Abstract
Successfully evaluating plastic lifetime requires understanding of the relationships between polymer dynamics and mechanical performance as a function of thermal ageing. The relatively high Tg (Tg = 110 °C) of poly(1,4-cyclohexylenedimethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate) (PCTT) renders it useful as a substituent for PET in higher temperature applications. This work links thermal ageing and mechanical performance of a commercial PCTT plastic after exposure to 40–80 °C for up to 2950 h. No chemical or conformational changes were found while pronounced physical ageing, measured as enthalpic relaxation, caused yield hardening (28% increase in yield strength) and embrittlement (80% decrease in toughness). Enthalpic relaxation increased with temperature and time to 3.8 J g−1 and correlated to the determined toughness and yield strength. Finally, a 9% increase in Young's modulus was observed independent of temperature and with no correlation to enthalpic relaxation. Enthalpic relaxation followed Vogel–Fulcher–Tammann behaviour, while yield strength and charpy v-notch toughness followed Arrhenius behaviour enabling prediction of the different properties with time and temperature.
Highlights
Increased awareness of environmental impact from plastics has intensi ed the research into bio-based and recyclable polymeric materials.[1,2,3,4] A class of plastics showing strong potential is polyesters
PCTT was characterised by comparing the nuclear magnetic resonance spectroscopy (NMR) spectra to analysis of the PCTT monomers, i.e. terephthalic acid (TPA), 1,4
The molecular masse might be valid if a surplus of CHDM was applied or if it was added later in the synthesis as both will result in only few TMCD terminated polymers. 1H and 13C NMR of heat-treated PCTT elements at 80 C (Table 1, Electronic supplementary information (ESI) Table S1–S3†) and ATRFTIR of heat-treated PCTT elements at 40, 60, and 80 C (Fig. 7, ESI Table S4 and S5†) showed no chemical or cis/trans conformational changes
Summary
Increased awareness of environmental impact from plastics has intensi ed the research into bio-based and recyclable polymeric materials.[1,2,3,4] A class of plastics showing strong potential is polyesters. Polyesters are challenged in high-temperature environments due to their vulnerability to thermally induced mechanical changes which limits service life. These mechanical changes can be subdivided into chemical and physical ageing. Chemical aging e.g. hydrolysis and oxidation can cause polymer cleavage and/or cross-linking.[5] Excessive cross-linking o en results in embrittlement where a reduction in molecular mass reduces mechanical, thermal and rheological properties rendering the plastic defective.[6] Physical ageing arises from super-cooling amorphous materials and is the subsequent time and temperaturedependent dri towards equilibrium.[7] It is a general effect, in uencing the amorphous phases in all polymer materials decreasing the ductile properties.[8,9,10] This polymeric compaction by segmental polymer movement results in reduced free volume, and increases density of the amorphous phases.[11,12] Physical ageing is only observed below the glass transition temperature (Tg, indicated as onset throughout this paper) where the amorphous phases are solidi ed out of equilibrium. Below Tg the driving force of physical ageing is the temperature difference between the exposed temperature (Te) and Tg14 implying that higher service temperature (below Tg) causes faster physical aging
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