We present an unusual temperature dependence of thermal strains in 4-(10-hydroxy)decyl benzoate (HDB) modified SWNT∕PS (SWNT—single wall carbon nanotube, PS—polystyrene) nanocomposites. The strain transfer from the matrix to nanotubes in these nanocomposites, inferred from the frequency change of the Raman active tangential modes of the nanotubes, is enhanced strongly below 300 K, whereas it is vanishingly small at higher temperatures. The increased strain transfer is suggestive of reinforcement of the HDB-SWNT∕PS nanocomposites at low temperatures. On the other hand, the pristine SWNTs couple weakly to the PS matrix over the entire temperature range of 4.5–410 K. We argue that the strain transfer in HDB-SWNT∕PS is determined by the thermomechanical properties of the interface region composed of polystyrene plasticized by the tethered alkanelike modifier.
Raman spectra were recorded on a Jobin Yvon S3000 spectrometers equipped with a LN-cooled charged coupled device (CCD). A long-working distance microscope objective 50× of Olympus 45 microscope was used for both to focus the laser beam to a spot with diameter on the sample surface and to collect the scattered light. The laser power density was kept below to prevent a substantial overheating of the sample at the laser spot.
Atactic polystyrene with weight-average molecular weight , polydispersity ratio was used for preparation of the composites. Composites were prepared by solution mixing appropriate quantities of pristine or functionalized SWNTs and polystyrene in toluene at room temperature. The solutions were dried extensively at room temperature and subsequently annealed at 180 °C in a vacuum oven for 24 h.
The RBM mode frequency in a SWNT is given by . From this expression it directly follows that , where is the strain along the circumference. On the other hand, under the reasonable approximation given in Ref. 4: , where is the Grüneisen parameter and to the nanotubes Poisson’s ratio. Given we finally obtain .
The axial stress on the nanotube due to the shrinkage of the matrix is , where , and and are the volume fractions of the nanotubes and the matrix. The 1.5 wt % HDB-SWNT∕PS nanocomposite was loaded with 1.5 wt % HDB-SWNTs that corresponds to a volume fraction and . For the calculation of we used the following values of the parameters: linear CTE (Ref. 26) and (Ref. 27) and the temperature dependence of taken from Ref. 28. Note that the volume CTE of PS is three times larger than the linear one (Ref. 26).
The ratio of the number of circumferential SWNTs and the total number of nanotubes in a bundle is given by , provided the number of the nanotubes across the bundle diameter is odd (Ref. 29). It is also reasonable to assume that only half of the surface of the nanotubes along the bundle circumference is exposed to the organic modifier. Given these premises it follows that bulk abundance of 1 functionalizing moiety to 66 carbon atom translates to surface coverage of a SWNT bundle, 60 nm thick. For even, the above expressions have different form (Ref. 29). The estimates for and , however, give very close results.