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New Concepts at the Interface: Novel Viewpoints and Interpretations, Theory and Computations
Study of the Interactions of Zwitterions and Carbon Nanotubes by Nonlinear Rheology in Aqueous Environment Lei Du, amin ghavaminejad, Mohammad Vatankhah-Varnoosfaderani, and Florian J. Stadler Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01778 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018
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Langmuir
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Study of the Interactions of Zwitterions and Carbon Nanotubes by Nonlinear Rheology in Aqueous Environment
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Lei Du1,2, Amin Ghavaminejad3,5, Mohammad Vatankhah-Varnoosfaderani4,5, Florian J.
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Stadler1,5*
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1
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Technology, Guangdong Research Center for Interfacial Engineering of Functional Materials, Shenzhen
1 2
College of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer Science and
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University, Shenzhen 518060, PR China
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2 Key
Laboratory of Optoelectronic Devices and System of Ministry of Education and Guangdong Province,
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College of Optoelectronic Engineering, Shenzhen University, Shenzhen 516080, People’s Republic of China
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3
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Toronto, 144 College Street, Toronto, Ontario, Canada M5S 3M2
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4
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5 Formerly: School of Semiconductor and Chemical Engineering, Chonbuk National University, Baekjero 567,
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Deokjin-gu, Jeonju, Jeonbuk, 561–756, Republic of Korea
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Abstract
Advanced Pharmaceutics & Drug Delivery Laboratory, Leslie Dan Faculty of Pharmacy, University of DSM Biomedical, 735 Pennsylvania Dr, Exton, PA 19341, USA
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Aqueous nanocomposite solutions of P(NIPAM) and P(NIPAM-co-N-(Methacryloxypropyl)-
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N,N-dimethyl-N-(3-sulfopropyl)ammonium Betaine) - a zwitterionic monomer with carbon
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nanotubes (CNT) as filler were synthesized and characterized rheologically. While the influence of
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P(NIPAM)-content and CNT-content can be considered relatively minor, the introduction of
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zwitterionic monomer (Zw) into the polymer leads to clear rheological traces of strong
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interactions between zwitterionic moieties and surface moieties on the CNTs, namely a
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significantly lower nonlinearity limit, a lower modulus at high Zw-contents but higher at
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intermediate contents – due to adsorption of zwitterionic moieties on the CNT-surface – as well
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as a significantly lengthened time for the sample to adjust itself to the applied deformation,
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suggesting that the adsorbed polymer chains need to reorganize themselves significantly to
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accommodate to the applied strain γ0. 1
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Keywords
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Large amplitude oscillatory shear (LAOS), zwitterions, carbon nanotubes, surface
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interactions
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Introduction
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Poly-N-isopropylacrylamide (PNIPAM) is one kind of intelligent thermo-sensitive polymer
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with the lower critical solution temperature (LCST) of 32°C in water and it is a very classical system
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with biocompatibility, hydrophilicity, and non-toxicity, and has been extensively studied. 1-4 For
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the past few years, a great number of studies have been conducted to examine thermo-sensitive
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polymers containing zwitterionic materials, such as carbobetaine and sulfobetaine, 5-11 consisting
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of both negative and positive charges in one monomer chain and these kinds of materials are
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advantageous in many applications such as biomedical sensors,
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protective coatings, 15-16 and oil field treatment fluids
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properties.
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12-13
environmental areas,
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due to their excellent physicochemical
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Zwitterionic materials have intra- and inter-molecular interactions due to the presence of
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ions with opposite charge, i.e., electrostatic interactions between moieties of opposite charge,
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which, as a result, can affect the phase transition behavior and their solution properties. The
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clusters formed by dipole-dipole electrostatic interactions play an important role in producing a
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physical network of zwitterionically modified polymers, and this eventually determined its
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solubility, transportability, and other specific properties. Introduction of the zwitterions into
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polymer makes it possible to use them in several new applications, such as resistance to non-
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specific protein adsorption and ultra-low fouling coatings. 18-19 The ultra-low fouling of zwitterionic
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materials arises from the high electrostatic hydration around the opposite charges 20 and the high
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energy barrier required to remove such hydration layer. 21
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Over the past decade, the rheological properties of zwitterionic copolymers and polymer
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mixtures containing zwitterions have been of increasing interest, and zwitterionic copolymers
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have also been reported to successfully make the water and brine viscous and reduce the brine-
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dragging effects 22 due to the interplay between positive and negative charges on the same group
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or in the backbone, between chains, and/or between chains and external electrolytes. Moreover, 2
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Langmuir
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zwitterionically modified polymers offer the possibility to control self-assembled structures by
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changing the total chain length, the hydrophilic/hydrophobic ratio, or by addition of a third
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component, and their rheological properties are dependent on the following molecular
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parameters: the number, the type, the distribution of charged groups in the chain, the
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hydrophilic/hydrophobic ratio, the distance of the charged moieties from the back bone, and the
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characteristic properties of the back bone.
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molecules can display a rich variety of rheological properties. In addition, the concentration of
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zwitterions can also adjust the physico-mechanical properties of the solutions containing
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zwitterionic copolymer. 7 While a particular type of interaction deserves attention in zwitterionic
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copolymer, which is involved in the formation of colloid or worm-like micelles.
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concentrations of zwitterionic comonomers, physical networks are formed depending on the
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degree of the interaction between the two components. These physical networks can be
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considered as polymer coils or colloids and play a role in significantly increasing the viscosity of
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the solutions. Hence, how to effectively characterize the various rheological properties of
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zwitterionic copolymer solutions, especially under large amplitude deformation, become
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increasingly important and interesting.
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As a result, diverse structures of zwitterionic
23-25
At high
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Rheological characterization of solutions started from self-assembling solutions such as
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worm-like micellar solutions 26-27 and block-copolymers, 28 which is important because rheological
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experiments could characterize the structure of the solution not only under equilibrium
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conditions, but also under strong mechanical stresses imposed by the rheometer itself. 29 While
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for a long time, rheology has been mostly performed in the linear viscoelastic regime (small
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amplitude oscillatory shear, SAOS), besides some early works by the groups of McSporran 30-31 and
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Giacomin 32. Only in the past 20 years, the nonlinear regime has been intensively explored by large
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amplitude oscillatory shear (LAOS)
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harmonics, 37 which have long been ignored in SAOS due to low magnitudes. The term LAOS refers
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to the test setup (large deformation in relation to the linear viscoelastic limit) and Fourier
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transform-rheology (FT-Rheology) refers to the dominating way to evaluate the obtained data.
33-36
and it leads to non-sinusoidal responses due to higher
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When analyzing LAOS-data, extracting meaningful information from higher-order harmonics
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is essential to interpret rheology of complex solution. For analyzing the complex non-sinusoidal
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responses, the current standard of analyzing LAOS-data is to deconvolute by Chebychev
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polynomials, 38 which ultimately leads to a series of moduli, i.e., n moduli Gn’, Gn” for each higher 3
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harmonic independently, of which usually only the odd ones are considered. Furthermore, the
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time and strain dependent stress shape, 37, 39 Fourier-transform 38, 40 and stress decomposition 41
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were also applied to the LAOS-data. Hence, a great deal of work on LAOS in the last 15 years was
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devoted to finding out how to represent the rather complex data in a similarly easy way. Recently,
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one rather popular method has been adopted to ignore the phase angles of the higher order
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moduli and only plot the relative intensities of the higher harmonics, i.e. the third order relative
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intensity (I3/1) instead of G3’ and G3”. 33 This idea has several advantages including a good signal-
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to-noise ratio, no negative numbers, and a rather straightforward plot-ability. LAOS-
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characterization of polymer containing zwitterions can provide a better understanding of the
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complex rheological behaviors and a deeper insight into microstructural changes. Moreover,
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exposing large deformations mimic real polymer processing and application conditions better.
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In this work, the zwitterionic copolymers of NIPAM and sulfobetaine were in-situ synthesized
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with CNTs, and CNTs were introduced to study the interactions of zwitterions in the copolymers
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with CNTs surfaces. Our motivation was to determine how a different graphenic filler (rod-like
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CNTs vs. sheet like graphene oxide and reduced graphene oxide with different levels oxygen on
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the surface
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additional bonding by zwitterions in the polymer chains leads to stronger interactions.
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Furthermore, the rheological properties of aqueous solutions of self-assemble, zwitterionic
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copolymers, especially the nonlinear rheological properties were studied and the work is also an
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extension of our work on the rheological behavior of NIPAM-co-Zw-copolymers (without fillers).
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5
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Experimental
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Materials
44)
influences the interaction patterns along with the question how introducing
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N, N, N’, N’-tetramethylethylenediamine (TMEDA, Fluka, >99%) and ammonium
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peroxodisulfate (APS, Aldrich, >98%) were used as received. NIPAM (98% Aldrich) was
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recrystallized from a 65:35 (v/v) mixture of hexane and benzene before use. Sulfobetaine
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zwitterions (Zw, Monomer N-(Methacryl-oxypropyl)-N, N-dimethyl-N-(3-sulfopropyl) ammonium
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Betaine) monomer was synthesized according to our previously published procedure.
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aqueous solutions were prepared using ultrapure water purified with a Milli-Q UV-Plus water
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All
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Langmuir
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purification system. Multiwalled carbon nanotubes (MWCNTs, purity >0.90) were obtained from
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Nano Solution Co. Ltd (Wolchul, Korea). The CNTs had a diameter of 10 nm, a length of over 10
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μm and contains a variety of oxygen-containing groups such as O–C=O, C=O, and C–O (Figure S1).
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Synthesis
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CNTs were dispersed in water and sonicated for 20 min to make a homogenous dispersion.
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The NIPAM and Zw monomers were dissolved in distilled water and predetermined amount of
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CNTs dispersion was gradually added to the solution to obtain homogeneous monomer-CNT
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solutions. The aqueous solution was stirred for 5 minutes, followed by removal of the dissolved
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oxygen with 30 min bubbling of nitrogen gas. Next, TMEDA and APS in H2O were added to the
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solution at 10°C under stirring. Free-radical polymerization was continued for 6 h under magnetic
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stirring. The as-prepared composite suspension was homogeneous without precipitates. Table 1
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shows the composition of each sample.
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Table 1: Sample compositions
Zw0CNT1
0.75
100
0
0
Total Polymer [g] 0.75
Zw1CNT1
0.7425
99
0.0075
1
0.75
1
8
8.6
Zw3CNT1
0.7275
97
0.0225
3
0.75
1
8
8.6
Zw10CNT1
0.675
90
0.075
10
0.75
1
8
8.6
Zw30CNT1
0.525
70
0.225
30
0.75
1
8
8.6
Zw0CNT1.5
0.75
100
0
0
0.75
1.5
8
8.6
Zw0CNT1.5L
0.5
100
0
0
0.50
1.5
8
5.9
Zw0CNT1L
0.5
100
0
0
0.50
1
8
5.9
Sample
NIPAM [g]
NIPAM [wt.%]a
Zw [g]
Zw [wt.%]b
CNT [wt.%] c
H2O [g]
1
8
Polymer concentration [wt.%] 8.6
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Zw: Zwitterionic monomer; Monomer N-(Methacryloxypropyl)-N, N-dimethyl-N-(3-sulfopropyl)
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ammonium Betaine;
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a, b
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c
NIPAM and Zw weight percent relative to total polymer mass, respectively;
CNT weight percent relative to total polymer mass.
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Based on the experience with materials synthesized under the same conditions but without
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CNTs,5 the molar masses of the polymers are somewhat above 1000 kg/mol. However,
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considering that the samples were synthesized in-situ with CNTs, it was not possible to determine 5
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the molar mass without damaging a gel permeation chromatography column. The determination
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of the molar mass by other means would also be impaired by CNTs in the sample. It was previously
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found that in-situ synthesis leads to strong (most likely covalent) interactions between PNIPAM
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(without zwitterions) and graphene oxide and reduced graphene oxide in aqueous solution, while
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simple blending of PNIPAM and graphene oxide leads to almost negligible effects in comparison
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to PNIPAM without any nano-fillers, 43-44 it is concluded that significant interactions between the
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polymer and CNTs are formed during synthesis, and the following zeta potential measurements
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also confirmed the interaction. Hence, the polymers are very high in molar mass and, thus, can
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be considered slightly entangled even at the low concentration of 5.9 wt.% used.
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Zeta potential and light scattering
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To investigate the change of the electro-kinetic surface potential of the CNTs before and
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after the copolymerization with zwitterionic copolymers, Zeta potential of the CNTs and
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Zw30CNT1 were measured by a Zetasizer Nano ZS (Malvern Instruments Inc., Massachusetts,
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USA). It should be noted that the composites were washed several times by centrifugation prior
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to the zeta potential measurements to remove any polymers not connected to the CNTs.
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Rheology
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The rheological experiments were performed using an strain-controlled TA Instruments ARES
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at room temperature (18°C) equipped with a 0.02 rad 50 mm cone plate setup. The experimental
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setup consisted of 10 oscillations (ω=1 rad/s) at 20 deformations (γ0) between 0.5% and 1428%
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(the maximum deformation possible by the current setup, corresponding to ±90° geometry
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deflection) to obtain a large amplitude oscillatory shear (LAOS) signal. γ00 indicates intracycle strain stiffening, and S0 represents intracycle shear thickening, and T100%,
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Zw10CNT1 does not show that behavior and instead rather approaches the behavior of Zw0CNT1,
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Zw1CNT1, and Zw3CNT1. This can be interpreted to be the trace of a rapid destructuring, leading
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to the sample crossing the percolation threshold at increasing γ0 due to the loss of structure.
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Hence, Zw10CNT1 has barely enough zwitterionic interactions to lead to a percolated network
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with CNTs, which is stable at small deformations γ0, but gets rapidly destructured at γ0>100%. The
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second nonlinearity peak in I3/1 and the continuously high values of S and T of Zw30CNT1 are the
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traces of the percolated CNT-network reinforced by polymers zwitterionically binding the CNTs
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together, which is slowly destructured by the increasing strain γ0.
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Both the strain-stiffening ratio (S) and shear-thickening ratio (T) show similar result for
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Zw0CNT1, Zw1CNT1, and Zw3CNT1 with linear limit around γ0=100% (within this regime, T, S 0).
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And with increasing strain further, S increased with value higher than 0 and T decreased with
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value lower than 0 as function of strain, which indicated that these three sample show similar
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intra-cycle strain-stiffening and shear-thinning behavior (Figure 4b, 4c). While for the samples
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with higher zwitterions content (Zw10CNT1, Zw30CNT1), both S and T start to diverge from 0 with
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the strain higher than 1%, which suggests that the linear limit range is very small or no linear range
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for these two samples (Figure 4b). Furthermore, S of both samples is higher than 0 with increasing
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strain, which indicates these two samples show intra-cycle strain-stiffening behavior, and the
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value of S for Zw30CNT1 is in higher level for the test strain range than that for Zw10CNT1, which
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might indicate that the nonlinear intensity of Zw30CNT1 is higher than Zw10CNT1 (Figure 4b). In
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addition, the T value of Zw10CNT1 and Zw30CNT1 is almost same and higher than 0 within
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intermediate strain range (γ0 0.5-30%) indicating intra-cycle shear-thickening behavior. While
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both samples show intra-cycle shear-thinning behavior with further increasing strain (γ0> 30%;
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T
