Nanoparticle role on the repeatability of stimuli-responsive nanocomposites

Abstract

Repeatability of the responsiveness with time is one important concern for effective durable functions of stimuli-responsive materials. Although the increase in the yield and tensile strength of the hybrid composite materials by nanoparticle (NP) incorporation has been reported, exact NP effect on stimuli-responsiveness is rarely reported. In this study, a set of nanoscale actuating system is demonstrated by a thermo-sensitive process operated by polyethylene glycol (PEG) linked by gold nanoparticle (AuNP). This designed nanocomposite exclusively provides an artificial on/off gate function for selective passages of permeate molecules. The results demonstrate high repetition efficiency with sharp responding in a timely manner. In terms of the morphology changes induced by repeated swelling-deswelling mechanics, the nanocomposite exhibits phase separation between AuNP clusters and PEG domains. This leads to a delay in responsiveness in a cumulative way with time. Acting as stable junction points in the nanocomposite network structures, the incorporated AuNPs contribute to maintain repeatability in responsiveness. This study contributes to new-concept smart material design and fundamental understanding on the hybrid nanomaterials for various applications in terms of a dynamic mechanical behavior.

Figure 1

(a) Gold nanoparticle (AuNP) and structure of the functional polyethylene glycol (PEG) for interlinking. Due to multi reactive sites of the AuNP to thiol group of PEGs various networked structures are generated. Binary functional PEGs having number of EO unit as of n = 77 is 2PEG 3400 and n = 227 is 2PEG 10000. Quaternary functional PEGs having EO unit as of n = 227 is 4PEG 10000 and n = 454 is 4PEG 20000. (b) Small angle X-ray scattering (SAXS) results of the designed AuNP-PEG nanocomposites in solution state in broad q ranges (two SDD distance conditions are combined) for 2PEG 3400, 2PEG 10000, 4PEG 10000 and 4PEG 20000 linked AuNP clusters. There is a critical q value (marked by q*) from which temperature-responsiveness is diversified indicating dual regions of the designed network: stable large scale domain and responsive small scale domain. In two systems of 2PEG 10000 and 4PEG 10000, definable size is determined according to the temperature. (c) Illustration of the dual-responsive network structures. Bold lines denote stable connection, while dotted lines indicates flexibly responsive chains by the external stimuli (left). Pore size variation of the nanocomposites embedded in PVA matrix induced by the stimuli-responsive PEGs through which permeates are transported. Swollen PEGs generates smaller path for permeate molecules [I], while shrunken PEGs allow relatively wide pathway for effective transport of permeates [II].

Figure 2

(a) Experimental set-up for mass-transport. Permeate molecules are loaded at the entrance of the diffusion cell and flow rate are carefully controlled. (b) Retention time (t_n) of each system is determined from which the permeate molecules are detected at the outlet flow. (c) Repetition cycles of the temperature triggering to the nanocomposite-loaded diffusion cell and molecular responding of permeate detection. The results are obtained by Rhodamine 6G passing through 2PEG 10000 nanocomposites and recorded in a minute interval. (d) Time-dependent repetition cycles for two nanocomposite systems of 2PEG 10000 and 4PEG 10000. Temperature triggering and permeate detection occur simultaneously for 2PEG 1000 system, while there is cumulative delay for 4PEG 10000 system. All the figures were created by the authors.

Figure 3

(a) TEM (top line), XNI (middle line) and XMI (bottom line) images before and after the repetition cycling for 2PEG 10000 and 4PEG 10000 system. (b) SAXS results after the repetition cycling for 2PEG 10000 and 4PEG 10000 systems.

Figure 4

(a) XMI image and schematic illustration of the nanocomposite before (left) and after (right) the cyclic repetition. This is a reversible procedure controlled by temperature control. (b) XMI images of nanocomposites obtained with mechanistic repetition from [I] to [IV]. (c) Suggested energy level and activation of the state [I] to [IV] in (b). Due to high activation energy, the state [III] and [IV] is reversible. (d) The wrinkled lines are drawn on the image [III] and [IV].