Materials Sciences and Applications, 2011, 2, 439-443
doi:10.4236/msa.2011.25058 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
The Effect of Annealing Treatments on Spherulitic
Morphology and Physical Ageing on Glass
Transition of Poly Lactic Acid (PLLA)
El-Hadi Ahmed Mohamed1,2
1Department of physics, Faculty of Applied Science, Umm al-Qura University, Mecca, Kingdom of Saudi Arabia; 2Department of
Basic Science, Higher Institute for Engineering and Technology, El Arisch, Nord Sinai, Egypt.
Received October 29th, 2010; revised December 24th, 2010; accepted May 9th, 2011.
Spherulitic morpho logy of pure poly lactic acid (PLLA) PLLA have investigated after thermal annea ling. The morphol-
ogy of spherulite of pure poly lactic acid (PLLA) PLLA have investigated after thermal annealing. The effect of both
annealing temperature and crystallization temperature on the formation of cracks was described by polarized optical
microscope (POM). Non banded spherulite (fibrils ) with cracks was detected in PLLA film after annealing at 160˚C
(180 min.) and isothermal crystallizatio n temperatures at 140˚C and 150˚C. With increasing temperature after anneal-
ing treatment the size of spherulite is increased and more cracks are formed. The maximum growth rate of spherulites
was found at 130˚C. The physical ageing was carried out by annealing the PLLA sample at room temperature for sev-
eral annealing time (ta) from 0 h to 720 h. The enthalpy relaxation has been studied by differential scanning calorimetry
(DSC) through analysis of the endothermic peak of glass transition temperature, which increased and shifted towards
higher temperatu re as the annealing time increased.
Keywords: Poly Lactic Acid (PLLA), Morphology, Non-Ban ded Spherulites, Annealing, Polarized Optical Microscopy,
Physical Ageing
1. Introduction
Most of the plastic materials from petrochemicals such as
polyethylene (PE), polypropylene (PP), poly vinyl chlo-
ride (PVC) and polystyrene (PS) are used in various
areas, such as household appliances, auto parts, building
materials and packaging of food, because of their excel-
lent processing, physical properties and durability. How-
ever there is a problem with the disposal of these prod-
ucts. It is making many environmental problems (pollu-
tion), such as the disposal of municipal solid waste by
burning. It leads to the rise of toxic gases associated with
rising temperature and increase in the concentration of
carbon dioxide in the atmosphere.
The last twenty years attracted the attention of many
scientists to this problem and they have been looking at
the production of bioplastics materials called green po-
lymers or environment polymers. The most important of
these new materials are poly(3-hydroxybutyrate) (PHB)
[1-3] and poly lactic acid (PLLA). PLLA [4] is polyester
aliphatic, insoluble in water, biodegradable thermo plas-
tic and transparent.
PLLA has a crystallinity of about 40%, glass transition
temperature of 50˚C - 70˚C and melting temperature of
173˚C - 178˚C. Although the lactic acid has been known
for more than a century. Its scientific and commercial
interest has increased only in recent years around the
world because of environmental pollution. Now, it is the
next generations alternative to synthetic polymers from
petrochemicals. Lactic acid can produce from renewable
resources (molasses from sugar cane and syrup date)
using bacterial fermentation (micro-organisms, bacteria),
remnants of food and dairy plants. PLLA can be used as
sutures and bone implants (screws, pins, plates, fixation
rods and clips, etc), pharmaceutical, cosmetic, textiles
industries and food packaging applications, shopping
bags, especially for deep drawing articles, thermoformed
products such as drinking cups, take-away food trays,
containers and plant boxes and diapers [4].
Advantages: PLLA is biodegradable and insoluble in
The Effect of Annealing Treatments on Spherulitic Morphology and Physical Ageing on
Glass Transition of Poly Lactic Acid (PLLA)
Copyright © 2011 SciRes. MSA
water, polyester, optics actives, crystalline and higher
melting temperature [4-11].
Disadvantages: PLLA is brittle, higher cost of produc-
tion, poor mechanical properties and slow crystallization
rate [8-11], compared to synthetic polymers like polys-
tyrene (PS), polypropylene (PP) and polyethylene tere-
phthalate (PET).
It is known that semi-crystalline polymers can form
spherulites when crystallized from solution or melts. The
morphology of PLLA is mainly spherulitic. The spheru-
lites made of thin fibrils are oriented along the radius of a
circle. With increasing the crystallization time, the sphe-
rulites grow and impinge with each other longitudinally.
The study of thermal stability and morphology is im-
portant [12-17] in polymer processing because control the
temperature of the end product during the processing like
injection molding or casting film by chill roll is necessary.
The physical properties of semi-crystalline polymers de-
pend upon the mechanisms of crystallization and mor-
phology during processing. Mechanisms of crystallization
is the key to the basic information for understanding the
relationship between manufacturing processes and fea-
tures observed in the final product of the polymer.
It is important to study the physical aging [18] to de-
sign and develop the polymers with long term durability,
i.e. understand the safe use of polymers. The aging
process is common in non-crystalline polymer and re-
ferred to as structural relaxation and is happening be-
cause these materials do not exist in the state of thermo-
dynamic equi- librium. Physical aging is a phenomenon
that is caused by material evolution towards thermody-
namic equilibrium. Physical ageing is called change in
amorphous phase by annealed at different temperature
(Ta) at near or lower than its glass temperature (Tg). This
leads to changing in physical properties such as density,
enthalpy, dielectric permittivity, electrical conductivity
and refractive index, also make changes in the mechani-
cal properties like Young’s modulus and yield stress.
This occurs during longer storage time by use of poly-
The aim of present work is to investigate the effect of
annealing treatments on the morphology of PLLA using
polarized optical microscopy (POM). Furthermore, the
physical ageing phenomenon and the glass transition
temperature were studied using differential scanning ca-
lorimetry (DSC).
The aim of this study is to investigate the effect of an-
nealing treatments of PLLA on morphology using pola-
rized optical microscopy (POM) and the physical ageing
phenomenon on the glass transition temperature using
differential scanning calorimetry (DSC).
2. Experimental
Poly (L-Lactic acid) PLLA, grade L9000 was obtained
from Biomer®, Germany. PLLA had initially a molar
mass of 220 000 g/mol.
PLLA as pellet was prepared by dissolving in hot
chloroform (0.04 g/ml), and then the solution was casted
on microscopy glass slide as a thin film (100 - 300 nm).
The casting films held for 24 hour to evaporate the sol-
vent and dried at 60˚C for 1 day to remove the solvent
completely. The casted film with glass slide was covered
with another glass slide. The casted film was placed be-
tween two microscopy glass slide as a sandwich and in-
serted to a hot stage and melted at 200˚C for 3 min to
erase their thermal history and cooled slowly to the iso-
thermal crystallization temperature (Tc). The sample was
held at both (Ta) and (Tc) for the required time to develop
The morphology of PLLA was studied by optical mi-
croscopy (Nikon Eclipse E600) equipped with hot-stage
(Instec STC200). The sample was heated on the hot-stage
from room temperature to 200˚C, held 3 min. for com-
plete melting of the polymer, and cooled slowly from
200˚C to annealing temperature (Ta = 160˚C) and iso-
thermal crystallization Tc of 140˚C and 150˚C where the
growing of spherulites started. A new sample was used
each time to keep away from any effects due to degrada-
tion processing by higher melting processing temperature
(Tmp = 200˚C).
Glass transition temperature (Tg), heat capacity (Cp)
and enthalpy of relaxation (ΔH) are determined by diffe-
rential scanning calorimetry (DSC). The sample was
heated from room temperature to 200˚C with the a rate of
10˚C·min1 and kept at ageing times 24 h, 48 h, 96 h and
720 h (first run). Whereas in the second run, the samples
were heated from 25˚C to 80˚C with a rate of 10˚C·min1
(second run). The enthalpy of relaxation was seen as an
endotherm near Tg.
3. Results and Discussion
3.1. Morphology and Spherulite Growth Rate
Spherulitic textures are morphological features. It ob-
served in semi crystalline polymers. A generation of
spherulite started from lamellar crystal, which is often
formed at the beginning of the crystal flakes (nucleation).
Moreover, spherulite can be formed in two or three di-
mensions (spherical), crystal circular that displays in
maltese cross between polaroids (polarizer and analyzer).
Figure 1 is shown the polarized optical micrographs of
neat PLLA spherulites with cracks. These cracks depend
on the crystallization temperature (Tc) and the annealing
temperature (Ta). Cracks appear after melting, annealing
The Effect of Annealing Treatments on Spherulitic Morphology and Physical Ageing on
Glass Transition of Poly Lactic Acid (PLLA)
Copyright © 2011 SciRes. MSA
(a) (a')
(b) (b')
Figure 1. Optical microscopy images of the crystallizations of PLLA after annealing for 180 min. and isothermal crystalli-
zation with optical polarizer at (a) 140˚C, (b) 150˚C; without optical polarizer (a'), (b'). The scale bar measures 50 µm.
temperature and then the crystallization temperature. We
have seen in Figures 1(a), (a') few cracks at the crystal-
lization temperature of 140˚C. By increasing the crystal-
lization temperature to 150˚C in Figures 1(b), (b'), this
leads to an increase the number of the cracks. These
cracks did not appear in the crystallization process with-
out annealing. Same results found in poly(3-hydroxy-
butyrate) PHB [3]. Keller and Hobbs [19] found large
spherulites with cracks and splitting around the center
circular in PHB spherulite. They attributed the formation
of cracks due to the cooling and difference in coefficients
of thermal expansion between the plastic film and glass
It is important to measure spherulite growth rates of
PLLA in the range from 90˚C to 150˚C. The radial
growth rate of the spherulite was determined by moni-
toring the spherulite radius R as a function of time during
isothermal crystallization in the hot-stage of a polarizing
microscope. The curve of growth rate of spherulite as
shown in Figure 2 shows clearly that the maximum
spherulite growth rate of PLLA is at 130˚C. The decrease
in spherulite diameter is due to reduction in the crystalli-
The Effect of Annealing Treatments on Spherulitic Morphology and Physical Ageing on
Glass Transition of Poly Lactic Acid (PLLA)
Copyright © 2011 SciRes. MSA
Figure 2. Spherulite growth rates of PLLA, isothermal
crystallization after cooling from the melt as a function of
zation temperature. The sharp peak at crystallization
temperature of 120˚C is similar to the results reported in
Refences [14,16]. It was also found that the crystalliza-
tion kinetics of the investigated PLLA is very slow. This
is due to the fact that the number of homogenous nuc-
leated is small at temperatures more than 130˚C, which
causing longer time period at the processing. We found
that the crystallization kinetics of the PLLA is very slow
because the number of homogenous nucleation are small
at more than 130˚C, causing longer time period at the
processing. This means that the spherulites are large and
make the material more brittle.
3.2. DSC Analysis
Physical aging in polymers can occurs at deferent time at
room temperatures below the glass transition temperature
(Tg). DSC thermograms were recorded on heating as a
function of aging time (ta). Figure 3 shows the relation
between heat flow and temperature. It was shown that,
the glass transition temperature (Tg) of pure PLLA ap-
peared at 60˚C, cold crystallization (Tcc) at 108˚C, melt-
ing point (Tm) at 174˚C. Figure 4 shows DSC curves for
the PLLA samples with different ageing time and the
glass transition temperature was determined. The result
the ageing of PLLA has also strong influence on the
glass transition temperature. The enthalpy relaxation was
found to be increase with ageing time and the glass tran-
sition shifted to a higher temperature. At beginning, the
value of Tg found to be 61˚C after 24 h and 96 h, it in-
creased to 62.5˚C after 15 and 30 days.
4. Conclusion
The effect of annealing after melting on morphology of
Figure 3. DSC second heating of PLLA after annealing at
room temperature for 360 h.
Figure 4. Enthalpy relaxation of the PLLA samples with
heating rate 10˚C·min1 after ageing time: (a) 24 h; (b) 96 h;
(c) 360 h; and (d) 720 h.
pure PLLA by POM has been discussed. Large non
banded spherulites with cracks are formed around the
center after annealed at higher temperature by longer
annealing time and isothermal crystallization. Changes
of relaxation enthalpy were detected by DSC after an-
nealing near glass transition temperature (Tg). The posi-
tion of glass transition shifted to higher values and
magnitude of the enthalpy increased by increasing the
ageing time.
5. Acknowledgements
The author thanks the research and consulting center and
institute of scientific research for part supporting this
project 43005001 (ministry of higher education of K.S.A).
The Effect of Annealing Treatments on Spherulitic Morphology and Physical Ageing on
Glass Transition of Poly Lactic Acid (PLLA)
Copyright © 2011 SciRes. MSA
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