Today's KNOWLEDGE Share : Why Amorphous Polymers Show Less Orientation and Anisotropy in Injection Molding

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 Today's KNOWLEDGE Share

Why Amorphous Polymers Show Less Orientation and Anisotropy in Injection Molding


Despite similar Melt Flow Index (MFI) values, amorphous polymers consistently demonstrate reduced molecular orientation and anisotropy following injection molding compared to semi-crystalline counterparts.

This phenomenon is attributed to three primary factors rooted in their distinct molecular structures and thermal behaviors.


Shorter Chain Lengths and Faster Relaxation Times:

Many engineering amorphous polymers incorporate bulky aromatic moieties, leading to a higher molecular weight per monomer unit. Consequently, for a given total molecular weight, amorphous polymers possess significantly shorter polymer chains than semi-crystalline polymers. Shorter chain lengths result in shorter relaxation times, allowing stress-induced orientation to dissipate more readily before solidification, leading to less locked-in molecular orientation in the final part.


Greater Temperature Difference Between Processing and "Freeze-Off":

The relative difference between the typical melt processing temperature and the material's "freeze-off" temperature significantly influences molecular relaxation. Amorphous polymers are considerably more viscous and necessitate processing at temperatures significantly above their Tg. This larger processing window means they remain in a highly mobile state for a longer duration during cooling in the mold. This extended period allows for greater molecular relaxation and a more complete dissipation of flow-induced orientation, contributing to lower overall locked-in anisotropy.


Absence of Crystallization-Induced Anisotropy:

The process of crystallization itself is a substantial contributor to anisotropy in semi-crystalline polymers. As polymer chains align and organize into crystalline structures, they impart unique anisotropic properties. This inherent contribution to anisotropy is entirely absent in amorphous grades, which by definition lack crystalline domains. Consequently, amorphous polymers avoid this additional source of orientation, further contributing to their lower overall mechanical properties anisotropy.

In a subsequent article, we will specifically address the higher anisotropic shrinkage observed in crystalline materials compared to amorphous ones.


source: Vito leo


#polymers #molecularorientation #rheology

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