Structure, dynamics and heterogeneity: solid-state NMR of polymers Jeremy Titman, School of Chemistry, University of Nottingham
Structure, dynamics and heterogeneity Structure Dynamics conformation, tacticity, cross linking, networks, crystallinity, polymorphism, packing effects, orientational distributions, polymer fibres and films, liquid-crystalline polymers mechanisms, rates, activation energies, mechanical relaxations, glass transition, inhomogeneous dynamics, correlation effects eterogeneity phase separation, domain sizes, polymer blends and block copolymers, morphology, semi-crystalline polymers ans W. Spiess
Crystallinity in PE by the γ-gauche effect Poly(ethylene) Poly(ethylene) is a semi-crystalline polymer comprising all-trans crystalline lamellae joined by conformationally disordered amorphous regions. The carbon-13 NMR spectrum shows a narrow peak at 33 and a broader one at 3. C C C with CP C The broader peak can be assigned to the more disordered and mobile amorphous regions. The lower chemical shift arises from the larger number of gauche conformations present in without CP these regions (the γ-gauche effect). The narrow peak can be ascribed to the more rigid crystalline regions. new crystalline form 5 4 gel spun 3 2 PE fibres 1 melt spun 5 4 3 2 5 4 3 2
Manufacturing conditions from carbon-13 spectrum Manufacturing Conditions The carbon-13 NMR spectrum of poly(ethylene) reveals information about the effect of manufacturing conditions on structure. Gel-spun ultra-high molecular weight poly(ethylene) is highly crystalline and shows a weak amorphous contribution compared to the more traditional meltspun poly(ethylene). C C C with CP C Note that gel-spun poly(ethylene) shows a new crystalline phase which gives a carbon-13 line at 35. 5 4 3 without CP 2 1 new crystalline form gel spun PE fibres melt spun 5 4 3 2 5 4 3 2
Polymer dynamics by solid-state NMR Most methods (dynamic mechanical spectroscopy, differential scanning calorimetry etc. ) used to characterize motions in polymers merely give an indication of the timescale; solidstate NMR provides additional information about geometry, amplitude and chemical location across 14 orders of magnitude in correlation time. Spin-lattice Relaxation. The observation of the minimum in T1 indicates the occurrence of motions in the Mz frequency range (τc < 1 µs). Lineshape Analysis. The observation of motional narrowing gives information about Timescale ps ns µs T 1 Temperature Relaxation Lineshape Analysis C 6 (Ph) local dipolar field motions in the kz range (τc < 1 ms), as well as information about the geometry of motions. Two-dimensional NMR. Exchange cross ms 2 1 Exchange NMR peaks indicate very slow motions (τc > 1 ms) and give detailed geometric information. s evolution mixing detection t 1! m t 2 " 1 " 2
Macromolecular dynamics and mechanical properties The changes in mechanical strength which occur as temperature is increased arise from the activation of different molecular motions The glass transition generally arises when large amplitude co-operative motions of main chain segments are activated leading to a large drop in mechanical strength. igh cross link densities impede the glass transition in elastomers. Mechanical relaxations at lower temperatures arise when local motions, often involving side chains, are activated. These motions give thermoplastic polymers impact strength. mechanical relaxation below the glass transition poly(vinyl chloride) thermoplastic Shear Modulus glass transition glass transition melting natural rubber elastomer decomposition 2 3 4 Temperature / K
Mechanical relaxations in PTFE by T1 Poly(tetrafluoroethylene) PTFE is a semi-crystalline thermoplastic with a melting temperature for the crystalline domains around 6 K a glass transition temperature for the amorphous matrix around 4 K. several mechanical relaxations below the glass transition temperature. Two contributions to the fluorine-19 spin-lattice and spinspin relaxation can be isolated, originating in the crystalline and amorphous parts of the polymer respectively. T 2 phase change crystalline regions T 1 amorphous regions 1 2 3 4 5 temperature / K
Mechanical relaxations in PTFE by T1 Amorphous regions The amorphous T2 increases rapidly above 22 K, while the amorphous T1 has a minimum at 273 K. This indicates the onset of molecular motion in the amorphous regions. 22 K τc ~ 2 µs. 1/τc must be of the order of the fluorine-19 linewidth to cause motional narrowing. T 2 273 K τc ~ 3 ns. 1/τc must be equal to the Larmor frequency (3 Mz in this case) at the T1 minimum. This motion occurs at too low a temperature to be ascribed to the glass transition. phase change Crystalline regions Both the crystalline relaxation times show a discontinuity at 292 K. This indicates a phase change in the crystalline regions. The crystalline T2 increases rapidly above 34 K, while the crystalline T1 has a minimum at 45 K. This indicates another motion starts in the crystalline regions below 34 K with τc ~ 3 ns at 45 K. T 1 crystalline regions amorphous regions 1 2 3 4 5 temperature / K
Glass transition in PMMA-d5 by lineshape analysis Poly(methylmethacrylate) is an amorphous thermoplastic polymer with a glass transition temperature of 398 K. The 2 lineshape is a sensitive probe of motion because of the large angular resolution which arises from the broad Pake powder pattern. Below the glass transition (38 K) the 2 spectrum of backbone labelled PMMA-d5 consists of two patterns arising from the (rigid) methylene and (rotating) methyl groups. Above the glass transition (43 K), the methyl group shows a lineshape which is characteristic of a motion with a broad distribution of reorientation angles and correlation times. At high temperatures (45 K), the signal consists of a slightly broadened isotropic line, characteristic of the fast motion limit (τc ~ 1 µs). temperature 45 K 44 K 43 K 418 K 38 K -15 15 kz S. C. Kuebler, D. J. Schaefer, C. Boeffel, U. Pawelzik and. W. Spiess, Macromolecules, 3, 6597 (1997).
Slow dynamics by exchange NMR During the mixing period of the two-dimensional exchange experiment molecules exchange orientations due to molecular motion. For a random jump motion with τc shorter than the mixing time a complete redistribution of intensity away from the diagonal occurs. For a well-defined jump angle intensity is redistributed into an elliptical ridge, the shape of which is determined by the geometry of the motion. well-defined jump " 1 " 2 powder pattern " 1 evolution mixing detection t 1 # m t 2! random jump " 2 " 1 " 1 " 2! " 2
Impact strength in POM Poly(oxymethylene) is a semi-crystalline thermoplastic with a highly mobile amorphous matrix with a glass transition temperature of 2 K. The crystals form helical structures in which 9 monomers occupy 5 turns of the helix. This implies that adjacent C2 groups along the helix are related by a 2 rotation. mixing time = 1s T = 335 K 2 3 1 C 2 helix axis 2 4 2 T = 36 K 9 5 helix Experiment Simulation
Impact strength in POM 252 K: two-dimensional exchange spectra with mixing times of up to 4 s show only diagonal intensity, indicating that any motions in the crystalline parts are slow on a timescale of 4 s. 335 K and 36K: two-dimensional exchange spectra with mixing times above 1 s show elliptical ridges. These can be assigned to jumps of 2, 4 and 6 respectively. T = 335 K mixing time = 1s T = 36 K Experiment Simulation ence the crystalline regions of poly(oxymethylene) have a well-defined jump motion with τc 1s which corresponds to a translation of the polymer chain.
Glass transition in PEMA Poly(ethylmethacrylate) is an amorphous thermoplastic polymer with a glass transition temperature of 338 K. The sample is 2 % enriched with carbon-13 at the carboxyl group, allowing investigation of the sidegroup motion through analysis of the lineshape of the carboxyl carbon. At 295 K: the spectrum shows a powder pattern with principal components ν11 = 268, ν22 = 15, ν33 = 113. Above Tg: lineshape changes indicate large-amplitude motions with rates exceeding 1 kz, the width of the powder spectrum. At 395 K: anisotropic chain motion leads to a motionally averaged axially symmetric tensor. This suggests that the ν 33 axis remains invariant during the motion which could result from either a rotation about this axis or by π flips of the side group. temperature 395 K 375 K 355 K 335 K! 22 ν33 ν22 295 K! 11! 33 ν11 3 2 1
Glass transition in PEMA Exchange NMR The exchange signal at ν33 is low compared to that between ν22 and ν11. 1 Experiment T = 295 K The exchange pattern can only be explained by assuming that π flips of the side group are accompanied by a rotation about the main chain. 2 Simulation The simulated spectrum was calculated for a rocking amplitude of ±2 coupled to the π flips and assuming that only 4% of the segments participate in the motion. This is indicative of a heterogeneous distribution of correlation times. 3 3 1 Simulation 2 ν33 1 ν22 2 ν11 3 3 2 1 A. S. Kulik,.W. Beckham, K. Schmidt-Rohr, D. Radloff, U. Pawelzik, C. Boeffel and. W. Spiess, Macromolecules, 27, 4746 (1994).
Side chain dynamics above Tg in PEMA At 355K: 17 K above Tg 85% of the side groups take part in the motion, leaving 15 % trapped in their local environments due to their bulky size. The motion involves a rocking amplitude of ±4 coupled to the π flips. ence, for PEMA even above Tg the molecular motion is anisotropic, with molecular geometry similar to that in the glass. At 365 K: 27 K above Tg the motion involves a rocking amplitude of ±5 coupled to the π flips and all segments participate in the motion. Experiment 1 T = 355 K 2 mixing time = 5 ms 3 3 2 1 1 T = 365 K 2 Simulation 1 2 3 3 2 1 1 2 mixing time = 5 ms 3 3 2 1 3 3 2 1
Main chain dynamics in PEMA by 2 exchange NMR PEMA-d5 This sample is enriched with deuterium, allowing investigation of the main-chain motion through analysis of the 2 exchange NMR spectrum. The spreading of exchange intensity with mixing time indicates increasing reorientation angles, suggesting planar rotational diffusion of main chain segments around the local chain axis. The equilibrium positions of the backbone rocking motions become increasingly imprecise after a large number of side chain flips, resulting in a slow rotation of the main chain itself. The simulations involve planar rotational diffusion with τc = 6 ms. mixing time " m = 2 ms! 1! 2 T = 355 K " m = 5 ms! 1 S. C. Kuebler, D. J. Schaefer, C. Boeffel, U. Pawelzik and. W. Spiess, Macromolecules 3, 6597 (1997).! 2
Coupling of α- and β-relaxations in PEMA Dielectric relaxation measurements detect the changes of the electric dipole moment caused by dynamics of the carboxyl side group and are therefore sensitive to the β-relaxation, as is 13 C NMR of the carboxyl carbon. Photon correlation spectroscopy is sensitive to main chain fluctuations and shows an almost temperature independent α-process with a distribution parameter βkww =.35, equivalent to a distribution of τ c approximately 3 decades wide. Side-Chain Motion Timescale probablility T = 298 K 9 reorientation angle / T = 355 K T = 365 K temperature T g = 338 K correlation time (s) 1 2 1 1-2 1-4 dielectric relaxation PCS 13 C NMR 2 NMR T g 1-6 1-8 2.4 2.6 2.8 3. 1 / T (K -1 ) 3.2
Dynamics in complex systems by MAS exchange NMR 1 CP Decoupling Decoupling -8-4 no mixing C 3-8 -4 mixing time = 1 s 13 C MAS rotor t 1 t 2 " m! 1 /! r 4 C 2 C! 1 /! r 4 8 75 5 25 8 75 5 25 normalized intensity.2.1.5 mixing time / s 1 1.5 2-5! 1 /! r 5 simulations for 12 jumps around the helix axis L. Shao and J. J. Titman, J. Chem. Phys., 12, 6457 (26); L. Shao and J. J. Titman, Macromol. Chem. Phys., 28, 255 (27).
Miscibility in PS-PVME blends by T1 Poly(styrene)-poly(vinylmethylether) blends The 5/5 PS-PVME mixture cast from toluene shows a single proton spin-lattice relaxation time which has a temperature variation intermediate between that for the pure components. This indicates that this blend is homogeneous down to a length scale of 2 nm. The 5/5 mixture cast from chloroform shows two separate proton spin-lattice relaxation times which have a temperature variation similar to that for the individual components. This indicates that the blend has phase separated into regions of poly(styrene)-rich and poly(vinyl methyl ether) rich material. Indirect observation PS-PVME blend cast from chloroform 1. PS T 1 / s.1 PVME PS-PVME blend cast from toluene 73 173 273 373 473 T / K
Direct probe of heterogeneity with spin diffusion There are three periods in a typical spin diffusion experiment: Selection of the magnetization from a part of a heterogeneous sample, using differences in mobility or chemical shift. During a mixing time spin diffusion down the resulting magnetization gradient redistributes the magnetization. Detection of the redistributed magnetization to probe the spin diffusion The rate of the observed spin diffusion can be related to the morphology of the heterogeneous sample and the sizes of the phase domains. Selection Spin diffusion Detection
Miscibility in PS-PVME blends by direct observation The 5/5 PS-PVME mixture cast from chloroform shows cross peaks between PS protons and between PVME protons. This indicates that this blend is phase separated. The 5/5 mixture cast from toluene shows cross peaks between PS and PVME protons. This indicates that the blend is homogeneous. The spin diffusion rate and hence the domain size can be extracted from the cross peak build-up. M-REV8 PS t 1 high resolution proton selection Direct observation PVME mixing spin diffusion M-REV8 t 2 acquire * high resolution proton detection chloroform toluene 4 4 8 8 8 4 8 4
Lamellae thicknesses in PS-PMMA block copolymers This experiment involves: PMMA block methyl proton selection according to chemical shift spin diffusion which redistributes magnetization to the PS blocks carbon-13 detection after TOSS for spectral simplification Note the increase in PS phenyl carbon intensity at longer mixing times. The equal lengths of both blocks in the symmetric copolymers ensures a lamellar structure which makes the quantitative analysis straightforward. The spin diffusion data yield domain sizes which are consistent with the scaling law Mn.66 where Mn denotes the molar mass of the blocks. Proton selection M-REV8 mixing time Signal intensity PMMA carbonyl 1 8 6 4 2 mixing CP CP PS phenyl PMMA methyl 2 1 statistical copolymer Carbon-13 detection Decoupling TOSS 1, gmol -1 3, gmol -1 75, gmol -1 3, gmol -1 blend 2 4 6 8 1 12 14 1/2! m / s 1/2
Orientational order by rotor-synchronized MAS With rotor-synchronized acquisition partially ordered materials result in MAS sidebands with distorted phases and amplitudes. This can be used as the basis of a new two-dimensional experiment which separates molecular structure via the chemical shift in ν2 and a sideband pattern in ν1 which depends on the degree of orientational order. trigger MAS rotor! R t 1 rotor phase CP CP Decoupling t 2 +1 +2 +3-3 -2-1! 1 /! R degree of orientational order! 2 resolution G. S. arbison, V.-D. Vogt and. W. Spiess, J. Chem. Phys., 86, 126 (1987).
Orientation distributions in liquid-crystalline polysiloxanes A homologous series of poly(siloxanes) shows changes in the degree of order on the (supposedly) disordered polymer backbone as a function of flexible spacer chain length and parity. carbon-13 NMR flexible spacer ordered mesogen disordered polymer backbone degree of orientational order silicon-29 NMR spacer: (C 2 ) 3 spacer: (C 2 ) 4-2 -2-1 -3 +1 +2 +3-12 -12-1 -3! 1 /! R spacer: (C 2 ) 5 spacer: (C 2 ) 6 +1 +2 +3-12 -12-1 +1 +2 +3-2 -2! 1 /! R -1 +1 +2 +3-3 -3! 1 /! R! 1 /! R +6 +5 +4 +3 +2 +1-1 -2-3 -4-5! 1 /! R degree of orientational order -4 +4-6! 2 /! R sideband separation J. J. Titman, D. L. Tzou, S. Féaux de Lacroix, C. Boeffel and. W. Spiess, Acta Polymer., 45, 24 (1994).
Summary Structure Dynamics conformation, tacticity, cross linking, networks, crystallinity, polymorphism, packing effects, orientational distributions, polymer fibres and films, liquid-crystalline polymers mechanisms, rates, activation energies, mechanical relaxations, glass transition, inhomogeneous dynamics, correlation effects eterogeneity phase separation, domain sizes, polymer blends and block copolymers, morphology, semi-crystalline polymers