2.6. Physical and thermal properties of the PLA fibres
Scanning electron microscopy (SEM) was used to assess the morphology and diameters of the fibres (analysed using Image-J).The glass transition temperature, crystallisation and melting behaviour of neat PLA and composite fibres were determined using differential scanning calorimetry (DSC Q2000, TA Instruments, UK) in a He atmosphere. DSC was used to assess the crystallinity of PLLA, both first and second heating curves were analysed to assess
the influence of drawing ratio on crystallinity and the capacity for nanocellulose to act as nucleation sites for PLA crystallisation. A heating-cooling-heating regime was used as follows, from room temperature (RT) to 200 C at 10C min1, held at 200 C for 1 min, cooled to RT at 50 C min1, held for 1 min then re-heating as aforementioned. The crystallinity of the nanocomposites was calculated as described in the literature [19,28], using a melting enthalpy of 100% crystalline PLLA of 93.7 J g1, and corrected for the amount of PLLA present. The only difference being that in order to determine the crystallinity for the fibres for the first heating curve induced by melt-spinning, the entropy of cold crystallisation was subtracted from the melting enthalpy. The crystallinity for the second heating curve was calculated using the method in [28]. Single fibre tensile tests were conducted to determine the mechanical properties of the fibres. Fibres were carefully mounted on card sample holders and bonded between further sections of card using epoxy resin (Araldite Rapide), to give a gauge length of 20 mm. As aforementioned, a section 10 mm in length was taken from each fibre in the post bonded region away from the test specimen for SEM analysis and diameter determination. Tensile tests were conducted at 21 C with a crosshead speed of 15 lm s1, according to BS ISO 11566:1996 using a TST 350 tensile testing rig (Linkam Scientific Instrument Ltd.) equipped with a 20 N load cell. The
gauge length was 20 mm. The displacement and load were recorded and converted to stress and strain, based on the actual fibre diameters determined via SEM/image analysis prior to testing. The modulus, ultimate tensile strength (UTS), strain at UTS, break tenacity, and strain at break were then calculated.