Heterogeneous polymer solids

Application to heterogeneous polymer solids, and elastic composites, is presented in the Section 7 (Gusev, Suter), which is followed by a summary and the outlook for the various methods reviewed here. It will be apparent to the reader that this review thus assembles several building blocks for the difficult task to bridge the gaps from the atomistic to the macroscopic scales in space and times for the simulation of polymeric materials. Integrating these building blocks into one coherent framework still is not fully solved and a matter of current research. 

Application of Atomistic Modeling Techniques to Heterogeneous Polymer Solids... 

A frequent complication in the use of an insoluble polymeric support lies in the on-bead characterization of intermediates. Although techniques such as MAS NMR, gel-phase NMR, and single bead IR have had a tremendous effect on the rapid characterization of solid-phase intermediates [27-30], the inherent heterogeneity of solid-phase systems precludes the use of many traditional analytical methods. Liquid-phase synthesis does not suffer from this drawback and permits product characterization on soluble polymer supports by routine analytical methods including UV/visible, IR, and NMR spectroscopies as well as high resolution mass spectrometry. Even traditional synthetic methods such as TLC may be used to monitor reactions without requiring preliminary cleavage from the polymer support [10, 18, 19]. Moreover, aliquots taken for characterization may be returned to the reaction flask upon recovery from these nondestructive... 

Figure 11. Maximum low temperature loss tangent (35 Hz) measured normal to mold flow vs. styrene block length heterogeneity. Circles— branched polymers, triangles—linear polymers, solid symbols—electron micrographs displayed.

Moreover, the sensitivity of the effect of a flame retardant to the ambient pressure should also be taken into account. Flame retardants that are active only in the gas phase usually fail to affect the composition of the volatile pyrolysis products and the coke yield. In this case whatever the nature of the polymer, the flame retarding element is released into the gas phase during combustion the type of oxidant (O /Nj, N O/N ) strongly affects the flammability. On the other hand the effect of flame retardants active in the solid phase depends on the polymer nature, but is independent of the nature of the oxidant. Variations of the pressure of the oxidative environment affect the rates of gas-phase as well as heterogeneous (interfacial solid-gas) reactions. 

The attention paid to the polymer solid state is minimized in favour of the melt and in this chapter the static properties of the polymer are considered, i.e. properties in the absence of an external stress as is required for a consideration of the rheological properties. This is addressed in detail in Chapter 3. The treatment of the melt as the basic system for processing introduces a simplification both in the physics and in the chemistry of the system. In the treatment of melts, the polymer chain experiences a mean field of other nearby chains. This is not the situation in dilute or semi-dilute solutions, where density fluctuations in expanded chains must be addressed. In a similar way the chemical reactions which occur on processing in the melt may be treated through a set of homogeneous reactions, unlike the highly heterogeneous and diffusion-controlled chemical reactions in the solid state. 

Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. 

Soluble polymer-bound catalysts can be expected to receive continued attention as they offer specific advantages. By comparison to aqueous two-phase catalysis, a range of substrates much broader with respect to their solubility can be employed. By comparison to heterogenization on solid supports, the selectivity and activity of homogeneous complexes can be retained better. However, it must also be noted that to date no system has been unambiguously proven to meet the stability and recovery efficiency required for industrial applications. 

The heterogeneity of the crystalline polymer solid is accentuated still more in the case of mechanical properties by the enormous mechanical anisotropy of the crystals and the large difference in the elastic moduli of the crystalline and amorphous components. With polyethylene, the elastic modulus of the crystals is 3452 or 2403 X 1010 dynes/cm2 in the chain direction (E ) and 4 X 1010 dynes/cm2 in the lateral direction (E ) (2, 3). The elastic modulus of the amorphous component (Ea) of polyethylene is 109-1010 dynes/cm2 (4). This is significantly less than Eu and Ebut at least 10 times the elastic modulus of a rubber that has about five monomers in the chain segments between the crosslinks. This is quite surprising since room temperature is far above the glass transition temperature of polyethylene (Tg is either —20°C or — 120°C), and therefore one would expect a fully developed rubbery... 

Since the 1960s position annihilation lifetime spectroscopy (PALS) has been used to measure free-volume cell size and/or its content in liquids or solids. The three chapters of Part III discuss correlations between the PALS experimental values and those computed from the S-S theory. Chapter 10, by Consolati and Quasso, considers free volume in amorphous polymers Chapter 11, by Dlubek, its distribution from PALS and Chapter 12, by Jamieson et al., the free volume in heterogeneous polymer systems. These state of the art texts offer intriguing observations on the structure of polymeric systems and its variation with independent variables. In all cases, good correlation has been found between the free-volume quantity measured by PALS and its variability computed from the S-S equation of state. 

In this chapter, we consider the sensitivity of the different NMR parameters to both molecular motions and spin dynamics. The first two parts illustrate the capability of NMR to study the local dynamics of bulk polymers at temperatures above and below the glass-transition temperature, Tg, respectively. The third part is devoted to the NMR investigation of the solid-state organization of heterogeneous polymer systems. In this last section, examples are taken mainly from the field of polymer blends. 

CNMR study of the molecular organization of some solid heterogeneous polymer systems... 

J. Clauss, K. Schmidt-Rohr, H.W. Spiess, Determination of domain sizes in heterogeneous polymers by solid-state NMR, Acta Polym. 44 (1993) 1—17. 

A. Buda, D.E. Demco, M. Bertmer, B. Blumich, B. Reining, H. Keul, H. Hocker, Domain sizes in heterogeneous polymers by spin diffusion using singlequantum and double-quantum dipolar filters. Solid State Nucl. Magn. Reson. 24 (2003) 39-67. 

---