The properties of synthetic polymers are critically dependent on both the chemical configuration and on the structure and morphology imposed during the processing stage. The dependence on the processing step is most pronounced in polymers which are usually crystalline at room temperature. Thermoplastics such as polyethylene and polypropylene, biomedically important polymers such as polycaprolactone and sustainable polymers such as polylactide, are semi-crystalline. The pioneering experiments of Keller and others showed that the crystals which form contain chains which are folded repeatedly to form a thin platelet or lamellar crystal with the chains lying normal to the large surface of the crystal as in Fig 1. The thickness of the crystal is ~10nm and the crystalline phase will typically form 50-80% of the material, the remainder is the amorphous phase with a structure broadly equivalent to the polymer melt. Much understanding has developed over the intervening years but the process by which a randomly coiled polymer chain in the melt reorganises to form a chain folded lamellar remain a subject of much debate and conjecture. For a simple liquid such as aluminium, it is straightforward to envisage a process by which individual atoms attach to a growth face.
For a long chain molecule, in contrast, crystallisation involves conformational rearrangements as well as the development of precise atom positioning. It could be as some have argued, that crystallisation is preceded by conformational changes, perhaps coupled to local density changes to yield an ordered but non-crystalline structure which subsequently transforms in to crystals. This is the basis of the model of crystallisation process proposed by Strobl. Experimental techniques which have been used to study crystallisation in polymers include in-situ time resolving small-angle and wide-angle x-ray scattering and morphology studies using electron and atomic force microscopy. There have been attempts to exploit computational modelling techniques, although in general the size of model required to accommodate realistic lamellar crystals means that an atomistic view is impracticable. It is fair to say that the mechanisms of crystallisation remain unclear. As a consequence, routes to improving the properties of polymers through the use of different processing conditions or nanoscale additives remains largely empirical. This programme seeks to take a fresh and innovative look at understanding crystallisation in synthetic polymers from a molecular viewpoint in order to allow the optimisation of both the design of the chemical configuration and the processing conditions and nano-additives exploited. Our strong confidence that we can make progress is due to the availability of new experimental procedures. The international pulsed neutron facility ISIS (UK) has been upgraded with a second target station and instrumentation especially designed for soft matter studies such as polymers. This facility houses a remarkable and unique instrument NIMROD which provides high quality neutron scattering data from |Q| from 0.01 to 100Å-1. This covers in a single quantitative data set, the three critical length scales for polymer crystallisation, namely a conformational change to produce segments with a regular conformation (~0.1nm) which form translationally ordered crystals (~1nm) which contains chain folded surfaces (~10nm). All three length scales are accessed in the NIMROD data. Moreover, we have already demonstrated that NIMROD is able to follow all three length scales in a time-resolving manner in a crystallising sample. Of course we need to consider how to exploit this hitherto unavailable scattering data. We have previously shown that precise quantitative structural information can be obtained from the broad Q (0.2-50Å-1) neutron scattering data from disordered polymers through tight coupling of the scattering data to computational modelling techniques. Such procedures provide quantitative information on the local conformation in terms of the distribution of conformers.
We will use fully atomistic models of the polymer chains to follow the conformational changes which accompany crystallisation using Monte Carlo sampling procedures. We will coarse grain the polymer chains to yield models of the lamellar crystals and the units cells to reveal whether these three structural scales evolve in a simultaneous or sequential manner. As all of the data is in a single quantitative function there is no ambiguity of intensity scale or time. We now wish to translate these preliminary studies in to a major project in order that we can radically improve our understanding of polymer crystallisation which can be used to design optimised polymer resins coupled with enhanced polymer processing and nano-additives to yield defined and improved polymer properties.