Polyethylene model backbone polymer

Molecular dynamics (MD) is an invaluable tool to study structural and dynamical details of polymer processes at the atomic or molecular level and to link these observations to experimentally accessible macroscopic properties of polymeric materials. For example, in their pioneering studies of MD simulations of polymers, Rigby and Roe in 1987 introduced detailed atomistic modeling of polymers and developed a fundamental understanding of the relationship between macroscopic mechanical properties and molecular dynamic events [183-186]. Over the past 15 years, molecular dynamics have been applied to a number of different polymers to study behavior and mechanical properties [187-193], polymer crystallization [194-196], diffusion of a small-molecule penetrant in an amorphous polymer [197-199], viscoelastic properties [200], blend [201,202] and polymer surface analysis[203-210]. In this article, we discuss MD studies on polyethylene (PE) with up to 120,000 atoms, polyethylproplyene (PEP), atactic polypropylene (aPP) and polyisobutylene (PIB) with up to 12,000 backbone atoms. The purpose of our work has been to interpret the structure and properties of a fine polymer particle stage distinguished from the bulk solid phase by the size and surface to volume ratio. 

What can ADMET offer in terms of tailoring the properties of a given polymer The answer lies in the clean chemistry of metathesis. If a metathesis active a,co-diene can be synthesized, then a known polymer can be produced. Few other polymerization techniques are so versatile, yet so precise. In recent years, our group has focused attention toward modeling polymers and copolymers made from ethylene in particular, we have been examining the effect of precise placement of alkyl and polar branches sequentially along tire backbone of polyethylene. 

The a bonds in the backbone of vinyl polymers should not be describable in terms of local states of small model molecules because of overlap of carbon atomic orbitals only 1.5X apart. This concept can be tested in polyethylene where the least bound C-C bond band widths have been calculated to be about 3 eV (.8). The energy loss function, Im(-l/e), for polyethylene is given in Figure 3 where and the real and imaginary parts of the... 

Recently, much attention has been devoted to modeling polyolefins and copolymers of ethylene and polar monomers. Eor example, polymers with regularly spaced methyl groups on a polyethylene backbone have been synthesized and display very interesting and surprising thermal properties and microstructure [12]. This represents a rational synthesis of branched polyethylene that cannot be achieved by any other means at this time (Scheme 6.12). 

The valence angle model, though more realistic than the freely jointed model, still underestimates the true dimensions of polymer molecules, because it ignores restrictions upon bond rotation arising from short-range steric interactions. Such restrictions are, however, more difficult to quantify theoretically. A simpler procedure is to assume that the conformations of each sequence of three backbone bonds are restricted to the rotational isomeric states that correspond to the potential energy minima such as those shown for n-butane in Fig. 2.3. For the simplest case of polyethylene and for vinylidene-type polymers, the application of the rotational isomeric state theory yields the following equation... 

We have used the tandem ADMET/hydrogenation method to produce a series of model EVA copolymers with pendant acetate functionality on every 19, 2V 23, and 2T carbon atom along the polyethylene backbone Figure 2, polymer 3, n = 18, 20, 22, and 26) [23]. Molecular weight analysis by GPC indicates that all of these polymers are in the range of 30,000 to 70,000 g/mol and have typical ADMET molecular weight distributions (PDI 2). Due to its low (23 °C), the copolymer with n = 18 lacks form stability at room temperature. However, the copolymers with n = 20, 22, and 26 all yield tough films and fibers from solution or the melt, and transparent or- translucent materials can be obtained from the melt. 

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