We are delighted to introduce this special section of physica status solidi (a) dedicated to Composites of Inorganic Nanotubes and Polymers (“COINAPO”). The papers included are contributions from members of the EU COST Action MP0902–COINAPO. Our objective in compiling this Topical Section was to provide an overview of the topics which help in understanding the structure and solid state properties of a range of inorganic nanoparticles, and to address the issues associated with the preparation of composites of these particles and polymeric materials. Several new types of inorganic nanotubes/nanowires/nanorods (INTs/IFs) were synthesised and composites of these prepared with a range of polymers. Various methodologies for achieving optimal INT/IF dispersion in polymers were developed and structural, optoelectronic, dielectric, ultrasonic, mechanical and tribological properties of the composite materials were studied. The effects of varying processing parameters on the final properties of the composites were investigated and various theoretical models of the response of the composites developed, including examining percolation effects and effective medium modelling of the dielectric function of model composite systems.
The first paper, an Invited Article by Tenne et al., provides an overview of the synthesis and mechanical properties of inorganic nanotubes (INTs) and inorganic fullerene-like nanoparticles (IFs) of WS2 , highlighting the potential reinforcing and lubrication applications of composites of INT-WS2 and IF-WS2 with polymers. The Feature Article by Petzelt et al. reviews the phenomenological models used to understand the complex dielectric spectra of weakly inhomogeneous, disordered conductors and composite conductor–dielectric materials . Uniquely, they report the dielectric/conductivity response of a range of composites, including those with WS2 and MoSI in the frequency range 10−5–1014 Hz, over some 19 orders of magnitude. On a similar theme, in the first contributed article, Svirskas et al. report the dielectric properties of composites of poly(ϵ-caprolactone) and MoSI nanowires . The authors identified a relaxation process at T < 100 K, which increased with increasing MoSI content.
It is anticipated that the many unique properties of INTs can be translated to polymer matrices to add functionality to resulting composite materials. The following contributions highlight some of the exciting properties of novel INTs/IFs and their composites with polymers. Di Luccio et al. describe the incorporation of WS2 nanotubes into poly(fluorene). The composite material obtained was tested in light-emitting devices realizing a new class of device with promising improved mechanical and thermal properties without affecting the optoelectronic performance of the device . Brnardić and co-workers report the synthesis, by a hydrothermal method, and functionalization of sodium titanate nanotubes (NaTi-NTs) . Addition of low concentrations of NaTi-NTs to an epoxy resin resulted in an increase in the glass transition temperature (Tg) and storage modulus (E′) of the epoxy, and interestingly, the combustion properties, specifically the heat release rate (HRR) of the epoxy was significantly reduced. Korzekwa et al. present a two-step synthesis method to prepare composite coatings of IF-WS2 and Al2O3 . The authors propose that such a composite layer could have improved anti-wear and anti-friction functionality.
Indeed, one of the earliest applications of INTs and IFs is as additives to enhance the tribological properties of polymeric materials. The following three articles describe the wear and friction behaviour of composites of both thermosets and thermoplastics on addition of INTs/IFs. In the article by Shneider et al., the coefficient of friction and wear of composites of an epoxy with WS2 were evaluated under dry contact conditions using loads from 0.25 to 50 MPa . The different improvements in the coefficient of friction obtained are a function of the shape of the WS2 particle as well as WS2 concentration and the experimental conditions employed. In the next contribution, Meier and co-workers report the tribological behaviour of composites of polyamide-6 (PA-6) with Mo6S2I8-nanowires, MoO(3–x)-nanowires and MoS2-INTs at different concentrations . Addition of the nanowires to PA-6 resulted in a reduced coefficient of friction and wear rates derived from the increased bulk shear strength of these composites. In contrast, the reduced coefficient of friction and wear rates achieved on addition of MoS2-INTs to PA-6 are associated with a lubricating effect at contact points. Remskar et al. also report a significant reduction in the coefficient of friction on addition of MoS2 to PVDF . Interestingly, the authors use Raman spectroscopy before and after friction testing and provide evidence for partial transformation of PVDF into a piezoelectric phase due to friction induced by drawing.
There continues to be intense debate over the effect of nanoparticle addition on the Tg of polymers. Reinecker et al. used dynamical mechanical analysis (DMA) to probe the effect of MoS2 addition on the Tg of poly(urea) elastomers . Computer simulations are used to explain the observed shift in Tg, reasoned to be a function of the retardation of polymer chain dynamics in the vicinity of the MoS2-INTs.
Rajh and co-workers report electro-optic properties of mixtures of 4-cyano-4′-pentylbiphenyl (5CB) thermotropic liquid crystal with four different 1D inorganic nanoparticles . The most profound effect was obtained for MnO2/5CB: a reduction of the threshold voltage by a factor of 1.7 and of the total switching time by a factor of 1.5. The observed modifications are associated with an increase in the dielectric anisotropy of the mixture and the relatively low aspect ratio of the MnO2.
In a further contribution, Varlec et al. recorded the photoluminescence spectra of novel composites of MoS2 nanotubes with poly(3-hexylthiophene) (P3HT) and revealed a quenching effect dependent on MoS2 concentration . The contribution by Kunzo et al. demonstrates the potential of composites of poly(aniline) and TiO2 nanoparticles for application as a chemiresistive gas sensor . An increased sensitivity to ammonia is reported. The final contribution by Samulionis et al. focuses on the ultrasonic properties of composites of poly(ε-caprolactone) with MoSI nanowires .
With this Topical Section besides reporting on some of the exciting achievements attained as part of the COST Action MP0902–COINAPO, we want also to highlight the tremendous opportunities and technological challenges that exist for composites of 1D and 2D inorganic materials and polymers. The results collected so far definitely show that “COINAPO materials” have a bright and prospective future.
WMG, University of Warwick, UK
Irena Drevenšek Olenik
Faculty of Mathematics and Physics, University of Ljubljana, and J. Stefan Institute, Ljubljana, Slovenia
Topical Section on Composites of Inorganic Nanotubes and Polymers
This publication is supported by COST
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