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Description
Overview
Polymer Composites
Polymer composites are a class of multiphase materials in which a polymer matrix is reinforced with either a fiber or filler phase. They are traditionally valued for their stiffness, high strength to weight ratio, and corrosion resistance. In emerging high-end applications, electronic, optical, thermal, chemical and barrier properties are additionally of interest, casting them now as multifunctional materials.
Polymer composite materials vary widely due to the many types of polymer matrices and reinforcement types that are possible, but may be most broadly classified by reinforcement type. Fiber reinforced plastics (FRPs), sometimes called "traditional composites", refers to materials in which the polymer matrices are reinforced with either continuous or chopped fibers. Fibers are most commonly glass, carbon, or aramid, but additional types such as natural fibers are now possible. FRPs are widely used in aerospace, automotive, marine, and construction applications, as well as specialty applications such as ballistic armor. Infrastructure applications are also becoming increasingly targeted. In these cases, the combination of excellent structural properties and light-weighting are paramount to their usage. The newer and still emerging area of polymer nanocomposites (PNCs) refers to materials in which the reinforcement phase has two or three dimensions that are less than 100 nm. These reinforcements are thus nanoparticles, nanosheets or nano-dimensional fibers such as carbon nanotubes or nanocellulose, amongst others. Nanocomposites differ from conventional composite materials by the much lower volume fraction, higher surface to volume ratio, and/or exceptionally high aspect ratio of the reinforcing phase, making the science of these materials in some respects different than the former.
Scientific Issues
While composite materials vary, their properties depend on essential features regardless of type and these become the objects of classification and investigation for the Materials Genome Initiative (MGI). In terms of thermomechanical properties, the first function of the reinforcement is to transfer loads from the weaker polymer phase to the more durable reinforcement. The material properties of the individual phases are therefore the entities of first order interest.
Beyond that, the properties of polymer composites materials are highly dependent on molecular scale interactions between the polymer matrix and filler. The first issue of fundamental importance is the interface where the matrix material comes into contact with the reinforcement. High-performance fibers give strength to composites via the interface between the matrix and the reinforcement surface. The stronger the interface, the greater the load transfer function. Interfacial bond strength, a measure of the strength of adhesion between the matrix and reinforcement, is a common design feature and critical property that must be engineered, controlled and measured. Structural properties such as interlaminar shear and flexural strength, as well as failure modes, are controlled by the interfacial bond strength. Thus, engineering the fiber surface chemistry to optimize the adhesion between fiber and matrix is of paramount importance. Optimized adhesion may also be achieved by means of chemical coupling agents which covalently bond the matrix and reinforcement.
The second issue of fundamental importance is the polymer/filler interphase. The presence of filler modifies the matrix properties, and the interphase is defined as the region in the vicinity of the filler where the matrix properties are modified from the bulk. The ability to tailor the polymer/filler interaction so as to optimize the impact of the interphase on macroscopic properties is another key to the intelligent design of composites. This is especially true for PNCs, where the surface to volume ratio is exceptionally high, making the interphase effectively a third component of the material.
In nanocomposites, another fundamental issue is filler dispersion. Mechanical properties are enhanced and optimized when dispersion is uniform and stable, and filler aggregates are minimized. At a molecular level, filler dispersion becomes an issue because strong van der Waal forces between the particles tend to produced high levels of aggregation in the absence of strategies to modulate these interactions. A common and widely adopted strategy for controlling interparticle interactions is to graft short molecules or polymer chains to the filler surface. However, this approach also affects the interphase, coupling the two problems. Finally, the polymer matrix itself also plays a role in dispersion (regardless of particle interaction properties) as dispersion is difficult to achieve in systems where the particle size is greater than polymer radius of gyration.
A final fundamental issue of fundamental importance is sorption of environmental components such as water, salts, or oxygen into the composite materials. While diffusion is slow in polymer matrices, it is finite, and environmental components indeed infiltrate the matrix usually finding home at polymer/reinforcement interface. Over time, this can lead to property degradation, loss of functionality, and promote failure. Measuring and controlling sorption is therefore an important component of composite research efforts.
MGI Related Composite Projects
This page describes NIST projects currently in place to understand the fundamental scientific issues related to polymer composite materials that are outlined above. Some of these are implemented in internal programs, others are taking place with external collaborators, most notably our partners at the Center for Hierarchical Materials Design (CHiMaD). CHiMaD collaborations are listed below. The goals is to increasingly coordinate these projects using MGI principles. The rising demand for multifunctionality makes the importance of progressively shifting composites design into the realm of the MGI even more imperative -- we expect that the combination of experimental, database, computational and machine learning tools will more and more provide a leveraged approach to the problem, decreasing the time and cost to develop new and better materials.
Major Accomplishments
THEORY & MODELING
Preface -- The most important issues related to the science of composite materials -- interface adhesion, interphase modifiction, filler dispersion and environmental sorption -- are fundamentally controlled by enthalpic and entropic interactions between filler and matrix which occurs at the molecular level. The need for classical molecular level models which enable fast and accurate simulation of such systems is therefore tacitly understood. In addition, experimental efforts aimed at using fluorescent dyes to indicate and measure damage at the polymer-reinforcement interface is also an active area of research. Density functional theory (DFT) tools are being used model fluorescence and bond breaking in candidate dye materials.
Molecular Modeling of Epoxy Matrix Materials
Summary -- Cross-linked epoxy thermosets, like all glass-forming viscoelastic materials, show both a temperature and rate dependence in their thermomechanical properties. However, accounting for rate effects on these properties using molecular dynamics (MD) simulations and making quantitative comparison with experimental measurements has proven to be a difficult task due to the extreme mismatch between experimental and computationally accessible cooling rates. For this reason, the effect of cooling rate on material properties in glass-forming systems (including epoxy networks) has been mostly ignored in computational studies, making quantitative comparison with experimental data nebulous. In this work, we investigate a strategy for modeling rate effects in an epoxy network based on an approach that uses theoretically informed simulation and analysis protocols in combination with material specific time–temperature superposition (TTSP) data obtained from experimental measurements.
References
- Ketan S. Khare and Frederick R. Phelan Jr., Quantitative Comparison of Atomistic Simulations with Experiment for a Cross-Linked Epoxy: A Specific Volume – Cooling-Rate Analysis Epoxy, Macromolecules, 51(2), pp. 564-575, (2018). DOI: 10.1021/acs.macromol.7b01303
- Ketan S. Khare and Frederick R. Phelan Jr., "Integration of Atomistic Simulations with Experiment using Time−Temperature Superposition for a Cross-linked Epoxy Network," submitted to ACS Macro Letters, (2018).
Contact: Frederick R. Phelan Jr. [frederick.phelan [at] nist.gov (frederick[dot]phelan[at]nist[dot]gov)]
Density Functional Theory Modeling of Mechanophore Dyes
Summary -- Mechanically activated dyes or mechanophores are being investigated as molecular sensors to indicate mechanical damage in advanced materials. Mechanophores rely on the cleavage of a labile covalent bond that activates a florescent dye. Density functional theory (DFT) is being used to calculate the force required to break such chemical bonds, which is highly dependent on the dye chemistry.
- Force required for mechanophore activation is about one-fourth that of typical carbon-carbon bond in polymers
- Absorption and emission characteristics of the activated dye agreed with experiments
- Multiscale modeling to integrate these results into classical simulations is underway
Contact: Frederick R. Phelan Jr. [frederick.phelan [at] nist.gov (frederick[dot]phelan[at]nist[dot]gov)]
EXPERIMENT
Mechanophore activation in fiber reinforced composites
Summary -- The structural failure of fiber-reinforced polymer composite components originates from the rupture of individual glass or carbon filaments. The local shock from these fiber breaks on the polymer matrix is extreme, but occurs on a nanosecond timescale, far out of reach of common measurement techniques. By sensitizing the matrix with a mechanophore that becomes fluorescent in response to deformation, we can gain insight into the chemical and physical damage processes that result from fiber fragmentation.
- The single fiber fragmentation test provides a model composite damage platform
- Resultant fluorescence intensity and lifetime indicate shock intensity and damage extent
- Results to date indicate highest mechanophore activation at locations far from the fracture
References
- G.A. Holmes, "The potential impact of DGEBA/amine reaction kinetics on interphase formation in glass fiber composites," Composites Science and Technology, in press, (2018). DOI: 10.1016/j.compscitech.2018.08.008
- E.D. McCarthy, J.H. Kim, N.A. Heckert, S.D. Leigh, J.W. Gilman, G.A. Holmes, The fiber break evolution process in a 2-D epoxy/glass multi-fiber array. Composites Science and Technology 121:73–81, (2015). DOI: 10.1016/j.compscitech.2014.10.013
- Jeremiah W. Woodcock, Ryan Beams, Chelsea S. Davis, Ning Chen, Stephan J. Stranick, Darshil U. Shah, Fritz Vollrath, Jeffrey W. Gilman, "Observation of Interfacial Damage in a Silk-Epoxy Composite, Using a Simple Mechanoresponsive Fluorescent Probe," Advanced Materials and Interfaces, 4(10):1601018, (2017). DOI: 10.1002/admi.201601018
Contact: Jeff Gilman [jeffrey.gilman [at] nist.gov (jeffrey[dot]gilman[at]nist[dot]gov), 301-975-6573]
Nanoparticle Enhanced Fiber Sizings
Summary -- Fiber-reinforced polymer composite materials maintain cohesion through covalent bonding at the fiber-matrix interface. The properties of the material at and near this interface have an outsized effect on the performance and durability of composite structures. We leverage recent work in nanocomposite design to spark a step change in the understanding and functional properties of engineering composites.
- Novel coating techniques yield uniform, controllable nanoparticle films
- Targeted performance changes possible through customization of nanoparticle parameters
- Electrochemical and electrophoretic processes present an opportunity for one-step carbon fiber treatments
References
- B. Natarajan, J.W. Gilman, "Bioinspired Bouligand cellulose nanocrystal composites: a review of mechanical properties," Philos Trans R Soc A Math Eng Sci 376(2112):20170050, (2018). DOI: 10.1098/rsta.2017.0050
Contact: Jeff Gilman [jeffrey.gilman [at] nist.gov (jeffrey[dot]gilman[at]nist[dot]gov), 301-975-6573]
Center for Hierarchical Materials Design (CHiMaD)
The Center for Hierarchical Materials Design (CHiMaD) is a NIST funded center of excellence (CoE) for advanced materials research focusing on developing the next generation of computational tools, databases and experimental techniques in order to enable the accelerated design of novel materials and their integration to industry, one of the primary goals of the U.S. Government's Materials Genome Initiative (MGI).
This section describes joint NIST-CHiMaD projects for the CHiMaD Polymer Matrix Materials Use Case group.
Sustainable Bioinspired Composites
Summary -- Exceptionally damage tolerant biological composites (bone, ivory, etc.) derive their toughness from the periodic, alternating arrangement of nanoscale hard (mineral) and soft phases. The emulation of this design in synthetic composites enables the simultaneous maximization of stiffness, toughness, and impact tolerance. We leverage the self-assembly behavior of sustainable nanocelluloses to develop novel biomimetic composites that elucidate the fundamental mechanisms underlying the superior mechanical performance of biological composites.
- Custom built casting chamber enables the controllable fabrication of self-assembled nanocellulose films.
- Hierarchically architectured composites with nanocellulosic blends display unprecedented mechanical performance.
- Coarse grained molecular dynamics simulations and micromechanical models reveal superior performance to arise from enhanced interaction length-scales between cellulose nanocrystals.
References
- Natarajan, B., Krishnamurthy, A., Qin, X., et al. (2018). Binary Cellulose Nanocrystal Blends for Bioinspired Damage Tolerant Photonic Films. Advanced Functional Materials, 1800032.
- Natarajan, B., & Gilman, J. W. (2018). Bioinspired Bouligand cellulose nanocrystal composites: a review of mechanical properties. Phil. Trans. R. Soc. A, 376(2112), 20170050.
- Natarajan, B., Emiroglu, C., Obrzut, J., et al. (2017). Dielectric characterization of confined water in chiral cellulose nanocrystal films. ACS applied materials & interfaces, 9(16), 14222-14231.
Contact: Jeff Gilman [jeffrey.gilman [at] nist.gov (jeffrey[dot]gilman[at]nist[dot]gov), 301-975-6573]
Development of Temperature Transferable Coarse-Grained Force-Fields for Glass-Forming Materials
Summary -- Computation at mesoscopic length scales between the atomistic and continuum for polymeric and similar soft materials requires coarse-graining (CG) techniques due to the overwhelming system size when described in atomistic terms. However, standard, validated coarse-grained force fields for polymeric materials are not available in the same sense as they exist for atomistic MD calculations. Most CG models exhibit faster dynamics and softer mechanical response relative to the atomistic representation. In addition, they lack temperature transferability and quickly lose accuracy at temperatures even moderately different from the state point at which the potential was derived. The loss of temperature transferability is catastrophic for the modeling of polymers and other amorphous materials which are typically processed from a high-temperature melt state to an end use solid state whose temperature lies below the glass transition. We are developing new approaches to CG modeling that accurately reflect the specific chemical properties of constituent polymers and particles, while maintaining accurate dynamics from the melt to the glassy state. This is especially important for composite systems where the specific nature of the polymer/filler interactions plays a fundamental role in the properties of the interphase.
- In a new approach to this problem called Energy Renormalization (ER), the nonbonded potential is renormalized as a function of temperature using the concept of temperature-dependent activation energy found in glass formation theories. This yields a coarse-grained force-field which replicates atomistic dynamics from the high-temperature melt state (Arrhenius regime), through the non-Arrhenius regime of incipient glass-formation, through to the amorphous state below the glass transition.
- The ER approach yields a simple functional form for the nonbonded potential which gives unique insight into the physics of the coarse-grained model. The potential may be quickly parametrized from data for density and the Debye-Waller Factor (DWF) as a function of temperature using fast picosecond timescale simulations.
- In a related approach under development, we similarly rescale the nonbonded potential vs. temperature using a Bayesian Calibration (BC) approach. In this method, there is no assumption of functional form and the method yields a tabular rather than an analytical form for the potential.
References
- Wenjie Xia, Jake Song, Cheol Jeong, David D. Hsu, Frederick R. Phelan Jr., Jack F. Douglas, Sinan Keten, "Energy-Renormalization for Achieving Temperature Transferable Coarse-Graining of Polymer Dynamics," Macromolecules, 50 (21), pp. 8787–8796, (2017). DOI: 10.1021/acs.macromol.7b01717
- Wenjie Xia, Jake Song, Nitin H. Krishnamurthy, Frederick R. Phelan Jr., Sinan Keten, Jack F. Douglas, "Energy Renormalization to Coarse-Graining of the Dynamics of a Model Glass-Forming Liquid," Journal of Physical Chemistry B, 122(6), pp. 2040-2045, (2018).
- Jake Song, David D. Hsu, Kenneth R. Shull, Frederick R. Phelan Jr., Jack F. Douglas, Wenjie Xia, Sinan Keten, Energy Renormalization Method for the Coarse-Graining of Polymer Viscoelasticity, Macromolecules, 51(10), pp. 3818-3827, (2018).
- Paul N. Patrone, Thomas W. Rosch, and Frederick R. Phelan Jr., "Bayesian calibration of coarse-grained forces: Efficiently addressing transferability," The Journal of Chemical Physics 144, 154101 (2016).
Contacts
- Jack Douglas [jack.douglas [at] nist.gov (jack[dot]douglas[at]nist[dot]gov)]
- Frederick R. Phelan Jr. [frederick.phelan [at] nist.gov (frederick[dot]phelan[at]nist[dot]gov)]
- Sinan Keten [s-keten [at] northwestern.edu (s-keten[at]northwestern[dot]edu)]
- Wenjie Xia [wenjie.xia [at] ndsu.edu (wenjie[dot]xia[at]ndsu[dot]edu)]