*2.2. Orientation*

Higher polymer crystallinity and chain orientation can be used to enhance mechanical properties in the direction of orientation [56]. Polymer chain orientation is a way to produce polymers with high strength and modulus. This can be achieved typically through coldand/or hot-drawing of the polymer materials by melt processing above a polymer glass transition temperature (Tg), but below the melting temperature (Tm) [57].

Processing methods, such as extrusion, compression, and injection moulding can be used to achieve chain orientation. It has been found that processing poly(D-lactide) by solid-state extrusion improved the bending strength and bending modulus of the material. The draw rate and temperature of extrusion are the key parameters that determine the mechanical properties of the polymer. Annealing of the polymers prior to solid state extrusion also allowed for relief of stress concentration and chain orientation [57], and the crystallinity of compression-moulded or extruded PLLA samples were increased through annealing, and sterilisation by ethylene oxide gas processing. Increased crystallinity is associated with an increase in the Young modulus; however, it is also accompanied with a decrease in tensile strength and elongation at break [58]. Optimisation of the injection moulding process and use of nucleating agents can result in an increase of crystalline content of commercial grade PLA from 5 to 42%. An added benefit is the concomitant decrease in processing time [59]. Despite the advantages of processing methods on some of the mechanical properties of material, the molecular weight does diminish with heat

treatment of polymers and should be taken into consideration, as this would, in turn, impact the mechanical behaviour of the polymer overall.

Orientation of polymer chains can also be achieved through enhanced entanglement between the long-branched chains. A poly(lactic acid)-branched-poly(lactide-cocaprolactone) (PLA-b-PLCL) was developed through two phase separated structures using long chained branches and demonstrated high tensile strength and modulus (172 MPa and 5.4 GPa respectively) showing potential for use in bone fixation [15].
