Characterise and control

Characterising the structure-function relationship of polymers is vital for improving production of these integral compounds – no easy task. Could a new approach to gel permeation chromatography allow better macromolecule design?

Even a very quick scan of the activities listed on the websites of universities and industrial research organisations around the world reveals the enormous breadth and complexity of the work taking place to develop novel polymers, and to extend the applications of those with which we are already familiar. While most everyday consumers of polymer-containing products probably have no interest in what enables a used plastic wrapper to degrade, how their shoe soles withstand repeated wear or what function is served by the coating on their medication, polymers of all types are integral to modern life.

Polymers may be said to offer solutions to many of our technological challenges, from food and transportation to construction, communication and medicine, and the term ‘polymer age’ is used by some to describe modern industrial times. Consequently, there is a commercial drive and academic funding, as evidenced for example by both the UK’s Engineering and Physical Sciences Research Council (EPSRC)¹ and the National Science Foundation (NSF) in the USA², to support polymer research across various scientific disciplines. With this comes an imperative for research tools that support and accelerate progress, and an onus on developers and manufacturers to deliver systems that enhance the research effort.

When developing new synthetic polymers, or modifying natural ones, being able to fully characterise materials allows a more complete understanding of the relationship between a polymer’s structure and its function. Correlating molecular characteristics with structure and properties provides a route to improved formulation and appropriate application.

While a wide range of analytical techniques are used in characterising polymers at various stages of development and production, gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), is considered fundamental. This long-established technique has generally been configured with a single detector to provide relative molecular weight distribution data for the development and control of polymer performance. However, the latest developments in fully integrated multi-detector systems, customisable to specific needs, offer the prospect of tailored, information-rich analysis from every sample and a means to accelerate polymer research. A brief exploration of single- versus multi-detector analysis illustrates the ramping power of such a detector array when applied in the development of new polymers and advanced polymerisation techniques. GPC/SEC is a two-stage analytical process in which dissolved macromolecules in a sample are first separated according to their molecular size (as opposed to molecular weight) and then passed through one or more detectors to measure physical properties. The ability to use a wide range of solvents makes this technique especially well-suited for different types of polymer.

It remains common practise to use just a single detector for post-separation analysis. Normally this is either a Refractive Index (RI) or a UV detector, which provides concentration measurements that directly reveal how much of each size fraction is present and allow the generation of a size distribution for the sample. However, since molecular weight (MW) and molecular weight distribution are defining parameters in polymer performance, these too must also be obtained. With the single-detector approach the only way to do this is by converting the size data into MW data, through referencing a calibration with an appropriate series of molecular weight standards. One of the major drawbacks when researching novel polymers is the unlikelihood of there being a comparable standard and the inaccuracy of results if the MW/size ratio of any standard used and the sample are not equivalent over the full MW range of interest. So not only does this fail to be a direct measurement, it is reliant on potentially error-prone calculations and limited by the availability of suitable standards.

By replacing a single detector with a multi-detector array, the information gained from each analysis increases immediately

Although single detector GPC/SEC remains useful in specific areas of polymer analysis, it has clear limitations in many of the field’s specialities, and especially in a research environment, where reliably obtaining maximum information from every single measurement helps accelerate development programs. Here, multi-detector GPC/SEC can play a transforming role. By replacing a single detector with a multi-detector array, the information gained from each analysis increases immediately. The detectors used can be tailored to meet the specific needs of the analysis, but typical arrays for polymer research might include: an RI detector (as in a single detector set-up) for concentration measurement; a UV/Vis detector for chromophore concentration; a light scattering detector to measure absolute molecular weight; and a viscometer for intrinsic viscosity (IV) measurement. Intrinsic viscosity in combination with MW reveals essential information about a polymer’s molecular structure, density and branching. A look at the synthesis of well-defined polymers gives an indication of how moving from single to multi-detector GPC/SEC translates into tangible research gains.

A variety of polymerisation techniques, many of which continue to evolve, are used to synthesise well-defined polymers

Extensive studies of the relationship between structure and properties have increasingly focused research towards the development of novel polymers whose molecular weight, dispersity and structure are well-defined. Here, for example, understanding the behaviour of a ‘perfect’ well-defined polymer in relation to its specific properties supports the development of behavioural models, which may then be used to get a better handle on ‘less than perfect’ industrial polymers. A variety of polymerisation techniques, many of which continue to evolve, are used to synthesise well-defined polymers. These include controlled free radical polymerisations, such as Atom Transfer Radical Polymerisation (ATRP), Reversible Addition-Fragmentation chain Transfer (RAFT) and Nitroxide Mediated Polymerisation (NMP), as well as living anionic or cationic polymerisations.

Living anionic polymerisation provides just one example of how GPC/SEC really supports polymer development. This technique can be used to synthesise polymers that are well-defined in terms of molecular weight, molecular weight distribution, microstructure and end-chain functionality. However, the choice of monomer, solvent, initiator and reaction conditions is critically important, and any impurities in the system can result in side products – that is, polymers with unwanted characteristics. Here, multi-detector GPC/SEC allows rapid discernment of whether the desired molecular weight and dispersity have been achieved, or if side reactions have produced unwelcome polymer chains. Accessing such comprehensive information in a single analysis allows efficient feedback for faster, more rational process refinements.

The concept of ‘well-defined’ also extends to branched polymers, where main chains are connected to side chains at branch points in the molecular structure. The physical properties of branched polymers may differ markedly from linear analogues of equivalent molecular weight. Understanding structure is essential in optimising both the polymerisation itself and performance of the product. Structural information can be obtained using multi-detector GPC/SEC analysis, where intrinsic viscosity data are collected alongside concentration, molecular weight and molecular weight distribution measurements, a combination not possible with conventional single detector set-ups.

[caption id="attachment_59106" align="alignnone" width="620"] To optimise polymerisation, the structure of the polymer must first be wholly understood.[/caption]

The data collected can be used to produce a Mark-Houwink plot. This powerful tool for detecting and quantifying structural differences between polymers presents MW as a function of IV. Intrinsic viscosity is indicative of the molecular density of a polymer, higher densities being associated with lower IV values. Those polymers with higher densities, resulting from their compact structure or conformation, exhibit a lower IV than less dense polymers of equivalent MW. Figure 1 shows how readily a Mark-Houwink plot points up clear differences in the structure of three different polyethylene glycol (PEG) polymers analysed. A single detector set-up will not provide all the data necessary to construct this plot, and in practice could incorrectly estimate the MW of the three different PEGs because they exhibit different MW-size ratios (i.e. different structures). A demonstration then, in comparison with single detector analysis, that a multi-detector array delivers information that can be more confidently relied upon to determine the effect of polymer branching on performance. The same approach helps in understanding if a synthesis has resulted in a linear polymer, a branched polymer, or a mixture of both.

[caption id="attachment_59095" align="alignnone" width="928"] Figure 1: Overlay of a Mark-Houwink plot of linear PEG (red line) and 4- and 8-arm PEG star polymers (orange and blue line respectively)[/caption]

A number of different coupling reactions can be applied to linear polymers to create branching

Understanding the structure-function relationships of well-defined polymers not only allows better macromolecule design, it also underpins the manufacturing of the commercial product, a process likely to involve primary polymerisation and subsequent modification to introduce functional groups, for example, or create branching. Where functionalisation is concerned, multi-detector GPC/SEC is used to monitor the well-defined polymer, which must remain monomodal, and can show whether or not the polymer is stable during the functionalisation reaction. A number of different coupling reactions can be applied to linear polymers to create branching. These change the chromatogram of the linear precursor and result in both structural and molecular weight changes. Mostly, the final product is a mixture of linear unreacted polymer and the desired branched polymers, and multi-detector GPC/SEC can be used to monitor the progress of reactions and to identify completion.

Figure 2 shows the results of monitoring a coupling reaction of a linear polymer to produce one that is hyperbranched. Samples taken at intervals were analysed using multi-detector GPC/SEC, to give a series of chromatograms. The main peak at high retention volume shows the linear precursor of the hyperbranched polymer. Peaks that increase with each subsequent sampling are the dimer, trimer, and so on as the reaction moves towards higher molecular weight material. For this reaction, both the light scattering (RALS) and RI detectors prove effective in monitoring its evolution. Reaction success is confirmed by observing the development of high MW species. When MW ceases to increase, the reaction can be considered complete.

[caption id="attachment_59096" align="alignnone" width="1102"] Figure 2: GPC/SEC chromatograms of sample taken at different times during the synthesis of a hyperbranched polymer resulting from the coupling reaction of a linear polymer (Reprinted with permission from³).[/caption]

Successful research leading to faster outcomes and early commercial benefits are perhaps some of the drivers in materials research today. Polymer science may hold the key to unlocking the solutions for many current challenges in technology and analytical tools that help speed research and development by extracting maximum information from every measurement have a key part to play. GPC/SEC has a proven track record and multi-detector systems, that can be customised for specific applications and help accelerate research programs are becoming more popular and it’s easy to see why.

Author: Dr Serena Agostini is a Product Technical Specialist (GPC/SEC) at Malvern PANalytical

References 1. EPSRC, Engineering and Physical Sciences Research Council https://www.epsrc.ac.uk/research/ourportfolio/researchareas/polymer/ Accessed 23 January 2017 2. NSF, National Science Foundation https://www.nsf.gov/funding/pgm_summ.jsp?pims_id=5357 Accessed 23 January 2017 3. Hutchings L. R., Agostini S., Hamley I. W., Hermida-Merino D. Macroml., 2015, 48 (24), 8806-8822