Morphology and deformation mechanisms and tensile properties of tetrafunctional multigraft (MG) polystrene-g-polyisoprene (PS-g-PI) copolymers were investigated dependent on PS volume fraction and number of branch points. The combination of various methods such as TEM, real time synchrotron SAXS, rheo-optical FTIR, and tensile tests provides comprehensive information at different dimension levels. TEM and SAXS studies revealed that the number of branch points has no obvious influence on the microphase-separated morphology of tetrafunction MG copolymers with 16 wt % PS. But for tetrafunctional MG copolymers with 25 wt % PS, the size and integrity of PS microdomains decrease with increasing number of branch point. The deformation mechanisms of MG copolymers are highly related to the morphology. Dependent on the microphase-separated morphology and integrity of the PS phase, the straininduced orientation of the PS phase is at different size scales. Polarized FT-IR spectra analysis reveals that, for all investigated MG copolymers, the PI phase shows strain-induced orientation along SD at molecular scale. The proportion of the PI block effectively bridging PS domains controls the tensile properties of the MG copolymers at high strain, while the stress-strain behavior in the low-mediate strain region is controlled by the continuity of PS microdomains. The special molecular architecture, which leads to the higher effective functionality of PS domains and the higher possibility for an individual PI backbone being tethered with a large number of PS domains, is proposed to be the origin of the superelasticity for MG copolymers.
The synthesis of well-defined multigraft copolymers having a polydiene backbone with polystyrene side chains is briefly reviewed, with particular focus on controlling branch point spacing and branch point functionality. Use of living anionic polymerization and chlorosilane linking chemistry has led to the synthesis of series of materials having regularly spaced trifunctional (comb), tetrafunctional (centipede), and hexafunctional (barbwire) branch points. The morphologies of these materials were characterized by transmission electron microscopy and small-angle X-ray scattering, and it was found that the morphologies were controlled by the local architectural asymmetry associated with each branch point. Mechanical properties studies revealed that such multigraft copolymers represent a new class of thermoplastic elastomers (TPEs) with superior elongation at break and low residual strains as compared to conventional TPEs
We
present the synthesis and characterization of a new class of
high temperature thermoplastic elastomers composed of polybenzofulvene–polyisoprene–polybenzofulvene
(FIF) triblock copolymers. All copolymers were prepared by living
anionic polymerization in benzene at room temperature. Homopolymerization
and effects of additives on the glass transition temperature (T
g) of polybenzofulvene (PBF) were also investigated.
Among all triblock copolymers studied, FIF with 14 vol % of PBF exhibited
a maximum stress of 14.3 ± 1.3 MPa and strain at break of 1390
± 66% from tensile tests. The stress–strain curves of
FIF-10 and 14 were analyzed by a statistical molecular approach using
a nonaffine tube model to estimate the thermoplastic elastomer behavior.
Dynamic mechanical analysis showed that the softening temperature
of PBF in FIF was 145 °C, much higher than that of thermoplastic
elastomers with polystyrene hard blocks. Microphase separation of
FIF triblock copolymers was observed by small-angle X-ray scattering,
even though long-range order was not achieved under the annealing
conditions employed. In addition, the microphase separation of the
resulting triblock copolymers was examined by atomic force microscopy.
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