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By Paul Clarke, Product Group Manager, Nanoparticle and Molecular Characterisation, Malvern Instruments
With ‘renewables’ a defining feature of today’s polymer industry Paul Clarke from Malvern Instruments considers the challenges inherent in engineering biopolymers towards market success and examines characterisation strategies that support these efforts.
The considerable advantages of biopolymers, polymers made from renewable resources such as cellulose and cornstarch, continue to fuel substantial growth in the industry and the ongoing substitution of conventional polymers with more environmentally benign alternatives. The ‘green’ credentials of biopolymers are extensive, from the nature of the feedstocks used through to the biodegradability of the resulting products, and are especially, but not uniquely, attractive to the packaging industry. Within this sector biopolymer use is becoming increasingly common, with polylactic acid (PLA) one of the most well-known and widely applied of this new generation of polymers.
The displacement of a conventional polymer with a bio-based alternative is a significant technical exercise that relies on being able to engineer a polymer with the properties required both for efficient processing and desirable end-use performance. Understanding biodegradability mechanisms is also essential to substantiate any claims made regarding end of life behaviour, a particularly important focus for products that often enjoy only single use, such as plastic bags. Meeting these requirements requires careful consideration of the best analytical tools for the job.
In this article we take a look at what modern gel permeation chromatography (GPC) technology and rheology can offer the biopolymer industry, focusing on how the techniques can be applied in tandem to generate the insight needed to develop successful products. Experimental studies are presented to show how multi-detector GPC extends analysis beyond molecular weight measurement to give structural information that supports the attainment of performance goals, and how rheology can be used to make lab-based assessments of degradation and compostability.
In 1990 California Attorney General John Van de Kamp led a law suit on behalf of the state against Mobil over unsubstantiated claims that their trash bags were biodegradable. The bags in question were made from a blend of corn starch and polyethylene and while the corn starch would indeed biodegrade, fragments of environmentally damaging non-biodegradable polyethylene remained behind.This legal action led to the federal trade commission imposing strict requirements on companies to prove their environmental claims and for their products to undergo rigorous scrutiny and testing.
There was a time when the cradle to grave lifecycle of a polymer was relatively uncomplicated. It began with a petroleum feedstock and finished in a land fill. However the increasingly unacceptable flaws within this system, its inefficient use of finite petroleum resources and growing levels of waste are becoming both environmentally and economically unsustainable. The resulting global need for new materials with a lower carbon footprint, has led to an on-going evaluation of what we should be making polymers from, stricter regulation on polymer disposability and, more generally, a pronounced shift in attitudes towards plastics. Today, over two decades since the Mobil law suit, Los Angeles has moved to completely ban plastic bags from shopping centres throughout the entire district. Clearly the materials landscape is undergoing a rapid change and increasingly the world is looking to polymer chemists for answers.
Replacing a conventional polymer with a biopolymer relies on tailoring both processing and end-use performance characteristics. Thus the development process requires an in depth understanding and manipulation of various molecular parameters. For example, the conventional polymer polyethylene terephthalate (PET) accounts for a significant proportion of all plastic bottles in circulation around the globe. Although recyclable, these often wind up as non-degradable waste within landfills making their replacement an on-going aim.
Such replacement demands the development of products that combine the defining characteristics of PET with the ability to break down within a landfill. This means mimicking the glass transition temperature, thermal conductivity, melting point and a plethora of other parameters that make the plastic so well-suited to this application, with a material that biodegrades more readily. In many instances the manipulation of molecular weight, weight distribution and structure hold the key to achieving this goal.
Measuring molecular weight information
Gel Permeation Chromatography (GPC) has been a staple polymer analytical tool for decades and its potential for supporting the biopolymer industry is considerable. GPC separates polymeric components in a sample on the basis of hydrodynamic size. Smaller species can enter within the pores of the columns packing material and elute at a slower rate than those that are larger, producing a size fractionated exit stream. This eluting stream can be analysed to determine a number of physical parameters including molecular weight and molecular weight distribution, as well as other properties, depending on the detector array applied.
Modern GPC systems incorporate a range of detectors to maximise experimental productivity and can be particularly valuable in development, when tailoring the physical and structural properties of new polymers. Refractive index (RI) is the most widely used detector and is often applied in isolation. The RI of an eluting sample is proportional to concentration and so the RI signal can be used to determine the amount of sample in each size fraction, to produce, via a calibration curve, a relative molecular weight distribution for the sample. Ultra violet absorption (UV) detectors may alternatively be used for concentration measurement, for polymers containing chromaphores, but can often add greater value when used as part of a multidetector array to determine, for example, the relative amount of different components of a copolymer.
Including a viscometer in the array most usefully produces values of intrinsic viscosity (IV), a property that directly correlates with molecular density and can provide molecular structural information. With a viscometer, the polymer solution flows through a capillary bridge and the pressure difference between the capillaries is measured to determine polymer intrinsic viscosity. In combination with an RI detector a viscometer enables the determination of molecular weight by Universal Calibration, which can be a very useful advantage when measuring novel biopolymers. However it is in combination with a light scattering detector that a viscometer can be most useful.
Light scattering detectors deliver absolute molecular weight measurement. A sample irradiated with a laser scatters the light with an intensity that is directly related to its molecular mass. Reliant as they are on the fundamental principles governing the behaviour of light, these detectors are independent of column calibration which is why they are described as providing absolute molecular weight and weight distribution data. For polymer scientists these data are valuable in their own right but in combination with IV values they can be used to gain a detailed understanding of internal structure. Such information can be vital particularly for biopolymers where structural characteristics can directly impact functionality.
The following case study demonstrates the application of multiple detector GPC to PLA, one of the most widely used biopolymers, and illustrates in greater detail how the detectors work in tandem to maximise analytical insight.
Case Study: Applying multiple detector GPC to polylactic acid
Of the many biopolymers currently in production PLA has proven to be one of the most versatile with an established track record as a viable alternative to traditional non degradable plastics for specific applications, such as bottle manufacture. PLA boasts the significant advantage of providing multiple end of life solutions as a bottle made of PLA may be recycled or incinerated, or finish its life on a land fill where, under the appropriate conditions, it will biodegrade into harmless components. Furthermore production of a PLA bottle will use approximately 60% less greenhouse gases and 50% less energy that is required for the conventional alternative.
Lactic acid can exist in the form of two enantiomers, two stereo isomers D and L that are a mirror image of each other, creating additional complexity in terms for polymerization. In PLA the arrangement of these D and L lactic acid isomers can vary considerably and have a major impact on the crystallinity of the polymer, and, consequently, the mechanical and structural properties of the product. Both the specific composition as well as the stereoregularity of the monomers within the chain is important.
For example, a random optical polymer conformation, one containing both enantiomers in uncontrolled amounts and an uncontrolled configuration, will have a larger glass transition range than an isotactic poly L or D lactide, one that contains either one enantiomer or the other. And a heterotactic PLA polymer, with strictly alternating enantiomers, will have a significantly lower glass transition than an LD block copolymer.
Understanding and fine tuning this critical aspect of PLA is therefore essential when tailoring the material to meet defined specifications, a process that can be supported by multi detection GPC.
Table 1 shows data produced during a GPC analysis of the three different types of PLA.
These data indicate that PDLLA has lower intrinsic viscosity than the alternative PLA samples. Viscosity is inversely proportional to the molecular density of the polymer and when used with absolute molecular weight measurements can reveal structural information about a polymer via a Mark-Houwink plot.
The Mark Houwink plot (Fig. 2) shows how molecular density changes as a function of molecular weight. More specifically parameters can be derived that relate to the density of the polymer backbone, from the intercept, MH Log K, and the way in which chains are added to it, MH a. In this way the plot enables the detailed comparison of different structures.
Here the linearity of the plots suggests a constant structure across the molecular weight distribution of each sample, minimal branching and a shared polydispersity across all three samples. The downward displacement of the PDLLA plot indicates that the molecular density of the PDLLA is appreciably higher that the other two samples. In other words, the molecules of the PDLLA are more tightly coiled in solution than the other two samples pointing to a difference in structure. Correlating these findings with polymer behaviour helps to progress development towards a successful endpoint on the basis of an understanding of the links between structure and performance.
Assessing biodegradability via rheological studies
As a result of the increasing standards and regulations relating to polymers, biodegradability has become a crucial parameter that requires accurate quantification. As a result concerted efforts are being made to establish techniques that can accurately measure compostability or degradability, within the lab. Rheological analysis potentially holds the answer with early experimental work suggesting that it can be successfully applied to simulate behaviour in the field.
Rheology tests measure the flow characteristics of a material and, in simple terms, can be applied to investigate whether it exhibits more viscous or more elastic properties or, to put it another way, whether it acts more like a solid or a liquid. A measure of the stiffness of a material is provided by the G modulus which can be further split into two components G’, the elastic modulus, and G’’, the storage modulus. These can be measured through oscillatory testing with a rotational rheometer, a technique that involves, as the name suggests, applying a sinusoidal stress or strain to a sample and measuring its response. The relative magnitude of the measured moduli defines the viscoeleasticity of the material and indicates whether it is exhibiting more solid- or liquid-like behaviour.
As biopolymers biodegrade they undergo internal changes in structure which can manifest themselves as changes in viscoelastic properties. PLA decomposes through hydrolysis and cleavage of the internal ester linkage. A key feature of rheological tests is that they are very sensitive to such small changes in the internal microstructure of viscoelastic materials, which explains their value for the study of this polymer breakdown. As a sample decomposes it undergoes micro-structural changes that can be measured as a function time via an oscillatory time sweep. The following case study illustrates this approach.
Case study: Using rheological tests to quantify the biodegradability of PLA samples
Research was carried out to determine whether rheological analysis could accurately detect polymer breakdown and provide a means of measuring the impact of different variables on degradability. Four random PLA samples (mixtures of both L and D lactic acid) of different molecular weight were exposed to solutions of tap water, water with a lactase enzyme and a slurry of starch in water created to simulate the varying conditions found within landfills. Tests were carried out over a period of 121 days at temperatures of 25 and 50°C. Each day a sample was extracted for oscillatory testing at a single frequency to ascertain the rate at which biodegradation was proceeding.
When all of the data for samples maintained at 50°C are plotted, they sit along a defined curve correlating percentage change in viscosity with time. This is regardless of PLA type, environment or sample thickness. This suggests that the degradation process proceeds via a single mechanism that is independent of the polymer type or environment.
In contrast the data for the samples held at 25°C suggests that sheet thickness is a determining factor in terms of the rate of degradation. When doing this, two distinct curves can be extrapolated both indicating the same correlation between change in viscosity to time. Therefore it is possible to conclude that the overriding dictating variable to degradation at lower temperatures is sheet thickness and that the hydrolytic degradation of PLA is diffusion controlled.
The significance of this experiment is that it shows how land fill environments can be simulated in a laboratory and how the rate of degradation can be clearly measured as a function of the change in viscoelasticity. With regulatory bodies becoming increasingly litigious in regards to the materials industry, charting biodegradability is not only key to development but a growing legal necessity.
A sustainable future
It seems clear that in the future the polymers industry will be increasingly shaped by ethical, economic and legal issues and the sustainability agenda. A growing biopolymer industry holds out the promise of advantageous solutions that address many of the issues faced. New analytical strategies are required to ensure delivery on this promise. Multi detection gel permeation chromatography and rheology testing have been shown to provide useful information both during development and when assessing the degradability of biopolymers and both therefore have valuable roles to play in advancing biopolymer application.