In a study published in the journal Materials today communication, analysis was performed on polymer composites with shape memory (SMPC) made from industrial substances previously used in space. The findings enabled the identification of recovery processes at the microscopic level crucial for shape memory capacity at the macroscopic level.
Shape-Memory Polymers – A mainstay in the future for several industries
Shape memory polymeric composites (SMPCs) belong to a class of smart material architectures that can distort and restore their original shape when stimulated.
In thermally stimulated architectures, the glass transition temperature (Tg) can be used to set the fixation phase and the restorative properties of the original shape. There is great interest in these smart materials in various industries, including biomedical and aeronautical areas, where the use of controls is crucial.
Shape Memory Behavior – Nobody likes polymers
Shape memory response (SM) in carbon fiber is often obtained by adding a polymeric framework with SM properties. Mold memory polymers (SMPs) can withstand larger deformations than shape memory alloys (SMAs), even when a smaller activation force is required to regain the undeformed baseline state.
Depending on the function required, a suitable polymeric framework can be used. In general, thermosets have superior shape memory properties compared to thermoplastics, and epoxy resins often provide the highest performance.
Epoxies have outstanding thermal and mechanical properties and are widely used in all production techniques used in high-impact sectors such as the aerospace and automotive industries.
The main objective of the study
The use of thermomechanical cycling to measure SM grades is a common approach to evaluating the shape memory behavior of SMPs.
The behavior of shape memory and the interactions between the distinct layers on a nanometric scale have not yet been studied. This research aims to thoroughly analyze these smart materials at a microscopic and a macroscopic level, with particular emphasis on the resulting mesostructures.
a) Schematic description of SMPC structures and b) Procedure for manufacturing SMPC samples. © Bellisario, D., et al (2022)
This work used compression molding of industrially available materials to create two distinct polymer composite structures with shape memory suitable for aeronautical use. A large carbon fiber casing made from commercially available thermosetting “prepregs” for the aerospace industry was evaluated for the first time for analysis of microscale shape memory behavior.
The contribution of the shape memory intermediate layer by virtue of its architecture was examined at the microscopic level. The proposed composite laminations linked the architectural properties of collaminations with the functionalities of SMP. The shape memory layer sandwiched between the two shape memory polymer composite structures differed so that one had a thin coating of shape memory epoxy resin, while the other contained a shape memory epoxy foam.
The two systems were built to test the suitability of a casting method in the aerospace industry, such as compression molding, to produce smart architectures for aircraft. This was a significant achievement for the research as most of the proposed new polymeric composite materials with shape memory in the existing literature could not be classified as aerospace quality materials.
Micro-CT scan and analysis of the SMPCs a) cross section, b) side view and c) analysis of SMPC with epoxy powder intermediate layer and d) cross section, e) side view and f) analysis of SMPC with thin layer of epoxy foam. © Bellisario, D., et al (2022)
The main results of the study
MicroCT and SEM imaging showed strong bonding between the carbon fiber reinforced prepreg layer (CFRP) and the epoxy interlayer, either in the form of a thin foam layer or a thin film. The homogeneity of the thin shape memory polymer intermediate layer created during composite lamination was emphasized by SEM imaging, with a very small level of porosity in the CFRP layer shown by MicroCT assessment.
DMA studies revealed that mold memory interlayers affected the transition region, which narrows further when mold memory resin is used. Nano-instrumented wells and micro-wells were used to analyze microscale and nanoscale shape restoration behavior.
This was the first time the nanoinstrumented approach was applied to this type of shape memory polymer composites, allowing the SM response temperature to be calculated over a small range for both SMPC forms.
The SM behavior of shape composite polymer composite structures was confirmed on a macro scale using ascending thermomechanical cycling and many thermomechanical cycles.
The SM polymer composite with the thin epoxy foam market showed better shape memory, as predicted, given the foam architecture. SM polymer composite with epoxy powder intermediate layer, on the other hand, showed lower but impressive shape memory properties.
Recurring thermomechanical cycling affected the SM behavior while maintaining the architectural integrity of the generated smart materials. In addition, a suitable design of the type and amount of shape memory epoxy interlayer could significantly increase the shape memory behavior.
Bellisario, D., Quadrini, F. et al. (2022). Microscopic testing of carbon fiber laminate with shape memory epoxy interlayer. Materials today communication. Available on: https://doi.org/10.1016/j.mtcomm.2022.103854
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