Comparison of Molecular Structure and Rheological Properties of

Article pubs.acs.org/Macromolecules

Comparison of Molecular Structure and Rheological Properties of Electron-Beam- and Gamma-Irradiated Polypropylene Dietmar Auhl,†,‡ Florian J. Stadler,†,§ and Helmut Münstedt*,† †

Institute of Polymer Materials, University Erlangen-Nürnberg, Martensstr. 7, D-91058 Erlangen, Germany Bio- and Soft Matter, Institute of Condensed Matter and Nanosciences,Université catholique de Louvain, Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium § School of Chemical Engineering, Chonbuk National University, Baekjero 567, Deokjin-gu, Jeonju, 561-756, Jeonbuk, Korea ‡

ABSTRACT: Polypropylene (PP) irradiated with electrons or electromagnetic waves (gamma-rays) undergoes chain scission, and the macroradicals generated form branched molecules as shown by sizeexclusion chromatography coupled with laser scattering. Information on the irradiation modification of the molecular structure of PP is still limited in the literature, especially with respect to the controlled generation of long-chain branches (LCB). This paper examines the branching structure of an electron-beam- and gamma-irradiated polypropylene. In general, appropriate irradiation conditions lead to the formation of LCB, which were analyzed in detail by rheological means. At smaller doses, the zero shear-rate viscosities η0 of the electron-beam-irradiated PP lie above the values for the unmodified PP with the same Mw but are distinctly lower than the linear reference at higher doses. This result can be interpreted by the change from a starlike to a treelike branching topography. The gamma-irradiation leads to η0 values smaller than those of the linear reference, giving rise to the assumption of the generation of treelike molecules. The measurements of the elongational viscosities support this molecular picture. An explanation of these findings is given along the line that the electron irradiation conducted at dose rates higher than the gamma-irradiation creates a larger concentration of short-living radicals, which will partially annihilate each other, but favor the growth of some long branches, which get branched themselves at higher doses. The structure of the gamma-irradiated samples is postulated to consist of a blend of linear and treelike molecules.

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INTRODUCTION The high interest in modifications of the molecular structure of thermoplastic materials with long-chain branching is due to its strong influence on processing as well as on some enduseproperties. Long-chain branched polypropylenes, for example, can be generated by electron-beam irradiation.1,2 In the literature, the influence of different doses on the molecular structure3−6 and rheological properties of a commercial polypropylene6 has been investigated in detail. Classical sizeexclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) indicated that the molecular weight decreased with increasing irradiation dose and the number of long-chain branches (LCB) got larger, whereas the polydispersity remained nearly unchanged.3,6−9 Measurements of the zero shear-rate viscosity η0 as a function of the absolute weightaverage molecular weight Mw and the determination of the elongational viscosity as a function of time at various elongation rates demonstrated that these rheological quantities are much more sensitive to the generation of LCB than the classical characterization methods.6 Moreover, some conclusions with respect to the architecture of the long-chain branches could be drawn from the rheological experiments.6 According to the literature,8,9 for gamma-irradiation polypropylene undergoes also simultaneous chain scission and growth comparable to electron-beam irradiation, especially in the presence of oxygen. The crystalline morphology, irradiation © 2012 American Chemical Society

conditions, and the comonomer content are the main factors influencing the result of the irradiation.7,10 A higher dose rate, i.e., the irradiation energy applied per time, leads to a stronger effect on the viscosity, which can be explained by an effect molecular weight and the LCB structure, as well.11 These results pose the question of how different kinds of irradiation, such as electrons or gamma-rays, affect the molecular structure of polypropylenes. In addition to fundamental concerns, the findings of such investigations may offer some insights into the molecular modification of polypropylenes with respect to their applications. As well as the effects on rheology, the end-use properties are also affected, which is due to the influence of irradiation on the crystallization kinetics.12 Therefore, the aim of this paper is to compare the change of the molecular structure and its reflection with respect to rheological properties by irradiating the same basic polypropylene with various doses of gamma-rays or electrons, respectively. The results of the electron-beam-irradiated samples were already published in Auhl et al.6 and are used in this paper for comparison with the gamma-irradiated specimens. Received: October 9, 2011 Revised: January 1, 2012 Published: February 3, 2012 2057

dx.doi.org/10.1021/ma202265w | Macromolecules 2012, 45, 2057−2065

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Table 1. Molecular Data of the Electron-Beam-Irradiated Samplesa PP PP1 PP2 PP5 PP10 PP20 PP60 PP100 PP150 PP300* PP100b a

d [kGy]

n [kGy]

Mw [kg/mol]

Mz [kg/mol]

Mw/Mn

λ̅LCB [LCB/1000 monomer]

0 1 2 5 10 20 60 100 150 300 100

0 1×1 1×2 1×5 1 × 10 2 × 10 6 × 10 10 × 10 15 × 10 30 × 10 100 × 1

669 604 565 473 444 384 285 279 268 339 189

1654 1445 1369 1135 1145 916 874 921 1240 1808

4.2 3.8 3.6 3.8 3.6 3.5 3.4 3.4 3.5 3.8 3.9

n.d. n.d. n.d. 0.05 0.07 0.12 0.30 0.58 0.82

M̅ S [kg/mol]

M̅ S/Me

354 232 158 71 39 22

n.a.

59 38.7 26.3 11.8 65 3.7

n.a.

n.a.

n: number and doses of irradiation steps, n.d.: not detectable; ∗: partially cross-linked; n.a.: data not available.

Table 2. Molecular Data of the Gamma-Irradiated Samples

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PP PP2γ PP20γ PP60γ PP100γ PP150γ

d [kGy]

Mw [kg/mol]

Mz [kg/mol]

Mw/Mn

λ̅LCB [LCB/1000 monomer]

M̅ S [kg/mol]

0 2.2 21 63 108 155

669 575 352 263 272 411

1654 1219 1064 1014 1067 4804

4.2 4.1 3.5 3.1 4.1 6.1

0.04 0.34 0.75 1.03 1.45

357 62 29 21 15

Molecular Characterization. The molecular characterization was performed according to the procedure already described in Auhl et al.6 For the matter of completeness it is sketched once more in the following. Briefly, the molecular data were measured by hightemperature SEC (Waters GPC 150) coupled with a MALLS device from DAWN EOS, Wyatt Technology, and a refractive index (RI) combined with an infrared (IR) detector. Further experimental details have been published elsewhere.6,16 By coupling SEC with MALLS, the absolute molecular weight MLS and the mean-square radius of gyration ⟨r2⟩1/2 of every fraction can be determined directly. The ratio of the mean-square radius of gyration of a branched polymer ⟨r2⟩br to that of a linear polymer ⟨r2⟩lin is the so-called Zimm−Stockmayer branching parameter g.17

MATERIALS AND METHODS

Sample Preparation. The commercial product Novolen PPH 2150 from Lyondell-Basell was used in this study. It is an isotactic polypropylene homopolymer (iPP) with a density at room temperature of 0.90 g/cm3. The granules were electron-beam-irradiated under a nitrogen atmosphere at atmospheric pressure and ambient temperature in a special vessel with 1.5 MeV electrons and with different total doses d up to 300 kGy using an accelerator of the type ELV-2 (Budker Institute of Nuclear Physics, Russia).6,13,14 The doses higher than 10 kGy were usually applied in n of steps on 10 kGy (c.f. Tab. 1) in order to minimize the temperature increase resulting from the irradiation and to avoid variations in the molecular structure by a heating effect.15 For the sample PP100b, a dose of 100 kGy was applied in 100 steps of 1 kGy in order to assess the effect of irradiation dose rate. After the irradiation process, the samples were first annealed for 30 min at 80 °C to allow for a sufficient migration of chain fragments to free radicals in order to form chain branches and, finally, for 60 min at 130 °C to deactivate the residual radicals. Both annealing steps were also carried out directly in the vessel under a nitrogen atmosphere. The samples are listed in Table 1, the numbers denominate the irradiation doses in kGy. The results of the electron-beam-irradiated samples have been partially published before elsewhere6 and are used as a reference in this article. For gamma-irradiation, the virgin material was irradiated with a 60 Co gamma-ray source under a nitrogen atmosphere at doses between about 2 and 150 kGy. Pellets were filled into 500 mL glass tubes on a turntable in a panorama unit with 12 sample tubes, orientated at angles of 30° each. After the irradiation, the material was annealed at 80 °C for 1 h and then for 2 h at 130 °C. The applied dose rate was measured by chlorobenzene vessels and determined to be 1.26 kGy/h, which was 1 order of magnitude below the electron-beam irradiation rate of around 100 kGy/h. The samples are listed in Table 2, and the total irradiation doses applied are indicated by their numbers. After irradiation the surface of the polypropylene pellets was powdered with a mixture of two sterically hindered phenolic antioxidants (0.2% Irganox 1010 and 0.2% Irganox 245) and one organophosphite stabilizer (0.2% Irgafos 38). These heat stabilizers were applied in order to prevent the samples from thermo-oxidative degradation during sample preparation and rheological measurements.

g=

⟨r 2⟩br ⟨r 2⟩lin

(1)

For a trifunctional randomly branched polymer of approximately equal lengths of the branches, g can be related to the number of branching points m along the molecule by17

⎡⎛ ⎤−0.5 ⎞0.5 m 4 m ⎥ g = ⎢⎜1 + ⎟ + 7⎠ 9π ⎥⎦ ⎢⎣⎝

(2)

The Zimm−Stockmayer theory has not yet been verified experimentally for multifunctional randomly branched polymers, and it is debatable whether the branching structures of the irradiated polypropylene discussed in this paper fulfill the assumptions of the theory. Nevertheless, it allows some kind of numerical description of the branching architecture and comparison between the different samples. From the number of branching points m, the number of LCB per 1000 monomer units λLCB can be determined for each fraction as λ̅LCB =

m · 1000· MM M

(3)

with MM being the molecular weight of the monomer unit and M the molecular weight of the branched polymer. The number of LCB per molecule also allows for calculating the segmental molecular weight M̅ S under the assumption that the branching is uniform for each fraction, i.e., that two or more differently branched species are not present and that they are statistically distributed. M̅ S can be determined from the 2058


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number of LCB per chain. As each molecule containing m branching points has 2m + 1 segments, M̅ S followsas

M̅ S(M ) =

M 2m + 1

Mw and the polydispersity index Mw/Mn are listed in Tables 1 and 2. Mw changes as a function of the dose for both methods of irradiation in a similar way. Mw/Mn decreases at already small doses for both irradiation methods but remains nearly constant at medium doses. The gamma-irradiation distinctly increases Mw/Mn at 100 and 150 kGy due to the pronounced high molecular weight tails (cf. Figure 1). Figure 2 presents the radius of gyration ⟨r2⟩0.5 as a function of molecular weight MLS in a double-logarithmic plot. The linear

(4)

Because of the mathematical nature of the determination of M̅ S, it is not possible to calculate a molecular weight independent value of this quantity. Instead, a slight decrease with increasing molecular weight is found, which is stronger at low long-chain branching levels. Therefore, the average values M̅ S are used in this paper, which were determined for the high molecular weight range available from light scattering and correspond to the values of M̅ S at a molecular weight of about the zaverage molecular weight Mz. However, there are more LCB in a statistically branched sample at higher molecular weights; it is sensible to analyze these highest molecular weights, for which the characteristic quantity is Mz. Rheological Characterization. The elongational experiments were carried out with a uniaxial extensional rheometer as has been described in detail elsewhere.6 Stressing experiments at 180 °C were conducted at different constant elongational rates ε̇0. The rheological measurements in shear were performed at 180 °C under a nitrogen atmosphere using a rotational rheometer. The zero-shear viscosity η0 was determined from creep experiments conducted with a constant stress rheometer (Gemini, Malvern Instruments). The applied constant shear stresses τ0 were in the order of 10 Pa, i.e., in the linear range of deformation. In the steady state of deformation, the creep compliance J(t) and η0 are related as follows:

lim

t

t →∞ J(t ) τ0 → 0

= η0 (5)

i.e., η0 can be determined in the linear range at sufficiently long creep times, when the spontaneous compliance and the viscoelastic part of deformation become very small compared to t/η0.18 The thermal stability of the samples was tested by measuring the time dependence of the storage modulus G′(t) at low angular frequencies. The time at which the storage modulus had changed by 5% was used as the criterion for the thermal stability. This time was sufficient for the different experiments performed.

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RESULTS AND DISCUSSION Molecular Characterization. Figure 1 shows the molar mass distributions of the electron-beam- and gamma-irradiated PP samples, which were modified with the identical doses of 20, 100, and 150 kGy in comparison with the neat PP. Figure 2. Mean radius of gyration ⟨r2⟩1/2 as a function of molar mass MLS for the linear precursor and the irradiated samples: (a) electronirradiated PP; (b) gamma-irradiated PPγ.

PP exhibits a linear relation with a slope of 0.58, which is in good agreement with the literature. The deviations of the irradiated samples from this line indicate a coil contraction that is related to long-chain branching. The deviation increases with growing irradiation dose and molecular weight. At the lowest dose of 2 kGy, the measured data lie within the scatter of the linear PP. The dependences of the radius of gyration on the molecular weight for the electron-beam- and gamma-irradiated products of PP indicates a significant coil contraction for molecular weights larger than 5 × 105 g/mol. This result implies the preferential presence of LCB for larger molecules (Figure 2). On the basis of the coil contraction, the concentration of long-chain branches per 1000 monomer units λLCB was determined according to eq 3. As these data showed a relatively large scatter, the average values, λ̅LCB, are discussed. They are

Figure 1. Differential molar mass distributions dW/d log M for the PP precursor and the irradiated samples. 2059


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normalized by the entanglement molecular weight M is proposed:46,47

represented in Table 1 and 2. The gamma-irradiated PP has a branching level λ̅LCB that is significantly higher (about twice as high) than that of the electron-irradiated PP, if compared at the same doses. In Figure 3, the data are plotted for a direct

⎛ Ma ⎞ κ ⎛ M ⎞ η0 ∝ ⎜ ⎟ exp⎜υ a ⎟ ⎝ Me ⎠ ⎝ Me ⎠

(7)

where ν and κ are parameters in the order of 1. Although it seems counterintuitive, η0 is hardly influenced by the number of arms but predominantly by their length. The validity of eq 7 has been proven for several different polymers (polystyrene, polybutadiene, hydrogenated polybutadiene, polyisoprene) over a wide range of molecular weights.48−51 For comblike polymers different and more complicated relations are found that depend on several parameters.52 Because the zero shear-rate viscosity as a function of Mw is a very sensitive quantity to be used to detect even low amounts of long-chain branching, this relationship is used to analyze the irradiated polypropylenes. In Figure 4, the zero-shear viscosities Figure 3. Average degree of long-chain branching λ̅LCB and molecular weight of segments M̅ S as a function of dose for the differently irradiated PP.

comparison. The clearly apparent differences reveal a stronger dose dependency of λ̅LCB in the case of gamma-irradiation. An increase of λ̅LCB with an exponent of about 0.8 is found for the electron-beam as well as for the gamma-irradiated series. Rheological Properties. Linear-Viscoelastic Properties in Shear. Rheological functions are not only an indicator of the flow properties of a polymer melt; they can also be used as a very sensitive means of assessing the molecular structure of polymers, especially with respect to long-chain branching.19−21 Because of the high sensitivity of the linear-viscoelastic properties on the molecular structure, these quantities are preferably taken into consideration. Several correlations between molecular and rheological quantities can be applied for a molecular analysis.22−31 Molecular Weight Dependence of the Zero Shear-Rate Viscosity. One of the important correlations for analyzing the branching structure is the relation between the zero shear-rate viscosity η 0 and the weight-average molecular weight Mw.16,22,32−34 For linear15,16,22,27,33,35,36 and to some degree also for short-chain branched polymers,18,37,38 the following correlation holds:

η0 = KMw a

Figure 4. Dependence of the zero shear-rate viscosity η0 at 180 °C on Mw (Mw determined from SEC-MALLS). The numbers at the data points refer to the irradiation dose in kGy.

of the irradiated samples are plotted as a function of the weightaverage molecular weights. As expected from the branching analysis using SEC-MALLS (cf. Figure 3), the electron-beamirradiated PP samples does not fulfill the η0−Mw correlation for linear iPP. The η0 values are higher than the zero-shear viscosity η0lin expected for linear iPP of corresponding Mw. For irradiation doses up to about 10 kGy, the deviation from the η0−Mw relation of linear polymers increases with increasing dose. The zero-shear viscosity increase factor η0/η0lin is clearly larger than 1 for small irradiation doses, which indicates the existence of long-chain branches.6,53 For doses of about 60 kGy, the zero shear-rate viscosity η0 agrees approximately to the relation for linear samples, while for higher irradiation doses, η0 decreases below the linear relation. The gamma-irradiated samples exhibited a totally different behavior (cf. Figure 4). PP2γ comes to lie on the line for the linear PP, indicating that the influence of the irradiation is negligible. All the other data points are below the relationship for the linear PP. The two highest irradiation doses are particularly surprising. Because of the increasing Mw, but still decreasing η0 with higher irradiation doses, η0(Mw) bends off. Such a behavior has not been reported in the literature before. Since the dose rate was different for the two series, data are added for another electron-beam-irradiated PP sample with 100 kGy (PP100b), but at a dose rate similar to that of the gamma-

(6)

For polymers with Mw below a critical molecular weight Mc which is about 2−3 times the entanglement molecular weight Me ≈ 6 kg/mol for isotactic PP,38,39 the exponent a is found to be around 1, whereas above Mc it is about 3.4−3.6.40 Auhl et al.6 found a = 3.54 and log K = −15.4 at 180 °C for isotactic PP, which is in good agreement with other findings in the literature.18,37 As the initial PP is one of these linear polymers, this relationship is used in this paper as a reference. Unlike most other structure−rheology correlations, eq 6 is found to be independent of the molecular weight distribution within the experimental accuracy.16,27,41−45 For star-shaped polymers with only one branching point per molecule, the following relationship between the arm length Ma 2060


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SEC-MALLS can have a profound impact on the rheological behavior.54 From Tables 1 and 2 it can be concluded that only a minor fraction of the molecules of the samples irradiated with 20 kGy or less was actually long-chain branched. For PP20 for example, each chain of M = Mw contains an average of 1.1 LCB. Hence, the samples with lower irradiation doses can be regarded as a mixture of linear and starlike chains of higher molecular weight. The molecules of higher molar mass contain more than one branching point. A growing irradiation dose increases the branched species at the cost of the linear ones. At around 60 kGy, corresponding to a number of 0.3 LCB per 1000 monomers (cf. Table 1), the effect of an increase of η0 was diminished. This number was already reached for the gamma-irradiation at a dose of 20 kGy, in accordance with the findings that η0 of this sample clearly comes to lie below the line of the linear PP. To investigate the influence of the length of the branches on the zero shear-rate viscosity η0, the data obtained from different irradiation doses and methods were compared to results from the literature. Because the exact molecular structure (position and distribution of the long-chain branches) is unknown in statistically branched polymers and cannot be assessed properly with analytical methods, the distribution is assumed to be uniform. Hence, the segmental molecular weightassuming that a molecule with m long-chain branches contains 2m + 1 molecular segments with a molecular weight M̅ Sis used for the analysis. This assumption means that the introduction of LCB leads to shorter molecular segments if the molecular weight is constant. Furthermore, it is assumed that the number and lengths of long-chain branches per molecule is identical for all chains of the same molecular weight. The electron-beam irradiation increased η0/η0lin up to doses of 20 kGy due to the increase in the number of branched molecules (cf. Table 1). Despite a further increase in the concentration of branched molecules with increasing irradiation dose, η0/η0lin decreases to less than 1. This finding can be explained by the assumption that the increase of the LCB concentration is accompanied by a decrease in the LCB length. Measurements and molecular models, e.g., on combs52,57,58 or stars,47,49,59 have shown exactly this effect for model architectures. For irradiation doses below 20 kGy, on average less than 1 LCB was present per molecule, which indicated the presence of a mixture of linear and predominantly starbranched chains. This conclusion is in agreement with the findings on various metallocene-synthesized LCB-mPE, which show the highest η0/η0lin for mixtures of linear and starbranched chains.25,31,34,60 All samples with an irradiation dose of 20 kGy or less (except PP20γ) show m(Mw) below about 1 (see Tables 1 and 2), which revealed the presence of at least some linear chains in all of these samples. The maximum in η0/η0lin(d) for the electronbeam-irradiated samples can be attributed to the nature of the following two opposing effects: an increasing concentration of branched chains and their decreasing arm length. The first effect increases η0 due to the increase of the fraction of branched chains, whereas the second effect decreases it (cf. eq. 7). The kind of irradiation, however, significantly affects η0/η0lin, as revealed by the difference between the electron-beam- and gamma-irradiated PP, as the gamma-irradiation predominantly leading to η0/η0lin < 1 (Figure 5). This finding indicated that the gamma-irradiation generated different branching structures compared to the electron-beam irradiation.

irradiation. A comparison shows that for the two electronbeam-irradiated PP-100 samples the molecular weight as well as η0(Mw) are quite similar, which indicates that the dose rate may not be the decisive factor for the branching structure. This indicates that the type of irradiation induces chain growth and simultaneously enhances the branching efficacy. The different effects of electron-beam and gamma-irradiation on η0 are once more revealed from the plot of η0/η0lin as a function of the irradiation dose d in Figure 5. The viscosity

Figure 5. Zero shear-rate viscosity increase factor η0/η0lin as a function of irradiation dose d.

ratio of the electron-beam-irradiated PP exhibits a maximum of "η0/η0lin" around 5 at around 10 kGy. However, no maximum of η0/η0lin occurred for the gamma-irradiation, but η0/η0lin(d) decreases monotonously to ∼0.04 at 150 kGy, compared to about 0.6 for the electron-beam-irradiated PP. This result gives further evidence that the type of irradiation distinctly influences the dose dependency of the zero shear-rate viscosity enhancement factor η0/η0lin(d). The dependence of this factor on the architecture of LCB implies that the two irradiation methods applied lead to different molecular structures. This conclusion is instructive insofar as the classical molecular analysis, the results of which are presented in Tables 1 and 2, does only show slight distinctions between the electron-beam and gamma-irradiation and gives no hint of any differences of the branching structure, except for a higher branching density for the gamma-irradiated series. From a comparison with results on polyethylenes with a branching structure known from the polymerization conditions, more detailed predictions of the structure of the irradiated polypropylenes can be obtained. For long-chain branched polyethylenes it was shown that slightly long-chain branched PE with a starlike structure lie above the line of linear PE, whereas a position beneath the line indicates a treelike topography.27,54−56 This anomaly was explained by the exponential dependence of η0 on the ratio of Ma/Me (cf. eq 7). Taking these findings into account, a treelike topography is likely for the electron-beam-irradiated samples with doses above 60 kGy and for the gamma-irradiated specimen already above 2 kGy.6 Some more detailed conclusions can be drawn from a comparison with the molecular analysis. The results of Figure 2 clearly showed the virtual absence of any LCB at small irradiation doses, but according to Figure 4, the η0−Mw relation for linear PP was not obeyed. This finding further demonstrates that a small amount of long-chain branching not detectable by 2061


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Stange et al.61 investigated blends of highly branched PP and linear PP, which also showed η0/η0lin < 1 despite a low degree of long-chain branching. Hence, the average degree of longchain branching alone cannot determine whether η0/η0lin will be larger or smaller than 1 as Bersted56 suggested. Stange et al.’s findings61 show many similarities to the PPγ series, which supports the conclusion that the gamma-irradiation leads to the introduction of long-chain branches by forming a few highly branched molecules within a rather linear matrix. Such a structure has to be distinguished from the existence of some few star-shaped molecules surrounded by linear ones. Uniaxial Elongational Flow. As it was shown in the literature, long-chain branching can significantly affect the elongational properties of polymer melts.62−68 Particularly, the branching structure is reflected by the elongational behavior. Therefore, stressing experiments at different constant elongational rates were performed on the differently irradiated polypropylenes. The results are presented in Figure 6a,b.

some of the elongational data for the electron-beam-irradiated PP.6 The time-dependent functions of the elongational viscosity ηE+(t,ε̇0) did not show any strain-hardening for PP2γ within the experimental accuracy, while PP20γ and PP60γ exhibited a distinct deviation from the linear curve. For elongations εH higher than 0.7, the samples are strain hardening. Strain hardening is very sensitive to the molecular structure. The so-called strain-hardening coefficient χE is defined by χE(t ) =

η+ E (t , ε̇0) 3η0+(t )

(8)

η0+(t)

with being the time-dependent shear viscosity in the linear range of deformation. χ is presented in Figure 7 as a

Figure 7. Strain-hardening factor χE as a function of strain-rate ε̇0 for electron-beam- and gamma-irradiated samples at a Hencky strain of 2.7.

function of the elongational rate. For the linear polypropylene PP, no strain hardening was observed at Hencky strains εH at least up to 2.7. This is the usual finding for linear samples with a unimodal molecular weight distribution.67 In contrast, most of the irradiated samples showed strain hardening of increasing severity with increasing irradiation doses. The exception was PP2γ, and the absence of any strain hardening indicated a very low branching efficiency. This observation is in agreement with the zero-shear viscosity as a function of Mw which lies on the curve for the linear polypropylenes (cf. Figure 4). PP100γ and PP150γ could not be characterized in elongation because their viscosities were too low. For the samples irradiated with 20 kGy, the strain hardening decreases with increasing strain rate. This effect was more pronounced for the electron-beam-irradiated sample than for the gamma-irradiated one. For the samples subjected to 60 kGy, the strain hardening increased with increasing strain rate. A decreased in strain hardening with increasing strain rate is usually associated with low degrees of long-chain branching of a starlike topography, while the opposite behavior is typical of a high degree of branching with a treelike topography.6,62,64,67,70−75 The polydispersity or, specifically, high molecular weight components can principally influence the elongational viscosity.76,77 However, the molecular weight distributions of the irradiated samples are not so different that a significant effect on the strain hardening has to be taken into account. Furthermore, the SEC measurements showed that the polydispersity of the samples characterized in elongation is

Figure 6. Transient elongational viscosity ηE+(t) as a function of time t at different Hencky strain rates ε̇0 for (a) electron-beam-irradiated PP and (b) the gamma-irradiated samples PP2γ to PP60γ.

For each irradiation dose, the respective curves superpose on the linear viscoelastic start-up curve at all strain rates up to a Hencky strain of about 0.7. Trouton’s relation69 ηE+(t) = 3ηS+(t), which relates the transient shear and elongational viscosity in the linear range of uniaxial flow, is fulfilled for deformations εH smaller than 0.7 for all samples and evidence the reliability of the experiments. We have previously published 2062


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that a change of architecture from starlike to treelike branches takes place with increased irradiation. A treelike structure is characterized by branches randomly distributed along the chains, resulting in a higher number of chains with values of Ma smaller than in the case of a molecule with few long-chain branches. The gamma-irradiation decreased Mw and η0 up to doses of 100 kGy. The difference with respect to the electron-beamirradiated series lies in the fact that η0/η0lin(d) monotonously decreased with no peak. In conjunction with the LDPE results, these findings can be interpreted as revealing that the samples contained long-chain branches in a treelike architecture. Elongational Rheology. The irradiated samples show a distinct strain hardening in elongational flow with different strain-rate dependences. Two conclusions can be drawn from the results. First, the strong effect of branching is clearly demonstrated. This conclusion is supported by the finding that strain hardening increased for the more strongly irradiated samples, despite the significant decrease in the molecular weight. The results from SEC-MALLS that the polydispersities of the polymers remained nearly constant after irradiation with various doses and that a high-molecular-weight component was not detectable demonstrate that the elongational viscosity can be used as a reliable indicator of the generation of long-chain branching in polypropylene by electron irradiation. Second, the rate dependence of strain hardening supports a conclusion with respect to the topography of the polypropylene molecules, if the well-established results on polyethylenes are taken into account. The decreasing strain hardening with increasing strain rate, as exhibited by the low irradiated samples, is indicative of a small degree of branching with high molecular weights of the branches. The increase in strain hardening with increasing strain rate indicates a high degree of branching with smaller arm molecular weights Ma as it is typical of LDPE. These conclusions concerning the topography of the long-chain branches in electron-beam-irradiated polypropylenes form an experimental basis for a comparison of models on radical reactions initiated by irradiation. Influence of the Type of Irradiation. The comparison of PP irradiated by the same doses of electrons or gamma-rays show a somewhat larger molecular weight reduction, but a higher degree of long-chain branching for the gammairradiation. The rheological properties differ significantly as the gamma-irradiated samples exhibit a lower zero shear-rate viscosity than the samples irradiated by electrons, when compared at the same molecular weight. While for the electron-beam-irradiated samples the viscosities lies above the reference line for the linear samples, i.e., η0/η0lin is larger than 1, η0 lay below this line for all the gamma-irradiated samples, i.e., η0/η0lin is larger 1. The elongational behavior of the electronbeam- and gamma-irradiated samples exhibited the same basic characteristics, namely strain hardening, but there were clear differences with respect to its level, which was higher for the electron irradiation at the same dose. These findings confirm that gamma- and electron-beam irradiation did not yield the same molecular structure. A possible explanation lies in the much lower dose rate of the gamma-irradiation, which leaves the radicals more time to react compared to the electron-beam irradiation. From Figure 4 and a comparison with literature results on polyethylenes, electron-beam-irradiated PP20 can be regarded as being roughly comparable to a long-chain branched mPE,28,60,67,78 i.e., with an architecture predominantly consist-

slightly decreased with irradiation and no bimodal molecular weight components were found. Hence, the results of the elongation behavior for the irradiated polypropylenes can be directly related to the branching structure The general trend of the strain-rate dependence of the strainhardening coefficient, χe, is comparable for electron- and gamma-irradiated PP, but the level of strain hardening for the gamma-irradiated products is lower and the dependence on the elongational rate weaker. Considering that the branching level λ̅LCB of PP60 is almost the same as that of PP20γ, the strainhardening coefficient as a function of strain rate χe(ε̇) could be expected to be comparable, if the LCB-structures were the same. However, PP60 shows the typical characteristic of a highly branched sample with χe(ε̇) increasing, while PP20γ exhibits the characteristic of a slightly branched sample with χe(ε̇) decreasing. This difference confirms the different branching structures of these two samples. Furthermore, it is obvious that PP20 shows a χE at εH = 2.7 distinctly decreasing with decreasing ε̇, while the strain-hardening coefficient for PP20γ is rather constant. Figure 7 clearly demonstrates that the strain-hardening coefficient decreases with increasing strain rates for the samples electron-beam-irradiated with 2−20 kGy.6,62,64,67,70−75 In contrast to the weakly irradiated samples, the strain hardening of PP-60 increases with increasing strain rate. Such a behavior is found for low-density polyethylene (LDPE) with a high amount of long-chain branching and a treelike molecular structure.67,71,72,75 Therefore, the differences of the strain-rate dependence of χE at εH = 2.7 support the conclusion that the branching structure changes with the irradiation dose.

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CONCLUSIONS The irradiation of PP under different conditions, i.e., electrons and gamma-rays, exhibits two general consequences: the molecular weight is reduced, and the number of long-chain branches was increased with increasing irradiation dose. The branching structure of the samples investigated is assumed as random, since the length of the branches and their position along the polymer backbone are of a statistical nature due to the irradiation process. The reduction of the molecular weight and, particularly, the long-chain branching significantly affected the viscoelastic properties of the melts. The generation of LCB was detected by usual molecular characterization methods. The shear and elongational properties were also strongly indicative of LCB of different amounts and topographies, which depend on the irradiation parameters. The various results are discussed below. Shear Rheology. The η0 values of the electron-beamirradiated polypropylenes were significantly increased compared to those of the untreated PP samples of the corresponding weight-average molecular weights. At small irradiation doses, i.e., at comparably low amounts of LCB the ratio of the viscosity of the branched sample to that of the linear one, η0/η0lin, reached a maximum, but approached unity again at higher LCB contents. This increase is explained by small amounts of long-chain branches with relatively high molecular weight of the arms Ma and, hence, a high ratio of Ma/Me, which affects η0 exponentially. The decrease of η0/η0lin at higher doses can formally be related to a decline of the molecular weight Ma of the branches. Two mechanism are postulated to explain this change. First, the molecular weight decreases with increasing irradiation dose, which shortens the length of the molecules attached to the backbone. Second, it is not unrealistic to assume 2063


Macromolecules

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Article

SUMMARY This work has compared the effects of electron-beam and gamma-irradiation on molecular and rheological properties of a polypropylene. While the differences between the two irradiation methods with respect to molecular properties measured by SEC-MALLS are rather small (gamma-irradiation led to higher degrees of long-chain branching and a high molecular weight tail), they are more pronounced regarding the rheological behavior. The gamma-irradiation leads to a much smaller zero shear-rate viscosity η0 than the electron-beam irradiation at comparable doses, although the differences of the weight-average molecular weights are not very pronounced. This behavior indicates the generation of different branching architectures by the two irradiation methods. Considering results on branched polyethylenes and blends of linear and long-chain branched polypropylenes, it can be concluded that the gamma-irradiation led to a branching structure, which may be similar to that of a slightly branched LDPE, while electronbeam irradiation led to branched molecules, which carry fewer but probably longer long-chain branches of a starlike structure at smaller irradiation doses and seem to approach a treelike structure with higher doses. This conclusion can be drawn from the findings that η0/η0lin becomes smaller than 1 for doses larger than 100 kGy (Figure 4) and that the strain hardening behavior resembles that of LDPE (cf. Figure 6a). Although details of the branching structures generated have to be left open, the main conclusion is, therefore, that electronbeam and gamma-irradiation effects in polypropylene do not follow the same reaction kinetics and, thus, generate different structures of long-chain branching.

ing of star-shaped and linear molecules, while PP20γ is rather comparable to an LDPE, if η0(Mw) is regarded. The branching structure has to be assumed somewhat different (less star-like and more linear and highly branched chains ), however, as the strain-hardening patterns of PP20γ and LDPE differ significantly. The differences found in the rheological behavior may go back to the different irradiation rates. Two reasons for these findings may be discussed. First, the type of irradiation causes different molecular reactions, and second, the different dose rates are responsible for the behavior observed. Spadaro and Valenza79 showed that increasing the dose and dose rate leads to a higher number of radicals in PP. The smaller irradiation dose rate and hence the lower concentration of radicals mean that the elimination of two radicals by recombination is reduced; i.e., the radicals have a longer lifetime and can cause a higher number of scission and re-formation reactions, which eventually leads to a treelike structure of long-chain branching. The strain hardening increasing with decreasing strain rate even at the highest doses are not in contrast to this assumption as the elongational viscosities of blends of a linear and a longchain branched PP demonstrate.61 For the components investigated by Stange et al.61 up to a content of 50% LCBPP a strain-hardening behavior similar to that of the gammairradiated PP is found. Therefore, the results on the gammairradiated PP can be interpreted in the way that it consists of a blend of linear and treelike chains. However, as only a low dose rate of 1.26 kGy/h was applied during gamma-irradiation and usually high dose rates of about 100 kGy/h for the electron-beam irradiation (while the total doses are the same for both sample series) in the present study, the extent to which the effects described in this article are due to the irradiation type or to the dose rate could not finally be determined. Structure−Property Relationships. The results of this work show that the rheological quantities in shear and elongational reacted more sensitively to the presence of longchain branches than the quantities measured by the classical size-exclusion chromatography. Especially, the zero shear-rate viscosity η0 was sufficiently sensitive to detect even low amounts of LCB and to provide a deeper insight into changes of the molecular structure. To summarize, the results support the conclusion that the irradiated polypropylenes investigated consist of molecular structures with long-chain branches, the number of which increases with increasing irradiation doses. Strong evidence is presented that this increase leads to a change of branching architectures from starlike to treelike structures. The conclusions are drawn taking the findings on long-chain branched polyethylenes into account; i.e., the lightly electronbeam-irradiated samples are similar to metallocene-catalyzed sparsely long-chain branched polyethylene, while the highly irradiated ones resemble highly branched LDPE. The results presented on the irradiated PP-samples broaden the experimental base of the relationship between rheological properties of polymer melts and their molecular structure, which can be exploited in two ways. First, the results strengthen the role of rheological measurements as an analytical tool, and second, they offer some suggestions for the method by which polypropylene can be modified by irradiation. These insights will be particularly valuable for tailoring polypropylene for special applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.J.S.); helmut.muenstedt@ww. uni-erlangen.de (H.M.).

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ACKNOWLEDGMENTS The authors thank the German Research Foundation (DFG) for financial support of this work. D.A. and F.J.S. would like to thank the EU Framework 7 program for the “SupraDyn” MarieCurie fellowship and “Human Resource Development (201040100660)” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), respectively. The contributions and useful discussions with J. Kaschta, J. Stange, I. Herzer (University Erlangen-Nuremberg), and S. Henning (MLU Halle-Wittenberg in Merseburg) are gratefully acknowledged. The work would not have been possible without the supply of irradiated PP samples by B. Krause, U. Lappan, and K. Lunkwitz (electron-beam irradiation, Leibniz-Institute of Polymer Research Dresden) as well as R. Godehard and G. Michler (gamma-irradiation, Martin-Luther University HalleWittenberg).

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REFERENCES

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Comparison of Molecular Structure and Rheological Properties of

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