TREF fractionation was combined with SEC‐FTIR analysis to measure the compositional heterogeneity within a commercial impact PP copolymer. The chemical composition of all fractions was determined as function of their molecular weight distribution. This approach proved to be highly successful at identifying different constituents within fractions exhibiting bimodal molecular weight distributions. Furthermore, the determination of ethylene and propylene crystallinity distribution across the molecular weight distribution confirmed the morphological nature of each of the components of the bimodal distribution. It is demonstrated that the combination of TREF and SEC‐FTIR provides a simple alternative to more time‐consuming conventional ways of characterising impact PP copolymers of complex heterogeneity.magnified image

ICPPs are complex materials that contain various ethylene/propylene copolymers in addition to the majority poly(propylene) phase. To understand their molecular structure, multiple fractionation and analysis techniques are used. In this study, TREF and CRYSTAF are used to study the difference in the thermo‐oxidative degradation of two grades of ICPPs with different ethylene contents. Direct correlations are drawn between the carbonyl index, molecular weight and crystallisability at various stages of degradation. The higher ethylene content sample shows increased thermal stability. This is due to the fact that the ethylene/propylene fraction is more stable due to the higher comonomer content and lower isotacticity as well as due to the higher amount of this fraction in the material.magnified image

Summary: Impact poly(propylene) copolymers (ICPP) are complex polymer systems containing various types of ethylene‐propylene copolymers as well as the majority poly(propylene) phase. In this study, multidimensional analytical techniques are applied to study the thermo‐oxidative degradation of these complex materials. The combination of size exclusion chromatography (SEC) and FTIR via an LC‐transform interface allows for the identification and tracking of the low molecular weight oxidized products. The degradation has a significant effect on the crystallisability of the material. DSC analysis shows that as the degradation proceeds, there is a significant decrease in the onset of the melt endotherm as well as the development of a double melt peak and peak broadening. Preparative Temperature Rising Elution Fractionation (TREF) is used to isolate the various fractions according to crystallisability during the polymer degradation. TREF‐SEC and TREF‐(SEC‐FTIR) allows for the isolation and identification of the polymer fractions undergoing oxidative degradation. It is shown that these multidimensional analytical techniques using crystallisability in the first dimensional fractionation provide more information on the mechanism and process of oxidative degradation than traditional bulk analysis methods.

The spatial degradation in impact PP copolymers with different ethylene contents is studied by FTIR microscopy and layer‐by‐layer milling of the sample surfaces, followed by ATR‐FTIR, SEC, and CRYSTAF analysis. FTIR allows for tracking of the rate of degradation, providing information on the depth profiling of the degradation. Results show that samples with lower ethylene content degrade faster at all depths than those with higher ethylene content. The latter show a more uniform degradation from the surface to the bulk of the material at longer degradation times. This is ascribed to the higher amorphous content which results in a larger oxygen diffusion to the centre of the material while its slower rate of degradation is ascribed to the lower tertiary carbon content.