The development of smart 4D materials utilising smart temperature responsive polymers

Abstract

This study involved the preparation and optimisation of novel temperature-sensitive hydrogel copolymer and terpolymer systems, and also the development and evaluation of smart hydrogels with the potential for 4D printing and shape-shifting behaviours. In this work, N-vinylcaprolactam (NVCL) was chosen as the base polymer. NVCL is a temperature-responsive polymer which is often used for biomedical applications due to its biocompatibility, solubility and non-toxic features. However, studies of the use of NVCL for developing 4D material have been limited. This polymer was bonded with different types and concentrations of photoinitiators, crosslinkers, and other polymers to obtain the optimum ratio, with the most suitable properties and performance for 3D printing. UV polymerisation, a process with similar mechanisms to Stereolithography (SLA) printing, was first employed for the preparation of samples. The cured polymers were characterised by the following techniques: The chemical structure of the hydrogels was confirmed using Fourier transform infrared spectroscopy (FTIR). Four techniques were used to determine the lower critical solution temperature (LCST) of the polymers in aqueous solution: i) cloud point analysis, ii) UV-spectrometry, iii) differential scanning calorimetry and iv) rheology. Pulsatile swelling studies were performed to investigate the effect of the transition temperature, monomer feed ratio and crosslinker content on the swelling size and quality. The mechanical properties of the polymers were detected by tensile tests. Goniometry was used to probe the water absorption capacity of the samples. Results indicated that the physically and chemically crosslinked NVCL based polymers exhibited a tuneable LCST by modifying the contents with regard to the material composition and concentration. The LCST could be raised to 54°C and was validated by four different techniques. FTIR showed that the samples were successfully synthesised. After incorporating with the monomer N, N-dimethylacrylamide (DMAAm), the mechanical properties improved. The copolymer P (70NVCL-30DMAAm) exhibited potential for the next stages of 4D printing trials. It exhibited outstanding swelling capabilities and was flexible to changes in size as the temperature increased or decreased. By changing structures that can be converted in a pre-programmed manner in response to a stimulus, 3D printed materials can be modified to impart flexibility and boost utility. Four-dimensional (4D) printing is a relatively new concept that was initially reported in 2013. 4D printing refers to the idea that the shape or properties of a printed object can be changed again when an external stimulus is applied. In this study, by using a modified P (NVCL-DMAAm) candidate formulation, containing 2 wt% H-Nu 400IL, 2 wt% PEGDMA, 30 wt% DMAAm and 70 wt% NVCL, 4D printing was achieved. The same characterisation tests were applied to the 3D printed samples to compare the differences in the properties of materials between samples prepared using SLA and those prepared with UV chamber photopolymerisation. Due to the superior properties of the shape memory polymers, a number of scholars have used the material to produce 4D printed objects in multi-material 3D printers. The printed objects are capable of demonstrating complex and magnificent changes when stimulated. However, smart hydrogels are only able to expand/contract in water. In addition, the printer used in this study was an inexpensive one and could only apply one material at a time for printing. It is difficult to develop flexible shape-shifting behaviours by using a single hydrogel material and a single material 3D printer. This study exploited a new method to utilise bilayer structures and a UV chamber system to allow hydrogel based materials to exhibit controllable shape shifting behaviours (such as self-curving and self-bending behaviours). Although other researchers have developed similar smart structures, none have use a NVCL based polymers developed in a UV chamber system. Based on the different ways of transformation, diverse demonstrators were developed In summary, by constantly modifying and improving the formulation of NVCL solutions, a new ‘NVCL-based resin’ that can be used in an SLA 3D printer has been successfully systhesised. In the presence of water, the 4D printed parts were able to switch their sizes as the temperature rises and falls. In addition, by using bilayer structures, the shape shifting behaviours have also been successfully developed for hydrogel-based materials. This research not only opens up new applications for NVCL materials, but also provides a simple and fast method for the preparation of 4D materials.