Session Y32: Polymer Nanocomposites III

Layered polymer nanocomposite films of type-specific single wall carbon nanotubes

Thin networks of single-wall carbon nanotubes (SWCNTs) on elastic polymer substrates show significant promise for applications in flexible electronics, but the modulus and conductivity of such films can degrade significantly under an applied strain. This softening occurs because strong van der Waals interactions between adjoining nanotubes promote coarsening into a preferred parallel alignment under even modest compression. We demonstrate that by capping the nanotube layer with a thin glassy polymer film, the mechanical properties of networks can be substantially improved, which we attribute to the stabilizing influence of excluded-volume interactions.   

Influence of Thermal History on Microphase Separation and Morphology of Elastomeric Polyureas

Polyureas are versatile elastomers consisting of alternating soft and hard segments. These polymers tend to form a nanophase-segregated morphology consisting of high aspect ratio hard domain ribbons in a low Tg matrix, the details of which are key in tailoring the unique characteristics of this family of materials. In the present work, bulk-polymerized polyureas were synthesized from a modified diphenylmethane diisocyanate and a polytetramethyleneoxide based diamine (1000 g/mol) and annealed at selected elevated temperatures. Various experimental probes (e.g. atomic force microscopy and small-angle X-ray scattering) reveal significant changes in hard domain ordering as a function of thermal history. Time-resolved synchrotron X-ray scattering was also conducted as a function of temperature to augment these findings.   

Optical characterization of isotactic polypropylene and carbon nanotube composites using spectroscopic ellipsometry

We report the dielectric properties of optically characterized isotactic polypropylene (iPP) and its composites with carbon nanotubes (CNTs) using spectroscopic ellipsometry. Characterization was performed at angles ranging from 50 to 70 degrees and for the spectral range between 300-1000 nm. CNT concentrations varied from 0 to 5 wt\% in the iPP/CNT composites investigated. Ellipsometry is a non-invasive and non-destructive technique that enabled us to determine the dielectric properties of the materials investigated. A concentration dependency on CNT wt\% was found to exist for both the refractive index and the extinction coefficient for the iPP/CNT composites. At higher concentrations however, this distinction was not very clear, suggesting that saturation levels were reached in the material. We will also discuss our efforts to separate the optical properties of bound CNT from the analyzed nanocomposites.   

Temperature dependent photoluminescence from polymer nanocomposites of size-puified silicon quantum dots

The photoluminescence (PL) of polydimethylsiloxane (PDMS) nanocomposites of size-purified silicon nanocrystals is measured as a function of temperature and nanoparticle size. The overall behavior is in agreement with the trends imposed by quantum confinement, where the temperature dependence of the nanocrystal bandgap is governed primarily by intrinsic electron-phonon coupling. The response of the PDMS nanocomposites provides a consistent measure of local temperature through intensity and lifetime in a polymer-dispersed morphology suitable for biomedical applications, and we exploit this to fabricate a small-footprint fiber-optic cryothermometer.   

Non-Bleaching Photoluminescent Magnetic Nanoparticles

We report a new type of photoluminescent magnetic nanoparticles produced by a very simple process. The nanoparticle consists of an ordinary magnetic nanoparticle as core and a non-toxic polymer shell. The biocompatibility is evaluated using in-vivo tests on mice. They are non-bleaching photoluminescent without any addition of fluorophores, such as quantum dots or fluorescent dyes that can be toxic and easily photobleached, respectively. This work provides a low-cost, bio-safe, non-bleaching alternative of conventional fluoroscent magnetic nanoparticles which covers a wide range of applications, from bio-imaging to biomedical diagnostics and therapeutics, such as hyperthermia.   

Electrically Percolating Clusters in Sheared Carbon Nanotube Composites

The electrical conductivity of polymer nanotube composites can be dramatically modified by processing flows and subsequent annealing. The mechanism is widely believed to be nanotube structural rearrangements that occur during flow and alter the percolating pathways. We seek to directly visualize these flow-induced three-dimensional percolating clusters through three-dimensional confocal microscopy and image analysis.   

Electrical properties of isotactic polypropylene loaded with carbon nanofibers

Nanocomposites have been obtained by dispersing vapor grown carbon nanofibers (VGCNF) within isotactic polypropylene (iPP) via melt mixing. VGCNFs were purified and disentangled before blending with iPP. The mixing was performed by using HAAKE Rheomix, at 180 $^{o}$C and 65 rpm for 9 minutes followed by an additional mixing at 90 rpm for 5 minutes (same temperature). The electrical properties of nanocomposites loaded with various amounts of VGCNFs (0{\%}, 1{\%}, 2.5{\%}, 5{\%}, 7.5{\%}, 10{\%}, 15{\%}, and 20{\%} wt.) have been investigated. DC electrical measurements revealed a percolation threshold at about 12 {\%} wt. VGCNFs. The DC electrical characteristics of the nanocomposites located above the percolation threshold were investigated in detail, in a wide temperature range starting from 20 K up to about 750 K. The investigations revealed small changes of the DC conductivity within the glass and melting transition range of the polymeric matrix. The dominant charge transport mechanism below the glass transition temperature as well as between the glass and melting transition temperature is the variable range hopping. Above the melting temperature an Arrhenius like dependence of the DC conductivity was noticed.   

Polyaniline-SnO$_2$ Nanocomposites for Better Sensitivity of NO$_{\mathrm{X}}$ gases at Lower Temperatures

We demonstrate that the sensor based on Polyaniline (PAni) nanofibers, simply prepared by the interfacial polymerization, has advantages of sensitivity, spatial resolution and rapid time response for NO$_2$ gas at room temperature. Although PAni is one of the most studied conducting polymers due of its good electrical conductivity, environmental stability and relative easier synthesis, yet due to poor solubility of PAni, it is difficult to form the film adopting conventional methods. Nonetheless, nanomaterials of conjugated polymers are found to exhibit superior performance as compared to conventional materials due to their larger exposed surface area. The objective of this work is to study the PAni doped SnO$_2$ nanocomposite as novel sensing system and to probe the NOx sensing characteristics of this sensor at room temperature. Here we focus on the effect of doping ratio of sensor material, gas flow time and response time. PAni with different amounts has been stirred with SnO$_2$ solution to obtain SnO$_2$/PAni mixture. In present work, sensors with different PAni doping ratio were prepared and characterized so as to ascertain the favorable conditions for higher sensitivity, selectivity and better gas sensing characteristics. The as-grown films characterized employing various techniques and revealed that PAni/SnO$_2$ nanocomposite show good gas sensitivity at 30-100 $^{\circ}$C.   

Self-healing of polymeric materials: The effect of the amount of DCPD confined within microcapsules

The self-healing SH) of polymers is based on the dispersion of a catalyst and of microcapsules filled with monomer within the polymeric matrix. Sufficiently large external stresses will rupture the microcapsule, releasing the monomer which will diffuse through the polymer and eventually will reach a catalyst particle igniting a polymerization reaction. The classical SH system includes first generation Grubbs catalyst and poly-urea formaldehyde microcapsules filled with DCPD. The polymerization reaction is a ring-opening metathesis. The size and the mechanical features of microcapsules are critical in controlling the SH process. Research was focused on the effect of DCPD on the size and thickness of microcapsules. Microscopy was used to determine the size of microcapsules (typically in the range of 10$^{-4}$ m) and the thickness of the microcapsules (ranging between 10$^{-6}$ to 10$^{-8}$ m). Research revealed a thick disordered layer over a thin and more compact wall. Raman spectroscopy confirmed the confinement of DCPD, TGA measurements aimed to a better understanding of the degradation processes in inert atmosphere, and mechanical tests supported the ignition of self-healing properties.   

Engineering Flame Retardant Biodegradable Nanocomposites

Cellulose-based PLA/PBAT polymer blends can potentially be a promising class of biodegradable nanocomposites. Adding cellulose fiber reinforcement can improve mechanical properties of biodegradable plastics, but homogeneously dispersing hydrophilic cellulose in the hydrophobic polymer matrix poses a significant challenge. We here show that resorcinol diphenyl phosphates (RDP) can be used to modify the surface energy, not only reducing phase separation between two polymer kinds but also allowing the cellulose particles and the Halloysite clay to be easily dispersed within polymer matrices to achieve synergy effect using melt blending. Here in this study we describe the use of cellulose fiber and Halloysite clay, coated with RDP surfactant, in producing the flame retardant polymer blends of PBAT(Ecoflex) and PLA which can pass the stringent UL-94 V0 test. We also utilized FTIR, SEM and AFM nanoindentation to elucidate the role RDP plays in improving the compatibility of biodegradable polymers, and to determine structure property of chars that resulted in composites that could have optimized mechanical and thermal properties.   

Designing high hard block Content TPU resins for composite application

Thermoplastic Polyurethanes (TPU) are linear block copolymers typically constructed of statistically alternating soft (SS) and hard (HS) segments. Due to their numerous industrial applications these materials have received considerable attention. We have recently investigated the phase behavior and morphology of a set of high hard block content polyurethanes. Using mainly calorimetry, scattering and microscopy techniques we were able to elucidate the origins of all the thermal events observed through differential scanning calorimetry and propose a new morphological model of the structure and the phase behavior of these high hard block content polyurethanes [A. Saiani et al. Macromolecules, 34, 9059-9068 (2001); 37, 1411-1421 (2004); 40, 7252-7262 (2007)]. We have now shown that these new materials can potentially be used as resins for designing fiber based composites and investigated the effect of processing on conditions the final properties of the composites   

Structure and Dynamics Characterization of HMDI- and MDI-based Poly(urethane urea) Elastomers via Solid- State NMR

High performance elastomers have recently gained considerable interest throughout DoD, particularly for their potential in ballistic impact protection and blast mitigation capabilities. Recent simulation results based on coarse-grained modeling have revealed the role of the intermolecular interaction and the flexibility of interface between hard and soft segments on the morphology and mechanical deformation behavior of poly(urethane urea), PUU, elastomers. In this work, we exploit solid-state nuclear magnetic resonance (NMR) techniques to investigate the influence of hard domain size on molecular dynamics by comparing the diisocyanate chemistry (aliphatic 4,4'-dicyclohexylmethane diisocyanate (HMDI) vs. aromatic 4,4'-diphenylmethane diisocyanate (MDI)) in PUU elastomers. Despite identical stoichiometry and soft segment chemical structure, large difference in the molecular dynamics, indicated by the $^{\mathrm{1}}$H dipolar dephasing time (T$_{\mathrm{d}})$, is observed. The T$_{\mathrm{d}}$ of HMDI-PUU is shorter and it exhibits higher activation energy, suggesting finer phase mixing. Results from $^{\mathrm{1}}$H spin echo measurements are also included for comparison.   

Thermal Boundary Resistance Across Solid-Fluid Interface

The recent advances in the field of nanotechnology, specially the advent of nanostractures and nanocomposite materials, have prompted an increased interest in the study of thermal transport across interfaces. When heat flows across an interface, the local temperature presents a discontinuity which is related to the thermal boundary resistance (TBR), also known as the Kapitza resistance. The investigation of Kapitza resistance has important technological applications in the improvement of the thermal performances of composite materials. The current theoretical understanding of TBR is primarily based on the ``acoustic mismatch theory'' or the ``diffusive mismatch model.'' Both these models consider only the bulk properties of the two materials, with no account being taken of the details of the material properties near the interface. Here, we investigate the thermal transport across a model solid-fluid interface using the technique of reverse non-equilibrium molecular dynamics simulations. The interaction potentials between the particles in our system are governed by the Lennard-Jones potential. We study the influence of pressure on the thermal boundary resistance for a range of mismatched interfaces and compare our results to the existing analytical models.