Chemical crosslinking enabling ferroelectric polymers for new memory applications

Chemical

Ferroelectric materials are signified by the existence of ferroelectricity enabling polarization reversal in response to external electric field, which is ideal for non-volatile memory. 1 Most ferroelectrics for memory applications are inorganic oxides such as lead-zirconium titanate [Pb(Zr,Ti)O 3 ] and hafnium dioxide (HfO 2 ); organic polymers represented by semicrystalline VDFbased fluoropolymers also show robust ferroelectricity which mainly arises from the crystalline domains. 2 With the increasing demand by recent development of portable and wearable electronics, it requires stretchable ferroelectrics which retain ferroelectricity under repeated large stretching field to develop next-generation nonvolatile memory.However, traditional ferroelectrics regardless of oxides or polymers show very limited stretchability.For instance, the strong Coulomb interactions and low yielding strain in rigid inorganic oxides result in a low mechanical stretching (usually <1%).Polyvinylidene fluoride (PVDF)-based ferroelectric polymers usually exhibit markedly larger elasticity when the content of amorphous regions is significantly increased.They can be elastomers as the intermolecular forces are pronouncedly reduced.Otherwise, an irreversible transition from elastic to plastic deformation always occurs in semicrystalline PVDF-based ferroelectric polymers with relatively high crystallinity.Consequently, it remains to be exploited that stretched ferroelectrics show robust ferroelectricity.
To address this challenge, Gao et al. 3 reported an elastic ferroelectric material based on poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), achieved through precise slight cross-linking of polymer chains in the amorphous regions (see Figure 1).Crosslinking technique has been utilized for exploration of memory applications using crosslinked P(VDF-TrFE) with the cross-linking agent of 2,4,4-trimethyl-1,6-hexanediamine 2 whereas the stretchability was not studied.They found that as the crosslinking density increases, the remnant polarization decreases considerably.Moreover, a ferroelectric field effect transistor (FeFET) based on crosslinked P(VDF-TrFE) copolymers was designed which shows significantly reduced gate leakage and reliable I-V hysteresis 2 .Moreover, Gao et al. measured ferroelectric instability under mechanical stretching.Surprisingly, slightly crosslinked P(VDF-TrFE) copolymers show elastic behavior rather than the plastic type at high strains >10%.As a result, they reported that crosslinked polymers exhibit stable ferroelectric properties even under 70% mechanical deformation,

Materials
Figure 1.Sketch of crosslinking mechanism between polyethylene glycol and P(VDF-TrFE) copolymers whereas the formation of crosslinking was elucidated at TrFE units 3 .The ferroelectric properties remain nearly unaffected by stretching, and the material retains its elasticity even when subjected to a strain that elongates the film by 70%.
enabling the potential memory applications that necessarily require elasticity (Figure 1).
Generally, the elastic properties of polymers are strongly dependent of the amount of the amorphous regions.By establishing a crosslinked network among the polymer chains within the amorphous regions, the crystallinity is found to decrease significantly.For instance, with no crosslinking the crystallinity of pristine P(VDF-TrFE) is about 65% while elastic P(VDF-TrFE) at a low crosslinking density of 1.4% shows a markedly lower crystallinity of only 35%.The weaker intermolecular interactions and lower possibilities of chain sliding and reordering during the stretching may provide reinforcement to induce elastic behavior, avoiding the emergence of plastic deformation. 4ndeed, elastomers with much lower intermolecular forces restrain molecular mobility, making materials more adept at accommodating deformation and external stresses, boosting elastic behavior.
Gao et al.'s results support that crosslinking may act a powerful platform to tailor mechanical properties of VDF-based fluoropolymers. 4,5Regarding the mechanism of cross-linking reactions, the reaction sites occurring in the amorphous or crystalline regions is crucial to tune the physical properties. 4,5ao et al. used the cross-linking agent of polyethylene glycol which is immiscible of crystalline regions of P(VDF-TrFE).Although the crystallinity is markedly reduced by crosslinking, ferroelectricity is not pronouncedly sacrificed accordingly, 3 which indicates that slight crosslinking approach enables precise control of mechanical properties without the expense of largely reduced remnant polarization.
In summary, Gao et al. developed a unique network structure in P(VDF-TrFE) through chemical crosslinking, which shows robust and nearly unchanged ferroelectric hysteresis loops upon mechanical stretching up to 70%.The elastic ferroelectrics developed by novel cross-linking structure open the door for ferroelectric devices for wearable electronics.