[4things]

Could somebody tell me if polymers with higher degree of polymerisation will burn more difficulty than polymers with lower degree of polymerisation?

I‘m wondering why cotton burns flawless and Viscose not

With best regards

4things

[Corribus]

Difficult question to answer generally because you haven't given too much information. By "more difficult" I assume you are referring to kinetics rather than thermodynamics. Kinetics with which a polymer burns (combusts) can vary quite a bit depending on the density, amount of branching, and etc. But in many cases the macroscopic form of the polymer can play a deciding role - example, for textile polymers, consider you have two spun polymers that are exactly the same chemically, but one has fiber diameter a tenth that of the other. The one with thinner fibers will probably burn (combust) much faster just because there's a higher surface area for interaction with oxygen in the air. Keep in mind that fiber and weave patterns can vary quite a bit among fabrics and can be quite complex on both the microscopic and macroscopic scales. Microscopic structure of the fibers will also probably make a difference for the same reason.

Also, most fabrics are treated in various ways or blended with other substances. Two fabrics may technically be "cellulose" but other substances added during manufacture (coatings, etc.) can influence flammability dramatically. For obvious reasons, most commercial textiles are treated to reduce flammability. Despite the way you've framed your question, viscose and cotton are both highly flammable in principle but in practice may not be equivalently flammable due to manufacturing differences - both physical/structure and chemical processes that produce the final textile material.

I'm no expert in textiles but I'd be thinking in those directions rather than molecular weight differences of the base polymers.

[Orcio_87]

He is not asking about speed but flawless (clearness).

Why does one polymer combusts to CO₂ and H₂O and second one - produces more elemental carbon as a soot (incomplete combustion).

[Corribus]

Question seems very ambiguous to me, but presuming this is what being asked (and we take for granted cotton is always burned more completely than viscose) - it is even a more complicated question to answer. Ultimately it still probably comes down to kinetics, because whether carbon is left or not once the reaction is done depends on competition between combustion (oxidation) and various other decomposition pathways. The kinetics for the various pathways would still largely depend on how much oxygen is near the fuel at any time, the density of the fuel, the amount of surface area available to interact with oxygen, how the energy is applied, the type of flow/turbulence in the vicinity of the fuel, and etc. Soot production is still poorly understood and difficult to model even for simple liquid fuels.

Were I to hazard a completely unsubstantiated guess, it would go something like this: if cotton has more surface area because it is less dense/thinner fibers/ etc, this could basically be viewed as a better mixed fuel/air system. Better mixing should favor more complete combustion because there will be less fuel present with no immediate access to oxygen. 

[rjb]

In my view Corribus has provided 2 excellent responses both worthy of mole snacks!

I don't have a lot of substance to add, but I can say that in this case an increased degree of polymerisation is not responsible for a decrease in flammability. Depending on which source you choose to accept, Cotton has a typical degree of polymerisation somewhere between 3000 and about 15,000. By contrast Viscose has an average DP of about 250-450, so the primary influence on flammability is unlikely to be DP. Another key structural difference in these cellulose based polymers is average degree of crystallinity. Cotton (at least un-mercerised cotton - primarily Cellulose I) has a much higher average crystallinity which lies somewhere between about 60 and 70%. Viscose (based on the Cellulose II allomorph) has a much lower average crystallinity which probably lies somewhere between about 40 and 60% (depending on the results/methods of analysis you choose to accept). Could this be responsible for the difference in flammability? Probably not.

If we are discussing 'naked' fibres, then personally I would back Corribus' view that the differences flammability are down to physical rather than chemical differences. Cotton is on average a thinner fibre (0.7-2.5 D) than Viscose (1.5-15 D) and I would imagine that its c shaped cross section would expose much more of its surface area, leading to more rapid burn. Viscose tends to have a crenelated cross section which although of greater surface area than say a plain round fibre, is still probably of lower surface area than a typical cotton fibre.

If we are discussing fabric rather than individual fibres, then a lot of this will be down to weave and fabric weight. Your nice new Levis are made of cotton fibres, but they are heavyweight and twill woven (a very closed weave), leading to low flammability. By contrast your nice new cotton T shirt consists of about 1/2-1/3 the number of fibres per unit area than your jeans and is probably plain weave or brushed cotton with a much more open structure meaning that its a lot more flammable.

Anyhow... I shall shut up now!

R

[Corribus]

@rjb

Thanks for putting in some numbers for context. I looked briefly for some but didn't find anything.