Grafting modification of carbon black by trapping macroradicals formed by sonochemical degradation

Abstract

Polymers were grafted onto the surface of carbon black (CB) by the trapping of macroradicals formed by sonochemical degradation of a polymer solution. Long chain radicals of poly(vinyl alcohol) (PVA) formed by sonochemical degradation were successfully trapped by CB and grafted onto its surface. Results showed the method to be suitable for all kinds of CB, even for furnace black with few functional groups. The crushing effect of the ultrasonic forces caused the PVA grafted CB to have a much smaller aggregate particle size and narrower particle size distribution than that of the original CB, as indicated by dynamic light scattering. The PVA grafted CB could be well dispersed in water and a polymer matrix. The compatibility between CB and polymer was significantly improved.

Introduction

Carbon black (CB) is widely used in industry as a filler of polymer materials. Compared with other fillers embedded, the primary particle size of CB is much smaller and the interparticle forces are so strong that aggregates and agglomerates form easily, which leads to its poor dispersion in polymer matrix and thus weakens the potential applications of CB. To solve this old issue, many researchers have concentrated on the surface modification of CB. Grafting of polymers onto the surface of CB has been proved to be an effective method. Tsubokawa [1], [2], [3] reported a series of approaches for grafting of polymers onto CB surface: (a) “grafting from” method: the surface grafting of polymer chains were initiated from initiating groups introduced onto the CB surface; (b) “grafting onto” method: the surface propagating polymer radicals were trapped by CB surface during the polymerization, initiated by a conventional radical initiator in the presence of CB; (c) “polymer reaction” method: polymers having terminal functional groups were reacted with functional groups on the CB surface. However, all of these methods have somewhat limitations, for example, methods (a) and (c) cannot be applied effectively to the grafting of polymers on furnace black and acetylene black with few functional groups. If these two methods were used, rigorous and complicate chemical pretreatment must be done on CB in order to introduce functional groups as grafting sites. Although method (b) was suitable for all kinds of CB unnecessary to be pretreated, more low-molecular weight initiator fragments than growing polymer radicals were trapped by CB. Owing to the problems existed in the method for grafting of polymer on CB, a approach to solve these problems will be proposed in the present study. This method mainly utilized the strong crushing and degradation effects of sonochemistry and the radical trapping nature of CB to realize the grafting of polymer on CB. While CB and polymer solutions were sonicated, the large agglomerates of CB were broken down and the macroradicals formed by sonochemical degradation were trapped by CB, which finally achieved the polymer grafted CB with smaller aggregate particle size.

The degradation of polymers using ultrasonic has been investigated by numerous researchers [4], [5], which was attributed to the cavitation process composed of formation, growth, and collapse of microbubbles. Polymer chains near the collapsing microbubbles were caught in a high-gradient shear field, causing the polymer segments in this shear field to move at a higher velocity than the chains father away from the collapsing cavity. This relative motion of the polymer segments and solvent produced stress on the polymer chain and caused shearing force [6], [7], which led to the breakage of macromolecular C–C bonds, and formed long chain radicals. Tabata and Sohma have detected the formation of macroradicals during ultrasonic irradiation of poly(methyl methacrylate), polystyrene and poly(vinyl acetate) using ESR techniques [8], [9]. If there were radical scavengers in the medium, they reacted with chain radicals.

It was well known that CB was a strong radical scavenger because of its polycondensed aromatic rings [10]. So it was possible to prepare polymer grafted CB by utilizing the polycondensed aromatic rings of CB as grafting sites to trap the macroradicals formed by the sonochemical degradation of polymer solutions. Just utilizing the intrinsic nature of CB, this grafting method could be applied effectively to all kinds of CB and avoided the rigorous pretreatment of CB. Besides, crushing effect that arose from cavitation in sonochemistry would make the large agglomerates of CB break down, for which CB with smaller particle size was obtained, as proved in our previous research [11].