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Physica B 385–386 (2006) 511–513

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Time-resovled SANS studies of the hot crystallisation of PET

Duncan Gordona, Andrew Ellisa, Ralf Schweinsb, Mike Jenkinsa,�

aDepartment of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UKbInstitut Laue-Langevin, B.P. 156, F-38042, Grenoble, CEDEX 9, France

Abstract

Time-resolved small-angled neutron scattering (SANS) was used to examine the hot crystallisation processes in a blend of deuterated

and hydrogenated poly(ethylene terephthalate) , over a temperature range of 220–230 1C. The scattering pattern was able to follow the

growth process of the crystallisation with a peak increasing in intensity over time at a scattering vector Q value of �0.04 A�1. Avarmi

analysis and half-life measurements were used to compare the SANS data with results from differential scanning calorimetry

experiments.

r 2006 Elsevier B.V. All rights reserved.

PACS: 81.05.Lg

Keywords: Poly(ethylene terephthalate); Crystalliation; Time-resolved sans; DSC

1. Introduction

Cooling a crystallisable polymer from a temperatureabove the melting point can result in the formationof crystallites. The kinetics of phase change duringcrystallisation can be described using the Avrami modeland this approach has been highly successful in describingcrystallisation in polymers [1,2]. The model relates theextent of crystallinity to time, such that

X ðtÞ ¼ 1� expðztnÞ, (1)

where z is the rate of phase change and is related to thecrystallisation half times t0.5 and n is a mechanisticconstant. Values for n of between 3 and 4 are commonfor polymers, indicating either pre-determined or sporadicgrowth of spherulitic crystallites. The Avrami analysis hasbeen widely applied to the study of polymer crystallisationusing differential scanning calorimetry (DSC) [3], but notsmall-angle neutron scattering (SANS). However, a num-ber of SANS studies have reported the development of apeak after crystallisation in the scattering vector Q,appearing at a maximum at around Q ¼ 0.05 A�1 [4,5].

e front matter r 2006 Elsevier B.V. All rights reserved.

ysb.2006.05.258

ng author. Tel.: +440121 414 2841;

14 5232.

ss: M.J.Jenkins@bham.ac.uk (M. Jenkins).

The purpose of this paper is to report a time-resolvedstudy of the crystallisation process in poly(ethyleneterephthalate) (PET) detected using SANS and also thecorresponding Avrami analysis of the data. In addition, acomparison is made between kinetic data obtained fromSANS and conventional DSC.

2. Experimental

Deuterated PET (D-PET) with MW ¼ 8140 was synthe-sized at The University of Durham, UK The hydrogenatedPET (H-PET) was a commercial grade of PET (DuPontLaser) with MW ¼ 36 400. A blend of 25wt% D-PET and75wt% H-PET was produced using a conical double-screwMinimixer at 280 1C at The University of Bayreuth,Germany. The blend was then vacuum dried at 150 1Cfor two days before the samples were prepared from it. Thesamples were prepared by putting 0.15 g of the blend in themiddle of an aluminium washers with external diameters of20mm, internal diameters of 10mm and a thicknesses of1mm, sandwiched between two poly(tetrafluoroethylene)-coated glass-fibre fabrics and heated to a temperature of280 1C. A pressure of 200–500 kg cm�2 was appliedand maintained for approximately 30 s. The sample,together with the fabrics, was then rapidly removed from

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0 4000 8000

0.0

0.2

0.4

0.6

0.8

1.0

0.02 0.03 0.04 0.05 0.06 0.07

0.2

0.4

0.6

0.8

1.0

1.2

1.4

I (Q

)

Q (Å-1)

Increasing Time

Xt

Time (secs)

Fig. 1. Crystallisation peak developing with time for D-PET/H-PET

sample crystallised at 224 1C. Inset shows the extent of crystallinity with

time for the same sample.

D. Gordon et al. / Physica B 385–386 (2006) 511–513512

the press and immediately quenched in liquid nitrogen toprevent crystallisation.

The SANS experiments were performed at the ILL,Grenoble, France, using the D11 diffractometer with asample-to-detector distance of 5m and a neutron wave-length of 6 A, giving the range of the scattering vector Q

between 0.0114 and 0.0684 A�1. The samples in theiraluminium washers were covered in aluminium foil to helpinsulate them and slotted into the centre of a hollowcylindrical electrically heated furnace of dimensions length:30mm; external diameter: 29mm; and internal diameter:21mm. This was controlled by a Linkam PR600 tempera-ture controller using a miniature PT100 RTD sensor. Thefurnace was clamped onto a specially built stand, whichwas placed between the neutron guides and the detector ofD11, so that the neutron beam passed through the samplein the middle of the furnace, illuminating a circular surfacewith a diameter of 8mm. The samples were heated fromroom temperature to the melt temperature of 280 1C at arate of 60 1C/min and held there for 2min. They were thenrapidly cooled down at the same rate to the selectedcrystallisation temperature, which was in the range of220–230 1C and held there over a period of time, from120min for the lower temperatures and up to 270min forthe higher temperatures. SANS measurements were thenperformed at 1min intervals in order to monitor thecrystallisation process at the selected crystallisationtemperature. The SANS data were radially averaged,normalised by using water as a calibration standard.Corrections were also made for variations in samplethickness, transmission and the background scattering ofthe furnace with an empty washer subtracted.

The crystallisation of the blend was also measured usingDSC according to a method previously described [3].

3. Results and discussion

Initially the first minute of measurement of the SANSdata, for the D-PET/H-PET samples, were plotted asZimm plots of I(Q)�1 vs. Q2. Linear least-square fit lineswere fitted to the data between 1.1� 10�4pQ2/A�2p9.8� 10�4. From these lines, the z-average radiusof gyration Rz

g and molecular weight MW were obtained byfitting the data to the equation:

IðQÞ�1 ¼ ð1þ ðQ2 Rz2g Þ=3ÞðCNMWDÞ

�1, (2)

with CN a constant given by

CN ¼ ðaH � aDÞ2X ð1� X Þ=NAr, (3)

where aH and aD are the coherent scattering lengthdensities of the labelled and unlabelled repeat units, NA isthe Avagadro’s number, r the density of the D-PET and isthe volume fraction of D-PET.

The average Rgz for the samples was found to be 102 A

and the average MW was found to be 5244 which is belowthe original value of 8140. These results show that duringthe melt processing and melt pressing, there was a drop in

the deuterated sequence length as transesterificationprocesses occurred [6]. However, no further transesterifica-tion was expected to take place following melt processing.This observation does highlight a limitation of meltblending H-PET and D-PET.The development of SANS with time during a melt

crystallisation of a D-PET/H-PET sample at a temperatureof 224 1C is shown in Fig. 1. The development of a peakcentred at a scattering vector Q value of around 0.04 A�1 isclearly apparent. The peak corresponds to a long period ofabout 16 nm and clearly relates to the growth of crystal-lisation as previous workers have reported the developmentof a peak after crystallisation appearing at a maximum ataround Q ¼ 0.05 A�1, which indicated the presence of along period of about 13 nm [4,5].The development of the peak with time at each crystal-

lisation temperature was expressed in terms of the relativecrystallinity Xt calculated from the following equation;

X t ¼ ðI t � I0Þ=ðI1 � I0Þ, (4)

where It, I0 and IN are the scattering intensities at time t,time zero and time infinity, respectively. The inset in Fig. 1shows a typical example of this for the variation of relativecrystallinity with time for the D-PET/H-PET samplecrystallised at 224 1C, as measured from the maximumpeak heights.The crystallisation half times were measured using both

DSC and SANS and the variation with crystallisationtemperature is shown in Fig. 2. The half times measuredusing DSC exhibit a similar temperature dependenceto those measured using SANS. This indicates that theSANS measures on a timescale similar to that observedusing calorimetry.The increase in crystallisation half time with decreasing

supercooling can be simply explained in terms of the

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220 222 224 226 2280

500

1000

1500

2000

2500

3000

220 222 224 226

250

300

350

400

450

Hal

f-lif

e (s

ecs)

Temperature (°C)

SANS

DSC

Fig. 2. Crystallisation half times as measured by SANS with crystal-

lisation half times as measured by DSC in inset graph.

2.0 2.5 3.0 3.5 4.0-3

-2

-1

0

1

Log

(-Ln

(1-X

t))

Log (time)

Fig. 3. Avrami double log plot for D-PET/H-PET sample crystallised at

224 1C.

D. Gordon et al. / Physica B 385–386 (2006) 511–513 513

competition between nucleation and growth, as describedby Turnbull and Fisher [7]. The crystallisation process iscontrolled by the nucleation density at temperaturesapproaching the melting point and by the diffusion ofchains to the crystal growth face at temperaturesapproaching the glass transition. Competition betweenthe two factors results in a minimum in the crystallisationhalf life approximately midway between the melting pointand the glass transition.

Treatment of crystallisation data using the Avramiapproach can provide additional insight into thecrystallisation mechanism. Fig. 3 shows an Avrami plotobtained from SANS data for a crystallisation temperatureof 224 1C.

The Avrami plot exhibits a linear section over which themechanistic n value can be calculated. An n value of 2–3indicates spherulitic growth of heterogeneous nuclei [3]. InDSC n values were in the range 2.4–2.8, whereas in SANSvalues were typically 2, showing the use of the techniquesis compatible for following the crystallisation kineticsof polymers.

Acknowledgements

We thank the EPSRC for the financial support of theresearch programme. We would also like to thankMr. Frank Biddlestone for his technical support to theproject, Mr. Michael Cannon and Dr. Lian Hutchingsfrom the IRC in Polymer Science and Technology,University of Durham, UK, for the synthesis of thedeuterated poly(ethylene terephthalate) for us andDr. Reiner Giesa from the Laboratory for MicroscaleProcessing, University of Bayreuth, Germany, for proces-sing the blend for us. We also acknowledge helpfuldiscussions with Dr. Stephen King, ISIS Science Division,Rutherford–Appleton Laboratory, UK for the interpreta-tions of our results.

References

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[3] X.F. Lu, J.N. Hay, Polymer 42 (2001) 9423.

[4] J.W. Gilmer, D. Wiswe, H.-G. Zachmann, J. Kugler, E.W. Fischer,

Polymer 27 (1986) 1391.

[5] M. Imai, K. Kaji, T. Kanaya, Y. Sakai, Phys. Rev. B 52 (1995) 12696.

[6] A.M. Kotliar, J. Polym. Sci. Macromol. Rev. 16 (1981) 367.

[7] D. Turnbull, J.C. Fisher, J. Chem. Phys. 17 (1949) 71.