Amorphous polymer solids

Influence of Chromophore Organization on Triplet Energy Migration in Amorphous Polymer Solids... 

The purpose of this chapter is to review the kinetics and mechanisms of photochemical reactions in amorphous polymer solids. The classical view for describing the kinetics of reactions of small molecules in the gas phase or in solution, which involves thermally activated collisions between molecules of approximately equivalent size, can no longer be applied when one or more of the molecules involved is a polymer, which may be thousands of times more massive. Furthermore, the completely random motion of the spherical molecules illustrated in Fig. la, which is characteristic of chemically reactive species in both gas and liquid phase, must be replaced by more coordinated motion when a macromolecule is dissolved or swollen in solvent (Fig. b). Furthermore, a much greater reduction in independent motions must occur when one considers a solid polymer matrix illustrated in Fig. Ic. According to the classical theory of thermal reactions the collisional energy available in the encounter must be suificient to transfer at least one of the reacting species to some excited-state complex from which the reaction products are derived. The random thermal motion thus acts as an energy source to drive chemical reactions. 

Summarizing, thermochromism only occurs above the glass temperature in amorphous polymer solids, and above the melting temperature for crystalline polymers [45], The crystallization of side chains, which may or may not occur, is not a necessary condition for the thermochromic transition. A more crucial requirement for thermochromism is that of the necessary level of regioregularity of the polymer. This will help define the local geometry of the polymer, which is crucial for the optical properties. Long-range order, like that found in crystalline phases, is not necessary for expression of the chromatic transitions. 

Simha, R., Amorphous polymer solids at low temperatures. Journal of Macromolecular Science -Physics, B5(2), pp. 331-334 (1971). 

In the polymerization of DL-lactide, temperatures in the range of 135-155 C are typical. Since DL-Pl. is an amorphous polymer, solid-state polymerization does not occur as with PG and L-PL. The DL-PL polymerization reaction is also normally catalyzed by stannous octoate or stannic chloride dihydrate. 

Figure 5 Schematic temperature dependence of the shear modulus of an amorphous polymer. Solid line for a polymer with a molecular weight /Wi > Me, dashed line for a polymer with the molecular weight M2> Mi.

Reactions in amorphous polymer solids are first characterized in terms of influences of molecular motion of matrix polymers and non-homogeneity of reaction sites. Specific features of photophysical processes, photoisomerization, photodimerization, chain scission and crosslinking reactions in polymer solids are then discussed separately. 

In this review article, after a description of the historical background of the present subject, some general features of reactions in amorphous polymer solids are summarized and then specific features of photo-physical processes, photoisomerization, photodimerization, chain scission, and crosslinking reactions are discussed separately. Typical theoretical treatments of solid-state reactions are reviewed in relation to the interpretation of experimental results. 

In contrast to topochemical reactions in the crystalline state where the crystalline structure and the distance between reacting chemical bonds are crucial the reactions in amorphous polymer solids are governed by the mobility and heterogeneity of reactive sites. Besides chemical reactivity, factors which affect the reactions in polymer solids are summarized in Table 1. 

As has been outlined in Chapt 3, the isomerization reactions in amorphous polymer solids are appreciably influenced by local mobility and heterogeneity of reactive sites, often leading to the deviation of reaction profiles from first-order kinetics. However, this situation allows us to obtain an insist into the microstructure of amorphous polymer solids, e.g. distribution of local free volume, by using photoisomerization reactions as molecular probes. Since the photochromic phenomena in polymer solids were reviewed by Smets in 1983, our discussion below will be limited to more recent advances, putting emphasis on the explanation of non-homo-geneous progress of reactions in terms of the distribution of local free volume in matrix polymers. 

A synthesis of comblike organoboron polymer/boron stabilized imidoanion hybrids was examined via reactions of poly(organoboron halides) with 1-hexylamine and oligo(ethylene oxide) monomethyl ether and subsequent neutralization with lithium hydride (scheme 8). The obtained polymers (10) were amorphous soft solids soluble in common organic solvents such as methanol, THF, and chloroform. In the nB-NMR spectra (Fig. 11), neutralization of the iminoborane unit with lithium hydride... 

Monnerie, L., Laupretre, F. and Halary,. L. Investigation of Solid-State Transitions in Linear and Crosslinked Amorphous Polymers. Vol. 187, pp. 35-213. 

In summary, it is clear that water absorbs into amorphous polymers to a significant extent. Interaction of water molecules with available sorption sites likely occurs via hydrogen bonding such that the mobility of the sorbed water is reduced and the thermodynamic state of this water is significantly altered relative to bulk water. Yet accessibility of the water to all potential sorption sites appears to be dependent on the previous history and physical-chemical properties of the solid. In this regard, the water-solid interaction in amorphous polymer systems is a dynamic relationship depending quite strongly on water activity and temperature. 

Besides the multiplicity of defects that can be envisaged, there is also a wide range of solid phases within which such defects can reside. The differences between an alloy, a metallic sulfide, a crystalline fluoride, a silicate glass, or an amorphous polymer are significant. Moreover, developments in crystal growth and the production of nanoparticles have changed the perspective of earlier studies, which were usually made on polycrystalline solids, sometimes with uncertain degrees of impurity present. 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

The two main transitions in polymers are the glass-rubber transition (Tg) and the crystalline melting point (Tm). The Tg is the most important basic parameter of an amorphous polymer because it determines whether the material will be a hard solid or an elastomer at specific use temperature ranges and at what temperature its behavior pattern changes. 

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