Internal Motions of Linear Chains and Spherical Microgels in Θ and Poor Solvents

Abstract

Internal motions of narrowly poly(N-isopropylacrylamide) (PNIPAM) linear chains and spherical microgels in a very dilute aqueous solution and dispersion were studied under the Θ and poor solvent conditions by dynamic laser light scattering (LLS) over a wide range of scattering angles. As expected, only one narrow peak related to the translational diffusion was observed in the line-width distribution G(Γ) of both linear chains and spherical microgels when x < 1, where x = (qR g ) 2 with q and R g the scattering vector and the radius of gyration, respectively. As x increases to approach 1, we start to see a second peak related to fast internal motions of linear chains, whose average line width is related to some of the internal motions (normal modes) predicted in the nondraining bead-and-spring model. However, for swollen microgels, the second peak related to the internal motions only appears when x is much higher (1/q ∼ 50 nm), indicating that thermal energy is not able to excite the entire microgel but only a small portion of the gel network. As the solvent quality changes from good to poor, the shrinking of a linear chain or subchains in the gel network gradually suppresses the internal motions, presumably due to stronger intersegment interaction. Surprisingly, we found that the relative contribution of the second peak makes a turning near the Θ temperature, leading to a new way to estimate the Θ condition of a given polymer solution.

The Swelling of Poly(Isopropylacrylamide) Near the θ Temperature: A Comparison between Linear and Cross‐Linked Chains

“…Microgels synthesized by precipitation polymerization display higher cross‐linking densities around the core region and weaker cross‐linking densities around the outer part of the microgels, where dangling chains are also present . This radial gradient in cross‐linking density, which usually becomes more significant for weakly cross‐linked microgels, is known to affect their equilibrium swelling properties .…”

“…Microgels synthesized by precipitation polymerization display higher cross-linking densities around the core region and weaker cross-linking densities around the outer part of the microgels, where dangling chains are also present. [37] This radial gradient in cross-linking density, which usually becomes more significant for weakly cross-linked microgels, is known to affect their equilibrium swelling properties. [107][108][109][110] Experiments by Acciaro et al [100] have established that microgels without a radial cross-linking gradient display R(298 K)/R(θ) values similar (but somewhat higher) to those of heterogeneously crosslinked microgels considered in this study.…”

Microgel particles: The structure‐property relationships and their biomedical applications

“…Above VPTT, the regions with longer subchains would collapse in front of the regions with shorter subchains because of the more intensive hydrophobic forces between the segments. Second, as shown in Figure , the transition of the microgel displays in a more gently way, telling from a much boarder transition temperature window and a smaller shrinkage extent in colloidal particle size . This is likely related to the inhomogeneities in the length of subchains and also derives from the structure character of the formed microgels, which is discussed as follows.…”

Functional Dynamics Inside Nano- or Microscale Bio-Hybrid Systems

“…Poly(N-isopropylacrylamide) (pNIPAM) nano- or microgels systems have been one of the most widely studied systems due to its unique thermal sensitivity. Nano- or microgels made of pNIPAM have a lower critical solution temperature (LCST) at approximately 32°C (Wu and Zhou, 1996; Dai and Wu, 2010). The nano- or microgels swell or shrink when temperature is below or above the LCST, with their size changing by more than an order of magnitude (Dai and Wu, 2010).…”

“…Nano- or microgels made of pNIPAM have a lower critical solution temperature (LCST) at approximately 32°C (Wu and Zhou, 1996; Dai and Wu, 2010). The nano- or microgels swell or shrink when temperature is below or above the LCST, with their size changing by more than an order of magnitude (Dai and Wu, 2010). The incorporation of other monomers, such as acrylic acid (AA) or methacrylic acid (MAA) further equips the system with pH and ionic strength responsiveness (Gan et al, 2009; Dai et al, 2015).…”