Glass Transition Temperature (Tg) Of Polymers - Protolabs

Amorphous vs. Semi-crystalline Polymers

Polymers fall into one of two classes: thermosets and thermoplastics. Thermoplastic polymers are then further divided into one of two camps: those that are amorphous such as polycarbonate (PC) and polystyrene (PS), and those that are semi-crystalline (polypropylene and acetal are two examples).

In order to take apart the watch on this, it’s necessary to revisit high school chemistry class. Don’t worry, though; it won’t take long. We’ll begin on the first day, when the teacher poses the question: what are polymers? The brainiac sitting up front who studies ancient languages for fun knows that “poly” and “mer” are the Greek words for “many parts” and therefore pipes up with “polymers are long chains of smaller molecules joined together through a process called polymerization, and whose molecular weight ranges from the hundreds to hundreds of thousands.” Congratulations.  

Polymer Structure

	Thermoplastic		Thermoset
	Amorphous	Crystalline
Chain Structure	Random/Disordered	Ordered/Stable	Crosslinked
Melting Point	None defined / softens gradually	Distinct/crystalline disassociation	No melting point
Shrinkage	Low	High	Low
Appearance	Transparent	Opaque	Varies
Chemical Resistance	Low	High	High
Examples	ABS, PC, PS	PP, PET, POM	Epoxy, LSR

 

Long Molecular Chains

But what’s molecular weight, anyway? More importantly, who cares? Anyone designing plastic parts should. Any polymer’s molecular weight determines the length of the “long chains” just mentioned and, therefore, its physical characteristics. For example, where a hydrogen molecule “weighs” just 1.01 g/mol (molar mass) and one of carbon weighs 12.01 g/mol, a single molecule of high-density polyethylene (HDPE)—which is nothing more than a linked series of these two molecules—can weigh in at 250,000 g/mol or more.

Whether it’s polyethylene terephthalate (PET) molecules that range from 8,000 to 31,000 g/mol or polystyrene (PS) molecules at a whopping 400,000 g/mol, these centipede-like chains of monomers—which are the building blocks of polymers—arrange themselves into either amorphous or semi-crystalline structures.

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Material Alternatives for Plastic Injection Molding

Navigating supply chain issues is a constant challenge, especially in times of material shortages. To help, we created a detailed guide to resin substitutes for commonly molded thermoplastics.

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Polymer Morphology

Amorphous polymers have random / disordered chain structure. Below Tg, they are hard and brittle. As heat is applied, they gradually begin to soften to a point they become leathery/rubbery. This transition is the glass transition. Continue to apply heat and they gradually grow molten (moldable), having passed through the Tg up to a temperature where the polymer begins to exhibit viscous flow. Common examples of amorphous polymers include hard, rigid materials such as polystyrene (PS) and polymethyl methacrylate (PMMA), which are used in their glassy state and well below their glass transition temperatures.

Semi-crystalline polymers have highly ordered crystalline regions along with amorphous regions.  The amorphous regions will exhibit the same behavior as just described.  However, with semi-crystalline materials, once the amorphous regions have passed through Tg the crystalline regions remain highly ordered and provide structure to the bulk material. Because of this, many semi-crystalline materials can be used well beyond their Tg. Semi-crystalline materials such as polypropylene (PP), which has a Tg around -20°C are used above their Tg in applications such as lawn furniture that display toughness and flexibility in the warm summer months but can become brittle in the depths of cold Northern winters.