16
Downloaded by UNIV OF ARIZONA on December 20, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch016
Morphology-Processing Relationship in Interpenetrating Polymer Blends Styrene-Ethylene/Butylene-Styrene Block Copolymer and Poly(ether ester) H. Verhoogt, J. van Dam, and A. Posthuma de Boer Faculty of Chemical Engineering and Materials Science, Department of Polymer Technology, Delft University of Technology, P.O. Box 5045, 2600 GA Delft, The Netherlands
Interpenetrating polymer blends, which show properties of their constituents as well as new properties, usually are formed in limited ranges of compositions. Blends of a styrene-ethylene/butylene-styrene (SEBS) block copolymer and a poly(ether ester) were studied as an example of systems that show co-continuity over a broader range of compositions. The blends were prepared on a two-roll mill. The morphology of the blends was studied by extraction experiments, scanning electron microscopy, and confocal laser scanning microscopy. Complete dual-phase continuity existed over a wide range of compositions (20-60-vol% SEBS) in blends quenched after blending. Compression molding or annealing at 200 °C narrowed the range of fully co-continuous compositions to 30-60 vol%. Extrusion of the annealed blends resulted in recovery of the broader range of full dual-phase continuity. The viscosity of the blends as measured by capillary rheometry showed a minimum in the intermediate composition range, possibly caused by interfacial slip.
WHEN
TWO POLYMERS ARE MECHANICALLY BLENDED the result usually is a heterogeneous system because of the positive free energy of mixing (I). The resulting morphology is most likely the dispersed-phase matrix type in which the minority phase can be spherical, fibrous, or lamella-like. In the intermedi-
0065-2393/94/0239-0333$06.00/0 © 1994 American Chemical Society
In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
Downloaded by UNIV OF ARIZONA on December 20, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch016
334
INTERPENETRATING POLYMER NETWORKS
ate composition range blends with dual-phase continuity may be formed. Such blends are frequently called thermoplastic interpenetrating polymer networks (IPNs) ( 2 - 7 ) , in accordance with the definition given by Gergen (8, 9): "a combination of two dissimilar polymers in which each polymer is topologically independent and has three-dimensional continuity". The term "network" in this definition is not restricted to networks on a molecular scale or on a phase-domain scale, and a distinction is sometimes difficult to make (3, 9). In the case of the co-continuous blend morphologies, the term "interpenetrating polymer blend" (IPB) (10) seems preferable, because any reference to molecular cross-links is avoided. The earliest examples of IPB s were mechanically blended combinations of polypropylene (PP) and ethylene-propylene-diene-monomer rubber ( E P D M ) (11). Since then there has been a lot of interest in this land of blend because of the large variety of applications for P P - E P D M blends (12-14). The properties of IPB s are combinations of the properties of the components, which retain their individual identities, and thus the properties of both components are fully expressed. The advantage of an IPB structure versus a dispersed structure is that the properties can generally be described by additive relationship (9, 15, 16). Additive behavior is reported for the dynamic modulus (8, 9) and for the tensile strength (17, 18) of IPBs. However, synergistic and antagonistic behavior with respect to the tensile strength is found as well (17-19). Two main routes can be employed to prepare IPBs: • Physical blending of two existing polymers, either in the molten state or in solution. The most common method is mechanical blending in the molten state. With this method various IPBs were obtained that consisted, for example, of a styrene-ethylene/butylene-styrene (SEBS) block copolymer and several thermoplastics like PP, nylon, and saturated polyesters, (8, 9, 15, 16); of polystyrene (PS) and cis-polybutadiene (PBD), of PS and poly(methyl methacrylate) ( P M M A ) , and of P M M A and ethylene-propylene rubber (EPR) (10); of sulfonated E P D M ionomers and several thermoplastics (PP, high-density polyeth ylene ( H D P E ) , SBS) (17, 18); of PP and E P R , and of PS and styrene-butadiene rubber (SBR) (20); and of styrene-isoprene-styrene (SIS) block copolymer and a polyethylene phase consisting of 75% linear low-density polyethylene ( L L D P E ) and 2 5 % low-density polyethylene ( L D P E ) (21). Solution blending (reference 1, Chapter 2) was used by Hsu et al. (19) to prepare IPBs that consisted of polyurethanes and polyvinyl alcohol).
In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
Downloaded by UNIV OF ARIZONA on December 20, 2012 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch016
16.
VERHOOGT ET AL.
Morphology-Processing Relationship in IPNs
335
• Chemical blending, for example, by swelling monomer 2 into polymer 1 and polymerization in situ. Phase separation during or after polymerization may result in a co-eontinuous morphol ogy. This method was used to synthesize IPBs that consisted of SEBS and a neutralized ionomer prepared from a random copolymer of styrene, methacrylic acid, and isoprene (SMAAI) (4-6). Nishiyama and Sperling (7) synthesized an IPB by combination of polyCn-butyl acrylate) and polystyrene; both polymers were cross-linked independently with acrylic acid anhydride. De-cross-linking of both polymers was carried out by hydrolysis, after which neutralization formed the ionomers. The chemically blended IPBs (4-7) were compared with their mechanically blended equivalents. For all IPBs it was found that the chemically prepared blends had better mechanical properties than the mechanically prepared blends. According to Gergen et al. (9) the most efficient mixing of two compo nents into an interpenetrating structure can be obtained when the viscosities and volume fractions of the two polymers are equal. Avgeropoulos et al. (22) reported that the critical volume fraction for the formation of dual-phase continuous structures was primarily determined by the viscosity ratio of the blend components. Paul and Barlow (23, 24) gave an empirical relation for the condition for the point of phase inversion (where dual-phase continuity exists) that was confirmed by Jordhamo et al. (25) and by Gergen et al. (9). This relation was generalized by Miles and Zurek (10) to %(Ύ)
^2(7)
=
Φι
(la)
Φ2
or φ
2
= 1/[1 + λ ]
(lb)
where η^'γ) is the viscosity of polymer i at the shear rate 7 that prevails in the mixing equipment during preparation of the blend, 7 is the shear rate, φ is the volume fraction of polymer i, and λ = % / % is the viscosity ratio. From filament instability considerations Metelkin and Blekht (26) derived for the point of phase inversion ί
φ
2
= 1/[1 + XF(k)]
(2)
where F(\) = 1 + 2.25 log λ + 1.81(log λ ) . According to Utracki (27), eqs 1 and 2 are reasonably close. By equating expressions for the viscosity of (hypothetical) systems of one component 2
In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
336
INTERPENETRATING POLYMER NETWORKS
dispersed in the other and vice versa, Utracki (28) derived the condition for phase inversion:
λ = [(Φ, -
