Hernlund_et_al_2005 - letters to nature than 10 mol has no...

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
than 10 mol% has no further influence on the shape-memory properties. After two cycles of UV irradiation, the clear film turned pale yellow, possibly owing to photo-Fries rearrangement 20 , but no changes of shape memory properties were observed (Fig. 3). Although the extent of photoreversibility of the photoresponsive chromophores is around 40%, it is likely that partial cleavage of the photoreversible crosslinks on the microscopic level can lead to the changes of macroscopic mechanical properties and to the shape recovery. Polymers from the IPN polymer system show an R f of 20–33% and an R r of more than 88%. Data obtained at 35 8 C (Table 1) are not much different from those obtained at 25 8 C. During the irradiation process, the temperature of the polymers does not change, indicating that heating is not the trigger of shape-memory behaviour. When the cyclic photomechanical experiment is per- formed at lower 1 max (such as 20%), higher values for R f are obtained. Large deformation will broadly distribute the photo- responsive groups within the specimen under test and make aggregation of these functional groups difficult, thereby influencing the photo-fixing reactions. The relaxation behaviour of IPN poly- mer networks was investigated in control tests, in which an IPN polymer film was examined in the cyclic mechanical test under the same conditions but without applying UV light. The extent of relaxation determined from these experiments is about 2% in a test with 1 max ¼ 20%, and 5% for 1 max ¼ 50%. Relative to thermoresponsive shape-memory polymers, the strain-recovery rate of photoresponsive shape-memory polymers is comparable, but the strain-fixity rate is much lower, because the mechanism is different. In photoresponsive polymers, the amor- phous permanent network itself has long and coiled segments between two netpoints. During the programming process, the coiled segments of the chains are stretched and elongated (Fig. 1C). The strain-fixation via light irradiation is due to for- mation of new chemical netpoints, rather than freezing the stretched chains as in the case of thermoresponsive shape-memory polymers. The elastic contraction of the stretched chain segments accounts for the lower strain-fixity rate of light-induced shape- memory polymers. However, the unique characteristics of the photoresponsive shape-memory polymers enable shape-recovery at ambient temperatures by using remote activation, and could eliminate the temperature constraints of thermoresponsive shape- memory polymers for medical and other applications arising from external sample heating. A Methods Synthesis As an example of the grafted polymer, the synthesis of BHCA(10,2,1) (see Table 1 for nomenclature) is described: a solution of 12.80 g (100 mmol) BA, 2.60 g (20 mmol) HEMA, 2.46 g (10 mmol) HEA-CA, 0.22 g (0.4 mmol) PPG2M560 and 0.05 g azo- isobutyronitril (AIBN) was polymerized between two glass sheets with a Teflon spacer of , 0.5 mm at 80 8 C for 12 h. The resulting films were swollen in chloroform to remove the residual monomers. SCAA was synthesized by reacting 4-arm star-poly(ethylene glycol)
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern