Interface Segregation Of Nanoparticles Induced By Solvent In All-polymer Nanocomposite Films

The incorporation of nanoparticles into polymers is an important approach to improve the performance of polymer materials.The properties of the polymer nanocomposites largely depend on the dispersion state of the nanofillers in the polymer matrix.However,there are many kinds of nanoparticles,such as silica nanoparticles,gold nanoparticles,C₆₀,etc.There are often special interactions between such nanoparticles and the polymer matrix,and the dispersion in the polymer matrix is complicated.Different from the above system,the all-polymer nanocomposites（all-PNCs）are obtained from the composite of polymer chains and single-chain nanoparticles（SCNPs）,because the latter components are basically the same as the polymer matrix,and the SCNPs have good compatibility in the polymer matrix.Different from bulk materials,polymer films often need to go through non-equilibrium processes such as spin coating and solvent evaporation.At the same time,there is a non-negligible interface effect at the substrate and the free surface during the film preparation process.In addition,the cross-linkers inside the SCNPs are difficult to be identical to the ordinary polymerized monomers.In the presence of the solution,the interaction between the cross-linkers and the polymeric monomers and the solvent are bound to be different.How the above factors affect the dispersion of nanoparticles in the process of all-PNC film formation is a key scientific issue.More importantly,how to realize the controllable preparation of the all-PNC film with nanoparticles under the premise of clarifying its mechanism has important potential application value for systems such as self-assembly film of block copolymers.In this paper,coarse-grained molecular dynamics simulations are used to study the dispersion state of the SCNPs.In particular,the structure and formation mechanism of the interface segregation of the SCNPs induced by solvent effect are discussed in detail.Further,a series of effects brought by the interface segregation of SCNPs on the polymer films are discussed.These studies mainly include the following three parts.（1）Interface segregation of SCNPs induced by solvent in all-polymer nanocomposite films.A coarse-grained model of the solvent evaporation of the all-PNC solutions was constructed to study the influence of solvent evaporation process and the solvent selectivity on the dispersion state of the SCNPs.Herein the solvent environmental differences refer to the differences in the affinities of the solvent for the polymerized monomers and the cross-linkers.The results show that the SCNPs can be kept uniformly dispersed in any solvent environment.With the evaporation,the SCNPs can remain uniformly dispersed only when the solvent is both good for polymeric monomers and cross-linkers.While for the poor affinity of the solvent for the cross-linkers,the SCNPs will gradually segregated to the substrate interface and free surface during evaporation.Moreover,when the solvent affinity difference is particularly large,the SCNPs will be completely segregated and phase separated from the polymer matrix,forming a heterogeneous film with sandwich-like structure.Such phenomena can be attributed to the intrinsic difference in the solvent density at an interface region from that in the bulk,which eventually results in the segregation of SCNPs at the interface region where the solvent particles are much less than in the bulk.At the interface region,cross-linkers in the SCNPs will have less contact with the solvent and,therefore,less enthalpy penalty than being located in the bulk region of the film.Mean-field theoretical calculations further confirm the simulation results.Moreover,the interface segregation of SCNPs can also affect the chain conformation near the interface.That is,in the uniform dispersion system,the polymer chains show an overall orientation along the interface,while in the segregation system,the polymer chains show more upright conformations due to the interface occupancy of the SCNPs.（2）Horizontal to perpendicular transition of lamellar and cylinder phases in block copolymer films induced by interface segregation of SCNPs.Based on the interface segregation of SCNPs in the research content（1）,we change the polymers from homopolymers to block copolymers,and study the effect of interface segregation of SCNPs on the self-assembly structure orientation of di-block copolymer films.The transition to the self-assembly orientation of the block copolymers is achieved by the interface segregation of SCNPs.The calculation of the interface tension distribution of the film along the normal direction show that the pure block copolymers tend to form structures parallel to the substrate during self-assembly due to the difference in the selectivity of the two blocks on the substrate interface and the free surface.And the interface segregation of SCNPs greatly reduces the two block interface selectivity difference and the overall interface tension,thus creating a non-selective substrate and upper surface for the block copolymer,making the lamellar and cylinder phases formed by the self-assembly of the block copolymer to reverse from parallel to perpendicular to the interface.These perpendicularly oriented self-assembled structures have important potential applications in the field of nanolithography and nanofiltration.（3）Inhibition of polymer film dewetting by the interface segregation of SCNPs.The dewetting behavior of the polymer chains on the substrate seriously affects the performance of the polymer film,and even makes the polymer film completely lose its due function.In this work,we investigated the law and mechanism of the interface segregation of SCNPs on inhibiting the dewetting behavior of polymer chains.A static droplet model and a dynamic pre-punched film model are constructed to study the influence of the doping amount of SCNPs and the interaction strength between the solvent and the cross-linkers on the inhibition of polymer chains dewetting.Accordingly,the droplet profile is identified by the convex hull algorithm and the contact angle is calculated,and the stability and wettability of the film are judged by calculating the evolution relationship of the hole area of the pre-punched film with the simulation time.In addition,through the analysis of the chain conformation,we give a phenomenological explanation for the inhibition of dewetting by the interface segregation of SCNPs.Namely in the composite film,SCNPs segregated to the substrate interface,the polymer chains near the interface can only take vertical orientation of part into the gap between the SCNPs,which limits the polymer chain movement along the horizontal direction,and inhibit the polymer chains to wetting behavior.In conclusion,we have investigated the effect of solvent evaporation on the dispersion of SCNPs in the film using the coarse-grained molecular dynamics simulations.The simulation results show that when the affinity between the solvent and the cross-linkers in the SCNPs becomes worse,the chemical potential of the solvent in the bulk phase increases as the evaporation proceeds.Through further simulation study,we found that this kind of interface segregation phenomenon can be well used to regulate the orientation of self-assembled structure in block copolymer films,especially in the system with large surface selectivity,the original parallel orientation can change from parallel to vertical;it can also inhibit the dewetting behavior of polymer film on the non-infiltrated substrate surface.Our simulation results are expected to provide a reliable theoretical reference for the preparation and regulation of related polymer thin film materials.