Atomic Layer Deposition of Al₂O₃ Films on Polyethylene Particles

Sequential exposures of Al(CH 3 ) 3 and H 2 O at 77 °C were used to encapsulate low-density polyethylene (LDPE) particles with an ultrathin Al 2 O 3 film. FTIR studies revealed that the nucleation of Al 2 O 3 atomic layer deposition (ALD) on the LDPE particles occurred primarily via adsorption of Al(CH 3 ) 3 onto the LDPE surface or absorption of Al(CH 3 ) 3 into the LDPE particle followed by the reaction with H 2 O. The FTIR spectra then revealed the progressive switching between AlCH 3 * and AlOH* species with alternating exposure to Al(CH 3 ) 3 and H 2 O. This nucleation of Al 2 O 3 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(CH 3 ) 3 and H 2 O led to an increase in Al 2 O 3 bulk vibrational modes. The increase of the absorbance for the Al 2 O 3 bulk vibrational modes was linear with the number of AB cycles. The presence of an Al 2 O 3 film on the LDPE particles was confirmed using transmission electron microscopy (TEM). The TEM images revealed that the Al 2 O 3 coating was very conformal to the LDPE particles. The Al 2 O 3 coating was also thicker than expected from typical Al 2 O 3 ALD growth rates. This thicker Al 2 O 3 coating was explained by the presence of hydrogen-bonded H 2 O on the Al 2 O 3 surface that increases the Al 2 O 3 growth rate during Al(CH 3 ) 3 exposures. On the basis of these results and additional investigations, a model is proposed for Al 2 O 3 ALD on polymers. Al 2 O 3 ALD should provide an effective gas diffusion barrier on temperature-sensitive polymeric materials such as LDPE.