Synthesis and properties of star-branched nylon 6 with hexafunctional cyclotriphosphazene core

Article abstract of DOI:10.1039/c5ra15598c

A novel star-branched nylon 6 with hexafunctional cyclotriphosphazene core was synthesized using hexa(4-carboxylphenoxy)cyclotriphosphazene (HCPCP) with six carboxyl groups as multifunctional agent in the hydrolytic ring-opening polymerization of ε-caprol

Full text of DOI:10.1039/c5ra15598c

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Synthesis and properties of star-branched nylon 6
with hexafunctional cyclotriphosphazene core†
Cite this: RSC Adv., 2015, 5, 88382
ab
a
b
b
b
Chunhua Wang, Feng Hu, Kejian Yang, Tianhui Hu, Wenzhi Wang,
b
b
a
Rusheng Deng, Qibin Jiang and Hailiang Zhang*
A novel star-branched nylon 6 with hexafunctional cyclotriphosphazene core was synthesized using
hexa(4-carboxylphenoxy)cyclotriphosphazene (HCPCP) with six carboxyl groups as multifunctional agent
in the hydrolytic ring-opening polymerization of 3-caprolactam, and then used for the investigation of
mechanical properties, crystallization and rheology behaviors. Star-branching structure and molecular
weight have great effects on its properties. Compared with linear nylon 6, star-branched nylon 6 has
lower relative viscosity and higher melt flow rate while its mechanical properties can be almost retained
by the use of star-branching and an appropriate molecular weight. Research on crystallization behavior
indicates that the degree of crystallinity (X
c
) of star-branched nylon 6 decreases slightly, but its crystal
structure still belongs to a form. The peak crystallization temperature (T
c
) and crystallization rate (1/t1/2
)
of star-branched nylon 6 are significantly higher than that of linear nylon 6 because of heterogeneous
nucleation induced by HCPCP core, which is beneficial for the use of rapid molding process. As the
c
molecular weight increases, the T and 1/t1/2 of star-branched nylon 6 first increase and then decrease.
Received 4th August 2015
Accepted 9th October 2015
Capillary rheometer measurement exhibits that the shear viscosity of star-branched nylon 6 decreases
with decreasing molecular weight, and the shear viscosity of star-branched nylon 6 with an appropriate
molecular weight shows a low value and little or no sensitivity to shear rate and temperature. Such
rheology behavior allows for processing at low temperature and low pressure and reduces system cost.
DOI: 10.1039/c5ra15598c
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molecular weight, especially lower viscosity indicating its
improved processability, which has been evidenced by Flory's
Introduction
7
Nylon, professionally named polyamide (PA), is a thermoplastics pioneering work on star-branched nylon 6.
polymer containing amide repeat units on the main chain, which
Recently, star-branched nylon has attracted increasing
serves as a very important engineering plastic because of excel- interests due to its special properties that set it apart from linear
1–6
lent properties and wide applications. With the rising demand nylon. For example, Fu et al. reported the synthesis of star-
for the precision and appearance of the part, the demand for the branched nylon 6 and nylon 12 using trimesic acid and cyclo-
processability of nylon material becomes higher. However, hexanonetetrapropionic acid as tri- and tetra-functional agents
traditional nylon is a linear polymer and possesses high viscosity and investigation of their viscosities, mechanical properties and
14–16
and poor melt owability, so that it is difficult to be molded into crystal morphologies.
Yuan et al. focused on the molecular
large part with thin wall, precision part or high ller content part. weights and molecular weight distributions of star-branched
Therefore, developing a new nylon with high owability has nylon 6s obtained by the polymerization of 6-aminocaproic
7–10
become a hotspot for research.
acid in the presence of trimesic acid, EDTA and cyclo-
It is well-known that star-branching decreases molecular hexanonetetrapropionic acid as multifunctional agents, and the
dimension of polymer and results in a decrease in viscosity and results showed that the molecular weights calculated were in
11–13
17,18
a rise in owability.
Theoretically, incorporation of star- good agreement with those measured by experiment.
Zhang
branched architecture into nylon molecule can result in some et al. prepared high-ow nylon 6 with star-branched structure
unusual properties compared to linear nylon with similar and dendritic polyamidoamine (PAMAM) core by in situ poly-
merization, and its melt ow rate was 70–90% higher than that
1
9–21
of linear nylon.
1 using the second generation polypropyleneimine dendrimer
PPI) as multi-functional agent, and investigated its isothermal
Wan et al. synthesized star-branched nylon
a
Key Laboratory of Polymeric Materials & Application Technology, Key Laboratory of
1
(
Advanced Functional Polymer Materials of Colleges of Hunan Province, College of
Chemistry, Xiangtan University, Xiangtan 411105, China. E-mail: zhl1965@xtu.edu.
cn
22,23
and nonisothermal crystallization kinetics.
b
Zhuzhou Times New Material Technology Co. Ltd., Zhuzhou 412007, China
Although many studies have been devoted to the synthesis,
mechanical property, crystallization behavior, relative viscosity
†
Electronic supplementary information (ESI) available: HCPCP characterizations
and TGA curves of star-branched nylon 6. See DOI: 10.1039/c5ra15598c
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and melt ow rate of star-branched nylon, little attention has Synthesis of hexa(4-formylphenoxy)cyclotriphosphazene
been paid to the systematic investigation of rheological (HAPCP)
behavior for star-branched nylon. Research on this aspect is
2 3
4-Hydroxy benzaldehyde (125 g, 1.02 mol), anhydrous K CO (140
very important because rheological behavior is one of the most
important properties for polymer processing and application.
Moreover, the multifunctional agents that have been reported
and used to synthesize star-branched nylons are limited. Some
multifunctional agent reported such as PAMAM is subjected to
thermally decompose or undergo a number of side reactions
g, 1.02 mol) and THF (500 mL) were added into a 1 L dried round-
bottom ask and stirred at room temperature. Hexa-
chlorocyclotriphosphazene (50 g, 0.14 mol) was dissolved in THF
(
100 mL) and added dropwise into this ask over 1 h. The reac-
ꢁ
tion mixture was heated to 65 C and then reuxed for 24 h. The
solution was concentrated by rotary evaporation and subse-
quently precipitated in distilled water. The crude product was
washed twice by distilled water. Weight: 114.8 g; yield: 92.8%.
ꢁ
24
even at a relatively low temperature (ꢀ120 C). For this reason,
the corruption and disassociation of PAMAM were doomed to
occur when 3-caprolactam and PAMAM were polycondensated
1
The synthetic route of HAPCP is shown in Scheme 1. H-NMR
ꢁ
at a high temperature (220–260 C) for a rather long time (>3 h).
(
DMSO-d6), d (ppm): 9.90 (s, 6H, –CHO); 7.76–7.78 (d, 12H, Ar-
Undoubtedly, the thermal stability of multifunctional agent is
extremely critical for the targeting star-branched nylon to
maintain structural integrity during high temperature melting
polycondensation process. It is inspiring to nd that multi-
functional cyclotriphosphazene such as HCPCP can well satisfy
this demand because of its excellent thermal stability caused by
rigid benzene ring and nitrogen and phosphorus six-member
31
H), 7.15–7.17 (d, 12H, Ar-H). P-NMR (DMSO-d6), d (ppm): 8.34
(
+
s, 3P, –N]P). Mass Spectrometry (MS) (m/z): [M + H] calcd for
C H N O P , 862.108; found 862.149.
4
2
31 3 12 3
Synthesis of hexa(4-carboxylphenoxy)cyclotriphosphazene
HCPCP)
(
ring structure. Furthermore, HCPCP has simple preparation HAPCP (50 g, 0.06 mol), NaOH (16 g, 0.40 mol), distilled water
method, easily controlled process, high yield and low cost. (300 mL) and THF (300 mL) were added into a 1 L beaker and
However, to our knowledge, few reports focus on star-branched stirred at room temperature. KMnO
4
(120 g, 0.76 mol) was
nylon with multifunctional cyclotriphosphazene core.
divided into several parts and added into this beaker in batches
In this study, we rst chose hexafunctional cyclo- until purple red did not fade. Aer removing the solvent THF,
triphosphazene HCPCP as multifunctional agent to synthesize vacuum ltration and acidication, the white solid product,
a series of star-branched nylon 6 with different molecular weights HCPCP was obtained. Weight: 53.7 g; yield: 96.7%. The
1
by the hydrolytic ring-opening polymerization of 3-caprolactam, synthetic route of HCPCP is shown in Scheme 1. H-NMR
and then systematically investigated its mechanical properties, (DMSO-d6), d (ppm): 12.99 (s, 6H, –COOH); 7.80–7.82 (d, 12H,
3
1
thermal properties, crystallization behaviors, owabilities, and in Ar–H), 6.96–6.98 (d, 12H, Ar–H). P-NMR (DMSO-d6), d (ppm):
+
particular rheological behaviors. Our results will demonstrate 8.79 (s, 3P, –N]P). Mass Spectrometry (MS) (m/z): [M + H] calcd
31 3 18 3
how star-branching and molecular weight inuence its proper- for C42H N O P 958.077, found 958.257.
ties by comparing with linear nylon 6.
Synthesis of star-branched nylon 6
Star-branched nylon 6 was synthesized with reference to the
1
general method for conventional nylon 6, but a multifunctional
agent HCPCP with six carboxyl groups was added into this reac-
tion system. A typical procedure is summarized as follows. 3-
Caprolactam, HCPCP and distilled water were added into a 5 L
Experimental
Materials
Hexachlorocyclotriphosphazene (HCCP, 98%) was purchased autoclave according to the feed compositions as shown in
from Lanyin Chemical Co., Ltd. 4-Hydroxy benzaldehyde (98%) Table 1. The mixture in the autoclave was deoxygenated by
was purchased from Aladdin. 3-Caprolactam (99%) was repeatedly pumping vacuum and purging with high purity N
provided by Yueyang Chemical Fiber Co., Ltd. All reagents were three times. The reaction system was heated to 250 C with stir-
2
ꢁ
used as received from commercial sources.
ring. The inner pressure reached 0.3–0.8 MPa due to the
Scheme 1 Synthetic route of HAPCP and HCPCP.
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Table 1 Feed compositions of star-branched and linear nylon 6
Aer titration, the number-average molecular weights (Mn,EDT) of
the samples were calculated according to the reported equation
based on Flory model and hypothesis.
Sample
Caprolactam (g)
HCPCP (g)
Water (g)
14–16
All the samples for mechanical properties tests were prepared
by injection molding. Tensile strength and elongation at break
were measured according to GB/T 1040.2-2006. Bending strength
and bending modulus were measured according to GB/T 9341-
SPA-1
SPA-2
SPA-3
SPA-4
SPA-5
LPA
2000
2000
2000
2000
2000
2000
94.1
70.6
56.5
37.6
28.2
0
60
60
60
60
60
60
2008. The charpy impact notched strength was measured
according to GB/T 1043.1-2008. The moisture contents of the
samples were regulated under standard conditions.
Thermal gravimetric analysis (TGA) was carried out on
a Mettler Toledo TGA/DSC1 1100SF instrument at a heating rate
spontaneous pressure generated by the water vapor from the
reaction system in the autoclave. Aer heating for 3 h, the inner
pressure was gradually reduced to atmospheric pressure about
ꢁ
ꢂ1
of 10 C min
2
under N . The crystallization and melting
behaviors of the samples were detected using DSC 204 F1
differential scanning calorimeter manufactured by Netzsch
Company. The temperature and heat ow were calibrated using
standard materials (indium and zinc). The scans were per-
0
.5–1 h. The reaction was then allowed to proceed for a further 2
ꢁ
h at 250 C under gradually reduced pressure up to ꢂ0.07 MPa.
Subsequently, the agitator was stopped and the autoclave was
ꢁ
ꢂ1
formed under N at the heating and cooling rate of 10 C min
.
2
lled with N . The product was drawn into long strand in water
2
The samples with typical mass of 3–10 mg were encapsulated in
sealed aluminum pans. The crystal structures of the samples
were measured by wide-angle X-ray diffraction (WXRD). The
WAXD patterns of melt-crystallized sample lms were recorded
on a Philips X' Pert Pro diffractometer with a 3 kW ceramic tube
as the X-ray source (Cu Ka) and an X' celerator detector in the 2q
bath and pelletized. These particles were extracted in boiled
water for 24 h and then dried under vacuum at 80 C for 12 h. Five
star-branched nylon 6 samples named SPA-1 to SPA-5 were
prepared using different concentrations of HCPCP as hexafunc-
tional agent. Linear nylon 6 (LPA) was also prepared using the
same preparation process for comparison.
ꢁ
ꢁ
ꢁ
range from 3 to 40 at a scanning step of 0.02 .
The relative viscosities of the samples were determined by an
Instrumentation
ꢂ1
Ubbelohde viscometer using 0.01 g mL
solution in 98%
ꢁ
FTIR spectra were measured with a Nicolet IS 10 Fourier trans- sulfuric acid in 25 C water bath. The melt ow rates (MFR) in g
form infrared spectrometer by the attenuated total reection per 10 min of the samples were measured by a melt ow rate
ꢁ
(
ATR) technology, except HCPCP which was measured by the test tester (ZRZ1452) made from MTS Company at 235 C and 0.325
1
31
mode of KBr pellet. H-NMR and P-NMR spectra were recorded kg load, according to GB/T3682-2000. Rheological behaviors of
on a Bruker ARX400 spectrometer at room temperature using the samples in melt were investigated by capillary rheometer
deuterated dimethyl sulfoxide (DMSO-d6) or deuterated tri- (RH7-D) made from Malvern Company with shear rates ranging
ꢂ1

uoroacetic acid (CF
3
CO
2
D) as the solvent and tetramethylsilane from 50 to 4000 s . The capillary diameter (D) was 1.0 mm, its
(
TMS) as the internal standard. GPC measurements were per- length (L) was 16.0 mm, and the L/D was therefore 16.0. The
formed on Agilent 1200 setup equipped with a column set con- experiments were carried out at 230, 240, 250 and 260 C.
ꢁ
sisting of two PL gel
5
mm MIXED-C columns using
ꢁ
trichloromethane as the eluent at 30 C at a ow rate of 1.0 mL
ꢂ1
min . Narrowly distributed polystyrene standards were utilized
Results and discussion
Synthesis of HCPCP
for calibration. The samples were dissolved in the mixed solvent
of 10% metacresol and 90% trichloromethane before GPC anal-
ysis. The weight-average molecular weights (Mw,SLS) of the samples As shown in Scheme 1, a hexafunctional agent HCPCP was
were assessed by static light scattering (SLS) measurements on successfully synthesized via two-step reaction with high yield and
a Brookhaven instrument equipped with a 100 mW solid-state low cost. First, HAPCP was synthesized by a nucleophilic substi-
laser emitting at 532 nm, a BI-200SM goniometer and a BI-9000 tution reaction between hexachlorocyclotriphosphazene and 4-
ꢁ
ꢁ
digital correlator scanning from 30 to 120 via Zimm extrapola- hydroxy benzaldehyde using anhydrous K CO as the deacid
tion method.
2
3
25,26
BI-DNDC differential refractometer was used to reagent. Then, HAPCP was oxidized by KMnO to form HCPCP.
4
determine the dn/dc of the samples solutions in 90% formic acid The chemical structure of HCPCP was conrmed by FTIR
ꢂ1
1
31
containing 1.0 mol L potassium chloride. Quantitative deter- (Fig. S1†), H-NMR (Fig. 1), P-NMR (Fig. S2†) and MS spec-
minations of the end groups of –COOH and –NH in the samples trometry (Fig. S3†). Before synthesizing star-branched nylon 6, the
2
were carried out by titration. One gram of each sample was dis- thermal stability of HCPCP was examined by TGA at a heating rate
ꢁ
ꢂ1
ꢁ
ꢂ1
ꢁ
solved in 50 mL benzyl alcohol at 150 C under N
2
. The resulting of 10 C min from room temperature to 800 C under N
2
. The
solution was titrated by 0.02 mol L KOH solution in methyl weight loss curves of HCPCP are shown in Fig. 2. The results
alcohol with phenolphthalein as indicator to determine the indicate that HCPCP has excellent thermal stability with the initial
ꢁ
concentration of –COOH. Similarly, the concentration of –NH
thermal decomposition temperature above 300 C. Therefore,
2
ꢂ1
was titrated by 0.01 mol L HCl solution with bromophenol blue HCPCP can be used as a multifunctional agent during high-
as indicator. The blank titration was carried out at the same time. temperature melting polycondensation process of nylon 6.
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1
Fig. 1 H-NMR spectra of HAPCP and HCPCP in DMSO-d6.
Fig. 3 FTIR spectra of star-branched and linear nylon 6.
Synthesis of star-branched nylon 6
Five star-branched nylon 6 samples with different molecular
weights were synthesized using different concentrations of
HCPCP as hexafunctional agent in the hydrolytic ring-opening
polymerization of 3-caprolactam. The feed compositions of all
the samples are listed in Table 1. The chemical structure of star-
branched nylon 6 with HCPCP core had been veried by FTIR
1
and H-NMR. FTIR spectra of star-branched and linear nylon 6
are shown in Fig. 3. The star-branched nylon 6 exhibits almost
the same spectral features as linear nylon 6, such as N–H
ꢂ1
ꢂ1
stretching at 3300 cm , C–H stretching 2935 and 2861 cm
,
ꢂ1
amide C]O stretching at 1640 cm , and N–H and C–N
ꢂ1
stretching at 1542 cm . But under careful observation, the
characteristic absorption band of P–O–C of star-branched nylon
ꢂ1
6
appears at 960 cm , and the intensity of this band increases
with increasing concentration of HCPCP in the feed composi-
1
1
tion. H-NMR spectra of star-branched nylon 6 (SPA-3 as Fig. 4 H-NMR spectra of star-branched and linear nylon 6 in
a typical example) and linear nylon 6 in CF CO D are shown in CF CO D.
3
2
3
2
Fig. 4. Compared with linear nylon 6, the characteristic
resonance peaks of the phenyl of star-branched nylon 6 are
presented at 7.80–7.82 and 6.96–6.98 ppm (denoted e and f,
respectively). Therefore, these data suggest the existence of star-
branched structure.
Molecular weight characterization data of star-branched and
linear nylon 6 obtained by end group titration, SLS and GPC are
listed in Table 2. As shown in Table 2, the molecular weight
obtained by GPC is obviously lower than that obtained by end
group titration and SLS due to the smaller hydrodynamic
volumes of star-branched nylon 6 in solution. However, all the
results indicate that the molecular weight of star-branched
nylon 6 mainly depends on the concentration of HCPCP, and
the molecular weight increases with decreasing concentration
of HCPCP, which are consistent with the results of previous
7,17,18
work.
As the concentration of HCPCP decreases to some
extent, molecular weight of star-branched nylon 6 can approach
and even exceed that of linear nylon 6.
ꢁ
ꢂ1
2
Fig. 2 TGA curves of HCPCP at heating rate of 10 C min under N .
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Table 2 Molecular weight characterization data of star-branched and linear nylon 6 obtained by end group titration, SLS and GPC
Paper
a
5
ꢂ1
a
5
ꢂ1
n,EDT ꢃ 10ꢂ4
a
w,SLS ꢃ 10ꢂ4
b
n,GPC ꢃ 10ꢂ4
c
c
Sample
[–COOH] ꢃ 10 mol g
[–NH
2
] ꢃ 10 mol g
M
M
M
PDI
SPA-1
SPA-2
SPA-3
SPA-4
SPA-5
LPA
33.75
26.83
18.76
12.92
7.33
3.42
2.91
2.84
2.48
2.52
5.47
1.18
1.45
1.82
2.37
3.01
1.86
2.47
3.05
3.72
4.51
5.64
3.98
0.97
1.18
1.36
1.58
1.82
1.67
1.98
2.03
1.87
2.10
2.28
2.16
5.29
a
The [–COOH] and [–NH
2
] values were determined by end group titration, and Mn,EDT ¼ bm/{[–COOH] + (b ꢂ 1)[–NH
2
]}, where m is the sample
b
weight for titration and b is the functionality of multifunctional agent. The Mw,SLS was obtained by SLS measurement and Zimm extrapolation
method. The Mn,GPC and PDI were measured by GPC and calibrated by narrowly distributed polystyrene standards.
c
Mechanical property
decreasing molecular weight, due to the more compact size and
the inability to undergo as much reorientation and molecular
The mechanical properties of injection molded samples of star-
branched and linear nylon 6 are shown in Fig. 5. Compared
with linear nylon 6, the tensile strength, bending strength,
bending modulus and impact strength of star-branched nylon 6
with high molecular weight do not markedly decrease. However,
these mechanical properties decrease dramatically when star-
branched nylon 6 has low molecular weight, such as SPA-1. The
poor mechanical properties of SPA-1 are mainly attributed to its
low molecular weight and short arms lengths. Furthermore, the
elongation at break of star-branched nylon 6 decreases with
8
“slippage”. When the molecular weight increases to some extent,
the arms lengths become long enough, the mechanical property
of star-branched nylon 6 remains at a certain level, approaching
that of linear nylon 6.
Thermal property and crystallization behavior
The thermal stabilities of star-branched and linear nylon 6 were
examined by TGA (Fig. S4†). The results shown in Table 3
indicate that star-branched and linear nylon 6 both have
excellent thermal stabilities with the initial thermal decompo-
ꢁ
sition temperatures (T
d
) above 380 C. The crystal structures of
star-branched and linear nylon 6 were investigated by WAXD.
Fig. 6 shows the WAXD patterns of star-branched and linear
ꢁ
nylon 6 aer melt-crystallization at 185 C. The diffraction
peaks of star-branched nylon 6 (SPA-3) locate at 2q ¼ 20.2 and
ꢁ
2
3.4 indexed as 200 and 002/202 reections, and the crystal
structure belongs to a form. It is the same as those of linear
nylon 6, indicating that star-branched nylon 6 has the same
crystal structure as that of linear nylon 6. Similar results were
found for other star-branched nylon 6 samples with different
molecular weights as well. Therefore, it can be concluded that
star-branching does not change the crystal structure, and
HCPCP core is excluded from the crystalline region and located
in the amorphous region.
The melting and crystallization behaviors of star-branched
and linear nylon 6 were researched by DSC. The DSC thermal
diagrams of star-branched and linear nylon 6 during the rst
ꢁ
ꢂ1
cooling and subsequent heating process at a rate of 10 C min
are shown in Fig. 7 and the results are listed in Table 3. Note
that the melting point (T ) of star-branched nylon 6 decreases
m
with decreasing molecular weight. The decrease in T
ascribed to the rising branched degree and the decreasing
hydrogen bond interaction. The degree of crystallinity (X ) was
m
is
c
determined from DSC analysis with the aid of the fusion
ꢂ1
enthalpy of 230.1 J g for the perfectly crystalline nylon 6. As
c
shown in Table 3, the X of star-branched nylon 6 is somewhat
lower than that of linear nylon 6, since HCPCP core and its
adjacent chains are suspected to be unable to crystallize.
Scheme 2 shows the schematic illustration of crystallizability
and crystallization rate of chains resided at different regions
Fig. 5 Mechanical properties of star-branched and linear nylon 6.
Error bars represent the standard deviations.
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Table 3 Characteristic parameters for thermal property and melt crystallization of star-branched and linear nylon 6
a
d
ꢁ
b
m
ꢁ
c
c
ꢁ
b
m
ꢂ1
d
c
e
ꢂ1
Sample
T
( C)
T
( C)
T
( C)
DH
(J g
)
X
(%)
1/t1/2 (min
)
SPA-1
SPA-2
SPA-3
SPA-4
SPA-5
LPA
387.4
388.9
390.5
391.7
392.1
392.6
217.6
218.4
219.7
219.9
220.0
222.1
173.8
175.6
178.3
182.5
180.1
173.5
48.6
49.5
51.5
53.2
52.7
53.8
21.1
21.5
22.4
23.1
22.9
23.4
0.77
0.86
1.01
1.38
1.23
0.64
a
ꢁ
ꢂ1
b
The initial thermal decomposition temperature T
d
was measured by TGA at a rate of 10 C min under N
2
.
m
The melting point T and the fusion
c
enthalpy DH
cooling process.
crystallization time when the relative degree of crystallinity (X
m
were obtained by DSC during the second heating process. The crystallization temperature T was obtained by DSC during the
c
d 0 0 e
ꢂ1
X
c
¼ DH /DH , DH
m m m
¼ 230.1 J g
.
1/t1/2 denotes the crystallization rate and is the reciprocal value of the half-
t
) reaches 50%.
branched nylon 6 is easy to nucleate and then crystallize at
a higher temperature than linear nylon 6 under the same cooling
rate. A similar heterogeneous nucleation phenomenon was also
28
reported in star-branched poly(3-caprolactone).
Another important parameter for crystallization behavior is
the half-crystallization time (t1/2), which is the time when
Fig. 6 WAXD patterns of star-branched and linear nylon 6 after melt-
ꢁ
crystallization at 185 C.
depending on the distance to HCPCP core of star-branched
nylon 6. The chains of star-branched nylon 6 can crystallize if
27
the free energy change DG is less than zero. As the chains are
approaching HCPCP core, the crystallization becomes ever
harder because of star-branching which restricts the chains
folding and increases the free energy to some extent, and
meanwhile the crystallization rate becomes ever slower as
a result of the chain motion increasingly inhibited by HCPCP
c
core. The X of star-branched nylon 6 increases with increasing
molecular weight. It is because star-branched nylon 6 with
higher molecular weight has less HCPCP core and uncrys-
tallizable chain and therefore is able to form more perfect
crystal. With the further increase of the molecular weight, the X
c
increases slowly and even reduces slightly due to the more
entanglements between the longer arms such as SPA-5.
As shown in Fig. 7, the peak crystallization temperature (T ) of
c
star-branched nylon 6 is higher than that of linear nylon 6. This
result indicates that central HCPCP core can help star-branched
nylon 6 to change the manner of nucleation from homogeneous
nucleation to heterogeneous nucleation. In other words, HCPCP
core plays a more important role than temperature in nucleation Fig. 7 DSC thermal diagrams of star-branched and linear nylon 6
and crystallization of star-branched nylon 6. Accordingly, star- during the first cooling (a) and the subsequent heating (b) at a rate of 10
ꢁ
ꢂ1
C min
.
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SPA-4 with relatively low molecular weights and high concen-
trations of HCPCP cores, their crystallization rates depend
mainly on crystal growth rates of crystallizable chains. From
SPA-1 to SPA-4, the molecular weight increases, the length of
crystallizable chain becomes longer, the restriction of HCPCP
core on the motion of crystallizable chain decreases, crystal
growth rate increases, and thus crystallization rate increases.
For SPA-5 with relatively high molecular weight and low
concentrations of HCPCP cores, nucleation rate has greater
effect on crystallization rate, and nucleation rate decreases
obviously due to low concentration of HCPCP core, so its crys-
tallization rate decreases. If the molecular weight continues to
increase, we can infer by theoretically that the crystallization
rate of star-branched nylon 6 will further decrease and approach
that of linear nylon 6. Molecular weight effect on crystallization
behavior of star-branched nylon 6 can be also observed from the
c c
T values listed in Table 3. The effect of molecular weight on T
is similar to that of 1/t1/2. For star-branched nylon 6, the fast
crystallization rate will contribute to the use of rapid molding
process.
Scheme 2 Schematic illustration of crystallizability and crystallization
rate of chains of star-branched nylon 6.
Flow property and rheological behavior
relative crystallinity (X
t
) reaches 50%. Its reciprocal value, 1/t1/2,
is oen used to represent crystallization rate. Fig. 8 shows the X
t
Usually, relative viscosity (RV) and melt ow rate (MFR) are used
in the course of nonisothermal crystallization for star-branched to rapidly evaluate the ow property of thermoplastic polymer.
and linear nylon 6 as a function of crystallization time at Fig. 9 shows the results of RV and MFR of star-branched and
ꢁ
ꢂ1
a cooling rate of 10 C min . All curves present the similar S- linear nylon 6. As shown in Fig. 9, the RV of star-branched nylon
shape, indicating that cooling rate has retardation effect on 6 decreases markedly with decreasing molecular weight, and
the crystallization. What's more, the curve of star-branched the RV of star-branched nylon 6 is signicantly lower than that
nylon 6 shis obviously to the shorter crystallization time of linear nylon 6 except SPA-5. This decrease in RV of star-
region in comparison with linear nylon 6, indicating the faster branched nylon 6 is attributed to the smaller hydrodynamic
crystallization rate. The dependence of crystallization rate on volume and less entanglement between arms, which was also
7,8,14–19
the molecular weigh of star-branched nylon 6 is shown in the reported by previous researchers.
The RV of SPA-5 is
inset of Fig. 8. With the increasing molecular weight, the 1/t1/2 slightly higher than that of linear nylon 6, because of its longer
of star-branched nylon 6 rst increases and then decreases. This arms lengths and more entanglements between the longer
may result from the opposite effect of HCPCP core concentra- arms. Similar ow property is also conrmed by the result of
tion on nucleation rate and crystal growth rate. For SPA-1 to MFR. When the molecular weight decreases, the MFR of star-
branched nylon 6 dramatically increases. The MFR of SPA-1 is
about six times higher than that of linear nylon 6, and the MFR
of SPA-5 is slightly lower than that of linear nylon 6. All of these
t
Fig. 8 Relative crystallinity (X ) of star-branched and linear nylon 6 as
a function of crystallization time. The inset shows the dependence of
crystallization rate 1/t1/2 on the molecular weigh of star-branched Fig. 9 Relative viscosity and melt flow rate of star-branched and linear
nylon 6.
nylon 6. Error bars represent the standard deviations.
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Fig. 10 Rheological curves of star-branched and linear nylon 6 at 230
C.
Fig. 11 Shear rate dependence of shear viscosity of star-branched and
linear nylon 6 at 230 C. The inset shows the dependence of shear
ꢁ
ꢁ
viscosity on the molecular weigh of star-branched nylon 6.
results indicate that star-branched nylon 6 with relatively low
molecular weight has better ow property than linear nylon 6,
and star-branching has a stronger effect on melt viscosity than
dilute solution viscosity.
shear viscosity of star-branched nylon 6 with relatively low
molecular weight is not sensitive to shear stress and shear rate.
Moreover, the n value of star-branched and linear nylon 6
increases with the rise of temperature. This result demonstrates
that the sensitivity of shear viscosity to shear stress and shear
rate weakens with the rising of temperature.
To systematically investigate rheological behaviors of star-
branched and linear nylon 6, capillary rheometry was applied.
Fig. 10 shows the rheological curves of star-branched and linear
ꢁ
nylon 6 at 230 C. As shown in Fig. 10, shear stress increases
Fig. 11 shows the dependence of shear viscosity of star-
with the increasing shear rate, but the relationship between
shear stress and shear rate is nonlinear. Therefore, the melts of
star-branched and linear nylon 6 exhibit non-Newtonian rheo-
logical behaviors.
Non-Newtonian index (n) is usually used to represent the
degree of deviation from Newtonian uid. The linear least
square method is applied to t lg s vs. lg g curve, and the slope
of the tting straight line is the n value. Table 4 lists the n values
of star-branched and linear nylon 6 at different temperatures.
All the n values are less than 1.0, which further indicates the
melts of star-branched and linear nylon 6 are pseudoplastic
ꢁ
branched and linear nylon 6 on shear rate at 230 C. The
shear viscosity of star-branched and linear nylon 6 reduces with
increasing shear rate, exhibiting a shear thinning behavior of
pseudoplastic uid. This is because molecular chains orient
along the ow direction and disentangle at high shear rate, and
the relative motions between molecules become easier.
Compared with linear nylon 6, the shear viscosity of star-
branched nylon 6 with relatively low molecular weight shows
a low value and little or no sensitivity to shear rate, which agree
well with the result of n. The dependence of shear viscosity on
the molecular weigh of star-branched nylon 6 is shown in the
inset of Fig. 11. The shear viscosity of star-branched nylon 6
increases with increasing molecular weight, due to the longer
arms lengths and the more entanglements between the longer
arms. When the molecular weight increases to some extent, the
shear viscosity of star-branched nylon 6 can reach and even
exceed that of linear nylon 6. This result is consistent with the
aforementioned result of relative viscosity.

uids. The value of n essentially reects the sensitivity of shear
viscosity to shear stress and shear rate. At the same tempera-
ture, the n value of star-branched nylon 6 reduces with
increasing molecular weight, which means the degree of devi-
ation from Newtonian uid increases accordingly, due to the
more entanglements between the longer arms. Compared with
linear nylon 6, the n value of star-branched nylon 6 with rela-
tively low molecular weight is higher and approaches 1.0, so the
Fig. 12 shows the temperature dependence of shear viscosity
ꢂ1
of star-branched and linear nylon 6 with shear rate at 1338 s
.
The shear viscosity reduces with increasing temperature. As the
temperature increases, the movement of chain segment
increases, the free volume of the melt increases, intermolecular
interaction weakens, and thus shear viscosity reduces. Flow
Table 4 Non-Newtonian indexes of star-branched and linear nylon 6
at different temperatures
Non-Newtonian index (n)
activation energy (DE ) reects the sensitivity of shear viscosity
to temperature. At the constant shear rate, the relationship
between shear viscosity and temperature follows Arrhenius
h
ꢁ
T ( C)
SPA-1
SPA-2
SPA-3
SPA-4
SPA-5
LPA
0.63
2
2
2
2
30
40
50
60
0.92
0.95
0.97
0.98
0.90
0.93
0.95
0.97
0.83
0.87
0.90
0.92
0.70
0.76
0.81
0.85
0.61
0.67
0.76
0.80
0.68 equation, so the DE
0.75 vs. T straight line. The DE
h
value can be calculated by the slope of lg h
ꢂ1
h
values of star-branched and linear
0.78
nylon 6 at different shear rates are listed in Table 5. As shown in
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GPC, and the molecular weight reduces when the concentration
of HCPCP increases. Compared with linear nylon 6, the viscosity
of star-branched nylon 6 signicantly decreases, but its
mechanical properties are almost retained by the use of star-
branching and an appropriate molecular weight. The crystal
structure of star-branched nylon 6 was researched by WAXD.
Star-branching does not change the crystal structure and the
crystal structure of star-branched nylon 6 still belongs to a form.
The crystallization and melting behaviors of star-branched
nylon 6 were investigated by DSC. The melting point (T ) and
m
the degree of crystallinity (X ) of star-branched nylon 6 decrease
c
slightly due to less perfect crystal caused by branching.
However, the peak crystallization temperature (T ) and the
c
crystallization rate (1/t1/2) of star-branched nylon 6 are obviously
higher than that of linear nylon 6 on account of heterogeneous
c
nucleation induced by HCPCP core. Moreover, the T and 1/t1/2
Fig. 12 Temperature dependence of shear viscosity of star-branched
ꢂ1
and linear nylon 6 with shear rate at 1338 s
.
of star-branched nylon 6 rst increase and then decrease with
increasing molecular weight resulting from the opposite effect
of HCPCP core concentration on nucleation rate and crystal
Table 5 Flow activation energies of star-branched and linear nylon 6 growth rate. The rheological behavior was investigated by
at different shear rates
capillary rheometer. As the molecular weight decreases, the
shear viscosity of star-branched nylon 6 decreases. The shear
viscosity of star-branched nylon 6 with appropriate molecular
weight shows a low value and little or no sensitivity to shear rate
and temperature. Such star-branched nylon 6 as an easy pro-
cessing nylon resin offers higher owability, faster crystalliza-
tion rate and lower processing temperature and pressure with
no signicant effect on mechanical properties compared to
traditional linear nylon 6, and the prospect of application will
be bright. Further studies on this new star-branched nylon 6
with HCPCP core are in progress, such as ame retardant
property, industrial research, functional application and so on.
ꢂ1
h
DE (kJ mol )
ꢂ1
g (s
)
SPA-1
SPA-2
SPA-3
SPA-4
SPA-5
LPA
2
1
2
4
59
58.03
53.22
50.56
48.14
64.29
58.45
54.76
52.03
68.32
64.07
61.28
58.65
73.52
68.28
65.16
62.39
75.65
71.80
67.26
64.42
73.74
70.25
67.76
65.18
338
314
000
Table 5, with the increase of shear rate, the DE value decreases,
that is to say the sensitivity of shear viscosity to temperature
h
weakens. The increase of shear rate is benet to disentangle-
ment and the number of entanglement points decreases Acknowledgements
accordingly, so the DE
h
value decreases. At the same shear rate,
This research was nancially supported by the National Science
technology Support Plan Project of China (2013BAE02B03)
and the Special Fund for the development of Strategic Emerging
Industry of China.
the DE value of star-branched nylon 6 increases with increasing
h
&
molecular weight, due to the more entanglement between long
arms. This phenomenon can be also observed in Fig. 12.
Compared with linear nylon 6, star-branched nylon 6 with
relatively low molecular weight shows a lower DE value, so its
h
shear viscosity is not sensitive to temperature as well. Such Notes and references
rheological behavior allows star-branched nylon 6 with appro-
priate molecular weight to process at low temperature and low
pressure and reduces system cost. When the molecular weight
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