journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
Available online at www.sciencedirect.com
www.elsevier.com/locate/jmbbm
Research Paper
Phase-structure and mechanical properties of isothermally
melt-and cold-crystallized poly (L-lactide)
Erlantz Lizundia, Susana Petisco, Jose-Ramon Sarasuan
Department of Mining-Metallurgy and Materials Science, School of Engineering, University of the Basque Country (UPV/EHU),
Alameda de Urquijo s/n. 48013 Bilbao, Spain
ar t ic l e in f o
abs tra ct
Article history:
The effects of crystallinity differences induced by isothermal melt- and cold-
Received 9 April 2012
crystallizations on thermal, mechanical and morphological behavior of poly (L-lactide)
Received in revised form
(PLLA) have been investigated. PLLA samples were crystallized from the melt and annealed
8 September 2012
from the glassy state at 80, 100 and 120 1C. The degree of crystallinity (Xc) and rigid
Accepted 12 September 2012
amorphous phase (RAP) of PLLA was found to increase by crystallizing the samples at
Available online 30 September 2012
higher temperatures. Dynamic mechanical analysis (DMA) results suggest the presence of
Keywords:
a rubber-like structure composed by both amorphous and crystalline phases for crystal-
Poly (L-lactide)
lized specimens. When samples are cold-crystallized, the structural integrity about Tg can
Brittle-elastic
be better kept, prompting to a smaller E0 reduction after glass transition. Improvements in
Viscoelastic
Young’s modulus from 1027 MPa for quenched PLLA to 1401 MPa for the sample melt
Hyperelastic
crystallized at 120 1C together with ductility reduction are obtained as the crystallization
Crystallization
temperature increases. The tensile stress–strain curves at a range of temperatures,
Rigid amorphous fraction
comprising below and above glass transition, have provided a mean for computing the
Atomic force microscopy (AFM)
mechanical properties ready for being used in linear elastic, visco-elastic and hyperelastic
Surface roughness
computing models. Polarized light optical microscopy (PLOM) and atomic force microscopy
(AFM) analysis revealed completely different morphologies for melt-crystallized and coldcrystallized samples. When PLLA was crystallized from the melt surface roughness
increases up to 566 nm, while the increase in spherulite diameter is accompanied by a
monotonous decrease of the nucleation density. However, when PLLA was cold-crystallized
the obtained semicrystalline structure is independent of the crystallization temperature
because nucleation occurred upon quenching.
& 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Polylactides belong to biodegradable and biocompatible thermoplastics that are interesting candidates for biomedical applications as antibiotic release systems or as regenerative medicine
substrates (Lizundia et al., 2012). Polylactides can be used as
n
Corresponding author. Tel.: þ34 94 601 4271; fax: þ34 94 601 3930.
E-mail address: jr.sarasua@ehu.es (J.-R. Sarasua).
1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jmbbm.2012.09.006
temporary fixation materials for regeneration of healing tissues.
Biodegradable polylactides present the advantage that once the
temporary function has been accomplished there will be no
need for surgical extraction. In addition, recent progress in
production of poly (L-lactide) (PLLA) at low cost has accelerated
its use as a commodity plastic (Gruber and ÓBrien, 2002).
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
It is well known that the crystalline morphology and
structure obtained during the thermoplastic conformation
process plays an important role on the physicomechanical
behavior of resulting PLLA (Turner et al., 2004), conditioning
its potential uses (Sarasua et al., 2005). In this way, crystallization can be seen as a successful approach for improving
physico-mechanical properties of polymers. Semicrystalline
polymers can crystallize from both molten and glassy state,
which usually are termed as melt-crystallization and coldcrystallization respectively. As could be expected for thermoplastic materials, crystallized PLLA might exhibit higher
stiffness and mechanical performance than non-crystallized
polymer. In this way, the suppression of molecular mobility
by crystallization reduces the abrupt decrease in modulus at
the glass transition temperature. Therefore, a detailed knowledge of how the crystal aggregates are arranged within the
bulk and affect several polymer properties is a key issue in
order to tailor the polylactide derived products for different
applications.
Previous studies on crystallization and melting behavior of
PLLA demonstrated that depending on the crystallization
conditions, PLLA can crystallize in a, b or g polymorphs. The
most common form usually encountered is the a polymorph
presenting orthorhombic unit cell (Miyata and Masuko, 1997;
De Santis and Kovacs, 1968) and is obtained from the melt, cold
and solution crystallization under normal conditions. b modification was produced under high drawing conditions and
high temperatures, presenting three chains packed into a
trigonal unit cell (Hoogsteen et al., 1990; Cartier et al., 2000).
The g polymorph (Cartier et al., 2000; Ho et al., 2003) was
obtained through epitaxial crystallization on special substrates
with organic solvents. Recently, the existence of a a0 crystal
modification has been reported, which can be identified as a
disordered a form, with the same 103 helical conformation but
different lateral packing (Meaurio et al., 2009).
A semicrystalline polymer can be considered as a heterogeneous material where the amorphous material presents
different mobility depending on the dimension and location
of the disordered domains in regard with crystalline domains.
In this way, the semicrystalline nature of PLLA can be
described with the theoretical framework known as a three
phase model which comprises the crystalline phase (Xc), the
rigid amorphous phase (RAP) and the mobile amorphous
phase (MAP) (Wunderlich, 2003). The RAP presents less
segmental mobility than the bulk MAP and is considered to
be the result of strong restrictions of amorphous chains
confined within the crystalline phase (Wurm et al., 2010). Those
three phases can be quantitatively measured by differential
scanning calorimetry (Magon and Pyda, 2009; Zuza et al., 2008).
In this study we have chosen poly (L-lactide) due to the fact that
we can modulate the presence of both MAP and RAP within
PLLA and in this way establish the link between its phasestructure and mechanical behavior.
The knowledge of how crystalline morphology affects the
thermal and mechanical properties of PLLA is of great interest (Engelberg and Kohn, 1991; Perego et al., 1996; Ikada and
Tsuji, 2000; Weiler and Gogolewski, 1996; Lizundia et al., 2009)
for prospective biomedical and packaging applications.
Despite the fact that up to now several studies on mechanical
properties of polylactides have been published, how the
243
crystallinity with resulting phase-structure affects its
mechanical properties remain insufficiently explored. In this
work, several melt- and cold-isothermal crystallizations of
PLLA were designed in order to investigate the effects of well
controlled phase by microstructures have on the thermal and
mechanical properties of PLLA.
2.
Experimental part
2.1.
Materials
In this study, poly (L-lactide) containing less than 0.2%
residual monomer and 2% D-lactyl moieties supplied
Boehringer Ingelheim (Biomers L9000, Germany) was used.
The number-average molecular weight (Mn) of 153.000 g/mol
and a polydispersity index (Mw/Mn) of 1.38 was determined
by GPC.
2.2.
Sample preparation
Samples were prepared by compression molding. First of all
PLLA pellets were dried overnight in an air circulating oven at
40 1C and 8% of RH. Then, PLLA pellets conformed as sheets
with thickness of 1000 mm in a hydraulic hot press by
compression molding 180 1C for 3 min under a pressure of
240 MPa. In order to obtain samples with different crystalline
and rigid amorphous fractions, three kinds of treatments
were done (quenching, annealing after quenching and meltcrystallization) at specific crystallization temperatures. For
the first heat-treatment, molten PLLA was solidified by sinking
it in an iced water bath (quenching treatment). In this way, quick
cooling of materials below the Tg was assured, preventing any
further development of crystallinity. For annealing heat treatment, quenched sheets were crystallized in an oven at 80, 100
and 120 1C during 100 min. Finally, for melt-crystallization treatment, crystallization was carried out from the melt in the same
hydraulic hot press at 80, 100 and 120 1C for 100 min and
afterward crystallized sheets were subsequently solidified by
sinking it in an iced water bath.
2.3.
Differential scanning calorimetry (DSC)
Differential scanning calorimetry was carried out on a DSC
2920 thermal analyzer (TA Instruments). Samples in the
range of 4–8 mg were sealed in an aluminum pan and heated
to 200 1C at a rate of 10 1C/min in order to determine the
thermal transitions and enthalpies. In order to avoid any
thermal oxidation of samples, all tests were carried out under
a nitrogen atmosphere. The crystalline fraction Xc (%) attributable to the PLLA crystallization during the corresponding
heat treatment (melt-crystallization, quenching and annealing)
was determined as follows:
Xc ð%Þ ¼
DHf DHc
DHf 0
100
ð1Þ
where DHf and DHc are respectively the enthalpy of fusion and
cold crystallization of the samples determined on the DSC.
DHf 0 ¼106 J/g was taken as the heat of fusion of an infinitely
thick PLLA crystal (Sarasua et al., 1998). Mobile amorphous
244
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
(XMA) and rigid amorphous phases (XRA) were determined
according to a previously reported procedure (Del Rio et al.,
2010) with the next equations
XMA ¼
DCp
ð2Þ
DC0p
XRA ¼ 1Xc XMA
ð3Þ
(DC0p)
are the measured specific heat changes
where (DCp) and
at Tg of PLLA and the fully amorphous PLLA respectively, the
later giving account for the specific heat change at Tg of the
unconfined mobile fully amorphous phase (DC0p ¼ 0.639 J/(g K))
(Pyda et al., 2004).
Since the density of the rigid amorphous fraction cannot be
measured experimentally, in order to transform weight fraction into volume fraction it is necessary assume that a
semicristalline polymer is a simple two-phase system and
that the volume of the polymer is a sum of the volumes of the
crystalline and amorphous phases. Since this model does not
completely represent the semicristalline nature of PLLA, in
this work only weight fractions have been used. We are aware
that only an estimate of volume fractions according to the 3
phase model could provide a more precise link of micromechanics at mesoscale level to the material strength at
macroscale.
2.4.
Dynamic mechanical analysis (DMA)
DMA was performed on a DMA/SDTA861 analyzer (MettlerToledo) in shear mode. Specimens of 1 mm thick and 10 mm
diameter approximately were cut from the compressionmolded sheets. Curves displaying storage modulus (E0 ) and
the energy loss (tan d) were recorded as a function of
temperature between 10 1C and 130 1C at a heating rate of
3 1C/min, a frequency of 1 Hz, force amplitude of 20 N and a
displacement of 0.5 mm.
2.5.
Polarized light optical microscopy (PLOM)
Nucleation and growth of PLLA spherulites was studied with
a polarizing microscope (Leica DMLM) coupled with a Mettler
FP90hot stage. Those samples were prepared using the
solvent casting technique with chloroform (Panreac). The
polymer was dissolved at a concentration of 1 wt% at room
temperature and the resulting solution was deposited on a
glass slide and dried at 30 1C for overnight. Films of 50 mm
were obtained. The same crystallization programs previously
reported were applied in order to compare the results.
Atomic force microscope (AFM)
Samples for the surface topology feature studies by AFM were
prepared by means of solvent casting technique. The polymer
was dissolved at a concentration of 1 wt% at room temperature and the resulting mixture was deposited into a Petri-dish
and dried at 30 1C for overnight in an oven. After obtaining
the as-cast films, a small sample was cut and adhered to an
AFM mounting disk. The same crystallization programs previously reported were applied in order to compare the
obtained results. In order to erase any effect of the substrate
on the crystallization and compare the obtained result with
the bulk crystallization, films of 50 mm were used.
Surface topology feature of the crystallized films was
studied at room temperature using a Nanoscope IIIa Multimode Scanning Probe Microscope from Veeco Instruments,
Santa Barbara, CA. Veeco NanoScope V531r1 was employed to
analyze the recorded images. All experiments were carried
out in contact mode. A Si3N4 cantilever (NP-10) with a
characteristic force constant of approximately 0.58 N/m and
tip radius approximately 10 nm was used. Scan rate of images
was 2 Hz and resolutions of 256 256 data points per image
were used for all scans.
In addition, with the aid of Veeco NanoScope V531r1 software the roughness of the surface topology feature of PLLA
films were quantified by Rq and Ra measurements parameters. Root mean square roughness (Rq) is the standard
deviation of the Z values within a given area and is calculated
as follows:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u N
uP
u
ðZi Zave Þ2
t
ð4Þ
Rq ¼ i ¼ 1
N
where Zave is the average Z value in the analyzed area, N is
the number of points employed and Zi is the current Z value.
The mean roughness (Ra) represents the arithmetic average of
the deviation from the center plane, determined by
Tensile testing
Tensile specimens were punched out from crystallized sheets
according to the ISO 527-2: 1993 type 5 A. The test specimens
were conditioned at 22 1C and 51% RH for overnight. The
static tensile experiments of the specimens were performed
in an Instron 5565 universal testing machine at 5 mm/min
and 22 1C according to UNE-EN ISO 527-3: 1996. Young’s
modulus (E), stress and strain at yield (sy, ey) and stress and
strain at break (sb, eb) were determined for each set of
samples as the mean value of at least 5 specimens.
2.6.
2.7.
N P
Zi Zcp Ra ¼
i¼1
N
ð5Þ
where Zcp is the Z value of the center plane. Those two
parameters are representative of the average vertical deviation of the surface profile from the centerline (Veeco, 2003).
The surface topology features of samples providing surface
changes occurred upon crystallization were quantified in two
steps. First, from images obtained from AFM, root mean
square roughness (Rq) and mean roughness (Ra) were determined for heat treated PLLA after completion of crystallization at the selected temperatures; these results are
presented in Fig. 6. Secondly, crystal aggregate diameters and
nucleation densities of heat treated PLLA were measured.
Table 3 reports the values obtained according to Eqs. (4) and (5).
3.
Results and discussion
Fig. 1 presents the DSC traces of heat treated PLLA samples.
The quenched PLLA shows the heat enthalpy jump corresponding to the glass transition temperature (Tg) at 56.6 1C,
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
followed by a typical cold crystallization peak at 119.9 1C
(signifying the rearrangement of amorphous phase into new
crystalline regions) and a melt fusion peak at 169.3 1C. The
development of partial crystallinity during the melt- and coldcrystallization treatments as a result of the high chain stereoregularity of PLLA (Sarasua et al., 2005; Miyata and Masuko,
1998) can be observed. It is remarkable the Tg increase of 5–8 1C
as melt-crystallization and cold-crystallization process is carried out, in addition to the continuous suppression of cold
crystallization behavior. The Tg increase in heat treated PLLA in
regard to quenched PLLA can be associated to the confinement
of polymer chains induced by the reduction of mobile amorphous phase as crystallization develops. When crystallization
occurs at 80 1C from the melt, the proximity to Tg results in a
hindered crystallization; therefore the sample remains quite
amorphous and presents a cold crystallization exothermic
peak during the subsequent DSC scan. As crystallization is
carried out at higher temperatures, the presence of a less
prominent specific heat change at the glass transition, DCp, and
an enlargement of crystallinity can be observed.
Only samples cold- and melt-crystallized at 100 1C show a
exothermal peak just before melting at temperatures close to
159 1C that has been previously reported to be a phase transition between a0 and a polylactide polymorphs (Meaurio et al.,
2009; Zhang et al., 2005; Zhang et al., 2008; Lizundia et al., 2011).
Relative Heat flow, exo>
a
b
c
d
f
g
h
20
40
60
80
100
120
T (ºC)
140
160
180
Fig. 1 – DSC curves of PLLA obtained by different heat
treatments: (a) quenching; (b) melt-crystallized at 80 1C; (c)
melt-crystallized at 100 1C; (d) melt-crystallized at 120 1C; (e)
cold-crystallized at 80 1C; (f) cold-crystallized at 100 1C and
(g) cold-crystallized at 120 1C.
245
As shown in Fig. 1, it is worth to note that while the melting
point of melt-crystallized samples progressively increases Tc up
to 176.2 1C for sample melt-crystallized at 120 1C, as compared
with 169.1 1C for the quenched PLLA, the Tm of samples crystallized from the glassy state remains constant at 173.3 1C. This
behavior can be interpreted in terms of crystalline perfection.
Indeed, for samples crystallized from the melt higher crystallization temperatures result in thicker lamella, which is accompanied by an increase of the fusion temperature of developed
crystals. During the quenching of the polymer, the creation of
some nucleating points does not allow the polymer to crystallize into a more packed and ordered structures, resulting in the
development of irregular crystal aggregates (Tsuji et al., 2005).
Table 1 summarizes the thermal properties of melt- and
cold-crystallized PLLA, showing the three phases comprising
a crystallinity fraction (Xc), a rigid amorphous fraction (XRA)
and a mobile amorphous fraction (XMA) calculated according
to Eqs. (1)–(3). The reduction of specific heat change (DCp)
from 0.553 for the quenched sample to a minimum value of
0.094 in PLLA melt-crystallized at 120 1C is indicative of the
coexistence of both RAP and MAP. While the quenched PLLA
shows a large mobile amorphous fraction of 0.865, as meltcrystallization and cold-crystallization process are carried
out, the reduction of MAP is accompanied by an enlargement
of both crystalline and rigid amorphous phases, which is in
line with other results obtained recently by our group (Zuza
et al., 2008). It is worth to note that annealed PLLA samples
show both higher crystallinity and lower mobile amorphous
phase than samples crystallized from the melt.
Polymer properties such as modulus and toughness are
narrowly dependant on the existing crystalline and amorphous fractions and also on crystalline structure of the
specimens. Both dynamic-mechanical analysis and tensile
tests of melt- and cold-crystallized PLLA have been performed in this work to correlate with phase behavior and
morphology. Fig. 2 reveals the effect of melt-crystallization
temperature on the appearance of specimens. As crystallization is carried out at higher annealing temperatures, the
obtained larger crystalline fraction and larger spherulites
composed by thicker lamellae tends to create opaque specimens due to light scattering on the numerous boundaries
between the crystalline and amorphous regions. For example,
it is well known that atactic polypropylene is amorphous and
transparent, while the semicrystalline syndiotactic polypropylene has opaque appearance (Peacock and Calhoun, 2006).
Fig. 3 shows the DMA curves with storage modulus (E0 ) and
tan d of quenched and heat treated PLLA as a function of
Table 1 – Thermal properties of quenched, melt-crystallized and annealed PLLA determined by DSC including crystalline
fraction (Xc), rigid amorphous fraction (XRA) and mobile amorphous fraction (XMA). M.C.¼ melt-crystallized; C.C. ¼coldcrystallized.
Sample
Tg (1C)
DCp (J/g 1C)
DHc (J/g)
DHm (J/g)
Xc
XRA
XMA
quenching
M.C. 80 1C
M.C. 100 1C
M.C. 120 1C
C.C. 80 1C
C.C. 100 1C
C.C. 120 1C
56.6
61.2
61.5
64.4
62.1
64.4
64.7
0.553
0.445
0.226
0.131
0.411
0.151
0.094
24.7
28.7
–
–
7.1
–
–
25.6
34.7
31.1
37.3
28.4
39.2
46.3
0.008
0.057
0.294
0.352
0.201
0.370
0.436
0.127
0.247
0.353
0.443
0.161
0.394
0.417
0.865
0.696
0.353
0.205
0.638
0.236
0.147
246
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
PLLA is annealed from the glassy state, the maximum tan d
peak is achieved at higher temperatures when compared
with melt-crystallized specimens. It is noteworthy that the
increase in the melt- and cold-crystallization temperature
produces a progressive broadening of tan d peak, changing
the full width at half maximum from 8.4 1C for quenched
sample to 26.8 and 36.4 1C for melt- and cold-crystallized
70
26ºC
60
37ºC
45ºC
50
55ºC
60ºC
σ (MPa)
temperature. As can be seen, quenched PLLA presents a
sharp glass transition at 62 1C as denoted by the abrupt
decrease of E0 and a prominent tan d peak. The high value
of tan d for quenched PLLA indicates that a large volume
fraction has been frozen in the amorphous state during the
quenching treatment. Both melt- and cold-crystallization of
PLLA result in higher glass transition temperature (taken as
the peak temperature of the DMA tan d curve) and a progressive lowering of tan d peak from 2.65 for the quenched
specimen to 0.26 for PLLA melt-crystallized at 120 1C due to
the restriction of the mobility polymer chains induced by the
increase of crystalline domains within the amorphous
regions. This effect is more prominent in cold-crystallized
samples since the already present crystalline domains in the
sample act as more effective physical cross-links. The differences in the dynamic mechanical properties can be explained
in terms of the obtained different phase-structure during
both melt- and cold-crystallization processes. In particular,
when samples are cold-crystallized, the structural integrity
about Tg can be better kept, prompting to a smaller E0
reduction after glass transition, indicating that an increased
chain stiffness is achieved. In addition, as a result of smaller
XMA developed for a given crystallization temperature when
75ºC
40
30
20
10
0
0
10
100
200
300
400
500
600
700
ε (%)
Fig. 4 – Engineering stress–strain curves for PLLA at different
temperatures at a constant crosshead speed of 5 mm/min.
Table 2 – The elastic modulus given for the 26, 37
and 45 1C tests correspond to Young’s modulus
and the elastic modulus for the 55, 60 and 75 1C
tests were calculated as the secant modulus for a
2% strain ().
Fig. 2 – Photograph showing specimens obtained by quenching and melt-crystalization at 80 1C, 100 1C and 120 1C.
Test temperature (1C)
E (MPa)
26
37
45
55
60
75
1027734
455715
365715
9575
3.9570.2
0.8870.04
2,75
1
quenching
M.C. at 80 ºC
M.C. at 100 ºC
M.C. at 120 ºC
C.C. at 80 ºC
C.C. at 100 ºC
C.C. at 120 ºC
2,50
2,25
2,00
1,75
E´
tan 0,1
0,01
quenching
M.C. at 80 ºC
M.C. at 100 ºC
M.C. at 120 ºC
C.C. at 80 ºC
C.C. at 100 ºC
C.C. at 120 ºC
1,50
1,25
1,00
0,75
0,50
0,25
0,00
20
30
40
50
60
70 80
T (ºC)
90
100 110 120 130
20
30
40
50
60
70 80
T (ºC)
90
100 110 120 130
Fig. 3 – Dynamic mechanical storage modulus vs. temperature (left) and tan d vs. temperature (right) plots for heat
treated PLLA.
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
247
Table 3 – Table showing main representative parameters of tensile test for crystallized PLLA. M.C.¼ melt-crystallized and
C.C.¼ cold-crystallized.
Sample
E (MPa)
ry (MPa)
ey (%)
rr (MPa)
er (%)
quenching
M.C. 80 1C
M.C. 100 1C
M.C. 120 1C
C.C. 80 1C
C.C. 100 1C
C.C. 120 1C
1027734
1088746
1202760
1401733
1185744
1212755
1381786
62.2771.16
63.9271.92
63.8471.02
40.1672.41
45.2172.29
43.8571.03
36.4874.04
9.3570.44
9.1670.32
8.0670.27
3.8170.26
6.8470.15
6.2570.33
4.3870.47
56.3271.42
62.8670.98
63.5170.99
40.1672.41
44.3272.09
42.4371.69
36.4874.06
11.6270.71
9.4570.76
8.1370.25
3.8170.26
7.0970.23
6.6170.64
4.3870.47
70
70
quenching
M.C. at 80 ºC
M.C. at 100 ºC
M.C. at 120 ºC
Stress (MPa)
50
quenching
C.C. at 80 ºC
C.C. at 100 ºC
C.C. at 120 ºC
60
50
Stress (MPa)
60
40
30
40
30
20
20
10
10
0
0
0
2
4
6
Strain (%)
8
10
12
0
2
4
6
Strain (%)
8
10
12
Fig. 5 – Stress–strain representative diagrams of quenched and melt-crystallized PLLA (left) and quenched and coldcrystallized PLLA (right).
samples, respectively. Those results are in good concordance
with previously reported DSC results (Zuza et al., 2008).
Fig. 4 shows the tensile stress-strain behavior of PLLA
tested at 26 1C, 37 1C, 45 1C, 55 1C, 60 1C and 75 1C. Since the
glass transition temperature of PLLA is located between 45 1C
and 60 1C, depending on the phase fractions present, this
temperature range provides a mean for determining the
mechanical properties of PLLA ready for being used not only
in linear elastic and visco-elastic models but also in hyperelastic computing models. As can be observed in Fig. 4 the
brittle to ductile transition is located between 37 1C and 45 1C
while the transition to a elastomeric type behavior is
observed above 55 1C.
Table 2 shows the values of elastic moduli of PLLA determined from the tensile stress–stress curves of Fig. 4 obtained at
different test temperatures. As can be observed, Young’s modulus continuously decreases from 1027 MPa to 365 MPa when
increasing the test temperature from 26 1C to 45 1C. The elastic
modulus determined at 55 1C or higher temperatures reveals a
elastomeric type behavior with deformations at break in excess
of 600%. In this case the secant modulus was determined for a
2% strain providing values of 95 MPa, 3.99 MPa and 0.88 MPa for
PLLA tested at 55 1C, 60 1C and 75 1C respectively.
Fig. 5 shows the stress–strain curves of quenched and heat
treated PLLA. They reveal a pronounced influence of heat
treatment on the mechanical properties. Quenched PLLA
shows a stiff and semi-ductile behavior as denoted by the
prominent linear slope at low deformations and the fracture
strain around 11.6%. On the contrary, the heat treatment at
temperatures above Tg increases the tensile modulus and
decreases elongation at break of PLLA.
As shown in Table 3, Young’s modulus evolves from
1027 MPa for quenched PLLA to 1401 MPa for the sample melt
crystallized at 120 1C. It is also noticeable that at low crystallization temperatures (at 80 and 100 1C) the improvement of
the Young’s modulus is higher when samples are coldcrystallized rather than melt-crystallized. This is due to the
formation of larger crystalline and rigid amorphous fractions.
The change in the mechanical properties is also manifested
by a progressive decrease in strain at break from 11.6% for the
quenched sample to 3.8% as the melt-crystallization proceeds
at 120 1C. Although lower ductility values are obtained when
samples are cold-crystallized at 80 and 100 1C with regard to
melt-crystallized samples, the sample crystallized form the
melt at 120 1C presents an elongation at break of 3.81% as
compared with 4.38% for the sample annealed by coldcrystallization at 120 1C. This behavior can be explained in
terms of differences on the obtained crystalline morphologies
in both crystallization processes. We can anticipate that,
indeed, the brittleness of the sample melt-crystallized at
120 1C may be due to the formation of large spherulites which
promote stress concentrations due to structural defects at
their boundaries (Tjong et al., 1996; KargerKocsis et al., 1997;
Grein et al., 2002), as will be discussed later.
248
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
The mechanical properties of PLLA are dependent on both
the morphology and the amount of crystalline fraction developed during heat treatments. When PLLA is crystallized from
the melt at 80 and 100 1C the proximity of the glass transition
temperature does not allow the polymer to crystallize. On the
contrary, when PLLA is cold-crystallized at 80 and 100 1C the
presence of a great deal of nucleating points developed during
the quenching process results in a rapid development of large
crystalline regions. At high crystallization temperatures, i.e.
120 1C, polymer chains have enough mobility to reorganize
themselves into crystalline domains, giving rise to the formation of large amount of spherulites. Thus, further improvement
of the Young’s modulus can be tailored.
It is well established that the obtained crystalline structure
and morphology influences the thermal, mechanical and optical properties of polymorphic polymers (Kristiansen et al., 2005;
Libster et al., 2007). In addition, in the case of biodegradable
polymers, it was shown that their biodegradation behavior also
depends on the obtained crystal modifications (Furuhashi et al.,
2003). In this way, PLOM was also employed for determining the
semicrystalline nature in heat treated PLLA. Micrographs were
taken during the crystallization process at different melt- and
cold-crystallization temperatures. Fig. 6 depicts the crystalline
microstructure of PLLA observed under polarized light during
the course of melt- and cold-crystallization at different temperatures. It can be observed that cold-crystallization produces
a larger amount of spherulite nuclei appearing at early stages of
crystallization. For instance, when cold-crystallized at 120 1C
3 min are required for the crystallization process, whereas in
the melt-crystallization at 120 1C, a complete crystallinity is
developed after 25 min. Indeed, when crystallization is carried
out from the melt instead from the quenched amorphous
glassy polymer the crystallization process is in overall delayed
because the reduction of nucleation points. Regardless the coldcrystallization temperature, the obtained microstructure
remains similar, giving rise to circular shape crystalline aggregates of about 1–4 mm. On the contrary, the semicrystalline
microstructure of PLLA when it is crystallized from the melt is
narrowly dependant on the melt-crystallization temperature as
will be discussed on Fig. 6. At high crystallization temperatures,
i.e. 120 1C, polymer chains can reorganize themselves in more
ordered crystalline domains, yielding large spherulites of about
40 mm with high perfection and large crystallinity index (Cheng
and Wunderlich, 1988; Kim et al., 2003). As temperature is
decreased, the increase of the nucleation points and the
increase of the medium viscosity creates numerous and smaller
spherulites.
The AFM is a very resourceful tool for determining the surface
topology of materials giving access to 3D morphology details. In
this work, AFM studies were also conducted to analyze the
surface topology feature and roughness of these differently
heat treated PLLA. Next Fig. shows the 3D height AFM images of
quenched, melt-crystallized and cold-crystallized PLLA at 80,
100 and 120 1C. For clarity, Z vertical scales were set at 3000 nm,
being the XY 50 mm. The crystallization procedure used for DSC,
tensile test, DMA analysis, PLOM and AFM samples are considered equivalent, hence the structure-property correlation
could be performed as targeted.
The first image of Fig. 7 is representative of the flat topology
obtained in nearly amorphous quenched PLLA sample. When
quenching a polymer from the melt to temperatures below Tg
polymer chains have not enough time to reorganize themselves
into a more ordered state, resulting in a predominantly amorphous and flat structure. As revealed by AFM, for meltcrystallized specimens, isothermal crystallizations at Tc4Tg
produces rough topologies due to the development of large
and scarce spherulites. At low crystallization temperatures, i.e.
80 1C, a few crystalline aggregates of about 7 mm are surrounded
by extensive amorphous regions. At higher crystallization temperatures voids between spherulites appear as pronounced
valleys whose depth ranges from 170 nm for Tc ¼ 80 1C to
1900 nm for Tc ¼ 120 1C. As shown by the remarkable increase
of crystalline aggregates in the height axis, increasing of Tc yields
spherulites evolving from a 2D or disk-like shape to a 3D or
Fig. 6 – PLOM micrographs taken during isothermal melt- and cold-crystallization at 80, 100 and 120 1C. Each micrograph
represents 717 532 lm2.
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
sphere-like shape. The dimensionality enhancement of the
crystal aggregates with the increase of the crystallization temperature reflected by AFM images for melt-crystallized PLLA
might be due to the presence of numerous nucleation points at
low crystallization temperatures which results in disc-like twodimensional crystal aggregates due to the early impingement of
crystals that cannot further grow. Besides of the experimental
observation of this dimensionality enhancement of crystal
aggregates developed during the crystallization by AFM, with
the Avrami’s theory it is possible to get an insight about the
Rq= 16nm
Ra= 13nm
Quenching
Rq= 74nm
Ra= 54nm
Rq= 71nm
Ra= 55nm
Rq= 95nm
Ra= 75nm
80 ºC
Rq= 237nm
Ra= 177nm
100 ºC
Rq= 566nm
Ra= 383nm
Rq= 61nm
Ra= 49nm
120 ºC
Fig. 7 – Contact-mode 3D AFM height images of PLLA
samples obtained by quenching, melt-crystallization (left
column) and cold-crystallization (right column) at 80 1C,
100 1C and 120 1C. All height scales are 3000 nm, being XY
scales 50 50 lm2.
249
crystal geometry (rods, discs or spheres) depending on the value
of n exponent (Avrami, 1939, 1940, 1941). The evolution of crystal
dimensionality growth from disc-like to sphere-like was also
demonstrated with Avramís theory in one of our previous works
with PLLA/SWCNT composites (Lizundia et al., 2009), where the
presence of single wall carbon nanotubes induced geometry
transformations on PLLA crystals.
However, when cold-crystallization is carried out, the obtained
surface topology remains similar after annealing regardless the
crystallization at different temperatures. This may be due to the
presence of numerous nucleation points obtained during the
quenching treatment in which the obtained glassy PLLA is not
completely amorphous, hence small crystal nuclei appear and
act as nucleating agent for recrystallization, constraining the
spherulite growth as previously revealed by PLOM images.
The mean average surface roughness values of heat treated
samples after completion of crystallization at the selected
temperatures are shown as well in Fig. 7. The report of surface
roughness parameters as a function of heat treatment allows
one to directly differentiate the effect of crystallization process
on PLLA surface topology feature. It is remarkable the influence
of heat treatments on the surface topology of crystallized
samples. Results reveal that while Rq and Ra values of the
melt-crystallized PLLA increased notably with crystallization
temperature, however, the value of those parameters remain
almost constant for the annealed (cold crystallized) samples.
For example, Rq progressively increases from 16 nm for
quenched PLLA to 566 nm for sample crystallized from the
melt at 120 1C. This increment of 35 times is indicative that
surface topology becomes much rougher. By means of meltcrystallizing the samples at higher temperature the enhanced
mobility of polymer chains develops rougher surfaces. The
surface roughness of cold-crystallized PLLA remains almost
constant close to 75 nm because quenching from the melt
produces a residual crystallinity where the created nuclei do
not allow the polymer to recrystallize into large spherulites. It
can be thought that this large change in surface topology
feature of samples following the different crystallization strategies could influence not only the mechanical properties as it
has been proved but also other important properties for
biomedical applications such as cell adhesion, proliferation
and differentiation.
It has been already demonstrated that in semicrystalline
polymers crystallized from the melt that higher Tc’s lead to
larger and less numerous spherulites as revealed by the AFM
images. Table 4 reports the values of spherulites diameter and
nucleation density of heat treated PLLA, both melt-crystallized
and cold-crystallized. After quenching no spherulites were
Table 4 – Crystal aggregate diameter and nucleation density of PLLA samples quenched in water from the melt, meltcrystallized and cold-crystallized at different temperatures.
Heat treatment
quenching
80 1C
100 1C
120 1C
Melt-crystallization
Cold-crystallization
Diameter (lm)
Density (mm2)
Diameter (lm)
Density (mm2)
–
–
30.000
4.335
2.400
–
1.3
1.9
1.8
–
284.000
226.000
235.000
7
16
40
250
journal of the mechanical behavior of biomedical materials 17 (2013) 242 –251
found. A monotonous increase in spherulite diameter from 7 mm
after completion of crystallization at 80 1C to 40 mm at 120 1C is
observed, which is accompanied by a monotonously decrease of
the nucleation density from 30,000 to 2.400 mm2. These results
indicate that average size diameter increases when PLLA crystallization is performed at lower undercoolings from the melt and
is leaded by a lowering of nucleation density in regard to coldcrystallized samples that have been recrystallized after quenching. On the contrary, for the cold-crystallized PLLA, the size of
crystalline aggregates remains constant in the range of
1.3–1.9 mm, with a very high density due to the crystallization
from many microcrystallites obtained during the quenching
process. Those specimens showed morphology consistent with
homogeneously distributed small crystal aggregates that will
favor the heterogeneous recrystallization during annealing.
4.
Conclusions
In this work the effects of melt- and cold-crystallization
(quenching and recrystallization by annealing) on thermal
and mechanical properties poly (L-lactide) have been studied.
Results proved that the obtained final properties of PLLA are
greatly influenced by heat treatment strategies. Increasing
both melt- and cold-crystallization temperatures larger crystalline and rigid amorphous fractions were obtained, together
with a continuous increase of Tg. Dynamic mechanical
analysis revealed coexisting amorphous and crystalline
phases for samples heat treated at 100 and 120 1C. When
samples are cold-crystallized, the structural integrity about
Tg can be better kept, prompting to a smaller E0 reduction
after glass transition, indicating that an increased chain
stiffness is achieved. An increase of the Young’s moduli at
the expense of ductility are found in heat treated samples
with regard to quenched samples suggesting that the presence of larger crystalline and rigid amorphous fraction leads
to a large improvement in stiffness of PLLA. The tensile
stress–strain curves determined at temperatures from below
to above glass transition revealed brittle to ductile and ductile
to elastomeric like transitions. Hence the mechanical properties derived from those tests can be used not only in linear
elastic and visco-elastic models but also in hyperelastic
computing models. Crystalline microstructure analyzed by
PLOM and AFM revealed large differences for the different
crystallization strategies investigated. While the crystal morphology of melt-crystallized specimens depends on the
temperature (increasing crystallization temperatures rougher
surfaces were obtained with larger and more sphere-like
crystal aggregates), when samples are crystallized from the
glassy state, and thus from predetermined nuclei, the obtained
morphology is independent of the crystallization temperature.
Obtained images reveal shorter crystallization times in coldcrystallized samples than in melt-crystallized ones.
Acknowledgments
The authors are thankful for funds of the Basque Government,
Department of Education, Universities and Research (GIC10/
152-IT-334-10) and Department of Industry (IE 10/276). E.L.
thanks the University of Basque Country (UPV/EHU) for a
postdoctoral fellowship.
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