Designing Temperature and pH Sensitive NIPAM Based Polymers
Product Manager
Sandra Forbes
Stimuli-responsive Polymers
Stimuli-responsive polymers, alternatively referred to as "smart" or "environmentally sensitive" polymers, undergo swift transformations in their micro-structure, shifting from hydrophilic to hydrophobic states, in response to environmental alterations. These environmental changes are prompted by external stimuli such as heat, pH levels, ionic strength, magnetic and electric fields, light, ultrasound, and various chemical species. Notably, the macroscopic changes exhibited are reversible, allowing the system to revert to its original state once the triggering stimulus is eliminated.
Stimuli-responsive Polymer Forms
Categories of stimuli-responsive polymers, classified according to their physical forms, include: (a) linear free chains that are dissolved in solution, (b) covalently cross-linked gels which can be either reversible or physical in nature, and (c) chains that are adsorbed or surface-grafted onto a substrate.
Figure 1. a) Collapse of linear free chains b) Swelling or shrinking of a gel c) Swelling or collapsing on surface
Applications of Stimuli-responsive Polymers
The distinctive characteristics of stimuli-sensitive polymers enable their diverse applications in chemical and biological processes, including controlled and self-regulating drug delivery systems, bio-conjugation techniques, tissue engineering methodologies, biosensor developments, and bio-separation procedures.
Figure 2. Applications of Stimuli-responsive Polymers
Drug release can be modulated by temperature using PNIPAM micelles, while cellular adsorption is enhanced with PNIPAM. This modulation occurs due to a physical change triggered by transitioning across the lower critical solubility temperature (LCST).
Temperature and PH Sensitive Polymers
Temperature and pH sensitivity are particularly intriguing properties of stimuli-responsive polymers due to the following reasons:
• Certain disease states are characterized by alterations in temperature and/or pH.
• It is straightforward to adjust the stimuli response to the desired temperature and pH range.
Poly(N-isopropylacrylamide) (PNIPAM) is the most extensively studied system for temperature/pH sensitivity. A range of temperature- and pH-responsive polymers based on PNIPAM has been developed. These polymers can be functionalized at the chain ends with carboxylic acid, NHS ester, amine, and maleimide groups. Copolymerization of NIPAM with methacrylic acid introduces pH sensitivity. Additionally, hydrogels sensitive to both pH and temperature have been prepared using NIPAM, acrylic acid, and a di-acrylamide crosslinker.
PNIPAM exhibits a lower critical solubility temperature (LCST) of approximately 32 °C in aqueous solution. This LCST can be readily adjusted by copolymerizing NIPAM with appropriate monomers.
Figure 3. Synthesis of Chain End Functionalized PNIPAM
Figure 4. Synthesis of Heat & pH Sensitive PNIPAM
Figure 5. Molecular weight can be controlled with the use of a chain transfer agent (CTA). Dependence of molecular weight on CTA concentration in the synthesis of PNIPAM-COOH [AIBN]/[NIPAM] = 0.1; Temperature 60 ºC, in THF.
Figure 6. Plot of transmittance as a function of temperature for P(N-isopropylacrylate-co-5% methacrylic acid), 1% aqueous solution, pH = 4.46
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