Shear Induced Demixing and Rheological Behavior of Aqueous Solutions of Poly(N‐isopropylacrylamide)

The interrelation between the phase separation behavior and the rheological performance of aqueous solutions of high molecular weight (M _w = 1 600 kg/mol) poly(N‐isopropylacrylamide) was investigated. The system demixes upon heating and the cloud point temperature, T_cp decreases steadily with rising polymer concentration up to 10 wt.‐%. The application of shear supports phase separation and reduces T_cp markedly. This observation is interpreted in terms of destruction of intersegmental clusters formed in the quiescent state owing to favorable interactions. Intrinsic viscosities and Huggins coefficients as well as the viscosities, η at higher polymer concentrations are closely connected with the thermodynamic conditions. [η] decreases by almost two orders of magnitude upon heating, whereas the corresponding increase of k_H is less pronounced. The η values (constant shear rate) of the moderately concentrated solutions as function of T pass a maximum at the corresponding phase separation temperatures. The existence of clusters also manifests in terms of stress overshoot and of particularities observed with solutions that are sheared for the first time.

Transmittance (ratio of intensities of the transmitted light and the incidence light) of a 2.0 wt.‐% PNIPAm solution in water as a function of temperature at different shear rates indicated. Heating rate is 0.22 °C/min.     

Phase Morphologies and Viscoelastic Relaxation Behaviors for an LCST‐Type Polymer Blend Composed of Poly(methyl methacrylate) and Poly[(α‐methyl styrene)‐co‐acrylonitrile]

Summary: Small amplitude oscillatory shear rheology is employed to investigate the linear viscoelastic behavior of the LCST‐type PMMA/α‐MSAN polymer blends as a function of annealing temperature and time. The results reveal that when temperature approaches the separation temperature, the blends exhibit some characteristic of complex thermorheological behavior, such as a shoulder in the dynamic storage modulus G′ or the linear relaxation modulus G(t), the appearance of a loss tangent (tan δ) peak and the additional relaxation in the relaxation spectrum H(τ). All of these can be attributed to the enhanced concentration fluctuations near the phase boundary. The anomalous pretransitional behavior can be quantified to yield the binodal temperature from the inflexion of variation and the spinodal temperature on the basis of the mean field theory. Furthermore, the linear viscoelastic properties of the phase‐separated PMMA/α‐MSAN (80/20) blends can be described well by either Palierne's or Bousmina's emulsion model in virtue of the calculated value in the whole frequency region.

Morphology images of 80/20 PMMA/α‐MSAN blends annealed for 4 h at 180 °C observed from phase contrast microscope.     

Nano‐Level Mixing of ZnO into Poly(methyl methacrylate)

A simple, facile and versatile approach is presented for the preparation of PMMA/ZnO nanocomposite materials, which possess high transparency, no color, good thermal stability, UV absorption and improved mechanical properties. The employed process involved mixing of ZnO nanoparticles dispersed in DMAc with the PMMA matrix dissolved in the same solvent. The effect of ZnO content on the physical properties of the PMMA matrix is studied. A significant improvement in mechanical properties was observed with the incorporation of 0.5 wt.‐% ZnO particles. The beauty of the described approach lies in the fact that despite being a simple and facile approach, it offers nano‐level (2–5 nm) mixing of ZnO nanoparticles into a polymer matrix.

     

Spinodal Decomposition in Binary Blend of Poly(methyl methacrylate)/Poly(α‐methyl styrene‐co‐acrylonitrile): Analysis of Early and Late Stage Demixing

Summary: The phase separation kinetics of a poly(methyl methacrylate), PMMA, and poly(α‐methylstyrene‐co‐acrylonitrile), PαMSAN, blend exhibiting a LCST‐type phase diagram have been investigated as functions of temperature and demixing time for the near critical composition (PαMSAN/PMMA = 25:75) using a time‐resolved light scattering technique. We found that the scattering data in the early stage spinodal decomposition (SD) can be well described by the linearized Cahn–Hilliard theory. Spinodal temperature T_s ～ 171 °C was determined from D_app versus T and qequation/tex2gif-stack-1.gif versus T based on the analysis of Cahn theory in the early stage SD, where D_app is the apparent diffusion coefficient and q_m is the scattering vector at the maximum scattering intensity. The value of T_s obtained from the analysis of light scattering data was in good agreement with the phase diagram obtained visually at the equilibrium condition. The LCST‐type phase diagram of this blend was also calculated using the Flory–Huggen theory. The estimated interaction parameter used for the calculation of the phase diagram was found to be composition and temperature dependent. The coarsening behavior of the blend at the late stage SD was also studied by analyzing the magnitudes of q_m and I_m at various demixing times and temperatures based on the nonlinear statistical theories. Both of q_m and I_m obtained at different times and temperatures can be superposed and reduced to the respective master curves by horizontal shifting when they are plotted against log (t/a_T) at a given reference temperature, where a_T is the temperature‐dependent shift factor. The shape of the phase separation domain structures as a function of time during the SD was also considered. The scaling structure function F(x,t;T) = q_m(t;T)³I(q,t;T) versus q/q_m was found to be time independent and falls onto a universal curve in the late stage SD as a result of the dynamical self‐similarity accompanying with the phase separation process.

Optical microscopy pictures for the structure development of spinodal decomposition for PαMSAN/PMMA = 25:75 blend at 180 °C for different demixing times.     

Effect of Blend Composition on Scratch Behavior of Polystyrene/Poly(2,6‐dimethyl‐1,4‐phenyleneoxide) Blends

The relationship between the scratch behavior and molecular aggregation states of polystyrene (PS), poly(2,6‐dimethyl‐1,4‐phenyleneoxide) (PPO), and their blends, is investigated based on differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), polarized optical microscopy (POM), and indentation and scratch tests. DSC reveals that all the PS/PPO blends show the single glass transition temperature (T_g) and the T_g monotonically increase and T_g breadth exhibits a maximum, with an increase in PPO content. Furthermore, density and intermolecular chain distance obtained by WAXD exhibits maximum and minimum values at near 50 wt% of PPO, respectively. It is evident that densification occurs by blending PS and PPO. The scratch coefficient of friction (SCOF) value of PS is the largest and PS exhibits a fish‐scale pattern after scratch testing, while the SCOF value of PPO is much smaller than PS and PPO exhibits smooth groove formation. The PS50/PPO50 and PS20/PPO80 blends exhibit superior scratch and indentation resistance than PS and PPO. Damage morphology observation by POM and indentation tests reveals that molecular orientation is more restricted, and resistance against indentation increases for blends. This is due mainly to densification of the blend system.     

The Control of Silica Nanoparticles on the Phase Separation of Poly(methyl methacrylate)/Poly(styrene‐co‐acrylonitrile) Blends

The influence of silica nanoparticles on the lower critical solution temperature (LCST) phase behavior and phase‐separation kinetics of a blend consisting of poly(methyl methacrylate) (PMMA) and poly(styrene‐co‐acrylonitrile) (SAN), is studied via a high‐throughput experimentation (HTE) approach, which combines a composition (φ) and a temperature (T) gradient. The evolution of the phase‐separation process is studied by optical microscopy (OM), small‐angle light scattering (SALS), and transmission electron microscopy (TEM). Depending on the specific interaction between the silica surface and the polymers, the distribution of silica particles during phase separation can be controlled to be either in one of the polymer phases or at the PMMA/SAN interface. The hydrophilic silica nanoparticles preferentially migrate to the PMMA phase, leading to a slow down of the coarsening rate, which may be related to a reduction of the mobility of PMMA due to an increase of the silica concentration. The hydrophobic silica nanoparticles are localized at the PMMA/SAN interface, and the inhibition of coalescence corresponds to the presence of a solid barrier (the nanoparticles) between the polymers, which prevents the coarsening process.

     

Enthalpy and Volume Relaxation of Core‐Crosslinked Star Polystyrene/Poly(methyl methacrylate) Blends

Blends of linear and core‐crosslinked star (CCS) polymers are prepared. The relaxation and thermal properties of the blends are determined using conventional and modulated‐temperature DSC and modulated‐temperature thermomechanometry. Addition of CCS polymers increase the glass transition temperature while decreasing the enthalpy and the linear coefficient of thermal expansion, suggesting that the hyperbranched polymers restrict matrix chain motions. Isothermal annealing increased the T_g and ΔH and decreased sub‐T_g α due to the reduced free volume and mobility within the polymer films. Volume relaxation during annealing is observed using mT‐TM, while a stretched‐exponential function is utilised to interpret the data.

     

A Miscible and Adaptive Poly(methyl acrylate)/Polystyrene Blend Formed by Multiple‐Responsive Host–Guest Interactions

Polymer blends play a significant role in polymer science due to their new and unique properties compared with the individual polymer components. However, most polymer blends are immiscible and not adaptive to environmental stimuli. Here, an enhancement of the compatibility of an immiscible poly(methy acrylate)/polystyrene blend based on multiple‐responsive benzo‐21‐crown‐7/dialkylammoniumsalt host–guest interactions is made. Owing to the multi‐responsiveness and reversibility of benzo‐21‐crown‐7/dialkylammoniumsalt host–guest interactions, the polymer blend recovers its compatibility after being damaged by environmental factors, indicating its adaptiveness.

     

Influence of Short‐Chain Branching of Polyethylenes on the Temperature Dependence of Rheological Properties in Shear

This contribution describes the influence of short‐chain branching on the temperature dependence of rheological properties of polyethylene (PE) melts in shear. The materials investigated are linear and short‐chain branched, metallocene‐catalyzed PEs of narrow molecular mass distribution. The linear viscoelastic properties are determined by dynamic‐mechanical analysis. Short‐chain branching (SCB) leads to an increase of the flow activation energy. The activation energy was found to increase linearly with rising weight comonomer content.

     

Microwave‐Induced Shape‐Memory Effect of Chemically Crosslinked Moist Poly(vinyl alcohol) Networks

MWs are explored as the driving force for an SMP based on chemically crosslinked moist PVA networks. The samples show rapid shape recovery under MW irradiation, whereas dry samples experience no shape changes when irradiated. This effect is attributed to the heat created by vibrating water molecules inside the samples that supplies the energy required for shape recovery. The rate of recovery rate is affected not only by the water content in the material but also by the applied MW power output. Automatic network analysis, DSC, and DMA are used to study the dielectric, thermal and mechanical properties of the samples. MW‐induced shape recovery offers advantages such as high and variable recovery rate and the absence of direct contact between the heating source and the material.

     

Miscibility Enhancement on the Immiscible Binary Blend of Poly(vinyl phenol) and Poly(acetoxystyrene) with Poly(ethylene oxide)

Summary: The miscibility and hydrogen‐bonding behaviors of ternary polymer blends of poly(ethylene oxide) (PEO)/poly(vinyl phenol) (PVPh)/poly(acetoxystyrene) (PAS) were investigated by using DSC and Fourier transform infrared spectroscopy (FTIR). The PEO is miscible with both PVPh and PAS based on the observed single T_g over the entire composition range. FTIR was used to study the hydrogen‐bonding interactions between PEO with PAS and PVPh, respectively. Quantitative analyses show that the strength of hydrogen‐bonding strength is of the order of the hydroxyl‐ether inter‐association of PVPh/PEO blend > the hydroxyl‐hydroxyl self‐association of pure PVPh > the hydroxyl‐carbonyl inter‐association PVPh/PAS blend at room temperature. Furthermore, the addition of PEO is able to enhance the miscibility of immiscible PVPh/PAS binary blends at lower (20 wt.‐%) or higher (60 and 80 wt.‐%) PEO content. However, there exists a closed immiscibility loop in the phase diagram at 40 wt.‐% PEO content due to the “Δχ” and “ΔK” effects in this hydrogen‐bonded ternary polymer system. Therefore, an interesting and unusual sandwich phase diagram has been observed in this ternary polymer blend.

Ternary phase diagram of the PEO/PAS/PVPh system.     

Kinetics of Shear Microphase Orientation and Reorientation in Lamellar Diblock and Triblock Copolymer Melts as Detected via FT‐Rheology and 2D‐SAXS

The microphase orientation/reorientation alignment kinetics behaviour of lamellar PS‐b‐PB diblock and PS‐b‐PB‐b‐PS triblock copolymers under LAOS conditions is studied. By online monitoring the degree of the mechanical nonlinearity during the orientation process, as determined via the higher harmonics in FT rheology [i.e: I_3/1(t), ?₃(t)], and investigation of the orientation distribution by 2D‐SAXS, the kinetics of the microphase alignment for different experimental shear conditions was followed. For di‐ and triblock copolymers, the shear orientation into parallel alignment is possible and the kinetics of orientation is strongly dependent on the strain amplitude.

     

Synthesis and Characterization of Poly(trimethylene oxide)‐block‐Polystyrene and Poly(trimethylene oxide)‐block‐Polystyrene‐block‐Poly(methyl methacrylate) by Combination of Atom Transfer Radical Polymerization (ATRP) and Cationic Ring‐Opening Polymerization (CROP)

Summary: Diblock copolymers, poly(trimethylene oxide)‐block‐poly(styrene)s abbreviated as poly(TMO)‐block‐poly(St), and triblock copolymers, poly(TMO)‐block‐poly(St)‐block‐poly(MMA)s (MMA = methyl methacrylate), with controlled molecular weight and narrow polydispersity have been successively synthesized by a combination of atom transfer radical polymerization (ATRP) and cationic ring‐opening polymerization using the bifunctional initiator, 2‐hydroxylethyl α‐bromoisobutyrate, without intermediate function transformation. The gel permeation chromatography (GPC) and NMR analyses confirmed the structures of di‐ and triblock copolymers obtained.

GPC curves of (a) poly(St); (b) diblock copolymer, poly(St)‐block‐poly(MMA) before precipitation; (c) poly(St)‐block‐poly(MMA) after precipitation in cyclohexane/ethanol (2:1); (d) triblock copolymer, poly(TMO)‐block‐poly(St)‐block‐poly(MMA).     

Competition Between Motion Constraint and Aggregation of Macromolecular Chains in Poly(vinyl methyl ether)/Poly(ethylene oxide) Aqueous Solution During Phase Transition

Resonance light scattering (RLS) technique was applied to study macromolecular entanglements in highly dilute poly(vinyl methyl ether) (PVME)/poly(ethylene oxide) (PEO) solution during phase transition process. Temperature dependences of RLS intensities of PVME, PEO and PVME/PEO solutions were recorded. In addition, simulated temperature dependence of RLS intensity of PVME/PEO solution was drawn supposing there was no interaction between PEO and PVME. Comparison between the measured with the simulated results indicated that there were obvious differences in RLS intensities and transition temperatures. The present work proved the existence of entanglements during phase separation in highly dilute solution. Moreover, a model was proposed to describe the entanglement behavior.     

Facile Surface Modification and Application of Temperature Responsive Poly(N‐isopropylacrylamide‐co‐dopamine methacrylamide)

The temperature‐responsive poly(N‐isopropylacrylamide) [PNIPAAm] has been exploited for various biomedical applications. In this work, poly(N‐isopropylacrylamide‐co‐dopamine methacrylamide) [P(NIPAAm‐co‐DMAAm)] was synthesized for facile surface modification and application to cell sheets. ¹H NMR, FT‐IR, and GPC confirmed the successful synthesis of P(NIPAAm‐co‐DMAAm). The lower critical solution temperature was measured to be ca. 29.2 °C by UV–Vis spectroscopy. AFM imaging clearly visualized the transient phase transition of the temperature‐responsive polymer bound on silicon substrate by coordination bond formation. Furthermore, the adhesive and temperature responsive P(NIPAAm‐co‐DMAAm) could be successfully applied to the facile preparation of NIH‐3T3 fibroblast cell sheets.     

Radically Crosslinkable Poly(acrylic acid)s From Poly[(tert‐butyl acrylate)‐co‐(3‐aminopropyl vinyl ether)]: Synthesis and Rheological Properties

Acrylic acid copolymers bearing radically polymerizable methacrylic and itaconic moieties are synthesized. Starting from copolymers of tBA and APVE, prepared at different ratios, methacrylic or itaconic amides are obtained via polymer‐analogous reaction. After acidic elimination of isobutylene, polymers with free carbon acids and staggered reactivity can be prepared. Varying amounts of attached acrylamide moieties allow the adjustment of network densities. Rheological properties in HEMA/water matrices during curing are evaluated.     

Crystallization of Poly(vinylidene fluoride) in Blends with Poly(methyl methacrylate‐co‐methacrylic acid) Copolymers

A series of poly(methyl methacrylate‐co‐methacrylic acid) (PMMA‐co‐MAA) random copolymers ranging in MAA content from 0–15 mol% is synthesized and blended with poly(vinylidene fluoride) (PVDF). Using infrared spectroscopy, it is observed that the absorption bands attributed to hydrogen‐bonded carbonyl groups increase in intensity as the amount of MAA in the copolymer increases. In DSC analysis, the crystallization temperature of the PVDF in the blend initially decreases by ca. 12 °C with MAA contents ranging from 0 to 5.5 mol%; however, a PVDF blend with a 15 mol% MAA copolymer has a crystallization temperature that is only ca. 3 °C below that of pure PVDF. Similarly, spherulitic growth rate analysis initially shows a decrease in radial growth rate for PVDF in blends with PMMA‐co‐MAA copolymers containing less than 5.5 mol% MAA. At higher MAA copolymer contents, the spherulitic growth rate approaches that of pure PVDF. It is concluded that the presence of the MAA comono­mer in the PMMA‐co‐MAA copolymer initially (<5.5 mol% MAA) increases the intermolecular interactions between the copoly­mer and the PVDF. However, as the MAA content of the copolymer rises above 5.5 mol%, intramolecular hydrogen bonding interactions within the PMMA‐co‐MAA copolymer cause the copoly­mer to be less compatible with PVDF.

     

Phase Behavior of Poly(methyl methacrylate)/Poly(vinylidene fluoride) Blends in the Presence of High‐Pressure Carbon Dioxide

Previous efforts have demonstrated that high‐pressure CO₂ can markedly influence the phase behavior of amorphous polymer blends. In this work, we examine the effect of high‐pressure CO₂ on the miscibility of blends composed of glassy poly(methyl methacrylate) (PMMA) and semicrystalline poly(vinylidene fluoride) (PVDF). Blends of this type are known to exhibit lower critical solution temperature (LCST) behavior with partial miscibility up to ≈50–60 wt.‐% PVDF at ambient conditions. Two miscible PMMA/PVDF blends have been systematically exposed to high‐pressure CO₂ at 35 °C and pressures below and above the critical pressure. Small‐angle X‐ray scattering reveals that the scattering intensity at high scattering angles shows little dependence on pressure at low CO₂ pressures, but increases substantially at relatively high CO₂ pressures. Transmission electron microscopy and differential scanning calorimetry analyses confirm that the blends are initially quasi‐homogeneous with diffuse PVDF‐rich dispersions and a single glass transition temperature. After exposure to relatively high CO₂ pressures, however, the PVDF is found to crystallize within the PMMA‐rich matrix. Thermal recycling of these blends promotes homogenization, indicating that such CO₂‐altered phase behavior is reversible.

SAXS patterns acquired from the 69/31 w/w PMMA/PVDF blend.     

Shear‐Induced Crystallization and Shear‐Induced Dissolution of Poly(ethylene oxide) in Mixtures with Tetrahydronaphthalene and Oligo(dimethyl siloxane‐b‐ethylene oxide)

Cloud point temperatures (T_cp) and crystallization temperatures (T_l/s) were measured at different constant shear rates for the ternary system tetrahydronaphthalene/poly(ethylene oxide)/oligo(dimethyl siloxane‐b‐ethylene oxide) using a rheo‐optical device and in the case of T_l/s additionally a viscometer. This system enables for the first time a joint investigation of both transitions with a given mixture. Shear favors the homogeneous liquid state and the formation of crystals. T_cp (liquid/liquid demixing, UCST) shifts to lower and T_l/s (liquid/solid, segregation of PEO) to higher temperatures by several degrees as the shear rate, , is increased up to 500 s^?1. The normalized shift in T_cp fits well into previous results for high molecular weight blends, oligomer mixtures, polymer solutions in single solvents and low molecular weight mixtures. A phase separated near critical blend was examined 1 K below its T_cp by means of a shear cell (Linkam) in the quiescent state and under shear with respect to its morphology. Upon an increase in one observes a transition from the co‐continuous structures existing in the quiescent state via deformed and oriented particles to string like morphologies. Finally, at sufficiently high shear rates the mixture becomes homogeneous and structures can no longer be seen under the microscope. The morphologies developing after the secession of shear are pointing to pronounced influences of the flow history of the system on the final structure of two phase blends.

Equilibrium phase diagram of the system THN/COP/PEO at the indicated temperatures as obtained from turbidimetric titration. The curve for 42 °C indicates the compositions under which the mixtures segregate the first solid PEO particles upon cooling. The curves for the higher temperatures denote the demixing of the homogeneous system into two liquid phases.     

Phase Separation Dynamics of Aqueous Poly [(2‐ethoxy) ethoxy ethyl vinyl ether] Solutions as Explored using the Laser T‐Jump Technique Combined With Photometry

Poly[(2‐ethoxy) ethoxy ethyl vinyl ether] (poly(EOEOVE)) is a representative thermoresponsive polymer in aqueous solution for which the time constants of the phase separation (PS) are determined with high accuracy. It is revealed that the PS dynamics of the polymer are entirely different from those of the poly(N‐isopropylacrylamide) (PNIPAM) system, which is an alternative representative thermoresponsive polymer. Poly(EOEOVE) exhibits complicated PS behavior that is described using double exponential functions. The PS of poly(EOEOVE) is much faster than the PS of PNIPAM in aqueous solutions and becomes faster with increasing concentration of the polymer. PS behavior that is particular to the present system is successfully understood within the framework of the aggregation mechanism.