A novel polymeric shape memory system of chemically cross-linked polycyclooctene (PCO) was developed and characterized. PCO was synthesized via ring-opening metathesis polymerization of cyclooctene using the dihydroimidazolylidene-modified Grubb's catalyst. After dicumyl peroxide was added to PCO, the mixture was compression-molded into a film and further cured through chemical crosslinking upon heating. The chemically cross-linked PCO samples were fully characterized using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and wide-angle X-ray scattering (WAXS) in order to gain insight into the rapid shape memory behavior. We observe that the transition temperature of PCO is tunable through the change of the trans/cis ratio of vinylene groups. A fast shape memory behavior was observed, where the primary stress-free shape was recovered within 1 s on immersion in hot water above the melting point of the crystalline PCO phase. In contrast with glassy shape memory polymers, chemically cross-linked PCO behaves as an elastomer capable of arbitrary shaping above the sharp melting temperature of the PCO crystalline phase and subsequent shape fixing during crystallization.
Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO's thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples' viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (Mw = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox-Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.
Thermally actuated shape memory polymers (SMPs) interest, both academically and industrially, due to their ability to memorize a permanent shape that is set during processing and a temporary shape that is later programmed by manipulation above a critical temperature, either Tg or Tm. However, the thermal triggering process for SMPs is usually retarded compared to that of shape memory alloys, because the thermal conductivity of polymers is much lower (<0.30 W/m.K). In the present study, we incorporated a highly thermal conducting filler into a shape memory matrix to increase its thermal conductivity and therefore, shorten the heat transfer progress. A mathematical was worked out that quantitatively relates the material's thermal conductivity with the heat transfer time, τ, also defined as a shape memory induction time. The model fit nicely with our experimental data. In addition, mechanical reinforcement was observed with the addition of this rigid thermal conducting filler.
In the evaluation of tissue adhesives, freshly harvested tissues are the most relevant to clinical application; however, it is difficult to obtain and test experimental samples immediately following surgery. The aim of the present investigation is to elucidate the effect of storage conditions on the ability of preserved tissue samples to mimic freshly harvested tissue. Porcine small intestine was used as a model tissue for the experiments. The treatment conditions were freshly harvested, 24-h aged in phosphate-buffered saline at 5 degrees C and formalin-fixed tissue. Adhesive performance of cyanoacrylate and a newly synthesized tissue adhesive on the three substrates was characterized by T-peel testing. Regardless of the adhesive used, the fresh tissue samples had the weakest bond strength. The 1-day aged tissue produced a bond that was approximately twice as strong, and the formalin-fixed intestine substrate had intermediate bond strength. Comprehensive tissue characterization using attenuated total reflection infrared (ATR-IR) spectroscopy, captive-bubble contact angle, Raman spectroscopy, bulk tissue water content measurement and histology were employed. The results from the different characterization techniques show that storage of tissue samples causes interfacial and chemical changes, limiting their ability to mimic freshly harvested tissue. In particular, the results show the importance of water detected by ATR-IR in mediating adhesion to the tissue samples.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.