PNIPAM-PEO

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

Figure 33 Simulation snapshots of the phase transfer process of the PNIPAM-PEO micellar shuttle in a water-ionic liquid system at different temperatures. Figure taken from Cesar et a I (2012).

The establishment of the living radical polymerization of NIPAM encouraged not only many polymer chemists but also polymer physicists to prepare functionalized NIPAM polymers with various controlled sequences and/or shapes, as discussed in this chapter. In the following parts, block, random, or graft copolymers will be simply designated by the acronym A-B for a diblock copolymer, A-B-A for an ABA-type triblock copolymer, A-B-C for an ABC-type triblock copolymer, A-co-B for a random copolymer, A-g-PNIPAM for grafting of NIPAM segments onto the polymer A. For example, PNIPAM-PEO stands for a diblock copolymer of PNIPAM and PEO. 

The formation of stable nanoparticles has been studied using various derivatives of thermosensitive PNIPAM, including diblock and graft copolymers, PNIPAM-b-PEO and PNIPAM-g-PEO [165-172], In these copolymers, the role of the PEO chains is to solubilise/stabilise collapsed PNIPAM at temperatures above its cloud point. Both the graft and the block copolymers, PNIPAM-g-PEO and PNIPAM-fr-PEO, form spherical core-shell structures in... 

Table 1 Characteristics of graft copolymers of poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene oxide) (PEO) [165-167,170]...

In all cases, the cloud-point temperature was slightly dependent on polymer concentration for a given copolymer it increased with decreasing concentration. This effect is enhanced with increasing number of PEO grafts per chain. Also, the PNIPAM collapse seemed to be less abrupt with decreasing concentration. Upon dilution of the solution the distance between polymer chains increases, which favours intrapolymeric interactions over in-terpolymeric attractions. Dilution also enhances the surface stabilisation of the polymer particles by PEO. 

Fig. 6 Formation of an aggregate and the dependence of its average hydrodynamic radius ((Rh ) on temperature. The polymer is PNIPAM-g-PEO-51, c= 1.0gI.. Top model describing the steps for the formation of an aggregate and its shrinking upon slow heating from a 20 °C to b 45 °C and to c 60 °C. (Adapted from Refs. [165,170])...

Fig. 7 Concentration dependence of (R ) (open symbols) and weight-average molar mass ((Mw)) (filled symbols) of the aggregates at 45 °C a PNIPAM-g-PEO-6 and b PNIPAM-g-PEO-7 [170]...

PNIPAM-co-GMA (Table 1). Thus, different distributions of substituents are possible, in principle. It was of interest to see (1) whether the distribution of the PEO grafts on the PNIPAM main chain influences the thermal properties of the polymer and (2) whether the polymer grafted at elevated temperature adopted the collapsed conformation in which it was synthesised when its aqueous solution was heated. 

The solubilising effect of PEO chains was also observed by micro calorimetry. The transition temperatures were 33.5 and 34.3 °C for PNIPAM-g-PEO-6 and PNIPAM-g-PEO-7, respectively. The enthalpy associated with the transition was higher in the case of PNIPAM-g-PEO-6 (2.5 kj mor1 of repeating NI-PAM unit) compared with that of aqueous PNIPAM-g-PEO-7 (1.3 kj mol-1). For both copolymers, it was significantly lower than the enthalpy of the phase transition of linear PNIPAM in water (approximately 7 kj mol-1). 

In this section, we review the properties of a series of PNIPAM-b-PEO copolymers with PEO blocks of varying length, with respect to the PNIPAM block. Key features of their solutions will be compared with those of PNIPAM-g-PEO solutions. PNIPAM-b-PEO copolymers were prepared by free-radical polymerisation of NIPAM initiated by macroazoinitiators having PEO chains linked symmetrically at each end of a 2,2/-azobis(isobutyronitrile) derivative [169,170]. The polydispersities of PEOs were low, enabling calculations of the number-average molar mass for each PNIPAM block from analysis of their H-NMR spectra (Table 2). 

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