Influence of cross-linking on the segmental dynamics in model polymer networks

6 pages, 6 figures, 3 tables.In an attempt to study the specific influence of cross-linking on the (alfa) relaxation in polymer networks, a series of model heterocyclic polymer networks (HPN) with well-defined cross-link densities and constant concentration of dipolar units were studied. Model HPN systems were prepared by simultaneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl isocyanate (HI). These HPN systems were characterized by dielectric relaxation spectroscopy in the 10–1 Hz<F<105 Hz frequency range and in the 123 K<T<493 K temperature interval. The (alfa) relaxation in these systems depends on network density and shifts toward higher temperatures as the cross-link density increases for high HMDI/HI ratios. Discussion of the (alfa)-relaxation shape in light of recent models indicates that segmental motions above the glass transition systematically experience a growing hindrance with increasing degree of cross-linking. Description of the temperature dependence of relaxation times according to the strong–fragile scheme clearly shows that fragility increases as polymer network develops.The authors are indebted to the DGICYT (Grant No. PB 94-0049) and to Comunidad de Madrid (07N/0063/1998), Spain, for generous support of this investigation. V.Yu.K. is thankful for the financial support of the NATO grant. I.S. thanks AECI (Spain) for the tenure of a fellowship.Peer reviewe

Thermophysical characterization of the deformation behavior of polyethylene/kaolin composites

13 pags., 5 figs., 2 tabs. -- Part IV of the series: Composition-Dependent Properties of Polyethylene/Kaolin Composites.The deformation behavior at room temperature of injection-molded and blow-molded samples of high-density polyethylene (HDPE) differing in orientation (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by the stretching calorimetry technique. Samples with longitudinal and transversal orientations were examined. Strain-softening and strain-hardening phenomena were observed, respectively, in the range of inelastic strains above the yield point. The depression of yield stresses for the filled composites compared to those for the corresponding neat polymers is associated with the onset of debonding of the matrix polymer from the filler surface. The analysis of the energy balance of the debonding process suggests that the fraction of a debonded interface is smaller in samples containing a coupling agent and larger filler particles.This work was supported by the BMBF through project UKR-031-96 and by the DGICYT, Spain (grant PB94-0049). Thanks are due to the Ministerio de Educacion y Cultura, Spain, for a sabbatical grant to V. P. P

Composition-Dependent Properties of Polyethylene/KaolinComposites. III. Thermoelastic Behavior of Injection-Molded Samples

8 pags., 5 figs., 1 tab.Injection‐molded samples of high‐density polyethylene (HDPE) differing in the orientation pattern (with respect to the melt flow direction) and in filler content (untreated and surface‐treated Kaolin, respectively) were characterized by wide‐angle X‐ray scattering, microhardness, and stretching calorimetry techniques. The crystallinity of the polymer matrix in filled samples shows the same value as that found for the neat polymer regardless, filler content, and/or filler surface treatment. The thermoelastic behavior of all samples in the strain interval below the apparent yield point ε* is quantitatively discussed in terms of classical equations for elastic solids. Analysis of thermoelastic parameters of the boundary interphase (BI) reveal an unusually stiff, highly oriented structure of the matrix polymer within BI. Discrepancies of experimental values for the internal energy increment in the inelastic strain interval above ε* between unfilled and filled samples is explained in terms of the filler debonding process. The latter process is discussed in light of the formation of a polymer‐free filler surface and of the concomitant inelastic deformation (plastic flow) of a polymer matrix in the interstitial space between filler particles. © 1999 John Wiley & Sons, Inc.BMBF. Grant Number: UKR‐031‐96 DGICYT. Grant Number: PB94‐0049 BMBF. Grant Number: UKR‐031‐96 DGICYT. Grant Number: PB94‐0049Peer reviewe

Structure-microhardness relationship in semi-interpenetrating polymer networks

Two series of single-phase, semi-interpenetrating polymer networks (semi-IPNs) based on the same linear polyurethane (LPU) and two different heterocyclic polymer networks (HPNs), respectively, were characterized by the room temperature microhardness H measurement. It is shown that the H value linearly increases with both the mass content of the stiffer component (HPN) and the glass transition temperature of a semi-IPN. The latter dependence can be justified on the assumption that the excess enthalpy of the glass with respect to a hypothetical melt state is a measure of the yield strength of the glassy quasi lattice.This work was supported by the DGICYT, Spain (grant PB94-0049). V. P. P. thanks the Ministerio de Educacion y Cultura, Spain, for the award of a sabbatical grant