DTIC ADA625631: Thermal Stability of Fluorinated Polydienes Synthesized by Addition of Difluorocarbene

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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Thermal Stability of Fluorinated Polydienes Synthesized by W911NF-10-1-0297
Addition of Difluorocarbene
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
611102
6. AUTHORS 5d. PROJECT NUMBER
Tianzi Huang, Xiaojun Wang, Thomas Malmgren, Kunlun Hong, Jimmy
W. Mays 5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAMES AND ADDRESSES 8. PERFORMING ORGANIZATION REPORT
NUMBER
University of Tennessee at Knoxville
1534 White Avenue
Knoxville, TN 37996 -1529
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The views, opinions and/or findings contained in this report are those of the author(s) and should not contrued as an official Department
of the Army position, policy or decision, unless so designated by other documentation.
14. ABSTRACT
Linear PCHD and polyisoprenes with different microstructures and molecular weights are synthesized and
chemically modi? ed to improve their thermal and chemical stability by forming a three-membered ring structure
containing two C  F bonds. Pyrolysis of these ? uorinated polydienes proceeds through a two-stage decomposition
involving chain scission, crosslinking, dehydrogenation, and dehalogenation. The pyrolysis leads to graphite-like
residues, whereas their polydiene precursors decompose completely under the same conditions. The ? uorination of
PCHD enhances its thermal stability. The stronger C  F bond along with high strain of the three-membered ring
15. SUBJECT TERMS
fluoropolymers, glass transition, thermal properties, degradation, thermogravimetric analysis
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 15. NUMBER 19a. NAME OF RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF PAGES Jimmy Mays
UU UU UU UU 19b. TELEPHONE NUMBER
865-974-0747
Standard Form 298 (Rev 8/98)
Prescribed by ANSI Std. Z39.18
Report Title
Thermal Stability of Fluorinated Polydienes Synthesized by Addition of Difluorocarbene
ABSTRACT
Linear PCHD and polyisoprenes with different microstructures and molecular weights are synthesized and
chemically modi? ed to improve their thermal and chemical stability by forming a three-membered ring structure
containing two C  F bonds. Pyrolysis of these ? uorinated polydienes proceeds through a two-stage decomposition
involving chain scission, crosslinking, dehydrogenation, and dehalogenation. The pyrolysis leads to graphite-like
residues, whereas their polydiene precursors decompose completely under the same conditions. The ? uorination of
PCHD enhances its thermal stability. The stronger C  F bond along with high strain of the three-membered ring
structure and formation of relatively stable free radicals play an important role in the thermal stability of ? uorinated
polydienes.
REPORT DOCUMENTATION PAGE (SF298)
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Continuation for Block 13
ARO Report Number 57790.4-CH
Thermal Stability of Fluorinated Polydienes Synt.h..esized by Addition of Difluorocarbene
Block 13: Supplementary Note
© 2012 . Published in Macromolecular Chemistry and Physics, Vol. 213 (1) (2012), (3 (1). DoD Components reserve a royalty-
free, nonexclusive and irrevocable right to reproduce, publish, or otherwise use the work for Federal purposes, and to authroize
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should not be construed as an official Department of the Army position, policy or decision, unless so designated by other
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Approved for public release; distribution is unlimited.
Macromolecular
Full Paper
Chemistry and Physics
Thermal Stability of Fluorinated Polydienes
Synthesized by Addition of Difl uorocarbene
Tianzi Huang, Xiaojun Wang, Thomas Malmgren, Kunlun Hong,
Jimmy W. Mays*
Linear PCHD and polyisoprenes with different microstructures and molecular weights are syn-
thesized and chemically modifi ed to improve their thermal and chemical stability by forming
a three-membered ring structure containing two C(cid:2) F bonds. Pyrolysis of these fl uorinated
polydienes proceeds through a two-stage decomposition involving chain scission, crosslinking,
dehydrogenation, and dehalogenation. The pyrolysis leads
to graphite-like residues, whereas their polydiene precursors F O F
F CF3 F F
dneactioomn poofs eP CcoHmDp elenthealyn cuensd ietrs tthhee rsmamale sctoanbdiliittiyo.n Tsh. Teh set rflo unogreir- n F O CF3 180 Co n
C (cid:2) F bond along with high strain of the three-membered :CF2
ring structure and formation of relatively stable free radicals n n
play an important role in the thermal stability of fl uorinated F F
polydienes.
1. Introduction (low dielectric constant) and modest intermolecular inter-
actions (low surface tension).[ 2 ]
Their unsaturated structures render polydienes less stable F luorination of polydienes by different approaches
than other polymers under harsh chemical (oxygen aging, has been previously reported and it has been reviewed
solvent swelling, moisture, etc.) and physical (heat, solar recently.[ 3 ] For example, Lodge and Hillmyer[ 4 ] described
radiation, etc.) conditions and thus limit their applications. radical grafting of perfl uoroalkyl iodides. Hydrosilyla-
Post-polymerization modifi cation is thus an important tion of perfl uoroalkylsilanes also has been used to modify
method used to alter the chemical nature of polydienes. polydienes.[ 5 ] It was recently reported that a mild, selec-
Such modifi cations can result in an enhancement of their tive, and quantitative fl uorination of polydienes could
properties and extension of their applications. Along with be achieved by in situ generated difl uorocarbenes (:CF 2 )
hydrogenation, halogenation including fl uorination, has from thermolysis of hexafl uoropropylene oxide.[ 6 ] We
been studied to improve the performance of polymers by also used this method to react the residual carbon–carbon
introducing C (cid:2) F bonds.[ 1 ] The high electronegativity of double bonds of poly(1,3-cyclohexadiene) (PCHD) with
fl uorine leads to strong, polar C (cid:2)F bonding, resulting in difl uorocarbene.[ 7 ]
better thermal stability and chemical inertness. Since the The addition of difl uorocarbene to the double bonds
ionic radius of fl uorine is small, the C(cid:2) F bonds have limited leads to a three-membered ring structure containing
ability to polarize, which leads to a low index of refraction two C(cid:2) F bonds. The fl uorination reaction schemes for
1,4-polyisoprene and 1,4-PCHD are shown in Scheme 1 .
Unlike fl uoropolymers, such as polytetrafl uoroeth-
T. Huang , X. Wang , T. Malmgren , J. W. Mays
ylene in which the majority of the chemical bonds of the
Department of Chemistry, University of Tennessee,
polymers are C (cid:2) F bonds, fl uorinated polymers obtained
Knoxville, TN 37996, USA
by chemical modifi cation of hydrocarbon polymers have
E-mail: j [email protected]
fewer C(cid:2) F bonds, while retaining different types of
K. Hong , J. W. Mays
Chemical Sciences Division and Center for Nanophase chemical bonds present in their precursors. These latter
Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, bonds will exhibit the same, or similar, responses when
TN 37831, USA they are exposed to chemical and thermal treatment as
Macromol. Chem. Phys. 2012, 213, 49−56
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com DOI: 10.1002/macp.201100548 49
Macromolecular
Chemistry and Physics T. Huang et al.
www.mcp-journal.de
F O F was performed under a nitrogen atmosphere with a heating rate
of 10 ° C min − 1, with the second heating being chosen for analysis
F CF3 F F in order to eliminate the thermal history of the sample.
O 180 F ourier transform infrared (FT-IR) spectra were obtained using
Co a Bio-Rad FTS 6000 instrument, via either transmission mode
n
F CF3 n (400–4000 cm − 1) , using fi lms cast from solution on silicon wafers
:CF2 or pellets pressed from mixtures with KBr, or in attenuated total
refl ectance (ATR) mode (700–4000 cm − 1 ) using solid samples. The
n n spectra were analyzed on the basis of published spectra[ 8 ] and the
standard infrared spectra of model organic molecules published
by Sadtler Research Laboratories. The elemental analysis was
F F
done by Galbraith Laboratory Inc. (Knoxville, TN).
Scheme 1. F luorination of polydienes with in situ generated
difl uorocarbene.
3. Results and Discussion
the chemical bonds in their precursors. Therefore, the D uring thermal treatment (pyrolysis), chemical bonds may
chemical and thermal stabilities of fl uorinated polymers break either homolytically or heterolytically, and bonds
are affected by the fl uorocarbon bonds as well as the may reform, which leads to polymer chain decomposition
hydrocarbon (and possibly other) bonds. Studies until and structural rearrangement. The breaking and reforming
today show that fl uorinated polydienes synthesized by of chemical bonds are affected by the strength of the
addition of difl uorocarbenes to their double bonds have chemical bonds, the kinetic and thermodynamic competi-
much higher glass transition temperatures ( Tg ) as com- tion between products generated, and the experimental
pared with their polydiene precursors.[ 6 , 7 ] The preliminary conditions such as heating rate, sample size, atmosphere,
results of thermal treatment of fl uorinated PCHD under a etc. [ 9 , 10 ] In this study, we thus maintained all the condi-
nitrogen purge revealed higher temperatures for the onset tions the same or at least minimized changes.
of decomposition, higher temperatures for the maximum G enerally, when polymer pyrolysis occurs under vacuum
rate of decomposition, and greater residue after thermal or under inert gas fl ow, the polymer chains go through
treatment as compared with their polydiene precursors.[ 7 ] three main types of decomposition processes: chain scis-
In this paper, detailed thermal stability studies on several sion, chain crosslinking, and chain-stripping reactions.[ 9 ]
fl uorinated polydienes and their precursors are reported. These three different decomposition processes lead to dif-
ferent decomposition products. Chain scission generates
volatile materials such as monomer, dimer, or oligomers.
2. Experimental Section
Chain crosslinking generates less volatile materials but
more unsaturated structures, such as aromatized materials
The synthesis, fl uorination, and characterization of linear PCHD or char. Chain stripping generates small volatile molecules
with controlled molecular weights, narrow polydispersites, and
by the loss of substituents (side group reactions) without
varied microstructures were previously reported.[ 7 ] Linear PCHD
affecting the main chain of the polymer. These three types
samples were hydrogenated using p -toluenesulfonylhydrazide
of decomposition processes may occur individually or in
(TSH) in toluene by gently refl uxing for 16 h under an argon
combination during pyrolysis.
atmosphere. The degree of functionalization was characterized
To understand the thermal stability of fl uorinated poly-
in deuterated chloroform on a Varian Mercury 300 MHz NMR
instrument. Polyisoprene with 93% 1,4-microstructure made by mers obtained by addition reactions of difl uorocarbenes,
anionic polymerization was fl uorinated with difl uorocarbene, as three different fl uorinated polydiene samples, two hydro-
reported by Ren et al.[ 4 , 6 ] genated polydiene samples, and their polydiene precur-
T hermal stability analyses and pyrolysis studies were sors were investigated. The chemical nature, molecular
performed on a thermogravimetric analysis instrument (TGA weights, and polydispersity indices, degrees of func-
Q50, TA Instrument Inc.) at a heating rate of 10 ° C min − 1 under tionality, and T of fl uorinated, and hydrogenated poly-
g
a nitrogen or air fl ow of 60 mL min − 1. The onset temperature of
diene samples, and their precursors are summarized in
decomposition was determined by the tangent of the weight
Table 1 . All the polydiene precursors were synthesized via
loss curve. The temperature of maximum rate of decomposition
anionic polymerization to obtain controlled microstruc-
was determined by the peak in the derivative of the weight
ture, molecular weights, and narrow molecular weight
loss curve. Isothermal treatment was performed by heating the
sample to a desired temperature at a heating rate of 10 ° C min − 1 distributions. The fl uorination and characterization
and then holding the temperature constant for a period of time methods were reported elsewhere. [ 7 ] The hydrogenation
that equals the time needed to heat the sample up to 600 ° C at of PCHD samples was performed in toluene with TSH and
the same heating rate. Differential scanning calorimetry (DSC) characterized as for the fl uorinated samples.
Macromol. Chem. Phys. 2012, 213, 49−56
50
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Macromolecular
Thermal Stability of Fluorinated Polydienes Synthesized by . . . Chemistry and Physics
www.mcp-journal.de
Table 1. C haracteristics of polydienes before and after chemical modifi cation.
Sample Nature of polymer Molecular weight PDI Degree of saturation T
g
[g mol − 1 ] [mol%] [ ° C]
A PCHD (high 1,4-) 16 200 1.12 – 120
B PCHD (50/50 1,4/1,2-) 19 700 1.12 – 138
FA fl uorinated PCHD 20 300 1.24 91 199
FB fl uorinated PCHD 15 400 1.12 71 229
HA hydrogenated PCHD – – 100 204
HB hydrogenated PCHD – – 95 228
C PI (high 1,4-) 76 800 1.09 – − 61a )
FC fl uorinated PI 99 500 1.27 98 43 a)
a) Literature value. [ 6 ]
3.1. Pyrolysis of PCHD
more detailed studies found that the low temperature
W eight losses (decompositions, under N ) over three dif- weight loss was attributed to degradation at the PCHD
2
ferent temperature ranges were observed for PCHD sam- chain end.[ 12 ] The mechanism of this unzipping of the
ples. The fi rst weight loss of around 10 wt% was observed chain ends is not clear. Thermal treatment of PCHD in
between 75–125 ° C for 1,4-PCHD with high-1,4-microstruc- bulk at 200 °C under argon eliminated this weight loss on
ture ( ≈9 5%). Continued heating led to a second weight loss subsequent reheating,[ 12 ] whereas thermal treatment in
starting at 328 °C , which reached a maximum decompo- benzene with inhibitor (2,6-di-t ert -butyl-4-methylphenol,
sition rate at 350 °C . A third temperature of maximum ≈0 .067 wt%) at 180 °C under argon in a Parr reactor, fol-
decomposition rate was observed at 440 °C , which resulted lowed by precipitation, did not eliminate this weight
in complete decomposition. The microstructure of PCHD loss.
did not signifi cantly affect the pyrolysis behavior. The I sothermal pyrolysis of PCHD with high 1,4-microstruc-
linear sample with about 50% 1,4-microstructure showed ture at the second maximum decomposition tempera-
similar weight loss patterns to the high 1,4-PCHD, but the ture (350 ° C) led to a yellowish solid material. The total
temperatures of maximum decomposition rate are higher weight loss was around 65%–70% after either isothermal
(365 and 460 °C , respectively) (Figure 1 ) . The decomposi- treatment at 350 ° C for 25 min or at 395 ° C for 20.5 min
tion profi les match previously reported data very well. [ 11 ] (Figure 2 ). The weight of the residue remained constant
T he weight loss at low temperature was initially after pyrolysis, which indicates that a thermally stable
believed to be caused by release of solvent that is “trapped material was formed. The residue was not soluble in
in” the condensed polymer during polymer recovery common solvents.
process after polymerization because it was located in The structure of the residue was analyzed via elemental
the vicinity of the glass transition of PCHD and the tem- analysis and FT-IR. Compared with its linear precursor,
perature seems to be too low for chain scission. However, the residue has similar carbon and hydrogen contents
400
100 b
100
350
a
80 80 300 C)
Weight (%) 4600 a b Weight (%) 4600 122505000omperature (
e
20 T
100
20
0 50
100 200 300 400 500 600 0 10 20 30 40 50 60
Temperature (oC) Time (min)
Figure 1. T GA curves for linear PCHD: (a) high-1,4-PCHD and Figure 2. I sothermal pyrolysis of linear PCHD at 350 °C for 25 min
(b) 50/50 1,4/1,2-PCHD. (a, dotted line) and at 395 °C for 20.5 min (b, solid line).
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Macromolecular
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www.mcp-journal.de
Table 2. T he elemental analysis results and empirical formula of samples before and after pyrolysis.
Sample Elemental analysis Recovered weight [%] Empirical formula
C [%] H [%] F [%]
A 89.18 9.44 – 0 C H a)
6 7.6
A-py@ 395 89.36 9.51 – 30–35 C H
6 7.6
FA 65.72 6.32 27.94 0 C H F b)
7 8.1 1.9
FA-py@395 80.99 6.30 13.00 15–20 C H F
7 6.5 0.7
FA-py@600 94.24 3.80 0.34 12–16 C H F
7 3.4 0.0
FC 60.20 6.25 35.33 0 C H F c)
6 7.3 2.2
FC-py@340 78.61 7.33 15.78 70–75 C H F
6 6.7 0.8
FC-py@600 91.32 3.37 1.08 20–23 C H F
6 2.6 0.0
a)T heoretical formula for linear PCHD is C H ; b )T heoretical formula for fl uorinated PCHD is C H F ; c )T heoretical formula for fl uorinated
6 8 7 8 2
polyisoprene is C H F .
6 8 2
and empirical formula (Table 2 , A: precursor; A-py@395: All the characterization data lead us to conclude that
residue) but exhibited a different FT-IR spectrum the thermal pyrolysis of PCHD involves two kinds of
(Figure 3 ). The typical signals after pyrolysis at 395 ° C decomposition mechanisms. One mechanism is chain
for 20.5 min, such as the C (cid:2)H stretching (3018 cm − 1 ), scission to lose volatile materials[ 11 ] (monomer, dimer,
the C (cid:2)H in-plane deformation (1312 cm − 1) and out- and oligomers) that were carried out by N fl ow during
2
plane deformation (974 cm − 1) from the olefi n groups heating; and the other mechanism is crosslinking caused
( (cid:2) CH (cid:3) CH (cid:2) ) in PCHD, and the breathing of cyclohexene- by addition to double bonds or coupling of free radicals
like ring systems (1069 cm − 1) decreased or disappeared; generated by homolytic bond cleavage. Chain crosslinking
while C (cid:2) H stretching (2922 and 2858 cm − 1 ) and deforma- results in a three-dimensional structure as indicated by
tion (1445 cm − 1 ) from (cid:2) CH (cid:2) were observed in all spectra. the higher T of the residue and insolubility in common
2 g
The changes in peak profi le of typical C (cid:3) C stretching solvent, leading to enhanced thermal stability as well as
(1646 cm − 1 ) suggested the emergence of different types retarded decomposition. The competition between chain
of carbon–carbon double bonds, such as unconjugated, scission and crosslinking is related to the conversion,
conjugated, and aromatic, after pyrolysis. A higher T of thickness of the sample, and the fl ow rate of nitrogen.
g
152 °C after pyrolysis as compared with one of 120 ° C for Chain rearrangement also occurred during this isothermal
the precursor indicates the more limited mobility of chain pyrolysis based on changes in the FT-IR spectra.
segments of the former polymers.
3 .2. Pyrolysis of Hydrogenated PCHD
Crosslinking during thermal pyrolysis of PCHD is mainly
attributed to the presence of carbon–carbon double bonds
and the generation of radicals. This assumption was veri-
fi ed by thermal pyrolysis of hydrogenated PCHD, in which
the double bonds were saturated by hydrogenation. Linear
PCHDs with different microstructures were hydrogenated
using TSH in toluene and the degree of hydrogenation
b
was characterized via 1 H NMR (Table 1 ) . DSC characteriza-
tion shows that hydrogenation leads to a much higher T
g
(Table 1 , HA and HB).
A s expected, hydrogenation enhanced the thermal sta-
a
bility of PCHD, which led to an elevated onset tempera-
ture of decomposition. The TGA curves of two hydrogen-
ated PCHD samples with different microstructures are
4000 3200 2400 1600 800
cm-1 shown in Figure 4 . Hydrogenated linear PCHD with 95%
1,4-microstructure started to decompose at 440 °C and the
sample completely decomposed over a narrow tempera-
Figure 3. F TIR spectra of linear PCHD (a) before and (b) after
pyrolysis at 395 °C for 20.5 min. ture range with maximum decomposition rate at 445 ° C.
Macromol. Chem. Phys. 2012, 213, 49−56
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Macromolecular
Thermal Stability of Fluorinated Polydienes Synthesized by . . . Chemistry and Physics
www.mcp-journal.de
100 100
80 80
%) %)
60 60
ht ( b a ht (
eig 40 eig 40
W W
a
20 20
b
0 0
100 200 300 400 500 600
100 200 300 400 500 600
Temperature (oC) Temperature (oC)
Figure 4. T GA curves of hydrogenated PCHD from precursors of Figure 5 . TGA curves of fl uorinated PCHD from precursors of high
high-1,4-PCHD (a, solid line) and 50/50 1,4/1,2-PCHD (b, dotted 1,4-PCHD (a, solid line) and 50/50 1,4/1,2-PCHD (b, dotted line).
line).
The weight loss at low temperature (75–125 ° C) observed decomposition occurred around 600 °C . The TGA trace
in PCHD was not observed after hydrogenation. The also shows two stages of weight loss, which suggests that
hydrogenated PCHD with 50% 1,4-microstructure started the thermal decomposition of fl uorinated PCHD pyro-
to decompose at 400 ° C and the temperature of maximum lyzed under air fl ow proceeds through a similar process to
decomposition was 470 ° C. The lack of two stages of pyrol- pyrolysis under a nitrogen fl ow.
ysis above 300 ° C is attributed to the absence of double Fluorinated PCHDs were isothermally pyrolyzed at
bonds in the hydrogenated PCHD, which resulted in only 395 ° C, which is the fi rst temperature of maximum
chain scission without crosslinking. decomposition rate under nitrogen (Figure 7 ). The weight
loss was around 80–85 wt% when the sample was held
isothermally at 395 °C for 20.5 min. The residual mate-
3.3. Pyrolysis of Fluorinated PCHD
rial was graphite-like and insoluble in common solvents.
Although fl uorination of linear PCHD with in situ generated The weight loss was larger than that for PCHD isother-
difl uorocarbene saturated the double bonds, the pyrolysis mally pyrolyzed at the same temperature and over the
of fl uorinated PCHD under an inert atmosphere still shows same time period. Although fl uorinated PCHD has a
decompositions over two different temperature ranges higher onset decomposition temperature (more thermally
(Figure 5 ) . The weight loss at low temperature (75–125 ° C)
observed in PCHD was eliminated by fl uorination. The
decomposition of fl uorinated PCHD started around 360 ° C,
100
and reached a maximum decomposition rate at 395 ° C,
which is 50 °C higher than for its PCHD precursor. Weight
80
loss continued above 400 °C and stopped around 500 ° C
yielding a shiny, graphite-like residues with a weight per- )
% 60
centage of 4.8 to 16.5% depending on the degree of fl uori- (
nation.[ 7 ] The microstructure of PCHD did not have much ht
g 40
infl uence on the thermal decomposition process (Figure 5 ). ei
W
For fl uorinated PCHD with 50% 1,4-microstructure, a sim-
ilar TGA curve was observed and the relatively low weight 20 a
percentage of residue was attributed to the relatively lower
b
0
degree of fl uorination (71 mol%) (Table 1 , FA, FB; Figure 5 ) .
O xygen lowered the thermal decomposition tempera-
150 300 450 600 750
ture and led to complete decomposition of fl uorinated
Temperature(oC)
PCHD. Figure 6 shows fl uorinated PCHD decomposed
under different atmospheres using the same heating
Figure 6 . TGA curves of fl uorinated PCHD from precursors of high
rate. The onset decomposition temperature of fl uorinated
1,4-PCHD under different atmosphere of nitrogen (a, solid line)
PCHD under air fl ow was around 310 oC and complete and air (b, dotted line).
Macromol. Chem. Phys. 2012, 213, 49−56
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Macromolecular
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www.mcp-journal.de
400 Elemental analysis and FT-IR characterization sug-
100
gest that not only crosslinking but also dehalogenation
350
occurred during the isothermal pyrolysis, which led to
80 300 C) an unsaturated carbon system with less fl uorine and
ht (%) 60 250oure ( hexytdernosgievne . dCeohnytidnruooguens aptyioronl yasnisd u dpe thoa 6lo0g0e °n Ca tciaouns eadn dm olerde
g 200 at
Wei 40 150 mper ttoru mm osrheo whisg hmlyu cahr osmtroantigzeerd Cm(cid:2) aH t esrtiraeltsc. hTinhge (F3T0-4IR4 csmpe − c 1- )
20 100 Te on unsaturated carbons, much stronger aromatized C (cid:2) H
out-of-plane deformation (745, 810, and 875 cm − 1 ), and
50
0 much stronger C(cid:3) C stretching of the aromatic type
0 10 20 30 40 50 60 (1601 cm − 1) together with non-conjugated type
Time (min) (1694 cm − 1) or even vinyl type (1906 cm − 1 ). Elemental
analysis shows the loss of most fl uorine atoms and more
Figure 7 . Isothermal pyrolysis of fl uorinated PCHD (395 ° C, hydrogen, which agrees with the FT-IR results very well
20.5 min).
(sample FA-py@600 in Table 2 ) .
T o eliminate the effect of carbon–carbon double bonds,
the remaining carbon–carbon double bonds in fl uori-
stable at low temperature), it tended to decompose com-
nated PCHD were saturated by hydrogenation with TSH
pletely after decomposition started. The reduced level
in toluene. 1 H NMR characterization proved the success
of carbon–carbon double bonds in fl uorinated PCHD
of this process. Hydrogenation did increase the onset
( ≈9 mol%, sample FA in Table 1 ) leads to a lower degree
decomposition temperature, but two stages of decom-
of crosslinking during isothermal pyrolysis; therefore, the
position pattern were still observed, as in fl uorinated
formation of a thermally stable, three-dimensional net-
PCHD. The hydrogen saturated fl uorinated PCHD has a
work is less likely.
lower decomposition onset decomposition temperature
DSC characterization did not show obvious glass tran-
as compared with hydrogenated PCHD, and the two-stage
sition type heat capacity changes for the isothermally
decomposition suggests that chain crosslinking occurred
pyrolyzed residue. Elemental analysis showed relatively
even though there were no carbon–carbon double bonds
higher ratios of carbon to hydrogen and carbon to fl uorine
left in this specifi c sample. The comparison of the pyrol-
(FA-py@395 in Table 2 ). The FT-IR spectra (Figure 8 ) shows
ysis behaviors of fl uorinated PCHD, hydrogenated PCHD,
the weakening of the C(cid:2) F stretching (1100–1350 cm − 1 )
hydrogenated and fl uorinated PCHD, leads to the conclu-
(Figure 8 b ), and the increasing C(cid:2) H stretching (3044 cm − 1 )
sion that the presence of a three-membered ring structure
on unsaturated carbons. The aromatic C(cid:3) C stretching
could trigger chain decomposition and cause crosslinking
(1601 cm − 1) and C (cid:2)H out-of-plane deformation (745, 810,
during pyrolysis by processes discussed below.
and 875 cm − 1) are clearly observed.
The high strain in the three-membered ring and the
polarized C(cid:2) F bonds made the C (cid:2) CF bonds in the ring
2
structure much weaker than other C(cid:2) C bonds. The homo-
lytic breakage of the C (cid:2) CF bonds generates two free rad-
2
icals, which can trigger a much more thorough decompo-
sition. The decomposition of fl uorinated PCHD consists of
c
chain scissions to generate volatile low-molecular-weight
materials (monomer, dimer, etc.), chain crosslinking, and
dehalogenation. A possible chain crosslinking mechanism
is shown in Scheme 2 .
b
3.4. Pyrolysis of Fluorinated Polyisoprene
a T he assumption of thermal cleavage of the three-membered
ring that triggers polymer decomposition was substanti-
4000 3500 3000 2500 2000 1500 1000 ated by the thermal decomposition study of fl uorinated
cm-1 polyisoprene synthesized by similar addition of i n situ
generated difl uorocarbenes. The TGA trace taken during
Figure 8 . FT-IR spectra of fl uorinated PCHD before (a) and after pyrolysis of polyisoprene under a nitrogen atmosphere
(b) pyrolysis (395 °C , 20.5 min) and (c) heated up to 600 ° C. with a heating rate of 10 ° C min − 1 shows a rapid, smooth
Macromol. Chem. Phys. 2012, 213, 49−56
54
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Macromolecular
Thermal Stability of Fluorinated Polydienes Synthesized by . . . Chemistry and Physics
www.mcp-journal.de
F F
F
n n
F F F F F
F F F F
n n n n
n n
F
Scheme 3 . The bond breakage of three-membered ring in fl uori-
F F F
n n nated polyisoprene.
F F
F F
F F F F The lower onset decomposition temperature of fl uori-
n n
nated polyisoprene is attributed to the weak C(cid:2) CF bond
2
Scheme 2 . Possible crosslinking mechanisms during thermal in the three-membered ring structure and the formation
pyrolysis of fl uorinated PCHD.
of relatively stable free radicals (Scheme 3 ) . These fl uorine
substituted free radicals are stabilized by the electronega-
weight loss curve beginning at 350 ° C with a maximum tive fl uorine atoms, and the tertiary carbon free radicals
decomposition rate at 380 °C (Figure 9 ) . Previous studies are stabilized by p - p orbital resonance. The high steric
found that the pyrolysis of polyisoprene under vacuum tension in the three-membered ring and the formation of
generates mostly isoprene monomers and dimers (such as relatively stable free radicals all thermodynamically favor
1-methyl-4-isopropenylcyclohexene, and its isomers) via a the bond breakage and trigger polymer decomposition.
mechanism of chain scission to form two free radicals, and The isothermal pyrolysis of fl uorinated polyisoprene
backbiting of the free radical involving one or two at 340 ° C for 26 min led to a brown, insoluble material
mono mer units. [ 13 , 14 ] The fl uorinated polyisoprene has a with 70–75 wt% of the starting mass (Figure 10 ). Below
lower onset temperature of decomposition at 325 ° C, and 200 °C , DSC characterization did not show an obvious
undergoes a two-stage decomposition (maximum decom- glass transition. The elemental analysis showed rela-
position temperatures of 340 and 450 ° C, respectively tively high carbon/hydrogen ratio and carbon/fl uorine
(Figure 9 ) . A graphite-like residue of 20 wt% was obtained ratios for the pyrolyzed residues (Sample FC-py@340 in
after pyrolysis up to 600 ° C. Oxygen lowered the onset tem- Table 2 ). Dehalogenation of fl uorinated polyisoprene in
perature of thermal decomposition to 300 °C and led to isothermal pyrolysis is less than that of fl uorinated PCHD
complete decomposition at 600 ° C. Three decomposition because of the lower temperature of isothermal pyrolysis
stages with temperatures of maximum decomposition rate (340 vs 395 ° C). Elemental analysis suggests that not only
at 320, 430, and 545 °C were observed. The fi rst two decom- crosslinking but also dehalogenation occurred during the
position stages with lower maximum decomposition tem- isothermal pyrolysis.
peratures are similar to those of fl uorinated polyisoprene The FT-IR spectrum of the isothermally pyro-
pyrolyzed under a nitrogen fl ow, and the third stage is the lyzed residues (Figure 11 b) shows the weakening of
complete oxidative decomposition of the residue that was the C (cid:2)F stretching (from 1100 to 1350 cm − 1) and the
formed during the fi rst two decomposition stages. increasing C (cid:2)H stretching on unsaturated carbons
(3040 cm − 1) and the typical (cid:2) C (cid:3) C (cid:2) olefi n stretching at
1700 (non-conjugated) and 1610 cm − 1 (conjugated or
100
80 350
100
300
60
%)
ght ( 40 c %) 90 250oe (C)
Wei 20 b Weight ( 80 125000 mperatur
0
a 100 Te
100 200 300 400 500 600
70 50
Temperature (oC)
0 10 20 30 40 50 60
Figure 9 . TGA curves of polyisoprene (a, solid line) and fl uorinated Time (min)
polyisoprene under a nitrogen fl ow (b, dotted line), and an air
fl ow (c, dashed line). Figure 10 . Isothermal pyrolysis of fl uorinated PI (340 °C , 26 min).
Macromol. Chem. Phys. 2012, 213, 49−56
55
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