Macromolecular Theory and Simulations

Cover: Responsive macromolecules are materials which undergo a physical change in a controllable, reproducible, and reversible manner in response to an external stimulus, for example, change of solvent, pH, or temperature. The temperature-responsive polymer poly(N-isopropylacrylamide), PNIPAM, is investigated in aqueous solution using molecular dynamics (MD) simulations in order to determine its lower critical solution temperature (LCST) and to study the physical properties below and above this temperature. A critical number of PNIPAM monomer units must be present before MD simulations are capable of detecting conformational changes. Further details can be found in the article by M. Alaghemandi and E. Spohr* on page 106.

Computer simulations are used to verify the mechanism of the OXT/THP copolymerization. In marked contrast to previous claims that the system is an example of a “living” and/or controlled pseudoperiodic copolymerization, it appears to be non-living and statistical in nature, leading to formation of THP-THP diads beside the dominating monads, separated by OXT sequences of various lengths.

Self-assembled supramolecular architectures depend on the characteristics and structures of the unimolecules. A typical species of these unimolecules is amphiphilic star block copolymers. A kinetic mechanism is described that allows one to design the dimensions and structures of amphiphilic star block copolymers. Supramolecular self assembly thus becomes controllable.

The computed properties of the interface between atactic polystyrene and water are compared for two different representations of the phenyl rings: as united atoms (UA) and as dummy hydrogen atoms (DHA). The structure of aPS and water near the interface reveals that the aPS surface modeled using UA is more hydrophobic than that modeled with DHA.

MD simulations of nanoparticles in a semi-flexible polymer melt are performed. When there are attractive but nonselective interactions, nanoparticles cluster in the bulk, whereas if all interactions are repulsive the nanoparticles are well-dispersed. In general, nanoparticles segregate to the substrate, but do so much less for systems with stiff chains compared to flexible chain systems.

The temperature-responsive polymer poly(N-isopropylacrylamide) is investigated in aqueous solution using molecular dynamics simulations in order to determine its lower critical solution temperature and to study the physical properties below and above this temperature. A critical number of PNIPAM monomer units must be present before MD simulations are capable to detect conformational changes.

In highly filled polymers melts, the observation of a secondary plateau on the loss modulus originates from the existence of agglomerates. The breaking of such objects causes dissipative processes characterized by a critical strain. Below this, experimental observations show that dissipation remains. This peculiar effect is attributed to internal re-organization of agglomerates.

Polymer translocation through a narrow pore is investigated using dissipative particle dynamics by incorporating electrostatic interactions under different solvent conditions. The simulation results demonstrate that solvent quality and electrostatic interactions exert considerable influences on the dynamics of translocation of polymers.

It is demonstrated that the critical filler volume fraction where a percolating network of carbon nanotubes is forming marks a “turning point” in the reinforcement efficiency. Expectations for the reinforcing effect of CNTs at concentrations above a percolating threshold within the limits of current technology are mostly unrealistic.