Rheology and morphology of polystyrene/polypropylene blends with in situ compatibilization
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- Huo, Y., Groeninckx, G. & Moldenaers, P. Rheol Acta (2007) 46: 507. doi:10.1007/s00397-006-0158-3
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Rheology and flow-induced morphology were studied in immiscible polypropylene (PP)/polystyrene (PS) blends with a droplet–matrix microstructure. Two reactive precursors, maleic anhydride grafted PP and amino terminated PS, were added during the melt-mixing process to form a graft copolymer. The effects of both the amount of compatibilizer and the shear history on the rheological and morphological behavior were investigated systematically. Small amplitude oscillatory experiments and scanning electron microscopy were used to study the phase morphology. Shear history has an important effect on the morphology of the uncompatibilized blends. The droplet size refines with increasing shear rate. The decrease of this effect with increasing degrees of in situ compatibilization is mapped out. The results are discussed in terms of interfacial tension and the interfacial coverage. It turns out that most of the conclusions that were previously obtained on physically compatibilized blends are also valid for chemically compatibilized ones.
KeywordsIn situ compatibilizationMorphologyRheologyPolymer blends
Blending of polymers is a common and economical means of creating new materials with improved properties. Most polymers are immiscible and hence produce two-phase blends. The weak interaction between the two components results in poor interfacial adhesion. In addition, the morphology is usually coarse and unstable, which causes bad mechanical properties. The addition of premade block or graft copolymers (physical compatibilization) can produce a finer morphology and increase the interfacial adhesion. The results of this compatibilization are influenced by molecular weight and architecture of the block copolymer (Macosko et al. 1996; Riemann et al. 1997; Van Hemelrijck et al. 2004). In situ formed block or graft copolymers, generated at the interface by a reaction during the melt-mixing process (chemical compatibilization), can act as efficient compatibilizers. In comparison with premade compatibilizers, these copolymers are more likely to be located at the interface between the two components (Milner and Xi 1996). Their compatibilization efficiency will also depend on their molecular weight and structure (Joen et al. 2004).
In in situ compatibilized polymer blends, the compatibilizer is generated using a polymer or precursor with functional groups. It can react with one of the components of the blend to form the copolymer at the interface. This is illustrated by the well-known reactive blends based on polyamide (PA). The reactive polymer contains functional groups such as maleic anhydride (Dedecker and Groenincks 1999) or acrylic acid (Jo and Kim 1992), which can react with the amine end group of the PA. Two reactive precursors can also be used to generate block or graft copolymers without involving any of the blend components (Moan et al. 2000; Harrats et al. 2004). The amount and the reactivity of the functional groups, the molecular weight, and the structure of the reactive chains and the diffusion kinetics of the precursors from their parent phases to the interface all affect the interfacial reaction (Yin et al. 2001, 2002; Harrats et al. 2004). Interfacial reactions were studied using bilayers of end-functional polymers to eliminate the effect of mixing (Jiao et al. 1999; Jones et al. 2003; Yin et al. 2003). The results indicate that the extent of reaction at polymer interfaces is related to the molecular weight and structure of functional polymers, reactant concentration, and thermodynamic interactions.
The rheological properties and the flow-induced morphology of immiscible blends are interconnected. This relationship was investigated intensively for uncompatibilized blends under steady shear flow, both for model systems at ambient temperature and blends of real molten polymers (e.g., Vinckier et al. 1996; Graebling et al. 1993; Gramespacher and Meissner 1992). In physically compatibilized blends, the rheological properties are influenced by the amount, molecular weight, and architecture of the added copolymers. Again data are available on model and real systems (Germain et al. 1994; Riemann et al. 1997; Velankar et al. 2004). Several groups also studied the rheological properties of in situ compatibilized blends. For example, Asthana and Jayaraman (1999) investigated PA 6/polypropylene (PP) blends with different extents of reaction at the interface. They reported that the interfacial tension, as expected, dropped progressively with increasing extent of reaction. Moan et al. (2000) studied the effects of a random terpolymer formed during the reactive blending on morphological and rheological properties of a PA 12/linear low-density polyethylene blend. In both studies an additional relaxation mechanism was observed, which did not exist in the uncompatibilized blends. The literature on the rheology–morphology relation in chemically compatibilized blends is, however, rather limited. This is, among others, related to the inherent difficulties associated with prolonged experiments at elevated temperatures. A systematic investigation of the rheology and the resulting flow-induced morphology in reactively compatibilized blends is the subject of the present study.
Materials and methods
The blend components were dry-blended at ambient temperature and fed to a DSM twin-screw midi extruder. The blending conditions were the same for all samples: a temperature of 215 °C, rotation rate 100 rpm, and mixing time of 5 min. The latter was adequate due to the high reactivity of MA and the amine groups (Harrats et al. 2004). During melt blending the mixing chamber was saturated with N2 gas to avoid oxidative degradation. The concentration of compatibilizer x is expressed as the weight percentage of the reactive mixture PP-g-MA + PS-NH2 on the total blend. The composition of the blends with the reactive polymers was 0.8(100 − x) wt% PP/0.2(100−x) wt% PS/x wt% compatibilizer with x varied from 0 to 5. The two reactive precursors were added at the stoichiometric anhydride/amine ratio. After melt blending, the extruded filaments were quenched in cold water and cut into pellets, which were squeezed into disc-shaped plates of 25 mm diameter and 1 mm thickness using a heating press (Collin) at 215 °C. These discs were used for the rheological measurements.
Rheological measurements and Palierne model
Rheological behavior and the resulting morphology were investigated in a systemic way with varying compatibilizer concentrations and shear histories. The rheological measurements were performed on a dynamic stress rheometer (Rheometric Scientific) with 25 mm diameter/0.1 rad cone and plate geometry at 205 °C in a nitrogen atmosphere. First, the sample was presheared at a given shear rate for a fixed duration in time. This generated a morphology, which is determined by the balance between break up and coalescence. More specifically, preshearing at 1.0, 1.5, and 2.0 s−1 was performed for 1,800 strain units. At lower shear rates of 0.5 and 0.1 s−1, the applied strain was 1,200 and 240, respectively. For uncompatibilized blends it was verified that 1,200 strain units were sufficient to reach steady state. It should be noted that for each experiment a new sample from the same batch was used to avoid degradation effects. After the flow was stopped, small amplitude oscillatory measurements were performed. The applied strain in the oscillatory tests was in the linear viscoelastic region and the morphology did not change during the dynamic measurements. The flow-induced morphology could then be probed by analyzing the frequency dependence of the storage moduli, using the Palierne model (Palierne 1990; Jacobs et al. 1999).
A simple version of the Palierne model has successfully been applied to obtain morphological information of uncompatibilized blends by several researchers (Graebling et al. 1993; Friedrich et al. 1995; Vinckier et al. 1996). For a blend with a droplet–matrix morphology, the storage moduli display a shoulder in the low-frequency region due to a relaxation mechanism that reflects the shape relaxation of the droplets. From fitting the Palierne model to the dynamic moduli, the ratio of the volume average radius (Rv) over the interfacial tension (α) can be derived. If the volume average radius is determined by an independent method and the droplet size distribution is not too large, the interfacial tension can be obtained in this manner. Inversely, the flow-induced droplet size can be determined for a blend with a known interfacial tension. For compatibilized blends, the more general version of the Palierne model has to be used (Jacobs et al. 1999). With some assumptions it can be reduced to a simplified form with only two parameters, α/Rv and β′′/Rv, in which the parameter β′′ is the interfacial shear modulus. The latter is related to the presence of the compatibilizer at the interface. The two-parameter version of the Palierne model, as discussed in detail by Van Hemelrijck et al. (2004), will be used in the present work.
After the rheological experiments the specimen was cooled using cold air and subsequently fractured in liquid nitrogen. A smooth surface was achieved by a microtome (Leica Ultracut Uct) equipped with a glass knife. The dispersed PS phase was extracted with chloroform at room temperature to enhance the contrast. The fractured surfaces of the etched samples were sputter-coated with gold. The blend morphology was observed by means of scanning electron microscopy (SEM, Philips XL30 FEG).
Results and discussion
Rheology and morphology of uncompatibilized blends
The shape relaxation time of the droplets can be determined by calculating the continuous relaxation spectrum based on the dynamic moduli. A nonlinear regression program (Honerkamp and Weese 1993) was used for that purpose. The relaxation spectra of the blends after different preshear histories, calculated from the moduli of Fig. 2a, are shown in Fig. 2b. For comparison, the relaxation spectrum of the matrix (PP) is added as well. A relaxation peak, reflecting the shape relaxation, can clearly be seen in all the spectra of the blend.
Rheology and morphology of compatibilized blends
Effect of compatibilizer concentration
Effect of shear history
Effect of compatibilizer concentration on droplet radii Rv, polydispersity d, fitting parameters α/Rv and β′′/Rv, the calculated interfacial tension α and interfacial shear modulus β′′, and the calculated interfacial coverage c0 (shearing at 1.0 s−1 for 1,800 strain units)
Effect of shear rate on droplet radii Rv, polydispersity d, fitting parameters α/Rv and β′′/Rv, the calculated interfacial tension α and interfacial shear modulus β′′, and the calculated interfacial coverage c0 for the blends with 1 and 5wt% compatibilizers
Shear rate (s−1)
The rheology and the flow-induced morphology of in situ compatibilized PP/PS blends were investigated using dynamic measurements and SEM. In uncompatibilized blends, the shear history has a pronounced effect on the morphology. The shear effect still exists in the compatibilized blends but it gradually disappears at higher compatibilizer concentration. Two relaxation mechanisms are observed at higher shear rates and lower compatibilizer contents. At higher compatibilizer levels, here 5wt%, the rheology and morphology hardly change with shear rates. The results are in line with those obtained on physically compatibilized systems. The interfacial tension and interfacial shear modulus were estimated by fitting the data with Palierne model. As for physically compatibilized blends, the interfacial shear modulus substantially increases with interfacial coverage.
Financial support from the Research Council, Katholieke Universiteit Leuven (grant GOA 03/06) and the FWO-Flanders (project G. 052304) are gratefully acknowledged. The authors wish to thank Prof. Jan Mewis for helpful discussions.