Discovering Polymers Through Visual Experiences

Abstract:
Polymeric materials are present in almost every aspect of our modern lives: packaging, textiles, household appliances, automobiles, electronic components, medical prostheses, and much more. However, the science behind their mechanical and thermal behaviour is complex and not usually accessible to the public or pre-university students. Within the framework of the e-PLASCOM research group at the Polytechnic University of Catalonia, we have designed a series of outreach activities for high school students, aiming to introduce fundamental concepts of polymer physics through visual and participatory activities. These activities have been carried out during open campus days, Science Week, and the Escolab program. The multisensory approach allows learning to take place through sight, touch, hearing, and even smell, transforming complex phenomena such as the glass transition or the thermodynamics of elastomers into concrete and memorable experiences. Three of these illustrative experiences, namely the mechanical properties, the glass transition and the elastocaloric effect of elastomers are presented below.

Key words: polymers, DIY tensile test, glass transition, elastocaloric effect, visual experiences, secondary school students.

1. Introduction

Polymers are present in numerous everyday objects, but their fundamental properties are little known to the public. To make these concepts accessible, we propose a series of experimental activities for secondary school students (ages 12–18), which can be carried out without the need of expensive machines or equipments. Through visual and participatory demonstrations, key phenomena of polymer behaviour are explored. The concept of some important mechanical properties such as the yield strength is introduced and visualized through a series of tensile tests. For this purpose, we present a low-cost, Do-It-Yourself experimental methodology for performing quasi-static tensile tests on common plastic items such as shopping bags using readily available equipment. The method employs dead-weight loading via known weights, simple mechanical grips, and manual strain measurement to obtain stress–strain behaviour, Young’s modulus (approximate) and yield strength. Results demonstrate that while absolute accuracy is lower than standardized ASTM or ISO methods, the approach provides reproducible trends suitable for educational purposes and comparative material analysis. Moreover, the glass transition (Tg) is illustrated by heating a polyethylene terephthalate (PET) sample under tensile load, changing its behaviour from glassy to rubbery when heated above its Tg. Finally, the elastocaloric effect in elastomers is discussed and illustrated by a simple experiment where a strip of rubber is quickly brought to the lips after being stretched and warmth is felt. These experiments allow for a tangible discovery of polymer properties, making the experience memorable.

2. Do-It-Yourself Tensile Test

Mechanical characterization of polymers is essential in materials science, product design, and sustainability studies. Tensile testing provides insight into elastic modulus, strength, ductility, and failure mechanisms. Standardized tensile testing (e.g., ISO 527 [1], ASTM D638 [2]) requires specialized equipment that may be inaccessible in home, classroom, or resource-limited settings. The objective of this work is to design and validate a simplified tensile testing experiment that can be conducted without a universal testing machine, using common household materials. The method is intended for educational use, preliminary material comparison, and sustainability-focused studies rather than high-precision certification testing.

In a tensile test a specimen is deformed until fracture, with a gradually increasing tensile load that is applied uniaxially along the long axis of the specimen. In this example we used a standardized tensile specimen, namely ASTM D638 type 2 specimen which is depicted in the Supplementary Information. This “dogbone” shape of the specimen assures that during testing, deformation is confined to the slender section and also to reduce the likelihood of fracture at the edges of the grips. Along the length of the slender section of the specimen the cross-section is uniform, therefore the engineering stress σ can be calculated according to:

$$\sigma = \frac{F}{A_0} = \frac{m \times a}{A_0}$$

The unit of engineering stress s is megapascals (MPa) or N/mm². F is the instantaneous load applied perpendicular to the specimen cross-section, in units of newtons (N), and A₀ is the original cross-sectional area before any load is applied (mm²). According to Newton’s Second Law (Force = mass x acceleration), we can calculate the applied forces from the known dead weights, assuming gravitational acceleration a of 9.81 m·s⁻².

The engineering strain is defined according to:

$$\varepsilon = \frac{L – L_0}{L_0} = \frac{\Delta L}{L_0}$$

In which L₀ is the original length before any load is applied and L is the instantaneous length. Sometimes the quantity L − L₀ is denoted as ΔL and is the deformation elongation or change in length at some instant, as referenced to the original length. Engineering strain is unitless, but millimetres per millimetre (mm/mm) is often used. Here, strain is expressed as a percentage, in which the strain value is multiplied by 100%.

Moreover, the tensile modulus E, more commonly known as stiffness, can be calculated using the following expression:

$$E = \frac{\sigma_2 – \sigma_1}{\varepsilon_2 – \varepsilon_1}$$

Where E is the tensile modulus of elasticity, in MPa, s₁ is the stress measured at a deformation e₁ and s₂ is the stress measured at a deformation e₂. According to ISO 527, typical values for e₁ and e₂ are 0.0005 mm/mm and 0.0025 mm/mm, respectively.

A schematic tensile stress­-strain curve for a semicrystalline polymer is shown in Figure 1. It typically features an initial linear-elastic region, a non-linear yield point, followed by plastic deformation, necking, and eventual failure. Unlike metals, polymers often exhibit significant, time-dependent elongation and, depending on the polymer, may exhibit either ductile, brittle, or elastomeric behaviour. The key regions of the stress-strain curve for plastics are:

• Linear elastic region: The initial, straight portion where the plastic deforms elastically; if load is removed, the material returns to its original shape. The slope represents the Young’s modulus.
• Yield strength s_y: The point where the curve deviates from linearity, indicating the onset of permanent, plastic deformation.
• Plastic deformation region: After yielding, the material undergoes molecular rearrangement. It may undergo cold drawing, where the plastic extends significantly at a roughly constant stress level.
• Ultimate tensile strength (UTS): The maximum stress the material can withstand.
• Necking and failure: The cross-sectional area decreases significantly (necking) before the material finally breaks at the fracture point.

Table 1. Experimental results of DIY tensile test 1.

Table 2. Results of yield strength and Young’s modulus

Table 3. Tensile properties determined with a universal testing machine.

3. Glass Transition

The glass transition temperature is denoted as:

$$T_g$$

4. Elastocaloric Effect

Temperature changes of up to 30°C can be observed during deformation and decreases of up to 13°C upon release.

5. CONCLUSIONS

We have presented three educational experiences that allow for the accessible and sensory introduction of key concepts in polymer science: basic mechanical properties such as the Young’s modulus and yield strength, the glass transition and the elastocaloric effect.

6. REFERENCES

[1] International Organization for Standardization. (2019). ISO 527-1:2019 Plastics — Determination of tensile properties — Part 1: General principles.
[2] ASTM D638-14, “Standard Test Method for Tensile Properties of Plastics,” ASTM International, West Conshohocken, PA, 2014.
[3] Callister, W. D., Jr., & Rethwisch, D. G. (2018). Materials science and engineering: An introduction (10th ed.). Wiley.
[4] M. C. Shen and A. Eisenberg, ‘Glass transitions in polymers’, Progress in Solid State Chemistry, vol. 3, pp. 407–481, Jan. 1967.
[5] J. P. Joule, ‘On some thermo-dynamic properties of solids’.
[6] HIKMICRO Viewer Software.