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The application of inorganic nanoparticles as reinforcement agent for polymer composites is constantly growing. Improving the performance of the material with desirable properties requires understanding of the interaction between polymer chains and nanoparticles and the properties of the interphase as well. Boehmite, a mineral of aluminum with basic unit of AlO(OH), is a novel and promising nanofiller which leads to enhanced performance of polymer composites. It has been recently reported that boehmite nanoparticles have reinforcing effect on epoxy matrix in carbon-fiber composites. It was primarily assumed that these improvements are due to very high Young’s modulus of boehmite particles. However, in our latest study we presented new values for the Young’s modulus of boehmite much lower than those reported earlier. This brings up the importance of interphase properties, e.g. the crosslink density, which can have the dominant role in the overall material property. Nevertheless, due to resolution limitations of conventional nanoprobing approaches, characterization of the interphase between individual particles and matrix is a challenge.
In this study, the main goal is to investigate the interphase of the epoxy/boehmite nanocomposites using AFM-based methods. We simplify the three-dimensional nanocomposite system to a two-dimensional horizontally layered sample with a large and easy to access interphase area. For this purpose, 1µm coatings of hydrothermally synthesized boehmite are prepared as the substrate on which the epoxy is later molded and cured. AFM surface potential and force maps were obtained on the cross-sectional cut of epoxy/ boehmite sample. The results show unexpectedly a large interphase area (approx. 1 µm) with different electrical and mechanical properties comparing to bulk epoxy. The average force-distance curves from this region showed more elastic behavior compared to bulk. The underlying mechanism of this influence is not fully understood yet. Thus, further investigation on the interphase region using the novel Nano-IR approach provides more information about the chemical characteristics. Numerical simulation will give complementary information to understand the effect of nanoparticles on the crosslinking density of the interphase. This can be different to bulk epoxy due to either different local temperature gradients or due to a surface selectivity of boehmite towards the monomer molecules.
The potential of nanocomposites, i.e. mixtures of nanoobjects with acceptable distribution in a polymer matrix, with respect to flame retardancy was discovered in an already early stage of nanocomposite reseach. Since then, a variety of nanocomposites often in combination with conventional flame retardants (FRs) and their effect on the burning behavior has been described.
Here, we show the application of this concept to enhance the flame retardancy of poly(butylene terephthalate) (PBT) and poly(butylene succinate) (PBS) using suitable phosphorus-containing polyesters as FR. The materials studied were prepared by melt compounding in a twin screw extruder using either a mixture of polymer matrix, phosphorus polymer and nanomaterial (direct compounding) or a mixture of a pre-formed batch of phosphorus polymer with nanomaterial and the polymer matrix (batch compounding). The second method has been shown to be very effective if batches prepared by in-situ nanocomposite synthesis via melt transesterification polycondensation were employed. Modified organoclay (montmorillonite, OMMT) as well as multiwalled carbon nanotubes were used as nanoobjects.
Organic modification of MMT resulted in better exfoliation and distribution within the poylmer matrix than observed with pure sodium MMT. However, modification with phosphorus-containing modifiers dis not support exfoliation due to high interaction between modifier and clay.
Analysis of the in-situ prepared nanocomposites of OMMT (Cloisite 30B) with the phosphorus polyester PET-P-DOPO by thermogravimetry, FTIR and pyrolysis-GC/MS showed that OMMT did not alter the principal decomposition pathway of the polyester, but shifted the onset of decomposition to lower temperature (due to the fast decomposition of the tertiary ammonium compound) and increased the amount of char. The fire behavir as observed by microscale combustion calorimetry (MCC) was altered and resulted in significant decrease of heat release capacity.
In-situ prepared batches of the phosphorus polyester PET-P-DOPO with 20 wt.-% OMMT and with MWCNT (1 wt.-%) were blended with PBT in the ratio 25/75 wt/wt to achieve a phosphorus concentration of 1.5 wt.-%. The nanocomposites were injection molded into plates for cone calorimeter measurements and examined using 50 kW/m2. In all samples, a reduction of toral heat evolved and total heat evolved/total mass loss (THE/TML) was observed. Addition of OMMT improved the char, which was even more pronounced in combination with PET-P-DOPO as illustrated in Figure 1. Blends containing PET-P-DOPO additionally showed intumescence. In all cases, a significant reduction of FIGRAmax was obtained. The addition of nanomaterials (both OMMT and MWCNT) to PBT/PET-P-DOPO reduced the effective heat of combustion (THE/TML) from 1.7 to 1.5 MJ/m2.