PNIPAM,

It turns out that in solutions of c < 0.1 gL 1 thermosensitive homopolymers, such as PNIPAM, PVCL, and PVME, themselves, form stable colloids in water at elevated temperature in the absence of additives or chemical modification [141-147]. The colloids remain stable upon prolonged heat treatment, without detectable aggregation or precipitation. Also, core-shell particles consisting of PNIPAM and a hydrophobic block are stable not only below but also above the LCST up to 50 °C, when the PNIPAM block is expected to be insoluble [185]. Factors that determine the colloidal stability as defined in Sect. 1.1 do not explain, it seems, their stability. In this review we have compiled a fist of all the reported instances where the formation of stable particles was detected in aqueous solutions of neutral thermosensitive neutral polymers at elevated temperature. We present studies of homopolymers, as well as their copolymers consisting of thermosensitive fragments and ei-... 

Turbidimetry is ideally suited to detect the temperature at which a transparent polymer solution turns opaque. The temperature corresponding to the onset of the increase of the scattered light intensity is usually taken as the cloud-point temperature, TcP, although some authors define the cloud point as the temperature for which the transmittance is 80% (or 90%) of the initial value. This technique is commonly known as the cloud-point method [199]. Turbidimetry was employed, for instance, to show that the cloud-point temperature of aqueous PNIPAM solutions does not depend significantly on the molar mass of the polymer [150]. 

A number of investigations on the thermal volume transition of PNIPAM have been performed [201-208]. PNIPAM chains carry two types of bound... 

Fig. 2 Typical thermogram obtained using conventional differential scanning calorimetry on PNIPAM solution the temperature of maximum heat capacity (Tmax), the width of the transition at half-height (AT1/2), the heat of transition (AH), the difference in the heat capacity before and after the transition (ACp), and the demixing temperature (Tdem). (Adapted from Ref. [200])...

Kujawa and Winnik [209] reported recently that other volumetric properties of dilute PNIPAM solutions can be derived easily from pressure perturbation calorimetry (PPC), a technique that measures the heat absorbed or released by a solution owing to a sudden pressure change at constant temperature. This heat can be used to calculate the coefficient of thermal expansion of the solute and its temperature dependence. These data can be exploited to obtain the changes in the volume of the solvation layer around a polymer chain before and after a phase transition [210], as discussed in more detail in the case of PVCL in Sect. 3.2.2. 

The reluctance of single PNIPAM globules to aggregate at elevated temperature may be ascribed qualitatively to the low probability of encounter of... 

The following mechanism emerges form the observation reported so far on the colloidal stability of multichain PNIPAM particles at elevated temperature. Let us note first that such PNIPAM particles are examples of mesoglob-ules, according to the definition of Timoshenko and Kuznetsov [215] ... 

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... 

Fig. 4 Formation of a core-shell sphere upon heating an aqueous PNIPAM-g-...

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. 

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