Temperature dependence of dynamics of solutions of triblock copolymer in a selective solvent

The temperature dependence of dynamic properties of solutions of a triblock copolymer with polystyrene outer blocks and a middle block of fully hydrogenated polybutadiene (M_w=7.0×10⁴ gmol⁻¹, mass fraction of PS 0.28) was studied in a selective solvent for the middle block, n-heptane. In dilute solutions, a gradual decomposition of the flower-like micelles was observed on heating. A more complex behavior was observed in semidilute solutions of the copolymer where three dynamic processes can be reliably extracted from the correlation functions. The fast mode corresponds to the collective diffusion mode of physically interconnected micelles, the broad middle mode is probably due to the tracer diffusion of polydispersed clusters (animal-like structures) formed by random association of the triblock copolymer and the slow mode is probably related to the diffusion of aggregates of insoluble polystyrene impurities. The crossover from relaxational to diffusive behavior was found for samples with c>5.5% (w/v). The scattered light intensity measurements revealed a phase transition for the solution with the concentration 4% (w/v) at a temperature of 79±2°C, which was related to the phase separation of polystyrene homopolymer present in a small amount in the copolymer used.     

Temperature dependence of probe diffusion in gels

We present a dynamic light scattering study of the temperature dependence of the dynamics of probe diffusion in polyacrylamide gels and pregel solutions as a function of crosslink content and scattering angle. Our results show that the spinodal transition in polyacrylamide gels affects the movement of the probe particles. The dynamical behaviour ranges from overdamped vibration of particles controlled by the gel elasticity (relaxational behaviour) at low temperatures to the free diffusion of particles controlled by the sol viscosity at higher temperature. The decay rate of the relaxational mode decreases upon approaching the spinodal temperature, and this decrease is even more pronounced than that of gels without probe particles. Our results suggest that the friction felt by the 50 nm probe particles as they move in the gel appears to diverge at the spinodal temperature, scaling as ≈(T−T_s)^−1.5. This behaviour is observed over the entire temperature range for probe diffusion in a well gelled sample, whereas in a sample closer to the gel threshold the strong temperature dependence is observed only at low temperatures, the higher temperature behaviour being the same as that in pre-gel solutions. We propose an interpretation of the experimental data in terms of a simple theory of Brownian particles harmonically bound to the gel network.     

Synthesis and characterization of temperature and pH-responsive pentablock copolymers

A family of amphiphilic ABCBA pentablock copolymers based on commercially available Pluronic^® F127 block copolymers and various amine containing methacrylate monomers was synthesized via Cu(I) mediated controlled radical polymerization. The block architecture and chemical composition of the pentablock copolymers were engineered to exhibit both temperature and pH responsive self-assembly by exploiting the lower critical solution temperature of the poly(ethylene oxide)/poly(propylene oxide) blocks and the polycationic property of the poly(amine methacrylate) blocks, respectively. In aqueous solutions, the pentablock copolymers formed temperature and pH-responsive micelles. Concentrated aqueous solutions of the copolymer formed a pH-responsive, thermoreversible gel phase. The controlled radical synthesis route yielded well-defined copolymers with narrow molecular weight distributions with the benefit of mild reaction conditions. Small angle X-ray scattering, laser light scattering, cryogenic transmission electron microscopy and dynamic mechanical analysis have been used to characterize the self-assembled structures of the micellar solution and gel phases of the aqueous copolymer system. These copolymers have potential applications in controlled drug delivery and non-viral gene therapy due to their tunable phase behavior and biocompatibility.     

Static and dynamic light scattering study of strong intermolecular interactions in aqueous solutions of PVP/C60 complexes

The investigations of the dilute aqueous solutions of polyvinylpyrrolidone/fullerene (PVP/C₆₀) complexes by static (SLS) and dynamic (DLS) light scattering showed that strong intermolecular interactions, effective on the distances of about 45-50 nm, take place in the solutions. Two concentration ranges are well distinguished in these solutions. Above a critical concentration (c_cr) the fluctuations in the solutions are hindered and only one, diffusive, mode is observed in DLS experiments. Upon dilution (c<c_cr) this unified structure divides into large fragments (domains) and the slow mode attributed to long-range concentration fluctuations, gradually appears. The angular and concentration dependencies of the diffusion coefficient of the slow mode indicate the existence of strong intermolecular interactions.     

Compatibilization efficiency of styrene‐butadiene block copolymers as a function of their block number

Effect of block number in linear styrene‐butadiene (SB) block copolymers (BCs) on their compatibilization efficiency in blending polystyrene (PS) with polybutadiene (PB) was studied. Di‐, tri‐, or pentablocks of SB copolymers as well as their combinations were blended with the mentioned homopolymers; supramolecular structure determined by small angle X‐ray scattering method (SAXS), morphology using scanning electron microscopy (SEM) combined with image analysis (IA), and stress transfer characteristics of the blends were chosen as criteria of compatibilization efficiency of the copolymers used. It was proved that the addition of SB BCs led to remarkably finer phase structure and substantially higher toughness of PS/PB blends. Triblock copolymer showed to be the compatibilizer with higher efficiency than diblock, pentablock, and the di/triblock copolymer mixture. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Comparison of the gelation dynamics for polystyrenes prepared by conventional and living radical polymerizations: a time-resolved dynamic light scattering study

The structure and dynamics for the two types of polystyrene gels prepared by conventional and living radical polymerizations have been investigated by time-resolved dynamic light scattering (TRDLS). The reaction was initiated with the monomer solutions with and without a reversible addition-fragmentation chain transfer (RAFT) agent and was monitored through the gelation process. In the absence of a cross-linker, the reaction process was characterized by the appearance of a maximum when the excess scattering intensity divided by concentration was plotted as a function of reaction time. The intensity maximum was ascribed to an increase in the molecular weight and subsequent suppression of the concentration fluctuations, which was qualitatively reproduced by the Flory-Huggins free energy formula. As the concentration became higher, the time-intensity correlation functions (ICFs) exhibited two modes, suggesting the formation of larger aggregates in addition to the cooperative mode. The intensities were successfully decomposed into the two components by taking account of the relative amplitude of the fast and slow modes. The difference of the time-course between the two types of polymerizations was clearly detected by combining TRDLS and gel permeation chromatography, GPC, data. For the cross-linking system, the kinetics was characterized by the universal gelation mechanism, namely: (1) divergence of the slow mode and (2) appearance of the power law in ICF around the gelation threshold regardless of the polymerization method. On the other hand, the real time intensity decomposition analysis revealed striking differences in the gelation mechanism between the two types of polymerization. For example, the slow mode appeared at an early stage of reaction for the conventional system and sustained its amplitude through the gelation process. For the living (RAFT) system, the appearance of the slow mode was limited in a narrower range of the reaction time with a much smaller amplitude.     

Ultrasonic studies of styrene-butadiene-styrene triblock copolymers

Ultrasonic measurements covering the frequency range 5–1000 MHz are reported on solid styrene-butadiene-styrene triblock copolymers and their solutions in toluene and cyclohexane. In the solid, two distinct relaxation processes were observed, corresponding to the glass transitions of the polystyrene and polybutadiene phases. Two distinct processes were observed also in the swollen solid, the relaxation peaks being shifted to lower temperatures with plasticization of the polymer by the solvent. Comparison of the changes observed with those detected in a similar mixture of the corresponding homopolymers confirms the importance of phase separation in the swollen copolymer. An increase in the high frequency attenuation of semi-dilute solutions can be associated with scattering of the sound wave by micelle structures. The temperature-concentration locus at which this scattering is first observed correlates with other observations of microphase separation in triblock copolymers. At these concentrations, the low frequency relaxation curves deviate from extrapolation from dilute solutions data indicative of significant polymer-polymer entanglement interaction.     

Synthesis,aggregation and adsorption behaviour of a thermoresponsive pentablock copolymer

Poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer (Pluronic F127) was modified by introducing poly(N‐isopropylacrylamide) (PNIPAM) at both the PEO ends, and the pentablock copolymer (PNIPAM₄₁–F127–PNIPAM₄₁, PN41) so prepared was characterized using gel permeation chromatography and ¹H NMR spectroscopy. The degree of polymerization of NIPAM blocks at the two ends was 41. The solution behaviour and microstructure of PN41 aggregates in water were examined using UV–visible spectroscopy, micro‐differential scanning calorimetry and small‐angle neutron scattering (SANS) and compared with F127. Two lower critical solution temperatures (LCSTs) were observed for the pentablock copolymer, corresponding to PPO and PNIPAM blocks, respectively. The adsorption of PN41 on thiol‐grafted hydrophobic gold surfaces at various temperatures was investigated using a quartz crystal microbalance. It was found that the adsorption behaviour and mechanism of PN41 were mainly determined by the interactions of the pentablock copolymers with different chain conformations in dilute aqueous solutions at various temperatures. SANS measurements were used to determine the temperature‐dependent structural evolution of polymer micelles in aqueous solution. A NOESY study revealed that above the LSCT of PNIPAM, the interaction of PPO and PNIPAM protons increases and the distance between PPO and PNIPAM decreases. © 2019 Society of Chemical Industry     

Dynamic light scattering of dilute PVA-borax aqueous solutions

Poly(vinyl alcohol)-borate (PVA-borate) complexes in dilute aqueous solutions were investigated by dynamic light scattering (DLS). Two relaxation modes obtained, were scattering vector-q² dependent and diffusive. The amplitude of slow mode was independent of borax concentration. In the very dilute PVA concentration regime ( [PVA] < 5 g/L), the PVA di-diol-borate complexation was dominated by intra-molecular reaction, and the fast mode correlation length ς_f which corresponds to the polymer chain dimension increased rapidly with borax concentrations lower than 0.06 M, and reached an asymptote in the higher borax concentration regime. However, for a dilute PVA aqueous solution with higher PVA content (i.e., [PVA] = 9 g/L), both intra-and inter-molecular PVA di-diol-borate complexations might happen. The chain expansion and shrinkage of PVA-borate complex with increasing borax concentration was observed due to the balance between the electrostatic repulsion of the charged diol-borate complexes and the intra-molecular crosslink induced by the intra-molecular di-diol-borate complexation.     

Diffusion and solution of gases and vapors in styrene-butadiene block copolymers

The diffusion, solution, and permeation behavior of a series of inert gases (helium, argon, nitrogen, krypton, and xenon) in S-B block copolymer films was studied by transient permeability measurements and by the equilibrium desorption method. The morphologies of most of the samples used in the measurements were (a) polystyrene rods dispersed in a polybutadiene matrix and (b) alternating lamellae of styrene and butadiene components. It was indicated, as far as the kinetic nature at lower temperatures is concerned, that the diffusion and permeation processes of gases, except for helium, are governed primarily by behavior in the polybutadiene matrix. At lower temperatures, it was shown that the transient method counts only the mobile penetrant in the polybutadiene matrix, while the equilibrium method counts less diffusive species in the polystyrene domains as well. The diffusion behavior in the copolymer films was compared with that in homopolybutadiene and discussed in terms of two impedance factors: the tortuosity and the chain immobilization factors. From the homopolymer-block copolymer comparison along with results obtained from diffusion experiments using n-hexane as the penetrant, it was indicated that segmental motions in the polybutadiene phase in the copolymers are restricted relative to motions in homopolybutadiene. Also, from data on gas sorption in samples of various styrene contents, involving both S-B block copolymers and binary mixtures with homopolystyrene, it was suggested that the partial mixing of component block chains occurs at the interface between the domains, resulting in rather diffuse domain boundaries.     

Sulfonamide functionalized amino acid-based pH- and temperature-sensitive biodegradable injectable hydrogels: Synthesis,physicochemical characterization and in vivo degradation kinetics

In this research, a novel pH and temperature-sensitive biodegradable oligomer serin-b-poly(lactide)-b-poly (ethylene glycol)-b-poly(lactide)-b- oligomer serin (OS-PLA–PEG–PLA-OS) pentablock copolymer was synthesized with modification of serine to sulfonamide group. The properties of the different pentablock copolymer structures were detail characterized. The copolymer solution easily flowed at high pH and changed to gel state at physiological conditions (37°C, pH 7.4). The sol to gel behavior of these block copolymer solutions were controllable by tuning the length of PEG segment, PLA/PEG block ratio, the molecular weight and the concentration of the pentablock copolymer. The in vivo gelation of the copolymer solution was investigated. The stable gel was formed after injection into the back of the adult male Mus musculus Swiss Albino mice. The degradation process was observed after 6 weeks and showed that the hydrogel depot was bioresorbable without any detection of inflammation sign. The results in this study suggest that the OS-PLA–PEG–PLA-OS pentablock copolymer has the potential application as an injectable delivery vehicle for therapeutic drugs that easily denatured at low-pH condition.     

Dynamics of ESIPT fluorescent methylmethacrylate-benzazole dye copolymers

The dynamics of fluorescent methylmethacrylate-benzazole dye copolymers were investigated in different concentration regimes by dynamic light scattering. In the dilute regime the polymer behaves as typical polydisperse linear chains in good solvent with a dynamics dominated by a single fast mode. In the semidilute regime, the cooperative diffusion coefficient, D_coop and the correlation length, ξ could be obtained. Above the semidilute regime the intensity autocorrelation functions show two-step decays, indicating the existence of low range correlations. The dye incorporation, even though small, affects the copolymer dynamics behavior in concentrated solutions if compared to PMMA, which is probably ascribed to a polymer-solvent interaction.     

Motional heterogeneity of polystyrene-block-polybutadiene: a spin probe study

Polystyrene-block-polybutadiene copolymers (SB) with 0.5 mass fraction of styrene were studied by electron spin resonance (ESR) of nitroxide spin probes. The influence of the block length ( and ) and the solvation power of casting solvents on the motional dynamics of spin probe were measured over a wide temperature range. Two nitroxide radicals as spin probes were selected: 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl benzoate (BzONO) and 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl (Tempol). Irrespective of the spin probe used two ESR spectral components differing in their motional properties above the phase transition of polybutadiene blocks (PB) were observed. The fast component was assigned to spin probes located in polybutadiene-rich domains and the slow component to spin probes in polystyrene-rich domains. The range of two spectral components and the phase transition of the slow ESR component, T_5mT, depend on the block length. The influence of the interphase and accumulation of free volume in the interphase on the Tempol probe motion was investigated by changing copolymer morphology in the films casted from selective and nonselective solvents. The analysis of the motional heterogeneity from the ratio of the fast and slow motional component presents evidence that in the selective solvent for polystyrene (PS) blocks (2-butanone) the most irregular structure with a large interphase is formed. The difference in fast motion of spin probes indicates that the motional dynamic is related to the change of domain structure.     

Small‐angle neutron scattering of poly(styrene)/poly(acrylic acid–ethyl acrylate) copolymers: The effect of the degree of hydrolysis of the poly(acrylic acid–ethyl acrylate) block

We report the results of systems based on polystyrene‐poly(ethyl acrylate) (PEA) diblocks, which self‐assemble in aqueous solutions to form spherical micelles. Previous work has shown that the rheological properties of these solutions, in particular the gel–liquid transition, can be tuned through the use of a simple hydrolysis reaction to convert PEA to poly(acrylic acid) (PAA). We studied the effect of the extent of hydrolysis on the self‐assembly and micellar interactions. Small‐angle neutron scattering (SANS) spectra were fit with a variety of models to determine the micelle structure. As more PEA was converted to PAA (i.e., as the corona became more charged and more hydrophilic), the micellar aggregation number decreased, analogous to observations of other polymeric micelles. This effect could impact the gel–liquid transition and rheology in this system and in similar micellar block copolymer gels. Finally, our SANS spectra qualitatively agreed with predictions for attractive colloidal glasses, confirming the idea that the elasticity of these gels arises from the jamming of micelles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 490–497, 2004     

Phase separation kinetics and morphology in a polymer blend with diblock copolymer additive

The phase diagram for a low molecular weight blend of deuterated polystyrene (PSD) and polybutadiene (PB) was determined by temperature jump light scattering (TJLS) measurements and phase contrast optical microscopy (PCOM). The PSD/PB blend exhibited upper critical solution temperature behavior, and the critical temperatures measured by these two techniques were consistent. Upon addition of 0 to 0.12 mass fraction of a comparable molecular weight PSD-PB symmetric diblock copolymer, a linear decrease in the phase transition temperature was observed with increasing diblock copolymer content. At a constant, shallow quench depth, the kinetics of phase separation via spinodal decomposition as measured by TJLS were greatly retarded by the presence of the copolymer. Additionally, the time dependence of the concentration fluctuation growth did not seem to follow a universal functional form anywhere in the accessible q range when the diblock was present. The results of morphology study of the blends in the late stage of phase separation by PCOM also indicated that the phase separated domain sizes did not grow to the same size for a given annealing time as diblock content increased.     

In situ graft copolymerisation: salami morphologies in PMMA/EP blends: part I

Rubber-modified polystyrene (PS) owes its impact resistance to a morphology of salami domains: in the PS matrix, polybutadiene (PB) domains are dispersed that, in turn, are strongly filled with PS subdomains. This unique structure is created in situ as styrene is polymerized in PB/styrene solutions. The salami domains are built up by graft copolymer chains PB-g-PS. It was suggested previously that these domains assume their characteristic architecture because strongly grafted chains stabilize them on the outside while weakly grafted chains provide their internal substructure. This model is extended in this paper that deals with the polymerisation of methylmethacrylate (MMA) in EP/MMA solutions of a poly(ethylene-co-propylene) copolymer (EP). Intermediate and final products were characterized by ¹H NMR spectroscopy, transmission electron microscopy, two-dimensional chromatography and statistical calculations, with a focus on the pivotal stage of phase inversion during the polymerisation where the salami domains are born.     

Dynamical and structural behavior of hydroxyethylcellulose hydrogels obtained by chemical gelation

Rheological, dynamical, de‐swelling, turbidity, and structural features during gelation with a chemical cross‐linking agent of semi‐dilute aqueous solutions of hydroxyethylcellulose (HEC) were investigated at different cross‐linker concentrations. A higher level of cross‐linker leads to faster gelation and stronger incipient hydrogel. The rheological results favor the percolation model. The cross‐linking reaction proceeds over a long time in the post‐gel region, and this produces gels with a solid‐like response. Dynamic light‐scattering experiments reveal a fast and a slow relaxation mode. The slow relaxation time increases and the sample becomes more heterogeneous in the course of gelation. In the post‐gel region, the gels shrink during a long period of time and this phenomenon is accompanied by strong turbidity enhancement. Small‐angle neutron scattering results from samples quenched at a certain stage in the post‐gel regime disclose growth of heterogeneity in the gel with increasing level of cross‐linker addition. At smaller length scales, no effect of cross‐linker addition could be detected on the structural organization, and scattered intensity in this domain suggests that the HEC chains are locally stretched. Copyright © 2006 Society of Chemical Industry     

Silica reinforced triblock copolymer gels

The effect of silica and polymer coated silica particles as reinforcing agents on the structural and mechanical properties of polystyrene-poly(ethylene/butylene)-polystyrene (PS-PEB-PS) triblock gel has been investigated. Different types of chemically modified silica have been compared in order to evaluate the influence of the compatibility between gel and filler.Time-resolved SANS and small-angle X-ray scattering (SAXS) shows that the presence of silica particles affects the ordering of the polystyrene domains during gelsetting. The scattering pattern of silica-reinforced gels reveals strong scattering at very low q, but no structure and formfactor information. However, on heating above the viscoelastic to plastic transition, the ‘typical’ scattering pattern of the copolymer gel builds-up. All reinforced gels are strengthened by the addition of the reinforcing agent. The transitions from a viscoelastic rubber to a plastic fluid and from a plastic fluid to a viscoelastic liquid are shifted to more elevated temperatures when silica is added to the triblock copolymer gel.     

Small angle neutron scattering from ionomer gels

Small angle neutron scattering intensities for sols and gels of the physically associating ionomer 1.39 mol% sodium sulphonated polystyrene with molecular weight 10⁵ g/mole in xylene have been obtained over a broad wavevector (q) and concentration range. In the low q and concentration range the scattering behaviour of this ionomer/solvent system can quite readily be interpreted using the open association aggregation model. In more concentrated solutions and at higher q, however, interpretation of the scattering behaviour for polymers associating via an open association mechanism (OAM) is more difficult, particularly if, as in this investigation, the single chains and aggregates have varying densities and fractal parameters. In this study various methods have been developed to interpret the low and high q scattering from systems whose extent of aggregation can be modelled using the OAM. Using these methods it has been possible to confirm that the open association model can be used to interpret the extent of aggregation of the above ionomer in xylene even after the solutions appear to be gelled. Single ionomer chains within both the dilute solutions and gels were found via modelling to have a radius of gyration of 60 Å, which compares with dimensions of 25 Å and 93 Å calculated for a solid sphere of polystyrene or an unperturbed polystyrene Gaussian coil, respectively. The aggregates, however, all had radii of gyration comparable with what would be expected for polystyrene of the aggregate molecular weight in an unperturbed state. These results suggest that gelation of ionomer solutions at particular concentration thresholds is not due to an abrupt change in the aggregate structures at some critical concentration but occurs as a result of interactions between the very large aggregates that the OAM predicts should gradually form as the ionomer concentration increases.     

Phase diagram of polymer blends containing block copolymers

The phase transition and phase separation behavior occurring in mixtures containing an A–B block copolymer and an A homopolymer is discussed. With a pure block copolymer an order-disorder transition can be induced by raising the temperature, whereby the ordered lattice of segregated microdomains becomes unstable and gives way to a homogeneous liquid structure. Small amounts of a homopolymer added to a block copolymer can be accommodated in the microdomains consisting of the same type of monomeric units, up to a solubility limit that depends on the relative lengths of the homopolymer and the copolymer block and on the temperature. The order-disorder transition temperature of the block copolymer is also affected by the added homopolymer. At the other extreme of concentration, spherical micelles of block copolymer are formed when a small amount of the copolymer is added in the bulk homopolymer, and the critical micelle concentration again depends on the relative lengths of the molecules and blocks involved and on the temperature. Measurements were made with light scattering and small-angle X-ray scattering techniques to determine the phase behavior of mixtures containing a styrene-butadiene block copolymer and either a polystyrene or a polybutadiene. The resulting phase diagram exhibits a fascinating complexity. Comparison with recent theories treating these phenomena shows that a good agreement is generally obtained on a qualitative or semi-quantitative level, but a quantitative agreement is often not attained.