Plant Oil-Based Supramolecular Polymer Networks and Composites

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Plant Oil-Based Supramolecular Polymer Networks and Composites for Debonding-On-Demand Adhesives Anselmo del Prado, Diana Kay Hohl, Sandor Balog, Lucas Montero de Espinosa, and Christoph Weder ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00175 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Polymer Materials

Plant Oil-Based Supramolecular Polymer Networks and Composites for Debonding-On-Demand Adhesives Anselmo del Prado,† Diana Kay Hohl, Sandor Balog, Lucas Montero de Espinosa, Christoph Weder* Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland

KEYWORDS:

supramolecular,

stimuli-responsive,

reversible

adhesive,

non-covalent

interactions, bonding and debonding on demand.

ABSTRACT: The stimuli-responsiveness of supramolecular polymers has recently been exploited for the development of adhesives that can be (de)bonded on demand when heated or exposed to UV-light. However, it remains difficult to combine competitive solid-state mechanical properties and very low melt viscosity in one material. Here we report a new supramolecular polymer adhesives platform based on soybean oil as a multifunctional lowmolecular weight monomer (~1500 g/mol) and isophthalic acid (IPA) groups that show hydrogen bonding and promote the formation of a reversible network. The polarity difference between the

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triglyceride backbone and the IPA groups leads to microphase separation, and the crystalline IPA domains act as physical cross-links. Heating the polymer above the melting temperature of the IPA-rich domains results in a dramatic viscosity reduction to 8 Pa·s at 120 ºC. Once cooled to room temperature, the material properties are fully recovered, as a result of the reassembly of the supramolecular network. Single lap joint adhesive tests performed at room temperature using glass and stainless steel substrates reveal shear strength values of 1.2 and 1.7 MPa, respectively, and heat and UV-light can be used as external stimuli to debond on command. In addition, composites were prepared by adding 5 or 10 wt%) microcrystalline cellulose (MCC), to the polymer and this led to an increase of strength and modulus below the glass transition by up to 80% and 170%, respectively. Because the introduction of MCC partially hinders the crystallization of the matrix, the the stiffness and tensile strength are reduced above the glass transition, while the elongation at break is significantly increased.

INTRODUCTION Reversible adhesives with on-demand bonding and debonding capabilities are considered useful for different applications, including the repair of aged complex structural bonded components,1-3 the simplification of recycling processes by facilitating the separation of different parts,4, 5 the temporary bonding of pieces for maintenance or processing purposes,6-8 and the painless removal of wound dressings9-13 and dental fixtures.14 Hot melt adhesives are the most widely employed type of materials that enable debonding-on-demand (DOD) solutions.15 However, they are based on high-molecular weight polymers and usually display high melt viscosity, which impacts processing and removal after use. DOD properties can also be imparted by combining highmolecular weight polymers via the introduction of heat-expandable fillers16-19 or thermally/light-


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induced and catalytically20 depolymerizable components.21-25 More recently, stimuli-responsive polymers have been considered as alternative to hot melt adhesives for DOD applications.14, 26-31 Despite the advances in the field, it has remained a challenge to create easily processable adhesives that combine excellent mechanical and adhesive properties during use, but which convert into an easily removable low-viscosity liquid when a specific stimulus is applied. However, recent studies on supramolecular polymers (SMPs) suggest that such a combination of properties should be accessible by this class of materials.32 SMPs are based on non-covalently connected monomers that are polymerized with the help of directional, non-covalent interactions.33, 34 The dynamic nature of these weak bonds allows for the reversible disassembly of SMPs on command, i.e., when a suitable stimulus is applied.35 This approach has served as the basis for the development of stimuli-responsive materials that display functions such as thermal and light-induced healing,36-39 shape memory behavior,40-42 or mechanochromism,43, 44 and more recently also for (de)bonding on demand applications.32 A common strategy for the design of reversible adhesives based on SMPs relies on the (dis)assembly of linear telechelic polymers that are end-functionalized with two supramolecular binding motifs; in such materials micro-phase separation into a soft phase formed by the telechelics’ backbones and a hard phase that is formed by the supramolecular binding motifs and which serves as physical cross-linker is observed.34,

40, 45, 46

The use of SMPs based on this

architecture as reversible adhesives is based on their capability to be thermally disassembled and the typically significant viscosity reduction facilitates debonding on demand. For instance, Bosman and coworkers prepared a telechelic poly(dimethylsiloxane) (PDMS) that was functionalized with the well-known self-complementary quadruple H-bonding ureidopyrimidone (UPy) motif.47 Lap joints formed bonding two glass slides with this polymer were able to hold a


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1 kg weight for hours, they could be debonded by heating, and the original adhesive properties could be restored by rebonding and cooling. Similarly, Weder and coworkers studied SMPs based on telechelic poly(ethylene-co-butylene) that had been modified with either UPy motifs48 or 2,6-bis(1′-methylbenzimidazolyl)-pyridine ligands, which allow for chain extension by chelating metal ions.36 In addition to thermal (de)bonding on demand, these SMPs were shown to enable light-induced de- and rebonding within seconds, on account of light-heat conversion by the binding motifs and optionally added light-heat converters.49 However, the telechelic nature of the above supramolecular monomers and the rubbery nature of the telechelic substrates used lead to relatively low strength, stiffness, and adhesive strength. In order to improve these characteristics, the groups of Long50 and later Weder51, 52 investigated poly(alkyl methacrylate)s with UPy side chains that function as reversible supramolecular cross-links; however, as the backbones were of high molecular weight, the melt-viscosity of such materials is high. Another possibility that was reported by Weder’s group is to create supramolecular networks based on multifunctional monomers featuring a low-molecular weight core and three or more supramolecular binding motifs, such as UPy or isophthalic acid groups, which can be assembled with different bipyridines.49, 53 Such monomers assemble into highly cross-linked SMP glasses with high storage modulus, but at the same time these materials are brittle and on account of the high content of binding motifs their viscosity is only significantly reduced at high temperatures, which limits their applicability and processability. Another strategy to combine mechanical strength and low melt viscosity, reported in a series of papers by Tournilhac, Leibler and coworkers, is the use of fatty acid-based supramolecular polymers either alone or blended with hot melt adhesives.54-56 Their general approach involves the use of mixtures of fatty acids that provide the basis for a soft phase, in combination with crystallizable blocks and weakly binding


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hydrogen-bonding motifs. In these, the crystalline parts provide competitive mechanical properties and freeze the dynamic bonds, while above the melting temperature monomer-like flow is observed as the weak binding motifs are no longer connected via the crystalline phase. Herein, we show that an interesting combination of properties can be accessed by SMP networks based on the self-assembly of a soybean oil-derived multifunctional monomer that carries only isophthalic acid as a H-bonding and π-π stacking motif. The soybean oil derivative used as starting material in this study is acrylated epoxidized soybean oil (AESO), which has been already used as component of different types of adhesives and composites, usually as crosslinker but to our best knowledge never as the main component in the formulation of supramolecular materials.57-59 We show that the combination of this low-glass transition branched structure, weak supramolecular cross-links, and nanophase segregation effects provides access to a material with adequate mechanical properties at room temperature, and a very low melt viscosity at 100-120 ºC. In addition, we show that it is possible to enhance the mechanical properties of this SMP by adding microcrystalline cellulose (MCC) as a reinforcing filler. Taking advantage of the weak bonds that maintain the network structure, the synthesized materials were used as adhesives and temperature and UV-light mediated (de)bonding was demonstrated.

EXPERIMENTAL SECTION Materials. All chemicals were purchased from Sigma-Aldrich and were used as received. 5Mercaptoisophthalic acid was prepared following a reported procedure,60 which was modified as described below. Methods. 1H NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were recorded on a Bruker 400 MHz spectrometer in DMSO-d6. Chemical shifts (δ) are reported in parts per million (ppm)


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relative to the tetramethylsilane, even though the signal of residual DMSO protons at 2.50 ppm were employed as internal. Mass spectrometry (MS) analyses were performed as high-resolution electrospray ionization (ESI) experiments on a Bruker FTMS 4.7T BioAPEX II. Fourier transform infrared spectroscopy (FTIR) spectra were acquired on a PerkinElmer Spectrum 65 spectrometer between 4000 and 600 cm−1 with a resolution of 4 cm−1 and 10 scans per data point, with an attenuated total reflectance (ATR) device using diamond/ZnSe as internal reflection element. Differential scanning calorimetry (DSC) studies were performed with a Mettler-Toledo STAR system under nitrogen atmosphere, heating/cooling rates of 10 °C/min, in the range −50 to +150 °C, and using a sample mass of approximately 5 mg. Thermal degradation studies were performed on a Mettler-Toledo TGA/DSC STARe System at a heating rate of 10 °C/min from 25 to 600 °C with samples of 10 mg in 40 μL aluminum crucibles. The melting points (Tm) reported are the minimum values of the corresponding heat capacity measured in the first heating cycle. The crystallization temperatures (Tc) reported are the maximum values in the heat capacity measured in the first cooling cycle. The glass transition temperatures (Tg) reported are the midpoints of the step change in the heat capacity measured in the first heating cycle. Wide- and small-angle X-ray scattering (WAXS, SAXS) experiments were conducted in vacuum and at ambient temperature using a NanoMax-IQ camera (Rigaku Innovative Technologies, Auburn Hills, MI). The scattering spectra (1d) shown have been processed using standard procedures, and data are presented as function of the momentum transfer q = 4πλ−1 sin(θ/2), in which λ = 0.1524 nm is the photon wavelength and θ the scattering angle. Rheological studies were performed on a TA Instruments AR-G2 rheometer with 40 mm parallel plates in oscillatory mode, using a Peltier-element setup for temperature control. The temperature-dependent viscosity measurements were conducted between 25 and 120 °C, with heating/cooling rates of 5 °C/min, at


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10 rad/s and 0.1% strain. Frequency sweep experiments were performed over a frequency range of ω = 0.05 - 100 rad/s at temperatures between 35 and 100 °C in 15 °C increments. Preparation of polymer films and composites. AESOIPA (1 g) and microcrystalline cellulose (0, 5 and 10 wt%) were mixed in 10 mL of EtOH, and sonicated for 30 min. The resulting mixtures were cast into Petri dishes and were dried in vacuum at room temperature overnight. Polymer films were produced by compression moulding the resulting solids in a Carver CE Press at 70 °C, and applying 4 tons of pressure during 3 minutes. The film thickness was controlled to 250-300 μm by using poly(tetrafluoroethylene) film spacers. These films were used for DMA and stress−strain experiments. Samples used in UV-light mediated thermal debonding experiments were based on materials that also contained the UV stabilizer 2-(5-chloro-2H-benzotriazole-2-yl)-6-(1,1-dimethylethyl)-4-methyl-phenol (available under the trademark Tinuvin 326 from BASF). This additive was added to the mixture of AESOIPA and microcrystalline cellulose in EtOH before sonication, and the materials were processed as indicted above. Mechanical testing. Dynamic mechanical analyses (DMA) and stress-strain measurements were conducted in a tension film clamp setup with a TA Instruments DMA Q800. DMA analyses were carried out at a heating rate of 5 °C/min, from 0 to 100 °C, at a frequency of 1 Hz, and an amplitude of 5 μm. Stress-strain analyses were carried out at a constant temperature of 10 and 40 ºC, with a constant strain rate of 5%/min until sample failure. The shape of the samples used for the mechanical analyses were rectangular with a length of ≈10 mm, a width of 5.3 mm, and a thickness of 0.25 mm. Creep experiments were carried out on the same equipment at 10, 25 and 40 °C. The strain was fixed to 0.04% for the experiment conducted at 10°C, and to 5% for the experiments carried out at 25 and 40 ºC. The decay of the stress under constant strain was then monitored as a function of time.


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Adhesive properties. Adhesive tests were carried out at ambient temperature on a Zwick/Roell Z010 tensile tester that was equipped with mechanical gripping clamps and a 10 kN load cell. A strain rate of 15 mm/min was applied. Single lap joints were fabricated using glass or stainless steel substrates with a thickness of 1 mm. The bond area was 20 x 25 mm in the case of glass and 10x10 mm in the case of stainless steel. For adhesive experiments, the thickness of the prepared thin films was reduced from 250-300 to ca. 90−100 μm, by melting 30 mg (glass substrates) or 10 mg (stainless steel substrates) of the respective material between the substrates. This was done by placing the sandwich on a hot stage maintained at 100 ºC, and the lap joint was removed from the hot stage and pressed while cooling to ambient temperature. Samples were tested within 1 hour after bonding. Debonding experiments were carried out on the same instrument. In this case, single lap joints were made with regular glass and stainless steel with bond areas of 10x10 mm. The applied stimuli for debonding were either heat – applied as hot air with a temperature of ca. 100 ºC (stainless steel) – or UV light (glass) – applied with a Hönle Bluepoint 4 Ecocure UV lamp equipped with an optical fiber (λ = 320-390 nm, 1600 mW/cm2). For these experiments the samples were subjected to a constant load of 20-23 N until debonding occurred. In the case of the UV light mediated debonding experiments, the surface temperature of the single lap joints was measured with an Optris PI Connect infrared camera model PI 160 from Roth AG. Synthesis of AESOIPA monomer. Dimethyl 5-((dimethylcarbamothioyl)oxy) isophthalate (1). In a 100 mL round-bottomed flask equipped with a magnetic stir bar, dimethyl 5-hydroxyisophthalate (5.00 g, 23.8 mmol) and 1,4diazabicyclo[2.2.2]octane (DABCO, 8.00 g, 71.4 mmol) were dissolved in DMF (60 mL). Dimethylthiocarbamoyl chloride (3.80 g, 29.76 mmol) was added to the solution and the reaction mixture was stirred for 5 h at room temperature. The yellowish solution was poured into distilled


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water (1 L) to precipitate a white powder, which was filtered off and washed with distilled water until the washing liquid was colourless and its pH neutral. The resulting solid was dried in vacuum at 130 ºC and the title product was obtained as a white solid (6.50 g, 92%). 1H NMR (400 MHz, DMSO-d6): δ= 8.35 (s, 1H, COOMe-C-CH-C-COOMe), 7.88 (s, 2H, CH-CO-CH), 3.90 (s, 6H, 2xCOOCH3), 3.36 (d, 6H, N(CH3)2). 13C NMR (100.6 MHz, DMSO-d6): δ= 185.59 (-OCSNMe2), 164.68 (2xCOOMe), 154.99 (CH-CO-CH), 131.16 (2xC-COOMe), 128.07 (CHCO-CH), 126.64 (COOMe-C-CH-C-COOMe), 52.68 (2xCOOCH3), 38.72 (N(CH3)2). HRMSESI: [M+Na]+= 320.0560. Tm: 119.7 °C. Dimethyl 5-((dimethylcarbamoyl)thio) isophthalate (2). In a 50 mL round-bottomed flask equipped with a magnetic bar that was purged with nitrogen and placed in a heating device, dimethyl 5-((dimethylcarbamothioyl)oxy) isophthalate (1, 3.00 g, 10.10 mmol) was heated to 215 ºC during 3 hours. The white solid melted and turned into a brownish oil. After cooling to room temperature, a dark brown solid was obtained. The product was purified by recrystallization from MeOH (300 mL) and the title product was obtained in the form of pale brown crystals (2.40 g, 80%). 1H NMR (400 MHz, DMSO-d6): δ= 8.46 (s, 1H, COOMe-C-CH-C-COOMe), 8.19 (s, 2H, CH-CS-CH) 3.91 (s, 6H, 2xCOOCH3), 3.00 (d, 6H, N(CH3)2).

13C

NMR (100.6

MHz, DMSO-d6): δ= 164.67 (2xCOOMe), 163.66 (-SCONMe2), 139.56 (CH-CS-CH), 131.02 (CH-CS-CH), 130.81 (2xC-COOMe), 129.78 (COOMe-C-CH-C-COOMe), 52.69 (2xCOOCH3), 36.66 (NCH3CH3), 36.47 (NCH3CH3). HRMS-ESI: [M+Na]+= 320.0557. Tm: 121.2 °C. 5-Mercaptoisophthalic acid (IPASH) (3). In a 50 mL round-bottomed flask equipped with a magnetic stir bar, dimethyl 5-((dimethylcarbamoyl)thio) isophthalate (2, 3.00 g, 10.10 mmol) was dissolved in EtOH (5 mL), and a 3 M aqueous NaOH solution (20 mL) was added. The mixture was heated to 60 ºC and stirred overnight. After cooling to room temperature, the


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solution was poured into a solution of 37% HCl and H2O (1:1 v/v, 100 mL) that was cooled in an ice bath, to precipitate a pale yellowish powder, which was filtered off and washed with distilled water until the washing liquid was pH neutral. The solid was then redissolved in diethylether, the solution was filtered in order to remove insoluble impurities, dried over MgSO4, the MgSO4 was filtered off, and the solvent was removed at reduced pressure. After drying in vacuum at 160 ºC overnight, the title product was obtained as a yellowish powder (1.80 g, 90%). 1H NMR (400 MHz, DMSO-d6): δ= 13.49 (broad band, -COOH), 8.33 (s, 1H, COOH-C-CH-C-COOH), 8.27 (s, 2H, CH-CSH-CH).

13C

NMR (100.6 MHz, DMSO-d6): δ= 165.62 (2xCOOH), 137.00 (CH-

CS-CH), 132.60 (2xC-COOH), 131.14 (CH-CS-CH), 128.97 (COOH-C-CH-C-COOH). HRMSESI: [M+Na]+= 220.9883. Tm: 282.8 °C. Additional IR data and 1H NMR comparison of compounds 1, 2, and 3 are shown in Supporting Information Figures S1-4. Isophthalic acid functionalized soybean oil (AESOIPA) (4). In a 50 mL round-bottomed flask equipped with a magnetic stir bar and a reflux condenser, acrylated epoxidized soybean oil (AESO, approximate structure shown in Scheme 1) (5.00 g, 4.04 mmol) was dissolved in EtOH (6 mL). IPASH (3, 2.20 g, 11.3 mmol) and Et3N (14 mL) were added, and the reaction mixture was stirred overnight at 40 ºC. The yellowish solution was then poured into a mixture of aqueous HCl (40 mL 6 M) and ethyl acetate (80 mL). The organic layer was separated off, and the aqueous phase was extracted with ethyl acetate (3 x 50 mL). The organic layers were combined, washed with distilled water, dried over MgSO4, filtered, and the solvent was removed at low pressure. The resulting solid was dried in vacuum at 70 ºC overnight, yielding the title product as a yellowish rubbery material (6.10 g, 85%). Due to the complexity of the 1H NMR spectra (resulting from the fact that the commercial AESO is a mixture of products), only the corresponding signals to the Michael addition in the 1H NMR spectrum were analyzed and


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compared to the commercial AESO (Supporting Information Figure S5). Commercial AESO contains ca. 2.45-2.65 mol of acrylate per mol of triglyceride. After the thiol-Michael addition of IPASH to AESO, the acrylate signals (6.5-5.8 ppm) disappeared, and the modified AESOIPA shows new resonances at 8.3-7.9 ppm, corresponding to the IPA motif, and in the range of 3.52.0 ppm, corresponding to the CH2 after the addition to the α,β-unsaturated ester (see Supporting Information). ESI: [M+Na]+= 1457 (Supporting Information Figure S6).

RESULTS AND DISCUSSION Isophthalic acid functionalized soybean oil (AESOIPA) was synthesized via thiol-Michael addition by reacting the commercially available acrylated epoxidized soybean oil (AESO) with 5-mercaptoisophthalic acid (IPASH), which was synthesized by making minor modifications to a previously reported protocol (Scheme 1, Experimental Section).60 Full conversion of the acrylate groups was indicated by the fact that the number of IPA motifs per triglyceride (determined by 1H

NMR spectroscopy, see Supporting Information Figure S5) matched the number of

acrylate groups in the parent AESO, i.e. AESOIPA had an IPA number of 2.6. In sharp contrast to AESO, which is a viscous oil, AESOIPA appears as a rubbery solid that can be readily be melted and compression-moulded into transparent, polymer-like films (Figure 1 and Supporting Information Figure S7).


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S COOMe

Cl

N

215 °C, 3h.

OH

DABCO MeOOC DMF, rt 5 h.

O

N2

N

MeOOC

S

1

COOH

60 ºC, on

O

S MeOOC

1) NaOH (aq), EtOH

COOMe

COOMe

2) pH < 4

N

2

HOOC

SH

3 IPASH

O O

O O

O

O O

OH

O

OH OH

O

O

O

COOH

O

EtOH, Et3N 40 °C, on

COOH

IPASH O

O HOOC

S

S

COOH

S

COOH

O O

O

O O

OH

O

OHOH

O

O

O

4

O

AESOIPA

COOH

Scheme 1: Synthesis of IPASH (3) and the IPA-functionalized acrylated epoxidized soybean oil AESOIPA (4). Approximate chemical structures are shown for AESO and AESOIPA as commercial AESO is a mixture of triglycerides. Small- and wide-angle X-ray scattering (SAXS-WAXS) experiments were carried out on AESOIPA, and for reference purposes on the parent AESO, in order to investigate the reason for this property contrast (Figure 1). The diffractogram of AESOIPA shows a broad diffraction peak between q = 0.3-2 nm-1, peaking at approx. 1.5 nm-1 which indicates microphase segregation with a characteristic length scale of approx. 4 nm, arguably on account of the formation of IPArich hard domains, which segregate from the non-polar soybean oil core. In addition, an additional sharp band at q = 18.5 nm-1 (3.4 Å) superimposing the so-called amorphous halo suggests π-π stacking of the IPA motifs within the hard phase, as previously observed in other IPA derivatives.61 The diffractogram of the parent AESO is void of these diffraction maxima. Thus, the SAXS results support the vastly changed mechanical properties of AESOIPA (relative


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to AESO), as such hard domains provide physical cross-links to the material (Figure 1).62,

63

Hydrogen bonding between the acid groups in the isophthalic acid motifs are well studied,61, 64, 65 and infrared spectra are often used to probe such interactions. In the infrared spectrum of the modified AESOIPA, the band corresponding to the carbonyl groups of the IPA motifs at 1685 cm-1 is shifted to 1700 cm-1, while the band corresponding to the carbonyl groups present in AESO, shifted from 1725-1740 to 1710-1730 cm-1. In addition, sharp bands appear in the region between 1575-1600 cm-1, which are characteristics of hydrogen bonds in carboxylic acids compounds (Supporting Figure S8).61 The nanophase segregation of the polar IPA motifs from the non-polar oily phase could also be confirmed by differential scanning calorimetry (DSC) of AESOIPA. A glass transition temperature (Tg) is observed at ca. 25 °C and the DSC trace also shows an endothermic transition at ca. 88 °C, which is ascribed to the melting point (Tm) of the IPA hard phase. This interpretation is supported by the fact that the DSC cooling trace shows a crystallization peak at 80 ºC upon cooling from the melt (Figure 2a, Table 1). The material displays excellent solubility in ethanol, which also enables solvent processing. In order to expand the property matrix of AESOIPA, several composites were prepared by mixing the supramolecular polymer with microcrystalline cellulose (MCC).66 MCC was anticipated to disperse well in AESOIPA on account of favourable interactions between its hydroxyl surface groups and the IPA motifs. Thus, AESOIPA/MCC composites were prepared by mixing AESOIPA and either 5 or 10 wt% MCC in ethanol, sonicating the mixture for 30 minutes and removing the solvent under reduced pressure. The composites were then compression moulded into homogeneous films, whose thermal and mechanical properties were investigated and compared to those of the neat AESOIPA (Figure 2). The DSC traces of the AESOIPA/MCC composites (Figure 2a-b, Table 1) are similar to those of AESOIPA. While the Tg values are in the same


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range (22-25 ºC), the melting transitions were observed at 65 °C and 62 °C for AESOIPA/MCC composites with 5 and 10 wt% MCC, respectively, i.e., at slightly lower temperatures. The crystallization temperatures (Tc) are also lower (at ca. 60 °C) than for the neat AESOIPA. The decrease (and broadening) of the melting and crystallization temperatures in the composites suggests some interactions between the IPA motifs and the hydroxyl groups of the MCC.67, 68

Figure 1: a) Schematic illustrating the formation of a supramolecular AESOIPA network and a picture of a film made from the material. b) SAXS-WAXS spectra of the parent AESO (green) and AESOIPA (black). After establishing that AESOIPA is stable when heated in air for 20 min to 150 °C (Supporting Information Figures S9-10), the mechanical properties of AESOIPA and its MCC composites were studied by dynamic mechanical analysis (DMA) and tensile testing (Figure 2c-d, Table 1). The neat AESOIPA is fairly rigid around and below ambient temperature, but the DMA trace reveals a drastic drop of the storage modulus (E´) from ca. 1.4 GPa to 100 MPa between 20-70 °C, which is associated with passing the glass transition, as observed by DSC. Further, a sharp modulus decrease above 70 °C and a failure temperature at ca. 80 °C are observed, which


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corresponds to the melting of the hard phase observed by DSC. The DMA trace does not show a flat rubbery plateau but displays instead a continuous modulus drop above Tg, which may indicate progressive dissociation of the IPA binding motifs. Overall, the DMA data are in agreement with the conclusions drawn from the DSC results, i.e., that the crystalline hard domains of AESOIPA maintain the structure of the polymer network above its Tg by establishing physical cross-links. The DMA traces of the AESOIPA/MCC composites show a very similar behavior, but compared to the neat AESOIPA, the storage moduli below Tg are considerably higher, which is explained by the reinforcement provided by the MCC. Consistent with the DSC data, the modulus drop and failure occur at lower temperatures than in the neat AESOIPA. A much steeper modulus drop is observed above Tg, reflecting again some interactions between the IPA motifs and the MCC hydroxyl surface groups and a reduced perfection of network assembly and phase segregation.


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Figure 2: Thermal and mechanical properties of AESOIPA (black) and AESOIPA/MCC composites with 5 wt% (blue) or 10 wt% (red) MCC. (a, b) DSC heating and cooling traces of the as-prepared materials recorded with heating/cooling rates of 10 °C/min under nitrogen atmosphere. Traces are vertically shifted for clarity. (c) DMA traces recorded with a heating rate of 5 °C/min. (d) Stress-strain curves recorded at 25 °C with a strain rate of 5%/min.


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Table 1: Thermal and mechanical properties of films prepared from AESOIPA and AESOIPA/MCC composites.

Tg (°C)a

Tc (°C)a

Tm (°C)a

E’ at 10°C (MPa)b,c

E’ at 25°C (MPa)b,c

Failure T (°C)b

AESOIPA

25

80

88

1390 ± 50

520 ± 70

75 - 85

AESOIPA/MCC 5 wt%

24

60

65

2090 ± 210

1200 ± 140

60 - 70

AESOIPA/MCC 10 wt%

22

60

62

2670 ± 200

1360 ± 190

60 - 70

a

Determined by DSC. b Determined by DMA. c Results are averages of 4 measurements, and

errors are standard deviations. Tensile tests were performed at room temperature (18-22 ºC), and because the Tg values of all materials are close to room temperature, where a modulus drop is observed, also at 10 °C (Table 2, Figure 2d, Supporting Information Figure S11). At 10 ºC, the neat AESOIPA displays a Young’s modulus of ca. 0.8 GPa, which increased to 1 GPa upon incorporation of 10 wt% MCC. The same effect was observed for the maximum stress, which was measured to be 1 MPa for the neat polymer and 2.4 and 2.7 MPa for the 5 and 10 wt% MCC composites, respectively. At 10 ºC, all materials were quite brittle, with an elongation at break of 1% or less. A significant decrease of the Young’s moduli was observed when the temperature was increased to 25 ºC, i.e. 130 MPa for neat AESOIPA, and 198 and 240 MPa for the composites containing 5 and 10 wt% MCC, respectively (Figure 2d, Table 2). At this temperature, the maximum stress determined for the neat AESOIPA (1.9 MPa) was higher than that of the MCC composites (0.9 and 1.4 MPa), reflecting again a lower density of cross-linking hard domains in the composites and revealing that this feature outweighs the reinforcing effect of the MCC. All three materials show


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a much higher elongation at break at 25 °C than at 10 °C, in accordance with a change from glassy to rubbery state. The MCC composites display a higher elongation at break (40-50%) than the neat AESOIPA (12%), perhaps on account of a larger fraction of a rubbery amorphous phase. Overall, the drop of the stiffness and tensile strength observed above the Tg in the AESOIPA/MCC composites point to low melt viscosities, which is interesting from a processing point of view and in the context of debonding-on-demand applications. On the other hand, these results suggest that in order to reinforce AESOIPA above its Tg a non-hydrogen bonding reinforcing filler such as carbon nanotubes or graphene would be preferable. Compared to previously reported fatty acid-based supramolecular polymers,54-56 AESOIPA and its MCC composites present a similar glassy modulus, but their rubbery modulus and tensile strength are somewhat lower, which, as discussed below, translates into particularly low melt viscosities appropriate for the debonding-on-demand application explored in this study. Table 2: Mechanical properties of films prepared from AESOIPA and AESOIPA/MCC composites. 10 ºC

25 ºC

Young’s Modulus (MPa)*

Maximum stress (MPa)

Elongation at break (%)

Young’s Modulus (MPa)*

Maximum stress (MPa)

Elongation at break (%)

AESOIPA

820 ± 130

1.0 ± 0.1

0.1 ± 0.04

130 ± 30

1.9 ± 0.2

12 ± 2

AESOIPA/ MCC 5wt%

980 ± 150

2.4 ± 0.2

0.4± 0.04

200 ± 90

0.9 ± 0.1

40 ± 9

AESOIPA/ MCC 10wt%

1000 ± 250

2.7 ± 0.5

1.1 ± 0.2

240 ± 40

1.4 ± 0.1

47 ± 7

*Calculated from the linear region in the strain regime of 0-0.4%. Results are averages of 4 measurements, and errors are standard deviations.


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Creep experiments were carried out on the neat AESOIPA films at 10, 25 and 40 °C to study the material’s behaviour in the glassy and elastic regime as well as in the transition phase. The strain was fixed to 0.04% for the experiment conducted at 10°C, and to 5% for the experiments carried out at 25 and 40 ºC; this resulted in maximum stresses of 0.3 ± 0.1 MPa, 1.2 ± 0.3 MPa, and 0.05 ± 0.02 MPa, respectively. The decay of the stress under constant strain was then monitored as a function of time (Supporting Information Figure S12). Interestingly, at all temperatures significant creep and rapid stress relaxation can be observed, which seems to suggest that despite the formation of a crystalline hard phase the network formation in AESOIPA is highly dynamic. The rheological behavior of AESOIPA and its MCC composites was analyzed with a temperature-controlled parallel plate setup. Figure 3, which shows the temperature dependence of the complex viscosity (*) at a constant frequency of 10 rad/s and an oscillatory strain of 0.1%, reveals a significant decrease in * upon heating for all three materials. These results mirror the modulus drop observed in DMA and are in agreement with the disassembly of a supramolecular polymer network into lower molecular weight species. AESOIPA displays a very low complex viscosity of 8 Pas at 120 °C, which suggests that it is largely dissociated into its low molecular weight triglyceride constituents. The composites display slightly higher viscosities (16 and 33 Pas) at the indicated temperature, on account of the presence of the cellulosic filler. While it is difficult to compare the melt viscosities of these materials with that of other previously reported fatty acid-based supramolecular polymers as the frequencies used for the measurement are different, it is possible to conclude that the melt viscosities of AESOIPA and its MCC composites are among the lowest reported. Indeed, these materials present viscosities between 8 and 33 Pas, that is, in the same order as for previously reported materials but in most cases at lower frequency (10 rad/s vs. 628 – 810 rad/s)55, 56 and temperature (120 °C


19


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vs. 180 °C).55 Comparing the melt viscosity of AESOIPA and its MCC composites with that of hot melt adhesives is not straightforward as the polymers used in hot melt formulations have moleculer weights of typically above 10’000 g/mol,15 whereas AESOIPA is a mixture of components with molecular weights below ca. 1’500 g/mol. Further, low-molecular-weight additives such as tackifiers and waxes are usually added to commercial hot melt formulations to lower melt viscosity,15,

69-71

and such complex mixtures cannot be compared to the neat

AESOIPA studied here. Perhaps a comparison with blends of a polyamide hot melt adhesive (Mn of ca. 20'000 g/mol) with a low-molecular weight supramolecular polymer (Mn of ca. 1'000 g/mol) is instructive.54 The latter displayed low viscosities between 1 and 5 Pa·s, but at a higher temperature (160 ºC vs. 120 ºC in this study) and much higher frequency (100 rad/s vs. 10 rad/s in this study), which suggests that the AESOIPA derivatives reported here have melt viscosities that are comparable these hot melt formulations.

Figure 3: Complex viscosity of AESOIPA (black) and AESOIPA/MCC composites with 5 wt% (blue) or 10 wt% (red) MCC at 50 °C and 120 °C. Data were recorded at 10 rad/s, 0.1% strain, and a heating rate of 5 °C/min.


20

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The complex viscosity (*), and the elastic (G’) and viscous (G’’) moduli were examined in dynamic mode as a function of the angular frequency (ω) in the range of 0.05 - 100 rad/s in the linear viscoelastic region at 0.1% strain. The time-dependent properties were investigated at 50 °C, i.e., in the rubbery regime and are shown in Figure 4. The * of AESOIPA decreases with increasing ω and the material behaves like a viscoelastic solid, where G’ dominates G’’ over the whole frequency range (Figure 4 left). Upon addition of MCC the material became more fluidlike, as evidenced by the observation of a crossover angular frequency ωc, where G’ = G’’ (Figures 4 middle-right), consistent with the reduction of the crystalline IPA phase (vide supra).67, 68 The 5 wt% AESOIPA/MCC composite behaves like a viscoelastic liquid, exhibiting terminal flow in a broad frequency range, and turning weakly elastic at a turnover frequency of ω = 50 rad/s. At low frequencies, the * of the 5 wt% MCC composite was about one order of magnitude lower than that of the neat AESOIPA, further indicating that the hard domains of the material are disrupted. The 10 wt% MCC composite was more elastic, with ω shifted to a lower frequency of ω = 5 rad/s, indicative of longer relaxation times at higher filler content. The * at low frequencies was higher compared to the lower filler content material, arguably due to the filler content. In addition, the 10 wt% composite showed a rubbery plateau at high frequencies, evidencing that on short timescales the material is partially cross-linked and behaves as a lightly entangled polymer. Additional temperature-dependent dynamic measurements confirm the solidlike behaviour at a lower temperature of 35 °C, where a plateau in the real part of the modulus and a lower imaginary part G’’ can be observed. At T > Tm, flow behaviour becomes evident, as G’’ is proportional to ω and G’, lower in value, shows a stronger ω-dependency (Supporting Information Figure S13). These observations are in agreement with the thermal properties of the


21


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materials, as the Tm of the hard phase of neat AESOIPA (88 °C) is well above the measuring temperature (50 °C), while those of the MCC composites (65 and 62 °C) are somewhat closer.

Figure 4: Frequency dependence of the elastic (●) and viscous (○) moduli and the complex viscosity (▪) of neat AESOIPA (black), and AESOIPA/MCC composites with 5 wt% (blue) or 10 wt% (red) MCC, all recorded at 50 °C. In order to investigate the usefulness of these plant oil-based supramolecular networks and composites as adhesives for (de)bonding on-demand using thermal or light-based activation, single lap joints were prepared by bonding two glass or stainless steel substrates. All samples were prepared by placing the polymer films between the substrates, heating the sandwiches to 100 ºC under compression with metallic clamps, and cooling them to room temperature. Overlap areas of 20 x 25 mm and 10 x 10 mm were used for glass and stainless steel substrates, respectively. The thickness of the adhesive layer in the bonded structure was checked with a micrometre (ca. 90-100 µm for all samples). The adhesive properties were quantitatively explored by lap shear tests at ambient temperature using a tensile tester to apply uniaxial stress (Table 3). The shear strength measured for the neat AESOIPA was 1.2 MPa for glass substrates and 1.7 MPa with stainless steel. Similar values were determined for the composites with 5 wt%


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MCC (1.0 MPa) and 10 wt% MCC (0.9 MPa) on glass, but the shear strength increased significantly when steel was used (2.3 and 2.2 MPa, respectively) (Supporting Information Figures S14-15). All samples failed through an adhesive mode (Supporting Information Figure S16), indicating that there is a priori room for improvement by formulation. In order to demonstrate the reversibility of these supramolecular networks and composites, failed measured samples were directly rebonded by applying the same heating/cooling process described above. Indeed, the shear strength remained constant through the two cycles for AESOIPA and the two MCC composites. Table 3: Shear test results of glass and stainless steel single lap joints bonded with AESOIPA and AESOIPA/MCC composites with 5 wt% and 10 wt% MCC.

Shear Strength (MPa)

Regular glass substrates

Stainless steel substrates

AESOIPA

AESOIPA/MCC 5 wt%

AESOIPA/MCC 10wt%

Thermally bonded

1.2 ± 0.2

1.0 ± 0.0

0.9 ± 0.1

Thermally rebonded

1.3 ± 0.1

1.0 ± 0.1

1.0 ± 0.1

Thermally bonded

1.7 ± 0.2

2.3 ± 0.2

2.2 ± 0.2

Thermally rebonded

1.7 ± 0.2

2.4 ± 0.4

2.1 ± 0.2

Samples were subjected to a shear-test experiment until the bonds failed, and then rebonded using the original bonding protocol without applying additional adhesive. Results are averages of 3 measurements, and errors are standard deviations.


23


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The debonding on demand of lap joints bonded with neat AESOIPA was investigated using heat and UV light (see Supporting Videos V1, V2, and V3). In a first qualitative experiment (Supporting Video V1), two metallic objects were thermally bonded, and multiple release/rebond cycles were realized without any apparent loss of properties (Table 3). In a second experiment (Supporting Video V2), a small amount of AESOIPA containing 0.2 wt% of the UV light absorber Tinuvin 326, which was added to increase the absorption in the UV regime,49, 51, 53, 72 was placed on a glass slide which was bonded with a metallic object by applying UV light (λ = 320-390 nm, 1600 mW/cm2) through the glass slide. After only 5 seconds of UV irradiation and a brief cooling period (3-4 seconds), the metallic object was firmly bonded to the glass support. The objects could be rapidly debonded by irradiation with the same UV light source and without yellowing of the supramolecular adhesive. In a third test (Supporting Video V3), the same AESOIPA/Tinuvin 326 mixture was used to thermally bond a single lap joint with two glass substrates (1 mm thickness). The lap joint could hold a 200 g weight (~2 N) and UV-light (λ = 320-390 nm, 1600 mW/cm2) triggered debonding could be achieved in ca. 30 seconds. In order to obtain quantitative data, single lap joint tests were performed with stainless steel and glass substrates to evaluate heat and UV light as debonding stimuli, and both neat AESOIPA and the AESOIPA/MCC composites were used as adhesives. For the thermal debonding test, hot air at a temperature of ca. 100 ºC was applied, whereas for debonding with UV light, a UV-light source (λ = 320-390 nm, 1600 mW/cm2) was used. Optical debonding experiments were carried out with the neat AESOIPA as well as with a blend containing 0.2 wt% of Tinuvin 326. All samples were mounted in a tensile tester and placed under an initial constant force of (20-23 N), before heat or light were applied to debond (Figure 5).


24

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Figure 5: Debonding-on-demand experiments of lap joints bonded with the neat AESOIPA (black) and AESOIPA/MCC composites with 5 wt% (blue) or 10 wt% MCC (red). An initially constant force of 20-23 N was applied, and the force was monitored as a function of time after a) heat was applied to the stainless steel substrates, and b) UV light (λ = 320-390 nm, 1600 mW/cm2) was focused on the lap joint bonded using adhesives with (dashed lines) and without (solid lines) 0.2 wt% of the UV-light absorber Tinuvin 326. A sample bonded with AESOIPA without applying any stimuli was measured as a reference (orange). The vertical dashed line marks the start of the application of heat or UV light. After the lap joints were mounted, the force was monitored over time, and the time when failure occurred was recorded and compared between the different materials. In the absence of an external stimulus, neither creep nor failure was observed within an hour. However, samples bonded with AESOIPA failed after ca. 17 seconds upon exposure to heat, while those bonded with the AESOIPA/MCC composites failed after 15 (5 wt% MCC) or 12 seconds (10 wt% MCC). A similar behaviour was observed in the UV light-mediated debonding experiments, where the debonding occurred after 30 - 45 seconds for the Tinuvin 326-free samples. The


25


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debonding time was reduced to 20-30 seconds when Tinuvin 326 was added. To monitor the temperature induced by UV light exposure, surface temperature profiles of the materials versus time were recorded with an IR camera (Figure 6). The comparison shows that after 20 s of UV irradiation, the temperature of the materials with Tinuvin 326 is around 20 ºC higher than that of the materials without Tinuvin 326, which illustrates the role of the additive in the debonding process. Overall, the debonding temperatures observed are comparable with only small differences due to the different response of the material to the external stimulus, and the corresponding heating rates.

Figure 6: Surface temperature profiles of the neat AESOIPA (black) and AESOIPA/MCC composites with 5 wt% (blue) and 10 wt% (red) MCC with 0.2 wt% of Tinuvin 326 (dashed lines) and without this light-heat converter (solid lines upon UV-irradiation (λ = 320-390 nm, 1600 mW/cm2) applied on glass substrates. The maximum values correspond to time points when the lamp was switched off.


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CONCLUSIONS Taking advantage of the reversible and reconfigurable features of supramolecular polymers, we have introduced a new materials platform that displays an interesting composition of adequate mechanical properties under ambient conditions and a very low melt-viscosity. This property combination was accessed by coupling multiple weakly interacting IPA motifs with a hydrophobic core and exploiting microphase segregation between the triglyceride backbone and the IPA groups. The domains formed by the latter act as physical cross-links and stabilize the weak hydrogen bonds. The resulting supramolecular networks are less brittle and show a lower melt viscosity than previously investigated supramolecular polymer networks,72, 73 and the IPA motif allows an effective dissociation at relatively low temperatures (70-100 ºC). The addition of microcrystalline cellulose to the material further increases strength and stiffness below the Tg. However, above the Tg the strength and stiffness decrease in the presence of microcrystalline cellulose but the toughness increases. Lap joint adhesive tests performed at room temperature using glass and stainless steel substrates revealed shear strength values between 0.9 and 2.4 MPa, and heat and UV-light were used as external stimuli to mediate de-bonding on command.


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ASSOCIATED CONTENT Supporting Information Analytic data (NMR, IR, and mass spectrometry) of compounds 1-4, additional SAXS-WAXS, DSC, rheology and adhesion data, and videos V1, V2, and V3. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (C.W.) Orcid: Anselmo del Prado: 0000-0003-2110-5621 Diana Kay Hohl: 0000-0001-5259-4478 Sandor Balog: 0000-0002-4847-9845 Christoph Weder: 0000-0001-7183-1790 Present Addresses † Current address: Departamento de Química Orgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Swiss National Science Foundation (precoR Grant 20PC21_161565 to C.W. and Ambizione Grant PZ00P2_154845 to L.M.E.) and the Adolphe Merkle Foundation.


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SYNOPSIS_TOC


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Plant Oil-Based Supramolecular Polymer Networks and Composites

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