Debonding on Demand with Highly Cross-Linked Photopolymers: A

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Debonding on Demand with Highly Cross-Linked Photopolymers: A Combination of Network Regulation and Thermally Induced Gas Formation Christian Gorsche,*,†,‡ Christoph Schnoell,†,‡ Thomas Koch,§ Norbert Moszner,‡,∥ and Robert Liska†,‡ †

Institute of Applied Synthetic Chemistry, Technische Universität Wien, Getreidemarkt 9/163 MC, 1060 Vienna, Austria Christian-Doppler-Laboratory for Photopolymers in Digital and Restorative Dentistry, Getreidemarkt 9, 1060 Vienna, Austria § Institute of Materials Science and Technology, Technische Universität Wien, Getreidemarkt 9/308, 1060 Vienna, Austria ∥ Ivoclar Vivadent AG, 9494 Schaan, Liechtenstein ‡

S Supporting Information *

ABSTRACT: Photopolymerizable glues and cements that offer debonding on demand (DoD) through an external stimulus are of great interest for the fields of recycling and repair. State-of-the-art DoD solutions often require a high-energy impulse (e.g., >200 °C, strong force), which is due to the typical glassy nature of such photopolymer networks. Herein, various blocked isocyanates (BICs) that enable thermally induced gas formation at temperatures far below 200 °C are studied. Thermally induced gas bubble formation is accomplished within a linear, thermoplastic poly(N-acryloylmorpholine) matrix above glass transition temperature, introducing porosity. The resulting porosity within the material then causes mechanical failure. However, highly cross-linked photopolymer networks remain unchanged due to their glassy nature at temperatures well above 150 °C. A BIC-based thermolabile photopolymerizable cross-linker is prepared in order to create a polymer network with cleavable cross-link. Additionally, a β-allyl sulfone-based chain transfer reagent is used to tune the final cross-linking density and thermomechanical properties of the material. Above the resulting sharp glass transition (>60 °C), plastic deformation becomes possible, thus allowing formation of porosity. This introduces a covalently cross-linked, thermolabile photopolymer with a tailored network architecture as potential glue for DoD at ∼150 °C.

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(i.e., hyperthermia8) and is not reliant on the transparency of the materials. Such concepts have also already been implemented in polymeric materials and served as inspiration for our research.9 The most prominent thermal DoD solutions (using moderate temperatures 200 °C), and have high hardness, rendering them insusceptible for rework. In recent years, research has focused on introducing reworkable functionalities (e.g., thermolabile4,5 or photoresponsive6,7 crosslinks) into such photopolymer-based adhesives to enable tractability. Focusing on highly cross-linked materials with high hardness, the external stimulus needed for rework is quite high and thus problematic when attempted in sensitive environments such as microelectronics. Moreover, photoresponsive glues have not been the focus of our research. We find the application of the stimulus challenging as light often cannot sufficiently penetrate highly filled or opaque glues. A thermal impulse can potentially be created locally by implementing microwaves or magnetic nanoparticles, which has been established in medicine © XXXX American Chemical Society

Received: November 3, 2017 Revised: December 21, 2017

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DOI: 10.1021/acs.macromol.7b02321 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

A promising concept for DoD using expanding materials (e.g., azo-compounds or expandable graphite) for thermally induced gas formation in adhesive compositions has been mentioned in the literature.19 Similarly, blocked isocyanates (BICs)20 could serve as thermally responsive compounds that undergo a deblocking reaction upon heating, which could ultimately yield decarboxylation. The thermal deblocking temperatures (Td) of numerous BICs have been extensively studied.21−23 Active methylene compounds such as diethylmalonate (DEM) or ethyl 2-acetoacetate (EAA) are reported to have a Td in the range of 80−120 °C24,25 and can also be easily functionalized with photopolymerizable units via their ester links. The proposed deblocking mechanism forming a reactive isocyanate is proven for BICs with phenol as blocking reagent (BL), which are commonly used in industry. Using active methylene compounds such as DEM or EAA as BLs might lead to cleavage through ketene instead of isocyanate formation.25 Nevertheless, a subsequent reaction with traces of water would lead to decarboxylation in both cases. Within this study we have focused on the realization of DoD for photocured polymeric networks that exhibit high stiffness and hardness comparable to state-of-the-art matrices used in industry (e.g., cross-linking dimethacrylates). A concept for DoD through thermally induced gas formation is developed, using blocked isocyanates as thermolabile compounds. Aside from the ideal BL, the isocyanate has also an influence on the derived Td. After selection of two possible isocyanates (i.e., phenyl isocyanate, ethyl isocyanate) BICs with DEM, EAA and phenol as BLs were synthesized (BIC1−4, Figure 2). The Tds of the respective BICs were evaluated by conducting Ba(OH)2 tests in solution. Moreover, the thermal deblocking and subsequently induced gas evolution within a linear, thermoplastic polymer matrix (poly(N-acryloylmorpholine), polyNAM), were studied by thermomicroscopy and DMTA. Based on 2-(acetoacetoxy)ethyl methacrylate (AAEM),26 a difunctional methacrylate with cleavable BIC units was synthesized as thermolabile photopolymerizable cross-linker (BIC5). After subsequent photopolymerization, the resulting photopolymer network prevents thermally induced gas formation within the polymer network due to its highly cross-linked nature. As a solution, a difunctional β-allyl sulfone was used as addition−fragmentation chain transfer (AFCT) reagent to tune the photopolymer network properties.27

Figure 1. Structural linkers enabling thermal or photolytic DoD: (a) azo-compounds,10 (b) hetero-Diels−Alder concept,11 and (c) ureido4-pyrimidone interaction as an example of a supramolecular system.15

Reversible structural units such as retro-Diels−Alder (Figure 1b) or supramolecular components (Figure 1c) serve as powerful tools in the fields of DoD or self-healing due to their dynamic exchange between closed cross-link and open structure. However, when thinking about fast removal in a sensitive environment, the back-reaction could disturb a smooth debonding procedure. The mentioned concepts provide promising DoD solutions especially for polymer networks with low cross-link densities or glass transition temperatures below 100 °C, as mobility of the reactive groups is critical for such approaches. However, this mobility is reduced within a highly cross-linked, glassy polymeric matrix. In a recent publication,18 the research group of Prof. Weder has addressed the issue of fast dynamic response within a rigid and stiff polymer network. A glassy, low molecular weight, dynamic supramolecular polymer network, based on ureido-4pyrimidone groups, has been synthesized and displays satisfactory mechanical properties, which are required for coating and adhesive applications. The glassy network exhibits high stiffness and undergoes self-healing upon UV-light irradiation, which can be achieved due to the lack of covalent cross-links in the material. Aside from the great advantages of optical healing and potential DoD, those materials were not photocured, which is a crucial requirement for precise and highly resolved materials.

Figure 2. (a) Proposed deblocking mechanism with decarboxylation, (b) studied blocking reagents (BLs) for the formation of thermolabile BICs (i.e., DEM, EAA, and phenol), (c) illustration of the BaCO3 formation during the Ba(OH)2 test, and (d) BICs tested (BIC1−4, including their respective Td(Ba)). B

DOI: 10.1021/acs.macromol.7b02321 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Darocur 1173 was chosen due to the relatively high exothermic polymerization of NAM, which causes premature gas evolution when the highly reactive photoinitiator BMDG is employed. The formulations were homogenized in an ultrasonic bath for 30 min. For thermomicroscopy experiments the respective formulations were dropped on a glass coverslip (50 mg, resulting in a layer thickness of ∼1 mm). A rectangular-shaped silicon mold (5 × 2 × 40 mm3) was filled with the formulations for DMTA. For DMTA a reference sample with 5 mol % DEP as plasticizer instead of a BIC was additionally prepared. All prepared formulations were then photopolymerized in an INTELLI-RAY 600 light oven from Uvitron International (irradiation power 600 W, UV-A: 125 mW cm−2; Vis: 125 mW cm−2). The emitted wavelength spectrum from the broadband mercury-based light source was ∼280−550 nm. First the samples were put into the light oven and cured under milder conditions (5 times 1 s irradiation at ∼23 mW cm−2 and 1 min break in between irradiation periods). Afterward, the specimens were cured for 10 min at ∼45 mW cm−2. This careful curing procedure prevented premature thermal cleavage of the BICs, caused by the highly exothermic photopolymerization of NAM. The final polymer specimens were sanded in order to remove the oxygen-inhibited top layer and ensure uniform shape. Highly cross-linked polymer matrices were tested via DMTA, tensile, and Dynstat test. BIC5 was tested as potential thermolabile cross-linker, and the difunctional monomer UDMA was used as reference compound. Both monomers were also mixed with 25 mol % AFCT reagent DAS to adjust the final photopolymer cross-link density. To all four formulations the photoinitiator BMDG (1 wt %) was added, and the formulations were homogenized in an ultrasonic bath for 30 min. Here the highly reactive visible-light photoinitiator BMDG could be used due to the less exothermic photopolymerization of methacrylates. The test specimens for mechanical characterization were cast in silicon molds (dumbbell-shaped with a total length of 35 mm and a parallel constriction region dimension of 2 × 2 × 12 mm3 for tensile test; rectangular-shaped 5 × 2 × 40 mm3 for DMTA and 10 × 2 × 15 mm3 for Dynstat impact test) and then photopolymerized in a Lumamat 100 light oven provided by Ivoclar Vivadent AG. Osram Dulux L Blue lamps were used as irradiation source (18 W, 400−580 nm). An Ocean Optics USB 2000+ spectrometer was used to measure a total intensity of approximately 20 mW cm−2 at the position of the silicone molds. All samples were irradiated for 2 × 10 min and flipped in between irradiation periods. The final polymer specimens were sanded in order to remove the oxygen-inhibited top layer and ensure uniform shape. Thermomicroscopy. The thermomicroscopy measurements were performed on an Axio Scope.A1 microscope by Zeiss with an integrated Linkam LTS 350 heating stage. The glass coverslips with the respective photopolymer samples were placed on the sample stage and heated to 100 °C (Tstart) with a heating rate of 5 °C min−1. The temperature was held for 5 min and a picture of the samples was taken (EC epiplan 5×/0.13 objective lens, Canon PowerShot G12 digital camera). Then the samples were heated (2 °C min−1, linear heating rate), and the temperature was recorded at which first gas formation was detected. This temperature was defined as the deblocking temperature Td(TM). Two additional temperatures were recorded (i.e., temperature where gas formation increased Ti and temperature of final gas evolution Tfinal) and for all three characteristic temperatures pictures of the samples were taken. Here it needs to be noted that Ti and Tfinal are not quantitatively defined and thus are only used to visualize the evolution of gas formation. All samples were measured in triplicate. Dynamic Mechanical Thermal Analysis (DMTA). An Anton Paar MCR 301 rheometer with a CTD 450 oven and an SRF 12 measuring system was used for DMTA. Rectangular-shaped polymer test specimens were measured in torsion mode with a frequency of 1 Hz and a strain of 0.1%. A constant heating rate (2 °C min−1) and normal force (−1 N) were applied, and the temperature was raised from 20 to 200 °C. The storage modulus G′ plot was recorded with the Rheoplus/32 V3.62 software from Anton Paar. Tensile Test. A Zwick Z050 testing machine, equipped with a 1 kN load cell, was used for tensile tests. For each sample, five dumbbell specimens were tested before and three specimens after temperature

This yields a polymer with a sharp glass transition in the respective DoD window (50 °C < X < 150 °C) without having to sacrifice high stiffness and hardness of the final material at ambient conditions. A first proof of concept for DoD, achieved via a matrix consisting of a thermolabile photopolymerizable crosslinker combined with a difunctional AFCT reagent as network regulator, is successfully shown.

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EXPERIMENTAL SECTION

Materials and General Methods. Phenyl isocyanate (SigmaAldrich), ethyl isocyanate (Sigma-Aldrich), diethyl malonate (DEM, Sigma-Aldrich), ethyl 2-acetoacetate (EAA, Fluka), phenol (SigmaAldrich), dibutyltin dilaurate (DBTDL, Fluka), (2-acetoacetoxy)ethyl methacrylate (AAEM, Wacker AG), the plasticizer diethyl phthalate (DEP, Loba Feinchemie GmbH), the monomer N-acryloylmorpholine (NAM, TCI Europe), 1,6-hexanediol dimethacrylate (HDDMA, Sigma-Aldrich), and the photoinitiator 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173; Ciba) were used as received unless otherwise mentioned. 2,2,4(2,4,4)-Trimethyl-1,6-diisocyanatohexane (TMDI, isomeric mixture ∼1:1), the reference monomer urethane dimethacrylate (UDMA, isomeric mixture; CAS: 72869-86-4), and the photoinitiator bis(4-methoxybenzoyl)diethylgermanium28 (BMDG) were kindly provided by the company Ivoclar Vivadent AG. The difunctional β-allyl sulfone (ethane-1,2-diylbis(oxy))bis(ethane-2, 1-diyl) bis(2-(tosylmethyl)acrylate) (DAS) was synthesized according to the literature.29 A Bruker AC 200 and a Bruker Avance DRX-400 spectrometer were used for NMR experiments (200 or 400 MHz for 1H; 50 or 100 MHz for 13C), and chemical shifts are reported in ppm using the solvent residual peak (CDCl3) as reference signal. Multiplicities are referred to as s (singlet), bs (broad singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). Coupling constants are given in hertz. Thin layer chromatography (TLC) was performed on TL-aluminum foils coated with silica gel 60 F245 (from Merck). For silica gel column chromatography a Büchi MPLC-system equipped with the control unit C-620, fraction collector C-660, and UV-photometer C-635 was used. All reagents and solvents for synthesis were used without further purification unless otherwise mentioned. A Netzsch Jupiter STA 449 F1 thermal analysis instrument with autosampler was used for DSC and TGA experiments. The respective temperature program (20−300 °C) was performed with a heating rate of 10 °C min−1 and a nitrogen flow rate of 40 mL min−1. The measured samples were accurately weighed (∼10 mg) into aluminum DSC pans. For data characterization the software Proteus Thermal Analysis version 5.2.1 from Netzsch was used. Ba(OH)2 Test. For the determination of Td via the Ba(OH)2 test, a 25 mL three-necked round-bottom flask equipped with a thermometer and a reflux condenser was purged with argon and charged with BIC (0.2 g), 10 mL of DMSO, and a water-saturated molecular sieve (4 Å, 2.5 g, 28 wt % H2O). A saturated Ba(OH)2 solution was prepared by dissolving 4 g of Ba(OH)2 octahydrate in 100 mL of water at 80 °C. The turbid solution was stirred at this temperature for 30 min and was then filtered to obtain a clear solution, which was stored under exclusion of air and cooled down to room temperature. A stream of argon was purged through the flask and guided through a gas washing bottle which was charged with the prepared saturated Ba(OH)2 solution. Subsequently, the reaction solution is heated up (heating rate ∼2 °C min−1) under magnetic stirring. At the point where the Td is reached the BIC dissociates, and the released reactive moiety (e.g., isocyanate or ketene) reacts with water to undergo decarboxylation. The formed CO2 is then transported by the argon stream and continuously bubbled through the Ba(OH)2 solution leading to precipitation of BaCO3. The temperature of the reaction solution at which the Ba(OH)2 solution turns turbid is then recorded as the Td(Ba) of the tested BIC. All conducted Ba(OH)2 tests were performed in triplicate with a standard deviation of less than ±1 °C. Preparation of Resin Formulations and Polymer Specimens. For tests in a linear polymer matrix the monofunctional monomer NAM was mixed with 5 mol % of BIC (4 mol % for cross-linkers BIC5 and HDDMA) and 1 wt % of photoinitiator Darocur 1173. Here, C

DOI: 10.1021/acs.macromol.7b02321 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules treatment (20 h at 120 °C). The respective measurement specimens were strained with a traverse speed of 5 mm min−1, and a stress−strain plot was recorded simultaneously. Dynstat Impact Test. Dynstat impact testing was performed according to DIN 53 435. The prepared polymer specimens were broken with a 10 kpcm (1 J) hammer. For p(BIC5-co-DAS) a smaller 2 kpcm (0.2 J) hammer was applied after temperature treatment (20 h at 120 °C) due to the fragile specimens. All measurements were conducted in triplicate, and the acquired impact resistance value was normalized to the width and thickness of the tested specimens. The impact resistance is determined by the ratio of work required to break the respective specimen to the cross section of the sample at the fracture site. Debonding on Demand Test. The polymer poly(BIC5-co-DAS) was applied as potential glue for DoD, and poly(UDMA) was tested as reference compound. 50 mg of the prepared formulations was put on a glass slide (7.5 × 2.5 × 0.1 cm3 with a 1.5 mm hole on one end). A thermocouple (RS Pro Thermoelement Typ K) was placed in that drop of formulation, and another glass slide was put on top (Figure S10). The resulting monomer−glass contact surface was ∼2 cm in diameter. This assembly was then put in a Lumamat 100 UV oven to photopolymerize the monomer solution as described before. The assembly was then fixed vertically on a laboratory stand using a copper wire, and the lower glass slide was additionally ballasted with a 200 g stainless steel weight, also using a copper wire. A heat gun (STEINEL HG 2120 E), fixed to a laboratory stand, was adjusted to aim directly at the center of the DoD test specimen (distance heat gun to specimen ∼15 cm). The thermocouple was plugged into a temperature control unit (HEJU JUCHHEIM SOLINGEN Temperaturregler LTR 350 dtrch) to directly monitor the temperature within the polymer network. To start the experiment the heat gun (level 6−9, depending on the targeted temperature) and the temperature control unit were turned on, and the assembly was heated continuously until the thermally induced gas formation in the polymer network caused the glass slides to separate. DoD tests were performed in triplicate.

(Figure 2c), and then application-oriented mechanical tests in a photopolymer matrix were designed (thermomicroscopy, DMTA, tensile test, Dynstat test). Characterization of the Deblocking Temperature Td in Solution. At first, the Tds of the tested BICs (BIC1−4, Figure 2d) were investigated in solution by a simple Ba(OH)2 test.31 The respective BICs were each dissolved in DMSO and combined in a flask with water-saturated molecular sieves. The flask was purged with argon, and the gas stream was guided through a Ba(OH)2 solution. During thermal deblocking of the BICs either reactive ketenes or isocyanates are formed,25 which cause decarboxylation upon reaction with water, thus making the performed Ba(OH)2 test a suitable method for the evaluation of Td. The temperature at which turbidity (i.e., formation of BaCO3) is detected is defined as Td(Ba) and was recorded for all investigated BICs (Table 1). Experiments were performed in Table 1. Deblocking Temperature for Neat BICs from Ba(OH)2 Test (Td(Ba)); Deblocking Temperature for BICs in PolyNAM Matrix Measured by Thermomicroscopy (Td(TM))

a

compound

Td(Ba) °C

Td(TM)a °C

BIC1 BIC2 BIC3 BIC4

113 105 135 45

116 113 136 174

5 mol % of BIC in polyNAM.

triplicate and showed good reproducibility (±1 °C). When looking at the evaluated Td(Ba)s for BIC1−2, it can be concluded that the less sterically stressed, nonaromatic BIC2 exhibits with 105 °C a lower Td. This goes against the trend for conventional hydroxy-blocked isocyanates stated in the literature, where aromatic groups tend to lower Td.32 The β-ketoester-blocked BIC3 exhibits with 135 °C a higher Td compared to the diesterblocked analogue. The phenol-blocked reference BIC4 showed by far the lowest Td of 45 °C. Additionally, simultaneous thermal analysis (TGA and DSC) of the neat BICs (BIC1−4) was conducted. The weight loss of BICs during thermal treatment gives an idea about their thermostability. However, only volatile deblocking products are detected with this technique (Figure S1). Decarboxylation is not favored in this case as thermal analysis was performed under inert conditions with no water present. All BICs tested exhibited no significant weight loss at temperatures