Molecular characterization of Polypropylene

Molecular Characterization of Maleic Anhydride-Functionalized Polypropylene

B. DE ROOVER,'* M. SCLAVONS,' V. CARLIER,' 1. DEVAUX,' R. LECRAS,' and A. MOMTAZ'

'Laboratoire de Physique et de Chimie des Hauts PolymPres, 1 Croix d u Sud, 6-1348 Louvain-la-Neuve, Belgium, 2SoIvay S.A. Direction Centrale des Recherches, Laboratoire Central, 31 0 rue de Ransbeek, B-1 120 Bruxelles, Belgium

SYNOPSIS

This work deals with the molecular characterization of maleic anhydride melt-functionalized polypropylene ( PP-g-MA) . The functionalization mechanism, the nature, the concentration, and the location of grafted anhydride species onto the polypropylene chain are discussed. The polypropylene functionalization was performed using a pre-heated Brabender Plas- tograph ( 190C, 4 min of mixing time). Several concentrations of maleic anhydride and organic peroxide were used for this study. In those experimental conditions, the organic peroxide undergoes an homolytic rupture and carries out a polypropylene tertiary hydrogen abstraction. The resulting macroradical undergoes a 0-scission leading to a radical chain end which reacts with maleic anhydride. When a termination reaction occurs at this first step a succinic type anhydride chain end is obtained. However, oligomerization of maleic anhydride is found to occur more frequently leading to poly( maleic anhydride) chain end. Concentration of both anhydride types and minimal length of the grafted poly (maleic anhydride) were determined. A fraction of maleic anhydride does not react with polypropylene or homopolymerize leading to nongrafted poly (maleic anhydride). 0 1995 John Wiley & Sons, Inc. Keywords: polypropylene 0-scission melt functionalization maleic anhydride poly( maleic anhydride)

INTRODUCTION

literature Survey

Maleic anhydride-functionalized polypropylene is of considerable importance for application as a copol- ymer precursor in polymer blends, as an adhesion promoter with glass or carbon fibers, and even as a processing aid for recycling of plastics waste '-three domains which received considerable attention in recent years. Generally reported functionalization procedure consists in grafting maleic anhydride in the presence of organic peroxide either in the melt '-lo or in the solid state, 11,12 or in ~olu t ion .~ . '~ Few particular methods were also reported: sus- pension method using water l4 or toluene, l5 ene- reaction process, l6 melt process where maleic anhydride and peroxide are solubilized in a s01vent.l~ Use of additive to overcome some side reactions is mentioned.l7-l9 Grafting onto atactic polypropylene

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 33, 829-842 (1995) 0 1995 John Wiley & Sons, Inc. CCC 0887-624X/95/050829-14

is also reported13 as well as functionalization of polypropylene during its synthesis." Nevertheless, those studies need previous complete characterization of the functionalization mechanism and of the structure of PP-g-MA to be significant. Finally, several articles concerning polyethylene and ethylene- propylene rubber maleic anhydride grafting can also be considered as pertinent referen~es.~l-~'

In any case, the most widespread method is the melt state process often called "reactive extrusion method." A definite molecular characterization of PP-g-MA resulting from the reactive extrusion method has not been realized thus far and some controversy remains about the concentration, location, and nature o the grafted anhydride species. Anhydride can, from a chemical viewpoint, be grafted at the chain end, along the chain or included in the backbone.

The occurrence of homopolymerization of maleic anhydride during the grafting of polypropylene is a particular subject of controversy. Homopolymeri- zation of maleic anhydride could lead to grafted or ungrafted poly (maleic anhydride) (PMA) .

829

830 DE ROOVER ET AL.

Gaylord et a1.9J8,39-45 propose mechanisms of homopolymerization of maleic anhydride and grafting of the PMA during the functionalization of polypropylene, polyethylene, and ethylene-propylene rubber. On the other hand, Russell46 invokes a thermodynamic argument based on ceiling temperature of PMA which would preclude any homopolymerization of maleic anhydride at temperature higher than 160C. The formation and consequently the grafting of PMA during maleation of polyolefins in the melt would therefore be impossible. But in both cases no explicit nor obvious experimental observations are given which prove the presence or the absence of PMA in the maleated polyolefins. This particular point will be discussed in detail herein.

From the literature, it can be deduced that the grafting mechanism, concentration, structure, and location of the grafted anhydride are to a large extent influenced by the grafting method (solution, melt state, or solid state) and the reaction conditions (temperature, pressure, concentration, solvent, additive, etc.) . In this work the organic peroxide structure, the polypropylene grade, the melt temperature, and the reaction time were kept constant. Only concentration of maleic anhydride and organic peroxide were varied. In this study, oxygen was assumed not to play an important role in the reactions because the experiments were realized under nitrogen at- mosphere. Although the present study will be strictly valid only for the experimental conditions used for this work it should be generalized providing those conditions are not modified too much.

General Mechanism

Figure 1 summarizes all the possible mechanisms reported in the literature for the grafting of maleic anhydride onto polypropylene in the presence of organic peroxide. Different pathways can correspond to different experimental conditions.

The first steps of grafting mechanism are rela- tively well established and can be summarized as follows: Homolytic scission of each organic peroxide produces two radicals. The decomposition rate (or half-life time of the peroxide) depends only on the temperature. Polypropylene pending hydrogen is abstracted by a radical attack which results in a new radical onto polypropylene. Hydrogen abstraction from one of the tertiary carbons of the polypropylene is generally mentioned for radical stability consid- e r a t i o n ~ . ~ ~

Different possibilities exist for the second steps. They are described in the following: The radical onto a polypropylene chain can lead to a @-scission (reaction A ) or to a maleic anhydride grafting (reaction B ) . The &scission is a fast intramolecular reaction

Hydrogen abstraction

+ Ro' -- + ROH * + AAA E l

pi + A E Depolymerization . Recombination

Or-OR

MA m

Figure 1. Possible reaction mechanisms for the grafting of maleic anhydride onto polypropylene in the melt state in the presence of organic peroxide. MA represents maleic anhydride.

and seems predominant in the melt state in the presence of organic per~xide.~'-~l Nevertheless, reaction B, maleic anhydride grafting on these pri- mary radicals, thus before @-scission, is sometimes sugge~ted.~*'' However, it seems that grafting in solution, or in solid state could favour this mecha- n i ~ m . ~ * " - ' ~ Reaction C seems unlikely as a conse- quence of the stability of the anhydride radical but sequence B + C is, in fact, undistinguishable from sequence A + H (see the following). Reaction D which could lead to the grafting of maleic anhydride by ene-reaction was already studied.16 Very severe conditions are needed to favour this reaction: very low polypropylene molecular mass (high concentration of double bond chain ends) very high concentration of maleic anhydride, high temperature and pressure, and long reaction time. Even maleic anhydride grafting onto polyisoprene by ene-reaction needs severe conditions and leads to moderate yields2' Depolymerization (reaction E ) seems very

MALEIC ANHYDRIDE-FUNCTIONALIZED PP 83 1

unlikely at 190C. Indeed, depolymerization of polypropylene becomes only significant at temper- atures above 300C.52 Moreover, no gaseous products were observed during processing of polypropylene at 190C even for long mixing time. Transfer and recombination (reactions F and G ) cannot auto- matically be discarded and need further discussions. Maleic anhydride end chain grafting (reaction H ) is generally ~ u g g e s t e d . ~ * ~ , ~ , ~ Recombinations ( reactions I and J ) and mainly grafting of poly (maleic anhydride) (reaction K ) are subjects of contro-

This work intends to contribute to the elucidation of the most probable mechanism for maleic anhydride grafting on polypropylene in our experimental conditions within all the possibilities described in Figure 1. This requires accurate molecular mass determination and determination of nature, location, and concentration of grafted anhydride.

versy.6.9,18,39-45

EXPERIMENTAL

Materials

Commercial maleic anhydride grafted polypropylene, pure polypropylene, maleic anhydride, and organic peroxide ( 1,3-di-tert-butylperoxyisopropyl benzene, Perkadox 14) were kindly supplied by Sol- vay & SA. n -0ctadecylsuccinic anhydride came from Alfa Product. It was purified by recrystallization from acetone solution. Its final purity (97% ) was verified by titration. Toluene (99% purity) and 1,2,4- trichlorobenzene (vacuum-distilled) were purchased from Merck-Belgolabo. Benzyl alcohol (99% purity) was provided from Janssen Chimica. Methanol was distilled from a technical grade. Poly ( maleic anhydride) (Belclene 200) was supplied by Ciba Geigy and was purified by reprecipitation. This product was analysed by NMR, SEC and MS. Belclene 200 was identified as mainly constituted of maleic anhydride oligomers containing less than 10 units.53

PP-g-MA Synthesis

Brabender Plasticorder

The Brabender Plasticorder was equipped with an electrically heated W50EH mixing device of 50 mL volume. Oxidation of mixed polymers was largely reduced by a nitrogen flow of 5 L/min above the mixing chamber. No mechanical pressure was applied on the polymer during mixing. The charge processed in the Brabender Plasticorder was 40 g. All the reactants (maleic anhydride, organic peroxide, and polypropylene) were dry mixed together before their fast (< 1 min) introduction in the pre-

heated Brabender Plasticorder. The mixing speed was fixed at 75 rpm, the reaction time duration at 4 min, and the temperature at 190C.

Powder Mixing

Some grafting experiments were performed on pow- ders obtained by dry mixing polypropylene, organic peroxide, and maleic anhydride. Then the heating was performed in glass sealed tubes in an electrically heated furnace. Temperature was regulated within 1C. Thermal treatments of PP-g-MA were also performed in such glass sealed tubes, the conditions (temperature, time) of which will be mentioned in the following.

Analytical Characterization

Titration of Anhydride Content

The anhydride concentrations present in raw, heated, and washed PP-g-MA samples were determined by titration of the acid groups derived from the anhydride functions. After dissolution of 1 g of PP-g-MA in 100 mL of toluene at boiling temperature, 200 pL of water were added to hydrolyze anhydride functions into carboxylic acid functions. The boiling temperature was maintained for 1 h. Car- boxylic acid concentration was determined directly by alkali titration using 0.025N potassium hydroxide in methanol/benzyl alcohol 1/9 (v/v) . The indi- cator used was a solution of 1% Phenolphthalein in methanol. The PP-g-MA was completely soluble at the boiling temperature and did not precipitate during titration. A blank was carried out by the same method.

FTIR Spectroscopy

FTIR spectra were recorded on a Perkin-Elmer FTIR Spectrometer 1760-X from 4000 to 400 cm- with a 0.5 cm- resolution. For some samples, absorption bands between 1880 and 1690 cm- were mathematically analyzed by an iterative curve-fitting software called IGOR provided from WaveMetrics and working on an Apple Computer. Each absorption band was approximated by a Lo- rentzian function.54 The iterative software defines location, amplitude and half-band width of each band to restore the original spectrum by the addition of all Lorentzian functions. Each Lorentzian function is considered to be the actual absorption band. Maximum values of these Lorentzian curves are considered as absorbance values for quantitative analyses.

Films of 50-100 pm thickness were obtained by compression-molding 0.1-0.2 g sample between

832 DE ROOVER E T AL.

2000

1600 - - E 9 1200- 2 g 800- i-

400 -

0

PTFE recovered aluminium sheets under 1 MPa pressure at 190C for 30 s. These films were dried at 12OOC for 20 h to evaporate the unreacted maleic anhydride as well as to perform complete cyclization of any diacid into the cyclic anhydride form. For accurate characterization of PP-g-MA it is of great importance to insure total elimination of all the unreacted maleic anhydride and complete cyclisation of carboxylic acid into carboxylic anhydride. Un- reacted maleic anhydride gives rise to absorption bands in the same region than grafted anhydride ( 1785 cm-' ) which prevents accurate determination of the grafted anhydride concentration. Elimination of unreacted maleic anhydride can be verified by the disappearance of a characteristic absorption band at 720 cm-' associated to the carbon/carbon double bond of maleic anhydride. Complete cyclization of carboxylic acid into a carboxylic anhydride form is of great interest to simplify the FTIR spectrum. This cyclization is assumed by the disappearance of the absorption band at 1715 cm-' , assigned to the carboxylic acid. To check that anhydride are really grafted on the polypropylene chains, some samples were purified by dissolution in boiling toluene followed by precipitation in acetone.

PP-g-MA model compounds were rz -0ctadecyl- succinic anhydride and poly (maleic anhydride). FTIR analysis of these model compounds was performed after incorporating various concentrations in melted polypropylene using the Brabender Plas- ticorder. This strategy was adopted to take into account the polypropylene effect onto carboxylic anhydride absorption bands. This procedure enables the attribution of absorption bands and the determination of molar absorption coefficients.

All analyses are performed for

this mixing time

1 *.. a s . . . . . . . . . .

' I ' I ' I . 0 '

Size Exclusion Chromatography (SEC)

Molecular weight measurements of PP-g-MA were carried out by high-temperature size exclusion chromatography. The chromatograph was a Waters 150C equipped with 2 Shodex columns AT-80M/S an 1 Shodex column Styragel 300 A. A differential refractometer detector coupled with a "microVAX Data Station" provided by Digital was used for re- cording and analyzing the signal. Permeation solvent was vacuum-distilled 1.2.4-trichlorobenzene stabilized with 2% Irganox 1010. Samples dissolution was achieved at 160C during 1 h. Concentrations were in the range of 6 to 8 mg/mL. Injection volume was 120 pL. The system was maintained at 135C during the analyses. Precautions similar to those described for FTIR were taken before molecular mass analyses by high-temperature SEC, viz., elimination of unreacted maleic anhydride and cyclization of maleic carboxylic acid. A polypropylene calibration was

used for SEC analyses. In the present case, the amounts of grafted anhydride remained low and it was assumed that the hydrodynamic volume in solution was not largely modified.

RESULTS AND DISCUSSION

Mixing Torque Determination

Figure 2 shows an example of the mixing torque obtained during melt reaction of polypropylene with maleic anhydride in the presence of organic peroxide. After a strong decrease of the torque due to the plas- ticization of the polypropylene the torque becomes nearly constant. In this work all the samples were characterized after 4 min of mixing time.

Figure 3 presents the torque measured after 4 min of mixing as a function of the concentration of maleic anhydride and organic peroxide introduced in the samples. It appears that, for the selected experimental conditions, the torque depends on organic peroxide content but not on maleic anhydride concentration.

Molecular Weight Determination

Figure 4 presents number-average molecular weights measured after 4 min of mixing as a function of the concentration of maleic anhydride and organic peroxide for all the samples. Figure 4 shows that the molecular weight actually depends on organic peroxide content but does not depend on the maleic anhydride concentration. Dispersion of the results corresponds to the reproducibility of the SEC analysis. The decrease of molecular masses is attributed to the well known p-s~iss ion .~~-~ '

Figure 5 details the mechanism of this p-scission.

0 1 2 3 4 5

Mixing time (min)

Figure 2. Example of torque vs. mixing time during the grafting of maleic anhydride onto polypropylene in the presence of organic peroxide. Conditions: 190C, 75 rpm, maleic anhydride 1% by weight, organic peroxide 1.2% by weight, 5 L/min Nz flow.

MALEIC ANHYDRIDE-FUNCTIONALIZED PP 833

1200

900 E cil v 2 600 + 4.80%MA g I- 300

0

0.0 0.5 1 .o 1.5 Peroxide weight percent

Figure 3. Torque after 4 min of mixing as a function of maleic anhydride and organic peroxide concentrations introduced in polypropylene. Conditions: 190C, 75 rpm, 5 L/min N2 flow. % MA: wt% of maleic anhydride used during the functionalization.

As it can be observed on Figure 5, p-scission gives rise to one radical chain end and one carbon-carbon double bond chain end. The molecular mass determination allows to calculate the efficiency of the organic peroxide to promote p-scission in polypropylene. Table I gives a comparison between the measured molecular masses of the samples and the calculated values considering that each peroxide radical induces one 0-scission. Calculation is performed using eq. ( 1 ) derived from demonstration in the following.

The difference between the number of polypropylene chains after and before peroxide treatment is equal to the number of polypropylene chains created by this treatment; thus:

1 / M, - 1 / M: = amount of polypropylene chains created by peroxide treatment (in mol/g)

80000

- - m m 60000

0 0.25OhMA A 1 .m MA -

.- + 4.80 V ~ M A i a & 40000 I=

20000 0.0 0.5 1 .o 1.5

Peroxide weight percent

Figure 4. M,, as a function of maleic anhydride and organic peroxide concentrations introduced in polypropylene. Conditions: 190C, 75 rpm, 5 L/min N2 flow. % MA: wt % of maleic anhydride used during the functionalization.

Figure 5. Detailed mechanism of polypropylene @-scission.

If each radical issued from the peroxide splitting induces one @-scission, then:

l / M n - 1/M: = [Rad]

with [ Rad] = the radical concentration (in mol/g) . Using [ PP ] = 1 /ME the initial polypropylene

concentration viz. the initial number of mol of polypropylene/ g, this relation can be transformed in eq. (1):

where [ PP] = polypropylene concentration (in mol/ g), [ Rad] = radical concentration (in mol/g), and ME = 60000 = initial number-average molecular weight (in g/mol).

In eq. ( I ) , [ PP] is the actual polypropylene concentration (in mol/g) taking into account the number of chains of polypropylene and not the number of structural units. For low concentrations of organic peroxide (0.01-0.05% ) the ratio between the calculated and experimental M,, lies close to unity ( 1.2 and 1.1 ) , while for medium to high concentrations of organic peroxide (0.25-1.25% ) , it decreases from 0.6 to 0.2. This behavior can be explained by the stoechiometric attack of the peroxide leading to p- scission for low peroxide (0.01-0.05% ) concentrations. For higher peroxide concentrations ( 0.25- 1.25% ) , the difference between calculated M,, and experimental M, may be due to radical recombination: more and more recombinations occur when organic peroxide concentration increases.

Those results are very important because they can lead to conclude that grafting mechanism is not occurring through path B of the Figure 1. Indeed, grafting of maleic anhydride (path B) onto the polypropylene chain competes with p-scission (path A ) and thus increasing concentration of maleic anhydride would progressively reduce the molecular weight decrease through p-scission and that is not the case.

Path F consisting of a transfer reaction seems not to occur to a large extent: the experimental molecular weight decrease is never significantly larger than the calculated thus increasing concentration of maleic anhydride would progressively reduce the molecular weight decrease through p-scission and that is not the case. Path F consisting of a transfer


Table I. Comparison between Experimental M,, and Calculated M,, of PP-g-MA.

Peroxide Experimental M,," Calculated M,,/ (wt %) (SEC) Calculated M,b Experimental M,,

0.00 60,000 60,000 1 0.01 46,000 55,000 1.2 0.05 42,000 44,000 1.1 0.25 36,000 21,000 0.6 1.25 27,000 6000 0.2

a Average value for all maleic anhydride concentrations. Obtained considering that one radical induces one 0-scission.

reaction seems not to occur to a large extent: the experimental molecular weight decrease is never significantly larger than the calculated values based on the steochiometric attack of the chain by the peroxide leading to @-scission.

Molecular Weight-Mixing Torque Relationship

In Figure 6 a linear relationship can be observed between the logarithm of the mixing torque and the logarithm of M,. Taking into account that Braben- der Plasticorder mixing torque is a function of melt viscosity, the slope of 3.2 does not disagree with rheological theory55 nor with similar results on other polymer^.^^-^^

Titration of Grafted Anhydride

Grafted Anhydride Concentration

Titrations were performed onto washed and dried PP-g-MA to determine the amount of grafted anhydride. Results are summarized in Figure 7 which shows that grafted anhydride concentrations in- crease with the concentration of maleic anhydride as well as with the concentration of organic peroxide used for the reaction.

8.6 - y = -27.9 + 3.2X

F k 0.87769

E .-

5.0

-1

3.2 10.1 10.5 10.9 11.3

Logarithm of Mn

Figure 6. of M,.

Logarithm of the mixing torque vs. logarithm

Grafted Anhydride Concentration and Chain Ends Concentration of the PP-g-MA

Following the reaction mechanism discussed before, it is expected that polypropylene undergoes a @- scission after the attack by organic peroxide and that grafting takes place a t the new radical chain end (paths A and H of Figure 1 ) . In this case it is of great interest to compare the grafted anhydride concentration onto polypropylene and the polypropylene radical chain ends concentration created by P-scission. Equation ( 2 ) allows the calculation of the concentration of radical chain ends which were generated by @-scission during grafting. This equation is easily deduced thanks to the demonstration of eq. (1)

where N = concentration of radical chain ends created during grafting (in peq/g), M; = number average molecular weight before grafting (in mol/g) , M, = number average molecular weight after grafting (in mol/g), and 106 = scaling factor converting mol / g into peq / g.

300

200

100

0 0.0 0.5 1 .o 1.5

Peroxide weight percent

Figure 7. Grafted anhydride concentrations determined by titration vs. maleic anhydride and organic peroxide concentrations introduced in polypropylene. Conditions: 190C, 75 rpm, 5 L/min N2 flow. % MA: wt % of maleic anhydride used during the functionalization.


Equation 2 takes into account the P-scission mechanism described in Figure 5 in which one radical chain end is always accompanied by a carbon- carbon double bond chain end. Consequently, concentration of radical chain ends is just the half of the concentration of chain ends concentration created during grafting.

In Figure 8 concentrations of grafted anhydride are plotted as a function of reciprocal M, for each sample. The straight line represents the concentration of radical chain ends generated during grafting and calculated using eq. ( 2 ) .

Figure 8 shows that, for most samples, the concentration of grafted anhydride is higher than the concentration of new radical chain ends created by the &scission mechanism. Referring to reaction paths summarized in Figure 1, several mechanisms can explain this behavior. The first one was already mentioned and is described by path B in Figure 1. It was previously discarded because no decrease of molecular weight during grafting of maleic anhydride would be observed if the reaction stops after grafting. No real variations of molecular weight versus grafted anhydride concentration is observed in our experimental conditions. Scission after grafting described in path C of Figure 1 could be invoked. Effectively it is important to noticed that paths B and C (grafting followed by scission ) give the same products and cannot be distinguished from paths A and H (0- scission followed by grafting). Nevertheless, the former sequence seems less probable because this mechanism requires a hydrogen abstraction by the anhydride radical. This radical should be stabilized by resonance effect5' as described in Figure 9.

Another explanation consisting in radical recombination is illustrated in reaction I of Figure 1. This

150 I I

100

50

0

0 0.050/oMA 0 0.25VoMA A l.ooo/oMA ' + 4.80 %MA

' A

1.5 3.0 4.5

llM, (rnol/g)

Figure 8. Grafted anhydride concentration determined by titration and radical chain ends concentration calculated from eq. ( 2 ) vs. reciprocal M,. Conditions: 190"C, 75 rpm, organic peroxide 0.01 to 1.2%, 5 L/min N2 flow. % MA: wt % of maleic anhydride used during the functionalization.

r

Figure 9. Radical stabilization by resonance effect.

one cannot occur to a large extent in the experimental conditions. Indeed, if this radical recombination took place, the molecular masses would be higher, the higher the amount of grafted maleic anhydride. As already pointed out, this behavior was not observed. Finally, as suggested by Gaylord the formation of poly (maleic anhydride) during functionalization of polypropylene (path B + H + K ) should be envisaged. This polymerization of maleic anhydride was actually observed. A future study will detail the homopolymerization of maleic anhydride in our experimental conditions53 and the presence of free poly (maleic anhydride) in highly grafted PP- g-MA. Indeed, maleic anhydride polymerization can explain that, for many samples of PP-g-MA, concentration of grafted anhydride is higher than the concentration of radical chain ends.

Infrared Analysis of PP-g-MA

FTIR Spectrum of PP-g-MA

Figure 10 shows a FTIR spectrum of a polypropylene processed in the presence of organic peroxide and

2000 1900 1800 1700 1600 ls00

Wave number (m-11

Figure 10. Infrared spectra of PP and PP-g-MA between 2000 and 1500 cm-'. (A) Polypropylene processed with organic peroxide and containing all processing additives except maleic anhydride. ( B ) PP-g-MA. Spectrum obtained after drying the samples at 120C during 20 h under vacuum.


processing additives but without maleic anhydride (blank) and a FTIR spectrum of a PP-g-MA. Drying at 12OOC for 20 h under vacuum was applied to elim- inate free maleic anhydride which could be confused with grafted anhydride and to transform carboxylic acid into carboxylic anhydride.

As it can be seen in Figure 10, the FTIR spectrum of PP-$-MA shows two intense overlapping absorption bands at 1784-1792 cm-' and weak absorption bands around 1850 cm-'. No absorption bands are observed at 1784-1792 cm-' for the polypropylene sample degraded in the presence of the same peroxide under the same experimental conditions. Ab- sorption bands at 1784-1792 cm-' and around 1850 cm-' can be assigned to grafted anhydride because five members cyclic anhydrides exhibit an intense absorption band near 1780 cm-' and a weak absorption band near 1850 cm-' due to symmetric and asymmetric C = 0 stretching respectively.60,61 As it

1840 1800 1760 1720

Wave numbex (cm-1)

1840 1800 1760 1720

Wavenumba(cm-I)

ID

1840 1800 1760 1720

Wave numbs (cm-1) 1040 1800 1760 1720

Wave numbs (cm-1)

1840 1800 1760 1720 Wave numbs (cm-1)

Figure 11. Curve fittings of infrared spectra of four industrial samples of PP-g-MA and of a polypropylene sample containing all processing additives except maleic anhydride (blank). (A) PP-g-MA 001, ( B ) PP-g-MA 002, (C) PP-g-MA 003, ( D ) PP-g-MA 004. See Table I11 for characteristics. (E) Polypropylene processed with organic peroxide and all processing additives except maleic anhydride.

2000 1900 1800 1700 16M) 1500

Wave numbex (cm-1)

Figure 12. Infrared spectra of n-octadecylsuccinic anhydride dispersed in polypropylene. ( A ) Polypropylene containing processing additives and n-octadecylsuccinic anhydride. ( B ) Polypropylene containing only processing additives.

can be seen on Figure 10, the absorption bands at 1784-1792 cm-' seem to result from the overlapping of two different absorption bands, which should therefore be assigned to two different anhydride species. To identify the two bands more clearly, curve-fitting of this spectrum was undertaken into a series of Lorentzian curves54 defined by their position, half bandwidth, and intensity. A curve fitting procedure restores the actual spectrum from these Lorentzian curves. Only these two intense absorption bands at 1784-1792 cm-' (symmetric stretching) will be taken into account in the present article for identification and calibration.

Figure 11 shows curve fittings of four industrial samples ( A-D ) and a fitting realized on the polypropylene sample containing organic peroxide and processing additives but no maleic anhydride (E) . As it can be seen in Figure 11, six absorption bands (or part of absorption bands) are found by curve fitting. Several of those absorption bands are due to polypropylene or processing additives [Fig. 11 (E) 3 . Absorption bands I, 11, and VI are due to polypropylene while absorption band V is due to a processing additive. Only absorption band I11 at 1792 cm-' with a half bandwidth of 10 cm-' and absorption band IV at 1784 cm-' with a half bandwidth of 25 cm-' correspond to anhydride functions. For the four samples, position and half bandwidth of those absorption bands are reproducible although concentrations of grafted anhydride range within one order of magnitude.

Absorption Band Assignment

With a view to identify the two new absorption bands detected in the PP-g-MA spectrum, some an-


Table II. Position and Half Bandwidth of the Anhydride C=O Stretching of Model Compounds Blended in Molten Polypropylene

Position Half Bandwidth Model Compound Formula (cm-' ) (cm-')

Citraconic anhydride 1780 10

10

10

Poly(ma1eic anhydride)

Maleic anhydride 1780 O f y 0

10

hydride model compounds were selected. For example, Figure 12 shows the FTIR spectra of 0.5 wt % of n -octadecylsuccinic anhydride dispersed in polypropylene and of the blank sample. Table I1 gives position and half bandwidth of anhydride absorption bands of these model compounds blended in molten polypropylene at low concentrations (0.5 wt % ) . As can be seen in Table 11, different positions are observed for the same symmetric stretching band of different anhydrides. Moreover, the half bandwidth of poly (maleic anhydride) is clearly larger than the others. In PP-g-MA, a band is observed at 1792 cm-' with a half bandwidth of 10 cm-' and a second band is observed at 1784 cm-' with a half bandwidth of 25 cm-'. It is thus proposed to asso- ciate this later one to poly ( maleic anhydride ) and the former one to succinic anhydride end-groups. Indeed, upon reaction a single maleic anhydride group undergoes a transformation into saturated anhydride (succinic type) as described in Figure 1.

Influence of Model Compound Concentrations and of Temperature

The influence of concentration and temperature on FTIR spectra of polypropylene/model compound blends was also studied. At high concentrations (e.g., 2 wt % of n-octadecylsuccinic anhydride) two overlapping absorption bands situated at about 1780 and 1792 cm-' are observed when FTIR analysis is performed at room temperature. When the temperature

is increased the intensity of the absorption band at 1780 cm-' decreased while the intensity of absorption band at 1792 cm-' increased. When the temperature reached 60C the absorption band at 1780 cm-' totally disappeared and the intensity of absorption band at 1792 cm-' reached its maximum. This behavior was attributed to physical interactions between anhydride dipoles. Those are only possible if the concentration of anhydride is high and if thermal agitation does not suppress the interactions.

The same experiment was undertaken with poly (maleic anhydride). For this model compound such a behavior was never observed absorption band at 1784 cm-' is not modified with concentration or temperature. Indeed, in poly (maleic anhydride ) the anhydride groups are chemically linked together and physical interactions between anhydride dipoles should be always possible even at high temperature or at low concentration.

FTIR analyses of PP-g-MA were performed at high temperature. The infrared spectra of PP-g-MA at 25C before heating ( A ) , at 180OC (B) , and at 25C after cooling ( C ) are reported in Figure 13. In Figure 13, it is shown that even at about 180C the two overlapping absorption bands of the cyclic car- bony1 anhydride at 1784 and 1792 cm-' are not modified (spectrum B ) . At this temperature, the FTIR polypropylene spectrum becomes similar to atactic polypropylene due to the melting. Conse- quently, it can be assumed that the two overlapping absorption bands at 1784 and 1792 cm-' observed


Figure 13. Infrared spectra of PP-g-MA at 25C before heating: (A) , at 180C (B) , and at 25C after cooling (C).

in PP-g-MA correspond effectively to two anhydride species. When the temperature decreases to 25C the usual aspect of semi-crystalline PP-g-MA FTIR spectrum appears again ( spectrum C ) .

In the Figure 1 those results support paths A + H + J (with R = hydrogen) leading to graft succinic anhydride and paths A + H + K leading to graft poly ( maleic anhydride). Further experiments will be undertaken in the following section with a view to corroborate the above assignments concerning PP-g-MA chemical structure.

Depolymerization of Crafted

Poly (maleic anhydride)

Poly(ma1eic anhydride) is reported to exhibit a ceiling temperature of about 150"C.62,63 This temperature seems quite low after examination of the results presented in this article and with those dealing with the oligomerization of maleic anhydride at 190C reported elsewhere.53 This can arise from the fact that the thermodynamic parameters used for the ceiling temperature calculation were not adapted to the polypropylene melt functionalization conditions. Indeed the AH, A S , and the ceiling temperature values were obtained for maleic anhydride homopolymerization in benzene solution.63 Extrap- olation for the maleic anhydride melt homopolymerization and for the polypropylene melt grafting conditions at high temperature is therefore certainly not obvious. Consequently, the ceiling temperature argument which was mentioned to preclude any homopolymerization of maleic anhydride may be inaccurate.

Prolonged heating of PP-g-MA at a temperature of 165C does not lead to poly( maleic anhydride) depolymerization and no decrease of the 1784 cm-' absorbance is observed. However, in another experiment, PP-g-MA 001 was heated at 300C for 20 h in a glass tube, continuously evacuated to a pressure

1900 1850 1800 1750 1700 1650

Wave number ( c m ~ l t

Figure 14. Infrared spectrum of PP-g-MA 001 ( A ) (see Table I11 for characteristics) and PP-g-MA 001 heated at 300C under high vacuum during 20 h (B) .

of about 1 mm Hg. Figure 14 shows anhydride region of FTIR spectra for the original PP-g-MA ( A ) and for the thermally treated PP-g-MA ( B ) . In Figure 14, the intensity of the 1784 cm-' infrared band [assigned to poly (maleic anhydride)] decreases with regard to the intensity of the 1792 cm-' infrared band. Figure 15 shows curve-fitting performed on these spectra.

Figure 15 shows that the ratio between poly ( maleic anhydride ) absorbance and succinic anhydride absorbance decreases from about 5 before heating to 2 after heating. The point quantitative analysis of this article enables the concentration calculation of single succinic anhydride and poly ( maleic anhydride) from curve-fitted spectra. Calculations performed following this calibration gave the absolute values 34.9 and 10.3 peq/g respectively for poly ( maleic anhydride) concentrations before and after heating while succinic anhydride concentration did not change significantly (6.9 to 6.2 peq/g). This result conclusively supports the assignment of the 1784 cm-' infrared band to poly (maleic anhydride).

I* n

1840 1800 1760 1720 1840 1800 1760 1 7 2 0

Wavcnurnba (cm-1) Wave number (cm-1)

Figure 15. Curve fitting of infrared spectra of PP-g-MA 001 (see Table I11 for characteristics) (A) and PP-g-MA 001 heated at 300C under high vacuum during 20 h (B) .


+

" 6 0

I

0

0

mechanism was performed during melt processing (Brabender Plasticorder ) by adding maleic anhydride a sufficiently long time after the organic peroxide to prevent any radical grafting: pure polypropylene was mixed in the Brabender Plasticorder ( 19O"C, 75 rpm) and 1 wt % of organic peroxide was added to promote @-scission and thus also unsaturated end groups formation as shown in Figure 5. Three minutes after the addition of the organic peroxide, the mixing torque was constant and corre- sponded to a M , of about 25,000 g/mol. One minute after the stabilization of the mixing torque, 5 wt % of maleic anhydride were added and allowed to react during 4 min. Infrared analysis were performed after this reaction time and very low amounts of grafted anhydride were detected. Those concentrations ( 1 wt percent of organic peroxide and 5 wt percent of maleic anhydride) are in fact the most favorable of all our experiments for the ene-reaction and even in this case ene-reaction mechanism remains negligible.

Consequently, the experimental conditions used for the melt maleic anhydride grafting of polypropylene with organic peroxide are, by far, too weak to promote the ene-reaction with high yield and path D in Figure 1 does not occur significantly in our experimental conditions.

Figure 16. Mechanism of the ene-reaction. Following this mechanism, more than one anhydride per polypropylene chain could be grafted.

ene-Reaction

Path D of the Figure 1 is called the ene-reaction. This path is detailed in Figure 16 which shows that more than one maleic anhydride for one polypropylene chain can be grafted by this mechanism. Fol- lowing the literature, 16,29 this reaction is possible but needs drastic conditions: very low molecular mass to promote a high concentration of unsaturated chain ends (e.g., M , = 1000) , a high concentration of maleic anhydride (e.g., 20 wt % ) , a very high temperature, and a long reaction time (e.g., 225C and 4 h using a mechanical stirrer a t 300 rpm) .

At first, an experiment was attempted in order to check the possibility of ene-reaction: a polypropylene with unsaturated end groups was first syn- thesised by peroxide treatment only ( 1 wt % ) . After reprecipitation, unsaturated end-groups were observed by their IR absorption band at 820 cm-'. This sample was heated at 190C in sealed glass tube in the presence of 25 wt % of maleic anhydride for 18 h. However, even in those severe conditions only limited amounts of grafted anhydride were detected.

Secondly, an attempt of grafting by ene-reaction

Quantitative Analysis of PP-g-MA

FTIR spectroscopy was used for quantitative analysis of PP-g-MA. Curve-fitting enables to separate the two overlapping absorption bands at 1792 cm-' (assigned to succinic anhydride) and at 1784 cm-' [assigned to poly (maleic anhydride ) ] . A calibration was realized by using representative model compounds, melt mixed in polypropylene: n -0ctadecyl- succinic anhydride and poly (maleic anhydride) at three concentrations. This calibration considers anhydride as well as carboxylic acid forms. Results of those calibrations are given by eqs. ( 3 ) and ( 4 ) .

For n -octadecylsuccinic anhydride:

[Anhydride] = 21.5 (Abs 1792 cm-'/

Abs 1100 cm-') + 24.5 (Abs 1715 cm-'/ Abs 1100 cm-l) ( 3 )

For poly ( maleic anhydride ) :

[Anhydride] = 51.3 (Abs 1784 cm-'/

Abs 1100 cm-') + 52.5 (Abs 1715 cm-'/ Abs 1100 cm-') (4)

where [anhydride] represent the anhydride concentration (in peq/g) , Abs 1792 cm-' is the absorbance


Table 111. Infrared and Titrations

PP-g-MA Characterization by SEC and Quantification of Grafted Anhydride by

Radical Chain Poly(ma1eic Total Total Ends Succinic Type anhydride) Anhydride Anhydride

MI Concentration Concentration Concentration Concentration" Concentrationb PP-g-MA (g/mol) (wq/g) (wq/g) (CLeq/g) (jleq/g) (Peq/g)

00 1 43,000 12.8 6.9 34.9 41.8 002 50,050 10.2 3.2 15.4 18.6 003 54,900 6.7 1.5 4.5 6.0 004 66,700 4.2 1.5 3.7 5.2

44 15 6 7

a Calculated by infrared spectroscopy. Calculated by titration.

of the succinic anhydride symmetric C = 0 stretch, Abs 1784 cm-' is the absorbance of the poly (maleic anhydride) symmetric C = 0 stretch, Abs 1715 cm-' is the absorbance of the carboxylic acid symmetric C = 0 stretch, and Abs 1100 cm-' is the polypropylene reference absorbance. Absorbance is the maximum values of the Lorentzian curves obtained by curve fitting.

The anhydride concentrations resulting from curve fitting and calibration are summarized in Ta- ble 111. Analyses were undertaken onto dried PP-g- MA films (24 h at 100C under vacuum) to decrease the carboxylic acid and to totally volatilize unreacted maleic anhydride. Concentrations based upon chemical titration are also reported.

The results reported in Table I11 show that the succinic anhydride concentration is always significantly lower than the poly (maleic anhydride) concentration. Titration results are coherent with the succinic and poly ( maleic anhydride ) concentrations determination by FTIR. Succinic and poly ( maleic anhydride) concentrations enables to calculate the average length of grafted poly (maleic anhydride) moieties. Indeed, radical chain end concentration generated during melt reaction can be estimated using eq. ( 2 ) which compares initial (before grafting) and final (after grafting) number-average molecular weight of polypropylene. Grafting of anhydride only at those polypropylene radical chain ends is dem- onstrated herebefore. Thus, both succinic anhydride and poly ( maleic anhydride) are grafted on polypropylene radical chain ends.

For each PP-g-MA the number of grafted succinic anhydrides can be subtracted from the total number of radical chain ends. Consequently, the average length of poly (maleic anhydride) moieties can be calculated by dividing the poly (maleic anhydride) concentration by the remaining amount of radical chain ends. For example, the approximate length of poly (maleic anhydride) is 6 in the case of PP-g-MA 001. Lower lengths are calculated for PP-g-MA 002,

003, and 004. This result suggests that all the radical chain ends were not actually functionalized by any kind of anhydride. Some of them should be elimi- nated by recombination (with low molecular weight radicals), transfer or dismutation.

CONCLUSIONS

This work consists in the characterization of PP-g- MA realized in the melt state in the presence of organic peroxide. Reaction mechanism as well as position, concentration, and nature of grafted anhydride present in PP-g-MA were studied. Melt grafting of maleic anhydride onto polypropylene in the presence of organic peroxide is always associated with molecular weight decrease. This molecular weight decrease depends mainly on organic peroxide concentration and seems not affected by maleic anhydride content. Grafting yield depends on maleic anhydride concentration as well as on organic peroxide concentration. Reaction mechanisms able to justify those experimental observations consist in grafting of maleic anhydride onto radical chain ends arising from the @-scission of polypropylene. However, concentrations of grafted anhydride determined both by chemical titration and FTIR spectroscopy exceed radical chain ends concentrations generated in polypropylene. Consequently, poly (maleic anhydride) grafting was taken into account to justify this observation. This assumption was confirmed by FTIR spectroscopy using model compounds.

Figure 17 summarizes the proposed overall mechanism for maleic anhydride grafting onto polypropylene in the melt state using organic peroxide. As a matter of fact, it consists only in the reaction routes A + H + ( K or J ) of Figure 1. The grafting starts with the homolytical scission of organic peroxide. The radical abstracts a tertiary hydrogen from the chain of polypropylene forming a macroradical.


Initiatipn reactions

Grafting reactions

ROOR - 2 RO' + RO'

H abstraction I I

+ ROH

0-scission

u. + dJ, End chain grafting and homopolymerization of MA

Termination reactions n = 0,1,2 ...

or qo +

or * Figure 17. Proposed PP-g-MA grafting mechanism.

This macroradical undergoes quickly a @-scission with the simultaneous formation of a radical chain end and a vinylidene chain end. Maleic anhydride grafting takes place on the radical chain end and does not take place before @-scission. Ene-reaction between vinylidene chain end of polypropylene and maleic anhydride does not occur in the present experimental conditions. After grafting of one maleic anhydride on the radical ended polypropylene chain, termination reaction or oligomerization of maleic anhydride can take place. Termination of radical reactions call out recombination with low molecular weight radicals or dismutation reactions. Possible recombination can involve for example, peroxide residues or process additives. Dismutation would lead to saturated (succinic) and unsaturated ( maleic) anhydride.

Determination of termination of radical reactions is always difficult and specially in this case owing to the very low concentrations of species which are, moreover linked to polypropylene chains. It can be noticed that the homopolymerization of maleic anhydride decreases the amount of termination reac-

tion per grafted anhydride. Grafting of poly ( maleic anhydride) onto polypropylene as proposed by Gay- lord et al. is now experimentally confirmed, both by direct IR determination and by calculation based on chain ruptures. Homopolymerization of maleic anhydride without grafting is also possible. Grafting of anhydride units along the chain seems to remain negligible in the conditions of reactive processing. However, this grafting, without decrease of molecular weight can be obtained with chemical deriva- tives of MA, for instance imides used as grafted sta- bilizing agents.64

The main difference between both case resides probably in the insolubility of maleic anhydride in molten p~lypropylene.~~ This insolubility could also explain the results reported by Lambla et al.65 on the influence of the nature of the peroxide (polar- i ty) on the grafted succinic anhydride/poly (maleic anhydride ) ratio. Following their polarity, perox- ides would dissolve preferentially into the maleic anhydride phase or into the polypropylene phase. A direct determination of solubility diagrams in molten polypropylene should be a definite confir- mation of these mechanisms but is beyond the scope of this article. This article will be followed by a second dealing with the occurrence of maleic anhydride homopolymerization in the conditions used for the polypropylene grafting53 and by a third concerning the characterization of several industrial PP-g-MA.66

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Received April 10, 1994 Accepted September 28, 1994

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Molecular characterization of Polypropylene