We study the scattering properties of a polymer gel uniaxially deformed by anisotropic swelling. Both the light-and neutron-scattering intensities appear to be greatly enhanced along the direction of maximum dilation. Using a scanning light-scattering device, we show that this effect is governed by the "quasistatic" fluctuations, in space, of the polymer concentration, and not by the "thermal" fluctuations. PACS numbers: 61.41.+e, 64.70.Pf, 82.70.Gg A large subgroup of the materials called gels are threedimensional "networks" made of polymer chains weakly cross-linked together by covalent bonds and comprising a large proportion of solvent [1 -3]. On one hand, therefore, a gel is an almost liquidlike binary mixture, in which some spatial fluctuations of the polymer concentration are constantly rebuilt, because of the thermal motion [3 -7] ("gel modes"). On the other hand, it is a soft solid, showing in particular a spectrum of "quasistatic" fiuctuations of concentration in space [6 -8]. The latter result presumably from local variations of the network structure: One may imagine the juxtaposition of harder than average regions, with a lower degree of swelling, and complementary soft regions, with a larger degree of swelling. However, the solidlike and the liquidlike characters are quite entangled and it is sometimes difficult to decide whether a given property of a gel is governed primarily by the first or the second feature. This question arises when considering uniaxially deformed gels, which behave as if they were driven close to a phase transition by the applied strain. It has indeed been found that the intensity of neutrons scattered by elongated gels at small angle (the solvent being labeled) is greatly enhanced along the direction of stretching [9]. One possible explanation [10] of the phenomenon is that the "thermal" fluctuations of concentration are amplified strongly along the direction of elongation. An opposite explanation [11 -13] would be that the observed behavior originates from a change of profile of the quasistatic fluctuations.This question is open and can be answered by an experiment.In this Letter, we report on light-scattering measurements which aim to discriminate between these two options. To this end, one wishes to separate the two contributions to the intensity I(q) scattered by a deformed gel: IF(q), associated with the dynamical concentration fiuctuations and IQ$(q), related to the quasistatic concentration fiuctuations [q denotes the scattering vector of amplitude q = 4m/A sin(8/2), A the wavelength in the medium, and 0 the scattering angle]. One can then write [6,7,14,15] l(q) = IF(q) + IQs(q). The method which is used involves the scanning of the studied sample in a laser beam [14,16], a technique which has recently been applied to gels [15,17 -20].Consider the static (i.e., time-averaged) scattering of a beam of coherent light, for a fixed value of q. Assume that the detector's size is less than one coherence area (the coherence area is a measure of the region over which the fiuc...

SYNOPSISClassically, the one-step synthesis of interpenetrating polymer networks ( IPNs ) and related materials requires noninterfering polymerization modes to achieve distinct networks, held together by only physical entanglements. For the combination of (meth) acrylic and allylic monomers, both polymerizable by free radicals, a new in situ sequential synthesis for obtaining semi-IPNs is proposed. Using specific initiators that decompose at two different temperatures, refractive index measurements, Fourier transform infrared spectroscopy, and dynamic mechanical analysis have shown that neat species are formed and that the two monomers do not copolymerize.

Classically, the simultaneous synthesis utilized to obtain interpenetrating polymer networks requires noninterfering polymerization modes to achieve distinct networks that are held together by only physical entanglements. In the absence of different mechanisms, a single copolymer network is formed, except for monomers with quite different reactivity ratios. Specific initiators that decompose at two different temperatures were used in an in situ sequential process to combine methyl methacrylate and diallyl carbonate of bisphenol A; both monomers are polymerizable under free-radical conditions. The monomer-to-polymer conversions were followed by Fourier transform infrared spectroscopy, and the absence of grafting between networks was deduced from electron spin resonance experiments.

We present several examples of an unexpected behaviour, occurring in different types of polymer networks bermanent or temporary) containing either small or large proportions of mobile species. The spatial fluctuations of local concentration of the mobile probes appear (through neutron scattering experiments) to be anisotropically enhanced (essentially parallel to the elongation axis) when the samples are uniaxially stretched. This behaviour may arise either from an anisotropic enhancement of the thermal fluctuations under strain or from the revealing of "quenched large scale heterogeneities. It has not been possible yet to decide which of these rather different explanations is the more satisfactory.