Mechanical hole burning spectroscopy in an SIS tri-block copolymer

We have previously developed a mechanical hole burning spectroscopy (MSHB) technique that promises to be a powerful tool for the investigation of dynamic heterogeneity of polymeric materials. This is because, unlike its dielectric analogue, MSHB can be used to characterize materials having a weak dielectric response, a particular feature of polymeric materials. However, while both mechanical and dielectric hole burning show behaviors that are consistent with dynamic heterogeneity in the materials, it is still unclear what the relationship between the hole properties, for example, frequency and amplitude, and the actual nature of the heterogeneity and particularly the length scale being probed. Here, we provide first evidence that a known length scale can be probed by the MSHB method by using a tri-block copolymer as a “calibration” sample. The heterogeneity of a styrene-isoprene-styrene copolymer was investigated in the vicinity of the order-disorder transition temperature (ODT). It was found that the amplitudes of the mechanical holes gradually decrease as the order-disorder transition is traversed from the region with ordered structures. In the disordered state of the block copolymer, no apparent mechanical holes were detected. In contrast, mechanical holes were burned in the heterogeneous region and the hole amplitude increased as the depth into the ordered structure increased. Hence, the MSHB technique provides a qualitative correlation between the amplitude of the burned holes and the corresponding heterogeneity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3277–3284, 2007     

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (T_g) on film thickness. Sizeable reduction of T_g was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of T_g on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the T_g or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Persistent spectral hole‐burning study on dendrimer porphyrins

We report the first results of persistent spectral hole burning of dendrimer porphyrins having three‐, four‐, or five‐layered aryl ether dendritic arrays. We evaluate structural relaxations of dendrimer framework around the porphyrin core at low temperatures. A large environmental change around the porphyrin core, as evaluated from the hole area, was suppressed in dendrimer porphyrins of higher generation numbers, whereas a small environmental change, as evaluated from hole width, showed no dependence on the number of generations. The dendrimer porphyrins showed sharp holes at 20 K, suggesting a long dephasing time and the suppression of spontaneous spectral diffusion. The results of dendrimer‐embedded polymer sample indicated that the structural relaxation of polymer chain outside the dendrimer does not have an influence on the resonant frequency of the porphyrin core. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 210–215, 2002     

Models for adhesion at weak polymer interfaces

The weak interfaces between immiscible polymer pairs typically fail through chain scission. The critical facture toughness for such interfaces is closely related to the density of intermolecular entanglements at the interface. From scaling analysis, a simple correlation between facture toughness and chain entanglement was developed. It predicts well the interfacial adhesion for many immiscible polymer pairs found in the literature. For an interface with block copolymer reinforcement, its critical fracture toughness comes from both intermolecular entanglements of homopolymers and copolymer bridges. In the chain scission regime (low copolymer coverage), the block copolymer contribution is found proportional to copolymer interfacial coverage, with the coefficient being the energy to stretch and break a copolymer chain. The chain‐breaking energy for different copolymers was evaluated and compared to literature data. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2313–2319, 2009     

The linear rheological responses of wedge‐type polymers

The linear rheological responses of a series of specially designed wedge‐type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, T_g. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at T_g ?60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 899–906     

Dynamics of polyethylene melts studied by Monte Carlo simulations on a high coordination lattice

A detailed comparison is made between the experiment, prior simulations by other groups, and our simulation based on a newly designed dynamic Monte Carlo algorithm, on the dynamics of polyethylene (PE) melts. The new algorithm, namely, noncross random two-bead move has been developed on a high coordination lattice (the 2nnd lattice) for studying the dynamics of realistic polymers. The chain length (molecular weight) in our simulation ranges from C40 (562 Da) to C324 (4538 Da). The effects of finite chain length have been confirmed and significant non-Gaussian statistics evidently results in nonstandard static and dynamic properties of short PE chains. The diffusion coefficients scale with molecular weight (M) to the −1.7 power for short chains and −2.2 for longer chains, which coincides very well with experimental results. No pure Rouse scaling in diffusion has been observed. The transitional molecular weight to the entanglement regime is around 1500 Da. The detailed mean square displacements of middle bead (g1) are presented for several chain lengths. The reptation-like slowdown can be clearly observed only above M ∼ 2400 Da. The slope 0.25 predicted by the theory for the intermediate regime is missing; instead a slope close to 0.4 appears, indicating that additional relaxation mechanism exists in this transitional region. The relaxation times extracted by fitting the autocorrelation function of end-to-end vectors with reptation model scale with M to 2.5 for long chains, which seemingly conflicts with the scaling of diffusion. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2556–2571, 2006     

The effect of polyethylene addition on the morphology of polystyrene/polyamide blends

The effect of a small admixture of high‐density polyethylene (HDPE) with a high or low viscosity to polystyrene/polyamide (PS/PA) blends of various compositions was studied. PS/PA blends with composition near 50/50 form sheet‐like or fiber‐like morphology at mixing that passes to the cocontinuous structure during compression molding. Ternary PS/PA/HDPE blends with PS/PA ratio about 50/50 show similar behavior. Generally, neither continuity nor shape of PS and PA phases was changed qualitatively by the addition of a small amount of HDPE. In agreement with existing rules for ternary blends, HDPE particles prefer a contact with PS phase to PA phase. On the other hand, none of these rules explains why a number of small HDPE subinclusions were dispersed into PS particles instead of HDPE‐PS core‐shell structure with a lower Gibbs free energy. Quantitative evaluation of the size of PA particles in blends with PS matrix showed that the previously proposed rule stating, that the addition of a small amount of a third immiscible component leads to a strong decrease in the size of dispersed particles, was not valid for the blends studied in this work. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2158–2170, 2009     

Effect of bending and torsion rigidity on self-diffusion in polymer melts: a molecular-dynamics study

Extensive molecular-dynamics simulations have been performed to study the effect of chain conformational rigidity, controlled by bending and torsion potentials, on self-diffusion in polymer melts. The polymer model employs a novel torsion potential that avoids computational singularities without the need to impose rigid constraints on the bending angles. Two power laws are traditionally used to characterize the dependence of the self-diffusion coefficient on polymer length: D proportional to N(-nu) with nu=1 for NNe (reptation regime), Ne being the entanglement length. Our simulations, at constant temperature and density, up to N=250 reveal that, as the chain rigidity increases, the exponent nu gradually increases towards nu=2.0 for NNe. The value of Ne is slightly increased from 70 for flexible chains, up to the point where the crossover becomes undefined. This behavior is confirmed also by an analysis of the bead mean-square displacement. Subsequent investigations of the Rouse modes, dynamical structure factor, and chain trajectories indicate that the pre-reptation regime, for short stiff chains, is a modified Rouse regime rather than reptation.     

Characterization of chemical heterogeneity in polymer systems using hydrolysis and tapping‐mode atomic force microscopy

Characterization of polymer coatings microstructure is critical to the fundamental understanding of the corrosion of coated metals. An approach for mapping the chemical heterogeneity of a polymer system using chemical modification and tapping‐mode atomic force microscopy (TMAFM) is demonstrated. This approach is based on the selective degradation of one of the phases in a multiphase polymer blend system and the ability of TMAFM to provide nanoscale lateral information about the different phases in the polymer system. Films made of a 70:30 polyethyl acrylate/polystyrene (PEA/PS) blend were exposed to a hydrolytic acidic environment and analyzed using TMAFM. Pits were observed to form in the PEA/PS blend films, and this degradation behavior was similar to that of the PEA material. Using these results, the domains in the 70:30 blend were identified as the PS‐rich regions and the matrix as the PEA‐rich region. This conclusion was confirmed by Fourier transform infrared‐attenuated total reflection analyses that revealed the hydrolysis of the PEA material. TMAFM phase imaging was also used to follow pit growth of the blend as a function of exposure time. The usefulness of the chemical modification/AFM imaging approach in understanding the degradation process of a coating film is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B Part B: Polym Phys 39: 1460–1470, 2001     

Stress overshoot of polymer solutions at high rates of shear; Polystyrene with bimodal molecular weight distribution

Overshoot of shear stress, σ, and the first normal stress difference, N₁, in shear flow was investigated for dilute solutions of polystyrene with very high molecular weight in concentrated solution of low M PS. In the case that the matrix was a nonentangled system, behavior of overshoot was similar to that of dilute solution of high M PS in pure solvent. The magnitudes of shear, γ_σm and γ_Nm, corresponding to the peaks of σ and N₁ lay on the universal functions of γ˙τ_R, respectively, proposed for dilute solutions in pure solvent. Here τ_R is the Rouse relaxation time for high M PS in the blend evaluated from dynamic modulus at high frequencies. In the case that the matrix was an entangled system, an additional σ peak was observed at high rates of shear at times corresponding to γ_σm = 2–3. This peak can be assigned to the motion of low M chains in entanglement network. When the matrix was entangled, stress overshoot was observed even at relatively low rates of shear, say γ˙τ_R < 10⁻². This is probably due to the motion of high M chains in entanglement of all the chains. In this case the γ_σm and γ_Nm values were higher than those expected for entangled chains of monodisperse polymer in pure solvent. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2043–2050, 2000     

Effect of chain entanglements on plastic deformation behavior of ultra‐high molecular weight polyethylene

Samples of ultra‐high molecular weight polyethylene, in which the chain topology within the amorphous component was altered using two‐stage processing, including crystallization at high pressure in the first step, were produced and their deformation behavior in the plane‐strain compression was studied. Deformation and recovery experiments demonstrated that the state of the molecular network governed by entanglement density is one of the primary parameters controlling the response of the material on the imposed strain, especially at moderate and high strains. Any change in the concentration of entanglements markedly influences the shape of the true stress–true strain curve. The strain hardening modulus decreases while the onset of strain hardening increases with a decrease of the entanglement density within the amorphous component. Density of entanglements also influences the amount of rubber‐like recoverable deformation and permanent plastic flow. In material of the reduced concentration of entanglements permanent flow appears easier and sets in earlier than in the material with a higher entanglement density, becoming a favorable deformation mechanism at moderate strains. As a result, strong strain hardening is postponed to higher strain when compared with the samples of equilibrium entanglement density. In the samples of an increased entanglement density the molecular network becomes stiffer, with a reduced ability of strain induced disentangling of chains. Consequently, there is a less permanent flow and strain hardening begins earlier than in the reference material of an unaltered chain topology. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 276–285, 2010     

Melt rheology of hyperbranched‐polystyrene synthesized with multisite macromonomer

Melt rheological behaviors of hyperbranched‐polystyrene (PS) copolymerized by dendric macromonomer technique are presented. The time–temperature superposition principle was applicable to the hyperbranched‐PS. The branched‐PS showed slightly lower zero‐shear viscosity in comparison with linear PS regardless of a presence of a number of branches expected from the dendric macromonomer technique. Although the influence of use of multimethacryloyl macromonomer in the polymerization process was marginal for linear viscoelastic regime, nonlinear shear and uniaxial elongational flows showed distinct differences between linear and branched‐PS. The strain dependence of the damping function became weak as increase of macromonomer content. The branched‐PS exhibited the growing elongational viscosity function comparing with linear PS. This prominent effect on the elongational flow behavior can be explained by the molecular architecture of the branched‐PS. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2226–2237, 2009     

Influences of three kinds of springs on the retraction of a polymer ellipsoid in dissipative particle dynamics simulation

The impacts of Rouse spring, Fraenkel spring, and one kind of finitely extensible nonlinear elastic spring (FENE‐PM spring) on the surface tension‐induced retraction of a polymer ellipsoid in a matrix were compared using dissipative particle dynamics. Using the same spring constant, obvious differences among the three kinds of springs were found. A fast retraction process was observed from the hard Fraenkel spring, a slow process from the soft Rouse spring, and an intermediate process from the FENE‐PM spring. The effects of varying the spring constant and the chain length were also investigated. The results indicate that the influence of increasing spring hardness for a given spring type was significant; whereas, the influence of chain length was minor after five bonds were reached. The effects of varying the FENE‐PM r_m parameter were also studied to provide a reliable value for this study. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Associated inter‐ and intrachain conformational transitions in polystyrene solutions

Understanding the conformational changes of polymeric chains in solutions is an essential and integral part of polymer physics. By increasing the concentration of polymer solutions from dilute to semidilute regime, the critical chain overlapping has been reported at the concentration termed as C*. In this study, the associated inter‐ and intrachain conformational transitions in polystyrene (PS) solutions are reported. By comparing the spectroscopic intensity ratio versus concentration for an intrachain PS system, a break point was observed in good solvent which coincided with the theoretically predicted C*. Moreover, the intrachain conformation showed no obvious change below C*, while significant collapse started to occur above C*. This result reveals a new insight in polymer physics, since traditionally the size of polymer chains is considered to decrease weakly regarding the concentration change in the semidilute regime. It is important to find such an abrupt intrachain conformational transition between the dilute and semidilute solutions and provide the first experimental observation that inter‐ and intrachain conformational transitions are correlated to one the other. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1373–1379     

Effect of nanotube functionalization on the properties of single‐walled carbon nanotube/polyurethane composites

A commercially available aliphatic thermoplastic polyurethane formulated with a methylene bis(cyclohexyl) diisocyanate hard segment and a poly(tetramethylene oxide) soft segment and chain‐extended with 1,4‐butanediol was dissolved in dimethylformamide and mixed with dispersed single‐walled carbon nanotubes. The properties of composites made with unfunctionalized nanotubes were compared with the properties of composites made with nanotubes functionalized to contain hydroxyl groups. Functionalization almost eliminated the conductivity of the tubes according to the conductivity of the composites above the percolation threshold. In most cases, functionalized and unfunctionalized tubes yielded composites with statistically identical mechanical properties. However, composites made with functionalized tubes did have a slightly higher modulus in the rubbery plateau region at higher nanotube fractions. Small‐angle X‐ray scattering patterns indicated that the dispersion reached a plateau in the unfunctionalized composites that was consistent with the plateau in the rubbery plateau region. The room‐temperature modulus and tensile strength increase was proportionally higher than almost all increases seen previously in thermoplastic polyurethanes; however, the increase was still an order of magnitude below what has been reported for the best nanotube–polymer systems. Nanotube addition increased the hard‐segment glass transition temperature slightly, whereas the soft‐segment glass transition was so diffuse that no conclusions could be drawn. Unfunctionalized tubes suppressed the crystallization of the hard segment; whereas functionalized tubes had no effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 490–501, 2007     

The dynamics of polymer melts as seen by neutron spin echo spectroscopy

Using the neutron spin echo spectroscopy, the internal segmental diffusion of chain molecules in polymer melts and concentrated solutions was studied. These investigations show that beyond a characteristic length d_t and after a cross over time τ_e(d_t) the segmental diffusion of the single chains is strongly impeded and deviates from the Rouse dynamics. d_t is polymer specific and depends on the temperature as well as on the polymer concentration. Within the framework of the reptation concept, where d_t is identified with the mean distance between intermolecular entanglements or with the tube diameter, the microscopically determined d_t-values agree quite well with those derived from related macroscopic measurements of the plateau modulus. A similar good agreement is also found with respect to the segmental friction coefficients obtained either from the Rouse regime of the NSE spectra or from Theological data of corresponding short chain systems, where entanglements are not yet effective.     

Terpene based sustainable methacrylate copolymer series by emulsion polymerization: Synthesis and structure‐property relationship

This work demonstrates the fabrication of terpene based sustainable methacrylate polymers by an environmentally benign emulsion polymerization method. The polymerization reaction has been found to be influenced by the side chain length of the methacrylate(s), which has been quantitatively calculated from the density functional theory. Apart from the analysis of the copolymer microstructure, various properties of the synthesized polymers have been studied and correlated with the structure of the methacrylate(s). The sub‐ambient glass transition temperature indicates rubbery nature of the synthesized copolymers. While the presence of residual unsaturations from the terpene moiety could act as an additional crosslinking site, the methacrylate group may facilitate the dispersion of polar additives. The completely new class of terpene‐based sustainable rubbery methacrylate polymers is thus envisaged as promising materials for polymer and allied industries. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2639–2649     

Orientational anisotropy of bead-spring star chains during creep process

The bead-spring model for star chains (Rouse–Ham model) is a fundamental model for the polymer dynamics. Despite the importance of this model, its dynamics under the stress-controlled condition was not analyzed so far. For completeness of the model, the equation of motion of the Rouse–Ham chain was solved to derive an analytical expression of the orientation function S(n,t) for the stress-controlled creep process. This expression indicated that the segments near the free end of the star arm exhibit overshoot of their orientational anisotropy to compensate for the slow growth of the anisotropy near the branching point and that the distribution of the anisotropy along the arm contour becomes more heterogeneous with increasing arm number f. This correlation/interplay of the segments at different locations along the arm, not seen under the strain-controlled condition, is a natural consequence of the constant-stress requirement during the creep process. The corresponding interplay was noted also for respective Rouse–Ham eigenmodes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3501–3517, 2006     

An efficient synthetic route to well‐defined theta‐shaped copolymers

A series of well‐defined θ‐shaped copolymers composed of polystyrene (PS) and poly(ε‐caprolactone) (PCL) with controlled molecular weight and narrow molecular weight distribution have been successfully synthesized without any purification procedure by the combination of atom transfer radical polymerization (ATRP), ring‐opening polymerization (ROP), and the “click” chemistry. The synthetic process involves two steps: (1) synthesis of AB₂ miktoarm star copolymers, which contain one PCL chain terminated with two acetylene groups and two PS chains with two azido groups at their one end, (α,α′‐diacetylene‐PCL) (ω‐azido‐PS)₂, by ROP, ATRP, and the terminal group transformation; (2) intramolecular cyclization of AB₂ miktoarm star copolymers to produce well‐defined pure θ‐shaped copolymers using “click” chemistry under high dilution. The ¹H NMR, FTIR, and gel permeation chromatography techniques were applied to characterize the chemical structures of the resultant intermediates and the target polymers. Their thermal behavior was investigated by DSC. The mobility decrease of PCL chain across PS ring in the theta‐shaped copolymers restricts the crystallization ability of PCL segment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2620–2630, 2009     

Linear viscoelastic studies on a transient network formed by host–guest interaction

A host–guest (HG) polymer was prepared through the radical polymerization of acrylamide monomers (AAm) with a small amount of host‐guest linkers, β‐cyclodextrin‐attached AAm (βCD‐AAm) and adamantane‐attached AAm (Ad‐AAm). The linear viscoelastic and swelling measurements indicated that the resulting HG polymer swollen in water was gel‐like, although the HG polymer is conceptually a linear chain having only temporary crosslinkings. NMR measurements indicated that half of the βCD units incorporated in the HG polymer do not form the inclusion complex with Ad. Rheological analysis of the HG polymer revealed that HG interaction retarded the Rouse modes of networks but did not affect the level of the plateau modulus, which was simply described by the entanglements of AAm chains. This result was confirmed with the reference experiment, in which Ad were capped by competitive βCD molecules. Furthermore, the PAAm polymer with only βCD units (no Ad) was found to exhibit gel‐like behavior. This behavior was attributed to the formation of a small amount of rotaxane structure, which act as permanent crosslinkings, based on 2D NMR data. The HG polymer is basically an entanglement network with temporary sticky points due to the HG interaction, and a few permanent branching points. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1109–1117