Thermal desorption of covalently bound fullerene C60 from poly-N-vinylpyrrolidone films

Article abstract of DOI:10.1023/B:RJAC.0000022449.49364.52

Kinetics of formation of thermolysis products in heating of thin films of poly-N-vinylpyrrolidone and of poly-N-vinylpyrrolidone with covalently bound fullerene C₆₀ was studied by thermal desorption mass spectrometry.

Full text of DOI:10.1023/B:RJAC.0000022449.49364.52

Russian Journal of Applied Chemistry, Vol. 76, No. 12, 2003, pp. 1965 1970. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 12, 2003,
pp. 2015 2021.
Original Russian Text Copyright
2003 by Pozdnyakov, Melenevskaya, Ratnikova, Ginzburg.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
Thermal Desorption of Covalently Bound Fullerene C60
from Poly-N-vinylpyrrolidone Films
A. O. Pozdnyakov, E. Yu. Melenevskaya, O. V. Ratnikova, and B. M. Ginzburg
Institute of Machine Science Problems, Russian Academy of Sciences, St. Petersburg, Russia
Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
Received July 15, 2003
Abstract Kinetics of formation of thermolysis products in heating of thin films of poly-N-vinylpyrrolidone
and of poly-N-vinylpyrrolidone with covalently bound fullerene C60 was studied by thermal desorption mass
spectrometry.
Since fullerene C60 (C60) exhibits biological activ-
ity, synthesis of its water-soluble forms and, in par-
ticular, polymer C60 systems, is a topical task [1 4].
These are two types of fullerene-containing polymers
in which C60 is bound to water-soluble macromolec-
ules by covalent or noncovalent bonds. Poly-N-vinyl-
pyrrolidone (PVP) is a nonionic water-soluble amphi-
philic polymer that can form complexes with molec-
ules of diverse structures and polarities [5]. Poly-N-
vinylpyrrolidone is produced commercially and is
widely used in medicine [6]. Fullerene C60 supported
by this polymer can be used in biological studies [7].
Some features of the interaction between C60 and PVP
are known. A radical ion donor acceptor complex in
the PVP C60 system has been detected by ESR spec-
troscopy [8]. The formation of donor acceptor bonds
between C60 and the carbonyl groups of PVP has been
confirmed by 13C NMR studies [9].
The high sensitivity of mass spectrometry and ther-
mal desorption mass spectrometry (TDMS) to a mol-
ecule flow in the gas phase allows determination of
the weight loss by the polymer C60 system in the
course of C60 desorption. The sensitivity of other
common methods of thermal analysis (e.g., thermo-
gravimetry) is substantially lower, and these methods
are unsuitable for this purpose. The type of poly-
mer fullerene bonding and the structural state of C60
(X-ray-amorphous or crystalline) were determined by
TDMS from the curves of thermal desorption of C60
and the thermolysis products of the polymeric matrix
[11 15]. In this study we examined thermal desorp-
tion of fullerene C60 from PVP C60 systems prepared
by different procedures and analyzed the thermolysis
products of PVP.
EXPERIMENTAL
The structure and properties of solutions of PVP
C60 systems are determined by the type of the PVP
C60 interaction and the nature of a solvent. As deter-
mined by the method of Rayleigh scattering, solutions
of PVP C60 complexes contain domains whose size
is determined by the balance of the attractive forces
between C60 molecules and repulsive force between
the macromolecules [10]. It should be noted that the
degree of heterogeneity of composite and nanocom-
posite polymer systems is an important parameter af-
fecting their properties. However, there are only a few
methods for measuring this parameter. At the same
time, this parameter is required to correlate the struc-
ture and the properties of these systems and to devel-
op techniques for their synthesis.
Samples of PVP and PVP containing C60 bound by
covalent bonds (PVP cov-C60) were prepared by rad-
ical polymerization in toluene, using asobis(isobutyro-
nitrile) (AIBN) of pure grade [TU (Technical Specif-
ication) 6-09-3840 74]. Trace amount of water were
removed from AIBN in a vacuum [16]. N-Vinylpyr-
rolidone (N-VP) was dried over KOH and distilled
under reduced pressure. The middle fraction was de-
gassed. A fullerene powder with 98% C60 was kept in
a vacuum at 100 C. The toluene solutions of C60 were
prepared by the procedure described in our previous
study [17]. Copolymerization of N-VP and C60 was
performed in toluene at 60 C in the presence of AIBN
as an initiator.
1070-4272/03/7612-1965 $25.00 2003 MAIK Nauka/Interperiodica

1966
POZDNYAKOV et al.
6
Properties of PVP and its complex with C6*0
sure 10 mm Hg) at 140 C for 10 15 h. The molec-
ular weights of PVP and PVP cov-C60 copolymer
were determined by microcolumn exclusion chroma-
tography (MEC) and by the method of Rayleigh scat-
tering (see table).
Sample
Properties
PVP
PVP cov-C60
1.6
C60 content, wt %
MWpeak (MEC)
MWLS in water
MWD
The glass transition point of PVP T 180 C [18].
>90 103
4.9
6
103
10 103
2.2
Complexes of PVP with C60, in which PVP is
bound to C60 by noncovalent bonds (PVP C60) were
prepared from a benzene solution of these components
[19]. The fullerene content in the complex thus ob-
tained may be higher than 1%. In this study, we used
the complex containing about 1.7% C60.
MWSD
103
*
MW
is the number-average molecular weight corre-
sponding to the maximum content of macromolecules in
the molecular-weight distribution (MWD) determined by
peak
exclusion chromatography; MW
is the weight-average
Samples of PVP, PVP cov-C60, and PVP C60 were
studied by TDMS. Thin films with a definite thick-
ness were prepared as follows. An aliquot of a dilute
solution of PVP cov-C60 or PVP C60 in distilled
water or toluene was applied to an oxidized tantalum
support-heater equipped with a thermocouple. To min-
imize the contact of the polymer with water, dried
PVP was applied by hatching a support with a sample.
The coated support was dried for 5 min at room tem-
perature under a reduced pressure ( 700 mm Hg) in
a VT6060T vacuum oven (Kendro Laboratory Prod-
ucts) and then placed in a vacuum chamber of MKh-
1320 magnetic mass spectrometer (energy of ionizing
electrons 75 eV). Thermal desorption experiments
were performed in a vacuum with residual pressure
LS
molecular weight determined from light scattering; and
MW is the number-average molecular weight determined
SD
by the sedimentation procedure.
Fullerene C60 traps free radicals and inhibits co-
polymerization. This leads to termination of in the
early stages in the case of simultaneous component
mixing. The monomer conversion is about 20%. To
increase the copolymer yield, AIBN was introduced in
two steps. Half of the whole amount of AIBN was
added at room temperature and the reaction mixture
was stirred for 1 h. Then a toluene solution of C60 was
added, and the reaction mixture was heated at 60 C
for 100 h. After that, the remaining portion of AIBN
was added, and the copolymerization was continued
for the same time. Then PVP cov-C60 was isolated.
7
of 10 mm Hg.
The composition of PVP complexes with C60 was
examined in our previous study by thin-layer chroma-
tography (THC) using toluene as an eluent [16]. The
presence of free fullerene was detected only in pure
C60 and PVP C60. The absence of free C60 in PVP
cov-C60 was confirmed by UV spectra of solutions of
C60 and PVP cov-C60 in chloroform [16]. Naturally,
PVP cov-C60 contains large amounts of C60 bound
to the initiator radicals formed after the second addi-
tion of AIBN.
The copolymer, formed in 70 90% yield was repre-
cipitated from diethyl ether, dissolved in distilled wa-
ter, and lyophilically dried. Polyvinylpyrrolidone pre-
pared by copolymerization in toluene in the presence
of AIBN and commercial fractionated PVP with mo-
lecular weight (MW) of about (10 12) 103 were
used as references. The PVP fraction with MW = 12
103 was additionally dried in a vacuum (residual pres-
It should be noted that, in these systems, C60 is
bound to radicals of the initiator and polymer by co-
valent bonds; their number may range from 1 to 3 4
[16]. We suggest that products are formed with both
polymer and initiator radicals, initiator radicals only,
and polymer radicals only. A study of PVP cov-C60
solution by two-detector exclusion chromatography
confirmed its nonuniformity (Fig. 1). As seen from
the chromatogram, the C60 content in the low-molec-
ular-weight fractions is high. Apparently, PVP with
higher molecular weight is formed after the second ad-
dition of the initiator, since the contribution of chain
termination reaction decreases owing to the lowering
Fig. 1. HPLC pattern of PVP cov-C solution in chlo-
60
roform measured with (1) refractometric detection and
(2) photometric detection ( = 260 nm, 0.5 400-mm
Ultrastyrogel 1000 column, chloroform as an eluent, elution
1
rate 3 l min ). (I) Intensity and (V ) retention volume.
R
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 12 2003

THERMAL DESORPTION OF COVALENTLY BOUND FULLERENE C60
of the activity of C60 that reacted in the first polymer-
1967
ization step. Thus, our chromatographic study demon-
strated that the polymerization product is not uniform
and contains PVP homopolymer, a series of fullerene-
containing PVPs with different numbers of macro-
molecules with different molecular weights, and struc-
tures containing radicals of both the polymer and ini-
tiator or only initiator radicals.
The mass spectrum of volatile thermolysis products
(VTP) contains signals of vinylpyrrolidone VP (m/z =
56100%, 11150%, 2847%, 2725%, 4118%), pyrrolidone Pr
(m/z = 85100%, 4242%, 4136%, 2832%, 3029%, 5616%
)
and trace amounts of water (m/z = 18100%). The main
lines of these compounds and their relative intensities
(lower index) are in parentheses. The ratios of VP and
Pr formed in thermolysis of PVP and PVP cov-C60
differ in a wide temperature range. A typical mass
spectrum of VTP of these samples is shown in Figs. 2a
and 2b, respectively. The intensity of the Pr lines in
the spectrum of VTP of PVP is the highest, which
indicates that elimination of side pyrrolidone groups
prevails. On the contrary, PVP cov-C60 is mainly ther-
molyzed by depolymerization to form VP monomer.
Fig. 2. Fragment of mass-spectrum obtained after thermal
treatment of (a) PVP and (b) PVP cov-C at 320 C.
60
(h) Relative peak intensity and (m/z) molecular weight
to charge ratio.
To gain deeper insight into thermolysis of these
samples, we recorded the temperature dependences
of the rate of formation of the VP monomer dNVP/d ,
pyrrolidone ring dNPr /d , and C60 molecules dNC /d
60
(Ni is the current amount of product liberated from
the sample; is the time). The kinetics of formation
of the thermolysis products was monitored by the line
of their molecular ions M+ 111, 85, and 720, respec-
tively. The samples were heated at a rate of about
Fig. 3. Rate of formation of (a) VP dN /d and (b) Pr
VP
dN /d vs. the temperature T of thermal treatment of
Pr
predried films of (1) PVP and (2) PVP cov-C
.
60
1
7 deg s .
1
mation in thermolysis of dry PVP is 170 kJ mol .
It is difficult to calculate the activation energy of PV
formation in the high-temperature step of PVP cov-
C60 thermolysis, since this step overlaps with the low-
temperature step.
The temperature dependences of the rate of libera-
tion of VP dNVP/d from predried PVP and PVP
cov-C60 are shown in Fig. 3. It should be noted that,
since different radicals are grafted to PVP cov-C60,
only a shift of Tmax of thermal desorption steps, but
not their contribution, can be interpreted. In the case
of PVP cov-C60, the low-temperature peak at Tmax
290 C is observed along with a high-temperature peak
The temperature dependence of the rate of Pr for-
mation, dNPr /d , in the course of thermolysis of PVP
cov-C60 films and predried PVP are shown in Fig. 3.
As can be seen from Fig. 3, Pr is formed in a wider
temperature range (100 700 C) than VP. It should
at Tmax
480 C.
The low-temperature peak is due to rupture of the be noted that, at low temperatures, liberation of Pr
PVP C60 covalent bond in PVP cov-C60. This bond from PVP predried in a vacuum and applied by the
is located at the -position with respect to the double dry procedure is less intense. Poly-N-vinylpyrrolidone
bond of C60 and is weak. A similar low-temperature contains trace amounts of N-VP and 2-pyrrolidone,
peak has been observed in thermolysis of polystyrene which are mainly removed by prolonged heating in
with grafted C60 groups [13]. The activation energy a vacuum [20]. It is natural to assume that pyrrolidone
of rupture of this bond, calculated from the tempera- is released from dry PVP owing to the presence of
ture dependence of the rate of monomer desorption in strongly bound water in the polymer [18]. We suggest
the low-temperature step (Tmax
80 kJ mol . The total activation energy of VP for- the pH of the aqueous PVP solution is 5.5. The hy-
290 C), is about that liberated VP undergoes acidic hydrolysis, since
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 12 2003

1968
POZDNYAKOV et al.
a toluene solution. In this case, C60 desorption starts
at temperatures lower than those of the onset of PVP
depolymerization (Fig. 3a). These data confirm that
the mechanisms of high- and low-temperature desorp-
tion of C60 differ.
The difference in the mechanisms of high- and low-
temperature desorption may be due to formation of
donor acceptor bonds of the fullerene both with
the macromolecule grafted to it and with free PVP
present in the sample. In addition, as we shown pre-
viously [19], donor acceptor bonds and radical ions
[8] are formed after exhaustive removal of the solvent
(toluene) from the PVP C60 system. Occurrence of
these processes in the course of PVP cov-C60 ther-
molysis cannot be ruled out a priori.
Fig. 4. Rate of C desorption, dN /d , vs. the tempera-
60
C60
ture T of thermal treatment of (1) 20-nm films of pure C
,
60
applied from a toluene solution to the metal surface, PVP
In concentrated aqueous solutions of PVP C60
complexes, aggregation to form fractal structures was
revealed by small-angle neutron scattering [22, 23].
At relatively high fullerene content in the complex
(more than 1%), phase separation occurs and micellar
structures composed of fullerene agglomerates with
a polymer shell are formed. At the same time, in tolu-
ene, which is bad solvent for PVP, its macromolecules
are contracted, which prevents contact between the
components. Hence, the probability of formation of
PVP C60 donor acceptor bonds in toluene is very
low, and the thermal desorption of C60 from a PVP
C60 film applied from toluene solutions should be
similar to that from other polymers, e.g., poly(meth-
ylmethacrylate).
cov-C films applied from (2) aqueous and (3) toluene
60
solutions, and (4) PVP C film.
60
drolysis continues until water is present in the system.
Water molecules form strong hydrogen bonds with
the oxygen atoms of the carbonyl groups and are, ap-
parently, retained in the polymer up to its complete
degradation.
The temperature dependences of the rate of C60 de-
sorption, dNC /d , from films applied from an aque-
60
ous solution of PVP C60, PVP cov-C60 films applied
from aqueous and toluene solutions, and 200- films
of C60 applied from saturated toluene solution of C60
to the metal support are shown in Fig. 4. The film
of pure C60 was studied for comparison, since thermal
desorption of C60 is not complicated in this case by
C60 diffusion in the polymeric matrix. The thermo-
gram of thermal desorption of C60 from PVP cov-
C60 contains a dome-shaped peak at 380 500 C and
a sharp peak at 500 530 C. Low-temperature desorp-
tion of C60 is accompanied by degradation of the mac-
romolecules to form VP and Pr (Figs. 3 and 4). In ad-
dition, a comparative kinetic study of C60 desorption
from a film of pure C60 on a metallic support and from
PVP cov-C60 film demonstrated that C60 molecules
are formly retained by the PVP matrix until they dif-
fuse to the polymer vacuum interface. The total ac-
tivation energy of low-temperature (380 500 C) de-
In this stage of the study, we suggest that the dif-
ference between the thermal desorption of C60 from
PVP cov-C60 films prepared from toluene and aque-
ous solutions is due both to different strengths of in-
teraction of these components and different degrees of
fullerene aggregation. The high-temperature peak at
500 530 C can be assigned to desorption of C60 mol-
ecules bound by donor-acceptor bonds or forming rad-
ical ions with appreciable charge transfer from the
carbonyl oxygen of PVP to C60 (negative charge on
C60 prevents its desorption from the polymer). The
content of these C60 molecules, roughly estimated by
integration of the desorption rate in the high-temper-
ature step, is about 10% of the total fullerene amount
in the sample.
1
sorption of C60 ( 110 kJ mol ) is close to that for
other polymer C60 complexes bound by covalent bonds
[21]. If the low-temperature desorption of C60 is
caused by rupture of the covalent bonds, the lower ac-
tivation energy of C60 desorption as compared to that
of PVP degradation should be due to the fact that
the PVP C60 covalent bond is weaker than the C C
bonds in a PVP macromolecule. The high-tempera-
ture peak (500 530 C) of C60 desorption is absent in
the thermogram of PVP cov-C60 films applied from
The fact that the temperature dependence of C60
formation in the course of thermolysis of PVP C60
film (Fig. 4, curve 4) shows only the high-temperature
peak (Tmax is slightly lower than that for PVP cov-
C60) and does not contain the dome-shaped peak at
380 500 C, which is present in the thermogram of
PVP cov-C60, confirms our assumption that the high-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 12 2003

THERMAL DESORPTION OF COVALENTLY BOUND FULLERENE C60
1969
temperature step is caused by rupture of donor ac-
ceptor bonds between C60 and the polymer. The ful-
lerene content in the complex is about 1.7%, i.e., is
higher than the critical concentration (1%) at which,
in accordance with the data of small-angle neutron
scattering [22, 23], dense micellar structures can be
formed. It should be noted that, at the temperatures of
desorption of the fullerene from PVP C60 film applied
from an aqueous solution [22, 23], the PVP C60 co-
valent bonds are ruptured and the degree of hetero-
geneity of the film is determined by the C60 content
in the system.
Although chemical transformations in solutions
that contain PVP and C60 and in films applied from
these solutions are complex, we suggest the following
simplified scheme describing the heterogeneous struc-
ture of PVP cov-C60 and PVP C60 films, as well as
the temperature transformations of this structure.
Fullerene desorption from PVP C60 agglomerates
is difficult, since they are encapsulated in the PVP
matrix, which is preserved up to complete thermolysis
of the polymer. As a result, the high-temperature peak
of C60 desorption appears at 500 530 C (Fig. 4).
The low-temperature peak (380 500 C) in the ther-
mogram of PVP cov-C60 is caused both by rupture of
C60 PVP covalent bonds and desorption of separate
nonbonded C60 molecules or C60 molecules present in
loose agglomerates. If the initial system becomes het-
erogeneous after formation from solution on the sup-
port, complex competitive processes determined by
the reactivity and interdiffusion of the components
will occur on further heating. The diffusion can be ac-
celerated at temperatures higher than the glass transi-
tion point Tg. The TDMS data discussed above sug-
gest heterogeneous structure of the examined films is
caused by agglomeration in the initial solutions is re-
tained up to the temperature of PVP thermal degrada-
tion, Tdegr. The heterogeneous structure of these sys-
tems will be studied in more detail in our further
works.
nylpyrrolidone matrix up to the decomposition tem-
perature of poly-N-vinylpyrrolidone. This interaction
is independent of the bonding type in the initial poly-
N-vinylpyrrolidone C60 system.
(3) The unusual high-temperature peak of C60 de-
sorption is due to rupture of the donor-acceptor bonds
between the fullerene and the carbonyl oxygen of
the pyrrolidone ring in the poly-N-vinylpyrrolidone
matrix.
(4) The mechanism of thermolysis of the systems
examined and the mechanism of thermal desorption of
C
60 from them is determined both by the type of poly-
N-vinylpyrrolidone-C60 bonds and the solvent used for
film preparation. The degree of aggregation of C60 and
its derivatives increases when water is used as the sol-
vent.
ACKNOWLEDGMENTS
We are grateful to the Humboldt Foundation for
placing vacuum oven at our disposal. This study was
financially supported by Urgent Directions in Physics
of Condensed Matter State research program ( Ful-
lerenes and Atomic Cluster direction), Russian Foun-
dation for Basic Research (project no. 03-03-32633),
and OEMMPU program no. IG-03 of the Russian
Academy of Sciences.
CONCLUSIONS
(1) Curves of thermal desorption of fullerene C60
from poly-N-vinylpyrrolidone bound to C60 by co-
valent bonds were recorded, as well as the curves of
formation of thermolysis products of poly-N-vinyl-
pyrrolidone films applied from different solvents.
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