28 Sep Thermosensitive Polymers as Smart Materials
Thermosensitive polymers are a class of “smart” materials that have the ability to respond to a change in temperature. Such polymers are uncommon as they are soluble in water at low temperature and become non-soluble when increasing the temperature. Because of their change of physical properties triggered by temperature stimulus, thermosensitive polymers are materials of interest in a wide range of applications such as drug delivery, surface coating, films and membranes, actuators or water treatment. The change of thermosensitive polymer physical state occurs at the LCST and depends on the polymer chemical structure and its interaction with water.
A – Below the LCST, the presence of hydrogen bonds between polymer and water as well as the creation of salvation cages (clathrates) result in a low enthalpy of mixing. In this case, Polymer-Water interactions are stronger than Polymer- Polymer interactions and thus the polymer is soluble in water.
B – Increasing the temperature above the LCST makes the hydrogen bonds break and favor the release of water out of the polymer chains. Both enthalpy and entropy of mixing increase, Polymer–Polymer interactions become dominant compared to Polymer-Water interactions which lead to the loss of solubility.
SPECIFIC POLYMERS synthesizes a broad array of monomers and polymers of interest in this area.
Oligo(ethylene glycol) methacrylates
A first class of thermosensitive polymers are based on oligo(ethylene glycol) methacrylate. Depending on the number of ethylene oxide units, LCST values from 25°C to more than 90°C can be obtained. 
SPECIFIC POLYMERS owns the expertise and the ressources to synthesis PEG based thermosensitive oligomers.
The best-known class of thermosensitive polymers are based on (meth)acrylamide monomers. Among them, N-iso-propylacrylamide (NiPAm) has been the most studied to synthesize Poly(NiPAM) Thermosensitive Polymer (LCST: 32°C). SPECIFIC POLYMERS offers a variety of (meth)acrylamide monomers and corresponding homopolymers that exhibit LCST values in different temperature domains and thus of interest for various applications.
Factors influencing polymer LCST values
LCST values depend on polymer concentration and homopolymer molecular weight. The concentration directly influence polymer-water interactions. Increasing homopolymers molecular weight is in favor of polymer-polymer interaction and thus lead to a decrease of the LCST.
Other factors can have a significant impact on the LCST :
- Chain end functionalized thermosensitive polymers: Hydrophilic functional groups will tend to increase the LCST value. This will be all the more pronounced for low molecular weight homopolymers. (See examples of chain-ends functional polyNiPAM below).
- Thermosensitive copolymers : Copolymerization of aforementioned monomers with hydrophilic or hydrophobic co-monomers can have significant impact on the LCST value. Hydrophlic co-monomers will increase the LCST and conversely. High ratio of co-monomers can even lead to the loss of thermosensitive properties. (See examples of thermosensitive copolymers below).
- Thermosensitive copolymers architecture : For block polymers, each block conserve its own LCST whereas, for statistical copolymers, resulting LCST value an average value of each corresponding homopolymers.
- Dispersity have an influence on the width of the transition from soluble to insoluble. To reduce polydispersity, Controlled Radical Polymerizations (CRP) can be used.
Telomerization or controlled radical polymerization allow preparing chain-ends functional thermosensitive polymers. Post modification can then give access to various functionalities (hydroxyles, amines, carboxylic acid, alkoxysilane, azide, NHS, etc.).
SPECIFIC POLYMERS can copolymerize aforementioned monomers in order to prepare functional thermosensitive copolymers. As for examples, NiPAM or NnPAM were copolymerized with MAPC1 leading to thermosensitive copolymers bearing phosphonic acid moieties. Increasing the ratio of phosphonated functional groups came with an increase of the LCST. Such polymers were shown to be of great interest for the treatment of metallic pollution in wastewater.