On the molecular weight determination by vapour pressure osmometry, 1. Consideration of the calibration function

A detailed study on the calibration of vapour pressure osmometry (VPO) using thirteen different samples (eight of them polystyrenes) in a range of molecular weights up to 50 000 as solutes and chloroform, dichloromethane, ethyl methyl ketone, cyclohexane, and acetone as solvents is carried out. The results show that contrary to the commonly used calibration equation M = K · (ΔV/c), the relationship between the measuring result (ΔV/C)_c=0 and molecular weight M obeys a function of the type M = K · (ΔV/c) in which the exponent a differs from 1. For each of the above mentioned solvents the values of K and a are determined. The limits of applicability of VPO with respect to polymers are discussed in view of these terms and recent theoretical considerations.     

Molecular weight determination of macromolecules by vapor pressure osmometry

A new vapor pressure osmometry (VPO) apparatus having a quite good sensitivity was constructed on the basis of a Hitachi 115 molecular weight measurement apparatus. In order to match a pair of thermistors, a conventional Wheatstone bridge circuit was modified by introducing a matching resistor, which enabled us to detect the temperature difference of ca. 6. 10^?6°C. By employing this apparatus the measurement of the temperature difference ΔT_s between a solution drop and a solvent drop at steady state was carried out as a function of concentration with solutions of the three normal alkanes (C₁₈, C₂₈ and C₃₆) dissolved in the normal alkanes (C₆, C₈ and C₁₀), and of eight atactic polystyrenes (the number-average molecular weights ranging from M?_n = 2030 to 4,1.10⁴) in benzene over a wide temperature range. The ratio of the calibration parameter K_sto the corresponding constant K_e(≡(RTV₀)/ΔH, R = gas constant, T₀ = measuring temperature in K, V₀ = molar volume of solvent, ΔH = partial molar heat of condensation), K_s/K_e, which represents the efficiency of measurement was found to be about 0,5, almost independent of the molecular weight of the solute, ranging from 200 to 20.10⁴. The maximum number-average molecular weight measurable by this apparatus with a precision of ±10% was estimated to be ca. 1·10⁵. The time necessary to reach a steady state was about 9 min regardless of the molecular weight.     

Osmotic coefficients of aqueous polyelectrolyte solutions at low concentrations, 2. dependence of polystyrenesulfonate osmotic coefficients on macroion molecular weight

The osmotic coefficients, Φ, of copper polystyrenesulfonates with various average molecular weights of the macroion, M? (from 40 000 to 520 000), were measured in aqueous solution and the dependence of Φ on M? was investigated. The Φ values are found to be nearly proportional to 1/M? at a molality of m_c=1.10^?3 mol/kg. The same solutions were measured with osmotic and cryoscopic methods and the good agreement of the results is regarded as a confirmation of the validity of the osmotic method. The experimental results were compared with the simple theory of F. Oosawa and a partial agreement with this theory was found. The activity coefficients, γ, of the counterions of sodium and copper polystyrenesulfonates (NaPSS and CuPSS) with different molecular weights M? were measured and a dependence on M? was found. From the concentration dependence of γ_Na for different M?, the concentration range m_c=0,1 to 1 mol/kg for NaPSS could be estimated as the beginning of a noticeable dependence of Φ and γ on the nominal molecular weight of macroions.     

On the molecular weight determination by vapour pressure osmometry, 4. Experimental proof of the deviation of MVPO from Mn

The kind of molecular weight average M_VPO resulting from vapour pressure osmometry (VPO) measurements on solutions of paucimolecular polymer mixtures of different compositions is studied with a Hewlett Packard Model 302 B. Recent theoretical calculations which predicted that M_VPO > M_n are confirmed. The experimentally found quotient M_VPOM_n is between 1,04 and 1,26, being the greater, the greater is the polydispersity of the mixture, which is agreement with the former mathematical derivations.     

Baker-Williams fractionation of high molecular weight polyisobutylene and its evaluation by gel permeation chromatography

A sample from commercially available polyisobutylene with a viscosity average molecular weight of M?_v = 320 000 is fractioned according to Baker-Williams. 9 Fractions are recovered and are characterized by viscometry and gel permeation chromatography (GPC). Molecularly very homogeneous fractions with an inhomogeneity U = (M?_w/M?_n)?1 ≈ 0,05 can be obtained in the range of molecular weights up to M?_v = 600 000. Inversion of molecular weight occurs in the last three fractions. A 3 wt.-% portion with M?_v>10⁶ is lost during the fractionation.     

Mechanischer Scherabbau von Polymeren in Lösung mittels turbulenter Strömung, 2. Die Abhängigkeit der Abbaugeschwindigkeit von der Polymergesamtkonzentration in guten Lösungsmitteln

Mechanical shear degradation of poly(decyl methacrylate), (viscosity average molecular weight in tetrahydrofuran M?_η = 1,3·10⁶, M?_w/M?_n = 5) in the thermodynamically good solvent tetrahydrofuran has been studied in turbulent flow through a capillary as a function of polymer concentration in the range from 0,22 to 8,9 g/100 cm³. Due to turbulent flow conditions the shear stress, shear rate and shear energy proved to be the same for all concentrations and remained constant during degradation, giving a general insight into mechanism of degradation. The rate of degradation has been followed using molecular weight distribution curves obtained by gel permeation chromatography. The reaction was found to be of first order. Rate constants determined for molecular weights from 3,2–9,5·10⁶ decreased with increasing concentration following a law of the type k_i = (K + b·c)^?1, K and b being constants for each molecular weight. Hydrodynamic volumes of polymer molecules have been calculated according to models of Rudin as function of molecular weight and concentration. It can be shown that rate constants of degradation and calculated hydrodynamic volumes are proportional for the whole range of molecular weight and concentrations up to 3,6 g/100 cm³. There is also a rather good proportionality between these rate constants and the volumes of polymer coils predicted by de Gennes. This result is an additional confirmation of the concept that hydrodynamic volume governs shear degradation of polymer solutions. Additional experiments show that this type of concentration dependence is also to be found for other polymers in other solvents.     

Molecular weight determination of isotactic polypropylene by high-sensitivity ebulliometry

The use of a new differential ebulliometer and the determination of the number average molecular weight of unfractionated isotactic polypropylenes is described; the results were reproducible and of satisfactory accuracy. Application of a thermopile with 150 junctions and of suitable devices for the regularity of boiling allowed measurements of number average molecular weights M?_n up to 100000 in good agreement with results obtained by other methods. In this way it is possible to measure M?_n directly, e.g. for reliable determinations of M?_w/M?_n also for polymer samples with low molecular weights and/or wide molecular weight distributions.     

Histamine reduces Cl− activity in surface epithelial cells of frog gastric mucosa

Intracellular chloride activity (a _c ^Cl ) and serosal as well as mucosal membrane potentials (V _cs andV _cm) were recorded in surface epithelial cells (SEC) of frog gastric mucosa during the resting state (cimetidine, 10^–4 mol/l) or during stimulation with histamine (10^–4 mol/l). Stimulation leads to a fall ina _c ^Cl from 18.7 SD±5.9 mmol/l (n=26) to 13.3 SD±4.9 mmol/l (n=33). Simultaneously both cell membranes hyperpolarize,V _cs from –56.0 SD±4.8 (n=42) to –62.8±7.6 (n=43) andV _cm from –39.6 SD±5.8 (n=42) to –47.9±7.6 (n=43), so that intracellular chloride remains elevated above electrochemical equilibrium at both cell membranes. Reduction or omission of chloride in the lumen perfusate does not affecta _c ^Cl , suggesting that the luminal cell membrane is virtually tight for chloride ions. Current induced hyperpolarization of the serosal cell membrane potential which simulates the electrical effects of stimulation, does not affecta _c ^Cl either; however, inhibition of gastric acid secretion by a benzimidazol derivative which is known to block the H⁺/K⁺ ATPase prevents the fall ina _c ^Cl in response to histamine. The same holds if the experimental solutions are gassed with 25% CO₂ which does not interfere with acid secretion but may block cell to cell communication via gap junctions. From these results we conclude that 1. the SEC are not involved in non-acidic Cl^– secretion, neither in resting nor in stimulated state, and 2. that the fall ina _c ^Cl of the surface epithelial cells under stimulation does not seem to reflect a direct action of histamine on the SEC but seems to reflect a fall ina _c ^Cl of the oxyntic cells which may be coupled to the surface epithelial cells by permeable cell-to-cell junctions.Part of this work has been presented at the SIBS-SIF-SINU-Congress, Abano-Terme, 1984This study was supported by CNR grant number 84.01734.04 and by Ministero della Pubblica Istruzione, Roma, Italy     

Osmotic coefficients of aqueous polyelectrolyte solutions at low concentrations, 3. Dependence on macroion molecular weight

Eight sodium salts of polystyrenesulfonic acid (poly[1-(sulfophenyl)ethylene]) of different molecular weights were prepared similarly and purified in the same way. Their aqueous solutions were thus chemically equivalent, and differed only in their molecular weights, which were determined viscosimetrically. The osmotic coefficients (Φ) of the acid and its sodium, thallium, calcium and cadmium salt solutions were measured by a membrane osmometer for counterion molalities m_c = 10^?2 to 5·10^?4mol/kg, at 25°C. In the same concentration range, the activity coefficients of hydrogen counterions (γ_H) of acid solutions were calculated from electromotive force measurements and the osmotic coefficients of the acid solutions were determined by the cryoscopic method for two different molecular weights. The values obtained are presented graphically as functions of the measured average molecular weight for m_c = 10^?3 and 10^?2 mol/kg and are in good mutual agreement within experimental error. This agreement of results could be assumed to verify experimentally that the colligative properties of polystyrene-sulfonate polyelectrolytes at low concentrations are dependent on the molecular weight of the polyion. The obtained results together with some literature data, allow us to evaluate very approximately the activity coefficients of monovalent counterions (γ_c) for m_c = 10^?2 mol/kg as a function of the degree of polymerization (n) of linear oligoions or polyions, for values of n up to 10⁴.     

Possibilities and limits of photon correlation spectroscopy in determining polymer molecular weight distributions

The effect of concentration and polydispersity on the collective diffusion coefficient D_c, evaluated using Photon Correlation Spectroscopy (PCS), has been investigated on poly(methyl methacrylate) (PMMA) in acetone solutions. The concentration dependence of the collective diffusion coefficient follows a linear regression law, the slope being fairly independent of polydispersity, molecular weight and temperature. The diffusion coefficient at infinite dilution D₀ obeys the scaling law D₀ = AM_w^–ν in the range from M_w = 10 000 to M_w = 800 000; the value of the scaling exponent, ν = 0.57, proves the good solvent quality of acetone. The inversion of the scattered intensity autocorrelation data by the regularization method CONTIN allowed the evaluation of the molecular weight distribution function of the polymeric samples. Although this algorithm gives valuable information on average quantities or on the width of the distribution, it has limited resolution power; therefore a comparison with the results obtained by Size Exclusion Chromatography (SEC) was carried out for a set of samples having monomodal and bimodal distribution functions.     

Dilute solution properties of high molecular weight poly(isobutyl methacrylate). III. Effect of centrifuging on various parameters

Three high molecular weight samples of poly(isobutyl methacrylate(designated as 937, SW 63/0082, and SW 62/0298 were obtained from Edgewood Arsenal, Maryland. Sample 937 has been fractionated and described^l,2) earlier. Initial investigations have shown that the solution viscosities were shear dependent. The three samples were subjected to centrifugation at constant velocity for various times. The intrinsic viscosity and molecular weight (M?_w) dropped slightly for sample 937, but dropped drastically for the other two samples. The number average molecular weight (M?_n) remained constant for each sample under these conditions. In this presentation we have also shown the necessity for taking measurements at low angles daring light scattering measurements in order to obtain the correct molecular weight (M?_w) for these high molecular weight species.     

Characterization of non-homogeneous polymers with size-exclusion chromatography by implementing an on-line viscometer

The viscosity distribution of a polymer sample can be obtained by using an on-line viscometer as a detector in size-exclusion chromatography. This newly defined viscosity distribution is closely related to the molecular weight distribution and expresses weight fraction times intrinsic viscosity of species i as a function of the corresponding molecular weight times intrinsic viscosity (w_i[η_i] vs. M_i[η_i]). The intrinsic viscosity ([η]) and number-average molecular weight (M?_n) can be obtained directly from a viscosity distribution. If the Mark-Houwink exponent a is known (or approximately known) for non-homogeneous polymer the M?_w/M?_n can be estimated from the viscosity distribution when the molecular weight distribution is approximated with a known distribution function. These estimates are independent of any other detector and are valid even for non-homogeneous polymer samples. The relation between the moments of the viscosity distribution and the M?_w/M?_n is presented for two widely used distribution functions, the Log-Normal and the Generalized Exponential Distributions. Polymer characterization based on the viscosity distribution is shown to be a robust technique. It is particularly attractive in characterizing non-homogeneous polymers since it is solely obtained from on-line viscometer.     

Mechanischer Scherabbau von Polymeren in Lösung mittels turbulenter Strömung, 1. Kinetik,Molekulargewichtsabhängigkeit und Frage des Bruchmechanismus

The mechanical shear degradation of poly(decyl methacrylate) with weight average molecular weights 1,0·10⁶ ≤ M?_w ≤ 1,7·10⁶, and molecular polydispersity ratio M?_w/M?_n = 5 in dilute solutions is studied in turbulent flow as a function of molecular weight using a special apparatus consisting of two vessels connected by a capillary. Shear stress and shear rate remained constant during degradation. The rate of degradation was followed up by molecular weight distribution curves using gel permeation chromatography and described by ?dc_i/dt = k_i·cⁿ, i being a high molecular weight species of the distribution. The reaction was found to be of the first order (n = 1) independent of solvent and of capillary length. Rate constants k_i in the molecular weight range from 3,2·10⁶ to 13,5·10⁶ proved to be proportional to the hydrodynamic volume of the polymer molecules expressed in terms of the product of intrinsic viscosity and molecular weight [η]_i·M_i. This corresponds to a linear relationship between k_i and M_i^1,75. Additional experiments show that this type of dependence on molecular weight holds only for turbulent flow; in laminar flow the result of the literature could be confirmed that there is a linear relation between k_i and M_i¹. Both results are independent of capillary length. As to the mechanism of breakage in turbulent flow it seems that in one step each macromolecule is broken simultaneously into several smaller parts.     

Synthesis and characterization of aliphatic-aromatic poly(ether amide)s

Four aromatic diamines containing aliphatic spacers and Meta and para oriented oxyphenylene rings, and their corresponding hydrochlorides, were combined with isophthaloyl chloride (IPC) and terephthaloyl chloride (TPC) to give high molecular weight polyamides by interfacial and low-temperature solution methods. The synthesis and characterization of monomers and polymers are reported, and the differences observed in polycondensation yields, molecular weights and molecular weight distributions, as a function of the method of synthesis, are discussed. Values of number-average molecular weight (M?_n) up to 8 × 10⁴ g/mol and weight-average molecular weight (M?_w) up to 1 × 10⁵ g/mol could be measured by gel permeation chromatography using aromatic polyamide standards, and values of M?_n up to 2 × 10⁵ g/mol and M?_w up to 3.6 × 10⁵ g/mol by using polystyrene standards.     

Electrophysiological identification of principal and intercalated cells in the rabbit outer medullary collecting duct

Intracellular microelectrode techniques were used together with inhibitors of Na⁺ transport (amiloride) and H⁺ transport (acetazolamide and SITS¹) to identify principal cells and intercalated cells in the outer stripe of the rabbit outer medullary collecting duct. The principal cell (n=9) had a basolateral membrane voltage (V _bl) of –64.7±3.2 mV, a fractional resistance of the apical membrane (fR _a=R _a/R _a+R _bl) of 0.82±0.02, and a K⁺-selective basolateral membrane. Luminal amiloride hyperpolarizedV _bl by 10.3±2.1 mV and increasedfR _a to near unity (n=7). Bath acetazolamide and SITS were without effect on these parameters. The intercalated cell (n=5) had aV _bl of –25.0±3.2 mV, afR _a of 0.99±0.01, and a Cl^–-selective basolateral membrane. Bath acetazolamide or SITS hyperpolarizedV _bl by 26.4±8.2 mV. Luminal amiloride did not alterV _bl of this cell. The differential effects of the inhibitors also indicate that the principal and intercalated cells are probably not directly coupled electrically.     

Continuous fractionation and solution properties of PVC, 6. Pressure dependence of the viscosity — influence of molecular weight and composition

Viscosities were measured as a function of pressure and temperature with solutions of PVC 75 000 in cyclohexanone (CHO) and polymer contents ranging from 0,6 to 12 wt.-%, by means of a Searle-type (≥3 wt.-%) and a rolling-ball viscometer (<3 wt.-%). Furthermore, the influence of molecular weight was determined with solutions of 8 wt.-% of PVC 20 000, PVC 37 000 and PVC 100 000. (The numbers in the codes of the PVC specimens are their approximate molecular weights.) For all concentrations and molecular weights, the viscosity increases in a more or less exponential manner with increasing pressure. The ratio f₁₀₀₀ of the viscosity of the solution at 1 000 and 1 bar can be varied by the change of the polymer content from 2,5 (the value of the pure solvent, index s) to 3,5 (12 wt.-% PVC 75 000) at t = 25°C and from 2,23 to 2,94 at t = 80°C. An increase of the molecular weight of the polymer raises f₁₀₀₀ in a similar manner as the polymer concentration. Using the reduced variables V^≠/V^≠ (ratio of the volumes of activation of the solution and the pure solvent) and c? (product of the polymer concentration and the intrinsic viscosity), all results obtained by variation of T, c and M_w can be represented by a master curve. This means that it is possible to calculate the pressure dependence of a given polymer solution of arbitrary polymer concentration from a mere measurement of the intrinsic viscosity at normal pressure. Criteria are presented which allow a forecast concerning the occurrence of minima in the concentration dependence of the energy of activation of the viscous flow E^≠ and V^≠.     

Intracellular potentials of microperfused human sweat duct cells

Intracellular potentials of cells from isolated segments of microperfused human sweat ducts were measured in order to determine the electrical profiles of these cells under resting, transporting, and inhibited conditions. Even though the cells are relatively small (ca. 6–8 m), continuous recordings of intracellular potentials from the same impalement were stable for up to 2 h. In the resting condition in normal Ringer's solution when the lumen of the duct was collapsed and not perfused, the intracellular potential measured across the basal membrane was 34.6±1.5 mV (n=31; mean±SE). In the same bathing medium, when the duct lumen was also perfused with normal Ringer's solution, the basolateral membrane potential (V _b), the apical membrane potential (V _a) and transepithelial potential (V _t) was –33.8±0.47 mV, –23.7±0.48 mV and –9.6±0.9 mV (n=73), respectively. The average input impedence (R _i) of these cells was 19.6±0.4 M (n=36). The frequency distribution ofV _b was unimodal suggesting that only one functional cell type exists in this tissue. Amiloride (0.1 mM) in the lumen hyperpolarized bothV _a andV _b by –40.5±3.6 mV and –33.2±3.7 mV (n=15), respectively, with a slight but significant increase inR _i (15%), while abolishingV _t. Removing luminal Cl^– depolarizedV _a by +37.0±4.2 mV and hyperpolarizedV _b by –19.0±4.2 mV (n=11). Removing Cl^– from the bath hyperpolarizedV _a by –3.3±2.3 mV and depolarizedV _b by +24.3±2.7 mV (n=15). Ouabain caused an initial fast depolarization (+8 mV) followed by a prolonged slow depolarization ofV _b, and an increase inR _i of about 84%. These results not only provide the first electrical profile of the human sweat duct tissue, but they also show that its cell membrane potentials are unusually low. This unusual property of this epithelium appears to be due to the combination of a significant Na⁺ conductance at the apical membrane and a remarkably high tissue Cl^– conductance.     

Heart rate overshoot at the beginning of muscle exercise

Summary A characteristic notch in the heart rate (f _c) on-response at the beginning of square-wave exercise is described in 7 very fit marathon runners and 12 sedentary young men, during cycle tests at 30% and 60% of maximal oxygen consumption (VO_2max). The (f _c) notch revealed af _c overshoot with respect to the (f _c) values predicted from exponential beat-by-beat fitted models. While at 30% of (VO_2max). all subjects showed af _c over-shoot, at 60% of (VO_2max). it occurred in the marathon runners but not in the sedentary subjects. The mean time of occurrence of thef _c overshoot from the onset of the exercise was 16.7 (SD 4.7) s and 12.2 (SD 3.2) s at 30% of (VO_2max). in the runners and the sedentary subjects respectively, and 23.8 (SD 8.8) s at 60% of (VO_2max). in the runners. The amplitude of the overshoot, with respect to rest, was 41 (SD 12) beats·min^–1and 31 (SD 4) beats·min^–1 at 30% of (VO_2max). in the runners and the sedentary subjects respectively, and 46 (SD 19) beats·min^–1 at 60% of (VO_2max). in the runners. The existence and the amplitude of thef _c overshoot may have been related to central command and muscle heart reflex mechanisms and thus may have been indicators of changes in the balance between sympathetic and parasympathetic activity occurring in fit and unfit subjects.     

Polymerization of lactams, 54. Anionic polymerization of 2-pyrrolidone accelerated with CO2, part 3

Intrinsic viscosities in m-cresol and weight average molecular weights, M?_w, were measured for samples of high molecular weight poly(2-pyrrolidone) (poly ( 1 )) prepared by anionic polymerization of 2-pyrrolidone ( 1 ) accelerated with CO₂. It was proved that the earlier found relationship [η] = 4 · 10^?2 · M^0,77 (in cm³ · g^?1) holds for M?M_w up to 8 · 10⁵ g. mol^?1. The probable reason for the formation of poly ( 1 ) with an exceptionally high molecular weight is discussed.     

Sex-specific causal factors and effects of common environment for symptoms of anxiety and depression in twins

Two thousand five hundred seventy intact pairs and 724 single responders from Norwegian twins aged 18–25 years completed questionnaires with information about anxiety and depression and perceived cotwin closeness. The aim of the study was the univariate estimation of sex-specific genetic and environmental effects on an index tapping symptoms of anxiety and depression. An index of social closeness between cotwins was significantly related to the cotwin correlation for anxiety/depression scores. MZ pairs were reported to be closer than DZ pairs, and like-sexed DZ pairs were closer than unlikesexed pairs. The symptom data were adjusted for this apparent violation of the equalenvironment assumption in twin studies, but the adjustment did not dramatically affect the parameter estimates of genetic and environmental effects on anxiety/depression. A model specifying male (a _M ) and female (a _F ) genetic additive effects, shared environment for males (c _M ), and individual environmental effects (e _M ande _F ) fitted the adjusted data very well. An alternative model, specifyinga _M =a _F ,c _M =c _F , ande _M =e _F , and no correlation between those environmental factors shared by brothers and those shared by sisters, fitted equally well. Estimated proportions of total variance from the first model werea _M ²=.30,a _F ²=.52, andc _M ²=.21.The estimates from the second model werea _M ²=a _F ²=.43 andc _M ²=c _F ²=.11.