Amorphous system glass-transition temperature

Further developments in this area have included the neparation of several additional N,N -diaryl indolo[3,2-h]carbazoles with substituents such as m-tolyl, ffi-anisoyl, or triarylamine-containing species. Like 221, these compounds, possessing excellent hole-transport properties, also occurred in stable amorphous states and displayed high glass-transition temperatures. LED devices involving these systems were also constructed and showed promising characteristics [OOSMO11-112)42]]. 

Heat capacity measurements at the glass transition temperature, Tg, are based on the same differential concept. The weight fraction of amorphous phase is calculated as the ratio of changes of heat capacity of the semi-crystalline sample ACp(S) over the change in heat capacity of the melt (ACp(m)) at the glass transition. For a two-phase system, the degree of crystallinity is given as ... 

In general, the miscibility between two polymers can be predicted by thermal characterization of the blends [36], One of the most simple and effective ways to predict miscibility between two polymers is to consider the behavior of the glass transition temperature in the blend systems, which is known as the Tg method. In miscible blend systems, only a single 7 g intermediate between two components appears in the amorphous state. Therefore, we studied the change of... 

Many polymer-salt complexes based on PEO can be obtained as crystalline or amorphous phases depending on the composition, temperature and method of preparation. The crystalline polymer-salt complexes invariably exhibit inferior conductivity to the amorphous complexes above their glass transition temperatures, where segments of the polymer are in rapid motion. This indicates the importance of polymer segmental motion in ion transport. The high conductivity of the amorphous phase is vividly seen in the temperature-dependent conductivity of poly(ethylene oxide) complexes of metal salts. Fig. 5.3, for which a metastable amorphous phase can be prepared and compared with the corresponding crystalline material (Stainer, Hardy, Whitmore and Shriver, 1984). For systems where the amorphous and crystalline polymer-salt coexist, NMR also indicates that ion transport occurs predominantly in the amorphous phase. An early observation by Armand and later confirmed by others was that the... 

For transport in amorphous systems, the temperature dependence of a number of relaxation and transport processes in the vicinity of the glass transition temperature can be described by the Williams-Landel-Ferry (WLF) equation (Williams, Landel and Ferry, 1955). This relationship was originally derived by fitting observed data for a number of different liquid systems. It expresses a characteristic property, e.g. reciprocal dielectric relaxation time, magnetic resonance relaxation rate, in terms of shift factors, aj, which are the ratios of any mechanical relaxation process at temperature T, to its value at a reference temperature 7, and is defined by... 

At low temperature the material is in the glassy state and only small ampU-tude motions hke vibrations, short range rotations or secondary relaxations are possible. Below the glass transition temperature Tg the secondary /J-re-laxation as observed by dielectric spectroscopy and the methyl group rotations maybe observed. In addition, at high frequencies the vibrational dynamics, in particular the so called Boson peak, characterizes the dynamic behaviour of amorphous polyisoprene. The secondary relaxations cause the first small step in the dynamic modulus of such a polymer system. 

The classic example of a NEAS is a supercooled liquid cooled below its glass transition temperature. The liquid solidifies into an amorphous, slowly relaxing state characterized by huge relaxational times and anomalous low frequency response. Other systems are colloids that can be prepared in a NEAS by the sudden reduction/increase of the volume fraction of the colloidal particles or by putting the system under a strain/stress. 

The mobility of the ions in polymer electrolytes is linked to the local segmental mobility of the polymer chains. Significant ionic conductivity in these systems will occur only above the glass transition temperature of the amorphous phase, Tg. Therefore, one of the reqnirements for the polymeric solvent is a low glass-transition temperature for example, Tg = —67°C for PEO. 

The model PBZT/ABPBI molecular composite system is limited since the rod and the matrix do not possess glass transition temperatures for subsequent post form consolidation. In an effort to improve the processability for molecular composites, thermoplastics were used as the host matrix. Processing from acidic solvents requires the thermoplastic host to be soluble and stable in meth-anesulfonic add. Thermoplastic matrices were investigated including both amorphous and semicrystalline nylons [71,72], polyphenylquinoxaline (PPQ) [73] and polyetheretherketone (PEEK) [74], Table 5 shows the mechanical properties obtained for various processed PBZT thermoplastic molecular composite systems. As an example, the PBZT/Nylon systems showed 50-300% improvement over uniaxially aligned chopped fiber composite of comparable compositions. However, the thermally-induced phase separation during consol-... 

These authors were the first FGSE workers to make extensive use of the concept of free volume 42,44) and its effect on transport in polymer systems. That theory asserts that amorphous materials (liquids, polymers) above their glass transition temperature T contain unoccupied volume randomly distributed and in parcels of sufficient size to permit jumps of small molecules — and of polymer jumping segments — to take place. Since liquids have a fractional free volume fdil typically greater than that, f, of polymers, the diffusion rate both of diluent molecules and (uncrosslinked and unentangled) polymer molecules should increase with increasing diluent volume fraction vdi,. The Fujita-Doolittle expression 43) describes this effect quantitatively for the diluent diffusion ... 

The homopolymers poly(methyl methacrylate) and poly-(ethyl methacrylate) are compatible with poly(vinylidene fluoride) when blended in the melt. True molecular com-patibility is indicated by their transparency and a single, intermediate glass transition temperature for the blends. The Tg results indicate plasticization of the glassy methacrylate polymers by amorphous poly(vinylidene fluoride). The Tg of PVdF is consistent with the variation of Tg with composition in both the PMMA-PVdF and PEMA-PVdF blends when Tg is plotted vs. volume fraction of each component. PEMA/PVdF blends are stable, amorphous systems up to at least 1 PVdF/I PEMA on a weight basis. PMMA/ blends are subject to crystallization of the PVdF component with more than 0.5 PVdF/1 PMMA by weight. This is an unexpected result. 

Mixtures of poly(vinylidene fluoride) with poly (methyl methacrylate) and with poly (ethyl methacrylate) form compatible blends. As evidence of compatibility, single glass transition temperatures are observed for the mixtures, and transparency is observed over a broad range of composition. These criteria, in combination, are acceptable evidence for true molecular intermixing (1, 19). These systems are particularly interesting in view of Bohns (1) review, in which he concludes that a compatible mixture of one crystalline polymer with any other polymer is unlikely except in the remotely possible case of mixed crystal formation. In the present case, the crystalline PVdF is effectively dissolved into the amorphous methacrylate polymer melt, and the dissolved, now amorphous, PVdF behaves as a plasticizer for the glassy methacrylate polymers. 

In addition to characterizing frozen systems intended to be freeze dried, it is important to characterize the freeze-dried product. This includes determination of the physical state of the dried product that is, crystalline, partially crystalline, or amorphous. It may also include identification of the polymorph of a crystallizing component which exhibits polymorphism and determination of whether the crystal form observed is affected by changes in formulation and processing conditions. For amorphous systems, the glass transition temperature of the amorphous solid, as well as the extent to which Ts changes with residual moisture, may be a critical attribute of the product with regard to both physical and chemical stability. 

---