Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1999-02-26
2002-07-02
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S409000, C428S520000, C428S522000, C428S315500, C525S223000
Reexamination Certificate
active
06413621
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to polymer articles having modified surfaces, such as an essentially hydrophobic polymer article having a hydrophilic surface resulting from entropically-enhanced migration of a miscible, hydrophilic component to the surface of the article.
A membrane having a hydrophobic core and a hydrophilic surface component is provided as well.
BACKGROUND OF THE INVENTION
Control of the surface chemistry of polymeric articles and compositions has technological relevance to a variety of commercially-important areas such as the medical devices industry, personal products, coatings, membranes, etc. Many polymeric articles and compositions that are useful in these areas are defined by a particular type of material because of economic considerations or mechanical requirements. For example, an essentially hydrophobic material might be used for structural reasons where it would be desirable to provide a different type of surface, for example a hydrophilic surface, on the article. Other examples involve imparting a chemical functionality to a surface such as a chelating functionality or other functionality that can selectively remove particular species from solution, or otherwise expose a desired chemical characteristic. While many techniques exist for modifying surface properties of polymers, many involve multi-step processes and/or do not result in thermodynamically or physically-stable incorporation of surface-modifying components.
It is often a goal in polymer chemistry to create a polymer article having a surface of high surface tension (surface energy) relative to the article as a whole, since higher surface tension typically corresponds to better wettability. However, in polymer blends including a higher surface energy component and a lower surface energy component the lower surface energy component (lower wettability component) tends to be present disproportionately at the surface since surface energy is characterized by inter-molecular attraction. That is, thermodynamic considerations result in the component with the higher inter-molecular attraction residing below the surface where it can be surrounded by a higher number of like molecules, while the lower surface energy component resides at the surface where a molecule is inherently surrounded by less like molecules. Techniques exist for creating polymeric materials having higher surface tension components at the surface, but a problem typically encountered with conventional methods is the tendency of the surface to reconstruct over time through chain reorientation where the lower surface tension component migrates to the surface of the polymer. (e.g., Wu, Supra; Garbassi, et al., Supra). Such reconstruction is consequently accompanied by an irrevocable loss of desired surface properties.
The control of surface properties of acrylate polymers has technological relevance to areas including biomedical devices, latex paints and other coatings, textiles, and recording media. However, conventional techniques for modification of acrylate polymer surface chemistry typically is achieved through kinetically-governed processes that allow little control over the final surface composition and structure. Plasma and flame treatments, commonly employed to oxygenate surfaces in order to improve wetting and/or adhesion, invoke reaction cascades of bond scission, fragmentation, and crosslinking, yielding poorly-defined surface compositions. Chemical oxidation by acid treatment typically causes pitting and solubilization that modifies surface morphology in an uncontrolled fashion (E.g., Wu,
Polymer Interface and Adhesion
(Marcel Dekker, Inc., New York, 1982); Garbassi, et al.,
Polymer Surfaces: From Physics to Technology
(John Wiley & Sons, West Sussex, 1994)). Grafting methods used to bond hydrophilic species like heparin or poly(ethylene glycol) to surfaces in order to improve biocompatibility typically yield low surface coverages (E.g., Pekna, et al.,
Biomaterials
, 14, 189 (1993); Harris, J. M., ed.,
Poly(ethylene glycol
)
Chemistry: Biotechnical and Biomedical Applications
(Plenum Press, New York, 1992)).
An alternative method of preparing a hydrophilic surface on a hydrophobic polymer article might be through the addition of a hydrophilic species to the polymer which selectively segregates to the surface upon processing, providing the desired surface hydrophilicity. This approach would be particularly useful if the hydrophilic additive were miscible with the polymer, so as not to adversely influence the bulk properties of the article, such as mechanical behavior or optical clarity. One such candidate additive might be poly(ethylene oxide), PEO, because of its high degree of hydrophilicity and well-known resistance to protein adsorption. PEO is known to be miscible in poly(methyl methacrylate) up to very high concentrations. It is also known, however, that the surface tension of PEO is somewhat higher than that of PMMA. From this, we would assume that a surface of an article prepared from a PMMA/PEO blend should be depleted with PEO, in order to reduce the surface energy. It has been reported that neither component is enhanced at the surface of such blends (Sakellariu, Polymer, 34, 3408, (1993)). However, in this study samples were annealed for only three hours at 170 C.
Membrane technology presents a particularly interesting challenge in connection with surface functionalization. The use of polymer membranes for water treatment has become increasingly widespread in the past thirty years in such applications as desalination of sea and brackish water, water softening, production of ultrapure water, and purification of industrial wastewater. Membrane processes have additionally been used to generate ultrapure water sources for the electronics and pharmaceutical industries, and to treat wastewater from such diverse industries as textiles and laundry, electroplating and metal finishing, petroleum and petrochemical, food and beverage, and pulp and paper.
Membrane processes offer significant advantages over conventional water treatment technologies. They require no phase change and are thus inherently less energy-intensive than distillation methods used for desalination. They provide an absolute filter for pollutants above a given pore size, and are hence more reliable than flocculation methods that can leave residuals in treated water if improperly performed. In addition, the modular and compact design of membrane filtration units offers great flexibility in the scale of operation. And because membranes can separate pollutants without chemical alteration, they allow for more cost-effective recovery of valuable components from wastewater.
However, membrane technologies suffer from critical materials-related drawbacks that limit their efficiency and lifetime, and hence cost-effectiveness in water treatment applications. In particular, membrane fouling is a major problem which results in reduced efficiency due to flux decline, high cleaning and maintenance costs, and low membrane lifetimes. The cleaning and replacement costs for ultrafiltration processes are estimated to account for 24% and 23%, respectively, of the total process costs. While careful system operation and flow-pattern design can reduce fouling by suspended particulates or precipitated salts, the adsorption of proteins onto membrane surfaces is more insidious, generating a monolayer film that provides a foothold for slower deposition processes which deteriorate membrane performance and lifetime substantially. Membranes used in reverse osmosis processes have additional materials-related limitations. While the cellulose acetate-based membranes most commonly found in this application exhibit high flux and good salt rejection, these polymers hydrolyze over time, generating physical holes in the membrane which reduces its useful lifetime. Clearly, the need exists for new membrane materials with improved fouling resistance and longer service lifetimes. Moreover, membranes with improved selectivity are sought for more cost-effective reco
Hester Jonathan F.
Mayes Anne M.
Walton David G.
Massachusetts Institute of Technology
Tarazano D. Lawrence
Thibodeau Paul
Wolf Greenfield & Sacks P.C.
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