Temperature-sensitive polymers properties

Poly(N-isopropylacrylamide) (polyNIPAAM), formed by a free radical polymerization of N-isopropylacrylamide, is a water soluble, temperature sensitive polymer. In aqueous solution, it exhibits a lower critical solution temperature (LCST) in the range of 30-35 C depending on the concentration and the chain length of the polymer. Thus, as the solution temperature is raised above the LCST, the polymer undergoes a reversible phase transition characterized by the separation of a solid phase which redissolves when the solution temperature is lowered below the LCST. Its physicochemical properties have been investigated by several laboratories (1-3). 

Large temperature rises due to viscous heating do indeed occur in melt capillary flow at moderate and high shear rates. These must be estimated and taken into account whenever temperature sensitive polymers are extruded and whenever the extrudate surface quality and extrudate properties are of critical importance. 

As a model to understand and to describe the processes during the response of a smart gel on changes of enviromnental properties, a two-step mechanism can be assumed (Fig. 8). In a first step, the stimulus which triggers the swelling/shrinking must permeate the gel. Heat transfer for temperature-sensitive polymers or mass transfer (ions, organic solvents) determine the rate of the first step. 

Stimuli-Responsive (SR) materials, also called smart materials have been attracting great interest within scientific community in the last few decades [1-4], They possess uitique properties that have made this class of materials very promising for several applications in the field of nanoscience. In particular, the smart materials undergo changes in response to small external variations in enviroiunen-tal conditions or to physical or biochenfical stimuli. In addition, there are dual SR materials that simultaneously respond to more than one stimulus [5-7]. For instance, temperature-sensitive polymers may also respond to pH changes [8-11]. 

In addition, the modification with thermosensitive polymers can provide liposomes with a temperature-sensitive surface property the liposome surface is covered with a layer of the hydrated polymer chains below the LCST. However, the dehydrated polymer chains shrink efficiently and are adsorbed on the liposome surface above this temperature. 

Azide polymers contain -N3 bonds within their molecular structures and burn by themselves to produce heat and nitrogen gas. Energetic azide polymers burn very rapidly without any oxidation reaction by oxygen atoms. GAP, BAMO, and AM-MOare typical energetic azide polymers. The appropriate monomers are cross-Hnked and co-polymerized with other polymeric materials in order to obtain optimized properties, such as viscosity, mechanical strength and elongation, and temperature sensitivities. The physicochemical properties GAP and GAP copolymers are described in Section 4.2.4. 

Although the crystalline poly(allyl isocyanate) polymers are reported to be stable, many of these polymers depolymerize upon heating to yield monomers and cyclic trimers. The level of temperature sensitivity is a strong function of the length of the side chain. Room temperature depolymerization occurs in polar solvents in the presence of an initiator. Interestingly, the solution properties of poly(alkyl isocyanates) display an unusual degree of chain stiffness which is attributed to their helical configuration (64). 

Size separation columns are available with silica, zirconium, and heavily cross-linked organic polymer backbones. The polymer columns show the same pressure and solvent fragility described for ion exchange columns. Silica size columns must be protected from pH changes like partition columns, which must be used with a pH between 2.5 and 7.5. Zirconium columns are not pH or temperature sensitive, but possess chelation properties that must be chemically masked to prevent interference with the size separation. 

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