A NEW CLASS OF SUPRAMOLECULAR THERMOPLASTIC
ELASTOMERS FOR SELF-HEALING COATINGS
M. M. Diaz 2,4,L. Voorhaar 1,4, T. Perkisas 3,4, A. Abakumov 3, G. Van Assche 2, B. Van Mele 2 and R. Hoogenboom 1
1 Supramolecular Chemistry Group, Department of Organic Chemistry, Ghent University
Krijgslaan 281 S4, B9000 Ghent, Belgium – e-mail: Lenny.Voorhaar@ugent.be
2 Department of Materials and Chemistry, Physical Chemistry and Polymer Science, Vrije
Universiteit Brussel,Pleinlaan 2, Brussels 1050, Belgium– e-mail: mdiazace@vub.ac.be
3 EMAT, University of Antwerp,Groenenborgerlaan 171, B-2020 Antwerp, Belgium –
e-mail:Tyche.Perkisas@ua.ac.be
4 SIM vzw, Technologiepark 935, B - 9052 Zwijnaarde, Belgium
Keywords: supramolecular polymer, thermoplastic elastomer, coatings, phase separation.
ABSTRACT
A supramolecular material exhibiting self-healing properties was studied in this work. The material is based on the mixture of two ABA-type oligomeric triblock copolymers, the first consisting of positively charged end blocks and the second one of negatively charged ones. Each oligomeric triblock consists of a soft middle block and soft end blocks. When mixed together the resulting material phase-separates in an electrostatically assembled phase with a high glass transition temperature (Tg) and
an uncharged phase with a low Tg. The mechanical and physical properties of the
resulting thermoplastic elastomer can be tuned by varying the size of the charged blocks, using different polymer architectures and using different monomers. The building block oligomers are synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization using a bifunctional chain transfer agent and purified by precipitation. Characterization was done by Modulated Differential Scanning Calorimetry (MDSC), Dynamic Mechanical Thermal Analysis (DMTA) and Transmission Electron Microscopy (TEM). The self-healing behaviour of the coating was studied under optical microscope proving self-healing abilities after scratching. 1. INTRODUCTION
Polymers are one of the most widely studied materials for self-healing (SH) applications. Several strategies have been approached in order to achieve materials that exhibit comparable performance to materials used today, with the extra capability of recovering its strength after being damaged. The use of supramolecular interactions as a SH mechanism are advantageous due to their reversible nature and their response to external factors such as temperature and concentration.[1] In addition, non-covalent reversible interactions undergo continuous dynamic exchange that can lead to intrinsic autonomous self-healing materials.[2][3][4]
The material studied in this work is based on ABA-type oligomeric triblock copolymers, consisting of a soft uncharged middle block and soft oppositely charged end blocks. When mixed together, the resulting material will phase-separate in an electrostatically assembled charged phase with a high Tg and an uncharged phase
with a low Tg. The electrostatic interaction between the charged blocks will provide
the strength of the material, resembling an electrostatic SBS rubber analogue.[5]
Polymers of similar design have been used previously for hydrogels.[6] The mechanical and physical properties can be tuned by varying the size of the charged blocks, using different polymer architectures and using different monomers.
Figure 1: Formation of a phase-separated material by mixing oppositely charged ABA-type triblock copolymers.
2. MATERIALS AND METHODS
The ABA-type triblock copolymers were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization using a bifunctional chain transfer agent and purified via precipitation. Molecular weights of the polymers were around 10000 g/mol with PDIs of around 1.2.
Figure 2: Two-step synthesis of ABA-type triblock copolymers via RAFT polymerization.
Thermal characterization of the materials was done in a TA instruments Q2000 DSC with MDSC option and an RCS cooling. Heat cooling cycles with the following conditions were done: a heating rate of 2°C/min, a period of 80s and amplitude of ±0.4°C was used. Dynamic Mechanical Analysis (DMTA) was done on a TA instruments Q800 at heating rate of 2.5°C/min and frequency of 1Hz.
A FEI Tecnai G2 electron microscope was used in order to study the morphology of the elastomer by Transmission Electron Microscopy (TEM). In order to enhance the contrast of the sample, it was stained in OsO4 vapors for 12 hours. The sample was also cooled to liquid nitrogen temperatures in order to avoid knock-on damage and heating of the sample by the electron beam.
3. RESULTS AND DISCUSSION
The individual block copolymers were characterized by MDSC to determine their thermal transitions. The observed result is shown in Figure 3(a), the ABA block copolymer exhibits one Tg at -41°C, these means that the two blocks are merged in
one phase. The same is observed for the second block copolymer CBC, which also exhibits one Tg at -35°C showing the miscibility of both blocks. The mixing of the two
materials results in a material with two glass transitions: a lower glass transition corresponding to the middle common block of both copolymers and a higher
transition resulting from the interaction between the charged blocks. In addition, the result shows that as both types of blocks associate a second phase is generated, giving an indication about the morphology of the mixture.
The thermomechanical characterization of the mixture on DMTA is shown in Figure 3(b). At low temperature around -30°C a maximum in the loss modulus can be observed indicating a glass transition. At this moment the modulus drops and a rubbery plateau is kept until 40°C when the storage modulus starts decreasing again. This last transition is agreement with the results obtained by DSC and show that the obtained material behaves like an elastomer.
Scanning Transmission Electron Microscopy (STEM) was performed upon the sample (see Figure 4). From diffractogram (see inset Figure 4) analysis it was found that the average domain size of the elastomer was nanometer sized, which is in agreement with the DSC measurements.
Figure 3: (a) MTDSC of the individual block copolymers and mixture (b) DMTA of the mixture of oligomers.
Figure 4: Scanning Transmission Electron Microscopy (STEM) image of the polymer mixture stained with OsO4 showing separate domains of the charged associated high
Tg zones in a soft matrix. The diffractogram (inset) shows evidence of nanometer
sized domains. 0 30 60 90 0,1 1 10 100 1000 10000 -90 -70 -50 -30 -10 10 30 50 70 90 D elt a / ° E ', E " / M P a T / °C E' E" Delta -35.05°C(T) 0.3695J/(g·°C) -41.23°C(T) 0.2879J/(g·°C) -41.78°C(T) 0.1761J/(g·°C) 26.08°C(T) 0.08249J/(g·°C) 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 R e v C p ( J/ (g ·° C )) -100 -50 0 50 100 Temperature (°C) C-B-C ––––––– A-B-A ––––––– MIX –––––––
Universal V4.5A TA Instruments
(a) (b)
4. CONCLUSIONS
When the oppositely charged ABA-type triblock copolymers are mixed together, a two-phase system is formed, giving indication of supramolecular interaction. This can be observed in MDSC by the appearance of a second Tg at 26°C and with TEM.
DMTA shows the elastomeric properties of the material. Further research to optimize the properties of the material is in progress.
ACKNOWLEDGEMENTS
This work was funded by the Special Research Fund (BOF) of Ghent University and the Strategic Initiative Materials (SIM).
REFERENCES
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