In order to obtain PVA/cellulose fibers composite films with different filler concentrations, the required amount of PVA corresponding to a final concentration of 10% in the aqueous medium and the calculated amount of L, U and H cellulose fibers suspensions to achieve 1, 3 and 5 wt% filler concentration in the final composite were mixed using a high speed stirrer (500 min-1). The stirring was performed at 800C for 3 hours, and the resulting mixture was degassed for approximately 15 minutes in an Elmasonic S40 H ultrasonic bath. The films were cast on a PET plate and were kept at room temperature for 2 days until they were completely dried and then removed from the PET plates and placed in a desiccator for two weeks before mechanical characterization. Neat PVA films were obtained under similar conditions.
1.3 Characterization methods
SEM micrographs of cellulose fibers were obtained with an environmental Quanta Scanning Electron Microscope 200, with a tungsten electron source in the following conditions: accelerating voltage 10 – 30 kV, no coating. The crystalline structure of cellulose fibers was determined by XRD with a DRON-UM diffractometer (horizontal goniometer Bragg – Brentano) using Co Ka radiation (wavelength A = 1.79021 A), scanning from the 20 value of 50 to 360 at a scanning rate of 0.050/5 sec. Samples were analyzed in reflection mode. Mechanical behavior of composite films at room temperature was tested in tensile mode according to ISO 527-1:1993 Part 1 and ISO 527-3:1995 Part 3 by an Instron 3382. The Universal Testing Machine was equipped with a video extensometer for strain measurement and with a load cell of 1 kN capacity. The following data: video axial strain at break (elongation at break), tensile stress at break (tensile strength), Young’s modulus (tensile modulus) and other optional features were automatically display using the software of Instron 3382, Bluehill 2. Young’s modulus on the strain channel with a start value of 0.05% and an end value of 0.25% was calculated by the system applying the least square fit algorithm to each region between the start and end value selecting the highest slope. The load cell of 1 kN capacity, used for PVA composites tensile properties measurements has a sensitivity of 0.001 N. Test specimens were cut to 110 mm (length) x 10 mm (width) x 0.0350.045 mm (thickness) from the films maintained in desiccators, 7 specimens being tested for each sample. Tensile modulus was determined at a crosshead speed of 2 mm/ min and tensile strength and elongation at 10 mm/ min. Lower crosshead speed was necessary for correct determination of tensile modulus as stipulated by ISO 527 Part 1. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed on a SDT Q600 V20.9 from TA Instruments under Heliom flow (100 mL/min). The samples weighing between 5 and 10 mg were packed in aluminium pans and placed in the DSC cell. The samples were tested from the ambient temperature to 6000C at a heating rate of 100C/min. The DSC glass transition temperature (Tg) was taken at the onset of the glass transition region. The PVA composite samples were conditioned for 24 h at 40 0C in a vacuum oven prior to thermal analysis.