In situ evaluation of structural changes in poly(ester-urethanes) during shape-memory cycles
Graphical abstract
Introduction
Linear segmented thermoplastic polyurethane (PU) elastomers are multiblock copolymers consisting of hard and soft segments that can separate to form hard and soft domains. The properties of these materials are dependent on phase morphology and the separation of hard and soft domains [1], [2], [3]. Recent methods have tailored the properties of PU to yield thermo-responsive materials that can be used to create smart devices capable of memorizing a permanent shape that is substantially different from the initial temporary shape [4]. These materials have been referred to as shape-memory polymers (SMP). In SMPs, the relative motion and rearrangement of molecular chains is the primary inelastic strain mechanism responsible for the shape-memory effect. The establishment and annihilation of a metastable structure allows for the storage of temporary shape and recovery from deformation during an SM cycle. For instance, to store a deformed shape in a stable manner, new chain positions must be fixed by new bonds (physical or chemical bonds). A fixed shape can be achieved by cooling the material below the transition temperature after significant chain alignment and chain slip occur. The newly formed bonds provide a storage mechanism for macroscopic stress and increased chain organization (lower entropy). To recover the original shape, the material is heated and temporary bonds are subsequently broken. At higher temperatures, the bonds are weakened and the low entropy state drives individual chains back to their original position, facilitating shape recovery [5], [6].
With the use of small-angle X-ray scattering, the objective of this study was to monitor morphology development during shape-memory thermomechanical cycles of polyurethanes with different amounts of hard segments and to describe the relationship between morphology evolution and observed shape-memory properties.
Section snippets
Polymer synthesis
Poly(caprolactone diol) (PCL – Mn = 1250, 2000 g mol−1), isophorone diisocyanate (IPDI), 2,2-bis(hydroxymethyl), propionic acid (DMPA) and dibutyl tin dilaurate (DBDLT) were obtained from Aldrich (St. Louis, MO). Triethylamine (TEA, 98%) and hydrazine (HZ, 25%) were purchased from Vetec (RJ, Brazil). All chemicals employed in this study were used without prior treatment.
PUs were prepared according to the prepolymer mixing method. PCL and DMPA were stirred in a glass reactor at 65 °C for 30 min.
Results
In DSC analysis, the width of the transition zone provides a qualitative measure of phase homogeneity and the variation in Tg indicates the degree of miscibility. Thus, the amount of hard segments dissolved in soft domains can be studied by monitoring Tg of soft domains [8]. In the first run, shown in Fig. 2(a) a first order transition around 45 °C was observed for PU–II and PU–III, corresponding to the melting of PCL-rich crystals. Neither the endothermic melting peak associated with the
Discussion
When a material is deformed, polymer chains orient along the direction of extension and induce crystallization. According to Table 4, deformation induced changes in the proportion of phases in PU-III, promoting crystallization and phase separation. However, crystallization was not observed in PU–I after step 3 of the SM cycle. Moreover, neither PU–I nor PU-III were efficient in recovering the original ratio of matrix to disperse phase (the disperse phase is formed by hard domains and crystals)
Conclusions
The development of phases during SM cycles applied to poly(ester-urethanes)was successfully monitored using in situ SAXS experiments. Recovery and fixation were driven by hydrogen bonding and crystallization. Poor SM results in PU-I may be related to the high degree of phase miscibility, which hinders polymeric chain movement and restricts crystallization during deformation. The SAXS results also showed that the ability to recover shape is related to the rapid re-establishment of specific
Acknowledgments
The authors acknowledge financial support from the following institutions: the National Council for Scientific and Technological Development (CNPq), a foundation linked to the Ministry of Science and Technology (MCT) of Brazil, the State of Minas Gerais Research Foundation (FAPEMIG) and the National Synchrotron Light Laboratory (LNLS-Brazil) for use of SAXS beamline facilities.
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