Cross-linking of Polymers in the Interphase by EUV emission from Nanoparticles

YOUNGWOOD, Pa. - Feb. 12, 2014 - PRLog -- .
Introduction
Nanocomposites comprising NPs embedded in a polymer are observed [1] to display significantly enhanced mechanical properties compared to the polymer without NPs. NP stands for nanoparticle. Enhanced properties are attributed to the interphase comprising a thin < 100 nm polymeric region that forms adjacent the NP surface. Since NPs are typically < 10 nm, the interphase controls the properties of nanocomposites.

Theories [2] of how the interphase explains observed macroscopic properties of the nanocomposites abound the literature, e.g., the models of Lewis, Tsagaropoulos, and Tanaka. All theories assume NPs embedded in a polymer somehow form the nanoscopic interphase adjacent the NPs having properties that explain the macroscopic properties of the nanocomposite. However, no theory identifies the origin of the interphase itself.

Experiments are required to determine the mechanical properties of the interphase, but tensile tests are difficult to perform because of the size of the specimens. Instead, atomistic MD simulations are generally used to derive interphase properties. MD stands for molecular dynamics. MD simulates polymer conformation [3] at the NP surface and cross-linking [4, 5] by reactions of chemical radicals. However, the validity of the MD simulations is questionable as assumptions for the interphase are usually unverifiable, e.g., the source of radicals in initiating cross-linking and the correctness of force-fields lack experimental support. Except for illustrating how the MD simulation would be performed if the assumptions of the interphase were indeed valid, the interphase otherwise remains uncharacterized.

Purpose
Propose the NPs in a polymer emit EUV radiation that cross-links the polymer adjacent to the NP surface to form the interphase thereby enhancing the mechanical properties of nanocomposites. EUV stands for extreme ultraviolet. In the thumbnail, the EUV emission from the NP is shown (green) entering the larger interphase region.

Theory
The EUV radiation emitted from NPs is a consequence of QM that precludes the atoms in NPs from having the heat capacity to conserve absorbed EM energy by an increase in temperature. QM stands for quantum mechanics and EM for electromagnetic. Instead, QED induces the NPs under TIR confinement to conserve absorbed EM energy by the emission of EM radiation. QED stands for quantum electrodynamics and TIR for total internal reflection. See diverse QED Applications at http://www.nanoqed.org , 2009 - 2014.

Applications
In nanocomposites, the EUV emission in cross-linking polymers finds origin in the thermal kT energy the atoms usually acquire from the temperature of the macroscopic surroundings, but cannot because of QM. Here, k stands for Boltzmann’s constant and T for absolute temperature.

The UV wavelength λ is given by the TIR confinement of the NP, where λ = 2 nD, n and D are the refractive index and diameter of the NP. The EUV fluence F is, F = 3NkT/2A , where N is the number of atoms in the NP, N = (D/d)³; d is the atom diameter; and A is the NP surface area, A = pi D². For 10 nm carbon NPs having n = 1.5, the wavelength λ = 30 nm and Planck energy = 41 eV. At 300 K, the carbon atom having d = 0.134 nm gives the steady EUV fluence F = 0.82 mJ/cm². During nanocomposite processing at temperatures T < 500 K, the fluence F may exceed 2 mJ/cm².  See “The Interphase in Nanocomposites” at Ibid, 2014.

The EUV radiation emitted from NPs is not new, having been known for some time as a major source of DNA damage that if not repaired may lead to cancer. See http://www.prlog.org/10219673-nanoparticles-by-emitting-e...

Characterization of the Interphase
Characterization of the mechanical properties of the interphase is proposed based on uniaxial tensile tests of EUV irradiated polymer specimens given by the following procedure:

1. For a particular nanocomposite application, determine the wavelength of the EUV emission expected from the NPs based on their diameter and refractive index. EUV sources similar to [6] are capable of providing fluences F < 70 mJ/cm².

2. Fabricate polymer tensile specimens, say < 1 mm diameter wires or 3 micron thick flat geometries. Irradiate the specimens at fluence F < 2 mJ/cm² at ambient temperature for various duration times. Perform uniaxial tensile tests of the EUV irradiated specimens similar to [7]. Determine the stress-strain curve , i.e., Young’s modulus and yield strength.

3. For the experimental Young’s modulus, derive the elastic stresses adjacent the NP surface using FEA programs. FEA stands for finite element analysis, e.g., ANSYS and COMSOL. In FEA, the RVE is used [8] to derive the nanoscopic properties of the interphase that give the macroscopic response of the nanocomposite. RVE stands for representative volume element. But like MD, the RVE lacks meaning because theYoung's modulus of the interphase is not known.

4. Similar to the FEA with RVE, perform MD simulations of the EUV irradiated polymer specimen to determine force-fields that give the experimentally derived Young’s modulus.

Conclusions
The origin of the interphase in nanocomposites is polymer cross-linking  from EUV radiation emitted by NPs. External EUV sources are not required.

Characterization of the mechanical properties of the interphase proceeds by experiment. Polymer specimens are irradiated at the appropriate EUV wavelength at low fluence < 2 mJ/cm². Tensile tests determine the Young’s modulus of the interphase.

References
[1] F. Deng and K. Van Vliet, “Prediction of elastic properties for polymer–particle nanocomposites exhibiting an interphase,” Nanotechnology 22, 165703, 2011.
[2] D. Pitsa and M. G. Danikas, “Interfaces features in polymer nanocomposites: a review of proposed models, NANO: Brief Reports and Reviews, 6, 497–508, 2011.
[3] J. S. Meth and S. R. Lustig, “Polymer interphase structure near nanoscale inclusions: Comparison between random walk theory and experiment,” Polymer 51, 4259-66, 2010.
[4] B. Arab and A. Shokuhfar, “Molecular Dynamics Simulation of Cross-Linked Epoxy Polymers: the Effect of Force Field on the Estimation of Properties.” J. Nano and Electronic Physics, 5, 01013, 2013.
[5] M. Amkreutz, et al., “Relating Mechanical Properties of UV-Cured Coatings to the Molecular Network: A New Approach to Predict Crosslinking of Coatings,” RadTech Conference Proceedings 18-23, 2011.
[6] H. Fiedorowicz, et al., “Nanostructured polymers by a compact laser plasma EUV source,” Synthesis and Photonics of Nanoscale Materials VII, Proc. of SPIE 7586, 75860G, 2010.
[7] U. Lang, et al., “Fabrication of a tensile test for polymer micromechanics,” Microelectronic Engineering 83, 1182-4, 2006.
[8] ANSYS - Finite Element Multiscale Homogenization and Sequential Heterogenization of Composite Structures