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Objective:
To establish a linkage between field and laboratory exposure using the reliability-based methodology for polymeric systems filled with nanoparticles.
Problem:
Nanostructured organic–inorganic composites are having significant commercial impacts. Considering the functionalities they may provide, e.g., light weight, more resistance to impact, UV stabilization, scratch and mar resistance, or anti-microbial, the field of nanocomposite materials is enormous. However, due to the extremely small size of the nanoparticles and their extensive interfacial area, the incorporation of nanoparticles into traditional polymeric materials presents many technological and scientific challenges. The metrologies for evaluating the dispersion of nanoparticles and the physical and chemical properties with nanoscale spatial resolutions are limited. No methodology for generating accurate, precise, and timely long-term performance estimates for nanostructured polymeric materials exposed in their intended in-service environment currently exists. The absence of such a methodology will continue to hinder innovation, product development, and acceptance of nanostructured polymeric materials into new applications.
Approach:
This project consists of four major thrusts:
• Characterization of the multiscale chemical and physical properties of the aged and unaged, nano-filled and unfilled polymers,
• Laboratory accelerated exposures,
• Outdoor exposures, and
• Linking laboratory and field exposure results via appropriate scientifically-based mathematical models.
Subtasks include 1) Development of metrologies for characterizing the nanoscale and macroscale physical and chemical properties of unaged and aged nanoparticulated systems, 2) Investigation of the effects of UV light (intensity and wavelength), temperature and relative humidity on the service life of these polymers exposed in the field and laboratory, 3) Investigation of the influence of nanoparticle-polymer interfacial interactions on the nanoparticle dispersion and the long-term performance of the nanocomposite systems, and 4) Development of models for service life prediction of nanocomposite materials.
Selected study materials include an epoxy polymer filled and unfilled with nanoscale metal oxide particles. The model epoxy polymer is chosen because 1) its chemical and physical changes during exposures to UV environments can be observed within reasonable short time, 2) its degradation mechanism is generally known, and 3) the Polymeric Materials Group has successfully linked field and laboratory exposure results for the neat epoxy system by using a reliability-based methodology. The metal oxide particles, such as TiO2, with diameters from 20 nm to 40 nm, will be added to the epoxy resin and curing agent system at different loading levels, using surfactants and dispersing agents as necessary.
The measurement techniques will be selected to characterize the chemical and physical properties of unaged and aged films. The chemical properties of the films will be measured using FTIR-attenuated total reflection (FTIR-ATR) and UV-visible reflection spectroscopies. Physical properties, including gloss, surface morphology, and mechanical properties, will be measured by gloss meter, AFM and confocal microscopy, and dynamic mechanical analysis, respectively. Surface nano-scale mechanical properties will be measured using a nanoindenter, and the thickness changes and mass loss during UV degradation will be assessed via atomic force microscopy (AFM) and high resolution scale, respectively. The particle dispersion will be characterized using AFM, confocal microscopy, scanning electron microscopy (SEM), UV-visible spectroscopy, and scattering techniques (light scattering, x-ray, and neutron scattering). The effects of nanoparticle loading and size on dispersion will be investigated.
Filled and unfilled polymeric materials will be exposed systematically on the NIST SPHERE to different combinations of panel temperature, relative humidity (RH), spectral UV wavelength, and spectral intensity. Laboratory experiments will be designed to assess the validity of both the additivity and reciprocity laws for both nanoparticle-filled and unfilled systems as a function of temperature and relative humidity. In future experiments, other independent material variables will be introduced into the experimental design including pigment photoreactivity, pigment particle size, and pigment dispersion. Field exposure experiments will be initially initiated only in Gaithersburg, MD but later experiments may include Madison, WI and Miami, FL or Phoenix, AZ. Specimens exposed outdoors will be characterized in the same manner that these materials are characterized in the laboratory.
The starting point for mathematically linking field and laboratory exposure results will be the model derived in the previous phase of the consortium by Professor William Meeker of Iowa State University. The validity of this model for both unfilled and filled nanocomposites will depend, in part, on whether the reciprocity and additivity laws are obeyed over a wide range of exposure conditions. If these laws are valid, then the derived model should be applicable to the study coatings. If not, modifications will have to be made accordingly.
For a better understanding of the nanoparticle-polymer interaction and its effect on the particle dispersion and the long-term performance of nanoparticulated polymeric materials, a model system of surface tailored nanoparticles and diblock copolymers with well-defined hydrophilic and hydrophobic segments will be selected. The distribution of the nanoparticles in the polymers, and the nanostructure of the particle-polymer interfacial regions will be monitored by AFM with fluorescence or Raman accessories. Effect of surface chemistry of nanoparticles on the interfacial interaction and the nanostructures of block copolymers will be assessed.
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