Papers

AVS2001 Session AS-WeM: Biomaterials and Polymers

Wednesday, October 31, 2001 8:20 AM in Room 134

Wednesday Morning

Start	Invited?	Item
8:20 AM		AS-WeM-1 Surface Characterization of Biomaterials for Medical Applications H.J. Mathieu (Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland) Biomaterials are non viable materials used as medical devices interacting with biological systems and are increasingly being applied as substitutes and/or sensors in human hosts. This paper describes the specific surface functionalization and characterization of biomaterials for Medical Applications by use of methods such as x-ray photoelectron spectroscopy (XPS) and imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS) as well as contact angle measurements and scanning force microscopies (SFM). Bio-molecules (peptides, polysaccharides, proteins, etc) are grafted to various types of materials ranging from metals, semiconductors to polymers. It is the bulk composition which determines the physical, mechanical and rheological properties, whereas surface chemistry and topography influence the response to a foreign implant. The control of chemistry, forces and topography of surfaces and thin films with femtomol sensitivity, nanometer in-depth information and submicron lateral resolution will be highlighted. Practical applications cover photo-grafting of hydrocarbons for the development of bio-sensors - glycoengineering -, plasma modification of polymers to reduce bacterial adhesion on endotracheal devices and cell adhesion on metallic surfaces. References: 1. D. Leonard and H.J. Mathieu, Fresenius' Journal Analytical Chemistry 365 (1999) 3-11 2. H. J. Mathieu, Surface and Interface Analysis 32 (2001) in print.
9:00 AM		AS-WeM-3 Micromechanical Properties of 'Smart' Gels: A Study of PNIPAAm by Scanning Force and Scanning Electron Microscopy T.R. Matzelle (Universite Libre de Bruxelles, Belgium); R. Reichelt (University of Muenster, Germany); N. Kruse (Universite Libre de Bruxelles, Belgium) PNIPAAm [poly-(N-isopropylacrylamide)] is one of the most interesting and promising 'smart' gels. It undergoes a reversible phase transition in response to external temperature changes. The PNIPAAm matrix, swollen in aqueous solution, collapses as the temperature is increased above the lower critical solution temperature (LCST), which is about 33°C. Due to this thermoresponsive ability, these gels are promising candidates for thermal switches, micro/nanoactuators or controlled-release systems. In order to provide information on the local structural and mechanical properties of PNIPAAm we employed scanning force microscopy (SFM) in air or in water at various temperatures below and above the LTSC. SFM images of the gel surface were compared with those obtained in dry, swollen, and collapsed states using field emission scanning electron microscopy (FESEM). Images of SFM and FESEM of the dry hydrogel surface revealed similar structural features. The surface is rather smooth except for small spherically shaped protrusions with a diameter and a height ranging from 10 to 50 nm and from 5 to 15 nm, respectively. FESEM of a cryogenically dried PNIPAAm sample swollen in water at 20°C revealed a coral-like structure with cavities of tda40 nm. Force vs. cantilever displacement curves were measured with both, spherical (µmm-sized) and commercial probes. Indentation of the hydrogel surface as a function of the probe load was evaluated using the Hertz model to determine the local elastic moduli at different temperatures. For the swollen state at 10°C Young's modulus was found to be 1.11 kPa, which is more than 100 times lower than for the collapsed state at 35°C. More generally, this modulus is significantly lower than the moduli measured for biological cells.
9:40 AM		AS-WeM-5 Tailored Polymer Surfaces Controlled by XPS J.-J. Pireaux (Facultés Universitaires Notre-Dame de la Paix, Belgium) If polymer materials are now so widely used that it is hard to imagine life without them - billions of kilograms of plastics are sold every year-, a significant set of applications rests on polymers in juxtaposition with another material, as in composites or (multi)layered structures. All properties of these ensembles depend on successful and controlled adhesion, a very complex technology indeed that encompasses various physico-chemical interactions between two surfaces. This presentation will review the different methods used in the laboratory, or at the production plant, to modify, if possible in a very controlled way, a polymer surface. To remove superficial contamination, to modify surface morphology, to tune hydrophobicity, to functionalize a polymer surface ... can be achieved by various chemical or physical methods. Surface treatment is particularly versatile when using a plasma discharge, a vacuum technique, while X-Ray Photoelectron Spectroscopy (XPS) appears a method of choice to control the tailored polymer surface. Potentials of the cold (reactive) plasma treatment will be shown; advantages and problems of the XPS characterization method will be pointed out; complementary information gained by FT-infrared and contact angle measurements will be illustrated. Two sample cases will be commented on: (1) plasma treatment of polyester, in various reactive gases that shows ageing (surface oxidized species slowly disappear with time), while an optimum amount of functionalization allows better adhesion of an evaporated aluminium layer (mechanical adhesion test); (2) some parameters governing the physico-chemical interactions at the SiO_x -functionalized polypropylene interface will be explained with the help of the acid-base concept.
10:20 AM		AS-WeM-7 Synthesis and Characterization of Poly(imidesiloxane) Copolymers Containing Two Siloxane Segment Lengths: Surface Composition and Its Role in Adhesion C.M. Mahoney (State University of New York at Buffalo); J.C. Rosenfeld (Occidental Chemical Corporation); J.A. Gardella, Jr. (State University of New York at Buffalo) Polyimidesiloxane (SIM) copolymers are extremely important materials for microelectronic applications due to their excellent adhesive properties, low dielectric constants and good overall thermal and mechanical properties. Hence it is of importance to study the surface and interfacial properties of this polymer system. A series of poly(imidesiloxane) (SIM) copolymers have been synthesized, where the total composition of PDMS was maintained at 10% (by weight) with two different PDMS segment lengths of different relative composition. (e.g. 5% PDMS containing 1 repeat unit, designated G-1 and 5% PDMS containing 9 repeat units, designated G-9 incorporated into the same polymer vs. 1% G-1 and 9% G-9 in the same polymer). Two main polymer series were synthesized, one containing G-1 and G-9 in varying ratios, and the other containing G-5 and G-9. Both of these series have been analyzed using angle dependent X-Ray Photoelectron Spectroscopy (XPS). The results suggest that there is preferential segregation of longer siloxane segment lengths to the surface. The angle dependent data was then used to obtain an in-depth profile by using a deconvolution process. From the profiles, it was determined that the thickness of the surface PDMS layer of all polymers containing both G-9 and G-1 were the same for all compositions studied, while that of the pure 10% G-1 was much less. The adhesion strengths of these polymers were measured using peel strength tests and the adhesion values were correlated to the XPS results. It was found that the adhesion of the pure 10% G-1 was much higher than that of any other polymer in the series. The remainder of the polymers in the series all had similar adhesion values. These results are consistent with a model of the surface, which has longer segment lengths preferentially segregating and dominating the adhesive properties.
10:40 AM		AS-WeM-8 Phase Contrast AFM Analysis of Polymers: Use of Correlative Classification Methods for Phase Identification J. Farrar, K. Artyushkova, J.E. Fulghum (Kent State University) Phase contrast AFM is increasingly utilized in the analysis of polymers and polymer blends. The phase contrast images potentially contain chemical information, although image interpretation can be challenging. In this study we evaluate methods for the correlation of XPS and AFM data, in order to facilitate chemical interpretation of phase contrast AFM images. Polymer grids are used to evaluate the image pre-processing required before the application of classification methods. Processing for image correlation includes resizing, image alignment and resolution matching. Considerations specific to each technique will also be discussed. Following the image processing, classification methods are used to correlate components present in the XPS and AFM images. After validating the approach on test samples, heterogeneous polymer blends are analyzed using image classification methods. This work has been partially supported by NSF ALCOM (DMR89-20147).
11:00 AM		AS-WeM-9 A Surface Chemistry Study of Laser Ablated Polymers Used for Microfluidic Devices D.L. Pugmire, E.A. Waddell, C.J. Taylor, L.E. Locascio, M.J. Tarlov (National Institute of Standards and Technology) Polymer substrates are being investigated for use in microfluidic devices because of their low cost, ease of fabrication, and wide range of materials properties. It is well established that the surface chemistry of a plastic substrate greatly influences the electroosmotic flow (EOF) behavior of microfluidic channels made from that material. Typical channel imprinting techniques do not offer direct control of surface chemistry. Laser ablation shows promise as a versatile method for directly forming a variety of microchannel geometries in plastics. In addition, we have demonstrated that surface chemistry, and, therefore, EOF behavior can be controlled by changing the atmosphere under which laser ablation of the plastic is performed. The surfaces of several plastics ablated in a variety of environments were studied with x-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR), and scanning electron microscopy (SEM). XPS results indicate that laser ablation generally resulted in an increase in the oxygen content of the polymers studied, regardless of ablation atmosphere. However, this oxygen uptake was often more pronounced when ablation was performed under O₂ as opposed to N₂ or Ar. Ablation of commercially obtained PVC, with an organotin stabilizer, resulted in concentration of tin species at the ablated surface. These results will be discussed and compared to EOF rates of ablated microchannels.
11:20 AM		AS-WeM-10 RBS-based Characterization of Hyper-Thin Silicon Compound Deposits on Polymers G. Dennler (Ecole Polytechnique de Montreal, Canada); A. Houdayer (University of Montreal, Canada); Y. Ségui (Université Paul Sabatier, France); M.R. Wertheimer (Ecole Polytechnique of Montreal, Canada) Rutherford Backscattering Spectroscopy (RBS) with 1 and 1.5 MeV alpha particles has been used to investigate the growth of SiO₂ and Si₃N₄ films deposited by Plasma-Enhanced Chemical Vapor Deposition (PECVD) on three different polymers, namely polyimide, polyethyleneterephthalate and polycarbonate. The thicknesses of the various films considered in this work vary from 0.1 to 50 nm. In the case of Kapton PI, using the IBM geometry at 150°, we verified that the ratio of Silicon to Carbon does not change during irradiation; this signifies that the specimen does not suffer a significant amount of damage. Thus, using this RBS geometry, we were able to follow the surface density of Si atoms versus time of deposition, t, that is, to measure film thickness, d, down to 1 Å. The calibration of d was accomplished using thicker samples, characterized by Variable Angle Spectroscopic Ellipsometry (VASE) and X-Ray Fluorescence (XRF); a perfectly linear relationship between d and t was observed over the entire range, for both coating types on PI. RBS was also used at near-grazing angle (95°) to investigate the interphase between SiO₂ and polymeric substrates. RUMP simulations predicted a precision of about 5 nm under these conditions. Thus, we investigated a 50 nm SiO₂ film on Kapton PI and found that the interphase thickness does not exceed 7 nm. The same methods, applied to deposits on the other two polymers, were unsuccessful because of serious modifications of the polymeric substrate, induced by the incident ion beam, even under conditions of very low beam current (1nA) : Scanning Electron Microscopy (SEM) allowed us to observe the damage caused at the surface by the volatile molecular fragments created in the irradiated bulk of the polymer during their escape through the coated sample surface. This work shows clearly that an interphase of the order of 5 nm can be observed in certain cases, but that this IBA technique is not generally applicable for all polymers, because of radiation damage.