Thermoplastics are defined as polymers that can be melted and recast for different use almost indefinitely. On a molecular scale, polymer chains are held together by weak non-covalent bonds. The chains themselves affect large-scale physical properties due to variety of number and weight-based averages in a sample. Thermoplastic stress-strain curves possess a distinctive "dip" corresponding to physical necking in response to a certain stress range. Common thermoplastics include acrylic (PMMA) and polyphenylene sulfide (PPS).
At cold temperature, thermoplastics are brittle (glassy). Upon heating, thermoplastics become soft and malleable, especially in the vicinity of their particular glass transition temperature. Further heating gradually transforms a thermoplastic mold into a viscous liquid. At that point, the plastic can be molded into a new shape for reuse. Thermoplastics are therefore easily recycled.
Polymer Chain Interaction
Thermoplastic properties are explained by their molecular structure. Long chains are held together by weak intermolecular forces (van-der-Waals forces) that are gradually overcome with higher temperature. This results in gradual softening and eventual polymer liquidity upon heating. Polymer backbones remain separate and comparatively inert. Thermoplastics are usually synthesized by addition reactions, where monomers continuously link on a chain "tail" to make the progressively longer polymer backbone.
Polymer Chain Characterization
Thermoplastic polymer chains are not monotonous carbon-hydrogen units. Chains of different lengths constitute a sample, with two numbers summarizing chain length distribution: number-average-molecular weight (Mn) and weight-average-molecular-weight (Mw). Weight-average-molecular-weight will always be higher than Mn, though the ratio of the two (polydispersity) can get close to one if manufactured very carefully. A polymer chain is built in simple series (single monomer), block or alternating patterns of several monomers. Chains can also possess different monomers throughout their lengths in a statistical/semi-random distribution.
Thermoplastics have a distinctive stress-strain curve. A smooth upward curve peaks and then goes down to some extent. This "dip" correlates to necking (narrowing). Necking mechanics in thermoplastics are such that in a limited domain, increased applied stress actually decreases material strain. As further stress is applied, material strain again increases--upward rise in the curve--until stress sufficient to break or "snap" the sample is achieved.
Product Example: Acrylic
Many common substances are thermoplastics, among them acrylic (polymethylmethacrylate, PMMA). Through clever synthesis techniques, this thermoplastic has exceptional transparency, corrosion resistance and impact resistance. Acrylic maintains these properties under temperature and humidity extremes. While not as scratch-resistant as traditional glass, acrylic combined with specially designed finishes and additives makes a lightweight, safe and recyclable alternative to glass.
Product Example: PPS
A general thermoplastic property is strength without excess weight. Polyphenylene sulfide (PPS) shares this property. Unlike acrylic, it is not a transparent substance at room temperature and has different applications. Thermoplastic PPS is used, among other places, as material for tubing within aircraft interiors. While lacking network-chain microstructure characteristic of thermosetting plastics, PPS contains inter-chain sulfur-based covalent "links" that render it especially impervious to flame and temperature extremes.
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