Structure of Solid (Amorphous and Crystalline) Polymers

1. STRUCTURE OF AMORPHOUS/ CRYSTALLINE SOLID POLYMERS CIPET AHMEDABAD

2. Structure of amorphous polymers 1.1 Molecular-structure models of amorphous polymers The degree of order in placement of substituents and more bulky side groups along the molecular chain characterized by the tacticity of the polymer chain governs the propensity for crystallization. While in atactic polymers a lack of long-range regularity along the chain does not permit crystallization, isotactic or syndiotactic polymers can also be made to override crystallization by rapid cooling to reach a glassy state with little or no order.

3. Here we consider first the glassy state of simple linear chain polymers, for the purpose of more definitive considerations of the small-strain and large-strain deformation of these polymers and their eventual fracture behavior. In Chapter 1, discussing the atomic structure of metallic glasses, some 2D structure models were considered in order to develop a useful simple conceptualization of more complex phenomena. This approach cannot lead to meaningful results for linear-chain polymers for clear topological reasons. There, the 3D packing of molecular segments is an essential feature to take into account for any reliable insight into the structure and its form of deformation.

4. Thus, it becomes essential to start by development of chemically specific 3D structure models of linear-chain polymers, which will be essential for exploration of their large-strain inelastic behavior. How such models of amorphous polymer micro-structure can be developed by a variety of modeling methods has been described by Gentile and Suter (1993) and summarized by Argon (2001).

5. 1.2 Chemically specific molecular-structure models of amorphous polymers The development of chemically specific polymer molecular-structure models employing primarily the static-energy-minimization (SEM) technique was pion-eered by Suter and co-workers (for a-polypropylene (PP) by Theodorou and Suter(1985), for a-polycarbonate (PC) by Hutnik et al. (1991), for i-polyvinylchloride (PVC) by Ludovice and Suter (1992), and for i-polystyrene (PS) by Robyret al.(1999)).

6. In these models a representative molecular structure of the dense glassy state, which is characteristic of a certain temperature (density), is obtained when a typical molecule and its identical images, with a given degree of polymerization starting from an initial random-walk conformation of the molecule in space, is confined within an appropriate-sized cubic simulation cell by systematic relaxation of its potential energy by a conjugate-gradient method.

8. 1.3 Chemically non-specific models of amorphous polymer structure In many cases the complexity of the more accurate chemically specific molecular-structure models has not been necessary for some applications and their derivation has been circumvented by using more structureless approximations to the polyethylene molecule. For example, in a so-called “polybead” model of a vinyl polymer, relaxation of the chemical accuracy permitted the inclusion of other components of intra-molecular energy due to bond- stretching and bond-angle-flexing that were ignored in the chemically specific models discussed above, even though the need for including them in the polybead model was questionable.

9. Two such models were considered, one developed by Brown and Clarke (1991) with MD-based energy relaxation and one by Chui and Boyce (1999) incorporating 250 molecular chains, each with 48 particles (beads) per chain. The latter is shown in Fig. 2.10, where only the bond vectors are presented, to permit viewing into the structure. Fig. 2.10 A Monte Carlo simulation of the random spatial distribution of molecules in a “polybead model” of a glassy polymer

24. POLYMER SINGLE CRYSTAL

31. Jaccodine’s report25 of single crystals of polyethylene in 1955 gained attention of several researchers who expanded on his work. In 1957, Till26, Keller27, and Fischer28 independently reported on the growth and identification of single crystals of polyethylene. Since these studies, lamellar crystal habit has been shown to be the dominant structural mode of crystallization for a large number of polymers. The various models proposed for the nature of these structures are:

39. Degree of crystallinity The fraction of the ordered molecules in polymer is characterized by the degree of crystallinity, which typically ranges between 10% and 80%. Most methods of evaluating the degree of crystallinity assume a mixture of perfect crystalline andtotally disordered areas; the transition areas are expected to amount to several percent. Thesemethods include density measurement, differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared spectroscopy and nuclear magnetic resonance (NMR). In addition to the above integral methods, the distribution of crystalline and amorphous regionscan be visualized with microscopic techniques, such as polarized light microscopy andtransmission electron microscopy.