Dynamic Mechanical Analysis Under Controlled Conditions of Temperature and Relative Humidity

Historically, dynamic mechanical analysis (DMA) measurements have focused on the analysis of materials as a function of an imposed temperature profile. However, most consumer products are subject to a variety of both temperature and relative humidity (RH) conditions during processing, storage, and end use. It is thus desirable to probe the combination of temperature and RH on the mechanical properties of materials to fully understand the environmental effects on structure and stability. DMA instruments often employ an inert purge to aid in temperature control and minimize thermal gradients. This purge effectively reduces the chamber relative humidity to very low levels, complicating the investigation of the effects of moisture or humid atmospheres on the mechanical behavior of materials. 

Figure 1 - Q800 DMA and DMA-RH accessory.

The DMA-RH accessory, for use with the Q800 dynamic mechanical analyzer (Figure 1) (TA Instruments, New Castle, DE) is an integrated unit that includes the following major hardware components:

Figure 2 - Operating range for the DMA-RH accessory.

Table 1    -    DMA-RH accessory operating specifications
  • The sample chamber mounts on the DMA in place of the standard furnace and completely encloses the sample. Peltier elements control the temperature to within ±0.1 °C. The DMA-RH sample chamber accommodates standard DMA clamps such as tension, cantilever, and three-point bending. The sample chamber is quickly and easily removed, allowing for rapid conversion back to the standard DMA furnace configuration.
  • A heated vapor transfer line is maintained at a temperature above the dewpoint temperature of the purge gas in order to avoid condensation.
  • The DMA-RH accessory cabinet contains the humidifier and associated electronics, which precisely controls temperature and humidity of the sample environment. The Q800 DMA communicates directly with the accessory and automatically controls all subsystems that are associated with generating and maintaining both temperature and humidity using Thermal Advantage instrument software.

The DMA-RH accessory provides a wide range of combined temperature and relative humidity, as shown in Figure 2. Table 1 provides the operating specifications. Operating modes include: isothermal/isohume, isothermal/step RH, isothermal/ramp RH, isohume/step temperature, isohume/ramp temperature, and step temperature/step RH. All of these operating modes are fully programmable using the Thermal Advantage software and associated method editor utility.

Results and discussion

Analysis of Nylon 6

Figure 3 - Isohume DMA-RH analysis of Nylon 6.

Nylon 6 is strongly plasticized by water; as such the mechanical properties of this polymer will be dependent on the surrounding relative humidity.1 The data in Figure 3 illustrate the effect of relative humidity on the glass transitions of Nylon 6 as measured on the Q800 DMA equipped with the DMA-RH accessory. In this experiment, a sample was analyzed in single cantilever mode at a frequency of 1 Hz over the Tg temperature range at a variety of constant RH conditions. As expected, the mechanical properties are influenced significantly by the imposed relative humidity. The glass transition peak in tan δ is suppressed nearly 40 °C over an imposed relative humidity range of 75% RH. 

Figure 4 - Isothermal (35 °C) DMA-RH analysis of Nylon 6.

The versatility of the DMA-RH accessory also allows for the Nylon 6 sample to be analyzed under isothermal conditions, with an applied RH ramp. The data in Figure 4 contain the results from this experiment in which the sample was held isothermal at 35 °C while the humidity was ramped at 0.2% RH/min from 5 to 95% RH. Under these conditions, the Tg is identified by the peak in tan δ at 50.6% RH. The lower Tg temperature relative to the previous plot likely indicates diffusion effects or hysteresis in the sample.

Analysis of gelatin capsule material

Gelatin is a naturally occurring material that is used in a wide array of applications, most typically in foods, and in pharmaceutical, photographic, and technical products. The use of gelatin in the manufacture of various pharmaceutical dosage forms dates back to the early nineteenth century and possibly earlier.2 When stored in an ambient, low-humidity environment, gelatin is remarkably stable. However, when combined with water, gelatin forms a semisolid colloid gel, and this can profoundly affect its mechanical properties. Particularly as it relates to storage and handling conditions, it is important to understand and quantify these effects in situ. 

Figure 5 - Relative humidity ramp of gelatin material at 25 °C and 40 °C.

The data in Figure 5 illustrate the effect of increasing relative humidity on a gelatin sample cut from the side wall of a two-piece capsule at both 25 °C and 40 °C. In each case, as the relative humidity is increased, the material undergoes a multistep transition, resulting in a significant decrease in modulus near 80% RH. The transition is resolved in both the modulus and tan δ signals. Pharmaceutical materials are often stored in bathrooms, which are typically the hottest and most humid locations in a dwelling. The combinations of temperature and RH can easily achieve those shown in the example, which could result in storage and stability concerns. These data suggest that even at ambient temperature, the storage conditions for gelatin capsules should be maintained below 70% RH to ensure mechanical integrity of the material.