Session C32: Polymer Nanocomposites I

Mechanical Properties of Cross-linked Epoxy - Carbon Nanotube Nanocomposites: Effect of Interfacial Interactions and Nanoconfinement

The effect of fillers on polymer nanocomposites is conceptually analogous to the effect of nanoconfinement of polymers by solid substrates. This assertion is tested in our work by studying the volumetric, structural, dynamic, and mechanical properties of nanocomposites of cross-linked epoxy and carbon nanotubes (CNTs) using molecular simulations. We use atomistically detailed models in our simulations, which allow us to explicitly account for the specific chemical interactions between the filler and the matrix. Our results show that the poor interfacial interactions between the filler and the matrix lead to a tendency for depression in the glass transition temperature (T$_{g}$) of the nanocomposite compared to the neat cross-linked epoxy. Functionalization of CNTs is expected to strengthen interfacial interactions between the filler and the polymer matrix, and thus can have a strong impact on the properties of the nanocomposite. The relationship between the T$_{g}$, molecular dynamics, and the mechanical properties of the nanocomposites can be non-intuitive. Results will be presented for the relationship between mechanical properties, molecular dynamics, and the T$_{g}$ of nanocomposites of cross-linked epoxy containing both, pristine and functionalized CNTs.   

Effect of Grafting Density and Curvature of Nanoparticle on Mechanical Properties of Polymer Nanocomposite

Polymer nanocomposites (PNCs) are materials obtained by dispersing nanoparticles in a polymer matrix. Due to the large surface-to-volume ratio between the nanoparticles and the polymer, substantial enhancement in dynamic and mechanical properties can be observed for relatively low concentrations of particles. One common approach for ensuring dispersion of the nanoparticles is to end-graft polymers that are miscible in the host polymer matrix to the surface of the nanoparticles in the PNC. In many applications, understanding the role that the nanoparticles with grafted chains have on the resulting mechanical properties of the PNC will be of central importance in the final applications as well as the processing of the original sample. In this talk, I will first introduce the coarse grain model we used to study various mechanical properties of polymer and PNC. By designing a model system where the nanoparticles with different radii are remain dispersed whether they are grafted with polymer chains or not, we are able to isolate the role that chain grafting has on various aspects of the mechanical response of the PNC. We provide a detailed picture of how the elastic constants, yield stress, and the strain hardening behaviors depend on the grafting density and the size of nanoparticles   

Mechanical properties of homogeneous nanofiber composites fabricated by electrospinning

A new composite that possesses uniformly dispersed polymeric nanofibers in different polymeric matrix was introduced by using electrospinning. Recently, nanofibers have been actively investigated for fillers for polymeric nano-composites to enhance the mechanical properties of the composites or to get highly functionalize polymer materials. Polyvinyl alcohol (PVA) nanofibers were selected as polymeric fillers and polydimethylsiloxane (PDMS) was used for polymeric matrix. Internally well-dispersed composites were fabricated by this new method, whereas rather anisotropic composites were also made by the traditional sandwich method. The morphology of the composites was analyzed by field emission scanning electron microscopy (FE-SEM). It was found that, in the new internally well-dispersed composites, PVA nanofibers existed from the both surfaces of the polymer matrix, uniformly dispersed in the composite. Isotropic mechanical properties were observed for internally well-dispersed composites, whereas relatively anisotropic characteristics could be observed for the traditionally-made composites.   

Nanoparticle synergies in modifying thermal conductivity for heat exchanger in condensing boilers

The heat exchanger we are using for condensing boilers is mainly made from aluminum alloys and stainless steel. However, the metal is relatively expensive and corrosion together with maintenance is also a big problem. Therefore, we have developed a new design and material which contain carbon black, carbon nanotube, aluminum oxide and graphene as additives in polypropylene. When multiple types of particles can be melt blended simultaneously and synergies can be achieved, imparting particles to the nanocomposite, achieved much higher thermal conductivity rather than single additive. Here we show the flame retardant nanocomposite which can pass the UL-94-V0 vertical burning test, perform nice in Cone Calorimetry Test and has relatively good mechanical properties. SEM images of the blend show that the Carbon nanobute and other additives well dispersed within the polymer matrix which match our computational calculation for getting the percolation to achieve thermal conductivity around 1.5W/m$\cdot$K rather than 0.23W/m$\cdot$K as pure polypropylene.   

Molecular Dynamics Simulations on the Mechanical Properties of Blend of Polymer and Polymer Grafted Nanoparticles

Grafting polymers onto the surface of NPs has become one of the most effective approaches to integrate NPs into polymer melts. It then becomes crucial to be able to understand the mechanical properties of the resulting composites. Using molecular dynamics simulations we investigated how the presence of grafted NPs changes mechanical responses of an entangled polymer matrix below its glass transition temperature under uniaxial tensile deformation. It is found that comparing to neat polymer melts adding fillers slightly increases the elastic modulus as well as the yielding stress of the composites. But changes in the fracture work (total work needed to pull samples to failure) are every dependent on the loading fraction and dispersion state of NPs in polymer matrix. At small loadings fracture work is found to be significantly reduced due to polymer crazing around NPs, which is induced by local triaxial stresses, while at higher loadings a big degree of enhancement is observed because of network forming among grafted NPs.   

Revealed nano-architecture and dynamics of bound polymer layers on nanofillers

It is known that the physical properties of adsorbed polymers on solids are often different from those of bulks. However, the mechanism associated with the structure and dynamics at the polymer/solid interfaces still remains unsolved, primarily due to the lack of suitable experimental tools. Recently, we used small-angle neutron scattering and neutron spin-echo spectroscopy which allow us to highlight adsorbed polymers on nanofillers with deuterated labeling. The system used was polybutadiene (PB) adsorbed on carbon black (CB) fillers in toluene. The CB (80 nm in diameter) was compounded into PB by using a Banbury mixer. The CB/PB compound was then dissolved in toluene, until the weight of the compound remained unchanged. To label the resultant un-dissolved PB layer on CB (i.e., about 3 nm in thickness based on TEM analysis) for the neutron scattering experiments, deuterated toluene, which has the nearly same scattering length density as that of CB, was used. We will highlight the unique structure and dynamics of the bound PB layer by comparing with a PB brush grafted on CB and further discuss geometric effects of solids (curvature or flat) on the nano-architectures at the polymer/solid interfaces.   

Studying the effect of the curvature of a polymer-grafted nanoparticle surface on equilibrium brush dimensions via small-angle neutron scattering (SANS) and polymer field theory

For polymer-grafted nanoparticles with a size that is on the order of the radius of gyration of the polymer chains, the curvature of the nanoparticle surface has a role in determining the equilibrium brush structure. The curvature can cause an increase in the conformational entropy of the brush chains relative to that of a flat surface, and hence has large implications for dispersion of nanoparticles within a polymer matrix. Here, small-angle neutron scattering (SANS) is performed to measure the radius of gyration of a poly(ethylene glycol) (PEG) brush that is grafted to the surface of gold nanorods (diameter x length: 10 x 30 nm) and nanospheres (diameter: 10 nm) in both solution and a within a polymer melt. To help interpret the SANS measurements, field theoretic simulations are employed to calculate density profiles for the brush polymer in solution and a polymer melt as a function of nanoparticle shape, radius, and brush grafting density .   

An interface controlled dynamic stiffening in polymer nanocomposites

Tunable interfaces between inorganic and organic phases determine the mechanical behavior of responsive and adaptive composites. We present that bonding/debonding of chains on nanoparticles can be modulated with extensive periodic strains. Mechanical response of an attractive model polymer composite, poly(methyl methacrylate) filled with silica nanoparticles of sizes 13 nm and 56 nm, is monitored in series of deformation-resting experiments allowing us to tune the interfacial strength of polymer. We show that this deformation process exhibit unusual stiffening of composites as the matrix polymer is bound to the surface stronger on removal of strain. Mechanical response during the recovery together with SANS and FTIR analysis of the composites at different states of deformation reveal that this behavior arises from enhancement in the entanglement of chains at interfaces. We studied the effects of strain amplitude, confinement parameter (ID/2Rg) and resting time and found that the stiffening is manifest only after large strains. This behavior offers an `on demand' reinforcement properties to polymer nanocomposites, implying that the composites with attractive interfaces can self-stiffen as needed.   

Thermoplastic Elastomers via polyolefin/Layered Silicate Nanocomposites

Here we report the synthesis of fully exfoliated polyolefin nanocomposites via Surface-Initiated Ring Opening Metathesis Polymerization (SI-ROMP). Montmorillonite (MMT) clay platelets were rendered hydrophobic through ion exchange with alkyl-ammonium surfactants terminated with norbornene. We were then able to form block copolymer brushes of (substituted) norbornenes and cyclopentene via SI-ROMP. Subsequent hydrogenation yielded highly crystalline polyethylene and rubbery saturated polynorbornenes, thus giving a thermoplastic elastomer. Nanocomposites were prepared with different nanofiller percentages and were characterized for morphological (XRD, TEM), thermal (TGA, DSC), and mechanical (DMA, Rheology) properties. Complete exfoliation of nanocomposites was confirmed by XRD and TEM. A fraction of the polymer brushes were subsequently removed from their substrate by reverse ion exchange and characterized in parallel with their corresponding nanocomposite analogs. In this way we were able to directly assess the role of the filler particle in the thermal properties, melt rheology, morphology, and tensile properties.   

Bound layer in polymer nanocomposites: nanoparticle size dependence and solvent effect

An interfacial polymer layer, or bound layer, has long been of interest in polymer nanocomposites (PNCs) since the divergent properties of the bound layer as compared to the bulk can have very important effects on PNCs properties. We study a system comprised of silica nanoparticles (NPs) in poly-2-vinylpyridine (P2VP) prepared by the solvent casting method. First, we determine by TGA the bound layer thickness $\delta $ in PNCs and show that $i) \delta $ decreases as NP radius decreases due to a decrease of polymer adsorbed amount and \textit{ii)} $\delta $scales as N$^{1/2}$, where N is the chain length, independent of NP size. This result qualitatively follows the theoretical prediction in term of loops distribution developed by Guiselin et al. for a flat surface. Here we note that the bound layer thickness is obtained by assuming that the polymer chains have a density corresponding to a dense melt -- this is clearly not a well-founded argument, and hence we measure a more realistic extent of the bound layer by using Dynamic Light Scattering in solution. Then, we investigate the influence of the solvent used to prepare the PNCs (in our case methylethylketone (MEK) or pyridine) on the bound layer and the final silica dispersion in PNCs. We show that pyridine prevents the P2VP adsorption leading to poor silica dispersion in the final PNCs while in MEK the good dispersion is related to the formation of a bound layer.   

Non-isothermal melt crystallization behavior of Poly(ethylene terephthalate)/graphene nanocomposites

Poly(ethylene terephthalate)(PET)/graphene nanocomposites were prepared by melt mixing with a goal of reduced gas permeability. With 2 wt{\%} of few layered graphene, PET/graphene composite films show more than 70{\%} decrease in N$_2$ gas permeation. Their non-isothermal crystallization were also investigated by differential scanning calorimetory (DSC). Crystallization temperature, Tc, of PET/graphene nanocomposites was more than 8 $^{\circ}$C higher than neat PET and the increment increased along with the concentration of graphene. This suggests that the nucleation effect of graphene enhanced with the increase in concentration of graphene. On the other hand, PET/graphene nanocomposites show shorter half crystallization time, t$_{1/2}$, than neat PET at lower concentrations, but t$_{1/2}$ increased along with concentration of graphene. From Raman spectroscopy, it was shown that PET chains in nanocomposites are strongly confined in the presence of an excess of graphene. Restricted mobility of PET chains slowed crystallization.   

Annealing polymer nanocomposite fibers and films with photothermal heating: effects on overall crystallinity and resultant mechanical properties

Metal nanoparticles embedded within polymeric systems can be made to act as localized heat sources thereby aiding in-situ polymer processing. This is made possible by the surface plasmon resonance (SPR) mediated photothermal effect of gold nanoparticles, wherein incident light absorbed by the nanoparticle generates a non-equilibrium electron distribution which subsequently transfers this energy into the surrounding medium, resulting in a temperature increase in the immediate region around the particle. The current research demonstrates this effect in polymer nanocomposite systems, electrospun nanofiber mats and thin films, which have been annealed at temperatures above the glass transition and below melting. A non-contact temperature measurement technique utilizing embedded fluorophores has been used to monitor the average temperature within samples. The effect of annealing methods (conventional and plasmonic), annealing conditions (temperature and duration) and cooling mechanisms on the morphology, crystallinity, and mechanical properties of polymeric nanocomposite systems will be discussed. The specificity of plasmonic heating coupled with the inside-outside approach of annealing presents a unique tool to thermally process polymers.   

DSC study of the isothermal crystallization of iPP-CNF nanocomposites

Nanocomposite materials have been obtained by dispersing vapor grown carbon nanofibers (VGCNFs) with diameters ranging between 60 and 100 nm and lengths between 30,000 and 100,000 nm supplied by Pyrograf Products, Inc (PR-24AG) within a polymer matrix - isotactic polypropylene (iPP) - type Marlex HLN-120-01 with density 0.906 g/cm$^{3}$ and melt flow rate at 230 $^{o}$C of 12 g/10 min, supplied by Philips Sumika Polypropylene Company. VGCNFs have been purified and disentangled by reflux in dichloromethane and deionized water followed by vacuum filtering (for 24 h) and drying at 110 $^{o}$C for 24h. The nanocomposites were obtained by melt mixing at 180 $^{o}$C for 9 minutes with a speed of 65 rpm followed by an additional mixing at 90 rpm for 5 minutes, using a HAAKE Rheomix, Nanocomposites loaded with various amounts of VGCNFs (0{\%}, 1{\%}, 2.5{\%}, 5{\%}, 7.5{\%}, 10{\%}, 15{\%}, and 20{\%} wt.) have been prepared and investigated by TA DSC Q-500. Isothermal crystallization was investigated in detail and analyzed by using an expression derived from the Avrami equation. The effect of the filler on the isothermal crystallization of iPP is discussed in detail. The research is focused on the effect of VGCNF on the degree of crystallization of iPP, crystallization rate, and dimensionality of the crystallization process.   

The Impact of Fullerenes on the Ordering of Polyacrylonitrile in Nanocomposites

The presence of nanoparticles can impact the crystallization and ordering of polymer chains in a nanocomposite. We have found that certain fabrication conditions of polyacrylonitrile (PAN) and fullerenes, as well as PAN and SWNTs, produce a SAXS peak. This SAXS peak is similar to a microphase separation peak and indicates a self-assembly of the nanocomposite on a length scale of $\sim$ 150 {\AA}. In order to identify the origin of this peak, we have completed dynamic light scattering, viscosity, small angle x-ray scattering and wide-angle x-ray scattering experiments to characterize the dispersion of C60 in solution and in the final solid nanocomposite. These results support a completely miscible solution. The interpretation of the SAXS \& WAXS results suggest that the addition of C60 directs the crystallization of PAN in the final nanocomposite by dramatically decreasing the amount of crystallinity while also affecting the packing structure and limiting the size of the PAN crystals.   

Polymer Lamellar Crystals Containing Precise Half-Folds Confined by Nano-Particles of Giant Molecular Shape Amphiphiles

Chain-folded polymer lamellar crystals with precise half-folds were first time experimentally observed in two specifically designed giant molecular shape amphiphiles. These molecules were synthesized via Click Chemistry to construct a nano-particle such as polyhedral oligomeric silsesquioxane (POSS) or [60]fullerene (C$_{60})$ tethered a poly(ethylene oxide) (PEO) chain as a tail. When these PEO tails crystallized, both integral folded crystals and crystals with the half-folds were identified experimentally. This is due to the fact that during the PEO tail crystallization, the nano-particles at ends of PEO chains build up double layered structures to impose the PEO chains to create specific number of stems in their crystals which must balance the cross-sections of the nano-particles and the stems.