Investigation of Chain Dynamics in Poly(n‐alkyl methacrylate)s by Solid‐State NMR: Comparison with Poly(n‐alkyl acrylate)s

PnAMAs exhibit a local nanophase separation associated with intriguing chain dynamics (Macromol. Chem. Phys. 2005 , 206, 142). PnAMAs of high molar mass, as determined by SEC and MHKS parameters, were investigated in the melt with a recently‐developed solid‐state NMR method (NOE with dipolar filter; Solid State Nucl. Magn. Res. 2005 , 28, 160). The correlation times are assigned to the relaxation of the alkyl nanodomains, as coupled motions of the main chain and hindered local modes in the side chain. Comparison with poly(n‐alkyl acrylates) shows a higher anisotropy of the main chain motions and a better organized local nanophase separation in PnAMAs.

     

Investigation of Polymer‐Filler Interactions in TiO2‐Filled Poly(n‐alkyl methacrylates) by Low‐Field NMR Relaxometry

Low‐field NMR relaxometry is sensitive to molecular mobility within well‐defined time windows and can thus be applied to study interactions between polymers and inorganic filler particles in composite materials. Volume fraction and dispersion of the filler influence the polymer chain dynamics, which also modify the macroscopic properties of the polymer composites. As model systems, TiO₂‐filled poly(n‐alkyl methacrylates) with different side‐chain lengths and filler contents are prepared by melt mixing. Several ¹H NMR relaxation parameters, which are sensitive and selective toward polymer–filler interactions, are investigated at a Larmor frequency of 20 MHz as a function of temperature. Both physical and empirical models are used to analyze the NMR data, and the results are compared with the aim of an unambiguous detection of polymer–filler interactions.

     

Chain Dynamics of Poly(oxyethylene) in Nanotubes of α‐Cyclodextrin by Solid‐State 2H NMR

Summary: Solid‐state ²H NMR spectroscopy was used to examine the chain dynamics of perdeuterated poly(oxyethylene) (d‐POE) inside α‐cyclodextrin (α‐CD) nanotubes. The nanotubes were prepared by aqueous self‐assembly of α‐CD onto either low‐molecular‐weight (1.5 kg/mol) or high‐molecular‐weight (25.8 kg/mol) monodisperse d‐POE. At a given temperature, POE chain segments exhibit faster dynamics when included inside the CD nanotubes as compared to the bulk. Even at 150 K, when no large‐angle dynamics are detected in bulk POE, evidence for chain motions in the nanotube‐confined POE is observed. The ²H line shapes representing this motion were modeled by a discrete 3‐site jump using a ln‐Gaussian distribution of correlation times. An activation energy of 15 ± 1 kJ/mol was determined and the motion envisioned as gauche defects propagating along the primarily trans chains included within CD nanotubes. As the nanotube length decreased, the jump angle became much less defined and more isotropic motions were observed for POE segments at elevated temperatures (>270 K).

     

Thermoresponsive Poly(2‐oxazoline) Molecular Brushes by Living Ionic Polymerization: Kinetic Investigations of Pendant Chain Grafting and Cloud Point Modulation by Backbone and Side Chain Length Variation

Molecular brushes of poly(2‐oxazoline)s were prepared by living anionic polymerization of 2‐iso‐propenyl‐2‐oxazoline to form the backbone and subsequent living cationic ring‐opening polymerization of 2‐n‐ or 2‐iso‐propyl‐2‐oxazoline for pendant chain grafting. In situ kinetic studies indicate that the initiation efficiency and polymerization rates are independent from the number of initiator functions per initiator molecule. This was attributed to the high efficiency of oxazolinium salt and the stretched conformation of the backbone, which is caused by the electrostatic repulsion of the oxazolinium moieties along the macroinitiator. The resulting molecular brushes showed thermoresponsive properties, that is, having a defined cloud point (CP). The dependence of the CP as a function of backbone and side chain length as well as concentration was studied.     

Molecular Motions in the Supramolecular Complexes between Poly(ε‐caprolactone)‐Poly(ethylene oxide)‐Poly(ε‐caprolactone) and α‐ and γ‐Cyclodextrins

The structure and molecular motions of the triblock copolymer PCL‐PEO‐PCL and its inclusion complexes with α‐ and γ‐cyclodextrins (α‐ and γ‐CDs) have been studied by solid‐state NMR. Different cross‐polarization dynamics have been observed for the guest polymer and host CDs. Guest–host magnetization exchange has been observed by proton spin lattice relaxation T₁, proton spin lattice frame relaxation T_1ρ and 2D heteronuclear correlation experiments. A homogeneous phase has been observed for these complexes. Conventional relaxation experiments and 2D wide‐line separation NMR with windowless isotropic mixing have been used to measure the chain dynamics. The results show that for localized molecular motion in the megahertz regime, the included PCL block chains are much more mobile than the crystalline PCL blocks in the bulk triblock copolymer. However, the mobility of the included PEO block chains is not very different from the amorphous PEO blocks of the bulk sample. The cooperative, long chain motions in the mid‐kilohertz regime for pairs of PCL‐PEO‐PCL chains in their γ‐CD channels seem more restricted than for the single PCL‐PEO‐PCL chains in the α‐CD channels, however, they are not influencing the more localized, higher frequency megahertz motions.     

Thermotropic Phase Behavior of a Liquid‐Crystalline Poly(ether ester) Derived from Hydroxydibenzoic Acid, 2‐Methyl‐1,3‐propanediol and R‐1,3‐Butanediol

The thermotropic phase behavior of a liquid‐crystalline poly(ether ester) derived from 4′‐hydroxybiphenyl‐4‐carboxylic acid and two different spacer diols (2‐methyl‐1,3‐propanediol and R‐1,3‐butanediol) has been analyzed by differential scanning calorimetry, real time synchrotron X‐ray diffraction and solid‐state ¹³C NMR. It has been found that the polymer develops a smectic mesophase that presents order inside the layers. This mesophase has a slow rate of formation, in such a way that it is possible to obtain either an amorphous glass or a liquid‐crystalline glass. These two phases exhibit different and independent glass transition temperatures: 95 °C for the amorphous one and 85 °C for the mesophase. The diffractogram of this mesophase shows two clear diffractions in the wide‐angle region and a long spacing (of low intensity) in the small‐angle region. All those features are characteristic of a mesophase of intermediate order, presumably of the type SmI, in contrast to the low‐ordered SmC_alt mesophase reported for the racemic polymer. Thus, the chirality of the spacer, which provides structural regularity, makes easier the packing of the chains, so that a more ordered mesophase is obtained. In addition, both the solid‐state ¹³C NMR line shapes and the value of are practically the same for the quenched and the annealed samples, indicating that the “ordered” phase obtained by annealing is not a three‐dimensional crystal, thus confirming its assignment as mesophase, with a considerable degree of conformational disorder.

     

Preparation and Characterisation of Hydrophobically Modified Hydroxypropylcellulose: Side‐Chain Crystallisation

Summary: Hydroxypropylcellulose has been hydrophobically modified by reaction with different amounts of palmitoyl chloride. When the degree of substitution is sufficiently high, the side chains are able to produce lateral crystallisation, which has been analysed by DSC and diffraction experiments, using both conventional and synchrotron radiation. The melting temperatures and enthalpies depend on the degree of substitution, in such a way that it is relatively easy to get melting temperatures close to that of the human body, although a rather wide melting region is obtained.

X‐ray diffractograms of the original HPC sample and of the modified specimens.     

Amorphous‐Smectic Glassy Main‐Chain LCPs, 1. Poly(ether esters) Derived from Hydroxybibenzoic Acid and (R,S)‐ and (R)‐2‐Methylpropane‐1,3‐diol

The synthesis and the characterization of main‐chain liquid‐crystalline poly(ether esters), derived from hydroxybibenzoic acid and (R,S)‐ and (R)‐2‐methylpropane‐1,3‐diol, are reported. These polymers show an interesting thermal behavior. They develop mesophases with a slow rate of formation, allowing the easy quenching of the melt into: a) the glassy amorphous state, b) the glassy liquid‐crystalline state, or c) a mixture of both, depending on the thermal treatment. The extent of the transformation and the symmetry of the different phases have been determined by means of calorimetric and X‐ray diffraction methods. Dielectric spectroscopy results provide additional evidence for the detection of distinct glass transitions. The results show that the racemic polymer forms a low‐ordered SmC_alt mesophase, while a more ordered phase is obtained in the case of the enantiomerically pure polymer. The comparison of the properties of the different states evidences the special behavior and properties of the glass transition (T_g) in these polymers. Emphasis is paid to the location of the T_g of the liquid‐crystalline state in comparison to the T_g of the amorphous state. It is found that the glass transition of the SmC_alt glass in R,S‐PBO3 (the poly(ether ester) derived from hydroxybibenzoic acid and (R,S)‐2‐methylpropane‐1,3‐diol) appears at lower temperatures than the glass transition of the amorphous state. However, in R‐PBO3 (the poly(ether ester) derived from hydroxybibenzoic acid and (R)‐2‐methylpropane‐1,3‐diol), where the more ordered phase is present, the glass transition follows the classical tendency of semicrystalline polymers.

     

Synthesis,Phase Behaviour and Mechanical Properties of Poly(2‐methyl‐1,3‐propanediol‐4,4′‐bibenzoate)

The synthesis and phase behaviour of PB32 is reported. The results indicate that PB32 develops a low‐ordered smectic mesophase of the type SmC_alt. The slow formation of such a mesophase allows the quenching of the isotropic melt into an amorphous state so that both amorphous glass and smectic glass can be obtained, which exhibit different glass‐transition temperatures. Mechanical tests indicate that the macromolecular chains in the mesophase of PB32 can be oriented either parallel or perpendicularly in relation to the streching direction, depending on the deformation conditions. Solid‐state ¹³C NMR spectroscopy results show that the mobility of the chains is rather similar in both phases.

     

Thermally Induced Crosslinking of Poly(N‐Propargyl Glycine)

As polypeptoids become increasingly popular, they present a more soluble and processable alternative to natural and synthetic polypeptides; the breadth of their potential functionality slowly comes into focus. This report analyzes the ability of an alkyne‐functionalized polypeptoid, poly(N‐propargyl glycine), to crosslink upon heating. The crosslinking process is analyzed by thermal analysis (differential scanning calorimetry and thermogravimetric analysis), Fourier‐transform infrared, electron paramagnetic resonance, and solid‐state NMR spectroscopy. While a precise mechanism cannot be confidently assigned, it is clear that the reaction proceeds by a radical mechanism that exclusively involves the alkyne functionality, which, upon crosslinking, yields alkene and aromatic products.

     

Synthesis and Optical Properties of a New Series of Side‐Chain Poly(amic acid)s With p‐π Conjugation

Summary: After the condensation polymerization of benzoguanamine (BGA) and pyromellitic dianhydride (PMDA) under microwave irradiation, the resulting p‐π conjugate poly(amic acid) was grafted via the azo coupling reaction. The obtained side‐chain polymers were further grafted with TDI‐aliphatic alcohol derivatives and TDI‐aniline derivatives. The third‐order NLO coefficient and response time of PAA and graft polymers were measured by degenerated four wave mixing (DFWM) technique and their fluorescent properties were also investigated. All of the graft polymers have larger NLO coefficients and film‐formability than PAA. Their fluorescent properties were also changed by the different electronic effect of the side‐chains. The influences of the introduction of side‐chains, the side‐chain length and the electronic effect of the substituting groups on the azobenzol side‐chain to both optical properties of the polymers by varying the conjugation degree were investigated.

     

Poly(methylenepyridinium)s by Poly(N‐alkylation) of Substituted Pyridines: Structural and Kinetic Study

The solution polymerization of 4‐bromomethylpyridine ( M1 ) and 3‐bromomethyl pyridine hydrobromides ( M2 ) was studied by NMR spectroscopy. A mechanism involving a series of bimolecular reactions of the monomer, dimer, and higher oligomers closely fits with the experimental variations of bromomethyl end group concentrations with time. M1 presents a higher reactivity than M2 and an unusual behavior, since the oligomers are more reactive than the monomer. An explanation based on a mesomeric phenomenon is proposed. The influence of the anion on the solubility and thermal stability of the poly(methylenepyridinium)s were studied after various anion exchanges. Bis(trifluoromethylsulfonyl)imide anion ( Tf ₂ N ) yielded the more stable and the more organosoluble polymers.

     

Control of Molecular Weight and Tacticity in Stereospecific Living Cationic Polymerization of α‐Methylstyrene at 0 °C Using FeCl3‐Based Initiators: Effect of Tacticity on Thermal Properties

The successful stereospecific living cationic polymerization of α‐methylstyrene (α‐MeSt) using FeCl₃‐based initiator systems at 0 °C is reported. Linear first‐order ln([M]₀/[M]) vs. time and linear molecular weight vs. conversion plots suggest that the polymerization is living in nature, which is further confirmed from successful chain‐extension experiments. Poly(α‐methylstyrene)s of varying syndiotacticities (59.1% to 79.2%) and controllable molecular weights (4300–32 100 g mol^?1) with moderately narrow polydispersity indices (PDIs ≈1.3) are synthesized simply by varying the monomer‐to‐initiator ratio ([M]₀/[I]₀). A possible mechanism for this stereospecific polymerization is proposed. The glass‐transition temperature and thermal‐decomposition temperature depend on the syndiotacticity of poly(α‐methylstyrene).

     

para‐Substituted Poly(styrene peroxide)s: Synthesis,Characterization, Thermal Reactivities,and Chain Dynamics Studies in Solution

The synthesis and spectroscopic characterization of three polymeric peroxides of styrene monomers with substituents in the para position are discussed. NMR spectroscopy revealed the alternating copolymer structure with labile peroxy bonds (—O—O—) in their main chain. The thermal reactivity of the polymers was studied by differential scanning calorimetry and thermogravimetry. The measured heat of degradation of these polymers is nearly the same as that of poly(styrene peroxide). The mechanism of the primary exothermic degradation has been substantiated by thermochemical calculations. The chain dynamics studies of these polyperoxides in terms of ¹³C spin‐lattice relaxation time (T₁) have been carried out to understand their main chain flexibility. The temperature dependence of correlation time has been utilized for the determination of the activation energy for the overall segmental motion and internal group rotation. Their flexibility in terms of glass transition temperature (T_g) have been examined. The flexibility trend observed in solution parallels that in the bulk.     

Study of Uniaxially Stretched Isotactic Poly(propylene) by 1H Solid‐State NMR and IR Spectroscopy

Changes in the amount of the rigid, semi‐rigid and soft fractions, molecular mobility and domain thickness of uniaxially stretched iPP were investigated as a function of temperature, draw ratio, drawing temperature and drawing rate. Correlations are established between the thicknesses of rigid domains and the amount of the rigid fractions of uniaxially stretched iPP. Also established are correlations between the thickness of rigid domains and the molecular mobility of the rigid fraction of uniaxially stretched iPP. The drawing temperature has an important effect on the strain‐induced transformation of the spherulitic morphology of iPP to a fibrillar one.

     

Poly(ester urethane) with Varying Polyester Chain Length: Polymorphism and Shape‐Memory Behavior

The swelling, viscoelastic, and mechanical behavior of phase‐segregated poly(ester urethane) (PEU) block copolymers, composed of 4,4′‐methylenediphenyl diisocyanate, 1,4‐butanediol as a chain extender, and crystallizable poly(1,4‐butylene adipate) (PBA) with molecular weights between 1330 and 4120 g mol^?1, are investigated. Wide‐angle X‐ray scattering (WAXS) is employed to study the overall PEU crystallinity, which increases from 8.6 to 13.6% at higher PBA contents. The existence of two crystalline, polymorphic PBA phases, a thermodynamically stable α phase and a metastable β phase, is confirmed by further WAXS measurements. Calorimetric and thermomechanical investigations give evidence for controllable PBA polymorphic behavior. The crystallization conditions, like the cooling rate, affect the emerging polymorphic mixture, whereas the storage conditions either promote or inhibit the polymorphic (β to α) transition. The introduced concepts represent a new approach for gaining control over programmable thermo­responsiveness, which may be transferable to other shape‐memory polymers with polymorphic switching segments.

     

Proton‐Conducting Properties of Acid‐Doped Poly(glycidyl methacrylate)‐1,2,4‐Triazole Systems

1,2,4‐triazole‐functional PGMA polymers have been synthesized and their anhydrous proton‐conducting properties were investigated after doping with phosphoric acid and triflic acid. PGMA was prepared by solution polymerization and then modified with 1H‐1,2,4‐triazole (Tri) and 3‐amino‐1,2,4‐triazole (ATri). FT‐IR, ¹³C NMR and elemental analysis verify the high immobilization of the triazoles in the polymer chain. Phosphoric‐acid‐doped polymers showed lower T_g and higher proton conductivities. PGMA‐Tri 4 H₃PO₄ showed a maximum water‐free proton conductivity of approximately 10^?2 S · cm^?1 while that of PGMA‐ATri 2 H₃PO₄ was 10^?3 S · cm^?1. The structure and dynamics of the polymers were explored by ¹H MAS and ¹³C CP‐MAS solid‐state NMR.

     

Glass‐Transition Temperature (Tg) of Free‐Radically Prepared Polyacrylonitrile by Inverse Gas Chromatography, 2. Molecular‐Weight Dependence of Tg of Two Different Types of Aqueous Polymers

The molecular‐weight dependence of the glass‐transition temperature (T_g) of a series of atactic polyacrylonitriles (PAN)s was studied by inverse gas chromatographic (IGC) analysis. PANs having different molecular weights were prepared by either; (i) the addition of isopropyl alcohol as a chain‐transfer agent, or (ii) a scission reaction induced by the addition of alkali (NaOH) to a solution (N,N‐dimethylformamide solution, at 25 °C) of the resulting polymer. The intrinsic viscosity [η] was in the range of 10.9–0.1 (dl · g^?1), which corresponds to a viscosity‐averaged molecular weight (M _v) of 1 590 000–3 000. As part of the results, a side reaction, which saw the conversion of the nitrile (CN) groups of PAN into amide (CONH₂) and/or carboxylic acids (COOH) groups by alkali, was found to occur. The typical molecular‐weight dependence of the T_g in free‐radically prepared PAN was discussed in connection with a chain‐transfer mechanism in an aqueous medium.

Molecular‐weight dependence of the T_g for PAN (WA). An error bar is given by a short vertical arrow.     

Synthesis of Poly(ester amide)s Derived from Glycolic Acid and the Amino Acids: β‐Alanine or 4‐Aminobutyric Acid

The polymerization of metal salts of N‐chloroacetyl‐β‐alanine and N‐chloroacetyl‐4‐aminobutyric acid was investigated. The former gives a mixture of polymer and a seven‐membered cyclic compound constituted of glycolic and β‐alanine units, and its reaction proceeds in the solid state. However, liquefaction is observed in the second case giving rise to a polymer with a moderate molecular weight. Condensation kinetics of both sodium and silver salts of N‐chloroacetyl‐β‐alanine have been studied by differential scanning calorimetry. Copolymers of glycolic acid and β‐alanine with a molar ratio of glycolic acid/β‐alanine varying from 0.5 to 1.0 have been synthesized by thermal reaction of co‐precipitated crystals of the sodium salts of chloroacetic acid and N‐chloroacetyl‐β‐alanine. NMR spectroscopy indicates that copolymers tend to have a random distribution. The resulting new poly(ester amide)s have been characterized by spectroscopy and thermal analysis.

DSC heating runs corresponding to different mixtures of the sodium salts of chloroacetic acid and chloroacetyl‐β‐alanine.     

Metastable States of Glassy Dynamics,Possibly Mimicking Confinement‐Effects in Thin Polymer Films

The impact of the preparation and thermal history on the dynamic glass transition in thin films of poly(methyl methacrylate) and poly(vinyl acetate) is systematically investigated by means of broadband dielectric spectroscopy. Metastable states of glassy dynamics are detected, exhibiting shifts in the mean‐relaxation time from the bulk values. These shifts ‐ possibly mimicking confinement‐effects in thin polymer films ‐ become weaker (or even disappear) after an extensive annealing.