Effect of Tacticity of Poly(N-isopropylacrylamide) on the Phase Separation Temperature of Its Aqueous Solutions

Abstract

An aqueous solution and cross-linked gel of poly-(N-isopropylacrylamide) (PNIPAM) undergo phase separation upon heating around 32 C. 1 Heskins and Guillet 2 studied the phase separation behavior of one unfractionated PNIPAM sample using a visible observation method. Later Fujishige et al.3 studied the phase transition temperature of an aqueous solution of fractionated PNIPAM samples using 500 nm wavelength light transmission and concluded that the phase separation temperature was independent of either the molecular weight (within the range of 50,000-8,400,000) or its concentration (within the range of 0.01-1%). However, Schild and Tirrell 4 observed the effect of the molecular weight and the concentration on the phase separation temperature of aqueous PNIPAM samples. Recently, using two well characterized samples (M w ¼ 49;400 with M w =M n ¼ 1:21 and M w ¼ 101;000 with M w =M n ¼ 1:23) studied over the wide concentration range of 0.58-70 wt %, Tong et al.5 reported that the phase separation temperature inversely depends on the molecular weight and concentration of PNIPAM. Thus, there have been relatively large numbers of papers dealing with the effects of molecular weights on the phase separation of PNIPAM. It has been reported earlier that the phase separation is due to the formation of hydrophobic bonding among the side chains of a polymer in an aqueous solution.6-9 However, there is no report about the effect of tacticity on the phase separation temperature of aqueous PNIPAM solutions. One of our groups recently reported the effect of tacticity on the phase transition temperature of PNIPAM. 10Very recently, one of our groups has also reported the synthesis of stereo and molecular weight controlled PNIPAM using the RAFT polymerization in the presence and absence of Lewis acids.11 Using the same technique, we prepared a series of PNIPAM samples with different meso diad (m) values in the range of 45-72% having the molecular weight (M n ) of 37;000 AE 3000 and polydispersity (M w =M n ) in the range of 1.2-1.3. These polymers were used for the determination of the phase separation temperature of aqueous PNIPAM solutions and the novel inverse dependency of the tacticity on the phase separation temperature was observed. EXPERIMENTAL N-Isopropylacrylamide (NIPAM) (Wako, > 98%) was recrystallized twice from hexane. AIBN (Kishida, 99%) was recrystallized from methanol. Y(OTf) 3 (Aldrich, 98%) and Sc(OTf) 3 (Aldrich, 98%) were dried under vacuum before use. Dehydrated methanol (Kanto, > 99:8%) and dehydrated toluene (Kanto, > 99:5%) were used as received. 1-Phenylethyl phenyldithioacetate (PEPD) 12 was synthesized according to the literature. The polymerizations were performed in a methanol-toluene mixture at 60 C using 2.23 M NIPAM monomer, 0.80 mM AIBN as the initiator and 8.94 mM PEPD as the RAFT agent in the absence y

Selective solute adsorption and partitioning around single PNIPAM chains

“…In our simulations, we study two cases of tacticity, the isotactic (meso-diad) PNIPAM polymer, with all the side chains located on the same side of the backbone, and the syndiotactic (racemo-diad) one, where the side chains alternate in the position along the chain. Experimentally it is known that tacticity, which can be controlled by polymerization procedures, significantly influences the hydration properties and volume transition temperature of the PNIPAM polymer [41,42]. The influence of the tacticity on the transition temperature has been reported also in computer simulations [43].…”

Living Radical Polymerization by the RAFT Process—A First Update

“…RAFT polymerization is compatible with the presence of Lewis acids which can be used to influence tacticity of homopolymers and/or the tendency for alternation in copolymerization. [4] Further studies of the use of rare earth metal triflates to control the stereochemistry of polymerization of acrylamide derivatives have been reported (NIPAM, [148] amino acid derived acrylamide derivatives, N-acryloyl-l-phenylalanine methyl ester [149] ). Studies on (alternating) copolymerization include styrene with MMA [150] and β-pinene with acrylonitrile.…”

Stereospecific living radical polymerization for simultaneous control of molecular weight and tacticity

“…Another application useful in material chemistry would be the control of the hydrophilicity of poly(NIPAM) by changing its tacticity. 37 A series of poly(NIPAM)s with different tacticities, but with almost the same molecular weights, were thus prepared to determine the effects of the steric structures on the property by removing the molecular weight effects. The phase separation (T PS ) temperature of the aqueous poly(NIPAM) solutions proved to be decreased by increasing the isotacticity, which may be useful for the design of poly(NIPAM)-based thermosensitive materials.…”