Polyethylene model structure

A family of ADMET model copolymers were synthesized and used to study the effects of regular microstructure on polymer properties, in particular functionahzed polyethylenes. The structure-property relationships of various ethylene copolymers can be clarified using these model systems. This is illustrated in Fig. 3 by the relationship of to functional group size. Future studies on these and similar systems should lead to fundamental discoveries concerning the class of materials known as polyethylenes and their physical properties. 

Polyethylene Properties of the different forms of polyethylene are reflected in their uses. Linear molecules of polyethylene can pack together very closely, as shown in the model of high-density polyethylene (HDPE). The branches of branched polyefhylene keep the molecules from packing fightly, as shown in the low-density polyethylene (LDPE) structure. The cross-links of cross-linked polyethylene (cPE) make it very strong. 

Typically, the temperature dependence of o is not large, but it may be either positive or negative. For example, 10 d In Rl Q/dTis —1.1 for polyethylene, 0.4 for atactic polystyrene, —0.1 to —0.3 for polyisobutylene, —0.2 for poly(ethyl acrylate), 2.2 for poly(hexyl methacrylate) and 0.5 for poly(vinyl alcohol). A large compendium of chain dimension parameters is available in the Polymer Handbook . A 1969 monograph by Flory gives a comprehensive summary of the application of the rotational isomeric state concept with near neighbor interactions to a great variety of real-chain model structures. 

Acrylonitrile polymerizes in the same way as ethylene. Notice that this polymer has the same structure as polyethylene, except that a CN group is attached to every second carbon atom, so the structure is a reasonable one. A line structure of polyacrylonitrile eliminates the clutter caused by the H atoms. A ball-and-stick model of the same polymer segment is included for comparison. 

To illustrate the utility of the MWBD method, a series of commercial polyvinyl acetates and low density polyethylenes are analyzed. Either kinetic models or 13c nuclear magnetic resonance results are used to estimate the branching structural parameter. 

At this point we return to the polymer which is simplest with respect to its chemical structure, namely polyethylene (PE). In addition, for this polymer, the experimental database is much more complete, and also simulations of chemically realistic models, such as those described by Eqs. (5.7)—(5.11), are possible at high temperatures (Fig. 5.2a). Thus the prospects are very good that more can be learnt about the merits, as well as the limitations, of this modeling approach. 

A rather crude, but nevertheless efficient and successful, approach is the bond fluctuation model with potentials constructed from atomistic input (Sect. 5). Despite the lattice structure, it has been demonstrated that a rather reasonable description of many static and dynamic properties of dense polymer melts (polyethylene, polycarbonate) can be obtained. If the effective potentials are known, the implementation of the simulation method is rather straightforward, and also the simulation data analysis presents no particular problems. Indeed, a wealth of results has already been obtained, as briefly reviewed in this section. However, even this conceptually rather simple approach of coarse-graining (which historically was also the first to be tried out among the methods described in this article) suffers from severe bottlenecks - the construction of the effective potential is neither unique nor easy, and still suffers from the important defect that it lacks an intermolecular part, thus allowing only simulations at a given constant density. 

It has also been inferred that differences found between crystallinities measured by density and those from heat of fusion by DSC area determination, as given for polyethylenes in the example of Figure 4 [72], may be related to baseline uncertainties, or not accounting for the temperature correction of AHc. Given that similar differences in crystallinity from density and heat of fusion were reported for isotactic poly(propylene) [43] and polyfaryl ether ether ketone ketone), PEEKK [73], other features of phase structure that deviate from the two-phase model may be involved in the crystallinity discrepancy. 

Fig. 11.2 Structural model of the composite of hydroxyapatite particles and high density polyethylene.

With the details associated with ADMET chemistry reasonably well understood, we have embarked on a study of the synthesis of well-controlled polymer structures via metathesis polycondensation chemistry [37]. A series of well-defined polyolefins have been designed to model the crystallization behavior of polyethylene and its related copolymers, including new materials synthesized by metallocene-based catalysts. This synthesis concept has been reduced to practice, and polymers that will aid in the understanding of branching within polyethylene itself have been produced. 

---