Malcolm B. Huglin

Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with acidic comonomers

Abstract Hydrogels have been prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with acrylic acid (AA), methacrylic acid (MAA) and 2-methyl-2-acrylamido-propanesulfonic acid (AMPS) as comonomers. For each copolymerization three different compositions were used, and for all systems three concentrations of a tetrafunctional crosslinker were employed. Swelling ratios, r = (mass hydrogel/mass dry polymer), were measured on heating from 20°C and then on cooling back to 20°C, and the swelling/deswelling was shown to be thermoreversible. Lower critical swelling temperatures (Tc) were exhibited except for copolymers of medium-high content of AMPS. The influences of composition and crosslinker concentration on r and Tc are discussed. The unusual behaviour of hydrogels containing MAA as comonomer is explained on the basis of attractive hydrophobic interactions.

Effect of composition on properties of copolymeric N-vinyl-2-pyrrolidonemethyl methacrylate hydrogels and organogels

Abstract Gamma irradiation has been used to copolymerize N-vinyl-2-pyrrolidone with methyl methacrylate to high conversion in both the presence and absence of ethyleneglycol dimethacrylate. The resultant solid xerogels were swollen to equilibrium in water and in dioxane to yield hydrogels and organogels respectively. The gels were characterized by uniaxial compression measurements and by determination of sol fraction, equilibrium solvent content, dimensions, effective crosslink density (ve) and polymer-solvent interaction parameter (χ). The Q-e scheme was used to compute instantaneous compositions of the terpolymer components at all stages of conversion, and compositional drift served to interpret the heterogeneous crosslinking at some compositions. No measurable effect of temperature on ve could be discerned. When the temperature was increased from 278 to 343 K, the hydrogels underwent deswelling with an increase in χ, whereas, over a similar increase in temperature, the opposite behavi was exhibited by the organogels.

Effect of pH on the swelling behaviour of hydrogels based on N‐isopropylacrylamide with acidic comonomers

Hydrogels have been prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with acrylic acid (AA), methacrylic acid (MAA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). For each copolymerization three comonomer feeds were used and for all systems three concentrations of a crosslinker were employed. For all of the 30 polymers previously obtained, swelling data in deionised water were extended here by corresponding data measured in aqueous media of pH = 1 and pH = 12. Starting at ca. 15 °C phase diagrams of swelling ratio r (= mass hydrogel/mass xerogel) were determined at several temperatures (T) up to ca. 70°C and reversibility in swelling/deswelling behaviour was verified on cooling back to 15 C. In some instances a lower critical swelling temperature (T c ) was located, whereas in others there was merely a continuous decrease in r with T. Swellability at pH = 1 is lower than that in pure water, the reduction in r increasing with content of acidic comonomer. At pH = 12 a similar reduction in r was observed in gels containing AA and AMPS, whereas with MAA as comonomer r is equal to or greater than the value in pure water. The findings are discussed on the basis of the influence of pH on the ionisation of carboxyl groups, hydrogen bonding, and chelation.

Properties of poly(N-vinyl-2-pyrrolidone) hydrogels crosslinked with ethyleneglycol dimethacrylate

Abstract N -Vinyl-2-pyrrolidone (VP) in the presence of ethyleneglycol dimethacrylate (EDMA) has been polymerized to high conversion by γ-irradiation. Poly( N -vinyl-2-pyrrolidone) (PVP) hydrogels were obtained by swelling the resultant solid xerogels to equilibrium in water. The hydrogels were characterized by differential scanning calorimetry, compression-strain measurements and determinations of sol fraction, water content and dimensions. For EDMA contents within the range 0.5–5.0 wt%, Youngs moduli and the polymer-water interaction parameter χ lay within the intervals 0.019–0.504 MN m −2 and 0.491–0.572, respectively, at 294 K. At a fixed EDMA concentration, an increase from 275 to 342 K resulted in a small increase and a small decrease in χ and in water content, respectively, but Youngs modulus and the effective crosslinking density ( v e ) remained practically independent of temperature. Inefficient crosslinking and inhomogeneity of the products are indicated by (i) the presence of large sol fractions and (ii) small values of v e relative to the theoretical crosslinking densities. The findings are supported by calculation of the compositional drift of unreacted monomer at all stages of the copolymerization (crosslinking) reaction. The water contents of the hydrogels were very high (∼ 95 wt%) and were reduced significantly only at a high content of EDMA. The ratio of the contents of non-freezing water to total water within the hydrogels increased with increasing extent of crosslinking.

Observations on some copolymerisations involving N-isopropylacrylamide

Abstract Free radical copolymerisations of N -isopropylacrylamide (NIPA) with acrylic acid (AA), methacrylic acid (MAA) and 2-methyl-2-acrylamidopropane sulphonic acid (AMPS) have been conducted over a range of conversions in ethanol, ethanol and water respectively. It was necessary to devise special methods for isolating the copolymers, since the common device of precipitation into a non-solvent could not be adopted here. Experimental procedures necessary to overcome rapid attainment of high conversion, crosslinking and irreproducibility for the systems containing AA and MAA as comonomers are discussed. For all three systems compositions of linear copolymers were determined by conductometric titration vs. aq. NaOH. The monomer reactivity ratios ( r ) were calculated from the initial monomer feed composition, the fractional conversion and the copolymer composition, by means of the extended Kelen-Tudős method. The 95% confidence ellipses were also computed. The results are: r 1 =14.0±1.9 and r 2 =0.07±0.09 for NIPA(1)/AA(2), r 1 =10.2±1.4 and r 2 =0.01±0.03 for NIPA(1)/MAA(2) and r 1 =2.4±0.8 and r 2 =0.03±0.02 for NIPA(1)/AMPS(2). Hence none of these systems undergoes azeotropic copolymerisation. The extent of compositional heterogeneity has been demonstrated by computed curves of instantaneous copolymer composition at different stages of conversion. Examination of the rather limited literature on linear copolymerisations involving NIPA discloses possible flaws and uncertainties in some of the procedures employed.

A new linear method to calculate monomer reactivity ratios by using high conversion copolymerization data: terminal model

Abstract A new linear method of calculation of monomer reactivity ratios is presented for copolymerizations up to very high conversion. It is a linear least-squares method involving several interations. The basis of calculation is the differential copolymerization equation and the Kelen-Tudős (K-T) method. The principle and the calculation of computer-simulated data indicate that this new method can correct completely the systematic errors due to high conversion. Calculation using data in the literature shows that this method can be applied in any real systems. The comparison of K-T and extended K-T methods is also discussed.

Complex formation between polyelectrolytes in dilute aqueous solution

Abstract Polyelectrolyte complexes (PEC) have been formed from mixing dilute aqueous solutions of the polyanion poly[sodium (2-acrylamido-2-methyl propane sulfonate)] (PAMPSNa) with the polycations poly(4-vinylpyridinium chloride) (P4VPHCl) and poly(2-vinylpyridinium chloride) (P2VPHCl). Simultaneous conductometric and potentiometric titrations indicated the electrochemical endpoint of each titration. In all cases, the endpoint occurred at a unit molar ratio (UMR) of polyanionic to polycationic groups between 0.9 and 1.1 which means that titrations exhibit approximate 1/1 stoichiometry. Moreover, the position of the titrimetric endpoint was in good accord with the UMR at which PEC particles were observed to flocculate rapidly. It was found that endpoints were not affected by: (1) changing from P4VPHCl to P2VPHCl; (2) using P2VPHCl samples of different molecular weights; (3) the addition of salt to the titrant or (4) by raising the reaction temperature from 298 K to 323 K. Elemental analysis of precipitates revealed, however, that PEC were deficient in polyanionic units. Potentiometric titrations performed at different pH values yielded strong evidence that complex composition is dependent primarily upon the degree of ionization of polycationic groups in the system and to the conclusion that elemental analysis data emphasize deviations in complex composition which are not apparent from titrimetric results. Turbidimetric measurements showed that the PEC particles were large (compared to the wavelength range scanned) and did not change size significantly in the range of UMR measured.

Thermoreversible swelling behaviour of hydrogels based on N-isopropylacrylamide with sodium acrylate and sodium methacrylate

Abstract Hydrogels were prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with sodium acrylate (SA) and sodium methacrylate (SMA) as comonomers. For each copolymerization three different compositions were used and for all systems three concentrations of a tetrafunctional crosslinker were employed. Swelling ratios, r = (mass hydrogel/mass dry polymer), were measured on heating from 20°C and then on cooling back to 20°C, and the swelling/deswelling was shown to be thermoreversible. Lower critical swelling temperatures (Tc) were exhibited in copolymers with low concentration of sodium salt. The type of comonomer did not affect the swelling–deswelling behaviour in hydrogels with low concentration of comonomer, but there were strong differences at higher concentrations. These variations were attributed to small differences in the degree of hydrolysis of the SA and SMA units within the hydrogel.

Some observations on the copolymerization of styrene with furfuryl methacrylate

Abstract The tendency for crosslinking to occur either during free radical polymerization in solution or to develop subsequently in air after precipitation of linear polymer has been observed for both polymerization of furfuryl methacrylate (FM) and copolymerization of FM with styrene (ST). Reactivity ratios rFM=0.33±0.08 and rST=0.44±0.02 have been determined. Thermo-oxidative stability is lowered by incorporation of FM moieties into poly-ST. Glass transition temperatures (Tg) of copolymers accord with the Fox equation and extrapolation affords a value of 64°C for the Tg of linear poly-FM. After admixture of solutions of poly(FM-co-ST) and bismaleimide (BM) at ambient temperature crosslinked gel is produced slowly by intermolecular Diels–Alder reaction between diene units (FM) and the dienophile (BM). The retro Diels–Alder reaction occurs rapidly on heating, regenerating linear copolymer and BM. The crosslinked gels exhibit considerable swelling in chloroform and toluene.

Copolymerizations involving N-vinyl-2-pyrrolidone

Abstract Low conversion copolymerizations have been conducted with N -vinyl-pyrrolidone (VP) as monomer-1 and the following reagents were used as monomer-2: acryloxymethylpentamethyldisiloxane (AMS), methacryloxymethylpentamethyl disiloxane (MMS), n-butyl acrylate (BA) and 2-hydroxyethylmethacrylate (HEMA). In the same order the derived monomer reactivity ratios ( r 1 , r 2 ) were (0.34, 1.57); (0.04, 4.92); (0.02, 0.80) and (0.05, 3.12). For a range of different feed compositions within each system integral curves were thereby computed for the instantaneous copolymer composition throughout all stages of conversion. These gave predictions on compositional heterogeneity which compared satisfactorily with the clear, translucent or opaque appearance of copolymers prepared to very high conversion via γ-irradiation. However, the VP-HEMA system, for which there is no azeotropic composition, yielded optically clear copolymers at all feed compositions. This finding is explained on the basis of the almost iso-refractive nature of the two homopolymers and/or strong thermodynamic compatibility among copolymers and homopolymers as evidenced by a single T g observed on cast films.