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Chapter6_poly (2) - Chapter 6 Polymeric Materials for Aero...

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Chapter 6 Polymeric Materials for Aero Applications
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Reading Material Callister, p. 489 - 565
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Structure and Properties of Polymers Major aerospace uses of polymers are in composites, adhesives and nonstructural applications such as adhesives and nonstructural applications such as seats and canopies Compared to metals, polymers have low density, low stiffness and strength and low conductivities stiffness and strength, and low conductivities. Service temperature is limited to about 300°C Dimensional stability over time is still uncertain P l t i ll h b t ti ll Polymers are typically amorphous but can partially crystallize with slow cooling E: 7 MPa -4 GPa (for metals: 50-400 GPa) UTS t 100 MP ( t l 4000MP ) UTS: up to 100 MPa (metals: 4000MPa)
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Classification Basic classification is according to the response to rising temperature: thermoplastics, thermosets and elastomers – Of course there are also: thermoplastic elastomers (TPE) and thermosetting elastomers (TSE) and even thermoplastic elastomers.
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Mer and Polymer Microstructure - Review Poly mer = many mers (mer is a structural entity repeated along the chain) C C C C C C H H H H H H mer C C C C C C H H H H H H mer C C C C C C H H H H H H mer H H H H H H Polyethylene (PE) Cl Cl Cl H H H Polyvinyl chloride (PVC) Polypropylene (PP) CH 3 H H H CH 3 CH 3 Covalent chain configurations and strength: secondary bonding Branched Cross-Linked Network Linear Direction of increasing strength
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Tg and Tm the presence of bulky side groups, polar side group,s double chain bond, and aromatic groups all increase Tg higher M (molecular weight) increases Tg b hi d li ki branching and crosslinking increase Tg thermal treatment history can thermal treatment history can affect Tg
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Molecular Structures of Polymers - i Review There are four types of molecular of molecular structures: linear (a), branched (b), cross linked (c) and network (d) 3-D framework
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Polymerization - Review Polymerization Review Two processes: addition polymerization or condensation polymerization • Addition: Bifunctional monomer units are attached on one at a – Bifunctional monomer units are attached on one at a time in chain-like fashions to form a linear macromolucule – Initiator/catalyst + monomers – Initiator/catalyst + monomers Condensation (with byproduct typically H 2 O) – The formation of polymer by stepwise intermolecular h i l ti chemical reaction – trifunctional monomer cross linked or network polymers Note polymer alloys can be created; e.g. polyimide-sulphones
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Thermoplastic Polymers – Soften when heated (and eventually liquify) and harden when cooled. The process is totally reversible and can be repeated a number of times (pop bottles become carpets etc) – With the increase of temperature, secondary bonding forces diminishes due to molecular motion relative forces diminishes due to molecular motion, relative movement of adjacent chains is made easy.
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