Shape memory polymers (SMPs) are a class of smart materials capable of fixing a temporary shape and recovering the permanent shape in response to environmental stimuli such as heat, electricity, irradiation, moisture, or magnetic field, among others. Recently, multi-shape SMPs, which are capable of fixing more than one temporary shape and recovering sequentially from one temporary shape to another and eventually to the permanent shape, have attracted increasing attention. In general, there are two approaches to achieve a multi-shape memory effect (m-SME): the first one requires the SMP to have a broad temperature range of thermomechanical transition, such as a broad glass transition. The second approach uses multiple transitions to achieve m-SME, most notably, using two distinct transition temperatures to obtain a triple-shape memory effect (t-SME). The recently reported approach for designing and fabricating triple-shape polymeric composites (TSPCs) provides a much larger degree of design flexibility by separately tuning the two functional components (matrix and fiber network) to achieve optimum control of properties. The triple-shape memory behavior demonstrated by a TSPC is studied in this paper. This composite is composed of an epoxy matrix, providing a rubber-glass transition to fix one temporary shape, and an interpenetrating crystallizable PCL fiber network providing the system the melt-crystal transition to fix a second temporary shape. A one-dimension (1D) model that combines viscoelasticity for amorphous shape memory polymers (the matrix) with a constitutive model for crystallizable shape memory polymers (the fiber network) is developed to describe t-SME. The model includes the WLF and Arrhenius equations to describe the glass transition of the matrix, and the kinetics of crystallization and melting of the fiber network. The assumption that the newly formed crystalline phase of the fiber network is initially in a stress-free state is used to model the mechanics of evolving crystallizable phases. Experiments including uniaxial tension, stress relaxation, and triple-shape memory testing were carried out for parameter identification. The model accurately captures t-SME exhibited in experiments. The stress and stored energy analysis during the shape memory cycle provides insight into the mechanisms of shape fixing for the two different temporary shapes, the nature of both recovery events, as well as a guidance on how to design transitions to achieve the desired behavior.

No abstract

No abstract

Intensity (ILS) and dynamic light scattering (DLS) experiments were performed on semidilute acid aqueous solutions of unmodified chitosan (UM-chitosan) and of hydrophobically modified chitosan (HM-chitosan) with two different degrees of C12-aldehyde substitution. Both the ILS and the DLS measurements suggest the formation of association structures. According to the ILS measurements in the range q > 1 (q is the wave vector and is the correlation length), all the systems have a fractal structure and the fractal dimension is about 2. The time correlation data obtained from the DLS experiments revealed, for all systems, the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. The value of the slow relaxation time increases with increasing concentration and hydrophobicity. This behavior reflects the importance of intermolecular associations and hydrophobic interactions. The reduced slow relaxation time exhibits a weak temperature dependence for both UM-chitosan and HM-chitosans. The fast mode is always diffusive, while the slow mode shows an approximately q 3 dependence at low concentrations and a q 2 dependence, characteristic of diffusion, at higher concentrations.

No abstract

Viscosity measurements have been carried out on dilute acid aqueous solutions of chitosan and of hydrophobically modified chitosan (HM-chitosan) with three different degrees of C12-aldehyde substitution. The experiments were performed in the presence of different amounts of the cationic surfactant cetyltrimethylammonium bromide (CTAB) (0−30 mm) and with and without salt addition. By using Huggins equation, the intrinsic viscosity [η], and Huggins constant (k‘) were evaluated at different conditions. The intrinsic viscosity was shown to decrease with increasing salt concentration. Addition of CTAB to polymer solutions without NaCl causes the values of the intrinsic viscosity to decline and the values of k‘ to rise. These observations are reminiscent of those reported from solutions of chitosan and HM-chitosans without surfactant but with increasing salinity. It was observed that [η] and k‘ dependency on the CTAB concentration is also influenced by the degree of hydrophobic substitution. In the presence of 10 mM NaCl, both [η] and k‘ seem to be independent of surfactant concentration for both the unmodified chitosan and the HM-chitosan. The features can be rationalized in terms of screening of electrostatic repulsion.

No abstract

No abstract