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MORPHOLOGY AND NON-ISOTHERMAL
CRYSTALLIZATION OF POLY(ETYLENE
TEREPHTHALATE) (PET)/GRAPHITE
OXIDE (GO) NANOCOMPOSITES
Sandra Paszkiewicz1*, Anna Szymczyk2, Zdenko Špitalský3,
Jaroslav Mosnáček4, Zbigniew Rosłaniec1
1
Institute of Materials Science and Engineering, West
Pomeranian University of Technology, Piastow 17 Av.,
Szczecin, 70-310, Poland
2
Institute of Physics, West Pomeranian University of
Technology, Piastow 48 Av., Szczecin, 70-310, Poland
3
Polymer Institute, Slovak Academy of Sciences, Dúbravská
cesta 9, Bratislava 45, 845 41, Slovakia,
4
Polymer Institute, Centre of Excellence FUN-MAT,
Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava
45, 845 41, Slovakia
*
[email protected]
Introduction
Incorporation of nanofillers to a polymer matrix gives
rise to a new class of materials known as polymer
nanocomposites, which have more potential for many
applications. Significant improvement in properties of the
composites depends mainly on: the size and shape of
particles nanofiller [1], surface area, degree of surface
development, surface energy and on the spatial distribution
of the polymer matrix of nanoparticles
Exfoliated and homogeneous dispersions of the layered
nanofillers, such expanded graphite or other graphene
derivatives, can be obtained straightforwardly when the
polymer contains functional groups, for example, amide or
imide groups. This is because the functionalized graphene
sheets have polar (hydroxyl, carboxyl) groups, which are
compatible with polymers containing polar functional
groups [2]. It has been shown [3] that for polyester-clay
nanocomposites modulus and HDT increase with clay
concentration as much as threefold. Furthermore, nonisothermal crystallization dynamics shows that the
nanocomposites of PET have a three times greater
crystallization rates than that of pure PET. The exfoliated
clay particles act as nucleating agents and have strong
interactions with PET molecular chains [3]. The increase of
the apparent crystallinity with filler content also appears to
result from the incidence of very rapid crystallization due to
the nucleating effect of nanofillers. It is considered that the
greater the interlamellar distance, the greater the interaction
of PET with layered nanofiller sheets. Graphene oxide (GO)
with functionalized groups on its surface has a larger
interlayer distance than graphite [2,4]. It can be readily
exfoliated into individual GO sheets in water and forms
stable dispersions after sonication. Polymer nanocomposites
with GO as filler have shown dramatic improvements in
properties such as elastic modulus, tensile strength,
electrical conductivity, and thermal stability [5,6].
In this work, the influence of graphite oxide (GO)
content on the morphology and non-isothermal
cristallization of poly(ethylene terephthalate) (PET) was
investigated. For this purpose PET/GO nanocomposites at
different nanofiller loadings were prepared by in situ
polymerization.
Experimental
Materials. For the in situ synthesis of poly(ethylene
terephthalate)/graphite oxide nanocomposites (PET/GO),
the following chemicals were used: dimethyl terephthalate
(DMT) (Sigma-Aldrich), ethane-1,2-diol (ED) (SigmaAldrich), zinc acetate Zn(CH3COO)2 (Sigma-Aldrich) -ester
exchange catalyst, antimony trioxide Sb2O3 (Sigma-Aldrich)
- polycondensation catalyst, Irganox 1010 (Ciba-Geigy,
Switzerland) as a thermal stabilizer.
Graphite oxide (GO) with flake size of about 5 μm was
provided by Polymer Institute of Slovak Academy of
Sciences, where the natural graphite was first converted to
intercalated or expandable graphite through chemical
oxidation in the presence of concentrated H2SO4 and HNO3
acids. Before adding nanofillers to the reaction mixture they
were combined with ethanediol in order to split
agglomerates and to improve further exfoliation.
Preparation of PET/GO nanocomposites
PET/GO nanocomposites were prepared by in situ
polymerization in the polycondensation reactor (Autoclave
Engineers, Pennsylvania, USA) capacity of 1000 cm3.The
process was conducted in two stages and followed the same
procedure as described in our previous publications [7-10].
Before polymerization, the dispersion of GO sheets was
prepared by dispersing the desired amount of graphite oxide
in ca. 250-300 ml of PDO through ultrasonication for 15
min using sonicator (Homogenizer HD 2200, Sonoplus, with
frequency of 20 kHz and 75% of power 200W) and next by
using for 15 min intensive mixing with high-speed stirrer
(Ultra-Turax T25). Additionally, to improve the
dispersion/exfoliation of GO in ED an ultra-power lower
sonic bath (BANDELIN, Sonorex digitec, with frequency of
35kHz and power 140W) was applied for 20 hours.
Characterization techniques
The inherent viscosity [η] of the polymers was
determined at 30 °C using a capillary Ubbelohde type Ic (K
= 0.03294), according to the procedure described elsewhere
[7-11]. The polymer solution had a concentration of 0.5 g/dl
in mixture phenol/1,1,2,2-tetrachloroethane (60/40 by
weight). The polymer nanocomposite solution was filtered
through 0.2 μm pore size polytetrafluoroethylene (PTFE)
filter (Whatman; membrane type TE 35) to ensure that the
intrinsic viscosity is not affected by present GO. After
filtration the polymer was precipitated and re-dissolved. The
Mark–Houwink equation [ ]  3.72  10 4  M 0.73 was used to
calculate the average viscosity molar mass of PET
homopolymer [12].
The melt volume rate (MVR) was measured using a
melt indexer (CEAST, Italy) as melt flow in cm3 per 10 min,
at temperature of 280oC, and at orifice diameter 1.050 mm
and under 2.160 kg load, according to ISO 1133
specification.
The density was measured at 23◦C on hydrostatic
balance (Radwag WPE 600C, Poland), calibrated according
to standards for which density is known.
The thermal properties of PET based nanocomposites
were studied with differential scanning calorimetry (DSC,
TA Instruments Q-100, USA, 2004) in cycle: heatingcooling-heating in the temperature range from -50 to 300oC
at a scan rate of 10oC/min. To determine the melting and
crystallization peaks, the second heating and cooling scans
were used. The glass transition temperature (T g) for the
prepared nanocomposites and pure PET was taken as the
midpoint of the change in heat capacity. The degree of
crystallinity (Xc, mass fraction) was calculated using the
formula:
Xc 
H m
100%
H m0
(1)
where H0m is 140 J/g.
Small angle X-ray scattering (SAXS) measurements were
performed at beam line A2 at HASYLAB (DESY,
Hamburg). The wavelength of the X-ray beam was = 0.15
nm. Scattering patterns were collected by a two-dimensional
MAR-CCD-165 detector placed at a distance of 2443 mm
from the sample. The scattering-vectors were calibrated
using a dray rat-tail tendon protein. Samples, mounted in
Mettler hot stage and encased between aluminium-foil
windows, were heated and cooled at heating rates of 5
o
C/min over a temperature range of 25–275 oC. During
cooling from the melt, data was collected with time
scanning 60 and accumulation time 20 s. The raw SAXS
intensity data was corrected for background scattering.
Changes of the long period (L) and the thicknesses of the
crystalline lamellae (lc) and amorphous layers (la) during
non-isothermal crystallization of the neat PET and
PET/0.5GO nanocomposite were determined from the
correlation function according to procedure reported in [13].
Results
A series of thermoplastic polyesters based on poly(ethylene
terephthalate) (PET) and graphite oxide (GO) were
synthesized by a catalyzed two-step reaction, involving
transesterification and polycondensation in the bulk.
Nanocomposites with 0.1, 0.3, 0.4 and 0.5wt.% of GO were
prepared. Additionally, for comparison purposes,
unmodified PET was synthesized and characterized in the
same manner as the nanocomposites. In order to prepare
high molecular mass polyesters, the polycondensation of
these oligomers ensued at higher temperature, with the
simultaneous application of high vacuum. The intrinsic
viscosity of PET that was prepared was 0.556 dl/g. The
presence of the GO in the polymerization mixture affected
the reaction, leading to variations of the final intrinsic
viscosities of the prepared nanocomposites. As shown in
Table 1, the intrinsic viscosity gradually decreased with the
increasing concentration of GO to 0.485 dl/g for
PET/0.5wt.% of GO. As can be also seen in Table 1, that the
synthesized nanocomposites have high molecular masses
with a slight decrease with the increasing content of GO
from 2.12·104 for PET to 1.85·104 for PET/0.5GO
nanocomposite. A slight increase in the MVR only for
nanocomposite with 0.1wt.% of graphene oxide and almost
no changes for PET/0.5GO was observed. The comparable
value of MVR for PET and PET/GO nanocomposites was
due to the polymer – functionalized graphene sheets
interactions. Nanocomposites in comparison to the neat PET
showed slightly higher density due to the presence of
nanofiller with higher density in PET matrix.
Morphology
of
PET
nanocomposites
containing
functionalized graphene sheets (GO) was investigated using
both SEM and TEM analysis. Figures 1a and b show SEM
micrographs of fracture surfaces of PET nanocomposites
with GO. SEM analysis of the fracture surfaces indicate
relatively homogenous dispersion of graphene sheets in
polymer matrix. Individual functionalized sheets and some
agglomerates of GO sheets, clearly embedded in the PET
matrix are observed on the surface (Figure 1a).
The ability of semicrystalline polymers (i.e. PET) to
crystallize is usually affected by nanofillers [14-15]. The
distributed nanoparticles have the influence on nucleation
and growth of crystallites in polymer matrix.
Table 1. Physical properties of PET/GO nanocomposites.
Sample
[η]
(For
polymer
matrix)
dL/g
Mv x
104
g/mol
d23
g/cm3
MVR
cm3/10 min
PET
0.536
2.12
1.330
72.8±1.9
PET/0.1GO
0.490
1.87
1.338
74.5±7.8
PET/0.3GO
0.490
1.87
1.340
73.9±3.9
PET/0.4GO
0.490
1.87
1.348
72.8±1.4
PET/0.5GO
0.486
1.85
1.351
72.4±2.3
[] -intrinsic viscosity; Mν– average viscosity molecular mass;
MVR - melt volume rate at 280 oC; d – density at 23 oC.
a)
b)
Figure 1. SEM micrographs of PET/GO nanocomposites:
a) 0.1GO wt.%, b) 0.5GO wt.%.
From TEM images (Fig. 2), it was observed that the
dispersion of the graphene flakes into the PET matrix was
homogeneous, while at higher concentrations of GO some
small aggregates were also formed. This was due to the
extremely high specific area of GO and the strong particlematrix interactions that take place.
a)
accompanied by the decrease of the la for neat PET and
nanocomposites was observed.
b)
a)
Exo up
5
PET/0.5GO
Heat Flow [W/g]
PET/0.4GO
PET/0.3GO
PET/0.1GO
PET
0
50
100
150
200
250
300
o
Temperature [ C]
b)
PET
0
PET/0.5GO
Heat Flow [W/g]
Figure 2. TEM micrographs of PET/GO nanocomposites:
a) 0.3GO wt.%, x 50 000, b) 0.5GO wt.%, x 150 000.
b)
In the prepared nanocomposites no significant effect on
degree of crystallization was observed and the values for
nanocomposites with the highest concentration of GO
(PET/0.4GO and PET/0.5GO) were comparable to the neat
PET (Table 2, Fig. 3). This was probably due to the fact that
the sizes of individual oxidized graphite nanoplatelets were
below the critical nucleation agents, as a result, may not
constitute active centers of growth of crystallites. However,
as seen in the crystallization traces in Fig. 3a, graphite oxide
played a nucleation role (change in T c form 214 oC for neat
PET to 221 oC for PET/0.5GO). The lack of changes in the
glass transition temperature (Tg) of semicrystalline PET
regardless of the GO content was also observed.
Nanocomposites with different amount of GO have very
slightly lower values of Cp at Tg than the neat PET.
PET/0.4GO
PET/0.3GO
PET/0.1GO
-5
Exo up
0
50
100
150
200
250
300
o
Temperature [ C]
Table 2. Transition temperatures and the degree of
crystallinity of PET/GO nanocomposites
Tg
C
Cp
J/g oC
Tm
o
C
PET
81
0.13
PET/0.1GO
82
0.11
Sample
o
Tc
C
xc
%
257
214
27.1
250
214
26.7
o
Figure 3. DSC thermograms obtained during cooling (a)
and 2nd heating (b) for PET/GO nanocomposites.
0,9
PET
PET/0.5GO
82
0.12
253
217
26.8
PET/0.4GO
82
0.12
254
218
27.2
PET/0.5GO
81
0.13
255
221
27.4
Tg - glass transition temperature; cp - heat capacity; Tm - melting
temperature; Tc - crystallizing temperature; xc – degree of crystallinity.
[a.u.]
PET/0.3GO
0,3
(x)
0,6
0,0
-0,3
-0,6
0
5
10
15
x
Effect of the presence of GO in PET matrix on the evolution
of the lamellar nanostructure was monitored by SAXS
during non-isothermal crystallization. The correlation
function γ(x) has been applied to depict the changes of the
nanostructure of PET/GO composites and neat PET during
crystallization from the melt. Figure 3 displays the
representative correlation functions for neat PET and
nanocomposite (PET/0.5GO) obtained after cooling from
the melt. According to procedure reported in [16] analysis of
the γ(x) allows to determine: L, lc, la and crystallinity within
the lamellar stacks (c). Figure 4 summarizes the results of
the
nanostructure
studies
during
non-isothermal
crystallization from the melt of neat PET and PET/GO
nanocomposites. The difference between L and lc gives la.
At higher temperatures a significant decrease of L
20
25
30
-1
[nm ]
Figure 3. One-dimensional correlation function for PET and
PET/0.5GO nanocomposite; samples after non-isothermal
crystallization
This is consistent with the appearance of lamellar stacks
filling in the sample volume. Finally, nanocomposites
showed almost the same values of the long period (L=11.4
nm for neat PET and L=11.5 nm for PET/0.5wt.% of GO).
Nanocomposites have comparable thickness of crystalline
lamellae as neat PET. Slightly higher value of lc for
nanocomposites with the highest concentration of nanofiller
was observed. According to an assumption that stacks of
lamellae are volume-filing in the semicrystalline polymer
the linear crystallinity determined by SAXS should be
identical to the bulk crystallinity measured with the DSC
method.
30
PET/0.5GO L
PET/0.5GO lc
PET/0.5GO la
PET L
PET lc
PET la
25
L , lc , la [nm]
20
cooling
15
10
5
0
0
50
100
150
200
250
o
Temperature [ C]
Figure 4 Changes of long period (L), crystalline and
amorphous lamellae thickness (lc, la) calculated from the
one-dimensional correlation function for neat PET and
PET/0.5GO nanocomposite during crystallization form the
melt.
Table 3. Parameters of nanostucture determined from
correlation function for PET and PET/0.5GO.
Sample
L [nm]
lc [nm]
la [nm]
E [nm]
c
PET
11.4
4.60
6.80
2.09
0.40
PET/0.5GO
11.5
4.63
6.87
1.97
0.40
L - long period; lc – thickness of crystalline lamellae; la – thickness of
amorphous layer; E – thickness of transition layer ; c – volume fraction of
crystallinity
The fully amorphous and crystal PET has density of 1.331
g/cm3 and 1.455 g/cm3 [17], respectively. Using the method
of relations for densities of amorphous and crystalline
phases, the volume fraction crystallinity were transformed
into a mass fraction crystallinity and compared with the
value obtained from DSC method. Here, the obtained for
neat PET value of volume fraction crystallinity is 0.4 and
it’s around 0.274 of mass fraction and is very close to the
calculated mass fraction of crystallinity by DSC (xc=0.271).
The obtained results for neat PET confirm that the model of
crystalline-amorphous two-phase lamellar system applied
here can correctly describe the real structure of investigated
PET and PET/GO nanocomposites
Conclusions
Nanocomposites based on PET containing from 0.1 to 0.5
wt.% of graphite oxide (GO) were obtained by in situ
polymerization. The prepared nanocomposites showed
uniform dispersion of GO sheets in polymer matrix (SEM.
TEM). In comparison to the neat PET, the glass transition
temperature and melting temperature of PET/GO
nanocomposites are not influenced by the presence of
graphite oxide sheets. The nucleating effect due to the
presence of GO hasn’t been observed, so only 7 oC change in
crystallization onset temperature for PET/0.5GO
nanocomposite in comparison to the neat PET was observed.
The degrees of crystallinity of the nanocomposites are
comparable to the neat PET and the results obtained for both
DSC and SAXS methods were almost the same (27.1 and
27.4 %). From SAXS data it was found that the long period
of PET/GO nanocomposites is similar to the neat PET.
This work is sponsored by European Project MNT ERA NET
2012 (project APGRAPHEL). The experiments performed at
A2 in HASYLAB (DESY, Hamburg) were done using the
beamtime of the proposal II-20110255EC
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