Georgios Staikos

Solution properties and phase behaviour of copolymers of acrylic acid with N-isopropylacrylamide: the importance of the intrachain hydrogen bonding

Abstract The solution properties and the phase behaviour of random copolymers of N -isopropylacrylamide (NIPAM) with acrylic acid (AA) have been investigated. The mole fraction of NIPAM in these copolymers ( x ) varies from 0 to 0.29. At pH=3.00, i.e. when the AA groups are in the undissociated form, the intrinsic viscosity decreases substantially as x increases. This chain shrinkage is attributed to the formation of intrachain hydrogen bonds between the two complementary groups, AA and NIPAM. The weakening of the hydrophilic character and the appearance of hydrophobic properties with increasing x is further supported by fluorescence probing studies and potentiometric measurements of the aqueous copolymer solutions. Moreover, the phase behaviour of these copolymers in salt solution changes dramatically with x . The copolymer with x =0.10 presents, like the homopolymer poly(acrylic acid), an Upper Critical Solution Temperature (UCST) behaviour. On the contrary, the copolymer with x =0.29 presents, similar to the homopolymer poly( N -isopropylacrylamide), a Lower Critical Solution Temperature (LCST) behaviour. Finally, the copolymer with the intermediate NIPAM content, x =0.17, presents both an UCST and an LCST behaviour.

A new approach to the determination of the statistical segment length of wormlike polymers

The Stockmayer-Fixman-Burchard (SFB) and the Dondos-Benoit (DB) equations have been applied to determine the unperturbed dimensions parameterKχ of wormlike polymers. An empirical relation between the Florys constant Φ and the Mark-Houwink-Sakurada (MHS) exponenta has been proposed. The Φ values found by this equation are lower than the value 2.5×1023 used in the case of flexible polymers and this deviation is attributed to the influence of the draining effect. From theKχ value and the so calculated value of Φ, we calculate the Kuhn statistical segment length of wormlike polymers. The obtained — for a great number of wormlike polymers — statistical segment lengths are almost the same as these calculated by the Yamakawa-Fujii and the Bohdanecky methods. The molecular mass regions in which the SFB, the DB, and the MHS equations are valid are explored. A criterion for the distinction between flexible and wormlike polymers is proposed based on the way of approach to the power law.

Chain conformation and intermolecular interaction of partially neutralized poly(acrylic acid) in dilute aqueous solutions

Abstract The isoionic dilution method was used to measure the intrinsic viscosity, [η], and the Huggins coefficient, K H , of two poly(acrylic acid) samples, 50% neutralized with NaOH, with molecular weights 5.0×10 3 (Na50PA5) and 4.8×10 4 (Na50PA48) at different ionic strengths. For Na50PA5 at low ionic strength, [η] is higher than the value predicted by a rigid-rod model. Assuming that the chain has reached its maximum extension, we have combined the rigid-rod model with the idea of effective dimensions that depend linearly on the Debye–Huckel screening length, κ −1 , to account for a primary electroviscous effect, as a means to describe the observed behavior. As a consistent approach, we have also combined the effective dimensions with the wormlike chain model to calculate the persistence length of the higher molecular weight sample, Na50PA48. The result is that the electrostatic persistence length, l e , is proportional to κ −1 , in the region of high to moderate ionic strength, as is usually observed experimentally for flexible polyelectrolytes, while at the lowest ionic strengths, l e tends to level off, which was predicted theoretically. Both the samples show a considerable increase in the Huggins coefficient as the ionic strength is lowered, with the sample having the lowest molecular weight giving the highest value at any particular ionic strength. This is attributed to the increasing intermolecular electrostatic interactions, which have a relatively larger importance in the case of Na50PA5, because the dimensions of the chain are smaller when compared with the range of the electrostatic interactions, as measured by κ −1 , than for Na50PA48.

Influence of annealing and casting solvent on the morphology of poly(ethylene oxide)-b-polystyrene-b-poly(ethylene oxide) triblock copolymer : compatibility effects

Abstract The influence of annealing on the morphology of solvent-cast films of poly(ethylene oxide)-b-polystyrene-b-poly(ethylene oxide) (PEO-b-PS-b-PEO) triblock copolymer was studied. The annealing temperature was chosen between the melting temperature of PEO and the glass transition temperature of PS. If the casting solvent is selective for PEO, microphase separation occurs and the crystallinity of the PEO is not affected by annealing. On the other hand, if the casting solvent is a good solvent for both blocks, a dramatic decrease in crystallinity of PEO is observed. This phenomenon is due to compatibility effects. The T g region of the PS-rich phase is shifted towards lower values. This behaviour was elucidated using differential scanning calorimetry combined with enthalpy relaxation experiments.

Rheological study of semidilute aqueous solutions of a thermoassociative copolymer

In this paper, the linear and nonlinear rheological behavior of semidilute aqueous solutions of a recently synthesized thermoassociative graft copolymer was investigated, as a function of temperature and polymer concentration. The polymer, namely CMC–g–PNIPAM, is based on a carboxymethylcellulose (CMC) backbone bearing thermosensitive poly(N-isopropylacrylamide) (PNIPAM) sidechains. The samples have been submitted to steady shear, oscillatory shear, and step-strain experiments, mainly at temperatures above the threshold temperature Tassoc to observe thermothickening. The linear and nonlinear rheological data clearly show the existence of two temperature regimes, separated by a transition temperature T′ > Tassoc. At temperatures below T′, the solutions behave like a soft critical gel, corresponding to weak PNIPAM segregation. At temperatures above T′, the solutions behave like a stiff critical gel, corresponding to strong PNIPAM segregation.

Template copolymerisation of N-isopropylacrylamide with a cationic monomer: influence of the template on the solution properties of the product

A copolymer containing 91 mol% N-isopropylacrylamide (NIPAM) units and 9 mol% N,N-(dimethylaminopropyl)methacrylamide (MADAP) units, copolymer II, was prepared using polyacrylate chains as a template for the positively charged MADAP units. The properties of this product were compared to the properties of a similar copolymer (copolymer I) prepared in the absence of the template. The molar mass of copolymer II was found much higher than that of copolymer I. Moreover, the cloud point of copolymer II is closer to the cloud point of the homopolymer poly(N-isopropylacrylamide) at various pH conditions, indicating that copolymer II contains longer NIPAM-sequences than copolymer I. This explanation is further supported by the fact that copolymer II forms stronger, more compact and more hydrophobic hydrogen-bonding interpolymer complexes with polyacrylic acid at low pH, as it was revealed by viscometry, fluorescence and turbidity studies.

Determination of the statistical segment of wormlike polymers using viscometric data

Abstract Using a new relation between the intrinsic viscosity and the molecular weight of wormlike polymers, we calculate the statistical segment of these polymers. Six wormlike polymers have been studied with this method and the results are in relatively good agreement with those already obtained by light scattering techniques or the Yamakawa-Fujii method. The best agreement with the results obtained by other methods is found in the case of DNA. We consider that the importance of the proposed method lies in its simplicity and on the possibility of its application for a very large range of molecular weights.

Thickening effect in soluble hydrogen-bonding interpolymer complexes. Influence of pH and molecular parameters

Linear and nonlinear viscoelastic properties of poly(acrylic acid) (PAA) and poly(acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid)-graft-poly(N,N-dimethylacrylamide) (P(AA-co-AMPSA)-g-PDMAM) mixtures have been investigated as a function of pH, the PDMAM content of the graft copolymer and the molecular weight of PAA. At pH < 3.75, strong hydrogen-bonding interpolymer complexation between PAA and PDMAM side chains in semidilute solution leads to the formation of a transient network, as the considerable increase in viscosity indicates. The sol/gel transition observed at pH=2.0 by increasing the graft copolymer composition in PDMAM is explained by a substantial increase in the number of the junctions (stickers) resulting from the PDMAAM/PAA hydrogen bonding complexation. Moreover, the thickening effect observed is further strengthened by increasing the molecular weight of PAA, due to the interconnection of more copolymer chains.Linear and nonlinear viscoelastic properties of poly(acrylic acid) (PAA) and poly(acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid)-graft-poly(N,N-dimethylacrylamide) (P(AA-co-AMPSA)-g-PDMAM) mixtures have been investigated as a function of pH, the PDMAM content of the graft copolymer and the molecular weight of PAA. At pH < 3.75, strong hydrogen-bonding interpolymer complexation between PAA and PDMAM side chains in semidilute solution leads to the formation of a transient network, as the considerable increase in viscosity indicates. The sol/gel transition observed at pH=2.0 by increasing the graft copolymer composition in PDMAM is explained by a substantial increase in the number of the junctions (stickers) resulting from the PDMAAM/PAA hydrogen bonding complexation. Moreover, the thickening effect observed is further strengthened by increasing the molecular weight of PAA, due to the interconnection of more copolymer chains.

Treatment of high molecular weight wormlike polymers as random coiled polymers

Abstract The possibility of treating a wormlike polymer as a random coiled polymer is examined. Taking from the literature intrinsic viscosity data concerning several wormlike polymers, we use two graphical methods which have been proposed for flexible polymers. These methods are only applicable in the region of molecular weights where the wormlike polymer presents a relatively great number of statistical segments. From the values of unperturbed dimensions obtained by the graphical methods, we obtain the statistical segment of wormlike polymers at Θ conditions.

A quantitative description of the viscometric behaviour of partially neutralized poly(acrylic acid) in aqueous solutions studied by the isoionic dilution method

Abstract The intrinsic viscosity, [η], and the Huggins constant, κ′, of a poly(acrylic acid) (PAA) sample have been measured at different degrees of neutralization, i, ranging from i=0.25 to i=1.00 and different ionic strengths, I, in the region 1.0 × 10−4 M