View Article Online
Green Chemistry
Paper
Directly after the periodate oxidation, the fibres partly con- taining dialdehyde cellulose were reduced with borohydride (complete reduction was concluded by a non-measurable reac- tion with hydroxylamine and further supported by the absence of a carbonyl peak in the FTIR spectra of the final papers; the spectra can be found in the ESI † ), which in the same way as the sodium periodate would be regenerated in an industrial process, 19 to dialcohol cellulose. According to our earlier findings, 6,7 this gives fibres in which the CNFs consti- tuting the fibre wall have been modified into a core – shell structure, i.e. as periodate works its way into the CNF, its surface is transformed into a shell of highly derivatised and amorphous cellulose surrounding the inert and highly ordered CNF core. Papers were rapidly made from the modi- fied fibres by a conventional laboratory technique and ana- lysed by XRD. Fig. 2 shows how the X-ray di ff raction and crystallite width decreased with increasing degree of oxi- dation, in line with the core – shell hypothesis. Fig. 2a also shows that the ordered structure was una ff ected by further hot pressing.
Thermoplasticity, pressability and structure Cellulose is an example of a technically non-thermoplastic polymer, i.e. it decomposes before it readily softens. However, there are cellulose derivatives that are thermoplastic, typically various cellulose esters, 1 but also dialcohol cellulose. 8,9,20 Therefore it is interesting to study how cellulose fibres partially transformed into dialcohol cellulose are a ff ected by tempera- ture. Fig. 3 shows DMTA of the di ff erent samples and clearly shows that the untreated material is more or less una ff ectedby temperature while the thermoplasticity increases with increas- ing degree of modification. For samples with a degree of oxi- dation of 24% or greater, Fig. 3 shows two rather sudden changes in storage modulus, one at 70 – 120 °C and one at 160 – 180 °C, where the first change can presumably be linked to the glass transition of dialcohol cellulose and the latter to its flow region. 9 Furthermore, Fig. 3 shows that hot pressing at 150 °C prior to the DMTA did not a ff ect the thermoplastic behaviour of the material, i.e. such a pre-treatment did not induce any significant permanent chemical or structural
Fig. 2 XRD data of oxidised-reduced fi bres; (a) di ff ractograms, (b) crys- tallinity index and crystallite width as functions of the degree of oxidation.
Fig. 3 DMTA of papers and fi lms made of oxidised-reduced fi bres; (a) storage modulus, (b) tan δ . Each curve is the average of four measurements.
Green Chem. , 2016, 18 , 3324 – 3333 | 3327
This journal is © The Royal Society of Chemistry 2016
Made with FlippingBook Ebook Creator