Fluorescence Studies of Solvent-Polymer Interactions at Surface Functionalized Polyethylene Films

Abstract

The effects of solvent on the accessibility and reactivity of an ester-bound fluorophore at a functionalized polyethylene-solvent interface have been studied. Fluorophore-labeled polyethylene surfaces were prepared by blending together a small amount of either a phenylpyrenylmethyl-terminated ethylene oligomer or a pyrene ester-terminated ethylene-ethylene glycol block cooligomer and a 20-200-fold excess of high molecular weight additive-free high-density polyethylene. Solution casting of 1,2dichlorobenzene solutions of these mixtures generated iilms containing the pyrene labels. Studies of the 11/13 values for these films in a series of solvents, studies of the extent of excimer formation, and studies of quenching of the amine fluorescence by soluble amines that are incompatible in bulk high-density polyethylene all suggest that the fluorophores are near or in the solvent-polymer interface. In contrast to soluble pyrene-labeled polymers, the extent of excimer formation for surface-bound pyrene labels was greatest with good solvents. Short grafts where the pyrene label was attached either at the graft origin or at the graft terminus produced significantly different behavior for the pyrene fluorophore label.Surface modification of polymers is important technologically and as a fundamental research p r~b l e m . l -~ Such chemistry has received increasing attention in the last few years because of the potential of such chemistry. For example, surface modification of polymers is often carried out to affect properties like permeability or a d h e~i o n .~,~ Polymer surface chemistry also affects a polymer's hydrophilicity and bio~ompatibility.~~~ In this paper, we describe ways pyrene labels can be used to study solvation changes that occur a t surface-functionalized polyethylenes.It is well recognized that the chemistry of functional groups located at polymer surfaces can be influenced by the medium the functional groups encounter a t the polymer-solvent interface. It is also becoming increasingly apparent that the term "surface" with respect to an organic polymer is a m b i g u o u~.~+~ The definition of what constitutes the "surface" of an insoluble but swellable polymer like polyethylene depends largely on the phenomena one chooses to study and on the experimental conditions of the determination. For example, water contact angles respond to functional groups in the top 5 8, of an otherwise hydrophobic polymer or hydrophobic substrate.1° X-ray photoelectron spectroscopy (XPS) detects groups in the outermost 50-100 8, of a surface.11J2 However, both of these assays involve chemistry where relatively well-defined phase boundaries are present. The situation is less clear for an organic polymer suspended in an organic solvent. For a polymer film suspended in a solvent, we would define the polymer surface as consisting of a solvent-polymer interfacial region where bound functional groups can interact with solvent or reagents in solution. The depth of this interface would depend on how the solvent interacts with the pol...

Release Kinetics Studies of Aromatic Molecules into Water from Block Polymer Micelles

“…We are not aware of any other experiments that support or refute this picture of a “fuzzy” PS−water interface. However, Bergbreiter et al have proposed a similar picture for the interface between high-density polyethylene and organic solvents based on the fluorescence quenching of surface pyrene groups …”

“…However, Bergbreiter et picture for the interface between high-density polyethylene and organic solvents based on the fluorescence quenching of surface pyrene groups. 39 We should point out that our measurement of a finite rate of pyrene migration out of our polymer micelles stands in contrast to the report of Creutz et al 12 In these papers the objective was to measure the rate of polymer chain exchange between micelles using naphthalene-tagged poly((dimethylamino)ethyl methacrylate)-b-poly(sodium methacrylate) mixed with untagged polymer micelles but containing dissolved pyrene. As part of their analysis they conclude that no pyrene diffuses out of their micelles over a 2 h period.…”

Synthesis and Characterization of Pyrene-Labeled Poly(ethylenimine)

“…Quenching of Pyrene by Amine Groups. Aromatic and aliphatic amines are known to be efficient quenchers of aromatic hydrocarbons, and they are believed to effect quenching through formation of an excited charge-transfer complex. ,, Before describing the fluorescence spectra of the pyrene-labeled polymer, we will anticipate the quenching of pyrene fluorescence by amino groups on the polymer. To build a basis for understanding these spectra, we first describe the quenching of pyrene monomer fluorescence in water by simple secondary and tertiary amines.…”

Studies of Two-Dimensional Morphology at Surface-Functionalized Polyethylene Films

“…At these higher concentrations of PEoiig-2, the excimer peak maximum of excimer emission intensity appears at 477 nm. 14 As noted above, the fluorescence spectrum of a bifunctional film containing both PEoiig-1 and -2 indicated some quenching of PEoiig-2 by PEoiig-1. This result shows that the functionalized surfaces contain a mixture of PEoiig-1 and -2 in which some of the pyrene groups of PEoiig-2 are accessible to the amine groups of some PEoiig-1.…”

“…The synthesis and characterization of the pyrene-terminated polyethylene oligomer 2 (PEoiig-2) used procedures reported elsewhere. 14 Codissolution of these oligomers with a large excess of host high molecular weight polyethylene in 1,2dichlorobenzene followed by solution casting yielded a surface-modified polymer via the process we call entrapment functionalization (Scheme 2).10 Using this process, we could prepare a number of films with varying molar ratios of surface-bound PEoug-l to PEoiig-2. Films containing a single functional group have been prepared in a similar manner and have been studied earlier by our group.…”