Spectroscopic studies of carbons. XVII. Pyrolysis of polyvinylidene fluoride

Materiuls

Chemistry

SPECTROSCOPIC

M.L.

STUDIES

O'SHEA,

Department

Received

and Physics,

26 (1990)

OF CARBONS.

and

M.J.D.

of Chemistry,

New

York

7,

accepted

1990;

PYROLYSIS

XVII.

C. MORTERRA

May

193

193-205

OF POLYVINYLIDENE

FLUORIDE*

LOW

University,

June

25,

New

York,

NY 10003

(U.S.A.)

1990

ABSTRACT Fourier examine stream

chars at

obtained polymer

provide

by heating

as well

characteristic

onset

of

up of largely

showing

temperature polyvinyl

profile chloride,

the degradation

occurs

in a complex aliphatic

of the hydrogen-containing a spectrum

300

to

about

the

groups

quite

features

different and

from

fluoride

of PVDF as suggested

of

the

those

the

The

pyrolysis residue

results

in

>2000 cm-l and a high observed

possible

by the spectroscopic

of

The loss of most

previously

chars.

original

evidence

in excess of 400°C

at wavenumbers

in a N2 spectra

a pyrolytic

structures.

to

a high thermal

Increasing and

used

(PVDF)

and spectroscopic

process

at temperatures

was

infrared

destruction

300°C.

and fluoro-aromatic

no vibrational

bromide

near

The

PVDF exhibits

fluorides,

degradation

fluoride

7OOOC.

as its carbonization. of polyvinyl

spectroscopy

polyvinylidene

from

information

degradation

results

beam deflection pure

ranging

direct

structure

temperature made

produced

photothermal

temperatures

stability, the

transform

for

mechanisms

evidence

the for

is discussed

in detail. INTRODUCTION The following fluoride

(PVDF)

characterization The analysis

investigation

of chars formed

of these

polymers

*: Part XVI: see ref. 39 #: to whom inquiries should

0254-0584/90/$3.50

of the thermal

is one of a series

of studies

degradation

which

from the pyrolysis represents

a portion

of polyvinylidene

focus on the spectroscopic of polyvinyl

halides

of an ongoing

[l-3].

examination

be directed.

0 Elsevier Sequoia/Printed

in The Netherlands

194

of the formation of chars or 'carbons' from various solid precursors [e.g., 4-

81.

Unlike the natural materials studied in the past, the polyvinyl halides

and polyvinylidene halides pyrolysis result

contain no constitutional oxygen and their vacuum

in chars free of any oxygen-containing functional groups.

The spectroscopic data obtained

from their pyrolyses has allowed

establish better the effect of oxygen-containing

us to

groups on the spectral

features characteristic of many high temperature chars [l].

Equivalently, the

polyvinylidene halides produce chars which, relative to the polyvinyl halide chars, are 'hydrogen-poor";through a comparison of the spectra obtained from the pyrolysis of the vinyl and vinylidene halides it is now possible to examine the effect of hydro~n-containing groups on the spectral features formed with the higher temperature chars. Besides providing information on the formation of chars, the direct infrared (IR)

spectroscopic examination of

the solid degradation products

obtained from the pyrolysis of PVDF over a wide temperature range should provide additional insight into the degradation process of

this

polymer.

Studies which have addressed the degradation of PVDF have, in general, relied upon indirect methods of analysis, such as weight loss data or the emission of volatile products in order to deduce the decom~sition reactions occurring within the solid itself.

Few IR studies of the solid itself have

been undertaken, and their emphasis has been only the determination of the very onset of degradation. It is hoped that the detailed spectroscopic description of the entire degradation process, up to the formation of a char, will greatly supplement these analyses.

EXPERINENTAL The spectra were recorded using IR Fourier transform Photothermal Beam Oeflection spectroscopy (PBD).

The PBD technique as wall as the apparatus has

been previously described in detail [4, g-101. Polysciences, Inc. (Warrington, PA);

the

The PVDF was supplied by

IR spectrum of the room temperature

polymer was that expected of pure PVDF. The PED spectrometer produces only single-beam spectra, S, which, unless noted, are compensated by ratioing against

the spectrum, So, of a standard

absorber (a high temperature carbon or platinum black) to produce double-beam spectra, S/S,.

pseudo

In the labeling of the figures the various samples

and spectra are identified by the abbreviation PVDF followed by a number which indicates the temperature in OC at which the virgin polymer was heated for a period of 2 hours. Degradation at temperatures less than 39D°C produced a pyrolyzate which was hard and solid and which could not be ground and/or packed into the sample

195

holder

in

the

pyrolyzates source

such

Spectra

manner

that

has

been

that

the

obtained

laser

probe

in this manner

grazed were

S/N ratio to the same-temperature It was discovered subject two

to pyrolysis

very

during

strong

bands

believed

that

etching

of the quartz

The

became

small

which the

amount

has decreased

of PVDF tube,

vibrations

[l-3].

through

heating

and

spectra

of

occurred

which

a flow

of

the

to

pyrolyzates

was also found

was

in a platinum

placed

period

of one hour.

during

the heating

with

the

initial

the oxidation

lL/min.

exposure

showed

that

point

Si-0

the

bands

this

little

first

ratio

of

to become

contamination

by

for the case

an open-ended

passed

air.

the

product,

specifically

where

the

within was

to

from

volatile

used was modified

N2 at

vibrations.

resulted

process,

To prevent

pyrolyzed

chars,

displayed

on the pyrolyzate

to enable

previously

cell,

during

platinum

the

sample's

IR

Subsequently-recorded

or

no

oxidation

of

the

char

of pyrolysis.

When it was desirable environ~nt

the burn-off

was

prevent

using this method

deposited

the oxidation

IR spectrum.

PVDF

cooling

were

during

process

virgin

stretching

primary

and

halides.

to Si-0

bands

IR

pyrolyzate.

of PVDF

vacuum

silica

these the

strength

a quartz

boat by PVDF's

sufficiently

of the

of the

that the spectra

these

at or near

pyrolysis

The

of

For under

in signal

chars of the polyvinyl

within

which

[4].

and positioned

similar

attributable

observable

of the char

the vacuum

were

of silica

char

the dominant

powder

presence

upon oxidation

silica,

described

cross-section

experimentation

cell and sample

spectroscopically

to silica

the

otherwise

and later oxidation

It was

HF.

previously

the sample was cut in half, width-wise,

to induce

oxidation

to be necessary. boat

and

Oxidation process.

charring

then

For this procedure

heated

of the char A77

spectra

temperature

of the PVDF char, a silica-free

within

resulted

from

sample

furnace

for

it's exposure

of the oxidized

appended

the char

a steel

samples

a

to air

are

labeled

by an OXT,

where

T designates

have

reported

in

The

which

temperature,

THE IR SPECTRA OF PVDF Several literature

normal

have observed those 151. this

coordinate

in the room

calculated

disorder,

that

of these PVDF,

commercial

[16].

spectrum

bands, are PVDF

(H-H) manner, The weak

of PVDF

based

can

bands

in

have

1453,

sample chain

I.

agree

cited

It has

a substantial

amount

incorporated

to the normal

1330,

and

678

bands

the

well

of

for been

chain

into the chain

head-to-tail

cm-l

[13-

above also

we

with

conformation

literature

Table

units being

in contrast at

TGTG

on the

presented

been

[ll-151.

of our

for the non-planar

with up to 10% of the monomer

in an head-to-head sequencing

of

forms

temperature

and observed

The assignments conformation

established

calculations

for the two crystalline

resulting

(H-T) from

196

these T-T and H-H isomers are, in fact, evident in our room temperature spectrum and are also listed in Table I.

Table I. The IR Spectrum of PVDF. cm-1

Assignment

3024 s

[13,15J

1153 m

Vs(CC)

[13,151

2981 s

Va(CH2) W CH,)

[13,153

1072 m

1450 w

E,f CH2CH2)

[13,161

Lla(CC) Vs(CC)

['"~:~~

1429 sh

b(CH,f

976 w [I3,151

t(CP2)

[13-151

1408 s

[13,151

945 w

WCH2)

t(CH2)

[I51

1384 s

N(CH2)

a75 s

[13,151

Va(CC)

[13,151

794 m

r(CH2)

[13-151

763 s

'i(CF2) w(CF2)

[I31 [I51

1330 w 1292 sh

Ref.

[I31

W(CH2CH2)

cm-1

Assignment

Ref.

[13,151

1219 s

Vs(CF2) Va(CF2f

[13,15]

670 m

tiCF2CF2)

[I51

1188 s

Us( CC)

Cl53 c13,151

617 m

w(CF2) 6(CF2)

Ki;]

Js( CF2)

S = Strong, m = medium, sh = shoulder, w = weak

RESULTSAWDDI!XlJSSIDtt Loss of original structure The zipping mechanism which is believed to characterize degradation in many of the

the onset of

polyvinyl and polyvinylidenes is illustrated in

Scheme 1 for the vinylidene system, where the "X" represents a halogen atom [17-231.

schem? 1 -(CH2CX2)m-CH2-CX2----> -(CH2CX2)m,b(CH=CX)n-CH2-CX2 + nHX For PVDF this reaction would result in the evolution of HF and, as in other polyvinyls, the formation of a conjugated chain. Spectroscopic evidence of the destruction of the original

polymer structure and the formation

conjugated system should be observable in the lowtem~rature

of the

chars of PVDF if,

in fact the polymer degrades primarily through the mechanism of Scheme 1.

197

The fingerprint spectroscopically temperature

of

pyrolyzate

The

region

similar 300°,

significant

signal

a change

which

cm-1 region of the spectrum

pyrolyzate

changes

CF2-related

bands

addition,

the loss of fluorine

occur

notable

1219,

at

in

at

T<300°C

is

but, by a charring

the

spectrum

of

the

In the 2000-500

structure.

change

763 and

along the polymer

of the CH2 deformation

obtained precursor

in the polymer's

the most

the main

the intensity

PVDF

to it's room temperature

is a loss of intensity

in

cm-'

In

613

(Fig.

chain results

band at 976 cm"

1, A-B).

in a decrease

in

and a loss of resolution

of the CH2 bands at 1384, and 1423 cm". At a pyrolysis a further region. are

loss

those

in

the at

higher

original

C-C

polymer

of the PVDF char exhibits

bands

in the

remaining

1400-600

at this

stretching

latter 875

is probably

v,(CF)

band

cm"

from

to

its

one

due to the

band

to

which

PVDF700

10

PVDF650

loss

it

was

coupled.

PVDFGOO

The persistence deformation

band

the presence

of a broad

at around

of abundant

the 300°C chars bending

cm-l

temperature

1072 and 883 cm-' (Fig. 1,

position

wavenumbers of an

with

spectrum

fluorine-sensitve

bands of the original

1189,

shift

original

of 325' the

original

associated at

The

*

the

Prominent

vibrations C)

temperature

of

1400

strong

cm-l

saturated

CH, PVDF550 PVDF525

indicates

CH groups

in

PVDFJOO

(Fig. 1, B-E) and, in fact, this

vibration

CH stretching

and

is paralleled

by bands

in the PVDF450

region.

PVDF400 The CH stretching region

the

structure

3024 and

saturated

2854

CH2

(Fig.

chars

2958 and

band

displays

typical

and

which

synmmtric

groups

2897

charring

wavenumber

cm"

cm-l

TW

less

are

also

temperature,

and

a

side. of

broadening

those

of

to

at

it's

PVDF300

bands

2854

the cm"

PVDF

high

of these

saturated

of

a fluorine

evident the

PVDF325

of

resp.,

intense

on

The frequencies

of bands

indicative

stretch,

nonadjacent

PVDF350

in

CH2 bands at

growth

are

PVDF390

polymer

is evident

of the original

2, B) C241.

3000

are

original

2981 cm-l, and in the

the asymmetric

at

In the CH stretching

the

300-350°C

decline

2924 and

atom

region of

in the

the gradual

at

loss

bands

cyclic

*'

Fig.

0

1560

lob0

1 cm'

1. Spectra of PVDF

198 hydrocarbons cm -' and

[24-261;

the

assigned

while

assigned

the

band

A further

evident

decrease

original

The

into

vCH3 stretching

the

325'C at

by a temperature in intensity, 3016

of the

3016 cm"

on its high wavenumber

PVDF325

of

PVDF300

the

3020 cm-l a decrease

0 cm" __ Frg. 2. Spectra of PVDF The.C_H Reqion.

charring

in

band

PVDF

and a shift

the a

further

and

side near

z!

now

The original

undergoes

results

PVDF350

2,

become

absorption

broadening

350°

is

have

Increasing to

broadening

325'C

is

(Fig.

band of PVDF at

of

PVDF390 of the

cm-I

to

an overall

cm".

temperature

char

2981

broad

PVDF400

be

bands

band at 2958 cm-l.

asyimnetric stretching

can

groups.

stretching

sufficiently

incorporated

cm-l

of CH

band

PVDF450

be

of CH3 groups,

2897

the

2958

can

in the intensity

of

original

at

cm-l

modes

CH

in spectra

decreased

to

2854

at

to the stretching

polymer's

-

absorption

near

to the v, and vs

resp.,

C)

the

shoulder

a buildup

3060 cm-l (Fig.

2, D). From our observations polyvinyl observed

bromide

(PVBr) and polyvinyl

in the 3100-3000

unsaturated

CH groups

In particular, presence

on the degradation

cm"

at these

temperatures,

structures,

part

va(CH2)

intensity

of the

band of the original

original 350°C

of the

absorption

char

must

buildup

of the initial

and/or

at

to

indicate the

formation

an aromatic

of

structure.

a band at 3050 cm-l implies

3016

still

the

present

(Fig. 2, D) [l-3,24-26].

cm-l

(at 3024 cm"),

would

primarily

(PVF) [l-3], the slight

CH absorption

band

polymer

at 2981 cm"

be due

(PVC),

while the broad absorption

at 3016 cm-' is in the region of alkenic While

chloride

is indicative

with an alkenic

charring

of some aromatic

fluoride

region

associated

of the polyvinyl

may

result

the absence

from

the

of the other

that the 3016 cm-l band in the

CH absorption

of

an

unsaturated

species.

The formation Although

of an olefinic the

elimination

proposed

to

[20,21],

spectroscopic

which,

in fact,

was observed the

lead

monomer

structure

to the

of

formation

evidence

establishes

the

in the CH saturated unit

fluorine

results

in

at

of a polyenic

suggests

that

predominant region, a

temperatures sequence

this

structure

325-350°C

as per

mechanism

containing

of the first mainly

is

Scheme

is not

the

of the pyrolyzate.

the elimination

pyrolyzate

of

1 one As

HF from

saturated

199

The

structures. aliphatic exceed

persistence

groups that

structure

at

at

which

is first

zipping

mechanism

extent

and

is

the

formed

these

occurs

and

which

polyenic

imply

that

the

PVDF400

only to a limited

accompanied

crosslinking

of

temperatures

by

widespread

branching

of

the

PVDF390

pyrolyzate. Evidence

for the initial

double-bonded can

structures

normally

cm -' range

be

chars

the

CH

polymeric

the

formation,

obtained

the

the C=C stretching While polyenic

Cftl -l,

with

formation

during

in which

to CF=CH

PVDF fitms,

polyenic

I

1700

Fig. 3.

1600

1500

Segments

cm-’

of spectra.

must

region

an

increase

the

of

bands

of

of electronegative

in the

initial

bands

the

at

stretching

stages

[29].

dehydrohalogenation

to the stretching in this

region,

in the 1600-1700

given

1620 and

C=C

in the

new

bands

degraded

bonds

CF=CF

T-T

structures

defects, units.

is clear

that

while Despite there

the

and

IR

1600

[211.

A the

band

was

C=C bonds

in

dehydrofluorinated The former,

arising the

cm-l

reported

1710 cm-l

bands to conjugated in chemically

of the CF=CH

1715

in vacuum the

a

An early

at

C=C

along

stretching

1700-1600

at 1710 and 1613 cm-l [30,31].

of CF=CH it

observed

1595 cm-l;

recent7y,

to and

groups

the

over the assignment

to conjugated

and the 1600 cm" More

of

of dehyrof~uorination.

was thermally

1710,

assigned of H-H

been

frequency

in air showed

former

two bands were observed was

have

thermally

the polymer

structure

band

attributed

of

evidence

Several

assignment

an aromatic

assignments

r

BOO

there is still some disagreement

of three

assigned

‘VDF

in order

spectral

of PVDF degraded

later study,

the bands

the band

the original

signs

causes

band(s).

of PVDF

weaker

to

it is known that the substitution

[24-281,

analysis

WDF310 ‘VDF300

in

bands.

sequence

stretching region

due

Therefore,

additional

from

PVDF 330

in

~nitored

with

first

PVDF 340

CH groups

unsaturated

region

structure.

to determine

mode

of

PVDF350

char

However,,

easily

associated

of

3000-3100

of the strong CH2 stretching

at 3020 cm-l

be

be

stretching

presence

chain

onset

cannot

the

the

absorb.

the

sequences

in

unsaturated

characteristically PVDF

within

sought

at which

formation

1613

cm-l

variation

is general

CIII'~region are due to the stretching

from band

the was

in exact

agreement

that

of C=C bonds.

200

In our sample of thermally degraded PVDF, a very weak band at 1710 cm-l is observable as early as 310°C, while at 330°C two new bands form at 1620 and 1597 cm-l (Fig. 3, D)

This latter doublet keeps growing and dominates the

region at temperatures in excess of 350°C, whereas at 390°C the 1710 cm-l band is no longer resolvable (Fig. 3, 6). As the growth of the doublet at 1600 cm" parallels the growth of aromatic structures, as suggested by the features observed in other regions of the spectra, we assign these bands to conjugated C=C bonds in an aromatic configuration. This doublet is similar both In it's initial formation and in it's growth to the 1600 cm"

band observed in the

chars of polyvinyl halides [l-3]. As for the 1710 cm-l band:

it is the first new band observable in the C=C

stretching region, and for this reason he assign it to CF=CH stretching of alkenic structures, and later to that of polyenic structures.

In the brevity

of it's appearance and it's overall weak intensity it is analogous to the 16301620 cm-I band which we have observed earlier in the char of PVF [3]. shift to a higher frequency

of

1700 cm-l is consistent

It's

with the C=C

assignments of fluorine-substitutedethlyene groups [25,26]. In addition to CF=C stretching vibrations, absorption in the region could

1700 cm-I

also result from the carbonyl stretching vibrations of oxygen-

containing impurities on the char's surface. 1710 cm-l region has previously

A weak absorption in the 1700-

been observed

with the chars of other

polyvinyls, especially with chars which contain a significant number of polyenic sequences which are particularly susceptible to oxidative attack. However, in oxidative studies of PVDF chars we have observed no significant increases in the 1710 cm"

absorption upon prolonged exposure to air at room

temperature and even at slightly elevated temperatures. These observations are consistent with other studies which have also reported no significant signs of oxidation in chars of PVDF left exposed to air for several days C321. The most intense bands associated with an olefinic structure are normally those associated with the CH out-of-plane bending vibrations. pyrolyzed in the 300-350°C range a band at 845 cm"

In PVDF chars

forms, grows, and

eventually declines in parallel to the C=C bands at 1710 and at 3020 cm" 1, B-F).

(Fig.

On the basis of this behavior and the bands position in the region of

CH wagging [25,27], we assign the 845 cm-' band to the out-of-plane wagging vibration of lone CH groups on a trisubstituted

olefin.

At the higher

charring temperatures, when there is no other spectral evidence supporting the persistence of olefinic structures within the char, the creases in intensity

until, at temperatures

incorporated into the broad absorption at 870 cm"

band at 844 cm"

in excess of 45OOC. (Fig. 1, F-I).

de-

it is

The formation While

of aromatic

spectral

evidence

crosslinking,

branching,

temperatures

of

suggests

and

400-600°C

polyaromatization assumes

rings and a polyaromatic the initial

polyene

precursor,

degradation

formation,

results

in

of the pyrolyzate.

a polyenic

structure

the

increasing

and

cyclization

below

involves

degradation

aromatization

One proposed

is illustrated

of PVDF

at

ultimately

mechanism,

which

[20].

v/-+Fw+"F Scheme

2

F

Contrary weight

loss

F

to the two distinct

mechanisms

studies

report

of

PVDF

[20,23,29,33,341.

A one-step

stability

like that of PVF,

of PVDF,

in which

the elimination

crosslinking the

and/or

second

fluorine

spectroscopic two fluorine between

atoms

them,

formation

presented

may well

the

first

and the second

temperature

(Fig.

bands

2, E).

By

very weak

band

the

of

possibly

shifted

a result

of

temperature affected

polymer

and

of

hydrogen

conjugated weak

region

is evident,

and

can

from chains (Fig.

CH

be

crosslinking

spectrum

and

stretching

olefinic

some

The

of

Yet, of

the gap

double-bond

and

chars

of

to

of any hydrogen

of

atoms

chars

absence

300-450°C

of of

spectra

of clarity

prominent

CH

[l-3] as

obtained

rings

the

fewer or

and

of

number no

at

are

of

pyrolysis the loss

in this

of PVF chars

are

structure

crosslinking

the CH stretching

the

original

of 450°C,

structure

groups,

polymer,

in

little

cm-l

absorption,

observed

vinylidene

band

3020

characteristics

and by a temperature

condensation

cm-'

aromatic

The in

at a

saturated

at

is the

than normally

results

region

For the purpose

3100

themselves.

residue

band

spectrum,

spectral

in fact

obtained

of the abundant

assigned

The

number

char

and ill-defined

the

the

frequency

in a complete

2, F-G).

the

CH stretching

of 400°C

atoms.

of 400°C;

increasing

of

of a broad

as is the decline

higher

pyrolysis

results

Fig. 2A for comparison.

elimination

confidently

of PVDF,

in excess the

cyclization.

the

to the rest

fluorine

chars

the

much

and

temperature

in the

in

temperatures

In the

subsequently

apparent

crosslinking

the formation

and of the weak

in the

of

with

with no real temperature

decreased

atoms

via

the elimination

together

involving

by the

hydrogen

process unit

processes

to a slightly

higher

degradation

from the monomer

be separate

mainly

of 390°C

substituted

greatly

atom

thermal

that

in relation

region

in a complex

process high

indicates

a charring

although

that the

(poly)aromatization.

in the 3100 cm-I region CH stretching

occurs

degradation

suggest

fluorine

1 and 2, most

below

The CH stretching region. pyrolysis

results

formation

in Schemes

one-step

would

via increasing

atom

evidence

a

degradation

of the first

polyene

described

of

already shown

in

region

of

202

PVDF has not been plotted so as to reflect it's actual relative intensity with respect to

the

pyrolyzates of PVF; the intensity of the CH stretching bands in the 390-450°C chars of PVDF are approximately

1/20th of that observed for

Pi ref

the same-temperature chars of PVF. The increasing

aromatization

can be

PVDF700

observed in the 4000-500 cm-I region of the PVDF650

single beam spectra (Fig. 4), as polyaromatization of the residue char brings about the

PVDFGOO

buildup of an extensive continuum in this region In PVDF

14951.

the

continuum

PVDF550

is first

observable in the spectrum of the 390°C char (Fig. 4, E). absorption

The presence implies

the

of this

formation

PVDF525

broad of

a

PVDFSOO 0

polyaromatic network [l-51. A further increase in the

charring

temperature

results

PVDF450

in an

PVDMOO

increase in the continuum absorption, until at a pyrolysis temperature of 700°C the single beam

PVDF390

spectrum closely resembles that of a standard

PVDF350

black body absorber such

as

carbon

black

(Fig. PVGF325

4, M, f-0. The finqerprint

reqion.

region of the spectra of

PVDF300

In the fingerprint PVDF char pyrolyzed at

PVDF

390°C one can observe a significant increase in the C=C stretching bands at 1620 and 1597 cm" (Fig. 1, D-E).

A doublet in the aromatic C-C

stretching region can sometimes be resolved with

I

4

IO

Fig. 4.

-

I

2000

cm-’

Single-beam spectra.

substituted benzenes [25], and in the 390°C spectra the doublet can be assigned to fluorinesubstituted C=C groups in an aromatic configuration [25,28,29]. The 1600 cm" bands are first observable at 325'C and are still evident as a single band with the

650°C char; however, they

reach their peak intensity around 4OO'C. Above

500°C the doublet is no longer resolvable (Fig. 1, I); this indicates the increasing polyaromatization and subsequent dehalogenation of the aromatic clusters, and it is also consistent with the broadening which is observed in the bands of all higher temperature

chars [4].

Increasing degradation

temperatures bring about a further broadening of the 1600 cm" absorption and a gradual decrease in it's intensity (Fig. 1, I-K).

Although in polyaromatic

chars the C=C ring stretching band is theoretically IR-inactive, the presence

203 of peripheral

groups

on the

polyaromatic

With the 390°C char several Most notable

region. on

the

broad

growing

at

1130

Although

both

stretching

cm-l

bands

within the narrower attached is

in

directly

close

substituted with

the

single could

are

1270-1100

with

vibrations

the

cm"

ring

of mixed

for mono- and n&a-substituted

at 883 cm"

(Fig. 1, t).

of the C-C stretching OC or slightly the

similar

1072 cm-l.

range,

absorbing

disubstituted

the other suggests

CH2

at

[24,25,27];

of the

C-C

in addition

wagging

883

cm-'

this

of

structure

400-450°C

spectra

for

would

above,

875-843

cm"

cm-'

range,

shifts

regions

of the

some

atoms while

naphthalene

Other changes intensity

associated

hydrogen

presence

600°C (Fig. 1, J-K).

an

at

1188

and

in the

be in

to the

part

=CH2

likely

due

groups

of

on

characterize

the abundant

a

the

However,

evidence

aromatic

in

structures formed

from

may be due to the weak C-

isolated

H-atom

benzene

a condensed

the absorptions

on

an

typically

aromatic

aromatic absorb

in

such

as

compound

are found in the 905-867

cm-l and

[25].

in the 400-600°C CH out-of-plane

20 cm-l

with

on a substituted with

bands

at 880 cm-l, and apparently

deformation

900-860

of 390

at temperatures

terminal

H out-of-plane

penta-substituted

bending

the zipping mechanism.

and the

also

of

of the two bands at 883 and 844 cm",

the

coincides

ring

from the frequency

stretching

may

vibration

that the broad band persisting

Isolated

cm-I

and

meta-

of

and, at a temperature

the coalescence

network.

atoms

[25].

to it,

chain formed during

temperatures

regions

1006

stretching

polymer

of the other

and/or

end groups of the polyenic at degradation

is

the 1130 cm-I band vibration

at

C-F

cm-I

with the 390°C char of PVOF is the absorption

the absorption

olefin

1130

it may still be due at least in part to this vibration,

persistence

325-390°C

band

CH

in which

at

for fluorine

This band is shifted only slightly

Alternatively,

strongly

the

the

structure.

region band

fluoro-benzenes

band of the original

greater,

cm-I

stretching

ring

The

in

aromatic

In addition,

C-F

1, E)

cm-l.

that the two bands

range reported

Similarly,

l-351.

frequency

an

cm-I

(Fig.

900

(both

frequency

[25,28]. cm"

to

suggests

1400-1000

higher

1117

asymmetry 1,361.

1500-500

1006 cm-1 1500

with

cm-l stretching

A band which is also evident

given

broad

in the

temperature

continuum)

the

the

fluorobenzenes 1000

this

from

associated

found,

to an aromatic

agreement

by

beam be

lie within

frequencies

sufficient

at 1130 and

stretching

evident

and the

1006

located

absorptions

network

region and

of

impart

of the 1600 cm-' band [cf 2

new bands are observable

are two bands

complex

aromatic

stretching

clusters

to insure the IR activity

to the structure

higher Weakly

chars

include

deformation

to 890 cm-l absorbing

890 cm-' band are too low in intensity

the gradual

band

near

at a degradation

870

decrease cm",

temperatures

bands on the low wavenumber and too inconsistent

in the

which of

also 550-

side of the

in frequency

to be

204

definitively assigned to other CH deformation out-ofplane deformation modes; yet, from a comparison of this region to that in the polyvinyl chars it can be seen that the overall shape of this region is quite different from those previously observed (Fig. 5 A-E). The CF stretching band at 1130 cm" maximum intensity at around

reaches a

450°C and begins

to

PVW800 g

PVDF550

decline upon increasing degradation; at degradation temperatures

in excess of

resolvable (Fig. 1, E-J).

plane deformation mode near 1000 cm" lost and/or

incorporated

PVF575

550°C it is no longer The corresponding CH in-

PVC585

is similarly

into the broad

buildup

centered around 1200-1300 cm". Increasing the degradation temperature to above 400°C also

results

in the eventual

loss of

the

saturated CH deformation buildup centered around 1450 cm -' (Fig.

1, F-I).

I( 10

PVB575 I 800 670cm'

Fig. 5. The Aromatic C-H deformation

region.

The decrease in the intensity of

this band at temperatures

in excess of 450 OC is

paralleled with the loss of structure observed in the CH stretching region (Fig. 2, 6). The broad absorption centered around 1300 cm-l and the band at a frequency of 1600 cm"

are the only vibrational structure observable in the spectra of

the high temperature chars.

This latter absorption is conznonto many of the

chars we have studied and in the past it has been ascribed to the residual activity of the quadrant ring stretching mode of the peripheral rings Cl, 361. Similarly, the absorption centered around 1300 cm-' has been attributed to a normally

inactive

lattice

mode

polyaromatization (as indicated by

1371.

At

these

advanced

stages

of

the strong continuum absorption in the

single-beam spectra of Fig. 4, G-L) it is now thought that the ring stretching modes, which are normally IR inactive, are made IR active by the presence of residual groups present on the peripheral rings of the polyaromatic network

C361. In the polyvinyl chars the residual groups which imparted the asymmetry necessary to insure IR activity were hydrogen containing; however, given the early loss of many of the bands ascribable to CH groups in the PVDF chars, it is possible that in PVDF some of this activity can be due to residual F atoms present on the outer rings.

This observation is consistent with the findings

of elemental analyses which report the presence of 5% by weight fluorine in a 600°C char [29,34]. The absence of the 1400 cm"

and 1200 cm"

absorptions in the 600°C chars

of PVDF results in an intermediate temperature char profile which is quite

different of

from

PVC,

oxygenated

and

or much

(Fig.

decreased

in-plane

observed

1450

stretching

modes,

ring the

the

700°C

hydrogen

spectra

not the

is

CH

1450 cm"

loss PVF620

ring PVC620

dependent in

evidence

of

been

observed

in

high

temperature

lack

PVB575

10 1560

to

residual

PVF = polyvinyl PVC = polyvinyl PVB = polyvinyl

normally band but

upon

lob0

bcm-'

’

Fig. 6. Chars of various in the fingerprint region

sufficient

1600 cm"

absorption

CELLULOSE 600

presence

charring,

of the

SARAN580

other

groups

to impart any activity

absorption

the

C-C

band

which

after

the

the

the

of

in with

normally

spectral

chars,

PVDFSOO

1200

all

that

has also

functionalities regain

of

of

Further

oxidation

band

absorption,

presence

and

the

semi-circular

relationship

chars;

of

of

absence,

However,

stretching

particular. this

The

is consistent

implies

chars

those

cellulose

or absence

cm-l with

upon

as

bands.

associated

this

such

the

as

6, A-F).

chars

hydrogen-related

of

well

intensity,

weakness

the

for

as

deformation

polyvinylidene

of

observed PVF

precursors

polycarbonate

cm -'

that

PVBr,

precursors

fluoride chloride bromide

oxidation

r31. The Oxidation

In order formation

of PVDF to

study

of oxidic

the

species

effect

of

during

a 500°C char of PVDF was undertaken from

the

oxidation

of

a

the

residual

high temperature

surface

oxidation,

and the resulting

similar-temperature

char

spectra

of

the

groups

the

on

the oxidation compared

polyvinyl

of

to those fluoride,

PVF. When

the

described

500°C

earlier,

Si-0 bands

These

bands

bands

the chars with the

oxidation

fluorine

similar

during

contaminant

PVDF,

in the

under

material

of

were observable

0x500-0x600). oxidized

char

the

same

pyrolysis

can only

PVF

still present

oxidized spectrum were

be due

chars

of

these

is

in the higher

This an

silica-free

a quartz

of the oxidized observed

chars,

to the reaction

vessel. and

also

in the within

Given the absence

conditions.

the oxidation of

pyrolyzed

was subsequently

indication

char

a 600°C

Fig. char

7, was

of any silica-containing

the

of

two strong

(cf.

appearance

of the residual

phenomenon

temperature

when

apparatus

cell,

of

was not observed the

amount

chars of PVDF.

these

fluorine

of

on

during reactive

When the same

206

Silica

3x600 0x550

0x200 in air

0x450

PVDFJOO

0x400 0x300 inpure0, 1400 PVDFSOO

1400

0

Fig. 8.

Fig. 7.

660 cm”

Oxidation

SOOcm*

Oxidation

in air.

in pure oxygen.

50C°C PVDF char is oxidized In air, in the Si-free apparatus described earlier, these bands do not appear and the spectral profiles of the oxidized char [Fig. 81 more closely resemble those of the oxidized PVF char [Fig. 91. However, in order to accurately compare the oxidation of PVF and PVDF chars for the purpose of examining the effect of residual hydrogen groups on the formation of surface oxidic species, it is necessary to carry out the oxidation of PVDF in an environment free of all outside sources of hydrogen. Therefore, the oxidation-in-air spectra of Fig. 8 could not be utilized due to the presence of H20 vapor in the air during

oxidation.

FigureslOandll

show

0x450 0x400

an enlargement of the carbonyl stretching region of Fig. 7 and the CH stretching region of the 5OO'C

PVDF

atmosphere;

char

oxidized

in

these

regions

are

a

pure

02

0x300

fortunately

unaffected by the silica bands near 1100 cm"

0x200

and 830 cm-l and allow direct comparison with the same frequency regions of the oxidized PVF

PM540

chars (Figs. 1OA and 1lA). The carbonyl stretching bands present in the oxidized chars of PVF are typical of those observed during

the oxidation

halide chars 1381.

of polyvinyl

At an oxidation temperature

2000 W Fig.

8OOcm9. Oxidation of PVF

207

0x450 0x450

0x400

0x400 0x300

0x300

0x200

0x200

PVDFSOO

PVF540

inpure OI

10 Fig.

.

10.

I’. Oxidation

3 cm-’ in pure

10

1550

Fig. 10 A. Oxidation of

oxygen.

PVF.

0x600 0x550 0x500

v

w

0x450

0x300

0x400 0x300

0x200

in pure 0,

------I

PVDFSOO

PVF540

35b0Ocmd Fig.

11.

Oxidation

in pure

Fig. 11 A. Oxidation of

oxygen.

PVF.

of 200 'C a band forms near 1700 cm-l that can be ascribed to the C=O stretch of simple aldehydic and/or ketonic oxidic groups

(Fig.

ItA).

At an oxidation

temperature of 300°C three other bands are present at 1850, 1774 and 1745 cm'l which can be assigned to the C=O stretching of cyclic anhydrides (1850 and 1774 cm-l) and carboxylic species (1745 cm"l)_

A weak absorption in the region of

carboxylic OH stretching confirms the presence of acidic surface groups (Fig. 11A) at thls oxidation temperature. of carboxylic

acids and therefore

In the oxidation of PVDF, the oxidic

species

the formation

formed

by their

208

condensation is hindered by the lack of available hydrogen and

the anhydride

and/or carboxylic-specificvibrations are not observable in either region (Fig. 10 and Fig. 11). The prominent bands in the C=O stretching region of PVDF500 oxidized at T>300°C are located at 1762 and 1726 cm-l.

These two absorptions are also

typical of those observed in the oxidation spectra of the polyvinyl halide The tHn, bands are normally attributed to the C=O stretching of

chars 1381.

lactonic species formed from the insertion of oxygen into the polyaromatic network.

The formation of lactonic species is not dependent upon the presence of

surface hydrogen groups and lactonic absorptions are normally observed during the advanced "burn off" stage of the oxidation process when the molecular oxidic (cyclic anhydride) species break amy

leaving a compact aromatic network

of surface lactones and aromatic groups linked by ether-like bridges. formation of these lactonic species and the loss of the 1845 cm" aldehydic bands can be observed in the spectra of the

The

and 1774 cm"

PVF char oxidized at

45ooc.

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