The crystallite orientation distribution functions of deformed polytetrafluoroethylene: Multiple-pulse 19F NMR spectroscopy

A moment analysis of dipolar-decoupled ¹⁹F NMR spectra of deformed polytetrafluoroethylene leads to an expansion which approximates the crystallite orientation distribution function. The resulting distributions are then interpreted in terms of this polymer's known mechanical properties, and an attempt is made to model these functions.     

Chain segment ordering in uniaxially strained networks: A deuterium NMR investigation

The deuterium NMR (²H-NMR) is used for probing the chain segment orientation in polymer networks under uniaxial stress. The method is based on the observation of an incomplete time averaging of quadrupolar interactions affixed to deuterated segments. The samples are end-linked polydimethylsiloxane networks. The ²H-NMR experiments are performed either on labelled network chains or an labelled probe polymer chains dissolved in the network. The basic results are the following: — The induced uniaxial order is related to a uniaxial dynamics of chain segments around the direction of the applied constraint. — A permanent orientation is observed on free polymer chains dissolved in the deformed networks. — The mean degrees of orientational order induced along short and long chains in bimodal networks are the same. These experimental facts appear as evidences for cooperative orientational couplings between chain segments in the deformed networks.     

Chain dimensions in plastically deformed semicrystalline polymers

The effect of plastic deformation on the chain dimensions of polymers in the semicrystalline state was investigated using linear hydrogenated polybutadiene (HPB), a model ethylene/butene-1 copolymer having about 40% crystallinity at room temperature. Dilute blends of deuterium-labeled chains with various molecular weights (20,000 ≤ M ≤ 214,000) in the same unlabeled matrix (M = 95,000) were uniaxially stretched at 25°C to extension ratios of α = 2.4 and 4.4. Radius of gyration normal to the stretch direction R_⊥ was measured for the labeled chains by small-angle neutron scattering. The molecular extension ratio inferred from these data α_m = R/R was significantly smaller than α for short chains (M < 50,000) but increased to the affine range α_m = α for M > 100,000. This variation in α_m/α closely parallels the molecular weight dependence of mechanical strength and ductility in HPB over the same range.     

Segment orientation in topologically constrained networks

Using the continous Edwards–chain model the simultaneous influence of harmonic–like constraints and of a stochastic order parameter field on the segment orientation in polymer networks is investigated. The harmonic potential reduces the orientation order parameter in comparison to the value for an unconstrained freely jointed chain. This reference value is approached from below in the case of strong topological constraints. The introduction of a stochastic order parameter field enlarges the orientation but the reference value remains the upper boundary.     

NMR of tracer molecules in networks

Liquid NMR techniques were used to elucidate the molecular organization and dynamics of poly[tetra(ethylene glycol)dimethacrylate] and poly[tetra(ethylene glycol)diacrylate] as a function of cure. The change in line width of the monomer was used to determine the extent of cure and indicate the mobility in the polymer matrix. Line widths of different parts of the monomer peaks broadened at different extents of cure. Non-reacting tracer molecules (dimethylsulphoxide, methyl acetate, decalin, cyclohexane and dodecane) were also introduced into the curing system at low concentrations and their line widths observed. It was found that they broadened at differing rates and persisted well into the later stages of the cure when the monomer peaks had broadened into the baseline. In typical cures the monomer peaks broadened simply but, in atypical cases, the line shapes showed that there were two monomer domains one constrained and the other unconstrained. All tracers became constrained with increasing cure, except dodecane. The line shape of pure monomer was also followed as a function of temperature and found to broaden in similar fashion to the monomer during a typical cure.     

Structural characterisation of poly(amidoamine) networks via high-resolution magic angle spinning NMR

A comprehensive structural characterisation of cross-linked insoluble poly(amidoamine) (PAA) networks was performed by high-resolution magic angle spinning (HRMAS) NMR spectroscopy. Model samples with 20%, 40% and 80% cross-linking degrees were prepared and the best conditions to obtain high-resolution spectra in the gel phase determined. Whereas the samples with 20% and 40% cross-linking degrees could be exhaustively resolved and described, the sample with 80% cross-linking degree could not be characterised by this technique owing to insufficient mobility of the polymer segments. Even with this limitation, the method developed in this study can be reasonably considered as a general one, which enables exhaustive characterisation of cross-linked PAA networks of biomedical interest.     

Relaxation of stress and orientation in thermo-reversible networks

4-(3,5-dioxo-1,2,4-triazolidin-4-yl) benzole (U4A) or -isophthalic acid (U35A) units randomly attached to a polybutadiene backbone form extended supramolecular hydrogen bond assemblies, which act as effective junction zones in unusual thermoreversible elastomer networks. The relaxation behavior of stress and the segmental orientation induced after a sudden strain, has been investigated by means of step-strain experiments and ²H-NMR spectroscopy.The stress relaxation behavior can be described as a two-step process in which the first step is very sensitive to the stability of the supramolecular aggregates. About 20 s after reaching the initial strain, stress is reduced to a value comparable to the yield-stress in stress-strain cycles. The long-time relaxation behavior is interpreted as a reduction of the effective cross-link density due to a gradual breakdown of the hydrogen bond clusters. The relaxation data of the functionalized polybutadienes can be superimposed to give master curves, which are characteristic for the type of junction zone. The degree of aggregate degradation is estimated as a function of the initial stress.In the same type of step-strain experiment, the ²H-NMR spectra show a decay of the line splitting as well as a change in the complete lineshape.     

Chain orientation and intrinsic anisotropy in birefringence-free polymer blends

We found 10 miscible pairs of dissimilar polymers that rendered birefringence-free polymer blends. Each pair consists of a positive birefringence polymer and a negative one. Zero birefringence in the blend is attained as the result of compensation of positive and negative contributions to the overall birefringence. This concept was confirmed by characterizing the chain orientation of individual chains in the blend using polarized Fourier-transform infrared spectroscopy and by estimating theoretically the intrinsic optical anisotropy of the chain molecules.     

Crystallite orientation in stretched polychloroprene networks. II

The orientation of crystallites grown isothermally in several drawn trans-polychloroprene networks is studied as a function of crystallization temperature t_x, degree of crystallinity ω, and elongation ratio α. The orientation distribution is particularly simple for this polymer since the crystallographic c axis (chain axis) orients preferentially along the stretching direction, while a and b are randomly arranged about c. Hence the parameter cos² χ_c adequately characterizes the distribution, where χ_c is the angle between the c axis and the fiber axis, and the average is taken over all crystallites. A treatment due to Krigbaum and Roe is utilized to obtain values of v (the number of statistical segments comprising the crystallization nucleus of critical size) through comparison of the average orientation of crystallites and amorphous statistical segments. The behavior observed falls into two categories. First, if the initial amorphous network is well oriented, 〈cos² χ_c〉 is independent of crystallinity during both crystallization and melting, and v varies with t_z (or the degree of supercooling) as predicted by nucleation theory. If different networks are to have the same crystallite orientation distribution, they must not only be crystallized at the same supercooling, but must also have the same distribution of amorphous segment orientations. Both the relative elongation and the network crosslink density affect the latter distribution. Next, we consider the second category. If the initial amorphous orientation is poor, 〈cos² χ_c〉 decreases linearly during crystallization and increases along approximately the same path during melting. Further, 〈cos² χ_c〉 for a given t_z yields v values which are too large. These two behaviors can be explained if, in the former case, nucleation involves the best oriented statistical segments of all network chains, while in the latter there is a selection according to the chain displacement vector orientation. Thus, if the amorphous orientation is poor, both the orientation and thermodynamic stability of the crystallites decreases with further crystallization. If this decreased stability is reflected in shorter fold lengths, the reversible variation of long period spacing with temperature reported earlier for an oriented polychloroprene network can also be explained as a preferential melting process.     

Fluorescence polarization measurements of orientation in labeled polyisoprene networks

Fluorescence polarization was used to measure segmental orientation in stretched polyisoprene networks. Stress and orientation of chemically bonded fluorescent labels were simultaneously recorded during stretching. Two deviations from the classical theory of rubber elasticity were observed: (i) an extra orientation of dry networks, which was interpreted by assuming the existence of weak nematiclike interactions between segments, and (ii) a saturation of orientation at high elongation which could be due to local conformational changes.     

Determining the solution state orientation of a Ti enolate via stable isotope labeling, NMR spectroscopy, and modeling studies

Our group has used Ti-promoted aldol additions with an oxazolidineselone as the chiral auxiliary with much success. In these reactions, the Se atom in the auxiliary both promotes stereospecific addition as well as reports on, through the use of 77Se NMR spectroscopy, the ratio of diastereomers produced and the geometry of intermediates as the reaction proceeds. Through stable isotope labeling and NMR spectroscopy, we are able to experimentally observe a Ti enolate in solution and gain insight into its structure and reactivity. Results from molecular modeling calculations are also presented for comparison with NMR data.     

Tricyclic orthoamides: Effects of lone-pair orientation upon NMR spectra

A series of novel tricyclic orthoamides has been synthesized. The stereochemical dependence of methine ¹H chemical shifts and ^IJ_CH are reported.     

Computer systems for laboratory networks and high-performance NMR

Modern computer technology is significantly enhancing the associated tasks of spectroscopic data acquisition and data reduction and analysis. Distributed data processing techniques, particularly laboratory computer networking, are rapidly changing the scientist's ability to optimize results from complex experiments. Optimization of nuclear magnetic resonance spectroscopy (NMR) and magnetic resonance imaging (MRI) experimental results requires use of powerful, large-memory (virtual memory preferred) computers with integrated (and supported) high-speed links to magnetic resonance instrumentation. Laboratory architectures with larger computers, in order to extend data reduction capabilities, have facilitated the transition to NMR laboratory computer networking. Examples of a polymer microstructure analysis and in vivo 31P metabolic analysis are given. This paper also discusses laboratory data processing trends anticipated over the next 5-10 years. Full networking of NMR laboratories is just now becoming a reality.     

Dynamic uniaxiality in strained networks: A deuterium NMR investigation

The orientational order generated in a polymer network by a uniaxial stress is probed using deuterium NMR. The experiments are performed on end-linked polydimethylsiloxane networks. When a uniaxial force is applied, the observed NMR spectra show that the chain segments reorient uniaxially around the external force direction. This experimental fact appears as new evidence for cooperative orientational interactions between chain segments in the deformed networks.     

Heteronuclear recoupling in solid-state magic-angle-spinning NMR via overtone irradiation

A heteronuclear dipolar recoupling scheme applicable to I-S spin pairs undergoing magic-angle-spinning (MAS) is introduced, based on the overtone irradiation of one of the coupled nuclei. It is shown that when I is a quadrupole, for instance (14)N, irradiating this spin at a multiple of its Larmor frequency prevents the formation of MAS dipolar echoes. The ensuing S-spin signal dephasing is significant and dependent on a number of parameters, including the I-S dipolar coupling, the magnitude of I's quadrupolar coupling, and the relative orientations between these two coupling tensors. When applied to a spin-1 nucleus, this overtone recoupling method differs from hitherto proposed recoupling strategies in that it involves only the +/-1> I(z) eigenstates. Its dephasing efficiency becomes independent of first-order quadrupolar effects yet shows a high sensitivity to second-order offsets. A constant-time/variable-offset recoupling sequence thus provides a simple route to acquire, in an indirect fashion, (14)N overtone spectra from rotating powders. The principles underlying this kind of S-(14)N experiments and different applications involving S = (13)C, (59)Co sites are presented.     

Brillouin scattering studies of the effect of orientation on mechanically deformed poly(ethylene terephthalate)

Brillouin scattering is used to study the internal structure of oriented films of poly(ethylene terephthalate). Splitting in the longitudinal spectrum is observed as the film is stretched, indicating that the crystalline region is developed gradually from the amorphous region. The hypersonic velocity data obtained from these two regions are used to draw directional maps of sound velocity propagation in different directions of the film. The results are discussed and correlated with a recently proposed model. The orientational parameter in the amorphous phase is calculated from the hypersonic velocity data as a function of stretch ratio. The results are found to be in good agreement with published values obtained by a different technique.     

Polymer networks via cationic ring-opening polymerization

Three methods for the formation of polymer networks from bifunctionally growing polymers obtained by cationic ring-opening polymerization are described. The first method is based on the irreversible inter-molecular termination reaction of thietane polymerizations. Starting from bifunctionally living poly(THF) a new kind of polymer structure consisting of ABA block copolymers with cross-linked A-segments is obtained. The second method is the direct coupling of active species with primary amines or ammonia. The third method consists in transformation of the living end groups of poly(THF) into triethoxysilane end groups, followed by cross-linking by addition of water and a trace of acid.