Session J4: Polymer-based Composite Materials

Tuesday, March 6, 2007

Significant advances have recently been made in the synthesis of nano-objects with well-defined functions. Various size and shape of nano-objects are now readily available. In order to find useful applications those nano-objects are often mixed with polymers. We investigated the effect of hard additives, i.e., \textit{interacting} magnetic nanoparticles (NPs), on the ordered morphology of block copolymers by varying NP concentration. In order to characterize the structural change of block copolymer associated with different NP loadings, small-angle X-ray scattering and transmission electron microscopy were employed. With the increase in NP concentration, domains of NP aggregates were observed. It is surprising to note that regular lattice-like aggregates with $\gamma $-Fe$_{2}$O$_{3}$ NPs induce an intriguing morphological transformation from the hexagonal cylinders to the body-centered cubic spheres via undulated cylinders of block copolymers, which does not show such morphological transition without NPs. These results are compared with the case where the interaction among NPs is relatively weak. In addition, we studied the effect of casting solvents and sample preparation conditions to confine such NPs in one of microphase separated domains. These results could add more flexibility in structural control and orientation of block templates in thin films opening up new applications.   

Tuesday, March 6, 2007

The structure and dynamics of polymer-tethered silica nanocomposites are examined here. Local dynamics of the hybrids suggest a significant increase in the glass transition temperature of the polymer chains, compared to the free polymer for the case of poly(butyl acrylate) systems. Mesoscale dynamics probed under quiescent conditions indicate a solid-like response for the nanocomposite, and this was seen to persist even upon dilution of the end-grafted chains with free PBA of approximately the same molecular weight. For the end-tethered hybrid, an ordered arrangement of the nanoparticles is observed using small angle x-ray scattering and transmission electron microscopy. Dilution results in a homogenous system of the hybrid with the free chains, and the resulting scaling of the correlation between hybrid domains suggest a fully penetrated brush system. In addition, linear viscoelastic characteristics of the blends were found to exhibit strong dependence on the hybrid volume fraction.   

Tuesday, March 6, 2007

In our studies of nanoparticles blended with linear polymer melts two unusual phenomena were noted; the viscosity was reduced upon nanoparticle addition and the nanoparticles remained dispersed despite the interparticle gap being smaller than the polymer radius of gyration. The viscosity decrease is not easily explained and it appears as if it is related to introduction of free volume created by the vast surface area generated by the nanoparticles and elimination of entanglements via constraint release generated by the fast diffusing nanoparticles. Further study is required to fully understand this phenomenon. This leads to the key point, unless nanoparticles are well dispersed in polymer melts then one would not expect any unusual phenomena to exist. Furthermore, one would expect the equivalent of depletion flocculation to occur at moderate volume fraction since the interparticle gap becomes so small in nano-systems and this simply does not occur. We suggest the dispersion driving force is due to an enthalpy gain the nanoparticles experience. Consider the pure nanoparticle phase, the van der Waals forces effectively propagate over a distance of order $\delta $, however, the interstices between the nanoparticles could be larger than this resulting in a reduced cohesive energy. Thus, when a nanoparticle is dispersed in a polymer melt it gains molecular contacts or enthalpy. This is a true nanoscale phenomenon since if the nanoparticles are too small then the interstices are quite small and the driving force for dispersion is reduced while if they are too large then there are not enough of them per unit volume to cause a significant enthalpy gain for a given volume fraction. An optimum exists at a radius of approximately 3-5 nm. This phenomenon and others will be discussed in the seminar.   

Tuesday, March 6, 2007

Microscopic liquid state theory has been employed to study the potential of mean force (PMF), statistical structure, and phase separation of spherical nanoparticles in a dense polymer melt over a wide range of interfacial chemistry, chain length, and filler size and volume fraction conditions. As interfacial cohesion strength increases the nanoparticle organization evolves from contact depletion aggregation, to well dispersed behavior associated with a thermodynamically stable polymer coating, to polymer-mediated bridging of a variable degree of tightness. Near linear scaling of the PMF with the particle/monomer diameter ratio is found, and the spatial range of the interfacial attraction is important in determining nanoparticle organization. Spinodal demixing calculations predict an entropy-driven fluid-fluid phase separation for weak interfacial attractions, and an enthalpically driven network or complex formation type of phase separation in the strong cohesion regime. A miscibility window exists at intermediate interfacial attraction strengths which systematically narrows, and is ultimately destroyed, as particle size and/or direct filler-filler van der Waals attractions increase. The length-scale dependent real space statistical structure is quantified via calculations of the polymer and filler intermolecular pair correlation functions and partial scattering structure factors. At high filler volume fractions interference between the polymer organization near nanoparticle surfaces induces significant changes of filler packing. The presence of bound polymer layers in miscible nanocomposites results in microphase- separation-like features in the small angle collective polymer structure factor. Implications of the theoretical results for the design of thermodynamically and/or kinetically well-dispersed polymer nanocomposites, and the formation of nonequilibrium networks, will be discussed. The theory has also been generalized to treat the consequences of soft intermolecular repulsions, and nonspherical fillers including rod, disk and compact molecule-like shapes.   

Tuesday, March 6, 2007

Polymers brushes grafted to the nanoparticle surface enable the precise positioning of particles within a block copolymer matrix by determining the compatibility of nanoparticles within a polymeric matrix and modifying the interfacial properties between polymers and inorganic nanoparticle. Short thiol terminated polystyrene (PS-SH), poly(2-vinylpyridine) (P2VP-SH) and PS-$r$-P2VP with the molecular weight (M$_{n})$ of 3 kg/mol were used to control the location of Au nanoparticles over PS-b-P2VP diblock copolymer template. We will discuss further the approach of varying the areal chain density ($\Sigma )$ of PS-SH brushes on the PS coated particles, which utilizes the preferential wetting of one block of a copolymer (P2VP) on the Au substrate. Such favorable interaction provides the strong binding of Au particles to the PS/P2VP interface as $\Sigma $ of PS chains on the Au particle decreases. We find that at $\Sigma $ above a certain value, the nanoparticles are segregated to the center of the PS domains while below this value they are segregated to the interface. The transition $\Sigma $ for PS-SH chains (M$_{n}$ = 3.4 kg/mol) is 1.3 chains/nm$^{2}$ but unexpectedly scales as M$_{n}^{-0.55}$ as M$_{n}$ is varied from 1.5 to 13 kg/mol. In addition, we will discuss changes in block copolymer morphology that occur as the nanoparticle volume fraction (\textit{$\phi $}) is increased for nanoparticles that segregate to the domain center as well as those that segregate to the interface, the latter behaving as nanoparticle surfactants. Small \textit{$\phi $} of such surfactants added to lamellar diblock copolymers lead initially to a decrease in lamellar thickness, a consequence of decreasing interfacial tension, up to a critical value of \textit{$\phi $} beyond which the block copolymer adopts a bicontinuous morphology. I thank my collaborators G. H. Fredrickson, J. Bang, C. J. Hawker, and E. J. Kramer as well as funding by the MRL as UCSB from the NSF-MRSEC-Program Award DMR05-20418.