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The effect of citrate ester plasticizers on the thermal and dynamic mechanical
properties of Poly (DL lactide) for use as medical implants
Imelda Harte
Department of Civil Engineering and Material Science, University of Limerick
Imelda.harte@ul.ie
Abstract
Poly (D,L lactide) (PDLLA) was blended with three
different citrate ester plasticizers and the resultant
blends were analyzed using differential scanning
calorimetry (DSC) and dynamic mechanical thermal
analysis (DMTA). Each plasticizer was miscible with
the polymer for all compositions and with increasing
plasticizer content a decrease in Tg was obtained.
1. Introduction
Developing degradable polymers as medical
implants is of great research interest. Polylactide (PLA)
is a thermoplastic polymer with hydrolytically labile
aliphatic ester linkages in its backbone. It is considered
safe, non-toxic and biocompatible for use as an
implantable biomaterial. However, PLA devices tend to
be susceptible to fracture when subjected to tension or
load bearing stresses during use. [1]
In order to modify its properties, PDLLA has been
blended with citrate esters plasticizers, which are nontoxic
and approved for use in medical plastics.
2. Methods
Poly (D,L lactide) (PDLLA) was blended with three
different citrate esters; triethyl citrate (TEC), tributyl
citrate (TBC) and acetyl tributyl citrate (ATBC) and the
resultant thermal and mechanical properties were
analyzed. DSC was preformed using a Perkin-Elmer
Pyris 1 DSC, while DMTA scans were carried out using
a Rheometric scientific mark 3.
3. Results
3.1. DSC
Each plasticizer was miscible with the polymer for
all compositions as only one Tg was noted. By
increasing plasticizer content a decrease in Tg occurs
which is true for all three plasticizers analyzed. The
smaller molecules of TEC are more effective at
enhancing chain mobility and this effectiveness is
reduced with the increasing molecular size of the
plasticizer.
3.2. DMTA
With increasing plasticizer content a decrease in Tg
is also obtained, but Tg values are higher then those
obtained by DSC. At temperatures above the Tg, the
185
storage modulus of each sample drops dramatically due
to the softening of the polymer specimens. Below Tg,
the storage modulus values are highest. With increasing
plasticizer content there is a decrease in storage
modulus values as the plasticizer increases the mobility
of the polymer chains.
Temp deg C
80
60
40
20
0
-20
0 10 20 30 40 50 60
% Plasticizer
TEC TBC TBAC
Figure Figure 1: Tg Tg Tg as as a a function function of of plasticizer plasticizer content
content
obtained obtained by by by DSC DSC DSC for for PDLLA PDLLA PDLLA samples samples plasticized plasticized
plasticized
with with different different citrate citrate esters
esters
E' (MPa)
1200
1000
800
600
400
200
0
-60 -10 40 90
Temp Deg. C
10% TBC
20% TBC
30% TBC
40% TBC
50% TBC
60% TBC
100%
PDLLA
Figure Figure 2: Sto Storage Sto
rage rage modulus modulus curves curves obtained obtained by
by
DMTA DMTA for for for PDLLA PDLLA and and PDLLA PDLLA PDLLA samples samples samples plasticized
plasticized
with with TBC
4. Conclusion
Each citrate ester was a suitable plasticizer for
PDLLA. With increasing plasticizer content a decrease
in Tg is obtained. The Tg values obtained by different
experimental techniques lead to different Tg values due
to the frequency of the analysis method.
8. References
[1] P.I.P. Park and S. Jonnalagadda, “Predictors of glass
transition in biodegradable polylactide and polylactide-coglycolide
polymers”, Journal of Applied Polymer Science,
2006, pp.1983-1987.