Differential scanning calorimetry (DSC) and Raman spectra are reported for NaNO3 bulk and for NaNO3 confined in porous silica with pore radii, rp=2.5, 5, 10, 20 nm. Raman spectra are also given for a 6 M solution of NaNO3. The melting transition for the confined NaNO3 exhibits a 1/rp dependence where rp is the pore radius for rp≳5 nm. No melting transition is observed for NaNO3 confined in 2.5 nm pores. Above this pore size, their appears to be a deviation in the melting transition dependence on rp. The internal modes observed in the Raman spectra for the confined material are in agreement with those of the bulk solid except for a feature observed on the low frequency side of the ν1 band. The external TO mode observed at 100 cm−1 and the librational mode at 175 cm−1 for NaNO3 both decrease in intensity and broaden as rp decreases and both bands disappear at rp=2.5 nm. An additional peak at 70 cm−1 not observed in the solution or bulk NaNO3 spectra appears in the spectra of confined NaNO3 and increases in intensity as rp decreases. We assign this band to a new phase of NaNO3 which is stabilized by the surface hydroxyl groups of the porous silica. For NaNO3 confined in pores, rp≤2.5 nm, we suggest that NaNO3 exists as disordered aggregates.
Despite the advantages of the non-flammable, good performance and low price, poly (vinyl chloride) (PVC) still suffer from poor thermal stability, restricting its melting process and applications. Although addition of some heat stabilizers can be used to improve the low thermal stability, so far, they normally compromise the environmental issues and smoke density of PVC during combustion. In this work, a series of La doping Mg-Al layered double hydroxides (LaLDHs) with different molar ratio of La3+ / Al3+ were successfully synthesized by coprecipitation-hydrothermal method and characterized by X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR), Scanning Electron Microscopy (SEM) and Transmission electron microscopy (TEM). The results showed that the as-prepared LaLDHs exhibit plate-like morphology with a lateral size around 100-180 nm. The different as-prepared LaLDHs were introduced into PVC as heat stabilizer to prepare PVC nanocomposites. The thermal stability and smoke suppression of PVC nanocomposites were investigated by TGA, thermal aging, Congo red and smoke density rating test (SDR), respectively. All the results demonstrated that PVC-LaLDHs2 nanocomposites containing 2% LaLDHs2 (the molar ratio of La3+ / Al3+ is 1 / 3) were optimized, which achieved the maximal T50% value of 337.2 oC, minimal SDR value of 45.6%, and prolonged the thermal aging time from less than 10mins to 90mins, respectively thermal stability time from 1242s to 2751s. In addition, the tensile strength and elastic modulus of PVC-LaLDHs2 respectively increased by 84.4% (56.6 MPa) and 75.5% (1019.4 MPa) with little affecting elongation at break of PVC. LDHs (layered double hydroxides); rare earth ions; thermal stability; smoke suppression; poly (vinyl chloride)
We have demonstrated that thermal and vibrational investigations of sodium nitrate (NaNO3) physically confined in the porous silica hosts is an effective approach to mimic the solute clustering effects in aqueous solution. The results show that the structure of the NaNO3 confined in nanopores can be divided into three regions, i.e., the regular solid phase region, solution-like molecular aggregates region, and a new solid-like phase region. Depending upon the pore size of the porous host, the physical range of the three regions varies accordingly. For pore size d <2.5 nm, only the solution-like molecular aggregates phase remains.
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