Abstract
Electrospun nanofibers of polybenzoxazines (PBzs) were fabricated using an electrospinning process and crosslinked by a sequential thermal treatment. Functionalization by the direct sulfonation process followed after the post-electrospinning modification treatment. The first stage of experiment determined the effects of varying the concentration of sulfuric acid as the sulfonating agent in the sulfonation reaction under ordinary conditions. The second stage examined the mechanism and kinetics of the sulfonation reaction using only concentrated H2SO4 at different reaction time periods of 3 h, 6 h, and 24 h. The mechanism of the sulfonation reaction with PBz nanofibers was proposed with only one sulfonic acid (–SO3H) group attached to each of the repeating units since only first type substitution in the aromatic structure occurs under this condition. The kinetics of the reaction exhibited a logarithmic correlation where the rate of change in the ion exchange capacity (IEC) with the reaction time increased rapidly and then reached a plateau at the reaction time between 18 h and 24 h. Effective sulfonation was confirmed by electron spectroscopy with a characteristic peak associated with the C–S bond owing to the sulfonate group introduced onto the surface of the nanofibers. ATR-FTIR spectroscopy also confirmed these results for varying reaction times. The SEM images showed that sulfonation has no drastic effects on the morphology and microstructure of the nanofibers but a rougher surface was evident due to the wetted fibers with sulfonate groups attached to the surface. EDX spectra exhibited sulfur peaks where the concentration of sulfonate groups present in the nanofibers is directly proportional to the reaction time. From surface wettability studies, it was found that the nanofibers retained the hydrophobicity after sulfonation but the inherent surface property of PBz nanofibers was observed by changing the pH level of water to basic, which switches its surface properties to hydrophilic. The thermal stability of the sulfonated nanofibers showed almost the same behavior compared to non-sulfonated nanofibers except for the 24 h sulfonation case, which has slightly lower onset temperature of degradation.
Highlights
Polybenzoxazines (PBz) are one of the recently developed polymers, which have been attracting research interests in the eld of polymers as superior alternatives to thermosetting polymers for high performance applications.[1]
The synthesis of PBz occurs via thermally induced ring-opening polymerization (ROP) of the 1,3-benzoxazine monomer, which results in crosslinked networks.[5,8]
Several works involve the blending of PBz with other polymers such as rubber,[13] polycarbonate (PC),[14] polyurethane (PU),[15] poly(3-caprolactone) (PCL),[14] and poly(S-rdiisopropenylbenzene) (SDIB),[16] which resulted in the modi cation of the functional properties derived from the synergy of polymer components
Summary
PBz, as a polymerization product, plays a special role among the class of highly-crosslinked polymers having benzoxazine (1,3-oxazine cycle condensed with a benzene ring) in their structures.[2,7] The synthesis of PBz occurs via thermally induced ring-opening polymerization (ROP) of the 1,3-benzoxazine monomer, which results in crosslinked networks.[5,8] This simple synthesis method for benzoxazine monomers led to the exploitation of addition of new functional groups, which allowed other possible molecular design for PBz.[1]. The thermal treatment of the nano bers a er electrospinning is a physical method of functionalization, which is an effective post-modi cation strategy for nano bers.[17] In some polymers such as PBz, application of thermal treatment causes crosslinking reaction in the polymer structure. Previous studies have investigated the functionalization of PBz by incorporating functional groups during synthesis or by blending and compositing with other polymers and materials This time, prior to functionalization, PBz was formed into the nano bers and subjected to successive modi cation treatment. This work explores the application of direct sulfonation process to electrospun PBz nano bers, which, to the best of our knowledge, is the rst time such an approach has been used for synthesizing thermally-crosslinked PBz nano bers. The possible interactions that took place, revealed on studying the mechanism and kinetics of sulfonation, provided a new perspective on the functionalization of PBz nano bers for applications in separation processes such as in oil–water mixtures
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