Abstract
Multi-Functional Polymer Composites (MFPCs) are active or "smart" materials that presents
mechanical responses to external stimuli such as temperature, light, water, or chemicals, which
promote Shape-Memory Effects (SME) in polymers. The selection of suitable Shape Memory
Polymers (SMPs) to be used as the matrix-phase material with various (In)organic filler
materials to achieve the required optimum multi-stimuli response in Multi-Functional Polymer
Composites (MFPC) systems is therefore critical in achieving the desired results. The selection
of these materials is based on their mechanical and physical properties as well as other
underlying factors such as cost, availability, shape recovery rate and aesthetic characteristics.
A lot of literature exist that investigates the performance of various SMPs and their
applications, a gap exists on available information on the ranking and selection of the best
performing polymer and nanoparticles to be used as matrix and filler materials respectively.
Therefore, the purpose of this study is to present the step-by-step selection of these materials
(SMPs and nanoparticles), examine their suitability based on their mechanical and physical
properties, investigate the design synthesis and characterization of the developed composite
for optimum performance.
In the first step of study, suitable polymer matrix-phase material and conductive fillers were
selected using Multi-Criteria Decision Making (MCDM) approach in which the entropy
method was applied to estimate the weightages of the various criteria while the Grey Relation
Analysis (GRA) and the Technique for Order Preference by Similarity to Ideal Solution
(TOPSIS) Multi-Criteria Decision Making (MCDM) approaches have been used to rank the
suitable matrix-phase polymer and filler materials for manufacturing the MFPC systems.
A total of eight alternative matrix-phase SMPs materials: Poly(methyl methacrylate) (PMMA),
polydimethylsiloxane(PDMS), PolyCarbonate (PC), Polystyrene(PS), Polyurethane elastomers
(TPU), Epoxy-Resin (EP), Polypropylene (PP) and Poly(Lactic Acid) (PLA) and eight filler
materials: Carbon Nanotubes (CNTs), Carbon Black (CB), Nickel powder (Ni), Silicon Carbide
(SiC), Iron Oxide (Fe2O3 ), Iron particles (Fe), Titanium Dioxide (TiO2), and Pure Titanium (Ti),
were selected based on a set of criteria from the literature and the best ranked alternatives were
used in the fabrication of the MFPC using open-casting method. The selection methodology and
the results obtained thereof have been illustrated in detail. The results obtained from TOPSIS
and GRA methods have been compared to conclude the effects of the material properties on the
ranking and the selection of the SMP and filler materials.
Among all the eight alternative polymer materials considered, Thermoplastic Polyurethane
(TPU) was found to be the best matrix-phase material whereas Carbon Nanotubes (CNTs) and
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Iron were found to be the best filler material in the GRA and TOPSIS methods employed
respectively. The material cost, resistivity, % elongation, and hydrophobicity present the most
influencing properties on the SMP material selection whereas density presented no effect on the
SMP matrix material selection. On the other hand, filler density, magnetization value, cost,
electrical resistivity and thermal conductivity presents the most influencing properties on the
MFPC filler material selection with refraction index presenting no or minimum effect. The
sensitivity analysis of the results was achieved using the TOPSIS methodology to validate its
reliability. It was revealed that the TPU, Polycarbonate, Polypropylene, and Epoxy-resin/PLA
respectively are the most dominant matrix-phase SMP material alternatives when entropy
weights of the primary evaluation criteria are applied.
The selected TPU/CNT/Fe matrix-filler materials were then used to fabricate high
performance composites using open molding method and then characterized through thermal
conductivity test, electrical conductivity test, water absorption test, thermogravimetric
analysis, a shore D hardness test and Scanning Electron Microscopy. The shore D hardness of
the TPU composites reinforced with functionalized CNT and Fe powder decreased with an
increase in the weight percentage of the filler nanoparticles while the reverse was the case for
the percentage water absorption that is the percentage water absorption of the composites
increased with an increase in the weight percentage of the filler material. The TPU/CNT
composites exhibited high percentage of water absorption (10.2%) compared to TPU/Fe
(8.78%) and TPU/CNT/Fe (8.8%) composites respectively. However, the TPU/CNT/Fe
composites exhibited high performance compared to TPU/CNTs and TPU/Fe composites. The
introduction of the CNTs and Fe powder, as conductive fillers, greatly improved the thermal
and electrical conductivity properties of the composites. The shore hardness and average
tensile strength of TPU/CNT composites were significantly improved by the curing of the
samples in an oven at glass transition temperature (Tg) for 72 hours while they decreased in the
case of TPU/Fe and TPU/CNT/Fe. Overall, a maximum of approximately 10.5 % remarkable
improvement was recorded in the TPU/CNT composites compared to 0.63% in TPU/CNT/Fe
composites and -1.3% in TPU/Fe composite. The SEM analysis revealed that the introduction of the
Fe powder increased the agglomeration of the fillers and the porosity of the composite thus
reducing the overall properties of the composites which could be an advantage in some typical
engineering applications. The functionalization of the nanoparticles reduced the agglomeration
of the particles in the TPU resin matrix thus enhancing particle dispersion to achieve uniformly
distributed properties of the composite. The novelty of this study is its contribution to the body
of knowledge for suitable matrix-phase and filler materials selection for development of
optimum high performance multi-stimuli polymer composites suitable for application in the
development of biomedical devices, sensors, actuators, and among other fields.