New Insight to the Mechanism of the Shear-Induced Macroscopic Alignment of Diblock Copolymer Melts by a Unique and Newly Developed Rheo–SAXS Combination

• Published in: Macromolecules 2012-01, Vol.45 (1), p.455-472
• Main Authors: Meins, T, Hyun, K, Dingenouts, N, Fotouhi Ardakani, M, Struth, B, Wilhelm, M
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma201492n

In-situ flow alignment kinetics of a self-assembled lamellar phase polystyrene-block-polyisoprene (PS-b-PI, M w = 26 500 g/mol, f PS = 51%) diblock copolymer melt has been investigated in detail under mechanical large amplitude oscillatory shear (LAOS) utilizing a unique Rheo–SAXS combination developed in cooperation with the German Electron Synchrotron (DESY) in Hamburg. This marks the first time that the strain and time dependence of the shear-induced macroscopic perpendicular orientation of the lamellar microstructure could be monitored with a time resolution of 10 s per frame. Two mechanical parameters were used to compare the structural evolution and dynamics with the mechanical response of the sample. The mechanical loss modulus G″, which was directly obtained from the in situ Rheo–SAXS experiments performed with a stress controlled rheometer, and the nonlinear parameter I 3/1, which was calculated by Fourier-transform-rheology (FT-rheology) from the raw stress data obtained from a strain controlled rheometer. Significant correlations between the mechanical response and the structural changes of the sample were detected. For example, the orientation times τ calculated from both the X-ray and the mechanical measurements showed a power law dependence with τ ∼ γ0 –1.6 (in situ SAXS) and ∼ γ0 –2 (FT-rheology). Furthermore, the quality of the macroscopic orientation at large shear amplitudes (γ0 = 2 and γ0 = 3) was found to be a function of the mechanical excitation time. A better macroscopic orientation for shorter mechanical excitation times was achieved, while longer experimental times caused an unexpected reduction in the degree of orientation. In these situations, ex-situ SAXS and TEM studies indicated that a stable biaxial distribution of the lamellar microstructure that was preferentially orientated both parallel and perpendicular was formed, causing a drastic change in the response of both the mechanical quantities G″(t) and I 3/1(t).