Architectural effects of poly(∈-caprolactone)s on the crystallization kinetics

Article abstract of DOI:10.1021/ma035387f

The architectural effects of poly(ε-caprolactone)s on the crystallization kinetics were examined. The nonisothermal crystallization exotherms of HPCLs and LPCL were measured by DSC and analysed by Avrami method. Steady shear melt viscosity measurements we

Full text of DOI:10.1021/ma035387f

Macromolecules 2004, 37, 3745-3754
3745
Architectural Effects of Poly(ꢀ-caprolactone)s on the Crystallization
Kinetics
†
,‡
J eon gsoo Ch oi a n d Seu n g-Yeop Kw a k *
Research Institute of Advanced Materials (RIAM), and School of Materials Science and Engineering,
Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-744, Korea, and
Hyperstructured Organic Materials Research Center (HOMRC), and School of Materials Science and
Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-744, Korea
Received September 17, 2003; Revised Manuscript Received February 12, 2004
ABSTRACT: A series of hyperbranched poly(ꢀ-caprolactone)s (HPCLs) with molecular architectural
variation, which refers to the different lengths of the linear backbone segments consisting of 5, 10, and
2
0 ꢀ-caprolactone monomer units (thereby referred to as HPCL-5, HPCL-10, and HPCL-20, respectively)
and the different numbers of branching points, were synthesized without significant variation in the
molecular weights, and linear poly(ꢀ-caprolactone) (LPCL) of the same chemical structure and similar
molecular weight was used as the linear counterpart to HPCLs for comparison. The specific molecular
1
architectures of these samples were characterized by H NMR end-group analyses. The nonisothermal
crystallization exotherms of HPCLs and LPCL were measured by DSC and further analyzed by the
modified Avrami method. Regardless of the samples, the Avrami exponents ranged from 2.4 to 3.3,
indicating that the nucleation and growth mechanisms of the samples are apparently identical. On the
other hand, the crystallization kinetics was found to be fairly affected by the molecular architectures. All
the kinetic parameters estimated from DSC exotherms such as the initial slope, S
Avrami analyses such as the corrected rate constant, K , and crystallization half-time, t1/2, indicated that
i
, and from the modified
c
HPCLs with longer linear segments and fewer branches showed the faster crystallization, whereas LPCL
exhibited intermediate crystallization rate between HPCL-10 and HPCL-20, i.e., HPCL-5 < HPCL-10 <
LPCL < HPCL-20. The slower crystallization was linked with the frequent presence of heterogeneous
branching points, hindering the regular chain packing, in the backbone of HPCLs with shorter linear
segments. In addition, the faster crystallization of HPCL-20 compared with LPCL was attributed to the
higher cooperative chain mobility in their melt state as evaluated by activation energy of flow. A number
of spherulites developed during isothermal process from the melt were observed for all samples by POM,
and the radial growth rates of these spherulites were evaluated to correspond well with the results of S
, and t1/2 from the nonisothermal crystallization analyses.
i
,
K
c
1
. In tr od u ction
architectures involving a large number of building
1
2-17
blocks,
it is difficult to tailor-make properties of
Macromolecules possessing complex molecular archi-
HBPs since only a few parameters can be adjusted and/
or modified.
tectures have received increasing attention during the
past decade. This interest is governed by the realization
that new and/or improved material properties can be
obtained by altering the architecture of the polymers,
which further enables development of tailor-made ma-
terials and to apply them to specific application where
high performance and/or novel functionality are de-
manded. One example in this context is the dendritic
macromolecules, which are mainly classified into den-
drimers with perfectly branched structure and hyper-
branched polymers (HBPs) with statistically branched
Recently, several approaches to the development of
HBPs or their derivatives with large versatilities through
the preparation of block copolymers or through the
introduction of controlled branching have been reported.
Among these approaches, the synthesis of hyper-
branched poly(ꢀ-caprolactone) (HPCL) is quite note-
worthy because this novel synthetic approach allows the
production of a homologous series of HPCLs with a
range of molecular architectural variation such as
different lengths of linear backbone segments and
1
,2
structure. Compared with dendrimers obtained from
1
8,19
multistep syntheses,3,4 HBPs are more advantageous for
different numbers of branching points.
However, to
our knowledge, there is no detailed study on the
correlation between these specific molecular architec-
tures of HPCLs and the resulting material properties
such as the crystallization behaviors.
The crystallization behaviors of polymeric materials
have been typically studied through both the isothermal
and the nonisothermal crystallization measurements.20
Numerous studies have been reported concerning the
different effects observed during the isothermal crystal-
lization since the theoretical analysis of the isothermal
crystallization data is relatively simple. In contrast,
there are a comparatively limited number of publica-
tions dealing directly with a quantitative evaluation of
the kinetic parameters for the nonisothermal crystal-
practical purposes since they can be easily synthesized
through one-step polymerization of ABx (x g 2) mono-
mers.5,6 Because of their highly functionalyzed globular
architectures, HBPs exhibit different properties from
those of their linear counterpart of the same molecular
weight, such as less entanglement in the solid state,
1
7
-9
high solubility in various solvents, low melt viscosity,
and fast molecular motion.10,11 Although this attractive
feature has led to the development of many different
†
Research Institute of Advanced Materials.
Hyperstructured Organic Materials Research Center.
‡
*
To whom correspondence should be addressed: e-mail
sykwak@snu.ac.kr.
1
0.1021/ma035387f CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/15/2004

3
746 Choi et al.
Macromolecules, Vol. 37, No. 10, 2004
lization because the treatment of dynamic crystalliza-
tion data is rather complicated.21,22 However, it is
without question that the study of the nonisothermal
crystallization is of great practical importance since this
type of crystallization approaches more closely to the
industrial conditions of polymer processing such as
Sch em e 1
extrusion, molding, melt spinning of synthetic fibers,
among others.2
3,24
In addition, a study dealing with the
isothermal and/or nonisothermal crystallization behav-
ior of the macromolecules possessing complex molecular
architecture as well as the effects of the specific molec-
ular architecture on the crystallization behavior is
essential for designing new materials with novel prop-
erties.
In this study, a series of HPCLs were synthesized to
incorporate various different lengths of linear homolo-
gous oligo(ꢀ-caprolactone) backbone segments consisting
of 5, 10, and 20 ꢀ-caprolactone monomer units and are
thereby referred to as HPCL-5, HPCL-10, and HPCL-
2
0, respectively. The difference in the macromolecular
architecture among HPCLs and LPCL was character-
ized by 1H NMR end-group analyses. The controlled
nonisothermal crystallization was performed with vari-
ous cooling rates by DSC and analyzed by the modified
Avrami method. Cooperative polymer chain mobility in
the melt state was characterized from the steady-shear
melt viscosity measurements, and the developed crystal
morphology as well as the spherulites growth during
isothermal process was observed using POM. The focal
point of this study is characterizing the crystallization
kinetics of HPCLs and LPCL in a rather quantitative
and comparative manner and, consequently, elucidating
the effects of the specific molecular architecture of
HPCLs and LPCL on the crystallization kinetics.
crystallization of HPCLs and LPCL. The samples were heated
to 100 °C under dry nitrogen gas to suppress the thermal
oxidation and kept for 3 min to eliminate thermal history.
Nonisothermal crystallization was performed by a controlled
cooling of the aforementioned melt samples at different cooling
rates, ranging from 2 to 10 °C/min.
2
. Exp er im en ts
.1. Ma ter ia ls. The initiator 1, 2,2-bis(hydroxymethyl)-
2
Steady-shear viscosity measurements were performed on a
TA Instruments AR 2000 constant-stress rheometer using a
cone-and-plate geometry with a 2° angle and 6 cm diameter
cone. The temperature was controlled by a Peltier lower plate
with a precision of 0.1 °C, and the measurements were
performed at regular intervals of 10 °C ranging from 60 to
120 °C. To minimize the effect of the thermal expansion of
the rheometer, the gap between the cone and the plate was
automatically controlled at all temperatures to 56 µm. The
propyl benzoate, for ring-opening polymerization of ꢀ-capro-
lactone was prepared by nucleophilic substitution of potassium
salt of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) with
2
benzyl bromide. Benzyl-protected AB macromonomers 2, 2,2-
bis[ω-hydroxyoligo(ꢀ-caprolactone)methyl]propyl benzoates, were
prepared to incorporate the various lengths of oligo(ꢀ-capro-
lactone) segments by ring-opening polymerization of ꢀ-capro-
lactone using a catalytic amount of tin(II) 2-ethylhexanoate
-
1
(
Sn(Oct)
group molar ratio ([ꢀ-CL]
hydrogenolysis under a hydrogen (H
palladium on activated carbon (Pd/C, 5 wt % Pd) catalyst to
produce R-carboxylic-ω-dihydroxy AB macromonomers 3, 2,2-
2
) with controlled monomer-to-initiating hydroxyl
/[-OH] ) and were followed by
) atmosphere using a
range of shear rate measured was from 0.1 to 100 s .
A Leica MPS30 polarized optical microscope equipped with
a Mettler Toledo FP82HT hot stage and a Mettler Toledo FP90
controller was used to observe the crystal morphology devel-
oped and the spherulites growth behavior during the isother-
mal process. The samples were first melted and kept on one
hot stage at 100 °C for 3 min and then rapidly transported to
another hot stage equipped in optical microscope and con-
trolled at the crystallization temperature of 40 °C. The digital
images were taken at desired crystallization times and further
analyzed using a homemade image processor to calculate the
average sizes of the growing spherulites.
0
0
2
2
bis[ω-hydroxyoligo(ꢀ-caprolactone)methyl]propionic acid. Fi-
nally, HPCLs were prepared by polycondensation of 3 in
the presence of p-toluenesulfonic acid (TSA) with continuous
water removal. To investigate and compare the role of hyper-
branched structure against conventional linear structure,
linear poly(ꢀ-caprolactone) (LPCL), whose chemical structure
and molecular weight are similar to those of HPCLs, was
commercially purchased and used as a linear counterpart of
HPCLs.
3. Resu lts a n d Discu ssion
2
.2. Mea su r em en ts. The chemical structures of synthesized
3
.1. Syn th esis an d Molecu lar Ar ch itectu r al Ch ar -
a ct er iza t ion . Hyperbranched poly(ꢀ-caprolactone)s
HPCLs) 4 were synthesized according to a reaction first
1
materials were analyzed by H NMR spectroscopy using a
Bruker Avance DPX-300 spectrometer with the tetramethyl-
silane (TMS) proton signal as an internal standard in CDCl .
3
H NMR spectra were also analyzed to determine the average
number of ꢀ-caprolactone monomer units incorporated in 2,
〈Nꢀ-CL〉, the average number of the AB macromonomer units
2
incorporated in 4, 〈NAB2〉, and consequently the number-
average molecular weights of 4.
(
1
9
1
developed by Trollsås et al. and recently modified in
our previous study, in which HPCLs had been prepared
by polycondensation of the AB2 macromonomers 3 with
2
5
continuous removal of water (Scheme 1).
The AB2 macromonomers were prepared to incorpo-
rate the three different lengths of homologous oligo(ꢀ-
caprolactone) segments by ring-opening polymerization
A Mettler DSC-30 differential scanning calorimeter was
used to investigate the overall kinetics of nonisothermal

Macromolecules, Vol. 37, No. 10, 2004
Poly(ꢀ-caprolactone)s 3747
Ta ble 1. Gen er a l Ch a r a cter istics a n d Sch em a tic Dr a w in gs of Hyp er br a n ch ed P oly(E-ca p r ola cton e)s a n d Th eir Lin ea r
Cou n ter p a r ts
a
b
ꢀ
-Caprolactone monomer-to-initiating hydroxyl group molar ratio. Average number of ꢀ-caprolactone units incorporated in AB2
1
c
1
macromonomers determined by H NMR. Average number of the AB2 macromonomer units incorporated in HPCLs determined by
NMR. Obtained from SEC-MALLS. Linear poly(ꢀ-caprolactone) (Aldrich Chemicals).
H
d
e
F igu r e 1. 1H NMR spectrum (300 MHz) of hyperbranched
poly(ꢀ-caprolactone) (HPCL-10).
F igu r e 2. Schematic representation of all crystallization
parameters during nonisothermal process.
with various ꢀ-caprolactone-to-initiating hydroxyl group
molar ratio ([ꢀ-CL]0/[-OH]0 ) 5, 10, and 20, respec-
tively) followed by hydrogenolysis deprotection and
accordingly referred to as AB2-5, -10, and -20, respec-
tively. Thus, HPCLs were prepared to have intrinsically
different lengths of branches and/or repeating backbone
segments by the utilizing different AB2 macromonomers
and accordingly referred to as HPCL-5, -10, and -20,
respectively. The average number of ꢀ-caprolactone
monomer units incorporated in the AB2 macromono-
mers, 〈Nꢀ-CL〉, indicating the length of linear oligomeric
backbone segments, was calculated from the ratio of the
integrated area of the peak corresponding to the repeat-
ing methylene units (-COCH2, δ 2.22) to the integrated
with the schematic drawings of HPCL molecules and
LPCL molecule. The average length of oligo(ꢀ-capro-
lactone) segments in these molecules, and the average
numbers of macromonomers in hyperbranched mol-
ecules were determined to be 〈Nꢀ-CL〉 and 〈NAB 〉 based
2
on the assumption that the reactivities of B parts in the
AB2 macromonomers are equal, which in turn caused
macromonomers to be randomly incorporated.
The molecular architectural variations among HPCLs
and LPCL in the present study, which are well reflected
in Table 1, are that in the order of HPCL-5, -10, and
-
20, the lengths of linear oligo(ꢀ-caprolactone) segments
appear to increase in length and the numbers of
branching points in HPCL molecules decrease in num-
ber, while LPCL is made of one long linear chain of
poly(ꢀ-caprolactone).
area of the peak corresponding to the chain ends
1
(
-CH2OH, δ 3.59) in H NMR spectra of the AB2
macromonomers, and in a similar manner, the average
number of the AB2 macromonomer units incorporated
3
.2. Non isoth er m a l Cr ysta lliza tion Kin etics. Fig-
in HPCLs, 〈NAB 〉, was calculated from the following
2
ure 2 shows a typical DSC curve for the nonisothermal
crystallization process during cooling from the melt.
equation based on the branching theories developed by
2
6
Flory: 〈NAB 〉 ) m/{2〈Nꢀ-CL〉 - m}, where m represents
2
the ratios of the integrated area of the peak correspond-
ing to the repeating methylene units (Figure 1, d) to
the integrated area of the peak corresponding to the
chain ends (Figure 1, c) of HPCLs. Here, it is worthy to
note that the average number of branching points
As shown in this figure, following useful parameters
can be obtained to describe the nonisothermal crystal-
lization behavior of samples: (1) the peak temperature,
Tp; (2) the initial slope of the exotherm at inflection on
the high-temperature side, Si; (3) the width at half-
2
7-30
existing in HPCLs is equal to the value 〈NAB 〉 - 1.
height of the exothermic peak, ∆W.
Figure 3
2
Listed in Table 1 are the general characteristics of
three HPCLs and their linear counterpart, LPCL, along
exhibits the exothermic curves during the nonisother-
mal crystallization for HPCL-5, HPCL-10, HPCL-20,

3
748 Choi et al.
Macromolecules, Vol. 37, No. 10, 2004
F igu r e 3. Nonisothermal crystallization exotherms of the HPCL-5 (a), HPCL-10 (b), HPCL-20 (c), and LPCL (d) measured at
various cooling rates.
Ta ble 2. Cr ysta lliza tion P a r a m eter s of Hyp er br a n ch ed
P oly(E-ca p r ola cton e)s a n d Th eir Lin ea r Cou n ter p a r ts
all samples as the cooling rate increased, indicating the
increase in supercooling. On the other hand, for a given
cooling rate, Tp for each HPCL shifted to a high-
temperature region with the increase in the length of
the incorporated linear backbone segments and the
decrease in the number of branching point. This indi-
cates acceleration of crystallization temperature caused
by the incorporation of the longer linear crystallizable
oligo(ꢀ-caprolactone) segments onto HPCL backbone. Tp
for LPCL was observed in the region between HPCL-
cooling rate (°C/min)
crystallization
sample
parameter
2
4
6
8
10
HPCL-5
Tp (°C)
Si
35.2 32.0 29.6 27.3 25.0
0.54 0.68 0.83 0.91 1.02
∆
W (°C)
2.8
3.1
3.5
3.8
4.1
∆Hc (J /g)
73.0 71.8 70.8 67.7 67.1
53.5 52.7 51.9 49.6 49.2
35.4 31.2 29.4 27.8 25.6
0.65 0.72 0.86 0.91 1.20
Xc (%)
Tp (°C)
Si
HPCL-10
HPCL-20
LPCL
1
0 and HPCL-20 for a given cooling rate, the origin of
∆
W (°C)
2.1
2.6
3.1
3.7
3.9
which will be discussed in detail in a later section. The
Si of the curves in Figure 3 was employed to express
∆Hc (J /g)
79.1 77.4 76.1 75.4 72.2
58.0 56.8 55.8 55.3 52.9
38.7 35.5 34.1 32.6 31.2
0.78 0.91 1.03 1.21 1.45
Xc (%)
Tp (°C)
Si
relative kinetics of the nonisothermal crystallization
27-30
process.
Higher absolute values of the initial slope,
∆
W (°C)
2.1
2.7
2.8
3.4
4.1
suggesting faster crystallization, were assigned to HPCL
with the longer linear backbone segments and fewer
number of branches. On the other hand, the absolute
value of Si for LPCL at a given cooling rate was between
those of HPCL-10 and HPCL-20, indicating that crys-
tallization rate of LPCL mediates between HPCL-10
and HPCL-20.
∆Hc (J /g)
82.9 82.2 80.6 78.8 73.8
60.7 60.2 59.1 57.8 54.1
36.0 33.2 31.6 29.6 26.4
0.75 0.79 0.94 1.18 1.23
Xc (%)
Tp (°C)
Si
∆
W (°C)
2.7
4.6
2.9
4.9
7.1
∆Hc (cal/g)
80.2 78.2 77.2 75.3 73.0
58.8 57.3 56.6 55.2 53.5
Xc (%)
As indicated in Table 2, the cooling rate also had a
profound effect on the crystallization kinetics. The
crystallization was faster for higher cooling rates, sug-
gesting the nucleation-dominated process as dictated in
many previous literatures. The ∆W is a measure of the
distribution of the crystal dimensions; i.e., the smaller
and LPCL at various cooling rates ranging from 2 to 10
°
C/min.
All DSC curves shown in Figure 3 were analyzed to
calculate the above-mentioned crystallization param-
eters, and the results are summarized in Table 2. The
experimental errors were examined with at least five
runs for each sample at each cooling rate under pre-
cisely controlled condition. The parameters Tp, Si, and
2
8-30
the ∆W, the narrower the distribution.
From ∆W
values calculated in Table 2, it was found that the
distributions of the crystal dimensions for HPCL-5 and
HPCL-10 are slightly broader than HPCL-20, which
might be the result of the slower crystallization rate
and little broader molecular weight distribution as
∆
W listed in Table 2 were found to be reproducible to
1 °C, (0.1, and (2 °C, respectively. It is clearly seen
in Figure 3 that Tp shifted to lower temperatures for
(

Macromolecules, Vol. 37, No. 10, 2004
Poly(ꢀ-caprolactone)s 3749
indicated in Table 1. In addition, the crystallinity, Xc,
of all samples were calculated using the following
formula
∆
Hc
X (%) )
× 100
(1)
c
∆
H100
where ∆Hc is the heat of crystallization of the sample
and ∆H100 is the heat of crystallization for a 100%
crystalline poly(ꢀ-caprolactone), which is taken as 136.4
J /g.31 The ∆Hc and corresponding Xc of HPCLs and
LPCL are also summarized in Table 2. Among HPCLs,
the Xc calculated for a given cooling rate increased with
the longer linear backbone segments and the fewer
branches in the molecules, whereas the Xc of LPCL is
in the middle of HPCL-10 and HPCL-20. To investigate
the nonisothermal crystallization kinetics in detail and
more quantitatively, the measured nonisothermal crys-
tallization exotherms in Figure 3 were further analyzed
by the modified Avrami method.
Integration of the exothermal peaks during the noniso-
thermal scan gives relative crystallinity, Xt, as a func-
tion of temperature, which can be transformed into a
function of time:3
2,33
t
∫
(dH
c
/dt) dt
/dt) dt
0
X )
(2)
t
∞
∫
(dH
c
0
Here, t0 and t∞ denote the onset and completion time of
crystallization, respectively, and dHc/dt is the rate of
heat evolution. Figure 4 shows the Xt variations as a
function of crystallization time for HPCLs and LPCL.
As seen in Figure 4, the lower the cooling rate, the
larger the time range at which crystallization occurs.
In addition, all curves are seen to have approximately
the same reverse S-shape, indicating that only the
retardation effect of cooling rate on the crystallization
can be observed in these curves.3
On the basis of the assumption that the crystallization
temperature is constant, the classical Avrami equation
was used for describing the primary stage of the
nonisothermal crystallization kinetics.32,36-38 In this
case, the Avrami equation can be expressed as follows
F igu r e 4. Plots of relative crystallinity, X
time, t, of HPCLs and LPCL.
t
, vs crystallization
the rate constants for HPCLs and LPCL were estimated,
respectively, and listed in Table 3. The derived Avrami
exponents for HPCLs and LPCL, ranging from 2.4 to
3.3 as listed in Table 3, were not affected much by either
the cooling rate or the molecular architecture of samples.
The Avrami exponents in this range are known to be
obtained with various combinations of heterogeneous vs
homogeneous nucleation, dimensionality of growth, and
4,35
n
27,42
X ) 1 - exp[-Kt ]
(3)
diffusion-restricted vs non-diffusion-restricted growth.
t
However, obtaining further insight into the nucleation
and crystal growth mechanism with these Avrami
exponents is not within the scope of research which is
focused more on the crystallization kinetic rate con-
stants and its correlation with the molecular architec-
ture for HPCLs.
where K is the rate constant containing the nucleation
and the growth parameter in the nonisothermal crystal-
lization process, and n is the Avrami exponent whose
values depend on the mechanism of nucleation and on
the nature of crystal growth.3
9-41
To facilitate the
evaluation of the Avrami parameters (n and K), the
double-logarithmic form is often taken to rearrange eq
Here, it is worthy to note that the K values in Table
3 are inadequate to characterize the nonisothermal
crystallization kinetics due to the influence of the
cooling rate and should be corrected adequately. As-
suming that the cooling rate is constant or approxi-
mately constant, the final form of the kinetic rate con-
stant, Kc, can be given by the following correction:3
3
as follows:
log[-ln(1 - X )] ) log K + n log t
(4)
t
5,43,44
Figure 5 shows the log[-ln(1 - Xt)] vs log t plots, so-
called Avrami plots, for HPCLs and LPCL. It is shown
in Figure 5 that the Avrami plots for the nonisothermal
crystallization of HPCLs and LPCL vary linearly with
one slope in a relatively large range of crystallization
time, though a slight deviation from the straight line
derived in the middle of the curves is also observed at
both ends of the curves. From the slope and the
intercept of the straight lines, the Avrami exponents and
log K
ø
log K )
(5)
c
The corrected rate constants for HPCLs and LPCL at
various cooling rates are shown in Figure 6. Among the
HPCLs at a given cooling rate, the Kc values are seen
to increase with the increase in the lengths of linear

3
750 Choi et al.
Macromolecules, Vol. 37, No. 10, 2004
c
F igu r e 6. Corrected Avrami rate constant, K , for HPCLs and
LPCL at various cooling rates.
Ta ble 3. Va lu es of Avr a m i Exp on en t, n , Ra te Con sta n t, K,
th e Cor r ected Ra te Con sta n t, Kc, a n d th e Cr ysta lliza tion
Ha lf-Tim e, t1/2, of Hyp er br a n ch ed P oly(E-ca p r ola cton e)s
a n d Th eir Lin ea r Cou n ter p a r ts a t Va r iou s Coolin g Ra tes
cooling rate (°C/min)
modified Avrami
sample
analysis
2
4
6
8
10
2.9
HPCL-5
n
2.9
2.9
2.6
2.5
K
0.16 0.47 1.19 1.99 4.41
0.40 0.83 1.03 1.09 1.16
1.21 0.94 0.86 0.83 0.84
Kc
t1/2
n
HPCL-10
HPCL-20
LPCL
3.3
3.1
2.8
2.7
3.1
K
0.18 0.78 1.59 2.30 5.69
0.42 0.94 1.08 1.11 1.19
1.16 0.91 0.85 0.84 0.84
Kc
t1/2
n
2.5
2.4
2.5
2.6
2.5
K
0.58 1.52 3.14 7.21 10.92
0.76 1.11 1.21 1.28 1.27
0.96 0.82 0.80 0.79 0.78
Kc
t1/2
n
3.3
3.1
3.3
3.1
2.5
K
Kc
t1/2
0.28 1.31 2.84 4.91 7.93
0.53 1.07 1.19 1.22 1.23
1.08 0.87 0.85 0.83 0.80
F igu r e 5. Plots of log[-ln(1 - X
t
)] vs log t for HPCLs and
LPCL.
the small AB2 (or A2B2, AB3, etc.) monomers in synthe-
ses bring about imperfection of the chains, consequently
deterring effective interaction and regular chain packing
for crystallization. However, in the case of HPCLs, the
incorporated linear oligo(ꢀ-caprolactone) backbone seg-
ments were quite long, and the branching points exist-
backbone segments and the decrease in the number of
branches, which are HPCL-5 < HPCL-10 < HPCL-20.
On the other hand, the Kc of LPCL showed an inter-
mediate value between those of HPCL-10 and HPCL-
2
0.
In addition, the crystallization half-time, t1/2, defined
ing in the backbone chain were only a few compared to
those in the other types of typical HBPs.1
8,19
These
as the time at which the extent of crystallization is
complete by 50%, was determined from the derived Kc
and n as follows3
architectural features of HPCLs may help the polymer
chains to interact more effectively and pack more
regularly, which accommodates the crystallization more
effectively. Moreover, we found that the molecular
architectural differences in the length of incorporated
linear segments and the number of branching points
altered the kinetic rates of the nonisothermal crystal-
lization of HPCLs. That is, the slower crystallization
rate was observed for HPCLs with shorter linear seg-
ments and with more branches as verified from the Kc
and 1/t1/2 in the nonisothermal crystallization for a given
cooling rate (HPCL-5 < HPCL-10 < HPCL-20). These
phenomena can be explained by the less favorable
thermodynamic condition of the HPCLs with the shorter
segments (HPCL-5 and HPCL-10) than the HPCLs with
the longer segments (HPCL-20). It is evident that there
exist more frequent heterogeneous branching points,
which act as an obstacle in regular chain packing, in
the backbone of the HPCLs with shorter segments.
However, LPCL comprising one long linear poly(ꢀ-
caprolactone) chain, which can be considered as in the
most favorable thermodynamic condition among the
samples, showed slower crystallization than HPCL-20.
5,43-45
ln 2 1/n
t1/2
)
(6)
(
)
Kc
and are also listed in Table 3. Considering that the
reciprocal of t1/2 corresponds to the overall crystallization
rate, the result of the overall crystallization rate for
HPCLs and LPCL was in good agreement with Kc result.
The overall crystallization of polymers can be envis-
aged as a two-step process: (1) the first step is the
cooperative movement of polymer chains in parallel
array, and (2) the second step is crystallization of these
ordered chain occurring by means of intermolecular
interactions with a concomitant decrease in free en-
4
2
ergy. Commonly, hyperbranched polymers (HBPs),
although they show a faster molecular motion, are
known to be more amorphous compared to their linear
counterpart having similar chemical backbone structure
and molecular weight.7-9 In the conventional HBPs,
numerous branching points resulting from the use of

Macromolecules, Vol. 37, No. 10, 2004
Poly(ꢀ-caprolactone)s 3751
Ta ble 4. Activa tion En er gy of F low , Eη, a n d Sp h er u litic
Ra d ia l Gr ow th Ra te, G, for Hyp er br a n ch ed
P oly(E-ca p r ola cton e)s a n d Th eir Lin ea r Cou n ter p a r ts
sample
HPCL-5
HPCL-10
HPCL-20
LPCL
Eη (kJ /mol)
G (µm/min)
34.4
1.5
35.4
4.9
37.1
15.3
39.8
11.4
Considering the overall crystallization as a two-step
process, it is obvious that the polymer chains of HPCLs
incorporating the shorter linear segments such as
HPCL-5 and HPCL-10 can move faster than the poly-
mer chains of HPCLs with the longer segments in
parallel array to crystallize in the first step. However,
many branching points existing in the HPCLs with the
shorter segments act as an obstacle in the interaction
and crystallization stage, as was explained in a previous
section. On the other hand, it is worthy to bear in mind
that HPCL-20 incorporates relatively long linear seg-
ments consisting or ca. 20 ꢀ-caprolactone monomer units
F igu r e 7. Steady shear viscosity as a function of shear rate
for HPCLs and LPCL measured at 90 °C.
(
ca. 120 carbons) and only a few branching points in the
backbone, which can be considered as the thermo-
dynamic condition acting in the later step of overall
crystallization of HPCL-20, is almost as favorable as
LPCL. Therefore, we could conclude that the polymer
chains of HPCL-20 move faster in parallel array com-
pared to those of LPCL in the first step of overall
crystallization due to faster chain mobility in the melt
state, which result in the faster crystallization rate as
determined by the Kc and 1/t1/2.
F igu r e 8. Temperature dependences of the viscosity for
HPCLs and LPCL.
3
.3. Mor p h ologica l Obser va tion a n d Sp h er u lite
To understand the nature of the faster crystallization
rate of HPCL-20 than LPCL, molecular motion of the
samples in their melt state, which affects the first step
of overall crystallization, was characterized by the
measurement of steady shear melt viscosity (This will
be discussed in the following section).
Ra d ia l Gr ow th . Crystal morphology developed during
melt quenched isothermal crystallization and radial
growth behavior of spherulites was characterized using
POM equipped with a hot stage. The morphological
forms obtained under controlled nonisothermal condi-
tions closely matched those reported for isothermal
crystallization as in other works and, hence, will not
be presented here. POM images captured during iso-
thermal crystallization of HPCLs and LPCL at 40 °C
at various crystallization times are shown in Figure 9.
2
7
3
.2. Stea d y Sh ea r Melt Viscosity a n d Activa tion
En er gy of F low . For HPCLs and LPCL, the viscosity
was measured under steady shear in the temperature
range 60-120 °C, where all samples are in their melt
state. Figure 7 shows the exemplary results of steady
shear viscosity for HPCLs and LPCL measured at 90
A characteristic pattern, which consists of a number
of spherulites exhibiting a Maltase cross, as well as
radial growth of spherulites with the crystallization time
was observed for all samples. As seen in Figure 9, the
number of growing crystals formed in the early stage
of crystallization for each sample was found to be
dependent on the molecular architecture as the number
of nuclei formed is in the order of HPCL-20 > LPCL >
HPCL-10 >HPCL-5. This phenomenon is correlated
with the different molecular architectures of samples.
Among HPCLs, the introduction of more branches in
the backbone of molecules (HPCL-20 < HPCL-10 <
HPCL-5) resulted in the decrease in chain regularity
and the increase in steric hindrance, which slowed the
formation of nuclei. Moreover, slower chain mobility of
LPCL compared to HPCL-20 resulted in the fewer
number of nuclei formed within the same crystallization
time.
°
C. For all HPCLs and LPCL, the viscosity was found
to be independent of shear rate. This is characteristic
of a Newtonian fluid.
The temperature dependences of the viscosity of
HPCLs and LPCL are illustrated in Figure 8. From this
figure, we can see that HPCLs and LPCL exhibit good
linear relationships between ln η and 1/T within the
temperature range employed, which is in good agree-
ment with the kinetic rate theory of flow as represented
by the Arrhenius-Frenkel-Eyring type equation as
follows:46
Eη
ln η ) A +
(7)
RT
The observed linearity of ln η vs 1/T relationships
indicates that the apparent activation energy of flow,
Eη, can be determined by the slopes of these plots. The
derived Eη values for HPCLs and LPCL are summarized
in Table 4. According to the kinetic theory of flow, the
lower activation energy of flow corresponds to the faster
In Figure 10, the average radii of spherulites for
HPCLs and LPCL are plotted against crystallization
time. The radii of spherulites increase linearly with
crystallization time up to the impingement of the
spherulites. From the slopes of these lines, the spheru-
litic radial growth rate during the isothermal process,
G, for HPCLs and LPCL was evaluated and are listed
in Table 4. As indicated in Table 4, the trend of G values
is in good agreement with the nonisothermal crystal-
4
7,48
cooperative diffusional motion of polymer chain.
As
indicated in Table 4, the cooperative diffusional motion
of polymer chains in their melt state is faster in the
order of LPCL < HPCL-20 < HPCL-10 < HPCL-5.

F igu r e 9. Polarized optical micrographs for HPCL-5 (a), HPCL-10 (b), HPCL-20 (c), and LPCL (d) crystallized at 40 °C from the melt after indicated crystallization time. The white
bars correspond to 50 µm.

Macromolecules, Vol. 37, No. 10, 2004
Poly(ꢀ-caprolactone)s 3753
packing regularly for the effective crystallization. How-
ever, the slower crystallization rate of LPCL compared
to that of the HPCL-20 was an unexpected behavior
from the above-mentioned point of view because LPCL
consisted of only one long chain with no chain hetero-
geneity. Steady shear melt viscosity measurements
performed at various temperatures revealed that the
cooperative chain mobility of the HPCL-20 is higher
than that of LPCL in their melt state, indicating a faster
ordering of polymer chains to further crystallize in the
first step of overall crystallization. In addition, the linear
segments incorporated in the HPCL-20 are relatively
long such that thermodynamic condition for crystalliza-
tion of ordered chains in parallel array in the second
step could be nearly equivalent to that of LPCL, causing
the overall crystallization of the HPCL-20 to occur at a
faster rate than LPCL. From the morphological obser-
vation during isothermal crystallization at 40 °C, a
number of spherulites exhibiting a Maltase cross were
observed for all samples. Moreover, the average radii
of spherulites increased linearly with isothermal crys-
tallization time, and the derived radial spherulites
growth rates for the HPCLs and LPCL were found
to be in good agreement with the K and 1/t results
F igu r e 10. Variation in the spherulite radius with crystal-
lization time for HPCLs and LPCL.
lization rate determined by the Kc and 1/t1/2 in the
previous section.
4
. Con clu sion s
In the present study, three hyperbranched poly(ꢀ-
caprolactone)s (HPCL-5, HPCL-10, HPCL-20) with the
molecular architectural variation, which are the differ-
ent lengths of homologous oligo(ꢀ-caprolactone) seg-
ments and the different numbers of branching points,
were prepared without significant changes in molecular
weights, and linear poly(ꢀ-caprolactone) (LPCL) whose
chemical structure and molecular weight are similar to
those of HPCLs was purchased and used as the linear
counterpart. From the molecular architectural charac-
c
1/2
that were estimated from the controlled nonisothermal
processes.
Ack n ow led gm en t. The authors are grateful to the
Ministry of Environment, Republic of Korea, for their
support of this study through the Eco-Technopia 21
project. The authors also wish to express their gratitude
to Sang-Wook Chun for performing DSC experiments.
1
terization performed by H NMR end-group analyses,
the lengths of the linear backbone segments were found
to be in the increasing order of HPCL-5 < HPCL-10 <
HPCL-20 by their 〈Nꢀ-CL〉 values, and the numbers of
branching points existing in the HPCL molecules were
found to be in decreasing order of HPCL-5 > HPCL-10
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