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Effects of temperature and salt addition on the association behavior in aqueous solutions of a series of charged thermosensitive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide)-block-poly(4-styrenesulfonic acid sodium) triblock copolymers (MPEG45-b-P(NIPAAM)n-b-P(SSS)22) with different lengths of the PNIPAAM block (n= 17, 48, and 66) have been studied with the aid of turbidity, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). Increasing temperature and salinity as well as longer PNIPAAM blocks are all factors that promote the formation of association structures. The SAXS data show that, for the copolymers with n= 48 and n= 66, increasing temperature and salt concentration induce interchain associations and higher values of the aggregation number, whereas no aggregation was observed for the copolymer with the shortest PNIPAAM chain. However, DLS measurements reveal the presence of larger association clusters. The cloud point is found to decrease with raising salinity and longer PNIPAAM block. The general picture that emerges is the delicate interplay between repulsive electrostatic forces and hydrophobic interactions and that this balance can be tuned by changing the temperature, salinity, and the length of the PNIPAAM block. [ABSTRACT FROM AUTHOR]

Abstract  Temperature-induced phase transition of three low-molecular-weight samples (M w  4) of poly(N-isopropylacrylamide) was studied with the aid of turbidimetry, dynamic light scattering, and rheology. We have demonstrated that the lower critical solution temperature depends on the length of the chain and the concentration of the polymer in the low molecular weight range. The turbidity results show a transition peak in the turbidity curve at intermediate temperatures. This peak, as well as the cloud point, is shifted toward lower temperatures when the molecular weight and the concentration of the polymer increase. The DLS measurements disclose a fast and a slow relaxation mode, which in both cases are found to be diffusive. The fast mode is linked to the diffusion of small species in the solution, and the slow mode is associated with the formation of large aggregates. The formation of these aggregates is less pronounced in solutions of polymers with low molecular weight and the incipient aggregation is shifted to higher temperatures. The shear viscosity measurements show the formation of weak aggregates, which are easily broken in solutions of short polymers. This effect is less pronounced when the molecular weight of the sample is increased. At certain shear rates, temperature-induced transition peaks of the viscosity are observed. [ABSTRACT FROM AUTHOR]

Summary??In this paper, the syneresis behaviors in the post-gel region of hydrogels of hydroxyethylcellulose (HEC) and its hydrophobically modified analogue (HM-HEC) were investigated by means of a new high precision swell-ratio-tester. The gels were prepared by cross-linking cellulose ether derivatives with divinyl sulfone (DVS) in alkaline solution at various cross-linker concentrations and temperatures. Increasing the cross-linker density promotes a faster shrinkage of the gel, and a more compressed gel. The compression of the 1?wt?% HEC gel starts at an earlier time at 40??C than at 25??C, because the increased mobility of the chains is more favorable to a faster formation of interpolymer cross-links. The results from the deswelling measurements show that the hydrophobic modification of the polymer yields less contracted gels. This novel finding is ascribed to the fact that some of the hydroxyl groups for the formation of intermolecular cross-links are deactivated through the incorporated hydrophobic groups at these sites. [ABSTRACT FROM AUTHOR]

We report energy transfer from polyfluorene containing binaphthyl, P(BiNDO-FO3), to the europium complex, Eu(dnm)3phen. The P(BiNDO-FO3) has an electroluminescence (EL) peak at 422?nm where as pure polyfluorene (PF) has an EL peak at 438?nm. We have doped both polymers and report energy transfer in P(BiNDO-FO3), which is not present in PF. This is due to overlap of the absorption spectrum in the Eu(dnm)3phen with the emission spectrum of the P(BiNDO-FO3) while there is no overlap with the emission spectrum of PF. [ABSTRACT FROM AUTHOR]