Materials- Polymers New Stuff Flashcards
As well as molecular mass and shape, what else do the physical characteristics of polymers depend on?
The structure of the molecular chains (they can be linear, branched or a network) which can be controlled by modern polymer synthesis.
Describe linear polymers
Repeat units are joined together end to end in single chains. The long chains are flexible with Van der Waals and possibly hydrogen bonds between chains.
Describe branched polymers
Side-branched chains connected to the main ones and result from side reactions that occur during the synthesis of the polymer. Reduces chain packing efficiency which lowers polymer density. UTS higher for higher density.
Describe network (or chemically cross linked) chains
Adjacent linear chains joined to one another at various positions by covalent bonds. Achieved either during synthesis or by non-reversible chemical reaction. Process is vulcanisation. Can form 3 dimensional networks. Example is rubber.
Describe thermoplastic polymers
They melt when heated and re-solidify when cooled. They are linear or branched. Ductile. Can be amorphous or semicrystalline (containing amorphous and crystalline phases). Under tension, highly entangled chains can be straightened. Can be recycled.
Describe thermoset polymers
Network polymers. High degree of cross-linking. Intractable once formed. Decompose instead of melting when heated. Hard and brittle. Under tension, tightly bound 3D network of polymer chains are prevented from moving apart by the cross links (apart from rubbers). Difficult to recycle but can be used at higher temperatures than thermoplastics and are often more chemically inert.
Describe elastomers
Lightly cross-linked. Easily stretched so have high extension ratio and show rapid recovery.
Describe stress strain graph for thermoplastic and thermoset polymers
Thermoplastic: linear then reaches a peak and curves down then back to linear until breaking.
Thermoset: linear until a little curvature then breaks
Things to consider when choosing a forming method for polymeric materials
If it is thermosetting or thermoplastic. If thermoplastic then at what temperature it softens
What is Tg?
The temperature region where the polymer transitions from a hard, glassy, brittle material to a soft, rubbery, ductile material. Only for amorphous thermoplastics
How are thermoplastic polymers formed?
Above their glass transition temperatures (Tg) if amorphous, or above their melting temperatures if semicrystalline. Applied pressure maintained as the piece is cooled so the formed article will retain its shape.
Stages of forming thermosetting polymers
Preparation of a linear polymer as a liquid having a low molecular mass.
Curing: occurs during heating and/or by the addition of catalysts often under pressure. Chemical and structural changes occur on molecular level. Network structure forms. Normally carried out in mold having desired shape.
Molding techniques for polymers
Include compression, transfer, blow, injection, extrusion molding. For each, a finely pelletised or granulated plastic is forced at an elavated temperature and by pressure to flow into, fill and assume the shape of a mold cavity.
How does extrusion work?
Polymer pellets fed in and heated and forced through a die so that it takes up a required shape. It continues to be forced along in a continuous process with lengths being cut as required. Used to manufacture products with constant CSA. Works for thermoplastics.
How does inject molding work?
Polymer pellets fed in and heated then injected into a mold. Can achieve complex shapes of various sizes and with fine details. Good dimensional accuracy, high production rate, but high tooling cost. Works for thermoplastics
