Abstract
The lack of specific knowledge of the network structure in shape memory polymers (SMPs) has prevented us from gaining an in-depth understanding of their mechanisms and limited the potential for materials innovation. This paper firstly reveals the unit-cell nanoscale morphological architecture of SMPs by simulation. The phase separated architecture of a segmented shape memory polyurethane (SMPU) with a 30 wt% hard segment content (HSC, 4,4’-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO)) showing good shape memory properties was investigated by dissipative particle dynamics (DPD) simulations. A linked-spherical netpoint-frame phase of MDI, a matrix-switch phase of polycaprolactone (PCL) and a connected-spider-like interphase for BDO were obtained for this SMPU. The BDO interphase can reinforce the MDI network. Based on these simulation results, a three-dimensional (3D) overall morphological architectural model of the SMPU can be established. This theoretical study has verified, enriched and integrated two existing schematic models: one being the morphological model deduced from experiments and the other the frame model for SMPs reported before. It can serve as a theoretical guide for smart polymeric materials design. This method for the simulation of polymer structure at the nanoscale can be extended to many areas such as photonic crystals where nanoscale self-assembly plays a vital role.
Highlights
Thermomechanical analysis (TMA) for a MDI-SMPU with 30 wt% hard segment content showing good shape memory properties
The dissipative particle dynamics (DPD) method developed by Hoogerbrugge and Koelman[33,34] is a mesoscopic simulation technique for complex fluids that can study systems over larger length and longer time scales than classical Monte Carlo (MC) and molecular dynamics (MD) simulations
Due to the structural complexity, we found that it is easy to compute a large amount of data from the DPD simulations of a shape memory polymer, but normally very difficult to interpret: What is the meaning of all individual figures? Is there any stable pattern in all computed figures? Is this pattern reasonable? Why is this pattern like this? In addition, how to present the pattern if existing? What is the exact size of this pattern? What is the relationship between the simulated structural data and shape memory effects? In present study, we will attempt to provide a 3D unit-cell model for a SMPU meeting the above requirements in the present work
Summary
For DPD simulations, several polymer-segments are systematically coarse-gained into a single simulation bead based on the molar volume of the monomers. In one way, these simulation outcomes are in good agreement with previous experimental results[29], in key aspects and can verify the characteristics of the morphological model, namely, representative phase separation between hard and soft segments deduced by multiple measuring techniques[29], (see Fig. 1d) In another way, it is interesting to note that, in this SMPU, the linked-spherical-phase structure of MDI are almost the same as the net-point-frame in SMP frame model which was proposed in our previous report[41], but our simulation provides more details including a sphere diameter of around 15 nm as eight net-points connecting each other by narrow rods to form a framework to a cubic unit cell of around 30 nm in each length dimension. The simulation method for polymer structure at nanoscale can be extended to many other applications where nanoscale self-assembly plays a vital role, such as photonic crystals for structural color materials
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