Bad Solvent Research Articles

A calorimetric study of the enthalpy of micelle formation by a polystyrene- b-polyisoprene block copolymer in n-hexadecane

A calorimetric study was made of micelle formation by a polystyrene- b-polyisoprene block copolymer in n-hexadecane which is a selectively bad solvent for polystyrene. A value of −44.0 ± 4.0 kJ mol −1 was obtained for the enthalpy of micellisation, in good agreement with a value of −40.7 ± 2.2 kJ mol −1 determined for this system in an earlier study from measurements of the temperature dependence of the critical micelle concentration. The agreement between the results supports the use of a simple mass-action model in accounting for the thermodynamics of micellisation of block copolymers in organic solvents.

Kinetics of collapse for a flexible coil

If a polymer chain is transferred abruptly from θ conditions to bad solvent conditions (e.g. by a shift of temperature ΔT) it collapses with a certain characteristic time τc. We propose a scaling structure for τc, based on an « expanding sausage model », and taking into account the hydrodynamic interactions. We find that τ c ≅ τR ΔT/θ where τR is the Rouse time of the θ chain : the time τc increases with the depth of quench.

A thermodynamic investigation of micelle formation by a polystyrene- b-polyisoprene block copolymer in n-hexadecane

The thermodynamics of micelle formation by a polystyrene- b-polyisoprene block copolymer in n-hexadecane, which is a selectively bad solvent for polystyrene, has been investigated. The temperature dependence of the critical micelle concentration (c.m.c.) was determined from measurements of scattering intensity against temperature carried out at seven concentrations within the range (1.24–3.25) × 10 −4 mol dm −3. A plot of 1n (c.m.c.) against T −1 gave a value for the standard enthalpy of micellisation of −40.7 ± 2.2 kJ mol −1 of copolymer chains. The standard entropy contribution ( −TΔS ⊖ ) to the standard free energy of micellisation was found to be positive. The results are in strong contrast to those reported for synthetic surfactants in aqueous media below 50°C for which a favourable standard entropy change is the factor mainly responsible for micelle formation.

The thermodynamics of micelle formation by a polystyrene-b-polyisoprene block copolymer inN,N′-dimethylacetamide

The thermodynamics of micelle formation by a polystyrene-b-polyisoprene block copolymer in N,N′-dimethylacetamide, which is a selectively bad solvent for polyisoprene, has been investigated. The micelles have a very narrow size distribution enabling the association process to be treated as a single-stage equilibrium. The temperature dependence of the critical micelle concentration was determined from measurements of scattering intensity against temperature carried out at six concentrations within the range 2.3 × 10−7 to 1.2 × 10−4 mol dm−3. A plot of 1n (c.m.c.) against T−1 gave a value for the standard enthalpy of micellisation (per mole of copolymer chains in the micelle) of − 153 ± 6 kJ mol−1. The standard entropy contribution (-TΔSΘ) to the standard free energy of micellisation was found to be positive. These results are in strong contrast to those reported for synthetic surfactants in aqueous media for which a favourable standard entropy change is the factor mainly responsible for micelle formation.

The concentration dependence of the angular dissymmetry of light scattered by micellar solutions

A light scattering investigation was made of micelles formed from a polystyrene-poly(ethylene/propylene) two-block copolymer in n-nexane. n-Hexane is a selectively bad solvent for polystyrene and so polystyrene blocks formed the cores of the micelles. The light scattering measurements were made at 25°C for concentrations up to 0.015 g cm −3. The dissymetry ratio ( I 45° I 135° ) was found to decrease with increase in concentration and it was below unity for measurements at c > 4.8 × 10 −3 g cm −3. The effect of concentration on the dissymmetry ratio was predicted quite well by assuming the micelle packing could be described by radial distribution functions for hard spheres.

Formation of worm-like micelles from a polystyrene–polybutadiene–polystyrene block copolymer in ethyl acetate

An investigation has been made of micelle formation by a polystyrene–polybutadiene–polystyrene block copolymer in ethyl acetate, which is a selectively bad solvent for polybutadiene; the number-average molecular weights of the three blocks were 12900, 66000 and 13700, respectively. As the temperature was raised dilute solutions of the copolymer in ethyl acetate showed a distinct cloud-point, which moved to higher temperatures with increases in concentration. On raising the temperature further the solutions gradually became clear again.Light scattering and electron microscopy studies showed that the onset of cloudiness was due to the formation of long worm-like micelles from spherical micelles. The clearing of solutions at higher temperatures occurred because the micelles dissociated to form copolymer molecules in the free-chain form. The cores of both types of micelle were composed of swollen polybutadiene blocks and the outer fringes of the micelles of polystyrene blocks. The spherical micelles had a fairly narrow size distribution. The worm-like micelles had a wide distribution of contour lengths but possessed a narrow size distribution in the radial direction.A second polystyrene–polybutadiene–polystyrene copolymer having shorter blocks than the first copolymer showed a similar type of solution behaviour at lower temperaturs.

Intramolecular excimer formation in polymers: Pyrene Labelled Polyvinylacetate

The rate constant for intramolecular excimer formation between pyrenyl side-groups, in a polyvinylacetate chain at a mean separation of 200 bonds, has been measured as a function of molecular weight, solvent viscosity and solvent thermodynamic power. Above M = 1 × 10 5, the rate constant is 1.4 × 10 7 sec −1 in low viscosity bad solvents. This value is about twenty times that for the rate constant of the analogous reaction between the two terminal groups in a chain with a mean end-to-end separation of 200 bonds. Increases of the viscosity and of the thermodynamic power of the solvent depress the rate constant, in agreement with the behaviour expected for a diffusion controlled reaction.

Inverse gas chromatography of poly(cyclohexyl methacrylate)

AbstractInverse gas chromatography is applied to determine the glass transition temperature Tg of poly‐(cyclohexyl methacrylate). Both good and bad solvents for the polymer are used as molecular probes. Although the transition is clearly detected by both types of probes, only the nonsolvents yield a Tg in quantitative agreement with the value determined by differential scanning calorimetry. The relative depth of penetration of the probe in the polymer phase is calculated from retention volume data. Also, the height equivalent to a theoretical plate is calculated from peak halfwidths. Both calculated magnitudes show a temperature dependence which significantly differs from good to bad solvents. Some kind of diffusion‐limited penetration of the probe in the bulk of the glassy polymer may be responsible for the difference.

Investigation of the size distribution of non-ionic micelles formed from a polystyrene–polyisoprene block copolymer in N,N-dimethylacetamide

Solutions of a polystyrene–polyisoprene block copolymer in N,N-dimethylacetamide were studied by gel permeation chromatography. The effect of temperature on the equilibrium between free copolymer chains and micelles was studied qualitatively from 26 to 110°C. Micelles studied in detail at T= 26°C were monodisperse in size, as judged by the reverse flow method. This result was supported by electron microscopy of micelles isolated from solution. Comparison with light scattering results showed that the universal calibration method of calibration was not applicable to the micelles.The application of gel permeation chromatography in a relatively bad solvent to the analysis of a mixture of a block copolymer with a component homopolymer was demonstrated.

Influence of the nature of the solvent on the lamellar crystalline structure of block copolymers with an amorphous block and a crystallizable poly(ethylene oxide) block

AbstractBlock copolymers of styrene and ethylene oxide, (SEO), and of butadiene and ethylene oxide, (BEO), in the presence of a preferential solvent of the amorphous block exhibit always a lamellar structure (LC) with crystallized and folded poly(ethylene oxide) chains at temperatures below about 50°C.To precise the influence of the nature of the solvent on the refolding of the poly(ethylene oxide) chains (PEO), we have studied by X‐ray diffraction and differential scanning calorimetry the LC structure of the copolymers BEO and SEO in the presence of different solvents for the amorphous block. The copolymers SEO were studied in the presence of xylene, toluene and diethyl phthalate. Diethyl phthalate being a bad solvent for polybutadiene, the copolymers BEO were studied only in the solvents xylene and toluene.The influence of the nature of the solvent on the extent of the domain where the LC structure exists, on the refolding, and on the crystallinity of the poly(ethylene oxide) blocks is demonstrated.

Effects of long range interactions on the conformational statistics of short polypeptide chains generated by a Monte Carlo method

Distributions of end-to-end distances and characteristic ratios are calculated for short poly-l-ala and poly-gly chains generated by a Monte Carlo method based on the procedure elaborated by Metropolis et al. Interactions between all the atoms of the chain are taken into account for the conformational energy calculations. The characteristic ratios obtained with the complete set of interaction terms are much smaller than those calculated according to the method of Flory. This collapse of the chain is interpreted as due to a bad solvent. Results in agreement with Flory’s are obtained by modifying the potential functions to simulate the behavior of the chain in a ϑ or even a good solvent. In this case an excluded volume effect appears for poly-gly chains, whereas poly-l-ala chains are too stiff to present this effect with less than 16 residues.

Configurational state of adsorbed chain molecules. Behaviour of terminally anchored chains

The configurational behaviour of an isolated terminally “anchored” chain molecule interacting with a surface has been investigated using a computational statistical technique. The technique involves the simulation of the canonical ensemble of the system by a method analogous to that of Metropolis et al. The present study assumes a lattice model with a square-well potential function for the segment/segment and segment/surface interactions. The solvent effect is incorporated in the model through the segment/segment interactions: it is assumed that good solvents produce repulsions between the segments, while bad solvents lead to attraction. The bonds in the chain assume three conformational states, viz. trans, gauche+ and gauche–, with the trans/gauche conformational energy difference equal to 1 kT. It was found that in a good solvent the molecule exists in an extended state. The outstanding feature of the configuration is long “tail” protruding into the solution phase, and there is a lower tendency for the formation of “loops”. A bad solvent, on the other hand, renders the molecule coiled and promotes the formation of “loops”.

X‐ray investigation of the crystallization behaviour of swollen polyvinyl alcohol

AbstractX‐ray measurements as function of temperature were made from pure polyvinyl alcohol (PVA) and concentrated PVA‐solutions in water, glycerine, and glycol. The solutions form gels at adequate low temperatures. As the formation of a chemically bound network can be excluded gel formation must be related to physical crosslinking. From the results of wide angle X‐ray measurements it is concluded that the gels are partially crystalline systems. In this case the polymer network is a physical one, consisting of crystalline regions connected by macromolecules in the amorphous state. At room temperature the used PVA is about 20% crystalline. By quenching from temperatures above the melting point down to temperatures far below the glass temperature we succeeded in producing approximately amorphous PVA.Water is the best and glycol is the poorest of the agents used for solving PVA. Crystallization and gel formation following from it take place at the lower temperatures the better the solvent is. The differences in the degree of crystallinity of PVA in these solvents is not caused, however, by the solvent itself but mainly by the different gel formation temperatures. Crystallisation and gel formation in bad solvents occur at higher temperatures than in good solvents. The degree of crystallinity of the gels is the same as that of pure PVA samples crystallized at the same temperatures. In the case of glycol the highest degree of crystallinity was found similar to the case of pure PVA, of about 42%.In the wide angle X‐ray diagrams of the gels investigated the positions of the crystal interferences are unchanged. Therefore it can be concluded that the solvents do not enter into crystals to an appreciable amount. The continuous scattering curves of the amorphous part, however, are changed during gel formation. In comparison with the pure polymer the medium distance of the amorphous part of the sample is enlarged and the distance distribution around the medium distance is broadened.

Thermodynamic parameters from osmotic studies on solutions of block copolymers of polyisoprene and polystyrene

A series of ten block copolymers of poly(isoprene:styrene) have been examined by osmometry over a range of temperatures. Several solvents were used; one was a good solvent for both sequences (toluene), another was a bad solvent for both sequences (methyl iso-butyl ketone) and another was a preferential solvent (cyclohexane) which is a good solvent for the polyisoprene sequence but a poor solvent for polystyrene. Flory (or theta) temperatures have been determined for all polymers in the three solvents; a comparison is made with values of θ determined by cloud point methods using methyl ethyl ketone as solvent. The interaction parameter χ 1 is separated into its enthalpic and entropic parts and examined in relation to composition and molecular weight. The molecular weight dependence of the second virial coefficient is also discussed. The inclusion of the composition parameter, in addition to those normally used for homopolymers in these systems, is not adequate for a complete description of solution properties of block copolymers, particularly in the case of preferential solvents where there is ample evidence for intramolecular phase separation.

Physicochemical studies of oligodextran. II. Intrinsic viscosity‐molecular weight relationship

AbstractThe viscosity behavior of dextran in two good solvents and four mixtures of a good and a bad solvent was studied over the molecular weight range of 410 < Mn < 32,000 at 25 and 50°C. In the molecular weight region larger than 2,000, the plot of log [η] versus log Mn gave the usual linear relationship for all solvents used. The constants K and a of the equation [η] = KMa were determined and their dependence on the concentration of the nonsolvent in mixed solvents and on the temperature were discussed. Using the unperturbed dimensions of the dextran molecule estimated from the Stockmayer‐Fixman plots, the value of {〈L〉/〈L〉}1/2 was calculated to be 1.7–1.8. The viscosity behavior for dextran with Mn < 2,000 was qualitatively discussed as a deviation from the randomly coiled model.

Fox‐Flory constant K obtained by viscometric measurements for poly(ethyl methacrylate) and poly(methyl methacrylate) systems

AbstractPoly(ethyl methacrylate) and poly(methyl methacrylate) prepared by benzoyl peroxide‐catalyzed polymerization were fractionated. The Fox‐Flory constant K was determined for these polymers by viscometry in several good and bad solvents. Application of some empirical methods for evaluation of K are also briefly discussed in relation to our results.

Dilute solution properties of the polyether from bisphenol ‘A’ and 4,4′ dichlorodiphenyl sulphone

Dilute solution properties of Union Carbide polysulphone have been measured by light scattering, osmometry, and viscometry; Mark-Houwink expressions have been determined in a thermodynamically good solvent, two thermodynamically bad solvents, and one solvent at the θ-temperature. The following relationships have been obtained: [ν] ( ml/gm)= 0·145× M ̄ W 0·50( dimethylsulphoxide, 105·5°) [ν] ( ml/gm)= 0·103× M ̄ W 0·55( dimethylformamide, 25°) [ν] ( ml/gm)= 0·079× M ̄ W 0·58( tetrahydrofuran, 25°) [ν] ( ml/gm)= 0·024× M ̄ W 0·72( chloform, 25°) The values of the unperturbed dimensions, ( < r o 2 > / M) 1 2 solvent-polymer interaction parameters ( B and Xi) and conformational parameter (σ) have been computed by applying the theories of Stock-mayer-Fixman-Burchard, Flory-Fox-Schaefgen, Kurata-Stockmayer-Roig, Ptitsyn and Inagaki-Suzuki-Kurata. Because of uncertainty in the values of the various bond angles in the polymer chain the exact value of σ cannot be given, but it must be close to unity. This low value is not unexpected, it is shared by other polymers containing X-Ph-X links.

Some properties of low molecular weight polybutadiene and polytetrahydrofuran

AbstractThe proposition, that low molecular weight polymer fractions in good solvents behave as if they were under ⊖ conditions, has been examined experimentally. Series of monodisperse hydroxy‐terminated polytetrahydrofuran (PTHF), 82% 1,4‐polybutadiene (PBD), and 30% 1,4‐PBD were prepared, and values of M̄n obtained by vapor‐pressure osmometry and endgroup analysis. The Mark–Houwink viscosity parameters K and ν were determined in a number of solvents. The general conclusion is that the proposition is invalid for these systems notwithstanding the fact that ν = 0.50 for one of them [82% 1,4‐PBD in methyl ethyl ketone (MEK) at 25°C]. For this particular case, the following evidence suggests that these are actually ⊖ conditions so that the apparent fulfilment of the proposition is fortuitous. (1) Cloud‐point precipitation yields ⊖ = 26 ± 3°C in MEK. (2) The value of K is close to that of K⊖ found elsewhere for PBD in a different solvent at a similar temperature. (3) Application of the Kurata‐Stockmayer iterative procedure for estimating K⊖ from data in good and bad solvents yields a reasonably small discrepancy (10%) between the K⊖ values from data in toluene and MEK at 25°C for this polymer and only a 3% difference in the unperturbed dimensions (〈r02〉/M)1/2 derived from them. Measured melting points Tm of PTHF (M̄n = 1000–13000), plotted as a function of chain length Z, viz., 1/Tm = 1/Tm0 + 2R/ZΔHf, yield 43 ± 3°C and 1.6 kcal/submole, respectively, for the limiting melting point Tm0 and the heat of fusion ΔHf. The former is in good agreement with the value obtained dilatometrically for high molecular weight polymer, while the latter indicates a degree of crystallinity of ca. 54%.

Polyethylmethacrylate in dilute solution

AbstractPolyethylmethacrylate (PEM) was prepared by bulk polymerization and fractionated using acetone/acetone‐water (2:1 V/V) mixture. The solution properties of fractionated as well as unfractionated polymers were investigated in various solvents (ethylacetabte, toluene, ethylacetate‐ethanol) by light scattering and viscosity at 35°C. Osmotic pressure measurements were also carried out on selected polymer samples for the evaluation of width of distribution. [η]–Mw and (r2)1/2–Mw relationships for these systems were established. Ethylacetate was found to be a bad solvent compared to toluene and methylethyketone. In the case of unfractionated PEM, the parameters like [η]2/3M1/3, Mw/[η], and Φ were corrected for molecular weight heterogeneity assuming SCHULZ‐ZIMM distribution. For the evaluation of FLORY's constant K, and the unperturbed dimension (r)1/2 of the polymer, FLORY‐FOX, KURATA‐STOCKMAYER, and STOCKMAYER‐FIXMAN methods were employed. The K and (r)1/2 values evaluated from the FLORY‐FOX method were almost similar to those measured in a Θ solvent, while the other two methods yielded higher values. A comparison of the measured (r2) and that calculated from KIRKWOOD‐RISEMAN and DEBYE‐BUECHE theories indicated the polymer coil as conforming to a statistical chain. The molecular weight dependence of A2, the second virial coefficient, for PEM/ethylacetate was established and the value of A2 compared well with that calculated from OROFINO‐FLORY equation.

Isotactic poly‐pentene‐1: Polymorphism and some physical properties

AbstractPreliminary results of a physical study of highly isotactic poly‐pentene‐1 are here reported.This polymer, although crystallizable to a high degree, can be fractionated by a normal fractional precipitation technique. This is due to its capacity of giving metastable solutions in bad solvents, and therefore a liquid‐liquid separation, owing probably to its relatively low crystallization rate. Fractions were thus prepared for further experiments.Isotactic poly‐pentene‐1 is found to be able to crystallize into two modifications: modification 2, so far known, having melting temperature at 80°C.; and the apparently more stable modification 1, with a melting temperature regularly affected by molecular weight, but exceptionally dependent on crystallization temperature. Its maximum melting temperature is estimated, although in a non definite way to be around 130°C.X‐rays diffraction diagram and volumetric and dilatometric data of the two pure modifications are given.The volume‐temperature relationship of the amorphous isotactic phase is eventually drawn from experimental data and three dilatometric isophasic transitions are individualized at −52°, −10° and +13°C.