Kinetics of Polymer Desorption from Colloids Probed by Aggregation

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Kinetics of Polymer Desorption from Colloid Probed by Aggregation-Induced Emission Fluorophore Feng Yan, Zhichao Zhu, Xiaobiao Dong, Chao Wang, Xiaohui Meng, Yue Xie, Guanxin Zhang, and Dong Qiu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04215 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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Kinetics of Polymer Desorption from Colloid Probed by AggregationInduced Emission Fluorophore Feng Yan†,‡, Zhichao Zhu†, Xiaobiao Dong†,‡, Chao Wang†,‡, Xiaohui Meng†, Yue Xie†, Guanxin Zhang† and Dong Qiu† ‡,* †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics

and Chemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100190, China

ABSTRACT: Polymer adsorption and desorption are fundamental in many industrial and biomedical applications. Here we introduce a new method to monitor the polymer desorption kinetics in situ, based on the behavior of aggregation induced emission. Poly(ethylene oxide) (PEO) and colloidal silica (SiO2) were used as a model system. It was found that the AIE method could be successfully used to determine the polymer desorption kinetics and the polymer desorption followed the first order kinetics. It was also found that polymer desorption rate constant decreased with the increasing molecular weight, which could be described by a power law function kd~M-0.28, close to that of the adsorption rate constant.

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1. INTRODUCTION Polymer adsorption/desorption on/from colloids has been extensively involved in both industrial and biomedical applications, such as flocculation,1-2 coating,3 and drug delivery.4 Differently from small molecules, polymer adsorption/desorption, especially those non-specific protein adsorption/desorption, usually does not produce much detectible spectroscopic changes; therefore, characterization of polymer adsorption/desorption on/from colloidal particles is often based on the consequences caused by the density distribution changes of polymer segments upon adsorption/desorption, such as changes in hydrodynamics, sedimentation, diffusion and scattering. Correspondently, several techniques have been successfully used to measure the polymer adsorption/desorption behavior in-situ, including light scattering (LS),5-6 solvent relaxation nuclear magnetic resonance (NMR)7 and small angle neutron scattering (SANS).8-10 Currently, most polymer adsorption/desorption studies are focused on the equilibrium state, i.e. measured after the adsorption/desorption has completed, except for those very slow process.11-14 Nevertheless, polymer adsorption/desorption kinetics is indeed of great significance to control the surface layer composition, especially in the cases of competitive adsorptions.15-16 For example, the understanding of the kinetics of protein adsorption and its prevention on the surface of the biocompatible material or drug carrier is very important for the design of materials interacting with biological systems.17 Therefore, new challenges have been raised on the rapid determination of polymer adsorption/desorption. Unfortunately, those characterization techniques usually take tenths minutes or hours for a single measurement, which are comparable with the time scale of most polymer adsorption/desorption and thus are not suitable for adsorption/desorption kinetics study. For example, on the state-of-art instruments, a typical NMR relaxation measurement on polymer adsorption will take around 20 min and SANS of adsorbed


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polymer layer on colloidal particles normally takes at least 1 hour. Therefore, a quick and versatile in-situ characterization method for polymer adsorption/desorption is the key for their kinetical study and is still highly demanding. Aggregation induced emission luminogen (AIEgen) may provide a practical solution for the above challenge, as they are sensitive to polymer adsorption state and can be quickly detected. In a previous study,18 a new method using AIEgen probe was successfully established and used to measure the adsorption of poly(vinyl alcohol) on silica particles, where a single measurement could be completed in as short as ~20 s with fairly good statistics. Therefore, this new method might be able to monitor the kinetics of polymer adsorption/desorption process in-situ. As the zero time point of polymer adsorption kinetics is difficult to set in practice, in the current work, kinetics of polymer desorption from colloidal particles was chosen for a tentative study. We wish to prove that this AIEgen based method is capable of monitoring the polymer desorption kinetics and also, to obtain the profile of polymer desorption kinetics. 2. EXPERIMENTAL SECTION 2.1. Materials Colloidal SiO2 dispersion LUDOX-TM40 with a quoted average diameter of 25nm was purchased from Sigma-Aldrich and used as received. Poly(ethylene oxide) (PEO) with average molecular weights of 1M g∙mol-1 , 100K g∙mol-1, 4.6K g∙mol-1 and 0.8K g∙mol-1 (Sigma-Aldrich) were used as received. Pure water (generated by an ELGA Purelab® system) with an electric resistance greater than 18.2 MΩ·cm was used for all samples. The AIEgen used was synthesized according to the reported procedures19 and its structure was shown in our previous study.18


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Formamide (MF) and methanol were purchased from Beijing Chemical Works and used as received. 2.2. Sample Preparation The mixture of silica and PEO solutions were prepared with pure water as solvent. These silicaPEO mixtures were shaken gently on a rolling device overnight to ensure adsorption equilibrium had been reached. 10 mM AIEgen stock solution was prepared with CH3OH as solvent. 25µL of AIEgen stock solution was added into 2.35mL of silica-PEO solution. Finally, 0.15mL of MF was added before the fluorescence measurements. For all the desorption measurements, the concentrations of PEO and silica were both fixed at 0.4 wt%. 2.3. Characterizations. NMR measurements. A Bruker AVANCE 600M NMR spectrometer, using the Carr-PurcellMeiboom-Gill (CPMG) sequence, was used to measure the spin-spin relaxation time constant of water 1H nuclei at 25℃. The separation between the 90° and 180° pulse was set as 1.0ms while the relaxation delay was about 5 times of the spin-lattice relaxation time constant. The relaxation rate constant (R2) is obtained from an exponential fit of the signal decay

M y (t ) = M y (0)e − R2t

(1)

where My(0) is the transverse magnetization immediately after the 90° pulse. To eliminate the effect of free water molecules, specific relaxation rate constant R2sp is used, defined as

R2 sp =

R2 −1 R20

(2)


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where the R20 is the relaxation rate constant of pure water, and the R2 is the relaxation rate constant of the investigated sample. Fluorescence spectra were collected on a Hitachi FP-6000 spectrometer (All the fluorescence spectra measurements were carried out in the presence of MF, and the fluorescence spectra in Figure 1b were immediately measured after the addition of MF to avoid the effect of desorption). UV-vis absorption spectra were recorded on a TU-1901 spectrophotometer. 3. RESULTS AND DISCUSSION The AIEgen used in this study was a derivative of tetraphenylethene, which is non-emissive in the solution state but is induced to emit intensively upon aggregates formation, owing to the restriction of intramolecular motions (RIM) in the aggregates.20-21 Based on this RIM mechanism, the AIEgens can be used to detect polymers adsorbed on colloidal particles, because the adsorbed polymer can exert a restriction on AIEgen.18 When the adsorbed amount of polymer on colloid particles is measured with the change of time, adsorption/desorption kinetics curves can be obtained. Previously, researchers have studied polymer desorption by using mixtures of two solvents, one of which is a displacer of the polymer.22-24 In the current study, we investigated the PEO desorption kinetics from colloidal silica particles by AIEgen probe, where the desorption was induced by using formamide (MF) as a displacer. The first question is whether this AIEgen can be used to characterize the adsorption of PEO on SiO2 or not. The key requirement is that the AIEgen should be able to distinguish free polymer chains from those adsorbed, which has been confirmed in the PVA-SiO2 system.18 From Figure 1a, we can see that both PEO aqueous solution and silica solution showed no UV-vis absorption band and were almost non-emissive. The mixture of PEO and AIEgen (PEO+AIE) showed an


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absorption peak (λmax) around 384nm, the same as AIEgen solution, indicating that AIEgen molecules are the only luminogen in the system. Figure 1b shows a very small fluorescence enhancement for AIEgen solution after the addition of PEO, suggesting that the free PEO chains would contribute very little to the AIE behavior, similar to the previous study.18 The mixture of SiO2 and AIEgen (SiO2+AIE) and the mixture of PEO, SiO2 and AIEgen (PEO+SiO2+AIE) showed UV-vis absorption peaks similar to that of PEO+AIE mixture and AIEgen solution (Figure 1a), indicting AIEgen was still the main luminogen in these two systems. As expected, the fluorescence emission (FL emission) intensity of AIEgen at ~600nm was enhanced by more than 10-fold in the presence of SiO2 particles (SiO2+AIE), due to the adsorption of AIEgen on the surface of SiO2 particles.18 The FL emission intensity of PEO+SiO2+AIE was further enhanced, more than twice of that in the SiO2+AIE system and 20 times of that in the PEO+AIE system, due to the additional restriction of AIEgen intramolecular rotation by adsorbed PEO on SiO2 particles.18 These observations suggest that the FL emission of this AIEgen could be used to detect the adsorbed PEO on SiO2 particles.

Figure 1. (a) UV adsorption and (b) FL emission spectra for SiO2, PEO4.6K, AIEgen, mixture of PEO4.6K and AIEgen (PEO+AIE), mixture of silica and AIEgen (SiO2+AIE), and mixture of


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PEO4.6K, silica and AIEgen (PEO+SiO2+AIE). (SiO2: 0.4 wt%, PEO4.6K:0.4 wt%, AIEgen: 0.1mM), λex=398nm. Since the FL emission intensity is only sensitive to adsorbed polymer segments in “train” conformation, which, for a nonionic polymer, usually saturates well before the full adsorption is achieved, the FL emission intensity is only linearly related to the absolute adsorption amount before adsorption saturation.[18] Therefore, an adsorption amount below the saturation adsorption would be better for desorption kinetics study. The adsorption isothermal of PEO-SiO2 system was established by measuring the AIEgen emission intensity at different PEO concentrations at equilibrium state (Figure S1). As seen in Figure S1, the FL emission intensity for 0.4 wt% SiO2 dispersion initially increased with added PEO amount, then reached the plateau at the PEO concentration of ~0.5 wt%. Therefore, we fixed the PEO concentration at 0.4 wt% for the following desorption kinetics studies. It is now clear that the AIEgen probe could be used to characterize PEO adsorption on colloidal SiO2, in the following, this method was used to monitor PEO desorption kinetics induced by adding MF. First of all, it is intuitive to ask whether the AIEgen probe method could reproduce desorption kinetical behavior measured by other well established methods, for example the solvent relaxation NMR. To verify this, both AIEgen and NMR methods were used to study the same system. However, desorption of PEO4.6K from silica surface may be too fast to be measured by NMR method. Therefore, PEO1M, with a higher molecular weight thus expected to have a slower desorption rate than PEO4.6K, is used to compare the kinetical curves generated by AIEgen and NMR methods (Figure 2). In NMR method, R2sp, is used to describe the adsorption amount, in analogue to the FL emission intensity in the AIEgen method. When adsorbed polymers gradually desorb from the particle surface, water molecules will exhibit a


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lower R2sp because water molecules at the interface become less constrained.25-26 As shown in Figure 2, the R2sp values as a function of time (t) can be well fitted to an exponential decay y∝ exp(-kd t) with kd being the desorption rate constant, and kd was found to be ~0.008. Similarly, the fluorescence intensity of the same sample was also successfully fitted to such an exponential decay (the typical characteristics of the first-order kinetics), with kd ~0.009, almost identical to that found by NMR method. This is rather expected, because both probes (water in NMR and AIEgen in the FL emission) are sensitive to the same conformation of adsorbed polymer (segments in “train” conformation), i.e. they are measuring the same event caused by adsorbed polymer on colloidal particles. This comparative study proves that the AIEgen method is capable of measuring polymer desorption kinetics.

Figure 2. Desorption kinetical curves of PEO1M on SiO2 established using the AIEgen method and the solvent relaxation NMR method. However, compared with the NMR method, the AIEgen method takes much shorter time for each measurement, thus is better for “snapshotting” the kinetical curves and is more suitable for


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fast desorption process. For example, for desorption of a lower molecular weight PEO (PEO100K) from colloidal SiO2 particles, NMR cannot record enough data points before the completion of desorption process (Figure S2), because a single measurement takes too long. For even smaller molecular weight, for example PEO4.6K, it can be imagined that NMR method will become more powerless. Nevertheless, AIEgen method works much better for these faster processes. The FL emission spectra of PEO4.6K on colloidal SiO2 at different intervals during the desorption process were summarized in Figure 3a. It can be seen that the FL emission intensity decreases with time elapsing, which can be described by an exponential decay function as well (Figure 3b), suggesting that the polymer desorption behavior was still a first-order kinetical process. For PEO4.6K, the desorption rate constant, kd, was found to be ~0.030 min-1, higher than that of PEO1M.

Figure 3. (a) FL emission spectra of PEO4.6K+SiO2+AIE with different time after adding MF. (b) FL emission intensity at λ=600nm with time at different PEO molecular weight (The solid lines are fits to exponential equation). (c) The desorption rate constant (kd) VS PEO molecular weight (the solid line is the fit to a power law function kd~M-a).


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The desorption kinetical profiles of PEO with two other molecular weights were also measured by the AIEgen method (Figure S3-S4). Both desorption profiles could be well represented by the exponential decay function (Figure 3b). All the obtained desorption rate constant (kd) and halflife (t1/2) of desorption were summarized in Table S1. It became evident that the desorption kinetics depends highly on the polymer molecular weight, similar to the adsorption kintetics.13 The desorption rate constant of PEO with lower molecular weight was found to be higher, qualitatively in agreement with other studies.11, 27 The half-life time (t1/2) of PEO100K, where half of the adsorbed polymer has desorbed, was ~50 min, which explains very well why NMR method did not work out well for this molecular weight. PEO0.8K has a t1/2 of ~10 min, even shorter than that of a single NMR measurement and can only be measured by this AIEgen method. The molecular weight dependence (a) of desorption rate constant was plotted in Figure 3c, which can be represented by a power law function

kd ~ M − a

(1)

a was found to be 0.28±0.05, very close to that of the adsorption rate constant (a~0.25),28 suggesting that the desorption and adsorption kinetics might be controlled by similar factors. It is notable that this result is different from the prediction made by the classical reptation predication (a=2)29-30 in polymer melts and concentrated solutions, probably due to different entanglement degrees. The mechanism of AIEgen method detecting the polymer desorption process can be illustrated as Figure 4. When PEO chains are adsorbed on the surface of SiO2 particles, the system shows


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strong FL emission because of the RIM effect exerted by adsorbed polymer chains. However, when PEO chains desorb from the particle surface, the constraint on AIEgen molecules becomes loosened, leading to the decrease in FL emission intensity. Once all polymer chains have desorbed, the system reaches the equilibrium, so does the FL emission intensity. Based on this understanding, this AIEgen method is a powerful tool to study the desorption kinetics of polymer from interface, and might be a universal method, because it does not require particular chemical interactions. For example, it can also be used to generate the desorption kinetics of PVA from colloidal silica (Figure S5). In principle, this method could also be used to measure the kinetics of polymer adsorption if the experiment is well designed, thus gaining deep insight in competitive adsorption processes. However, the selection of AIEgen is vital, which should have distinctly different emission behaviors in different states, such as in solution, in complex with polymer, in adsorption at interface etc., thus, emission intensity can be directly related to polymer adsorbed amount.

Figure 4. Schematic illustration of AIEgen FL emission intensity changes during the polymer desorption process.

4. CONCLUSION


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In summary, we have investigated the desorption kinetics of PEO on SiO2 particles using AIEgen as a probe, by monitoring the FL emission intensities during the desorption process. It was found that AIEgen method could follow the change of adsorbed polymer amount in time during desorption process thus generates the desorption kinetics profiles. The PEO desorption rate constant was found to be dependent on molecular weight, similar to polymer adsorption rate constant. This AIEgen method is advantageous in its much shorter measuring time thus is ideal for monitoring fast adsorption/desorption process of polymer on/from colloid particles. It is based on physical interactions, therefore might be universal. ASSOCIATED CONTENT Supporting Information. “This material is available free of charge via the Internet at http://pubs.acs.org.” Figure S1 showing the adsorption isotherm of PEO4.6K on colloidal SiO2 established using an AIEgen probe approach. Table S1 showing the desorption rate constant (kd) and half-life time (t1/2) of different molecular weights PEO from SiO2 particles. Figure S2 showing the desorption kinetics profile of PEO100K on silica established using the solvent relaxation NMR method. Figure S3 showing the FL emission spectra of PEO0.8K+SiO2+AIE with different time after adding MF. Figure S4 showing the FL emission spectra of PEO100K+SiO2+AIE with different time after adding MF. Figure S5 showing the desorption kinetics of PVA from SiO2 established using the AIEgen method. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Project No. 21474122), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020300) and the National Basic Research Program (2017YFC1103300). REFERENCES (1) Boluk, M.; Van de Ven, T. Effects of polyelectrolytes on flow-induced deposition of titanium dioxide particles onto a cellophane surface. Colloids and surfaces 1990, 46 (2), 157176. (2) Hogg, R. The role of polymer adsorption kinetics in flocculation. Colloids Surf., A 1999, 146 (1), 253-263. (3) Soga, I. Polydispersity and functional group distribution of dispersant polymer: Adsorption properties and magnetic paint dispersion. J. Colloid Interface Sci. 2001, 240 (2), 622629. (4) Gref, R.; Domb, A.; Quellec, P.; Blunk, T.; Müller, R.; Verbavatz, J.; Langer, R. The controlled intravenous delivery of drugs using PEG-coated sterically stabilized nanospheres. Adv. Drug Delivery Rev. 2012, 64, 316-326. (5) O’Shaughnessy, B.; Vavylonis, D. Non-equilibrium in adsorbed polymer layers. J. Phys.: Condens. Matter 2004, 17 (2), 63. (6) Cattoz, B.; de Vos, W. M.; Cosgrove, T.; Crossman, M.; Prescott, S. W. Manipulating interfacial polymer structures through mixed surfactant adsorption and complexation. Langmuir 2012, 28 (15), 6282-6290. (7) Cooper, C. L.; Cosgrove, T.; van Duijneveldt, J. S.; Murray, M.; Prescott, S. W. The use of solvent relaxation NMR to study colloidal suspensions. Soft Matter 2013, 9 (30), 7211-7228. (8) Cosgrove, T.; Heath, T. G.; Ryan, K.; Crowley, T. L. Neutron scattering from adsorbed polymer layers. Macromolecules 1987, 20 (11), 2879-2882. (9) Qiu, D.; Flood, C.; Cosgrove, T. A small-angle neutron scattering study of adsorbed polymer structure in concentrated colloidal dispersions. Langmuir 2008, 24 (7), 2983-2986. (10) Flood, C.; Cosgrove, T.; Qiu, D.; Espidel, Y.; Howell, I.; Revell, P. Influence of a surfactant and electrolytes on adsorbed polymer layers. Langmuir 2007, 23 (5), 2408-2413. (11) Frantz, P.; Granick, S. Kinetics of polymer adsorption and desorption. Phys. Rev. Lett. 1991, 66 (7), 899. (12) Kawaguchi, M.; Anada, S.; Nishikawa, K.; Kurata, N. Effect of surface geometry on polymer adsorption. 2. Individual adsorption and competitive adsorption. Macromolecules 1992, 25 (5), 1588-1593. (13) Dijt, J.; Cohen Stuart, M.; Fleer, G. Kinetics of adsorption and desorption of polystyrene on silica from decalin. Macromolecules 1994, 27 (12), 3207-3218.


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(14) Geffroy, C.; Labeau, M.; Wong, K.; Cabane, B.; Stuart, M. C. Kinetics of adsorption of polyvinylamine onto cellulose. Colloids Surf., A 2000, 172 (1), 47-56. (15) Cooper, C. L.; Cosgrove, T.; van Duijneveldt, J. S.; Murray, M.; Prescott, S. W. Colloidal particles in competition for stabilizer: a solvent relaxation NMR study of polymer adsorption and desorption. Langmuir 2012, 28 (48), 16588-16595. (16) Fu, Z.; Santore, M. M. Kinetics of competitive adsorption of PEO chains with different molecular weights. Macromolecules 1998, 31 (20), 7014-7022. (17) Fang, F.; Satulovsky, J.; Szleifer, I. Kinetics of protein adsorption and desorption on surfaces with grafted polymers. Biophys. J . 2005, 89 (3), 1516-1533. (18) Zhu, Z.; Dong, X.; Zhang, G.; Xiang, J.; Qiu, D. Characterizing the adsorption of poly (vinyl alcohol) on colloidal silica with aggregation-induced emission fluorophore. Langmuir 2016, 32 (9), 2145-2150. (19) Hu, F.; Zhang, G.; Zhan, C.; Zhang, W.; Yan, Y.; Zhao, Y.; Fu, H.; Zhang, D. Highly Solid‐State Emissive Pyridinium‐Substituted Tetraphenylethylene Salts: Emission Color‐ Tuning with Counter Anions and Application for Optical Waveguides. Small 2015, 11 (11), 1335-1344. (20) Mei, J.; Hong, Y.; Lam, J. W.; Qin, A.; Tang, Y.; Tang, B. Z. Aggregation‐Induced Emission: The Whole Is More Brilliant than the Parts. Adv. Mater. 2014, 26 (31), 5429-5479. (21) Feng, G.; Kwok, R. T. K.; Tang, B. Z.; Liu, B. Functionality and versatility of aggregation-induced emission luminogens. Applied Physics Reviews 2017, 4 (2), 021307. (22) Stuart, M. C.; Fleer, G.; Scheutjens, J. Displacement of polymers. I. Theory. Segmental adsorption energy from polymer desorption in binary solvents. J. Colloid Interface Sci. 1984, 97 (2), 515-525. (23) Van der Beek, G.; Cohen Stuart, M.; Fleer, G.; Hofman, J. A chromatographic method for the determination of segmental adsorption energies of polymers. Polystyrene on silica. Langmuir 1989, 5 (5), 1180-1186. (24) Van der Beek, G.; Stuart, M. C.; Fleer, G.; Hofman, J. Segmental adsorption energies for polymers on silica and alumina. Macromolecules 1991, 24 (25), 6600-6611. (25) Flood, C.; Cosgrove, T.; Espidel, Y.; Howell, I.; Revell, P. Effects of surfactants and electrolytes on adsorbed layers and particle stability. Langmuir 2008, 24 (14), 7323-7328. (26) Cooper, C. L.; Cosgrove, T.; van Duijneveldt, J. S.; Murray, M.; Prescott, S. W. Competition between Polymers for Adsorption on Silica: A Solvent Relaxation NMR and SmallAngle Neutron Scattering Study. Langmuir 2013, 29 (41), 12670-12678. (27) Johnson, H. E.; Douglas, J. F.; Granick, S. Topological influences on polymer adsorption and desorption dynamics. Phys. Rev. Lett. 1993, 70 (21), 3267. (28) De Gennes, P. Polymers at an interface; a simplified view. Adv. Colloid Interface Sci. 1987, 27 (3-4), 189-209. (29) Lodge, T.; Rotstein, N.; Prager, S. Dynamics of entangled polymer liquids: Do linear chains reptate? Adv. Chem. Phys. 1992, 79, 1-1. (30) Rotstein, N.; Lodge, T. Tracer diffusion of linear polystyrenes in poly (vinyl methyl ether) gels. Macromolecules 1992, 25 (4), 1316-1325.


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Kinetics of Polymer Desorption from Colloids Probed by Aggregation

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