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Journal of Chromatography A, 1154 (2007) 319–330

Characterization of high molecular weight polyethylenes using hightemperature asymmetrical flow field-flow fractionation withon-line infrared, light scattering, and viscometry detection

E.P.C. Mes a,∗, H. de Jonge a, T. Klein b, R.R. Welz b, D.T. Gillespie c

a Analytical Sciences Terneuzen, Dow Benelux B.V., 446 Bldg, P.O. Box 48, 4530 AA, Terneuzen, The Netherlandsb Postnova Analytics, Max-Planck-Strasse 14, DE-86899 Landsberg/Lech, Germany

c Polyolefins Research, Dow USA Texas Operations, B1470 Bldg, 2301 Brazosport Boulevard, Freeport, TX 77541-3257, USA

Received 9 January 2007; received in revised form 9 March 2007; accepted 30 March 2007Available online 4 April 2007

bstract

High temperature asymmetrical flow field-flow fractionation (HTAF4) coupled to infrared (IR), multi-angle light scattering (MALS), andiscometry (Visc) detection is introduced as a tool for the characterization of high molecular weight polyethylenes. The high molecular weightraction strongly affects the rheological behaviour and processability of polyethylene materials and can often not be accurately resolved by currentechnology such as high temperature size-exclusion chromatography (HTSEC). Molecular weight (M), radius of gyration (Rg), and intrinsiciscosity [η] of linear high density polyethylene (HDPE) and branched low density polyethylene (LDPE) samples are studied in detail by HTAF4nd are compared to HTSEC. HTAF4 showed a better separation and mass recovery than HTSEC for very high molecular weight fractions inDPE and LDPE samples. As no stationary phase is present in an HTAF4 channel, the technique does not show the typical drawbacks associatedith HTSEC analysis of high molecular weight polyethylenes, such as, exclusion effects, shear degradation, and anomalous late elution of highlyranched material. HTAF4 is applied to study the relation between the molecular weight and the zero shear viscosity η0 for high molecular weightDPE. It was found that the zero shear viscosity values predicted from HTAF4 results are in good qualitative agreement with measured values

btained from dynamic mechanical spectroscopy (DMS) experiments, whereas η0 values predicted from HTSEC do not show a strong correlation.he low molecular weight cutoff of HTAF4 is approximately 5 × 104 as a result of relatively large pores in the HTAF4 channel membrane. HTAF4

s, therefore, currently not suited to analyze low molecular weight materials.2007 Elsevier B.V. All rights reserved.

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eywords: High temperature asymmetrical flow field-flow fractionation; High

. Introduction

Although it has long been recognized that the moleculareight distribution (MWD) and the long chain branching dis-

ribution (LCBD) strongly affect the rheological and physicalroperties of polyethylene, their accurate analysis has remainedchallenging area of research. One important area particu-

arly difficult to deal with is the separation and analysis of

igh molecular weight components in polyethylenes. The mostidely-used separation method for the determination of theolecular weight distribution of polyethylenes is high tempera-

∗ Corresponding author. Tel.: +31 115673973; fax: +31 115673729.E-mail address: emes@dow.com (E.P.C. Mes).

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021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.chroma.2007.03.116

rature size-exclusion chromatography; Polyethylene

ure size-exclusion chromatography (HTSEC) [1,2]. Separationn HTSEC is based on differences in hydrodynamic volume.y using column calibration and understanding the relation-

hip between hydrodynamic volume and molecular weight, theolecular weight distribution of a sample can be calculated from

ts retention time. With the advent of on-line light scattering (LS)nd viscometry (Visc) detection the characterization capabilitiesave been greatly expanded; allowing the direct measurementf molecular weight (M), radius of gyration (Rg), and intrin-ic viscosity [η] independent of the separation mechanism [3].oupling of multiple detectors such as LS and Visc in com-

ination with a concentration detector such as refractive indexRI) to HTSEC has resulted in a powerful tool to determineot only the molecular weight distribution but also the longhain branching distribution in polyolefins [3–7]. This being

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20 E.P.C. Mes et al. / J. Chrom

aid, HTSEC typically falls short when it comes to analyz-ng high- or ultrahigh molecular weight material in polyolefinesins [3,7–12]. The most important drawbacks being: (1) poly-lefins with molecular weight distributions with tails that extendeyond 106–107, i.e., near or at the exclusion limit of mostEC columns, cannot be effectively separated and quantified;2) large (branched) structures can shear degrade or elute laterom the column; (3) microgels that can be present in theample can also shear degrade or might build-up on the SEColumn; (4) branching calculations can be hampered by anoma-ous late elution of large highly branched structures. Some ofhe aforementioned drawbacks such as shear degradation andate elution can likely be minimized by running the HTSECxperiment under “low shear” conditions, e.g., at very low flowates [12–14]. But these conditions result in long run times.ecause there is a strong need for an accurate and sensitive

echnique that is able to characterize high molecular weightolyethylene resins, it was decided to study the application ofeld-flow fractionation (FFF) [15,16]. In several studies it haseen shown that FFF can offer a higher resolution than SECor high molecular weight material [17–20]. FFF comprises aumber of techniques all based on a one-phase system whereeparation is achieved in a thin channel without a stationaryhase by applying an external field perpendicular to a lami-ar solvent flow. In practice, a carrier liquid is forced to flownder laminar conditions through a thin channel without pack-ng material. A sample is injected and is taken along with theiquid stream through the channel. Perpendicular to the liquidow a force is applied, acting on the solutes to be separated,

hat makes the injected components move towards a channelall (the so-called accumulation wall). This is counteracted byormal diffusion, which results in an equilibrium distributionf the components to be separated close to the accumulationall. The separation in the axial direction is the result of theelocity gradient of the carrier liquid near the wall, where theetention of solutes is determined by the distance from the wall.n the so-called normal mode of FFF, the elution order is theeverse of SEC, i.e., small components elute before large. Asresult of the absence of a stationary phase, FFF can be used

or dissolved polymers as well as for colloids and particles [16].broad range of FFF applications can be found in the perti-

ent literature [21]. FFF techniques are subdivided based uponhe type of field employed and some well-known commerciallyvailable FFF sub-techniques are sedimentation FFF [22], ther-al FFF [23], and flow FFF [24]. The latter two techniques

ave been described in literature for high temperature analysisf polyethylene and will be briefly discussed in the followingection.

.1. Thermal FFF

A thermal FFF channel consists of two highly polishedhromium-plated copper bars clamped together with a spacer

andwiched in between. The cross-force in thermal FFF is gen-rated by heating one of the walls and cooling the other one.he retention depends on the temperature difference between

he walls and on the so-called Soret coefficient, i.e., the ratio of

mapm

r. A 1154 (2007) 319–330

he thermal diffusion coefficient and the normal diffusion coef-cient (DT/D) of the solute in the solvent [23]. Unfortunately,

hermal diffusion is still an ill understood phenomenon, and theack of a predictive model of thermal diffusion has hindered tocertain extent the widespread implementation of thermal FFF.onetheless, thermal FFF has been used for a wide variety of

amples. The technique is most suited to analyze high moleculareight lipophilic polymers in organic solvents [23,25], includ-

ng microgels [26]. Furthermore, coupling of thermal FFF toight scattering detection has resulted in a powerful techniqueor the determination of ultrahigh molecular weight polymersnd microgels [27]. Several studies on high temperature ther-al FFF have been reported [28–33]. Brimhall et al. already

howed the feasibility of using high temperature thermal FFF in981 [28]. By using tetrachloroethylene as a solvent in a pres-urized channel, they were able to retain polyethylene (PE) andolypropylene (PP) samples. In a second study by Brimhall etl. the thermal diffusion of polystyrene (PS) in ethylbenzene atifferent temperatures was investigated [29]. In 1995 Pasti et al.ublished a paper on the development of a new high temperaturehermal FFF instrument [30]. A modified FFFractionation T100nstrument combined with evaporative light scattering detectionas used to analyze PE and PS in dichlorobenzene. The results

orresponded well with HTSEC measurements. Melucci et al.sed the same instrument to study the retention behaviour [31]nd the Soret coefficient [32] of PS in decalin. In 2002 anothertudy by Pasti et al. on the high temperature thermal FFF anal-sis of PS and PP in decalin was published [33]. Unfortunately,he retention of PP was low, indicating that the thermal diffusionf PP in decalin is low.

.2. Flow FFF

Flow FFF is regarded as the most universal FFF technique24]. The universality comes from the fact that the viscous dragf the cross-flow is exerted equally on any object in its path. FlowFF has been thoroughly studied and the mechanism that gov-rns the separation is relatively simple and well understood. Inow FFF the cross-force is generated by a cross-flow that perme-tes through one or both channel walls. Retention is dependentn the cross-flow and on the diffusion coefficient of the soluteccording to [34]

R = t0 Vc w2

6D V 0 (1)

here t0 is the void time, Vc the cross-flow rate, w the chan-el height, D the diffusion coefficient, and V0 the volume ofhe channel. Since its introduction in 1976 several types of flowFF have been developed [35]. The most often used being sym-etrical flow FFF [36] and asymmetrical flow FFF [37]. A flowFF channel consists of two blocks clamped together, forming

he walls of the channel. Between the blocks are sandwiched apacer and a porous membrane to prevent the passage of sample

aterial. In symmetrical flow FFF both walls are permeable, in

symmetrical flow FFF only one wall (the accumulation wall) isermeable. The pore size of the membrane determines the lowolecular weight cutoff and is usually in the range of 103–104,

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E.P.C. Mes et al. / J. Chrom

hich limits information deliverable in this MWD range. FlowFF techniques can be effectively used to fractionate ultrahigholecular weight material and have been used mostly for aque-

us samples. The coupling of flow FFF to light scattering haseen very successful and many papers have appeared exploit-ng this technique to study large polymeric [37–39], colloidal40], and particulate matter [41]. A study on the development of“universal” symmetrical flow FFF instrument was published

y Miller and Giddings [42]. This system could operate withqueous and nonaqueous solvents at ambient and elevated tem-eratures. In their paper, retention data of PS in xylene at 140 ◦Cre shown. It was claimed that data from the separation of PEould be presented elsewhere. Unfortunately, to our knowledge,o data on PE separation has been published.

Although it has been shown that it is possible to apply highemperature FFF for polyolefin analysis, the development of aobust and reliable instrument turns out to be difficult and cur-ently available commercial instrumentation is still limited tombient temperature. In this study, the application of a newlyeveloped high temperature asymmetrical flow field-flow frac-ionation (HTAF4) instrument with multiple detectors for theharacterization of high molecular weight polyolefin resins wille demonstrated. We designed a HTAF4 channel that is com-atible with trichlorobenzene (TCB) at 145 ◦C. The elutingpecies were monitored by a triple detector array, consisting ofn infrared detector (IR), a multi-angle light scattering detec-or (MALS), and a viscometry detector (Visc). The HTAF4eparation of high density and low density polyethylenes wille presented. To illustrate the possibilities and limitations ofTAF4, the technique was applied to commercial LDPE (LDPE-), to tubular LDPE standard reference material (SRM) 1476,nd to a set of HDPE resins; all containing relatively smallmounts of very high molecular weight material. The LDPE-material is a branched resin with a broad MWD. SRM 1476

s a well-known standard reference material from NIST that isrequently used as a calibrant of HTSEC systems [2,11]. It con-ains a small amount of very high molecular weight and highlyranched material that is very difficult to analyze with the cur-ent techniques. For both LDPE resins the HTAF4 results will beompared to HTSEC data. The HDPE sample set was includedn this study in order to try to reconcile observed differenceshen correlating triple detection HTSEC data and melt rheol-gy data. It was suspected that the cause of this inconsistencyas the presence of ultrahigh molecular weight material that

annot be accurately analyzed or quantified by HTSEC. In thistudy we will compare the correlation of HTAF4 and HTSECesults with experimentally determined values from dynamicechanical spectroscopy (DMS).

. Experimental

.1. Instrumentation

All HTAF4 experiments were performed on an AF2000T instrument from Postnova Analytics (Landsberg/Lech, Ger-any). A new stainless steel channel was designed for use with

rganic solvents at elevated temperatures. A trapezoidal chan-

(da(

r. A 1154 (2007) 319–330 321

el space with a tip-to-tip length of 27.8 cm was cut from aylar spacer (Postnova Analytics). The length of the triangu-

ar tapered inlet was 1.5 cm and the maximum breadth at thend of the inlet was 2 cm. The nominal thickness of the spaceras 350 �m. A flexible ceramic foil (Postnova Analytics) with aore size of approximately 10 nm was used as an accumulationall membrane. The membrane had a thickness of ≈100 �m

nd consisted of a stainless steel mesh covered with a thinayer of ceramic material. The channel was placed in a WatersPCV2000 instrument (Waters Corp., Milford, MA, USA) set

t 145 ◦C. The channel assembly proved to be very robust. Evenfter running samples on a channel for over 8 months and cool-ng down the instrument from 145 ◦C to ambient temperatureeveral times, we did not experience any leaks or a decrease ineparation performance. Furthermore, visual inspection of theembrane after opening the channel showed no indications of

ample adsorption. The HTSEC measurements were also carriedut at 145 ◦C using the above-mentioned Waters GPCV2000nstrument. Two column sets differing in separation range andxclusion limit were used. The first set consisted of three PLgelixed B columns (10 �m particles; 7.5 mm × 300 mm; Polymeraboratories Ltd. (Church Stretton, UK). The second columnet consisted of four PLgel mixed A columns (20 �m particles;.5 mm × 300 mm; Polymer Laboratories Ltd.). The stationaryhase of both column sets was a highly cross-linked porousolystyrene/divinylbenzene material. Each column containedmixture of different pore size materials in order to cover a

road molecular weight range. The PLgel mixed B and mixed Aolumns had, according to the supplier, a linear operating rangepolystyrene-equivalent molecular weight, measured in tetrahy-rofuran (THF)) of 5 × 102 to 1 × 107 and 2 × 103 to 4 × 107,espectively. It should be noted that the column operating rangeor PE is shifted towards lower molecular weights compared toS because at equal molecular weight the hydrodynamic volumef polyethylene in TCB is more than double the hydrodynamicolume of PS in TCB [43].

In Fig. 1 a schematic diagram is given of the dual-modeTAF4/HTSEC set-up. By installing three Rheodyne six-portalves (Rheodyne LLC, Rohnert Park, CA, USA) in the WatersPCV2000 we could easily switch between the HTAF4 chan-el and the HTSEC columns without the need to cool downhe instrument. Furthermore, this set-up also ensures that whenither the HTAF4, HTSEC, or both are not used, a constant lowolvent flow is maintained through the system at all times. Foroth the HTAF4 and the HTSEC experiments the injection vol-me was 200 �L. The flow rates for the HTSEC mixed B andixed A columns were 0.7 mL/min and 1 mL/min, respectively

unless otherwise noted). The HTAF4 channel outlet flow rateas set at 0.7 mL/min. The inlet flow, cross-flow, and focusingow rates during the injection/focusing step were 0.7 mL/min,mL/min, and 2 mL/min, respectively. The injection/focusing

ime was 4 min. Cross-flow programming was used in all HTAF4xperiments. The used cross-flow gradients are given in Fig. 2

see also Section 3.1). Detection was achieved by using an IR4etector from Polymer ChAR (Paterna, Spain), equipped withCH-sensor; a Wyatt Technology Dawn DSP MALS detector

Wyatt Technology Corp., Santa Barbara, CA, USA), equipped

322 E.P.C. Mes et al. / J. Chromatogr. A 1154 (2007) 319–330

/HTS

waWwGGBabba

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Fig. 1. Schematic illustration of the dual-mode HTAF4

ith a 30-mW argon-ion laser operating at λ = 488 nm; andWaters three-capillary viscometry detector. Occasionally aaters RI detector was used. The viscosity and RI detectorere both located in the column compartment of the WatersPCV2000. The MALS detector was connected to the WatersPCV2000 by using a heated transfer line provided by Wyatt.oth the MALS cell and the transfer lines were maintained at

temperature of 145 ◦C. A heated interface accessory suppliedy Polymer ChAR was used to maintain the connecting linesetween the IR4 detector and the Waters GPCV2000 at a temper-ture of 145 ◦C. The temperature of the IR4 cell was controlled

ig. 2. Influence of the cross-flow gradient on the retention of HDPE 120; (A)TAF4 fractograms, (B) cross-flow gradients. Channel outlet flow rate was.7 mL/min; the inlet flow, cross-flow, and focusing flow rates during the injec-ion/focusing step were 0.7 mL/min, 2 mL/min, and 2 mL/min, respectively; thenjection/focusing time was 4 min.

tCisoatwi[

2

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EC set-up. The set-up is depicted in the HTSEC mode.

y the internal heater of the detector and was also set at 145 ◦C.he MALS detector was calibrated by measuring the scattering

ntensity of the solvent TCB. Normalization of the photodiodesas done by injecting SRM 1483, a high density polyethyleneith weight-average molecular weight (Mw) of 32100 and poly-ispersity (PD) of 1.11. A specific refractive index incrementdn/dc) for polyethylene in TCB of −0.110 mL/g was used.

Rheological experiments were done on a RMS-800 rheome-er from Rheometric Scientific (now TA Instruments, Inc., Newastle, DE, USA) equipped with 25-mm parallel plate tool-

ng, using small amplitude oscillatory shear measurements. Theamples were compacted into disks at 175 ◦C for 5 min. Thescillatory experiments were carried out at 190 ◦C in nitrogentmosphere. The range of frequencies employed was 0.1 rad/so 100 rad/s, and the strain amplitude was ca. 1%. The dataere collected at the lowest oscillating frequencies first. A mod-

fied Carreau-Yasuda (CY) equation was used to model the data44,45].

.2. Materials and methods

Distilled “Baker Analyzed”-grade 1,2,4-trichlorobenzeneJ.T. Baker, Deventer, The Netherlands) containing 200 ppm 2,6-i-tert-butyl-4-methylphenol (Merck, Hohenbrunn, Germany)as used as the solvent for the sample preparation as well

s for the HTAF4 and HTSEC experiments. Two high den-ity polyethylenes were used as reference materials in thistudy; HDPE 161 (Mw: 1.61 × 105, PD: 1.16; Elf Aquitaine,rance) and HDPE 120 (Mw: 1.20 × 105, PD: 2.45; Theow Chemical Company, Midland, MI, USA). LDPE SRM476 and HDPE SRM 1483 were obtained from the U.S.ational Institute of Standards and Technology (Gaithersburg,D, USA). LDPE-1 and HDPE-A, HDPE-B, HDPE-C, andDPE-D are resins produced by The Dow Chemical Com-any. PE solutions were prepared by dissolving the samplesnder gentle stirring for 3 h at 160 ◦C. Unless otherwise noted,he sample concentrations for the HTSEC experiments were.1 mg/mL for high-polydispersity materials and 0.2 mg/mL for

ow-polydispersity materials. The sample concentrations for theTAF4 experiments were typically 3.0 mg/mL and 0.6 mg/mL

or high- and low-polydispersity materials, respectively. Foroth HTAF4 and HTSEC too high sample load can lead to shifted

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E.P.C. Mes et al. / J. Chrom

nd distorted peaks due to overloading phenomena. Especiallyor HTAF4, overloading can be an issue when analyzing higholecular weight materials [46,47]. The absence of overload-

ng of both the HTAF4 and HTSEC was verified for the higholecular weight samples by performing the runs at different

oncentrations.

.3. Data processing

A MALS detector measures the scattered signal from poly-ers or particles in a sample under different scattering angles

. The basic light scattering equation using the Berry formalisman be written as [48]

K c

Rθ

=√

1

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here Rθ is the excess Rayleigh ratio, K an optical constanthich is, among other things, dependent on the specific refrac-

ive index increment (dn/dc), c the concentration of the solute,the molecular weight, Rg the radius of gyration, and λ the

avelength of the incident light. Calculation of the molecu-ar weight and radius of gyration from the light scattering dataequire extrapolation to zero angle [49]. This is done by plot-

ing (Kc/Rθ)12 as a function of sin2(θ/2) in a so-called Debye

lot. The molecular weight can be calculated from the interceptith the ordinate and the radius of gyration from initial slopef the curve. The Berry method was used for all data. The sec-nd virial coefficient was assumed to be negligible. The bestolynomial fit degree was determined by minimizing the errorn the calculated M and Rg values ensuring that the lowest fitossible was used. A first order fit gave the best results for theelatively low molecular weight material and a third order fit forhe ultrahigh molecular weight material (the four highest angles27–148◦ were not included in the calculations). It should beoted that the choice of fitting method can, especially in the casef ultrahigh molecular weight material, have a large impact onhe results in terms of accuracy. More information on fit methodsor MALS detection is described elsewhere [48,50]. The intrin-ic viscosity numbers were calculated from both the viscometrynd concentration detector signals by taking the ratio of the spe-ific viscosity and the concentration at each elution slice [51].STRA 4.72 (Wyatt Technology Corp.) software was used to

ollect the signals from the concentration detector (IR or RI),he viscometer, and the MALS detector. Data processing wasone with ASTRA and Microsoft EXCEL.

. Results and discussion

.1. Optimization of HTAF4; cross-flow programming

Retention in flow FFF, and thus HTAF4, depends on both thepplied cross-flow and the diffusion coefficient of the sample.

nlike SEC, flow FFF has no confined elution window and has,

onsequently, a much higher peak capacity. In SEC all peakslute between the total exclusion volume (interstitial volume)nd permeation volume (interstitial volume plus the pore vol-

atlm

r. A 1154 (2007) 319–330 323

me), whereas in flow FFF there is no concrete upper boundo retention volume. The efficiency is, however, very low com-ared to SEC, but this is especially in the case of high moleculareight species largely compensated by the very selective reten-

ion mechanism of flow FFF. The limiting value of the so-calledeight-based selectivity Sm = |d log tR/d log M| (tR is retention

ime and M is molecular weight) for flow FFF is equal to ≈0.618]. This compares favorably to the maximum Sm value of0.22 found for SEC [18]. Furthermore, it should be added

hat the selectivity of SEC readily drops off near the column’sxclusion limit, resulting in a low selectivity for high molecu-ar weight species. Flow FFF, however, approaches the highestelectivity in the limit of high retention, which means in practicehigh selectivity for high molecular weight species. The high

electivity of flow FFF can, however, lead to excessively longetention times for polydisperse samples, as tR ∝ M0.6 [15]. Theigh cross-flow which is required to separate relatively smallomponents would excessively retain the large components. Forhis reason, we applied gradient cross-flow programming in allur HTAF4 experiments. The effect of the cross-flow on the sep-ration is illustrated in Fig. 2. Fig. 2A depicts the fractionationf HDPE 120 using four different cross-flow gradient programs,iven in Fig. 2B. From Fig. 2 it is clear that the fractionatingower can easily be adjusted by changing the cross-flow gra-ient. Here, fractionating power is defined as the resolution ofolymers whose molecular weights differ by the relative incre-ent dM/M [15]. Note that HTAF4 is in this respect much moreexible than HTSEC. For our measurements gradient C pro-ided sufficient fractionation in a reasonable analysis time andas used in all experiments mentioned hereafter.

.2. HTAF4 versus HTSEC

FFF in general should be largely regarded as a complementaryechnique to SEC (although there is a very substantial overlapn the molecular weight range that is attainable to both tech-iques). In a theoretical study Stegeman et al. have shown bysing adequate models for selectivity and efficiency that FFFerforms best for high molecular weight species, whereas SECs clearly superior in the lower molecular weight range (M < 105)18]. Apart from fundamental restrictions, a more technical con-traint of the instrument presented in this study also limits its useor small polymers. The low molecular weight limit of HTAF4s determined by the size of the pores in the channel membrane.he pore size of the membrane used in this study was in the

ange of 10 nm. This corresponds to a polyethylene low molec-lar weight cutoff of approximately 5 × 104. This means thator polydisperse samples, relatively small material below thisutoff is lost via the pores in the membrane, which leads to aow recovery of low molecular weight species. For example, theecovery found for HDPE 120 shown in Fig. 2, which has aelatively low Mw of 1.20 × 105 and a polydispersity of 2.45,as only 43%. Because only the low molecular weight species

re partially lost through the pores, this will lead to an overes-imation of the number-average molecular weight Mn, and to aesser degree of the weight-average molecular weight Mw, of low

olecular weight samples. For higher molecular weight mate-

324 E.P.C. Mes et al. / J. Chromatogr. A 1154 (2007) 319–330

Fw

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htsa1fpaicsau

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ig. 3. Elution curves and molecular weight plots of HDPE 161; (A) separationith HTAF4 and (B) separation with HTSEC mixed B columns.

ials and low dispersity materials, these values are generally inetter agreement with HTSEC values. To overcome the limita-ion discussed above, new membranes with smaller pore sizeseed to be developed.

In Fig. 3 the HTAF4 fractogram, the HTSEC chromatogram,nd corresponding molecular weights of HDPE 161 are plot-ed. The molecular weight moments found by HTAF4 were Mn:.45 × 105 and Mw: 1.58 × 105. These values are in good agree-ent with the values given by the supplier (Mn: 1.49 × 105 andw: 1.61 × 105). The HTAF4 recovery for this low disperse

ample was 98%. We note that apart from the reversed elu-ion order, no significant differences between the HTAF4 andTSEC traces are observed. Therefore, it may be concluded

hat for this sample both techniques provide essentially the samenformation.

The advantage of using HTAF4 for the determination of ultra-igh molecular weight material becomes clear when we comparehe HTAF4 and HTSEC results of a typical LDPE sample. Fig. 4hows the elution curves and molecular weight plots of HTAF4nd HTSEC (mixed A and mixed B columns) of LDPE-1. HDPE20 was added for reference purposes. First, a significant dif-erence between the shape of the HTAF4 and HTSEC elutionrofiles can be seen. Both the HTSEC traces of LDPE-1 showclear shoulder at the high molecular weight end, whereas this

s not observed for HTAF4. This shoulder in the HTSEC traces

an be attributed to high molecular weight material with littleize separation as the exclusion limit of the HTSEC column ispproached and reached. A significant amount of the high molec-lar weight material is excluded from the stationary phase pores

stss

ig. 4. Elution curves and molecular weight plots of LDPE-1 and HDPE 120;A) separation with HTAF4, (B) separation with HTSEC mixed A columns, andC) separation with HTSEC mixed B columns.

nd coelutes at ≈24 min from the HTSEC mixed A columnsnd at ≈22 min from the HTSEC mixed B columns. Exclu-ion is, as expected, less pronounced on the mixed A columnshan on the mixed B columns. The molecular weight obtainedor the partly excluded high molecular weight shoulder is theeighted average of all coeluting material. This will result in

rroneous weight-average molecular weight Mw and especially-average molecular weight Mz values by HTSEC column cali-ration methods depending upon the amount and the distributionf the excluded material. Although a light scattering detectoran still be used to measure a correct Mw value for the whole

ample, it can only measure the weight-average property acrosshe highest molecular weight and thus Mz values from lightcattering will be underestimated for this sample. Moreover,tructural information will be lost when plotting either radius

atogr. A 1154 (2007) 319–330 325

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E.P.C. Mes et al. / J. Chrom

f gyration or intrinsic viscosity versus M across the sectionsf the chromatogram where separation is limited. It should benally realized that since these plots become the basis of manyranching calculations, attempts to calculate long chain branch-ng distributions as a function of molecular weight will also failcross excluded sections of the chromatogram. In contrast tohe HTSEC results, the HTAF4 fractogram depicted in Fig. 4oes not show a shoulder at the high molecular weight end.rom the HTAF4 results we may conclude that the LDPE-1ample contains a substantial amount of very high moleculareight material, with molecular weights up to roughly 3 × 108

hat cannot be separated by the HTSEC columns used in thistudy.

Another phenomenon that is frequently observed in HTSECf LDPE materials is late elution of a small fraction of pre-umably the high molecular weight branched material. This isbservable as a slight upward curvature in the HTSEC molec-lar weight plots (in this example; Fig. 4B from approximately2 min and Fig. 4C from approximately 29 min). This upwardurvature in the molecular weight calculated from light scatter-ng data may be the result of coelution of a small amount ofigh molecular weight branched material which contaminateshe bulk property of the low molecular weight species. Severalxplanations for the abnormal elution effect have been proposedn literature [52–55]. In a systematic study by Podzimek et al. its suggested that late elution is caused by entanglement of pri-

arily branched polymers in the stationary phase particle pores55]. In Figs. 5 and 6 the conformation plots (Rg versus M)nd the Mark-Houwink plots ([η] versus M) of LDPE-1 andDPE 120 are given. Both plots can provide important struc-

ural information of polyolefins and can be used to calculate longhain branching (LCB) numbers. It can be seen that the effectf late eluting material is more pronounced in the conformationlot. Especially the conformation plot of the mixed B columnsFig. 5C) is significantly curved over a broad molecular weightange (approximately 104–106). The upward shift towards lowerolecular weight is the result of the high local dispersity. The

ypical curvature of the conformation plot towards the lowerolecular weight as often observed for highly branched sam-

les is because MALS measures the z-average radius of gyrationnd the weight-average molecular weight at each elution slice.he measured Rg value is more sensitive to high moleculareight species than the measured M value. The upswing ofg at low molecular weights significantly hampers the accu-

ate calculation of the LCB distribution of LDPE’s from theonformation plot. The effect is less pronounced for the HTSECixed A columns. Presumably less material elutes late from theixed A columns compared to the mixed B columns. The con-

ormation plot from HTAF4 is virtually linear with a slope of.45. This value is in good agreement with the results foundor highly branched polymers reported in literature [56]. TheTSEC Mark-Houwink plots of LDPE-1, given in Fig. 6, do not

how a strong curvature at low M as [�] is less sensitive to the late

luting large (most likely branched) species. At the high molecu-ar weight end the plots diverge and break up near the exclusionimit of the columns. In contrast, the HTAF4 Mark-Houwinklot of LDPE-1 is not disturbed by exclusion phenomena and

ists

ig. 5. Comparison of the conformation plots of LDPE-1 and HDPE 120; (A)TAF4, (B) HTSEC mixed A columns, and (C) HTSEC mixed B columns.

hows a Mark-Houwink plot with a slightly decreasing slope ofpproximately 0.26 up to a molecular weight of approximately× 108.

For the sake of completeness the conformation plots andark-Houwink plots of HDPE 120 obtained by HTAF4 andTSEC have been included as well in Figs. 5 and 6. In contrast

o the previous results, there is very good agreement betweenTAF4 and HTSEC results for this mid-range molecular weight

linear) HDPE. A slope of 0.58 was found for the conforma-ion plots of both the HTAF4 and HTSEC. The slope of the

ark-Houwink plots varied between 0.71 and 0.73. The slopef the conformation plots corresponds well with the theoret-

cal limiting value of 0.588 for a linear polymer in a goodolvent [57] and the slope of the Mark-Houwink plots is consis-ent with values typically found for linear polyethylene in goodolvents [58].

326 E.P.C. Mes et al. / J. Chromatogr. A 1154 (2007) 319–330

FH

3

NhabAdwiSwwebaw

Fa

FHmirmpaotcs

ig. 6. Comparison of the Mark-Houwink plots of LDPE-1 and HDPE 120; (A)TAF4, (B) HTSEC mixed A columns, and (C) HTSEC mixed B columns.

.3. Analysis of SRM 1476

Next we investigated the tubular LDPE SRM 1476. ThisIST standard is known to contain a small amount of ultra-igh molecular weight material which severely complicates itsccurate analysis. The material has been studied extensivelyy HTSEC and off-line low-angle light scattering (LALS).

wide range of molecular weight moments has been found,epending on the technique used. Beer et al. lists the moleculareight moments that have been reported in literature [11]. Typ-

cal weight-average molecular weight Mw values reported forRM 1476 are approximately 1.00 × 105 by HTSEC combinedith low-angle light scattering (HTSEC–LALS) and 2.15 × 105

hen measured by off-line LALS (see Beer’s study and ref-

rences therein). DeGroot and Hamre studied the discrepancyetween the HTSEC–LALS and off-line LALS measurementsnd their results strongly indicated that shear degradation occurshen SRM 1476 migrates through the HTSEC column [12].

tbiM

ig. 7. Separation of SRM 1476; (A) HTAF4, (B) HTSEC mixed A columns,nd (C) HTSEC mixed B columns.

ig. 7 shows the elution profiles of SRM 1476 obtained byTAF4 (Fig. 7A), HTSEC mixed A (Fig. 7B), and HTSECixed B (Fig. 7C). The HTSEC mixed B 90◦ light scatter-

ng signal shows a small pre-peak around 23 min. This peakepresents a very small amount of highly branched ultrahigholecular weight material. The IR signal barely shows a second

eak, whereas the viscometry signal shows a very small shouldert 23 min. These observations reflect the different sensitivitiesf the detectors: the IR signal is proportional to the concentra-ion c; the MALS signal is proportional to the product of theoncentration and molecular weight, cM; and the viscometryignal is proportional to cMα, where α is the exponent fromhe Mark-Houwink equation. The low concentration and the

ranched character of the high molecular weight material resultsn a low sensitivity of the IR and viscometer compared with the

ALS detector. The upper molecular weight found by using

atogr. A 1154 (2007) 319–330 327

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tHrmpme≈sppAtaoFuHto

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3

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E.P.C. Mes et al. / J. Chrom

he HTSEC mixed B column set is approximately 3.5 × 106. Aignificantly higher maximum molecular weight of 1.4 × 107 isound by using mixed A columns. An upward curvature at lateetention in the molecular weight measurement can be observedor both column sets. This again suggests the late elution of largeresumably branched structures (Fig. 7B and C).

The HTAF4 results given in Fig. 7A clearly show an improvedeparation and detection for the high molecular weight LDPEpecies compared to HTSEC. The uppermost molecular weightound by HTAF4 is approximately 2.9 × 108. As mentioned ear-ier, it has been shown by comparison of HTSEC–LALS andff-line LALS measurements that shear degradation probablyccurs when SRM 1476 migrates through the HTSEC column12]. This most likely explains the observed differences in theaximum molecular weight found by HTSEC and HTAF4. Theeight fraction of material obtained by HTAF4 with a molecu-

ar weight higher than 3.5 × 106 is only 0.45% and the amountbove 1.4 × 107 is 0.29%. Assuming that all the high molec-lar weight material elutes from the HTAF4 channel, we canack-calculate that less than 0.5% of the sample is not measur-ble by the HTSEC mixed B columns and less than 0.3% byhe HTSEC mixed A columns. Yet even a small amount of high

olecular weight material such as this is, in general, consideredo be important as it can affect rheological (e.g. melt strength)nd physical properties (e.g. optics) of the material.

It can be seen in Fig. 7B and C that the pre-peak, whichs caused by large highly branched structures, is considerablyigher in area in case of the mixed A columns (20 �m particleize) than in the case of the mixed B columns (10 �m parti-le size). However, the pre-peak area is not nearly as large ashe post-peak observable in the HTAF4 run. Further gains inTSEC molecular weight area recovery can be made by reduc-

ng flow rate (such as in the work of DeGroot and Hamre [12])t the expense of both plate count and run time. HTAF4 does notequire a reduced flow rate to allow the analysis of these fragiletructures.

Comparison of the conformation plots of SRM 1476 showshat there is reasonable agreement between HTAF4 and theTSEC mixed A columns in a relatively broad molecular weight

ange of approximately 2 × 105–3 × 106 (Fig. 8A). Up to aolecular weight of 1 × 106 the slopes of log Rg versus log M

lot of both techniques are between 0.45 and 0.475. Above aolecular weight of 1 × 106 the Rg value from HTAF4 lev-

ls slightly off, but keeps steadily increasing up to a value of175 nm (very large structures in the sub-micron range). This

uggests that the second high-M peak in the fractogram is com-osed of material with a higher degree of branching than the maineak. Also the log Rg versus log M curve of the HTSEC mixedcolumns levels off above a molecular weight of 1 × 106, but

o a larger degree than the curve obtained by HTAF4 and reachesplateau value of approximately 90 nm. The conformation plotbtained with the HTSEC mixed B column set is depicted inig. 8B. Here the slope of the conformation plot below a molec-

lar weight of 1 × 106 is lower than the slopes obtained withTAF4 or with the HTSEC mixed A columns. Moving towards

he low molecular weight end in the conformation plot it can bebserved that the conformation plot starts curving up; indicative

(dtm

ig. 8. Comparison of conformation plots of SRM 1476; (A) HTAF4 vs. HTSECixed A columns and (B) HTAF4 vs. HTSEC mixed B columns.

f anomalous late elution of high molecular weight branchedaterial. For M > 1 × 106 the conformation plot also appears

o reach a plateau value of, in this case, approximately 50 nm.he HTSEC Rg values corresponding to very high molecu-

ar weights appear to be completely independent of moleculareight (flattening of the conformation plot). Theory, however,redicts for the most compact homogeneous spherical structurecube root dependence for the radius of gyration on moleculareight [49]. This number has indeed been experimentally con-rmed for nearly compact spheres of hyperbranched polymers59]. The plateauing of Rg above a molecular weight of approx-mately 1 × 106 can therefore not be explained by branchinglone, and is more likely the result of the lack of separation inTSEC.

.4. HTSEC under “low shear” conditions

It has been shown in literature that high molecular weightolyethylene is susceptible to shear degradation when passinghrough a HTSEC system [12]. Probably the column station-ry phase, the column frits, and the in-line filters can allnduce shear degradation [12–14]. DeGroot showed that poly-

er degradation can be reduced and higher molecular weightaterial can be recovered when working at very low flow rates

0.1–0.2 mL/min). In a study on high molecular weight PS stan-ards Aust et al. further investigated the experimental conditionso minimize shear degradation [13] and Parth et al. applied this

ethodology to partially cross-linked PE [14]. We have analyzed

328 E.P.C. Mes et al. / J. Chromatog

Fig. 9. Elution curves and molecular weight plots of LDPE-1 on HTSEC mixedAdr

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rtldso

3u

aoiosη

κ

eibtmmdhmdrtpHnoattvfiin(svtHeb

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η

columns obtained under (A) “low shear” and (B) “standard” HTSEC con-itions; (A) flow rate: 0.2 mL/min, sample concentration: 0.5 mg/mL, (B) flowate: 1 mL/min, sample concentration: 1.1 mg/mL.

DPE-1 with HTSEC at “low shear” conditions and comparedt with HTSEC at “standard” conditions. Fig. 9 shows the chro-

atograms of LDPE-1 obtained at flow rates of 0.2 mL/min andmL/min. The overall molecular weight distribution obtainedt both flow rates are in good agreement. However, small dif-erences at the high molecular weight end of both the elutionurve and the molecular weight distribution can be observed. Itppears that reducing the flow rate to 0.2 mL/min does result in alightly better separation and recovery of high molecular weightpecies. But even under these conditions shear degradation ofarge fragile structures cannot be ruled out as we still observe aignificant difference between the high molecular weight mate-ial recovered by HTAF4 and “low shear” HTSEC (cf. Fig. 4).t the low molecular weight end it can be observed that late

lution of high molecular weight structures also occurs at a flowate of 0.2 mL/min. Furthermore, it is possible that some of theltrahigh molecular weight material remains even longer in theolumn or does not elute at all. We may conclude that perform-ng HTSEC at low flow rates can offer certain advantages inerms of minimizing HTSEC artifacts, such as shear degrada-ion. However, this comes at the expense of both plate count andspecially run time. Triple detection HTSEC at a flow rate of

.2 mL/min using four mixed A columns results in a run timef approximately 5 h. One can argue that the run time could beeduced by using fewer columns, but this would reduce the plateount and ultimately the fractionating power (which is already

tcb

r. A 1154 (2007) 319–330

elatively low for high molecular weight species). We note fur-her that reducing the flow rate does not increase the exclusionimit of the column. In other words, even at “low shear” con-itions, the HTSEC separation of very high molecular weightpecies in polyethylene is still restricted by the exclusion limitf the column.

.5. Relating molecular weight and zero shear viscosity forltrahigh molecular weight HDPE

The rheological behaviour of polymeric materials is stronglyffected by the molecular weight and branching distribution. Inrder to predict the processability of polymers, it is of paramountmportance to understand the effect molecular parameters haven the viscoelastic properties. It has been shown that above theo-called critical molecular weight Mc, the zero shear viscosity0 of most linear polymers is proportional to κ · Ma

w, whereand a are empirical parameters [60,61]. The “power-law”

xponent a for linear HDPE materials above Mc is typicallyn the order of 3.4–3.6. Zero shear viscosity is of importanceecause it can be used to predict rheological behaviour. However,o understand the rheological properties we need the detailed

olecular structure data from for example HTSEC measure-ents. Although η0 can be predicted from the molecular weight

istribution obtained by HTSEC for many linear polymers, weave observed that this is often difficult for very broad higholecular weight HDPE materials. It is suspected that this is

ue to the inability of HTSEC to properly quantify and sepa-ate the high molecular weight material. In order to investigatehis, four very broad high molecular weight HDPE samples (allossessing high molecular weight tails) have been analyzed byTSEC and HTAF4, and the weight-average molecular weightumbers have been determined (see Table 1). The Mw valuesbtained by the HTSEC and HTAF4 techniques differ consider-bly. Mw values obtained by HTAF4 are 2–7 times higher thanhe Mw values from the HTSEC mixed B columns. In addi-ion, we can observe considerable differences between the Mwalues obtained with the mixed A and mixed B columns. Per-orming HTSEC at a flow rate of 0.2 mL/min had only a minornfluence the weight-average molecular weight (although signif-cantly higher z-average molecular weight values were found,ot shown here). The loss of low molecular weight materialbelow ≈5 × 104) through the HTAF4 membrane will lead tolightly higher Mw values. However, this cannot account for theery large differences between HTAF4 and HTSEC. Also withinhe sample set the HTAF4 data show significant differences withDPE-A having by far the largest Mw and HDPE-C the small-

st Mw. The HTSEC data show in contrast only minor variationetween the samples.

An often used power law for linear HDPE that relates Mw to0 is given by the following equation [62]:

0 = 5.8 × 10−14M3.41w (3)

In Fig. 10 the zero shear viscosity numbers calculated withhe power law given in Eq. (3) from HTAF4 and HTSEC data areompared with estimated zero shear viscosity numbers obtainedy DMS experiments. The η0 values calculated from the HTAF4

E.P.C. Mes et al. / J. Chromatogr. A 1154 (2007) 319–330 329

Table 1Weight-average molecular weight values of high molecular weight HDPE samples obtained by HTAF4, HTSEC on mixed A columns, HTSEC on mixed A columnsat a low flow rate of 0.2 mL/min, and HTSEC on mixed B columns

Sample name Mw (HTAF4) Mw (HTSEC mixed A) Mw (HTSEC mixed A at low flow) Mw (HTSEC mixed B)

HDPE-A 1.72 × 106 4.23 × 105 4.42 × 105 2.40 × 105

HDPE-B 1.11 × 106 4.52 × 105 5 5

HDPE-C 6.95 × 105 4.66 × 105

HDPE-D 1.18 × 106 4.72 × 105

Fig. 10. Comparison of zero shear viscosity numbers obtained from DMSena

eftDltmucvmbfitcpilvl

4

lamd

dcHbc[ssrmi

HbeHHlec

ammcomtcn

A

Cto

R

xperiments with numbers from Eq. (3) with weight-average molecular weightumbers obtained with HTAF4, HTSEC mixed A columns, mixed A columnst flow rate 0.2 mL/min, and mixed B columns.

xperiments are in good qualitative agreement with the valuesound from melt rheology data. Overall, the zero shear viscosi-ies predicted by HTAF4 are slightly higher than the values fromMS. This might be partly due to the earlier mentioned loss of

ow molecular weight material in the HTAF4. It is also likelyhat the correlation in Fig. 10 would improve with true “creep”

easurements (beyond the scope of this paper). The η0 val-es predicted from HTSEC data (both mixed A and mixed Bolumns) do not show a good correlation with the zero sheariscosity estimations from DMS measurements. HTSEC experi-ents at low flow give slightly higher zero shear viscosity values,

ut fail to predict the correct trend. The reason for the HTSECailure is likely the loss of high molecular weight material dur-ng the HTSEC measurement. As mentioned before, it appearshat the high molecular weight material either remains on theolumn or is shear degraded. HTAF4 on the other hand is aromising tool to analyze ultrahigh molecular weight materialn HDPE. Further study is required to fully explore the corre-ation between predicted η0 values from HTAF4 and from η0alues from melt rheology for high molecular weight polyethy-ene.

. Conclusions

The use of HTAF4–IR–MALS–Visc to analyze high molecu-

ar weight polyethylenes has been demonstrated. HTAF4 showsbetter separation and mass recovery than HTSEC for very higholecular weight fractions in LDPE and HDPE as it is not hin-

ered by exclusion effects and anomalous late elution that can

4.85 × 10 2.96 × 104.46 × 105 3.51 × 105

5.03 × 105 2.56 × 105

istort the MWD obtained by HTSEC. Shear degradation, whichan be a problem when analyzing large fragile structures withTSEC, has not been observed for HTAF4. HTAF4 in com-ination with IR, MALS, and viscometry detection allows thealculation of the molecular weight and corresponding Rg andη] distributions; which in turn can be used to compare moleculartructures. By comparing HTAF4 and HTSEC results, we havehown that great care must be taken when interpreting HTSECesults for very broad high molecular weight polyethylenes asass recovery issues and areas of limited separation can lead to

mproper conclusions.The zero shear viscosity of very broad high molecular weight

DPE samples was predicted from Mw values obtained byoth HTAF4 and HTSEC. Good qualitative agreement betweenxperimental η0 values and predicted values was found forTAF4 whereas no predictive correlation was found for bothTSEC column sets studied. The apparent loss of high molecu-

ar weight material by HTSEC leads to the assumption that it isither shear degraded during the HTSEC run or remains on theolumn.

The lower limit of the molecular weight range that can benalyzed with HTAF4 is determined by the pore size of theembrane. This in the apparatus presented here was approxi-ately 5 × 104. The technique as such is, therefore, not suited to

haracterize low molecular weight material. Furthermore, lossf small molecular species will, especially for relatively lowolecular weight samples, lead to an overestimation of Mn, and

o a lesser degree Mw, of polydisperse polyethylene. To over-ome this limitation, new membranes with smaller pore sizeeed to be developed.

cknowledgements

The authors express their sincere appreciation to The Dowhemical Company for permission to publish this work. We

hank Joe Huang of The Dow Chemical Company for the rheol-gy measurements.

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