Aging miscible blends

Figure 14.5. DSC thermograms for aged polymer blends (a) polyvinylchloride/poly isopropyl methacrylate, immiscible blend, aged at a temperature of 60°C, and (b) polyvinyl chloride/polymethylmethacrylate, miscible blend, aged at 80°C. Time of aging, t in hours, is shown alongside each curve. Broken lines represent the un-aged samples for comparison.

The first comprehensive study of physical aging in a miscible blend system using enthalpy relaxation was reported by Cowie and Ferguson... 

C.G. Robertson, G.L. Wilkes, Physical aging behavior of miscible blends ofpoly(methyl methacrylate) and poly(styrene-co-acrylonitrile). Polymer 42 (2001) 1581-1589. 

Her current research interests include studies of miscibility and physical aging in blends, nanophase separation in polymers with long side-chains, polymer dynamics, liquid crystalline polymers, composites, and systems containing nanoparticles. A common feature of these studies is the use of scattering techniques, especially neutron scattering, to study the local structure, conformation, and dynamics in polymers. She has written various reviews and book chapters in this area and has served on selection panels to allocate beam time at neutron facilities. 

The thermograms shown in Fig. 13.4a demonstrate clearly the development of two distinct enthalpy recovery peaks that increase with increasing aging time ta- Good separation of the peak maxima is obtained as tends to increase with aging time for both components but at different rates. This indicates that phase separation has occurred and the enthalpy recovery peaks are characteristic of each distinct phase in the mixture. A miscible blend of PVC and atactic PMMA was also treated in a similar fashion, but only one enthalpy recovery peak could be detected (Fig. 13.4b) indicating a single-phase system. 

The first comprehensive study of physical aging in a miscible blend system using enthalpy relaxation was reported by Cowie and Ferguson (1989) who followed the enthalpic relaxatirm in a series of blends of PS and poly(vinyl methyl ether), PVME. ComparisOTi of the blend behavior with that of the two components by analyzing the data oti the basis of both the P-M and C-F models led to the conclusions that the blends aged more slowly than PVME when aging was carried... 

The key to the equilibrium phase behavior of the PE/PE blends is that in the molten state - upon solidification the polymers crystallize or vitrify. Both of these states are affected by kinetics and aging. Molten blends are either miscible or immiscible with UCST or LCST after the phase separation via SD or NG mechanism. Extensive experimental studies of blended model polymers have been published. 

The time-temperature superposition principle, t-T, is not valid even in miscible blends, e.g., in PS/PVME, where the deviation was evident in tan5 vs. co plot. It was postulated that the number of couplings between the macromolecules varies with concentration and temperature. Thus, even in miscible blends, as either orT changes, the chain mobility is differently affected. Thus, the relaxation spectra of the polymeric components have different temperature dependencies, what make the t-T principle invalid. In immiscible blends, the t-T principle does not hold. Two processes must be taken into account the t-T superposition, and the aging time — at test temperatures, the polymeric components are at different distances from their respective Tg s, T - Tgi T - Tg2. 

PVC is well known for its efficiency to form miscible systems with various low- or high-molecular-weight polymers, which act as plasticisers. Miscible blends of PVC include its blends with nitrile-butadiene rubber (NBR), chlorinated PE and epoxidised natural rubber (ENR). In PVC/NBR blends, NBR acts as a permanent plasticiser for PVC in applications such as wire and cable insulation, food containers and pond liners. Simultaneously, PVC improves the ozone and chemical resistance as well as thermal ageing characteristics of NBR [a.l04]. 

Robertson, C.G. and Wilkes, G.L. (2000) Physical aging behavior of miscible blends containing atactic polystyrene and poly(2,6-dimethyl-l,4-phenylene oxide). Pdymer, 41, 9191. 

The materials analyzed were blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in various ratios. The two components are miscible in all proportions at ambient temperature. The photooxidation mechanisms of the homo-polymers PS and PVME have been studied previously [4,7,8]. PVME has been shown to be much more sensitive to oxidation than PS and the rate of photooxidation of PVME was found to be approximately 10 times higher than that of PS. The photoproducts formed were identified by spectroscopy combined with chemical and physical treatments. The rate of oxidation of each component in the blend has been compared with the oxidation rate of the homopolymers studied separately. Because photooxidative aging induces modifications of the surface aspect of the material, the spectroscopic analysis of the photochemical behavior of the blend has been completed by an analysis of the surface of the samples by atomic force microscopy (AFM). A tentative correlation between the evolution of the roughness measured by AFM and the chemical changes occurring in the PVME-PS samples throughout irradiation is presented. 

A group of new, fully miscible, polymer blends consisting of various styrene-maleic anhydride terpolymers blended with styrene-acrylonitrile copolymer and rubber-modified versions of these materials have been prepared and investigated. In particular the effects of chemical composition of the components on heat resistance and the miscibility behavior of the blends have been elucidated. Toughness and response to elevated temperature air aging are also examined. Appropriate combinations of the components may be melt blended to provide an enhanced balance of heat resistance, chemical resistance, and toughness. 

S/MA/MM). Miscibility behavior of typical matrix pairs is discussed as a function of composition, and discussion of material behavior in this work emphasizes response to elevated temperatures and to stress at temperature extremes. Thus, glass transition temperature (Tg), distortion temperature under load (DTUL), and tensile deformation properties as a function of temperature are reported for various typical blend combinations. Also discussed are the results of elevated temperature air aging studies on a typical blend compared to ABS. 

A more comprehensive study of this system has been carried out by Cowie and Ferguson [1991]. These authors followed the enthalpy and stress relaxation of a series of PMMA/SAN blends with SAN compositions spanning the miscibility window, i.e., from 13.3 to 30 wt% AN. It was found that the blends relaxed faster than either of the components, when aging temperatures were (Tg - T ) = 10°C, but that this was no longer the case at (Tg - T ) = 20°C where blend aging was intermediate to both components. The data were analyzed using both the P-M and C-F approaches and examples of AH(t, T ) vs. log t plots are shown in Figure 14.6. The C-F model... 

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