Pulsed n.m.r. of cis-polyisoprene: 1

Proton spin-spin (T₂), and spin-lattice (T₁) relaxation time measurements are reported for six monodisperse cis-polyisoprenes ($\text{M}\text{?}{}_{\mathrm{n}}$ from 2000 to 200 000) over the temperature range from ?50° to 170°C. At low temperatures (?30° to 10°C) T₁ and T₂ are determined by the short range segmental motions but above 10°C T₂ is sensitive to the long range motions. When $\text{M}\text{?}{}_{\mathrm{n}}\text{? 30 000 T}{}_2$ becomes influenced by the presence of entanglements which produce a transient network structure and this confers on the spin-spin relaxation a pseudo-solid-like response. Similar behaviour is observed in crosslinked networks produced by irradiation. The results are discussed in terms of the types of motion occurring in amorphous polymers above T_g and the analogy with dynamic mechanical measurements is discussed.     

Nuclear magnetic relaxation studies of coal

The techniques of both pulsed nuclear magnetic resonance (n.m.r.) and electron spin resonance (e.s.r.) were applied to clarify the nature of coal. The spin-lattice relaxation time (T_1n) and the spin-spin relaxation time (T_2n) for protons were measured. At room temperature T_1n varies with rank of coal and shows a maximum value at about 86% carbon of coal. The values of T_1n for the samples selected increase in the order of extraction residue, parent coal and pyridine extract respectively. The variation of T_1n during solvent treatment reflects the different circumstances of protons in such a heterogeneous system as coal. From spin-spin relaxation time (T_2n) measurements two relaxation regions, that is an immobile region and a relatively free one, are deduced.     

N.m.r. studies on the effect of water on the glass transition of polystyrene

These investigations are concerned with water-polymer interactions in polymer latices. It is known that water can act as a plasticizer for many solid polymers and cause a reduction in the glass transition temperature, T_g, of the amorphous regions. Experiments were carried out to determine whether pulsed n.m.r. techniques could be used to study the T_g of a polymer suspension and hence the influence of water and electrolyte on it. From T₁ and T₂ proton relaxation measurements as a function of temperature on polystyrene latex systems it was shown that the presence of water lowers the T_g of the polymer particles (by about 10°C), the effect being slightly greater in the presence of concentrated electrolyte. The extent of electrolyte penetration into the particles was deduced by studying relaxation as a function of particle diameter in latices containing paramagnetic Mn²⁺ ions. Using simple theories of relaxation and spin diffusion it was concluded that for all but the smallest particles electrolyte penetration is restricted to a very thin shell of the order of 1 nm. These conclusions were supported by the results of similar measurements on PTFE particles.     

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.     

Characterization and behaviour of epoxy-based diaminodiphenylsulphone networks

We have analysed the polycondensation of the system based on diglycidyl ether of bisphenol A (DGEBA) crosslinked with 4,4′-diaminodiphenylsulphone (DDS) in the absence of or presence of a catalyst, benzyldimethylamine or BF₃ complex. Chemical data obtained with Fourier transform infra-red spectroscopy (FTi.r.) and solid state ¹³C n.m.r. with CP MAS have enabled the various chemical species in the network to be qualitatively detected and quantitatively assayed. The molecular results related to reaction mechanisms, obtained as a function of the per cent catalyst present, were correlated with thermal (T_g) dielectric (β relaxation) and moisture absorption data.     

Pulsed n.m.r. studies of molecular motion in wet nylon-6,6 fibres

The influence of water molecules on molecular motion in commercial nylon-6,6 fibres has been investigated by pulsed n.m.r. techniques. Transient n.m.r. signals, T₂ and T₁ relaxation times for the fibre protons were measured as a function of temperature and moisture (D₂O) content. Above the glass transition temperature, T_g, of the fibre, separation of the signal into two components, a ‘rigid’ and ‘non-rigid’ fraction, was possible. For wet fibres, the temperature at which the ‘non-rigid’ or ‘mobile’ component appeared was reduced as the water content was increased and the ‘mobile fraction’ increased with temperature. This behaviour is explained in terms of the mobilization of amorphous chain segments above the T_g and their ability to be ‘plasticized’ by water molecules. The effect of D₂O molecules and paramagnetic Mn²⁺ ions on T₁ relaxation of rigid and mobile segments provided further information on the properties of accessible chain segments between and on the crystallite surfaces. A chain folded model of semi-crystalline morphology has been adopted throughout the discussion.     

Kinetics of gelation of aqueous gelatin solutions

The kinetics of gelation of gelatin + water solutions have been investigated by several experimental methods, in particular optical rotation, ¹H nuclear magnetic resonance and rheological measurements. During quenching and isothermal annealing of the solutions, the gelatin chains undergo a conformational coil→helix transition which is responsible for the network growth. The amount of helix (derived from optical rotation measurements) shows a time evolution which could be analysed as the combination of two processes, the first obeying an Avrami equation, the second proceeding logarithmically in time. The water ¹H n.m.r. measurements reveal that the spin-spin relaxation time T₂ is affected by these processes, thus implying that a fraction of water contributes to the network structure. Finally, the sol-gel transition could be followed with a Weissenberg rheogoniometer in oscillatory shear, for a suitable range of temperatures, when the system changes from a purely viscous liquid (the shear modulus G′=0), to an almost purely elastic solid (the loss modulus G″=0).     

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.     

Nuclear magnetic relaxation in linear and branched polyethylene

An assessment is made of the information provided by n.m.r. second moment, fourth moment and T₂ measurements on oriented samples of branched and linear polyethylene. Detailed theoretical comparisons are made with the previously reported experimental results of a number of workers in order to derive information about the molecular motions which underlie the α, β and γ relaxations in the two forms of the polymer. Plausible motional models are presented in each case. Particular attention has been given to the β relaxation which is explained in terms of lamellar fold motion. The relative sensitivity of the various n.m.r. measurements to molecular anisotropy and motion in the polymer is discussed.     

Network properties: 3. A pulsed n.m.r. study of molecular motions in some AB-crosslinked polymers

This paper presents the results of a pulsed n.m.r. study of molecular motions in poly (methyl methacrylate) (PMMA) and poly (methyl acrylate) (PMA) chains in a series of multicomponent network polymers consisting of poly (vinyl trichloroacetate) (PVTCA) crosslinked with PMMA and with PMA, with emphasis on segmental motions. Results of ancilliary broad line n.m.r. and dilatometric studies are included; the latter demonstrate that in PMA containing polymers microphase separation of the components is complete while in PMMA containing polymers a mixed microphase of PVTCA and PMMA and a pure PMMA microphase are formed. α-Methyl group rotations in PMMA chains and segmental motions in both PMMA and PMA chains are modified with respect to those in the corresponding homopolymers. Modifications to the segmental motions in the crosslinking chains are attributed to the fact that their chain ends are attached to PVTCA chains. It is considered that the comparative rigidity of PVTCA chains (T_g ～ 60°C) reduces segmental motions in at least portions of the PMA chains (T_g ～ 5°C) while the comparative mobility of PVTCA enhances segmental motions in PMMA (T_g ～ 100°C). Thus the molecular mobility of chains of one polymer is to some extent transmitted to chains of another polymer to which it is attached.     

Diluent effects on carbonate mobility in bisphenol-A polycarbonate in the solid state

The effect of miscible low molecular weight additives on the mobility of the carbonate group in bisphenol-A polycarbonate (BPAPC) has been studied using n.m.r. and dielectric relaxation experiments in the solid state. Proton-enhanced dipolar-decoupled carbon-13 n.m.r. spectra of BPAPC, isotopically enriched at the carbonate position, are obtained without magic-angle sample spinning. The resolved chemical shift anisotropy allows study of nuclear spin relaxation for the carbonate groups in the polymer that have different orientations relative to the static magnetic field in the laboratory frame. The spin-lattice relaxation time in the rotating frame (T_1?) is measured at a motional-probe frequency of 50 kHz for the undiluted polymer and for BPAPC-diluent blends containing either dibutylphthalate or dinitrobiphenyl. The T_1? exhibits some dependence on orientation in all systems studied. In the blend containing dibutylphthalate (DBP), T_1? is decreased by a factor of two for all orientations of the carbonate group. This implies that DBP substantially increases the spectral density of 50 kHz motions in the carbonate region of the polymer at ambient temperature. In contrast, dinitrobiphenyl does not significantly alter the Fourier component of thermal fluctuations at 50 kHz. Dielectric relaxation measurements at 10 kHz reveal that the primary (T_g) and secondary (β) motional processes in BPAPC are affected by low molecular weight additives. An intermediate relaxation process appears in the temperature interval between the glass transition temperature (T_g) and the sub-T_g β-relaxation (T_β) in the polymer-diluent blends. The n.m.r. spin-lattice relaxation rate in the rotating frame, T^?1_1?, correlates well with the relative magnitude of the dielectric dissipation factor (tan δ_ε) between T_g and T_β.     

Structural mobility of molecular bottle-brushes investigated by NMR relaxation dynamics

The structural mobility of monomeric units of molecular bottle-brushes was studied by a systematic evaluation of NMR relaxation dynamics. The spin-spin relaxation time (T₂) was determined by Carr-Purcell-Meiboom-Gill (CPMG) NMR spectroscopic measurements. T₂ for protons that reside on the exterior and interior of the bottle-brush macromolecules varied with the grafting density and side chain length in bottle-brush copolymers. Poly((2-(2-bromopropionyloxy)ethyl methacrylate-stat-methyl methacrylate)-graft-butyl acrylate) (poly((BPEM-stat-MMA)-graft-PBA) was studied as a model brush copolymer. The T₂ values for protons of MMA units in the brush backbone significantly decreased with increasing side chain length and grafting density of PBA. The mobility and relaxation times T₂ for the side chain PBA protons decreased with grafting density. However, after initial increase, the relaxation times eventually decreased with PBA side chain length.     

Effect of molecular oxygen on the n.m.r. relaxation times of some aromatic polymers

Proton spin-lattice (T₁) and spin-spin (T₂) relaxation times are reported for poly(2-vinylpyridine), poly(1-vinylanthracene) and polycarbonate in air, oxygen and in vacuo. The results substantiate earlier findings that oxygen complexes with the aromatic rings giving rise to T₁ minima which are not intrinsic to the polymers. This effect appears to be fairly general for aromatic containing polymers. For those polymers containing low temperature relaxations intrinsic to the polymer, the oxygen paramagnetic effect can complicate the relaxation behaviour and, in some cases, totally mask the intrinsic processes. The transitions in T₁ and T₂, due to the torsional oscillation of the anthracene side group in poly(1-vinylanthracene), is much better defined than the corresponding transitions for previously reported aromatic vinyl polymers. The activation energy for the motion of the side group is comparable to that of polystyrene and poly(N-vinylcarbazole).     

1H N.m.r. relaxation and molecular motion of polyethylene in solution as studied by the isotopic dilution technique

The molecular motion of polyethylene has been studied in decalin-d₁₈ solution by ¹H n.m.r. relaxation with the isotopic dilution technique which can separate the proton spin-lattice relaxation times into the intra- and intermolecular contributions, intra T₁ and inter T₁. The temperature dependences of intra and inter T₁s show that the intra- and intermolecular motions are in the extreme narrow region. These results, also, were compared with the results of molten polyethylene reported in our previous work.     

Rotating frame 1H n.m.r. spin-lattice relaxation studies of modified isotactic polypropylene

Rotating frame ¹H n.m.r. spin-lattice relaxation times T_1ϱ have been used to study relaxation processes in partially crystalline polypropylene and polypropylene modified by a copolymer of ethylene and alkylaminoacrylate. Measurements were carried out within the temperature range of 250–420 K. Three relaxation times T_1ϱ were detected over the whole temperature range. α_c relaxation in the crystalline regions of the polymer, relaxation processes β(U) and β(L), associated with an apparent double glass transition, and α_a relaxation process, which was ascribed to a free reorientation of the whole chains in the amorphous regions, were observed by temperature dependences of these relaxation times. In addition to the α_c relaxation, another relaxation process with relatively low molecular mobility was found in crystalline regions. The influence of the polymer modifier on the observed relaxation processes greatly depends on its amount.     

Pulsed n.m.r. relaxation study of a polystyrene/poly(ethylene oxide) diblock copolymer: evidence for interaction at the phase boundary

Pulsed nuclear magnetic resonance relaxation measurements are presented for a hydrophobic-hydrophilic diblock copolymer of polystyrene and poly(ethylene oxide) along with results for the individual homopolymers. Although observation of the thermal transitions of the homopolymer components in the copolymer reveals a gross incompatibility, the n.m.r. data suggests strong interfacial effects. From the T₁ and T₂ data below the melting point of PEO (T_m = 335K), it was concluded that polystyrene creates localized defects which reduce PEO domain crystallinity to 75% from 93% in the homopolymer. Above T_m in the copolymer three motional domains can be distinguished, isolated rigid polystyrene, isolated molten PEO and an intermediate domain. The composition of the intermediate domain suggests that 23% of the polystyrene is plasticized whereas 80–90% of the molten PEO is motionally constrained. The results are interpreted in terms of a spatial segregation of the homopolymer components with a forced interaction at the phase boundary as a result of the covalent linkages tying the two components together.     

Study of three-dimensional polystyrene networks containing linear chains: 2. Relaxation spectrum of linear chains

A study of the relaxation phenomenon in polystyrene networks containing linear polystyrene chains of high molecular weight has been carried out using measurements in methylcyclohexane and in the dry state by dynamic sinusoidal deformation and static stress relaxation. It is shown that the relaxation of the free chains contained in the network is (i) proportional to (1 ? v²₂) where v₂ is the volume fraction of the crosslinked network in the dry state, and (ii) depends very little on their concentration in the network. The distribution of relaxation time shows a peak related to the glass transition of the networks and a second peak corresponding to the spectrum of the linear chains of high molecular weight.     

N.m.r. study of the ferroelectric phase transition in a 7030mol% copolymer of vinylidene fluoride (VF2) and trifluoroethylene (TrFE)

Nuclear magnetic resonance (n.m.r.) studies of ¹⁹F nuclei in a $\text{70}\text{30}\text{mol}\text{\%}$ random copolymer of vinylidene fluoride and trifluoroethylene were performed at 9.14 MHz and 20.0 MHz. The free induction decays (FIDs) were analysed in terms of two T₂ components attributed to the amorphous and crystalline portions of the polymer. The changes in crystallinity as well as the effects of the ferroelectric transition were observed during cycles of heating and cooling between 20°C and 140°C. The crystalline component of the FID lengthens by a factor of 2 at 100°C on heating and decreases by this factor at 60°C on cooling, thus exhibiting the thermal hysteresis of this ferroelectric transition. The spin-lattice relaxation was also investigated. From measurements at 9.14 MHz the observed longitudinal relaxation time T₁ appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well defined minima of T₁ were observed, which are associated with the ferroelectric transition (especially after repeated annealing of the samples). Results are discussed in terms of the crystalline phase structure, which appears dynamically disordered above the ferroelectric phase transition. An analogy is considered with the plastic phase transitions encountered in molecular crystals.     

Selective saturation as an aid for carbon-13 CP/MAS n.m.r. analysis of coals at high frequencies

A selective saturation experiment is described and applied to selective elimination of ¹³C high resolution solid state n.m.r. signals and of their spinning sidebands, as well for sharp lines as for broad bands. A proper adjustment of experimental conditions permits selective saturation of the aromatic carbon signals of coal samples and eliminates unwanted overlapping of aromatic sidebands with the aliphatic region of the spectrum at high working frequencies. The described procedure seems to be especially useful for qualitative and quantitative high frequency n.m.r. investigations of coals having a distribution of short T₂ relaxation times.     

Molecular motion of poly(methyl methacrylate), polystyrene and poly(propylene oxide) in solution studied by 13C n.m.r. spin-lattice relaxation measurements: effects due to distributions of correlation times

The ¹³C n.m.r. spin lattice relaxation times and Nuclear Overhauser Enhancements have been measured for solutions of poly(methyl methacrylate) (PMMA) in o-dichlorobenzene, polystyrene (PS) in pentachloroethane and poly(propylene oxide) (PPO) in CDCI₃ as a function of temperature. For PS and PMMA a T₁ minimum is observed close to ambient temperature. The single correlation time theory of relaxation is inadequate to explain the relaxation data, but within experimental error, the data can be interpreted in terms of either the Cole-Cole distribution of correlation times, the log ?χ² distribution or a conformational jump model of chain dynamics.