Molecules 2023 , 28 , 7984
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5.2. Characterization and Conformation So far, various experimental methods have been adopted to characterize the SPBs. For example, the morphology of SPBs can be characterized by cryogenic transmission electron microscopy (Cryo-TEM) [154–157], small-angle neutron and X-ray scattering [158–161]. The particle size can be measured by atomic force microscopy (AFM) [162–164] and dynamic light scattering (DLS) [165–168]. The molecular weight and distribution of polymer brushes can be characterized by gel permeation chromatography (GPC) [169–171], and then surface grafting density is calculated [172,173]. Curie E P K et al. [174] studied the polymer brush of hydrogenated PS-b-PEO diblock copolymer by the neutron reflection technique. It was found that chain density changed from step distribution to parabola distribution with the increase in grafting density. Guo x et al. [175] studied the hydrodynamic radius RH of micellar polyelectrolyte brush consisting of PS core (50~100 nm) and PAA brushes at different KCl concentration (10 − 3 ~3 m) by DLS. It was found that R H was inversely proportional to the ionic strength, which was consistent with the Daoud–Cotton model. Prucker O et al. [176] studied a PS brush with different grafting densities grafted on silicon wafer by AFM. The results showed that the structure of the brush was dependent on the grafting density. When the grafting density was low, the surface was uneven because of the aggregation of PS chain. On the contrary, the collapse of the PS chain could form a uniform coating. In order to explore the conformation of SPBs, many theoretical models have been explored [177]. According to the complexity of models, it can be divided into the following categories: scale theory [178–180], numerical self-consistent field theory (NSCFT) [181–183], analytical self-consistent field theory (ASCFT) [184,185], molecular dynamics (MD) [186], Brownian dynamics (BD) [187,188], and so on. Hariharan R et al. [189] studied an SPB system using the Daoud–Cotton model. Using the Monte Carlo model, Luo MB [190] studied the relationship between the conformational parameters of polymer brushes and the stretching volume of random and restricted chains. The results showed that the effects of the extended volume (EV) of two types of chains on the mean square end distance <R 2 > and the mean square radius of rotation <S 2 > are basically identical. In addition, research on the fundamental nature of SPBs has been reported. Con- sidering the special constitution of SPBs, the influence of external factors (ionic strength, pH) on the conformational parameters of polyelectrolyte brush can also be monitored by physical test methods. For example, Ballauff M [191] investigated the influence of pH value and ionic strength on brush thickness in a PS-PAA system. They found that the pH value of the system affected the dissociation degree of polyacrylic acid brush. In the acid condition, acrylic acid was difficult to dissociate. When NaOH was added to the system, OH - was neutralized with H + , causing the presence of carboxylate anion on the acrylic acid chain. Due to the increase in the electrostatic repulsion force, the swelling of chains happens. Moreover, the effect of electrolyte concentration on the morphology of PAA brush on microspheres was mainly due to the electrostatic effect. The higher electrolyte concentration was, the more obvious the electrostatic shielding effect was, which led to the shrinkage of the polymer brush. Yu [192] found that the brush chain in the spherical PSS brush could fully swell in the water phase, and the brush collapsed after adding La 3+ , which may be caused by the decrease in osmotic pressure. 5.3. Application in Papermaking As in our previous work (see Figure 8), cationic spherical polyacrylamide (CSPAM) brushes were synthesized by grafting manniched polyacrylamide (PAM) from the surface of γ -methacryloxypropyl trimethoxy-silane-modified SiO 2 nanoparticles [193]. The retention effect of CSPAM revealed that the highest first-pass retention was 71.1% when the dosage of CSPAM was 3.5 mg · g − 1 .
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