# Star-Shaped Thermoplastic Elastomers Prepared via RAFT Polymerization
## 3 Results
### Characterization of SIS Linear and Star Triblock Copolymers
#### 3.3.2 Morphology  
Block copolymers formed by styrene with butadiene and isoprene will spontaneously exhibit topographic composition phase phenomenon in the polymer matrix state which can be directly observed by AFM, SFM and SEM [57]. It is known from the linear SIS AFM images obtained by the natural volatile drying method of film formation using xylene as the solvent (Figure 5) that L1 (Figure 5a,b) exhibits a compact phase separation structure because of its higher molecular weight. L2 (Figure 5c,d) shows a typical spherical hard segment morphology, and the higher hard segment content also resulted in larger phase domains, possibly forming larger phase domains (Figure 5e,f). When using chloroform as the solvent and spin-coating the film with a homogenizer, the AFM diagram shows a bicontinuous phase structure and a significant reduction in the size of phase domains (Figure S3). Similar conclusions could be drawn from the AFM phase images and TEM images in Figures S4 and S5 as well, which prove that the linear SIS has a strong microscopic phase separation capability.  
Figure 5: Atomic force microscope images and 3D height diagram of linear SISs (film-dried naturally). L1: (**a**,**b**); L2: (**c**,**d**); L3: (**e**,**f**).  
Regarding the microscopic shape of the star SIS in this work (Figure 6), the AFM images (Figure 6a-c and Figure S6) show no obvious phase separation morphology, but a structure similar to that of free particles indicates that the presence of the star structure affected the ability to split the phase. The same nanoparticle morphology was observed in the TEM images, and the presence of nanoparticles of various scales in the system was illustrated in the DLS (Figure 6d-f). If the isoprene soft segment content was too high, the core region might be completely encapsulated in it, making it difficult to observe the non-discolored core region (Figure 6e).  
The presence of hollow nanoparticles can also be observed in the figure, which can be explained in Figure 7. For a normal core crosslinked star polymer, each arm contributes about 30% to the formation of the core region. However, in this work, isoprene was involved in the polymerization and the arm molecules possessed isolated double bonds; thus, the crosslink monomer intentionally expanded the crosslinked region during the star-forming reaction, making the original core region much larger and eventually making it exhibit nanoparticle properties. Meanwhile, osmium tetroxide dye is a double-bond-specific dye, and only the isoprene soft segment could be greatly stained in the crosslinked particles, while the core region consisting of styrene and DVB was left unstained or lightly stained. Therefore, the system exhibited the microscopic morphology of hollow nanoparticles.  
To better illustrate the phase separation structure of the SIS, linear and star-shaped SISs with similar hard segment ratios were selected in this work for a comparative study of SAXS in Figure 8. The scattering peaks at 0.3 nm\({}^{-1}\) indicate that the linear system still had a strong phase separation ability, and the calculated phase domains were 17 nm and 19 nm for L1 and L2, respectively, from the equation 2\(\pi\)/d = q. The corresponding scattering peaks in the star group were shifted to the right and the peak shape became wider, indicating that the scale of the sequence structure became smaller, and the interface of phase separation tended to be blurred, proving that the star SISs had a weaker phase-splitting ability.  
Figure 6: AFM images (**a-c**) and TEM images (**d–f**) of S5: (**a,b**) AFM images under different microscale; (**c**) 3D height diagram; (**d**) TEM images; (**e**) microphase separation in agglomerated particles; (**f**) particle size distribution from DLS.