Motivation

水分侵入による無機・有機界面でのプロトン移動と接着耐久性の低下

Adhesive bonding è Key technology in various manufacturers
For Lighter-weight body

Trends in Joining Processes of Automotive80%(in Tech. Roadmaps, by Smith etal, 2018)
gt weldin
nce spoResistae
Adhesiv

50%

Multimaterial

尾形 修司，浦長瀬 正幸
名古屋工業大学 物理工学

Compositematerial0%

2018

2025

2030

Degradation of various materials by ubiquitous water
We address the industrially important problem of“Weakening (by half) of Interface Bonding by Moisture”
through large-scale simulation and calculation.

13:20-13:40, 2021.12.08「富岳」成果創出加速プログラム物質・材料系課題 合同研究会, Zoom

è Unveiling electronic-level microscopic mechanisms

Revising simulation approach
Former approach: simple insertion of H2O’s to presumed interface
Adhesion strength

Water diffusion

strainEpoxyresin

QM

AlOx
(~15Å)

InsertH2O’sPart 1: Degradation of cured epoxy resin
under moist conditions

DFT(N~3500)

DFT(N~1500)

H 2O

Resin

(1)

Al

量子領域
Hybrid QM-CL simulations …
but, short time-scale
Our new approach(1) First, determine thermodynamically stable interface
under moist conditions(2) Second, calculate the strengths of interface based on (1)(2)
Surface oxide

Metal

In experiments under moist conditions:l Breakings at both regions (1) and (2)are often observed.
l Particularly in acid environment,
breaking of amine-cured epoxy resin
at region (1) is significant
In Part 1, we consider about region (1).

Cured epoxy resin: protonation/deprotonation of N ?
Epoxy molecule x 5Amine Curing agent+

Vertical energy-gap method for deprotonation free-energy
Deprotonation half reactions• NR ! H• H! OCuring(Ring-openingof epoxide G.)""$% → NR ! $% + H$% → H# O $% + H$""($) + ∆+′

Method by M. Sulpizi et al.
(2008)($) + ∆+′′"• H# O $% → OH $% + H ($) + ∆+′′′
11∴ NR ! H " $% + OH $ $% = NR ! $% + H! O" $% + ∆+22

Calculate deprotonation half reaction free-energy with DFT&∆+ = / 01 2'() deprotonated − 2'() ) *Remember“ammonia water”:
NH! + H" O ↔
NH# $ + OH %Is N(–R)3 really OK ?
Protonation of N(–R)3 in water ?

Results: Protonated vs. deprotonated states of cured epoxy

Same # of electrons
Average over DFT-MD trajectoryusing artificial Hamiltonian ℋ* = 1 − 1 ℋ + 1ℋ+,-./0/120,+(“deprotonated” = zero charge on the dummy proton)&Note: ' & = − ln ∫ -Γ / %'ℋ! ;')*(,)),

Shortenedresin model

NR ! H " $% +=∫ /0(ℋ"#$%&'&()'#"%ℋ) 1*+ℋ!

*+ℋ!
∫ /01

Reasons: Protonated vs. deprotonated states of cured epoxy

Embedded in water of 1.0 g/cm3 at 300 K

N(2)
N(1)

deprotonation degree: &%OHRadial distribution between
H of NH(1,2) and O of H2O

l Two DFT calc. at eachtimestep.
l About 1600 atoms intotal.
l In-house DFT code:GGA(PEB) functional.

11
OH $ $% = NR ! $% + H! O" $% + ∆+22

For N(1): ∆" = −0.03 eV ∴ N(−R)! H " is stable in water !
For N(2): ∆" = 0.12 eV ∴ N(−R)! H " is unstable è H " is thermodynamicallysubtracted by water

For N(1): N(−R)! H " is stable in water
For N(2): N(−R)! H " is unstable

N(1)-H is thermodynamically“protected” by –OH groups ofdeformed epoxy

Possible weakening of cured epoxy resin due to pH7 water

Bond-breaking barrier energies of cured epoxy in DRY conditionl CO-bond breaking of ether-group:

Investigating effects of• Protonation of N(1)• Involvement of H2O

Ether group

DRY = unprotonated& no H2O

N(3)l CN-bond breaking of unprotonated tertiary amine-group… Returning back to epoxide-group after breaking

N(2)

N(1)
Aminegroup

Bond-breaking barrier energies of cured epoxy in WET conditionl CO-bond breaking of ether-group

WET = H2O involvement &amin-N(1) is protonatedRelation to experiment: Protonated CN-bond breaking
Probability of bond breakage per unit time: ! = # exp(−)! /+" ,)Setting , = 300 K and # = 10$& s #$)! = 1.94 eV è ! ≈ 10#$% year #$… Still too small !!

Lowering by
H2O à OH + Hl CN-bond breaking of protonated amine-group… No lowering by H2O à OH + H
Substantial
lowering ofbond-breaking
barrier energyby protonation

Stress may be magnified near the interface because ofinterfacial inhomogeneity.

The stress may further lower the energy barrier, resulting in asignificantly increased breaking probability, p.
èIn experiment, breaking of amine-cured epoxy resin
near the interface (particularly in acid environment)

Part 2: Degradation of silane couplingunder moist conditions

Perfluoro polyether compound as SCA, connecting to HO-SiO2
Perfluoro polyether compound

Famous Silane Coupling to connect organic and inorganic materialsX

Si (–O–CH3)3 Silane Coupling Agent (SCA)

HO–
organic

X

Si (–OH)3
HO–inorganic+ H 2O

– H2O(dehydration)– H 2OX

Si– O–(–OH)2

organicinorganicHO–l Coating of glass by perfluoro (element F) polymers.
l Good water and oil repellent. Anti-fingerprint.

Silane Coupling

pH-dependent degradation of SCA under moist conditions
Dental composite

corrosion

Resin Silica

l Still, increased anti-rubbing strength is expected.

“Which HO–” deprotonates first in waterComposite
Resin + Silica
connected
with SCA

sugar,
virus

Long-term test of dental composite under moist conditions
3-MPTS as SCA:
3-(Methacryloyloxy)
PropylTrimethoxySilane

After 14 days (Cilli etal, 2012):
In pH = 1
è No degradation

days
H2Oç

In pH = 13
è Hardness loss by 50%
due to SC destruction

Deprotonation free-energy calculations with VEGM
l pKa = 2.4 ± 1.3 and ~ 8.5 for HO-SiO2 è Partial deprotonation in pH 7
(Two types of HO– for HO-SiO2)(Experimental confirmed by the vib. spectra and the isoelectric point)
l pKa = 8.7 ± 1.0 for HO-SCA1 and SCA2 è No deprotonation in pH 7

Barrier energies for protonated HO-SiO2 (i.e., pH < 7)

Barrier energies for 50%-deprotonated HO-SiO2 (i.e., pH ≥ 7)

l –Si-O– breaks firstcompared to –Si-C–l –Si-O– breaking without H2O
è Barrier 2.85 eVl –Si-O– breaking with an H2O(H2O = H +OH)
è Barrier 1.88 eV

Reason of barrier lowering in 50%-deprotonated HO-SiO2

l –Si-O– breaking without H2O
è Barrier 1.66 eVl –Si-O– breaking with an H2O(H2O = H +OH)
è Barrier 0.77 eV

DFT-MD simulation of pulling SCA with protonated HO-SiO2

At bond-breaking transition states; –Si-O– breaking without an H2O
Protonated HO-SiO2

50%-deprotonated HO-SiO2Chargedensity

Termination of breaking–Si–O– bond assisted
by surface electrons

Chargedensity

Eigenfunction
at -0.20 eV

Applied work up to thebreaking stage: 2.0 eV(Calculated barrier: 1.88 eV)

DFT-MD simulation of pulling SCA with 50%-deprotonated HO-SiO2

Summary
Part 1: Degradation of amine-cured epoxy resin under moist conditions
l Deprotonation free-energy calculation with DFT.
l Protonation of an amine-G occurs in pH = 7 water.
l CN-bond breaks easily for protonated amine-G in pH ≤ 7,
which explains the breaking of strained resin under moist
condition in the experiment.

Part 2: Degradation of silane coupling (SC) under moist conditions
l Deprotonation free-energy calculation with DFT.
l Deprotonation of HO-SiO2 surface occurs in pH ≥ 7.

Applied work up to thebreaking stage: 1.0 eV(Calculated barrier: 0.77 eV)

Substantial diffusion ofprotons on surfacel –Si–O– of SC breaks easily for deprotonated HO-SiO2 in pH ≥ 7,
which explains the SC breaking in pH ≥ 7 in the experiment.