Effect of chemistry on thermo-mechanical shape-memory properties of acrylate networks
Shape-memory acrylate networks are novel materials for both biomedical and industrial applications. The purpose of this study is to understand how the chemical structure dictates thermo-mechanical properties of shape-memory acrylate networks, specifically strain to failure and toughness. The strain to failure is useful because it is pivotal to know how much recovery strain the material experiences. To understand how the structure is related to mechanical properties, such as strain to failure, materials of differing chain stiffness ratio, C∞, are compared at varying percentages of cross-linker. First, a set of networks is characterized to understand the trends in the basic thermo-mechanical properties of the monomers once cross-linked. Thirty-one acrylates are separated into two groups: linear chain builders and cross-linkers. The networks are systematically synthesized by varying the linear chain builders with poly(ethylene glycol) dimethacrylate Mn ∼550 (PEGDMA550) as the cross-linker, and varying the cross-linker while holding tert-butyl acrylate constant as the linear chain builder. A dynamic mechanical analyzer evaluates the glass transition temperature and rubbery modulus. Subsequently, strain to failure tests are performed at the glass transition temperature of each respective mixture. The linear chain builders with PEGDMA550 have glass transition temperatures ranging from -29 to 112 °C, and rubbery moduli from 2.75 to 17.5 MPa. The cross-linkers co-polymerized with tert-butyl acrylate have glass transition temperatures ranging from -3 to 98 °C, and rubbery moduli from 6 to 130 MPa. Materials can be selected to independently vary the glass transition temperature and rubbery modulus. Based upon the initial screening results, networks with different C∞ are formed at varying percentages of cross-linker. C∞ values typically apply only for pure linear chain builders, not networks, and here we demonstrate how chemical cross-linking alters the impact of C∞ on strain to failure. The comparison of these networks yields insight into the relationship between chemical structure and mechanical properties leading to a relationship between C∞, percentage cross-linker, and strain to failure.
