𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞 : 𝐃𝐢𝐝 𝐲𝐨𝐮 𝐧𝐨𝐭𝐢𝐜𝐞 𝐭𝐡𝐚𝐭 𝐬𝐞𝐦𝐢-𝐜𝐫𝐲𝐬𝐭𝐚𝐥𝐥𝐢𝐧𝐞 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐬 𝐜𝐨𝐧𝐬𝐢𝐬𝐭𝐞𝐧𝐭𝐥𝐲 𝐬𝐡𝐨𝐰 𝐚 𝐡𝐢𝐠𝐡𝐞𝐫 𝐝𝐞𝐠𝐫𝐞𝐞 𝐨𝐟 𝐬𝐡𝐫𝐢𝐧𝐤𝐚𝐠𝐞 𝐚𝐧𝐢𝐬𝐨𝐭𝐫𝐨𝐩𝐲 𝐝𝐮𝐞 𝐭𝐨 𝐦𝐨𝐥𝐞𝐜𝐮𝐥𝐚𝐫 𝐨𝐫𝐢𝐞𝐧𝐭𝐚𝐭𝐢𝐨𝐧 ?

𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞

𝐃𝐢𝐝 𝐲𝐨𝐮 𝐧𝐨𝐭𝐢𝐜𝐞 𝐭𝐡𝐚𝐭 𝐬𝐞𝐦𝐢-𝐜𝐫𝐲𝐬𝐭𝐚𝐥𝐥𝐢𝐧𝐞 𝐩𝐨𝐥𝐲𝐦𝐞𝐫𝐬 𝐜𝐨𝐧𝐬𝐢𝐬𝐭𝐞𝐧𝐭𝐥𝐲 𝐬𝐡𝐨𝐰 𝐚 𝐡𝐢𝐠𝐡𝐞𝐫 𝐝𝐞𝐠𝐫𝐞𝐞 𝐨𝐟 𝐬𝐡𝐫𝐢𝐧𝐤𝐚𝐠𝐞 𝐚𝐧𝐢𝐬𝐨𝐭𝐫𝐨𝐩𝐲 𝐝𝐮𝐞 𝐭𝐨 𝐦𝐨𝐥𝐞𝐜𝐮𝐥𝐚𝐫 𝐨𝐫𝐢𝐞𝐧𝐭𝐚𝐭𝐢𝐨𝐧 ?

One could think that amorphous polymers (often more viscous, so with a longer relaxation time) should produce as much, if not more, orientation effects. But, at least as far as shrinkage anisotropy is concerned, they typically don't.

In a semi-crystalline matrix the much faster freezing induced by the phase change and the intricately mixed morphology of the resulting two-phase system (amorphous and crystalline regions), leads to an overall much higher locked-in orientation of the amorphous regions, easily visualized by annealing induced post-shrinkage.

Of course the crystalline phase will also have its own anisotropy, for instance in terms of preferred crystalline orientation, possibly observed by X-ray scattering experiments. This strongly contributes to the much higher elastic modulus observed in the flow direction for semi-crystalline polymers.

Despite some orientation, amorphous polymers will show almost perfectly isotropic modulus with possibly just a smallish increase in Yield Stress in the flow direction.

In practice, we will see more warpage driven by anisotropy when molding PP vs. say PC.

Birefringence will however confirm that some orientation is also present in PC, but with far less significant consequences on shrinkage and warpage.

source : Vito leo