Glass-transition temperature dependence

A report130 of DSC measurements on polybenzimidazole fibers describes important differences for the glass transition temperature depending on die mechanical treatment of the fiber. An as-spun fiber exhibits a Tg at 387°C instead of 401°C for a drawn fiber free to shrink or 435°C for a drawn fiber widi fixed length. 

For blends consisting of components with sufficiently different glass transition temperatures, like PS/PPE (Tg(PS) = 105 °C, Tg(PPE) = 220 °C),two phases (two glass transition temperatures) can still be detected for the blended powder. However, the melt and the solid obtained from the melt are only composed of a single phase (with only one glass transition temperature, depending on the composition of the blend). 

This new development in the microstructural architecture of polybutadiene has opened the door for the preparation of various block copolymers made from the same monomer. For example, one can use this concept to prepare various polybutadiene rubbers in which the chain segment contains various glass transition temperatures, depending on its microstructural arrangements. Similarly, manipulating the polymerization temperature using the same modifier and... 

Table I. Soft-Segment, Glass-Transition-Temperature Dependence on Hard-Segment Content for PTMO/MDI/BD(ET) - and PTMA/MDI/BD (ES) -Segmented Copolymers...

Fig. 1. Glass transition temperature dependence on poly(isobutylene-co-p-Pinene) Composition...

Particularly for freeze-dried products, formulation and process are interrelated. Properties of the formulation, in particular the collapse temperature, will have a significant impact on the ease of processing. An efficient process is one that runs a high product temperature. However, the temperature cannot be too high or product quality will be compromised. As the glass transition temperature depends on chemical composition of the amorphous phase, Tg and collapse temperature are strongly formulation dependent. Collapse temperatures for common excipient systems vary from less than —50°C to around —10°C (Table 2). 

Copovidone forms soluble films, independently of the pH, regardless of whether it is processed as a solution in water or in organic solvents, or as a powder. It differs from povidone as a film-forming agent in that it is less hygroscopic (see Section 4.2.4.4) and has greater plasticity and elasticity. At the same time the films are also less tacky. The glass transition temperature depends on the moisture content, and at 103°C for dry copovidone, is also below that of dry povidone K 30 (i68°C). 

Fig. 4. Glass transition temperature dependence on the EBBA content for PC-EBBA system (curve 1). For comparison the plots Tg vs. contents of chlorinated terphenyl, chlorinated diphenyl and dibutyl phthalate in PC also are presented (curves 2, 3 and 4 respectively).

Since water is a plasticizer for starch, the glass transition temperature depends on the water content. DSC measurements were used to determine the glass transition temperature as a function of the water content. Fig 3 shows the results of these measurements. Della Valle et al. have collected data on the dependency of the Tg on the water content from several sources. The area between the dashed lines in the figure represents the band in which this data can be found. From the figure it is obvious that the present measurements, represented by the solid line, folly agree with this data. 

The presence of stresses in the adhesive layer alters the polymer s physical and chemical properties. The glass-transition temperature depends on the internal stresses that appear in the polymer. For example, the glass-transition temperature is known to decrease with increase in internal stresses in films based on polystyrene [169] and other pol5rmers. Note that restriction of the macromolecular mobility... 

However, the glass transition temperature dependence on plasticizer volume fraction is only linear for a relatively small range of plasticizer concentrations and equation [10.15] gives large differences between experimental and predicted values. Other departures are observed when the glass transition temperatures of the polymer and the plasticizer are close to each other (see Figure 10.34, curve 3). 

The numerical value of the glass-transition temperature depends on the rate of measurement (see Section 10.1.2). The techniques are therefore subdivided into static and dynamic measurements. The static methods include determinations of heat capacities (including differential thermal analysis), volume change, and, as a consequence of the Lorentz-Lorenz volume-refractive index relationship, the change in refractive index as a function of temperature. Dynamic methods are represented by techniques such as broad-line nuclear magnetic resonance, mechanical loss, and dielectric-loss measurements. Static and dynamic glass transition temperatures can be interconverted. The probability p of segmental mobility increases as the free volume fraction / Lp increases (see also Section 5.5.1). For /wlf = of necessity, p = 0. For / Lp oo, it follows that p = 1. The functionality is consequently... 

The glass-transition temperature depends on the mobility of the chain segments and can therefore be raised by stiffening the chain (see Section 10.5.3). Thus, a-methyl styrene forms a polymer that, in contrast to poly-(styrene), does not deform at lOC C, because of a glass-transition temperature of 170°C. However, since the thermodynamic ceiling temperature for for the polymerization/depolymerization equilibrium is also simultaneously lowered (see Section 16.3), poly(a-methyl styrene) degrades more easily than poly(styrene), so that it is not so easy to work by injection molding. 

The behavior of a thermoplastic material above its glass transition temperature depends on its level of crystallinity. As a noncrystalline (amorphous) polymer is slowly heated from a temperature below its Tg, it displays a large decrease in modulus as the glass transition temperature is reached. As one heats a semicrystalline plastic from a temperature below its Tg, it displays a relatively small modulus change at the glass transition temperature, followed by a plateau and then a decreasing modulus as the temperature increases and approaches the crystalline melting point. 

Nano particles, such as aluminum powder, quartz and multiwalled carbon nanotubes were added to aniline prior to its polymerization." Glass transition temperature depended on whether nucleating agent was present or not." Addition of nucleating particles also decreased degradation rate to various degrees depending on the particle size and its specific surface area." ... 

Figure 6. Glass transition temperature depending on DS for a S-PEEK sample. Reprinted from [18] with permission from Elsevier.

So far the discussion has implicitly assumed that the time (for static) or frequency (for dynamic) measurements of Tg were constant. In fact the observed glass transition temperature depends very much on the time allotted to the experiment, becoming lower as the experiment is carried out slower. 

The glass transition temperature depends strongly on molecular mass up to mass -10,000. Note that the lower limit of the molecular mass of a polymer is generally considered to be about 10,000, so that the Tg of a polymer shows little to no dependence on the molecular mass. Such a dependence is shown in Fig. 2.26 for polystyrene of narrow molecular mass distribution (Tg was measured by DSC at ACp). 

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