Temperature dependence of spherical micelles of PS3000-b-PAA5000 studied by in-situ small angle X-ray scattering

Abstract

Amphiphilic block copolymer has a character that it spontaneously self-assembles into various micellar morphologies when dissolved in selective solvents with different proportions. Amphiphilic block copolymer has wide potential applications in drug delivery such as the targeting delivery, controlled release, molecular recognition, etc. Poly (styrene)-block-poly (acrylic acid) (PS-b-PAA) is a representative amphiphilic block copolymer whose self-assembly in the selective solvents has been widely studied during the past years. Micellar morphology of PS-b-PAA sensitive to temperature, and temperature effect of PS-b-PAA are of great importance for the drug delivery. However, the micellar morphologies of PS-b-PAA have been investigated mainly at the room temperature so far. The understanding is still limited to micellar morphology of PS-b-PAA in the varying temperature processes. In the present work, an investigation of the relationship between micellar morphology of PS-b-PAA and the temperature is conducted by using in-situ small-angle X-ray scattering (in-situ SAXS). The SAXS experiments are performed on the BL19U2 beamline of Shanghai Synchrotron Radiation Facility. The energy is selected to be 10 keV and the wave length is 0.1033 nm. The two-dimensional (2D) SAXS patterns are recorded by Pilatus 1 M with a pixel size of 172 m172 m. A sample-to-detector distance of 5340 mm is chosen, giving access to a range of scattering vectors q of 0.11-0.89 nm-1. The temperatures of the specimens are monitored by using a Linkam thermal stage THMS600 (Linkam Scientific Instruments). One-dimensional (1D) integrated intensity curves are obtained from the 2D SAXS patterns by employing the Fit2D software. The PS-b-PAAs (PS:PAA=3000:5000) is purchased from Sigma-Aldrich Inc and used directly (without any treatment prior to experiment). The PS-b-PAA is dissolved in solvents of N, N-Dimethylformamide and H2O with various proportions. The concentration of solution of PS-b-PAA is 10 mg/mL. The experiments show that the sizes of micelle particles in PS3000-b-PAA5000 solution are grown with water content increasing, and double scattering peaks (qpeak1=0.418 nm-1, qpeak1=0.456 nm-1) appear for the solution with 10% water. A temperature-dependent change of SAXS intensity is demonstrated by in-situ SAXS. The intensities of peak 1 and peak 2 vary in a contrary waywith the sample's warming up, but the positions of the peaks are independent of temperature. The double peaks in SAXS profiles suggest that the size of micelle particles in the solution is not homogeneous but the micelles with two close sizes coexist. It is interesting that the number of two-sized particles changes at the same rate in the heating process although there is a significant difference between the initial number and the final number of micelles.