Quantifying Micro-Scale Mechanical and Fracture Properties in Nanocomposite Materials

Objective:

Develop measurement methods to quantify the mechanical properties, at multiple length scales, of polymeric nanocomposites relevant to the building and construction industry. The goals related to this objective are to • Extend metrologies to quantify the mechanical and viscoelastic properties of nanoparticle filled polymeric materials at reduced length scales (bulk ® micrometer ® sub-micrometer) . • Support reliability-based service life prediction by developing metrologies to link bulk and surface mechanical properties to concomitant chemical and appearance changes in nanocomposite coatings exposed to UV, hydrolytic, or thermal degradation. • Develop methodologies and metrologies to determine the effect of nanoparticles on fracture properties at the micrometer length scale.

Problem:

Traditional micrometer size fillers such as titanium dioxide, aluminum oxide, silica, and elastomeric rubbers, have historically been added to polymeric building materials to enhance toughness, increase stiffness, and extend durability. Nanoparticles have revolutionized composite behavior because the size of the particle, less than 100 nm, results in a large surface area to volume ratio. Therefore, the particle-matrix interface dominates composite behavior, such that improvements in multiple aspects of composite performance are achieved with much less filler material. Currently, the challenge for the industrial and academic community has been to reliably obtain uniform size distributions of filler particles that maintain a uniform dispersion in the matrix. The lack of absolute control over dispersion and interfacial properties has not slowed the incorporation of nanoparticles into materials used for consumer, construction, and transportation products. As more and more nanocomposite materials are developed, questions concerning the limits of environmental longevity will become more urgent. This project focuses on three aspects of nanocomposite durability:

• At what length scale do nanoparticles influence composite mechanical properties?
• What is the long term mechanical durability for the next-generation of composite materials containing nanoparticles during environmental exposure? How do mechanical signals of degradation relate to appearance and chemical cues of degradation?
• How does the addition of nanoparticles alter failure modes (fracture, yielding) in model thermoplastic and thermoset materials?

Approach:

The goal of this project is to develop linkages between composite mechanical performance metrics such as stiffness, toughness and controllable variables such as nanoparticle characteristics, interphase properties, matrix properties, and particle dispersion. In order to accomplish this goal efficiently, research tasks are developed to couple with other projects within the Service Life Prediction Program.

Materials: Model matrix materials (thermoplastic and thermoset) will be combined with functionalized inorganic or organic at differing particle volume concentrations. The surface of the particles will be functionalized for each matrix to maximize dispersion and interfacial strength. Pigment loading will range between 0 % vol. to 20 % vol. Guidance on sample preparation will come from industrial, academic, and NIST collaborations.

Methods: Mechanical and viscoelastic properties are characterized using bulk axisymmetric tests such as compression and tension. The workhorse for small-scale mechanical property characterization will be instrumented indentation testing (IIT). IIT allows for the measurement of mechanical properties of different sample volumes by changing tip shape and indentation depth. The bulk measurements serve as a baseline for comparison to indentation measurements of mechanical and viscoelastic properties.

Projects:
1. One thermoplastic and one thermoset nanocomposite material that contains TiO2 (UV-active anatase) or SiO₂ nanoparticles at filler levels of 0 %vol., 2 %vol., and 5 % vol. will be used to investigate particle effects on mechanical properties. Particle dispersions will be measured using a combination of LSCM (Project 861-6021), AFM, and SEM. Bulk and micrometer-scale mechanical properties will be measured and modeled analytically.

2. The thermoplastic composites will be characterized chemically using FT-IR ATR. The TiO₂ particles are photoreactive, while the SiO₂ particles are not. The fundamental photoreactivity studies of project 861-6020 will be used to quantify the reactivity of the TiO₂ particles and understand the reactive species created at the TiO₂ interface. The materials will be subjected to UV exposure (SPHERE, project 861-6019), with a sample set in the dark serving as the control. The mechanical properties (bulk and microscale), dispersion (Project 861-6021), and chemical properties will be tracked with time to fully determine how degradation of the particle-matrix interface diminishes mechanical and chemical properties.

3. Dog bone samples will be fabricated from the thermoplastic composites and tested to failure in tension. The onset of plastic deformation (yielding) as a function of UV exposure will be measured. This will be compared to the onset of plastic deformation observed for the unexposed and exposed samples tested using indentation with different shaped probes. Ultimately, the ability of different the indenter tips to induce yield behavior in the polymer and the comparison to bulk measurements will be used to determine the effect of surface damage on durability.

Potential Collaborations: These projects relate to relevant studies in polymer indentation behavior occurring at the Army Research Laboratory and MSEL at NIST. It is anticipated that the results for the nanocomposite materials will be of particular importance to these Laboratories. In addition, indentation techniques will be used to extend the reliability metrology for the degradation of other filled materials under investigation in project 861-6019.