Ceramides are known to be a key component of the stratum corneum, the outermost protective layer of the skin that controls barrier function. In this work, molecular dynamics simulations are used to examine the behavior of ceramide bilayers, focusing on non-hydroxy sphingosine (NS) and non-hydroxy phytosphingosine (NP) ceramides. Here, we propose a modified version of the CHARMM force field for ceramide simulation, which is directly compared to the more commonly used GROMOS-based force field of Berger (Biophys. J. 1997, 72); while both force fields are shown to closely match experiment from a structural standpoint at the physiological temperature of skin, the modified CHARMM force field is better able to capture the thermotropic phase transitions observed in experiment. The role of ceramide chemistry and its impact on structural ordering is examined by comparing ceramide NS to NP, using the validated CHARMM-based force field. These simulations demonstrate that changing from ceramide NS to NP results in changes to the orientation of the OH groups in the lipid headgroups. The arrangement of OH groups perpendicular to the bilayer normal for ceramide NP, verse parallel for NS, results in the formation of a distinct hydrogen bonding network, that is ultimately responsible for shifting the gel-to-liquid phase transition to higher temperature, in direct agreement with experiment.

We present the results of discontinuous molecular dynamics simulations of a “coloring” reaction performed on A-type homopolymers having length ranging from 100 to 300 units in implicit solvents. The transformation of selected A-type monomers to B-type units along the macromolecule produces A1−x -co-B x random copolymers, where x is the mole fraction of B (= degree of “coloring”). We demonstrate that for a fixed A−B interaction strength the comonomer distribution of A and B units in A1−x -co-B x can be tuned to range from “purely random” to “random−blocky” by adjusting both the degree of “coloring” and the solubility of the A and B segments with respect to the implicit solvent. In general, increasing the solubility of the A-type homopolymer or the degree of coloring results in a decrease in blockiness in the comonomer distribution. In addition, decreasing the solubility of the B species in the implicit solvent increases the tendency of the A1−x B x copolymer to form “random−blocky” sequences.

We present the results of discontinuous molecular dynamics simulations of mechanically assembled monolayers in good solvent. Polymers of chain lengths 5-100 were end-grafted to surfaces at low density and then compressed laterally at varying rates. Data for brush thickness and end-monomer density were collected as a function of surface density; they were shown to correspond well with theoretical predictions and simulation results performed at constant surface density. Brush thickness for all chain lengths could be controlled by judicious choice of the compression rate. Defects in the brush layer were dependent on chain length; it was shown that quick compression for relatively short chains allowed the layer no time to relax into coil form. Quick compression on long chain systems led to entanglement in the brush layer since the longer-chained system was not being afforded the long relaxation time required to form a fully relaxed brush. Hysteresis effects were examined by allowing the brush to relax to a lower surface density, and it was shown that higher surface compression/relaxation rates led to an increase in disparity between brush thickness found during the compression and relaxation stages; in large part, this disparity was due to inadequate equilibration time. Last, results from nonuniform compression in good solvent show negligible effects on monolayer height and structure.

We report on establishing the polydispersity in chemical composition (PCC) and polydispersity in monomer sequence distribution (PMSD) in random copolymers of poly(styrene-co-4-bromostyrene) (PBr x S), where x = (0.385 ± 0.035) is the mole fraction of the 4bromostyrene units (4-BrS), prepared by electrophilic substitution of bromine in the para-position of the phenyl ring of the parent polystyrene. Upon fixing the total number of repeating units, we tune the distribution of styrene and 4-BrS segments in PBr x S by carrying out the bromination reaction on polystyrene homopolymers in different solvents. While PBr x S with relatively random comonomer distribution is prepared in 1-chlorodecane, random-blocky sequences of 4-BrS in PBr x S are achieved by carrying out the bromination reaction in 1-chlorododecane. The PCC in both copolymers is established by fractionating both polymers using interaction chromatography (IC) and determining the chemical composition of the individual fractions by neutron activation analysis (NAA). The NAA data along with IC experiments reveal that the random-blocky sample possesses a narrowed PCC relative to a specimen with a more random comonomer sequence distribution. The full width at halfmaximum (fwhm) in the chemical composition profile from IC is used to quantify PCC; the random mother sample possessed a 25% fwhm, while the random blocky mother sample has a fwhm equal to 8.7%. The change in the adsorption enthalpy per brominated segment due to adsorption is determined to be ≈1.5 times greater for the random-blocky than the relatively random sample, proving that more pronounced cooperative adsorption occurs in the case of the random-blocky sample relative to the random copolymer sample. Computer simulation employing the discontinuous molecular dynamic scheme further reveals that the distribution of comonomer sequences, that is, PMSD, in the random-blocky copolymer is narrower than that in the copolymer with a random distribution of both monomers.

Postpolymerization chemical modification ("coloring") of homopolymer brushes made of A units using B chemical moieties produces surface-anchored random copolymers (RCPs) A(1-x)B(x), where x is the degree of "coloring". We employ discontinuous molecular dynamics to study the "coloring" process in macromolecular tethers made of various lengths grafted at low and high densities on flat impenetrable surfaces. We demonstrate that the comonomer distribution in the A(1-x)B(x) RCPs depends on the interplay among (1) the length and the grafting density of the A-based homopolymer anchors, (2) the solubility of the parent homopolymer, and (3) the solubility of the B coloring units. Chemical modification of sparsely spaced chains on the surface leads to nearly random comonomer distribution in the A(1-x)B(x) RCPs regardless of the solubility of A and B. In contrast, the distribution of A and B units in A(1-x)B(x) RCPs prepared from homopolymers tethered at high grafting densities depends on the solubility of the parent homopolymer. Chemical modification of well-solvated A homopolymer grafts results in comonomer distributions that resemble those of diblock copolymers, comprising lightly modified blocks near the surface and heavily "colored" blocks at the top of the grafts. The relative lengths of the two blocks can be tuned by varying the solubility of B. Under poor solvent conditions, the distribution of A and B in the A(1-x)B(x) RCP is more complex; it is governed by the conformation of the parent A macromolecular anchors that form collapsed clusters before the coloring reaction.