73 The experiments aiming at the incoherent scattering of PEO protons (TOF and back scattering spectrometer (BSS)) were performed with hPEO/dPMMA samples with 25%, 35%, and 50% PEO. Up to now PEO/PMMA is the only highly asymmetric blend that has been investigated by neutron time of flight (TOF), backscattering, and NSE spectroscopy. This relation explains data pertaining a spatial scale from 7 to 18 Å. The fast process found is insensitive to mixing and resembles one of the pure polymer, whereas the characteristic times of the slow process follow a Vogel–Fulcher relation if the Vogel–Fulcher temperature is changed by the T g difference according to the self-concentration inserted into the Fox equation as above. 72 Incoherent scattering measured by direct time-of-flight neutron spectroscopy reveals two relaxation processes in the ps range and on the monomeric length scale.
This is partly true also for the dynamic features observed by inelastic neutron scattering on PEO/PMMA.
The calorimetric observation of two class transitions and their dependence on composition can be reasonably well explained by the above Ansatz. T g eff ( Φ ) = T g ( Φ eff ), T g ( Φ ) = Thus, an estimate for the self-concentration is The volume of influence approximately has the size of the Kuhn length l K. 68,69 These findings are rationalized using the self-concentration concept of Lodge and McLeish 70 which takes into account that the chain connectivity leads to an enhanced probability to find a segment of the same chain close to a given segment. Careful calorimetric measurements reveal the occurrence of two glass transitions in the mixtures. Among the known miscible polymer systems (see, e.g., introduction of Reference 68 for a compilation) PEO/poly(methylmethacrylate) (PMMA) is among the best investigated ones and exhibits a large dynamical asymmetry as expressed by the huge difference of the glass transition temperatures T g(PEO) ≅ 200 K and T g(PMMA) ≅ 400 K. However, the underlying combination rules of mixing have to be understood to fully exploit the blending method. This may be utilized to adjust the mechanical properties. The grafted SAN acts as a dispersing agent.īlends of different polymers with very different glass transition temperatures T g are expected to exhibit new dynamical effects due to the close interpenetration of chains with a high variation of mobility. Some of the styrene–acrylonitrile–copolymer (SAN) copolymer molecules are grafted to the rubber during polymerization. ABS copolymers consist of a dispersion of polybutadiene rubber particles in a matrix of styrene–acrylonitrile copolymer. Very attractive blends have been envisaged: inorganic fillers dispersed in rubber of plastic materials, wood flower dispersed in polyolefines, and glass fibers as reinforcement for tires. A large class of compatibilizers has been developed.
The surface-active properties of block and graft copolymers have made them useful as dispersants, emulsifiers, foam stabilizers, and wetting agents. They may be absorbed at various surfaces: liquid/gas, liquid/liquid, solid/liquid, etc. Block and graft copolymers act as oil in oil emulsifiers. In order to do so, their blocks or branches should be different enough and with a real affinity for those phases. In a dispersed system, the graft and/or block copolymers self-organize at the interface and modify interfacial properties to a great extent, thus enhancing phase compatibility. At similar compositions, both block copolymers and graft copolymers should be subjected to similar thermodynamic driving forces for phase separation, phase-size constraint, and compatibility with homopolymer mixture.īlock and graft copolymers are amphiphilic molecules because they may have affinity for two different types of environments. They are blends and the “blends” issue is open to many choices. Styrene–methyl methacrylate copolymers with only a difference of 5% in styrene content become incompatible. Most of the copolymers are actually blends of various copolymers with different chemical compositions. M cop T g = M a T gaa + M b T gbb + M a + M b T gab
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