Semi-Crystalline Polymers: Structure vs Properties

The combination of chain length, molecular weight, monomer unit combinations, tacticity, additive content, extent of branching and/or crosslinking, and lattice constants combine to determine a polymer’s molecular/physical properties. Each category can be tweaked by adjusting reactor temperatures, pressures, and catalyst composition. During injection molding and extrusion processes, the injection speed (or extruder output rate), melt temperature, mold/die temperature, and process pressure can influence the long range order and vary the degree of crystallinity. In general, the macromolecular form that polymer products take post processing can be referred to as the morphological structure; these exist as either amorphous, semi-crystalline, or liquid crystalline. The strongest correlation that leads to polymers with high crystallinity is the regularity of the structure of the polymer backbone and the flexibility of the polymer backbone. For these reasons atactic isomeric structures will have very low crystallinity while isotactic and syndiotactic materials will more readily recrystallize. This article will be focused on a few effects that degree of crystallinity and crystal size distribution can have on a semi-crystalline polymer’s chemical and mechanical properties. Polymer crystallinity is controlled by several parameters:

• Rate of cooling post injection varies with cross sectional area as polymer chains align into a repeated structure.
• Monomer complexity: crystallization is less likely in complex structures while simple polymers, such as polyethylene, crystallize relatively easily.
• Chain configuration: linear polymers crystallize relatively easily while polymers with branches prevent rapid crystallization.
• Copolymerism: less energy is required to crystallize if monomer units are more consistently spaced – alternating and block copolymers can crystallize more rapidly as compared to random and graft copolymers.
• In general increased crystallinity brings higher density, more strength, and higher resistance to dissolution by chemical solvents, and higher resistance to softening by heating.

Determination of crystallinity

Degree of crystallinity can be determined with many analytical techniques. Below we show examples for DSC and density calculations.

Melt/Mold Temperature Effects

While spherulite radius growth is generally considered linear with time at a constant temperature, it is important to note that the injection molding thermal process is a constant fluctuation during the molten phase injection and subsequent cooling. During injection, molecular chains can become oriented and act as nucleation zones that other polymer chains build around. The initial re-crystallization from the melt will be slow until the material is below the melting point but above the glass transition temperature.

Monomer Sub-unit Structure

Monomer sub-units can alter the crystallinity for many polymers. The addition of altered monomer units changes the long range order that dispersion forces play on the polymer. This in turn reduces the crystallinity and eventually melting point. 

The addition of atactic PP to the matrix of isotactic PP reduces the melting point and radial growth rate of spherulites. This ultimately drives down the percent crystallinity.

Impact Strength Modification

As the density and percent crystallinity of olefinic elastomers increase, the efficiency of impact modification is reduced. Lower crystalline regions allow for mechanical energy to be absorbed by the mobility of chains in the polymer structure and prevent breakage. It is important to note that the operating temperature relative to an amorphous plastics’ Tg (glass transition) will influence the relative impact/elongation performance. Materials that are operating well below their Tg will behave in a glassy or brittle manner while materials above their TG will have much lower mechanical strength while improving elongation at break.

Lattice Size / Optics

Nucleation can affect the size and overall uniform geometry of crystals within a molded product. Rather than crystal formation occurring randomly, the nucleation sites act to initiate crystallization such that most spherulites are of the same order of magnitude (size). This helps reduce variations in the index of refraction and can help reduce haze in random copolymer PP and PE.

Exothermic recrystallization with nucleation agents helps to increase the onset temperature and peak recrystallization temperature for a polymer sample. This ultimately can reduce cycle time and reduce haze in resulting parts.

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