Photocrosslinking of poly(N-vinylcarbazole): Implementing a complementary
set of techniques to characterize the three-dimensional network formation
P.-O. Bussie
`
re
a
, B. Mailhot
a
,
b
,A.Rivaton
a
, M.-F. Barthe
d
, J.-L. Gardette
a
, M. Baba
c
,
*
a
Laboratoire de Photochimie Mole
´
culaire et Macromole
´
culaire, Universite
´
Blaise Pascal, CNRS, UMR 6505, Ensemble Universitaire des Ce
´
zeaux,
24, avenue des Landais, 63170 Aubie
`
re, France
b
IUT, Univ Clermont 1, Campus Universitaire des Ce
´
zeaux, BP 86, 63172 Aubie
`
re, France
c
Laboratoire de Thermodynamique des Solutions et des Polyme
`
res, Universite
´
Blaise Pascal, CNRS, UMR 6003, Ensemble Universitaire des Ce
´
zeaux,
24, avenue des Landais, 63170 Aubie
`
re, France
d
Centre d’Etudes et de Recherches par Irradiation, CNRS, UPR 33, 3a rue de la Fe
´
rollerie, 45071 Orle
´
ans, France
article info
Article history:
Received 18 January 2008
Received in revised form 14 March 2008
Accepted 27 March 2008
Available online 3 April 2008
Keywords:
PVK (poly(N-vinylcarbazole))
Photoageing
Thermoporosimetry
Positron annihilation spectroscopy
Swelling
IR spectroscopy
abstract
The crosslinking of poly(N-vinylcarbazole) PVK resulting from UV irradiation (
l
> 300 nm) was studied
by combining various techniques including infrared spectroscopy, size exclusion chromatography (SEC),
differential scanning calorimetry (DSC), swelling ratio, positron annihilation spectroscopy (PAS) and
thermoporosimetry. The results obtained from all of these techniques were correlated and allowed
a complete monitoring of the crosslinking during irradiation, from the creation of the first crosslinks to
the formation and densification of the three-dimensional network. From SEC analysis and determination
of swelling ratios, it was demonstrated that crosslinking was the predominant mechanism during the
first stages of irradiation. PAS and DSC analyses revealed a good correlation between variations of free
volume and increase of glass transition (T
g
) of the bulk material. In addition, the development of the
polymer architecture shown by the growth of T
g
was nicely correlated with the modifications of
the chemical structure observed by infrared analysis. Thermoporosimetry allowed the determination of
the mesh size distributions within the polymer for the longest irradiation times. Numerical relationships
were established and their validity was checked.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
The UV light ageing of polymeric materials generates dramatic
modifications of the chemical structure, which induces a loss of the
engineering properties of the polymer. The modifications of the
structure result from various reactions including rearrangements of
the chemical structure, formation of oxidation products, cross-
linking and chain scission. At the molecular scale, the experimental
method most used to determine the photodegradation mechanism
of polymers is spectroscopy. Infrared and UV–visible spectroscopies
are indeed very useful to monitor the rupture of the chemical bonds
and the chain scission by showing the formation of oxidation
photoproducts as chain ends or as low molecular weight products.
Conversely, the more difficult problem of assessment of cross-
linking has often been ignored because of the lack of direct char-
acterization methods. In fact, the formation of new covalent bonds
that lead to the crosslinking of the polymer is not easily observable
by optical spectroscopy. This work participates in redressing this
imbalance. At the macroscopic scale, measurements of insoluble
fraction and chromatographic methods such as size exclusion
chromatography (SEC) also give molecular information about the
modification of the chemical structure, but these analytical tech-
niques are limited to the period when the polymer is not totally
insoluble [1]. When the polymer becomes totally insoluble, its
three-dimensional network can be probed and analyzed by ther-
moporosimetry [2–5], swelling gel methods [6,7] and positron
annihilation spectroscopy (PAS) [8]. In this paper we attempted to
test and validate these methods for the study of crosslinking in
a polymeric material.
Poly(N-vinylcarbazole) (PVK) appeared as a good candidate
since the mechanisms of photoageing of this polymer have been
previously studied in detail [9]. It has been shown that the photo-
degradation mechanism of PVK was predominantly governed by
crosslinking reactions (Scheme 1).
It was also shown, on the basis of spectroscopic analysis, that the
absorption of light (
l
> 300 nm) by the polymer provokes the
rupture of the C–N bond. This rupture leads to the formation of
a carbazolyl radical accompanied by the creation of a macro-radical
on the polymer chain. Then, the abstraction of a hydrogen atom on
the macromolecule by the carbazolyl radical generates molecular
*
Corresponding author.
E-mail address: mohamed.baba@univ-bpclermont.fr (M. Baba).
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
Polymer Degradation and Stability 93 (2008) 1376–1382
Contents lists available at ScienceDirect
Polymer Degradation and Stability
journal homepage: www.elsevier.com/locate/polydegstab
0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2008.03.025