Structure and Dynamics of Polyhedral Oligomeric Silsesquioxane (POSS) and Poly(Ethylene Glycol) (PEG) Based Amphiphiles as Langmuir Monolayers at the Air/Water Interface

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Virginia Tech

Throughout the study of polymeric Langmuir monolayers at the air/water (A/W) interface, the Wilhelmy plate and Langmuir-Blodgett (LB) techniques along with Brewster angle microscopy (BAM) have been identified as key methods for acquiring structural, thermodynamic, rheological and morphological information. These techniques along with surface light scattering (SLS), a method for probing a monolayer's dynamic dilational rheological properties, will be used to characterize homopolymers, poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG), and a new class of novel polymeric surfactants, telechelic (POSS-PEG-POSS) and hemi-telechelic (POSS-PEG) polyhedral oligomeric silsesquioxane (POSS) derivatives of PEG.

PEO with number average molar mass, Mn > ~ 18 kg·mol-1 form stable spread Langmuir films at the A/W interface, while oligomeric PEG have ï -A isotherms that deviate from high molar mass PEO. Nonetheless, SLS reveals that the dynamic dilational viscoelastic properties of any Mn PEG(PEO) only depend on ï and not Mn. Likewise, POSS-PEG-POSS telechelics exhibit molar mass dependent ï -A isotherms, where low ï regimes (ï < 1 mN·m-1) have PEG-like behavior, but high ï regimes were dominated by POSS-POSS interactions. SLS studies reveal that the dynamic dilational moduli of POSS-PEG-POSS are greater than either PEO or an analogous POSS compound, trisilanolcyclohexyl-POSS. The ability to control rheological properties and the hydrophilic-lipophilic balance even allows one POSS-PEG-POSS (PEG Mn = 1 kg·mol-1) to form Y-type LB-multilayer films.

For POSS-PEG systems, comparisons at comparable POSS:PEG ratios reveal short PEG chains (PEG Mn ~ 0.5 kg·mol-1) yield similar viscoelastic properties as POSS-PEG-POSS (PEG Mn ~ 1 kg·mol-1), while longer PEG chains (PEG Mn ~ 2 kg·mol-1) yield lower modulus films than comparable POSS-PEG-POSS. These differences are attributed to brush-like PEG conformations in short POSS-PEG versus mushroom-like PEG conformations in long POSS-PEG at the A/W interface.

These results provide insight for designing PEG-based amphiphilic nanoparticles with controlled interfacial rheology.

Viscoelasticity, Langmuir monolayers, Poly(ethylene oxide) (PEO), Poly(ethylene glycol) (PEG), Hemi-telechelic POSS-PEG, Telechelic POSS-PEG-POSS, Polyhedral Oligomeric Silsesquioxane (POSS)