Induction forces in polymers

Electrostatic forces also result from slight displacement of electrons and nuclei in covalent molecules from proximity to electrostatic fields associated with the dipoles from other molecules. These are induced dipoles. The displacements cause interactions between the induced dipoles and the permanent dipoles creating forces of attraction. The energy of the induction forces, however, is small and not temperature-dependent. 

Due to this hydrogen bonding, nylon 11 melts at 184—187 C and is soluble only in very strong solvents. Linear polyethylene, on the other hand, melts at 130-134 C and is soluble in hot aromatic solvents. 

The energy of dipole interactions, ( k) can be calculated from the equation [1]  

This flexibility of the four carbon segment in poly(ethylene adipate) contributes significantly to the lowering of the melting point. 

Linear polymers that possess only single bonds between atoms in their backbones, C-C, or C-O, or C-N, can undergo rapid conformational changes [5]. Also ether, imine, or cA-double bonds reduce energy barriers and, as a result, soften the chains, causing the polymer to become more rubbery and more soluble in various solvents. 

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As already mentioned molecules cohere because of the presence of one or more of four types of forces, namely dispersion, dipole, induction and hydrogen bonding forces. In the case of aliphatic hydrocarbons the dispersion forces predominate. Many polymers and solvents, however, are said to be polar because they contain dipoles and these can enhance the total intermolecular attraction. It is generally considered that for solubility in such cases both the solubility parameter and the degree of polarity should match. This latter quality is usually expressed in terms of partial polarity which expresses the fraction of total forces due to the dipole bonds. Some figures for partial polarities of solvents are given in Table 5.5 but there is a serious lack of quantitative data on polymer partial polarities. At the present time a comparison of polarities has to be made on a commonsense rather than a quantitative approach. 

Types of AppI ied Stress. Mechanical behavior of polymer-based materials depends on composition, structures, and interactions at molecular and super-molecular levels (5-7). The structures are much dependent on primary chemical (mostly covalent) bonding inside the chains and secondary bonding (dispersion van der Waals, induction, electrostatic, and hydrogen bonding, the last being the strongest in this category) forces in between chains (8). The composition often includes additives aimed at an improvement of a particular property. 

Unlike the case for metals, secondary bonds are of great importance in polymers. These bonds are much weaker than covalent bonds, but for even moderate chain length polymers these bonds have a significant impact on the molecular and bulk properties of these materials. These intermolecular bonds are based on electrostatic interactions and are due to either attractions between permanent dipoles, quadmpoles, and other multipoles, or between a permanent multipole and an induced charge on a second molecule (or moiety, in the case of a polymer), or between transient multipoles. All such secondary bonds can be considered van der Waals forces, but many texts use van der Waals to denote induced and/or transient multipole interactions only. The induced interaction is sometimes referred to as polarization, or sometimes induction bonding. The transient interaction is very weak and is known as dispersion or London dispersion forces, and arises from electrostatic interactions between two molecules due to temporary inhomogeneous electron density distributions in the outermost electron shells of these molecules. 

Polymer molecules interact with each other because of secondary bondings due to dipole forces, induction forces, and/or hydrogen bonds. The dipole forces arise when there are polar substituents on the polymer chain, as, for example, in polyvinyl chloride (PVC). Because of the substituent chlorine, the Tg value of PVC is considerably higher than that of polyethylene. Sometimes, forces are also induced due to the ionic nature of substituents (as in polyacrylonitrile, for example). The cyanide substituents of two nearby chains can form ionic bonds as follows ... 

Intermolecular forces will determine the behavior of all materials in every phase in which they exist. Intermolecular forces can be classified into (1) dispersion, (2) dipole, (3) induction, and (4) hydrogen bonding. The relative strength of these forces can be stated as dispersion < dipole < induction < hydrogen bonding. Owing to the low polarizability of the C—F bond, the dominant intermolecular force is often dispersive in character. The extension to more dominant forces should become obvious as more complicated molecules are discussed. The discussion here can be confined to simple pair-wise interactions between two molecules or polymer chains that contain C—F bonds. 

Gerasimov et al. have reported that poly-p-PDA Et is obtained quantitatively at 170 - 4.2 K and that the activation energy is 1600 300 eal/mol at 170 - 100 K and close to zero (<20 cal/mol) at 90 — 4.2 K, respectively. From the outstanding reactivity of p-PDA Et at an extremely low temperature, the barrier to the reaction in the monomer crystals has been attributed to the force of the crystal lattice and classified into the region of negative values of the potential energy. In addition the observed induction period at 4.2 K has been attributed to the growth period of crystal defects (see Sect. IV.a.) In the case of DSP, quantitative conversion of monomer to polymer crystals has been achieved by photoirradiation at — 60°C26). 

Surface tension of polymers can be divided into two components—polar (yP) and dispersion (y )— to account for the type of attraction forces at the interfaces. The chemical constitution of the surface determines the relative contribution of each component to the surface tension. The polar component is composed of various polar molecular interactions including hydrogen bonding, dipole energy, and induction energy, while the dispersion component arises from London dispersion attractions. The attractive forces (van der Waals and London dispersion) are additive, which results in the surface tension components to be additive y = y + y. ... 

Table 59.3 is based primarily on the Zisman critical surface tension of wetting and Owens and Wendt approaches because most of the polymer data available is in these forms. The inadequacies of equations such as Eq. (59.7) have been known for a decade, and newer, more refined approaches are becoming established, notably these of van Oss and coworkers [24]. A more limited number of polymers have been examined in this way and the data (at 20 °C) are summarized in Table 59.4. is the component of surface free energy due to the Lifshitz-van der Waals (LW) interactions that includes the London (dispersion, y ), Debye (induction), and Keesom (dipolar) forces. These are the forces that can correctly be treated by a simple geometric mean relationship such as Eq. (59.6). y is the component of surface free energy due to Lewis acid-base (AB) polar interactions. As with y and yP the sum of y and y is the total solid surface free energy, y is obtained from... 

One problem that can occur in the solid state that does not occur in the melt is that a large portion of the antioxidant that was soluble in the melt becomes insoluble in the solid state, where it blooms to the surface and is lost to its surroundings. This process is controlled by the solubility and diffusion of the antioxidant in the polymer, and it can be characterized by a TGA technique developed by Roe et al. (1974). The Roe-Bair-Gieniewski (RBG) method involves analyzing a concentration profile across a stack of polyethylene sheets through which the antioxidant has been forced to diffuse (Bair 1997). The level of antioxidant in the polyethylene sheets was determined by TGA based on an induction time calibration curve (Bair 1973). In this study... 

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