NANOPARTICLES *
CHAPTER 5 CHAPTER 5
5.2 SYNTHESIS AND CHARACTERISTICS
5.2.3 POLYMER NANOPARTICLES FOR SEMICONDUCTORS
105 5.2 SyNThESIS ANd ChARACTERISTICS
106 CHAPTER 5 NANOPARTICLES
(bottom–up) approach would provide a methodology compared to the top–down lithographic method.
Moreover, the bottom–up approach would provide access to structures smaller and with higher 3D control than lithographic techniques such as EBL (Skaff et al., 2008).
5.2.3.1 Polymer nanoparticle blends
Mixing polymer materials and nanoparticles is the simplest method of composite formation, where it is accompanied by several aggregation problems. Moreover, the disparate nature of polymers and nano- particles generally precludes homogenous particle dispersion such as the entropic gain associated with random dispersion is overwhelmed by enthalpy issues. Moreover, phase separation would destroy the nanoscopic integrity of nanoparticle, which would lead to a reduction in properties for the polymers.
Many methods were considered to overcome and control the phase separation that occurs during the mixing processes (Skaff et al., 2008).
“Alivisatos and coworkers explored mixtures of CdSe nanoparticles and Poly-3-hexylthiophene (P3HT). A p-type conducting polymer for photovoltaic applications.” P3HT and CdSe are dissolved in a mixture of chloroform and pyridine, with good solvents for the polymer and nanoparticle. Composite films would be obtained by spin casting these solutions to afford CdSe-based composites with particles contacts that are surmised to function as a percolating network for charge transport. Photovoltaic cells would gener- ate from these materials an external quantum efficiency of 54% and a monochromatic power conversion efficiency of 6.9%. Also, efficiencies are lower than those of pure inorganic solar cells (Skaff et al., 2008).
The polymer structure would be improved to better match the structure of conventional and aliphatic chain covered CdSe nanoparticles. Bawendi and coworkers blended poly(laurylmethacrylate) with (CdSe)ZnS core shell nanocrystals encapsulated with TOP. The aliphatic nature of the lauryl dode- cylester side chains of this polymer serve to solubilize the nanoparticles by interaction is manifest in the optical clarity of the composites obtained upon blending, which suggests the likelihood of unaggregated nanocrystals in the composite material. Solid state quantum yields of these composites ranged from 22% to 44%.
Skaff et al., 2008.
Another set of experiments were done by Schrock and coworkers. Functional polymers are prepared with pendant coordination sites capable of nanocrystal passivation.
Monomeric norbornene derivatives are prepared with pendant phosphine oxides, where they are copoly- merized with methltetracyclododecane through ring opening metathesis polymerization (ROMP) using molybdenum alkylidene catalyst to from a diblock copolymer with pendant coordination sites. This pol- ymer is then dissolved in THF with CdSe nanocrystals to give optically clear solutions as the polymer coordinates to the nanocrystals. When this self assembly is performed in the presence of nanocrystals, the resulting TEM images show complete segregation of the nanocrystals into the coordinating block.
Skaff et al., 2008.
5.2.3.2 Clay–nanoparticle polymer composites
The use of clay that comes from hydrated silicates of aluminum in combination with polymer mate- rials would provide significant advantages in the physical properties compared to polymers alone.
Other advances in composites in terms of melt processing would allow these materials to be prepared in the absence of any organic solvent (Skaff et al., 2008). Polypropylene–montmorillonite composites were prepared by varying the volume fraction of montmorillonite nanoparticles such as:
107 5.2 SyNThESIS ANd ChARACTERISTICS
montmorillonite nanoparticles and varying block lengths of polypropylene-styrene or polypropylene- poly(methylmethacrylate) copolymers to probe mechanical and thermal properties. For example, a 3% inorganic loading into polypropylene gave a 30% increase in Young’s modulus and a 30°C increase in heat deflection temperature when compared to native polypropylene.
Polymer-Nanoparticle Composites Part 1 (Nanotechnology), 2010.
Nonetheless, significant challenges are associated with blending polymers and nanoparticles in order to afford homogeneous, well-dispersed inorganic material within the polymer. In order for dispersion to be achieved, “the entropic penalty associated with addition of the nanoparticles must be balanced by favorable enthalpy interactions” (Polymer-Nanoparticle Composites Part 1 (Nanotechnology), 2010).
For this reason, polymer–clay hybrids composed of layered nanoparticles, such as silicates talc and mica which are aggregated to some degree, as the immiscibility of clay in the polymer leads to a very close proximity of sheets to one another. It is very important to mention that the degree of dispersion in these composites is generally referred to as:
● Unmixed highly aggregated
● Intercalated minimally aggregated
● Exfoliated well dispersed.
In intercalated cases, the polymer chains interpenetrate would be stacked silicate layers with small separation distances between layers. For the exfoliated or delaminated morphology, the silicate layers are well dispersed inside the polymer (Fig. 5.8). Exfoliation could be achieved by using the polar polymers, by the addition of a surfactant to the material, typically a long-chain alkylammonium salt. However, for nonpolar polymers such as poly(ethylene) and poly(propylene), the addition of a surfactant is not suffi- cient to overcome the entropic penalty. Thus a functional comonomer such as methyl methacrylate must be incorporated into the nonpolar polymer to allow nanoparticle dispersion within the matrix. Advances in processing have also led to decreased aggregation in clay–polyethylene materials, such as the use of supercritical CO2 during polyethylene extrusion. Because the physical properties of these composites depend on the ability to produce controlled intercalation or exfoliation, the interest in small molecule and polymeric additives tuned to this target is growing significantly (Fig. 5.12).
FIGURE 5.12
depiction of three types of clay–polymer hybrid materials showing different levels of particle dispersion.
From Polymer-Nanoparticle Composites Part 1 (Nanotechnology). (2010, May 25).
Retrieved 13.04.11, from What-When-How In depth information: http://what-when-how.com/nanoscience- and-nanotechnology/polymer-nanoparticle-composites-part-1-nanotechnology/.
108 CHAPTER 5 NANOPARTICLES