Polycaprolactone grafted-starch by Ring Opening Polymerization
D. Rutot, Ph. Degée, R. Gouttebaron, M. Hecq, Ph. Dubois
University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, BELGIUM
Aim of this work
Synthesis of the composites
Polymer blends and composites of starch (plasticized or granular) and aliphatic polyesters,e.g.,
polycaprolactone, are of great interest as new biodegradable materials. However, conventional
melt-processing usually provides the starch-based composites with very poor mechanical
properties, mainly due to thermal decomposition of starch, strong water absorption and poor
interfacial adhesion. In order to overcome these drawbacks, physical and chemical modifications
of starch have been performed.
In this work, we study the chemical surface treatment of starch granules so as « in situ » ROP of
ε-CL (δ-VL) monomers is promoted directly from the starch phase and results in covalent grafting
of polyester chains onto starch.
Plsch OH + AlEt3
Plsch O
p
AlEt3-p + p C2H6
1. ε -CL or δ-VL
Plsch O p AlEt3-p
2. H2O
O
Plsch O C
Polymerizations are initiated by the modified/activated hydroxyl groups of amylose and
amylopectin constituents of starch (Plsch – OH). This modification is carried out by reaction
of the OH groups with triethylaluminum. Drying of starch is a key-step of the process. The
quantity of residual water has been determined by Karl-Fisher analysis. The best way is drying
-3
of granular starch under reduced pressure (10 mm Hg) at 90-120°C (residual water : 0.13%)
The polymerization is then performed in bulk or in 10 wt% toluene suspension. (Scheme 1).
(CH2)m O H
n
where m = 4 or 5
Scheme 1
First step : Fixation of the initiation species
Characterization
Granulometry (Laser Scattering)
In granulometry analysis, we can verify the fixation of aliphatic polyesters onto the starch surface by checking the
increase of the diameter of the coated particles. Starch granules have a mean diameter of 13.45 µm. The diameter of
the particles obtained with a grafting efficiency of 31% PCL and a grafting of 53% PVL increases up to 27.30 and
31.41 µm, respectively (Fig.3).
X-ray Photoelectron Spectroscopy
In order to check the effective fixation of the initiator onto the starch surface, AlEt3 (0.5 wt%Al) was added to a
suspension of starch in dried toluene. The treated starch was isolated by filtration and washed twice with dried
toluene. In the XPS spectrum (Fig.1) of the treated starch, aluminum characteristic peaks were detected. It has been
shown that the surface aluminum content decreases from 11% down 9% when the reaction is carried out with 0.1
wt%Al.
a)
b)
a)
Fig. 3. Particle size distibution of : a) starch-g-PCL and b) starch-g-PVL
Secondary Ion Mass Spectrometry
Presence of PCL and PVL grafted chains has been evidenced by SIMS (Fig. 4). Typical fragments, including
monomeric molecular ones, are detected not only for grafted homopolyester chains, but also sequentially
copolymerized PVL-b-PCL grafts onto starch granules (Fig. 4c).
Peak
O1s
C1s
Center
529.5
282.5
Peak area
93667
53593
Concentration (%)
38.46
60.49
a)
b)
b)
c)
Peak
O1s
C1s
Al2p
Centre
528.4
280.9
70.2
Peak area
90670.1
39938.3
5127.9
Concentration (%)
39.54
47.88
11.00
Fig. 4. SIMS spectra of : a) starch-g-PCL ; b) starch-g-PVL ; c) starch-g-[PVL-b-PCL]
Scanning Electron Microscopy
Fig.1. XPS spectra of : a) unreacted starch and b) after treatment with AlEt3 (0.5 wt%Al)
SEM images of composites attest for the very good interfacial adhesion between starch and aliphatic polyesters, after
(Fig.5) and before (Fig.6) selective extraction experiment in toluene. In Fig. 6 a, we can also verify the total lack of
adhesion between the two components in a simple starch/polycaprolactone melt blend (50/50 wt/wt).
Second step : Synthesis of the composites
Polymerization/grafting yields
a)
After ε-CL polymerization, selective extraction in toluene was performed onto the as-recovered composite to
determine the monomer conversion and the grafting efficiency. The grafting efficiency is defined as follow :
Grafting efficiency =
% PCL grafted
% PCL grafted + % PCL non grafted
~ 50-95 %
b)
c)
The insoluble part contains PCL-grafted onto starch and the soluble one, non grafted PCL chains.
Differential Scanning Calorimetry
Determination of the Tm and the Tg of the aliphatic polyester which is grafted onto starch is realized by DSC
thermograms. A thermogram is shown at Fig.2. We can also conclude that the molecular weight of the PCL grafted is
high enough to allow the polyester chains crystallization.
Fig.5. SEM images of : a) untreated starch ; b) insoluble part of starch-g-PVL and c) insoluble part of
starch-g-PCL
a)
b)
c)
Fig.6. SEM images of a) melt blend of starch and PCL (50/50wt%) ; b) starch-g-PVL and
c) starch-g-PCL
Fig.2. DSC thermogram of PCL-grafted starch (26 wt% PCL)
Future works
Kinetics of ε-CL/δ-VL ring opening polymerization initiated by starch surface aluminum alkoxide groups will be investigated and compared to data available for ROP promoted by soluble aluminum alkoxides. Sequential copolymerization and extension
to polylactide grafting will be carried out as well. Thermal, mechanical and morphological properties of the related starch-g-polyester composites will be studied and compared to their counterparts obtained by melt blending of untreated starch and
preformed polyesters.