Glass Transition Temperature T

Because of the perceived importance of the T in relation to physical or chemical stability of amorphous pharmaceutical formulations [16], experimental data are available for a large number of excipients and drug-excipient combinations including most systems chosen for MD simulation. Therefore, published MD simulations of glasses often include comparisons of values from MD simulation and experiment. As the T does not reflect a first-order phase transition such as the melting point for 

Systematic studies of the effect of cooling rate on for a number of low-molecular- 

The effects of plasticizers such as water on have been examined in numerous experimental studies [26a, 28] and a few MD simulations. Such comparisons provide a means of assessing the reliability of MD simulations for predicting possible effects of moisture on stability as influenced by molecular mobility associated with structural relaxation. 

Typically, the effect of water sorption on the T for an amorphous solid is estimated using the Gordon-Taylor equation [28a]  

For water uptake, the Simha-Boyer rule (i.e., Aa T -constant) is often applied to 

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Glass Transition. The glass-transition temperature T reflects the mechanical properties of polymers over a specified temperature range. 

Improved Hot—Wet Properties. Acryhc fibers tend to lose modulus under hot—wet conditions. Knits and woven fabrics tend to lose their bulk and shape in dyeing and, to a more limited extent, in washing and drying cycles as well as in high humidity weather. Moisture lowers the glass-transition temperature T of acrylonitrile copolymers and, therefore, crimp is lost when the yam is exposed to conditions requited for dyeing and laundering. 

The glass-transition temperature, T, of dry polyester is approximately 70°C and is slightly reduced ia water. The glass-transitioa temperatures of copolyesters are affected by both the amouat and chemical nature of the comonomer (32,47). Other thermal properties, including heat capacity and thermal conductivity, depend on the state of the polymer and are summarized ia Table 2. 

Polymer systems have been classified according to glass-transition temperature (T), melting poiat (T ), and polymer molecular weight (12) as elastomers, plastics, and fibers. Fillers play an important role as reinforcement for elastomers. They are used extensively ia all subclasses of plastics, ie, geaeral-purpose, specialty, and engineering plastics (qv). Fillets are not, however, a significant factor ia fibers (qv). 

Determination of the glass-transition temperature, T, for HDPE is not straightforward due to its high crystallinity (16—18). The glass point is usually associated with one of the relaxation processes in HDPE, the y-relaxation, which occurs at a temperature between —100 and —140° C. The brittle point of HDPE is also close to its y-transition. 

The value of the glass-transition temperature, T, is dependent on the stereoregularity of the polymer, its molecular weight, and the measurement techniques used. Transition temperatures from —13 to 0°C ate reported for isotactic polypropylene, and —18 to 5°C for atactic (39,40). 

Strong-Acid Catalysts, Novolak Resins. PhenoHc novolaks are thermoplastic resins having a molecular weight of 500—5000 and a glass-transition temperature, T, of 45—70°C. The phenol—formaldehyde reactions are carried to their energetic completion, allowing isolation of the resin ... 

A plasticizer is a substance the addition of which to another material makes that material softer and more flexible. This broad definition encompasses the use of water to plasticize clay for the production of pottery, and oils to plasticize pitch for caulking boats. A more precise definition of plasticizers is that they are materials which, when added to a polymer, cause an increase in the flexibiUty and workabiUty, brought about by a decrease in the glass-transition temperature, T, of the polymer. The most widely plasticized polymer is poly(vinyl chloride) (PVC) due to its excellent plasticizer compatibility characteristics, and the development of plasticizers closely follows the development of this commodity polymer. However, plasticizers have also been used and remain in use with other polymer types. 

The iatroduction of a plasticizer, which is a molecule of lower molecular weight than the resia, has the abiUty to impart a greater free volume per volume of material because there is an iucrease iu the proportion of end groups and the plasticizer has a glass-transition temperature, T, lower than that of the resia itself A detailed mathematical treatment (2) of this phenomenon can be carried out to explain the success of some plasticizers and the failure of others. Clearly, the use of a given plasticizer iu a certain appHcation is a compromise between the above ideas and physical properties such as volatiUty, compatibihty, high and low temperature performance, viscosity, etc. This choice is appHcation dependent, ie, there is no ideal plasticizer for every appHcation. 

Polycarbonates are an unusual and extremely useful class of polymers. The vast majority of polycarbonates are based on bisphenol A [80-05-7] (BPA) and sold under the trade names Lexan (GE), Makrolon (Bayer), CaUbre (Dow), and Panlite (Idemitsu). BPA polycarbonates [25037-45-0] having glass-transition temperatures in the range of 145—155°C, are widely regarded for optical clarity and exceptional impact resistance and ductiUty at room temperature and below. Other properties, such as modulus, dielectric strength, or tensile strength are comparable to other amorphous thermoplastics at similar temperatures below their respective glass-transition temperatures, T. Whereas below their Ts most amorphous polymers are stiff and britde, polycarbonates retain their ductiUty. 

Since successful commercialization of Kapton by Du Pont Company in the 1960s (10), numerous compositions of polyimide and various new methods of syntheses have been described in the Hterature (1—5). A successful result for each method depends on the nature of the chemical components involved in the system, including monomers, intermediates, solvents, and the polyimide products, as well as on physical conditions during the synthesis. Properties such as monomer reactivity and solubiHty, and the glass-transition temperature,T, crystallinity, T, and melt viscosity of the polyimide products ultimately determine the effectiveness of each process. Accordingly, proper selection of synthetic method is often critical for preparation of polyimides of a given chemical composition. 

Fig. 2. Glass-transition temperature, T, for two commercially available, miscible blend systems (a) poly(phenylene oxide) (PPO) and polystyrene (PS) (42) ...

Snow and wet traction are highly dependent on the tread pattern. Although the tread pattern overwhelms the compound properties in significance, the latter can play a role in optimizing snow traction. Compounds using polymers with low glass-transition temperature, T (—40 to —OS " C), remain more flexible at low temperatures. Tread compounds with low complex modulus at 0—20°C have better snow traction. 

Thermal Properties. Spider dragline silk was thermally stable to about 230°C based on thermal gravimetric analysis (tga) (33). Two thermal transitions were observed by dynamic mechanical analysis (dma), one at —75° C, presumed to represent localized mobiUty in the noncrystalline regions of the silk fiber, and the other at 210°C, indicative of a partial melt or a glass transition. Data from thermal studies on B. mori silkworm cocoon silk indicate a glass-transition temperature, T, of 175°C and stability to around 250°C (37). The T for wild silkworm cocoon silks were slightly higher, from 160 to 210°C. 

Glass-Transition Temperatures. The glass-transition temperature, T, of fully hydrolyzed PVA has been determined to be 85°C for high molecular weight material. The glass transition in case of 87—89% hydrolyzed PVA varies according to the following formula (59) ... 

Oxyhalide Glasses. Many glasses contain both oxide and haUde anions. The introduction of haUdes into an oxide glass typically serves to reduce the glass-transition temperature, T, and to increase the coefficient of thermal expansion. Oxyfluorophosphates have been investigated as laser host... 

Glass-Transition Temperature. When a typical Hquid is cooled, its volume decreases slowly until the melting point, T, where the volume decreases abmpdy as the Hquid is transformed into a crystalline soHd. This phenomenon is illustrated by the line ABCD in Eigure 3. If a glass forming Hquid is cooled below (B in Eig. 3) without the occurrence of crystallization, it is considered to be a supercooled Hquid until the glass-transition temperature, T, is reached. At temperatures below T, the material is a soHd. 

FZ Characterization. FZ elastomer is a translucent pale brown gum with a glass-transition temperature, T of —68 to —72 C. The gum can... 

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