Composites surface free energy

The broken bond approach has been extended by Nason and co-workers (see Ref. 85) to calculate as a function of surface composition for alloys. The surface free energy follows on adding an entropy of mixing term, and the free energy is then minimized. 

The effect of different types of comonomers on varies. VDC—MA copolymers mote closely obey Flory s melting-point depression theory than do copolymers with VC or AN. Studies have shown that, for the copolymers of VDC with MA, Flory s theory needs modification to include both lamella thickness and surface free energy (69). The VDC—VC and VDC—AN copolymers typically have severe composition drift, therefore most of the comonomer units do not belong to crystallizing chains. Hence, they neither enter the crystal as defects nor cause lamellar thickness to decrease, so the depression of the melting temperature is less than expected. 

In alloys, the component with the lower surface free energy segregates to the surface, making the surface composition different from that of the bulk (Scheme 5.1). 

Discuss the role of the surface free-energy in phenomena such as alloy segregation, surface reconstruction, faceting and sintering of small particles. How does a gas environment (air, vacuum) affect the surface composition of alloys ... 

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

There has been considerable recent interest in the determination by XPS of the surface composition of co-polymers whose components differ considerably in their surface free energy (51). When the individual components of block co-polymers or interpenetrating networks can be detected and individually identified by XPS, a fairly rapid co-polymer composition determination is possible as well as the determination of the composition of... 

At high crystallisation temperatures, the high molar mass polymer crystallised alone. Data for the fold surface free energy obtained from linear growth rate data supported the view that the nature of the fold surface of the dominant lamellae was related only to the molar mass of the crystallising component and was not affected by the composition of the melt. 

The solid carbonate can also present several potential difficulties in solubility studies. These can be broken down into two major areas heterogeneity in composition, and excess free energy associated with lattice strain or defects and surface free energy. The problem of solid heterogeneity presents itself in most sedimentary carbonates, and is especially important in biogenic carbonates such as magnesian calcites. The problem of lattice strain and high defect density is most... 

The surface free energy (s.f.e.) of a material is a measure for the work required to enlarge its surface (mJm-2). At constant pressure, constant temperature and if the surface composition remains constant, die s.f.e. equals the surface tension (ysv) of the material against its vapor [26], To determine the value of ysv and hence of the s.f.e. of a solid, contact angles with several liquids with known s.f.e. (yLv) are required. Thomas Young s equation relates the surface free energies and the contact angle (9) based on the force balance at the three-phase boundary [27] ... 

Surface segregation phenomena and the surface composition of bimetallic catalysts are controlled by the surface free energies of the constituents of the bimetallic particles. The deposition of metal on metal in relation to bimetallic catalysts has been discussed by Dodson [63]. 

Graphite-based materials are nowadays involved in a very wide range of systems. They are used as bulk products or are combined with other components to form composite materials. The improvement of the properties of these systems mostly depends on the modifications that occur at the surface for example, the mechanical behavior is dependent on the quality of the fiber/matrix interface and the water repellency is improved when the surface free-energy is reduced. 

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