Thermal Behaviors of Biomedical Materials

Introduction

Thermoanalytical techniques play a crucial role in characterizing the properties of materials as they undergo temperature changes. The analysis of these properties is essential for assessing the suitability of various materials for practical applications. In this experiment, we employ two thermoanalytical techniques, Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), to examine the properties of polymers.

In dynamic TGA, a sample is heated over a range of temperatures while its weight is continuously recorded. At higher temperatures, the sample may undergo thermal decomposition and react with the surrounding atmosphere, resulting in a decrease in its weight.

The data collected by the TGA can be presented as a thermogravimetric (TGA) curve, which provides information about the temperature at which the material begins to lose weight, or as a differential thermogravimetric (DTG) curve, which indicates the rate of mass loss with respect to temperature. These pieces of information are critical for characterizing the thermal stability of different materials (_{MLE Lab Manual, 2019}).

In DSC, a sample and a thermally inert reference material are simultaneously heated, and the heat flow into the materials is recorded.

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To ensure both materials experience the same temperature increase, varying amounts of heat flow between them are required. This difference in energy input is measured as a function of temperature, resulting in a DSC curve. During the heating process, the sample undergoes physical transformations such as endothermic or exothermic phase transitions, which require varying amounts of energy from the system to maintain the same temperature change. These transformations are reflected in the DSC curve as deviations from the baseline.

Examples of such transitions include glass transition, crystallization, and melting (_{MLE Lab Manual, 2019}).

The glass transition is a second-order transition during which an amorphous or semi-crystalline material transitions from a hard and brittle state to a soft and rubbery state. This transition involves a significant change in mechanical properties and heat capacity. DSC exploits this change in heat capacity to determine the glass transition temperature, denoted as T_g, at which the glass transition occurs (_{Hutchinson, 2009}).

Another important physical property of a polymer is its degree of crystallinity. Directly measuring the degree of crystallinity enables the prediction of fundamental physical properties of a material. DSC allows for the determination and analysis of the heat required to melt the crystalline portion of a polymer, which in turn provides the degree of crystallinity. The objective of this experiment is to utilize TGA and DSC techniques to examine various physical and chemical properties of different polymer samples (_{MLE Lab Manual, 2019}).

Methodology

For TGA, a clean pan was first tared within the TGA system (a TA Instruments TGA Q500 was used). Subsequently, a sample of 0.0158g of Poly(lactic-co-glycolic acid) (PLGA) was added to the pan. The loaded pan was then placed back into the TGA system, as illustrated in Figure 1a. The TGA system was set to operate at a heating rate of 20°C/min within a temperature range of 30°C to 650°C, with a flowing air rate of 60ml/min and a nitrogen flow rate of 40ml/min. The thermogravimetric (TG) and Differential Thermogravimetric (DTG) curves were obtained by subjecting the sample to the specified temperature range. This procedure was repeated for 0.0149g of Polytetrafluoroethylene (PTFE).

For DSC, samples of polymethylmethacrylate (PMMA), low-density polyethylene (LDPE), and high-density polyethylene (HDPE) were examined. All samples were crimped in a tared sample pan and weighed. The initial mass of the samples was 7.4mg for PMMA, 5.8mg for LDPE, and 5.1mg for HDPE, respectively. These samples were placed into a TA Instruments DSC 25 System along with an empty pan serving as a reference, as depicted in Figure 1b. For PMMA, the heating rate was set at 10°C/min, with a nitrogen flow rate of 50ml/min. An initial heating run from 20°C to 160°C was performed to eliminate any pre-existing thermal history of the sample. Subsequently, the sample was cooled back to room temperature, and a second heating run was conducted, during which the heat flow into the sample was recorded as a function of temperature across the same temperature range. For HDPE and LDPE, the heating rate was set at 10°C/min within a temperature range of -50°C to 160°C, with a nitrogen flow rate of 50ml/min to obtain DSC curves for these samples.

Results

TGA Results

The procedural decomposition temperature (PDT), which marks the onset of deviation from the baseline weight/temperature behavior, as well as the temperatures where 30% (T30) and 50% (T50) of the weight of the polymer was lost, are summarized in Table 1 below. Notably, the PTFE sample exhibited superior performance compared to the PLGA sample, with higher PDT, T30, and T50 values.

DSC Results

PMMA Glass Transition Temperature Results

A distinct 'downward' step change in the curve is observed between 105.53°C and 116.42°C, indicating an increase in the heat capacity of the sample. This increase in heat capacity corresponds to the glass transition of the polymer, which was determined to occur at approximately 111.03°C. The onset and endpoint of the glass transition were observed at 105.53°C and 116.42°C, respectively. These values fall within the literature's Tg range of 85°C to 165°C, with a Tg value of 105°C for atactic PMMA (_{Ashby, 2017}).

LDPE and HDPE

The DSC curves resulting from the controlled heating of LDPE and HDPE show that the endothermic peaks in both graphs correspond to the melting of the polymer, with the enthalpy change of this phase transition calculated using the formula: ΔH = AK/m, where A is the area of the DSC curve, m is the sample mass, and K is a calibration constant (_{MLE Lab Manual, 2019}). The enthalpy change of melting, as indicated in Figure 4, is 137.4 J/g for LDPE and 174.1 J/g for HDPE.

The degree of crystallinity is calculated by determining the ratio between the heat required to melt the crystalline portion of the sample and the heat needed to melt a 100% crystalline sample of the same polymer. These calculations are summarized in Table 2, revealing that HDPE exhibits a higher degree of crystallinity than LDPE.

Discussion

TGA Analysis of the Thermal Stability of PTFE vs. PLGA

Higher values of PDT, T30, and T50 indicate that a material requires a higher temperature before undergoing thermal decomposition, indicating higher thermal stability. PTFE demonstrated superior thermal stability compared to PLGA, as evidenced by its higher PDT (538.98°C), T30, and T50 values.

The variation in thermal stability observed between these polymers can be attributed to differences in their chemical structure and properties. The chemically inert C-F bonds in PTFE do not readily react with the atmosphere, even at high temperatures, as demonstrated in our experiment. In contrast, PLGA contains more reactive carboxyl groups, making it more susceptible to reactions with the atmosphere, leading to lower thermal stability. It is essential to note that while the material may begin to decompose around PDT, other physical transitions may occur at lower temperatures, significantly altering the material's properties (_{Beyler & Hirschler, 2002}). For instance, the glass transition temperature sets an upper limit for the temperature at which a polymer can be used effectively, as mechanical properties change above this temperature, rendering it unsuitable for certain applications.

Determination of the Tg of PMMA through DSC

The glass transition is a second-order transition observed in amorphous and semi-crystalline materials. Above the glass transition temperature range, polymer chains gain mobility, leading to soft and rubbery mechanical properties, while below this range, a 'glassy state' with hindered molecular rearrangements is formed. The increase in mobility above the glass transition temperature results in a higher specific heat capacity, which is indicative of the glass transition process. The glass transition temperature (Tg) is determined from a DSC curve.

It is important to note that the glass transition does not occur at a single temperature but over a range of temperatures. However, conventionally, a single value is reported as the glass transition temperature (_{Epotek, 2012}). To determine these values precisely, tangent lines are drawn at the slopes of the baselines before and after the glass transition. Another tangent is drawn at the slope of the step change associated with the glass transition, marked as line 2. The intersection of tangent lines 1 and 3 with tangent line 2 provides unambiguous values for the onset and endpoint of the glass transition, while the midpoint of tangent line 2 is considered the glass transition temperature.

Structural Features and Resulting Degree of Crystallinity of HDPE and LDPE

In polyethylene, polymer chains can align themselves in well-ordered, closely packed arrangements known as spherulites. These regions coexist with amorphous regions in a semi-crystalline structure. A higher degree of crystallinity, representing the percentage of crystalline material, contributes to greater overall density and impacts various polymer properties, including modulus, stiffness, and permeability (_{Callister, 2013}). As shown in section 3.2.2, HDPE displayed a significantly higher degree of crystallinity, around 60%, compared to LDPE at 47%. These differences can be explained by examining the chemical structure of the two polymers.

LDPE possesses a higher degree of branching in its polymer chains compared to HDPE. These branches hinder the crystallization process, preventing chains from stacking neatly and compactly. Moreover, the branches reduce interchain forces of attraction, leading to a less dense molecular arrangement. Consequently, LDPE exhibits a lower degree of crystallinity, as polymer chains occupy more space. Conversely, HDPE features linear chains with fewer branches, resulting in less hindrance to the formation of crystalline regions. With stronger interchain forces of attraction and fewer side chains disrupting crystallization, HDPE achieves a higher degree of crystallinity, leading to a less spacious molecular arrangement and, ultimately, a lower density.

Error and Data Analysis

While the experiment's results generally aligned with accepted literature values, it is important to acknowledge potential sources of error in both the TGA and DSC experiments. These sources of error can be attributed to the wide range of literature values, which may vary due to differences in the physical and chemical properties of polymer samples based on their manufacturing process and application history.

Systematic errors in the TGA and DSC experiments might have arisen from calibration inaccuracies, including errors in balance, furnace calibration, and thermocouple settings. Furnace cleanliness could also impact results, although this possibility was minimized through thorough furnace cleaning prior to the experiment. Additionally, variations in operating variables, such as heating rates and sample atmospheres, can lead to different PDT values for PLGA and PTFE. This is because the thermal decomposition of polymers is influenced by heating rates and the reactivity of the surrounding atmosphere (_{Beyler & Hirschler, 2002}).

Conclusion

Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) are valuable techniques for characterizing how a polymer's physical properties change with temperature. TGA provides insights into a polymer's thermal stability, while DSC enables the determination of the glass transition temperature and degree of crystallinity in different polymers. These properties play a crucial role in assessing the suitability of various polymers for a wide range of applications.

References

• Ashby, M. F. (2017). Materials selection in mechanical design. Amsterdam: Butterworth-Heinemann.
• Beyler, C. L., & Hirschler, M. M. (2002). Thermal Decomposition of Polymers. In SFPE handbook of fire protection engineering (3rd ed.). Quincy, MA: National Fire Protection Association.
• Callister, W.D. (2013). Materials Science and Engineering: An Introduction 9th Edition. Salt Lake City, Utah, United States of America: Wiley.
• Epotek. (2012). Tech 23 Tip T - Glass Transition Temperature for Epoxies. Retrieved February 28, 2019, from http://www.epotek.com/site/files/Techtips/pdfs/tip23.pdf.
• Hutchinson, J. (2009). Determination of the glass transition temperature. Journal of Thermal Analysis and Calorimetry, 98(3), 579-589. doi: 10.1007/s10973-009-0268-0.
• MLE3101 MLE Lab Manual (2019); Experiment H: Thermal Behaviors of Biomedical Materials. National University of Singapore, Materials Science and Engineering, Singapore.