Researchers have developed the first polymeric material that exhibits a rubbery-to-glassy transition with increasing temperature.
When plastic bottles and plastic bags are heated, their general polymers are stiff and soft at respectively low and high temperatures. The soft, easily deformable state is referred to as a ‘rubbery state’, whereas the hard, less-deformable state is called a ‘glassy state’. As the temperature is elevated beyond the transition temperature, known as the ‘glass transition temperature (Tg)’, the elastic modulus, comparable to hardness, drops by approximately three orders of magnitude around the Tg, corresponding to the softening of plastics.
In general, this three-digit modulus change is almost independent on the chemical species of polymer. In other words, this ‘rubbery-to-glassy transition at elevated temperature’ is the universal and intrinsic property of polymers. The softening property is very important in the manufacturing process of plastic products (such as injection moulding), however it has still restricted some specific applications of polymers in a high-temperature environment.
In order to create a polymer that shows a rubbery-to-glassy transition with increasing temperature, contrary to the polymer-intrinsic property, the team focused on hydrogels that exhibit a spinodal decomposition type of thermal phase separation. Phase separation is a phenomenon in which a homogeneous, single-phase substance is divided into two or more phases. In the case of polymer-based hydrogels, the homogenous state, where water and polymer network are uniformly distributed, changes to two phases, consisting of dehydrated polymer dense and water-swollen, dilute regions, above a critical temperature.
When the gel becomes phase-separated, the locally formed polymer dense phase with low water content has more intermolecular interactions, making the gel stiffer in macroscale. You can easily imagine this phenomenon as foods get harder with drying. It has been reported that some polymer gels such as poly(N-isopropylacrylamide) (PNIPAm) gel exhibit phase separation at high temperatures, but the polymer dense region in these gels remains in a rubbery state due to the weak phase separation, resulting in a modulus jump before and after the phase separation that is single digit at best.
Designing a phase-separation polymeric gel
To achieve strong phase separation, the research team studied the structure of thermophilic proteins. Thermophiles are a kind of organism that inhabits hot springs and deep-sea hydrothermal vents and have adapted to life in high-temperature environments of up to 120˚C. Since thermophilic proteins have more amino acids forming electrostatic interactions and hydrophobic interactions to stabilise their conformations than organisms that live in moderate environments such as humans, they do not denature at such high temperature. In addition, according to the Coulomb’s law, the strength of ionic bond is inverse proportionally to relative permittivity of the surrounding environment. Namely, the ionic bonds in thermophilic proteins must be more enhanced in hydrophobic environments than those of organisms living in a normal environment.
Utilising this principle, the team designed a phase-separation polymeric gel that allows ionic crosslinks to be incorporated from a water-rich environment into the polymer dense phase of the phase-separated gel above a demixing temperature, resulting in that polymer dense region becoming finally varied from a rubbery state to glassy state.
Specifically, the hydrogel can be easily prepared from conventional, non-toxic and inexpensive poly(acrylic acid) (PAAc) gel equilibrated in calcium acetate (CaAc) solution. Both chemicals have been used as a food additive. A pristine PAAc gel doesn’t show thermal phase separation, while the PAAc/CaAc gel forms an ion complex between carboxyl group of PAAc side chain and CaAc ions that is capable of phase separation. The gel is transparent and has a soft rubbery state at low temperatures.
With an increasing temperature, the gel quickly becomes cloudy and changes to an extremely hard glassy state accompanying phase separation. A folded gel sheet with just 1mm thickness cannot lift a 10kg weight at low temperatures, but an 1,800-fold increase in modulus occurs after the phase separation, and the weight can now be supported. This modulus jump is comparable to turning a soft jelly into a hard plastic.
This thermal response is completely reversible, and the vitrification temperature can be tuned in the range of 40-100˚C, depending on the concentrations of PAAc and CaAc. Conventional phase separation generally accompanies large shrinking after phase separation because of dehydration. However, the phase separation of PAAc/CaAc hydrogel is isochoric process and forms water micro clusters in the gel matrix. Moreover, because the gel has not only phase-separable PAAc-CaAc complex moiety but also hydrophilic carboxyl group of PAAc side chain, the water can be encapsulated to be micro cluster by carboxyl groups in the bulk and doesn’t have to go out from the gel matrix. The moving distance of water in PAAc/CaAc hydrogel is extremely smaller than that of conventional phase separation gels. Thus, the phase separation is rather fast and volume constant.
Whilst taking advantages of the thermo-stiffening and endothermic behaviour of the phase separation gel, the team conducted two practical demonstrations. First, researchers prepared a thermo-responsive smart protector that can protect body and clothes when hardened by frictional heating in situations of traffic and sports incidents. When the phase separation gel/glass fibre fabric composite was dragged against the asphalt surface for five seconds at a rate of 80km/h, the surface temperature of the composite immediately rose to 90˚C, high enough to cause the hardening transition, and the composite had less damage as a result.
A second application is as a heat-absorbing material. This polymer gel undergoes a large endothermic process during phase separation. If a gel sheet is attached to a window, it can absorb heat from the sun light, causing a cooling effect within the room. This demonstration shows a 15˚C difference in the room temperature, compared to the state without the gel sheet. Of course, since the phase separation is a reversible process, it can repeatedly absorb heat. In terms of global warming, we can expect this gel to be used in conjunction with applications such as energy-saving air conditioning systems to maintain low indoor temperatures.
This hydrogel is the first polymeric material that exhibits a rubbery-to-glassy transition with increasing temperature, opposing the intrinsic nature of conventional polymers. This achievement is expected to contribute to the development of basic and applied researches on such novel temperature-responsive polymers.
Jian Ping Gong
Laboratory of Soft & Wet Matter (LSW)
Faculty of Advanced Life Science, Hokkaido University, Japan
+81 (0) 11 706 9011
Please note, this article will feature in the March 2020 issue of our publication.