Immiscible blends, properties modulus

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

Coherently, as expected for immiscible blends, Tg values measured by DSC show very small variations with respect to the pure components while the mechanical properties degrade with respect to neat sPS. In particular, for minimum polyolefin contents <40 wt%, uniaxial tensile tests revealed a decrease in Young s modulus, elongation at break and energy to break. For higher contents, a phase inversion of the morphology occurs and the blend properties approach progressively those of the pure polyolefins. 

One exception to the above mentioned trend in physical properties of immiscible blends is in the utilization of liquid crystal polymers as a reinforcement for a more flexible thermoplastic polymer. The fact that LCP s can act as reinforcing agents in a blend has led some workers to model the mechanical behavior of the blends using theories of composites. Thus, Dutta et al. (1990) showed that the moduli values of highly drawn melts that contain liquid crystal polymers can be treated effectively by a simple rule of mixtures. That is, the modulus of a blend is given by... 

In 1977 Idemitsu Petrochemicals announced that immiscible blends of LDPE with HOPE, with EPDM, or with an atactic polypropylene, aPP, are suitable for the production of soft, thin films, with improved properties for packaging (Sakane et al. 1979). Similar blends, LDPE with HOPE, PP, and EP block copolymer, were proposed by Shin-Kobe Electric Machinery (1984) for films with good modulus, tear strength, and no sagging behavior. Also, Showa Denko (1983) found that immiscible, but lightly compatibilized, blends of LDPE with either HOPE or MDPE are suitable for films with uniform thickness and anisotropic tensile strength. 

Fang et al. (2005) studied the thermal and rheological properties of two types of m-LLDPEs, two LDPEs, and their blends. The C2+6 m-LLDPE-1 was immiscible, whereas the C2+8 m-LLDPE-2 was miscible with the LDPEs, indicating that increasing the length of SCB in m-LLDPEs promoted miscibility with LDPE. The Palieme (1990, 1991) emulsion model provided good predictions of the linear viscoelastic behavior for both miscible and immiscible blends. The low-frequency data showed an influence of the interfacial tension on the elastic modulus of the blends for the immiscible blends. 

The viscoelastic properties of polymer blends determined by dynamic mechanical analysis to yield E, E" and tand has been reviewed in Section 5.2. The modulus-temperature behavior of polymer blends is a strong function of the phase behavior. In Fig. 6.2, the generalized modulus-temperature behavior of miscible versus immiscible blends is compared for the case of two amorphous polymers with different glass transition temperatures. The phase separated blend exhibits a modulus plateau between the TgS of the components with the plateau position dependent upon the composition. The miscible blends show single Tg behavior, with the Tg position dependent upon the composition. 

Figures 20.13 and 20.14 describe the effect of dibutyltin dilaurate (DBTDL) on the tensile strength and tensile modulus for the 25/75 LCP/PEN blend fibers at draw ratios of 10 and 20 [13]. As expected, the addition of DBTDL slightly enhances the mechanical properties of the blends up to ca. 500 ppm of DBTDL. The optimum quantity of DBTDL seems to be about 500 ppm at a draw ratio of 20. However, the mechanical properties deteriorate when the concentration of catalyst exceeds this optimum level. From the previous relationships between the rheological properties and the mechanical properties, it can be discerned that the interfacial adhesion and the compatibility between the two phases, PEN and LCP, were enhanced. Hence, DBTDL can be used as a catalyst to achieve reactive compatibility in this blend system. This suggests the possibility of improving the interfacial adhesion between the immiscible polymer blends containing the LCP by reactive extrusion processing with a very short residence time.

Miscibility is identified as an existence of a single phase thus, the term refers to liquid systems solutions and melts (some authors treat co-ciystallization as a solid-state miscibility). Most polymer blends available oti the market are immiscible, but with adequate interactions across the interphase. For example, by 1980 ca. 42 % film producers used immiscible LLDPE/LDPE blends, where LLDPE improved modulus and strength and LDPE enhanced processability and ductility. Properties of LLDPE/LDPE blends have been described in several publications (Utracki and Schlund 1986,1987 Schlund and Utracki 1987 Zahavich and Vlachopoulos 1997). Blends of LLDPE with PP were also studied (Dumouhn et al. 1987, 1988, 1991 Dumoulin and Utracki 1990). Similarly, blends of PE with PC were described (Utracki and Sammut 1989, 1990b). 

Cycloolefin copolymer (COC) is an amorphous, clear metallocene product of norbomene and ethylene with a spectrum of attractive performance characteristics. Thus, COC (MFI at 190 °C and 2.16 kg = 1.7 dg min, p = 1,020 kg m ) was blended with C2+6 LLDPE (MFI at 190 °C and 2.16 kg = 3.2 dg min , p = 920 kg m ). The mechanical properties of the blends indicate immiscibility, despite the increased LLDPE crystallinity. The presence of COC improved the thermo-oxidative stability. Quasi-static tensile tests showed that increasing fraction of COC in the blends accounts for an enhancement of the elastic modulus and a decrease in the strain at break, while tensile strength passes through a minimum. A significant reduction of the creep compliance of LLDPE could be achieved only for the COC fractions exceeding 20 wt% (Dorigato et al. 2010). 

Poly(3-hydroxybutyrate) (PHB) 8 was a commercially available, biodegradable, non-linear polyester. Kumagai and Doi established that this polymer (M = 652,000, M /M =l.8) is immiscible with PCL (M =68,000, M /M =1.9) when solvent cast from chloroform [113]. Samples studied by DSC showed two glass-transition temperatures, identical with those of the individual components and invariant with composition. Mechanical properties of the blends were poor and tensile modulus and strength were minimal at 50 wt % of the components. PCL had a complex and accelerating influence on the rate of enzymatic degradation of PHB, the kinetics of which were correlated with scanning electron microscopy observations. 

PCL was also blended with poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymers on a small two-roll mill for 5 min at 140 °C (12% HV) or 128 °C (20% HV). The blends were injection moulded to produce tensile test samples. Samples were prepared in the presence of 1 wt % hydroxyapatite as a crystal nucleating agent and to reduce sample preparation time. For both copolymers, the addition of PCL (10 wt %) reduced the initial modulus and tensile strength (Table 12). Samples containing 90 wt % of PCL exhibited properties very similar to those of PCL and the effects were attributed to phase separation from the blends brought about by PHBV crystallisation at temperatures exceeding T for PCL. That is, the samples show immiscibility. Even blends containing 10 wt % PCL samples aimealed in aqueous media or in an oven showed the presence of crystalline PCL. 

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