Flow induced crystallisation in polymers: from molecules to processing

Lead Research Organisation: University of Nottingham
Department Name: Sch of Mathematical Sciences

Abstract

Polymer processing is a multi-billion pound, world-wide industry, manufacturing products used by virtually every person in the developed world (and beyond) on a daily basis. This vital sector of the UK economy will gain a significant competitive advantage from a molecular understanding of how polymers crystallise during processing, as it will enable stronger, lighter, more durable and more easily recycled plastic products. In this proposal we will overcome the key experimental, simulation and numerical issues in understanding polymer crystallisation to deliver a molecular based, predictive platform for the processing of semi-crystalline polymers. We will tightly integrate a family of progressively coarse-grained simulations and models, covering all relevant lengthscales within a single project. This will displace the current sub-optimal semi-empirical approaches in polymer processing and enable molecular design of polymer products, through choice of processing conditions. By facilitating the manufacture of polymer products with tailored properties this program will provide a critical competitive advantage to this important industry.

Polymers are long-chain molecules, formed from connecting together a large number of simple molecules. These long-chain molecules are at the heart of the multi-billion pound plastics industry. Semi-crystalline polymers make up a very significant fraction of the worlds production of synthetic polymers. Unlike simple molecules, the connectivity of polymer molecules means they crystallise into a composite structure of crystalline and amorphous regions. The proportion of amorphous and crystalline material, along with the arrangement and orientation of the crystals, is collectively known as the morphology. The crystal morphology strongly influences strength, toughness, permeability, surface texture, transparency, capacity to be recycled and almost any other property of practical interest. Furthermore, polymer crystallisation is radically influenced by the flows that are ubiquitous in polymer processing. Flow drastically enhances the rate at which polymers crystallise and has a profound effect on their morphology. Flow distorts the configuration of polymer chains and this distortion breaks down the kinetic barriers to crystallisation and directs the resulting morphology.

Understanding polymer crystallisation is a formidable problem. The huge range of relevant lengthscales ranges from the size of a monomer (nm) up to near macroscopic crystals (micro-metres). The range of timescales is even wider, ranging from the monomer relaxation time (ns) to nucleation (hours at low under-cooling). Our project will involve extensive multiscale modelling, supported at each level by experiments specifically designed to address key modelling issues. Our experiments will involve controlled flow geometries, the systematic variation of molecular weight and the probes of both nucleation and overall crystallisation. Close integration of experiments and all levels of modelling is a key feature.

We will develop an interrelated hierarchical family of multiscale models, spanning all relevant lengthscales and delivering results where piecewise approaches have been ineffective. Each technique will be tightly integrated with its neighbours, retaining the molecular basis of the models while progressively addressing increasingly challenging systems. This will cumulate with the low-undercooling and high-molecular weights that are characteristic of polymer processing. Each simulation will use a rare event algorithm to dramatically increase the nucleation rate, the cause of the very long timescales. Insight from the most detailed models will guide the development of faster modelling. At the highest coarse-graining, the program will derive models suitable for computational modelling of polymer processing. Using these models in cutting-edge finite element code, we will compute FIC behaviour in polymer processing geometries.

Planned Impact

Flow-induced crystallization in polymers is a fascinating example of an externally driven, nonequilibrium phase transition, controlled by kinetics. Furthermore, FIC is ubiquitous in industrial processing of semicrystalline polymers, the largest group of commercially useful polymers. Hence the research described herein will be impact significantly on both fundamental and industrial polymer science and engineering. Furthermore, the experimental, simulation and modelling techniques developed in the project will impact on the many fields where molecular motion determines macroscale properties of practical importance.

Academic
The most important academic impacts from this project will arise from the novel results from experiments, simulation and modelling of polymer FIC. Furthermore the tools for rare-event simulation, coarse-graining and multiscale modelling developed herein will impact strongly across the numerous fields that depend on molecular modelling. Full details of academic impacts are given in the Academic Beneficiaries section.

Industrial and technological
Industrial polymer processors will benefit significantly from the control and customisation of polymer processing and products that our modelling will deliver. In particular:
- Practical computational tools from the project will accurately predict crystal morphology from processing conditions, this will enable process engineers to design and optimise the properties of polymer products by tailoring processing speed, temperature, molecular weight and geometry.
- The ability to control crystal morphology will allow process engineers to manufacture products that are lighter, stronger, more transparent, less permeable and easier to recycle using processes that are faster, more efficient and less prone to instabilities, all of which will increase profitability.
- The ability to optimise solid-state properties will make polymer products suitable for a greater range of product lines, opening new markets and revenue streams to the polymer industry.
- Software developers for polymer processing will benefit from the predictive ability of our models, combined with our techniques in computational fluid dynamics. The resulting improved speed, accuracy and predictive ability will lead to wider uptake and exploitation of their polymer process modelling software.

Societal and general public
Polymer processing is a multi-billion pound, world-wide industry, manufacturing products used by virtually every person in the developed world (and beyond) on a daily basis. This vital sector of the UK economy will gain a significant competitive advantage from a molecular understanding of how polymers crystallise during processing, as it will enable stronger, lighter, more durable and more easily recycled plastic products. There will be environmental benefit from more efficient packaging and greater recycling.

Wider academic impacts
Beyond the project's central aim there are numerous impacts from the cutting-edge molecular simulation and modelling techniques developed herein. Molecular simulation and modelling techniques can predict a wide-range of physical properties, relevant to numerous academic and technological fields. Examples include viscosity, heat capacity, latent heat, solubility and diffusion rates. Our novel simulation tools provide a flexible framework for highly efficient molecular simulation of phase transitions and barrier crossing. This will facilitate rapid characterisation of many molecular systems, with the ability to efficiently explore a wide range of temperature being an especially attractive feature. Examples of fields this will impact on include polymers, colloids, protein folding, drug delivery and molecular self-assembly.
 
Description We have extended a detailed method to simulate polymer crystallisation to longer polymer chains, moving this technique into the range that is relevant for industrial polymer processing. From this technique we have extracted the nucleation rate under flow for long polymer chains.

We have developed and validated a constitutive equation that describes the stress response of polydisperse linear polymers under strong flow. The resulting predictions of the stress allow us to predict how polymer will flow during polymer processing. As the model works for polydisperse polymers, it can connect directly to the types of polymers used in commercial polymer processing. In particular, it can predict how changing the spread of lengths of polymer chains in a melted plastic will alter, and potential enhance, polymer processing.

We have developed and validated a model of how flow (during polymer processing) increases the rate of crystallisation in polymer. In particular, the model predicts how different chain lengths deform differently and work co-operatively to dynamically create a stable crystal nucleus. This model will enable polymer processors to design the final crystal properties of plastic products by tailoring the processing conditions and spread on chain lengths.
Exploitation Route Our results can be used to inform models of polymer crystallisation that can be used to model polymer processing and 3D printing. We have made our new models available via a software platform that is widely used by industrial polymer scientists/engineers (see software section). We also held an industrial-academic workshop to showcase and train users on our new software in January 2020 (see engagement section).
Sectors Aerospace, Defence and Marine,Chemicals,Manufacturing, including Industrial Biotechology

 
Description Our work has been translated into software that is used by key-industrial stakeholders. This will allow them access to results and knowledge from these models. Full details of the economic impact of this translation industry is still emerging.
First Year Of Impact 2020
Sector Chemicals,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Description Flexible vs stiff polymers for 3D printing: Understanding crystallisation for enhanced properties.
Amount £5,935 (GBP)
Funding ID IES\R3\183003 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2019 
End 03/2020
 
Description Semi-crystalline Materials in Additive Manufacturing: fellowship for Claire McIlroy
Amount £150,000 (GBP)
Organisation Royal Commission for the Exhibition of 1851 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2017 
End 09/2020
 
Description Collaboration with Julie Kornfield at CalTech 
Organisation California Institute of Technology
Country United States 
Sector Academic/University 
PI Contribution An exchange of experimental and theoretical expertise between our project and the Kornfield group at CalTech.
Collaborator Contribution An exchange of experimental and theoretical expertise between our project and the Kornfield group at CalTech.
Impact Exchange of knowledge and a paper is being planned.
Start Year 2016
 
Title New module in the rheological software RepTate to plot and model flow-induced crystallisation experiments 
Description New software to give end-users rapid and straightforward access to the latest models of flow-induced crystallisation in polymers. 
Type Of Technology Software 
Year Produced 2020 
Open Source License? Yes  
Impact The software was introduced to key industrial users in a workshop in January 2020 
URL http://reptate.readthedocs.io
 
Description Academic-industrial software/experimental workshop on flow-induced crystallisation in polymers 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Flow-induced crystallisation in polymers is a central problem in both polymer physics and industrial polymer processing. The field has seen rapid recent progress in experiments, molecular simulations and modelling.

Our Industrial-Academic workshop on flow-induced crystallisation in polymers, which involved presentations, software and demonstration lab experiments. The workshop took place between Monday 20th January and Wednesday 22nd January and was jointly hosted by the Universities of Leeds and Bradford.

The contributions from Nottingham, Leeds and Bradford were be based around our EPSRC-funded project, Flow induced crystallisation in polymers: from molecules to processing. Participants tried out our new crystallisation module in the rheological modelling package RepTate, along with and a hands-on experimental session at the labs in Bradford.
Year(s) Of Engagement Activity 2020
URL https://www.nottingham.ac.uk/mathematics/events/workshops/workshop-on-flow-induced-crystallisation-i...