We have characterized the effect of molecular weight distribution on slip of linear 1,4-polybutadiene samples sandwiched between cover glass and silicon wafer. Monodisperse polybutadiene samples with molecular weights in the range of 4–195 kg/mol and their binary mixtures were examined at steady state in planar Couette flow using tracer particle velocimetry. Slip velocity was measured at shear rates over the range of ∼0.1–15 s–1. Our results revealed that weakly entangled short chains play a crucial role in wall slip and flow dynamics of linear polymer melts. It was found that the critical shear stress for the onset of the transition to the strong slip regime is significantly reduced when a small amount of weakly entangled chains is added to a sample of highly entangled polymer. Within the same range of shear stresses, binary mixtures of long and short chains exhibit significantly enhanced slip compared to the moderate slip of the individual long chains. This is attributed to the reduced friction coefficient at the polymer–solid interface which is likely a complicated function of the nature of entanglement, chain adsorption, and relaxation dynamics of chains at the interface.
Thermal, rheological, and mechanical properties of a commercial carbon fiber epoxy prepreg, Cycom 977-2 UD, were obtained for isothermal cure temperatures ranging from 149 C to 182 C. For each cure profile, an encapsulated-sample rheometer (ESR) was used to measure the storage modulus. Each ESR cure profile was followed by the glass transition temperature (T g ) test. The degree of cure () during cure and the heat of reaction of the prepreg were obtained using a differential scanning calorimeter (DSC). Combined loading compression (CLC) and short-beam shear (SBS) tests were performed to obtain compressive properties and SBS strength, respectively. It was observed that the compressive properties did not vary significantly for the studied isothermal cure temperatures; likewise, the compressive failure mode was the same for all the CLC specimens. However, the SBS strength for the specimens cured at 149 C was approximately 10% less than that of those cured at isothermal cure temperatures ranging from 160 C to 182 C. Further, the failure mode of the SBS specimens cured at 149 C was also different from other specimens. The storage modulus of the ESR sample cured at 149 C also showed a 10% decrease compared to other ESR samples. The SBS strength exhibited a good correlation with the storage modulus and a weak correlation with T g and .
We characterized wall slip of tridisperse linear 1,4-polybutadiene on a silicon wafer in a parallel plate shear cell and tracer particle velocimetry. Tridisperse mixtures of fixed weight-average molecular weight M w and varying number-average molecular weight M n were prepared from nearly monodisperse polybutadienes. Their steady state slip behavior was examined at shear rates over the range of ∼0.1−15 s −1 . The results show that the slip behavior in the transition regime depends on M n at constant M w . This study also revealed that weakly entangled and unentangled chains in the mixtures influence wall slip differently: mixtures containing moderate amounts of weakly entangled chains exhibited enhanced slip while those containing unentangled chains did not. We explain this observation using the tube theory through the slip disentanglement mechanism proposed by Brochard and de Gennes in 1992 and conclude that the slip behavior is changed because of the force balance between the mobile and adsorbed chains and the reduced entanglement density and coil stiffness related to the incorporation of short chains of different lengths.
Here, we propose a new method for estimating the extent of curing of thermosetting prepregs. In the proposed method, the extent of curing is estimated with the curing index (C i ), defined as the ratio of the glasstransition temperature (T g ) to the ultimate glass-transition temperature of the material. The advantages of this new method over the conventional degree of conversion (a) for estimating the extent of curing of thermosetting prepregs are discussed in detail. C i and a of a toughened epoxy prepreg (977-2 unidirectional) were obtained for a wide range of isothermal curing temperatures with a differential scanning calorimeter. The ultimate heat of reaction varied inconsistently with decreasing curing temperature; this resulted in erratic behavior of a. However, C i provided a more consistent estimate of the extent of curing because T g , unlike a, did not need to be modified on the basis of the curing history of the material and was measured directly with the heat-flow data from differential scanning calorimetry. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: [883][884][885][886][887][888][889][890][891] 2011
We investigate the effect of short chains on slip of highly entangled polystyrenes (PS) during thin film dewetting from non-wetting fluorinated surfaces. Binary and ternary mixtures were prepared from monodisperse PS with weight average molecular weights 5 < Mw < 490 kg/mol. Flow dynamics and rim morphology of dewetting holes were captured using optical and atomic force microscopy. Slip properties are assessed in the framework of hydrodynamic models describing the rim height profile of dewetting holes. We show that short chains with Mw below the polymer critical molecular weight for entanglements, Mc, can play an important role in slip of highly entangled polymers. Among mixtures of the same Mw, those containing chains with M < Mc exhibit larger slip lengths as the number average molecular weight, Mn, decreases. The slip enhancement effect is only applicable when chains with M < Mc are mixed with highly entangled chains such that the content of the long chain component, φL, is dominant (φL < 0.5). These results suggest that short chains affect slip of highly entangled polymers on non-wetting surfaces due to the physical or chemical disparities of end groups, and any associated dynamical effect their presence may have, as compared to the backbone units. The enhanced slip in this regard is attributed to the impact of chain end groups or short chain enrichment on the effective interfacial friction coefficient. Accordingly, for entangled PS, a higher concentration of end groups or short chains at the interface results in a lower effective friction coefficient which consequently enhances the slip length.
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