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Radiation Physics and Chemistry 63 (2002) 185191
Radiation synthesis of poly(2-vinylpyridine) gels
and their swelling characteristics
N. Acar*
Department of Chemistry, Faculty of Science & Letters, Technical University of Istanbul, 80626 Maslak, Istanbul, Turkey
Received 19 June 2000; accepted 17 March 2001
Abstract
The effect of irradiation under vacuum on thermal properties, and swelling behaviour on Poly(2-vinylpyridine)
(P2VP) has been investigated with respect to their swelling properties and network structures. The gel percent of the
irradiated P2VP samples was determined by Soxhlet extraction, and UV spectroscopy was used to determine sol
percent. The change in glass transition temperature (Tg) was followed by Differential Scanning Calorimetry before and
after soxhlet extraction.# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Irradiation; Poly(2-vinylpyridine); Gel; Characterization
1. Introduction
A gel is a crosslinked polymer or copolymer, which is
capable of imbibing a considerable quantity of liquid up
to swelling equilibrium. The final liquid content depends
on the chemical composition and also, in the majority
cases, on the crosslinked density (Huglin and Zakaria,
1986). Interest in the preparation of gels with various
properties has increased considerably in recent years,
due to their applications in many fields (Dole, 1986).
Crosslinked gels are generally prepared from monomer
with or without comonomers, in the presence of
crosslinking agents or treating the polymer with radia-tion. This technique is generally termed a clean
technique, not requiring any extra chemicals or leaving
some unwanted residues (G .uuven and Sen, 1991; Huglin
et al., 1987).
Polyvinylpyridines have drawn much attention in the
past and gained wide practical use because of their
properties (Reilly Report, 1983).The vinylpyridine mono-
mers are similar to styrene except the presence of
nitrogen atom on the pyridyl ring. P2VP would be
expected to combine high radiation resistance with its
thermal properties because of its aromatic content.
Knowledge of the changes in thermal properties of the
polymer on irradiation is important for its application in
many fields such as advanced composites and as an
insulating materials for wire and cables in nuclear power
plants.
In the present work P2VP samples were irradiated and
their thermal properties were determined after irradia-
tion. The gels prepared were characterised with respect
to their swelling properties and network structures.
2. Experimental
2.1. Materials
P2VP was purchased from Polyscience Inc. The
measured value of the limiting viscosity number in
methanol at 298 K (114.99 dm3 kg1), in conjuction with
the MarkHouwink constants gave a molecular weight
of 28.4 104 gmol1 (Fuoss et al., 1958). P2VP was
dissolved in ethanol and then solvent cast in a petri dish
to give thin film of P2VP. However, the film thus formed
strongly adhered to the petri dish and it was very brittle
*Fax: +90-212-285-6386.
E-mail address:[email protected] (N. Acar).
0969-806X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
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so that it could not be separated from the petri dish
without fracturing. For this reason, it was impossible to
prepare uniform strip specimen. Powdered P2VP sam-
ples were dried in vacuum at 343 K before irradiation.
2.2. Irradiation
The glass ampoules (7 cm in length and 3 cm in
diameter) which contained P2VP samples were put on a
vacuum rig and evacuated for 4 h, after which they were
sealed. The glass ampoules were irradiated at a 4.5 MeV
Dynamitron electron accelerator at Isotron plc. to six
different doses (0.20; 0.61; 1.03; 1.43; 1.84; 2.19 MGy).
There was no discernible increase in temperature during
the process, with each of the samples receiving a 32 kGy
per pass. The ampoules were at room temperature
before entering the irradiation cell for each pass.
2.3. Gel determination
Each irradiated sample was weighed into a soxhlet
thimble (Winitial) and extracted with ethanol for 15 h to
determine the insoluble part in the samples gravime-
trically (Wgel). This procedure was also used for the
unirradiated samples. After extraction, amount of
polymer dissolved in ethanol (Wsol) was determined
spectrophotometrically (Champbell and White, 1989;
Skurlatov et al., 1982). P2VP can be detected by UV
spectrophotometry (Molyneux, 1982). P2VP had an
absorbance peak at 264 nm. The absorbance of series
of standard solutions of P2VP was measured at this
wavelength by using a Hewlett-Packard 8452 A UV
Spectrophotometer and found to obey the Lambert
Beer Law. The extinction coefficient was determined to
233.9 m2 mol1. At the end of the experiment, the sum of
Wgel andWsol was equal to Winitial. After extraction, the
gel percent in the irradiated samples was calculated as
follows:
Gel% wi=wo100; 1
where wo represents the weights of dry irradiated
samples (before Soxhlet extraction that contains both
sol and gel part) andwi represents the dry insoluble (gel)
part.
2.4. Swelling measurements
To measure swelling properties of powdered P2VP
samples tea bags were used. First the tea bag was wetted
with ethanol and blotted quickly with absorbent paper,
then dried P2VP sample (contains only gel part) of
known weight, was put into a tea bag. The tea bag that
contained P2VP sample was immersed in ethanol. The
swelling proceeded at 208C. At various times the tea
bags were removed, blotted quickly to remove ethanol
attached to the surface and weighed in a stoppered
weighing bottle. Equilibrium was reached after 24 h,
then tea bags were removed, the surface dried and
the sample weighed. The ethanol uptake percent was
calculated as follows:
Ethanol uptake% ws wd
ws
100; 2
where Wd and Ws represents the weights of dry and
swollen irradiated samples, respectively.
For a P2VP gel, swelling data from the gravimetric
analysis were used to calculate the volume fraction, V2m,
and equilibrium degree of swelling, qv, of polymer in a
given gel sample swollen to equilibrium in ethanol
(Baysal et al., 1997; Sen et al., 1999).
qv 1 r qw 1
d
; 3
whereqwis the ratio of the weights of the network in the
swollen state and dry state, r and dare the densities ofP2VP and ethanol, respectively. The equilibrium degree
of swelling, qv, was defined as qv 1=V2m. Using thevalues ofr 0:95gml1 andd 0:786gml1,qvvalueswere calculated.
2.5. Measurements of glass transition temperatures
DSC measurements were carried out using DSC
Mettler TA 3000. Samples of20 mg of the irradiated
P2VP were used for measurements. Indium, zinc and
nickel standards were utilised to calibrate the tempera-
ture scale. The measurements were carried out in N2atmosphere at a heating rate of 10 deg min1. Tg of
irradiated and unirradiated samples was determined.
After soxhlet extraction all samples were dried under
vacuum at 318 K andTg was determined in the gel part.
3. Results and discussion
3.1. Gel determination
The formation of intermolecular crossslinks is one of
the most important changes brought about radiation,
since crosslinking of polymers leads to beneficialchanges in some of their properties, such as heat
resistance etc. Therefore, the gelled part, i.e. the
insoluble part, in the samples having different degrees
of crosslinking was determined. Results are given in
Fig. 1, that shows the relation between the gel percent
and irradiation dose for P2VP samples in Fig. 1 error
bars at 1 standard deviation are shown. Initially, the gel
percent increase with increasing irradiation dose up to
0.61 MGy, at higher doses increase in gel percent is
small.
Much effort is expended on measuring the relation-
ship between radiation dose and gel fraction, since this
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determines the efficiency of crosslinking, the G value of
crosslinking, Gx and the G value of scission, Gs. In
many instances, radiation induces simultaneous cross-
linking and scission reactions. In these cases, the yields
for each reaction can be determined by Charlesby
Pinner relationship (Charlesby and Pinner, 1959).
s s1=2 50N
MnGx
1
D
GS
Gx; 4
where s=sol fraction of the irradiated polymer,
Mn=number average molecular weight of polymer
prior to irradiation, D=absorbed dose, Gs=scission
yield and Gx=crosslinking yield. To permit the calcula-
tion of crosslink yields, a determination of scission
concurrent with crosslinking is necessary. Results are
shown in Fig. 2 and Table 1. In Fig. 2, s s1=2 is plotted
against 1/dose for irradiated P2VP samples. Extrapola-
tion to 1/dose=0 gives value for Gs=Gx, the scission to
crosslinking ratio, of 0.279 for irradiated P2VP samples.Extrapolation to complete solubility (s s1=2 2)
permits an estimate of the gel points. For a calculation
of both crosslinking and scission yields, the gel point
dose, rg, and the scission to crosslinking ratios may be
substituted in Eq. (5) (Charlesby, 1960).
w
Mw rg Gx
Gx
2
5
in which w=molecular weight of repeating unit,
rg=6.855 103 MGy, Mw=28.4 10
4 gmol1. Substi-
tuting Gs=Gx 0:279 gives: w=Mw rg 0:861Gx andGx is found 0.063. The scission yield is Gs=0.279
(0.063)=0.018. In literature, polystyrene (PS) was
chosen for the study because it was easy to prepare
and characterize as a linear polymer and it crosslinked
by radiation with few side reactions (Graessley, 1964).
On the basis of some literature values for Gx and Gs,
namely, approximately 0.043 and 0, respectively, for
irradiation in vacuum, Gx decreased from 0.043 in
vacuum to 0.022 in air, there being a concominant
increase in Gs from 0 to 0.022 (ODonnell et al., 1979).
In theg-radiation induced crosslinking of PS in vacuum,
Shimizu and Mitsui (1979) investigated that the gel
fraction increased with increasing irradiation time by the
irradiation beyond the critical time for incipient gel
formation and the rate of gel formation decreased with
time. They found that at 308C, Gx 0:035 and Gs 0:01 when temperature was increased to 708CGx andGsvalues were 0.042 and 0.051, respectively. Parkinson
et al. (1965) described measurements of crosslinking in
PS with g-radiation and with the mixed g-neutron field
Fig. 1. Influence of dose on gel percent of P2VP (The solid
curve is the best fitting curve to the experimental data).
Fig. 2. Solubility of irradiated samples in ethanol as a function
of the reciprocal of dose.
Table 1
Determination of solgel fractions of irradiated samples by UV spectroscopy
Sample no. Dose (MGy) 1/Dose wsol (g) wgel (g) (wsol wgel)a (g) ssol fraction s
1=2 s s1=2
1 0.196 5.102 0.031 0.410 0.441 0.071 0.267 0.338
2 0.614 1.629 0.035 0.562 0.597 0.059 0.243 0.302
3 1.027 0.973 0.029 0.473 0.501 0.057 0.239 0.297
4 1.432 0.698 0.027 0.469 0.496 0.055 0.233 0.288
5 1.841 0.543 0.026 0.472 0.498 0.052 0.229 0.281
6 2.188 0.457 0.021 0.383 0.404 0.052 0.228 0.280
a
=initial weight of sample.
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of a reactor. G value for crosslinking PS is smaller than
the value for P2VP. It shows that PS is a more radiation
resistant polymer than P2VP.
The vinylpyridine monomers are somewhat more
reactive than styrene; this observation is consistent with
the methyl affinities of the respective monomers. Methyl
affinity (relative reactivity of monomer towards methylradical) of styrene, 2-vinylpyridine and 4-vinylpyridine is
792,1360 and 1360, respectively. This can explain why
polystyrene is more resistant to irradiation than poly-
vinylpyridines although their similar chemical structure
(Encyclopedia of Polymer Science and Engineering,
1989; Davidson, 1984).
3.2. Swelling measurements
Fig. 3 represents the ethanol uptake of P2VP as a
function of dose. It can be seen that the ethanol uptake
decreases with increasing dose in accordance with theincreasing degree of crosslinking, which normally results
in lower degree of swelling (Graessley, 1964; Hegazy
et al., 1995).
3.3. Determination of Mc values of gels
The gels thus prepared were characterized with respect
to their swelling properties and network structures. One
of the basic parameters that describes the structure of a
gel network is the molecular weight between crosslinks,
Mc for swollen networks. This describes the average
molecular weight of polymer chains between twoconsecutive junctions. These junctions may be chemical
crosslinks, physical entaglements, crystalline regions or
even polymer complexes (Peppas, 1991; Peppas and
Klier, 1991; Philippova et al., 1996). Several theories
have been proposed to calculate the molecular weight
between crosslinks in a gel. Probably the most widely
used of these theories is that of Flory and Rehner (Flory
and Rehner, 1943a,b). This theory describes the
equilibrium swelling characteristics of a crosslinked
polymer system where the polymer chains have reacted
in a solid state. The theory deals with neutral polymer
chains within the polymer gel. From the swelling
expression (Eq. (3)), the average molecular weightbetween consecutive crosslinks, Mc can be expressed
by Eq. (5). This equation has been widely used to
characterize a variety of networks. It was used when the
network was prepared from polymer, not from mono-
mer or monomer mixtures.
1
Mc
2
Mn %vv=V1
ln 1 V2m V2m wV22m
V1=32m V2m=2
: 6
Here, Mc is the number average molecular weight of
starting polymer, %vvis the specific volume of polymer, V1
is the molar volume of the swelling agent, V2m is thepolymer volume fraction in the equilibrium-swollen
system and w is the Flory polymer-solvent interaction
parameter. wis 0.487 for P2VP-ethanol system andV1 is
58.3gcm3 for ethanol. The Mc values thus determined
from Eq. (5) are given in Table 2. The results obtained
show that the average molecular weight between cross-
links decreases by increasing dose.
When a non-ionic polymeric network is placed in a
swelling agent, there are two contributions to the free
energy of the system, mixing and elastic-retractive free
energies as expressed as DGmix and DGel, respectively
(Flory, 1953). It is assumed that the change in total free
energy is the sum ofDGmix and DGel, thus,
DG DGmix DGel: 7
The state of equilibrium is obtained when the two
changes balance each other. Mathematically this state is
expressed as:
@DG
@n1
T;P
@DGmix
@n1
T;P
@DGel
@n1
T;P
0; 8
where n1 is the number of molecules of swelling agent,
subscripts T,P indicate that the differentiantions made at
constant temperature and pressure.
Fig. 3. Effect of dose on the percentage swelling of P2VP gels in
ethanol (the solid curve is the best fitting curve to the
experimental data).
Table 2
Values ofMc for gel systems
Dose (MGy) Mc (g mol1) qv
0.20 46382.35 16.83
0.61 18005.07 8.44
1.03 15232.79 7.58
1.43 7448.20 5.03
1.84 4487.86 4.32
2.19 3650.66 3.89
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Performing the differentiations indicated in Eq. (7):
ln1 V2m V2m wV22m
V1r
McV
1=32m V2m=2 0
9
or, adopting the terminology
V1r
Mc N1 10
where N is the number of segments in a network chain,
r is the density of polymer network.
ln1 V2m V2m wV22m N
1V1=32m V2m=2: 11
The left hand member in this equation represents the
lowering of the chemical potential owing to mixing of
polymer and swelling agent; that on the right gives the
increase from the elastic region of the network. As
expressed before, at swelling equilibrium, 1=V2m may be
replaced byqv. At large Mc values of 10,000 or more, qvin a good solvent will exceed 10. Then, V2m=2 isconsiderably smaller than V
1=32m and can be neglected.
Also higher terms in the series of expansion of the first
term member of Eq. (8) may be neglected. The swelling
equilibrium equation may then be solved for V2m
1=qv. The FloryRehner equation relating volumeswelling ratio to density of crosslinks has been simplified
by Charlesby to the form Charlesby, 1953):
q5=3v Mc 0:5 w
rV1: 12
Since Mc
is proportional to the reciprocal of dose,
a loglog plot ofqv vs 1/dose should have a slope of 0.6.
The data are presented in this manner in Fig. 4 with the
line constrained to a slope of 0.64.
3.4. Glass transition temperature measurements
Glass transition temperature of P2VP samples were
measured before and after Soxhlet extraction. The
results in Table 3 shows that Tg for P2VP shifts to
higher temperature with dose. Boyer (1963) suggested
that Tg is related to the chemical structure of polymers.
In general, factors that increase the energy required for
the onset of molecular motion increase Tg [such as
intermolecular forces (Boyer, 1954), bulky stiff side
groups (Gibbs, 1956), interchain steric hidrance (Karazsand MacKnight, 1968)]; that decrease the energy
reqirements lower Tg [such as flexible side groups
(Dunham et al., 1963; Rogers and Mandelkern, 1957)
and symmetrical substitution (Barb, 1959)]. Tg is also
governed by the presence and absence of crosslinks
(Glans and Turner, 1981; Sasuga et al., 1987; Hegazy
et al., 1992; Seguchi et al., 1999). In this study, the
increase in Tg with dose suggests that crosslinking is
probably important mechanism in P2VP.
Tg is an important property of polymeric systems,
which determine their temperature range of applicabil-
ity. The Fox equation has been proposed for predictingTg dependence on composition for polymer blends
(Fox, 1956), the equation used here for calculations
Table 3
Changes in Tg of irradiated P2VP samples with gel percent, before and after Soxhlet extractiona
Dose (MGy) Gel % Tgm (8C) Tg1 (8C) Tg1 (Calcd.)b (8C) DTg1
c (8C)
0 0 103.96 } } }
0.196 92.91 0.03 105.98 105.60 106.14 0.54
0.614 94.10 0.02 105.72 107.55 105.83 1.72
1.027 94.27 0.04 106.10 109.61 106.23 3.38
1.432 94.55 0.01 108.53 110.05 109.37 0.68
1.841 94.78 0.02 109.79 111.33 110.12 1.21
2.188 94.83 0.01 109.79 111.51 110.11 1.40
aError limits were estimated using standard deviations with the statistical parameter t=% 95 (Skoog et al., 1992).bCalculated by Foxs Equation.c
DTg1 Tg12
Tg1 (Calcd.).
Fig. 4. The loglog plot of the equilibrium volume swelling
ratio qv of the P2VP gels vs. reciprocal of dose.
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of Tg values of samples before and after Soxhlet
extraction.
1
Tgm
w1
Tg1
w2
Tg2; 13
where Tgm is the Tg of irradiated sample beforeseparation of gel and sol part, Tg1 is theTg of irradiated
P2VP sample after Soxhlet extraction (contains only gel
part), Tg2 is the Tg of original P2VP sample before
irradiation (139.68C),w1 andw2 refer to weight fraction
of gel and sol part, respectively. Results are shown in
Table 3. As shown from table, the observed Tg1 values
for samples, except first sample, were higher than the
calculated values. This increase is proportional to the
number of specific interactions existent in gel part and
can be interpreted as the contribution of the effective
crosslinks (Kwei, 1984).
4. Conclusions
In this study, P2VP samples were irradiated under
vacuum and the effect of irradiation on Tg and swelling
behaviour was investigated. Tg shifts to higher tempera-
ture with dose and the gelled part increases with dose
resulting in lower swelling degree. The average mole-
cular weight between two consecutive crosslinks was
determined by swelling experiments according to Flory
Rehner equation. The results obtained showed that
the average molecular weight between crosslinks was
decreased with increasing dose.
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