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A. Jayaraman et al.
to the purely entropically driven PNCs. Moreover, in PNCs with χ GM > 0, close to
the particle surface, there is an increased concentration of graft monomers likely
due to collapse of grafted layer; this occurs to minimize energetically unfavorable
graft-matrix contacts. Similarly, brush heights, plotted as vertical dotted lines in
Fig. 10a show reduced values for PNCs with χ GM > 0 compared to the purely entropic
χ GM = 0 case. This is further confirmed by the probability distribution of end-end
distance for graft chains in Fig. 10b showing a shift in R ee distribution to lower R ee
values with increasing χ GM . A simulation snapshot rendered using visual molecular
dynamics (VMD)
157 in Fig. 10c shows the grafted layer structure with a graft chain
in a collapsed chain conformation for PNC with χ GM = 0.1 in the absence of A-D
attraction.
Next, to elucidate how introducing h-bonds affects the above trends in unfavorable graft-matrix PNCs, we use our CG model described above to introduce directionally attractive A-D interactions (ε AD = 13kT) between graft-matrix monomers.
Figure 10d shows an increased penetration of matrix chains into the grafted layer
compared to the purely entropic case in the presence of attractive A-D interactions. A
similar increase in grafted layer brush height is also noted implying that graft chains
extend into the matrix to make favorable A-D contacts. The graft chain conformations plotted in Fig. 10e confirm this behavior, showing a higher probability for larger
R ee values implying extended graft chain conformations. This is also seen visually in
a simulation snapshot showing the grafted layer structure (Fig. 10f) for PNCs with
unfavorable graft-matrix interactions and attractive h-bonding interactions between
acceptors and donors on graft and matrix polymers.
We show only a few results in Fig. 10 and direct the reader to our recent work
[154], where we show simulation results for varying grafting density, graft-matrix
interactions, h-bonding A-D interaction strength, matrix chain length versus graft
chain length using the above CG model.
4.5 Limitations and Future Directions
The model described above in Sect. 4.2 incorporates CG acceptor and donor beads
on CG monomer beads of chosen graft and matrix polymer chemistry and through
simple isotropic bonded and non-bonded interactions mimics directional and specific
interactions between polymers in PNCs. Our results show that in PNCs the model
successfully captures the enhanced wetting of grafted layer by matrix chains due to
h-bonding interactions even in cases where the graft-matrix interactions are unfavorable. Nonetheless, there are certain assumptions that require careful consideration before the model can be used to describe specific interactions in any polymeric system. First, to ensure specificity in h-bonds, the interactions between similar
acceptor/donor bead types (i.e., A-A and D-D) in the model are defined as purely
repulsive through WCA potential. A drawback of this approach is that certain
chemistries with a higher propensity to form intra-molecular h-bonds and other
intra-molecular directional associations cannot be described by the model in its
A. Jayaraman et al.
to the purely entropically driven PNCs. Moreover, in PNCs with χ GM > 0, close to
the particle surface, there is an increased concentration of graft monomers likely
due to collapse of grafted layer; this occurs to minimize energetically unfavorable
graft-matrix contacts. Similarly, brush heights, plotted as vertical dotted lines in
Fig. 10a show reduced values for PNCs with χ GM > 0 compared to the purely entropic
χ GM = 0 case. This is further confirmed by the probability distribution of end-end
distance for graft chains in Fig. 10b showing a shift in R ee distribution to lower R ee
values with increasing χ GM . A simulation snapshot rendered using visual molecular
dynamics (VMD)
157 in Fig. 10c shows the grafted layer structure with a graft chain
in a collapsed chain conformation for PNC with χ GM = 0.1 in the absence of A-D
attraction.
Next, to elucidate how introducing h-bonds affects the above trends in unfavorable graft-matrix PNCs, we use our CG model described above to introduce directionally attractive A-D interactions (ε AD = 13kT) between graft-matrix monomers.
Figure 10d shows an increased penetration of matrix chains into the grafted layer
compared to the purely entropic case in the presence of attractive A-D interactions. A
similar increase in grafted layer brush height is also noted implying that graft chains
extend into the matrix to make favorable A-D contacts. The graft chain conformations plotted in Fig. 10e confirm this behavior, showing a higher probability for larger
R ee values implying extended graft chain conformations. This is also seen visually in
a simulation snapshot showing the grafted layer structure (Fig. 10f) for PNCs with
unfavorable graft-matrix interactions and attractive h-bonding interactions between
acceptors and donors on graft and matrix polymers.
We show only a few results in Fig. 10 and direct the reader to our recent work
[154], where we show simulation results for varying grafting density, graft-matrix
interactions, h-bonding A-D interaction strength, matrix chain length versus graft
chain length using the above CG model.
4.5 Limitations and Future Directions
The model described above in Sect. 4.2 incorporates CG acceptor and donor beads
on CG monomer beads of chosen graft and matrix polymer chemistry and through
simple isotropic bonded and non-bonded interactions mimics directional and specific
interactions between polymers in PNCs. Our results show that in PNCs the model
successfully captures the enhanced wetting of grafted layer by matrix chains due to
h-bonding interactions even in cases where the graft-matrix interactions are unfavorable. Nonetheless, there are certain assumptions that require careful consideration before the model can be used to describe specific interactions in any polymeric system. First, to ensure specificity in h-bonds, the interactions between similar
acceptor/donor bead types (i.e., A-A and D-D) in the model are defined as purely
repulsive through WCA potential. A drawback of this approach is that certain
chemistries with a higher propensity to form intra-molecular h-bonds and other
intra-molecular directional associations cannot be described by the model in its