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Objective:
• To develop non-destructive optical scattering techniques for characterizing micro- and nano-filler dispersed in liquid (uncured) and in solid (cured) polymeric matrices over multiple size scales, and
• To relate dispersion and physical properties to the appearance and weathering service life of polymeric materials.
Problem:
The overwhelming majority of commercial polymeric products contain pigmentary particles while new materials containing nanofillers recently have been introduced into the marketplace. Poor filler dispersion is qualitatively known to adversely affect the properties and ultimately the appearance, service life, and mechanical performance of polymeric systems. Current industrial methods for assessing filler dispersion are subjective, unreliable, and limited to dilute suspensions of micron-size particles. No reliable metrologies have been identified for characterizing filler dispersion in both cured and uncured coatings and for monitoring the formation of filler dispersion and microstructure during the curing process. Such metrologies are required to improve the dispersion and to optimize of performance of fillers in a polymeric product. In material performance evaluation, gloss retention and color change are common performance attributes used in the coatings and plastics industry. However, current commercial color and gloss measurement methods are neither adequate to fully characterize the optical properties of a filled polymer system, nor to provide sufficient data to correlate physical properties with different dispersion states. Lack of both accurate optical property measurements and an understanding of the linkage between optical and physical properties makes it difficult to control appearance quality for new materials, manufacturing processes.
Approach:
Two non-destructive scattering metrologies will be applied to characterize particle dispersion in uncured and cured coatings: small angle neutron scattering (SANS) and light scattering (LS). SANS measurements will be performed in the cold neutron facility in the NIST Center for Neutron Research. SANS is a powerful tool for measuring the structure of polymeric materials but, for obvious reasons, this metrology cannot be easily applied in industry. A practical alternative to SANS is light scattering. These measurements will be performed in BFRL’s light scattering materials characterization (LSMC) laboratory.
A series of experiments, using two custom-designed, state-of-the-art light scattering instruments in LSMC Lab, will be carried out to study the relationship between optical performance properties (e.g. color and gloss), surface morphology, microstructure, and filler dispersion in a polymer nanocomposite. Our research efforts will focus on metal oxide, nanoparticle systems due to their widespread commercial importance. The developed metrologies, however, should have generic application to any filler system. This project will establish the metrological framework for determining structure-property relationships in polymer nanocomposites. Measurement protocols for characterizing multi-scale structure and filler dispersion as a function of processing conditions in both uncured and cured coatings will be developed. Angular-resolved static light scattering (SLS) and time-resolved dynamic light scattering (DLS) measurements will be used for characterizing particulate and cluster size and the degree of dispersion in liquid suspensions (solvent, surfactant, polymer solution), while the angular-resolved scattering profiles for cured films (or in solid polymeric matrices) will be determined using forward and backward scattering using multi-wavelength light scattering approach. The structure and spatial correlation of particle/nanoparticles can be then deduced from scattering profiles. By correlating SANS measurements and microcopy measurements, an accurate and quantitative filler dispersion measurement using light scattering metrology can be developed.
In addition to developing metrologies for characterizing filler dispersion and micro/nano-structure of polymeric materials, we also have focused on developing advanced measurement methods for accurately quantifying the appearance properties. Using the fully automated, five-axis goniometric optical scattering instrument at LSMC Lab, we can quantitatively obtain an angular-resolved optical scattering profile (including specular and off-specular reflections) with a wide range of incident illumination wavelengths (320 nm - 800 nm). This scattering profile provides crucial data for understanding the link between appearance and micro-physical properties (surface roughness, particle dispersion, and subsurface microstructure) of a material. A series of samples containing filler of different sizes and dispersion states, obtained from researchers in SLP core project or industrial partners, will be studied. The effect of filler dispersion on the optical properties and durability (via UV degradation) of a filled-polymer system will be investigated and characterized using optical scattering, scanning microscopy (LSCM and AFM), neutron scattering, and other methods. Optical scattering data as a function of exposure time will be correlated to the corresponding microphysical properties of the test samples. The trend and scaling behavior in the data for different degradation times will be analyzed. Methodology and measurement protocols will be developed to generate accurate and useful optical scattering data for predicting service life of a filler-polymer system using following approach: We will provide optical scattering data to compare with the calculated optical scattering data obtained from modeling and computer rendering communities for predicting the optical scattering profile for a given surface morphology/microstructure or a given filler dispersion state. Ultimately, by integrating measurements and models in a computer rendering system, we can provide sufficient scattering/physical property data to help to create an accurate virtual representation of the appearance of objects, and predict service life of a filler-polymer system.
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