Molecular motion of syndiotactic poly(α-methylstyrene) in solution studied by carbon-13 n.m.r. relaxation measurements

The carbon-13 n.m.r. spin-lattice relaxation times, nuclear Overhauser enhancement factor (NOE), and line widths have been measured for a syndiotactic poly(α-methylstyrene) in solutions in toluene-d₈ and o-dichlorobenzene-d₄ as a function of temperature from 40° to 163δC. The single correlation time model of relaxation is inadequate to explain the data of spin-lattice relaxation time and NOE. But, within experimental error, these relaxation data in two solvents over a full temperature range can be interpreted in terms of either the Cole-Cole or the log-χ² distributions of correlation times, or a conformational jump model proposed by Monnerie et al. The internal rotation of the methyl group is relatively rapid, while that of the phenyl ring is slow and practically overshadowed by the backbone segmental reorientation over the temperature range examined. The solvent dependence of relaxation data was discussed.     

N.m.r. study of cellulose-water systems: Water proton spin-lattice relaxation in the rotating reference frame

A pulsed n.m.r. study of relaxation in the rotating reference frame was performed on cellulose samples with moisture contents (MC) ranging from 20.5% to 218%. The dependence of the spin-lattice relaxation time (T_1ρ) in the rotating frame on temperature in a sample hydrated to approximately the fibre saturation point (FSP) indicates that in this case water exists in two different environments or phases. The first phase consists of a small number of water molecules (<1% of adsorbed water molecules in sample) tightly bonded to cellulose whereas the second phase consists of the remaining adsorbed water molecules which are more loosely attached. At room temperature exchange occurs between these phases at an intermediate exchange rate. For samples with MC >FSP a third water phase was identified. From the dependence of T_1ρ on the rotating field strength information about water molecule dynamics was deduced.     

N.m.r. study of molecular dynamics in chemically crosslinked polyethylene

Results of measurements of the proton spin-lattice relaxation time T₁ in chemically crosslinked low-pressure polyethylene are presented. The dicumyl peroxide used for crosslinking changed the α-process dynamics without disturbing the β -process or the motion of the methyl groups. Classical relaxation processes (which are analogous to the case of pure polyethylene) have been observed for samples with 0.5–2.5 wt% crosslinking.     

N.m.r. of solid biopolymers

The dynamical behaviour of four solid polypeptides (α-chymotrypsin, insulin, lysozyme, ribonuclease A) has been investigated by proton magnetic relaxation. The interpretation was supplemented by measurements on the constituent amino acids, and on dipeptides, tripeptides and homopolypeptides. The following motions were identified and characterized: methyl group reorientation, segmental motion, sidechain motion, proline ring puckering, reorientation of water molecules in the structure. Slow motions were investigated through measurements of the dipolar relaxation time T_ID.     

Deuteron n.m.r. study of chain motion in solid polyethylene

The chain motion in the amorphous regions in linear polyethylene is studied by exploiting various ²H n.m.r. pulse methods. The experimental techniques employed are described in considerable detail. In particular, the solid echo- and the spin alignment spectra are recorded over a wide range of temperatures (123–393 K) as a function of the pulse spacings. In addition the spin-lattice relaxation times of both polymerization and alignment are given for the crysralline as well as the amorphous regions. The data are analysed in terms of highly constrained conformational motions which generate an exchange of C-H bond directions between 2, 3, or all 4 tetrahedral sites on a diamond lattice. Thus in the temperature interval from the γ-relaxation up to ～200 K only 2 sites are accessible, whereas a third site is appreciably populated at room temperature. Between 330 K and the melting point the number of conformations accessible to the chain must be high enough to lead to an increasingly isotropic 4 site exchange. The motion remains highly constrained and localized, however, up to the melting region as shown by the analysis of the spin-alignment spectra. The intensity ratio of the deuteron n.m.r. signals from the rigid crystalline and the mobile amorphous regions, respectively, yields the crystallinity of the sample as a function of temperature in remarkable agreement with the results obtained by X-rays. Finally, frequency dependent relaxation of spin alignment due to torsional oscillations of the chains in the crystalline regions is observed and analysed.     

Secondary relaxation processes in polyethylene terephthalate-additive blends: 1. N.m.r. investigation

High-resolution carbon-13 and deuterium n.m.r. experiments have been used to examine the effects that low-molecular weight additives have on the secondary relaxation processes in polyethylene terephthalate (PET). These experiments have revealed that both the phenyl rings and the carbonyl groups in PET undergo small angle oscillations at temperatures below the glass transition temperature. It would, however, appear from the deuterium n.m.r. experiments that the phenyl rings also undergo rapid 180 flips, which appear to be closely related to the relaxation peak that is observed in dynamic mechanical experiments. Extremely little motion has, however, been observed in the ethylene glycol units and it has therefore been concluded that this group does not contribute to the secondary relaxation processes. Finally it has been observed that by blending low-molecular weight additives into the polymer it is possible to significantly reduce the number of the phenyl ring flips in the polymer and thereby suppress the relaxation peak.     

1H n.m.r. spectroscopy of the complexation of polybutadienyllithium by N,N,N′,N′-tetramethylethylenediamine

Partially deuterated oligomeric butadienyllithium was prepared both by capping a d-6 chain with d-4 monomer (1,1,3,4-tetra-deuterobutadiene) and by direct oligomerization of the d-4 monomer. The initiator, sec-butyllithium, was used at low concentration to minimize the level of vinyl addition and the N,N,N′,N′-tetramethylethylenediamine (TMEDA) was added only after polymerization was complete. The introduction of TMEDA causes the γ proton resonances of the allylic carbanion to move upfield and the β resonances downfield, corresponding to an increase in the extent of charge delocalization. With ratios of TMEDA: Li below 1:1 the carbanions having the cis conformation are complexed preferentially.     

Molecular motion of polycarbonate in the vicinity of −60°C as revealed by broad-line n.m.r., magic angle spinning n.m.r. and conformational analysis

On the basis of correlation of data obtained by broad-line n.m.r. and magic angle spinning n.m.r. analysis as well as conformational analysis, conclusions have been drawn concerning the nature of molecular motion occurring in a polycarbonate chain in the vicinity of ?60°C. It is postulated that this motion is connected with conformational reorientation of bisphenol in unordered chains with a random conformation.     

N.m.r. studies of molecular motion in modified polypropylene

N.m.r. proton spin-lattice relaxation measurements were performed on modified polypropylene at 88 MHz over a wide range of temperatures. The modification by ethylene acrylate copolymer produced compatibility between the polymer fibres and the dyestuff molecules. The influence of the modifier on the segmental motion and on the methyl group relaxation regions was studied, and a slight shift of the high frequency glass transition to lower temperatures and a change in the form of T₁ minima were found. Both effects are discussed in terms of a qualitative n.m.r. relaxation model.     

N.m.r. and macromolecular migration in a melt or in concentrated solutions

A procedure of analysis of n.m.r. measurements is proposed as an approach to the observation of a single chain diffusion in a melt. The dynamic screening effect is applied to the definition of a temporary submolecule; this is used as a semi-local probe to investigate collective motions of all parts of a polymer chain. Measurements performed on ¹³C nuclei or protons lead to similar conclusions. The best agreement with experimental results is obtained by combining a M³ dependence of the terminal relaxation time (M is the chain molecular weight) with a multiple-mode relaxation spectrum; all modes have the same statistical weight.     

Anionic polymerization of isoprene by the complexes oligoisoprenyllithium/tertiary polyamines in cyclohexane: 3. Structural study of the polymers by 1H n.m.r. at 100 MHz

The microstructure of polyisoprene prepared anionically in the presence of catalytic amounts of N,N,N′,N′-tetramethylethylenediamine (TMEDA) and pentamethyldiethylenetriamine (PMDT) has been studied as a function of $\text{r =}\text{[}\text{TMEDA}\text{]}\text{[}\text{living ends}\text{]}$ and $\text{r =}\text{[}\text{PMDT}\text{]}\text{[}\text{living ends}\text{]}$. No significant effect is observed for r ? 0.5. The structure varies drastically for r > 0.5. The 4,3 addition increases mainly at the expense of the cis4,1 addition and a plateau is reached for r ? 1.0. The results are discussed on the basis of the nature of the living species.     

Configurational structures of lactic acid stereocopolymers as determined by 13C{1H} n.m.r.

Investigations are made of the ¹³C{¹H} n.m.r. spectra of a series of poly(lactic acid) stereocopolymers obtained by different means, i.e. ring-opening polymerization of l-, rac- and meso-lactides initiated by powdered Zn, or copolymerization of l- and rac-lactides in different proportions, or condensation polymerization of rac-lactic acid. Resonances arising from the stereosensitivity of the carbonyl carbon atoms have been resolved using the resolution enhancement technique. The fine structures thus obtained are discussed in terms of configurational sequences. Comparison is made of the peak intensities with theoretical stereosequence distributions obtained when assuming single and pair additions of repeating units according to the Bernoullian statistics. Neither of the models for triad, tetrad and pentad stereosequences agrees with the area of the experimental lines. It is shown that redistribution of stereosequences in poly(lactic acid) occurs because of transesterification at the ester bonds in the melt. The final stereosequence distribution in the polymers obtained by ring-opening polymerization of lactides is discussed in relation to the pair-addition mechanism and to the redistribution due to transesterification reactions.     

On the solution conformation of poly(ethylene oxide). An FT-pulsed field gradient n.m.r. self-diffusion study

Frictional coefficients for short-chain poly(ethylene oxide) have been determined by spin-echo n.m.r. measurements in chloroform, benzene and water. The results do not support an anomalous helical conformation in aqueous media but rather a common structure in all three solvents. Diffusion coefficients for PEO in dextran in NaPSS solutions have been determined. It is found that PEO transport is related to the macroscopic viscosity of the system, i.e. to both the concentration and molar mass of the matrix polymer.     

Small and large scale segmental motion in polymers: estimating cooperativity length by ordinary relaxation experiments

We derive a suitable expression for estimating the size of the cooperatively rearranging regions (CRRs) in supercooled polymer melts by fitting data worked out by ordinary relaxation experiments carried out in isothermal conditions. As an example, the average CRR size in poly(n‐butyl methacrylate) in proximity to the glass transition temperature is derived from a stress relaxation experiment performed by means of an atomic force microscopy setup. Good agreement is found with results in the literature derived from measurements of temperature fluctuations (the so‐called Donth method). The temperature dependence of the CRR size is explored for poly(butadiene); in this case the segmental relaxation function is derived through a novel method for the analysis of the efficiency with which free induction decay echoes are refocused in ¹H NMR experiments. It is found that the CRR size increases upon cooling. The results derived from the analysis of the NMR data are found to be in satisfactory agreement with those worked out from broadband dielectric spectroscopy data in the literature. © 2015 Society of Chemical Industry     

N.m.r. study of copolymer of styrene with methyl methacrylate prepared in presence of zinc chloride

In the n.m.r. spectrum of a styrene (Sty)-methyl methacrylate (MMA) copolymer prepared in presence of zinc chloride at 60°C the phenyl protons of the styrene unit appeared as one peak at 3τ, the methoxy protons of methyl methacrylate unit appeared as three peaks at 6.4–6.8τ, 7.1–7.3τ and 7.5τ. The position of these peaks indicated an alternate copolymer. This was further confirmed by estimation of copolymer composition from n.m.r. spectrum and element analysis. The value of ‘σ’ which is the probability of alternating MMA and Sty units having same configuration was also determined and found to be 0.865 ±0.006. It was found that the value of ‘σ’ was a direct function of concentration of zinc chloride.     

H n.m.r. study on the sequence length of polysulphide linkages in the copolymer of chloroprene and sulphur

The sulphur rank distribution and the structures of chloroprene units in the chloroprene-sulphur copolymer have been investigated in detail by proton nuclear magnetic resonance spectroscopy (¹H n.m.r.). Two factors, the sulphur rank and the structures of chloroprene units, which complicate the spectrum, were elucidated by modification of the copolymer structure through partial and full reductions of the polysulphide linkages. The populations of the chloroprene units with different structures adjacent to the sulphur units were found to be different from those of the homosequence. It was found that the major sulphur unit in the copolymer is octasulphide, the next disulphide and the least common are the other polysulphides composed of 3–7 sulphur atoms. No evidence of monosulphide was found.     

Local composition heterogeneities occurring during crystallization in PVF2/PMMA blends as studied by high-resolution solid-state 13C n.m.r.

High-resolution solid-state ¹³C n.m.r. has been used to study the amorphous phase of PVF₂/PMMA blends. The decrease of the carbon magnetization as a function of the contact duration in a cross-polarization sequence was analysed in terms of the local composition heterogeneities occurring during PVF₂ crystallization. The influence of the composition and thermal history of the samples is discussed.     

The effect of polydispersity on measuring polymer self-diffusion with the n.m.r. pulsed field gradient technique

The echo attenuation in the n.m.r. pulsed field gradient technique for polydisperse samples ($\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}\text{= 1.06, 2.0}$ and 10) was calculated, which, for the smallest polydispersity, is already markedly nonexponential. The echo attenuations of solutions of a polydisperse polystyrene, mixtures of monodisperse polystyrenes and the melt of a commercial polyethylene were measured. In the solutions, depending on the concentration regime, the echo attenuations were considerably less nonexponential than calculated and often exponential within experimental error giving one averaged self-diffusion coefficient. An averaging of the self-diffusion process does exist which is not in accordance with the reptation concept. In the entangled melt the echo attenuation is the sum of the contributions of the different molar masses, where no averaging is present, but, due to the strong dependence of the nuclear relaxation times on molar mass, the smaller molar masses are more strongly weighted and a dependence of the echo attenuation on the diffusion time arises.