Melt grafting copolymerization of glycidyl methacrylate onto acrylonitrile-butadiene-styrene (ABS) terpolymer

2.1 Materials

ABS was purchased from Taiwan Qimei Co., Ltd (Taiwan, China). ABS B338 (GMA carrier) was purchased from General Electric Co. (MI, USA). GMA was purchased from Taian Chemical Trading Company, Shangdong (China), and St from Tianjing Fine Chemistry Institute (China). The free radical initiator DCP was purchased from Shanghai Lingfeng Chemical Solvent Ltd. Co. (China). All other chemicals and solvents were of general purpose grade.

2.2 Synthesis of the grafted copolymers

The grafting of ABS was carried out by mixing ABS, GMA, St, and GMA carrier with a constant DCP content in a twin screw extruder of L/D ratio 40 and screw diameter of 21.7 mm. The temperature profile along the extruder was 170°C, 185°C, 200°C, 215°C, 225°C, and 200°C and the rotor speed was 300 rpm. The rod extrudate was cooled and pelletized. The grafted ABS samples were dissolved in 1,2-dichloroethane and then precipitated in ethanol. The purified ABS-g-GMA copolymers were dried in a vacuum oven at 80°C to constant weight.

2.3 Characterization

The samples were cast into films (0.010- to 0.015-mm thickness) with chloroform as a solvent. Infrared (IR) spectroscopy information on ABS and grafted ABS was obtained on a NEXUS570 Fourier transform IR (FTIR) spectrophotometer (USA). No significant changes were observed in the FTIR spectrum of the grafted ABS after further purification, and this indicated that the procedure was effective.

¹H nuclear magnetic resonance (NMR) spectra were recorded on a JEOL ECX-500 spectrometer at 500 MHz at room temperature. CDCl₃ was used as solvent and tetramethylsilane as an internal standard.

Elemental analysis of the samples was done on a Vario MICRO element analyzer.

The molecular weights (Mws) of ABS-g-GMA were determined by gel permeation chromatography on a Viscotek TDA 302 multidetector. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 ml/min at 30°C.

2.4 Determination of the GD of GMA

The relative GD of GMA was calculated from FTIR spectra. Figure 1 shows the FTIR spectra of pure ABS (ABS) and modified ABS (ABS-g-GMA). Peaks located at 1710 and 2237 cm^-1 are assigned to units of carbonyl (C=O) and acrylonitrile (AN) in grafted copolymer, respectively. The intensity ratios (Ra) of the absorbance peak of C=O at 1710 cm^-1 and a reference peak of AN at 2237 cm^-1 were calculated, which showed the relative amount of grafted GMA.

Figure 1

Infrared spectra of ABS, ABS-g-GMA, and ABS-g-GMA/St. (A) ABS, (B) ABS-g-GMA, (C) ABS-g-GMA/St.

(1)Ra=A1710A2237 (1)

where A₁₇₁₀ and A₂₂₃₇ are the intensities of absorbance peaks at 1710 and 2237 cm^-1, respectively.

Otherwise, the GD and grafting efficiency of GMA were calculated from elemental analysis results and are defined as follows:

(2)GD=monomer grafted on ABSweight of ABS×100% (2)

(3)Grafting efficiency=grafted monomerweight of monomer×100% (3)

3.1 Characterization of grafting

Figure 1 shows the IR spectra of ABS, ABS-g-GMA, and ABS-g-GMA/St. Compared with the FTIR spectra of ABS, ABS-g-GMA, and ABS-g-GMA/St copolymers, the carbonyl vibrations at 1710 cm^-1 and the C-O-C vibrations at 1221 cm^-1 were observed in ABS-g-GMA and ABS-g-GMA/St copolymers besides the inherent vibrations of ABS, which indicates that GMA has been grafted onto ABS and that the melt grafting process is an effective method for the grafting copolymerization of GMA onto ABS. The GD of GMA was calculated from the FTIR spectrum. Peaks located at 2237 cm^-1 are assigned to acrylonitrile (AN) in the grafted copolymer. The intensity ratios (Ra) of the absorbance peak of C=O at 1710 cm^-1 and a reference peak of AN at 2237 cm^-1 were calculated and may be defined as the relative GD.

To further confirm that GMA was grafted on the backbones of ABS, the ¹H NMR spectra of pure ABS, ABS-g-GMA, and ABS-g-GMA/St are shown in Figure 2. Compared with the ¹H NMR spectra of pure ABS, ABS-g-GMA, and ABS-g-GMA/St, a new peak at 3.77 ppm, which could be assigned to the hydrogen proton on the -CH₂-O from GMA, was observed in ABS-g-GMA and ABS-g-GMA/St besides the expected signals for the block copolymer. From the results of FTIR and ¹H NMR spectra, we can conclude that GMA was successfully grafted on the backbones of ABS.

Figure 2

¹H NMR spectra of pure ABS (A) and ABS-g-GMA, ABS-g-GMA/St (B).

Considering the structures of ABS, ABS-g-GMA, and ABS-g-GMA/St, it is possible to determine GD by elemental analysis of the oxygen content of the ABS-g-GMA or ABS-g-GMA/St copolymers. Table 1 shows that the oxygen content of the ABS-g-GMA/St and ABS-g-GMA is apparently more than that of pure ABS, which means that the GMA monomer has been grafted on ABS.

3.2 Reaction mechanism

The reaction mechanism is shown in Scheme 1. From Scheme 1, it is seen that the reaction is very complicated, which includes the production of radicals, the grafting reaction, the cross-linking reaction, and the termination reaction.

Scheme 1

Scheme of the reaction mechanism.

Figures 3 and 4 show the variation of Mw and GD with GMA concentration. For ABS/GMA or ABS/GMA/St, there were grafting reaction and cross-linking reaction. The GD of ABS-g-GMA gradually increased with the increase in GMA concentration, but Mw decreased with increasing GMA concentration, reaching a minimum at 3 phr, and then increased with increasing GMA concentration. The GD of ABS-g-GMA/St gradually increased with the increase in GMA concentration. However, Mw initially increased with increasing GMA concentration, reaching a maximum at 3 phr, and then decreased with further increase in the GMA concentration.

Figure 3

Variation of Mw and GD of ABS-g-GMA with GMA concentration.

Figure 4

Variation of Mw and GD of ABS-g-GMA/St with GMA concentration.

The above results will be discussed on the basis of a mechanism for the free radical grafting of GMA onto polyolefin ABS (Scheme 1) [17–19]. Graft copolymerization is initiated by the generation of free radical species, for instance, by the thermal decomposition of a peroxide, ROOR, into primary alkoxy radicals (RO^·), which may subsequently decompose to secondary radicals. When primary radicals attacked ABS chains, polymer radicals were formed. As can been seen from Figure 3, when GMA concentration was low, many polymer radicals were generated because of low reactive GMA monomers. Thus, degradation reactions of polymers might occur [20, 21]. However, the number of radicals consumed by GMA increased with the increasing of its concentration, stopping many radicals from attacking ABS chains. Thus, grafting and cross-linking reactions of polymers were the main reactions at high GMA concentration. From Figure 4, the increasing of GMA concentration and addition of high reactive St comonomers reduced the number of radicals attacking ABS chains. Grafting reaction and cross-linking reaction were competitive. When GMA concentration was over 3 phr, macromolecules were formed and even gel was produced; thus, the Mw of samples was not true and was lower than that of graft copolymers.

3.3 Effect of GMA concentration

As to the nature of the backbone, the reactivity of the monomer also has an important effect on the grafting reaction. Figure 5 shows that the GD increased with the increasing of GMA concentration in the absence of St and reached a constant after 4 phr. This is because the increase in GMA content leads to higher GD due to the higher probability of a free radical reacting with a GMA molecule before being terminated by the collapse with another free radical. However, the extent of the grafting reaction is governed by the monomer number diffusing throughout the reaction medium, reaching the polymer backbone, and further acting as a trap for radicals. When the grafting process was carried out in the presence of St, the GD increased with increasing GMA concentration. The St acts as a reactive linker between the modifier and the polymer, enhancing the free radical grafting efficiency of GMA on polymers. The curves in Figure 6 have a similar trend with those in Figure 5, which indicates that the values calculated are close to those obtained by FTIR and elemental analysis. In addition, we find that St did not enhance the GD of GMA until its concentration was to the extent.

Figure 5

Effect of GMA concentration on the GD by FTIR.

Figure 6

Effect of GMA concentration on the GD by elemental analysis.

3.4 Effect of initiator concentration

The effect of initiator concentration on the GD of GMA was investigated through IR absorbance ratio. Figure 7 gives the GD of GMA as a function of initiator concentration. As can be seen, the GD of the ABS-g-GMA/St copolymer increased with the increasing of initiator concentration, which might be because the number of free radicals, formed by the decomposition of the initiator, increased along with increasing of the initiator. In addition, from Figure 7, we can also find that a higher concentration of initiator is not favorable to obtain the high GD because when the concentration of initiator reached a certain value, homopolymer was formed more readily than grafting copolymer. In other words, higher levels of DCP favored the chain-radical formation, and the combination of the two radicals will lead to cross-linking.

Figure 7

Effect of initiator concentration on the GD.

The impact strength of ABS-g-GMA/St decreased as the initiator concentration increased, as shown in Figure 8, which may be explained by the grafting reaction occurring most likely in the butadiene region of ABS, and thus the contents of the butadiene region (rubber region) are decreased with increased initiator concentration, which is also reported in Rao et al.’s work [22] and Qi et al.’s work [23].

Figure 8

Effect of initiator concentration on Izod impact strength for ABS-g-GMA/St.

3.5 Effect of GMA carrier

The effect of ABS B338 concentration on the grafting polymerization is shown in Figure 9. We can see that the highest GD was up to 1.35 when ABS B338 was 25 wt%. It is well known that the grafting polymerization depended on the number of active sites available, and the GD increased with the increasing of the grafting sites as a double bond in ABS, so the GD increased with the increasing of ABS B338 concentration and reached the highest value when ABS B338 was 25 wt%. However, if ABS B338 concentration was much higher, the viscosity of the reaction medium was remarkably increased, and the diffusion and mobility of GMA, St, and DCP onto the ABS backbone became difficult, leading to the decreasing of GD.

Figure 9

Effect of ABS B338 concentration on the grafting polymerization.

3.6 Effect of the comonomer

To minimize side reactions, it is important to trap radicals on the ABS backbone as rapidly as possible. Some monomers are more effective in trapping such radicals because of their relatively high solubility in the ABS melt or the inherent reactivity of the monomers. As a comonomer, St can effectively improve the grafting reaction of GMA with ABS. Figure 10 gives the influences of the binary system of St and GMA with various compositions on the grafting of GMA onto ABS. The relative GD (Ra) initially increased with the increasing of St content and reached a maximum (Ra=1.13) at a comonomer composition of 1 (St/GMA molar ratio). Then, it decreased with further increasing of St content. This result showed that a synergistic effect on the grafting process of an St and GMA binary mixture onto ABS occurred, and the St content in the binary monomer mixtures had a considerable influence on the relative GD in the grafting process.

Figure 10

Effect of St concentration on GD (GMA).