Transparent conductors as solar energy materials: A panoramic review

The glass transition temperature of an amorphous pharmaceutical solid is a critical physical property which can dramatically influence its chemical stability, physical stability, and viscoelastic properties. Water frequently acts as a potent plasticizer for such materials, and since many amorphous solids spontaneously absorb water from their surroundings the relationship between the glass transition temperature and the water content of these materials is important. For a wide range of amorphous and partially amorphous pharmaceutical solids, it was found that there is a rapid initial reduction in the glass transition temperature from the dry state as water is absorbed, followed by a gradual leveling off of the response at higher water contents. This plasticization effect could generally be described using a simplified form of the Gordon–Taylor/ Kelley–Bueche relationships derived from polymer free volume theory. Most of the systems considered showed a nearly ideal volume additivity and negligible tendency to interact. This is consistent with the hypothesis that such mixtures behave as concentrated polymer solutions and indicates that water acts as a plasticizer in a way similar to that of other small molecules and not through any specific or stoichiometric interaction process(es).

The relationship between the glass transition temperature and the water content of amorphous pharmaceutical solids.

▪ Abstract Applications of state-of-the-art atomic force microscopy methods to the elucidation of the surface and near-surface structure of polymeric solids are described. Contact, tapping, force modulation, frictional force, and other modes of atomic force microscopy are described, and recent results are summarized. Conformational and chain order, crystalline order, polymer crystals, lamellar structures, lamellar surfaces, fold surfaces, and fibers and films with highly oriented molecules all yield important information. Controlled deformation of polymer surfaces, both reversible and irreversible, with the atomic force microscope, provides a wealth of information about mechanical properties on a nanometer scale. The observation of phase-separated regions and of polymer crystals lying below a smooth surface shows that not only topography but also elastic inhomogeneity can be observed in great detail with the atomic force microscope. This is a rapidly developing field, and some indications of future develo...

Characterization of polymer surfaces with atomic force microscopy

Interpenetrating polymer networks (IPN’s) are a unique type of polyblend, synthesized by swelling a crosslinked polymer (I) with a second monomer (II), together with crosslinking and activating agents, and polymerizing monomer II in situ (Sperling, 1974–1975 ; Sperling and Friedman, 1969). The term IPN was adopted because, in the limiting case of high compatibility between crosslinked polymers I and II, both networks could be visualized as being interpenetrating and continuous throughout the entire macroscopic sample.* As with other types of polyblends, if components I and II consist of chemically distinct polymers, incompatibility and some degree of phase separation usually result (Sperling, 1974–1975; Sperling and Friedman, 1969; Sperling et al., 1970a,b,c; 1971). Even under these conditions, the two components remain intimately mixed, the phase domain dimensions being on the order of hundreds of angstroms. If one polymer is elastomeric and one polymer is plastic at the use temperature, the combination tends to behave synergistically, and either reinforced rubber or impact-resistant plastics result, depending upon which phase predominates (Curtius et al., 1972; Sperling and Mihalakis, 1973; Sperling et al., 1971; Huelck et al., 1972). Among the other kinds of polymer blends discussed in this monograph, the graft-type copolymers are the ones most closely related to the IPN’s.

Interpenetrating Polymer Networks

Visualization of macromolecules--a first step to manipulation and controlled response.

Electronic origin of scanning tunneling microscopy images and carbon skeleton orientations of normal alkanes adsorbed on graphite

Variable temperature magic angle spinning 13C NMR experiments on linear alkanes and low molecular weight polyethylene allowed the observation of up to six carbon atoms at the chain ends near the surface of the lamellar crystallites. Conformational order-disorder and intermolecular packing effects are reflected by the spectra. The effects are discussed with regard to the peculiar phase behaviour of the n-alkanes and the fringed micella model for low molecular weight polyethylene. The solid-solid transitions of nonadecane and hexatriacontane appear to be correlated with the occurrence of specific non-liquid-like conformational disordering. Thus, conformational defects were observed for the “rotator phase” which depend on the chain length and which vary along the chain. Whilst the center part of the chain remains all-trans ordered, significant gauche defects were observed for the bonds at the chain ends. For a polydisperse low molecular weight linear polyethylene as well as for monodisperse octahexacontahectane reversible liquid-like conformational disordering of the terminal chain segments could be detected when the melting point was approached. Hence, in this case the NMR spectra indicate pre-melting of the lamella surface, while in case of the shorter n-alkanes the occurrence of end-gauche defects is correlated with solid-solid transitions.

Phase transitions and defect structure in the lamellar surface of polyethylene and n-alkane crystallites. Magic angle spinning 13C NMR studies†

STM study of molecular order and defects in the layers of cycloalkanes (CH2)48 and (CH2)72 adsorbed on graphite

An attempt is made to extend the model of Leibler for the emulsifying activity and interfacial properties of A-b-B diblock copolymers in incompatible blends of the homopolymers A and B-which are identical with the respective copolymer components- to enthalpically interacting C-b-D diblock copolymers, the block C being thermodynamically compatible with A and D with B. Due to the attractive enthalpic interaction the A/C-b-D/B compatibilized blends are promising for optimum phase adhesion (bold types for thermodynamically compatible partners). Thus, the extended model for a plane interfacial layer includes the enthalpic interaction of the compatible polymer pairs beside the entropic effects. The approach starts with the equillibrium supposition, not taking into consideration enthalpy driven migration effects of the block copolymer from the bulk to the interface, The model confirms a dominant role of the enthalpic interaction between blocks of the diblock copolymer and the respective homopolymers to the compatibilization of incompatible blend components. It is applicable also for blends compatibilized with block copolymers of unfavourable repulsive type interaction, A/C-b-D/B, and for blend systems with mixed type interactions, e. g. A/C-b-B/B or A/C-b-D/B.

Enthalpic interaction of diblock copolymers with immiscible polymer blend components

In a previous publication a new concept has been presented for the composition dependence of the glass transition in compatible polymer blends. The concept starts with the idea that besides conformational energy barriers, contacts due to interaction are responsible for both conformation and “free” volume distribution. In accordance with the Flory-Huggins lattice theory, the contact probability has been related to the volume fraction of the components, corrected for the different volume expansion coefficients. The concept accounts for both chainspecific and interaction-specific contributions. It is concluded that local chain orientation due to the increased hetero-contact interaction may explain the observed molecular weight influences on the composition dependence of Tg of polymer blends. Using experimental data for poly(vinyl methyl ether)-polystyrene (PVME-PS) blends it is shown that this local chain orientation due to hetero-interaction is extremely strong in blends of the low molecular weight PS with high MW PVME. Thus, apparently, the stiffer component is dominating the Tg behaviour of compatible polymer blends.

Hans-Joachim Cantow

Papers

Electronic origin of scanning tunneling microscopy images and carbon skeleton orientations of normal alkanes adsorbed on graphite

Phase transitions and defect structure in the lamellar surface of polyethylene and n-alkane crystallites. Magic angle spinning 13C NMR studies†

STM study of molecular order and defects in the layers of cycloalkanes (CH2)48 and (CH2)72 adsorbed on graphite

Enthalpic interaction of diblock copolymers with immiscible polymer blend components

Approach to the composition dependence of the glass transition temperature of compatible polymer blends, 2 The effect of local chain orientation†‡