Lower Critical Solution Temperature Of Polymers Research Articles

Shear-induced mixing and demixing in poly(styrene- co-maleic anhydride)/poly(methyl methacrylate) blends

The effects of shear flow on the phase behavior of a polymer blend with high glass transition temperature (Tg) constituents possessing a large viscosity difference were investigated using shear and capillary rheometry, complemented by differential scanning calorimetry and analysis of the extrudates with scanning electron microscopy. The blend is a lower critical solution temperature polymer mixture of a random copolymer of styrene and maleic anhydride, SMA (Tg=178 °C), and poly(methyl methacrylate), PMMA, (Tg=105 °C). Depending on temperature, both shear-induced mixing, typically, at very high shear rates, and shear-induced demixing, typically, at moderate shear rates, were observed. In the former case, extrudates were optically transparent, yielding one Tg and were thermorheologically simple at all temperatures below and up to the capillary extrusion one. On the other hand, extrudates related to shear-induced demixing were opaque, yielding two distinct Tg’s and were thermorheologically complex. The experimental methodology presented here for the determination of the shear-phase diagram in a flowing polymer blend should be applicable to any industrial mixture, and it is of particular value for assessing the effects of strong shear flow, relevant in polymer processing applications. Finally, the method of blend preparation, i.e., solution-cast versus melt-mixed samples, affected slightly the rheologically determined demixing temperatures.

Temperature-dependent adsorption/desorption behavior of lower critical solution temperature (LCST) polymers on various substrates

We have been studying adsorption and retention (resistance to desorption) behavior of temperature sensitive LCST polymers on different substrates as a function of temperature. According to our studies with Poly 64 (a copolymer of 60% (mol) NIPAAm and 40% (mol) NnBAAm, LCST = 8.5 degrees C in water), the copolymer retention depends on the rinse temperature. When the rinse temperature is above the LCST, the polymer adheres well to most surfaces. On the contrary, at rinse temperatures below the LCST, most of the adsorbed polymer is easily rinsed off. These studies are relevant to our work on the thermally reversible adsorption of LCST polymers conjugated to peptides and proteins, such as affinity ligands, for uses in immunoassays and affinity separations. The interaction between the LCST polymer and most hydrophobic polymer surfaces is mainly due to hydrophobic interactions, and the critical surface tension (gamma c) and the solubility parameter (delta) of the solid polymer substrate are the most important factors which influence the LCST polymer adsorption and retention. The critical surface tension appears to correlate best with the LCST polymer adsorption levels on different substrates, while the solubility parameter correlates best with the retention of the adsorbed polymer. According to our preliminary study, n-butyl groups probably interact more strongly with the substrates than isopropyl groups because of the greater hydrophobic surface area of the former groups.

Interactions of liposomes and hydrophobically-modified [formula omitted]: an attempt to model the cytoskeleton

The interactions of small unilamellar vesicles (SUV) and water-soluble copolymers were studied by fluorescence spectroscopy, differential scanning calorimetry (DSC) and quasi-elastic light scattering (QELS). The anchoring onto liposomal bilayer membranes of copolymers of N- isopropylacrylamide , N-(2-(1- naphthyl) ethyl)-N- n- octadecylacrylamide and or N-[4-(1- pyrenyl) butyl]-N- n- octadecylacrylamide (0.5 mol% of the octadecylacrylamide comonomer) was monitored by non-radiative energy transfer between excited naphthalene and pyrene. The anchoring process occurred on zwitterionic lecithin liposomes and on negatively charged phosphatidic acid liposomes, whether the bilayer was in the crystalline or the liquid-crystalline phase. Insertion of the copolymer octadecyl groups within crystalline bilayers was attributed to the presence of packing defects. Aqueous solutions of poly-(N- isopropylacrylamide) and of its hydrophobically-modified copolymers exhibit a lower critical solution temperature (LCST). The coil to globule collapse of the polymer chains which is known to occur as the aqueous solution is heated through the LCST, also took place when the copolymers were anchored onto vesicular bilayers. The copolymers remained anchored during this collapse and the liposomes were not destroyed. The process was thermo-reversible. Detailed aspects of the reversibility of the phenomenon depended on the relative values of the phase transition temperatures of the liposomes and of the polymer LCST.

Controlling the swelling characteristics of temperature-sensitive cellulose ether hydrogels

Novel temperature-sensitive hydrogels; have been synthesized by chemically crosslinking cellulose ethers; hydroxypropyl cellulose, methyl cellulose and hydroxypropylmethyl celluloses. These gels are highly swollen at low temperatures, gradually deswelling as temperature rises, usually sharply over a limited temperature range. Linear polymer type, crosslink density and ionic content have been varied in order to determine the effects of controllable network parameters on the important properties (degree of swelling, transition temperature and sharpness of transition) of the gels obtained. The transition temperature itself is about that of the observed linear polymer lower critical solution temperature (LCST). Producing gels from increasingly hydrophilic cellulose ethers raises the transition temperature and increases the swelling degree at comparable crosslink densities. However, decreasing crosslinking has little effect on swelling above the transition temperature. In contrast, increasing ionic content increases swelling at all temperatures. The transition temperature is not affected by the degree of crosslinking, but increases with increasing ionic content. The volume transition of the hydroxypropyl cellulose gel is significantly sharper than for the other gels. Crosslinking and ionic content have little effect on transition sharpness. The trends observed with cellulose ether gels are consistent with those observed in gels produced by conventional copolymerization/crosslinking synthesis methods, though some differences are noted.