Comparison of High-Temperature HPLC, CRYSTAF and TREF for the Analysis of the Chemical Composition Distribution of Ethylene-Vinyl Acetate Copolymers

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Comparison of High-Temperature HPLC,CRYSTAF and TREF for the Analysis of theChemical Composition Distribution ofEthylene-Vinyl Acetate Copolymers

Andreas Albrecht, Robert Brull,* Tibor Macko, Frank Malz, Harald Pasch

The chemical composition distribution (CCD) is a fundamental molecular parameter ofcopolymers. High-temperature interactive liquid chromatography (HT-HPLC) has recentlyemerged as a new analytical technique for determination of the CCD of semicrystallinecopolymers of ethylene and polar comonomers. With the aim of comparing the results of HT-HPLC with those from the traditionally used temperature rising elution fractionation (TREF)and crystallization analysis fractionation (CRYSTAF) techniques, three ethylene-vinyl acetate(EVA) copolymers were fractionated by TREF and the fractions were analyzed by HT-HPLC. HT-HPLC-Fourier transform-infrared (FT-IR) spec-troscopy showed that individual fractions ofdifferent VA-content coelute in the HPLC. Whilethe separation in TREF and CRYSTAF is mainly theresult of the overall effect of alkyl branches andVA-comonomer units, in HT-HPLC it is the polarcomonomer that selectively contributes to theadsorption. Thus, HT-HPLC leads to a much moredetailed knowledge of the distribution of thestructured units; in addition, it saves time.

Introduction

The determination of the chemical composition distribu-

tion (CCD) of polymer materials is of paramount impor-

A. Albrecht, R. Brull, T. Macko, F. Malz, H. PaschGerman Institute for Polymers (DKI), Schlossgartenstr. 6, 64289Darmstadt, GermanyE-mail: [email protected]. Albrecht, T. MackoDutch 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. 2009, 210, 1319–1330

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tance for understanding the catalyst performance, as well

as for process optimization and elaborating structure-

property relationships. The CCD of semicrystalline poly-

olefins is routinely analyzed using temperature rising

elution fractionation (TREF) or crystallization analysis

fractionation (CRYSTAF).[1–5] Both techniques separate

the polymers according to crystallization of the macro-

molecules fromahot solution. For ethylene copolymers, the

fractionation mechanism in CRYSTAF and TREF is based on

differences in the crystallization of the longest ethylene

sequences (LES) of the polymer chains.[5] Due to the fact

that comonomer units interrupt the chain regularity, the

ability of the chains to orientate themselves into a crystal

will be lower in copolymers. As a consequence, semicrystal-

line copolymers can be fractionated according to their

DOI: 10.1002/macp.200900135 1319

A. Albrecht, R. Brull, T. Macko, F. Malz, H. Pasch

1320

comonomer content. Wild and Kelusky analyzed the CCD

of ethylene-vinyl acetate (EVA) copolymers containing

9–42wt.-% VA by TREF.[6,7] They found that copolymers

with a higher content of polar comonomer (>30wt.-%) are

totally amorphousand thus cannotbe separatedbyTREFor

CRYSTAF.

Using high-temperature size-exclusion chromatogra-

phy–Fourier transform-infrared (HT-SEC-FT-IR) spectro-

scopy, the average composition along the molar mass axis

can be determined for these copolymers.[8–10] However,

macromolecules that have different chemical composition

for the same hydrodynamic volume are not separated by

SEC-FT-IR.

Interactive chromatography is an alternative technique

to fractionate polymer samples according to their chemical

heterogeneity. The separation is based on interactions

between the polymer molecules and the stationary phase.

Besides reducing the analysis time, an additional advan-

tage over the crystallization techniques is the possibility of

applying different chromatographic modes that are selec-

tive forparticular structural features in themacromolecule-

like end-groups of the polymer chain, block structures or

chemical composition. In the literature various examples

for analyzing the chemical heterogeneity of polymers or

polymer blends by chromatographic methods have been

described.[11–13]

Themajority of published chromatographic analyses are

for polymers that are soluble at room temperature.

Polyolefins and many olefin copolymers like EVA are

soluble only at high temperature (50–150 8C). The first

examples of analysis by HT-HPLC have recently been

described in detail by Pasch et al.[14–18] The use of HPLC

systems for interactive chromatography of samples com-

posed of ethylene and polar comonomers were published

by our group. Liquid chromatography under critical

conditions (LCCC) for polystyrene (PS) at a temperature of

140 8C was used to separate blends of PS and polyethylene

(PE) and to analyze the styrene block length of ethylene-

styreneblock copolymers.[14] ByusingLCCC forpoly(methyl

methacrylate) (PMMA) at 140 8C ethylene-methyl metha-

crylate copolymers were analyzed.[15] Ethylene-methyla-

crylate or ethylene-butylacrylate copolymers were

successfully separated according to their chemical compo-

sition in a gradient of decalin-cyclohexanone or decalin-

dibenzylether.[16] Finally, a separation of EVA at a

temperature of 140 8C according to the polar comonomer

content has been described.[17] This separation is based on

full adsorption and subsequent desorption of EVA by

the gradient decalin-cyclohexanone. The separation

according to the vinyl acetate content was verified by

coupling the interactive chromatography with FT-IR

spectroscopy via the LC-Transform interface.[17] The use

of similar systems for HT-HPLC has also been claimed by

Petro et al. [19]



In this paper, we compare the results obtained by HT-

HPLC with the results from CRYSTAF and TREF. For a better

understanding of the results and the mechanisms of the

techniques used, the EVA samples were fractionated by

TREF. Three EVA samples were analyzed by HT-HPLC and

CRYSTAF. The TREF fractions were characterized by

standard analytical techniques like HT-SEC, FT-IR and

NMR spectroscopy, as well as by HPLC and hyphenated

HPLC-FT-IR spectroscopy. EVA copolymers with a low

average content of VA were deliberately selected due to

the fact that EVA with a higher content of VA are mainly

amorphous, that is, they cannot be adequately (or at all)

separated by CRYSTAF and TREF.

Experimental Part

High-Temperature Chromatograph PL 220

A high temperature chromatograph PL GPC 220 (Polymer

Laboratories, Varian Inc, Church Stretton, UK) was used for the

determination of the molar mass distribution. The temperature of

the injection sampleblockand the columncompartmentwas set at

140 8C. The mobile phase flow rate was 1mL �min�1. A refractive

index (RI) was used as detector. The copolymers were dissolved for

2 h in1,2,4-trichlorobenzene (TCB)at a concentrationof1mg �mL�1

and a temperature of 150 8C. 200mL of each polymer solution was

injected. Polystyrene (PS) standards were used for calibration.

High-Temperature Chromatograph PL XT-220

A high-temperature gradient HPLC system PL XT-220 (Polymer

Laboratories, Varian Inc, Church Stretton, UK) was used.[20]

Dissolution and injection of samples were performed using the

robotic samplehandlingsystemPL-XTR (PolymerLaboratories). The

temperature of the sample block, injection needle, injection port

and the transfer line between the autosampler and the column

compartment was set at 140 8C. The mobile phase flow rate was

1mL �min�1. The copolymers were dissolved for 2 h in decalin at a

concentrationof1–1.2mg �mL�1anda temperatureof140 8C.50mLof each polymer solution was injected. The column outlet was

connected either to an evaporative light scattering detector (ELSD;

model PL-ELS 1000, Polymer Laboratories) or to an LC-Transform

FT-IR interface (Series 300, Lab Connections, Carrboro, USA). The

ELSD was run at a nebulisation temperature of 160 8C, an

evaporation temperature of 270 8C and with an air velocity of

1.5 L �min�1. At the LC-Transform the stage temperature was

150 8C.The temperature for thenozzlewassetat129 8Cfor theHPLC

and 154 8C for the SEC experiments. The Germanium disc rotation

speedwas set at 10 deg �min�1. FT-IR spectroscopy of the deposited

eluatewasperformedusingaNicoletProtege460 (ThermoElectron,

Waltham, USA). Compilation of a molar mass calibration for the

GPC-FT-IR measurements was done by spraying PS-standards on

the Germanium disc and calculating a calibration curve from the

resulting elution volumes. The WinGPC-Software (Polymer Stan-

dards ServiceGmbH,Mainz, Germany)was used for data collection

and processing.

DOI: 10.1002/macp.200900135

Comparison of High-Temperature HPLC, CRYSTAF and TREF for . . .

Table 1. Weight average molar mass (Mw), polydispersity index(PDI), and vinyl acetate (VA) content of the polymer samples.

Sample Producer Mwa) PDa) VAb)

kg �mol�1 wt.-%

Crystallization Analysis Fractionation (CRYSTAF)

A CRYSTAF apparatus Model 200 (PolymerChar, Valencia, Spain)

was used for the fractionations at a cooling rate of 0.1 8C �min�1.

20mg of the samplewas dissolved in 40mL of 1,2-dichlorobenzene

(ODCB). IR detectors were used to monitor the absorption of the

C�H and the carbonyl stretching vibrations.

EVA 1 150 4.0 9/9.0c)

EVA 2 DuPont 444 7.8 9.5/9.8c)

EVA 3 437 8.7 7.5/7.7c)

EVA 4 Exxon Mobile 270 4.8 12

a)Data from our SEC measurements; b)Data obtained from produ-

cers; c)Data from our NMR measurements.

Temperature Rising Elution Fractionation (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. Subsequently, the

polymer solution was cooled to room temperature at a cooling

rate of 0.1 8C �min�1. The following elution stepwas donewith the

same solvent at heating rates of 20 8C �min�1 and the fractions

were collected at temperatures 35, 50, 65, 75, and 100 8C. Thesefractionswere precipitatedwithmethanol, separated by filtration,

and dried in vacuum at 50 8C.

13C NMR Spectroscopy

The spectra were acquired using a Mercury-VX 400 NMR spectro-

meter (9.4 T; Varian, Inc., Palo Alto, USA) with a 5mm four nuclei

probe (direct detection). The 13C NMR spectra (Larmor frequency of

100.6MHz) were recorded using a 908 pulse with 1H decoupling

during the acquisition time. The acquisition of the spectra was

set at 64 000 data points (corresponding to an acquisition time of

1.3 s at a spectralwidthof 25 000Hz), a relaxationdelay of 7 s, anda

total of 50 000 scans. Fourier transformation was done after zero

filling the data to 64000 time domain points and exponential

filtering of 1.0Hz.

TheEVAsampleswerepreparedas15wt.-%polymersolutions in

amixtureofbenzene-d6andTCB (1:6). ThesamplesEVA1andEVA2

were measured at 80 8C and EVA 3 at 110 8C.

FT-IR Spectroscopy

FT-IR spectroscopy of the samples was performed in attenuated

total reflectance (ATR) mode using a Nicolet Nexus 670 (Thermo

Electron, Waltham, USA). Peak areas at the wavenumber of the

carbonyl band (1 730 cm�1), the methylene (1 450 cm�1) and the

methylbands (1 371 cm�1)wereused for aquantitativeevaluation.

Stationary Phases

A Perfectsil 300 column (25�0.46 cm I.D., particle diameter 10mm,

MZ Analysentechnik, Mainz, Germany) packed with bare silica gel

wasused for interactive chromatographyanalysis. FourcolumnsPL

MixedA (25� 0.8 cm I.D.), 20mm, Polymer Laboratories, Varian Inc,

Church Stretton, England were chosen for SEC analysis.

Mobile Phases

1,2,4-trichlorobenzene (TCB), 1,2-dichlorobenzene (ODCB), decalin,

tetrachloroethylene, and cyclohexanone, all of synthetic quality



(Merck, Darmstadt, Germany), were used for preparing themobile

phases. Decalin and cyclohexanone used for the HPLC-FT-IR

analysis were purified by vacuum distillation. Methanol of

synthetic quality (Merck, Darmstadt, Germany) was used for the

precipitation of the TREF fractions.

Polymer Samples

Samples of the EVA copolymers were obtained from DuPont

(Geneva, Switzerland) and Exxon Mobile-Chemicals (Meerhout,

Belgium). The compositional data given by the producer and the

molar mass data of the copolymers are summarized in Table 1.

Results and Discussion

SEC-FT-IR

The combination of SEC and FT-IR spectroscopy gives

information on the distribution of comonomer or micro-

structural parameters along themolarmass axis. In the LC-

transform instrument, the eluate from the chromatograph

is deposited on a rotating Germanium disc and the mobile

phase evaporated under vacuum. The obtained polymer

film is then analyzed off-line by FT-IR.[8,9] Figure 1 shows

a) themolarmass distributions and b) the SEC-FT-IR results

of samples EVA 1–3. While the molar mass distribution of

EVA1 ismono-modal, thedistributions of EVA2and3show

a shoulder in the highmolar mass area. The Gram-Schmidt

plots (Figure 1b) corresponding to the total sample

concentration are very much comparable to the refractive

index (RI) traces in Figure 1a. TheVA content is presented as

the relative peak area ratio of the carbonyl band

(1 730 cm�1) to the methylene band (1 450 cm�1). The

distribution of the vinyl acetate is homogeneous over the

largest part of the molecular mass distribution (MMD). In

the lowmolarmass region,up to20 000g �mol�1, adecrease

of the VA content is observed with increasing molar mass

for EVA 2 and 3. Additionally the average VA content of

the samples decreases in the same order, as expected from

www.mcp-journal.de 1321


1000000100000100001000

0,0

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0,4

0,5

0,6

0,7

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W(lo

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)

Molar Mass in g/mol

EVA 1 EVA 2 EVA 3

1E71000000100000100001000

0

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15

20

25

30

35

40

45

0,0

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VA content EVA 1 EVA 2 EVA 3

Gra

m S

chm

idt

Molar Mass in g/mol

Gram Schmidt EVA 1 EVA 2 EVA 3

A [1

730

cm-1] /

A [1

450

cm-1]

a) b)

Figure 1. Overlay of the results from: a) SEC, and, b) SEC-FT-IR spectroscopy. Experimentalconditions: stationary phase: four PL mixed A columns; mobile phase: TCB; temperature:140 8C; detectors: a) refractive index (RI), and, b) FT-IR; sample solvent: TCB.

1322

the compositional data given by the producer. However,

SEC-FT-IR spectroscopy does not deliver information about

the CCD of the EVA copolymers, since the obtained peak

area ratios (Figure 1b) are only average values of the

corresponding molar masses.

Figure 2. Overlay of the chromatograms of EVA copolymers.Experimental conditions: stationary phase: Perfectsil 300; mobilephase: gradient decalin/cyclohexanone (dotted line); tempera-ture: 140 8C; detector: ELSD; sample solvent: decalin.

HT-HPLC

Inorder to investigate theirCCD, thesampleswereanalyzed

witha liquid chromatographicmethod,whichwas recently

developed by our group.[17] The following gradient elution

protocol was applied: after starting with 100% decalin for

2min the volume fraction of cyclohexanone was increased

linearly to 2 vol.-% within 3min and then to 20 vol.-%

within 2min; then, the cyclohexanone content was held

constant for 2min and afterwards the initial conditions

were re-established (Figure 2). According to the determined

void volume of the column (3.21mL) and the dwell volume

of the chromatographic system (3.04mL), the gradient

reaches the detector in 8.3min.[17] As a reference, EVA 4

containing 12wt.-% VA was chosen for comparing the

elution behavior of these EVA copolymers to a well-

described EVA copolymer.

All sampleselute in threepeaks, a small one ranging from

2.5 to 3.4mL and a large one from 9.2 to 11.6mL. This

indicates that a portion of the sample does not or only

2520151050

0,0

0,5

1,0

1,5

2,0

2,5

VA content by NMR in wt.-%

Pea

k ar

e ra

tio C

arbo

nyl/C

H2 (

FT

IR)

201816141210864

0

20

40

60

80

100

120

Gra

m S

chm

idt

Gram Schmidt EVA 1 EVA 2 EVA 3

VA content EVA 1 EVA 2 EVA 3

Elution Volume in mL

0

2

4

6

8

10

12

14

VA

content in wt.-%

a) b)

Figure 3. a) Correlation between the VA content measured by NMR spectroscopy and therelative VA content by FT-IR spectroscopy measurements. b) HPLC-FT-IR spectroscopyanalysis of samples EVA 1–3.

weakly adsorbs on the stationary phase

under these conditions. For EVA 1 and 2, a

bimodality of the samples in the main

peak (9.2–11.6mL) is observed, indicating

chemical heterogeneity. A sharp peak at

9.20mL and a second one at 10.90mL

(EVA 1) and 10.95mL (EVA 2) are found.

Interestingly this system is able to

differentiate a very small variation

(0.5wt.-% VA) in the average chemical

compositionbetweenEVA1andEVA2. In

contrast to EVA 1 and 2, EVA 3 shows a

trimodal peak between 9.2 and 11.6mL,

with a sharp signal at 9.20mL, and two



further ones at 9.95 and 10.90mL. How-

ever, the elugrams do not allow a further

identification of these fractions and,

therefore, the high-temperature gradient

HPLC has to be coupled to methods like

FT-IR- or NMR-spectroscopy. This was

achieved by coupling the high-tempera-

ture gradient HPLC with FT-IR spectro-

scopy using the LC-Transform interface,

as previously demonstrated for EVA

and EMA copolymers.[15,17] In order

to obtain absolute values of the VA

content, a calibration was carried out

as previously described for EMA copoly-

mers (Figure 3a).[16] The Gram-Schmidt plots and the

calculated VA contents are shown in Figure 3b.

All samples elute in two peaks: the first one between 4.4

and 7.0mL with an VA content from 0–4wt.-% and the

second peak between 11 and 16mL with an increased VA

content between 5 and 13wt.-%, that means separation

according to the chemical composition takes place and

DOI: 10.1002/macp.200900135


8001000120014001600180095

100

Tra

nsm

issi

on (

%)

Wavenumber in cm-1

PE

8001000120014001600180090

95

100

Tra

nsm

issi

on (

%)

Wavenumber in cm-1

EVACopolymer

a) b)

Figure 4. FT-IR spectra of the sample EVA 3 at elution volumes of: a) 5, and, b) 5.5 mL.

100806040200

0

1

2

3

4

5

6

7

dW/d

T

Temperature in °C

EVA 1 EVA 2 EVA 3

Figure 5. Overlay of the 1st derivatives of the CRYSTAF traces of theEVA copolymers measured in ODCB.

Table 2. Peak crystallization temperatures (Tc), the size of eachfraction (in brackets, w/w) and the soluble fraction, as obtainedwith CRYSTAF.

the analyzed samples are chemically inhomogeneous. The

peaks fromHPLC-FT-IR spectroscopy are broader than those

from ELSD detection (see Figure 2 and 3) as a result of the

spraying process in the LC-transform. The bimodality of

EVA 1 and 2, however, is well reflected. EVA 3 shows a

trimodal peak structure with signals at 11.7, 13.2 and

14.7mL, which is in good agreement with the results from

the ELSD detection. The amount of VA increases with the

elution volume between 5–7 and 10–12mL (Figure 3b).

After a maximum value at 12.2mL, the VA content

decreases slightly for the samples EVA 1 and 3 and passes

a minimum at 13.2mL, whereafter it again increases.

Beyondthechemical composition, themolarmass[17,24] and

the microstructure should also be taken into consideration

to interpret this chromatographic behavior. However, a

proper explanation for the different VA contents at the

same elution volume is not possible at this point.

Owing to theVAcontent (0wt.-%) at 5.0mL for EVA2and

3 obtained by HPLC-FT-IR spectroscopy, it can be assumed

that the samples are blends of EVA copolymers and

polyethylene (PE). To confirm the presence of PE, the IR

spectra of EVA 3 at elution volumes of 5.0 and 5.5mL were

studied in detail. For the elution volume of 5.0mL, only a

very small signal of the carbonyl absorption band at

1 730 cm�1 was detected (Figure 4a), while for 5.5 ml the

signal in the FT-IR spectrum (Figure4b) is clearlydetectable,

that is, EVA 3 contains PE homopolymer and is therefore a

blend of PE and EVA with low VA content. The outcome of

this is the absence of interaction between the stationary

phase and the eluting polymer fractions between 5 and

5.5mL. The absorption band at 1 375 cm�1 in the PE fraction

revealsCH3groups,[21]whichcanoriginateeither fromalkyl

branchesor chainend-groups.Thus, thePE-fraction inEVA3

can be linear low-molecular-weight PE or branched PE.
Sample Tc Soluble
fractionPeak 1 Peak 2 Peak 3

-C -C -C wt.-%

EVA 1 – 40.5 (21.5%) 20.5 (51.5%) 27

EVA 2 – 39 (24.5%) 26 (50.5%) 16

EVA 3 87 (4.5%) 42.5 (29.5%) 31 (56.5%) 11

CRYSTAF

CRYSTAF is the most commonly used technique for the

analysis of the CCD of semicrystalline olefin copolymers.

However there are only a few reports on the analysis of

copolymers of ethylene and polar comonomers in the

literature.[15,16] The CRYSTAF-traces of the EVA copolymers



are presented in Figure 5. The peak

crystallization temperatures and the

amount of soluble fraction are summar-

ized in Table 2. All samples crystallize

between 50 and 20 8C in bimodal peaks

indicating compositional heterogeneity.

EVA 3 has the highest Tc and the smallest

amountof soluble fraction.Only forEVA3

an additional crystallization peak at

87 8C is observed. The broadness of the

peak at 87 8C indicates branched PE or

EVA copolymer with a very low VA

content. This correlates very well with the HPLC and

HPLC-FT-IRspectroscopymeasurementsofEVA3indicating

aVA-content between 0–3.5wt.-% in the peakwhich elutes

first (Figure 2 and 3).

Comparison of the results presented in Figure 2, 3 and 5

shows that CRYSTAF and HPLC detect compositional

heterogeneity and highly crystalline fractions. As men-

tioned in the introduction, the crystallization process in

CRYSTAF is driven by both the content of polar comonomer

and the content of alkyl branches. In order to evaluate

the contribution of both units on the crystallization, the

CRYSTAF was equipped with a band-pass filter, which

was selectively transparent for the carbonyl vibration.



806040200

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2

4

6

8

10

12

14

16

EVA 3

dW/d

T

Temperature in °C

EVA 1 EVA 2 EVA 3

EVA 2

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2

4

6

8

10

12

14

16

dW/d

T

Temperature in °C

EVA 1 EVA 2 EVA 3

EVA 2

a) b)

Figure 6. Overlay of the 1st derivatives of: a) the concentration profile, and, b) thecarbonyl profile of the CRYSTAF traces of the EVA copolymers measured in tetrachloro-ethylene.

Table 3. Peak crystallization temperatures and the size of each fraction (in brackets, w/w) obtained with CRYSTAF measured intetrachloroethylene.

Sample Concentration profile Carbonyl profile

TC Peak 1 TC Peak 2 TC Peak 1 TC Peak 2

-C -C -C -C

EVA 1 23.8 (94.0%) – 23.7 (90.0%) –

EVA 2 23.2 (95.0%) 75.5 (2.5%) 23.3 (92.0%) 76.2 (8.0%)

EVA 3 27.0 (89.5%) 72.1 (10.5%) 27.0 (98.0%) –

Table 4. The fractions obtained by TREF of the EVA copolymers.

Fraction T EVA 1 EVA 2 EVA 3

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

1 35 13.97 9.55 5.06

1324

Tetrachloroethylene was used as solvent as it shows

optimum transparency in the carbonyl region. The traces

of the C�H stretching sensor and the carbonyl sensor are

shown in Figure 6a and b, respectively. The corresponding

peak crystallization temperatures (Tc) are summarized in

Table3.Uniformcrystallizationpeaksbetween40and10 8Care obtained in tetrachloroethylene as solvent. This is in

contrast to the crystallization from ODCB, where bimodal

peaks are detected (see Figure 6) between 10 and 50 8C.While the shift of the crystallization temperature can be

explained by different solvation power, as discussed

by Glockner,[22] the reason for the mono-modality cannot

be explained. The effect of the solvent on the shape of the

crystallization curve has, until now, not been discussed in

the literature. EVA 2 and 3 show crystallization peaks

between 60 and 80 8C in their concentration profiles. This

observation is contrary to the crystallization from ODCB,

where only for EVA 3 could a peak between 70 and 90 8C be

detected.However, only for EVA2wasanadditionalpeakat

76.2 8C identified with carbonyl-detection, while EVA 3 did

not reveal this peak.

2 50 18.13 11.07 6.99

3 65 62.92 73.07 76.66

4 75 2.05 5.96 6.68

5 100 2.94 0.35 4.61

TREF

To understand the factors that influence the separation

in the techniques used – HPLC and CRYSTAF – in a deeper



way, the samples need to be fractionated

according to crystallinity. Therefore, all

sampleswere fractionated by preparative

TREF. The results of the fractionation are

summarized inTable 4. The fractionswere

subsequently analyzed by SEC, HPLC and

IR spectroscopy. The major portions of

samples EVA 1–3 eluted between 50 and

65 8C in the third fraction. Additionally, a

highly crystalline fraction was obtained

for all samples. The peak area ratios of the

carbonyl (1 730 cm�1) to CH2 bands

(1 450 cm�1) and the CH3 (1 375 cm�1) to

CH2 bands obtained for the TREF fractions

by FT-IR spectroscopy are summarized in

Table 5. In all samples, the first fractions have the highest

values (carbonyl/CH2), indicating thehighest amountofVA

and (CH3/CH2) the highest branching content, which

includes the CH3 from the VA as well as from the alkyl

branches. A decrease of the carbonyl/CH2 and the CH3/CH2

ratio with an increase of the elution temperature is

observed. This proves that the TREF separation is mainly

based on the presence of both carbonyl groups and alkyl

branches. Fractions 2 and 3 show similar carbonyl- and

methyl-indices, that is, these fractions are very similarwith

regard to their VA and branching content. In addition to the

alkyl andVAbranches, the differences in the crystallization

temperatures can also be caused bymicrostructural effects,

suchasblockiness.However, using the FT-IR spectroscopy it

DOI: 10.1002/macp.200900135


Table 5. Peak area ratios of the carbonyl to CH2 (1 730 cm�1/1 450 cm�1) and CH3 to CH2 (1 371 cm�1/1 450 cm�1) of the TREF fractions.

Fraction EVA 1 EVA 2 EVA 3

CH3/CH2 Carbonyl/CH2 CH3/CH2 Carbonyl/CH2 CH3/CH2 Carbonyl/CH2

1 0.40 0.96 0.41 1.05 0.32 0.81

2 0.35 0.84 0.36 0.90 0.29 0.71

3 0.36 0.84 0.37 0.87 0.30 0.72

4 0.30 0.66 0.36 0.84 – –

5 0.10 0.14 0.25 0.62 0.08 0.09

1E71000000100000100001000

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Molar Mass in g/mol

fr. 1

EVA 1

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Molar Mass in g/mol

fr. 1

EVA 2

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a)

b)

c)

is not possible to distinguish the methyl groups of the VA

units from those of alkyl branches. Therefore NMR

spectroscopy is necessary.

The fifth fraction of all samples clearly shows a carbonyl

vibration in the FT-IR spectra and therefore contains VA.

This explains the relatively broad crystallization peak

(88–75 8C) in the CRYSTAF profile for EVA 3 (Figure 5). The

carbonyl index of 0.62 for EVA2 could be an explanation for

the crystallization peak measured with the carbonyl

detector in tetrachloroethylene. These results are in good

agreement with the results of hyphenated HPLC-FT-IR

spectroscopy, which identified a VA content of 0–3.0wt.-%

(EVA 1 and 3) and 3.0–4.0wt.-% (EVA 2) in the first eluting

fractions between 5 and 7mL.

Themainparameters that influence the crystallizationof

polyolefins are the chemical composition and the number

of alkyl branches.[8,23] The influence of the molar mass on

the crystallization profile is observed only for low molar

masses (Mw < 8 kg �mol�1).[24] Due to the presence of low

molar mass fractions (see Figure 1) this parameter should

not be neglected. The molar mass distributions of the TREF

fractions are shown in Figure 7 and the calculated average

molarmasses are summarized in Table 6. Both number and

weight-average molar masses (Mn and Mw) increase until

fraction 3 in all samples. However, a general correlation

between the fractionation temperature and themolarmass

is not observed. It should be noted that the lowmolarmass

portion of EVA1–3 is exclusively found in thefirst twoTREF

fractions. This explains that the lowmolarmass fractions of

the samples have an increased VA-content, as observed by

SEC-FT-IR spectroscopy (Figure 1). It is also of interest that

the 5th TREF fractions of EVA 3 and 1 do not contain

molecules with a molar mass more than 1 000 kg �mol�1.

The high molar mass fractions are exclusively in fraction 3

and 4 of the TREF fractions in all analyzed samples.

1E71000000100000100001000

Molar Mass in g/mol

Figure 7. Overlay of the molar mass distributions of the bulksamples and the fractions of: a) EVA 1, b) EVA 2, and, c) EVA 3. Forexperimental conditions see Figure 1.

HT-HPLC of the TREF Fractions

ThepreviouslydescribedHPLC systemwasused to separate

the TREF fractions according to the VA content. The


� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 1325


Table 6. Average molar masses and polydispersity indices of the TREF fractions (due to the low amounts of fraction 5 of EVA 2 and fraction 4of EVA 3, no SEC could be run for these fractions).

Fraction EVA 1 EVA 2 EVA 3

Mn Mw PD Mn Mw PD Mn Mw PDI

kg �mol�1 kg �mol�1 kg �mol�1 kg �mol�1 kg �mol�1 kg �mol�1

1 12.7 44.1 3.48 13.0 60.9 4.67 9.4 16.7 1.78

2 36.2 85.4 2.36 51.6 267 5.17 25.7 54.6 2.12

3 64.0 239 3.74 87.4 471 5.40 78.8 454 5.76

4 53.8 169 3.15 88.7 470 5.30 – – –

5 80.0 242 2.80 – – – 59.5 160 2.68

1326

corresponding chromatograms are shown in Figure 8.

Fractions 1–4 elute in two peaks. The first one elutes

between 3 and 4mL and the second one between 8.9 and

11.5mL with the gradient, that is, a part of each fraction

does not or only very weakly adsorbs on the stationary

phase and the rest elutes with increasing desorption

strength of the gradient. The 5th fraction of EVA 1 and 3

elutes with a main peak between 2 and 3.2mL and an

additional small peak at 9.0mL. This indicates that the

fraction 5 contains either copolymer with a very low VA

content or polyethylene which cannot adsorb on the

stationary phase. In this context it is of interest to note

that the 2nd TREF fractions possess a higher amount of

the later-eluting peak in HPLC compared to the 1st TREF

fractions. The first peak has a larger peak area than the

second one. The influence of the parameters concentration,

molar mass and chemical composition of the analyte and

the composition of themobile phase on the response of the

evaporative light scattering detector (ELSD) used has been

shown previously.[17] Therefore, it can be speculated that

the TREF fractions obtained at 35, 50, 65 and 75 8C contain

macromolecules with substantially different VA content.

The fractionation in TREF is influenced not only by the

comonomer content (VA groups) but also by the number of

alkyl branches, themicrostructure effects (e.g., thedegreeof

blockiness) and the molar mass.

To study the influence of the chemical composition,

molar mass and microstructure on adsorption in HPLC and

the crystallization in TREF, HPLC-FT-IR spectroscopy of the

five fractions of EVA 1were carried out. The Gram-Schmidt

plots of theTREF fractions of EVA1,which correspond to the

sample concentration and the calculated VA content are

shown in Figure 9. With increasing elution temperature, a

decrease in theVAcontent could be observed. Interestingly,

similar elution volumes are obtained for all fractions,

especially for the second peak (8–12mL).

The FT-IR spectra of the TREF fractions 1–4 at 9.7mL are

shown in Figure 10. For fractions 2 and 3, similar VA



contents are observed at equivalent elution volumes

(Figure 9 and 10). Fractions 1 and 4, which also elute at

the same volume, have relatively higher and lower VA

content, respectively, than fractions 2 and 3. In the used

gradient the separation is based on adsorption and

desorption, so a higher elution volume (Ve) could be

expected to be the result of either a higher VA content or a

higher molar mass of the copolymer.[12] This leads to the

assumption that the lower molar mass in fraction 1

(Mn ¼ 13 kg �mol�1) and the absence of molar masses

<10kg �mol�1 in fraction4 (seeFigure7) result in co-elution

of these copolymers even though they have different VA

contents. The observed molar mass dependence is in

agreement with the literature.[26,27]

The average VA content at the peak maximum, the VA

distributions by HPLC-FT-IR spectroscopy and the average

VA content of the TREF-fractions of EVA 1 measured by1HNMR spectroscopy (spectra not shown) are summarized

in Table 7. The average VA contents of the fractions 1–4

obtainedbyHPLC-FT-IRspectroscopyare ingoodagreement

with the VA contents measured by 1H NMR spectroscopy;

only for fraction 5 is a discrepancy between the results

found, which could be explained by low signal-to-noise

ratios for both spectroscopic techniques. It is also important

to mention that for fractions 2–4 both elution peaks show

an increase of the VA content with the elution volume (see

Figure 10 and Table 7). This observation contradicts the

expectation that the TREF fractions have anarrowchemical

compositiondistribution. It canbespeculatedthat thiseffect

is either the result of the contribution of alkyl branches,

which do not contribute to retention in the HPLC but do

influence the crystallization in TREF, or an incomplete

separation in the TREF-fractionation, that is, co-crystal-

lization. Anantawaraskul et al. found an increasing

tendency of cocrystallization with decreasing DTc in blends

of ethylene/1-olefin copolymers.[25] In order to verify this,

TREF fractions 2 and 3 of EVA 1 and 2 were analyzed by

CRYSTAF. The crystallization profiles obtained by CRYSTAF

DOI: 10.1002/macp.200900135


EVA 3 EVA 2 EVA 1

Fr. 1

1210864200,0

0,2

0,4

0,6

0,8R

espo

nse

ELS

D in

Vol

t

Elution Volume in mL121086420

0,0

0,2

0,4

Res

pons

e E

LSD

in V

olt


0,0

0,5

1,0

1,5

2,0

2,5

Res

pons

e E

LSD

in V

olt


Fr. 2

1210864200,0

0,2

0,4

0,6

0,8

Res

pons

e E

LSD

in V

olt


0,0

0,2

0,4


Res

pons

e E

LSD

in V

olt

121086420

0,0

0,5

1,0

1,5

2,0

2,5

Res

pons

e E

LSD

in V

olt


Fr. 3

1210864200,0

0,2

0,4

0,6

0,8

Res

pons

e E

LSD

in V

olt


0,0

0,1

0,2

0,3

0,4

Res

pons

e E

LSD

in V

olt


0,0

0,5

1,0

1,5

2,0

2,5

Res

pons

e E

LSD

in V

olt


Fr. 4

1210864200,0

0,2

0,4

0,6

0,8

Res

pons

e E

LSD

in V

olt


0,0

0,2

0,4

Res

pons

e E

LSD

in V

olt


not measured

Fr. 5

1210864200,0

0,2

0,4

0,6

0,8

Res

pons

e E

LSD

in V

olt


not measured

121086420

0,0

0,5

1,0

1,5

2,0

2,5

Res

pons

e E

LSD

in V

olt


Figure 8. Elugrams of the TREF fractions of EVA 1–3 (Table 4). For the experimental conditions see Figure 2.

are presented in Figure 11. All fractions show amorphous

portions (which do not crystallize). For fractions 2 and 3 of

EVA 1, bimodal crystallization peaks that crystallize over a

range of 35–40 8C are observed. For TREF fractions 2 and 3 of

EVA 2, a narrower crystallization range of 15–20 8C is

obtained. These results confirm the assumption that the



fractionation is not complete and the TREF fractions still

display considerable compositional heterogeneity.

In order to evaluate the role of alkyl branches and the

microstructure in the TREF-fractionation, the fractions

were analyzed by NMR spectroscopy. The 13C NMR spectra

of EVA 1 and 2 are shown in Figure 12 and the calculated



16141210864

0

20

40

60

80

100VA content

Fr.1 Fr.2 Fr.3 Fr.4 Fr.5

Gram Schmidt Fr.1 Fr.2 Fr.3 Fr.4 Fr.5


Gra

m S

chm

idt

0

2

4

6

8

10

12

14

16

18

20

VA

content in wt.-%

Figure 9. Overlay of the HPLC-FT-IR spectroscopy analysis of theTREF fractions of EVA 1. For experimental conditions see Figure 3.

1900180017001600150014001300

0,0

0,1

0,2

0,3

0,4

0,5 Fr. 1 Fr. 2 Fr. 3 Fr. 4

Ext

inct

ion

Wavenumber in cm-1

Figure 10. Overlay of the normalized FT-IR-spectra at 9.7 mL of theTREF fractions of EVA 1.

Table 7. Average VA content and VA distribution obtained byLC-FT-IR spectroscopy and the average VA content measured by1H NMR spectrocopy.

Fraction Average VA

contentin the

main peak

Range of VA

content (FT-IR

spectroscopy)

FT-IR

spectroscopy

NMR

spectroscopy

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

1 10.6 10.5 9.5–11.0

2 8.4 8.9 5.0–10.0

3 8.2 8.6 4.5–9.5

4 6.5 7.5 1.0–8.0

5 0.3 3.1 0.0–1.0

1328

triads and branching contents of EVA 1–3 and the TREF

fractions of EVA 1 are summarized in Table 8 and 9. The

assignments and calculations are based on published

procedures.[28] No resonance signals for EVV and VVV

triads are present in the 13C NMR spectra of any sample.

Therefore it can be concluded that the influence of the

blockiness is neglible. This means that the crystallization

806040200-1

0

1

2

3

4

5

6

7

8

9

dW/d

T

Temperature in °C

EVA 1 Fr.2 EVA 1 Fr.3

806040200

-2

0

2

4

6

8

10

12

dW/d

T

Temperature in °C

EVA 2 Fr. 2 EVA 2 Fr. 3

a) b)

Figure 11. Overlay of the 1st derivatives of the CRYSTAF traces of the TREF fractions 2 and3 of: a) EVA 1, and, b) EVA 2.

mainlydepends onalkyl branches and the

VA content. EVA 2 shows the highest total

alkyl branch content, followed by EVA 1

and then EVA 3. The high alkyl branch

content of EVA 2 could be one reason for

the lower TC of the second peak (see Table

3) of EVA 2 compared to EVA 1. Methyl

branches are only detected by 13C NMR

spectroscopy in EVA 1, indicating the use

of propylene as a chain transfer agent.[28]

As expected from Flory’s theory, the

amount of branches decreases in the order

of the elution temperature in TREF for the

analyzed fractions.[29] The compositional



broadness of the fraction with regard to the VA content as

obtained by the HPLC-FT-IR spectroscopy can be explained

by the fact that TREF separates according to the overall

effect of all branches including VA and alkyl. Therefore,

interactive chromatography presents the possibility of

separating these copolymer fractions, which are narrow

with regard to their crystallization temperature (Tc), into

fractions with different comonomer content (see Figure 9).

Analysis of the same EVA copolymers with crystal-

lization techniques (CRYSTAF, TREF) and gradient HPLC has

shown not only the advantages of gradient HPLC but also

its drawbacks. Besides reducing the analysis time, themain

advantage of the gradient HPLC compared to TREF and

CRYSTAF is that the separationdependsmainly on thepolar

VAgroups that interactwith the stationary phase. By using

HPLC it is possible to separate TREF fractions that are

narrow distributed with regard to their Tc into fractions

with different VA content. This behavior can be explained

by the contribution of alkyl branches and co-crystallization

effects in the preparative TREF. Additionally, amorphous

portions, which cannot be fractionated by crystallization

DOI: 10.1002/macp.200900135


Figure 12. 13C NMR spectra of: a) EVA 1, and, b) EVA 2.

techniques, are well separated according to their comono-

mer content. The only drawback of the gradient HPLC is the

molar mass-dependence of the elution behavior of copo-

lymer with molar masses <20 kg �mol�1. In CRYSTAF or

Table 8. Fractions of triads of EVA copolymers. TREF fractions 4 and 5 could not be meinsufficient size.

Sample Fraction o

EEE VEE VEV

mol-% mol-% mol-%

EVA 1 92.5 5.6 0

EVA 2 93.3 5.2 0

EVA 3 94.8 4.3 0

EVA 1 (Fraction 1) 92.3 6.0 0

EVA 1 (Fraction 2) 91.3 5.9 0

EVA 1 (Fraction 3) 92.2 5.4 0



TREF, the crystallization temperature

depends on the molar mass to only a

minor degree (<8 kg �mol�1).

Conclusion

In this study, three EVA copolymerswere

analyzed by HPLC, CRYSTAF and TREF, as

well as by HPLC-FT-IR spectroscopy. With

all three fractionation techniques, het-

erogeneities in the chemical composition

of the samples could be detected. While

CRYSTAF and TREF determine the Tc that

depends on the sum of alkyl and vinyl

acetate branches, HT-HPLC-FT-IR spectro-

scopy determines the distribution of the

polar comonomer, VA. By fractionating

these samples by TREF, fractions with a

narrow CCD should be obtained. Further

analysis of the TREF-fractions by HPLC-

FT-IR spectroscopy reveals the broadness

of the VA content of the individual

fractions. This could be explained by

the contribution of alkyl branches in

TREF and their non-contribution in the

interactive chromatography. Addition-

ally the broadness is the result of an

incomplete separation, as confirmed by

the broad CRYSTAF peaks.

It is shownthatHT-HPLC-FT-IR spectro-

scopy is a fast and synergistic method to

the routinely used fractionation techni-

ques, CRYSTAF and TREF, for the deter-

mination of CCD. In contrast to CRYSTAF and TREFHT-HPLC

offers selectivity for the polar comonomer and also the

possibility of analyzing amorphous EVA copolymers

according to their VA-content.

asured with 13C NMR spectroscopy because of

f triad

EVE EVV

mol-% mol-%

2.4 0

2.0 0

1.5 0

3.0 0

3.0 0

2.7 0



Table 9. Alkyl branch distribution of the EVA copolymers.

Sample Branch type (number per 1 000C)

Methyl Ethyl Butyl Amyl HexylR BranchTotal

EVA 1 3.5 – 6.4 2.4 4.4 16.5

EVA 2 – – 9.6 3.7 9.4 23.0

EVA 3 – – 7.0 1.7 3.8 12.5

EVA 1 (Fraction 1) 3.9 – 8.9 2.5 7.1 22.4

EVA 1 (Fraction 2) 3.7 – 6.3 1.9 4.7 16.6

EVA 1 (Fraction 3) 3.3 – 6.2 1.9 3.5 14.9

1330

Acknowledgements: This research is part of the research pro-gramme of the Dutch Polymer Institute (DPI) under Project # 642/643, in addition to the DPI affiliation of the researchers concerned(Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven,The Netherlands). The authors acknowledge C. Brinkmann formeasuring the molar masses of the EVA copolymers and C. Hockfor the FT-IR spectroscopy measurements.

Received: March 26, 2009; Revised: June 9, 2009; Accepted:June 15, 2009; Published online: July 21, 2009; DOI: 10.1002/macp.200900135

Keywords: CRYSTAF; ethylene-vinyl acetate copolymers; FT-IR;liquid chromatography; polyolefins; TREF

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Documents

Comparison of High-Temperature HPLC, CRYSTAF and TREF for the Analysis of the Chemical Composition Distribution of Ethylene-Vinyl Acetate Copolymers