Synthesis and characterization of new polyester-starch composites
D. Rutot, Ph. Degée, 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 starchbased 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 (hydrophobic coating,
cross-linking, addition of external plasticizers) and chemical (grafting reactions and substitution of the hydroxyl with
functional groups like esters, ethers, isocyanates, anhydrides) 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.
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. The
effective fixation of the initiator has been evidenced by X-Ray Photoelectron Spectroscopy in which aluminum
characteristic peaks are detected. Drying of starch is a key-step of the process as we can see in Table 1. The quantity
of residual water has been determined by Karl-Fisher analysis. The best way is drying of granular starch under
-3
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).
Table 2 reports some of the results obtained under different experimental conditions.
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 follows :
⇒
Starch
PCL
Grafting efficiency =
% PCL grafted
% PCL grafted + % PCL non grafted
The insoluble part contains PCL-grafted onto starch and the soluble one, non grafted PCL chains.
Starch coating
with PCL
Plsch OH + AlEt3
Plsch O
Plsch O p AlEt3-p
1. ε-CL or δ-VL
2. H2O
O
p
Plsch O C
Table 1 : Effect of starch drying
AlEt3-p + p C2H6
(CH2)m O H
n
where m = 4 or 5
Drying
Residual water
(%)
1.19
2.14
0.33
0.31
0.50
0.13
1
2
3
4
5
6
Al content (wt%)
0.1
0.2
0.1
0.5
0.5
0.3
Μonomer
conversion (%)
<5
0
<5
<5
24
54
Grafting
efficiency (%)
<5
0
<5
<5
33
52
Scheme 1
Table 2 : Synthesis of PCL or PVL/starch (50/50 wt%)
Monomer
Temperature
(°C)
0.33
ε-CL
0.36a)
ε-CL
0.10
ε-CL
0.55
δ-VL
a) On 10 wt% suspension in toluene
-1
1) Under 7.7 10 mbar at 90°C for 16 h
-2
2) Under 6.10 mbar at 90°C for 16 h
-1
3) At 90°C in a ventilated oven for 16 h and then under 7.7 10 mbar at 90°C for 70 h
-1
4) At 90°C in a ventilated oven for 16 h then under 7.7 10 mbar at 90°C for 5 h and at 120°C for 70 h
-2
5) Under 6.10 mbar at 90°C for 21 h and then at 120°C for 16 h
-2
-3
6) At 100°C in a ventilated oven for 24 h then at 5.10 mbar at 90°C for 7 h and under 10 mbar at 120°C for 16 h
Differential Scanning Calorimetry
Al content
(wt%)
Characterization
65
65
90
65
Reaction
time (min)
95
300
3
30
Monomer
conversion
(%)
54
6
99
68
Grafting
efficiency
(%)
52
33
90
56
Granulometry (Laser Scattering)
In granulometry analysis, we can verify the fixation of aliphatic polyesters onto the starch surface by checking the increase of
the coated particles diameter. 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 56% PVL increases up to 27.30 and 31.41 µm, respectively (Fig.3).
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 in Fig.2. We can also conclude that the molecular weight of the PCL grafted is high enough to allow
the polyester chains crystallization.
a)
b)
Fig. 3. Particle size distribution of : a) starch-g-PCL ; b) starch-g-PVL
Fig.2. DSC thermogram of PCL-grafted starch (26 wt% PCL)
Scanning Electron Microscopy
Secondary Ion Mass Spectrometry
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).
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).
a)
a)
b)
b)
c)
Cc
Fig.5. SEM images of : a) untreated starch ; b) insoluble part of starch-g-PVL and c) insoluble part of
starch-g-PCL
a)
c)
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. 4. SIMS spectra of : a) starch-g-PCL ; b) starch-g-PVL ; c) starch-g-[PVL-b-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.