Insight into dynamics of polyelectrolyte chains in salt-free solutions by laser light scattering and analytical ultracentrifugation

We have investigated the dynamics of xanthan aqueous solutions with and without added salt (NaCl) by using laser light scattering (LLS) and analytical ultracentrifugation (AUC) via sedimentation velocity (SV). The fast and slow modes are observed in salt-free and low-salt xanthan solutions by dynamic light scattering (DLS). The scattering ratio (KC/R_s(q)) and apparent diffusion coefficient (D_s,app(q)) of the slow mode is linearly related to scattering vector (q²), indicating that it is related to the diffusion of scattering objects. The intensity contribution (α_s) of the slow mode is independent of scattering angles, indicating that the slow mode is not related to some scattering objects larger than the LLS observation length. However, the slow mode disappears in SV experiments, indicating that it arises from the temporal aggregates due to long range electrostatic interactions between chains, which can be destroyed in centrifugal field. The diffusion coefficient measured by SV is close to that of the fast mode in DLS measurements, indicating that it is the coupling diffusion of macroions and counterions. The present studies also demonstrate that the chain stiffness does not change the characteristics of the dynamics of polyelectrolyte in solutions.     

Solvents effect on the physical properties of semi-dilute poly(N-isopropyl acryl amide) solutions

The physical properties of 5 wt% poly(NIPAM) (M_v=3.22×10⁵) semi-dilute solutions in H₂O, D₂O, and THF (tetrahydrofuran) solvents were studied using dynamic light scattering (DLS) and dynamic shear viscosity (DSV) measurements. The DLS data showed that there were poly(NIPAM) slow mode inter-polymer chains associations in H₂O and D₂O solvents. However, no DLS slow mode was observed in poly(NIPAM)/THF solutions. The DSV data showed that there are shear thickening behavior in these three poly(NIPAM) solutions, resulting in a maximum shear viscosity η_peak in the viscosity η′(ω) versus shear frequency ω curve. The slow mode hydrodynamic radius 〈R_hs〉 of DLS measurements and the zero shear rate viscosity η₀ and maximum viscosity η_peak data of DSV measurements from poly(NIPAM)/H₂O and poly(NIPAM)/D₂O solutions show two critical transition temperatures with T_cr1=30-32 °C and T_cr2=32-34 °C. Poly(NIPASM)/D₂O has higher T_cr1 and T_cr2 than poly(NIPASM)/H₂O. However, no transition temperatures of poly(NIPAM)/THF solution were observed. The different temperature dependencies of these three solutions were attributed to the ‘solubility’ and ‘hydrogen bonding’ effects between poly(NIPAM) with H₂O, D₂O, and THF solvents. Without considering the polymer-solvent hydrogen bonding, the solubility of poly(NIPAM) in solvents decreases in the following sequence: THF>H₂O>D₂O and the degree of polymer-solvent hydrogen bonding increases in the following sequence: THF<H₂O<D₂O. The effects of the degree of ‘hydrogen bonding’ and the ‘solubility’ of polymer in solvents on the physical properties of poly(NIPAM) solutions are discussed.     

Coagulation Mechanism in Wet Spinning of Fibres

Spinning baths are divided into three types as a function of the coagulating (desolvating) power: hard, soft, and ultrasoft. The concentration of precipitant c_p in hard spinning baths is two times higher than the critical (threshold) precipitation concentration c_cr, i.e., c_p > 2c_cr. Precipitation takes place with formation of a boundary line (precipitation front) which coincides with the critical supersaturation line — spinodal. The fibre is characterized by large fibril size and high porosity. Soft baths are realized when the concentration of precipitant in the bath is less than two and more than one critical precipitation concentration, i.e., c_cr > c_p > 2c_cr. Precipitation takes place according to a frontal mechanism with initial formation of a liquid segment in the spun fibre. In spinning into soft baths, the optimum conditions are created for formation of fibres with a small-fibril structure and elevated physicomechanical indexes. At a concentration of precipitant below c_cr but above c_e (equilibrium), the precipitant accumulates on the fibre and precipitation takes place over the entire volume, without formation of a precipitation front. The fibre obtained is characterized by high porosity. __________ Translated from Khimicheskie Volokna, No. 4, pp. 26–31, July–August, 2005.     

Phase separation in randomly charged polystyrene sulphonate ionomer solutions

The dynamics of randomly charged polystyrene caesium-sulfonate ionomers in semi-dilute solutions were studied using a combination of dynamic light scattering (DLS), small angle neutron scattering (SANS), and bulk rheology. The samples were studied in toluene solutions where the aggregation of the dipolar groups is favoured. Evidence of aggregation in dilute solution is found using DLS and SANS with both the hydrodynamic and static radius of gyration indicating that there is a contraction of the chains due to intra-chain attractive forces. SANS experiments demonstrate the evolution of the aggregates into a network structure as a function of polymer concentration. The association process is caused by the dipolar attraction between the charged groups and introduces two static correlation lengths in the mesh structure of the network; the standard semi-dilute mesh size (ξ=1.12c^{−0.72±0.03}) and an inhomogeneity length (Ξ=24c^0.58±0.05) due to micro-phase separation. The scaling of the amplitudes of the correlation lengths I₁(0)∼c^{−0.33±0.07} and I₂(0)∼c^2.0±0.4 are consistent with good solvent conditions and micro-phase separation, respectively. An imposed shear causes the break up of the micro-phase separated micellar system with a characteristic yield stress for the Bingham step-like shear thinning.     

Dilute-solution structure of charged arborescent graft polymer

The solutions of charged G1 arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers in methanol-d4 and D₂O were investigated over a dilute concentration range ?=0.005-0.05 (?: mass fraction) using small-angle neutron scattering (SANS). Upon addition of acid (HCl) arborescent graft polymers became charged and a peak appeared in SANS data. The interparticle distance (d_exp) calculated from a peak position corresponded to the expected value (d_uni) for a uniform particle distribution. This indicates the formation of liquid-like ordering due to long-range Coulombic repulsions. The smaller dielectric constant of methanol-d4 resulted in long-range electrostatic repulsions persisting to lower polymer concentration than in D₂O. The slow mode scattering was observed by dynamic light scattering measurements for the same polymer solutions, indicating the presence of structural inhomogeneity in the solutions. Both the peak and slow mode disappeared by addition of NaCl or excess HCl into the solutions due to the screening of electrostatic interactions. The G1 polymer grafted with longer P2VP chains (M_w∼30,000 versus 5000 g mol) formed a gel on addition of HCl. This result reveals that molecular expansion is more significant for arborescent polymers with longer (M_w∼30,000) linear polyelectrolyte branches, resulting in gelation for ?>0.01. Upon addition of NaCl or excess HCl a gel transformed back to a liquid resulted from the screening of electrostatic interactions.     

Influence of molecular weight of polymer matrix on the structure and rheological properties of graphene oxide/polydimethylsiloxane composites

The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (C_cr). Moreover, at GO concentration just above C_cr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (M_c). However, positive ΔN is found in samples with PDMS molecular weight above M_c. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.     

Electrochemical study of the intermolecular electron transfer to Pseudomonas aeruginosa cytochrome cd1 nitrite reductase

The kinetics of electron transfer reaction between cytochrome cd₁ nitrite reductase (NiR) from Pseudomonas aeruginosa and various physiological/non physiological redox partners was investigated using cyclic voltammetry at the pyrolytic graphite electrode. While NiR did not exchange electron with the electrode, cytochrome c₅₅₁ and azurin, both from Ps. aeruginosa, behaved as fast electrochemical systems. The intermolecular electron transfers between NiR and cytochrome c₅₅₁ or azurin as electron shuttles, in the presence of nitrite, were studied. Second order rate constants of 2×10⁶ and 1.4×10⁵ M⁻¹ s⁻¹ are calculated for cytochrome c₅₅₁ and azurin, respectively. The dependence of the second-order rate constant on ionic strength and pH is discussed. Finally, the effect of the global charge of the electron shuttles was explored using differently charged species (proteins or small ions). The experimental results suggest involvement of polar interactions as well as of hydrophobic contacts in the protein recognition prior to the intermolecular electron transfer. As the cross-reaction between Ps. nautica cytochrome c₅₅₂ and Ps. aeruginosa NiR was shown to be as efficient as the catalytic reaction involving the physiological partners, it is concluded to a ‘pseudo-specificity’ in the recognition between NiR and the electron donor.     

Aggregation behavior of new cyclic saturated copolymers synthesized via ring-opening metathesis polymerization

Cyclic saturated copolymers were prepared from 8-methyl-8-methoxycarbonyltetracyclo[4.4.0.1^2,5.1^7,10]dodec-3-ene (MMT) with polar ester group and dicyclopentadiene (DCP) without polar group. This procedure consisted of ring-opening metathesis polymerization (ROMP) followed by hydrogenation. Monomer reactivity of DCP was higher than that of MMT; the monomer reactivity ratio r_DCP/r_MMT varied from 2.135 to 1.159 in a temperature range from 80 to 130 °C. These kinetic results indicated that the copolymer had distribution of DCP composition in a macromolecule chain, which could provide the interesting aggregation behavior. The aggregation behaviors of the hydrogenated copolymer and the homopolymer in various solvents were also examined using dynamic light scattering (DLS) and static light scattering (SLS). DLS analysis indicated that the fast mode in each polymer is attributed to the diffusive motion of each single polymer chain, while the slow mode in the copolymer is caused by aggregated polymer. The aggregation degree of the copolymer decreased with increasing hydrophobicity of solvent, decreasing polymer concentration, decreasing molecular size of solvent and increasing temperature. Based on these findings, the mechanism of aggregation behavior was clarified that the DCP-rich unit in a macromolecule might be acting as core to give the aggregation in poor solvent.     

Chloride-induced corrosion of steel in cracked concrete—Part II: Corrosion rate prediction models

Chloride-induced corrosion rate (i_corr) prediction models for RC structures in the marine tidal zone that incorporate the influence of crack width (w_cr), cover (c) and concrete quality are proposed. Parallel corrosion experiments were carried out for 2¼ years by exposing one half of 210 beam specimens (120 × 130 × 375 mm long) to accelerated laboratory corrosion (cyclic wetting and drying) while the other half underwent natural corrosion in the tidal zone. Experimental variables were w_cr (0, incipient crack, 0.4, 0.7 mm), c (20, 40 mm), binder type (PC, PC/GGBS, PC/FA) and w/b ratio (0.40, 0.55). The two proposed models (one each for accelerated and natural i_corr) can aid not only in quantifying the propagation phase, but also provide a novel way to select c, w_cr and concrete quality.     

Dynamic viscoelastic behavior of triple helical Lentinan in water: Effects of concentration and molecular weight

The dynamic viscoelasitc behavior of Lentinan, one triple helical β-(1 → 3)-d-glucan from the fruiting body of Lentinus edodes, in water was investigated as a function of concentration and molecular weight at 25 °C by using dynamic rheology. It was revealed that the shear storage moduli (G′), viscous loss moduli (G″), and the dynamic complex viscosity (η^∗) exhibited strong dependence on concentration and molecular weight. At low concentrations, the Lentinan/water systems displayed liquid-like behavior with G′ lower than G″ at low frequencies and crossing-over at high frequencies. With increasing concentration, the elastic response of the Lentinan/water system was stronger than the viscous response, leading to the conclusion that the Lentinan/water systems displayed a predominantly solid-like behavior. The gel point (c_gel) was determined from Winter-Chambon method (frequency-independence of tan δ). The most important point is that the c_gel was much lower than some synthesized polymers and other flexible polysaccharides, which may be attributable to the high stiffness of triple helical Lentinan and strong intra- and intermolecular interactions among polysaccharide chains. Furthermore, a decrease in molecular weight leads to a sharp increase of c_gel. The dynamic strain sweep measurements proved that the gelation of Lentinan in water is induced by the extremely entangled and stiff triple helices forming continuous network, and the Lentinan gel is structurally more like a solution that is unable to flow within a timescale of usual observation.     

Dynamics of thermoresponsive PNIPAM-g-PEO copolymer chains in semi-dilute solution

The effects of concentration and temperature on dynamics of poly(N-isopropylacrylamide)-graft-poly(ethylene oxide) (PNIPAM-g-PEO) chains in semi-dilute aqueous solution were studied by static and dynamic laser light scattering. The intensity-intensity time correlation function shows a fast and a slow relaxation mode, with line widths Γ_f and Γ_s, respectively. Γ_f is scaled to the scattering vector (q) as Γ_f ∝ q², revealing that it is due to the cooperative diffusion of the subchains between two neighboring entangled points. As the concentration increases, the slow relaxation becomes slower and contributes more to the total scattered light intensity, indicating that the slow relaxation is related to the chain entanglement. On the other hand, when the solution temperature increases, the PNIPAM chain backbone shrinks, but the fast relaxation remains and the slow mode slows down with a minimum rate at ∼33 °C. It indicates that the slow mode arises from inter-chain clustering, which is gradually suppressed by the intra-chain shrinking. The sample position independence of the time-averaged scattered light intensity 〈I〉_T reveals that the solution is homogeneous and the clustering is transient.     

An observation of the coexistence of multimers and micelles in a nonionic surfactant C10E4 solution by dynamic light scattering

The bulk phase of nonionic surfactant C₁₀E₄ solution was monitored by a dynamic light scattering (DLS) system at 20 °C in a narrow range of concentration near the cmc. Two particle aggregations were observed. The DLS data show (i) there exist premicellar multimers (or called sub-micelles) and (ii) micelles coexist with multimers. The C₁₀E₄ sub-micelles have a narrow size distribution with an averaged hydrodynamic diameter (D_h) of 1.35 nm. The D_h of the micelles is around 10.5 nm at 1.0 × 10⁻⁶ mol/mL and increases slightly with C₁₀E₄ concentration. It is illustrated from the DLS data that (i) at C = 0.78-0.82 μmol/mL, monomers and premicellar multimers coexist and (ii) at C = 0.84-0.92 μmol/mL, monomers + submicellar multimers + micelles coexist. At more elevated concentrations, only the signals from the micelles are detected by DLS.     

Synthesis and viscoelastic behavior of water-soluble polymers modified with strong hydrophobic side chains

A new series of associating polymers were prepared by grafting highly hydrophobic side chains: poly(n-butyl acrylate), PNBA; poly(n-butyl methacrylate), PNBMA; and poly(N-(tert-butyl)acrylamide), PTBA of different sizes onto a poly(sodium acrylate), PAANa, backbone. Due to the strong hydrophobic character of the stickers, the dynamics of the associations is very slow as compared to more conventional water-soluble polymers modified with short alkyl chains and the physical associations mainly behave as chemical ones in the experimental conditions. As a consequence, all the copolymers readily self-assemble in aqueous solution forming clusters in very dilute conditions and then gels at higher concentrations. From dynamic measurements, it was shown that the copolymer solutions follow the same scaling relation η^∗ ∼ c^a, where a is a frequency dependent exponent. In these conditions, all the copolymer solutions exhibit a sol-gel transition which obeys the main rules of the percolation theory. For each copolymer, the critical gel concentration c_g depends strongly on the hydrophobic character of the stickers and a single master curve can be drawn by plotting the complex viscosity vs. the reduced concentration, c/c_g. Although the temperature dependence of the viscoelastic properties is very weak, due to the slow dynamics of the associations, it was clearly evidenced that the alkyl acrylamide derivative (PAANa-g-PTBA) exhibits a slight thermothickening behavior which contrasts with the thermothinning behavior of alkyl(meth)acrylate derivatives (PAANa-g-PNBA and PAANa-g-PNBMA). The opposite type of behavior is explained by the presence of the amide function which is known to play an important role in the LCST (lower critical solution temperature) phase diagram of N-alkyl derivatives.     

Pulsed electron beam irradiation of dilute aqueous poly(vinyl methyl ether) solutions

A dilute aqueous solution of the temperature-sensitive polymer, poly(vinyl methyl ether) (PVME), was irradiated by a pulsed electron beam in a closed-loop system. At temperatures, below the lower critical solution temperature (LCST), intramolecular crosslinked macromolecules, nanogels, were formed. With increasing radiation dose D the molecular weights M_w increase, whereas the dimensions (radius of gyration R_g, hydrodynamic radius R_h) of the formed nanogels decrease. The structure of the PVME nanogels was analyzed by field emission scanning electron microscopy (FESEM) and globular structures with d=(10-30) nm were observed. The phase-transition temperature of the nanogels, as determined by cloud point measurements, decreases from T_cr=36 °C (non-irradiated polymer) to T_cr=29 °C (c_p=12.5 mM, D=15 kGy), because of the formation of additional crosslinks and an increase in molecular weights. The same behavior was observed for a pre-irradiated PVME (γ-irradiation) with higher molecular weight due to intermolecular crosslinks. After pulsed electron beam irradiation the molecular weight again slightly increases whereas the dimension decreases. Above D=1 kGy the calculated ρ-parameter (ρ=R_g/R_h) is in the range of ρ=0.5-0.6 that corresponds to freely draining globular structures.     

Fouling mechanisms of alginate solutions as model extracellular polymeric substances

Extracellular polymeric substances (EPS) are viewed as major fouling components during filtration of biological feeds in industry. However, very few studies investigated the fouling mechanisms of polysaccharides, one major group of macromolecules in EPS. In this work, dead end unstirred filtration of sodium alginate, a model polysaccharide, was carried out and analysed using classic filtration laws and the combined pore blockage and cake formation model. It was found that the cake model appears to fit the entire range of the ultrafiltration data while the consecutive standard pore blocking model and cake model are more applicable to microfiltration membranes (0.2 m track-etched and 0.22 m PVDF membranes). The specific cake resistance (α_c) (approximately 1×10¹⁶ m/kg) is similar to α_c of bovine serum albumin (BSA) and actual EPS in MBR systems reported in the literature, and higher than that of many colloidal particles. α_c increases with the increase of alginate concentration but slightly decreases with the presence of BSA in the feed solution. The alginate cake obtained during filtration features both low compressibility and low porosity despite some differences between different membranes. The lack of a full flux recovery following filtration interruption implies the existence of strong intermolecular attractive interactions within the alginate fouling layer.     

Light and neutron scattering studies of excess low-angle scattering in moderately concentrated polystyrene solutions

The light scattering (LS) and small-angle neutron scattering (SANS) behaviour of semi-dilute solutions of polystyrene has been determined in both ‘good’ and ‘theta’ solvents. Above a critical concentration related to chain overlap, an excess small-angle scattering component is in evidence for scattering vectors, q, such that qR_g < 1. Application of a number of recent solution scattering theories fails to account for the small-angle scattering observed. The inter- and intramolecular scattering functions are measured experimentally through characterization of the SANS behaviour of solutions containing mixtures of polystyrene and perdeuteropolystyrene. The resultant intermolecular scattering functions depend on the fraction of labelled chains, indicating clearly that the solutions contain large scale fluctuations. LS studies support this hypothesis and further show that the presence of these fluctuations is reproducible, yet dependent on the solution preparation procedure. Similar behaviour is observed in screening length measurements. The excess low-angle scattering is well characterized by the Debye—Bueche random two-phase model, which is subsequently used to estimate the characteristic dimensions of the long-range fluctuations.     

Conformation of oligo(ethylene glycol) grafted polystyrene in dilute aqueous solutions

Temperature induced conformational changes of poly(p-oligo(ethylene glycol) styrene) (POEGS) in aqueous solutions were investigated by small angle neutron scattering (SANS), neutron transmission and dynamic light scattering (DLS). The molecular weight of the polymer studied was 9400 g/mol with a polydispersity index of 1.18 and each repeat unit of the polymer had four ethylene glycol monomer segments. The polymer was water soluble due to the hydrophilicity of the OEG side chains and these solutions showed lower critical solution temperature (LCST) depending on the concentration of the polymer. Measurements of solution behavior were made as a function of temperature in the range of 25-55 °C for three polymer concentrations (0.1 wt%, 0.3 wt%, and 1.8 wt%). Neutron transmission measurements were used to monitor the amount of polymer which precipitated or remained in solution above the cloud point temperature (T_CP). DLS revealed the presence of large clusters in all solutions both below and above T_CP while SANS provided information on the structure and interactions between individual chains. It was found that in the homogeneous region below T_CP the shape of individual polymers in solution was close to ellipsoidal with the dimensions R_a = 37 Å and R_b = 14 Å and was virtually independent of temperature. The SANS data taken for the most concentrated solution studied (1.8 wt%) were fit to the ellipsoidal model with attractive interactions which were approximated by the Ornstein-Zernike function with a temperature-dependent correlation length in the range of 24-49 Å. The collapse of individual polymers to spherical globules with the radius of 15 Å above T_CP was observed.     

Intra- and Intermolecular Hydrogen Bonding in Miscible Blends of CO2/Epoxy Cyclohexene Copolymer with Poly(Vinyl Phenol)

In this study, we synthesized a poly(cyclohexene carbonate) (PCHC) through alternative ring-opening copolymerization of CO₂ with cyclohexene oxide (CHO) mediated by a binary LZn₂OAc₂ catalyst at a mild temperature. A two-dimensional Fourier transform infrared (2D FTIR) spectroscopy indicated that strong intramolecular [C–H···O=C] hydrogen bonding (H-bonding) occurred in the PCHC copolymer, thereby weakening its intermolecular interactions and making it difficult to form miscible blends with other polymers. Nevertheless, blends of PCHC with poly(vinyl phenol) (PVPh), a strong hydrogen bond donor, were miscible because intermolecular H-bonding formed between the PCHC C=O units and the PVPh OH units, as evidenced through solid state NMR and one-dimensional and 2D FTIR spectroscopic analyses. Because the intermolecular H-bonding in the PCHC/PVPh binary blends were relatively weak, a negative deviation from linearity occurred in the glass transition temperatures (T_g). We measured a single proton spin-lattice relaxation time from solid state NMR spectra recorded in the rotating frame [T_1ρ(H)], indicating full miscibility on the order of 2–3 nm; nevertheless, the relaxation time exhibited a positive deviation from linearity, indicating that the hydrogen bonding interactions were weak, and that the flexibility of the main chain was possibly responsible for the negative deviation in the values of T_g.     

Thermal response of low molecular weight poly-(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) polymers in aqueous solution

Temperature-induced phase transition of three low-molecular-weight samples (M _w  < 1.2 × 10⁴) 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.     

Thermo- and pH-sensitivity of aqueous poly(N-vinylpyrrolidone) solutions in the presence of organic acids

The phase transition in poly(N-vinylpyrrolidone) (PVP) aqueous solutions is shown to occur at heating upon addition of organic acids such as isobutyric, isovaleric, and, especially, trichloroacetic (TCA) ones. The cloud point temperature (T_c) of PVP solutions drops from 70 to 6 °C when the TCA concentration rises from 0.2 to 0.3 mol/l. A decrease in T_c is even more drastic when HCl is also added though HCl addition to the system without TCA does not result in phase separation. These phenomena are explained by the reversible coordination between the non-ionized form of TCA and PVP units via hydrogen bonding. An increase in the medium acidity depresses TCA dissociation, resulting in an increase in PVP-TCA associate concentration. Calculations based on the pK_a values of TCA confirm this suggestion. The similar behavior is observed with poly(N-vinylcaprolactam) systems. The amount of TCA bound to PVP has been determined by means of separation of the precipitate by centrifugation at temperatures above T_c and subsequent titration of TCA in the polymer with NaOH. It is shown that the precipitate contains one TCA molecule per 3-6 VP units, this value decreasing down to 1.25-2 upon HCl addition to the system.