Alan E. Tonelli

High-resolution solid-state 13C nuclear magnetic resonance study of isotactic polypropylene polymorphs

Abstract High-resolution 13C nuclear magnetic resonance spectra are reported for three solid samples of isotactic polypropylene (i-PP): (i) the α-crystalline form (monoclinic), (ii) the β-crystalline form (hexagonal), and (iii) the mesomorphic or smectic form of i-PP. These spectra were obtained using magic angle spinning, high-power proton dipolar-decoupling and cross-polarization techniques, and for the β- and smectic forms of i-PP constitute the first reported high-resolution solid-state spectra. The spectrum of the α-crystalline form shows well resolved splittings (1 ppm) of the methyl and methylene carbon resonances, as was reported previously by Bunn et al. These splittings are absent in the spectra observed for the β- and smectic forms of i-PP. Based on a comparison of the chemical shifts and T1 relaxation times observed for the carbon resonances in these three forms of solid i-PP, we reach the following conclusions: (i) i-PP adopts the same 31 helical conformation in each crystalline polymorph, (ii) the packing of i-PP helices in the β-form crystal is closer to the more distant of the two interhelical packings in the α-crystalline form, and (iii) the local packings of 31 helices are very similar in the β- and smectic forms of i-PP.

Calculation of the Intramolecular Contribution to the Entropy of Fusion in Crystalline Polymers

The intramolecular contribution to the entropy of fusion of crystalline polymers is treated through calculation of the change in configurational entropy on melting of a single isolated polymer chain. Perfect backbone conformational order is assumed in the crystalline polymer chain, and the configurational entropy of the molten amorphous polymer chain is obtained through calculations based on the rotational isomeric state model of polymers. Comparison of the calculated change in intramolecular configurational entropy on melting with experimental results is achieved by assuming the entropy of fusion consists of two independent contributions: the entropy of fusion due to volume expansion (intermolecular contribution), and the entropy of fusion at constant volume, which is identified as the change in intramolecular configurational entropy. Several linear polymers are treated in the above fashion. The agreement between the calculated and experimentally determined intramolecular contribution to the entropy of f...

Experimental and calculated conformational characteristics of the bicyclic heptapeptide phalloidin

Abstract Several conformations generated from approximate potential energy calculations are presented for the bicyclic heptapeptide phalloidin which are consistent with the conformation-dependent information obtained from proton nuclear magnetic resonance measurements performed on phalloidin in dimethylsulfoxide solution. In each conformation, the cysteine amide proton is intramolecularly hydrogen bonded, the tryptophan amide is internally buried and the methyl group of the alanine residue preceding tryptophan is shielded by the tryptophan ring. Thus, phalloidin appears to be a relatively rigid molecule in solution.

Conformational characteristics of poly(N-vinyl pyrrolidone)

Abstract Conformational energies are calculated for poly( N -vinyl pyrrolidone) (PVP) chains as a function of stereosequence using semiempirical potential functions appropriate to peptides and n-alkanes. The planar pyrrolidone side groups are permitted to adopt both conformations which result in an eclipsed arrangement of the pyrrolidone NCH 2 or N(CO) and the C a H a bonds. Solvent interactions were considered in the manner used to treat other vinyl polymers bearing planar side groups. Dimensions and dipole moments were calculated using the RIS model developed for PVP from the conformational energies considering both the effects of stereosequence and temperature. Dipole moments were measured for three PVP samples with molecular weight ranging from 10 000 to 360 000. The dimensions and dipole moments calculated for atactic PVP chains agree with the dimensions reported in the literature and the dipole moments measured here.

Study of the correlation between the backbone conformation and the electronic structure of polydiacetylenes by solid state 13C n.m.r.

Abstract The relationship between the conformation and the electronic structure of the backbones in polydiacetylene (PDA) has been investigated by high resolution, solid state 13C nuclear magnetic resonance (n.m.r.). We have studied four different PDAs: PTS in its blue phase, the red phase of PTCDU, PTCDU in the stress-induced blue phase, and the thermochromically related blue and red phases of PETCD and P4BCMU. We have not detected the presence of a stable butatrienic structure in any of these PDAs. Instead, we have found that there are two kinds of backbone conformations, or structures, one which is unique to the blue phase and the other unique to the red phase. For all the blue phase PDAs studied here, the C resonance appeared at c. 107 ppm, while for all the red phase PDAs the same resonance occurred at c. 103 ppm. The backbone structure of the blue phase was found to be more planar than that of the red phase based on the overall C chemical shift positions. These backbone structures appear to be general for PDAs. This study shows that the chemical shift position of C can be utilized as a fingerprint for the extent of planarity, or conjugation length, of any PDA system.

Intramolecular and Intermolecular Contributions to the Fusion of Linear Aliphatic Polyesters and Polyamides and Their Effects on the Observed Differences in Polyester and Polyamide Melting Temperatures

The inter‐ and intramolecular contributions to the entropy and energy of fusion are calculated for several linear aliphatic polyesters and polyamides assuming the fusion process consists of two independent contributions: the volume expansion (intermolecular contribution) and the increase in the conformational freedom of each polymer chain on melting (intramolecular contribution). The intramolecular entropy and energy contributions are obtained from the configurational partition function and its temperature coefficient calculated for an isolated, unperturbed polymer chain using the rotational isomeric state approximation. Subtraction of the calculated changes in the conformational entropy and energy upon melting from the measured entropy and enthalpy of fusion, respectively, provides an estimate of the intermolecular contributions to these two quantities if the small volume change accompanying melting is ignored and the enthalpy and energy of fusion are assumed to be nearly identical. Application of this p...

On the Existence of Local Segmental Order in Undiluted Bulk Polymers

The mean‐square end‐to‐end distance and its temperature coefficient, the change in the intramolecular conformational entropy on melting, and the optical anisotropy, as manifested by the strain birefringence, and its temperature coefficient are calculated in the rotational isomeric‐state approximation for unperturbed linear polyethylene chains containing short stretches of skeletal bonds exclusively in the all trans or planar zigzag conformation. The results are compared with the corresponding quantities calculated for a polyethylene chain free of such conformational constraints and with experimental data taken from the literature which were obtained on undiluted bulk polyethylene. This comparison shows that the optical properties of polyethylene are more sensitive to the local segmental order, as defined in this study, than are its dimensions and their temperature coefficient or the change in intramolecular conformational entropy on melting. An estimate of the upper limit of local segmental order in bulk ...

A Kerr effect and dielectric study of α, ω‐dibromoalkanes

We report the molar Kerr constants mK and dipole moments squared <μ2≳ of the α, ω‐dibromoalkanes Br(CH2)n−1Br (n−1 = 3–20) in cyclohexane. We calculate the average mK and <μ2≳ on the basis of the rotational isometric state theory with nearest‐neighbor interactions and the optical valence scheme. The dipole moments at each end of the alkane chain are assumed to be (a) along the C–Br bond only, (b) the sum of a moment along the C–Br bond and an induced moment along the adjoining C–C bond. We obtain good agreement between calculated and experimental mK and <μ2≳ by assuming that the gauche states of the first and last rotatable bonds are φg± = ±80° to ±100° and not ±120°. Better agreement with experimental results is obtained by using assumption (b) for the dipole moment. We calculate the effect of dipole–dipole interactions between the C–Br dipoles on mK and <μ2≳ and find them to be negligible. Finally we use Onsager’s theory of the reaction field to calculate solvent effects on mK and <μ2≳ and find the theo...

Polyethylene and polytetrafluoroethylene crystals: chain folding, entropy of fusion and lamellar thickness

Abstract Chain folding in polyethylene (PE) and in polytetrafluoroethylene (PTFE) crystallites is simulated on the computer in an attempt to determine if the large disparity in their lamellar thicknesses has its origin in any differences between their inherent abilities to fold. PE and PTFE chains of 6 to 15 carbon atoms are permitted to adopt each f their many rotational isomeric state ( RIS ) conformations, and each is checked for the presence of a chain fold. The conformational energetics of both polymers are used to calculate the probabilities of generating both randomly and adjacently re-entering chain folds. Depending on the RIS model adopted for PTFE, calculated probabilities for generating randomly re-entering folds are found to be comparable in PE and PTFE. Adjacently re-entering folds, however, are much more easily formed in PE than in PTFE. Despite the greater ease of folding PE chains, the relative energies required to create a unit area of chain fold are estimated to be nearly the same for both polymers in agreement with results obtained from the kinetic theory of chain folding. Consequently, in agreement with the recent proposal of Bassett and Davitt, it is concluded that the greater lamellar thickness of PTFE crystals is attributable exclusively to their lower entropy of fusion. An analysis of the possible reasons for the low entropy of fusion of PTFE is also presented.

Effects of crosslink density and length on the number of intramolecular crosslinks (defects) introduced into a rubbery network

Abstract Estimates are presented for the effects of the crosslink density, or the molecular weight of polymer chains between crosslinks, and the length of crosslinks, or crosslinking agents, upon the expected ratio of internal or intramolecular (possibly elastically ineffective) to external or intermolecular (elastically effective) crosslinks introduced in rubber networks crosslinked in both the dry and dissolved states. Model calculations are performed on cis -1,4-polyisoprene with the following results: (i) in rubber networks formed by crosslinking in the dry state, the number of possibly inactive, intramolecular crosslinks introduced is negligibly small; and (ii) the relative number of intramolecular crosslinks introduced may become appreciable for those networks formed by lightly crosslinking low molecular weight rubber molecules in solutions, where the volume fraction of rubber present is small, using initiators that produce short crosslinks.