Polymers in amorphous state

Gedde, U. W. (2001), The Glassy Amorphous State in Polymer Physics, 4th ed., Kluwer Academic, Amsterdam, Chapter 5. 

Since polymer chains are largely immobilized below Tg, if they are cooled rapidly through T to below Tg, it is sometimes possible to obtain a metastable amorphous state in polymers which at equilibrium would be crystalline. As long as the material is held below Tg, this metastable amorphous state will persist indefinitely. When annealed above Tg and below T ,), the polymer will crystallize, as the chains gain the mobility necessary to pack into a lattice. 

Blends of enzymatically synthesized poly(bisphenol-A) and poly(p-r-butylphenol) with poly(e-CL) were examined. FT-IR analysis showed the expected strong intermolecular hydrogen-bonding interaction between the phenolic polymer with poly(e-CL). A single 7 was observed for the blend, and the value increased as a function of the polymer content, indicating their good miscibility in the amorphous state. In the blend of enzymatically synthesized poly(4,4 -oxybisphenol) with poly(e-CL), both polymers were miscible in the amorphous phase also. The crystallinity of poly(e-CL) decreased by poly(4,4 -oxybisphenol). 

G. Mitchell, Order in the Amorphous State of Polymers, Plenum Press, New York (1986). 

The diffusion coefficient D is inversely related to the cross-link density of vulcanized rubbers. When D is extrapolated to zero concentration of the diffusing small molecules, it is related to the distance between the cross-links. Thus, as the cross-link density increases D becomes smaller, as expected. Further, the diffusion coefficient is less for crystalline polymers in comparison with the same polymer except in the amorphous state. In fact, this can be roughly stated as follows. 

The rubbery amorphous state of polymers has the greatest correspondence with the liquid state of organic compounds. So it may be expected that the molar volume per structural unit of polymers in this state can be predicted by using the averaged values of the group contributions mentioned in Table 4.5 (Van Krevelen and Hoftyzer, 1969). 

There seems to be no limit to the types of pharmaceutical systems that can be isolated in the amorphous state. In the literature, samples of sugars, acids, bases, polymers, buffers, inorganics, salts, natural products, proteins, and low-molecular-weight APIs have all been reported to exist in an amorphous form. Likewise, pharmaceutical raw materials, intermediates, and final products that include these amorphous materials are widespread and varied (Table 1). 

In amorphous state, solid polymers retain the disorder characteristic for liquids, except that the molecular movement in amorphous solid state is restrained. The movement of one molecule versus the other is absent, and some typical liquid properties such as flow are absent. At low stress, polymers display elastic properties, reverting to a certain extent to the initial shape in a relaxation process. However, they can be irreversibly deformed upon application of appropriate force. The deformation and flow of polymers is very important for practical purposes and is studied by a branch of science known as rheology (see e.g. [1]). The combination of mechanical force and increased temperature are commonly applied for polymer molding for their practical applications. The polymers that can be made to soften and take a desired shape by the application of heat and pressure are known as thermoplasts, and most linear polymers have thermoplastic properties. 

For the above reasons, it is probably preferable that the samples be totally quenched to obtain a fully amorphous sample, and then to anneal the samples to achieve the correct crystallization profile. This is of course possible only for a polymer that can be quenched to its fully amorphous state. In some cases, the rate of crystallization for the polymers may be so high that some amount of crystallization is unavoidable. Most thermal analysis methods used for measuring crystallinity, such as the DSC or DMA methods, preclude the availability of data relating the heat of fusion or the storage modulus against the crystallinity [2]. Such data may not be always available especially for polymers that either do not fully crystallize or cannot be fully amorphous. An alternative method to determine the absolute crystallinity value would be to use the X-ray diffraction technique [. ]. 

Chain flexibility also affects the crystalhzabihty of a polymer. Excessive flexibility in a polymer chain, as in natural rubber and polysiloxanes, gives rise to difficulty in chain packing, with the result that such polymers remain almost completely in the amorphous state. In the other extreme, excessive rigidity in polymers due to extensive cross-hnking, as in thermosetting resins like phenol-formaldehyde and urea—formaldehyde, also results in an inabihty to crystallize. 

The amorphous state is the characteristic of all polymers at temperatures above their melting points (except under special circumstances where liquid crystals may form). If a molten polymer retains its amorphous nature on cooling to the solid state, the process is called vitrification. In the vitrified amorphous state, the polymer resembles a glass. It is characteristic of those polymers in the solid state that, for reasons of structure, exhibit no tendency toward crystallization. The amorphous solid state is characterized by glass transition (Tg), which is described in a later section. We consider below only the behavior of polymer melt. 

Boyer, R. R, Evidence from Tll and related phenomena for local structure in the amorphous state of polymers, in Order in the Amorphous State, Miller, R. L., and Rieke, J. K., Eds., Plenum Press, New York, 1987. 

Boyer RF (1987) In Keinath SE, Miller RL, Reieke JK (Eds) Order in the amorphous state of polymers, Plenum Press, New York, p 135 Pechhold W (1968) Kolloid-Z Z Polym 228 1... 

In MD, too, demands for the future should be obvious from the discussion in Sections 3, 6, and 7.1. To obtain realistic simulations of the dynamics, not only is it essential to simulate considerably longer real-time relaxations but also to include an adequate number of particles in the simulation box. This is particularly important for modeling amorphous states of polymer systems for which the demands of computer storage and speed are critical and at present inadequately met. 

At the Tg, a material change occurs from a solid rigid glassy state to a soft amorphous state. The polymer molecules are transformed from a somewhat ordered state to a random state of high molecular motion. The change in CTE at the can be two to five times greater than its value before... 

Semicrystalline polymers are polymers that contain both crystalline and amorphous states. In general, the major effect of irradiation, either electron beam or y-rays, on the crystalline region is to cause some imperfections. At high levels of irradiation the original crystalline structure tends to be progressively destroyed and is nearly always accompanied by a drop in the crystalline melting point, Tm. An example is that of poly(ethylene terephthalate), which shows a decrease in melting point of approximately 25 °C after irradiation (20 MGy) [51]. 

A. Bernes and C. Locahanne, in S. E. Keithwath, ed.. Order in the Amorphous State of Polymers, Plenum Publishing Corp., New York, 1987, p. 305. 

The term random coil is often used to describe the unperturbed shape of the polymer chains in both dilute solutions and in the bulk amorphous state. In dilute solutions the random coil dimensions are present under Flory 0-solvent conditions, where the polymer-solvent interactions and the excluded volume terms just cancel each other. In the bulk amorphous state the mers are surrounded entirely by identical mers, and the sum of all the interactions is zero. Considering mer-mer contacts, the interaction between two distant mers on the same chain is the same as the interaction between two mers on different chains. The same is true for longer chain segments. 

Figure 5.5 Models of the amorphous state in pictorial form, (a) Flory s random coil model the (b) Privalko and Lipatov randomly folded chain conformations (c) Yeh s folded-chain fringed-micellar model and (d) Pechhold s meander model. Models increase in degree of order from (a) to (d). References, (a) P. J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953. (b) V. P. Privalko and Y. S. Lipatov, Makromol. Chem., 175, 641 (1972). (c) G. S. Y. Yeh, J. Makoromol. Scl. Phys., 6, 451 (1972). (cf) W. Pechhold, M. E. T. Hauber, and E. Liska, KolloIdZ. Z. Polym., 251, 818 (1973). W. Pechhold, lUPAC Preprints, 789 (1971).

---