Glassy-rubbery phase transition

Amorphous, glassy polymers, used far below their Tg, are cold-brittle if no other mechanisms are active an example is PS. If a polymer has been improved in impact strength by the addition of a rubbery phase (high-impact PS or PVC, ABS etc.), then the cold-brittleness temperature is related to the Tg of the added rubber. If the polymer shows a secondary transition in the glassy region (such as PC), then this governs the brittleness temperature. 

In the analysis of polymer blends the identification and subsequently the characterization of the distribution of the components of the blend (surface coverage and dispersion) is of prime interest. The already mentioned materials contrast in various AFM modes provides a straightforward differentiation of, e.g. glassy and rubbery phases (after considering the effect of frequency on the corresponding transition temperatures). As different components in a blend tend to possess different surface... 

The volume resistivity/temperature curve clearly illustrates the strong difference in the temperature dependency of the volume resistivity of a polymer in its glassy phase and in its rubbery phase. The Tg-value, obtained by drawing two tangents near this glass-rubber transition, is determined at 1000/T 3.58 or 6°C. This Tg-value is a real static Tg-value and its hypothetical frequency [f(h)l in the frequency/temperature plane will be lower than the f(h) = lxB-2 to lxE-4 claimed by Phillips [12] for dilatometric experiments. A good fit on the Arrhenius plot was obtained assuming an f(h) of lxE-6 for this Tg-value, see below. 

The glass transition is exhibited by amorphous polymers or the amorphous regions of partially crystalline polymers when a viscous or rubbery state is transformed into a hard, brittle, glass-like state. The glass transition is neither a first- nor a second-order thermodynamic phase transition since neither the glassy state nor the viscous state is an equilibrium state. Relaxation phenomena are observed above and below the glass transition temperature (Tg). The glass transition of polymers is observed by DSC as... 

Glass transition Change of state of an amorphous or semi-crystalline polymer from a rubbery (or viscous) state to a glassy state. The glass transition is not a thermodynamic first- or second-order phase transition. It is a relaxation phenomenon which is characterized by a general enhancement of molecular motion in the polymer at the glass transition temperature. 

As found in previous observations [8], polyisocyanurate networks display two transitions with respect to their composition the first is low-temperature transition associated with devitrification of the rubbery phase, and high-temperature related to devitrification of the glassy phase. Hence, both transitions are temperature-shifted towards one another, and the progress of such shift depends on the microphase composition. Therefore, Figure lb presents separately thermomechanical curves of polyurethane isoeyanurate samples related to the low-temperature area. 

For blends, at a molecular level during phase transition the molar volume and bulk density changes along with the heat capacity. This implies that ASg, the entropy of mixing in the glassy state, is different from ASJn, the entropy of mixing in the rubbery phase. The mixed system entropy is given by... 

Many polymer blends that are used in industrial practice have been found to be partially miscible. Examples are PVC/SAN, PC/SAN at certain AN compositions, PET/PHB, etc. The entropic difference model was developed by taking into account the change in entropy of mixing at glass transition in the Couchman model. Equation (6.15) quadratic expression for blend TgS is obtained upon Taylor approximation of the relation obtained by equating the entropy of the blend in glassy phase with the entropy of blend in rubbery phase at glass transition of the blend. 

The glass transition generally occurs over a relatively narrow temperature span and is similar to the solidification of a liquid to a glassy state it is not a phased transition. Not only do hardness and brittleness undergo rapid changes in this temperature region, but other properties, such as the coefficient of thermal expansion and specific heat, also change rapidly. This phenomenon has been called second-order transition, rubber transition, and rubbery transition. The word transformation has also been used instead of... 

On the other hand, an amorphous polymer does not crystallize under any conditions. The phase transition for this type of polymer occurs from tiie glassy state to rubbery state at a temperature termed the glass transition temperature and often designated Tg. 

Synthetic binder 2 (Figure 10.2) exhibits a behaviour equivalent to synthetic binder 1 it shows the same 3 regions of the mechanical spectrum as a function of temperature. At low temperatures, the transition from the glassy to rubbery phase is observed. In this interval, a crossover between the linear viscoelastic functions is present, as is a displacement of the crossing-point towards higher frequencies as the temperature... 

The polymer used by Koros and co-workers (4,15,16) is a PMMA with a rather high molecular weight (Mw 600000 kg/kmol), and with a relatively high glass transition temperature (Tg 120°C). Its density at 35 C was not reported and thus has been estimated by considering the experimental specific volume of the rubbery phase at T=120°C, after Olabisi and Simha (20), and the cubic expansion coefficient for the glassy phase as indicated in (27). At 35 C the (tensity of the glassy PMMA is estimat to be 1.176 kg/m. ... 

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