“Surrounding matters” is a phrase that has become more significant in recent times when discussing polymeric materials. Although regular polymers do respond to external stimuli like softening of material at higher temperatures, that response is gradual and linear in nature. Smart polymers (SPs) or stimuli-responsive polymers (SRPs) behave differently to those external stimuli, as their behavior is more rapid and nonlinear in nature and even a small magnitude of external stimulus can cause noticeable changes in their shape, size, color or conductivity. Of these SRPs, two types of SPs with the ability to actively change can be differentiated: shape-memory polymers and shape-changing polymers. The uniqueness of these materials lies not only in the fast macroscopic changes occurring in their structure but also in that some of these shape changes are reversible. This paper presents a brief review of current progress in the area of light activated shape-memory polymers and shape-changing polymers and their possible field of applications.
Poly(ethylene
oxide) (PEO) is a polymer of great interest due to
its prevalence in biomedical, pharmaceutical, and ion conductive systems.
In this study, the crystallization behaviors of a PEO with 22 monomer
units (PEO22) and a PEO having the same degree of polymerization
but with an additional 1,4-disubstituted 1,2,3-triazole ring in central
position of the chain (PEO11-TR-PEO11) are investigated.
PEO11-TR-PEO11 shows one type of lamella crystal
after cooling to T = 0 °C, but structural changes
during heating below their final melting are detected by WAXS, DSC,
POM, and solid-state NMR spectroscopy. The lamella thickness increases,
but simultaneously the helix–helix distance decreases and an
additional Bragg reflection appears at 2θ = 22.1°. A model
is proposed which explains these structural changes by incorporation
of the TR ring into the crystals which are additionally stabilized
by attractive C–H···π interactions of
the TR rings. Additionally, two different types of extended chain
lamella crystals are found in PEO22 by SAXS which are discussed
in the context of fractionation caused by the molar mass distribution
obtained from MALDI-ToF data.
Well-defined poly(ethylene glycol) (PEG) networks were synthesized using copper(I)-catalyzed azide− alkyne cycloaddition (CuAAC). Two types of PEG network structures were prepared (i) by linking two three-arm star PEG oligomers together and (ii) by connecting three-arm PEG star units with bifunctional linear PEG oligomers of different molar masses. End-group functionalization of PEG oligomers to azide and alkyne moieties was performed while for CuAAC the catalytic system of CuSO 4 and sodium ascorbate in aqueous environment was used. The successful conversion of the precursors and the formation of networks were confirmed by 13 C-MAS NMR and FTIR spectroscopy. Network defects like multiple links and dangling chain ends were quantitatively investigated by 1 H double quantum (DQ) NMR spectroscopy.
Polymer electrolytes are of tremendous importance for applications in modern lithium-ion (Li + -ion) batteries due to their satisfactory ion conductivity, low toxicity, reduced flammability, as well as good mechanical and thermal stability. In this study, the Li + -ion conductivity of well-defined poly(ethylene oxide) (PEO) networks synthesized via copper(I)-catalyzed azidealkyne cycloaddition is investigated by electrochemical impedance spectroscopy after addition of different lithium salts. The ion conductivity of the network electrolytes increases with increasing molar mass of the PEO chains between the junction points which is completely opposite to the behavior of their respective uncrosslinked linear precursors. Obviously, this effect is directly related to the segmental mobility of the PEO chains. Furthermore, the ion conductivity of the network electrolytes under investigation increases also with increasing size of the anion of the added lithium salt due to a weaker anti-plasticizing effect of the more bulky anions.
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