Chemistry Reference
In-Depth Information
5.5 Evidence of Grafting Copolymerization
5.5.1
Infrared Spectra
Infrared spectroscopy is an important approach to identify the characteristic groups
and prove the existence of a special chemical group. For the gum-g-copolymer, the
FTIR spectra can be used as the evidence of forming a graft polymer. Typically, the
spectrum of the gum-g-copolymer shows a set of strong absorption bands which is
absent in the spectrum of raw gum, and the new characteristic bands are ascribed to
the polymerization of monomers. The position of absorption bands is decided by
the type of functional groups of monomers. The grafted product was usually washed
by large amount of water and then soaked or extracted with ethanol (or methanol,
acetone, etc.) to remove the unreacted gum and the formed homopolymers. After
these treatments, the interference can be suppressed and the FTIR spectra are
convincible and can be as an evidence for graft copolymers. For instance, Biswal
et al. [
174
] proved the acrylamide was grafted onto guar gum backbone by FTIR
spectra. The appearance of absorption bands of gum at 1,670 and 1,635 cm
1
(amide-I (C
O stretching) and amide-II (N-H bending)) indicated the graft of
poly(acrylamide) chains onto the guar gum backbone. Another approach to prove
the graft reaction by FTIR spectra is to remove the gum backbone through chemical
reaction and to determine the FTIR spectra of the residual moiety. For example,
Chowdhury et al. [
78
] hydrolyzed AG by acid in the grafted polymers and then
determined the FTIR spectra of residues. Results indicate that the FTIR spectra of
the residues are identical with the spectra of PMMA, but are different from the
FTIR spectra of graft copolymer. It proved the occurrence of graft reaction between
AG and MMA.
ΒΌ
5.5.2 Thermogravimetric and Differential Scanning Calorimetric
Analysis
Thermal analysis is also a conventional method to provide an evidence for graft
reaction. Besides the gum backbone, the grafted polymer chains may generate various
interactions with each other (such as condensation and cyclizing reaction) to affect the
thermal behavior during the process of thermal decomposition [
116
,
120
,
121
]. So,
different grafted monomers (or functional groups) and graft efficiency may cause
different thermal stability of the corresponding graft copolymers. However, it is true
that the thermal stability and endothermic-exothermic behaviors of graft copolymer
are clearly better than the matrix gum due to the increase of molecular weight and
number of functional groups. The raw gum may rapidly thermal decompose by a one-
step process, but the graft copolymer enhances the thermal decomposition tempera-
ture and usually exhibited 2-4 steps thermal decomposition. For example, the graft
