Polymer blends glass transition

The determination of T of a blend is one of the calorimetric techniques used to elucidate the miscibility or partial miscibility in the amorphous phase of binary polymer blends. Glass transition temperature is the temperature at which the transition from the glassy to the rubbery state of the bulk material takes place. The establishment of miscibility using is based on the degree of dispersion of the second component in the amorphous region of the first component and that the size of the disperse phase domain is < 15 nm (Silvestre et al., 1996 Shultz and Young, 1980). It is noteworthy that blends which exhibit a are miscible whereas... 

An Example Calculation of Polymer Blend Glass Transition Temperatures... 

Consider a non-cristalline binary polymer system for which the glass transition temperatures of the pure components significantly differ. A homogenous blend shows only one glass transition, located between those of the pure polymers. Thus, at a given measurement temperature T, the distance to the blend glass transition Tg depends... 

It is noteworthy that even in miscible polymers of similar molecular structure, viz. 1,4-polyiso-prene with 1,2-polybutadiene, the time-temperature superposition fails. The polymers having glass transition temperatures separated by 60°C preserve their different dynamics in the blends [Kan-nan and Komfield, 1994]. Thus, even miscible systems can be rheologicaUy complex. The rheological behaviors of blends in the vicinity of the phase separation are of great fundamental importance. They will be discussed in Part 7.4.3. 

Immiscible Polymer Blends. These blends have large-size domains of dispersed phase and poor adhesion between them. If the blend is formed by two polymers, two glass transition temperatures will be observed. 

The second example considers a blend formed by LDPE, with 30% crystallinity, and PVC. The polymer matrices examined are pure LDPE, the blends LDPE (80%)-PVC (20%) and LDPE (50%)-PVC (50%), and pure PVC, with toluene as the penetrant. Experimental data by Markevich etalS report solubiUty of toluene in the above blends, at the temperature of 30° C, while toluene solubiUty in pure PVC was taken from Berenst l The glassy transition temperature is equal to —25° C for LDPE and to +75° C for pure PVC. Therefore, pure PVC is a glass at 30° C however, due to the large swelling and plasticization of the polymer induced by toluene sorption, it can be seen that the sorption of toluene lowers the glass transition of PVC to temperatures below 30° C, already at relatively low toluene activities. That is also confirmed by the sorption isotherm which is concave to the concentration axis as is typical of rubbery polymers. The glass transition temperatures for the blends are estimated to be — 10°C for the 80% LDPE blend and +17° C for the 50% LDPE blend, all below the temperature of the sorption experiment. The crystalline fraction of LDPE is assumed, as is usual, not to contribute to the sorption process, therefore we consider only the amorphous fraction of LDPE in the sorption calculations based on EoS. For the sake of simplicity, we present here only the results obtained with the LF equilibrium model. 

A polymer blend that obeys equation (12.7) consists of equal fractions by weight of two polymers with glass-transition temperatures 50 and 190°C. Calculate the glass-transition temperature of the blend. 

Growing numbers of commercial materials are blends of two or more polymers in which at least one of the components is a crystalline polymer. The crystallization in miscible blends is restricted to temperatures between the blend glass transition temperature and the equilibrium melting point, T ,e, i.e., to the crystallization temperature, Tc < Tm e- The difference, Dc = T ,e - Tc, depends on the cooling rate and the nucleation process. There are three mechanisms of the crystallization nucleation (Utracki 1989) ... 

Rather than blending for example an SBR and a polybutadiene or polyisoprene, copolymers or terpolymers based on styrene and butadiene, butadiene and isoprene, styrene and isoprene, or styrene, butadiene, and isoprene can be prepared having a dual Tg. Using n-butyllithium catalyst and h/, h/, N, N -tetramethylethylenediamine (TMEDA) as a modifier, a range of polymer dual glass transition temperatures can be obtained (Table 4.6). 

When a plasticizer is blended into a polymer, the glass transition temperature (Tg) of the polymer decreases. The variation Tg for a mixture of polyfvinyl chloride) (PVC) and dioctyl phthalate... 

The optically pure polylactides L-PLA (or PLLA) and D-PLA (or PDLA) are semicrystalline polymers with glass transition and melting temperatures of about 60 °C and 180 °C, respectively. In contrast, DL-PLA (or PDLLA) is an amorphous polymer that consists of racemic lactate units and is crystalline only when the D and L unit sequence is completely alternating such a polymer is termed poly (meso-lactide) [44]. The blending of PLAs can result in new materials that present with unexpected synergisms, such as those observed in the equimolar blends between the optically pure polylactides, PLLA and PDLA. 

Jungnickel B J (1996) Poly(vinylidene fluoride) (overview) in Polymeric Materials Encyclopedia, (Ed. Salamone J C) CRC Press, Boca Raton, Vol. 9, pp. 7115-7122. 37. Fakirov S, Balta Calleja F J and Boyanova M (2003) On the derivation of microhardness of amorphous blends of miscible polymers from glass transition temperature values, J Mater Sci Lett 22 1011-1013. 

For a binary miscible blend (A amorphous polymer, B crystalline polymer) the glass transition is intermediate between those of plain components (Tg, Tje) and thus the crystallization range—and the crystallization behavior— will depend on the glass transition of the amorphous component (Tja) (Fig. 10.2) [19]. In fact, if FgA is lower than T,b, the glass transition of the crystallizable polymer, the crystallization window of the blend (T°-Tg), where T° represents the equilibrium melting point of B in the blend, is larger than that of the neat crystallizable component (T b -TgB), and the ability to crystallize is enhanced. On the contrary, if TgA... 

Alloys exhibit physical properties, the values of which are typically the weighted average of those of its constituents. In particular, the blend exhibits a single glass-transition temperature, often closely obeying semitheoretically derived equations. Blends of two compatibiLized immiscible polymers exhibit physical properties which depend on the physical arrangement of the constituents and thus maybe much closer to those of one of the parent resins. They will also typically exhibit the two glass-transition temperatures of their constituent resins. 

Before providing such an explanation it should first be noted that progressive addition of a plasticiser causes a reduction in the glass transition temperature of the polymer-plasticiser blend which eventually will be rubbery at room temperature. This suggests that plasticiser molecules insert themselves between polymer molecules, reducing but not eliminating polymer-polymer contacts and generating additional free volume. With traditional hydrocarbon softeners as used in diene rubbers this is probably almost all that happens. However, in the... 

In a fundamental sense, the miscibility, adhesion, interfacial energies, and morphology developed are all thermodynamically interrelated in a complex way to the interaction forces between the polymers. Miscibility of a polymer blend containing two polymers depends on the mutual solubility of the polymeric components. The blend is termed compatible when the solubility parameter of the two components are close to each other and show a single-phase transition temperature. However, most polymer pairs tend to be immiscible due to differences in their viscoelastic properties, surface-tensions, and intermolecular interactions. According to the terminology, the polymer pairs are incompatible and show separate glass transitions. For many purposes, miscibility in polymer blends is neither required nor de-... 

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