III Results and discussions
III.6.1 Reactive processing of PLA, PBAT and PLA/PBAT/Joncryl blends
To compensate for such chain scission reactions, a chain extender, named Joncryl ADR®, is in-corporated. According to the literature, the epoxide groups of Joncryl ADR® can theoretically re-act with both hydroxyl and carboxyl groups of the polyesters. Japon et al allow differentiating be-tween the reaction of carboxylic acid and hydroxyls groups with epoxide, since epoxide groups are known to react differently with –COOH and –OH groups (Table II-5) [18-52-53].
Wave number (cm-1)
C-H
groups
C=O C-O
(CH2)4
C=C
O CH CH3
O n O
O O O
OH O O
H H
O
O CH2 (a)
JONCRYL O
O R5
O C
O
OH CH2 C
Acid End Groups
(b) JONCRYL
OH CH2 O PLA
Degradation
+
Vinylic terminated ester
Table II-5 Comparison between the reaction rate of Carboxylic acid/Epoxide and Hydrox-yl/Epoxide
Reactive pair Reaction rate
Carboxylic acid/epoxide 18
Primary Hydroxyl/epoxide 1.2
Secondary hydroxyl/epoxide 1
In the case of polyesters, glycidyl esterification of carboxylic acid end groups precedes hydroxyl end group etherification. This latter reaction competes with etherification of secondary hydroxyl groups and main chain transesterification. The resultant couplings involve epoxy ring-opening re-actions and the creation of covalent bonds via hydroxyl side group formation. The proposed mechanism of reaction is shown here (Figure II-9). The resultant (polymer-GMA functions) sys-tem represents a complex set of concurrent reactions due to the degradation/chain exten-sion/branching balance.
a) Predicted mechanism of the reaction between PLA and Joncryl
O
b) Predicted mechanism of the reaction between PBAT and Joncryl
Figure II-9 Predicted reaction between polyesters and epoxy functions
III.6.1.1 Rheological investigations of modified PLA and PBAT systems
Figure II-10 Eta*(t)/Eta° (t=0) evolution versus time for (a) neat and modified PLA (b) neat and modified PBAT at 180°C
As discussed before, the results of rheological tests of neat PLA and PBAT at 180°C demonstrated a monotonous decreasing viscosity over time, highlighting therefore the oc-currence of thermal degradation. This latter phenomenon is related to the decrease of intri n-sic viscosity and the average molecular weight. Figure II-10 shows that the incorporation of
0 1000 2000
polymers, at 0,25 and 0,5%wt, the viscosity takes less time to stabilize compared to the 1%wt of Joncryl where a drastic increase of relative viscosity was observed.
This could indicate significant branched structures in addition to linear ones. The initial vi s-cosity further increases until remains constant, confirming that no significant change takes place during the rheological tests.
For both polymers, the complex viscosity modulus gradually decreases with angular frequency which is typical shear-thinning behavior, as it can be seen in Figure II-11. Thus, the incorporation of Joncryl ADR® ratio enhance the shear thinning behavior of both PLA and PBAT and conse-quently shifts the Newtonian plateau to lower angular frequencies [36].
Figure II-11 Complex viscosity versus angular frequency (at 180°C) for the neat PLA and PBAT Moreover, respective viscosities converge at high angular frequency. Chain orientation and ther-mal energy dissipation would be responsible for these close results. In addition, an enhancement of storage modulus G’ values steadily over the whole frequency range was observed, more pro-nounced at lower angular frequency. The introduction of chain branches due to chain extending reactions could explain the improvement of melt elasticity. According to the literature simple star or linear structures cannot result in such a pronounced increase on elasticity of material [7-55-56].
The work is in progress to investigate the real chains topology of modified PLA and PBAT. Stor-age modulus become less shear sensitive when chain extension/branching Stor-agent content is in-creased thus revealing a cohesive network viscosity (cf. Figure II-12).
1 10 100
1000 10000
Complex viscosity Eta* (Pa.s)
Angular frequency (rad/s) PLA4032D
PBAT
Figure II-12 (a) The complex viscosity and (b) the storage modulus angular frequency depend-ence at 180°C for neat and modified stable PLA with chain extender
Using a chain extender, as Joncryl ADR®, increases the viscosity and the storage modulus. It fa-cilitates further processing since high melt viscosity and elasticity are required in processes such as thermoforming and foaming. Furthermore, the weight relaxation spectrums (λH (λ)) were also plotted to probe the effect of chain extension/branching agent on the linear viscoelastic behavior of our modified polyesters. The linear relaxation spectrum H (λ), using the plots of G’and G" da-ta’s, is plotted as follow (Eq. 6) [36]:
Where ω is the frequency and λ is the relaxation time. The time distribution of chains relaxation for neat and modified PLA and PBAT is reflected in Figure II-13. It is observed that the neat pol-ymers present only one relaxation time, corresponding to this of one component, which is the pure polymer. However, in the case of modified polymers, two main relaxation times can be clearly identified, which were interpreted by the simultaneous occurrence of two relaxation processes.
1 10 100
The same double time relaxation distribution has already been observed by Wood-Adams for long chain branched PE [57]. The high values of relaxation time λ correspond to the relaxation of the long, heavy and branched chains due to the presence of epoxy functions which restricts the chain mobility of neat polymers and retards the overall relaxation of modified polymers. The lower ones are related to the relaxation of short chains. The presence of branching, the improvement of mo-lecular weight M and intrinsic viscosity could explain the remarkable increase of λ.
III.6.2 Molecular weight measurements and rheological investigation of modified PLA