Decomposition Characteristics of SF6 under Thermal Fault ...
Further Developments in Modeling the Thermal Decomposition ...
Transcript of Further Developments in Modeling the Thermal Decomposition ...
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Further Developments in Modeling the Thermal Decomposition of Polymers
Marc R. NydenBuilding and Fire Research Laboratory
National Institute of Standards and TechnologyGaithersburg, MD 20899
Stanislav I. Stoliarov and Phillip R. WestmorelandDept of Chemical EngineeringUniv. Massachusetts Amherst
Amherst MA 01003-3110
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Molecular Level Understanding of Materials Flammability
Sensing Film
Thermometer Plate
SiO2 Base LayerHeater
Contact Pads
Micro-Hotplate Heating Experiments
Current (mA)
2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Tem
pera
ture
(o C)
0
100
200
300
400
500
600
Col 1 vs Ps # 2 Col 1 vs PS/Clay #1
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Principles of Classical Molecular Dynamics
,,dtdp- =
qH
dtdq =
pH i
i
i
i ∂∂
∂∂
The molecular dynamics algorithm consists of numerical solution ofHamilton’s equations of motion:
where qi, pi, and mi are the coordinates, momenta, and masses of atoms
),,...,,( 321 Ni
2i
3N
i
qqqV + m2p = H ∑
V is defined by The Consistent Valence Force Field:
Vn
Vn
Vn
Vn = V bondnon
pairs
torsion
torsions
angle
angles
bond
bonds
+ + + −∑∑∑∑
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Morse Potential
Bond Lenth
V
Bond Length
DissociationEnergy
5. Dauber-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler, A. T.; Structure, Function and Genetics 1988, 4: 31.
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MD Model of Thermal Degradation in Polymers
The feature that distinguishes MD_REACT from other MD codes is that it allows for the formation of new bonds from freeradical fragments that are generated when bonds in the polymer break and, thereby, accounts for the chemical reactions that play a major role in the thermal degradation process.
This is achieved using the IPC protocol to send bonding information back and forth between MD_REACT and Discover.
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Reactive Force Field
D]))r-(r(-exp-D[1 = V 2eb −α
)-(kS(ij)S(jk) = V 2ea θθθ
)]-(ncos+[1kS(kl)S(ij)S(jk) = V et φφφ
r])
rr2( - )
rr[( = V ji6
*12
*
nbδδ
ε +
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Reactive Force Field
>−
≤=
eb
e
rr)ij(D
)ij(Vrr1
)ij(S
• The atoms participating in covalent bonds become radicals when the corresponding bond orders become less than a pre-determined value.
• The program sorts through all possible bonds between these free radicals and retains those corresponding to the lowest energy subject to the constraints imposed by atomic valence rules.
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Available Reaction ChannelsR CH2
R
R
CHR RR +
RR
HH R
RR
R
H
R
HR
R
R
RR + H2
R CH2 +CH2R
H
+
R
CH3CH3
CH3 CH3
CH3
CH3 CH3CH3
CH4
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Random Scission Beta - Scission
H - TransferElimination
CyclizationCrosslinking
Recombination
CH3
H
CHR
R
CH3
R
CH3
H
RR
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Interface to Discover 95
GUI
Polymerizer
ElectronicStructure
Discover 95
MolecularMechanics
MD_REACT
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Thermal Degradation of PP
pp_3000_pbc_1
Still frame from an MD simulation of the thermal degradation of a modelpolypropylene consisting of 6 polymer chains, each with 21 monomers.
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Thermal Degradation of PP
Figure 1. Major Reaction Channels in the Thermal Degradation ofPolypropylene.
RRecombination
Random ScissionCH3
+R Rn CH2
n'
CH2
R
H
H - Transfer
H
RCH2
Beta - ScissionRCH2R +
CH3
n'Rn
First OrderTermination
CH3CH3 CH3
CH3
R
CH3
CH3 CH3
CH2R
RR
H
RR
CH33
CH3
CH3
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Kinetic Model
)())(][)(2()()(
1 TZTkRTkdt
tdmtm Ii +−=
,)()(2][
0
0
mTkdTkR
t
i=
0
0
0
0 )(
)(
))()((
)()(
mdTk
Tk
mdTkTk
TkTZ
I
p
It
p →+
=
+−=
)()(
)()(2)()(
1TkTk
TZTkdt
tdmtm t
pi
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Simple Kinetic Model of Thermal Degradation Random Scission
Ea = 323 kJ/mol A = 1.5 x 1015 s-1
1/T (K)-10.00032 0.00034 0.00036 0.00038 0.00040 0.00042 0.00044 0.00046
ln(k
(T))
-11
-10
-9
-8
-7
-6
-5
Molecular dynamics dataRegression Lineslope = -38871 Kintercept = 7.308
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Simple Kinetic Model of Thermal Degradation Propagation
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Simple Kinetic Model of Thermal Degradation Propagation
Ea = 77 kJ/mol A = 5.6 x 1012 s-1
1/T (K)-14.8e-4 5.0e-4 5.2e-4 5.4e-4 5.6e-4 5.8e-4 6.0e-4 6.2e-4 6.4e-4 6.6e-4 6.8e-4
ln(k
(T))
-5
-4
-3
-2
Molecular dynamics dataRegression Lineslope = -9263 Kintercept = 1.720
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Simple Kinetic Model of Thermal Degradation Termination
Ea = 163 kJ/mol A = 3.1 x 1015 s-1
1/T (K)-14.8e-4 5.0e-4 5.2e-4 5.4e-4 5.6e-4 5.8e-4 6.0e-4 6.2e-4 6.4e-4 6.6e-4 6.8e-4
ln(k
(T))
-6
-5
-4
-3
-2
-1
Molecular dynamics dataRegression Lineslope = -19563 Kintercept = 8.053
TSimple Kinetic Model of Thermal
DegradationRate of Mass-loss from Degrading Polypropylene
)237528exp(103.5)()(
1 12
RTx
dttdm
tm−−=
Experimental Values:
Ea = 220 kJ/mol A = 19 x 1012 s-1
Bockhorn et. al., Journ Anal. and Appl. Pyrolysis 46, 1998, 1
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New Features of MD_REACT• A new mechanism that allows simultaneous rupture and
formation of bonds was introduced.• Every type of chemical reaction that involves rupture and/or
formation of σ and/or π bonds (with the exception of conjugated and aromatic systems) was included.
• All the chemical transformations are treated in the unified fashion based on the competition between energies of interatomic interactions. This approach results in more realistic model of reactions involving π bonds and eliminates the necessity of usage of special “effective” potentials for β- scission reactions.
• The CVFF forcefield is updated to accommodate decomposition of oxygen-containing polymers (in particular, PMMA). The new bond dissociation energies are obtained from CBS-QB3 calculations performed on model molecules.
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New Algorithm of MD_REACTThe following cycle is performed at every time step:
DISCOVER Integrator of Equatons of Motion
Bond-formationModule
Potential Energy and Gradient Calculation
Module
IPC
IPC
♦Effective bond order of each bond iscomputed as:
♦If [BO < Dissociation Criterion] then:BO = 0atom type = radical (R)
EnergyonDissociatiVBO bond=
DISCOVER Integrator of Equatons of Motion IPC
Bond-dissociationModule
Bond-formationModule
♦All possible covalent interactionsbetween radicals are examined.
♦The lowest energy bonds are retained:
number of bonds = valency + 1
♦If [BO > Disociation Criterion] for all the valent bonds of a radical then:
radical = regular atom type
number of bonds = valency
DISCOVER Integrator of Equatons of Motion IPC
Bond-dissociationModule
Potential Energy and Gradient Calculation
Module
For every covalent interaction with BO < Dissociation Criterion
∑∑×+ V + VBO VV torsion
torsionsangle
angles
bond =corrected
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New Algorithm of MD_REACTThe following cycle is performed at every time step:
DISCOVER Integrator of Equatons of Motion
Bond-dissociationModuleIPC
IPC
Bond-formationModule
Potential Energy and Gradient Calculation
Module
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MD Simulations
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Thermal Degradation of PMMA
pmma_2000K
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Thermal Degradation of PMMA
[CH2 monomer
3
CH3
C O
C ]
O CH33
n CH2C
CH33OC OCCH33
R. .+CH3
C O
O C3
H3
R’
CH2C=
CO
O
C
3
H3
R’ CH2
+
CH3
.
CHC=CH2
CH
.
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BE’s in MA and MMA
CH3O
OC
R
CH3
CH2C91.5
78.779.4
100.3
86.9
195
99.9
CH3O
OC
R
H
CH2C R79.7
87.5
92.5R
99.9
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Thermal Degradation of PMA
pmma_2000K
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Conclusions• The differences in the thermal degradation
chemistries of PMMA and PMA appear to be due to higher initiation temperatures required for PMA.
• Molecular modeling can be useful tool for the investigation of thermal degradation and materials flammability and the development of new and more fire resistant materials. This role will continue to expand as advances in computer technology make it possible to extend the range of applicability of molecular modeling to more complex systems.
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AcknowledgementsMD_REACT was developed as part of a CRADA betweenNIST and Accelrys Inc. (CN 1241)
Partial support for this work was provided by the FAA underInteragency Agreement DTFA0003-92-Z-0018 monitored byDr. Richard E.Lyon
MOLECVIEW was written by Dr. Glenn Forney (BFRL/NIST)
Dr. Robert Bohn (ITL/NIST) performed CBS-QB3 calculationson MMA