Background on Polymer Mechanical Properties

In this thesis, efforts were made to measure the impact of protein nanofibres on PVOH mechanical properties since promising results have been found by introducing other nanostructures in polymer systems (Aoi et al. 2000a, Aoi et al.

2000b, Bhattacharyya et al. 2006, Bhattacharyya et al. 2005, Safadi et al. 2002, Zhang et al. 2003). The mechanical properties of interest generally comprise the polymer strength, elongation, modulus and toughness, which measure how well a material resists deformation. To measure these properties, the composite has to be placed under stress and its response to the stress characterised. This can be done by tensile strength measurement and thermo-mechanical analysis, which are widely employed to measure the mechanical properties of composites (Aoi et al. 2000a, Bhattacharyya et al. 2006, Bhattacharyya et al. 2005, Cadek

et al. 2004, Chang and Kim 2007, Mi et al. 2007, Park et al. 2001, Peng et al.

2007, Ryan et al. 2006, Safadi et al. 2002, Zhang et al. 2003).

Strength is measured by the force that is needed to break a sample, and more specifically tensile strength refers to the force required when the material is under tension (Sionkowska 2005). The tensile strength in this study was measured on an Instron Universal Testing Machine (UTM) (Figure 4.11), where the machine stretches the sample and it measures the amount of force that it is exerting and the stress (force/cross sectional area) that the sample is experiencing.

Figure 4.11 Image of Instron (UTM) testing machine showing a film strip attached for

uniaxial elongation (green arrow).

Attached strip of film

The UTM continues to increase the amount of force, and stress, on the sample until it breaks. The force needed to break the sample is the tensile strength of the material. The elongation is a type of deformation that a material undergoes understress and the toughness is the amount of energy that is needed to break a sample, measured by the area under the stress-strain curves. Figure 4.12 shows a force-extension curve, normally obtained by an UTM, demonstrating 3 types of curves with distinctive material properties. The curve shows that a strong material can be identified by a large force and a minor extension while a strong material may possess a large force and extension. Weak materials are signified by a small increase in force with a large extension. Figure 4.12 also indicates that the dynamic mechanical analysis (DMA) testing (described below and in chapter 5) is done at low force regions.

DMA

Figure 4.12 Nominal force-extension plot showing several materials of contrasting

strength and toughness from (Rosenthal 1999).

The mechanical properties of nanocomposites are dependent on the interface between nanostructures and their matrix (Adam and Alan 2001). This is because the nanostructures have a high surface area. Studies have shown that some types of CNTs provide better nanotube-matrix adhesion than others (Manocha 2006). A good adhesion allows effective stress transfer from the nanostructures to the polymer, for example in PVOH/CNT composites (Cadek

the PVOH + protein fibril films may also depend on the extent of stress transfer between the PVOH and the protein.

In preliminary experiments, the direct impact of the protein (0.2%-0.6%) on the PVOH polymer was evaluated by the tensile strength and strain to failure on an UTM. The stress-strain curve provides information on the strength, elongation and toughness of the films and forms the preliminary discussion in this chapter (sections 4.7.1, 4.9 and 4.10.2) and provides baseline data for the DMA work. More detailed information about the behaviour of the films was obtained by DMA analysis (Figure 4.13). DMA testing is relevant to the initial slope of the stress-strain test profile obtained from the UTM (Figure 4.12) (Staiger, pers. comm.). The slope of the initial curve represents the elastic modulus of the material, which under tension is the Young’s modulus (Nielsen 1974). DMA analysis is most useful to study polymer viscoelasticity, because polymers are partially elastic and partially viscous (Nielsen 1974). They exist in 2 distinct states with different physical properties: at low temperatures they exhibit the glass state, the material is hard and rigid, but at high temperatures a rubbery state is exhibited, which is flexible and extendible. Thus DMA is carried out over a range of temperatures.

Figure 4.13 A schematic of a DMA showing the test arrangement in tension mode,

During the DMA test, an oscillating force is applied continuously to a sample and the resulting displacement of the sample is measured. Sample deformation under oscillating load is monitored against time, temperature or frequency of oscillation and the sample storage modulus (E′) and loss modulus (E″) and loss factor Tan δ are obtained (Figure 4.14). E′ is directly associated with elastic response and is a measure of the stiffness of the material (Yang et al. 2004) and E″ measures the energy dissipated per cycle of vibration. The ratio of E″/E′

is the loss Tangent (Tan δ), which describes the damping properties of a material. A high Tan δ is characteristic of a material with non-elastic behaviour, while a low value of Tan δ is characteristic of a more elastic material. Since Tan

δ measures the ratio of energy dissipated to maximum energy stored, a decrease in Tan δ means more energy is stored in the material (Yang et al.

2004). The maximum value of the Tan δ or E″ modulus is the glass transition temperature Tg (Ehrenstein et al. 2004), which defines the limit between the

glassy and the rubbery state (Roos and Karel 1991).

Figure 4.14 A diagram showing the typical DMA curves for an amorphous polymer,

from (Ehrenstein et al. 2004).

Thus DMA was used to obtain information as to whether protein addition affects the mechanical relaxation processes of the PVOH films. The impact of both insulin and crude crystallin proteins was determined at a level of 0.6%, in a 2.5% PVOH film. Experiments were conducted by increasing the DMA temperature at 2°C/min at a constant deformation frequency of 1 Hz, under stress in tension (stretching) mode. Initial tests were carried out at a wide

temperature range of -150°C to 150°C but were narrowed to 25°C to 150°C since the desired PVOH profiles were obtainable within this range. The DMA data were supplemented by characterisation with differential scanning calorimetry-thermogravimetric analysis (DSC-TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) imaging.