15 “Polymer–Oxide” Micro-/Nanocomposites: Background and Promises
255
unbleached and ozone-bleached hardwood kraft pulps as well as monocarboxyl
cellulose in NaOH solution was studied in the work [61]. The spectra were obtained
in alkaline media, so the dissociation of carboxyl groups as well as the ionization
followed by the isomerization of keto- and aldo-groups took place. The carried
out study had shown that cellulose demonstrates six intensive absorption bands
and the last ones were proposed to assign as related with absorption transitions in
chromophore composition of oxypolysaccharides as follows: carboxyl and carbonyl
groups absorbing at 200–220 and 270–290 nm, respectively, heteroaromatics of
furan and pyron type absorbing additionally at 230–250 and 290–320 nm, as well
as conjugated heteroaromatics absorbing above 300 nm with maximums at 350–370
and 430–450 nm due to the charge-transfer complexes.
In some cases dependence of the luminescent characteristics (the PL intensity and
peak positions) on the cellulose source was found. Thus, the bleached hardwood
papers chemically modified to increase the carboxylic content in the anhydroglucose, Avicel, cotton linters, sisal holocellulose, and sisal cellulose were made. The
cellulose samples were acetylated in homogeneous solution (DMAc/LiCl) to obtain
soluble polymers in dichloromethane. These soluble samples allowed comparison
of the fluorescence behavior in the solid and liquid states. It was found that the
presence of carboxylic groups in the anhydroglucose units shifts the emission
maxima by 10 nm to shorter wavelength and increases their intensity. Oxidizing
decreased the cellulose PL intensity by creating carbonyl groups. This effect is
common for various cellulose samples regardless of their source [62]. At the same
time, it was observed that position of the PL band (λ max ) is not correlated with
either the average molecular weight (M vis ) or with the crystallinity index. In this
regard the following order was observed: λ max cotton linters > λ max mercerized sisal
cellulose > λ max microcrystalline cellulose whereas M vis mercerized sisal cellulose
> M vis cotton linters> M vis microcrystalline cellulose and the PL intensity (I) for
the microcrystalline cellulose I MCC > I CL cotton linters > I SC sisal cellulose. This
indicates that anhydroglucose units emit as single chromophores without strong
interactions between them.
Some reasons were also discussed as those influenced the PL intensity via
emission quenching. For example, cotton linter pulps, wood pulps, and several filter
papers have been evaluated as substrates for room temperature phosphorescence in
the work [45]. The authors concluded that the difference in performance between the
poorest paper and the best paper is considerably less than an order of magnitude.
In contrast, some other researchers stated that aldehyde and ketone carbonyl
groups have a quenching effect on fluorescence, and the formation of carbonyl
groups after oxygen and ozone lignification decreases pulp fluorescence (see also
above noted), while sodium borohydride is able to recover the PL by reducing
carbonyl groups in the cellulose [62, 63].
The sources of the cellulose PL can be related with external factors. We
emphasize here those related with the pulp processing and treatment. Really, the
residual traces of the proteins or lignin, dityrosine, and coumarin acids were
regarded as possible sources of the cellulose PL [62, 64].
255
unbleached and ozone-bleached hardwood kraft pulps as well as monocarboxyl
cellulose in NaOH solution was studied in the work [61]. The spectra were obtained
in alkaline media, so the dissociation of carboxyl groups as well as the ionization
followed by the isomerization of keto- and aldo-groups took place. The carried
out study had shown that cellulose demonstrates six intensive absorption bands
and the last ones were proposed to assign as related with absorption transitions in
chromophore composition of oxypolysaccharides as follows: carboxyl and carbonyl
groups absorbing at 200–220 and 270–290 nm, respectively, heteroaromatics of
furan and pyron type absorbing additionally at 230–250 and 290–320 nm, as well
as conjugated heteroaromatics absorbing above 300 nm with maximums at 350–370
and 430–450 nm due to the charge-transfer complexes.
In some cases dependence of the luminescent characteristics (the PL intensity and
peak positions) on the cellulose source was found. Thus, the bleached hardwood
papers chemically modified to increase the carboxylic content in the anhydroglucose, Avicel, cotton linters, sisal holocellulose, and sisal cellulose were made. The
cellulose samples were acetylated in homogeneous solution (DMAc/LiCl) to obtain
soluble polymers in dichloromethane. These soluble samples allowed comparison
of the fluorescence behavior in the solid and liquid states. It was found that the
presence of carboxylic groups in the anhydroglucose units shifts the emission
maxima by 10 nm to shorter wavelength and increases their intensity. Oxidizing
decreased the cellulose PL intensity by creating carbonyl groups. This effect is
common for various cellulose samples regardless of their source [62]. At the same
time, it was observed that position of the PL band (λ max ) is not correlated with
either the average molecular weight (M vis ) or with the crystallinity index. In this
regard the following order was observed: λ max cotton linters > λ max mercerized sisal
cellulose > λ max microcrystalline cellulose whereas M vis mercerized sisal cellulose
> M vis cotton linters> M vis microcrystalline cellulose and the PL intensity (I) for
the microcrystalline cellulose I MCC > I CL cotton linters > I SC sisal cellulose. This
indicates that anhydroglucose units emit as single chromophores without strong
interactions between them.
Some reasons were also discussed as those influenced the PL intensity via
emission quenching. For example, cotton linter pulps, wood pulps, and several filter
papers have been evaluated as substrates for room temperature phosphorescence in
the work [45]. The authors concluded that the difference in performance between the
poorest paper and the best paper is considerably less than an order of magnitude.
In contrast, some other researchers stated that aldehyde and ketone carbonyl
groups have a quenching effect on fluorescence, and the formation of carbonyl
groups after oxygen and ozone lignification decreases pulp fluorescence (see also
above noted), while sodium borohydride is able to recover the PL by reducing
carbonyl groups in the cellulose [62, 63].
The sources of the cellulose PL can be related with external factors. We
emphasize here those related with the pulp processing and treatment. Really, the
residual traces of the proteins or lignin, dityrosine, and coumarin acids were
regarded as possible sources of the cellulose PL [62, 64].