In this study, pulped cellulose fibres were
pre-treated with aqueous morpholine prior to mechanical disruption in the production of cellulose nanofibrils (CNF). The properties of the morpholine pretreated CNF (MCNF) were closely compared with
CNF obtained from carboxymethylation (CMCNF)
and TEMPO-oxidation (TCNF) pre-treatment methods. An investigation of the swelling behaviours of
cellulose in varying concentrations of morpholine
revealed that there is a synergistic behaviour between
morpholine and water in its ability to swell cellulose.
As a result, cellulose pulp dispersed in 1:1 mole ratio of
morpholine to water was well swollen and readily
fibrillated by mechanical shear. Surface chemistry
analyses indicated that the surface of the MCNF
remained unmodified, compared to the CMCNF and
TCNF which were modified with anionic groups. This
resulted in only a slight decrease in crystallinity index
and a minimal effect on the thermal stability of MCNF,
compared to CMCNF and TCNF which showed
marked decreases in crystallinity indices and thermal
stabilities. The average widths of MCNF, CMCNF and
TCNF, as measured from electron microscopic images,
were broadly similar. The higher polydispersity of
MCNF widths however led to a differential sedimentation and subsequent lower aspect ratio in comparison
with CMCNF and TCNF as estimated using the sedimentation approach. Also, the presence of electrostatic repulsive forces, physical interactions/
entanglements and lower rigidity threshold of the
CMCNF and TCNF resulted in higher storage moduli
compared to the MCNF, whose elasticity is controlled
by physical interactions and entanglements. Aqueous
morpholine pre-treatment can potentially be regarded
as an ecologically sustainable process for unmodified
CNF production, since the chemical reagent is not
consumed and can be recovered and reused.

• Aqueous morpholine pre-treatment in cellulose nanofibril (CNF) production: comparison with carboxymethylation and TEMPO oxidisation pre-treatment methods