Pseudohexagonal Crystallinity and Thermal and TensileProperties of Ethene–Propene Copolymers

GAETANO GUERRA,1 ODDA RUIZ DE BALLESTEROS,1 VINCENZO VENDITTO,1 MAURIZIO GALIMBERTI,2

FRANCO SARTORI,2 RACHELE PUCCIARIELLO3

1 Dipartimento di Chimica, Universita di Salerno, via S. Allende, I-84081 Baronissi (SA), Italy

2 Montell Polyolefins, G.Natta Research Center, P.le G.Donegani 12, I-44100, Ferrara, Italy

3 Dipartimento di Chimica, Universita della Basilicata, Via N.Sauro 85, I-85100, Potenza, Italy

Received 4 August 1998; revised 3 November 1998; accepted 4 November 1998

ABSTRACT: Structural (X-ray diffraction), melting (differential scanning calorimetry),as well as mechanical (tensile tests) characterizations on uncrosslinked ethene–pro-pene copolymer samples, obtained using a metallocene-based catalytic system andhaving an ethene content in the range 80–50% by mol, are reported. Samples with anethene content in the range 80–60% by mol present a disordered pseudohexagonalcrystalline phase, whose melting moves from ' 40°C down to ' 220°C as the ethenecontent is reduced. The dramatic influence of the crystalline phase on tensile propertiesof uncrosslinked ethene–propene copolymers is shown. In particular, highest elongationat break values are obtained for samples being essentially amorphous in the un-stretched state and partially crystallizing under stretching. On the other hand, lowesttension set values (most elastic behavior) are observed for samples presenting, alreadyin the unstretched state, microcrystalline domains acting as physical crosslinks in aprevailing amorphous phase. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37:1095–1103, 1999Keywords: ethene–propene copolymers; pseudohexagonal crystallinity; thermal be-havior; tensile properties

INTRODUCTION

The crystallinity, which is present in ethene–pro-pene (EP) copolymers from traditional vanadium ortitanium-based catalysts as well as from new met-allocene based catalysts, has been deeply studied byX-ray diffraction.1–13 The larger part of these crys-tallinity studies has been devoted to copolymersuseful for production of industrially relevantcrosslinked elastomers, presenting ethene contentgenerally lower than 85% by mol.

It is well established that propene units enterinto the lattice of orthorhombic polyethylene,14

gradually increasing the disorder in the crystal-line phase but leaving substantially unaltered thetrans-planar conformation of the chains. In fact,the dimension of the a axis of the unit cell ofpolyethylene increases almost proportionally tothe propene content of the copolymer, whereasthe b and the c axes practically retain the dimen-sions found in polyethylene.6,8,9 For high propenecontent, a becomes nearly equal to b =3 and,hence, the unit cell becomes pseudohexagonal.6,13

X-ray diffraction studies on oriented EP copoly-mer samples have shown that this pseudohexago-nal form presents a long-range order only in thehexagonal arrangement of the axes of nearlytrans-planar chains and a large packing disorder(rotational, translational as well as conforma-

Correspondence to: V. VendittoJournal of Polymer Science: Part B: Polymer Physics, Vol. 37, 1095–1103 (1999)© 1999 John Wiley & Sons, Inc. CCC 0887-6266/99/111095-09

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tional) associated with the inclusion of methylgroups of propene comonomer units in the crys-talline phase.13

It is also well established that ethene-basedcopolymers with an ethene content lower than70–65% by mol, can be amorphous under normalconditions,6,8 whereas they can crystallize understretching in the pseudohexagonal form.6,9,13 Thiskind of crystallinity disappears almost completelyupon removal of stress, whereas it appears if thesamples, immediately after the removal of stress,are examined at low temperatures (' 230°C) andsome axial orientation of the crystalline phase isretained.6

Several studies relative to the crystallinity ofEP copolymers have been also performed by dif-ferential scanning calorimetry (DSC).7,11,12,15–17

The EP copolymers presenting some crystallinityby X-ray diffraction analyses at room tempera-ture, present broad endothermic peaks, whosemaximum is positioned in the temperature range30–100°C.7,11,15,16 EP copolymers being amor-phous at room temperature can present, however,very broad endothermic peaks centered belowroom temperature.17 To our knowledge, the na-ture of these broad endothermic peaks has notbeen elucidated.

The mechanical properties of EP uncrosslinkedcopolymers present a large variability, dependingnot only on the copolymer composition but also onthe comonomer distribution. This was demon-strated by investigating model copolymers9 aswell as materials prepared with vanadium andtitanium based catalysts.8

The recent discovery of single center metallo-cene/alumoxane-based catalytic systems18 has al-lowed the preparation of ethene–propene copoly-mers19–23 with quite narrow distribution of mo-lecular mass and inter- and intramolecular distri-bution of chemical composition. Moreover, the avail-ability of metallocenes endowed with dramaticallydifferent structures allows to control the product ofreactivity ratios in the range from about 0.01 tomore than 3, and the regioregularity (from onlyprimary (1, 2) insertions to up to 20% of secondary(2, 1) insertions) and stereoregularity (from sub-stantially only m dyads to comparable amount of mand r dyads) of the propene sequences.

The metallocene-based samples presented inthis manuscript were prepared with an isospecificmetallocene, rac-EBTHIZrCl2, first synthesizedby Brintzinger, and employed by Kaminsky24 forthe preparation of isotactic polyolefins. The mi-crostructure of this copolymer could be considered

as a limit one. In fact, the propene sequences arehighly regioregular and stereoregular, more ex-actly, isotactic. These features of the 1-olefin se-quences allow to regard this copolymer as a “true”ethene–propene bipolymer, each chain being com-posed of ethene and 1,2-propene units insertedwith the same enantioface. Let us recall that inthe case of the vanadium-based copolymers, allthe possible insertions of the propene units mayoccur: 1, 2 or 2, 1, with the two possible enantio-faces.

A thorough statistical analysis allowed to iden-tify a second-order Markovian model as the mostsuitable one to describe ethene–propene copoly-merization from rac-EBTHIZrCl2. Although fourdifferent reactivity ratios are implied in the sec-ond-order Markovian model, in a first approxima-tion the intramolecular distribution of thecomonomers can be described by using the prod-uct of r1 and r2. For the samples used in thepresent article, values for r1 3 r2 in the rangefrom 0.4 to 0.6 were determined.

This family of EP copolymers was discoveredand reported to be suitable to give rise to thermo-plastic elastomeric behavior in the uncrosslinkedstate.20a,b Some samples, for instance, show largeextension at break (.2000%) and low tension set(residual strain after a 200% elongation, ,10%).It is reasonable to expect that this elastomericbehavior, as often occurs for thermoplastic elas-tomers,25 is possibly associated with some kind ofmicrocrystallinity.

In this article, comparative structural (X-raydiffraction), melting (DSC), as well as mechanical(tensile tests) characterization on uncrosslinkedEP copolymer samples, with ethene content in therange 60–80% by mol, characterized by theabove-reported microstructural features, are pre-sented. The study is aimed first to establish thepossible presence in unoriented EP copolymersamples of a low-temperature crystalline phaseand its possible relation with broad endothermicDSC peaks located below room temperature. Themain aim is to study possible relationships be-tween this residual crystallinity and the largevariability of the tensile properties of the un-crosslinked EP copolymers with comonomer com-position.

EXPERIMENTAL

Polymerization Procedures

rac-EBTHIZrCl2 was purchased from WITCO.Methylalumoxane (MAO) was purchased from

1096 GUERRA ET AL.

WITCO as a 10% toluene solution. Ethene–pro-pene copolymer samples were prepared in solu-tion according to the following general procedure.In a 4.3-L steel autoclave, equipped with mechan-ical stirrer, external jacket for thermostating thepolymerization bath, valves for the introductionof liquids and gases, hexane, ethene, and propenewere introduced after purging with hot mono-mers. The relative amount of the comonomerswas tuned as a function of the target copolymercompositions. The temperature of the polymeriza-tion solution was brought to 50°C, time waswaited until a constant pressure was achieved,and the catalyst solution containing rac-EBT-HIZrCl2 and MAO in a few milliliters of toluenewas injected. Ethene was fed during the polymer-ization to keep constant the pressure. The poly-merization was stopped by quickly degassing themonomers and by injecting a few milliliters ofacetone. The copolymers were recovered by pre-cipitation in a large excess of acetone. The co-polymer compositions were determined by 13C-NMR analyses.

Compression Molding

The material was molded in a double heatedplates press at 200°C. It was allowed to melt for 4min without any applied pressure, and it wasthen kept for 4 min under an applied pressure of200 atm. Cooling was made in a second press withthe plates cooled at 20°C at 200 atm. Sheet di-mension were: 120 3 120 mm, with a thickness of1.5 mm. Polytetrafluoroethylene sheets were usedto separate the iron plates from the materials toprevent any contamination and adhesion.

Differential Scanning Calorimetry (DSC)Measurements

Copolymer samples (8–16 mg) closed in standardAl cells were measured, using DSC7 apparatusmanufactured by Perkin–Elmer. The compres-sion-molded samples, as well as the room temper-ature-annealed samples, were first cooled fromroom temperature to 2120°C at a rate of 210°C/min, and then measured in the range 2120,100°C with heating rate of 10°C/min. The meltingtemperatures have been evaluated as the maximaof the appropriate endotherms and the heats offusion calculated from the peaks areas.

X-ray Diffraction Measurements

Samples used for the X-ray diffraction measure-ments, obtained by compression molding, had a

thickness in the range 1–2 mm. X-ray diffractionpatterns at different temperatures were obtainedwith an automatic Philips powder diffractometer(Ni-filtered Cu Ka radiation), with a temperaturecontrol of 61°C.

Mechanical Tests

Mechanical tests were performed on compression-molded samples, kept at 23°C for 24 h. Tensiletests were carried out according to the DIN 53504method, on a rectangular specimen 50 mm longand 6 mm wide, having a span of 30 mm, cut froma 1.5 mm-thick compression-molded sheet. Thecross head rate was fixed at 500 mm/min.

Tension set measurements were performed ac-cording to the ASTM D412 method. The specimenhad a 1.5-mm thick, 50 mm-long and 2 mm-widespan. It was held at 200% elongation for 1 min.The tension was then released and measuredaccording to the following formula: 100 3 ((Lf2 Li)/Li), where Lf and Li are the final and initiallengths of the specimen, respectively.

RESULTS AND DISCUSSION

Thermal Behavior

The thermal behavior of the considered EP co-polymer samples is qualitatively similar to thatone of the commercial samples from vanadium-based catalysts characterized in ref. 15. In fact,the DSC curves of all the samples are nearlyidentical above ' 60°C, as could be expected foramorphous rubbery samples having a similarchemical structure.15 At lower temperatures, thecurves generally exhibit endotherms; size andtemperature of the maximum of these endo-therms increase with the ethene comonomer unitcontent.

The DSC scans of freshly compression-moldedcopolymer samples with ethene molar content of63.6, 65.2, and 73.8% are shown in Figure 1(A), asrepresentative examples.

The samples with ethene molar content equalto 63.6% [curves a of Fig. 1] can be considered astruly amorphous. In fact, their DSC curves do notpresent endothermic peaks but only a well-de-fined glass transition nearly located at 245°C.Differences between the relative specific enthalp-ies of the individual samples and the amorphoussamples can be regarded as a melting enthalpy(DHm) and as a relative measure of crystallinity.

ETHENE–PROPENE COPOLYMERS 1097

The temperature of the maximum of the endo-therms (Tm) and the corresponding DHm values,for freshly compression-molded samples, are re-ported vs. the copolymer composition in Figure2(A) and (B), respectively.

The DSC scans of compression-molded copoly-mer samples with ethene molar content of 63.6,65.2, and 73.8% [the same samples of Fig. 1(A)]after 1 year of storage at room temperature areshown in Figure 1(B).

The DSC scans of aged samples with ethenecontent higher than 70% by mol [like curve c ofFig. 1(B)] show, besides the broad endothermicpeak already present in the fresh samples [curvec of Fig. 1(A)], a narrow higher temperature en-dothermic peak.

The temperatures of the maximum of the broadlower temperature peak (Tm1) and of the narrowhigher temperature peak (Tm2) are reported vs.the copolymer composition in Fig. 3(A). Tm2 val-ues are essentially independent of the copolymercomposition and close to 45°C. Tm1 increases reg-ularly with the ethene content for the propene-rich copolymers for which the high-temperatureendothermic peak is not observed (64% by mol, ethene , 70% by mol), whereas it remains

nearly constant for samples with larger ethenecontents.

The overall melting enthalpy DHm of the twotransitions for the aged samples [Fig. 3(B)] is notsignificantly different with respect to the meltingenthalpy of the freshly molded samples; hence,the annealing at room temperature and the asso-ciated formation of sharp endothermic peaks donot significantly change the degrees of crystallin-ity. It is also worth noting that these meltingenthalpy values are similar to those observed forethene–propene copolymers of analogous compo-sition obtained by traditional vanadium-basedcatalysts.7,15

Determination of the degree of crystallinity ismade difficult by the lack of knowledge of thespecific melting enthalpy and of its dependence ontemperature, on the structure defects, on the se-quence structure, as well as on the surface en-ergy. Anyway, limiting lower values of the degreeof crystallinity of ethene–propene copolymer sam-ples could be obtained using the melting enthalpyof an infinite size crystal of the orthorhombicphase26 (DHm

0 5 280 J/g). For instance, for thesample with an ethene molar content of 73.8%(curves c in Fig. 1), whose X-ray diffraction pat-terns will be presented in the next section (Fig. 4),the overall degree of crystallinity is of 9.0%.

Figure 2. Plots of the temperatures of the maximumof the endotherms Tm (A) and of the correspondingDHm values (B) vs. the copolymer composition, for com-pression-molded EP copolymer samples, from DSCscans analogous to those of Figure 1(A).

Figure 1. DSC scans of compression-molded EP co-polymer samples with ethene molar content of (a)63.6%, (b) 65.2%, and (c) 73.8%: (A) not annealed, (B)stored for 1 year at room temperature.

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In summary, the examined EP copolymer sam-ples show broad melting endothermic peaks,whose position is shifted from above room tem-perature down to 220°C, as the ethene content,and hence, the average length of crystallizablesequences is reduced. At room temperature, thelower melting samples (ethene content lower than70% by mol) are fully melted and, in reasonabletimes, no crystallization occurs; as a consequence,their DSC behavior is not influenced by storage.On the other hand, the higher melting samples(ethene content higher than 70% by mol) are an-nealed at room temperature and a higher meltingTm2 peak is produced by storage.

X-ray Diffraction Characterization

The X-ray diffraction patterns at room tempera-ture of all freshly prepared (compression molded)samples, whose thermal behavior is shown in Fig-ures 1(A) and 2, present essentially only a broadamorphous halo centered at 2u ' 19° [Fig. 4(A)].

Substantial differences, instead, occur for theX-ray diffraction patterns collected at low temper-atures. All the copolymer samples with ethenemolar content lower than 64%, whose DSC scansdo not present endothermic peaks, at low temper-atures still present, as at room temperature, onlya broad amorphous halo whose maximum isshifted towards high 2u values. On the otherhand, the copolymer samples with larger ethenecontents, whose DSC scans present endothermicpeaks, at low temperatures show a weak diffrac-tion peak superimposed on the amorphous halo.The X-ray diffraction pattern at 2100°C of thecopolymer sample with ethene molar content of73.8% is shown, for instance, by curve B of Figure4, together with the two calculated Gaussian com-ponents (dotted lines) centered at 2u 5 21.3° and19.8°, representing the low temperature diffrac-tion of the crystalline and amorphous phase, re-spectively. A crystallinity index could be evalu-ated taking the areas of these two dotted curvesas proportional to the amorphous and crystallineFigure 3. Plots of the temperatures of the maximum

of endotherms Tm (A) and the corresponding DHm val-ues (B) vs. the copolymer composition, for aged EPcopolymer samples, from DSC scans analogous to thoseof Figure 1(B). When two partially superimposed endo-thermic peaks are present, both temperatures corre-sponding to the two maxima are indicated (Tm1 E andTm2 h) in (A) while the overall melting enthalpy isplotted in (B).

Figure 4. X-ray diffraction patterns of the compres-sion-molded EP copolymer sample with an ethene con-tent of 73.8% by mol: (A) freshly prepared [whose ther-mal behavior is shown by curve c of Fig. 1(A)], patternat room temperature; (B) freshly prepared, pattern at2100°C; (C) aged for 1 year at room temperature[whose thermal behavior is shown by curve c of Fig.1(B)], pattern at room temperature. In Figure 4(B) and(C) the calculated Gaussian components representingthe diffraction of the amorphous and crystalline phaseare also shown (dotted lines).

ETHENE–PROPENE COPOLYMERS 1099

fractions of the copolymer, respectively. For in-stance, for the copolymer with an ethene molarcontent of 73.8% (curve B of Fig. 4), the crystal-linity index is 7.5%, in qualitative agreementwith the value obtained by DSC measure-ment (9.0%).

The X-ray diffraction patterns at room temper-ature of the annealed samples, whose DSC scansshow a sharp endothermic peak at T ' 45°Cbesides the low-temperature broad one [curve c inFig. 1(B)], present superimposed to the amor-phous halo a well-defined diffraction peak. TheX-ray diffraction profile of the copolymer samplewith ethene molar content of 73.8% after nearly 1year of storage at room temperature is shown, forinstance, by curve C of Figure 4, together with thetwo calculated Gaussian components (dottedlines) centered at 2u 5 20.7° and 19.2° represent-ing the diffraction, at room temperature, of thecrystalline and amorphous phase, respectively.The halo centered at 2u 5 19.2° corresponds, ofcourse, to the diffraction of the amorphous phase[cf. Fig. 4(A)], while the broad diffraction peakcentered at 2u 5 20.7° corresponds to the mainpeak (100) of the pseudohexagonal form.6,13

The corresponding crystallinity index is 1.6%and compare well with the degree of crystallinityevaluated on the basis of the melting enthalpy ofthe high-temperature DSC peak of curve c of Fig-ure 1(B) (DHm 5 3 J/g) using the method de-scribed in the previous section (1.8%).

It is also reasonable to assume that the diffrac-tion peak observed at 2u 5 21.3° in the patterncollected at T 5 2100°C [Fig. 4(B)], for samplesbeing amorphous at room temperature [Fig. 4(A)],is due to a low-temperature shift of the (100) peakof the pseudohexagonal form.

Hence, these data suggest that, in unorientedcopolymer samples with ethene content in therange of 64–80% by mol, the pseudohexagonalform is present in the imperfect crystals, whichare formed at low temperatures and whose melt-ing corresponds to broad DSC endothermic peaks[like those of curves b and c of Fig. 1(A)], as wellas in the annealed crystalline phase, whose melt-ing corresponds to the sharp endothermic peaklocated close to 45°C (like that one of curve c ofFig. 1(B)]. This is confirmed by the X-ray fiberdiffraction patterns of these samples, after adrawing of 500–1500%, which are similar to thosereported in Figures 1 and 2 of ref. 13, and aretypical of the pseudohexagonal form.

Tensile Characterizations

The stress–strain tensile behavior of the consid-ered EP copolymer samples presents a large vari-ability with copolymer composition. The stress–strain curves at room temperature of copolymersamples having an ethene molar content of 62.5,66.5, and 74.6%, and inherent viscosity values of3.0, 3.8, and 4.0, respectively, are reported, asrepresentative examples, in Figure 5(a)–(c). It isapparent that the work to break, i.e. the areaunder the stress–strain curves, and the elasticmodulus of ethene–propene copolymers, arelargely influenced by the composition. In particu-lar, the elastic modulus values increase with in-creasing of ethene content, whereas the work tobreak values are particularly large for copolymershaving an ethene content of 66.5% by mol (curveb) and decrease abruptly as the ethene content isreduced (curve a).

The elongation at break of all the consideredsamples is reported vs. the copolymer compositionin Figure 6. As generally occurs for polymers thatare above their glass transition temperature, theelongation at break depends on the molecularmasses. For this reason, two sets of data, forwhich the inherent viscosity is in the ranges 1.8–3.0 or in the range 3.6–6.0, are plotted with dif-ferent symbols in Figure 6.

The data of Figure 6 can be divided into threegroups: (1) for copolymers with ethene contentlarger than 70% by mol, the elongation at breakvalues are higher for samples of lower molecularmasses, and for both series of samples are nearlyconstant up to 80% by mol; (2) for the range of

Figure 5. Stress–strain curves at room temperatureof EP copolymer samples with inherent viscosity valuesin the range 3.0–4.0 with different ethene molar con-tents: (a) 62.5%; (b) 66.5%; (c) 74.6%.

1100 GUERRA ET AL.

ethene content from 70% down to 64% by mol, theelongation at break values can be much larger (upto 2400–2800%); (3) as the ethene molar contentbecomes smaller than 64%, the elongation atbreak decreases abruptly to values smaller than200%, for all the considered samples.

By comparison of the results of Figures 5 and 6with the DSC and X-ray diffraction data of theprevious sections it is apparent that, at room tem-perature, the drawability of EP copolymer sam-ples, presenting crystalline domains already inthe unstretched state (ethene content .70% bymol; melting at Tm2 ' 45°C), is relatively small,while it is much larger for samples being essen-tially amorphous in the unstretched state andpartially crystallizing under stretching (60%, ethene content , 70% by mol).

This can be rationalized in terms of reductionof size and perfection of crystalline domains act-ing as physical crosslinks in the drawing process.

The drawability becomes abruptly small whenthe ethene comonomer content becomes smallerthan 64% by mol, that is, when the samples arefully amorphous at room temperature and unableto crystallize under stretching. Hence, the lowdrawability of propene-rich samples can be ac-counted for by their inability to form microcrys-talline domains acting as physical crosslinks.

This interpretation of the data of Figures 5 and6 is confirmed by drawing tests at low tempera-tures. For instance, the copolymer sample with65.2% by mol of ethene presents an increase of theelongation at break from 200% up to 1000–1500%as the drawing temperature decreases from room

temperature down to 220°C. In fact, this sampleat low temperatures present a microcrystallinitywhose melting occurs below room temperature, asshown by the endothermic peaks of curves b ofFigure 1(A) and (B).

The tension sets data for the considered EPcopolymer samples are reported vs. the copolymercomposition in Figure 7. As for Figure 6, two setsof data, for which the inherent viscosity is in theranges 1.8–3.0 or 3.6–6.0, are plotted with differ-ent symbols.

As expected, more elastic behavior (lower ten-sion set values) are observed for samples withlonger macromolecular chains (larger inherentviscosity values). Independently of the molecularmasses of the samples, lower tension set values atroom temperature are observed for samples withan ethene content in the range of 70–75% by mol.

This is easily rationalized in terms of the de-scribed microcrystallinity of the samples. In fact,low tension set values, that is, more elastic be-havior, are generally related for uncrosslinkedsamples to the presence of physical crosslinks inthe unstretched state. For EP copolymers, physi-cal crosslinks are the microcrystalline domains,which are present at room temperature already inthe unstretched state, for ethene molar contentlarger than 70%. The observed increase of tensionset values, for ethene molar content larger than75%, could be rationalized in terms of increase insize and perfection of the crystalline domains,reducing their efficiency as physical crosslinks.

Figure 7. Tension set values of EP copolymer sam-ples of different inherent viscosity reported vs. the co-polymer composition: (h) 1.8 , h , 3.0; (E) 3.6 , h, 6.0.

Figure 6. Elongation at break values of EP copoly-mer samples of different inherent viscosity reported vs.the copolymer composition: (h) 1.8 , h , 3.0; (E) 3.6, h , 6.0.

ETHENE–PROPENE COPOLYMERS 1101

CONCLUSIONS

DSC scans on unoriented uncrosslinked EP copol-ymer samples with ethene content in the range80–64% by mol, from a metallocene-based cata-lytic system, present broad endothermic peaks,whose maximum moves from ' 40°C down to' 220°C as the ethene content is reduced. Com-parisons between DSC scans and X-ray diffrac-tion patterns of unoriented samples indicate thatthese broad endothermic peaks correspond to themelting of imperfect crystallites, which arepresent when the ethene molar content is largerthan 64%. Indeed, for samples with an ethenemolar content larger than 70%, annealing at roomtemperature produces reorganization phenom-ena, which improve the quality (perfection andsize) of these imperfect crystallites, as deducedfrom the appearance of a narrower melting peak(at about 45°C) and from the appearance, in theX-ray diffraction patterns collected at room tem-perature, of a well-defined peak at 2u ' 20.7°.

X-ray diffraction patterns of unoriented andoriented samples indicate that these imperfectcrystallites obtained on cooling, as well as thoserecrystallized by long-term annealing at room tem-perature, are in the pseudohexagonal form.6,8,13

These disordered microcrystalline domainshave a strong influence on the tensile propertiesof uncrosslinked EP copolymer samples. In par-ticular, the elongation at break values at roomtemperature of EP copolymer samples presentingcrystalline domains already in the unstretchedstate (e.g., at room temperature, ethene . 70% bymol) is relatively small, and it is also smaller forfully amorphous samples noncrystallizing understretching (ethene , 64% by mol). Instead, it ismuch larger for samples being essentially amor-phous in the unstretched state and partially crys-tallizing under stretching (e.g., at room tempera-ture, 64% , ethene , 70% by mol). Moreover,the presence already in the unstretched stateof microcrystalline domains, acting as physicalcrosslinks in prevailingly amorphous samples,would account for the low tension set values (ther-moplastic elastomeric behavior), observed forsuitable copolymer compositions.

We thank Prof. P. Corradini and Dr. F. Auriemma ofthe University of Naples for useful discussions, and Dr.Simona Antinucci of the University of Salerno for ex-perimental support. The financial support of NationalResearch Council of Italy and of “Ministero dell’ Uni-

versita’ e della Ricerca Scientifica e Tecnologica” arealso acknowledged.

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