Oxidation of polyethylene with chromic acid/sulfuric acid generates a material (PE-C02H) having a high density of carboxylic acid and ketone functionalities in a thin surface layer on the polymer. This paper determines the extent of ionization of the surface and near-surface carboxylic acid groups of these materials in contact with water as a function of pH using three experimental techniques: measurement of attenuated total reflectance infrared (ATR-IR) spectra, measurement of contact angles, and direct potentiometric titration. On the basis of correlations between results obtained by these three techniques, we propose an equation relating the contact angle of an aqueous solution having a given value of pH and the extent of ionization (a;) of those carboxylic acid groups that are directly exposed to the solution. These carboxylic acid groups have broad titration curves and have C02H groups that are less acidic than soluble carboxylic acids. The initial ionization of these carboxylic acid groups occurs when the solution is approximately pH 6. The detailed structures of these oxidized polymer surface layers and the nature of the interactions between the carboxylic acid and carboxylate ions in them are still not completely defined. Salt effects on the extent of ionization a; at a particular value of pH are unexpectedly small and suggest that charge-charge interactions between carboxylate ions may not dominate the titration curves. This work demonstrates the usefulness of contact angle in following chemical changes occurring in organic functional groups on surfaces. Comparisons of wetting behavior of buffered and unbuffered solutions establishes the importance of using experimental protocols for measuring contact angles in which the reactive groups present in the interfacial region do not outnumber the reactive groups present in the small drop of dilute aqueous solution.

799heterogeneity of a solid, which is characterized by the adsorption potential distribution function X ( A ) , on the enthalpy and entropy of adsorption. In these expressions the characteristic curve 8JA) and the adsorption potential distribution X ( A ) are expressed by the integral eq 19 and 22, respectively, which contain the micropore distribution J(x) characterizing the structural heterogeneity of a microporous solid. It is noteworthy that eq 22 describes the relationship between energetic and structural heterogeneity of microporous solids, which are defined, respectively, by the distribution functions X ( A ) and J(x).Special cases of the general expressions given by eq 12 and 13 were discussed for the discrete and y-type micropore distribution functions. It has been shown that the y-type distribution, viz., eq 30, produces a simple equation for the characteristic Curve &(A), viz., eq 31, which is useful for microporous solids. To illustrate practical utility of this equation, the benzene adsorption isotherm on activated carbon AC12 was interpreted in order to evaluate the adsorption parameters, which are necessary to calculate the thermodynamic functions. This example shows that in the case of adsorption on a nonuniform microporous solid, besides the adsorption potential distribution function X ( A ) , which characterizes its energetic heterogeneity and is used to calculate the thermodynamic functions, the micropore distribution function J(x) should be also evaluated in order to characterize the solid structural heterogeneity.Acknowledgment. I thank Drs. J. Choma and J. Piotrowska for assistance in numerical calculations and helpful discussions.Oxidation of low-density polyethylene film with aqueous chromic acid results in a material (PE-C02H) having hydrophilic carboxylic acid and ketone groups in a thin oxidatively functionalized interface. This interface is indefinitely stable at room temperature. On heating under vacuum, it rapidly becomes hydrophobic and similar in its wettability to unfunctionalized polyethylene film. The progression of the contact angle with water from the initial value (55') to the final value (103') follows kinetics that suggest that the polar functional groups disappear from the interface by diffusion. The magnitude of the apparent diffusion constant derived from these studies can be described approximately by an Arrhenius equation over a significant portion of the temperature range explored, with an Arrhenius activation energy of diffusion of -50 kcalfmol. Comparison of the properties of interfaces composed of carboxylic acid groups with those containing other species demonstrates that the structure of the interfacial groups also significantly influences the rate of reconstruction. In particular, reconstruction is slow when the interfacial functional groups are large and polar (e.g., esters of poly(ethy1ene glycol)) and when they have structures that result in low solid-air interfacial free energies (e.g., CF3 moieties). Studies of reconstruction carried out with PE-C02H in con...

The level of acetate-group surface segregation in poly(vinyl alcohol-co-vinyl acetate) (PVA−PVAc) films was found to depend markedly on the functional group distribution along the backbone (blockiness). PVA−PVAc polymers with both random and blocky distributions were prepared at levels between 2 and 12 mol % acetate and cast into films from aqueous solution. Films from polymers with blocky distributions showed significantly higher levels of acetate at the surface than in the bulk, while polymers with random distributions of acetate functionality exhibited little or no surface segregation. The level of acetate surface segregation was determined by both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Blocky PVA−PVAc films containing 4 mol % were seen to have approximately 40 mol % PVAc at the outermost surface by XPS at low take-off angle. Bulk acetate levels and acetate group distributions were determined by NMR. Variations in the weight average molecular weight (M w) between 40k and 155k, modifying the casting solvent, or annealing the films above T g did not effect the level of surface segregation, suggesting that the segregation is not simply a kinetic phenomenon.

We have used video-microscopy to observe the behavior of liquid crystal (LC) droplets within nematic droplet-polymer films (NCAP) as the droplets respond to an applied electric field. The textures observed at intermediate fields yielded information about the process of liquid crystal orientation dynamics within droplets.The nematic droplet-polymer films had low LC content (less than 1 percent) to allow the observation of individual droplets in a 2-6Rm size range. The aqueous emulsification technique was used to prepare the films as it allows the straightforward preparation of low LC content films with a controlled droplet size range. Standard electro-optical (E-O) tests were also performed on the films, allowing us to correlate single droplet behavior with that of the film as a whole. Hysteresis measured in E-O tests was visually confirmed by droplet orientation dynamics; a film which had high hysteresis in E-O tests exhibited distinctly different LC orientations within the droplet when ramped up in voltage than when ramped down in voltage.Ramping the applied voltage to well above saturation resulted in some droplets becoming "stuck" in a new droplet structure which can be made to revert back to bipolar with high voltage pulses or with heat.

Dineopentylbis(triethylphosphine)platinum(II) reacts with H2 (34 psi) at 32 °C in hydrocarbon solvents and yields neopentane and tra«i-dihydridobis(triethylphosphine)platinum(II). The addition of triethylphosphine (L) changes the rate-limiting step of the reaction. When [L] = 0 M, the overall rate-limiting step is the dissociation of triethylphosphine from L2PtR2: no isotope effect is observed on substitution of D2 for H2, and the rate is independent of the pressure of H2. When [L] > 0.1 M, phosphine loss is reversible, and a later step, either addition of H2 to platinum or (more probably) elimination of neopentane from platinum, is rate limiting: the rate of reaction depends on the first order of the H2 pressure, and an isotope effect of kH/kD sa 1.9 is observed on substitution of H2 by D2. Activation parameters obtained at 0.0, 0.1, and 0.5 M triethylphosphine are presented. These data are useful in understanding the differences in rates of interand intramolecular oxidative additions to platinum(ll). The rate of reaction of H2 with several structurally related bis(phosphine)dialkylplatinum(II) compounds was surveyed under similar reaction conditions. In general, bulky substituents on platinum accelerate the reaction. This observation suggests that phosphine dissociation is a general feature of reaction of bis(phosphine)dialkylplatinum(II) compounds with dihydrogen.The formation and cleavage of bonds between carbon, hydrogen, and transition metals is important in many catalytic processes.3"6 This paper describes a study of the mechanism of the homogeneous hydrogenolysis of dineopentylbis(triethylphosphine)platinum(II) (l).7 The reaction proceeds under mild conditions (32 °C, 34 psi of H2) and yields neopentane and Zrwts-dihydridobis(triethylphosphine)platinum(II) (5) quantitatively (Scheme I). In Scheme I and throughout this paper L refers to triethylphosphine, R to CH2C(CH3)3, and LD to P(CD2CD3)3. We use [L] to refer to the concentration of L resulting from added triethylphosphine (that is, triethylphosphine added to the solution rather than that present as a result of dissociation from L2PtR2). Thus, [L] = 0.0