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Strain-induced crystallization (SIC) prevalently strengthens, toughens, and enables an elastocaloric effect in elastomers. However, the crystallinity induced by mechanical stretching in common elastomers (e.g., natural rubber) is typically below 20%, and the stretchability plateaus due to trapped entanglements. We report a class of elastomers formed by end-linking and then deswelling star polymers with low defects and no trapped entanglements, which achieve strain-induced crystallinity of up to 50%. The deswollen end-linked star elastomer (DELSE) reaches an ultrahigh stretchability of 12.4 to 33.3, scaling beyond the saturated limit of common elastomers. The DELSE also exhibits a high fracture energy of 4.2 to 4.5 kJ m-2 while maintaining low hysteresis. The heightened SIC and stretchability synergistically promote a high elastocaloric effect with an adiabatic temperature change of 9.3°C. [ABSTRACT FROM AUTHOR]

Two all-aromatic, high-performance poly(etherimide)s ODPA-P3 ( I ) and BPDA-P3 ( II ) are studied as a function of SWCNT loading. Polyetherimide I is amorphous but SWCNTs nucleate crystallization during imidization to produce a nanocomposite (nc- I ) with SWCNTs embedded in the crystals. This crystalline interphase, with a radial thickness of ∼8 nm, results in a substantial reinforcement efficacy. Melting the interphase and cooling nc- I yields an amorphous nanocomposite with an interphase thickness (3.6 nm) that is smaller and has a lower reinforcement efficacy. Polyetherimide II was used as a reference since SWCNTs reside in the amorphous fraction of its nanocomposite. Consequently, the reinforcement efficiency of the semi-crystalline nc- II and its amorphous analog (prepared by heating above the crystal melting point and cooling) remains unchanged. A crystalline interphase surrounding SWCNT-reinforced poly(etherimide)s appears to be critical in obtaining high reinforcement efficacies. Most high-performance polyimides and poly(etherimide)s are prepared using a thermal imidization step. Our work emphasizes that care has to be taken in selecting the final imidization temperature to avoid melting the crystalline interphase if one is targeting high modulus materials. [ABSTRACT FROM AUTHOR]

In this study we contrast the behavior of an amorphous polyetherimide (ODPA-P3) with that of a semi-crystalline one (BPDA-P3) on introducing single-wall carbon nanotubes (SWCNTs; 0.1-4.4 vol.%). The SWCNTs act as a nucleating agent and induce crystallinity (up to 45%) in amorphous ODPA-P3; in BPDA-P3, the SWCNTs lower the onset of crystallization during thermal imidization, but decrease the overall degree of crystallinity. Tensile tests show that the yield strength is the same for both polyimides up to 0.3% SWCNT loadings. Above this concentration the yield strength for the BPDA-P3 nanocomposites remains constant whereas in the ODPA-P3 nanocomposites it increases from 80 to 126 MPa (1.2 vol.%). In the amorphous PEI, SWCNTs enhance the thermo-mechanical properties (Tg, storage- and elastic modulus, and yield strength), but in the semi-crystalline PEI there are little or no effects on the mechanical performance. [ABSTRACT FROM AUTHOR]

We have selected two amorphous all-aromatic poly(etherimide)swith similar chemical structures but with different backbone geometriesas matrices for SWCNT-based nanocomposites. Up to 4.4 vol %, SWCNTscould be incorporated using an in situpolymerizationmethod. Nanocomposites prepared from aBPDA-P3, a nonlinear matrixpolymer with a Tgof 230 °C, remainsamorphous, and the presence of the SWCNTs reduces the Tgby 11 °C. No effect on E′or stress–strain behavior was observed. When ODPA-P3 was usedas the matrix, the SWCNTs appear to be highly compatible with thismore linear polymer host. The SWCNTs act as a nucleating agent atconcentrations as low as 0.1 vol %. XRD and TEM measurements showthat the SWCNTs become embedded in a highly crystalline polymer matrix.The result is a significant change in thermomechanical properties.The polymer Tgwas increased by 12 °C,from 196 to 208 °C, and due to the induced crystallinity, themodulus above Tgshowed a dramatic increase.The neat polymer fails at Tg, but the4.4 vol % nanocomposite shows a storage modulus of 1 GPa at 280 °C.Stress–strain measurements show a noticeable improvement instrain and toughness at low SWCNT loadings (0.1–0.3 wt %),which is indicative of good stress transfer between the SWCNTs andpolymer matrix. At higher loadings the yield strength increases from80 to 126 MPa at 4.5% strain. Our findings show that the poly(etherimide) backbone geometry determines whether the polymer is good hostfor SWCNTs. The more linear ODPA-P3 is able to maximize its interactionwith the SWCNT surface. To the best of our knowledge, this is thefirst time that an amorphous polymer has been shown to develop a semicrystallinemorphology in the presence of SWCNTs. Steric factors in aBPDA-P3 seemto inhibit favorable π–π interactions and preventthe polymer chains from adapting low-energy conformations that readilyinteract with the SWCNT surface. [ABSTRACT FROM AUTHOR]