Chapter 10
Domain Structures and Viscoelastic Properties of Immiscible Polymer Blends Under Shear Flow
Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0597.ch010
Yoshiaki Takahashi and Ichiro Noda Department of Applied Chemistry, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan
The relationship between domain structures and viscoelastic properties of a 1:1 blend of two immiscible polymers under steady shear flow and after step increase of shear rate were studied. The sample was a mixture of polydimethylsiloxane and polyisoprene which have almost the same viscosity. At steady states, domains were elipsoidal and their diameters were rather uniform and inversly proportional to shear rate. The shear stress and first normal stress difference were almost proportional to shear rate. After a step increase of shear rate, domains in the initial steady state were extremely elongated, which then ruptured to shorter ones and gradually changed to the final steady-state structure. An undershoot of shear stress and an overshoot of normal stress followed by a gradual change to steady-state values were observed during these processes. These results can be qualitatively explained by the change in the distribution of unit normal vectors of the interface. In immiscible polymer blends, there exist various domain structures of component polymers, which deform, rupture and aggregate under flow fields, so that their viscoelastic properties become very complicated. Many experimental and theoretical studies on viscoelastic properties of immiscible polymer blends have been carried out with reference to suspension and emulsion systems, which were first theoretically studied by Einstein (1) and Taylor (2), respectively. Most of the studies have been recently summarized by Utracki (3). Despite many studies, the relationship between domain structures and viscoelastic properties of immiscible polymer blends are not elucidated especially when the compositions of the blends are close to 1:1, that is, concentrated blends. Further experimental and theoretical studies on concentrated blends are needed. Recently, two such theories have been proposed independently.
0097-6156/95/0597-0140$12.00/0 © 1995 American Chemical Society
In Flow-Induced Structure in Polymers; Nakatani, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
10.
Immiscible Polymer Blends Under Shear Flow
TAKAHASHI & NODA
Onuki (4) presented a theory on the viscoelastic properties of phase-separating fluids near the critical point taking into account of interfacial tension. In the weak shear case, he predicted that enhancements of the shear stress, Δσ , and first normal stress difference, N are proportional to shear rate, Y, and the excess viscosity, Δη(=Δσ Α), and normal viscosity (=N /Y) become of order ηφ for ellipsoidal domains, where φ is volume fraction of domains. 12
1 ?
12
1
Δη/ηφ = 1-2 (Ν /Ϋ)/ηφ = 3~6
(1) (2)
Downloaded by VIRGINIA TECH on February 27, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0597.ch010
1
Here the viscosities, η, of the existing two phases are assumed to be the same for near-critical fluids. It is to be noted that N of phase-separating fluids is much larger than for a one-phase system. Onuki also noted that these ideas should be applicable to other systems which have domain structures such as polymer blends even far from critical point, as long as η of the two phases are the same. Doi and Ohta (5, 6) proposed a constitutive equation of textured materials by considering the time evolution of the area and orientation of the interface and the interfacial tension opposing the deformation of 1:1 mixtures of immiscible Newtonian fluids with the same viscosity. They predicted interesting scaling relations, of which two typical relations are described below. For a steady shear flow, they predicted that both shear stress, σ , and first normal stress difference, N , are proportional to the shear rate, Y. x
12
x
σ
12
Ni
oc
γ
oc [Υ
(3)
I
(4)
These relations are the same as those predicted by Onuki (equations 1 and 2). Another scaling relation was presented for a step change of shear rate, where the shear rate is changed from an initial shear rate, Y to a final shear rate, Y , at time t=0. In this case, they predicted that plots of the transient stresses, σ(ί;Υ ;Y divided by σ(Υ j) against the strain, Y ft, yield a universal curve independent of Y , as long as the shear rate ratio, Y is maintained constant, i.e., i?
f
{
{
[9
c^y ydf^d
