Molecular dynamics of poly(ATRIF) homopolymer and poly(AN-co-ATRIF) copolymer investigated by dielectric relaxation spectroscopy

Aiming to develop new dielectric polymers containing CN and F groups with strong dipole moments, a novel copolymer of acrylonitrile (AN) and 2,2,2-trifluoroethyl acrylate (ATRIF) was synthesized in acetonitrile by free radical process as well as the respective homopolymer (poly(ATRIF)). The copolymer’s composition and microstructure were analyzed by FTIR, ¹H and ¹³C NMR spectroscopy and SEC. The molar incorporation of AN determined in the copolymer by NMR was 58 mol%. Thermogravimetric analysis of poly(AN-co-ATRIF) copolymer showed good thermal stability comparatively to the fluorinated homopolymer.Both copolymer, poly(AN-co-ATRIF), and homopolymer, poly(ATRIF), were dielectrically characterized over a frequency range from 10⁻¹ to 10⁶ Hz, and in a temperature range from 223 to 393 K. The dominating relaxation process detected in both materials is the α-relaxation, associated with the dynamic glass transition. A VFTH temperature dependence of the relaxation times (τ) was found for both materials, as characteristic of cooperative processes, from which the respective glass transition temperatures (T_g(τ = 100 s)) were estimated, which differ ∼40 K, the one of the copolymer being higher (307 K) in accordance to the calorimetric analysis. This effect was attributed to a higher stiffness of the backbone in the copolymer originated by the inclusion of the acrylonitrile groups. Both relaxation functions have the same breath of relaxation times allowing constructing a single master curve, indicating similar non-exponential character. A less fragile behavior was found for the copolymer. This was rationalized in a more straightforward way by the free volume approach instead from a correlation between fragility and intermolecular coupling. It was found that in the copolymer the free volume increases at a lower rate with the temperature increase. It was inferred from the VFTH temperature dependence of the dc conductivity and low values of the decoupling index that ion motion is significantly influenced by the dynamics of the α-process.     

Molecular dynamics in fluorinated side-chain maleimide copolymers as studied by broadband dielectric spectroscopy

A series of alternating maleimide (MI) copolymers with fluorinated side chains have been investigated using broadband dielectric spectroscopy. The side chains consist of fluoroalkane (–C _x F_2x+1, x=1, 7, 9) end groups connected to the main chain via methylene spacers. The experiments were carried out in a frequency range of 0.1 Hz to 10 MHz and at temperatures between 120 K and 500 K. The fluorinated MI copolymers show a fast sub-T _g (β) relaxation characterized by an Arrhenius-type temperature dependence with activation energy in the range of 30–37 kJ/mol. Two more processes (α and δ-like) are observed, corresponding to independent relaxations of the main chain and the fluoroalkane domains respectively. For shorter side chains, the δ-like process is not observed but instead another relaxation process, α _S , occurs at temperatures higher than either the α and δ-like processes. When compared with unfluorinated MI copolymers, the fluorinated MI copolymers show the δ-like process and a slower β-relaxation unlike their unfluorinated counterparts. A model to explain the molecular origin of the four processes is proposed, supplemented by differential scanning calorimetry and published WAXS/SAXS data.     

Molecular dynamics of amorphous/crystalline polymer blends studied by broadband dielectric spectroscopy

The molecular dynamics of poly(vinyl acetate), PVAc, and poly(hydroxy butyrate), PHB, as an amorphous/crystalline polymer blend has been investigated using broadband dielectric spectroscopy over wide ranges of frequency (10⁻² to 10⁵ Hz), temperature, and blend composition. Two dielectric relaxation processes were detected for pure PHB at high and low frequency ranges at a given constant temperature above the T_g. These two relaxation peaks are related to the α and α′ of the amorphous and rigid amorphous regions in the sample, respectively. The α′-relaxation process was found to be temperature and composition dependent and related to the constrained amorphous region located between adjacent lamellae inside the lamellar stacks. In addition, the α′-relaxation process behaves as a typical glass relaxation process, i.e., originated from the micro-Brownian cooperative reorientation of highly constraints polymeric segments. The α-relaxation process is related to the amorphous regions located between the lamellar crystals stacks. In the PHB/PVAc blends, only one α-relaxation process has been observed for all measured blends located in the temperature ranges between the T_g’s of the pure components. This last finding suggested that the relaxation processes of the two components are coupled together due to the small difference in the T_g’s (ΔT_g = 35 °C) and the favorable thermodynamics interaction between the two polymer components and consequently less dynamic heterogeneity in the blends. The T_g’s of the blends measured by DSC were followed a linear behavior with composition indicating that the two components are miscible over the entire range of composition. The α′-relaxation process was also observed in the blends of rich PHB content up to 30 wt% PHB. The molecular dynamics of α and α′-relaxation processes were found to be greatly influenced by blending, i.e., the dielectric strength, the peak broadness, and the dielectric loss peak maximum were found to be composition dependent. The dielectric measurements also confirmed the slowing down of the crystallization process of PHB in the blends.     

Self-aggregation of dendritic poly(benzyl ether)–poly(acrylic acid), an amphiphilic block copolymer, studied by 1H NMR

The dynamic properties of the micelles of a novel synthesized amphiphilic block copolymer, dendritic poly(benzyl ether)–poly(acrylic acid) (Dendr.PBE-PAA), formed in aqueous solutions were studied by the ¹H self-diffusion coefficient, relaxation measurements and 2D nuclear Overhauser enhancement spectroscopy. The experimental results show that Dendr.PBE-PAA molecules self-aggregate in aqueous solution. The dynamic properties of the Dendr.PBE-PAA micelles vary with their total concentration in the solution. The motion of the molecules in the micelles of a concentrated solution is more restricted than that in a less concentrated one. The main chains of PAA are densely packed in the surface layer of the hydrophobic core with the carboxyl side chain pointing to the aqueous medium and the hydrophobic phenoxy rings stay in the interior. The self-aggregate becomes larger as the degree of polymerization of PAA increases. However the phenoxy rings situated in the interior of the hydrophobic core become more loosely packed. n-Hexadecane is solubilized in the micelles. The optimal position of n-hexadecane is between the phenoxy rings next to the PAA chains. Received: 25 January 2001 Accepted: 18 July 2001     

Macromolecular dynamics of sulfonated poly(styrene-b-ethylene-ran-butylene-b-styrene) block copolymers by broadband dielectric spectroscopy

Macromolecular dynamics of sulfonated poly(styrene-b-ethylene-ran-butylene-b-styrene) (sSEBS) triblock copolymers were investigated using broadband dielectric spectroscopy (BDS). Two main relaxations corresponding to the glass transitions in the EB and S block phases were identified and their temperature dependences were VFT-like. T_g for the S block phase shifted to higher temperature due to restrictions on chain mobility caused by hydrogen bonded SO₃H groups. While the EB block phase T_g appeared to remain constant with degree of sulfonation in DMA experiments, it shifted somewhat upward in BDS spectra. A low temperature relaxation beneath the glass transition of the EB block phase was attributed to short range chain motions. The Kramers–Krönig integral transformation was used to calculate conductivity-free loss permittivity spectra from real permittivity spectra to enhance true relaxation peaks. A loss permittivity peak tentatively assigned to relaxation of internal S-EB interfacial polarization was seen at temperatures above the S block phase glass transition, and the temperature dependence of this relaxation was VFT-like. The fragilities of the EB and S block domains in sulfonated SEBS decreased after sulfonation. The temperature dependence of the dc conduction contribution to sSEBS loss spectra also followed VFT-like behavior and S block segmental relaxation time correlated well with conductivity according to the fractional Debye–Stokes–Einstein equation.     

A well-defined diblock copolymer of poly-(ethylene oxide)-block-poly (4-vinylpyridine) for separation of basic proteins by capillary zone electrophoresis

A series of well-defined diblock copolymers, poly(ethylene oxide)-block-poly(4-vinylpyridine) (PEO-b-P4VP) used as physical coating of capillaries, were synthesized by atom transfer radical polymerization (ATRP). EOF measurement results showed that all synthesized PEO-b-P4VP diblock copolymer-coated capillaries in this report could suppress EOF effectively compared to the bare fused-silica capillaries, and efficient separations of basic proteins were achieved. The effects of the molecular weight of P4VP block in PEO-b-P4VP and buffer pH on the separation of basic proteins for CE were investigated in detail. Moreover, the relationships between morphologies of PEO-b-P4VP diblock copolymers in buffer, which were studied by transmission electron microscopy, and the separation efficiencies of basic protein with PEO-b-P4VP diblock copolymers coatings were discussed.     

(14)N NMR and two-dimensional suspension (1)H and (13)C HRMAS NMR spectroscopy of ionic liquids immobilized on silica

A variety of popular ionic liquids have been synthesized and characterized, including by optimized (14)N NMR spectroscopy of the neat and dissolved ionic liquids. Ionic liquids incorporating Si(OEt)(3) groups have been immobilized on silica in a well-defined manner with the imidazolium moiety remaining intact. This has been proved by optimized one- and two-dimensional (1)H and (13)C HRMAS NMR spectroscopy of the materials suspended in suitable solvents.     

The magnetic properties of myoglobin as studied by NMR spectroscopy

Deoxymyoglobin has been investigated by NMR spectroscopy to determine the magnetic anisotropy through pseudocontact shifts and the total magnetic susceptibility through Evans measurements. The magnetic anisotropy values were found to be Deltachi(ax)=-2.03+/-0.08 x 10(-32) m(3) and Deltachi(rh)=-1.02+/-0.09 x 10(-32) m(3). The negative value of the axial susceptibility anisotropy originates from the z tensor axis lying in the heme plane, unlike all other heme systems investigated so far. This magnetic axis is almost exactly orthogonal to the axial histidine plane. The other two axes lie essentially in the histidine plane, the closest to the heme normal being tilted by about 36 degrees from it, towards pyrrole A on the side of the proximal histidine. From the comparison with cytochrome c' it clearly appears that the position of the one axis lying in the heme plane is related to the axial histidine orientation. Irrespective of the directions, the magnetic anisotropy is smaller than that of the analogous reduced cytochrome c' and of the order of that of low-spin iron(III). The magnetic anisotropy of the system permits the measurement of residual dipolar couplings, which, together with pseudocontact shifts, prove that the solution structure is very similar to that in the crystalline state. Magnetic measurements, at variance with previous data, demonstrate that there is an orbital contribution to the magnetic moment, micro(eff)=5.5 micro(B). Finally, from the magnetic anisotropy data, the hyperfine shifts of iron ligands could be separated in pseudocontact and contact components, and hints are provided to understand the spin-delocalisation mechanism in S=2 systems by keeping in mind the delocalisation patterns in low-spin S=1/2 and high-spin S= 5/2 iron(III) systems.     

Miscibility and dynamics of the poly(vinylimidazole-co-methyl methacrylate)-silica hybrids studied by solid-state NMR

Poly(vinylimidazole-co-methyl methacrylate)-silica hybrids, bonded through hydrogen bond (PVM-SiO₂) or chemical bond (PVM(5)-SiO₂) between organic and inorganic units, were prepared and characterized. The characterization of PVM-SiO₂ and PVM(5)-SiO₂ hybrids were confirmed by IR, ¹³C and ²⁹Si NMR spectra. The intermolecular interaction between copolymer chains was studied by the spin-lattice relaxation time in the rotating frame (T^H_1ρ), and that between copolymer and silica was evaluated by the time constant for energy change between ¹H and ²⁹Si spin system (T_SiH). T^H_1ρ and T_SiH values in PVM-SiO₂ hybrids were consistent with those in PVM(5)-SiO₂ hybrids, and those were independent of the silica content. Moreover, the T^H_1ρ values are in order of poly(methyl methacrylate)-silica hybrids (PMMA-SiO₂) ≧ PVM-SiO₂ ≒ PVM(5)-SiO₂ > polyvinylimidazole-silica hybrids (PVI-SiO₂), while those of T_SiH are in reverse order PMMA-SiO₂ ≦ PVM(5)-SiO₂ < PVI-SiO₂.     

The molecular dynamics of thermoreversible networks as studied by broadband dielectric spectroscopy

Polybutadienes modified by a small number of 4-phenyl-1,2,4-triazoline-3,5-dione form thermoreversible networks via hydrogen bonding between the polar stickers. The molecular dynamics of systems with different contents of polar stickers are investigated by broadband dielectric spectroscopy in the frequency regime of 10^–1–10⁹ Hz. Unmodified polybutadiene shows two relaxation processes, the -relaxation which is correlated to the dynamic glass transition of the polybutadiene, and a -relaxation corresponding to a local relaxation of polybutadiene segments. In the polar functionalized systems, besides these two relaxations, an additional relaxation process (called ^*) is observed, which occurs at lower frequencies than the -process. While the -relaxation remains unaffected by the functionalization the cooperativity of the -relaxation increases by the formation of reversible junctions and slows down considerably. This indicates a decreased mobility of the polymer matrix. At the same time the dipole moment of relaxing units contributing to the -relaxation is increased by free phenyl urazole units. The ^* is assigned to the local complex dynamics resulting from the dissociation and formation of dimeric contacts. Hence, for this dynamic process, the absolute value of the dipole moment fluctuates with time and causes a dielectric absorption. This interpretation is in agreement with the hindered reptation model of Leibler, Rubinstein and Colby and simultaneous measurements of infrared dichroism and birefringence.     

The micellization of the diblock copolymer of ethylene oxide and styrene in benzene and the effect on the preparation of the star ABC triblock copolymer of ethylene oxide, styrene and methyl methacrylate

In the preparation of the ABC star triblock copolymer of ethylene oxide, styrene and methyl methacrylate (MMA), the photo-induced charge-transfer complex (CTC) was used to initiate the polymerization of the third monomer MMA. The CTC was composed of the diblock copolymer of poly(ethylene oxide) (PEO) and polystyrene (PS), PEO-b _i -PS, with an aromatic imino group at the conjunction point and benzophenone (BP). It was confirmed that the kinetic behavior of this macromolecular initiation system is nearly the same with a general small radical initiator: the polymerization rate R _p ∝ [PEO-b _i -PS]^0.48[BP]^0.45[MMA]^0.97. Moreover, if the molecular weight of the PEO block is fixed, R _p is independent of the molecular weight of the PS block.  By means of measurements of viscosity and fluorescence, it was found that the micelles of the diblock copolymer PEO-b _i -PS were formed in benzene. The aromatic imino groups were located on the boundary surfaces of the micelles and were fully exposed, and so the BP and MMA molecules easily approached them and affected the charge-transfer polymerization of MMA. Received: 18 August 1998 Accepted in revised form: 25 November 1998     

Relaxation dynamics of blends of PVDF and zwitterionic copolymer by dielectric relaxation spectroscopy

Relaxation dynamics of PVDF blended with a random zwitterionic copolymer (r-ZCP) of methyl methacrylate and zwitterionic sulfobetaine-2-vinylpyridine (PMMA-r-SB2VP) were investigated using dielectric relaxation spectroscopy. FTIR spectroscopy was used to determine the PVDF crystal phase of compression molded blends. Adding 25 wt% of r-ZCP promoted the formation of the polar β and γ crystals over the nonpolar α phase. A structural model is proposed where the r-ZCP biases the PVDF to form polar crystal phases. Boyd's model was used to calculate the room temperature dielectric constants and led to good agreement with our measurements. Dielectric spectra of neat r-ZCP showed two relaxation peaks attributed to PMMA units, with no additional relaxations present from the zwitterions. Blends of PVDF with r-ZCP were dominated by the α_c relaxation associated with the crystalline phase of PVDF, which showed an Arrhenius temperature dependence. Analysis of the conductivity spectra shows a larger DC conductivity in the blends than in either r-ZCP or homopolymer PVDF. Blends show an additional peak in the loss tangent, absent in the copolymer or PVDF attributed to space-charge polarization. Higher DC conductivity and space-charge polarization indicate that the combination of zwitterions and unique microstructure affects charge transport properties.     

Aggregation behavior of amphiphilic poly(2-alkyl-2-oxazoline) diblock copolymers in aqueous solution studied by fluorescence correlation spectroscopy

The diffusional behavior of amphiphilic poly(2-oxazoline) diblock copolymers in aqueous solution is studied using photon correlation spectroscopy (PCS) and fluorescence correlation spectroscopy (FCS). The polymers were synthesized by living cationic polymerization and were fluorescence-labeled with tetramethyl rhodamine isothiocyanate either at the end of the hydrophilic or the hydrophobic block. Temperature-resolved PCS showed that, at room temperature, large metastable aggregates are present along with unimers and micelles. An annealing above ~40 °C resulted in stable equilibrium micellar solutions. By means of FCS, the hydrodynamic radii of the unimers and the micelles were measured simultaneously in a broad concentration range, and the critical micelle concentration could be determined. Comparison of the results from conventional PCS measurements with this first FCS study showed excellent agreement and the high potential of the FCS technique.Dedicated to Prof. Dr. Erhard W. Fischer on the occasion of his 75th birthdayAn erratum to this article can be found at     

Microstructure of poly(methyl methacrylate-co-isopropyl acrylate) studied by 13C NMR

The distribution of configurational–compositional sequences of poly(methyl methacrylate-co-isopropyl acrylate) (PMMA/iPrA) has been determined from the carbonyl and β-CH₂ signals in the 100?MHz ¹³C NMR spectra of the copolymer. The carbonyl signal provided information on configurational–compositional sequences up to heptads, whereas β-CH₂ signals offered complementary information on even sequences up to hexads. The assignment of the sequences to the respective signals was based on a comparison with the spectra of respective homopolymers, that is, PMMA and PiPrA followed by a computer simulation applying an incremental calculation of chemical shifts of the individual sequences.     

Molecular dynamics of hydrophilic poly(propylene imine) dendrimers in aqueous solutions by 1H NMR relaxation

The local dynamics of three poly(propylene imine) dendrimers with hydrophilic triethylenoxy methyl ether terminal groups were studied in D₂O by the measurement of the ¹H NMR relaxation times, which were treated with the Lipari–Szabo model‐free approach. The results showed that the overall mobility increased with temperature and decreased with increasing dendrimer size. An Arrhenius trend was observed for both overall and local motions. The activation energy of overall tumbling increased from 11.3 to 17.5 kJ/mol with the dendrimer size. The local mobility decreased from the outer part to the inner part of the dendrimer and with the dendrimer size. The spatial restriction of local motions decreased with increasing temperature up to 55 °C and remained constant above 55 °C. Local motions were more restricted when the dendrimer size increased. The results showed that the hydrophilic end groups of the dendrimers were located preferentially at the periphery of the molecules and were extended in the aqueous environment. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2969–2975, 2003     

Spectral assignment of poly(vinyl acetate) by one and two dimensional NMR spectroscopy: Revisited

Poly(vinyl acetate) (PVA) prepared by photopolymerization was studied by a combination of one and two dimensional NMR spectroscopy. The ¹³C{¹H} and ¹H NMR spectra of the homopolymer (PVA) were assigned to the configurational pentads (CH region) and tetrads (CH₂ region). These assignments were substantiated by the use of two dimensional heteronuclear single quantum correlation (HSQC), heteronuclear single quantum correlation‐total correlation spectroscopy (HSQC‐TOCSY) and double quantum filtered correlation spectroscopy (DQFCOSY) experiments. The results obtained by the analysis of the area under the resonance signals confirmed that PVA obeys Bernoullian statistics. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 551–556, 1999     

Influence of dT20 and [d(AT)10]2 on cisplatin hydrolysis studied by two-dimensional [1H,15N] HMQC NMR spectroscopy

The influence of the presence of DNA on the kinetics of cisplatin (cis-[PtCl2(NH3)2]) aquation (replacement of Cl- by H2O) and anation (replacement of H2O by Cl-) involved in the hydrolysis of cisplatin have been determined by two-dimensional [1H,15N] HMQC NMR spectroscopy. Single-stranded dT20 and double-stranded [d(AT)10]2 oligonucleotides were used as DNA models, avoiding guanines which are known to react rapidly with aquated cisplatin forms. Reactions starting from cis-[PtCl2(15NH3)2], or from a stoichiometric mixture of cis-[Pt(15NH3)2(H2O)2]2+ and Cl- (all 0.5 mM Pt(II); in ionic strength, adjusted to 0.095 M or 0.011 M with NaClO4, pH between 3.0 and 4.0) were followed in an NMR tube in both the absence and presence of 0.7 mM dT20 or [d(AT)10]2. In the presence of dT20, we observed a slight and ionic-strength-independent decrease (15-20 %) of the first aquation rate constant, and a more significant decrease of the second anation rate constant. The latter was more important at low ionic strength, and can be explained by efficient condensation of cis-[Pt(15NH3)2(H2O)2]2+ on the surface of single-stranded DNA, in a region depleted of chloride anions. At low ionic strength, we observed an additional set of [1H,15N] HMQC spectral signals indicative of an asymmetric species of PtN2O2 coordination, and we assigned them to phosphate-bound monoadducts of cis-[Pt(15NH3)2(H2O)2]2+. Double-stranded [d(AT)10]2 slowed down the first aquation step also by approximately 15 %; however, we could not determine the influence on the second hydrolysis step because of a significant background reaction with cis-[Pt(NH3)2(H2O)2]2+.