Photon Factory Activity Report 2019 #37 (2020)

BL-6A/2019G160

Toughening of thermoplastic elastomers via introduction of dynamic bonds
Shintaro NAKAGAWA1,*, Saki KAWANA1, and Naoko YOSHIE1
1
Institute of Industrial Science, The University of Tokyo,
4-6-1 Komaba, Meguro, Tokyo 153-8505, Japan
1 Introduction
Thermoplastic elastomers (TPEs) are a physicallycrosslinked polymer network that is elastic at ambientconditions but becomes malleable at elevated temperature.
One of the most widely used types of TPE is a hard-softhard triblock copolymer, where the hard blocks aggregate
into a domain which serves as a physical crosslink for thesoft middle block. It is important to enhance themechanical toughness of TPE, as it directly determines thematerial's reliability and lifecycle. Introduction of the
dynamic bonds, which are weaker than covalent bonds andcan reversibly associate/dissociate, is the key strategy toincrease the toughness of polymeric materials. However,
the effects of introducing such dynamic bonds into harddomain and/or soft matrix of TPE are not clear. In thisstudy, we selectively introduce hydrogen bonds (H-bonds)
into either or both of hard and soft blocks of TPE andcompare their microphase-separated structure andmechanical property, to clarify the capability of H-bonds
in each phase on toughening of TPEs.
2 Experiment
The samples were ABA triblock copolymers synthesized
by sequential ring-opening metathesis polymerization offunctionalized norbornene monomers. The A blocks
mainly consisted of the phenyl-bearing monomers,As
thedynamicbonding
functionalgroup,
ureidopyrimidinone (UPy)-bearing monomers wereselectively introduced to either or both of the A and Bblocks. While the number of UPy monomers in each block
was varied, the degree of polymerization the blocks waskept the same among all the samples, by which we could
observe the effects of dynamic bonds only. The samples
are denoted as TPE(x-y-x) where x and y are the number of
UPy monomers introduced in the A and B blocks,
respectively. The microphase-separated structure of thetriblock copolymers was examined by small-angle X-ray
scattering (SAXS) measurements performed at BL-6A,
Photon Factory, as well as by atomic force microscopy(AFM) on a thin film sample. Mechanical properties wereinvestigated by uniaxial tensile tests.
3 Results and Discussion
Fig. 1a shows the SAXS profile of the bulk TPEs and AFM
phase image of the corresponding thin films. There is anintense peak in the SAXS profile in all the samples. All thesamples show the peak at similar q position, indicating thatthe structures formed in these TPEs are similar in size. The
AFM images show densely packed nanometer-scale spots,
which is typical of a discrete microphase separated
structure formed by block copolymers. Also shown in thefigure is the Fourier-transformation of the AFM phase
image. Both methods show a correlation peak at a similar
length scale, indicating that both methods probe the samestructure. By a combination of AFM and SAXS results weconclude that the synthesized TPEs had sphericalmicrophase-separated structure with a similar size among
the samples.
The mechanical property of the TPEs wascharacterized by tensile tests (Fig. 1b). Introduction of UPy
in the soft B blocks (from TPE(0-0-0) to TPE(0-20-0))
increases the stress in an entire strain region while that in
the hard A blocks (TPE(0-0-0) to TPE(5-0-5)) does notaffect the curve at small deformation. However, if UPy isadded to the A blocks in addition to UPy in the B block(TPE(0-20-0) to TPE(5-20-5)), the stress is enhanced evenat small deformation. This suggests that the introduction ofUPy in both the hard and soft phases is more than just theaddition of each effect: there is a synergistic interaction
between UPy in the hard domains and soft matrix thatfurther enhances the mechanical resilience of the TPE.

Fig. 1: SAXS profiles and AFM phase images (a) and thestress-strain curves (b) of the TPEs.
Acknowledgement
This work has been financially supported by JSPS
Grants-in-Aid for Scientific Research (grant No.
17H03111).
Research Achievements
1. Kawana, S.; Nakagawa, S.; Nakai, S.; Sakamoto, M.;
Ishii, Y.; Yoshie, N.; "Interphase synergistic effects ofdynamic bonds in multiphase thermoplasticelastomers", J. Mater. Chem. A, 7, 21195-21206(2019).
* snaka@iis.u-tokyo.ac.jp