Couple of questions about crosslinking and its effect on mechanical properties..
1) I understant that crosslinking results in bond forming between chains which decreases chain mobility hence resulting in decreased tensile strength, My question is how does cross linking affect the storage modulus of a material? I understand that cross linking increases the storage modulus, my question is how?
2) What is the relationship between tensile strength and storage modulus? Is it possible for a sample to show reduced tensile strength, but increased storage modulus after cross linking?
3) From what I read online storage modulus is the ability to store deformation energy in an elastic manner, does elastic manner mean a material with a high elastic modulus? If yes, how come crosslinking increases storage modulus? Crosslinking would decrease chain mobility hence decreasing the elasticity...
I am a student and a lot of the technical jargon goes above my head so relatively simple answers would be great...
Hi Fatima,
1) The decrease in chain mobility means that chains can't flow past one another upon deformation of the bulk. The chain reaches its mobility limit in smaller deformations, causing the external load to strain the chemical bonds (trying to "bend" them). In an un-crosslinked material (especially elastomers), the modulus is driven by the difficulty of chains to flow past one another due to entanglements (think noodles) and also, a thermodynamic drive which we can go into detail if you want (you wanted simple). Crosslinks form a more rigid, non-flowing bulk compared to entanglements.
2) Strength and Modulus sometimes correlate but the relationship is an artifact of how we present this data (Stress-strain curves in a static test). The definitions are: Tensile strength is the maximum stress the material can withstand before undergoing irreversible deformation or break. It is the highest point on the stress-strain curve (usually, without splitting hairs). The storage modulus is in a dynamic test, not a static test. In these tests, we oscillate a certain strain in a sample and measure the energy lost (heat, loss modulus E''), the energy recovered (elastic recovery, storage modulus E') and their ratio (tan(delta)). A high storage modulus means that more energy will be recovered.
However, the tensile modulus, in a static tensile test is the slope of the stress-strain curve in the linear region (early on, before the onset of unrecoverable deformation).
Now when I say static test, I mean the classic tensile test when a sample is pulled in a constant rate and the forced is measured over time.
3) They will often correlate but remember that it is all relative. Comparing two very different materials may be tricky... "Elasticity" is often confusing when used in science/engineering and in everyday life. The "Elastic region" of the stress-strain curve is when recoverable deformation occurs (nothing is permanent). And high elasticity means high elastic modulus (slope of the curve in that region). In everyday life, we sometimes say "a very elastic material" to describe a material with large, recoverable, elongations (strain).
Hope this helps.
Interlinking by any means(atoms, electrons or ions)between different long backbone chain in a polymer is called cross linking. This enhances the binding the chains in such a three dimensional way the mobility or movement, sliding, breaking of long chain by external condition is prevented.
Crosslinking always enhances the storage modulus. Nanoindentation is the right technique to identify changes in storage modulus due to changes in cross-linking.
The storage modulus in polymer materials measures the stored energy i.e., elastic portion, and the energy. Cross linking brings decrease in chain mobility. What you understood that cross linking increases the storage modulus that is correct because with cross linking the interconnection between different long back bone chains increase the elastic energy(stress applied and strain)or storage modulus of the polymer.
As an addition to this question: What effect will increase in chain mobility have on tensile strength and elongation at break for a material?
Hi dear Fatima
check the following paper please:
Article The Effect of Cross-Linking on the Elastic Modulus of Polythene
Best regards
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