Polyethylene terephthalate (PET) is one of the most widely used engineering and packaging polymers, and its structural integrity changes significantly during thermal, hydrolytic, photochemical, and biological decomposition. Comparative structural analysis of PET before and after decomposition provides critical evidence of chain scission, crystallinity evolution, end-group formation, and degradation product generation. This article systematically reviews the sample types, testing items, characterization methods, and industrial applications involved in PET structural comparison studies before and after decomposition.
Technical Overview
Polyethylene terephthalate (PET) is a semi-crystalline, thermoplastic polyester composed of repeating units containing an aromatic terephthalate ring and an ethylene glycol segment linked through ester bonds. The ester linkage along the backbone is the primary reactive site for virtually all decomposition pathways, including hydrolysis, thermal-oxidative degradation, photolysis, glycolysis, methanolysis, and enzymatic depolymerization. Understanding how the molecular and supramolecular structure of PET changes from its initial state to the decomposed state is essential for failure analysis, recyclate quality control, biodegradation research, and the development of chemical recycling processes.
Before decomposition, virgin PET typically exhibits a number-average molecular weight in the range of 15,000 to 25,000 g/mol, a glass transition temperature of approximately 75 to 80 degrees Celsius, a melting temperature of 250 to 260 degrees Celsius, and a degree of crystallinity between 30 and 40 percent for oriented products. After decomposition, the polymer undergoes chain scission at ester bonds, generating carboxylic acid, hydroxyl, and vinyl ester end groups, along with oligomers and monomeric species such as terephthalic acid (TPA), mono(2-hydroxyethyl) terephthalate (MHET), bis(2-hydroxyethyl) terephthalate (BHET), ethylene glycol, and acetaldehyde. A rigorous before-and-after comparison therefore requires a multi-technique analytical strategy that spans molecular weight, chemical functionality, thermal behavior, crystalline morphology, and surface topography.
Testing Samples
Virgin and Processed PET Resins
Pelletized PET resins, both virgin and extrusion- or injection-molded, serve as baseline samples. These are compared against thermally treated counterparts that have undergone controlled decomposition at elevated temperatures, typically between 250 and 400 degrees Celsius under air or nitrogen atmospheres.
Packaging Materials
Biaxially oriented PET films, beverage bottles, and food trays represent the largest commercial application class. Samples before and after accelerated weathering, retort sterilization, or long-term storage are compared to assess hydrolytic and photochemical stability.
Recycled and Reprocessed PET
Mechanically recycled flakes, regranulated PET, and multi-cycle reprocessed materials are compared with their feedstock to quantify degradation accumulated during reprocessing cycles and to evaluate solid-state polymerization (SSP) restoration effects.
Biodegradation Test Coupons
Standardized PET films or powder specimens exposed to enzymatic systems such as PETase and MHETase, or to composting and soil environments, are analyzed against unexposed controls to quantify biodegradation progress.
Chemically Depolymerized Products
Reaction mixtures from glycolysis, methanolysis, hydrolysis, or aminolysis are compared with the starting polymer to confirm depolymerization pathways, conversion efficiency, and product purity.
Testing Items
The comparative analysis covers the following core structural parameters, each measured on both the pre-decomposition and post-decomposition states:
- Molecular weight parameters: number-average molecular weight (Mn), weight-average molecular weight (Mw), z-average molecular weight (Mz), and polydispersity index (PDI)
- Chemical structure and end groups: carbonyl index, hydroxyl index, carboxyl end-group concentration, vinyl ester end groups, and intrinsic viscosity (IV)
- Degradation products and oligomers: TPA, MHET, BHET, ethylene glycol, acetaldehyde, cyclic oligomers such as CHET, and linear oligomers
- Thermal properties: glass transition temperature, melting temperature, melting enthalpy, cold crystallization behavior, crystallinity, and thermal decomposition onset temperature
- Crystalline structure: degree of crystallinity, crystallite size, and crystal form transitions determined by wide-angle X-ray scattering
- Surface morphology: erosion features, cracks, cavities, pitting, and roughness changes
- Optical and colorimetric properties: yellowing index, transmittance, and haze, which reflect chromophore formation during oxidative decomposition
- Mechanical properties: tensile strength, elongation at break, and impact strength as macroscopic indicators of structural integrity loss
Testing Methods
Gel Permeation Chromatography (GPC/SEC)
Gel permeation chromatography quantifies the reduction in molecular weight caused by chain scission, which is the most fundamental indicator of PET decomposition. Hexafluoroisopropanol (HFIP) or HFIP with sodium trifluoroacetate is commonly used as the eluent to dissolve both intact and partially degraded PET. A typical thermal-oxidative degradation event reduces Mn by 40 to 80 percent, while the polydispersity index often shifts from approximately 2.2 to below 1.8 for random scission-dominated processes or above 2.5 when secondary crosslinking or oligomer accumulation occurs.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR, in ATR or transmission mode, tracks functional group evolution. The characteristic bands of PET include the ester carbonyl stretch near 1715 cm⁻¹, the aromatic C=C ring vibration near 1409 cm⁻¹, the C-O stretching bands between 1240 and 1095 cm⁻¹, and the ethylene glycol CH₂ wagging doublet near 728 cm⁻¹. After decomposition, new or intensified absorptions appear: a broad O-H stretch around 3200 to 3500 cm⁻¹ from hydroxyl and carboxylic acid end groups, carboxylate C=O absorption between 1680 and 1700 cm⁻¹, vinyl ester carbonyl near 1685 cm⁻¹, and trans-vinylene C-H out-of-plane bending near 966 cm⁻¹ in thermally degraded samples. The carbonyl index, calculated as the ratio of the 1715 cm⁻¹ band area to a reference band such as 1409 cm⁻¹, provides a normalized measure of oxidative damage.
Nuclear Magnetic Resonance Spectroscopy (NMR)
Solution ¹H NMR in deuterated trifluoroacetic acid or HFIP-d₂ resolves end-group structures with high specificity. Chemical shifts near 4.7 and 4.9 ppm correspond to methylene protons adjacent to ester and hydroxyl end groups, respectively, while aromatic proton signals between 8.1 and 8.3 ppm track terephthalate species. Quantitative integration of end-group signals against backbone signals allows calculation of number-average molecular weight and end-group concentration. ¹³C NMR further distinguishes free carboxylic acid carbonyl (approximately 167 ppm) from esterified carbonyl (approximately 165 ppm), enabling differentiation between hydrolytic and thermal-vinyl-ester-forming degradation mechanisms.
Differential Scanning Calorimetry (DSC)
DSC compares the thermal transitions before and after decomposition. Common observations after degradation include a decrease or broadening of the melting peak, reflecting shorter chains and less perfect crystallites; changes in glass transition temperature, which decreases with molecular weight reduction according to the Fox-Flory relationship; and intensified or suppressed cold crystallization exotherms, indicating altered chain mobility. The degree of crystallinity, calculated from melting enthalpy normalized against the 100 percent crystalline PET reference value of approximately 140 J/g, frequently increases after hydrolytic or biodegradation because amorphous regions are preferentially attacked, leaving an enriched crystalline phase.
Thermogravimetric Analysis (TGA)
TGA under nitrogen and air atmospheres establishes the decomposition onset temperature, maximum degradation rate temperature, and residue yield. Intact PET begins rapid decomposition near 380 to 420 degrees Celsius in nitrogen with a single major mass-loss step, whereas pre-degraded samples show reduced onset temperatures and, under air, additional low-temperature mass-loss steps attributable to oxidative chain scission. Derivative TGA curves reveal whether decomposition follows a single-step random scission pathway or a multi-step pathway involving volatile oligomer evaporation.
Wide-Angle X-ray Diffraction (WAXD/XRD)
X-ray diffraction quantifies crystalline structure changes. Virgin oriented PET shows characteristic triclinic reflections, while degraded samples may exhibit sharpened diffraction peaks due to chemi-crystallization of released chain segments or diminished crystallite coherence after severe degradation. Comparing diffraction intensity ratios before and after decomposition distinguishes amorphous-phase-selective degradation from bulk random degradation.
Scanning Electron Microscopy (SEM)
SEM visualizes surface structural damage that molecular analyses cannot directly capture. Undegraded PET surfaces are smooth and featureless, whereas hydrolytically or enzymatically degraded surfaces display progressive erosion, pits, cavities, and crystalline lamellar exposure. In thermally degraded samples, surface cracking and embrittlement features dominate. Quantitative image analysis of surface roughness complements gravimetric mass-loss data.
Chromatographic and Mass Spectrometric Product Analysis
HPLC with UV detection at 240 to 254 nm separates and quantifies TPA, MHET, and BHET released during hydrolytic or enzymatic decomposition. Gas chromatography, often coupled with mass spectrometry, detects volatile products including acetaldehyde, ethylene glycol, and benzene-derived fragments. MALDI-TOF mass spectrometry identifies the full oligomer distribution, distinguishing cyclic from linear oligomers and revealing the specific scission chemistry governing decomposition.
Intrinsic Viscosity Measurement
Intrinsic viscosity, determined by Ubbelohde viscometry in a phenol/tetrachloroethane mixture at 25 degrees Celsius, remains the industry-standard quality metric. The Mark-Houwink relationship converts IV to molecular weight, and a drop from a typical bottle-grade value of 0.72 to 0.84 dL/g to below 0.60 dL/g signals significant degradation requiring solid-state polymerization before reuse in food-contact applications.
Applications
Comparative structural analysis of PET before and after decomposition supports decision-making across multiple industries:
- Recycling industry: assessing degradation accumulated across mechanical recycling loops and validating the effectiveness of solid-state polymerization in restoring molecular weight for bottle-to-bottle applications
- Chemical recycling development: confirming depolymerization pathways and quantifying monomer yields in glycolysis, methanolysis, and enzymatic recycling processes
- Packaging and beverage sector: evaluating hydrolytic stability during hot-filling, retort sterilization, and long-term storage, and controlling acetaldehyde migration relevant to food safety
- Textile industry: monitoring UV and hydrolytic degradation of PET fibers in outdoor and high-humidity applications
- Biodegradation research: quantifying enzymatic depolymerization efficiency of engineered PETase and MHETase variants and characterizing intermediates in biorecycling workflows
- Failure analysis: identifying root causes of embrittlement, discoloration, or mechanical failure in service-exposed PET components through end-group and crystallinity fingerprinting
- Academic and regulatory studies: providing standardized evidence of environmental persistence or degradability for compliance with ecodesign and recyclability regulations
FAQ
What are the main decomposition products of PET?
Depending on the pathway, PET decomposes into terephthalic acid, ethylene glycol, MHET, BHET, acetaldehyde, vinyl-terminated oligomers, and cyclic oligomers. Hydrolysis and enzymatic degradation primarily yield TPA, MHET, BHET, and ethylene glycol, while thermal-oxidative degradation preferentially generates acetaldehyde and vinyl ester end groups.
Does the crystallinity of PET increase or decrease after decomposition?
In most hydrolytic, enzymatic, and weathering scenarios, measured crystallinity increases because amorphous regions are attacked preferentially, enriching the crystalline fraction. However, severe thermal decomposition at high temperatures can destroy crystalline order and broaden or lower the melting endotherm, so the direction of change depends on the degradation mechanism and severity.
How can hydrolytic degradation be distinguished from thermal-oxidative degradation?
The end-group fingerprint is decisive. Hydrolysis produces paired carboxylic acid and hydroxyl end groups detectable by NMR and titration, whereas thermal degradation produces vinyl ester end groups accompanied by acetaldehyde release, identified through FTIR bands near 1685 cm⁻¹, the trans-vinylene band at 966 cm⁻¹, and GC-MS detection of acetaldehyde.
What is the carbonyl index and why is it used?
The carbonyl index is the ratio of the integrated intensity of the carbonyl absorption near 1715 cm⁻¹ to a stable internal reference band such as the 1409 cm⁻¹ aromatic vibration. It normalizes thickness and contact variations in ATR-FTIR measurements and serves as a convenient single-number metric for tracking oxidative decomposition progression.
Which technique should be prioritized when budget is limited?
Intrinsic viscosity measurement combined with ATR-FTIR offers the highest information-to-cost ratio, delivering molecular weight estimates and functional group evidence simultaneously. DSC and TGA can then be added when thermal property changes or decomposition kinetics are relevant to the investigation.
How sensitive is GPC for detecting early-stage PET decomposition?
HFIP-based GPC reliably detects molecular weight changes below 5 percent relative standard deviation, making it suitable for early degradation stages where visual and mechanical changes are not yet apparent. Cross-validation with NMR end-group analysis further improves confidence at incipient degradation levels.