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The effect of chain topology on (i) the peptide secondary structure, (ii) the nanophase self-assembly, and (iii) the local segmental and global peptide relaxations has been studied in a series of model diblock and 3-arm star copolypeptides of poly(epsilon-carbobenzyloxy-L-lysine) (PZLL) and poly(gamma-benzyl-L-glutamate) (PBLG) with PZLL forming the core. Diblock copolypeptides are nanophase separated with PBLG and PZLL domains comprising alpha-helices packed in a hexagonal lattice. Star copolypeptides are only weakly phase separated, comprising PBLG and PZLL alpha-helices in a pseudohexagonal lattice. Phase mixing has profound consequences on the local and global dynamics. The relaxation of the peptide secondary structure speeds up, and the helix persistence length is further reduced in the stars, signifying an increased concentration of helical defects. more...

We report on the combined use of fluorescence correlation spectroscopy (FCS) and 1H and 13C NMR spectroscopy to detect the size and type of peptide secondary structures in a series of poly-Z-L-lysine functionalized polyphenylene dendrimers bearing the fluorescent perylenediimide core in solution. In dilute solution, the size of the molecule as detected from FCS and 1H NMR diffusion measurements matches nicely. We show that FCS is a sensitive probe of the core size as well as of the change in the peptide secondary structure. However, FCS is less sensitive to functionality. A change in the peptide secondary conformation from beta-sheets to alpha-helices detected by 13C NMR spectroscopy gives rise to a steep increase in the hydrodynamic radii for number of residues n > or = 16. Nevertheless, helices are objects of low persistence. more...

The melting behaviour and transport properties of straight chain alkanes mono- and difunctionalized with phosphonic acid groups have been investigated as a function of their length. The increase of melting temperature and decrease of proton conductivity with increasing chain length is suggested to be the consequence of an increasing ordering of the alkane segments which constrains the free aggregation of the phosphonic acid groups. However, the proton mobility is reduced to a greater extent than the proton diffusion coefficient indicating an increasing cooperativity of proton transport with increasing length of the alkane segment. The results clearly indicate that the "spacer concept", which had been proven successful in the optimization of the proton conductivity of heterocycle based systems, fails in the case of phosphonic acid functionalized polymers. Instead, a very high concentration of phosphonic acid functional groups forming "bulky" hydrogen bonded aggregates is suggested to be essential for obtaining very high proton conductivity. Aggregation is also suggested to reduce condensation reactions generally observed in phosphonic acid containing systems. On the basis of this understanding, the proton conductivities of poly(vinyl phosphonic acid) and poly(meta-phenylene phosphonic acid) are discussed. Though both polymers exhibit a substantial concentration of phosphonic acid groups, aggregation seems to be constrained to such an extent that intrinsic proton conductivity is limited to values below sigma = 10(-3) S cm(-1) at T = 150 degrees C. The results suggest that different immobilization concepts have to be developed in order to minimize the conductivity reduction compared to the very high intrinsic proton conductivity of neat phosphonic acid under quasi dry conditions. In the presence of high water activities, however, (as usually present in PEM fuel cells) the very high ion exchange capacities (IEC) possible for phosphonic acid functionalized ionomers (IEC >10 meq g(-1)) may allow for high proton conductivities in the intermediate temperature range (T approximately 120 -160 degrees C). more...

Different procedures based on parameters of the wideline NMR absorption spectrum are presented to obtain localized molecular mobility contrast for imaging of solid polymers. For this purpose a 1H-NMR imaging technique with magic sandwich echoes is used for acquiring localized wideline spectra. With samples composed of polystyrene and high impact strength polystyrene, and polycarbonate and low density polyethylene a spatial difference in NMR absorption spectrum lineshape and linewidth is displayed. Furthermore, the spatial distribution of rigid and mobile domains in a heterogeneous polymer can be derived from the NMR spectral components. It is demonstrated that a van Vleck moment analysis can be performed from spatially resolved magic echo decays. The second (M2) and fourth (M4) moments of the rigid components show considerable variation with the spatial composition of the investigated samples. more...

A solid-state 1H nuclear magnetic resonance (NMR) imaging technique based on magic sandwich echoes (MSE) is described for obtaining spatial projections of solid polymer samples. The modification of MSE multiple echo detection is combined with short gradient pulses applied during the sandwich windows. This allows proton homonuclear decoupling in solids with dipolar couplings up to 50 kHz. For a rapid gating of the gradients a fast gradient pulse driver was constructed with switching times between 550 and 910 ns giving a maximum gradient strength of 330 mT/m. With our detection technique the spectral width can be doubled. One-dimensional projections and spatially resolved spectra of rigid polymer phantoms are presented. For contrast enhancement the use of the T1 relaxation time and the number of the magic sandwiches functioning as an adjustable magnetization filter discriminating between different strengths of the dipolar coupling is demonstrated. more...

NMR imaging with protons is becoming a more and more versatile tool for material research. Two different approaches are presented to obtain spatial resolution in elastomers and solids. The first, magic-sandwich-echo-imaging, belongs to the group of multiple-pulse techniques which are applied to overcome the strong dipolar couplings in solids. A driver for fast gradient pulses was constructed, and the technique was used to measure one-dimensional projections and spatially resolved spectra of rigid polymers. The second approach uses surface coils. In this way large objects can be investigated from the surface at optimum signal to noise. The drawback of the inhomogeneous B1 field resulting in signal attenuation across the image can be overcome by imaging an NMR parameter like the relaxation time T2. This was applied to image a phantom of different polyurethane foams. Furthermore spin-echo images of signal intensity maps around conducting wires are presented for the analysis of current distributions. more...

The size of micro-separated domains of polyether (PEO), -[CH2CH2O]n-, and polycarbonate (PC), -[(C6H4)-C(CH3)2-(C6H4)-OCO2]m-, in the dialysis membrane 'Gambrane' have been determined using an advanced solid-state NMR technique which exploits differences in 1H spin diffusion. The characteristic diameter of a PEO domain is 4.8 +/- 1.4 nm and that of PC is 5.2 +/- 1.4 nm with a mixed phase region of 0.8 +/- 0.5 nm. more...

The two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphous polymers in the glass transition range. Acquisition of the time domain data with a four-pulse sequence leads to new characteristics in the corresponding 2H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis-1,4-polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to Tg, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF) equation. In addition, the width of the correlation time distribution is found to decrease with increasing temperature. [ABSTRACT FROM AUTHOR] more...

Theoretical as well as experimental examples concerning the evolution of the two-time distribution S2|0 (ω1,ω2 ;tm) as a function of the mixing time tm are presented, where S2|0 is identical with the two-dimensional (2D) absorption spectrum rendered by 2D exchange NMR spectroscopy of static powder samples. The model calculations comprise standard models like isotropic rotational diffusion or overall isotropic reorientation combined with discrete internal rotational jumps to simulate the chain dynamics of polymers. In any case, the 2D spectrum directly reflects the main aspects of the underlying motional mechanism. An axially symmetric coupling (η=0) between spin and lattice is assumed throughout. Thus, the angular information contained in a 2D spectrum is completely specified by a one-dimensional jump angle distribution supplied with each spectrum. In connection with the simulations the numerical mapping of a discrete distribution function into a space of new variables is discussed. In the experimental section 2H NMR spectra of chain deuterated linear polystyrene above its glass transition temperature are shown, which are compared with the model calculations. [ABSTRACT FROM AUTHOR] more...

We present a new multidimensional NMR exchange experiment, three dimensional difference correlated exchange spectroscopy (DICO), which allows one to extract detailed information about the time scale as well as the geometry of rotational processes simultaneously. In contrast to experiments presently available the geometric information specified by a dynamic order parameter is independent of a specific model. Among others full isotropisation can be directly seen from the spectrum via a symmetry criterion. We apply this method to study the chain motion of polystyrene slightly above the glass transition. The DICO spectra prove that a typical chain segment performs small angle fluctuations as well as large angle jumps which can neither be described as random jumps nor tetrahedral jumps. A typical jump angle can be estimated to be of the order of 30°-40°. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR] more...