Dewetting and surface properties of ultrathin films of cellulose esters

Surface properties of ultrathin films of cellulose esters deposited onto silicon wafers have been investigated by means of contact angle measurements and atomic force microscopy (AFM). Cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) films adsorbed or spin-coated onto Si wafers were annealed up to one week. Film stability was monitored by AFM. Dewetting has been observed for CA and CAP. Only CAB films with lower degree of esterification presented dewetting, CAB films with high degree of butyrate were stable even after one week annealing. Surface energy of CA, CAP, and CAB was indirectly determined by contact angle measurements using drops of water, formamide and diiodomethane. The surface energy decreased as the size of alkyl ester group or the degree of esterification increased because van der Waals interactions became weaker. Effective Hamaker constant AeffAeff was calculated for CA, CAP, and CAB onto Si wafers in air. Negative values of AeffAeff were found for CA, CAP, and lower butyrate content CAB, which are related to instability and agree with dewetting phenomena observed by AFM. In contrast, a positive AeffAeff was determined for higher butyrate content CAB, corroborating with experimental observations.

Surface energy of cellulose ester films deposited onto Si wafers decreased with the size of alkyl ester group. Dewetting of cellulose ester films was observed by AFM and explained with basis on the Hamaker constant values.Figure optionsDownload as PowerPoint slide