Material Behaviour

Effect of branching characteristics of ethylene/1-butene

copolymers on melt flow index

N. Fazeli a,b, H. Arabi b,*, Sh. Bolandi b

a Islamic Azad University, Science and Research Campus, Polym. Eng. Group, P.O. Box 14155/4933, Tehran, Iranb Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran

Received 3 August 2005; accepted 12 September 2005

Abstract

Five different samples of ethylene/1-butene copolymers with relatively similar weight average molecular weight and molecular

weight distribution, but different branching characteristics, were investigated. The co-monomer content of the samples was

measured by 13C NMR technique. Then, the samples were fractionated by a step-wise crystallization method in DSC and the

relative amount of each fraction was compared between the samples. By measuring the Melt Flow Index (MFI) of the samples, a

qualitative relationship between the branching structure and MFI of the samples is proposed.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: LLDPE; Short chain branching; Short chain branching distribution; Fractionation; Melt flow index

1. Introduction

Linear low density polyethylene (LLDPE) produced

by the copolymerization of ethylene with a-olefins over

either Ziegler–Natta or metallocene catalysts, is a

copolymer which possesses short chain branches (SCB)

due to the incorporation of co-monomer into the

backbone. The thermal and mechanical properties of

LLDPEs are strongly dependent upon the structural

characteristics of polymer chains, such as molecular

weight (Mw), molecular weight distribution (MWD),

co-monomer content and short chain branching

distribution (SCBD). Therefore, the characterization

of LLDPE samples and relating the structural para-

meters to the final properties of the samples is of great

interest [1–7].

Techniques such as 13C NMR and FTIR have long

been used for the determination of overall short chain

0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymertesting.2005.09.008

* Corresponding author.

E-mail address: n.fazeli@ippi.ac.ir (N. Fazeli).

branching levels in LLDPE samples [8–11]. But, since

most commercial LLDPE resins exhibit broad, multi-

modal SCBD, they cannot be adequately described by an

average number of branches per 1000 backbone carbons

and it is necessary to use a fractionation technique.

Temperature rising Elution Fractionation (TREF) is a

technique that fractionates polymers according to their

crystallizability and has been widely used for the

characterization of SCBD of semi-crystalline polymers

[12–16]. Since TREF is a time-consuming technique and

has high cost of implementation, Crystallization Anal-

ysis Fractionation (CRYSTAF) was developed as a good

alternative to TREF [17–22].

Recently, the use of Differential Scanning Calori-

metry (DSC) for the characterization of molecular

heterogeneity has also been proposed [23–33]. As for

TREF, the method consists of a step-wise crystal-

lization of the PE chains by successive annealing at

decreasing temperatures, starting from the melt, and

subsequent analysis by DSC of the melting behavior of

the treated sample.

Polymer Testing 25 (2006) 28–33

www.elsevier.com/locate/polytest

Table 1

Measured characteristics of the samples

Sample Mwa PDIa SCBb MFIc (g/10 min) Tm(8C)d Density (g/cm3)

A (TCP) 155,000 6.4 24.3 0.9 125 0.920

B (TCP) 163,000 7.3 21.2 0.78 124.5 0.920

C (SABIC) 141,000 4.7 24 1 123.33 0.918

D (SABIC) 112,000 4.6 28.1 2 121.67 0.918

E (SABIC) 135,000 1.45 9 5 125.83 0.935

a Measured by SEC.b SCB/1000C atoms measured by C NMR.c At 190 8C and 2.16 Kg.d Measured by DCS.

N. Fazeli et al. / Polymer Testing 25 (2006) 28–33 29

In this paper, five different samples of LLDPE

produced by different manufacturers are investigated.

The Mw, MWD and co-monomer content of the samples

were measured using SEC and CNMR techniques.

Then, the samples were fractionated by DSC and the

extent of each fraction compared between the samples.

By measuring the Melt Flow Index (MFI) of the

samples as a critical parameter in polymer processing, a

qualitative relation-ship between the extent of each

fraction and MFI of the samples is proposed.

2. Experimental

2.1. Materials

All the commercial LLDPE samples used in this

study were Ziegler–Natta catalyst ethylene–1-butene

copolymers supplied by Tabriz Petrochemical Com-

pany (TPC) and Saudi Arabia’s Basic Industries

Corporation (SABIC). The measured characteristics

of the samples are listed in Table 1.

2.2. Fractionations using DSC

All the DSC runs were obtained in a DSC (Mettler

Toledo Stare system) apparatus 822 Module, interfaced to

a digital computer equipped with Stare software. For

comparison purposes, DSC runs for un-fractionated

samples were carried out under the following conditions:

The sample was first heated to 160 8C and

maintained at this temperature for 10 min. Then, the

sample was cooled from 160 to 0 8C at a rate of

10 8C/min. Finally, the sample was re-heated from 0 to

160 8C at a rate of 10 8C/min. and the melting point of

the un-fractionated sample was measured from the

obtained endotherm. All the steps were performed

under nitrogen atmosphere.

The annealing steps in the DSC measurements were

performed in the following way:

The sample was first heated to 160 8C at a rate of

10 8C/min. and maintained at this temperature for

10 min. (in order to eliminate any effect of previous

thermal history). The sample was then rapidly cooled to

the first isothermal crystallization temperature at a rate

of 200 8C/min. and the isothermal crystallization was

followed for 45 min. The temperature stages were

separated from each other by 5 8C. The annealing

temperatures covered the range of 135–70 8C. The

cooled sample was heated at 10 8C/min. and melting

endotherms were recorded.

2.3. C NMR

13C NMR spectra of the samples were obtained with

a BRUKER AQS-300 MHz AVANCE spectrometer

operating at 75.489 MHz. at 100 8C. Samples were

prepared in 5 mm o.d. tubes at 60% (w/w) concen-

tration in 1,2,4 trichlorobenzene (TCB). The samples

were dissolved at 150 8C for 30 min before analyzing in

the NMR spectrometer. The conditions for measure-

ment were as follows: pulse interval 3 s, acquisition

time 1.82 s, pulse width 9 ms, spectral width 17,985 Hz,

number of scans 500.

3. Results and discussion

In ethylene copolymer chains, crystallizable seg-

ments with a certain length will crystallize at a

corresponding temperature. According to the Gibbs–

Thomson relation, if the crystallization time is

sufficiently long, all the ethylene segments with a

fixed length have the chance to crystallize completely.

In multiple step isothermal crystallization, segments

of various lengths can, therefore, crystallize separately

to form lamellae of different thicknesses, whereas

short chain branching (SCB) is rejected at the

interface between the crystalline and amorphous

regions. During a subsequent DSC heating run, these

Fig. 1. DSC thermograms of the samples.

N. Fazeli et al. / Polymer Testing 25 (2006) 28–3330

lamellae with different thicknesses melt at different

temperatures. Therefore, a DSC thermogram with

multiple melting peaks is obtained. Fig. 1 shows the

thermograms of the samples after segregation frac-

tionation in DSC.

According to the Gibbs–Thomson equation, it is

possible to determine the lamellar thickness of different

lamellae [1,2,28,32–35]:

Tm Z T8mð1 � 2de=DH:LcÞ

where Tm is the observed melting point, T8m is the

equilibrium melting point of an infinite polyethylene

crystal (418.7 8K), DH is the enthalpy of fusion per unit

volume (285!106 J/m3), de is the surface energy of a

polyethylene crystal (90!10K3 J/m2) and Lc is the

thickness of the lamellae with melting point Tm.

The distribution and type of SCB has a deep

influence on physical and mechanical properties of a

LLDPE sample. Using the above equation, Hosoda

et al. [23] has proposed the following relationship

between melting peaks and SCB for ethylene/1-butene

copolymers, which has also been used by other

0

2

4

6

8

10

12

SCB

% S

CB

C D A B E

34 31 28 25 22 19 16 13

Fig. 2. SCBD in samples after fractionation in DSC.

researchers [26,28,31]:

Tð8CÞ ZK1:6 � SCB C136

With the help of the above equation, the correspond-

ing SCB of each peak is determined in the DSC

thermograms. Then, by measuring the relative area

under the peaks, it was possible to compare the relative

amount of each fraction of the samples. As can be seen

in Fig. 2, the sample E (roto-molding grade) has the

lowest amount of branched chains so the highest

amount of linear ones. Moreover, in other samples,

which are all film blowing grade, the amounts of

branched chains in the two SABIC samples (C and D)

are higher than the TCP ones (A and B).

The Melt Flow Index (MFI) of the samples is

considered as a critical parameter in polymer proces-

sing and industrial designs. MFI of a PE resin refers to

the rate at which it extrudes from a capillary die under a

standard set of conditions. The MFI is reflected by the

average dimensions of the molecules in a resin and their

entanglements with one another so it depends on

molecular characteristics (Mw and MWD) and

NMR Data

R2 = 0.9991

0

1

2

3

4

5

6

0 5 10 15 20 25 30

br carbons/1ooo CH2

MF

I

Fig. 3. The variation of MFI against SCB/1000C (obtained by C NMR).

DSC Data

R2 = 0.9795

0

1

2

3

4

5

6

0 5 10 15 20 25 30

% branched chains

MF

I

Fig. 4. The variation of MFI against percent branched chains

(obtained from DSC).

R2 = 0.9256

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1 1.2

amount of chains with SCB=28

MF

I

R2 =0.8885

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3

Amount of chains with SCB=22

MF

I

R2 = 0.9943

0

1

2

3

4

5

6

0 4 8Amount of chains with SCB=16

MF

I

2 6

Fig. 5. The variation of MFI against the am

N. Fazeli et al. / Polymer Testing 25 (2006) 28–33 31

branching characteristics (SCB and SCBD) of the

sample. The MFI of the studied samples were measured

and are shown in Table 1. Since the weight average

molecular weight and molecular weight distribution of

the samples are relatively close together (except for the

sample E which has a narrow PDI), the difference in

MFI of the samples is related to their branching

characteristics.

In Fig. 3, the MFI of the samples is plotted against

their SCB/1000C atoms of the main chain (measured by

C NMR). As it can be seen, with increasing the degree

of branching (SCB/1000C), the MFI of the sample goes

R2 = 0.8851

0

1

2

3

4

5

6

0 0.5 1 1.5 2amount of chains with SCB=25

MF

I

R2 =0.9012

0

1

2

3

4

5

6

0 2 4

Amount of chains with SCB=19

MF

I

R2 =0.9912

0

1

2

3

4

5

6

0 4 10 12Amount of chains with SCB=13

MF

I

2 6 8

1 3 5

ount of a certain degree of branching.

N. Fazeli et al. / Polymer Testing 25 (2006) 28–3332

through a minimum. It seems that, before the minimum

point, the increase in SCB will cause more entangle-

ments between the chains (inter-molecular entangle-

ment) and therefore will impede the flow. However,

after the minimum point, increase in the SCB/1000C

will cause the chains to have a more compact molecular

profile (more intra-molecular entanglements instead of

inter-molecular ones), so the chains will cause less

hindrance to the flow of other chains. Such effect has

been seen in the viscosity of molten ultra high

molecular weight polyethylene, which may be greatly

reduced if it has been previously precipitated from

dilute solution [36]. Dissolving PE in a large excess of

solvent greatly reduces the overlap between adjacent

random coils, thereby decreasing the number of

intermolecular entanglements along the length of each

chain. When the PE chains are precipitated by a rapid

reduction of temperature they have insufficient time to

re-entangle. The net result is a material with relatively

few chain entanglements in comparison to melt

crystallized samples. Therefore, the viscosity is greatly

reduced (i.e. MFI is increased).

Predicting a quantitative relation between MFI and

the amount of branched chains is not possible because

there is still another parameter affecting on MFI and

that is SCBD, i.e. the amount of chains with a certain

degree of branching. This data can be obtained from

the area under each of the multiple peaks in the DSC

thermograms. In Fig. 4, the variation of MFI against

the whole amount of branched chains obtained from

DSC data is shown. As can be seen, the same result is

obtained: by increasing the amount of branched

chains (and decreasing the amount of linear chains),

the MFI of the sample goes down until a minimum is

reached. After that, further increase in the amount of

branched chains will cause the MFI to rise. If the area

under each of the multiple peaks in the DSC

thermograms is measured and plotted against MFI,

it is expected that the changes of MFI with the

amount of each fraction will go through a minimum.

By increasing the number of SCB, this minimum

should occur at lower amounts.

In Fig. 5, the variation of MFI against the amount of

a certain degree of branching is shown according to the

data obtained from DSC thermograms. As can be seen,

all the plots show a minimum and, as expected, by

increasing the degree of branching this minimum

occurs in fewer amounts. In other words, the degree

of branching (number of branching per 1000C atoms of

the main chain) and the amount of each fraction (the

number of chains which have the same degree of

branching) have the same effect on MFI.

4. Conclusions

With increasing of the degree of branching in

LLDPE samples, the MFI goes through a minimum.

Before the minimum point, the increase in SCB will

cause more entanglements between the chains (inter-

molecular entanglement) and, therefore, will impede

the flow. However, after the minimum point, increase in

the SCB will cause the chains to have a more compact

molecular profile (more intra-molecular entanglements

instead of inter-molecular ones), so the chains will

cause less hindrance to the flow of other chains. The

same effect will be observed when the amount of

branched chains with a certain degree of branching is

increased: the MFI will go down until a minimum is

reached. After that, further increase in the amount of

branched chains will cause the MFI to rise. In other

words, the degree of branching (number of branching

per 1000C atoms of main chain) and the amount of

branching (the number of chains which have the same

degree of branching), have the same effect on MFI. By

increasing the degree of branching, this minimum

occurs at lower amounts.

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