Situation: A film made of viscoelastic polymers (composite of Nylon 6 and LLDPE) placed in a weatherometer overtime ( 5000+hrs ) looses (20-25)% of its elongation at yield and increases both its yield strength by (40-45)% and Young’s modulus by (60-65)%. Additionally it loses 50% in both ultimate ( strength and elongation).
Questions for discussion:
1. Can one conclude that this is the result of cross linking of the polymer chains?
2. Is this cross linking similar to the one deliberately induced via peroxides, radiation, silanes, etc? in other words are the mechanical effect similar? Building intermolecular chain bridges to limit chain mobility and slippage?
3. If the mechanical properties of interest are yield strength and creep resistance can one claim that in reality UV induced degradation is “temporarily and on its way to failure” improving properties of interest? Like:
4. Higher creep resistance due to reduced chain mobility and ability to slip?
5. Improved resistance to permanent deformation as yield strength has increased?
6. Improved thermal stability as the chain mobility has been reduced, consequently better creep and tensile resistance at higher temperatures?
Dear Ricardo,
Much of the phenomena you observed appears in polyetehylen films even without polyamide addition.
http://www.scientific.net/AMR.830.49
http://www.sciencedirect.com/science/article/pii/S1877705811002670
http://egmrs.powweb.com/EJS/PDF/vo242/141.pdf
http://repository.ksu.edu.sa/jspui/bitstream/123456789/7736/3/Mechanical%20deterioration%20of%20polyethylene%20greenhouses%20covering%20under%20arid%20conditions.pdf
http://my.civil.queensu.ca/Research/Environmental/R-Kerry-Rowe/publications/documents/2011%20Islam%20Gross%20and%20Rowe%20GeoFrontiers%202065-2072.pdf
UV irradiaton does produce radical, which may recombine or may lead to chain scission. Chain scission dgerades molecular weight, resulting in a loss of ultimate properties, including elongation at break. Changes in crystallinity may contirbute to higher modulus. You will also have a high degree of photo-oxidation, especially on the surface which can be easily detected by FTIR (transmission or especially ATR-FTIR). Presence of corsslinking (whihc is less probable, only if really high energy irraditon is used) can be detected by gel content tests. (Extractio of sol content by proper solvent and measuring the residual weight - in your case it is complicated by the fact that you will have to use two solvents, perhaps subsequently - you may check the effectiveness of yur approach on the initial samples).
Thanks for your interest and your answer
In PEs normally higher modulus is associated to higher crystallinity. Higher crystallinity PE is definitely "harder" than the more amorphous ones.
How do you view he the UV inducing changes in crystallinity? Could one not associate crosslinking reactions with the formation of a denser polymer network, hence a higher degree of crystallinity evidenced by a decrease in chain mobility?
Gyorgy, thank you for the articles. I had read the North Algerian one and the degradation process appeared more "smooth' as time progressed, the SS curves fell nicely one below the others. My results indicate a drastic increase in yield strength ( 40%) which definitely can be associated with higher crystallinity but the only mode I envisioned was via the formation of a "crosslinked network"
Dear Ricardo
I believe that the increased crystallinity may be due to the degradation of the amorphous phase (photo-oxidative scission). the presence or absence of the crosslinking process should be checked by gel content measurement. If you have access to wide angle X-ray and/or DSC you may check the changes in crystallinity by alternative methods. WAXD may also help to identify changes in crystalline modifications (especially nylon). Crosslinking by itself decreases rather than increases crystallinity. The attached papers are only rough indications what may happen in an icompatible blend with two semicrystalline component. So far I do not search for such papers - probably you did. I just wanted to help with the basic ideas, the full interpretation ofyour data would take a longer time and much mor effort - which does not mean that I am against it, if you wish and if you are ready for joint publication. If it is an industrial problem, probably you will not get the consent to publish it. Anyway, if you are interested in such a cooperation, I would need much more detailed data. Best personal regards Gyorgy
Consider also possibility of improved stress transfer between LLDPE and Ny6 phases as a result of some free radical induced formation of LLDPE-Ny6 graft copolymers in the interfacial region. Such copolymers can act as LLDPE - Ny6 compatibilizers, reducing LLDPE mobility and facilitating Ny6 phase loading.
Michael interesting concept . The PE/Nylon interface is a PE/MAH graft. The reduced mobility definitely could help explain the increase in tensile yield of the whole structure, 30%??, but is there enough energy to do this grafting? Are you thinking decomposition peroxides somehow intervening in the formation of theses links?
Thinking of Chemi-crystallization. Scissioned amorphous chains crystallizing , while Nylon tie molecules being cut and snapping back into the crystal structures, further increasing crystallinity. Thinking the increase in crystallinity can be associated with the increase in tensile yield and modulus as well as with loss of elasticity.
Anyone familiar with Chemi-crystallization appearing in UV degradation of nylon 6 stock?
Thanks
The increase in tensile yield after artificial weathering suggested a crosslinking phenomenon. Tests of creep deformation before and after artificial weathering show a 2X+ reduction in creep deformation.
Conclusions is that scissioned chains formed crosslinks. The increase in "reticulation" responsible for reduced ability of chains to slip past each other hence increased resistance to creep deformation..
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