Role of Light Scattering in Production of Performance Polymers and Other Functional Materials

Summary

The majority of modern performance polymers are designed to enhance a particular material property. A number of high-profile engineering plastics, resins, and gels are made from these kinds of precursors.  Light scattering can be used to solve many of the unique problems encountered when working with unusual functional plastics and associated polymeric precursors.

Areas of interest: Novel materials development, circular economy, plastics recycling

Synonyms: Performance Materials, Functional Materials, Specialty Polymers

What are Performance Polymers?

Performance polymers have had several different definitions since the inception of modern Materials Science.  This term is commonly used to refer to materials or material precursors with novel properties or functions, where these properties are often controllable by external stimuli. Many are designed to enhance, and often maximize, a particular material property (e.g., heat resistance, crystallinity, optical clarity, mechanical resistance, etc.). The building blocks of all these functional materials are synthetic polymers engineered to satisfy specific industry needs. The exact definition has, changed over the years.

polymer pyramid

Many previously novel manufacturing and prototyping plastics were initially classified as performance materials but have since become commonplace. High profile examples include chemically resistant materials such as PEEK and Nylon, machining plastics such as Delrin, and bioplastics like PLA. Recent examples include prototyping plastics for 3D printing, other thermoplastics, bioplastics, dental resins, composites, ceramics, thermoset adhesives, slip resistant coatings, ion-exchange resins, flocculants, and miscellaneous toughened or chemically resistant engineering plastics. 

Light Scattering as a tool for characterizing novel polymers

There are few techniques better suited for measuring polymer molecular weight than static light scattering.  In contrast to small molecules, synthetic polymers are nearly always polydisperse. This intrinsic polydispersity with respect to chain length and molecular weight is unavoidable. In the case of many advanced materials these are often functional, leading directly to some of the more desirable properties observed. Light scattering can be used with polydisperse materials such as polymers to give an average molecular weight with no need for chemical separation or additional purification. Other parameters that can be obtained include chain length, degree of polymerization, radius of gyration, and persistence length. All these macromolecular properties directly impact the bulk properties of resulting blends or plastics, making these properties critical when producing novel polymers.    

Sulfonated Polymers & Resins as a Case Study

One interesting class of specialized materials are strong-ion exchange resins, often made from cross-linked polymers with strong permanent charge including polycations and polyanions.  One such example are sulfonated polymers, which despite having hydrophobic backbones can be made extremely water soluble due to their high linear charge density.  These polyanions bind extremely strongly to cationic molecules, including small-ions, colloids, and various other macroions.  In addition to being used as the building blocks of ion-exchange resins, sulfonated polymers are used in a number of other highly specialized applications, including plasticizers, industrial water softening agents, and as the functional component in proton exchange membranes in commercial fuel cells. 

Preparation of Sulfonated Polystyrene for Light Scattering

An unusually high molecular weight polystyrene sulfonate, NaPSS, was selected as a it is representative of a common commercial polymer.  This commercial polymer was prepared by sulfonation of 1 MDa polystyrene. The exact degree of sulfonation is unknown but is assumed to be greater than 99%. A common consideration in light scattering is the removal of dust, however as long-chain polyelectrolytes are both strongly interacting and tend to take on extended dimensions, passing such polymer solutions through a filter is problematic. To get around this, all stock solutions were carefully filtered prior to dissolution of polymer.  These samples were analyzed using the BI-200SM Research Goniometer.

BI-200SM for performance polymer testing
The BI-200SM has been a staple of advanced polymer research for almost 4 decades

Determination of molecular weight by static light scattering requires construction of a Zimm Plot, which involves measurements made at a series of different polymer concentrations, Cp, and scattering angles, θ.  To facilitate this measurement 4-5 concentrations were prepared, each at three different ionic strengths, resulting in a total of 14 separate polymer solutions.  Mass concentrations were determined to at least 4 trailing decimal places, in order to accurately extrapolate to zero concentration (eg., 0.93 g/L is actually 9.3020 x 10-01 g/L).

performance polymers polystyrene sulfonate
Figure 1 – Zimm Plot of 1 MDa NaPSS in near zero ionic strength. Note the uncharacteristically high curvature of this plot leading to a failure of the double extrapolation (concentration and angular extrapolation differ wildly and are over-estimated by several orders of magnitude). A total of nine scattering angles were used between θ = 30˚ to 150˚ for each concentration.
Figure 1 – Zimm Plot of 1 MDa NaPSS in near zero ionic strength. Note the uncharacteristically high curvature of this plot leading to a failure of the double extrapolation (concentration and angular extrapolation differ wildly and are over-estimated by several orders of magnitude). A total of nine scattering angles were used between θ = 30˚ to 150˚ for each concentration.
Figure 2 – Zimm plot of 1 MDa NaPSS in moderate salt (I = 32 mM NaCl). Much of the curvature seen in low salt is absent, except at the highest concentrations.
Figure 2 – Zimm plot of 1 MDa NaPSS in moderate salt (I = 32 mM NaCl). Much of the curvature seen in low salt is absent, except at the highest concentrations.
Figure 3 – Zimm plot of 1 MDa NaPSS in high salt salt (I = 128 mM NaCl). Curvature is fully suppressed in the presence of excess screening electrolyte (salt), resulting in a conventional Zimm Plot and an apparent MW of approximately 1.4 MDa. In contrast to figures 1 and 2, the total number of scattering angles was reduced to span only θ = 90˚ to 150˚.
Figure 3 – Zimm plot of 1 MDa NaPSS in high salt salt (I = 128 mM NaCl). Curvature is fully suppressed in the presence of excess screening electrolyte (salt), resulting in a conventional Zimm Plot and an apparent MW of approximately 1.4 MDa. In contrast to figures 1 and 2, the total number of scattering angles was reduced to span only θ = 90˚ to 150˚.

As shown in figures 1 and 2, the extreme charge density of this sulfonated polymer complicates the double extrapolation (concentration and angle) required for construction of a Zimm Plot.  In low salt, none of the long-range electrostatics between, and within, polymer chains are screened.  This is most obvious in the absence of any background electrolyte (no salt) as shown in figure 1 where the double extrapolation fails to produce a physically meaningful molecular weight. Even in the presence of moderate salt (Figure 2), some long-range intermolecular interactions persist at the highest concentrations. With a sufficient amount of added salt, these long-range electrostatics are neutralized, resulting in a more conventional Zimm Plot and a reasonable molecular weight estimate (Figure 3). 

Conclusions:

One of the challenges of working with performance polymers is that they are often engineered to have unusual physiochemical properties, meaning that these highly practical industrial samples, often make for challenging measurements that require careful consideration of theoretical limits to each method.  As shown in the example of sulfonated polymers, the exact parameter that makes them desirable, i.e., high linear charge density, makes them difficult to analyze. Static light scattering is a highly useful experimental technique for the study of synthetic polymers and by recognizing that long-range electrostatics can be screened by high-salt, we are able to obtain useful results. 

Recap:

  • Performance polymers are a broad class of specialized commercial polymers, and are often used as precursors for functional materials, plastics, and other specialty chemicals. 
  • Static Light Scattering is an ideal method for determining the molecular weight of intrinsically polydisperse samples such as synthetic polymers.
  • Interactions with strong ion-exchange resins are screened by high-salt, a fact usually exploited to elute charged molecules and macroions. The same effect is exploited to screen out long-range intermolecular interactions in solutions of sulfonated polymers.
Applications: ManufacturingMaterialsMolecular WeightPolymers
Instruments: NanoBrook SeriesBI-200SM
Posted on: Dec 17, 2020

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