This study investigated the physical aging of a blend of deuterated and hydrogenated poly(ethylene terephthalate) (PET) using differential scanning calorimetry (DSC) and small-angle neutron scattering (SANS). DSC showed an endothermic peak developed during aging and increased with aging time, indicating an increase in enthalpy. SANS revealed the radius of gyration decreased during aging, suggesting a change in molecular conformation. However, no difference was observed between aged and de-aged samples by SANS. The study thus showed physical aging of PET leads to changes in enthalpy and molecular conformation over time.

1. Physica B 385–386 (2006) 514–516
Physical ageing studies of poly(ethylene terephthalate) using
SANS and DSC
Andrew Ellisa
, Duncan Gordona
, Stephen Kingb
, Mike Jenkinsa,Ã
a
Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
b
ISIS Spallation Neutron Source, Rutherford-Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK
Abstract
The process of physical ageing in a blend of deuterated and hydrogenated poly(ethylene terephalate) has been investigated using a
combination of differential scanning calorimetry (DSC) and small-angle neutron scattering (SANS). The development of an endothermic
peak on the glass transition has been shown using DSC. Furthermore, the radius of gyration was found to decrease during physical
ageing.
r 2006 Elsevier B.V. All rights reserved.
PACS: 81.05.Lg
Keywords: Poly(ethylene terephthalate); Physical ageing; SANS; DSC
1. Introduction
On cooling a polymeric liquid, the molecular relaxation
processes are sufficiently rapid to maintain the molecular
conformations at equilibrium. However, a temperature is
eventually reached at which the relaxations become too
slow for the time scale of cooling and the molecular
conformations become fixed at this temperature. The liquid
then converts to a glass. This description of the glass
forming process is kinetic in nature and the measured glass
transition temperature, Tg, is cooling rate dependant and is
a thermally activated process.
Below the glass transition temperature, the glass will
have excess thermodynamic quantities over equilibrium
values such as volume, enthalpy and entropy and there will
be a driving force to reduce these towards the equilibrium
values at that temperature. If a glass is stored at a
temperature below Tg this driving force to attain equili-
brium changes the molecular conformations at rates which
are temperature dependant [1]. As a result, there is a
corresponding change in material properties, such as
enthalpy and volume. This process is called enthalpic
relaxation or physical ageing and is also kinetic in nature.
Physical ageing of amorphous poly(ethylene terephalate)
(PET) has been found to increase the rate of crystallization
on heating to above the glass transition [2]. Since crystal-
lization involves conformational change, the purpose of
this work was to investigate the development of any
conformational change during the ageing process of PET
as detected by small-angle neutron scattering (SANS).
2. Experimental
Deuterated poly(ethylene terephthalate) (D-PET) with
MW ¼ 8140 was synthesized at The University of Durham,
UK. The hydrogenated poly(ethylene terephthalate)
(H-PET) was commercial PET (DuPont Laser) with
MW ¼ 36 400. A blend of 25 wt% D-PET and 75 wt%
H-PET was obtained by processing the material in a
conical double screw Minimixer at 280 1C at The Uni-
versity of Bayreuth, Germany. The blend was vacuum
dried at 150 1C for several days before the samples were
prepared from it. The samples were prepared by putting
0.15 g of the blend in the middle of aluminium washers,
with internal diameters of 10 mm and thicknesses of 1 mm,
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doi:10.1016/j.physb.2006.05.259
ÃCorresponding author. Tel.: +44 0121 414 2841;
fax: +44 0121 414 5231.
E-mail address: M.J.Jenkins@bham.ac.uk (M. Jenkins).

2. sandwiched between two poly(tetrafluoroethylene) coated
glass-fibre fabrics and heated to a temperature of 280 1C.
A pressure of 200–500 kg cmÀ2
was applied and maintained
for approximately 30 s. The sample, together with the
fabrics, was then quickly removed from the press and
immediately quenched in liquid nitrogen to prevent
crystallization. The samples were then aged in an oven at
a temperature of 63 1C, 5 1C below Tg, for various times
between 30 min and 29 h or at a temperature of 58 1C, 10 1C
below Tg, for various times between 3 and 127 h.
The glass transition temperature and the development
of physical ageing were measured using a Perkin Elmer
differential scanning calorimeter (DSC), model 2B which
was interfaced to a personal computer. The thermal
response of the instrument was calibrated from the
enthalpy of fusion of a known mass of indium (99.999%
pure). The temperature scale of the calorimeter was
calibrated using the melting points of indium, tin and
lead. Plots of actual against experimental melting
points were linear and used to calibrate the calorimeter
temperature directly after correcting for thermal lag.
Corrections were made for thermal lag by extrapolation
to zero heating rate.
Samples in the form of discs (1 mm thick and 3 mm in
diameter) were contained within aluminium pans, and an
empty pan was used as a reference. Plots of heat capacity
against temperature were obtained from the DSC at
different heating rates. The glass transition temperatures
were determined directly from a plot of heat capacity
against temperature using the method outlined by Ri-
chardson and Savill [3].
The aged samples were then cooled to room temperature
prior to reheating through the glass transition at 10 K minÀ1
in the DSC. A baseline was obtained by repeating this
procedure but storing at the ageing temperature for zero
time. The recorded baseline was subtracted from the
original trace of the aged sample and the area under the
endotherm was considered to be the enthalpy of ageing.
The SANS data for the samples, were collected at
room temperature using the LOQ diffractometer at
the ISIS pulsed neutron source, at the Rutherford-
Appleton Laboratory, Oxfordshire, UK, with a neutron
wavelength range of 2.2–10 A˚ , giving the range of the
scattering vector Q between 0.007 and 0.283 A˚ À1
. Calibra-
tion of the scattered neutron intensity in terms of scattering
cross section, I(Q), was made using the scattering
from mixtures of deuterated and hydrogenous polystyrene
of known molecular weight and composition. Data
were radially averaged, normalized for variations in
sample thickness and transmission and the background
scattering subtracted, calculated from measurements on
amorphous H-PET and D-PET samples. The data
collection time was 20 min. The samples were then de-aged
by placing them on a hotplate (Bibby Sterilin Heat-
Stir CB162) at 78 1C, 10 1C above Tg, for 1 min and
the SANS measurements were repeated on all of the
samples as before.
3. Results and discussion
The development of an ageing peak following storage at
58 1C and its disappearance after de-ageing as detected by
DSC is shown in Fig. 1. It is clear that the extent of ageing
increases with increasing storage time. Calculation of
the enthalpy change with time at 58 and 63 1C, as shown
in Fig. 2, shows the characteristic trend of physical ageing.
However, when the corresponding SANS data were
analysed, no difference between aged and de-aged samples
was observed as shown in Fig. 3.
The SANS data for the D-PET/H-PET samples
were plotted as Zimm plots of I(Q)À1
vs. Q2
. Linear
least-square fit lines were fitted to the data between
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60 62 64 66 68 70 72 74 76 78 80 82 84 86
-3.50
-3.25
-3.00
-2.75
-2.50
-2.25
Aged
De-aged
Powerdifference
Temperature (°C)
Fig. 1. Development of an ageing peak from storage at 58 1C for 72 h and
its subsequent disappearance after de-ageing as followed by DSC
measurements.
0 1000 2000 3000 4000 5000
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
58°C
63°C
ΔH
Time (mins)
Fig. 2. Enthalpy change with time with storage at 58 and 63 1C as
measured by DSC.
A. Ellis et al. / Physica B 385–386 (2006) 514–516 515

3. 4.9 Â 10À4
pQ2
/A˚ À2
p9.6 Â 10À4
. From these lines, the
z-average radius of gyration Rg
z
and molecular weight
MW were obtained by fitting the data to the equation:
IðQÞÀ1
¼ ð1 þ ðQ2
Rz2
g Þ=3ÞðCNMWDÞ À 1 (1)
with CN a constant [4].
Presentation of the SANS data for aged and de-aged
samples in this form revealed a decrease in Rg
z
during
ageing. This is illustrated for an ageing temperature of
58 1C in Fig. 4. The data for the samples aged at 63 1C gave
similar results. The change in Rg
z
occurs in the same time
scale as the enthalpy changes measured by DSC and can be
associated with the ageing process.
Acknowledgements
We thank the EPSRC for the financial support of the
research programme. We would also like to thank Mr.
Frank Biddlestone for his techical support to the project,
Mr. Michael Cannon and Dr. Lian Hutchings from the
IRC in Polymer Science and Technology, University of
Durham, UK for the synthesis of the deuterated poly
(ethylene terephthalate) for us and Dr. Reiner Giesa from
the Laboratory for Microscale Processing, University of
Bayreuth, Germany for processing the blend for us.
References
[1] L.C.E. Struik, Physical Ageing of Amorphous Polymers and Other
Materials, Elsevier, New York, 1978.
[2] M.J. Jenkins, J.N. Hay, Comput. Theor. Polym. Sci. 11 (2001) 283.
[3] M.J. Richardson, N.G. Savill, Polymer 16 (1975) 753.
[4] G.D. Wignall, Physical Properties of Polymers, American Chemical
Society, Washington, DC, 1993.
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0.00 0.05 0.10 0.15 0.20 0.25 0.30
0
1
2
3
4
0.0000 0.0004 0.0008 0.0012 0.0016 0.0020
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Q2
(Å-2
)
I(Q)
-1
(cm)
Aged for 127 hours
Unaged
I(Q)(cm-1)
Q (Å-1)
Fig. 3. SANS results (I(Q) vs. Q) for D-PET/H-PET sample aged at 58 1C
for 127 h and then de-aged. The inset shows the equivalent data plotted as
Zimm plots (I(Q)À1
vs. Q2
).
30
0 20 40 60 80 100 120 140
32
34
36
38
40
42
44
46
48
50
RadiusofGyration(Å)
Ageing Time (Hours)
Fig. 4. Radius of gyration of D-PET/H-PET samples aged for various
times at 58 1C.
A. Ellis et al. / Physica B 385–386 (2006) 514–516516