J.P. Cohen-Addad

Strongly entangled polymer chains in a melt. Description of n.m.r. properties associated with a submolecule model

Abstract A model is proposed to illustrate properties of the transverse magnetic relaxation function, G ( t ), of proton pairs linked to strongly entangled polymer chains in a melt. According to this model, any polymer molecule is described as a freely jointed chain and it is divided into submolecules of equal contour length L v e . Every link is supposed to carry a proton pair; dipolar spin couplings between different proton pairs are neglected. The disentanglement relaxation time is supposed to be much longer than any characteristic time of the spin system; consequently, any submolecule observed on an n.m.r. time scale is supposed to have fixed ends. It is considered that the residual spin-coupling energy resulting from such a constraint governs the magnetic relaxation process. The free induction decay is expressed as a contour length function; its time evolution is shown to exhibit two ranges, which might be characterized by two relaxation times. The model is easily extended to rotating methyl groups. Theoretical results are compared with magnetic relaxation properties observed on entangled real chains: polydimethylsiloxane (PDMS) and cis -1,4-polybutadiene (PB). An attempt to adjust the contour length value to experimental results leads to the determination of average submolecule molecular weights M v e equal to 8200 and 2000 for PDMS and PB, respectively; the values usually obtained from viscoelastic plateau modulus measurements are 8100 and 1900, respectively.

Mechanically induced sorption of siloxane on silica: Experimental and theoretical investigations of chain binding, collective behaviour and multiple-aggregate processes

Abstract The purpose of the present paper is to propose a two-parameter model to describe the sorption of siloxane chains onto silica particles immersed in the polymeric liquid at room temperature. Random adsorbed systems were obtained from mechanical mixing. One parameter of the model is the probability θ B that a polymer chain is simultaneously bound to two silica particles; the other one, π, attempts to describe the possible collective character of the binding of a single chain. The model was compared with values of the residual amount of polymer Q r measured siloxane-silica mixtures after removal of all free polymer chains. The number of available chain binding sites per unit mass of silica was varied by using a silica surface treated to a variable extent. Typical relative numbers of silicon chains ( M n ⋍ 3.6 × 10 5 ) simultaneously bound to two or three particles were found to be 0.21 and 0.49, respectively, in a mixture determined from an initial siloxane-silica weight-to-weight ratio equal to 2.5. Also, it was shown that a chain binding site can be divided into four subsites; on average, any chain is torn off from the surface whenever it occupies less than two subsites.

Nuclear magnetic resonance approach to fractal chain structure in molten polymers and gels: Characterization method of the spin-system response

Abstract By observing protons attached to polymer chains, pulse sequences are proposed to disclose tranverse magnetization properties specific to polymer network structures. It is shown that a peculiar relaxational behaviour originates from the existence of a residual energy h Δ r of spin-spin interactions closely associated with the existence of chain coupling junctions. Properties are illustrated by observing vulcanized polybutadiene and polyethylene, at room temperature and 418 K, respectively. Furthermore, it is shown that these properties are clearly modified when coupling junctions in a melt fluctuate within time intervals shorter than Δ−1r. Finally, a pulse sequence is proposed to eliminate both the residual energy and diamagnetic inhomogeneities occurring in most polymeric systems. This elimination thus reveals effects of monomericunit high-frequency motions.

Entangled linear polymer chains in melts: n.m.r. and Rouse or reptation models; stress relaxation

Abstract This study deals with detailed descriptions proposed to analyse slow chain diffusional process effects on n.m.r. properties of nuclei linked to long linear polymer molecules in melts or in concentrated solutions. It aims to predict relationships among macroscopic viscoelastic properties and semi-local dynamical properties observed from n.m.r. These relationships are discussed considering two main models of low-frequency chain fluctuations in melts: a multiple relaxation-mode spectrum analogous to a Rouse model or a single relaxation-mode spectrum analogous to a reptation model. The two descriptions are compared with each other. The Rouse model was chosen because n.m.r. properties might be sensitive to the equilibration process of conformational fluctuations within a so-called ‘tube’ as well as to the reptation in a ‘tube’. Also, the Rouse model is more easily handled than the reptation one. Local molecular properties are transferred to the unusual n.m.r. semi-local space scale ( A ) through the submolecule concept: every monomeric unit is supposed to have a uniform average orientational order within a given submolecule. Semi-local dynamical properties are analysed from the transition of the spin-system response from a pseudo-solid behaviour to a liquid-like one, induced by shortening polymer molecules or by slightly diluting them. This transition is currently observed from the transverse magnetic relaxation function of nuclei linked to entangled polymer chains. This spin-system response is contrasted to the longitudinal response only sensitive to local high relaxation frequency motions. Also, it is shown how stress-relaxation processes induced on a macroscopic scale can be monitored on a semi-local space scale from n.m.r. Relaxation rates of the transverse magnetization are shown to depend strongly upon both the initial stretching ratio λ of the polymer sample and the structure of its chain relaxation spectrum.

Correlation between ionic conductivity and7Li-NMR of polyether-polyurethane networks containing lithium perchlorate

SummaryThe effect of temperature on the ionic conductivity and the7Li NME line width of a polyether-polyurethane network containing lithium perchlorate was studied. A treatment of the results based on free-volume principles yielded a linear correlation between the logarithm of the reduced conductivity, бT /бT0, and the logarithm of the reduced7Li NMR line width, δT/δT0, for a given reference temperature, To=323 K.

Nuclear magnetic resonance approach to the characterization of siloxane-silica network-like structures

This work deals with mesh size properties of network-like structures associated with random siloxane-silica mixtures. The transverse magnetic relaxation function of protons is shown to obey a superposition property controlled by the silica concentration CSi, in the range 0⩽CSi⩽0.50 (w/w). Correspondingly, its invariant mathematical structure is governed by the average residual energy hΔe of dipole-dipole interactions only. The parameter Δe is given the role of a timescale shift factor; it is related to the average mesh size of a given network. Nuclear magnetic resonance measurements show that silica aggregates do not induce any strong deviation from the primary entanglement system established in a pure siloxane melt. Whether infinite siloxane-silica clusters are swollen by polymer chains or whether finite size clusters are diluted in a siloxane melt, the resulting networks are described by similar distribution functions of elementary chains connecting neighbouring entanglements; these functions are shifted towards smaller mesh sizes upon addition of silica. These properties hold for several high molecular weight samples: Mw = 6.1 × 105, 2.4 × 105 and 1.6 × 105, respectively.

Polyisobutylene: fractional precipitation from binary solvent induced by non-solvent addition

Abstract Large fractions of polyisobutylene chains may be obtained from precipitation induced by acetone added to solutions of polymer in a binary mixture (chloroform and dichloromethane). Fractions were characterized by weight average molecular weights, M w , and average radii of gyration, R 2 G , obtained from light-scattering measurements performed on a good solvent (cyclohexane); the product M w × R 2 G was found to vary according to the formula: M w × R 2 G = 1.33 × M m w with m = 2.22; while the intrinsic viscosity of fractions was found to vary as: [keta;] = 1.11x10 -4 × M m w with n = 0.76. Considering that m = 2v + 1 and n = 3v − 1, where v is the critical exponent associated with the description of properties of polymer chain solutions; values of v derived from m and n were found to be 0·61 or 0·58, respectively; the theoretical value is known to be 0·588. Polydispersity indices, l, were obtained from g.p.c. measurements: 1.1

Proton magnetic relaxation observation of weak gel stretching

SummaryUniaxially elongated crosslinked polybutadiene networks were observed from the transverse magnetization of protons attached to polymer chains. The residual energy of dipole-dipole interactions characterizing the relaxation function was shown to vary as a linear function of the expression λ2 — λ−1; the stretching ratio λ was varied from 1 to 3. This result gives evidence for affine deformation of crosslinks. The orientational order of monomeric units perceived from NMR proved more sensitive to variations of crosslinking concentrations and/or chain molecular weights than corresponding elastic moduli.