pre-polymer by extending the chain with MG and by maintaining overall NCO/OH ratio unity by addition of another isocyanate, IPDI. The second diisocyanate, IPDI is aliphatic, hence less reactive and thus the controlling the reaction is easy. Further, the reactivity of two isocyanate groups of IPDI is different and hence the reaction was occurring 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 PE branching unit with equal number of functionality as DAGP is to judge the effect of bio-based polyol unit on performance of HBPUR. On the other hand, LPUR was prepared without using any branching unit and thus the chain extender, BD was used in place of 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 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 content was kept 42-43.5 wt%, while to obtain the biodegradability as well as to replace the synthetic petrochemical about 23-25 wt% appropriate bio-based content was used (Table 1).


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Chemical structure. The FT-IR spectra of the pre-polymer proposed in Scheme 1 and synthesized PURs are shown in Figure S1 (ESI) and Figure 1b. Absence of band for free NCO group near 2270-2250 cm-1 clearly indicates that the completion of the reaction. The band appearing near 3436 cm-1 is due to N-H stretching vibration of urethane bond coupled with O-H stretching vibration. A small band near 1735 cm-1 is assigned to carbonyl stretching of ester group, while the sharp band at 1633 cm-1 is attributed to carbonyl stretching of amide group of urethane bond. The small band near 1496 cm-1 corresponds to N-H bending vibration. Again, the band in the range 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 sp3 hybridized C-H bond. The small band in the region 1400-1460 cm-1 represents C-H bending vibration. Further the proposed structure of HBPUR is confirmed from NMR spectral analyses, as discussed underneath. In general, it is very difficult to predict the exact structure of pre-polymer and final polymer because of high reactivity and high molar mass. However, 1H and

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C NMR spectral analyses

support the proposed structure of pre-polymer and HBPUR1 as shown in Scheme 1b. 1H and 13C NMR spectra of the pre-polymer were displayed in Figure S2 (ESI) and confirmed the presence of DAGP, PCL and TDI moieties in the structure. In 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 DAGP moiety with respect to ester group. Peak appeared at 1.96 (i) ppm is due to the protons of DAGP ring adjacent to the


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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 MG moiety. The peaks at 3.56 ppm (m) are due to proton adjacent to the amide bond in 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 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 appears at 6.9-7.1 (s) and 5.26 (r) ppm. The –NH urethane peaks of IPDI and TDI were assigned at 8.668.77 (t) and 9.42 ppm (u). In

13

C NMR spectrum (Figure 3b), peaks appeared at 14.47 (a) and

17.60 (b) ppm are due to methyl carbon of MG, 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, long chain fatty acids of MG and PCL moieties were found at 24.60 (f), 27.13 (i), 29.27, 29.65 (g), 31.4 (h) ppm. Peaks at 63.98 (j) and 68.74 (k) ppm are associated with the methylene carbons. 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 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 13

C NMR spectrum of HBPUR1 represented the carbon atoms of tetra-, tri-, di and mono

substituted DAGP moiety. 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). DB= (D1 + D2+ T1)/ (D1 + 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 moiety in the following equation.15


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DB= (D1 + D2+ D3+D4+ T1+ T2)/ (D1 + D2+ D3+D4+ T1+ T2+L1+L2)----------------(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 ESI) even though they have almost same PCL content.16 These results clearly indicate the influence of other moieties on the crystallinity of PURs. But the crystallinity nature of PCL moiety remained intact in the polymer. Further, the study revealed 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, but a minute shift in diffraction peaks of HBPURs from LPUR was observed. This may be due to the fact that the branching architecture resulting less crystallinity in HBPURs than LPUR. Physical properties. The physical properties of HBPURs are provided in Table S1 of ESI. HBPURs were soluble in polar aprotic solvents such as tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, etc. as well as halogenated solvent such as chloroform, dichloromethane etc. and even in acetone but insoluble in hexane, ethanol, etc. This high solubility in different types of solvent 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 bio-based PURs.17 The average molar mass was found to be in 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 little less, compare to their molar mass which may be due to the globular structure of HBPURs. Most


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significantly, the polydispersity index (PDI) of the synthesized HBPURs lies within 1.32 -1.57 which indicates low dispersity and hence expected to show well defined property (Table S1 of ESI).14 Mechanical properties. Molar mass of PUR and its soft segment, flexibility and rigidity in the chain, physical cross-linking, 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 4 (b). 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, higher will be the chain entanglement. 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 is also depends on the rate of applied external forces. Lower the rate of applied external forces, higher will be the elongation as polymer chain get sufficient time for relaxation. The rigidity of all PURs is assigned by chain entanglement, intermolecular attractions, crystallinity and presence of aromatic moiety in TDI and fused ring in IPDI moiety. Good strength and exceptional elongation result very high toughness and makes it mechanically tough (Table 2). Bio-based HBPUR reported by the 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) due to the same reason, i.e. high flexibility and good strength of the synthesized PURs. Exactly similar


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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 degree of rigidity or softness and the values were found to be moderately high for all PURs. Moreover HBPURs displayed (Table 2) better mechanical property compared to LPUR due to the presence of 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 plastic and metal substrates (Table 2). The adhesive strength of PURs with non-polar substrates arises from the mechanical interlocking due to penetration of polymer chains in the pores of the substrates. However, as the physico-chemical interactions such as H-bonding, 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 cellulosic substance is also much higher than the same for plastic and metal substrates. Similar results are also observed in vegetable oil-based HBPUR system.18 Thermal properties. Thermal properties of the synthesized PURs were studied from TGA and DSC analyses (Figure 5 a, b and c, and Table S2 of ESI). Generally 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 5 a and b). 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


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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. aromatic unit of TDI. Due to the presence of 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 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 ESI). 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 bio-based content in the synthesized PURs the degradation temperature increases. HBPUR3 showed maximum degradation temperature (TMAX) 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 ESI. The Tg was observed in between the range -55 to -50°C due to presence of PCL moiety as a soft segment of PURs, while the Tm was found in the range between -3.2 °C to 4.2 °C. Low Tm of the synthesized PUR attributed to the plasticization effect of MG and low degree of crystallinity (Table S2 of ESI).12 Self-healing properties. The healing process of PURs at fractured surface involves five stages such as segmental surface rearrangements, surface approach, wetting, diffusion, and randomization (Figure 6a).20,21 Applying MW (450W) on the fractured surface some polar


17


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groups absorb energy and started to oscillate its 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, hard segment of PURs help to retain its original shape. As self-healing is achieved by rearrangement and diffusion of soft segment of PURs thus healing of 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 same even after 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 compare to PE based HBPUR4. The minimum healing time with 100 % retention in mechanical strength make the PURs more 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, high molecular mass of the soft segment are the favorable factors resulted easy diffusion of the polymer chain in the cracks upon exposure of the stimulus. In addition presence of large number of polar functional groups result high amount of secondary interactions once the stimulus is removed. Thus thermoplastic adhesive bonding is really help in repeatable self-healing ability of PUR


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Chemical resistance. All synthesized PURs exhibited good chemical resistance in 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 ESI). The results are supported by reported data of similar bio-based 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 hydrolysable 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 test to evaluate the weather resistance property. But natural weathering tests are more time consuming, thus artificial weathering conditions are adopted to stimulate the natural weathering process.23 UV-aged films showed slight decrease in elongation at break but the strength of the strips remains almost constant (Table S4 of ESI). It indicates that long time UV exposure creates additional crosslinking density or molecular recombination as well as chain scission from oxidation reaction which reduces the flexibility of the material. As these two effects on mechanical strength are cancelling each other so the resulted strength remains almost same. HBPURs possess better UV resisting power than LPUR due to hyperbranched structure with confined geometry. However in all HBPURs as the UV-active component was same so the effect was almost same. Biodegradation study. Biodegradation of PURs by P. aeruginosa and Bacillus sp. was studied by plotting optical density (OD) against time of exposure (Figure 7 b). Kay et al. (1991) reported that P. aeruginosa act as effective bacteria for bio-degradation 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


19


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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 biodegradation process is a combination of physical, chemical and biological actions as shown in Figure 7 c. Huang and Robay (1986) observed that the degradation proceed in amorphous region preferentially than the crystalline region.25 The structure of HBPUR1 contains less crystalline region (18.37%), resulting more biodegradation than other HBPURs. All HBPURs undergoes more degradation compare to LPUR having high crystalline region (Table S2 of ESI). Also the well expanded hyperbranched architecture of HBPURs is responsible for attachment of bacterial strain in high extent compare to LPUR. From bio-degradation study it is also evident that degradation decreases with decrease in wt% of bio-based content from HBPUR1 to LPUR (Figure 7b). LPUR showed least degradation which is clearly displayed in the SEM images as it contains lowest bio-based content (Figure 7 c and d).

CONCLUSION High molar mass, extremely flexible thermoplastic hyperbranched polyurethane (HBPUR) elastomer with desired bio-degradability can be successfully synthesized from bio-based multifunctional macroglycol by the conventional pre-polymerization technique using 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 provides high thermostability along with significant amount of UV-resistance ability. Most interestingly the flexibility of the synthesized PURs found to be extremely high, which was not reported so far for any polymers. In addition this bio-based PURs showed very good microwave


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responsive repeatable self-healing ability. The bio-based macroglycol containing branching unit has much more influence on the performance of HBPUR than petroleum based chain extender containing branching unit. Thus studied bio-based, bio-degradable HBPURs have remarkable potential as a sustainable material.

ASSOCIATED CONTENT Supporting Information Experimental section for preparation of monoglyceride, chemical resistance test and biodegradation test; FT-IR and NMR spectra of pre-polymer, 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.

AUTHOR INFORMATION Corresponding Author *N. Karak. Email: [email protected]. Tel: +91-3712-267009. Fax: +91-3712-267006. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Funding Department of Science & Technology (DST), New Delhi, India.


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ACKNOWLEDGMENT 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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Figure 1. FT-IR spectra of (a) DA and DAGP, and (b) HBPUR1, HBPUR2, HBPUR3, HBPUR4 and LPUR


26

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


27


Figure 3. (a) 1H NMR and (b)

13

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C NMR spectra of HBPUR1 and (c) Dendritic, terminal and

linear units of HBPUR2, HBPUR3 and HBPUR4


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


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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


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


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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 bio-degradation and (d, e) represents the SEM images of HBPUR1 and LPUR of bio-degraded surface.


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Scheme 1. Synthetic routes of (a) DAGP and (b) HBPUR1


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Table 1. Composition of the reactants for the synthesized HBPURs and LSPUR with other important parameters. Reagent(mM)/Parameter

HBPUR1

HBPUR2 HBPUR3

HBPUR4 LPUR

PCL

3

3

3

3

3

DAGP

0.3

0.15

0.075

-

-

PE

-

0.15

0.225

0.3

-

BD

-

-

-

-

0.6

MG

6.9

6.9

6.9

6.9

6.9

TDI

4.5

4.5

4.5

4.5

4.5

IPDI

6

6

6

6

6

NCO/OH ratio

1

1

1

1

1

Soft segment (wt%)

57.75

57.35

56.88

56.60

57.04

Hard segment (wt%)

42.24

42.65

43.12

43.4

42.96

Branching unit (wt%)

2.09

1.24

0.81

0.38

-

Bio-based content (wt%)

24.75

23.90

23.47

23.04

23.01


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Table 2. Mechanical properties and adhesive strength of PURs Property

HBPUR1

HBPUR2

HBPUR3

HBPUR4

LPUR

Tensile strength (MPa)

8.7±0.3

8.2±0.5

9.5±0.2

8.4±0.5

7.7±2

Elongation at break (%)

2549±50

2400±100

3496±60

3160±50

2449±100

Scratch hardness (kg)

4.5±0.1

4.0±0.03

4.0±0.02

4.5±0.05

3.6±0.01

Impact resistance (kJ.m-1)

15.7± 0.3

14.6± 0.2

17.3± 0.6

16.6± 0.4

13.22± 0.3

Toughness (MJ.m-3)

132.7±1.3

102.4±1.1

205.5±1.6

189.84±2.4

128.2±1.5

Hardness (Shore A)

59± 0.5

54± 0.3

57± 0.6

57± 0.4

53± 0.8

Adhesive strength (W-W)(kPa)

5.8± 0.4

4.2± 0.3

6.7± 0.5

3.3± 0.2

2± 0.6


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For Table of Contents Use only

Synopsis A sustainable smart thermoplastic hyperbranched polyurethane elastomer with rapid and repeated self-healing ability was reported.


36

Bio-based multifunctional macroglycol containing smart thermoplastic

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