Nucleating effect and crystal morphology controlling based on binary phase behavior between organic nucleating agent and poly(l-lactic acid)

Article abstract of DOI:10.1016/j.polymer.2015.04.062

A new kind of nucleating agent for poly(l-lactic acid) (PLLA), N,N'-bis(benzoyl) hexanedioic acid dihydrazide (BHAD), was synthesized. Differential scanning calorimetry (DSC), polarization optical microscopy (POM), wide-angle X-ray diffraction (WAXD) and rheometer were used to study the crystallization behavior and crystal morphology of PLLA nucleated by BHAD. In order to ensure the same thermal history of the samples characterized by DSC, POM and XRD, these samples were all thermally treated by DSC. In nonisothermal crystallization, BHAD showed excellent nucleating ability to PLLA. However, it was different from the traditional heterogeneous nucleation mechanism. POM observation showed that morphologies of PLLA crystals varied with the concentration of BHAD and can be classified into three types. Through the systematic investigation of DSC, POM and rheological analysis, it was clearly demonstrated that BHAD had solubility in PLLA matrix, depending on its concentration and processing temperature. Consequently, a highly schematic binary phase diagram of the PLLA/BHAD system was presented, accompanied with a reasonable mechanism of how the PLLA crystal morphologies controlled by BHAD.

Full text of DOI:10.1016/j.polymer.2015.04.062

Polymer 67 (2015) 63e71
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Nucleating effect and crystal morphology controlling based on binary
phase behavior between organic nucleating agent and poly(
acid)
L-lactic
Yinqing Fan , Jun Zhu , Shifeng Yan , Xuesi Chen b, Jingbo Yin a
a
a
a
a
,
, *
a
Department of Polymer Materials, Shanghai University, 200444, Shanghai, China
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022, Changchun, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2015
Received in revised form
A new kind of nucleating agent for poly(L-lactic acid) (PLLA), N,N'-bis(benzoyl) hexanedioic acid dihy-
drazide (BHAD), was synthesized. Differential scanning calorimetry (DSC), polarization optical micro-
scopy (POM), wide-angle X-ray diffraction (WAXD) and rheometer were used to study the crystallization
behavior and crystal morphology of PLLA nucleated by BHAD. In order to ensure the same thermal
history of the samples characterized by DSC, POM and XRD, these samples were all thermally treated by
DSC. In nonisothermal crystallization, BHAD showed excellent nucleating ability to PLLA. However, it was
different from the traditional heterogeneous nucleation mechanism. POM observation showed that
morphologies of PLLA crystals varied with the concentration of BHAD and can be classified into three
types. Through the systematic investigation of DSC, POM and rheological analysis, it was clearly
demonstrated that BHAD had solubility in PLLA matrix, depending on its concentration and processing
temperature. Consequently, a highly schematic binary phase diagram of the PLLA/BHAD system was
presented, accompanied with a reasonable mechanism of how the PLLA crystal morphologies controlled
by BHAD.
15 April 2015
Accepted 20 April 2015
Available online 25 April 2015
Keywords:
Poly(L-lactic acid)
Crystallization
Binary phase behavior
©
2015 Elsevier Ltd. All rights reserved.
1. Introduction
and other inorganic powders [14,15] have showed nucleating
ability for PLLA. By adding small amount of Poly( -lactide) (PDLA), a
D
Poly( -lactic acid) (PLLA) is one of the most promising biode-
L
polylactide stereocomplex is formed between PLLA and PDLA. The
stereocomplex is able to nucleat PLLA in cooling [16e19]. Mean-
while, organic compounds, such as benzenetricarboxylamide de-
rivatives [20,21] and benzoylhydrazide derivatives [22,23], have
also been reported as effective nucleating agents for PLLA. Gener-
ally, organic nucleating agents are preferred because of the ad-
vantages of better dispersion in polymer matrix and higher
efficiency even when the addition is at low level. Moreover, it is of
great interest and broad significance to study the crystal mor-
phologies of polymers controlled by nucleating agents [24e27]. For
example, Fu's group noted that, by adding a rare earth nucleating
gradable polymer materials, which is devoted to solve the problem
of white pollution [1,2]. Contributing to the renewable sources, the
cycle of PLLA will not produce any extra carbon dioxide into the
atmosphere [3e5]. This characteristic make PLLA very attractive as
alternative for petroleum-based polymers [6,7]. However, PLLA
does have some inherent shortcomings, which greatly restrict its
development and application. One of the main problem is its slow
crystallization rate during processing, which largely affects its
performance.
The additives of nucleating agent (NA) is one of the most
effective way to improve the crystallization ability of semi-
crystalline polymer. With appropriate nucleating agent added,
puzzle of slow crystallization rate can be easily conquered. Talc
agent (WBG), b-form isotactic polypropylene (i-PP) can be gener-
ated into three morphologies upon different molten temperatures
[24]. By controlling the crystal morphologies, the toughness of iPP
can be improved, which is very important for potentially industrial
[
8e10], montmorillonite [11,12], layered metal phosphonate [13]
0
00
applications. Later, they also reported that N,N ,N -tricyclohexyl-
,3,5-benzenetricarboxylamide (TMC-328) had noticeable effects
1
on controlling the crystal morphologies of PLLA, such as cone-like,
shish-kebab-like, and needle-like macroscopic structures [21]. The
*
Corresponding author.
E-mail address: jbyin@oa.shu.edu.cn (J. Yin).
http://dx.doi.org/10.1016/j.polymer.2015.04.062
032-3861/© 2015 Elsevier Ltd. All rights reserved.
0

64
Y. Fan et al. / Polymer 67 (2015) 63e71
ꢀ
supermolecular structure in semi-crystalline polymers is of great
significance in the study of polymer crystallization and further
applications. However, the underlying formation mechanism for
different crystal morphologies of polymers with nucleating agents
still remains unknown.
dried at 100 C in vacuum over night, the melting point of which
ꢀ
was 246.6 C (by DSC). The structure of BHAD is shown in Fig. 1.
1
H NMR (DMSO, 500 MHz) d: ppm; 10.29 (s, 2H, CONH), 9.85 (s,
2H, CONH), 7.88 (d, 4H, ArH), 7.58 (t, 2H, ArH), 7.50 (t, 4H, ArH), 2.20
(t, 4H, COCH ), 1.58e1.35 (m, 4H, CH ).
2
2
There are complex interactions between polymer and nucle-
ating agents, especially for organic ones. Firstly, there are several
growth patterns of polymer crystals on the surface of nucleating
agents, such as spherulites, transcrystalline layers and shish-kebab
structures [17,28,29]. These growth patterns are related to the
shape and size of nucleating agents, as well as their surface con-
dition [28]. Secondly, nucleating agents also affect the crystal
2
.3. Preparation of PLLA/BHAD blends
ꢀ
PLLA was dried for 8 h at 80 C under vacuum before use. PLLA/
BHAD blends were prepared using an XSS-300 torque rheometer
ꢀ
(Shanghai Ke Chuang Co., Ltd.) at 200 C. The blends were first
mixed at 32 r/min for 4 min, followed by 64 r/min for another
4
0
structures of polymers. By adding
a
-nucleating agents or
b
-nucle-
min. The concentrations of BHAD in blends were 0.1, 0.2, 0.3, 0.4,
.5, 0.75, 1, 1.5 and 2 wt%. The samples are abbreviated as PLLA/
ating agents, i-PP crystals can be induced into
a
-form or
b-form,
respectively [30e32]. Polymer with different crystal structures
exhibits different morphologies. Last but not the least, some
organic nucleating agents have solubility in polymer matrix. They
can dissolve in polymers when heating to molten temperature, and
then separate and self-organize into new crystals in subsequent
cooling run [21,24]. However, the solubility of nucleating agent
largely depends on the molten temperature (processing tempera-
ture) and concentration. Magnus Kristiansen et al. proposed a
classical binary system of i-PP/Bis(3,4-dimethylbenzylidene)sorbi-
tol, coming up with a schematic of four relevant composition
ranges [27]. Thus, it can be concluded that the formation of
different crystal morphologies of polymer is related to the in-
teractions between polymer and nucleating agents, including the
size, structure and solubility of nucleating agents.
BHAD-x, where x is the mass fraction of BHAD in blends. PLLA/
BHAD samples were hot-pressed at 200 C under 20 MPa for 3 min
to obtain 0.4 mm thicknesses sheets, followed by cold-pressed at
room temperature under 20 MPa for 10 min.
ꢀ
2
2
.4. Characterization
.4.1. 1H nuclear magnetic resonance
The 1H nuclear magnetic resonance was recorded on Brucker
AVANCE 500 spectrometers. Dimethyl sulfoxide-D6 (DMSO-D6)
was used as solvent.
2.4.2. Differential scanning calorimetry measurement (DSC)
0
In this paper, N,N -bis(benzoyl) hexanedioic acid dihydrazide
Thermal analysis of samples was carried out by DSC Q2000 (TA
Instruments-Waters LLC, USA). Temperature and enthalpy calibra-
tion had already been done with indium. Weight of all the samples
varied between 4 and 6 mg. Samples were first heated to the set
(
BHAD), a kind of benzoylhydrazide derivative, was synthesized
and studied as nucleating agent for PLLA. We presented a detailed
study on the unique morphological evolution of BHAD over a
concentration range, which will ultimately influence the crystalli-
zation behaviors of PLLA. Based on the analysis, a highly schematic
binary phase diagram of the PLLA/BHAD system is proposed.
f
temperature (called “final melting temperature”, T ) and main-
tained at that temperature for 5 min to establish an initial state.
ꢀ
Then samples were cooled to 40 C at different cooling rates (1, 5,10
ꢀ
and 20 C/min).
2
. Experimental section
2
.4.3. Thermal treatment of PLLA/BHAD blends
In order to ensure the same thermal history of the samples
2.1. Materials
characterized by DSC, POM and XRD, all these samples were ther-
mally treated by DSC. For POM observation, samples were first hot
pressed between two small glass sheets. Then, they were put into
aluminum crucible, reproducing the same treatment as DSC
applied. For XRD characterization, 4.0 ꢁ 4.0 ꢁ 0.4 mm samples
were put into aluminum crucible, followed by DSC treatment. The
process of thermal treatment is the same as DSC measurement
mentioned above.
Hexanedioic acid, thionyl dichloride, triethylamine, methanol,
N,N-dimethylformamide (DMF), dichloromethane (DCM) and
benzoyl hydrazine were purchased from Guo Yao Chemical Re-
agents Company, China. PLLA (trade name 4032D) was purchased
from Nature Works LLC, USA. The number-averaged and weight-
averaged molecular weight of 4032D is about 120 kg/mol and
2
00 kg/mol, respectively.
0
2
.2. Synthesis of N,N -bis(benzoyl) hexanedioic acid dihydrazide
2.4.4. Polarization optical microscopy (POM)
The crystal morphology of PLLA was observed on a polarized
0
.1 mol hexanedioic acid was added into 150 mL thionyl
optical microscopy (Leica DM2500P, Germany) equipped with a hot
stage (Linkam-THM600, U.K.). For in situ observation, samples were
hot pressed between two glass sheets in the hot stage. Firstly, they
dichloride with some drops of DMF. Then they were refluxed at
ꢀ
8
0 C for 4 h. After reaction, rotary evaporator was used to remove
the excessive thionyl dichloride to obtain hexanedioyl chloride.
Residue was then dissolved in 100 mL DCM to eliminate the re-
sidual thionyl dichloride by azeotropy in rotary evaporator. DCM
azeotropic operation was repeated for three times to purify the
hexanedioyl chloride.
ꢀ
ꢀ
were heated to 230 C at 5 C/min, and subsequently cooling to
ꢀ ꢀ
4
0 C at 5 C/min. Pictures were captured in the heating and cooling
process. For the samples treated by DSC, pictures were captured
directly.
0
.22 mol Benzoyl hydrazine and 0.1 mol triethylamine were
dissolved in DMF. 0.1 mol hexanedioyl dichloride was slowly
dropped into the mixture in ice water bath. After the addition, the
ꢀ
mixture was heated to 80 C for 2 h. Then the solution was poured
into deionized water and stirred. The resulting precipitate was
collected by suction filtration and rinsed with deionized water and
hot methanol for 3 times respectively. The resultant powder was
Fig. 1. Structure of N, N-Bis(benzoyl) Hexanedioic Acid Dihydrazide (BHAD).

Y. Fan et al. / Polymer 67 (2015) 63e71
65
2.4.5. Wide-angle X-ray diffraction (WAXD)
c
higher crystallization peak temperature (T ), which indicates the
WAXD experiments were performed on an X-ray Diffractometer
excellent nucleating ability of BHAD to PLLA. There are two
mechanisms of nucleation for polymer crystallization, homoge-
neous nucleation and heterogeneous nucleation. For neat polymer,
homogeneous nucleation is predominant. However, in the presence
of nucleating agent, heterogeneous nucleation plays a major role.
(
1
D/Max 2550, Rigaku, Japan) using Cu K
.54 Å) in the range of 5e50 with a scanning rate of 5 /min.
a
radiation (wavelength,
ꢀ
ꢀ
Samples had already been thermally treated by DSC.
2.4.6. Rheological
c
Under the circumstance, polymer crystallization rate (or T ) in-
The rheological experiments were carried out by a rotational
rheometer (DHR-3, TA Instruments) with 25 mm diameter and
creases with the concentration of nucleating agent until saturated
[18,19,33,34]. However, here for BHAD, situation is quite different.
1
2
mm gap parallel plates. Each sample of disk shape was heated to
30 C and maintained for 5 min. Then storage modulus evolutions
At first, T
c
of PLLA is elevated as the increased concentration of
of PLLA decreases unex-
ꢀ
BHAD. When BHAD exceeds 0.4 wt%, T
c
of the samples were monitored during the cooling process at the
rate of 5 C/min. The applied strain and oscillation frequency were
set at 1% and 2 rad/s, respectively. In order to minimize degrada-
tion, all rheological measurements were performed under nitrogen
atmosphere.
pectedly. This phenomenon implies that the heterogeneous
nucleation mechanism of BHAD is different from traditional het-
erogeneous nucleating agents (i.e. layered metal phosphonate
[34]).
ꢀ
3.1.2. Crystal morphology of PLLA/BHAD after nonisothermal
3
. Results and discussion
crystallization
With the aid of POM, the distinctly different morphologies of
PLLA after nonisothermal crystallization are observed along with
the concentration of BHAD. At the range of BHAD concentration,
morphologies of PLLA can be classified into three types. When
BHAD concentration is 0.1 wt%~0.2 wt%, PLLA crystal morphology
in PLLA/BHAD is almost the same as that of neat PLLA, which can be
viewed as numerous imperfect spherulites. When BHAD
3
.1. Nonisothermal crystallization of PLLA/BHAD and crystal
morphology
3.1.1. DSC curves of PLLA/BHAD at different cooling rates
Fig. 2 shows nonisothermal crystallization curves of PLLA con-
taining BHAD. Compared to neat PLLA, nucleated samples have
¼ 200 ꢀC): a. 1 C/min; b. 5 C/min; c. 10 C/min; d. 20 C/min.
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 2. Nonisothermal crystallization of PLLA/BHAD at different cooling rates (T
f

66
Y. Fan et al. / Polymer 67 (2015) 63e71
concentration is 0.3 wt%~0.5 wt%, the morphology is changing into
some pattern of branches and leaves. The crystal fibers of BHAD
serve as bifurcate branches, meanwhile PLLA crystallizes on these
fibers, acting as leaves. When BHAD concentration is 0.75 wt%~2 wt
%, the nucleating agent forms needle crystals, while PLLA crystal-
lizes around them. The shift of T in nonisothermal crystallization
may be related to the variation of PLLA crystal morphology, which is
dominated by the BHAD morphology. The crystallization process of
c
Fig. 3. POM images of PLLA/BHAD as BHAD concentration from 0 wt% to 2 wt% (T
f
¼ 200 ꢀC, cooling rate ¼ 5 ꢀC/min, prepared in DSC).

Y. Fan et al. / Polymer 67 (2015) 63e71
67
PLLA depends on two factors, nucleation and crystal growth
34,35]. The addition of BHAD enhances nuclei density of PLLA.
Table 1
T
c
f
of PLLA/BHAD after melting at different T .
[
ꢀ
However, it can be noticed from Fig. 3 that, only when the BHAD is
at an appropriate concentration can PLLA crystals have adequate
place to growth on the surface of nucleating agent. It still remains a
question that how BHAD changes its crystal morphology with the
concentration. It will be discussed in next sections.
T / C
200
98.4
95.9
98.9
106.8
131.4
125.2
126
210
98.7
95.8
94.3
101.6
122.7
132.6
125.4
125.4
127.4
127.5
220
98.5
95.8
94.8
230
99.7
95.1
95.7
93.2
96.0
110.7
132.8
135.5
135.3
134.1
f
ꢀ
T
c
/ C
neat PLLA
I
PLLA/BHAD-0.1
PLLA/BHAD-0.2
PLLA/BHAD-0.3
PLLA/BHAD-0.4
PLLA/BHAD-0.5
PLLA/BHAD-0.75
PLLA/BHAD-1
PLLA/BHAD-1.5
PLLA/BHAD-2
II
96.4
102.1
133.4
134.9
132.9
130.5
125.7
3
3
.2. The solubility of BHAD in PLLA and their binary phase behavior
III
127
128.3
127.4
.2.1. The influence of T
It is found that T of sample has influence on T
indicates that the solubility of BHAD in PLLA matrix is related to
temperature as well as BHAD concentration. T of the samples are
f
on the crystal morphologies of BHAD
f
c
(Fig. 4), which
c
dimethylbenzylidene)sorbitol/iPP system. Kristiansen et al. [27]
suggested that very low content additive would be inactive due
to “complete solubility” in the polymers. So the crystal mor-
phologies of PLLA/BHAD-0.1 and PLLA/BHAD-0.2 are similar to
that of neat PLLA (Fig. 3).
summarized in Table 1, and the influences are concluded into three
types, which is consistent with the crystal morphology diversity.
For the first type, the BHAD concentration is from 0.1 wt% to
0
c
.2 wt%, T almost remains unchanged, which is the same as neat
PLLA. This concentration range is defined as region I. In this
For the second type, the BHAD concentration is from 0.3 wt% to
range, BHAD completely dissolves in PLLA when temperature is
above 200 C. And in the cooling, BHAD cannot separate from
0.5 wt%, T
defined as region II. In this range, BHAD is partially dissolved in
PLLA matrix. When T is low, BHAD concentration will slightly
c f
is in inverse proportion to T . This concentration range is
ꢀ
PLLA. Compared to neat PLLA, T
c
of PLLA/BHAD-0.1 and PLLA/
f
BHAD-0.2 slightly depress. Under the circumstance, BHAD plays
a role of solvent to PLLA and has no nucleating effect on PLLA.
exceed its saturated solubility. Those undissolved BHAD crystals
become nuclei for the dissolved BHAD and for PLLA in cooling.
Similar
phenomenon
has
been
found
in
bis(3,4-
f
However, if T rises, the solubility of BHAD will increase. The more
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 4. Nonisothermal crystallization of PLLA/BHAD at 5 C/min from different T
f
: a. 200 C; b. 210 C; c. 220 C; d. 230 C.

68
Y. Fan et al. / Polymer 67 (2015) 63e71
BHAD will tend to dissolve in PLLA, the fewer BHAD nuclei will be
left in the melt. In this way, T of PLLA will drop when T increases.
For the third type, the BHAD concentration is from 0.75 wt% to
wt%, T is in direct proportion to T , which is opposite to region II.
c
f
2
c
f
This concentration range is defined as region III. In order to
investigate the difference, optical microscope with hot stage was
used for in situ observation (Fig. 5). In the heating process, a lot of
ꢀ
needle-like BHAD crystals can be seen at 200 C, which means the
BHAD concentration at this temperature is far beyond its saturated
solubility. As the temperature rises, the amount of needle-like
BHAD will decrease. Also, liquid phase separation can be seen
ꢀ
ꢀ
when temperature is over 220 C. At 230 C, almost all the BHAD
needle-like crystals disappear. In the cooling process, BHAD begins
to crystallize. Interestingly, the crystal morphology of BHAD is
c
changing into dendritic structures. The increasing of T is related to
the BHAD morphological change, although the deep mechanism
needs further study. A similar dissolution and recrystallization
process of nucleating agent was found in WBG/i-PP [24]. Needle-
like WBG dissolved and then self-organized into flower-like
Fig. 6. Rheology measurement of PLLA/BHAD.
structure with i-PP b-crystallites growing on its surface.
The solubility of BHAD in PLLA is strongly influenced by tem-
perature. This means there is a complex binary phase behavior
between PLLA and BHAD. It needs to be emphasized that PLLA/
BHAD-0.2 locates at the boundary between region I and region II,
meanwhile PLLA/BHAD-0.5 is between region II and region III.
below the melting temperature of PLLA. Therefore, the transition is
mainly attributed to the crystallization of PLLA. In this range, the
dissolved BHAD separates from the PLLA matrix and turns into
nucleating agent for PLLA. Then, PLLA crystallizes almost simulta-
neously with the separated BHAD. For BHAD concentration in range
of 0.75 wt%~2 wt%, two transitions are found in the curves. The high
temperature transition is attributed to the phase separation of
BHAD, and the low temperature transition belongs to the subse-
quent crystallization of PLLA. Similar observations have been also
reported previously, which indicates that the high temperature
transition is related to the network formed by nucleating agents
3.2.2. Rheological analysis
The solubility of BHAD in PLLA and binary phase behavior can
also be demonstrated by rheology (Fig. 6). Phase separation and
crystallization of BHAD is reflected by evolution of the storage
modulus. For PLLA/BHAD-0.1 and PLLA/BHAD-0.2, the curves are
similar with the neat PLLA. No transition is found in the curves,
which means BHAD are totally dissolved in PLLA and cannot
separate from the matrix. This is in accordance with the crystal
[21,27,36].
3.2.3. Binary phase behavior scheme
morphology and T
tration in range of 0.3 wt%~0.5 wt%, only one storage modulus
transition can be seen in the curves. The transition temperature is
f
influence discussed before. For BHAD concen-
Base on the data from DSC, POM and rheology, a schematic bi-
nary phase behavior of PLLA and BHAD is proposed in Fig. 7. For
region I, in which BHAD concentration is below 0.2 wt%, BHAD is
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 5. The morphology evolution of PLLA/BHAD-1 as temperature increasing from 200 C to 230 C, and then cooling to 140 C, both at 5 C/min (Bright field image).

Y. Fan et al. / Polymer 67 (2015) 63e71
69
most of the BHAD is undissolved in PLLA. When the temperature
increases, some BHAD will dissolve in PLLA matrix, leaving the rest
changing into liquid, followed by the liquid phase separation be-
tween PLLA and BHAD. In the cooling, BHAD separates and crys-
tallizes long before the crystallization of PLLA.
3
3
.3. Crystal morphology evolution of PLLA/BHAD
.3.1. The different BHAD crystals formed in PLLA
It still remains a question that the relationship between
different morphologies of BHAD crystals is unknown. Three typical
morphologies of needle-like, furcated-fiber-like, and dendrite-like
patterns can be observed. As is discussed above, crystal
morphology of BHAD is associated with its additive concentration
f
and T . It is supposed that, these two variables can determine the
residual BHAD crystals persisted in the PLLA melt, which subse-
quently influence the BHAD morphologies. It implies that, for the
same sample, different types of morphologies of BHAD can be ac-
Fig. 7. Binary phase behavior scheme of PLLA/BHAD.
f
quired by changing T from higher to lower. Take PLLA/BHAD-1 as
ꢀ
completely dissolved in PLLA and unable to separate from the
matrix. In this region, BHAD is not effective to nucleate PLLA. For
region II, in which BHAD concentration is between 0.2 wt% and
an example, when T is 200 C, most of the BHAD is undissolved in
f
PLLA. A dynamic balance between dissolved and undissolved BHAD
is achieved. In this case, imperfect crystals dissolve and crystallize
on the undissolved crystals. Finally, crystal sizes become uniform,
as shown in Fig. 8a . For the reason of high nuclei density, BHAD
crystals will not grow large, but form needle-like crystals. When Tf
0
.5 wt%, BHAD will dissolve in the PLLA when temperature is
0
elevated. In the cooling, BHAD is able to separate from the matrix,
and at the same time, PLLA crystallizes on the surface of the embryo
BHAD. For region III, in which BHAD concentration is above 0.5 wt%,
ꢀ
is up to 220 C, most of the BHAD dissolve or turn into liquid phase.
Fig. 8. Three types of BHAD crystal morphologies of PLLA/BHAD-1 (prepared in DSC) crystallized from different T
polarized images and picture (a , b , c ) are their bright field images.
f
: a, T
f
¼ 200 ꢀC; b, T
f
¼ 220 ꢀC; c, T
f
¼ 230 ꢀC. Picture (a, b, c) are
0
0
0

70
Y. Fan et al. / Polymer 67 (2015) 63e71
0
0
Fig. 9. PLLA crystals grow perpendicularly on BHAD (prepared in POM). Picture (a) is polarized image, and picture (a ) is bright field image. Picture (b) and (b ) are the magnified
images of (a) and (a ).
0
Residual concentration of BHAD crystals is quite low. However, they
still retain the needle-like morphology. In cooling, oversaturated
BHAD will separate and crystallize along the length direction on
these residual crystals. With the increase of the length, BHAD
will be dominated. It is supposed that, the high nucleating effi-
ciency of the dendrite-like BHAD may be related to the shish-
kebab structure between PLLA and BHAD. There are many fine
fiber structures at the end of the highly branched BHAD. These
fibers tend to form the shish structure and later nucleate PLLA to
form the kebab, which is clearly illustrated in Fig. 11. However,
this is still a speculation, which needs to be further studied in
future.
0
ꢀ
crystals begin to furcate (Fig. 8b ). When T is 230 C, all the BHAD
f
crystals dissolve or turn into liquid phase, which means no BHAD
crystals exist in the PLLA melt. Upon the cooling process, BHAD
homogeneously nucleates and grows in the form of furcated
branch. Finally, several dendrite-like BHAD crystals can be clearly
0
seen in the image (Fig. 8c ). Thus, three morphologies of BHAD
4. Conclusions
crystals are obtained from PLLA/BHAD-1, and they are determined
by the residual concentration of BHAD crystals at the highest
temperature in the heating process.
0
In summary, N,N -bis(benzoyl) hexanedioic acid dihydrazide
was synthesized as nucleating agent for PLLA, which showed
excellent nucleating effect as well as the ability to control the
crystal morphology of PLLA. By changing BHAD concentration or
3.3.2. The PLLA crystals grow on the BHAD
Based on the analysis of Fig. 8, we concludes that when BHAD
concentration exceeds 0.2 wt%, the nucleating agent will form
needle-like, furcated-fiber-like, or dendrite-like crystal frameworks
in PLLA matrix. It is interesting to investigate the crystal growth
pattern of PLLA on BHAD. Fig. 9 shows the polarized and bright field
images of PLLA/BHAD-1 at the beginning stage of nonisothermal
crystallization. It is clear to see that PLLA is no longer spherulite
morphology. They grow perpendicularly on BHAD frameworks,
which become the transcrystalline regions [37e39]. This unique
combination creates a supermolecular structure of PLLA and BHAD,
which will probably provide different properties compared with
PLLA spherulite [24,40]. However, the detailed differences, such as
mechanical properties, is out of our concern in this paper, and will
be studied later. From XRD data in Fig. 10, it is clearly seen that PLLA
keeps
a structure in all samples [23]. Although the morphology of
PLLA is totally changed, the crystal structures are kept same.
0
00
Hongwei Bai found that an organic nucleating agent, N,N ,N -
tricyclohexyl-1,3,5- benzenetricarboxylamide (TMC-328), formed
shish-kebab structure with PLLA [21]. Eric D. Laird noted that, by
inducing foreign fibers, polymer would form shish-kebab or
transcrystalline layer with them, which was decided by the fiber
diameter [28]. With large fiber diameter, transcrystalline layer is
preferred. And when fiber diameter is quite small, shish-kebab
Fig. 10. XRD of PLLA/BHAD samples.

Y. Fan et al. / Polymer 67 (2015) 63e71
71
Fig. 11. Supermolecular structure of PLLA/BHAD.
T
f
, different morphologies of PLLA crystal can be obtained. It is
[12] Yu ZY, Yin JB, Yan SF, Xie YT, Ma J, Chen XS. Polymer 2007;48:6439e47.
[13] Wang SS, Han CY, Bian JJ, Han LJ, Wang XM, Dong LS. Polym Int 2011;60:
found that BHAD has solubility in PLLA. The blend of PLLA/BHAD
can be viewed as a binary phase system, which is related to
BHAD concentration and the processing temperature. Three re-
gions have been found in this binary phase system at the BHAD
concentration less than 2 wt%. Mechanism for PLLA morphology
controlling by BHAD is given at viewpoint of binary phase
behavior. It has been demonstrated that, PLLA crystal
morphology is determined by the residual concentration of
BHAD crystals at the highest temperature in the heating process.
In cooling, dissolved BHAD gradually separates from PLLA and
accumulates on those undissolved BHAD crystals. Then they
form needle-like, furcated-fiber-like, or dendrite-like crystal
frameworks. PLLA crystals grow perpendicularly on these
frameworks, which will become the transcrystalline regions.
This unique combination creates a supermolecular structure of
PLLA and BHAD, which is of great value to be further studied
later.
2
84e95.
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