Styrene-Assisted Free Radical Grafting of GlycidylMethacrylate onto Polyethylene in the Melt

HERVE CARTIER, GUO-HUA HU

Laboratoire d’Extrusion Reactive, ECPM-Departement Polymeres, 4 rue Boussingault, 67000 Strasbourg, France

Received 26 January 1998; accepted 27 April 1998

ABSTRACT: Glycidyl methacrylate (GMA) is a very useful monomer as it bears an epoxygroup which is capable of reacting with various other functional groups. However, itsmelt free radical grafting reactivity onto a polymer backbone is low. In this study, weshow that the use of styrene (St) as a comonomer greatly promotes both GMA’s graftingyield and grafting rate onto polyethylene (PE). It is proposed that, in the presence of St,the dominant mechanism of the free radical grafting of GMA onto PE is that St reactsfirst with PE secondary macroradicals and the resulting styryl macroradicals thencopolymerize with GMA leading to grafted GMA. We also show that the contribution ofSt is not related to an improved solubility of GMA in the molten PE. © 1998 John Wiley& Sons, Inc. J Polym Sci A: Polym Chem 36: 2763–2774, 1998Keywords: free radical grafting; glycidyl methacrylate; polyethylene; styrene; reac-tive extrusion

INTRODUCTION

Vinyl monomers containing functional groups suchas maleic anhydride (MA), ricinoloxazoline malein-ate (OXA), and glycidyl methacrylate (GMA) areoften grafted onto otherwise chemically inert and/orapolar polymer backbones. However, their free rad-ical grafting reactivity is often low. This can beunderstood from the inherent reaction mechanisminvolved. A free radical grafting system often con-tains three types of reactants: a polymer, unsatur-ated molecules like the vinyl monomer (M), and afree radical initiator. Irrespective of their nature, atypical free radical grafting scheme can be repre-sented in Figure 1.1

The first step of the reaction is the formationof primary radicals (RO*) as a result of thethermal decomposition of a free radical initiator(usually a peroxide). The abstraction of a hydro-gen atom from the polymer substrate by a pri-

mary radical leads to the formation of a macro-radical. This latter may then follow one of thetwo competing routes: it either reacts with themonomer, which is desirable, or undergoesstructural change, which is often undesirable.The structural change can be chain crosslinkingif the macroradical tends to react with anotherone (this is the case for a secondary macroradi-cal) or chain fragmentation by b-scission (this isso for a tertiary macroradical). The monomermay also homopolymerize in the presence ofRO*.

This simplified reaction scheme implies thatthe success of a free radical grafting depends verymuch on the competition between the graftingand side reactions (polymerization and chaincrosslinking or fragmentation), which in turn isassociated with the following factors:

1. The rate of formation of the primary radi-cals, which is characterized by the half-life-time (t1/ 2) of the free radical generator used.A good free radical generator should have ahalf-lifetime which is about one-fifth of thereaction time so that once the reaction is

Correspondence to: G.-H. Hu, EEIGM-INPL, LSGC, 1 rueGrandville, BP 451, 54001 Nancy, FranceJournal of Polymer Science: Part A: Polymer Chemistry, Vol. 36, 2763–2774 (1998)© 1998 John Wiley & Sons, Inc. CCC 0887-624X/98/152763-12

2763

over, the free radical generator is also ex-hausted.

2. The ability of the primary radicals to ab-stract hydrogen atoms from the polymerbackbone to form macroradicals.

3. The reactivity of the macroradicals towardthe monomer.

4. The stability of the macroradicals (recombi-nation between or fragmentation of macro-radicals).

5. The tendency of the monomer to homo-polymerize.

Although all the factors listed above can affectthe overall free radical grafting reactivity, thethird one is probably the most important one. Infact, the reactivity of a macroradical toward avinyl monomer is usually low. This is because, onthe one hand, a macroradical of a hydrocarbonpolymer backbone would be nucleophilic and pre-fer to react with an electropositive double bondand, on the other hand, the accessibility of a mac-roradical is much reduced by the bulkiness of themacromolecular chain surrounding it. This is par-ticularly so for vinyl monomers which are bulkyas well. The above analysis implies that the freeradical reactivity of MA should have been highbecause its double bond is highly electropositive.However, this favorable electronic effect is coun-terbalanced by its bulkiness. As for GMA, bothelectronic and steric effects disfavor its free radi-

cal grafting reactivity.The best way to favor grafting against side

reactions is to promote the reactivity between themonomer and macroradicals so that these latter,once formed, will react with the monomer beforethey undergo recombination or fragmentation.One way of doing so is to add a second monomer(or comonomer) which has a high reactivity to-ward the macroradicals, and the resulting macro-radicals are capable of reacting (or copolymeriz-ing) with the grafting monomer of interest. Forexample, addition of styrene (St) as a comonomerimproves markedly the grafting yield of GMA andMA onto polypropylene (PP).1–7 It is proposedthat St reacts first with PP tertiary macroradicalsand the resulting styryl radicals then copolymer-ize readily with GMA or MA.7 In other words,GMA is not grafted directly onto PP macroradi-cals but via St and more specifically styryl mac-roradicals. Since a fraction of MA also forms acomplex with St by an electron transfer mecha-nism (charge transfer complex, CTC; see Fig. 2)under grafting conditions, it could be that thisfraction of MA is grafted onto PP simultaneously

Figure 1. Simplified scheme of a free radical graftingprocess.

Figure 2. Formation of a complex between maleicanhydride (MA) and styrene (St) by an electron trans-fer mechanism (charge transfer complex, CTC).

Figure 3. Illustration of the styrene-assisted freeradical grafting of maleic anhydride (MA) and glycidylmethacrylate (GMA) onto polypropylene (PP).

2764 CARTIER AND HU

with St in the form of the CTC.1,5,6 For steric andelectronic reasons, the St part of the CTC wouldlikely be connected to PP chains. Figure 3 illus-trates the St-assisted free radical grafting ofGMA and MA onto PP.

The objective of this study is to examine theefficiency of this comonomer concept for the freeradical grafting of GMA onto PE. An importantdifference between PP and PE lies in that thehydrogen abstraction from PP chains leads to ter-tiary macroradicals while that from PE yieldssecondary macroradicals. Therefore, the effi-ciency of using St as a comonomer for the PE/GMA/peroxide system is expected to depend onwhether or not the reactivity of St toward PEsecondary macroradicals is greater than GMA.

EXPERIMENTAL

Materials

In this study, a powdery high-density polyethyl-ene free of additives (PE) was used. It had adensity of 0.957 g/cm3 and a melt flow index of 0.2g/10 min at 190°C under a 2.16 kg load. Glycidylmethacrylate (GMA) and styrene (St) were pur-chased from Aldrich and used as received. Unless

specified otherwise, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (DHBP) was used as free radicalinitiator. The grafting efficiency of three addi-tional peroxides was also compared. Table Ishows the chemical structures and half-lifetimesof these four peroxides.

Grafting Procedures

Most grafting experiments were carried out in abatch mixer of Rheocord Haake type which had achamber of 50 cm3 and in which two sigma rotorsrotated in opposite directions to ensure mixing.Liquid monomers (GMA and/or St) and peroxidewere premixed with the PE (40 g) at room tem-perature for about 15 min so as for them to beabsorbed by the polymer powder. The mixturewas then charged to the mixing chamber whichwas preheated to a desired temperature (180°C,unless specified otherwise). After 15 min of mix-ing, samples were taken out of the mixing cham-ber and quenched immediately in liquid nitrogento stop the reaction. For kinetic study purposes,samples were also taken at different time inter-vals. Some experiments were carried out in acorotating self-wiping twin screw extruder ofWerner & Pfleiderer ZSK 30 type in order to com-

Table I.

Code 160ƒC 180ƒC 200ƒC

Half-Lifetime (s)

Chemical Structure

DYBP

DHBP

DCP

BP

128

120

116

71

85

34

29

1.2

15.5

6

5.2

0.3

t©C›H·©OO©C©CH¤CH¤©C©OO©t©C›H·

CH‹ CH‹

CH‹ CH‹

t©C›H·©OO©C©CßC©C©OO©t©C›H·

CH‹

CH‹ CH‹

CH‹

©C©OO©C©

CH‹ CH‹

CH‹ CH‹

©C©OO©C©

O O

STYRENE-ASSISTED FREE RADICAL GRAFTING 2765

pare the grafting performance of both types ofreactors. Figure 4 shows the screw and tempera-ture profiles used. For extrusion experiments, thePE and other reagents were premixed in a tum-bler before they were fed to the extruder. Sampleswere taken quickly from ports A and B and at thedie exit during the extrusion process and thenquenched immediately in liquid nitrogen to stopthe reaction. More detailed information concern-ing the grafting experimental procedures in thebatch mixer and the twin screw extruder can befound elsewhere.2,3

Sample Purification and Grafting YieldMeasurement

A GMA-modified PE sample contained three dif-ferent types of GMA species: grafted GMA,(co)polymerized GMA, and nonreacted GMA (re-sidual monomer). To quantify the amounts ofthose different species, samples were purified ac-cording to the following procedure: They werefirst dissolved in hot xylene and then precipitatedin either methanol or acetone. The use of metha-nol allowed one to precipitate all polymer speciesinvolved in the system including the modified PEand polymerized GMA species (homopolymer ofGMA or copolymer of GMA and St). When acetonewas used, the PE precipitated whereas the ho-mopolymer of GMA or the copolymer of GMA andSt remained soluble. Those two precipitation pro-cedures allowed one to determine the amounts ofgrafted and polymerized monomers. One purifica-tion was found enough to remove residual GMAand/or St monomers. FTIR was used to measure

the grafting yields of GMA and St. To that end,purified samples were pressed into thin films(100–150 mm) at 200°C under 150 bars betweentwo Teflon sheets. Figure 5 shows the IR spec-trum of a GMA-modified PE. In this study, thepeak at 1368 cm21 corresponding to the bendingof the methyl group in the PE was chosen as thereference and that at 1730 cm21 corresponding tothe stretching of the carbonyl group of GMA wasused to determine the amount of grafted GMA.The one at 700 cm21 corresponding to the stretch-ing of the hydrogen atoms of the monosubstitutedaromatic ring of St was chosen for measuring theamount of grafted St. Note that the peak at 700cm21 is somewhat overlapped with a neighboringpeak, which might cause some inaccuracy to the

Figure 4. Screw and temperatures profiles used forthe free radical grafting of GMA onto PE. Screw profile:14/14(1)//42/42(2)//28/28(3)//20/20(14)//KB90/5/28(1)//L20/10(1)//42/42(6)//28/28(8)//(20/20(5)//14/14(5). x/y(z):zright-hand screw elements each being x mm long witha screw lead of y mm. KB90/5/28 (1): one kneadingblock which is 28 mm long and contains 5 discs whichare assembled together 90° one with respect to theother. This type of screw element is good at mixing.L20/10 (1): one left-handed screw element which is 10mm long having a screw lead of 20 mm. This type ofscrew element generates a negative pressure and oftenfollows up a KB as melt seal.

Figure 5. Infrared spectrum of a GMA-modified PE.

Figure 6. IR calibration curves for determining theamounts of grafted St and GMA onto the HDPE. (E) IRcalibration curve for grafted St; it was done using mix-tures of PE and polystyrene containing differentamounts of polystyrene. (F, ‚) IR calibration curve forgrafted GMA; it was done using GMA- and St-modifiedPE samples of which the amounts of grafted GMA weremeasured separately by element analysis (F) and 1H-NMR at 80°C using dichloroethane-d2 as solvent (‚).

2766 CARTIER AND HU

measurement of the amount of grafted St. TheFTIR calibration curve was built up using modi-fied HDPE samples whose GMA and St contentswere measured separately by elemental analysisand 1H-NMR (See Fig. 6).

RESULTS

During the free radical grafting process, glycidylmethacrylate (GMA) is converted to grafted andpolymerized species. The amounts of these spe-cies will be denoted as [GMA]g and [GMA]p, re-spectively. When St is added to the PE/GMA/peroxide system, it is also transformed to graftedand polymerized species, and their amounts willbe denoted as [St]g and [St]p, respectively. Inwhat follows, the contribution of St to GMA’sgrafting will be characterized mainly by GMA’sgrafting kinetics and yield. For the sake of clarity,the grafting systems of PE/GMA/peroxide, PE/St/peroxide, and PE/GMA 1 St/peroxide will be de-noted as GMA system, St system, and GMA 1 Stsystem, respectively.

Contribution of St to GMA’s Grafting Kinetics

Figure 7 shows the grafting kinetics of the PE/GMA/DHBP, PE/St/DHBP, and PE/GMA 1 St/DHBP systems. The free radical grafting of GMAonto the PE seems to proceed progressively as the

amount of the grafted GMA increases with time,although the increase goes relatively fast at thebeginning and then slows down. By contrast, thegrafting of St onto the PE proceeds much morerapidly: the ultimate grafting yield is reachedwithin less than 2 min. These 2 min include notonly the time of the free radical grafting itself butalso the time necessary for melting the PE andhomogenizing the reacting system. One also notesthat on the same amount basis (6.0 phr GMA orSt, which corresponds to 6 g of GMA or St per100 g of PE), the amount of grafted St is muchhigher (4.4 phr) than that of grafted GMA (1.6phr) after 15 min of reaction. In the case of thePE/GMA 1 St/DHBP system ([St]i/[GMA]i 5 1.0mol/mol), both GMA and St are grafted rapidlyonto the PE. The grafting of GMA now proceedsas fast as St in the PE/St/DHBP system. Further-more, GMA’s grafting yield is substantially in-creased.

Polymerized GMA is hardly detected in thePE/GMA/DHBP system. Some GMA is foundpolymerized in the PE/GMA 1 St/DHBP sys-tem, but the amount of polymerized GMA is nothigh (0.9 phr) compared to that of grafted GMA(3.7 phr).

Contribution of St to GMA’s Grafting YieldIncrease

The contribution of St to GMA’s grafting yieldincrease can also be appreciated in Figure 8, in

Figure 8. Influence of adding St on GMA’s graftingyield: [St]i/[GMA]i 5 0 (without St) or 1 mol/mol (withSt), [DHBP]i 5 0.4 phr, temperature 5 180°C; (ƒ)[GMA]g, PE/GMA/DHBP system; (F) [GMA]g, PE/GMA1 St/DHBP system; (Œ) [St]g, PE/GMA 1 St/DHBPsystem; (E) [GMA]p, PE/GMA 1 St/DHBP system; (‚)[St]p, PE/GMA 1 St/DHBP system; (E) [GMA]p, PE/GMA 1 St/DHBP system; (‚) [St]p, PE/GMA 1 St/DHBP system. Polymerized GMA is not detected in thePE/GMA/DHBP system.

Figure 7. Influence of the use of St as a comonomeron GMA’s grafting kinetics and grafting yield: (ƒ)[GMA]g, PE/GMA/DHBP system, [GMA]i 5 6.0 phr; ({)[St]g, PE/St/DHBP system, [St]i 5 6.0 phr, (F) [GMA]g,PE/GMA 1 St/DHBP system, [GMA]i 5 6.0 phr, [St]i/[GMA]i 5 1.0 mol/mol; (Œ) [St]g, PE/GMA 1 St/DHBPsystem; (E) [GMA]p, PE/GMA 1 St/DHBP system; (‚)[St]p, PE/GMA 1 St/DHBP system. No polymerizedGMA was found in the PE/GMA/DHBP system. In allcases, [DHBP]i 5 0.4 phr; temperature 5 180°C.

STYRENE-ASSISTED FREE RADICAL GRAFTING 2767

which GMA’s grafting yield is plotted as a func-tion of the initial amount of GMA in the presenceor absence of St. For the PE/GMA/DHBP system(without St), the amount of grafted GMA firstincreases with increasing initial amount of GMAand then levels off when the initial amount ofGMA is beyond, say, 3 phr. Under the reactionconditions specified in Figure 8, the maximumGMA’s grafting yield is about 1.7 phr, regardlessof the initial amount of GMA charged. By con-trast, when St is added to the PE/GMA/peroxidesystem, much more GMA is grafted. This is par-ticularly so for high GMA loads. For example, thedifference is insignificant between the PE/GMA/DHBP and PE/GMA 1 St/DHBP grafting systemsin terms of GMA’s grafting yield when GMA’sload is low, say, below 2 phr. This is because, inboth cases, the conversions of GMA to graftedspecies are high (not far from 100%) and conse-quently a further increase in GMA’s grafting yieldbecomes impossible. However, when 10 phr GMAis charged, GMA’s grafting yield has reached 5.1phr in the presence of an equimolar amount of Stas opposed to only 1.7 phr for the PE/GMA/DHBPwithout St. In other words, under these reactionconditions, the addition of St has brought about a3-fold increase in GMA’s grafting yield. It is alsointeresting to note that, in the presence of St,GMA’s grafting yield is no longer limited by aplateau but increases with increasing initialamount of GMA. This implies that the use of St asa comonomer provides an additional freedom tocontrol GMA’s grafting yield.

Again, polymerized GMA is not found in thePE/GMA/DHBP system, whatever the initialamount of GMA. On the other hand, some GMA is

polymerized in the PE/GMA 1 St/DHBP system.The amounts of polymerized GMA and St in-crease with increasing initial amount of GMA.Nevertheless, the amount of polymerized GMAremained weak with respect to that of graftedGMA: the former is approximately one-fifth of thelatter.

Figure 9 shows the influence of the initialamount of St on GMA’s grafting yield for a giveninitial amount of GMA (6.0 phr). One notes that,depending on the amount of St charged, GMA’sgrafting yield can vary to a great extent. It is 1.6phr without St and reaches 3.8 phr upon additionof St for [St]i/[GMA]i 5 1.0 mol/mol. Apparently,the contribution of St to GMA’s grafting yieldincrease decreases with increasing [St]i/[GMA]i.This is because, at high [St]i/[GMA]i, the conver-sions of GMA to grafted and polymerized speciesare so high (over 80%) that the effect of furtherincreasing the amount of St is no longer detect-able.

It is also seen from Figure 9 that adding St pro-motes both the grafting and polymerization ofGMA. Nevertheless, under the specified reactionconditions, the amount of polymerized GMA israther small with respect to that of grafted GMA,although it increases with increasing [St]i/[GMA]i.

Effects of the Nature and Concentrationof Peroxide

In addition to DHBP, DYBP, DCP, and BP weretested. DHBP, DYBP, and DCP are commonly

Figure 9. Influence of the molar ratio [St]i/[GMA]i onGMA’s grafting yield: [GMA]i 5 6.0 phr, [DHBP]i 5 0.4phr, temperature 5 180°C; (■) [GMA]g; (F) [St]g; (h)[GMA]p; (E) [St]p.

Figure 10. Comparison of the grafting efficiency interms of [GMA]g between various peroxides at differentconcentrations for the PE/GMA/peroxide and PE/GMA1 St/peroxide systems: [GMA]i 5 6.0 phr, [St]i/[GMA]i5 0 (without St) or 1.0 mol/mol (with St), temperature5 180°C; (h) DHBP, PE/GMA/peroxide system; (E) BP,PE/GMA/peroxide system; (■) DHBP, PE/GMA 1 St/peroxide system; (‚) DCP, PE/GMA 1 St/peroxide sys-tem; (ƒ) DYBP, PE/GMA 1 St/peroxide system; (F) BP,PE/GMA 1 St/peroxide system.

2768 CARTIER AND HU

used for free radical grafting processes due totheir appropriate half-lifetimes (see Table I). Asshown in Figure 10, GMA’s grafting yield in-creases with increasing peroxide concentrationfor all peroxides studied. Moreover, they havesimilar grafting yields except BP. The weak freeradical grafting efficiency of BP is probably due tothe fact that its rate of decomposition is too fastcompared to the overall rate of grafting. (Thehalf-lifetime of BP is only 1.2 s at 180°C.)

It is also interesting to note that GMA’s graft-ing yield of the PE/GMA (6.0 phr) system withoutexternal peroxide is 0.56 phr. When St is added([St]i/[GMA]i 5 1.0 mol/mol), it is increased to 1.6phr. These results may be explained by the factthat the grafting system in the batch mixer isexposed to air, which can generate some free rad-icals at high temperatures (180°C). This is partic-ularly so when the polymer substrate is free ofadditives, which is the case for the PE used in thisstudy.

Effect of Temperature

Figure 11 shows the amounts of grafted andpolymerized GMA at three different grafting tem-peratures: 160, 180, and 200°C for the PE/GMA/DHBP system. GMA’s grafting yield does not varymuch with temperature, although the half-life-times of the peroxide are supposed to be quitedifferent, ranging from a few minutes to a fewseconds. On the other hand, the amount of poly-merized GMA decreases with increasing temper-ature. There is virtually no polymerized GMAfound in the system when the grafting tempera-ture is higher than 180°C. Obviously, this is re-

lated to the ceiling temperature of GMA’s poly-merization system, which is expected to be be-tween 150 and 220°C, like other methacrylates.

In the PP/GMA 1 St/DHBP system (see Fig.12), the amount of grafted GMA is a weak func-tion of temperature. But this time it decreasesslightly with increasing temperature. This maybe due to the fact that, under normal graftingconditions, there is some loss of GMA and St (theboiling points of GMA and St are 189 and 145°C,respectively). The loss of St is expected to be moreimportant because of its lower boiling point. Asfor polymerized GMA and St, their amounts alsodecrease with increasing temperature for the rea-son stated above. When the grafting temperatureis close to 200°C, the concentrations of polymer-ized GMA and St are quite small.

DISCUSSION

Mechanism of St-Assisted GMA Grafting onto PE

It has been shown above that, similar to what wasobserved for the PP/GMA/peroxide system,1,2,3,4,7

the addition of St as a comonomer to the PE/GMA/peroxide system promotes both GMA’s grafting

Figure 11. Influence of temperature on the amountsof grafted and polymerized GMA in the PP/GMA/DHBPsystem: [GMA]i 5 6.0 phr, [DHBP]i 5 0.4 phr; (E)grafted GMA; (F) polymerized GMA.

Figure 12. Influence of temperature on the amountsof grafted and polymerized GMA and St for the PP/GMA 1 St/DHBP system: [GMA]i 5 6.0 phr, [DHBP]i5 0.4 phr, [St]i/[GMA]i 5 1.0 mol/mol; (F) grafted GMA;(Œ) grafted St; (E) polymerized GMA; (‚) polymerizedSt.

Figure 13. Illustration of the styrene-assisted freeradical grafting of GMA onto polyethylene.

STYRENE-ASSISTED FREE RADICAL GRAFTING 2769

yield and the grafting rate. This suggests that themechanism proposed for the PP/GMA 1 St/perox-ide system also works for the PE/GMA 1 St/per-oxide system, as shown in Figure 13. St reactsfirst with the secondary PE macroradicals, andthe resulting styryl radicals then copolymerizewith GMA. Some GMA molecules may also begrafted directly onto PE chains.

How many GMA monomers are grafted ontoPE via a styryl macroradical? In the PP/GMA1 St/peroxide, it was found that, on the average,one styryl macroradical was allowed to graft oneGMA monomer.7 In other words, if the increase inthe amount of grafted GMA as a result of addingSt, [GMA]g

1, is defined as the difference betweenthe amount of grafted GMA obtained in the pres-ence of St, [GMA]g

St, and that obtained without St,[GMA]g

0, and if the amount of GMA’s grafted di-rectly onto PP is the same as [GMA]g

0, then thefollowing relationship held:

@GMA#g1 5 @GMA#g

St 2 @GMA#g0 5 @St#g

Figure 14 plots [GMA]g1 against [St]g for the

PE/GMA 1 St/peroxide system using the experi-mental data of Figures 8 and 9. Clearly the rela-tionship [GMA]g

1 5 [St]g is not valid. The experi-mentally obtained GMA grafting increase issmaller than the prediction (the dashed diagonalline). This implies that all styryl macroradicalsare not copolymerized with GMA. This may beexplained by the fact that the conversion of GMAto grafted species is much lower in the PP/GMA1 St/peroxide system (; 30%) than in the PE/

GMA 1 St/peroxide system (; 60%). In otherwords, the ratio between residual GMA andgrafted St in the PP/GMA/St/peroxide system ismuch higher than that in the PE/GMA 1 St/peroxide system. Thus, the probability for styrylmacroradicals to react with GMA in the PP/GMA1 St/peroxide system is much greater.

Polymerization of GMA in the Melt Free RadicalGrafting System

In a melt free radical grafting process, the mono-mer of interest undergoes not only grafting butalso polymerization. For methacrylates like GMAand methyl methacrylate (MMA), there exists athermodynamic equilibrium between polymerand monomer, which is given by

Tc 5DHp

DSp5

DHp

DS p° 1 R ln[M]

where Tc is the so-called ceiling temperature,DHp and DSp are enthalpy and entropy changesin the polymer propagation reaction, respectively,and DSp

° is the standard entropy change for [M]5 1 mol/L (strictly for unit activity of monomer).Note that the monomer concentration is Tc de-pendent.

The above equation helps one to understandthe experimental facts that, in the PE/GMA/DHBP system, there is practically no homopoly-merized GMA in the temperature ranging from160 and 200°C. Since the values of DHp and DSp

°

are not available for the poly(GMA)/GMA sys-tem, those of the PMMA/MMA system (DHp5 2 56,800 J/mol and DSp 5 2 117 J/(Kmol)1) are used to estimate the concentration ofnonreacted GMA in the PE/GMA/DHBP system.It is found that the concentration of residual GMAin the poly(GMA)/GMA system is 26, 52, and 98g/L at 160, 180, and 200°C, respectively. Assum-ing that 1 L of the PE/GMA/DHBP system isabout 900 g, then the amount of residual GMA inthe system at these three temperatures is 2.9, 5.8,and 10.9 phr, respectively. Since, under the graft-ing conditions, the amounts of grafted GMA at160, 180, and 200°C are 1.5, 1.6, and 1.65 phr,respectively (see Figure 11), the maximumamount of polymerized GMA would be 1.6 phr at160°C and zero at 180 or 200°C. These predictionsare in semiquantitative agreement with the ex-perimental data: 0.4, 0.04, and 0.02 phr at 160,180, and 200°C, respectively.

Figure 14. Quantitative analysis of the contributionof adding St to GMA’s grafting yield increase: experi-mental data versus prediction: ( z z z ) [GMA]g

1

5 [GMA]gSt 2 [GMA]g

0 5 [St]g; (E) data from Figure 8;(‚) data from Figure 9. Note that all the quantities arein “mhr”, which stands for moles per 100 g of resin (PE).

2770 CARTIER AND HU

In principle, the above analysis also applies tothe PE/GMA 1 St/DHBP system. However, no di-rect or indirect thermodynamic data are availablefor the poly(GMA 1 St)/(GMA 1 St) system. Nev-ertheless, it is conceivable that the ceiling temper-ature of the poly(GMA 1 St)/(GMA 1 St) systemwould be higher than that of the poly(GMA)/GMAsystem. Consequently, the amount of polymerizedGMA is expected to be more important. This is inline with the experimental results (see Figures 11and 12).

Grafting-Induced Molecular Change of the PE

During the free radical grafting process, PEchains may become branched and eventuallycrosslinked. When branching and/or crosslinkingoccur, there would be an increase in the viscosityof the PE and, thus, a torque increase in the batchmixer. Figure 15 shows the evolution of thetorque as a function of time for the PE/GMA/DHBP system in the presence of varying amountsof St. First of all, it should be noted that thetorque of the PE/GMA/DHBP system without Stis higher than that of the pure PE. This likelyresults from the recombination of PE secondarymacroradicals. Also, in the PE/GMA 1 St/DHBPsystem, there appears a second peak at about 2 or3 min. Its height increases with increasingamount of St while its location remains almostunchanged. This peak is believed to be a conse-quence of the recombination between styryl mac-roradicals, between GMA macroradicals, or be-tween styryl and GMA macroradicals, as shownin Figure 16. While all these three modes of therecombination of macroradicals can lead to

branching or crosslinking, the one between GMAmacroradicals may be unimportant. This is sup-ported by the fact that the above mentioned peakobserved in the PE/GMA 1 St/DHBP system doesnot exist in the PE/GMA/DHBP system.

What remains to be known is whether modifiedPE chains are only branched or partiallycrosslinked. Apparently, none of the modified PEsamples obtained in this study are crosslinkedsince they are all soluble in hot xylene. This alsoagrees with the fact that the thermal properties ofthe PE are little altered by the free radical graft-ing. For example, the melting and crystallizationtemperatures of the modified PE are pretty muchthe same as those of the pure PE (see Table II).Nevertheless, the free radical grafting bringsabout a slight decrease in the crystallinity of thePE. This might be a consequence of PE chainbranching.

When DHBP is replaced by DCP or DYBP, thesecond peak is also observed in the torque histo-gram of the PE/GMA 1 St/peroxide system (Fig.17). It is interesting to note that the locations ofthis peak are not the same for these three perox-ides (DCP, DHBP, and DYBP). There seems to bea correlation between the peak location and therate of decomposition of the peroxide, which ischaracterized by the half-lifetime. The longer thehalf-lifetime of the peroxide (see Table I), the

Figure 16. Recombination of macroradicals leadingto branching and/or crosslinking of polyethylene duringthe melt free radical grafting.

Figure 15. Influence of the amount of St on thetorque histogram of the PE/GMA 1 St/DHBP system:[GMA]i 5 6.0 phr, [DHBP]i 5 0.4 phr, [St]i/[GMA]i 5 0,0.3, 0.5, or 1.0 mol/mol, temperature 5 180°C.

STYRENE-ASSISTED FREE RADICAL GRAFTING 2771

later the peak appears. However, this peak is notobserved when BP is used as free radical initiator.This is because its half-lifetime is too short at thereaction temperature (1.2 s at 180°C). This is alsoin agreement with the fact that the grafting effi-ciency of BP is much lower than those of the otherperoxides used in this study (see Fig. 10).

If there is indeed a correlation between thepeak location and the half-lifetime of the perox-ide, one would expect a shift of the peak positionto a longer time when decreasing the graftingtemperature. This is what we have observed ex-perimentally (see Fig. 18). The peak is located at300, 200, and 100 s when temperature is at 200,180, and 160°C, respectively.

Keep in mind that the results above concern anonstabilized polyethylene free of any additives.When an industrial stabilizing package is added,the second peak is much smaller or disappearscompletely. This is because some of the species inthe stabilizing package are capable of capturingfree radicals so that the concentrations of macro-radicals in the grafting system are much lowerthan those in the nonstabilized PE. Therefore, the

probability of the recombination between them islargely reduced.

Despite what has been said above, two pointsremain difficult to understand. The first one isthat the torque of the PE/GMA 1 St/DHBP sys-tem goes through a maximum, but its ultimatevalue is close to that of the PE/GMA/DHBP sys-tem. Similar observation is made for the PP (freeof additives)/GMA 1 St/DHBP system. Work isunderway to better understand this phenomenon.

The second point is associated with the locationof the second peak. As discussed above, it may berelated to the half-lifetime of the peroxide used.But why is there a mismatch between the locationof the peak and the half-lifetime shown in TableI? For example, the peaks for DCP, DHBP, andDYBP at 180°C are located at 160, 200, and 250 s,respectively (see Fig. 17). But their half-lifetimesare 29, 34, and 85 s, respectively. Also, when thegrafting temperature is 160, 180, and 200°C, thehalf-lifetime of DHBP is 120, 34, and 6 s, respec-tively, while the peak of the torque is located at300, 200, and 100 s, respectively. This mismatchmay be explained by the fact that the values of the

Figure 18. Influence of temperature on the torquehistogram of the PE/GMA 1 St/DHBP system: [GMA]i5 6.0 phr, [St]i/[GMA]i 5 1.0 mol/mol, [DHBP]i 5 0.4phr.

Table II. Melting Temperature, Crystallization Temperature, and Crystallinity ofthe Pure and Modified Polyethylene (PE) Samplesa

Tm (°C) Tc (°C) Hm (J/g) Hc (J/g) Xc (%)

PE 133.9 116.2 205.2 197.3 70PE modified with a mixture of 6 phr GMA

and 0.4 phr DHBP 134.1 116.6 194.0 190.2 66PE modified with a mixture of 6 phr GMA,

6.15 phr St, and 0.4 phr DHBP 136.0 116.7 191.0 188.1 65

a To determine the crystallinity of the PE, Xc, the enthalpy of fusion of 100% crystalline linear polyethylene is taken as 293 J/g.8

Figure 17. Influence of the type of peroxide on thetorque histogram of the PE/GMA 1 St/peroxide system:[GMA]i 5 6.0 phr, [St]i/[GMA]i 5 1.0 mol/mol, [perox-ide]i 5 0.2 phr, temperature 5 180°C.

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half-lifetimes of the peroxides reported in Table Iare measured under conditions which are verydifferent from the conditions in which the freeradical grafting is carried out.1 The half-lifetimesof the peroxides in viscous media like ours couldbe longer. Another explanation would be that thehalf-lifetime of a peroxide is a measure of the rateof formation of primary radicals. Since the peak ofthe torque is believed to be related to the recom-bination of macroradicals, it is understandablethat there is a correlation between half-lifetime ofa peroxide and location of the peak. However,there is no reason to expect a perfect match be-tween them.

Effect of the Solubility of GMA in the PE Matrix

The solubility of GMA in the molten PE may below because of their disparity in polarity. Thiscould be a limiting factor for its weak grafting ontoPE. In order to verify the point, trials were carriedout upon adding ethylbenzene or xylene to the PE/GMA/DHBP system. It was found that none ofthese chemicals allowed an improvement in GMA’sgrafting yield (see Fig. 19). This implies that thesolubility of GMA is not a problem for its graftingonto PE and that the contribution of St is not re-lated to an accrued solubility of GMA in the moltenPE. Similar experiments were done by replacingstyrene with a-methylstyrene. Again, GMA’s graft-ing yield was not improved either. This can be ex-plained by the fact that, on the one hand, the freeradical reactivity of a-methylstyrene toward PE ismuch smaller than St because of an accrued steric

hindrance effect and, on the other hand, it does notcopolymerize with GMA as easily as St.

Comparison between a Batch Mixer and a TwinScrew Extruder

GMA’s grafting yield was compared between thebatch mixer and the twin screw extruder. Asshown in Figure 20, GMA’s grafting yields ob-tained in the batch mixer are higher than those inthe twin screw extruder. This is probably relatedto air-induced free radical grafting. As pointedout earlier, the batch mixer is a semi-open systemwhile the twin screw extruder is more or lesssealed. Moreover, the former was without nitro-gen purge while a nitrogen stream was ensured atthe hopper of the twin screw extruder. Thus thecontribution of air-induced free radical grafting isexpected to be more important in the batch mixer.This is particularly so when the polymer sub-strate is free of additives, which is the case for thePE used here. A greater air-induced GMA graft-ing in the batch mixer is also related to a muchlonger residence time (15 min in the batch mixerand 2.5 min in the twin screw extruder). As forthe St-assisted GMA grafting induced by the per-oxide per se, both reactors should yield the sameresult as the reaction is over in less than 2 min inthe batch mixer or just after the melting zone inthe twin screw extruder (see Fig. 4).

CONCLUSION

In this paper, we have reported the melt freeradical grafting of glycidyl methacrylate (GMA)

Figure 19. Comparison between St and othercomonomer or solvent in terms of GMA’s grafting yield:(■) styrene; (h) a-methylstyrene; (E) xylene; (F) ethyl-benzene; [GMA]i 5 6.0 phr, [DHBP]i 5 0.4 phr,[comonomer or solvent]i 5 6.15 phr, temperature5 180°C.

Figure 20. Comparison of GMA’s grafting yield be-tween a Haake Rheocord batch mixer and a corotatingself-wiping twin screw extruder with or without St:closed symbols, batch mixer (residence time 5 15 min);open symbols, twin screw extruder (mean residencetime 5 2.5 min); [DHBP]i 5 0.1 phr, [St]i/[GMA]i 5 0(h, E) or 1.0 mol (■, F), temperature 5 180°C.

STYRENE-ASSISTED FREE RADICAL GRAFTING 2773

onto polyethylene (PE). More specifically, we haveshown that the use of styrene (St) as a comonomergreatly promotes both GMA’s grafting yield andgrafting rate.

It has been proposed that in the presence of St,the dominant mechanism of the free radical graft-ing of GMA onto PE is that St reacts first with PEsecondary macroradicals and the resulting styrylmacroradicals then copolymerize with GMA lead-ing to grafted GMA.

The contribution of St cannot be explained bythe argument that its presence in the moltenGMA improves the solubility of GMA and conse-quently increases GMA’s grafting yield becausethis latter is not affected by adding other goodsolvents or comonomers like ethylbenzene, xy-lene, and a-methyl styrene. Temperature does nothave much influence on GMA’s grafting yield butdoes reduce the amount of polymerized GMA.

The authors greatly appreciate the financial and tech-nical support of Borealis of Norway.

REFERENCES AND NOTES

1. G. H. Hu, J. J. Flat, and M. Lambla, Free-radicalgrafting of monomers onto polymers by reactiveextrusion: principles and applications, Chapter 1 inthe book Reactive Modifiers for Polymers, ed. S.Al-Malaika, Ed., Thomson Science & Professional,London, 1997, chapter 1.

2. Y. J. Sun, G. H. Hu, and M. Lambla, Angew. Mak-romol. Chem., 229, 1–13 (1995).

3. Y. J. Sun, G. H. Hu, and M. Lambla, J. Appl.Polym. Sci., 57, 1043–1054 (1995).

4. H. Cartier and G. H. Hu, Poly. Eng. Sci. 38, 177–185 (1998).

5. G. H. Hu, J. J. Flat, and M. Lambla, Makromol.Chem., Macromol. Symp., 75, 137 (1993).

6. J. J. Flat, Doctorate thesis, EAHP, UniversiteLouis Pasteur de Strasbourg, Strasbourg, France,1991.

7. H. Cartier, and G. H. Hu, J. Polym. Sci., Part A:Polym. Chem., 36, 1053 (1998).

8. B. Wunderlich and C. M. Cormier, J. Polym. Sci.,Polym. Phys. Ed., 5, 987 (1967).

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