Analysing the Chemical Composition Distribution of Ethylene-Acrylate Copolymers: Comparison of HT-HPLC, CRYSTAF and TREF

Full Paper

Analysing the Chemical CompositionDistribution of Ethylene-Acrylate Copolymers:Comparison of HT-HPLC, CRYSTAF and TREF

Andreas Albrecht, Robert Brull,* Tibor Macko, Pritish Sinha, Harald Pasch

HT-HPLC is an attractive technique to analyse CCD of olefin copolymers and offers thepossibility to shorten analysis times compared to fractionation techniques based on crystal-lisation. We have found that HT-HPLC enables the selective elution of EMA and EBA copo-lymers according to their content of the polar co-monomer. This fractionation could be confirmedby coupling the gradient HPLC system with FT-IRspectroscopy. The CCDs obtained by this newmethod were then compared to the results fromCRYSTAF. Both methods, HT-HPLC and CRYSTAF,can discriminate between sets of samples havinga narrow or a broad CCD. They can also prove thepresence of acrylate-poor fractions.

Introduction

The determination of the chemical composition distribu-

tion (CCD) is crucial for an in depth understanding of

structure-property relationships. It is also important to

develop process-structure relationships and, thereby, to

understand the influence of reaction parameters or

catalyst properties on the structure of the resulting

polymer. To analyse the compositional heterogeneity,

semicrystalline polyolefins are commonly fractionated

from dilute solution using Temperature Rising Elution

Fractionation (TREF) or Crystallisation Analysis Fractiona-

A. Albrecht, R. Brull, T. Macko, P. Sinha, H. PaschDeutsches Kunststoff-Institut (German Institute for Polymers),Schlossgartenstr. 6, 64289 Darmstadt, GermanyE-mail: [email protected]. AlbrechtDutch Polymer Institute (DPI), PO Box 902, 5600 AX Eindhoven,The NetherlandsH. PaschUniversity of Stellenbosch, Institute for Polymer Science, PrivateBag X1, 7602 Matieland, South Africa

Macromol. Chem. Phys. 2008, 209, 1909–1919

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tion (CRYSTAF).[1–5] TREF has been introduced into polymer

analysis in the late 1970s while CRYSTAF has been

developed byMonrabal in the early 1990s. Both techniques

have since then been used to fractionate copolymers and

blends of semicrystalline polyolefins and are based on the

crystallisation of the macromolecules from a hot solution.

Consequently they cannot be applied to amorphous

samples. Another drawback is the long duration of the

analysis, which is the result of the required slow cooling

rates and typically ranges between 8 and 48 h.

Interaction chromatography presents an attractive alter-

native to fractionate polymer samples according to their

chemical composition. It has beenwidely applied to analyse

the CCD of polymers which are soluble at room tempera-

ture, or to separate blends of different polymers.[6–8]

The separation is based on the interaction of the polymer

molecule with the stationary phase. Advantages over

crystallisation techniques are the shortening of analysis

time, and the possibility to apply chromatographic modes

which are selective for particular structural features in the

macromolecule, such as end-groups, block structures or the

chemical composition. Taking these aspects into account,

DOI: 10.1002/macp.200800223 1909

A. Albrecht, R. Brull, T. Macko, P. Sinha, H. Pasch

1910

it is an important task to develop chromatographic

systems which can be used at temperatures which are

required for the dissolution of semicrystalline polyolefins

(100–150 8C).Only a few examples exist for interaction chromato-

graphy at high temperatures (HT). Apart from instru-

mental problems, the main reason behind this is that the

choice of solvents having sufficiently high boiling points

for the dissolution of the sample is limited. The first

examples for interaction chromatography of semicrystal-

line polyolefins above temperatures of 100 8C have been

published recently. Liquid chromatography at critical

conditions (LC-CC) for polystyrene at 140 8C in decalin/

cyclohexanone was used to separate blends of polyethylene

and polystyrene.[9] Critical conditions for poly(methyl

methacrylate) (PMMA) at 140 8C were also established

and copolymers of ethylene and MMA were analysed.[10]

Within gradient chromatography, the mechanism can be

based either on precipitation/dissolution or adsorption of

the macromolecules to the stationary phase. Gradient

liquid chromatography using a combination of 1,2,4-

trichlorobenzene (TCB) and ethylene glycol monobutyl

ether as mobile phase and silica gel as the stationary

phase was recently used to separate blends of poly-

ethylene and poly(propylene).[11,12] It could also be shown

that these chromatographic systems can separate ethyl-

ene-propylene copolymers according to their chemical

composition at a temperature of 140 8C.[13] These separa-

tions are based on the precipitation and redissolution of

the individual polymer fractions. The first example for

gradient liquid adsorption chromatography of polymers

at elevated temperatures has been reported by Lyons,

who fractionated ethylene-styrene copolymers according

to their composition at temperatures between 30 and

80 8C.[14] An example for selective adsorption of poly-

olefins from TCB on zeolithes at 140 8C has been reported

by Macko et al.[15] However, the desorption of the

samples was very difficult or even impossible. The first

example of gradient liquid chromatography based on full

adsorption and desorption operating at temperatures

above 100 8C has been published recently by Albrecht

et al. Using gradients of decalin/cyclohexanone or TCB/

cyclohexanone it was possible to separate ethylene-vinyl

acetate copolymers according to their chemical composi-

tion.[16] This separation was based on full adsorption

and desorption of the copolymers at temperatures as high

as 140 8C.In this paper we will demonstrate that high tempera-

ture gradient chromatographic systems can be tailored for

the analysis of copolymers of ethylene with different

esters of acrylic acid and then be applied over a wide range

of their chemical composition. We also want to show for

the first time a comparison of the results from HT-HPLC

with those from CRYSTAF and TREF.



Experimental Part

High-Temperature Chromatograph PL XT-220

A prototype of the high-temperature gradient HPLC system PL XT-

220 (Polymer Laboratories, Varian Inc, Church Stretton, England)

was used.[12] Dissolution and injection of samples were performed

using a robotic sample handling system PL-XTR (Polymer

Laboratories). The temperature of the sample block, injection

needle, injection port and the transfer line between the auto

sampler and the column compartment was set to 140 8C. Themobile phase flow rate was 1 mL �min�1. The copolymers were

dissolved for 2 h in TCB or decalin at a concentration of 1–

1.2 mg �mL�1 and a temperature of 140 8C. 50 mL of the polymer

solutions were injected. The column outlet was connected either

to an evaporative light scattering detector (ELSD, model PL-ELS

1000, Polymer Laboratories) or to a LC-Transform FT-IR Interface

(Series 300, Lab Connections, Carrboro, USA). The ELSDwas run at a

nebulisation temperature of 160 8C, an evaporation temperature

of 270 8C and with an air flow of 1.5 mL �min�1. The stage

temperature in the LC-Transform was 150 8C. The temperature for

the nozzle was set to 129 8C. The Germanium disc rotation speed

was set on 10 degree �min�1. FT-IR spectroscopy of the deposited

eluate was performed using a Nicolet Protege 460 (Thermo

Electron, Waltham, USA). For data collection and processing, the

WinGPC-Software (Polymer Standards Service GmbH, Mainz,

Germany) was used.

High-Temperature Chromatograph PL 220

A high temperature chromatograph PL 220 (Polymer Laboratories,

Varian Inc, Church Stretton, England) was used for determining

the molar mass distribution. The temperature of the injection

sample block and of the column compartment was set to 140 8C.The mobile phase flow rate was 1 mL �min�1. The copolymers

were dissolved for 2 h in TCB at a concentration of 1mg �mL�1 and

a temperature of 150 8C. 200 mL of the polymer solutions were

injected. Narrowly distributed polystyrene standards (Polymer

Standards Service GmbH, Mainz, Germany) were used for

calibration.

CRYSTAF

A CRYSTAF apparatus Model 200 (PolymerChar, Valencia,

Spain) was used for the fractionations at a cooling rate of

0.1 K �min�1. 20 mg of the sample were dissolved in 40 mL

1,2-dichlorobenzene (ODCB). An IR detectormonitoring the

absorption of the C–H stretching vibration was used.

TREF

A preparative TREF instrument (model PREP, PolymerChar,

Valencia, Spain) was used for the fractionation of the

polymers. The polymer samples were dissolved in ODCB at

130 8C in a stainless steel container of the TREF apparatus.

DOI: 10.1002/macp.200800223

Analysing the Chemical Composition Distribution of . . .

Subsequently the polymer solution was cooled to room

temperature at a cooling rate of 0.1 K �min�1. The

following elution was done with the same solvent at

heating rates of 20 K �min�1, collecting fractions at 35, 50,

65, 75 and 100 8C. These fractions were precipitated with

methanol, separated and dried in vacuum at 50 8C.

13C NMR

13C NMR (100 MHz) spectra were measured on a 400 MHz

spectrometer AVANCE (Bruker BioSpin GmbH, Rheinstet-

ten, Germany) on 15 wt.-% polymer solution in benzene-d6and TCB (1/6, v/v) at 80 8C. The pulse program zgig 30 (zero

go inverse gated) with an acquisition time of 1.366 s and a

relaxation delay of 7 s was used.

FT-IR

FT-IR spectroscopy of the samples was performed in

attenuated total reflectance (ATR) modus using a Nicolet

Nexus 670 (Thermo Electron, Waltham, USA).

Stationary Phases

The following columns packed with bare silica gels were

used: Perfectsil 300 A (particle diameter 5 mm, pore volume

1.05 mL � g�1, void volume V0¼ 3.21 mL, theoretical plates/

column¼ 20410) and Polygosil 1 000 A (particle diameter

10 mm, V0¼ 3.15 mL, theoretical plates/column¼ 17 087)

both from MZ Analysentechnik, Mainz, Germany. Column

size was 250� 4.6 mm i.d. A PLgel Mixed A column set,

column size 250� 8 mm i.d. (particle diameter 20 mm,

Polymer Laboratories, Varian Inc., Church Stretton, Eng-

land), was chosen for SEC analysis. The specifications of all

column packings were given by their producers. The

determination of the void volume was described else-

where.[16]

Mobile Phases

TCB, decalin, cyclohexanone and dibenzyl ether, all of

synthesis quality (Merck, Darmstadt, Germany), were used

as components of the mobile phases. Cyclohexanone was

purified by vacuum distillation.

Polymer Samples

Linear polyethylene (PE) standards with weight-average

molar masses (Mw) in the range of 2–126 kg �mol�1

(Mw=Mn ¼ 1.12–1.59) and poly(n-butyl acrylate) (PBA)

standards with a Mw of 5.11–70.8 kg �mol�1



(Mw=Mn ¼ 1.04–1.25) were obtained from Polymer Stan-

dards Service (Mainz, Germany). Samples of the ethylene-

methyl acrylate (EMA) and ethylene-butyl acrylate (EBA)

copolymers were obtained from Exxon-Mobil Chemical

(Meerhout, Belgium), Du Pont (Geneva, Switzerland) and

Arkema (Paris, France). The compositional data given by

the producer and the molar mass data of the copolymers

are summarised in Table 1.

Results and Discussion

Taking into account that copolymers of ethylene and

esters of acrylic acid are a combination of polar and non-

polar units, one can expect that the acrylate comonomer

selectively interacts with a polar stationary phase, while

the ethylene units do not contribute to retention. With

regard to the mobile phase, TCB, decalin, cyclohexanone

and dibenzyl ether were identified as solvents for the

homopolymers, PE and PBA, and the copolymers at 140 8C.Therefore, a chromatographic method which has been

developed to fractionate ethylene-vinylacetate copoly-

mers[16] using a gradient decalin/cyclohexanone (Figure 1)

and bare silica gel as stationary phase was first tested.

Startingwith 100% decalin for 5.5min, the volume fraction

of cyclohexanone was increased linearly to 50% within

20 min and then increased to 100% in 2 min. This

composition was then held constant for 2 min. Finally, the

initial chromatographic conditions were re-established.

Due to the column void volume and the dwell volume of

the chromatographic system the gradient reaches the

detector with a delay of 6.26 min, i.e., the gradient reaches

the detector at 11.76 min. The procedure for the

determination of the void volume and the dwell volume

has been described elsewhere.[16] Figure 1 shows the

elugrams of EBA (a) and EMA (b) samples.

All samples elute in order of increasing polarity

(Figure 1a): First, the least polar EBA 1 (11.8 wt.-% BA)

elutes followed by themore polar EBA 3 (31.7 wt.-%), EBA 4

(58.4 wt.-%) and EBA 2 (59.3 wt.-%). Using this system,

EBA 1, eluting at 1.9–3.3 mL, cannot be distinguished from

the PE homopolymers (elution volume: 2.2 mL for

Mp¼ 126 kg �mol�1 and 3.2 mL for Mp¼ 2.03 kg �mol�1).

Furthermore, EBA 2–4 elute at 11.85, 12.00 and 12.05 ml,

respectively, and are therefore not sufficiently separated.

EMA copolymers can be better separated using this

system. As can be seen in Figure 1b, EMA 1 (23.5 wt.-%

MA) and EMA 2 (27 wt.-%) elute in the order of increasing

MA content. It is interesting to note that EMA 10 (24wt.-%)

and EMA 11 (25 wt.-%) show a broader chemical

distribution.

To achieve a better separation of EBA copolymers and a

stronger adsorption of EBA 1, Perfectsil 300 was chosen

as stationary phase. The gradient was modified by

www.mcp-journal.de 1911


Table 1. Weight average molar mass (Mw), polydispersity index (PDI), and methyl acrylate (MA) and butyl acrylate (BA) content, respectively,given by the producers.

Sample code Producer Mwa) PDIa) MA or BAb)

kg �molS1 wt.-%

EMA 1 Exxon-Mobil 282 4.9 23.5

EMA 2 183 3.7 27

EMA 3 Arkema 279 6.8 9b)/6.8c

EMA 4 264 6.5 14b)/11.4c)

EMA 5 289 7.2 18b)/18.5c)

EMA 6 250 7.2 28b)/29c)

EMA 7 DuPont 240 5.8 9

EMA 8 197 4.7 18

EMA 9 196 4.7 15

EMA 10 235 4.6 24

EMA 11 245 5.3 25

EBA 1 Arkema 381 13.6 11.8c)

EBA 2 96 4.2 59.3c)

EBA 3 285 8.5 31.7c)

EBA 4 196 8.3 58.4c)

EBA 5 375 12.9 7

EBA 6 232 6.6 17

EBA 7 294 7.5 17

EBA 8 114 5.4 28

EBA 9 DuPont 266 5.8 17

EBA 10 302 7.1 17

EBA 11 192 5.1 17

a)Data from our SEC measurements; b)Data from the producers; c)Data from NMR measurements.

1912

introducing a second slope where the fraction

of cyclohexanone is linearly increased to 20% within

10 min (see Figure 2).

Figure 2 clearly shows a separation of EBA copolymers

from the poly(n-butyl acrylate) (PnBA) homopolymer

samples, which elute at 26.2 mL (Mw ¼ 5.11 kg �mol�1),

28.2 mL (Mw ¼ 31.3 kg �mol�1) and 28.3 mL (Mw ¼70.8 kg �mol�1), respectively. Therefore, the elution volume

of PnBA is a function ofmolarmass for the lowmolarmass

sampleswhile for highmolarmass it becomes increasingly

independent. Further improvements of the separation, in

particular regarding the resolution of copolymers with low

BA content can be achieved when dibenzyl ether is used as

desorption promoting solvent (Figure 3).

As shown in Figure 3, the separation of the EBA

copolymers occurs according to the polarity of the samples.



The least polar EBA 1 (11.8 wt.-% BA) elutes before EBA 6

and EBA 9 (both having 17 wt.-% BA) and EBA 8 (28 wt.-%

BA). Themost polar EBA 2with 59wt.-% BA elutes last. It is

interesting to note that EBA 1 elutes in two peaks ranging

from 1.7–4.0mL and from 12.7–13.2mL. This indicates that

a portion of the sample either does not or only very weakly

adsorb on Perfectsil 300 from decalin. EBA 9 (17 wt.-% BA)

and EBA 10 (17wt.-% BA) elute in bimodal peaks indicating

a broad chemical composition distribution. For EBA 10 a

second peak at 1.6 mL can be observed. This peak can be

assumed to be either EBA copolymer with a very low BA

content or PE homopolymer. However, this cannot be

decided from the elugrams. The sharp peak, which is

observed for the EMA samples at 11.8 mL (Figure 1) and for

the EBA samples at 14.8 mL (Figure 2) or 12.8 mL (Figure 3)

can be explained byweakly adsorbing copolymer fractions

DOI: 10.1002/macp.200800223


Figure 1. Overlay of the chromatograms of EBA (a) and EMAcopolymers (b); stationary phase: Polygosil 1 000, mobile phase:gradient decalin/cyclohexanone (dotted line); temperature:140 8C; detector: ELSD; sample solvent: decalin.

Figure 3. Overlay of the chromatograms of EBA copolymers;stationary phase: Perfectsil 300; experimental conditions seeFigure 1.

with a low acrylate content, which can be desorbed by a

small amount of the desorption promoting solvent, e.g.

cyclohexanone or dibenzyl ether, which have a high

affinity to the stationary phase. By reducing the desorption

strength of the eluent this peak could be reduced, compare

Figure 2. Overlay of the chromatograms of EBA copolymers;stationary phase: Perfectsil 300; experimental conditions seeFigure 1.



Figure 1–3. To avoid this problem, the difference in the

polarity of the adsorption and desorption promoting

solvent should be smaller. In this context it should be

noticed that the gradient itself, only injecting the sample

solvent decalin, leads to a small peak at the beginning of

the gradient.

For the separation of EMA copolymers, a step gradient of

decalin/cyclohexanone, which has been described earlier,

was applied, using Perfectsil 300 as stationary phase. The

chromatograms of EMA samples from different producers

are shown in Figure 4.

The EMA copolymers containing 9–28 wt.-% MA are

separated with regard to their MA content (see Figure 4a).

Clear differences in the elution behaviour of the EMA

copolymers fromdifferent producers can be observed. EMA

1–6 elute in relatively narrow peaks while the EMA

copolymers 7, 9 and 10, from a different producer, elute in

broader peaks, indicating broad CCD. EMA 4, EMA 7 and

EMA 9, show additional peaks between 1.5 and 2.5 mL

(EMA 4) and 2.5 and 3 mL (EMA 7 and EMA 9), which are

close to the exclusion volume of the column (v0¼ 3.21 mL).

This indicates for EMA 4 the presence of a non-adsorbing

fraction and for EMA 7 and EMA 9 of a very weakly

adsorbing fraction in the samples. Due to the increase in

the gradient slope at 28.7 mL (see Figure 2) the desorption

power of the eluent increases, thereby speeding up the

elution and in turn leading to narrower peaks, compared to

the gently inclined gradient used before (e.g. the peak of

EMA 10 at 29 mL).

It is known that the response of the ELSD depends, at

constant instrumental parameters (flow rate, temperature,

sample loop volume, etc.), on the concentration of the

analyte and the composition of the mobile phase, and is

not strictly independent of the structure of the analyte.[17–20]

The influence of parameters such as concentration,

molar mass and chemical composition of the analyte, as



Figure 4. Overlay of the chromatograms of EMA copolymers (aand b); stationary phase: Perfectsil 300; experimental conditionssee Figure 1.

Figure 5. Influence of the experimental parameters on theresponse of the ELSD detector; temperature: 140 8C; (a) specificresponse of PE 60 kg �mol�1 as function of mobile phase com-position; stationary phase: PLgel Mixed A Column; mobile phase:decalin/dibenzyl ether; sample solvent: mobile phase; (b) cali-bration of the chromatographic system decalin/dibenzyl etherdescribed in Figure 3 with PE 60 kg �mol�1 (~) and EBA 4 (D); (c)calibration of the chromatographic system decalin/cyclohexa-none described in Figure 4 with PE 60 kg �mol�1 (~), EMA 3(!), EMA 5 (&) and EMA 6 (*).

1914

well as the composition of the mobile phase on the

response of the detector, were described for the gradient

system decalin/cyclohexanone in our previous publica-

tion.[16] The influence of the composition of the mobile

phase in the gradient system decalin/dibenzyl ether

(Figure 5a) and the calibrations for the quantification of

the detector response for EBA 4, EMA 3, 5 and 6 and PE

homopolymer (Mw ¼ 60 kg �mol�1) in the used chromato-

graphic systems (Figure 5b and c) are shown.

As seen in Figure 5a, the detector response decreases in

an exponential way with the dibenzyl ether content in the

mobile phase (75% decrease by changing the mobile phase

from100% to 90%decalin). This pronounced dependence of

the detector response on the mobile phase composition is

in good agreement with our previous observation[16] and

the ref.[17–20] The corresponding calibration of the peak

areas of the copolymer EBA 4 shows a linear behaviour.

The sensitivity of the detector for the analysed copolymer

is about 10 times smaller than for PE homopolymer

(Figure 5b). We conclude that the response of the ELSD is

strongly influenced by the composition of the mobile

phase and that changes as a result of the gradient

substantially complicate the quantitative evaluation of

the ELSD response for the copolymers.

The elugrams obtained using the ELSD do not allow an

identification of the chemical composition of these eluted

fractions. For this purpose the high-temperature gradient



HPLC was coupled with FT-IR spectroscopy using a LC-

Transform interface. In this approach the eluate from the

chromatograph is deposited on a rotating Germanium disc

and the mobile phase removed under vacuum.[21–23] In

order to obtain a homogeneous deposition of the polymer

on the Germanium disc through the gradient elution, the

evaporation rate of the solvent was adjusted by tuning the

spray temperature.[22] Suitable traces of the polymer were

formed with this technique when the gradient decalin/

DOI: 10.1002/macp.200800223


Figure 6. Overlay of the results from HPLC-FT-IR analysis of EMA1–6 (a) and EMA 7, 8, 10 and 11 (b) and the correlation between theMA content measured by NMR and the peak area ratio of thecarbonyl group (1 730 cm�1) to the CH2-group (1 450 cm�1)measured by FT-IR (C).

Table 2. Average MA content and MA distribution obtained by LC-FT-IR and the average MA content measured by NMR.

Sample Average MA (FT-IR) MA distribution (FT-IR) Average MA (NMR)

wt.-% wt.-% wt.-%

EMA 1 20.5 18.9–21.4 –

EMA 2 23.4 22.0–24.7 –

EMA 3 9.7 9.5–10.0 6.8

EMA 4 13.2 13.2–13.6 11.4

EMA 5 14.6 12.4–16.0 18.5

EMA 6 30.1 29.0–32.5 29



cyclohexanone was used. A calibration was carried out to

obtain absolute values for theMA content. For this purpose

a series of EMA samples was analysed by NMR-spectro-

scopy. The absolute MA contents thus obtained were then

correlated to the peak area ratios from IR analysis of the

bulk samples deposited on the Germanium disc (Figure 6c).

Figure 6a and b show the Gram-Schmidt (GS) plots, which

reflect the sample concentration, and the MA content as

measured from the ratio of the carbonyl vibration band vs.

the vibration band of the methylene groups along the

elution volume.

An increase of theMA content with the elution time can

be found for all samples, except EMA 3 and 4, which show

no increase in the main peak (Figure 6a and b). This means

that separation according to the chemical composition

really takes place and that the analysed samples are

chemically inhomogeneous. From these results two sets of

samples can be distinguished: The first one (EMA 7, 8, 10

and 11) with a broad CCD where the co-monomer content

spans a range of� 50% around the average value. The

second one (EMA 1–6) has a narrow CCD and the MA

content scatters about 5 wt.-% around the average

(Table 2), and among these EMA 3 and 4 show the most

homogenous chemical composition around an average of

9.7 wt.-% and 13.2 wt.-% MA along the elution volume,

respectively. It should be noted here that for the samples

having a broad CCD (Figure 6a) different MA contents are

found for fractions eluting at the same elution volume but

stemming from different samples, particularly at higher

elution volumes. This hints at either the effect of

branching or molar mass on the elution behaviour.

However, this cannot be decided from the HPLC-FT-IR

results alone. The average values of the EMA samples with

a narrow CCD and their maximum and minimum MA

content are summarised in Table 2.

In both sets of samples copolymers with a second

elution peak between 4 and 5mL, namely EMA4 and 7, can

be identified. The MA content in the second peak ranges

between 1 and 2.5 wt.-% (EMA 7) and from 0–2 wt.-%



Figure 7. FT-IR spectra of the sample EMA 4 at elution volume 4mL (a) and 5 mL (b).

Figure 8. Relationship between the elution volume and the con-tent of MA in the copolymer (a) and the resulting plot of the MAcontent versus the detector intensity (b).

1916

(EMA 4). The presence of the PE homopolymer in sample

EMA 4 can be verified by FTIR spectra at selected elution

volumes (Figure 7a). At an elution volume of 4 mL, no

carbonyl absorption band (1 740 cm�1) was detected

(Figure 7a), while in the FT-IR spectrum at 5 ml the

carbonyl absorption band can be clearly identified

(Figure 7b).

The band at 1 376 cm�1 indicates the presence of CH3

groups,[24] which can either originate from branches or

chain end groups. Thus the PE fraction in EMA 4 can be

linear PE wax or branched PE. Plotting the elution volume

at the peak maximum as a function of the calculated

average MA content for samples EMA 1–6, a linear

relationship between 16 and 28 mL elution volume

(Figure 8a) is obtained. With this linear relationship the

resulting elugrams can be plotted against the MA content

(Figure 8b).

EMA 1 (23.5 wt.-%) and EMA 10 (24 wt.-%) have similar

average chemical compositions but show different elution

behaviour (Figure 6a, b). This is caused by the different

chemical composition distribution, but can additionally be

the result of differing polymer architectures, i.e. degree of

blockiness. Therefore these samples were studied by

quantitative 13C NMR spectroscopy. The NMR spectrum

of EMA 1 is shown in Figure 9 and the calculated triads of

EMA 1 and EMA 10 are summarised in Table 3. The



assignments and calculations are based on published

procedures.[25–27]

Both samples have a similar total branching content per

1 000 carbons (Table 3). The results shown in Table 3 do not

reveal significant differences in the microstructure. So it

could be assumed that the differences in the elution

behaviour are primarily the results of the CCD. However, it

is an important future task to study the effect of the

microstructure on the elution behaviour with regard to the

analysis of copolymers from new, e.g. organometallic

catalysts. The presence of microblocks in both samples can

be explained by the copolymerisation parameters

(r1¼ 0.045 and r2¼ 5.3 for EMA) which favour the cross

propagation reaction for the ethylene radical at the

polymer chain end over the homo-propagation reaction

for the ending acrylate radical.[28–30]

CRYSTAF is the technique commonly applied for the

analysis of the CCD of olefin copolymers. However, there

are no reports on the analysis of copolymers of ethylene

and polar comonomers in the literature. CRYSTAF traces of

selected EMA and EBA copolymers are presented in

Figure 10.

A broad, multimodal crystallisation peak could be

observed for EMA 7. Similar results were also found for

EMA 8–10. For EMA 7–10 the amount of the amorphous

fraction increases with decreasing ethylene content of the

DOI: 10.1002/macp.200800223


Figure 9. 13C NMR spectrum of EMA 1.

copolymers. On the contrary, EMA 3–5 crystallise prevai-

lingly in narrow peaks (Figure 10a). The decrease in the

peak crystallisation temperature correlates with the MA

content starting from 33 8C (EMA 3with 9 wt.-%MA), 25 8C(EMA 4, 14 wt.-% MA), over 10 8C (EMA 5, 18 wt.-% MA) to

the completely amorphous EMA 1, 2 and 6 (23.5, 27 and 28

wt.-% MA), which are not shown in Figure 10. Similar

results are obtained for the EBA copolymers. EBA 9 and 11

(Figure 10b) crystallise in a broad peak between 55 8C and

5 8C. EBA 5 and EBA 7 show narrow crystallisation peaks

compared to the broader ones from EBA 9 and EBA 11. EBA

8with a BA content of 28% is a totally amorphousmaterial.

An additional crystallisation peak can be detected for EMA

4 and EBA 9 at 85 8C (EMA 4) and 83 8C (EBA 9). This

indicates that these samples contain either slightly

branched PE homopolymer or copolymer having very

low acrylate content, since a crystallisation peak at 87 8C is

expected for HDPE.[31]

When comparing the results from CRYSTAF or TREFwith

interaction chromatography, one has to keep in mind that

the first ones are based on the crystallisation of the longest

ethylene sequences,[31] while the chromatographic separa-

Table 3. Triads (mol-%) and total branch content per 1 000 C of EMA copolymers.

Sample Total branch/1 000 C Triads

EEE MEE MEM EME EMM

mol-% mol-% mol-% mol-% mol-%

EMA 1 11.7 74.4 15.6 3.2 7.5 0.4

EMA 10 13.1 68.5 14.7 4 7.4 0.9

Figure 10. Overlay of the CRYSTAF traces of the EMA copolymers(a) and EBA copolymers (b).



tion is the result of the interaction of

the polar component with the sta-

tionary phase. Therefore, it is worth-

while to note that both methods yield

comparable results regarding the CCD

of the analysed samples. More speci-

fically, two sets of samples can be

differentiated one with rather narrow

and onewith broad CCD. Additionally,

the presence of an acrylate-poor frac-

tion can be identified by both meth-

ods. Further work with suitable chro-

matographic modes and model

polymers is necessary to identify the

influence of the microstructure on the

elution behaviour.



Figure 11. FT-IR spectra of the TREF-fraction (100 8C) from EMA 4(a) and EBA 9 (b).

1918

To study the highly crystalline fraction in EMA 4 and

EBA 9 in detail, the samples were fractionated by TREF. The

fraction eluting between 75–100 8C was isolated and

analysed by IR spectroscopy (Figure 11).

The FT-IR spectra in Figure 11 clearly show a carbonyl

vibration and, therefore, prove the presence of MA or BA,

respectively. This confirms the results from the HPLC-FT-IR

analysis, which identified MA (0–2 wt.-%) in the first

eluting fraction of EMA 4.

Conclusion

Two gradient HPLC methods have been tailored for the

analysis of EMA and EBA copolymers at a temperature of

140 8C. The copolymer sampleswere separatedwith regard

to the content of the polar comonomer. The separation in

the systems silica gel/decalin/cyclohexanone or dibenzyl

ether is based on full adsorption and the subsequent

controlled desorption of the polymers by a solvent

gradient. The separation of the EMA copolymers according

to the chemical composition was confirmed by coupling

the HPLC to FT-IR spectroscopy via the FT-IR interface. As a

result, a gradient in the methyl acrylate content along the

elution volume was identified, revealing heterogeneity in

the chemical composition of the copolymer samples.



For the first time results from CRYSTAF and TREF

measurements have been comparedwith data obtained by

the gradient HPLC and gradient HPLC-FT-IR analysis. All

three techniques identified two sets of copolymers

according to the producer which differ in the broadness

of the CCD. Moreover acrylate-poor fractions can be

identified by both methods.

Acknowledgements: This research forms part of the researchprogram of the Dutch Polymer Institute (DPI), Project # 642/643.The authors acknowledge C. Brinkmann for measuring the molarmasses of the EMA and EBA copolymers and C. Hock for the FT-IRmeasurements.

Received: April 28, 2008; Accepted: June 3, 2008; DOI: 10.1002/macp.200800223

Keywords: fractionation of polymers; FT-IR; high performanceliquid chromatography (HPLC); polyolefins

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