SOLVENTS FOR COTTON BASIC INFORMATION AND TUTORIALS

Cotton Solvents
What can be used as solvents for cotton?

Cellulose is s oluble only in unusual and com plex s olvent systems. Solvents for cellulose are c entral to the rayon and cellulose film industries, but are also necessary for s olubilizi ng cotton for the determination of molecular weight and degree of polymerization ( DP) by chromatographic m ethods.

These solvents fall into several categories. The solvents discussed do not include processes w here cellulose is converted t o a derivative that i s subsequently dissolved in a nother m edium. For e xample, cellulose acetate i s solubl e in acetone, but this is not a solut ion of cellulose. H ow ever, t he viscose process that f orm s a cell ulose xanthate deri v at ive, f rom which c el lul ose is readil y regenerated, is gene rally c onsidered to use a cellulose solution because s olvation and derivatization oc cur simultaneously.

The viscose process is the most important method for m aking cellul ose solutions for industrial use. Alkali cellulose ( pulp s wollen in NaOH) i s pressed a nd aged to reduce m olecula r weight. Xanthation (a reaction with CS2 ) takes place in a vessel t hat c ontains an inert atmosphere (CS2 –air mixtures ar e explosive).

The orange xanthate i s s ubsequently dissolved in aque ous alkali to make the spinni ng dope. The dope is pumped t hrough spinnerets in which t here are f rom 14 t o 40,000 holes. The spun dope is converted back to c ellulose by the sulfuric acid i n t he coagul ating bath. Another s ystem with simultaneous derivitization and dissolution uses dimethyl sulfoxide and f ormaldehyde [ 133].

Several solvent s, such as cuprieth ylenedia mine (C UEN) hy droxide, de pend on the formation of metal–i on c omplex es with cellulose. Wh ile not as wi despread in use as the viscos e process , CUEN and its relative s with diff erent metals and ammoni um hyd roxide find substantial indust rial use [131]. The cad mium comp lex, cadoxen , is now the so lvent of choice in laborat ory work [134].

Aqueous salt solut ions such as satur ated zinc chloride or calci um thiocy anate can dissol ve limit ed amounts of cellulose [131] . Two nona queo us salt solut ions wi th a lengthy hist ory are ammoni um thiocy ana te=ammon ia and dimethylac etam ide =lithium chlori de (DMAc =LiCl). Soluti ons up to abou t 15% can be prepared with these solvent s. DMA c–LiC l has be en used for mo lecular weight determinat ions of cotton.

Trifluoroacet ic acid–m ethyl ene chlorid e and N -methyl mo rpholin e N -oxide mon ohydrate (NMMO) [136–138] are two other solvent systems that have been studied [139]. The new generic class of regenerated cellulose fibers, lyocell (e.g., Tencel [Courtaulds Fibres Limited, London, England]), is spun from aqueous solutions of NMMO [140].

Lyocell is an alternative to the generic name ‘‘rayon’’ for a subcategory of rayon fibers where the fiber is composed of cellulose precipitated from an organic solution in which no substitution of the hydroxyl group takes place and no chemical intermediates are formed.

Lyocell may have a different crystalline structure (a mixture of cellulose II and cellulose III [141]) than other rayons and cotton cellulose. No information has been published on cotton molecular weight determinations using NMMO as the solvent.

STEM CELLS INFORMATION Q&A

What’s left to learn about stem cells?
We know that stem cells are present at all stages of our life. Stem cells found in early embryos have the potential to become different types of cell, while adult stem cells are more specific. The questions we are trying to answer are: can we identify all stem cells? Can we grow them in large numbers in the lab? Can we make them give rise to any cells we wish? Can we use stem cells to treat cancer, ageing and degenerative diseases?

Does every multicellular organism have stem cells?
Yes. In mammals, there are two main types of stem cells: embryonic, which are generated from early embryos, and adult, which are found in various tissues and contribute to the repair and replenishment of our tissues.

For a long time it was thought that once the stem cells changed to form the various cells that make up our organs, it was impossible to make them revert back to the initial stem cell state.

However, the Nobel prize winner Shinya Yamanaka reported in 2006 that adult cells can be turned back to the embryonic stage by simple genetic manipulation.

Who first discovered stem cells?
The concept of stem cells was fi rst mentioned by Valentin Haecker and Theodor Boveri in the 19th century. In parallel, Artur Pappenheim, Alexander Maksimov, Ernst Neumann and others used it to describe a proposed origin of the blood system.

As the field progressed, the term ‘stem cell’ has been used to describe the capacity of stem cells for self-renewal as well as the ability to give rise to all cell types that make up our bodies.

Do stem cells have to be prompted in some way to repair the body?
Adult stem cells need prompting if a quick repair is needed, and we can achieve this in the lab. Stem cell prompting in the body is a bit more tricky, but can occur in response to specific stress or injuries.