Polymer Classification - Morphology

This is a continuation of the discussion started last Wednesday.  Once we determine whether our polymer is thermoplastic or thermoset, then we want to know some things about something called, "morphology."  The term morphology refers to the structure of the molecules as they make up the polymer matrix.  This is more relevant to thermoplastics, but there are some thermosets where the morphology plays a role in the properties of the resin.  The two primary morphological states of interest to us, as engineers, are crystalline and amorphous.  Well, crystalline is a relative term as polymers tend to be more "semi-crystalline" than truly crystalline the way a metal is. 

If one were to imagine a pile of sticks.  Pulling on one of the sticks will lead the puller to the conclusion that the pieces are entangled.  The same is true of amorphous materials such a polycarbonate.  The side-chains on the molecules create entanglements that help add to the strength of the polymer where crystallinity is lacking and would otherwise provide strength.  Also like the stick pile, the molecules in an amorphous material are porous on the molecular level.  This tends to make them especially susceptible to chemical attack.  Clear polymers are generally known for being especially sensitive to solvents when under stress - and more sensitive to tensile stress than compressive.  Some amorphous materials are known for being brittle (i.e. Polystyrene) while others are known for their toughness (i.e. Polycarbonate).  This is generally due to the differences in intermolecular forces (i.e. van der Waals forces). 

Some examples of amorphous materials are:  Polycarbonate, Polystyrene, Polymethylmethacrilate (PMMA - Acrylic), Acrilonitrile Alloys (SAN, ABS, etc), Polyvinylchloride (PVC), Transparent Nylon 12,  and many others.

On the other hand, if one were to imagine the image of a plate of spaghetti, we have an image that's more descriptive of a crystalline material.  The polymer chains are narrow and long.  Now if we imagine that our strands of spaghetti were made of tiny little magnetic balls, then we might get a mental image of how they would want to fold and form crystals as they cool.  When it comes to the flow of a polymer, crystalline polymers tend to align and then flow very freely past one another where amorphous polymers tend to be thicker with a less distinct fluid transition.  The morphology of crystalline polymers is affected by the shear history during flow.  The higher the shear rate, the more nuclei that will form and thus the greater number of crystals of smaller size.  Also, the rate of cooling will secondarily affect the crystalline morphology of polymers.  The slower the cooling, the larger the crystals.  Because of this, crystalline polymers tend to be more dense than amorphous polymers and the appearance of crystalline polymers without colorants will generally be a milky translucent white, and never clear at room temperature as opposed to amorphous polymers which can (as natural, uncolored resins) be clear at room temperature.

Some examples of crystalline polymers are: Polyethylene (PE) - including LDPE, HDPE, & UHMWPE, Polypropylene (PP), Polyoxymethylene (POM - Acetal - Celcon, Celstran, Delrin), Polyamide (Nylon 6, 6/6, etc), Polyethersulphone (PES), Polyesters (PBT, PET, PCT), Syndiotactic Polystyrene, etc.

So when you're thinking about crystalline and amorphous polymers the analogy to a pile of sticks or a plate of spaghetti works well in thinking about the differences in their physical properties and their flow behaviors.