Study Of The Relationship Between Structure And Mechanical Properties Of Polyamide 12 And Its Elastomers

In this work, the relationship between the structure and mechanical properties of polyamide 12 (PA 12) and its elastomers (polytetrahydrofuran-PA 12 segmented copolymers) were systematically studied. In the yield stage, a transient phase namedα''phase was induced. It disappeared when the stress increased. Deformation inducedα''phase also occurred when drawing polyamide 12 (PA 12) in its segmented copolymers form with polytetrahydrofuran (PTHF). In these segmented copolymers, which contain a high content of PTHF, the metastableα"phase can exist at room temperature. When the temperature and content of PA 12 increase, theα"phase is a transient phase, only survived in a widow of stress and temperature.The structural evolution of PA 12 during tensile test was studied using in-situ wide X-ray scattering (WAXS) below and above glass transition temperature (Tg). During the yield stage,γphase was transformed to a transientα''phase, whose lifetime decreased with the increase of temperature. When the stress increased at temperature below Tg, the transientα''phase was transformed to mesomorphic state. However, above Tg, the final state wasγ'phase. The different final state is due to the competition and coupling between external work and thermal activation. At temperature below Tg, the external work is the main reason why the transient phase appears and further transforms to mesomorphic state, while thermal activation drives the transition from the transientα''phase toγ'phase at temperature above Tg. The phase transitions occur in the plastic deformation region, which shows a close correlation between the mechanical response and the structural evolution during tensile deformation.Deformation-induced phase transition behavior of polyamide 12 (PA 12) in its segmented copolymers form with polytetrahydrofuran (PTHF) was studied with in-situ WAXS at different temperatures. In these segmented copolymers, which contain a high content of PTHF, a transformation from the stableγphase to a metastableα"phase was observed during tensile deformation at room temperature. The deformation-inducedα"phase is not a thermodynamic stable phase but arises from kinetic origins. After the release of tensile force following deformation, the metastableα"phase can partially transform back to the initial phase. The reversible phase nature may contribute somewhat to the elasticity of PTHF-PA 12 systems as a result of this enthalpic contribution. Upon increasing the content of PTHF in the copolymer, the critical stress required to induce the newα"phase increases. Theα"phase will also disappear in samples that possess a particularly high content of PA 12. Higher temperatures also prevent theγphase from transforming to theα"phase.