A numerical and experimental investigation of the effects of thermal history on the structure/property relationship of PPS/carbon fiber composites

Abstract

The purpose of this investigation was to examine the effects of thermal history during cooling from the melt on the degree of crystallinity, morphology, and mechanical properties of (polyphenylenesulfide) PPS/carbon fiber composites. Three thermal treatments were employed in this study: isothermal crystallization from the melt at 140,160,180,200, and 220°C, quenching from 315° C and then annealing at 160 and 200° C, and nonisothennal crystallization from the melt at rates varying from 0040 C/minute to 68° C/second. The effect of varying the thermal history of the sample on the degree of crystallinity developed in the matrix polymer was determined using differential scanning calorimetry. The effect of thermal history on and the resulting matrix morphology was examined by scanning electron microscopy. The subsequent effects of the degree of crystallinity and the morphology on the mechanical behavior of the samples were monitored by transverse tensile tests and flexural tests. In all cases, the transverse tensile and flexural moduli increased as the amount of crystallinity in the samples increased. However, samples with greater amounts of crystallinity did not always yield higher transverse tensile or flexural strengths. Upon examination of the composite samples by electron microscopy, it was observed that trends in the values of the transverse tensile and flexural strengths could be correlated with structural changes in the matrix.

This paper is concerned with the simulation of the development of crystallinity and morphology (both amount of crystallinity and the size of spherulites) which arise during the cooling of a slab of a semicrystalline polymer reinforced with long continuous carbon fibers. This situation is commonly found during the processing of semicrystalline thermoplastic composites. Whereas published attempts at simulating this process have treated the composite material as a continuum and thereby used mass averaged physical properties (such as thermal conductivity, density, and specific heat), we use a quasi-continuum approach in which locally we consider the properties of the matrix and fiber separately. Once a temperature distribution is calculated using the continuum approach, the fmite element method is applied locally at various points in the slab to calculate the amount of crystallinity and the size of the developing spherulites. This is done by using the Avrami equation and the Hoffman and Lauritzen radial growth equation. The amount of crystallinity and the spherulite size are predicted as a function of fiber spacing and packing geometry, and the predictions are found to be in good agreement with experimental results obtained on polyphenylenesulfide/carbon fiber composites. The advantages of our approach over the continuum approach is that a relatively accurate prediction of the spherulite size is possible due to constraints imposed by the fiber on the spherulitic growth.