Extent Of Gradient Research Articles

Programmable Mechanically Assisted Geometric Deformations of Glassy Two-Stage Reactive Polymeric Materials

Thiol-isocyanate-methacrylate two-stage reactive network polymers were developed and used for fabrication of well-defined surface patterns as well as functional geometric shapes to demonstrate a new methodology for processing of "smart materials". The dynamic stage I networks were synthesized in base-catalyzed thiol-isocyanate cross-linking reactions to yield tough, glassy materials at ambient conditions. Methacrylate-rich stage I networks, incorporating photoinitiator and photoabsorber, were irradiated with UV light to generate stage II networks with intricate property gradients. Upon directional straining and subsequent temperature-dependent stress relief of the predefined gradient regions, the desired surface or bulk geometric transformations were achieved. Depending on the gradient extent in conjunction with photoorthogonal initiators, the introduced deformations were shown to be easily erasable by heat or permanently fixable by bulk polymerization.

The bending and failure of sandwich structures with auxetic gradient cellular cores

We describe the bending and failure behaviour of polymorphic honeycomb topologies consisting of gradient variations of the horizontal rib length and cell internal across the surface of the cellular structures. The novel cores were used to manufacture sandwich beams subjected to three-point bending tests. Full-scale nonlinear Finite Element models were also developed to simulate the flexural and failure behaviour of the sandwich structures. Good agreement was observed between the experimental and FE model results. And the validated numerical model was then used to perform a parametric analysis on the influence of the gradient core geometry over the mechanical performance of the structures. It was found that the aspect ratio and the extent of gradient (i.e. the horizontal rib length growth rate or the internal angle increment) have a significant influence on the flexural properties of the sandwich panels with angle gradient cores.

Drawing of polypropylene filaments on a gradient heater

High modulus and high tenacity polypropylene filament has been prepared by drawing as-spun filament on a heater having a gradient of temperature. The fiber properties are significantly affected by gradient heater settings, that is, nature and extent of gradient and end temperatures. Very high draw ratios are obtained through gradient drawing, resulting in high crystal perfection and crystallinity, along with very low void fraction. The gradient-drawn filaments showed superior mechanical as well as thermal properties when compared to filaments drawn over a constant temperature heater. Fibers having initial modulus of 17.5 GPa and tenacity of 750 MPa have been prepared in the process. Very low shrinkage of 7% at 150°C indicates excellent thermal stability of the gradient-drawn filaments.