Enhanced molecular alignment in poly-L-lactic acid nanotubes induced via melt-press template-wetting Michael Smith, Cathrin Lindackers, Kevin McCarthy and Sohini Kar-Narayan* Department of Materials Science & Metallurgy, University of Cambridge, CB3 0FS, UK. ABSTRACT Molecular ordering in polymers can have a drastic effect on their properties, and can be used to induce or enhance functionality. In the case of poly- L-lactic acid (PLLA), which is a widely used polymer in biomedicine, sensors and actuators, preferential orientation of chains can lead to significantly enhanced electromechanical properties. In this context, template-wetting is a straightforward method of producing polymer nanostructures, which can lead to some degree of molecular order in the polymer. Template-wetting of PLLA has not been fully explored, especially in terms of morphological and/or structural characterisation. In this work, PLLA nanotubes are grown via a modification of the template-wetting process, referred to here as meltpress template-wetting. The nanotubes are thoroughly characterised with wide-angle X-ray diffraction, isothermal differential scanning calorimetry and polarised light optical microscopy. This characterisation indicates that the polymer chains in these PLLA nanotubes are aligned parallel to the cylindrical axis of the nanotube, which may be beneficial in certain applications. 1 1. Introduction The properties of polymers depend strongly on the degree of molecular alignment present in them. It is possible to control this molecular alignment at the nanoscale, through finely tuned nano-fabrication processes. Template-wetting is a versatile and facile technique that has been widely used in many separate fields to create nanostructures of different types of materials, including polymers 1–4. The process involves infiltrating a nano-porous template with the desired material and subsequently removing the template to reveal the nanostructures that have formed within the pores. The template-wetting method was first reported by Martin and co-workers 5–7 while synthesizing nanotubes of conducting polymers such as polypyrrole in porous anodised aluminium oxide (AAO) and track-etched polymer membranes. The technique has subsequently been used to produce nanostructures from many different polymeric materials and is the subject of numerous recent review articles 2,4,8. The template-wetting process can have significant implications on the structure and properties of polymers 8–10 . Preferential orientation of the polymer chains has been reported in a number of polymer systems, such as polyvinylidene fluoride (PVDF) and its copolymers 16 , Nylon-11 11–15 , polystyrene 17–19 , and poly(ethylene oxide) 20. In this work, we report how the template-wetting process can influence molecular chain alignment in the polymer poly-L-lactic acid (PLLA), which is a biocompatible polymer that is widely used in bio-medical applications. Molecular alignment can strongly influence both the mechanical and electromechanical properties of this material, and therefore the ability to control and/or enhance polymer chain orientation in PLLA through processing parameters is particularly interesting. In this context, despite the relative popularity of both PLA and template-wetting, the two have seldom been combined. [PLA refers to the general name poly lactic acid. The lactic acid monomer unit is chiral and therefore it is 2 possible to produce optically pure stereoisomers poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA). PLA may refer to either PLLA or PDLA, or a mixture of the two.] Reports of PLA template-wetting are either incomplete references 3,21 or are presented without any material characterisation 22. There is one detailed report of the crystallisation kinetics of PLA contained within the pores of an AAO template 23 , which only briefly discusses the case of polymer chain orientation. Previous work in this group has shown how solution template-wetting of PLLA leads to the formation of nanowire structures which express shear piezoelectric properties 24 , which serves as an indirect observation of the polymer chain alignment. Furthermore, while template-wetting is able to produce both solid nanowires as well as hollow nanotubes 1,11 , the above references only report solid nanowire structures. Nanowire and nanotube polymer structures are both interesting, but one geometry may be preferable over the other for a particula