Donald E. Gregonis

Contact angles at the solid—water interface

Abstract The study of polymer—water interfaces by contact angle methods can be accomplished directly at the polymer—water interface. Using two water-immiscible liquids or a liquid and a vapor, one can deduce the dispersion and polar components of the hydrated solid surface free energy and the solid—water interfacial free energy. The theory is presented and a numerical analysis procedure is developed to solve the equations in the general case. The special case of n -octane and air is also presented. Data and results are given for poly(hydroxyethyl methacrylate-methoxyethyl methacrylate) copolymers of varying composition and equilibrium water contents. The results show that the hydrophilic component dominates the polymer—water interfacial properties, even at relatively low hydrophilic component compositions. The method presented should be useful for the study of polymer—water interfaces, particularly for hydratable or mobile polymers which can reorient to equilibrate differently with a water environment than with the air or vapor environment commonly used in contact angle studies.

Polymer Surface Dynamics

Classical surface chemistry assumes that solid surfaces are rigid, immobile, and at equilibrium. These assumptions allow one to probe adsorption and wetting or contact angle processes purely from the point of view of the liquid phase, because one assumes that the solid phase does not in any way respond, reorient, or otherwise change in the different liquid environments. Although such assumptions may be partially correct for truly rigid solids, they are generally inappropriate for polymers (see also Chapter 7).

The Contact Angle and Interface Energetics

Information on the outermost few angstroms of solid surfaces is very difficult to obtain. One of the most sensitive methods known for obtaining true surface information is solid/liquid/vapor (S/L/V) or solid/liquid/liquid (S/L/L) contact angles. These methods are unique in that the equipment required is relatively simple and inexpensive. Although interpretation of the results obtained is dependent on a number of assumptions, each of which is somewhat controversial, a first-order interpretation is possible and has proven to be very useful in practically all areas of surface science and engineering. Most of the surface science texts briefly referred to in Chapter 1 contain one or more chapters on surface tension, capillarity, or contact angle methods. In addition, a number of the monographs and review serials cited in Chapter 1 also contain chapters on the contact angle technique.

Fibroblast cell proliferation on charged hydroxyethyl methacrylate copolymers

Abstract Two fibroblastic cell lines, 3T3 and 3T12, were grown on hydrophilic hydrogel substrates containing various amounts of positive or negative charge. The materials were copolymers of hydroxyethyl methacrylate (HEMA) and methacrylic acid (MAA), N,N -dimethylaminoethyl methacrylate hydrochloride (DMAEMA-HCl), or trimethylaminoethyl methacrylate chloride (TMAEMA-Cl). The samples were prepared as spun cast films on ultraclean glass microscope slides. The cells were grown in 10% fetal bovine serum supplemented Dulbeccos modified medium (DMM). Both cell lines survived and proliferated on the positively charged DMAEMA-HCl or TMAEMA-Cl copolymers with HEMA. Cells did not survive on the negative MAA-HEMA copolymers nor on the neutral HEMA homopolymers. Proliferation did not correlate with water content of the gel materials. Attachment as well as proliferation did correlate well with zeta potential and mole percentage positive charge as determined by bulk titration data.

Wettability and ζ potentials of a series of methacrylate polymers and copolymers

Polymers and copolymers of different methacrylates were synthesized and coated on glass slides. The surfaces of the polymer films were characterized by their water contact angles and potentials using the Wilhelmy plate technique and streaming potential measurements, respectively. From contact-angle measurements information was also obtained about mobility of surface polymer chains. Receding contact angles of methyl methacrylate (MMA) copolymers containing hydrophilic or charged units were decreased as compared to the MMA homopolymer. When charged hydroxyethyl methacrylate (HEMA) copolymers were compared with the HEMA homopolymer, the advancing contact angles increased, probably due to reorientation of surface polymer chains. The receding contact angles of poly(alkyl methacrylates) first increased and then decreased with increasing side-chain lengths. These changes were related to the mobility of the different polymers. Incorporation of positively or negatively charged groups in MMA or HEMA polymers accordingly changed the potential of the polymers.

Interfacial tensions at acrylic hydrogel-water interfaces

Abstract The hydrogel-water interface is a highly mobile transition region, perhaps on the order of 100 A in depth, which interfaces the bulk gel network with the surrounding aqueous solution. Consideration of the adsorption characteristics at such an interface requires information on the interfacial free energy or interfacial tension. Such information is generally difficult to obtain by most conventional methods. In this paper, we have applied underwater contact angle techniques (sessile bubbles) as probes of fully hydrated interfaces. Utilizing work of adhesion and contact angle assumptions, and contact angle data using captive air, octane, and dodecane at the gel-water interface, we have obtained the hydrated gel surface free energy and the gel-water interfacial free energy. The interfacial free energy drops rapidly to near zero as the equilibrium water content of the gel network approaches 30 wt%. Apparently, the surface, even with relatively low water contents, has sufficient mobility to achieve a molecular orientation which minimizes the interfacial tension. Data are presented on copolymers of methyl methacrylate-hydroxyethyl methacrylate, ethyl methacrylate-hydroxyethyl methacrylate, methoxyethyl methacrylate-hydroxyethyl methacrylate, methacrylic acid-hydroxyethyl methacrylate, and dimethylaminoethyl methacrylate-hydroxyethyl methacrylate, all prepared in our laboratories, and a commercially prepared polyacrylamide gel with varying crosslink density. These systems cover the equilibrium water content range from roughly zero to in excess of 90% wt%. Brief discussions of the possible effects of surface and interfacial tension-induced deformation of the gel substrate and the effects of charge density at the interface are also included.

Preparation and properties of stereoregular poly(hydroxyethyl methacrylate) polymers and hydrogels

Abstract Linear poly(hydroxyethyl methacrylate) (PHEMA) has been synthesized in highly syndiotactic and highly isotactic configurations. The high syndiotactic PHEMA prepared by u.v. photolysis at −40° C was found by 13C n.m.r. to have a tactic triad content of 84% syndio, 16% hetero and 0% iso. High isotactic PHEMA was prepared by anionic polymerization of benzoxyethyl methacrylate in toluene followed by selective hydrolysis of the benzoate ester, and was observed by 13C n.m.r. to have a 5% syndio, 15% hetero and 80% isotactic triad content. A linear PHEMA polymer formed by radical polymerization at 60°C in ethanol solvent was found to have a tactic triad content of 58% syndio, 42% hetero and 0% iso. These polymers have been crosslinked with hexamethylene diisocyanate and their water swelling properties determined as functions of temperature and crosslinker concentration. Isotactic PHEMA exhibited greater aqueous swelling below 30° C than the syndiotactic PHEMA samples. The stereochemistry of the polymer chain is shown to be a factor in determining the swelling behaviour of hydrophilic methacrylate gels.

Streaming potential investigations: Polymer thin films

Abstract Streaming potential evaluations were carried out on a wide variety of biopolymer and synthetic polymer thin films supported on glass microscope slides. The film-forming and streaming evaluation techniques are sufficiently accurate to provide reproducible results between different investigators. Films evaluated to date include various silanes, albumins, agarose, and synthetic polymers based on hydroxyethyl methacryle (HEMA), n -butyl methacrylate ( n -BMA), and methylacrmethylate (MMA). The effect of negative and positive charge incorporation upon the streaming potential and thus the ζ potential was studied via incorporation of methacrylic acid (MAA) and both HC1 and CH 3 C1 salts of dimethylaminoethyl methacrylate (DMAEMA) into the neutral polymer chains of HEMA, n -BMA, and MMA via copolymerization. Thin films of polystyrene, polydimethyl siloxane, polyvinyl chloride, and avcothane were also evaluated successfully.

Clinical Evaluation of a Raman Scattering Multiple Gas Analyzer for the Operating Room

A Raman spectrometer multiple gas analyzer was used to monitor inspired and expired concentrations of oxygen (O2), nitrogen (N2), carbon dioxide (CO2), nitrous oxide (N2O), halothane, and isoflurane in 10 patients. The Raman spectrometer and a dedicated mass spectrometer were connected to each patient to provide a comparison of the two instruments. Results show that readings from the Raman spectrometer are within 0.62 vol% of known gas standards for O2, N2, N2O; within 0.03 vol% for CO2; and within 0.04 vol% for halothane, enflurane and isoflurane. Clinical results show that Raman spectrometer readings are within 1.36 vol% of the mass spectrometer readings for O2, N2, N2O; within 0.01 vol% for CO2; and within 0.22 vol% for halothane and isoflurane. The clinical and laboratory results indicate the Raman spectrometer monitors airway gases and vapors as accurately as a dedicated mass spectrometer.

Dedicated monitoring of anesthetic and respiratory gases By Raman scattering

The monitoring of respiratory and anesthetic gases in the operating room is important for patient safety. This study measured the accuracy and response time of a multiplegas monitoring instrument that uses Raman light scattering. Measurements of oxygen, carbon dioxide, nitrogen, nitrous oxide, halothane, enflurane, and isoflurane concentrations were compared with a gas mixer standard and with measurements made with an infrared anesthetic agent analyzer. Correlation coefficients were all greater than 0.999, and probable errors were less than 0.43 vol% for the gases and less than 0.03 vol% for the volatile anesthetics. Response time was 67 ms with a sample flow rate of 150 ml/min. There was some signal overlap between nitrogen and nitrous oxide and between the volatile anesthetic agents. Such overlap can be compensated for by linear matrix analysis. The Raman instrument promises a monitoring capability equivalent to the mass spectrometer and should prove attractive for the monitoring of respiratory and anesthetic gases in the operating room.