No abstract

This paper examines the relationship between the composition of two-component self-assembled monolayers (SAMs) of alkanethiolates on gold and the composition of the solutions from which they were formed. The SAMs were prepared by competitive adsorption of a long-chain alkanethiol (HS(CH2)21CHs) and a short-chain alkanethiol (HS(CH2)110H) from solutions in ethanol. Under conditions in which the alkanethiolates in a SAM and the alkanethiols in solution are close to equilibrium, the relationship between the composition of the solution and the composition of the SAM suggests that the monolayer tends, thermodynamically, to exist as a single phase predominantly composed of either long-chain or short-chain thiolates. A derivation of the thermodynamic relationship between the compositions of the SAM and solution is described that includes intermolecular interactions between components in the SAM; theory and experiment agree qualitatively. This analysis concludes that, for a two-component system of alkanethiolates on gold well-equilibrated with alkanethiols in solution, a single phase is preferred at equilibrium; phase-separated, two-component monolayers of the sort extensively studied in Langmuir systems are not observed. Introduction'Mixed SAMs" on gold-self-assembled monolayers (SAMs) comprising two alkanethiolates prepared by m-chemisorption from a solution containing two alkanethiols-are useful in studying phenomena involving organic surface^.^-'^ A number of techniques can establish the average composition of a mixed SAMa2-11 X-ray photoelectron spectroscopy, XPS, can ascertain the compositionover an area thesizeofthe X-ray spot (approximately 1 mmz), and scanning electron microscopy can sometimes detect heterogeneities in composition of a SAM on the dimension of the order of 1 pm;14 phase separation has not been observed on these scales. In short, neither has the extent to which the components of a mixed SAM segregate into separate phases within a SAM been established nor has the question of the relationship between the composition of the solution used to form the SAM and its heterogeneity been addressed experimentally or theoretically. The problem is a difficult one experimentally, because there are few techniques well adapted for direct visualization of phase-separated regions with small sizes in an organic monolayer, especially on optically opaque substrates. Scanning probe microscopies (especially atomic and lateral force microscopies)17 are techniques that are applicable but only to certain types of SAMs.17-19In this paper, we investigate the phase behavior of twocomponent SAMs from both theoretical and experimental perspectives. The approach we have taken is thermodynamicto search for characteristic signatures of phase separation in the dependence of average composition on temperature, concentration, and time-rather than spectroscopic and based on imaging. We focused on the question of whether two-component SAMs form phase-separated domains. In the theoretical section, we derive a thermodynamic relationship b...

No abstract

The relationship between the fracture toughness (G c) and the areal density of diblock copolymer at the interface (Σ) was investigated for joints between polypropylene (PP) and polyamide-6 (PA6), two incompatible, semicrystalline polymers. Diblock copolymers were formed in situ by reaction between succinic acid groups terminally grafted onto 5% of the PP chains and the NH2 ends of the PA6 chains. Fracture toughnesses were measured using an asymmetric double cantilever beam test (ADCB). After the bulk PA6 had been completely rinsed from an adhered sample, X-ray photoelectron spectroscopy (XPS) was used to measure the areal density of copolymer at the interface. Above the melt temperature of PP, but below that of PA6, reaction at the interface was limited by diffusion of the reactive PP chains (D = 1.58 × 10-11 cm2 s-1 at 213 °C). By controlling the temperature and the time of formation, G c could be varied in the range from 5 to 100 J/m2. Dissipation during fracture was observed to occur by plastic deformation in the PP, and failure of the joint was determined to occur by chain scission of the PP part of the copolymer. The fracture toughness was found to vary as the square of the areal density of copolymer at the interface, a relationship similar to that observed and predicted for glassy polymers. This scaling behavior suggests that the stresses in the fracture are concentrated over a distance on the order of 10 nm at the head of the crack.

No abstract

We present an investigation of the mechanisms of mechanical reinforcement at interfaces between polypropylene (PP) and polyamide-6 (PA6), associated with the incorporation of a small amount of maleic anhydride functionalized PP (PP-g-MA) which reacts with the NH2 groups of the PA6 to form a copolymer in situ. In a previous study we have demonstrated, for one molecular weight of PP-g-MA, that diblock copolymer molecules were indeed formed at the interface, with an areal density Σ, controlled by the reaction temperature and the reaction time, and that the measured fracture toughness of the interface scaled as G c ∝ Σ2, regardless of the reaction temperature, but for similar sample cooling conditions. We report here the behavior of the same system for a higher molecular weight PP-g-MA:  at a reaction temperature above 220 °C, very close to the melting point of the PA6, and above a given Σ, the measured G c becomes 4 times higher than that for reaction temperatures below 220 °C, where the observed G c values are identical to what has been measured for the low molecular weight PP-g-MA. G c is therefore no longer uniquely dependent on Σ. Crystallographic analysis on the PP side of the interface showed a correlation between the presence of the PP β-phase in the 20 μm near the interface and a high toughness; this crystalline phase was not present in the samples prepared at T ≤ 220 °C or with the low molecular weight PP-g-MA which always exhibited a low toughness even for samples prepared above 220 °C. It is argued that the presence of this β phase of the PP is the main factor responsible for the very high fracture toughness, first evidence of the influence of the crystallinity of a semi crystalline polymer on its adhesive properties.

Micromachining allows the formation of micrometer-sized regions of bare gold on the surface of a gold film supporting a self-assembled monolayer (SAM) of alkanethiolate. A second SAM forms on the micromachined surfaces when the entire system—the remaining undisturbed gold-supported SAM and the micromachined features of bare gold—is exposed to a solution of dialkyl disulfide. By preparing an initial hydrophilic SAM from HS(CH 2 ) 15 COOH, micromachining features into this SAM, and covering these features with a hydrophobic SAM formed from [CH 3 (CH 2 ) 11 S] 2 , it is possible to construct micrometer-scale hydrophobic lines in a hydrophilic surface. These lines provide new structures with which to manipulate the shapes of liquid drops.

No abstract