Session S21: Focus Session: Polymer Nanocomposites I - Active Particles and Dynamics

Engineering polymer-fullerene thin films and solar cells with external fields

Trace amounts of nanoparticles, including fullerenes, can impart stability to thin polymer films against dewetting by the combined effects of pinning the contact lines of dewetting holes and by effectively altering the polymer-substrate interaction. Polymer nanocomposite (meta)stable thin films can yield well-defined morphologies from uniform to spinodal-like, via spontaneous polymer-nanoparticle phase separation and crystallization. Confinement breaks the structural isotropy and generally causes (partial) segregation of components orthogonally to the film surface. Surface energy patterning can thus modulate composition and morphology, both in plane and normal to the surface. Further, UV-visible, and even background, light exposure, in both solutions and melts, is shown to tune the solution stucture and morphology of dewetting and phase separating polymer-fullerene thin films. Neutron reflectivity allows us to locate the various constituents within the film. We find a coupling of fullerene photo-sensitivity and both self-assembly processes which results in controlled pattern formation, and we illustrate the potential with a model polymer-fullerene circuit pattern. We then translate this approach into the directed assembly of energy harvesting bulk heterojunctions thin films. Indeed, a key challenge to the commercialization of organic solar cells remains the achievement of morphological stability, particularly under thermal stress conditions. The directed assembly a blend polymer:PC$_{60}$BM solar cells via a simple light processing step results in a 10-100 fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerisation of PC$_{60}$BM that effectively hinders diffusion and crystallization in blends. This effect appears to be general and promises to be an effective and cost-effective strategy to optimize fullerene-based solar cell performance.   

Spatial temperature mapping in polymer nanocomposites due to ultrafast photothermal heating of gold nanorods

In pulsed laser irradiance, extremely high peak powers (low average powers) can be attained due to short bursts of energy. This property can be exploited for photothermal heating of polymers using gold nanorods in which the incident radiation can be efficiently converted into heat in short pulses. This leads to extreme localization of heat energy which does not affect the global polymer temperature significantly. In this work, we describe the effect of using pulsed laser to generate photothermal heat within polymer matrices doped with gold nanorods, and novel optical techniques to determine the corresponding temperature distribution. The rotation of the nanorods are studied to monitor the temperature of the polymer melt immediately surrounding the nanorods and the polarized fluorescence of probe molecules* are used to determine the temperatures of concentric volumes of polymer far away from the nanorods. The experimental techniques discussed provide simple tools to monitor the ensemble behavior of the nanorods and map the temperature distribution due to pulsed heating. The pulsed photothermal effect enables nanoscale thermal manipulations without altering the bulk temperature or morphology of the polymer. *S. Maity et al. Adv. Funct. Mater. 22, 5259 (2012)   

Photothermal heating and mechanical properties of Au/PEO and Ag/PEO nanocomposites

In the current study, the photothermal effect of gold (Au) and silver (Ag) nanoparticles in poly(ethylene oxide) is investigated. Both Au and Ag nanoparticles were synthesized in-house and were characterized by dynamic light scattering, UV-Visible spectroscopy and transmission electron microscopy experiments. The average size of the Au and Ag nanoparticles was found to be on average 8.9 and 8.4 nm, respectively. The Au/PEO and Ag/PEO nanocomposites containing 0.01--2{\%} nanoparticles (by weight) were prepared via solution mixing. Mechanical and thermo-mechanical properties were investigated by static and dynamic tests. The results indicate that the Young's modulus increases with increasing nanoparticle concentration, however, the modulus values reached a plateau at high concentrations. Both nanocomposites were heated via laser radiation at appropriate wavelengths and via traditional heating (using a heating stage). The temperature variations were measured through Raman spectroscopy experiments and by correlating Raman and traditional heating experiments.   

Annealing polymer nanofibrous nanocomposite mats via photothermal heating: effects on overall crystallinity, morphology, and 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 mediated photothermal effect of metal 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. Here we demonstrate this effect in polyethylene oxide-gold nanoparticle electrospun nanofibrous mats, which have been annealed at temperatures above the glass transition. A non-contact temperature measurement technique utilizing embedded fluorophores (perylene) has been used to monitor the average temperature within samples. The effect of annealing methods (conventional and photothermal) and annealing conditions (temperature and time) on the fiber morphology, overall crystallinity, and mechanical properties is discussed. In conclusion we demonstrate that the specificity of plasmonic heating coupled with the inside-outside approach of annealing presents a unique tool to improve crystallinity, and therefore mechanical properties, of the polymer mats while maintaining the unique nanofibrous morphologies.   

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the re-growth. Specifically, functionalized nanorods localize at the new interface and initiate Atom Transfer Radical Polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material.   

Translational and Rotational Motion of Nanocrystals in Rubber

We present the observation of translational and rotational dynamics of carbon-black nanocrystals in styrene-butadien rubber using coherent X-ray scattering. X-ray photon correlation spectroscopy (XPCS) exploits the partial coherence of X-rays to provide the information of microscopic dynamics. In diffracted X-ray tracking (DXT) measurement, the motion of diffraction spots from single nanocrystals is monitored to track their rotational motion. A combination of XPCS and DXT reveals the detailed translational and rotational motion of nanocrystals in a medium. Experimentally XPCS requires a monochromatic beam whereas DXT requires a wide energy range to increase the probability of diffraction spots being on the Ewald sphere shells. This experimental incompatibility can be overcome by using an intense pink beam X-ray that is available using a helical undulator at synchrotron facilities.   

Directed Assembly of Polymeric Films Filled with Gold Nanoparticles

Incorporation of nanoparticles (NPs) into polymer matrices has been explored extensively as an efficient way to fabricate novel functional materials, such as photonic bandgap materials, nanostructured solar cells, and high-density magnetic storage media. Towards that end, it is essential to disperse NPs in a well-controlled manner. We applied our unique dynamic thermal field processing method to gold nanopaticles with PS corona (AuNPs) into PS-b-PMMA cylinder forming block copolymer (c-BCP) thin films, and observed a sharp transition from vertical to horizontal cylinder orientation with AuNP loading fraction increasing. This transition is attributed to enrichment of AuNPs at the substrate side and favorable interaction of PMMA chains with gold cores. Furthermore, we investigated the dynamics of phase separation behavior of AuNP filled PMMA films as a function of time. It is intriguing that the homogeneous one-phase distribution of AuNPs transformed into a two-phase state upon thermal annealing accompanied with film surface undulating. Moreover, the phase separation phenomenon was effectively suppressed when confined with an elastomeric overlayer, thus leading to excellent dispersion.   

Dynamic gold nanoparticle, polymer-based composites

Artificial polymer-based biomembranes may serve as a foundational architecture for the integration and spatial organization of metal nanoparticles forming functional nanocomposites. Nonionic triblock copolymer (PEO-PPO-PEO), lipid-based gels, containing Au nanoparticles (NPs) can be prepared by either external doping of the preformed nanoparticles or by in-situ reduction of Au $^{3+}$. Neutron reflectivity on quartz supported thin films of the Au NP --doped polymer-based biomembranes was used to determine the location of the Au. The nanoparticles were found to preferentially reside within the ethylene oxide chains located at the interface of the bulk water channels and the amphiphile bilayers. The embedded Au nanoparticles can act as localized heat sinks, inducing changes in the polymer conformation. The collective, thermally-triggered expansion and contraction of the EO chains modulate the mesophase structure of the gels. Synchrotron X-ray scattering (SAXS) was used to monitor mesophase structure as a function of both temperature and photo-irradiation. These studies represent a first step towards designingexternally-responsive polymer-nanoparticle composites.   

Self-Assembly of Supramolecular Nanocomposite in Thin Film: A Kinetic Study

The comprehensive studies on the thermodynamics in block copolymer-based nanocomposites have paved the way for hierarchically structured materials with unique collective properties. We extend the investigation to the assembly kinetics to gain further control over the 3D spatial organization of nanoparticles (NPs) in thin films of supramolecular nanocomposites. Our studies reveal that, by simply controlling the solvent fraction (f$_{s}$) in the film during solvent annealing, the thermodynamic driving force for defect elimination, the chain mobility, and the activation energy for interdomain diffusion can be modulated to tailor 3D NP assemblies in thin films. At a low f$_{s}$, the anisotropy in the local diffusion coefficient results in NP arrays normal to the surface. As f$_{s}$ reaches an optimal value, the solvent and the small molecules effectively reduce the activation energy for interdomain diffusion and the T$_{g}$ of the supramolecule. This leads to rapid formation of highly ordered 3D NP arrays in seconds. The nanocomposite eventually undergoes an order-disorder transition as f$_{s}$ increases substantially. The fundamental insights gained from these studies lay the foundation for the rational design of functional nanocomposites with tunable macroscopic properties.   

Segmental dynamics and atomistic motions in PMMA/SWNT composites

The addition of single wall nanotubes (SWNT) to polymers has been repeatedly shown to have a significant impact on the macroscopic properties of the host polymer, including enhanced mechanical properties and shifts in the glass-transition temperatures, $T_{g} $. These properties usually result from collective structural and dynamical interactions of the polymer chains in the composite. Here, we investigate the effect of nanotubes on the polymer dynamics in PMMA composites with 10 wt{\%} SWNT. Neutron spin echo (NSE) and backscattering (BS) are used in probing local polymer dynamics in deuterated samples and atomistic hydrogen motions in hydrogenated samples, respectively. NSE data, collected at Q-values corresponding to inter-chain correlations and at $T > T_{g} $, indicate an order of magnitude increase in the chain relaxation time in the SWNT composite relative to pure PMMA. BS data support this observation and show suppressed atomistic motions in the composite in the same temperature range. A peculiarly opposite trend, however, is observed below $T_{g} $, indicating that the presence of SWNTs promotes polymer mobility in the glassy state, in contrast with previous reports on PMMA/C60 composites which exhibit suppressed mobility at all temperatures below and above $T_{g} $.   

Fast Electromechanical Response in Liquid Crystal Elastomer Nanocomposites

Liquid crystal elastomers (LCEs) combine the elasticity of polymer networks with the fluidity and responsiveness of liquid crystals. LCEs can respond to a variety of external stimuli -- heat, light, electric and magnetic fields -- with large and reversible shape-changes. However, the response can be slow and/or require large external fields. Here, we present our recent work with LCE bilayers and LCE composite materials that demonstrates LCEs can respond quickly and with 3-D shape changes. Nematic LCE bilayers are prepared by depositing a PS film on top of a nematic LCE, and the bilayers exhibit reversible wrinkling, folding, and curling with temperature. The shape change of LCE bilayers is quantitatively predicted using finite-element modeling. Next, we show that a fast response to an electric field is achieved in nematic LCE composites. While typical nematic LCEs are relatively unresponsive to electric fields, LCE composites with 2 wt {\%} carbon black can reversibly contract and expand in response to a 40 V electric field. The response time ( 0.1 -- 10 Hz) and amplitude of shape change (1 -- 20 {\%}) depends on the external field and carbon black content. These composites may be useful for biomedical applications, such as substrates for dynamic cell culture and biocompatible scaffolds for heart tissue regeneration. Neonatal rat ventricular myocytes remain viable on LCE-carbon black bilayer substrates, and aligned myocyte cell sheets were successfully grown on LCE-composite bilayers.   

Dynamics of responsive polypeptide composite particle suspended in a liquid crystal matrix and jamming

The emerging field of polypeptide composite particles, PCPs, has received an increased interest in the last years because of many opportunities open to a variety of applications. A PCP made of a silica core and a homopolypeptide shell resembles unique properties that cannot be achieved by the core or soft polymer shell alone. PCPs are responsive to external stimuli (e.g. light, electric and magnetic field) by incorporating fluorescent dyes and magnetic nuggets inside the silica core. Beside the responsive core, the polypeptide shell undergoes conformational transitions as a function of pH, temperature and solvent. Magnetic PCPs coated with a sparse polypeptide corona can be used as platforms to study colloidal self-assembly under jamming conditions. They are also reliable models to investigate the dynamics of complex fluids consisting of PCPs suspended in a liquid crystal matrix.   

Untangling colloidal caging in entangled PEG solutions

Using fluorescence microscopy, we record motion of colloids of size intermediate between correlation length of a polymer solution and size of a polymer molecule in entangled regime. The analysis of trajectory points of colloids show a transition from ``caged'' localization to diffusive randomization. The size and time spent in each individual cage is quantified using several statistical methods to give a distribution that is remarkably well-behaved and whose averages are consistent with values obtained from ensemble-average methods of trajectory analyses.