Self-assembly Of Nanorod/Nanoparticle Mixture In Polymer Brushes

Recently, many studies have shown that the mixing of nanorods and nanospheres possess better performance than nanorods or nanospheres, which mainly results from the additional orientation entropy and depend on their transversal and longitudinal sizes, and thus obtained a great deal of attention. For example, it may be possible to simultaneously and independently optimize photoelectric properties by varying the NR diameter and length; in medicine, NR-NP junction can be used to construct highly sensitive surface enhanced Raman scattering (SERS) aptasensors and design various plasmatic nanodevice. While all these applications are based on ordered alignment of nanorods, and thus it become the key to the problem on how to regulate the nanorods alignment. Zhang studied the the binary nanoparticles in polymer brushes and indicated that hexagonal-closed packed-like structure is obtained for the small nanoparticles. Inspired by his working, we studied the conformation of the nanorods combined with nanospheres in polymer brushes. By regulating the size of nanorods and the interaction between polymer brushes and nanoparticles, we not only obtained the ordered structure of the nanorods and nanospheres, but also help us to understand the reason of the transforming from disordered to ordered of nanorods.In this paper, molecular dynamics is employed to study the conformation of NPs/NRs in polymer brushes and the competition between the attractive interaction and the orientation entropy of the NRs. In our model, the NRs/NPs and polymer brushes are confined between two parallel plates surface to which the polymer brushes grafted on the lower plate, and NRs/NPs are randomly generated above the polymer brushes. the NRs/NPs have an attractive interaction with the polymer brushes and a periodicity is applied in the x-y directions. we found that the aggregation of NRs and NPs depends on the attractive interaction between polymer brushes and NRs/NPs, and scattered conformation of NRs/NPs will transform to gathered one and to ordered structure by increasing the attractive interaction. Meanwhile, to maintain the attractive interaction to a certain value, some phase separations will occur combined with a mixing state of NRs/NPs. i.e. when the NRs are relatively smaller than NPs, the NRs will be incorporated by NPs, combined with a mixing phase to another separated pahse along with the increasing of size ratio. We also use a number of other parameters to describe the system, such as the radial density function, site order parameter, structure factor, bond orientation order parameter and so on.1.in the case of a small value of size ratio, the NRs of the peak of normalized radial probability density profiles is on the left, while the NPs’ is on the right, indicting NRs are incorporated by NPs; with the increasing of size ratio, the nanoinclusion will in a mixing phase which can be reflected by almost coincide normalized radial probability density profiles; keeping on the increasing of size ratio, the phase separation would occur again, while the NPs are surrounded by the NRs. These changes can be attributed to the free volume, for the polymer brushes in order to maximize the free volume, the nanoparticles with larger size will be expelled to the edge of the nanoinclusions.2.when the NRs are surrounded by the NPs, NRs will change from a disordered state to an ordered state by increasing the attractive interaction. However, the degree of ordering of NRs will decrease while the attractive interaction exceeds a critical value, for the lager attractive interaction leads to make the NRs aggregation more spherical thereafter leads to a decline of the site order parameter.3.for the case of0.1of the attractive interaction, the NPs are all dispersed for all ratio size while this is not the case for the NRs. When the ratio size is small, the NRs are gathering and will chage to dispersed phase while the ratio size up to a certain value, for the NRs play a leading role at small size ratio due to the orientation entropy.4.increasing the attractive interaction will make NPs change from a disordered to ordered under the condition of NPs included by NRs. This Can be reflected from the nearest neighbor of12reaching a to98.6percents and the value of Q6 corresponding to the two peaks are0.55and0.48. So the main structure of NPs is face-centered cubic structure combined with a hexagonal-like structure. This structure is most favorable to maximization of the free volume of the polymer brushes under the condiation of existing NRs.This study will insight into the self-assembly of nanoinclusions and provide a new way to control the self-assembly of nanoparticles. By regulating tye phase separation and ordering degree of the NRs, our results may provide a new avenue to process superior optical and mechanical properties and construct a better macroscopic performance of the material.