Dear Lila, yes definetely there exist a relation between the degree of crystallinity and mechanical properties. If you are after a mathematical expression I do not know. However, qualitatively speaking the greater the crystallinity the harder the polymer, or the greater the strength as compared to the totally amorphous structure of a given average molecular weight. This is so, as the crystallinity reduces the degree of freedom for the molecular chains to move.

I am sorry but I did not understand the second part of your question. So no comment.

Dear all,

the crystalline phase is ordered and with high cohesion, contrary to the amorphous phase which is in disorder and less cohesive. So, any applied stress will be much concentrated on the weak phase. This means that ones the DENSITY and PERFECTION of the crystalline zone is increased, less is left for the stress concentration, so one may expect an improvement of the mechanical properites compared to the more amorphous polymer. Quantitative models and theories exist and are found in many textbooks. Regards

Then why Glass having material properties better than Polypropylene, as glass is totally amorphous and PP is semicrystalline?

Dear all;

basically, you are comparing two different materials which belong to two different families from the physics and chemistry point of views. The nature of cohesion energy and intensity is not the same. In polymers (linear) most of the cohesion comes from the intermolecular secondary forces and sometimes entanglements if the molecular weight goes over a threshold value. For glass which is an inorganic material the cohesion arises from ionic interactions. If you want to get a better insight compare bond energies involved in each case.

Also not all the poperties of glass are better than PP, take the impact resistance, tensile, flexurer, and compression stresses. May be the only physical property that is in the side of glass is the hardness. Regards

My simple answer is by way of example: Upon comparison between HDPE (highly crystalline) & LDPE (less crystalline), the first has Strength To Weight Ratio(in kN-m/kg) equal to 21 to 63 while the second has 8. Ultimate Tensile Strength(in MPa) for the 1st is 20 to 80 while it is 7 for the 2nd.

Dr. Abdelkader BOUAZIZ explained correctly why mechanical properties improve upon increase of crystallinity.

In PE the degree of crystallinity is responsible for all mechanical properties: strength, toughness, creep resistance, crack propagation resistance and seal strength.

High level of crystalline phase: High tensiles ( yield and ultimate) and creep resistance. Lower toughness as elongation to rupture is decreased and lesser seal strength( lower elasticity and molecular diffusion).

The amorphous phase absorbs energy hindering propagation of cracks, presents better chain diffusion and molten entanglement to offer greater seal strength. More amorphous PEs exhibit lower Ty,Tu and creep resistance, but better resistance to impact due their greater elongation to rupture

Dear All,

I tried to refrain from giving a second comment on the question, expecting that someone from the field gives a more comprehensive answer. I am sorry to see that no such answer has so far emerged. So, I will further comment by saying the following:

The polymers are a group of materials in which covalent bonding dominates. This is the very strong bond that exist between the molecular chains of carbon. The bonds, on the other hand, between the carbon chains are weak type such as Van der Waals bonds. Having understood the bond type we can actually envisage how the polymer behaves mechanically.

When a polymer is subjected to external forces there exist two degrees of freedom operating and leading to elastic deformation: 1) The rotational freedom of the zig-zag bond between the carbon atoms (which makes about 109 degree). This means the C-C bond can rotate and allow elastic deformation. 2) The spagetti-like (in your plate, i.e. cooked) bendings that exist is long chains of carbon that are easy to straighten under external forces and to recoil upon releasing the force.

When it comes to the permanent deformation, that is plastic deformation, we are NOT breaking, or not even stretching the very strong C-C bonds. After the elastic deformation capacity is exhausted, the external force starts sliding the molecular chains over one another. This deformation is not recoverable. At the end, when a polymer is ruptured, we still have not broken any C-C bonds, but simply have come to the end of the sliding motion of the molecular chains over each other. This movement, needless to say becomes more difficult as the average molecular chain lenght (i.e. average molecular weight) is increased. For the same average chain length, if the C-C chains have large side atoms or pendant groups attached (this means now we have a different polymer, not the same one), again the sliding movement of the chains over each other becomes more difficult. And we read it as an increased strength once again.

How does the crystallinity enter into this picture and what does it mean? First of all it should be borne in mind that complete crystallinity in polymers is not achievable. Crystallinity in polymers means, instead of having a spagetti-like chain structure (which we call amorphous) we now have, at least for some segment lengths of a group of chains sitting in order over each other (like those segments are laid on top of each other regularly). Such limited-size regions are crystallites. The longer the average molecular chain length (or the larger the pendant atoms or pendant groups) the harder to create such configurations. In such regions, it is understandably more difficult for the chains to slide past one another due to greater friction forces. hence, the greater strength levels as the degree of crystallinity increased (I do not mean the temperature by the word 'degree').

Thanks for your attention.

I am quoting a sentence from my comment above that needs correction. I apologize for this mistake. I should have read it before posting it. Although the rest of the text shows that it was a mistake during writing, still the error must be corrected. The word "between" should actually read as "within":

The polymers are a group of materials in which covalent bonding dominates. This is the very strong bond that exist between (WITHIN) the molecular chains of carbon.

Thank you for your understanding.

Regards,

AAK

Dear all,

Thanks for sharing.

Regards,