Title

Modification of polymeric surface using block copolymer in supercritical carbon dioxide

Date of Completion

January 2002

Keywords

Engineering, Chemical|Engineering, Materials Science|Plastics Technology

Degree

Ph.D.

Abstract

A novel polymeric surface modification method using block copolymer was suggested in this thesis. As a nontoxic and easily removable solvent, supercritical carbon dioxide (scCO2) was used for the modification. Poly(styrene) (PS) was used as a selective surface and poly(styrene-b-dimethylsiloxane) (P(S-b-DMS)) copolymer was used to modify the PS surface in scCo2. The solution pressure and temperature were varied between 10 MPa and 50 MPa, and between 30°C and 60°C, respectively. The modification not only coated the outermost surface of PS substrate with the PDMS block of the copolymer but also impregnated block copolymer molecules in PS bulk. The impregnation was due to swollen structure of PS in scCO2. The swollen PS structure also induced collapsed PDMS brushes on modified PS surface resulting in the brush thickness of 1.6 nm in maximum. The thickness was approximately 30% of a fully stretched dimension. For better understanding of the modification method, phase behavior of poly(dimethyl-siloxane) (PDMS) in scCO2 was studied first. The PDMS with molecular weights of 2,000–20,000 g/mol exhibited theta dimensions or collapsed dimensions under the conditions of P ≤ 30 MPa and T = 50°C. Below cloud points, PDMS single chains had collapsed dimensions, and part of the single chains aggregated to form precipitants. The pressure-dependent phase behavior could be explained with respect to the Daoud and Jannink's general description on polymer solution, and the theta pressure (which was an analogical term of the theta point at atmospheric pressure) of PDMS at 50°C was estimated at 31.3 MPa. Since the highest safe operating pressure in the study was approximately 30 MPa, a complete coil-globule transition could not be observed. Under the same conditions of P ≤ 30 MPa and T = 50°C, the solubility of P(S-b-DMS) (2.7K-b-2.8K) copolymer in scCO2 was less than 6.4 × 10−5 g/cm3 due to the limited solvent quality of the scCO 2 for PDMS. ^