Thermochimica Acta 522 (2011) 110 117
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Thermochimica Acta
journa l h o me page: www.elsevier.com/locate/tca
Crystal polymorphism of poly(l-lactic acid) and its influence on thermal
properties
Maria Laura Di Lorenzo", Mariacristina Cocca, Mario Malinconico
Istituto di Chimica e Tecnologia dei Polimeri (CNR), c/o Comprensorio Olivetti, Via Campi Flegrei, 34, 80078 Pozzuoli (NA),Italy
a r t i c l e i n f o a b s t r a c t
Article history:
The influence of crystal polymorphism on the thermal properties of poly(l-lactic acid) (PLLA) is discussed
Received 15 October 2010
in this contribution. Crystallization of PLLA at high temperatures yields the stable form, whereas at low
Received in revised form
temperatures the metastable modification develops, which is characterized by slightly larger lattice
16 December 2010
dimensions compared to the counterpart, and by some degree of conformational disorder. Quantitative
Accepted 22 December 2010
analysis with conventional and temperature-modulated calorimetry revealed a three-phase structure
Available online 12 January 2011
of PLLA composed of a crystal phase and two amorphous fractions with different mobility, for all the
analyzed thermal histories. A higher coupling of the amorphous chain segments with the crystal phase
Keywords:
was found in the presence of crystals, probably due to the slightly larger lattice dimensions and the
Poly(l-lactic acid)
looser arrangements of PLLA chains in the structure. Some peculiarities in the thermal behavior were
Polymorphism
rationalized, like an unusual frequency-dependence of the reversing apparent heat capacity upon the
Cold crystallization
Rigid amorphous fraction solid solid transition from the to the crystals. Devitrification of the rigid amorphous segments seems
Thermal analysis
also to be differently affected by the coupled crystal structure for the two analyzed crystal modifications
of PLLA.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction the molecular chains of the form crystals have a nearly hexago-
"
nal packing, as the b/a is very close to 3 [3]. Puiggalí et al. later
Poly(l-lactic acid) (PLLA) is a biodegradable and biocompati- suggested that the -form rests on a frustrated packing of three 31
ble polyester that can be produced by renewable resources, like helix chains in a trigonal unit cell with parameters a = b = 1.052 nm,
corn. Being non-toxic to human body, PLLA is used in biomedical c = 0.880 nm, Û = Ç = 90ć%, = 120ć%, with a space group P32 [5]. This
applications, like surgical sutures, bone fixation devices, or con- frustrated structure seems to be formed to accommodate the ran-
trolled drug delivery. Moreover, the good mechanical properties dom up-down orientation of neighbor chains associated with the
and easy of processability make PLLA a good candidate to substitute rapid crystallization under stretching [5].
petroleum-based polymers in selected and commodity application, The form is obtained via epitaxial crystallization on hex-
with the added value of biodegradability. amethylbenzene substrate. It is characterized by two antiparallel
Similar to other biodegradable polyesters, PLLA displays crystal helices with 31 conformation packed in an orthorhombic unit cell
polymorphism, as three main different crystal modifications can with a = 0.995 nm, b = 0.625 nm, c = 0.880 nm [6]. The a (0.892 nm)
develop, named , , and forms, depending on preparation con- and b (0.886 nm) axes of hexamethylbenzene crystals are close to
ditions. The form of PLLA grows upon melt or cold crystallization, the chain axis repeat distance of the form of PLLA in the 31 heli-
as well as from solution. The form has two antiparallel chains cal conformation (0.880 nm). This matching favors the epitaxial
in a left-handed 103 helical conformation (or distorted 103 helix) growth of form crystals of PLLA on hexamethylbenzene crystal
packed in an orthorhombic (or pseudo-orthorhombic) unit cell with surface.
a = 1.066 nm, b = 0.616 nm, c = 2.888 [1 3]. Hot-drawing melt-spun Besides these three main crystal polymorphs, a disordered mod-
or solution-spun PLLA fibers to a high-draw ratio leads to the ification of the form, named form, was recently proposed for
form. An orthorhombic unit cell with six chains in the 31 helical PLLA. The WAXD patterns of the and forms of PLLA are very
conformation, with axes a = 1.031 nm, b = 1.821 nm and c = 0.900 nm similar, with small differences seen in the shift to higher 2 val-
was first proposed for the modification [4]. Similar to crystals, ues of the two strongest reflections, assigned to the (1 1 0)/(2 0 0)
and (2 0 3) planes, and in the appearance of a weak reflection at
2 = H"24.5ć% in the modification. This corresponds to a similar
packing of the two polymorphs, as, analogous to the form, the
" PLLA chains in the modification have a 103 helix conformation
Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address: [e-mail] (M.L. Di Lorenzo). and orthorhombic (or pseudo-orthorhombic) unit cell [7 9]. The
0040-6031/$  see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.12.027
M.L. Di Lorenzo et al. / Thermochimica Acta 522 (2011) 110 117 111
lattice spacings for the (1 1 0)/(2 0 0) and (2 0 3) planes of form period, a pressure of 150 bar was applied for 2 min. Successively
crystals are somewhat larger than those of their counterparts, the press plates, equipped with cooling coils, were quickly cooled
indicating that the form has slightly larger lattice dimensions to room temperature by cold water.
[7,10]. Upon melt or cold crystallization conditions, the form The as-prepared PLLA films were crystallized in oven at different
ć%
is known to grow at low temperatures, whereas crystallization at crystallization temperatures (Tc = 85, 95, 105, 115, 125, 145, 165 C)
ć%
high temperatures leads to formation of the modification. The for 18 h. At low Tc (85 C) the crystallization time was extended to
exact temperature range where each of the two polymorphs pre- 66 h because of the slow crystallization rate, as discussed below.
vails depends on the specific PLLA grade. Upon heating, the less
stable crystals transform to the form, which results in the
2.2. Wide angle X-ray analysis
appearance of multiple endotherms and possible exotherms when
PLLA is analyzed by calorimetry [8 10].
The crystalline structure of PLLA crystallized at different Tc
Crystal polymorphism is known to have a large influence on
was investigated by wide-angle X-ray diffraction analysis (WAXS).
thermal properties of semicrystalline polymers. The variation in
WAXS investigations were carried on PLLA films by means of a
melting behavior caused by different thermal stability of the crystal
Philips (PW 1050 model) powder diffractometer (Ni-filtered CuK
modifications, and the possible interconversion among the various
crystal forms, as reported for PLLA, are the most commonly ana- radiation) equipped with a rotative sample holder. The high voltage
was 40 kV and the tube current was 30 mA.
lyzed effects. In some cases, a variation in the crystal modification
may affect not only the crystal phase, but also the thermal prop- The degree of crystallinity (wC) of PLLA films was evaluated
according to the Hermans Weidinger method, as wC is given by
erties of the amorphous segments. This is the case, for instance,
of isotactic poly(1-butene) (PB-1), as the spontaneous transforma- the ratio between the diffraction due to the crystalline phase (Ic)
and the total diffraction intensity arising from both the amorphous
tion of the metastable form II to the more stable form I results
(Ia) and crystal parts [18]:
in a slight increase of the glass transition temperature and in a
large increase of the rigid amorphous to mobile amorphous ratio,
Ic
despite an unchanged crystallinity [11]. These effects are due not
wC = (1)
only to a shrink of the crystals associated to the solid solid phase Ic + Ia
transformation, caused by a much higher density of form I packing,
The crystallinity values shown below are averaged from seven
but also to the different mobility of PB-1 chains within the crystals,
as the large-amplitude intramolecular chain motion of the tetrag- different PLLA films for each Tc.
onal form II makes it a conformational disordered (condis) crystal
[12].
2.3. Calorimetry
Some varied degree of order of the different crystal polymorphs
was also proposed for poly(l-lactic acid): the molecular packing
The thermal properties of PLLA films were measured with a
within the unit cell of form PLLA is looser and disordered, with
Perkin Elmer Pyris Diamond DSC, equipped with Intracooler II as
larger lattice dimension and weaker interchain interaction [7,9,10].
cooling system and with a Mettler DSC 822e calorimeter equipped
A preliminary analysis by Zhang et al. [13] suggested that the chain
with a liquid-nitrogen cooling accessory. Both the instruments
conformation of and crystal modifications are somewhat dif-
were calibrated in temperature with high purity standards (indium
ferent, but quantitative results have not been reported yet. The
and cyclohexane) and in energy with heat of fusion of indium. Dry
disorder of the chains within the crystals is conformational,
nitrogen was used as purge gas at a rate of 48 ml/min. To obtain
which makes this crystal modification a mesophase (condis crys-
precise heat capacity data, each measurement was accompanied
tal) [14]. As discussed in this contribution, the varied disorder of the
by an empty pan run, and a calibration run with sapphire under
crystal packing in PLLA affects the thermal properties not only of
identical conditions [19]. All the measurements were repeated at
the crystal phase, but also of the coupled amorphous portions. Two
least three times to improve accuracy.
types of amorphous fractions are usually present in semicrystalline
The conventional differential scanning calorimetry (St-DSC)
polymers: a mobile amorphous phase (MAF), made of the poly-
analyses were conducted with the Perkin Elmer Pyris Diamond
mer chains that mobilize at the glass transition temperature (Tg),
ć%
DSC at the scanning rate of 20 C/min. Temperature-modulated
and a rigid amorphous fraction (RAF), made of the polymer chains
ć%
calorimetry (TMDSC) at the underlying heating rate of 2 C was
coupled with the crystal phase that usually devitrify at higher tem-
conducted with the Perkin Elmer Pyris Diamond DSC using a
peratures [15,16]. The influence of crystal polymorphism on the
ć%
modulation amplitude of 0.4 C and periods of temperature oscilla-
relative ratio of the crystal and of the two amorphous fractions
tions ranging from 60 to 120 s. Quasi-isothermal TMDSC data were
is also analyzed in this contribution. As the thermal properties of
gained with the Mettler DSC 822e calorimeter, using a sawtooth
poly(lactic acid) are highly affected by the stereochemistry of the
ć%
oscillation with a temperature amplitude of 0.4 C and a modula-
repeating unit [17], a polymer with a very high amount of l-lactic
tion period of 60 s about a base temperature To, which was raised
acid was used.
ć%
stepwise in temperature increments of 5 C after 16 min at each To.
From TMDSC measurements the reversing specific heat capac-
ity was obtained from the ratio of the amplitudes of modulated
2. Experimental
heat flow rate and temperature, both approximated with Fourier
series [20,21]. The reversing specific heat capacity values reported
2.1. Materials and sample preparation
in this contribution were obtained from the first harmonics of the
Fourier series. Similar to conventional DSC analyses, each TMDSC
Poly(l-lactic acid) (PLLA), Biomer L9000, was purchased from
measurement was accompanied by an empty pan run, and a cali-
Biomer Biopolyesters, Germany. Before use PLLA was dried in a vac-
ć%
bration run with sapphire under identical conditions [19]. The good
uum oven at 60 C for 24 h to avoid hydrolysis of the polymer during
agreement between the experimental data and the thermodynamic
melt-processing.
heat capacity of solid and liquid PLLA [22] proves that the modula-
After drying, the PLLA chips were compression-molded with a
ć%
tion periods used are long enough to be corrected satisfactorily by
Carver Laboratory Press at a temperature of 185 C for 4 min, with-
the calibration with sapphire.
out any applied pressure, to allow complete melting. After this
112 M.L. Di Lorenzo et al. / Thermochimica Acta 522 (2011) 110 117
(110/200)
a
85°C 66 h
2.5
95°C 18 h
8
105°C 18 h (203)
2.0
115°C 18 h
125°C 18 h
85°C
145°C 18 h
6
1.5
95°C
165°C 18 h
105°C
4
50 100 150 115°C
125°C
145°C
2
165°C
50 100 150
10 15 20 25 30
Temperature (°C) 2¸ (°)
Fig. 1. Specific heat capacity of PLLA after isothermal cold crystallization at the indi-
cated temperatures. The dashed lines are the solid and liquid specific heat capacities
b (010) (015)
(204)
of PLLA, as taken from Ref. [22].
(016)
(206)
Ä…
Ä…'
(115)
(004)/(103)
(018)
(207)
3. Results and discussion
85°C
The thermal analysis of poly(l-lactic acid) after isothermal cold
95°C
crystallization at various temperatures is shown in Fig. 1. The
apparent heat capacity (cp) data measured upon heating at the con- 105°C
ć%
stant linear rate of 20 C/min are compared to thermodynamic cp
115°C
values of solid and liquid PLLA, as taken from Ref. [22]. The multiple
125°C
melting and recrystallization behavior of PLLA is largely affected by
the thermal history, which in turn determines its polymorphism 145°C
ć%
[8 10,23 25]. At high crystallization temperatures (Tc e" 145 C)
165°C
only the form is present, as proven by the WAXS data shown
10 12 14 20 22 24 26 28 30
below, and one single melting peak is observed, as the material
goes on fusion directly from the fully ordered crystal to the melt, 2¸ (°)
without changing its crystal modification [8,10,24]. PLLA films crys-
Fig. 2. (a) WAXS profiles of PLLA samples crystallized at different Tc. (b) Enlarged
tallized at lower temperatures, where either and forms coexist,
WAXS profile of PLLA samples crystallized at different Tc.
or is the only crystal modification present in the film before the
DSC scan, display multiple thermal events, the most notable ones
include a major exotherm after partial melting, followed by a large where the initial crystals transform into the structure during
endothermic peak. This complex melting behavior is to be linked to heating, as seen in Fig. 1, as well as because of the lack of precise data
metastability of crystals, that convert to the stable modification on enthalpy of fusion of both the polymorphs and of the enthalpy
during heating [10]. of transition from the metastable to the stable crystal modification.
The polymorphic composition of PLLA in dependence of ther- Besides the conventional DSC analyses exhibited in Fig. 1,
mal history was determined by wide-angle X-ray diffraction. Fig. 2 TMDSC experiments were conducted for all the analyzed crys-
shows the WAXS patterns of PLLA after crystallization at various tallization temperatures. Specific examples are presented for two
temperatures. For easier comparison, all the diffraction patterns selected samples, containing only one of the two analyzed poly-
were normalized using the strongest (2 0 0)/(1 1 0) reflection inten- morphs, to illustrate the different properties of the two crystal
sity [24]. Indexing of the observed reflections is based on the crystal modifications. Fig. 3a reports the St-DSC and TMDSC analyses of
ć%
structure reported for the ordered modification [26,27]. With PLLA after isothermal crystallization at 85 C for 66 h. On the same
ć%
increasing Tc the reflections of (2 0 0)/(1 1 0) and (2 0 3) planes shift plot, the St-DSC analysis of PLLA crystallized at 85 C for 18 h is also
to higher 2 , together with an increase of (0 1 0) and (0 1 5) reflec- presented, to show that at this temperature crystallization of PLLA
tions intensities, evidenced in the enlarged WAXS profiles reported for 18 h is largely incomplete. This is confirmed by the much larger
in Fig. 2b. Moreover, small diffraction peaks at 2 = 12.5ć%, 20.8ć%, heat capacity step at the glass transition, that indicates a higher
24.1ć%, and 25.1ć% appear at high Tc, which are assigned to the reflec- mobile amorphous fraction, as well as by the broad exotherm that
ć% ć%
tions of (0 0 4)/(1 0 3), (2 0 4), (1 1 5), (0 1 6), and (2 0 6) planes of extends from about 85 90 C up to 145 C, that reveals large crys-
crystals, respectively, while they are absent in the samples crystal- tallization during heating. It is worth to note that in the poorly
ć%
lized at Tc d" 95 C. At low Tc a reflection at 2 = 24.6ć%, characteristic crystallized PLLA the glass transition of the MAF is located at lower
of crystals, can be detected [2,28]. These results suggest that at temperatures, compared to the polymer maintained at Tc for much
ć%
Tc d" 95 C the analyzed PLLA grade crystallizes only in the form; longer times, which reveals the marked influence of the semicrys-
ć% ć% ć%
at 105 C d" Tc d" 125 C both and forms coexist; at Tc e" 145 C talline structure on the amorphous PLLA chain segments.
only the modification is present, which is in good agreement with An enlargement of the PLLA data gained after crystallization at
ć%
the available literature data on the temperature-dependence of for- 85 C for 66 h is illustrated in Fig. 3b. Below the glass transition
mation of the two different polymorphs of PLLA [7,8,10,13,24,29]. region and above completion of melting, St-DSC and TMDSC exper-
The WAXS data shown in Fig. 2 were used to determine the imental data well agree with thermodynamic cp of solid and liquid
crystal fraction of PLLA after each thermal treatment. This proce- PLLA, respectively. The specific heat capacity of PLLA, measured by
dure was preferred to integration of the DSC melting endotherms St-DSC, starts to deviate from thermodynamic cp of solid PLLA at
ć%
because of the complex melting behavior of PLLA, especially in cases around 60 C, in correspondence of the onset of the glass transition
p
c
[J/(K g)]
Intensity (a.u.)
Intensity (a.u.)
M.L. Di Lorenzo et al. / Thermochimica Acta 522 (2011) 110 117 113
1.88
a
St-DSC 20°C/min
8
TMDSC p=60s
TMDSC p=90s
TMDSC p=120s
TMDSC Q-Iso
6
1.86
Tc=85°C 18 h
4
1.84
2
5 10
50 100 150
Time (min)
Temperature (°C)
Fig. 4. Time dependence of the reversing specific heat capacity of PLLA during quasi-
ć%
isothermal TMDSC analysis at 100 C.
b
St-DSC 20°C/min
TMDSC p=60s this thermal event may be interpreted as either a second glass tran-
2.5
TMDSC p=90s sition, followed by a weak and broad exotherm that extends from
ć%
TMDSC p=120s
about 100 to 130 135 C, or as a weak and broad endotherm cen-
ć%
TMDSC Q-Iso
tered around 100 C. The appearance of a double glass transition
in PLLA was reported in a number of papers, on the basis of St-
2.0
DSC or dynamical mechanical analyses [33 35]. In some cases, this
second relaxation was ascribed to mobilization of the rigid amor-
phous fraction. From above the glass transition temperature up to
ć%
about 100 C the apparent cp curve measured by St-DSC increases
1.5
beyond the cp level that corresponds to vitrified rigid amorphous
fraction. This may be connected to a partial devitrification of the
RAF, as seen by comparison of the St-DSC trace with the base-
line heat capacities drawn in Fig. 3b on the basis of the two-phase
model, that accounts for the crystal phase and a single amorphous
50 100 150
fraction, and of the three-phase model, that takes into account
Temperature (°C)
the crystal and two amorphous fractions with different mobility,
ć% respectively. Quantitative analysis by TMDSC in Fig. 3b shows a
Fig. 3. (a) Specific heat capacity of PLLA after cold crystallization at 85 C for 66 h.
The thick black solid line is the total heat capacity by St-DSC, the red, green and blue frequency-dependence of the reversing heat capacity, starting from
ć%
lines are the reversing specific heat capacity measured by TMDSC at modulation
80 C, which may indicate some reversing exchange of latent heat
periods p = 60, 90, 120 s, respectively, the yellow circles represent the reversing heat
in this temperature range. This interpretation may be not unique,
capacity measured in quasi-isothermal mode of modulation, the dashed black lines
since during devitrification the reversing cp is also affected by the
are the solid and liquid specific heat capacities, as taken from Ref. [22]. The St-DSC
ć%
periodicity of temperature oscillation [16], as also seen in the tem-
data of PLLA after cold crystallization at 85 C for 18 h are also shown as thin black
solid line. (b) Enlargement of the plot shown in (a) in the area of changing baseline
perature range of the glass transition of the MAF. However, the
cp. (For interpretation of the references to color in this figure legend, the reader is
two processes (fusion and devitrification) have different response
referred to the web version of this article.)
to small oscillations of the temperatures, and may be distinguished
by quasi-isothermal TMDSC analysis, which usually provides differ-
of the mobile amorphous fraction. In the temperature region of the
ent outputs in the time domain when a polymer is analyzed in the
glass transition, a minor frequency-dependence of the reversing
glass transition or in the melting range. In the first case the revers-
heat capacity can be observed. The dynamic Tg, i.e. the glass transi-
ing cp remains practically constant with time, whereas a slow decay
tion originating from temperature modulation and obtainable from
is observed upon reversing melting [16].
the reversing cp curve, is observed at temperatures slightly higher
The time-dependence of the reversing cp of the quasi-isothermal
ć%
than the devitrification process deriving from linear heating (ther-
TMDSC analysis of Fig. 3 at 100 C, i.e. at the peak temperature
mal glass transition). This can be explained considering that the
of the apparent small endotherm, or at the end of the apparent
frequencies related to the ordinary linear heating rates are different
Tg in the St-DSC plot of Fig. 3b, is exhibited in Fig. 4. The slight
from those used in TMDSC measurements, the latter being gener-
decrease of the reversing cp with time reveals the occurrence of
ally higher [30 32]. The experimental data of Fig. 3 were used to
some reversing melting, and that the frequency-dependence of the
ć%
determine the three-phase composition of PLLA. The heat capac-
TMDSC curves measured at the underlying heating rate of 2 C/min
ity step at Tg accounts for a mobile amorphous phase content (wA)
is to be linked to latent heat exchanges that cause an increase of
of 0.43. The crystal fraction, measured by WAXS, is wC = 0.33. The
the computed reversing cp beyond the reversible cp values [36 38].
rigid amorphous fraction is quantified by difference using Eq. (2):
The thermal event under analysis can therefore be linked to fusion
of smaller and/or more defective crystals, probably grown under
wC + wA + wRA = 1 (2)
secondary crystallization, followed by crystallization of additional
ć%
which yields a value of wRA = 0.24 for PLLA after cold crystallization chain segments above 100 C.
ć%
at 85 C for 66 h. No quantitative information on devitrification of the RAF cou-
A notable thermal event appears in Fig. 3b a few degrees above pled with crystals can be derived from the data of Fig. 3, due to
completion of the glass transition. On the basis of St-DSC data only, the overlapping of partial melting of crystals and transformation
p
c [J/(K g)]
p
c [J/(K g)]
p
c [J/(K g)]
114 M.L. Di Lorenzo et al. / Thermochimica Acta 522 (2011) 110 117
of the metastable structure into the more stable crystals. The
a 10
quasi-isothermal TMDSC data intersect the two-phase baseline at
p=60s (exp)
ć%
135 C, but the temperature at which the reversing heat capacities
p=60s (calc)
reaches the value expected for full devitrification of the RAF is prob-
ably affected by other simultaneous thermal events, which may 5
increase the level of the measured reversing cp. Therefore, it is likely
that devitrification of the RAF reaches completion in temperature
range of the main melting endotherm [39].
0
Another noteworthy feature of the plots shown in Fig. 3a is the
unusual frequency-dependence of the reversing heat capacity in
ć%
the temperature range around 150 C, in correspondence of the
exothermic peak visible in the St-DSC traces. This uncommon trend
-5
was observed for all analyzed crystallization temperatures, where
ć%
some amounts of crystals are present (85 d" Tc d" 125 C). As men-
tioned above, in correspondence of polymer melting, a decrease of
modulation frequency (or an increase of amplitude of temperature
140 150 160
oscillation), usually leads to a higher apparent reversing heat capac-
Temperature (°C)
ity, because a decrease in the frequency of modulation permits
a larger percentage of crystalline material to follow the modula- b 10
tion within a single temperature cycle [11,35,40 46]. Similarly, an
p=120s (exp)
increase in modulation amplitude implies that a higher fraction of p=120s (calc)
the crystallites that is involved in the melting process is added to
5
the reversing signal. This kind of dependency of the reversing cp on
the frequency of modulation is seen in the data of Fig. 3a, except
ć%
around 150 C, where the data gained at lower modulation period
display a higher apparent reversing cp. In order to clarify the ori-
0
gin of this unusual trend, the raw modulated heat flow data were
analyzed.
Fig. 5 reports the modulated heat flow rate of PLLA isothermally
ć%
-5
crystallized at 85 C for 66 h, analyzed by TMDSC at the underlying
ć%
heating rate of 2 C/min and at modulation periods of 60 and 120 s.
These data are compared in Fig. 5 with the modulated heat flow
rate with the same modulation parameters, without distortions
140 150 160
caused by the occurrence of thermal processes. The latter curves
were obtained by computer-simulation from the experimental raw
Temperature (°C)
data taken above completion of melting, i.e. in absence of thermal
Fig. 5. Experimental and simulated heat-flow rates of PLLA, obtained after isother-
events, using the procedure detailed in Refs. [38,47]. Comparison
ć%
mal crystallization at 85 C for 66 h: (a) p = 60 s and (b) p = 120 s.
of experimental and simulated heat flow rate data allows to deter-
mine the latent heat exchanged during each oscillation period.
ć%
Above 160 C, in the region of the main melting peak, large dis- of long period of oscillation, p = 120 s, despite the larger percentage
tortions in the experimental curves can be observed in both their of crystalline material that follows the modulation within a single
endothermic and exothermic parts, and the effect is much larger temperature cycle, the neat latent heat that is exchanged in each
at higher modulation period. As a result, the modulated heat-flow- modulation cycle (endothermic minus exothermic heat) is lower
rate amplitude, and in turn the reversing cp, increases with the than when lower periods of temperature oscillation are used. This
period of temperature oscillation. At lower temperatures, around results in a lower amplitude of modulated heat flow rate, when the
ć%
150 C, the modulated heat-flow-rate curves are deformed to a experimental data are approximated with a Fourier series in each
lower extent, but still both endothermic and exothermic events modulation cycle, and in turn in a lower apparent reversing cp, as
ć%
can be detected, which are linked to partial melting and to the seen in Fig. 3a around 150 C.
ongoing phase transformation from the metastable structure to The thermal analysis of PLLA after isothermal cold crystalliza-
ć%
the stable form. In the area of interest, highlighted by the arrow tion at 145 C for 18 h is presented in Fig. 6a, with an enlargement of
in Fig. 5b, in the experimental curve gained at p = 120 s endother- the cp data shown in Fig. 6b. As revealed by the WAXS plots of Fig. 2,
ć%
mic events take place during the heating segment around 150 C. this thermal history leads to development of the crystal modifica-
The initial increase in the heat-flow-rate, caused by the switch to tion only, and a single major melting endotherm appears in the DSC
ć%
a different scanning rate overlapping partial melting, is followed plots of Fig. 6. The glass transition of the MAF is centered at 64 C,
ć%
by exothermal effects, as revealed by comparison with the sim- a few degrees below the Tg of the polymer crystallized at 85 C for
ć%
ulated data. The initial increase of amplitude of modulated heat 66 h (Tg = 66 C). This slight decrease of the Tg of PLLA at increas-
flow rate linked to latent heat release is followed by a decrease ing crystallization temperatures, very close to the experimental
of the oscillation amplitude, as the experimental modulated heat uncertainty, is in agreement with literature data [48]. From the
flow rate curve falls below the simulated plot before the switch heat capacity step at Tg a mobile amorphous fraction wA = 0.31 is
to the next oscillation segment. Such a decrease of the experi- measured, which, compared to the wA = 0.43 computed after crys-
ć%
mental data below the level corresponding to the simulated curve tallization at 85 C for 66 h, indicates that crystallization at higher
is not seen in the curve gained at p = 60 s, shown in Fig. 5a, due temperatures leads to a reduction of the MAF content. The crys-
ć%
to the short modulation period. Similarly, in the preceding half- tal fraction measured by WAXS after crystallization at 145 C is
cycle at p = 120 s, the exotherm overlaps endothermic latent heat wC = 0.45, which leads to a rigid amorphous content wRA = 0.24.
exchange, and again, crosses the simulated curve before the end of Above completion of the glass transition, the St-DSC and the
the modulation half-period. The overall result is that, in the case TMDSC data of Fig. 6, including the quasi-isothermal analysis, over-
Modulated
Åš
(W/g)
Modulated
Åš
(W/g)
M.L. Di Lorenzo et al. / Thermochimica Acta 522 (2011) 110 117 115
a
wC
St-DSC 20°C/min
0.6 wA
TMDSC p=60s
8
TMDSC p=90s wRA
TMDSC p=120s
TMDSC Q-Iso
6
0.4
4
2
0.2
50 100 150
80 100 120 140 160
Temperature (°C)
Tc (°C)
b
St-DSC 20°C/min Fig. 7. Crystalline (wC), mobile amorphous (wA), and rigid amorphous (wRA) frac-
tions of PLLA after isothermal cold crystallization at various Tc.
TMDSC p=60s
2.5
TMDSC p=90s
TMDSC p=120s
ć%
TMDSC Q-Iso
measured after crystallization at 85 C compared to crystallization
ć%
at 95 C. This decrease is probably related to the extended crystal-
2.0
ć%
lization time at 85 C (66 h) compared to the other Tc (18 h), which
may induce a larger extent of secondary crystallization, with possi-
ble insertion of thin lamellae in the interlamellar amorphous layer
or interlamellar stacks [54]. The broad and weak endotherm in the
1.5 ć%
DSC data of Fig. 3 around 100 C, discussed above, confirms the exis-
tence of a small population of thin and defective lamellae with very
ć%
poor thermal stability in PLLA crystallized at 85 C for 66 h.
It is interesting to note that a decrease of the crystallization tem-
ć%
50 100 150
perature from 95 to 85 C corresponds to a considerable increase of
the rigid amorphous fraction. Upon melt crystallization, it has been
Temperature (°C)
often reported that a higher rate of crystal formation, like upon
ć%
Fig. 6. (a) Specific heat capacity of PLLA after cold crystallization at 145 C for 18 h. ć% ć%
crystallization at Tc = 95 C compared to Tc = 85 C [50], leads to
The black line is the total heat capacity by St-DSC, the red, green and blue lines
short times for the adjustment of the crystals into the locally ener-
are the reversing specific heat capacity measured by TMDSC at modulation periods
getically most favorable states. Internal stresses are not released
p = 60, 90, 120 s, respectively, the yellow circles represent the reversing heat capacity
measured in quasi-isothermal mode of modulation, the dashed lines are the solid during crystal growth, and concentrate at the interface between the
and liquid specific heat capacities, as taken from Ref. [22]. (b) Enlargement of the plot
crystal and amorphous phases, resulting in a large rigid amorphous
shown in (a) in the area of changing baseline cp. (For interpretation of the references
fraction [16]. The formation of secondary crystals upon prolonged
to color in this figure legend, the reader is referred to the web version of this article.)
ć%
crystallization at 85 C may also be linked to an increased coupling
between the amorphous and crystalline areas: rearrangements of
ć%
lap up to 130 C, in correspondence of the onset of the melting the amorphous regions localized in proximity of the growing sec-
endotherm, which starts at temperatures slightly lower than Tc, ondary lamellae are subjected to geometrical restrictions, in which
probably due to some residual crystallization during cooling to the melt undergoes larger constraints, with consequent forma-
room temperature. The overlapping of reversing and total cp data tion of rigid amorphous phase. This may confirm the hypothesis
reveals that negligible latent heat exchanges take place up to the often appeared in the literature, of a connection between secondary
onset of melting and that the increase of the experimental cp values crystallization and vitrification of the rigid amorphous segments
up to the beginning of melting is to be linked to devitrification of [40 46]. A reduction in crystallization temperature corresponds
the rigid amorphous segments of PLLA. Unfortunately, the overlap- also to lower chain mobility, especially in the temperature range
ping of reversing melting to the cp increase due to devitrification, under consideration, that is very close to the glass transition of
before the intersection of the experimental reversing cp data with the mobile amorphous fraction, which may complicate rearrange-
the two-phase baseline, does not allow to estimate the exact point ments of the chains at the crystal-amorphous boundary, leading to
of full devitrification of the RAF of PLLA coupled with the crystals, an increased fraction of amorphous material under local stress at
which however seems to attain full mobility at temperatures close the crystal surfaces.
ć%
to the onset of crystal melting. The wRA vs. Tc plot of Fig. 7 displays a maximum at 125 C. At
The three-phase composition of PLLA after isothermal cold high temperatures wRA decreases with Tc, as commonly reported
crystallization at various temperatures is illustrated in Fig. 7. Crys- in the literature for a number of semicrystalline polymers [16]. At
ć% ć%
tallinity increases with the crystallization temperature in the whole 95 C d" Tc d" 125 C the opposite trend can be observed. An increase
ć%
analyzed range, with a discontinuity around 110 120 C, as often in crystallization temperature in this range corresponds to a larger
reported in the literature [49 53]. This irregular trend is to be linked fraction of to crystals, as shown in Fig. 2 [55]. It is likely that
to growth of PLLA crystals in the two different polymorphs and crystallization of PLLA into the ordered modification leads to a
the corresponding varied crystallization kinetics. The mobile amor- larger coupling of the amorphous and crystalline chain segments,
phous fraction decreases with Tc for all the analyzed crystallization compared to the conformationally disordered arrangement. A
temperatures, with the only exception of a slightly lower wA value similar influence of the varied order in the crystal structure was
p
c [J/(K g)]
C
A
RA
w , w , w
p
c [J/(K g)]
116 M.L. Di Lorenzo et al. / Thermochimica Acta 522 (2011) 110 117
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