Improving crystallization and processability of PBS: Via slight cross-linking

Article abstract of DOI:10.1039/c6ra13488b

To improve the processability and crystallization of poly(butylene succinate) (PBS), a cross-linkable comonomer containing an alkynyl group, named 5-(2-(trimethylsilyl)ethynyl)isophthalate (DTS) was synthesized and copolymerized with dimethyl succinate and 1,4-butanediol to prepare a series of slightly cross-linked PBS copolyesters (PBDTSx). Due to the very low cross-linking degree, PBDTSx do not form gels when the DTS molar content is lower than 1.0%, and this made them keep good solubility and reprocessability. The cross-linking degree of DTS in PBDTSx is determined by NMR spectra. GPC data show that the M_w values of the copolyesters are higher than 23 × 10⁴ g mol^-1, and increase with the increase of DTS content. The thermal stability, crystallization, rheological behaviours, and mechanical properties of PBDTSx were investigated. Compared to neat PBS, PBDTSx have greatly increased crystallization rates because of promoted nucleation of the cross-linking domains. The elongation at break of PBDTSx drops slightly, while the Young's modulus increases. The rheological behaviours indicate that PBDTSx have higher melt viscosity than neat PBS even at a high shear rate, meaning that they will have better blow moulding processability. This work demonstrates that via controlling tiny cross-linking during the polymerization of PBS, the properties and processability of PBS can be improved, and this method overcomes the defects caused by traditional post cross-linking.

Full text of DOI:10.1039/c6ra13488b

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Improving crystallization and processability of PBS
via slight cross-linking†
Cite this: RSC Adv., 2016, 6, 68942
Gui-Cheng Liu, Wen-Qiang Zhang, Shui-Lian Zhou, Xiu-Li Wang*
and Yu-Zhong Wang*
To improve the processability and crystallization of poly(butylene succinate) (PBS), a cross-linkable
comonomer containing an alkynyl group, named 5-(2-(trimethylsilyl)ethynyl)isophthalate (DTS) was
synthesized and copolymerized with dimethyl succinate and 1,4-butanediol to prepare a series of slightly
cross-linked PBS copolyesters (PBDTSx). Due to the very low cross-linking degree, PBDTSx do not form
gels when the DTS molar content is lower than 1.0%, and this made them keep good solubility and
reprocessability. The cross-linking degree of DTS in PBDTSx is determined by NMR spectra. GPC data
4
ꢁ1
show that the M
w
values of the copolyesters are higher than 23 ꢀ 10 g mol , and increase with the
increase of DTS content. The thermal stability, crystallization, rheological behaviours, and mechanical
properties of PBDTSx were investigated. Compared to neat PBS, PBDTSx have greatly increased
crystallization rates because of promoted nucleation of the cross-linking domains. The elongation at
break of PBDTSx drops slightly, while the Young's modulus increases. The rheological behaviours
indicate that PBDTSx have higher melt viscosity than neat PBS even at a high shear rate, meaning that
they will have better blow moulding processability. This work demonstrates that via controlling tiny
cross-linking during the polymerization of PBS, the properties and processability of PBS can be
improved, and this method overcomes the defects caused by traditional post cross-linking.
Received 24th May 2016
Accepted 14th July 2016
DOI: 10.1039/c6ra13488b
www.rsc.org/advances
1
2–18
the compound containing more than one double bond.
Introduction
13
Bahari et al. found that the melt strength of PBS was improved
Due to the increasing concerns on the environment and sustain- by electron-beam radiation cross-linking and biodegradable PBS
able energy development, bio-based polymers derived from foam can be easily prepared. Via ultraviolet cross-linking the
renewable resources have attracted much attention. Poly(butylene modied PBS shows improved thermal stability and dynamic
1
4
16
succinate) (PBS) is considered to be a promising bio-based poly- mechanical property. The results reported by Kim et al. and
17
mer, which can compete with conventional polypropylene and Ma et al. revealed that the crystallization temperature and the
polyethylene owing to its good thermal stability and mechanical mechanical property of cross-linked PBS were enhanced. The
1
properties. However, it still has some disadvantages, such as aforementioned cross-linking modication is achieved by post-
insufficient mechanical properties, low melt strength/viscosity processing, and extra equipment is needed. Furthermore, to
and a narrow processing temperature window, which limit its prevent pollution caused by the electron beam, rigorous safe-
wide applications. To resolve these problems, many methods, guarding is also needed. At the same time, in this way a lot of
2,3
such as enhancing molecular weight by chain extension, phys- gels are formed, which will inuence the reprocessability and
4–7
8–11
ical blending and copolymerization, are used.
biodegradability of PBS. If we make PBS slightly cross-linking at
Cross-linking is also an effective way to improve the proper- the polycondensation step instead of post-processing, its prop-
ties of PBS. Usually the cross-linking reaction can occur under erties are accordingly improved, while the crosslinking degree is
irradiation or thermal processing in the presence of peroxide or so low as not to affect PBS further thermal processing.
It has been demonstrated that alkynyl group is prone to
ꢂ
ꢂ
cross-linking reaction around 200 C to 350 C, depending on
Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), its chemical structure. This cross-linking group has been
College of Chemistry, State Key Laboratory of Polymer Materials Engineering,
introduced into
carbosilane,
phenylquinoxalines
a
variety of polymers, such as poly-
National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan),
Sichuan University, 29 Wangjiang Road, Chengdu 610064, China. E-mail:
xiuliwang1@163.com; yzwang@scu.edu.cn; Fax: +86-28-85410755; Tel: +86-28-
19–21
22–24
thermosetting
polyimide,
poly-
25,26
27,28
and poly(ethylene terephthalate),
8
5410755
etc. to improve their thermal stability, adhesive strength and
†
Electronic supplementary information (ESI) available: DSC curves of rst ame retardancy. As far as we know, alkynyl group has not been
heating scan and the second heating scan for monomer DTS. See DOI:
0.1039/c6ra13488b
introduced into PBS yet.
1
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In this study, dimethyl 5-(2-(trimethylsilyl)ethynyl)- detailed procedures are shown as follows: the samples were
ꢂ
isophthalate (DTS), a comonomer containing alkynyl group, is kept at 140 C for 5 min to eliminate the thermal prehistory, and
ꢂ
ꢁ1
ꢂ
synthesized and introduced into PBS main chain via melt then they were cooled at a rate of 10 C min to 40 C to create
polymerization. The effect of DTS contents on thermal proper- a “standard thermal history”. Aer that, the samples were
ties, crystallization properties, mechanical properties and heated again at the same rate up to self-nucleation temperature
rheological behaviours of PBS were investigated in detail. It is (T
s
, usually in the melting range) and kept at that temperature
expected that the properties of PBS can be improved via for 5 min to allow the samples to partially melt. At last, the
ꢂ
ꢁ1
ꢂ
changing its chemical structure.
samples were cooled at 10 C min from T down to 40 C.
s
During that nal cooling step, the fraction of the samples
initially molten at T will crystallize by using the unmelted
crystal fragments as the self-nuclei.
s
Experimental section
Materials
The non-isothermal crystallization processes were examined
ꢂ
by DSC. The samples were rst heated to 140 C and held for
1
,4-Butanediol (BDO, AR grade), and zinc acetate (Zn(OAc)
grade) were purchased from Kelong Chemical Corporation
Chengdu, China). Dimethyl succinate (DSA, AR grade) was
2
, AR
ꢂ
5
min to eliminate the thermal history, and then cooled to 40 C
ꢂ
ꢁ1
ꢂ
ꢁ1
at different cooling rates of 20 C min , 10 C min , and
(
ꢂ
ꢁ1
5
C min
.
purchased from Alfa Aesar Chemical Corporation (Tianjin,
China). Tetrabutyl titanate (TBT), was also provided by Kelong
Chemical Corporation, and dissolved in anhydrous toluene to
The isothermal crystallization kinetic of samples was also
studied by DSC. The treatment on the sample was as following:
ꢁ1
the thermal history was removed by heating the sample up to
prepare 0.2 g mL solution. Iodine (I
periodate (NaIO , AR grade), dimethyl isophthalate (AR grade),
ethynyltrimethylsilane (AR grade), copper iodide (CuI, AR grade)
and bis(triphenylphosphine)-palladium(II)chloride (Pd(PPh
Cl , AR grade) were purchased from Yinuokai Chemical Corpo-
ration (Beijing, China). Sulfuric acid (H SO , 96%), methylene
2
, AR grade), sodium
ꢂ
1
40 C and held for 5 min, and then it was quickly cooled to the
4
ꢂ
predetermined crystallization temperature (94 C), where it was
le to crystallize until saturation.
3 2
) -
Polarized optical microscope (NIKON ECLIPSELV100POL)
equipped with a temperature controller (HSC621V) was used to
observe the crystallization morphology of samples. The samples
2
2
4
chloride (CH Cl ), sodium bicarbonate (NaHCO ), sodium sulte
2
2
3
ꢂ
were rst melted at 140 C for 3 min to diminish any thermal
(
Na SO ), triethylamine (TEA), tetrahydrofuran (THF), methanol
2 3
ꢂ
history, and then quickly cooled to 94 C to investigate their
and ethanol were purchased from Kelong Chemical Corporation.
crystalline morphology.
WAXD measurements were carried out at room temperature
on the Philips X'Pert X-ray diffractometer using the Cu-Ka
Intrinsic viscosities of polyester were measured with an Ubbe- radiation with a scan rate of 2 min scanning from 5 to 40 .
Characterization
ꢂ
ꢁ1
ꢂ
ꢂ
ꢁ
1
lohde viscometer at a concentration of 2.0 g dL in chloroform
Dumbbell-shaped samples with the thickness and width of
0.5 mm and 4 mm were prepared by hot pressing. Tensile
ꢂ
at 30 ꢃ 0.1 C.
The molecular weights of copolyesters were determined by testing for samples was measured on a Sansi Universal Testing
GPC, using a Waters apparatus equipped with a model 1515 Machine (CMT, Shenzhen, China) at room temperature at
ꢁ
1
pump, a Waters model 717 auto samplers, and a 2414 refractive a stretching rate of 50 mm min . The average values of each
index detector. Chloroform and monodisperse polystyrene were sample were obtained from at least ve measurements.
used as the eluent and standard, respectively. The concentra-
tion of sample and the ow rate of eluent were 2.5 mg mL and measured on a TADHR-2 rheometer equipped with a 25 mm
The dynamic rheological behaviour of samples was
ꢁ
1
ꢁ
13
1
ꢂ
1
.0 mL min , respectively.
parallel plate. The measurements were carried out at 130 C,
H and C-NMR spectra were recorded using a Bruker AC-P and dynamic oscillation frequency sweeps from 0.01 to 100 Hz,
00 MHz spectrometer in CDCl
solution using tetramethylsi- strain ¼ 1%.
The capillary measurement was conducted by
1
4
3
lane as an internal reference.
a
Thermogravimetric analysis for samples (5 mg) was per- RosandRH7D capillary rheometer. The diameter of the circular
ꢂ
formed on a NETZSCH Instrument (TG209 F1, Germany). The die was 1 mm and L/D was 16 : 1, with a 90 entrance angle. The
ꢂ
ꢁ1
ꢂ ꢂ
thermograms were recorded at a heating rate of 10 C min
measurements were performed at 130 C and 140 C. Aer
ꢁ1
under a nitrogen ow of 50 mL min , within a temperature being added into the barrel, the PBS pellets were compacted
ꢂ
range of 40–550 C.
and preheated for 1 min.
The thermal transition behaviour of samples was examined
by a TA Instrument (DSC-Q200). Samples (5 mg) were rst
ꢂ
Synthesis of dimethyl 5-iodoisophthalate
heated to 130 C (the rst heating scan), and held 3 min to
ꢂ
eliminate thermal history. Aer that, they were cooled to ꢁ70 C Dimethyl 5-iodoisophthalate was synthesized according to the
ꢂ
ꢁ1
31
at a cooling rate of 10 C min (cooling scan), and nally the literature, and the detailed processes are shown as follows:
ꢂ
ꢂ
samples were heated again to 130 C at the heating rate of 10 C 250 mL 96% sulfuric acid, iodine (46.66 g, 184 mmol) and
min (the second heating scan).
ꢁ
1
sodium periodate (13.54 g, 63 mmol) were placed in a 500 mL
The self-nucleation experiments were carried out by a TA round-bottom ask. Aer stirring 30 min at room temperature,
29,30
Instrument (DSC-Q200), according to the literature.
The the reaction mixture became black. Aer that, dimethyl
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temperature for 16 h. The mixture was then ltered, and the
solution was dried by rotary evaporation. The obtained residue
was recrystallized with ethanol. The yellow solid (16.55 g, 57
mmol) with yield of 76% was nally got.
1
H-NMR (CDCl
3
, 400 MHz) d (ppm): 8.61 (t, J ¼ 1.6 Hz, 1H),
13
8
.30 (d, J ¼ 1.6 Hz, 2H), 3.95 (s, 6H), 0.29 (s, 9H). C-NMR (101
MHz, CDCl ) d (ppm): 165.7, 137.1, 131.0, 130.5, 124.4, 102.9,
6.9, 52.8, 0.24.
3
9
Scheme 1 Synthesis route of DTS.
Synthesis of PBS copolyesters containing DTS (PBDTSx)
The copolyesters (PBDTSx) contain DTS were synthesized by
isophthalate (55.70 g, 287 mmol) was added, and the reaction
mixture was stirred at room temperature overnight. Finally, the
reaction mixture was poured into crushed ice and the precipi-
tates were obtained. The resulting precipitation was dissolved
in 1 L methylene chloride, and washed with sodium bicar-
bonate. Aer neutralization was complete, sodium sulte was
added until the purple color of the organic layer disappeared.
The organic layer was collected by rotary evaporation, and the
white needle-like products with yields of 76% were obtained by
a
two-step melt polymerization (esterication and poly-
condensation), and their chemical structures are shown in
Scheme 2. In the abbreviations, x refers to the molar percentage
(mol%) of DTS units relative to the total diesters. In the rst
esterication step, diesters (the compositions of DSA and DTS
mixtures are shown in Table 1) and BDO with the molar ratio of
1
: 1.06, as well as the transesterication catalyst Zn(OAc) (0.2
2
wt%, based on the total reactants) were added into a three-
necked round-bottom ask equipped with a mechanical
stirrer, water separator, and nitrogen inlet pipe. The reactor was
recrystallized with methanol.
1
H-NMR (400 MHz, CDCl
3
) d (ppm): 8.64 (t, J ¼ 1.5 Hz, 1H),
ꢂ
heated to 175 C and maintained for 4 h, and then the catalyst
13
8
.55 (d, J ¼ 1.5 Hz, 2H), 3.95 (s, 6H). C-NMR (101 MHz, CDCl
3
)
TBT (0.1 wt%, based on the total reactants) was added to the
d (ppm): 164.7, 142.4, 132.1, 129.8, 93.5, 52.7.
ꢂ
reaction system. The polycondensation was carried out at 230 C
for 4 h under vacuum of 20–70 Pa. The obtained products were
puried by dissolving in chloroform and then precipitating in an
excess methanol in order to remove oligomers and unreacted
monomers. Finally, the ltered powder products were washed
Synthesis of dimethyl 5-(2-(trimethylsilyl)ethynyl)isophthalate
32
(
DTS)
Scheme 1 presents the synthesis route for DTS, and the detailed
procedures are shown here: dimethyl 5-iodoisophthalate (24.00
ꢂ
with methanol and dried under vacuum at 60 C for 48 h.
g, 75 mmol), CuI (0.45 g, 2.36 mmol), and Pd(PPh
3 2 2
) Cl (0.53 g,
0
.76 mmol) were added into a 500 mL round-bottom ask, and
Results and discussion
Synthesis and chemical structure characterization
then the system was evacuated and purged with N for three
times. Aer that, dry THF (50 mL), triethylamine (150 mL) and
2
ethynyltrimethylsilane (7.37 g, 75 mmol) were added. The The monomer DTS can be successfully introduced into PBS
resulting solution was stirred under nitrogen at room main chain via melt polymerization. All the copolyesters show
Scheme 2 Synthesis route of PBDTSx copolyesters.
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Table 1 Composition, molecular weight and cross-linking data of PBDTSx copolyesters
a
f (mol%)
Molecular weight
0
b
ꢁ1
c
n
ꢁ1
c
w
ꢁ1
c
Sample
f
DTS
f
DSA
D
e
(%)
D
f
(%)
Dx (%)
D x (%)
[h] (dL g
)
M
(g mol
)
M
(g mol
)
PDI
4
4
4
4
4
PBS
0
100
0
0
0
0
0.81
0.78
0.86
d0.99
7.40 ꢀ 10
6.76 ꢀ 10
5.76 ꢀ 10
16.25 ꢀ 10
23.88 ꢀ 10
26.72 ꢀ 10
2.2
3.5
4.7
4
4
4
PBDTS0.6
PBDTS0.8
PBDTS1.0
PBDTS1.5
0.6
0.8
1.0
1.5
99.4
99.2
99.0
98.5
52.4
64.2
d59.7
53.1
66.3
d57.4
52.8
65.3
d58.6
31.7
52.2
d58.6
9.13 ꢀ 10
36.40 ꢀ 10
4.0
d
d
d
a
b
ꢂ
c
d
Molar fraction of diesters in the feed. Intrinsic viscosity measured in chloroform at 30 C. Obtained by GPC. Not determined because of gel
formed in chloroform.
e
f
high molecular weight, whose intrinsic viscosities are in the ppm (H ), and 8.60 ppm (H ) (Fig. 1(b)) aer magnication. It is
ꢁ
1
e
f
range of 0.78–0.99 dL g (Table 1). Except PBDTS1.5, PBDTSx found that the peaks of H and H are splitted into two parts due
have good solubility in chloroform, and their molecular weights to the different chemical environments.
can be determined by GPC, and the data are also shown in Table
. From table we can see that the M
It was found that DTS monomer can cross-link around at
values of PBDTSx are 225–255 C (Fig. S1†), so these splits are caused by the cross-
ꢂ
1
w
4
greater than that of PBS, which all are higher than 23 ꢀ 10 g linking reaction of alkynyl group in DTS. It is the cross-
ꢁ
1
w
mol . Their M values are enhanced with the increase of DTS linking reaction occurring during the preparation that makes
content, indicating that introducing DTS favours improving the PBDTSx show high PDI values. Based on the integral peak area
molecular weights of PBS. PBDTS1.5 does not dissolve well in of splitting peak, we can calculate the cross-linking degree of
chloroform, and some gels are obtained, which makes its the comonomer DTS in the copolyesters according to the
molecular weight unable to be determined. Besides, their PDI following eqn (1):
values are also increased when DTS is introduced. It has been
SHe0
D
D
e
¼ SHe
ꢀ 100%
ꢀ 100%
demonstrated that the higher molecular weights and PDI values
will benet enhancing the melt viscosity of polymer, indicating
that PBDTSx has better processability, which will be discussed
in the later part of this work.
þ SHe0
SHf0
(1)
f
¼ S
Hf
þ SHf0
1
Fig. 1(a) shows H-NMR spectra of neat PBS and PBDTSx in
the range of 0–6 ppm. From the gure, we can see that there is
a little difference between them. For pure PBS, the peak
D or D refers to the cross-linking degree of the comonomer
e
f
f
e
DTS, SH
f
or SH is the integral peak area of H or H (the uncross-
e
a
appearing at 2.63 ppm (H ) belongs to the methine proton of
0
0
linked part of DTS), and S
H
f
or S
H
is the integral peak area of
e
b
0
0
e
DSA, and the chemical shis at 4.12 ppm (H ) and 1.71 ppm
f
H or H (the cross-linked part of DTS). Based on the results of
c
(
H ) assign to the methine protons of BDO. For PBDTSx, besides
D
D
e
, and D
f e
, we can get Dx (eqn (2), the average values of D and
the above peaks, some new peaks can be observed. The peak
f
), which refers to the average cross-linking degree of the
comonomer DTS.
d
3
appearing at 0.28 ppm (H ) is ascribed to the Si–CH of DTS.
The chemical shis for phenyl proton of DTS are found at 8.30
1
Fig. 1 H-NMR spectra in the range of 0–6 ppm (a), 8.1–8.7 ppm (b) of neat PBS and PBDTSx.
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Fig. 3 DSC curves of (a) second heating run and (b) cooling run of neat
ꢂ
ꢁ1
.
Fig. 2 TGA (a) and DTG (b) curves of neat PBS and PBDTSx.
PBS and PBDTSx at a rate of 10 C min
D
e
þ D
f
0
ðD xÞ for copolyesters can be obtained, in which PBDTS1.0
D
x
¼
(2)
0
2
shows the highest value. Because D x can reect the cross-
linking density of the copolyesters, PBDTS1.0 has the highest
Besides, the relative average cross-linking degree ðD xÞ of cross-linking density than the others.
0
DTS in the copolyesters can be obtained by eqn (3).
Thermal stability of PBDTSx copolyesters
D
e
þ D
f
0
D
x
¼
ꢀ x
(3)
2
Fig. 2 shows the thermal stability of neat PBS and PBDTSx
measured by TGA. All the TG curves almost overlap together,
illustrating the thermal stability of PBDTSx is as same as PBS.
The thermal decomposition temperatures such as the temper-
ature at 5% weight loss (T5%), the temperature at the maximum
decomposition rate (Tmax), as well as the residue weight at 550
in which, x refers to the molar percentage (mol%) of DTS units
relative to the total diesters of PBDTSx, for example, the x value
for PBDTS1.0 is 1.0. The detailed results of D , D , Dx, and D x are
0
e
f
listed in Table 1.
From the table we can see that the Dx shows the highest value
when 0.8 mol% DTS is introduced into PBDTSx. This means
that DTS comonomer in PBDTS0.8 has the highest cross-linking
degree, which made it have the highest PDI value. In view of
DTS content in PBS, the relative average cross-linking degree
ꢂ
C are listed in Table 2. And all the data are almost as same as
pure PBS, demonstrating that the introduction of a little DTS
does not inuence the thermal stability of PBS.
Crystallization and thermal transition behaviours of PBDTSx
The thermal transition behaviour is very important for a certain
polymer, which can inuence its properties and determine the
Table 2 TGA data of neat PBS and PBDTSx

elds where the polymer can be used in. DSC was used to
ꢂ
ꢂ
ꢂ
Sample
T
5% ( C)
T
max ( C)
Residue at 550 C (%) evaluate the thermal transition behaviours of PBDTSx. The
heating and cooling curves are shown in Fig. 3, while the
detailed thermal transition data are summarized in Table 3.
PBS
332
328
334
332
393
392
395
392
1.5
1.1
1.6
2.0
PBDTS0.6
PBDTS0.8
PBDTS1.0
Due to the high crystallinity (X , discussed in later part), their
c
g
glass transition temperatures (T ) can be not see clearly from its
DSC curves. So, the magnied DSC curves are provided as
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Table 3 DSC data, NE value and isothermal crystallization data of PBS and PBDTSx
Cooling
Second heating
ꢂ
ꢁ1
ꢂ
ꢂ
ꢁ1
a
c
Sample
T
c
( C)
DH
c
(J g
)
T
g
( C)
T
m
( C)
DH
m
(J g
)
X
(%)
NE
n
t1/2 (min)
PBS
71.1
73.8
74.9
81.3
62.9
68.9
65.3
67.5
ꢁ40.1
ꢁ39.2
ꢁ39.1
ꢁ39.4
114.0
114.2
113.5
113.1
78.9
84.7
75.9
57.7
77.3
83.0
74.4
56.6
0
2.6
1.7
2.3
1.7
14.7
7.3
7.8
2.4
PBDTS0.6
PBDTS0.8
PBDTS1.0
5.6
8.0
21.2
a
0
m
0
m
ꢁ1 35
The degree of crystallinity (X
c
) was calculated according to X
c
¼ DH
m
/DH
, where DH
of perfectly (100%) crystalline PBS is 102 J g
.
Fig. S2,† and from this gure, we can see that their T
as same as that of pure PBS. Besides, the melting temperatures PBS and PBDTSx. From gure we can see that both the T
g
are almost isothermal crystallization temperature versus cooling rate of
of
) of PBDTSx are almost unchanged compared to pure PBS PBDTSx and PBS decrease with the cooling rate quickened up,
Table 3). This means that although the introduction of DTS and PBDTSx always have higher T than the neat PBS at the
c
(T
m
(
c
reduces the segment regularity of PBS chain, the content is so same cooling rate. It is widely known that the crystallization
low to generate great effect on its chain mobility. Interestingly, process of the melting polymer can be divided into two steps,
the crystallization temperatures (T ) of PBDTSx are higher than nucleation and crystal growth. Though the cross-linking may
c
that of neat PBS. Especially for PBDTS1.0, its T is 10 degrees prevent segment motion to crystal growth, the cross-link points
c
c
higher than that of PBS. The dependence of T on cooling rate can be regarded as crystal nucleus in the nucleation step.
was investigated by DSC. Fig. 4(a) shows the plots of non- Therefore, it facilitates PBDTSx crystallization, and makes T
c
increase.
Fig. 4 (a) Non-isothermal crystallization temperature versus cooling
rate of neat PBS and PBDTSx, and (b) the neat PBS after partial or total Fig. 5 (a) Relative crystallinity versus time plot, and (b) Avrami plots of
ꢂ
melting at various temperatures.
PBS and PBDTSx at crystallization temperature of 94 C.
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ꢂ
Fig. 6 POM images of neat PBS and PBDTSx, isothermal at 94 C.
ꢂ
Based on the value of DH
m
, the crystallinity (X
c
) is calculated decreases. When the temperatures decrease to 119 C, the
and the detailed data are shown in Table 3. It can be seen that X
c
crystallization temperatures no longer move, thus, the optimum
ꢂ
is rst increased and then decreased with the increase of DTS self-nucleation temperature Tcmax is determined as 119 C. The
content. This states that although the cross-link points can be detailed NE values of PBDTSx are also given in Table 3. With the
regarded as crystal nucleus favouring crystallization, with the increase of DTS content, the NE value rises from 5.6 of
increase of cross-linking degree, the segment integrity and PBDTS0.6 to 21.2 of PBDTS1.0. Based on our previous NMR
mobility is reduced, which makes the X
c
decreased.
results, it is found via introducing alkynyl group into PBS main
To estimate the nucleating efficiency (NE) of PBDTSx, the chain, some cross-linking reaction occurs. The cross-linking
29,30
0
following method put forward by Fillon et al. is used.
density ðD xÞ is enhanced with the increase of DTS content.
NE value variation trend demonstrates that higher cross-linking
density is benecial to high nucleating efficiency, which will
lead to a higher crystallization temperature and fast crystalli-
zation. To make clear how slightly cross-linking affects the
crystallization process of PBDTSx, isothermal crystallization
processes were investigated by DSC. The relative crystallinity vs.
crystallization time plots are depicted in Fig. 5(a). From the
T
cNA ꢁ T
c
NE ¼ T
ꢀ 100
(4)
is
cmax ꢁ T
c
where TcNA is the crystallization temperature of PBDTSx, T
c
the crystallization temperature of neat PBS, and Tcmax is the
optimum self-nucleation temperature of neat PBS. NE is
expressed as a percentage where 0 represents no nucleating
action and 100 stands for the ideal nucleating action. This
efficiency scale is satisfactory than mere comparisons between
peak crystallization temperatures upon dynamic cooling from
the melt, since it can be used as a reference not only for the neat
polymer but for the polymer that is self-nucleated to

gure, it can be seen clearly that the enhancing crystallization
effect of DTS is pronounced. All the copolyesters complete 100%
crystallization in a shorter time than PBS. Avrami equation was
employed to analyze the isothermal crystallization kinetics:
33,34
n
saturation.
1 ꢁ X ¼ exp(ꢁkt )
(5)
t
Fig. 4(b) shows the DSC cooling curves of neat PBS aer
t
partial or total melting at various temperatures. It is found that where X is the relative crystallinity, t is the crystallization time,
the crystallization peak shis to high temperature as T
s
k is a rate constant depending on nucleation and crystalline
growth rate, and n is the Avrami exponent which denotes the
6
nature of the nucleation and growth process. The equation can
be rewritten as:
log[ꢁln(1 ꢁ X
t
)] ¼ log k + n log t
(6)
A plot of log[ꢁln(1 ꢁ X )] versus log t (Fig. 5(b)) gives
t
a straight line from which both the Avrami exponent and the
Table 4 Mechanical properties of neat PBS and PBDTSx
Tensile strength
(MPa)
Elongation at
break (%)
Young's
modulus (MPa)
Sample
PBS
33.7 ꢃ 0.9
30.5 ꢃ 1.0
32.0 ꢃ 0.4
32.7 ꢃ 2.1
331 ꢃ 16
221 ꢃ 17
221 ꢃ 24
263 ꢃ 25
286 ꢃ 20
310 ꢃ 25
333 ꢃ 15
337 ꢃ 10
PBDTS0.6
PBDTS0.8
PBDTS1.0
Fig. 7 WAXD patterns of PBS and PBDTSx.
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ꢂ
ꢂ
rate constant can be determined. By calculation, the n values of strong characteristic diffraction peaks at around 19.6 , 21.7 ,
ꢂ
11
PBS and PBDTSx are 2.6, 1.7, 2.3, and 1.7 (shown in Table 3), and 22.5 , which are ascribed to (020), (021) and (110) planes.
suggesting that the crystallization of the neat PBS corresponds For PBDTSx, the WAXD patterns share similar diffraction peak
to a three dimensional growth with heterogeneous nucleation, positions with the neat PBS, suggesting that they remain the
while the PBDTSx corresponds to a two dimensional growth same crystal structure. This indicates that although cross-
34
with heterogeneous nucleation. It may owe to the decreased linking enhanced the crystallization rate, it has no inuence
segment integrity and mobility caused by the cross-linking, and on crystallization structure of PBS.
the segments tend to be gathered on a two dimensional axial
axe.
Tensile properties of PBDTSx
Half-time of crystallization (t1/2) is another important
parameter, which means the time needed to achieve 50% of the
The mechanical properties play an important role in the
application of materials, and the tensile properties of the

nal crystallinity. The t1/2 values of PBS and PBDTSx are 14.7,
copolyesters are provided in Table 4, while the stress–strain
curves of neat PBS and PBDTSx are given in Fig. S3.† The tensile
strength of PBDTSx is close to neat PBS while the elongation at
break decreases a little and the Young's modulus increases with
the increase of DTS content. For PBDTS1.0, its Young's modulus
is raised from 286 of PBS to 337 MPa, illustrating the rigidity of
PBS is improved. This is ascribed to the networks formed
between polymer chains, which are caused by cross-linking.
And these networks reduce the segment mobility, so the elon-
gation at break decreases and the rigidity increases. The
networks are not strong enough when the molar percentage of
DTS units is less than 1.0%, therefore, the tensile strengths of
PBDTSx are close to neat PBS.
7
.3, 7.8, and 2.4 min (shown in Table 3), respectively, illus-
trating PBDTSx crystallizes faster than PBS, especially for
PBDTS1.0. This demonstrates again via promoting nucleation
the slightly cross-linking can accelerate PBS crystallization
greatly.
Meanwhile, POM was used to investigate the crystalline
ꢂ
morphology of PBS and PBDTSx at 94 C (Fig. 6). For PBDTS1.0,
only aer 90 s the small crystals are full of the view. For other
PBDTSx their crystallizations are also speeded up compared to
PBS, which is accord to the isothermal crystallization result.
People can see completely spherical morphology from the POM
image of pure PBS, however, the crystals for PBDTSx are tiny
and imperfect, especially for PBDTS1.0.
Fig. 7 exhibits the WAXD patterns of neat PBS and PBDTSx.
The crystal unit cell of PBS is monoclinic and it shows three
0
0
Fig. 8 Dynamic rheological (a) complex viscosities vs. frequency and Fig. 9 Dynamic rheological (a) loss modulus G vs. frequency and (b)
0
0
00
(
b) storage modulus G vs. frequency for neat PBS and PBDTSx.
storage modulus G vs. loss modulus G for neat PBS and PBDTSx.
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0 00
The slopes of the G vs. G curves can be regarded as an
Rheological behaviour of PBDTSx
36–38
evidence of change in polymer microstructure.
From
The dynamic rheological behaviours of the neat PBS and slightly
Fig. 9(b), we can obviously see that the curves of PBDTSx are
almost superimposed and deviated with that of neat PBS, sug-
gesting that some microstructure change occurred in PBDTSx.
This demonstrates again that with the introduction of DTS
content, a small amount of cross-linking network are formed at
polycondensation step.
The capillary rheological behaviours of the neat PBS and
PBDTS1.0 are shown in Fig. 10. The capillary rheological
behaviours are similar to the dynamic rheological behaviours,
i.e. the shear viscosity of PBDTS1.0 is always higher than PBS at
the whole shear region. The shear viscosity of PBDTS1.0 is
enhanced from 200 Pa s of PBS to 1300 Pa s at a shear rate of 50
cross-linked PBDTSx were studied comparably, and their
complex viscosity–frequency, storage modulus G -frequency,
0
0
0
0
loss modulus G -frequency and storage modulus G -loss
00
modulus G curves are depicted in Fig. 8 and 9. The neat PBS
acts as a Newtonian behaviour, and presents low complex
viscosity. With the introduction of DTS, the complex viscosity
increases signicantly, which owed to the formation of network
and an increase in molecular weight. The increase extent of
complex viscosity has great relationship with the cross-linking
degree of PBDTSx, and PBDTS1.0 always shows higher
viscosity than the others.
0
As we know, the storage modulus G is connected with the
elasticity of microstructure, while the loss modulus G is related
ꢁ
1
ꢂ
0
0
s
when the testing temperature is xed at 130 C. Further
ꢁ1
enhancing the shear rate to 3000 s , the shear viscosity of
PBDTS1.0 and PBS are around 150 Pa s and 50 Pa s, respectively.
Even the testing temperature is increased to 140 C, PBDTS1.0
to the molecular mobility and molecular interactions. From
0
00
Fig. 8(b) and 9(a), it is clear that both G and G increases with
ꢂ
0
the increase of frequency, in which PBDTSx shows higher G
00
still remains higher shear viscosity than PBS. The high melt
viscosity will benet the blow moulding processing.
and G than the neat PBS.
This phenomenon means the PBDTSx have improved elastic
response, molecular interactions and reduced chain mobility,
which is due to the formation of cross-linking network caused Conclusions
by introducing DTS into its polymer chains.
As we expected, the functional monomer containing alkynyl
group can cross-link during the preparation of PBS, so a series
of PBS copolyesters (PBDTSx) having slightly cross-linking are
prepared successfully. PBDTSx show good solubility in chloro-
form and exhibit increased molecular weights when the molar
percentage of DTS units is less than 1.0%. Based on NMR
spectra, it is found that PBDTS1.0 has the highest cross-linking
density than the others. PBDTSx still remain the similar
thermal stability with the neat PBS. The cross-linking points in
the PBS chains act as crystal nucleus in the nucleation step, so
c
PBDTSx copolyesters have higher T and fast crystallization
rates, especially for PBDTS1.0 with high relative cross-linking
density. The elongation at break of PBDTSx decreases a little
and the Young's modulus improves compared to pure PBS.
With the introduction of DTS, the complex viscosity of PBDTSx
increases signicantly, and the elastic response is also
enhanced. The capillary rheological behaviours of PBDTSx
illustrate that the shear viscosity of PBDTS1.0 is always higher
than PBS at the whole shear region and the testing tempera-
tures. In this work, we provide a new feasible method to
improve the processability and properties of PBS by adjusting
its chemical chain structures during the preparation instead of
post cross-linking modication.
Conflict of interest
The authors declare no competing nancial interest.
Acknowledgements
This work was supported nancially by the National Science
Foundation of China (51273123 and 51121001), and the
Program for Changjiang Scholars and Innovative Research
Teams in University of China (IRT 1026).
Fig. 10 Capillary rheological shear viscosities vs. shear rate (a) at 130
C and (b) at 140 C for neat PBS and PBDTS1.0.
ꢂ
ꢂ
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2
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