Toughening of a Polysilsesquioxane Network by Simultaneous Incorporation of Short and Long PDMS Chain Segments

• Published in: Macromolecules Vol. 37; no. 4; pp. 1455 - 1462
• Main Authors: Zhu, Bizhong, Katsoulis, Dimitris E, Keryk, John R, McGarry, Frederick J
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24.02.2004
• Subjects: Applied sciences; Exact sciences and technology; Inorganic and organomineral polymers; Physicochemistry of polymers; Properties and characterization; Dimethylsiloxane polymer; Strengthening; Telechelic polymer; Mechanical properties; Crosslinking; Dispersion reinforced material; Silsesquioxane copolymer; Experimental study; Mechanism; Thermal stability; Dynamic mechanical properties; Morphology; Concentration effect
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma0353843

An effective toughening approach is described in this report. This approach uses a combination of both short and long PDMS segments, simultaneously incorporated into a polysilsesquioxane-based rigid network through chemical bonding at the terminals of these segments. Upon curing, the short chain PDMS remains molecularly dispersed (phase I PDMS) and the long chain PDMS segregates to form silicone rubber particles in situ (phase II PDMS). Proper combinations of phase I with phase II PDMS toughen the network 7−9 times more effectively than the phase I alone at the same total PDMS loading level, while the phase II PDMS alone deteriorates the mechanical properties. The effectiveness of the phase I/II combinations is dependent on the particle size. Submicron-sized particles are more effective than particles of a few microns in diameter. Particles larger than a few tens of microns become ineffective. The size of the particles can be controlled by changing these parameters:  the precoupling reaction conditions, the amount of phase II PDMS, the phase II PDMS chain length, and the chain length ratio of phase I PDMS segment to phase II PDMS segments. With appropriate phase I/II combinations, the K Ic is increased by up to 220% and the G Ic by up to 900%, with less loss of elastic modulus as compared with toughening by the phase I alone. It is proposed that the effective engagement of high cross-link density domains into the deformation process by submicron-sized rubbery particles is responsible for the increased fracture toughness.