Polymer blends amorphous phase

Some of the most difficult heterophase systems to characterize are those based on hydrocarbon polymers such as mbber-toughened polypropylene or other blends of mbbers and polyolefins. Eecause of its selectivity, RuO staining has been found to be usehil in these cases (221,222,230). Also, OsO staining of the amorphous blend components has been reported after sorption of double-bond-containing molecules such as 1,7-octadiene (231) or styrene (232). In these cases, the solvent is preferentially sorbed into the amorphous phase, and the reaction with OsO renders contrast between the phases. 

Miscible Blends. Sometimes a miscible blend results when two polymers are combined. A miscible blend has only one amorphous phase because the polymers are soluble in each other. There may also be one or more crystal phases. Simple theory (26) has supported the empirical relation for the permeabihty of a miscible blend. Equation 18 expresses this relation where is the permeabihty of the miscible blend and ( ) and are the volume fractions of polymer 1 and 2. 

Since interactions at the molecular level between polymer components in the blends occur only in the amorphous phase, it is reasonable to assume that these effects are due to kinetic factors and, in particular, to the influence of a polymer component on the nucleation or crystallization kinetics of the other one. 

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

For the case of the crystallization from the amorphous phase, the blending with PPO for lower contents (less than 30wt%) favours the obtainment of the a" ordered modification with respect to the a disordered modification, which is obtained for the unblended polymer. For higher PPO contents the obtainment of the p form is favored [105]. This behavior would be simply due to the increases of the glass transition temperature, and hence of the crystallization temperature on heating, which correspond to increased PPO contents in the blends [105],... 

Blends of enzymatically synthesized poly(bisphenol-A) and poly(p-r-butylphenol) with poly(e-CL) were examined. FT-IR analysis showed the expected strong intermolecular hydrogen-bonding interaction between the phenolic polymer with poly(e-CL). A single 7 was observed for the blend, and the value increased as a function of the polymer content, indicating their good miscibility in the amorphous state. In the blend of enzymatically synthesized poly(4,4 -oxybisphenol) with poly(e-CL), both polymers were miscible in the amorphous phase also. The crystallinity of poly(e-CL) decreased by poly(4,4 -oxybisphenol). 

The main limitation of birefringence is that it only provides an averaged orientation value without any discrimination between amorphous and crystalline phases, or between the components in polymer blends, copolymers, and... 

Thermal Properties. A typical dsc thermogram of an HPL/PVA blend (Fig. 4) shows a single Tg and Tm (10). Differences in the shape of the melting endotherms of PVA(96), (88), and (75) can be attributed to different degrees of crystallinity in the three polymers. Changes in crystalline structure of polymer blends usually result from polymer-polymer interactions in the amorphous phase. Such interactions result in a reduction of crystallinity, thereby reducing the enthalphy of the phase change (16,17). The observed reductions in melt endotherm area of HPL blends with PVA (> 0) may therefore indicate the existence of polymer-polymer interactions between the two types of macromolecules. 

Specific interactions between PCL and PVC are clearly indicated. In the solid state (Figure 5.9a) the spectrum of neat PCL indicates the presence of crystalline (1724 cm 1) and amorphous (1737 cm"1) bands. At mole ratios up to 2 1 of PVC to PCL, the spectra indicate that in the solid state the blends consist of crystalline and amorphous phases. As the PVC concentration increases, a parallel increase of the intensity of the amorphous band is observed. Moreover, the frequency shifts observed for both the crystalline and amorphous bands as a function of the composition of the blend suggests that specific interactions between the two polymers occur. No shift is observed in the carbonyl stretching vibration of PPL/PVC blends, in the molten state or in the solid state over the entire range of compositions and the two polymers are incompatible [28]. 

The phase behavior of polymer blends comprising amorphous polymers is experimentally well accessible in a window which is bounded at high temperatures by the thermal decomposition temperature of the polymer components and at low temperatures by the glass transition temperature of the system (cf. Fig. 1). Below the glass transition temperature the phase behavior can be estimated only tentatively. 

The basic issue confronting the designer of polymer blend systems is how to guarantee good stress transfer between the components of the multicomponent system. Only in this way can the component s physical properties be efficiently used to give blends with the desired properties. One approach is to find blend systems that form miscible amorphous phases. In polyblends of this type, the various components have the thermodynamic potential for being mixed at the molecular level and the interactions between unlike components are quite strong. Since these systems form only one miscible amorphous phase, interphase stress transfer is not an issue and the physical properties of miscible blends approach and frequently exceed those expected for a random copolymer comprised of the same chemical constituents. 

Studies performed on the amorphous blends by DSC show the presence of only one glass-transition temperature in the blends which is intermediate between those of PBT and PET, changing in value with composition (Figure 2). This is the first evidence we have of the compatibility of both polymers in the amorphous phase. 

Thus, although substitution generally increases the proportion of amorphous phase and leads to more disordered crystalline phases, the reverse effect can be obtained using specific molecular interactions between side groups and conjugated chains. This suggests that interaction of a CP chain with another, nonconjugated polymer could be used to orient the former. In a sense, this is what is achieved in blends (see Section II.E). 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

The phase angle shift can be used to obtain contrast due to local differences in energy dissipation as a consequence of different surface characteristics related to materials properties. These different properties allow one to differentiate materials with different adhesion [110] or widely different Young s moduli, if these differences are related to differences in energy dissipation [111-115]. Hence the amorphous and crystalline phases in semicrystalline polymers can be clearly differentiated, as discussed in Sect. 3.2, as well as different phases in polymer blends or filled systems (see below). As an example, we show in Fig. 3.52 an intermittent contact AFM phase image of a block copolymer thin film on silicon [116]. 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

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