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standard. This thickness value was then used together with the area and the mass of the test piece to calculate the material density. Dynamic mechanical thermal analysis DMTA was performed with a TA Instruments Q800 operating in the tensile mode. The oscillation frequency and amplitude were 1 Hz and 10 μ m, respectively, and temperature scans were performed at a rate of 3 °C min − 1 in the temperature range of 20 – 300 °C (or until sample failure). For each degree of modifi- cation, four replicates were tested; using test pieces that had an approximate width of 3 mm, a thickness of 100 – 180 μm and a distance between the clamps of about 8 mm. Tensile and peel testing Tensile and t -peel testing were performed with an Instron 5944, equipped with a 500 N load cell, in a controlled climate of 23 °C and 50% RH. For tensile testing, test pieces, 5 mm wide and 100 – 180 μm thick, were clamped with a free span of 20 mm and strained at a rate of 2 mm min − 1 . The strain was determined by measur- ing the grip displacement; the Young ’ s modulus was calcu- lated as the initial linear slope of the stress – strain curve, and the yield point was determined by the o ff set method, using an o ff set of 0.3%. 12 A total of ten pressed and ten non-pressed test pieces were tested at each degree of modification. Prior to t -peel testing, two 20 mm wide strips of modified cellulose were hot pressed as described above, fusing them in the middle and leaving four free ends (see ESI † for further information). The fused strips were then cut in half to give two T-shaped test pieces with a 20 mm wide and approximately 30 mm long fused area. t -Peel testing was performed using a strain rate of 20 mm min − 1 . A total of four test pieces were evaluated. Electron microscopy A Hitachi S-4800 high-resolution field-emission scanning elec- tron microscope (SEM) was used to acquire micrographs of the pressed and non-pressed papers. In order to suppress speci- men charging during imaging, the specimens were sputtered (208 HR Cressington Sputter Coater) for 20 – 30 s using a plati- num – palladium target. Cross-sections of modified samples were prepared using a microtome (RMC MTXL, Boeckeler Instruments Inc., AZ, USA) equipped with a glass knife. The papers were first clamped between two 0.8 mm polystyrene plates and the assembly was then “ polished ” by cutting 50 nm thin sections for a total thickness of a few micrometres. This procedure could unfortu- nately not be used for the reference papers due to their high porosity and low sti ff ness and a sharp knife was therefore used for cross-sectioning of these samples. To study whether the hot pressing damages the macro- scopic fibres, papers were soaked in water and gently strained until failure. After drying, the failure zone was then imaged as top view micrographs.
Optical properties The optical properties of the papers were studied with a Shi- madzu UV-2550 UV-vis spectrophotometer equipped with an integrating sphere accessory. Each sample was studied at three random positions, and three non-pressed and two pressed samples were evaluated for each degree of modification. Oxygen permability The oxygen permeability was evaluated on 5 cm 2 samples using a MOCON OX-TRAN 2/21 according to the ASTM D3985 standard. The oxygen permeability measurements were per- formed at 23 °C and 50% RH or 80% RH, using the same rela- tive humidity on both sides of the sample. For samples exhibiting measurable barrier properties, four samples were evaluated at each relative humidity.
Results and discussion Molecular and supra-molecular characterisation
Non-beaten and beaten cellulose fibres were oxidised with sodium periodate, which is known to selectively oxidise vicinal diols, for up to 24 h. Fig. 1 shows that the rate of conversion was relatively slow and remained fairly constant throughout the studied time frame, decreasing only slightly as the degree of oxidation increased. This is well in accordance with other studies on the periodate oxidation of cellulose. 6,13 – 15 The results also indicate that the oxidation rate is slightly higher in beaten fibres, which is plausible since beating is known to soften the fibre wall and to increase the swelling and hence the accessibility of cellulose molecules inside the fibre wall. 15,16 It should, however, also be stressed that periodate oxidation is significantly faster and more industry-applicable at high temperature, and that the sodium periodate is not to be considered as a consumable, but would in an industrial process be regenerated. 14,17,18
Fig. 1 Degree of oxidation as a function of reaction time at room temp- erature for beaten and non-beaten fi bres.
3326 | Green Chem. , 2016, 18 , 3324 – 3333
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