Part I. Optical measurement of polyethylene spherulite growth rates in three growth regimes. Part II. Influence of matrix properties on the single fiber fragmentation technique.

摘要

Part I. Much of nucleation theory has been developed and evaluated based on studies of narrow fractions of linear polyethylene. However, polyethylene crystallizes very quickly with growth rates spanning five orders of magnitude in just 10{dollar}spcirc{dollar}C of undercooling and has a high heat of fusion which can serve to limit obtaining large undercoolings owing to self heating. Currently, the presence of three crystallization regimes in polyethylene is inferred from calorimetry and small droplet measurements. In this study optical growth measurements are made at larger undercoolings than previously observed by making the samples very thin and using a flowing liquid to remove the heat of fusion. Narrow fractions of {dollar}rm Msb{lcub}w{rcub}{dollar} = 32.1k and {dollar}rm Msb{lcub}w{rcub}{dollar} = 74.4k were used. Regime III crystallization was confirmed for these fractions and growth in all three regimes was observed. The regime transitions occurred in good agreement with nucleation theory and the K{dollar}rmsb{lcub}g{rcub}{dollar} value ratios approached the theoretical value of 2.0. Banded spherulites were observed at higher undercoolings. The band spacings decrease with increased undercooling and show a remarkable molecular weight dependence.; Part II. Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T{dollar}rmsb{lcub}g{rcub} = 166spcirc C{dollar}) are altered by annealing below T{dollar}rmsb{lcub}g{rcub}{dollar}. Neat resin samples with identical thermal histories are tested. All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.