(Amorphous vs semi-crystalline) Using words and visuals, describe the amorphous and semi-crystalline arrangements of polymer chains when forming a...

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Mechanical Engineering

(Amorphous vssemi-crystalline)
Using words andvisuals, describe the amorphous and semi-crystalline arrangementsof polymer chains when forming a solid. Using examples of specificpolymers, explain why some types of polymer chains do not readilyalign and tend to be amorphous while others readily formsemi-crystalline structures.

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Polymers are composed of long molecular chains which formirregular entangled coils in the melt Some polymers retain such adisordered structure upon freezing and thus convert into amorphoussolids In other polymers the chains rearrange upon freezing andform partly ordered regions with a typical size of the order of 1micrometer Although it would be energetically favorable for thepolymer chains to align parallel such alignment is hindered by theentanglement Therefore within the ordered regions the polymerchains are both aligned and folded Those regions are thereforeneither crystalline nor amorphous and are classified assemicrystallineExamples of semicrystalline polymers are linear polyethylenePE polyethylene terephthalate PET polytetrafluoroethylenePTFE or isotactic polypropylene PPSemi crystalline polymers form spherulite structuresIt is for arelatively rapid growthWhen compared to semicrystalline thermoplastics of a similargrade amorphous plastics tend to have better dimensional stabilityand impact resistance Drawbacks include amorphous plastics havingpoor fatigue resistance and are prone to stress crackingAdvantages of semicrystalline plastics are that they areexcellent for bearing wear and structural applications Whencompared to amorphous thermoplastics these semicrystallines tendto have better chemical resistance electrical properties and alower coefficient of friction Drawbacks include semicrystallineplastics being difficult to thermoform difficult to bond have asharp melting point and only average impact resistanceNucleationNucleation starts with small nanometersized areas where as aresult of heat motion some chains See Answer

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