A.C. Su

Functionalization of polypropylene via melt mixing

Abstract Grafting of maleic anhydride (MAH) onto isotactic polypropylene (PP) was performed by melt mixing in a laboratory-scale internal mixer with the addition of dicumyl peroxide (DCP) as an initiator. The efficiency of the graft reaction (estimated by use of Fourier-transform infra-red spectroscopy) tends to decrease with increasing MAH or DCP content; the optimal composition for the present reaction system appeared to be at a low DCP level and a moderate MAH loading. Concurrent with the grafting reaction, PP undergoes significant chain scission as indicated by the decrease in mixing torque and supported by the dynamic rheological evidence. The first addition of MAH to the reaction mixture enhanced the DCP-initiated chain scission; upon further addition of MAH, the scission reaction became less severe. These observations were explained in terms of competition between chain transfer/β scission and bimolecular coupling of radical intermediates.

Damping behaviour of dynamically cured butyl rubber/polypropylene blends

Abstract Dynamically cured blends of butyl rubber (IIR; isoprene-isobutylene rubber) and anhydride-grafted polypropylene (PP) of various component ratios and curative loadings were prepared by melt mixing. Evidence from dynamic mechanical analysis and phase-contrast microscopy suggested that these blends were heterogeneous, with the PP phase being continuous. Damping characteristics of these blends in the temperature range of −60 to 0°C were strongly affected by a liquid-state relaxation peak of the IIR phase. It was observed that the presence of a continuous PP phase tends to suppress the intensity of this characteristic loss peak and shift it towards lower temperatures; on the other hand, the crosslinking of IIR has little effect on the intensity of the broad loss peak but tends to shift it towards higher temperatures. These observations were interpreted in terms of effects of thermal misfit stress and crosslinking on the liquid-liquid transition of the IIR phase.

Effect of interleaving on the impact response of a unidirectional carbon/epoxy composite

Abstract Instrumented drop-weight impact response of a unidirectional carbon/epoxy composite and that of the corresponding laminate interleaved with poly(ethylene-co-acrylic acid) at the midplane were studied. The impact penetration energy of the unidirectional composite beam increased strongly upon interleaving. Realtime observations using a microscope in a static flexure test indicated a dramatic change of failure mode from the dominantly compressive fracture of the baseline laminate to the dominantly tensile fracture process upon interleaving, in accordance with the corresponding fractographic features of the impact specimens. These observations were attributed to the change in stress distribution upon introduction of the interleaf layer. The relevance of the present observations to the role of high-strain, low-modulus adhesive layers in the toughening of composites is discussed.

Morphology of poly(p-phenylene sulfide)/polyethylene blends

Abstract Blends of poly(p-phenylene sulfide) (PPS) and polyethylene (PE) of various component ratios were prepared via melt mixing in a laboratory-scale internal mixer. Cryogenically fractured surfaces of these blends were examined by means of scanning electron microscopy. General morphological features including the domain size and the phase inversion process were discussed in terms of droplet dispersion during melt mixing and phase coarsening after cessation of flow. Interestingly, the surface of dispersed PE particles showed distinct features of protruding fibrils whereas the surface of dispersed PPS particles was characterized by an orange-peel appearance. These were explained in terms of the sequential crystallization of the two phases and the loose packing of PPS crystallites in the intermediate stage of solidification.

Vibration Damping of Interleaved Carbon Fiber-Epoxy Composite Beams

Vibration-damping behavior of unidirectional and symmetric angle-ply carbon fiber-epoxy laminates as well as their interleaved counterparts with a layer of poly(ethylene-co-acrylic acid) (PEAA) at the mid-plane was examined. The introduction of the PEAA layer significantly improved the damping capability. The effectiveness of interleaving increased with the flexural modulus of the outer layers. In the case of unidirectional laminates, calculations based on a sandwich structure of isotropic layers quantitatively reproduced this trend. In the case of angle-ply laminates, however, the model predicted only part of the improvement experimentally observed. This was explained in terms of the bending of the angle-ply laminates in the transverse direction which would induce additional deformations in the interleaf layer and was not accounted for by the present model.

Kinetics of polycyclotrimerization of 4,4′-thiodiphenylcyanate

Abstract Polycyclotrimerization of 4,4′-thiodiphenylcyanate was studied by means of differential scanning calorimetry. Samples of different impurity levels or different catalyst loadings (n-nonylphenol (NP), 1 to 10 phr) were studied. In the absence of added catalyst, the sample with higher impurity level (most likely residual water) underwent polycyclotrimerization at a lower temperature range than the sample of comparatively higher purity, although both samples shared the same apparent activation energy (Ea = 93 kJ mol−1) and exhibited similar autocatalysed first-order characteristics. The lowering of the reaction temperature range was more significant in the presence of NP. Interestingly, kinetic features in this case were quite different from the uncatalysed counterpart: in addition to the expected decrease in the apparent activation energy (Ea = 75 kJ mol−1, independent of the catalyst concentration), the cure reaction followed an autocatalysed second-order kinetics. These observations are explained in terms of a mechanistic scheme in which the competition between hydroxyl-catalysed and autocatalytic paths is considered. In particular, formation of NP aggregates ((NP)m, where m ≈ 7) is proposed to account for the less-than-additive catalytic capacity with increasing NP level.

Spherulites of cis-1,4-polybutadiene: molecular weight effects

Abstract Morphological features of melt crystallized cis -1,4-polybutadiene were examined by means of polarized light microscopy. Samples similar in branching characteristics but different in molecular weight ( MW ) were studied. High MW samples exhibited features of dwarf spherulites. Samples of lower MW formed spherulites of radial texture at low crystallization temperatures ( T c ), spherulites of banded texture at intermediate T c , and spherulites of complex birefringence at the high T c end. Microscopic observations during subsequent heating of the latter spherulites indicated that they were composed of a mainframe of the banded texture and interstitial crystallites with different orientation. The interstitial crystallites melted at a lower temperature range whereas the mainframe persisted to higher temperatures. The melting temperature of slowly cooled or cold-crystallized samples decreased first but then remained approximately constant with increasing MW . These observations are discussed in terms of the entanglement effects during melt crystallization.

Suppression of side reactions during melt functionalization of ethylene-propylene rubber

Abstract Grafting of maleic anhydride (MAH) onto ethylene-propylene rubber (EPR) was performed by melt mixing in a laboratory-scale internal mixer with the addition of an initiator (dicumyl peroxide, DCP) and an ingredient (stearamide, SA) for the suppression of side reactions of cross-linking and chain scission. Effective suppression of side reactions was realized for systems of moderate or low DCP contents upon addition of SA, but no apparent changes could be identified at high DCP loadings. Within the confidence level of Fourier-transform infrared spectroscopic measurements, the extent of grafting appeared to be little affected by the addition of SA. Possible reaction paths in the present system are discussed. In particular, SA is considered as a chain transfer agent which tends to suppress the more populated reaction path, whether it be cross-linking or chain scission.

Effect of intramolecular cycles on the formation of rigid polycyanate networks

Abstract On the basis of the recursive method of Macosko and Miller ( Macromolecules , 1976, 9 , 199), a theoretical scheme with due considerations on the formation of intramolecular cycles is developed for polycyanate networks. Experimental observations on gelation behaviour of a typical aromatic dicyanate, 2,2-bis(4-cyanatophenyl)propane, are compared with the theoretical predictions. Results indicate that deviations from the classical behaviour (including the sluggish molecular weight build-up during the pre-gelation regime, the delayed gelation, and the steeper increase of gel fraction in the post-gelation stage) can be explained in terms of effects of intramolecular cyclisation, which is negligible in the early stage of curing, becomes more important with increasing conversion, and is then counterbalanced (presumably by loop-interlocking) in the post gelation regime.