Supporting Information

Transcription

1 Supporting Information Atomic Force Microscopy Nanomechanical Mapping Visualizes Interfacial Broadening between Networks Due to Chemical Exchange Reactions Changfei He, Shaowei Shi, *, Xuefei Wu, Thomas P. Russell, *,,,, and Dong Wang *, Beijing Advanced Innovation Center for Soft Matter Science and Engineering & State Key Laboratory of Organic Inorganic Composites, Beijing University of Chemical Technology, Beijing, , China Polymer Science and Engineering Department, University of Massachusetts Amherst, Massachusetts, 01003, United States Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720,United States Advanced Institute for Materials Research, Tohoku University, Katahira, Aoba, Sendai , Japan S1

2 Experimental Section Materials. 1,5,7-Triazabicyclo [4,4,0] dec-5-ene (TBD, 98% J&K), Adipic acid (99.5%, Yuanye Ltd.), Dodecanedioicacid(99%, Aladdin), diglycidyl ether of bisphenol A (DGEBA, Sigma-Aldrich, D.E.R. 332, M.W= g/mol), octadecyltrichlorosilane(ots, 98% J&K) were used without any purification. Sample preparation. We prepared vitrimer 1 (V1) by reacting DGEBA with adipic acid and vitrimer 2 (V2) by DGEBA with dodecanedioic acid, using TBD as a transesterification catalyst as reported previously. 1 The reactions are shown in Scheme S1. The vitrimer mixture was prepared by mixing the curing agent with different composition as a crosslink agent by the same way of the pure vitrimer preparation. Scheme S1. Preparation of vitrimer 1 and vitrimer 2. Taking vitrimer 1 as an example, stoichiometric amounts of DGEBA (3.4 g, 0.01 mmol) and adipic acid(1.46 g, 0.01 mmol) were mixed in a vial. The mixture was heated to 160 o C manually stirred. Then, TBD (2.5 mol % to the COOH) was introduced and stirred manually until the mixture was homogeneous. The viscous solution was rapidly poured onto a silicon wafer that was treated by octadecyltrichlorosilane (OTS), sandwiched with a second treated silicon wafer, to ensure smooth surfaces. The sandwiched samples were heated to 160 o C for 6 h to cross-link the epoxy. The thickness of the epoxy vitrimer was mm. The sandwiched sample was emerged in liquid nitrogen to separate the vitrimer film and the silicon wafers. The surface roughness, R q, of the vitrimer slabs was measured to be ~ 5 nm over a 5 5 µm 2 area. Subsequently, two vitrimer slabs were laid over each other to form a bilayer and clamped between two silicon wafers. The bilayer was then placed into a vacuum oven, preheated to a specific temperature, for a pre-determined period of time. The bilayers were then rapidly removed and quenched in liquid nitrogen to stop the interfacial broadening and to separate the film from silicon wafer. The prepared bilayers were ultramicrotomed using a Leica EM FC7 (Leica Microsystems GmbH, Wetzlar, Germany) at 50 o C to obtain a flat surface for AFM characterization. The temperature was set at 50 C to freeze-in the structure and ensure that the ultramicrotomy resulted in a brittle fracture to minimize the surface roughness. Moreover, the cutting was done normal to the interpenetration direction to minimize influence of the interface morphology. AFM Nanomechanical Mapping Measurement. AFM Nanomechanical mapping measurements were performed in PeakForce QNM (Quantitative Nano-Mechanics) mode on a Bruker MultiMode AFM at ambient conditions. The oscillation frequency of the Z-piezo was 1.0 khz and the peak force amplitude was set at 150 nm. The samples were scanned using OMCL-AC200TS-R3 cantilevers (Olympus Micro Cantilevers) with a nominal spring constant of 9.0 N/m. The actual spring constant was measured by a thermal tuning method. The stress between the tip and the sample was designed to be much smaller than the yield stress of the vitrimers, and only elastic S2

3 deformation was involved in the measurement. The tip geometry, before and after imaging, was checked by tapping mode imaging of a Nioprobe tip-check sample (Aurora Nanodevices, Canada) at scan rate of 1 Hz. The scan area was µm 2 and the number of pixels were The Bruker NanoScope Analysis software was used to estimate the tip geometry. In PeakForce QNM mode, the reduced Young s modulus, E*, is obtained by fitting the unloading curve using the Derjaguin, Muller, Toropov (DMT) model, 2 which is described by E * = 3(F tip F adh ) 4 Rd 3 (1) where F tip is the force on the tip, F adh is the adhesive force, R is the tip radius, and d is the tip-sample separation. E* has the relationship to sample Young s modulus, E s, as follows, E s = (1 ν s 2 )E * (2) where ν s is the Poission s ratio. Thus, a combination of equation (1) and (2) gives E s. Different Scanning Calorimetry (DSC). Samples were heated at a rate of 20 o C/min from room temperature to180 o C to erase thermal history, cooled to 0 o C at 20 o C/min, and then heated to 180 o C at 10 o C/min. All data shown are taken from the second heating ramp. Dynamic Mechanical Thermal Analyses (DMTA). The thermomechanical properties of vitrimer 1 was measured from 0 to 160 o C and for vitrimer 2-50 to 100 o C at a rate of 3 o C/min and a frequency of 1 Hz. Sample geometry is 1 mm (T) 10 mm (W) 60 mm (L). Stress Relaxation Analysis (SRA). SRA was tested utilizing rectangular films (ca. 1 mm (T) 5 mm (W) 20 mm (L)). The SRA experiments were performed in a strain control at specified temperature (for vitrimer 1 is o C and for vitrimer 2 is o C). The samples were allowed to equilibrate at this temperature for approximately 5 min. To ensure the good contact, a constant normal strain of 0.1% was applied. Subsequently, each sample was subjected to an instantaneous 3% strain. The stress decay was monitored, while maintaining a constant strain (3%), until the stress relaxation modulus had relaxed to at least 37% (1/e) of its initial value, which is relaxation time τ determined from the experiment. Swelling experiments. Swelling experiments were performed in 20 ml chlorobenzene with a samples weight of ~110 mg. The temperature was increased to 120 o C and was kept constant for 24 h. Then, the solvent was carefully removed using a syringe and replaced twice using MeOH solvent to remove residual chlorobenzene. Finally, the samples were dried in vacuum oven, first at 40 o C overnight and then at 120 o C for 2 h to ensure a complete removal of the residual solvent. The gel fraction was calculated. Young s modulus measurement. The bulk samples of V1, V2 and their different composition mixtures were made in a dog-bond shape (25 mm 4 mm 1 mm) to make the mechanical test samples. Tensile tests were performed according to the ISO with a strain rate of 10 mm/min using an Instron 3345 tensile machine. The Young s modulus for film samples were tested using the AFM nanomechanical mapping method as described previously. S3

4 Figure S1. DMA curves of (a) vitrimer 1 (V1) and (b) vitrimer 2 (V2). Figure S2. DSC curves of V1 and V2 S4

5 Sample s Table S1. Thermal and thermomecanical properties of V1 and V2 T g ( o C) (DSC) T g ( o C) (DMA) E at glass state (MPa) E at 150 o C (MPa) M c (kg/mol) V V Note 1. Calculation of cross-linking density Cross-linking density, or the average molecular weight between cross-links (M c ) was calculated by the following equation using storage modulus of rubbery plateau. 3 Mc = 2(1 +ν) ρrt / E ν ρ R (1) : Poissons ratio (for epoxy resin=0.33) : density (epoxy resin: 1.56 g/cm -3 ) : universal gas constant E : storage modulus of rubbery plateau Using the equation above, the value of M c for V1 and V2 at 150 o C are calculate to be: V1( E =1.4MPa): 10.4 kg/mol V2( E =1.5MPa): 9.7 kg/mol Figure S3. Normalized stress relaxation curves of (a) V1 and (b) V2 at different temperatures. The inset is the fitting of the relaxation times to the Arrhenius equation. S5

6 Note 2. Calculation of activation energies (E a ) and topology-freezing transition temperatures (T v ) Activation energies (E a ) and topology-freezing transition temperatures (T v ) were determined using the methodology reported in literature 1.The measured relaxation time τ were fit to the Arrhenius law in eq(2). τ( T) = τ e Ea/ RT 0 (2) (R is the universal gas constant) Eq(2) can be transformed to eq(3) Ea ln τ( T) = lnτ0 + RT (3) According to the inset fitting line in Figure S3, eq(3) for V1 and V2 are transformed into eq(4) and eq(5) respectively ln τ ( T) = T V1: (4) 1000 ln τ ( T) = T V2: (5) E a /R=13.44 for V1 and for V2, so the activation energy of V1 and V2 are calculated to be kj/mol and 98.4 kj/mol, respectively. T v is defined to be the temperature at which the material reaches a viscosity of Pa s. The relation of the viscosity η and the characteristic relaxation time τ is known as the Maxwell relation (eq(6)), E η = G τ = τ 2(1 + ν ) (6) where G is the shear modulus, is the Poissons ratio and E is the storage modulus. Using the Poissons ratio=0.5, eq(6) is transformed to eq(7) τ E η = 3 ν (7) The storage modulus of V1 from 90 to 110 is 1.5 MPa and from 70 o C to 90 o C for V2 is 1.7 MPa. Thus, the relaxation time τ of V1 is s and τ of V2 is s at T v because of =10 12 Pa s. Using this value and eq(4) and eq(5), the T v of V1 and V2 is calculated to be 37.2 o C and 13.0 o C respectively. η S6

7 Figure S4. a) DSC curves of mixtures for different vitrimer compositions. X 1 represents the mole fraction of vitrimer V1. b) 1/T g as a function of the mole fraction of V1(x 1 ). Figure S5. The Young s modulus has a linear relationship with vitrimer composition for both tensile testing and nanomechnical mapping. S7

8 Table S2. Sol fraction of V1, V2 and their different composition mixtures in chlorobenzene at 120 for 24 h. Sample V1 fraction (x1) Before (mg) After (mg) Sol fraction* (%) 1 (V1) (V2) *sol fraction = m m dry intial 100% Note 3. In AFM nanomechanical mapping measurements, the Young s modulus is obtained by fitting the unloading curve using some contact models, such as DMT, JKR, etc. These models contain several parameters such as the tip radius, the adhesive force between the sample surface and AFM tip. If the material is single component, then the measured Young s modulus will be fine. When the sample consists of two or multi components, however, the interactions between the surface and tip vary from one component to another, making the contact models is not perfectly fit for all components, which will give measurement errors for some components. S8

9 Figure S6. Converted concentration profile annealing for 40 minute at different temperature. Note 4. Definition of transesterification rate We define the transesterification rate in the transition region as the following eq (8) dx k = dt (8) (k is the transesterification rate, x is V1 mole fraction in the transition region, t is the diffusion time) Figure S7. Schematic profile to explain the relationship between V1 fraction(x) and transition region width (w). As shown in the Figure S7, from concentration profile, the value of x can be described in eq(9) 1 dx = dw w (9) Using the eq(8) and eq(9), we obtain the eq(10) dw k = / dt w (10) S9

10 To integral the eq(10), so the transesterification rate can be defined as following eq(11) w ln( ) w = kt 0 (11) References 1. Yang, Y.; Pei, Z.; Li, Z.; Wei, Y.; Ji, Y. J. Am. Chem. Soc. 2016, 138, Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. J. Colloid Interface Sci. 1975, 53, David Roylance. Engineering viscoelasticity. 2011, p S10