La transizione vetrosa è una delle più importanti proprietà chimico-fisiche dei materiali amorfi e semi-cristallini, come, ad esempio, vetri, metalli (amorfi), polimeri, ingredienti farmaceutici e alimentari e definisce l’intervallo di temperatura in cui le proprietà meccaniche die materiali variano da duro e fragile a più morbido, malleabile o gommoso. Molti polimeri, come, termoplastici, termoindurenti, elastomeri, presentano generalmente parti con strutture amorfe e parti con strutture cristalline. In questi materiali ritroveremo sia la temperatura di transizione vetrosa (Tg) che la temperatura di fusione. LaTg è solitamente più bassa della temperatura di fusione dello stesso materiale cristallino.
- Determinazione della temperatura di transizione vetrosa con la Calorimetria Differenziale a Scansione (DSC)
La temperatura di transizione vetrosa, Tg, di un materiale caratterizza l’intervallo di temperature all’interno del quale avviene la transizione vetrosa. Risulta essere sempre inferiore alla temperatura di fusione del materiale cristallino (se esiste). In questo range di temperature i polimeri si trasformano da materiali duri e rigidi a più morbidi e flessibili. Alla Tg si verifica un incremento notevole della mobilità delle catene polimeriche. I materiali termoindurenti, come il polistirene (PS) e il poli(metilmetacrilato) (PMMA) sono solitamente utilizzati al di sotto delle loro Tg, quindi nel loro stato „vetroso“; mentre gli elastomeri, come il poli-isoprene e il polibutadiene (BR) vengono utilizzati al di sopra della loro Tg, ed appaiono morbidi e flessibili.
Determination of the glass transition temperature is a tool for material identification. The glass transition temperature (Tg) also determines the application field of a material. For example, a rubber tire (car) is soft and ductile because at normal operating temperatures it is well above its glass transition temperature. If its glass transition temperature were higher than its operating temperature, it would not have the flexibility needed to grip the pavement.
Other polymers operate below their glass transition temperature, e.g., a stiff plastic handle. If the plastic handle had a glass transition temperature below its operating temperature, it would be too flexible.
In DSC measurements, the glass transition can be observed by a step in the baseline of the measurement curve (Fig.1). It is characterized by its onset, midpoint, inflection and endset temperature. The step height corresponds to Δcp and is given in J/(g⋅K). The evaluation procedure is described in e.g., ASTM E1356-08. DSC can be used for solids, powders and liquids.
Sorbitol is used as a sugar substitute in many sweets, diet products and medications. A proportion of 10% water in sorbitol effectuates a decrease in the glass transition temperature of approx. 24 K (mid temperatures) relative to anhydrous sorbitol. Both samples remain completely amorphous after the rapid cooling from the molten state (which took place prior to the displayed heating step).
The measurements were carried at a heating rate of 10 K/min in nitrogen atmosphere. The sealed sample pans made of aluminum were closed with a pierced lid. The sample masses amounted to approximately 12 mg ± 1mg.
The DMA technique (e.g., ASTM E1640-09) is a very sensitive technique for the determination of the glass transition temperature (e.g., 1640-94). It provides an alternative procedure in the determination of the glass transition to the use of differential scanning calorimetry (DSC) (ISO 11357‑2). In DMA measurements, Tg can be observed in the extrapolated onset of the sigmoidal change in the storage modulus E’, the peak of the loss modulus E’’ and the peak of tanδ.
DMA can be used for unreinforced and filled polymers, foams, rubbers, adhesives and fiber-reinforced plastics/ composites. Different modes (e.g. flexure, compression, tension) of dynamic mechanical analysis can be applied, as appropriate, to the form of the source material.
Dynamic Mechanical Analysis (DMA) records a material’s temperature-dependent visco-elastic properties (stiffness, E’ and loss modulus, E’’, measure for the oscillation energy), and determines its modulus of elasticity and damping values (tanδ) by applying an oscillating force to the sample.
The glass transition temperature, Tg, of a hydrogenated acrylonitrile butadiene rubber (HNBR) was determined in tension mode by means of dynamic mechanical analysis, DMA. The measurement was performed at a heating rate of 2 K/min, a frequency of 1 Hz and an amplitude of ±20µm in the temperature range between -90°C and 40°C. The extrapolated onset determined in the storage modulus E’, the peak in the loss modulus E’’ and the peak in the tanδ curve all correspond to the glass transition temperature, Tg, of this rubber material (through application of the respective evaluation conventions).
In dilatometer (DIL) and thermomechanical analyzers (TMA, both described in ASTM E 473 – 11a), the glass transition corresponds to the inflection in the dimensional change (e.g., ASTM E1545. It is recorded as the extrapolated onset of kink in the experimental DIL/TMA curve and displayed as a function of temperature. To make this definition reproducible, one should specify the cooling or heating rate. E.g., ASTM E1545 describes the determination of the glass transition by means of TMA.
DIL measurement on a natural rubber material between -120°C and 20°C at a heating rate of 3 K/min in helium atmosphere. the sample length amounted to 2 mm. The extrapolated onset temperature of -62°C corresponds to the glass transition (Tg). In amorphous materials such as rubber, it is a reversible transition. The material changes from a hard and relatively brittle state into a soft or rubbery state.