Controlling the Surface Properties of Binary Polymer Brush-Coated

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Controlling the Surface Properties of Binary Polymer Brush Coated Colloids via Targeted Nanoparticles Masoumeh Ozmaian, Brayden Freitas, and Rob D. Coalson J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b05520 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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The Journal of Physical Chemistry

Controlling the Surface Properties of Binary Polymer Brush Coated Colloids via Targeted Nanoparticles Masoumeh Ozmaian 1†, Brayden Andrew Freitas2, and Rob D. Coalson1* Affiliations: 1 Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA 2 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA † Current address: Department of Chemistry, University of Texas at Austin, Austin, TX 78712, USA

Abstract: This paper explores a novel mechanism for controlling the surface properties of polymer coated colloids using targeted (“sticky”) nanoparticles which attract monomers of certain polymer species. In our study, colloids are coated by two types of tethered polymer chains having different chemical properties. Attraction of nanoparticles to the monomers of one polymer type causes these polymer chains to contract towards the grafting surface, rendering the other type more exposed to the environment. Thus, the effective surface properties of the colloid are dominated by the intended polymer type. We use Coarse Grained Molecular Dynamics (CGMD) simulation to demonstrate that introducing nanoparticles which interact preferentially with certain types of polymers makes it possible to switch between different surface properties of the colloid. This mechanism can in principle be exploited in drug delivery systems and self-assembly applications. ACS Paragon Plus Environment

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Introduction: Polymers have been extensively used to modify the surface properties of colloidal materials1,2 because of their chemical versatility. When polymer chains are tethered to a surface at high grafting density, they extend into a polymer brush conformation3,4. Polymer brushes have been the subject of intensive study during the past few decades5–8. Stimulated by progress in methods for fabricating nanomaterials and polymer brushes, novel applications have emerged9–11. These include functionalizing colloidal particles by grafting polymer brushes on their surfaces 12–14. Polymer chains anchored on the outer surface of colloids can prevent the colloids from agglomerating, or be used to tune properties of the surface including wettability and permeability, as well as their adhesive and adsorptive characteristics15. This motif can be exploited in a wide variety of applications such as Janus particles, drug delivery systems, and self-assembled structures15–20. Of particular relevance to this research area is controlling the adhesion of colloids to different surfaces by surface modification, which is a challenge in numerous applications, including achieving the targeted delivery of nanocarriers for drug delivery19. A variety of physical and chemical stimuli such as light9, temperature21, pH22 and sound waves16,23 can be used to control the conformational properties of polymer chains. For example, ultrasound has been utilized to activate anti-cancer drug-loaded nanoemulsions for targeted drug delivery applications24. pH has also been used extensively as a trigger for activating polymeric chains grafted onto nanoparticles, especially in the vicinity of acidic tumors25,26. Mixed polymer brushes, i.e., brushes with two or more grafted polymer components, have been synthesized and utilized in a variety of applications where alteration of the surrounding solvent switches the surface characteristics of the coated colloid27–31. Theoretical and computational analyses have also contributed to our understanding of these processes15,32,33. The motif we present here is based on controlling the surface properties of a coated colloid using targeted nanoparticles that interact with mixed polymer chains grafted on the colloid’s surface. We have shown in previous Coarse Grained Molecular Dynamics (CGMD) simulation work how this motif can be utilized to control molecular transport through polymer-functionalized nanochannels17. Using appropriately chosen numbers of nanoparticles in the solution phase environment, we present here another application of this mechanism, namely, for adjusting the surface properties of colloids that are coated with polymer brushes of two different types. By contracting the polymer chains with certain properties (denoted here as grey chains), the surface properties of the coated colloid become dominated by the other type of polymer chain (pink chains), which are exposed to the solution. This mechanism can be exploited in applications where a surface-functionalized nanoparticle needs to exhibit variable surface properties. Our method for suppressing polymer chains is to insert into the system nanoparticles that interact with grey polymer chains, causing the grey chains to contract towards the surface of the sphere. The degree of polymer chain contraction can ACS Paragon Plus Environment

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be reversibly controlled by changing the nanoparticle concentration in the solvent, which then changes the adhesion strength of the coated polymer, for example, to an exterior surface which preferentially attracts grey or pink polymer monomers. In previous work, we showed that adding “sticky” nanoparticles into solution modifies the morphology of grafted polymers for both planar and inside-grafted cylindrical brushes. Infiltration of nanoparticles into the polymer brush due to attractive nanoparticle-polymer interactions can cause a dramatic compression of the polymer layer at a threshold concentration of nanoparticles17,34–36. Utilizing this effect, we can construct colloid particles that are coated with mixed polymer chains having different characteristics, and use these to design multi-functional devices with desired properties. In contrast to the process of switching solvents to induce morphological change in a polymer brush, the use of targeted nanoparticles for this purpose has the potential advantage of being more feasible and less toxic, for example within a biological cell37. Results and Discussion In order to illustrate some basic features of the spherical polymer brushes used in our study, we first investigate the case where a colloidal sphere is coated with a single polymer type (grey chains) whose monomers attract nanoparticles present in the solvent. We employ a coarse-grained bead spring simulation model for the polymer chains38. In addition to imposing a repulsive truncated Lennard-Jones (LJ) potential between all pairs of particles in the system, adjacent monomers in the same polymer chain are connected by a FENE spring potential39,40. Full details of the coarse grained model and simulation procedure can be found in the Supporting Information (SI) and our previous papers17,36,41. Note that unless otherwise stated the monomer of each polymer chain that connects to the colloid is rigidly attached to a designated spot on the colloid surface38. We will designate this as a static bond.

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Figure 1. (a) Cross sectional snapshot of a polymer coated colloid at an early stage of the MD simulation. (b) Equilibrium monomer density profile  (r ) [arbitrary units] of polymer chains on the colloid. The surface of the colloid lies at R0=10 in units of the monomer diameter (dashed orange line).

All mobile particles in the system are coupled to a Langevin thermostat in order to establish a canonical ensemble distribution of particle configurations corresponding to a specified system temperature. MD simulations are performed using LAMMPS38. The masses of all particles in the system are taken to be equal, since we are only interested in the equilibrium distribution of configurations that arises under long time steady state conditions. Choosing an appropriate energy unit ε (see Supporting Information), we then set the absolute temperature 𝑇 = 𝜀/𝑘𝐵 , so that all energies are in units of 𝑘𝐵𝑇, where kB = Boltzmann’s constant. We also measure lengths in units of the monomer diameter. Our system consists of a rigid spherical colloid with radius R0= 10 coated with 200 grafted polymer chains, randomly distributed over the surface of the colloid. Each polymer chain has a chain length of L0=100 monomers, which corresponds to an average surface grafting density of S0=0.16. The colloid itself is a shell consists of 1020 smaller particles. Each small particle is an LJ type sphere, possessing an excluded volume size corresponding to 1 monomer diameter. The system is confined in a cubical box with the dimension of 240 along each of its three dimensions. Consider first the case that there are no targeted nanoparticles interacting with the polymer chains in the system and these grafted polymer chains can freely fluctuate. Fig. 1a shows a snapshot of the system near time t=0, and Fig. 1b shows the equilibrated monomer density profile  (r ) as a function of radial distance r. The brush height r0 is defined as the value of r at which, on average, 99.5% of the monomers have r < r0. The brush height is ca. 46 here. The monomer density profile illustrated in Fig. 1 is in good ACS Paragon Plus Environment

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qualitative agreement with results for outside-grafted spherical polymer brushes found in previous theoretical and computational studies42,43. When nanoparticles which attract (“stick to”) grey polymer monomers are added to the system, they spontaneously partition into the polymer brush. Due to their enthalpic interaction, polymer chains wrap around the nanoparticles to maximize energetically favorable contacts, which causes the brush to contract and form a dense polymer layer at distances not far from the surface of the colloid particle. The same effect has been observed previously for other geometries (planar and cylindrical brushes)17,34–36. Higher concentration of sticky nanoparticles in the solvent causes stronger contraction of polymer chains up to a point. (At very high nanoparticle concentrations, not considered here, the polymer brush swells to incorporate more nanoparticles.) Another parameter affecting the polymer chain morphology is the strength of the nanoparticle-polymer interaction, 𝜀𝑏. A stronger nanoparticle-polymer interaction energy induces a similar compressive effect on the polymer brush. These observations, which have been described extensively in previous studies34–36, extend to the spherical brush case under consideration here. For concreteness, we fix the nanoparticle-polymer interaction constant as 𝑏 = 2 in the present study, and choose the nanoparticle diameter to be equal to that of a polymer monomer. (See SI for details of the pair potentials used in these simulations.)

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Figure 2. Polymer chains become more compressed as the number of sticky nanoparticles in the system is increased from 1000 to 12000 (top panel; sticky nanoparticles not shown). Bottom panel shows  (r ) , the monomer density profile of the coated colloid [arbitrary units], as a function of the radial distance from the colloid for different numbers of sticky nanoparticles in the system. Orange dashed line indicates the location of the surface of the colloid at r=R0, where the polymer chains are grafted.

Fig. 2 compares the degree of compression of the polymer chains at different concentrations of sticky nanoparticles in solution. As can be seen, by increasing the concentration of nanoparticles over the indicated range, the maximum density of polymer beads increases by ca. x4 (from ~2 to ~8), and the length of chains decrease by ca. x1.5 (from ~46 to ~29). Polymer chains become more compressed around the colloid surface, and thus the effective radius of the coated colloid decreases. This behavior can also be utilized to suppress polymers with undesirable characteristics when two or more types of polymer chain are grafted to the colloidal sphere, as demonstrated next.


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Figure 3. Nanoparticles (yellow) interacting with grey polymer chains contract these chains, which effectively increases the exposure of pink chains. (a) The hairy chain-coated colloid ball before infiltration of nanoparticles into it. (b) The hairy ball after contracting the grey polymer chains via sticky nanoparticles. (c) Average monomer density profile of the polymer chains (grey and pink taken together) as a function of the radial distance from the colloid before introducing the nanoparticles to the system (d) Monomer density profile of the pink and grey polymer chains (separately resolved), and the nanoparticle density as a function of the radial distance from the colloid after introducing 8000 nanoparticles into the system.

In order to build a multi-faceted “hairy ball” (polymer coated colloid sphere), we randomly graft to the colloid two types of polymer chains (grey and pink): 100 grey chains and 100 pink chains. Grey monomers attract the nanoparticle additives, while pink monomers do not. After introducing these sticky nanoparticles into the simulation system, grey polymer chains collapse and consequently the surface properties of the hairy ball are dominated by pink polymer chains. (Movie1 in SI illustrates this behavior.) Figure 3 demonstrates the initial structure and equilibrated configuration of the hairy ball in the presence of 8000 nanoparticles. As can be seen in the snapshots in Figs. 3a, b, once ACS Paragon Plus Environment

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equilibration is attained, grey chains are hidden and wrapped around nanoparticles, while pink chains are exposed to the outside solution. Fig. 3c shows the density profile for the monomers (pink and grey chains taken together) before nanoparticles are added to the system. Fig. 3d shows density profiles for pink monomers and for grey monomers (separately resolved), as well as the nanoparticle density profile. In the absence of sticky nanoparticles, the pink and grey chains are physically equivalent, hence they give rise to the same equilibrium distribution. When (yellow) nanoparticles that stick to grey monomers are introduced into the system, these infiltrate into the brush layer and contract the grey brush chains against the surface of the colloid. The pink polymer chains are virtually unaffected by the addition of these nanoparticles, since there is no attraction between nanoparticles and monomers of the pink chains. Next we put the coated colloid between two external flat surfaces, one of which attracts grey polymer chains, and the other one attracts pink chains, as illustrated in Fig. 4. The two surfaces are constructed out of spherical particles having the same diameter as that of a polymer monomer, close packed into a lattice one particle thick, and constrained in place during a simulation in which the hairy nanoball system is allowed to diffuse freely within the simulation box. Pink/grey monomers are attracted to the particles of the pink/grey wall. Pink/grey polymers are repelled from the particles of the grey/pink wall by truncated LJ forces. In the absence of sticky nanoparticles there is a 50% chance that the colloid will adhere to either of the two surfaces. (Movie2 showing this effect can be found in the SI.) As can be seen in Fig. 4, when nanoparticles that stick to grey polymer chains are introduced into solution the coated colloid will stick preferentially to the pink surface, which attracts the exposed pink chains. (This is illustrated in Fig. 4 and Movie3 in the SI.) Thus, by adding sticky nanoparticles to the system, we determine which surface the diffusing colloid will adhere to.

Figure 4. Left panel: Nanoparticles (yellow) that attract monomers of the grey chains cause these chains to contract, leaving pink chains exposed to the solution. Right Panel: Coated colloid now adheres preferentially to the surface which attracts the pink polymer chains. ACS Paragon Plus Environment

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The only force between the colloidal sphere and the surface is a repulsive LJ force. Due to the presence of polymer chains anchored on the surface of the rigid colloid, the interaction between the colloid and the surface in our system can be categorized as a soft particle-rigid surface interaction44,45. When the polymer coated colloid approaches a surface with exposed polymer chains that stick to this surface, the colloid core equilibrates at a distance very close to the surface and the sticky polymer chains spread out along the wall. To illustrate how the strength of the attachment between the soft colloid and the surface can be controlled, we have studied the system at different concentrations of nanoparticles. We expect that at lower nanoparticle concentrations, the repulsive polymer (grey chains) will be more exposed and thus the contact with the pink wall (attractive to the pink chains) will be weaker. It has been shown in other studies that anchoring different types of polymer chains on the surface of a colloid can be used to manipulate the interaction of the colloid with its environment, which can be a surface or a polymer matrix46,47. To evaluate the stickiness between the polymer grafted colloid and the wall more quantitatively, we have calculated the Potential Mean Force (PMF)48 of the center of the sphere for three systems with different numbers of sticky nanoparticles. (Details of these calculations may be found in the SI.) In these simulations the pink monomer - surface particle binding energy is b=0.5. As shown in Fig. 5(a), as the number of nanoparticles in the system increases from 0 to 8000, the equilibrium distance between the center of the colloid sphere and wall decreases monotonically. (a)


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Figure 5. (a) The equilibrium distance between colloid and wall decreases as the number of sticky nanoparticles in the system increases. Horizontal lines show the location of the center of the colloid sphere for systems with 0, 2000 and 8000 sticky nanoparticles. (Nanoparticles not shown.) (b) PMF of the colloid center of mass coordinate as a function of distance from the wall for these three systems. (c) Number density of the monomers of the pink polymer chains (arbitrary units) as a function of distance from the wall.

If the colloid moves freely in the box with no constraints, it will occupy the conformations with lower free energy content with higher probability. Therefore, the shape of the PMF curves determines the equilibrium distance between the colloid and wall, and its depth indicates the binding strength between them. As Fig. 5(b) shows, the binding well of the PMF becomes deeper as the number of sticky nanoparticles in the system increases. Insight into the mechanism by which this process occurs can be gleaned from Fig. 5(c) which shows the number density of sticky monomers vs. the distance from the wall. As the grey polymer chains become more collapsed by increasing the number of nanoparticles in the system, the equilibrium distance between the colloid and wall decreases and the peak of the monomer density, which occurs near the adhesion surface, becomes sharper. This corresponds to attachment of a larger number of pink monomers to the surface. More contacts between the surface and pink monomer lead to a deeper PMF well and stronger attachment between the colloid and surface particles. In addition to the number of sticky nanoparticles, the interaction strength between each monomer and the surface particles also affects the stickiness between the colloid and surface as measured by the depth of the PMF cure. To see this effect, we repeated the simulation of the colloid in the presence of 2000 sticky nanoparticles for three different binding energies b=0.25, 0.5 and 0.625 between the pink monomers and the surface particles. The resultant PMF curves and monomer densities are shown in Fig. 6.


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Figure 6. (a) PMF of the colloid center of mass coordinate as a function of distance from the wall for three systems with different pink monomer – surface particle binding energies b=0.25, 0.5 and 0.625. There are 2000 nanoparticles in each system. (b) Number density of the monomers of the pink polymer chains as a function of distance from the wall for these three systems.

Another observation one can make from these plots is that the stickiness of the spherical brush to the surface is weakened by entropic forces that the free chains exert on the colloid. For instance, we can calculate the interaction energy of each pink monomer residing on the flat surface, which is comprised of small particles of the same size as monomers ordered in a close-packed structure. We determine the binding or sticking energy (enthalpy) by summing the surface-monomer interaction energy of all pink monomers and we find the largest sticking energy to be ~ 35.0, which suggests a far more attractive interaction than the minimum PMF value of 15 for the b=0.5 system. The difference between these two numbers provides some estimation of the influence of thermal fluctuations and also of the loss of configurational entropy of the polymer chains that occurs when the colloid is located directly adjacent to the surface. Both effects promote detachment of the sphere from the surface. (a)

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Figure 7. Mechanism of detachment of polymer chains from the colloid and sticking to the surface is shown in (a), (b). Thermal fluctuations cause detachment of all pink chains, after which the colloid moves away from the surface (c). ACS Paragon Plus Environment

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So far, we have assumed that the bond between the polymer chains and the colloid at the grafting point is strong enough that it can be considered permanent. However, for some applications, it might be useful to consider the situation where the strength of the bond between the polymer chains and colloid is comparable to the other interaction energies in the system. In this case the chain-colloid bonds are dynamic (breakable). For example, as a potential drug delivery application, we can consider a system where some polymer chains are attached to the colloid via dynamic bonds. Fig. 7 shows snapshots from the time evolution of a system comprised of 100 pink chains and 100 grey chains. Each grey chain is grafted to a random position on the colloid surface via a static bond, while each pink chain is grafted to the colloid surface via a dynamic bond. The dynamic bond between the end monomer of a pink chain and the colloid is modeled via the same functional form employed for the polymer-nanoparticle pair potential, as described in the Supplemental Information. The strength of the dynamic bonds can be adjusted according to the desired release rate of the pink chains. Strong dynamic bonds take a longer time to break via thermal energy fluctuations, which have a typical strength of a few 𝑘𝐵𝑇. In order to observe the detachment of the colloid from the adhesion surface in a feasible simulation time scale for the system considered in Fig. 7, we set the strength of the grafting bond between the colloid and the pink polymer chains to be 6 kBT. With this grafting bond strength, we did in fact observe some pink chains eject from the colloidal sphere before and after the latter attached to the adhesion surface. For visual clarity, ejected chains are not shown in Fig. 4 or Movie 4 (described below). Pink chain ejection can be eliminated to any desired degree by increasing the binding strength of the pink polymer chains to the colloid appropriately. This will of course slow down the eventual detachment of the colloid from the adhesion surface. For simplicity, we consider the situation where there are no nanoparticles in the system. (Similar effects will be seen in a system containing sticky nanoparticles.). As can be seen in Fig. 7, once the colloid approaches the wall, pink polymer chains whose interaction with the wall is stronger than their bond to the colloid detach from the colloid and absorb onto the surface. After the colloid sheds most of the pink polymer chains, thermal fluctuations of the remaining chains drive the colloid away from the surface. A movie illustrating this mechanism is provided in the SI (Movie4).


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Figure 8. Effect of thermal fluctuations of inert, non-detachable polymer chains (grey) on detachment of colloid from the surface. Panels (a,b,c) pertain to a colloidal sphere grafted with 0,50,100 non-detachable chains, respectively, plus 100 detachable (pink) chains in all three systems. There are no nanoparticles in these systems, and the three snapthots are taken at the same time. For the cases of 0 and 50 inert polymer chains, thermal fluctuations of the colloid are insufficient to detach it from the surface on the simulated time scale. Larger numbers of inert polymer chains accelerate this process. For the case of 100 inert chains, panel (c), detachment of the colloid from the surface does occur on the simulation time scale.

In this system, the grey polymer chains help the colloid detach from the surface due to their large thermal fluctuations. To further illustrate this effect, we have simulated three brush systems with different numbers of inert polymer chains, from zero to one hundred. In all three cases shown in Fig. 8, the pink chains and their interaction parameters remain unchanged from the system of Fig. 7. The snapshots shown in Fig. 8 were taken at the same time for each of the three systems shown. We did not observe detachment of the colloid from the surface over our simulation time scale for the two systems with no grey chains and with half of the number or the original (Fig. 7) system. Clearly, the strength of the thermal fluctuations of the grafted polymer chains depends on several different parameters, including the number of chains, the proportion of chains of different types in a mixed polymer brush coating, and the polymer chain lengths, all of which can be used to control the average time that the colloid stays adsorbed onto the surface. This feature may prove useful for applications where it is desirable to release the payload compounds inside the carrier at a specific rate49,50. Depending on the application, compounds can be loaded via different techniques into the polymer-modified colloid system considered in the present work. The colloid itself can be a shell in which drugs are encapsulated and transferred to the targeted tissue, with the polymer chains coated on its surface facilitating its transport and delivery51,52. Alternatively, the drugs can be bound to the polymers53,54, which in some cases might have advantages over the encapsulation method in terms of efficiency55,56. Since our model is a generic one and is not restricted to a specific material, the results presented here can be generalized to different types of materials with different interactions between them, including hydrophilic and hydrophobic materials53,57. The colloid particle in our model can also represent a nanoparticle comprised of a material ACS Paragon Plus Environment

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with special optical properties, e.g., gold57 or conjugated polymers52. In this case, light emission by the colloid particle can be used to track its location within a cell or tissue region. In addition to the interaction between the polymer coated colloid and the surface, one might be interested in interactions between multiple colloids. For such applications, the mechanism demonstrated here could serve as a means for controlling colloidal selfassembly 58–61. In conclusion, CGMD simulations were performed to demonstrate a novel mechanism for controlling the surface properties of a polymer coated colloid by adding nanoparticles that interact preferentially with a particular type of surface grafted polymer chain. Results obtained from simulations show that by wrapping the “undesired” polymer chains around sticky nanoparticles that penetrate into the brush region, the polymer type with “desired” properties is left more exposed to the environment, and the surface properties of the colloid sphere become dominated by the properties of the desired polymer chains. We also showed that the strength of attachment between the polymer coated colloid and surface can be tuned by varying the number of sticky nanoparticles in the system, as well as the ratio of desired/undesired polymer chains in the brush coating. This motif can in principle be used for drug delivery and self-assembly applications. Acknowledgments: This work was supported by NSF grant CHE-1464551.


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Controlling the Surface Properties of Binary Polymer Brush-Coated

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