Chapter 1
Poly(N‐isopropylacrylamide): Physicochemical Properties and Biomedical Applications

Marzieh Najafi, Erik Hebels, Wim E. Hennink and Tina Vermonden

Department of Pharmaceutics, Faculty of Science, Utrecht Institute for Pharmaceutical Sciences (UIPS), Utrecht University, 3508 TB Utrecht, The Netherlands

1.1 Introduction

Poly(N‐isopropylacrylamide) (PNIPAM) (Figure 1.1) has attracted a lot of attention during the past decades because of its thermoresponsive behavior in a biomedically interesting temperature window. This polymer exhibits inverse solubility in aqueous media and precipitates upon increasing the temperature [1, 2]. The temperature at which this polymer converts from a soluble state to an insoluble state, known as the cloud point (CP) or the lower critical solution temperature (LCST), is 32 °C [3]. The first study on the PNIPAM phase diagram was reported by Heskins and Guillet [2] Since then this polymer has been known as a thermosensitive polymer. PNIPAM has been prepared by a wide range of polymerization techniques such as free radical polymerization (FRP) [4], redox polymerization [5], ionic polymerization [6], radiation polymerization [7], and living radical polymerization [8].

Figure 1.1 Chemical structure of poly(N‐isopropylacrylamide) (PNIPAM).

The focus of this chapter is on polymerization techniques, and examples are given addressing PNIPAM's potential applications as biomaterial in drug and gene delivery and bioseparation. For other applications of PNIPAM in, e.g. membranes, sensors, thin films, and brushes, the reader is referred to reviews published elsewhere [9–12].

After introducing the general physicochemical properties of PNIPAM, an overview of the most frequently used polymerization techniques (free and living radical polymerization) is given, and a variety of copolymers and structures obtained by these methods are highlighted. Copolymerization with other monomers or conjugation/grafting of PNIPAM with other stimuli‐responsive polymers/materials results in dual responsive materials, of which the physical properties can be changed by several stimuli, e.g. changes in pH or redox conditions, light, and magnetic field. Examples of these systems along with the effect of copolymer composition on the LCST of PNIPAM are provided in this chapter. In addition, different methods of chemical and physical crosslinking and their effects on properties of the final materials are discussed.

Also, the potential of designing complex bioconjugates provided by recent developments in polymerization methods is discussed. Conjugation of responsive polymers to biomolecules (e.g. proteins, peptides, and nucleic acids) is a sophisticated method because the attached PNIPAM imparts responsiveness to these biomolecules. Furthermore, conjugation to biomolecules induces changes in stability and bioactivity as a result of altering the (surface) properties and solubility of materials. Here, we will review examples of grafting PNIPAM to biomolecules or growing polymeric chains from their surfaces. Finally, the future prospects of PNIPAM in biomedical and pharmaceutical applications are outlined.

1.2 PNIPAM as Thermosensitive Polymer

Thermosensitive polymers are by definition polymers whose physical properties can change in response to temperature changes, usually occurring in aqueous media [13]. This transition is most often drastic and follows upon passing a certain threshold that may be, in context of miscibility in a solvent, either an upper critical solution temperature (UCST) or lower critical solution temperature (LCST). LCST behavior indicates the temperature above which the polymer will no longer be soluble, while UCST behavior indicates the temperature below which immiscibility is reached. It should be noted that in literature the terms CP and LCST are often mixed up. The CP of a polymer solvent mixture is the temperature at which separation into a polymer‐rich and polymer‐poor phase occurs. The LCST is defined as the minimum of the CP in a temperature versus polymer concentration plot. So by definition, below the LCST, only one phase is observed independent of the polymer concentration (see Section 1.3) [14].

PNIPAM is an especially interesting thermosensitive polymer for application in biomedical and pharmaceutical sciences because of its sharp LCST of 32 °C in aqueous media. This transition is reversible, and PNIPAM solubilizes again when the temperature drops below its LCST [3].

The exact mechanism by which PNIPAM self‐assembles in water above the LCST is still not fully clear but believed to be because of the entropic gain of water molecules that dissociate from the hydrophobic isopropyl side‐chain moieties above the LCST. The enthalpy gain of water molecules associated via hydrogen bonds with the amide groups of the polymer becomes smaller than the counter effect of entropic gain of the system with water being dissociated when passing the LCST [3]. Since the extent of hydration of polymers is dependent on the characteristics of the monomer units, the LCST of PNIPAM may be varied by copolymerizing NIPAM with monomers differing in hydrophobicity or hydrophilicity. Furthermore, hydrophobic interactions between the polymer segments themselves have also been suggested to be crucial to the LCST transition from isolated extended coils of PNIPAM to collapsed chains [3, 15, 16].

Water molecules form hydrogen bonds with the carbonyl group, accepting two hydrogen bonds, and the nitrogen atom of the amide group can donate one hydrogen bond in the hydrated state below LCST [16]. During this transition, it has been shown that the number of hydrogen bonds between PNIPAM and water is reduced and intra‐chain hydrogen bonds are formed instead, of which some remain, even when cooled again below LCST. This explanation is used to rationalize why the aggregated chains swell upon cooling and do not immediately dissociate slightly below the LCST and hence cause hysteretic behavior [17]. Computer simulations confirmed that besides a reduction of intermolecular hydrogen bonds, there is a substantial decrease in the solvent accessible surface area, and it has been even suggested that a decrease in torsional energy of the isopropyl groups occurs during this thermal transition. The model also predicted the decrease in LCST upon copolymerizing with hydrophobic tert‐butylacrylamide (tBAAM), which is in line with experimental results [18].

The carbon backbone has shown to play an important role in the hydrophobic contribution of phase transition. To investigate this effect, Lai and Wu [19] used N‐isopropylpropionamide (NIPPA) as a small molecular model compound for PNIPAM. They observed that at high concentration (40 wt%), the NIPPA solution shows a higher LCST of 39 °C with a broader phase transition temperature range. They explained that the carbonyl group in the small molecule of NIPPA has more interaction with water molecules, which explains the higher LCST. Yet, the presence of the hydrophobic main chain in PNIPAM interferes with hydrogen bonding between the carbonyl groups and water molecules [19]. On the other hand, the presence of α‐methyl groups in the main chain (poly(N‐isopropylmethacrylamide) (pNIPMAM)) results in increased hydrophobicity; however, surprisingly the LCST of this polymer is not lower than that of PNIPAM but even increased by about 15 °C. The authors speculated that the higher CP for pNIPMAM is due to the methyl groups that induce steric hindrance for the hydrophobic groups to self‐assemble in the most favorable manner [20].

1.3 Physical Properties of PNIPAM

This section briefly describes some of the physical properties of PNIPAM by highlighting the effect of composition of the media on its phase transition temperature.

1.3.1 Phase Behavior of PNIPAM in Water/Alcohol Mixtures

In water/organic solvent mixtures (e.g. alcohols/acetone), the LCST of PNIPAM is dependent on the type of cosolvent and its volume fraction. In general, first a decrease in a CP is found upon increasing the volume fraction of organic solvent, while after a certain volume ratio an increase in a CP is observed. The less polar the cosolvent, the lower the volume fraction at which the increase in the transition temperature occurs. For example, for acetone the minimum transition temperature is found at a molar fraction of 0.15, while for methanol this mole fraction is 0.34 (see Figure 1.2). At low volume ratios, the cosolvent molecules and PNIPAM compete for water molecules, resulting in less hydration of PNIPAM and thus a lower CP. Upon increasing the volume fraction of a cosolvent, these solvent molecules interact with the polymer chains and increase their solubility. Remarkably, for some alcoholic cosolvents such as ethanol and 1‐propanol, a coexistence of LCST and UCST behavior is observed. In contrast, UCST behavior is not observed in water only or methanol–water mixtures [15, 21, 22].

Figure 1.2 Comparison between phase transition temperatures of PNIPAM in water–methanol (open symbols) and water–acetone (filled symbols) solutions.

Source: Costa and Freitas 2002 [22]. Reproduced with permission of...