Grafting of Functional Monomers on Biomaterials - Biointerfaces: Where Material Meets Biology - page 316

The source of radiation can be either ultraviolet (UV) or high-energy ions
such as an electron beam or gamma rays. While UV radiation is confined to
the surface, electron beam and gamma rays will penetrate the entire ma-
terial. Upon exposure of polymeric materials to radiation the radicals that
form in the material will, if they are not initiating a graft co-polymer, either
undergo radical combination to form cross-links or disproportionation to
give scission and thus the bulk properties will be altered. The overall effect of
ionising radiation depends on the polymer substrate and the radiation dose
and these can be characterised with a G-value that quantifies the yield of an
event. In general, G-values are described for cross-linking G(X), scission G(S)
and gas evolution G(g). 20 Many polymers can undergo both chain scissoring
and cross-linking but depending on the polymer type as well as the absorbed
dose, one mechanism dominates. 19,21
The advantage of the simultaneous radiation method, when using gamma
ray or electron beam, is that it is a simple one-pot, one-step process. In
contrast to gamma rays, electron beam needs much shorter time of exposure
of the substrate to the irradiation source and as such has less of an effect on
the bulk properties. 17 When UV is used as the radiation source, the grafting
of monomers is a two-step process. In the first step a photo-initiator, such as
benzophenone (BP; a Norrish type II photo-initiator) is irradiated with the
substrate and in the second step, this pre-treated and washed substrate is
irradiated in the presence of a monomer. Since this process uses a photo-
initiator as well as other chemicals to obtain high eciency of the grafting
reaction concerns of complete removal of such additives post-grafting arise.
The radical concentration in the polymer substrate is affected by the ir-
radiation process and depends on the type of radiation employed. The
reader is referred to comprehensive reviews on UV radiation 22,23 as well as on
high energy irradiation 19,24,25 for in-depth details on this topic. The radical
concentration generated on (and in) a polymer substrate will furthermore be
dependent on the polymer chemistry and crystallinity as radicals form pref-
erentially in the amorphous or amorphous/crystalline interface regions. 24,25
While electron beam radiation has been used for simultaneous grafting,
most studies involve UV or gamma radiation induced grafting and examples
of these are listed in Table 11.1. It is evident that the acrylate acrylic acid (AA)
has been studied extensively for grafting onto various substrates; however, the
phosphate-containing monomers monoacryloxyethyl phosphate (MAEP) and
2-(methacryloyloxy) ethyl phosphate (MOEP) have also been the focus of a
number of studies. The monomer reactivity affects the grafting outcome with
some being highly reactive (e.g., AA) while other are more dicult to graft
[e.g., 2-aminoethyl methacrylate (AeMA) 18 ]. There are a couple of important
characteristics to note about some of the monomers listed in Table 11.1:

d n 3 r 4 n g | 2

.

AeMA can undergo rearrangement (intra- and inter-molecular amida-
tion) and degradation (hydrolysis) reactions in alkaline media, 26 and it
is thus important that the grafting process is carried out at neutral or
weakly acidic pH.