Atomic Layer Deposition of Al2O3 Films on Polyethylene Particles

Sequential exposures of Al(CH3)3 and H2O at 77 °C were used to encapsulate low-density polyethylene (LDPE) particles with an ultrathin Al2O3 film. FTIR studies revealed that the nucleation of Al2O3 atomic layer deposition (ALD) on the LDPE particles occurred primarily via adsorption of Al(CH3)3 onto the LDPE surface or absorption of Al(CH3)3 into the LDPE particle followed by the reaction with H2O. The FTIR spectra then revealed the progressive switching between AlCH3* and AlOH* species with alternating exposure to Al(CH3)3 and H2O. This nucleation of Al2O3 ALD did not require the existence of specific chemical functional groups on the polymer. The FTIR spectra also demonstrated that the sequential exposures of Al(CH3)3 and H2O led to an increase in Al2O3 bulk vibrational modes. The increase of the absorbance for the Al2O3 bulk vibrational modes was linear with the number of AB cycles. The presence of an Al2O3 film on the LDPE particles was confirmed using transmission electron microscopy (TEM). The TEM images revealed that the Al2O3 coating was very conformal to the LDPE particles. The Al2O3 coating was also thicker than expected from typical Al2O3 ALD growth rates. This thicker Al2O3 coating was explained by the presence of hydrogen-bonded H2O on the Al2O3 surface that increases the Al2O3 growth rate during Al(CH3)3 exposures. On the basis of these results and additional investigations, a model is proposed for Al2O3 ALD on polymers. Al2O3 ALD should provide an effective gas diffusion barrier on temperature-sensitive polymeric materials such as LDPE.